Physical Chemistry Of The Hydriding And Dehydriding Reaction In Magnesium-based Storage Hydrogen Alloys

Hydrogen storage and transportation is a key issue in the application of hydrogen energy. The Mg-based alloys as hydrogen storage material are attractive for researchers in the world because of its high capacity, abundant resource and low cost. However, their practical application has been limited due to its high working temperature, relatively poor hydriding/dehydriding (H/D) kinetics and some special requirement in activation and heating treatment. In order to obtain some proper hydrogen storage materials with a large amount of hydrogen capacity and fast absorbing and desorbing rate at a low temperature environment, various attempts have been made in our lab to overcome these difficulties mentioned above.In my thesis, the physicochemical characteristics in H/D reaction for seventeen kinds of alloys in five systems were carefully investigated by means of several neoteric experimental analytical instruments including isovolumetric method, pressure-composition isotherm (PCI), X-ray diffraction (XRD), differential scanning calorimeter (DSC), thermogravimetric analysis (TG), scanning electron microscopy (SEM) and transmission electron microscope (TEM). Firstly, the method of orthogonal test designing has been used to optimize the technique of hydriding combustion synthesis (HCS) for Mg-La-Ni ternary alloys, then the Mg2Ni, Mg-Ag-Ni and Mg-Al-Ni systems were prepared by using the technique of keeping temperature constant in one-step. Besides, Mg-La-Ni ternary alloys and Mg-Mm(NiCoMnAl)5 composite were also obtained by the optimized mechanical alloying method (MA) and the Mg2Ni and Mg1.9Al0.1Ni alloys were produced by the same way either. In the present communication, our attention was paid to the thermodynamics, kinetics, structure and morphology for clarifying the relationships between these characteristics. It is worth mentioning that the new Mg-LaNix composite is one of the most promising hydrogen storage materials recently.Comparing the characteristics of hydrogenation of Mg2Ni and those of Mg2-xMxNi(M=Ag,Al;x=0.05,0.1,0.2,0.5)alloys, it can be seen that after the suitable substitution of Ag(x<0.2) or Al(≤0.1) for Mg in Mg2Ni alloy, the equilibrium plateaus of the alloys increase, the temperature of hydrogen absorption and desorption decreases, the H/D rates become faster but the structure is still hexagonal. But excessive additive in Mg2Ni (x>0.2) will result in the change of the structure and the decrease of the hydrogen storage capacity due to the non-absorbing phase. The same amount of silver or aluminum was added to substitute Mg in Mg2Ni prepared by HCS, e.g. Mg1.9Ag0.1Ni and Mg1.9Al0.1Ni, they can absorb 3.32 and 2.79 w[H] at 553K and 3MPaH2. Compared with their H/D properties, it can be concluded that the properties of the alloy prepared by MA is better than that by HCS.A serial of Mg-8at.%LaNi0.5, Mg-6at.%LaNi and Mg-4at.%LaNi1.5 alloys prepared by MA or HCS were investigated and it is found that the mechanically alloyed Mg-8at.%LaNi0.5 composite material is the best one among the five systems in my research, that can absorb 3.86~6.29 w[H] under 3MPa hydrogen pressure and desorb 3.14~6.18 w[H] under 0.0133 MPa in 10 minutes above 423K without any activation. However, the same constituent alloy obtained by HCS can only reach the amount of 4.80 w[H] under the 3MPaH2 and desorb 4.50 w[H] under the 0.0133MPa in 15 minutes at 553K. The difference of the H/D characteristics between Mg-8at.%LaNi0.5 prepared by HCS and MA can be explained through the results obtained from XRD and TEM. The presence of the nanocrystalline produced in the mechanical alloying process is beneficial to the solubility of hydrogen atom in alloy and the diffusion of hydrogen atom through hydrides that results in the increase of the real hydrogen storage capacity being larger than the theoretic values, e.g. MgHx(x>2). Moreover, the multiphase structure(nano-/amorphous) and a catalytic effect of LaH3 and Mg2Ni formed in the ball-milling process can also accelerate the H/D rate for the composite material of mechanically alloyed Mg-8at.%LaNi0.5.Thermodynamic properties are the most important parameters for predicting the direction, limitation and maximal output ratio of H/D reactions. Although a great progress of the experimental techniques have been made, it is still impossible to obtain all relevant thermodynamic data in this field only relying on the experimental method, especially for the multicomponent systems. The unique feasible solution is that, calculating thermodynamic properties with theoretical model based on some limited experimental data. At present there are a few models used to calculate the thermodynamic properties for Mg-based multicomponent systems, however, these models possess many defects such as difficult to collect the value for parameters, very complicated expression and introducing large errors. In my thesis, based on the method of stepwise regression and considering the parameters of microcosmic atomic structure, a new semi empirical mathematical model has been established for predicting the heat of formation(?H0) and the amount of hydrogen absorbed(C) of multicomponent Mg2-xMxNi1-yMly alloys for the first time,they are, The proposed models have investigated the interaction of the individual microcosmic parameters and revealed the inner relationship between the macroscopic properties of hydrogenation and the microcosmic parameters of structure for Mg2-xMxNi1-yMly alloys, which indicates that the most important factors affecting the thermodynamic properties of Mg-based alloys should be the difference among electronegatives of constituent elements ?X2,the electron concentration (e/a)2/3,the electron density ?n2/3,the ratio of the number of valance electron to the radius of metallic atom Z/R as well as temperature. These models have successfully been applied to predict the enthalpy of formation of hydrides and the hydrogen content for Mg2-xMxNi1-yMly alloy with the maximum absolute error of±5kJ/mol and±0.3 w[H], respectively.The kinetics of hydrogen absorption/desorption (A/D) is one of key points in the field of hydrogen storage materials, in which the theoretical study is far behind the experiment research. At present there are two major methods to describe the kinetic behavior for hydrogenation, namely, semiempirical models and the method of solving a group of differential equations. The former one is using a series of mathematic formula to fit experimental data, from which to select the best one. This method is actually a kind of data fitting and cannot give a clear physical meaning for many parameters. Though the latter one is a kind of rigorous solution, that is too complicated only relying on numerical calculation and difficult to perform a theoretical analysis for the practical system. As a result, most of experimental researchers couldn't follow it and prefer to do a point-by-point data description rather than doing this kind of complicated calculation.In my thesis, a new model has been proposed after introducing an approximation assumption for the mechanism of absorption/desorption of hydrogen. This new formulae is an analytic solution expressing the reacted fractionξas a function of time t, temperature T and particle size R0.Since it is an explicit function that is easy to use and perform a theoretical analysis. Besides, a new physical conception, namely, the"characteristic absorption/desorption time of reacted fraction of hydrogen"has been introduced to the derivation of this new model, which not only simplifies the expression of formulae but also offers some significant physical meanings that will be useful for future theoretical discussion. The application of this new model to the Mg-based hydrogen storage alloy shows that this new model works very well. When this model is used to other systems offered by other authors, a good agreement has also been obtained.