Oxygen Reduction Electrocatalysts Based On Metal Oxide-modified And Heteroatom-doped Carbon Supports

The cathodic oxygen reduction reaction（ORR）,the crucial step in fuel cell,is kinetically sluggish than anodic hydrogen oxidation on electrodes and requires far more platinum（Pt）catalyst.Nowadays,Pt and Pt-based catalysts are regarded as the most active electrocatalysts for ORR,but they are low in reserves and overpriced as well as offering poor stability.Thus,it is of great importance to develop newly low cost and highly active ORR electrocatalysts for improving the performance and popularity of fuel cell.Therefore,the research content of this paper is divided into the following three parts:At first,this paper synthesized a compound support material C-Ti_0.7Nb_0.3O₂,by generating physically and chemically stable Nb-doped Ti O₂ on EC300J carbon black in situ,which was used to load Pt nanoparticles（NPs）to get 40%Pt/C-Ti_0.7Nb_0.3O₂catalysts in order to improve the ORR activity and stability of Pt-based catalysts.By doping Nb,the conductivity of pure Ti O₂ was to be increased.Loading the Nb-doped Ti O₂ onto carbon black will help to solve the problem of the low conductivity and insufficient specific surface area of the pure metal oxide.More importantly,the strong metal-substrate interaction（SMSI）between Pt NPs and the substrate was supposed to enhance the dispersion of Pt NPs,in turn to improve the activity and stability of catalysts.The results showed that when the molar ratio of Ti/Nb was 7/3 and the mass ratio of carbon/oxide was 2,the obtained catalyst displayed the best stability and activity,better than commercial Pt/C catalyst with the same load.Secondly,to develop highly effective non-precious metal ORR catalysts（NPMCs）,this paper presented facile and green template synthesis of Fe/N/S co-doped hierarchically porous carbon（Fe NS/HPC）through an in situ generated dual-template method to catalyze ORR.Heteroatoms doping was contributed to produce more catalytic sites in carbon materials while the hierarchically porous structure with abundant micropores,mesopores and macropores can help to increase the active site density and speed up the mass transfer of NPMCs.In this methode,sucrose,thiourea and ferric chloride were pyrolyzed in the existence of Na Cl submicro-crystallites with Na Cl crystallites formed during the precursor lyophilization process as the primary template to generate～500 nm macropores with ultrathin graphene-like carbon-layer walls,and Fe₃O₄ nanoparticles formed during the high-temperature carbonization process as the secondary template to produce mesopores on the walls of macropores.As well as the coexistence of graphitic-N,pyridinic-N,and thiophene-S which are all beneficial to ORR,the resulted Fe NS/HPC possesses a highly graphitized and interconnected hierarchical porous structure,giving a specific surface area as high as938 m² g^-1.As a result,it exhibits excellent four-electron ORR performance in both alkaline and acid electrolytes.The in situ generation and facile solution removal make the present template method a promising way for scale-up preparation of active porous carbon materials for various applications.Finally,this paper synthesised a Pt single atom ORR catalysts with ultra high Pt loading of 5%with the hierarchically porous carbon（Fe NS/HPC）mentioned above as the support material.By adjusting the reaction temperature,the Pt existed as 1 nm clusters and its electrochemical performance was also studied.The results show that both single atom Pt and Pt clusters are not so good for ORR but when the growth of crystal structure is more advantageous to promote ORR catalyst.