Electrochemical CO 2 Reduction into Chemical Feedstocks: From Mechanistic Electrocatalysis Models to System Design

Products containing more C-H and C-C bonds such as ethylene and ethanol are more sensitive to electricity prices compared to CO and formate, assuming similar energy efficiency and selectivity.

“Due to their higher energy requirements, products containing more C-H and C-C bonds such as ethylene and ethanol, are more sensitive to electricity prices compared to CO and formate, assuming similar energy efficiency and selectivity (Figure .1d-g).”

• AssumptionThe authors state this as an assumption: For comparing sensitivity to electricity prices, energy efficiency and selectivity are similar across products.

The production cost for all products shows a strong dependence on Faradaic efficiency.

“As shown in Figure 1d-g, the production cost for all products show a strong dependence on FE. [18]”

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The current density must be greater than 300 mA/cm2 for industrial application.

“However, as highlighted in our TEA, the current density must be greater than 300 mA/cm 2 for industrial application.”

Gas phase electrolyzer with bipolar membrane has decreased liquid product loss.

“decreased liquid product loss”

Batch H cell has a maximum current of less than 100 mA/cm2.

“<100 mA/cm 2”

Gas phase electrolyzer with bipolar membrane causes acidification of cathode.

“acidification of cathode”

Gas phase electrolyzer with anion exchange membrane has a maximum current of 100s of mA/cm2.

“100s of mA/cm 2”

Gas phase electrolyzer with bipolar membrane has pH balance in cell.

“pH balance in cell”

The TEA reveals that the most economically compelling target products include CO, formate, C2H4 and C2H5OH.

“The TEA reveals that the most economically compelling target products include CO, formate, C2H4 and C2H5OH.”

Liquid phase electrolyzer achieves high current densities.

“high current densities”

Batch H cell has low currents.

“low currents”

Gas phase electrolyzer with cation exchange membrane has competition in selectivity between HER and CO2RR.

“selectivity between HER and CO2RR”

The combination of the PCET model with the CHE model provides means of determining the chemical potential of protons and electrons by avoiding the explicit treatment of solvated protons.

“The combination of the Proton-Coupled Electron-Transfer (PCET) model with the Computational Hydrogen Electrode (CHE) model provides means of determining the chemical potential of protons and electrons by avoiding the explicit treatment of solvated protons.[39]”

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Ethanol and ethylene are good targets compared to methanol and methane because of their higher market price per ton.

“Ethanol and ethylene are good targets compared to methanol and methane in view of their higher market price per ton (Figure .1a).”

Electricity costs are the determining factor for the economic feasibility of CO2RR products in most technoeconomic models.

“As in hydrogen production from water electrolysis, electricity costs are the determining factor for the economic feasibility of CO2RR products in most technoeconomic models.[17][18][19]26,27]”

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Based on market prices and performance metrics, CO and formate appear the most promising target products for CO2RR.

“In summary, based on market prices and performance metrics, CO and formate appear the most promising target products for CO2RR .”

Early DFT computations assumed that the reaction took place in vacuum, without explicitly considering water molecules in aqueous electrolytes or the electric field induced by ions.

“Early DFT computations assumed that the reaction took place in vacuum, without explicitly considering the water molecules in aqueous electrolytes or the electric field induced by the ions.”

Later studies revealed inaccuracies in calculating reaction energies in the vacuum approach resulting from the omission of electric field and solvation effects during the simulation.

“However, later studies revealed inaccuracies in calculating reaction energies in this approach.[34-37][38] resulting from the omission of electric field and solvation effects during the simulation.”

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The CO2 electrolyzer must meet the preceding performance targets (current density, Faradaic efficiency, and energy efficiency) over a prolonged period of time, typically 60,000-80,000 hours.

“The CO2 electrolyzer must meet the preceding performance targets (current density, FE, and EE) over a prolonged period of time, typically 60,000-80,000 hours.”

Multiple-electron-transfer products including methanol, methane, ethanol, ethylene, and propanol have so far been produced with lower selectivity.

“Multiple-electron-transfer products, including methanol, methane, ethanol (C2H5OH), ethylene (C2H4), and propanol (C3H8O), have so far been produced with lower selectivity.”

The selectivity of CO and formate depends on the initial binding mode of the first intermediate of CO2 reduction, i.e. *COOH or *HCOO.

“The selectivity of these products depends on the initial binding mode of the first intermediate of CO2 reduction, i.e. *COOH or *HCOO (Figure 2, orange and green pathways, respectively), where * indicates the atom bound to the catalyst.”

DFT efficiently calculates the reaction energetics based on the adsorption energies of different reaction intermediates.

“Among different methods developed for quantum chemistry calculations, density function theory (DFT) efficiently calculates the reaction energetics based on the adsorption energies of different reaction intermediates.”

Larger market size, energy dense and long-chain hydrocarbon and oxygenate liquid products, such as ethanol and ethylene, are promising targets to address the issues associated with CO and formate production.

“Larger market size, energy dense and long-chain hydrocarbon and oxygenate liquid products, such as ethanol and ethylene, are therefore promising targets to address the issues associated with CO and formate production.”

To include solvation effects, the adsorption energies calculated in the vacuum approach were corrected by 0.1 eV for CO and CHO, 0.45 eV for OH, and 0.25 eV for R-OH.

“To include solvation effects, such as hydrogen bonding, the adsorption energies calculated in this approach were corrected by 0.1 eV for CO and CHO, 0.45 eV for OH, and 0.25 eV for R-OH, where R denotes a hydrocarbon.[31][32][33]”

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Cheng et al. using an explicit water model showed that CO formation proceeds from physisorbed CO2 to chemisorbed CO2 (*CO2δ-), with a free energy barrier of 0.43 eV.

