Unlocking Full State‐of‐Charge of Polyoxometalate for High‐Energy‐Density Redox Flow Batteries via Concerted Proton‐Electron Transfer

Abstract Polyoxometalates (POMs) exhibit exceptional multi‐electron transfer capacity for next‐generation high‐energy‐density redox flow batteries (RFBs), while their operable state‐of‐charge (SoC, ≤33.3%) is commonly limited by universal highly reduced metastable states under proton‐starved conditions. Herein, by establishing a proton‐coupled electron transfer (PCET) paradigm for [P 2 W 18 O 62 ] 6− ({P 2 W 18 }) cluster, we reveal that protonation at oxygen sites stabilizes reduced tungsten sites via concerted proton‐electron transfer (CPET). Marcus theory combined with DFT calculations quantifies the thermodynamic driving force and kinetic barrier for region‐selective CPET processes, and operando analyses by pH monitoring and Raman spectroscopy further confirm this proton‐coupled reversible redox mechanism. Guided by these findings, we engineer the high‐proton‐activity H 6 {P 2 W 18 } negolyte paired with a VOSO 4 ‐based posolyte and stepwise charging‐discharging protocol that enables stable full SoC operation. The resulting RFBs achieve unprecedented performance, which maintains 95.04 Ah L −1 without decay over 600 cycles (over 1020 h) at 66.7% SoC of 0.3 M H 6 {P 2 W 18 }, and 141.75 Ah L −1 at 100% SoC of 0.3 M H 6 {P 2 W 18 }, as well as delivers a record‐breaking 236.03 Ah L −1 and 239.02 Wh L −1 at 100% SoC of 0.5 M H 6 {P 2 W 18 }. This work unlocks full SoC of {P 2 W 18 } by translating CPET mechanistic insights into actionable electrolyte design, establishing a generalizable pathway toward high‐energy‐density POM‐RFBs.