Development of a new cathode for aqueous rechargeable batteries : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Chemistry at Massey University, Palmerston North, New Zealand

Abstract

The demand for low/cost energy storage is a current issue. Existing batteries are unable to meet this constraint due to the high raw material prices, in particular the metal content. The risk of fluctuating metal prices and future availability will not meet the market demand and therefore alternative materials need to be considered.

The focus of this project was to develop a non/metal based battery electrode specifically for stationary battery systems. This study presents fundamental concepts required to form a rechargeable electrochemical storage device utilising hydrogen peroxide as the electroactive species. This involved two key aspects: immobilisation of hydrogen peroxide in order to prevent self/discharge and catalytic regeneration of hydrogen peroxide from hydroxide ions. Although the construction of the device was not within the scope of this project, the chemical and electrochemical analysis of potential compounds were evaluated at a molecular level. In particular, the synthesis and molecular behaviour of a urea/ based ’binder‘ that will immobilise hydrogen peroxide, and an oxoammonium ’catalyst‘ to reform hydrogen peroxide during recharge of the battery. Additionally, the attachment of these compounds to a surface was also evaluated.

Analysis of the interactions between substituted ureas (‘binder’) and hydrogen peroxide proved challenging. Although these findings suggest that adduct formation is occurring, the methods undertaken were not able to determine the equilibrium constant or strength of binding. They did however give an indication of the quantity of hydrogen peroxide in the synthesised adducts and this methodology can be applied to the range of hydrogen peroxide adducts. Additionally, functionalisation of surfaces with a diazonium/containing substituted urea was achieved and is a viable method for attaching the ‘binder’ to the electrode substrate material.

The second key step is to incorporate a rechargeable aspect to the battery system. An oxoammonium cation was proposed to act as a ‘catalyst’ to replenish H2O2 during charging. Details regarding the synthetic methodology for synthesis of nitroxide/containing compounds, incorporating diazonium functionalities for attachment to a surface, were obtained. Of particular interest was the amido/ TEMPO structure that was electrochemically attached to a carbon surface electrode. Evaluation of its ability to form H2O2 was achieved using a bi/potentiostat where a current density of 0.21 A m/2 was observed. This novel idea shows promise and demonstrates the ability for the catalyst to replenish H2O2 in an aqueous battery system.