Understanding the pseudocapacitance of RuO2 from joint density functional theory

TL;DR

This study uses Joint Density Functional Theory to simulate and explain the pseudocapacitive behavior of RuO2, revealing how hydrogen adsorption influences capacitance and providing a first-principles understanding of transition-metal oxide pseudocapacitance.

Contribution

It introduces a novel JDFT simulation approach to model pseudocapacitance in RuO2, linking hydrogen coverage to capacitance changes and advancing fundamental understanding.

Findings

JDFT accurately reproduces experimental CV redox peaks.

Hydrogen adsorption explains the transition from double-layer to pseudocapacitive charge storage.

Hydrogen coverage increases surface restructuring, enhancing capacitance.

Abstract

Pseudocapacitors have been experimentally studied for many years in electric energy storage. However, first principles understanding of the pseudocapacitive behavior is still not satisfactory due to the complexity involved in modeling electrochemistry. In this paper, we applied a novel simulation technique called Joint Density Functional Theory (JDFT) to simulate the pseudocapacitive behavior of RuO2, a prototypical material, in a model electrolyte. We obtained from JDFT a capacitive curve which showed a redox peak position comparable to that in the experimental cyclic voltammetry (CV) curve. We found that the experimental turning point from double-layer to pseudocapacitive charge storage at low scan rates could be explained by the hydrogen adsorption at low coverage. As the electrode voltage becomes more negative, H coverage increases and causes the surface structure change, leading to bended OH bonds at the on-top oxygen atoms and large capacitance. This H coverage-dependent capacitance can explain the high pseudocapacitance of hydrous RuO2. Our work here provides a first principles understanding of the pseudocapacitance for RuO2 in particular and for transition-metal oxides in general.