DIOXIDE

Cross-Reference to Related Applications

[0001] This application claims priority from United States Application No. 62/558505 filed 14 September 2017. For purposes of the United States, this application claims the benefit under 35 U.S.C. §119 of United States Application No. 62/558505 filed 14 September 2017 and entitled SYSTEM AND METHOD FOR REDUCING C0 ₂ which is hereby incorporated herein by reference for all purposes.

Field

[0002] This application relates to electrochemical cells and to electrochemical methods for reduction of carbon dioxide. The invention has example application to the electrochemical reduction of carbon dioxide to carbon monoxide.

Background

[0003] Carbon dioxide (C0 ₂ ) may be used as a feedstock for producing useful chemicals such as carbon monoxide, formate, methanol, ethylene and longer chain alkanes. One approach to produce such useful chemicals is to chemically reduce C0 ₂ using an electrocatalytic reaction. Catalysts and other reaction conditions may be selected to yield a desired product. Electrocatalytic reduction of C0 ₂ is of particular current interest because it offers a way to mitigate the rapid rise in atmospheric C0 ₂ levels using clean electricity.

[0004] One product that may be produced by electrocatalytic reduction of C0 ₂ is carbon monoxide (CO). CO serves as an important chemical precursor for a number of industrial processes, including Fischer-Tropsch chemistry that could effectively enable the conversion of electricity generated from sunlight or wind into liquid fuels. Electrocatalysts capable of converting C0 ₂ to CO with high selectivity are now known.

[0005] A problem which needs to be addressed to make electrochemical reduction of C0 ₂ economically viable on an industrial scale is that current systems are not sufficiently efficient, selective, and robust. A particular challenge is to provide C0 ₂ electrolyzers that can operate at higher current densities (e.g. current densities in the range of about 200 mA/cm ² or more).

[0006] The following references describe results of research into electrochemical systems including various approaches to electrochemical reduction of carbon dioxide:

1. D. T. Whipple and P. J. A. Kenis, J. Phys. Chem. Lett., 2010, 1 , 3451-3458.

2. Y. Hori, in Handbook of Fuel Cells - Fundamentals, Technology and

Applications, eds. W. Vielstich, H. A. Gasteiger, A. Lamm and H. Yokokawa, John Wiley & Sons, Ltd, 2010, pp. 1-14.

3. C. W. Li, J. Ciston and M. W. Kanan, Nature, 2014, 508, 504-7.

4. M. Liu, Y. Pang, B. Zhang, P. De Luna, O. Voznyy, J. Xu, X. Zheng, C. T.

Dinh, F. Fan, C. Cao, F. P. G. de Arquer, T. S. Safaei, A. Mepham, A.

Klinkova, E. Kumacheva, T. Filleter, D. Sinton, S. O. Kelley and E. H. Sargent, Nature, 2016, 537, 382-386.

5. J. He, K. E. Dettelbach, D. A. Salvatore, T. Li and C. P. Berlinguette, Angew.

Chemie Int. Ed., 2017, 56, 1-6.

6. Y. Hori, in Modern Aspects of Electrochemistry, ed. C. Vayenas, Springer, 2008, pp. 89-189.

7. J. L. Dimeglio and J. Rosenthal, J. Am. Chem. Soc, 2013, 135, 8798-8801.

8. S. Verma, B. Kim, H. R. M. Jhong, S. Ma and P. J. A. Kenis, ChemSusChem,

2016, 9, 1972-1979.

9. J. Durst, A. Rudnev, A. Dutta, Y. Fu, J. Herranz, V. Kaliginedi, A. Kuzume, A.

A. Permyakova, Y. Paratcha, P. Broekmann and T. J. Schmidt, Chim. Int. J. Chem., 2015, 69, 769-776.

10. Y. Yoon, A. S. Hall and Y. Surendranath, Angew. Chemie - Int. Ed., 2016, 55, 15282-15286.

1 1. Wuttig, M. Yaguchi, K. Motobayashi, M. Osawa and Y. Surendranath, Proc.

Natl. Acad. Sci., 2016, 1 13, E4585-E4593.

12. Xie, C. Chen, Y. Yu, J. Su, Y. Li, G. A. Somorjai and P. Yang, Nano Lett.,

2017, acs.nanolett.7b01 139.

13. S. Lin, C. S. Diercks, Y.-B. Zhang, N. Kornienko, E. M. Nichols, Y. Zhao, A. R.

Paris, D. Kim, P. Yang, O. M. Yaghi and C. J. Chang, Science, 2015, 1 , 1-1 1.

14. Y. Chen, C. W. Li and M. W. Kanan, J. Am. Chem. Soc, 2012, 134, 19969- 19972.

15. Q. Lu, J. Rosen, Y. Zhou, G. S. Hutchings, Y. C. Kimmel, J. G. Chen and F.

Jiao, Nat. Commun, 2014, 5, 1-6. R. Kas, K. K. Hummadi, R. Kortlever, P. de Wit, A. Milbrat, M. W. J. Luiten- Olieman, N. E. Benes, M. T. M. Koper and G. Mul, Nat. Commun., 2016, 7, 10748.

S. Rasul, D. H. Anjum, A. Jedidi, Y. Minenkov, L. Cavallo and K. Takanabe, Angew. Chemie - Int. Ed., 2015, 54, 2146-2150.

J. L. DiMeglio and J. Rosenthal, J. Am. Chem. Soc, 2013, 135, 8798-801. B. A. Rosen, A. Salehi-khojin, M. R. Thorson, W. Zhu, D. T. Whipple, P. J. A. Kenis and R. I. Masel, Science., 201 1 , 334, 643-644.

M. Carmo, D. L. Fritz, J. Mergel and D. Stolten, Int. J. Hydrogen Energy, 2013, 38, 4901-4934.

Y. C. Li, D. Zhou, Z. Yan, R. H. Goncalves, D. A. Salvatore, C. P. Berlinguette and T. E. Mallouk, ACS Energy Lett., 2016, 1 , 1149-1 153.

E. J. Dufek, T. E. Lister and M. E. Mcllwain, J. Appl. Electrochem., 201 1 , 41 , 623-631.

M. R. Thorson, K. I. Siil and P. J. a. Kenis, J. Electrochem. Soc, 2013, 160, F69-F74.

S. Verma, X. Lu, S. Ma, R. I. Masel and P. J. A. Kenis, Phys. Chem. Chem. Phys., 2015, 18, 7075-7084.

B. Kim, F. Hillman, M. Ariyoshi, S. Fujikawa and P. J. A. Kenis, J. Power Sources, 2016, 312, 192-198.

H. Yang, J. J. Kaczur, S. D. Sajjad and R. I. Masel, J. C02 Util., 2017, 20, 208-217.

E. J. Dufek, T. E. Lister, S. G. Stone and M. E. Mcllwain, J. Electrochem. Soc, 2012, 159, F514-F517.

B. Jahne, G. Heinz and W. Dietrich, J. Geophys. Res. Ocean., 1987, 92, 10767-10776.

C. Delacourt, P. L. Ridgway, J. B. Kerr and J. Newman, J. Electrochem. Soc, 2008, 155, B42-B49.

C. Shen, R. Wycisk and P. Pintauro, Energy Environ. Sci., 2017, 10, 1435- 1442

M. B. McDonald, S. Ardo, N. S. Lewis and M. S. Freund, ChemSusChem, 2014, 7, 3021-3027.

N. M. Vargas-Barbosa, G. M. Geise, M. A. Hickner and T. E. Mallouk,

ChemSusChem, 2014, 7, 3017-3020. 33. D. A. Vermaas and W. A. Smith, ACS Energy Lett., 2016, 1 , 1 143-1 148.

