Electrode, electrocatalyst and electrolyte development for hybrid redox flow batteries

Abstract

The development of new electrochemical devices and their subsequent widespread adoption alongside our energy grids is critical to support the incoming wave of renewable energy generation and the transition to net zero. Hybrid redox flow batteries are a promising alternative energy storage technology that could ultimately provide inexpensive and sustainable large scale energy storage solutions compared to their lithium-based counterparts. Technological improvements rely on improved understanding of the processes occurring in these devices. This thesis focuses on three aspects essential to all electrochemical devices – the electrode, electrocatalyst and electrolyte. First, electrochemical and tomographic imaging analyses are combined to understand the effect of electrode microstructure properties on the performance of a non-aqueous redox flow battery. With the help of a one dimensional model, mass-transfer coefficients are gathered from the experimentally determined polarisation curves and affirm the results positively linking electrode permeability to overall cell performance. Second, the synthesis and optimisation of a multifunctional, nitrogen-doped carbon based electrocatalyst is outlined. The active site of the electrocatalyst is examined using a suite of physical characterisation techniques and is shown to have intriguing versatility across the pH spectrum towards catalysing both oxygen and hydrogen fundamental reactions. This catalyst is then tested, with a variety of success, in two fuel cell devices and, alongside the electrodes tested in the first section, in a hydrogen-manganese hybrid redox flow battery. Finally, the aqueous liquid electrolyte involved in this hybrid redox flow battery is scrutinised. Two in-operando analytical techniques are developed, providing improved handles on real-time solution-phase manganese species concentration in the electrolyte. The production of solid manganese oxides significantly alters solution equilibria making the deconvolution of electrochemical and adjacent, simultaneous chemical processes complex.