Solid superionic conductor membranes are being considered as alternatives to polymer-based membranes for use in redox flow batteries (RFBs) due to their superior abilities to mitigate reactant crossover,¹ and enable the deployment of aqueous electrolytes comprising small, inorganic earth-abundant reactants.²,³ Much however remains to be understood about the evolution of the electrochemical performance and microstructural stability of these membranes while they are immersed in aqueous electrolytes.

In this work, we evaluate the suitability of von Alpen sodium superionic conductor (NaSICON) as a prospective RFB membrane material by examining its resistance, permeability and interfacial morphology over time as a function of electrolyte pH and composition, as well as temperature. NaSICON is found to have a stable resistance profile for several weeks while immersed in neutral to strongly alkaline ([OH^-] = 3 M) aqueous electrolytes. Its permeability to polysulfide-based and permanganate-based small-molecule RFB reactants is negligible compared to that of Nafion. Its area-specific resistance falls dramatically with increasing temperature and decreasing membrane thickness; we project that a membrane with a thickness of 100 μm or lower, if operated slightly above ambient temperature (~ 40 °C), can enable power densities comparable to or better than those of conventional polymer membrane-containing RFBs. Nevertheless, the glassy phase of the NaSICON microstructure at the membrane-electrolyte interface was found to undergo small amounts of etching while in contact with aqueous electrolytes containing sodium ions; this etching became significantly more extensive when potassium ions were present in the electrolyte, leading in certain instances to complete disintegration of the membrane.

The extraordinary high stability of NaSICON in strongly alkaline electrolyte permits the construction of flow cells containing a positive electrolyte based on permanganate, a high-potential, inexpensive reactant with high volumetric capacity (> 100 Ah/L). The flow cells had open-circuit voltages 1.2 V and greater, along with negligible reactant crossover and very low capacity fade (< 0.02 %/day). This work highlights the promise of ceramic membranes as effective separators in RFBs operating under neutral pH to strongly alkaline pH conditions. It also points to the need for further research on the long-term stability of the membrane and its interface with the electrolyte in solid-state membranes under investigation as separators in aqueous RFBs.

(1) Yu, X.; Gross, M. M.; Wang, S.; Manthiram, A. Aqueous Electrochemical Energy Storage with a Mediator-Ion Solid Electrolyte. Advanced Energy Materials 2017, 7 (11), 1602454, https://doi.org/10.1002/aenm.201602454. DOI: https://doi.org/10.1002/aenm.201602454 (acccessed 2021/03/12).

(2) Wei, X.; Xia, G.-G.; Kirby, B.; Thomsen, E.; Li, B.; Nie, Z.; Graff, G. G.; Liu, J.; Sprenkle, V.; Wang, W. An aqueous redox flow battery based on neutral alkali metal ferri/ferrocyanide and polysulfide electrolytes. J. Electrochem. Soc. 2016, 163 (1), A5150-A5153. DOI: 10.1149/2.0221601jes].

(3) Colli, A. N.; Peljo, P.; Girault, H. H. High energy density MnO4-/MnO42- redox couple for alkaline redox flow batteries. Chem Commun (Camb) 2016. DOI: 10.1039/c6cc08070g.