Patent Application
20240271294
The technical field generally relates to electrocatalytic conversion in electrolyzers, and more particularly to electroreduction process and system combining a bipolar membrane and a stationary catholyte layer facilitating efficient conversion of CO2 and/or CO into multicarbon (C2+) products.
Gigatons of CO2 need to be avoided or removed from the atmosphere each year (see reference 1). When powered by renewable energy sources, the fixation of captured carbon via electrochemical reduction of CO2 (CO2RR) offers a route to net-negative-emission production of multi-carbon (C2+) chemicals (see reference 2). However, CO2RR in electrolyzers operating both with alkaline and neutral electrolytes incur significant CO2 loss to carbonate formation and crossover, leading to low CO2 utilization.
The industrial implementation of the CO2RR for C2+ production requires the simultaneous achievement of high production rates, high energy efficiencies, and high carbon efficiencies (see references 3 and 4). Known CO2RR electrolyzers based on alkaline bulk electrolytes (e.g. alkaline flow cell,
For CO2RR, a catholyte having a high local pH (>12) near the cathode can be used to favour the CO2RR reaction with respect to the competing hydrogen evolution reaction (HER), to enhance the selectivity towards C2+ products(see references 2, 5 and 7 to 9). To maintain such a high local pH, many present-day CO2RR electrolyzers use a flowing alkaline electrolyte/catholyte reservoir (see references 2, 4 and 5). For the same reason, MEAs typically use strong alkaline anion-exchange membranes (AEM) and anolytes(see reference 18). However, locally alkaline conditions absorb CO2 to form carbonates:
CO2+2OH−→CO32−+H2O [1]
CO2+OH −→HCO3− [2]
Thus, at a steady-state, CO2 reacts with hydroxides ions to form carbonate or bicarbonate ions, and both reactant and electrolyte are lost, corresponding to low carbon efficiency in the utilization of CO2 feedstock. Recovering CO2 from carbonate/bicarbonate can consume as much as 60% to 70% of the energy input (see references 11 and 12).
Certain flow cells and MEAs have been designed to use neutral electrolytes (e.g., KHCO3) rather than strong alkaline ones in order to reduce CO2 absorption. Neutral media flow cells and MEAs have lower CO2 absorption than do alkaline cells, and yet, since the reaction drives up the local pH and creates locally alkaline conditions, carbonate and bicarbonate formation remain a problem (see reference 11). Carbonate/bicarbonate ions migrate to the anode via the AEM, to combine with protons provided from the anode oxygen evolution reaction, thereby releasing CO2 into the anode gas stream (SI1). To date, a single pass CO2 utilization (SPU, the fraction of the CO2 feed been transformed to products) of C2+ producing electrolyzers has remained in the range 3% to 30% (Table S1) (see references 6, 11, 13 to 16).
The present techniques relate to carbon oxides-to-C2+ electrochemical reduction strategies that overcome previously-observed limits of carbon efficiency by designing an electroreduction system that inhibits carbon oxides crossover from cathode to anode and reverts formed carbonate/bicarbonate ions to carbon oxides via acidification of the catholyte. Carbon oxides as encompassed herein are selected from selected from CO, CO2 or any mixture thereof.
More particularly, in a first aspect, there is provided an electroreduction system for converting carbon oxides into multicarbon (C2+) products; the system comprising:
For example, the thickness of the stationary catholyte layer can be between 20 μm and 250 μm as measured by a spiral micrometer; preferably between 40 μm and 200 μm; and more preferably between 50 μm and 150 μm; and even more preferably between 65 and 125 μm. The solid porous support can be sandwiched between the catalyst layer of the cathode and the CEL for direct contact therewith.
In some implementations, the cathodic compartment further comprises a solid porous support in between the CEL and the catalyst layer, the solid porous support being configured to be saturated with the catholyte solution to form the stationary catholyte layer.
In an embodiment, the solid porous support can include polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polycarbonate, nylon, cellulose acetate, cellulose nitrate, polypropylene, alumina, or any combinations thereof.
In an embodiment, the solid porous support can have a mean pore diameter (i.e., a mean pore size) between 0.05 and 50 μm as determined by scanning electron microscopy (SEM), optionally between 0.2 and 1 μm, and further optionally of 0.45 μm.
In an embodiment, the stationary catholyte layer can have a liquid content between 5 and 50 μL·cm−2 preferably between 6 and 40 μL·cm−2; more preferably between 8 and 30 μL·cm−2; even more preferably between 8 and 30 μL·cm−2; most preferably between 10 and 25 μL·cm−2; even most preferably between 10 and 20 μL·cm−2 or between 12 and 18 μL·cm−2; when the solid porous support is saturated with the catholyte solution; the liquid content being determined by weighting. For example, the stationary catholyte layer can have a liquid content about 10 μL·cm−2.
In some implementations, the stationary catholyte layer has a thickness that is selected to maximize a mass transport of regenerated CO2 or CO to the catalyst layer of the cathode while maintaining a resistance to compression of the solid porous support.
In some implementations, the catholyte solution has a concentration of cations between 0.25 M and 3 M; preferably between 0.4 M and 4 M; and more preferably between 0.5 M and 2 M, and for example about 1M.
For example, the catholyte solution can be a solution of K2SO4 with a K+ concentration equal to or greater than 0.5M.
For example, the cations in the catholyte solution can be one or more selected from K+, Na+, Cs+, Rb+, NH4+, Mg2+, Ca2+, Al3+.
For example, the catholyte solution is a solution with a cations concentration equal to or greater than 0.5M.
In some implementations, the catholyte solution is a buffered solution. For example, the buffered solution can be a solution comprising one or more selected from KHCO3, K3PO4, K2HPO4, KH2PO4, the buffered solution is a mixture of glycine and sodium hydroxide, or a mixture of H3BO3 and sodium hydroxide.
In some implementations, the catholyte solution is a non-buffered solution. For example, the non-buffered solution can be or comprise K2SO4, KCl or any other combinations of the Cl− anions or SO42− anions with Na+, Cs+, Rb+, NH4+, Mg2+, Ca2+, or Al3+ cations.
The anolyte solution can have an anolyte concentration between 2.0 M and 0.01M, optionally about 0.1 M. In some implementations, the anolyte solution is a neutral solution. For example, the anolyte solution can have a pH between 7 and 10. In the context of the invention the anolyte solution is a neutral solution hen having a pH between 7 and 10. The anolyte (neutral) solution can be a KHCO3, K2SO4, or K2HPO4 solution.
In some implementations, the anolyte solution is an acidic solution. For example, the anolyte solution has a pH between 1 and 4. For example, the anolyte solution has a bulk pH between 1 and 4. The acidic solution can be a H3PO4 solution, H2SO4 solution or a combination thereof.
In some implementations, the catalyst layer of the cathode comprises copper (Cu), silver (Ag), platinum (Pt), carbon (C), or any combination thereof.
In some implementations, the cathode further comprises a gas diffusion layer for contacting the stream of CO, CO2 or any mixture thereof, and the catalyst layer is deposited onto the gas diffusion layer. For example, the gas diffusion layer can be a hydrophobic carbon paper, or a copper sputtered hydrophobic PTFE layer. In the context of the invention hydrophobic means a water contact angle following ISO 19403-6:2017 of at least 30°.
In some implementations, the anode comprises an anodic catalyst layer and an anodic current collector layer. For example, the anodic catalyst layer can include one or more selected from IrO2, Pt, Pd, Ni, NiOx, CoOx. For example, the anodic current collector layer can include Ti felt, hydrophilic carbon paper, or Ni foam. In the context of the invention hydrophilic means a water contact angle following ISO 19403-6:2017 below 30°.
In some implementations, the interfacial layer of the bipolar membrane comprises a water dissociation catalyst. The water dissociation catalyst can be present as nanoparticles. The water dissociation catalyst can comprise one or more selected from TiO2, IrO2, NiO, SnO2, graphene oxide, CoOx, ZrO2, Al2O3, Fe(OH)3, MnO2, Ru, Rh, RuPt alloy, Ptlr alloy, Ir, Pt. In some implementations, the AEL is a membrane comprising poly(aryl piperidinium), polystyrene methyl methylimidazolium, or polystyrene tetramethyl methylimidazolium. The CEL can comprise or consist of a sulfonated tetrafluoroethylene based fluoropolymer-copolymer.
In some implementations, the system can include a temperature controller configured to maintain an operating temperature between 20° C. and 50° C., optionally about 35° C.
A single-pass utilization of the stream of CO, CO2 or any mixture thereof can be of at least 50% for an inlet flowrate between 1 sccm and 15 sccm. The single-pass utilization of the stream of CO, CO2 or any mixture thereof can be of at least 60% for an inlet flowrate between 1 sccm and 8 sccm. The system can have a Faradeic Efficiency (FE) for conversion into the C2+ products of at least 20% during at least 20 hours of operation and under an applied current density between 100 and 400 mA·cm−2. For example, the FE for conversion into the C2+ products can be of at least 25% during at 30 hours of operation and the applied current density of 350 mA·cm−2. In another aspect, there is provided a carbon oxides electroreduction process for converting CO, CO2 or any mixture thereof into C2+ products. The process includes:
In some implementations, the process can include maintaining an operating temperature between 20° C. and 50° C., and optionally about 35° C.
In some implementations, supplying CO, CO2 or any mixture thereof to the cathodic compartment is performed at an inlet flowrate between 1 sccm and 15 sccm.
In some implementations, the process can include providing the cathode with an applied current density between 100 and 400 mA·cm−2.
In some implementations, the process can include forming the stationary catholyte layer by providing a solid porous support between the cathode and the CEL, and saturating the solid porous support with the catholyte solution. For example, the saturating can be performed to reach a liquid content of the stationary catholyte layer between 5 and 50 μL·cm−2, optionally between 10 and 20 μL·cm−2, and further optionally about 10 μL·cm−2 when the solid porous support is saturated with the catholyte solution.
In some implementations, the catholyte solution can be supplied with a concentration of cations between 0.25 M and 3 M, and optionally between 0.5 M and 2 M, and further optionally about 1M.
In some implementations, the stationary catholyte layer can be formed with a thickness between 20 μm and 250 μm as measured by a spiral micrometer; preferably between 40 μm and 200 μm, more preferably between 50 μm and 150 μm.
In some implementations, the process can include utilizing CO, CO2 or any mixture thereof according to a single-pass utilization of the stream of CO, CO2 or any mixture thereof of at least 50% for an inlet flowrate between 1 sccm and 15 sccm. For example, the process can include utilizing CO, CO2 or any mixture thereof according to a single-pass utilization of the stream of CO, CO2 or any mixture thereof of at least 60% for an inlet flowrate between 1 sccm and 8 sccm.
In some implementations, the process can include producing the C2+ products according to a Faradeic Efficiency (FE) for conversion into the C2+ products that is of at least 20% during at least 20 hours of operation and under an applied current density between 100 and 400 mA·cm−2. For example, the process can include producing the C2+ products according to the FE for conversion into the C2+ products that is of at least 25% during at 30 hours of operation and the applied current density of 350 mA·cm−2.
