Synthesis And Electrocatalysis Studies Of Biomass-derived Carbon-based Catalysts For Oxygen Reduction Reaction

Fuel cells（FCs）are regarded as the most potential device for energy conversion to solve energy crisis and environmental pollution.Nevertheless,oxygen reduction reaction（ORR）in cathode heavily relies on the costly Pt-based catalysts,which results in the increasing cost for whole cell and a short life due to the poor long-term stability and poison resistance.These reasons seriously limit the large-scale application of FCs.In recent years,biomass-based carbon catalysts with low cost,abundant reserve and simple preparation have attracted a lot of attention.But the ORR activity is still very low so that energy output cannot meet the requirements of practical applications for FCs.It is very meaningful to explore high-efficiency and stable carbon catalysts with anti-toxicity for improving the industrialization of FCs.Besides,as a green technology,microbial fuel cell（MFC）can not only directly convert the organic energy into electricity,but also treat wastewater simultaneously,which has gradually attracted a strong interesting.In order to prepare efficient and stable biomass-based carbon catalysts for ORR and meet the requirements of MFC capacity,the abundant and cheap biomass waste as precursor and various strategies were carried out in this paper.Based on structural and chemical regulations for activity,various biomass-based carbon catalysts were synthetized.Further,the physical characteristics,electrochemical properties and MFC performances were analyzed to verify the feasibility of biomass-based carbon catalysts prepared via different strategies for ORR and MFC.This work mainly was as follows:（1）Basing on the diversity of biomass source,various biomass materials were selected to be used as precursors to prepare natural biomass-based carbon catalysts with special structure and composition via a simple pretreatment and direct pyrolysis.Additionally,their intrinsic advantages were analyzed in terms of structure and composition in catalysts.（2）To enhance the ORR activity of natural biomass-based carbon catalysts,Fe and N doped carbon catalysts were synthetized via combining liquefaction of biomass,preparation of epoxy-amide resin as precursor,atom doping and final pyrolysis.Finally,the structure-function relationship based on preparation process,doping source and properties was analyzed.（3）Furthermore,abundant N-containing biomass waste could be selected as raw material,while dual-doped（Fe/N and P/N）and tri-doped（Fe/N/P）carbon catalysts were prepared by oxidization pretreatment,exogenous doping and high-temperature pyrolysis.Similarly,the structure-function relationship was demonstrated according to the two regulatory factors including doping source and pyrolysis temperature.（4）In order to enhance the applicability of biomass as the precursor to prepare ORR catalysts,lignocellulose was used as the raw material to be oxidized and mixed with NH₄Cl to prepare N,O dual-doped carbon catalysts at high-temperature pyrolysis.Naturally,the structure-function relationship was investigated by analyzing preparation process,doping ratio and pyrolysis temperature.Basing on above experiments,the main conclusions could be obtained as follows:（1）Cube carbon,sheet carbon,self-doped nitrogen carbon and activated carbon-based catalysts can be obtained via alkali boiling,pre-carbonization pretreatment and high-temperature pyrolysis using coconut chaff（CC）,pomelo peel（PP）,shrimp shell（SS）and coconut fiber（CF）as raw materials,respectively.Among all biomass-based carbon catalysts,CC catalyst and PP catalyst have high specific surface area,SS catalyst holds large nitrogen-containing active sites.The three catalysts display a 4e^-path for ORR process,but they have lower catalytic activity than that of Pt/C.Besides,CF catalyst shows poor catalytic activity with a 2e^-path for ORR due to the lowest specific surface area and active sites.Therefore,with 3.75 mg cm^-2 of catalyst loading as air cathode,CC-MFC,PP-MFC,SS-MFC and CF-MFC obtain the maximum power density corresponding to 445,435,470 and 238 m W m^-2,respectively.The large specific surface area and more active site in biomass-based carbon catalysts improve the ORR activity,but they always have lower MFC performance than that of Pt/C with a value of 552 m W m^-2.（2）The N-C and Fe/N-C catalysts with sheet structure were synthetized using epoxy-amide resin derived from banana stem as precursor.In alkaline medium,Fe/N-C exhibits a slightly higher ORR activity than that of Pt/C.For a neutral medium,Fe/N-C has a larger ORR activity than that of Pt/C,while N-C holds a comparable performance with Pt/C.They also confirm a 4e^-path for ORR.The excellent ORR activity in Fe/N-C benefits from the larger specific surface area,more N and Fe active sites and better stability than that of N-C.With 3.75 mg cm^-2 of catalyst loading as cathode,Fe/N-C-MFC,N-C-MFC and Pt/C-MFC achieve maximum power density corresponding to 485±15 m W m^-2,497±5 m W m^-2and 501±7 m W m^-2.They have similar MFC pefermance.Furthermore,the anti-toxicity analysis shows that N-C has better poison resistance than that of Fe/N-C,which results in the weaken catalytic activity for Fe/N-C in electrolyte containing nutrient solution.Hence,Fe/N-C-MFC can not obtain the best MFC performance.（3）P/N-SS,Fe/N-SS and P/Fe/N-SS were prepared by doping P and Fe into N-containing derived from SS.Based on different doping sources for excellent ORR activity,phosphorus and iron doping catalysts respectively are suitable for alkaline and acidic mediums,and the synergy effect of P and Fe facilitates the highest ORR activity for P/Fe/N-SS 900 in neutral medium.Especially,doping Fe promotes the long-term stability.Interestingly,P/N-SS shows the best ORR activity in alkaline condition with an extremely poor stability,which can be solved by further introducing Fe into P/N-SS with a negligible attenuation of ORR activity corresponding to 0.01 V and 0.01 m A cm^-2for half-wave potential and limiting current.In MFC application,P/Fe/N-SS 900-MFC achieves a maximum power output of 590±11m W m^-2 and a cell voltage of 612 m V,which are 96.7%and 91.5%of Pt/C-MFC(610±22m W m^-2 and 669 m V).（4）In order to provide a strategy for various biomass sources,the lignocellulose derived from CF was selected as raw material to in situ dope N,while oxidization pretreatment was carried out to improve the surface activity on lignocellulose and then reacted with NH₄Cl during pyrolysis via an amine-carboxyl condensation reaction.The oxidization pretreatments as well as optimized doping ratio and pyrolysis temperature ignificantly facilitates the specific surface area and N-containing active sites in CON-2-900.Therefore,comparing with commercial 20%Pt/C,the CON-2-900 exhibits a comparable catalytic activity with a 4e^-path for ORR and higher operational stability and poison resistance.When CON-2-900 is used as cathodic catalyst to fabricate MFC,CON-2-900-MFC obtains a maximum power density of 1014±23 m W m^-2 and a cell voltage of 0.49 V,which are close to 1062±14 m W m^-2 and 0.50 V in Pt/C-MFC,indicating the alternative for biomass-based carbon catalysts to commercial Pt/C.