Synthesis And Hydrogen Storage Properties Of Mg-Based Ultralfine Composite Materials

Mg-based alloys is one of the most promising hydrogen storagematerials owing to its lightweight, great abundance, low cost and highhydrogen storage capacity as compared with other hydrogen storage materials.However, the difficulty in preparation reduces the possibility of its practicalapplications as well as poor hydrogen absorption and desorption properties.In this paper, ultrafine Mg powders were prepared by DC arc plasma inH₂+Ar atmospheres using Mg blocks, while Mg-Nd ultrafine compositematerials were prepared in Ar atmospheres using Mg-Nd alloys. Mg-Ni,Mg-Fe and Mg-Ti ultrafine composite materials were synthesized by DC arcplasma in H₂+Ar atmospheres using Mg powders cold pressing with Nipowders , Mg powders cold pressing with Fe powders, Mg powders coldpressing with Ti powders. The phase component, morphology, size,composition and hydrogen sorption properties of the prepared powders wereanalyzed by using X-ray diffraction (XRD), transmission electronmicroscopy (TEM), inductive coupled plasma emission spectrometer(ICP),pressure-composition- temperature (PCT) and thermogravimetry / differentialscanning calorimetry spectroscopy (TG/DSC) techniques.It is revealed that most of the particles in the ultrafine composite materials are hexagonal in shape with particle size in the range of50nm-700nm. In the Mg-Ni ultrafine composite materials, Mg2Ni particlesattached to the surface of Mg particles with their size ranging from 10nm to50nm. This special microstructure of Mg-Ni composites will help improvingthe performance of hydrogen sorption properties. There wereα-Fe andγ-Fephases in Mg-Fe ultrafine composite materials, but only existedα-Fe in thecomposites after hydrogen absorption, indicating thatγ-Fe was trasformedinto Mg₂FeH₆. X-ray diffraction(XRD) and selected area electron diffraction(SAED) showed MgO existing in the ultrafine composite materials, and theMgO layer at the surface of the magnesium particles can effectively preventthe further oxidation or even burning of the Mg particles when exposed to air.Hydrogen storage test showed that the hydrogen absorption anddesorption platforms of magnesium-based ultrafine composite materials werestable, the plateau pressures had little hysteresis, the plateau pressure andhydrogen content increased with increasing temperature. The PCT curvesMg-Ni and Mg-Fe ultrafine composite materials presented low and highplatforms, and in low platform area of Mg-Ni composites, MgH₂ andMg2NiH4 were generated during hydrogen absorption. The maximumhydrogen absorption capacity of pure Mg at 400℃reached 6.14wt%. Thehydrogenation enthalpy of pure Mg powders was calculated to be -78.6kJ/mol H₂ from the PCT curves of hydrogen absorption plateau pressureaccording to the Van't Hoff equation. The hydrogenation enthalpy of Mg-Ni,Mg-Nd,Mg-Fe and Mg-Ti ultrafine composite materials were more positivethan that of pure Mg powders, indicating that the addition of Ni, Nd, Fe or Tican effectively improve the hydrogen sorption thermodynamic properties ofMg. DSC results showed that the dehydriding endothermic peak temperatureof pure Mg hydride was 447.5℃, while the two peaks of Mg-Ni hydride were370℃and 395℃, respectively, and the peak of Mg-Nd, Mg-Fe or Mg-Tihydrides were all lower than that of Mg hydride. These results proved that thedesorption temperature of MgH₂ can be reduced by alloying.