CO-ELECTROLYSIS CELL DESIGN FOR EFFICIENT CO2 REDUCTION FROM GAS PHASE AT LOW TEMPERATURE

Abstract

A membrane electrode assembly for an electrochemical cell, in particular a co-electrolysis cell for CO.sub.2 reduction reaction, can overcome the problem of parasitic CO.sub.2 pumping from cathode to anode side and, at the same time, maintain good Faradaic efficiency towards CO.sub.2 reduction reaction in a co-electrolysis system where pure or diluted gaseous CO.sub.2 is used. The assembly includes an MEA, having an anode, a cathode, a polymer ion exchange membrane between cathode and anode, an additional ion exchange polymer film between the cathode and the polymer ion exchange membrane and a discontinuous interface formed between the additional polymer film located at the cathode side and the ion exchange membrane.

Claims

1-9. (canceled)

10. An electrochemical co-electrolysis cell for the reduction of carbon dioxide, the cell comprising a membrane electrode assembly (MEA) including: a) an anode electrode layer, a cathode electrode layer, and a cation exchange polymer membrane disposed between said anode electrode layer and said cathode electrode layer; b) said cathode electrode layer including a mixture of a cathode catalyst material and an anion exchange ionomer, and a distribution of said anion exchange ionomer within said cathode electrode layer causing said anion exchange ionomer to: (i) form a discontinuous contact interface with said cation exchange polymer membrane, and (ii) separate said cathode catalyst material from said cation exchange polymer membrane.

11. A membrane electrode assembly (MEA), comprising: an anode electrode layer; a cathode electrode layer; and an ionic conductive polymer membrane disposed between said anode electrode layer and said cathode electrode layer; said ionic conductive polymer membrane being formed of two layers of different ionic conductive polymers forming a discontinuous polymeric interface between said two layers of different ionic conductive polymers.

12. The membrane electrode assembly according to claim 11, wherein: a) one of said two layers of different ionic conductive polymers is an anionic conductive polymer being in contact with a cathode catalyst layer included in said cathode electrode layer; and b) the other of said two layers of different ionic conductive polymers is a cationic conductive layer being in contact with said anode electrode layer.

13. The membrane electrode assembly according to claim 12, wherein said cathode electrode layer has a side in contact with said anionic polymer membrane and has a 3D porous structure including a catalytic active powder and an anionic conductive polymer.

14. The membrane electrode assembly according to claim 12, wherein: said anionic conductive layer is deposited on said cathode catalyst layer; and said anionic conductive layer has a thickness equal to approximately 10% of a thickness of said cationic polymer layer and takes a porosity of said cathode catalyst layer.

15. An electrochemical device capable of transforming CO.sub.2 into fuels or other chemical molecules including CO, HCOO, CH.sub.4, C.sub.2H.sub.4 and alcohols using electricity, water and CO.sub.2 gas, the device comprising an MEA according to claim 10.

16. An electrolyzer for hydrogen production, the electrolyzer comprising an MEA according to claim 10.

17. A fuel cell, comprising an MEA according to claim 10.

18. A process for fabricating a membrane electrode assembly (MEA), the process comprising the following steps: i) fabricating a cathode gas diffusion electrode by coating a mixture of cathode catalyst material and anion exchange ionomer onto one side of a gas diffusion layer; ii) applying an additional coating of anion exchange ionomer onto the cathode gas diffusion electrode; iii) bringing the anion exchange ionomer-coated side of the cathode gas diffusion electrode in contact with one side of a cation exchange polymer membrane; and iv) bringing an opposite side of the cation exchange polymer membrane in contact with an anode electrode layer.

Description

[0039] Preferred embodiments of the present invention are hereinafter described in more detail with reference to the attached drawing which depicts in:

[0040] FIG. 1 a schematic of the MEA of a novel co-electrolysis system design for electrochemical CO.sub.2 reduction from the gas phase;

[0041] FIG. 2 the electrochemical cyclic voltammetry performances of the different cell configurations tested for CO.sub.2 reduction from gas phase;

[0042] FIG. 3 the MS ion current corresponding to CO.sub.2 mass fraction (m/z=44) detected at the anode of a co-electrolysis cell using an alkaline membrane with different anode configurations: Pt/C anode catalyst fed with H.sub.2 (blue), and IrTiO.sub.2 anode catalyst fed with highly humidified N.sub.2 (black);

[0043] FIG. 4 the MS ion current corresponding to CO.sub.2 mass fraction (m/z=44) detected at the anode of a co-electrolysis cell once with an alkaline membrane (black), and once with a bipolar membrane (red); in both cases, the anode electrodes contain IrTiO.sub.2 catalyst for oxygen evolution and the anode side is fed with highly humidified N.sub.2;

[0044] FIG. 5 the MS ion current corresponding to CO.sub.2 mass fraction (m/z=44) detected at the anode of a co-electrolysis cell once using an alkaline membrane (black), and once constructed in the new cell configuration according to the invention (red); in both cases, the anodes consist of Pt/C electrodes fed with H.sub.2 gas.

