Liquid water visualization in PEM fuel cells: A review

international journal of hydrogen energy 34 (2009) 3845–3857

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Review

Liquid water visualization in PEM fuel cells: A review A. Bazylak Department of Mechanical & Industrial Engineering, Microscale Energy Systems Transport Phenomena Laboratory, University of Toronto, 5 King’s College Road, Toronto, Ontario, Canada, M5S 3G8, Canada

article info

abstract

Article history:

Over the past few years, the importance of water management to the successful operation

Received 22 January 2009

of polymer electrolyte membrane (PEM) fuel cells has stimulated an extensive research

Received in revised form

focus on liquid water transport and its effect on performance and durability. Empirical

21 February 2009

methods employed to investigate water transport in the fuel cell have the potential to

Accepted 23 February 2009

provide useful feedback for developing empirical correlations and validating numerical

Available online 3 April 2009

models for fuel cell research and development. In this paper, a literature review is provided for the experimental techniques that have been applied to visualize liquid water in oper-

Keywords:

ating hydrogen PEM fuel cells and flow fields. The main hypotheses that have been

PEM fuel cell

proposed to describe liquid water transport in the gas diffusion layer (GDL) and current

Liquid water transport

challenges will also be discussed.

Visualization

ª 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.

Flooding Flow field Gas diffusion layer Gas diffusion medium Porous transport layer Review

1.

Introduction

Effective water management is vital for the successful operation, performance and durability of the polymer electrolyte membrane (PEM) fuel cell; however achieving an optimal level of water requires a delicate balance in the hydrogen PEM fuel cell, which typically operates between 60  C and 80  C [1]. Water produced from the electrochemical reactions combined with water from humidified inlet gases contribute towards necessary membrane hydration. However, the accumulation of excess liquid water in the gas diffusion layer (GDL) and gas flow channels leads to oxidant starvation and performance

loss [2]. In addition to these mass transport limitations, excess liquid water can also lead to non-homogeneous current density [3], ineffective heat removal, and membrane swelling [4]. This excess water may also lead to the delamination of fuel cell components during the thermal cycling associated with freeze/thaw processes [5]. Without a sufficient level of water, the membrane dehydrates, resulting in performance degradation [6,7]. This dehydration may lead to temporary performance losses in addition to permanent material damage [7]. A common component of the hydrogen PEM fuel cell is the GDL, which is also known as the gas diffusion medium (GDM) or porous transport layer (PTL). Herein this material will be

E-mail address: [email protected] 0360-3199/$ – see front matter ª 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2009.02.084

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2.

Fig. 1 – SEM images of (a) carbon cloth (Avcarb 1071) and (b) carbon paper (Avcarb P50) materials at 503 magnification. Length bars indicate 500 mm.

In this paper, an overview will be provided for the following visualization techniques: nuclear magnetic resonance (NMR) imaging, beam interrogation, and direct optical photography. Liquid water transport in an operating PEM fuel cell is a challenging phenomenon to study using in situ visualization techniques, due to the opaque nature of traditional GDL and bipolar plate materials. NMR imaging [14–26] and beam interrogation techniques, such as neutron imaging [4,7,27–47], electron microscopy [48,49], and X-ray techniques [50–54], enable the in situ measurement of liquid water distributions in operating PEM fuel cells through materials that would otherwise be opaque to optical access. However, the direct optical visualization of liquid water in PEM fuel cells and PEM fuel cell materials [8,55–73] has the potential to provide high temporal and spatial resolution information about water transport in the gas flow channels and upper layers of the GDL. Using fluorescence microscopy, liquid water can be detected in the throughplane direction of the GDL, but limited in depth to several fiber diameters due to the opacity of the material [8]. Table 1 summarizes the reported state-of-the-art spatial and temporal resolutions of the methods discussed in this paper. An overview is provided for the prominent visualization techniques for measuring the liquid water distribution in operating PEM fuel cells and PEM fuel cell materials. It is also noteworthy that in addition to these visualization techniques, various other methods have been applied to measure PEM fuel cell water content, such as real time gas analysis [74,75], ionic resistance [76], infrared absorption [77], and residence time distribution [78,79].

