US20210079538A1 - Membrane-Coupled Cathode for the Reduction of Carbon Dioxide in Acid-Based Electrolytes Without Mobile Cations - Google Patents

Membrane-Coupled Cathode for the Reduction of Carbon Dioxide in Acid-Based Electrolytes Without Mobile Cations Download PDF

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US20210079538A1
US20210079538A1 US16/629,728 US201816629728A US2021079538A1 US 20210079538 A1 US20210079538 A1 US 20210079538A1 US 201816629728 A US201816629728 A US 201816629728A US 2021079538 A1 US2021079538 A1 US 2021079538A1
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cathode
space
anode
exchange membrane
ion exchange
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Bernhard Schmid
Christian Reller
Günter Schmid
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Siemens AG
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    • C25B9/10—
    • C—CHEMISTRY; METALLURGY
    • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
    • C25B9/00—Cells or assemblies of cells; Constructional parts of cells; Assemblies of constructional parts, e.g. electrode-diaphragm assemblies; Process-related cell features
    • C25B9/17—Cells comprising dimensionally-stable non-movable electrodes; Assemblies of constructional parts thereof
    • C25B9/19—Cells comprising dimensionally-stable non-movable electrodes; Assemblies of constructional parts thereof with diaphragms
    • C25B9/23—Cells comprising dimensionally-stable non-movable electrodes; Assemblies of constructional parts thereof with diaphragms comprising ion-exchange membranes in or on which electrode material is embedded
    • C—CHEMISTRY; METALLURGY
    • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
    • C25B1/00—Electrolytic production of inorganic compounds or non-metals
    • C25B9/02—
    • C—CHEMISTRY; METALLURGY
    • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
    • C25B9/00—Cells or assemblies of cells; Constructional parts of cells; Assemblies of constructional parts, e.g. electrode-diaphragm assemblies; Process-related cell features
    • C25B9/17—Cells comprising dimensionally-stable non-movable electrodes; Assemblies of constructional parts thereof
    • C25B9/19—Cells comprising dimensionally-stable non-movable electrodes; Assemblies of constructional parts thereof with diaphragms
    • C—CHEMISTRY; METALLURGY
    • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
    • C25B9/00—Cells or assemblies of cells; Constructional parts of cells; Assemblies of constructional parts, e.g. electrode-diaphragm assemblies; Process-related cell features
    • C25B9/60—Constructional parts of cells
    • C25B9/63—Holders for electrodes; Positioning of the electrodes

Definitions

  • the present disclosure relates to electrolysis of CO 2 .
  • Various embodiments include methods of electrolysis, electrolysis systems comprising an electrolysis cell.
  • Table 1 states Faraday efficiencies (FE) (in [%]) of products formed in carbon dioxide reduction at various metal electrodes. The values reported are applicable to a 0.1 M potassium hydrogencarbonate solution as electrolyte. As apparent from table 1, the electrochemical reduction of CO 2 at solid-state electrodes in aqueous electrolyte solutions offers a multitude of possible products.
  • FE Faraday efficiencies
  • they may take the form of electrically conductive catalyst particles bound to polymers, for example of an extruded or calendered film, which corresponds to an all-active-catalyst gas diffusion electrode, or of a porous, catalytically inactive but conductive electrode, for example in the form of carbon fiber gas diffusion layers impregnated with a small amount of active catalyst particles.
  • some embodiments of the teachings herein include a method of electrolysis of CO 2 , wherein an electrolysis cell comprising a cathode space comprising a cathode; a first ion exchange membrane which contains an anion exchanger and/or anion transporter and adjoins the cathode space, where the cathode forms direct contact with the first ion exchange membrane; an anode space comprising an anode; a first separator membrane; and a salt bridge space, where the salt bridge space is disposed between the first ion exchange membrane and the first separator membrane, is used; wherein CO 2 is reduced at the cathode, wherein the electrolyte in the salt bridge space consists of a liquid acid and/or a solution of an acid.
  • an electrolysis cell comprising a cathode space comprising a cathode; a first ion exchange membrane which contains an anion exchanger and/or anion transporter and adjoins the cathode space, where the cathode forms direct contact with the first ion exchange membrane; and an anode space comprising an anode, where the anode space adjoins the first ion exchange membrane is used; wherein CO 2 is reduced at the cathode, wherein the electrolyte in the anode space consists of a liquid acid and/or a solution of an acid.
  • the second ion exchange membrane is selected from an ion exchange membrane containing a cation exchanger, a bipolar membrane and a diaphragm.
  • the anode space comprises an anolyte comprising a liquid and/or dissolved acid, preferably wherein the anolyte and/or the acid in the salt bridge space does not comprise any mobile cations except for protons and/or deuterons, especially any metal cations.
  • the anode lies against the first ion exchange membrane.
  • the electrolysis is conducted with a current density of more than 50 mAcm ⁇ 2 , more than 100 mAcm ⁇ 2 , of 150 mAcm ⁇ 2 or more, 170 mAcm ⁇ 2 or more, 250 mAcm ⁇ 2 or more, 400 mAcm ⁇ 2 or more, or 600 mAcm ⁇ 2 or more.
  • an acid of the electrolyte in the salt bridge space has a pK A of 6 or less, 5 or less, 3 or less, 1 or less, or 0 or less, wherein the liquid and/or dissolved acid is selected from dilute or neat H 2 SO 4 , a solution of H 2 N—SO 2 —OH, dilute or neat HClO 4 , a solution of H 3 PO 4 , dilute or neat CF 3 —COOH, dilute or neat CF 3 —SO 2 —OH, a solution of (CF 3 —SO 2 ) 2 —NH, a solution of HF, dilute or neat HCOOH, dilute or neat CH 3 —COOH, a solution of HCl, a solution of HBr, a solution of HI, and/or mixtures thereof.
  • some embodiments include an electrolysis cell comprising: a cathode space comprising a cathode; a first ion exchange membrane which contains an anion exchanger and/or anion transporter and adjoins the cathode space, where the cathode forms direct contact with the first ion exchange membrane; an anode space comprising an anode; and a diaphragm that adjoins the anode space; further comprising a salt bridge space, wherein the salt bridge space is disposed between the first ion exchange membrane and the diaphragm, wherein the diaphragm is non-ion-conductive.
  • the anode is in contact with the diaphragm, and/or wherein the anode and/or the cathode is in contact with a conductive structure on the side remote from the salt bridge space.
  • the cathode and/or the anode takes the form of a gas diffusion electrode, of a porous bound catalyst structure, of a particulate catalyst on a support, of a coating of a particulate catalyst on the first and/or second ion exchange membrane, of a porous conductive carrier impregnated with a catalyst, and/or of a noncontinuous sheetlike structure.
  • the cathode takes the form of a gas diffusion electrode, of a porous bound catalyst structure, of a particulate catalyst on a support, of a coating of a particulate catalyst on the first and/or second ion exchange membrane, of a porous conductive carrier impregnated with a catalyst, and/or of a noncontinuous sheetlike structure, containing an anion exchange material and/or anion transport material, and/or wherein the anode takes the form of a gas diffusion electrode, of a porous bound catalyst structure, of a particulate catalyst on a support, of a coating of a particulate catalyst on the first and/or second ion exchange membrane, of a porous conductive carrier impregnated with a catalyst, and/or of a noncontinuous sheetlike structure, containing a cation exchange material.
  • the first ion exchange membrane and/or the diaphragm are hydrophilic.
  • the electrolyte in the salt bridge space consists of a liquid acid and/or a solution of an acid, preferably wherein an acid in the electrolyte in the salt bridge space has a pK A of 6 or less, preferably 5 or less, further preferably 3 or less, even further preferably 1 or less, especially preferably 0 or less, further preferably wherein the liquid and/or dissolved acid is selected from dilute or neat H 2 SO 4 , a solution of H 2 N—SO 2 —OH, dilute or neat HClO 4 , a solution of H 3 PO 4 , dilute or neat CF 3 —COOH, dilute or neat CF 3 —SO 2 —OH, a solution of (CF 3 —SO 2 ) 2 —NH, a solution of HF, dilute or neat HCOOH, dilute or neat CH 3 —COOH, a solution of HCl, a solution of HBr, a solution of HI, and/or mixtures thereof.
