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ChemE/Materials for Energy Conversion and Storage, ChemE/O&O groep

Electrochemical reduction of CO2 presents an attractive way to store renewable energy in chemical bonds in a potentially carbon-neutral way. However, the available electrolyzers suffer from intrinsic problems, like flooding and salt accumulation, that must be overcome to industrialize the technology. To mitigate flooding and salt precipitation issues, researchers have used super-hydrophobic electrodes based on either expanded polytetrafluoroethylene (ePTFE) gas-diffusion layers (GDL’s), or carbon-based GDL’s with added PTFE. While the PTFE backbone is highly resistant to flooding, the non-conductive nature of PTFE means that without additional current collection the catalyst layer itself is responsible for electron-dispersion, which penalizes system efficiency and stability. In this work, we present operando results that illustrate that the current distribution and electrical potential distribution is far from a uniform distribution in thin catalyst layers (~50 nm) deposited onto ePTFE GDL’s. We then compare the effects of thicker catalyst layers (~500 nm) and a newly developed non-invasive current collector (NICC). The NICC can maintain more uniform current distributions with 10-fold thinner catalyst layers while improving stability towards ethylene (≥ 30%) by approximately two-fold., ChemE/Materials for Energy Conversion and Storage

Immobilizing molecular catalysts on electrodes is vital for electrochemical applications. However, creating robust electrode-catalyst interactions while maintaining good catalytic performance and rapid electron transfer is challenging. Here, without introducing any foreign elements, we show a bottom-up synthetic approach of constructing the conjugated C−C bond between the commercial Vulcan carbon electrode and an organometallic catalyst. Characterization results from FTIR, XPS, aberration-corrected TEM and EPR confirmed the successful and uniform heterogenization of the complex. The synthesized Vulcan-LN4−Co catalyst is highly active and selective in the oxygen reduction reaction in neutral media, showing an 80 % hydrogen peroxide selectivity and a 0.72 V (vs. RHE) onset potential which significantly outperformed the homogenous counterpart. Based on single-crystal XRD and NMR data, we built a model for density functional theory calculations which showed a nearly optimal binding energy for the *OOH intermediate. Our results show that the direct conjugated C−C bonding is an effective approach for heterogenizing molecular catalysts on carbon, opening new opportunities for employing molecular catalysts in electrochemical applications., ChemE/Materials for Energy Conversion and Storage

The electrochemical reduction of carbon dioxide (CO2) to value-added chemicals is a promising strategy to mitigate climate change. Metalloporphyrins have been used as a promising class of stable and tunable catalysts for the electrochemical reduction reaction of CO2 (CO2RR) but have been primarily restricted to single-carbon reduction products. Here, we utilize functionalized earth-abundant manganese tetraphenylporphyrin-based (Mn-TPP) molecular electrocatalysts that have been immobilized via electrografting onto a glassy carbon electrode (GCE) to convert CO2 with overall 94 % Faradaic efficiencies, with 62 % being converted to acetate. Tuning of Mn-TPP with electron-withdrawing sulfonate groups (Mn-TPPS) introduced mechanistic changes arising from the electrostatic interaction between the sulfonate groups and water molecules, resulting in better surface coverage, which facilitated higher conversion rates than the non-functionalized Mn-TPP. For Mn-TPP only carbon monoxide and formate were detected as CO2 reduction products. Density-functional theory (DFT) calculations confirm that the additional sulfonate groups could alter the C−C coupling pathway from *CO→*COH→*COH-CO to *CO→*CO-CO→*COH-CO, reducing the free energy barrier of C−C coupling in the case of Mn-TPPS. This opens a new approach to designing metalloporphyrin catalysts for two carbon products in CO2RR., ChemE/Materials for Energy Conversion and Storage, ChemE/O&O groep, QN/Kavli Nanolab Delft

The electrochemical CO2 reduction reaction (CO2RR) is an attractive method to produce renewable fuel and chemical feedstock using clean energy sources. Formate production represents one of the most economical target products from CO2RR but is primarily produced using post-transition metal catalysts that require comparatively high overpotentials. Here a composition of bimetallic Cu–Pd is formulated on 2D Ti3C2Tx (MXene) nanosheets that are lyophilized into a highly porous 3D aerogel, resulting in formate production much more efficient than post-transition metals. Using a membrane electrode assembly (MEA), formate selectivities >90% are achieved with a current density of 150 mA cm−2 resulting in the highest ever reported overall energy efficiency of 47% (cell potentials of −2.8 V), over 5 h of operation. A comparable Cu-Pd aerogel achieves near-unity CO production without the MXene templating. This simple strategy represents an important step toward the experimental demonstration of 3D-MXenes-based electrocatalysts for CO2RR application and opens a new platform for the fabrication of macroscale aerogel MXene-based electrocatalysts., ChemE/Materials for Energy Conversion and Storage, ChemE/Catalysis Engineering, ChemE/O&O groep

To explore the effects of solvent-ionomer interactions in catalyst inks on the structure and performance of Cu catalyst layers (CLs) for CO2 electrolysis, we used a “like for like” rationale to select acetone and methanol as dispersion solvents with a distinct affinity for the ionomer backbone or sulfonated ionic heads, respectively, of the perfluorinated sulfonic acid (PFSA) ionomer Aquivion. First, we characterized the morphology and wettability of Aquivion films drop-cast from acetone- and methanol-based inks on flat Cu foils and glassy carbons. On a flat surface, the ionomer films cast from the Aquivion and acetone mixture were more continuous and hydrophobic than films cast from methanol-based inks. Our study’s second stage compared the performance of Cu nanoparticle CLs prepared with acetone and methanol on gas diffusion electrodes (GDEs) in a flow cell electrolyzer. The effects of the ionomer-solvent interaction led to a more uniform and flooding-tolerant GDE when acetone was the dispersion solvent (acetone-CL) than when we used methanol (methanol-CL). As a result, acetone-CL yielded a higher selectivity for CO2 electrolysis to C2+ products at high current density, up to 25% greater than methanol-CL at 500 mA cm-2. Ethylene was the primary product for both CLs, with a Faradaic efficiency for ethylene of 47.4 ± 4.0% on the acetone-CL and that of 37.6 ± 5.5% on the methanol-CL at a current density of 300 mA cm-2. We attribute the enhanced C2+ selectivity of the acetone-CL to this electrode’s better resistance to electrolyte flooding, with zero seepage observed at tested current densities. Our findings reveal the critical role of solvent-ionomer interaction in determining the film structure and hydrophobicity, providing new insights into the CL design for enhanced multicarbon production in high current densities in CO2 electrolysis processes., Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public., ChemE/Materials for Energy Conversion and Storage