34. X. Zhou, R. Liu, K. Sun, Y. Chen, E. Verlage, S. A. Francis, N. S. Lewis and C.

Xiang, ACS Energy Lett., 2016, 1 , 764-770.

35. R. D. L. Smith, M. S. Prevot, R. D. Fagan, Z. Zhang, P. A. Sedach, M. K. J.

Siu, S. Trudel and C. P. Berlinguette, Science, 2013, 340, 60-63.

36. C. C. L. McCrory, S. Jung, I. M. Ferrer, S. M. Chatman, J. C. Peters and T. F.

Jaramillo, J. Am. Chem. Soc, 2015, 137, 4347-4357.

37. R. D. L. Smith, M. S. Prevot, R. D. Fagan, S. Trudel and C. P. Berlinguette, J.

Am. Chem. Soc, 2013, 135, 1 1580-1 1586.

38. R. D. L. Smith and C. P. Berlinguette, J. Am. Chem. Soc, 2016, 138, 1561- 1567.

39. D. K. Bediako, Y. Surendranath and D. G. Nocera, J. Am. Chem. Soc, 2013, 135, 3662-3674.

40. M. Dinca, Y. Surendranath and D. G. Nocera, Proc. Natl. Acad. Sci., 2010, 107, 10337-10341.

41. M. W. Louie and A. T. Bell, J. Am. Chem. Soc, 2013, 135, 12329-12337.

42. F. D. Speck, K. E. Dettelbach, R. S. Sherbo, D. A. Salvatore, A. Huang and C.

P. Berlinguette, Chem, 2017, 2, 590-597.

43. L. Trotochaud, S. L. Young, J. K. Ranney and S. W. Boettcher, J. Am. Chem.

Soc, 2014, 136, 6744-6753.

44. S. Ma, J. Liu, K. Sasaki, S. M. Lyth and P. J. A. Kenis, Energy Technol., 2017, 5, 1-3.

45. J. Qiao, Y. Liu, F. Hong and J. Zhang, Chem. Soc. Rev., 2014, 43, 631-675.

46. J. R. Hudkins, D. G. Wheeler, B. Pena and C. P. Berlinguette, Energy Environ.

Sci., 2016, 9, 3417-3423.

47. H. R. Q. Jhong, F. R. Brushett and P. J. A. Kenis, Adv. Energy Mater., 2013, 3, 589-599.

[0007] Despite the current depth of knowledge in the field of electrochemistry there remains a need for new practical and cost efficient ways to convert C0 ₂ into useful materials such as CO.

Summary

[0008] Various alternative embodiments and examples are described herein. These embodiments and examples are illustrative and should not be construed as limiting the scope of the invention.

[0009] This invention has a number of aspects. These include, without limitation:

• apparatus for electrochemical reduction of carbon dioxide;

• electrochemical cells;

• membrane electrode assemblies for electrochemical cells;

• methods for electrocatalytic reduction of carbon dioxide to useful materials;

• methods for maintaining hydration of a cathode side of a membrane in an

electrocatalytic cell having a gaseous cathode feed; and

• methods for maintaining hydration of the cathode side of a membrane in an electrocatalytic cell.

[0010] One aspect of the invention provides a membrane electrode assembly comprising an anode, a gas diffusion cathode, a bipolar membrane located between the anode and the cathode, and a hydration layer located between the bipolar membrane and the cathode.

[0011] In operation, an electrical potential is applied between the anode and the cathode. Carbon dioxide is supplied to the cathode. At the cathode the carbon dioxide is reduced to carbon monoxide. At the bipolar membrane, water is dissociated into protons (H ⁺ ) and hydroxide (OH ^" ). The protons migrate toward the cathode and the hydroxide migrates toward ^" the anode.

[0012] Another aspect of the invention provides a method for electrocatalytically reducing carbon dioxide. The method comprises: delivering a gas comprising carbon dioxide to a cathode of a membrane electrode assembly. By way of non-limiting examples, the gas may be air, pure carbon dioxide gas, flue gas, combustion products or the like. The cathode comprises a gas diffusion layer and a cathode catalyst. The membrane electrode assembly comprises: an anode comprising an anode catalyst; the cathode, a bipolar membrane between the anode and the cathode and a liquid permeable support layer between the bipolar membrane and the cathode. The bipolar membrane comprises an anion exchange layer facing the anode and a cation exchange layer facing the cathode. The method comprises: maintaining an electrical potential difference between the cathode and the anode of the membrane electrode assembly; delivering water or an aqueous electrolyte solution to the support layer so that the support layer contains liquid water (which may be, for example in the form of pure water or an aqueous solution) and allowing the liquid water of the support layer to hydrate the cation exchange layer and to carry cations from the bipolar membrane toward the cathode. The electrical potential between the anode and the cathode electrochemically reduces the carbon dioxide in the gas and causes a current density at the cathode of at least 100 mA/cm ² .

[0013] Some embodiments comprise humidifying the gas before delivering the gas to the cathode. Delivering the water to the support layer may comprise allowing water from the humidified gas to enter the support layer and to collect in the support layer as liquid water.

[0014] In some embodiments, delivering the gas to the cathode comprises passing a flow of the gas through a flow plate that is in fluid communication with a gas diffusion layer of the cathode.

[0015] In some embodiments the support layer comprises a solid porous support. The solid porous support may, for example comprise glass fibers.

[0016] In some embodiments the water in the aqueous support layer is in a solution comprising a dissolved electrolyte. The solution may, for example have a pH near neutral such as a pH of 7±1.5.

[0017] In some embodiments the electrolyte comprises NaHC0 ₃ .

[0018] In some embodiments the cathode catalyst comprises one or more metals selected from the group consisting of Ag, Au, Cu, Co, Zn, Co, Cd, Pb, Pd, Fe, Mn, W, and Sn.

[0019] Some embodiments comprise delivering a basic anolyte such as KOH or NaOH to the anode.

[0020] In some embodiments the anode catalyst is operable to promote an oxygen evolution reaction at the anode.

[0021] In some embodiments the anode comprises a foamed metal such as, for example a nickel foam that serves as a gas diffusion layer and as the anode catalyst.

[0022] In some embodiments the anode catalyst comprises a material selected from the group consisting of Ni, Fe, Co, Mn, W, and FeNiOx.

[0023] In some embodiments the anode catalyst comprises a late transition metal selected from the group consisting of Ir, Pt, and Ru.

[0024] Some embodiments comprise, at the cathode, electrochemically reducing the carbon dioxide to yield carbon monoxide. The carbon monoxide may be collected.

[0025] Some embodiments comprise, at the cathode, generating hydrogen gas and collecting a mixture of the carbon monoxide and the hydrogen gas.

[0026] The gas may have any of a wide range of carbon dioxide contents. For example, in various embodiments the gas has a carbon dioxide content of: at least 1 % or less than 2% or in the range of 1 % to 100%.

[0027] Another aspect of the invention provides an electrolytic cell operable for electrochemical reduction of carbon dioxide. The cell comprises a membrane electrode assembly comprising: an anode comprising an anode catalyst; a cathode comprising a gas diffusion layer and a catalyst, a bipolar membrane between the anode and the cathode, the bipolar membrane comprising an anion exchange layer facing the anode and a cation exchange layer facing the cathode, and a liquid permeable support layer between the bipolar membrane and the cathode.

