Probing the pseudocapacitance and energy-storage performance of RuO2 facets from first principles

The energy density of ruthenia (${\mathrm{RuO}}_{2}$) pseudocapacitor electrodes is critically dependent on their surface structure. To understand this dependence, we simulate the electrochemical response of ${\mathrm{RuO}}_{2}$(110), ${\mathrm{RuO}}_{2}$(100), and ${\mathrm{RuO}}_{2}$(101) in aqueous environments using a self-consistent continuum solvation (SCCS) model of the solid-liquid interface. The insertion of protons into the ${\mathrm{RuO}}_{2}$(110) sublayer is found to profoundly affect the voltage-dependent characteristics of the system, leading to a sharp transition from a battery-type to capacitor-type response. The calculated charge-voltage properties for ${\mathrm{RuO}}_{2}$(101) are in qualitative agreement with experiment, albeit with a pseudocapacitance that is significantly underestimated. In contrast, the ${\mathrm{RuO}}_{2}$(100) facet is correctly predicted to be pseudocapacitive over a wide voltage window, with a calculated pseudocapacitance in close agreement with experimental voltammetry. These results establish the SCCS model as a reliable approach to predict and optimize the facet-dependent pseudocapacitance of polycrystalline systems.