| With the fast development of the global economy, the deficiency of fossil fuel re-sources and the deterioration of ecological environment present a tremendous challenge to long-term development of human society. The research and development of a clean and reproducible energy source are of great significance. Hydrogen is believed to be the most promising candidate as an energy carrier to replace the conventional fossil fuel because of its abundance, convenience, and non-polluting nature while the storage and transportation of hydrogen is one of stumbling block to its practical applications. In or-der to meet the requirements of US Department of Energy (DOE) for on-board fuel cell vehicle, it is essential to develop new technologies and materials with high volumetric and gravimetric energy densities. Based on the review of the progress in hydrogen stor-age materials, the NaAlH4 was selected as the major object owning to its high hydrogen contents. However, there still remains problems such as high de-/re-hydrogenation tem-perature, poor kinetics and short cycling life in the pristine NaAlH4 system. To over-come the obstacle mentioned above, a novel space-confined system where NaAlH4 is confined into the ordered mesoporous silica (OMS) or the ordered mesoporous carbon (OMC) was developed. The morphologies, microstructures and element distribution of the samples were systematically investigated by means of X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Nitrogen adsorption/desorption iso-therms, Scanning electron microscopy (SEM), High-resolution transmission electron microscopy (HR-TEM). The de-hydrogenation thermodynamics, de-/re-hydrogenation kinetics, cycling stability and retention of materials were characterized by Thermogravi-metry (TG)-Differential scanning calorimetry (DSC) technique and Pressure-compo-sition-temperature (p-c-T) de-/re-hydrogenation isotherms and cycling test.Firstly, the ordered mesoporous silica SBA-15 was synthesized through the hydro-thermal method. Then the NaAlH4 was loaded into the pore of the SBA-15 by the means of wet-impregnation technique. It was found that as-synthesized SBA-15 possesses a large numbers of reactive silanol groups at the surfaces which can react with the NaAlH4 to produce water and hydrogen, leading to the reduction of the actual hydrogen capac-ity of NaAlH4. After the surface modification of SBA-15 with suitable silane coupling agent, the silanol groups at the surface was decreased markedly, depressing the influences on the NaAlH4. The space confined NaAlH4 in the modified SBA-15 shows fine distri- bution of NaAlH4 particle and the de-hydrogenation product in pore of SBA-15,lower de-hydrogenation temperature of 150℃from the pristine NaAlH4 of 185℃and faster de-hydrogenation kinetics under lower temperature. Moreover, without any catalysts, the re-hydrogenation in a de-hydrogenated space-confined NaAlH4 system was achieved even under the temperatures of 125-150℃and the hydrogen pressures of 3.5-5.5 MPa. By the physical limitation of the pores of SBA-15, the NaAlH4 particles, as well as the re-sulting Na3AlH6, Al and NaH phases, were maintained within nanoscale, and have larger surface area and higher reactivity, preventing the phase segregation and agglomeration of Al element during de-/re-hydrogenation cycles. This is responsible for the improvement of thermodynamics and kinetics of the NaAlH4 space-confined in SBA-15.Secondly, the ordered mesoporous carbon CMK-3 was synthesized using orderd mesoporous silica SBA-15 as the template and sucrose as the carbon source. Then the space-confined system was conducted by loading the NaAlH4 into the pore of CMK-3. The de-hydrogenation temperature of this system was decreased from 180℃to 150℃, and the activation energy of the system also reduced from 120 kJ/mol to 46 kJ/mol compared with pristine NaAlH4. At 180℃the space-confined NaAlH4/CMK-3 sam-ple evolved about 5.0 wt.% hydrogen in 90 min, and had the capacity retention of>80% after fifteen de-/re-hydrogenation cycles. These remarkable improvements on de-/re-hydrogenation cycling stability were mainly attributed to the nano-confinement by the CMK-3 which controled the particle size within nanoscale and prevented the phase seg-regation and agglomeration of elements. Furthermore, the catalysis of carbon itself was also the key factor for the improved performance by comparison with NaAlH4/graphite sample. Therefore, the synergistic effects of both nano-confinement and catalysis of porous materials provided a new avenue for improvement of the hydrogen storage prop-erties of complex hydrides. |
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