Reversible Structural Evolution of NiCoO_xH_y during the Oxygen Evolution Reaction and Identification of the Catalytically Active Phase

Operando Raman spectroscopy and electrochemical techniques were used during the oxygen evolution reaction to identify the composition and local structure of electrodeposited CoO_xH_y and NiCoO_xH_y catalyst films. Before evaluation, the electrodeposited samples were subjected to a variety of thermal and electrochemical conditioning procedures, which generated unique initial catalyst structures and crystallinities. During oxygen evolution, Ni-modified CoO_xH_y films having lower initial crystallinity underwent substantial structural evolution that began with an irreversible transformation of a spinel local structure to an amorphous CoO structure at low anodic potentials (0.2 V vs Ag/AgCl). Increasing anodic polarization to greater than 0.3 V vs Ag/AgCl facilitated additional structural conversion from the amorphous CoO structure to a complex phase best described as an amalgamation of NiOOH and layered CoO₂ motifs (NiOOH-h-CoO₂) under elevated oxygen evolution rates. The formation of this NiOOH-h-CoO₂ active structure was correlated with improved OER activity, which at 0.35 V overpotential is 100% greater than that of the catalyst where Ni was coordinated in a spinel structure. Independent of the initial cobalt oxide structure, the same NiOOH-h-CoO₂ structure was formed during oxygen evolution, which suggests the active phase identified herein could be the universally active structure for NiCoO_xH_y materials.