Efficient Electrocatalytic Water Oxidation by Fe(salen)-MOF Composite: Effect of Modified Microenvironment.

An efficient and robust heterogeneous electrocatalyst, ((Fe-(salen)(OH) + H[SiWO]·HCl)@ZIF-8) for oxygen evolution reaction (OER) at the neutral pH, was developed by coencapsulation of Fe-salen (i.e., Fe(salen)Cl) and SiW (i.e., H[SiWO]) inside the cavity of zeolitic imidazolate framework-8 () material by an in situ synthesis. Here functions as a host, Fe-salen as the active catalyst, and SiW helps in the charge transport by lowering the overall electrical resistance of the resulting composite system. High turnover frequency (∼5 s) and high Faradaic efficiency (∼94%) make the concerned composite an efficient catalyst toward water oxidation. This is the first report of one of the simplest known metal complexes, Fe-salen, to perfrom electrocatalytic OER as a heterogeneous catalyst in the neutral pH. This work also highlights the benefits of coencapsulation of the Keggin polyoxometalate (POM) along with the active catalyst Fe-salen species. Encapsulation of SiW results in (i) faster formation of composite, (ii) higher loading of Fe-salen, and, most importantly, (iii) lowering of required overpotential for electrochemical OER by more than 150 mV. The Keggin POMs, located as discrete molecular oxides inside the cavity of as well as on the surface of ZIF-8, facilitate electrical charge conduction in the matrix and lower the overall charge-transfer resistance.