Tuning the inner- and outer-sphere electron transfer behavior of aqueous {CoW₁₂} polyoxometalate clusters for redox flow batteries exceeding 1.5V

Redox active species as the key component of redox flow batteries (RFBs) directly determine battery performance. Herein, we report a unique redox active specie, {CoW12}, exhibits both outer-sphere electron transfer of the {CoO4} core and inner-sphere electron transfer of the {W12O36} shell in one cluster, which is capable of constructing a symmetric {CoW12}-based RFB with pH-controlled operating voltage output. The DFT calculation discloses the proton-coupled electron transfer process of the {W12O36} shell in thermodynamically and kinetically, explaining the negative shift of the redox potential of {W12O36} shell as the pH increasing from 1 to 4. Interestingly, the redox potential of outer-sphere controlled {CoO4} core is unchanged, which leads to an extension of voltage window from 1.31 to 1.56 V. Consequently, the cycling measurement of the symmetrical battery proves that low capacity decay rate of 0.009 % and high Coulombic efficiency over 99 % can be retained in 400 cycles at pH 4.