Anal. Chem. 2010, 82, 9221–9224
Near-Infrared Imaging of Water in a Polymer Electrolyte Membrane during a Fuel Cell Operation Shigeaki Morita,*,† Yuki Jojima,‡ Yasushi Miyata,§ and Kuniyuki Kitagawa† Division of Energy Science, EcoTopia Science Institute and Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan, and Nagoya Municipal Industrial Research Institute, 3-4-41, Rokuban, Atsuta-ku, Nagaya 456-0058, Japan A novel technique of spectroscopic imaging using a nearinfrared (NIR) laser sheet beam was developed for visualization of liquid water in a proton-exchange membrane (PEM) sandwiched between two opaque electrodes set in a polymer electrolyte fuel cell (PEFC). In-plane twodimensional distribution of water in the thin membrane was clearly visualized during the fuel cell operation. Under the condition of fuel feeding into the PEFC without humidification, water was generated by the fuel cell reaction in the whole electrode area. In contrast, under the condition of fuel feeding with humidification, the PEM got wet in the vicinity of a gas flow field locally. Water in a polymer electrolyte fuel cell (PEFC) plays important roles in the operation.1 When a fuel of hydrogen and an oxidizer of oxygen are fed into a PEFC, the following fuel cell reactions occur. Oxidation half-cell reaction at the anode: 2H2 f 4H+ + 4e-
(1)
Reduction half-cell reaction at the cathode: O2 + 4H+ + 4e- f 2H2O
(2)
Not only are water molecules generated by the electrode reaction at the cathode but also they act as a proton carrier and are transported in a proton-exchange membrane (PEM) from the anode side to the cathode side. Due to the phenomena abovementioned, water is enriched at the cathode side while drained at the anode side during the PEFC operation. In order to prevent the PEM from being dried at the anode side, the fuel of hydrogen is generally humidified before being fed into the fuel cell. However, excess cell reaction and/or humidification lead to clogging of gas flow field in the fuel cell.2 Due to the reasons * To whom correspondence should be addressed. E-mail: smorita@ nagoya-u.jp. † Division of Energy Science, EcoTopia Science Institute, Nagoya University. ‡ Department of Applied Chemistry, Graduate School of Engineering, Nagoya University. § Nagoya Municipal Industrial Research Institute. (1) Eikerling, M.; Kornyshev, A. A.; Kucernak, A. R. Phys. Today 2006, 59, 38–44. (2) Li, H.; Tang, Y. H.; Wang, Z. W.; Shi, Z.; Wu, S. H.; Song, D. T.; Zhang, J. L.; Fatih, K.; Zhang, J. J.; Wang, H. J.; Liu, Z. S.; Abouatallah, R.; Mazza, A. J. Power Sources 2008, 178, 103–117. 10.1021/ac101525z 2010 American Chemical Society Published on Web 10/21/2010
mentioned above, water management in the PEFC is among the key issues for the practical application.3,4 Properties and behavior of water in PEFCs have extensively been studied by theoretical and numerical computations,5,6 gas chromatography,7 X-ray scattering,8 fluorescence microscopy,9 neutron imaging,10,11 and so on. However, actual distribution and transportation of water in the PEM during the PEFC operation are still not clear because of analytical difficulty. Matic et al. reported one-dimensional distribution of water at an edge of the PEM in a working H2/H2 pump cell in the direction of the membrane thickness, which corresponds to that of the proton transfer in the membrane, using micro-Raman spectroscopy.12 Tsushima et al. successfully visualized the distribution of water in the direction of the PEM thickness under the PEFC operation using magnetic resonance imaging.13,14 However, in-plane two-dimensional distribution of water in the PEM during the PEFC operation has still not been visualized directly. Near-infrared (NIR) spectroscopy is a spectroscopic method which mainly observes overtone and combination modes of molecular vibrations.15 NIR absorptions by functional groups containing X-H bonds (X ) C, N, O, etc.) are