Catalyst support in oxygen electrocatalysis: A case study with CoFe alloy electrocatalyst

Abstract

Synthesis of a highly efficient oxygen electrocatalyst is of significant interest for the development of energy storage devices. The electronic and surface structure, shape, size, and catalyst support largely control the electrocatalytic activity. We demonstrate the synthesis of bifunctional CoFe alloy electrocatalysts encased in nitrogen-doped graphitic carbon (N-C-CoFe) and supported on nitrogen-doped reduced graphene oxide (N-rGO-CoFe) for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) and the influence of catalyst support on the electrocatalytic performance. N-C-CoFe and N-rGO-CoFe were synthesized from a single-source precursor, potassium cobalt hexacyanoferrate multimetal complex, by thermal annealing. Both catalysts efficiently catalyze ORR and OER at low overpotential. The benchmark current density of 10 mA/cm² for OER is obtained at very low overpotentials of 0.22 and 0.29 V with N-rGO-CoFe and N-C-CoFe, respectively. The N-C-CoFe catalyst is highly durable toward OER and ORR, whereas N-rGO-CoFe has limited durability. N-C-CoFe could retain >98% of its initial ORR activity even after repeated 2000 cycles. The chemical nature of nitrogen and degree of graphitization of the catalyst support largely control the overall performance of the catalyst. A highly graphitized carbon (sp² C–C) support containing a large amount of graphitic nitrogen highly favors ORR. The effective encapsulation of the active catalyst into nitrogen-doped graphitic carbon and the presence of nitrogen-doped graphitic carbon nanostructures ensure high durability of the catalyst in a harsh environment. The graphitized carbon with a large amount of graphitic nitrogen is an ideal catalyst support for bifunctional CoFe alloy catalysts.