<p>Electrochemical CO<sub>2</sub>
reduction has recently reached current densities as high as 1 A cm<sup>-2</sup>,
enabled by improving diffusion of CO<sub>2</sub> 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.</p>