Fuel cell studies with neutrons at the PSI's neutron imaging facilities

Fuel cell studies with neutrons at the PSI's neutron imaging facilities

ARTICLE IN PRESS Nuclear Instruments and Methods in Physics Research A 605 (2009) 123–126 Contents lists available at ScienceDirect Nuclear Instrume...

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ARTICLE IN PRESS Nuclear Instruments and Methods in Physics Research A 605 (2009) 123–126

Contents lists available at ScienceDirect

Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima

Fuel cell studies with neutrons at the PSI’s neutron imaging facilities E.H. Lehmann a,, P. Boillat b, G. Scherrer b, G. Frei a a b

Spallation Neutron Source Division, Paul Scherrer Institut, Switzerland Electrochemistry Laboratory, Paul Scherrer Institut, Switzerland

a r t i c l e in fo

abstract

Available online 5 February 2009

The aim of the current study is on water distribution inside and near the gas diffusion layers around the membrane (‘‘in-plane’’ study). Because this region is normally only about 200 mm wide, a very high spatial resolution is required for this kind of studies. PSI managed to build a neutron imaging setup with a stationary detection system providing bestpossible conditions for high-resolution investigations [8]. However, the field of view is adapted accordingly to 27 mm only. Using a special arrangement by tilting the detection plane in one direction, it ends up with nominal 2.5 mm per pixel. The real resolution was found to be about 20 mm in the relevant direction across the fuel cell plane [9]. This work will present this new setup and summarize the first promising results from fuel cell studies around the membrane region. Despite these good results with high spatial resolution, other options in fuel cell research can be used at the PSI neutron imaging facilities. Due to the large beam diameter (40 cm circular) full size cells can be studied under various conditions with moderate special and time resolution. & 2009 Elsevier B.V. All rights reserved.

Keywords: Fuel cell Neutron imaging Water distribution Spatial resolution Quantification

1. Introduction Fuel cells are considered to be a very promising future option with respect to energy conversion processes from chemical compounds towards electricity and in addition offer the possibility of environmental benigness. Although the main concepts for fuels cells are well known for decades, there is need for further research in order to improve the performance, to understand all operational behavior under different conditions and to make it cost competitive for market entry. One major aspect is the use of fuel cells for automotive applications. Despite other concepts, the most promising fuel cell type in this respect is the polymer electrolyte fuel cell (PEFC), which is based on a proton-conducting membrane working at moderate temperatures (typ. 70–80 1C). Hydrogen and oxygen are electrochemically combined to water as the reaction product, which is either withdrawn from the reaction zone with the gas flow or accumulated in liquid phase inside the cell. On the one hand, membrane hydration is required to maintain the proton conductivity. On the other hand, too much water will impede the access of gaseous reactants and this will limit the performance. Therefore, water management is a critical topic for the advance of the PEFC technology.

 Corresponding author. Tel.: +41 563 1029 63; fax: +41 563 1031 31.

E-mail address: [email protected] (E.H. Lehmann). 0168-9002/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2009.01.143

A non-invasive investigation of fuel cells under operation with respect to the water distribution is difficult because of the missing transparency for light of the structure elements (graphite plates, metal structures, sealing). Therefore, the option of neutron imaging is very helpful and powerful for two reasons: structural materials can be transmitted relatively easily by neutrons, and water delivers a high transmission contrast, even in small amounts. Taking advantage of these unique properties, the Electrochemistry Laboratory together with the Neutron Imaging & Activation Group at PSI has been using neutron radiography since several years for imaging the water distribution in operating fuel cells.

2. Options for fuel cell studies In the beginning, the research interest has been focused on through plane imaging (cell perpendicular to the beam), from which several interesting observations could be made [1–4]. These experiments allowed not only observing, but also quantifying liquid water in gas flow channels as well as gas diffusion media with accuracy better than 1 ml cm2, with typical exposure times of a few seconds and spatial resolution of approximately 0.2 mm (Fig. 1). This through plane configuration can be used with single cells of technical size, or with short stacks of a few cells, though it will in the later case not be possible with a single radiogram to distinguish between the different cells.

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about 300 times longer than single transmission scans—and the cell has to be kept in stationary conditions for hours [4,5].

3. Neutron imaging aspects and detection techniques

Fig. 1. Principle setup for studies of the water distribution over the cell area using through plane imaging.

Fig. 2. Principle setup for studies of the water distribution across the cell structure using in-plane imaging.

