Article pubs.acs.org/cm
Hydrogen Storage Material Composed of Polyacetylene and LiH and Investigation of Its Mechanisms Akihiro Yoshida,* Takashi Okuyama, Yoshinori Mori, Naoki Saito, and Shuichi Naito* Department of Material and Life Chemistry, Kanagawa University, 3-27-1 Rokkakubashi, Kanagawa-ku, Yokohama 221-8686, Japan S Supporting Information *
ABSTRACT: The hydrogen storage/release phenomena on a lithium hydride-polyacetylene composite (LiH-PA) are reported. LiH-PA reversibly releases 2.7 wt % hydrogen. Isotopic experiment and Raman spectroscopy reveal that hydrogen release/storage reaction proceeds via electron transfer between H− and polyacetylene. This is the first report that employs electron transfer in conjugated macromolecules for hydrogen storage.
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INTRODUCTION The development of highly efficient hydrogen storage systems is the key technology for realizing a hydrogen energy society in the near future. For achieving dense storage of hydrogen without applying cryogenic and/or highly pressurized conditions, nonmetal hydrides such as hydrocarbons, ammonia, and ammonia borane, as well as metal hydrides, especially of light metallic elements, have been investigated for conventional storage media.1,2 Among various metal hydrides, complex metal hydrides composed of lithium, aluminum, magnesium, and sodium are regarded as suitable candidates because they can reversibly store hydrogen.3−9 However, all these hydrides are inadequate for practical usage because of the following problems: (a) requirement of high temperatures hydrogen release, (b) slow rates of hydrogen release and storage reactions, and (c) inability to restore hydrogen under moderate conditions. Addition of carbonaceous materials such as graphite and carbon nanotubes to hydrogen storage systems is one idea to improve these problems and develop new hydrogen storage systems.10−15 While investigating the hydrogen storage/release properties of the composites of carbonaceous materials with lithium, we found that the lithium−fullerene composite (C60Lin) reversibly stores/releases hydrogen under practical conditions.16 NMR analyses revealed that molecular hydrogen was stored as H+ incorporated with C 60n−, and as H− incorporated with Li+. Stabilization of negative charges on a C60 molecule by its conjugation system is the characteristic feature of this material. This work motivated our further investigation of hydrogen storage materials composed of metal hydrides with conjugated molecules. Polyacetylene is an extremely simple conjugated hydrocarbon macromolecule. Its capabilities as an electroconductive polymer have generated a great deal of interest.17−19 Several reports have described the potential of this material for application to hydrogen storage materials. Shirakawa et al. reported that cation-doped polyacetylene activates molecular hydrogen, enabling the H2−D2 isotopic exchange reaction to proceed.20 This report shows clearly that cation-doped polyacetylene can © 2014 American Chemical Society
activate molecular hydrogen. The other attractive feature of this material is its high thermal stability. Although most organic macromolecules are stable only below 250 °C, polyacetylene is stable at 350 °C in the absence of oxygen.21 In spite of the features described above, only a single report explains an investigation of the possibility of using polyacetylene as a hydrogen storage material. Ihm et al. conducted theoretical investigations of Ti-decorated and Sc-decorated polyacetylene, demonstrating that 2−3 molecules of hydrogen can be stored on each Ti or Sc atom on polyacetylene.22,23 However, no experimental investigations have been described in the literature to date. This report describes reversible hydrogen storage/release phenomena on a composite of lithium hydride with transpolyacetylene (LiH-PA) and presents an investigation of their mechanism. LiH-PA released 2.71 wt % of hydrogen at 573 K. It was rehydrogenated with 3 MPa of hydrogen at 523 K. The isotopic tracer experiment employing deuterated polyacetylene and Raman spectroscopy revealed that hydrogen release might occur as a result of electron transfer from hydride ions of LiH to polyacetylene chains. As far as we know, this is the first report that employs electron transfer in conjugated macromolecules for hydrogen storage.
