Mg-Promoted LiH−LiNH2 Hydrogen Storage System Synthesized by

J. Phys. Chem. C 2007, 111, 8389-8396

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Mg-Promoted LiH-LiNH2 Hydrogen Storage System Synthesized by Using the Mechanochemical Method Tetsuya Ikeda,* Yasuhiro Mikami, and Takashi Haruki Hiroshima Research and DeVelopment Center, Mitsubishi HeaVy Industries, Ltd., Kan-on-shin-machi 4-6-22, Nishi-ku, Hiroshima 733-8553, Japan ReceiVed: February 13, 2007; In Final Form: April 11, 2007

Hydrogenation-dehydrogenation properties in a Mg-added LiH-LiNH2 hydrogen storage system were examined by using a newly developed gas chromatography technique with hydrogen pulse detection as 2LiNH2 + tMg(surface) rf tMg(LiNH2)2 rf tMg(LiNH)2 + H2, tMg(LiNH)2 + 2LiH rf t Mg(Li2N)2 + 2H2 (6.96 mass %). The improved performance of hydrogen storage was found by preparing a Mg additive instead of MgH2 or Mg(NH2)2. A high plateau pressure of >0.1 MPa at 175 °C, which was obtained from PCT measurements, yielded lower dehydrogenation temperatures of 169 and 193 °C in Mg/ LiH/LiNH2 ) 0.5/1/1 and 0.25/1/1 (DTA results), respectively. The reversibility was confirmed from the results of redehydrogenation after rehydrogenation at a pressure of 1 MPa in the temperature range 150250 °C over 1-3 h, but dehydrogenation properties showed little difference. It was concluded that the change in the dehydrogenation kinetics may be caused by the difference in temperature between hydrogenation and dehydrogenation.

1. Introduction Hydrogenation-dehydrogenation in Li-based amide-imide reactions such as LiH-LiNH2 mixtures, exhibiting high performance as a hydrogen storage material, is very attractive to hydrogen storage chemists for practical vehicular applications. Among them, the following reactions have been reported by Chen et al.1

Li3N + H2 rf Li2NH + LiH LiNH2 + LiH rf Li2NH + H2

(1)

It is noted that the second step in reaction 1 yields a hydrogen storage capacity of 6.5 mass %, in spite of the fact that the plateau pressure of hydrogenation is about 0.1 MPa at 285 °C. Thus far, it is well-known that NaAlH4 theoretically releases hydrogen up to 5.6 mass %; however, its reaction kinetics and partial reversibility are not completely satisfactory for practical use.2-4 The Li-based amide-imide reactions that are composed of other light elements such as mixtures of Mg(NH2) 2 and LiH, LiNH2, and MgH2 have been proposed, promising much lower temperatures of hydrogenation-dehydrogenation, and the total reversible capacity is quite high.5-10 Although most of the experimental results indicate the participation of Li+, Mg2+, or NH3 in hydrogen-release in the LiH-Mg(NH2)2 system,5,11 the mechanism of hydrogenation-dehydrogenation has not yet been fully clarified. By introducing Mg for improvement of the Libased amide-imide reaction, only the addition of Mg compounds such as Mg(NH2)2 and MgH2 have been achieved, because no other starting materials have been found. To examine hydrogenation-dehydrogenation kinetics and a new phase in the Mg added Li-based amide-imide reaction, kinetic information about the effects of hydrogenation-dehydrogenation temperature and chemical composition has not been well-established * Address correspondence to this author. E-mail: [email protected].

