Abstract
Energy storage is a solution to the problem of renewable energy intermittency. The vanadium redox flow battery (VRFB) is one such energy storage technology, allowing energy to be stored electrochemically in vanadium electrolytes until required. The advantages of VRFBs include independently scalable energy and power characteristics, high reliability and long life time. The broad aim of this work was to investigate the main problems VRFBs currently present: 1. temperature stability of the vanadium electrolyte, and 2. chemical stability and vanadium cation permeability issues of the ion permeable separator membrane. It was determined that the concentrations of vanadium and sulfuric acid in the electrolyte have a greater impact on thermal stability than the presence of additives. Decreasing the sulfuric acid concentration, improves cold temperature stability of the vanadium electrolyte without impeding stability at elevated temperature. The testing of commercial VRFB membranes indicated that there is an upper limit to beneficial ionic conductivity. With this in mind, several radiation grafted ion exchange membranes were synthesised and characterised. Results show that the higher the degree of grafting (functionality) and associated ionic conductivity, the higher the susceptibility to vanadium cation cross-over. In order to lower vanadium cation permeability, a more crosslinked and dense membrane structure was found to be advantageous. It was shown that the highly oxidising vanadium(V) species causes significant chemical degradation (via oxidation) of many different membrane functional groups. Amine-functionalised anion exchange membranes show high susceptibility to oxidation by vanadium(V). Carboxylic acid groups potentially show higher chemical stability and lower vanadium cation permeability than the amine-containing functional groups. A VRFB cell was commissioned and its key operating conditions were optimised. A low current density was required to allow effective charge-discharge cycling. The novel use of a conductive graphite ink (Electrodag) between the felt electrode and current collector, achieved a fivefold reduction in the internal resistance of the VRFB due to improved electrical contact.