Fig. 3. Differential fuel cell for investigations in the ‘‘in-plane’’ mode; the real dimensions of the components are subject of optimization and must be hidden therefore.

In order to enhance the contrast and the detection sensitivity for the water distribution in the channels of the cell, an ‘‘referencing’’ procedure is applied, where the radiogram of the operating cell is divided by the radiogram of the dry cell, thus allowing to separate the attenuation of water from the attenuation of the structural materials. A drawback of through plane imaging is that it does not allow discriminating between the different layers of the cell structure. Therefore, in-plane imaging (cell parallel to the beam) has been developed recently. Because the typical thickness of gas diffusion layers is 200 mm, a very high spatial resolution is required for this kind of studies (Fig. 2). Studies using in-plane imaging have been focused on differential cells, which are cells of small dimension operated on high stoichiometries, reproducing the local operating conditions expected to be found in a full size technical cell. The focus of this paper is related to this aspect (see Section 3). The setup of such a ‘‘differential cell’’ is shown in Fig. 3. Attempts to use tomography for fuel cell studies might be challenging in principle because the three-dimensional moisture distribution can be observed. However, the structure has to be transparent from all directions, which is difficult for typical cells. On the other hand, to take all needed tomography projections is

All neutron imaging studies with fuel cell components were done until 2005 at the NEUTRA facility, providing a thermal spectrum with 25 meV average energy. Since the ICON facility [6] was completed, most of the measurements have been done there due to the availability of cold neutrons. As shown in Fig. 4, cold neutrons deliver higher image contrast for water (larger slope) and higher detection probability therefore. These results also demonstrate that the attenuations in water layers up to at least 0.3 and 0.7 mm, follow the Lamberts Law, which enables a quite simple quantitative evaluation. For time-dependent fuel cell studies, the detectors based on CCD are limited by the exposure time and the readout performance too. Both are in the order of seconds under best ICON conditions. If an amorphous Si flat panel is used [7], the frame rate can be higher, but there are limitations in the image quality and the size of a sequence (by the internal memory). A time resolution of 1–10 s is, however, usually enough for studying water dynamics in fuel cells. Another aspect is the spatial resolution in the neutron imaging studies, in particular when in-plane studies have to be performed. Common detection systems (camera based, imaging plates-IP) have about 0.1 mm resolution, which is not sufficient when the reaction zone of the PEMFC (about 0.4 mm thick, see Fig. 5) should be studied. Several attempts were made to overcome this limitation. A first step was an IP-scanner with higher readout density (0.0125 mm nominal), but it was no stationary setup and a referencing procedure was extremely difficult. In particular for the ICON conditions (as part of the project), a camera-based system which consists of a perfect optical system with respect to resolution and light efficiency in conjunction with a very thin scintillator screen [7] was developed. The final step for the moment for fuel cell studies in in-plane configuration is to tilt the detector by about 801 with respect to the perpendicular ‘‘normal’’ configuration. In this way, the image is stretched and a higher resolution is obtained. Although the effective scintillator thickness becomes thicker in this way and some more blurring can occur, the ‘‘magnification effect’’

Fig. 4. Experimental results from transmission measurements for water layers with thermal and cold neutrons (courtesy D. Kramer, PSI).

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Fig. 5. Transmission neutron image of a small test cell (dry-left) and the water distribution (dark areas) measured during operation, measured with the tilted option.

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Fig. 6. (a–c) In-plane neutron radiograms of a PEFC showing the water distribution at 1 A/cm2 for different inlet relative humidity at the anode/cathode: (a) 60/0%, (b) 110/40%, (c) 90/110% and (d) relationship between average water content and cell performance. The cell’s dimension corresponds to the numbers in Fig. 5.

dominated in this technique. As shown in Fig. 5, structural details of the cell become very visible. Applying the now possible referencing process, water distribution can be studied with high-resolution.