It should be noted that the process can include using the system according to all implementations as defined herein.
The inventors have thus discovered that a cation effect can be allowed at the cathode surface to enable carbon oxide reduction in the acidified catholyte solution. For example, the electroreduction system includes a bipolar membrane and a stationary catholyte layer that maintains a catholyte solution within a cathodic compartment, to facilitate the participation of regenerated CO2 in CO2RR reactions. The presently designed electroreduction system showed a single-pass CO2 utilization of more than 60%, representing twice the previously reported state-of-art designs that produced C2+. Owing to its high single-pass CO2 utilization (SPU), the presently proposed electroreduction system minimizes the energy input associated with CO2 recovery, while enabling comparable performance and stability to the benchmark alkaline and neutral media AEM-based flow cell or MEA electrolyzers.
In an embodiment 1, the invention provides an electroreduction system for converting carbon oxides selected from CO, CO2 or any mixture thereof into multicarbon (C2+) products, the system comprising:
In an embodiment 2, the system according to embodiment 1, wherein the cathodic compartment further comprises a solid porous support in between the CEL and the catalyst layer, the solid porous support being configured to be saturated with the catholyte solution to form the stationary catholyte layer.
In an embodiment 3, the system according to embodiment 2, wherein the solid porous support comprises polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polycarbonate, nylon, cellulose acetate, cellulose nitrate, polypropylene, alumina, or any combinations thereof.
In an embodiment 4, the system according to embodiment 2 or 3, wherein the solid porous support has a mean pore size between 0.05 and 50 μm, optionally between 0.2 and 1 μm, and further optionally of 0.45 μm.
In an embodiment 5, the system according to any one of embodiments 2 to 4, wherein the stationary catholyte layer has a liquid content between 5 and 50 μL·cm−2, optionally between 10 and 20 μL·cm−2, and further optionally about 10 μL·cm−2, when the solid porous support is saturated with the catholyte solution.
In an embodiment 6, the system according to any one of embodiments 2 to 5, wherein the stationary catholyte layer has a thickness that is selected to maximize a mass transport of regenerated CO2 or CO to the catalyst layer of the cathode while maintaining a resistance to compression of the solid porous support.
In an embodiment 7, the system according to embodiment 6, wherein the thickness of the stationary catholyte layer is between 20 μm and 400 μm, optionally between 20 μm and 300 μm, further optionally between 20 μm and 125 μm.
In an embodiment 8, the system according to any one of embodiments 2 to 7, wherein the solid porous support is sandwiched between the catalyst layer of the cathode and the CEL for direct contact therewith.
In an embodiment 9, the system according to any one of embodiments 1 to 8, wherein the catholyte solution has a concentration of cations between 0.25 M and 3 M, and optionally between 0.5 M and 2 M, and further optionally about 1M.
In an embodiment 10, the system according to embodiment 9, wherein the catholyte solution is a solution of K2SO4 with a K+ concentration equal to or greater than 0.5M.
In an embodiment 11, the system according to any one of embodiments 1 to 10, wherein the cations in the catholyte solution are one or more selected from K+, Na+, Cs+, Rb+, NH4+, Mg2+, Ca2+, Al3+.
In an embodiment 12, the system according to any one of embodiments 1 to 11, wherein the catholyte solution is a buffered solution.
In an embodiment 13, the system according to embodiment 12, wherein the buffered solution is a solution comprising one or more selected from KHCO3, K3PO4, K2HPO4, KH2PO4, the buffered solution is a mixture of glycine and sodium hydroxide, or a mixture of H3BO3 and sodium hydroxide.
In an embodiment 14, the system according to any one of embodiments 1 to 11, wherein the catholyte solution is a non-buffered solution.
In an embodiment 15, the system according to embodiment 14, wherein the non-buffered solution is or comprises K2SO4, KCl or any other combinations of the Cl− anions or SO42− anions with Na+, Cs+, Rb+, NH4+, Mg2+, Ca2+, or Al3+ cations.
In an embodiment 16, the system according to any one of embodiments 1 to 15, wherein the anolyte solution is a neutral solution.
In an embodiment 17, the system according to embodiment 16, wherein the anolyte solution has a pH between 7 and 10.
In an embodiment 18, the system according to embodiment 16 or 17, wherein the neutral solution is a KHCO3, K2SO4, or K2HPO4 solution.
In an embodiment 19, the system according to any one of embodiments 1 to 15, wherein the anolyte solution is an acidic solution.
In an embodiment 20, the system according to embodiment 19, wherein the anolyte solution has a bulk pH between 1 and 4.
In an embodiment 21, the system according to embodiment 19 or 20, wherein the acidic solution is a H3PO4 solution, H2SO4 solution or a combination thereof.
In an embodiment 22, the system according to any one of embodiments 1 to 21, wherein the anolyte solution has an anolyte concentration between 2.0 M and 0.01M, optionally about 0.1 M.
In an embodiment 23, the system according to any one of embodiments 1 to 22, wherein the catalyst layer of the cathode comprises copper (Cu), silver (Ag), platinum (Pt), carbon (C), or any combination thereof.
In an embodiment 24, the system according to any one of embodiments 1 to 23, wherein the cathode further comprises a gas diffusion layer for contacting the stream of CO, CO2 or any mixture thereof, and the catalyst layer is deposited onto the gas diffusion layer.
In an embodiment 25, the system according to embodiment 24, wherein the gas diffusion layer is a hydrophobic carbon paper, or a copper sputtered hydrophobic PTFE layer.
In an embodiment 26, the, the system according to any one of embodiments 1 to 25, wherein the anode comprises an anodic catalyst layer and an anodic current collector layer.
In an embodiment 27, the system according to embodiment 26, wherein the anodic catalyst layer comprises one or more selected from IrO2, Pt, Pd, Ni, NiOx, CoOx.
In an embodiment 28, the system according to embodiment 26 or 27, wherein the current collector layer comprises Ti felt, hydrophilic carbon paper, or Ni foam.
In an embodiment 29, the system according to any one of embodiments 1 to 28, wherein the interfacial layer of the bipolar membrane comprises a water dissociation catalyst.
In an embodiment 30, the system according to embodiment 29, wherein the water dissociation catalyst is present as nanoparticles.
In an embodiment 31, the system according to embodiment 29 or 30, wherein the water dissociation catalyst comprises one or more selected from TiO2, IrO2, NiO, SnO2, graphene oxide, CoOx, ZrO2, Al2O3, Fe(OH)3, MnO2, Ru, Rh, RuPt alloy, Ptlr alloy, Ir, Pt.
In an embodiment 32, the system according to any one of embodiments 1 to 31, wherein the AEL is a membrane comprising poly(aryl piperidinium), polystyrene methyl methylimidazolium, or polystyrene tetramethyl methylimidazolium.
In an embodiment 33, the system according to any one of embodiments 1 to 32, wherein the CEL comprises or consists of a sulfonated tetrafluoroethylene based fluoropolymer-copolymer.
In an embodiment 34, the system according to any one of embodiments 1 to 33, further comprising a temperature controller configured to maintain an operating temperature between 20° C. and 50° C., optionally about 35° C.
In an embodiment 35, the system according to any one of embodiments 1 to 34, having a single-pass utilization of the stream of CO, CO2 or any mixture thereof of at least 50% for an inlet flowrate between 1 sccm and 15 sccm.
In an embodiment 36, the system according to any one of embodiments 1 to 34, having a single-pass utilization of the stream of CO, CO2 or any mixture thereof of at least 60% for an inlet flowrate between 1 sccm and 8 sccm.
In an embodiment 37, the system according to any one of embodiments 1 to 36, having a Faradeic Efficiency (FE) for conversion into the C2+ products of at least 20% during at least 20 hours of operation and under an applied current density between 100 and 400 mA·cm−2.
In an embodiment 38, the system according to embodiment 37, wherein the FE for conversion into the C2+ products is of at least 25% during at 30 hours of operation and the applied current density of 350 mA·cm−2.
In an embodiment 39, the invention provides a carbon oxides electroreduction process for converting CO, CO2 or any mixture thereof into C2+ products, the process comprising:
In an embodiment 40, the process of embodiment 39, comprising maintaining an operating temperature between 20° C. and 50° C., and optionally about 35° C.
In an embodiment 41, the process of embodiment 39 or 40, wherein supplying the stream of CO, CO2 or any mixture thereof to the cathodic compartment is performed at an inlet flowrate between 1 sccm and 15 sccm.
In an embodiment 42, the process of any one of embodiments 39 to 41, comprising providing the cathode with an applied current density between 100 and 400 mA·cm−2.
In an embodiment 43, the process of any one of embodiments 39 to 42, comprising forming the stationary catholyte layer by providing a solid porous support between the cathode and the CEL, and saturating the solid porous support with the catholyte solution.
In an embodiment 44, the process of embodiment 43, wherein the saturating is performed to reach a liquid content of the stationary catholyte layer between 5 and 50 μL·cm−2, optionally between 10 and 20 μL·cm−2, and further optionally about 10 μL·cm−2 when the solid porous support is saturated with the catholyte solution.
In an embodiment 45, the process of any one of embodiments 39 to 44, wherein the catholyte solution is supplied with a concentration of cations between 0.25 M and 3 M, and optionally between 0.5 M and 2 M, and further optionally about 1M.
In an embodiment 46, the process of any one of embodiments 39 to 45, wherein stationary catholyte layer is formed with a thickness between 20 μm and 400 μm, optionally between 20 μm and 300 μm, further optionally between 20 μm and 125 μm.
In an embodiment 47, the process of any one of embodiments 39 to 46, comprising utilizing the stream of CO, CO2 or any mixture thereof according to a single-pass utilization of at least 50% for an inlet flowrate between 1 sccm and 15 sccm.
In an embodiment 48, the process of any one of embodiments 39 to 46, comprising utilizing the stream of CO, CO2 or any mixture thereof according to a single-pass utilization of at least 60% for an inlet flowrate between 1 sccm and 8 sccm.
In an embodiment 49, the process according to any one of embodiments 39 to 48, comprising producing the C2+ products according to a Faradeic Efficiency (FE) for conversion into the C2+ products that is of at least 20% during at least 20 hours of operation and under an applied current density between 100 and 400 mA·cm−2.
In an embodiment 50, the process according to embodiment 49, comprising producing the C2+ products according to the FE for conversion into the C2+ products that is of at least 25% during at 30 hours of operation and the applied current density of 350 mA·cm−2.
In an embodiment 51, the process of any one of embodiments 39 to 50, further comprising using the system as defined in any one of embodiments 2 to 38.
The attached figures illustrate various features, aspects and implementations of the technology described herein.
Under all the acidic conditions, the CO2 concentration in the anode gas was below the detection limit.
