WO2024197118A2

WO2024197118A2 - Electrolysis cell systems including proton conducting oxide membranes

Info

Publication number: WO2024197118A2
Application number: PCT/US2024/020857
Authority: WO
Inventors: Lucas COHEN; Daniel V. ESPOSITO
Original assignee: The Trustees Of Columbia University In The City Of New York
Priority date: 2023-03-21
Filing date: 2024-03-21
Publication date: 2024-09-26
Also published as: WO2024197118A3

Abstract

Water electrolysis systems are provided with ultra-thin, porous oxide-compound membranes for the production of hydrogen and oxygen product effluent streams. The electrolysis systems include one or more membrane-electrode assemblies with oxide-compound membranes fabricated using atomic layer deposition and wet chemical processes. The membrane-electrode assemblies also include gas diffusion layer and porous transport layer electrodes, and porous hydrogen evolution reaction and oxygen evolution reaction catalyst layers disposed between the electrodes and the membrane. The oxide-compound membranes, composed of SiO_x, TiO_x, and/or WO_x, can increase electrolysis efficiency by 20% at high current densities compared to conventional systems utilizing Nafion. The enhanced performance is in part due to the lower ionic resistance of the oxide-compound membranes that are several orders of magnitude thinner than conventional Nafion membranes (< 1 μM), e.g., potential ionic resistances below 0.2 V at current densities of 5 A/cm² or higher from reduced ohmic losses.

Description

ELECTROLYSIS CELL SYSTEMS INCLUDING PROTON CONDUCTING OXIDE MEMBRANES

CROSS REFERENCE TO RELATED APPLICATION(S)

[0001] This application claims the benefit of U.S. Provisional Application Nos. 63/453,671, filed March 21, 2023, and 63/567,613, filed March 20, 2024, which is incorporated by reference as if disclosed herein in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] This invention was made with government support under Grant Number DE- AR0001567 awarded by the U.S. Department of Energy. The government has certain rights in the invention.

BACKGROUND

[0003] Because of its flexibility as an energy carrier, hydrogen (H2) from water electrolysis is gaining significant international and domestic attention for energy storage applications. Making hydrogen renewably also reduces fossil fuel use, as most of the industrial hydrogen supply is from reforming of natural gas. The hydrogen produced from electrolysis can be injected into the natural gas pipeline (thus making that energy carrier greener), or used in the production of high-volume chemicals such as ammonia, in upgrading of methanizati on-produced biogas, as a transportation fuel, etc. Such “power-to-gas” system concepts are also being investigated in the United States, where Nel Hydrogen is partnered with Southern California Gas and the National Renewable Energy Laboratory in a ground-breaking demonstration of renewable hydrogen for both pipeline injection and bio-methanation, as well as heavy duty vehicle fueling. In addition, electrolysis can also provide ancillary services to the grid such as frequency regulation and load shifting.

[0004] Water electrolysis powered by renewable energy is an attractive approach to generating carbon-free, energy-dense H2 fuel that is expected to be an enabler of industrial decarbonization. Globally, H2 usage is currently below 1 million metric tons/year but projected to grow to 7 million tons/year by the year 2030. In the United States, projected revenue from H2 has been estimated to be about $140 billion/year by 2030 and to support about 700,000 jobs. However, large-scale deployment of so-called “green hydrogen” produced by electrolysis is hindered by its relatively high levelized cost of hydrogen (LCOH, $3-$8/kg H2) compared to “blue H2” produced from CCh-emitting steam methane reforming (SMR) ($0.7-2.1/kg H2). Presently, the price of electricity remains the single largest cost contributor to LCOH from water electrolysis, creating a need to increase the energy efficiencies of electrolyzers. Fortunately, reductions in electricity prices from solar and wind are helping to reduce the electricity cost contribution to LCOH, although the availability of low cost electricity (< 2 /kWh) from these variable renewable energy (VRE) generators is generally less than 45%. As a result, electrolyzers operated in areas of high VRE deployment can be expected to be hamstrung by lower capacity factors, creating a desire to decrease the capital costs (Capex) of electrolyzers.

[0005] Electrolyzer Capex is already decreasing thanks to economies of scale as the industry grows, but more rapid decreases in Capex through advances in manufacturing and improved electrolyzer performance, i.e., higher current density and/or efficiency, are needed. The exact performance and Capex targets for electrolyzers to compete with SMR are dependent on the price and availability of electricity, but it is generally understood that next generation water electrolyzers capable of operating at higher current densities and energy efficiencies would allow for green hydrogen to reach parity with blue H2.

[0006] Today’s electrolyzer industry typically utilizes two technologies: alkaline and polymer electrolyte membrane (PEM) electrolyzers. Alkaline electrolyzers typically use about 30 wt% KOH(aq) as the electrolyte and operate with current densities between 0.1-0.4 A/cm² with HHV stack efficiencies of 75-80 %. PEM electrolyzers utilize thin Nafion solid electrolyte membranes that are sandwiched between porous anode and cathode layers. The so- called “zero-gap” geometry and active water management of PEM electrolyzers enables lower ionic resistances, allowing them to operate at 1-2 A/cm² current densities with comparable efficiencies to alkaline electrolyzers. While the more mature alkaline technology is still expected to be important in the near term, the majority of research and development efforts have shifted towards advancing PEM electrolyzer concepts as the technology of the future thanks to their ability to operate at 2 A/cm², and their potential to operate at even higher current densities.

However, the gap between electrodes in these “zero-gap” PEM electrolyzers is not actually zero; rather, the Nafion membranes typically have a thickness of 125-250 pm. Even at these thicknesses, ohmic resistance associated with ion transport across the membrane becomes the dominant loss mechanism at high current densities (>2 A/cm²). [0007] Researchers have attempted to reduce these losses by using thinner Nafion membranes, but limited gains have been observed due to decreased manufacturing yields and mechanical failure associated with swelling and creeping phenomena in many polymeric membranes like Nafion when they are in contact with water. Furthermore, the proton conductivity of Nafion decreases for thinner membranes. It is likely that, even with development, it will be difficult to achieve membrane thicknesses much below 50 pm.

