Effects of flow field and diffusion layer properties on water accumulation in a PEM fuel cell

Transcription

1 International Journal of Hydrogen Energy 32 (2007) Effects of flow field and diffusion layer properties on water accumulation in a PEM fuel cell J.P. Owejan a,, T.A. Trabold a, D.L. Jacobson b, M. Arif b, S.G. Kandlikar c a General Motors Fuel Cell Activities, 10 Carriage Street, Honeoye Falls, NY , USA b National Institute of Standards and Technology (NIST), 100 Bureau Drive, Gaithersburg, MD , USA c Department of Mechanical Engineering, Rochester Institute of Technology, 76 Lomb Memorial Drive, Rochester, NY 14623, USA Received 4 April 2007; received in revised form 31 May 2007; accepted 31 May 2007 Available online 7 September 2007 Abstract Water is the main product of the electrochemical reaction in a proton exchange membrane (PEM) fuel cell. Where the water is produced over the active area of the cell and how it accumulates within the flow fields and gas diffusion layers, strongly affects the performance of the device and influences operational considerations such as freeze and durability. In this work, the neutron radiography method was used to obtain two-dimensional distributions of liquid water in operating 50 cm 2 fuel cells. Variations were made of flow field channel and diffusion media properties to assess the effects on the overall volume and spatial distribution of accumulated water. Flow field channels with hydrophobic coating retain more water, but the distribution of a greater number of smaller slugs in the channel area improves fuel cell performance at high current density. Channels with triangular geometry retain less water than rectangular channels of the same cross-sectional area, and the water is mostly trapped in the two corners adjacent to the diffusion media. It was also found that cells constructed using diffusion media with lower in-plane gas permeability tended to retain less water. In some cases, large differences in fuel cell performance were observed with very small changes in accumulated water volume, suggesting that flooding within the electrode layer or at the electrode-diffusion media interface is the primary cause of the significant mass transport voltage loss International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. Keywords: PEMFC; Flow channels; GDL; Neutron radiography; Water management; Diffusion layer 1. Introduction Hydrogen fuel cells are being developed as highly efficient and cost effective energy conversion devices that potentially have less environmental impact than internal combustion engines. The proton exchange membrane fuel cell (PEMFC) is the subject of the majority of fuel cell research, as it can be operated at low temperatures, and thus can be constructed of relatively low cost materials. This will enable the PEMFC to compete in automobile and stationary power generation markets which generally have very stringent cost targets. As PEMFC technology is further refined, it is recognized that several major hurdles must be overcome before current research-scale units are robust enough for commercialization. Corresponding author. Tel.: ; fax: address: jon.owejan@gm.com (J.P. Owejan). The focus in the present work is management of the water that is produced in the cathodic oxygen reduction reaction. Because a PEMFC operates at temperatures below 100 C, liquid water can form throughout the system due to condensation in the porous gas diffusion layers (GDLs) and gas delivery channels. Under steady-state conditions, liquid water accumulation can be minimized by controlling parameters such as inlet relative humidity, temperature, and pressure. However, these parameters must be optimized to ensure that a sufficient amount of water is present to maintain membrane and ionomer hydration required for adequate proton conductivity [1]. For automotive applications in particular, the fuel cell stack will rarely be at a steady-state condition, and the power delivery throughout the drive cycle will be quite dynamic. These constant changes in fuel cell power output can cause brief temperature variations, thus influencing the amount of liquid water in the system. For this reason, it is believed that mass transport losses due to /$ - see front matter 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi: /j.ijhydene

