XAFS studies on Ni (II) hydroxide nanoparticles obtained by eletrochemical synthesis applied as electrode material for rechargeable batteries

L. ANDRINI; F.J. RODRÍGUEZ NIETO; S.J.A. FIGUEROA

Campinas

Workshop; Reunião Anual de Usuários (23º RAU); 2013

CNPEM

Its widely known that the Nickel Hydroxide is an important and functional material with multiple applications, principally in battery technology as the positive electrode of so called Cd/Ni, Zn/Ni,H2/Ni, Fe/Ni and MH/Ni cells, also as a precursor of catalysts, and in electrochromic devices. Particular emphasis is placed on the structural (and textural) characterization of the phases involved in nickel hydroxide electrodes as a prerequisite to a rational approach to their redox behavior and understand the relationship between the structural features and the charge storage capacity. Nickel hydroxide has a hexagonal layered structure with two polymorphs forms, namely αand β. The α -form is isostructural with hydrotalcite (HT)- like compounds and consists of a stacking of positively charged Ni(OH)x-2 layers, with intercalated anions (e.g., carbonate, nitrate, etc.) and water molecules in the interlayer space to restore charge neutrality; if the slabs are oriented randomly with respect to one another, leading to turbostratic disorder. The β-form possesses a brucite like Mg(OH)2 structure and does not contain any intercalated species. In this structure, the OH-ions are hexagonally packed and the nickel ions occupy alternate rows of octahedral sites leading to layered structure which can be described as an ordered stacking of charge neutral layers of Ni(OH)2. The HT and the brucite structures are not very much different. In most cases, the active material of positive electrodes belongs to the β-form. It is generally accepted that α-nickel hydroxide will exhibit superior electrochemical properties compared to the β-form. However, the α-nickel hydroxide is a metastable phase and is difficult to synthesize because it changes rapidly to the β-form during synthesis or on storage in a strong alkali. Actually its recognized that the overall performance of nickel cathodes depends on the microstructure, textural characteristics, and the crystallite size of the active material. In this work we explore la possibility of obtain α and/or β nickel hydroxide at semi micro level using a high area platinum mesh as cathode and following the changes with Co hydroxide doping.