Techniques described herein relate to an electroreduction system that can be used to convert carbon oxides selected from CO, CO2 or any mixture thereof into multicarbon products with an enhanced single pass utilization (SPU) of CO2 or CO by comparison to know flow cells or membrane electrode assemblies (MEAs). Multiple factors can affect the SPU in electroreduction systems. The present electroreduction system particularly includes a stationary catholyte layer being configured to facilitate mass transfer via diffusion of regenerated CO2 or CO across the stationary catholyte layer and back to an adjacent cathode of the system.
It should be noted that the system and related process implementations that are described herein in relation to CO2 electroreduction can be applied to CO electroreduction, or the electroreduction of a mixture of CO2 and CO, without departing from the scope of the present techniques.
In contrast to known flow cell electrolyzers, the catholyte solution of the present electroreduction system is not provided flowing in and out of a cathodic compartment but rather remains within the cathodic compartment as a stationary catholyte layer between a cathode and a membrane separating the cathodic compartment from an adjacent anodic compartment. More particularly, referring to
In an embodiment, the catalyst layer of the cathode comprises copper (Cu), silver (Ag), platinum (Pt), carbon (C), or any combination thereof. In an embodiment, the cathode further comprises a gas diffusion layer for contacting the CO2 stream, and the catalyst layer is deposited onto the gas diffusion layer. For example, the gas diffusion layer is hydrophobic. For example, the gas diffusion layer is a hydrophobic carbon paper, or a copper sputtered hydrophobic PTFE layer. In the context of the invention, hydrophobic means a water contact angle following ISO 19403-6:2017 of at least 30°.
In an embodiment, the anode comprises an anodic catalyst layer and an anodic current collector layer. For example, the anodic catalyst layer comprises one or more selected from IrO2, Pt, Pd, Ni, NiOx, CoOx. For example, the current collector layer comprises Ti felt, hydrophilic carbon paper, or Ni foam. In the context of the invention hydrophilic means a water contact angle following ISO 19403-6:2017 below 30°.
Referring to
The stationarity of the catholyte layer as defined herein thus prevents the regenerated CO2 to be flushed away from the cathodic compartment and allows the CO2 to diffuse back into the cathode for conversion thereof into value-added products. In other words, being stationary means that the catholyte is not flowing out of the cathodic compartment during CO2 conversion and that the volume of the catholyte solution contained in the stationary catholyte layer remains between the bipolar membrane and the cathode during operation of the electroreduction system. One skilled in the art will understand that the catholyte solution can flow/move within the stationary catholyte layer according to various mass transport mechanisms (diffusion, migration, convection if any).
The nature of the catholyte solution of the stationary catholyte layer can be further selected to reduce the diffusion layer thickness. For example, it was noted from the experimentation discussed herein that a non-buffered catholyte solution can shorten the CO2 path length (including the diffusion layer thickness) in comparison to a buffered catholyte solution, such as KHCO3.
To enhance the mechanical robustness of the stationary catholyte layer, the stationary catholyte layer can include a solid porous support having pores that are saturated with the catholyte solution. For example, the porous solid support is or comprises polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polycarbonate, nylon, cellulose acetate, cellulose nitrate, polypropylene, alumina, or any combinations thereof. The solid porous support has a mean pore diameter (i.e., a mean pore size) between 0.05 and 50 μm as determined by scanning electron microscopy; for example, between 0.08 and 25 μm; for example, between 0.10 and 10 μm; for example, between 0.15 and 5 μm; for example, between 0.20 and 1.0 μm; for example, between 0.30 and 0.80 μm. For example, the solid porous support has a mean pore size of 0.45 μm. The porous solid support can be provided to contact the cathode surface at one side and the cation exchange layer of the bipolar membrane at another side thereof. The thickness of the stationary catholyte layer can thus be equal to the thickness of the solid porous support.
The thickness of the stationary catholyte layer can be at most 280 μm as measured by a spiral micrometer .; preferably, at most 250 μm; preferably, at most 220 μm; preferably at most 200 μm; preferably at most 180 μm; preferably, at most 150 μm; preferably at most 140 μm; preferably, at most 130 μm; and more preferably at most 125 μm.
The thickness of the stationary catholyte layer can be at least 20 μm as measured by a spiral micrometer; preferably, at least 25 μm; preferably, at least 30 μm; preferably at least 40 μm; preferably at least 45 μm; preferably, at least 50 μm; preferably at least 55 μm; preferably, at least 60 μm; and more preferably at least 65 μm.
The thickness of the stationary catholyte layer can be ranging from 20 to 280 μm as measured by a spiral micrometer; for example, from 20 to 250 μm; for example, from 30 to 220 μm; for example, from 40 to 200 μm; for example, from 45 to 180 μm; for example, from 50 to 150 μm; for example, from 55 to 140 μm; for example, from 60 to 130 μm; for example, from 65 to 125 μm. The thickness of the stationary catholyte layer is measured by a spiral micrometer. For example, the thickness of the stationary catholyte layer can be about 125 μm, when measured by a spiral micrometer.
However, the thickness of the stationary catholyte layer is not to be bound to these values and is selected to minimize the diffusion layer thickness while maintaining the mechanical robustness of the stationary catholyte layer. The mechanical robustness can refer herein to a resistance to compression that can be estimated for example via a compression-stress test. In other words, the stationary catholyte layer is configured to resist compression an maintain sufficient thickness to avoid direct contact of the BPM with the cathode. For example, the thickness of the stationary catholyte layer could be inferior to 125 μm if a solid porous support having such thickness is used and yet maintain robustness.
When the solid porous support is saturated with the catholyte solution, the stationary catholyte layer has a liquid content between 5 and 50 μL·cm−2; for example, between 8 and 35 μL·cm−2; for example, between 10 and 20 μL·cm−2. For example, the stationary catholyte layer has a liquid content of about 10 μL·cm−2.
When analyzing CO2 crossover in known neutral media electrolyzers (see section SI1 of Supplemental Information), it was concluded that achieving high carbon efficiency can be achieved based on two requirements. Firstly, the carbonate/bicarbonate ions that are formed from CO2 absorption in locally alkaline catholyte solution should not reach the anode compartment side. Secondly, the carbonate/bicarbonate ions that were formed near the cathode should revert back to CO2, and participate in CO2RR (and not mix with CO2RR products).
In known neutral media flow cells, BPMs (e.g. Fumasep FBM) can be used to convert carbonate/bicarbonate back to CO2 and to block CO2 crossover to the anode (see references 10 and 14). A conventional BPM can consist of a cation-exchange layer (CEL) laminated with an anion-exchange layer (AEL). With the CEL facing the cathode side, the concentration (and hence conductivity) of carbonate/bicarbonate anions in the BPM is substantially reduced due to the Donnan effect (see reference 17). The BPM also generates protons and hydroxide ions via water dissociation at the junction between the CEL and AEL(see references 18 and 19) under appropriate external potential. The protons are driven to the cathode surface, where, with judicious device engineering, they can intercept carbonate/bicarbonate, reverting it to CO2.
However, in the BPM-based flow cells, the catholyte reacts with and absorbs CO2. Even at steady-state (catholyte is saturated by CO2 absorption), the CO2 regenerated at the catholyte/BPM interface can be flushed away from the cathode surface due to catholyte flow, thereby releasing CO2 with gaseous CO2RR products in the cathodic compartment, rather than diffusing back to the cathode to participate in CO2RR (see reference 14). As a result, the CO2 loss due to carbonate/bicarbonate formation is ca. twice the amount transformed to products, similar to AEM-based electrolyzers.
It was thus hypothesized that using a BPM in a device without a flowing catholyte solution, such as a membrane electrode assembly (MEA), could prevent CO2 crossover to the anode, minimize CO2 loss via absorption to the bulk electrolyte, avoid flushing CO2 away, and thereby maximize the SPU. However, another issue was observed, i.e. acidification of the cathode (see references 1 and 20 to 22). The herein discussed experiments (
Previous reports (see references 22 and 23) have suggested inserting a solid porous support layer (a buffer layer) saturated with DI water or a KHCO3 solution as the catholyte solution between the cation-exchange membrane and an Ag catalyst layer (cathode) to improve the selectivity of CO2-to-CO reactions over HER. The presently described system was then developed to evaluate the potential of BPM-based MEA for producing C2+, using a Cu catalyst, and with the goal of a minimum of CO2 loss through engineering the catholyte solution. The analysis provided in section SI2 of the Supplemental Information reveals that, in principle, the in-situ CO2 recovery in a BPM-based MEA can potentially become energy-efficient and may also aid on projected capital costs.
Here is demonstrated that BPM-based MEAs incorporating a stationary catholyte layer between catalyst and BPM can produce C2+ with significantly reduced CO2 loss (
Thus, the BPM of the present disclosure comprises a cation-exchange layer (CEL) in cation communication with the catholyte solution to provide protons into the catholyte solution; an anion-exchange layer (AEL) in anion communication with the anolyte solution to provide hydroxide ions at a surface of the anode; and an interfacial layer defined between the cation-exchange layer and the anion-exchange layer for splitting water into the protons and the hydroxide ions. The interfacial layer can comprise a water dissociation catalyst; with preference that the water dissociation catalyst comprises one or more selected from TiO2, IrO2, NiO, SnO2, graphene oxide, CoOx, ZrO2, Al2O3, Fe(OH)3, MnO2, Ru, Rh, RuPt alloy, Ptlr alloy, Ir, Pt. More preferably, the water dissociation catalyst can be a combination of IrO2 on the AEL side) and NiO on the CEL side. In an embodiment, the water dissociation catalyst is present as nanoparticles.
In an embodiment, the AEL is a membrane comprising poly(aryl piperidinium), polystyrene methyl methylimidazolium, or polystyrene tetramethyl methylimidazolium.
In an embodiment, the CEL is a Nafion™ membrane (CAS number 31175-20-9)
In an embodiment, the system further comprises a temperature controller configured to maintain an operating temperature between 20° C. and 50° C., for example, 25° ° C. and 45° C.; for example, 30° C. and 40° C. optionally about 35° C.
In an embodiment, the system is having a single-pass utilization of the CO2 stream of at least 50% for a CO2 inlet flowrate between 1 sccm and 15 sccm. In an embodiment, the system is having a single-pass utilization of the CO2 stream of at least 60% for a CO2 inlet flowrate between 1 sccm and 8 sccm. In an embodiment, the system is having a Faradeic Efficiency (FE) for conversion into the C2+ products of at least 20% during at least 20 hours of operation and under an applied current density between 100 and 400 mA·cm−2. In an embodiment, the FE for conversion into the C2+ products is of at least 25% during 30 hours of operation and the applied current density of 350 mA·cm−2. The present disclosure also relates to a CO2 electroreduction process for converting CO2 into C2+ products, the process comprising:
For example, the process uses the system described above.
