(703d) Modeling of the Electrochemical Double Layer during CO2 Electrolysis

An attractive approach to help minimize carbon emissions is the electrochemical conversion of CO₂ into value-added products, such as syngas. However, a critical challenge for this technology is the ability to control the selectivity of the reaction. For CO₂ reduction (CO₂R), the selectivity depends not only on the catalyst identity, but also on the electrolyte environment adjacent to the electrode surface (i.e., the electrochemical double layer). Despite its importance, the structure and composition of the double layer during CO₂ electrolysis remains elusive, largely because of the complex bicarbonate buffer reaction system. Previously published models of the CO₂R double layer often neglect the bicarbonate buffer reactions and do not include model validation. Therefore, there is a need to develop a comprehensive, multiphysics model of the electrochemical double layer during CO₂ electrolysis in order to identify the structure of the double layer and to understand how it impacts CO₂R kinetics.

This talk will present our recent efforts to simulate the electrochemical double layer during CO₂ electrolysis by using a one-dimensional multiphysics continuum model that accounts for the bicarbonate buffer reactions, species diffusion and migration, steric forces, local electrostatic and physical interactions between ions, and the 2^nd Wien effect. This model is validated by comparing experimental and simulated capacitance versus potential curves and CO product polarization curves from a Ag catalyst. The model shows how the CO₂R double layer structure (species concentrations and the electrostatic potential) changes as a function of applied potential and electrolyte concentration. Moreover, we explore how the identity of the cation (Cs⁺, K⁺, Na⁺, vs Li⁺) impacts the double layer structure and, consequently, CO₂R kinetics. Our model predicts that at CO₂R potentials the Outer Helmholtz Plane (OHP) is dominated by cations and that cations with a small, hydrated radius can pack very tightly at the OHP, thereby increasing the electric field within the Stern layer. We propose that this Stern layer electric field can impact the reaction kinetics by polarizing the neighboring water molecules and facilitating CO₂R. This work offers key insight into the structure of the double layer during CO₂ electrolysis and its role in modulating reaction kinetics, a critical step toward improving control over reaction selectivity.