Controlling product selectivity in electrocatalytic CO₂ reduction is critical for achieving an economically viable process due to costly downstream separations of valuable products. Thermodynamically, several possible products of CO₂ reduction, as well as the undesired (in this case) H⁺ reduction reaction, reside within a few tenths of a Volt. Because most CO₂ reduction reactions utilize H⁺ as a reactant, the products are also a strong function of pH. As the local pH varies due to the consumption of protons, the local product changes at a fixed potential. The majority of CO₂ electrocatalyst development occurs in a buffered, liquid electrolyte half-cell in a configuration that isn’t feasible for upscaling. Aqueous-based cathode systems are limited by minimal water solubility limits of CO₂, which at ~30 mM can only support current densities[1] around 30 mA/cm². Such current densities are approximately an order of magnitude too low to be considered for an industrially-relevant device. The product selectivity and efficiency of any catalyst measured in aqueous half-cells will undoubtedly be different in high rate (current) devices. A new analytical solution is required for screening materials designed to utilize CO₂ at a scale commensurate with which it is currently being produced. In this talk, we will present an overview of our approach that seeks to manipulate CO₂ reduction reaction dynamics at the triple phase boundary, where high rates of conversion are possible.

Through a robust fuel cell and electrochemical engineering research program, the National Renewable Energy Laboratory has developed a core capability in fabricating and characterizing fuel cells and electrolyzers from components to functional devices. This expertise in technologies that rely on optimization of the triple phase boundary is leveraged to develop a CO₂ electrolyzer utilizing a gas-fed cathode and aqueous alkaline oxidation anode assembly. The electrodes are separated by an in-house synthesized bipolar membrane with an engineered 3-D interfacial layer. Accelerating the water dissociation reaction at the interface of the cation and anion exchange layers in the bipolar membrane is necessary for achieving and maintaining high current densities in our advanced CO₂ electrolyzers. Initial results from this endeavor will be presented. We will also describe our progress in designing and fabricating new automated test stands, with real-time online reduction product analysis, capable of performing in-situ electrochemical diagnostics on dynamic processes in these complex systems. The ultimate goal of this project is to establish a platform to evaluate and benchmark promising CO₂ electrocatalysts under conditions common to scalable architectures.