2.1. referred to as the GDL. It is a microstructured fibrous threedimensional structure [8,9], with a thickness on the order of 100 mm and pore sizes on the order of 10 mm [9]. Fig. 1 shows two examples of GDL materials: (a) carbon cloth (Avcarb 1071) and (b) carbon paper (Avcarb P50). The GDL serves several purposes in the PEM fuel cell: it must provide mechanical support to the catalyst layers in the membrane electrode assembly (MEA) of the fuel cell. It must also provide pathways for heat, electronic, gaseous and liquid water transport. The GDL is typically treated with a hydrophobic coating of polytetrafluoroethylene (PTFE) to enhance liquid water removal from the fuel cell. Despite this hydrophobic treatment, liquid water tends to accumulate within the GDL and gas flow channels of operating fuel cells. Current continuum models of fuel cell transport phenomena commonly reduce the GDL to a one-dimensional interface. Due to the lack of realistic twophase flow data, these models often rely on empirical measurements of water transport in unconsolidated sand [10]. However, there have been recent developments in measuring water retention curves for GDL materials [11–13]. Still, significant improvements in the PEM fuel cell are contingent on advancements in water management, particularly in the GDL. Over the past decade, a variety of novel techniques have been reported for visualizing the liquid water accumulation in the hydrogen PEM fuel cell.

Liquid water visualization

Nuclear magnetic resonance (NMR)

In nuclear magnetic resonance (NMR) imaging, otherwise known as magnetic resonance imaging (MRI), the quantum mechanical magnetic properties of an atom’s nucleus are exploited to provide image contrast. In NMR imaging, specific atomic nuclei with non-zero spin moments, such as 1H, are excited by a static magnetic field and a radio-frequency signal. The excited nuclei absorb this radio-frequency energy and resonate at a detectable frequency that is proportional to the strength of the applied magnetic field [6]. By measuring the emitted radio-frequency signal from excited 1H nuclei in a fuel cell, the presence of water molecules can be detected. NMR imaging provides a useful tool for measuring the liquid water distribution of an operating fuel cell in situ, where liquid can be detected under the gas channel and land areas, in contrast to direct optical photography. Tsushima et al. [14] reported the use of NMR imaging to measure the spatial distribution of water in a 340 mm thick Nafion membrane of an operating PEM fuel cell. They correlated this water content with the output current density of the cell at steady-state operation. The authors performed NMR imaging of the PEM fuel cell through-plane and observed less water at the anode compared to the cathode side, which they attributed to the dominance of the electroosmotic drag of water over back diffusion.

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Table 1 – Summary of best reported resolution capabilities for liquid water visualization. Method

Spatial resolution

Temporal resolution

Merits

NMR

50 mm [14]

50 s [14]

Compatible with operating fuel cell; can detect water under land areas

Neutron imaging

25 mm [47]

5.4 s [41]

Synchrotron X-ray

3–7 mm [51]

4.8 s [51]

Microtomography

10 mm [50]

0.07 s [50]

Compatible with operating fuel cell and carbon materials Compatible with operating fuel cell and carbon materials; signal intensity can be correlated to through-plane water content Through-plane resolution available

Optical photography

10 mm [57]

0.06 s [62]

Fluorescence microscopy

5.38 mm [8]

0.3 s [8]

Compatible with operating fuel cell; high temporal and spatial resolution High spatial and temporal resolutions; signal intensity can be correlated to through-plane water content

In the same year, Feindel et al. reported the use of NMR imaging to examine the distribution of water in an operating PEM fuel cell [15]. In contrast to the work of Ref. [14], Feindel and co-workers imaged the in-plane direction of the PEM fuel cell. They observed a radial gradient of water distribution, and reported the diffusion of liquid water from the MEA into the surrounding Nafion membrane. Tsushima et al. employed NMR imaging to investigate the performance of a PEM fuel cell with a direct liquid water supply to the membrane [16]. They found that the use of a direct water supply to the membrane resulted in an increase in cell voltage, which they attributed to the decrease in membrane resistance. From NMR imaging, a deformation in the PEM was observed where the water supply came into contact with the membrane. They attributed this deformation to the localized swelling of the membrane. This group also measured membrane water content to investigate the effect of varying membrane materials [17,26] and membrane thickness [18]. In a separate work [19], these authors employed their NMR imaging results of the PEM through-plane to validate the development of a one-dimensional model for PEM water transport. Minard et al. [20] employed NMR imaging to the in-plane direction of a PEM fuel cell and observed the formation and slow propagation of a dehydration front from the gas inlet side to the gas outlet side of the cell over 11 h of continuous operation. Feindel and co-workers [21–23] also performed further investigations of in-plane water distributions in the PEM. They found that the water content and performance of the fuel cell were sensitive to the gas flow configuration of the cell. In particular, they reported that counter-flow configurations resulted in more uniform PEM water distributions [21]. Fig. 2 illustrates the apparatus employed by Feindel et al. [23] to investigate the in-plane water accumulation in a PEM fuel cell. These authors also demonstrated the employment of hydrogen–deuterium exchange as a method of providing contrast in NMR imaging to investigate changes

