Preparation And Electrochemical Hydrogen Storage Properties Of TiVNi Composites

Ti-V-Ni alloys with icosahedral quasicrystalline phase manifest great success as negative electrodes for Ni-MH batteries.However,the fast capacity degradation seriously restricts their applications in the electrochemical area.Thus,this paper is focus on enhancing the electrochemical hydrogen storage properties of Ti_1.4V_0.6Ni and preparing new composites through surface modification and adding compounds with high hydrogen storage capacity.The main contents can be summarized as follows:The Ti_1.4V_0.6Ni+xLiAlH₄?x=0.5,1,1.5,2,2.5,wt.%?composites are prepared.LiAlH₄ partially decomposes during ball-milling because LiAlH₄is metastable at room temperature.In addition,the hydrogenation of the Ti_1.4V_0.6Ni alloy occurs during ball-milling.The coating contains LiAlH₄and its decomposition products.When the electrode is immersed in the electrolyte,LiOH is distributed on the defective positions of the alloy surface,which protects the alloy not to be oxidized and enhances the cycle life of the battery.In addition,the dissolution of coating will generate some pores on the surface of the alloy,with the result that the hydrogen diffusion progress is accelerated.Li or Li⁺can make the rate of ion migration faster.Therefore,the alloy unfolds the good electrochemical properties.Graphene exhibits high specific area and electrical conductivity,which make it ideal as a dispersive phase designing the composite material.Hydrogen can also adsorb on graphene surfaces as sp³ C-H bonds,which enables the viability of graphene for hydrogen storage.Additionally,graphene is hydrophobic due to the non-polar covalent double bonds,which prevent hydrogen bonding with water and has potential as an ultrathin protective coating especially in protection of alloys from corrosion in the electrolyte.Addition of multi-walled carbon nanotubes?MWNTs?is effective at enhancing the hydrogen diffusion process and cycling stability of hydrogen storage alloys.The MWNTs can also prevent the aggregation of as-milled alloys and make the smaller powder particles more stable.Ti_1.4V_0.6Ni with carbon addition composites are prepared by ball-milling method.Different amount of carbon materials are coating on the alloys by controlling the content of adding carbon materials.The coating layer obviously inhibits the dissolution of V in the electrolyte.When coating with 10 wt.%graphene,the cycling stability after 50charging/discharging cycles is increased by 6.5%,compared with the Ti_1.4V_0.6Ni electrode.Composites of Ti_1.4V_0.6Ni alloy and MWNTs are able to deliver outstanding maximum discharge capacities of 295.5 mAh g^-1 enhanced by 30%in comparison to Ti_1.4V_0.6Ni alloy,and higher cycling stability.The surface oxidation and pulverization of the alloys are availably inhibited.Transition metal carbide?TMC?exhibits catalytic property analogous to platinum group metals owing to the unique d-band electronic structure.Among TMC materials,molybdenum carbide?Mo₂C?and tungsten carbide?WC?are especially promising candidates to be used in hydrogen evolution reaction?HER?.Mo₂C or WC has been proposed as highly effective electrocatalyst for HER.Composites of Ti_1.4V_0.6Ni coated with TMC are prepared through ball-milling,and their electrochemical properties are discussed as negative electrode materials in a Ni-MH battery.Mo₂C and WC disperse adequately on the surface of the Ti_1.4V_0.6Ni alloy to form Ti_1.4V_0.6Ni@Mo₂C and Ti_1.4V_0.6Ni@WC,respectively.The effect of different adding content of TMC on the electrochemical behavior of alloys has been investigated.The cycling stability of composites has been enhanced.The capacity conservation rate of Ti_1.4V_0.6Ni@WC is 67.4%after 200 cycles,and it increases by 30%compared with Ti_1.4V_0.6Ni alloy.TMC coating can inhibit the formation of oxide film and suppress the V dissolution and volume change.MOFs with high porosity and designability have received much attention in a wide range of application.The MOFs with porous nanostructure successfully in situ grow on the surface of Ti_1.4V_0.6Ni alloy.Self-polymerization of dopamine is used to form a thin PDA film on the surface of Ti_1.4V_0.6Ni particles.After a simple coprecipitation of Co ions and 2-methylimidazolate,ZIF-67 is in situ grown on the PDA film.Finally,Ti_1.4V_0.6Ni@C@Co₃O₄ composite is obtained by the thermal treatment.After electrochemical measurement,the PDA derived C film can enhance the electrical conductivity and decrease the charge transfer resistance and inhibit the dissolution of active constituent.The electrochemical kinetics of the electrode can be catalyzed by Co₃O₄ and the hydrogen diffusion channels are increased,leading to the faster hydrogen diffusion process.As a result,the capacity conservation rate of Ti_1.4V_0.6Ni@C@Co₃O₄ composite electrode after 300 charging/discharging cycles is enhanced by 120%.