CA3219519A1

CA3219519A1 - System for electrocatalytic conversion of carbon oxides to multicarbon products using a stationary catholyte layer and related process

Info

Publication number: CA3219519A1
Application number: CA3219519A
Authority: CA
Inventors: David Sinton; Edward Sargent; Ke XIE; Rui Kai MIAO
Original assignee: University of Toronto; TotalEnergies Onetech SAS
Current assignee: University of Toronto; TotalEnergies Onetech SAS
Priority date: 2021-06-15
Filing date: 2022-06-10
Publication date: 2022-12-22
Also published as: EP4355932A1; WO2022263316A1

Abstract

An electroreduction system for converting carbon oxides selected from CO, CO2 or any mixture thereof into multicarbon (C2+) products, the system comprising a cathodic compartment having a reactant inlet and comprising a cathode, the cathode comprising a catalyst layer that is contactable with a catholyte solution; an anodic compartment having a product outlet to release the C2+ products, the anodic compartment comprising an anode and being configured to accommodate a flowing anolyte solution; and a bipolar membrane being positioned between the cathodic compartment and the anodic compartment, the bipolar membrane comprising an interfacial layer defined between a cation-exchange layer and a anion-exchange layer; wherein the cathodic compartment is configured to accommodate a stationary catholyte layer between the catalyst layer of the cathode and the CEL, the stationary catholyte layer comprising the catholyte solution..

Description

SYSTEM FOR ELECTROCATALYTIC CONVERSION OF CARBON OXIDES TO
MULTICARBON PRODUCTS USING A STATIONARY CATHOLYTE LAYER AND
RELATED PROCESS
TECHNICAL FIELD
The technical field generally relates to electrocatalytic conversion in electrolyzers, and more particularly to electroreduction process and system combining a bipolar membrane and a stationary catholyte layer facilitating efficient conversion of CO2 and/or CO into multicarbon (C2 ) products.
BACKGROUND
Gigatons of CO2 need to be avoided or removed from the atmosphere each year (see reference 1). When powered by renewable energy sources, the fixation of captured carbon via electrochemical reduction of CO2 (CO2RR) offers a route to net-negative-emission production of multi-carbon (02+) chemicals (see reference 2). However, CO2RR
in electrolyzers operating both with alkaline and neutral electrolytes incur significant CO2 loss to carbonate formation and crossover, leading to low CO2 utilization.
The industrial implementation of the CO2RR for 02+ production requires the simultaneous achievement of high production rates, high energy efficiencies, and high carbon efficiencies (see references 3 and 4). Known CO2RR electrolyzers based on alkaline bulk electrolytes (e.g. alkaline flow cell, Figure 1A) have achieved 02+ partial current densities greater than 1 A cm-2 with Faradeic Efficiency (FE) towards 02+ products exceeding 70%
(see references 2, 4 and 5). For example, zero-gap electrolyzers based on alkaline or neutral bulk electrolytes (e.g. membrane electrode assembly (MEA, Figure 1B) can deliver about 100 mA cm-2 with total C2+ FEs of more than 70% (see references 3, 4 and 6).
For CO2RR, a catholyte having a high local pH (>12) near the cathode can be used to favour the CO2RR reaction with respect to the competing hydrogen evolution reaction (HER), to enhance the selectivity towards C2+ products(see references 2, 5 and 7 to 9).
To maintain such a high local pH, many present-day CO2RR electrolyzers use a flowing alkaline electrolyte/catholyte reservoir (see references 2, 4 and 5). For the same reason,

2 MEAs typically use strong alkaline anion-exchange membranes (AEM) and anolytes(see reference 18). However, locally alkaline conditions absorb CO2 to form carbonates:
CO2 + 20H- ¨> 0032- + H20 [1]
CO2 + OH- ¨> H003- [2]
Thus, at a steady-state, CO2 reacts with hydroxides ions to form carbonate or bicarbonate ions, and both reactant and electrolyte are lost, corresponding to low carbon efficiency in the utilization of CO2 feedstock. Recovering CO2 from carbonate/bicarbonate can consume as much as 60% to 70% of the energy input (see references 11 and 12).
Certain flow cells and MEAs have been designed to use neutral electrolytes (e.g., KHCO3) rather than strong alkaline ones in order to reduce CO2 absorption. Neutral media flow cells and MEAs have lower CO2 absorption than do alkaline cells, and yet, since the reaction drives up the local pH and creates locally alkaline conditions, carbonate and bicarbonate formation remain a problem (see reference 11).
Carbonate/bicarbonate ions migrate to the anode via the AEM, to combine with protons provided from the anode oxygen evolution reaction, thereby releasing CO2 into the anode gas stream (5I1). To date, a single pass CO2 utilization (SPU, the fraction of the CO2 feed been transformed to products) of 02+ producing electrolyzers has remained in the range 3% to 30%
(Table Si) (see references 6, 11, 13 to 16).
SUMMARY
The present techniques relate to carbon oxides-to-C2+ electrochemical reduction strategies that overcome previously-observed limits of carbon efficiency by designing an electroreduction system that inhibits carbon oxides crossover from cathode to anode and reverts formed carbonate/bicarbonate ions to carbon oxides via acidification of the catholyte. Carbon oxides as encompassed herein are selected from selected from CO, CO2 or any mixture thereof.
More particularly, in a first aspect, there is provided an electroreduction system for converting carbon oxides into multicarbon (02+) products; the system comprising:
- a cathodic compartment having a reactant inlet for receiving CO, CO2 or any mixture thereof, and comprising a cathode, the cathode comprising a catalyst layer that is in contact with a catholyte solution:

3 an anodic compartment, the anodic compartment comprising an anode and accommodating a flowing anolyte solution;
a bipolar membrane being positioned between the cathodic compartment and the anodic compartment, the bipolar membrane comprising:
a cation-exchange layer (CEL) in cation communication with the catholyte solution to provide protons into the catholyte solution;
an anion-exchange layer (AEL) in anion communication with the anolyte solution to provide hydroxide ions at a surface of the anode; and an interfacial layer defined between the cation-exchange layer and the anion-exchange layer for splitting water into the protons and the hydroxide ions;
wherein the cathodic compartment and/or the anodic compartment have a product outlet to release the C2+ products;
remarkable in that the cathodic compartment accommodates a stationary catholyte layer between the catalyst layer of the cathode and the CEL, the stationary catholyte layer comprising the catholyte solution; and in that the thickness of the stationary catholyte layer is at most 280 pm as measured by a spiral micrometer.
For example, the thickness of the stationary catholyte layer can be between 20 pm and 250 pm as measured by a spiral micrometer; preferably between 40 pm and 200 pm; and more preferably between 50 pm and 150 pm; and even more preferably between 65 and 125 pm. The solid porous support can be sandwiched between the catalyst layer of the cathode and the CEL for direct contact therewith.
In some implementations, the cathodic compartment further comprises a solid porous support in between the CEL and the catalyst layer, the solid porous support being configured to be saturated with the catholyte solution to form the stationary catholyte layer.
In an embodiment, the solid porous support can include polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polycarbonate, nylon, cellulose acetate, cellulose nitrate, polypropylene, alumina, or any combinations thereof.
In an embodiment, the solid porous support can have a mean pore diameter (i.e., a mean pore size) between 0.05 and 50 pm as determined by scanning electron microscopy (SEM), optionally between 0.2 and 1 pm, and further optionally of 0.45 pm.

4 In an embodiment, the stationary catholyte layer can have a liquid content between 5 and 50 pLcm-2 preferably between 6 and 40 pL.cm-2; more preferably between 8 and pL.cm-2; even more preferably between 8 and 30 pL.cm-2; most preferably between 10 and 25 pLcm-2; even most preferably between 10 and 20 pL.cm-2 or between 12 and 18 pL.cm-2; when the solid porous support is saturated with the catholyte solution; the liquid content being determined by weighting. For example, the stationary catholyte layer can have a liquid content about 10 pL.cm-2.
In some implementations, the stationary catholyte layer has a thickness that is selected to maximize a mass transport of regenerated CO2 or CO to the catalyst layer of the cathode while maintaining a resistance to compression of the solid porous support.
In some implementations, the catholyte solution has a concentration of cations between 0.25 M and 3 M; preferably between 0.4 M and 4 M; and more preferably between 0.5 M
and 2 M, and for example about 1M.
For example, the catholyte solution can be a solution of K2SO4 with a KE
concentration equal to or greater than 0.5M.
For example, the cations in the catholyte solution can be one or more selected from K+, Nat, Cs, Rb+, NH4, Mg2+, Ca2+, Al3+.
For example, the catholyte solution is a solution with a cations concentration equal to or greater than 0.5M.
In some implementations, the catholyte solution is a buffered solution. For example, the buffered solution can be a solution comprising one or more selected from KHCO3, K3PO4, K2HPO4, KH2PO4, the buffered solution is a mixture of glycine and sodium hydroxide, or a mixture of H3B03 and sodium hydroxide.
In some implementations, the catholyte solution is a non-buffered solution.
For example, the non-buffered solution can be or comprise K2SO4, KCI or any other combinations of the Cl- anions or S042- anions with Nat, Cs, Rb+, NH4, Mg2+, Ca2+, or Al3+
cations.
The anolyte solution can have an anolyte concentration between 2.0 M and 0.01M, optionally about 0.1 M. In some implementations, the anolyte solution is a neutral solution.
For example, the anolyte solution can have a pH between 7 and 10. In the context of the invention the anolyte solution is a neutral solution hen having a pH between 7 and 10. The anolyte (neutral) solution can be a KHCO3, K2804, or K2HPO4 solution.

In some implementations, the anolyte solution is an acidic solution. For example, the anolyte solution has a pH between 1 and 4. For example, the anolyte solution has a bulk pH between 1 and 4. The acidic solution can be a H3PO4 solution, H2804 solution or a combination thereof.

5 In some implementations, the catalyst layer of the cathode comprises copper (Cu), silver (Ag), platinum (Pt), carbon (C), or any combination thereof.
In some implementations, the cathode further comprises a gas diffusion layer for contacting the stream of CO, CO2 or any mixture thereof, and the catalyst layer is deposited onto the gas diffusion layer. For example, the gas diffusion layer can be a hydrophobic carbon paper, or a copper sputtered hydrophobic PTFE layer. In the context of the invention hydrophobic means a water contact angle following ISO 19403-

6:2017 of at least 30 .
In some implementations, the anode comprises an anodic catalyst layer and an anodic current collector layer. For example, the anodic catalyst layer can include one or more selected from 1r02, Pt, Pd, Ni, Ni0x, CoOx. For example, the anodic current collector layer can include Ti felt, hydrophilic carbon paper, or Ni foam. In the context of the invention hydrophilic means a water contact angle following ISO 19403-6:2017 below 30 .
In some implementations, the interfacial layer of the bipolar membrane comprises a water dissociation catalyst. The water dissociation catalyst can be present as nanoparticles. The water dissociation catalyst can comprise one or more selected from TiO2, 1r02, NiO, Sn02, graphene oxide, CoOx, ZrO2, A1203, Fe(OH)3, Mn02, Ru, Rh, RuPt alloy, PtIr alloy, Ir, Pt.
In some implementations, the AEL is a membrane comprising poly(aryl piperidinium), polystyrene methyl methylimidazolium, or polystyrene tetramethyl methylimidazolium. The GEL can comprise or consist of a sulfonated tetrafluoroethylene based fluoropolymer-copolymer.
In some implementations, the system can include a temperature controller configured to maintain an operating temperature between 20 C and 50 C, optionally about 35 C.
A single-pass utilization of the stream of CO, CO2 or any mixture thereof can be of at least 50% for an inlet flowrate between 1 sccm and 15 sccm. The single-pass utilization of the stream of CO, CO2 or any mixture thereof can be of at least 60% for an inlet flowrate between 1 sccm and 8 sccm. The system can have a Faradeic Efficiency (FE) for conversion into the C2, products of at least 20% during at least 20 hours of operation and under an applied current density between 100 and 400 mA.cm-2. For example, the FE for conversion into the 02+ products can be of at least 25% during at 30 hours of operation and the applied current density of 350 mA.cm-2.
In another aspect, there is provided a carbon oxides electroreduction process for converting CO, CO2 or any mixture thereof into C2, products. The process includes:
supplying a catholyte solution and CO, CO2 or any mixture thereof to a cathodic compartment comprising a catalyst layer in contact with the catholyte solution:
flowing an anolyte solution through an anodic compartment having a product outlet to release the C2, products, the anodic compartment comprising an anode;
providing a bipolar membrane between the cathodic compartment and the anodic compartment, the bipolar membrane comprising:
a cation-exchange layer (CEL) in cation communication with the catholyte solution to provide protons into the catholyte solution;
an anion-exchange layer (AEL) in anion communication with the anolyte solution to provide hydroxide ions into the anolyte solution; and an interfacial layer defined between the cation-exchange layer and the anion-exchange layer for splitting water into the protons and the hydroxide ions; and retaining a portion of the catholyte solution as a stationary catholyte layer between the catalyst layer of the cathode and the CEL and in contact with the CEL
wherein the thickness of the stationary catholyte layer is at most 280 pm as measured by a spiral micrometer.
In some implementations, the process can include maintaining an operating temperature between 20 C and 50 C, and optionally about 35 C.
In some implementations, supplying CO, CO2 or any mixture thereof to the cathodic compartment is performed at an inlet flowrate between 1 sccm and 15 sccm.
In some implementations, the process can include providing the cathode with an applied current density between 100 and 400 mA.cm-2

7 In some implementations, the process can include forming the stationary catholyte layer by providing a solid porous support between the cathode and the CEL, and saturating the solid porous support with the catholyte solution. For example, the saturating can be performed to reach a liquid content of the stationary catholyte layer between 5 and 50 pL.cm-2, optionally between 10 and 20 pL.cm-2, and further optionally about 10 pL.cm-2 when the solid porous support is saturated with the catholyte solution.
In some implementations, the catholyte solution can be supplied with a concentration of cations between 0.25 M and 3 M, and optionally between 0.5 M and 2 M, and further optionally about 1M.
In some implementations, the stationary catholyte layer can be formed with a thickness between 20 pm and 250 pm as measured by a spiral micrometer; preferably between 40 pm and 200 pm, more preferably between 50 pm and 150 pm.
In some implementations, the process can include utilizing CO, CO2 or any mixture thereof according to a single-pass utilization of the stream of CO, CO2 or any mixture thereof of at least 50% for an inlet flowrate between 1 sccm and 15 sccm. For example, the process can include utilizing CO, CO2 or any mixture thereof according to a single-pass utilization of the stream of CO, CO2 or any mixture thereof of at least 60% for an inlet flowrate between 1 sccm and 8 sccm.
In some implementations, the process can include producing the C2, products according to a Faradeic Efficiency (FE) for conversion into the C2, products that is of at least 20%
during at least 20 hours of operation and under an applied current density between 100 and 400 mA.cm-2. For example, the process can include producing the 02+
products according to the FE for conversion into the C2, products that is of at least 25% during at hours of operation and the applied current density of 350 mA.cm-2.
25 It should be noted that the process can include using the system according to all implementations as defined herein.
The inventors have thus discovered that a cation effect can be allowed at the cathode surface to enable carbon oxide reduction in the acidified catholyte solution.
For example, the electroreduction system includes a bipolar membrane and a stationary catholyte layer 30 that maintains a catholyte solution within a cathodic compartment, to facilitate the participation of regenerated CO2 in CO2RR reactions. The presently designed

8 electroreduction system showed a single-pass CO2 utilization of more than 60%, representing twice the previously reported state-of-art designs that produced C2+. Owing to its high single-pass CO2 utilization (SPU), the presently proposed electroreduction system minimizes the energy input associated with CO2 recovery, while enabling comparable performance and stability to the benchmark alkaline and neutral media AEM-based flow cell or MEA electrolyzers.
In an embodiment 1, the invention provides an electroreduction system for converting carbon oxides selected from CO, CO2 or any mixture thereof into multicarbon (C2+) products, the system comprising:
a cathodic compartment having a reactant inlet for receiving a stream of CO, or any mixture thereof and comprising a cathode, the cathode comprising a catalyst layer that is contactable with a catholyte solution:
an anodic compartment having a product outlet to release the C2, products, the anodic compartment comprising an anode and being configured to accommodate a flowing anolyte solution;
a bipolar membrane being positioned between the cathodic compartment and the anodic compartment, the bipolar membrane comprising:
a cation-exchange layer (CEL) in cation communication with the catholyte solution to provide protons into the catholyte solution;
an anion-exchange layer (AEL) in anion communication with the anolyte solution to provide hydroxide ions at a surface of the anode; and an interfacial layer defined between the cation-exchange layer and the anion-exchange layer for splitting water into the protons and the hydroxide ions;
wherein the cathodic compartment is configured to accommodate a stationary catholyte layer between the catalyst layer of the cathode and the CEL, the stationary catholyte layer comprising the catholyte solution.
In an embodiment 2, the system according to embodiment 1, wherein the cathodic compartment further comprises a solid porous support in between the CEL and the catalyst layer, the solid porous support being configured to be saturated with the catholyte solution to form the stationary catholyte layer.

9 In an embodiment 3, the system according to embodiment 2, wherein the solid porous support comprises polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polycarbonate, nylon, cellulose acetate, cellulose nitrate, polypropylene, alumina, or any combinations thereof.
In an embodiment 4, the system according to embodiment 2 or 3, wherein the solid porous support has a mean pore size between 0.05 and 50 pm, optionally between 0.2 and 1 pm, and further optionally of 0.45 pm.
In an embodiment 5, the system according to any one of embodiments 2 to 4, wherein the stationary catholyte layer has a liquid content between 5 and 50 pL.cm-2, optionally between 10 and 20 pLcm-2, and further optionally about 10 pL.cm-2, when the solid porous support is saturated with the catholyte solution.
In an embodiment 6, the system according to any one of embodiments 2 to 5, wherein the stationary catholyte layer has a thickness that is selected to maximize a mass transport of regenerated CO2 or CO to the catalyst layer of the cathode while maintaining a resistance to compression of the solid porous support.
In an embodiment 7, the system according to embodiment 6, wherein the thickness of the stationary catholyte layer is between 20 pm and 400 pm, optionally between 20 pm and 300 pm, further optionally between 20 pm and125 pm.
In an embodiment 8, the system according to any one of embodiments 2 to 7, wherein the solid porous support is sandwiched between the catalyst layer of the cathode and the CEL
for direct contact therewith.
In an embodiment 9, the system according to any one of embodiments 1 to 8, wherein the catholyte solution has a concentration of cations between 0.25 M and 3 M, and optionally between 0.5 M and 2 M, and further optionally about 1M.
In an embodiment 10, the system according to embodiment 9, wherein the catholyte solution is a solution of K2SO4 with a K+ concentration equal to or greater than 0.5M.
In an embodiment 11, the system according to any one of embodiments 1 to 10, wherein the cations in the catholyte solution are one or more selected from K+, Nat, Cs, Rb+, NH4, Mg2+, Ca2+, Al3+.

