(124b) Mass Transport Controls Product Selectivity in Electrocatalytic CO2 Reduction on Copper

Due to the mass transfer effect inherent in the nature of electrocatalysis, a profound understanding of processes occurring at multiple scales is critical in the demonstration of a high-performing catalyst, and it is important to broaden catalyst design principles beyond the transition state theory to include fundamentals of mass and charge transfer. As an attempt to control and directly observe the effect of mass transfer on electrochemical CO₂ reduction, we have developed a gas-tight cell compatible for the use of rotating cylinder electrode. This design enables CO₂ electrocatalysis and on-line quantification of gaseous products under well-defined hydrodynamics by systematically controlling the thickness of concentration boundary layer. Herein, we report for the first time, a direct experimental evidence that the mass transport property has a significant control over the product selectivity of the electrocatalytic CO₂ reduction on copper in bicarbonate buffer electrolyte. The results show that the production of hydrogen and formate is favored under higher rotation speed that allows the access of more bicarbonate anions closer to the electrode surface. The rate of carbon monoxide generation also increases an order of magnitude as the electrode rotation speed increases from 100 rpm to 800 rpm, which could be due to the facilitated desorption and diffusion of produced carbon monoxide away from the electrode at higher rotation speeds. On the other hand, the production of methane and C₂₊ products decrease as the electrode rotates faster. This could be explained by the enhanced diffusion of key intermediate species on the pathway to C₂₊ products away from the electrode surface, suppressing the C-C coupling mechanism between intermediates to form multi-carbon products. In addition, we have synthesized cubic nanostructures on the copper rotating cylinder electrode to study the relative contributions of bulk mass transport and internal pore diffusion on the selectivity of electrocatalytic CO₂ reduction.