Challenges Incompatible with carbon materials; limited spatial and temporal resolutions Limited spatial and temporal resolutions; limited availability Limited temporal resolution; limited availability

Has yet to be demonstrated with an operating fuel cell; vulnerable to artifacts; limited sensitivity to water Transparent window requires substitution materials for operating fuel cell Has yet to be demonstrated with an operating fuel cell

in PEM water distributions during steady-state conditions [24]. NMR imaging has provided valuable information about the water content in the gas channels and membrane; however some drawbacks include the limited temporal resolution (50 s [14]), limited in-plane spatial resolution (400 mm  25 mm [14]), and invasiveness due to changes in fuel cell materials (nonmagnetic current collectors, such as acrylic resin [18]). The limiting size of the magnet-core for fuel cell housing is also a drawback [6]. For instance, typically small single cells accompany NMR based investigations, such as that utilized by Tsushima et al., where a 2.0 cm2 was employed [14]. Furthermore, NMR imaging cannot be used to resolve water content in the GDL due to the rapid attenuation of the signal in the carbon layer [23,50].

2.2.

Beam interrogation

2.2.1.

Neutron imaging

Neutron imaging relies on the measurement of an attenuated signal after a sample has been bombarded with a neutron beam. The attenuation of the signal is proportional to the material composition. This method is particularly useful for PEM fuel cells due to the neutron’s sensitivity to hydrogen containing compounds, such as water [27], and insensitivity to common fuel cell materials (for example, aluminum and graphite) [22]. Due to high costs and limited availability of neutron imaging, this research has been concentrated amongst a few groups worldwide [6], including the Laboratoire Le´on Brillouin at the Orphe´e Nuclear Reactor in Saclay, France [4], National Institute of Standards and Technology’s Center for Neutron Research (NIST CNR) [27,32,33– 45,47], the Penn State Breazeale Nuclear Reactor [7,30,31,34,36], the Paus Scherrer Institut (PSI) [28,29,35], and recently the Neutron Radiography Facility (NRF) at Hanaro, within the Korea Atomic Energy Research Institute (KAERI) [46]. Through this non-invasive method, these groups have

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Fig. 2 – 1H NMR microscopy apparatus for PEM fuel cell investigation and sample images reported by Feindel et al. [23]: (a) PEM fuel cell schematic, (b) 500 mm slice image containing the MEA, (c) 750 mm slice image containing a water filled flow field, and (d) photograph of the fuel cell cross-section, Ref. [23] – Reproduced by permission of the PCCP Owner Societies.

been able to quantify liquid water accumulation in operating PEM fuel cells. In 1996, Mosdale et al. [4] reported the first use of neutron imaging to measure the water profile across the membrane in a PEM fuel cell. The authors employed small-angle X-ray (SAXS) and neutron scattering (SANS) techniques to detect regions of the membrane that experienced swelling from excess water accumulation. In 1999, Bellows et al. [38] demonstrated the ability to measure the response of water content in the membrane with respect to changes in gas humidification levels. Xu et al. [80] also performed imaging along the fuel cell through-plane to investigate water distribution in the membrane. In 2004, Satija et al. [27] employed in-plane neutron imaging of an operating PEM fuel cell and produced a time series of images to evaluate the water management of a fuel cell system. Neutron imaging has since been used extensively to visualize water accumulation along the in-plane direction to observe a two-dimensional distribution of water [30,32,40,81,82]. Pekula et al. [30] reported the tendency for liquid water to accumulate at the 90 bends in the gas channels. The authors attributed this to the change in momentum of the gas flow, which resulted in localized pressure variations. Similar results were reported by Trabold et al. [32], as they also observed the accumulation of liquid water at the channel bends, which they attributed to the decreasing channel-to-channel pressure gradients. Hickner and co-workers [40] investigated the water content in the PEM fuel cell as a function of current density and temperature. They reported that water content decreased with increasing temperature, and they attributed this to the evaporation of liquid water at higher temperatures. Recently, Kim and Mench [47] employed neutron imaging to visualize the phase-change-induced flow of liquid water in a PEM fuel cell. They found that at low currents, the phase-changeinduced flow of liquid water (from hot to cold) in the porous media dominated over the thermo-osmostic flow of water (cold to hot) in the membrane [47]. Li et al. [82] employed neutron imaging in the through-plane to evaluate flow channel designs. They based their flow channel designs on the