The nitrogen reduction reaction (NRR) is a promising pathway toward the decarbonization of ammonia (NH3) production. However, unless practical challenges related to the detection of NH3 are removed, confidence in published data and experimental throughput will remain low for experiments in aqueous electrolyte. In this perspective, we analyze these challenges from a system and instrumentation perspective. Through our analysis we show that detection challenges can be strongly reduced by switching from an Hcell to a gas diffusion electrode (GDE) cell design as a catalyst testing platform. Specifically, a GDE cell design is anticipated to allow for a reduction in the cost of crucial 15N2 control experiments from €100−2000 to less than €10. A major driver is the possibility to reduce the 15N2 flow rate to less than 1 mL/min, which is prohibited by an inevitable drop in mass-transport at low flow rates in H-cells. Higher active surface areas and improved mass transport can further circumvent losses of NRR selectivity to competing reactions. Additionally, obstacles often encountered when trying to transfer activity and selectivity data recorded at low current density in Hcells to commercial device level can be avoided by testing catalysts under conditions close to those in commercial devices from the start., ChemE/Materials for Energy Conversion & Storage

Continued advancements in the electrochemical reduction of CO 2 (CO 2RR) have emphasized that reactivity,selectivity, and stability are not explicit material properties butcombined effects of the catalyst, double-layer, reaction environ- ment, and system configuration. These realizations have steadily built upon the foundational work performed for a broad array of transition metals performed at 5 mA cm−2, which historically guided the research field. To encompass the changing advancements and mindset within the research field, an updated baseline at elevated current densities could then be of value. Here we seek to re-characterize the activity, selectivity, and stability of the five most utilized transition metal catalysts for CO2 RR (Ag, Au, Pd, Sn, and Cu) at elevated reaction rates through electrochemical operation, physical characterization, and varied operating parameters to provide a renewed resource and point of comparison. As a basis, we have employed a common cell architecture, highly controlled catalyst layer morphologies and thicknesses, and fixed current densities. Through a dataset of 88 separate experiments, we provide comparisons between CO-producing catalysts (Ag, Au, and Pd), highlighting CO-limiting current densities on Au and Pd at 72 and 50 mA cm−2, respectively. We further show the instability of Sn in highly alkaline environments, and the convergence of product selectivity at elevated current densities for a Cu catalyst in neutral andalkaline media. Lastly, we reflect upon the use and limits of reaction rates as a baseline metric by comparing catalytic selectivity at 10 versus 200 mA cm−2. We hope the collective work provides a resource for researchers setting up CO 2RR experiments for the first time., ChemE/Materials for Energy Conversion & Storage

Finding alternative ways to tailor the electronic properties of a catalyst to actively and selectively drive reactions of interest has been a growing research topic in the field of electrochemistry. In this Letter, we investigate the tuning of the surface electronic properties of electrocatalysts via polymer modification. We show that when a nickel oxide water oxidation catalyst is coated with polytetrafluoroethylene, stable Ni-CFx bonds are introduced at the nickel oxide/polymer interface, resulting in shifting of the reaction selectivity away from the oxygen evolution reaction and toward hydrogen peroxide formation. It is shown that the electron-withdrawing character of the surface fluorocarbon molecule leaves a slight positive charge on the water oxidation intermediates at the adjacent active nickel sites, making their bonds weaker. The concept of modifying the surface electronic properties of an electrocatalyst via stable polymer modification offers an additional route to tune multipathway reactions in polymer/electrocatalyst environments, like with ionomer-modified catalysts or with membrane electrode assemblies., ChemE/Materials for Energy Conversion & Storage, ChemE/Inorganic Systems Engineering, ChemE/Chemical Engineering

The electrochemical reduction of bicarbonate to renewable chemicals without external gaseous CO2 supply has been motivated as a means of integrating conversion with upstream CO2 capture. The way that CO2 is formed and transported during CO2-mediated bicarbonate reduction in flow cells is profoundly different from conventional CO2 saturated and gas-fed systems and a thorough understanding of the process would allow further advancements. Here, we report a comprehensive two-phase mass transport model to estimate the local concentration of species in the porous electrode resultant from homogeneous and electrochemical reactions of (bi)carbonate and CO2. The model indicates that significant CO2 is generated in the porous electrode during electrochemical reduction, even though the starting bicarbonate solution contains negligible CO2. However, the in situ formation of CO2 and subsequent reduction to CO exhibits a plateau at high potentials due to neutralization of the protons by the alkaline reaction products, acting as the limiting step toward higher CO current densities. Nevertheless, the pH in the catalyst layer exhibits a relatively smaller rise, compared to conventional electrochemical CO2 reduction cells, because of the reaction between protons and CO32- and OH- that is confined to a relatively small volume. A large fraction of the CL exhibits a mildly alkaline environment at high current densities, while an appreciable amount of carbonic acid (0.1-1 mM) and a lower pH exist adjacent to the membrane, which locally favor hydrogen evolution, especially at low electrolyte concentrations. The results presented here provide insights into local cathodic conditions for both bicarbonate cells and direct-CO2 reduction membrane electrode assembly cells utilizing cation exchange membranes facing the cathode., ChemE/Materials for Energy Conversion & Storage

The ever-growing level of carbon dioxide (CO2) in our atmosphere, is at once a threat and an opportunity. The development of sustainable and cost-effective pathways to convert CO2 to value-added chemicals is central to reducing its atmospheric presence. Electrochemical CO2 reduction reactions (CO2RRs) driven by renewable electricity are among the most promising techniques to utilize this abundant resource; however, in order to reach a system viable for industrial implementation, continued improvements to the design of electrocatalysts is essential to improve the economic prospects of the technology. This review summarizes recent developments in heterogeneous porphyrin-based electrocatalysts for CO2 capture and conversion. We specifically discuss the various chemical modifications necessary for different immobilization strategies, and how these choices influence catalytic properties. Although a variety of molecular catalysts have been proposed for CO2RRs, the stability and tunability of porphyrin-based catalysts make their use particularly promising in this field. We discuss the current challenges facing CO2RRs using these catalysts and our own solutions that have been pursued to address these hurdles., ChemE/Materials for Energy Conversion & Storage, ChemE/Product and Process Engineering

Achieving operational stability at high current densities remains a challenge in CO2 electrolyzers due to flooding of the gas diffusion layer (GDL) that supports the electrocatalyst. We mitigated electrode flooding at high current densities using a vacuum-assisted infiltration method to embed 200-400 nm-sized polytetrafluoroethylene (PTFE) particles at the interface of the microporous layer (MPL) and carbon cloth in a commercial GDL. In CO2 electrolysis to CO over a silver nanoparticle catalyst on the GDL, the PTFE-embedded GDL not only just exhibited less than 10% of the electrolyte seepage rates observed in untreated GDLs at a current density of 300 mA·cm-2 but also expanded the electrochemical active area across the testing conditions. The PTFE-embedded GDL also maintained a Faradaic efficiency for CO2 electrolysis to CO above 80% for more than 100 h at 100 mA·cm-2, which was a 50-fold improvement in the stable operation time of the electrolyzer., Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public., ChemE/Materials for Energy Conversion and Storage

