| Hydrogen has the advantages of abundant reserves,high fuel efficiency and no pollution,so it is considered as one of the most promising alternative fuels for fossil fuels in the future.Conventional high voltage storage and cryogenic storage are not the most ideal methods for actual transportation due to their hazardous properties and their low volumetric energy density.Therefore,searching for a feasible,efficient and safe hydrogen storage technology is the key to the commercial application of hydrogen energy.In recent years,Light-metal based H storage materials are considered as an efficient and convenient alternative to store hydrogen because of their high gravimetric and volumetric hydrogen contents,which also is a relatively safe method of solid-state storage and transport for hydrogen.Li3N-H2 hydrogen storage system has drawn a high degree of attention around the world since it was first reported by Chen et al in 2002,and the LiNH2-LiH system as a classical hydrogen storage system has been also widely investigated because of its high hydrogen storage capacity(6.5 wt%)and low enthalpy of reaction(-45,kJ/mol).Nevertheless,the actual temperature of adsorption/desorption of hydrogen in LiNH2-LiH system is higher,and there is the release of ammonia.Therefore,the system can not meet the requirements of modern commercial hydrogen storage.It has been found by scientists that doping other atoms into LiNH2 lattice or doping other compounds into Li-N-H system is one of the effective methods to improve the hydrogen storage performance of Li-N-H system.In recent years,Light-metal based H storage materials are considered as an efficient and convenient alternative to store hydrogen because of their high gravimetric and volumetric hydrogen contents,which also is a relatively safe method of solid-state storage and transport for hydrogen.Li3N-H2 hydrogen storage system has drawn a high degree of attention around the world since it was first reported by Chen et al in 2002,and the LiNH2-LiH system as a classical hydrogen storage system has been also widely investigated because of its high hydrogen storage capacity(6.5 wt%)and low enthalpy of reaction(-45 kJ/mol).Nevertheless,the actual temperature of adsorption/desorption of hydrogen in LiNH2-LiH system is higher,and there is the release of ammonia.Therefore,the system can not meet the requirements of modern commercial hydrogen storage.It has been found by scientists that doping other atoms into LiNH2 lattice or doping other compounds into Li-N-H system is one of the effective methods to improve the hydrogen storage performance of Li-N-H system.And,we not only considered the effect of doped high efficiency additives on the hydrogen storage performance of LiNH2-LiH system,but also referred to the interaction between ions and CO2/H2 mixing as hydrogen source directly,when we designed the ideas for improving the hydrogen storage performance of LiNH2-LiH system.In this paper,we successfully doped different additives in LiNH2-LiH system by mechanical milling.The influence of the type and content of the additives on the kinetics,mechanism structure and cycle performance of the samples was investigated respectively by using test methods such as XRD,FTIR,TG-MS,and PCT.There are three main parts for discussion:1?Synergistic effects of K+,Ti+ and F-on hydrogen storage performance of Li-N-H systemBy studying the influence of K2TiF6 as additive on hydrogen storage performance of LiNH2-LiH system,we found that the addition of 5 mol%K2TiF6 in the LiNH2-LiH system has a significant effected on improving the dehydrogenation temperature of the system.At the heating rate of 10 ?/min,the lower temperature of dehydrogenation onset and peak temperatures were obtained.The onset temperature of dehydrogenation of LiNH2-LiH-5 mol%K2TiF6 sample is 75?,which is about 124 0Clower than that of LiNH2-LiH sample.The peak temperature of dehydrogenation of LiNH2-LiH-5 mol%K2TiF6 sample is 233?,which is about 66 ?lower than that of LiNH2-LiH sample.Moreover,doping 5 mol%K2TiF6 sample can greatly reduce the reaction activation energy of LiNH2-LiH system,which is also one of the important reasons for the decrease of dehydrogenation temperature.In addition,in the high pressure abs-/desorption experiments of hydrogen,we found that the stable stable cycle performance of the 5 mol%K2TiF6 doped LiNH2-LiH system was improved.Through the reaction mechanism and structural characterization,we determined that the synergistic effects of K+,Ti+ and F-can improve the hydrogen storage performance of Li-N-H system.2?Effect of alkali metal carbonate on hydrogen storage property of Li-N-H systemCurrently,95%of H2 is produced from hydrocarbon based feedstocks(steam reforming of CH4,coal gasification and partial oxidation of light oil residues),with CO2 as a byproduct.And for continuous production processes,CO2/H2 can provide sustainable driving force,which is of low cost and high value.It is found that the doped alkali metal carbonate(Li2CO3,Na2CO3 and K2CO3)can greatly reduce the dehydrogenation temperature of Li-N-H system,we obtained the lowest dehydrogenation starting temperature(74.1?)and peak temperature(238.3?)by doping 5 mol%K2CO3.The LiNH2-LiH-5 mol%L2CO3 sample has no advantage in improving the dehydrogenation temperature,but it can stably recycle for 17 times,which is better than the LiNH2-LiH-5 mol%Li2CO3 sample.It is worth noting that the LiNH2-LiH-5 mol%Li2CO3 sample can stably recycle for 26 times,so we can conclude that the doping of alkali metal carbonate can obviously improve the cycle performance of Li-N-H system.In addition,the activation energy of the reaction decreases with the addition of alkali metal carbonate,especially for the LiNH2-LiH-5 mol%K2CO3 sample,the activation energy of reaction is 44.98 kJ/mol,which is about 58.12 kJ/mol lower than that of LiNH2-LiH system.We determined that alkali metal carbonate,as an excellent additive,can optimize the hydrogen storage properties of Li-N-H system.3?Comparative study of effects of rubidium compounds on hydrogen storage performance of Li-N-H systemThe influence of rubidium compounds(mainly R2CO3,RbF)as the main additive on the hydrogen storage properties of the LiNH2-LiH system was investigated,when the heating rate was 5 °C/min,the dehydrogenation temperature of LiNH2-LiH system was obviously decreased by the doping of rubidium compound.By comparing the different additives,we found that the peak temperature of dehydrogenation for 5 mol%doped Rb2CO3 sample is the lowest(TPeak= 173.2 °C),and there are two peaks.The dehydrogenation peak temperature of LiNH2-LiH-5 mol%Rb2CO3 sample is about 115.4 ? and 37.8 °C lower than that of LiNH2-LiH system and LiNH2-LiH-5 mol%RbF sample,respectively.Through isothermal dehydrogenation under different temperature we found that doping rubidium compounds can improve the dehydrogenation rate of LiNH2-LiH system.After dehydrogenation of 1h,the dehydrogenation rate of LiNH2-LiH-5 mol%RbF sample is 2.7 times of LiNH2-LiH,and the dehydrogenation rate of LiNH2-LiH-5 mol%Rb2C03 sample is 3.7 times of that of LiNH2-LiH at 200 ?.In addition,the reaction energy of dehydrogenation can be greatly reduced by doping rubidium compounds,and the results show that the LiNH2-LiH-5 mol%Rb2CO3 sample is 41.62 kJ/mol.Therefore,the hydrogen storage performance of Li-N-H system can be effectively improved by doping rubidium compounds,and the improvement effect of Rb2CO3 is more obvious than that of RbF. |
PDF Full Text Request