| Greenhouse gases,acid gases and particulate matter will be released and lead to environmental pollution when fossil fuel provides energy for human beings.Developing new sustainable clean energy is the only way for sustainable development of human society.Fuel cell is a new power generation technology,which directly converts the chemical energy of fuel into electrical energy.Metal fuel cell is a new concept fuel cell with advantages of low cost,non-toxic,no pollution,stable discharge voltage,high specific energy and power.However,the cathodic oxygen reduction(ORR)of fuel cell is a four electron transfer process with complex mechanism and slow reaction speed.The by-product,hydrogen peroxide,can cause serious corrosion to electrode materials.It is the key step for large-scale commercial application of fuel cells to design catalysts to accelerate selectively the formation of hydroxyl ions through the four electron transfer path of cathode oxygen.Pt-based catalysts are the most active ORR catalysts at present,but the stability,durability and fuel cross tolerance are poor.Moreover,Pt is very expensive and rare.Therefore,the development of cheap and readily available catalyst has become an important research field.A large number of biomass is abandoned in the world every year,which resource utilization will bring huge economic and environmental benefits.Biomass carbon structures have stable chemical and mechanical properties and contain a certain amount of heteroatoms,which are suitable for electrocatalytic carbon materials or supporter.In recent years,a variety of carbon materials from biomass for ORR catalysts have been studied,but the performance are far from practical application.Based on this,three kinds of biomass carbon materials were prepared by reasonable design and synthesis process in this paper.Their structures and properties were fully characterized,and the structures of active sites were speculated.The contents are as follows:1.A new type of nano-N-doped carbon sheets,NACC,was constructed by hydrothermal reaction assisted by dilute sulfuric acid,pyrolysis and nitridation.NACC has specific surface area of 1726 m2 g-1,abundant micropores,appropriate pore volume,2.98%nitrogen content and lots of carbon defects.The results show that the material exhibits high activity for ORR in alkaline condition with better durability,stability and methanol resistance than those of 20%Pt/C.The excellent properties are attributed to high specific surface area and pore volume,high pyridine nitrogen and graphite nitrogen content,large amount of carbon defects and three-dimensional interpenetrating network structure.2.A novel nitrogen doped biochar material(BMNC)with carbon quantum dots(CDS)embedded in carbon nanosheets(CSS)was prepared by hydrothermal with the aid of dilute phosphoric acid,pyrolysis and nitrogen doping.The spectroscopic analysis showed that there were strong UV-visible absorption peaks in the wavelength range of 200-800 nm,and three fluorescence emission peaks at 404,463 and 534 nm,indicating the existence of large conjugated structure of CDs in the carbon structure.The carbon material has 2260 m2 g-1 specific surface area,abundant micropores,a large number of carbon defects and three different kinds of doped nitrogen:pyridine N,pyrrolidine N and graphite N.BMNC showed high ORR catalytic activity,high stability,good methanol resistance and high durability.In alkaline condition,the onset potential and half wave potential are 0.91 V and 0.83 V respectively,close to 20%Pt/C.Comparing the structure changes of the materials before and after reaction,it can be seen that carbon defects are the main active sites for ORR.Zn-air battery was assembled with this material,which showed excellent performance.3.CoxN and N codoped carbon nanomaterials were synthesized from cattail catkins by hydrothermal method in the presence of cobalt acetate,and further pyrolysis and nitrogen doping.By adjusting the pyrolysis temperature,three kinds of materials with different compositions were obtained:Co1.12NBC,Co0.80NBC and Co0.25NBC.The three materials have good catalytic performance for ORR,and Co0.80NBC is the best one.The onset potential and half wave potential of ORR on Co0.80NBC were 0.96 and 0.84 V,respectively.The current retention rate was 96.8%after 22 hours i-t test,and 98.8%when running in the 1M methanol for 1600s.The DFT calculation shows that the synergistic effect of Co2N nanoparticles and N-doped carbon sheet is the main reason for its high catalytic performance.The zinc air battery assembled with this material has good performance.The results of these research provide a reference value for the preparation and application of biomass carbon materials,and a reference path for the rational design of ORR catalyst structure. |
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