determination of an appropriate pressure drop along the flow channel so that all water was removed or evaporated, and the gas stream was maintained at a saturated condition to prevent membrane dehydration [82]. Chen et al. [42] employed neutron imaging to measure the liquid water content at four places in an operating PEM fuel cell. They observed that the water content in the GDL decreased with increasing current densities and increasing stoichiometric values. Park et al. [45] employed neutron imaging to measure the water content in a PEM fuel cell with a single serpentine channel. They observed the tendency for liquid water to accumulate in the GDL directly below the cathode gas flow channel, whereas the GDL region directly below the land area remained relatively free of water. They attributed the relative decrease in GDL saturation below the land area to high air cross leakage flows. They also observed that little liquid water flooding was detected at a cell operating temperature of 80  C, compared to significant liquid water flooding at 60  C. Park et al. [45] varied the current density from 0.05 A/cm2 to 0.75 A/cm2 and observed a significant hysteresis in performance at the same operating conditions due to the varying accumulation of liquid water in the system. Several authors have also employed neutron imaging to specifically focus on measuring the liquid water distribution in the cathodic flow fields and GDL [29,31,33–35,41,42]. Kramer et al. [29] studied the effect of varying flow field geometries on water distribution, and found that a single serpentine channel resulted in less liquid water accumulation compared to a fiftychannel interdigitated design. These authors also employed neutron imaging to investigate the liquid water accumulation in a direct methanol fuel cell (DMFC) [28]. Zhang et al. [35] employed neutron imaging to investigate the effect of changing the cathodic GDL material on liquid water accumulation in a single cell. They reported that a cloth GDL tended to accumulate less water than paper GDLs. Kowal and co-workers [31] quantified the liquid water distribution in a PEM fuel cell under varying flow rates, humidities, and currents for both paper and cloth GDLs, and they found that paper GDLs held 174% more water per volume under the land area in comparison to cloth GDLs.

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In the second part of the work published by Trabold et al. [32], Owejan et al. [33] employed neutron imaging to a PEM fuel cell with an interdigitated flow field to investigate the accumulation of liquid water in the cathode with respect to inlet reactant gas humidity and inlet/outlet header differential pressure. Among several findings, they observed that the GDL accumulated liquid water up to a critical mass, filling approximately 44.2% of the available void space. Turhan et al. [34] visualized water in the channels and GDL, and observed the tendency for water to accumulate under the land areas in a PEM fuel cell with a parallel flow field design, after which it was difficult to remove water from these locations. They proposed that this difficulty in removing liquid water from under the land could be attributed to the high heat removal rate and restricted mass transfer at the landings. Ludlow et al. [39] extended the use of neutron imaging from quantifying liquid water accumulation in the gas flow channels to isolating the water content in the MEA and membrane alone by analyzing images before and after purging flooded gas channels. Siegel et al. [37] employed neutron imaging over four continuous days of PEM fuel cell testing as they explored the accumulation of liquid water in both the anode and cathode gas channels. Kim et al. [46] employed neutron imaging to investigate methods of discharging water in a flooded PEM fuel cell. They injected the PEM fuel cell cathode with distilled water, and they recommended a combination of compressed air and heating to remove water from the gas flow channels and MEA. Recently, Turhan et al. [36] employed neutron imaging to a 50 cm2 active area PEM fuel cell with seven different flowfield patterns to investigate the effect of varying the channel size, landing size, and land to channel ratio (L:C ). They generally found that larger L:C ratios led to increased residual water. They also observed that increasing the number of channels, and thus increasing the number of channel-GDL contacts, resulted in an increase in water accumulation. Fig. 3 from Ref. [36] is a neutron image of this increase in liquid water accumulation that comes with increasing the number of channel-GDL interfaces. Similar to Pekula et al. [30] and

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Trabold et al. [32], Turhan et al. [36] also observed liquid water accumulation at the channel bends. In the past, neutron imaging typically provided a spatial resolution of 100 mm  100 mm, and was insufficient to analyze the through-plane evolution of water transport. Difficulties in distinguishing the presence of water in the GDL from water in the flow channels have been reported [31,83]. Furthermore, typical integration times (1.0 s) for this technique are also insufficient to measure the microscale transport of droplet movement in a fuel cell, which has associated time scales of milliseconds [6]. However, the National Institute of Standards and Technology (NIST) has recently demonstrated the capability of imaging with a spatial resolution of 25 mm [44], and work conducted at the Penn State Breazeale Nuclear Reactor as well as the NIST CNR was reported with frame rates up to 30 frames/s [30,39].

