Synthesis And Electrocatalytic Performance Of Heteroatom-Doped Three-Dimensional Porous Graphene

Fuel cells?FCs?,as a device that directly convert chemical energy into electrical energy,have attracted worldwide attentions because of their non-polluting emissions and high energy conversion efficiency.Among all the components of the FCs,people pay more attention to the cathode catalyst because the oxygen reduction reaction?ORR?occurring at the cathode is the most important electrochemical reaction in FCs,which profoundly affects the work efficiency of FCs.Currently,Pt and Pt-based materials are generally considered to be an efficient ORR catalyst,but these materials'high cost,limited reserves,poor resistance to methanol and poor stability make it impossible to meet large-scale commercial requirements.Therefore,it is needed to develop a low-cost and novel efficient Pt-free catalyst to replace precious ones.Recent studies have shown that introducing heteroatoms?N,F,P,etc.?into carbon materials can destroy atomic charge density and spin density,which can result in adjusting the electronic alignment and improving the ORR performance of the carbon materials.Therefore,the ORR performance for different heteroatoms doped three-dimensional graphene are investigated in this paper,and the specific research process and conclusions are as follows:?1?Graphene oxide?GO?,prepared by the modified Hummers method,is used as a precursor of graphene,urea is used as a nitrogen source,ascorbic acid is used as a reducing agent and a crosslinking agent.The mixed reduced graphene oxide hydrogel can be obtained by thermal reduction process,and the N-doped three-dimensional reduced oxide graphene?N-3DrGO?can be obtained after dialysis,freeze-drying and pyrolysis.XPS test demonstrates that N atoms can be successfully incorporated into the carbon skeleton,and SEM images show that the original GO is a large nanosheet without pores,while samples exhibit a typical three-dimensional porous structure after N-doping.When the mass ratio of GO to urea is 1:4,the specific surface area of the obtained sample reaches 162.07 m²g^-1.Electrochemical tests showed that all the N-3DrGO samples have a good stability and moderate ORR performance,and N4-3DrGO has the best ORR activity in all as-prepared materials,which may be due to its large specific surface area and high content of active N.However,results demonstrate that the ORR process of N4-3DrGO is mainly existed in 2 electron transfer,so ORR performance of N4-3DrGO is much lower than that of Pt/C.?2?In order to improve the ORR performance of the catalyst,the second heteroatom F is introduced and N,F-co doped three-dimensional reduced graphene oxide?NF-3DrGO?are obtained.Effect of different mass ratios of urea to F sources on the ORR activity has been investigated.Results show that both N and P can be successfully incorporated into the carbon framework,and the obtained products are porous structure.Electrochemical test illustrates that the ORR performance for NF-3DrGO is significantly stronger than that of N-3DrGO.The ORR activity for NF-3DrGO can be improved by simply adjusting the mass ratio of urea to F source.When the mass ratio of urea to F sourc is 1:5?NF5-3DrGO?,the ORR activity of the obtained material is optimal,and the average transfer electron number of NF5-3DrGO is 3.85,which indicates that the ORR process is mainly existed in 4 electrons,and close to that of commercial Pt/C.However,it also can be observed from the LSV curves that the ORR performance for NF-3DrGO is limited only by adjusting the mass ratio of urea and F source.?3?In order to further improve the electrochemical activity of materials,a new heteroatom P,by using green and low-cost hydrogen phosphate as the precursor,is introduced and N,P-co doped three-dimensional reduced graphene oxide?NP-3DrGO?has been prepared.Effect of pyrolysis temperature on ORR performance for NP-3DrGO has been investigated.Results show that the material is a typical three-dimensional porous structure,and both N and P are successfully incorporated.Furthermore,Pyrolysis temperature has a significant influence on ORR activity for NP-3DrGO,and the higher pyrolysis temperature may contribute to the better ORR performance.When the pyrolysis temperature is 1100°C,in our experiments,the ORR performance for NP-3DrGO is the best.The calculated average number of transferred electrons for NP-3DrGO-1100 is 4,which is equal to that of Pt/C.Futher study illustrate that the methanol resistance and stability of NP-3DrGO-1100 is better than that of Pt/C,which indicates that the NP-3DrGO-1100 can be acted as a promising ORR catalyst in FCs.