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

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 [P₂W₁₈O₆₂]^6- ({P₂W₁₈}) 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 H₆{P₂W₁₈} negolyte paired with a VOSO₄-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 H₆{P₂W₁₈}, and 141.75 Ah L^-1 at 100% SoC of 0.3 M H₆{P₂W₁₈}, 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 H₆{P₂W₁₈}. This work unlocks full SoC of {P₂W₁₈} by translating CPET mechanistic insights into actionable electrolyte design, establishing a generalizable pathway toward high-energy-density POM-RFBs.