In an embodiment 12, the system according to any one of embodiments 1 to 11, wherein the catholyte solution is a buffered solution.
In an embodiment 13, the system according to embodiment 12, wherein the buffered solution is a solution comprising one or more selected from KHCO3, K3PO4, K2HPO4, 5 KH2PO4, the buffered solution is a mixture of glycine and sodium hydroxide, or a mixture of H3B03 and sodium hydroxide.
In an embodiment 14, the system according to any one of embodiments 1 to 11, wherein the catholyte solution is a non-buffered solution.
In an embodiment 15, the system according to embodiment 14, wherein the non-buffered

10 solution is or comprises K2SO4, KCI or any other combinations of the CI-anions or S042 anions with Nat, Cs, RID, NH4, me, c_2-F, a or Al3+ cations.
In an embodiment 16, the system according to any one of embodiments 1 to 15, wherein the anolyte solution is a neutral solution.
In an embodiment 17, the system according to embodiment 16, wherein the anolyte solution has a pH between 7 and 10.
In an embodiment 18, the system according to embodiment 16 or 17, wherein the neutral solution is a KHCO3, K2SO4, or K2HPO4 solution.
In an embodiment 19, the system according to any one of embodiments 1 to 15, wherein the anolyte solution is an acidic solution.
In an embodiment 20, the system according to embodiment 19, wherein the anolyte solution has a bulk pH between 1 and 4.
In an embodiment 21, the system according to embodiment 19 or 20, wherein the acidic solution is a H3PO4 solution, H2SO4 solution or a combination thereof.
In an embodiment 22, the system according to any one of embodiments 1 to 21, wherein the anolyte solution has an anolyte concentration between 2.0 M and 0.01M, optionally about 0.1 M.
In an embodiment 23, the system according to any one of embodiments 1 to 22, wherein the catalyst layer of the cathode comprises copper (Cu), silver (Ag), platinum (Pt), carbon (C), or any combination thereof.

11 In an embodiment 24, the system according to any one of embodiments 1 to 23, wherein the cathode further comprises a gas diffusion layer for contacting the stream of CO, CO2 or any mixture thereof, and the catalyst layer is deposited onto the gas diffusion layer.
In an embodiment 25, the system according to embodiment 24, wherein the gas diffusion layer is a hydrophobic carbon paper, or a copper sputtered hydrophobic PTFE
layer.
In an embodiment 26, the, the system according to any one of embodiments 1 to 25, wherein the anode comprises an anodic catalyst layer and an anodic current collector layer.
In an embodiment 27, the system according to embodiment 26, wherein the anodic catalyst layer comprises one or more selected from 1r02, Pt, Pd, Ni, Ni0x, CoOx.
In an embodiment 28, the system according to embodiment 26 01 27, wherein the current collector layer comprises Ti felt, hydrophilic carbon paper, or Ni foam.
In an embodiment 29, the system according to any one of embodiments 1 to 28, wherein the interfacial layer of the bipolar membrane comprises a water dissociation catalyst.
In an embodiment 30, the system according to embodiment 29, wherein the water dissociation catalyst is present as nanoparticles.
In an embodiment 31, the system according to embodiment 29 or 30, wherein the water dissociation catalyst comprises one or more selected from TiO2, 1r02, NiO, Sn02, graphene oxide, CoOx, ZrO2, A1203, Fe(OH)3, Mn02, Ru, Rh, RuPt alloy, PtIr alloy, Ir, Pt.
In an embodiment 32, the system according to any one of embodiments 1 to 31, wherein the AEL is a membrane comprising poly(aryl piperidinium), polystyrene methyl methylimidazolium, or polystyrene tetramethyl methylimidazolium.
In an embodiment 33, the system according to any one of embodiments 1 to 32, wherein the CEL comprises or consists of a sulfonated tetrafluoroethylene based fluoropolymer-copolymer.
In an embodiment 34, the system according to any one of embodiments 1 to 33, further comprising a temperature controller configured to maintain an operating temperature between 20 C and 50 C, optionally about 35 C.

12 In an embodiment 35, the system according to any one of embodiments 1 to 34, having a single-pass utilization of the stream of CO, CO2 or any mixture thereof of at least 50% for an inlet flowrate between 1 sccm and 15 sccm.
In an embodiment 36, the system according to any one of embodiments 1 to 34, having a single-pass utilization of the stream of CO, CO2 or any mixture thereof of at least 60% for an inlet flowrate between 1 sccm and 8 sccm.
In an embodiment 37, the system according to any one of embodiments 1 to 36, having a Faradeic Efficiency (FE) for conversion into the C2+ products of at least 20%
during at least 20 hours of operation and under an applied current density between 100 and 400 mA.cm-2.
In an embodiment 38, the system according to embodiment 37, wherein the FE for conversion into the C2, products is of at least 25% during at 30 hours of operation and the applied current density of 350 mA.cm-2.
In an embodiment 39, the invention provides a carbon oxides electroreduction process for converting 00, 002 or any mixture thereof into 02, products, the process comprising:
supplying a catholyte solution and a stream of CO, CO2 or any mixture thereof to a cathodic compartment comprising a catalyst layer in contact with the catholyte solution:
flowing an anolyte solution through an anodic compartment having a product outlet to release the C2, products, the anodic compartment comprising an anode;
providing a bipolar membrane between the cathodic compartment and the anodic compartment, the bipolar membrane comprising:
a cation-exchange layer (CEL) in cation communication with the catholyte solution to provide protons into the catholyte solution;
an anion-exchange layer (AEL) in anion communication with the anolyte solution to provide hydroxide ions into the anolyte solution; and an interfacial layer defined between the cation-exchange layer and the anion-exchange layer for splitting water into the protons and the hydroxide ions; and

13 retaining a portion of the catholyte solution as a stationary catholyte layer between the catalyst layer of the cathode and the CEL and in contact with the CEL.
In an embodiment 40, the process of embodiment 39, comprising maintaining an operating temperature between 20 C and 50 C, and optionally about 35 C.
In an embodiment 41, the process of embodiment 39 01 40, wherein supplying the stream of CO, CO2 or any mixture thereof to the cathodic compartment is performed at an inlet flowrate between 1 sccm and 15 sccm.
In an embodiment 42, the process of any one of embodiments 39 to 41, comprising providing the cathode with an applied current density between 100 and 400 mA.cm-2 In an embodiment 43, the process of any one of embodiments 39 to 42, comprising forming the stationary catholyte layer by providing a solid porous support between the cathode and the CEL, and saturating the solid porous support with the catholyte solution.
In an embodiment 44, the process of embodiment 43, wherein the saturating is performed to reach a liquid content of the stationary catholyte layer between 5 and 50 pL.cm-2, optionally between 10 and 20 pLcnn-2, and further optionally about 10 pLcnn-2 when the solid porous support is saturated with the catholyte solution.
In an embodiment 45, the process of any one of embodiments 39 to 44, wherein the catholyte solution is supplied with a concentration of cations between 0.25 M
and 3 M, and optionally between 0.5 M and 2 M, and further optionally about 1M.
In an embodiment 46, the process of any one of embodiments 39 to 45, wherein stationary catholyte layer is formed with a thickness between 20 pm and 400 pm, optionally between 20 pm and 300 pm, further optionally between 20 pm and125 pm.
In an embodiment 47, the process of any one of embodiments 39 to 46, comprising utilizing the stream of CO, CO2 or any mixture thereof according to a single-pass utilization of at least 50% for an inlet flowrate between 1 sccm and 15 sccm.
In an embodiment 48, the process of any one of embodiments 39 to 46, comprising utilizing the stream of CO, CO2 or any mixture thereof according to a single-pass utilization of at least 60% for an inlet flowrate between 1 sccm and 8 sccm.
In an embodiment 49, the process according to any one of embodiments 39 to 48, comprising producing the 02, products according to a Faradeic Efficiency (FE) for

14 conversion into the C2+ products that is of at least 20% during at least 20 hours of operation and under an applied current density between 100 and 400 mA.cm-2.
In an embodiment 50, the process according to embodiment 49, comprising producing the 02+ products according to the FE for conversion into the C2, products that is of at least 25% during at 30 hours of operation and the applied current density of 350 mA.cm-2.
In an embodiment 51, the process of any one of embodiments 39 to 50, further comprising using the system as defined in any one of embodiments 2 to 38.
BRIEF DESCRIPTION OF THE DRAWINGS
The attached figures illustrate various features, aspects and implementations of the technology described herein.
Figures 1A and 1B schematically illustrate carbonate formation and CO2 crossover mechanisms in known alkaline and neutral media CO2RR electrolyzers (prior art): Figure 1A: conventional AEM-based flowing-electrolyte electrolyzer, Figure1B: zero-gap gas-phase electrolyzer (B);
Figures 1C and 1D schematically illustrate carbonate formation and CO2 crossover mechanisms in the electroreduction system that is proposed herein:
Figure 1C:
bipolar membrane (BPM)-based stationary catholyte layer (SC)-MEA, Figure 1D:
mechanism of cation effects: potassium ions form an electrochemical double layer on the surface of a Cu catalyst, which modulates the local pH and prohibits the proton adsorption, and thereby enhances the selectivity of CO2RR and suppress that of HER (see references 1 and 33). In SC-MEA, the carbonate/bicarbonate ions generated by the cathode reactions are reverted back to CO2 by the protons generated from water dissociation inside the BPM.
The CO2 crossover in the present SC-MEA electrolyzer is thereby greatly reduced in comparison to known systems of Figure 1A and 1B.
Figures 1E and IF are scanning electron microscopy (SEM) images of the cathode electrode (1E) and a cation exchange layer (CEL) / anion exchange layer (AEL) interface of the customized BPM used for SC-MEA in neutral anolyte (1F). The customized BPM consisted of a NafionTm 212 as CEL, Piperion as AEL, and a TiO2 nanoparticle layer sandwiched in between as a water dissociation catalyst.

Figures 2A to 2F are graphs resulting from experimental investigations on the CO2RR performance of the SC-MEA using neutral anolyte:
Figure 2A is a graph showing gas product FEs (`)/0) for the SC-MEA for zero gap (direct contact) and four catholyte solutions based on an applied current density producing 5 maximum ethylene FE (marked on the top of each column) and at an operating temperature of 20 C. The dependence of cell voltage and gas FE on the current density can be found in Figures 7A to 70;
Figure 2B is a graph showing the dependence of cell voltage on current density at various operating temperatures;

Figure 2C is a graph showing the dependence of cathode gas products FEs on current density at an operating temperature of 35 C;
Figure 20 is a graph showing CO2/02 ratio in an anode gas for a conventional electrolyzer (black squares) and the present SC-MEA (grey squares) at various current densities. 02 and CO2 flow rates in the present SC-MEA are also indicated. The plots

15 represent the data obtained after 1 hour of continuous electrolysis at each current density;
Figure 2E is a graph showing the dependence of the cathode products FEs on an inlet CO2 flow rate (sccm);
Figure 2F is a graph showing an SPU of CO2 versus the inlet CO2 flow rate. The ideal SPU values of the conventional electrolyzers are marked for comparison.
'CO ideal' and 'C2, (excl. acetate) ideal refer to AEM-based MEAs that produce 100% CO FE
and 100% 02, FE except for acetate (ideal SPU = 36.4%), respectively. The ' Simulated ideal' refers to the simulated upper limit of SPU in an AEM-based MEA, assuming the AEM-based MEA has the same product distribution to the present BPM-based SC-MEA
operating at a CO2 flow rate of 1 sccm, and based on the assumption that one extra CO2 is lost to carbonate per two OH- generated. The justification of the ideal SPU
simulation can be found in SI1 of Supplemental Information.
Figures 3A to 30 are graphs resulting from experimental investigations on a CO2RR performance of the present BPM-based SC-MEA using an acidic (pH = 2.37) anolyte at an operating temperature of 35 C. The CO2RR performance at lower pHs and various temperatures are shown in Figures 14A, 14B and 15 in Supplemental Information.

16 Under all the acidic conditions, the CO2 concentration in the anode gas was below the detection limit.
Figure 3A is a graph showing the dependence of cell voltage and cathode gas products FEs on current density;
Figure 3B is a graph showing the dependence of the cathode products FEs on the inlet CO2 flow rate;
Figure 3C is a graph showing the SPU of CO2 versus the inlet CO2 flow rate.
'CO
ideal' and 'C2, (excl. acetate) ideal' refer to the ideal SPU values of AEM-based MEA
electrolyzers that produce 100% FE CO and 100% C2, except for acetate (ideal SPU =
36.4%), respectively. The ' Simulated ideal' refers to the simulated upper limit of SPU in AEM-based MEA electrolyzers, assuming it has the same product distribution to the present BPM-based SC-MEA operating at an inlet CO2 flow rate of 1 sccm, and based on the assumption that one extra CO2 is lost to carbonate per two OH- generated.
The justification of the ideal SPU simulation can be found in SI1 of Supplemental Information;
Figure 3D is a graph showing ethylene concentration at the outlet stream for the electrolyzers operated at different pH and inlet CO2 flow rates.
Figures 4A to 4C are graphs showing an extended CO2RR performance of the present BPM-based SC-MEA:
Figure 4A is a graph of ethylene FEs and cell voltages versus operating time using a neutral anolyte at an operating temperature of 35 C, an inlet CO2 flow rate of 9.0 sccm, and an applied current density of 250 mA.cm-2;
Figure 4B is a graph of ethylene FEs and cell voltages versus operating time using an acidic anolyte at an operating temperature of 35 C, an inlet CO2 flow rate of 9.0 sccm and an applied current density of 350 mA.cm-2; and Figure 4C is a graph of the FE distribution and SPU versus operating time for the present BPM-based SC-MEA using an acidic anolyte (pH = 2.37) at an operating temperature of 35 C and an inlet CO2 flow rate of 1.0 sccm.
Figures 5A to 5C are schematic representations of a hypothesized pH gradient over layers of various cell configurations: AEM-based MEA (Figure 5A); SC-MEA
with a

17 pH = 10 buffer catholyte (Figure 5B); and SC-MEA with a non-buffer salt catholyte (Figure 5C). The extent of the gradients in pH are not precisely known and are not drawn to scale.
Figure 6 is a schematic illustration of CO2 regeneration in a stationary catholyte layer for a rough estimation of the impacts of a thickness of the stationary catholyte layer on the present BPM-based SC-MEA performance.
Figures 7A to 7C are graphs showing the FE and full cell voltage for the SC-MEA
for various catholyte solution comprised in the stationary catholyte layer:
(Figure 7A) DI-water; (Figure 7B) 0.25M K2SO4; (Figure 70) 2M KCI. In all cases, the anolytes are 0.1M
KHCO3. The measurements were performed at 2000 with a CO2 inlet flow rate of 15 sccm.
Figure 8 is a graph showing ethylene FE and cell voltage with extended operating hours for the present BPM-based SC-MEA with a catholyte solution of 1M KHCO3 in the stationary catholyte layer and an anolyte solution of 1M KHCO3 as the anolyte.
The measurement was carried out at 20 'C with a CO2 flow of 9 sccm and a current density of 100 mA cm-2.
Figures 9A to 9C relate to investigations on the CO2RR performance of an SC-MEA based on a CEL (Nafion XL) with a catholyte solution of 0.5 M K2SO4 in the stationary catholyte layer and an anolyte solution of 0.5 M K2SO4 plus 0.1 (pH = 2.37) or 0.5 M (pH
= 1.84) H3PO4 as the anolyte. All the measurements were performed at an operating temperature of 20 'C and a CO2 flow rate of 15 sccm:
Figure 9A is a schematic representation of the present stationary catholyte layer in combination with an MEA;
Figure 9B is a graph showing the full cell voltage of the OEM-based SC-MEA at various current densities;
Figure 9C is a graph showing gas products FEs of the CEM-based SC-MEA at the same various current densities.
Figure 10 is a graph showing full cell voltage of a BPM-based SC-MEA at various current densities, for a water-splitting BPM including a layer of TiO2 nanoparticles as water dissociation catalyst sandwiched by CEM and AEM (customized BPM), a commercially available BPM (Fumasep), and a membrane with OEM and AEM simply compressed

18 together. A 5 cm2 Pt/C loaded hydrophilic carbon paper and a 5 cm2 Ir02 loaded Ti felt were used as cathode and anode of the MEA, respectively.
Figure 11 is a graph showing a gas product FE and full cell voltage for the SC-MEA based on commercially available BPM (Fumasep) and 0.1 M KHCO3 neutral anolyte.
Figure 12A is a graph showing the gas products FE and full cell voltage for the SC-MEA based on customized BPM and 0.1 M H3PO4 + 0.5 M K2SO4 acidic anolyte, and Figure 12B is a graph showing the cell voltage versus operating time diagram of the BPM-based SC-MEA.
Figures 13A and 13B are graphs showing the dependence of gas products FE on the current density of the BPM-based SC-MEA with 1M K of catholyte solution in the stationary catholyte layer and a solution of 0.1M KHCO3 (pH = 8.20) as the anolyte for the operating temperatures of 20 C (Figure 13A) and 50 C (Figure 13B), with the CO2 inlet flow rate of 15 sccm in both cases. Their cell voltages are shown in Figure 2B.
Figures 14A and 14B are graphs showing the dependence of gas products FE
(columns) and cell voltage (plot) on the current density of the BPM-based SC-MEA with 1M K in the stationary catholyte layer and 0.1M H3PO4 + 0.5M K2SO4 (pH =
2.37) as acidic anolyte for the operating temperatures of 20 C (Figure 14A) and 50 C
(Figure 14B), with the CO2 inlet flow rate of 15 sccm in both cases.
Figure 15 is a graph showing the dependence of gas products FE (columns) and cell voltage (plot) on the current density of the BPM-based SC-MEA with a catholyte solution of 1M K+ in the stationary catholyte layer and a solution of 0.5M
H3PO4 + 0.5M
K2SO4 (pH = 1.84) as acidic anolyte for the operating temperatures of 35 C
with the CO2 inlet flow rate of 15 sccm.
Figure 16 is a graph showing a CO2: 02 ratio in an anode gas phase for the SC-MEA using the presently customized BPM with a 0.1 M K2SO4 anolyte solution (black), or using a commercially available BPM (Funnasep) and a 0.1 M KHCO3 anolyte solution (red).
The catholyte solution is 1M K+ in the stationary catholyte layer in both cases. All the gas samples were recorded after the cell operating for 1 hour at each current density_ In K2SO4 anolyte, the CO2: 02 ratio is close to that in KHCO3 anolyte with the customized BPM, indicating that the detected anode CO2 flow in the customized BPM/KHCO3 system (Figure 2D in the main text) was not ascribed to the acidification of bicarbonate. In an SC-MEA

