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Hydrogen permeation of tungsten phosphate glass thin films
Solid State Ionics 180 (2009) 556–559
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Solid State Ionics j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s s i
Hydrogen permeation of tungsten phosphate glass thin films Hiromasa Tawarayama a,⁎, Hiroshi Kawazoe b, Hideo Hosono a,c a b c
Materials and Structures Laboratory, Tokyo Institute of Technology, Nagatsuta-cho 4259, Midori-ku, Yokohama 226-8503, Japan Kawazoe Frontier Technologies Corporation, Kuden-cho 931-113, Sakae-ku, Yokohama 247-0014, Japan Frontier Research Center, Tokyo Institute of Technology, Nagatsuta-cho 4259, Midori-ku, Yokohama 226-8503, Japan
a r t i c l e
i n f o
Article history: Received 22 January 2008 Received in revised form 15 July 2008 Accepted 4 December 2008 Keywords: Tungsten phosphate glass Mixed conductor Thin film Hydrogen separation
a b s t r a c t Thin films of tungsten phosphate glasses were deposited on a Pd substrate by a pulsed laser deposition method and the flux of hydrogen passed thorough the glass film was measured with a conventional gas permeation technique in the temperature range 300–500 °C. The glass film deposited at low oxygen pressure was inappropriate for hydrogen permeation because of reduction of W ions due to oxygen deficiency. The membrane used in the hydrogen permeation experiment was a 3-layered membrane and consisted of Pd film (~ 20 nm), the glass film (≤300 nm) and the Pd substrate (250 µm). When the pressure difference of hydrogen and thickness of the glass layer were respectively 0.2 MPa and ~ 100 nm, the permeation rate through the membrane was 2.0 × 10− 6 mol cm− 2 s− 1 at 500 °C. It was confirmed that the protonic and electronic mixed conducting glass thin film show high hydrogen permeation rate. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Protonic and electronic mixed conductors have been expected as hydrogen separation membrane materials because of their high selectivity and permeability of hydrogen [1–6]. Hydrogen is a promising energy carrier in hydrogen energy technology, and the demand will remarkably increase with the spread of fuel cells in the near future. At the present large scale production technology of hydrogen is steam reforming of hydrocarbons due to its cost competitiveness and, therefore, processes of hydrogen separation from byproducts gases are indispensable. Especially, highly pure hydrogen is needed for polymer electrolyte fuel cells because contaminant carbon monoxide is very poisonous to the catalysts for dissociation of hydrogen. Tungsten phosphate glasses react with hydrogen and water vapor at the ambient temperatures below the glass transition temperature (Tg) and W6+ ions in the glasses are reduced to W5+ [7,8]. The reduced tungsten ions, W5+, show strong intraionic and intervalence optical absorptions in the visible to near infrared region and the color of the glass changes to dark blue. Concentration profile of W5+, measured as the plot of optical absorption intensities as a function of the depth from the glass surface, is completely reproduced by assuming the reduction reaction being diffusion controlled. The diffusing species was assumed to be a redox pair of proton and electron trapped on W6+.
⁎ Corresponding author. Materials and Structures Laboratory, Tokyo Institute of Technology, Nagatsuta-cho 931-113, Midori-ku, Yokohama 247-0014, Japan. Tel.: +81 45 924 5128; fax: +81 45 924 5127. E-mail address: [email protected] (H. Tawarayama). 0167-2738/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2008.12.008
Existence of the charge compensating proton was supported by the observation that hydrogen gas thermally desorbed from the darkblue-colored glass was quantitatively agreed with simultaneously oxidized W5+ [9]. It was also found that the hydrogen-reduced glass was also found to show significant protonic and electronic conductivity in the order of 10− 3 S cm− 1 at 500 °C and the value of the transport number of proton at 300 °C was evaluated as 0.67 from DC polarization experiment [10]. Therefore, high selectivity hydrogen separation is expected by utilizing the dense glass film with no grain boundary. Pulsed laser deposition (PLD) is widely utilized in many fields because of its suitability for fabrication of dense films with the stoichiometry of the target and a high degree of adhesion to the substrate [11]. In the present study we report that fabrication of tungsten phosphate glass thin films by a PLD method and the hydrogen permeation of the glass films. 2. Experimental Chemical compositions of the tungsten phosphate glasses used in the present study are 30PO5/2·10WO3·25NbO5/2·10BaO·25NaO1/2 (bulk glass A) and 37PO5/2·6WO3·18NbO5/2· 9BaO·30NaO1/2 (bulk glass B) in mol. A stoichiometric mixture of Na2CO3, NaPO3, Ba(PO3)2, WO3 and Nb2O5 was melted with a Pt crucible at 1200–1300 °C for 2 h in air. The melt was poured into a carbon mold and was placed in an electric furnace preheated at the Tg of the glasses. The glasses obtained were annealed in air to bleach the purple color due to W5+ [7]. In order to estimate flux of hydrogen passed through the glass film we firstly evaluated the diffusion coefficient of the redox pair in the form of W5+ and H+ (W5+/H+) and the concentration equilibrated
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557
difference of hydrogen ranged up to 0.2 MPa and the flux of atmospheric hydrogen passed through the membrane was measured using a mass flow meter. 3. Results and discussion 3.1. Estimation of hydrogen flux through the glass film In our reaction model [7,9], the reversible reaction of the glass with hydrogen is expressed by the following equations, Fig. 1. Schematic diagram of a gas permeation cell used in the hydrogen permeation experiment. The area for hydrogen permeation is approximately 0.25 cm2. Pressure difference of hydrogen ranged up to 0.2 MPa and the flux of atmospheric hydrogen passed through the membrane was measured using a mass flow meter.
