Aqueous Manganese-Based Electrolytes for Redox Flow Batteries

Redox flow batteries (RFBs) are an emerging technology suitable for large-scale, stationary energy storage applications, including grid storage. Redox flow batteries are capable of storing high amounts of both energy (MWh) and power (MW).¹ One principle advantage of flow batteries is the ability to decouple the energy density and power density of the system, and scale both independently. The all-vanadium RFB represents the current state-of-the-art in flow battery technology, and features several advantages, including rapid response times, high depth of discharge, long cycle life, and good cell performance metrics (i.e. efficiencies). Anolyte and catholyte cross-contamination is eliminated through the use of the same active metal cation in both compartments of the cell.

Issues associated with the vanadium RFB include a low cell voltage (1.26 V), the use of expensive membranes including Nafion®, and the relatively high cost of vanadium, leading to expensive electrolytes.² The development of anolytes with higher reversible potentials than VO²⁺/VO₂⁺, along with electroactive species featuring higher natural abundance and lower cost than vanadium, represents an attractive alternative to vanadium-based anolytes. Flow battery  anolytes based on the Mn²⁺/Mn³⁺ redox couple have been reported in the literature, and the high standard electrode potential of Mn²⁺/Mn³⁺ (1.51 V) has been utilized in manganese anolyte (Mn²⁺/Mn³⁺)/vanadium catholyte (V²⁺/V³⁺) redox flow batteries, featuring a theoretical open circuit voltage of 1.77 V.^2,3 The usage of manganese anolytes can lead to higher cell voltages (Figure 1) and cheaper anolytes, but the disproportionation reaction of Mn³⁺ to Mn²⁺ and MnO₂ is a technical issue that needs to be resolved in order for manganese-based anolytes to find widespread utility in redox flow batteries.²

This presentation will disclose our investigations on the development of manganese-based anolytes for redox flow batteries. The effect of various additives on electrolyte stability and electrochemical kinetics for the Mn²⁺/Mn³⁺ redox couple will be quantified using cyclic voltammetry, RDE measurements, electrochemical impedance spectroscopy, overpotential measurements, and full-cell testing (constant current charge/discharge) of Mn/V redox flow batteries (Figure 2).

References

1) Wang, W. et al. J. Power Sources. 2012, 216, 99.

2) Xue, F.-Q. et al. Electrochimica Acta. 2008, 53, 6636.

3) Hong, T.; Xue, F. “Investigation on manganese (Mn²⁺/Mn³⁺)-vanadium (V²⁺/V³⁺) redox flow battery.” 2009 Asia-Pacific Power and Energy Engineering Conference (APPEEC).