Preparation And Electrocatalystic Oxygen Reduction Performance Of Nano-Carbon-Based Composite Materials

Fuel cells are highly efficient and clean energy conversion devices.However,the slow kinetics of the cathode oxygen reduction reaction（ORR）is a key factor affecting the efficiency of fuel cells.Therefore,efficient catalysts are needed to accelerate the ORR reaction.At present,commercial Pt-based electrocatalysts show excellent activity in catalyzing ORR,but the scarcity and high cost of precious metal Pt limit their large-scale application in the field of fuel cells.In comparison,non-Pt-based precious metal and metal-free catalysts have recently received extensive attention due to their large specific surface area and strong stability.For metal-free carbon-based catalysts,heteroatom（such as N）doping is generally used to adjust the electron cloud distribution of carbon atoms to creates more catalytically active sites.However,N doping usually destroys the sp² structure of carbon materials.The conductivity of carbon-based materials is seriously damaged and the catalytic activity is reduced.Therefore,creating a carbon-based ORR catalyst that is rich in nitrogen-doped active sites and has high conductivity is still an urgent problem to be solved.Therefore,this thesis takes the design and preparation of ORR-based catalysts with high stability and high catalytic activity as the research goal.Through the composite of carbon-based nanomaterials and the support of different forms of non-metallic materials,efficient electrocatalytic ORR reactions are realized.The research content is as follows:（1）Mesoporous phenolic resin microspheres（mPF）were grown in situ on oxidized carbon nanotubes（OCNT）to obtain a carbon nanotube support（mPF@OCNT）composite.Using the composite as a precursor and melamine as a nitrogen source,a nitrogen-doped mesoporous carbon sphere composite（N-MCN@CNT）supported by carbon nanotubes with hierarchical porous structure was prepared through a one-step pyrolysis method.The composite has a high specific surface area,rich pore structure and excellent electrical conductivity.In the alkaline electrolyte,the N-MCN@CNT catalyst exhibits excellent oxygen reduction catalytic activity,where the onset potential(E_onset)and half-wave potential(E_1/2)reach about-65 and-192mV,which are close to commercial Pt/C catalysts,and are a four-electron transfer catalytic reaction process.N-MCN@CNT catalyst not only has good ORR catalytic activity,but also shows long-term cycle stability and good methanol tolerance,which is better than commercial Pt/C catalyst.（2）On the basis of the above work,with mPF@OCNT as the precursor,the transition metal ions Fe³⁺and Co²⁺were adsorbed by the microporous adsorption method,and the transition metal supported nitrogen doped was prepared by pyrolysis under the condition of melamine blending.Heteroporous carbon-based（Fe/N-MCN@CNT and Co/N-MCN@CNT）composite catalysts.The two metal-nitrogen-doped composite catalysts retain the large specific surface area,hierarchical pore structure and high conductivity of N-MCN@CNT.At the same time,the introduction of highly active metal-N active species into the carbon framework further reduces the ORR reaction Starting potential.The results of electrochemical studies found that the onset and half-wave potentials of Fe/N-MCN@CNT are-15 and-134mV,respectively,and the onset and half-wave potentials of Co/N-MCN@CNT are-46 and-141mV,these are comparable to commercial Pt/C（-19mV,-126mV）.And calculated by RRDE and K-L curves,the ORR reaction process of the two catalysts are both efficient four-electron transfer pathways.In addition,the non-noble metal-supported carbon-based catalyst has good catalytic stability（the half-wave potential only shifts to the left by 6mV after 1000 cycles of stability testing）.（3）Using MCM-22 molecular sieve as the hard template and sucrose as the carbon source,mesoporous graphene sheets（MGN）were prepared by the hard template method,and a simple chemical reduction method is used to stably load small-sized Pd Cu alloy nanoparticles on a carbon support（MGN）to form a Pd₃Cu₁/MGN catalyst.Pd₃Cu₁/MGN has a 3D porous network nanostructure formed by interweaving ultra-flaky nanosheets,which has a high specific surface area and a small-sized alloy nanoparticle load.The Pd₃Cu₁/MGN catalyst exhibits excellent oxygen reduction activity,and its catalyzed low yield of H₂O₂ in ORR reaction.In addition,Pd₃Cu₁/MGN catalyst also exhibited good catalytic activity in methanol electrocatalytic oxidation reaction（MOR）due to the synergistic effect of the dispersed small-size alloy loading and the porous carbon structure of the molecular sieve template,where the mass activity of 1.19 A/mg_Pdwas 2.76 times higher than that of the commercial Pd/C catalyst.