2.2.2.

X-ray

In X-ray microtomography, a material is exposed to an X-ray beam, and the intensity of the signal is attenuated as it travels through the material. The transmitted signal is measured to provide a three-dimensional map of adsorption variation within a sample. Sinha et al. [50] demonstrated the use of this technique to measure the liquid water saturation distributions in a GDL during gaseous purges at high in-plane spatial resolution (10 mm  10 mm) with a through-plane resolution of 13.4 mm. Lee et al. [52] demonstrated the potential for X-ray imaging to be used for in situ PEM fuel cell water accumulation measurements. Synchrotron X-ray radiography has recently been demonstrated by Manke et al. [51] as a high spatial resolution measurement (3–7 mm) technique for water distribution in an operating fuel cell. With an image acquisition time of 4.8 s, Manke and co-workers employed this technique to observe the dynamic water transport behaviour in an operating PEM fuel cell with some through-plane resolution. They observed an eruptive transport mechanism [51], which they described as the quick and periodic ejection of droplets from the GDL into the gas channel. The same group also employed synchrotron radiation to investigate the liquid water accumulation in

Fig. 3 – Neutron images for a PEM fuel cell reported by Turhan et al. [36] with (a) 48 channel-GDL interfaces and (b) 12 channel-GDL interfaces at a current density of 0.2 A/cm2 [36] showing the increase in liquid water accumulation associated with a higher number of channel-GDL interfaces. Reprinted from Ref. [36] with permission from Elsevier.

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a PEM fuel cell in the cross-section [54]. They observed that at high current densities (io ¼ 420 mA/cm2), liquid water tended to accumulate in the cathode. At higher current densities (io > 500 mA/cm2), liquid water accumulated at the anode and cathode, near the channel ribs and near the microporous layer (MPL), as shown in Fig. 4. Mukaide et al. [53] employed synchrotron X-ray radiography to investigate the distribution of water in a PEM fuel cell during operation, with a spatial resolution of 12 mm and a temporal resolution of 1 frame/s. They captured images showing the water distribution in their PEM fuel cell in both the in-plane and through-plane directions. When they employed a GDL with a pinhole array running transversely through the material, they observed water accumulation in the pinhole regions. In the absence of a pinhole array, water appeared to distribute unevenly within the PEM fuel cell, and the authors attributed this to the uneven pore distribution of the GDL. X-ray radiography is a promising technology newly applied to investigate water transport in PEM fuel cells. High spatial and temporal resolutions have been achieved, but separately. The challenge remains to combine high temporal and spatial resolution to capture the rapid evolution of liquid droplets within the GDL.

2.2.3.

Electron microscopy

Nam and Kaviany [48] employed an environmental scanning electron microscope (ESEM) to visualize condensed water droplets on GDL fibers in the absence of bulk liquid water transport at low temperature and vapour pressure. Fig. 5 is a time series of ESEM images showing the evolution of smaller liquid water droplets to larger water droplets in a GDL. These authors employed consecutive images of water droplet growth on the GDL for the development of a one-dimensional water saturation model for the GDL. Gurau et al. [84] employed scanning electron microscopy (SEM) to visualize water droplet formation on a Toray GDL to illustrate that the internal contact angle is necessary to quantify the capillary forces

acting on the water inside the GDL pores; however due to the complex structure of the GDL material and droplet formation within the pores, the internal contact angle of the GDL cannot be measured directly with goniometry [84]. SEM imaging provides a high spatial resolution technique to investigate droplet formation at the microscale; however the ability to simulate fuel cell operating conditions has not yet been reported in the literature.

2.3.