“A detailed study at pH 7 was performed by Cheng et al. using an explicit water model to study the CO formation pathway.[38] In contrast with previously proposed models, CO formation was shown to proceed from physisorbed CO2 to chemisorbed CO2 (*CO2 δ-), with a free energy barrier of 0.43 eV (Figure 2, top orange pathway).”

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Normalizing price to chemical energy stored makes CO, formate, and propanol promising targets.

“This analysis makes CO, formate, and propanol promising targets.”

Industrial water electrolyzers have demonstrated stable performance with operation times over 60,000-80,000 hours.

“Industrial water electrolyzers have demonstrated stable performance with operation times over 60,000-80,000 hours, and a similar bar will be applied to CO2 electrolyzers.[19]”

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• AssumptionThe authors state this as an assumption: A similar stability bar (60,000-80,000 hours) from industrial water electrolyzers applies to CO2 electrolyzers.

Two-electron-transfer products such as CO and formate are readily achieved on a variety of catalysts with high selectivity.

“Two-electron-transfer products, such as carbon monoxide (CO) and formate (formic acid in acidic media), are readily achieved on a variety of catalysts with high selectivity.”

A high current density (usually >300 mA/cm2) is required to minimize capital investment for a desired CO2RR production rate.

“Thus, a high current density (usually >300 mA/cm 2 ) is required to minimize capital investment for a desired CO2RR production rate.”

The Sn(S)/Au catalyst exhibited FEHCOO- up to 93% at -0.75 V vs RHE for over 40 hours.

“The Sn(S)/Au catalyst exhibited a FEHCOO -up to 93% at -0.75 V vs RHE for over 40 hours.”

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Group 1 catalysts weakly bind the *CO2- intermediate, resulting in selective production of formate.

“Group 1 catalysts weakly bind the *CO2 -intermediate, resulting in the selective production of formate.”

CO2RR to formate has achieved the target performance metrics (overpotential, current density) to be economically viable.

“In brief, CO2RR to formate has achieved the target performance metrics (i.e., overpotential, current density) to be economically viable.”

In situ GIWAXS and XAS studies reveal preferential exposure of highly active Bi (110) facets in the oxyhalide-derived catalyst are the source of high selectivity.

“In situ grazing-incidence wide-angle X-ray scattering (GIWAXS) and X-ray absorption spectroscopy (XAS) studies reveal the preferential exposure of highly active Bi (11 ̅ 0) facets in this oxyhalide-derived catalyst are the source of the high selectivity.”

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Microkinetic models are needed to use DFT-calculated activation energy barriers to determine reaction rates, catalytic activity, product distribution, and current density under real experimental conditions.

“However, microkinetic models are needed to use the DFT calculated activation energy barriers to determine the reaction rates, the catalytic activity, the product distribution, and the current density under real experimental conditions.”

High pH conditions shift the selectivity towards multi-carbon products by shutting down C1 pathways.

“Thus, high pH conditions shift the selectivity towards multi-carbon products by shutting down C1 pathways.”

By altering the reaction environment through increased electric fields, increased CO coverage, and/or increased catalyst strain, it is possible to improve the catalytic activity towards multi-carbon products.

“Thus, by altering the reaction environment through increased electric fields (e.g.larger cations in the electrolyte), increased CO coverage (e.g. higher CO2 pressure), and/or increased catalyst strain (e.g.engineering a core-shell structure [63] ) it is possible to improve the catalytic activity towards multi-carbon products.”

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From DFT calculations, the onset potential and the potential determining step can be ascertained.

“From these calculations, the onset potential and the potential determining step can be ascertained.”

Cation-induced electric fields reduce the *CO dimerization barrier.

“Moreover, cation-induced electric fields have been shown to reduce the *CO dimerization barrier.[33]”

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High-throughput DFT simulations were used to screen Ag-based catalysts for efficient CO production.

“For instance, highthroughput DFT simulations were used to screen Ag-based catalysts for efficient CO production.[65]”

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Tafel analysis showed faster kinetics of the first electron transfer for Au nanoneedles compared to Au nanorods and Au nanoparticles.

“Tafel analysis showed faster kinetics of the first electron transfer for the Au nanoneedles as compared to Au nanorods and Au nanoparticles.”

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Machine learning can leverage growing datasets of adsorption energies of reaction intermediates on different catalysts to train models for predicting more active and selective CO2RR catalysts without running massive DFT computations across catalyst type, reactants, and reaction environment.

“Using machine learning, one can leverage the ever-growing datasets for the adsorption energies of different reaction intermediates on different catalysts, to train models for predicting more active and selective CO2RR catalysts without running massive DFT computations in a large variable space, including catalyst type, reactants and reaction environment.”

• AssumptionRests on an assumption: Accurate electrochemical models are too complex for high-throughput computational screening of the large parameter space of CO2RR variables.

Several nanostructuring strategies have been reported to enhance selectivity and reduce overpotential of Au and Ag by increasing surface area, edge sites, strain, or low coordinated sites.

“Several nanostructuring strategies have been reported to enhance the selectivity and reduce the overpotential of Au and Ag, by increasing their surface area, edge sites, strain, or low coordinated sites.”

Controlling the surface water concentration is expected to be critical in tuning the ethylene and ethanol selectivity.

“Thus, controlling the surface water concentration is expected to be critical in tuning the ethylene and ethanol selectivity.”

The ability of a catalyst to reduce the energy barrier depends on the binding energies of specific reaction intermediates along the pathway to different CO2RR products, thereby directly influencing the final product distribution.

“The ability of a catalyst to reduce the energy barrier depends on the binding energies of specific reaction intermediates along the pathway to the different CO2RR products, thereby directly influencing the final product distribution.”

