CO.SUB.2.RR-OOR electrolyser system and related process for facilitating the capture and conversion of CO.SUB.2 .in gas mixture streams

Abstract

The present disclosure relates to a MEA electrolyser comprising a cathodic compartment operating CO2 reduction reactions (CO.sub.2RR) of CO.sub.2 from a gaseous CO.sub.2-containing stream, an anodic compartment operating all-liquid organic oxidation reactions (OOR), an ionic exchange membrane in between. A CO.sub.2RR-OOR system can further include a gas-liquid separation unit in fluid communication with the anodic compartment to receive the anodic product mixture and separate gaseous CO.sub.2 from the anodic product mixture to produce a CO.sub.2-depleted liquid product stream and a recovered pure gaseous CO.sub.2 stream. The system can further include a recycle line in fluid communication with the gas-liquid separation unit to redirect the recovered pure gaseous CO.sub.2 stream to the cathodic compartment of the MEA electrolyser as a portion of the gaseous CO.sub.2-containing stream. The present disclosure also concerns a process for electrochemically converting the gaseous CO.sub.2-containing stream to multi-carbon products in such a MEA CO.sub.2RR-OOR electrolyser.

Claims

1. A process for electrochemically converting a gaseous carbon dioxide stream to multi-carbon products having at least two carbon atoms in a carbon dioxide reduction reaction/organic oxidation reaction (CO.sub.2RR/OOR), the process is characterized in that it comprises: a) providing a CO.sub.2RR/OOR system being a catholyte-free system and comprising: an anodic compartment comprising an anode and configured to operate the organic oxidation reaction; and a cathodic compartment comprising a cathode with a CO.sub.2 reduction reaction catalyst being or comprising copper and being configured to operate carbon dioxide reduction reactions; b) providing a solution comprising an anolyte and an organic liquid-phase precursor of an organic oxidation reaction; c) supplying the solution to the anodic compartment of the CO.sub.2RR/OOR system to operate the organic oxidation reaction and generate an anodic product mixture comprising OOR liquid-phase products; d) supplying a gaseous CO.sub.2-containing stream to the cathodic compartment of the CO.sub.2RR/OOR system to operate the reduction of a first portion of CO.sub.2 and generate a cathodic product mixture comprising multi-carbon products, wherein a second portion of CO.sub.2 is transferred to the anodic compartment by an ionic exchange to produce a crossover CO.sub.2; e) recovering the anodic product mixture from the anodic compartment, the anodic product mixture comprising the crossover CO.sub.2; f) separating the crossover CO.sub.2 from the anodic product mixture to produce a CO.sub.2-depleted product stream and a recovered pure gaseous CO.sub.2 stream.

2. The process according to claim 1 is characterized in that the CO.sub.2 concentration of the gaseous CO.sub.2-containing stream is ranging between 5 vol. % and 95 vol. %, based on the total volume of the gaseous CO.sub.2-containing stream; or between 10 vol. % and 90 vol. %.

3. The process according to claim 1 is characterized in that the process is carried out at a temperature ranging between 30 C. and 50 C.

4. The process according to claim 1 is characterized in that the gaseous CO.sub.2-containing stream is a by-product CO.sub.2 stream produced from an industrial upstream process.

5. The process according to claim 4 is characterized in that the industrial upstream process is fermentation of glucose to ethanol.

6. The process according to claim 1 is characterized in that the OOR liquid-phase products comprise gluconate, glucuronate, glucarate, formate, tartarate, tratronate or any mixture thereof.

7. The process according to claim 1 is characterized in that it further comprises redirecting the recovered pure gaseous CO.sub.2 stream to the cathodic compartment as a portion of the gaseous CO.sub.2-containing stream to maximize CO.sub.2 utilization.

8. The process according to claim 1 is characterized in that it comprises redirecting the recovered pure gaseous CO.sub.2 stream as a feedstream to another electrolyser being a solid oxide electrolyser cell, a membrane electrode assembly electrolyser, an alkaline flow cell or any combination thereof.

9. The process according to claim 1 is characterized in that the anolyte is selected from KHCO.sub.3, K.sub.2CO.sub.3, NaHCO.sub.3, Na.sub.2CO.sub.3 and any mixture thereof.

10. The process according to claim 1 is characterized in that the organic liquid-phase precursor is or comprises one or more selected from glucose, glycerol, furfural, 5-hydroxymethylfurfural, ethanol, n-propanol, iso-propanol, methanol, benzyl alcohol, starch, cellulose, lignin and any mixtures thereof.

11. The process according to claim 1 is characterized in that the solution comprising the anolyte and the organic liquid-phase precursor has a bulk pH between 4 and 9.

12. The process according to claim 1 is characterized in that the multi-carbon products are or comprise ethylene.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Implementations of the CO.sub.2RR-OOR system and related process are represented in and will be further understood in connection with the following figures.

(2) FIG. 1 illustrates the performance of an all-liquid anode enabling CO.sub.2 recycling and low energy intensity for producing ethylene, including the mass balance of the electrochemical process in the conventional CO.sub.2RR-OER electrolysers.

(3) FIG. 2 illustrates the performance of an all-liquid anode enabling CO.sub.2 recycling and low energy intensity for producing ethylene, including the energy intensity of ethylene production in benchmark systems from literature (Neutral MEA-1.sup.5, -2.sup.6, -3.sup.4; acidic flow cell.sup.14; acidic MEA.sup.21) versus this work.

(4) FIG. 3 is a schematic representation of a conceptual design of CO.sub.2-ORR MEA electrolyser operating organic oxidation reaction like GOR and reduction of CO.sub.2 contained in a stream of pure or diluted CO.sub.2, and allowing recovery of crossover CO.sub.2 as a pure gaseous CO.sub.2 stream.

(5) FIG. 4 is a schematic process flow diagram of a solid oxide electrolyser cell (SOEC) that is fed with the pure gaseous CO.sub.2 stream generated in the anodic side of the system of FIG. 3 to produce CO at high-temperature (i.e., above 350 C.), in series with an MEA electrolyser or alkaline flow cell to produce multi-carbon products.

(6) FIG. 5 is a schematic process flow diagram of a low temperature (i.e., below 100 C.) MEA electrolyser that is supplied with the pure gaseous CO.sub.2 stream generated in the anodic side of the system of FIG. 3 to produce CO, in series with another MEA electrolyser or alkaline flow cell to produce multi-carbon products (COR products).

(7) FIG. 6 is a schematized process flow diagram of a CO.sub.2RR MEA electrolyser that is supplied with the pure gaseous CO.sub.2 stream generated in the anodic side of the system of FIG. 3 to generate multi-carbon products at low-temperature (i.e., below 100 C.).

(8) FIG. 7 is a schematic representation of a CO.sub.2RR-OOR MEA electrolyser operating all-liquid anodic reactions facilitating gaseous CO.sub.2 recycling and enabling low energy intensity for producing ethylene.

(9) FIG. 8 illustrates the performance of an all-liquid anode enabling CO.sub.2 recycling and low energy intensity for producing ethylene, including the operating principle of the CO.sub.2RR-OOR electrolysis system that combines low-energy input and high-carbon utilization in CO.sub.2-to-C.sub.2+ conversion. The system uses an anolyte composed of KHCO.sub.3 and liquid organic precursors. The cathode chamber is fed with humidified CO.sub.2.

(10) FIG. 9 is a schematic representation of the main mechanism of the electrochemical glucose oxidation reaction (GOR).

(11) FIG. 10 illustrates an electron microscopy characterization of MEA electrolyser catalysts including scanning electron microscopy (SEM) and transmission electron microscopy (TEM, inset) images of the cathodic catalyst: Cu nanoparticles/PFSA composite.

(12) FIG. 11 illustrates an electron microscopy characterization of MEA electrolyser catalysts including SEM images of the anodic catalyst: Pt/C loaded on hydrophilic carbon fibres.

(13) FIG. 12 illustrates an electron microscopy characterization of MEA electrolyser catalysts including SEM images of the Pt/C catalyst.

(14) FIG. 13 illustrates an energy-dispersive X-ray spectroscopy (EDS) elemental mappings of carbon for Pt/C catalyst.

(15) FIG. 14 illustrates an energy-dispersive X-ray spectroscopy (EDS) elemental mappings of overlap for Pt/C catalyst.

(16) FIG. 15 illustrates the performance of the CO.sub.2RR-GOR electrolysis system including copper cathode catalyst and anode platinum catalyst with the mass loading of Cu: 0.5 mg cm 2 and Pt: 2 mg cm.sup.2, including the linear scan voltammetry (LSV) of the CO.sub.2RR-GOR electrolysis system with various glucose concentrations (0 M refers to CO.sub.2RR-OER on a high-surface-area IrO.sub.xTi catalyst) at 20 C. All the profiles were recorded at a scanning rate of 5 mV s.sup.1 immediately after three cycles of voltammetry scanning.

(17) FIG. 16 illustrates the performance of the CO.sub.2RR-GOR electrolysis system including copper cathode catalyst and anode platinum catalyst with the mass loading of Cu: 0.5 mg cm.sup.2 and Pt: 2 mg cm.sup.2, including the linear scan voltammetry (LSV) of the CO.sub.2RR-GOR electrolysis system with 1 M glucose at various temperatures. All the profiles were recorded at a scanning rate of 5 mV s.sup.1 immediately after three cycles of voltammetry scanning.