2 4490 J.P. Owejan et al. / International Journal of Hydrogen Energy 32 (2007) liquid water accumulation in the various fuel cell components is an inevitable problem, regardless of the operating conditions selected for steady-state load operation. The liquid water handling characteristics of the membrane, electrodes, GDL, and flow field reactant delivery channels must be well understood, so that each of these components can be optimized individually and as an integrated fuel cell system. In this study, liquid water accumulation in the carbon fiber GDL and reactant distribution channels was investigated with neutron radiography imaging of 50 cm 2 active area fuel cells. Neutron radiography is ideal for these experiments, because it is a noninvasive diagnostic that allows the test cell to be examined without changing the thermal, electrical or mechanical characteristics of the typical design. Because neutrons interact with the nucleus of an atom, and not the electrons as X-rays do, many common materials such as aluminum are relatively transparent to neutrons while hydrogenous materials like water are highly attenuating. Therefore, neutron radiography is well suited for imaging water within the metallic or carbon-based structure of a PEMFC [2 9]. The neutron images obtained in this study have been used to qualitatively examine the regions of liquid water accumulation, and to quantify the volume of water present based on a system calibration relating attenuated neutron beam intensity to water thickness. The focus of these experiments was to quantify the impact of GDL and flow field channel properties on water accumulation, and the attendant effect on cell performance. These two components are known to have a strong effect on mass transport losses in PEMFCs (e.g., [10,11]), but little experimental evidence exists which demonstrates the localized impact of GDL and flow field channel properties. Attempts have been made to acquire such data using more standard imaging methods, but it is necessary to alter the thermal properties of the flow field for optical access of such visualization systems [12 14]. 2. Experimental The hardware for the fuel cell test section was specifically designed to optimize the quality of neutron images and to facilitate post-process data analysis. A commercial test stand, as described elsewhere [15 17], was used to control operation of the fuel cell while acquiring neutron image data with an independent data acquisition system. The test cells were constructed in a consistent manner with particular consideration of compression, material integrity, and alignment. The test conditions were chosen such that liquid water was known to be present, although they were representative of conditions that can exist during an automotive drive cycle Fuel cell hardware and flow field design The test hardware design was critical for obtaining the highest resolution neutron images in the active area of the fuel cell. The compression end plates were slightly modified from the most common configuration used for single cell 50 cm 2 testing to be more compatible with neutron imaging. The heater rods were moved to the outer edges of the cell, beyond the Fig. 1. Serpentine flow field pattern. Fig. 2. Cross-sectional views of flow channel geometries. electrochemically active area, and the temperature control thermocouple was also moved outside the active area. The bolt pattern and dimensions were kept the same as the standard hardware to avoid any variability in the compression distribution. The cross-sectional thickness of hardware and material also remained constant. The gas inlet and outlet port locations were repositioned from the standard hardware to accommodate space constraints within the radiation enclosure and the test stand position relative to the neutron source. Both the anode and cathode flow field plates used in this study had serpentine designs as illustrated in Fig. 1. The five channel, five pass pattern had long enough flow paths to elucidate water management phenomena similar to those present in fullscale fuel cell bipolar plate hardware. With a large hydraulic diameter and a minimal number of turns, this flow field pattern yielded a low pressure drop between the inlet and the outlet. Two different cross-sectional geometries were tested using the serpentine channel pattern: rectangular with 1.37 mm width and 0.38 mm depth; and isosceles triangular with 1.37 mm width and 0.76 mm depth (Fig. 2). The cross-sectional area was kept

3 J.P. Owejan et al. / International Journal of Hydrogen Energy 32 (2007) energy (i.e., increased hydrophobicity) of the gold plated aluminum surfaces, the plates were coated with an ionically bonded polytetrafluoroethylene (PTFE), provided by TUA Systems (Merritt Island, Florida, USA). 1 The coating was found to be very uniform with an average thickness of less than 2 μm, and was applied to two cathode flow fields consisting of each of the two cross-sectional geometries described above (rectangular and triangular) GDL selection Fig. 3. Assembled channel orientation (black = anode; red = cathode). constant between these channel designs to maintain a consistent mean velocity in both channel geometries. The hydraulic diameters of the rectangular and triangular cross-sections were 0.68 and 0.71 mm, respectively, so a small difference in frictional pressure differential existed between the two flow field designs. Previous work by our research group using neutron radiography to study PEM fuel cells [15,16] established a need to unambiguously distinguish the anode flow field from the cathode flow field. It was thus determined that arranging the flow fields orthogonally would make it possible to discriminate water in the anode from water in the cathode when viewing two-dimensional neutron radiographs. This approach was first demonstrated in the thesis by Owejan [4]. Fig. 3 is a schematic of the flow channel orientation, with the observer looking through the anode toward the cathode. Reference channels outside of the active area are also incorporated into the design. These reference channels were used to verify the calibration method applied to quantify liquid water volume within the active area of the running cell. Because the reference channels were not covered by GDL, the thickness of a retained water slug was precisely known and could be used in water quantification for verification of measurement precision. The pattern in Fig. 3 is in the same orientation as all neutron images that were taken of the running fuel cell. Hence, a water slug in a vertical anode channel (black) will be easily distinguished from a water slug in a horizontal cathode channel (red). An important parameter in the present study was the surface energy of the flow field plates. To achieve a decreased surface Three commercially available GDLs were investigated in this study, as summarized in Table 1. The first material tested was T060 from Toray Industries (Tokyo, Japan), which was treated with PTFE, but did not have a microporous layer (MPL). Two additional materials studied were 20BC and 21BC manufactured by SGL Carbon (Wiesbaden, Germany), each with a PTFE treatment and MPL applied to the substrate. The in-plane gas permeability values were obtained by forcing a controlled air flow through a hole in the center of a disk-shaped sample of GDL that was sealed between two plates, while measuring the upstream and exit pressures. Porosity was calculated based on fiber size, binder volume fraction, and manufacturing process, as disclosed by the manufacturers. It is shown in Table 1 that the Toray and SGL materials have large differences in throughplane thermal resistance, but that the two SGL materials differ significantly only in their values of in-plane gas permeability. Although there are a variety of other material properties that may affect fuel cell performance, it was presumed at the outset that these two properties would have the most appreciable influence on the accumulated water volume, as they control the amount of convective flow through the GDL, and the effective temperature gradient between the flow field plates and MEA. For each fuel cell build, the same GDL material was used on both the anode and cathode sides. The membrane electrode assemblies (MEAs) used in all tests were supplied by W.L. Gore & Associates (Newark, Delaware, USA), and fabricated from 25 μm thick Nafion membranes with 0.4/0.4mg/cm 2 loading of carbon supported platinum in ionomer, hot-pressed on both the anode and cathode sides Neutron imaging system Experiments were conducted at the neutron source operated by the Center for Neutron Research at the National Institute of Standards and Technology (NIST), in Gaithersburg, Maryland, USA. Thermal neutron beam line BT-6 was utilized with an aperture of 1 cm and a resulting L/d ratio of 400. A complete description of the BT-6 neutron imaging facility is provided by Hussey et al. [17]. 1 Certain trade names and company products are mentioned in the text or identified in illustrations in order to adequately specify the experimental procedure and equipment used. In no case does such identification imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the products are necessarily the best available for the purpose.