Performance enhancements have been reported on oxide-derived (OD) nanostructured Ag catalyst.

“Similarly, performance enhancements have been reported on oxide-derived (OD) nanostructured Ag catalyst.[77]”

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Group 2 catalysts bind *CO2- and *COOH intermediates strong enough for subsequent reduction to *CO.

“Group 2 catalysts bind with the *CO2 -intermediate and the following *COOH intermediate strong enough for subsequent reduction to *CO.”

p-block dopants modulate reaction energetics by imposing partial covalency into the Ag catalyst, thereby enhancing catalytic activity.

“It was found that p-block dopants modulate reaction energetics by imposing partial covalency into the Ag catalyst, thereby enhancing catalytic activity.”

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Zheng et al. reported a three-fold enhancement in formate selectivity using a sulfur-modulated Sn catalyst (Sn(S)/Au) compared to a Sn nanoparticle/Au catalyst.

“For example, Zheng et al. reported a three-fold enhancement in formate selectivity using a sulfurmodulated Sn catalyst (Sn(S)/Au) as compared to a Sn nanoparticle/Au catalyst, [70]”

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Gao et al. reported a partially oxidized Co 4 atomic layer catalyst that achieved FEHCOO- up to 90% at -0.24 V vs RHE for over 40 hours.

“Gao et al. reported a partially oxidized Co 4atomic layer catalyst that achieved FEHCOO -up to 90% at -0.24 V vs RHE for over 40 hours.[72]”

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CO is considered a key intermediate in the formation of multi-carbon hydrocarbons and oxygenates from CO2RR because similar product distributions result from the direct reduction of CO.

“CO is considered a key intermediate in the formation of multi-carbon hydrocarbons and oxygenates from CO2RR because similar product distributions result from the direct reduction of CO.”

In the early CO dimerization mechanism, the rate-determining step involves an electron transfer to couple two CO molecules.

“In this mechanism, the rate-determining step involves an electron transfer to couple two CO molecules.”

The competition between CO and formate formation occurs in the first electron-transfer step.

“Thus, the competition between CO and formate formation occurs in the first electron-transfer step.”

Garcia de Arquer et al. demonstrated a 2D Bi-based catalyst derived from a BiOBr template for highly selective (>90% FE) and stable CO2 reduction at 200 mA/cm2.

“Recently, Garcia de Arquer et al. demonstrated a 2D Bi-based catalyst derived from a BiOBr template for highly selective (>90% FE) and stable CO2 reduction at 200 mA/cm 2 .[71]”

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Weak binding of the *CO intermediate results in production of CO from Group 2 catalysts.

“Weak binding of the *CO intermediate results in the production of CO from Group 2 catalysts.”

Several microkinetic models on Ag and Cu have been proposed.

“Several microkinetic models on Ag [69] and Cu [34] have been proposed”

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Strategies such as structuring and catalyst doping have been employed to improve the selectivity of Group 1 catalyst materials.

“Strategies such as structuring and catalyst doping have been employed to improve the selectivity of these catalyst materials.”

Early CO dimerization was proposed as a pathway for CO reduction to ethylene, ethanol, and acetaldehyde.

“Consequently, early CO dimerization was proposed as a pathway for CO reduction to ethylene (Figure 2, red pathways), ethanol (Figure 2, blue pathways), and acetaldehyde.[55]”

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Electrocatalyst materials can be divided into three main groups based on their tendency to bind various CO2 reaction intermediates and generate certain products.

“As a result, electrocatalyst materials can be divided into three main groups based on their tendency to bind various CO2 reaction intermediates and generate certain products.”

The Tafel slope of 44 mV dec-1 suggested the enhanced CO2 adsorption capability of the layered catalyst accelerated CO2 activation.

“The Tafel slope of 44 mV dec -1 suggested the enhanced CO2 adsorption capability of the layered catalyst accelerated CO2 activation.”

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Klinkova et al. synthesized Pd nanoparticles containing an abundance of high-index facets and surface kinks to weaken CO binding affinity.

“To circumvent the CO poisoning effect, Klinkova et al. synthesized Pd nanoparticles containing an abundance of high-index facets and surface kinks to weaken the CO binding affinity.[74]”

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Group 1 catalysts include p-block metals (Sn, In, Pb, Ti, Hg, Cd, Bi) which are selective for formate production.

“Group 1 catalysts include p-block metals (e.g., Sn, In, Pb, Ti, Hg, Cd, and Bi) which are selective for formate production.”

Working in the potential range of -0.6 to 0.85 V is beneficial for ethylene formation.

“These findings suggest that working in the potential range of -0.6 to 0.85 V is beneficial for ethylene formation.”

GBs may also create high step densities that are catalytically active for CO2RR.

“Distinct from dislocation-induced strain effects, GBs may also create high step densities that are catalytically active for the CO2RR.”

Wu et al. reported a nitrogen-doped carbon nanotube (NCNT) catalyst which obtained a FECO of 80% at an overpotential of 0.26 V.

“Wu et al. reported a nitrogen-doped carbon nanotube (NCNT) catalyst which obtained a FECO of 80% at an overpotential of 0.26 V. [87]”

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Further studies on NCNTs revealed that catalytic activity is very dependent on the density and type of nitrogen defect.

“Further studies on NCNTs revealed that the catalytic activity is very dependent on the density and type of nitrogen defect.[88]”

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GBs create catalytically active strained regions in polycrystalline Au by stabilizing dislocations.

“It was revealed that GBs create catalytically active strained regions in polycrystalline Au by stabilizing dislocations.”

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The introduction of small organic molecules on the surface of Ag nanoparticles was reported to be an effective way to improve CO2RR activity.