(18) FIG. 17 illustrates the performance of the CO.sub.2RR-GOR electrolysis system including copper cathode catalyst and anode platinum catalyst with the mass loading of Cu: 0.5 mg cm 2 and Pt: 2 mg cm.sup.2, including the full-cell potential of the CO.sub.2RR-GOR at various temperatures.

(19) FIG. 18 illustrates the performance of the CO.sub.2RR-GOR systems with various cathode and anode catalyst loadings, measured at 50 C., showing the dependences of cell voltage on current density.

(20) FIG. 19 illustrates the performance of the CO.sub.2RR-GOR systems with various cathode and anode catalyst loadings, measured at 50 C., showing the dependences of oxygen FE on current density.

(21) FIG. 20 illustrates the performance of the CO.sub.2RR-GOR systems with various cathode and anode catalyst loadings, measured at 50 C., showing the CO.sub.2RR gas product distributions at different current densities for an anode catalyst loading of 2.0 mg/cm.sup.2.

(22) FIG. 21 illustrates the performance of the CO.sub.2RR-GOR systems with various cathode and anode catalyst loadings, measured at 50 C., including the CO.sub.2RR gas product distributions at different current densities for an anode catalyst loading of 0.5 mg/cm.sup.2.

(23) FIG. 22 illustrates the FE distributions toward gas-phase CO.sub.2RR products at various current densities, showing measurements at 35 C.

(24) FIG. 23 illustrates the FE distributions toward gas-phase CO.sub.2RR products at various current densities, showing measurements at 20 C.

(25) FIG. 24 illustrates the performance of the CO.sub.2RR-GOR electrolysis system including copper cathode catalyst and anode platinum catalyst with the mass loading of Cu: 0.5 mg cm-2 and Pt: 2 mg cm.sup.2, including the cathodic FE distributions at 50 C. and various current densities.

(26) FIG. 25 illustrates the performance of the CO.sub.2RR-GOR electrolysis system including copper cathode catalyst and anode platinum catalyst with the mass loading of Cu: 0.5 mg cm.sup.2 and Pt: 2 mg cm.sup.2, including the CO.sub.2 and O.sub.2 flow rates (normalized by electrode geometric area) in the anodic gas streams at 50 C. The simulated CO.sub.2 is assessed by the stoichiometry of generated OH.sup. and transferred electrons, assuming CO.sub.2 is converted to CO.sub.3.sup.2:

(27) FIG. 26 is a graph showing the anolyte pH as a function of operating temperature. The anolyte contains 1 M KHCO.sub.3 and 1 M glucose.

(28) FIG. 27 illustrates the performance of the CO.sub.2RR-GOR electrolysis system including copper cathode catalyst and anode platinum catalyst with the mass loading of Cu: 0.5 mg cm.sup.2 and Pt: 2 mg cm.sup.2, including the recovery rates and purities of CO.sub.2 at the anodic product stream at various current densities and 50 C. Recovery rates are defined by dividing the CO.sub.2 flow rate from measurement by that from prediction.

(29) FIG. 28 illustrates the performance of the CO.sub.2RR-GOR electrolysis system including copper cathode catalyst and anode platinum catalyst with the mass loading of Cu: 0.5 mg cm 2 and Pt: 2 mg cm.sup.2, including the FE distributions of liquid products of GOR at various current densities at 50 C.

(30) FIG. 29 illustrates the performance of the CO.sub.2RR-GOR electrolysis system including copper cathode catalyst and anode platinum catalyst with the mass loading of Cu: 0.5 mg cm.sup.2 and Pt: 2 mg cm.sup.2, including the FE distributions of gas products of CO.sub.2RR at various temperatures and current densities.

(31) FIG. 30 illustrates the FE toward liquid product distributions in CO.sub.2RR-GOR electrolyser in the cathodic (solid) and anodic (patterned) streams, showing FE (in %) at 35 C.

(32) FIG. 31 illustrates the FE toward liquid product distributions in CO.sub.2RR-GOR electrolyser in the cathodic (solid) and anodic (patterned) streams, showing FE (in %) at 50 C. At 50 C., only <6% FE of CO.sub.2RR products crosses over to the anolyte for all the current densities studied.

(33) FIG. 32 is a graph showing a weight ratio between the liquid products of CO.sub.2RR (ethanol, acetate and propanol) and the target products of GOR (gluconate, glucuronate and glucarate) at the temperature of 35 C. and 50 C.

(34) FIG. 33 illustrates the performance of the CO.sub.2RR-GOR system under low CO.sub.2 availability. The mass loadings on the cathode and anode electrodes are Cu: 0.5 mg cm.sup.2 and Pt: 2 mg cm.sup.2, including the CO.sub.2 carbon efficiency for total CO.sub.2RR and CO.sub.2-to-C.sub.2H.sub.4 (mole ratio of the input CO.sub.2 converted to C.sub.2H.sub.4) at various CO.sub.2 input flow rates. The experiments are performed at a current density of 100 mA cm.sup.2. The flow rate of the CO.sub.2 supplied is normalized by the geometric area of the electrodes.

(35) FIG. 34 illustrates the performance of the CO.sub.2RR-GOR system under low CO.sub.2 availability. The mass loadings on the cathode and anode electrodes are Cu: 0.5 mg cm.sup.2 and Pt: 2 mg cm.sup.2, including the FE distributions at various CO.sub.2 input flow rates. The experiments are performed at a current density of 100 mA cm.sup.2. The flow rate of the CO.sub.2 supplied is normalized by the geometric area of the electrodes.

(36) FIG. 35 illustrates the performance of the CO.sub.2RR-GOR system under low CO.sub.2 availability. The mass loadings on the cathode and anode electrodes are Cu: 0.5 mg cm.sup.2 and Pt: 2 mg cm.sup.2, including the long-term electrolysis performance with a CO.sub.2 input flow rate of 0.36 sccm cm.sup.2. The experiments are performed at a current density of 100 mA cm.sup.2. The flow rate of the CO.sub.2 supplied is normalized by the geometric area of the electrodes.

(37) FIG. 36 shows peaks of a 1H NMR spectra corresponding to the liquid products of CO.sub.2RR at the cathodic or anodic stream at 100 mA cm.sup.2.

(38) FIG. 37 shows peaks of a 1H NMR spectra corresponding to the glucose oxidation reaction (GOR) products in the anodic stream at 100 mA cm.sup.2.

(39) FIG. 38 illustrates the performance of an all-liquid anode enabling CO.sub.2 recycling and low energy intensity for producing ethylene, including the operating principle of the conventional CO.sub.2RR-OER electrolysis.

(40) FIG. 39 illustrates an electron microscopy characterization of MEA electrolyser catalysts including the scanning transmission electron microscopy image for Pt/C catalyst.

(41) FIG. 40 illustrates an electron microscopy characterization of MEA electrolyser catalysts including the energy-dispersive X-ray spectroscopy (EDS) elemental mappings of platinum for Pt/C catalyst.

(42) FIG. 41 shows X-ray photoelectron spectroscopy (XPS) measurements for copper nanoparticles (Cu NPs) on Cu/PTFE gas diffusion electrode.

(43) FIG. 42 shows X-ray photoelectron spectroscopy (XPS) measurements for a PtC on hydrophilic carbon cloth gas diffusion electrode.

DEFINITIONS

(44) For the disclosure, the following definitions are given:

(45) As used herein, the term C#hydrocarbons, wherein # is a positive integer, is meant to describe all hydrocarbons having #carbon atoms. C#hydrocarbons are sometimes indicated as just C#. Moreover, the term C#+ hydrocarbons is meant to describe all hydrocarbon molecules having #or more carbon atoms. Accordingly, the expression C.sub.2+ hydrocarbons is meant to describe a mixture of hydrocarbons having 2 or more carbon atoms.

(46) The term transition metal refers to an element whose atom has a partially filled d sub-shell, or which can give rise to cations with an incomplete d sub-shell (IUPAC definition). According to this definition, the transition metals are Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, La, Hf, Ta, W, Re, Os, Ir, Pt, Au, Hg, Ac, Rf, Db, Sg, Bh, Hs, Mt, Ds, Rg, and Cn. The metals Ga, In, Sn, TI, Pb and Bi are considered as post-transition metal.

(47) The yield to particular chemical compounds is determined as the mathematical product between the selectivity to said particular chemical compounds and the conversion rate of the chemical reaction. The mathematical product is expressed as a percentage.

(48) The terms comprising, comprises and comprised of as used herein are synonymous with including, includes or containing, contains, and are inclusive or open-ended and do not exclude additional, non-recited members, elements or method steps. The terms comprising, comprises and comprised of also include the term consisting of.

(49) The recitation of numerical ranges by endpoints includes all integer numbers and, where appropriate, fractions subsumed within that range (e.g., 1 to 5 can include 1, 2, 3, 4, 5 when referring to, for example, a number of elements, and can also include 1.5, 2, 2.75 and 3.80, when referring to, for example, measurements). The recitation of endpoints also includes the recited endpoint values themselves (e.g., from 1.0 to 5.0 includes both 1.0 and 5.0). Any numerical range recited herein is intended to include all sub-ranges subsumed therein.

(50) The particular features, structures, characteristics or embodiments may be combined in any suitable manner, as would be apparent to a person skilled in the art from this disclosure, in one or more embodiments.

