Examining the charge storage mechanisms of nanoscale nickel hydroxide

Abstract

The aim of this work was to develop mechanistic understanding, as well as techniques to study the charging and discharging behaviour of a material which has controversially been described as either a battery or a pseudocapacitor electrode material. Nickel hydroxide, Ni(OH)2, was chosen as the material of choice due to its long history as a secondary battery cathode material, as well as recent interest in it as a high rate supercapacitor. This recent interest is mainly due to advances in nanoscaling the material architecture. After a review of the literature (Chapter 1) and a summation of the methods used in this thesis (Chapter 2), the experimental work began by studying the specific capacitance (Caps), the specific capacity (Cs) and the areal capacity (CA) of nickel hydroxide nanoparticles in aqueous 1.0 mol dm-3 potassium hydroxide electrolyte. Particles were fabricated using hydrothermal methods, in the diameter range of 3–450 nm (Chapter 3). All metrics were consistent with surface only charging down to a critical diameter of around 20 nm at which point there was a deviation. This suggested the novel idea that a charging “skin”, which was interacting below this diameter, was preventing further charging. This charging “skin” was investigated using ToF-SIMS with isotopic labelling (Chapter 4) to follow the uptake of deuterium ions during charging cycling. Ingress of deuterium into the material was dependent on charge cycling number, and the depth was found to approximately match that observed in the nanoparticle study. The impedance behaviour of the Ni(OH)2 was studied in order to separate the different contributions of non-Faradaic double layer capacitance, surface Faradaic pseudocapacitance, and intercalation reactions to the charge storage (Chapter 5). Studying the capacitance at different DC potentials and frequencies allowed these mechanisms to be deconvoluted, while the best fitting circuit model gave insight into the porous structure of the electrode. Using a semi-infinite pore model, the porous electrode data would not map onto the flat electrode data, showing the diffusion into the pores does not follow a semi-infinite model, as often assumed in literature. The evidence gathered suggests that the charging mechanism of Ni(OH)2 is a complex combination of both capacitive surface dependant charging and intercalation into the bulk of the structure.