Mg(NH2)2−2LiH is considered a promising hydrogen storage composition because of its reversibility, moderate operating conditions, and favorable thermodynamics. Two barriers which remain for this ...system are reducing (eliminating) ammonia release which occurs in parallel with hydrogen desorption and further reducing the desorption temperature toward its thermodynamically predicted value (∼90 °C). Here, we evaluate the Mg(NH2)2−2LiH composition in the presence of various amounts of its decomposition product, Li2Mg(NH)2. From this screening study, we demonstrate that seeding the composition with the addition of 10 wt % excess Li2Mg(NH)2 is indeed effective for decreasing the peak hydrogen desorption temperature by 40 °C (to 180 °C) relative to the unseeded composition. This kinetic enhancement is quantified by the corresponding reduction in activation energy (E a) from 88.0 to 76.2 kJ/mol for the unseeded and seeded forms, respectively. Similarly, temperature-programmed desorption mass spectrometry (TPD−MS) data reveals that the improved hydrogen desorption temperature also translates to a significant reduction in the amount of ammonia liberated. The effect of cycling a fresh sample containing product seed is also determined and reveals that after 13 charge/discharge cycles, a 25% decrease in performance is observed. Finally, isothermal desorption kinetics for the seeded sample show rapid hydrogen desorption within minutes at 220 °C (whereas the unseeded form requires 2 h) and full desorption at 180 °C within 2 h (whereas the unseeded form is only 50% complete in the same time frame).
•Rotating Fluidized Bed design parameters are investigated.•Small diameter designs show potential for process intensification.•A fluidization quality metric was applied to evaluate device ...performance.
Design of a small scale rotating fluidized bed (RFB) with diameter of 4–6cm to fluidize 40–80μm diameter particles was investigated using computational fluid dynamics (CFD). Simulations explored the impact of the following components on pressure drop and fluidized behavior of the device: particle size, particle density, outer diameter, solids loading, height, number of inlet slots, inlet slot width, angle of inlet slots, chimney diameter, chimney number of slots, chimney slot width, chimney slot angle, and position of the chimney relative to the inlets. Fluidized behavior was evaluated based on a “fluidization quality” metric yielding information about the distribution of particles in the device. Although additional work is required to elucidate design guidelines for small scale RFBs for fine particles, the initial designs evaluated in this work indicate potential for developing a fluidized bed of relatively small diameter, presenting opportunities for process intensification for numerous potential applications.
Self-Catalyzing Hydrogen-Storage Material Yang, Jun; Sudik, Andrea; Siegel, Donald J ...
Angewandte Chemie (International ed.),
01/2008, Letnik:
47, Številka:
5
Journal Article
Recenzirano
Greater than the sum of its parts: Hydrogen storage in complex hydrides is accelerated by using the ternary composite 2 LiNH2/LiBH4/MgH2, which exhibits a “self‐catalyzing” reaction pathway that ...results in faster H2 desorption, lower desorption temperatures, and suppression of NH3 release in comparison to the constituent binary composites (see diagram). The enhanced properties arise from the incorporation of an ionic liquid phase (Li4BH3H10) and from ancillary‐reaction seeding of a reversible H2 storage reaction.
