Preparation Of Multi-doped Graphene And Its Electrocatalytic Performance For Oxygen Reduction Reaction

Fuel cells and zinc-air batteries have received widespread attention owing to their advantages of environmental protection,high energy density and other respective characteristics.There is an important oxygen reduction reaction in both batteries whose reaction rate is directly connected to the overall performance of the battery.As a widely used cathode catalyst at present,Pt has some disadvantages such as low content,high price and easy toxicity.Therefore,many researchers turn their attention to low-carbon metal-free catalyst with low cost and high stability.Carbon based materials have no intrinsic catalytic activity,but the introduction of doped atoms can effectively improve the oxygen reduction activity.The earliest and most in-depth studies were conducted on nitrogen atom doping,followed by the gradual emergence of double or ternary doping.The spin density and charge distribution of the carbon atom will be changed with the introduction of heteroatoms because of differences in atomic radius,electronegativity and other electronic properties between the doped atom and the carbon atom.At this time,doped atoms or carbon atoms become the catalytic centers for adsorbing oxygen atoms,and the synergistic effect of different heteroatoms in multi-doping can further boost the oxygen reduction activity of the catalyst.In this paper,graphene as a carbon-based carrier was used to prepare nitrogen-phosphorus double doping and nitrogen-phosphorus-sulfur tripledoping catalysts through hydrothermal and high-temperature treatment,and the effects of different factors on oxygen reduction activity were discussed.The main research contents are as follows:（1）With graphite phase carbon nitride（g-C₃N₄）as nitrogen source,phytic acid as phosphorus source and graphene as carbon-based carrier,N-Gr with macroporous structure was prepared by secondary heat treatment under the action of hydrogen bond and π-π conjugation between g-C₃N₄ and GO,and then nitrogen-phosphorus doped graphene catalyst was obtained by introducing phosphorus-rich liquid phytic acid which can fully infiltrate the pore structure.Structural characterization and electrochemical performance tests were carried out.The results show that the catalyst doped with N before P can achieve high phosphorus content（2.61 at%）,while retaining a certain amount of nitrogen（0.73 at%）,which presents a large specific surface area(935 m² g^-1),excellent oxygen reduction catalytic activity(Onset potential 1.005 V and limiting current density-5.98 mA cm^-2),stability and methanol resistance,and the performance of zinc-air battery test is better than that of commercial Pt/C.（2）Trithiocyanuric acid was used as the nitrogen-sulfur source,triphenylphosphine as the phosphorus source,and graphene as the carbonbased carrier.The polymerization of trithiocyanuric acid to sulfur-doped C₃N₄ under high temperature treatment and as a template to promote the generation of hierarchical porous structures.At the same time,triphenylphosphine reacts with hydroxyl groups in GO to promote the introduction of phosphorus,and finally nitrogen,phosphorus and sulfur ternary doped graphene catalyst was obtained.The characterization and electrochemical performance test results show that the catalyst successfully realizes the doping of P（2.54 at%）and S（1.08 at%）atoms with large radius,and the doping of N,P and S elements is uniform,which is beneficial to realize the synergistic effect among heteroatoms in the catalyst.The material exhibits the limit current density(5.33 mA cm^-2)close to that of commercial Pt/C(-5.45 mA cm^-2),better catalytic stability and methanol tolerance,and higher power density and discharge stability than Pt/C when applied to zinc-air batteries.Therefore,the cathode catalyst exhibits great application potential.