Design And Construction Of Manganese/Carbon Based Materials And Their Applications In Rechargeable Zinc-air Batteries

Rechargeable zinc-air batteries meet the requirements for the current battery technology due to its high energy density,high power density,safety and environmental friendly,which are considered as one of the most feasible and effective new pollution-free energy storage and conversion equipment in the 21st century.It is expected to be widely used in electric vehicles,stationary energy stations,flexible wearable devices and other fields.However,oxygen reduction reaction（ORR）and oxygen evolution reaction（OER）as the core reactions of rechargeable metal-air batteries are faced with the severe situation of high overpotential and slow kinetic process,which directly leads to the loss of energy and efficiency in the charging and discharging process.Platinum（Pt）,ruthenium（Ru）,iridium（Ir）and their oxides as commercial catalysts are widely used in ORR and OER.However,its disadvantages such as high cost,scarce resources and weak stability restrict its large-scale application.Therefore,to design and construct highly efficient,stable and cheap bi-functional oxygen electrode with favorable crystal structure and surface interface properties is the focus and hot spot in the research of the rechargeable zinc-air batteries.Recently,carbon-supported transition metal oxides（TMOs/C）have shown strong bifunctional catalytic potential and have attracted extensive attention,due to its abundant source,good stability and rich chemical valences.In this work,based on the unique advantages,a series of carbon-supported manganese-based metal oxide bifunctional catalysts were prepared by a mild hydrothermal method with the engineering strategies of crystal structure/morphology/particle size and cation doping.The growth rule,electrocatalytic performance,stable decay mechanism and the structure-activity relationship between the physical intrinsic structure and electrochemical properties of the catalysts were studied.In addition,a large area homogeneous nickel foam self-supporting electrode was prepared by electrodeposition-electrochemical oxidation coupling method.The above-mentioned materials,zinc and electrolyte were used in rechargeable zinc-air batteries and were tested.Finally,based on the related electrochemical reaction model,the evolution of the active sites and the catalytic mechanism of the catalysts were elucidated by the density functional theory（DFT）calculation.The main research contents and conclusions in the work are as following aspects:（1）Different Mn O₂ precursors were prepared by hydrothermal method and changing reaction reductant.The results show that different reductant form Mn O₂ with different morphologies,thus exposing different active regions and leading to different catalytic activities.Among them,the nanowires Mn O₂ precursors was prepared by oxalic acid,and the hydrangea Mn O₂ precursors was prepared by melamine,which show good electrochemical performance.Then,adding CNTs into the second-step hydrothermal process to prepare two different Mn O₂/CNTs₃₅₀ composite catalysts,respectively.The Mn O₂/CNTs₃₅₀ hybrid catalyst prepared by the oxalic acid showed excellent ORR/OER activity,due to its unique 3D intertwine network“bamboo”structure,which is beneficial to the adsorption and dissociation of the reactants/products,forms a plurality of reaction buffer zones,and the process of accelerating the catalytic reaction.The electron transfer number of ORR is 3.89,which is the four-electron reaction path.Moreover,the tafel slope of OER was only 69 m V dec^-1 and the TOF was up to 3×10^-2 s^-1,showing a rapid kinetic parameter.Because of the formation of interpenetrating network structure and strong coupling between metal oxide and CNTs,the Mn O₂/CNTs₃₅₀ composite catalyst prepared by melamine exhibited better dual-functionality（△E:0.838V）than that of the catalyst prepared by melamine,because of the formation of interpenetrating network structure and the strong coupling between metal oxides and CNTs.Through the XRD SEM and TEM characterization of the samples after the stability test,Mn O₂ would complete the transformation fromα-toβ-crystal structure during the stability test.The results show that the unique interpenetrating network structure formed can be used as an effective carrier and buffer layer in catalytic reactions,and further inhibiting the transformation/folding/aggregation of the morphology and structure for the sample.（2）Cation doping has been proved to be an effective method to improve the activity of catalysts.These different cations doped samples of Fe-Mn O₂/CNTs,Co-Mn O₂/CNTs and Ni-Mn O₂/CNTs catalysts were successfully constructed by adding iron nitrate,cobalt nitrate and nickel oxalate in simple hydrothermal method.XRD and TEM showed that,among these catalysts,the doping of Fe and Ni cations greatly changed the morphology of the Mn O₂/CNTs,especially Ni cation reduced the crystallinity of the sample.However,Co doping not only maintains the morphology and structure of the samples,but also regulates the crystal plane structure of the original Mn O₂/CNTs and creates more defects in the crystal plane.By testing the three-electrode system,the sequence of ORR activity was:Co-Mn O₂/CNTs>Fe-Mn O₂/CNTs>Ni-Mn O₂/CNTs,and OER:Co-Mn O₂/CNTs>Ni-Mn O₂/CNTs>Fe-Mn O₂/CNTs.The results show that Co cation doping can improve the activity of ORR and OER.However,compare to Ni cation,Fe cation increased the ORR of the catalyst,and Ni cation was beneficial to the enhancement of the activity of OER.In addition,the decrease of catalyst activity is mainly due to the aggregation of metal oxides and the destruction of catalyst structure.The DFT calculation results show that Mn1/Co₃O₄@Mn O₂（110）and Mn2/Co₃O₄@Mn O₂（110）were the active sites of the composites by elucidating the different theoretical model.The active sites of Mn in Mn O₂/CNTs were activated rapidly by doping with Co,thus showing better electrochemical activity.