with atmospheric hydrogen gas using the Pd-coated bulk glass A. Detail of the experiment was described in the separate paper [7]. Thin layer of Pd was deposited on a single surface of the mirror-polished and bleached glass sample with 20 mm × 20 mm × 10 mm in size, and the glass plate was heated in H2 ambient. The depth profile of W5+ concentrations from the Pd-coated surface was obtained by measuring the optical absorption intensities at different positions in the sliced sample. Same experiment was also performed in a non-Pd coated bulk glass A. Thin film of the glasses was deposited on a mirror-polished Pd (250 µm in thickness) or a-SiO2 (0.5 mm in thickness) substrate by a PLD (ULVAC, ULP-1000-2C6) method at room temperature. The bulk glass B was used as a target to have similar chemical composition between the resulting film and the bulk glass A. ArF excimer laser was utilized at a repetition rate of 10 Hz for ablation of the target and a power density was ~4 J cm− 2 per pulse. Oxygen gas was introduced into the PLD chamber and its pressure was kept at 1–10 Pa. The glass films were characterized using a UV–vis-NIR spectrometer (Hitachi, U4000) and an X-ray photoelectron spectrometer (VG Microtech, MT500). The morphology was investigated with a surface plofilometer (Sloan, Dektak3), an optical microscope (Carl Zeiss) and a SPM (JEOL, JSPM-5200). Flux of hydrogen passed through sample membranes was measured at 300–500 °C using a gas permeation cell shown in Fig. 1. The area for hydrogen permeation is approximately 0.25 cm2. Pressure
5+
Fig. 2. The depth profiles of W concentrations in the Pd coated (●) and non-Pd coated (■) bulk glass A heat-treated at 500 °C for 3 h in H2. The symbols show the experimentally observed intensity profiles and the solid lines denote the results calculated using Eq. (3).
H2 X 2HS 6 +
WG
ð1Þ 5 +
+ HS X WG
+
+ HG ;
ð2Þ
where HS is hydrogen atom formed at the surface of the glass and WG6+, WG5+ and HG+ denote the species dissolved in the glass. The hydrogen atom on the glass surface dissolved in the glass by reducing W6+ to W5+. A proton is located around a nearby W5+ for charge compensation. A redox pair of proton and electron trapped on W6+ diffuses into inside of the glass via hopping between W6+ and W5+ by gradient of the concentration (chemical potential). Hydrogen is reversibly released from the surface of the reduced glasses under lower partial pressures of hydrogen. The chemical reaction by Eq. (1) is extremely promoted by coated Pd, which works as a catalyst for dissociation and recombination of hydrogen, on the surface of the glass. The diffusion coefficient of the redox pair and the concentration equilibrated with atmospheric hydrogen gas were determined by the following experiment. Fig. 2 shows the depth profile of W5+ concentrations in the Pd coated bulk glass A (Tg = 570 °C) heat-treated at 500 °C for 3 h in H2. The symbol (●) shows the experimentally observed intensity profile. Assuming the profile is one-dimensional diffusion of the redox pair in the form of W5+/H+ with constant surface concentration (C0), the concentration of the redox pair (C(x)) as a
Fig. 3. (a) The optical absorption spectra of the films deposited on the SiO2 substrate when a pressure of oxygen gas during the deposition were 10 Pa (films A and B) and 1 Pa (film C). The thickness of the films A, B and C is 100, 200 and 100 nm, respectively. For comparison, the spectrum of SiO2 substrate was also shown in the figure. (b) Difference spectrum between films A and C. Several absorption bands were induced in the film C.
