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Electrochemical gas sensor using a novel gas permeable electrode modified by ion implantation
Surface & Coatings Technology 201 (2007) 8116 – 8119 www.elsevier.com/locate/surfcoat
Electrochemical gas sensor using a novel gas permeable electrode modified by ion implantation K. Okamura a,⁎, T. Ishiji a , M. Iwaki b , Y. Suzuki b , K. Takahashi b a b
Riken Keiki Co. Ltd., 2-7-6 Azusawa, Itabashi-ku, Tokyo, 174-8744, Japan RIKEN (The Institute of Physical and Chemical Research) 2-1 Hirosawa, Wako-shi, Saitama, 351-0198, Japan Available online 12 March 2007
Abstract It is important to develop a reliable gas sensor for the detection of H2 that is expected for clean energy. Expanded polytetrafluoroethylene (ePTFE) membrane is a chemically stable substance and has high gas permeability without permeation of aqueous electrolytes. A study has been made of ion implantation of various kinds of ions into ePTFE membranes with fluenses of 1 × 1013, 1 × 1014, 1 × 1015, and 5 × 1015 ions/cm2. After Au ion plating to ion-implanted ePTFE, we produced the electrochemical H2 gas sensor using the gas permeable electrode. The electrochemical gas sensor using a gas permeable electrode modified by ion implantation showed a good sensitivity and selectivity for H2 detection. Especially the sensor used N+-, N+2 -, O+-, and O+2 - implanted ePTFE membrane with fluences of above 1 × 1015 ions/cm2 showed significant effect, which was more than 20 times higher than that of control. Morphology change of ePTFE by ion implantation was examined by SEM and chemical bonding structure of the irradiated ePTFE surface was analyzed by FT-IR–ATR spectroscopy. With ion-implanted ePTFE, internodal distance, density between the nodes, and C_C bonds induced by ion implantation were major factors influencing the H2 detection current. © 2007 Elsevier B.V. All rights reserved. Keywords: Electrochemical gas sensor; Hydrogen; Expanded-polytetrafluoroethylene; Ion implantation
1. Introduction Although some kinds of chemical sensors for H2 gas detection have been used for industrial usages, development of the H2 gas sensors having high sensitivity and reliability are still desired because of increasing purposes in technology of the H2 economy. The H2 gas sensors are essential for detection or monitoring of leakage of H2 for keeping the safety, for a control of H2 gas supplied into fuel cell systems, and so on. The H2 gas sensor based on electrochemical amperometry shows better characteristics than a resistance measurement method based on the resistance change induced by the heat of catalytic reaction of H2 gas. The former sensor needs lower operation energy than that of the later, which is better to make portable type and general usage sensors.
⁎ Corresponding author. Tel.: +81 3 3966 1123; fax: +81 3 3966 0249. E-mail address: [email protected] (K. Okamura). 0257-8972/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.02.065
The electrochemical H2 gas sensor is constructed by a three electrode system. The working electrode is a gas permeable electrode made by Au ion-plated expanded polytetrafluoroethylene (ePTFE) [1,2]. The detection gas permeates to the working electrode through the gas permeable membrane, and the gas, H2, is oxidized at the Au electrode. In this case, the permeability of the gas, reactivity of the gas on the electrode, and physicochemical nature of the gas/electrode/electrolytic solution interface influence on the sensor property. In this study, we apply an ion implantation for the modification of the ePTFE membrane. It is known that the ion implantation is effective to control the surface properties of ePTFE and some organic polymers. The gas permeation of polyimide [3], surface morphology of ePTFE [4], surface wettability of polymers [5], and chemical compositions of polymer surface [4] have been studied, which suggest that the ion implantation will be effective to control the nature of the membrane electrode. We compared conditions of the ion implantation, such as ion species and fluences, on the sensitivity of the sensors used the ion-implanted ePTFE membranes for the H2 gas and some other gases.
