Heteroatom-Doped Carbon And Its Composites As Oxygen Electrocatalysts For Zn-Air Battery Cathodes

Due to its high energy density,low cost,zero emission and system safety,rechargeable zinc-air battery(ZAB)has been considered as a promising next-generation energy storage and conversion system,and can be applied in large-scale energy storage equipment and electric vehicles.The slow kinetics of the oxygen reduction reaction(ORR)and oxygen evolution reaction(OER)limits the performance of the cathode of ZAB.The catalyst play a vital role in improving the performance of ZAB.Carbon,as a catalyst for air electrode,possesses many advantages,such as low cost,abundant reserves,high electronic conductivity,high specific surface area,etc.,but its catalytic activity and stability are insufficient.Catalytic activity of carbon-based catalysts can be enhanced by introducing heteroatoms(e.g.,N,P,S,B,O,etc.)and transition metals(e.g.,Fe,Co,Ni,etc.)into carbon.Besides,the structure design of carbon-based materials is also very important.The porous structure with reasonable design is beneficial for the effective transfer of electrolyte and oxygen,promoting the reaction rate of oxygen at the triple phase,thereby improving the performance of batteries.In view of the above problems of carbon-based catalysts,we improve the activity and stability of carbon materials as a cathode catalyst for ZAB by selecting green carbon precursors,using environmentally-friendly preparation methods,and designing heteroatom doping at the molecular level and compounding them with transition metals.This paper focus on using yeast,onium salt,lysine and cotton as precursors,and introducing heteroatoms and transition metals into the carbon crystal lattice through pyrolysis and the help of new templates.The catalytic activity,selectivity,stability of ORR/OER and performance of ZAB for the prepared materials are systematically discussed.Based on the structureactivity relationship study and theoretical calculations,the catalytic mechanism of the prepared materials on ORR/OER was explored.The research contents and results of this paper are as follows:(1)Preparation and catalytic performance of N/P/B ternary co-doped carbon(NPB-C)core-shell hollow materials.Heteroatom-rich yeast cells were selected as precursors of C,N,P,and sodium tetraphenylborate was used as precursor of additional doping element B.The NPB-C with hollow coreshell porous structure was prepared by one-step pyrolysis.The additional B-doping can not only change the structure of yeast cells,but also effectively regulate the N and P content of the yeast cell body.Thanks to the high specific surface area,abundant active sites and the synergy between different doping elements,the synthesized NPB-C shows high ORR electrocatalytic activity and good stability.This work provides a new way to prepare porous carbon and multiple doping from biological precursors.(2)Preparation and catalytic performance of N/P/S ternary co-doped graphene-like carbon(NPS-G).The NPS-G catalysts were prepared by pyrolysis using urea and ionic onium salts as precursors.By changing the content of one of the heteroatoms S,not only the specific surface area of the catalyst can be increased,but also the total content of heteroatoms can be effectively controlled.The prepared NPS-G catalyst can achieve a higher heteroatomic content,resulting in abundant and high active sites.Therefore,it exhibits excellent electrocatalytic activity(E1/2=0.85 V)and stability of ORR,and good discharge performance(power density of 0.151 W cm-2 and specific capacity of 686 mAh gZn-1)when applied to ZAB.DFT theoretical calculations show that the synergistic effect of N,P,S promotes the chemical adsorption of oxygen and its reaction intermediates on the catalyst surface,and promotes the kinetics of oxygen reduction.This work proposes a novel onium salt pyrolysis method to prepare multi-doped carbon materials,and a high amount of heteroatoms can be obtained through effective control.(3)Preparation and catalytic performance of N-doped carbon hollow cubes(NCHCs)catalyst.3D porous NCHCs were prepared using NaCl cubic crystals as sacrificial template and green L-lysine monohydrochloride as precursor.Due to the easy removal of NaC1 template and the environmentallyfriendly characteristics of L-lysine monohydrochloride,this work provides a method for large-scale preparation of heteroatom-doped carbon.The prepared carbon hollow cubes have the characteristics of 3D porous network structure,high surface area,high graphitization degree,and uniform N doping.Therefore,it exhibits excellent electrocatalytic activity for ORR and OER in alkaline media(ΔEgap=0.92 V).