Study On Hydrogen Storage Properties And Mechanism Of Ti Based Multi-principal Element Alloys

The rapid development of human society has led to a rapidly increasing demand of energy sources.The overuse of fossil fuels has resulted in kinds of environmental problems.Therefore,development of clean energy with high energy density to replace fossil fuels is an alternative solution to alleviate energy crisis and environmental pollution.Hydrogen energy has received great attention because of its high energy density and renewability.Hydrogen storage technology with high safety and efficiency is a key component for efficient utilization of hydrogen energy.Rare-earth series alloys,Ti-based alloys,V-based solid solution alloys,Mg-based alloys are the most investigated hydrogen storage materials.However,these materials suffer from high desorption temperature,sluggish kinetics as well as low hydrogen storage capacity.Their practical application remains limited.Therefore,it is necessary to develop materials with enhanced hydrogen storage properties.Recently,multi-principal element alloys（MPEAs）have been regarded as promising hydrogen storage materials due to their high hydrogen storage capacity.Ti-based MPEAs with high hydrogen storage capacity and fast kinetics are the most investigated MPEAs for hydrogen storage.However,there still lacks understanding about the hydrogen storage mechanism of MPEAs.In this study,a series of MPEAs with different element compositions are designed,synthesized and investigated.The effects of lattice parameters,mean enthalpy of hydrogen solution,mixing entropy,and microstructure etc.,on the hydrogen storage properties are revealed.The main works and results are displayed below.1.A series of body-centered cubic（BCC）Ti-based MPEAs are synthesized.The effects of mean enthalpy of hydrogen solution,valence electron concentration（VEC）,lattice parameters and mixing entropy etc.,on hydrogen storage properties are investigated.The results show that lattice parameters and mean enthalpy of hydrogen solution influence the hydrogen storage capacities.Mean enthalpy of hydrogen solution plays key role in the hydrogen storage capacities of MPEAs.The higher the mean enthalpy of hydrogen solution,the lower the hydrogen storage capacities.Besides,mixing entropy of MPEAs is found to have significant influences on the hydrogen diffusion rate.High mixing entropy will lead to low hydrogen diffusion rate.In addition,the thermal stability of MPEA hydrides is influenced by VEC.2.The hydrogen storage properties of Ti Zr Hf_（1-x）Ta Nb Al_x MPEAs are investigated.The results show that the hydrogen storage capacities of Ti Zr Hf_（1-x）Ta Nb Al_x decrease with the increasing Al content.The hydrogen absorption kinetics is also reduced with increasing Al content.The decreased hydrogen storage capacity and decreased hydrogen diffusion rate are mainly caused by the enhanced element fluctuation in nanometer scale and micrometer scale.In addition,the distribution of hydrogen is influenced by other metal elements,which is primarily affected by the mean enthalpy of hydrogen solution and distribution of charge density.3.Noble metals,such as platinum,palladium and gold,are doped in TiZrHfMoNb to modify its hydrogen storage properties.It turns out that the activation performance and hydrogen storage capacities are improved after platinum,palladium and gold doping.Noble metals doping is believed to aid in destabilizing the oxide on the surface of MPEA,and it results in the improved activation performance.In addition,direct imaging of interstitial hydrogen atoms in MPEAs using a recently developed integrated differential phase contrast（i DPC）technique with an aberration-corrected scanning transmission electron microscope is achieved.The results show that hydrogen occupancy in TiZrHfMoNb MPEA is changed by gold doping.Hydrogen atoms only occupy tetrahedral interstitial sites in TiZrHfMoNb hydride.However,hydrogen atoms occupy both tetrahedral and octahedral interstitial sites in TiZrHfMoNb Au_0.0025 hydride.This may be resulted from the enhanced element fluctuations in atomic scale caused by gold doping.4.The thermal stability and structure of TiZrHfMoNb MPEA hydrides with different hydrogen content are investigated.The occupation of deuterium atoms in the MPEA deuterides with different deuterium content are determined by neutron powder diffraction（NPD）.The as-cast MPEA has a BCC structure.As the hydrogen/deuterium content increases,the introduction of hydrogen/deuterium atoms induces cell volume expansion.After the MPEA is completely hydrogenated,the MPEA hydride transforms into a single face-centered cubic（FCC）structure.Besides,the thermal stability of MPEA hydrides decreases with the increasing hydrogen content.More important,the heat for MPEA hydrides desorption decreases linearly with the increasing cell volume in BCC structure.It is indicated that the cell expansion caused by the introduction of hydrogen atoms plays key role in the thermal stability of MPEA hydrides.The NPD results show that deuterium atoms prefer to occupy both tetrahedral and octahedral interstitial sites,and tetrahedral interstitial sites are dominated in the BCC structure.In addition,all the deuterium atoms occupy tetrahedral interstitial sites in the FCC structure.