SUMMARY

[0008] Aspects of the present disclosure are directed to a water electrolyzer including one or more membrane-electrode assemblies. In some embodiments, the assemblies include an oxidecompound membrane, a gas diffusion layer (GDL), a porous transport layer (PTL), a hydrogen evolution reaction (HER) catalyst layer disposed between the GDL and the oxide-compound membrane, and an oxygen evolution reaction (OER) catalyst layer disposed between the PTL and the oxide-compound membrane. In some embodiments, the membrane has a thickness less than about 1 pm. In some embodiments, the oxide-compound membrane includes SiO_x, TiO_x, WO_X, or combinations thereof. In some embodiments, the oxide-compound membrane is doped with one or more ion exchange groups (lEGs). In some embodiments, the oxide-compound membrane is doped with one or more sulfonate lEGs, one or more PO_X compounds, or combinations thereof. In some embodiments, the PO_X compound includes diethyl phosporamidate, trimethylphosphate, triethylphosphite, trimethylphosphite, or combinations thereof. In some embodiments, the dopant is present at a concentration of less than about 10% by weight. In some embodiments, the H2 leakage rate across the oxide-compound membrane is below about 10 pg H2/cm²/min. In some embodiments, at least one of the HER catalyst layer and the OER catalyst layer includes platinum, IrO2, or combinations thereof.

[0009] In some embodiments, the oxide-compound membrane is a continuous layer having substantially uniform thickness. In some embodiments, the GDL includes carbon paper. In some embodiments, the PTL includes sintered porous Ti. In some embodiments, the PTL includes a macroporous transport layer having an average pore size above about 1 pm and a microporous transport layer having an average pore size below about 1 pm. In some embodiments, the microporous transport layer is positioned between the macroporous transport layer and the OER catalyst layer. In some embodiments, the PTL has a gradient porosity. In some embodiments, the GDL and PTL have a thickness of about 250-500 pm. In some embodiments, the oxide-compound membrane has a roughness factor between about 1 and about 30. In some embodiments, the oxide-compound membrane is fabricated by, alternatingly, a plurality of oxide-compound deposition steps followed by one or more additional deposition steps, e.g., PO_X compound deposition steps. In some embodiments, the area of the oxidecompound membrane is between about 80 cm² and 1600 cm².

[0010] Aspects of the present disclosure are directed to a water electrolysis system, including a water flowstream, a first flow channel in fluid communication with the water flowstream, a second flow channel, and one or more membrane-electrode assemblies positioned between the first flow channel and the second flow channel. In some embodiments, the assemblies include an oxide-compound membrane, the oxide-compound membrane having a thickness less than about 1 pm, and including SiO_x, TiO_x, WO_X, or combinations thereof. In some embodiments, the assemblies include a GDL, a PTL, a HER catalyst layer disposed between the GDL and the oxide-compound membrane, and an OER catalyst layer disposed between the PTL and the oxide-compound membrane. In some embodiments, a power supply is in electrical communication with the GDL and the PTL. In some embodiments, an oxygen gas effluent flow stream is in fluid communication with the first flow channel. In some embodiments, a hydrogen gas effluent flow stream is in fluid communication with the second flow channel.

[0011] In some embodiments, at least one of the HER catalyst layer and the OER catalyst layer includes platinum, IrO2, or combinations thereof. In some embodiments, the GDL includes carbon paper having a gradient porosity and a thickness of about 250-500 pm. In some embodiments, the PTL includes sintered porous Ti and a thickness of about 250-500 pm. In some embodiments, the PTL includes a macroporous transport layer having an average pore size above about 1 pm and a microporous transport layer having an average pore size below about 1 pm, In some embodiments, the microporous transport layer is positioned between the macroporous transport layer and the OER catalyst layer.

[0012] Aspects of the present disclosure are directed to a method of making a membraneelectrode assembly, including providing a first porous electrode substrate; depositing a first catalyst layer on the first porous electrode substrate; positioning one or more oxide-compound membrane layers on the first catalyst layer via an atomic layer deposition (ALD) fabrication process, a wet chemical process, or combinations thereof; providing a second porous electrode substrate; depositing a second catalyst layer on the second porous electrode substrate and/or the oxide-compound membrane layers; and positioning the second porous electrode substrate on the first porous electrode substrate such that the second catalyst layer is positioned between the second porous electrode substrate and the oxide-compound membrane layer. In some embodiments, at least a first oxide-compound membrane layer is positioned via an ALD fabrication process and at least a second oxide-compound membrane layer is positioned via a wet chemical process. In some embodiments, at least one of the first porous electrode substrate and the second porous electrode substrate include a macroporous transport layer having an average pore size above about 1 pm and a microporous transport layer having an average pore size below about 1 pm, and a catalyst layer is positioned on the microporous transport layer In some embodiments, positioning one or more oxide-compound membrane layers on the first catalyst layer includes depositing a first oxide-compound layer and electrodepositing at least a second oxide-compound layer on the first oxide-compound layer.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The drawings show embodiments of the disclosed subject matter for the purpose of illustrating the invention. However, it should be understood that the present application is not limited to the precise arrangements and instrumentalities shown in the drawings, wherein:

[0014] FIG. l is a schematic representation of a water electrolyzer according to some embodiments of the present disclosure;

[0015] FIG. 2 is a schematic representation of a water electrolysis system according to some embodiments of the present disclosure;

[0016] FIG. 3 is a chart of a method of making a membrane-electrode assembly according to some embodiments of the present disclosure;

[0017] FIG. 4 is a graph showing the relative ohmic drops of traditional Nafion membranes versus oxide-compound membranes according to some embodiments of the present disclosure;

[0018] FIG. 5A is a graph showing the dependency of membrane resistance on membrane thickness in the context of traditional Nafion membranes and oxide-compound membranes according to some embodiments of the present disclosure;

[0019] FIG. 5B is a graph showing the dependency of electrolyzer efficiency on current density in the context of traditional Nafion membranes and oxide-compound membranes according to some embodiments of the present disclosure;

[0020] FIG. 6A is a graph portraying linear sweep voltammograms for samples based on bare Pt and 5 nm thick SiO_x-Pt constructs; and [0021] FIG. 6B is a graph portraying i-V curves for an electrolysis cell based on a 30 nm thick SiC>2 membrane and Pt thin film electrodes.

