Mechanism Of Electrodeposition Of Hydrogen Storage Alloy Mg-Ni/Ce-Mg-Ni By Hydrogen Solution

Hydrogen energy is an ideal new green energy with rich resources,wide sources,zero pollution,no geographical restrictions,various forms of existence and application,low development cost,high combustion heat capacity,good combustion performance,clean,envir onmentally friendly,and renewable.However,there are many defects in the storage and transportation of hydrogen energy in China.Mg-Ni（Re-Mg-Ni）alloy has the advantages of high hydrogen storage capacity and density,small volume,strong safety and convenient storage and transportation.Large-scale long-distance safe transportation can be realized by conventional means such as cars,trucks,cargo ships,etc.In this thesis,amorphous Mg-Nihydrogen storage alloy coating was prepared by constant current induced co-deposition with soluble nickel plate as anode and 99.9%pure copper plate as cathode in NiCl ₂-MgCl₂ acidic aqueous solution system.Base d on this process,CeCl ₃ was added to prepare Ce-Mg-Nialloy coating,which effectively improved the hydrogen absorption and desorption kinetics and corrosion resistance of the alloy and prolonged the service life of the alloy.Different electrodeposition process conditions will affect the material content ratio,microstructure,and current efficiency of hydrogen storage alloy coating.The electrochemical reaction mechanism of nickel ion,magnesium ion and cerium ion reduction process in different acidic baths was studied by electrochemical three electrode system.The results show that:（1）In the acidic aqueous solution of NiCl ₂-MgCl₂,using nickel plate as anode and copper plate as cathode,the effects of electrodeposition process conditions on the material content,co-deposition current efficiency and hydrogen evolution kinetics of magnesium-nickel alloy coating were studied by single variable method.The optimum process conditions are as follows:current density 45 A/dm²,the electrodeposition temperature is 70°C,the nickel ion concentration in the bath is 0.05 mol/L,the magnesium ion concentration is 0.1mol/L,the boric acid concentration is 37 g/L,and the deposition time is 15 min.The current efficiency is up to 61.27%,and the Mgcontent is up to 2 0 wt%.The main diffraction peak of XRD is amorphous Mg₂Niphase alloy.The co-deposition process is that Ni²⁺is first reduced and attached to the surface of the copper substrate,and then magnesium and nickel are co-deposited to form a nodular magnesium-nickel alloy.The hydrogen evolution reaction kinetics test in 1 mol/L Na OH solution showed that the Tafel hydrogen evolution potential was-0.553 V,the exchange current density j⁰ was 1.73×10^-5 A/cm²,the Tafel slope was 210.96 m V/dec,and the charge tra nsfer resistance R_ct was 12.8Ω/cm²,indicating that the alloy coating had good hydrogen evolution and storage performance.（2）Electrochemical tests were carried out in a three-electrode system of NiCl2-basic plating solution（NiCl ₂-MgCl₂-basic plating solution）.The pure copper substrate was used as working electrode and the saturated calomel electrode（SCE）was used as the reference electrode.Cyclic voltammetry analysis showed that the reduction potential of nickel is-0.967 V,and the co-deposition potential of magnesium and nickel is-1.299 V.The reduction reaction process of Ni²⁺and Mg²⁺is a one-step two-electron irreversible process controlled by diffusion.The average charge transfer coefficient of Ni²⁺isα=0.031,and the average charge transfer coefficient of Mg²⁺isα=0.023.The diffusion coefficients are D_Ni2+=4.314×10^-7 cm2·s-1、D_Mg2+=1.930×10^-12 cm2·s-1。（3）On the basis of the optimal conditions for the preparation of Mg-Nialloy,Ceelement was added to induce the co-deposition of Ce-Mg-Nihydrogen storage alloy coating.The optimum process conditions were obtained as follows:current density 7.5 A/dm²,electrodeposition temperature 30°C,cerium ion concentration 0.01 mol/L,deposition time 60 min.At this time,the content of cerium is 25 wt%and the content of magnesium is 16 wt%.With the addition of Ceelement,the microstructure of the alloy changes from blocky and nodular to nearly spherical and partially elongated,and the grain refinement is significant.The main alloy phase in the Ce-Mg-Nihydrogen storage alloy coating is CeMg₂Ni₉.The Tafel hydrogen evolution potential is-0.293 V,j⁰ is7.35×10^-4 A/cm²,Tafel slope is 101.74 m V/dec,R_ct=2.58Ω/cm ²,which is significantly higher than that of Mg-Nialloy.In the potentiodynamic polarization curve,the Ce-Mg-Nialloy material has an obvious corrosion passivation zone,and the self-corrosion potential is positively shifted by 0.387V compared with the Mg-Nialloy,and the self-corrosion current density is reduced by an order of magnitude to 1.387×10^-7 A/cm²,which has better corrosion resistance.（4）Three-electrode electrochemical test was carried out in NiCl ₂-MgCl₂-CeCl₃-basic plating solution system.Cyclic voltammetry analysis showed that the reduction potential of cerium-magnesium-nickel induced co-deposition is-1.155 V,which is 0.144 V higher than that of magnesium-nickel co-deposition.The reduction reaction process of Ce³⁺is a one-step three-electron irreversible process controlled by diffusion.The red uction mechanism is Ce³⁺+3e^-→Ce,the average charge transfer coefficient of Ce³⁺is 0.017,and the diffusion coefficient is 7.310×10^-11 cm²·s^-1.