DETAILED DESCRIPTION

(51) Electrochemical reduction of CO.sub.2 to multi-carbon products (C.sub.2+), when powered using renewable electricity, offers a route to valuable chemicals and fuels. In conventional zero-gap, neutral-media CO.sub.2-to-C.sub.2+ devices, over 70% of input CO.sub.2 crosses the cell to the anodic side where CO.sub.2 is mixed with produced oxygen at the anode to form a gaseous product mixture. This amount of CO.sub.2 that migrates to the anodic side can be referred to as the crossover CO.sub.2. Recovering CO.sub.2 from the formed gaseous product mixture (which contains CO.sub.2 and oxygen) incurs a significant energy penalty.

(52) There is thus proposed herein a liquid-to-liquid anodic system and related processes that can be implemented to facilitate the capture of crossover CO.sub.2 without additional energy input. The techniques encompassed herein can be used to achieve a high carbon efficiency while being compatible with highly-performing CO.sub.2 reduction reaction catalysts and electrolysers that are already developed to work optimally in neutral and alkaline electrolytes. For example, the performance of the proposed system can be characterized by a low full-cell voltage of about 1.9 V and a total carbon efficiency of about 48%, for achieving production of about 259 GJ/tonne ethylene, with a 30% reduction in energy intensity compared to state-of-art CO.sub.2-to-C.sub.2+ systems.

(53) More particularly, it is proposed herein to pair a CO.sub.2 reduction reaction (CO.sub.2RR) with an all-liquid anodic reaction (e.g., organic oxidation reaction (OOR)) in a neutral electrolyte to achieve high carbon efficiency and low energy input in the electrosynthesis of renewable chemicals and fuels. The present techniques enable recovery of the crossover CO.sub.2 as a stream of pure gaseous CO.sub.2 which can be used in various ways including (1) being stored, (2) being recycled to the cathode for utilization in the CO.sub.2RR-OOR electrolyser or (3) being fed into any other electrolyser for the production of CO, C1 products, C2+ products, or any combinations thereof. Examples of combinations of electrolysers are described further below with reference to FIGS. 3 to 6. Alternatively, the stream of pure CO.sub.2 can be used as an industrial source of CO.sub.2 or transported for geological storage.

(54) Referring to FIG. 3, the CO.sub.2RR-OOR system can include an anodic compartment (A) sustaining all liquid oxidation reactions (OOR) and a cathodic compartment (C) sustaining CO.sub.2 reduction into multi-carbon products, with the anodic compartment and the cathodic compartment being separated by an anion exchange membrane (AEM). The process includes feeding the gaseous CO.sub.2-containing stream, that can be a pure or dilute CO.sub.2 stream, to the cathodic compartment to generate a cathodic product mixture comprising multi-carbon products via electrochemical reduction from the cathodic compartment, and an anodic product mixture comprising CO.sub.2 and OOR products from the anodic compartment. The process further includes gas-liquid separation of the anodic product mixture into a CO.sub.2-depleted liquid stream and a pure gaseous CO.sub.2 stream (corresponding to the crossover CO.sub.2). It should be noted that the CO.sub.2-depleted liquid stream can be recycled as a portion of the anolyte.

(55) For example, the CO.sub.2RR-OOR electrolyser can be a zero-gap CO.sub.2RR-OOR electrolyser or a one-gap CO.sub.2RR-OOR electrolyser (flow cell).

(56) Experimentation in operating the proposed CO.sub.2RR-OOR electrolyser and related system implementations demonstrated a high carbon efficiency by returning the recovered crossover CO.sub.2 to the cathodic gaseous CO.sub.2-containing stream, thereby achieving a high CO.sub.2 conversion of up to 75%. It was further shown that the proposed CO.sub.2RR-OOR electrolyser can achieve a low full-cell potential of 1.90 V at a current density of 100 mA cm.sup.2 and stable electrosynthesis of C.sub.2+ products for over 80 hours while maintaining a high CO.sub.2 conversion of 45%. Accounting for the total electricity and downstream separation energy costs, the present techniques achieve a total energy intensity of 259 GJ per ton of ethylene produced, approximately 30% lower than that of known CO.sub.2RR electrolysers.

(57) Process and System Design to Facilitate Recovery of Crossover CO.sub.2

(58) The techniques described herein facilitate direct recovery of pure CO.sub.2 from the anodic product mixture that is generated from a neutral/alkaline electrolyte media and can apply to a CO.sub.2RR-OOR electrolyser including a cathodic compartment comprising a cathode supporting CO.sub.2 reduction reactions, an anodic compartment comprising an anode supporting organic oxidation reactions in a neutral/alkaline media containing an organic liquid-phase precursor, and an anionic exchange membrane (AEM) ensuring anionic exchange between the two compartments.

(59) For example, the present system can be a zero-gap CO.sub.2RR-OOR MEA electrolyser. Referring to FIG. 7, the cathodic compartment of the CO.sub.2RR-OOR MEA electrolyser is continuously supplied with CO.sub.2 via the gaseous CO.sub.2-containing stream. FIG. 7 further provides the mass balance of the electrochemical process in the CO.sub.2RR-OOR MEA electrolyser. The anodic compartment is configured to receive a near-neutral anolyte (e.g. 1 M KHCO.sub.3) containing the organic liquid-phase precursor that can be electrochemically oxidized to value-added liquid-phase products according to organic oxidation reactions (OOR), and being thereof referred to as OOR liquid-phase products. The MEA-type electrolyser uses a Cu-loaded gas diffusion electrode as the cathode, and a Pt/C loaded hydrophilic carbon cloth as the anode, an anion-exchange membrane (AEM) as a solid-state electrolyte. At the cathode, a small portion of CO.sub.2 (<25 vol. %) is electrochemically converted to the CO.sub.2RR products, and a significant fraction of CO.sub.2 (50-75 vol. %) is converted to carbonate/bicarbonate due to its reaction with locally produced hydroxide (OH) ions (see the study of Larrazbal G. O., et al, entitled A comprehensive approach to investigate CO.sub.2 reduction electrocatalysts at high current densities (Acc. Mater. Res., 2021, 2, 220-229). The carbonate/bicarbonate ions then migrate to the anode through the AEM. At the anode, the organic liquid-phase precursor is electrochemically oxidized to value-added product(s) in the near-neutral anolyte and generates protons. The protons combine with the carbonate/bicarbonate ions, regenerating crossover CO.sub.2 as the only gas-phase product at the anodic product stream.

(60) Other examples of near-neutral anolyte are anolytes selected from K.sub.2CO.sub.3, NaHCO.sub.3, Na.sub.2CO.sub.3 and any mixture thereof.

(61) CO.sub.2RR products refer herein to multi-carbon products having at least 2 carbon atoms. CO.sub.2RR products for example include ethylene.

(62) The AEM separates the cathode and the anode and further provides highly alkaline conditions favourable for CO.sub.2RR. Referring to FIGS. 1 and 7, both the present system (FIG. 7) and conventional AEM-based zero-gap CO.sub.2RR electrolyser system (FIG. 1) allow a large portion of the input CO.sub.2 (e.g., 70 vol. %) crossing over the AEM from the cathode to the anode under the form of carbonate and bicarbonate ions. Such crossover ions can further combine with the protons generated from the reaction at the anode (anodic reaction) to regenerate gaseous CO.sub.2. The present techniques include controlling the anodic reaction to being all liquid in naturei.e., to avoid any O.sub.2 evolution from CO.sub.2to avoid contamination of the anodic product stream (including CO.sub.2) with O.sub.2. The anodic product mixture is a gas-liquid mixture with the CO.sub.2 making up for substantially all the gas phase and the OOR products comprised in the liquid phase.

(63) The process can include separating the anodic product mixture into a pure gaseous CO.sub.2 stream (purity >99%) and a CO.sub.2-depleted liquid stream (that can be recycled as liquid anolyte remainder). Referring to FIG. 8, the pure gaseous CO.sub.2 stream can be recovered by gas-liquid separation of the anodic product stream in a gas-liquid separator serving as an anolyte reservoir, and the pure gaseous CO.sub.2 stream can further be directly recycled to the cathodic compartment (see recycle line for circulating recovered 99% CO.sub.2 stream in FIG. 8) to yield ethylene as part of a cathodic product mixture. Thus, controlling the anodic reaction to remain in liquid phase allows achieving higher CO.sub.2 utilization than the conventional 25% CO.sub.2 utilization threshold, and avoiding an energy consumption penalty associated with supplemental anodic gas mixture separation, without incurring penalties to a full-cell voltage or a selectivity to ethylene.

(64) All-Liquid-Phase Anodic Process

(65) The present techniques allow all-liquid-phase anodic reactions that produce protons (or consume hydroxides) and operate in near-neutral media. Candidate anode reactions include water-to-hydrogen peroxide, chloride-to-hypochlorite, and a wide range of organic oxidation reactions (OORs). However, known catalysts for hydrogen peroxide and hypochlorite production can result in gaseous by-products.

(66) Coupling electrochemical CO reduction with OOR has been demonstrated in an MEA electrolyser. However, prior systems that employed OOR as an anodic reaction did not focus on overall carbon efficiency: recent gas-CO.sub.2-fed CO.sub.2RR-OOR systems operated in strong alkaline electrolytes (pH>14), causing a severe energy penalty associated with the regeneration of (bi) carbonate back to alkaline and CO.sub.2.

(67) The process thus includes controlling the anodic reaction to favour OORs at the anode at a neutral/alkaline pH. A neutral/alkaline anolyte/electrolyte/media refers herein to an anolyte/electrolyte/media having a neutral/alkaline pH, i.e., a pH between 4 and 9, optionally between 4 and 8, and further optionally between 4.5 and 7.5. The OORs that are encompassed herein include the oxidation of glucose, glycerol, furfural, 5-hydroxymethylfurfural, starch, cellulose, lignin, and alcohols.

