POM Anolyte for All‐Anion Redox Flow Batteries with High Capacity Retention and Coulombic Efficiency at Mild pH

A highly soluble Li5 BW12 O40 cluster delivers 2 e- redox reaction with fast electron transfer rates (2.5 × 10-2 cm s-1 ) and high diffusion coefficients (≈2.08 × 10-6 cm2 s-1 ) at mild pH ranging from 3 to 8. In-operando aqueous-flowing Raman spectroscopy and density functional theory calculations reveal that Raman shift changing of {BW12} clusters is due to the bond length changing between W-Ob -W and W-Oc -W at different redox states. The structure changing and redox chemistry of Li5 BW12 O40 are highly reversible, which makes the Li5 BW12 O40 cluster versatile to construct all-anion aqueous redox flow batteries (RFBs). The cation-exchange Nafion membrane will also repel the cross permeability of the anion redox couples. Consequently, by coupling with Li3 K[Fe(CN)6 ] catholyte, the aqueous RFB can be operated at pH 8 with a capacity retention up to 95% and an average Coulombic efficiency more than 99.79% over 300 cycles within 0 to 1.2 V. Meanwhile, Li5 BW12 O40 cluster can also be paired with LiI catholyte to form aqueous RFBs at pH 7 and pH 3, the capacity retention of 94% and 90% can be realized over 300 cycles within 0 to 1.3 V.