Design Of Carbon-based Electrocatalysts With Large Specific Surface Area And Their Electrocatalytic Performance

With the increasing exhaustion of non-renewable energy sources,energy crisis can't cope with the rapid economic development.Sustainable and efficient energy sources act the important role for the social development in the future.Metal-air batteries,using metals(Li,Mg,Al,Zn,etc.)as the negative electrode and air(O2)as the positive electrode are attractive owing to their environmental friendly,non-toxic,light characters and high specific energy density.However,oxygen reduction reduction is supressed due to it's sluggish reaction kinectics during the energy convension system.As known,Oxygen reduction reaction(ORR)can be enhanced under the role of desirable electrocatalysts,which are very important for developing the clean and renewable energy devices.Platinum or its alloys,which have been demonstrated to be the best cathode catalysts for ORR,are very expensive and their limited supply pose a serious problem in commercialization of metal-air battery technology.Thus,the development of cost-effective and durable catalysts for ORR have become the most attractive topics in the metal-air battery field.Among these,many carbon materials appear to be very promising,owning to their large specific surface area,good conductivity and high stability.In this paper,graphene and ordered mesoporous carbon(CMK-3)are used as substrates to prepare N,S co-doped carbon-based catalysts and graphene-supported alloy nanoparticle catalysts.Compared with the catalytic performance of the obtained catalysts,the excellent catalysts were obtained by adjusting the components and synthetic conditions,and the catalytic mechanism was analyzed.The results show that pyridine N plays an important role in the four-electron transfer path of the oxygen reduction reaction in the nitrogen-doped functional group.Also,pyridine N and thiophene S are the main catalytic active units in the nitrogen-sulfur co-doped systems.Among them,the heteroatom-doped CMK-3 samples(N-C'MK-3,NS-CMK-3),12.5-NS-CMK-3 showed the excellent oxygen reduction catalytic activity(?onset=0.92 V vs.RHE,n=3.91,H2O2?4%,k=-60.4 mVdec-1),and it's catalytic performance is significantly better than other that of N-CMK-3,suggesting that the reasonable composition design and the synergistic effect between N and S atoms can be significantly improve the oxygen reduction catalytic activity of the samples.In addition,among the heteroatom-doped graphene samples(N-GO,HN-GO and NS-GO),HN-GO showd the high nitrogen content and pyridine N using urea as a doping source.HN-GO showed the better catalytic performance than those of N-GO or NS-GO,such as 200-HN-GO-180(Ntotal=14%,?onset=0.92 V vs.RHE,n=3.86,H2O2?8%)Metal nanoparticles obtained from the ultrasonic irradiation demonstrate a much smaller size distribution and more active centers,which both contribute to the enhanced catalytic activity.Graphene-supported metal nanoparticles have an average particle size of 3 nm to 5 nm and have high dispersibility.In the Co-Fe-Pd alloy system,Co5Fe2Pd1/rGO showed better ORR catalytic activity than those of Co5/rGO,Fe2/rGO,Co5Fe2/rGO,Fe2Pd1/rGO and Co5Pdi/rGO,indicating that the ternary alloy particles have larger lattice compression than that of the binary and unit nanoparticles,leading to the more active sites.A low cost of Co-Sn-Pd alloy nanoparticle can be obtained by reducing the Pd content with an alternative Sn.The ORR catalytic performance of Co-Sn-Pd alloy can be enhanced gradually with the increasing Sn content.Overall,Co-Sn-Pd/rGO had the best ORR catalytic activity(?onset=1.05 V vs.RHE,n=3.93,H2O2?%,k=-66.3 mVdec-1)when the molar ratio of Sn and Pd equal to 1(Co5Sn0.5Pd0.5/rGO),indicating that the alloy particles had the largest lattice compression and thus produced the most active units.At the same time,the nanoparticles also exhibited an excellent oxygen extraction reaction(OER)performance,suggesting that Co5Sn0.5Pd0.5/rGO is a promissing candidate using as a dual-functional catalyst.