[0028] In some embodiments the membrane electrode assembly is between a first flow plate engaging the cathode and a second flow plate engaging the anode.

[0029] Some embodiments comprise a source of water connected to deliver water to the support layer. Some embodiments comprise a conduit arranged to carry a gas containing carbon dioxide to the cathode wherein the source of water comprises a humidifier connected to humidify the gas passing through the conduit, In some embodiments the source of water comprises a pump connected to deliver water directly to the support layer.

[0030] In some embodiments, the cell comprises at least two passages in fluid communication with the support layer wherein the pump is operable to flow water continuously or intermittently through the support layer by way of the passages.

[0031] In some embodiments the support layer comprises a mat of glass fibers, hydrophilic polytetrafluoroethylene (PTFE), expanded PTFE, porous PTFE, silica beads, ceramic beads or porous sheets.

[0032] In some embodiments the support layer has a thickness of 1 mm or less.

[0033] In some embodiments the anode is in contact with a basic anolyte. [0034] Some embodiments comprise a pump connected to circulate the basic anolyte past the anode.

[0035] Some embodiments provide a system for electrochemically reducing gaseous carbon dioxide comprising a cell having any combination of cell features described herein and a power supply connected to supply a potential difference between the anode and the cathode wherein the potential difference is sufficient to maintain a current density at the cathode of at least 100 mA/cm ² .

[0036] Further aspects and example embodiments are illustrated in the

accompanying drawings and/or described in the following description.

Brief Description of the Drawings

[0037] The accompanying drawings illustrate non-limiting example embodiments of the invention.

[0038] Fig. 1 is a block diagram of a system for electrochemical reduction of carbon dioxide according to an example embodiment of the invention.

[0039] Fig. 1A is a schematic diagram of a membrane electrode assembly (MEA) for a bipolar membrane-based C0 ₂ electrolyzer cell according to an example

embodiment.

[0040] Fig. 1 B is a block diagram illustrating a bipolar membrane-based C0 ₂ electrolyzer cell including ancillary equipment according to an example embodiment.

[0041] Fig. 2 is a schematic diagram indicating electrochemical reactions that occur within a bipolar membrane-based C0 ₂ electrolyzer cell according to an example embodiment.

[0042] Fig. 3A is an exploded perspective view of an example prototype bipolar membrane-based C0 ₂ electrolyzer cell. Fig. 3B is a schematic diagram depicting a cross-section of the example prototype bipolar membrane-based C0 ₂ electrolyzer cell shown in Fig. 3A. Fig. 3C is an exploded top view of the example prototype bipolar membrane-based C0 ₂ electrolyzer cell shown in Fig. 3A.

[0043] Figs. 4A-C are schematic diagrams depicting some example flow fields that can be used in a bipolar membrane-based C0 ₂ electrolyzer cell according to an example embodiment.

[0044] Fig. 5 is a schematic diagram indicating the dimensions of the example prototype bipolar membrane-based C0 ₂ electrolyzer cell shown in Fig. 3.

[0045] Fig. 6A shows cyclic voltammograms for the example prototype bipolar membrane-based C0 ₂ electrolyzer cell shown in Fig. 3 operating with gas phase and liquid phase C0 ₂ cathode feeds.

[0046] Fig. 6B is a graph showing Faradaic efficiency for CO production at different current densities between 20 to 100 mA cm ^"2 , with and without an aqueous support layer, for the example prototype bipolar membrane-based C0 ₂ electrolyzer cell shown in Fig. 3. When an aqueous support layer was present the aqueous support layer contained a NaHC0 ₃ solution.

[0047] Fig. 6C is a graph showing Faradaic efficiencies for CO and H ₂ production after 700 seconds of humidified gas-phase C0 ₂ electrolysis with an NaHC0 ₃ aqueous support layer in the example prototype bipolar membrane-based C0 ₂ electrolyzer cell shown in Fig. 3.

[0048] Fig. 6D is a graph showing Faradaic efficiencies for CO and H ₂ production after 700 seconds of humidified gas-phase C0 ₂ electrolysis, with either an NaHC0 ₃ aqueous support layer or an H ₂ 0 support layer, in the example prototype bipolar membrane-based C0 ₂ electrolyzer cell shown in Fig. 3.

[0049] Fig. 6E is a graph showing Faradaic efficiencies 60-1 and chronopotentiometry data 60-2 for the bias (V) required to convert non-humidified C0 ₂ to CO in the example prototype bipolar membrane-based C0 ₂ electrolyzer cell shown in Fig. 3.

[0050] Fig. 6F is a graph showing Faradaic efficiencies 61 -1 and chronopotentiometry data 61-2 for the bias (V) required to convert hydrated C0 ₂ to CO in the example prototype bipolar membrane-based C0 ₂ electrolyzer cell shown in Fig. 3.

[0051] Fig. 6G is a comparison of Faradaic efficiency as a function of current density for three flow plate geometries.

Detailed Description

[0052] Throughout the following description, specific details are set forth in order to provide a more thorough understanding of the invention. However, the invention may be practiced without these particulars. In other instances, well known elements have not been shown or described in detail to avoid unnecessarily obscuring the invention. Accordingly, the specification and drawings are to be regarded in an illustrative, rather than a restrictive sense.

Definitions

[0053] "Bipolar membrane" or "BPM" is a membrane comprising plural layers including an anion exchange layer on one side and a cation exchange layer on another side. A bipolar membrane may comprise one or more layers between the anion exchange layer and the cation exchange layer. For example, an intermediate layer may comprise a catalyst which facilitates dissociation of water into protons and hydroxide ions. The anion exchange layer may conduct hydroxide ions. The cation exchange layer may conduct protons. An example bipolar membrane is Fumasep FBM available from FUMATECH BWT GmbH.

[0054] "Current density" is total current divided by the geometric surface area of an electrode. For example, an electrode having an area of 100 cm ² carrying an electrical current of 20 Amperes would have a current density of 200 mA/cm ² .

[0055] "High current density" means a current density greater than or equal to 100 mA/cm ² . 200 mA/cm ² is an example of a high current density.

[0056] "Faradaic efficiency" (FE) is a measure of the efficiency with which an electron transfer reaction generates a desired product. Faradaic efficiency can be reduced by side reactions which create undesired products or by further reactions which consume the desired product after it is produced. Reduction of C0 ₂ to CO may proceed, for example, by the half reaction:

C0 ₂ + H ₂ 0 + 2e ^" ==> CO + 20H ^"

The theoretical yield of this half reaction is one molecule of CO for every two electrons consumed. FE for a gaseous product k may be determined in accordance with Equation 1.

(Eq. 1) where n _k is the number of electrons exchanged, F is Faraday's constant (F = 96,485 C/mol), x _k is the mole fraction of the gas k in the gaseous mixture analyzed, F _m is the molar flow rate in mol/s, and / is the total current in A. The molar flow rate may be derived from the volume flow rate F _v by the relation F _m = pFJRT, with p being the atmospheric pressure in Pa, R the ideal gas constant of 8.314 J/mol K and T the temperature in Kelvin.

Example Embodiments

[0057] Embodiments described herein related to apparatus, systems and methods for electrochemical conversion of C0 ₂ to useful products such as CO.

