Study On Preparation And Catalytic Effects Of Transition Metal Composites For Hydrogen Storage Of MgH₂

In response to the issues of high thermodynamic stability and slow kinetic performance of MgH₂,a series of transition metal composites have been prepared and doped into MgH₂.The underlying hydrogen storage mechanisms have been explored.The main research conclusions are as follows:（1）The flower-like Ni@C additive was prepared by the coprecipitation method and high-temperature calcination treatment and was introduced into MgH₂ to obtain the MgH₂-Ni@C composite by mechanical balling.The initial dehydrogenation temperature of the MgH₂-10 wt%Ni@C was reduced to 165℃,while the dehydrogenation amount reached6.10 wt%.The dehydrogenated MgH₂-Ni@C could rapidly absorb about 6.54 wt%H₂ within300 s at 380℃ and 3 MPa.Mechanism study has shown that the flower-like structure was beneficial to improve the contact sites between the additive and MgH₂ and the carbon structure was conductive to prevent material aggregation.In addition,Mg₂Ni/Mg₂NiH₄formed during the cycling process was beneficial to reduce the bonding barrier of hydrogen molecules on the particle surface and improve the hydrogen ab/desorption performance.（2）Two-dimensional ultra-thin Ni-MOF@Pd hybrid nanosheets were obtained by the wet-medical method,and the active sites of noble metals were activated by subsequent heat treatment,and then were introduced as additives into MgH₂ via mechanical balling.It was found that the initial dehydrogenation temperature of MgH₂-10 wt%Ni-MOF@Pd was significantly reduced by nearly 230℃compared to the pristine MgH₂,and the dehydrogenation amount was approximately 6.36 wt%H₂ at 300℃.The MgH₂-Ni-MOF@Pd sample exhibited rapid ad/desorption kinetics,releasing 6.00 wt%H₂ within 30min at 325℃.The fully-dehydrogenated MgH₂-Ni-MOF@Pd could absorb approximately6.06 wt%H₂ within 300 s at 150℃ and 3 MPa.Structural analysis has shown that Mg₂Ni/Mg₂NiH₄ in the hydrogen absorption and desorption products acted as a“hydrogen pump”,providing channels for the rapid transfer of hydrogen atoms.Moreover,the nano-Mg₆Pd formed in the dehydrogenation process promoted the instability of the system and improved the hydrogen desorption kinetics.In addition,Ni-MOF nanosheets were torn into thin-walled carbon sheets during ball milling,which effectively prevented the aggregation and growth of particles.（3）The Fe-Ni@NC-CNTs hybrid composites were prepared by one-pot solvothermal method and subsequent calcination treatment and mixed with MgH₂ to obtain MgH₂-Fe-Ni@NC-CNTs composites.The results have shown that the optimized MgH₂-10 wt%Fe-Ni@NC-CNTs began dehydrogenation at 188℃.The de/rehydrogenation apparent activation energy of MgH₂-10 wt%Fe-Ni@NC-CNTs was 50.76±5.86 k J·mol^-1 and 61.74±2.11 k J·mol^-1,respectively,which was much lower than that of un-doped MgH₂.The MgH₂-Fe-Ni@NC-CNTs sample could release 6.67 wt%H₂ within 30 min at 325℃.The fully-dehydrogenated MgH₂-Fe-Ni@NC-CNTs could absorb 4.38 wt%H₂ at 150℃,3 MPa,and 300 s.The structure analysis has shown that the phase transition of Mg₂Ni/Mg₂NiH₄ was beneficial to increase the hydrogen diffusion rate,and the in-situ Fe phase was functioned as the catalytic active substance to promote the absorption and desorption of hydrogen molecules.In addition,the nitrogen-doped carbon matrix effectively avoided the aggregation and growth of Mg/MgH₂.