Preparation And ORR Catalytic Performances Of Heteroatom-Doped Porous Carbon Catalysts

In modern society,energy crisis and environmental pollution brought by fossil fuel depletion have become increasingly prominent.To get rid of the dependence on fossil fuels,it is urgently needed to exploit new types of energy resources or to develop new energy conversion technologies.Fuel cell?FC?is an environmentally friendly energy conversion technology with high energy efficiency but with few emissions.However,the sluggish kinetics of the oxygen reduction reaction?ORR?at the cell cathode has significant impacts on the energy efficiency as well as its practical applications.More importantly,the high cost,poor stability and easy to methanol poisoning of traditional platinum?Pt?or Pt-based catalysts have seriously hampered the industrial application of FC.It is therefore important to develop novel ORR catalysts with low-cost,high catalytic activity together with high electrochemical stability.In this thesis,based on the research progress on preparation and ORR catalytic performances of heteroatom-doped carbon catalysts,three heteroatom-doped porous carbon catalysts including B-doped,N-doped,and N/B-codoped carbons have been prepared by separate pyrolysis of single polymeric precursor.In addition,their ORR performances in alkaline electrolyte were also systematically investigated.The main results are shown as follows:?1?Preparation and ORR performance of the B-doped porous carbon catalyst.In this chapter,hyper-crosslinked porous polymer was first synthesized by Friedel-Crafts polymerization of 2,4,6-triphenylboroxin monomer.The as-obtained porous polymer was referred to as BPN and had a specific surface area of 714 m² g^-1.The B-doped porous carbon?BC-900?catalyst was then fabricated by the direct pyrolysis of BPN precursor at 900°C under an inert atmosphere.The B and O contents of the BC-900catalyst were found to be 1.88 at%and 9.03 at%,respectively,while the BET surface area and pore volume were found to be 1481 m² g^-1 and 1.21 cm³ g^-1,in which the porosity was consisted of a large proportion of micropores and mesopores together with a very small proportion of macropores.In alkaline media,the BC-900 catalyst exhibited excellent activity towards ORR with an onset potential?E_o?of?0.013 V?vs.Ag/AgCl,the same hereinafter?,a half-wave potential(E_1/2)of?0.176 V,along with a limiting current density?J_L?of 5.24 mA cm^-2,which was very close to those of Pt/C catalyst(E_o=0.000 V,E_1/2=-0.138 V,J_l=5.21 mA cm^-2).Moreover,BC-900catalyst also showed good methanol tolerance and durability.The excellent ORR activity of the BC-900 catalyst is possibly due to the synergistic effect of its high specific surface area,developed porous structure,and relatively high proportion of B-containing and O-containing ORR active species.?2?Preparation and ORR performance of the N-doped porous carbon catalyst.A triazine-based,quaternary bipyridinium-type porous aromatic framework?TBPF?was firstly synthesized by nucleophilic substitution polycondensation of cyanuric chloride and 4,4'-bipyridine monomers.The as-synthesized TBPF with a specific surface area of 172 m² g^-1 was then subject to pyrolysis at 900°C to give N-doped porous carbon?NC-900?catalyst.The as-prepared NC-900 catalyst is truly metal-free catalyst due to that any metal-containing raw materials were not used throughout the entire synthesis procedure.The NC-900 catalyst had a brain-like architecture together with a specific surface area and a total pore volume of 684 m² g^-1 and 0.38 cm³ g^-1,respectively.The N content of this catalyst was as high as 5.40 at%,of which the relative contents of pyridinic-N and graphitic-N that proved to be ORR active reached up to 91.77%.Due to the synergistic effects,the NC-900 catalyst displayed outstanding ORR catalytic activity in alkaline electrolyte with an onset potential?E_o?of 0.004 V and a half-wave potential?E1/2?of-0.112 V,which was higher than those of Pt/C catalyst?Eo=0.000V,E1/2=-0.138 V?.Moreover,the NC-900 catalyst also exhibited excellent durability as well as tolerance towards MeOH and CO.Finally,Li-O2 batteries with the NC-900 cathode or the Pt/C cathode were assembled and tested for their discharge–charge at 100 mA g^-1,it was found that the initial discharge overpotential for NC-900 catalyst was 0.28 V,which was very close to that of Pt/C based cell?0.27V?.?3?Preparation and ORR performance of the N/B-doped porous carbon catalyst.In this chapter,porous copolymer?PNB?with a specific surface area of 89 m² g^-1 was first synthesized by oxidative copolymerization of aniline and m-aminophenylboronic acid comonomer using ammonium persulfate as the oxidant.The N/B-codoped porous carbon?NBC-900?catalyst was then prepared by directly pyrolyzing PNB precursor at 900°C under an inert atmosphere.The NBC-900 catalyst with a hierarchical pore structure?micro-,meso-,and macropores?had a BET surface area and a pore volume of 967 m² g^-1 and 0.70 cm³ g^-1,respectively.The N and B contents of the NBC-900catalyst were found to be 7.37 at%and 5.01 at%,respectively,of which the relative contents of pyridinic-N?48.87%?,graphitic-N?34.69%?,BC₂O?61.00%?,and BCO₂?39.00%?that proved to be ORR active were pretty high.Synergetic effect of both hierarchically porous structure and N/B-codoping endows the NBC-900 catalyst with outstanding ORR catalytic activity.Compared to the Pt/C catalyst,NBC-900 catalyst showed an ORR onset potential that exceeded the value measured with the Pt/C by ca.15 mV,while the ORR half-wave potential and limiting current density for NBC-900catalyst were very close to those of Pt/C catalyst.The electron transfer number of the NBC-900 catalyst was calculated to be ca.3.88 in alkaline solution,indicating that the ORR process on the NBC-900 catalyst was dominated by a 4e reduction pathway.In addition,the long durability and tolerance towards MeOH and CO of the NBC-900catalyst were much better than those of the commercial Pt/C catalyst.