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Hydrogen Storage in Metal and Complex Hydrides: from Possible Niche Applications towards Promising High Performance Systems
Full text linkA promising alternative to hydrogen storage in high pressure cylinders and cryogenic tanks is the hydrogen storage in solid form as metal hydrides or complex metal hydrides. However, much research is still necessary in this direction. In particular, the optimal pressure and temperature of operation for the use of a hydride-based tank in a PEM fuel cell-powered vehicle should remain in the of 1-10 atm and 25-120 °C, respectively. The further difficulty is related to the weight of the storing materials, which is still too high for efficient mobile applications compared to the amount of stored hydrogen.
The work reported in this thesis deals with systems belonging to several hydride classes such as complex hydrides, interstitial metal hydrides and magnesium-based hydrides. Improvements from the point of view of thermodynamic and kinetic properties are proposed and discussed on systems almost ready for niche applications and on other very promising systems but still far from hydrogen storage applications. Among the classical interstitial hydrides-forming alloys, the interaction of hydrogen with TiCr1.78-xMnx alloys, one of the most promising for the use in high pressure-solid state hybrid tanks, has been studied. Among the Mg-based hydrides, a compacted Mg-Al-based material is proposed in order to overcome the degradation problems found during the scaling up of MgH2 as hydrogen storage medium. Concerning the complex hydrides, the kinetics improvement of the Li-Mg-N-H system by high energy ball milling processing is discussed and the problem of reversibility and slow decomposition kinetics of LiBH4 has been faced.Una promettente alternativa allo stoccaggio dell’idrogeno in bombole ad alta pressione e in contenitori criogenici è lo stoccaggio dell’idrogeno allo stato solido utilizzando idruri metallici o idruri complessi. In ogni caso, molta ricerca è ancora necessaria in questa direzione. In particolare, la pressione e la temperatura di lavoro ottimali per un serbatoio da utilizzare in una vettura basata su celle a combustibili PEM dovrebbero rimanere rispettivamente negli intervalli 1-10 atm e 25-120 °C. L’ulteriore difficoltà è legata al peso dei materiali assorbenti, che è ancora troppo elevato rispetto alla quantità di idrogeno stoccata per applicazioni veicolari efficienti.
Il lavoro riportato in questa tesi riguarda sistemi appartenenti a diverse classi di idruri come idruri complessi, idruri metallici interstiziali, idruri a base di magnesio. Vengono proposti e discussi miglioramenti dal punto di vista termodinamico e cinetico apportati a sistemi ormai quasi pronti ad applicazioni di nicchia e ad altri molto promettenti ma ancora lontani da applicazioni per lo stoccaggio dell’idrogeno. Per quanto riguarda i classici idruri interstiziali, è stata studiata l’interazione dell’idrogeno con le leghe TiCr1.78-xMnx, tra le più promettenti per l’utilizzo in serbatoi ibridi ad alta pressione. Riguardo gli idruri a base di magnesio, un materiale basato su Mg-Al compattato in pastiglie viene proposto per ovviare ai problemi legati allo “scaling-up” del MgH2. Per quanto riguarda la classe degli idruri complessi, viene discusso il miglioramento delle cinetiche di assorbimento/desorbimento di idrogeno grazie al trattamento con macinazione ad alta energia e vengono affrontati i problemi della reversibilità e della lenta cinetica di decomposizione del LiBH4
The problem of solid state hydrogen storage
No full textAbstract
A short review of the materials under investigation suitable for solid state hydrogen storage is presented, with particular reference to the experimental activity carried out at the laboratory of Hydrogen Group of Padova University
Ball-milling and AlB2 addition effects on the hydrogen sorption properties of the CaH2+MgB2 system
No full textAmong the borohydrides proposed for solid state hydrogen storage, Ca(BH4)2 is particularly interesting because of its favourable thermodynamics and relatively cheap price. Composite systems, where other species are present in addition to the borohydride, show some advantages in hydrogen sorption properties with respect to the borohydrides alone, despite a reduction of the theoretical storage capacity. We have investigated the milling time influence on the sorption properties of the CaH2 + MgB2 system from which Ca(BH4)2 and MgH2 can be synthesized by hydrogen absorption process. Manometric and calorimetric measurements showed better kinetics for long time milled samples. We found that the total substitution of MgB2 with aluminum boride in the starting material can improve the sorption properties significantly, while the co-existence of both magnesium and aluminum borides in the starting mixture did not cause any improvement. Rietveld refinements of the X-ray powder diffraction spectra were used to confirm the hypothesized reactions
A study of the LiNH2 - MgH2 system for solid state hydrogen storage
No full textAbstract
The influence of different high energy milling times and of the addition of catalysts such as Nb2O5, TiCl3 and graphite on the hydrogen absorption/desorption (A/D) kinetics of a mixture of 2LiNH2 + 1.1MgH2 has been studied in the temperature range 220–240 °C. It is found that a prolonged milling time is effective in improving the A/D kinetics, irrespective of the presence or not of any kind of tested additive. The enthalpy of decomposition reaction results to be about 40.4 kJ/mol, as derived from van’t Hoff plot using the values of the plateau pressures measured in desorption mode. This thermodynamic parameter fits well with the current literature data
Recent achievements on materials for hydrogen storage
No full textAfter a brief introduction on the problems related to hydrogen storage, recent trends of the research on hydrogen storage materials are presented and discussed: metal hydrides; nanostructured magnesium-based hydrides; nanocomposites based on mixtures of amides and hydrides, amides and alanates, and borohydrides and hydrides; chemical hydrides; and nonhydride systems. The aim of the paper is to show that, even if none of these studied materials satisfies all the requirements for a very wide practical use, some niche applications are already feasible
Structure and hydrogen storage properties of MgH2 catalysed with La2O3
No full textAbstract
The catalytic effect of the addition of lanthanum oxide (La2O3), in the range 0.5–2.0 mol%, on the hydrogen storage properties of MgH2 prepared by ball milling has been studied. The addition of La2O3 reduces the formation during milling of the metastable orthorhombic γ-MgH2 phase. The desorption rate of samples with 1 and 2 mol% La2O3 comes out to be about 0.010 wt% per second at 573 K under an hydrogen pressure of 0.3 bar, better than for sample with 0.5 mol% La2O3. The presence of LaH3 after hydrogenation/dehydrogenation cycles has been observed in all samples. The sample with 1 mol% of La2O3 gives a lower hysteresis factor compared with sample with 2 mol%
Improvement of dehydrogenation kinetics of LiBH4 dispersed on modified multi-walled carbon nanotubes.
No full textThe dehydrogenation kinetics of LiBH4 dispersed on multi-walled carbon nanotubes (MWCNTs) by the solvent infiltration technique has been studied. Commercial MWCNTs were ball-milled for different milling times in order to increase the specific surface area (SSA) as measured by the BET technique. Thermal programmed desorption measurements have been performed using a Sievert's apparatus on samples with different SSA of MWCNTs and different LiBH4 to MWCNT ratio. Pressure composition isotherms (PCI) have been obtained at different temperatures in order to estimate the ΔH and ΔS of dehydrogenation. It has been observed that the dispersion of LiBH4 on MWCNTs leads to a lower dehydrogenation temperature compared to pure LiBH4. Moreover, the dehydrogenation temperature further decreases with increasing MWCNT surface area. An interpretation of the kinetic effect is proposed
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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