J. Phys. Chem. C 2010, 114, 16801–16805
16801
Stability of the LiBH4/CeH2 Composite System Determined by Dynamic pcT Measurements Philippe Mauron,*,† Michael Bielmann,† Arndt Remhof,† Andreas Zu¨ttel,† Jae-Hyeok Shim,‡ and Young Whan Cho‡ Empa. Swiss Federal Laboratories for Materials Science and Technology, DiVision “Hydrogen and Energy”, ¨ berlandstrasse 129, 8600 Du¨bendorf, Switzerland, and Materials/DeVices DiVision, Korea Institute of Science U and Technology, Seoul 136-791, Republic of Korea ReceiVed: May 10, 2010; ReVised Manuscript ReceiVed: July 15, 2010
We determined the stability of the LiBH4/CeH2 composite system by dynamic pcT (pressure, composition, temperature) measurements by using different constant hydrogen flows and by extrapolating ln(pdes/p0) linearly to equilibrium at zero flow. During desorption, the reaction 6LiBH4 + CeH2 f 6LiH + CeB6 + 10H2 occurs, leading to a theoretical hydrogen capacity of the destabilized system of 7.4 mass %. Within the model used and by applying the Van 't Hoff equation, the following thermodynamic parameters were determined for the desorption: enthalpy of reaction ∆rH ) (58 ( 3) kJ mol-1 H2 and entropy of reaction ∆rS ) (113 ( 4) J K-1 mol-1 H2, leading to a decomposition temperature Tdec ) (240 ( 32) °C at a hydrogen pressure of p0 ) 1.01325 bar, compared with ∆rH ) 74 kJ mol-1 H2 and ∆rS ) 115 J K-1 mol-1 H2 (Tdec ) 370 °C) for pure LiBH4.1 1. Introduction A technically applicable hydrogen storage material should possess a high reversible hydrogen storage capacity, fast sorption kinetics, and a low hydrogen release temperature. LiBH4 has a promising, high gravimetric hydrogen density of 18.4 mass %,2-4 making it an interesting hydrogen storage material for mobile applications. LiBH4 thermally decomposes in two steps. First, it releases 13.9 mass % of hydrogen according to
LiBH4 f LiH + B + 3/2H2
(1)
at a decomposition temperature Tdec ) 370 °C,1 which is well above its melting point Tm ) 268 °C.2 The second step, the decomposition of LiH f Li + 1/2H2, is far too stable for practical applications (Tdec > 727 °C)5 (∆fH0(LiH) ) -181 kJ mol-1 H2).6 The slow kinetics7 of hydrogen sorption for LiBH4 and the relatively high desorption temperature hinder practical applications as a hydrogen storage material. To exploit the hydrogen capacity of LiBH4, additives have to be found to destabilize the system as well as catalysts to improve the kinetics. A lower desorption temperature may lead to the emission of diborane (B2H6) in the decomposition process. The less stable borohydrides such as Mn(BH4)2,8,9 Al(BH4)3,9 or the recently discovered mixed cation systems such as LiZn2(BH4)510 are known to emit significant amounts of diborane during decomposition. The occurrence of diborane as a byproduct depends on the decomposition temperature, as shown in figure 4 of ref 11. Borohydrides with decomposition temperatures above 250 °C desorb mainly hydrogen, and borohydrides with decomposition temperatures below 250 °C also desorb a considerable amount of diborane.11 This can be explained by the fact that diborane thermally decomposes at ∼250 °C.11,12 * To whom correspondence should be addressed. E-mail:
[email protected]. † Empa. Swiss Federal Laboratories for Materials Science and Technology. ‡ Korea Institute of Science and Technology.
Diborane is a highly undesirable byproduct. Because it is a volatile gas, it acts as a boron-sink, strongly reducing the reversibility of the system. Furthermore, diborane is highly toxic. Therefore, the emission of diborane during decomposition for the less stable or destabilized borohydrides must be circumvented. The first and probably the most investigated destabilization of LiBH4 by mixing with MgH2 was proposed by Vajo et al.13,14 and Barkhordarian et al.15,16 The addition of MgH2 leads to the formation of MgB2 during the desorption, reducing the enthalpy of desorption and chemical binding of the boron. In other words, the formation of MgB2 effectively destabilizes LiBH4 (see the energy diagram in figures 1 and 2 in ref 17) and, equally as important, suppresses the development of diborane. The following two-step reaction mechanism has been observed by synchrotron XRD by Bo¨senberg et al.:18 2LiBH4 + MgH2 T 2LiBH4 + Mg + H2 T 2LiH + MgB2 + 4H2 with a hydrogen back pressure between 1 and 5 bar. The hydrogen storage capacity of this mixture is 11.7 mass %, which is comparable to the first desorption step of pure LiBH4 (13.9 mass %). Pinkerton et al.19 plotted a phase diagram for the Li-Mg-B-H system and could show that depending on the choice of the reaction parameters (temperature and applied hydrogen pressure), two- and one-step reactions without formation of Mg are possible. From the phase diagram, it can be seen that the pressure-temperature domain for the direct reaction without formation of Mg is relatively narrow. Other metal hydrides apart from MgH2 may be used as well to destabilize LiBH4. Jin et al.20 investigated the reversible hydrogen storage in 6LiBH4 + MH2 composite systems with M ) Ca, Ce. The following destabilization reaction was proposed
