High Capacity Redox-Flow Batteries with High Density Suspensions of Spiky Nanostructured Particles

Self-assembled complex nanoparticles with spiky surfaces can accommodate significant amounts of excess charge, which can enable various energy storage and conversion technologies. Their combination of high charge storage capacity, high dispersibility, and synthetic simplicity renders them attractive for use in redox-flow batteries. Here we show that hedgehog-like FeSe₂ particles (HPs) are effective charge carriers in aqueous redox-flow batteries for long-duration energy storage. The spikes reduce particle-to-particle attraction, engendering stable aqueous dispersions. Shear thinning behavior of the spiky particles observed in this work for the first time facilitates their utilization in redox-flow batteries. HP suspensions exhibited a half-wave potential (E_1/2) of 0.45 V vs RHE (-0.47 vs Hg/HgO) at high HP loadings under strongly alkaline conditions (pH 14). A compositionally asymmetric flow cell comprising FeSe₂ HPs in the negative electrolyte and ferro/ferricyanide in the positive electrolyte displayed an open circuit voltage of ∼1.0 V. Up to 1.4 mol e/L (∼36.4 Ah/L) of volumetric capacity in the negative electrolyte was attained. Both the spiky shapes of these particles and their high densities in dispersion were responsible for capacity increase relative to nonspiky particles. The slow formation of iron hydroxide-species was responsible for capacity fade at 0.6-5.8%/cycle. Such capacity fade may be mitigated in future work through conformal particle coatings and judicious adjustments to electrolyte composition.