Optical photography

Optical photography has been employed extensively to visualize liquid water transport in transparent PEM fuel cells [55– 60,62,64,65,67,68] and in flow fields [58,61,64,69,71,73]. By using this method, one’s spatial and temporal resolution capabilities are only limited by the employed microscope magnification and camera speed, respectively. However, optical photography is only possible when traditional fuel cell materials are replaced by transparent substitutes. Tu¨ber et al. [55] visualized liquid water transport in the cathode gas channel of a transparent PEM fuel cell. They operated their fuel cell at 30  C, and found that using a hydrophilic cathode GDL resulted in increased current density, which they attributed to a more uniformly hydrated membrane. Spernjak et al. [67] also investigated the effects of varying GDL materials and hydrophobicity. They found that PEM fuel cells with untreated GDLs were more prone to film and slug formation in the cathode gas channel. Weng et al. [65] observed the beneficial effects of high cathode gas flow rates for water removal; however unhumidified cathode gas streams at high stoichiometry resulted in membrane dehydration. Ge and Wang [60] visualized water droplet formation in the anode gas channels, and they observed that droplets tended to form on the gas channel walls when a hydrophobic GDL was employed, whereas hydrophilic GDLs tended to wick water from the channel into the GDL. Liu et al. [62] investigated the liquid water accumulation in the cathode gas channels of PEM fuel cells with three different flow

Fig. 4 – Cross-sectional synchrotron radiation images of a PEM fuel cell operating at various current densities reported by Hartnig et al. [54]. As the current density increases, water clusters begin to form first at the cathode on the outer edge of the MEA, followed by water clusters appearing at the anode. Arrows denote the preferential condensation regions. Reprinted with permission from Ref. [54]. Copyright [2008], American Institute of Physics.

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Fig. 5 – Time-sequenced ESEM images of smaller liquid water droplets agglomerating to form larger droplets in a GDL. Images courtesy of Massoud Kaviany.

field configurations: parallel, interdigitated, and cascaded. At low operating temperatures (25  C) and ambient pressure, they observed that the parallel flow field was the most unsuitable flow field design for water removal, resulting in the worst performance of the three fuel cells. In all cases, the authors observed liquid water accumulation at the downstream locations, and they attributed this to the diminished oxygen flow rates at downstream locations of the fuel cell. Yang et al. [57] visualized water droplet emergence from the GDL surface and behaviour in the gas channel. These authors observed a variety of phenomena in the gas channel, including the intermittent emergence of droplets from the GDL surface, film formation along the channels, and channel clogging. Hakenjos et al. [56] also employed a transparent fuel cell and attempted to correlate the appearance of flow field flooding with the spatial temperature distribution. Sugiura and co-workers [59] employed a transparent fuel cell to evaluate and successfully demonstrate the mitigation of flow field flooding with the use of a water absorption layer. Borrelli et al. [58] employed a flow field to image water droplet transport through a GDL into a flow field. Similarly to Tu¨ber et al. [55] and Yang et al. [57], these authors observed droplet emergence at preferential locations. However, an explanation of these phenomena was not provided. Theodorakakos et al. [64] also utilized a flow field to capture side-view droplet detachment images for input to their computational fluid dynamics (CFD) volume of fluid (VOF) simulations. Kumbur et al. [61] employed a simulated flow channel apparatus to study the effects of hydrophobicity, channel geometry, droplet chord length and height, and air

flow rate on droplet formation and instability. They also presented an analytical force balance model to predict the droplet characteristics at instability. Kimball et al. [71] measured the critical hydrostatic pressure head for liquid water breakthrough for various GDL materials, and they obtained photographs of the GDL at breakthrough. They also varied the orientation of a singlechannel fuel cell to observe liquid water motion and its effect on the local current density. Lu et al. [72] employed an ex situ apparatus to investigate the water transport (slug flow, annular/film flow, and mist flow) in a test section composed of eight flow fields. The authors investigated the effects of air flow rates on the two-phase flow, and presented photographs of liquid water accumulation in the flow fields. Recently, Gao et al. [73] visualized unstable water flows in GDL materials using confocal microscopy. Similar to Litster et al. [66] and Bazylak et al. [8,69], the authors injected dyed liquid water into one side of the GDL and visualized the dynamic water movement from a plane of view. Furthermore, Gao et al. visualized the GDL fibers and liquid water simultaneously, and captured three-dimensional imaging of static liquid water in the GDL. Similar to Bazylak et al. [8], Gao et al. also observed preferential flow locations of the GDL and the recession of water pathways upon breakthrough. The authors proposed that liquid water flows in the GDL can be described by column flow models for soils. Transparent fuel cells provide useful information on how flow field flooding corresponds to current density degradation; however, the spatial resolution necessary to analyze the transport of liquid water within the GDL has not yet been

