Vanadium-oxygen cell for positive electrolyte discharge in dual-circuit vanadium redox flow battery

The “dual-circuit redox flow battery” takes advantage of a conventional all-vanadium redox flow battery (VRFB) combined with a separated catalytic hydrogen evolution reactor. Depending on demand, the VRFB can be conventionally discharged or supply H2 by using the catalytic reactor. However, during hydrogen generation, protons are consumed and a state-of-charge imbalance is created. Therefore, the corresponding discharge of the positive compartment is necessary to maintain proton and charge balance. Water oxidation is a suitable reaction providing regeneration of the positive electrolyte (reduction of VO2+ to VO2+) and also protons. VO2+/VO2+ does not possess a potential high enough to spontaneously drive the oxygen evolution reaction thus an additional energy input is required. We introduce the V–O2 cell, a secondary flow cell to integrate into the “dual-circuit redox flow battery” that provides a high rate of conversion while minimizing the energy required. This was tested in a complete dual-circuit system and required 140 mW cm−2 of additional energy at 110 mA cm−2. The total efficiency of the coupled system in term of electricity to hydrogen was between 42 and 62% depending on the current density and stable operation was demonstrated during extended tests.