[0058] Fig. 1 is a block diagram illustrating a system 100 according to an example embodiment. System 100 comprises an electrochemical cell 1 10. Cell 110 comprises a cathode 12 and an anode 14 separated by a BPM 16. Cathode 12 is a gas diffusion electrode. An electrical potential is applied between cathode 12 and anode 14 from a power supply 1 12. Power supply 1 12 may be configured to maintain a desired electrical potential difference between cathode 12 and anode 14. Electrical power may be supplied to power supply 1 12 from any suitable source including solar cells, mains electricity or the like.

[0059] A gaseous feed 1 15 comprising C0 ₂ is supplied from a feed source 114 to cathode 12. Gaseous feed 1 15 may be any suitable source of C0 ₂ . Fig. 1 shows optional feed processing stage 1 16. Feed processing stage 1 16 may perform one or more of: filtering particles or other contaminants from feed 1 15, increasing

concentration of C0 ₂ in feed 1 15, controlling the pressure of feed 1 14, controlling the temperature of feed 1 15, and humidifying feed 1 15.

[0060] In some embodiments feed processing stage 1 16 comprises a carbon capture system. The carbon capture system may use any suitable technology for capturing C0 ₂ from the atmosphere or any other source containing C0 ₂ . For example, a carbon capture system may capture C0 ₂ by dissolving the C0 ₂ in a liquid.

[0061] Cathode 12 comprises one or more catalyst materials that promote

electrochemical reduction of C0 ₂ to CO or another desired product.

[0062] Feed 1 15 now carrying the desired product is carried to an extraction stage 1 18 where the product is taken off or used. Extraction stage 1 18 may comprise a selective membrane or other technology for separating the desired products from the flow of feed 115 exiting cell 1 10. A system 100 as illustrated in Fig. 1 may, for example be operated to generate CO from C0 ₂ taken from the atmosphere, flue gas, byproduct of an industrial process or other sources of C0 ₂ . [0063] System 100 may be adapted for applications where the source of C0 ₂ is a source such as air that contains only a low concentration of C0 ₂ in various ways including:

• Coupling the cathode inlet stream to a system that increases C0 ₂

concentration such as an atmospheric carbon capture platform that uses known C0 ₂ capture technology, e.g, aqueous solvent capture to produce a pure or enhanced-concentration stream of C0 ₂ (for example as described by Keith, et al. Climatic Change 2006, 74, 17; Lackner, et al. Science 2003, 300, 1677;

• Incorporate C0 ₂ -absorbing porous layers into the cathode. For example C02- absorbing porous layers may be used to provide a porous gas diffusion layer that also supports the cathode catalyst; or

• Optimize the cell (e.g. by selection of catalyst, amount of catalyst and cathode size) for reducing C0 ₂ present in a dilute stream.

[0064] System 100 can be optionally scaled to include multiple cells 1 10 each connected to receive C0 ₂ from gaseous feed 1 15. Cells 1 10 may, for example, be arranged in stacks. Stacks of cell 1 10 can be connected in parallel such that a single gas stream is split to simultaneously feed multiple cells 1 10, in series where each subsequent cell receives a feed containing reduced concentrations of C0 _2, or in a configuration comprising a combination of parallel and series connections.

[0065] Fig. 1A is a schematic diagram illustrating components of a membrane electrode assembly (MEA) 1 1. Cell 1 10 may, for example, comprise a MEA like MEA 1 1. MEA 1 1 comprises a cathode 12 and an anode 14 separated by a BPM 16. A support layer 18 is provided between cathode 12 and BPM 16.

[0066] Cathode 12 is configured to receive C0 ₂ in the form of a gas. For example, cathode 12 may comprise a gas diffusion electrode. In the illustrated embodiment, a gaseous cathode feed 21 containing C0 ₂ is delivered through a cathode feed inlet 25 to a gas diffusion layer 12A of cathode 12. Cathode feed inlet 25 may be coupled to an atmospheric carbon capture platform (e.g. an aqueous solvent capture system).

[0067] Cathode feed 21 may, for example, be pure C0 ₂ , humidified C0 ₂ , air, atmospheric air, air in which the concentration of one or more constituents other than C0 ₂ has been reduced, air in which the concentration of C0 ₂ has been enhanced, industrial process gas containing C0 ₂ , flue gas or other combustion gases containing C0 ₂ , or the like.

[0068] Gas diffusion layer 12A may comprise porous materials. For example, layer 12A may comprise a carbon felt, carbon paper, carbon cloth, a sintered gas diffusion layer, etc.

[0069] Cathode 12 comprises a catalyst suitable for promoting the reduction of C0 ₂ to CO or other desired product. In preferred embodiments cathode 12 operates under conditions that are near to neutral pH (pH 7). If pH at the cathode is too low (too acidic) then the relative rate of reduction of C0 ₂ to hydrogen evolution is reduced (thereby reducing Faradaic efficiency). On the other hand if pH at the cathode is too high then undesired reactions producing carbonates CO3 ^2" become more favoured. These reactions can undesirably yield solids which can impair operation of the cathode and reduce the overall single pass conversion efficiency of the desired product (e.g. CO).

[0070] The cathode catalyst may, for example, comprise any late first (or second) row transition metal catalyst, post-transition metals (e.g. bismuth), alloys of suitable metals, suitable metal oxides, etc. Some embodiments use bi and tri-metal mixed metal materials as catalysts. A highly active cathode catalyst may be selected to promote C0 ₂ reduction. An example of a suitable cathode catalyst is silver (Ag). Silver catalysts advantageously provide high selectivity for converting C0 ₂ to CO.

[0071] Cathode catalyst 12B may comprise an electrocatalyst ink. The electrocatalyst ink may optionally comprise a dispersion of silver nanoparties, a conductive ionomer, PTFE to control water content, etc.

[0072] Anode 14 comprises an anode catalyst. In a preferred embodiment the anode operates under basic conditions (i.e. pH in the range of 7 to 14). In basic conditions, efficient and earth-abundant transition metal catalysts may be used as the anode catalyst. Such catalysts tend to perform well only under basic conditions. Examples of suitable anode catalysts are Ni, and FeNiO _x . Precious metals such as Pt or Ir may also be used as anode catalysts. In an example embodiment the anode comprises a layer of porous nickel foam.

[0073] In some embodiments, anode 14 comprises a diffusion layer. The anode diffusion layer and the anode catalyst may be made from the same or different materials. In an example embodiment, anode 14 comprises a layer of a porous metal such as a porous nickel foam that acts as a catalyst and is formed to provide a diffusion layer.

[0074] Basic conditions may be maintained at anode 14 by providing an anode feed that comprises a basic aqueous solution. For example, the anode feed may be an anolyte 41. Anolyte 41 may comprise an aqueous solution of potassium hydroxide (KOH), sodium hydroxide (NaOH), suitable carbonates, suitable bicarbonates or the like. The anolyte may comprise, for example KOH or NaOH at a concentration of approximately 1 mole/litre (1 M). Anolyte 41 may be delivered to the anode through anode feed inlet 45. Anolyte 41 may be circulated using a pump or other

mechanisms. Flow of the anolyte 41 may carry anode-side reaction products out of the cell.

[0075] BPM 16 comprises an anion exchange layer 16A facing anode 14, a cation exchange layer 16C facing cathode 12 and a junction 16B. Junction 16B optionally comprises an intermediate layer. Junction 16B may comprise a catalyst that promotes the dissociation of water. Bipolar membrane 16 accommodates an oxygen evolution reaction (OER).

