Abstract: Pseudocapacitors are energy storage devices characterized by fast and reversible redox reactions near the surface of an electrode that allows them to store large amounts of energy at fast rates. Much is unknown about these materials because of the complex electrochemical reaction that occurs at the interface between the electrode and solvent.

A theoretical modeling approach is developed focusing on ruthenium dioxide (RuO₂), a prototypical pseudocapacitive electrode material, for analyzing the electrochemical response of an electrode under realistic conditions in order to identify the factors that control the performance. Electronic-structure methods are used in combination with a self-consistent continuum solvation model to generate a complete dataset of free energies for varying amounts of proton coverage on the surface. The dataset is used in conjunction with a grand canonical Monte Carlo sampling technique that computes hydrogen-adsorption isotherms and the charge-voltage response of the system.

Close agreement is found with experimental results of the RuO₂ (110) surface under optimal surface charging conditions. It is found that the intrinsic double-layer contribution represents a small fraction of the overall electrochemical response of the electrode but controls to a large extent the onset of pseudocapacitive reactions by influencing the change in the surface dipole. At variance with RuO₂ (110), the double-layer capacitance of RuO₂ (100) is found to vary linearly across a significant portion of the voltage range. This range of variation is also well captured by first-principles calculations of the double-layer capacitance for different coverages.

Additionally, current research on the MXene material will be presented that proves to be a promising electrode in pseudocapacitive applications. The newly developed model provides a widely applicable computational method to help identify novel pseudocapacitive materials.