6LiBH4 + MH2 T 6LiH + MB6 + 10H2
(2)
The theoretical hydrogen capacities are 7.4 and 11.7 mass % for the Ce and Ca systems, respectively. Both systems form
10.1021/jp104222j 2010 American Chemical Society Published on Web 09/03/2010
16802
J. Phys. Chem. C, Vol. 114, No. 39, 2010
LiH and the metal hexaboride MB6 as the dehydrogenation products. The reaction is fully reversible for the first reabsorption and is obtained after a treatment for 20 h at a hydrogen pressure of 100 bar and temperature of 350 °C (Ce system) or 400 °C (Ca system). For the Ce system, Jin et al. calculated ∆H ) 44.1 kJ mol-1 H2 and ∆S ) 99.4 J K-1 mol-1 H2, leading to a decomposition temperature Tdec ) 171 °C. Because of the fact that CeH2 and CaH2 are very stable (∆fH0 ) -193 kJ mol-1 H2 and ∆fH0 ) -177 kJ mol-1 H221), there is no formation of Ce or Ca, and it is supposed that reaction 2, for the Ce system, is a one-step reaction without competing reactions when performed at pressure-temperature conditions within the stability domain of LiBH4. (See Figure 1 in ref 20.) The calculations showed that the stability of the Ca system is only slightly lower than the stability of LiBH4; therefore, competing reactions (e.g., reaction 1) could occur. The disadvantage of the Ce system, however, is the lower gravimetric hydrogen density compared with LiBH4/MgH2, LiBH4/CaH2, and pure LiBH4. Shim et al.22 investigated the influence of a hydrogen back pressure for the dehydrogenation of the Ce and Ca systems (reaction 2) and of 4LiBH4 + YH3 T 4LiH + YB4 + 7.5H2 (8.5 mass %). They showed that a hydrogen back pressure (higher than 3 bar) promotes the dehydrogenation reaction and formation of the metal boride, whereas under static vacuum, desorption is incomplete, even after 24 h at 350 °C. Under these conditions, the destabilization reaction did not take place, and only some LiBH4 decomposed. They explained this behavior under static vacuum by the formation of layers of amorphous B or Li2B12H12 enveloping the metal hydride particles and preventing the direct contact of liquid LiBH4 with the metal hydride, therefore hindering the destabilization reaction. Under back-pressure conditions, LiBH4 does not directly decompose but is in direct contact with the metal hydride, and the destabilization reaction can take place. According to the calculations of Jin et al.20 for the LiBH4/ CeH2 system, the pressure-temperature domain where only the one-step reaction 2 takes place is relatively broad; therefore, we have determined the enthalpy ∆rH and entropy ∆rS of the desorption reaction experimentally by dynamic pcT measurements. We can determine ∆rH and ∆rS by plotting the logarithm of the equilibrium pressure peq divided by the standard pressure p0 (1.01325 bar) as a function of the inverse temperature in the so-called Van 't Hoff plot defined by the following equation3
( )
ln
∆rH peq ∆ rS ) p0 RT R
The ideal gas constant is R ) 8.31447 J mol-1 K-1. The slope of the straight line is proportional to ∆rH, and the y-axis intercept is proportional to ∆rS. (In this Article, ∆rH and ∆rS are defined as positive values because the desorption is an endothermic process.) By setting peq ) p0, the desorption temperature can be determined by Tdec ) ∆rH/∆rS. In the dynamic pcT method, pressure composition isotherms are recorded at different flow rates. The equilibrium state is thereby determined by the extrapolation to zero flow. The logarithms of the measured plateau pressures ln(pdes/p0) are linearly extrapolated to zero flow to assess the equilibrium pressures peq. The linear dependence of ln(pdes/p0) on the flow is in accordance with a kinetics model of hydrogen absorption and desorption in Ti-doped NaAlH4.23 The dynamic pcT method was successfully used to determine the stabilities of LiBH41 and NaBH4.24 In the present study, we apply the dynamic pcT method for the first time to a two-phase system in which liquid LiBH4 and solid CeH2 react together.
Mauron et al. 2. Experimental Section CeH2 was synthesized by reacting cerium (99.9%, SigmaAldrich) chips under a hydrogen (purity 99.9999%) pressure of 90 bar in a closed reactor at 350 °C for 2 h. For the synthesis of the LiBH4/CeH2 composite, LiBH4 (95%, Acros) and the synthesized CeH2 were used. A 3 g powder mixture of 6LiBH4 + CeH2 was prepared, and the mixture was charged into a 140 mL hardened steel bowl together with 13 12.7 mm and 24 7.6 mm diameter Cr-steel balls and then sealed with a lid having a Viton O-ring. The powder mixture was ball-milled at 650 rpm for 12 h in a Retsch PM 200 planetary ball mill. The whole process of sample mixing and loading was carried out inside an argon-filled glovebox where oxygen and water vapor levels were kept below 1 ppm. For the pcT measurements, 100 mg of the ball-milled LiBH4/ CeH2 mixture was loaded in a stainless steel autoclave, sealed from air, and transferred to the pcT apparatus. Prior to the measurement, the sample was evacuated at room temperature to a vacuum of
