Evaluation of Asymmetric Flow Rates for Better Performance Vanadium Redox Flow Battery

Abstract

Electrolyte imbalance caused by water and ion crossover is one of the main factors affecting the capacity of vanadium redox flow battery system over cycling. Ion crossover and the associated water transfer towards the positive half-cell is mainly caused by the transfer of the vanadium ions, its bounded water and the water driven by osmosis. The different viscosity of the vanadium (III) ions in the negative half-cell builds inconsistent pressure profiles at both half-cells during charge-discharge cycling and therefore magnifies ions and water transfer tendency. To mitigate the effect of electrolyte imbalance, herein we report an experimental study on the effect of using asymmetric flow rates in the negative and positive half-cells. Over different current densities of 50, 100, and 150 mA cm−2, the use of one magnitude flow factor lower in the negative half-cell and one magnitude flow factor higher in the positive half-cell is superior to the symmetric flow rates on reducing electrolyte imbalance and subsequently, improving the capacity retention. At 100 mA cm−2, a gain of 5 % in discharge capacity coupled with energy efficiency improvement of around 3 % is achieved using a cell with asymmetric flow rates compared to a cell with the symmetric flow rates after 50 cycles. Low magnitude of flow rate asymmetry can sufficiently compensate for the half-cells pressures imbalance, higher increment in flow rate asymmetry showed that positive-to-negative crossover become more favourable.