The specific identity of electrolyte cations has many implications in various electrochemical reactions. However, the exact mechanism by which cations affect electrochemical reactions is not agreed upon in the literature. In this report, we investigate the role of cations during the electrochemical reduction of CO2 by chelating the cations with cryptands, to change the interaction of the cations with the components of the electric double layer. As previously reported we do see the apparent suppression of CO2 reduction in the absence of cations. However, using in situ-SEIRAS we see that CO2 reduction does indeed take place albeit at very reduced scales. We also observe that cations play a role in tuning the absorption strengths of not only CO2 as has been speculated, but also that of reaction products such as CO., Green Open Access added to TU Delft Institutional Repository 'You share, we take care!' - Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public., ChemE/Materials for Energy Conversion and Storage, ChemE/Inorganic Systems Engineering

Electrochemical conversion of gaseous CO2 to value-added products and fuels is a promising approach to achieve net-zero CO2 emission energy systems. Significant efforts have been achieved in the design and synthesis of highly active and selective electrocatalysts for this reaction and their reaction mechanism. To perform an efficient conversion and desired product selectivity in practical applications, we need an active, cost-effective, stable, and scalable electrolyzer design. Membrane-electrode assemblies (MEAs) can be an efficient solution to address the key challenges in the aqueous gas diffusion electrodes (GDE), e.g., ohmic resistances and complex reactor design. This review presents a critical overview of recent advances in experimental design and simulation of MEAs for CO2 reduction reaction, including the shortcomings and remedial strategies. In the last section, the remaining challenges and future research opportunities are suggested to support the advancement of CO2 electrochemical technologies., Green Open Access added to TU Delft Institutional Repository 'You share, we take care!' - Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public., ChemE/Materials for Energy Conversion and Storage

The electrochemical reduction of carbon dioxide (CO2) to value-added materials has received considerable attention. Both bulk transition-metal catalysts and molecular catalysts affixed to conductive noncatalytic solid supports represent a promising approach toward the electroreduction of CO2. Here, we report a combined silver (Ag) and pyridine catalyst through a one-pot and irreversible electrografting process, which demonstrates the enhanced CO2conversion versus individual counterparts. We find that by tailoring the pyridine carbon chain length, a 200 mV shift in the onset potential is obtainable compared to the bare silver electrode. A 10-fold activity enhancement at -0.7 V vs reversible hydrogen electrode (RHE) is then observed with demonstratable higher partial current densities for CO, indicating that a cocatalytic effect is attainable through the integration of the two different catalytic structures. We extended the performance to a flow cell operating at 150 mA/cm2, demonstrating the approach's potential for substantial adaptation with various transition metals as supports and electrografted molecular cocatalysts., ChemE/Materials for Energy Conversion and Storage, ChemE/O&O groep

Salt precipitation is a problem in electrochemical CO2 reduction electrolyzers that limits their long-term durability and industrial applicability by reducing the active area, causing flooding and hindering gas transport. Salt crystals form when hydroxide generation from electrochemical reactions interacts homogeneously with CO2 to generate substantial quantities of carbonate. In the presence of sufficient electrolyte cations, the solubility limits of these species are reached, resulting in "salting out"conditions in cathode compartments. Detrimental salt precipitation is regularly observed in zero-gap membrane electrode assemblies, especially when operated at high current densities. This Perspective briefly discusses the mechanisms for salt formation, and recently reported strategies for preventing or reversing salt formation in zero-gap CO2 reduction membrane electrode assemblies. We link these approaches to the solubility limit of potassium carbonate within the electrolyzer and describe how each strategy separately manipulates water, potassium, and carbonate concentrations to prevent (or mitigate) salt formation., ChemE/Materials for Energy Conversion and Storage

The electrochemical reduction of CO2 (CO2RR) on silver catalysts has been demonstrated under elevated current density, longer reaction times, and intermittent operation. Maintaining performance requires that CO2 can access the entire geometric catalyst area, thus maximizing catalyst utilization. Here we probe the time-dependent factors impacting geometric catalyst utilization for CO2RR in a zero-gap membrane electrode assembly. We use three flow fields (serpentine, parallel, and interdigitated) as tools to disambiguate cell behavior. Cathode pressure drop is found to play the most critical role in maintaining catalyst utilization at all time scales by encouraging in-plane CO2 transport throughout the gas-diffusion layer (GDL) and around salt and water blockages. The serpentine flow channel with the highest pressure drop is then the most failure-resistant, achieving a CO partial current density of 205 mA/cm2 at 2.76 V. These findings are confirmed through selectivity measurements over time, double-layer capacitance measurements to estimate GDL flooding, and transport modeling of the spatial CO2 concentration., ChemE/Materials for Energy Conversion and Storage, ChemE/O&O groep

Integrating carbon dioxide (CO2) electrolysis with CO2 capture provides exciting new opportunities for energy reductions by simultaneously removing the energy-demanding regeneration step in CO2 capture and avoiding critical issues faced by CO2 gas-fed electrolysers. However, understanding the potential energy advantages of an integrated process is not straightforward due to the interconnected processes which require knowledge of both capture and electrochemical conversion processes. Here, we identify the upper limits of the integrated process from an energy perspective by comparing the working principles and performance of integrated and sequential approaches. Our high-level energy analyses unveil that an integrated electrolyser must show similar performance to the gas-fed electrolyser to ensure an energy benefit of up to 44% versus the sequential route. However, such energy benefits diminish if future gas-fed electrolysers resolve the CO2 utilisation issue and if an integrated electrolyser shows lower conversion efficiencies than the gas-fed system., ChemE/Materials for Energy Conversion and Storage

Electrolysis of water, CO2, and nitrogen-based compounds presents the opportunity of generating fossil-free fuels and feedstocks at an industrial scale. Such devices are complex in operation, and their performance metrics are usually reported as electrode-averaged quantities. In this work, we report the usage of infrared thermography to map the electrochemical activity of a gas-diffusion electrode performing water and CO2reduction. By associating the heat map to a characteristic catalytic activity, the presented system can capture electrochemical and physical phenomena as they occur in electrolyzers for large-scale energy applications. We demonstrate applications for catalyst screening, catalyst-degradation measurements, and spatial activity mapping for water and CO2electrolysis at current densities up to 0.2 A cm-2. At these current densities we report catalyst temperature increases (>10 K for 0.2 A cm-2) not apparent otherwise. Furthermore, substantial localized current density fluctuations are present. These observations challenge assumed local conditions, providing new fundamental and applied perspectives., ChemE/Materials for Energy Conversion and Storage