19 using KHCO3 anolyte, the CO2 crossover through Fumasep is similar to the case through customized BPM.
Figure 17: Investigations on the CO2RR performance of the SC-BPMEA using catholyte thicknesses of 250, 125, 65 and 16 pm. All the results are collected at 35 oC
with a CO2 flow rate of 10 sccm cm-2 (normalized by the geometric area of the cathode), a catholyte of 0.5 M K2SO4, and an anolyte of 0.1 M KHCO3. The dependence of cell voltages on current density. (b) Distribution of voltage losses measured in the SC-BPMEA
with 65 pm thick catholyte operating at 200 mA cm-2 (cell voltage = 3.82 V).
Figure 18: Investigations on the cell voltage of the SC-BPMEA using the custom BPM. The ohmic resistance (black) and the contribution to voltage loss (grey) of the stationary catholyte layer with various thicknesses. The errors in these measurements are below 1%.
Figure 19 Investigations on the CO2RR performance of the SC-BPMEA using catholyte thicknesses of 250, 125, 65 and 16 pm. The dependence of the CO2RR
gas products FE on the current density for the SC-BPMEAs with the catholyte thickness of 250 (a), 125 (b), 65 pm (c) and 16 pm (d).
Figure 20 The exploration of the CO2RR performance and energy intensity of SC-BPMEA with restricted reactant availability. All the measurements were conducted at 35 C and 200 mA cm-2, and the data were collected after 2 h of continuous operation. Carbon balance in SC-BPMEA with 65 pm 0.5 M K2SO4 at different input CO2 flow rates.
See Supplementary Fig. 13b for plots on a logarithmic scale.
Figure 21 The exploration of the CO2RR performance and energy intensity of SC-BPMEA with restricted reactant availability. All the measurements were conducted at 35 C and 200 mA cm-2, and the data were collected after 2 h of continuous operation. The total CO2 single-pass utilization (the 002-to-ethylene single-pass conversion see Fig. 18) for the SC-BPMEAs with different catholyte thickness and input CO2 flow rates.
Figure 22 The exploration of the CO2RR performance and energy intensity of SC-BPMEA with restricted reactant availability. All the measurements were conducted at 35 C and 200 mA cm-2, and the data were collected after 2 h of continuous operation. (a-c) The FE distributions and the CO2 requirements (total CO2 converted to products) of the SC-BPMEAs with different catholyte thickness and input CO2 flow rates (sccm normalized by electrode area). Ci refers to CO, formate and methane. C2, refers to ethylene, ethanol, acetate and n-propanol.
Figure 23 (a) The FE and full cell voltage for the SC-BPMEA based on 125 pm 0.5 5 M K2SO4, custom BPM and 0.1 M H3PO4 + 0.5 M K2SO4 anolyte. (b) The cell voltage versus operating time diagram of the cell.
Figure 24 Measurements of CO2 SPU of the SC-BPMEAs with 125 pm 0.5 M
K2SO4, operating at 300 mA cm-2 (a, b) or 200 mA cm-2 (c, d). The operating anolytes are (a, b) 0.1 M H3PO4 + 0.5 M K2SO4 (pH = 2.3) and (c, d) 0.1 M KOH (pH = 13.3).
All the 10 experiments were conducted at 35 C. (a, c) The FE distributions at different input CO2 flow rates. (b, d) The total CO2 SPU and CO2-to-ethylene conversion at different input flow rates.
DETAILED DESCRIPTION
Techniques described herein relate to an electroreduction system that can be used to 15 convert carbon oxides selected from CO, CO2 or any mixture thereof into multicarbon products with an enhanced single pass utilization (SPU) of CO2 or CO by comparison to know flow cells or membrane electrode assemblies (MEAs). Multiple factors can affect the SPU in electroreduction systems. The present electroreduction system particularly includes a stationary catholyte layer being configured to facilitate mass transfer via

20 diffusion of regenerated CO2 or CO across the stationary catholyte layer and back to an adjacent cathode of the system.
It should be noted that the system and related process implementations that are described herein in relation to CO2 electroreduction can be applied to CO
electroreduction, or the electroreduction of a mixture of CO2 and CO, without departing from the scope of the present techniques.
In contrast to known flow cell electrolyzers, the catholyte solution of the present electroreduction system is not provided flowing in and out of a cathodic compartment but rather remains within the cathodic compartment as a stationary catholyte layer between a cathode and a membrane separating the cathodic compartment from an adjacent anodic compartment. More particularly, referring to Figure 1C, the electroreduction system

21 includes a bipolar membrane (e.g., including a cation-exchange layer (CEL), an interfacial layer comprising a water dissociation catalyst, and anion-exchange layer (AEL)) that separates an anodic compartment from a cathodic compartment. The bipolar membrane is used to dissociate water, thereby providing hydroxide ions to the anodic compartment side and protons to the cathodic compartment side. The electroreduction system further includes a cathode (e.g., gas diffusion layer plated with Cu catalyst) that is positioned in the cathodic compartment, and an anode (e.g. hydrophilic electrode such as Ir02 catalyst coated on a support of Ti felt) that is positioned in the anodic compartment.
As seen in Figure 10, CO2 can be provided to the cathode via an inlet of the cathodic compartment so as to be converted into gas products (021-14, CO, H2) at a surface of the cathode. A
portion of the CO2 can be lost to carbonate formation. The present electroreduction system further includes the stationary catholyte layer sandwiched between the bipolar membrane and the cathode, with the stationary catholyte layer comprising a catholyte solution that receives the carbonate/bicarbonate ions derived from the lost CO2 portion. As protons are provided from the bipolar membrane into the catholyte solution, these protons can combine with the carbonate/bicarbonate ions in the catholyte solution to regenerate the portion of CO2 that did not serve to form gas products. The regenerated CO2 can then diffuse back to the cathode surface to form the gas products that will be recovered in a cathode gas mixture. In summary, the bipolar membrane can favor the conversion of carbonate/bicarbonate ions back to CO2 and further prevent ions crossover between the anodic and cathodic compartments. The stationary catholyte layer further enables local alkalinity to promote CO2RR in (bulk) acidified catholyte solution, and facilitates thereby that the regenerated CO2 participates in CO2RR reactions.
In an embodiment, the catalyst layer of the cathode comprises copper (Cu), silver (Ag), platinum (Pt), carbon (C), or any combination thereof. In an embodiment, the cathode further comprises a gas diffusion layer for contacting the CO2 stream, and the catalyst layer is deposited onto the gas diffusion layer. For example, the gas diffusion layer is hydrophobic. For example, the gas diffusion layer is a hydrophobic carbon paper, or a copper sputtered hydrophobic PTFE layer. In the context of the invention, hydrophobic means a water contact angle following ISO 19403-6:2017 of at least 30 .
In an embodiment, the anode comprises an anodic catalyst layer and an anodic current collector layer. For example, the anodic catalyst layer comprises one or more selected from Ir02, Pt, Pd, Ni, Ni0x, CoOx. For example, the current collector layer comprises Ti

22 felt, hydrophilic carbon paper, or Ni foam. In the context of the invention hydrophilicmeans a water contact angle following ISO 19403-6:2017 below 30 .
Referring to Figure 6, the thickness of the stationary catholyte layer can be selected to enhance the mass transport via diffusion of the regenerated CO2 to the cathode surface.
The stationary catholyte layer can be characterized as including a boundary where CO2 is regenerated and from which regenerated CO2 can diffuse towards the cathode surface (Cu) or the CEL, this diffusional phenomenon being driven by a concentration gradient.
The distances from this boundary to the cathode surface and CEL are noted as Li and L2, respectively. The portion of the stationary catholyte layer between the boundary and the cathode surface can be defined as a diffusion layer having a diffusion layer thickness Li.
The thickness of the stationary catholyte layer of the present system is selected to minimize the diffusion layer thickness, Li, and facilitate regenerated CO2 mass transport while providing a mechanically robust stationary catholyte layer. The diffusion layer thickness L1 can be estimated based on physical properties of protons and carbonates.
For example, one can determine the position where protons and carbonates meet each other by estimation of encounter problems, and considering that the speed is proportional to their mobility. More precise determination can be via a cross-platform finite element analysis, solver and multiphysics simulation software such as COMSOL .
The stationarity of the catholyte layer as defined herein thus prevents the regenerated CO2 to be flushed away from the cathodic compartment and allows the CO2 to diffuse back into the cathode for conversion thereof into value-added products. In other words, being stationary means that the catholyte is not flowing out of the cathodic compartment during CO2 conversion and that the volume of the catholyte solution contained in the stationary catholyte layer remains between the bipolar membrane and the cathode during operation of the electroreduction system. One skilled in the art will understand that the catholyte solution can flow/move within the stationary catholyte layer according to various mass transport mechanisms (diffusion, migration, convection if any).
The nature of the catholyte solution of the stationary catholyte layer can be further selected to reduce the diffusion layer thickness. For example, it was noted from the experimentation discussed herein that a non-buffered catholyte solution can shorten the CO2 path length (including the diffusion layer thickness) in comparison to a buffered catholyte solution, such as KHCO3.

23 To enhance the mechanical robustness of the stationary catholyte layer, the stationary catholyte layer can include a solid porous support having pores that are saturated with the catholyte solution. For example, the porous solid support is or comprises polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polycarbonate, nylon, cellulose acetate, cellulose nitrate, polypropylene, alumina, or any combinations thereof. The solid porous support has a mean pore diameter (i.e., a mean pore size) between 0.05 and 50 pm as determined by scanning electron microscopy; for example, between 0.08 and 25 pm; for example, between 0.10 and 10 pm; for example, between 0.15 and 5 pm; for example, between 0.20 and 1.0 pm; for example, between 0.30 and 0.80 pm. For example, the solid porous support has a mean pore size of 0.45 pm. The porous solid support can be provided to contact the cathode surface at one side and the cation exchange layer of the bipolar membrane at another side thereof. The thickness of the stationary catholyte layer can thus be equal to the thickness of the solid porous support.
The thickness of the stationary catholyte layer can be at most 280 pm as measured by a spiral micrometer.; preferably, at most 250 pm; preferably, at most 220 pm;
preferably at most 200 pm; preferably at most 180 pm; preferably, at most 150 pm; preferably at most 140 pm; preferably, at most 130 pm; and more preferably at most 125 pm.
The thickness of the stationary catholyte layer can be at least 20 pm as measured by a spiral micrometer.; preferably, at least 25 pm; preferably, at least 30 pm;
preferably at least 40 pm; preferably at least 45 pm; preferably, at least 50 pm; preferably at least 55 pm; preferably, at least 60 pm; and more preferably at least 65 pm.
The thickness of the stationary catholyte layer can be ranging from 20 to 280 pm as measured by a spiral micrometer.; for example, from 20 to 250 pm; for example, from 30 to 220 pm; for example, from 40 to 200 pm; for example, from 45 to 180 pm; for example, from 50 to 150 pm; for example, from 55 to 140 pm; for example, from 60 to 130 pm; for example, from 65 to 125 pm. The thickness of the stationary catholyte layer is measured by a spiral micrometer. For example, the thickness of the stationary catholyte layer can be about 125 pm, when measured by a spiral micrometer.
However, the thickness of the stationary catholyte layer is not to be bound to these values and is selected to minimize the diffusion layer thickness while maintaining the mechanical robustness of the stationary catholyte layer. The mechanical robustness can refer herein to a resistance to compression that can be estimated for example via a compression-

24 stress test. In other words, the stationary catholyte layer is configured to resist compression an maintain sufficient thickness to avoid direct contact of the BPM with the cathode. For example, the thickness of the stationary catholyte layer could be inferior to 125 pm if a solid porous support having such thickness is used and yet maintain robustness.
When the solid porous support is saturated with the catholyte solution, the stationary catholyte layer has a liquid content between 5 and 50 pLcm-2; for example, between 8 and 35 pL.cm-2; for example, between 10 and 20 pL.cm-2. For example, the stationary catholyte layer has a liquid content of about 10 pLcm-2 When analyzing CO2 crossover in known neutral media electrolyzers (see section SI1 of Supplemental Information), it was concluded that achieving high carbon efficiency can be achieved based on two requirements. Firstly, the carbonate/bicarbonate ions that are formed from CO2 absorption in locally alkaline catholyte solution should not reach the anode compartment side. Secondly, the carbonate/bicarbonate ions that were formed near the cathode should revert back to CO2, and participate in CO2RR (and not mix with CO2RR products).
In known neutral media flow cells, BPMs (e.g. Fumasep FBM) can be used to convert carbonate/bicarbonate back to CO2 and to block CO2 crossover to the anode (see references 10 and 14). A conventional BPM can consist of a cation-exchange layer (CEL) laminated with an anion-exchange layer (AEL). With the CEL facing the cathode side, the concentration (and hence conductivity) of carbonate/bicarbonate anions in the BPM is substantially reduced due to the Donnan effect (see reference 17). The BPM
also generates protons and hydroxide ions via water dissociation at the junction between the CEL and AEL(see references 18 and 19) under appropriate external potential.
The protons are driven to the cathode surface, where, with judicious device engineering, they can intercept carbonate/bicarbonate, reverting it to 002.
However, in the BPM-based flow cells, the catholyte reacts with and absorbs CO2. Even at steady-state (catholyte is saturated by CO2 absorption), the CO2 regenerated at the catholyte/BPM interface can be flushed away from the cathode surface due to catholyte flow, thereby releasing CO2 with gaseous CO2RR products in the cathodic compartment, rather than diffusing back to the cathode to participate in CO2RR (see reference 14). As a result, the CO2 loss due to carbonate/bicarbonate formation is ca. twice the amount transformed to products, similar to AEM-based electrolyzers.
It was thus hypothesized that using a BPM in a device without a flowing catholyte solution, such as a membrane electrode assembly (MEA), could prevent CO2 crossover to the 5 anode, minimize CO2 loss via absorption to the bulk electrolyte, avoid flushing CO2 away, and thereby maximize the SPU. However, another issue was observed, i.e.
acidification of the cathode (see references 1 and 20 to 22). The herein discussed experiments (Figure 2A) showed that a Cu catalyst can produce 100% hydrogen at current densities from 50 to 200 mA cm-2 when contacted with a cation exchange layer (CEL) of the BPM.
10 Previous reports (see references 22 and 23) have suggested inserting a solid porous support layer (a buffer layer) saturated with DI water or a KHCO3 solution as the catholyte solution between the cation-exchange membrane and an Ag catalyst layer (cathode) to improve the selectivity of CO2-to-CO reactions over HER. The presently described system was then developed to evaluate the potential of BPM-based MEA for producing C2+, using 15 a Cu catalyst, and with the goal of a minimum of CO2 loss through engineering the catholyte solution. The analysis provided in section 612 of the Supplemental Information reveals that, in principle, the in-situ CO2 recovery in a BPM-based MEA can potentially become energy-efficient and may also aid on projected capital costs.
Here is demonstrated that BPM-based MEAs incorporating a stationary catholyte layer 20 between catalyst and BPM can produce 02+ with significantly reduced CO2 loss (Figure 1C). After a series of optimizations on the stationary catholyte layer, it was discovered that cations (Figure 1D) enable ongoing CO2-to-02+ production on the Cu catalyst.
The BPM
was used as a proton source to regenerate CO2 in-situ and then reduce it to 02+ products on a copper (Cu) catalyst (Figure 10). By allowing the catholyte solution to be provided in

25 a stationary catholyte layer, the regenerated CO2 has the opportunity to participate in CO2RR. The presently described system can be referred to as a stationary-catholyte MEA
(SC-MEA) or a BPM-based SC-MEA. CO2RR on Cu catalyst with an SPU of at least 60%
(02, FE of 26%) was achieved, which twice the value of the best prior known electrolyzers producing 02+ (see references 2,4, 15 and 24) and, as a result, the theoretical upper limit of SPU for neutral/alkaline C2, systems previously demonstrated was also surpassed.
When run at a total current density of 350 rnA.cm-2, the present system maintains an ethylene FE at a steady rate of above 30% for more than 30 hours of continuous operation.

