Preparation And Application Of Biomass-derived Carbon-based Oxygen Reduction Catalysts

With the rapid development of the economy,the great demand for energy is increasing year by year.In the past one hundred years,fossil fuels have been the main source of energy.Fossil fuels are non-renewable energy and global storage is decreasing with the increase of exploitation.Meanwhile,the enormous usage of fossil fuels will also lead to a range of ecological problems such as environmental pollution and climate warming,so it is urgent to explore and develop renewable energy sources.As a kind of renewable energy,biomass resources are extremely abundant and extensively distributed all over the world.If biomass resources can be effectively employed,it will aleviate the energy shortage problem in the world today.The fuel cell is a device that directly converts the chemical energy of the fuel into electric energy.It is considered an energy conversion device with great potential to ease the energy crisis.On the one hand,microbial fuel cell（MFC）can not only achieve biological productivity but also realize sewage purification.On the other hand,due to its low cost,zero emissions,high energy density,and high safety,zinc-air battery（ZAB）has also attracted extensive attention.At present,the oxygen reduction reaction（ORR）of the cathode is still highly dependent on the precious metal platinum-based catalysts.However,the poor toxicity resistance and working instability of metal platinum-based catalysts will not only reduce the service life but also increase the production cost for the battery.In recent years,carbon-based catalysts derived from biomass with abundant reserves,wide sources,low prices,and simple preparation have shown excelent potential for practical application.Therefore,the development of carbon-based catalysts derived from biomass with excelent performance is of great significance to accelerate the application of batteries.In this work,biomass waste and biomass derivatives are used as raw materials,and their structure and composition are regulated by doping different types of non-metallic and transition metal elements to prepare carbon-based catalysts,which own high activity,high stability,and high toxicity resistance.The relationships between the structure and efficiency of electrocatalysts prepared under different strategies are analyzed by combining the physicochemical and electrochemical properties,as well as the practical application capabilities in MFC and ZAB.Based on the above discussion,the main research contents of the dissertation are as follows:（1）N-doping carbon-based catalysts with different structures are prepared by demineralization,depigmentation,and high-temperature calcination of crustacean biomass wastes such as shrimp shels（SS）,crayfish shels（CFS）,and crab shells（CS）,respectively.Among them,the catalyst derived from CS manifests the best catalytic performance in acidic media:the corresponding half-wave potential(E_1/2)exhibits 0.50V,and the limiting diffusion current（J_L）is 4.10 m A cm^-2.Furthermore,using waste CS as raw material to prepare N/C precursors,doped with Fe transition metal for hydrothermal carbonization（160℃,24 h）and high-temperature pyrolysis（900℃,2h）,finally,Fe/N co-doped catalyst（Fe-CS）is obtained.Fe-CS exhibits good ORR activity with an E_1/2 of 0.893 V in alkaline medium,which is higher than that of 20%Pt/C（0.858 V）.And the E_1/2 of Fe-CS in an acidic medium is 0.782 V,which is close to0.832 V for 20%Pt/C.By combining TEM,XPS and synchrotron radiation test techniques,it is proved that the prepared Fe-CS contains dispersed Fe-N₄ single-atom configuration,in which the Fe atom and adjacent N atoms possess charge transfer to form a coordination structure,which is the main reason for the high activity of catalyst.（2）The waste SS is used as both carbon and nitrogen sources to directly synthesize N and P co-doped carbon networks with abundant mesopores and high specific surface area by simple acid pretreatment and carbonization.Using exogenous phosphorus as a dopant,the prepared catalyst（PA-SS 900）favorably possesses a high ORR activity regarding E_1/2（0.82 V）and J_L(4.47 m A cm^-2),which approached those of commercial20 wt%Pt/C.For practical application in MFC,the PA-SS 900 achieves a maximum power density（MPD）of 802 m W m^-2 and an open circuit voltage（OCV）of 653 m V,which also are close to that based on 20%Pt/C as cathode catalyst(892 m W m^-2 and752 m V).Remarkably,the synthetic catalyst has a better long-term stability than that of 20%Pt/C in alkaline medium.These results demonstrate that N/P co-doped PA-SS900 is an accessible and efficient ORR catalyst in air-cathode MFC.（3）Using sodium carboxymethyl celulose（CMC）,a derivative of lignocellulose,as a precursor.Herein,a strategy is developed to optimize the catalyst Fe _SA/Fe _AC-NC with nitrogen-coordinated Fe single atoms（SAs）and closely surrounding Fe atomic clusters（ACs）towards efficient ORR.The influences of calcination temperature,doping source,and metal loading on catalytic activity are also analyzed.The synergistic effect of Fe SAs and Fe ACs on catalytic activity is proved experimentally.Fe _SA/Fe _AC-NC 900 catalyst delivers extraordinary ORR activity with an E_1/2 in both acidic（0.80V）and alkaline（0.90 V）media,as well as superior stability.Theoretical calculations further reveal that the synergistic effects of Fe-based SAs and ACs along with porous structures can decrease the overall energy barrier in the ORR process.Besides,the Fe _SA/Fe _AC-NC 900-based ZAB manifests a high peak power density(214.3 m W cm^-2),a high-specific capacity(773.6 m A h g^-1),and excellent long-term durability.This work provides a new perspective to highly optimize the synergistic interaction of SAs and ACs for boosting activity and energy storage.