Hydrogen Absorption and Desorption Behavior of Magnesium Hydride

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Hydrogen Absorption and Desorption Behavior of Magnesium Hydride ( hydrogen-absorption-and-desorption-behavior-magnesium-hydrid )

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1162 N. Takeichi, Y. Sakaida, T. Kiyobayashi and H. T. Takeshita Before the measurement of the hydrogenation and dehydrogenation curves, the sample was subjected to 3 cycles of dehydrogenation and hydrogenation to reduce the amount of unreacted Mg. The sample was first evacuated at room temperature, then it was heated to 663 K. After 2 h, the hydrogen gas was introduced and the hydrogen pressure was kept at 2MPa for 2h. The temperature was kept at 663K during the dehydrogenation and rehydrogenation processes. The particle size distribution was measured by the dry method using a Nikkiso SPR-7340. The average particle sizes of the raw MgH2 and activated MgH2 were 34.8 and 34.9 μm, respectively. The specific surface areas of the raw MgH2 powders and activated MgH2 powders, as determined by the Brunauer-Emmet-Teller (BET) method, were 0.36 and 3.2 m2 g11, respectively. 2.2 Measurement of hydrogen absorption and desorp- tion curves The hydrogen absorption and desorption curves were measured using a Sieverts’ type instrument (Suzuki Shokan Co., Ltd.). Figure 1 shows a schematic diagram of the Sieverts’ type instrument for measurement of the hydrogen absorption and desorption curves. A sample cell with a valve can be separated from the system without exposing the sample to air. A thermocouple was placed in the sample in the cell in order to control and measure the sample temperature. A two hundred mg sample of MgH2 was stored in a cell in a glove box filled with Ar. The reservoir tank and the cell with total inner volume of approximately 90cc were used for the measurements. The apparatus had a working pressure range from 0.01 and 10 MPa with a precision of 0.005 MPa. The error was estimated to be nearly 0.01 H/M and 0.05 mass% for the amount of released hydrogen. Before starting the measurements, each sample was subjected to 3 cycles of dehydrogenation and rehydrogena- tion. The conditions of the dehydrogenation and rehydroge- nation are described in the previous section. The hydrogen desorption behavior was measured in the temperature and pressure ranges of 653­683 K and 0.5­1.7 MPa, respectively. The hydrogen pressure was recorded every 10 s. The hydrogen desorption curve measurement was as follows: (1) Sample was heated to the measurement temperature and the hydrogen pressure was maintained at 2 MPa to avoid any hydrogen desorption. (2) Value 3b, which is connected to the reservoir vessel and sample cell as shown in Fig. 1, was closed, then valve 3c, which is connected to the reservoir vessel and rotary pump, was opened and the hydrogen gas in the reservoir tank was evacuated. (3) Value 3c was closed and valve 3a, which is connected to the reservoir vessel and hydrogen source, was opened and the hydrogen gas was introduced to a certain hydrogen pressure, Ps. Valve 3a was then closed. (4)Valve 3b was opened to measure the hydrogen desorption behavior. 2 p 3a 3c 5 1 3b 4 Fig. 1 Schematic diagram of instrument for measurement of hydrogen absorption and desorption curves; 1. Reservoir tank, 2. Pressure gauge, 3a, 3b, 3c. Valve, 4. Sample cell, 5. Rotary pump, 6. Thermocouple. The hydrogen pressure of the reservoir vessel, Ps, was estimated using the equilibrium hydrogen pressure at the specified temperature, inner volume of the instrument apparatus and amount of the sample stored in the cell. This value corresponds to the initial pressure listed in Table 1 when valve 3b was opened to measure the hydrogen desorption behavior. The Gibbs energy of reaction, "rG, is listed in Table 1 together with the initial and equilibrium hydrogen pressures. "rG was used as the measure of the driving force, "G, defined as the difference between the free energy of the initial state, Gi, and free energy of the equilibrium state, Ge. "rG can be expressed as follows: rG 1⁄4 rG þ RT ln Pi 1⁄4 RT lnPe þRT lnPi 1⁄4 RT lnðPi=PeÞ ð1Þ Here, eq. (1) is used for the formation of Mg and H2 from MgH2. Pi and Pe are the hydrogen pressures of the initial state and equilibrium state, respectively. Based on the enthalpy ("rH) and the entropy ("rS) for Mg­Hydrogen system, Pe was calculated for each condition by using the van’t Hoff equation. R (= 8.31447 J K11·mol11) is the gas constant and T is the measurement temperature. For Pi = 1.0 MPa and Pe = 1.52 MPa at 663 K, "rG is 12.3 kJ·mol11. The Gibbs energy of reaction was calculated for each condition, and the values are summarized in Table 1. 2.3 Analysis of hydrogen absorption and desorption The hydrogen absorption and desorption behavior was analyzed using the fraction, f, of MgH2 and pure Mg in the sample, respectively. The fractions of MgH2 and pure Mg were evaluated from the amounts of absorbed hydrogen and released hydrogen, respectively. The incubation period, which is discussed in Section 3.2, was estimated using the fraction of pure Mg. The incubation period was estimated as follows: (1) The amount of hydrogen released from MgH2 was evaluated every 10s. (2) The fraction of pure Mg, f, was calculated as follows: f 1⁄4 hydrogen content released from MgH2 at t s: ð2Þ expected hydrogen content released from MgH2 in the sample cell 6

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