DETAILED DESCRIPTION

[0022] Referring now to FIG. 1, some embodiments of the present disclosure are directed to a water electrolyzer 100 including one or more membrane-electrode assemblies 100 A. In some embodiments, membrane-electrode assembly 100 A includes one or more oxide-compound membranes 102. In some embodiments, oxide-compound membrane 102 has a thickness less than about 1.5 pm. In some embodiments, oxide-compound membrane 102 has a thickness less than about 1 pm. In some embodiments, oxide-compound membrane 102 has an average thickness less than about 1 pm. In some embodiments, oxide-compound membrane 102 has an average thickness between about 25 and about 500 nm. In some embodiments, oxide-compound membrane 102 has an average thickness variation below about 25%. In some embodiments, oxide-compound membrane 102 has a substantially uniform thickness. In some embodiments, oxide-compound membrane 102 is a continuous layer. The area of oxide-compound membrane 102 can be any suitable size. In some embodiments, the area of oxide-compound membrane 102 is between about 1 cm² and 10 cm². In some embodiments, the area of oxidecompound membrane 102 is about 1 cm², 5 cm², 10 cm², 50 cm², 80 cm², 90 cm², 100 cm², 200 cm², 300 cm², 400 cm², 500 cm², 600 cm², 700 cm², 800 cm², 900 cm², 1000 cm², 1100 cm², 1200 cm², 1300 cm², 1400 cm², 1500 cm², 1600 cm², or greater. In some embodiments, the area of oxide-compound membrane 102 is between about 80 cm² and 1600 cm².

[0023] In some embodiments, oxide-compound membrane 102 includes SiO_x, TiO_x, WO_X, or combinations thereof. In some embodiments, oxide-compound membrane 102 has a roughness factor between about 1 and about 30. In some embodiments, oxide-compound membrane 102 has a roughness factor of about 1.

[0024] In some embodiments, oxide-compound membrane 102 is doped with one or more ion exchange groups. In some embodiments, the dopant is present at a concentration of between about 1 and about 10% by weight. In some embodiments, the dopant is present at a concentration of less than about 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, or 1% by weight. In some embodiments, the dopant is a sulfonate compound. In some embodiments, the dopant is a PO_X compound. In some embodiments, the PO_X compound includes diethyl phosporamidate, trimethylphosphate, triethylphosphite, trimethylphosphite, or combinations thereof. [0025] In some embodiments, oxide-compound membrane 102 includes two or more layers. In some embodiments, a first layer 102A has a minimum thickness to achieve a continuous layer. In some embodiments, additional layers, e.g., 102B, have the same or greater thickness than this first layer. In some embodiments, first layer 102 A and at least one additional layer 102B have substantially the same composition, e.g., are composed of the same oxide-compound material. In some embodiments, oxide-compound membrane 102 is a bilayer. In some embodiments, layers of the oxide-compound membrane 102 have different electrical properties relative to adjacent layers. In some embodiments, oxide-compound membrane 102 includes an H⁺ conducting titanium oxide TiCL / SiC>2 bilayer.

[0026] In some embodiments, oxide-compound membrane 102 is prepared using atomic layer deposition (ALD), wet-chemical (WC) processes, or combinations thereof, as will be discussed in greater detail below. In some embodiments, at least first layer 102A is formed via ALD. In some embodiments, at least additional layer 102B is formed via WC. In some embodiments, the bilayered membrane includes a WC oxide on an ALD oxide. In some embodiments, oxide-compound membrane 102 includes a plurality of thinner barrier layers separated by thicker conductive layers. In some embodiments, oxide-compound membrane 102 includes thinner (10-50 nm) ALD SiO_x barrier layers followed by thicker, more conductive SiO_x layers deposited by WC. In some embodiments, oxide-compound membrane 102 includes a plurality of alternating oxide-compound layers and one or more dopant layers, e.g., of sulfonate compound, PO_X compound, etc.

[0027] In some embodiments, membrane-electrode assembly 100 A includes one or more pairs of electrodes, e.g., 104 A and 104B. In the exemplary embodiment shown in FIG. 1, electrode 104 A is shown and described as a cathode, and electrode 104B is shown and described as an anode, however, this present disclosure is not intended to be limiting in this regard, as electrolyzer 100 may also be constructed with electrode 104A as an anode and electrode 104B as an cathode.

[0028] In some embodiments, electrode 104A is a gas diffusion layer (GDL). In some embodiments, the GDL includes carbon paper. In some embodiments, electrode 104B is a porous transport layer (PTL). In some embodiments, the PTL includes one or more layers. In some embodiments, the PTL includes a first layer having a first porosity and at least a second layer having a second porosity. In some embodiments, the first layer is a macroporous transport layer having an average pore size above about 1 pm. In some embodiments, the microporous transport layer has an average pore size between about 1 pm and about 30 pm. In some embodiments, the microporous transport layer has an average pore size of about 10 pm. In some embodiments, at least one of the second layers is a microporous transport layer having an average pore size below about 1 pm. In some embodiments, the PTL has a gradient porosity. In some embodiments, the PTL includes sintered porous Ti. In some embodiments, electrode 104 A and/or electrode 104B have a thickness of about 250-500 pm. In some embodiments, oxidecompound membrane 102 and/or electrodes 104A/104B include an ionomer coating.

[0029] In some embodiments, oxide-compound membrane 102 is deposited on one or more catalyst layers. In some embodiments, membrane-electrode assembly 100 A includes a hydrogen evolution reaction (HER) catalyst layer 106. In some embodiments, HER catalyst layer 106 is disposed between the GDL, e.g., electrode 104A, and oxide-compound membrane 102. In some embodiments, HER catalyst layer 106 includes platinum, IrCL, or combinations thereof.

[0030] In some embodiments, membrane-electrode assembly 100 A includes an oxygen evolution reaction (OER) catalyst layer 108. In some embodiments, OER catalyst layer 108 is disposed between the PTL, e.g., electrode 104B, and oxide-compound membrane 102. In some embodiments, a microporous transport layer of the PTL is positioned between a macroporous transport layer of the PTL and OER catalyst layer 108. In some embodiments, OER catalyst layer 108 includes platinum, IrO2, or combinations thereof.

