A V-Mn Redox Flow Battery for Concomitant Energy Storage and Hydrogen Production
Authors: Reynard Danick , Maye Sunny , Nagar Bhawna , Girault Hubert 
Published: July 2021

Redox flow battery (RFB) has drawn considerable attention past years for large-scale energy storage applications. However, RFB’s suffer from low energy density and premature degradation failures were reported due to uncontrolled electrolyte imbalance, limiting their commercialization. To address those concerns, we proposed in 2014 an innovative concept called ''dual-circuit redox flow battery’’[1]. This system is distinct from the conventional RFB in that the former includes a secondary energy platform, in which electrical energy can be converted into hydrogen via mediated water electrolysis. In fact, the positive and the negative charged redox species can be circulated in external catalytic bed reactors (out of the electrochemical cell), where they will respectively act as electron mediators (donor and acceptor) to carry out water oxidation and proton reduction over the catalysts. The dual-flow circuit RFB has the advantage to store energy beyond the capacity of the conventional RFB due to the higher volumetric capacity of the hydrogen energy carrier. Furthermore, mediated water electrolysis gives the opportunity to decouple oxygen evolution and hydrogen evolution reactions in time and in space, unlike conventional technologies (e.g alkaline and polymer electrolyte membrane electrolysers). This feature enables to avoid O2 and H2 recombination, addressing materials degradation and safety concerns. Additionally, the temporal/spatial separation of water splitting reactions allows us to design bed reactors using less active and cheaper electrocatalysts and facilitates the electrolytes rebalancing.
In this work, a complete proof-of-concept of a novel dual-flow circuit based on a vanadium-manganese RFB has been demonstrated (Figure 1). First, we studied the influence of Ti(IV) or V(V) additives on Mn(III) stability in high acidic medium. V(V) was observed to enhance the stability of Mn(III) as compared to Ti(IV), which is of high interest for improving the performances and the cyclability of the system [2]. Then, the chemical discharges of Mn and V electrolytes over Mo2C and RuO2 electrocatalysts were demonstrated. Additionally, we elaborated a kinetic model as a predictive tool for the vanadium-mediated hydrogen evolution on Mo2C electrocatalyst [3]. Finally, the V-Mn redox flow battery was designed at lab-scale and was operated up to 50 cycles between 20 and 80% SOC at 50 mA/cm–
2. The catalytic bed reactors were designed using an innovative approach for fast and cheap catalyst preparation. Hydrogen production was carried out at 10 bars and the system achieved an overall efficiency of 70%.
[1] V. Amstutz et al., « Renewable hydrogen generation from a dual-circuit redox flow battery », Energy Environ. Sci., vol. 7, no 7, p. 2350-2358, juin 2014, doi: 10.1039/C4EE00098F.
[2] D. Reynard, S. Maye, P. Peljo, V. Chanda, H. H. Girault, et S. Gentil, « Vanadium–Manganese Redox Flow Battery: Study of MnIII Disproportionation in the Presence of Other Metallic Ions », Chemistry – A European Journal, vol. 26, no 32, p. 7250-7257, juin 2020, doi: 10.1002/chem.202000340.
[3] D. Reynard, G. Bolik-Coulon, S. Maye, et H. H. Girault, « Hydrogen production on demand by redox-mediated electrocatalysis: A kinetic study », Chemical Engineering Journal, p. 126721, août 2020, doi: 10.1016/j.cej.2020.126721.

Figure
1
Schematic of the dual-circuit V-Mn redox flow battery for concomitant energy storage and hydrogen production

Figure 1