  • some embodiments include an electrolysis system comprising an electrolysis cell as described above.
  • a recycling device connected to an outlet from the salt bridge space and an inlet to the cathode space, which is set up to return a reactant from the cathode reaction that can be formed in the salt bridge space to the cathode space.
  • FIGS. 1 and 2 show a graphic representation of the cathodic half-cell of the above-described transport model of ions of salts and acids in an AEM adjoining a cathode.
  • FIG. 3 shows a schematic of an example of an electrolysis system with an electrolysis cell as employed in some methods incorporating the teachings herein.
  • FIGS. 4 and 5 show schematics of further examples of electrolysis cells with which various methods incorporating the teachings herein may be executed.
  • FIGS. 6 and 7 show schematic graphic representations of the different release of CO 2 in the case of use of a salt electrolyte ( FIG. 6 ) and an acid electrolyte ( FIG. 7 ).
  • FIG. 8 shows a schematic of an electrolysis system of the invention with an AEM diaphragm cell incorporating the teachings herein in which methods incorporating the teachings herein can be conducted.
  • FIG. 9 shows a schematic diagram of an AEM bipolar double membrane cell in which methods incorporating the teachings herein can likewise be conducted.
  • FIG. 10 shows a schematic of the experimental setup in example 1.
  • FIG. 11 shows experimental results of example 1, wherein the Faraday efficiency has been plotted against the current density applied.
  • FIG. 12 shows a schematic of the experimental setup in the present comparative example 1.
  • FIG. 13 shows the experimental results obtained thereby, again by a plot of the Faraday efficiency against the current density applied.
  • FIG. 14 compares the experimental results from example 1 (solid lines) with those from comparative example 1 (dotted lines).
  • FIG. 15 shows a schematic diagram of the experimental setup in comparative example 2.
  • FIG. 16 shows the experimental results thus obtained, again by a plot of the Faraday efficiency against the current density applied.
  • FIG. 17 compares the experimental results from example 1 (solid lines) with those from comparative example 2 (dotted lines).
  • FIG. 18 shows a comparison of the gas chromatograms obtained in comparative example 2 (solid line; w/o AEM) and example 1 (dotted line; w/AEM) at a current density of 150 mAcm ⁇ 2 .
  • FIGS. 19 and 20 each show schematics of the experimental setup in reference examples 1 and 2.
  • FIGS. 21 and 22 show the experimental results obtained therein.
  • CO 2 can be converted effectively in the presence of a liquid and/or dissolved acid at a membrane facing the cathode space and containing an anion exchanger and/or anion transporter, for example in a salt bridge space and/or in the anolyte, to products utilizable further in an economically viable manner, and the formation of hydrogen can surprisingly be suppressed.
  • the elucidations that follow are applicable to the above systems, for example. On the basis of these considerations, it is possible to create a charge transport model for CO 2 electrolysis as follows:
  • Solid-state electrolytes in electrolysis cells are, for example, membranes made from polymers modified with charged functionalities.
  • AEMs anion exchange membranes
  • the cationic functional groups are at fixed locations.
  • the charge transport in this case can therefore typically only be by HCO 3 ⁇ ions.
  • this process can more particularly only be employed when the anode is also directly connected to the membrane.
  • the supply of the HCO 3 ⁇ ions to the anode is undesirable since the CO 2 formed there is released again by neutralization.
  • a formal double salt system can exist in which, for example, the anionic part is taken entirely by HCO 3 ⁇ , while the cationic part is taken partly by M + ions and partly by the cationic functional groups of the polymer. It is thus also possible for the penetration of M + to be limited but not entirely prevented by an AEM in the presence of a salt electrolyte.
  • a salt electrolyte In corresponding laboratory studies—as specified in comparative example 1 below—it was possible to observe crystallization of MHCO 3 on the reverse side (gas side) of the electrode. However, the phenomenon is significantly attenuated compared to direct contact between cathode and electrolyte.
  • HCO 3 ⁇ in the charge transport can be distinctly increased compared to a mode of operation without AEM and can be determined, for example, to be ⁇ 50 mol %, for example by CO 2 measurement by gas chromatography, but is still limited.
  • the cause of this is the low mobility of hydrogencarbonate anions as stated above and apparent from table 2 below, taken from Current Separations 18:3 (1999), Conductance Measurements, Part 1: Theory, Lou Coury, p. 91-96.
  • HCO 3 ⁇ cannot function as counterion for “H + ”, the only cations present in the electrolyte. It is thus not possible for a double salt situation to exist, as with alkali metal salt electrolytes. Presence of “H + ” ions in the AEM is therefore possible only when the acid anions (e.g. SO 4 2 ⁇ ) of the electrolyte are also present in the AEM. If these are displaced from the AEM by a sufficiently high ion current, a high pH can be established in the cathode-AEM composite in spite of an acid electrolyte. The only other charge transport pathway is the conduction of OH ⁇ via the Grotthuss mechanism through the membrane swollen in H 2 O, or hopping transport of HCO 3 ⁇ from localized polymer-bound cation to localized cation.
  • FIGS. 1 and 2 illustrate this difference in the use of various electrolytes 1 that adjoin the anion exchange membrane AEM, and which pass ions to the cathode K.
  • FIG. 1 shows the variant with a salt M + X ⁇ as electrolyte 1 by way of example
  • FIG. 2 shows the variant with an acid H+X ⁇ as electrolyte 1 .
  • teachings of the present disclosure include a method of electrolysis of CO 2 , wherein an electrolysis cell comprising
  • a method of electrolysis of CO 2 wherein an electrolysis cell comprising:
  • an electrolysis cell comprises:
  • the salt bridge space is disposed between the first ion exchange membrane and the diaphragm, wherein the diaphragm is nonconductive.
  • an electrolysis system comprising the electrolysis cell described above, and/or include the use of the electrolysis cell or of the electrolysis system for electrolysis of CO 2 .
  • Gas diffusion electrodes in general are electrodes in which liquid, solid and gaseous phases are present, and where, in particular, a conductive catalyst catalyzes an electrochemical reaction between the liquid phase and the gaseous phase.
  • a conductive catalyst catalyzes an electrochemical reaction between the liquid phase and the gaseous phase.
  • They can be constructed in different ways, for example as a porous “all-active-material catalyst”, optionally with auxiliary layers for adjustment of hydrophobicity, in which case it is possible to produce, for example, a membrane-GDE composite, e.g.
  • AEM-GDE composite as a conductive porous carrier to which a catalyst may be applied in a thin layer, in which case it is likewise again possible to produce a membrane-GDE composite, e.g. AEM-GDE composite; or as a catalyst which is porous in the composite and which may, optionally with additive, be applied directly to a membrane, e.g. an AEM, and in that case can form a CCM in the composite.
  • hydrophobic is understood to mean water-repellent. Hydrophobic pores and/or channels are those that repel water. In particular, hydrophobic properties are associated in accordance with the invention with substances or molecules having nonpolar groups.
  • hydrophilic is understood to mean the ability to interact with water and other polar substances.
  • Electro-osmosis includes an electrodynamic phenomenon in which a force in the cathode direction acts on particles having a positive zeta potential that are present in solution, and a force in the anode direction on all particles having a negative zeta potential. If a conversion takes place at the electrodes, i.e. if there is galvanic current flow, there is also a stream of matter of the particles having positive zeta potential toward the cathode, irrespective of whether or not the species is involved in the conversion. The same is also true of a negative zeta potential and the anode. If the cathode is porous, the medium is also pumped through the electrode. This is also referred to as an electro-osmotic pump.
  • the streams of matter that result from electro-osmosis can also flow counter to concentration gradients. Diffusion-related currents that compensate for the concentration gradients can be overcompensated as a result.
  • FIGS. 3 to 5 Illustrative different modes of operation of a double-membrane cell and a single-membrane cell with which the methods of the invention can be conducted are shown in FIGS. 3 to 5 —in FIG. 3 also in conjunction with further constituents of an electrolysis system, also with regard to the method of the invention.
