The Electrolyte Monitoring of a Vanadium Redox Flow Battery

Abstract

A series of carbon felts was examined by cyclic voltammetry to investigate their suitability as an electrode material for the all-vanadium redox flow battery. A commercially supplied vanadium electrolyte of unknown concentration was analysed by UV-visible spectroscopy and was shown to contain VO2+ and V3+ ions in a ratio of 4.8:1 with a total vanadium concentration of 1.5 mol dm-3 in 4 mol dm-3 H2SO4. A unit laboratory redox flow battery (100 cm2 membrane area) was assembled using carbon felts and planar carbon feeders as the positive and negative electrodes with a Nafion® 115 proton exchange membrane. Its performance was examined using both 1.5 mol dm-3 and 1.1 mol dm-3 vanadium electrolytes at volumetric flow rates in the range of 0.5 to 3 cm3 min-1. At a vanadium concentration of 1.1 mol dm-3, a suitable volumetric flow rate was found to be in the range1.5 – 2.0 cm3 min-1. At a constant current density charge and discharge of 100 mA cm-2 for 45 minute half-cycles (and <30 cycles), the typical voltage efficiencies were 65 %.