Carbon dioxide (CO2) electrolysis is a promising route to utilise captured CO2 as a building block to produce valuable feedstocks and fuels such as carbon monoxide and ethylene. Very recently, CO2 electrolysis has been proposed as an alternative process to replace the amine recovery unit of the commercially available amine-based CO2 capture process. This process would replace the most energy-intensive unit operation in amine scrubbing while providing a route for CO2 conversion. The key enabler for such process integration is to develop an efficient integrated electrolyser that can convert CO2 and recover the amine simultaneously. Herein, this review provides an overview of the fundamentals and recent progress in advancing integrated CO2 conversion in amine-based capture media. This review first discusses the mechanisms for both CO2 absorption in the capture medium and electrochemical conversion of the absorbed CO2. We then summarise recent advances in improving the efficiency of integrated electrolysis via innovating electrodes, tailoring the local reaction environment, optimising operation conditions (e.g., temperatures and pressures), and modifying cell configurations. This review is concluded with future research directions for understanding and developing integrated CO2 electrolysers., ChemE/Materials for Energy Conversion and Storage

Continued advancements in the electrochemical reduction of CO 2 (CO 2RR) have emphasized that reactivity,selectivity, and stability are not explicit material properties butcombined effects of the catalyst, double-layer, reaction environ- ment, and system configuration. These realizations have steadily built upon the foundational work performed for a broad array of transition metals performed at 5 mA cm−2, which historically guided the research field. To encompass the changing advancements and mindset within the research field, an updated baseline at elevated current densities could then be of value. Here we seek to re-characterize the activity, selectivity, and stability of the five most utilized transition metal catalysts for CO2 RR (Ag, Au, Pd, Sn, and Cu) at elevated reaction rates through electrochemical operation, physical characterization, and varied operating parameters to provide a renewed resource and point of comparison. As a basis, we have employed a common cell architecture, highly controlled catalyst layer morphologies and thicknesses, and fixed current densities. Through a dataset of 88 separate experiments, we provide comparisons between CO-producing catalysts (Ag, Au, and Pd), highlighting CO-limiting current densities on Au and Pd at 72 and 50 mA cm−2, respectively. We further show the instability of Sn in highly alkaline environments, and the convergence of product selectivity at elevated current densities for a Cu catalyst in neutral andalkaline media. Lastly, we reflect upon the use and limits of reaction rates as a baseline metric by comparing catalytic selectivity at 10 versus 200 mA cm−2. We hope the collective work provides a resource for researchers setting up CO 2RR experiments for the first time., ChemE/Materials for Energy Conversion and Storage

Finding alternative ways to tailor the electronic properties of a catalyst to actively and selectively drive reactions of interest has been a growing research topic in the field of electrochemistry. In this Letter, we investigate the tuning of the surface electronic properties of electrocatalysts via polymer modification. We show that when a nickel oxide water oxidation catalyst is coated with polytetrafluoroethylene, stable Ni-CFx bonds are introduced at the nickel oxide/polymer interface, resulting in shifting of the reaction selectivity away from the oxygen evolution reaction and toward hydrogen peroxide formation. It is shown that the electron-withdrawing character of the surface fluorocarbon molecule leaves a slight positive charge on the water oxidation intermediates at the adjacent active nickel sites, making their bonds weaker. The concept of modifying the surface electronic properties of an electrocatalyst via stable polymer modification offers an additional route to tune multipathway reactions in polymer/electrocatalyst environments, like with ionomer-modified catalysts or with membrane electrode assemblies., ChemE/Materials for Energy Conversion and Storage, ChemE/Inorganic Systems Engineering, ChemE/Chemical Engineering

The electrochemical reduction of bicarbonate to renewable chemicals without external gaseous CO2 supply has been motivated as a means of integrating conversion with upstream CO2 capture. The way that CO2 is formed and transported during CO2-mediated bicarbonate reduction in flow cells is profoundly different from conventional CO2 saturated and gas-fed systems and a thorough understanding of the process would allow further advancements. Here, we report a comprehensive two-phase mass transport model to estimate the local concentration of species in the porous electrode resultant from homogeneous and electrochemical reactions of (bi)carbonate and CO2. The model indicates that significant CO2 is generated in the porous electrode during electrochemical reduction, even though the starting bicarbonate solution contains negligible CO2. However, the in situ formation of CO2 and subsequent reduction to CO exhibits a plateau at high potentials due to neutralization of the protons by the alkaline reaction products, acting as the limiting step toward higher CO current densities. Nevertheless, the pH in the catalyst layer exhibits a relatively smaller rise, compared to conventional electrochemical CO2 reduction cells, because of the reaction between protons and CO32- and OH- that is confined to a relatively small volume. A large fraction of the CL exhibits a mildly alkaline environment at high current densities, while an appreciable amount of carbonic acid (0.1-1 mM) and a lower pH exist adjacent to the membrane, which locally favor hydrogen evolution, especially at low electrolyte concentrations. The results presented here provide insights into local cathodic conditions for both bicarbonate cells and direct-CO2 reduction membrane electrode assembly cells utilizing cation exchange membranes facing the cathode., ChemE/Materials for Energy Conversion and Storage

The ever-growing level of carbon dioxide (CO2) in our atmosphere, is at once a threat and an opportunity. The development of sustainable and cost-effective pathways to convert CO2 to value-added chemicals is central to reducing its atmospheric presence. Electrochemical CO2 reduction reactions (CO2RRs) driven by renewable electricity are among the most promising techniques to utilize this abundant resource; however, in order to reach a system viable for industrial implementation, continued improvements to the design of electrocatalysts is essential to improve the economic prospects of the technology. This review summarizes recent developments in heterogeneous porphyrin-based electrocatalysts for CO2 capture and conversion. We specifically discuss the various chemical modifications necessary for different immobilization strategies, and how these choices influence catalytic properties. Although a variety of molecular catalysts have been proposed for CO2RRs, the stability and tunability of porphyrin-based catalysts make their use particularly promising in this field. We discuss the current challenges facing CO2RRs using these catalysts and our own solutions that have been pursued to address these hurdles., ChemE/Materials for Energy Conversion and Storage, ChemE/Product and Process Engineering

The nitrogen reduction reaction (NRR) is a promising pathway toward the decarbonization of ammonia (NH3) production. However, unless practical challenges related to the detection of NH3 are removed, confidence in published data and experimental throughput will remain low for experiments in aqueous electrolyte. In this perspective, we analyze these challenges from a system and instrumentation perspective. Through our analysis we show that detection challenges can be strongly reduced by switching from an Hcell to a gas diffusion electrode (GDE) cell design as a catalyst testing platform. Specifically, a GDE cell design is anticipated to allow for a reduction in the cost of crucial 15N2 control experiments from €100−2000 to less than €10. A major driver is the possibility to reduce the 15N2 flow rate to less than 1 mL/min, which is prohibited by an inevitable drop in mass-transport at low flow rates in H-cells. Higher active surface areas and improved mass transport can further circumvent losses of NRR selectivity to competing reactions. Additionally, obstacles often encountered when trying to transfer activity and selectivity data recorded at low current density in Hcells to commercial device level can be avoided by testing catalysts under conditions close to those in commercial devices from the start., ChemE/Materials for Energy Conversion and Storage

