Design Of F And N Co-Doped Carbon Based Electrocatalysts And Their Mechanism Study

The development of sustainable energy storage and conversion devices,such as fuel cells and metal-air cells,is one of the ideal solutions to energy and ecological problems.In a typical proton exchange membrane fuel cell（PEMFC）,the anode reaction is HOR reaction and the cathode reaction is oxygen reduction reaction.They all use Platinum（Pt）-based electrocatalysts to accelerate the reaction rate.However,the reaction kinetics at the cathode is about five orders of magnitude slower than the anode,so the cathode requires more catalyst to speed up the reaction rate.High prices and low reserves of the precious metal Pt have hampered PEMFC’s commercial development.In addition,Pt-based catalysts have poor stability in acidic environments and are prone to CO poisoning and deactivation.Therefore,it is significant to develop low-cost ORR catalysts with excellent catalytic performance to replace Pt-based catalysts.In this paper,the expensive pt-based catalysts are replaced by the development of non-precious metal and metal-free catalysts.Transition metal doped carbon materials can be used to adjust the morphology and porosity of catalysts,thus exposing more active sites conducive to electrocatalysis,has a good application prospect.Metal-free electrocatalysts can avoid"Fenton reaction"and have great potential to replace Pt-based catalysts in PEMFC.In this paper,2-methylimidazole/melamine/thiourea as N source or S source,with PTFE as F source,the formation of single/bimetallic active center or heteroatomic doping structure,the preparation of a series of high-performance carbon-based catalysts,mainly have the following:（1）Non-precious metal tris（Fe/N/F）-doped particulate porous carbon materials were synthesized by a one-step method using 2-methylimidazole,PTFE and Fe Cl₃ as raw materials.The resulting catalyst M₁P₂-Fe₁-850 has superior ORR electrocatalytic performance(E_1/2=0.85 V)than commercial Pt/C in alkaline media.In addition,it has better stability and methanol resistance than Pt/C.This is mainly attributed to the effect of the tris（Fe/N/F）-doped carbon material on the morphology and chemical composition of the catalyst,thus improving the oxygen reduction performance of the catalyst.This work provides a reference method for the mild and rapid synthesis of carbon materials co-doped with highly electronegative heteroatoms and transition metals.（2）Fe or Zn-doped graphene-like nanoribbons were prepared in a controlled manner.This structure is conducive to fully exposing the active site and forming efficient ORR catalysts.The results showed that Fe-N/C-F catalyst had better ORR electrocatalytic performance than commercial Pt/C catalyst,and the half-wave potential was 40 m V higher than that of commercial Pt/C catalyst.Density functional theory calculation shows that:（i）During the ORR catalytic process,the F-doped graphene nanocrystalline bands can facilitate the adsorption of O₂ by Pauling adsorption model and reduce the RDS energy barrier.（ii）F doping can weaken the bonding energy of Fe N₄ site 3d orbital,reduce the RDS barrier,and facilitate the adsorption and desorption of*OH.（3）Here,a N,F,S triple-doped metal-free ORR electrocatalyst is synthesized by thiourea and PTFE using a one-step heat treatment method.The catalyst has a graphene-like network structure.In addition,the ORR half-wave potential(E_1/2)of catalyst T₁P₁-950 was 16 m V higher than that of Pt/C in alkaline media(E_1/2:0.861 vs.0.845 V).In acidic media,the ORR half-wave potential of this catalyst differs from that of Pt/C by only 80 m V(E_1/2:0.751 V vs.0.831 V).T₁P₁-950 is superior to Pt/C in methanol resistance and durability.The reasons for these can be attributed to:（i）has a graphene-like network structure,which facilitates the transport of electrons in the ORR reaction.（ii）The incorporation of N,F and S was conducive to the occurrence of synergistic effect between elements and the generation of highly active sites.（iii）The proportion of active functional groups（6.1at.%）and nitrogen content（7.1at.%）were high in the catalyst,which was conducive to the formation of many active sites.This work provides a reference method for the production of a series of ultra-thin mesh carbon materials doped with heteroatoms.