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Fig. 4. The XPS spectra of W4f5/2 and W4f7/2 in the films A and C. Additional bands peaked at lower binding energies in the film C were clearly discernible, compared with the film A.
function of distance from the surface (x) is given by the following equation [12], x C ðxÞ = C0 1 − erf pffiffiffiffiffiffi ; 2 Dt
ð3Þ
where D and t denote a diffusion coefficient and a heat-treatment time, respectively. An excellent fit was obtained between the observed and the calculated (solid line), and the diffusion coefficient and the surface concentration were calculated as 1.4 × 10− 6 cm2 s− 1 and 9.3 × 1018 cm− 3, respectively. According to Fick's first low, F= −D
@C ; @x
ð4Þ
flux of hydrogen (F) passed through the glass film with 100 nm in thickness is expected to be 1.1 × 10− 6 mol cm− 2 s− 1 at 500 °C if partial
Fig. 5. The hydrogen permeation rate of the 3-rayered membrane B as a function of thickness of the glass film. Measuring temperature and the pressure difference of hydrogen were 500 °C and 0.2 MPa, respectively. The permeation rate was proportional to reciprocal thickness of the glass film t− 1.
Fig. 6. Temperature dependence of the hydrogen permeation rate of membranes A and B. The pressure difference of the hydrogen and the thickness of glass films were 0.2 MPa and ~ 100 nm, respectively. The symbols show the experimentally observed permeation rates of the membranes A (●) and B (■) and the Pd substrate (♦) that is shown for comparison. Thermally activated type dependence was observed in all membranes.
pressures of hydrogen at the feed- and permeate-side are respectively 0 and 0.1 MPa (ΔP = 0.1 MPa). To our surprise, a similar depth profile of W5+ concentration (■) was observed in the non-Pd coated bulk glass. Although the surface concentration (4.0× 1018 H atom cm− 3) was different from that of the Pd-coated glass, the diffusion coefficients calculated by Eq. (3) were the same with each other. Surface concentration is determined by a balance of the rates of hydrogen dissociation at the glass surface and diffusion into the glass. Therefore, the result means that the rate of hydrogen dissociation is comparable to that of hydrogen diffusion in the bulk glass. However, in the glass film, surface concentration is thought to be much lower because the rate of hydrogen diffusion will be higher with decreasing the thickness of the glass because of high concentration gradient ∂C/∂x.
Fig. 7. The hydrogen permeation rate of the membrane B at 500 °C as a function of the difference of hydrogen partial pressures between the feed- and permeate-side (ΔP). The thickness of the glass film was ~ 100 nm. The hydrogen permeation rate was proportional to ΔP0.85.
H. Tawarayama et al. / Solid State Ionics 180 (2009) 556–559
3.2. Characterization of the glass film According to the reaction model described above, reduction of W ions due to oxygen deficiency should affect its hydrogen permeability of the glass films. Valence of W ions in the as-deposited film changed with the oxygen pressure during the deposition. Fig. 3a shows the optical absorption spectra of the films deposited on the SiO2 substrate when a pressure of oxygen gas during the deposition was 10 Pa (films A and B) and 1 Pa (film C). The thickness of the films A, B and C is ~100, ~200 and ~100 nm, respectively. For comparison, the spectrum of SiO2 substrate was also shown in the figure. Apparent absorption peaking at around 650 nm in the film A is not intrinsic absorption but due to thin film interference because the peak position is shifted to around 1300 nm in the film B with a double thickness. On the other hand, from a difference spectrum between films A and C (Fig. 3b), several induced bands were found for the film C. The induced broad absorption at the wavelength more than 500 nm was almost the same as that due to W5+ in the hydrogen-reduced bulk glass [7]. However, the origin of the induced absorption observed a shorter wavelength side was unknown. Both absorptions were completely bleached by annealing at 500 °C in air and the spectrum was exactly the same as that of the film A. The existence of the reduced W ions in the film C was also supported by the XPS spectra of W4f5/2 and W4f7/2 shown in Fig. 4. Additional bands peaked at lower binding energies in the film C were clearly discernible, compared with the film A. Therefore, W ions in the film C were thought to be reduced by oxygen deficiency, and we used the film A in the hydrogen permeation experiment. Molar ratio of cations in the film A determined from the XPS measurement was approximately 28P:9W:23Nb:11Ba:29Na, which is close to that of the bulk glass A. 3.3. Hydrogen permeability of glass thin film membrane In the deposition at relatively high pressure of oxygen gas, droplets of molten materials from the target are weakly connected with each other and adhesion between the resulting film and a substrate is weak [13]. Consequently, the as-deposited film was inappropriate for hydrogen permeation experiment. Hydrogen feed is in direct contact with the Pd substrate and permeates to the permeate-side. In order to improve the drawbacks, the film was annealed at 590 °C (~Tg) for 1 h in N2 atmosphere. The film heat-treated was amorphous and sufficiently flat and smooth. Surface roughness and texture changed during the heattreatment, and average roughness of the heat-treated film was ~1.2 nm. In the heat-treatment at 