Catholyte Solution Engineering for High SPU of CO2 Feedstock in a CO2-to-C2+ Electrolyzer
In contrast with previous works that used 420 to 800 μm glass microfiber filters (see references 22 and 23), the present system can include a porous stationary catholyte layer having a thickness that is below 400 μm, e.g. about 125 μm and including a solid porous support that is configured to be saturated with the catholyte solution. The solid porous support can be, for example, a PVDF filter membrane having a mean pore size (i.e. diameter) of 0.45 μm and configured to receive a liquid content of about 10 μL·cm−2. In addition to apparent benefits such as lower CO2 absorption capacity (ca. 0.1 mmol cm−2) and lower ohmic resistance, the lower thickness of this porous stationary layer allows for improving the mass transport efficiency of the in-situ recovered CO2 to catalyst (quantitatively simulated in section SI3 of Supplemental Information provided further below) in comparison to known systems.
It was further discovered that the composition of the stationary catholyte layer greatly impacts CO2RR performance in SC-MEA (analyzed and rationalized in sections SI2 and SI3 of Supplemental Information provided further below). Rather than DI-water or KHCO3 as used in previous studies (see references 22 and 23), the catholyte solution can be designed as a non-buffered catholyte solution, e.g. K2SO4, in order to introduce cation effects as a means to promote selectivity to CO2RR over HER (see references 25 and 26). Under external potential, cations such as K+ can form an electrochemical double layer on the catalyst surface (see
Therefore, the cation concentration of the catholyte solution is ranging from 0.25 M to 6.00 M; for example, from 0.50 M to 3 M; for example, from 0.50 M to 2.80 M, for example from 0.75 M to 2.50 M, for example from 1.00 M to 2 M; for example, the cation concentration of the catholyte solution is about 1 M. In an embodiment, the cations in the catholyte solution are one or more selected from K+, Na+, Cs+, Rb+, NH4+, Mg2+, Ca2+, Al3+. For example, the catholyte solution can be a solution of K2SO4 having a K+ concentration equal to or greater than 0.5M; preferably equal to or greater than 1.0 M, more preferably equal to or greater than 1.5 M. For example, the catholyte solution can be a solution of K2SO4 having a K+ concentration of at most 3.00 M.
In an embodiment, the catholyte solution is buffered. For example, the buffered solution is a solution comprising one or more selected from deionized (DI) water, KHCO3, K3PO4, K2HPO4, KH2PO4 or the buffered solution is a mixture of glycine- and sodium hydroxide, or a mixture of H3BO3 and sodium hydroxide.
In another embodiment, the catholyte solution is non-buffered. For example, the non-buffered solution is or comprises K2SO4, KCl or any other combinations of the Cl− anions or SO42− anions with Na+, Cs+, Rb+, NH4+, Mg2+, Ca2+, or Al3+ cations.
The anolyte solution has an anolyte concentration between 2.0 M and 0.01M; for example, between 1.5 M and 0.05M; for example, between 1.0 M and 0.08 M; for example, between 0.5 M and 0.1 M. For example, the anolyte solution has an anolyte concentration of about 0.1 M.
In an embodiment, the anolyte solution is neutral. For example, the anolyte solution has a pH between 7.0 and 10.0; preferably a pH between 7.5 and 9.5. For example, the anolyte (neutral) solution is a KHCO3, K2SO4, or K2HPO4 solution.
In another embodiment, the anolyte solution is acidic. For example, the anolyte solution has a pH between 1.0 and 4.0; for example, between 1.5 and 3.5; for example, between 2.0 and 3.0. For example, the acidic solution is an H3PO4 solution, H2SO4 solution or a combination thereof.
The mechanism of preventing CO2 crossover in the SC-MEA is depicted in
Proton-induced CO2 regeneration could also be accomplished by coupling a cation-exchange membrane and acidic anolyte since the anodic OER can supply protons. It was observed that the CO2 crossover in this system is below the detection limits. However, the experimental and theoretical analyzes showed that this system does not operate continuously because of co-ion transport and water balance issues (discussed in SI3 of Supplemental Information).
To compare the CO2 crossover in the present BPM-based SC-MEA with the known AEM-based neutral media electrolyzers, the CO2RR performance of the present BPM-based SC-MEA was firstly measured, using a flowing neutral anolyte solution (0.1 M KHCO3, PH ˜8.2), and the results are summarized in
The operating temperature on CO2RR performance was then optimized and it was discovered that 35° C. can be the optimal temperature in the presently designed BPM-based SC-MEA (see
A CO2/O2 ratio of an anodic gas mixture (also referred to as an anode gas) was further studied to evaluate the capability to prohibit CO2 crossover in the BPM-based SC-MEA, the key to achieving a high SPU (SI1) (see references 13 and 14). In agreement with previous studies (see references 13 and 14), the AEM-based MEA showed CO2/O2 ratios very close to 2 for current densities from 100 to 300 mA·cm−2 (
Conversely, the CO2/O2 ratio in the anode gas produced in the present BPM-based SC-MEA is one order of magnitude lower than that in AEM-based MEA, evidencing the prevention of CO2 crossover. The detected anode CO2 flow is not ascribed to the acidification of KHCO3, as supported by the performed control experiments (
By decreasing the inlet CO2 flow rate, it was further demonstrated that a BPM-based SC-MEA operating in neutral (pH=8.20) anolyte solution and at 35° C. can achieve an SPU of about 61%, which is a significant improvement in carbon efficiency compared to the known neutral-media AEM-based CO2RR electrolyzers.
When using a neutral anolyte solution in the present BPM-based SC-MEA, the CO2 crossover—despite being significantly reduced—was not down to zero. In the neutral anolyte solution, some of the CO2 generated in the stationary catholyte layer may diffuse through the BPM's CEL, combining with hydroxide ions in the AEL and migrate to the anode. When using an acidic anolyte solution, it was hypothesized that the hydroxide ions in the AEL could be partially replaced by the anionic species in the anolyte solution, such as H2PO4−, SO42−, which might inhibit carbonate/bicarbonate formation and crossover. It was indeed observed that when operating in an acidic anolyte solution with a bulk pH of 2.37 (0.1 M H3PO4+0.5 M K2SO4), the CO2 content at the anodic gas stream was below the detection limit in the operating current density range from 100 to 400 mA cm−2.
The impacts of operating temperature and anolyte acidity on the performance of the present BPM-based SC-MEA (
37 ± 0.5
a All the evaluated devices are flowing-catholyte-free MEA cells operating in neutral or acidic anolytes, of which the CO2 absorption is negligible. Therefore, a 4.3 GJ/Ton CO2 separation energy consumption (see reference 29) was implemented based on the amine capture process from mixed gas for all the unutilized CO2 from both cathode and anode.
Table 1 summarizes and compares the CO2 recovery energy consumptions of the SC-MEA (acidic anolyte), and the literature benchmark neutral media AEM-based MEA electrolyzer (see reference 15). Coupling the advantages of minimized CO2 crossover (enabled by BPM) and acidic bulk anolyte solution, the present BPM-based SC-MEA can allow a 86% and 72% reduction in energy penalty associated with CO2 recovery compared to the simulated MEA electrolyzer and literature benchmark neutral media electrolyzer15, respectively. These results highlight the need for high-SPU CO2RR devices, i.e., lower energy consumption.
The stability of the proposed BPM-based SC-MEA operating under optimized conditions (in terms of ethylene FE) (
Several alternative implementations and examples have been described and illustrated herein. The implementations of the BPM-based SC-MEA described above are intended to be exemplary only. A person of ordinary skill in the art would appreciate the features of the individual implementations and the possible combinations and variations of the components. A person of ordinary skill in the art would further appreciate that any of the implementations could be provided in any combination with the other implementations disclosed herein. It is understood that the developed design may be embodied in other specific forms without departing from the central characteristics thereof (e.g., CO2 crossover limitation, regeneration of absorbed CO2, and control of the diffusion layer in the stationary catholyte layer). The present implementations and examples, therefore, are to be considered in all respects as illustrative and not restrictive, and the system and process proposed herein are not to be limited to the details given herein. Accordingly, while the specific implementations have been illustrated and described, numerous modifications can come to mind.
For example, the cathode of the system could be further designed to include a macroporous gas diffusion layer, microporous gas diffusion layer, a metallic layer containing Cu with/without Al and/or Zn and/or Mg in any form (ionic or reduced or nanoparticles), short-side-chain ionomers (e.g., Nafion or similar ionic polymers), an organic molecular compound (e.g., pyridine) either in free form or grafted into any of the above layer.
A single-pass CO2 utilization of at least 60% at a production rate of 91 mA·cm−2 toward C2+ products was achieved. When run at a total current density of 350 mA·cm−2, the present BPM-based SC-MEA electrolyzer delivered an average ethylene Faradaic Efficiency (FE) of 35% for over 30 hours. CO2 loss due to crossover was inferior to 0.1%.
Phosphoric acid (H3PO4, 85%), potassium sulfate (K2SO4, 99%), potassium bicarbonate (KHCO3, 99.7%), potassium chloride (KCl, 99%), potassium hydroxide (KOH, 99.95%), copper nanoparticles (25 nm), Nafion™ 1100W (5 wt. % in a mixture of lower aliphatic alcohols and water) and isopropanol (IPA, 99%) were purchased from Sigma Aldrich and used as received. Titanium oxide nanoparticles (TiO2, Aeroxide P25) and PVDF membrane filter (0.45 μm pore size, 125 μm thickness) were purchased from Fisher Scientific and used as received. Nafion™ 212, Nafion™ XL, Fumasep (FAS-PET-130) and titanium (Ti) felt were purchased from Fuel Cell Store. Iridium(IV) chloride hydrate (Premion®, 99.99%, metals basis, Ir 73% min) was purchased from Alfa Aesar. The water used in this study was 18 MΩ Milli-Q deionized-(DI-) water. Nafion membranes were activated through the following procedure: 1 hour in 80° C. 1M H2SO4—1 hour in 80° C. H2O2—1 hour in 1M H2SO4—stored in DI-water. Fumasep was used as received and stored in 1M KCl. Piperion (40 μm) was purchased from W7Energy and stored in 0.5M KOH.
The water dissociation catalyst layer was fabricated following a similar procedure in a previous report (see reference 18). TiO2 nanoparticles ink were prepared by sonicating the mixture of TiO2, DI-water, and IPA with the weight ratio of 1:833:2833 for 30 minutes. TiO2 nanoparticle ink was spray-coated onto a Nafion 212 membrane, of which the edges were sealed by Kapton tapes. The exposed membrane dimension was 2.2 cm×2.2 cm. The nominal loading of TiO2 is 0.2 mg cm−2. The TiO2-coated Nafion was immediately used for assembling electrolyzers once prepared.
For the CO2RR, cathode gas diffusion electrodes (GDEs) were prepared 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 hydrophobic carbon paper. The mass loading of Cu NPs in the GDE was kept at 1.5 mg/cm2. The GDEs were dried in the air overnight before experiments.
