A Vanadium-Cerium redox flow battery has been developed with a secondary circuit where the vanadium (II) can be chem. oxidised to produce hydrogen and where the cerium (IV) can be chem. reduced to produce oxygen. This redox flow battery can be used as an indirect water electrolyzer even when the battery is under elec. charging. This battery has been designed to help to regulate the grid in Switzerland and to absorb the load of so-called junk electricity, i.e. The electricity overprodn. from renewable sources. In such a battery, the electrolytes play a key role to optimize the energy efficiency. Indeed, the redox potential of cerium (IV) in particular can be tuned by optimizing the electrolyte compn. [on SciFinder(R)]