Design Of N-doped Carbon-based Non-noble Metal Catalysts For Enhanced Oxygen Reduction Reaction

The depletion of fossil fuels and the deterioration of the natural environment have triggered significant research on sustainable energy storage and conversion systems,such as fuel cells.However,the slow kinetics of the oxygen reduction reaction（ORR）at the cathode has limited the widespread commercialization of these renewable energy systems.To date,Pt and its alloys are considered as the most efficient catalysts for ORR.Unfortunately,the limited reserve and prohibitive cost of Pt-based materials limit their large-scale application.Moreover,their low durability and poor tolerance to methanol result in a loss of performance.Accordingly,increased efforts are focused on developing nonprecious metal catalysts with a low-cost and a comparable performance to that of Pt.Therefore,Transition metal–nitrogen/carbon（M–N/C,M=Fe,Co,Ni…）nanocomposites have emerged to replace Pt and other noble metals as ORR electrocatalysts.In recent few decades,a more portable and cost-efficient synthesis mehod for massive M–N/C composites has been progressed by immediately pyrolyzing the mixture of separate metal salts,nitrogen precursors and carbon subtrate.This simle approach put forward new probability for more funthional ORR catalysts.It is a great improvement to design and construct efficient M–N/C nanocomposites with modified nanosturctures.Designing and modifying the catalysts structures（like pore and surface area structures and N content）to expose more active sites and to the utmost usage of these active sites during catalytic process are consummately controlled.（1）Inspired by the g-C₃N₄ and zeolitic imidazolate frameworks（ZIFs）,we demonstrate a novel strategy for controlling the structures and dimensions of nitrogen-doped carbon-based electrocatalysts.g-C₃N₄ nanosheets was used not only as the nitrogen source but the template actracting Zn²⁺/Co²⁺on the surface.Followed by a coordination with dimethyl imidazole,zeolitic imidazolate frameworks（ZIFs）arrays were in-situ grown on the both sides of the g-C₃N₄ nanosheets.After a pyrolysis process,three different catalysts were ontained.1D nitrogen-doped bamboo-like CNTs encapsulated Co nanoparticles,2D nitrogen-doped porous carbon nanosheets and 3D nitrogen-doped carbon nanoframeworks encapsulated Co nanoparticles materials were obtained,respectively.The resulting 1D materials have a better ORR performance than the other two materials,with a better half-wave potential than 20%Pt/C catalysts（0.83 V vs.RHE）in 0.1M KOH solutions.What’s more,the stability and better methanol-tolerance of the 1D materials are even superior to those of the Pt/C catalyst in alkaline solution.The enhanced ORR performance of 1D catalysts are likely attributed to the well-dispersed N dopants,suitable surface area,and the encapsulated Co nanoparticles.This work offers new strategy in designing low-price and high-activity ORR catalysts.（2）We rationally designed and originally developed a strategy to fabricate 2D hierarchical porous Fe-N-C nanosheets electrocatalysts with the different pyrolysis temperature.In this work,we utilize g-C₃N₄ as nitrogen source and template attracting Fe ions on the surface followed by PDA coatings.TEM and SEM images demonstrate that the samples have a sheet-like structure with Fe₃C nanoparticles evenly loaded on it.What’s more,the average thickness of Fe-N-C-900 nanosheet is around 3 nm,which contains about 8 atomic layers.Compared with 20%Pt/C catalyst,the Fe-N-C-900 possesses comparable catalytic activity,which exhibits an onset potential of 0.98 and a half-wave potential of 0.86 V.These values are even superior to those of Pt/C（0.97 and0.84 V）,and much more positive than those of the Fe-N-C-800（0.95 and 0.82 V vs.RHE）and Fe-N-C-1000（0.93 and 0.80 V vs.RHE）.better tolerance to methanol as well as superior durability,showing great potential in alkaline fuel cell application.It has been investigated that the excellent catalytic performance of Fe-N-C-900 is probably due to the suitable degree of graphitization,high N content,and Fe₅C encapsulated inside of carbon nanosheets.The method greatly simplifies the experimental details involved,guaranteeing high control over fabricating the electrocatalyst,which provide a new strategy to design highly active non-noble metal catalysts for ORR.