For the OER, the anode electrodes were prepared following a recipe described in Ozden, A., Li, F., García De Arquer, F.P., Rosas-Hernández, A., Thevenon, A., Wang, Y., Hung, S.F., Wang, X., Chen, B., Li, J., et al. (2020). High-rate and efficient ethylene electrosynthesis using a catalyst/promoter/transport layer. ACS Energy Lett. 5, 2811-2818; and Gabardo, C.M., O'Brien, C.P., Edwards, J.P., McCallum, C., Xu, Y., Dinh, C.T., Li, J., Sargent, E.H., and Sinton, D. (2019). Continuous Carbon Dioxide Electroreduction to Concentrated Multi-carbon Products Using a Membrane Electrode Assembly. Joule 3, 2777-2791 (see references 6 and 15). The electrode preparation procedure involves: etching the Ti felt in hydrochloric acid at 70° C. for 40 min; rinsing the etched Ti felt with DI water; immersing the Ti felt into an Ir(IV) chloride hydrate solution; drying and sintering the Ir(IV) loaded Ti felt. The loading, drying, and sintering steps were repeated until a final Ir loading of 1.5 mg cm−2 was achieved.
The MEA set (5 cm2) was purchased from Dioxide Materials. A cathode was cut into a 2.1 cm×2.1 cm piece and placed onto the MEA cathode plate with a flow window with a dimension of 2.23 cm×2.23 cm. The four edges of the cathode were sealed by Kapton tapes, which also made the flow window fully covered. The exposed cathode area was measured every time before the electrochemical tests, which was in the range of 3.1 to 4.2 cm2. Onto the cathode, a PVFD filter membrane (2 cm×2 cm) saturated with desirable electrolyte (sonicate in electrolyte for 15 minutes to degas) was carefully placed. This PVDF layer serves as the ‘stationary catholyte layer.’ Note that the BPMs used in neutral and acidic conditions were a customized one and a commercially available one
(Fumasep), respectively, to achieve a better compromise of CO2RR performance and stability. The considerations of membrane selection can be found in SI4 of Supplemental Information. When using customized BPM, a TiO2 coated Nafion was placed onto the stationary catholyte layer with the TiO2 layer facing up, then covered by a Piperion (5 cm×5 cm) membrane. When using Fumasep BPM, the membrane was placed with its cation-exchange layer (CEL) facing the cathode side. An IrO2 loaded Ti felt (2.5 cm×2.5 cm) was placed onto the anion-exchange layer (AEL) of the BPM.
Images of cathode and customized BPM were captured by an FEI Quanta FEG 250 environmental SEM (see
Throughout all experiments, the cathode side was flowed by CO2 with a flow rate of 15 sccm unless otherwise specified, while the anode side was fed with neutral or acidic electrolyte at 10 mL/min by a peristaltic pump. The electrochemical measurements were performed with a potentiostat (Autolab PGSTAT204 with 10A booster). The cell voltages reported in this work are not iR corrected. The exemplified flow rate of anolyte should not be taken as a limitation and different values (other than 10 mL/min) would provide an SPU of at least 60% as encompassed herein.
The CO2RR gas products, oxygen, and CO2 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:
Where xi is the volume fraction of the gas product i, V is the outlet gas flow rate in L s−1, P is atmosphere pressure 101.325 kPa, R is the ideal gas constant 8.314 J mol−1 K−1, T is the room temperature in K, ni is the number of electrons required to produce one molecule of product F is the Faraday Constant 96485 C mol−1, and J is the total current in A.
The liquid products from the cathode side of the SC-MEA were collected using a cold trap cooled to 0° C. The collected liquid was combined with anolyte (some crossover liquid product) for quantifying by the proton nuclear magnetic resonance spectroscopy (1H NMR) on an Agilent DD2 500 spectrometer in D2O using water suppression mode and dimethyl sulfoxide (DMSO) as the internal standard. 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:
Where mi is the quantity of the liquid product i in mole, t is the duration of product collection (1800 seconds).
The CO2 SPU calculation is detailed in section SI1 of Supplemental Information provided below.
SI1 Analysis and Comparison of CO2 Single-Pass Utilization Among State-of-Art CO2RR Electrolyzers
The ordinary CO2RR electrolyzers suffer from an upper limit of CO2 utilization, depending on their product distributions, as analyzed below:
In the presence of hydroxide ions, CO2 molecules react with OH− (reactions [1] and [2]) faster than being electrochemically reduced because the reaction kinetics are more favorable (see references 11, 13 and 14).
CO2+2OH−→CO32−+H2O [1]
CO2+OH−→HCO3− [2]
Meanwhile, the major cathode reactions in neutral or alkaline media include:
CO2+H2O+2e−→CO+2OH− [3]
CO2+H2O+2e−→HCOO−+OH− [4]
2CO2+5H2O+8e−→CH3COO−7OH− [5]
2CO2+8H2O+12e−→C2H4+12OH− [6]
2CO2+9H2O+12e−→C2H5OH+12OH− [7]
CO2+6H2O+8e−→CH4+8OH− [8]
3CO2+13H2O+18e−→C3H7OH+18OH− [9]
2H2O+2e−→H2+2OH− [10]
All these reactions generate hydroxide, of which the rate (in mole per second, MOH) is:
Where J is the current in amps, FE[i] is the faradaic efficiency of the specific reaction [3-10], F is the Faraday constant, and k[i] is the number of OH− generated per electron transferred in the specific reaction [3-10]. For the cathode reactions [4] and [5], the k[i] values are 0.5 and 0.875, respectively; for the other cathode reactions the k[i] values are 1.
Based on the analysis above, the inlet CO2 (Cin) is balanced by four parts: the CO2 in outstream (C1), the electrochemically reduced CO2 (C2), the absorbed CO2 (C3), and the crossover CO2 (C4). In other words, the mass balance of CO2 (in mole per second) is:
The carbon utilization efficiency is evaluated by single-pass utilization (SPU):
In conventional flow cells and MEAs, some studies have demonstrated that C1 can be negligible compared to C2 and C4 by carefully tuning Cin. When the CO2 absorption in the system reaches a steady-state, C3 is almost zero. Therefore, the upper limit of SPU is:
Where J is the current in amp, FE[i] is the faradaic efficiency of the specific reaction [3-9], n[i] is the number of electrons transferred per consumed CO2 in the specific reaction [3-9], and F is the Faraday constant. In neutral media, C4 ranges from 0.5 to 1 times MOH, depending on the species of charge carrier cross the AEM. To evaluate the upper limit of the SPU, C4=0.5 MOH. Substituting (1) and (5) into (4) gives:
Therefore, in the conventional flow cells and MEAs operating in neutral media, the upper limits of SPU depend on their product distributions. For example, the SPU upper limits of the systems that produce 100% FE of CO (n[i]=2; k[i]=1) or 100% FE of ethylene (n[i]=6; km=1) are 50% or 25%, respectively.1 Notably, HER does not contribute to C2 but still generates hydroxide that can drive CO2 crossover. Accordingly, the CO2RR performances of the electrolyzers that show state-of-art SPU in the references were identified and summarized in Table S1. None of the reported electrolyzers can achieve an SPU exceeding 30% for C2+ production, and 44% for CO production.
The SPU measurement missed 10-13% of the product FE likely because some liquid product was trapped in the stationary catholyte layer or migrated and got oxidized on the anode. Therefore, the SPU reported in this work are the minimum values. The upper limit of CO2 SPU was also simulated for the SC-MEA under various conditions and are listed in Table S1. The upper limit SPU values without considering the missing FE are indicated in the brackets.
Nevertheless, the missing FE is taken into account for calculating the upper limit SPU of the electrolyzer to make a conservative comparison of SPU. The missing FE can be ascribed to three groups of liquid products, i.e., formate, acetate, and ethanol/propanol. However, formate FE is only 5% of the liquid product in the SC-MEA operating under all the circumstances, and ascribing the missing FE to formate will result in a total CO2 consumption exceeding the inlet CO2 amount; the missing FE to acetate, ethanol, and propanol is ascribed here. This simulation gives the ranges of upper limit SPU for the SC-MEA under different conditions (Table S1). In the main text, these ranges are depicted in
7
0.07
0.2
0.45
0
0.175
0
0
0
24%
Alkaline
FC
5
50
0.07
0.06
0.7
0.02
0.1
0.05
0
0
0.5%
Alkaline
FC
6
2
0.59
0.00
0.24
0.08
0.09
0.025
0.015
0.01
30%
Neutral
MEA
9
45
0.05
0.15
0.45
0.02
0.2
0.04
0.02
0.04
Neutral
FC
3
45
0.08
0.18
0.42
0.02
0.15
0.03
0.03
0.05
Neutral
FC
SI2 Analysis of Intrinsic Energy Consumption Associated with In-Situ CO2 Regeneration in SC-MEA and the Comparison with AEM-Based MEAs
With the application of an appropriate external potential, water dissociation: H2O→H++OH−, occurs at the interface of the CEL/AEL, and the protons and hydroxides serve as charge carriers in CEL and AEL, respectively. Under standard conditions (25° C., 1 atm, with activities of H+ and OH− at 1 M in the CEL and AEL, respectively), the electric potential across the BPM is ˜0.83 V at equilibrium. The electric potential energy difference for H+ and OH− across the BPM exactly compensates for the difference in activity such that the electrochemical potential is the same everywhere at equilibrium (see reference 18). For net current to flow, an additional electric potential must be applied across the membrane causing a deviation from the open-circuit value of ˜0.83 V. This deviation is typically called the water dissociation overpotential and represents the losses associated with generating H+ and OH− and transporting it out of the interfacial layer between CEL and AEL and out of the BPM. Often, it is stated that a BPM induces a “thermodynamic” voltage loss of 0.83 V—however as discussed above, this is incorrect—the losses can be quite small. For example, with appropriate materials and operating conditions, the cell voltage of a BPM-based water electrolyzer can be lower than that of an AEM-based electrolyzer at the current density up to 500 mA cm−2 (see reference 18). BPM electrolyzers can begin to split water with a total voltage of <2 V, which would be impossible if there were an intrinsic 830 mV penalty for using the BPM.
With a pH=8.20 anolyte, the SC-MEA has a cell voltage of 3.7 V at 100 mA cm−2 and 50° C. (
In the present SC-MEA design, the stationary catholyte layer is a 125 μm-thick 0.5 M K2SO4 solution (conductivity 0.15 S cm−1). Although the total ionic conductivity of this catholyte is large, the H+/OH+/HCO3/CO32− conductivity is very small. Because these are the relevant ionic charge carriers in carbon dioxide electrolysis at steady state, a large pH gradient between Cu and the bulk catholyte is induced. Establishing this pH gradient is a source of an additional concentration overpotential. As shown in
In the present SC-MEA (
In other words, SC-MEA is likely to save energy compared to ex-situ CO2 capture (3.5 to 4.7 GJ per ton -see reference 29), especially as the costs of renewable electricity decrease, and if cross-over of the buffer ions can be minimized or eliminated.
