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

Metal (oxy)­hydroxides
(MOxHy, M
= Fe, Co, Ni, and mixtures thereof) are important materials in electrochemistry.
In particular, MOxHy 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 Ni1−δCoδOxHy 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 NiOxHy 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 NiOxHy, which dramatically reorganize under electrochemical conditions
to form nanoparticle assemblies, restructuring is not found for Ni1−δCoδOxHy with a high Co content. Ni0.8Fe0.2OxHy 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 MOxHy materials and provide insight into how nanostructure and composition
affect structural dynamics at the nanoscale.