4. Results of recent investigations Two kinds of investigations based on high-resolution in-plane imaging are reported here as examples for illustration of the possibilities of the new setup. Firstly, a study of the relationships between the local relative humidity conditions in the gas channels, the water distribution in the gas diffusion layers and the performance of the cell was realized. The relative humidity of anode and cathode gas inlets was varied independently, while operating the cell with a constant current density of 1 A/cm2. The operating temperature and pressure were, respectively, 70 1C and 2 bar. The water distribution corresponding to some selected conditions can be seen in Fig. 6a–c. In Fig. 6d, the cell performance as a function of the average water content is displayed, clearly illustrating the negative impact of either too dry or too wet local operating conditions. It must be noted that even in the case of an anode gas flow oversaturated with water (Fig. 6b), the anode side GDL can be free of water. A further interesting possibility is to take advantage of the isotopic sensitivity of neutron radiography. The cross-section of deuterium (2H) atoms being approximately 10 times lower than that of light hydrogen (1H), heavy water is virtually invisible in comparison to light water. This allows in principle to trace the origin of accumulated water, by operating the cell using deuterium gas (2H2) or by humidifying either of the gas flows using heavy water (2H2O). When operating the cell using 2H2, it is intuitive to consider that the water produced electrochemically will be heavy water. However, recent experiments [10] have shown that this assumption has to be put in perspective, when taking into account the exchange processes occurring in the membrane and at its interfaces. As observed in Fig. 7, the rate of replacement of light water with heavy water when switching the anode gas from 1H2 to 2H2 is virtually independent of the current density. In the case of a low current density, this rate is much higher than can be explained by the electrochemical production of heavy water, emphasizing the importance of the above-mentioned exchange processes.

Fig. 7. Replacement of water by heavy water (displayed as a light shade) observed in a thick (180 mm) Nafion membrane, after switching the anode gas from hydrogen to deuterium. Current density is 0.02 A/cm2 (a) and 0.80 A/cm2 (b).

5. Further improvements and activities The neutron spectrum at ICON is not only ‘‘cold’’ but also has some contributions from thermal and epithermal regions, given by the setup of the beam line. In order to raise the contrast for water and to have higher transparency for the structural materials, the narrowing of the neutron energy band may be an important step. As shown in Table 1, the ratio between the attenuation coefficients of water and aluminum (a typical fuel cell structural material) is improved when using neutrons with low energy. In practice, either the Be filtered option can be used or an turbine-type energy selector. Although the number of neutrons is reduced in both cases and the exposure time has to be increased accordingly, the gain in contrast might justify such approach in some cases. Optimization procedures are still needed to find out the best-possible conditions for the particular setup. Concerning further improvements in resolution, an option is to increase the tilting angle of the detector. Recently, preliminary experiments have demonstrated the possibility of reducing this way the pixel size to approximately 1 mm, resulting in an effective spatial resolution (FWHM) of approximately 10 mm.

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Table 1 Neutron attenuation behavior under different spectral conditions at the PSI imaging beam lines. Attenuation coefficients (cm NEUTRA ICON ICON Be filtered

1

)

Al

H2O

Ratio (H2O/Al)

0.097 0.088 0.075

3.27 4.17 6.01

33.7 47.4 80.1

Another aspect will be to come back to other type of fuel cells. When the direct methanol cell (DMFC) is considered, an opposite behavior will be observed—the exchange of the methanol by the produced CO2. High sensitivity and good resolution in space and time were already demonstrated [1,2].

6. Conclusion Several years of experimentations have proved the potential of neutron imaging for understanding and further improving water

management. Recently, the ability of neutron imaging for highresolution in-plane imaging has been demonstrated. Due to the higher contrast of cold neutrons for hydrogen, thanks to the very high spatial resolution of the detection system, the ICON facility is a favorite for many demanding investigations in basic and applied studies. Further methodical improvements will again raise this position.

References [1] A. Geiger, et al., Fuel Cells 2 (2) (2002) 92–98. [2] D. Kramer, Mass Transport Aspects of PEMFC Under Two-phase Flow Conditions, Dissertation, Freiberg, 2007. [3] D. Kramer, et al., Electrochim. Acta 50 (13) (2005) 2603. [4] J. Zhang, et al., Electrochim. Acta 51 (13) (2006) 2715. [5] I. Manke, et al., Appl. Phys. Lett. 90 (2007) 184101. [6] G. Ku¨hne, et al., Swiss Neutron News 28 (December 2005) 20. [7] M. Estermann, et al., Paul Scherrer Institut Annual Report III 188 (2003). [8] E.H. Lehmann, G. Frei, G. Ku¨hne, P. Boillat, Nucl. Instr. and Meth. A 576 (2007) 389. [9] P. Boillat, et al., Electrochem. Commun. 10 (2008) 546. [10] P. Boillat, et al., Electrochem. Commun., 2008, in press.