Preparation And Performance Research Of Biomass-derived Porous Carbon-based Oxygen Electrode Catalysts

At present,as the next generation of green energy,hydrogen energy has attracted much attention.Fuel cells and water electrolysis units are the key to the sustainable development of hydrogen energy.However,the slow reaction kinetics,high overpotential and the necessity of noble metal catalysts have greatly limited their development.Therefore,the development of oxygen electrode catalysts with low development cost,high electrocatalytic activity and good stability are great significance.Among the non-platinum catalysts,the heteroatom and transition metal doped carbon-based catalysts exhibit superior catalytic performance and stability,therefore it receives much attention.Porous carbon materials have excellent characteristics such as abundant source,low price,large specific surface area and diverse pore structure.The hierarchical porous structure with micropores,mesopores and macropores can promote the rapid mass transfer of the reaction.Thus,it is widely used in fuel cells,metal-air batteries,supercapacitors and adsorption and other fields.However,the carbon materials themselves have low catalytic activity,and doping can effectively improve the performance of carbon materials.And multiple heteroatom doping can improve the charge density distribution and increase the electrocatalytic active site by a synergistic effect,thereby improving the catalytic activity.The local electronic structure can be further modified by loading transition metal on the doped base.Considering the cost,biomass materials in nature have a wide range of sources,low price,and high composition of heteroatoms,which is a good raw material for preparing doped carbon materials.In this thesis,the preparation of catalysts with low cost,high performance and simple preparation process is taken as the research object.A series of low-cost biomass-derived porous carbon-based catalysts with higher specific surface area,different morphologies and different pore structures were prepared from biomass materials,by hydrothermal method,supramolecular self-assembly method combined with high-temperature pyrolysis.The effects of precursor structure and composition and preparation conditions on catalyst structure were investigated.The synergistic effect of the doping atoms and the effect between the specific surface area,pore volume and doping atom content of the catalyst material are discussed.1.A nitrogen-sulfur self-doped nanoporous carbon catalyst was prepared using green and inexpensive onions as a single C,N and S source for the first time.The catalyst samples has a specific surface area up to 1558 g cm^-2,and the larger specific surface area can expose more active sites.XPS analysis combined with electrical performance analysis shows that the NSC-A2 sample with high graphite-N and pyridine-N content has the best electrocatalytic performance.The half-wave potential of NSC-A2 is 80 mV lower than the platinum carbon catalyst.The oxygen reduction reaction is a four-electron reaction.The stability and methanol tolerance of NSC-A2 is better than the platinum carbon catalyst.2.Supramolecular aggregates formed by intermolecular self-assembly of chitosan and phytic acid were used as precursors to obtain N,P co-doped hierarchical porous carbon oxygen reduction catalysts by pyrolysis at high temperature.The effect of different carbonization temperatures on materials properties was discussed.The specific surface area of NPC-1000 was 1117.2 g cm^-2,and the degree of graphitization is the highest.The hierarchical porous structure facilitates mass transfer and increases the reaction rate.Electrocatalytic results show that the N and P co-doping can significantly improve the oxygen reduction catalytic activity of carbon catalyst samples.The half-wave potential is 50 mV lower than that of Pt/C catalyst.The oxygen reduction reaction is a four-electron reaction.The stability and methanol tolerance of NPC-1000 is better than the platinum carbon catalyst.This is also related to the synergistic effect between specific surface area,pore volume,doping atom chemical bond structure and content.3.Gelatin with high nitrogen content was selected as the nitrogen source,and supramolecular aggregates formed by intermolecular self-assembly of gelatin and phytic acid were used as the precursor.N and P co-doped graded porous carbon and oxygen reduction catalysts were obtained through high temperature pyrolysis,and the structure and atomic composition of the catalysts were optimized.Pore structure and XPS analysis showed that the proportion of the mesopores are increased,and the total amount of heteroatom doping was significantly increased.Electrocatalytic results show that N,P co-doped NPC-1000 has good electrocatalytic activity,the half-wave potential is 40 mV lower than that of Pt/C catalyst.The oxygen reduction reaction is a four-electron reaction and the stability and methanol tolerance is better than that of Pt/C catalyst.The high catalytic activity is due to the increased doping of heteroatoms and the synergistic effect of high specific surface area and more mesopores.4.A small amount of transition metals such as Fe,Co,etc.,can coordinate with heteroatoms in the carbon matrix,especially Co-N-C,which can effectively modify the local electronic structure and significantly improve the activity.In view of this,based on the above studies,we introduce a transition metal Co into carbon.The effects of co-doping of Co,N and P on the oxygen reduction and oxygen evolution catalytic activity of carbon materials were investigated.In addition,the metal phosphide exhibits good oxygen evolution characteristics.During the high-temperature carbonization,the P-O bond of the phytic acid is broken,and the released P combines with the cobalt ion to form a CoP.Such an in-situ synthesis method is simpler than the conventional phosphating method,and the CoP is embedded in a Co-N,P-C hierarchical porous carbon material with high specific surface area,which not only improves the specific surface area of CoP,but also the oxygen reduction characteristics and oxygen precipitation characteristics are combined.Electrocatalytic results show that CoP/CoNPC-900has good bifunctional catalytic performance.