(4)Preparation and catalytic performance of N-doped porous carbon nanosheets modified by transition metal nanoparticles(TM/TM-NC,TM=Fe,Co,Ni).The TM/TM-N-C composites were prepared by the template method,and we systematically study the effects of different transition metal elements on the prepared catalysts,finding that the structure,elctrocatalytic activity and long-term durability of the resulting TM/TM-N-C are greatly dependent on the transition metal choice.Among them,Ni/Ni-N-C with Ni nanocube particles embedded in double-layer carbon nanosheet structure presents the best OER activity,and the potential at 10 mA cm-2 can be as low as 1.536 V.More importantly,Ni/Ni-NC achieves long-term bifunctional stability in a wide potential range of 0.1-1.9 V,which is mainly attributed to the effective Ni2+/Ni3+ oxidation-reduction,thereby protecting the carbon from high potentials oxidized.Benefiting from its high OER activity and long-term durability,the rechargeable ZAB assembled with it as a cathode catalyst exhibits excellent electrochemical performance.Combining XAS analysis and DFT theoretical calculations,the catalytic mechanism of TM/TM-N-C for ORR and OER is clarified.The simple and massproducible strategy in this work can expand the preparation of nitrogen-doped carbon and metal particle composite materials and their applications in other fields.(5)Preparation and catalytic performance of N-doped CNT jungle embedded with FeNi alloy nanoparticles(FeNi@CNT-CP).A series of CNT jungle with a uniform tube diameter and encapsulated uniform alloy nanoparticles is grown on a porous substrate by a self-injection gas phase method in a furnace.Compared to the heterogeneous FeNi@NCNT obtained by direct pyrolysis,the FeNi@NCNT-CP jungle-like composite exhibits excellent bifunctional performance for ORR(E1/2=0.85 V)and OER(Ej=10=1.59 V)as well as extremely small potential gap of ORR-OER(ΔEgap=0.74 V).When applied to rechargeable ZAB,it possesses a highest power density of 200 mW cm-2 and stable cycle life over 250 hours.The results show that Fe and Ni in the alloy play a key role in their ORR and OER activity and stability,respectively.(6)Preparation and catalytic performance of N-doped graphene-like full carbon air electrode with Orich functional group(NO-G@CP).Porous cotton pad was used as substrate,and heteroatom-doped graphene can grow on the surface of the substrate by urea pyrolysis.Thereby synthesizing 3D porous NOG@CP with high specific surface area and can still maintain good flexibility.The resultant full carbon air electrode presents good ORR/OER bifunctional electrocatalytic activities,and can be directly used as the cathode electrode of all solid ZAB.The results show that an open-circuit voltage up to 1.328 V,a maximum power density of 65.1 mW cm-2 and a low voltage gap of 0.43 V for the flexible ZABs based on NOG@CP are obtained.DFT theoretical calculations revealed the key roles of graphitic-N,pyridinic-N,and C=O groups of 3D graphene in ORR and OER.This work proposes a new strategy for preparing flexible carbon materials,expands the synthesis method of self-standing air electrodes,and is conducive to promoting the development of flexible zinc-air batteries.Based on above study,we deeply discussed the heteroatom-doped and transition metal-modified porous carbon materials as electrocatalyst for ZAB.Through the selection of precursor materials,the optimization of dopping elements and the reasonable design of structure for carbon-based catalyst,it obtained excellent electrocatalytic activity and stability.The mechanism of optimizing the structure of materials by synthesis method and improving the electrocatalytic performance by heteroatom doping and transition metal modification was explored.These research results are of great significance for the synthesis of efficient and stable carbon based catalysts and their application in ZAB.Based on above studies,we systematically and deeply explore the effects of the choice of carboncontaining precursors,the optimization of doping elements,and the design of the material structure on the electrocatalytic performance and stability of heteroatom-doped carbon,as well as their effects on application in ZAB.A strategy for preparing efficient and stable carbon-based catalysts is proposed,which provides an experimental and theoretical basis for the development of air electrode catalysts for rechargeable ZAB.