Preparation And The Catalytic Performance For Oxygen Reduction Reaction Of Copper/N-doped Graphene Catalysts

The oxygen reduction reaction(ORR)kinetics of the fuel cells cathode is slow,requiring the use of platinum catalyst to accelerate the reaction rate.However,the shortcomings such as scarcity of platinum reserves and high prices have seriously hampered the commercial application of fuel cells.Therefore,the development of nonprecious metal catalysts with abundant reserves,low cost,high activity and high stability is imminent.Recently,graphene-based materials have been regarded as one of the most promising ORR catalysts because of their good electrical conductivity and rich doping properties.In this paper,a highly active and stable nitrogen-doped graphene and copper nitrogen co-doped graphene ORR catalyst were designed and prepared based on graphene,and the active sites of the catalysts were explored.Firstly,natural graphite was used as raw material,and Hummers method and improved Hummers method were used to prepare graphene oxide(GO).The results showed that the GO prepared by the improved Hummers method had the advantages of simple method and high oxidation degree.Further,the mixture of GO and urea was treated by high temperature heat treatment,and nitrogen-doped graphene(N-G)catalysts were successfully prepared.Studies have shown that N-G prepared at a temperature of 1000°C has a good ORR catalytic performance when the ratio of GO to urea is 1:15.The ORR half-wave potential of the catalyst in the 0.1 mol L-1 KOH electrolyte was 0.862 V(vs.RHE),29 m V higher than the commercial Pt/C catalyst(0.833 V).The onset potential and the limiting diffusion current density were 0.982 V,5.46 m A cm-2,respectively.Tafel slope is 70.62 m V decade-1,indicating that the ORR performance of N-G catalyst is superior to that of commercial Pt/C.The ORR electron transfer number is close to 4,and has the same 4 electrons as Pt/C transfer process.It also has long-term stability and methanol resistance.Through the study of the active sites of N-G catalyst,the results show that the heat treatment temperature can regulate the contents of pyridine nitrogen,pyrrolic nitrogen,and graphite nitrogen in the N-G catalyst.When the heat treatment temperature increases,the content of graphite nitrogen in N-G catalyst increases,while the total nitrogen content,pyridine nitrogen,and pyrrole nitrogen content decrease,but the catalytic activity of ORR increases,indicating that graphite nitrogen is the active component of ORR.The total nitrogen content is not a critical factor in determining ORR activity.By DFT calculation and analysis,it was further confirmed that the graphite nitrogen is the ORR active component,oxygen reduction intermediate products tend to adsorb on the adjacent carbon atoms of the graphite nitrogen nitrogen atom,and the active site is not the nitrogen atom of the graphite nitrogen,while the carbon atom adjacent to a nitrogen atom.Trace amounts of copper were added to the preparation of N-G catalyst,and the copper-nitrogen co-doped graphene(Cu-N-G)was prepared.When the ratio of GO,urea and copper acetate mixture is 1:20:0.075 and the heat treatment temperature is900°C,the prepared Cu-N-G catalyst has excellent ORR performance with an ORR half-wave potential of 0.872 V(vs.RHE).It is 38 m V higher than commercial Pt/C catalyst(0.834 V);The onset potential and the limiting diffusion current density were1.001 V,5.63 m A cm-2,respectively.Tafel slope is 62.42 m V decade-1.Hydrogen peroxide yield is below 5%.It shows that the ORR performance of the Cu-N-G catalyst is better than that of the N-G catalyst;The ORR reaction is a 4-electron process.The Cu-N-G catalyst also has better stability and methanol resistance than the Pt/C catalyst.Through TEM,XPS,XRD characterization and DFT calculation,the active sites of Cu-N-G catalyst were experimentally demonstrated and theoretically analyzed,and the reason why the ORR performance of Cu-N-G catalysts was excellent was confirmed.The main reasons are as follows:(1)Under high temperature conditions,Cu is embedded in the graphene lattice to form highly-dispersed Cu-Nx active sites.This active site is the most critical factor for the improvement of ORR activity.(2)Copper acts as a catalyst during the pyrolysis process,promoting the formation of N-C active sites with high catalytic activity;(3)Copper particles supported on graphene have a certain role in promoting ORR activity.The three active sites of Cu-NX embedded in the graphene lattice,graphite nitrogen,and copper nanoparticles supported on nitrogen-doped graphene contribute to the improvement of ORR activity in the order of Cu-NX > graphite nitrogen > copper nanoparticles.