Experimental Study of Capacity Fade in Large Vanadium Redox Flow Battery Cells

One of the not so well understood aspects of vanadium redox flow battery cells is the capacity fade that happens during continuous charge–discharge cycles. While several modeling and experimental studies have been reported, reliable data are not available in the open literature. In the present work, long‐duration charge–discharge cycling studies are reported in cells of industrial size having an electrode area of about 400 and 1800 cm 2 . The continuous current and cell voltage measurements have been supplemented by periodic measurement of concentration of the electro‐active vanadium ions in the positive and the negative electrolytes. The weight of the electrolyte tanks and the open circuit voltage of the cell have also been monitored. The mutual self‐consistency of these data has been verified by post‐test analyses. Data from over thirteen different cases spanning a range of current densities and electrolyte circulation rates show that the capacity fade is about (0.15 ± 0.05)% per cycle. In all cases, an asymptotic capacity fade pattern is established in which the concentration of V(V) increased and V(IV) decreased steadily on the positive side, and that of V(III) decreased steadily but slightly on the negative side.