K. Okamura et al. / Surface & Coatings Technology 201 (2007) 8116–8119
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2. Experimental 2.1. Ion implantation The ePTFE membranes used were 0.3 μm on pore size, 34% on porosity, and 0.2 mm of thickness. The membranes, 30 mm square, were implanted with H+, He+, N+, N2+, O+, O2+, Ne+, and Kr+ at an energy of 150 keV with fluences from 1 × 1013 to 5 × 1015 ions/cm2 at room temperature. The beam current density was kept from 0.05 to 0.1 μA/cm2 to prevent from increasing the temperature and the gas pressure. 2.2. Preparation of the working electrode The Au ion-plating (BMC800, SHINCRON, Japan) was carried out on the ion-implanted ePTFE membrane surfaces to make the gas permeable electrodes. The plating rate was 0.1 nm/s under Ar gas pressure of 2.3 × 10− 2 Pa. The thickness of Au ionplated was about 370 nm.
Fig. 2. Relationship between current ratio, Ig/I0, for H2 measured by sensors using various ion-implanted ePTFE membrane and fluences of ion implantation for ePTFE membrane.
(100 ppm), and NO2 (10 ppm) were measured as interference gases to examine the selectivity of H2 against other gases. 2.5. Surfaces analysis
2.3. Structure of the sensor The sensor structure is shown in Fig. 1. The gas permeable working electrode used was ϕ28 m in diameter. The counter electrode and the reference electrode were Au-black electrodes, and the electrolyte solution was 9 mol/dm 3 H2SO4 and the applied electrode potential was 50 mV vs. reference electrode.
The morphology of the ion-implanted ePTFE membrane was observed by SEM (JED6330F, JEOL, Japan). The chemical bonding structure of the membranes was analyzed by FT-IR– ATR spectroscopy (NEXUS470, Nicolet, France). 3. Results and discussion 3.1. Sensitivity of the sensors for H2
2.4. Measurement The flow rate of the sample gas was 0.25 dm3/min. The current–time curves were recorded by a pen recorder and the variation of the current at two minutes after introduction of the gas into the sensor was measured as the characteristic current, Ig, for the sample gas. Very low residual current was continuously observed on the chart, so the Ig was assumed to be the height of reaction current with subtraction of the residual current. The current response for H2 (2500 ppm) gas was measured as a sample gas, and that for H2S (0.9 ppm), SO2 (15 ppm), NO
Fig. 1. Schematic diagrams of the electrochemical gas sensor (a) and the ionimplanted gas-permeable-electrode (b).
To know the effect of the ion implantation on the current response of H2, the currents, Ig, were measured for the sensors using ion-implanted membrane and non-implanted one. Fig. 2 shows the relationship between current ratio, Ig/I0, and fluences of ions, where I0 is the characteristic current of the control (nonimplanted one) for each gases. The figure shows that the Ig for H2 is remarkably increased by N+, N2+, O+, and O2+ implantation with fluences of 1 × 1015 and 5 × 1015 ions/cm2. The highest value of Ig/I0 reaches approximately 26, which suggests that the ion beam modification of the ePTFE membrane is effective to enhance the sensitivity for the H2 sensing.
Fig. 3. Current ratio, Ig/I0, for various gases measured by sensors using N+-, N+2-, O+-, and O+2-implanted ePTFE membranes with fluences of 1 × 1015 and 5 × 1015 ions/cm2.
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3.2. Sensitivity of the sensors for interference gases Fig. 3 shows the current ratio, Ig/I0, of the ion-implanted sensors for H2 and interference gases, H2S, SO2, NO, and NO2, where ion implantations were carried out with fluences of 1 × 1015 and 5 × 1015 ions/cm2. The H2 shows very high current ratio, from 9 to 26, however, the current ratios for the interference gases are less than 4. It is clear suggests that the ion implantations of N+, N2+, O+, and O2+ were markedly effective to enhance the selectivity of H2 against the interference gases. 3.3. SEM observation Fig. 4 shows the SEM image of ePTFE membrane after N+ implantation with a fluence of 5 × 1015 ions/cm2 and nonimplanted one. The pore size of the ion-implanted surface becomes larger, and the shape of the surface is more complicate than those of the non-implanted one. Therefore, it is conceivable that the increase of current ratio can be due to the increase of the effective surface area for the reaction at the working electrode surface (Au/ ePTFE interface). However, in this case, the current increase is expected for all sorts of gases, it means that the enhancement of the selectivity for H2 cannot be explained. 3.4. FT-IR–ATR spectroscopy Fig. 5 shows the IR spectra of O+-implanted ePTFE with fluences from 1 × 1013 to 5 × 1015 ions/cm2 and nonimplanted one. Fig. 6 also shows the IR spectra of O+-, Ne+-, and Kr+- implanted ePTFE with a fluence of 1 × 1015 ions/cm2. The characteristic peaks are observed at 1150 cm− 1 , 1205 cm− 1, and 1720 cm− 1. The peaks of 1150 cm− 1 and 1205 cm− 1 are assigned C–F bond, and 1720 cm− 1, as C_C bond. The absorbance peak of C–F is depressed with increase of the fluence of O+ implantation, as shown in Fig. 5. From Fig. 6,
Fig. 4. SEM images of non-implanted ePTFE membrane and N+-implanted one with a fluence of 5 × 1015 ions/cm2.
