| Complex hydrides (such as NaAlH4, LiAlH4and LiBH4) have been considered as one of the most promising materials for hydrogen storage due to their potential for acceptably high gravimetric capacities, as well as an ability to be engineered for favorable hydrogen release kinetics. Among the complex hydrides, NaAlH4has been widely studied after the pioneering research by Bogdanovic and Schwickardi because of its relative large hydrogen storage capacities (5.6wt%) and moderate temperatures for reversible hydrogen release/uptake. However, NaAlH4suffers from inherent limitations as a hydrogen storage media related to low kinetics and poor reversibility. Based on the review of the research and development of complex hydrides for hydrogen storage materials, Ti doped NaAlH4was selected as the subject of this work. All the research contents and results are as follows:(1)Nanocrystalline titanium dioxide/carbon composite (TiO2/C) was synthesized through a direct solution-phase carburization using tetrabutyl titanate (Ti(OBu)4) and resol as precursors. The prepared TiO2/C composite was mainly in the anatase structure with an average particle size under20nm, which was then introduced in NaAlH4as a catalyst through ball milling. The desorption curves show that both nanocrystalline TiO2/C and TiO2can obviously improve the kinetics of NaAlH4, while NaAlH4with3mol%TiO2/C exhibits better cycling stability than NaAlH4with3mol%TiO2. The hydrogen storage capacity of NaAlH4with TiO2/C remains stable after5th cycle, and about94%of initial hydrogen is released, while the capacity of NaAlH4with TiO2decreases continuously during cycling, and only88%of initial hydrogen is released after10th cycle. Furthermore, NaAlH4with3mol%TiO2/C exhibits good reversibility at relatively low hydrogen pressures, and it can reload4.16and1.63wt%hydrogen at50and30bar hydrogen pressures, respectively.(2)Combination of nanoconfinement and catalyst addition is a promising strategy to enhance the kinetics and reversibility of hydrogen storage in complex hydrides. Ti loaded high ordered mesoporous carbons (Ti-OMCs) were directly synthesized through a solvent evaporation induced self-assembly method (EISA) with in situ crystallization and carbonation technology using phenolic resols, tetrabutyl titanate (Ti(OBu)4) and triblock copolymer F127as organic carbon sources, Ti sources and templates, respectively. The obtained Ti-OMCs exhibit uniform pore sizes (4nm), high specific surface area (427.9m2g-1) and large pore volumes (0.3cm3g-1), which were used to combine catalyst addition and nanoconfinement to improve hydrogen storage properties of NaAlH4by melt infiltration. The hydrogen desorption curves show that NaAlH4with Ti-OMCs exhibit better kinetics properties than both nanocrystalline TiO2catalysted NaAlH4and melt-infiltrated NaAlH4with high ordered mesoporous carbons (OMCs). The hydrogen-release onset temperature of NaAlH4with Ti-OMCs is reduced to less than60℃, and80%hydrogen is released in less than20min. In addition, NaAlH4with Ti-OMCs exhibit good reversibility and cycling stability, and the optimum rehydrogention temperature is120℃.(3)Nanocrystalline titanium dioxide loaded carbon spheres (Ti-CSs) with10wt%TiO2were synthesized through an easy one-step method using phenolic resols, titania nanoparticles and Pluronic F-127as organic carbon sources, inorganic precursors and surfactant, respectively. The results show that the as-prepared Ti-CSs composite is spherical shape with a diameter ranging from0.3to2μm, and rutile TiO2nanoparticles are distributed on the surface of the carbon spheres. Then the kinetics of NaAlH4was improved through depositing it on the surface of as-prepared Ti-CSs by melt infiltration. The results show that NaAlH4with Ti-CSs exhibits better hydrogen desorption kinetics than TiF3or nanocrystalline TiO2catalysted-NaAlH4, and it starts to release hydrogen at about40℃and releases about25%of the hydrogen content during heating to60℃. The results from SEM and XPS show that hydrogen storage properties of NaAlH4were considerably improved due to the formation of special structure during melt infiltration and the nanocrystalline TiO2and/or amorphous phase Ti-Al clusters near the subsurface sites, which succeed in combining catalyst addition (TiO2nanoparticles) and nanoconfinement to improve the kinetics of NaAlH4-.(4) XRD patterns and XPS spectra were used to observer the evolution of Ti addition in NaAlH4+10mol%TiF3and NaAlH4+10mol%TiO2during ball milling and during hydrogen absorption/desorption cycling. The results show that TiF3and TiO2are reduced to amorphous Ti0gradually during milling, and Al3Ti intermetallic is formed gradually during cycling. Then active species are separated directly by dissolution and filtration from Ti doped-NaAIH4using tetrahydrofuran (THF) as solvent. TEM results show that the average particle size of the obtained Al-Ti nanoparticles is30-50nm, and Ti element is uniformly distributed on the Al matrix. The hydrogen desorption experiment indicates the separated Al-Ti nanoparticles exhibit good catalytic effect on NaAlH4for hydrogen storage. To the best of our knowledge, this is the first report on separation and characterization of the Al-Ti nanoparticles from Ti-doped NaAlH4More importantly, the reported method should offer a general approach to obtained active species for other complex hydride such as LiAlH4, NaBH4and LiBH4, which is very useful to investigate the catalytic mechanism of these doped complex hydrides for hydrogen storage. (5) First-principles calculation was used to investigate hydrogen adsorption, dissociation and diffusion on clean and Ti doped Al(111) surfaces. The results show that substitutional Ti in the surface layer of Al(111) is not the energy favorable state, while the formation energy(Ef) of Ti atom in the subsurface and second subsurface layer of Al(111) are higher than Ti in the surface layer by0.19and0.4eV, respectively. In addition, the transition research results show that the H2dissociation energy barrier for clean Al surface is1.28eV, and the H diffusion energy barrier for Ti doped in the surface of Al(111) is0.57eV. While, Ti doped in the subsurface of Al(111) exhibit the highest catalytic activity because it provided an optimal trade-off of moderate dissociation and diffusion energies. Combining results of experiments and calculations, a hydrogen desorption/absorption model of Ti doped NaAlH4was proposed and catalysis mechanism was discussed.(6) Based on a number of experiments, NaAlD4was synthesized by D2desorption/absorption cycles, and hydrogen isotope effect of Ti-doped NaAlD4was investigated. The Mass spectra and Raman results show that relative purity97.4%of NaAlD4is obtained after three deuterium desorption/absorption cycles. In addition, the hydrogen isotope desorption kinetics curvers show that kinetics of deuterium desorption/absorption is slower than hydrogen desorption/absorption. The measurement of hydrogen isotope effect show that the NaAlHxD4-x exhibit hydrogen isotope effect during5cycles, and the deuterium desorption is slower than hydrogen desorption at initial stage, while the kinetics of deuterium desorption is increased gradually. The hydrogen isotope separation factors at initial (al) and final (a2) stage are0.93and1.12, respectively. |
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