[0031] As discussed above, the area of the membrane 102/electrode 104/catalyst 108 combined construct can be any suitable size. As discussed above, the area of the membrane 102/electrode 104/catalyst 108 combined construct is between about 1 cm² and 10 cm². In some embodiments, the area of membrane 102/electrode 104/catalyst 108 combined construct is about 1 cm², 5 cm², 10 cm², 50 cm², 80 cm², 90 cm², 100 cm², 200 cm², 300 cm², 400 cm², 500 cm², 600 cm², 700 cm², 800 cm², 900 cm², 1000 cm², 1100 cm², 1200 cm², 1300 cm², 1400 cm², 1500 cm², 1600 cm², or greater. In some embodiments, the area of the membrane 102/electrode 104/catalyst 108 combined construct is between about 80 cm² and 1600 cm².

[0032] In some embodiments, electrodes 104 A and 104B are in electrical communication with a power supply 110 for supplying a desired voltage across the electrodes and thus oxidecompound membrane 102.

[0033] In some embodiments, electrolyzer 100 includes a stack of greater than about 10 membrane-electrode assemblies 100 A. In some embodiments, electrolyzer 100 includes a stack of between 2 and about 10 membrane-electrode assemblies 100 A. In some embodiments, the electrolyzer includes a stack of between about 3-5 membrane-electrode assemblies 100 A.

[0034] As discussed above, electrolyzers 100 include proton-conducting membranes, e.g., SiO_x membranes 102. SiO_x materials have an ability to conduct protons (H⁺), but lower conductivities relative to Nafion at low temperatures have hindered its consideration as membrane materials in conventional catalyst-coated membranes (CCMs). However, the lower H⁺ conductivity can be mitigated if the thickness of the membrane is reduced.

[0035] In some embodiments, lower thicknesses are achieved with the thin oxide membranes via higher densities that can suppress gas crossover, as can occur through pores, and high mechanical strength and resistance to deformation compared to polymers. In some embodiments, H2 leakage rate across oxide-compound membrane 102 is below about 10 pg H2/cm²/min, equivalent to about 1% crossover rate for electrolysis current density of 1.5 A/cm². In some embodiments, oxide-compound membrane 102 exhibits above about 10% (absolute) efficiency gain relative to CCMs at z = 1.7 A cm'². In some embodiments, oxide-compound membrane 102 exhibits above about 20% (absolute) efficiency gain at z = 4.25 A cm'². In some embodiments, oxide-compound membrane 102 exhibits degradation rates less than 50 mV 1000 hrs'¹. In some embodiments, oxide-compound membrane 102 exhibits degradation rates less than 5 mV 1000 hrs'¹ for times exceeding 5000 hours. This is comparable to the typical degradation rate observed for emerging polymer electrolyte membrane (PEM) electrolyzers based on alkaline exchange membranes (AEMs). In some embodiments, efficiency gains for electrolyzer 100 are calculated relative to the efficiency of a conventional PEM electrolyzer based on Nafion- 117 membrane operated under the same temperature and catalyst loading.

[0036] In some embodiments, the electrolyzer is incorporated into existing electrolyzer systems, e.g., replaces traditional PEM electrolyzers in existing systems. In some embodiments, electrolyzer 100 is operated at temperatures between about room temperature and about 200°C. In some embodiments, electrolyzer 100 is operated at temperatures between about room temperature and about 80°C. In some embodiments, electrolyzer 100 is operated at temperatures below about 100°C. In some embodiments, electrolyzer 100 is operated at temperatures between about 80°C and about 150°C. Without wishing to be bound by theory, operating at higher temperature would enable higher efficiency gains owing to enhanced membrane conductivity and decreased kinetic overpotential losses.

[0037] Referring now to FIG. 2, some embodiments of the present disclosure are directed to a water electrolysis system 200 including one or more influent flow streams F' and one or more effluent flow streams F". In some embodiments, influent flow streams F' is a water flowstream 202. Water flowstream 202 includes water from any suitable water source, e.g., industrial effluent, natural water, etc., or combinations thereof. In some embodiments, water flowstream 202 is in fluid communication with the suitable water source.

[0038] In some embodiments, water electrolysis system 200 includes one or more membrane-electrode assemblies 204. In some embodiments, membrane-electrode assembly 204 is consistent with the description of membrane-electrode assembly 100A described above. In some embodiments, membrane-electrode assembly 204 includes one or more oxide-compound membranes 204A. In some embodiments, oxide-compound membranes 204A has an average thickness less than about 1 pm. In some embodiments, oxide-compound membranes 204A has an average thickness between about 25 and about 500 nm. In some embodiments, oxidecompound membranes 204A includes SiO_x, TiO_x, WO_X, or combinations thereof. In some embodiments, oxide-compound membrane 204A is doped with one or more ion exchange groups, e.g., with a sulfonate compound, PO_X compound including diethyl phosporamidate, trimethylphosphate, triethylphosphite, trimethylphosphite, or combinations thereof, etc.

[0039] The oxide-compound membranes consistent with embodiments of the present disclosure are appropriate for reducing membrane resistance to target levels at least because (i) oxides are more suitable for suppressing cross-over at a given membrane thickness due to their higher densities than polymeric membranes, (ii) oxides can tolerate elevated temperatures during manufacturing and operation, (iii) oxides are not susceptible to swelling and creep (deformation) issues that are found in lower density, hydrated polymeric membranes, and (iv) thin conformal oxide coatings can be readily achieved on rough surfaces with modem fast ALD methods. Related to benefit (i), and without wishing to be bound by theory, it is noted that oxides like SiCh conduct protons via a facilitated transport mechanism that involves H⁺ hopping between silanol/bridging oxygen sites, vs. Nafion-type membranes that rely primarily on H⁺ diffusion through water channels that form in the pores of Nafion. This difference in transport mechanisms gives an advantage for using dense oxide membranes, rather than relatively thin (< 50 pm) porous polymeric membranes.