yet.12-14 Pressure-composition-temperature (PCT) experiments are a powerful tool for studying equilibrium behavior of hydrogen storage systems.1,7-10,15 It has been shown that the determination of equilibrium properties may be very difficult to discuss in terms of final equilibrium state, because the rate of hydrogenation-dehydrogenation may be too slow to obtain equilibrium information.12,13 Even if thermal gravimetric analysis gives the total amount of a weight loss up to a preset temperature, it does not always indicate the net amount of hydrogen released, e.g., production of NH3. Thus, the exact equilibrium properties for hydrogen storage cannot be discussed. It was found that PCT properties of the metal Mg-promoted LiH-LiNH2 system yield a high plateau pressure at 175 °C and may be revealed in the kinetic behavior of hydrogenationdehydrogenation. In this paper, we present the results of experiments on PCT, thermal gravimetric analysis, and kinetic properties of a time-resolved dehydrogenation obtained with the metal Mg-added LiH-LiNH2 hydrogen storage system prepared by using the mechanochemical method. 2. Experimental Section Lithium amide (powder, 95%) and lithium hydride (powder, 30 mesh, 95%) were purchased from Sigma-Aldrich Co. and were used without further purification. Mg powder (80 mesh) and TiO2 nanoparticles (ST-01, average diameter 20-50 nm) were purchased from Koujyundo Kagaku Co. and Ishihara Sangyo Co., respectively. To improve hydrogen storage properties in the present study, TiO2 nanoparticles were selected as the Ti-based additive, whose catalytic properties have been shown to be nearly the same as those obtained for TiCl3.16 TiO2 was added to the LiH-LiNH2 system until the hydrogen-release and ammonia-generation properties reach the values obtained for TiCl3 (1 mol % for LiNH2) and the amount of TiO2 was determined to be 5 mol % for LiNH2. All samples were handled in a glovebox filled with Ar gas to avoid oxidation and water

10.1021/jp071223f CCC: $37.00 © 2007 American Chemical Society Published on Web 05/19/2007

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Figure 1. (a) Schematic view of the pulse detected gas chromatograph. Closing the air driven valve (1) (dashed line), the hydrogen released in the cell at a preset temperature was stored in the cell for a period of 2 min. When valve (1) was opened (solid line), the hydrogen released was introduced into the gas chromatograph with Ar gas flow. Controlling valves (1) and (2), the gas chromatograph of hydrogen was calibrated with H2 gas flow through the calibration tube. (b) The time-resolved hydrogen peaks obtained in the gas chromatograph.

absorption, using a gas recycling purification system (Miwa Mfg. Co., DB0-1KP). Mechanochemical milling experiments for samples were carried out with use of a rocking mill (Seiwa Giken Co., RM05). In the milling process, ∼500 mg of mixed powder and 20 pieces of Cr steel balls with a diameter of 7 mm were placed in a steel pot and milled at about 600 rpm under atmospheric pressure of Ar at room temperature. To avoid an increase in temperature of samples during milling, the milling process was interrupted over 30 min for the milling time of 1 h.17 Thermal gravimetric analysis (TGA) and differential thermal analysis (DTA) were performed with a DTGA-60 (Shimadzu Co.) at atmospheric pressure with purified Ar in the glovebox. Hydrogenation-dehydrogenation of samples was measured in a hydrogen atmosphere, using a pressure-compositiontemperature (PCT) measuring apparatus (Lesca Co.). The equilibrium pressure and hydrogen storage amount were determined when a pressure variation was held within 0.002 MPa over 7 min. A schematic view of the newly developed gas chromatography technique with time-resolved hydrogen pulse detection is shown in Figure 1a. To achieve exact analysis of the kinetic behavior of hydrogen-release, air-driven valves, which divide the desorbed hydrogen into small amounts, were newly installed into a gas chromatograph (Shimadzu Co., GC-14B). The cell filled with the sample after mechanochemical milling was installed in a temperature-controlled bath. The temperature of the sample was raised in a stepwise manner from room temperature to 250 °C and the desorbed hydrogen was released into the cell at a preset temperature, as shown in Figure 1a. When the air-driven valve in Figure 1a was opened, Ar gas

