Fundamental Studies Of Some High-capacity Hydrogen-Storage And Lithium-Storage Materials

Development of high energy-density power sources is urgently needed in many high-tech applications for solving the problems of environmental pollution and gradual depletion of fossil fuels. From the theoretical point of view, H or Li has the highest energy density and, therefore, development of the battery systems based on H or Li anodes have become the highlight in electrochemical researches. However, at the present state of art, all the commercialized H or Li storage materials do not deliver satisfactory high electrochemical capacity and therefore cannot meet the requirements of related high technology applications. This Ph.D work was aimed at developing a new generation of high capacity batteries through exploring new H or Li storage materials and studying their electrochemical properties and associated applied problems. The main results and new findings are summarized as follows:1. A new approach was proposed to generate H₂ from alkaline NaBH₄ solution by using nickel boride as hydrolysis catalyst for construction of high H₂ storage systems. The kinetic properties of H₂ generation from NaBH₄ hydrolysis were investigated under wide temperature range and OH" concentrations. The experimental results showed that high-purity H₂ could be generated from NaBH₄ hydrolysis at room temperature by use of both Ru and NixB catalyst. However, compared with Ru catalyst reported previously in the literature, NixB catalyst prepared in this work not only has very low cost, but also exhibit superior catalytic activity and operational stability with increasing alkaline concentration, showing a great promise for practical applications. Moreover, it was found that the available H₂ storage capacity by this new technique could reach >6 wt% even at room temperature and 20% NaOH concentration, exceeding all the hydrogen storage systems being currently reported. In addition, a small prototype H₂ storage reactor based on Kipp gas generator was constructed to demonstrate a safe and controllable H₂ generation with reversible self-actuating and pressure-controlled on-off mechanism, which provides an ideal model for commercial reactor design.2. In order to solve the BH₄^- crossover problem in direct borohydride fuel cell (DBFC), a novel battery structure was developed by employing MnO₂ cathode catalyst, which was found to have sufficient electrocatalytic activity for oxygen reduction but not any catalyticactivity for the electrooxidation and hydrolysis of BR,'ions. Thus, it is possible to construct a direct and high efficiency borohydride fuel cell without the need of expensive polymer electrolyte membrane and noble metal catalysts. In this work, the electrooxidation and hydrolysis behaviors BET ions at typical metal electrodes, such as Ni, Pt and Au, were investigated and a reaction mechanism was proposed to deal with the complicated experimental phenomena observed on the anodic process of BJLf. In this reaction scheme, the BR?' hydrolysis is conjugated with the electro-oxidation of BH^'and the electro-reduction of H2O. As a result, what extent BH4" is oxidized to is closely related to the polarization status of the electrode surface. In the potential range of H adsorption, BH4" electro-oxidation is mainly involved in some intermediates like H or H", which inevitably leads to the hydrogen production. Otherwise, in the range of O adsorption, the electro-oxidation mechanism of BPLf changes dramatically, resulting in no hydrogen generation and complete 8 electron reaction. This new mechanism can explain all phenomena in our experiment including the 4-electron oxidation of BH4" on the Ni electrode and more than 4-electron reaction on the Pt and Au electrode. Furthermore, it was observed that some additives capable of shielding the active site of hydrogen adsorption would inhibit the H2 generation not only from BH4" hydrolysis but also from BPL}'electrooxidation.3. Fe-Si alloy was studied as a new generation of anode materials for Li-ion batteries for replacement of the carbonaceous materials because of their intrinsic low capacity, slow dynamics and safety problems of the carbon anodes. To improve the electrochemical performances of Fe-Si alloy, a mechanical milling process was adopted to prepare Fe-Si/C composite by coating carbon on the surface of Fe-Si alloy. Structure analysis shows that Fe-Si alloy and graphite has formed a sandwich structure with the alloy particles as middle cores and the graphite layer as outer shells, which can effectively inhibit the aggregation of Si and buffer the volumetric change to avoid the pulverization of Fe-Si/C composite during charge-discharge cycles. Ex-situ XRD results prove that FeSi2 only serves as an inert and conductive matrix to buffer the expansion of Si. Raman spectra indicate that the large capacity loss due to active carbon atoms on the grain boundaries of graphite is the main cause for the low efficiency in the initial cycle of Fe-Si/C composite. Thus, those methods that can improve the first charge-discharge efficiency of graphite will also work in the case of Fe-Si/Ccomposite. Electrochemical impedance spectroscopy demonstrates that the instability of SEI film and the accumulation of Li in the composite are the major reasons for the observed low efficiency of the Fe-Si/C anode in prolonged charge-discharge cycles.4. To search for highly cycleable silicide anodes, a Ba-Fe-Si/C composite was synthesized and investigated as Li inserting compound because of its relatively lower content of active Si and larger buffering volume of inert components. Charge-discharge experiments showed that Ba-Fe-Si/C composite has high initial discharge capacity (424 mAh/g), suitable charge-discharge plateau (0.2-0.5 V vs. Li+/Li) and favorable cycleability (capacity retention >95% at 25th cycles), exhibiting great practical applicability in the future.