Study On Hydrogen Storage Mechanism Of Coupling Between Magnesium And Carbon

Hydrogen, as a clean and superior energy source, still has many major issues about hydrogen storage technology which need to be addressed in hydrogen application nowadays. Magnesium is a promising candidate for hydrogen storage because of its high hydrogen capacity, low cost and wide resource. However, the sluggish kinetics and stable thermodynamic property limit its application. Carbon materials attract the attention of researchers because of its high hydrogen capacity and gases insusceptible.The hydrogen storage properties of magnesium and carbon materials can be improved after they milled together. In this work, the coupling mechanism between magnesium and carbon was studied by experiment and first-principles calculation.Crystalline carbon was achieved by demineralization and carbonization for anthracite. Then it was milled with magnesium to prepare magnesium-carbon composite. Scanning electron microscope (SEM),transmission electron microscope (TEM), X-ray diffraction (XRD) and Fourier transform infrared spectrometer (FT-IR) were used to analyze the morphologies, structures and hydrogen storage properties of magnesium,crystalline carbon and magnesium-carbon composite. Experimental results show that magnesium powder can hardly store hydrogen under 2.0 MPa H2 atmosphere at 420 ? in 10 h,and crystalline carbon can't store hydrogen under 2.0 MPa H2 atmosphere at 360 ? either. Whereas, magnesium and crystalline carbon can store amount of hydrogen during ball milling in H2 atmosphere. After milled for 3 h, the hydrogen storage amount of magnesium is 5.5 wt%, and many C-H bonds appears in crystalline carbon at the same time. This means the coupling between magnesium and carbon improves their hydrogen storage properties. Furthermore, the activation energy of magnesium-carbon composite is lower than that of bulk magnesium hydride, which means the hydrogen desorption performance of composite is also enhanced.The influence of vacancy, element doping and line defect on hydrogen adsorption, dissociation and diffusion were researched by first-principles calculation to study the coupling mechanism between magnesium and carbon. Calculation results show that, the introduction of vacancy is against for the H2 adsorption performance on Mg(0001) plane and graphite(0001)plane. However, the barrier energies of H2 dissociation on these two plane are reduced with the appearance of vacancy. At the same time, the barrier energy of H2 diffusion on Mg(0001) plane will reduce to 26.56 kJ/mol and will increase to 294.54 kJ/mol when it comes to graphite(0001) plane.Element doping also against the H2 adsorption performance of Mg(0001)plane and graphite(0001) plane. Nevertheless, the performance of H2 dissociation on Mg(0001) and graphite(0001) plane with element doping is improved. The slipped Mg(0001) plane with C doping will against the H2 adsorption performance but will improve the H2 dissociation performance.The barrier energy of H2 dissociation on slipped Mg(0001) plane with C doping is reduced to 92.73 kJ/mol.The coupling mechanism of hydrogen storage between magnesium and carbon can be described as follows. On the crystalline carbon surface,the vacancy defect structure can be formed with C atom split off. The C atom from crystalline carbon comes into the crystal structure of magnesium and form C-doping structure. The changes of crystalline carbon and magnesium structure can improve their hydrogen storage properties. At the same time, with the splitting off of Mg atom, the vacancy defect structure can form in magnesium and Mg-doped structure can form in crystalline carbon. These changes of crystalline carbon and magnesium structure can make the transfer of H atoms from magnesium to crystalline carbon much easier than before.