600 °C, the film crystallized. We fabricated two types of membrane for hydrogen permeation experiment. Membrane A is a 2-layered membrane of the glass film and the Pd substrate. Membrane B is a 3-layed membrane which coated with thin layer of Pd (~20 nm in thickness) on the surface of the glass film side of the membrane A (sandwich structure). Pd substrate side of the membrane was always mounted on the permeate-side in the permeation cell. Fig. 5 shows hydrogen permeation rate of the 3-layered membrane B with different thicknesses of the glass film (≤300 nm). Measuring temperature and the pressure difference of hydrogen were 500 °C and 0.2 MPa, respectively. The hydrogen permeation rate was found to be proportional to reciprocal thickness of the glass film t− 1. The result strongly suggests that diffusion of redox pair in the glass film governs the permeation rate in the thickness of ≥100 nm. Fig. 6 shows the hydrogen permeation rate of membranes A and B as a function of reciprocal absolute temperature T − 1. The pressure difference of the hydrogen and the thickness of glass films were 0.2 MPa and ~ 100 nm, respectively. The symbols show the experimentally observed permeation rate of the membranes A (●) and B (■) and the Pd substrate (♦) that is shown for comparison. The permeation rate of the membrane A was 2.0 × 10− 7 mol cm− 2 s− 1 at 500 °C, which is 7% of that in the Pd substrate. This low permeation
559
rate is due to the fact that the rate of dissociation of hydrogen on the surface of the glass film is much smaller than that of diffusion in the membrane, as noted in Section 3.1. On the other hand, the permeation rate of the membrane B (2.0 × 10− 6 mol cm− 2 s− 1 at 500 °C) was 10 times larger than that of the membrane A. This result implies that hydrogen atoms in the coated Pd reacted with the glass film (Eq. (2)) and the redox pair diffused in the glass film. Thermally activated type dependence was observed in all membranes and the activation energies for permeation of the membranes A and B and the Pd substrate were 23, 18 and 16 kJ mol− 1, respectively. We reported that activation energy for electrical conduction of the bulk glass heattreated in 3.5%H2–96.5%N2 ambient for different times decreased from 107 kJ mol− 1 to 35 kJ mol− 1 with increase in concentration of the redox pair [10]. Therefore, in the glass film heated at the hydrogen partial pressure of ≥0.1 MPa, the activation energy for conduction is thought to be considerably low. Although hydrogen permeation rate according to ambipolar diffusion is limited by minor conductivity, its evaluation under such severe condition is extremely difficult. On the other hand, in the membrane A, the activation energy for permeation is thought to be governed by hydrogen dissociation on the surface of the glass film. However, measurement of the rate is also hard at current stage. It is established that the hydrogen permeation rate of a membrane is proportional to the nth power of the difference of hydrogen partial pressures between the feed- and permeate-side (ΔP) at a constant temperature [14], where n is a constant and ranges from 0.5 to 1. The values of 0.5 and 1 mean that the rate determining process is diffusion of hydrogen through the membrane and reaction of hydrogen with the membrane, respectively. The hydrogen permeation rate of the membrane B at 500 °C as a function of ΔP is shown in Fig. 7. The thickness of the glass film was ~100 nm. It was found that the hydrogen permeation rate was proportional to ΔP0.85 indicating that the rate of hydrogen reaction process is comparable to that of hydrogen diffusion. In addition, the permeation rate was 1.1 × 10− 6 mol cm− 2 s− 1 when ΔP was 0.1 MPa, which agrees with the estimated value in Section 3.1. 4. Conclusions (1) The thin film (~ 100 nm) of the mixed conducting glass fabricated by the PLD method showed high hydrogen permeation rate (2.0 × 10− 6 mol cm− 2 s− 1 at 500 °C when ΔP was 0.2 MPa). (2) Valence state of W ions in the film changed from +6 to +5 due to oxygen deficiency with decreasing oxygen partial pressure during deposition. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
J. Guan, S.E. Dorris, U. Balachandran, M. Liu, Solid State Ionics 110 (1998) 303. T. Norby, Y. Larring, Solid State Ionics 136–137 (2000) 139. X. Qi, Y.S. Lin, Solid State Ionics 130 (2000) 149. H. Matsumoto, T. Shimura, T. Higuchi, H. Tanaka, K. Katahira, T. Otake, T. Kudo, K. Yashiro, A. Kaimai, T. Kawada, J. Mizusaki, J. Electrochem. Soc. 152 (2005) A488. C. Zuo, T.H. Lee, S.E. Dorris, U. Balachandran, M. Liu, J. Power Sources 159 (2006) 1291. G.Q. Lu, J.C. Diniz da Costa, M. Duke, S. Giessler, R. Socolow, R.H. Williams, T. Kreutz, J. Colloid Interface Sci. 314 (2007) 589. H. Tawarayama, F. Utsuno, H. Inoue, H. Hosono, H. Kawazoe, Chem. Mater. 18 (2006) 2810. H. Tawarayama, F. Utsuno, H. Inoue, S. Fujitsu, H. Kawazoe, J. Power Sources 161 (2006) 129. H. Tawarayama, S. Sugata, H. Inoue, H. Hosono, H. Kawazoe, Chem. Mater. 19 (2007) 4385. H. Tawarayama, H. Kawazoe, S. Sugata, H. Inoue, H. Hosono, Ceram. Trans. 198 (2006) 63. M. Miyakawa, K. Hayashi, M. Hirano, Y. Toda, T. Kamiya, H. Hosono, Adv. Mater. 15 (2003) 1100. J. Crank, The Mathematics of Diffusion, 2nd ed., Oxford Univ. Press, New York,1975, p.11. E. Joanni, M.C. Ferro, C.C. Mardare, A.I. Mardare, J.R.A. Fernandes, S.C.A. Pina, Mater. Res. 7 (2004) 431. R.C. Hurlbelt, J.O. Konecny, J. Chem. Phys. 34 (1961) 655.