(68) In some implementations, controlling the anodic reaction can include favouring a glucose oxidation reaction (GOR) in a neutral/alkaline anolyte. Coupling the CO.sub.2RR with GOR is demonstrated herein as a suitable liquid-phase anodic process strategy for high-carbon efficiency and low-energy intensity in CO.sub.2-to-C.sub.2+ conversion. Favouring the GOR includes providing glucose as a liquid precursor in the anolyte.

(69) Glucose is abundant in biomass, with an average market price of $400-500 ton-1, mainly produced from starch. In 2017, over 5 million tons of glucose were produced in the United States. Electrochemical oxidation of glucose mainly produces gluconate, glucuronate, and glucarate (FIG. 9), which command a higher market price per ton than does the input chemical glucose, for they function as feedstocks for the production of biopolymers and pharmaceuticals. The market price of gluconic acid reaches $1,500 ton-1. Glucaric acid is a high-value-added biomass-derived commodity chemical. The projected market sizes of gluconic acid and glucaric acid are $1.9 billion (2028) and $1.3 billion (2025). The recent techno-economic assessment estimated that the separation process of the GOR product requires 3.6 to 4.5 GJ per ton of input glucose, acceptable at $60 to $75 per ton assuming an electricity price of $0.06 kWh-1 compared to the market price of the GOR products.

(70) The GOR that is selected herein avoids gaseous products, thereby facilitating the recovery of pure gaseous CO.sub.2 from the anodic product mixture via direct gas-liquid separation. The selected GOR can outcompete the oxygen evolution reaction (OER) at industrially relevant reaction rates in electrolytes having a pH between 4 and 9, between 4 and 8, or between 4.5 and 7.5. The selected GOR also offers electrolysis energy savings, with a thermodynamic potential of 0.05 V, significantly lower than that of the OER (1.23 V). A large supply of each reactant, CO.sub.2 and glucose, is available and co-located in industrial bioethanol plants. In these operations, glucose ferments to ethanol and CO.sub.2 is emitted. A 2012 report estimated that 14.8 tons of CO.sub.2 is emitted in producing 1 ton of bioethanol. The CO.sub.2RR-GOR electrolyser can convert waste CO.sub.2 and available glucose to chemicals, providing additional product streams and reducing the overall/net carbon footprint of bioethanol production if it used low-carbon electricity.

(71) Catalyst Characterization

(72) The cathodic compartment of the system includes a cathode that catalyzes the CO.sub.2RR. The cathode comprises a catalyst that can be referred to as a CO.sub.2RR catalyst. The CO.sub.2RR catalyst comprises one or more transition metals, for example, Cu, Ag, Pb, Co, Sn, Zn, and alloys thereof, and/or any combinations thereof. The CO.sub.2RR catalyst comprises one or more transition metals in addition to copper, for example, Ag, Pb, Co, Sn, Zn, and alloys thereof, and/or any combinations thereof. For example, the CO.sub.2RR catalyst can comprise one or more phthalocyanines of said one or more transition metals. In some implementations, the cathode is a gas diffusion electrode (GDE) that includes hydrophobic porous support. For example, the hydrophobic porous support can comprise polytetrafluoroethylene (PTFE) and/or hydrophobic carbon paper.

(73) Optionally, the cathode can further include an ionomer layer that comprises a perfluorinated sulfonic acid ionomer. The ionomer layer is co-sprayed with catalyst nanoparticles (e.g., copper nanoparticles).

(74) For example, the perfluorinated sulfonic acid ionomer can be Fumion, Sustainion, Aquivion, Pention, or PiperION. For example, the perfluorinated sulfonic acid ionomer can include perfluorosulfonic acid (PFSA), sulfonated tetrafluoroethylene based fluoropolymer-copolymer (such as Nafion or 1,1,2,2-Tetrafluoroethene; 1,1,2,2-tetrafluoro-2-[1,1,1,2,3,3-hexafluoro-3-(1,2,2-trifluoroethenoxy) propan-2-yl]oxyethanesulfonic acid), SSC, Aciplex, Flemion, 3M-perfluorinated sulfonic acid ionomer, Aquivion, an ionene, or a combination thereof.

(75) In some implementations, the cathode can be produced by depositing copper nanoparticles and a perfluorosulfonic acid (PFSA) ionomer on a hydrophobic porous polytetrafluoroethylene (PTFE) support, thereby being referred to as a PTFE gas diffusion electrode. Optionally, the production of the cathode can include pre-sputtering a layer of copper to improve the electrical conductivity thereof. For example, experimental results provided further below include experiments with a cathode being prepared by steps including pre-sputtering a 200 nm-thick polycrystalline Cu layer to improve electrical conductivity (see Experimental Results for details). Referring to the scanning and transmission electron microscopy (SEM and TEM, respectively) of FIG. 10, the produced cathode has a surface morphology composed of copper nanoparticles bonded by several tens of nm-thick PFSA ionomer films.

(76) The anodic compartment of the system includes an anode that comprises a catalyst that can be referred to as an anodic catalyst. For example, the anodic catalyst can include Pt, IrO.sub.2, Pd, Au, Ni.sub.3P, NiFe alloys or any combinations thereof. In some implementations, the anode can further include a hydrophilic and porous support. For example, the hydrophilic and porous support can include, without being limited to, a hydrophilic and highly porous carbon fiber cloth substrate, Ti felt, Ni mesh, Cu mesh, or any combination thereof. In some implementations, the anode can further include an ionomer provided as a layer or film to bond the catalyst particles. For example, the anode can be prepared in accordance with the details provided in the Experimental Results section, to comprise a homogeneous blend of Pt/C nanoparticles and PFSA ionomer on a hydrophilic and highly porous carbon fibre cloth substrate. As seen in the SEM image of FIG. 11, the anode can be composed of macroporous carbon fibres that are homogeneously covered by Pt/C nanoparticles and PFSA composites (inset in SEM image of FIG. 11). As seen in the TEM image of FIG. 12, the diameter of Pt nanoparticles can be in the range of 5 to 10 nm. Referring to the energy-dispersive X-ray spectroscopy (EDS) elemental mapping shown in FIGS. 13 and 14, Pt is shown to be evenly distributed on the surface of the carbon nanoparticles.

(77) In some implementations, the cathode can be prepared by depositing copper nanoparticles and a perfluorosulfonic acid (PFSA) ionomer on a hydrophobic porous carbon paper.

(78) In some implementations, the anode can be prepared by depositing metal nanoparticles onto above-mentioned hydrophilic and highly porous substrates via electrochemical deposition or solvent-thermal deposition.

(79) On Engineering Cathode and Anode to Facilitate Ethylene Faradaic Efficiency (FE) and Reduce/Prevent Oxygen FE Simultaneously

(80) In some implementations, controlling the anodic reaction can include favouring the OOR by selecting the organic liquid-phase precursor of the anolyte in the group consisting of glucose, glycerol, furfural, 5-hydroxymethylfurfural, alcohols, starch, cellulose lignin, and any mixtures thereof. The alcohols can include ethanol, n-propanol, iso-propanol, methanol or benzyl alcohol, or any mixtures thereof. For example, the liquid precursor of the anolyte can be glucose and controlling the anodic reaction includes favouring a glucose oxidation reaction (GOR). Optionally, the anodic reaction can be further controlled by adjusting an active organic concentration of glucose in the anolyte.

(81) The present techniques allow maintaining a low OER FE to facilitate/maximize GOR FE, and thereby achieving recovery of an anodic gaseous stream being substantially pure CO.sub.2. In the present CO.sub.2RR-GOR system, the cathodic and anodic catalysts can be tailored to the CO.sub.2 recovery strategy.

(82) In some implementations, controlling the anodic reaction to avoid production of gaseous O.sub.2 from the crossover CO.sub.2 can include at least one of adjusting a catalyst loading of the anode, and adjusting a catalyst loading of the cathode. For example, favouring OOR instead of OER at the anode can include balancing a catalyst loading between the anode and the cathode. For example, the catalyst loading can be a metal loading of the electrode.

(83) In some implementations, controlling the anodic reaction to avoid O.sub.2 production by OER can include adjusting the catalyst loading of the anode. For example, the catalyst loading of the anode can be adjusted between 0.1 mg/cm.sup.2 and 10 mg/cm.sup.2, preferably between 0.2 mg/cm.sup.2 and 9.5 mg/cm.sup.2, more preferably between 0.4 mg/cm.sup.2 and 9 mg/cm.sup.2, or between 0.5 mg/cm.sup.2 and 5 mg/cm.sup.2. The catalyst loading thus depends on the surface area of the anode catalyst. The catalyst loading of the cathode can amount to a range between 20% and 30% of the catalyst loading of the anode, optionally between 22% and 28%. For example, the catalyst loading of the cathode can amount to 25% of the catalyst loading at the anode.

(84) Referring to graphs of FIGS. 15 and 16, linear scan voltammetry (LSV) measurements were conducted to investigate the electrochemical response of the CO.sub.2RR-GOR system as encompassed herein. The tested CO.sub.2RR-GOR system included a cathode having a copper loading of 0.5 mg cm 2 and an anode having a platinum loading of 2 mg cm.sup.2. Without adding glucose to the anolyte (CO.sub.2RR-OER), the electrolyser delivered a current density of 94 mA cm.sup.2 at a full-cell voltage of 3 V. At 100 mA cm.sup.2, when glucose was introduced as the liquid precursor of the anolyte, with a glucose concentration gradually increasing from 0.1 M to 0.5 M and 1 M, the full-cell voltage decreased from 2.90 V to 2.18 and 2.23 V. The proximity between the full-cell voltages for the glucose concentrations of 0.5 M and 1 M is attributable to an electrokinetic limitation of the anode. A further increase in the glucose concentration to 2 M increased full-cell voltage, i.e., 2.40 V at 100 mA cm.sup.2 due to the excess coverage of Pt with glucose and oxidation intermediates. For example, a 1 M glucose concentration was adopted for further performance investigations.

