Effect Of Additives And Microwave Radiation On The Absorption/Desorption Properties Of Metal-N-H System

Security, economic and efficient hydrogen storage technology is a n importantbut also bottleneck link in the large-scale application of hydrogen energy. Aftermany years of research, a variety of hydrogen storage materials have been developed.Among them, LiH-LiNH2(Li-N-H) and2LiH-Mg(NH2)2(Li-Mg-N-H) compositeswith high reversible hydrogen storage capacity and moderate hydrogen sorptionconditions have become the most prospective hydrogen storage materials forapplication. However, higher hydrogen sorption temperatures and poor sorptionkinetics prevent them from utilization. Aiming at solving these problems, in thispaper, the latest research progress of Metal-N-H composite systems were firstlysummarized, then the hydrogen storage performances and improvement mechanismsof LiBH4added Li-N-H systems and Mg(BH4)2added Li-Mg-N-H systems werestudied respectively. Finally, the effect of the microwave irradiation on thedesorption properties of Li-N-H/LiBH4and Li-Mg-N-H/Mg(BH4)2systems wereinvestigated to further improve their desorption properties.The effect of LiBH4on the hydrogen sorption properties of Li-N-H system wasstudied. In particular,10mol%LiBH4added sample (0.1LiBH4) started to desorbhydrogen at192oC and ended at331oC, which were lower than those of Li-N-Hsystem by35oC and55oC, respectively. The release of NH3was also obviouslyinhibited.0.1LiBH4sample could reversibly store~4.8wt.%H2at250oC and2MPa of hydrogen pressure, while the pure Li-N-H could only reversibly store2.6wt.%H2. Results of XRD, FTIR and DSC indicate that the improved hydrogensorption properties of LiBH4doped Li-N-H system may be contributed to(LiNH2)x(LiBH4)(1-x)formed by the reaction between LiBH4and LiNH2during ballmilling, which yields a ionic liquid phase and could transfer LiNH2from solid stateto molten state with a weakened N-H bond. The effect of Mg(BH4)2on the hydrogen sorption properties of Li-Mg-N-Hsystem was investigated. It was found that the onset and peak temperatures forhydrogen desorption of10mol%Mg(BH4)2added sample (0.1Mg(BH4)2) decreasedto150oC and190oC, which was25oC and30oC lower than those of pureLi-Mg-N-H system, respectively.0.1Mg(BH4)2sample could reversibly store4.9wt.%of H2at180oC and5MPa of hydrogen pressure, which is3times of the pureLi-Mg-N-H system under the same condition. XRD, FTIR and DSC results showedthat Mg(BH4)2could react with LiH and transform into LiBH4during ball milling. Inthe subsequent heating process, LiBH4reacted with the intermediate phase LiNH2toform (LiNH2)x(LiBH4)(1-x), which promoted the hydrogen sorption reaction of2LiH-Mg(NH2)2. Furthermore，(LiNH2)x(LiBH4)(1-x)has a weakened N-H bond,which promotes the reaction between LiH and N-H group. These two factors maycontribute to the improved hydrogen storage performances of Li-Mg-N-H system.The effect of the microwave irradiation on the hydrogen desorptionperformances of Li-N-H/LiBH4was studied.10mol%LiBH4added sample could beheated to500oC within5min and achieved completely hydrogen desorption underthe microwave irradiation with a power of400W. Further research showed that itwas (LiNH2)x(LiBH4)(1-x)generated during ball milling that made the sample bequickly heated by microwave irradiation. By microwave heating,0.1LiBH4samplecompletely released4.95wt.%of H2at250oC within30min, while only1.44wt.%H2was released from pure Li-N-H by conventional heating, indicating that bothLiBH4additive and microwave have significant effects on the improvment of thehydrogen desorption performances of Li-N-H system.The effect of the microwave irradiation on the hydrogen desorptionperformances of Li-Mg-N-H/Mg(BH4)2was studied.10mol%Mg(BH4)2addedsample could be heated to325oC in10min and realized completely hydrogendesorption under the microwave irradiation with a power of400W. It was found thatLiBH4formed during ball milling and (LiNH2)x(LiBH4)(1-x)formed during heating process acted as microwave absorbing media. Hydrogen content of0.1Mg(BH4)2sample released under microwave at180oC within30min was2.5times more thanthat of conventional heating. Under microwave the sample could desorb1.7wt.%H2at100oC in30min, while no hydrogen could be dsorbed under conventional heating.The results indicated that the microwave irradiation could accelerate the desorptionkinetics and reduce the desorption temperature, suggesting that the microwaveheating is an effective technology for improving the hydrogen desorption kinetics ofmetal-N-H system.