  • a reduction of CO 2 to CO is assumed.
  • the method is not limited to this reaction but can also be used for any other products, such as hydrocarbons, etc., e.g. in gaseous and/or liquid form.
  • FIG. 3 shows, by way of example, a 2-membrane setup for CO 2 electroreduction with an acidic anode reaction.
  • the cathode K is provided in the cathode space I and the anode A in the anode space III, with a salt bridge space II formed between these spaces, which is divided from the cathode space I by a first ion exchange membrane, here in the form of an AEM, and from the anode space III by a first separator membrane, here in the form of a CEM, for example in the form of a cation and/or proton exchange membrane.
  • a first ion exchange membrane here in the form of an AEM
  • a first separator membrane here in the form of a CEM
  • catholyte k to supply the cathode with substrate, for example H 2 O-saturated gaseous CO 2 , electrolyte s in the salt bridge space comprising liquid and/or dissolved acid that couples the half-cells to one another, and anolyte a for supply of the anode with substrate, e.g. HCl and/or H 2 O, and also a recycle conduit R for CO 2 .
  • substrate for example H 2 O-saturated gaseous CO 2
  • anolyte a for supply of the anode with substrate, e.g. HCl and/or H 2 O, and also a recycle conduit R for CO 2 .
  • FIG. 3 and also in the analogous FIGS. 8, 9, 10, 12, 15, 19 and 20 are customary fluidic connection symbols.
  • FIGS. 4 and 5 additionally show further constructions of an electrolysis cell as can be employed in a method incorporating the teachings herein.
  • No salt bridge space is provided in the two-chamber setup, and so the anode space III directly adjoins the AEM, and it is possible here for the anode, as shown in FIGS. 4 and 5 , to be present anywhere in the anode space III.
  • Corresponding configurations of the anode space are also possible in a method having a set up as shown in FIG. 3 , where the anode A thus does not adjoin the CEM.
  • the electrolysis cells shown in FIGS. 4 and 5 can likewise be used in the electrolysis system shown in FIG. 3 . It is also possible for the different half-cells from FIGS.
  • the cathode K forms direct, especially also ionic, contact with the first ion exchange membrane containing an anion exchanger and/or anion transporter.
  • the space adjoining the first ion exchange membrane either the salt bridge space II in FIG. 3 or the anode space III in FIGS. 4 and 5 —contains a liquid and/or dissolved acid.
  • the methods herein have the particular feature of the use of a liquid and/or dissolved acid in the salt bridge space or in the anode space, specifically by comparison with strongly acidic ion exchanger packages or similar solid apparatuses:
  • gas bubbles that form from the reaction in the salt bridge space or anode space can be transported away unhindered through the fluid medium, which enables a simple mode of operation.
  • salt bridge space or the anode space are not particularly restricted provided that they correspondingly adjoin the first ion exchange membrane.
  • the term “salt bridge space” is used here with regard to its function of acting as a bridge between the anode arrangement and cathode arrangement, and in that respect of including cations and anions which, however, need not necessarily form salts in the present context. Since a liquid or dissolved acid is present in the salt bridge space in the present context, this could also be called acid bridge space or ion bridge space. However, since this term is not in common use, the space is referred to in accordance with the disclosure as salt bridge space even if no salt need be present therein in the conventional sense.
  • an electrolyte in the salt bridge space if present—that can assure electrolytic ionic connection between cathode arrangement and anode arrangement.
  • This electrolyte is also referred to as salt bridge and includes a liquid and/or dissolved acid.
  • the salt bridge thus serves here as electrolyte, preferably with high ion conductivity, and serves to establish contact between the anode and cathode.
  • the salt bridge also enables the removal of waste heat.
  • the salt bridge can serve as reaction medium for the anodically and cathodically generated ions such as protons or hydroxide or hydrogencarbonate ions.
  • the technical teaching consists in the construction and operation of the cathodic half-cell.
  • the latter consists of a gas-permeable electrically connected catalyst layer in direct contact with an AEM, the opposite face of which is adjoined by an acid-based electrolyte, preferably without alkali metal cations, especially without metal cations.
  • the acid here is not particularly restricted, provided that it is in the form of a liquid and/or in solution, i.e. the acid is able to flow through the salt bridge space and/or the anode space.
  • the acid is water-soluble and/or is in the form of a solution in a suitable solvent such as water, alcohols, aldehydes, esters, carbonates, etc., and/or mixtures, especially water, e.g. double-distilled or demineralized water.
  • a suitable solvent such as water, alcohols, aldehydes, esters, carbonates, etc., and/or mixtures, especially water, e.g. double-distilled or demineralized water.
  • an acid in the electrolyte in the salt bridge space has a pK A of 6 or less, 5 or less, 3 or less, 1 or less, or 0 or less, where the liquid and/or dissolved acid may be selected from dilute or neat H 2 SO 4 , a solution of H 2 N—SO 2 —OH, dilute or neat HClO 4 , a solution of H 3 PO 4 , dilute or neat CF 3 —COOH, dilute or neat CF 3 —SO 2 —OH, a solution of (CF 3 —SO 2 ) 2 —NH, a solution of HF, dilute or neat HCOOH, dilute or neat CH 3 —COOH, a solution of HCl, a solution of HBr, a solution of HI, and/or mixtures thereof.
  • the acid electrolyte is notable for the absence of mobile cations—as will be defined further down, especially metal cations, except for “H + ” or “D + ”. Rather than H + or D + , reference is made hereinafter solely to H + or protons.
  • the electrolyte thus may not contain any mobile cations except for “H + ”, especially any metal cations.
  • sulfuric acid especially dilute sulfuric acid (H 2 SO 4 )
  • H 2 SO 4 dilute sulfuric acid
  • strong acids with nonoxidizing anions such as H 2 N—SO 2 —OH, HClO 4 , H 3 PO 4 , CF 3 —COOH, CF 3 —SO 2 —OH, (CF 3 —SO 2 ) 2 —NH, etc.
  • weak acids in relatively high concentrations, for example greater than 10% or 20% by weight, for example greater than 30% by weight, or at their respective conductivity maximum, e.g. HF, HCOOH, CH 3 —COOH.
  • this acid is identical to the cathodic product from the CO 2 electrolysis, for example in the case of formic acid or acetic acid.
  • the acids may be present in a concentration up to 30% by weight, up to 50% by weight, up to 70% by weight, or up to 100% by weight. It is also possible to use other acids, especially in the case of demonstrable compatibility with the electrode catalysts, for example dissolved HCl, HBr, HI.
  • a salt electrolyte typically adjoining the first ion exchange membrane may be replaced by an acid.
  • the advantage of the CO 2 -free anode and the partial removal of the CO 2 excess in the salt bridge space continues to exist, as shown in schematic form in FIGS. 6 and 7 .
  • the CO 2 is released not at the interface between CEM and electrolyte but, as shown in FIG. 7 , at the interface between AEM and acid 3.
  • an acid is also present here in the anode space.
  • a diaphragm is also possible as the first separator membrane, as detailed further hereinafter, and is consequently also possible to use a corresponding electrolysis cell of the invention, for example an AEM diaphragm cell—as detailed further hereinafter—in the method of the invention.
  • the cathode space, the anode space and any salt bridge space present, in the methods and also in the electrolysis cell discussed hereinafter, are not particularly restricted in terms of shape, material, dimensions, etc., provided that they can accommodate the cathode, the anode and the first ion exchange membrane and any first separator membrane.
  • the two or three spaces may be formed, for example, within a common cell, in which case they may be separated correspondingly by the first ion exchange membrane and optionally the first separator membrane.
  • this may be supplied to the cathode in solution, as a gas, etc.—for example in countercurrent to an electrolyte stream in the salt bridge space in the three-chamber setup or in the anode space in a two-chamber setup (without first separator membrane).
  • this may be supplied to the cathode in solution, as a gas, etc.—for example in countercurrent to an electrolyte stream in the salt bridge space in the three-chamber setup or in the anode space in a two-chamber setup (without first separator membrane).
