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A combined method of small-angle neutron scattering and neutron radiography to visualize water in an operating fuel cell over a wide length scale from nano to millimeter
ARTICLE IN PRESS Nuclear Instruments and Methods in Physics Research A 605 (2009) 95–98
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
A combined method of small-angle neutron scattering and neutron radiography to visualize water in an operating fuel cell over a wide length scale from nano to millimeter H. Iwase a,b, S. Koizumi a,, H. Iikura c, M. Matsubayashi c, D. Yamaguchi a, Y. Maekawa b, T. Hashimoto a a
Advanced Science Research Center, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan Quantum Beam Science Directorate, Japan Atomic Energy Agency, Takasaki, Gunma 370-1292, Japan c Quantum Beam Science Directorate, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan b
a r t i c l e in fo
abstract
Available online 5 February 2009
In order to visualize water generated in an operating polymer electrolyte fuel cell (PEFC), a neutron radiography (NR) apparatus, composed of a scintillator, optical mirrors and a CCD camera, was installed at a sample position of the focusing and polarized neutron small-angle scattering (SANS) spectrometer (SANS-J-II) at research reactor JRR-3 at Japan Atomic Energy Agency, Tokai, Japan. By combining SANS and NR, we aim to cover a wide length scale from nanometer to millimeter. The new method succeeded in detecting a spatial distribution of the water generated in individual cell elements; NR detected the water in a gas diffusion layer and a flow field, whereas SANS quantitatively determines the water content in a membrane electrode assembly (MEA). Published by Elsevier B.V.
Keywords: Small-angle neutron scattering Neutron radiography Polymer electrolyte fuel cell In-situ observation
1. Introduction A fuel cell has attracted a lot of interests as a next-generation power supply. Particularly, a polymer electrolyte fuel cell (PEFC) is promising for its applications to automotive and portable buttery. During an operation of PEFC, water is generated in individual cell elements (a polymer electrolyte membrane (PEM), catalysts, gas diffusion layers (GDLs) and flow fields) as a result of the electrochemical reaction, transportation and exclusion of water. Since a spatial distribution of the water in fuel cells directly affects an operation performance of PEFC, it is crucial to determine simultaneously and in-situ the distribution and diffusion of the water appeared in the individual cell elements belonging to a given single fuel cell under operation. So far, many of in-situ observations on water in an operating PEFC have been attempted by several methods [1–12]. In the case of Nafion used as a PEM, a characteristic size of its ion-cluster is about several nanometers. Small-angle X-ray scattering (SAXS) [1] and small-angle neutron scattering (SANS) [2,9] have successfully observed a scattering maximum, the so-called ‘‘ionomer peak’’ [13], due to the ion-cluster swollen by water [1,2,13,14]. On the other hand, the water distribution accumulated in the flow field plates is macroscopic. An imaging technique, i.e. neutron radiography (NR), is highly sensitive to hydrogen, and is one of the
Corresponding author. Tel.: +81 29 284 3511; fax: +81 29 282 5939.
E-mail address: [email protected] (S. Koizumi). 0168-9002/$ - see front matter Published by Elsevier B.V. doi:10.1016/j.nima.2009.01.165
potential methods to visualize the water distribution in the macroscopic length scale [4–8,10–12]. As mentioned above, the length scale concerning the water distribution in an operating PEFC is ranged widely from a microscopic (nanometer) to a macroscopic (millimeter) length scale. A single experimental method, such as SANS or NR, is limited to investigate the water in a whole range of the length scale in PEFC. To overcome this limitation, we aimed to combine NR with SANS. Fig. 1 schematically shows our combined SANS and NR method. A neutron imaging system is installed behind the fuel cell placed at a sample position of the SANS spectrometer. When we observe SANS from PEFC, a scintillator sheet and the first optical mirror (defined as a NR set), which are necessary for NR, are removed out from a beam line, and SANS are acquired by a 2D detector. By replacing the NR set in and out intermittently and by synchronized data acquisition of NR and SANS during the operation of PEFC, we intended to observe both the water distribution in the flow-field plate by NR and that in a membrane electrode assembly (MEA) by SANS.
