Modified Nano-fe <sub> 3 </ Sub> O <sub> 4 </ Sub> Preparation And Hydrogen Storage Properties Of Hydrogen Storage Materials

Hydrogen (H2) has been recognized as an attractive alternative energy carrier, which is lightweight, nonpolluting, highly efficient, and easily derived. In the near future it will become a significant fuel. Finding a safe and reliable hydrogen storage technology and a convenience hydrogen production technology is the biggest challenges to the application of hydrogen energy a new hydrogen storage technology is proposed and developed. Pure H2 without carbon monoxide can be supplied to polymer electrolyte fuel cell (PEFC) vehicles through this method. The process can be explained by following reaction: Fe3O4+4H2=3Fe+4H2O or Fe2O3+3H2=2Fe+3H2O (step 1.chemical storage of H2), 3Fe+4H2O= Fe3O4+4H2 (step 2. recovery of H2). The first step Fe3O4 (or Fe2O3) was reduced into active Fe and then it reacted with vapor to release equivalent H2 in the second step through the water splitting process. Through this reaction the amount of H2 storage is theoretically 4.8wt%-Fe that is closely approach the (IEA) standard 5wt%.The mophplogy will affect the material performance in hydrogen storage. So we did some reaserch in the synthses of Fe3O4. Monodisperse Fe3O4 microspheres assembled by a number of nanosize tetrahedron subunits have been selectively synthesized through the hydrothermal process. The synthesized Fe3O4 microspheres have good dispersibility. The subunits made up of the microspheres were uniform in size and liked-tetrahedron in shape. A series of experiments had been carried out to investigate the effect of reluctant, precipitator and reaction time on the formation of Fe3O4 microsphere and tetrahedron subunits. The results show that ascorbic acid as reductant and urea as precipitator supplied a relative steady environment during the synthesis process and led to the formations of Fe3O4 tetrahedron subunit and monodisperse Fe3O4 microspheres. As the reaction time increased from 3 to 24h, the Fe3O4 microspheres trended towards dispersion. The subunits formed Fe3O4 microspheres varied from spheroid to tetrahedron and from a small nanoparticle (20-30nm) to a large one (90-110nm). A reasonable explanation for the formation of the Fe3O4 microsphere and the tetrahedron subunit was proposed by Ostwald ripening and the attachment growth mechanism, respectively.Fe3O4 with novel nanostructure and uniform size was synthesized by hydrothermal method in a mild chemistry strategy created by ascorbic acid and urea. This Fe3O4 compared with Fe2O3 brought in market played more excellent performance as a hydrogen storage material in both hydrogen storage capability and cycle stability. Ce, Zr, Mo, Al cations were respectively added in our system by hydrothermal method and impregnation method in order to get modified matterial. Effects of various metal additives in the modified samples were investigated. For hydrothermal modified sample, among all the metal cations, Al additive was the most effective one. The temperature of H2 formation for Fe3O4-Al decreased from 400℃to 236℃in average in four cycles comparing with Fe3O4 without additives. At the same time no obvious deactivation was observed in following 3 cycles. The addition of Al not only accelerates the reaction of water decomposition but also improve the stability of the modified sample during redox cycling. In addition, the capacities of hydrogen storage reached 4.5wt% averagely in 4 cycles. A novel mass transfer mechanism was introduced to describe the hydrogen formation process. Sintering may be major reason for the deactivation of some samples as we concluded. We also put forwarded the modification mechanism in the H2 generation process. For mpregnation method Mo cation was the most effective one, the peak temperature keep steady at 313℃and the H2 generation temperature were also relatively low at about 286℃. The amount of H2 storage are all over 4.5wt% in 4cycles. We conclude the reason that Mo Oxide is a relatively intensity delocalization formation, this characteristic make the electron of active Fe trend to join the delocalization thus the active Fe may express some positive charge which can attract the oxygen atom of H2O molecules for the charge attracting. The active energy of Fe react H2O may decrease, so H-O band is easier to break and H2 is much easier to generate at relatively low temperature.