Non-precious Metal-based Electrocatalysts Derived From MOFs For Oxygen Reduction Reaction

Because of the significant advantages,such as high energy conversion efficiency,environmental friendliness,quick start-up,and simple design,fuel cells are considered to be one of the ideal clean energy conversion technologies in the future.At present,platinum-based catalysts are widely used to reduce the activation energy of cathode oxygen reduction reaction in fuel cells.However,the scarcity and high cost of platinum severely limit the commercialization of fuel cells.Recently,high-efficiency,inexpensive and non-precious metal electrocatalysts have received extensive attention.Metal-organic frameworks?MOFs?have the advantages of high specific surface area,large pore volume,easy surface functionalization,etc.The porous carbon material synthesized from MOFs precursors is a highly potential electrocatalytic,which can be used as a substitute for oxygen reduction reaction platinum-based catalyst.Based on the MOF of ZIF-8,a zeolite imidazole skeleton structure,two M-N-C materials were prepared as high-performance non-precious metal oxygen reduction catalysts by adding a transition metal source and calcining at high temperature:First,by doping copper into zeolitic imidazolate framework-8?ZIF-8?as a precursor and then calcining at 900?under argon atmosphere,we have successfully prepared Cu-N-C.When the molar ratio of Cu to Zn is 3:1,the obtained 3-Cu-N-C shows the highest activity after electrocatalytic oxygen reduction reaction test:the initial potential of 3-Cu-N-C is 0.917.V?vs.RHE?,the half-wave potential at 1600rpm is 0.843 V?vs.RHE?,the limiting current density is 5.52 mV cm^-2,and the electron transfer number is 3.98,which indicates a higher catalytic activity than commercial Pt/C,its methanol tolerance is also more effective than commercial Pt/C.In addition,after 20,000 s chronoamperometry,the stability is 29.3%higher than commercial Pt/C.Its high-efficiency oxygen reduction reaction activity is mainly due to the large specific surface area and the pore volume of the sample as well as the high content of reactive nitrogen species.Secondly,after the synthesis of ZIF-8,the nitrogen-carbon carrier is prepared by heat treatment at 900°C under argon atmosphere,followed by absorbing SnO₂particles,and finally the Sn-N-C material was obtained by heat treatment under ammonia atmosphere at 900°C.When the mass of SnO₂ is 10%of the mass of nitrogen carbon,the synthesized 10%Sn-N-C shows the best electrocatalytic oxygen reduction reaction activity:the initial potential of 10%Sn-NC catalyst reaches0.923 V?vs.RHE?,the half-wave potential at 1600 rpm is 0.836 V?vs.RHE?,the limiting current density is 5.30 mV cm^-2,and the electron transfer number is 3.95,which shows slightly higher catalytic activity than commercial Pt/C,besides its methanol tolerance is significantly higher than commercial Pt/C;after 20,000s chronoamperometry,stability is 26.9%better than commercial Pt/C.The high-efficiency oxygen reduction reaction activity is mainly due to the large specific surface area,large mesoporosity of the sample and the high density of active sites.