[0040] In some embodiments, membrane-electrode assembly 204 includes one or more pairs of electrodes, e.g., 206A and 206B. In some embodiments, electrode 206A is a gas diffusion GDL. In some embodiments, the GDL includes carbon paper. In some embodiments, electrode 206B is a PTL. As discussed above, in some embodiments, the PTL includes one or more layers. In some embodiments, the PTL includes a first layer having a first porosity and at least a second layer having a second porosity. In some embodiments, the first layer is a macroporous transport layer having an average pore size above about 1 pm. In some embodiments, the microporous transport layer has an average pore size between about 1 pm and about 30 pm. In some embodiments, the microporous transport layer has an average pore size of about 10 pm. In some embodiments, at least one of the second layers is a microporous transport layer having an average pore size below about 1 pm. In some embodiments, the PTL has a gradient porosity. In some embodiments, the PTL includes sintered porous Ti. In some embodiments, electrode 206A and/or electrode 206B have a thickness of about 250-500 pm.

[0041] In some embodiments, membrane-electrode assembly 204 includes an HER catalyst layer 208. In some embodiments, HER catalyst layer 208 is disposed between the GDL and oxide-compound membrane 204A. In some embodiments, HER catalyst layer 208 includes platinum, IrCh, or combinations thereof.

[0042] In some embodiments, membrane-electrode assembly 204 includes an oxygen evolution reaction (OER) catalyst layer 210. In some embodiments, OER catalyst layer 210 is disposed between the PTL and oxide-compound membrane 204A. In some embodiments, a microporous transport layer of the PTL is positioned between a macroporous transport layer of the PTL and OER catalyst layer 210. In some embodiments, OER catalyst layer 210 includes platinum, IrO2, or combinations thereof.

[0043] In some embodiments, water electrolysis system 200 includes a first flow channel 212 in fluid communication with water flowstream 202. In some embodiments, first flow channel 212 is positioned and configured to provide water from water flowstream 202 to membrane-electrode assemblies 204. In some embodiments, first flow channel 212 is also positioned and configured to remove one or more products evolved at membrane-electrode assemblies 204 via one or more effluent flow streams F". In an exemplary embodiment, an oxygen gas product in an oxygen gas effluent flow stream 214 is removed through first flow channel 212. In some embodiments, oxygen gas effluent flow stream 214 is removed through a flow channel separate from first flow channel 212 (not pictured).

[0044] In some embodiments, water electrolysis system 200 includes a second flow channel 216. In some embodiments, second flow channel 216 is positioned and configured to collect products evolved at membrane-electrode assemblies 204 and diffuse across, e.g., oxidecompound membrane 204A. In some embodiments, second flow channel 216 is also positioned and configured to remove one or more products evolved at membrane-electrode assemblies 204 via one or more effluent flow streams F". In an exemplary embodiment, a hydrogen gas product in a hydrogen gas effluent flow stream 218 is removed through second flow channel 216. In some embodiments, hydrogen gas effluent flow stream 218 is removed through a flow channel separate from second flow channel 216 (not pictured).

[0045] In some embodiments, a power supply 220 is in electrical communication with the GDL and the PTL to provide the necessary voltage to facilitate water electrolysis on water flowstream 202 and generate the oxygen gas effluent flowstream 214, e.g., in first flow channel 212, and hydrogen gas effluent flow stream 218, e.g., in second flow channel 216.

[0046] In some embodiments, water electrolysis system 200 includes a plurality of “cells” for receiving and electrolyzing water from a water source, e.g., a plurality of flow channels 212, membrane-electrode assemblies 204, etc. In some embodiments, water electrolysis system 200 includes greater than about 10 cells. In some embodiments, water electrolysis system 200 includes between 2 and about 10 cells. In some embodiments, water electrolysis system 200 includes between about 3-5 cells. As discussed above, the area of the cells in water electrolysis system 200 can be any suitable size. In some embodiments, the cells in water electrolysis system 200 have a cell area between about 1 cm² and 10 cm². In some embodiments, the cells in water electrolysis system 200 have a cell area of about 1 cm², 5 cm², 10 cm², 50 cm², 80 cm², 90 cm², 100 cm², 200 cm², 300 cm², 400 cm², 500 cm², 600 cm², 700 cm², 800 cm², 900 cm², 1000 cm², 1100 cm², 1200 cm², 1300 cm², 1400 cm², 1500 cm², 1600 cm², or greater. In some embodiments, the cells in water electrolysis system 200 have a cell area between about 80 cm² and 1600 cm².

[0047] Referring now to FIG. 3, some embodiments of the present disclosure are directed to a method 300 of making a membrane-electrode assembly. In some embodiments, at 302, a first porous electrode substrate is provided. At 304, a first catalyst layer is deposited on the first porous electrode substrate.

[0048] At 306, one or more oxide-compound membrane layers are positioned on the first catalyst layer via ALD, WC, or combinations thereof. In some embodiments, the composition of the membrane deposited at step 306 is consistent with the description of oxide-compound membrane 102 and/or oxide-compound membrane 204A described above. In some embodiments, the membrane has an average thickness less than about 1 pm. In some embodiments, the membrane has an average thickness between about 25 and about 500 nm. In some embodiments, the membrane includes SiO_x, TiO_x, WO_X, or combinations thereof. In some embodiments, the membrane is doped with one or more ion exchange groups, e.g., with a POx compound including diethyl phosporamidate, trimethylphosphate, triethylphosphite, trimethylphosphite, or combinations thereof.

[0049] Substitution of ultrathin oxide membranes for Nafion in conventional PEM electrolyzers can be difficult because the thicker Nafion membrane serves as the structural backbone of the CCM. In some embodiments of step 306, ALD deposits continuous oxidecompound membranes onto large area porous electrode substrates. In some embodiments, a polymeric backing layer, e.g., ionomer, is incorporated to serve as a backing layer to support the continuous oxide compound during step 306, and can be subsequently removed. As discussed above, in some embodiments, the oxide-compound membranes include two or more layers. In these exemplary embodiments, step 306 can include forming at least a first layer via ALD, and then additional layers via ALD, WC, or combinations thereof. In some embodiments, positioning 306 includes depositing a first oxide-compound layer and electrodepositing at least a second oxide-compound layer on the first oxide-compound layer. In these embodiments, the additional oxide material can be used to plug pinholes and/or cracks in a preceding oxidecompound layer, thereby reducing cross-over of undesired species therethrough. In some embodiments, the manufacturing of the membrane is continuous.

