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

The spinel NiCo 2 O 4 and rock-salt NiCoO 2 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 NiCo 2 O 4 and NiCoO 2 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 NiCoO 2 enable its faster redox reaction kinetics and greater charge transfer, when compared to the spinel NiCo 2 O 4 . The in-depth understanding of crystal structure-property relationship here will guide rational optimization and selection of appropriate electrodes for advanced supercapacitors.