was introduced into the cell for a period of 1 min, and the desorbed hydrogen was carried with Ar into the gas chromatograph. Closing the valve, the hydrogen was analyzed as one hydrogen peak for 2 min. Figure 1b shows a typical signal obtained as a time-resolved hydrogen pulse. The hydrogen can be analyzed until 600 pulses and the final signal was usually too small to detect. The integrated amount of hydrogen released was estimated from the volume of the hydrogen pulse. By using pulse detected gas chromatography, since the hydrogen was divided into small volumes, a high-quality chromatograph can be obtained to examine the kinetic process of time-resolved dehydrogenation and the hydrogen pulse can be analyzed without saturation of the detector, compared with mass spectroscopy. In the present system, ammonia cannot be measured because the hydrogen column (Molecular Sieves 5A) absorbs ammonia completely. X-ray diffraction measurements of products after ball milling and hydrogen-release were performed with Cu KR radiation (Rigaku, RINT-2100). 3. Results and Discussion A. Hydrogenation-Dehydrogenation in the (Mg)LiHLiNH2 System. Mechanochemical milling experiments were performed for Mg-added LiH and LiNH2 mixtures with TiO2 catalyst (5 mol % for LiNH2), i.e., a (Mg)LiH-LiNH2 system, using a rocking mill for a milling time of 20 h. Performances of hydrogenation-dehydrogenation in the (Mg)LiH-LiNH2 system were examined for two mole fractions, Mg/LiH/LiNH2 ) 0.5/1/1 and 0.25/1/1. In addition to these systems, LiH/LiNH2 ) 1/1 (Mg-free), which was of the same system as reported by Chen et al.,1 was also examined.

Mg-Promoted LiH-LiNH2 Hydrogen Storage System

Figure 2. Typical spectra of TGA and DTA obtained in (a) the (Mg)LiH-LiNH2 systems for Mg/LiH/LiNH2 ) 0.25/1/1 and 0.5/1/1 and (b) the LiH-LiNH2 (Mg-free) system for LiH/LiNH2 ) 1/1.

Typical spectra of TGA and DTA in the (Mg)LiH-LiNH2 system obtained at temperature rates of 2 and 5 deg/min are shown in Figure 2a, and those in the LiH-LiNH2 system (Mgfree) obtained at a temperature rate of 5 deg/min are shown in Figure 2b. Comparing the two cases in the absence and presence of Mg, it was found that the addition of Mg to the (Mg)LiHSCHEME 1

J. Phys. Chem. C, Vol. 111, No. 23, 2007 8391 LiNH2 system shifts the weight loss of the samples in TGA to a lower temperature. The temperature showing a peak in the DTA spectrum in Figure 2 was determined to be 226 °C in the absence of Mg and 193 and 169 °C for the Mg/LiNH2 ratios of 0.25 and 0.5, respectively. Broadening of the DTA peak was observed for the (Mg)LiH-LiNH2 system in Figure 2a. It was also found in Figure 2a that the addition of Mg particles (0.5/ 1/1) shifts the peak temperature in DTA to a lower temperature and that the minimum temperature in DTA was determined to be nearly 150 °C at 2 deg/min. From a comparison of the DTA profiles for the temperature rates of 2 and 5 deg/min, it seems that the rate of hydrogen-release was too slow. Taking into account that the improved dehydrogenation indicates dependence on the amount of Mg additive in Figure 2a and that dehydrogenation at a lower temperature is because of the formation of the Mg-LiNH2 complex as discussed in earlier reports,12,17 it is suggested that Mg participates in reactions with hydrogen on the surface of Mg particles. First, let us consider the reaction scheme of hydrogenationdehydrogenation on Mg particle surfaces in the (Mg)LiHLiNH2 system as shown in Scheme 1, where tMg, t Mg(LiNH2)2, tMg(LiNH)2, tMg(Li2N)2, and tMgNLi denote the surface state of Mg particles, the inserted state of Mg that formed durning mechanochemical milling, the first dehydrogenated state, the second dehydrogenated state in the presence of LiH, and the second dehydrogenated state in the absence of LiH, respectively. When a system is heated after milling, dehydrogenation followed by Mg insertion reaction 2 begins, and reactions 3 and 4 accompanied by dehydrogenation occur in series. The theoretical value of hydrogen storage for reactions 2-4 is 6.96 mass %. For the above mechanism, the system of Mg/LiH/LiNH2 ) 0.5/1/1 yields 6.38 mass %, when the addition of TiO2 is 5 mol %. This is a little higher than that obtained in the TGA value in Figure 2a. If reactions 2-4 exist, the Mg composition ratio with LiH and LiNH2 should be set to 0.5/1/ 1. If Mg in the (Mg)LiH-LiNH2 system is lower than Mg/LiH/ LiNH2 ) 0.5/1/1, such as Mg/LiH/LiNH2 ) 0.25/1/1, production