(85) Referring to the graphs of FIGS. 16 and 17, the LSV and chronopotentiometry measurements were performed to investigate a voltage-current density dependence at various temperatures. Elevating the operating temperature from 20 C. to 35 C. lowers the full-cell voltage by 0.1-0.3 V in a wide range of current densities from 80 mA cm.sup.2 to 160 mA cm.sup.2 (see FIGS. 16 and 17), attributed to accelerated electrochemical kinetics. A similar full-cell voltage reduction was observed as the operating temperature increased from 35 C. to 50 C. For example, the process can include adjusting an operating temperature between 20 C. and 50 C.

(86) Known catalysts have typical mass loadings that include a cathode Cu loading and an anode Pt loading of 1 mg cm.sup.2 and 0.5 mg cm.sup.2, respectively. Referring to the graph of FIG. 18, when applying these typical mass loadings in the presently encompassed CO.sub.2RR-GOR system, high full-cell voltages of >3.4 V were obtained when seeking to operate above 100 mA cm.sup.2, showing little advantage over CO.sub.2RR-OER systems. Referring to the graph of FIG. 19, the high full-cell voltage appeared to degrade the selectivity of GOR over OER, leading to an anodic O.sub.2 FE of >8%.

(87) The techniques described herein include adjusting a catalyst mass loading on at least one of the cathode and anode to maximize CO.sub.2RR product selectivity and minimize anodic OER selectivity simultaneously. Consequently, upon separation of the anodic product mixture, an anodic gaseous stream can be directly recovered with a high purity of >99% in CO.sub.2.

(88) Controlling the anodic reaction to avoid O.sub.2 production by OER can include adjusting the catalyst loading of the anode. For example, referring to FIG. 18, optimizing the Pt loading at the anode was shown to reduce the full-cell voltage to <2.4 V, and consequently the O.sub.2 FE to <1% (see FIG. 19) at the current density of 120 mA cm.sup.2. However, operating at this current density, the CO.sub.2RR selectivity toward ethylene was found to be about 30% (see the graph of FIG. 20), which is significantly lower than the 40-45% benchmark for a copper catalyst at the cathode. To achieve this benchmark, the system was operated at 200 mA cm.sup.2 (see FIG. 20) with a full-cell voltage of 3.23V and O.sub.2 FE of 7% (see FIGS. 18 and 19). FIG. 20 illustrates the performance of the CO.sub.2RR-GOR systems with an anode catalyst loading of 2.0 mg/cm.sup.2 while in FIG. 21, the anode loading is 0.5 mg/cm.sup.2.

(89) Controlling the anodic reaction to avoid O.sub.2 production by OER can further include adjusting the catalyst loading of the cathode. Still referring to FIG. 18, one can see that increasing the CO.sub.2RR selectivity toward ethylene cannot be achieved by further increasing the Pt loading, and thus it is proposed herein to adjust both the anode catalyst loading and the cathode catalyst loading, for example in accordance with one another. It was observed that the electrochemical surface area of Pt had reached its maximum at the Pt loading of 2 mg cm-2. Tuning the Cu loading in accordance with the Pt loading changed the current density required to maximize the ethylene FE. In some implementations, the copper loading of the cathode can be 0.5 mg cm.sup.2 and the platinum loading of the anode can be 2 mg cm.sup.2 to achieve maximum ethylene FE and low oxygen FE simultaneously at industrial-relevant current densities.

(90) The anode can further have a carbon loading with the carbon serving as a conductor and/or substrate for the metal catalyst, such as Pt. For example, the carbon loading of the anode can be further adjusted between 0.5 mg/cm.sup.2 and 60 mg/cm.sup.2, preferably between 1 mg/cm.sup.2 and 50 mg/cm.sup.2. For example, the anodic carbon to catalyst ratio can be ranging between 2 and 10, between 3 and 9, or between 4 and 8.

(91) Recycling CO.sub.2

(92) The present techniques facilitate the use of a dilute stream of CO.sub.2 as the gaseous CO.sub.2-containing stream being the CO.sub.2RR-OOR electrolyser feedstream, and recovering the crossover CO.sub.2 as a stream of pure CO.sub.2. The stream of pure CO.sub.2 can be further fed to an electrolyser to produce CO and/or other multi-carbon products (C.sub.2+). A combination of electrolysers can be referred to herein as an assembly of electrolysers.

(93) FIG. 3 illustrates the zero-gap CO.sub.2RR-OOR electrolyser as encompassed herein which can be combined in an assembly with any known downstream electrolyser systems, as shown for example in FIGS. 4 to 6. FIG. 8 illustrates the recycling of the recovered crossover CO.sub.2 to the same electrolyser as part of the CO.sub.2-containing stream. These example configurations of electrolysers eliminate the need for a stream of pure gaseous CO.sub.2 as the initial electrolyser feedstream to produce industrial chemicals which offers significant cost advantage (i.e., CO.sub.2 capture cost is avoided) and improved overall carbon footprint and life-cycle energy efficiency of such chemicals.

(94) FIG. 3 represents a conceptual design of CO.sub.2-ORR operating GOR as an organic oxidation reaction which results in the generation of H.sup.+ at the anode. A portion of CO.sub.2 present in the gaseous CO.sub.2-containing stream serving as a cathode feed stream can react electrochemically to produce CO.sub.2RR products while the remaining fraction reacts with OH ions forming carbonate and bicarbonate ions. 50-70 vol % of CO.sub.2 fed into the cathode based on the total volume of the gaseous CO.sub.2 stream gets converted into carbonates/bicarbonates (see the study of Larrazbal G. O., et al., entitled A comprehensive approach to investigate CO.sub.2 reduction electrocatalysts at high current densities (Acc. Mater. Res., 2021, 2, 220-229). The anion-exchange membrane (AEM) allows the transport of carbonate or bicarbonate ions to the anode where such ions can react with generated H.sup.+ and regenerate the pure stream of CO.sub.2.

(95) Referring to the combination of electrolysers as shown in FIGS. 3 and 4, the assembly further includes in series a downstream SOEC (solid oxide electrolyser cell) and any COR electrolyser, such as an MEA (Membrane Electrode Assembly) or another alkaline flow cell (also referred to as a one-gap electrolyser). The process can include feeding the pure gaseous CO.sub.2 stream, which was recovered from the anodic product mixture of the CO.sub.2RR-OOR electrolyser to the SOEC to produce CO. The assembly can further include any carbon oxide reduction reaction (CORR) electrolyser, such as an MEA (Membrane Electrode Assembly) or another alkaline flow cell, to generate additional multi-carbon products and enhance CO.sub.2 utilization. The SOEC can be operated at high temperature, for example at a temperature of at least 350 C. to produce the CO. For example, the SOEC can be operated at a temperature ranging between 350 C. and 800 C., between 400 C. and 800 C., between 450 C. and 800 C., or between 500 C. and 800 C.

(96) Referring to the combination of electrolysers as shown in FIGS. 3 and 5, the assembly further includes in series a first MEA type electrolyser and any COR electrolyser, such as a second MEA or another alkaline flow cell. The process can include feeding the pure gaseous CO.sub.2 stream, which was recovered from the anodic product mixture of the CO.sub.2RR-OOR electrolyser to the MEA electrolyser to produce CO, and further feeding the CO to the second MEA/alkaline flow cell to generate multi-carbon products. The first MEA electrolyser is configured to be operated at a temperature below 100 C. to produce CO which can be utilized as the feed to any CO-electrolyser. For example, the first MEA electrolyser can be operated at a temperature between 0 C. and 100 C., between 0 C. and 90 C., between 0 C. and 80 C., or between 0 C. and 70 C.

(97) Referring to the combination of electrolysers as shown in FIGS. 3 and 6, the assembly can further include an MEA-type electrolyser that is supplied with the pure gaseous CO.sub.2 stream (crossover CO.sub.2) produced from the CO.sub.2RR-OOR electrolyser. The MEA-type electrolyser is configured to be operated at a temperature below 100 C. to produce products of electrochemical reductions such as C.sub.2+ products. For example, the MEA-type electrolyser can be operated at a temperature between 0 C. and 100 C., between 0 C. and 90 C., between 0 C. and 80 C., or between 0 C. and 70 C.

(98) Referring to FIG. 8, the assembly can include the zero-gap CO.sub.2RR-OOR electrolyzer as described herein being an MEA electrolyzer. The process includes recovering the anodic product mixture from the anodic compartment, the anodic product mixture comprising the crossover CO.sub.2; separating the crossover CO.sub.2 from the anodic product mixture to produce the CO.sub.2-depleted liquid stream and the recovered pure gaseous CO.sub.2 stream. The process further includes redirecting the recovered pure gaseous CO.sub.2 stream to the cathodic compartment of the MEA electrolyzer via a recycle line as a portion of the gaseous CO.sub.2-containing stream to maximize CO.sub.2 utilization. For example, the MEA electrolyzer can be operated at a temperature between 0 C. and 100 C., between 10 C. and 90 C., between 20 C. and 80 C., or between 30 C. and 70 C.

