Electrolytic CO₂ Reduction in a Flow Cell

ConspectusElectrocatalytic 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 (<i>J</i>) 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 (<i>J</i> > 200 mA cm^–2) for CO₂ reduction. One strategy common to both reactor architectures for improving <i>J</i> 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