| Hydrogen has the potential to replace the carbon-based fuels and become the future energy carrier because of its abundant supply,high energy content and pollution-free conversion.Developing a compact,safe,reliable and cost-effective hydrogen-storage medium is a major challenging issue for establishing a hydrogen economy.One of the main approaches is to store hydrogen in the condensed-phase hydride materials,especially chemical hydrides consisting of light elements.Ammonia borane(NH3BH3)has recently received great interest owing to its high theoretical high capacity of 19.6 wt%and low releasing temperature of H2.Solid NH3BH3 releases 12.5 wt%of H2 at 150?,however,practical applications of NH3BH3 are hindered by unfavorable dehydrogenation kinetics,along with the formation of impurity gases such as ammonia,borazines.To address the above problems,this study synthesized amidoboranes by substituting protic hydrogen on the N atom of NH3BH3 with metal ions.Moreover,the reaction kinetics and thermodynamics of the system were tuned by the addition of metal hydrides.The main contents of this thesis as follows:(1)Lithium amidoborane(LiNH2BH3)was synthesized by high-energy ball milling the mixture of lithium hydride(LiH)and ammonia borane in stoichiometric ratio.The results show that lithium amidoborane can be prepared at 400 rpm and in the ball-to-material ratio of 50:1 under an argon atmosphere.It should be noted that the relative contents of LiNH2BH3,?-LiNH2BH3 and ?-LiNH2BH3,in the post-milled samples change with the increase of the milling speeds.The initial hydrogen evolution temperature(61?)of a-LiNH2BH3 is about 15? lower than that of ?-LiNH2BH3(77.5?),and the activation energy(157 KJ·mol-1)of the hydrogen evolution activity of ?-LiNH2BH3 phase is significantly lower than that of P-LiNH2BH3 phase hydrogen activation energy(272 KJ·mol-1).The hydrogen release rate constant of the a-LiNH2BH3 phase(1.422 × 100)is significantly greater than that of the ?-LiNH2BH3 phase(1.023 × 10-1).(2)A comparative study of the hydrogen release properties of a-LiNH2BH3 with MH0 added(M=Li,Ca and Mg)was investigated.The results show that adding 0.1 MH to a-LiNH2BH3 will improve the hydrogen release kinetics of the systems and at the same time the loss of hydrogen storage capacity of the system is the least.It is demonstrated that the order of the dehydrogenation temperatures of the composite systems from high to low is Ts(MgH2)>Ts(CaH2)>Ts(LiH);Tp(MgH2)>Tp(CaH2)>Tp(LiH),indicating that the addition of 0.1 LiH exhibits the best performance in terms of thermodynamic properties.Besides,The hydrogen release rates of the composite systems from high to low is vde(LiH)>vde(CaH2)>vde(MgH2).(3)Li2Mg(NH2BH3)4 and Li2Ca(NH2BH3)4 were synthesized successfully by high energy ball milling with stoichiometric ratio of reaction raw materials(LiH,NH3BH3,MgH2,CaH2).The phase structure and compositions and chemical bonds of samples were characterized by X-ray diffraction spectroscopy(XRD)and Fourier transform infrared spectroscopy(FTIR).The thermal decomposition behaviors of samples were investigated by differential scanning calorimetry(DSC),thermogravimetry(TG)and isothermal hydrogen release measurements.The results show that the enthalpy of dehydrogenation reaction of Li2Mg(NH2BH3)4 was about 2.8 KJ·mol-1 and the reaction is endothermic,however,the spent fuel cannot absorb hydrogen at 200? under 7 MPa hydrogen pressure.Moreover,Li2Mg(NH2BH3)4 releases 9.79 wt%of hydrogen in 2 hours at 200? under the vacuum condition.On the other hand,the enthalpy of dehydrogenation reaction of Li2Ca(NH2BH3)4 was approximately-1.81KJ·mol-1 and the H2-release from Li2Ca(NH2BH3)4 takes place in a stepwise manner with about one-third of its total hydrogen content liberated in each release step.Li2Ca(NH2BH3)4 releases 7.89 wt%of hydrogen in 24 hours at 150? under the vacuum condition.It is worth mentioning that there is the evolution of the toxic products in this hydrogen release process. |
PDF Full Text Request