26 Thus, the BPM of the present disclosure comprises a cation-exchange layer (CEL) in cation communication with the catholyte solution to provide protons into the catholyte solution; an anion-exchange layer (AEL) in anion communication with the anolyte solution to provide hydroxide ions at a surface of the anode, and an interfacial layer defined between the cation-exchange layer and the anion-exchange layer for splitting water into the protons and the hydroxide ions. The interfacial layer can comprise a water dissociation catalyst; with preference that the water dissociation catalyst comprises one or more selected from TiO2, 1r02, NiO, Sn02, graphene oxide, CoOx, ZrO2, A1203, Fe(OH)3, Mn02, Ru, Rh, RuPt alloy, PtIr alloy, Ir, Pt. More preferably, the water dissociation catalyst can be a combination of 1102 on the AEL side) and NiO on the CEL side. In an embodiment, the water dissociation catalyst is present as nanoparticles.
In an embodiment, the AEL is a membrane comprising poly(aryl piperidinium), polystyrene methyl methylimidazolium, or polystyrene tetramethyl methylimidazolium.
In an embodiment, the CEL is a NafionTm membrane (CAS number 31175-20-9) In an embodiment, the system further comprises a temperature controller configured to maintain an operating temperature between 20 C and 50 C, for example, 25 C and 45 C;
for example, 30 C and 40 C optionally about 35 C.
In an embodiment, the system is having a single-pass utilization of the CO2 stream of at least 50% for a CO2 inlet flowrate between 1 sccm and 15 sccm. In an embodiment, the system is having a single-pass utilization of the CO2 stream of at least 60%
for a CO2 inlet flowrate between 1 sccm and 8 sccm. In an embodiment, the system is having a Faradeic Efficiency (FE) for conversion into the 02, products of at least 20% during at least 20 hours of operation and under an applied current density between 100 and 400 nriA.cnn-2. In an embodiment, the FE for conversion into the C2+ products is of at least 25%
during 30 hours of operation and the applied current density of 350 mA.cm-2.
The present disclosure also relates to a CO2 electroreduction process for converting CO2 into C2+ products, the process comprising:
supplying a catholyte solution and CO2 to a cathodic compartment comprising a catalyst layer in contact with the catholyte solution:

27 flowing an anolyte solution through an anodic compartment having a product outlet to release the C2, products, the anodic compartment comprising an anode;
providing a bipolar membrane between the cathodic compartment and the anodic compartment, the bipolar membrane comprising:
a cation-exchange layer (CEL) in cation communication with the catholyte solution to provide protons into the catholyte solution;
an anion-exchange layer (AEL) in anion communication with the anolyte solution/anode?
to provide hydroxide ions into the anolyte solution; and an interfacial layer defined between the cation-exchange layer and the anion-exchange layer for splitting water into the protons and the hydroxide ions;
retaining a portion of the catholyte solution as a stationary catholyte layer between the catalyst layer of the cathode and the CEL and in contact with the CEL
For example, the process uses the system described above.
Catholyte solution engineering for high SPU of CO2 feedstock in a CO2-to-C2.
electrolyzer Figure 1C schematically illustrates a BPM-based SC-MEA as encompassed herein with related reactional and diffusional mechanisms. Figure 1E is an SEM photograph of a cathode that was prepared by spraying Cu nanoparticles onto a hydrophobic carbon paper (gas diffusion layer) for CO2RR. For example, the anode can be Ir02 supported on titanium felt to support the oxygen evolution reaction (0ER). Figure 1F is an SEM
photograph of a customized BPM under reverse bias that was installed with the anion-exchange layer (AEL) contacting the anode, and the cation-exchange layer (CEL) contacting the stationary catholyte layer. The cathode was then compressed onto the solid porous support of the stationary catholyte layer, and anodic and cathodic flow-field plates sandwiched the system.
In contrast with previous works that used 420 to 800 pm glass microfiber filters (see references 22 and 23), the present system can include a porous stationary catholyte layer having a thickness that is below 400 pm, e.g. about 125 pm and including a solid porous support that is configured to be saturated with the catholyte solution. The solid porous

28 support can be, for example, a PVDF filter membrane having a mean pore size (i.e.
diameter) of 0.45 pm and configured to receive a liquid content of about 10 pLcm-2. In addition to apparent benefits such as lower CO2 absorption capacity (ca. 0.1 mmol cm-2) and lower ohmic resistance, the lower thickness of this porous stationary layer allows for improving the mass transport efficiency of the in-situ recovered CO2 to catalyst (quantitatively simulated in section SI3 of Supplemental Information provided further below) in comparison to known systems.
It was further discovered that the composition of the stationary catholyte layer greatly impacts CO2RR performance in SC-MEA (analyzed and rationalized in sections SI2 and SI3 of Supplemental Information provided further below). Rather than DI-water or KHCO3 as used in previous studies (see references 22 and 23), the catholyte solution can be designed as a non-buffered catholyte solution, e.g. K2SO4, in order to introduce cation effects as a means to promote selectivity to CO2RR over HER (see references 25 and 26).
Under external potential, cations such as K can form an electrochemical double layer on the catalyst surface (see Figure 1D), introducing changes in polarity, absorption preference, local pH, and local CO2 concentration, as observed and modeled before (see references 25 and 26). Increasing K+ concentration from 0 to 2 M in the stationary catholyte layer was seen to enhance CO2RR selectivity. At optimized current densities (referring to Figures 7A to 7C further detailed in Supplemental Information), the ethylene FE was observed to increase from 0.5% (0 M) and 2.5% (0.5 M) to 25% (1 M) and 27% (2 M). The enhancement of CO2RR selectivity (Figures 2A and 7A to 7C) as K concentration increases, implies a predominant role of cation effects in promoting CO2RR in the present BPM-based SC-MEA. Despite higher ethylene FE and lower hydrogen FE, 2 M lc in the stationary catholyte also resulted in carbonate salt precipitation at the back of the carbon paper of the cathode, which caused the loss of K+ and obstructed the mass transport of CO2 over time (see reference 27).
Therefore, the cation concentration of the catholyte solution is ranging from 0.25 M to 6.00 M; for example, from 0.50 M to 3 M; for example, from 0.50 M to 2.80 M, for example from 0.75 M to 2.50 M, for example from 1.00 M to 2 M; for example, the cation concentration of the catholyte solution is about 1 M. In an embodiment, the cations in the catholyte solution are one or more selected from K+, Na, Cs, Rb+, NH4, Mg2+, Ca2+, Al3+.
For example, the catholyte solution can be a solution of K2SO4 having a K+
concentration

29 equal to or greater than 0.5M; preferably equal to or greater than 1.0 M, more preferably equal to or greater than 1.5 M. For example, the catholyte solution can be a solution of K2804 having a K+ concentration of at most 3.00 M.
In an embodiment, the catholyte solution is buffered. For example, the buffered solution is a solution comprising one or more selected from deionized (DI) water, KHCO3, K3PO4, K2HPO4, KH2PO4 or the buffered solution is a mixture of glycine- and sodium hydroxide, or a mixture of H3B03 and sodium hydroxide.
In another embodiment, the catholyte solution is non-buffered. For example, the non-buffered solution is or comprises K2SO4, KCI or any other combinations of the CI- anions or S042- anions with Nat, Cs, Rb+, NH4, me, c_2 , a or Al3+ cations.
The anolyte solution has an anolyte concentration between 2.0 M and 0.01M; for example, between 1.5 M and 0.05M; for example, between 1.0 M and 0.08 M; for example, between 0.5 M and 0.1 M. For example, the anolyte solution has an anolyte concentration of about 0.1 M.
In an embodiment, the anolyte solution is neutral. For example, the anolyte solution has a pH between 7.0 and 10.0; preferably a pH between 7.5 and 9.5. For example, the anolyte (neutral) solution is a KHCO3, K2SO4, or K2HPO4 solution.
In another embodiment, the anolyte solution is acidic. For example, the anolyte solution has a pH between 1.0 and 4.0; for example, between 1.5 and 3.5; for example, between 2.0 and 3Ø For example, the acidic solution is an H3PO4 solution, H2SO4 solution or a combination thereof.
The mechanism of preventing CO2 crossover in the SC-MEA is depicted in Figure 1C.
Under applied potential, the protons generated at the CEL/AEL interface of the BPM
migrate to the cathode, while the carbonate/bicarbonate ions generated at the cathode migrate to the GEL side of the BPM. The carbonate/bicarbonate ions are reverted to CO2 when being intercepted by the protons near the interface between the stationary catholyte layer and CEL (at the boundary of the diffusion layer) and subsequently diffuse back to the cathode to participate in CO2RR.
Proton-induced CO2 regeneration could also be accomplished by coupling a cation-exchange membrane and acidic anolyte since the anodic OER can supply protons.
It was observed that the CO2 crossover in this system is below the detection limits.
However, the experimental and theoretical analyzes showed that this system does not operate continuously because of co-ion transport and water balance issues (discussed in SI3 of Supplemental Information).
BPM prevents CO2 crossover in SC-MEA using neutral anolyte 5 To compare the CO2 crossover in the present BPM-based SC-MEA with the known AEM-based neutral media electrolyzers, the CO2RR performance of the present BPM-based SC-MEA was firstly measured, using a flowing neutral anolyte solution (0.1 M
KHCO3, pH
-8.2), and the results are summarized in Figures 2A to 2F.
The operating temperature on CO2RR performance was then optimized and it was 10 discovered that 35'C can be the optimal temperature in the presently designed BPM-based SC-MEA (see Figures 2B, 20, 13A and section SI5 of Supplemental Information).
A CO2/02 ratio of an anodic gas mixture (also referred to as an anode gas) was further studied to evaluate the capability to prohibit CO2 crossover in the BPM-based SC-MEA, the key to achieving a high SPU (SI1) (see references 13 and 14). In agreement with 15 previous studies (see references 13 and 14), the AEM-based MEA showed 002/02 ratios very close to 2 for current densities from 100 to 300 mA.cm-2 (Figure 2D), indicating that the majority of the anionic charge carrier in AEM-based MEA is 0032-, which causes ca.
one molecule of CO2 loss per two electrons transferred.
Conversely, the CO2/02 ratio in the anode gas produced in the present BPM-based SC-20 MEA is one order of magnitude lower than that in AEM-based MEA, evidencing the prevention of CO2 crossover. The detected anode CO2 flow is not ascribed to the acidification of KHCO3, as supported by the performed control experiments (Figure 16 in SI6 of Supplemental Information). The CO2/02 ratio of the anode gas decreased as the operating current density increased, which was assigned to the fact that higher current 25 density (also higher cell voltage) decreases the pH at the CEL surface and lowers the effective diffusion coefficient of CO2 and HCO3-/003- in the CEL where they must move against the outward flow of hydrated H+ (see references 19 and 28). Given that in the SC-MEA, CO2 crossover is greatly reduced, and the catholyte solution is provided in the stationary catholyte layer, the CO2 that was regenerated from carbonate/bicarbonate ions

30 can diffuse back to the cathode for participating in CO2RR. Therefore, the present BPM-based SC-MEA is shown to present high CO2 SPU at a low inlet CO2 flow rate.

31 By decreasing the inlet CO2 flow rate, it was further demonstrated that a 6PM-based SC-MEA operating in neutral (pH = 8.20) anolyte solution and at 35 C can achieve an SPU of about 61%, which is a significant improvement in carbon efficiency compared to the known neutral-media AEM-based CO2RR electrolyzers. Figure 2E shows FEs of gas products at the cathode within a given range of CO2 feed rates (Figure 2C). Lowering the CO2 flow rate from 15 (Figure 3a) to 8 sccm (Figure 3b) was shown not to cause a significant change in the gas product distribution. However, a further decrease in CO2 flow rate led to the domination of HER over the CO2RR, likely due to the limited CO2 mass transport (see reference 15). At the CO2 flow rates investigated, it was observed that 10% of the CO2RR
FE was 'missing', which can be attributable to the liquid products being oxidized at the anode and/or being trapped in the stationary catholyte layer and thus not found in the analysis. The SPU was calculated by substituting the products FE values into Equation (3) in section 511 of Supplemental Information and is reported in Figure 2F.
As the flow rate decreases, the SPU of SC-MEA increases from 23% (8 sccm) to 61% (1 sccm), exceeding the upper limit of the SPU for the ordinary electrolyzers producing fully CO
(50%) or fully C2, (excluding acetate) products (25%), and is also higher than the state-of-art reported SPU (30%) for producing 02+ products (see reference 15). The upper limit of SPU for an AEM-based electrolyzer was also simulated. Assuming that the AEM-based electrolyzer had a similar product distribution and one extra CO2 is lost to carbonate per two OH- generated, the upper limit of the resulting SPU should be in the range of 13% to 15% (red zone in Figure 2F, simulation details in section 511 of Supplemental Information).
Acidic anolyte further suppresses CO2 crossover When using a neutral anolyte solution in the present BPM-based SC-MEA, the CO2 crossover ¨ despite being significantly reduced ¨ was not down to zero. In the neutral anolyte solution, some of the CO2 generated in the stationary catholyte layer may diffuse through the BPM's CEL, combining with hydroxide ions in the AEL and migrate to the anode. When using an acidic anolyte solution, it was hypothesized that the hydroxide ions in the AEL could be partially replaced by the anionic species in the anolyte solution, such as H2PO4-, S042-, which might inhibit carbonate/bicarbonate formation and crossover. It was indeed observed that when operating in an acidic anolyte solution with a bulk pH of 2.37 (0.1 M H3PO4+ 0.5 M K2SO4), the CO2 content at the anodic gas stream was below the detection limit in the operating current density range from 100 to 400 mA
cm-2.

32 The impacts of operating temperature and anolyte acidity on the performance of the present BPM-based SC-MEA (Figure 3A, 14A, 14B, 15 and SI5) was studied. Under optimized operating conditions (e.g., 35 C, anolyte pH = 2.37), the BPM-based SC-MEA
exhibited an ethylene FE of 38% at an applied current density of 350 mA cm-2.
The dependence of cathode FE on the inlet CO2 flow rate is shown in Figure 3B. In an acidic anolyte solution, the CO2 mass transport limitation occurs when the CO2 inlet flow rate is reduced from 4 sccm to 2 sccm, which is lower than that in a neutral anolyte solution. This effect also leads to a higher maximum outlet ethylene concentration in the acidic anolyte solution (8.6%) than that in the neutral anolyte solution (6.2%), both of which were achieved at an inlet CO2 flow rate of 4 sccm (Figure 3D). These results are ascribed to the better efficiency (as seen from the anode gas analysis) of CO2 recycling in the acidic anolyte solution ¨ this compensates for the low CO2 feeding when mass transport limits set in. At an inlet CO2 flow rate of 1 sccm, the BPM-based SC-MEA achieved an SPU of 60% (Figure 3C): which is twice the highest experimental SPU reported for known AEM-based MEA that produced C2, (see reference 15). As a reference, a simulated AEM-based MEA showed an SPU of 15 1%.
Table 1: Summary of energy penalty associated with CO2 recovery energy consumption for SC-MEA, simulated AEM-based MEA (Figure 30), and benchmark neutral media AEM-based MEA electrolyzer from literature.
SC-MEA (acidic, SC-MEA (acidic, Reference 15 1sccm) 4 sccm) MEA
CO2 SPU (%) 60 3 39 3 30 Ethylene FE ( /0) 14 2 37 0.5 25 Inlet CO2 flow rate (sccm) CO2 recovery (GJ per 2.8 0.3 6.6 0.7 10 Ton utilized CO2) a

33 CO2 recovery (GJ per Ton produced ethylene) 20 3 46 3 60 a a All the evaluated devices are flowing-catholyte-free MEA cells operating in neutral or acidic anolytes, of which the CO2 absorption is negligible. Therefore, a 4.3 GJ/Ton CO2 separation energy consumption (see reference 29) was implemented based on the amine capture process from mixed gas for all the unutilized CO2 from both cathode and anode.
Table 1 summarizes and compares the CO2 recovery energy consumptions of the SC-MEA (acidic anolyte), and the literature benchmark neutral media AEM-based MEA

electrolyzer (see reference 15). Coupling the advantages of minimized CO2 crossover (enabled by BPM) and acidic bulk anolyte solution, the present BPM-based SC-MEA can allow a 86% and 72% reduction in energy penalty associated with CO2 recovery compared to the simulated MEA electrolyzer and literature benchmark neutral media e1ectr01yzer15, respectively. These results highlight the need for high-SPU CO2RR devices, i.e., lower energy consumption.
CO2RR stability of the present BPM-based SC-MEA
The stability of the proposed BPM-based SC-MEA operating under optimized conditions (in terms of ethylene FE) (Figures 4A and 4B) was then investigated. In both neutral (Figure 4A) and acidic (Figure 4B) anolyte solutions, the SC-MEA was fed with 9 sccm CO2 and exhibited stable cell voltages at around 5 V and ca. 30% to 40%
ethylene FE
throughout 25 hours (neutral) or 30 hours (acidic) of continuous operation.
The operating time of the SC-MEA under the conditions that enable high SPU (i.e., anolyte bulk pH of 2.37, the operating temperature of 35 C, and inlet CO2 flow rate of 1 sccm) was also extended. Under this condition, the SC-MEA was less stable than the ones fed by 9 sccm CO2 ¨ the C2, FE dropped by ca. 37% after 5 hours accompanied by decreased SPU. In the SC-MEA fed by 9 sccm CO2 flow, the products could be carried out by unreacted CO2.
In the case of 1 sccm CO2 inlet flow rate, the mass exchange efficiency can be lower than the 9 sccm cases, which can lead to the over-accumulation of CO2RR products and consequently lower the activity of Cu catalyst (see references 30 to 32). This over-

