| Mg-base hydrogen storage material is one of the most promising candidates of hydrogenstorage materials. However, the dehydrogenation temperature is still too high and thedehydrogenation rate remains slow. In this thesis, the improvements in recent years onMg-base hydrogen storage material were firtly reviewed. It can be pointed out that addingcatalysts or nano-structuring could improve the kinetics of Mg/MgH2. For nano-structuring,the problem of the structure stability must be considered. For adding catalyst, it indeed needsto improve the catalytic efficiency which is relatively low by traditional method for addingcatalyst. In terms of thermodynamic tuning, although the enthalpy change is given attention,the effect of dehydrogenation entropy has rarely reported. It is indeed important to find waysto avoid the negative effects of the decrease of entropy (absolute value–all following isdefined as it), and theoretically calcutate for the entropy change.Hence, this thesis investigates the tunning methods for enhance the structure stability,dynamic performance and thermodynamic properties of Mg-based hydrogen storage materials.The kinetics, cycylic and thermodynamics properties of AAO nano-confined Mg andTM-based catalyst coating on Mg particle surfaces were studied. The structure tunning bypreparation process and its impact on kinetics, cyclic and thermodynamics properties werealso studied. The influence of nano-size and the catalysts coating on the dehydrogenationentropy of MgH2based on the thermodynamic models and the experiments are investigated.A new approach has been developed to successfully load Mg into the nanometre-sizedpores of an Anodic Aluminium Oxide (AAO) template for realizing the nano-confinement ofMg. Mg nano-particles were nucleated along the AAO pore walls. Mg-AAO can absorbhydrogen within25mins and desorb hydrogen within30mins under300°C. Afterhydrogenation, Mg and Mg17Al12phases transformed into MgH2and Mg3Al2. Although smallamount of MgO and Mg17Al12formed as by-products, the effective filling was about35wt%which is higher than that reported by other groups in Mg nano-comfinement. The confinedMg/MgH2shows favourable kinetics with high stability. Furthermore, the slight reduction inhydrogen desorption enthalpy and entropy of MgH2is found from74.4kJ mol-1to73.2kJmol-1and131.0J mol-1K-1to130.1J mol-1K-1, respectively, in the presentnano-confinement.The core-shell structured Mg with Ti-based catalyst (denoted as Mg-Ti) is prepared bythe chemical reaction between Mg powders and TiClxin THF solution, which is of~10nm in thickness and contains multiple phases and valences. Compared with Mg-TiCl3by traditionalball-milling method, the Mg-Ti system has superior dehydrogenation properties, which canstart to release H2at about175oC and release5wt%H2within15mins under250oC. And thecyclic kinetics is relatively stable as the kinetics performance from the3rdcycle to the10thcycle maintains well. The deh drogenation reaction entrop (ΔS) of the system is changedfrom130.5J K-1mol-1H2to136.1JK-1mol-1H2, which reduces the Tplateauto279oC from300oC at equilibrium pressure of1bar. A new mechanism has been proposed that the mulitiplevalence Ti sites act as the intermediate for electron transfers between Mg2+and H-, whichenables recombination of H2on Ti easier.Mg is coated by different transition metals (TM: Ti, Nb, V, Co, Mo, or Ni) with a crystalsize of nano-scale (less than10nm) to form a core (Mg)-shell (TM) nano-structure by areaction of Mg powders in THF solution with TMClx. It is experimentally confirmed that thesignificance of catalytic effect on the dehydrogenation is in a sequence of Mg-Ti, Mg-Nb,Mg-Ni, Mg-V, Mg-Co and Mg-Mo. This may be contributed to the decrease ofelectro-negativity (χ) from Ti to Mo. However, Ni shows a special case with high catalyticeffect in spite of the electro-negativity. It is supposed that the formation of Mg2Ni compoundmay play an important role to enhance the hydrogen de/hydrogenation of Mg-Ni system. Itcan also be found that the larger formation enthalpy, the worse dehydrogenation kinetics.On the basis of the above, Mg with different sizes (40nm,500nm,1μm) coated withTi-based catalyst is compared both in structure and properties. Although the particle size ofmicro-sized Mg-Ti is larger than nano-sized Mg-Ti, the micro-sized still show betterdehydrogenation kinetics than nano-sized Mg-Ti. It is suggested that the key factor effect onthe kinetics could be the surface conditions (the contents of positive catalyst and negativeimpurity element) of Mg/MgH2while the particle-size is under2μm.The particle/grain size and catalysts effect on dehydrogenation entropy of MgH2arecalculated by a theoretical model. The excess volume of Mg and MgH2is increased alongwith the decrease of particle/grain size. Thus, the translational and vibrational entropy of Mgand MgH2are both increased, while the increase in MgH2is larger than Mg. Hence, thedehydrogenation entropy decreases with the decrease of the particle/grain size. Besides, theconcentration of dissociative H on TM surface is considered as the catalyst can dissociate H2more easily. The dehydrogenation entropy increases with the concentration of dissociative H.The the experimental results support our theoretical calculations. |
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