The local conditions inside a gas diffusion electrode (GDE) pore, especially in the electrical double layer (EDL) region, influence the charge transfer reactions and the selectivity of desired CO₂ER products. Most GDE computational models ignore the EDL or are limited in their applicability at high potentials. In this work, we present a continuum model to describe the local environment inside a catalytic pore at varying potentials, electrolyte concentrations and pore diameters. The systems studied in this work are based on an Ag catalyst in contact with KHCO₃ solution. Our study shows that steric effects dominate the local environment at high cathodic potentials (≪−25 mV vs pzc at the OHP), leading to a radial drop of CO₂ concentration. We also observe a drop in pH value within 1 nm of the reaction plane due to electrostatic repulsion and attraction of OH⁻ and H⁺ ions, respectively. We studied the influence of pore radii (1-10 nm) on electric field and concentrations. Pores with a radius smaller than 5 nm show a higher mean potential, which lowers the mean CO₂ concentration. Pores with a favourable local environment can be designed by regulating the ratio between the pore radius and Debye length.

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Electrochemical reduction of CO2 heavily depends on the reaction conditions found near the electrode surface. These local conditions are affected by phenomena such as electric double layer formation and steric effects of the solution species, which in turn impact the passage of CO2 molecules to the catalytic surface. Most models for CO2 reduction ignore these effects, leading to an incomplete understanding of the local electrode environment. In this work, we present a modeling approach consisting of a set of size-modified Poisson–Nernst–Planck equations and the Frumkin interpretation of Tafel kinetics. We introduce a modification to the steric effects inside the transport equations which results in more realistic concentration profiles. We also show how the modification lends the model numerical stability without adopting any separate stabilization technique. The model can replicate experimental current densities and faradaic efficiencies till −1.5 vs. SHE/V of applied electrode potential. We also show the utility of this approach for systems operating at elevated CO2 pressures. Using Frumkin-corrected kinetics gels well with the theoretical understanding of the double layer. Hence, this work provides a sound mechanistic understanding of the CO2 reduction process, from which new insights on key performance controlling parameters can be obtained.@en