34 accumulation issue is a newly discovered challenge in high-SPU electrolyzers, calling for innovations of system design and catalyst in the future. Nevertheless, throughout the 5 hours of continuous operation, SC-MEA maintained SPUs greater than that of the neutral and alkaline media electrolyzers.
Several alternative implementations and examples have been described and illustrated herein. The implementations of the BPM-based SC-MEA described above are intended to be exemplary only. A person of ordinary skill in the art would appreciate the features of the individual implementations and the possible combinations and variations of the components. A person of ordinary skill in the art would further appreciate that any of the implementations could be provided in any combination with the other implementations disclosed herein. It is understood that the developed design may be embodied in other specific forms without departing from the central characteristics thereof (e.g., CO2 crossover limitation, regeneration of absorbed CO2, and control of the diffusion layer in the stationary catholyte layer). The present implementations and examples, therefore, are to be considered in all respects as illustrative and not restrictive, and the system and process proposed herein are not to be limited to the details given herein.
Accordingly, while the specific implementations have been illustrated and described, numerous modifications can come to mind.
For example, the cathode of the system could be further designed to include a macroporous gas diffusion layer, microporous gas diffusion layer, a metallic layer containing Cu with/without Al and/or Zn and/or Mg in any form (ionic or reduced or nanoparticles), short-side-chain iononners (e.g., Nafion or similar ionic polymers), an organic molecular compound (e.g., pyridine) either in free form or grafted into any of the above layer.
EXPERIMENTAL RESULTS
A single-pass CO2 utilization of at least 60% at a production rate of 91 nnA.cnn-2 toward C2+
products was achieved. When run at a total current density of 350 mA.cm-2, the present BPM-based SC-MEA electrolyzer delivered an average ethylene Faradaic Efficiency (FE) of 35% for over 30 hours. CO2 loss due to crossover was inferiorto 0.1%.
Materials Phosphoric acid (H3PO4, 85%), potassium sulfate (K2SO4, 99%), potassium bicarbonate (KHCO3, 99.7%), potassium chloride (KCI, 99%), potassium hydroxide (KOH, 99.95%), copper nanoparticles (25 nnn), NafionTM 1100W (5 wt.% in a mixture of lower aliphatic alcohols and water) and isopropanol (IPA, 99%) were purchased from Sigma Aldrich and 5 used as received. Titanium oxide nanoparticles (T102, Aeroxide P25) and PVDF
membrane filter (0.45 pm pore size, 125 pm thickness) were purchased from Fisher Scientific and used as received. NafionTM 212, NafionTm XL, Fumasep (FAS-PET-130) and titanium (Ti) felt were purchased from Fuel Cell Store. Iridium(IV) chloride hydrate (Premion , 99.99%, metals basis, Ir 73% min) was purchased from Alfa Aesar.
The water 10 used in this study was 18 MO Milli-Q deionized- (DI-) water. Nafion membranes were activated through the following procedure: 1 hour in 80 C 1M H2SO4 ¨ 1 hour in 80 C
H202 ¨ 1 hour in 1M H2SO4 ¨ stored in DI-water. Fumasep was used as received and stored in 1M KCI. Piperion (40 pm) was purchased from VV7Energy and stored in 0.5M
KO H.
15 Fabrication of a water dissociation catalyst layer of the customized bipolar membrane (BPM) The water dissociation catalyst layer was fabricated following a similar procedure in a previous report (see reference 18). TiO2 nanoparticles ink were prepared by sonicating the mixture of TiO2, DI-water, and IPA with the weight ratio of 1: 833: 2833 for 30 minutes.
20 TiO2 nanoparticle ink was spray-coated onto a Nafion 212 membrane, of which the edges were sealed by Kapton tapes. The exposed membrane dimension was 2.2 cm x 2.2 cm.
The nominal loading of TiO2 is 0.2 mg cm-2. The TiO2-coated Nafion was immediately used for assembling electrolyzers once prepared.
Anode and cathode preparation 25 For the CO2RR, cathode gas diffusion electrodes (GDEs) were prepared by spray-depositing a catalyst ink dispersing 1 mg mL-1 of Cu nanoparticles and 0.25 mg mL-1 of NafionTM 1100W in methanol onto a hydrophobic carbon paper. The mass loading of Cu NPs in the GDE was kept at 1.5 mg/cm2. The GDEs were dried in the air overnight before experiments.
30 For the OER, the anode electrodes were prepared following a recipe described in Ozden, A., Li, F., Garcia De Arquer, F.P., Rosas-Hernandez, A., Thevenon, A., Wang, Y., Hung, S.F., Wang, X., Chen, B., Li, J., et al. (2020). High-rate and efficient ethylene electrosynthesis using a catalyst/promoter/transport layer. ACS Energy Lett.
5, 2811-2818; and Gabardo, C.M., O'Brien, CF., Edwards, J.P., McCallum, C., Xu, Y., Dinh, CT., Li, J., Sargent, E.H., and Sinton, D. (2019). Continuous Carbon Dioxide Electroreduction to Concentrated Multi-carbon Products Using a Membrane Electrode Assembly.
Joule 3, 2777-2791 (see references 6 and 15). The electrode preparation procedure involves:
etching the Ti felt in hydrochloric acid at 70 C for 40 min; rinsing the etched Ti felt with DI
water; immersing the Ti felt into an Ir(IV) chloride hydrate solution; drying and sintering the Ir(IV) loaded Ti felt. The loading, drying, and sintering steps were repeated until a final Ir loading of 1.5 mg cm-2 was achieved.
Assembly of the stationary catholyte membrane electrode assembly (SC-MEA) The MEA set (5 cm2) was purchased from Dioxide Materials. A cathode was cut into a 2.1 cm x 2.1 cm piece and placed onto the MEA cathode plate with a flow window with a dimension of 2.23 cm x 2.23 cm. The four edges of the cathode were sealed by Kapton tapes, which also made the flow window fully covered. The exposed cathode area was measured every time before the electrochemical tests, which was in the range of 3.1 to 4.2 cnn2. Onto the cathode, a PVFD filter membrane (2 cm x 2 cm) saturated with desirable electrolyte (sonicate in electrolyte for 15 minutes to degas) was carefully placed. This PVDF layer serves as the 'stationary catholyte layer.' Note that the BPMs used in neutral and acidic conditions were a customized one and a commercially available one (Fumasep), respectively, to achieve a better compromise of CO2RR performance and stability. The considerations of membrane selection can be found in SI4 of Supplemental Information. When using customized BPM, a TiO2 coated Nafion was placed onto the stationary catholyte layer with the TiO2 layer facing up, then covered by a Piperion (5 cm x 5 cm) membrane. When using Fumasep BPM, the membrane was placed with its cation-exchange layer (CEL) facing the cathode side. An Ir02 loaded Ti felt (2.5 cm x 2.5 cm) was placed onto the anion-exchange layer (AEL) of the BPM.
Scanning electron microscopy (SEM) Images of cathode and customized BPM were captured by an FEI Quanta FEG 250 environmental SEM (see Figures lE and 1F).
Electrochemical measurements Throughout all experiments, the cathode side was flowed by CO2 with a flow rate of 15 sccm unless otherwise specified, while the anode side was fed with neutral or acidic electrolyte at 10 mL/min by a peristaltic pump. The electrochemical measurements were performed with a potentiostat (Autolab PGSTAT204 with 10A booster). The cell voltages reported in this work are not iR corrected. The exemplified flow rate of anolyte should not be taken as a limitation and different values (other than 10 mUmin) would provide an SPU
of at least 60% as encompassed herein.
Product analysis The CO2RR gas products, oxygen, and CO2 were analyzed by injecting the gas samples into a gas chromatograph (Perkin Elmer Clarus 590) coupled with a thermal conductivity detector (TCD) and a flame ionization detector (FID). The gas chromatograph was equipped with a Molecular Sieve 5A Capillary Column and a packed Carboxen-1000 Column with argon as the carrier gas. The volumetric gas flow rates in and out of the cell were measured with a bubble column. The FE of a gas product is calculated as follows:
FEi = xi x rev.; x njiF
Where x, is the volume fraction of the gas product i, V is the outlet gas flow rate in L s', P
is atmosphere pressure 101.325 kPa, R is the ideal gas constant 8.314 J mol-1 K-1, T is the room temperature in K, n, is the number of electrons required to produce one molecule of product F is the Faraday Constant 96485 C m01-1, and J is the total current in A.
The liquid products from the cathode side of the SC-MEA were collected using a cold trap cooled to 0 C. The collected liquid was combined with anolyte (some crossover liquid product) for quantifying by the proton nuclear magnetic resonance spectroscopy CH NMR) on an Agilent DD2 500 spectrometer in D20 using water suppression mode and dimethyl sulfoxide (DMS0) as the internal standard. For each plot of liquid product quantification, fresh anolyte was used, and the duration of the collection is 30 minutes. The FE of a liquid product is calculated as follows:
FE i = rn x ft Where m, is the quantity of the liquid product i in mole, t is the duration of product collection (1800 seconds).

The CO2 SPU calculation is detailed in section 511 of Supplemental Information provided below.
Sll Analysis and comparison of CO2 single-pass utilization among state-of-art CO2RR electrolyzers The ordinary CO2RR electrolyzers suffer from an upper limit of CO2 utilization, depending on their product distributions, as analyzed below:
In the presence of hydroxide ions, CO2 molecules react with OH- (reactions [1]
and [2]) faster than being electrochemically reduced because the reaction kinetics are more favorable (see references 11, 13 and 14).
CO2 + 20H- ¨> C032- + H20 [1]
CO2 + OH- ¨> HCO3- [2]
Meanwhile, the major cathode reactions in neutral or alkaline media include:
CO2 + H20 + 2e- ¨> CO + 20H- [3]
CO2 + H2O + 2e- ¨> HC00- + OH- [4]
2CO2 + 5H20 + 8e- ¨> CH3C00- + 70H- [5]
2CO2 + 8H20 + 12e- ¨> C21-14+ 120H- [6]
2CO2 + 9H20 + 12e- ¨> C2H5OH + 120H- [7]
CO2 + 6H20 + 8e- ¨> CH4+ 80H- [8]
3CO2 + 13H20 + 18e- ¨> C3H7OH + 180H- [9]
2H20 + 2e- ¨> H2+ 20H- [10]
All these reactions generate hydroxide, of which the rate (in mole per second, MOH) is:
J x FE[i] x k[i]
MOH (1) Where J is the current in amps, FEN is the faradaic efficiency of the specific reaction [3-10], F is the Faraday constant, and kfil is the number of OH- generated per electron transferred in the specific reaction [3-10]. For the cathode reactions [4] and [5], the kid values are 0.5 and 0.875, respectively; for the other cathode reactions the kfij values are I. Figures 1A and 1B show that, in neutral media, the in-situ generated hydroxide reacts with CO2 to form carbonate and/or bicarbonate, which migrate to anode, combine with protons (generated by the oxygen evolution reaction), and release CO2 into anode gas stream. This phenomenon is known as CO2 crossover (see references 11 to 13).
Therefore, the 002/02 ratio in anode gas provides insight into the identity of the anionic charge carrier(s) that combine with the I-I+ generated on the anode (see reference 14),Ideally, if the charge carrier is H003- or 0032-, the 002/02 ratio in the anode gas stream is 4 0r2, respectively (see reference 14). While the other charge carriers like OH-, HC00-or CH3000- do not release CO2 by acidification at the anode.
Based on the analysis above, the inlet CO2 (Cm) is balanced by four parts: the CO2 in outstreann (CO, the electrochemically reduced CO2 (C2), the absorbed CO2 (C3), and the crossover CO2 (Ca). In other words, the mass balance of CO2 (in mole per second) is:
= + C2 C3 C4 (2) The carbon utilization efficiency is evaluated by single-pass utilization (SPU):

SPU = ¨ (3) Cin In conventional flow cells and MEAs, some studies have demonstrated that C/
can be negligible compared to C2 and C4 by carefully tuning Cm. When the CO2 absorption in the system reaches a steady-state, C3 is almost zero. Therefore, the upper limit of SPU is:

SPUurnit = _______________________________________________________ (4) C2 = >jX FE[i]
(5) n[i] F
Where J is the current in amp, FEN is the faradaic efficiency of the specific reaction [3-9], n111 is the number of electrons transferred per consumed CO2 in the specific reaction [3-9], and F is the Faraday constant. In neutral media, C4 ranges from 0.5 to 1 times MoH, depending on the species of charge carrier cross the AEM. To evaluate the upper limit of the SPU, C4 = 0.5 MOH. Substituting (1) and (5) into (4) gives:

FE
Etl n[i]
(6) SP Ulimtt ___________________ 1 Eli' FEN X (- 0.5k[i]) n[i]
Therefore, in the conventional flow cells and MEAs operating in neutral media, the upper limits of SPU depend on their product distributions. For example, the SPU
upper limits of the systems that produce 100% FE of CO (n = 2; kfil = 1) or 100% FE of ethylene ON =
6; kg = 1) are 50% or 25%, respectively.' Notably, HER does not contribute to C2 but still 5 generates hydroxide that can drive CO2 crossover. Accordingly, the CO2RR
performances of the electrolyzers that show state-of-art SPU in the references were identified and summarized in Table Si. None of the reported electrolyzers can achieve an SPU
exceeding 30% for 02, production, and 44% for CO production.
The SPU measurement missed 10-13% of the product FE likely because some liquid 10 product was trapped in the stationary catholyte layer or migrated and got oxidized on the anode. Therefore, the SPU reported in this work are the minimum values. The upper limit of CO2 SPU was also simulated for the SC-MEA under various conditions and are listed in Table S1. The upper limit SPU values without considering the missing FE are indicated in the brackets.
15 Nevertheless, the missing FE is taken into account for calculating the upper limit SPU of the electrolyzer to make a conservative comparison of SPU. The missing FE can be ascribed to three groups of liquid products, i.e., formate, acetate, and ethanol/propanol.
However, formate FE is only 5% of the liquid product in the SC-MEA operating under all the circumstances, and ascribing the missing FE to formate will result in a total CO2 20 consumption exceeding the inlet CO2 amount; the missing FE to acetate, ethanol, and propanol is ascribed here. This simulation gives the ranges of upper limit SPU
for the SC-MEA under different conditions (Table S1). In the main text, these ranges are depicted in Figure 2F and 30.
25 Table S1 Summary of the CO2 single-pass utilization (SPU) of the flow cells (FC) and membrane electrode assembly (MEA) in literature operating at the current densities over 100 mA cm-2. The stationary catholyte MEA (SC-MEA) operating under different conditions are listed for comparison. The upper limit SPU is simulated by substituting the reported FE distribution into Equation (6). The experimental SPU is calculated from Equation (3).
The electrolyzers producing C2, are indicated by references are bold.
FE ____________________________ distribution Experimental CO2 flow SPU SPU
Cell type/
: Upper Limit (sccm) H2 CO C2H4 CH4 Et0H Acetate Formate Propanol Upper Limit Experimental SPU
references Alkaline 7 0.07 0.2 0.45 0 0.175 0 0 0 -24% -Alkaline 50 0.07 0.06 0.7 0.02 0.1 0.05 0 0 -0.5% - FC9 Alkaline 17 0.36 0.64 0 0 0 0 0 0 - 21% -Neutral 100 0.02 0.95 o o o o o o - 7% -MEM
Neutral 2 0.59 0.00 0.24 0.08 0.09 0.025 0.015 0.01 - 30% -Neutral 15 0.23 0.77 0 0 0 0 0 0 - 44% -Alkaline 100 0.28 0.72 0 0 0 0 0 0 - 40% -MEAll Neutral 45 0.05 0.15 0.45 0.02 0.2 0.04 0.02 0.04 4% FC3 Neutral 45 0.08 0.18 042 0.02 0.15 0.03 0.03 0.05 - 3% -FC
(BPM)2 13.7 Neutral 1 0.62 0.007 0.12 0.001 0.052 0.046 0.008 0.012 1.0% 61%
4.5 SC-MEA
(9.1%) a (this work) 15.5 Acidic 1 0.56 0.002 0.14 0.007 0.03 0.09 0.004 0.002 1.1% 60%
3.9 SC-MEA
(10.0%) a (this work) 25.0 Acidic 4 0.21 0.042 0.37 0.002 0.11 0.085 0.012 0.031 1.0% 39%
1.6 SC-MEA
(this work) SI2 Analysis of intrinsic energy consumption associated with in-situ CO2 regeneration in SC-MEA and the comparison with AEM-based MEAs Bipolar membranes losses With the application of an appropriate external potential, water dissociation:
H20 ¨> H+ +
OH-, occurs at the interface of the CEL/AEL, and the protons and hydroxides serve as charge carriers in CEL and AEL, respectively. Under standard conditions (25 C, 1 atm, with activities of H+ and OH- at 1 M in the CEL and AEL, respectively), the electric potential across the BPM is -0.83 V at equilibrium. The electric potential energy difference for H+
and OH- across the BPM exactly compensates for the difference in activity such that the electrochemical potential is the same everywhere at equilibrium (see reference 18). For net current to flow, an additional electric potential must be applied across the membrane causing a deviation from the open-circuit value of -0.83 V. This deviation is typically called the water dissociation overpotential and represents the losses associated with generating H+ and OH- and transporting it out of the interfacial layer between CEL and AEL and out of the BPM. Often, it is stated that a BPM induces a "thermodynamic" voltage loss of 0.83 V - however as discussed above, this is incorrect - the losses can be quite small. For example, with appropriate materials and operating conditions, the cell voltage of a BPM-based water electrolyzer can be lower than that of an AEM-based electrolyzer at the current density up to 500 mA cm-2 (see reference 18). BPM electrolyzers can begin to split water with a total voltage of <2 V, which would be impossible if there were an intrinsic 830 mV penalty for using the BPM.
With a pH = 8.20 anolyte, the SC-MEA has a cell voltage of 3.7 V at 100 mA cm-2 and 50 C (Figure 2B in the main text), very close to the AEM-based MEA (3.4 to 3.5 V) operating under similar conditions and the same anolyte (see reference 16). The 0.2 to 0.3 V cell voltage gap is likely ascribed to two factors: the ohmic loss due to thicker BPM (ca. 50 pm CEL + 40 pm AEL) than AEM (ca. 40 pm); the cathode pH gradient (see Figures 5A
to 5C).
Energy loss in stationary catholyte layer In the present SC-MEA design, the stationary catholyte layer is a 125 pm-thick 0.5 M
K2SO4 solution (conductivity 0.15 S cm-1). Although the total ionic conductivity of this catholyte is large, the H+ / OH- / HCO3 / 0032- conductivity is very small.
Because these are the relevant ionic charge carriers in carbon dioxide electrolysis at steady state, a large pH gradient between Cu and the bulk catholyte is induced. Establishing this pH
gradient is a source of an additional concentration overpotential. As shown in Figure 5A, there is no cathode pH gradient in AEM-based MEA as long as fresh base (e.g. alkaline KOH) is fed to the cathode to react with and capture CO2 (of course this induces a different efficiency loss, namely the need to generate base externally to the system).
In the present SC-MEA (Figures 5B and 5C), the local pH of Cu is considered to be high (see reference 38) (>13) due to the continuous generation of OH- from cathode reactions.
As shown in Figure 5B, if the catholyte is a pH = - 10 buffer solution (e.g., glycine-sodium hydroxide or potassium carbonate/bicarbonate), ApH is likely - 3, which would increase the cell voltage by ca. 0.18 V. Accordingly, the extra energy (Gex) consumed in SC-MEA
with pH = 10 buffer catholyte from carbonate or bicarbonate is calculated as follows:
Gõ = 0.059 x ApH xF x J
In other words, SC-MEA is likely to save energy compared to ex-situ CO2 capture (3.5 to 4.7 GJ per ton -see reference 29), especially as the costs of renewable electricity decrease, and if cross-over of the buffer ions can be minimized or eliminated.
Previous techno-economic analysis has concluded that (see reference 39) for an AEM-based MEA, even under the optimistic evaluations (zero-ohmic loss, cell voltage 2 V, SPU
40%, 90% 02+ FE, half the CO2 loss and current density 1 A cm-2), the total cost for producing 1 ton of ethylene from CO2RR is $1,300, higher than the ethylene market price ($1,000), of which $750 is spent for electricity and $520 is spent for ex-situ 002 recovery.
Using the same optimistic metrics (except for SPU, which can be optimized to 100% in SC-MEA), the in-situ 002 recovery in SC-MEA (if using pH = 10 buffer catholyte) can cut the CO2 capture cost ($520 ¨> $0), with a 9% (0.18 V to 2 V) increase to electricity cost ($750 ¨> $817), making the production cost for 1 ton of ethylene to $847.
Therefore, SC-MEA has the potential to make the 002-to-ethylene electrolysis economically feasible.
Nevertheless, the buffer catholyte is not used in the present SC-MEA. In the buffer catholyte SC-MEA, CO2 is generated at the boundary of CEL and catholyte and diffuses across the catholyte layer (125 pm) to Cu. Previous simulation works have suggested that such a long diffusion path cannot support a high rate (>100 mA cm-2) CO2RR
(see reference 38). This effect also explains the experimental observation when using KHCO3 (with some buffer capability) as the stationary catholyte, as shown in Figure 8 discussed in section SI3. In this work, the primary purpose is to demonstrate that the SC-MEA can effectively prohibit CO2 crossover and promote high SPU in conjunction with a BPM. A
non-buffered catholyte like K2SO4 can shorten the regenerated CO2 diffusion path. As shown in Scheme 1C, the migration of protons from the CEL to Cu acidifies the stationary catholyte, making the diffusion path shorter than that in a buffer catholyte.
However, this phenomenon also creates a larger pH gradient than that in buffer catholyte SC-MEA. The pH gradient is greater at higher current densities, as needed to drive the larger proton/hydroxide fluxes.
In the future, by adopting strategies from membrane science and materials engineering, the catholyte thickness can be reduced to below 10 pm and a buffer catholyte can be used with minimum cross-over that will minimize the pH gradients developed within the system and thus the concentration overpotential losses associated with them. Coupling with the efforts of BPM materials and catalysts development, it is expected that the SC-MEA may be a high-total-energy-efficient system for CO2-to-C2, production.
SI3 Additional information for the catholyte engineering towards high SPU

With the present BPM-based SC-MEA, the CO2 consumed for CO2RR is provided by two sources: the inlet CO2 flow (gas), and the regenerated CO2 (dissolved form) in the stationary catholyte layer. As the upper limit of the CO2 SPU for most of C2, is 25%, the CO2 regeneration procedure in the SC-MEA will supply 75% of the CO2 consumption if the target is to achieve 100% SPU. Thus, the mass transport effectiveness of the regenerated CO2 is an important consideration in the SC-MEA, which is determined by the thickness of the stationary catholyte layer because a too thick catholyte layer cannot effectively deliver the regenerated CO2 to Cu catalyst, as analyzed below.
At steady-state, the net current in the stationary catholyte layer of the SC-MEA should be primarily driven by the electromig ration of protons/hydroxide and carbonate/bicarbonate ions simultaneously generated from water dissociation of BPM and reactions [1]/[2], respectively. Referring to Figure 6, the protons and carbonate/bicarbonate ions combine somewhere in the stationary catholyte layer, forming a virtual boundary where CO2 is regenerated and diffuses towards Cu and CEL driven by the concentration gradient. The distances from this boundary to the Cu and CEL are noted as Li and L2, respectively. The zones between the boundary and Cu/CEL are noted as Zone 1 and Zone 2 for convenient discussion. Before the electrolysis, the CO2 concentration is zero everywhere in the stationary catholyte layer.

