Enhanced hydrogen uptake/release in 2LiH–MgB 2 composite with titanium additives

Abstract

The influence of different titanium additives on hydrogen sorption in LiHeMgB 2 system has been investigated. For all the composites LiHeMgB 2 eX (X ¼ TiF 4 , TiO 2 , TiN, and TiC), prepared by ball-milling in molar ratios 2:1:0.1, Þve hydrogen uptake/release cycles were performed. In-situ synchrotron radiation powder X-ray diffraction (SR-PXD) and attenu-ated total reßection infrared spectroscopy (ATR-IR) have been used to characterize crystal phases developed during the hydrogen absorptionedesorption cycles.All the composites with the titanium additives displayed an improvement of reaction kinetics, especially during hydrogen desorption. The LiHeMgB 2 eTiO 2 system reached a storage of about 7.6 wt % H 2 in w1.8 h for absorption and w2.7 h for desorption. Using in-situ SR-PXD measurements, magnesium was detected as an intermediate phase during hydrogen desorption for all composites. In the composite with TiF 4 addition the formation of new phases (TiB 2 and LiF) were observed. Characteristic diffraction peaks of TiO 2 , TiN and TiC additives were always present during hydrogen absorptionedesorption. For all as-milled composites, ATR-IR spectra did not show any signals for borohydrides, while for all hydrogenated composites BeH stretching (2450e2150 cm 1 ) and BeH bending (1350e1000 cm 1 ) bands were exactly the same as for commercial LiBH 4 . IntroductionHydrogen can be one of the alternative energy carriers, which should replace the traditional fossil fuels in the near future. One of the promising materials for hydrogen mobile applica-tion which has been studied approximately for 10 years is LiBH 4 [1]. Having high gravimetric and volumetric hydrogen density, this material, though, exhibits unfavorable kinetics and thermodynamics for real application in fuel cells. Recently, it was found that LiBH 4 can be destabilized by the addition of MgH 2 , showing better decomposition kinetics with respect to the pure compound [2]. A detailed analysis of the reversible interaction between LiBH 4 and MgH 2 was made in [3] and can be summarized as follow: 2LiBH 4 þ MgH 2 4 2LiBH 4 þ Mg þ H 2 4 2LiH þ MgB 2 þ 4H 2 (1)The direct reactions (1) take place at w400 C. Opposite reactions, with simultaneous formation of LiBH 4 and MgH 2 under 50 bar of H 2 , was conÞrmed at the temperatures 250e300 C [3]. It was observed that suitable additives might decrease reaction temperatures and improve kinetics of Eq. (1). Experimental evidence of kinetic improvement for reversible middle-temperature Na, Li and Al based complex hydrides doped by titanium additives appeared in 1997 [4]. It has also been reported that the kinetic improvement of the reaction (1) was reached by addition of 1 mol% of TiF 3 [5]. The property enhancement arising upon this additive persists well in the subsequent hydrogen uptake/release cycles. Another prom-inent example of the additives effect was the composite LiBH 4 eMgH 2 eTi{OCH(CH 3 ) 2 } 4 mixed in molar ratio 2:1:0.1 [6]. After ball-milling TiO 2 anatase was found and during 1-st hydrogen desorptio...

A novel three-step method for preparation of a TiB2-promoted LiBH4–MgH2 composite for reversible hydrogen storage

“…Recent studies found that addition of a small amount of transition metal (TM) compounds can markedly shorten or even eliminate the incubation period. [28][29][30][31][32][33][34][35][36][37][38][39][40][41] According to Bo ¨senberg and coworkers, the various TM additives may share a common promoting mechanism, that is, formation of TM borides which act as heterogeneous nucleation sites for the MgB 2 product. [31][32][33][34] These studies elucidated a seeding approach for improving the reversible dehydrogenation property of the 2LiBH 4 -MgH 2 composite.…”

Modeling the kinetic behavior of the Li-RHC system for energy-hydrogen storage: (I) absorption

“…This reaction is generally accepted [22,23,[25][26][27][28][29][30][31][32] as representative of the absorption process and proceeds as a one-step reaction [25,28]. However, it was first suggested by Vajo et al [22] that between 400 and 450 °C a two-plateau region should exist.…”

“…The kinetic properties of the system are influenced fundamentally by temperature and pressure. However, they are also affected by many other factors, including, for example, the reacted fraction, use of additives (catalysts) [22,[25][26][27][28][29][30][31][32]34], microstructure, particle size distribution [35], cycle number [9], and degree of compaction [36], among others.…”

Superior Catalytic Effects of Transition Metal Boride Nanoparticles on the Reversible Hydrogen Storage Properties of Li‐Mg‐B‐H System

“…However, the real kinetics of the undoped Li‐Mg‐B‐H system is sluggish and high temperature is needed to give an acceptable dehydrogenation rate. Thereby, great efforts have been devoted to exploring efficient additives, which can be classified as chlorides, fluorides, and oxides . Undoubtedly, these additives improved the kinetics of RHC systems.…”

“…Yet, almost all of these dopants will generate some by‐products, which will take up some weight in the system, causing a decrease of reversible hydrogen storage capacity . Besides, to get an improved kinetics, activation process is needed . Moreover, the kinetics and cycling stability should be further improved for its practical application.…”