[0076] At BPM 16, water is dissociated into protons (H+) and hydroxide ions (OH-). The protons migrate toward cathode 12 and the hydroxide ions migrate toward anode 14. These ions help to maintain stability of pH at the anode and cathode, with anode 14 operating under basic conditions and cathode 12 operating under neutral or acidic conditions.

[0077] BPM 16 may comprise materials that have properties including, but not limited to: high proton conductivity by cation exchange layer 16C, high hydroxide conductivity by anion exchange layer 16A, high resistance to electrons, impermeability to carbon products, long-term chemical stability, long-term thermal stability and/or high mechanical robustness.

[0078] It is desirable to operate a cell as described herein at a high current density. As described in more detail elsewhere herein, supplying C0 ₂ to cathode 12 in the form of a gas as opposed to in solution helps to improve the efficiency of the cell. However, it is a significant problem that the cathode side of BPM 16 can become dehydrated, especially when the cell is operated at high current densities.

Dehydration of BPM 16 tends to reduce efficiency of the cell. [0079] MEA 11 includes a support layer or hydration layer 18 located between cathode 12 and BPM 16. Support layer 18 is configured to contain liquid water or a water-containing solution such as an aqueous solution of sodium bicarbonate

(NaHCOs).

[0080] The purpose of support layer 18 is to provide a reservoir of water that is available to maintain hydration of the cathode side of BPM 16 while providing a pathway for protons to pass from BPM 16 to cathode 12. Support layer 18 comprises a thin layer of a medium that is water permeable. For example, support layer 18 may comprise a matrix of fibers or particles. The fibers or particles may be chemically inert.

[0081] Example materials for support layer 18 are a mat of glass fibers, hydrophilic polytetrafluoroethylene (PTFE), expanded PTFE, porous PTFE, silica beads, ceramic beads or porous sheets or the like. A non-limiting example of a material suitable for support layer 18 is a Whatman® glass microfiber filter (Grade GF/D). This material has a dry weight of 121 g/m ² and a water permeability under gravity of approximately 10.7 ml cm ^"2 min ^"1 .

[0082] Water may be introduced into support layer 18 in various ways. These include:

• direct introduction of water into support layer 18 and/or

• humidifying the gas provided to cathode 12 and allowing water from the gas to be transported into support layer 18 from cathode 12.

It is beneficial to maintain support layer 18 fully saturated with water while cell 1 10 is operating.

[0083] The material of support layer 18 may optionally comprise a hygroscopic coating to enhance water transport. Support layer 18 may optionally be formed of the same or similar material as cathode 12. Support layer 18 may be integrated with cathode 12. In such integrated embodiments, the pore size within the portion cathode 12 that provides support layer 18 may be different from that in the portion of cathode 12 that provides gas diffusion layer 12A. These pore sizes may be set to allow the accumulation of liquid water in support layer 18 while gas diffusion layer 12A remains unobstructed by liquid water.

[0084] In some embodiments, transport of water into support layer 18 is promoted by modifying water transport properties of all or part of a cathode gas diffusion layer (e.g. by adjusting the content of hydrophobic additives such as PTFE in an electrocatalyst ink applied to the gas diffusion layer). A reduced PTFE content may be chosen to facilitate water transport from the humidified stream to support layer 18.

[0085] The embodiment illustrated in Fig. 1A shows a humidifier 50 located upstream from cathode 12. Humidifier 50 humidifies the C0 ₂ - containing gas being supplied to cathode 12. The C0 ₂ - containing gas may be humidified sufficiently to replenish support layer 18 (e.g. the humidity may be 70% RH or more, for example ~90% RH, in some embodiments. Humidifier 50 may be of any suitable type. In some

embodiments, C0 ₂ - containing gas is bubbled through water heated to a sub-boiling temperature (e.g. 85°C).

[0086] In some embodiments, water or an aqueous solution is continuously or periodically fed to replenish support layer 18. In some embodiments a continuous or intermittent flow of water or an aqueous solution is maintained in support layer 18. For example, a pump may be connected to deliver a flow of water to support layer 18. In some embodiments, it may be desirable to maintain continuous flow within support layer 18 to assist with temperature control of cell 1 10 as well as maintaining BPM 16 in a hydrated state. In such embodiments plural passages may be provided to transport water into and out of support layer 18.

[0087] Support layer 18 is advantageously thin while being thick enough to maintain its integrity and to make water available over the surface of BPM 16. In some embodiments support layer 18 has a thickness on the order of about 1 mm. In some embodiments support layer 18 has a thickness that does not exceed 1 mm. In some embodiments support layer 18 has a thickness in the range of about 300 μηη to about 1.5 mm. In an example prototype embodiment, support layer 18 has a thickness of 675 μηη (or 500 μηη to 800 μηη in some embodiments).

[0088] A cell as described herein may be equipped with various ancillary systems. Fig. 1 B is a block diagram illustrating a cell 1 10 which includes a number of such systems including pumps 150A, 150B, and 150C which are respectively connected to deliver flows of anolyte 41 , water or electrolyte solution 51 , and cathode feed 21 to anode 14, support layer 18, and cathode 12 respectively. In some embodiments an output stream from cathode 12 is processed by a filter/separator 1 17 to deliver desired products (e.g. CO or a mixture of CO and hydrogen or another desired product) to collector 1 18. Some of cathode feed 21 that has passed through cathode 12 is optionally recirculated to cathode 12 by way of pump 150C. Anolyte 41 and/or water 51 may optionally be recirculated through pumps 150A, 150C or other mechanisms.

[0089] Fig 1 B also shows a controller 160. Controller 160 controls one or more of:

• power supply 21 ,

• pumps 150A, 150B, and/or 150C,

• a valve or other device that is operative to control how much of cathode feed 21 is recycled,

• a composition of the cathode feed (e.g. via control of a feed processing system 1 16 (see Fig. 1);

• a humidity of the cathode feed (e.g. via control of a humidifier 50);

• etc.

based on inputs from sensor(s) 161.

[0090] Sensor(s) 161 monitor one or more or any combination of:

• cell temperature,

• current supplied by power supply 1 12,

• voltage supplied by power supply 1 12,

• composition, pressure and/or temperature of anolyte 41 entering cell 1 10,

• humidity of cathode feed 21 entering cell 1 10;

• composition, pressure and/or temperature of cathode feed 21 entering cell 1 10,

• composition of cathode feed 21 leaving cell 1 10,

• flow rate of any one or more of cathode feed 21 , anolyte 41 and water or electrolyte solution 51 ;

• an amount of cathode feed 21 that is being recycled;

• etc.

[0091] Some non-limiting examples of functions that may be performed by controller 160 include:

• increasing the feed rate of and/or decreasing a temperature of water or

electrolyte solution 51 being flowed through support layer 18 in response to the temperature of cell 1 10 being above a desired operating temperature;

• decreasing a voltage and/or current being supplied by power supply 1 12 in response to detecting more than a desired amount of side-reaction products (such as, for example, hydrogen and/or carbonates in the cathode feed leaving cell 1 10)

• increasing recycling of cathode feed 21 in response to a concentration of carbon dioxide in cathode feed 21 leaving cell 1 10 exceeding a threshold

• regulating voltage and/or current being supplied by power supply 1 12 to

maintain a desired balance of carbon monoxide to hydrogen in cathode feed 21 leaving cell 1 10

• etc.

