(650e) Carbon Dioxide Mass Transfer in the Presence of Hydroxide Production in Flow Reactors

In the use of flow reactors for electrochemical CO₂ reduction, the mass transfer of CO₂ to the cathode surface strongly influences the resulting CO₂ conversion rate. For the CO evolution reaction, however, OH^- molecules are produced by the surface reaction that consumes CO₂, and these OH^- molecules can react away CO₂ molecules in the bulk liquid, ultimately suppressing the mass transfer of CO₂ to the cathode surface. Motivated by this process, we consider analytical approximations of the dimensionless governing advection-diffusion-reaction equations to explore how OH^- production inhibits CO₂ mass transfer, and we use numerical simulations to confirm our theoretical results. Specifically, we derive a simple approximation of the Sherwood number, i.e., the mass flux of CO₂ into the cathode, as a function of the Péclet number, surface Damköhler number, and bulk Damköhler number, representing the strength of the flow, surface reaction rate, and bulk reaction rate, respectively. In the limit of strong surface reaction, we derive simple expressions for the “reaction factor” when the bulk reaction is strong and weak, representing the amount that the Sherwood number is reduced by the presence of bulk reaction. The model problem that we solve represents the simplified problem of CO₂ saturated water undergoing the CO evolution reaction, however, because CO₂ reduction is rarely performed using pure water in practice, we also show how a common buffered electrolyte, KHCO₃, affects our results.

This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.