Preparation And Characterization Of M-N_x/C Non-noble Metal Oxygen Reduction Electrocatalyst

Among many clean energy sources,fuel cell is considered as the most effective and clean sustainable energy source because of its high efficiency,and low pollutant emission.However,in practical applications,the oxygen reduction reaction（ORR）process on the cathode side of the fuel cell is slow,and more Pt catalyst is needed to reduce the reaction overpotential and promote oxygen reduction.The price of precious metal Pt is high,the total content is scarce,and the stability is poor,which severely limits the commercial application of fuel cells.Therefore,there is an urgent need to develop low-cost,high-efficiency non-precious metal catalysts.M-N_X/C type electrocatalyst is a non-precious metal oxygen reduction catalyst with excellent oxygen reduction performance,stability,and methanol tolerance.In this paper,a series of M-Nx/C catalysts are designed by simpler methods using cheap and readily available nitrogen-containing molecules as raw materials.The prepared catalysts were characterized in alkaline solution,and the kinetics of oxygen reduction was analyzed.Specific research results include:（1）A nitrogen-rich precursors（PPYTZ）with a plurality of pyridine rings and chelating sites was prepared by chemical reaction,and PPYTZ/C and PPYTZ-Fe/C catalysts were prepared using carbon black as a support.The results showed PPYTZ-Fe/C has better electronic conductivity and lower charge transfer resistance.Compared with PPYTZ/C,the limiting current density of PPYTZ-Fe/C is 4.98 m A cm^-2,is twice times as many as as PPYTZ/C.And it has a higher electron transfer number and lower hydrogen peroxide yield.In addition,PPYTZ-Fe/C showed excellent resistance to methanol toxicity and stability.（2）Polyhexamethylene guanidine（PHMG）was used as a nitrogen-containing precursor,mixed with cobalt salt and carbon black in a"one-step"method to obtain a series of catalysts.The characterization shows that the catalyst exhibits a PHMG-coated carbon structure.Co-PHMG-2@C showed the best catalytic activity in the prepared catalyst,which is close to Pt/C.Moreover,Co-PHMG-2@C has lower H₂O₂yield and a high electrocatalytic efficiency dominated by 4e.In addition,Co-PHMG-2@C has better long-term durability and higher methanol tolerance than Pt/C.（3）A new type of cobalt-based polymer coordinated with transition metal was obtained through polymerization reaction.Four kinds of catalysts of HAT-CN-Co/C,HAT-CN-Co/C-700,HAT-CN-Co/C-800 and HAT-CN-Co/C-900 were prepared by one-step pyrolysis.The heat-treated catalyst has large specific surface area,high porosity and high density of active sites.Among them,HAT-CN-Co/C-800 exhibited the ORR activity similar to Pt/C,which is a typical 4e electron process.More importantly,compared with Pt/C,HAT-CN-Co/C-800 catalyst has better stability and methanol tolerance.（4）The previously reported ligand HTPP was chemically polymerized to obtain PHTPP,which was mixed with cobalt salts,carbon black was used as a carrier,and four catalysts（Co-PHTPP/C,Co-PHTPP/C-600,and Co-PHTPP/C-700,Co-PHTPP/C-800）were obtained by heat treatment.On the one hand,the heat treatment process increased the specific surface area,optimized the pore structure,and simultaneously prepared a catalyst containing N and S elements.S has a higher electronegativity than N and can change the structure of the active center.Electrocatalyst tests show that Co-PHTPP/C-700 has the best ORR performance.The onset and half-wave potential are comparable to Pt/C.It has a high electron transfer number,low H₂O₂yield,and it has a high electrocatalytic efficiency dominated by 4e.At the same time,Co-PHTPP/C-700 exhibits better methanol tolerance and long-term operational stability than commercial Pt/C.