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 [1]. Much is unknown about these materials due to 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 [2]. 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. The newly developed model provides a widely applicable computational method to help identify novel pseudocapacitive materials.

[1] V. Augustyn. P. Simon, and B. Dunn, Energy and Environ. Sci. 7, 1597 (2014)

[2] N. Keilbart, Y. Okada, A. Feehan, S. Higai, and I. Dabo, Phys. Rev. B 95, 115423 (2017)