[0045] The electrochemical reduction of CO.sub.2 results in products such as CO and H.sub.2, known as syngas that can be further converted to fuels and chemicals via industrial processes like Fischer-Tropsch, methane, ethylene, alcohols or other useful chemicals. The selectivity of the CO.sub.2RR mainly depends on the type of electrocatalyst. The co-electrolysis cell disclosed according to the present invention can be used for the production of various types of chemicals, not exclusively syngas.

[0046] The invention comprises a membrane electrode assembly comprising the following parts as shown in FIG. 1: [0047] a) a cathode based on powder electrocatalyst active for CO.sub.2RR; the powder electrocatalyst is mixed with an anion exchange ionomer (which can be in OH.sup., HCO.sub.3.sup., CO.sub.3.sup.2, or other anionic form) and solvents to form a slurry or ink; [0048] b) an electrically conductive porous gas diffusion substrate (carbon gas diffusion layer, titanium, etc.) onto which the slurry/ink is deposited on one side to form a gas diffusion electrode structure containing the anion exchange ionomer within the electrocatalyst layer; [0049] c) an additional thin film of anion exchange ionomer deposited, e.g. by spray coating, directly on top of the cathode catalyst layer of the gas diffusion electrode prepared in steps a) and b). This ionomer thin film covers the surface of the cathode catalyst layer towards the membrane, thus avoiding direct contact between the cathode catalyst and the membrane. Furthermore, this ionomer thin film adjusts to the rough surface morphology of the cathode catalyst layer; [0050] d) a cation exchange membrane between the cathode side, including the additional anion exchange ionomer thin film of step c), and the anode side; [0051] e) an anode based on electrocatalyst active for an oxidative counter reaction for the CO.sub.2RR , such as IrO.sub.2, RuO.sub.2, or a mix thereof for the oxygen evolution reaction (OER), or Pt-based catalysts for the hydrogen oxidation reaction (HOR).

[0052] This specific rational design of the cathode is expected to combine several advantages: Firstly, high catalytically active surface area for the CO.sub.2RR is provided by the alkaline anion exchange ionomer within the cathode catalyst layer providing an optimal alkaline environment for the reduction of CO.sub.2, which is supplied directly from gas phase through the cathode gas diffusion layer (CGDL) to the cathode catalyst layer. Secondly, the additional thin film of alkaline anion exchange ionomer protects the cathode catalyst from direct contact with the acidic cation exchange membrane. This prevention of direct contact is highly important, because, due to the high proton activity of the acidic membrane, direct contact would result in an increased fraction of hydrogen evolution, thus strongly deteriorating the Faradaic efficiency towards CO.sub.2RR.

[0053] Thirdly, the cation exchange membrane guarantees that the carbonate/bicarbonate anions from the alkaline cathode ionomer are stopped from being transferred to the anode side, thus avoiding parasitic CO.sub.2 pumping from cathode to anode side. Instead, carbonate/bicarbonate anions are transported within the cathode alkaline ionomer only to the interface between the additional alkaline ionomer thin film and the acidic membrane where they react with protons to form H.sub.2O and CO.sub.2 according to reactions 7 and 8 above. Since the morphology of the alkaline ionomer thin film is adjusted to the rough surface of the cathode catalyst layer, its interface with the flat two-dimensional acidic membrane is established by discontinuous local contact areas. The spot-like character of this interface is highly beneficial, because CO.sub.2 and H.sub.2O, formed there according to reactions 7 and 8, can laterally escape in plane at the perimeter of these interface spots back to the cathode electrode pore structure. In this way, both delamination of the alkaline/acidic interface and significant CO.sub.2 diffusion to the anode side are prevented. Finally, the use of a cation exchange membrane provides lower electrical resistance than state-of-the-art anion exchange membranes or bipolar membranes enabling increased energetic efficiency of the proposed co-electrolysis cell design.

EXAMPLE 1

MEA Manufacture

Cell New Configuration According To Invention

[0054] Cathode electrode assemblies were fabricated by spraying on a GDL substrate (Sigracet 24 BC) an ink comprising gold black nanoparticles (Sigma Aldrich), an anion exchange ionomer (Fumasep) in HCO.sub.3.sup. form (10 wt % w.r.t. Au catalyst mass) and Milli-Q water. The final loading of Au nanoparticles on the electrode was approx. 3 mg/cm.sup.2. The electrode was dried under an air flow for several hours.

