Green Hydrogen Production Using Manganese – Vanadium Redox Flow Batteries

The redox flow batteries (RFBs) can be tuned to allow mining of surplus energy capacity and supplement green hydrogen economy as a one-stop station [1, 2].With this concept, an RFB can undergo two sequential discharging modes: i) electrochemical discharge of redox active species, and ii) chemical discharge to produce hydrogen gas, when surplus electricity is available, on the surface of electrocatalysts contained in additional external tanks (i.e., dual circuit RFB) [3, 4]. Herein, electrolytes with Mn 3+ /Mn 2+ (RFB catholyte ~ 1.5 V vs. SHE) and V 3+ /V 2+ (RFB anolyte ~ −0.26 V vs. SHE) redox species are flown to spatially separated external catalytic reactors to drive redox mediated water electrolysis. The fully charged V 2+ redox mediator in the anolyte provides electrons to achieve hydrogen evolution reaction (HER) and thereby itself gets discharged to V 3+ redox state before circulating back to flow battery tanks. Considering the additional layer of redox-mediated water electrolysis, it is imperative to identify benchmark cycling performance for a robust dual circuit RFB operation. In the first phase of our study, electrochemical behavior of Mn 3+ /Mn 2+ redox couple in the favorable presence of V 5+ as an additive was elucidated using ultramicroelectrode voltammograms. Despite significant performance enhancement, capacity fade during galvanostatic cycling could not be fully eliminated owing to Mn 3+ disproportionation reaction and manganese crossover. Also, charge/discharge cycling profiles suggest that the inevitable Mn 3+ disproportionation affects the true state-of-charge in electrolytes which in turn hinders their potential to generate hydrogen gas. Working towards hydrogen generation, our recent efforts have been to evaluate kinetics for hydrogen evolution reaction (HER) on different carbon substrate – electrocatalyst combinations in acidic V 3+ /V 2+ environment. Overall, this study aims to provide a profound understanding of reliable operating conditions to establish a synergy between Mn 3+ disproportionation during the conventional energy storage mode and the total volume of hydrogen gas produced during the secondary mode. References Reynard, D. and H. Girault, Combined hydrogen production and electricity storage using a vanadium-manganese redox dual-flow battery. Cell Reports Physical Science, 2021. 2 (9): p. 100556. Amstutz, V., et al., Renewable hydrogen generation from a dual-circuit redox flow battery. Energy & Environmental Science, 2014. 7 (7): p. 2350-2358. Peljo, P., et al., All-vanadium dual circuit redox flow battery for renewable hydrogen generation and desulfurisation. Green Chemistry, 2016. 18 (6): p. 1785-1797. Piwek, J., et al., Vanadium-oxygen cell for positive electrolyte discharge in dual-circuit vanadium redox flow battery. Journal of Power Sources, 2019. 439 : p. 227075.