Previous techno-economic analysis has concluded that (see reference 39) for an AEM-based MEA, even under the optimistic evaluations (zero-ohmic loss, cell voltage 2 V, SPU 40%, 90% C2+ FE, half the CO2 loss and current density 1 A cm−2), the total cost for producing 1 ton of ethylene from CO2RR is $1,300, higher than the ethylene market price ($1,000), of which $750 is spent for electricity and $520 is spent for ex-situ CO2 recovery. Using the same optimistic metrics (except for SPU, which can be optimized to 100% in SC-MEA), the in-situ CO2 recovery in SC-MEA (if using pH=10 buffer catholyte) can cut the CO2 capture cost ($520→$0), with a 9% (0.18 V to 2 V) increase to electricity cost ($750→$817), making the production cost for 1 ton of ethylene to $847. Therefore, SC-MEA has the potential to make the CO2-to-ethylene electrolysis economically feasible.
Nevertheless, the buffer catholyte is not used in the present SC-MEA. In the buffer catholyte SC-MEA, CO2 is generated at the boundary of CEL and catholyte and diffuses across the catholyte layer (125 μm) to Cu. Previous simulation works have suggested that such a long diffusion path cannot support a high rate (>100 mA cm−2) CO2RR (see reference 38). This effect also explains the experimental observation when using KHCO3 (with some buffer capability) as the stationary catholyte, as shown in
In the future, by adopting strategies from membrane science and materials engineering, the catholyte thickness can be reduced to below 10 μm and a buffer catholyte can be used with minimum cross-over that will minimize the pH gradients developed within the system and thus the concentration overpotential losses associated with them. Coupling with the efforts of BPM materials and catalysts development, it is expected that the SC-MEA may be a high-total-energy-efficient system for CO2-to-C2+ production.
With the present BPM-based SC-MEA, the CO2 consumed for CO2RR is provided by two sources: the inlet CO2 flow (gas), and the regenerated CO2 (dissolved form) in the stationary catholyte layer. As the upper limit of the CO2 SPU for most of C2+ is 25%, the CO2 regeneration procedure in the SC-MEA will supply 75% of the CO2 consumption if the target is to achieve 100% SPU. Thus, the mass transport effectiveness of the regenerated CO2 is an important consideration in the SC-MEA, which is determined by the thickness of the stationary catholyte layer because a too thick catholyte layer cannot effectively deliver the regenerated CO2 to Cu catalyst, as analyzed below.
At steady-state, the net current in the stationary catholyte layer of the SC-MEA should be primarily driven by the electromigration of protons/hydroxide and carbonate/bicarbonate ions simultaneously generated from water dissociation of BPM and reactions [1]/[2], respectively. Referring to
After the electrolysis starts and proceeds, the CO2 concentration at the boundary gradually arises, driving the generated CO2 (dissolved form) to diffuse towards Cu and CEL. The CO2 (dissolved form) that diffuses deeper into the stationary catholyte layer towards the CEL is not consumed by CO2RR but accumulates in Zone 2 until reaching a concentration close to that at the boundary (then no driving force). In the other direction, the CO2 diffuses towards Cu is consumed for CO2RR, forming a concentration gradient in Zone 1, which creates a continuous CO2 diffusion flux from boundary to Cu. Note that the real CO2 (dissolved form) concentration distribution in Zone 1 deviates from linear due to the local pH gradient. Zone 1 can be described as a diffusion layer (see reference 40). The diffusion layer thickness, L1, has great impacts on the CO2 mass transport (see reference 40). Since the electric field of the stationary catholyte layer can be considered homogeneous (see reference 41), L1 and L2 are known to be proportional to the mobility of the corresponding ions. Given that L1+L2 is the total thickness of the stationary catholyte layer, a thinner stationary catholyte layer has a thinner CO2 diffusion layer (L1), which is beneficial for CO2 mass transport from bulk to catalyst (see reference 40). In the future, thinner, yet mechanically robust porous layers, can be employed to improve the mass transport of the regenerated CO2, as discussed in SI2.
A previous work (see reference 22) showed that in an MEA cell, inserting a solid porous supporting layer saturated with DI water in between Ag catalyst and the cation-exchange layer of the BPM can improve the FE for converting CO2 to CO. In the system, this strategy was also attempted, and the electrolyzer shows a >94% FE towards hydrogen, and a high full-cell voltage of 6-9 V, even under optimized conditions, as shown in
The K2SO4 catholyte in the stationary catholyte layer will gradually be partially transformed to KHCO3 over time owing to reactions [1] and [2] as well as the slow leakage of SO42− to anolyte. To study the impact of such transformation, the CO2RR performance of SC-MEA was measured when a 1 M KHCO3 buffer electrolyte is used as catholyte at the beginning. It was found that using 1 M KHCO3, the SC-MEA shows an expectable C2H4 FE of ca. 23% at the current density of 100 mA cm−2. However, its stability is poor in that the FE gradually decreases by >50% after the initial 5 hours. This experiment has been repeated three times, and a typical FEs and cell voltage vs. time curve is displayed in
To this end, the stationary catholyte layer was engineered in SC-MEA to a 125 μm thick, 0.5 M non-buffer K2SO4 solution.
Since it was discovered that the cation effect enables the CO2RR even in an acidified environment, it was attempted to extend the stationary catholyte layer strategy in an MEA cell using CEM and an acidic anolyte pH<2.37, expecting a lower cell voltage than the BPM-based cells while maintaining high SPU. The configuration of this system is shown in
This configuration shows lower full cell voltage (
Meanwhile, it has a reasonable CO2RR selectivity over HER because the K+ in the anolyte can migrate to the Cu surface to induce the cation effect. However, this design was not amenable to steady state operation without continuous addition of acid and salt to the anolyte and catholyte as the initial pH gradient will be eliminated due to co-ion transport and neutralization. In fact, the CO2RR selectivity of this system gradually drops over time, and after ca. 3 hours, this system produces 100% hydrogen at the cathode. In addition, it was observed that this design periodically eject electrolyte from the cathode flow channel, probably due to a water balance issue:
On the anode, the OER generates one proton per one electron transfer:
2H2O—4e−→4H++O2
Since a CEM is used in this system, the charge carriers across the membrane are K+ and H+. K+ is adsorbed at the Cu surface to form an electrochemical double layer, and the migration of K+ terminates until reaching the equilibrium between electric potential and chemical potential, which usually takes tens of seconds (see reference 19). H+ migrates to the cathode side combines with OH− (or CO32−/HCO3−), which produces water at the cathode side. The water balance for different cathode reactions ([3-9] in section SI1) was accordingly calculated:
As seen, most of the cathode reactions generate water on the cathode side. The generated water keeps diluting and pushing out the electrolyte in the stationary catholyte layer, of which the volume is small (ca. 10 μL per cm2 electrode area). This phenomenon results in the flooding (also confirmed by experimental observation) of the cathode and continues loss of K+ in the system, degrading CO2RR performance because the concentration of K+ is critical to CO2RR performance for the catalyst (
In a neutral anolyte solution, a customized BPM was adopted, which consisted of a layer of TiO2 nanoparticles as a water dissociation catalyst sandwiched by a CEM and an AEM. In an acidic anolyte solution, a commercially available BPM (Fumasep) was used to achieve the best compromise among CO2RR performance, cell voltage, and stability.
Water splitting measurement was firstly conducted to compare the resistance of customized BPM and Fumasep.
When been used in the SC-MEA with neutral anolyte (
For the SC-MEA using acidic anolyte, the customized BPM also promotes lower cell voltage than Fumasep (
Elevating the operating temperature reduced the full cell voltage at given current densities (
Referring to
The present invention founds that the composition and thickness of the catholyte layer influence the local pH, the efficiency of CO2 regeneration and, thereby, the overall cell performance. A one-dimensional multiphysics model in COMSOL® was applied to investigate the catholyte layer in BPM-based CO2RR electrolyzers.
The CO2 reactant is provided by two sources: the inlet CO2 flow (gas) and the regenerated CO2 (dissolved form, aq.) in the catholyte. To achieve high SPU, it is necessary to restrict the gaseous CO2 feed. Under a restricted gaseous CO2 availability, the cathode CO2 supply relies more on regeneration: in an ideal case with 100% SPU and 100% C2+ selectivity, regeneration contributes 75% of the consumed CO2. Thus, the mass transport of regenerated CO2 is most critical, and that transport is governed by catholyte composition and thickness.
At steady-state, electrolysis creates a pH gradient through the catholyte layer: the pH is high near the cathode and low near the CEL. The protons and (bi)carbonate ions recombine in the catholyte, forming CO2 (aq.) that diffuses, in response to a concentration gradient, to the Cu catalyst.
Simulations resolve the local cathode environment as a function of dimensions, electrolyte and running conditions. The modeled thicknesses of 250, 125, 65 and 16 μm were selected to correspond to commercially available materials.
Use of a buffering catholyte (e.g. KHCO3) leads to a thick CO2 (aq.) diffusion layer—close to the catholyte thickness, since the CO2 (aq.) is generated near the CEL surface. This effect reduces the CO2 (aq.) mass-transfer efficiency.
In contrast, with a non-buffering catholyte layer (e.g. 0.5 M K2SO4) with thicknesses of 250, 125, and 65 μm, the local pH values near the cathode are greater than 11, which is sufficient to promote selectivity towards CO2RR over HER. Reducing the SC-layer thickness to 16 μm results in a cathode pH of 8.7, implying a lower selectivity toward CO2RR.
Guided by the above analysis, the inventors focused on a stationary catholyte bipolar membrane electrode assembly (SC-BPMEA) electrolyzer and incorporated a judiciously-designed catholyte layer and BPM.
The cathode was prepared by spraying Cu nanoparticles onto a hydrophobic carbon gas-diffusion layer for CO2RR. The anode was IrO2 supported on Ti felt for the oxygen evolution reaction (OER). A BPM under reverse bias was employed with the AEL contacting the anode and the CEL contacting the SC-layer (porous support saturated with electrolyte). The cathode was compressed onto the porous layer, and the anode and cathode flow-field plates sandwiched the system.
The BPM employed in this work sandwiched TiO2 nanoparticles as the water dissociation catalyst (see reference 18). This custom BPM can lower the cell voltage by ˜1 V compared with commercial BPMs (e.g. Fumasep). The full cell voltage of such custom BPM-based electrolyzers is close to that of AEM systems.
Measurements of the CO2/O2 ratio in the anode gas stream show that the SC-BPMEA effectively prevents CO2 crossover, as required for high SPU (see references 13 and 14). In agreement with the previous studies (see references 13 and 14), the AEM-based MEA (AEMEA) showed an anode CO2/O2 ratio of ˜2 for current densities ranging from 100 to 300 mA cm−2. In conventional AEMEAs, the anionic charge carriers are CO32−, and thus suffer the loss of one molecule of CO2 for every two electrons transferred. The anode CO2/O2 ratio in the SC-BPMEA (0.06 at 200 mA cm−2) is one order of magnitude lower. Control experiments confirm that the CO2 detected in the anode is not due to acidification of anolyte (using 0.1 M K2SO4 instead of 0.1 M KHCO3 resulted in a similar CO2/O2 ratio). The anode CO2/O2 ratio decreases as the operating current density increases, an effect that was ascribed to an increased flux of protons toward the cathode. This flux decreases the pH at the CEL surface and reduces the diffusion of CO2 and HCO3−/CO3− in the CEL (see references 19 and 28).
