Aqueous Colloid Flow Batteries Based on Redox-Reversible Polyoxometalate Clusters and Size-Exclusive Membranes

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Aqueous redox flow batteries (ARFBs) exhibit great potential for large-scale energy storage, but the cross-contamination, limited ion conductivity, and high costs of ion-exchange membranes restrict the wide application of ARFBs. Herein, we report the construction of aqueous colloid flow batteries (ACFBs) based on redox-active polyoxometalate (POM) colloid electrolytes and size-exclusive membrane separators. The aqueous suspensions of POM clusters, such as [N(C 3 H 7 ) 4 ] 4 [H 12 (VO 2 ) 12 (C 6 H 5 PO 3 ) 8 ]· x H 2 O and [N(C 3 H 7 ) 4 ] 4 [H 12 (VO 2 ) 12 (4-FC 6 H 4 PO 3 ) 8 ]· x H 2 O, deliver good reversibility, high redox kinetics, and long cycling life. The nanoscale sizes of POM clusters make them compatible with cheap commercial dialysis membrane separators to replace expensive ion-exchange membranes, thus inhibiting the cross-contamination of active species via size exclusion. The ACFBs achieve a high energy efficiency of ∼90% and an ultralow capacity fade rate of 0.004% per cycle. This work highlights the great potential of ACFBs based on redox-reversible POM clusters and size-exclusion membrane separators toward grid-scale and sustainable energy storage applications.