The Hydriding And Dehydriding Kinetics Study Of La-Ni Series Alloys Based On The Modified-varying-size Model

La-Ni series metal hydrides have become a widely studied hydrogen storage material due to the advantages of mild hydrogen absorption and desorption conditions,fast kinetic rate,and easy activation.However,the radius and density of La-Ni-based metal hydrides particles changed non-linearly with reaction fraction during hydrogenation and dehydrogenation,which results in a large deviation between the experimental data and existing kinetics models.Therefore the nonlinear modified varying-size model?M-VSM?was proposed based on varying-size model?VSM?,which modified the particles radius and density nonlinearly to calibrate the kinetics equations precisely.Both differential and integral kinetics equations of the three rate-controlling steps have been deduced during hydrogen absorption and desorption.In this paper,the hydriding and dehydriding kinetics properties of four La-Ni alloys(La Ni₅,La Ni_4.5Al_0.5,La Ni_4.5Fe_0.5 and La Ni₄Fe_0.5Co_0.5)were investigated.The conditions of hydriding kinetics experiments were 20?,40?,60?under 0.4 MPa,0.8 MPa and 1.2MPa,respectively.The dehydrding kinetics experiments were carried out under various working temperatures of 90?,110?,130?,150?and barometric pressure.By comparing the effects of different temperatures and pressures on the kinetics of hydrogenation and dehydrogenation,it is concluded that the reaction rate and hydrogen storage capacity perform excellent under the experimental conditions of low temperature and high pressure.In the dehydriding reaction,the high temperature is obviously beneficial to the reaction.We compared the kinetics of hydrogen absorption and desorption of La-Ni-based alloys doped with Al,Fe,and Co metal elements,it is found that La Ni_4.5Fe_0.5has outstanding hydrogen absorption and desorption performance,and has relatively small restrictions on temperature and pressure,which could be applied in many fields widely.The experimental data was fitted with M-VSM,and the rate-controlling steps of the hydrogenation and dehydrogenation were both internal diffusion of hydrogen atoms.The model accuracy of M-VSM was investigated and compared with other conventional models,and the results indicated that M-VSM owned the highest model precision than any other models including varying-size model,shrinking-core model,Johnson-Mehl-Avrami model and Jander diffusion model.Furthermore,the simplified hydriding/dehydriding equations were also derived,whose correlation coefficient R² could reach 0.98 and 0.99,respectively.The simplified M-VSM kinetics models were applied to La Ni_4.5Al_0.5,La Ni_4.5Fe_0.5,and La Ni_4.0Fe_0.5Co_0.5.It was found that the experimental data fit the model well,indicating that M-VSM is still applicable for the metal hydrides with little amount of other elements.Therefore,M-VSM has high accuracy and application convenience,and will show its huge potential applications in the future.