(Invited) Electrocatalytic Reduction of CO 2 in Gas-Phase Electrolyzers: Moving Towards a Relevant Use of CO 2

• Published in: Meeting abstracts (Electrochemical Society) Vol. MA2019-01; no. 31; p. 1651
• Main Authors: Deutsch, Todd G, Chen, Yingying, Klein, Ellis, Bender, Guido, Neyerlin, K C
• Format: Journal Article
• Language: English
• Published: 01.05.2019
• ISSN: 2151-2043; 2151-2035
• DOI: 10.1149/MA2019-01/31/1651

Controlling product selectivity in electrocatalytic CO 2 reduction is critical for achieving an economically viable process due to costly downstream separations of valuable products. Thermodynamically, several possible products of CO 2 reduction, as well as the undesired (in this case) H + reduction reaction, reside within a few tenths of a Volt. Thus, product selection cannot be manipulated by controlling the potential but rather must occur via engineering catalyst-intermediate kinetics. Because most CO 2 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 CO 2 electrocatalyst development occurs in a buffered, liquid electrolyte half-cell with a configuration that portrays a misleading parameter set. Aqueous-based cathode systems are limited by minimal water solubility limits of CO 2 , which at ~30 mM can only support current densities 1 around 30 mA/cm 2 . Such current densities are at least 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 CO 2 at a scale commensurate with which it is currently being produced. In this talk, we will present an overview of our approach for transforming the CO 2 reduction reaction from an aqueous environment to an industry-relevant gas-phase environment, where high rates of conversion are possible. Through a robust fuel cell and electrochemical engineering research program, the National Renewable Energy Laboratory has developed considerable expertise in transforming gas-phase reactions from utilizing material properties to a functioning device level. This expertise is leveraged to develop a CO 2 electrolyzer utilizing a gas phase cathode, and alkaline oxidation anode assembly. The electrodes are separated by a 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 CO 2 electrolyzer assembly. Initial results of 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 CO 2 electrocatalysts subject to conditions relevant for incorporation into a viable solution for efficient industrial reduction of CO 2 . Acc. Chem. Res. 2018, 51, 910−918