[0050] ALD equipment can incorporate high deposition rates at reduced process temperatures to maximize process efficiency and minimize cost, while delivering uniform surface conformal thin films even on highly textured surfaces. In some embodiments, the ALD process at step 306 is performed at low temperatures, e.g., down to 80°C. In some embodiments, the ALD process at step 306 is performed at high deposition rates up to 12 nm/min. Catalyzing reactions for induced surface process have been prepared for SiCL (SiCL/CRISP) that can be deposited between 70-450°C that is greater than 95% conformal with high dielectric breakdown voltage, but still capable of conducting small ions like H⁺. In some embodiments of the present disclosure, ALD is used to deposit continuous silicon dioxide (SiCL) layers onto a catalyst- coated PTL and GDL substrates, which can replace the membrane as the structural component of the CCM membrane-electrode assembly. In some embodiments, roll-to-roll (R2R) or plate-to- plate manufacturing is utilized. In some embodiments, atmospheric R2R ALD is used for high throughput, inline, economical manufacturing.

[0051] Still referring to FIG. 3, at 308, a second porous electrode substrate is provided. In some embodiments as discussed above, at least one of the first porous electrode substrate provided at step 302 and the second porous electrode substrate provided at step 308 include one or more layers. In some embodiments, the substrate includes a first layer having a first porosity and at least a second layer having a second porosity. In some embodiments, the first layer is a macroporous transport layer having an average pore size above about 1 pm. In some embodiments, the microporous transport layer has an average pore size between about 1 pm and about 30 pm. In some embodiments, the microporous transport layer has an average pore size of about 10 pm. In some embodiments, at least one of the second layers is a microporous transport layer having an average pore size below about 1 pm. In some embodiments, the second layer is applied to the first layer via any suitable process. In an exemplary embodiment, a macroporous layer of titanium is provided. Then, a titanium slurry is prepared and spread over a surface of the macroporous layer and subsequently sintered, resulting in a porous electrode substrate having a macroporous layer and a microporous layer disposed thereon.

[0052] At 310, a second catalyst layer is deposited, e.g., on the second porous electrode substrate, on the oxide-compound membrane layer(s), or combinations thereof. At 312, the second porous electrode substrate is positioned on the first porous electrode substrate such that the second catalyst layer is positioned between the second porous electrode substrate and the oxide-compound membrane layer, consistent with the constructions of membrane-electrode assembly 100A and/or membrane-electrode assembly 204 described above.

[0053] Referring now to FIG. 4, oxide-based membranes with nanoscopic thickness according to embodiments of the present disclosure were prepared and tested for use in low temperature water electrolysis. Oxide membranes can be orders of magnitude thinner than conventional PEM membranes, enabling lower ohmic voltage drop. Without wishing to be bound by theory, this approach is based on the relationship between membrane ionic resistance (R_m, in units of mQ-cm²) and its thickness (t_m):

R_m =(t_m p)/RF = t_m /( o RF) (1) where p is the membrane H⁺ resistivity, o (1/p) is the membrane H⁺ conductivity, and RF is the membrane roughness factor (RF), which is defined as the ratio of the 3D area of the membrane to the 2D cross-sectional area of the electrolysis cell. Using this relationship and Ohm’s law, the ohmic drop associated with H⁺ transport across the membrane can be calculated as a function of t_m and c, as shown in FIG. 4 for a constant target current density of i=5 A cm'² and RF=1 (conventional thick membranes are relatively flat). This figure shows that the total ohmic drop across a t_m=l pm membrane can be reduced below 0.2 V even if its H⁺ conductivity is 1-2 orders of magnitude below that of Nafion. If a continuous oxide membrane with t_m = 100 nm could be realized with o = 10'³ S cm'¹, the ohmic drop across the membrane could be reduced to about 30 mV, nearly eliminating the largest loss mechanism in PEM electrolyzers at high current density.

[0054] Referring now to FIGs. 5 A-5B, the deposition of membranes as conformal 3D coatings onto porous GDLs or PTLs consistent with embodiments of the present disclosure are shown to be characterized by high RF. Equation 1 above shows that R_m varies inversely with RF, meaning that depositing a thin membrane conformally onto a rough substrate characterized by high RF is an additional factor that can reduce R_m compared to conventional membranes that have near unity RF. Additionally, the higher 3D surface area of a conformal thin membrane provides additional interfacial sites for dispersing electrocatalytic nanoparticles, with possible benefits for reducing kinetic and concentration overpotential losses. Specifically referring to FIG. 5A, R_m is plotted as a function of t_m for 4 different RF values and <5=1 O'⁴ S cm’¹, showing how increases in RF can be expected to be very helpful to achieving target R_m values. FIG. 5B extends the resistance modeling further to show how decreases in R_m (80% below that of Nafion) can translate to efficiency gains during electrolyzer operation.

[0055] Table 1 below shows a comparison of the technical metrics for an electrolyzer consistent with embodiments of the present disclosure, compared to a CCM. The conventional commercial PEM electrolyzer was based on Nafion-117 membrane, which had a thickness of 178 pm and area-based membrane-resistance of about 180 mQ-cm². Meanwhile, the exemplary electrolyzer was based on a sub-micron oxide-compound membrane with a resistance of 30 mQ-cm² (83% reduction), which can reduce ohmic drops associated with proton transport to below 0.2 V at current densities up to 5 A/cm². In Table 1, electrolyzer operations were considered for two operating modes: a “standard” current density (z) of 1.7 A/cm² and a high current density of z = 4.25 A/cm². Using a 1 -dimensional (ID) electrolyzer model that assumes constant temperature and kinetic overpotential losses between the two types of electrolyzers, the gain in electrolyzer efficiency (T|E) was then computed, with the results shown in the last column of Table 1. The reduction in membrane resistance led to efficiency gains in both operating modes, but gave the largest advantage for the high current density operation for which the membrane ohmic losses normally dominate polarization behavior. The high current density, i.e., high capacity, operating mode is most attractive for a renewable energy future where low-cost electricity can be sourced from wind and/or solar but might only be available 30-40% of the time. These efficiency gains at high current density can translate to reductions in electrolyzer stack costs from current values between about $40 /kW and about $70 /kW. Achieving $l/kg EE would allow H2 from water electrolysis to undercut H2 from steam methane reforming (SMR), and directly compete with the price of gasoline at current market values.