Contents lists available at ScienceDirect
Solid State Ionics j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s s i
Hydrogen permeation of tungsten phosphate glass thin films Hiromasa Tawarayama a,⁎, Hiroshi Kawazoe b, Hideo Hosono a,c a b c
Materials and Structures Laboratory, Tokyo Institute of Technology, Nagatsuta-cho 4259, Midori-ku, Yokohama 226-8503, Japan Kawazoe Frontier Technologies Corporation, Kuden-cho 931-113, Sakae-ku, Yokohama 247-0014, Japan Frontier Research Center, Tokyo Institute of Technology, Nagatsuta-cho 4259, Midori-ku, Yokohama 226-8503, Japan
a r t i c l e
i n f o
Article history: Received 22 January 2008 Received in revised form 15 July 2008 Accepted 4 December 2008 Keywords: Tungsten phosphate glass Mixed conductor Thin film Hydrogen separation
a b s t r a c t Thin films of tungsten phosphate glasses were deposited on a Pd substrate by a pulsed laser deposition method and the flux of hydrogen passed thorough the glass film was measured with a conventional gas permeation technique in the temperature range 300–500 °C. The glass film deposited at low oxygen pressure was inappropriate for hydrogen permeation because of reduction of W ions due to oxygen deficiency. The membrane used in the hydrogen permeation experiment was a 3-layered membrane and consisted of Pd film (~ 20 nm), the glass film (≤300 nm) and the Pd substrate (250 µm). When the pressure difference of hydrogen and thickness of the glass layer were respectively 0.2 MPa and ~ 100 nm, the permeation rate through the membrane was 2.0 × 10− 6 mol cm− 2 s− 1 at 500 °C. It was confirmed that the protonic and electronic mixed conducting glass thin film show high hydrogen permeation rate. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Protonic and electronic mixed conductors have been expected as hydrogen separation membrane materials because of their high selectivity and permeability of hydrogen [1–6]. Hydrogen is a promising energy carrier in hydrogen energy technology, and the demand will remarkably increase with the spread of fuel cells in the near future. At the present large scale production technology of hydrogen is steam reforming of hydrocarbons due to its cost competitiveness and, therefore, processes of hydrogen separation from byproducts gases are indispensable. Especially, highly pure hydrogen is needed for polymer electrolyte fuel cells because contaminant carbon monoxide is very poisonous to the catalysts for dissociation of hydrogen. Tungsten phosphate glasses react with hydrogen and water vapor at the ambient temperatures below the glass transition temperature (Tg) and W6+ ions in the glasses are reduced to W5+ [7,8]. The reduced tungsten ions, W5+, show strong intraionic and intervalence optical absorptions in the visible to near infrared region and the color of the glass changes to dark blue. Concentration profile of W5+, measured as the plot of optical absorption intensities as a function of the depth from the glass surface, is completely reproduced by assuming the reduction reaction being diffusion controlled. The diffusing species was assumed to be a redox pair of proton and electron trapped on W6+.
