Titanium and native defects in LiBH<sub>4</sub>and NaAlH<sub>4</sub>

Łodziana, Zbigniew; Züttel, Andreas; Zieliński, Piotr

doi:10.1088/0953-8984/20/46/465210

Cited by 37 publications

(35 citation statements)

References 44 publications

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“…Interestingly, the sorption reaction is only reversible under technically applicable conditions, if the material is doped with transition-metal compounds, most efficiently with titanium compounds [68][69][70]. A multitude of experimental and theoretical methods have been applied to unravel the mechanism of the formation and decomposition process of NaAlH 4 -so far without a conclusive model [71][72][73][74][75][76][77][78][79][80][81]. Furthermore, similar effective additives for other complex hydrides e.g.…”

Section: Sorption Mechanism Of Complex Hydrides Probed By Vibrationalmentioning

confidence: 99%

Hydrogen Dynamics in Lightweight Tetrahydroborates

Remhof¹,

Gremaud²,

Buchter³

et al. 2010

Zeitschrift Für Physikalische Chemie

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Hydrogen . Dynamics . TetrahydroboratesThe high hydrogen content in complex hydrides such as M[AlH 4 ] x and M [BH 4 ] x (M = Li, Na, K, Mg, Ca) stimulated many research activities to utilize them as hydrogen storage materials. An understanding of the dynamical properties on the molecular level is important to understand and to improve the sorption kinetics. Hydrogen dynamics in complex hydrides comprise long range translational diffusion as well as localized motions like vibrations, librations or rotations. All the different motions are characterized by their specific length-and timescales. Within this review we give an introduction to the physical properties of lightweight complex hydrides and illustrate the huge variety of dynamical phenomena on selected examples.

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Section: Sorption Mechanism Of Complex Hydrides Probed By Vibrationalmentioning

confidence: 99%

Hydrogen Dynamics in Lightweight Tetrahydroborates

Remhof¹,

Gremaud²,

Buchter³

et al. 2010

Zeitschrift Für Physikalische Chemie

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“…However because of no change of the limiting rate neither for hydrogen absorption (contracting volume model) nor for desorption (interfaced-controlled one-dimensional growth) [8], induced by the additives, the latter do not show catalytic behavior. The theoretical work [9] has shown that thermodynamic stability of point defects in complex hydrides deÞnes the ground or inter-mediate states or the driving force for atomic motion. Incor-poration of Ti cations in LiBH 4 is energetically unfavorable suggesting that only surface effect takes place.…”

Section: Introductionmentioning

confidence: 99%

“…In case of RHCs, based on lithium borohydride, the nium additives on hydrogen sorption of LiHeMgB2 composite additives likely react with LiBH4 in similar way. However, in order to Þnd suitable chemical structure and optimal MgH2 plays an additional role in the kinetic improvement [9]. amount of the proposed dopant.…”

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confidence: 99%

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

Saldan

Campesi

Zavorotynska

et al. 2012

International Journal of Hydrogen Energy

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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...

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“…Previous calculations have addressed the energetics of defects in NaAlH 4 using static total energies as obtained from density-functional theory (DFT) calculations. 24,30 We go beyond these studies by incorporating finite-temperature vibrational free energies and the free energy of gaseous hydrogen. We also present DFT calculations of defects in Na 3 AlH 6 , a product phase of Reaction 1, which to the best of our knowledge have not been previously published.…”

Section: ' Introductionmentioning

confidence: 99%

Native Defect Concentrations in NaAlH₄ and Na₃AlH₆

Michel

Ozoliņš

2011

J. Phys. Chem. C

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Titanium-doped sodium alanate has been shown to have potentially useful properties for storing hydrogen in fuel cell vehicles. To quantitatively explain the kinetic rates and activation energies for hydrogen release and absorption, it is necessary to calculate the rate of metal diffusion that is required for forming the dehydrogenation products, Na3AlH6 and Al. The bulk defects existing in large concentrations will likely play a dominant role in the transport of metal species. In the first of a series of papers, we use first-principles density functional theory (DFT) calculations to determine the formation free energies and concentrations of native defects in NaAlH4 and Na3AlH6 as functions of temperature, including vibrational and H2 gas-phase entropy contributions. We find that at low temperatures the largest concentrations of native defects in NaAlH4 are positively charged AlH4 vacancies and negatively charged Na vacancies, which can be thought of as the primary defect types for NaAlH4. At high temperatures (near 100 °C), neutral AlH3 vacancies, positively charged AlH4 vacancies, and negatively charged interstitial hydrogen ions and hydrogen vacancies have the largest concentrations at an interface between NaAlH4 and Al. At all temperatures, an interface between NaAlH4 and Na3AlH6 remains predominantly populated by positively charged AlH4 vacancies and negatively charged Na vacancies. In Na3AlH6, the highest defect concentrations belong to negatively charged Na vacancies and positively charged H vacancies throughout the entire temperature range considered. Inclusion of the gas-phase free energy of hydrogen is shown to be crucial for obtaining quantitative estimates of defect free energies and concentrations.

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Titanium and native defects in LiBH₄and NaAlH₄

Cited by 37 publications

References 44 publications

Hydrogen Dynamics in Lightweight Tetrahydroborates

Hydrogen Dynamics in Lightweight Tetrahydroborates

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

Native Defect Concentrations in NaAlH₄ and Na₃AlH₆

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Titanium and native defects in LiBH4and NaAlH4

Cited by 37 publications

References 44 publications

Hydrogen Dynamics in Lightweight Tetrahydroborates

Hydrogen Dynamics in Lightweight Tetrahydroborates

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

Native Defect Concentrations in NaAlH4 and Na3AlH6

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Titanium and native defects in LiBH₄and NaAlH₄

Native Defect Concentrations in NaAlH₄ and Na₃AlH₆