Conspectus Electrocatalytic CO2 conversion at near ambient temperatures and pressures offers a potential means of converting waste greenhouse gases into fuels or commodity chemicals (e.g., CO, formic acid, methanol, ethylene, alkanes, and alcohols). This process is particularly compelling when driven by excess renewable electricity because the consequent production of solar fuels would lead to a closing of the carbon cycle. However, such a technology is not currently commercially available. While CO2 electrolysis in H-cells is widely used for screening electrocatalysts, these experiments generally do not effectively report on how CO2 electrocatalysts behave in flow reactors that are more relevant to a scalable CO2 electrolyzer system. Flow reactors also offer more control over reagent delivery, which includes enabling the use of a gaseous CO2 feed to the cathode of the cell. This setup provides a platform for generating much higher current densities (J) by reducing the mass transport issues inherent to the H-cells. In this Account, we examine some of the systems-level strategies that have been applied in an effort to tailor flow reactor components to improve electrocatalytic reduction. Flow reactors that have been utilized in CO2 electrolysis schemes can be categorized into two primary architectures: Membrane-based flow cells and microfluidic reactors. Each invoke different dynamic mechanisms for the delivery of gaseous CO2 to electrocatalytic sites, and both have been demonstrated to achieve high current densities (J > 200 mA cm–2) for CO2 reduction. One strategy common to both reactor architectures for improving J is the delivery of CO2 to the cathode in the gas phase rather than dissolved in a liquid electrolyte. This physical facet also presents a number of challenges that go beyond the nature of the electrocatalyst, and we scrutinize how the judicious selection and modification of certain components in microfluidic and/or membrane-based reactors can have a profound effect on electrocatalytic performance. In membrane-based flow cells, for example, the choice of either a cation exchange membrane (CEM), anion exchange membrane (AEM), or a bipolar membrane (BPM) affects the kinetics of ion transport pathways and the range of applicable electrolyte conditions. In microfluidic cells, extensive studies have been performed upon the properties of porous carbon gas diffusion layers, materials that are equally relevant to membrane reactors. A theme that is pervasive throughout our analyses is the challenges associated with precise and controlled water management in gas phase CO2 electrolyzers, and we highlight studies that demonstrate the importance of maintaining adequate flow cell hydration to achieve sustained electrolysis.

Electrochemically reducing CO2 using renewable energy is a contemporary global challenge that will only be met with electrocatalysts capable of efficiently converting CO2 into fuels and chemicals with high selectivity. Although many different metals and morphologies have been tested for CO2 electrocatalysis over the last several decades, relatively limited attention has been committed to the study of alloys for this application. Alloying is a promising method to tailor the geometric and electric environments of active sites. The parameter space for discovering new alloys for CO2 electrocatalysis is particularly large because of the myriad products that can be formed during CO2 reduction. In this Minireview, mixed‐metal electrocatalyst compositions that have been evaluated for CO2 reduction are summarized. A distillation of the structure–property relationships gleaned from this survey are intended to help in the construction of guidelines for discovering new classes of alloys for the CO2 reduction reaction.

The compositions of iron-nickel oxy/hydroxide oxygen evolution reaction (OER) catalysts were investigated before and after electrolysis at different current densities and pH values. Chronopotentiometric measurements showed nominal changes in electrochemical activity, but the physical analysis showed a dramatic change in iron content at both the anode and cathode. These experiments highlight the challenges that need to be solved before these types of films can be used on a commercial scale.

Electrolyzers are now capable of reducing carbon dioxide (CO 2 ) into products at high reaction rates but are often characterized by low energy efficiencies and low CO 2 utilization efficiencies. We report here an electrolyzer that reduces 3.0 M KHCO 3 (aq) into CO(g) at a high rate (partial current density for CO of 220 mA cm −2 ) and a CO 2 utilization efficiency of 40%, at a voltage of merely 2.3 V. These results were made possible by using: (i) a reactive carbon solution enriched in KHCO 3 as the feedstock instead of gaseous CO 2 ; (ii) a cation exchange membrane instead of an anion exchange membrane, which is common to the field; and (iii) the hydrogen oxidation reaction (HOR) at the anode instead of the oxygen evolution reaction (OER). The voltage reported here is the lowest reported for any CO 2 to CO electrolyzer that operates at high current densities (i.e., a partial current density for CO greater than 200 mA cm −2 ) with a CO 2 utilization efficiency of greater than 20%. This study highlights how the choice of feedstock, membrane, and anode chemistries affects the rate and efficiency at which CO 2 is converted into products.

Electrocatalytic palladium membrane reactors (ePMRs) use electricity and water to drive hydrogenation reactions without forming H2 gas.