The production of value added C1 and C2 compounds within CO2 electrolyzers has reached sufficient catalytic performance that system and process performance-such as CO2 utilization-have come more into consideration. Efforts to assess the limitations of CO2 conversion and crossover within electrochemical systems have been performed, providing valuable information to position CO2 electrolyzers within a larger process. Currently missing, however, is a clear elucidation of the inevitable trade-offs that exist between CO2 utilization and electrolyzer performance, specifically how the faradaic efficiency of a system varies with CO2 availability. Such information is needed to properly assess the viability of the technology. In this work, we provide a combined experimental and 3D modelling assessment of the trade-offs between CO2 utilization and selectivity at 200 mA cm-2 within a membrane-electrode assembly CO2 electrolyzer. Using varying inlet flow rates we demonstrate that the variation in spatial concentration of CO2 leads to spatial variations in faradaic efficiency that cannot be captured using common 'black box' measurement procedures. Specifically, losses of faradaic efficiency are observed to occur even at incomplete CO2 consumption (80%). Modelling of the gas channel and diffusion layers indicated that at least a portion of the H2 generated is considered as avoidable by proper flow field design and modification. The combined work allows for a spatially resolved interpretation of product selectivity occurring inside the reactor, providing the foundation for design rules in balancing CO2 utilization and device performance in both lab and scaled applications. This journal is, ChemE/Materials for Energy Conversion & Storage, ChemE/O&O groep

Advancing reaction rates for electrochemical CO2 reduction in membrane electrode assemblies (MEAs) have boosted the promise of the technology while exposing new shortcomings. Among these is the maximum utilization of CO2, which is capped at 50% (CO as targeted product) due to unwanted homogeneous reactions. Using bipolar membranes in an MEA (BPMEA) has the capability of preventing parasitic CO2 losses, but their promise is dampened by poor CO2 activity and selectivity. In this work, we enable a 3-fold increase in the CO2 reduction selectivity of a BPMEA system by promoting alkali cation (K+) concentrations on the catalyst's surface, achieving a CO Faradaic efficiency of 68%. When compared to an anion exchange membrane, the cation-infused bipolar membrane (BPM) system shows a 5-fold reduction in CO2 loss at similar current densities, while breaking the 50% CO2 utilization mark. The work provides a combined cation and BPM strategy for overcoming CO2 utilization issues in CO2 electrolyzers., ChemE/Materials for Energy Conversion & Storage

CO 2 electroreduction offers a route to net-zero-emission production of C 2H 4—the most-produced organic compound. However, the formation of carbonate in this process causes loss of CO 2 and a severe energy consumption/production penalty. Dividing the CO 2-to-C 2H 4 process into two cascading steps—CO 2 reduction to CO in a solid-oxide electrolysis cell (SOEC) and CO reduction to C 2H 4 in a membrane electrode assembly (MEA) electrolyser—would enable carbonate-free C 2H 4 electroproduction. However, this cascade approach requires CO-to-C 2H 4 with energy efficiency well beyond demonstrations to date. Here, we present a layered catalyst structure composed of a metallic Cu, N-tolyl-tetrahydro-bipyridine, and SSC ionomer that enables efficient CO-to-C 2H 4 in a MEA electrolyser. In the full SOEC-MEA cascade approach, we achieve CO 2-to-C 2H 4 with no loss of CO 2 to carbonate and a total energy requirement of ~138 GJ (ton C 2H 4) −1, representing a ~48% reduction in energy intensity compared with the direct route., Accepted Author Manuscript, ChemE/Materials for Energy Conversion & Storage

The deployment of gas diffusion electrodes (GDEs) for the electrochemical CO2 reduction reaction (CO2RR) has enabled current densities an order of magnitude greater than those of aqueous H cells. The gains in production, however, have come with stability challenges due to rapid flooding of GDEs, which frustrate both laboratory experiments and scale-up prospects. Here, we investigate the role of carbon gas diffusion layers (GDLs) in the advent of flooding during CO2RR, finding that applied potential plays a central role in the observed instabilities. Electrochemical characterization of carbon GDLs with and without catalysts suggests that the high overpotential required during electrochemical CO2RR initiates hydrogen evolution on the carbon GDL support. These potentials impact the wetting characteristics of the hydrophobic GDL, resulting in flooding that is independent of CO2RR. Findings from this work can be extended to any electrochemical reduction reaction using carbon-based GDEs (CORR or N2RR) with cathodic overpotentials of less than -0.65 V versus a reversible hydrogen electrode., ChemE/Materials for Energy Conversion & Storage

The electrochemical reduction of carbon dioxide (CO2RR) requires access to ample gaseous CO2and liquid water to fuel reactions at high current densities for industrial-scale applications. Substantial improvement of the CO2RR rate has largely arisen from positioning the catalyst close to gas-liquid interfaces, such as in gas-diffusion electrodes. These requirements add complexity to an electrode design that no longer consists of only a catalyst but also a microporous and nanoporous network of gas-liquid-solid interfaces of the electrode. In this three-dimensional structure, electrode wettability plays a pivotal role in the CO2RR because the affinity of the electrode surface by water impacts the observed electrode reactivity, product selectivity, and long-term stability. All these performance metrics are critical in an industrial electrochemical process. This review provides an in-depth analysis of electrode wettability's role in achieving an efficient, selective, and stable CO2RR performance. We first discuss the underlying mechanisms of electrode wetting phenomena and the foreseen ideal wetting conditions for the CO2RR. Then we summarize recent advances in improving cathode performance by altering the wettability of the catalyst layer of gas-diffusion electrodes. We conclude the review by discussing the current challenges and opportunities to develop efficient and selective cathodes for CO2RR at industrially relevant rates. The insights generated from this review could also benefit the advancement of other critical electrochemical processes that involve multiple complex flows in porous electrodes, such as electrochemical reduction of carbon monoxide, oxygen, and nitrogen., ChemE/Materials for Energy Conversion & Storage

The electrochemical reduction of carbon dioxide (CO2RR) requires access to ample gaseous CO2and liquid water to fuel reactions at high current densities for industrial-scale applications. Substantial improvement of the CO2RR rate has largely arisen from positioning the catalyst close to gas-liquid interfaces, such as in gas-diffusion electrodes. These requirements add complexity to an electrode design that no longer consists of only a catalyst but also a microporous and nanoporous network of gas-liquid-solid interfaces of the electrode. In this three-dimensional structure, electrode wettability plays a pivotal role in the CO2RR because the affinity of the electrode surface by water impacts the observed electrode reactivity, product selectivity, and long-term stability. All these performance metrics are critical in an industrial electrochemical process. This review provides an in-depth analysis of electrode wettability's role in achieving an efficient, selective, and stable CO2RR performance. We first discuss the underlying mechanisms of electrode wetting phenomena and the foreseen ideal wetting conditions for the CO2RR. Then we summarize recent advances in improving cathode performance by altering the wettability of the catalyst layer of gas-diffusion electrodes. We conclude the review by discussing the current challenges and opportunities to develop efficient and selective cathodes for CO2RR at industrially relevant rates. The insights generated from this review could also benefit the advancement of other critical electrochemical processes that involve multiple complex flows in porous electrodes, such as electrochemical reduction of carbon monoxide, oxygen, and nitrogen., ChemE/Materials for Energy Conversion and Storage