  • the respective feed may be provided either in continuous or discontinuous form, for example in pulsed form, etc., for which pumps, valves, etc. may correspondingly be provided in an electrolysis system of the invention, and also cooling and/or heating devices, in order to be able to correspondingly catalyze desired reactions at the anode and/or cathode.
  • the materials of the respective spaces or of the electrolysis cell and/or of the further constituents of the electrolysis system may also be suitably adapted here to desired reactions, reactants, products, electrolytes, etc.
  • at least one power source per electrolysis cell is of course also included.
  • electrolysis cells or electrolysis systems may be provided in the electrolysis system of the invention or the electrolysis cell.
  • these individual cells are used to construct a stack comprising 2-1000 or 2-200 cells and the operating voltage thereof may be in the range of 3-1500 V or 200-600 V.
  • a reactant gas formed in the salt bridge space for example CO 2 that may also contain H 2 and/or CO, is recycled back in the direction of the cathode space.
  • the cathode is not particularly restricted and may be matched to a desired half-reaction, for example with regard to the reaction products, in that it forms direct contact with the first ion exchange membrane, i.e. is in direct contact with the first ion exchange membrane at at least one point, wherein the cathode is in direct contact essentially in two dimensions with the first ion exchange membrane.
  • the cathode thus directly adjoins the first ion exchange membrane at least in one region.
  • a cathode for reduction of CO 2 and optionally CO may include, for example, a metal such as Cu, Ag, Au, Zn, Pb, Sn, Bi, Pt, Pd, Ir, Os, Fe, Ni, Co, W, Mo, etc., or mixtures and/or alloys thereof, e.g. Cu, Ag, Au, Zn, Pb, Sn, or mixtures and/or alloys thereof, and/or a salt thereof, where suitable materials may be matched to a desired product.
  • the catalyst may thus be chosen according to the desired product.
  • the catalyst may be based on Ag, Au, Zn and/or compounds thereof, such as Ag 2 O, AgO, Au 2 O, Au 2 O 3 , ZnO.
  • Ag, Au, Zn and/or compounds thereof such as Ag 2 O, AgO, Au 2 O, Au 2 O 3 , ZnO.
  • Cu or Cu-containing compounds such as Cu 2 O, CuO and/or copper-containing mixed oxides with other metals, etc. may be used.
  • catalysts based on Pb and/or Cu, especially Cu are possible.
  • the cathode is the electrode at which the reductive half-reaction takes place. It may be in single-part or multipart form and take the form, for example, of a gas diffusion electrode, porous electrode, or be directly in a composite with the AEM, etc. At least the following embodiments, for example, are possible here:
  • cathode Various combinations of the above-described electrode structures are also possible as cathode.
  • the corresponding cathodes here too may contain materials that are customary in cathodes, such as binders, ionomers, for example anion-conductive ionomers, fillers, hydrophilic additives, etc., which are not particularly restricted.
  • the cathode may thus, in particular embodiments, contain at least one ionomer, for example an anion-conductive or anion-transporting ionomer (e.g.
  • anion exchange resin anion transport resin
  • anion exchange resin anion transport resin
  • a carrier material for example a conductive carrier material (e.g.
  • a metal such as titanium
  • at least one nonmetal such as carbon, Si, boron nitride (BN), boron-doped diamond, etc.
  • at least one conductive oxide such as indium tin oxide (ITO), aluminum zinc oxide (AZO) or fluorinated tin oxide (FTO)—for example as used for production of photoelectrodes, and/or at least one polymer based on polyacetylene, polyethoxythiophene, polyaniline or polypyrrole, as, for example, in polymer-based electrodes; nonconductive carriers, for example polymer meshes, are possible given adequate conductivity of the catalyst layer, binders (e.g.
  • hydrophilic and/or hydrophobic polymers e.g. organic binders, for example selected from PTFE (polytetrafluoroethylene), PVDF (polyvinylidene difluoride), PFA (perfluoroalkoxy polymers), FEP (fluorinated ethylene-propylene copolymers), PFSA (perfluorosulfonic acid polymers), and mixtures thereof, especially PTFE), conductive fillers (e.g. carbon), nonconductive fillers (e.g. glass) and/or hydrophilic additives (e.g. Al 2 O 3 , MgO 2 , hydrophilic materials such as polysulfones, e.g.
  • organic binders for example selected from PTFE (polytetrafluoroethylene), PVDF (polyvinylidene difluoride), PFA (perfluoroalkoxy polymers), FEP (fluorinated ethylene-propylene copolymers), PFSA (perfluorosul
  • polyphenylsulfones polyimides, polybenzoxazoles or polyetherketones, or generally polymers that are electrochemically stable in the electrolyte, polymerized “ionic liquids”, and or organic conductors such as PEDOT:PSS or PANI (camphorsulfonic acid-doped polyaniline), which are not particularly restricted.
  • the cathode e.g. in the form of a gas diffusion electrode, for example bonded to the first ion exchange membrane, or present in the form of a CCM, in particular embodiments, contains ion-conductive components, especially an anion-conductive component.
  • ion-conductive components especially an anion-conductive component.
  • Other cathode forms are also possible, for example cathode constructions as described in US2016 0251755-A1 and U.S. Pat. No. 9,481,939.
  • the anode is not particularly restricted either and may be matched to a desired half-reaction, for example with regard to the reaction products.
  • the anode which is electrically connected to the cathode by means of a power source to provide the voltage for the electrolysis, the oxidation of a substance takes place in the anode space.
  • the anode material is not particularly restricted and depends primarily on the reaction desired.
  • Illustrative anode materials include platinum or platinum alloys, palladium or palladium alloys, and glassy carbon, iron, nickel etc.
  • anode materials are also conductive oxides such as doped or undoped TiO 2 , indium tin oxide (ITO), fluorine-doped tin oxide (FTO), aluminum-doped zinc oxide (AZO), iridium oxide, etc. optionally, these catalytically active compounds may also have been merely superficially applied by thin-film methodology, for example on a titanium and/or carbon carrier.
  • the anode catalyst is not particularly restricted.
  • Catalysts used for production of O 2 or Cl 2 also include, for example, IrO x (1.5 ⁇ x ⁇ 2) or RuO 2 . These may also be in the form of a mixed oxide with other metals, e.g.
  • TiO 2 and/or have been supported on a conductive material such as C (in the form of conductive carbon black, activated carbon, graphite, etc.).
  • a conductive material such as C (in the form of conductive carbon black, activated carbon, graphite, etc.).
  • catalysts based on Fe—Ni or Co—Ni for production of O 2 .
  • the construction described below with a bipolar membrane is suitable.
  • the anode is the electrode at which the oxidative half-reaction takes place. It may likewise take the form of a gas diffusion electrode, porous electrode or all-active electrode or solid electrode, etc. At least the following embodiments are possible:
  • the anode here may follow on from the acid electrolyte or else directly adjoin the first ion exchange membrane, for example an AEM, for example in the form of a sheetlike structure (e.g. fine-mesh coated grid), such that there is no salt bridge space.
  • AEM a sheetlike structure
  • the cathode is coupled to the anodic half-cell via the liquid acid, for example in the salt bridge space or in the anode space, for example in the salt bridge space.
  • the corresponding anodes may likewise contain materials that are customary in anodes, such as binders, ionomers, for example including cation-conducting ionomers, for example containing sulfonic acid and/or phosphonic acid groups, fillers, hydrophilic additives, etc., which are not particularly restricted, which have also been described above, for example, with regard to the cathodes.
  • electrolyte it is possible to combine the electrodes mentioned by way of example above with one another as desired.
  • electrolyte it is also possible for electrolyte to be present in the anode space and cathode space, which are also respectively referred to as anolyte and catholyte, but it is not ruled out in that no electrolytes are present in the two spaces and they are correspondingly supplied, for example, solely with gases for conversion, for example CO 2 only, optionally also as a mixture with, for example, CO and/or H 2 O, which may optionally also be in liquid form, for example as an aerosol, but with gaseous H 2 O to the cathode and/or water or HCl to the anode.
