On the Role of the Interfacial Capacitance in Grand-Canonical Simulations of Electrocatalysis

Common models of the electrochemical interface suggest that the potential response of electron transfer energetics scales linearly with the interfacial capacitance. However, grand-canonical simulations of electrocatalytic adsorption energies, and their dependence on the applied potential, remain remarkably similar when the interface is described by a bare implicit solvent or by a complete water layer, even though these setups change the interfacial capacitance by up to a factor of 3. Herein, we address this conundrum using constant-potential implicit-solvent simulations of the reductive electrosorption of CO2 and Na+ on Au(111). We manually tune the interfacial capacitance over more than an order of magnitude by independently varying the dielectric constant and the solvent cavity size. We show that adsorption energetics can vary drastically despite equal interfacial capacitances; while changes in the bulk dielectric constant lead to a behavior consistent with the classical predictions, reaction energetics are more robust against capacitance variations arising from the size and shape of the implicit solvent cavity. We rationalize the behavior based on the coupled interplay between the interfacial capacitance, the adsorption-induced shift in the potential of zero charge, and a local capacitance mismatch between the clean and adsorbate-covered surface. These contributions can partially compensate, rationalizing the observed insensitivity to the interfacial description while clarifying when simplified models succeed or fail.