Tenacious Mass Transfer Limitations Drive Catalytic Selectivity during Electrochemical Carbon Dioxide Reduction

Abstract

Over the past decade, electrochemical carbon dioxide reduction has become a thriving area of research with the aim of converting electricity to renewable chemicals and fuels. New life has been breathed into the field as nanostructured electrodes have been deployed as electrocatalysts, which have significantly improved the catalytic selectivity. However, evaluation of the performance of these electrocatalysts has been complicated, not only due to their intricate surface morphology and mesoscopic structure, but also by immense concentration gradients existing between the electrode surface and bulk solution. In this work, by using in-situ surface enhanced infrared absorption spectroscopy (SEIRAS) and computational modelling, we explicitly show that commonly used phosphate buffers cannot sustain the interfacial pH during CO 2 electroreduction on copper electrodes at relatively low current densities,<10 mA/cm 2 . The pH near the electrode surface was observed to be as much as 5 pH units higher compared to bulk solution in 0.2 M phosphate buffer at potentials relevant to the formation of hydrocarbons (-1 V vs RHE), even on smooth polycrystalline copper electrodes. Drastically increasing the buffer capacity did not stand out as a viable solution for the problem on polycrystalline and copper nanowire electrodes as the concurrent production of hydrogen increased dramatically, which resulted in a breakdown of the buffer in a narrow potential range. These unforeseen results imply that most of the aqueous CO 2 electroreduction studies in H-cells, if not all, were evaluated under mass transport limitations on smooth and rough copper electrodes. We underscore that the large concentration gradients on electrodes with high local current density (i.e. Nanostructured) have important implications on the selectivity, activity, and kinetic analysis, and any attempt to draw structure-activity relationships must rule out mass transport effects.