Hierarchical trimetallic Co-Ni-Fe oxides derived from core-shell structured metal-organic frameworks for highly efficient oxygen evolution reaction

Metal-organic frameworks (MOFs) have recently emerged as promising precursors to construct efficient non-noble metal electrocatalyst for oxygen evolution reaction (OER). Herein, a Co-Ni-Fe spinel oxide-carbonitrides hybrids (CoNiFeOx-NC) electrocatalyst with hierarchical structure was synthesized from Fe-MIL-101-NH2 through a unique ion-exchange based strategy. The ion exchange of Fe-MIL-101-NH2 with both Ni and Co ions induced a hierarchically structured 2-D ternary metal MOF shell layer encapsulated 3-D octahedral MOF crystals as a core. This prevents the collapse of MOF frameworks during the air calcination process and affords highly porous structure and large surface area. Additionally, the unique combination of Co-Ni-Fe in spinel oxides derived from calcination of the hierarchically structured core-shell MOF provides a favorable electronic environment for the adsorption of OER intermediates, which was further verified by the XPS characterizations and DFT calculations. DFT study revealed the Ni-Co coordinated Oh sites in the MFe2O4 reverse spinel structures as the main active sites, which tuned the binding strength of oxygen species with a catalyst through electron transfer of Fe→Co→Ni, thereby lowered the energy barriers for OER. As a result, the rationally designed CoNiFeOx-NC catalyst manifests superior OER performance with a low overpotential of 265 mV at 50 mA cm−2 and a decent Tafel slope of 64.1 mV dec-1. The ion-exchange based strategy may serve as a versatile platform for rational design and synthesis of multi-metallic MOF derived electrocatalysts.