(99) The present CO.sub.2 recycling strategy requires a high CO.sub.2 recovery rate (defined as the fraction of the recovered CO.sub.2 flow rate to the rate of CO.sub.2 crossover). Referring to the graph of FIG. 27, one can see that the amount of CO.sub.2 collected at the anode is in agreement with the stoichiometry of OH generated and electrons transferred 16, indicating a CO.sub.2 recovery rate approaching 100% (see FIG. 28). Additionally, the anodic CO.sub.2 flow rate is three orders of magnitude larger than that of O.sub.2 (see FIG. 27), indicating the anodic gas stream is at least 99% CO.sub.2 (see FIG. 28). This low/absent level of O.sub.2 enables direct recycling of this anode gas stream to the cathode, without the need for separation and associated energy costs. Experimental observations are in good agreement with the mass balance analysis provided in FIG. 7 and indicate the potential for high carbon efficiency without any energy penalty in zero-gap, neutral media CO.sub.2RR MEA electrolysers.

(100) Suppressing CO.sub.2RR liquid products and subsequent crossover.

(101) Ethylene production via CO.sub.2RR is accompanied by cathodic liquid-phase products such as ethanol, acetate and propanol, much of which can cross the AEM to join the anodic product mixture. Cathode-to-anode crossover of liquid products remains a challenge in CO.sub.2RR systems as this liquid products risk oxidation and dilution in the anolyte.

(102) In some implementations, adjusting the temperature of the electrolyser can control, e.g. reduce, the crossover of cathodic liquid products. Increasing the temperature from 20 C. to 50 C., the FEs toward the major gas products of CO.sub.2RR (C.sub.2H.sub.4 and CO) were found to be increased from 48% to 56% at a constant current density of 100 mA cm.sup.2 (see FIGS. 22, 23, 24 and 29), and the FE toward the liquid products of CO.sub.2RR was seen to decrease from 24% to 9%. Thus, in some implementations, adjusting the temperature can reduce the crossover of cathodic liquid products, such as ethanol and n-propanol (see FIGS. 30 and 31) to the anode side, attributable to a higher rate of evaporation into the cathode gas product stream. As a result, referring to FIG. 32, the weight ratio of the liquid CO.sub.2RR products to the GOR target products in the anolyte stream was <1% at 50 C., in contrast to 1.4% at 35 C. Thus, operating at modestly elevated temperatures, for example between 30 C. and 50 C., can benefit the CO.sub.2RR-GOR system by reducing full-cell voltage and by suppressing the formation and crossover of liquid CO.sub.2RR products.

(103) It should be understood that any one of the above-mentioned implementations of the CO.sub.2RR-OOR system and related process may be combined with any other of the aspects thereof unless two aspects clearly cannot be combined due to their mutual exclusivity.

EXPERIMENTAL RESULTS

(104) Materials

(105) Potassium bicarbonate (KHCO.sub.3, 99.7%), D-glucose (99.5%), copper nanoparticles (25 nm), Nafion 1100W (5 wt. % in a mixture of lower aliphatic alcohols and water) and Pt/C (40 wt. % Pt on Vulcan XC72) were purchased from Sigma Aldrich and used as received. Aquivion D79-25BS ionomer was purchased from Fuel Cell Store. Piperion (40 m) was used as the anion-exchange membrane, purchased from W7Energy and stored in 0.5M KOH. The water used in this study was 18 MO Milli-Q deionized-(DI-)water.

(106) Electrodes

(107) For the CO.sub.2RR, we prepared the gas diffusion electrodes (GDEs) by spray-depositing a catalyst ink dispersing 1 mg mL-1 of Cu nanoparticles and 0.25 mg mL-1 of Nafion 1100W in methanol onto a PTFE substrate that pre-sputtered with a 200 nm thick polycrystalline Cu layer. The Cu sputtering procedure was described in detail in the previous reports. The mass loading of Cu NPs on the GDE was tuned between 0.5 to 1.0 mg/cm.sup.2. The GDEs were dried in the air overnight prior to experiments.

(108) For the GOR anode electrodes, a commercially available Pt/C was first physically mixed with an ionomer (Aquivion D79-25BS) in a glass beaker and then sonicated for 1 h. The resulting catalyst ink was then spray-coated on both sides of the hydrophilic carbon cloth until the Pt loading of 0.5 to 2.0 mg cm.sup.2 was achieved.

(109) Characterizations

(110) Scanning Electron Microscopy

(111) Scanning electron microscopy (SEM) images of the cathode and anode were captured by an FEI Quanta FEG 250 environmental SEM.

(112) Transition Electron Microscopy

(113) Transition electron microscopy (TEM) images and elemental mappings were acquired by an FEI Titan 80-300 kV TEM microscope.

(114) X-Ray Photoelectron Spectra

(115) X-ray photoelectron spectra (XPS) of the electrodes were determined by a model 5600, PerkinElmer using a monochromatic aluminum X-ray source.

(116) .sup.1H Nuclear Magnetic Resonance

(117) 1H NMR spectra were determined by the Agilent DD2 500 spectrometer.

(118) High-Performance Liquid Chromatography (HPCL)

(119) The by-products of the GOR were measured by high-performance liquid chromatography (UltiMate 3000 HPLC) equipped with an Aminex HPX-87H column (Bio-Rad) and a reflective index detector. The eluent was 0.05 M H.sub.2SO.sub.4, and the column was kept at 60 C.

(120) Assembling of the CO.sub.2RR-GOR System

(121) The MEA set (5 cm.sup.2) was purchased from Dioxide Materials. A cathode was cut into a 2.5 cm2.5 cm piece and placed onto the MEA cathode plate with a flow window with a dimension of 2.23 cm2.23 cm. The four edges of the cathode were sealed by copper tapes and then Kapton tapes, and make sure the tapes did not cover the flow window. A Piperion AEM (3 cm3 cm) was carefully placed onto the cathode. A gasket with a 2.23 cm2.23 cm window was placed on the cathode. The Pt/C loaded carbon cloth anode (2 cm2 cm) was placed onto the AEM.

(122) Electrochemical Measurements

(123) The cathode side of the MEA was fed with CO.sub.2 flow (0.18 to 10 sccm per cm.sup.2 of electrode area, 10 sccm cm.sup.2 if not specified) that comes from both CO.sub.2 feedstock and anodic gas stream. The anode side was circulated with a solution containing 1M KHCO.sub.3 and glucose with various active organic concentrations (0 to 2M) at 10 mL/min by a peristaltic pump. A gas-tight glass bottle with four in/out channels (gas inlet, gas outlet, liquid inlet and liquid outlet) was used as the anolyte reservoir and gas-liquid separator. In typical CO.sub.2RR-GOR performance evaluations, the gas inlet channel was sealed, and the gas outlet channel was connected to a Y shape tubing connector.

(124) Since the anolyte reservoir/gas-liquid separator is gas tight, the CO.sub.2 pressure between the feedstock stream and the anodic stream will eventually balance and promote a steady flow rate from both sides. The electrochemical measurements were performed with a potentiostat (Autolab PGSTAT204 with 10A booster). All the performance metrics were recorded after at least 1000 seconds of stabilization at a specific condition. The full-cell voltages reported in this work are not iR corrected.

(125) Product Analysis

(126) The CO.sub.2RR gas products, oxygen, and CO.sub.2 were analyzed by injecting the gas samples into a gas chromatograph (Perkin Elmer Clarus 590) coupled with a thermal conductivity detector (TCD) and a flame ionization detector (FID). The gas chromatograph was equipped with a Molecular Sieve 5A Capillary Column and a packed Carboxen-1000 Column with argon as the carrier gas. The volumetric gas flow rates in and out of the cell were measured with a bubble column. The FE of a gas product is calculated as follows:

(127) ${\mathrm{FE}}_i=x_i\frac{\mathrm{VP}}{\mathrm{RT}}\frac{n_{i}F}{J}$

(128) Where x.sub.i is the volume fraction of the gas product i, V is the outlet gas flow rate in L s.sup.1, P is atmosphere pressure 101.325 kPa, R is the ideal gas constant 8.314 J mol.sup.1 K.sup.1, T is the room temperature in K, n; is the number of electrons required to produce one molecule of product F is the Faraday Constant 96485 C mol.sup.1, and J is the total current in A. To analyze the anodic gas stream component, the gas outlet channel of the anolyte reservoir was disconnected from the tubing for circulating to the cathode. A 20 sccm argon flow was input from the gas inlet channel of the anolyte reservoir as the carrier gas to promote the accurate analysis of CO.sub.2 and O.sub.2 components in the anode gas.

(129) The liquid products from the cathode side of the SC-MEA were collected using a cold trap cooled to 0 C. The collected liquid from the cathode side and the anolyte were quantified separately by the proton nuclear magnetic resonance spectroscopy (1H NMR) on an Agilent DD2 500 spectrometer in D20 using water suppression mode and dimethyl sulfoxide (DMSO) as the internal standard. Typical 1H NMR spectra can be found in FIGS. 12 and 13. For each plot of liquid product quantification, fresh anolyte was used, and the duration of the collection is 30 minutes. The FE of a liquid product is calculated as follows:

(130) ${\mathrm{FE}}_i=m_i\frac{n_{i}F}{\mathrm{Jt}}$ Where m.sub.i is the quantity of the liquid product i in mole, t is the duration of product collection (1800 seconds).
CO.sub.2RR-GOR System Performance

(131) When run at 100 mA cm.sup.2 and 50 C., the CO.sub.2RR-GOR system provides a full-cell voltage of 1.800.1 V, representing a 1.6 V lower voltage than the conventional CO.sub.2RR-OER system at the same current density and temperature. This low full-cell voltage can be attributed to the lower thermodynamic potential of GOR than OER (0.05 vs. 1.23 V) and the anodic catalyst's high activity toward GOR. Such a low full-cell potential significantly reduces electricity demand (Table 1). At 100 mA cm.sup.2, the system delivers ethylene FEs of 42%, 48%, and 44% at 20 C., 35 C., and 50 C. (see FIGS. 22 to 24 and tables 2 and 3).