Fig. 5. FT-IR–ATR spectra of non-implanted ePTFE membrane and O+implantated one with fluences of 1 × 1013, 1 × 1014, and 1 × 1015 ions/cm2.
the absorbance of the peaks shows the order of non-implanted NKr+ N Ne+ N O+, which suggests that the smaller ion is effective to chemical modification of ePTFE surface. There are two patterns for forming new structures, one caused by the electronic stopping power and the other by the nuclear stopping power. The electronic stopping powers are related to the energy transfer from energetic ions to the electrons surrounding the nuclei in the sample. It is thought that O+ implantation decomposed chemical bonds comparing with Kr+ implantation, because O+ implantation possessed the electronic stopping powers more than that of Kr+ and this kind of energy transfer ionized atoms in the sample and induced decomposition of original chemical bonds, carbonization. It is expected that the chemical bonding structure influences on solution–polymer interaction. The wettability of the membrane is essential factor to control the electrochemical reaction properties of the gas permeable electrode. According to the previous studies [4,5], the change of C–F and C_C bonds induced by the ion implantation enhances the wettability of the
Fig. 6. FT-IR–ATR spectra of non-implanted ePTFE membrane and O+-, Ne+-, and Kr+-implanted one with a fluence of 1 × 1015 ions/cm2.
K. Okamura et al. / Surface & Coatings Technology 201 (2007) 8116–8119
ePTFE surface, which contributes to improve the sensitivity of the sensor. It is also expected that the chemical bonding structure change of ePTFE, especially the pore surfaces, influences the interactions between gas molecules and polymer surface, which induces the permeability change of the gases. Doping of nitrogen or oxygen atoms into the polymer by N+ and O+ implantation is probably effective to introduce high affinity of the pore surface to H2 molecules. The effect on H2 selectivity may be due the surface affinity change induced by the ion implantations. 4. Summary ePTFE membranes of the gas permeable electrode for the amperometric sensor was modified by ion implantations of various kinds of ions. The H2 detection current is 9–26 times enhanced by the N+, N2+, O+, and O2+ implantation with fluences above 1 × 1015 ions/cm2. In contrast, the detection current for the interference gases, H2S, SO2, NO, and NO2, were enhanced less than 4 times, which gives higher selectivity for H2 sensing. It is concluded that those ion implantation effects on surface properties of ePTFE resulted in the improvements of the H2 sensor characteristics.
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Morphology change of ePTFE surface was analyzed by FTIR–ATR spectroscopy. With ion-implanted ePTFE, internodal distance, density between the nodes, and C_C bonds induced by ion implantation were major factors influencing the H2 detection current. One possibility may be that gas sieving-like effects induced by the surface carbonization due to N+, N2+, O+, and O2+ implantation which can provide a high degree of H2 permeation in compared with permeability of H2S, SO2, NO, and NO2. Another possible explanation for the increase of H2 detection current of the ion-implanted ePTFE could be the surface wettability changes which improve H2 gas affinity induced by ion implantation. Further studies are required to clarify these wettability effects in more detail. References [1] T. Ishiji, T. Iijima, K. Takahashi, Denki Kagaku (1996) 1304. [2] N. Nakano, S. Ogawa, Sens. Actuators, B 21 (1994) 51. [3] M. Iwase, A. Sannomiya, S. Nagaoka, Y. Suzuki, M. Iwaki, H. Kawakami, Macromol 37 (2004) 6892. [4] Y. Ono, T. Tsukamoto, N. Takahashi, T. Yotoriyama, Y. Suzuki, M. Iwaki, Trans. Mat. Res. Soc. Jpn. 29 (2004) 599. [5] M. Kusakabe, Y. Suzuki, A. Nakao, M. Kaibara, M. Iwaki, M. Scholl, Polym. Adv. Technol. 12 (2001) 453.
