Preparation Of Biomass Carbon-Based Oxygen Catalysts For The Application In Zinc-air Batteries

The increasing global energy crisis and the serious environmental issues have impelled extensive research on renewable energy conversion/storage technologies.Among them,fuel cells,water electrolysis and zinc-air batteries(ZABs)are considered ideal options for clean energy conversion technologies.With the in-depth study of the above technologies,people gradually realize that the keys to driving these technologies are two half reactions,namely:oxygen reduction reaction(ORR)and oxygen precipitation reaction(OER).However,the sluggish ORR/OER reaction kinetics and high overpotential have greatly hindered their development.Although noble metal-based materials(such as Pt,Ru,and Ir)have been extensively investigated for their excellent catalytic activity,high cost and scarcity largely limited their application.Therefore,it is critical to explore low-cost,highly active,and stable non-noble metal oxygen catalysts.In this thesis,various high-efficiency and low-cost oxygen catalysts derived from biomass/non-precious metal precursor materials were prepared with simple process,such as pyrolysis,in-situ reduction,codeposition,electrospinning.We also analyzed and discussed their active sites and catalytic mechanism through STEM,EXAFS,and In-situ Raman and other characterization methods,and further studied their charge and discharge performance in ZABs.The main contents and innovative points are listed as follows:1.Single Fe Atom on Hierarchically Porous S,N-Codoped Nanocarbon Derived from Porphyra Enable Boosted Oxygen Catalysis for Rechargeable Zn-Air BatteriesComprehensive experiments and density functional theory calculations(DFT)have demonstrated that the Fe-Nx structure in Fe-N-C materials can effectively optimize the ORR intermediate adsorption,results in an outstanding ORR activity.However,Fe-N-C materials generally exhibit an unenlightened OER activity due to the excessively strong adsorption of the intermediates and the difficult desorption of the final product oxygen(O2).Here,we innovatively designed and fabricated a high-efficiency bifunctional S-doped Fe-N-C(Fe-NSDC)electrocatalyst utilizing a scalable porphyra precursor pyrolysis strategy,characterized the single atom dispersed Fe-N sites by STEM,EXAFS and other methods.The main significances and novelties of our work can be summarized as follows:First,the dry weight percentages of vegetable proteins and taurine in porphyra reached up to 30%and 1.4%which could introduce high N and S supply,respectively.Second,unlike traditional heterogeneous Fe-N-C catalysts,taurine,as the main S-source in porphyra cells,can be ultrasound-assisted extracted from the porphyra easily,making it possible to verify the role of S species in ORR/OER process.Thirdly,the inherent advantages of organics homogeneously dispersed in the biomass cells were beneficial to generate the atomic dispersed active sites.Then the deeper mechanism analysis results showed that when the N atoms are replaced by the S atoms with weak electronegativity(N(3.04)>S(2.58)?C(2.55)),the desorption of O2 becomes very easy,resulting in significant OER activity.Finally,benefiting from the plentiful atomically dispersed active sites,porous nanostructures and synergistic effect between doped sulfur and Fe-Nx,the resultant Fe-NSDC electrocatalyst exhibited ultra-high ORR activity(E1/2=0.84 V)and OER performance(Ej=10=1.64 V),and high stability.The bifunctional electrocatalyst was assembled into a reversible zinc-air battery(RZABs)device and its battery performance was tested.The assembled Zn-air battery device comprising this bifunctional catalyst showed higher power density(225.1 mW cm-2)and lower charging-discharging overpotential(1.00 V,100 mA cm-2),even better than Pt/C+RuO2 based batteries(180 mW cm-2 and 1.09 V).The novel design and mechanism analysis of this porphyra-derived bifunctional catalyst will guide the synthesis of more efficient heteroatom-doped carbon-based oxygen catalysts.2.Three-Dimensional Interconnected Core-Shell Networks with Ni(Fe)OOH and M-N-C Active Species Together as High-Efficiency Oxygen Catalysts for Rechargeable Zn-Air BatteriesNiFe alloys and metal-nitrogen-carbon materials(M-N-C,M=Ni,Fe,Co etc.)have been extensively studied recently due to their intrinsic OER and ORR activity,respectively.In this study,we innovatively integrated the NiOOH and Ni/Fe-N-C active species into a robust bifunctional Ni2Fei@PANI-KOH900 electrocatalyst utilizing a scalable PANI-encapsulated-Ni2Fe1 3D core-shell precursor pyrolysis strategy accompanied by effective KOH activation.The results showed that the three-dimensional chain-like NiFe alloy cores prepared by in-situ reduction of sodium borohydride(NaBH4)not only exhibited excellent OER catalytic activity,but also could provide abundant Fe and Ni sources and interact with the atoms in the carbon shell in the subsequent pyrolysis process,thus producing efficient Ni/Fe-N-C active sites.Graphitized carbon derived from polyaniline(PANI)not only improved the electrical conductivity of Ni2Fei@PANI-KOH900,but also promoted the electrochemical stability of the catalyst.Meanwhile,KOH activator could tailor the carbon shell to precisely control the thickness of the shell and produce numerous micropores-mesoporous structures that can expose more alloy cores.Finally,benefiting from the plentiful active sites and 3D interconnected core-shell networks with porous carbon shell,our low-cost electrocatalyst displayed superhigh activity and durability in both OER and ORR reactions.Subsequently,the assembled rechargeable Zn-air battery comprising with Ni2Fei@PANI-KOH900 as the air cathode exhibited better rechargeability and longer cycle life than the Pt/C+RuO2 mixed catalyst.Our work provides a new idea for the rational design and synthesis of highly efficient and stable bifunctional oxygen electrocatalysts in RZABs.3.Ultrathin NiFe-LDH Nanowalls Vertically Anchored on Soybean-Derived Fe-N-C Substrates as Efficient Bifunctional Catalysts for Rechargeable Zn-Air BatteriesNiFe layered double hydroxide(NiFe-LDH)nanosheets have been extensively studied for their excellent OER catalytic performance.Unfortunately,the inherent low conductivity and self-agglomeration as well as their laggard and monofunctional OER catalytic activity without ORR performance severely hamper their application in reversible Zn-air batteries.In this study,we first innovatively synthesized a surface-oxidized Fe-N-C substrate(FeSoy-CNSs-A)derived from biomass soybean by one-step pyrolysis method followed by HNO3 treatment.Then,nanometer-sized NiFe-LDH nanosheets were vertically anchored on carbon substrates utilizing an in-situ coprecipitation strategy.The dry weight percentages of soy proteins in soybean cells reached up to 40%,which could introduce high N source and generate the atomic dispersed N defects after pyrolysis.Meanwhile,N defects sites could serve as the adsorption and nucleation centers for the Ni2+and Fe3+cations,and form strong interaction between N defects and metal ion(C-N…Ni-O and C-N…Fe-O),which would regulate the vertical growth of NiFe-LDH nanosheets with smaller size and thinner thickness.Meanwhile,soy proteins as the main N source could be ultrasound-assisted extracted from the soybean easily,making it possible to verify the role of N defects.Subsequently,our further research found that the existence of N defects and vertically anchored nanowalls structure with strong interaction between them resulting in favorable electron/mass transport and improved electrical conductivity,were responsible for the state-of-the-art OER and ORR performances.The assembled rechargeable Zn-air battery comprising with NiFe-LDH/FeSoy-CNSs-A as the air cathode exhibited unprecedented low charge/discharge voltage gap(?Egap=0.74 V)at a high current density of 100 mA cm-2 and highly durable cycle life c.This study will have important implications for the preparation of combining carbon-based materials with non-precious metal composites.4.Preparation of Fe/N Co-doped Carbon Nanofibers Derived from Biomass Gelatin and Its Performance of Oxygen Reduction and Zinc-Air BatteryFe-N-C nanofibers(Fe-GDC-CNFs)with three-dimensional interconnect structure were prepared with electrospinning technology and gelatin precursor.Compared with traditional catalysts,this synthesis method showed great advantages,such as simple preparation process,wide source of precursor materials,controllable morphology.In addition,the high content of protein and amino acids in gelatin could directly generate high-density N self-doping sites after pyrolysis at high temperature.Finally,benefiting from the synergy between the high Fe-Nx sites,the three-dimensional interconnection structure and good conductivity,Fe-GDC-CNFs exhibited excellent ORR catalytic activity(Eonset=1.05V vs.RHE and E1/2=0.92 V vs.RHE)and stability.And the ZABs assembled with this catalyst showed high open circuit voltage,stable discharge voltage and large specific capacity.In short,the high-efficiency Fe-N-C catalyst derived from natural biomass is expected to promote the large-scale commercial development of new energy devices by minimizing their cost.This thesis is based on the research of biomass-derived non-precious metal materials to improve the catalytic activity and stability of ORR/OER,hoping to provide some guidance and reference for the design of efficient and cheap oxygen catalysts.