2. Focusing and polarized neutron ultra-small-angle scattering spectrometer (SANS-J-II) Fig. 2(a) shows a schematic view of the focusing and polarized neutron ultra-small-angle scattering spectrometer (SANS-J-II) at research reactor (JRR-3) at Japan Atomic Energy Agency (JAEA), Tokai, Japan [15]. A sample position locates at the middle
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Fig. 1. Schematic illustration of a combined method of neutron radiography (a) and small-angle neutron scattering (b), installed at the SANS-J-II spectrometer in JAEA.
In the case of ultra-small-angle neutron scattering, covering q ¼ 3 104–3 103 A˚1, a high-resolution position-sensitive scintillation detector (a cross-wired position-sensitive photomultiplier (5 in. in size with 0.5 mm spatial resolution) coupled with a ZnS/6LiF scintillator (0.2 mm thickness) is utilized. By employing pinhole SANS and focusing SANS on SANS-J-II, a wide range of q from 3 104 to 2 A˚1, corresponding to several micrometers to nanometers in real space, is covered.
3. Monochromatic cold neutron radiography
Fig. 2. (a) Schematic diagram of a focusing and polarized neutron small-angle scattering spectrometer (SANS-J-II) and (b) Photograph of the neutron imaging system, newly installed at SANS-J-II.
(10 m from the first aperture) of the total spectrometer with length of 20 m. At the upstream, cold neutron is monochromatized by a velocity selector, a usually selected wavelength l ¼ 6.5 A˚ and Dl/l ¼ 0.13. The monochromatized cold neutron is collimated by beam apertures made of B4C inside a collimator chamber (the first and second apertures appear at the upper stream and before the sample position, as denoted by D1 and D2 in Fig. 2(a), respectively). We can choose an aperture size (D) for both D1 and D2 from 1 1 to 50 50 mm2. In the case of SANS measurements, D1 ¼ 20 20 mm2 and D2 ¼ 10 10 mm2 were employed. In order to detect an ultra-small-angle scattering region of q104 A˚1, where q is a magnitude of scattering vector given by q ¼ (4p/l)sin(y) with 2y being the scattering angle, we are able to utilize focusing lenses (biconcave lens made up of MgF2 or sextupole permanent magnetic lens) in the collimator chamber. Two position-sensitive detectors (PSDs) are installed in a vacuum flight tube. A 3He main PSD, whose diameter and spatial resolution are 0.58 m and about 5 mm, respectively, is utilized for conventional pinhole SANS, covering from q ¼ 3 103 to 2 A˚1.
To combine NR with SANS, we installed a neutron imaging system at the sample position of SANS-J-II. Fig. 2(b) shows a photograph of the NR system installed on SANS-J-II. The imaging system, allocated behind a fuel cell, is composed of a sheet of ZnS/6LiF scintillator (0.2 mm thickness), two optical mirrors, an optical lens (MicroNikkor, 55 mm, Nikon Co.), and a full frame transfer type CCD camera (C4880, Hamamatsu Photonics Co. Ltd.). The CCD camera (1024 1024 pixels) is placed far from a direct neutron beam to be protected from radiation damage. A monochromatic cold neutron beam (50 50 mm2), passing through a fuel cell, is converted to a visible light by the scintillator, reflected by the double mirrors and finally transmitted to the CCD camera by the optical lens. The scintillator and the first optical mirror are mounted on a vertical elevating bench, in order to move them out from a neutron beam when the SANS measurements were conducted. For NR measurements, we selected D1 ¼ 20 20 mm2, D2 ¼ 50 50 mm2, and L ¼ 5000 mm (Fig. 1), giving L/D ¼ 250, which is matched with the spatial resolution of the imaging system of 184 mm.
4. Fuel cell We utilized a standard single fuel cell provided by Japan Automobile Research Institute (JARI) [16]. On a JARI standard single fuel cell, an active area of MEA is originally 50 50 mm2 and a flow channel for H2 and air gases is a single parallel serpentine; the width, depth and pitch of which are 1 mm, each. Based on the JARI standard single fuel cell and by replacing cell elements, we prepared a single fuel cell specific for the combined method of SANS and NR. Graphite flow-field plates and copper current collectors, which cause strong small-angle scattering, were replaced with those made of aluminum and further coated with a gold thin layer (1 mm thickness) to achieve chemical stability and a low electric resistance. Clamping plates were also made of a thinner aluminum. Rubber sheet heaters were placed not to cover a MEA area of 50 50 mm2. Transmission of the modified cell with those replacements described above was dramatically improved from 0.01 to 0.86 for a cold neutron
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Fig. 3. Neutron radiography images (52 52 mm2) obtained for PEFC with a single parallel serpentine flow field, operated at current densities: (a) 40 mA/cm2, (b) 340 mA/ cm2, and (c) 640 mA/cm2. Anodic and cathodic inlets for H2 and air supplies are shown by arrows.
