Sequential Synthesis of 3 d–3 d, 3 d–4 d, and 3 d–5 d Hybrid Polyoxometalates and Application to the Electrocatalytic Oxygen Reduction Reactions

The Co-7(AlePy)(2) polyoxometalate, which encloses a {(PW9)(2)Co-7(II)} core covalently bound to two free aminopyridine groups through bisphosphonate ligands (AlePy), has been isolated. It can be used as a precursor, allowing the synthesis of heterometallic hybrid compounds, as illustrated by the characterization of cobalt/zinc (Co-7(AlePyZn)(2)), cobalt/palladium (Co-7(AlePyPd)(2)), and cobalt/platinum (Co-7(AlePyPt)(2)) species. A composite based on the water-insoluble precious metal-free Co-7(AlePyZn)(2) compound and the low-cost carbon material Vulcan XC-72 has been selected as a cathode material (Co7Zn/C) for oxygen reduction reaction studies. The electrocatalytic performances of the Co7Zn/C hybrids were assessed at neutral and basic pH, showing that Co7Zn/C exhibits high selectivity for the four-electron reduction of O-2. Moreover, its durability is superior to that of a commercial Pt/C catalyst with 20% loading. Also, comparative studies performed in the presence of methanol have indicated that Co7Zn/C has a much better tolerance to the crossover effect than Pt/C. Altogether, these results indicate for the first time that, even in neutral media, polyoxometalate/carbon composites can represent low-cost oxygen reduction catalysts that can function stably, for a long time, and with high performance.