dominant because of vibrational anharmonicity.15 Thus, water has intense NIR absorptions, whereas it is expected that tetrafluoroethylene based PEM has almost no NIR absorptions. Furthermore, extinction coefficients of NIR absorptions are generally much smaller than those of fundamentals observed in the mid-infrared region.15 For (3) Prater, K. B. J. Power Sources 1994, 51, 129–144. (4) Van Nguyen, T.; Knobbe, M. W. J. Power Sources 2003, 114, 70–79. (5) Springer, T. E.; Zawodzinski, T. A.; Gottesfeld, S. J. Electrochem. Soc. 1991, 138, 2334–2342. (6) Wang, C. Y. Chem. Rev. 2004, 104, 4727–4765. (7) Mench, M. M.; Dong, Q. L.; Wang, C. Y. J. Power Sources 2003, 124, 90– 98. (8) Gebel, G.; Diat, O. Fuel Cells 2005, 5, 261–276. (9) Litster, S.; Sinton, D.; Djilali, N. J. Power Sources 2006, 154, 95–105. (10) Satija, R.; Jacobson, D. L.; Arif, M.; Werner, S. A. J. Power Sources 2004, 129, 238–245. (11) Pekula, N.; Heller, K.; Chuang, P. A.; Turhan, A.; Mench, M. M.; Brenizer, J. S.; Unlu, K. Nucl. Instrum. Methods Phys. Res., Sect. A 2005, 542, 134– 141. (12) Matic, H.; Lundblad, A.; Lindbergh, G.; Jacobsson, P. Electrochem. SolidState Lett. 2005, 8, A5–A7. (13) Teranishi, K.; Tsushima, S.; Hirai, S. J. Electrochem. Soc. 2006, 153, A664– A668. (14) Tsushima, S.; Hirai, S. In Magnetic Resonance Microscopy: Spatially Resolved NMR Techniques and Applications; Wiley-VCH: Weinheim, Germany, 2009, pp 421-433. (15) Siesler, H. W.; Ozaki, Y.; Kawata, S.; Heise, H. M. Near-Infrared Spectroscopy, Principles, Instruments, Applications; Wiley-VHC: Weinheim, Germany, 2002.
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Figure 1. (a) Schematic design of the PEFC, (b) photo of the anode separator, (c) schematic diagram of the NIR imaging system. (A) PEM (Nafion), (B) GDL, (C) platinum catalyst, (D) gasket, (E) anode separator, (F) cathode separator, (G) gas flow channel, (H) air-intake, (I) NIR laser, (J) cylindrical lens, (K) slit, and (L) NIR linear image sensor. (x, y) and (X, Y) are the PEFC and the laboratory coordinate systems, respectively.
example, the extinction coefficient of the first overtone of O-H stretching mode for liquid water observed at around 6900 cm-1 is ca. 380 times smaller than that of fundamentals at around 3400 cm-1.16 Therefore, NIR spectroscopy has widely been applied to nondestructive analyses for agricultural and industrial products as well as laboratory use.17 Spectroscopic imaging based on NIR absorption has also been progressed recently.18,19 In the present study, a novel imaging technique based on NIR spectroscopy using a NIR laser sheet beam is proposed for visualization of water in the PEM. The small extinction coefficient of NIR absorption enables the laser sheet beam to pass through the hydrated membrane along the in-plane direction having a long path length without undetectable light intensity attenuation. Inplane two-dimensional distribution of water content in the PEM sandwiched between two-opaque electrodes visualized by the method is reported. EXPERIMENTAL SECTION Figure 1a shows a schematic design of a homemade PEFC developed in the present study. A commercially available ionexchange membrane of Nafion (NE-1110, DuPont) having a thickness of 254 µm was used as a PEM. A platinum catalyst of ca. 0.1 g · m-2 was deposited onto a one-side surface of porous carbon paper and the platinum/carbon electrode was used as a gas diffusion layer (GDL). Edges of the GDL were sealed by a gasket made of silicon rubber. The PEM was put between (16) Bertie, J. E.; Lan, Z. D. Appl. Spectrosc. 1996, 50, 1047–1057. (17) Ozaki, Y.; McClure, W. F.; Christy, A. A. Near-infrared Spectroscopy in Food Science and Technology; John Wiley & Sons: Hoboken, NJ, 2006. (18) Morita, S.; Hattori, E.; Kitagawa, K. Appl. Spectrosc. 2008, 62, 1216–1220. (19) Awa, K.; Okumura, T.; Shinzawa, H.; Otsuka, M.; Ozaki, Y. Anal. Chim. Acta 2008, 619, 81–86.