l ¼ 6.5 A˚. It was confirmed that a current–voltage curve well reproduced before the modification. A MEA element was prepared by Eiwa Co. Ltd., Japan, using a Nafion 212 (DuPont) sandwiched with carbon-supported catalysts of Pt (0.5 mg/cm2) and Pt–Ru (0.5 mg/cm2) for the cathode and anode, respectively. Carbon papers were used as the GDL (190 mm thickness) for the anode and cathode. PEFC was operated by a PEFC operating system (HPE-1000), provided by Eiwa Co. Ltd., Japan. This system controls humidity and mass flow of H2 and air gases and temperature of PEFC. An electronic loader (PLZ164WA, Kikusui Electronics Co., Japan) is also connected with HPE-1000 to control an electric current. Hydrogen gas (99.99%) was supplied from a hydrogen generator (OPGU-2200, HORIBA STEC Co. Ltd., Japan). In the case of leakage of H2 gas, an interlock system stops the supply of H2 gas.
5. Simultaneous and in-situ observation In this study, both the fuel-cell temperature and the dewpoints of the anode and the cathode were controlled at 80 1C. The flow rates of hydrogen for the anode and that of air for the cathode were set at 180 and 800 ml/min, respectively. The current density is increased step-wise from 0 to 640 mA/cm2 with a duration of 5 min at each current density. Before starting PEFC in operation, PEFC was purged with dry nitrogen for 15 min, and it was confirmed by NR that the no water was left inside PEFC. Fig. 3 shows NR images obtained for the modified JARI cell operated at current densities: (a) 40, (b) 340, (c) 640 mA/cm2. The area size of images is 52 mm 52 mm, which corresponds to the active area in the flow-field plates. Note that an anodic and a cathodic inlet were lower and upper left corners of the images, respectively. The each exposure time was 210 s. After subtracting a dark image, the all images are normalized by those of the dried cell obtained before operation. At the current density of 40 mA/cm2 (Fig. 3(a)), a NR image in all observable area is almost transparent, indicating that water does not appear in the flow-field plates. At 340 mA/cm2 (Fig. 3(b)), on the other hand, at the upper left portion, corresponding to air inlet, the water starts to appear as a shadow in the image. The dark shadow clearly shows a single parallel serpentine. At 640 mA/cm2 (Fig. 3(c)), the water is accumulated widely from upper to middle parts of the fuel cell. In particular, in comparison with that at 340 mA/cm2, a shadowed area is observed in 901 turn region, which indicates a generation of many water droplets in the flow fields. In contrast, the entire area irradiated by neutron becomes slightly dark, which originate from the accumulated water distributed in MEA or GDL.
Fig. 4. SANS profiles obtained for an operating PEFC at steady state with increasing a current density from 0 to 640 mA/cm2.
We performed SANS measurements on the same operating PEFC investigated by NR. The monochromatic cold neutron (l ¼ 6.5 A˚) was exposed at a center of the PEFC, the beam size was collimated by the first aperture D1 ¼ 20 20 mm2 and the second one D2 ¼ 14 mmf. The incident neutron beam was focused on to the detector plane by a stack of 40 MgF2 lenses so as to gain a large flux of the beam at a sample position. We obtained a gain by a factor of 2.5, as compared to a conventional pinhole SANS setup (D1 ¼ 20 20 mm2 and D2 ¼ 8 mmf) [15]. Fig. 4 shows SANS profiles obtained by increasing current densities from 0 to 640 mA/cm2, where SANS profiles from (1) 0 to (2) 640 mA/cm2 are superposed for varying the current densities. The measuring time spent for each profiles was 80 s. The scattering profiles can be divided into two regions; scattering appeared in q ¼ 0.003–0.02 A˚1 is attributed to the carbonsupported catalysts packed in MEA and that in q ¼ 0.035–0.2 A˚1 to MEA of Nafion. The scattering maximum (ionomer peak) appeared at around q ¼ 0.15 A˚1. As the current density increases, we founded that: the scattering intensity at q ¼ 0.005 A˚1 slightly decreases; whereas the scattering intensity maximum (the ionomer peak) at around q ¼ 0.15 a˚1 systematically increases,
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and its q-value shifts to a lower value. Both findings are attributed to appearance of the water in MEA in the length scale of nanometer. Especially, the shift of peak-positions depends on swelling of the ion-clusters during operation of PEFC.