We have recently reported the synthesis and properties of a novel hydrogen storage composition comprised of a 2:1:1 molar ratio of three hydride compounds: lithium amide (LiNH2), lithium borohydride ...(LiBH4), and magnesium hydride (MgH2). This new ternary mixture possesses improved hydrogen (de)sorption attributes (relative to the individual compounds and their binary mixtures), including facile low-temperature kinetics, ammonia attenuation, and partial reversibility. Comprehensive characterization studies of its reaction pathway revealed that these favorable hydrogen storage properties are accomplished through a complex multistep hydrogen release process. Here, we expound on our previous findings and determine the impact of MgH2 content on the resulting hydrogen storage properties by examining a series of (LiNH2)2−LiBH4−(MgH2) X reactant mixtures (i.e., 2:1:X molar ratio) where X = 0, 0.15, 0.25, 0.40, 0.50, 0.75, and 1.0. Specifically, we characterize each starting composition (after ball-milling) using powder X-ray diffraction (PXRD) and infrared spectroscopy (IR) and find that addition of MgH2 facilitates a spontaneous milling-induced reaction, introducing new species (Mg(NH2)2 and LiH) into the hydride composition. We additionally measure the relative hydrogen and ammonia release amounts for each mixture using temperature-programmed desorption mass spectrometry (TPD−MS) and find that ammonia liberation is suppressed for increasing values of X (<0.1 wt % NH3 for X = 1). Kinetic hydrogen desorption data reveal a low-temperature reaction step (centered at ∼160 °C) for all MgH2-containing samples which grows in intensity for larger values of X (up to ∼4.0 wt % H2 for X = 1). Finally, we characterize desorbed samples to investigate the dependence of X (MgH2 amount) on the resulting distribution of observed product phases. These data are used to understand how MgH2 contributes to and impacts the low- and high-temperature hydrogen release events through comparing theoretical (based on the previously proposed reaction set) and observed desorption data for these reactions.
We have investigated the ternary mixture of complex hydrides with stoichiometry 2LiNH2+LiBH4+MgH2, and have identified a set of novel hydrogen storage reactions. One of these reactions involves the ...known reversible reaction Mg(NH2)2+2LiH{left-right arrow}Li2Mg(NH)2+2H2. Previous studies have shown that initiating this reaction from the binary mixture 2LiNH2+MgH2 results in poor hydrogen desorption kinetics and a small amount of NH3 release. In contrast to this behavior, here we demonstrate that by starting from the ternary mixture 2LiNH2+LiBH4+MgH2, the above reaction can proceed at lower temperatures and with improved kinetics, while maintaining reversibility. The advantage of starting with the ternary mixture can be traced to the subsequent formation, melting, and reaction of Li4BH4(NH2)3 with MgH2 to form the mixed imide phase Li2Mg(NH)2, which acts as a seed for the reversible reaction, and is at least partly responsible for the improved kinetic response.
We have investigated the ternary mixture of complex hydrides with stoichiometry 2LiNH
2
+
LiBH
4
+
MgH
2, and have identified a set of novel hydrogen storage reactions. One of these reactions ...involves the known reversible reaction Mg(NH
2)
2
+
2LiH
↔
Li
2Mg(NH)
2
+
2H
2. Previous studies have shown that initiating this reaction from the binary mixture 2LiNH
2
+
MgH
2 results in poor hydrogen desorption kinetics and a small amount of NH
3 release. In contrast to this behavior, here we demonstrate that by starting from the
ternary mixture 2LiNH
2
+
LiBH
4
+
MgH
2, the above reaction can proceed at lower temperatures
and with improved kinetics, while maintaining reversibility. The advantage of starting with the ternary mixture can be traced to the subsequent formation, melting, and reaction of Li
4BH
4(NH
2)
3 with MgH
2 to form the mixed imide phase Li
2Mg(NH)
2, which acts as a seed for the reversible reaction, and is at least partly responsible for the improved kinetic response.
Mehr als die Summe der Teile: Das ternäre Komposit 2 LiNH2/LiBH4/MgH2 unterstützt einen „selbstkatalysierenden“ Reaktionspfad, der eine schnellere H2‐Desorption, niedrigere Desorptionstemperaturen ...und eine effektivere Unterdrückung der NH3‐Freisetzung als bei den binären Komponenten bewirkt (siehe Diagramm). Die verbesserten Eigenschaften werden auf die Entstehung einer ionischen Flüssigphase (Li4BH3H10) zurückgeführt, die die Bildung von Impfkeimen für eine zusätzliche reversible Wasserstoffspeicherreaktion vermittelt.
