Hierarchical Porous Fe-N-S-Doped Carbon For Efficient Oxygen Reduction Reaction

Renewable energy technologies,such as metal-air batteries and fuel cells rely on oxygen electrochemistry.The oxygen reduction reaction(ORR)is a part of oxygen electrode reactions,which is took place in the cathode of metal-air batteries and fuel cells.However,the ORR is recognized as kinetically limiting component of these devices based on oxygen electrochemistry to practical levels.Although the platinum based catalysts has been considered to be the most effective solution to overcome the sluggishness of ORR kinetics;platinum suffer from the disadvantages of scarcity,prohibitive high cost and poor stability in global supply.Development of non-precious metal catalysts seems to be necessary for replace platinum catalysts.The treatment producing heteroatoms and transition metal doped porous carbon catalysts obtained through high temperature pyrolysis beyond 700? one of promising class of materials,showing high catalytic activity and excellent stability in both acidic and alkaline conditions.However,well distributed porous structure,high contents and intrinsic active sites of doped heteroatom remained a big challenge.In the present work,heteroatoms and transition metal doped porous carbon catalysts were synthesized via one step pyrolysis of thiourea as sole precursor of nitrogen and sulfur,and glucose as cheap green carbon source in the presence of transition metal(iron or cobalt)nitrate and the macroporous silica as template.Then all prepared catalysts were characterized and evaluated using electrochemical tests.The choice of heteroatoms(N and S)and transition metals(Fe and Co)is not fortuitous,because both nitrogen and sulfur changed the electronic structure of carbon by inducing charge and sin density.As for transition metals(Fe and Co),they enabled the introduction of heteroatom(N and S),promoted graphitization during pyrolysis.In fact,the encapsulated transition metal species make active surrounding graphitic carbon toward ORR.Numerous of catalysts were synthesized by varying the synthesis conditions and precursors used to increase the activity of heteroatoms and transition metal doped porous carbon catalysts.First,iron-nitrogen-sulfur/doped porous carbon catalysts were synthesized with different composition mass ratio of thiourea/glucose,following base leaching of Si02 to evaluated the effect of mass ratio on pore structure,surface area and electrocatalytic activity,in order to obtain an optimal composition.Second,the as-prepared catalyst hasundergone an acid leaching of inorganic Fe2O3 species to give iron-nitrogen-sulfur doped hierarchical porous carbon exhibiting high electrocatalytic activity and stability.The aim of successive removal of SiO2 template and in-situ generated inorganic Fe2O3 species using base and acid leaching,respectively,was not only to investigate fully the contribution of Fe on pore structure,surface area and active site,but also to avoid the use of dangerous reactant(HF).The excellent electrocatalytic performance is attributed to following reasons:(1)High content of nitrogen mainly in form of pyridinic-/graphitic-N species,co-doping thiophene-S and encapsulated iron nanoparticles,providing the highly active sites.(2)The hierarchical porous mesh structures with micro-,meso-and macro-porosity,accelerating the mass-transfer and facilitating the full utilization of active sites.(3)The high specific surface area composite with graphitic carbon,assuring large interface and rapid electron conduction for ORR.A comparative study of surface transition metal species was conducted between Fe and Co,as well as the effect of heat treatment by varying the pyrolysis temperature from 700 to 1000?.Moreover,the importance of used sulfur has been studied using urea to replace thiourea.Finally our excellent electrocatalyst was compared to the commercial platinum catalyst in term of activity,selectivity and stability andwas applied as cathode electrode for discharging zinc-air battery.The results of the present work provide a promising non precious metal oxygen reduction catalyst,which can replace platinum catalyst,bringing us a step closer to full commercialization of renewable energy device such as for metal-air batteries and fuel cells.