Structural Evolution of Metal (Oxy)hydroxide Nanosheets during the Oxygen Evolution Reaction

Abstract

Metal (oxy)hydroxides (MO _xH _y, M = Fe, Co, Ni, and mixtures thereof) are important materials in electrochemistry. In particular, MO _xH _y are the fastest known catalysts for the oxygen evolution reaction (OER) in alkaline media. While key descriptors such as overpotentials and activity have been thoroughly characterized, the nanostructure and its dynamics under electrochemical conditions are not yet fully understood. Here, we report on the structural evolution of Ni_1-δCo_δO _xH _y nanosheets with varying ratios of Ni to Co, in operando using atomic force microscopy during electrochemical cycling. We found that the addition of Co to NiO _xH _y nanosheets results in a higher porosity of the as-synthesized nanosheets, apparently reducing mechanical stress associated with redox cycling and hence enhancing stability under electrochemical conditions. As opposed to nanosheets composed of pure NiO _xH _y, which dramatically reorganize under electrochemical conditions to form nanoparticle assemblies, restructuring is not found for Ni_1-δCo_δO _xH _y with a high Co content. Ni_0.8Fe_0.2O _xH _y nanosheets show high roughness as-synthesized which increases during electrochemical cycling while the integrity of the nanosheet shape is maintained. These findings enhance the fundamental understanding of MO _xH _y materials and provide insight into how nanostructure and composition affect structural dynamics at the nanoscale.