Due to the negative environmental influences arising upon using fossil fuel as well as its diminishing reserves, finding alternative energy is becoming increasingly important for achieving sustainable development. Among the various potential solutions, the clean and recyclable energy carrier --- hydrogen is highly promising. Utilization of hydrogen energy involves mass production, storage/transportation and combustion (fuel cells) of hydrogen. Among them, the intermediate “hydrogen storage” step is generally regarded as the most challenging one. This is evidenced by the lack of on-board applications even after decades of intensive worldwide research efforts on hydrogen storage system/method. Magnesium (Mg) has been attracting considerable interest as a viable hydrogen storage medium due to its high H-capacity (with a theoretical value of 7.6 wt%), high abundance and low cost. However, its practical application has been largely hindered by its high operation temperature and sluggish sorption kinetics. In the process of hydrogen storage in Mg-based system, the particle size effect and heterogeneous interfacial catalysis play key roles in the H2 molecule dissociation, H atom diffusion and Mg-H bonding destabilization. In the past decades, tremendous efforts have been made for improving hydrogen storage performance in Mg-based system.
The aim of this project is to design and fabricate novel efficient nano-structured Mg-based composite with various catalysts to realize magnesium as one of promising candidates for hydrogen storage application under a moderate temperature range with fast sorption kinetics. Based on the fundamental understanding of size effect and catalytic effect, we attempted to design and prepare some promising nano-structured Mg-based composites including MgH2 ball milled with activated carbon (AC) supported nano multiple-component catalysts (e.g. Pd-VOx/AC and Ni-VOx/AC) and MgH2@CMK-3 nanoconfinement system by physical ball milling, wet chemistry and chemical “guest/host” nanoconfinement strategies.
The first part of this program, MgH2-AC composites with different AC adding amount were prepared by ball milling technique to confirm the effect of AC on Mg de/hydrogenation, as AC will be further used as catalyst support in the following study. The optimal hydrogen storage properties of Mg are achieved by adding 5 wt% AC, revealing the important roles of both total surface area and grain size of Mg in the enhanced hydrogen storage properties.
In the second session of this project, wet impregnation nanotechnology was employed to synthesize AC supported nano multiple-component catalysts (e.g. Pd-VOx/AC and Ni-VOx/AC) in order to enhance the efficiency of Mg/catalysts interfacial catalysis as well as reduce the fabrication cost. In the case of MgH2+Ni-VOx/AC composites, an exciting low temperature hydrogen storage performance with fast absorption rate was successfully achieved (absorbed 6.2 wt% hydrogen in only 1 minute at 150 °C). The fundamental understanding of the relationship between crystal size and catalytic effect in AC supported nanocatalysts was further revealed. The experiment results also indicate that the synergistic effects of different catalytic roles in multi-component catalysts. In addition, fabrication cost was reduced by the decrease of both adding level of catalyst to 5 wt% and milling time to 2 h.
In the last part of this thesis, a series of new MgH2@CMK-3 nanoconfiment systems were designed and prepared for improving the thermodynamics of MgH2 to challenge the liberation of hydrogen at ambient temperature. Firstly, MgH2@CMK-3 nanoconfiment system with a high loading ratio of nano MgH2 (ca. 37.5 wt%) encapsulated in monodisperse porous carbon framework (CMK-3) has been synthesized by chemical “guest/host” strategy. It is observed that the size of confined MgH2 is uniform in the range of 1-2 nm. More importantly, MgH2@CMK-3 composite realized the hydrogen release starting at only 50°C due to the significant reduction of the reaction enthalpy and entropy. According to DFT calculations, we conclude that the above observed improvement is a synergistic result of size effect and interface confinement. However, it is also noted that the hydrogen sorption kinetics is still slow in this confined system, underlining the need to further improve the kinetics of dehydrogenation for nanoconfined MgH2 in CMK-3 by functionalizing the surface of CMK-3 scaffold. In this regard, modified host frameworks (N doped CMK-3 and Ni/CMK-3) were continuously investigated and the relevant results are summarized in the last main chapter.