Understanding the crystal structure-dependent electrochemical capacitance of spinel and rock-salt Ni–Co oxides via density function theory calculations

Abstract

The spinel NiCo₂O₄ and rock-salt NiCoO₂ have been well established as attractive electrodes for supercapacitors. However, what is the intrinsic role of the congenital aspect, i.e., crystal structure and the surface and/or near-surface controlled electrochemical redox behaviors, if the acquired features (i.e., morphology, specific surface area, pore structure, and so on) are wholly ignored? Herein, we purposefully elucidated the underlying influences of unique crystal structures of NiCo₂O₄ and NiCoO₂ on their pseudocapacitance from mechanism analysis through the density function theory based first-principles calculations, along with the experimental validation. Systematic theoretical calculation and analysis revealed that more charge carriers near the Fermi-level, stronger affinity with OH⁻ in the electrolyte, easier deprotonation process, and the site-enriched characteristic for low-index surfaces of NiCoO₂ enable its faster redox reaction kinetics and greater charge transfer, when compared to the spinel NiCo₂O₄. The in-depth understanding of crystal structure–property relationship here will guide rational optimization and selection of appropriate electrodes for advanced supercapacitors.