(670g) Development of Reaction-Transport Kinetics Model for Electrochemical CO2 Reduction on Copper: Moving Away from Tafel Kinetics to Incorporate Mass Transfer Effects

The realization of technologies to electrify catalytic processes of transforming small molecules such as CO₂, CO, NO₃^-/NO₂^-, and H₂O into value-added products necessitates the development of practical reaction-transport kinetics that can be used for scale-up and operation of electrolyzers. The first step to the development of such kinetic expressions is the recognition that what we measure is reactor kinetics, not intrinsic kinetics. Decoupling the effect of mass transport on observed reactor kinetics and revealing true surface kinetics is enabled by interpreting a large experimental dataset obtained under a broad range of well-defined transport characteristics in the gastight rotating cylinder electrode reactor. In this talk, we present the reaction-transport kinetics model for electrochemical CO₂ reduction on copper developed based on the continuously-stirred tank reactor approximation of the reaction front at the electrode/electrolyte interface. Dimensionless Sherwood and Damköhler numbers are incorporated in the model to represent transport characteristics of supplying reactants to the reaction front and the residence time of intermediates generated on the electrode surface. Using the same reaction-transport kinetics obtained from electropolished copper, we have further investigated the effect of internal pore diffusion on the electrochemical CO₂ and NO₃^- reduction on nanoporous copper. Simulated effectiveness factor for pore utilization of reactants and intermediates under different Thiele modulus explains changes in observed reactor kinetics when catalysts with the same surface kinetics but of different porosity are used. Analogous to thermal catalysis, observed Tafel slopes can change dramatically under different mass transfer resistance regimes. Therefore, reaction kinetics should incorporate transport effects in electrocatalysis where the concept of the differential reactor is not readily available. Both the models discussed in this talk, capturing the effects of convective mass transfer and internal pore diffusion, show why the Tafel analysis fails to describe complicated reactions involving intermediates and multiple products.