The thickness of the stationary catholyte was found to have a major impact on cell voltage. The cell voltage of the SC-BPMEA decreases as the thickness of the SC-layer decreases (
The simulations indicate that the thicker SC-layer results in longer transport distances for dissolved CO2. The CO2 regeneration rate inside the SC-layer also depends on the current density, and for thicker SC-layers (e.g. >125 μm), CO2 bubbles are more prone to form near the CEL. These bubbles obstruct ion migration, increasing the ohmic resistance of the SC-BPMEA. Electrochemical impedance spectroscopy measurements also support this finding. An applied current of 200 mA cm−2 resulted in an insignificant change to the high-frequency resistance (HFR) of the SC-BPMEA with a 65 μm-thick SC-layer; while, in contrast, the HFR of the SC-BPMEA with a 125 μm-thick SC-layer increased by 120% after applying 200 mA cm−2 for 20 min, leading to a cell voltage 0.6 V higher than for the 65-μm SC-layer.
The cell voltage of the SC-BPMEA with a 65 μm SC-layer operating at 200 mA cm−2 is 3.8 V, comparable to the AEM-based neutral-media MEAs operating at similar conditions (difference<±0.05 V) (see references 5, 6 and 15). This result demonstrates that the cell voltage of a BPM-based CO2RR electrolyzer can be as low as that of an AEM-based electrolyzer with a current density of up to 200 mA cm−2, while suppressing unwanted crossover and providing high SPU.
The thickness of the SC-layer also affects selectivity towards CO2RR. With thicknesses of 65, 125 and 250 μm, the H2 Faraday efficiencies (FEs) are consistent (˜20% at 200 mA cm−2,
By suppressing the crossover of CO2 (e.g. <0.5% of total CO2 input at 200 mA cm−2,
As the inlet CO2 flow rate decreased, the C2+ FE of the SC-BPMEA at 200 mA cm−2 decreased, accompanied by an increase in the H2 FE (
The stationary catholyte thickness affects the SPU of the SC-BPMEA. The SPU gradually increases up to 21, 61 and 78% for the SC-BPMEAs with SC-layer thicknesses of 250, 125 and 65 μm, respectively (
For a given CO2 flow rate, a thicker SC-layer produces a lower SPU (
The experimental trends are generally consistent with those of the simulations. The SC-BPMEA with a dissolved CO2 diffusion layer thicker than 75 μm (representing a 250-um SC-layer) fails to surpass the SPU limit because of insufficient mass transfer. In contrast, a 65-um SC-layer facilitates efficient mass transport of the regenerated CO2 (diffusion layer thickness of 12 μm) and simultaneously promotes high local cathode pH.
It was found (see
The SC-BPMEA shows>50-h stability operating at 200 mA cm−2 with limited CO2 availability (CO2 input flow rate of 1.42 sccm cm−2). This operating stability is competitive with that of the neutral-electrolyte-based CO2-to-C2+ electrolyzers.
The inventors attempted to extend the SC-layer strategy in a CEM-based MEA cell (i.e. SC-CEMEA,
SC-CEMEA shows a lower full cell voltage (
It was also observed that the SC-CEMEA design periodically ejects electrolyte from the cathode flow channel, likely due to poor water balance. On the anode, the OER generates one proton per one electron transfer. The charge carriers across the CEM are primarily H+, although neutral ion pairs will diffuse as well. At the cathode K+ makes up the electrochemical double layer at the Cu surface, and the steady-state K+ profiles are governed by the electric and chemical-potential gradients that develop under operation, which usually takes tens of seconds (see reference 19). H+ migrates to the cathode and combines with OH (or CO32−/HCO3), producing water at the cathode. The protons also drag water molecules (˜1 per proton) by electro-osmosis. It was accordingly calculated the water balance for different cathode products as listed in Table 2. The water generated and transported to the cathode appears to dilute and push out the electrolyte in the stationary catholyte layer, of which the volume is small (ca. 10 μL per cm2 electrode area). This phenomenon results in flooding of the cathode (as confirmed experimentally) and loss of supporting electrolyte, thus degrading performance. In the BPMEA design, it is likely that the BPM slows co-ion transit across the membrane, compared to the CEM, by the large outward flux of OH− and H+ from the water dissociating junction.
Energy Assessment of the SC-BPMEA with Optimal SC-Layer
The energy costs (measured in gigajoules per tonne of the target product, GJ/t) for a CO2-to-C2+ electrolyzer include the electrolysis electrical energy, cathodic stream separation, and anodic stream separation. CO2RR performance metrics of importance include cell voltage, target product FE, SPU and CO2 crossover (see reference 12). High SPU and high energy efficiency have not been accomplished simultaneously in C2+ electroproduction. In SC-BPMEAs, a higher SPU reduces the energy required for cathode separation, but the accompanying decrease in the ethylene selectivity (
0 b
0 b
0 b
0 b
a The energy intensities of reference CO2-to-ethylene devices operating under the reported conditions are calculated, and those that provide the lowest energy intensity are presented in this table.
b Crossover of CO2 in the acidic flow cell, acidic MEA and SC-BPMEA are each lower than 0.5% of input CO2. Therefore, we assume the anodic separation energy to be 0.
(1) Huang, J. E. et al. CO 2 electrolysis to multi-carbon products in strong acid. Science 372, 1074-1078 (2021)
(2)O'Brien, C. P. et al. Single Pass CO2Conversion Exceeding 85% in the Electrosynthesis of Multicarbon Products via Local CO2Regeneration. ACS Energy Lett. 6, 2952-2959 (2021)
In such systems, CO2 and OH− react to form carbonate continuously. This carbonate has to be recovered to maintain the CO2RR performance of such a system, consuming 5.5 GJ per tonne CO2. In the alkaline CO2RR electrolyzers, ca. 63 tonne of CO2 transforms to carbonate to produce 1 tonne of ethylene, representing an energy penalty of 350 GJ. This costs at least $1,900 per tonne of ethylene, while its market price is $800-1000 per tonne. The alkaline electrolyzers thus do not allow for ethylene electrochemical production to be yet profitable.
In neutral-media CO2RR electrolyzers, recovering the CO2 from the anodic gas stream results in significant energy costs. In the context of highly selective conversion (i.e., CO2-to-ethylene with unity selectivity), the recovery process requires an energy input of 52 GJ to produce every tonne of product. In practice, due to non-unity product selectivity, the process is even more prohibitive, i.e., requiring an energy penalty of 80-130 GJ for producing one tonne of ethylene.
As the SPU increases from 4 to 35%, we found a dramatic decrease in energy associated with cathode separation—from 85 to 15 GJ/t ethylene (Table 3), with the ethylene FE reduced by only 2%. Further increasing the SPU beyond 35% does not substantially reduce the energy cost associated with cathodic separation. This finding agrees with a recent energy analysis that in a (bi)carbonate-free CO2-to-C2+ electrolyzer, improving SPU over 40% offers an insignificant benefit to the downstream separation cost. Pursuing an SPU>35% decreases ethylene FE by more than 4% when using the SC-BPMEA, and thus the increased input electricity cost exceeds the savings in the cathodic separation (Table 3). Therefore, 35% SPU is the most favourable condition for the present SC-BPMEA.
The energy intensity of producing ethylene in SC-BPMEA is ˜30% lower than that in conventional neutral-electrolyte-based CO2 electrolyzers (Table 3). In conventional neutral-electrolyte CO2-to-ethylene electrolyzers, the CO2 crossover (at least 70%) costs 60-90 GJ per ton of ethylene to recover CO2 from the anodic O2 stream5. Notably, this energy penalty cannot readily be reduced, independent of optimizing catalysts and operating conditions (e.g. input CO2 flow rates, reaction rates, operating temperature and pressure). In contrast, crossover CO2 in SC-BPMEA is <0.5% of the total CO2 input, minimizing the energy cost of anodic separation.
Recently, CO2-to-ethylene conversion has been achieved in acidic electrolytes in both flow cell and MEA configurations. These systems enabled CO2 SPUs exceeding 75% and also mitigated the energy cost associated with anodic separation (Table 3). Owing to the strongly acidic environment, the flow cell enables an ethylene FE of 28% at a full-cell potential of 4.2 V. The acidic MEA used an anion-exchange ionomer coating on the catalyst layer to promote CO2RR over HER. The modification of the surface with the anion exchange ionomer resulted in a higher ohmic loss, and thus the cell required potentials of 3.8 V and 4.4 V at 100 mA cm−2 and 200 mA cm−2, respectively. These devices thus eliminated the anodic CO2/O2 separation energy but at the penalty of larger cell voltages and/or lower ethylene FEs. In contrast, SC-BPMEA shows a cell voltage of 3.8 V at 200 mA cm−2 with an ethylene FE of 42%—voltages and selectivities comparable to the best conventional neutral-electrolyte CO2-to-ethylene MEAs (see reference 15). Compared to acidic systems, the energy intensity of the SC-BPMEA is 36% and 12% lower than acidic flow cell and acidic MEA, respectively (Table 3).
The inventors have demonstrated a BPM-based CO2-to-C2+ MEA, with a judiciously-designed SC-layer between catalyst and BPM, that overcomes the (bi)carbonate-formation reactant loss issue without compromising performance. The composition and thickness of the SC-layer determine the CO2RR performance and SPU via a strong influence on the local pH and the chemistry and transport of CO2. The buffering capacity and the thickness of the SC-layer determine the efficiency of the regeneration, the transport, and the availability of reactant CO2. These effects were predicted in simulations and supported by experiments. The SC-BPMEA design largely eliminates the energy penalty associated with the CO2 loss in electrochemical CO2 reduction.
The performance of the SC-BPMEA might be further improved using, for example, ionic liquid or other organic salts as the catholyte, and by optimizing the porosity, structure and hydrophobicity of the porous support layers. The CO2RR performance of the SC-BPMEA might be improved with new cathodic catalysts, optimizing the loading and processing of the catalyst layer, and by implementing BPMs with further-lowered water-dissociation voltage loss. Broadly, the SC-BPMEA is a useful platform for evaluating CO2RR catalysts operating with high CO2 utilization. The strategy and findings presented here are also relevant to the electrochemical systems such as nitrate reduction and (bi)carbonate reduction, where controlling dissimilar microenvironments near each electrode is useful, and the exchange/transport of species (other than OH− or H+) between cathode and anode is problematic.