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EXPERIMENTAL METHODS
Materials. Mg powder (Wako Pure Chemical Industries Ltd.), THF (dehydrated grade, Wako Pure Chemical Industries Ltd.), Cp2TiCl2 (TCI), P(OEt)3 (TCI), LiH (Aldrich), CaC2 (Wako Pure Chemical Industries Ltd.), and D2O (Acros Organics) were used as purchased. MS4A (Wako Pure Chemical Industries Ltd.) was calcined at 573 K for 3 h before use. Graphite (Tokablack #3800, Tokai Carbon Co., Ltd.) and polyethylene (average Mw = 3 000 000−6 000 000, Aldrich) were evacuated at 473 K before use. Acetylene (Toho Acetylene Co., Ltd.) was used for polymerization after purification using a glass line equipped with a gasbag and a liquid N2 trap. The Received: January 7, 2014 Revised: May 16, 2014 Published: May 29, 2014 4076
dx.doi.org/10.1021/cm500042c | Chem. Mater. 2014, 26, 4076−4081
Chemistry of Materials
Article
gasbag, charged with acetylene, was connected to the liquid N2 trap. After most of the acetylene was collected into the trap, the glass line was evacuated. The trap connected to the gasbag was warmed to room temperature to gasify the acetylene. This procedure was conducted several times to remove contaminated oxygen, nitrogen, and solvents. Synthesis of trans-Polyacetylene. Polyacetylene was synthesized by polymerization of gaseous acetylene with Ti complex, according to the procedures reported by Takeda et al., with minor modifications.24 To obtain powdered polymer instead of film polymer, we modified the concentration of Ti complex. All procedures were conducted with Schlenk technique under an argon atmosphere. Mg powder (0.155 g, 6.36 mmol), MS4A (2.20 g), and Cp2TiCl2 (0.625 g, 2.51 mmol) were placed in a Schlenk tube and evacuated for 10 min at 373 K. After cooling to room temperature, THF (25 mL) and P(OEt)3 (0.86 mL, 5.12 mmol) were added successively with stirring. The color of the solution changed from orange to green; then it became dark brown. After 3 h of stirring at room temperature, the remaining Mg powder and MS4A were filtered off. The filtrate was placed in a 200 mL three-necked round-bottomed flask. Acetylene at atmospheric pressure was introduced into the three-necked flask with stirring of the solution. The reaction mixture was stirred continuously for 48 h. After a certain period, the remaining acetylene was removed; then the obtained black powder was washed successively with THF. The obtained polyacetylene was evacuated at 473 K for 2 h (yield: 3.10 g). IR: 1011 cm−1 (C−H out-of-plane deformation), 13C CP-MAS NMR: 136.4 ppm. Preparation of LiH−trans-Polyacetylene Composite. A lithium hydride−polyacetylene composite (LiH-PA) was prepared using ball-milling. 0.187 g of polyacetylene (14.4 mmol of carbon atoms) and 0.114 g of LiH (14.3 mmol) were placed in a 40 mL stainless steel mill-pod with 18 pieces of zirconia balls (10 mm diameter). The mill-pod was charged with 1.0 MPa of argon to prevent contamination of air with subsequent rotation using a planetary ballmill apparatus (Pulverisette P-7; Fritsch GmbH) at 250 rpm for 5 h. Hydrogen Storage and Desorption. Hydrogen storage was conducted using the following procedures. After 0.050 g of the sample was placed in a U-shaped stainless steel reactor, the reactor was evacuated and 3.0 MPa of hydrogen was introduced. After hydrogen introduction, the reactor temperature was increased by 5 K/min to 523 K and was held for 12 h. After cooling to ambient temperature, the remaining hydrogen was released slowly, followed by evacuation of the reactor. A temperature-programmed desorption (TPD) experiment of hydrogen was conducted in a U-shaped stainless steel reactor by increasing the reactor temperature at 5 K/min with a 50 mL/min of He flow. The amount of desorbed hydrogen was monitored using a quadrupole mass analyzer (QME200; Pfeiffer Vacuum GmbH). XRD, Raman, and NMR Measurement. XRD measurements were performed using a diffractometer (MultiFlex; Rigaku Corp.) with Cu Kα radiation. Raman measurements were performed using a spectrometer (NRS-1000; Jasco Corp.) equipped with a 532 nm YAGlaser source. The samples were placed on dry ice to prevent heat decomposition. Solid state 7Li NMR measurements were performed on a JEOL JNM-270 spectrometer equipped with a 5 mm MAS probe. A sample was placed in an O-ring sealed sample tube. Spectra were recorded at 105.00 MHz. 5 kHz of MAS speed and proton decoupling was applied for all the measurements. The values of the chemical shifts were referenced to solid LiCl (0 ppm).