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Figure 3. X-ray diffraction patterns of the samples for Mg/LiH/LiNH2 ) 0.25/1/1: (a) dehydrogenated by raising the temperature to 200 °C, (b) dehydrogenated by raising the temperature to 250 °C, and (c) dehydrogenated by raising the temperature to 350 °C; (b) Mg(LiNH)25, (4) LiH, (0) Li2O, (]) LiOH, and (O) MgNLi.

of Li2NH, i.e., the second reaction in (1), should be taken into account, and can be written as

/2LiNH2 + 1/4Mg + 1/2LiH rf 1/4Mg(Li2N)2 + 3/4H2 (6)

1

/2LiNH2 + 1/2LiH rf 1/2Li2NH + 1/2H2

1

(7)

The above reactions are summarized as follows:

LiNH2 + 1/4Mg + LiH rf 1/4Mg(Li2N)2 + 1/2Li2NH + 5

/4H2 (6.76 mass %) (8)

Reaction 8 in the Mg/LiH/LiNH2 ) 0.25/1/1 system may yield hydrogen storage capacities of 6.76 mass % and also 6.1 mass % when the addition of TiO2 is 5 mol %. This indicates that TGA values near 350 °C obtained in Figure 2b are consistent with those estimated from the theoretical formula, justifying the proposed mechanism. Figure 3 shows X-ray diffraction patterns obtained for dehydrogenated samples at 200, 250, and 350 °C of the (Mg)LiH-LiNH2 ) 0.25/1/1 system. The samples that were dehydrogenated by raising the temperature to 200 and 250 °C showed the appearance of a single phase LiMgN-like ternary imide,5 Mg(LiNH)2, as shown in Figure 3, parts a and b, respectively. The initially added LiH was also found in both the dehydrogenated samples. In Figure 3b, the remaining peaks were assigned to LiOH including a low-angle range. After calcinations at 350 °C, MgNLi and Li2O were formed. However, no phases

Figure 4. (a) PCT obtained for (Mg)LiH-LiNH2 ) 0.5/1/1 (TiO2 of 5 mol %) at 175 and 200 °C. (b) PCT obtained for (Mg)LiH-LiNH2 ) 0.25/1/1 (TiO2 of 5 mol %) at 175 and 200 °C.

of Li2NH, Mg(NH2)2, or Mg(Li2N)2 were found. Thus, it was confirmed that ball-milling Mg with LiH and LiNH2 is sufficient to promote the dehydrogenation in the proposed mechanism. Taking into account both the theoretical hydrogen storage amount of 6.1 mass % and the disappearance of the Mg(LiNH)2 phase at 350 °C in the XRD experiment, it appears that an intermediate state still remains for hydrogen-release at 200250 °C. B. PCT Characterization of Hydrogenation-Dehydrogenation. Figure 4a shows PCT profiles obtained for the Mg/ LiH/LiNH2 ) 0.5/1/1 system at 175 and 200 °C in hydrogen atmosphere. As is apparent from this figure, a plateau pressure was obtained at 3.209 MPa and 175 °C. The value of the plateau pressure obtained in the present study may be much higher than that at 0.1 MPa at 285 °C (Chen et al.1). This indicates that the hydrogen-release performance may be very much improved. Although the data analysis program runs until a maximum pressure of 5 MPa in equilibrium, this maximum pressure is not sufficient to ensure that the sample is in thermodynamic equilibrium with maximum hydrogen absorption at the preset pressure (Figure 4a). This is evident from the TGA experiments by decreasing the weight loss as H2 content and reaching near 5 mass %. Figure 4b shows PCT profiles obtained for the Mg/ LiH/LiNH2 ) 0.25/1/1 system at 175 and 200 °C. The plateau pressure at 175 °C in Figure 4b is less than that obtained in the Mg/LiH/LiNH2 ) 0.5/1/1 system in Figure 4a. As is apparent from this figure, it is suggested that the variation in the profiles in the system Mg/LiH/LiNH2 ) 0.25/1/1 is because of a twostep hydrogenation-dehydrogenation. In the PCT values, hydrogenation-dehydrogenation reached equilibrium with hydrogen pressure variation within 0.0003 MPa/min. Thus, it remains a possibility that hydrogenation-dehydrogenation is

