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

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journal contribution
posted on 09.12.2019, 17:54 by Subhabrata Mukhopadhyay, Olivia Basu, Aranya Kar, Samar K. Das
An efficient and robust heterogeneous electrocatalyst, FSWZ-8 ((Fe-(salen)­(OH) + H4[SiW12O40]·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 SiW12 (i.e., H4[SiW12O40]) inside the cavity of zeolitic imidazolate framework-8 (ZIF-8) material by an in situ synthesis. Here ZIF-8 functions as a host, Fe-salen as the active catalyst, and SiW12 helps in the charge transport by lowering the overall electrical resistance of the resulting composite system. High turnover frequency (∼5 s–1) 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 SiW12 results in (i) faster formation of FSWZ-8 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 ZIF-8 as well as on the surface of ZIF-8, facilitate electrical charge conduction in the ZIF-8 matrix and lower the overall charge-transfer resistance.