Article pubs.acs.org/JPCC
Hydrogen Desorption Kinetics in Metal Intercalated Fullerides Philippe Mauron,*,† Mattia Gaboardi,‡ Daniele Pontiroli,‡ Arndt Remhof,† Mauro Riccò,‡ and Andreas Züttel†,§ Division “Hydrogen and Energy”, Empa - Swiss Federal Laboratories for Materials Science and Technology, Ü berlandstrasse 129, 8600 Dübendorf, Switzerland ‡ Dipartimento di Fisica, Scienze della Terra, Via G. P. Usberti 7/a, 43124 Parma, Italy § Ecole Polytechnique Fédérale de Lausanne (EPFL), Institut des Sciences et Ingénierie Chimiques, 1015 Lausanne, Switzerland †
ABSTRACT: For different hydrogenated metal intercalated fullerides (Na10C60-H, Li12C60-H, and Li28C60-H) the activation energies for hydrogen desorption were determined by DSC. The Vyazovkin advanced method (VA) was used for the calculation of the reaction model free activation energy as a function of the extent of conversion α. Activation energies are highest for low α and decrease for increasing α, between around 200−145 and 245−175 kJ/mol for the Na and Li compounds, respectively. The decrease of activation energy as a function of the extent of conversion can be explained by an increasing charge transfer to the C60H36+y cage during desorption. Na intercalation leads to a significant thermodynamic destabilization for hydrogen desorption. Dehydrogenation enthalpies of 52 (Na10C60-H), 66 (Li12C60-H), and 69 kJ/mol H2 (Li28C60-H) were determined. These values are lower compared to literature values for desorption of pure C60H36 (74 kJ/mol H2). The onsets of hydrogen desorption are 185 °C (Na10C60-H), 260 °C (Li12C60-H), and 250 °C (Li28C60-H) compared to >400 °C for pure C60H36.
1. INTRODUCTION Metal intercalated fullerides represent a new class of compounds for reversible hydrogen storage,1−7 in which up to 3.5 (Na10C60),1,5 5 (Li6C60, Li12C60),3,2 and 5.9 mass % (Pd decorated Li6C60)6 can be stored. By intercalating alkali or alkaline earth metals in to fullerene, the so-called fullerides are formed, where charges from the metal atoms are transferred to the fullerene cages. A large charge transfer to C60 of up to three electrons per molecule can also be obtained by the hybridization between a C60 monolayer on Cu(111).8 Yamada et al.9 investigated the hydrogen absorption on such electron-doped C60 monolayer on Cu(111) and found that most of the hydrogen is dissociatively chemisorbed on the C60 monolayer. By Fourier transform infrared (FT-IR) spectroscopy, X-ray diffraction (XRD), and neutron powder diffraction (NPD), we also found that hydrogen is chemisorbed on the intercalated C60, and the following reaction mechanism was proposed:1,2
metallic Li, this would result in a hydrogen storage amount of 6.6 mass % (x = z, y = 0). In order to facilitate the readability in the following, we abbreviate the hydrogenated compounds as MzC60-H (e.g., Li12C60-H for Li12−xC60H36+y + xMH). Hydrogen desorption of Na10C60-H started at approximately 200 °C and at 275 °C for Li12C60-H, which is considerably lower than for pure C60Hw, which shows desorption only above 400 °C.1,11 At higher temperatures also fragmentation of the C60 occurs,12,13 making the sorption irreversible. For Na10C60-H it has been shown that above a temperature of 300 °C and a pressure of 200 bar the compound is unstable and dehydrogenates. This is not the case for Li12C60-H under similar conditions, where hydrogen remains stably chemisorbed on C60, at least up to 350 °C, meaning that Na10C60-H is less stable upon heating compared to Li12C60-H.1,2 The energy diagram of reaction I for Na6C60 is shown in Figure 1; for simplicity it is supposed that the reaction is completed (x = z), and C60H36 is taken as the final product (y = 0). On the left-hand side the heat of formations of C60, Na6C60, and C60H36 are shown, and on the right-hand side the levels of additionally 6 NaH. The numerical values for the heat of formation of NaH,14 C60,15 Na6C60,16 and C60H3617 were taken from the literature. The heat of reaction of C60 + 18H2 is −74 kJ/mol H2 (green arrow), and for Na6C60 + 18H2 the value is
