Effect Of Transition Metal-based Nanocatalysts On The Hydrogen Storage Properties Of MgH₂

Contemporary times have witnessed the burgeoning prominence of energy scarcity and environmental pollution,thus intensifying the urgency to develop green and sustainable energy sources.Hydrogen,with its wide accessibility,high energy density,and ability to function as an ideal carrier of clean energy,has emerged as a promising candidate.However,efficient and safe storage and transportation of hydrogen remains a primary bottleneck for its practical implementation.Amongst the array of high-safety solid-state hydrogen storage materials,magnesium(Mg)has garnered significant attention as a viable option due to its abundance and environmentally friendly nature.In particular,the hydride of Mg(Mg H2)achieves a theoretical hydrogen storage density of up to 7.6 wt.%.However,the thermodynamic and kinetic properties of Mg H2 are not satisfactory,with slow dehydrogenation rate and actual dehydrogenation temperature over 350 ℃,which seriously limits its practical application.The addition of catalyst to Mg H2 has effectively improved the hydrogen storage performance of Mg H2,but the dehydrogenation temperature of modified Mg H2 is still higher than 225 ℃.Moreover,the synthesis process of most high-performance catalysts is complicated,with expensive raw materials,making it difficult for mass production.The efficient synthesis of inexpensive catalysts of superior catalytic performance is a significant challenge.In this article,based on the existing inexpensive catalysts,an efficient plasma modification method and a promising approach to design catalysts are introduced to synthesize ultrafine Ni nanoparticles as well as Ti O2-based solid-solution type catalysts.This study systematically investigates the impact of catalysts on the performance of Mg H2 absorption and dehydrogenation,while also revealing the underlying mechanism of the catalysts’ action.Plasma reduction offers a novel and innovative approach for the preparation of ultrafine transition metal catalysts.In this study,an independent plasma treatment device was constructed to synthesize ultrafine Ni nanoparticles and uniformly deposit them onto the surface of Mg H2.The precursor Ni(acac)2 can be converted into ultrafine Ni nanoparticles of 2～6 nm in about 40 min by the strong reducing property of H2 plasma.Among them,the Mg H2-10Ni(acac)2composites can dehydrogenate more than 6.0 wt.% in 25 min at 250 ℃,with a final dehydrogenation of 6.56 wt.%,or even complete dehydrogenation at 225 ℃.The completely dehydrogenated samples exhibit favorable reversibility,absorbing more than 5.0 wt% of hydrogen within 20 min at 100 °C.Structural analyses indicate that during the dehydrogenation process,the Ni nanoparticles are transformed into uniformly dispersed ultra-fine Mg2 Ni H0.3,which acts as an efficient "hydrogen pump" to promote the dissociation and diffusion of H2.Therefore,the hydrogen storage performance of Mg H2 is effectively improved.TiO2 exhibits a catalytic effect similar to that of ultrafine nano-Ni,however,it has the disadvantage of being less effective in adsorbing and dissociating H2.In this work,the same inexpensive transition metal oxide Ti O2 was modified to address the shortcomings of Ti O2 which is difficult to adsorb and dissociate H2.The solid solution catalyst of Nb-doped Ti O2 is obtained by hydrothermally doping the lattice of Ti O2 with Nb,which is easy to adsorb and dissociate H2.Ti(30Nb)O2 lowered the starting dehydrogenation temperature of Mg H2 to180 ℃,lower than that of commercial Mg H2 by 100 ℃.It allows Mg H2 to dehydrogenate more than 6 wt.% of H2 at 225 °C for 12 min.At a pressure of 5 MPa and room temperature,the dehydrogenated Mg H2 exhibited an explosive uptake of hydrogen,with a hydrogen uptake exceeding 5 wt.% in just one minute.Structural analysis and theoretical calculations show that the Nb d orbitals of Ti(Nb)O2 can hybridize significantly with H 1s orbitals,effectively reducing the difficulty of Ti(Nb)O2 adsorption and dissociation of H2.Moreover,numerous wellcontacted Ti(30Nb)O2/Mg interfaces serve as H-atom diffusion channels for efficient transfer of H atoms during the process of hydrogen uptake and dehydrogenation.The combined effect of the above two significantly enhanced the rate of Mg H2 uptake and dehydrogenation.Changing the energy level distribution by doping to modulate the catalyst performance provides a promising direction for the subsequent development of high-performance catalysts.Based on above,the N,Nb co-doped solid solution catalyst P-Ti(Nb)O2 was obtained by NH3 plasma treatment of solid solution catalyst Ti(30Nb)O2,which can enhance the lowtemperature dehydrogenation performance of Mg H2.The starting dehydrogenation temperature of Mg H2 is reduced to 155 °C catalyzed by P-Ti(Nb)O2,and 6.4 wt.% of H2 can be dehydrogenated in only 6 min at 225 °C.The dehydrogenation amount can exceed 6 wt.% in20 min at low temperature of 200 °C,whereas more than 5.0 wt.% of hydrogen absorption is still possible in 1 min at room temperature.XRD and TEM results confirm that P-Ti(Nb)O2 and numerous well-contacted P-Ti(Nb)O2/Mg H2 interfaces are the basis for maintaining excellent hydrogen absorption and dehydrogenation properties.DFT calculations indicate that N doping is responsible for the enhanced hydrogen storage performance and that N atoms in P-Ti(Nb)O2consistently reduce the difficulty of dissociation of H2.N atoms between the PTi(Nb)O2(101)/Mg H2(110)interface change the charge distribution of Mg and H atoms,thus weakening the Mg-H interaction.The simple and efficient plasma treatment effectively enhances the catalyst activity.To further enhance the cyclic stability of the composite hydrogen storage materials,the present work introduced multi-walled carbon nanotubes in the solvent heat process to prepare loose and friable 20CNTs-Ti(30Nb)O2 hollow spheres,which effectively improved the uniformity of ball milling.Among them,Mg H2-Ti(30Nb)O2@20C removes 6.0 wt.% H2 in 8min at 225 °C and 6.0 wt.% H2 in only 23 min at 200 °C.The hydrogen uptake exceeds 5 wt.%in only 10 s in room temperature environment and up to 6.15 wt.% in 30 min.The introduction of CNTs remarkably improved the cycling stability of Mg H2,with only 0.03 wt.% decay in dehydrogenation capacity after 20 cycles.The dehydrogenation capacity was as high as 6.18 wt.% after 50 cycles,with a capacity retention rate of 93.8%.The TEM and EDS results confirm that the highly catalytically active Ti(30Nb)O2,numerous well-contacted Ti(30Nb)O2/Mg H2 interfaces are the basis of its excellent hydrogen storage performance.The coating of Mg H2 particles with CNTs fragmented by ball milling prevents the agglomeration and growth of Mg H2/Mg particles,which is the intrinsic mechanism of its high cyclic stability.