Sustainable Synthesis of Co@NC Core Shell Nanostructures from Metal Organic Frameworks via Mechanochemical Coordination Self-Assembly: An Efficient Electrocatalyst for Oxygen Reduction Reaction

Herein, a new type of cobalt encapsulated nitrogen-doped carbon (Co@NC) nanostructure employing ZnxCo1-x(C3H4N2) metal-organic framework (MOF) as precursor is developed, by a simple, ecofriendly, solvent-free approach that utilizes a mechanochemical coordination self-assembly strategy. Possible evolution of ZnxCo1-x(C3H4N2) MOF structures and their conversion to Co@NC nanostructures is established from an X-ray diffraction technique and transmission electron microscopy analysis, which reveal that MOF-derived Co@NC core-shell nanostructures are well ordered and highly crystalline in nature. Co@NC-MOF core-shell nanostructures show excellent catalytic activity for the oxygen reduction reaction (ORR), with onset potential of 0.97 V and half-wave potential of 0.88 V versus relative hydrogen electrode in alkaline electrolyte, and excellent durability with zero degradation after 5000 potential cycles; whereas under similar experimental conditions, the commonly utilized Pt/C electrocatalyst degrades. The Co@NC-MOF electrocatalyst also shows excellent tolerance to methanol, unlike the Pt/C electrocatalyst. X-ray photoelectron spectroscopy (XPS) analysis shows the presence of ORR active pyridinic-N and graphitic-N species, along with CoNxCy and CoNx ORR active (M-N-C) sites. Enhanced electron transfer kinetics from nitrogen-doped carbon shell to core Co nanoparticles, the existence of M-N-C active sites, and protective NC shells are responsible for high ORR activity and durability of the Co@NC-MOF electrocatalyst.