Oxygen Evolution Reaction Electrocatalytic Improvement in POM@ZIF Nanocomposites: A Bidirectional Synergistic Effect

Abstract

A bidirectional synergy involving Co-based polyoxometalates (POMs) incorporated in the Co-containing zeolitic imidazolate framework, ZIF-67, that promoted a notable improvement in the oxygen evolution reaction (OER) performance was unveiled for the first time. Two POM@ZIF nanocomposite materials—SiW9Co3@ZIF-8 and SiW9Co3@ZIF-67—were successfully prepared via a room temperature “in situ” approach, by the immobilization of POM anions [SiW9Co3(H2O)3O37]10– (SiW9Co3) in the cages of two isostructural ZIFs: ZIF-8 and ZIF-67, formed with Zn2+ and Co2+, respectively. Despite both SiW9Co3@ZIF-8 and SiW9Co3@ZIF-67 revealing similar and low occupancy degrees (ca. 6 POM units per 100 ZIF-cages), an intense synergy has been detected in the ZIF-67 based nanocomposite, while no synergistic interaction was found for POMs and ZIF-8. Due to this synergistic effect, the OER activity of SiW9Co3@ZIF-67 is significantly enhanced in comparison with that of ZIF-67: ∼110 mV decreased overpotential and double current density. Additionally, this nanocomposite exhibits excellent stability in the alkaline electrolyte. Insights into the nature and magnitude of the SiW9Co3–ZIF-67 synergy have shown up its interesting ZIF ↔ POM bidirectional nature that involves activation of the Co-based active sites in POM clusters as a consequence of a ZIF-67 → SiW9Co3 electron transfer, along with POM-induced generation of more active unsaturated Co-nodes in the ZIF-67 frameworks. These findings represent a promising “proof of concept” for the development of more efficient POM@MOF-based electrocatalysts in the future.