4 4492 J.P. Owejan et al. / International Journal of Hydrogen Energy 32 (2007) Table 1 Physical properties of gas diffusion layers Thickness (μm) MPL Porosity (%) Substrate PTFE In-plane gas Through-plane thermal (mass%) permeability a,b resistance c (m 2 K/W) (μm 2 ) (darcy) 190 No E Yes E Yes E 04 a In-plane gas permeability measurements made with GDL samples under 1.38 MPa compression. b SGL measurements of through-plane gas permeability for 21BC and 20BC are 2.35 and 0.65 cm 3 /(cm 2 s), respectively, using a Gurley model cm 3.0.1m 2 orifice ( SIGRACET GDL 20 & 21 series gas diffusion layer). c Thermal resistance measurements made with GDL samples under 1.64 MPa compression. The neutron imaging device used here was a neutron scintillator/ccd (charge coupled device) system. The neutron scintillator creates a two dimensional light image of the distribution in the beam. This light image is then captured by a CCD camera. To acquire an image with this device the CCD camera opens a built in shutter and integrates light for a period of time called the image exposure time. The shutter is then closed while the image is transferred to a computer. Next, the CCD image on the device is deleted allowing for the shutter to be opened for the next integration period. Although the image exposure time is selectable the transfer time is fixed and in this case it was about 4 5 s. This transfer time is a dead period in which no data are being acquired. The image exposure time was set to 1 s after preliminary experiments were conducted to optimize the neutron image contrast. Ideally, the exposure time must be minimized to visualize transient behavior within the cell. Conversely, a short exposure time does not provide enough light to expose each image with the desired contrast. Each pixel value was saved to a fits (formatted image transport system) file in 16-bit double precision format. The CCD imaging chip was a array of pixels, and with binning set at 2, the images were saved as pixel arrays. In the present study, a series of 300 images was taken once the fuel cell operating point was considered to be at steady state condition. These images were later averaged to increase the signal-to-noise ratio. Images were also analyzed individually to verify that the liquid water profile was constant throughout the averaging period. Liquid water content was quantified using a calibration based on the Lambert Beer exponential attenuation law, as described in [15]. The macroscopic neutron cross-section was determined experimentally to be (2.958 ± 0.010) cm 1.

