Transition Metal Based Nano-materials For Electrochemical Energy Storage And Conversion

In recent years,with the burning of fossil fuels,environmental pollution and resources shortage problems become more and more serious,developing new and efficient energy and conversion devices to reduce the reliance on fossil fuels is imminent.Generally,the requirements for energy storage devices are high energy density,power density and safety.Compared with lithium battery,aqueous storage devices,including supercapacitor,NiFe battery,etc.,due to the improved intermal conductivity,tend to perform at a higher rate characteristics,power density,cycle stability,safety and lower production cost,thus attracting wide research interests and attentions.On the other hand,designing highly active electrocatalyst to improve the efficiency of electrochemical energy conversion reactions,such as hydrogen evolution reaction（HER）,oxygen evolution reaction（OER）and oxygen reduction reaction（ORR）,which play decisive roles in the field of hydrogen energy and fuel cells,is another important way to alleviate the energy crisis problem.Therefore,the electrochemical energy storage and conversion is the development direction of moderm energy technology,and the transition metal based compounds are the core working materials.Here,we focuse on the transition metal based electrode design for the aqueous energy storage device of supercapacitor and NiFe battery,as well as the fuel cell and water splitting system.For aqueous energy storage electrode,we combined the nanoarray construction with the mesoporous design by sacrifice template method,introducing the advantages of high specific surface area,high porosity,fast electron transfer process,shortened ion diffusion distance,thus greatly improved the capacity and stability of the electrode materials.For the functional design of oxygen electrocatalytic electrode,our study focuses on exploring three-dimensional structure,surface active component effect,single atom catalyst design and magnetic field interaction,which helps to the efficient electrocatalyst design and catalytic mechanism revelation.The research contents and conclusions are shown as following:1.A "self-generated sacrificial template approach" was developed to prepare hierarchical mesoporous NiO nanoarray electrode（NiO-HMAs）.Firstly,ZnO/NiO hierarchical nanoarrays were prepared by hydrothermal co-deposition and calcination processes,and then they were immersed in strong alkali solution to remove ZnO template.Finally,hierarchical mesoporous NiO nanoarrays were obtained.NiO-HMAs showed a capacitance of 3114 F g^-1(5 mA cm^-2)and exhibited good cycling performance（4000 cycles）.The excellent energy storage performance is mainly attributed to the nanoarray structured design and the construction of multi-level mesopores,making the utilization of active materials close to 100%,which greatly enhances both the faradaic and double-layer capacitances.A hybrid supercapacitor with NiO-HMAs as the positive electrode and graphene aerogel as the negative electrode shows an energy density of up to 67.0 W h kg^-1(320 W kg^-1)and good cycle stability（6000 cycles）.2.The "self-generated sacrificial template approach,was extended to the synthesis of porous Fe₃O₄-C nanoarrays as negative electrode for Ni-Fe battery with an ultrahigh specific capacitance(292.4 mA h g^-1 at the current density of 5 mA cm^-2)and high stability.Similarly,the synthesis of porous Fe304 nanoarrays involve the preparation of hierarchical ZnO/Fe₃O₄ composites by co-deposition of Zn²⁺ and Fe²⁺ and the removal of ZnO by an alkali etching process to construct mesoporous structure.Then,the material was carbon-coated by glucose carbonization process to improve its structural stability,and the Fe₃O₄-C nanoarray electrode was obtained.The ultrahigh capacitance of Fe₃O₄-C nanoarrays is ascribed to the high utilization of Fe₃O₄ in the unique mesoporous nanoarrays architecture.Moreover,the optimized Ni-Fe battery fabricated by using NiO-HMNAs as the positive electrode and porous Fe₃O₄-C nanoarrays as the negative electrode has demonstrated a high energy density of 213.3 Wh kg^-1 at a power density of 657.9 W kg^-1(113.9 Wh kg^-1 at 20727.1 W kg^-1)with a maximum voltage of 1.5 V and outstanding cycleability（capacitance retention of 81.7%after 5000 cycles）.3.A versatile strategy to synthesize ultrathin IrO_x nanodendrites（NDs）was developed by a one-pot hydrothermal coreduction method to prepare IrCo NDs,following electrochemical dealloying（ECD）process to obtain the Irox NDs（IrCo-ECD NDs）.The alloyed dendritic structure of the as-prepared nanocrystals and the composition of IrO_x NDs were fully characterized.It is demonstrated that the Co doping not only adjust the branched morphology of IrCo NDs,but also introduce more highly active IrOH sites during the electrochemical Co etching process.Thus,the as-prepared IrCo-ECD NDs deliver excellent electrocatalytic activity toward OER in acidic condition with an onset potential at 1.42 V versus reversible hydrogen electrode（RHE）and very low Tafel slope of 57 mV decade^-1,which are much better than those of IrO_x samples derived from Ir NDs（Ir-ECD NDs）,IrCo nanoparticles（IrCo-ECD NPs）,and commercial Ir/C.Additionally,IrCo-ECD NDs also exhibit good stability with the current retention of higher than 85%after 12 h chronoamperometry test.This strategy of the present work may be extended to the preparation of other Ir-based nanocrystals with dendritic structure and optimized surface composition,which is conducive to design highly active catalysts.4.The catalyst with isolated single-atom Mn supported on nitrogen-doped porous carbon（Mn-ISAs/CN）was prepared by carbonization of metal-organic framework（MOF）precursor.Atomic-resolution high-angle annular dark-field scanning transmission electron microscope（HAADF-STEM）and extended X-ray absorption fine structure（EXAFS）measurements were used to prove that Mn was distributed in single atom state.The loading of Mn was up to 1.68 wt%.The Mn-ISAs/CN sample exhibited excellent ORR catalytic activity,with a half-wave potential（E1/2）of 0.91 V,superior to commercial Pt/C and most non-precious metal catalysts reported so far.In addition,it also exhibited good methanol tolerance and stability,and showed broad commercial application prospects.5.The oxygen evolution reaction（OER）catalyzed by nanoarray electrodes was studied in the presence of an external magnetic field.When thin films of NiFe-LDH arrays on nickel foam was used as working electrode,the adding magnetic field increases the current density by roughly 44.5%under a potential of 2.0 V vs.RHE.We demonstrated that the high current enhancement maybe chiefly attributed to the effect of magnetic fields（MFs）on the arrangement of surface spins,which lowered the adsorption energy of O2 on the catalyst surface,hence facilitate the gas releasing and enhance the electrocatalytic efficiency.These findings present a potential revolution of traditional electrocatalysts by simply applying an external magnetic field to enhance the catalytic performance without material replacement and structural modification.