⁎ Corresponding author. Materials and Structures Laboratory, Tokyo Institute of Technology, Nagatsuta-cho 931-113, Midori-ku, Yokohama 247-0014, Japan. Tel.: +81 45 924 5128; fax: +81 45 924 5127. E-mail address: [email protected] (H. Tawarayama). 0167-2738/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2008.12.008
Existence of the charge compensating proton was supported by the observation that hydrogen gas thermally desorbed from the darkblue-colored glass was quantitatively agreed with simultaneously oxidized W5+ [9]. It was also found that the hydrogen-reduced glass was also found to show significant protonic and electronic conductivity in the order of 10− 3 S cm− 1 at 500 °C and the value of the transport number of proton at 300 °C was evaluated as 0.67 from DC polarization experiment [10]. Therefore, high selectivity hydrogen separation is expected by utilizing the dense glass film with no grain boundary. Pulsed laser deposition (PLD) is widely utilized in many fields because of its suitability for fabrication of dense films with the stoichiometry of the target and a high degree of adhesion to the substrate [11]. In the present study we report that fabrication of tungsten phosphate glass thin films by a PLD method and the hydrogen permeation of the glass films. 2. Experimental Chemical compositions of the tungsten phosphate glasses used in the present study are 30PO5/2·10WO3·25NbO5/2·10BaO·25NaO1/2 (bulk glass A) and 37PO5/2·6WO3·18NbO5/2· 9BaO·30NaO1/2 (bulk glass B) in mol. A stoichiometric mixture of Na2CO3, NaPO3, Ba(PO3)2, WO3 and Nb2O5 was melted with a Pt crucible at 1200–1300 °C for 2 h in air. The melt was poured into a carbon mold and was placed in an electric furnace preheated at the Tg of the glasses. The glasses obtained were annealed in air to bleach the purple color due to W5+ [7]. In order to estimate flux of hydrogen passed through the glass film we firstly evaluated the diffusion coefficient of the redox pair in the form of W5+ and H+ (W5+/H+) and the concentration equilibrated
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difference of hydrogen ranged up to 0.2 MPa and the flux of atmospheric hydrogen passed through the membrane was measured using a mass flow meter. 3. Results and discussion 3.1. Estimation of hydrogen flux through the glass film In our reaction model [7,9], the reversible reaction of the glass with hydrogen is expressed by the following equations, Fig. 1. Schematic diagram of a gas permeation cell used in the hydrogen permeation experiment. The area for hydrogen permeation is approximately 0.25 cm2. Pressure difference of hydrogen ranged up to 0.2 MPa and the flux of atmospheric hydrogen passed through the membrane was measured using a mass flow meter.
with atmospheric hydrogen gas using the Pd-coated bulk glass A. Detail of the experiment was described in the separate paper [7]. Thin layer of Pd was deposited on a single surface of the mirror-polished and bleached glass sample with 20 mm × 20 mm × 10 mm in size, and the glass plate was heated in H2 ambient. The depth profile of W5+ concentrations from the Pd-coated surface was obtained by measuring the optical absorption intensities at different positions in the sliced sample. Same experiment was also performed in a non-Pd coated bulk glass A. Thin film of the glasses was deposited on a mirror-polished Pd (250 µm in thickness) or a-SiO2 (0.5 mm in thickness) substrate by a PLD (ULVAC, ULP-1000-2C6) method at room temperature. The bulk glass B was used as a target to have similar chemical composition between the resulting film and the bulk glass A. ArF excimer laser was utilized at a repetition rate of 10 Hz for ablation of the target and a power density was ~4 J cm− 2 per pulse. Oxygen gas was introduced into the PLD chamber and its pressure was kept at 1–10 Pa. The glass films were characterized using a UV–vis-NIR spectrometer (Hitachi, U4000) and an X-ray photoelectron spectrometer (VG Microtech, MT500). The morphology was investigated with a surface plofilometer (Sloan, Dektak3), an optical microscope (Carl Zeiss) and a SPM (JEOL, JSPM-5200). Flux of hydrogen passed through sample membranes was measured at 300–500 °C using a gas permeation cell shown in Fig. 1. The area for hydrogen permeation is approximately 0.25 cm2. Pressure
5+
Fig. 2. The depth profiles of W concentrations in the Pd coated (●) and non-Pd coated (■) bulk glass A heat-treated at 500 °C for 3 h in H2. The symbols show the experimentally observed intensity profiles and the solid lines denote the results calculated using Eq. (3).
H2 X 2HS 6 +
WG
ð1Þ 5 +
+ HS X WG
+
+ HG ;
ð2Þ
where HS is hydrogen atom formed at the surface of the glass and WG6+, WG5+ and HG+ denote the species dissolved in the glass. The hydrogen atom on the glass surface dissolved in the glass by reducing W6+ to W5+. A proton is located around a nearby W5+ for charge compensation. A redox pair of proton and electron trapped on W6+ diffuses into inside of the glass via hopping between W6+ and W5+ by gradient of the concentration (chemical potential). Hydrogen is reversibly released from the surface of the reduced glasses under lower partial pressures of hydrogen. The chemical reaction by Eq. (1) is extremely promoted by coated Pd, which works as a catalyst for dissociation and recombination of hydrogen, on the surface of the glass. The diffusion coefficient of the redox pair and the concentration equilibrated with atmospheric hydrogen gas were determined by the following experiment. Fig. 2 shows the depth profile of W5+ concentrations in the Pd coated bulk glass A (Tg = 570 °C) heat-treated at 500 °C for 3 h in H2. The symbol (●) shows the experimentally observed intensity profile. Assuming the profile is one-dimensional diffusion of the redox pair in the form of W5+/H+ with constant surface concentration (C0), the concentration of the redox pair (C(x)) as a
Fig. 3. (a) The optical absorption spectra of the films deposited on the SiO2 substrate when a pressure of oxygen gas during the deposition were 10 Pa (films A and B) and 1 Pa (film C). The thickness of the films A, B and C is 100, 200 and 100 nm, respectively. For comparison, the spectrum of SiO2 substrate was also shown in the figure. (b) Difference spectrum between films A and C. Several absorption bands were induced in the film C.