（3）On the basis of single cationic doping,the catalyst samples with different double cationic doping Mn O₂/CNTs were constructed by adding iron nitrate/cobalt nitrate,cobalt nitrate/nickel oxalate and iron nitrate/nickel oxalate respectively in the hydrothermal process.Before preparing the double-cation-doped Mn O₂/CNTs catalyst,we prepared different carbon-loaded bimetal oxide precatalysts,and conducted effective regulation in terms of morphology,particle size and composition,as well as the electrochemical/battery performance were tested.In the carbon-supported bimetallic oxide pre-catalysts,the particle size of the Ni Co₂O₄/CNTs is 110 nm.The Ni:Co ratio is 1:1,the activity of the Ni Co₂O₄/CNTs catalyst is the highest.The maximum power density of Ni Co₂O₄/CNTs catalyst in the zinc-air battery was 227m W cm^-2,and the time of charge and discharge cycles was 115 hours.The Co Fe₂O₄/CNTs composite catalyst has two different crystal structures:crystalline and amorphous;wherein the amorphous structure in the catalyst is favorable for ORR;the crystallinity is beneficial to the OER;the maximum power in the zinc-air battery was 333.7 m W cm^-2,and the time of charge-discharge cycles was 210 hours.The quantum dots formation of Ni_xFe_yO₄/CNTs_（T,t）composite is related to the Fe/Ni ratio.The increase of nickel content is beneficial to the formation of catalyst quantum dots.For example,the Fe/Ni atomic ratio of Ni_xFe_yO₄ quantum dots is 2 to 2.68（2≤Fe/Ni≤2.68）.The formation of quantum dots shows excellent electrochemical activity and battery performance of Ni Fe₂O_4（QDs）/CNTs composites,and it is worth noting that the active site of the catalyst is located at the Ni site of the contact between Ni/Fe metal oxide and CNTs.In addition,based on using nickel foam instead of carbon paper as diffusion electrode and charging and discharging test,the results showed the charge-discharge stability of the battery can be effectively improved by changing the air electrode substrate（the charge-discharge cycle time is more than 800 hours）.Finally,on the basis of the above studies,the double-cation-doped Mn O₂/CNTs samples were constructed.Moreover,the physical spectral characterization and electrochemical/battery test of the samples were conducted.The results showed that,compared with single-cation doping,double-cation doping could further improve the electrocatalytic activity and performance of Mn O₂/CNTs in battery applications.In addition,it is easy to generate nanoparticles with small size（40-50 nm）,and loaded on the surface of the Mn O₂/CNTs by Ni/Co double-cation doped.The sample is easy to obtained large-size（180-200nm）particle by Ni/Fe double-cation-doped,and the Mn O₂/CNTs structure is seriously damaged.In particular,the catalyst morphology is destroyed by Co/Fe double-cation-doped,and affected the crystallinity of the catalyst.Among these,Ni/Co double-cation-doped Ni/Co-Mn O₂/CNTs exhibited excellent ORR/OER activity and battery performance(the power density:487 m W cm^-2,and the charge/discharge time:360 hours).（4）The carbon-free and binder-free flexible bifunctional self-supported electrode（Ni Co₂O₄@Ni-foam）is obtained by a simple and stable electrodeposition-electrochemical in-situ oxidation method.The Ni Co₂O₄ nanosheets array in-situ grow in the nickel foam suface.The results of BET,XPS and TEM showed that The self-supported Ni Co₂O₄@Ni-foam electrode has a large specific surface area,with a pore structure with medium and large pores,and a rich oxygen defect.The results of three-eletrode test displayed that these self-supported electrodes shows excellent ORR/OER activity（ΔE=0.57V）and stability（the ORR activity is attenuated by 4.3%in 48 hours,and the OER activity is attenuated by 3.5%）,due to the large specific surface area,the mesopores and large pore structure,and the rich oxygen defect.It is to be noted that the self-supported electrode,zinc foil and anion-exchange membrane were used in large-area flexible zinc-air batteries and exhibits excellent battery performance(the power density:73.8 m W cm^-2,the specific capacity:567m Ah g^-1and the voltage drop of charge-discharge:0.56V).The results showed that the electrochemical activity and stability of self-supported Ni Co₂O₄@Ni-foam electrose are mainly due to the rich pore structure,unique transmission network,and the high-efficiency coupling of Ni Co₂O₄nanosheet and nickel foam.The addition of no binder and no carbon further improves the conductivity and the stability of the catalyst.