Gel-like State of Nickel Hydroxide Created by Electrochemical Aging under Alkaline Conditions

Nickel-containing electrocatalysts are being developed for the electrochemical transformations of organic species and used in gas evolution reactions, such as the oxygen evolution reaction (OER). Nickel-based materials are sought after in part for their lower cost relative to precious metal catalysts (e.g., Pt, Ru, Ir). To develop more durable materials and to better understand the mechanisms involved in electrochemical transformations on nickel-based materials, it is essential to understand how these materials evolve and age as a result of electrocatalytic processes. In this study, we preserve and analyze the hydrated form of nickel electrocatalysts prepared by electrochemical aging under alkaline conditions relevant to the OER. A series of electrocatalysts were prepared to also evaluate differences in the aging of electrocatalysts with nanoscale grains versus those with microscale grains. These materials were each prepared with similar nanoscale roughness. The series of Ni electrocatalysts were aged by potential cycling and their hydrated forms subsequently preserved by immersion in liquid nitrogen. After freeze-drying, the preserved state of these samples was analyzed by scanning electron microscopy techniques while held at cryogenic temperatures. The surfaces of aged Ni electrocatalysts were all observed to contain an electrochemically active layer with a gel-like form. Through the use of transmission electron microscopy analyses, it was determined that these gel-like layers contained predominantly nanocrystalline β nickel hydroxide (β-Ni(OH)2). The formation of the gel-like layer covering these electrocatalysts has implications for dynamic processes taking place at their interface with the electrolyte. Processes influenced by the form of this active layer include rates of diffusion of electrolyte, the mechanism of O2 bubble nucleation, and the mechanics of bubble release. The results of these studies also have implications for the electrocatalytic activity and stability of other types of electrocatalysts.