[0092] Controller 160 may be implemented using specifically designed hardware, configurable hardware, programmable data processors configured by the provision of software (which may optionally comprise "firmware") capable of executing on the data processors, special purpose computers or data processors that are specifically programmed, configured, or constructed to perform one or more steps in a method as explained in detail herein and/or combinations of two or more of these. Examples of specifically designed hardware are: logic circuits, application-specific integrated circuits ("ASICs"), large scale integrated circuits ("LSIs"), very large scale integrated circuits ("VLSIs"), and the like. Examples of configurable hardware are: one or more programmable logic devices such as programmable array logic ("PALs"),

programmable logic arrays ("PLAs"), and field programmable gate arrays ("FPGAs")). Examples of programmable data processors are: microprocessors, digital signal processors ("DSPs"), embedded processors, graphics processors, math coprocessors, general purpose computers, server computers, cloud computers, mainframe computers, computer workstations, and the like. For example, one or more data processors in a controller for a cell 1 10 or system of cells 1 10 may implement methods as described herein by executing software instructions in a program memory accessible to the processors. In some embodiments controller 160 comprises a suitably-programmed commercially available process controller.

[0093] Fig. 2 is a schematic diagram depicting electrochemical reactions that occur in cell 1 10 during operation of cell 1 10 according to an example embodiment. Cathode side 20 and anode side 40 of cell 10 are separated by BPM 16.

[0094] As mentioned above, when an electrical potential is applied between anode 14 and cathode 12 water is dissociated by an electrochemical reaction at junction 16B of BPM 16. Hydroxide ions produced in this reaction migrate through anion exchange layer 16A to anode 14.

[0095] At anode 14, the OH ^" ions are converted to oxygen (0 ₂ ), and water (H ₂ 0). This reaction liberates electrons. Water migrates from anode 14 through anion exchange layer 16A to junction 16B. The water maintains hydration of anion exchange layer 16A and also becomes available for dissociation at junction 16B. Oxygen released at anode 14 diffuses through anode 14 and is carried out of cell 1 10 with a flow of anolyte 41 through anode feed channel 42 by way of an anode outlet 46.

[0096] On cathode side 20 of cell 10, C0 ₂ from cathode feed 21 combines with water and electrons to yield CO and hydroxide ions. The CO is carried out of cell 1 10 in the flow of cathode feed 21 through cathode feed channel 22 by way of cathode outlet 26. The hydroxide ions liberated at cathode 12 combine with protons from the water dissociation reaction that occurs in BPM 16 to yield water. The water hydrates BPM 16, replenishes the supply of water in support layer 18 and is available for

dissociation.

[0097] Any features described herein as being present or optionally present in any embodiment may optionally be present in any other embodiment. Any embodiment of the invention described herein may be varied, for example, by one or more of:

• selecting a suitable cathode catalyst. Any cathode catalyst that promotes

electrochemical reduction of carbon dioxide to a desired chemical or a precursor of a desired chemical may be used;

• selecting a suitable anode catalyst. Any anode catalyst that promotes an

anode-side oxygen evolution reaction or other reaction that supports the desired catalyst-side reduction of carbon dioxide my be used;

• selecting a suitable anolyte. Those skilled in the art will recognize a wide range of anolytes suitable for use in the context of the invention including those described elsewhere herein;

• selecting a suitable cathode gas diffusion layer. Many specific designs and compositions of gas diffusion layer that may be used in a cathode in the context of the present invention are described in the electrochemistty literature and/or are commercially available.

• selecting a suitable anode diffusion layer. Many specific designs and

compositions of diffusion layer that may be used in an anode in the context of the present invention are described in the electrochemistry literature and/or are commercially available.

• selecting a suitable source of and composition of carbon dioxide cathode feed.

• selecting a specific bipolar membrane. A wide range of bipolar membranes are commercially available from Fumasep and other vendors and/or described in the academic and patent literature.

• There are a wide variety of design possibilities for a support structure / housing that may be provided to support one or more MEAs.

Prototype Embodiment

[0098] Fig. 3A is an exploded view of a prototype bipolar membrane-based C0 ₂ electrolyzer cell 200 that has been made and used to verify the operation of cells as described herein. Fig. 3B is a schematic diagram depicting a cross-section of cell 200. Cell 200 comprises membrane electrode assembly (MEA) 210. MEA 210 has the general structure illustrated in Fig. 1A. MEA 210 comprises cathode 212, anode 214, bipolar membrane 216, and support layer 218. In the prototype each of the cathode and anode had dimensions of 2.5 cm ^χ 2.5 cm of which a 2 cmx 2 cm area was exposed for an active area of 4 cm ² .

[0099] In the prototype embodiment, cathode 212 comprises a silver

nanopowder/Nafion™ catalyst mixture deposited on a 2.5 x 2.5 cm carbon paper gas diffusion layer (GDL). The GDL has a high surface area and enables rapid mass transport of gas phase C0 ₂ under locally acidic or neutral conditions. In the prototype, anode 214 comprises a 2.5 ^χ 2.5 cm nickel foam layer which acts as both a diffusion layer and as an OER catalyst in basic conditions. The nickel foam was model EQ- BCNF-16m available from MTI Corp of Richmond California USA.

[0100] Cathode 212 may be prepared, for example, using an ultrasonic spray coating method, a hand coating method and/or an airbrush method. In the prototype embodiment, cathode 212 comprises a GDL and a cathode catalyst prepared by mixing 32 mg of silver nanopowder (Sigma, trace metal basis, >99%), 800 μΙ_ of deionized water, 800 μΙ_ of isopropyl alcohol and 60 μΙ of Nafion 1 17 solution (Sigma, 5 wt% in a mixture of lower aliphatic alcohols and water). In some embodiments, cathode 212 can be prepared by spray-coating a catalyst ink on a 4-cm ² area of carbon cloth (Fuel Cell Store, GDL-CT) and drying the catalyst ink under a gentle air stream. In some embodiments, a mask (e.g. Kapton tape) can be applied to avoid depositing catalysts outside the active area of the GDL of cathode 212. Cathode 212 may, for example, comprise a catalyst loading of 1.5 ± 0.2 mg/cm ² .

[0101] In the prototype, bipolar membrane 216 is provided by a Fumatech BPM.

[0102] Support layer 218 comprises a 3 cm ^χ 3 cm layer of glass fibers impregnated with NaHC0 ₃ solution or H ₂ 0. In the prototype, support layer 218 was provided by a sheet of Whatman® glass microfiber filter (Grade GF/D) having a thickness of 675 μηη.

[0103] MEA 210 is sandwiched between cathode flowplate 223 and anode flowplate 243. The assembly comprising MEA 210, cathode flowplate 223 and anode flowplate 243 is in turn clamped between cathode housing 220 and anode housing 240.

Gaskets 222A, 222B, 242A, and 242B seal cell 200.

[0104] Cathode housing 220 includes ports 225, 226 connected to deliver cathode feed to cathode 212 by way of cathode flow field 224 in cathode flowplate 223 and to receive reaction products such as CO formed at the cathode of cell 200. Anode housing 240 includes ports 245, 246 connected to deliver anolyte to anode 214 by way of anode flow field 244 in anode flowplate 243 and to recover product (e.g. oxygen gas) formed at the anode of cell 200.

[0105] Cathode housing 220 and/or anode housing 240 may be made from suitable materials such as stainless steel or other materials that are chemically inert to anode and/or cathode feeds. Cathode housing 220 and anode housing 240 can be made from the same or different materials.

