High energy density MnO₄⁻/MnO₄²⁻ redox couple for alkaline redox flow batteries

Abstract

Redox flow batteries (RFBs) are receiving wide attention for large-scale energy storage due to their many attractive advantages including decoupling of energy storage and power output, high coulombic (CE) and energy (EE) efficiencies, and scalability. Among the various options considered, RFBs have become one of the most promising electricity-storage systems to address the intermittency issues of renewable energy sources such as wind and solar. [1][2][3][4][5][6] However, they are limited by their low energy density. To solve that problem, the active species are required to have reversible redox behaviour with a redox potential within the electrochemical potential window of the solvent, and large solubility in the electrolytes. Existing RFBs are mainly based on metal ions dissolved in acidic solutions. They suffer capacity losses due to membrane crossover, electrode corrosion and the occurrence of undesired secondary reactions during battery cycling, which translate into high operational and maintenance costs. Nowadays research efforts focus on cheaper materials, higher energy densities and better energy efficiencies.In this perspective, alkaline RFBs have the advantages of low self-discharge and low hydrogen evolution rate.7 Alkaline solutions allow the use of materials which are cheaper and less susceptible to corrosion compared to those required in acidic condition and, to compensate a lower conductivity of the electrolyte, they can tolerate the use of more conductive metallic three dimensional electrodes, allowing an overall higher energy efficiency. . It can be seen that the potential energy density could be much higher than existing RFB chemistries. The redox couple was evaluated at room temperature with NaOH or KOH supporting electrolytes by cyclic voltammetry, rotating disc electrode method on a carbon or platinum disk electrode, and by charge/discharge cycling.The electrochemical oxidation of manganate(VI) to permanganate(VII), reaction (1), is carried out industrially, but has not before been considered for energy storage applications. Typically, this reaction is performed in an undivided cell with an electrolyte containing potassium hydroxide and potassium manganate at 60 1C at an anode made from nickel or monel, while the cathode is iron or steel. A few studies have been carried out in order to obtain permanganate from manganate in a divided cell made with a Nafion 423 ion exchange membrane 17 or more recently Nafion 417. 18 In both cases, no stability problems were detected.A second redox couple corresponding to reaction (2) is observed at slightly lower potentials. Both processes can be seen in Fig. 3(A). It was found that the rate of the electrode reaction (1) is higher than that of electrode reaction (2) under comparable conditions.

Recent progress in organic redox flow batteries: Active materials, electrolytes and membranes

“…1 . With the electrolytes stored in the external tanks, one of the most attractive advantages of the RFBs is the flexibility in decoupling of power and energy [10][11][12][13][14] . In a RFB system, the amount of stored energy scales with the volume of the tank while the power rating scales with the size of cell stack [15][16][17][18][19] .…”

Compact and General Strategy for Solving Current and Potential Distribution in Electrochemical Cells Composed of Massive Monopolar and Bipolar Electrodes

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Vanadium–Manganese Redox Flow Battery: Study of MnIII Disproportionation in the Presence of Other Metallic Ions

“…Among the most common oxidation states (+II, +III, +IV, +VI, and + VII), Mn II is the most stable. Higher oxidation states are strong oxidizing agents such as permanganate (+VII; MnO 4 − ) or manganate anions (+VI, MnO 4 2− ), or known catalysts for water oxidation (+IV, MnO 2 ) . The high standard redox potential of the Mn III /Mn II couple (+1.51 V vs. SHE) has attracted interest for the investigation of V/Mn RFBs .…”