Figure 1. TPD profiles from 300 to 823 K for (a) LiH-PA composite, (b) trans-polyacetylene, and (c) LiH.
which were derived respectively from methane and ethylene, were not observed to any marked degree. It is notable that hydrogen release from LiH was not enhanced by the addition of 10 mol % of TiH2, suggesting that undetectable amounts of remaining titanium catalysts on PA for polymerization did not participate in the enhancement of hydrogen release on LiH-PA. Although evolution of hydrogen was observed in the TPD analysis of polyacetylene alone, its shape was almost flat below 600 K and comparable amount of methane and ethylene to hydrogen were released at 750−823 K (Figure 1b). Lithium hydride did not release hydrogen below 750 K (Figure 1c). Therefore, hydrogen evolution from the composite of LiH-PA was clearly not a result of simple heat decomposition of polyacetylene or lithium hydride. Cyclic Hydrogen Release/Storage Capability of LiHPA. To examine the recyclability of the LiH-PA composite (Li:C = 1:1), the hydrogen-released sample at 573 K for 5 h was rehydrogenated under 3.0 MPa of hydrogen at 523 K for 12 h, followed by TPD measurements. The TPD measurements were performed at ranges from 300 to 573, 623, or 823 K with subsequent hydrogenation at 523 K. In TPD measurements up to 573 and 623 K, the sample temperature was subsequently held for 5 h after raising the temperature. These TPD and hydrogenation procedures were repeated for 5 cycles. The TPD profiles and the amount of released hydrogen are presented respectively in Figure 2 and Table 1. At the first cycle, hydrogen release amounts were increased when the final temperature of TPD were raised. However, on the second cycle, only the sample heated to 573 K exhibited almost identical hydrogen release capability, although significantly lower hydrogen release amounts were observed in the other two samples. Those results represent clearly that the reversible hydrogen release reactions proceed at 573 K, although irreversible reactions proceed at temperatures higher than 623 K. In the cyclic experiment conducted with hydrogen release at 573 K, 2.43 wt % of hydrogen was released even after five cycles, which corresponded to 90% of the initial amount. This fact indicates that the conditions of 573 K for release and 523 K with 3.0 MPa of hydrogen for storage were suitable for reversible hydrogen storage/release cycles. Slight decrease of the hydrogen release amount in five cycles was probably because the irreversible reaction proceeded even at 573 K to a small extent. The hydrogen pressure of 3.0 MPa in the storage
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RESULTS AND DISCUSSION TPD Analyses of LiH-PA Composite. The hydrogen release capability of the LiH-PA composite (elemental ratio is Li:C = 1:1) prepared using a ball-milling technique was examined using TPD measurements obtained in conditions of 300 to 823 K. The TPD profiles for the as-milled sample are presented in Figure 1a. Hydrogen started to release at around 500 K, and peaked at 675 K. The hydrogen release amount calculated from the integration of the m/z = 2 signal from 300 to 823 K was 7.77 wt %. Other signals, such as m/z =16 and 28, 4077
dx.doi.org/10.1021/cm500042c | Chem. Mater. 2014, 26, 4076−4081
Chemistry of Materials
Article
Table 1. Amount of Released Hydrogen in the Hydrogen Storage/Release Cyclic Experiments in Different Hydrogen Release Temperatures amount of released hydrogen (wt %) cycle 1 2 3 4 5
573 K 2.71 2.52 2.74 2.63 2.43
(100) (93) (101) (97) (90)
623 K 5.29 1.87 1.58 1.45
(100) (35) (30) (27)
823 K 6.06 (100) 0.74 (12)
Table 2. Amount of Released Hydrogen and Particle Size of LiH on LiH-PA, LiH-Graphite, and LiH-Polyethylene Composites
LiH-PA LiH-graphite LiH-polyethylene a b
amount of released hydrogena (wt %)
particle sizeb (nm)
2.71 0.44 1.33
36 48 33
Determined by the TPD measurements from 300 to 573 K. Estimated by the Scherrer equation.
