A combined in situ monitoring approach for half cell state of charge and state of health of vanadium redox flow batteries

Efficient vanadium redox flow battery operation requires reliable half cell specific state of charge and capacity information at each point in time. This study uses long term correlations of electrolyte densities with negative half cell capacities and half cell potentials in Nernst equations for a joint in situ measurement approach. In principle, no ex situ reference data are needed for the calibration since a combination of Coulomb counting and half cell electrolyte potentials enables a self consistent calibration during battery operation cycles. By this way state of charge results of a negative half cell feasibility study are confirmed for long term data and extended to the positive half cell. The explicit consideration of H+ concentration and acid dissociation for the positive half cell leads to consistent half cell measurements with lower errors. Experiments have been carried out with two different cation exchange membranes achieving comparable measurement errors in the few per cent range. (C) 2020 Elsevier Ltd. All rights reserved.