  Pseudocapacitors are energy-storage devices characterized by fast and
reversible redox reactions that enable them to store large amounts of
electrical energy at high rates. We simulate the response of pseudocapacitive
electrodes under realistic conditions to identify the microscopic factors that
determine their performance, focusing on ruthenia (RuO2) as a prototypical
electrode material. Electronic-structure methods are used together with a
self-consistent continuum solvation (SCCS) model to build a complete dataset of
free energies as the surface of the charged electrode is gradually covered with
protons under applied voltage. The resulting dataset is exploited to compute
hydrogen-adsorption isotherms and charge-voltage responses by means of
grand-canonical sampling, finding close agreement with experimental
voltammetry. These simulations reveal that small changes on the order of 5
{\mu}F/cm2 in the intrinsic double-layer capacitance of the
electrode-electrolyte interface can induce variations of up to 40 {\mu}F/cm2 in
the overall pseudocapacitance.
