Investigation of hydrogen based redox flow batteries

Abstract

Large scale energy storage is crucial for effective integration of intermittent renewable energy sources such as solar and wind. Redox flow batteries are a promising grid scale energy storage technology and understanding their losses is key to further optimisation. This thesis investigates a novel class of RFBs which utilises a conventional liquid based cathode in combination with a hydrogen anode, similar to that used in a Proton Exchange Membrane Fuel Cell (PEMFC).

Firstly, a new Reference Electrode (RE) positioning method was developed in order to decouple anode and cathode processes in RFBs. Unlike other methods presented in literature, the approach described here can generate reliable measurements, without noticeably affecting the performance of the cell. This set-up was used to further understand the Regenerative Hydrogen-Vanadium Fuel Cell (RHVFC), which revealed that cathode diffusion and ohmic losses were the limiting processes of this cell. Catholyte crossover was also observed, which resulted in vanadium adsorption onto the catalyst of the hydrogen electrode.

Secondly, the feasibility of a novel Regenerative Hydrogen-Cerium Fuel Cell (RHCFC) was validated. Using the same RE set-up, further investigation was carried out on an optimised cell, which yielded energy efficiencies in the range of 70 to 85 \% when charging/discharging at current densities up to 20 mA/cm2. A combination of electrochemical impedance spectroscopy and polarisation tests allowed the decoupling of many of the processes occurring at each electrode. In addition, imaging and surface characterisation techniques revealed the presence of carbon deposits on the fresh electrode surface, which increased with use; the nature of these deposits and their implication on cell operation will be the subject of further investigation. Finally, it was established that the performance of the cell is currently limited by the cerium cathode, which suffers from poor kinetics and large diffusion losses.