6. Conclusions We developed a new simultaneous and in-situ observation method of an operating polymer electrolyte fuel cell, by combining small-angle neutron scattering and neutron radiography. By using this technique, we succeeded in selective, simultaneous and in-situ observation of the water widely distributed in the operating fuel cell. NR is suitable for visualization of the water in a flow field in millimeter-to-micrometer in scale, whereas SANS quantitatively detects the water content in a polymer electrolyte membrane in nanometer scale.
Acknowledgements The authors would like to thank Prof. Tsutsumi (Ibaraki University and FC Development Co. Ltd., Japan) for helpful
supports on operation of a fuel cell. They would like to thank Prof. Takenaka (Kobe University) for helpful discussion. They also acknowledge T. Yamaki, R. Yasuda, and R. Motokawa (Japan Atomic Energy Agency) for helpful discussion and support. This work was supported by research and development of polymer electrolyte fuel-cell technology project of New Energy and Industrial Technology Development Organization (NEDO), Japan. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
A. Okawara, et al., Polymer 33 (1992) 1579. R. Mosdale, et al., J. Membr. Sci. 118 (1996) 269. R.J. Bellows, et al., J. Electrochem. Soc. 146 (1999) 1099. R. Satija, et al., J. Power Sources 129 (2004) 238. N. Pekula, et al., Nucl. Instr. and Meth. A 542 (2005) 134. D.J. Ludlow, et al., J. Power Sources 162 (2006) 271. D. Kramer, et al., Electrochim. Acta 51 (2006) 2603. J. Zhang, et al., Electrochim. Acta 51 (2006) 2715. K.W. Feindel, et al., J. Am. Chem. Soc. 128 (2006) 14193. F. Xu, et al., J. Electrochem. Soc. 154 (2007) B1389. T. Kim, et al., Appl. Radiat. Isot. 66 (2008) 593. T. Ueda, et al., J. Power Energy Syst. 2 (2008) 997. T.D. Gierke, et al., J. Polym. Sci. Polym. Phys. 19 (1981) 1687. J.-C. Perrin, et al., J. Phys. Chem. C 111 (2007) 3393. S. Koizumi, et al., J. Appl. Crystallogr. 40 (2007) s474. Y. Hashimasa, J. Power Sources 155 (2006) 182.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
A combined method of small-angle neutron scattering and neutron radiography to visualize water in an operating fuel cell over a wide length scale from nano to millimeter H. Iwase a,b, S. Koizumi a,, H. Iikura c, M. Matsubayashi c, D. Yamaguchi a, Y. Maekawa b, T. Hashimoto a a
Advanced Science Research Center, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan Quantum Beam Science Directorate, Japan Atomic Energy Agency, Takasaki, Gunma 370-1292, Japan c Quantum Beam Science Directorate, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan b
a r t i c l e in fo
abstract
Available online 5 February 2009
In order to visualize water generated in an operating polymer electrolyte fuel cell (PEFC), a neutron radiography (NR) apparatus, composed of a scintillator, optical mirrors and a CCD camera, was installed at a sample position of the focusing and polarized neutron small-angle scattering (SANS) spectrometer (SANS-J-II) at research reactor JRR-3 at Japan Atomic Energy Agency, Tokai, Japan. By combining SANS and NR, we aim to cover a wide length scale from nanometer to millimeter. The new method succeeded in detecting a spatial distribution of the water generated in individual cell elements; NR detected the water in a gas diffusion layer and a flow field, whereas SANS quantitatively determines the water content in a membrane electrode assembly (MEA). Published by Elsevier B.V.