“Likewise, the introduction of small organic molecules on the surface of Ag nanoparticles was reported to be an effective way to improve the CO2RR activity.[85]”

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The NCNT performance was attributed to an optimum binding energy, yielding strong *COOH adsorption and *CO desorption.

“This performance was attributed to an optimum binding energy, yielding strong *COOH adsorption and *CO desorption.”

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Pyridinic N is found to be the most active site in carbon matrix for CO2RR as reported in a number of studies.

“Pyridinic N is found to be the most active site in carbon matrix for CO2RR as reported in a number of studies.”

Detailed DFT studies suggest the attachment of amine molecules to Ag nanoparticles destabilizes hydrogen binding, and thereby suppresses HER.

“Detailed DFT studies suggest the attachment of amine molecules to Ag nanoparticles destabilizes hydrogen binding, and thereby suppresses HER.[85]”

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OD Cu enhances CO2RR activity at lower overpotential.

“Similar to Au and Ag, a number of studies have explored the use of OD Cu for the enhancement of CO2RR activity at lower overpotential (Figure . 5C,D).[90,94,95]”

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The presence of microstrain in GB-rich Cu nanoparticles was revealed using TEM studies and Williamson-Hall analysis of X-ray diffraction patterns.

“The presence of microstrain in GB-rich Cu nanoparticles was revealed using transmission electron microscopy (TEM) studies and Williamson-Hall analysis of the X-ray diffraction patterns.”

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These findings enable high selectivity towards C2 products to be attributed to the residual Cu+.

“These findings enable high selectivity towards C2 products to be attributed to the residual Cu + .[101]”

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OD Au catalysts prepared by electrochemically reducing a thick Au oxide film can exhibit high FECO (>96%) at low overpotentials (0.24 V) compared to polycrystalline Au counterparts.

“Moreover, OD Au catalysts, prepared by electrochemically reducing a thick Au oxide film, can also exhibit a high FECO (>96%) at low overpotentials (0.24 V) compared to their polycrystalline Au counterparts (Figure .5A). [78]”

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This result indicates that unmodified Cu can be highly selective for C2+ products.

“This result indicates that unmodified Cu can be highly selective for C2+ products.”

Mariano et al. verified high CO2RR selectivity for GBs using scanning electrochemical cell microscopy.

“Recently, Mariano et al. verified the high CO2RR selectivity for GBs using scanning electrochemical cell microscopy.[80]”

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Differences in the oxidation state of Cu is another potential factor that may alter CO2RR activity.

“Differences in the oxidation state of Cu is another potential factor that may alter CO2RR activity.[92,93]”

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Pure CO has a small market.

“Considering the fact that pure CO does have a small market (Figure .1a), it would be worth pursuing a two-step tandem electrochemical system wherein the produced CO can further be electrochemical reduced (CORR) [91] to more energy value fuels.[26]”

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OD catalysts prepared by electrochemically reducing Ag2O demonstrated higher activity for CO2RR to CO than polycrystalline Ag.

“OD catalysts prepared by electrochemically reducing Ag2O demonstrated higher activity for CO2RR to CO than polycrystalline Ag.”

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The reduction in overpotential is attributed to the OD Ag's low-coordination surface sites, which enhance stabilization of the *COOH intermediate.

“The reduction in overpotential is attributed to the OD Ag's low-coordination surface sites, which enhance the stabilization of the *COOH intermediate.”

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The presence of chloride ions on the surface of Ag nanocorals more than doubles the FECO compared to chloride-free Ag nanocorals.

“The presence of chloride ions on the surface of Ag nanocorals more than doubles the FECO as compared to chloride-free Ag nanocorals.”

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OD nanostructuring of the catalyst has been demonstrated to be an effective strategy to enhance activity and reduce overpotential for CO production.

“In summary, OD nanostructuring of the catalyst has been demonstrated to be an effective strategy to enhance the activity and reduce the overpotential for CO production.”

Cu can produce up to 16 different products, including various hydrocarbons and alcohols, due to its ability to bind and further reduce the *CO intermediate.

“As the only metal catalyst in Group 3, Cu can produce up to 16 different products, [24] including various hydrocarbons and alcohols, due to its ability to bind and further reduce the *CO intermediate.”

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High selectivity towards multi-carbon products has been demonstrated on OD Cu at low overpotentials.

“While high selectivity towards multi-carbon products has been demonstrated on OD Cu [96] at low overpotentials, the underlying mechanism is yet to be well understood, and is often pointed to crystallite sizes and facets, strains, GBs, and rise in local pH.”

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The OD Ag required 0.49 V less overpotential than polycrystalline Ag (0.89 V) to achieve a FECO of 80%.

“The OD Ag required 0.49 V less overpotential than polycrystalline Ag (0.89 V) to achieve a FECO of 80%.”

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Hsieh et al. fabricated a Ag nanocoral electrocatalyst in the presence of aqueous chloride anions which demonstrated a FECO of 95% at an overpotential of 0.37 V.

“For instance, Hsieh et al. fabricated a Ag nanocoral electrocatalyst in the presence of aqueous chloride anions which demonstrated a FECO of 95% at an overpotential of 0.37 V. [83]”

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Further studies on OD Au demonstrated a large presence of grain boundaries (GB), which was linearly correlated with surface-area-normalized activity for CO2RR.

“Further studies on OD Au demonstrated a large presence of grain boundaries (GB), which was linearly correlated with surface-area-normalized activity for CO2RR (Figure .5B). [79]”

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Metal-free carbon-based materials have received attention due to their electrocatalytic activity for CO2RR.

“In an effort to reduce catalyst expense, metal-free carbon based materials have received attention due to their electrocatalytic activity for CO2RR.[86]”

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Mandal et al. did not detect CO2RR products as long as Cu2O was present at the catalyst surface.