Mg-Promoted LiH-LiNH2 Hydrogen Storage System

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Figure 5. PCT obtained for (Mg)LiH-LiNH2 ) 0.25/1/1 (TiO2-free) at 175, 200, and 250 °C.

not complete in the time allowed at 175 and 200 °C under the present experimental conditions. To examine the effect of additives of TiO2, PCT experiments were performed in the absence of TiO2. The PCT profiles for Mg/LiH/LiNH2 ) 0.25/1/1 (TiO2-free) at 175, 200, and 250 °C are shown in Figure 5. From the comparison of Figures 4a and 5, it was found that the plateau pressure for the presence of TiO2 is the same as that for the absence of TiO2, but the amount of hydrogenation at 175 °C in Mg/LiH/LiNH2 ) 0.25/ 1/1 (TiO2-free) is less than that in Mg/LiH/LiNH2 ) 0.25/1/1. With the absence of TiO2 as shown in Figure 5, it seems that conditions are not sufficient to equilibrate for hydrogenation. Therefore, it is quite clear from this figure that hydrogenation does not reach the maximum value for hydrogen absorption and that PCT in the present study is not well characterized because of slow hydrogenation without the TiO2 catalyst. Next, the thermodynamic quantities were examined by using the plateau pressures at 175 and 200 °C for the values in Figure 4a,b. The van’t Hoff equation can be used to estimate the hydrogenation-dehydrogenation equilibrium in (Mg)LiHLiNH2 systems of Mg/LiH/LiNH2 ) 0.5/1/1 and 0.25/1/1. The values of the plateau pressure in equilibrium at 175 and 200 °C were 3.209 and 4.985 MPa for Mg/LiH/LiNH2 ) 0.5/1/1 and 0.1244 and 0.2705 MPa for Mg/LiH/LiNH2 ) 0.25/1/1, respectively. In the determination of plateau pressure, the final point of the plateau pressure was chosen for estimations,1,15 because the PCT data obtained do not show a clear plateau pressure. The van’t Hoff plots are shown in Figure 6. In this figure, the temperature range used has taken into account the high reproducibility of PCT, avoiding thermal denaturations and phase changes above 200 °C. From the straight lines in this figure, the standard enthalpy change ∆H and the standard entropy change ∆S for Mg/LiH/LiNH2 ) 0.5/1/1 were determined to be -31.02 kJ/mol of H2 and -78.94 J/mol of H2K, respectively. On the other hand, it was also found from this figure that the standard enthalpy change ∆H and the standard entropy change ∆S for Mg/LiH/LiNH2 ) 0.25/1/1 were -54.7 kJ/mol of H2 and -104.8 J/mol of H2K. These values are slightly less than those obtained in other works15 for LiH and LiNH2 systems, ∆H ) -64.5 kJ/mol of H2, ∆S ) -118 J/mol of H2K. C. Kinetic Properties of Hydrogenation-Dehydrogenation. To examine the kinetic properties of hydrogenationdehydrogenation, the (Mg)LiH-LiNH2 system was prepared with use of a rocking mill for milling times of 20 and 42 h. Performances of dehydrogenation in the (Mg)LiH-LiNH2 and LiH-LiNH2 systems were examined for mole fractions Mg/

Figure 6. The van’t Hoff plots of the systems (Mg)LiH-LiNH2 ) 0.5/1/1 and 0.25/1/1.

Figure 7. Integrated amount of hydrogen in the (Mg)LiH-LiNH2 and LiH-LiNH2 systems, which was released as the temperature rose from room temperature to 250 °C and evaluated from 20 hydrogen peaks of the gas chromatogram for each temperature.