Keywords: Small-angle neutron scattering Neutron radiography Polymer electrolyte fuel cell In-situ observation
1. Introduction A fuel cell has attracted a lot of interests as a next-generation power supply. Particularly, a polymer electrolyte fuel cell (PEFC) is promising for its applications to automotive and portable buttery. During an operation of PEFC, water is generated in individual cell elements (a polymer electrolyte membrane (PEM), catalysts, gas diffusion layers (GDLs) and flow fields) as a result of the electrochemical reaction, transportation and exclusion of water. Since a spatial distribution of the water in fuel cells directly affects an operation performance of PEFC, it is crucial to determine simultaneously and in-situ the distribution and diffusion of the water appeared in the individual cell elements belonging to a given single fuel cell under operation. So far, many of in-situ observations on water in an operating PEFC have been attempted by several methods [1–12]. In the case of Nafion used as a PEM, a characteristic size of its ion-cluster is about several nanometers. Small-angle X-ray scattering (SAXS) [1] and small-angle neutron scattering (SANS) [2,9] have successfully observed a scattering maximum, the so-called ‘‘ionomer peak’’ [13], due to the ion-cluster swollen by water [1,2,13,14]. On the other hand, the water distribution accumulated in the flow field plates is macroscopic. An imaging technique, i.e. neutron radiography (NR), is highly sensitive to hydrogen, and is one of the
Corresponding author. Tel.: +81 29 284 3511; fax: +81 29 282 5939.
E-mail address: [email protected] (S. Koizumi). 0168-9002/$ - see front matter Published by Elsevier B.V. doi:10.1016/j.nima.2009.01.165
potential methods to visualize the water distribution in the macroscopic length scale [4–8,10–12]. As mentioned above, the length scale concerning the water distribution in an operating PEFC is ranged widely from a microscopic (nanometer) to a macroscopic (millimeter) length scale. A single experimental method, such as SANS or NR, is limited to investigate the water in a whole range of the length scale in PEFC. To overcome this limitation, we aimed to combine NR with SANS. Fig. 1 schematically shows our combined SANS and NR method. A neutron imaging system is installed behind the fuel cell placed at a sample position of the SANS spectrometer. When we observe SANS from PEFC, a scintillator sheet and the first optical mirror (defined as a NR set), which are necessary for NR, are removed out from a beam line, and SANS are acquired by a 2D detector. By replacing the NR set in and out intermittently and by synchronized data acquisition of NR and SANS during the operation of PEFC, we intended to observe both the water distribution in the flow-field plate by NR and that in a membrane electrode assembly (MEA) by SANS.
2. Focusing and polarized neutron ultra-small-angle scattering spectrometer (SANS-J-II) Fig. 2(a) shows a schematic view of the focusing and polarized neutron ultra-small-angle scattering spectrometer (SANS-J-II) at research reactor (JRR-3) at Japan Atomic Energy Agency (JAEA), Tokai, Japan [15]. A sample position locates at the middle
ARTICLE IN PRESS 96
H. Iwase et al. / Nuclear Instruments and Methods in Physics Research A 605 (2009) 95–98
Fig. 1. Schematic illustration of a combined method of neutron radiography (a) and small-angle neutron scattering (b), installed at the SANS-J-II spectrometer in JAEA.
In the case of ultra-small-angle neutron scattering, covering q ¼ 3 104–3 103 A˚1, a high-resolution position-sensitive scintillation detector (a cross-wired position-sensitive photomultiplier (5 in. in size with 0.5 mm spatial resolution) coupled with a ZnS/6LiF scintillator (0.2 mm thickness) is utilized. By employing pinhole SANS and focusing SANS on SANS-J-II, a wide range of q from 3 104 to 2 A˚1, corresponding to several micrometers to nanometers in real space, is covered.
3. Monochromatic cold neutron radiography
Fig. 2. (a) Schematic diagram of a focusing and polarized neutron small-angle scattering spectrometer (SANS-J-II) and (b) Photograph of the neutron imaging system, newly installed at SANS-J-II.
