The Preparation And The Oxygen Electroreduction Performance Of Mulit-doped Mesoporous Nonprecious Metal Carbon-based Materials

In the process of modern industrialization,the large-scale and long-term use of fossil fuels has led to the increasingly energy crisis and environmental pollution,which seriously threaten the human living environment.Development of eco-friendly new energy is a necessary option to alleviate the energy crisis and environmental pollution to maintain the sustainable development of human society.Among many green energy sources,fuel cell is a promising technology capable of turning chemical energy into a more environmentally friendly energy source,and has aroused much concern for its high efficiency,scalability,low carbon emission,reliably delivered,low noise,fast start up,no pollution,high energy density and long life.However,the high cost and scarce of Pt-based catalysts which used for oxygen reduction reaction(ORR)at the cathode of fuel cells restrict the large-scale commercialization of fuel cells,which necessitate the development high efficient non-noble metal catalysts to supersede platinum-based catalysts for ORR.Numerous studies have shown that the activity of electrochemical catalysts strongly depends on the performance of the catalytic active center,the density of accessible active sites,the mass transport capability and electric conductivity of the catalyst.Based on these conclusions,in order to develop an ORR catalyst to replace the noble metal,it is also important to improve the density of accessible active sites,the mass transport capability and electric conductivity of the catalyst,while looking for highly efficient active center.Carbon-based mesoporous materials possess large specific surface area and porous structure,which is benefit for the distribution of the surface active sites and the mass transport during the oxygen reduction reaction.The electric conductivity of the carbon-based mesoporous material can be controlled by its degree of graphitization and selection of carbon-based precursor materials.Therefore,we prepared and systematically studied the mulit-doped mesoporous nonprecious metal carbon-based catalysts with different active centers.The investigation results were as follows:1.A high-performance Co-based carbon electrocatalyst(Co-N-GC)for ORR is prepared by a simple wet-impregnation nanocasting method using SB A-15 as hard templates and water-soluble cobalt porphyrin as precursors.The prepared catalysts with Co-Nx moieties have ordered mesoporous channels,and high special surface area and the degree of graphitization.In 0.1 M HClO4 medium,the ORR over Co-N-GC prepared at the optimized heat-treatment temperature(800?)exhibits positive half-wave potential(0.79 V)and higher ORR current density(5.6 mA cm-2 at 0.2 V)comparable to commercial Pt/C(20 wt%)catalysts.In 0.1 M KOH,the ORR half-wave potential reach 0.81 V,which is only 10 mV lower than that over the Pt/C catalysts.Moreover,the prepared Co-N-GC materials possess the intrinsic long-time stability and the excellent methanol resistance toward ORR in both acidic and alkaline media.This is the best result of the similar materials reported in the literature.2.Ordered mesoporous Fe-Co-N-doped graphite-based catalyst(Fe-Co-N-GC)with the embedding Fe-N,Co-N,Fe-C=N-Co active sites has been prepared by alternate dipping and drying processing using{[Co(bpy)2]3[Fe(CN)6]2}[Fe(CN)6]1/3 compound as precursor and ordered mesoporous silica SBA-15 as hard templates.The prepared catalyst has high special surface area,high degree of graphitization and highly efficient active site Fe-C=N-Co.In 0.1 M KOH,the ORR onset potential,half-wave potential over the prepared Fe-Co-N-GC-700 reach 1.07 V and 0.88 V,which are 80 mV and 30 mV higher than those over the Pt/C catalysts,respectively.In 0.1 M HClO4,it revealed that the onset potential and current density over Fe-Co-N-GC reached 0.98 V and 5.4 mA cm-2,which are 40 mV and 0.4 mA cm-2 higher than those over Pt/C catalysts.Moreover,the prepared Fe-Co-N-GC-700 catalysts exhibit long-time stability toward ORR as well as the excellent methanol resistance in both media.Such excellent activity of ORR catalyst have not yet been reported in the literature.3.Using the specially chosen(Ph3P)2Fe(CO)3 compound containing Fe-P coordination centers as precursor,an efficient ordered mesoporous Fe-P-C electrocatalyst with the embbeding Fe-P,active sites was obtained by a wet-impregnation nanocasting method.In 0.1 M KOH and 0.1 M HClO4,the ORR half-wave potential over the prepared Fe-P-C reach 0.80 V and 0.64 V,which are 20 mV and 17 mV higher than those over the Pt/C catalysts,respectively,and higher than those over the similar materials reported previously.These results indicate that the Fe-P-C catalyst has the high ORR activity in both alkaline and acidic media can be successfully prepare by this strategy.The high oxygen reduction reaction activity of this catalyst may be attribute to the Fe-P and P-C structures in Fe-P-C catalysts which were confirmed by Fourier transform infrared spectroscopy(FT-IR)and X-ray photoelectron spectroscopy(XPS).4.A series catalysts with high accessible surface area,concentrated active centers,eminent electron conductivity,and porous structure for the ORR is prepared by using porous graphene foam hybrid carbon nanotubes as carbon matrix and different N,P sources as precursors.Synergistic effect and catalytic activity of N-C,P-C,and P-N was evaluated.The investigation results testify that the catalyst(PN-GF-CNTs)prepared by using hexamethylphosphoric triamide(HMPA)as P and N source possesses highest content of P and N,and also containing P-N active site in the carbon-matrix.In 0.1 M KOH,the onset potential and current density are 50 mV and 0.4 mA cm-2 higher than those over Pt/C catalysts.The half-wave potential over the prepared PN-GF-CNTs reach 0.84 V,which is only 10 mV lower than Pt/C catalysts.In 0.1 M HClO4,the onset potential is 50 mV higher than than Pt/C catalysts.The half-wave potential over the prepared PN-GF-CNTs reach 0.78 V,which is only 40 mV lower than Pt/C catalysts,which should be the best P-N-C catalysts as reported previously.5.A series of multi-atom doping ordered mesoporous carbon-based catalysts were synthesized under the same preparation conditions by using SBA-15 as hard templates.By analyzing the chemical composition and the bonding structures of the catalysts,the catalytic activity of mesoporous N-C,P-C,P-N-C,Fe-N-C,Fe-P-C,Fe-P-N-C was compared,and revealed the significant variation of catalytic activity influenced by chemical composition.The results show that the catalytic activity sequence is in alkaline medium,Fe-P-N-C>Fe-N-C>Fe-P-C>P-N-C>N-C>P-C,in acidic medium,Fe-P-N-C>P-N-C>Fe-N-C>Fe-P-C>N-C>P-C,which provide the theoretical and experimental basis for the design of high efficiency catalysts as promising substitutes to Pt.