MzC60 + ((36 + x + y)/2)H 2 ↔ Mz − xC60H36 + y + x MH (I)
where M is an alkali metal (Li, Na). During hydrogenation M atoms deintercalate from the MzC60 phase and form MH in which hydrogen is ionically bound to M and at the same time in the remaining M-depleted phase Mz−xC60 hydrogen covalently binds to C60. During desorption, the reverse process takes place and MH reacts with the Mz−xC60H36+y phase by reintercalating M released from MH and forming MzC60 again. For Li as high doping levels as z = 28 were shown10 without segregation of © 2015 American Chemical Society
Received: November 6, 2014 Revised: January 8, 2015 Published: January 21, 2015 1714
DOI: 10.1021/jp511102y J. Phys. Chem. C 2015, 119, 1714−1719
Article
The Journal of Physical Chemistry C
can also be applied to noisy data without smoothing, which is difficult for differential methods. A determination of the activation energy as a function of extent of conversion can deliver more insights into the reaction mechanism, e.g., sequential reaction steps showing different activation energies.
2. EXPERIMENTAL SECTION Lithium fullerides (Li12C60, Li28C60) were synthesized by ball milling a stoichiometric mixture of pure C60 (MER Corp., 99.9%) with granular lithium (Sigma-Aldrich, 99%) in an inert argon atmosphere for 15 min in a planetary ball mill (Fritsch, Pulverisette 7). After the milling the sample was heated to 270 °C for 36 h. Sodium fulleride (Na10C60) was synthesized by mixing sodium azide (NaN3) with C60 and long annealing at 450 °C, as explained in ref 1. Although it is possible that the intercalation reaction is not completed, the compounds are labeled with their stoichiometries used for their synthesis. Hydrogen absorption of the samples was measured volumetrically in a pcT (pressure, composition, temperature) instrument.20 The three different samples were hydrogenated under the following conditions Na10C60: 225 °C/185 bar/50 h, Li12C60: 350 °C/180 bar/50 h, and Li28C60: 350 °C/170 bar/50 h. The hydrogenation was started at room temperature up to the final temperature with a heating rate of 10 °C/min. Additionally, Li12C60 was also measured under isothermal condition for three different temperatures (250, 300, and 350 °C) at 175 bar for 4 h. The samples were solely handled in an argon glovebox (H2O and O2 levels 6).25 This can explain the constant activation energy for higher extent of conversion (0.4−0.9), e.g., clearly seen for Na10C60-H. For Li28C60-H more Li is intercalated for the same extent of conversion compared to Li12C60-H; therefore the activation energy is already constant at lower extent of conversion (α > 0.5), and the activation energy is smaller for α < 0.5 because already more charge is transferred. Supposed that at a high extents of conversion (α = 0.925), the intercalated fullerides also end up with Mz−xC60H2 the activation energy of 149 kJ/mol (Na10C60-H) and 191/200 kJ/mol (Li12C60-H/Li28C60-H) are considerably reduced compared to the value of 257 kJ/mol for the dehydrogenation of pure C60H2. At low extent of conversion (α = 0.025) the values of 245 and 235 kJ/mol for z = 12 and z = 28, respectively, for the Li compounds are comparable to the values of pure C60H2. During dehydrogenation thermodynamically stable hydrides such as LiH (Tm = 680 °C, Tdec = 943 °C) and NaH (Tm = 425 1717
DOI: 10.1021/jp511102y J. Phys. Chem. C 2015, 119, 1714−1719
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The Journal of Physical Chemistry C
