In situ bulk magnetization measurements reveal the state of charge of redox flow batteries.

We report two methods that use either NMR spectroscopy or direct magnetic susceptibility measurements for in situ (strictly online) determination of the state of charge of redox flow batteries. These methods are demonstrated on the inorganic, redox-active potassium ferro/ferri cyanide catholyte cycled against 2,6-dihydroxyanthraquinone as the anolyte in a full cell, and should be applicable to a wide range of redox couples, provided that the magnetization of the electrolyte solution depends on its oxidation state.C.P.G. acknowledges support from Centre of Advanced Materials for Integrated Energy Systems (CAM-IES) via EPSRC grant number EP/P007767/1, and support from Shell. E.W.Z. acknowledges the start-up support from Institute for Molecules and Materials, Radboud University Nijmegen. The authors acknowledge Prof. Dominic Wright from the University of Cambridge for helpful discussions, and Dr. Chris Brackstone from the physical teaching laboratory for lending the Evans balance