  • an anolyte is present, which may differ from or correspond to the salt bridge, i.e. the electrolyte of the salt bridge space, which includes a liquid and/or dissolved acid—if present, for example with regard to solvents, acids present, etc. If no salt bridge is present, the anolyte comprises a liquid and/or dissolved acid.
  • a catholyte here is the electrolyte flow around the cathode and, in particular embodiments, serves to supply the cathode with substrate or reactant.
  • the embodiments which follow are possible, for example.
  • the catholyte may take the form, for example, of a solution of the substrate (CO 2 ) in a liquid carrier phase (e.g. water) and/or of a mixture of the substrate with other gases (e.g. CO+CO 2 ; water vapor+CO 2 ). It is likewise possible for recycled gases such as CO and/or H 2 to be present as a result of recycling. It is also possible as described above, for the substrate to be present as a pure phase, e.g. CO 2 . If the reaction gives rise to uncharged liquid products, these may be washed out by the catholyte and may subsequently also optionally be removed in a corresponding manner.
  • An anolyte is an electrolyte flow around the anode or at the anode and, in particular embodiments, serves to supply the anode with substrate or reactant and optionally to transport anode products away.
  • the embodiments that follow are possible, for example.
  • the anode space comprises an anolyte comprising a liquid and/or dissolved acid, preferably wherein the anolyte and/or the acid in the salt bridge space or the electrolyte in the salt bridge space does not include any mobile cations except for protons and/or deuterons, especially any metal cations.
  • the acid in the salt bridge space does not comprise any mobile cations except for protons and/or deuterons, especially any metal cations.
  • the anolyte does not comprise any mobile cations except for protons and/or deuterons, especially any metal cations.
  • Mobile cations here are cations that are not bonded to a support by a chemical bond and/or especially have an ion mobility of more than 1 ⁇ 10 ⁇ 8 m 2 /(s ⁇ V), especially of more than 1 ⁇ 10 ⁇ 10 m 2 /(s ⁇ V).
  • the anodic half-reaction does not release or produce any mobile cations except for “D + ”, “H + ”, especially any metal cations. In such a case, therefore, for the specific case of the evolution of O 2 at the anode, water (especially in the case of the CCM anode) or acids with non-oxidizable anions are possible anolytes or reagents.
  • halogen-hydrogen acids HCl, HBr or HI are correspondingly suitable, and halide salts may not be suitable in the case of use of a diaphragm as the first separator membrane, but may be used in the case of use of a bipolar membrane as the first separator membrane. It is also possible to use SO 2 in the anolyte for preparation of sulfuric acid, or H 2 O for preparation of H 2 O 2 , etc.
  • the anolyte is an aqueous electrolyte, where appropriate reactants that are converted at the anode may optionally be added to the anolyte.
  • the addition of reactants here is not restricted.
  • the addition of reactants on supply to the cathode space is likewise not restricted.
  • CO 2 can be added to water outside the cathode space, or can also be added via a gas diffusion electrode, or can also be supplied solely as a gas to the cathode space.
  • the reactants used for example water, HCl, etc., and the desired product.
  • the first ion exchange membrane which contains an anion exchanger and/or anion transporter or an anion transport material and which adjoins the cathode space is not particularly restricted in accordance with the invention. In some embodiments, it separates the cathode from the salt bridge space, or, in the method of the second aspect, it separates the cathode from the anode space, so as to result in, from the direction of the cathode space comprising CO 2 in the electrolyte direction, the sequence of cathode/first ion exchange membrane/salt bridge space (first aspect) or cathode/first ion exchange membrane/anode space.
  • an anion exchanger which, in the zero-current state, is in the form of an acid anion salt, preferably corresponding to the acid of the salt bridge, and further may be converted to the hydrogencarbonate/carbonate form over and above a minimum current density.
  • the first ion exchange membrane is an anion exchange membrane and/or anion transport membrane.
  • the first ion exchange membrane may have a hydrophobic layer, for example on the cathode side, for better contacting with gas.
  • the anion exchange membrane and anion transport membrane additionally functions as a cation blocker (albeit in traces, for example), and especially as a proton blocker.
  • an anion exchanger and/or anion transporter with cations bound in a fixed manner may constitute a blockage here for mobile cations by coulombic repulsion, which can additionally counteract separation of salts, especially within the cathode.
  • the enrichment of the electrolyte cations in the region of the interface is typically attributable to electroosmosis.
  • a concentration gradient cannot simply be dissipated here on the electrode side since a catalyst-based cathode configured as set out above, for example a gas diffusion electrode or a CCM, usually has only very poor anion conductivity.
  • the integration of anion-conducting components here can distinctly improve the anion conductivity.
  • the electrolyte contains solely protons.
  • anion transporters especially anion transport resins, may be used as binder material or additive in the electrode itself and/or in an anion exchanger layer adjoining the cathode in order to rapidly lead off or partly buffer OH ⁇ ions that form, for example, such that the reaction with CO 2 and the associated formation of hydrogen-carbonates can be reduced or the anion transport resins conduct HCO 3 ⁇ themselves.
  • anion transport can be effected by anion exchangers.
  • an integrated anion exchanger again specifically constitutes a blockage for cations, for example including traces of metal cations, which can additionally counteract separation of salts and contamination of the electrode. In the case of protons, the formation of hydrogen can be suppressed.
  • the first ion exchange membrane for example from the cathode side adjoining the salt bridge in the method of the first aspect, may thus contain, for example, an anion exchanger and/or anion transporter in the form of an anion exchanger and/or transporter layer, in which case further layers such as hydrophobizing layers may be present to improve contact with the gas, for example CO 2 .
  • the first ion exchange membrane is an anion exchange membrane and/or anion transport membrane, i.e., for example, an ion-conductive membrane (or else in the broader sense a membrane having an anion exchange layer and/or anion transport layer) with positively charged functionalizations, which is not particularly restricted.
  • charge transport takes place through anions in the anion exchange layer and/or anion transport layer or an anion exchange membrane and/or anion transport membrane.
  • the first ion exchange membrane and anion exchange layer and/or anion transport layer therein or an anion exchange membrane and/or anion transport membrane serves to provide anion transport along positive charges at fixed locations. It is possible here to reduce or completely prevent the penetration of a proton-containing electrolyte into the cathode, for example, which is promoted by electro-osmotic forces.
  • the ion exchanger present in the membrane in particular embodiments, especially in operation, can be converted to the carbonate/hydrogencarbonate form and hence suppress the passage of protons through the membrane to the cathode.
  • a suitable first ion exchange membrane for example anion exchange membrane and/or an ion transport membrane, in particular embodiments, shows good wettability by water and/or acids, especially aqueous acids, high ion conductivity, and/or tolerance of the functional groups present therein to high pH values, especially does not show any Hoffmann elimination.
  • An illustrative AEM of the invention is the A201-CE membrane sold by Tokuyama which is used in the example, the “Sustainion” sold by Dioxide Materials, or an anion exchange membrane sold by Fumatech, for example Fumasep FAS-PET or Fumasep FAD-PET.
  • the first separator membrane is not particularly restricted, if present, i.e., for example, in the methods described herein.
  • the first separator membrane (adjoining the salt bridge, viewed from the anode side) is selected from an ion exchange membrane containing a cation exchanger, a bipolar membrane, where the cation-conducting layer in the case of the bipolar membrane may be oriented toward the cathode and the anion-conducting layer toward the anode, and a diaphragm.
  • a suitable first separator membrane for example a cation exchange membrane or a bipolar membrane, contains, for example, a cation exchanger that may be in contact with the electrolyte in the salt bridge space. It may contain, for example, a cation exchanger in the form of a cation exchanger layer, in which case further layers such as hydrophobizing layers may be present. It may likewise take the form of a bipolar membrane or of a cation exchange membrane (CEM).
  • CEM cation exchange membrane
  • the cation exchange membrane or cation exchange layer may be, for example, an ion-conductive membrane or ion-conductive layer having negatively charged functionalizations.
  • An illustrative mode of charge transport into the salt bridge in such a first separator membrane is through cations.
  • commercially available Nafion® membranes are suitable as CEM, or else the Fumapem-F membranes sold by Fumatech, Aciplex sold by Asahi Kasei, or the Flemion membranes sold by AGC.