LiH/LiNH2 ) 0.25/1/1 and 0.5/1/1 and LiH/LiNH2 ) 1/1, respectively. Now, the amounts of hydrogen storage in the systems (Mg)LiH-LiNH2 ) 0.25/1/1, (Mg)LiH-LiNH2 ) 0.5/1/1, and LiH-LiNH2 (Mg-free) after the mechanochemical milling were determined directly from the amount of hydrogen released. When the temperature of the system was raised stepwise to 100, 150, 175, 200, and 250 °C, the amount of hydrogen released at each step was analyzed continuously as 20 hydrogen peaks of the gas chromatograph as shown in Figure 1b. Figure 7 shows the integrated amounts of hydrogen released up to each temperature level, wherein the amounts of hydrogen released were evaluated from 20 hydrogen pulses at each temperature. As is apparent from Figure 7, hydrogen-release for Mg-added LiH-LiNH2 mixtures was detected slightly from a temperature of 100 °C, and dehydrogenation profiles in the (Mg)LiH-LiNH2 ) 0.25/1/1 and 0.5/1/1 systems may shift to a lower temperature. The maximum values of the integrated amount of the hydrogenrelease up to 250 °C were determined to be 4.71 mass % for (Mg)LiH-LiNH2 ) 0.25/1/1, 3.33 mass % for (Mg)LiHLiNH2 ) 0.5/1/1, and 5.22 mass % for LiH-LiNH2 ) 1/1 (Mgfree). As shown in Figure 7, since the difference in the presence and absence of Mg was found in the temperature range of 100-

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Figure 8. (a) The amount of hydrogen released after mechanochemical milling as a function of the temperature of hydrogen-release, where the temperature was maintained at 175 °C. (b) Integrated amounts of hydrogen released under a constant temperature of 175 °C as a function of temperature of rehydrogenation. Mg/LiH/LiNH2 ) 0.25/1/1 (O) and 0.5/1/1 (20 h) (0), 0.5/1/1 (42 h) (4), and 1/1 (Mg-free) (b).

175 °C, it is clear that the hydrogen-release from the LiHLiNH2 (Mg-free) mixtures can be enhanced by the addition of Mg. To estimate the amount of total hydrogen-release, the temperature of the sample was maintained at 250 °C, and the amounts of hydrogen released were measured continuously until the hydrogen concentration was negligibly small. The integrated amounts of hydrogen released in the temperature range of 100250 °C obtained from the gas chromatogram were determined to be 5.04 mass % for (Mg)LiH-LiNH2 ) 0.25/1/1, 4.70 mass % for (Mg)LiH-LiNH2 ) 0.5/1/1, and 5.52 mass % for LiHLiNH2 (Mg-free), and are also shown in Figure 7. When one compares the amount of hydrogen released for (Mg)LiH-LiNH2 ) 0.25/1/1 and 0.5/1/1, the amounts of hydrogen released for 0.5/1/1 at the low temperature are less than those at the high temperature. Then, both total amounts were summarized as almost equivalent when the temperature was maintained at 250 °C. The amount of ammonia-release was measured previously with temperature-programmed desorption mass spectroscopy,17 and it was found that the concentration of ammonia was negligibly small at the present TiO2 concentration. To obtain the total amount of hydrogen storage, hydrogenrelease experiments under a constant temperature were carried out for the same samples as those in Figure 7 after mechanochemical milling. Figure 8a shows the time-resolved amount of hydrogen-release as a function of the number of hydrogenrelease pulses (3 min/pulse), where the temperature of all samples was maintained at 175 °C. It was confirmed from this figure that the amount of hydrogen-release decreased with an increase in divided pulse number and decreased exponentially at the end of most of the hydrogen-release. Figure 8b shows

Ikeda et al.