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two platinum/carbon electrodes, and the membrane electrode assembly was sandwiched between anode and cathode separators made of SUS304 with the gaskets as shown in Figure 1a. A gas flow channel having a linear shape was cut in the anode separator as shown in Figure 1b. Nine holes for air-intake were made on the cathode separator. The PEFC was operated with a 3 Ω external resistor at an ambient temperature of ca. 22 °C. A fuel of hydrogen was dried by passing through calcium chloride or humidified by bubbling through liquid water and then was fed into the anode at a flow rate of 0.11 cm3 · s-1. As a result, a linear flow rate of the fuel in the anode channel (cross section of 1 mm × 2 mm) was 55 mm · s-1. Relative humidity (RH) of the fuel was controlled by dew-point temperature. An oxidizer of oxygen was diffusively provided from an ambient air via the air-intake in the cathode separator. Figure 1c illustrates a schematic diagram of an experimental setup assembled for the NIR spectroscopic imaging. A continuous wave NIR diode laser tuned to be a wavelength of 1470 nm with an output power of 11.2 mW was obtained from FiberLabs Inc., Japan (FLG-S-1470-10). The collimated laser beam was formed in a thin sheet of ca. 13 mm in height and ca. 0.1 mm in thickness through two cylindrical lenses and a slit. The resulting laser sheet beam was introduced into the one-side edge of the PEM set in the PEFC. Light intensities of the transmitted laser sheet beam through the plane of the PEM were detected by a NIR linear image sensor consisting of 256 pixels InGaAs photodiode array developed by Hamamatsu Photonics K.K., Japan (G9211-256S). Each pixel has a 0.05 mm width, and a total acceptable surface size of the detector is 12.8 mm × 0.5 mm. Two different coordinate systems of a PEFC coordinate system (x, y) and a laboratory coordinate system (X, Y) were independently defined as illustrated in Figure 1b,c, respectively. The x-axis is parallel to the direction of gas flow field in the anode channel, and the y-axis is perpendicular to the x-axis. The X-axis is parallel to the traveling direction of the laser sheet beam, and the Y-axis is perpendicular to the X-axis and opposite to the direction of gravity. Locations of the zeroth and 255th pixels in the NIR linear image sensor are defined as Y ) 0.0 mm (bottom) and 12.8 mm (top), respectively. One-dimensional distribution of water content in the PEM along the Y-axis was obtained from the Beer-Lambert law.
( )
A(Y) ) -log10
I(Y) I0(Y)
(3)
where I and I0 are light intensities of the laser sheet beam with and without water in the PEM. The beam profile of the laser sheet without water I0(Y) was measured after enough dried the membrane by nitrogen gas flow in the fuel cell. Two-dimensional distribution of water content in the PEM was obtained by the following procedure: A one-dimensional distribution perpendicular to the gas channel field A(y) was measured as (X, Y) ) (x, y). After that, the PEFC was rotated 90° counterclockwise as (X, Y) ) (-y, x), and then, a one-dimensional distribution parallel to the gas flow channel A(x) was independently measured. Finally, a two-dimensional distribution of water was calculated from the two one-dimensional distributions as
A(x, y) ) AT(x) · A(y)
(4)
where AT is a transpose of the A. Here, let us consider a model distribution consisting of a small matrix size of 5 × 5 elements.