Electrochemical gas sensor using a novel gas permeable electrode modified by ion implantation K. Okamura a,⁎, T. Ishiji a , M. Iwaki b , Y. Suzuki b , K. Takahashi b a b
Riken Keiki Co. Ltd., 2-7-6 Azusawa, Itabashi-ku, Tokyo, 174-8744, Japan RIKEN (The Institute of Physical and Chemical Research) 2-1 Hirosawa, Wako-shi, Saitama, 351-0198, Japan Available online 12 March 2007
Abstract It is important to develop a reliable gas sensor for the detection of H2 that is expected for clean energy. Expanded polytetrafluoroethylene (ePTFE) membrane is a chemically stable substance and has high gas permeability without permeation of aqueous electrolytes. A study has been made of ion implantation of various kinds of ions into ePTFE membranes with fluenses of 1 × 1013, 1 × 1014, 1 × 1015, and 5 × 1015 ions/cm2. After Au ion plating to ion-implanted ePTFE, we produced the electrochemical H2 gas sensor using the gas permeable electrode. The electrochemical gas sensor using a gas permeable electrode modified by ion implantation showed a good sensitivity and selectivity for H2 detection. Especially the sensor used N+-, N+2 -, O+-, and O+2 - implanted ePTFE membrane with fluences of above 1 × 1015 ions/cm2 showed significant effect, which was more than 20 times higher than that of control. Morphology change of ePTFE by ion implantation was examined by SEM and chemical bonding structure of the irradiated ePTFE surface was analyzed by FT-IR–ATR spectroscopy. With ion-implanted ePTFE, internodal distance, density between the nodes, and C_C bonds induced by ion implantation were major factors influencing the H2 detection current. © 2007 Elsevier B.V. All rights reserved. Keywords: Electrochemical gas sensor; Hydrogen; Expanded-polytetrafluoroethylene; Ion implantation
1. Introduction Although some kinds of chemical sensors for H2 gas detection have been used for industrial usages, development of the H2 gas sensors having high sensitivity and reliability are still desired because of increasing purposes in technology of the H2 economy. The H2 gas sensors are essential for detection or monitoring of leakage of H2 for keeping the safety, for a control of H2 gas supplied into fuel cell systems, and so on. The H2 gas sensor based on electrochemical amperometry shows better characteristics than a resistance measurement method based on the resistance change induced by the heat of catalytic reaction of H2 gas. The former sensor needs lower operation energy than that of the later, which is better to make portable type and general usage sensors.
⁎ Corresponding author. Tel.: +81 3 3966 1123; fax: +81 3 3966 0249. E-mail address: [email protected] (K. Okamura). 0257-8972/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.02.065
The electrochemical H2 gas sensor is constructed by a three electrode system. The working electrode is a gas permeable electrode made by Au ion-plated expanded polytetrafluoroethylene (ePTFE) [1,2]. The detection gas permeates to the working electrode through the gas permeable membrane, and the gas, H2, is oxidized at the Au electrode. In this case, the permeability of the gas, reactivity of the gas on the electrode, and physicochemical nature of the gas/electrode/electrolytic solution interface influence on the sensor property. In this study, we apply an ion implantation for the modification of the ePTFE membrane. It is known that the ion implantation is effective to control the surface properties of ePTFE and some organic polymers. The gas permeation of polyimide [3], surface morphology of ePTFE [4], surface wettability of polymers [5], and chemical compositions of polymer surface [4] have been studied, which suggest that the ion implantation will be effective to control the nature of the membrane electrode. We compared conditions of the ion implantation, such as ion species and fluences, on the sensitivity of the sensors used the ion-implanted ePTFE membranes for the H2 gas and some other gases.
K. Okamura et al. / Surface & Coatings Technology 201 (2007) 8116–8119
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2. Experimental 2.1. Ion implantation The ePTFE membranes used were 0.3 μm on pore size, 34% on porosity, and 0.2 mm of thickness. The membranes, 30 mm square, were implanted with H+, He+, N+, N2+, O+, O2+, Ne+, and Kr+ at an energy of 150 keV with fluences from 1 × 1013 to 5 × 1015 ions/cm2 at room temperature. The beam current density was kept from 0.05 to 0.1 μA/cm2 to prevent from increasing the temperature and the gas pressure. 2.2. Preparation of the working electrode The Au ion-plating (BMC800, SHINCRON, Japan) was carried out on the ion-implanted ePTFE membrane surfaces to make the gas permeable electrodes. The plating rate was 0.1 nm/s under Ar gas pressure of 2.3 × 10− 2 Pa. The thickness of Au ionplated was about 370 nm.