  • the first separator membrane prevents the passage of anions, especially HCO 3 ⁇ , into the anode space.
  • the first separator membrane may take the form of a diaphragm, which means that the cell can be configured in a less complex and cheaper manner.
  • the diaphragm essentially separates the anode space and the salt bridge space, for example to an extent of more than 70%, 80% or 90%, based on the interface between anode space and salt bridge space, or separates the anode space and the salt bridge space, i.e. to an extent of 100%, based on the interface between anode space and salt bridge space.
  • the use of the liquid acid in the salt bridge space can prevent HCO 3 ⁇ ions from getting into the anode space. In this respect, it is thus possible to dispense with a cation exchange layer in the first separator membrane.
  • the diaphragm here is not particularly restricted and may be based, for example, on a ceramic (e.g. ZrO 2 or Zr 3 (PO 4 ) 3 ) and/or a swellable functionalized polymer, e.g. PTFE. It is also possible for binders (e.g.
  • hydrophilic and/or hydrophobic polymers for example organic binders, for example selected from PTFE (polytetrafluoroethylene), PVDF (polyvinylidene difluoride), PFA (perfluoroalkoxy polymers), FEP (fluorinated ethylene-propylene copolymers), PFSA (perfluorosulfonic acid polymers), and mixtures thereof, especially PTFE), conductive fillers (e.g. carbon), nonconductive fillers (e.g. glass) and/or hydrophilic additives (e.g. Al 2 O 3 , MgO 2 , hydrophilic materials such as polysulfones, e.g. polyphenylsulfones (PPSU), polyimides, polybenzoxazoles or polyetherketones, or polymers that are generally electrochemically stable in the electrolyte to be present.
  • organic binders for example selected from PTFE (polytetrafluoroethylene), PVDF (polyvinylid
  • the diaphragm is porous and/or hydrophilic. Since it is itself non-ion-conductive, it should preferably be capable of swelling in the acid electrolytes used. Furthermore, it constitutes a physical barrier to gases and cannot be penetrated by gas bubbles. For example, it is a porous polymer structure, where the base polymer is hydrophilic (e.g. PPSU). By contrast with CEM or bipolar membrane, the polymer does not comprise any charged functionalizations.
  • the diaphragm may further preferably contain hydrophilic structure-imparting components such as metal oxides (e.g. ZrO 2 ) or ceramics, as set out above.
  • a suitable first separator membrane for example a cation exchange membrane, a bipolar membrane and/or a diaphragm, in particular embodiments, shows good wettability by water and/or acids, high ion conductivity, stability to reactive species that can be generated at the anode (which is the case for perfluorinated polymers, for example), and/or stability in the pH regimes required, especially to the liquid acid in the salt bridge space.
  • the first ion exchange membrane and/or the first separator membrane are hydrophobic, such that they form a CCM with the electrodes, at least on the side facing the electrodes, such that the reactants for the electrodes are in gaseous form.
  • the anode and/or cathode are at least partly hydrophilic.
  • the first ion exchange membrane and/or the first separator membrane are wettable with water. In order to assure good ion conductivity of the ionomers, swelling with water may be used. It has been found in the experiment that poorly wettable membranes can lead to distinct worsening of the ionic binding of the electrodes.
  • the presence of water is also advantageous, e.g.:
  • the anode and/or cathode may also have sufficient hydrophilicity. This can optionally be adjusted by hydrophilic additives such as TiO 2 , Al 2 O 3 , or other electrochemically inert metal oxides, etc.
  • first separator membranes In some embodiments, it is especially possible to use at least one of the following first separator membranes:
  • FIG. 8 A corresponding construction of an illustrative electrolysis system with diaphragm DF is shown in FIG. 8 , where the further system constituents here correspond to those in FIG. 3 .
  • FIG. 9 An illustrative specific construction with a bipolar membrane is shown in FIG. 9 , which, by way of example, shows a 2-membrane setup for CO 2 electroreduction with AEM on the cathode side and bipolar membrane (CEM/AEM) on the anode side, where, as in FIGS. 1 to 3 as well, the supply of catholyte k, electrolyte s with liquid and/or dissolved acid (electrolyte for the salt bridge space) and anolyte a, and also recycling R of CO 2 , are shown here and, by way of example, water is oxidized on the anode side.
  • the further reference numerals correspond to those in FIG. 3 .
  • a bipolar membrane may be executed, for example, as a sandwich of a CEM and of an AEM. In this membrane, however, there are typically not two superposed membranes, but rather a membrane having at least two layers.
  • the diagram in FIG. 9 with AEM and CEM serves here merely for illustration of the preferred orientation of the layers.
  • the AEM or anion exchange layer points toward the anode, and the CEM or cation exchange layer toward the cathode.
  • These membranes are virtually impassable both to anions and cations.
  • the conductivity of a bipolar membrane is accordingly not based on transport capacity for ions.
  • the ions are instead typically transported via acid-base dissociation of water in the middle of the membrane. As a result, two oppositely charged charge carriers are generated, which are transported away by the electrical field.
  • the conductivity of the bipolar membrane is based on the separation of charges in the membrane, a higher voltage drop is typically to be expected.
  • the advantage of such a construction may lie in the decoupling of the electrolyte circuits since, as already mentioned, the bipolar membrane is virtually impermeable to all ions.
  • Some embodiments in the case of use of a bipolar membrane as the first separator membrane, also include the use of bases, for example a hydroxide base, as anolyte when an acid is used in the salt bridge.
  • bases for example a hydroxide base
  • the advantage here is that significantly less costly anode catalysts can be used in basic anolyte, for example based on Ni/Fe.
  • anode and/or cathode in particular embodiments, have sufficient hydrophilicity. This can optionally be adjusted by hydrophilic additives such as TiO 2 , Al 2 O 3 , or other electrochemically inert metal oxides, etc.
  • the cathode and/or anode takes the form of a gas diffusion electrode, of a porous bound catalyst structure, of a particulate catalyst on a carrier, of a coating of a particulate catalyst on the first and/or second ion exchange membrane, of a porous conductive carrier impregnated with a catalyst, and/or of a noncontinuous sheetlike structure.
  • the cathode takes the form of a gas diffusion electrode, of a porous bound catalyst structure, of a particulate catalyst on a carrier, of a coating of a particulate catalyst on the first and/or second ion exchange membrane, of a porous conductive carrier impregnated with a catalyst, and/or of a noncontinuous sheetlike structure containing an anion exchange material and/or anion transport material.
  • the anode takes the form of a gas diffusion electrode, of a porous bound catalyst structure, of a particulate catalyst on a carrier, of a coating of a particulate catalyst on the first and/or second ion exchange membrane, of a porous conductive carrier impregnated with a catalyst, and/or of a noncontinuous sheetlike structure, containing a cation exchange material and/or coupled and/or bound to a bipolar membrane.
  • the various embodiments of the cathode and anode are combinable here with one another as desired.
  • the anode is in contact with the first separator membrane as already described above by way of example. This enables good binding to the salt bridge space. In this case, in addition, no charge transport through the anolyte is necessary and the charge transport pathway is shortened. It is thus also possible to avoid electrical shadowing effects by support structures between the anode and first separator membrane.
  • the anode and/or the cathode is in contact with a conductive structure on the side remote from the salt bridge space.
  • the conductive structure here is not particularly restricted.
  • the anode and/or the cathode in particular embodiments, is thus in contact via conductive structures on the side remote from the salt bridge.
  • These are not particularly restricted. These may, for example, be carbon nonwovens, metal foams, metal knits, expanded metals, graphite structures or metal structures.
  • the electrolysis is conducted with a current density of more than 50 mAcm ⁇ 2 , more than 100 mAcm ⁇ 2 , of 150 mAcm ⁇ 2 or more, 170 mAcm ⁇ 2 or more, 200 mAcm ⁇ 2 or more, 250 mAcm ⁇ 2 or more, e.g. 300 mAcm ⁇ 2 or more, 400 mAcm ⁇ 2 or more, or 600 mAcm ⁇ 2 or more.