“For example, using surface sensitive in-situ Raman spectroscopy with selected-ion flow tube mass spectrometry, Mandal et al. did not detect CO2RR products as long as Cu2O was present at the catalyst surface (Figure.7).[104]”

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An ion-exchange membrane prevents CO2RR products from crossing over to the anode where they can be oxidized back into CO2.

“First, it prevents CO2RR products from crossing over to the anode where they can be oxidized back into CO2.”

Liu et al. reported a B- and N-co-doped nanodiamond catalyst, with very high ethanol selectivity (FE of 93.2%) at -1.0 V vs. RHE.

“Liu et al. reported a B-and N-co-doped nanodiamond catalyst,, with very high ethanol selectivity (FE of 93.2 %) at -1.0 V vs. RHE.[119]”

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Eilert et al. recently performed in situ APXPS and TEM-EELS to reveal the presence of residual surface oxygen (e.g., Cu2O or Cu(OH)2) and trapped subsurface oxygen during reaction.

“Eilert et al. recently performed in situ ambient pressure X-ray photoelectron spectroscopy (APXPS) and TEM electron energy loss spectroscopy (TEM-EELS) to reveal the presence of residual surface oxygen (e.g., Cu2O or Cu(OH)2) and trapped subsurface oxygen during reaction.[106]”

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Alternative electrolyzer architectures are being pursued for large-scale implementation.

“Thus, alternative electrolyzer architectures are being pursued for large-scale implementation.”

Ren et al. tuned the Zn content in a series of OD CuxZn catalysts to achieve a FEC2H5OH of 29.1%.

“Ren et al. tuned the Zn content in a series of OD CuxZn catalysts to achieve a FEC2H5OH of 29.1%.[113]”

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The Ag doped phase-blended catalysts exhibited a three-fold higher ethanol selectivity (FEC2H5OH 34.15%) than that of the Cu2O without Ag dopant (10.5%).

“The Ag doped phase-blended catalysts exhibited a three-fold higher ethanol selectivity (FEC2H5OH 34.15%) than that of the Cu2O without Ag dopant (10.5%).”

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Faradaic efficiency of 92% for acetate production has been demonstrated at -0.8 to -1.0 V vs RHE.

“Faradaic efficiency of 92% for acetate production has been demonstrated at -0.8 to -1.0 V vs RHE.”

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For ionic CO2RR products, using the opposite type of ion exchange membrane (e.g., a CEM for an anionic species like formate) can prevent product crossover and loss.

“For ionic CO2RR products, using the opposite type of ion exchange membrane (i.e., a CEM for an anionic species, like formate) can prevent product crossover and loss.[122]”

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Liu et al. reported that nitrogen doped nanodiamond (NDD) can serve as a promising catalyst for the formation of acetate.

“Liu et al. reported that nitrogen doped nanodiamond (NDD) can serve as a promising catalyst for the formation of acetate (Figure.8 c,d).[120]”

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The high selectivity was attributed to the synergistic effect of the nanostructuring, yielding highly exposed edge sites, and the heteroatom N doping.

“The high selectivity was attributed to the synergistic effect of the nanostructuring, yielding highly exposed edge sites, and the heteroatom N doping (Figure 8 a,b).[118]”

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The choice of ion-exchange membrane (CEM, AEM, or BPM) depends on the products of interest.

“The choice of ion-exchange membrane, cation exchange membrane (CEM), anion exchange membrane (AEM), or bipolar membrane (BPM) depends on the products of interest.”

In general, the conclusions drawn from bulk sensitive techniques support the presence of residual Cu+ in OD Cu catalysts, which is credited with the enhanced CO2RR activity.

“In general, the conclusions drawn from bulk sensitive techniques support the presence of residual Cu + in OD Cu catalysts, which is to credited with the enhanced CO2RR activity.”

Metal-free carbon electrode materials have also been reported for CO2RR to hydrocarbons and oxygenates.

“Metal-free carbon electrode materials have also been reported for CO2RR to hydrocarbons and oxygenates.[86]”

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Cu-based metal alloys have also garnered interest due to their ability to tune product selectivity by modulating the stability of key reaction intermediates.

“Cu-based metal alloys have also garnered interest due to their ability to tune product selectivity by modulating the stability of key reaction intermediates.”

Jingjie et al. reported a N-doped graphene quantum dot catalyst with a selectivity towards ethylene and ethanol of 45%.

“Jingjie et al. reported a N-doped graphene quantum dot catalyst with a selectivity towards ethylene and ethanol of 45%.”

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In H cells, the current density is limited (typically under 100 mA/cm2) by the available CO2 dissolved in the electrolyte.

“In H cells the current density is limited (typically under 100 mA/cm 2 ) by the available CO2 dissolved in the electrolyte, which is only 34 mM at room temperature.[121]”

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An ion-exchange membrane restricts evolved O2 at the anode from crossing over to the cathode and stealing electrons for ORR that could otherwise be used for CO2RR.

“In addition, ion-exchange membranes restrict evolved O2 at the anode from crossing-over to the cathode and stealing electrons for ORR that could otherwise be used for CO2RR.”

The concentration of CO2 dissolved in the electrolyte in H cells is 34 mM at room temperature.

“In H cells the current density is limited (typically under 100 mA/cm 2 ) by the available CO2 dissolved in the electrolyte, which is only 34 mM at room temperature.[121]”

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Cavalca et al. determined that the stabilization of subsurface oxygen under the reaction conditions is possible due to the abundant defects present in the OD Cu electrodes.