Figure 9. (a) The amount of redehydrogenation after rehydrogenation as a function of the number of pulses (3 min/pulse), where the temperature was maintained at 175 °C. (b) Integrated amounts of hydrogen released under a constant temperature of 175 °C as a function of the temperature of rehydrogenation. Mg/LiH/LiNH2 ) 0.25/1/1 (O) and 0.5/1/1 (20 h) (0), 0.5/1/1 (42 h) (4), and 1/1 (Mg-free) (b).

the integration of the amount of hydrogen released, where the values were estimated from the sum of hydrogen volume in Figure 8a. The total amount of hydrogen-release shown in Figure 8b was determined to be 4.2 mass % for (Mg)LiH-LiNH2 ) 0.25/1/1, 2.8 mass % for (Mg)LiH-LiNH2 ) 0.5/1/1, and 5.1 mass % for LiH-LiNH2 ) 1/1 (Mg-free), where hydrogen pulses were accumulated until the 200th pulse. As the net amount of hydrogen released can be found by extrapolation of the experimentally obtained exponential curve, it was found that the total values listed above are less than the saturated values. It is seen from this figure that the rates of hydrogen-release increase with the addition of Mg particles, but the total amount of hydrogen-release of nearly 2.8 mass % for 0.5/1/1 is much less than that obtained for 0.25/1/1 and 1/1 (Mg-free). Therefore, it is concluded that Mg may improve the hydrogen storage performance of the LiH-LiNH2 system in spite of no addition of Mg compounds, such as Mg(NH2)2. Next, the dehydrogenated samples were rehydrogenated at 175 °C and a pressure of 1 MPa for 3 h. Figure 9a shows the amount of hydrogen released after the rehydrogenation as a function of the number of pulses (3 min/pulse), where the temperature was maintained at 175 °C. Figure 9b also shows the integrated amount of hydrogen-release obtained in Figure 9a. When one compares the amount of hydrogen-release in Figure 8a, the variation profiles of hydrogen released for rehydrogenation in each sample may be similar to those obtained after the ball milling. It seems that the dehydrogenation after rehydrogenation has a slow reaction although the sample with

Mg-Promoted LiH-LiNH2 Hydrogen Storage System

Figure 10. (a) The amount of redehydrogenation after rehydrogenation at the constant condition of retention times of 1, 2, and 3 h as a function of the number of pulses (3 min/pulse), where the temperature was maintained at 175 °C. The system Mg/LiH/LiNH2 ) 0.25/1/1 (after milling) (O), the retention times of 1 h (b), 2 h (0), and 3 h (4), and the system of LiH-LiNH2 after the milling (+). (b) The amounts of rehydrogenation after rerehydrogenation under the condition of retention temperatures of 150 (b), 175 (4), 200 (0), and 250 °C (]) (retention time of 3 h) in the (Mg)LiH-LiNH2 system and of 175 °C in the LiHLiNH2 system as a function of the number of pulses (3 min/pulse), where the temperature was maintained at 175 °C.

Mg addition (0.5/1/1) shows almost the same variation profile as that after rehydrogenation. When one compares the kinetic processes for 0.5/1/1 and 0.25/1/1 in Figures 8 and 9, it is clear that the rate of hydrogen-release from 0.5/1/1 is faster than that from 0.25/1/1 and that it is effective in kinetics, although the addition of Mg is disadvantageous in the hydrogen storage amount. Furthermore, it was found that a hydrogen pressure of 1 MPa and rehydrogenation for 3 h may be sufficient for reversibility, and that this reversibility may be improved by the addition of Mg. To confirm the reversibility of the (Mg)LiH-LiNH2 system, two conditions were examined at a hydrogen pressure of 1 MPa: (i) for retention times of 1, 2, and 3 h at 175 °C and (ii) for a retention time of 3 h at 150, 175, 200, and 250 °C. Hydrogen-release experiments were carried out for samples after rehydrogenation at a constant temperature. Figure 10a shows the amount of hydrogen released after hydrogenation for condition (i) as a function of the number of hydrogen-release pulses (3 min/pulse), where the temperature was maintained at 175 °C. From a comparison of the rates of dehydrogenation in Figure 10a, dehydrogenation may be promoted significantly by the addition of Mg. As shown in Figure 10a, hydrogen-release is delayed with a decrease in retention time of rehydrogenation. Figure 11a shows integrated amounts of hydrogenation obtained from the sum of 600 hydrogen pulses in Figure 10a. The integration of the amounts of hydrogen-release was estimated