CO 2 electroreduction offers a route to net-zero-emission production of C 2H 4—the most-produced organic compound. However, the formation of carbonate in this process causes loss of CO 2 and a severe energy consumption/production penalty. Dividing the CO 2-to-C 2H 4 process into two cascading steps—CO 2 reduction to CO in a solid-oxide electrolysis cell (SOEC) and CO reduction to C 2H 4 in a membrane electrode assembly (MEA) electrolyser—would enable carbonate-free C 2H 4 electroproduction. However, this cascade approach requires CO-to-C 2H 4 with energy efficiency well beyond demonstrations to date. Here, we present a layered catalyst structure composed of a metallic Cu, N-tolyl-tetrahydro-bipyridine, and SSC ionomer that enables efficient CO-to-C 2H 4 in a MEA electrolyser. In the full SOEC-MEA cascade approach, we achieve CO 2-to-C 2H 4 with no loss of CO 2 to carbonate and a total energy requirement of ~138 GJ (ton C 2H 4) −1, representing a ~48% reduction in energy intensity compared with the direct route., Accepted Author Manuscript, ChemE/Materials for Energy Conversion and Storage

The production of value added C1 and C2 compounds within CO2 electrolyzers has reached sufficient catalytic performance that system and process performance-such as CO2 utilization-have come more into consideration. Efforts to assess the limitations of CO2 conversion and crossover within electrochemical systems have been performed, providing valuable information to position CO2 electrolyzers within a larger process. Currently missing, however, is a clear elucidation of the inevitable trade-offs that exist between CO2 utilization and electrolyzer performance, specifically how the faradaic efficiency of a system varies with CO2 availability. Such information is needed to properly assess the viability of the technology. In this work, we provide a combined experimental and 3D modelling assessment of the trade-offs between CO2 utilization and selectivity at 200 mA cm-2 within a membrane-electrode assembly CO2 electrolyzer. Using varying inlet flow rates we demonstrate that the variation in spatial concentration of CO2 leads to spatial variations in faradaic efficiency that cannot be captured using common 'black box' measurement procedures. Specifically, losses of faradaic efficiency are observed to occur even at incomplete CO2 consumption (80%). Modelling of the gas channel and diffusion layers indicated that at least a portion of the H2 generated is considered as avoidable by proper flow field design and modification. The combined work allows for a spatially resolved interpretation of product selectivity occurring inside the reactor, providing the foundation for design rules in balancing CO2 utilization and device performance in both lab and scaled applications. This journal is, ChemE/Materials for Energy Conversion and Storage, ChemE/O&O groep

Advancing reaction rates for electrochemical CO2 reduction in membrane electrode assemblies (MEAs) have boosted the promise of the technology while exposing new shortcomings. Among these is the maximum utilization of CO2, which is capped at 50% (CO as targeted product) due to unwanted homogeneous reactions. Using bipolar membranes in an MEA (BPMEA) has the capability of preventing parasitic CO2 losses, but their promise is dampened by poor CO2 activity and selectivity. In this work, we enable a 3-fold increase in the CO2 reduction selectivity of a BPMEA system by promoting alkali cation (K+) concentrations on the catalyst's surface, achieving a CO Faradaic efficiency of 68%. When compared to an anion exchange membrane, the cation-infused bipolar membrane (BPM) system shows a 5-fold reduction in CO2 loss at similar current densities, while breaking the 50% CO2 utilization mark. The work provides a combined cation and BPM strategy for overcoming CO2 utilization issues in CO2 electrolyzers., ChemE/Materials for Energy Conversion and Storage

The deployment of gas diffusion electrodes (GDEs) for the electrochemical CO2 reduction reaction (CO2RR) has enabled current densities an order of magnitude greater than those of aqueous H cells. The gains in production, however, have come with stability challenges due to rapid flooding of GDEs, which frustrate both laboratory experiments and scale-up prospects. Here, we investigate the role of carbon gas diffusion layers (GDLs) in the advent of flooding during CO2RR, finding that applied potential plays a central role in the observed instabilities. Electrochemical characterization of carbon GDLs with and without catalysts suggests that the high overpotential required during electrochemical CO2RR initiates hydrogen evolution on the carbon GDL support. These potentials impact the wetting characteristics of the hydrophobic GDL, resulting in flooding that is independent of CO2RR. Findings from this work can be extended to any electrochemical reduction reaction using carbon-based GDEs (CORR or N2RR) with cathodic overpotentials of less than -0.65 V versus a reversible hydrogen electrode., ChemE/Materials for Energy Conversion and Storage

Electrochemical CO2 electrolysis to produce hydrocarbon fuels or material feedstocks offers a renewable alternative to fossilized carbon sources. Gas-diffusion electrodes (GDEs), composed of solid electrocatalysts on porous supports positioned near the interface of a conducting electrolyte and CO2 gas, have been able to demonstrate the substantial current densities needed for future commercialization. These higher reaction rates have often been ascribed to the presence of a three-phase interface, where solid, liquid, and gas provide electrons, water, and CO2, respectively. Conversely, mechanistic work on electrochemical reactions implicates a fully two-phase reaction interface, where gas molecules reach the electrocatalyst's surface by dissolution and diffusion through the electrolyte. Because the discrepancy between an atomistic three-phase versus two-phase reaction has substantial implications for the design of catalysts, gas-diffusion layers, and cell architectures, the nuances of nomenclatures and governing phenomena surrounding the three-phase-region require clarification. Here we outline the macro, micro, and atomistic phenomena occurring within a gas-diffusion electrode to provide a focused discussion on the architecture of the often-discussed three-phase region for CO2 electrolysis. From this information, we comment on the outlook for the broader CO2 electroreduction GDE cell architecture., Accepted Author Manuscript, ChemE/Materials for Energy Conversion & Storage

The valorization of CO2 to valuable products via microbial electrosynthesis (MES) is a technology transcending the disciplines of microbiology, (electro)chemistry, and engineering, bringing opportunities and challenges. As the field looks to the future, further emphasis is expected to be placed on engineering efficient reactors for biocatalysts, to thrive and overcome factors which may be limiting performance. Meanwhile, ample opportunities exist to take the lessons learned in traditional and adjacent electrochemical fields to shortcut learning curves. As the technology transitions into the next decade, research into robust and adaptable biocatalysts will then be necessary as reactors shape into larger and more efficient configurations, as well as presenting more extreme temperature, salinity, and pressure conditions., BT/Bioprocess Engineering, ChemE/Materials for Energy Conversion & Storage