After the electrolysis starts and proceeds, the CO2 concentration at the boundary gradually arises, driving the generated CO2 (dissolved form) to diffuse towards Cu and CEL. The CO2 (dissolved form) that diffuses deeper into the stationary catholyte layer towards the CEL is not consumed by CO2RR but accumulates in Zone 2 until reaching a concentration 5 close to that at the boundary (then no driving force). In the other direction, the CO2 diffuses towards Cu is consumed for CO2RR, forming a concentration gradient in Zone 1, which creates a continuous CO2 diffusion flux from boundary to Cu. Note that the real CO2 (dissolved form) concentration distribution in Zone 1 deviates from linear due to the local pH gradient. Zone 1 can be described as a diffusion layer (see reference 40).
The diffusion 10 layer thickness, Li, has great impacts on the CO2 mass transport (see reference 40). Since the electric field of the stationary catholyte layer can be considered homogeneous (see reference 41), Li and L2 are known to be proportional to the mobility of the corresponding ions. Given that L1+L2 is the total thickness of the stationary catholyte layer, a thinner stationary catholyte layer has a thinner CO2 diffusion layer (Li), which is beneficial for CO2 15 mass transport from bulk to catalyst (see reference 40). In the future, thinner, yet mechanically robust porous layers, can be employed to improve the mass transport of the regenerated CO2, as discussed in SI2.
A previous work (see reference 22) showed that in an MEA cell, inserting a solid porous supporting layer saturated with DI water in between Ag catalyst and the cation-exchange 20 layer of the BPM can improve the FE for converting CO2 to CO. In the system, this strategy was also attempted, and the electrolyzer shows a >94% FE towards hydrogen, and a high full-cell voltage of 6-9 V, even under optimized conditions, as shown in Figure 7A. On the other hand, the cation effect plays an important role in the SC-MEA, which can suppress the HER and promote CO2RR under acidified environment near the Cu catalyst, as shown 25 in Figure 7B, 7C, and 12A.
The K2SO4 catholyte in the stationary catholyte layer will gradually be partially transformed to KHCO3 over time owing to reactions [1] and [2] as well as the slow leakage of S042- to anolyte. To study the impact of such transformation, the CO2RR performance of SC-MEA
was measured when a 1 M KHCO3 buffer electrolyte is used as catholyte at the beginning.
30 It was found that using 1 M KHCO3, the SC-MEA shows an expectable C2I-14 FE of ca. 23%
at the current density of 100 mA cm-2. However, its stability is poor in that the FE gradually decreases by > 50% after the initial 5 hours. This experiment has been repeated three times, and a typical FEs and cell voltage vs. time curve is displayed in Figure 8. HCO3-has a higher buffer capacity than S042-, and the pH gradient built up in KHCO3 is thus expected to be smaller than that in K2SO4. The CO2 diffusion layer thickness in KHCO3 is thus likely greater than that in 0.5 M K2804 catholyte (analyzed above). With the thicker CO2 diffusion layer, the regenerated CO2 may gradually accumulate at the boundary (Figure 6) because the concentration gradient-driven CO2 diffusion flux is lower than the CO2 generation rate. When the accumulated aq. CO2 reaches the saturated concentration, it could bubble out periodically and physically damage the catalyst layer and/or stationary catholyte layer. This phenomenon may also be the reason for the periodical voltage fluctuation in Figure 8.
To this end, the stationary catholyte layer was engineered in SC-MEA to a 125 pm thick, 0.5 M non-buffer K2504 solution.
Since it was discovered that the cation effect enables the CO2RR even in an acidified environment, it was attempted to extend the stationary catholyte layer strategy in an MEA
cell using CEM and an acidic anolyte pH <2.37, expecting a lower cell voltage than the BPM-based cells while maintaining high SPU. The configuration of this system is shown in Figure 9A. Some previous studies have confirmed that, in neutral electrolytes, using a cation exchange membrane (CEM) instead of AEM even increases the amount of CO2 crossover to the anode gas (see reference 13). In contrast, it was found that in the acidic MEA cell, the CO2 crossover was eliminated. The anode gas CO2 contents were below the detection limit of the GC for the current density in the range between 100 to 300 mA
cm-2. This observation should be ascribed to the lower pH near the stationary catholyte layer/CEM interface, as shown in Figure 9A.
This configuration shows lower full cell voltage (Figure 9B) in comparison to the BPM-based SC-MEA presented in the main text in part due to the low resistance of the CEM.
Meanwhile, it has a reasonable CO2RR selectivity over HER because the Win the anolyte can migrate to the Cu surface to induce the cation effect. However, this design was not amenable to steady state operation without continuous addition of acid and salt to the anolyte and catholyte as the initial pH gradient will be eliminated due to co-ion transport and neutralization. In fact, the CO2RR selectivity of this system gradually drops over time, and after ca. 3 hours, this system produces 100% hydrogen at the cathode. In addition, it was observed that this design periodically eject electrolyte from the cathode flow channel, probably due to a water balance issue:

On the anode, the OER generates one proton per one electron transfer:
2H20 ¨ 4e- ¨> 4H+ + 02 Since a OEM is used in this system, the charge carriers across the membrane are K+ and H. K+ is adsorbed at the Cu surface to form an electrochemical double layer, and the migration of K+ terminates until reaching the equilibrium between electric potential and chemical potential, which usually takes tens of seconds (see reference 19) H+
migrates to the cathode side combines with OH- (or 0032-/HCO3-), which produces water at the cathode side. The water balance for different cathode reactions ([3-9] in section SI1) was accordingly calculated:
Table S2. The water balance in a CEM-based SC-MEA.
Water balance in the cathode Water balance in the anode Product (Mole) (Mole) (1 Mole) Consumed Generated Net Consumed CO 1 2 +1 1 CH3000- 5 7 +2 4 02H4 8 12 +4 6 C2H5OH 9 12 +3 6 CH4 6 8 +2 4 As seen, most of the cathode reactions generate water on the cathode side. The generated water keeps diluting and pushing out the electrolyte in the stationary catholyte layer, of which the volume is small (ca. 10 pL per cm2 electrode area). This phenomenon results in the flooding (also confirmed by experimental observation) of the cathode and continues loss of K+ in the system, degrading CO2RR performance because the concentration of K is critical to CO2RR performance for the catalyst (Figures 7A to 70).
SI4 Additional considerations to BPM in SC-MEA

In a neutral anolyte solution, a customized BPM was adopted, which consisted of a layer of TiO2 nanoparticles as a water dissociation catalyst sandwiched by a CEM and an AEM.
In an acidic anolyte solution, a commercially available BPM (Fumasep) was used to achieve the best compromise among CO2RR performance, cell voltage, and stability.
Water splitting measurement was firstly conducted to compare the resistance of customized BPM and Fumasep. Figure 10 shows that the BPM with TiO2 water dissociation catalyst (black plots) has lower resistance than the one without water dissociation catalyst (blue plots) and commercially available Fumasep BPM.
When been used in the SC-MEA with neutral anolyte (Figure 11), Fumasep shows similar product distributions and a slightly better CO2 crossover inhibiting capability (Figures 2D, and 16) but higher cell voltage comparing with the one based on customized BPM, which is expectable. Therefore, the customized BPM was adopted in the neutral anolyte studies.
For the SC-MEA using acidic anolyte, the customized BPM also promotes lower cell voltage than Fumasep (Figure 12A). However, this system always fails within 4-6 hours of continuous operating due to the short circuit issue, and a typical voltage versus operating time diagram is shown in Figure 12B. It is suspected that this is caused by the growth of Cu dendrites that physically penetrated through BPM and contact with the anode. Cu could be partially dissolved by acid and electrochemically re-deposited onto the catalyst layer, forming sharp dendrites (see reference 42). Differently, it was found that the Fumasep-based SC-MEA is more stable (Figure 4B in the main text) probably because Fumasep is mechanically rigid, so Cu dendrites cannot penetrate. To this end, Fumasep was adopted in the acidic anolyte studies.
S15 Temperature and pH effects on CO2RR performance in SC-MEA
Elevating the operating temperature reduced the full cell voltage at given current densities (Figure 2B in the main text), which is attributed to accelerated water dissociation, reduced cross-membrane ohmic loss, and improved electrode kinetics (see reference 18).
As displayed in Figure 2C, 13A and 13B, with increasing operating temperature, the optimal ethylene FE first increased from 25% (20 C, 200 mA cm-2) to 35% (35 C, 250 mA
cm-2) and then decreased to 30% (50 C, 150 mA cm-2). This trend is attributed to the trade-off between CO2RR and HER kinetics as well as the CO2 solubility (detrimental to FE) (see references 1, 43 and 44). It is thus concluded that 35 C can be an optimal temperature in the present device design.

SI6 Control experiment of CO2 crossover Referring to Figure 16, all the gas samples were recorded after the cell operating for 1 hour at each current density. In K2SO4, the CO2: 02 ratio is close to that in KHCO3 for customized BPM, indicating that the detected anode CO2 flow in the customized BPM/KHCO3 system (Figure 2D in the main text) was not ascribed to the acidification of bicarbonate. In an SC-MEA using KHCO3 anolyte, the CO2 crossover through Fumasep is similar to the case through customized BPM.
Further results regarding the thickness and the composition of the stationary catholyte ¨(SC) layer Finite-element numerical simulations of the stationary catholyte (SC)-layer The present invention founds that the composition and thickness of the catholyte layer influence the local pH, the efficiency of CO2 regeneration and, thereby, the overall cell performance. A one-dimensional multiphysics model in COMSOLO was applied to investigate the catholyte layer in BPM-based CO2RR electrolyzers.
The CO2 reactant is provided by two sources: the inlet CO2 flow (gas) and the regenerated CO2 (dissolved form, aq.) in the catholyte. To achieve high SPU, it is necessary to restrict the gaseous CO2 feed. Under a restricted gaseous CO2 availability, the cathode supply relies more on regeneration: in an ideal case with 100% SPU and 100%
C2, selectivity, regeneration contributes 75% of the consumed CO2. Thus, the mass transport of regenerated CO2 is most critical, and that transport is governed by catholyte composition and thickness.
At steady-state, electrolysis creates a pH gradient through the catholyte layer: the pH is high near the cathode and low near the CEL. The protons and (bi)carbonate ions recombine in the catholyte, forming CO2 (aq.) that diffuses, in response to a concentration gradient, to the Cu catalyst.
Simulations resolve the local cathode environment as a function of dimensions, electrolyte and running conditions. The modeled thicknesses of 250, 125, 65 and 16 pm were selected to correspond to commercially available materials.
Use of a buffering catholyte (e.g. KHCO3) leads to a thick CO2 (aq.) diffusion layer¨close to the catholyte thickness, since the CO2 (aq.) is generated near the CEL
surface. This effect reduces the CO2 (aq) mass-transfer efficiency.

In contrast, with a non-buffering catholyte layer (e.g. 0.5 M K2504) with thicknesses of 250, 125, and 65 pm, the local pH values near the cathode are greater than 11, which is sufficient to promote selectivity towards CO2RR over HER. Reducing the SC-layer thickness to 16 pm results in a cathode pH of 8.7, implying a lower selectivity toward 5 CO2RR.
Fig. 1c and ld show the simulated concentration profiles of CO2 (aq.) in the non-buffering SC-layer. At steady-state, the CO2 (aq.) is continuously supplied to the cathode to participate in CO2RR, forming a concentration gradient (the boundary was defined here as the position where CO2 concentration is 1% lower than the saturated concentration) to 10 the cathode surface. Prior studies have termed the zone between the cathode and this boundary the diffusion layer (see reference 40). The thickness of the diffusion layer controls the efficiency of CO2 (aq.) mass transport (see reference 40).
According to the simulations, the thicknesses of the diffusion layers are 75, 35, 12 and 5 pm for the catholyte layers with the thicknesses of 250, 125, 65 and 16 pm, respectively.
For 15 reference, the CO2 (aq.) diffusion layer thickness in H-cells (all CO2 supplied in dissolved form) is typically 40-100 pm, and this does not support current densities exceeding 100 mA cm-2. It was expected that diffusion layers <40 pm, and a corresponding catholyte thickness <150 pm, are required for sufficient mass transport in a non-buffering catholyte.
To achieve similar mass transport in a buffering catholyte, the total thickness could not 20 exceed 12 pm, and the cathodic pH would not be sufficiently alkaline for selective CO2RR.
The simulation results suggest the following design principles for the catholyte layer in a BPM-based electrolyzer: the local cathode pH and the diffusion layer thickness of the regenerated CO2 increase as the catholyte thickness increases; the buffering capacity of the catholyte increases the diffusion layer thickness and reduces transport.
Precise control 25 of the thickness of a non-buffering catholyte should thus offer a route to high SPU, CO2RR
selectivity and reaction rate.
System design for high SPU of CO2 feedstock Guided by the above analysis, the inventors focused on a stationary catholyte bipolar 30 membrane electrode assembly (SC-BPMEA) electrolyzer and incorporated a judiciously-designed catholyte layer and BPM.
The cathode was prepared by spraying Cu nanoparticles onto a hydrophobic carbon gas-diffusion layer for CO2RR. The anode was 1102 supported on Ti felt for the oxygen evolution reaction (OER). A BPM under reverse bias was employed with the AEL contacting the anode and the CEL contacting the SC-layer (porous support saturated with electrolyte).
The cathode was compressed onto the porous layer, and the anode and cathode flow-field plates sandwiched the system.
The BPM employed in this work sandwiched TiO2 nanoparticles as the water dissociation catalyst (see reference 18). This custom BPM can lower the cell voltage by -1 V compared with commercial BPMs (e.g. Fumasep). The full cell voltage of such custom BPM-based electrolyzers is close to that of AEM systems.
Measurements of the 002/02 ratio in the anode gas stream show that the SC-BPMEA
effectively prevents CO2 crossover, as required for high SPU (see references 13 and 14).
In agreement with the previous studies (see references 13 and 14), the AEM-based MEA
(AEMEA) showed an anode CO2/02 ratio of -2 for current densities ranging from 100 to 300 mA cm-2. In conventional AEMEAs, the anionic charge carriers are C032-, and thus suffer the loss of one molecule of CO2 for every two electrons transferred.
The anode CO2/02 ratio in the SC-BPMEA (0.06 at 200 mA cm-2) is one order of magnitude lower.
Control experiments confirm that the CO2 detected in the anode is not due to acidification of anolyte (using 0.1 M K2SO4 instead of 0.1 M KHCO3 resulted in a similar CO2 /02 ratio).
The anode 002/02 ratio decreases as the operating current density increases, an effect that was ascribed to an increased flux of protons toward the cathode. This flux decreases the pH at the CEL surface and reduces the diffusion of CO2 and H003-/003- in the CEL
(see references 19 and 28).
Impact of the thickness of the SC-layer on CO2RR.
The thickness of the stationary catholyte was found to have a major impact on cell voltage.
The cell voltage of the SC-BPMEA decreases as the thickness of the SC-layer decreases (Fig. 17) from 250 pm (5.1 V, 200 mA cm-2) to a minimum at 65 pm (3.8 V, 200 mA cm-2).
Further thinning the catholyte to 16 pm resulted in higher voltage (4.4 V, 200 mA cm-2) -an effect of the lower-porosity support layer used in the 16 pm case (< 20%
vs. > 70% for the thicker layers). A longer ion migration path and higher ohmic resistance partially explain the 0.67 V cell voltage increase as the stationary catholyte thickness increases from 65 to 125 pm. Based on the independently measured ohmic resistance (Supplementary Fig. 18), increasing the SC-layer thickness from 65 to 125 pm imposes an ohmic voltage increase of merely 0.07 V at 200 mA cm-2. Similarly, compared to 65 pm, the 250 pm SC-layer increases the ohmic voltage loss by 0.24 V at 200 mA
cm-2, while the cell voltage increases by 1.3 V.
The simulations indicate that the thicker SC-layer results in longer transport distances for dissolved 002. The CO2 regeneration rate inside the SC-layer also depends on the current density, and for thicker SC-layers (e.g. > 125 pm), CO2 bubbles are more prone to form near the CEL. These bubbles obstruct ion migration, increasing the ohmic resistance of the SC-BPMEA. Electrochemical impedance spectroscopy measurements also support this finding. An applied current of 200 mA cm-2 resulted in an insignificant change to the high-frequency resistance (HFR) of the SC-BPMEA with a 65 pm-thick SC-layer;
while, in contrast, the HFR of the SC-BPMEA with a 125 pm-thick SC-layer increased by 120%
after applying 200 mA cm-2 for 20 min, leading to a cell voltage 0.6 V higher than for the 65-pm SC-layer.
The cell voltage of the SC-BPMEA with a 65 pm SC-layer operating at 200 mA cm-2 is 3.8 V, comparable to the AEM-based neutral-media MEAs operating at similar conditions (difference < 0.05 V) (see references 5, 6 and 15). This result demonstrates that the cell voltage of a BPM-based CO2RR electrolyzer can be as low as that of an AEM-based electrolyzer with a current density of up to 200 mA cm-2, while suppressing unwanted crossover and providing high SPU.
The thickness of the SC-layer also affects selectivity towards CO2RR. With thicknesses of 65, 125 and 250 pm, the H2 Faraday efficiencies (FEs) are consistent (- 20% at 200 mA
cm-2, Fig. 19a-19c), confirming that high local pH conditions are maintained the cathode in these cases. However, reducing the thickness to 16 pm increases the H2 FE
to 88% at 200 mA cm-2 (Fig. 19d), consistent with a cathodic pH that is reduced due to fast proton transport through a thin SC-layer. Without restricting CO2 availability (the performance in Fig. 19a-19d was recorded at a CO2 flow rate of 10 sccm cm-2), the SC-BPMEAs with the SC-layer thickness of 65, 125 and 250 pm show similar ethylene FE of 35-43%.
Assessment of SPU in SC-BPMEA.
By suppressing the crossover of CO2 (e.g. <0.5% of total CO2 input at 200 mA
cm-2, Fig.
2d and 20), the SC-BPMEA surpasses the SPU of conventional CO2-to-C2, electrolyzers, in which carbonate is the dominant charge carrier. Measuring the CO2 SPUs with a restricted CO2 flow rate is a direct approach to determining the upper bound of SPU in the CO2RR electrolyzers.
As the inlet CO2 flow rate decreased, the C2+ FE of the SC-BPMEA at 200 mA cm-decreased, accompanied by an increase in the H2 FE (Fig. 4a, 4b and 4c). With SC-layer thicknesses of 65 pm (Fig. 22c), as the input 002 flow rate decreases from 1.17 to 0.58 and 0.29 sccm cm-2, the 02+ FE decreases from 49% to 48% and 34%, while the H2 FE
increases from 23% to 31% and 64%. This shift is consistent with a 002 mass transport limitation (see references 6 and 15).
The stationary catholyte thickness affects the SPU of the SC-BPMEA. The SPU
gradually increases up to 21, 61 and 78% for the SC-BPMEAs with SC-layer thicknesses of 250, 125 and 65 pm, respectively (Fig. 21). These results demonstrate that high CO2 conversion efficiencies are possible using SC-BPMEAs with SC-layer thicknesses of 125 and 65 pm.
For a given CO2 flow rate, a thicker SC-layer produces a lower SPU (Fig. 21).
In the SC-BPMEA, reactant CO2 is available from the inlet gas stream and regeneration in the SC-layer. With unrestricted CO2 supply (Fig. 19a-Fig 19c), the H2 FEs are similar for different stationary cathode layer thicknesses, indicating that both the CO2 availability and local pH
are unaffected by catholyte thickness under excess supply conditions. The simulations suggest that the thicker SC-layer results in a lower dissolved CO2 flux to the cathode due to the smaller concentration gradient. Compared to the SC-BPMEAs with thinner SC-layers, CO2 availability with thicker SC-layers decreases more significantly with reducing CO2 flow rate, leading to a more dramatic increase in H2 FE (Fig. 22a-22c).
The experimental trends are generally consistent with those of the simulations. The SC-BPMEA with a dissolved CO2 diffusion layer thicker than 75 pm (representing a 250-pm SC-layer) fails to surpass the SPU limit because of insufficient mass transfer. In contrast, a 65-pm SC-layer facilitates efficient mass transport of the regenerated CO2 (diffusion layer thickness of 12 pm) and simultaneously promotes high local cathode pH.
It was found (see Fig. 23a-b and Fig. 24a-d) that SC-BPMEAs using acidic and alkaline electrolytes achieve carbon efficiencies comparable to those using neutral electrolytes.
The compatibility of SC-BPMEAs with a range of electrolytes offers flexibility in the selection of cathode and anode catalysts. In contrast, in prior art, acidic 002-to-C2+

