Hydrogen Desportion Performances Of The Novel LiBH₄/2LiNH₂ Complex Hydrogen Storage System

On the basis of the review on the research progress of the novelborohydrides/amides system for hydrogen storage, the LiBH₄/2LiNH₂ complexsystem was selected as the object of this thesis. By means of XRD, SEM, TPD DSCanalyses and hydrogen desorption performance evaluations, the dehydrogenationreaction and the kinetic properties of the LiBH₄/2LiNH₂ complex system wassystematically investigated for developing the novel hydrogen storage material withhigh capacity and excellent performance. The study includes the thermodynamic andkinetic properties of the complex system in the dehydrogenation reaction and thekinetic mechanism of the dehydrogenation reaction. Furthermore, thedehydrogenation performances of the Cu- and Co-catalyzed LiBH₄/2LiNH₂ complexsystem were also studied and the catalysis mechanism of the Cu- and Co-catalyzedsystem was clarified.The hydrogen desorption performances and structures of LiBH₄/2LiNH₂complex system were systematically investigated. The results show that the post-36hmilled sample started to release hydrogen at about 190℃, and the hydrogendesorption accelerated at about 250℃. As the temperature heated up to 355℃, thehydrogen desorption finished and the total amount of hydrogen desorption was about10.8 wt% with a dehydrogenated product of Li₃BN₂. From SEM micrograph of thedehydrogenated sample of LiBH₄/2LiNH₂, the plate-like dehydrogenated productscan be distinctly seen. which suggests a 2D growth mechanism of thedehydrogenated product, namely Li₃BN₂. in the present study. The dehydrogenatedproduct of the LiBH₄/2LiNH₂ sample is composed of a large number of small thinplate-like particles with a thickness of 0.2-0.4μm. The isothermal hydrogendesorption behaviors showed that the sample achieve to full dehydrogenation in 800min at 250℃and only 250 min at 270℃. The kinetic mechamism of thedehydrogenation reaction was analyzed by using the Johnson-Mehl-Avrami (JMA)equation and a diffusion-controlled dehydrogenation kinetic mechanism wasdeduced. The high apparent activation energy (128 kJ/mol) of the sample is responsible for its high operation temperature.Then, the effects of Cu addition on the hydrogen storage performances of theLiBH₄/2LiNH₂ system were studied systematically. It was found that The onsettemperature for hydrogen desorption of the sample with 5wt% Cu was decreased to200℃, a～50℃reduction with respect to the undoped sample, since the startingtemperature was still at about 190℃. About 10.4 wt% of hydrogen can be releasedfrom the Cu-additive sample as it was heated up to 345℃. Differential ScanningCalorimetry (DSC) examination indicated that the melting temperature of theCu-doped sample remains unchanged, but the operating temperature for hydrogendesorption was decreased after melting. It indicates that the additive Cu facilitatedthe formation of the B-N bond and served as the nuclei sites for dehydrogenatedproducts, consequently inducing the improvement of dehydrogenation kinetics.However, the dehydrogenation temperature is still high because of the highmelting-point temperature of theα-Li₃BN₂H₈ phase. The activation energy of thesample with Cu additive determined is about 106 kJ/mol, slightly lower than that ofthe undoped sample.The hydrogen desorption properties and mechanisms of the Co-dopedLiBH₄/2LiNH₂ system were further investigated. It is found that the Co-doped samplestart to dehydrogenate at 150℃and finish at about 270℃. A～85℃reduction wasattained in the starting temperature for hydrogen desorption relative to the pristinesample, indicating that Co is an effective catalyst for the dehydrogenation of theLiBH₄/2LiNH₂ system. XRD results revealed the presence of metallic Co before andafter dehydrogenation. Isothermal dehydrogenation showed the Co-doped samplereleased 10.4 wt% of hydrogen at 190℃within 600 min. DSC examination showedthat the melting-point temperature of the Co-doped sample was lowered to 150℃.Mechanistic analyses revealed that the introduction of metallic Co into theLiBH₄/2LiNH₂ system decreased the melting temperature ofαphase. catalyzed theformation of B-N bonds and facilitated nucleation and growth of the dehydrogenatedproduct. As a consequence. a lower apparent activation energy (-94.8 kJ/mol) for thesample with Co additive was obtained. This lower activation energy is responsible for the significant enhancement of the dehydrogenation kinetics. Besides, the Co-dopedsystem achieves the partial hydrogen storage and the reversible amount is about 1.4wt%.