Modeling the Local Environment inside a GDE Pore for CO2 Electrochemical Reduction

Abstract

Porous Gas Diffusion Electrodes (GDE) have gained a lot of attention in recent years for their ability to overcome the solubility and mass transfer limitations of a planar electrode assembly in CO2 electrochemical reduction (CO2ER) systems. In this work, we present a continuum scale modeling technique based on a set of size-modified Poisson-Nernst-Planck (SMPNP) equations to accurately account for the local environment including the Electric Double Layer (EDL) inside a catalytic nanopore. Most GDE models for CO2ER ignore the EDL or are limited in their applicability at relevant applied potentials due to numerical instability. The local conditions found inside an EDL heavily influence the charge transfer reactions and thus the productivity of desired CO2ER products. Our SMPNP model is able to solve for the concentration and electric field profiles at practical cathodic potentials. We highlight the variation of the predicted CO2 concentration which is often assumed to remain uniform along the radii of the catalytic pores even at high applied potentials. The predicted pH profile also varies significantly along the pore radius, influencing both the CO3 2- balance and the charge transfer reactions. Geometrical parameters such as the diameter and length of the pore, influence the electric field and can potentially be used as control parameters for increasing the selectivity of desired products. We also discuss the implications of double layer overlap as a consequence of small pore radii and low electrolyte concentration. Overall, using the SMPNP model we gain insights into key performance-controlling parameters inside a catalytic nanopore. Furthermore, the model can be used to inform boundary conditions in larger-scale GDE models.