Despite substantial progress in the electrochemical conversion of CO2 into value-added chemicals, the translation of fundamental studies into commercially relevant conditions requires additional efforts. Here, we study the catalytic properties of tailored Cu nanocatalysts under commercially relevant current densities in a gas-fed flow cell. We demonstrate that their facet-dependent selectivity is retained in this device configuration with the advantage of further suppressing hydrogen production and increasing the faradaic efficiencies toward the CO2 reduction products compared to a conventional H-cell. The combined catalyst and system effects result in state-of-the art product selectivity at high current densities (in the range 100-300 mA/cm2) and at relatively low applied potential (as low as-0.65 V vs RHE). Cu cubes reach an ethylene selectivity of up to 57% with a corresponding mass activity of 700 mA/mg, and Cu octahedra reach a methane selectivity of up to 51% with a corresponding mass activity of 1.45 A/mg in 1 M KOH., ChemE/Materials for Energy Conversion & Storage

Electrochemical CO2 reduction has received an increased amount of interest in the last decade as a promising avenue for storing renewable electricity in chemical bonds. Despite considerable progress on catalyst performance using nanostructured electrodes, the sensitivity of the reaction to process conditions has led to debate on the origin of the activity and high selectivity. Additionally, this raises questions on the transferability of the performance and knowledge to other electrochemical systems. At its core, the discrepancy is primarily a result of the highly porous nature of nanostructured electrodes, which are vulnerable to both mass transport effects and structural changes during the electrolysis. Both effects are not straightforward to identify and difficult to decouple. Despite the susceptibility of nanostructured electrodes to mass transfer limitations, we highlight that nanostructured silver electrodes exhibit considerably higher activity when normalized to the electrochemically active surface in contrast to gold and copper electrodes. Alongside, we provide a discussion on how active surface area and thickness of the catalytic layer itself can influence the onset potential, selectivity, stability, activity and mass transfer inside and outside of the three dimensional catalyst layer. Key parameters and potential solutions are highlighted to decouple mass transfer effects from the measured activity in electrochemical cells utilizing CO2 saturated aqueous solutions., ChemE/Materials for Energy Conversion & Storage, Large Scale Energy Storage

Electrochemical CO2 electrolysis to produce hydrocarbon fuels or material feedstocks offers a renewable alternative to fossilized carbon sources. Gas-diffusion electrodes (GDEs), composed of solid electrocatalysts on porous supports positioned near the interface of a conducting electrolyte and CO2 gas, have been able to demonstrate the substantial current densities needed for future commercialization. These higher reaction rates have often been ascribed to the presence of a three-phase interface, where solid, liquid, and gas provide electrons, water, and CO2, respectively. Conversely, mechanistic work on electrochemical reactions implicates a fully two-phase reaction interface, where gas molecules reach the electrocatalyst's surface by dissolution and diffusion through the electrolyte. Because the discrepancy between an atomistic three-phase versus two-phase reaction has substantial implications for the design of catalysts, gas-diffusion layers, and cell architectures, the nuances of nomenclatures and governing phenomena surrounding the three-phase-region require clarification. Here we outline the macro, micro, and atomistic phenomena occurring within a gas-diffusion electrode to provide a focused discussion on the architecture of the often-discussed three-phase region for CO2 electrolysis. From this information, we comment on the outlook for the broader CO2 electroreduction GDE cell architecture., Accepted Author Manuscript, ChemE/Materials for Energy Conversion and Storage

The valorization of CO2 to valuable products via microbial electrosynthesis (MES) is a technology transcending the disciplines of microbiology, (electro)chemistry, and engineering, bringing opportunities and challenges. As the field looks to the future, further emphasis is expected to be placed on engineering efficient reactors for biocatalysts, to thrive and overcome factors which may be limiting performance. Meanwhile, ample opportunities exist to take the lessons learned in traditional and adjacent electrochemical fields to shortcut learning curves. As the technology transitions into the next decade, research into robust and adaptable biocatalysts will then be necessary as reactors shape into larger and more efficient configurations, as well as presenting more extreme temperature, salinity, and pressure conditions., BT/Bioprocess Engineering, ChemE/Materials for Energy Conversion and Storage

Despite substantial progress in the electrochemical conversion of CO2 into value-added chemicals, the translation of fundamental studies into commercially relevant conditions requires additional efforts. Here, we study the catalytic properties of tailored Cu nanocatalysts under commercially relevant current densities in a gas-fed flow cell. We demonstrate that their facet-dependent selectivity is retained in this device configuration with the advantage of further suppressing hydrogen production and increasing the faradaic efficiencies toward the CO2 reduction products compared to a conventional H-cell. The combined catalyst and system effects result in state-of-the art product selectivity at high current densities (in the range 100-300 mA/cm2) and at relatively low applied potential (as low as-0.65 V vs RHE). Cu cubes reach an ethylene selectivity of up to 57% with a corresponding mass activity of 700 mA/mg, and Cu octahedra reach a methane selectivity of up to 51% with a corresponding mass activity of 1.45 A/mg in 1 M KOH., ChemE/Materials for Energy Conversion and Storage

Electrochemical CO2 reduction has received an increased amount of interest in the last decade as a promising avenue for storing renewable electricity in chemical bonds. Despite considerable progress on catalyst performance using nanostructured electrodes, the sensitivity of the reaction to process conditions has led to debate on the origin of the activity and high selectivity. Additionally, this raises questions on the transferability of the performance and knowledge to other electrochemical systems. At its core, the discrepancy is primarily a result of the highly porous nature of nanostructured electrodes, which are vulnerable to both mass transport effects and structural changes during the electrolysis. Both effects are not straightforward to identify and difficult to decouple. Despite the susceptibility of nanostructured electrodes to mass transfer limitations, we highlight that nanostructured silver electrodes exhibit considerably higher activity when normalized to the electrochemically active surface in contrast to gold and copper electrodes. Alongside, we provide a discussion on how active surface area and thickness of the catalytic layer itself can influence the onset potential, selectivity, stability, activity and mass transfer inside and outside of the three dimensional catalyst layer. Key parameters and potential solutions are highlighted to decouple mass transfer effects from the measured activity in electrochemical cells utilizing CO2 saturated aqueous solutions., ChemE/Materials for Energy Conversion and Storage, Large Scale Energy Storage

Using renewable energy as an input, Power-to-X technologies have the potential to replace fossil fuels and chemicals with dense-energy carriers that are instead derived out of thin air. In this work, we put into context what the industrial-scale production of chemicals from ambient CO2 using CO2 electrolysis means in terms of future required operating conditions and the device and catalyst scales that will be needed for the technology to assume its role in our global energy system., Accepted Author Manuscript, ChemE/Materials for Energy Conversion & Storage, ChemE/Transport Phenomena