(10 m from the first aperture) of the total spectrometer with length of 20 m. At the upstream, cold neutron is monochromatized by a velocity selector, a usually selected wavelength l ¼ 6.5 A˚ and Dl/l ¼ 0.13. The monochromatized cold neutron is collimated by beam apertures made of B4C inside a collimator chamber (the first and second apertures appear at the upper stream and before the sample position, as denoted by D1 and D2 in Fig. 2(a), respectively). We can choose an aperture size (D) for both D1 and D2 from 1 1 to 50 50 mm2. In the case of SANS measurements, D1 ¼ 20 20 mm2 and D2 ¼ 10 10 mm2 were employed. In order to detect an ultra-small-angle scattering region of q104 A˚1, where q is a magnitude of scattering vector given by q ¼ (4p/l)sin(y) with 2y being the scattering angle, we are able to utilize focusing lenses (biconcave lens made up of MgF2 or sextupole permanent magnetic lens) in the collimator chamber. Two position-sensitive detectors (PSDs) are installed in a vacuum flight tube. A 3He main PSD, whose diameter and spatial resolution are 0.58 m and about 5 mm, respectively, is utilized for conventional pinhole SANS, covering from q ¼ 3 103 to 2 A˚1.
To combine NR with SANS, we installed a neutron imaging system at the sample position of SANS-J-II. Fig. 2(b) shows a photograph of the NR system installed on SANS-J-II. The imaging system, allocated behind a fuel cell, is composed of a sheet of ZnS/6LiF scintillator (0.2 mm thickness), two optical mirrors, an optical lens (MicroNikkor, 55 mm, Nikon Co.), and a full frame transfer type CCD camera (C4880, Hamamatsu Photonics Co. Ltd.). The CCD camera (1024 1024 pixels) is placed far from a direct neutron beam to be protected from radiation damage. A monochromatic cold neutron beam (50 50 mm2), passing through a fuel cell, is converted to a visible light by the scintillator, reflected by the double mirrors and finally transmitted to the CCD camera by the optical lens. The scintillator and the first optical mirror are mounted on a vertical elevating bench, in order to move them out from a neutron beam when the SANS measurements were conducted. For NR measurements, we selected D1 ¼ 20 20 mm2, D2 ¼ 50 50 mm2, and L ¼ 5000 mm (Fig. 1), giving L/D ¼ 250, which is matched with the spatial resolution of the imaging system of 184 mm.
4. Fuel cell We utilized a standard single fuel cell provided by Japan Automobile Research Institute (JARI) [16]. On a JARI standard single fuel cell, an active area of MEA is originally 50 50 mm2 and a flow channel for H2 and air gases is a single parallel serpentine; the width, depth and pitch of which are 1 mm, each. Based on the JARI standard single fuel cell and by replacing cell elements, we prepared a single fuel cell specific for the combined method of SANS and NR. Graphite flow-field plates and copper current collectors, which cause strong small-angle scattering, were replaced with those made of aluminum and further coated with a gold thin layer (1 mm thickness) to achieve chemical stability and a low electric resistance. Clamping plates were also made of a thinner aluminum. Rubber sheet heaters were placed not to cover a MEA area of 50 50 mm2. Transmission of the modified cell with those replacements described above was dramatically improved from 0.01 to 0.86 for a cold neutron
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Fig. 3. Neutron radiography images (52 52 mm2) obtained for PEFC with a single parallel serpentine flow field, operated at current densities: (a) 40 mA/cm2, (b) 340 mA/ cm2, and (c) 640 mA/cm2. Anodic and cathodic inlets for H2 and air supplies are shown by arrows.
l ¼ 6.5 A˚. It was confirmed that a current–voltage curve well reproduced before the modification. A MEA element was prepared by Eiwa Co. Ltd., Japan, using a Nafion 212 (DuPont) sandwiched with carbon-supported catalysts of Pt (0.5 mg/cm2) and Pt–Ru (0.5 mg/cm2) for the cathode and anode, respectively. Carbon papers were used as the GDL (190 mm thickness) for the anode and cathode. PEFC was operated by a PEFC operating system (HPE-1000), provided by Eiwa Co. Ltd., Japan. This system controls humidity and mass flow of H2 and air gases and temperature of PEFC. An electronic loader (PLZ164WA, Kikusui Electronics Co., Japan) is also connected with HPE-1000 to control an electric current. Hydrogen gas (99.99%) was supplied from a hydrogen generator (OPGU-2200, HORIBA STEC Co. Ltd., Japan). In the case of leakage of H2 gas, an interlock system stops the supply of H2 gas.
