Synthesis And Hydrogen Sorption Properties Of Mg-based Nanocomposites

Magnesium hydride is considered to be one of the most promising hydrogen storage materials due to its high hydrogen storage capacity（7.6 wt%）, environment friendliness and low cost. However, the sluggish hydrogenation and dehydrogenation kinetics at practical operating temperatures and the high desorption temperature limit its practical application. Reducing Mg particles to nanoscale and doping with various catalysts are considered as efficient approaches for improving the hydrogen storage properties of Mg/Mg H₂. In this work, Mg nanoparticles were successfully synthesized through the nanoconfinement method and adapted Rieke method, respectively. It is found that the hydrogen storage propreties of Mg nanoparticles preprared through the adapted Rieke method is better than that preprared through the nanoconfinement method. Based on the adapted Rieke method, the Mg-TM（TM=Ni, Ti, Fe, Co, V, Pd） nano-composites were co-precipitated from a homogeneous THF solution containing anhydrous magnesium chloride, transition metal chlorides and lithium naphthalide as the reducing agent. XRD, TEM, STEM, EDS, DSC and PCT techniques were used to characterize the microstructure, morphology and hydrogen storage properties of the nanocomposites systematically.The investigation on Mg nanoparticles confined in the carbon aerogel shows that: Mg H₂ particles are dispersed well in the carbon aerogel pore networks, and the size of most Mg H₂ particles is smaller than 10.0 nm. The thermodynamic properties and hydrogen absorption kinetics of confined Mg nanoparticles are slightly promoted by “nano-size effect”. The loading rate of Mg in the carbon aerogel is improved to be 22.5 wt%. However, the measured sorption capacity is less than 2.35 wt%, which dose not fit for the demand of practical application.The study of pure Mg prepared through the adapted Rieke method shows that: plate-shaped pure Mg particles are stacked together, forming a shape like “chicken claws” structure. Compared with the nano-confinement method, pure Mg prepared through the adapted Rieke method displays faster hydrogen absorption kinetics at lower temperatures and lower hydrogen desorption temperature. For instance, pure Mg can absorb 85% of its maximum hydrogen capacity within 52 s at 250 oC, and the onset dehydrogenation temperature of the hydrogenated Mg is reduced to 311.1 oC. In addition, the absorption and desorption apparent activation energies are determined to be 73.1 k J/mol H₂ and 147.4 k J/mol H₂, respectively.The investigations on the Mg-TM（TM=Ni, Ti, Fe, Co, V or Pd） coprecipitated from solution show that: the co-precipitated TM is homogeneously distributed on the surface or inside Mg particles. Due to the addition of Ni, Ti, Fe, Co or Pd, high density defects are likely to form in the nano-composites, and still remain after hydrogenation at low temperatures, leading to the phase transformation from β-Mg H₂ to γ-Mg H₂. The Mg-TM nano-composites not only maintain higher hydrogen capacity, but also display superior hydrogen storage properties. For instance, the hydrogenation and dehydrogenation enthalpies of the Mg-Ni nano-composite are determined to be around 70.0 k J/mol H₂, slightly lower than those for the reduced pure Mg. The Mg-Ti nano-composite shows the best hydrogen absorption property: it absorbs hydrogen up to 6.2 wt% within 2 h at room temperature.The hydrogen desorption temperature of the hydrogenated Mg-Pd nano-composite can be reduced to 216.8 oC, and the desorption apparent activation energy is also decreased to 93.8 k J/mol H₂. High catalytic effectiveness of the co-precipitated TM depends not only on its intrinsic activity, but also on its distribution state, which may be entirely different from previous composites prepared through physical routes. For the Mg-Fe and Mg-Co nano-composites, nano-crystalline state of Fe or Co palys a key role in impoving the hydrogen absorption kinetics at low temperatures and decreasing the hydrogen desorption temperature. For the Mg-Pd, Mg-V and Mg-Ti nano-composites, transition metal hydrides Pd H0.706, γ-TiH₂ and V₂H formed during hydrogenation process act as hydrogen pumps leading to superior sorption properties. The excellent hydrogen sorption properties of the Mg-Ni nano-composite can be attributed to the gateway effect of Mg₂NiH_0.3 formed in the composite after hydrogenation/dehydrogenation cycles.