  • a current density of more than 50 mAcm ⁇ 2 , more than 100 mAcm ⁇ 2 , of 150 mAcm ⁇ 2 or more, 170 mAcm ⁇ 2 or more, 200 mAcm ⁇ 2 or more, 250 mAcm ⁇ 2 or more, e.g. 300 mAcm ⁇ 2 or more, 400 mAcm ⁇ 2 or more, or 600 mAcm ⁇ 2 or more.
  • the methods of the invention provide comparatively low demands on the chemical stability of the first ion exchange membrane, for example an AEM.
  • the stability and hence the usability of AEMs in particular is currently limited mainly by two degradation mechanisms, firstly by the often inadequate stability of the functional groups to concentrated bases, e.g. KOH (Hoffmann elimination of quaternary ammonium ions), and secondly by the destruction of the polymer backbone by anodic oxidation. Since only acid electrolytes are used in contact with the first ion exchange membrane in the electrolysis systems introduced here, the first ion exchange membrane, for example an AEM, is never exposed to concentrated bases.
  • the anode preferably does not directly adjoin the first ion exchange membrane, for example an AEM, which also rules out anodic damage to this membrane.
  • the electrolyte used is an acid-containing electrolyte, especially pure acid.
  • formic acid e.g. dilute formic acid
  • electrolyte in the salt bridge which can be concentrated by the electrolysis, which is promoted by an appropriate electrical conductivity of the formic acid as apparent from table 5.
  • a 10% by weight formic acid is used as the initial charge, which is concentrated in operation to 60-70% by weight, for example. Then the electrolyte is drawn off down to a residue which is utilized to re-establish the starting concentration of 10% by weight.
  • Electrodes such as those based on or composed of tin or lead.
  • the HCO 3 ⁇ transport that occurs demonstrates that a high pH exists in the region of the cathode. Since formic acid has a lower pK A than CO 2 , it is in the form of formate in the region of the cathode.
  • anions are then, for example, transported away through the first ion exchange membrane, e.g. AEM, into the salt bridge (first aspect) or the anolyte (second aspect) and reprotonated by the acid therein. This is regenerated by the protons that pass over from the anodic half-cell or are present in the anolyte. There is no likelihood of the formic acid exiting on the side of the electrode remote from the salt bridge space, if present.
  • AEM first ion exchange membrane
  • a first ion exchange membrane e.g. AEM diaphragm cell
  • the components are less costly and the electrical resistance of the cell is lower.
  • a double-membrane cell with an acid salt bridge is also advantageous for such applications in which exchange of anions between the salt bridge and anolyte is to be avoided, for example
  • CO 2 is electrolyzed by the method of the invention, although it is not ruled out that a further reactant such as CO is present alongside CO 2 on the cathode side, which can likewise be electrolyzed, i.e. there is a mixture comprising CO 2 and also, for example, CO.
  • a reactant contains, on the cathode side, at least 20% by volume of CO 2 , for example at least 50% or at least 70% by volume of CO 2 , and the reactant on the cathode side is especially 100% by volume of CO 2 .
  • an electrolysis cell comprising
  • the salt bridge space is disposed between the first ion exchange membrane and the diaphragm.
  • This electrolysis cell can be used to perform the methods described herein. Consequently, all the features discussed with regard to the methods of the invention are also applicable in the case of the electrolysis cell of the invention. Particularly the cathode space, the cathode, the first ion exchange membrane, the anode space, the anode, diaphragm and the salt bridge space have already been discussed with regard to the methods of the invention. The corresponding features may thus be detailed in accordance with those discussed above in the electrolysis cell of the invention.
  • the electrolysis cell and the electrolysis system thus especially find use in the methods described herein for electrolysis of CO 2 , and therefore aspects that are discussed in connection therewith above and hereinafter also relate to the electrolysis cell and to the electrolysis system. Correspondingly, aspects associated with the electrolysis cell and/or electrolysis system may also relate to the methods described herein.
  • an electrolysis cell comprising:
  • the anode is in contact with the diaphragm.
  • the anode and/or cathode is in contact with a conductive structure on the side remote from the salt bridge space.
  • the cathode and/or the anode takes the form of a gas diffusion electrode, of a porous bound catalyst structure, of a particulate catalyst on a support, of a coating of a particulate catalyst on the first and/or second ion exchange membrane, of a porous conductive carrier impregnated with a catalyst, and/or of a noncontinuous sheetlike structure.
  • the cathode takes the form of a gas diffusion electrode, of a porous bound catalyst structure, of a particulate catalyst on a support, of a coating of a particulate catalyst on the first and/or second ion exchange membrane, of a porous conductive carrier impregnated with a catalyst, and/or of a noncontinuous sheetlike structure, containing an anion exchange material and/or anion transport material
  • the anode takes the form of a gas diffusion electrode, of a porous bound catalyst structure, of a particulate catalyst on a support, of a coating of a particulate catalyst on the first and/or second ion exchange membrane, of a porous conductive carrier impregnated with a catalyst, and/or of a noncontinuous sheetlike structure, containing a cation exchange material.
  • the first ion exchange membrane and/or the diaphragm is hydrophilic.
  • the salt bridge space comprises a liquid and/or dissolved acid, where an acid in the liquid and/or dissolved acid in the salt bridge space has a pK A of 6 or less, 5 or less, 3 or less, 1 or less, or 0 or less, where the liquid and/or dissolved acid may be selected from dilute or neat H 2 SO 4 , a solution of H 2 N—SO 2 —OH, dilute or neat HClO 4 , a solution of H 3 PO 4 , dilute or neat CF 3 —COOH, dilute or neat CF 3 —SO 2 —OH, a solution of (CF 3 —SO 2 ) 2 —NH, a solution of HF, dilute or neat HCOOH, dilute or neat CH 3 —COOH, a solution of HCl, a solution of HBr, a solution of HI, and/or mixtures thereof.
  • the electrolyte in the salt bridge space consists of a liquid and/or dissolved acid and any
  • the anode space contains an acid which may be identical to the electrolyte in the salt bridge, especially if the second membrane takes the form of a diaphragm.
  • an electrolysis system comprises the electrolysis cells described above.
  • the corresponding embodiments of the electrolysis cell and also further illustrative components of an electrolysis system have already been discussed above and are thus also applicable to the electrolysis system.
  • an electrolysis system comprises a multitude of electrolysis cells, although it is not ruled out that other electrolysis cells are present in addition.
  • the electrolysis system further comprises a recycling device connected to an outlet from the salt bridge space and an inlet to the cathode space, which is set up to return a reactant from the cathode reaction that can be reformed in the salt bridge space, especially a gaseous reactant or one immiscible with the electrolyte, to the cathode space, such as CO 2 , where this may also contain CO and/or H 2 .
  • a recycling device connected to an outlet from the salt bridge space and an inlet to the cathode space, which is set up to return a reactant from the cathode reaction that can be reformed in the salt bridge space, especially a gaseous reactant or one immiscible with the electrolyte, to the cathode space, such as CO 2 , where this may also contain CO and/or H 2 .
  • the electrolysis system further comprises an external device for electrolyte treatment, especially an apparatus for removal of dissolved gases from an acid which is particularly used to treat the anolyte and/or the electrolyte in the salt bridge space, in order to remove gases such as CO 2 or O 2 , for example, and hence to enable recycling of anolyte and/or the electrolyte in the salt bridge space.
  • an external device for electrolyte treatment especially an apparatus for removal of dissolved gases from an acid which is particularly used to treat the anolyte and/or the electrolyte in the salt bridge space, in order to remove gases such as CO 2 or O 2 , for example, and hence to enable recycling of anolyte and/or the electrolyte in the salt bridge space.
  • both are pumped out of a common reservoir, i.e. there is just one common anolyte/electrolyte for the salt bridge space reservoir, i.e. the anolyte and the electrolyte in the salt bridge space are identical.
  • the electrolysis system comprises two separate circuits for anolyte and electrolyte in the salt bridge space, which may optionally have separate devices for electrolyte treatment, especially apparatuses for removal of dissolved gases from an acid, or where only the circuit for the electrolyte in the salt bridge space has a corresponding device.