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Fig. 6 – Time-sequenced digital images of fluorescent dye transport in the GDL with three-dimensional renderings reported by Litster et al. [66]. Reprinted from Ref. [66] with permission from Elsevier.

reported in the literature. Furthermore, for the exception of Borrelli et al. [58], Liu et al. [62] and Theodorakakos et al. [64], who achieved image acquisition times of milliseconds, transparent fuel cells have not been employed at sufficiently high temporal resolutions to investigate the dynamic behaviour of droplets.

2.3.1.

Fluorescence microscopy

Fluorescence microscopy in conjunction with optical photography provides a method to visualize the microscale transport of liquid water in the surface of the GDL. Litster et al. [66] directly visualized the through-plane liquid water transport in the GDL using a fluorescent tracer. A dye solution was injected through the GDL of a PEM fuel cell, and fluorescence microscopy was employed to visualize the transport of liquid water through this fibrous structure to the extent that the opaque fiber structure would allow. This novel technique allowed for the tracking of time-evolving gas/liquid interfaces, as shown in Fig. 6, and provided unique insight into the dynamics of liquid water flow through distinct pathways. From the same group, Bazylak and co-workers investigated the liquid water transport behaviour in Toray GDL materials under the influence of compression [8] and the dynamic water

droplet behaviour emerging from the GDL into a flow field [69]. Bazylak et al. [8] observed that compressed regions of Toray GDL materials provided preferential pathways for liquid water transport leading to breakthrough in an ex situ test apparatus. The authors studied the irreversible damage to the GDL caused by compression, and concluded that fiber and PTFE breakage led to localized regions of hydrophilic surfaces. In Ref. [69], Bazylak et al. found that with an initially dry GDL and flow field apparatus, the emergence and detachment of individual droplets were followed by slug formation and flow field flooding. Droplets commonly pinned to the surface of the GDL, accelerating the formation of slugs. The authors also reported an interesting finding whereby breakthrough locations changed with time, suggesting a dynamic and interconnected network of water pathways within the GDL. Fig. 7 from Ref. [69] is a time series of fluorescence microscopy images capturing the change in breakthrough location accompanied by the recession of an old breakthrough location. The interaction of a water droplet on the GDL surface with the channel wall was investigated with an ex situ test apparatus by Bazylak et al. [70]. The stability of the droplet as a function of plate wetting properties was investigated by quasistatically translating a solid surface towards a water

Fig. 7 – Time series of fluorescence images reported by Bazylak et al. [69] showing the recession of a droplet formed above a previous breakthrough location. A new breakthrough location has emerged in the lower right hand corner of the field of view. Length bars represent 0.5 mm.

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droplet and capturing the liquid water behaviour using simultaneous side view and top view imaging. As shown in Fig. 8, a hydrophilic top plate encouraged liquid water trapping under the land area. This work provided insight into the dependence of land surface wettability on droplet behaviour. Fluorescence microscopy is a useful tool for investigating droplet behaviour near and on the surface of the GDL, and the spatial and temporal resolutions are only limited by the microscope optics and digital camera capabilities. However, due to the opaque nature of the GDL, this method has not been employed to elucidate the through-plane transport of the GDL to a great extent. The applicability of this method for in situ investigations has also yet to be shown.

2.4.

Hypothesis

Although efforts have been made to model water transport in the PEM fuel cell [85–88], until recently, little attention has been paid to develop models for microscale liquid water

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transport in the porous GDL. Due to the lack of two-phase flow correlations required for closure in macroscopic models, multiphase PEM fuel cell continuum models often rely on empirical measurements of water transport in unconsolidated sand [10] for transport in the GDL. Nam and Kaviany [48] proposed a branching-type geometry, where water vapour condenses on GDL fiber surfaces to produce micro-droplets, as shown in Fig. 9. Micro-droplets agglomerate to form macro-droplets, followed by water flowing preferentially towards larger pores. The authors hypothesized that water is distributed in a branching geometry where large streams act as the backbone for macrotransport, and smaller streams transport water from micro-droplets to macro-droplets. Pasaogullari and Wang [89] also hypothesized the formation of a tree-like liquid water percolation in the GDL after condensation begins. Litster et al. [66] proposed that water transport at the surface was found to be dominated more by fingering and channeling (Fig. 10), accompanied by highly dynamic eruptive

Fig. 8 – Images showing the time evolution of a 1.25 mL droplet with a hydrophilic glass slide moving from right to left on a Toray TGP-H-060 10 wt.% PTFE GDL reported by Ref. [70]. Side view images are shown along the left column, and top view fluorescence images are shown along the right column. The vertical dashed line indicates the location of the glass wall. For top view images, the contact line of the entrapped liquid has been outlined with a dashed line. Images are separated by 20.5 s intervals.