Fig. 2. Relationship between current ratio, Ig/I0, for H2 measured by sensors using various ion-implanted ePTFE membrane and fluences of ion implantation for ePTFE membrane.
(100 ppm), and NO2 (10 ppm) were measured as interference gases to examine the selectivity of H2 against other gases. 2.5. Surfaces analysis
2.3. Structure of the sensor The sensor structure is shown in Fig. 1. The gas permeable working electrode used was ϕ28 m in diameter. The counter electrode and the reference electrode were Au-black electrodes, and the electrolyte solution was 9 mol/dm 3 H2SO4 and the applied electrode potential was 50 mV vs. reference electrode.
The morphology of the ion-implanted ePTFE membrane was observed by SEM (JED6330F, JEOL, Japan). The chemical bonding structure of the membranes was analyzed by FT-IR– ATR spectroscopy (NEXUS470, Nicolet, France). 3. Results and discussion 3.1. Sensitivity of the sensors for H2
2.4. Measurement The flow rate of the sample gas was 0.25 dm3/min. The current–time curves were recorded by a pen recorder and the variation of the current at two minutes after introduction of the gas into the sensor was measured as the characteristic current, Ig, for the sample gas. Very low residual current was continuously observed on the chart, so the Ig was assumed to be the height of reaction current with subtraction of the residual current. The current response for H2 (2500 ppm) gas was measured as a sample gas, and that for H2S (0.9 ppm), SO2 (15 ppm), NO
Fig. 1. Schematic diagrams of the electrochemical gas sensor (a) and the ionimplanted gas-permeable-electrode (b).
To know the effect of the ion implantation on the current response of H2, the currents, Ig, were measured for the sensors using ion-implanted membrane and non-implanted one. Fig. 2 shows the relationship between current ratio, Ig/I0, and fluences of ions, where I0 is the characteristic current of the control (nonimplanted one) for each gases. The figure shows that the Ig for H2 is remarkably increased by N+, N2+, O+, and O2+ implantation with fluences of 1 × 1015 and 5 × 1015 ions/cm2. The highest value of Ig/I0 reaches approximately 26, which suggests that the ion beam modification of the ePTFE membrane is effective to enhance the sensitivity for the H2 sensing.
Fig. 3. Current ratio, Ig/I0, for various gases measured by sensors using N+-, N+2-, O+-, and O+2-implanted ePTFE membranes with fluences of 1 × 1015 and 5 × 1015 ions/cm2.
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K. Okamura et al. / Surface & Coatings Technology 201 (2007) 8116–8119
3.2. Sensitivity of the sensors for interference gases Fig. 3 shows the current ratio, Ig/I0, of the ion-implanted sensors for H2 and interference gases, H2S, SO2, NO, and NO2, where ion implantations were carried out with fluences of 1 × 1015 and 5 × 1015 ions/cm2. The H2 shows very high current ratio, from 9 to 26, however, the current ratios for the interference gases are less than 4. It is clear suggests that the ion implantations of N+, N2+, O+, and O2+ were markedly effective to enhance the selectivity of H2 against the interference gases. 3.3. SEM observation Fig. 4 shows the SEM image of ePTFE membrane after N+ implantation with a fluence of 5 × 1015 ions/cm2 and nonimplanted one. The pore size of the ion-implanted surface becomes larger, and the shape of the surface is more complicate than those of the non-implanted one. Therefore, it is conceivable that the increase of current ratio can be due to the increase of the effective surface area for the reaction at the working electrode surface (Au/ ePTFE interface). However, in this case, the current increase is expected for all sorts of gases, it means that the enhancement of the selectivity for H2 cannot be explained. 3.4. FT-IR–ATR spectroscopy Fig. 5 shows the IR spectra of O+-implanted ePTFE with fluences from 1 × 1013 to 5 × 1015 ions/cm2 and nonimplanted one. Fig. 6 also shows the IR spectra of O+-, Ne+-, and Kr+- implanted ePTFE with a fluence of 1 × 1015 ions/cm2. The characteristic peaks are observed at 1150 cm− 1 , 1205 cm− 1, and 1720 cm− 1. The peaks of 1150 cm− 1 and 1205 cm− 1 are assigned C–F bond, and 1720 cm− 1, as C_C bond. The absorbance peak of C–F is depressed with increase of the fluence of O+ implantation, as shown in Fig. 5. From Fig. 6,
Fig. 4. SEM images of non-implanted ePTFE membrane and N+-implanted one with a fluence of 5 × 1015 ions/cm2.