electrolyzers have only been demonstrated with precious metal anodes. Indeed For the SC-BPMEA using acidic anolyte, the custom BPM enables a lower cell voltage than Fumasep. However, this system always fails within -4-6 h of continuous operation due to an apparent short-circuit issue, and the typical voltage versus operation duration is shown in Fig. 23b. We suspect this is caused by the growth of Cu dendrites that physically penetrated through BPM and contact with the anode. Cu could be partially dissolved by acid and electrochemically re-deposited onto the catalyst layer, forming sharp dendrites.
Differently, itw as found that the Fumasep-based SC-BPMEA is more stable, probably because Fumasep is mechanically reinforced and thus more rigid, so Cu dendrites cannot easily penetrate.
The SC-BPMEA shows > 50-h stability operating at 200 mA cm-2 with limited CO2 availability (CO2 input flow rate of 1.42 sccm cm-2). This operating stability is competitive with that of the neutral-electrolyte-based CO2-to-C2, electrolyzers.
Can a cation-exchange membrane replace the BPM in SC-BPMEA?
The inventors attempted to extend the SC-layer strategy in a CEM-based MEA
cell (i.e.
SC-CEMEA, Fig 9a) using an acidic anolyte with pH <2.4, expecting a lower cell voltage than the SC-BPMEA while maintaining high SPU. It was found that in the SC-CEMEA, the CO2 crossover was essentially eliminated. This observation is ascribed to the lower pH
near the stationary catholyte layer/CEM interface, as shown in Fig. 9a.
SC-CEMEA shows a lower full cell voltage (Fig. 9b) compared to the SC-BPMEA
presented, partly due to the lower resistance of the CEM and the absence of water dissociation overpotential. Meanwhile, it has a reasonable CO2RR selectivity over HER
(Fig. 9c) due to the cation effect and high local pH induced by the presence 1K+ in the SC-layer (Fig. 9a). However, this design is not amenable to steady-state operation without continuous addition of acid and salt to the anolyte, as the initial pH
gradient will be eliminated due to co-ion transport and neutralization. We found the CO2RR
selectivity decreases over time and approaches 100% H2 after -3 h.
It was also observed that the SC-CEMEA design periodically ejects electrolyte from the cathode flow channel, likely due to poor water balance. On the anode, the OER
generates one proton per one electron transfer. The charge carriers across the CEM are primarily H+, although neutral ion pairs will diffuse as well. At the cathode K+ makes up the electrochemical double layer at the Cu surface, and the steady-state 1K+
profiles are governed by the electric and chemical-potential gradients that develop under operation, which usually takes tens of seconds (see reference 19). H+ migrates to the cathode and combines with OH- (or C0321HCO3-), producing water at the cathode. The protons also drag water molecules (-1 per proton) by electro-osmosis. It was accordingly calculated the water balance for different cathode products as listed in Table 2. The water generated 5 and transported to the cathode appears to dilute and push out the electrolyte in the stationary catholyte layer, of which the volume is small (ca. 10 pL per cm2 electrode area).
This phenomenon results in flooding of the cathode (as confirmed experimentally) and loss of supporting electrolyte, thus degrading performance. In the BPMEA
design, it is likely that the BPM slows co-ion transit across the membrane, compared to the OEM, by 10 the large outward flux of OH- and H+ from the water dissociating junction.
Table 2. The cathode-anode water balance in an SC-CEMEA.
Product Water balance in the cathode (mol) Water balance in the anode (mol) (1 mol) Consumed Generated Dragged in Net Consumed and dragged out CO 1 2 2 +3 3 HC00- 1 1 1 +1 2 CH3C00- 5 7 7 +9 11 C2H4 8 12 12 +16 18 C2H5OH 9 12 12 +15 18 CH4 6 8 8 +10 12 H2 2 2 2 +2 15 Energy assessment of the SC-BPMEA with optimal SC-layer The energy costs (measured in gigajoules per tonne of the target product, GJ/t) for a 002-to-02+ electrolyzer include the electrolysis electrical energy, cathodic stream separation, and anodic stream separation. CO2RR performance metrics of importance include cell voltage, target product FE, SPU and CO2 crossover (see reference 12). High SPU
and 20 high energy efficiency have not been accomplished simultaneously in 02+
electroproduction. In SC-BPMEAs, a higher SPU reduces the energy required for cathode separation, but the accompanying decrease in the ethylene selectivity (Fig.
22c) elevates the specific energy requirement. Atotal energy assessment of the SC-BPM EA was carried out and other state-of-art CO2-to-ethylene electrolyzers and summarized the results in 25 Table 3.
Table 3 Comparison of the energy intensity between various 002-to-ethylene electrolyzers and this work. All the energy costs are normalized per ton of ethylene produced. The energy intensities of the SC-BPMEA operating at other CO2 input flow rates are listed in Table S3 of Supplementary Information.
This work This work Neutral- Acidic Acidic MEAa Metrics (10 sccm (1.17 sccm MEA a 15 flow cell a (1) 1(2) CM -2) cm-2) Cell type MEA Flow cell MEA Sc-SC-BPMEA
Electrolyte Neutral Acidic Acidic Neutral Neutral Full cell voltage (V) 3.75 4.20 3.80 3.82 3.82 Ethylene FE (`)/0) 45 28 36 42 Current density (mA cm-2) 150 1200 100 200 Input CO2 flow rate 8 3 0.8 10 1.17 (sccm cm -2) Total CO2 SPU (%) 3 78 34 4.1

35 CO2-to-ethylene (%) 12 28 7.6 2.0 Demonstrated stability 100 14 12 52 Energy intensity (GJ per ton ethylene) Electrolyzer electricity 345 620 436 385 Cathode separation 38 17 28 85 Anode separation 116 0 b 0 b 0 b 0 b Overall energy 499 637 465 470 ' The energy intensities of reference CO2-to-ethylene devices operating under the reported conditions are calculated, and those that provide the lowest energy intensity are presented in this table.
b Crossover of CO2 in the acidic flow cell, acidic MEA and SC-BPMEA are each lower than 0.5% of input CO2. Therefore, we assume the anodic separation energy to be 0.
0) Huang, J. E. et al. CO 2 electrolysis to multi-carbon products in strong acid.
Science 372, 1074-1078 (2021) (2) O'Brien, C. P. et al. Single Pass CO2Conversion Exceeding 85% in the Electrosynthesis of Multicarbon Products via Local CO2Regenerati on. ACS
Energy Lett. 6, 2952-2959 (2021) In such systems, CO2 and OH- react to form carbonate continuously. This carbonate has to be recovered to maintain the CO2RR performance of such a system, consuming 5.5 GJ
per tonne CO2. In the alkaline CO2RR electrolyzers, ca. 63 tonne of CO2 transforms to carbonate to produce 1 tonne of ethylene, representing an energy penalty of 350 GJ. This costs at least $1,900 per tonne of ethylene, while its market price is $800-1000 per tonne.
The alkaline electrolyzers thus do not allow for ethylene electrochemical production to be yet profitable.

In neutral-media CO2RR electrolyzers, recovering the CO2 from the anodic gas stream results in significant energy costs. In the context of highly selective conversion (i.e., CO2-to-ethylene with unity selectivity), the recovery process requires an energy input of 52 GJ
to produce every tonne of product. In practice, due to non-unity product selectivity, the process is even more prohibitive, i.e., requiring an energy penalty of 80 -130 GJ for producing one tonne of ethylene.
As the SPU increases from 4 to 35%, we found a dramatic decrease in energy associated with cathode separation - from 85 to 15 GJ/t ethylene (Table 3), with the ethylene FE
reduced by only 2%. Further increasing the SPU beyond 35% does not substantially reduce the energy cost associated with cathodic separation. This finding agrees with a recent energy analysis that in a (bi)carbonate-free CO2-to-C2, electrolyzer, improving SPU
over 40% offers an insignificant benefit to the downstream separation cost.
Pursuing an SPU > 35% decreases ethylene FE by more than 4% when using the SC-BPMEA, and thus the increased input electricity cost exceeds the savings in the cathodic separation (Table 3). Therefore, 35% SPU is the most favourable condition for the present SC-BPMEA.
The energy intensity of producing ethylene in SC-BPMEA is -30% lower than that in conventional neutral-electrolyte-based CO2 electrolyzers (Table 3). In conventional neutral-electrolyte 002-to-ethylene electrolyzers, the CO2 crossover (at least 70%) costs 60-90 GJ per ton of ethylene to recover CO2 from the anodic 02 stream'.
Notably, this energy penalty cannot readily be reduced, independent of optimizing catalysts and operating conditions (e.g. input CO2 flow rates, reaction rates, operating temperature and pressure). In contrast, crossover CO2 in SC-BPMEA is < 0.5% of the total CO2 input, minimizing the energy cost of anodic separation.
Recently, CO2-to-ethylene conversion has been achieved in acidic electrolytes in both flow cell and MEA configurations. These systems enabled CO2 SPUs exceeding 75% and also mitigated the energy cost associated with anodic separation (Table 3). Owing to the strongly acidic environment, the flow cell enables an ethylene FE of 28% at a full-cell potential of 4.2 V. The acidic MEA used an anion-exchange ionomer coating on the catalyst layer to promote CO2RR over HER. The modification of the surface with the anion exchange ionomer resulted in a higher ohmic loss, and thus the cell required potentials of 3.8 V and 4.4 V at 100 mA cm-2 and 200 mA cm-2, respectively. These devices thus eliminated the anodic CO2/02 separation energy but at the penalty of larger cell voltages and/or lower ethylene FEs. In contrast, SC-BPMEA shows a cell voltage of 3.8 V
at 200 mA cm-2 with an ethylene FE of 42% - voltages and selectivities comparable to the best conventional neutral-electrolyte CO2-to-ethylene MEAs (see reference 15).
Compared to acidic systems, the energy intensity of the SC-BPMEA is 36% and 12% lower than acidic flow cell and acidic MEA, respectively (Table 3).
The inventors have demonstrated a BPM-based CO2-to-02, MEA, with a judiciously-designed SC-layer between catalyst and BPM, that overcomes the (bi)carbonate-formation reactant loss issue without compromising performance. The composition and thickness of the SC-layer determine the CO2RR performance and SPU via a strong influence on the local pH and the chemistry and transport of CO2 The buffering capacity and the thickness of the SC-layer determine the efficiency of the regeneration, the transport, and the availability of reactant CO2. These effects were predicted in simulations and supported by experiments. The SC-BPMEA design largely eliminates the energy penalty associated with the CO2 loss in electrochemical CO2 reduction.
The performance of the SC-BPMEA might be further improved using, for example, ionic liquid or other organic salts as the catholyte, and by optimizing the porosity, structure and hydrophobicity of the porous support layers. The CO2RR performance of the SC-BPMEA
might be improved with new cathodic catalysts, optimizing the loading and processing of the catalyst layer, and by implementing BPMs with further-lowered water-dissociation voltage loss. Broadly, the SC-BPMEA is a useful platform for evaluating CO2RR
catalysts operating with high CO2 utilization. The strategy and findings presented here are also relevant to the electrochemical systems such as nitrate reduction and (bi)carbonate reduction, where controlling dissimilar microenvironments near each electrode is useful, and the exchange/transport of species (other than OH- or H+) between cathode and anode is problematic.
Methods Materials Phosphoric acid (H3PO4, 85%), potassium sulfate (K2SO4, 99%), potassium bicarbonate (KHCO3, 99.7%), potassium chloride (KCI, 99%), potassium hydroxide (KOH, 99.95%), copper nanoparticles (25 nm), NafionTm 1100W (5 wt. % in a mixture of lower aliphatic alcohols and water) and isopropanol (IPA, 99%) were purchased from Sigma Aldrich and used as received. Titanium oxide nanoparticles (TiO2, Aeroxide P25) were purchased from Fisher Scientific and used as received. The porous supports were also purchased from Fisher Scientific: 125 pm PVDF (0.45 pm pore size), 65 pm PTFE (0.44 pm pore size) and 16 pm PC (0.4 pm pore size). NafionTM 212, NafionTM XL, Fumasep (FAS-PET-130) and titanium (Ti) felt were purchased from Fuel Cell Store. Iridium(IV) chloride hydrate (Premion , 99.99%, metals basis, Ir 73% min) was purchased from Alfa Aesar.
The water used in this study was 18 MO Milli-Q deionized- (DI-) water. Nafion membranes were activated through the following procedure: 1 h in 80 C 1M H2SO4¨ 1 h in 80 C
H202¨ 1 h in 1 M H2SO4 ¨ stored in DI-water. Fumasep was used as received and stored in 1 M
KCI. Piperion (40 pm) was purchased from W7Energy and stored in 0.5 M KOH.
Fabrication of water dissociation catalyst layer of the custom bipolar membrane (BPM) The water dissociation catalyst layer was fabricated following a similar procedure in a previous report17. TiO2 nanoparticles inks were prepared by sonicating the mixture of TiO2, DI-water, and IPA with the weight ratio of 1: 833: 2833 for 30 min. TiO2 nanoparticle ink was spray-coated onto a Nafion 212 membrane, of which the edges were sealed by Kapton tape. The exposed membrane dimension was 2.2 cm x 2.2 cm. The nominal loading of TiO2 is 0.2 mg cm-2. The TiO2-coated NafionTM was immediately used for assembling electrolyzers once prepared.
Electrode preparation For the CO2RR, we prepared the gas diffusion electrodes (GDEs) by spray-depositing a catalyst ink dispersing 1 mg mL-1 of Cu nanoparticles and 0.25 mg mL-1 of NafionTM 1100W
in methanol onto a hydrophobic carbon paper. The mass loading of Cu NPs in the GDE
was kept at 1.5 mg/cm2. The GDEs were dried in the air overnight prior to experiments.
The OER electrode preparation procedure involves: etching the Ti felt in hydrochloric acid at 70 C for 40 min; rinsing the etched Ti felt with DI water; immersing the Ti felt into an Ir(IV) chloride hydrate solution; drying and sintering the ft-loaded Ti felt.
The loading, drying, and sintering steps were repeated until a final Ir loading of 1.5 mg cm-2 was achieved.