Subsequently, a solution of anion exchange ionomer (Fumasep) in HCO.sub.3.sup. form (5 wt % in ethanol) was sprayed on top of the gold catalyst layer and dried under air flow for one hour. The anode side was a commercial gas diffusion electrode (GDE) Pt/C (Johnson Matthey) with a catalyst loading of 0.4 mg.sub.pt/cm.sup.2.

[0055] The cathode electrode assembly and the anode GDE were placed on both sides of a Nafion XL 100 membrane, with their respective catalyst layers facing towards the membrane, and tested in an electrochemical laboratory cell. The active cell area was 0.5 cm.sup.2. For comparison, two other cells were built with the following compositions:

Cell Alkaline Membrane

[0056] Cathode: Au black catalyst layer containing 10 wt % of anion exchange ionomer (Fumasep) deposited on a GDL substrate

[0057] Membrane: Anion exchange membrane used in carbonate form (Fumasep AA 30)

[0058] Anode: Pt/C GDE (Johnson Matthey)

Cell Bipolar Membrane

[0059] Cathode: Au black catalyst layer containing 10 wt % of anion exchange ionomer (Fumasep) deposited on a GDL substrate

[0060] Membrane: Bipolar membrane (Fumasep 130 m) with the anion exchange side in contact with the cathode and the cation exchange side in contact with the anode

[0061] Anode: Pt/C GDE (Johnson Matthey)

Operation Conditions

[0062] The cell was operated at 40 C. and ambient pressure. The cathode was fed with a 50/50 vol % mixture of CO.sub.2/Ar at 10 ml/min and the anode side was fed with pure H.sub.2 at 50 ml/min. Both gases were 100% humidified. In this operation mode, the anode serves both as counter electrode and as reference electrode, corresponding to a pseudo-reversible hydrogen electrode (pseudo-RHE). A Biologic SP 300 potentiostat was used for all electrochemical measurements. Polarization curves were recorded with the cathode as working electrode using cyclic voltammetry in potentiostatic mode. Galvanostatic measurements at fixed currents were also performed in order to analyze the cell efficiency and short term stability.

Product Gas Analysis

[0063] The exhaust cathode gases were analyzed by on-line mass spectrometry (MS). FIG. 2 shows the electrochemical cyclic voltammetry performances of the different cell configurations and Table 1 summarizes the CO selectivities and cell voltages obtained in galvanostatic mode at different fixed current densities. The CO selectivity values were calculated based on MS analysis of the exhaust cathode gases after 15 min of operation at each current density (i.e. 50 mA/cm.sup.2, 100 mA/cm.sup.2, 200 mA/cm.sup.2). The cell voltages given in Table 1 are not iR-corrected in order to be able to assess the different membrane configuration performances.

TABLE-US-00001 TABLE 1 Cell performances obtained in galvanostatic mode at various fixed current densities. Current density CO selectivity* Cathode potential Cell configuration [mA/cm.sup.2] [%] [V vs. pseudo-RHE] Alkaline 50 12.2 1.875 membrane 100 4.3 2.24 200 Bipolar 50 13.6 1.857 membrane 100 4.2 2.216 200 2 3.2 New 50 11 1.278 configuration 100 6 1.44 200 3.6 1.820 *CO selectivity = mol % CO/(mol % CO + mol % H.sub.2)

[0064] The CO selectivities were very similar for all three cell configurations. This preservation of CO selectivity demonstrates that the additional anion exchange ionomer thin film between the cathode catalyst layer and the acidic membrane in the new cell configuration was effective to prevent increased H.sub.2 evolution by preventing direct contact between the cathode catalyst and the acidic membrane. At the same time, in the case of the new configuration, the energetic efficiency was highly improved: At 50 mA/cm.sup.2 a cathode potential of only 1.3 V vs. pseudo-RHE was required for the new configuration compared to 1.9 V vs. pseudo-RHE for both other configurations.