(132) TABLE-US-00001 TABLE 1 (1/2) Energy assessment comparison between the state-of-art CO.sub.2-to-ethylene CO.sub.2RR devices. This work (max Neutral- Acidic Acidic carbon Metrics MEA .sup.a flow cell .sup.a MEA efficiency) Reaction pair CO.sub.2R- CO.sub.2R- CO.sub.2R- CO.sub.2R- OER OER OER GOR Cell type MEA Flow cell MEA MEA Electrolyte Neutral Acidic Acidic Neutral Full-cell voltage (V) 3.75 4.20 3.80 1.90 Ethylene FE (%) 45 28 34 26 Anode O.sub.2 FE (%) 100 100 100 Anode GOR FE (%) 0 0 0 Current density (mA cm.sup.2) 120 1200 100 100 Input CO.sub.2 flow rate 10 3 0.8 0.18 (sccm cm.sup.2) CO.sub.2 carbon efficiency (%) 3.6 78 31 78 CO.sub.2-to-ethylene carbon 1.2 28.4 10.5 36 efficiency (%) Demonstrated stability (Hours) 100 .sup.b 14 .sup.c 8 .sup.d Energy distributions (GJ per ton ethylene) Electrolyser electricity 345 620 462 302 Cathode gas separation 38 17 23 17 Anode gas separation 71 0 0 0 Overall energy 499 637 485 319 .sup.a All the data sets from references are the ones that consume the least overall energy for producing one ton of ethylene. .sup.b Recorded with the CO.sub.2 carbon efficiency indicated in the same column above. .sup.c Recorded with a CO.sub.2 carbon efficiency of 20%. .sup.d Recorded with a CO.sub.2 carbon efficiency of 1.8%. .sup.e The sum of the GOR product FEs obtained from NMR (FIG. 37) and HPLC

(133) TABLE-US-00002 TABLE 1 (2/2) Energy assessment comparison between the state-of-art CO.sub.2-to-ethylene CO.sub.2RR devices This work This work (min Metrics (max FE) energy) Reaction pair CO.sub.2R- CO.sub.2R- GOR GOR Cell type MEA MEA Electrolyte Neutral Neutral Full-cell voltage (V) 1.80 1.90 Ethylene FE (%) 45 32 Anode O.sub.2 FE (%) 0.6 Anode GOR FE (%) 94.sup.e Current density (mA cm.sup.2) 100 100 Input CO.sub.2 flow rate 10 0.36 (sccm cm.sup.2) CO.sub.2 carbon efficiency (%) N/A 48 CO.sub.2-to-ethylene carbon efficiency (%) 1.1 23 Demonstrated stability (Hours) 80 .sup.b Energy distributions (GJ per ton ethylene) Electrolyser electricity 165 246 Cathode gas separation 147 16 Anode gas separation 0 0 Overall energy 312 262 .sup.b Recorded with the CO.sub.2 carbon efficiency indicated in the same column above. .sup.eThe sum of the GOR product FEs obtained from NMR (FIG. 37) and HPLC

(134) TABLE-US-00003 TABLE 2 The CO.sub.2RR product distribution of the copper-based catalyst in the MEA at various current densities at 35 C. Current Cell Faradaic efficiency (%) density voltage n- J.sub.ethylene J.sub.CO2RR (mA cm.sup.2) (V) C.sub.2H.sub.4 EtOH PrOH Acetate CO CH.sub.4 H.sub.2 (mA cm.sup.2) (mA cm.sup.2) 80 2.00 0.05 42.1 7.2 4.9 3.9 2.9 <0.1 24.1 33.7 48.8 100 2.21 0.06 48.0 10.6 6.2 4.4 1.9 <0.1 33.0 48.0 71.1 120 2.30 0.04 41.8 11.3 4.3 5.0 1.4 <0.1 43.4 50.2 76.6 140 2.43 0.06 33.3 9.1 2.8 5.1 1.4 <0.1 46.4 46.7 72.5 160 2.55 0.03 26.9 8.9 2.6 4.1 1.1 <0.1 60.6 43.0 69.7 MEA operating conditions: anolyte: 1M KHCO.sub.3 + 1M glucose; anolyte flow rate: 20 mL/min; CO.sub.2 inlet flow rate: 10 sccm cm.sup.2; cell temperature: 35 C.; and cell pressure: atmospheric pressure.

(135) TABLE-US-00004 TABLE 3 The CO.sub.2RR product distribution of the copper-based catalyst in the MEA at various current densities at 50 C. Current Cell Faradaic efficiency (%) density voltage n- J.sub.ethylene J.sub.CO2RR (mA cm.sup.2) (V) C.sub.2H.sub.4 EtOH PrOH Acetate CO CH.sub.4 H.sub.2 (mA cm.sup.2) (mA cm.sup.2) 80 1.76 0.02 33.9 4.7 3.4 2.4 8.3 <0.1 33.9 27.1 43.2 100 1.81 0.04 43.5 5.9 2.8 2.0 12.0 <0.1 43.5 43.5 64.5 120 2.08 0.02 41.0 5.3 1.9 2.2 11.1 <0.1 41.0 49.2 72.2 140 2.26 0.04 38.6 5.0 1.1 2.2 6.6 <0.1 38.6 54.1 72.4 160 2.40 0.03 34.1 4.7 1.0 2.0 3.9 <0.1 34.1 54.6 72.4 MEA operating conditions: anolyte: 1M KHCO.sub.3 + 1M glucose; anolyte flow rate: 20 mL/min; CO.sub.2 inlet flow rate: 10 sccm cm.sup.2; cell temperature: 50 C.; and cell pressure: atmospheric pressure.

(136) In addition, the selectivity of the GOR was investigated for a wide range of current densities (from 80 mA cm.sup.2 to 160 mA cm.sup.2) and operating temperatures (see FIG. 25, tables 4 and 5).

(137) TABLE-US-00005 TABLE 4 The glucose oxidation reaction (GOR) product distribution of the PtC catalyst in the MEA at various current densities and 35 C. Current Cell density voltage Faradaic efficiency (%) J.sub.gluconate (mA cm.sup.2) (V) Gluconate Formate Glucarate Glucuronate Total (mA cm.sup.2) 80 2.00 0.05 34.2 6.8 6.1 18.7 65.8 27.4 100 2.21 0.06 43.6 3.9 5.4 13.6 66.5 43.6 120 2.30 0.04 48.1 3.2 4.8 11.2 67.3 57.7 140 2.43 0.06 51.4 2.5 3.9 7.4 65.2 72.0 160 2.55 0.03 46.7 2.4 2.7 4.3 56.1 74.7 MEA operating conditions: anolyte: 1M KHCO.sub.3 + 1M glucose; anolyte flow rate: 20 mL/min; cell temperature: 35 C.; and cell pressure: atmospheric pressure.

(138) TABLE-US-00006 TABLE 5 The glucose oxidation reaction (GOR) product distribution of the PtC catalyst in the MEA at various current densities. Current Cell density voltage Faradaic efficiency (%) J.sub.gluconate (mA cm.sup.2) (V) Gluconate Formate Glucarate Glucuronate Total (mA cm.sup.2) 80 1.76 0.02 37.1 7.3 5.7 21.2 71.3 37.1 100 1.81 0.04 48.6 4.7 4.9 15.9 74.1 48.6 120 2.08 0.02 52.3 3.9 5.0 12.7 73.9 62.8 140 2.26 0.04 56.3 3.3 3.8 6.2 69.6 78.8 160 2.40 0.03 56.2 3.0 2.1 3.5 64.8 89.9 MEA operating conditions: anolyte: 1M KHCO.sub.3 + 1M glucose; anolyte flow rate: 20 mL/min; cell temperature: 50 C.; and cell pressure: atmospheric pressure.

(139) With the temperature increasing from 20 C. to 50 C., we detected a slight increase in anolyte pH (from pH 7.9 to 8.3, FIG. 26), attributable to the lower solubility of CO.sub.2 in warmer anolyte. Gluconate was detected as the major GOR product (>49% FE), achieving a plateau of 56% at 140 mA cm.sup.2. The FEs toward oxygen remained <3% at current densities from 80 mA cm.sup.2 and 160 mA cm.sup.2 (<1% at 100 mA cm.sup.2 and a full-cell voltage of 1.80 V) owing to the sluggish kinetics of OER (see FIG. 15).

(140) Carbon efficiency upper limits in the proposed CO.sub.2RR-OOR system were studied. A common approach to determine carbon efficiency upper limits is restricting the CO.sub.2 availability at the cathodic stream and measuring a ratio between [CO.sub.2 converted to products] and [total CO.sub.2 feeding].

(141) Decreasing an input CO.sub.2 flow rate increases the carbon efficiency (see Table 6 and FIG. 33).