5 J.P. Owejan et al. / International Journal of Hydrogen Energy 32 (2007) Results and discussion All experiments were conducted using a consistent set of test parameters (Table 2),which were chosen specifically to ensure that liquid water would be present during fuel cell operation. Four cathode flow field configurations were investigated: rectangular and triangular geometry (Fig. 2), each with and without the ionic PTFE coating which increased the surface static contact angle. The anode flow field was constant through all tests (rectangular channels, with no PTFE coating), and water accumulation in the vertically traversing anode channels should not be confused with the water accumulation of interest in the horizontally traversing cathode channels. It was consistently observed that the anode channels contained large, stagnant water slugs which were present throughout the entire measurement sequence. This level of anode water is attributed primarily to condensation which occurs as the hydrogen fuel is consumed in the fully humidified gas stream, and also to liquid water that is periodically introduced at the inlet due to condensation upstream of the fuel cell. In either case, once water enters the anode channels, it cannot be continuously purged by the low density hydrogen gas flowing at relatively low velocity. However, it was observed that fuel cell performance was minimally influenced by the presence of water in the anode channels. Table 2 Fuel cell parameters for neutron radiography experiments Parameter Value Active area 50 cm 2 Membrance thickness 25 μm Catalyst loading 0.4/0.4mgPt/cm 2 Anode fuel Hydrogen Cathode oxidant Air Cell temperature 80 C Back pressure 200 kpa Inlet humidification 100%/100% Stoichiometric ratio 2/2 Polarization curve 0,0.1,0.5,1.0,1.2, 1.5A/cm 2 Start-up 1 h at 0.6V 3.1. Effects of GDL properties on water accumulation In this section, results are presented to contrast the water accumulation behavior of fuel cells run with three GDLs having varying physical properties, as summarized in Table 1. Throughout the following discussion, neutron radiographs are presented as either gray scale images, or as RGB (red, green, blue) images in which the top of the color scale (red) corresponds to a water thickness of 0.30 mm, which is close to the rectangular channel depth of 0.38 mm. Portions of the active area quantified by colors in the orange to red range thus represent water thicknesses that can only exist inside a flow field channel. A deep green to yellow color is representative of liquid water contained primarily in the GDLs and MEA. At the outset, it is important to note that under the fully humidified conditions investigated in this study, the Toray diffusion media experienced significant mass transport loss and could not be run beyond 1.0A/cm 2.InFig. 4, itisshown that the cell voltage with Toray 060 was below 0.4V at this current density, while both SGL materials performed well out to 1.5A/cm 2 with relatively little mass transport voltage loss. This is due to the use of a MPL that is known to improve water transport from the MEA into the diffusion medium [18]. In Fig. 5, a direct comparison is made among the water distributions for Toray 060, SGL 20BC and SGL 21BC at 0.1 and 1.0A/cm 2. Several trends are apparent in this figure. First, the large stagnant water slugs present in the anode channels of all cells at 0.1A/cm 2 are largely eliminated with a 10-fold increase in gas flows at 1.0A/cm 2. Secondly, it appears that the quantity of water within the softgoods (i.e., GDL and MEA) increases with increasing current density, as indicated by visually Fig. 4. Performance comparison among GDL materials. comparing the relative gray scales of non-channel portions of each image pair. It is also important to note that the GDL/MEA immediately adjacent to the reactant inlets (upper right hand corner of each image) are relatively dry in comparison to the rest of the active area. The land areas between the first pass of the anode and cathode channels show appreciable gradients in water content, as if water is removed from the GDL where the gas velocity is greatest (i.e., in the first pass of the serpentine flow field). The overall dry trend near the anode inlet, which is most pronounced at 1.0A/cm 2, is attributed to lack of membrane hydration in this location. Because the conditions in this area are less accommodating to facilitate proton conduction, the corresponding cathode reaction on the opposite side of the MEA is suppressed. Without an efficient cathode reaction, the local current density is accordingly lower, and hence less liquid water is produced in this region over an averaged period of operation. The observed gradient in water content over the land regions of the flow field is a function of gas permeability and in-plane pressure drop; this observation will be discussed further later in this section.