EXAMPLE 2PARASITIC CO.SUB.2 .PUMPING TO THE ANODE SIDE

[0065] Various cell configurations (similar to the ones described in Example 1) were tested to investigate the parasitic CO.sub.2 pumping to the anode side. The exhaust anode gas analysis was done by MS for the following cell configurations: [0066] 1. Au cathode//alkaline anion exchange membrane//Pt/C GDE anode. The cathode compartment was fed with pure CO.sub.2 gas at a flow of 10 ml/min and 100% relative humidity (RH), while the anode compartment was fed with pure H.sub.2 at 10 ml/min and 100% RH. [0067] 2. Au cathode//alkaline anion exchange membrane//IrTiO.sub.2 anode (approx. 7 mg.sub.IrTiO2/cm.sup.2). The cathode compartment was fed with pure CO.sub.2 gas at a flow of 10 ml/min and 100% RH, while the anode compartment was fed with pure N.sub.2 at 10 ml/min and minimum 100% RH. [0068] 3. Au cathode//bipolar membrane//IrTiO.sub.2 anode (approx. .sup.7 mg.sub.IrTiO2/cm.sup.2). The bipolar membrane was used with the acidic side of the membrane facing towards the anode and the alkaline side of the membrane facing towards the cathode. The cathode compartment was fed with pure CO.sub.2 gas at a flow of 10 ml/min and 100% RH, while the anode compartment was fed with pure N.sub.2 at 10 ml/min and minimum 100% RH. [0069] 4. New configuration according to invention: Au black cathode including anion exchange ionomer//anion exchange ionomer thin film//Nafion XL membrane//Pt/C GDE anode. The cathode compartment was fed with pure CO.sub.2 gas at a flow of 10 ml/min and 100% RH, while the anode compartment was fed with H.sub.2 at 10 ml/min and 100% RH.

[0070] All the investigated cells have a geometric active surface of 0.5 cm.sup.2.

For investigating parasitic CO.sub.2 pumping with the different cell configurations, galvanostatic current steps were applied from 2 mA to 50 mA with each current maintained for 2 min. The exhaust anode gas composition was analyzed by on-line MS.

[0071] FIG. 3 compares the results obtained for cells with an alkaline membrane, Au-based cathode and two different anodes: Once with a Pt/C GDE anode fed with H.sub.2 (cell configuration 1) and once with an anode containing IrTiO.sub.2 catalyst for oxygen evolution fed with highly humidified N.sub.2 (cell configuration 2). The latter configuration corresponds to a full co-electrolyser cell. When no current is applied to the electrochemical cell, a small background signal is detected on the CO.sub.2 channel of the MS (m/z=44). When electrical current is applied to the cell, a correlated increase of the CO.sub.2 signal is detected with the MS for both cell configurations. In configuration 1, the Pt/C anode fed with H.sub.2 acts as a pseudo-RHE reference electrode with a potential close to 0 V vs. RHE. At such low potential, no electrochemical oxidation of the carbonate/bicarbonate species is expected (see reactions 5 and 6). However, a small increase in the CO.sub.2 signal at the anode side is observed also in this configuration. In this case, the CO.sub.2 is formed according to reactions 7 and 8 of the carbonate/bicarbonate species of the anion exchange membrane with the protons resulting from the hydrogen oxidation reaction at the anode (HOR: H.sub.2.fwdarw.2H.sup.++2e.sup.). In the full co-electrolyser cell configuration 2, a significant amount of CO.sub.2 is detected at the anode. In this case, the anode potential is increased and the electrochemical reactions 5 and 6 occur resulting in a significant amount of undesired CO.sub.2 release. These results prove that, when an alkaline membrane is used in a co-electrolyser, a significant amount of CO.sub.2 is pumped from cathode to anode side with the consequence that such a system is not efficient for the purpose of electrochemical CO.sub.2 reduction.

[0072] The CO.sub.2 release at the anode side in full co-electrolyser cell configuration 3 using a bipolar membrane was also tested and compared with the results for full co-electrolyser cell configuration 2 using an alkaline membrane. These results are shown in FIG. 4. When a bipolar membrane is used, the CO.sub.2 release at the anode is significantly reduced compared to the system with an alkaline membrane. As explained above, in the case of a bipolar membrane the carbonate/bicarbonate species produced at the cathode are neutralized at the internal alkaline/acidic interface of the bipolar membrane (see reactions 7 and 8). The CO.sub.2 detected at the anode side is a result of the diffusion of CO.sub.2, formed in this way at the internal membrane interface, through the acidic part of the membrane to the anode side. Thus, using a bipolar membrane is only partially effective to prevent the CO.sub.2 pumping from cathode to anode side.

[0073] FIG. 5 shows the MS measurement of CO.sub.2 release at the anode obtained with the new cell configuration 4 in comparison with the cell configuration 1 using alkaline membrane. In the case of the new cell configuration 4, no CO.sub.2 was detected at the anode side, showing the efficiency of this cell configuration to prevent the parasitic CO.sub.2 pumping.