Study On Construction Of Hierarchically Porous Carbon Materials Derived From Agricultural And Forest Wastes And Oxygen Electrocatalytic Performance

The oxygen reduction reaction（ORR）as a crucial step occurs at the cathode of fuel cells.The performance of fuel cells were restricted by the the slow kinetics of ORR process,and a high-activity electrocatalyst is the key to promoting the kinetic of ORR.So far,platinum（Pt）-based materials have been adopted as as efficient ORR catalysts in fuel cells.However,the large-scale commercial applications of Pt-based catalysts is severely hampered by their high cost,low storage,poor toxicity resistance and other problems.It is still a serious challenge for researchers to develop non-noble metal catalysts with low cost and high performance.The ORR performance depends heavily on its electrochemical reaction kinetics and mass transfer efficiency.Rapid reaction kinetics requires highly active catalysts,while good mass transport demands efficient transfer efficiency.In this thesis,a series of low-cost and high-performance ORR catalysts were prepared by using agricultural and forest wastes as carbon sources and coordinating the electrochemical reaction kinetics and mass transfer efficiency.The main achievements are listed as belows:（1）Simultaneous pore-making and doping method to increase reaction surface areas and improve ORR activity.A high-level nitrogen-doped（11.55 at%）,high-surface-area(1978.4 m² g^–1)hierarchically micro/mesoporous carbon derived from coconut mesocarp powder by a solvent-free ball-milling process using melamine as nitrogen dopant and potassium bicarbonates as activating agent,followed by one-step pyrolysis process.By comparing the configuration changes of N atoms in materials before and after ORR,the active site is the C atom adjacent to pyridine N,so the highest content of pyrrolic N（49.7 at%）could provide abundant active sites.This material has similar ORR catalytic performance and in alkaline electrolyte to Pt/C catalyst,and exhibits better long-term stability and methanol tolerance.（2）Long-range order,short-range strategy to engineer one-dimensional flow channel and three dimensional reaction sites s to balance and optimize activity and mass transfer.A nitrogen-doped carbon derived from coir fibers with‘long-range order,short-range disorder’hierarchically porous structures was constructed by utilizing the natural structure of fibers.This materials not only has abundant micro/mesoporous structures and high content nitrogen,which provides a large number of reaction sites,but also has ordered macroporous tubular channels to reduce the mass transfer resistance and optimize the mass transfer process.In addition,the influences of mass transfer on ORR performance were investigated by four-probe and EIS impedance test.The results proved that an excellent ORR performance requires not only a large number of catalytic active sites and rough reaction interface,but also high charge transfer efficiency and fast mass transfer channel.Therefore,it has excellent ORR performance,stability and methanol tolerance in alkaline electrolyte.In addition,the maximum power density of the zinc-air fuel cell with the material as cathode catalyst is higher than that of the Pt/C zinc-air fuel cell,indicating it also has high ORR catalytic activity in zinc-air fuel cell.（3）Revealing the tendency towards convergence of diverse tissues of biomass and achieving high ORR performance in both acidic and alkaline electrolytes.A series of nitrogen-doped porous carbon materials derived from different tissues of hibiscus were developed by one-step pyrolysis.The influences of the different tissues as precursor on the final product with the physical structure and ORR catalytic activity were investigated.In addition,the effects pyrolysis temperatures on the structure and ORR catalytic performance of the materials were researched,and it was concluded that the influence of pyrolysis temperature was higher than that of different biomass tissues.A hierarchically porous nitrogen-doped carbon material with large specific surface area and nitrogen atom content was prepared by optimizing the temperature.The onset potential（alkaline:0.98 V vs.RHE;acidic:0.83 V vs.RHE）and half-wave potential（alkaline:0.83 V vs.RHE;acidic:0.67 V vs.RHE）and limiting current density(alkaline:5.95 m A cm^–2;acidic:5.45 m A cm^–2)of this material were higher than those of commercial Pt/C catalysts.（4）Practical applications of hierarchically porous carbon materials in acidic and alkaline fuel cells.A nitrogen doped hierarchically porous carbon was constructed through hot pressure and synchronous activation doping method.The spatial confinement effect of hot pressure process could promote activation and doping efficiency,which increased the specific surface areas and nitrogen content of carbon,leading to good ORR performance in different electrolytes.In alkaline solution,the onset potential of this material（1.02 vs.RHE）is 50 m V higher than that of commercial Pt/C catalyst（0.97 V vs.RHE）.Its current density(5.95 m A cm^–2)was 0.45 m A cm^–2higher than Pt/C(5.50 m A cm^–2).In acidic solution,this material also has higher onset potential（0.85 V and 0.82 V vs.RHE）and limit current density(5.55 m A cm^–2 and 5.30m A cm^–2)than Pt/C.The maximum output power density of alkaline zinc-air fuel cell using this material as cathode catalyst(132.5 m W cm^–2)is also higher than Pt/C counterpart(105.4 m W cm^–2).In addition,a MEA electrode with this material as cathode catalyst and assembled a proton exchange membrane fuel cell.By optimizing the cathode catalyst loading,nafion content and test conditions,the maximum output power density of the cell reached 320.3 m W cm^–2,proving that the ORR performance of the materials is excellent in both acidic and alkaline fuel cells.