LiH-polyethylene were fewer than LiH-PA. This fact indicates that emergence of enhanced hydrogen release are not a result of simple decomposition of nanosized LiH decorated with carbonaceous or polymer materials, but may be a result of involvement of PA’s characteristic properties. Presumably, the hydrogen release from the LiH-PA composite takes place in accordance with the reaction shown as eqs 1 and 2. Equation 1 is the electron transfer reaction between hydride ion and polyacetylene to form Li-doped polyacetylene. Equation 2 is the acid−base reaction by which hydrogen atoms on polyacetylene chains are abstracted by hydride ions of LiH. When the hydrogen release reaction proceeds in accordance with eqs 1 and 2, the maximum hydrogen release amounts on the LiH-PA composite are, respectively, 2.40 and 9.60 wt %. Hydrogen release amounts observed in the cyclic experiments at 573 K are close to the maximum amount expected for eq 1, suggesting that the hydrogen release reaction proceeded in accordance with eq 1. However, in the hydrogen release at higher than 623 K, the hydrogen release amounts exceeded the maximum amount expected for eq 1, suggesting that the reaction shown in eq 2 proceeded in a certain degree. It is presumed that eq 1 is reversible and that eq 2 is irreversible because recyclability was good in hydrogen release at 573 K and was poor above 623 K. Reversibility of the reaction shown as eq 1 was confirmed by 7 Li MAS NMR spectroscopy and the hydrogen storage/release experiment employing Li-doped PA as a hydrogen release state. 7 Li MAS NMR spectrum of hydrogenated Li-doped polyacetylene was almost identical to that of LiH in the LiH-PA composite, indicating formation of LiH by hydrogenation of Lidoped PA (see Figure S1 in the Supporting Information). In the TPD measurement from 300 to 573 K, hydrogenated Lidoped PA exhibited a similar hydrogen release profile (see Figure S2 in the Supporting Information). Those observations clearly indicate that eq 1 is reversible. Although eq 1 suggests that the 1:2 ratio of Li:C is consistent with the stoichiometry, the hydrogen release/storage capacity of the 1:2 (Li:C) composite was inferior to the 1:1 composite; the hydrogen release amounts of the 1:2 (Li:C) composite in
Figure 2. Hydrogen release profiles for LiH-PA from 300 to (a) 573, (b) 623, and (c) 823 K.
process is lower than well-known NaAlH4-based systems, which require more than 10 MPa.6−9 This is one of the beneficial points of this material in a practical viewpoint. LiH-graphite and LiH-polyethylene were prepared by the same procedure as LiH-PA to compare the hydrogen release properties. The amount of released hydrogen and the particle size of LiH on those composites are summarized in Table 2. Although the particle sizes were not so much different in these composites, the hydrogen release amounts of LiH-graphite and 4078
dx.doi.org/10.1021/cm500042c | Chem. Mater. 2014, 26, 4076−4081
Chemistry of Materials
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cycle 1 to 3 were 1.60, 1.26, and 0.91 wt %, respectively. This inferiority was probably caused by the consumption of LiH due to irreversible side reactions between LiH and polymer termini. To achieve the nearly theoretical amount of hydrogen release, addition of excess amount of LiH is necessary to compensate consumed LiH by side reactions. On the other hand, the 2:1 (Li:C) composite released 1.97 wt %, which is identical amount to eq 1. This result indicates that excess amount of LiH is not responsible for hydrogen release in the present conditions. n LiH + (C2H 2)n → [Li n(C2H 2)n ] + n/2H 2
(1)
2n LiH + (C2H 2)n → (C2Li 2)n + 2nH 2
(2)
Mechanistic Investigation of Cyclic Hydrogen Release/Storage on LiH-PA. To elucidate the hydrogen release mechanisms, hydrogen release experiments were carried out on the composite of LiH-(C2D2)n prepared using the same procedure as that for LiH-PA. When the hydrogen release reaction proceeds in accordance with eq 1, H2 is expected to be the main product in released hydrogen. However, when the hydrogen release reaction proceeds in accordance with eq 2, HD is expected to be the main product. Figure 3 presents the
Figure 4. Raman spectra of LiH-PA composite: (a) as-milled, (b) after hydrogen release at 573 K, (c) after rehydrogenation, and (d) after hydrogen release at 823 K.
1298, and 1127 cm−1 which were assigned to Raman-active stretching vibrations of conjugated C−C bonds for an infinite planar polyene chain, although their positions were slightly shifted to higher wavenumber side than those of transpolyacetylene (Figure 4a).25 This observation suggests that the structure of polyacetylene was retained after ball-milling with LiH. The shift of the bands was probably caused by partial electron transfer from H− to a polyene chain. After hydrogen release at 573 K, large bands were observed at around 1120 and 1500 cm−1 in a similar manner to the as-milled LiH-PA, suggesting again that the polyacetylene structure was retained, although the significantly shifted peak was observed at 1577 cm−1 (Figure 4b). In the Raman spectra for Na-doped and Kdoped polyacetylene, it was reported that the peaks appeared at around 1580−1600 cm−1.25,26 Therefore, it is expected that the structural and electronic environment of polyacetylene chains on hydrogen released LiH-PA is similar to that of cation-doped polyacetylene: accepting an electron from H− to form an anionic polyacetylene chain interacting with Li +. The impedance measurements also support the occurrence of electron transfer. The electrical conductivities measured by the AC impedance method of bear polyacetylene, as-milled, hydrogen released, and rehydrogenated samples were 2.1 × 10−8, 3.1 × 10−9, 1.5 × 10−6, and
