Functionalization Of Carbon-Based Catalytic Materials And Their Applications In Zinc-Air Batteries

Because of the merits of low cost,high safety,and high specific energy density,zinc-air batteries become ideal candidates as clean and sustainable energy devices in the next generation.However,the kinetics of oxygen reduction reaction(ORR)during discharging and oxygen evolution reaction(OER)during charging in the cathode of zinc-air battery is relatively slow,severely restricting its commercialization.The development of bifunctional electrocatalysts to improve the ORR/OER activity has become the key to improve the overall performance of zinc-air batteries.At present,precious metals with excellent catalytic activity,such as platinum,palladium,and iridium,have become the mainstream catalysts.But their wide applications are hindered by the low content,high price,poor durability,and serious polarization phenomenon.Therefore,it is of great significance to search for catalysts with sufficient raw materials,low price,and high activity to replace the traditional precious metal catalysts.Carbon-based materials have attracted much attention because of their chemical stability,high electrical conductivity,and adjustable porous structure and composition,while its preparation and application are still in the trial and error stage,and the material performance have a large room for improvement.Therefore,the development of efficient design strategies for carbon-based materials to control the location,structure,and chemical composition of active centers at the atomic level will greatly accelerate the preparation and optimization of the next generation of carbon-based materials.Based on the above problems,this paper aims to prepare highly active and highly stable carbon-based materials via metal composite design,doping design,vacancy design,and composite design of above three strategies,and realize their application in zinc-air battery.The interface structure,charge distribution,and the active site of catalyst can be accurately controlled on the atomic scale.Density functional theory calculations are used to carry out the thermodynamic study on the electrochemical reaction process to achieve deep understanding of the catalytic reaction mechanism and the dynamic evolution process of ORR and OER on the cathode.In situ X-ray diffraction(XRD)and in situ Raman characterization methods are used to analyze the thermodynamic mechanism of interfacial reaction of zinc-air battery.The relationship between the physicochemical properties of catalytic materials and their electrochemical reaction thermodynamics can be systematically understood,and the structure-activity relationship between the optimization effect of the catalyst microstructure and the macroscopic electrochemical performance can be established,so as to provide universal design strategies of high-performance carbon-based materials for zinc-air batteries.The main research contents and results of this paper are as follows:1.Metal composite design of carbon-based catalyst was conducted for zinc-air batteries.A carbon-based material loaded with highly activity PtNi alloy was prepared by a modified solvothermal reaction method.The Ni-based component was not only the active site of OER,but also can promote the adsorption of O2 at the Pt site,thus enhancing the ORR activity of PtNi and endowing the material with excellent bifunctional catalytic activity.The potential difference between the ORR and the OER was 0.75 V.In situ XRD results show that the O2 chemisorption covered the(111)crystal plane of the alloy and blocked the diffraction signal.The O2 coverage was the highest on the alloy surface with ideal Pt/Ni ratio,demonstrating that the reaction process was greatly promoted.The zinc-air battery assembled with this material can output an open circuit voltage of 1.49 V and a power density of 154.1 mW cm-2,and showed excellent cycling performance with more than 500 discharge-charge cycles.2.Doping design of carbon-based catalyst was conducted for zinc-air batteries.Graphene framework was exfoliated from the commercial carbon fiber paper to serve as the substrate for the growth of metal-organic framework-derived mesoporous nanoparticles,affording Co,N co-doped carbon-based materials.Excellent electrical conductivity enabled it to be directly used as a working electrode.The highly exposed active sites of Co-N and N-C were beneficial to the bifunctional catalytic activity of the material,and their potential difference was as low as 0.79 V.The integrated electrode can be directly used as cathode in zinc-air batteries,which delivered a high open circuit voltage of 1.51 V and a high power density of 154.4 mW cm-2.In situ XRD tests showed that heteroatomic doping was beneficial to the chemisorption of oxygen molecules and oxygen-containing intermediates on carbon-based materials.After 630 cycles for 105 h,the round-trip efficiency of the zinc-air battery remained almost unchanged(55.8%),and its excellent discharge-charge reversibility was further confirmed by in situ Raman tests.3.Vacancy design of carbon-based catalyst was conducted for zinc-air batteries.A nitrogen-doped carbon-based material with abundant oxygen-containing vacancies was prepared by inducing the formation of oxygen-containing vacancies in the annealing process due to the abundant oxygen atoms in the precursor.Density functional theory calculations showed that the oxygen-containing vacancy was a kind of highly effective active site,which played a key role in improving the catalytic performance of the air cathode.The material with the highest vacancy concentration and the most effective doping structure provided the highest electrocatalytic activity during the electrochemical test,with ORR half-wave potential of 0.86 V and OER overpotential of 0.47 V.The obvious intensity changes of carbon peak in the in situ XRD patterns showed that the oxygen-containing vacancy in the catalyst was beneficial to the chemisorption of oxygen molecules and oxygen-containing intermediates on the carbon material,which greatly promoted the electrochemical reaction of the cathode.The zinc-air battery assembled with this material can achieve a long cycle life of more than 500 discharge-charge cycles at a stable current density of 25 mA cm-2,and the round-trip efficiency remained at 57.9%after 500 cycles.4.Based on the above three studies,we conducted cooperative design strategy of metal composite,doping,and vacancy construction of carbon-based catalyst for zinc-air batteries.A single atom Fe-doped carbon material with a certain concentration of Fe vacancy was prepared through precise etching of the single atom Fe in the Fe-N-C structure.Density functional theory calculations showed that when single atom Fe was etched into a vacancy to expose pyridinic-N,the charge delocalization at the adjacent single atom Fe site occured and its valence states increased,which greatly enhanced the activity of this Fe site.The material with the ideal Fe-N4/pyridinic-N ratio exhibited the best electrochemical performance,and the potential difference between the ORR and the OER was as small as 0.76 V.The zinc-air battery assembled with this material can output a stable open circuit voltage of 1.49 V and an excellent power density of 154.4 mW cm-2.It also afforded superior cyclic performance of over 1000 discharge/charge cycles with negligible increase in the low voltage gap.