“Moreover, Cavalca et al. determined that the stabilization of subsurface oxygen under the reaction conditions is possible due to the abundant defects present in the OD Cu electrodes.[108]”

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In brief, rationally designed Cu-based metal alloys have shown great promise in enhancing the selectivity of CO2RR towards multi-carbon alcohols.

“In brief, rationally designed Cu-based metal alloys have shown great promise in enhancing the selectivity of CO2RR towards multi-carbon alcohols.”

Ma et al. reported high selectivity (>60%) towards C2 chemicals using phase-separated CuPd alloys.

“Further, Ma et al. reported high selectivity (>60%) towards C2 chemicals using phase-separated CuPd alloys.[114]”

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DFT studies further support that subsurface oxygen enhances the adsorption energy, and consequently, the surface coverage of *CO on Cu (100).

“DFT studies further support that subsurface oxygen enhances the adsorption energy, and consequently, the surface coverage of *CO on Cu (100).[107]”

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Removing the membrane and relying on laminar electrolyte flow can mitigate cross-over in liquid phase electrolyzers.

“Another approach is to remove the membrane, and rely on laminar electrolyte flow to mitigate cross-over, akin to microfluidic fuel cells.[124,125]”

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It is difficult to maintain the laminar flow barrier when scaling membrane-free systems up in size, manifold length, and current density.

“In addition, it is difficult to maintain the laminar flow barrier when scaling these systems up in size, manifold length, and current density.”

Removing the membrane decreases capital costs.

“Removing the membrane reduces ohmic loss, simplifies the system requirements, decreases capital costs, and alleviates salting out issues.[126,127]”

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Gas phase electrolyzers involve less electrolyte pumps and flow fields than liquid phase electrolyzers.

“Gas phase electrolyzers have several advantages over their liquid phase counterparts as they involve less electrolyte pumps and flow fields, can be pressurized easily, and have lower ohmic losses.”

AEM gas phase electrolyzers typically have less competition from HER owing to decreased proton availability.

“Owing to the decreased proton availability, AEM systems typically have less competition from HER. [158]”

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Controlling proton transport and the local catalyst environment is key to higher CO2RR selectivity in membrane systems.

“Controlling proton transport and the local catalyst environment is the key to higher CO2RR selectivity in these membrane systems.”

Good selectivity and current densities have been demonstrated in liquid phase electrolyzers.

“While good selectivity and current densities have been demonstrated, further advances in gas diffusion electrode design (recent examples in section 5.2) are needed to increase the stability of these liquid phase electrolyzers.”

Reduced proton availability in AEM systems can influence CO2RR product distribution.

“In addition, reduced proton availability in the AEM system can influence the CO2RR product distribution.”

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Acidification of the cathode compartment over time can be an issue in CEM gas phase electrolyzers, especially at higher current densities, leading to increased HER and decreased CO2RR.

“On the other hand, the acidification of the cathode compartment over time can be an issue in these systems, especially at higher current densities, leading to increased HER and decreased CO2RR.[155,156]”

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Changes in product selectivity in gas phase CEM electrolyzers have been attributed to the influence of proton transport within the membrane, with poor proton transport favoring products requiring less proton transfers.

“Moreover, changes in product selectivity have been attributed to the influence of proton transport within the membrane, with poor proton transport favoring products requiring less proton transfers.[154]”

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BPMs solve the problem of pH gradient development in monopolar membrane systems by maintaining a constant pH in each side of the cell.

“BPMs solve the problem of pH gradient development in monopolar membrane systems, which can result in voltage fluctuations and instability over time, by maintaining a constant pH in each side of the cell.[170]”

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Removing the membrane simplifies the system requirements.

“Removing the membrane reduces ohmic loss, simplifies the system requirements, decreases capital costs, and alleviates salting out issues.[126,127]”

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Liquid phase electrolyzers have achieved high current densities toward CO, formate, and multi-carbon hydrocarbons/oxygenates.

“Liquid phase electrolyzer has achieved high current densities toward CO, [128][129][130][131][132][133] formate, [134][135][136] and multi-carbon hydrocarbons/oxygenates. [93,111,114,137,138]”

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• cites [93] sourceNot checked yet

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In direct comparisons with liquid phase electrolyzers, gas phase CEM electrolyzers have shown improved performance attributed to improved CO2 availability.

“In direct comparisons with liquid phase electrolyzers, gas phase CEM electrolyzers have shown improved performance attributed to improved CO2 availability.”

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The type of membrane used can influence product selectivity in CO2RR.

“Similarly, type of membrane used can also influence product selectivity.[160,161]”

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Including a buffer layer between the catalyst and membrane can circumvent cathode acidification in these systems.

“Researchers have managed to circumvent this acidification by including a buffer layer between the catalyst and membrane, however, these layers may not be effective for long run timescales, increase the resistance of the cell, and negate some of the most attractive features of gas phase electrolyzers.[157]”

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With control of fluid streams on both sides of the electrode, the liquid phase electrolyzer configuration allows for precise control and optimization of the reaction environment.

“With control of fluid streams on both sides of the electrode, this configuration allows for precise control and optimization of the reaction environment (electrolyte tuning is discussed in section 5.3).”

Removing the membrane alleviates salting out issues.

“Removing the membrane reduces ohmic loss, simplifies the system requirements, decreases capital costs, and alleviates salting out issues.[126,127]”

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The membrane-free laminar flow approach is not suitable for electrolyzers that generate liquid products while operating with recirculating electrolyte.

“However, this approach is not suitable for electrolyzers that generate liquid products while operating with recirculating electrolyte.”

Forcing CO2 in alkaline electrolytes can lead to bicarbonate/carbonate formation and salt precipitation.

“In addition to increasing ohmic resistance of electrolytes through bubble production, forcing CO2 in alkaline electrolytes can lead to bicarbonate/carbonate formation and salt precipitation, which reduce electrolyte conductivity, alter electrolyte pH, and block gas diffusion electrode/membrane pores.”

