Considerations for Gas- and Liquid-Fed Electrocatalytic Coupling: A Case Study with Carbon Dioxide and Nitrate

There are several electrocatalytic reactions where co-feeding liquid- and gas-phase reactants is advantageous through the use of a gas diffusion electrode (GDE) system. A common reason to use a GDE is to avoid or reduce transport limitations due to low solubility of reactants in liquids, but other advantages exist as well. We discuss some of the example reactions where this type of electrochemical reactor is used, highlighting the coupling reaction of carbon dioxide and nitrate as a case study. The concentration gradients that appear in these GDEs are analogous to porous catalyst pellets, and we discuss how the internal transport limitations in GDEs impact selectivity and activity using a simple reaction−diffusion model. Because of the importance of internal transport limitations, understanding the structure of the catalyst layer in a GDE is imperative for interpreting reaction rates. We identify key challenges in characterizing these GDE systems, in particular the ability to spatially map the catalyst (typically a metal) and polymer/ionomer (used as a binder or ionic conductor) simultaneously. Lastly, we discuss how the selectivity of coupling reactions can change with reactant conversion, similar to the selectivity dependence of cascade reactions.