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Fig. 4. The XPS spectra of W4f5/2 and W4f7/2 in the films A and C. Additional bands peaked at lower binding energies in the film C were clearly discernible, compared with the film A.
function of distance from the surface (x) is given by the following equation [12], x C ðxÞ = C0 1 − erf pffiffiffiffiffiffi ; 2 Dt
ð3Þ
where D and t denote a diffusion coefficient and a heat-treatment time, respectively. An excellent fit was obtained between the observed and the calculated (solid line), and the diffusion coefficient and the surface concentration were calculated as 1.4 × 10− 6 cm2 s− 1 and 9.3 × 1018 cm− 3, respectively. According to Fick's first low, F= −D
@C ; @x
ð4Þ
flux of hydrogen (F) passed through the glass film with 100 nm in thickness is expected to be 1.1 × 10− 6 mol cm− 2 s− 1 at 500 °C if partial
Fig. 5. The hydrogen permeation rate of the 3-rayered membrane B as a function of thickness of the glass film. Measuring temperature and the pressure difference of hydrogen were 500 °C and 0.2 MPa, respectively. The permeation rate was proportional to reciprocal thickness of the glass film t− 1.
Fig. 6. Temperature dependence of the hydrogen permeation rate of membranes A and B. The pressure difference of the hydrogen and the thickness of glass films were 0.2 MPa and ~ 100 nm, respectively. The symbols show the experimentally observed permeation rates of the membranes A (●) and B (■) and the Pd substrate (♦) that is shown for comparison. Thermally activated type dependence was observed in all membranes.
pressures of hydrogen at the feed- and permeate-side are respectively 0 and 0.1 MPa (ΔP = 0.1 MPa). To our surprise, a similar depth profile of W5+ concentration (■) was observed in the non-Pd coated bulk glass. Although the surface concentration (4.0× 1018 H atom cm− 3) was different from that of the Pd-coated glass, the diffusion coefficients calculated by Eq. (3) were the same with each other. Surface concentration is determined by a balance of the rates of hydrogen dissociation at the glass surface and diffusion into the glass. Therefore, the result means that the rate of hydrogen dissociation is comparable to that of hydrogen diffusion in the bulk glass. However, in the glass film, surface concentration is thought to be much lower because the rate of hydrogen diffusion will be higher with decreasing the thickness of the glass because of high concentration gradient ∂C/∂x.
Fig. 7. The hydrogen permeation rate of the membrane B at 500 °C as a function of the difference of hydrogen partial pressures between the feed- and permeate-side (ΔP). The thickness of the glass film was ~ 100 nm. The hydrogen permeation rate was proportional to ΔP0.85.
H. Tawarayama et al. / Solid State Ionics 180 (2009) 556–559
3.2. Characterization of the glass film According to the reaction model described above, reduction of W ions due to oxygen deficiency should affect its hydrogen permeability of the glass films. Valence of W ions in the as-deposited film changed with the oxygen pressure during the deposition. Fig. 3a shows the optical absorption spectra of the films deposited on the SiO2 substrate when a pressure of oxygen gas during the deposition was 10 Pa (films A and B) and 1 Pa (film C). The thickness of the films A, B and C is ~100, ~200 and ~100 nm, respectively. For comparison, the spectrum of SiO2 substrate was also shown in the figure. Apparent absorption peaking at around 650 nm in the film A is not intrinsic absorption but due to thin film interference because the peak position is shifted to around 1300 nm in the film B with a double thickness. On the other hand, from a difference spectrum between films A and C (Fig. 3b), several induced bands were found for the film C. The induced broad absorption at the wavelength more than 500 nm was almost the same as that due to W5+ in the hydrogen-reduced bulk glass [7]. However, the origin of the induced absorption observed a shorter wavelength side was unknown. Both absorptions were completely bleached by annealing at 500 °C in air and the spectrum was exactly the same as that of the film A. The existence of the reduced W ions in the film C was also supported by the XPS spectra of W4f5/2 and W4f7/2 shown in Fig. 4. Additional bands peaked at lower binding energies in the film C were clearly discernible, compared with the film A. Therefore, W ions in the film C were thought to be reduced by oxygen deficiency, and we used the film A in the hydrogen permeation experiment. Molar ratio of cations in the film A determined from the XPS measurement was approximately 28P:9W:23Nb:11Ba:29Na, which is close to that of the bulk glass A. 3.3. Hydrogen permeability of glass thin film membrane In the deposition at relatively high pressure of oxygen gas, droplets of molten materials from the target are weakly