Investigating Manganese–Vanadium Redox Flow Batteries for Energy Storage and Subsequent Hydrogen Generation

Dual-circuit redox flow batteries (RFBs) have the potential to serve as an alternative route to produce green hydrogen gas in the energy mix and simultaneously overcome the low energy density limitations of conventional RFBs. This work focuses on utilizing Mn3+/Mn2+ (∼1.51 V vs SHE) as catholyte against V3+/V2+ (∼ −0.26 V vs SHE) as anolyte redox mediators capable of indirect water splitting in an external chemical reactor, i.e., chemical discharge of charged species (Mn3+ and V2+) to harvest hydrogen gas from the anolyte. However, Mn3+ is prone to rapid chemical disproportionation, yielding Mn4+ which precipitates as MnO2(s) responsible for severe capacity fade in RFB. The primary objective of this study is to investigate the electrochemical behavior of Mn3+/Mn2+ in the presence of an additive using three different electrodes–graphite sheet, graphite rod, and carbon disk ultramicroelectrode. Cyclic voltammograms using macro electrodes provide direct experimental evidence for competing redox reactions, whereas UME exhibits strong hysteresis and flattening of faradaic peaks in the favorable presence of V5+ as an additive. These results indicate an increase in effective electrode surface area, possibly due to the growing diffusion layer from MnO2(s) deposition. Further, full cell Mn–V RFB cycling studies with Nafion 212 reveal a continuous fluctuation in coulombic efficiency up to 30 cycles, owing to rapid MnO2(s) passivation and Mn crossover. Optimizing the true state of charge is of utmost importance because there exists a trade-off between the extent of Mn disproportionation within the flow cell during the primary mode of energy storage and the volume of hydrogen gas produced during the secondary mode.