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Fig. 9 – Converging capillary tree water transport mechanism proposed by Nam and Kaviany [48] whereby micro-droplets agglomerate with other liquid water bodies to create flowing macro-droplets. Reprinted from Ref. [48] with permission from Elsevier.

water transport processes. From the extended work of this group, Bazylak et al. proposed that the behaviour of liquid water in the GDL can be further characterized with preferential pathways [8] that also evolve over time [69]. These insights suggest some difference compared to the converging capillary

tree mechanism suggested in the prior work of Nam and Kaviany [48] and Pasaogullari and Wang [89]. In the hypothesis proposed by Litster and co-workers, numerous ‘‘dead ends’’ occur where water transport recedes when adjacent breakthrough channels form. Recent in situ visualizations using synchrotron X-ray radiography [51] confirm the eruptive transport process observed by Litster et al. [66] and Bazylak et al. [8,69] and show that this occurs during operation of some localized areas of the cell, while in others areas, GDL pores fill continuously following the capillary tree-like process [48,89]. It is expected that there are indeed several transport mechanisms involved in the accumulation and transport of liquid water within the complex GDL material, including those already proposed [8,48,66,69,89].

3.

Fig. 10 – Channeling liquid water transport mechanism proposed by Litster et al. [66]. Reprinted from Ref. [66] with permission from Elsevier.

Conclusions

The purpose of this review was to provide an overview of the recent developments in visualizing liquid water within PEM fuel cells. Over the past decade, several novel methods have been reported including NMR imaging, beam interrogation, and direct optical photography. Through the novel application of NMR (or MRI), neutron imaging, X-ray tomography and synchrotron radiography, valuable information regarding liquid water accumulation and transport behaviour has been discovered in operating PEM fuel cells. NMR imaging provides high contrast images, while neutron imaging has the benefit of being insensitive to common fuel cell materials. X-ray tomography and synchrotron radiography are also unique methods that have provided new insight into water transport behaviour in the GDL and an operating PEM fuel cell, respectively. Although valuable, NMR and beam interrogation techniques are somewhat limited in availability and currently limited in temporal resolution. Optical photography is a readily available and flexible visualization technique that can be applied to both flow fields, GDLs, and operating PEM fuel cells. Furthermore, the spatial and temporal resolutions associated with this method are only limited by the available

international journal of hydrogen energy 34 (2009) 3845–3857

microscope magnification and digital camera speed. However, a major limiting factor to consider when dealing with an operating fuel cell is that transparent materials must be substituted for traditional materials. Alternate materials may result in changes to thermal conductivities and surface properties, resulting in varying heat transfer and liquid water transport behaviour. Additionally, optical access into the GDL is currently restricted due to the opaque nature of commercially available materials. Each method discussed in this paper has its associated strengths and weaknesses, but combined these methods have provided the means to gather valuable insight into the behaviour of liquid water accumulation and transport within the PEM fuel cell. Obtaining high temporal and spatial resolution in both the in-plane and especially the through-plane directions of the GDL and MEA continue to be major challenges to visualizing and understanding the dynamic microscale transport of liquid water in the PEM fuel cell and its effect on performance and durability. Furthermore, these techniques have only been applied to single cells, while directly visualizing liquid water in stacks still remains a challenge. It is expected that the methods discussed in this paper will continue to play an important role in the future; however innovative techniques will be required for more in-depth investigations of liquid water management in PEM fuel cells for model development and for the exploration of new materials, designs, and operating conditions for improved PEM fuel cell performance.

Acknowledgments Financial support from the following is gratefully acknowledged: the Natural Sciences and Engineering Research Council (NSERC) of Canada, the Bullitt Foundation, and the University of Toronto. The author would also like to thank Mr. Zachary Fishman for providing the SEM images shown in Fig 1.

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