Preparation Of Carbon-based Transition Metal Oxide Composite Catalysts And Their Regulation On The Hydrogen Storage Performance Of MgH₂

Mg-based hydrogen storage materials have internal advantages and broad application prospects in the field of energy storage,but the practical application is hindered by their poor kinetic properties and stable thermodynamic properties.In order to improve the hydrogen storage performance of MgH₂,a series of carbon-based transition metal oxide composite catalysts were prepared by using wet chemical and high-energy ball milling methods,and then they were introduced into MgH₂ by high-energy ball milling.The effect of carbon-based transition metal oxide composite catalysts on the hydrogen storage performance of MgH₂ was systematically studied by means of XRD,TEM and XPS,and the catalytic mechanism of the catalysts on the hydrogen absorption/desorption MgH₂was analyzed.First,a low-cost carbon supported manganous oxide（MnO@C）composite was successfully prepared by wet chemical and heat treatment methods using cation exchange resin and manganese acetate tetrahydrate as raw materials.It was found that after the addition of MnO@C composites,MgH₂ could start to absorb hydrogen at room temperature,and the dehydrogenation/absorption apparent activation energies of it reduced by 59.9 k J mol^-1/46.1 k J mol^-1,respectively.The MnO@C composite also can improve the hydrogen absorption and desorption cycle stability of MgH₂,which benefit from the buffering effect of carbon atoms in the hydrogen absorption process of Mg and suppresses the lattice expansion of Mg.According to the mechanism analysis,MnO in MnO@C was transformed into Mn and Mg_0.9Mn_0.1O after the first dehydrogenation,and Mn and Mg_0.9Mn_0.1O can promot the breaking of Mg-H bonds,which improves the hydrogen storage performance of MgH₂.In addition,Nb₂O₅@C composites were prepared by the same methods.It was found that the in situ formed Nb O and Mg O and Nb₂O₅ nanoparticles form a multiphase interface with Mg/MgH₂,which provides more catalytic sites and hydrogen atom diffusion channels for the composite material and reduces the nucleation energy of MgH₂,thereby enhancing the hydrogen storage performance of MgH₂.To further improve the hydrogen storage performance of MgH₂,N-Nb₂O₅@Nb₂C composites were successfully prepared by wet chemical method using Nb₂Al C as the precursor.It was found that MgH₂-N-Nb₂O₅@Nb₂C sample can absorb 5.0 wt%hydrogen within 1.95 min at 90℃and within 5 h at 30℃.The addition of N-Nb₂O₅@Nb₂C can effectively delay the grain growth rate of MgH₂ in the process of hydrogen absorption and desorption cycle.Under the modification of N-Nb₂O₅@Nb₂C,the capacity retention rate of MgH₂ is still 92.3%ever after 500 cycles of hydrogen absorption and desorption.While the Nb N and Nb₂O₅ formed in situ during the introduction of N-Nb₂O₅@Nb₂C into MgH₂ can weaken the interaction between Mg and H,and increases the"capturing ability"of Mg composite for H₂,thus effectively promoting the reversible hydrogen storage performance of MgH₂.Finally,in order to further study the catalytic effect of carbon-based transition metal oxide composites on MgH₂,Nitrogen-doped graphene supported titanium monoxide（TiO@N-C）nanocomposites as catalysts.It was found that TiO@NC composites have an excellent catalytic effect on the hydrogen storage performance of MgH₂,and the catalytic effect of TiO@NC（TTONC）is better than that of Ti@NC（TNC）and TiO₂@NC（TONC）composites for MgH₂.The TiO@N-C formed nano-Ti,Ti₂O₃ and TiO₂ in situ during the introduced into MgH₂.The in-situ formed nanoparticles can reduce the nucleation energy of MgH₂ and improve the hydrogen absorption properties of the samples.In addition,Ti/TiH₂has a"hydrogen pump effect"in the process of hydrogen absorption and desorption,and carbon structural defects in nitrogen-doped graphene serve as nucleation sites to enhance hydrogen diffusion.Under the synergistic effect of the"hydrogen pump effect"and carbon structural defects,the hydrogen absorption and desorption kinetics and cycling stability of MgH₂ are significantly improved.