Investigation On The Desorption Thermodynamic Performance And Hydrogen Diffusion Behavior Of ZrCo-based Hydrogen Storage System

The massive consumption of fossil fuels causes serious environmental pollution,and thus it has become the consensus of human society to develop clean energy.Hydrogen energy is a kind of renewable energy with the advantages of high combustion calorific value,abundant reserves.Therefore,hydrogen energy attracts a lot of attention from domestic and international research.However,the safe and efficient storage of hydrogen is a difficult problem which prevents large-scale application of hydrogen energy.As a typical Zr-based hydrogen storage material,ZrCo alloy possesses excellent hydrogen storage performance,such as a mass storage fraction of 1.96 wt%and excellent kinetics of hydrogen absorption and desorption.However,the hydrogen induced disproportionation reaction is prone to occur under high temperature and pressure in ZrCo alloy.As a result,disproportionation causes the capacity attenuation and deteriorated cycling performances.It was found that the disproportionation reaction was related to interstitial site in the hydride.In other words,the hydrogen atoms tended to form disproportionation products when they occupied unstable interstitial sites rather than diffusing outward.In this study,the dehydrogenation performance and anti-disproportionation ability of ZrCo alloy were investigated from both thermodynamics and hydrogen diffusion behavior.Firstly,the most stable structures of the intermediate phase ZrCoH_x（x=2,2.25,2.50,and 2.75）were predicted by first-principles calculations.It was discovered that the hydrogen atoms occupying the 8f₁ interstitial site were preferentially released during the desorption process.As the hydrogen content in ZrCoH_x decreased,the enthalpy of formation increased,implying a decrease in thermodynamic stability.Then,based on the structure of the ZrCoH_x,the two-step dehydrogenation（ZrCoH₃→ZrCoH_xand ZrCoH_x→ZrCo）was investigated.The enthalpy change,entropy change and equilibrium pressure of each reaction were obtained by thermodynamic relations.The enthalpy changes of the first step reaction appeared to decrease as the hydrogen content in ZrCoH_x decreased,but the opposite trend was observed for the second step reaction.This was due to the fact that the first step reaction involved shrinkage and slight deformation of hydride cells,while the second step reaction underwent the lattice restructuring from orthorhombic to cubic.In addition,ZrCoH₃→ZrCoH₂ possessed the higher equilibrium pressure and exhibited excellent dehydrogenation performance compared with other reactions.Finally,the effects of applied strain on the structural properties and dehydrogenation thermodynamics of hydrides were investigated.After the biaxial compression strain was applied to hydride,the lattice stability of ZrCoH_xdeteriorated,but it decreased rapidly when the strain exceeded-1.5%.Meanwhile,the compressive strain reduced the enthalpy change of dehydrogenation and improved the equilibrium pressure,which enhanced the hydrogen release capacity.The dehydrogenation process of ZrCo hydride was a typical gas-solid reaction,and the diffusion occurring at the atomic scale limited the overall reaction rate.Therefore,the occupancy and diffusion behavior of hydrogen atom in ZrCoH₃ was deeply investigated in this study.The analysis of hydrogen vacancy formation energy indicated that the hydrogen atom occupying the 8f₁ interstitial site was released easily,then a vacancy formed.Afterwards,the effect of Ti substitution and the applied strain on the diffusion of hydrogen atom along 8e interstitial site to the 8f₁ and 4c₂ interstitial sites was analyzed.The results demonstrated that the Ti substitution for Zr and the applied compressive strain favored the reduction of the diffusion barrier of hydrogen atom,promoting the migration of hydrogen atom from the 8e interstitial site to the stable 8f₁ and 4c₂ interstitial site.Consequently,this avoided the hydrogen atom remaining in the 8e interstitial site and participating in the disproportionation to form ZrH₂.In addition,when the Ti content was increased to 15%and the strain with a magnitude of-3%,the dehydrogenation kinetics of the hydride obtained the best improvement.The mechanism of Ti element substitution as well as strain on hydrogen diffusion was further investigated by electronic properties.The results of density of states indicated that Ti substitution and compressive strain weakened the lattice stability and covalency of hydrides,but promoted the formation of 8f₁ and 4c₂ vacancies.On the other hand,the analysis of the local chemical bonding properties of the 8e interstitial site revealed that the bond strength between metal atom and hydrogen atom was weakened after Ti element substitution and the application of compressive strain,facilitating the reduction of the hydrogen diffusion barrier along the paths 8e→4c₂ and8e→8f₁.