Preparation And Thermal Conductivity On Working Condition Of Magnesium-based Hydrogen Storage Material

The problem of lacking safe and efficient hydrogen storage and transportation technology was one of the key factors restricting the large-scale commercial application of hydrogen energy.High-capacity hydrogen storage materials were the effective means to solve this problem.Mg-based hydrogen storage materials have become one of the most attractive hydrogen storage materials due to its high hydrogen storage capacity,abundant resources,and low price.However,due to the high enthalpy of the hydrogen absorption and desorption reaction of the Mg-based hydrogen storage material and the poor thermal conductivity of the powder,it is urgent to improve the heat and mass transfer performance of the bed to meet the application needs.At present,the calculation models for the heat and mass transfer performance of Mg-based hydrogen storage materials simplified the effective thermal conductivity of the Mg-based hydrogen storage material bed to a constant and ignore the significant changes under the working conditions(hydrogen pressure,temperature,hydrogen content),resulting in deviation between the calculated value and the actual results under application conditions,which affects the accuracy of the model calculation.Therefore,there was an urgent need to systematically study the effective thermal conductivity of the Mg-based hydrogen storage material beds with working condition parameters,such as hydrogen pressure,temperature and hydrogen content to provide key data support for the optimization of heat and mass transfer in the system.Based on the above problems,we first optimized the Mg/Ti-Cr-V hydrogen storage composites with high hydrogen storage capacity and good dynamic performance through mechanical ball milling as the research object.X-ray diffraction(XRD),scanning electron microscopy(SEM)and laser particle size analysis method were used to characterize the phase composition,microscopic morphology and particle size distribution of the composites.The Sievert’s volume method was applied to determine the hydrogen absorption/desorption kinetic properties of the composites.The hydrogenated Mg/Ti-Cr-V hydrogen storage composites(MH)was mixed with expanded graphite(EG)in a certain proportion,and the MH/EG compact was prepared by uniaxial die pressing method to explore the effect of expanded graphite addition and molding pressure to the performance of MH/EG compact.The dynamic thermal conductivity test platform was built used the transient plane heat source method,and studied the influence of working conditions on the thermal conductivity of the MH/EG compact,such as hydrogen pressure,temperature,and hydrogen content.The empirical formula for the relationship between the thermal conductivity of the MH/EG compact with the working conditions such as temperature,hydrogen pressure and hydrogen content were obtained and discussed the corresponding mechanism,which provided important basic data for the design and calculation of the heat and mass transfer performance of solid-state hydrogen storage devices.The in-situ hydrogenation reaction ball milling method was used to prepare the Mg-x wt%Ti0.16Cr0.24V0.6 hydrogen storage composites(x=0,3,5,10).The results show that the Mg-3 wt%Ti0.16Cr0.24V0.6 hydrogen storage composites has the most excellent hydrogen storage capacity and better dynamic performance.At 300℃,the hydrogen desorption capacity under 0.1 MPa is 7.08 wt%within 80 min,and the hydrogen absorption capacity under 2 MPa is 6.95 wt%under 30 min.The 200 g per batch preparation of Mg-3 wt.%Ti0.16Cr0.24V0.6 hydrogen storage composites was realized through the scale-up experiment.The optimized ball milling process conditions are:the ball-to-material ratio is 20:1,the number ratio of φ10 and φ7 stainless steel balls is 1:3,forward ball milling is 30 minutes,pause for 5 minutes,and reverse ball milling for 30 minutes,and the speed is 500 r/min.The hydrogen storage composites prepared in batches have good kinetic properties and the consistency of phase,particle size and microscopic morphology.In order to explore the influence of the preparation on the Mg-based hydrogen storage composite compact,the MH/EG compact are prepared by uniaxial die pressing method under a pressure of 273 MPa with expanded graphite addition of 0 wt%,5 wt%,10 wt%,15 wt%,20 wt%firstly.Then fixed expanded graphite addition of 10 wt%under pressures of 39 MPa,117 MPa,195 MPa,273 MPa,and 351 MPa to prepare the MH/EG compact.The results show that increasing the addition of expanded graphite will lead to the more obvious grid structure in the MH/EG compact,which will increase the thermal conductivity and apparent density of the MH/EG compact,decrease the hydrogen permeability and mass hydrogen storage capacity,and has no effect on the porosity.The MH/EG compact prepared by adding 10 wt%expanded graphite has ideal thermal conductivity and hydrogen absorption/desorption rate while sacrificing part of the hydrogen storage capacity.Increasing the molding pressure will cause the MH/EG compact to have higher compaction density and more obvious lamellar tendency of expand the graphite layer flakes,which will increase the thermal conductivity and apparent density of the MH/EG compact,decrease the porosity and hydrogen permeability,and have no significant effect on the mass hydrogen storage capacity and kinetics.The compact prepared under 351 MPa has the highest thermal conductivity based on the hydrogen desorption/absorption speed.The Mg-3 wt%Ti0.16Cr0.24V0.6/10 wt%EG(MH/EG)compact is selected as the research object,and systematically measure the thermal conductivity of the MH/EG compact under different atmospheres,hydrogen pressure,temperature and hydrogen content.The results show that the MH/EG compact has higher thermal conductivity under hydrogen atmosphere,and the thermal conductivity shows logarithmic increasing trend with increasing hydrogen pressure.This is because when the temperature and gas type was fixed,as the pressure gradually increases,the mean free path of hydrogen molecules begins to decrease,and collisions between gas molecules intensify.The energy exchange between particles is proportional to the concentration of gas molecules and the thermal conductivity of the filling gas.The thermal conductivity of MH/EG compact shows linear decreasing trend with increasing temperature.This is because as the temperature increases,the average number of phonons in the MgH2 crystal lattice increases,the collision frequency between phonons increases,and the free path decreases.As a result,the thermal conductivity of MgH2 crystals decreases with increasing temperature.The thermal conductivity of the MH/EG compact shows "S" shaped trend with hydrogen content when desorption.After the MH/EG compact starts to desorption,the thermal conductivity first increases slightly.When the amount of hydrogen desorption reaches about 3 wt%,the thermal conductivity increases significantly.When the hydrogen desorption ends,the thermal conductivity is basically unchanged.At the same time,the thermal conductivity of the MH/EG compact decreases with the increase of temperature under same hydrogen content.This is because the MgH2 particles in the MH/EG compact gradually transform into metallic Mg particles when hydrogen desorption.Mg in the form of free electron heat conduction has higher thermal conductivity than MgH2 in the form of lattice heat transfer,and the metal Mg particles may be locally sintered at high temperatures.The fitted empirical formula of the thermal conductivity of the MH/EG compact with temperature,hydrogen pressure and hydrogen content is obtained,which will provide important basic data for the design and calculation of the heat and mass transfer performance of the solid-state hydrogen storage device.The in-situ thermal conductivity test of the high-capacity Mg-based compact will provide important reference for future research.