(651c) Decoupling of Intrinsic Kinetics of Electrochemical CO2 Reduction on Flat and Porous Copper Via Dimensionless Characterization of External and Internal Mass Transfer

Understanding and decoupling mass, heat and charge transfer effects involved in the electrocatalytic upgrading of CO₂ is the bottleneck towards the realization of these technologies at industrial scales. We have previously demonstrated the use of a gastight rotating cylinder electrode (RCE) cell for the characterization of electrochemical systems using dimensionless Sherwood and Damköhler numbers. Herein, we report the decoupling of intrinsic kinetics from mass transport effects for the electrocatalytic CO₂ reduction reactions on polycrystalline copper electrodes.

The extraction of kinetic parameters is achieved through the systematic study of an atomically flat catalyst under standardized conditions of mass transfer, surface potential and concentration of species at the electrode/electrolyte interface. Over 200 individual experiments were carried out to systematically change the concentration of species at the electrode surface as well as the residence time of CO. Elucidation of a first-principle reaction model based on the large experimental dataset shows that CO residence time and local pH near the electrode surface are the critical descriptors of reaction selectivity which can be tuned by changing the Sherwood number. In addition, we have synthesized cubic nanostructures to study the relative contributions of external mass transfer and internal pore diffusion on the selectivity of electrocatalytic CO₂ reduction on copper. Compared to catalyst with a flat surface geometry, selectivity shifted towards C₂₊ products on nano-porous copper. We show that the change in selectivity is the result of longer CO residence times inside the pores rather than due to the existence of sites with intrinsically higher C-C coupling activity. The dominance of governing internal mass transfer is parameterized by the effectiveness factor for pore utilization of reactants and intermediates. Observations and insights obtained in the RCE cell can be readily translated to other systems via dimensionless quantities, thus bridging the gap between fundamental studies and industrial-scale applications.