Development and characterisation of advanced energy storage devices for stationary applications

Abstract

The growing demand for energy and increasing attention on environmental challenges outline the requirement to optimise the electrical grid, and gradually replace the current energy sources with sustainable and renewable alternatives. The electrical grid at present forms an enormous infrastructure to instantaneously transmit the primary generated energy to the end users. However, due to the lack of storage capabilities to store the primary energy, the existing grid must conform to the oscillations due to the changes in customer demand. Grid-scale energy storage devices such as redox flow batteries (RFBs) have emerged as key technologies to accommodate the transition from finite fossil fuels to renewable energies and improve the sustainability of the electricity generation sector. This thesis focuses on characterisation and development of novel RFB systems for energy storage applications. Firstly, a novel Regenerative Fuel Cell (RFC) utilising inexpensive manganese electrolyte in the cathode and facile hydrogen in the negative electrode has been examined. To understand the impact of different materials on the performance of the Regenerative Hydrogen Manganese Fuel cell (RHMnFC), various membrane electrode assemblies are tested. it was found that carbon felt as the liquid half-cell electrode, Carbon paper with Pt loading of 0.3 mg/cm2 as the hydrogen electrode and Nafion 117 as the membrane yield the highest performance in the cell. This configuration of the membrane electrode assembly yields energy efficiencies in the range of 77% and 90%, while charging and discharging the cell at current densities in the range of 20 mA/cm2 to 100 mA/cm2. Furthermore, viability of scaling-up is studied, where a techno economic study has been carried out to explore the feasibility of this novel chemistry compared to the conventional energy storage devices, with an estimated 37% reduction in the levelized cost of storage compared to the all-vanadium RFB system. RFBs with manganese redox active species have hitherto been little investigated for energy storage applications due to the instability of Mn3+. To improve the lifespan of the novel RFC, an electrolyte composition, consisting of manganese as the redox active species and Ti4+ as an additive that supresses the Mn3+ disproportionation, is presented. The performance of this electrolyte composition is tested, to identify the impact of the operating conditions, such as the operating temperature of the cell, rest time between half-cycles and overcharging the electrolyte, on the stability of the RHMnFC system. This set of experiments reveals that, although the presence of Ti4+ supresses the Mn3+ disproportionation reaction, precipitation of MnO2 is an unavoidable phenomenon. Following these findings, a method to regenerate the inevitable precipitation of MnO2 in the electrolyte is proposed and the practicality of the method is experimentally tested and proven. Secondly, an in-situ method was developed using X-ray radiography and tomography techniques to enable the visualisation and characterisation of electrodeposited zinc (Zn) in Zn-RFBs. Zn-Based RFBs are promising technologies for energy storage applications. However, there are a number of challenges that must overcome prior to the commercialisation of these systems. The main obstacle is the dendritic growth of Zn on the anode electrode. This part of the thesis focuses on developing a method to investigate the mechanisms which effect the morphology of the zinc deposit. This method consists of designing a novel three-electrode cell with the capability to operate under different conditions, to investigate the effect of current density and electrolyte flow on the morphology of the deposited Zn. By monitoring the real-time formation of Zn deposits and reconstructing the morphology of the deposits, the mechanisms which supresses the dendritic growth of Zn deposits have been found and analysed. Quantitative analysis showed that operating under dynamic flow improved the morphology of the electrodeposited Zn and gave a compact deposit.