Table 1: Technical metrics of exemplary ultrathin oxide membranes and associated electrolyzer systems compared to metrics for a CCM based on standard Nafion-117 membrane. Analysis is based on a ID electrolyzer model that assumes constant temperature and catalyst loadings for both types of electrolyzers.

[0056] In addition to achieving the combination of H⁺ conductivities and t_m described above, some embodiments of the present disclosure are directed to nanoscopic oxide membranes that can (i.) be sufficiently insulating to minimize electrical leakage current between the cathode and anode, (ii) be continuous and possess low enough O2 and H2 permeabilities to prevent high cross-over rates, (iii) be deposited as continuous layers onto rough, mesoporous electrodes over large area (> 100 cm²), or combinations thereof. Four primary test platforms (TPs 1-4) were prepared with varying degrees of complexity to systematically evaluate oxide membranes consistent with embodiments of the present disclosure and assess the performance of the associated electrolyzers. TP1 and TP2 were chip-scale (1-5 cm²) platforms that rapidly quantified the ionic and electrical transport properties of oxide membranes deposited on smooth, well-defined thin film electrodes. TP2 possessed the characteristic “zero-gap” design of a PEM electrolyzer. Oxide membranes consistent with embodiments of the present disclosure were deposited on rough, porous electrodes in TP3 and TP4. While TP3 could be used for small-scale half-cell measurements focusing on studying the performance of a single porous electrode, TP4 was a full electrolysis cell representing an exemplary final POM electrolyzer. Within TP4, the membrane-electrode assembly included an oxide-compound membrane deposited on a porous cathode, followed by deposition of anode electrocatalyst ink and deposition/application of a gas diffusion layer. As discussed above, membrane-coated anodes could be fabricated using similar steps, but with the PTL/anode acting as the structural base of the membrane-electrode assembly rather than the GDL/cathode. [0057] Referring now to FIG. 6A, membrane functionality of thin SiO? coatings from TP1 is shown, including a 5 nm thick SiO? coating deposited onto a Pt thin film by a low-temperature photochemical deposition process. Representative atomic force microscopy (AFM) images of this TP1 sample verified that the coating is continuous without visible pinholes or cracks. Linear sweep voltammograms carried out in the presence and absence of Ch-purged 0.5 M FhSO^aq) showed that the SiCh-coated Pt electrode greatly suppresses O2 reduction reaction (ORR) signal (0.0-0.8 V vs. RHE) while having negligible effect on the HER signal (< 0.0 V vs. RHE) compared to a bare Pt control. The difference in the ORR limiting current densities for the SiO2- Pt and bare Pt samples was used to estimate an O2 permeability of 2xl0'⁹ cm²/s, which is 2 orders of magnitude lower than O2 in hydrated Nafion and 2 orders of magnitude lower than H⁺ permeabilities for identical SiO2 membranes evaluated in dilute acid. These results highlight the ability of thin SiO2 membranes to conduct H⁺ (the reactant for HER) while blocking diffusion of molecular O2 (the limiting reactant for ORR), which minimizes O2 crossover to the cathode of an electrolyzer.

[0058] Referring now to FIG. 6B, to measure the through-plane electrical conductivity of SiO2 membranes and demonstrate a functional electrolyzer according to embodiments of the present disclosure at room temperature, a zero-gap “mini POM” cell based on TP3 was fabricated by depositing a 30 nm thick SiO2 membrane made by wet chemical processing onto a Pt thin film followed by deposition of a 0.1 mm² Pt top contact. Current-voltage (i-V) curves were then measured in the presence and absence of H2SO4 electrolyte, exposed to the anode top contact. The black curve was measured in the absence of H2SO4(_aq) electrolyte, showing that a 30 nm thick SiO2 membrane is resistive enough to suppress electrical leakage and lacks large pinholes, which would have led to electrical shorting between the top/bottom contacts. From the slope of this dry i-V curve, an electrical resistivity of 5xl0⁹ Q-cm was obtained. In contrast to the ohmic behavior under dry conditions, the red curve measured in the presence of the H2SO4 exhibited the characteristic i-V curve shape of an electrolyzer, with the additional electrolysis current at voltages above 2 V greatly exceeding the “dry” electronic leakage current.

[0059] Systems and methods of the present disclosure advantageously include oxide-based membranes with nanoscopic thickness for efficient, high current density water electrolysis. Hydrogen production based on these ultrathin oxide membranes can increase electrolysis efficiency by 20% (absolute) at high current densities compared to conventional PEM electrolyzers. The enhanced performance of these electrolyzers can be enabled, at least in part, by the lower ionic resistance of the dense oxide-based membranes that are 2-4 orders of magnitude thinner than conventional Nafion membranes and the potential to reduce ionic resistances below 0.2 V at current densities of 5 A/cm² or higher, in part by reducing ohmic losses. Electrolyzers based on an oxide-compound membranes of the present disclosure have ionic resistance that is > 80% lower than that of Nafion-117. It is projected that the capital cost of these electrolyzer stacks will be 80% lower than today’s PEM electrolyzers, thereby enabling green production of clean, domestically-produced EE fuel at less than 1 $/kg that can compete with fossil-derived EE and gasoline.

[0060] These oxide-based membranes can be integrated into 3-dimensional membrane electrode assembly structures, e.g., in short stack electrolyzers. These systems and methods demonstrate large area batch-2-batch manufacturing with fast atomic layer deposition, producing high quality, pinhole-free layers on large area parts and substrates. This inverted “membrane coated electrode” process is well-suited for scalable and low-cost roll-to-roll or plate-to-plate manufacturing schemes. Given the large differences in membrane material properties and assembly methods compared to conventional low-temperature electrolyzers, the embodiments of the present disclosure are positioned to increase electrolysis efficiencies and current densities to levels that would lead to step change reductions in LCOH and Capex.

