Performance enhancement of vanadium redox flow battery by optimized electrode compression and operational conditions

Two different commercial carbon felt electrodes for vanadium redox flow battery, based on polyacrylonitrile and rayon, are tested and compared in order to evaluate the effect of felt compression on the battery performance in terms of cell polarization and pressure drop through the felt. The following properties are measured under different felt compression ratios: electrical and mechanical properties of the felt, pressure drop through the felt and performance in lab-scale single-cell. Optimum compression ratio (30% for polyacrylonitrile and 60% for rayon) is evaluated as a minimum of total power losses caused by cell polarization and electrolyte pumping. At the optimized felt compression the effect of electrolyte flow rate and operating temperature on the cell performance is studied at different states of charge by combination of electrochemical impedance spectrometry and load curve measurements. The minimum cell resistance, evaluated from load curves, is found around 50% state of charge for charging and around 70% state of charge for discharging for both electrode materials. The optimized compression together with optimized thermal activation of the electrodes and proper membrane selection enabled us to achieve outstanding peak power densities over 1.1 W cm(-2), which is by far the highest value published for a simple flow-through design.