Water Activity Regulates CO2 Reduction in Gas-Diffusion Electrodes

Electrochemical CO 2 reduction has recently reached current densities as high as 1 A cm -2 , enabled by improving diffusion of CO 2 from the gas phase to the electrocatalyst by use of gas-diffusion electrodes (GDEs) and by improving electrolyte ionic conductivity with concentrated hydroxide electrolytes (7 M KOH). Despite such high solute concentrations, the dilute electrolyte assumption is commonly used to evaluate the thermodynamics of the system, specifically reaction equilibrium potential and reaction rate expression. Here we establish a paradigm shift by demonstrating how to properly include the activity of water and solutes and highlighting corrections to associated reaction thermodynamics.