Fabricating a Functionalized Polyoxometalate with ZIF-8: A Composite Material for Water Oxidation in a Wide pH Range

Designing an efficient oxygen evolution reaction (OER) electrocatalyst is a challenging task because molecular oxygen is essential to sustain life. Herein, we have synthesized and structurally characterized a {Bi­(OH2)2}3+-functionalized Keggin polyoxometalate (POM) compound, K5[Bi­(H2O)2SiW11O39]·13H2O (K51·13H2O). Keeping in mind that the {Bi­(OH2)2}3+ species on the surface of the POM has the potential to act as an active site for electrocatalytic water oxidation, we encapsulated the functionalized Keggin POM anion [Bi­(OH2)2SiW11O39]5– inside the cavities of a well-known zeolitic-type framework material ZIF-8, a zinc imidazole-containing metal–organic framework, because compound K51·13H2O per se is water-soluble and unstable in the oxidation window of water oxidation. The resulting host–guest-type composite material H5[Bi­(H2O)2SiW11O39]@ZIF8 (H51@ZIF8) functions as an efficient electrocatalyst for water oxidation. Detailed electrochemical analyses have established that the title OER catalyst, H51@ZIF8, works in a wide pH window, ranging from alkaline pH 13.0 to neutral pH 7.0 through an acidic pH 4.0. Diverse controlled experiments have been performed to infer that the bismuth–aqua complex is the functional site responding to the OER, with a catalytic turnover frequency of 1.24, 5.90, and 0.93 s–1 of pH 13.0, pH 7.0, and pH 4.0 solutions, respectively, at the respective overpotentials of 375.0 mV [vs the reversible hydrogen electrode (RHE)], 585.21 mV (vs the RHE), and 830.10 mV (vs the RHE).