(142) TABLE-US-00007 TABLE 6 The CO.sub.2RR product distribution of the copper-based catalyst in the MEA at 100 mA cm.sup.2 at various CO.sub.2 flow rates. CO.sub.2 Cell Faradaic efficiency (%) flow rate voltage n- Carbon CO.sub.2-to-C.sub.2H.sub.4 (sccm cm.sup.2) (V) C.sub.2H.sub.4 EtOH PrOH Acetate CO CH.sub.4 H.sub.2 efficiency conversion 0.18 1.9 0.1 25.7 5.8 1.1 1.3 5.1 4.8 46.1 75.3 36.2 0.36 1.9 0.1 32.2 8.4 1.1 3.5 7.1 3.4 42.2 48.5 23.0 0.96 1.9 0.1 36.3 10.4 1.9 1.6 8.1 2.3 37.6 20.3 6.8 1.46 1.9 0.1 39.4 10.9 1.8 1.6 9.3 1.5 33.2 14.4 4.5 MEA operating conditions: anolyte: 1M KHCO.sub.3 + 1M glucose; anolyte flow rate: 20 mL/min; cell temperature: 50 C.; and cell pressure: atmospheric pressure.

(143) At an inlet CO.sub.2 flow rate of 0.18 sccm cm.sup.2 (flow rates are normalized by electrode area), the system delivered a total C.sub.2+ FE of 44% at a constant current density of 100 mA cm.sup.2 and a full-cell voltage of 1.90 V, corresponding to a carbon efficiency of 75% toward all CO.sub.2RR products (total carbon efficiency, see FIGS. 33 and 34), exceeding a typical upper limit of carbon efficiency in neutral media CO.sub.2RR electrolysers. At these conditions the ethylene FE stabilizes at 26%, corresponding to a CO.sub.2-to-ethylene carbon efficiency of 36% (FIG. 33). This carbon efficiency is 1.4-fold greater than the theoretical upper limit of 25% in CO.sub.2-to-ethylene conversion in conventional, neutral-media, AEM-based electrolysers. Restricting the flow rate results in a significant increase in the hydrogen FE (FIG. 34), which can be attributed to the mass transfer limitation of CO.sub.2. At the anode, GOR can maintain consistent selectivity and productivity, independent from CO.sub.2 availability in the cathodic gas stream (see FIG. 34 and Table 7).

(144) TABLE-US-00008 TABLE 7 The glucose oxidation reaction (GOR) product distribution of the PtC catalyst in the MEA at 100 mA cm.sup.2 at various CO.sub.2 flow rates. CO.sub.2 Cell flow rate voltage Faradaic efficiency (%) J.sub.gluconate (sccm cm.sup.2) (V) Gluconate Formate Glucarate Glucuronate Total (mA cm.sup.2) 0.18 1.9 0.1 57.6 6.6 6.5 15.6 86.3 57.6 0.36 1.9 0.1 52.9 7.1 6.3 13.1 80.8 52.9 0.96 1.9 0.1 55.0 5.9 5.8 12.7 79.4 55.0 1.46 1.9 0.1 56.9 6.0 5.9 14.4 83.2 56.9 MEA operating conditions: anolyte: 1M KHCO.sub.3 + 1M glucose; anolyte flow rate: 20 mL/min; cell temperature: 50 C.; and cell pressure: atmospheric pressure.

(145) A trade-off between carbon efficiency and ethylene FE is typical of CO.sub.2-to-ethylene electrolysis (FIGS. 33 and 34). A higher carbon efficiency can reduce the energy demand for cathode separation, but the accompanying decrease in ethylene FE increases the specific electrolyser energy demand. To reconcile these metrics, the total input energy (electricity, cathode separation and anode separation per ton of ethylene produced) of various CO.sub.2RR approaches can be assessed, based on a previously established model (see Experimental Results section). The present CO.sub.2RR-GOR system was assessed to achieve the lowest energy consumption of 259 GJ per ton of ethylene, with the input CO.sub.2 flow rate being 0.36 sccm cm.sup.2, and the total carbon efficiency being 48% toward all CO.sub.2RR products. The FE toward C.sub.2+ and ethylene were shown to be 45% and 32%, respectively (Table 1).

(146) Compared to state-of-art conventional CO.sub.2-to-ethylene systems (i.e., MEAs based on AEM and neutral electrolyte), the present CO.sub.2RR-GOR system eliminates the anodic separation energy (>57 GJ per ton ethylene, see FIG. 2, Table 1). When run under restricted CO.sub.2 availability (0.36 sccm cm.sup.2), the CO.sub.2RR-GOR system can further save about 57 GJ for cathodic separation per ton of ethylene produced. The overall energy intensity of ethylene production is 30% less than the most energy-efficient prior CO.sub.2RR systems among neutral and acidic CO.sub.2-to-ethylene electrolysers (Table 1).

(147) Stability with High Carbon Efficiency

(148) Stability is a prerequisite for the industrial application of CO.sub.2RR. However, long-term operation of CO.sub.2RR with a high carbon efficiency (e.g., CO.sub.2 carbon efficiency >40%) has not been achieved to date. The best CO.sub.2 carbon efficiency achieved for a run duration of 100 hours was <4%.

(149) Extended CO.sub.2RR operation was performed under conditions that enable the lowest energy intensity of ethylene production. The CO.sub.2RR-GOR system achieved stable electrosynthesis of cathodic C.sub.2+ and anodic products for over 80 hours at a current density of 100 mA cm.sup.2, comparable to the stability of conventional MEAs. The system maintained an average full-cell voltage of 1.900.1 V, an average total C.sub.2+FE of 42%, and an average carbon efficiency of about 45% toward all CO.sub.2RR products (see FIG. 35 and Table 8).

(150) TABLE-US-00009 TABLE 8 The CO.sub.2RR product distribution of the copper-based catalyst in the MEA during extended operation. Faradaic efficiency (%) Carbon Time n- J.sub.ethylene J.sub.CO2RR efficiency (hour) C.sub.2H.sub.4 EtOH PrOH Acetate CO CH.sub.4 H.sub.2 (mA cm.sup.2) (mA cm.sup.2) (%) 15 32.2 8.4 1.1 3.5 7.1 3.4 42.2 32.2 55.7 48.0 28 32.7 9.1 1.2 1.9 6.7 3.6 42.9 32.7 55.2 46.4 42 31.3 9.4 1.1 2.5 6.1 3.8 43.6 31.3 54.2 44.9 56 30.8 8.6 1.0 1.8 5.8 4.1 44.1 30.8 52.1 42.6 71 30.1 9.1 0.9 1.6 5.6 4.4 44.5 30.1 51.7 41.7 MEA operating conditions: anolyte: 1M KHCO.sub.3 + 1M glucose; anolyte flow rate: 20 mL/min; cell temperature: 50 C.; CO.sub.2 inlet flow rate: 0.36 sccm cm.sup.2; and cell pressure: atmospheric pressure.

(151) Similarly, stable GOR productivity was detected throughout (See FIG. 35 and table 9).

(152) TABLE-US-00010 TABLE 9 The glucose oxidation reaction (GOR) product distribution of the PtC catalyst in the MEA during extended operation. Time Faradaic efficiency (%) J.sub.gluconate (hour) Gluconate Formate Glucarate Glucuronate Total (mA cm.sup.2) 15 49.7 7.6 8.1 14.4 79.8 49.7 28 47.3 8.2 8.9 12.8 77.2 47.3 42 45.0 8.1 9.5 13.5 76.1 45.0 56 52.5 8.4 10.3 13.4 84.6 52.5 71 45.0 7.2 11.6 13.8 77.6 45.0 MEA operating conditions: anolyte: 1M KHCO.sub.3 + 1M glucose; anolyte flow rate: 20 mL/min; CO.sub.2 inlet flow rate: 0.36 sccm cm.sup.2; cell temperature: 50 C.; and cell pressure: atmospheric pressure.

(153) Notably, the present CO.sub.2-to-C.sub.2+ system demonstrates high stability while maintaining high carbon efficiency.

(154) The cathodic and anodic liquid-phase products were evaluated from the 1H NMR spectra of catholyte and anolyte, respectively. The typical 1H NMR spectra are shown in FIGS. 36 and 37. Notably, formate can be detected in the anolyte which is majorly ascribed to the oxidation of glucose. However, some of the formate may also come from the CO.sub.2RR. In our previous studies, the Cu nanoparticle usually shows a low formate FE of <1.5%.

(155) Energy Assessment

(156) Energy consumptions for electrolyser electricity, cathodic separation, and anodic separation were evaluated for ethylenethe world's most produced feedstock. State-of-the-art CO.sub.2RR systems, including alkaline flow-cell electrolysers, neutral MEA electrolysers, acidic flow-cell and acidic MEA electrolysers were considered. This consideration is based on the performance metrics including selectivity, productivity, and full-cell voltage-combination of them in turn reflect as energy intensity of producing multi-carbon products (i.e. ethylene). The proximity of these performance metrics will help refine the effect of anodic and cathodic separation on the energy requirement for producing ethylene. Input parameters to the model for all the systems are summarized. The energy assessment model as well as the assumptions are based on a previous work. The majority of these input parameters listed in Table 1 are from the literature. The model considers a production rate of 1 ton/day, with the assumptions of H.sub.2 and O.sub.2 are the only products at the anodic and cathodic streams. The details of calculations for the carbon regeneration (for alkaline flow cell), cathodic separation (for all the electrolysers), and anodic separation (for neutral MEA electrolyser) can be found in the previous work. For acidic flow-cell and MEA electrolysers, no energy cost was assumed to be associated with the anodic separation considering no CO.sub.2 availability at the anodic gas stream.