Analysis of a Gel-like State of Nickel Hydroxide Created By Electrochemical Aging

Abstract

As global energy demands increase, the need for clean energy and energy storage is evident. Water electrolysis has shown promise as a clean energy storage solution and can be coupled with other clean energy techniques (e.g., wind, solar) to store chemical energy in the form of hydrogen. Alkaline systems are of particular interest as they are more cost effective compared to acidic systems that require precious metal-based catalysts. Alkaline water electrolysis suffers from inefficiencies that include the oxidative half reaction known as the oxygen evolution reaction (OER). Nickel-based electrocatalysts are being developed for the electrochemical transformations of organic species and used in gas evolution reactions, such as the OER. Nickel-based materials are sought after in part for their lower cost relative to precious metal catalysts (e.g., Pt, Ru, Ir), but they lack the higher electrochemical activity achieved by their precious metal counterparts. To develop more active and 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 use.In this study, we have preserved the hydrated form of Ni electrocatalysts aged under alkaline conditions relevant to the OER. We prepared a series of electrocatalysts with nano- and micro-scale grains that were polished to a similar nano-scale roughness. First, we electrochemically aged Ni electrocatalysts with nanoscale and microscale grains by potential cycling techniques (i.e., cyclic voltammetry). Following electrochemical aging, the Ni electrocatalysts were preserved by immersion in liquid nitrogen and sublimed in a lyophilizer. After this freeze-drying process, the electrocatalysts were imaged under cryogenic conditions using scanning electron microscopy (SEM) techniques. A comparison was made to aged electrocatalysts where freeze-drying was implemented and those that were allowed to air dry. The surfaces of aged Ni electrocatalysts were all observed to contain an electrochemically active layer with a gel-like form. When the catalysts were air-dried, the layer appeared to have a collapsed, web-like texture. Through the use of transmission electron microscopy analyses, it was determined that these gel-like layers contained predominantly nanocrystalline β-phase nickel hydroxide (β-Ni(OH)2), which likely formed due to relaxation of the OER active beta-phase nickel oxyhydroxide [β-NiOOH] prior to the imaging process. 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 gel-like form of this active layer include the 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. Further, this work can be extended for the design of new electrocatalysts for a variety of electrocatalytic processes, such as the hydrogen evolution reaction and other gas evolution reactions. Figure 1