different activation energies are summarized, the values from the two methods are rather similar. The present investigation shows that the determination of the activation energy as a function of the extent of conversion is very useful and can give valuable additional information for interpreting the reaction mechanism taking place. For the reverse reaction, the hydrogenation, a reverse behavior could be expected: increasing activation energy for increasing extent of conversion. The isothermal hydrogenation for Li12C60 shown in Figure 2 already gives an indication for this because the reaction is fast at the beginning and then slows down after some time (being aware that the reaction mechanism is not necessarily the same). As seen in the Introduction, the values Φ(Eα) − 12 from the minimization ideally should be zero. It can be seen that they are rather low (0.001−0.05); by trend they are the highest (up to 0.09) at low (0.9) α values. By integration of the DSC peaks the enthalpy of reaction Δhm was calculated, and the mean value of all heating rates is given in Table 1. With the measured hydrogen amounts from the desorption measurements the enthalpy of reaction per mol H2, ΔHm is calculated. For Na10C60-H with a value of 52 kJ/ mol H2 the enthalpy of reaction is considerably lower than the value of 74 kJ/mol H2 reported by Karpushenkava17 for the pure C60H36. With a value of ΔS = 126 J/(mol H2 K)17 this leads to equilibrium temperatures (Tcalc = ΔHm/ΔS) of 140 °C for Na10C60-H and 314 °C for pure C60, respectively. For Li12C60-H and Li28C60-H an enthalpy of reaction of 66 kJ/mol H2 (251 °C) and 69 kJ/mol H2 (275 °C) was obtained. In the case of LizC60-H the DSC starting temperature of desorption (260 °C for z = 12 and 250 °C for z = 28) correspond therefore well to the calculated values (Table 1). In summary, for Na a considerable thermodynamic destabilization of the intercalated C60 occurs, and for Li a lower destabilization is seen as suggested by the thermodynamic considerations made in the introduction. At the example of sodium we have seen that compared to the hydrogenation of C60 the hydrogenation of Na6C60 is reduced by the difference of the heat of reaction of Na with C60 to Na6C60 and the heat of formation of NaH.
Figure 7. Activation energy as a function of the extent of conversion α for (a) Na10C60-H, (b) Li12C60-H, and (c) Li28C60-H calculated with the extents of conversion α from Figure 6 according to the Vyazovkin advanced method (VA) (full symbols) and the corresponding Φ(Eα) − 12 (open symbols).
°C, Tdec = 462 °C) (calculated from ref 14) undergo a reaction. These temperatures are all above the dehydrogenation temperatures of the investigated compounds. The mean value of activation energy over the whole α-range is plotted as a straight line. For comparison, Kissinger plots19 (nth order reaction model) of the measured DSC traces are made and plotted in Figure 8. Because these values are taken at the maximum of the peaks, where the rate is the maximum, they are compared with values from the VA method also taken at the maximum rate. As can be seen in Table 1, where the
4. CONCLUSION Different metal intercalated fullerides were synthesized (Na10C60, Li12C60, and Li28C60). The synthesis was complete for Na10C60 and Li12C60 whereas for Li28C60 excess Li could be identified by XRD which led to some inert LiH decreasing the hydrogen storage capacity. Hydrogen absorption was performed at 225 °C for the Na compound and at 350 °C for the Li compound at hydrogen pressures between 170 and 180 bar. The highest reversible hydrogen desorption of 5.1 mass % was attained with Li12C60. The extent of conversion α dependent activation energies for desorption between around 200−145 and 245−175 kJ/mol for the Na and Li compounds, respectively, were found. The decrease of activation energy as a function of the extent of conversion can be explained by the increasing charge transfer to the C60H36+y cage during desorption. Dehydrogenation enthalpies of 52 (Na10C60-H), 66 (Li 12C 60-H), and 69 kJ/mol H2 (Li 28C 60 -H) were determined by DSC.
Figure 8. Kissinger plots19 of the hydrogen desorption of MzC60-H determined by DSC. 1718
DOI: 10.1021/jp511102y J. Phys. Chem. C 2015, 119, 1714−1719
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Corresponding Author
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[email protected] (P.M.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the SNF Synergia project “Smart carbon-based materials for hydrogen storage” under Contract CRSII2_130509/1.
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REFERENCES
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