Nitrogen-doped Porous Carbon Materials: Study On Their Preparation By Porogen Approach And Their ORR Performance

The rapid depletion of fossil fuels is one of the serious problems in China,which is the largest energy consumer in the world.What's more,the quality of environment is getting worse and worse because of the consumption of traditional energy?coal,oil,etc.?.Fuel cells?FCs?are considered as promising candidates for next generation energy conversion systems because of their low pollution,high energy conversion efficiency?as high as 83%?and rich fuel sources,which can greatly reduce mankind's dependence on fossil energy.Therefore,fuel cells have attracted extensive attention since 1880s.At present,major challenges impeding the large-scale application of fuel cells reside in sluggish kinetic process of oxygen reduction reaction?ORR?on the cathode,which is five order of magnitudes more sluggish than that of hydrogen oxidation reaction on anode.Currently,Pt-base materials are the most electroactive catalysts for ORR owing to their high current density and low overpotential.However,there are still some obstacles that fuel cells have to overcome for practical application.1)Pt-base catalysts suffer from very limited reserves,high cost?accounting for 49%of the cost of fuel cells?;2)Pt-base catalysts are sensitive to the poisoning of trace CO in the fuel,which seriously reduce the working efficiency of fuel cells;3)Pt-base catalysts have no immunity towards methanol crossover,which reduce the service life of the battery;4)Pt-base catalysts have poor stability.In this regard,it is highly desirable to develop low-cost and highly active materials with excellent selectivity and stability for ORR as alternative cathode catalysts.Heteroatom-doped porous carbon materials have been considered as the most promising ORR catalysts because of their unique pore structure,large specific surface area,good mechanical properties,tailorable structure,high conductivity and stability.The preparation strategy has significant influence on its morphology,versatility of pore size,active sites and graphitization degree,etc.Based on these,we have carried out following works mainly revolving around the influence of porogen on preparation of N-doped porous carbon materials and their ORR catalytic performance.?1?Study on the preparation of N-doped micro/mesoporous carbon materials by single-porogen approach and their ORR catalytic performances.The N-doped micro/mesoporous carbon materials?NDC-T?are prepared by pyrolyzing the mixture of ZnCl₂ and poplar catkins,where ZnCl₂ as the only porogen,poplar catkins as carbon and nitrogen source.The representative sample NDC-800 not only shows a comparable ORR electrocatalytic activity with Pt/C catalyst,but also displays outstanding cycling stability,excellent resistance to methanol cross-over and CO poisoning.?2?Study on the preparation of N-doped hierarchically porous carbon materials by dual-porogen approach and their ORR catalytic performance.Fluffy N-doped carbon materials with micro-,meso-and macropores are constantly fabricated from 20kinds of cheap daily biomass even biowaste?such as carrot,long bean and ginkgo leaf,etc.?by dual-porogen approach,which are tested to be excellent ORR catalysts.Here,ZnCl₂ and basic magnesium carbonate?BMC?behavior as dual porogens,and urea as nitrogen source.The representative sample(N_0.54-Z₃/M₁-900)delivers superior ORR performance in alkaline medium,i.e.a positive half-wave potential,favorable durability and selectivity,to those of both commercial Pt/C and the materials prepared with single porogen.The precursor can be easily extended to 20 kinds of daily biomass even biowaste substantially confirming that the combination of ZnCl₂ and BMC as porogen is a vivid universal protocol to prepare fluffy N-doped carbon materials as ORR catalysts from low-cost biomass,where the underlying formation mechanism may be based on the congenerous pore-creating functions of ZnCl₂ and BMC.Such synergetic dual-porogen method holds promising potential as a general approach for synthesizing desirable ORR catalysts from widely available biomass in nature,and it is essential to positively drive FCs commercialization scale-up in terms of ORR catalysts.?3?Study on the preparation of N-doped hierarchically porous carbon materials by single-porogen approach and their ORR catalytic performance.The N-doped hierarchically porous carbon materials?NMC-1?are prepared via high-temperature pyrolysis,using carbonate as single porogen,glucose as carbon source and urea as nitrogen source.Electrochemical characterization indicates that NMC-1 delivers superior ORR catalytic performance to that of Pt/C catalyst in alkaline medium in terms of catalytic activity,cycling stability,resistance to methanol cross-over and CO poisoning.