Assembly of the stationary catholyte membrane electrode assembly (SC-BPMEA) The MEA set (5 cm2) was purchased from Dioxide Materials. A cathode was cut into a 2.1 cm x 2.1 cm piece and placed onto the MEA cathode plate with a flow window with a 5 dimension of 2.2 cm x 2.2 cm. The four edges of the cathode were sealed by Kapton tape, which also made the flow window fully covered. The exposed cathode area was measured every time before the electrochemical tests, in the range of 3.1 to 4.2 cm2.
Onto the cathode, a porous support layer (2 cm x 2 cm with various thicknesses, 250 pm was stacking two 125 pm-thick PVDF) saturated with desirable electrolyte (sonicated in 10 electrolyte for 15 min to degas) was carefully placed. This porous support layer serves as the 'stationary catholyte layer (SC-layer).' The considerations of membrane selection can be found in SI2 and SI4 of the Supplementary Information. When using the custom BPM, a TiO2-coated Nafion membrane was placed onto the SC-layer with the TiO2 layer facing up, then covered by a Piperion (5 cm x 5 cm) membrane. When using Fumasep BPM, the 15 membrane was placed with its cation-exchange layer (CEL) facing the cathode side. An Ir02 loaded Ti felt (2 cm x 2 cm) was placed onto the anion-exchange layer (AEL) of the BPM.
Scanning electron microscopy (SEM) 20 Images of cathode and custom BPM were captured by an FEI Quanta FEG 250 environmental SEM.
Electrochemical measurements Throughout all experiments, CO2 flowed to the cathode side at 10 sccm cm-2 unless 25 otherwise specified, while the anode side was fed with neutral 0.1 M
KHCO3 at 10 mUrnin by a peristaltic pump unless otherwise specified. The electrochemical measurements were performed with a potentiostat (Autolab PGSTAT204 with 10A booster). The cell voltages reported in this work are not iR corrected. The system was allowed to stabilize at the specific conditions for > 1000 seconds before recording the results. All the error bars 30 represent standard deviations based on three measurements.
Product analysis The CO2RR gas products, oxygen, and CO2 were analyzed by injecting the gas samples into a gas chromatograph (Perkin Elmer Clarus 590) coupled with a thermal conductivity detector (TCD) and a flame ionization detector (FID). The gas chromatograph was equipped with a Molecular Sieve 5A Capillary Column and a packed Carboxen-1000 Column with argon as the carrier gas. The volumetric gas flow rates in and out of the cell were measured with a bubble column. The FE of a gas product is calculated as follows:
VP niF
FEi = xi x x (1) Where x, is the volume fraction of the gas product i, V is the outlet gas flow rate in L s', P
is atmosphere pressure 101.325 kPa, R is the ideal gas constant 8.314 J mo1-1 K-1, T is the room temperature in K, n, is the number of electrons required to produce one molecule of product F is the Faraday Constant 96485 C mo1-1, and J is the total current in A.
The liquid products from the cathode side of the SC-BPMEA were collected using a cold trap cooled to 0 C. The collected liquid was combined with anolyte (some crossover liquid product) for quantifying by the proton nuclear magnetic resonance spectroscopy CH NMR) on an Agilent DD2 500 spectrometer in D20 using water suppression mode and dimethyl sulfoxide (DMSO) as the internal standard. For each plot of liquid product quantification, fresh anolyte was used, and the duration of the collection was 30 min. The FE
of a liquid product is calculated as follows:
FEi = mi xn.; (2) Where m, is the quantity of the liquid product i in mole, t is the duration of product collection (1800 seconds).
COMSOL one-dimensional modeling The electrochemical reaction model was performed by COMSOL Multiphysics version 5.5.
This simulation was built upon previous modeling work. The local pH and different species concentrations were simulated for different catholyte thicknesses (16 pm, 65 pm, 125 pm, and 250 pm). Two different catholytes (K2SO4 and KHCO3) were used in the simulation.
All the chemical reactions between species were considered in this one-dimensional modeling. The simulation included a 50 pm thick gas diffusion layer (GDL), a 0.1 pm thick Cu cathode catalyst (CL), a catholyte region with various thicknesses indicated above, and a cation exchange layer (CEL) boundary.

Constant concentration (Dirichlet) boundary conditions were used.
Specifically, a constant concentration 37.8 mM of CO2 was assumed within the GDL layer, as this region is in direct contact with the input CO2 flow and thus assumed to be at equilibrium with gas phase CO2 over this region for the purposes of the simulation. The BPM was interpreted as a boundary with a constant species concentration (1 M H30 at the CEL
surface), because it was assumed to generate protons as the dominant ionic charge carrier at a constant rate under constant current density (200 mA cm-2).
A user-controlled mesh is employed in the COMSOL simulation. Edge type of mesh is used for GDL, CL, catholytes, respectively. Specifically, the mesh distribution is predefined with an interval of 500 nm for GDL and catholytes, and an interval of 5nm for CL.
Five different electrode reactions were considered at the cathode catalyst layer in this simulation. Specifically, the hydrogen evolution reaction and CO2 reduction reactions to CO, CH4, 02H4, and C2H5OH occurred at the cathode catalyst layer. In SC-BPMEA, the catalyst layer is immersed in a catholyte. Thus the simulation considers no gas-phase transport in the catalyst layer. The carbonate equilibrium reactions, corresponding catholyte buffer reactions, and a water dissociation reaction were considered in the catholyte region. The electrochemical reaction rates of the specific products were determined from experimental results. They are calculated based in the same manner as previous work17.
The electrochemical reactions at cathode catalyst layer:
2H20 + 2e- 112 20H-CO2+ H20 +2e- ¨> CO +20H-CO2 H2 0 8e- ¨> CH4 + 80 H-2CO2+ 8H20 +12e- C2H4 + 120H-2CO2+ 9H20 +12e- C2H5OH + 12011-The heterogenous electrochemical reaction rates are determined by the following equations:
¨ * _______________________________ (3) niF Lcarcayst 'total (-FEco FE'cH4 FEc2H4 FEczlison * _____________ rCO2 =
(4) F \ 2 8 12 12 Lcata/yst total r0H¨ = (5) F Lcatalyst Where /i represents the partial current density for CO, CH4, C2H4, and C2H5OH
occurred at the cathode catalyst layer, respectively. ni represents the number of electrons transferred per mole reactant. F represents faraday's constant. /roral represents the total current density. The FEs for the specific product is determined by the experimental results.
E represents the catalyst porosity value. Lcataiyõ represents the cathode catalyst length.
The chemical reactions at the catholyte region and the corresponding forward kr rate constants and reverse kr rate constants taken from the literature:
Reaction kf kr CO2 +H20 H + HCO 0.036 [s-1] 7.83x104 HCO3 44 11+ + co 2.5 [s-1] 5x1019 [M-1s-1]
CO2 + OH- HCO 2.23x103[M-1s-1] 4.85x10-HCO + 0H CO + H20 6x109[M-1s-1] 1.2 [s-1]
K2504 21C+ + S0,21- 1x107 [s-1] 7.96x107 [M-1-120 44 H + OH- 108 [s-1] 1019 [M-1s-1]
The Transport of Diluted Species physics model was used. The Nernst-Planck set of equations governed the species diffusion, and they were calculated in the same manner as previous work.13,14 Migration was ignored for simplicity as the experiments were performed in the concentrated electrolyte. The ion species transport is thus calculated by solving the two equations below.
8ci aft + = Ri (6) ji = Daiaci (7) Di = * (8) TF,i = ET," (9) Where Lis the molar flux, and ri represents the heterogeneous electrode reactions for CO2 reduction that were modelled at the cathode catalyst layer. RI represents the rates of the homogeneous reactions indicated above. The Millington and Quirk model is used to determine the effective diffusivity, D1. Ep represents porosity coefficient.
TF,, represents tortuosity coefficient.
The porosity value of 0.6 was used for the cathode catalyst and the porosity value of 1 for the catholyte region. The species diffusion coefficients are listed below.
Species Diffusion coefficients Di(10-9 m2s-1) CO2 1.91 H20 2.57 K2SO4 1.39 KHCO3 1.20 K+ 1.98 H+ 9.31 OH- 5.26 HCO3- 1.185 C032- 0.923 S042- 1.07 Henry's law and sets of Sechenov equation are applied to calculate the CO2 concentration.
The concentration of CO2 in electrolytes depends on temperature and pressure.
It is estimated in the same manner as previous work. The Sechenov coefficients are listed below.37 Species Sechenov coefficients hc,o,c02 -0.0172 h-r,c02 -0.000338 hK 0.0922 hoH 0.0839 hiHco3 0.0967 hc03 0.1423 Energy assessment We evaluated the energy consumptions for electrolyzer electricity, cathodic separation, and anodic separation in the context of ethylene. We consider the state-of-the-art CO2RR
systems from the literature, including alkaline flow-cell electrolyzers, neutral MEA
5 electrolyzers, acidic flow-cells and MEAs. This consideration is based on the performance metrics, including selectivity, productivity, and full-cell voltage ¨ the combination reflects as energy intensity of producing multi-carbon products (i.e. ethylene). The proximity of these performance metrics will help refine the effect of anodic and cathodic separation on the energy requirement for producing ethylene. We summarize the input parameters to 10 the model for all the systems. The energy assessment model, as well as the assumptions, are based on the previous work. Ideally, it will be interesting to use experimental/modelling data corresponding to the exact gas composition from the CO2-to-C2, device.
However, at present, there is a gap in published literature. We therefore employed one of the most widely used models (i.e. biogas upgrading) as the best approximation for evaluating the 15 energy cost associated with cathode gas separation. The details of calculations for the carbon regeneration (for alkaline flow cell) and cathodic separation (for all the electrolyzers), can be found in previous work. The anodic separation (for neutral MEA
electrolyzer) is modelled based on an alkaline capture solvent. The amount of crossover to the anode is calculated for one tonne of ethylene produced. The energy 20 required to separate the CO2/02 mixture is calculated based on a recent report by Carbon Engineering, in which 5.25 GJ/tonne CO2 thermal energy and 77 kWh/tonne CO2 are reported to be required to capture CO2 and release at 1 bar. This energy consumption is a typical value for the alkaline capture process. For acidic flow-cell and MEA
electrolyzers, we assume no energy cost associated with the anodic separation considering no 25 availability at the anodic gas stream.
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Claims

CLAIMS (clean)

1. An electroreduction system for converting carbon oxides selected from CO, CO2 or any mixture thereof into multicarbon (C2+) products, the system comprising:
- a cathodic compartment having a reactant inlet for receiving a stream of CO, CO2 or any mixture thereof, and comprising a cathode, the cathode comprising a catalyst layer that is in contact with a catholyte solution:
- an anodic compartment, the anodic compartment comprising an anode and accommodating a flowing anolyte solution;
- a bipolar membrane being positioned between the cathodic compartment and the anodic compartment, the bipolar membrane comprising:
a cation-exchange layer (CEL) in cation communication with the catholyte solution to provide protons into the catholyte solution;
an anion-exchange layer (AEL) in anion communication with the anolyte solution to provide hydroxide ions at a surface of the anode; and an interfacial layer defined between the cation-exchange layer and the anion-exchange layer for splitting water into the protons and the hydroxide ions;
wherein the cathodic compartment and/or the anodic compartment have a product outlet to release the C2, products;
characterized in that the cathodic compartment accommodates a stationary catholyte layer between the catalyst layer of the cathode and the CEL, the stationary catholyte layer comprising the catholyte solution; in that the thickness of the stationary catholyte layer is at most 280 pm as measured by a spiral micrometer; and in that the catholyte solution is a non-buffered solution.

2. The system according to claim 1, characterized in that the thickness of the stationary catholyte layer is between 20 pm and 250 pm as measured by a spiral micrometer;
preferably between 40 pm and 200 pm, more preferably between 50 pm and 150 pm.

3. The system according to claim 1 or 2, characterized in that the cathodic compartment further comprises a solid porous support in between the CEL and the catalyst layer, and in that the solid porous support is saturated with the catholyte solution to form the stationary catholyte layer.

4. The system according to claim 3, characterized in that the solid porous support comprises polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polycarbonate, nylon, cellulose acetate, cellulose nitrate, polypropylene, alumina, or any combinations thereof.

5. The system according to claim 3 or 4, characterized in that the solid porous support has a mean pore diameter between 0.05 and 50 pm as determined by scanning electron microscopy; preferably, between 0.2 and 1 pm.

6. The system according to any one of claims 3 to 5, characterized in that the stationary catholyte layer has a liquid content between 5 and 50 pLcm-2; with preference, between and 20 pLcm-2, when the solid porous support is saturated with the catholyte solution;
the liquid content being determined by weighting.

7. The system according to any one of claims 3 to 6, characterized in that the solid porous support is sandwiched between the catalyst layer of the cathode and the CEL
for direct contact therewith.

8. The system according to any one of claims 1 to 7, characterized in that the catholyte solution has a concentration of cations between 0.25 M and 3 M; preferably, between 0.5 M and 2 M.

9. The system according to claim 8, characterized in that the catholyte solution is a solution of K2SO4 with a K+ concentration equal to or greater than 0.5M.

10. The system according to any one of claims 1 to 9, characterized in that the cations in the catholyte solution are one or more selected from K+, Na+, Cs+, Rb+, NH4+, Mg2+, Ca2+, Al3+.

11. The system according to claim 10, characterized in that the catholyte solution is a solution with a cations concentration equal to or greater than 0.5M.

12. The system according to any one of claims 1 to 11, characterized in that the non-buffered solution is or comprises K2SO4, KCI or any other combinations of the Cl- anions or S042- anions with Na+, Cs+, Rb+, NH4+, me, cr 2+, a or Al3+ cations.

13. The system according to any one of claims 1 to 12, characterized in that the anolyte solution has a pH between 7 and 10.

14. The system according to claim 13, characterized in that the anolyte solution is a KHCO3, K2SO4, or K2HP0.4 solution.

15. The system according to any one of claims 1 to 14, characterized in that the anolyte solution is an acidic solution and has a pH between 1 and 4.

16. The system according to claim 15, characterized in that the acidic solution is a H3PO4 solution, H2SO4 solution or a combination thereof.

17. The system according to any one of claims 1 to 16, characterized in that the anolyte solution has an anolyte concentration between 2.0 M and 0.01M.

18. The system according to any one of claims 1 to 17, characterized in that the catalyst layer of the cathode comprises copper (Cu), silver (Ag), platinum (Pt), carbon (C), or any combination thereof.

19. The system according to any one of claims 1 to 18, characterized in that the cathode further comprises a gas diffusion layer for contacting the stream of CO, 002 or any mixture thereof, and the catalyst layer is deposited onto the gas diffusion layer.

20. The system according to any one of claims 1 to 19, characterized in that the anode comprises an anodic catalyst layer and an anodic current collector layer.

21. The system according to claim 20, characterized in that the anodic catalyst layer comprises one or more selected from 1r02, Pt, Pd, Ni, Ni0x, CoOx.

22. The system according to any one of claims 1 to 21, characterized in that the interfacial layer of the bipolar membrane comprises a water dissociation catalyst.

23. The system according to claim 22, characterized in that the water dissociation catalyst is present as nanoparticles.

24. The system according to claim 22 or 23, characterized in that the water dissociation catalyst comprises one or more selected from Ti02, 1r02, NiO, SnO2, graphene oxide, CoOx, Zr02, A1203, Fe(OH)3, Mn02, Ru, Rh, RuPt alloy, Ptlr alloy, lr, Pt.

25. The system according to any one of claims 1 to 24, characterized in that the AEL is a membrane comprising poly(aryl piperidinium), polystyrene methyl methylimidazolium, or polystyrene tetramethyl methylimidazolium.

26. The system according to any one of claims 1 to 25, characterized in that the CEL
comprises or consists of a sulfonated tetrafluoroethylene based fluoropolymer-copolymer.

27. The system according to any one of claims 1 to 26, characterized in that it further comprising a temperature controller configured to maintain an operating temperature between 20 C and 50 C.

28. A carbon oxides electroreduction process for converting CO, 002 or any mixture thereof into C2+ products, the process comprising:
supplying a catholyte solution and a stream of CO, CO2 or any mixture thereof to a cathodic compartment comprising a catalyst layer in contact with the catholyte solution;
and having a product outlet to release the C2+ products:
flowing an anolyte solution through an anodic compartment , the anodic compartment comprising an anode;

providing a bipolar membrane between the cathodic compartment and the anodic compartment, the bipolar membrane comprising:
a cation-exchange layer (CEL) in cation communication with the catholyte solution to provide protons into the catholyte solution;
an anion-exchange layer (AEL) in anion communication with the anolyte solution to provide hydroxide ions into the anolyte solution; and an interfacial layer defined between the cation-exchange layer and the anion-exchange layer for splitting water into the protons and the hydroxide ions;
and retaining a portion of the catholyte solution as a stationary catholyte layer between the catalyst layer of the cathode and the CEL and in contact with the CEL; wherein the thickness of the stationary catholyte layer is at most 280 pm as measured by a spiral micrometer and wherein the catholyte solution is a non-buffered solution.

29. The process of claim 28, comprising maintaining an operating temperature between 20 C
and 50 C.

30. The process of claim 28 or 29, characterized in that supplying the stream of CO, 002 or any mixture thereof to the cathodic compartment is performed at an inlet flowrate between 1 sccm and 15 sccm.

31. The process of any one of claims 28 to 30, characterized in that comprising providing the cathode with an applied current density between 100 and 400 mA.cm-2.

32. The process of any one of claims 28 to 31, characterized in that comprising forming the stationary catholyte layer by providing a solid porous support between the cathode and the CEL, and saturating the solid porous support with the catholyte solution.

33. The process of claim 32, characterized in that the saturating is performed to reach a liquid content of the stationary catholyte layer between 5 and 50 pL.cm-2, optionally between 10 and 20 pLcm-2 when the solid porous support is saturated with the catholyte solution; the liquid content being determined by weighting.

34. The process of any one of claims 28 to 33, characterized in that the catholyte solution is supplied with a concentration of cations between 0.25 M and 3 M, and optionally between 0.5 M and 2 M.

35. The process of any one of claims 28 to 34, wherein stationary catholyte layer is formed with a thickness between 20 pm and 250 pm as measured by a spiral micrometer;
preferably between 40 pm and 200 pm, more preferably between 50 pm and 150 pm.