Influence Of CO₂ Impurity On Hydrogen Storage Performance Of Magnesium

Magnesium-based hydrogen storage materials are considered as one of the most promising hydrogen storage materials because of their high hydrogen capacity,abundant resources,low price and low density.There are many kinds of impurities in hydrogen,and all kinds of impurities can affect the hydrogen storage performance of magnesium in varying degrees.CO2 impurity as one of the most common impurity gases in industrial hydrogen are selected.In this work,the hydrogen absorption properties of the Mg was determined by using the a high-pressure and-temperature gas-absorption analyzer and differential scanning calorimeter when hydrogen containing different concentration of CO2 impurity are employed as gas source.X-ray diffraction,scanning electron microscopy,transmission electron microscopy and X-ray photoelectron spectroscopy characterization techniques were used in this research.The effect of CO2 impurity gas on hydrogen storage performance of magnesium was studied analyzing results from structure morphology and DFT simulation calculations..Under incremental pressure hydrogenation mode,when hydrogen containing 0.1 mol%CO2 impurity is used as the gas source,the hydrogen adsorption capacity of 4.59 wt.%with an increased hydrogen adsorption plateau pressure,whiles a hydrogen capacity of 7.04wt.%is recorded when pure hydrogen is used as gas source.When hydrogen containing 1.0 mol%CO2 impurity is used,a further reduction of the hydrogen adsorption platform is observed and the hydrogen absorption capacity is reduced to 1.09 wt.%.Under isobaric hydrogenation mode,the effect of CO2 impurity was not significant during the initial stage of hydrogenation,as the reaction proceeds further,the hydrogen absorption rate decreases,and the hydrogen absorption capcity of Mg decreases from 7.29wt.%to 5.57wt.%within 5 h when hydrogen containing 0.1 mol%is used as the gas source.When CO2 impurity reaches 1.0 mol%,the hydrogen absorption kinetics changes obviously,with descreasing absorption rate.With the increase of time,the amount of hydrogen absorbed in 5 hours is only 2.12 wt.%.DSC tests were carried out directly on the samples of isobaric hydrogenation mode.Kissinger formula was used to calculate the activation energies of hydrogen desorption in pure hydrogen and hydrogen containing 0.1 mol%and 1.0 mol%CO2 impurity gas,which were 137.89 kJ/mol,140.79 kJ/mol and 158.74 kJ/mol,respectively.The XRD characterization shows that CO2 reacts with Mg and results in crystal MgO in both hydrogenation modes,and the HRTEM observation shows the MgO film tightly adheres to the surface of Mg particles.XPS analysis shows that besides MgO,carbon is formed,and there are C-O and C=O groups chemisorbed on the surface of Mg particles.The thermodynamics calculation reveals that in incremental hydrogenation mode,CO2 reacts with Mg to form dense MgO film at pressure below the hydrogen plateau,which prevents Mg from reacting with H2 to form MgH2 at high pressure.In contrast,in isobaric hydrogenation mode,CO2 and H2 competitively react with Mg when the initial hydrogen pressure is higher than plateau pressure,and it results in higher hydrogen capacity than that in incremental pressure mode.The hydrogen absorption curves of magnesium show that the hydrogen absorption kinetics of Mg decreases with the increase of CO2 content and cycle times.When hydrogen containing 0.1 mol%CO2,the hydrogen capacity of Mg in 60 minutes decreased to 2.3 wt.%after 20 cycles.After 20 cycles,Mg no longer absorbed hydrogen when the impurity content in hydrogen increased to 1.0 mol%.In the isothermal dynamic cycling test,the passivation layer on the surface of the material becomes dense with the increase of concentration and cycling times,which hinders the dissociation and diffusion of hydrogen on the surface of Mg and reduces its dynamic performance.The most stable adsorption site of CO2 molecule on Mg(001)surface is bridge site,and the adsorption energy is 0.06 eV,which is physical adsorption.The adsorption energy of CO2 on MgH2 surface is only 0.01 eV,and on MgO(100)surface is 0.48 eV,which is chemical adsorption.The distance between O atom in CO2 molecule and MgO(100)surface is 1.46 A,indicating that new chemical bonds can be formed between CO2 and MgO(100)surface.Based on the experimental results and simulation analysis,both MgO and MgH2 are formed on the surface of Mg during the initial cycle.The formation of MgO film hinders the reaction between Mg and H2,while the existence of MgH2 provides a channel for the diffusion of H.Because CO2 is easier to chemisorb on MgO surface than on Mg surface,CO2 molecules adsorb on MgO surface and react with Mg to form MgO through overflow reaction,which further reduces the hydrogenation channels.The existence of CO2 molecule makes the MgO content increase with the formation of new surface,while the corresponding MgH2 production decreases,that is,the hydrogen absorption decreases.With the increase of CO2 concentration in hydrogen,the shielding effect of MgO becomes more serious.