[0106] Flow plates 223 and 243 respectively provide electrical connections between the negative output of a power supply (not shown in Figs. 3A, 3B, 3C) and cathode 212 and the positive output of the power supply and anode 214. To this end flow plates 223 and 243 are in electrical contact with the electrically conductive diffusion layers of anode 212 and cathode 214 respectively. Flow plates 223 and 243 respectively deliver cathode feed to cathode 212 and anolyte to anode 214 by way of corresponding flow fields 224, 244. In the prototype, flow fields 224, 244 were each made up of serpentine channels 1.5 mm wide and 1.5 mm deep with 1-mm ribs.

[0107] Flow fields 224, 244 may have any of a wide variety of patterns. Figs. 4A to 4C are schematic Illustrations showing a few non-limiting example patterns for anode flow field 244 and cathode flow field 224. Fig. 4A depicts a serpentine flow field. Fig. 4B depicts a flow field comprising parallel channels. Fig.4C depicts a flow field comprising an interdigitated pattern. At higher current densities an interdigitated pattern may desirably provide improved Faradaic efficiency (see Fig. 6G). Anode flow field 244 and cathode flow field 224 may have the same or different designs.

[0108] Cathode flowplate 223 and anode flowplate 243 may be made from the same or different materials. Cathode flowplate 223 may comprise materials that are chemically inert to the cathode reactant (e.g. C0 ₂ gas), stable in acidic conditions, electrically conductive, and/or unreactive towards the C0 ₂ reduction reaction. Anode flowplate 243 may comprise materials that are chemically inert to the anode electrolyte, stable in basic conditions, electrically conductive, and/or unreactive toward the OER. In the prototype embodiment, cathode flowplate 223 and anode flowplate 243 are made from grade 2 titanium and 316 stainless steel respectively.

[0109] Gaskets 222A, 222B, 242A, and 242B may be made from the same or different materials. Gaskets 222A, 222B, 242A, and 242B may comprise materials with good chemical inertness and/or high compressibility to maintain gas-tight and liquid-tight seals between different layers of cell 200. In the prototype embodiment, gaskets 222A, 222B, 242A, and 242B comprise 1.5-mm thick chemical resistant compressible polytetrafluoroethylene (PTFE).

[0110] Holes formed in gaskets 222A, 242A facilitate liquid and/or gas delivery between the ports on housings 220, 240 and flowplates 223, 243. 2 ^χ 2 cm square cut outs in gaskets 222B and 242B expose active areas of anode 212 and cathode 224 to the corresponding flow fields 224, 244.

Experimental Electrolysis and Product Analysis.

[0111] Fig. 5 is a schematic diagram describing the dimensions (cm) of various components of the prototype cell described above. In experiments using this prototype cell, pure C0 ₂ was supplied as a cathode feed. The C0 ₂ flow rate to the cathode was set to 100 seem by a flow controller (Matheson; 7531-602) and humidified to a relative humidity of >90% (confirmed by a hygrometer; Neoteck) by passing the gas through a water bath prior to entering the cathode. 1 M NaOH or KOH was recirculated through the anode compartment at a flow rate of 10 ml/min using a peristaltic pump (McMaster-Carr; 43205K1 1). The electrolyzer outlet was introduced into a condenser before being vented directly into the gas-sampling loop of a gas chromatograph (e.g. Perkin Elmer; Clarus 580 GC). The flow rate of C0 ₂ was measured prior to each GC run.

[0112] It was found that anolyte flow rates in the range of about 2.5 to 125

mL/min cm ² of anode area and cathode gas flow rates in the range of about 2.5 to 100 sccm/cm ² of cathode area provided acceptable performance.

[0113] The GC was equipped with a packed MolSieve 5 A column and a packed HayeSepD column. Argon (Praxair, 99.999%) was used as the carrier gas. A flame ionization detector with methanizer was used to quantify CO concentration and a thermal conductivity detector was used to quantify hydrogen concentration.

[0114] Electrochemical measurements were conducted at room temperature and pressure using a potentiostat (CH instruments 660D with a picoamp booster) through two-electrode cell measurements.

[0115] Anodes were prepared by cutting as-purchased nickel foam to size. A standard cleaning procedure as described in reference 46 was used to clean both the carbon GDL and nickel foam. The support layer (Whatman GF/D, 2.7 m pore size) was prepared by soaking in H ₂ 0 for 1 min. The BPMs (FuMA-tech; Fumasep FBM) were stored in 1 M NaCI solution prior to assembly in the cell. A fresh cathode, anode, intermediate layer and BPM were used for each electrolysis test.

[0116] Gas phase vs. liquid phase electrolysis. Cyclic voltammograms over the range of -1.0 V to -3.0 V were collected for both liquid- and gas-phase flow cell reduction of C0 ₂ (Fig. 6A). The cell architectures were the same for both cells (BPM-separated nickel GDL and silver-coated carbon GDL), and the anode feed was 1.0 M NaOH. For liquid-phase electrolysis the cathode feed was C0 ₂ -saturated 0.5 M NaHC0 ₃ ; for gas- phase electrolysis the cathode feed was a humidified gaseous stream of C0 ₂ . Both experiments demonstrated an increase in J with increasing potential (V); however, the gas-fed stream achieves a J approximately double that of the liquid-fed system at -3.0 V (100 mA/cm ² vs. 48 mA/cm ² ). These differences in current densities, while holding all other experimental parameters at parity, provide a clear demonstration of the potential to overcome mass-transport limitations in liquid-fed C0 ₂ electrolyzer systems by using a gas-phase feedstock.

[0117] Catalyst stability. The physical structure of silver-based cathodes before and after gas-phase electrolysis was studied by scanning electron microscopy (SEM), X- ray fluorescence (XRF) and powder X-ray diffraction (XRD) techniques. The cathodes were subjected to 3000 s chronopotentiometric experiments from 20 up to 200 mA/cm ² in 20-mA increments amounting to ca. 10 hours of cumulative electrolysis. The SEM images indicate that the ca. 100 nm nanoparticles remain unchanged during electrolysis on the basis that there is no apparent loss in surface coverage or change in morphology. XRF analysis of the cathode shows nominal differences in metal loading before and after electrolysis experiments. Powder XRD d iff ractog rams show superimposable reflections corresponding to the silver signals indicating no change in the crystalline structure of the catalysts. These collective results show that the catalytic components of the cell are robust during gas phase C0 ₂ electrolysis on the timescale of the experiments.

[0118] Solid-support layer and cell humidification. GC measurements of the expelled gases were used to evaluate the FE values for CO and H ₂ production in the gas- phase flow cell system described above. FEs were calculated according to Equation 1 for J values between 20 and 100 mA/cm ² after 700 s of electrolysis (Fig. 6B). The results show an FE of only 25% for CO at low current densities (J = 20 mA/cm ² ), and diminish to below 10% with increasing J up to 100 mA/cm ² . A dramatic improvement in CO selectivity was observed for all J values (Fig. 6B), with moderate FE for CO (67%) retained at J = 100 mA/cm ² . At higher J values (200 mA/cm ² ), there were only a minor decrease in FE for CO (45% at 200 mA/cm ² ) (Fig. 6C). These results demonstrate the ability to perform C0 ₂ to CO reduction in a flow cell at the high current densities relevant to industrial electrolysis. The inclusion of the support layer clearly facilitates high CO selectivity in this flow cell configuration.

