Thermodynamics Reaserch Of The Hydriding And Dehydriding Reaction In Vanadium-Based Solid Solution Hydrogen Storage Alloys V₃TiNi_0.56M_x (M=Al, Cr)

Vanadium-based solid solution hydrogen storage alloys have been attractive for their potential application in the negative electrode materials of Ni-MH battery because of their hight hydrogen storage,excellent diffusion rate in Vanadium-based hydride and hydrogen absorption-desorption at room temperature.Numerous research works on the properties of Vanadium-based solid solution hydrogen storage alloys such as crystal structure,hydrogen absorption/desorption behaviour and thermodynamics of dissolved hydrogen have been studied widely both experimentally and theoretically at present.however, the quantitative explanation for the phenomena and laws during hydrogenation process is still a important and hardship problem. therefore,the influence of V3TiNi0.56Mx(M=Al,Cr;x=0.1,0.3) alloys microstructure on enthalpy of formation during hydrogenation process were studied. The thermodynamics equilibrium process during hydrogenation reaction was researched with electrochemical method and calculate their thermodynamic function change value. A mathematical model of enthalpy of formation for V-based hydrogen storage Alloys was established and tested in this paper. The results show that:(1) V3TiNi0.56Alx(x=0.1,0.3) alloys consisit of Vanadium-based solid solution main phase,TiNi phase and AlV3 phase,which are precipitated and formated a three-dimensional network along grain boundary of Vanadium-based solid solution main phase and Al element distribute mainly in grain boundary; but V3TiNi0.56Crx(x=0.1,0.3) consisit of Vanadium-based solid solution main phase,TiNi phase and Cr element also distribute mainly in grain boundary.(2) The results show that the equilibrium hydrogen pressure is more lower with the unit cell volume increased.With the equilibrium hydrogen pressure descreased,the enthalpy of formation is more lower ,and the hydrides more stable for V3TiNi0.56Mx(M=Al,Cr;x=0.1,0.3) system because the hydrogen atom is more easily diffused into the interstitial space of crystals.(3) The enthalpy of formation of V3TiNi0.56Al0.1,V3TiNi0.56Al0.3,V3TiNi0.56Cr0.1,V3TiNi0.56Cr0.3 were tested during hydrogen absorption/desorption Period using electrochemical method at 298K. The testing process as follows: The potential of them were tested during hydrogenation reaction Period,then plotting the P-C-T cuvers of electrode Alloys according to the equilibrium hydrogen pressure which be calculation by the nernst equation.and then experimental data were calculated by using Van't Hoff equation. the measurement results were -39.38 KJ.mol-1H2,-41.54 KJ.mol-1H2,-41.11 KJ.mol-1H2,-40.72 KJ.mol-1H2。At the same time, the enthalpy of formation of them were -34.13 KJ.mol-1H2 , -37.05 KJ.mol-1H2 , -42.02 KJ.mol-1H2 , -40.2 KJ.mol-1H2,which be calculated by the semi-empirical mathematical model. comparison with the experimental data, which basically the same.the result show that the semi-empirical mathematical model of enthalpy of formation can be applied to composition design for new types hydrogen storage alloys in theory.(4) the detail different for enthalpy of formation between the electrochemical process of charging-discharging and hydrogen absorption-desorption in solid-gas reaction were obtained.But, in essence, the hydrogen storage alloy enthalpy of formation througt electrochemical measurements and by solid-gas sievert method is exactly the same. It fully shows that enthalpy of formation of electrode alloys and performance of the hydrogen storage alloys have a inevitable relationship which can be used for characterization of electrochemical properties of hydrogen storage alloys.A hermal-electrochemical equation of a closed cell system: was obtained theoretically. It relation the the standard enthalpy during the charging-discharging period with the battery system's reaction heat, terminal voltage and internal resistance of the battery.So that,it can easily to characterization the internal relation betweent thermodynamic properties and electrochemical properties of hydrogen storage materials.Activation capacity of hydrogen storage electrode is mainly influented by their internal resistance and enthalpy changes values during hydrogen absorption-desorption process, the lower internal resistance of the battery and enthalpy change values, the activation performance more excellent.(5) The relation between formation enthalpy in dehydriding for Vanadium-based solid solution hydrogen storage alloys and the atomic parameters can be express as a semi-empirical mathematical model :Calculated values using stepwise regression method show a fairly good agreement with those of the experiment values with maximum deviation in heat of formation of hydrides about 5 kJ/mol. It pointed out that the main factors of the order of significance which influence on thermodynamic properties of V-based hydrogen storage alloys,is electron concentration factor((e/a)2/3) >electrongativaity difference factor(ΔX2)>atomic size factor(δ2). Electrongativaity difference factor, atomic size factor and electron concentration factor of V3TiNi0.56Mx alloys are all decreased with the contents of addition Al element increased.The enthalpy of formation of alloys decreased due to equilibrium hydrogen pressure droped. The lower electronegativity difference, the lower electron concentration and the larger atomic size with the increased of Cr content,which tend to make the equilibrium hydrogen pressure hinghter and the enthalpy of formation lower ,and the hydrides more stable for v-based solid solution alloys.