Fig. 5. FT-IR–ATR spectra of non-implanted ePTFE membrane and O+implantated one with fluences of 1 × 1013, 1 × 1014, and 1 × 1015 ions/cm2.
the absorbance of the peaks shows the order of non-implanted NKr+ N Ne+ N O+, which suggests that the smaller ion is effective to chemical modification of ePTFE surface. There are two patterns for forming new structures, one caused by the electronic stopping power and the other by the nuclear stopping power. The electronic stopping powers are related to the energy transfer from energetic ions to the electrons surrounding the nuclei in the sample. It is thought that O+ implantation decomposed chemical bonds comparing with Kr+ implantation, because O+ implantation possessed the electronic stopping powers more than that of Kr+ and this kind of energy transfer ionized atoms in the sample and induced decomposition of original chemical bonds, carbonization. It is expected that the chemical bonding structure influences on solution–polymer interaction. The wettability of the membrane is essential factor to control the electrochemical reaction properties of the gas permeable electrode. According to the previous studies [4,5], the change of C–F and C_C bonds induced by the ion implantation enhances the wettability of the
Fig. 6. FT-IR–ATR spectra of non-implanted ePTFE membrane and O+-, Ne+-, and Kr+-implanted one with a fluence of 1 × 1015 ions/cm2.
K. Okamura et al. / Surface & Coatings Technology 201 (2007) 8116–8119
ePTFE surface, which contributes to improve the sensitivity of the sensor. It is also expected that the chemical bonding structure change of ePTFE, especially the pore surfaces, influences the interactions between gas molecules and polymer surface, which induces the permeability change of the gases. Doping of nitrogen or oxygen atoms into the polymer by N+ and O+ implantation is probably effective to introduce high affinity of the pore surface to H2 molecules. The effect on H2 selectivity may be due the surface affinity change induced by the ion implantations. 4. Summary ePTFE membranes of the gas permeable electrode for the amperometric sensor was modified by ion implantations of various kinds of ions. The H2 detection current is 9–26 times enhanced by the N+, N2+, O+, and O2+ implantation with fluences above 1 × 1015 ions/cm2. In contrast, the detection current for the interference gases, H2S, SO2, NO, and NO2, were enhanced less than 4 times, which gives higher selectivity for H2 sensing. It is concluded that those ion implantation effects on surface properties of ePTFE resulted in the improvements of the H2 sensor characteristics.
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Morphology change of ePTFE surface was analyzed by FTIR–ATR spectroscopy. With ion-implanted ePTFE, internodal distance, density between the nodes, and C_C bonds induced by ion implantation were major factors influencing the H2 detection current. One possibility may be that gas sieving-like effects induced by the surface carbonization due to N+, N2+, O+, and O2+ implantation which can provide a high degree of H2 permeation in compared with permeability of H2S, SO2, NO, and NO2. Another possible explanation for the increase of H2 detection current of the ion-implanted ePTFE could be the surface wettability changes which improve H2 gas affinity induced by ion implantation. Further studies are required to clarify these wettability effects in more detail. References [1] T. Ishiji, T. Iijima, K. Takahashi, Denki Kagaku (1996) 1304. [2] N. Nakano, S. Ogawa, Sens. Actuators, B 21 (1994) 51. [3] M. Iwase, A. Sannomiya, S. Nagaoka, Y. Suzuki, M. Iwaki, H. Kawakami, Macromol 37 (2004) 6892. [4] Y. Ono, T. Tsukamoto, N. Takahashi, T. Yotoriyama, Y. Suzuki, M. Iwaki, Trans. Mat. Res. Soc. Jpn. 29 (2004) 599. [5] M. Kusakabe, Y. Suzuki, A. Nakao, M. Kaibara, M. Iwaki, M. Scholl, Polym. Adv. Technol. 12 (2001) 453.