5. Simultaneous and in-situ observation In this study, both the fuel-cell temperature and the dewpoints of the anode and the cathode were controlled at 80 1C. The flow rates of hydrogen for the anode and that of air for the cathode were set at 180 and 800 ml/min, respectively. The current density is increased step-wise from 0 to 640 mA/cm2 with a duration of 5 min at each current density. Before starting PEFC in operation, PEFC was purged with dry nitrogen for 15 min, and it was confirmed by NR that the no water was left inside PEFC. Fig. 3 shows NR images obtained for the modified JARI cell operated at current densities: (a) 40, (b) 340, (c) 640 mA/cm2. The area size of images is 52 mm 52 mm, which corresponds to the active area in the flow-field plates. Note that an anodic and a cathodic inlet were lower and upper left corners of the images, respectively. The each exposure time was 210 s. After subtracting a dark image, the all images are normalized by those of the dried cell obtained before operation. At the current density of 40 mA/cm2 (Fig. 3(a)), a NR image in all observable area is almost transparent, indicating that water does not appear in the flow-field plates. At 340 mA/cm2 (Fig. 3(b)), on the other hand, at the upper left portion, corresponding to air inlet, the water starts to appear as a shadow in the image. The dark shadow clearly shows a single parallel serpentine. At 640 mA/cm2 (Fig. 3(c)), the water is accumulated widely from upper to middle parts of the fuel cell. In particular, in comparison with that at 340 mA/cm2, a shadowed area is observed in 901 turn region, which indicates a generation of many water droplets in the flow fields. In contrast, the entire area irradiated by neutron becomes slightly dark, which originate from the accumulated water distributed in MEA or GDL.
Fig. 4. SANS profiles obtained for an operating PEFC at steady state with increasing a current density from 0 to 640 mA/cm2.
We performed SANS measurements on the same operating PEFC investigated by NR. The monochromatic cold neutron (l ¼ 6.5 A˚) was exposed at a center of the PEFC, the beam size was collimated by the first aperture D1 ¼ 20 20 mm2 and the second one D2 ¼ 14 mmf. The incident neutron beam was focused on to the detector plane by a stack of 40 MgF2 lenses so as to gain a large flux of the beam at a sample position. We obtained a gain by a factor of 2.5, as compared to a conventional pinhole SANS setup (D1 ¼ 20 20 mm2 and D2 ¼ 8 mmf) [15]. Fig. 4 shows SANS profiles obtained by increasing current densities from 0 to 640 mA/cm2, where SANS profiles from (1) 0 to (2) 640 mA/cm2 are superposed for varying the current densities. The measuring time spent for each profiles was 80 s. The scattering profiles can be divided into two regions; scattering appeared in q ¼ 0.003–0.02 A˚1 is attributed to the carbonsupported catalysts packed in MEA and that in q ¼ 0.035–0.2 A˚1 to MEA of Nafion. The scattering maximum (ionomer peak) appeared at around q ¼ 0.15 A˚1. As the current density increases, we founded that: the scattering intensity at q ¼ 0.005 A˚1 slightly decreases; whereas the scattering intensity maximum (the ionomer peak) at around q ¼ 0.15 a˚1 systematically increases,
ARTICLE IN PRESS 98
H. Iwase et al. / Nuclear Instruments and Methods in Physics Research A 605 (2009) 95–98
and its q-value shifts to a lower value. Both findings are attributed to appearance of the water in MEA in the length scale of nanometer. Especially, the shift of peak-positions depends on swelling of the ion-clusters during operation of PEFC.
6. Conclusions We developed a new simultaneous and in-situ observation method of an operating polymer electrolyte fuel cell, by combining small-angle neutron scattering and neutron radiography. By using this technique, we succeeded in selective, simultaneous and in-situ observation of the water widely distributed in the operating fuel cell. NR is suitable for visualization of the water in a flow field in millimeter-to-micrometer in scale, whereas SANS quantitatively detects the water content in a polymer electrolyte membrane in nanometer scale.
Acknowledgements The authors would like to thank Prof. Tsutsumi (Ibaraki University and FC Development Co. Ltd., Japan) for helpful
supports on operation of a fuel cell. They would like to thank Prof. Takenaka (Kobe University) for helpful discussion. They also acknowledge T. Yamaki, R. Yasuda, and R. Motokawa (Japan Atomic Energy Agency) for helpful discussion and support. This work was supported by research and development of polymer electrolyte fuel-cell technology project of New Energy and Industrial Technology Development Organization (NEDO), Japan. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
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