Membranes have been functionalized to tune selectivity and other performance parameters in the absence of a liquid electrolyte stream.

“In the absence of a liquid electrolyte stream with which to control the local reaction environment, membranes have been functionalized to tune selectivity and other performance parameters.[162]”

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In gas phase systems, catalysts that have poor CO2RR selectivity in liquid phase electrolyzers (Pt, Fe, Fe-Co, Pt-Ru, etc.) can produce methanol, isopropanol, and >C5 long chain products, although in small quantities.

“Researchers have also used catalysts that have poor CO2RR selectivity in liquid phase electrolyzers (Pt, Fe, Fe-Co, Pt-Ru, etc.) to produce products not typically observed in other flow cell systems, such as methanol, [146][147][148][149] isopropanol, [150,151] and >C5 long chain products, although in small quantities.[152,153]”

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Removing the membrane reduces ohmic loss.

“Removing the membrane reduces ohmic loss, simplifies the system requirements, decreases capital costs, and alleviates salting out issues.[126,127]”

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Li et al. determined that a 30% PTFE loading yielded high CO2 permeability in liquid phase electrolyzers.

“In liquid phase electrolyzers, Li et al. determined that a 30% PTFE loading yielded high CO2 permeability, [180]”

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Salvatore et al. improved the product selectivity of a BPM system through the addition of a solid-supported aqueous NaHCO3 layer between their BPM and Ag catalyst.

“Salvatore et al. improved the product selectivity of this type of system through the addition of a solid-supported aqueous NaHCO3 layer between their BPM and Ag catalyst.”

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As pH is increased, the selectivity of CO decreases in favor of formate formation on Ag catalysts.

“however, as the pH is increased, the selectivity of CO decreases in favor of formate formation. [47,131]”

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Kim et al. obtained the highest currents for CO production using a PTFE loading of 20% at the same cell voltage.

“while Kim et al. obtained the highest currents for CO production using a PTFE loading of 20% at the same cell voltage.[178]”

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Salvatore et al.'s modified BPM system ran for over 24 hours at 100 mA/cm2 and 65% FECO.

“With this modification, their system ran for over 24 hours at 100 mA/cm 2 and 65% FECO.[172]”

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Jhong et al. found that cathodes that were airbrushed outperformed hand-painted and screen-printed cathodes in terms of current density.

“Comparing different deposition methods, Jhong et al. found that cathodes that were airbrushed outperformed hand-painted and screen-printed cathodes in terms of current density.[132]”

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Flow cells operating under alkaline conditions have achieved improved selectivity for C2H4 at reduced overpotentials.

“Flow cells operating under alkaline conditions have achieved improved selectivity for C2H4 at reduced overpotentials.[138,208]”

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Kim et al. found notably higher current densities towards CO formation at a fixed cell voltage when using a thin, 60 μm macroporous substrate in a liquid phase CO2 electrolyzer.

“Using a liquid phase CO2 electrolyzer, Kim et al. studied the effect of macroporous substrate thickness where notably higher current densities towards CO formation were achieved at a fixed cell voltage when using a thin, 60 µm substrate.[178]”

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Replacing carbon black with TiO2 as the catalyst support, both at 40 wt%, allowed Ma et al. to nearly double the CO2RR partial current at the same cell voltage.

“Moreover, by replacing carbon black with TiO2 as the catalyst support, both at 40 wt%, allowed Ma et al. to nearly double the CO2RR partial current at the same cell voltage.[133]”

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The pH of the electrolyte is central to reaction selectivity and overpotentials in CO2RR.

“The pH of the electrolyte is central to reaction selectivity and overpotentials.”

Using Sn catalysts in liquid phase electrolyzers, one study found 20 wt% Nafion to yield the highest CO2RR partial currents.

“Using Sn catalysts in liquid phase electrolyzers, one study found 20 wt% Nafion to yield the highest CO2RR partial currents,”

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Li et al. demonstrated the reduction of CO2 to syngas using a BPM configuration with a Bi/ionic liquid catalyst for 14 h at 80 mA/cm2.

“For example, Li et al. demonstrated the reduction of CO2 to syngas using a BPM configuration with a Bi/ionic liquid catalyst for 14 h at 80 mA/cm 2 .[171]”

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For the production of formate, various studies in both H cells and flow cells have found that a bulk pH in the range of 2-4 yields the optimal selectivity with p-block metal catalysts.

“As for the production of formate, various studies in both H cells and flow cells have found that a bulk pH in the range of 2 -4 yields the optimal selectivity with p-block metal catalysts.[125,202,203]”

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In H cells, solutions with weak buffering abilities have been shown to increase the ratio of C2H4 to CH4.

“Thus in H cells, solutions with weak buffering abilities have been shown to increase the ratio of C2H4 to CH4. [205][206][207]”

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Unlike AEMs, BPMs inhibit the loss of cathodic liquid products to the anode via electromigration, diffusion, and/or electroosmotic drag, resulting in higher concentrations of recovered products.

“Moreover, unlike AEMs, BPMs inhibit the loss of the cathodic liquid products to the anode via electromigration, diffusion, and/or electroosmotic drag, resulting in higher concentrations of recovered products.[173]”

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Ma et al. later produced a fourfold increase in partial current density of CO when integrating carbon foam as the supporting material in place of carbon black.

“In a later report, they produced a fourfold increase in partial current density of CO when integrating carbon foam as the supporting material in place of carbon black.[185]”

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Larger cations Cs+ and K+ can greatly increase the total current density in CO2RR.

