Quantitative Understanding of the Reactivity of Bicarbonate and the Role of Cations in Gas Diffusion Electrode Flow Cells for CO ₂ Reduction

Bicarbonate (HCO3–) is ubiquitous in aqueous CO2 reduction (CO2RR), where it influences the local pH and reaction pathways despite not being the primary reactant. Its poorly understood role complicates strategies to enhance CO2RR reaction rates and selectivity. Here, we measured bicarbonate reduction kinetics and transport on silver gas diffusion electrodes (GDEs) with Na+, K+, and Cs+ electrolytes. Using linear sweep voltammograms (LSVs) with varied scan rates, we extracted mass transport (limiting current densities) and kinetic parameters (exchange current density and charge transfer coefficients) for bicarbonate and water reduction. We show that the mass transport and kinetics of bicarbonate reduction are cation-dependent, with both increasing from Na+ to Cs+. To probe this effect, we developed a simple-reaction diffusion model that demonstrates that the cation identity controls the flux of bicarbonate to the electrode surface by mitigating the repulsion between bicarbonate and the negatively charged electrode. The measured cation-dependent bicarbonate transport explains why certain cations more effectively buffer the local pH for CO2 reduction, enhancing selectivity. Our framework enables deconvolution of mass transport and kinetic contributions for different species, advancing ion-specific electrocatalyst design in GDE systems.