Phosphoric acid (H3PO4, 85%), potassium sulfate (K2SO4, 99%), potassium bicarbonate (KHCO3, 99.7%), potassium chloride (KCl, 99%), potassium hydroxide (KOH, 99.95%), copper nanoparticles (25 nm), Nafion™ 1100W (5 wt. % in a mixture of lower aliphatic alcohols and water) and isopropanol (IPA, 99%) were purchased from Sigma Aldrich and used as received. Titanium oxide nanoparticles (TiO2, Aeroxide P25) were purchased from Fisher Scientific and used as received. The porous supports were also purchased from Fisher Scientific: 125 μm PVDF (0.45 μm pore size), 65 μm PTFE (0.44 μm pore size) and 16 μm PC (0.4 μm pore size). Nafion™ 212, Nafion™ XL, Fumasep (FAS-PET-130) and titanium (Ti) felt were purchased from Fuel Cell Store. Iridium(IV) chloride hydrate (Premion®, 99.99%, metals basis, Ir 73% min) was purchased from Alfa Aesar. The water used in this study was 18 MΩ Milli-Q deionized-(DI-) water. Nafion membranes were activated through the following procedure: 1 h in 80° C. 1M H2SO4—1 h in 80° C. H2O2—1 h in 1 M H2SO4—stored in DI-water. Fumasep was used as received and stored in 1 M KCl. Piperion (40 μm) was purchased from W7Energy and stored in 0.5 M KOH.
The water dissociation catalyst layer was fabricated following a similar procedure in a previous report17. TiO2 nanoparticles inks were prepared by sonicating the mixture of TiO2, DI-water, and IPA with the weight ratio of 1:833:2833 for 30 min. TiO2 nanoparticle ink was spray-coated onto a Nafion 212 membrane, of which the edges were sealed by Kapton tape. The exposed membrane dimension was 2.2 cm×2.2 cm. The nominal loading of TiO2 is 0.2 mg cm−2. The TiO2-coated Nafion™ was immediately used for assembling electrolyzers once prepared.
For the CO2RR, 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 hydrophobic carbon paper. The mass loading of Cu NPs in the GDE was kept at 1.5 mg/cm2. The GDEs were dried in the air overnight prior to experiments. The OER electrode preparation procedure involves: etching the Ti felt in hydrochloric acid at 70° C. for 40 min; rinsing the etched Ti felt with DI water; immersing the Ti felt into an Ir(IV) chloride hydrate solution; drying and sintering the Ir-loaded Ti felt. The loading, drying, and sintering steps were repeated until a final Ir loading of 1.5 mg cm−2 was achieved.
The MEA set (5 cm2) was purchased from Dioxide Materials. A cathode was cut into a 2.1 cm×2.1 cm piece and placed onto the MEA cathode plate with a flow window with a dimension of 2.2 cm×2.2 cm. The four edges of the cathode were sealed by Kapton tape, which also made the flow window fully covered. The exposed cathode area was measured every time before the electrochemical tests, in the range of 3.1 to 4.2 cm2. Onto the cathode, a porous support layer (2 cm×2 cm with various thicknesses, 250 μm was stacking two 125 μm-thick PVDF) saturated with desirable electrolyte (sonicated in electrolyte for 15 min to degas) was carefully placed. This porous support layer serves as the ‘stationary catholyte layer (SC-layer).’ The considerations of membrane selection can be found in SI2 and SI4 of the Supplementary Information. When using the custom BPM, a TiO2-coated Nafion membrane was placed onto the SC-layer with the TiO2 layer facing up, then covered by a Piperion (5 cm×5 cm) membrane. When using Fumasep BPM, the membrane was placed with its cation-exchange layer (CEL) facing the cathode side. An IrO2 loaded Ti felt (2 cm×2 cm) was placed onto the anion-exchange layer (AEL) of the BPM.
Images of cathode and custom BPM were captured by an FEI Quanta FEG 250 environmental SEM.
Throughout all experiments, CO2 flowed to the cathode side at 10 sccm cm−2 unless otherwise specified, while the anode side was fed with neutral 0.1 M KHCO3 at 10 mL/min by a peristaltic pump unless otherwise specified. The electrochemical measurements were performed with a potentiostat (Autolab PGSTAT204 with 10A booster). The cell voltages reported in this work are not iR corrected. The system was allowed to stabilize at the specific conditions for >1000 seconds before recording the results. All the error bars represent standard deviations based on three measurements.
The CO2RR gas products, oxygen, and CO2 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:
Where x, is the volume fraction of the gas product i, V is the outlet gas flow rate in L s−1, P is atmosphere pressure 101.325 kPa, R is the ideal gas constant 8.314 J mol−1 K−1, T is the room temperature in K, ni is the number of electrons required to produce one molecule of product F is the Faraday Constant 96485 C mol−1, and J is the total current in A.
The liquid products from the cathode side of the SC-BPMEA were collected using a cold trap cooled to 0° C. The collected liquid was combined with anolyte (some crossover liquid product) for quantifying by the proton nuclear magnetic resonance spectroscopy (1NMR) on an Agilent DD2 500 spectrometer in D2O using water suppression mode and dimethyl sulfoxide (DMSO) as the internal standard. For each plot of liquid product quantification, fresh anolyte was used, and the duration of the collection was 30 min. The FE of a liquid product is calculated as follows:
Where mi is the quantity of the liquid product i in mole, t is the duration of product collection (1800 seconds).
The electrochemical reaction model was performed by COMSOL Multiphysics version 5.5. This simulation was built upon previous modeling work. The local pH and different species concentrations were simulated for different catholyte thicknesses (16 μm, 65 μm, 125 μm, and 250 μm). Two different catholytes (K2SO4 and KHCO3) were used in the simulation. All the chemical reactions between species were considered in this one-dimensional modeling. The simulation included a 50 μm thick gas diffusion layer (GDL), a 0.1 μm thick Cu cathode catalyst (CL), a catholyte region with various thicknesses indicated above, and a cation exchange layer (CEL) boundary.
Constant concentration (Dirichlet) boundary conditions were used. Specifically, a constant concentration 37.8 mM of CO2 was assumed within the GDL layer, as this region is in direct contact with the input CO2 flow and thus assumed to be at equilibrium with gas phase CO2 over this region for the purposes of the simulation. The BPM was interpreted as a boundary with a constant species concentration (1 M H3O+ at the CEL surface), because it was assumed to generate protons as the dominant ionic charge carrier at a constant rate under constant current density (200 mA cm−2).
A user-controlled mesh is employed in the COMSOL simulation. Edge type of mesh is used for GDL, CL, catholytes, respectively. Specifically, the mesh distribution is predefined with an interval of 500 nm for GDL and catholytes, and an interval of 5 nm for CL. Five different electrode reactions were considered at the cathode catalyst layer in this simulation. Specifically, the hydrogen evolution reaction and CO2 reduction reactions to CO, CH4, C2H4, and C2H5OH occurred at the cathode catalyst layer. In SC-BPMEA, the catalyst layer is immersed in a catholyte. Thus the simulation considers no gas-phase transport in the catalyst layer. The carbonate equilibrium reactions, corresponding catholyte buffer reactions, and a water dissociation reaction were considered in the catholyte region. The electrochemical reaction rates of the specific products were determined from experimental results. They are calculated based in the same manner as previous work17.
The electrochemical reactions at cathode catalyst layer:
2H2O+2e−→H2+2OH−
CO2+H2O+2e−→CO+2OH−
CO2+H2O+8e−→CH4+8OH−
2CO2+8H2O+12e−→C2H4+12OH−
2CO2+9H2O+12e−→C2H5OH+12OH−
The heterogenous electrochemical reaction rates are determined by the following equations:
Where Ii represents the partial current density for CO, CH4, C2H4, and C2H5OH occurred at the cathode catalyst layer, respectively. ni represents the number of electrons transferred per mole reactant. F represents faraday's constant. Itotal represents the total current density. The FEs for the specific product is determined by the experimental results. ε represents the catalyst porosity value. Lcatalyst represents the cathode catalyst length.
The chemical reactions at the catholyte region and the corresponding forward kf rate constants and reverse kr rate constants taken from the literature:
The Transport of Diluted Species physics model was used. The Nemst-Planck set of equations governed the species diffusion, and they were calculated in the same manner as previous work.13,14 Migration was ignored for simplicity as the experiments were performed in the concentrated electrolyte. The ion species transport is thus calculated by solving the two equations below.
Where Ji is the molar flux, and ri represents the heterogeneous electrode reactions for CO2 reduction that were modelled at the cathode catalyst layer. Ri represents the rates of the homogeneous reactions indicated above. The Millington and Quirk model is used to determine the effective diffusivity, Di. εp represents porosity coefficient. τF.i represents tortuosity coefficient.
The porosity value of 0.6 was used for the cathode catalyst and the porosity value of 1 for the catholyte region. The species diffusion coefficients are listed below.
Henry's law and sets of Sechenov equation are applied to calculate the CO2 concentration. The concentration of CO2 in electrolytes depends on temperature and pressure. It is estimated in the same manner as previous work. The Sechenov coefficients are listed below.37
We evaluated the energy consumptions for electrolyzer electricity, cathodic separation, and anodic separation in the context of ethylene. We consider the state-of-the-art CO2RR systems from the literature, including alkaline flow-cell electrolyzers, neutral MEA electrolyzers, acidic flow-cells and MEAs. This consideration is based on the performance metrics, including selectivity, productivity, and full-cell voltage—the combination reflects 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. We summarize the input parameters to the model for all the systems. The energy assessment model, as well as the assumptions, are based on the previous work. Ideally, it will be interesting to use experimental/modelling data corresponding to the exact gas composition from the CO2-to-C2+ device. However, at present, there is a gap in published literature. We therefore employed one of the most widely used models (i.e. biogas upgrading) as the best approximation for evaluating the energy cost associated with cathode gas separation. The details of calculations for the carbon regeneration (for alkaline flow cell) and cathodic separation (for all the electrolyzers), can be found in previous work. The anodic separation (for neutral MEA electrolyzer) is modelled based on an alkaline capture solvent. The amount of CO2 crossover to the anode is calculated for one tonne of ethylene produced. The energy required to separate the CO2/O2 mixture is calculated based on a recent report by Carbon Engineering, in which 5.25 GJ/tonne CO2 thermal energy and 77 kWh/tonne CO2 are reported to be required to capture CO2 and release at 1 bar. This energy consumption is a typical value for the alkaline capture process. For acidic flow-cell and MEA electrolyzers, we assume no energy cost associated with the anodic separation considering no CO2 availability at the anodic gas stream.
The following references are incorporated herein by reference:
28. Bui, J.C., Digdaya, I., Xiang, C., Bell, A.T., and Weber, A.Z. (2020). Understanding Multi-Ion Transport Mechanisms in Bipolar Membranes. ACS Appl. Mater. Interfaces 12, 52509-52526.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2022/065823 | 6/10/2022 | WO |
| Number | Date | Country | |
|---|---|---|---|
| 63210675 | Jun 2021 | US |
