Preparation And Electrocatalytic Performance Of Co-N-C/graphene Composites

Combined abundant transition metals with non-metallic nitrogen and carbon（TM-N-C,TM=Co,Fe,Ni）forms a composite material that shows excellent catalytic performance in electrocatalysis and it is expected to be a substitute catalyst for noble metals.The d orbital of the outermost layer of transition metal is incompletely filled,so its electrochemical reaction can easily accept/donate electrons.In addition,the introduction of graphene as a substrate can overcome the shortcomings of transition metal compounds such as small surface area and easy aggregation,thereby improving the electrocatalytic activity of oxygen reduction reaction（ORR）,oxygen evolution reaction（OER）and nitrogen reduction reaction（NRR）.The combination of transition metal compound and graphene（especially doped graphene）can improve the electronic structure of transition metal compounds.At the same time,the strong bridging bonds between graphene and transition metal compounds can be built through interface engineering,which can accelerate the charge transfer rate and enhance the electrocatalytic performance.In this paper,the polypyrrole（PPy）microspheres were synthesized by the co-precipitation method.Co-N-C/rGO composite powders were prepared by pyrolyzing the ZIF-67/PPy precursor,and the reduced graphene oxide（rGO）was used as the substrate.X-ray diffraction（XRD）,scanning electron microscopy（SEM）,transmission electron microscopy（TEM）,nitrogen adsorption and desorption test and other characterization methods are used to further study the crystal structure and morphology of the catalyst.The electrochemical performance tests of the Co-N-C/rGO catalyst material were performed by Rotating Disk Electrode（RDE）technique,such as cyclic voltammetry,linear scanning voltammetry,AC impedance,etc.,and the electrocatalytic activity was optimized by changing the content of cobalt and introducing reduced graphene oxide（rGO）as the substrate.The results show that when the Co content is 5%of the Co-N-C/rGO composite,and the reduced graphene oxide（rGO）was used as the substrate,that is,5%Co₄N/Co₂C@rGO sample has the highest ORR and OER catalytic activity.At the same time,the controlled variable method was used to explore the best NRR test conditions of Co-N-C/rGO composite powder.Studies have shown that the introduction of graphene can increase the specific surface area of the catalyst,which is conducive to the adsorption of N₂ and the exposure of the active site,and can also increase the conductivity of the catalyst.The NRR electrochemical tests indicate that the 5%Co₄N/Co₂C@rGO sample can obtain the highest ammonia yield and faradaic efficiency at-0.1V（vs.RHE）for NRR test.Data analysis results show that Co₄N（111）crystal plane adopts Mars-van Krevelen（M-v K）reaction mechanism,and Co₂C（101）crystal plane is more conducive to NRR.