Preparation Of Nb-based Catalysts And Their Effects On Hydrogen Absorption/desorption Properties Of MgH₂

Developing renewable energy technologies is important for a clean and sustainable future due to the increasing air pollution and global warming problems,as well as the shortage of fossil fuels.Hydrogen,one of the most abundant elements on the Earth,is a potential clear energy carrier.However,a significant bottleneck of the practical use of hydrogen is storing it in a safe,efficient and economic manner.With7.6 wt%of hydrogen capacity,MgH₂ has attracted much attention as one of the most promising hydrogen storage media along with its good reversibility,natural abundance,low cost and environmental benignity.However,hydrogen release from MgH₂ usually requires high temperature and suffers from sluggish kinetics caused by its high thermodynamic stability and kinetic barriers,which largely limits its use in the practical on-board hydrogen storage.In this work,the nanometer-sized Nb-based catalysts with different morphologies and compositions have been synthesized successfully.The catalytic effects of the as-synthesized Nb-based compounds on de/hydrogenation behaviors of MgH₂ were systematically investigated,and the corresponding mechanisms were discussed.First,an Nb₂O₅ hollow microsphere?o-Nb₂O₅?was successfully synthesized through a solvothermal method.The o-Nb₂O₅ is spherical particles with diameter of5-10?m.The hydrogen storage properties of the o-Nb₂O₅-added MgH₂ were systematically investigated.It was found that adding a minor amount of o-Nb₂O₅significantly improved the hydrogen storage performance of MgH₂.The onset dehydrogenation temperature was decreased to 207°C for the MgH₂-7 wt%o-Nb₂O₅sample,i.e.93°C lower than that of pristine MgH₂.The dehydrogenated sample absorbed approximately 6.1 wt%of hydrogen while it was heated to 190?C under 50bar of hydrogen,exhibiting a good reversibility.The activation energy of MgH₂-7 wt%o-Nb₂O₅ was calculated to be 101±5 kJ mol^-1 for hydrogen desorption,which is reduced by 27%in comparison with the pristine MgH₂,while the enthalpy change remained almost unchanged.The compositional and structural characterization revealed that Nb₂O₅ was reduced to NbO during the ball milling process,and further reduced to NbO/Nb and Nb upon dehydrogenation,which is the most important reason for the improved de/hydrogenation performance of MgH₂.A nanocrystalline NiNb₂O₆ catalyst was synthesized by means of a sol-gel process with nickel nitrate hexahydrate as the nickel source.The effects of the addition of NiNb₂O₆ on hydrogen storage behaviors of the MgH₂ were further investigated.The results showed that the MgH₂-5 wt%NiNb₂O₆ sample could release6.95 wt%of H₂ at the temperature range of 215°C to 315°C,which were reduced by85°C and 45°C relative to those of the additive-free MgH₂,respectively.The rehydrogenation was completed at 160°C under 50 bar of hydrogen pressure,representing an excellent hydrogenation performance.However,only 5.6 wt%of hydrogen could be absorbed.The onset dehydrogenation temperature was further decreased after the first de/hydrogenation cycle.Theoretical calculations showed that Ni²⁺was reduced to elemental Ni during ball milling,which is further combined with Mg to form Mg₂Ni in the first cycle,leading to the decrease of hydrogen storage capacity of MgH₂.The Nb element in NiNb₂O₆ only acted as a catalyst for the MgH₂.Subsequently,the TiNb₂O₇ was prepared as a catalyst and its effects on hydrogen storage behaviors of MgH₂ were investigated.The onset dehydrogenation temperature of the MgH₂-7 wt%TiNb₂O₇ sample was decreased from 300?C?the pristine MgH₂?to 211?C,and further decreased to 177?C after the first de/hydrogenation cycle.At250?C,approximately 5.5 wt%of hydrogen was released from the activated MgH₂-7wt%TiNb₂O₇ sample within 10 min.Further isothermal hydrogenation measurement revealed that approximately 5.2 wt%of hydrogen was rapidly absorbed by the dehydrogenated sample within 5 s at 200°C,showing rapid hydrogenation kinetics.After 10 cycles,this sample still retained a hydrogen storage capacity of 6.29 wt%,which showed the improved cycle stability.Further XRD and XPS analyses indicated that the Ti and Nb in TiNb₂O₇ additive were reduced to TiO₂,Ti,NbO₂ and NbO after the first cycle.Multivalent Ti and Nb effectively promoted charge transfer between Mg and H₂,accelerated hydrogen absorption and desorption,and significantly improved hydrogen storage properties of MgH₂-7 wt%TiNb₂O₇.Afterward,a worm-like N-Nb₂O₅ catalyst was prepared via a solvothermal process with niobium ethoxide as the niobium source and triethyl amine as the nitrogen source.The catalytic effects of N-Nb₂O₅ on the de/hydrogenation behavior of MgH₂ were systematically studied.The results revealed that N-Nb₂O₅ exhibited a remarkable catalytic activity for MgH₂.The activated MgH₂-10 wt%N-Nb₂O₅ started releasing hydrogen at 170°C,which was 130°C lower than the pristine MgH₂.The dehydrogenated sample began to absorb hydrogen at room temperature under 50 bar of hydrogen pressure,and the hydrogen absorption was 6.3 wt%at 120°C.Even at70°C,the dehydrogenated sample can be completely hydrogenated within 6 h,exhibiting good cycling stability.Results of XPS and TEM analyses indicated that a new catalytic active ternary NbN_0.9O_0.1 was formed after activation,and remained stable in subsequent cycles.DFT calculations indicated that the Nb-N-O clusters adsorbed on the surface of MgH₂ induced the distortion of the MgH₂ lattice due to the elongated Mg-H bonding,which not only changes the charge distribution of Mg and H but also weakens the Mg-H bonding.Moreover,the presence of N facilitates a positive effect for the contribution of Nb in the DOS around the Fermi level,which reasonably explains the much higher catalytic activity of NbN_0.9O_0.1.Finally,a graphene-supported N-Nb₂O₅ nanorod composite was obtained with a solvothermal process with graphene as support.The effect of graphene on the catalytic activity of as-synthesized catalyst was investigated.The N-Nb₂O₅ nanorods were measured to be approximately 20-30 nm in diameter and 1-2?m in length.The operation temperatures for dehydriding and hydriding of N-Nb₂O₅@C-modified MgH₂ were significantly reduced.The post-activated MgH₂-10 wt%N-Nb₂O₅@C was determined to release 5.0 wt%of hydrogen even at temperature as low as 175°C while measured by thermogravimetry.Even at room temperature,more than 6.0 wt%of hydrogen can be absorbed within 42 h.NbN_0.9O_0.1 was also formed after the first cycle and remained stable in subsequent cycles.DFT calculations and COOP analyses revealed that the graphene-supported Nb-N-O had a much higher catalytic activity,which was due to the synergestic effect between NbN_0.9O_0.1 and graphene.The synergistic effect induced the extended bond length and the weakened bond strength of Mg-H bonding in MgH₂,thus improving the hydrogen absorption and release performance of the system.