Morphology Dynamics of Single-Layered Ni(OH)₂/NiOOH Nanosheets and Subsequent Fe Incorporation Studied by <i>in Situ</i> Electrochemical Atomic Force Microscopy

Nickel (oxy)­hydroxide-based (NiO_<i>x</i>H_<i>y</i>) materials are widely used for energy storage and conversion devices. Understanding dynamic processes at the solid–liquid interface of nickel (oxy)­hydroxide is important to improve reaction kinetics and efficiencies. In this study, <i>in situ</i> electrochemical atomic force microscopy (EC-AFM) was used to directly investigate dynamic changes of single-layered Ni­(OH)₂ nanosheets during electrochemistry measurements. Reconstruction of Ni­(OH)₂ nanosheets, along with insertion of ions from the electrolyte, results in an increase of the volume by 56% and redox capacity by 300%. We also directly observe Fe cations adsorb and integrate heterogeneously into or onto the nanosheets as a function of applied potential, further increasing apparent volume. Our findings are important for the fundamental understanding of NiO_<i>x</i>H_<i>y</i>-based supercapacitors and oxygen-evolution catalysts, illustrating the dynamic nature of Ni-based nanostructures under electrochemical conditions

Influence of Electrolyte Cations on Ni(Fe)OOH Catalyzed Oxygen Evolution Reaction

Iron-doped, nickel oxyhydroxide (Ni­(Fe)­OOH) is one of the best catalysts for the oxygen evolution reaction (OER) under alkaline conditions. Due to Ni­(Fe)­OOH’s layered structure, electrolyte species are able to easily intercalate between the octahedrally coordinated sheets. Electrolyte cations have long been considered inert spectator ions during electrocatalysis, but electrolytes that penetrate into the catalyst may play a major role in the reaction process. In a joint theoretical and experimental study, we report the role of electrolyte counterions (K⁺, Na⁺, Mg²⁺, and Ca²⁺) on Ni­(Fe)­OOH catalytic activity in alkaline media. We show that electrolytes containing alkali metal cations (Na⁺ and K⁺) yield dramatically lower overpotentials than those with alkaline earth cations (Mg²⁺ and Ca²⁺). K⁺ and Na⁺ lower the overpotential because they have an optimal acidity and size that allows them to not bind too strongly or alter the stability of reaction intermediates. These two features required for intercalated cation species provide insight into selecting appropriate electrolytes for layered catalyst materials, and enable understanding the role(s) of electrolytes in the OER mechanism