( )
0 0 Amodel(x, y) ) 1 0 0
0 1 2 1 0
0 1 3 1 0
0 1 2 1 0
0 0 1 0 0
(5)
Figure 2. Time-dependent NIR spectra of a hydrated Nafion membrane measured under a drying process. The broken line is the spectra of the membrane almost dried. The arrow indicates the corresponding wavelength of the NIR laser of 1470 nm.
One-dimensional distributions along the x- and y-axes are, respectively, obtained as
()
0 3 AT(x) ) 9 , 3 0
A(y) ) (1 4 5 4 1 )
(6)
In the actual case, these two distributions consisting of 256 elements are measured by the NIR linear image sensor. Finally, the two-dimensional distribution is calculated from the two onedimensional distributions.
()
0 3 Acalc(x, y) ) 9 × (1 4 5 4 1 ) ) 15 × 3 0
(
0.0 0.2 0.6 0.2 0.0
0.0 0.8 2.4 0.8 0.0
0.0 1.0 3.0 1.0 0.0
0.0 0.8 2.4 0.8 0.0
0.0 0.2 0.6 0.2 0.0
)
(7)
The calculated distribution is not completely the same as the model distribution. However, the calculated two-dimensional distribution by eq 4 is almost proportional to the real distribution within numerical limitation like an optical defocusing. All the NIR absorption spectra were measured using a Fourier transform infrared spectrometer (Varian FTS-3000) equipped with a calcium fluoride beam splitter for the NIR range detection (FTNIR). A total of 64 scans were coadded to obtain each spectrum at a resolution of 4 cm-1. RESULTS AND DISCUSSION Figure 2 shows time-dependent NIR absorption spectra of a hydrated Nafion membrane measured using the FT-NIR spectrometer under a drying process. Two NIR absorption bands located at 6880 and 5184 cm-1 assigned to ν1 + ν3 and ν2 + ν3 modes of water, respectively, are identified.20 Intensities of these two bands decrease with time. Although non-Beer’s law behavior in the mid-infrared region is reported for a hydrated Nafion-Na membrane,21 the NIR absorbance variations are (20) Buijs, K.; Choppin, G. R. J. Chem. Phys. 1963, 39, 2035–2041. (21) Wang, Y. Q.; Kawano, Y.; Aubuchon, S. R.; Palmer, R. A. Macromolecules 2003, 36, 1138–1146.
Figure 3. One-dimensional distributions of water in the PEM (a) perpendicular and (b) parallel to the gas flow field in the anode channel.
more or less in accordance with the Beer’s law, because of the small extinction coefficient. In contrast, almost no spectral contribution is detected in the region from the almost dried Nafion membrane (broken line) because of tetrafluoroethylene based copolymer. These results demonstrate that the NIR absorptions at around 6880 and 5184 cm-1 are usable as an index of water content in the membrane. The laser wavelength of 1470 nm used in the present study is represented by an arrow in Figure 2. We chose the NIR absorption band at 6880 cm-1 for the detection of water in the membrane, since the extinction coefficient of the band at 6880 cm-1 is smaller than that at 5184 cm-1. The advantage of this choice will be discussed later. Figure 3a,b depicts typical one-dimensional distributions of water in the PEM when Y ) y and Y ) x, respectively. The distribution of water perpendicular to the gas flow field (Figure 2a) has maximum peak close to the gas flow channel located at around y ) 6.4 mm. In contrast, that parallel to the gas flow field (Figure 2b) shows almost flat distribution. The minimum absorbance of ca. 0.2 and the maximum absorbance of ca. 1.6 represent, respectively, the maximum transmittance T ) 10-A of ca. 63% and the minimum transmittance of ca. 3%, indicating that ca. 37-97% of the laser light intensities are extinct by water in the PEM. Such the small extinction of the incident light intensity is successfully detected by choosing the NIR absorption band at 6880 cm-1, since the extinction coefficient of which is smaller than that at 5184 cm-1. Figure 4 shows a two-dimensional image of water content distribution in the PEM constructed from the one-dimensional distributions shown in Figure 3 by eq 4. A broken line square shown in the figure illustrates the position of the gas flow channel in the anode separator. Flow direction of the fuel in the channel is left-to-right. It is clearly visualized in the image that water in the PEM locally exists in the vicinity of the gas flow channel. Analytical Chemistry, Vol. 82, No. 22, November 15, 2010
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Figure 4. A two-dimensional distribution of water in the PEM constructed from the one-dimensional distributions shown in Figure 3 by eq 4.
