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

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.