Preparation And Electrochemical Performance Study Of Co/n-doped Carbon Catalysts For Oxygen Reduction Reaction

The large-scale use of fossil energy leads to rapid economic growth,while it also brings about problems such as global warming,unbalanced energy shortages,and environmental damage.Fuel cells and Zn-air batteries have attracted extensive attention due to their high energy density,favourable energy conversion efficiency,environment friendly and high reliability.To solve the problems of the insufficient activity and poor stability of non-precious metal oxygen reduction catalysts in practical application environments,this research synthesis several low-cost,high-efficiency and stable ORR catalysts by improving the dispersion of metal particles and construct species structures of the catalysts.Co/N-CNs catalyst with uniform two-dimensional nanosheet structure was synthesized by a simple Na Cl-assisted method with glucose,urea,and cobalt metal salts as carbon,nitrogen,and cobalt sources.Compared with Co/N-C catalyst prepared without the use of Na Cl template,the Co/N-CNs catalyst reveals the properties with better dispersion of Co nanoparticles,larger specific surface area and more defect structure.Besides,Co/N-CNs catalyst exhibits higher ORR catalytic activity than Co/N-C.Specifically,the ORR half-wave potential of the Co/N-CNs catalyst is increased by 40 m V,and the limiting current density is increased by 0.8 m A cm^-2in 0.1 mol L^-1 KOH electrolyte.Calcination temperature has significant effects on the morphology and structure of the catalyst.The Co/N-CNs-800 catalyst calcined at 800 ^oC exhibits the best ORR performance,in which Co/N-CNs-800 catalyst shows the half-wave potential of0.83 V and 7 m V decay after 5000 cycles.Co@hNCTs catalyst with one-dimensional hierarchical micro-nanostructure was synthesized by using the template of polypyrrole nanotubes,in which ZIF-67 was in-situ growth on the surface of PPy.PPy template reduces the particle size of ZIF-67 and inhibits its agglomeration during calcination process.Besides,PPy template improve the catalytic activity of catalyst by enriching the pore structure,promoting mass and charge transfer,and increasing the concentration of the active sites.The ORR half-wave potential of Co@h NCTs catalyst is increased by 30 m V compared with the Co NPs catalyst prepared without PPy template.Calcination temperature affects the cobalt nanoparticle size,pore structure and graphitic degree of the catalyst,and dosage of cobalt source affects the cobalt loading.The best calcination temperature and amount of cobalt is verified to be 800 ^oC and 2 mmol.Co@h NCTs-800 catalyst shows the half-wave potential of 0.87 V and 7 m V decay after 5000 cycles.Besides,the Zn-air battery assembled with Co@h NCTs-800 catalyst shows the maximum discharge power density of 149 m W cm^-2,which is better than that of 120 m W cm^-2 assembled with Pt/C and Ru O₂ catalysts.g-SA-Mn catalyst with atomically dispersed Mn was synthesized by a vapor deposition method using ZIF-8 as the support.Compared with the catalyst prepared by ordinary liquid phase method,g-SA-Mn shows larger manganese loading and more surface-loaded metal.Atomically dispersed Mn-N₄ sites is determined to be the mainly active sites in the catalyst.The g-SA-Mn-900catalyst calcined at 900 ^oC exhibits the best ORR performance,in which g-SA-Mn-900 catalyst shows better half-wave potential of 0.90 V than commercial Pt/C.Besides,the Zn-air battery assembled with g-SA-Mn-900 catalyst shows the maximum discharge power density of 147 m W cm^-2,which is better than another assembled with Pt/C catalysts.On this basis,the co-doped Co Mn-NC catalyst was prepared by the combination methods of chemical doping and liquid-phase adsorption.The synergistic effect of bimetallic sites enhanced the ORR catalytic performance and durability of the catalyst.The as-prepared Co（4）Mn-NC catalyst exhibit the half-wave potential of 0.90 V and 5 m V decay after 5000 cycles,which is better than the single-metal-doped Co-NC and Mn-NC catalyst.