Preparation Of Nitrogen-doped Carbon-based Non-noble Metal Catalysts For Oxygen Reduction Reaction

The catalytic reduction of oxygen is an important reaction of advanced energy technologies.Fuel cells and metal-air cells typically utilize the reduction reaction of O₂ at cathode and the oxidation reaction of the fuel or metal at anode to convert chemical energy into electrical energy.Oxygen reduction reaction（ORR）requires a catalyst with high selectivity and energy conversion efficiency to facilitate the reaction with rapid speed.A stable,cheap and efficient ORR catalyst has yet to be found to replace the traditional Pt-based catalysts.Nitrogen-doped carbon（NC）has attracted much attention in the ORR field since it was first reported in 2009.As doping of nitrogen atoms at carbon rings causes the redistribution of the electronic structure and the O₂ adsorption pattern changes on NC surface,the N-doped carbon can achieve high ORR catalytic activity.In addition,NC nanomaterial can be an ideal carrier for metal or metal oxide as catalyst with synergistic catalytic effect.In recent studies,the simultaneous heat treatment of transition non-precious metals,nitrogen and carbon can obtain metal-nitrogen/carbon（M-NC）ORR catalysts with promising catalytic performance.Through the rational design of M-NC nanostructure,the catalytic activity can be improved effectively,and thus its electrocatalytic ORR performance could be optimized accordingly.In the study,we utilize N-doped graphene as supporting medium for metal oxides and controlthe Fe distribution in NC to enhance the ORR activity at cathode effectively.The results are as follows:（1）Nitrogen-doped graphene-loaded cobalt oxide nanospheres（Co₃O₄/N-r GO）are synthesized via three-step reactions.Co₃O₄/N-r GO exhibites excellent electrochemical activity for ORR catalysis.In electrochemical tests,it shows that the ORR activity of Co₃O₄/r GO composite is significantly improved,due to synergistic effect.Followed by nitrogen doping,the ORR catalytic activity of the Co₃O₄/N-r GO is significantly enhanced,with 140 m V improvement of half-wave potential and a favourable 4-electron transfer.The full zinc-air battery assembled by Co₃O₄/N-r GO as cathode has the maximum power density of 80 m W cm^-2 and the specific capacity of 805 m Ah g^-1,which are much higher than Co₃O₄/r GO with the power density of 52 m W cm^-2 and the specific capacity of 613m Ah g^-1.（2）ZIF-8 crystals with two different scale are synthesized using iron p-toluenesulfonate as iron source and cetyl trimethyl ammonium bromide（CTAB）cationic surfactant as the control agent.The ZIF-8 without CTAB addition is about 1μm with dodecahedron crytalline structure（ZIF-8@Fe-1000）.With CTAB addition,the grain size of ZIF-8 is reduced into 50 nm with hexahedron structure（ZIF-8@Fe-50）.From TEM results,it reveals that the iron nanoparticles derived from ZIF-8@Fe-50are encapsulated by graphene after pyrolysis.Electrochemical tests show that ZIF@Fe-50-derived Fe-NC-2 exhibits better ORR catalytic activity,which has a half-wave potential of 0.81 V,as compared with ZIF-8@Fe-1000-derived Fe-NC-1 and Fe-free NC.The catalytic performance of Fe-NC-2 is comparable to commercialplatinum/carbon.Due to the carbon-encapsulated structure,both Fe-NC-2 and Fe-NC-1 show excellent stability in alkaline electrolyte.In addition,Co Ni-NC catalysts are prepared via the similar synthesis methodology.The catalytic activity of Co Ni-NC is comparable to that of Pt/C,which is much higher than that of catalysts with monometal-loading under alkaline condition.After 10,000 CV cycles,the half-wave potential only negatively shifts 10 m V,it means the Co Ni-NC is a potential alternative to replace Pt/C.（3）As a chelating agent is added during the synthesis procedure of ZIF-8@Fe,the highly dispersed Fe-atom clusters are formed.And a large amount of atomically dispersed Fe is present around Fe-atom clusters.Electrochemical performance tests show that Fe-C/NC exhibits the highest ORR activity with a half-wave potential of up to 0.866 V,even higher than Pt/C 0.815 V.The kinetic current density of Fe-C/NC at 0.85 V is three times higher than that of the Pt/C catalyst.Moreover,the electron transfer number of Fe-C/NC is higher than 3.96 at the potential range from 0.2 to 0.8 V.The half-wave potential only negatively shifts 4 m V after 5000 cycles.It indicates the durability of Fe-C/NC much better than the commercial Pt/C ones.As the cathode material in Zn-Air battery,the Fe-C/NC behaves the maximum power density of 140 m W cm^-2 and the specific capacity of 791m Ah g^-1,which are much better than Pt/C fabricated Zn-Air battery.