Advanced framework-modified POM@ZIF-67 nanocomposites as enhanced oxygen evolution reaction electrocatalysts

Abstract

The smart assembly of Co-containing polyoxometalates (POMs) and Co-based zeolitic imidazole frameworks (ZIF-67) is a promising tool to produce active electrocatalysts for the oxygen evolution reaction (OER). The encapsulation of SiW₁₁Co inside ZIF-67 cavities via ‘in situ’ synthesis produced two novel POM@ZIF-67 nanocomposites with low and high POM loadings—SiW₁₁Co@ZIF-67 and SiW₁₁Co[h]@ZIF-67, respectively. Other materials involving the analogue tri-substituted silicotungstate, SiW₉Co₃@ZIF-67 and SiW₉Co₃[h]@ZIF-67, have been prepared for comparison reasons. Regardless of the POM substitution degree, we demonstrated that the bidirectional synergistic POM-ZIF-67 interaction found for the already reported material, SiW₉Co₃@ZIF-67, is applicable to the other first-time prepared nanocomposites. Remarkably, this synergy is radically enhanced in the case of the two high POM loading samples, with these nanostructured materials being very active OER electrocatalysts, exhibiting 150–160 mV decreased overpotentials and much faster kinetics (∼35% decreased Tafel slopes) in relation to pristine ZIF-67. Moreover, they reveal intrinsic OER activities higher than those developed by the expensive commercial references (RuO₂ and IrO₂ powders) especially in the case of SiW₉Co₃[h]@ZIF-67. This important electrocatalytic improvement can be related to the higher POM loadings achieved, as well as to the formation of an open defect-enriched phase wrapping the ‘standard’ ZIF-67 framework in these nanomaterials.