Tuning Redox Active Polyoxometalates for Efficient Electron‐Coupled Proton‐Buffer‐Mediated Water Splitting

We present strategies to tune the redox properties of polyoxometalate clusters to enhance the electron‐coupled proton‐buffer‐mediated water splitting process, in which the evolution of hydrogen and oxygen can occur in different forms and is separated in time and space. By substituting the heteroatom template in the Keggin‐type polyoxometalate cluster, H(6)ZnW(12)O(40), it is possible to double the number of electrons and protonation in the redox reactions (from two to four). This increase can be achieved with better matching of the energy levels as indicated by the redox potentials, compared to the ones of well‐studied H(3)PW(12)O(40) and H(4)SiW(12)O(40). This means that H(6)ZnW(12)O(40) can act as a high‐performance redox mediator in an electrolytic cell for the on‐demand generation of hydrogen with a high decoupling efficiency of 95.5 % and an electrochemical energy efficiency of 83.3 %. Furthermore, the H(6)ZnW(12)O(40) cluster also exhibits an excellent cycling behaviour and redox reversibility with almost 100 % H(2)‐mediated capacity retention during 200 cycles and a high coulombic efficiency >92 % each cycle at 30 mA cm(−2).