Alleviating Manganese – Vanadium Redox Flow Batteries for Energy Storage and Subsequent Hydrogen Generation
Authors: Chaurasia Shabdiki B. , Aravamuthan Sundar Rajan , Sullivan Connor S. , Ross Michael B. , Agar Ertan 
Published: September 2023

Dual-circuit redox flow batteries (RFBs) consist of two discharging modes which has the potential to store energy beyond the capacity of the electrolytes in the form of hydrogen generation [1-3]. Inspired from the recent exploratory study by Reynard and Girault et al., this work focuses on utilizing Mn3+/Mn2+ (RFB catholyte ~ 1.51 V vs. SHE) in conjunction with V3+/V2+ (RFB anolyte ~ −0.26 V vs. SHE) as indirect redox mediators to drive mediated electrolysis of water in spatially separated external catalytic reactors [3]. The advantage of an additional layer to existing RFB is that a fully charged battery could potentially be chemically discharged particularly when green electricity is available in abundance to provide a secondary resource for energy such as hydrogen gas or other value-added commodity chemicals. However, Mn3+ is prone to rapid chemical disproportionation yielding Mn4+ which precipitates as MnO2(s) particles in acidic media, responsible for severe capacity fade during cycling. Studies in the literature suggest that oxygen scavenging additives such as V5+ is possibly the best defense mechanism to prevent MnO2 formation [4].

In this study, the electrochemical behavior of Mn3+/Mn2+ couple in the favorable presence of V5+ is elucidated using cyclic voltammetry and ionic speciation diagrams. The voltammograms obtained under systematically varied conditions provide direct experimental evidence for competing redox reactions at the electrode surface. Nonetheless, the response measured from a macroelectrode mainly describes the quasi reversibility of the redox couple and gives little insight on electrode-electrolyte interface phenomenon. To obtain a more detailed picture of the diffusion layer, we employ carbon disk ultramicroelectrode (UME) for in-depth qualitative analysis. We anticipate that any deviation from the sigmoidal shape is an indication of a growing diffusion layer due to MnO2 passivation on the surface of electrode. Lastly, galvanostatic charge discharge profiles are obtained for Mn-V based redox flow battery with an aim to reveal the optimum state of charge because there exists a trade-off between the extent of Mn disproportionation within RFB during energy storage mode and volume of hydrogen gas produced during secondary mode. The knowledge obtained throughout the study will provide a fundamental understanding required for the rational design and development of aqueous electrochemical flow systems using Mn as the redox active species.

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