The deployment of gas diffusion electrodes (GDEs) for the electrochemical CO 2 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 N 2 RR) with cathodic overpotentials of less than −0.65 V versus a reversible hydrogen electrode.

Over the past decade, electrochemical carbon dioxide reduction has become a thriving area of research with the aim of converting electricity to renewable chemicals and fuels. Recent advances through catalyst development have significantly improved selectivity and activity. However, drawing potential dependent structure–activity relationships has been complicated, not only due to the ill-defined and intricate morphological and mesoscopic structure of electrocatalysts, but also by immense concentration gradients existing between the electrode surface and bulk solution. In this work, by using in situ surface enhanced infrared absorption spectroscopy (SEIRAS) and computational modeling, we explicitly show that commonly used strong phosphate buffers cannot sustain the interfacial pH during CO2 electroreduction on copper electrodes at relatively low current densities, <10 mA/cm2. The pH near the electrode surface was observed to be as much as 5 pH units higher compared to bulk solution in 0.2 M phosphate buffer at potentials relevant to the formation of hydrocarbons (−1 V vs RHE), even on smooth polycrystalline copper electrodes. Drastically increasing the buffer capacity did not stand out as a viable solution for the problem as the concurrent production of hydrogen increased dramatically, which resulted in a breakdown of the buffer in a narrow potential range. These unforeseen results imply that most of the studies, if not all, on electrochemical CO2 reduction to hydrocarbons in CO2 saturated aqueous solutions were evaluated under mass transport limitations on copper electrodes. We underscore that the large concentration gradients on electrodes with high local current density (e.g., nanostructured) have important implications on the selectivity, activity, and kinetic analysis, and any attempt to draw structure–activity relationships must rule out mass transport effects.

Advancing reaction rates for electrochemical CO 2 reduction in membrane electrode assemblies (MEAs) have boosted the promise of the technology while exposing new shortcomings. Among these is the maximum utilization of CO 2 , 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 CO 2 losses, but their promise is dampened by poor CO 2 activity and selectivity. In this work, we enable a 3-fold increase in the CO 2 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 CO 2 loss at similar current densities, while breaking the 50% CO 2 utilization mark. The work provides a combined cation and BPM strategy for overcoming CO 2 utilization issues in CO 2 electrolyzers.

Results of a 2-D transport model for a gas diffusion electrode performing CO2 reduction to CO with a flowing catholyte are presented, including the concentration gradients along the flow cell, spatial distribution of the current density and local pH in the catalyst layer. The model predicts that both the concentration of CO2 and the buffer electrolyte gradually diminish along the channels for a parallel flow of gas and electrolyte as a result of electrochemical conversion and non-electrochemical consumption. At high single-pass conversions, significant concentration gradients exist along the flow channels leading to large local variations in the current density (>150 mA/cm 2 ), which becomes prominent when compared to ohmic losses. In addition, concentration overpotentials change dramatically with CO2 flow rate, which results in significant differences in outlet concentrations at high conversions. The outlet concentration of CO attains a maximum of 80% along with 5 % CO2 and 15% H2, although the maximum single-pass conversion is limited to below 60% due to homogenous consumption by the electrolyte. Fundamental and practical implications of our findings on electrochemical CO2 reduction are discussed with a focus on the tradeoff between high current density operation and high single-pass conversion efficiency.

Do self-standing porous electrodes truly offer better selectivity, activity, stability and lower onset potentials?

Typically, anion exchange membranes (AEMs) are used in CO 2 electrolyzers, but those suffer from unwanted CO 2 crossover, implying (indirect) energy consumption for generating an excess of CO 2 feed and purification of the KOH anolyte. As an alternative, bipolar membranes (BPMs) have been suggested, which mitigate the reactant loss by dissociating water albeit requiring a higher cell voltage when operating at a near-neutral pH. Here, we assess the direct and indirect energy consumption required to produce CO in a membrane electrode assembly with BPMs or AEMs. More than 2/3 of the energy consumption for AEM-based cells concerns CO 2 crossover and electrolyte refining. While the BPM-based cell had a high stability and almost no CO 2 loss, the Faradaic efficiency to CO was low, making the energy requirement per mol of CO higher than for the AEM-based cell. Improving the cathode–BPM interface should be the future focus to make BPMs relevant to CO 2 electrolyzers.

No abstract

The electrochemical reduction of bicarbonate to renewable chemicals without external gaseous CO 2 supply has been motivated as a means of integrating conversion with upstream CO 2 capture. The way that CO 2 is formed and transported during CO 2 -mediated bicarbonate reduction in flow cells is profoundly different from conventional CO 2 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 CO 2 . The model indicates that significant CO 2 is generated in the porous electrode during electrochemical reduction, even though the starting bicarbonate solution contains negligible CO 2 . However, the in situ formation of CO 2 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 CO 2 reduction cells, because of the reaction between protons and CO 3 2− 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-CO 2 reduction membrane electrode assembly cells utilizing cation exchange membranes facing the cathode.