Study On Non-noble Metal Catalysts With Hierarchical Porous Structure For Oxygen Reduction Reaction

Fuel cells?FCs?are considered as one of the most promising energy technologies because of their high energy density and zero emission of pollutants.However,their high cost has seriously hampered their large-scale commercialization by using scarce and expensive platinum-based catalysts,especially for the sluggish oxygen reduction reaction.Therefore,it is important to develop non-precious metal catalysts to substitute platinum-based materials as ORR catalysts.In recent years,many materials have been investigated as non-precious metal catalysts.Among them,carbon-based materials have been regarded as the most promising catalysts for ORR.However,the low active sites density of carbon-based materials leads to their ORR activity far away from that of platinum-based catalysts.Therefore,increasing the active sites density and meanwhile exposing these active sites effectively become the main research directions for carbon-based catalysts.To construct a 3D hierarchical porous structure is believed to be beneficial to achieve the creative and expose more accessible active sites.Based on this,we have developed two effective methods to develop the 3D hierarchical porous structure for carbon-based ORR catalysts.?1?In this paper,a silica colloidal template method was developed to prepare a Co/NCNTs catalyst compounded with cobalt nanoparticles which contained an internal structure that electron path was connected by carbon tube and gas-liquid mass transfer channels was connected by hierarchical porous channel.The approach involves infusion of ZIF-8 and ZIF-67 precursors into the crevices formed by stacks of silica spheres,followed by pyrolysis at 900°C under a N₂ atmosphere,silica leaching,and secondary pyrolysis to remove unstable amorphous carbon.It is found that the leaching of silica yields interconnected macropores as main channels of mass transport while the vapourable Zn element induces the formation of meso-/micro-pores in N-doped carbon.After tuning the Co/Zn molar ratio,the optimal 3D carbon nanotube/nanoparticle network exhibits comparable ORR activity in alkaline medium which showed that the half-wave potential of the catalyst was 0.841 V vs.RHE.?2?Considering the instability of transition metals in oxygen reduction environments,a porous carbon catalyst with 3D nanosheets network was synthesized by using zinc oxide?ZnO?and sodium chloride?NaCl?as the mesoporous and macroporous templates respectively,using polyacrylonitrile?PAN?as nitrogen sources and carbon sources.During the process of the high temperature carbonization,the ZnO reacted with C to form CO₂ and Zn,the microporous was formed due to the vapourable CO₂ while the mesoporous was formed by leach the residue of ZnO and Zn in N-doped carbon.The NaCl is mixed well with the precursor by the ball milling,and after carbonization process,NaCl is removed by the subsequent water washing to construct macropores.Moreover,NaCl not only promotes the formation of macroporous structures of the catalyst,but also makes the catalyst heated more uniform,reduces the structure collapse,fixs and protects the catalyst's morphology during the carbonization process.The test results in acidic medium showed that the half-wave potential of the catalyst which was prepared by adjust the mass ratio of PAN/ZnO/NaCl was 0.755 V vs.RHE,this performance is in the world advanced level in the field of metal-free nitrogen-carbon catalysts.