  • the construction of the electrolysis apparatus in example 1 is based on the construction shown in FIG. 3 and is shown in schematic form in FIG. 10 .
  • a three-chamber cell was used.
  • the cathode used was a carbon GDL coated with silver particles: Freudenberg HL 23 .
  • the particles were precipitated by means of NaBH 4 from AgNO 3 in ethanol as follows: AgNO 3 (3.4 g, 20 mmol) was dissolved in ethanol (250 ml).
  • NaBH 4 (3 g, 80 mmol) was dissolved in NaOH-saturated methanol (100 ml), and this solution was added dropwise. Once all the silver had been precipitated (no black color at the site of dropwise addition), the addition was stopped.
  • the precipitate was transferred to a frit (P4) and washed 4 ⁇ with 50 ml each time of ethanol and 1 ⁇ with 50 ml of diethyl ether. Subsequently, the powder was dried under reduced pressure. Yield: 2.88 g of borate-stabilized particles.
  • the particles (90 mg) were used to produce a dispersion comprising the ionomer AS-4 (anion exchanger, Tokuyama) (180 mg of 5% solution in n-PrOH) (n-propanol)) in n-PrOH (2.8 g).
  • a dispersion comprising the ionomer AS-4 (anion exchanger, Tokuyama) (180 mg of 5% solution in n-PrOH) (n-propanol)) in n-PrOH (2.8 g).
  • Three layers of this dispersion were applied to a 60 cm 2 piece of the GDL.
  • a 10 cm 2 piece of this cathode was pressed mechanically onto an A201-CE AEM (Tokuyama) and the cathode was contacted by a titanium frame.
  • the anode used was an IrO 2 -coated expanded Ti metal with mesh size 1 ⁇ 2 mm.
  • the CEM used was a Nafion N115 membrane that was pressed directly onto the expanded metal. In order to assure sufficient mechanical contact pressure, five polymer meshes with a mesh size of 0.5 mm were integrated into the cell.
  • the electrolyte used in the salt bridge space II and in the anode space III was 0.1 M H 2 SO 4 .
  • the cell was run in at 4 V for 20 minutes. Subsequently, the cell was run in at 10 mAcm ⁇ 2 for a further 30 min. Thereafter, both the amount and the composition of the gases in gap I and gap 2 were determined at 10, 50, 100 and 150 mAcm ⁇ 2 .
  • the experimental setup used in comparative example 1 is shown in FIG. 12 and corresponds essentially to that of example 1 and is identical with regard to the apparatus constituents except that the acid in the salt bridge space II has been replaced by a KHCO 3 salt electrolyte.
  • the selectivity falls with rising current density. This is caused by the increased passage of alkali metal cations through the electrode and the associated partial flooding of the electrode.
  • a comparison between comparative example 1 (dotted lines) and the working example (solid lines) in FIG. 14 shows the advantages of the method of the invention at elevated current densities by comparison with the conventional salt electrolyte.
  • FIG. 15 A schematic diagram of the experimental setup in comparative example 2 is shown in FIG. 15 .
  • the AEM was omitted in order to show that it is essential, with the further experimental setup corresponding to that of example 1.
  • the cathode still contains an anion exchange ionomer corresponding to the polymer basis of the AEM.
  • Titanium corrosion is confirmed here as the cause of the blue color by means of chronotropic acid, and cathodic corrosion is detected in control experiments.
  • the permeate liquids (if present at all) have low or zero electrical conductivity in the arrangement of the invention presented here or in the methods of the invention.
  • the contacts are nevertheless exposed to a strongly negative potential, but not subjected to ionic contact. Consequently, such corrosion phenomena occur to a significantly limited degree, if at all. Since any liquid that occurs on the reverse side of the electrode is water, this does not contain any ions that have to be returned to the electrolyte. This liquid can therefore simply be discarded. Any corrosion products of the contacts that occur are correspondingly not washed into the electrolytes.
  • the oxidation potential of water to oxygen is dependent on the pH of the electrolyte.
  • an acid except in the case of use of a bipolar membrane
  • anolyte is chosen as anolyte. This at first does not seem very advantageous from the point of view of the cell voltage, since this course of action leads to a high water oxidation potential.
  • thermodynamic considerations according to the Nernst equation
  • the thermodynamic considerations are applicable only to the “onset” region, (i.e. the region of minimum current densities). At high current densities, the same cell voltage was observed for an acidic anode and a pH-neutral to slightly basic anode.
  • FIG. 19 is an adaptation of an alkali electrolysis cell for CO 2 electrolysis.
  • the replacement of the cation exchange membrane by a diaphragm was dispensed with for reasons of comparability.
  • the anolyte in the construction according to FIG. 20 does not contain any anions of stable acids. Therefore, the imposition of a locally low pH, as would be possible, for example, in the case of Na 2 SO 4 , is likewise ruled out here.
  • FIGS. 21 and 22 show the comparison of the UI characteristics with the measurements with the construction according to FIG. 19 with filled squares and the measurements with the construction according to FIG. 20 with open circles, with FIG. 21 showing the “onset” region of the characteristic (especially on the left) and FIG. 22 showing the complete characteristic up to 200 mAcm ⁇ 2 .
  • CO 2 is released from HCO 3 ⁇ in the cell.
  • this takes place in the salt bridge, and in the case of the construction according to FIG. 20 in the anode space.
  • four times the volume of CO 2 is released compared to the oxygen generated at the anode.
  • the situation is different for the anode.
  • the voltage for the acid anolyte is actually lower, which is attributable not least to the high gas load on the anode (in both cases a non-continuous sheetlike structure with catalyst coating).
  • liquid and/or dissolved acids can be used as electrolytes for CO 2 electrolysis at high current densities and simultaneously high Faraday efficiencies.
  • first ion exchange membrane and diaphragm for example an AEM diaphragm double-separator cell
  • a new cell type has been introduced.

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PCT/EP2018/065854 WO2019011577A1 (fr) 2017-07-12 2018-06-14 Cathode couplée à une membrane destinée à la réduction de dioxyde de carbone dans un électrolyte à base acide dépourvu de cations mobiles

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US20210140056A1 (en) * 2018-04-11 2021-05-13 University Of Delaware Electrochemical generation of carbon-containing products from carbon dioxide and carbon monoxide
US20230010993A1 (en) * 2021-07-12 2023-01-12 Dioxycle Carbon dioxide extraction electrolysis reactor
WO2023110198A1 (fr) * 2021-12-17 2023-06-22 Siemens Energy Global GmbH & Co. KG Concept de cellule pour l'utilisation de milieux d'extraction à conductivité non ionique
WO2023233587A1 (fr) * 2022-06-01 2023-12-07 日本電信電話株式会社 Membrane électrolytique et procédé de fabrication de membrane électrolytique

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DE102018212409A1 (de) 2017-11-16 2019-05-16 Siemens Aktiengesellschaft Kohlenwasserstoff-selektive Elektrode
DE102017223521A1 (de) 2017-12-21 2019-06-27 Siemens Aktiengesellschaft Durchströmbare Anionentauscher-Füllungen für Elektrolytspalte in der CO2-Elektrolyse zur besseren räumlichen Verteilung der Gasentwicklung
DE102018210303A1 (de) 2018-06-25 2020-01-02 Siemens Aktiengesellschaft Elektrochemische Niedertemperatur Reverse-Watergas-Shift Reaktion
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AT525988B1 (de) * 2022-12-12 2023-10-15 Gig Karasek Gmbh Anlage zur Reduktion von Kohlenstoffdioxid und Elektrolysezelle hierfür

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US20230010993A1 (en) * 2021-07-12 2023-01-12 Dioxycle Carbon dioxide extraction electrolysis reactor
WO2023110198A1 (fr) * 2021-12-17 2023-06-22 Siemens Energy Global GmbH & Co. KG Concept de cellule pour l'utilisation de milieux d'extraction à conductivité non ionique
WO2023233587A1 (fr) * 2022-06-01 2023-12-07 日本電信電話株式会社 Membrane électrolytique et procédé de fabrication de membrane électrolytique

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