[0061] The electrolyzers and associated membranes consistent with embodiments of the present disclosure are useful for electrochemical CO2 conversion / hydrogenation reactions / electroorganic synthesis, operation as a fuel cell, N2 reduction to NH3, other chemistries, or combinations thereof. H2 produced by the systems and methods of the present disclosure can be used, e.g., for the chemical and materials industry, fuel for forklifts / fuel cell vehicles, weather balloons, and more. The ability to operate at higher current densities also makes these embodiments more suitable for dynamically operated electrolysis facilities. These facilities primarily use electricity from variable renewable energy generators like solar and wind; in hybrid grid connected, grid-scale storage systems, this may also allow for price arbitrage when there is time of use electricity pricing.

[0062] Although the invention has been described and illustrated with respect to exemplary embodiments thereof, it should be understood by those skilled in the art that the foregoing and various other changes, omissions and additions may be made therein and thereto, without parting from the spirit and scope of the present invention.

Claims

CLAIMS What is claimed is:

1. A water electrolyzer, comprising: one or more membrane-electrode assemblies, the assemblies including: an oxide-compound membrane, wherein the membrane has a thickness less than about 1 pm; a gas diffusion layer (GDL); a porous transport layer (PTL); a hydrogen evolution reaction (HER) catalyst layer disposed between the GDL and the oxide-compound membrane; and an oxygen evolution reaction (OER) catalyst layer disposed between the PTL and the oxide-compound membrane.

2. The electrolyzer according to claim 1, wherein the oxide-compound membrane includes SiO_x, TiO_x, WO_X, or combinations thereof.

3. The electrolyzer according to claim 1, wherein the oxide-compound membrane is doped with one or more ion-exchange groups.

4. The electrolyzer according to claim 3, wherein the dopant is present at a concentration of less than about 10% by weight.

5. The electrolyzer according to claim 1, wherein the oxide-compound membrane is a continuous layer having substantially uniform thickness.

6. The electrolyzer according to claim 1, wherein the GDL includes carbon paper.

7. The electrolyzer according to claim 1, wherein the PTL includes sintered porous Ti.

8. The electrolyzer according to claim 1, wherein the GDL and PTL have a thickness of about 250-500 pm.

9. The electrolyzer according to claim 1, wherein the PTL includes: a macroporous transport layer having an average pore size above about 1 pm; and a microporous transport layer having an average pore size below about 1 pm, wherein the microporous transport layer is positioned between the macroporous transport layer and the OER catalyst layer.

10. The electrolyzer according to claim 9, wherein the PTL has a gradient porosity.

11. The electrolyzer according to claim 1, wherein the oxide-compound membrane has a roughness factor between about 1 and about 30.

12. The electrolyzer according to claim 1, wherein the area of the oxide-compound membrane is between about 80 cm² and 1600 cm².

13. The electrolyzer according to claim 1, wherein the EE leakage rate across the oxidecompound membrane is below about 10 pg H2/cm²/min.

14. The electrolyzer according to claim 1, wherein at least one of the HER catalyst layer and the OER catalyst layer includes platinum, IrO2, or combinations thereof.

15. A water electrolysis system, comprising: a water flowstream; a first flow channel in fluid communication with the water flowstream; a second flow channel; one or more membrane-electrode assemblies positioned between the first flow channel and the second flow channel, the assemblies including: an oxide-compound membrane, wherein the membrane has a thickness less than about 1 pm, wherein the oxide-compound membrane includes SiO_x, TiO_x, WO_X, or combinations thereof; a gas diffusion layer (GDL); a porous transport layer (PTL); a hydrogen evolution reaction (HER) catalyst layer disposed between the GDL and the oxide-compound membrane; and an oxygen evolution reaction (OER) catalyst layer disposed between the PTL and the oxide-compound membrane, a power supply in electrical communication with the GDL and the PTL; an oxygen gas effluent flow stream in fluid communication with the first flow channel; and a hydrogen gas effluent flow stream in fluid communication with the second flow channel.

16. The water electrolysis system according to 15, wherein at least one of the HER catalyst layer and the OER catalyst layer includes platinum, IrO2, or combinations thereof.

17. The water electrolysis system according to 15, wherein: the GDL includes carbon paper having a gradient porosity and a thickness of about 250-500 pm; and the PTL includes sintered porous Ti and a thickness of about 250-500 pm.

18. The water electrolysis system according to claim 17, wherein the PTL includes: a macroporous transport layer having an average pore size above about 1 pm; and a microporous transport layer having an average pore size below about 1 pm, wherein the microporous transport layer is positioned between the macroporous transport layer and the OER catalyst layer.

19. A method of making a membrane-electrode assembly, comprising: providing a first porous electrode substrate; depositing a first catalyst layer on the first porous electrode substrate; positioning one or more oxide-compound membrane layers on the first catalyst layer via an atomic layer deposition (ALD) fabrication process, a wet chemical process, or combinations thereof; providing a second porous electrode substrate; depositing a second catalyst layer on the second porous electrode substrate, the one or more oxide-compound membranes layers, or combinations thereof; and positioning the second porous electrode substrate on the first porous electrode substrate such that the second catalyst layer is positioned between the second porous electrode substrate and the oxide-compound membrane layer, wherein the overall thickness of the oxide-compound membrane layers is less than about 1 pm.

20. The method according to claim 19, wherein at least a first oxide-compound membrane layer is positioned via an ALD fabrication process and at least a second oxide-compound membrane layer is positioned via a wet chemical process.

21. The method according to claim 19, wherein the oxide-compound membrane includes SiO_x, TiO_x, WO_X, or combinations thereof.

22. The method according to claim 19, wherein at least one of the first porous electrode substrate and the second porous electrode substrate include a macroporous transport layer having an average pore size above about 1 pm and a microporous transport layer having an average pore size below about 1 pm, and a catalyst layer is positioned on the microporous transport layer.

23. The method according to claim 19, wherein positioning one or more oxide-compound membrane layers on the first catalyst layer includes: depositing a first oxide-compound layer; and electrodepositing at least a second oxide-compound layer on the first oxidecompound layer.