connected with each other and adhesion between the resulting film and a substrate is weak [13]. Consequently, the as-deposited film was inappropriate for hydrogen permeation experiment. Hydrogen feed is in direct contact with the Pd substrate and permeates to the permeate-side. In order to improve the drawbacks, the film was annealed at 590 °C (~Tg) for 1 h in N2 atmosphere. The film heat-treated was amorphous and sufficiently flat and smooth. Surface roughness and texture changed during the heattreatment, and average roughness of the heat-treated film was ~1.2 nm. In the heat-treatment at 600 °C, the film crystallized. We fabricated two types of membrane for hydrogen permeation experiment. Membrane A is a 2-layered membrane of the glass film and the Pd substrate. Membrane B is a 3-layed membrane which coated with thin layer of Pd (~20 nm in thickness) on the surface of the glass film side of the membrane A (sandwich structure). Pd substrate side of the membrane was always mounted on the permeate-side in the permeation cell. Fig. 5 shows hydrogen permeation rate of the 3-layered membrane B with different thicknesses of the glass film (≤300 nm). Measuring temperature and the pressure difference of hydrogen were 500 °C and 0.2 MPa, respectively. The hydrogen permeation rate was found to be proportional to reciprocal thickness of the glass film t− 1. The result strongly suggests that diffusion of redox pair in the glass film governs the permeation rate in the thickness of ≥100 nm. Fig. 6 shows the hydrogen permeation rate of membranes A and B as a function of reciprocal absolute temperature T − 1. The pressure difference of the hydrogen and the thickness of glass films were 0.2 MPa and ~ 100 nm, respectively. The symbols show the experimentally observed permeation rate of the membranes A (●) and B (■) and the Pd substrate (♦) that is shown for comparison. The permeation rate of the membrane A was 2.0 × 10− 7 mol cm− 2 s− 1 at 500 °C, which is 7% of that in the Pd substrate. This low permeation
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rate is due to the fact that the rate of dissociation of hydrogen on the surface of the glass film is much smaller than that of diffusion in the membrane, as noted in Section 3.1. On the other hand, the permeation rate of the membrane B (2.0 × 10− 6 mol cm− 2 s− 1 at 500 °C) was 10 times larger than that of the membrane A. This result implies that hydrogen atoms in the coated Pd reacted with the glass film (Eq. (2)) and the redox pair diffused in the glass film. Thermally activated type dependence was observed in all membranes and the activation energies for permeation of the membranes A and B and the Pd substrate were 23, 18 and 16 kJ mol− 1, respectively. We reported that activation energy for electrical conduction of the bulk glass heattreated in 3.5%H2–96.5%N2 ambient for different times decreased from 107 kJ mol− 1 to 35 kJ mol− 1 with increase in concentration of the redox pair [10]. Therefore, in the glass film heated at the hydrogen partial pressure of ≥0.1 MPa, the activation energy for conduction is thought to be considerably low. Although hydrogen permeation rate according to ambipolar diffusion is limited by minor conductivity, its evaluation under such severe condition is extremely difficult. On the other hand, in the membrane A, the activation energy for permeation is thought to be governed by hydrogen dissociation on the surface of the glass film. However, measurement of the rate is also hard at current stage. It is established that the hydrogen permeation rate of a membrane is proportional to the nth power of the difference of hydrogen partial pressures between the feed- and permeate-side (ΔP) at a constant temperature [14], where n is a constant and ranges from 0.5 to 1. The values of 0.5 and 1 mean that the rate determining process is diffusion of hydrogen through the membrane and reaction of hydrogen with the membrane, respectively. The hydrogen permeation rate of the membrane B at 500 °C as a function of ΔP is shown in Fig. 7. The thickness of the glass film was ~100 nm. It was found that the hydrogen permeation rate was proportional to ΔP0.85 indicating that the rate of hydrogen reaction process is comparable to that of hydrogen diffusion. In addition, the permeation rate was 1.1 × 10− 6 mol cm− 2 s− 1 when ΔP was 0.1 MPa, which agrees with the estimated value in Section 3.1. 4. Conclusions (1) The thin film (~ 100 nm) of the mixed conducting glass fabricated by the PLD method showed high hydrogen permeation rate (2.0 × 10− 6 mol cm− 2 s− 1 at 500 °C when ΔP was 0.2 MPa). (2) Valence state of W ions in the film changed from +6 to +5 due to oxygen deficiency with decreasing oxygen partial pressure during deposition. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
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