The environment of a CO2 electroreduction (CO2ER) catalyst is intimately coupled with the surface reaction energetics and is therefore a critical aspect of the overall system performance. The immediate reaction environment of the electrocatalyst constitutes the electrical double layer (EDL) which extends a few nanometers into the electrolyte and screens the surface charge density. In this study, we resolve the species concentrations and potential profiles in the EDL of a CO2ER system by self-consistently solving the migration, diffusion and reaction phenomena using the generalized modified Poisson–Nernst–Planck (GMPNP) equations which include the effect of volume exclusion due to the solvated size of solution species. We demonstrate that the concentration of solvated cations builds at the outer Helmholtz plane (OHP) with increasing applied potential until the steric limit is reached. The formation of the EDL is expected to have important consequences for the transport of the CO2 molecule to the catalyst surface. The electric field in the EDL diminishes the pH in the first 5 nm from the OHP, with an accumulation of protons and a concomitant depletion of hydroxide ions. This is a considerable departure from the results obtained using reaction-diffusion models where migration is ignored. Finally, we use the GMPNP model to compare the nature of the EDL for different alkali metal cations to show the effect of solvated size and polarization of water on the resultant electric field. Our results establish the significance of the EDL and electrostatic forces in defining the local reaction environment of CO2 electrocatalysts., ChemE/Materials for Energy Conversion & Storage, ChemE/Algemeen, ChemE/Inorganic Systems Engineering

Electrocatalytic CO 2 reduction has the dual-promise of neutralizing carbon emissions in the near future, while providing a long-term pathway to create energy-dense chemicals and fuels from atmospheric CO 2 . The field has advanced immensely in recent years, taking significant strides towards commercial realization. Catalyst innovations have played a pivotal role in these advances, with a steady stream of new catalysts providing gains in CO 2 conversion efficiencies and selectivities of both C1 and C2 products. Comparatively few of these catalysts have been tested at commercially-relevant current densities (∼200 mA cm -2 ) due to transport limitations in traditional testing configurations and a research focus on fundamental catalyst kinetics, which are measured at substantially lower current densities. A catalyst's selectivity and activity, however, have been shown to be highly sensitive to the local reaction environment, which changes drastically as a function of reaction rate. As a consequence of this, the surface properties of many CO 2 reduction catalysts risk being optimized for the wrong operating conditions. The goal of this perspective is to communicate the substantial impact of reaction rate on catalytic behaviour and the operation of gas-diffusion layers for the CO 2 reduction reaction. In brief, this work motivates high current density catalyst testing as a necessary step to properly evaluate materials for electrochemical CO 2 reduction, and to accelerate the technology toward its envisioned application of neutralizing CO 2 emissions on a global scale., ChemE/Materials for Energy Conversion & Storage

Electrocatalysis of carbon dioxide can provide a valuable pathway towards the sustainable production of chemicals and fuels from renewable electricity sources. One of the main challenges to enable this technology is to find suitable electrodes that can act as efficient, stable and selective CO2 reduction catalysts. Modified silver catalysts and in particular, catalysts electrochemically derived from silver-oxides, have shown great promise in this regard. Here, we use operando EXAFS analysis to study the differences in surface composition between a pure silver film and oxide-derived silver catalysts-a nanostructured catalyst with improved CO2 reduction performance. The EXAFS analysis reveals the presence of trace amounts of oxygen in the oxide-derived silver samples, with the measured oxygen content correlating well with experimental studies showing an increase in CO2 reduction reactivity towards carbon monoxide. The selectivity towards CO production also partially scales with the increased surface area, showing that the morphology, local composition and electronic structure all play important roles in the improved activity and selectivity of oxide-derived silver electrocatalysts. Earlier studies based on X-ray photoelectron spectroscopy (XPS) were not able to identify this oxygen, most likely because in ultra-high vacuum conditions, silver can self-reduce to Ag0, removing existing oxygen species. This operando EXAFS study shows the potential for in situ and operando techniques to probe catalyst surfaces during electrolysis and aid in the overall understanding of electrochemical systems., Accepted Author Manuscript, ChemE/Materials for Energy Conversion & Storage

Using renewable energy as an input, Power-to-X technologies have the potential to replace fossil fuels and chemicals with dense-energy carriers that are instead derived out of thin air. In this work, we put into context what the industrial-scale production of chemicals from ambient CO2 using CO2 electrolysis means in terms of future required operating conditions and the device and catalyst scales that will be needed for the technology to assume its role in our global energy system., Accepted Author Manuscript, ChemE/Materials for Energy Conversion and Storage, ChemE/Transport Phenomena

The environment of a CO2 electroreduction (CO2ER) catalyst is intimately coupled with the surface reaction energetics and is therefore a critical aspect of the overall system performance. The immediate reaction environment of the electrocatalyst constitutes the electrical double layer (EDL) which extends a few nanometers into the electrolyte and screens the surface charge density. In this study, we resolve the species concentrations and potential profiles in the EDL of a CO2ER system by self-consistently solving the migration, diffusion and reaction phenomena using the generalized modified Poisson–Nernst–Planck (GMPNP) equations which include the effect of volume exclusion due to the solvated size of solution species. We demonstrate that the concentration of solvated cations builds at the outer Helmholtz plane (OHP) with increasing applied potential until the steric limit is reached. The formation of the EDL is expected to have important consequences for the transport of the CO2 molecule to the catalyst surface. The electric field in the EDL diminishes the pH in the first 5 nm from the OHP, with an accumulation of protons and a concomitant depletion of hydroxide ions. This is a considerable departure from the results obtained using reaction-diffusion models where migration is ignored. Finally, we use the GMPNP model to compare the nature of the EDL for different alkali metal cations to show the effect of solvated size and polarization of water on the resultant electric field. Our results establish the significance of the EDL and electrostatic forces in defining the local reaction environment of CO2 electrocatalysts., ChemE/Materials for Energy Conversion and Storage, ChemE/Algemeen, ChemE/Inorganic Systems Engineering

Electrocatalytic CO 2 reduction has the dual-promise of neutralizing carbon emissions in the near future, while providing a long-term pathway to create energy-dense chemicals and fuels from atmospheric CO 2 . The field has advanced immensely in recent years, taking significant strides towards commercial realization. Catalyst innovations have played a pivotal role in these advances, with a steady stream of new catalysts providing gains in CO 2 conversion efficiencies and selectivities of both C1 and C2 products. Comparatively few of these catalysts have been tested at commercially-relevant current densities (∼200 mA cm -2 ) due to transport limitations in traditional testing configurations and a research focus on fundamental catalyst kinetics, which are measured at substantially lower current densities. A catalyst's selectivity and activity, however, have been shown to be highly sensitive to the local reaction environment, which changes drastically as a function of reaction rate. As a consequence of this, the surface properties of many CO 2 reduction catalysts risk being optimized for the wrong operating conditions. The goal of this perspective is to communicate the substantial impact of reaction rate on catalytic behaviour and the operation of gas-diffusion layers for the CO 2 reduction reaction. In brief, this work motivates high current density catalyst testing as a necessary step to properly evaluate materials for electrochemical CO 2 reduction, and to accelerate the technology toward its envisioned application of neutralizing CO 2 emissions on a global scale., ChemE/Materials for Energy Conversion and Storage