[0119] Experiments were conducted to compare performance of the prototype cell with the support layer filled with water to the performance of the prototype cell with the support layer filled with a NaHC0 ₃ solution (all other experimental parameters were unchanged). The FE for CO production was measured to be within 15% for all J values up to 200 mA/cm ² in this experiment with a solid-supported water devoid of NaHC0 ₃ (Fig. 6D). This result indicates that the presence of water in the support layer that is most important in improving cell performance.

[0120] The influence on cell performance of insufficient water in the support layer was tested by running the prototype cell continuously at 100 mA/cm ² with a low-humidity C0 ₂ inlet stream. A 15% relative humidity for the C0 ₂ inlet stream can be achieved by bypassing the C0 ₂ inlet water bath (passing C0 ₂ through the water bath reaches a relative humidity of 90%). The result of using a low-humidity C0 ₂ inlet was a rapid decay in cell performance within 2 h, as indicated by the sharp rise in required bias above 4 V (Fig. 6E). This decline in performance occurred concomitantly with dehydration of the aqueous support layer. Replenishing the support layer with water resulted in a full recovery of J and FE for CO production (relative to the initial values). Adequate hydration during sustained electrolysis can be achieved by simply supplying the flow cell with humidified C0 ₂ on the basis that we measured stable C0 ₂ to CO electrolysis at 65% FE and 100 mA/cm ² (Fig. 6F).

[0121] Experiments with a prototype cell having the general structure of cell 200 of Fig. 3 have demonstrated that the cell can perform gas-phase reduction of C0 ₂ to CO at high current densities (e.g. J≥ 100 mA/cm ² ). The prototype cell provided stable performance for 24 h.

[0122] The high performance of the cell is believed to result from the combination of the general architecture of the cell as shown in Fig. 3 together with:

• provision of C0 ₂ to the cathode in the gas phase; and

• hydration of BPM 16 by way of support layer 18.

[0123] Delivery of C0 ₂ to the cathode in the gas phase was found to nearly double the current density when compared to delivery of C0 ₂ in a C0 ₂ -saturated aqueous solution when all other experimental parameters are held at parity. It is believed that this improvement results primarily from avoiding the mass transport limitations associated with aqueous C0 ₂ chemistry.

[0124] The presence of aqueous support layer 18 is also thought to inhibit carbonate- forming reactions between hydroxide ions (which may, for example be produced in a carbon dioxide reduction reaction) and carbon dioxide at cathode 12 by ensuring the availability of water at cathode 12. The water can undergo a hydrogen-forming reaction at cathode 12 which competes with reactions that would create carbonates. The hydrogen-forming reaction can also beneficially cause cell 110 to produce a mixture of CO and hydrogen gas (this mixture is known as syngas, which is useful as a fuel).

[0125] The mass transport limitation in liquid phase C0 ₂ electrolyzers originates from the low solubility of C0 ₂ in aqueous electrolyte solutions (~30 mM in H ₂ 0 at atmospheric pressure) as well as a low diffusion coefficient of C0 ₂ in water (0.0016 mm ² /s). Gas-phase electrolysis therefore has the potential to increase mass transport and achieve current densities several orders of magnitude greater than liquid phase electrolysis at atmospheric pressure.

[0126] The inventors have discovered that achieving stable performance of a electrocatalytic cell reducing gaseous C0 ₂ at a reasonable FE while operating at high current densities is facilitated by providing a support layer 18 which retains water for hydration of BPM 16. Providing support layer 18 helped to achieve cell stability of more than 24 h at a current density of 100 mA/cm ² . A solid-supported aqueous layer comprising water or an aqueous solution such as 1.0 M NaHC0 ₃ can enhance the FEs and facilitate current densities for gas phase C0 ₂ electrolysis on the order of 200 mA/cm ² with a FE for CO production of 45%.

Interpretation of Terms

[0127] Unless the context clearly requires otherwise, throughout the description and the claims:

• "comprise", "comprising", and the like are to be construed in an inclusive

sense, as opposed to an exclusive or exhaustive sense; that is to say, in the sense of "including, but not limited to";

• "connected", "coupled", or any variant thereof, means any connection or

coupling, either direct or indirect, between two or more elements; the coupling or connection between the elements can be physical, logical, or a combination thereof;

• "herein", "above", "below", and words of similar import, when used to describe this specification, shall refer to this specification as a whole, and not to any particular portions of this specification;

• "or", in reference to a list of two or more items, covers all of the following

interpretations of the word: any of the items in the list, all of the items in the list, and any combination of the items in the list;

• the singular forms "a", "an", and "the" also include the meaning of any

appropriate plural forms.

[0128] Words that indicate directions such as "vertical", "transverse", "horizontal", "upward", "downward", "forward", "backward", "inward", "outward", "vertical", "transverse", "left", "right", "front", "back", "top", "bottom", "below", "above", "under", and the like, used in this description and any accompanying claims (where present), depend on the specific orientation of the apparatus described and illustrated. The subject matter described herein may assume various alternative orientations.

Accordingly, these directional terms are not strictly defined and should not be interpreted narrowly.

[0129] For example, while processes or blocks are presented in a given order, alternative examples may perform routines having steps, or employ systems having blocks, in a different order, and some processes or blocks may be deleted, moved, added, subdivided, combined, and/or modified to provide alternative or

subcombinations. Each of these processes or blocks may be implemented in a variety of different ways. Also, while processes or blocks are at times shown as being performed in series, these processes or blocks may instead be performed in parallel, or may be performed at different times.

[0130] While steps or blocks are at times shown as being performed sequentially, they may instead be performed simultaneously or in different sequences. While steps or blocks are described as being performed continuously such steps or blocks may be performed intermittently and/or operation of such steps or blocks may be interrupted on occasion.

[0131] Where a component (e.g. a cathode, diffusion layer, port, power supply, assembly, device, circuit, etc.) is referred to above, unless otherwise indicated, reference to that component (including a reference to a "means") should be interpreted as including as equivalents of that component any component which performs the function of the described component (i.e., that is functionally equivalent), including components which are not structurally equivalent to the disclosed structure which performs the function in the illustrated exemplary embodiments of the invention.

[0132] Specific examples of systems, methods and apparatus have been described herein for purposes of illustration. These are only examples. The technology provided herein can be applied to systems other than the example systems described above. Many alterations, modifications, additions, omissions, and permutations are possible within the practice of this invention. This invention includes variations on described embodiments that would be apparent to the skilled addressee, including variations obtained by: replacing features, elements and/or acts with equivalent features, elements and/or acts; mixing and matching of features, elements and/or acts from different embodiments; combining features, elements and/or acts from embodiments as described herein with features, elements and/or acts of other technology; and/or omitting combining features, elements and/or acts from described embodiments.

[0133] Various features are described herein as being present in "some

embodiments". Such features are not mandatory and may not be present in all embodiments. Embodiments of the invention may include zero, any one or any combination of two or more of such features. This is limited only to the extent that certain ones of such features are incompatible with other ones of such features in the sense that it would be impossible for a person of ordinary skill in the art to construct a practical embodiment that combines such incompatible features. Consequently, the description that "some embodiments" possess feature A and "some embodiments" possess feature B should be interpreted as an express indication that the inventors also contemplate embodiments which combine features A and B (unless the description states otherwise or features A and B are fundamentally incompatible).

[0134] It is therefore intended that the following appended claims and claims hereafter introduced are interpreted to include all such modifications, permutations, additions, omissions, and sub-combinations as may reasonably be inferred. The scope of the claims should not be limited by the preferred embodiments set forth in the examples, but should be given the broadest interpretation consistent with the description as a whole.