“Studies have demonstrated that larger cations, Cs + and K + , can greatly increase the total current density, [209,210]”

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Delacourt et al. found that catalysts with 20 wt% acetylene black support exhibited higher CO2RR selectivity compared to their unsupported counterparts.

“Similarly, in CO2RR, Delacourt et al. found that catalysts with 20 wt% acetylene black support exhibited higher CO2RR selectivity compared to their unsupported counterparts.[157]”

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On pressurized Ni and Fe, the selectivity towards longer chain hydrocarbon products increased with temperature up to 80°C.

“On pressurized Ni and Fe, the selectivity towards longer chain hydrocarbon products increased with temperature up to 80°C.[228,248]”

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Some catalysts (Fe, Co, Rh, Ni, Pd, Pt, and C) with predominantly H2 selectivity at 1 atm can generate CO or HCOOH when pressurized.

“In addition, some catalysts (Fe, Co, Rh, Ni, Pd, Pt, and C) with predominantly H2 selectivity at 1 atm can generate CO or HCOOH when pressurized.[140,141,143,223,[227][228][229]”

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Verma et al. discovered that the best selectivity and lowest overpotentials for CO production were obtained in the order OH- > HCO3- > Cl-.

“Verma et al. discovered that the best selectivity and lowest overpotentials for CO production were obtained in the order: OH -> HCO3 -> Cl -. [190]”

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Dufek et al. demonstrated an 18% improvement in their EE by increasing the temperature of their flow cell from 18 to 70 °C.

“Here, the most comprehensive example is that of Dufek et al. which demonstrated an 18% improvement in their EE by increasing the temperature of their flow cell from 18 to 70 °C.[142]”

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When the CO2 concentration is high enough in H cells to negate the decrease in solubility of CO2, increasing the temperature can have net positive effects on product selectivity and current density.

“Therefore, when the CO2 concentration is high enough in H cells to negate the decrease in solubility of the CO2, increasing the temperature can have net positive effects on product selectivity and current density.”

In H cell reactors, the increased CO2 solubility achieved at lower temperatures improves the selectivity for CO2RR as compared to the HER.

“In these systems, the increased CO2 solubility achieved at lower temperatures (Figure .13) improves the selectivity for CO2RR as compared to the HER.[239][240][241][242][243][244]”

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Ohmic losses may be prohibitively large in the case of less conductive electrolytes, such as 0.1 M KHCO3 (conductivity of 10 mS/cm compared to 1.0 M KOH which has a conductivity of 220 mS/cm), where the energy efficiency would be greatly impacted.

“Ohmic losses may be prohibitively large in the case of less conductive electrolytes, such as 0.1 M KHCO3 (conductivity of 10 mS/cm as compared to 1.0 M KOH which has a conductivity of 220 mS/cm), [218,219] where the energy efficiency would be greatly impacted”

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Pb operating at a constant pressure and temperatures of 60-80 °C yielded significant increases in both current density and formate selectivity.

“Moreover, Pb operating at a constant pressure and temperatures of 60 -80 °C yielded significant increases in both current density and formate selectivity.[225,249]”

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Larger cations Cs+ and K+ can suppress the HER.

“suppress the HER, [205,211]”

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To produce hydrocarbons on pressurized Cu, the current density must be increased.

“To produce hydrocarbons on pressurized Cu, the current density must be increased.[206,222]”

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For Cu catalysts, increasing pressure typically results in increased CO production at the same applied potential due to stimulated desorption of intermediates from the surface.

“For Cu catalysts, increasing pressure typically results in increased CO production at the same applied potential [222,231] due to stimulated desorption of intermediates from the surface.[232,233]”

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The primary benefit of elevated pressure is enhanced CO2 solubility in aqueous electrolytes, which allows for higher CO2RR current densities.

“The primary benefit of elevated pressure is well-evidenced in H cell reactors, the enhanced CO2 solubility in aqueous electrolytes, which allows for higher CO2RR current densities [221,222] .”

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Larger cations Cs+ and K+ can improve selectivity towards multi-carbon products, especially C2H4.

“and improve selectivity towards multi-carbon products, especially C2H4.[212,213]”

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Lee et al. found that elevating the operating temperature from 30 to 70 °C doubled their CO2RR partial current density at the same cell voltage.

“For example, Lee et al. found that elevating the operating temperature from 30 to 70 °C doubled their CO2RR partial current density at the same cell voltage.[145]”

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CO2RR is often performed in a different environment than the optimal OER conditions, introducing increased membrane resistances or other full cell kinetic and polarization losses, which are not typically captured in reported metrics.

“In addition, CO2RR is often performed in a different environment than the optimal OER conditions, introducing increased membrane resistances or other full cell kinetic and polarization losses, which are not typically captured in reported metrics.”

In many cases, high FEs for products of interest have been achieved with high overpotentials that are detrimental to the overall EE and the ultimate applicability.

“In many cases, high FEs for products of interest have been achieved with high overpotentials that are detrimental to the overall EE and the ultimate applicability.”

A remaining barrier to commercialization of this technology is the stability of the catalyst and/or the electrodes in these systems.

“A remaining barrier to commercialization of this technology is the stability of the catalyst and/or the electrodes in these systems.”

The ultimate commercial viability of strategies to enhance selectivity will require catalyst synthesis routes that are scalable.

“In addition, the ultimate commercial viability of these strategies will require catalyst synthesis routes that are scalable.[252]”

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Gutiérrez-Guerra et al. employed a H3PO4-doped polybenzimidazole polymer electrolyte membrane, which enabled electrolyzer operation at 110 °C.

“Notably, a recent work by Gutiérrez-Guerra et al. employed a H3PO4-doped polybenzimidazolepolymer electrolyte membrane, which enabled electrolyzer operation at 110 °C.[251]”

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