Preparation And Electrochemical Properties Of Transition Metal Based Micro/Nano Hierarchical Arrays

Among the many investigated energy storage and conversion devices,supercapacitors and water electrolysis hydrogen production devices have attracted more and more attention.For supercapacitors or water electrolysis,the main challenge lies on the preparation of high-performance electrode materials.Transition metal-based materials possess many advantages such as low price and abundant resources.Nevertheless,with regard to practical applications,this type of materials still suffers from poor conductivity and limited active sites.To address these issues,three-dimensional transition metal-based materials with nanoarray structure on metal substrates are designed and constructed in this thesis.Moreover,through structural modifications of the 3D hierarchically structured materials are modified to produce electrode materials with improved electrical conductivity and more active sites are produced,which are carefully investigated in supercapacitors and water electrolysis.The contents of the thesis are summarized as below.(1)An etching-induced co-precipitation approach coupled with reduction and dehydration is developed,through which the Ni-modified MnO(Ni/MnO)nanosheet arrays can be directly used as binder-free electrode for pseudocapacitors.With the introduction of Ni nanoparticles,the conduction of the electrons as well as their accumulation and release at the Ni/MnO interfaces is efficiently promoted,which leads to the reduced charge transfer resistance for the Ni/MnO composite electrode.Meanwhile,the porous structure of the Ni/MnO nanosheet arrays gives rise to the larger contact area between the active material and the electrolyte.The Ni-modified MnO nanosheets electrode can offer excellent electrochemical performances with a high specific capacitance of 1235.6 F g^-1 achieved at 0.5 A g^-1 and a high-rate capacitance of1075.6 F g^-1 delivered at 10 A g^-1.Moreover,it is found that the capacitance of the composite electrode does not decay,but gets an increase of over 19%after 15000 cycles at a high rate of 10 A g^-1,the result is that the Ni nanoparticles on the MnO nanosheets and the Ni substrate are gradually be oxidized during the charge/discharge process,which could participate in the electrochemical charge storage processes and exert extra positive effects on the improvement of the capacitance for the composite material.In addition,the etching-induced co-precipitation method can be extended to other transition metal elements,such as Fe,Co,etc.,to prepare their hydroxides arrays.(2)We report the fabrication of CuO/Mn₃O₄ hierarchical nanoarrays on Cu substrate via an oriented catalytic oxidation combined with subsequent annealing.Based on an ingenious oriented catalytic oxidation process,abundant CuO/Mn₃O₄ nanosheets have been directionally modified on the surface of Cu(OH)₂ nanorod arrays.Through subsequent annealing treatment in N₂ atmosphere,CuO/Mn₃O₄ nanorod arrays are fabricated with tuned oxygen vacancies.The 3D hybrid nanoarray structure with advantages of enhanced electron transfer,large exposed surface area,and robust structure stability can efficiently address the drawbacks associated with powder-form materials which require binder for fabrication of electrodes and sufer from poor conductivity.As a result,the as-prepared CuO/Mn₃O₄ nanoarray electrode demonstrates excellent electrochemical performances,delivering a high specific capacitance of 433m F cm^-2 at 5 m A cm^-2(1732 F g^-1 at 20 A g^-1),and achieving a long cycling lifespan with 144%capacitance retention at 20 m A cm^-2 after 20000 cycles.Interestingly,this facile oriented catalytic oxidation strategy can be extended to prepare other metal oxides/hydroxides(including Ni²⁺,Co²⁺,Cu²⁺,Zn²⁺or mixed ions based)on the surface of Cu(OH)₂ nanorod templates respectively,manifesting its versatility for preparing CuO-based hybrid nanorod arrays on Cu substrate.(3)Hierarchical Fe-Ni(OH)₂/Co(OH)F composite nanoarrays are synthesized via the induction of Fenton reaction(H₂O₂+Fe²⁺).Firstly,Ni(OH)₂/Co(OH)F nanoarrays are synthesized through hydrothermal method,and then the Fe-Ni(OH)₂/Co(OH)F composite nanoarrays are prepared by Fenton reaction at room temperature.And their catalytic performance for oxygen evolution reaction(OER)is investigated.As expected,the hierarchical Fe-Ni(OH)₂/Co(OH)F composite nanoarrays electrode exhibits a low overpotential of 255 mV at 100 m A cm^-2 and can stably work for 115 h.At the same time,Fe-Ni(OH)₂ and Fe-Co(OH)F arrays are also prepared by this method,respectively.The results show that the introduction of Fe can increase the active site and electrical conductivity of the materials,thus improving the catalytic performance of the catalysts.