Figure 6. Number of hydration of the Nafion side chain terminal SO3group plotted as a function of time.
In the case of fuel feeding into the PEFC with humidification (Figure 5b), a large amount of water in the membrane compared with that in the case without humidification (Figure 5a) is observed. Furthermore, in the case with humidification, the distribution of water in the membrane is localized near the gas flow field, whereas in the case without humidification, that spreads in the whole electrode area. These results suggest that the humidification of the fuel makes the membrane wet in the vicinity of the flow field, while the fuel cell reaction occurs in the whole electrode area, although the fuel was fed from the narrow and localized gas flow channel. Figure 6 plots the number of hydration to the sulfonate group on the side chain terminal of the Nafion ionomer as a function of time under the different RH conditions. The number of hydration was estimated from the two-dimensional image of the water content and calibrated by weight fraction of water in the membrane measured by a gravimetric method. The number of hydration suddenly increases with starting the fuel feeding at RH 75.1%, whereas that has an induction period of ca. 240 s for the increase at RH 0.0%. The number of hydration at 1200 s at RH 75.1% is approximately two times larger than that at RH 0.0%. The temporal change in the hydration at RH 53.3% indicates intermediate behavior between that at RH 75.1% and that at RH 0.0%. These results demonstrate that humidification of the fuel is the essential technique for the initial startup of the PEFC. Figure 5. Two-dimensional distributions of water in the PEM aligned as a function of time. (a) Dried hydrogen or (b) humidified hydrogen of RH 75.1% was fed into the PEFC.
Figure 5a,b shows two-dimensional distributions of water in the PEM aligned as a function of time with different RH of 0.0% and 75.1%, respectively. The initial time of 0 s is defined as the starting time of the fuel feeding into the PEFC. Before the PEFC operation, the PEM was sufficiently dried by a nitrogen gas flow though the anode channel. In the case of fuel feeding into the PEFC without humidification (Figure 5a), the two-dimensional distribution of water looks almost flat in the plane. In particular, the distribution gradually spreads left-to-right along the direction of the gas flow field in the channel with time. This spreading of the water distribution is much slower than the linear flow rate of the fuel of 55 mm · s-1. Similar behavior was observed when the PEFC was rotated 180° from (X, Y) ) (-y, x) to (X, Y) ) (y, -x) (figures are not shown here). These results represent that water generation by the fuel cell reaction in the vicinity of the fuel inlet is larger than that of the outlet, suggesting consumption of the fuel along the flow field. 9224
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CONCLUSION In-plane two-dimensional distributions of water in a PEM were successfully visualized during the PEFC operation. These were revealed that the generating water at the cathode catalyst by the fuel cell reaction made the membrane wet in the whole electrode area, while the humidification water with the fuel feeding made the membrane wet near the gas flow channel in the anode separator. It is suggested that this method is applicable to other systems, e.g., chemical analysis of a laminate film, etc. ACKNOWLEDGMENT This work was supported by Grant-in-Aid for Scientific Research (B) (20360429) from Japan Society for the Promotion of Science (JSPS).
Received for review June 9, 2010. Accepted October 10, 2010. AC101525Z