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Figure 11. (a) Integrated amount of hydrogen released at 175 °C after rehydrogenation as a function of the retention time of hydrogenation in the (Mg)LiH-LiNH2 system of 0.25/1/1 and the LiH-LiNH2 system of 1/1. (b) The integrated amount of hydrogen released at 175 °C after rehydrogenation as a function of the retention temperature of hydrogenation.

from the sum of the hydrogen volume and that of the end of hydrogen-release was estimated from an extrapolation through the amount of hydrogen in each pulse. The increase in retention time accelerates the dehydrogenation rate, but the integrated amount of hydrogen-release is nearly in agreement with the values obtained for other retention times. This suggests the completion of rehydrogenation within 3 h. It is also seen from Figure 11a that the values of hydrogen-release are almost consistent with those obtained from each retention time and that previously obtained from the theoretical formula. Figure 10b shows the amount of hydrogen released after rehydrogenation for condition (ii) as a function of the number of pulses (3 min/pulse), where the temperature was maintained at 175 °C. When one compares variation profiles of the hydrogen-release in this figure, a high temperature of rehydrogenation may delay the hydrogen-release. This suggests that rehydrogenation at a long retention time and high temperature of 200-250 °C causes a decomposition of Mg(LiNH2)2 and suppresses the performance of dehydrogenation in the (Mg)LiH-LiNH2 system. It is considered that the difference between the hydrogenation and dehydrogenation temperatures may be attributed to the change in the affinity in hydrogenationdehydrogenation, e.g., the formation of another path in the proposed mechanism. Figure 11b also shows the integrated amount of hydrogen, which released 600 hydrogen pulses after the rehydrogenation. As can be seen from this figure, the integrated amount of hydrogen released for rehydrogenation at each temperature may be nearly the same as that obtained in the temperature range 150-250 °C of the rehydrogenation. This fact also indicates that a hydrogen pressure of 1 MPa and

8396 J. Phys. Chem. C, Vol. 111, No. 23, 2007 rehydrogenation for 3 h may be completely reversible in a temperature range of 150-250 °C except for a delay in the rate of dehydrogenation. In this figure, the maximum amount of hydrogen-release for the reabsorption at 150 °C and for the release at 150 °C was nearly 5.0 mass %. The value obtained is the same as the theoretical value for reaction 6, taking into account the addition of TiO2. Therefore, the rehydrogenated amount of hydrogen becomes about 5.0 mass %. D. Conclusions. As shown in the present study, one can conclude that reversible hydrogenation-dehydrogenation occurs easily in the Mg-added lithium amide-imide system of Mg/ LiH/LiNH2 ) 0.5/1/1. Improved performances of hydrogen storage were found by using Mg as the additive instead of MgH2 or Mg(NH2)2. A high plateau pressure of >0.1 MPa at 175 °C, which was obtained from PCT measurements, yields lower dehydrogenation temperatures of 169 and 193 °C in Mg/LiH/ LiNH2 ) 0.5/1/1 and 0.25/1/1 (DTA results), respectively. The role of Mg in the present hydrogenation-dehydrogenation can be summarized as follows: The metal Mg additives make N-H binding in Mg(LiNH2)2 unstable (by the first hydrogen-release) and then make Mg(LiNH)2 unstable and LiH can be dehydrogenated at a lower temperature by formation of Mg(Li2N)2. The addition of TiO2 causes an acceleration of hydrogenationdehydrogenation. In the present system, Mg(LiNH)2 may exist as an intermediate state in hydrogenation-dehydrogenation as discussed in the proposed mechanism. To obtain kinetic information on hydrogenation-dehydrogenation, dehydrogenating behaviors were investigated with use of a newly developed gas chromatography technique with hydrogen pulse detection. The reversibility was confirmed from the results of redehydrogenation after the samples were rehydrogenated at a pressure of 1 MPa and temperature range of 150-250 °C over 1-3 h. We concluded that the change in the dehydrogenation kinetics may be caused by the difference in temperature between hydrogenation and dehydrogenation.

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