6 4494 J.P. Owejan et al. / International Journal of Hydrogen Energy 32 (2007) Fig. 5. Effect of GDL type on water distributions at 0.1 and 1.0A/cm 2. In Fig. 6, neutron radiographs are presented for SGL 20BC operated at all five current densities. For each condition, the water distribution is first shown as a time-averaged gray scale image, and then as a colorized RGB scale image with the computed water mass, based on the macroscopic neutron crosssection determined via calibration as described above. This computation considers only water present within the square 50 cm 2 active area, and does not include the reference channels in the upper right-hand corner, nor the outlet ports in the lower left-hand corner. From this series of images, one can extract a clear qualitative indication that with increasing current density, the amount of channel-level water decreases while the water content within the softgoods increases. It is also notable, from the data presented in both Figs. 5 and 6, that the effect of the MPL on the SGL gas diffusion media is to produce a more even water distribution than the Toray material which does not have an MPL. By summing the individual pixel water thickness values and multiplying by the active area, the total average water mass was determined and plotted for all three GDL materials (Fig. 7). The cell constructed with Toray material shows a trend of a slightly increasing then decreasing water mass with a change in current density from 0.1 to 1.0A/cm 2. Conversely, the SGL materials display the opposite trend: decreasing water mass from 0.1 to 0.5A/cm 2, with a generally increasing water mass from 0.5 to 1.5A/cm 2. This figure does not show a trend of monotonically increasing water mass with current density because the values plotted also include the water slugs in the channels for 0.5A/cm 2. To develop a better understanding of how accumulated water mass varies with fuel cell operating conditions, an image processing procedure was applied to enable separate analysis of water in the channels from that in the softgoods. An interpolation routine was derived to exclude the water in the channels for each thickness value in the original thickness matrix. Several methods were considered, but the most sophisticated would have to preserve the thickness gradient in the MEA and GDM over the channel area. The image was examined manually at all areas consisting of only GDM and MEA (i.e., aligned with flow field lands), and a maximum thickness was determined from the grid of intersecting land areas (see Fig. 8). Then an algorithm was developed to replace areas with water slugs with a thickness value equal to the maximum thickness value in the GDM and MEA in the grid. The assumption made with this interpolation was that the portion of the softgoods adjacent to a channel water slug would contain the same amount of liquid as that in the neighboring land area. After applying this algorithm, line plots were generated from horizontal slices of the thickness matrix (1 1024). By observing the magnitude of thickness values before and after the interpolation it could be determined if the interpolation was sacrificing the integrity of the thickness values across the flow field lands. These areas were to remain unchanged through the interpolation, and if clipping of the thickness signal was observed in these land areas, the maximum thickness in only GDM and MEA was re-evaluated. Fig. 9 illustrates the effect of the interpolation routine for a particular test point plot; note that the color values were rescaled to a broader range of the colorbar spectrum in the interpolated plot. Horizontal line plots of the top, middle, and bottom of the active area were also generated for each interpolated data array. The line plots were used to ensure that the interpolation only clipped the data array in areas consisting of a channel with a water slug inside. Interpolating such that the average water mass values did not include the water slugs in the channels enabled conversion of the data in Fig. 7 into the plots shown in Figs. 10 and 11. By excluding the volume of water slugs in the channels, a trend of increasing water accumulation with load was observed for all three GDL materials. Furthermore, by subtracting the water mass values in Fig. 10 from the total water mass in Fig. 7, a trend of decreasing water in the channels with increased load can be observed. The water mass profiles for the three GDL types can be compared to the performance comparison illustrated in Fig. 4. An obvious correlation between accumulated water mass and cell voltage is observed, where the Toray GDL demonstrated largest mass transport loss. The performance of the two SGL GDM samples were similar as the average amount of liquid water accumulated at each test point was comparable and consistently lower than for Toray. The MPL on the SGL gas diffusion media samples likely plays a key role in optimizing the fuel cell water management. This is accomplished by facilitating transport of product water away from the MEA and distributing the water produced in the electrode layer more evenly over the active area.

7 J.P. Owejan et al. / International Journal of Hydrogen Energy 32 (2007) Fig. 6. Water distributions and computed masses for SGL 20BC.

8 4496 J.P. Owejan et al. / International Journal of Hydrogen Energy 32 (2007) Fig. 7. Total water mass variation with current density. Aside from the well-documented benefit of a MPL on water management (e.g., [18]), it is believed that two properties of the GDL substrate play a key role in the water accumulation within fuel cells operating at the same nominal conditions: in-plane gas permeability which influences the amount of convective flow through the GDL, and through-plane thermal conductivity which affects the temperature gradient from the MEA to the flow field plate, and therefore the local relative humidity of the reactant gases. The in-plane gas permeabilities of the three GDLs (Table 1) had a strong effect that was observed in each radiograph in the form of accumulated water gradients across the land areas of the flow field. The Toray GDL had the highest permeability, and hence the least resistance to gas transport over the lands. SGL 21BC and 20BC had lower permeability, with that of the 20BC being the lowest. In Fig. 12 a plot is presented of the Fig. 8. Discrimination of water in softgoods and channels. Fig. 9. Example result of water separation procedure.

9 J.P. Owejan et al. / International Journal of Hydrogen Energy 32 (2007) Fig. 10. Water mass in softgoods only, resulting from water separation procedure. Fig. 11. Water mass in channels only, resulting from water separation procedure. water thickness gradient over a cathode land near the reactant inlets (upper right-hand corner of the radiographs) at a current density of 1.0A/cm 2. This specific location and test condition demonstrated the most pronounced effect, but the general trend was observed throughout the analysis. Fig. 12 clearly shows that a higher in-plane permeability in the GDL yields more effective gas transport over the land area. This effect may be associated with anode channel water slugs observed at lower current densities in the Toray test cell, where at the same condition, SGL test cells did not retain water in the anode channels. The significant increase in the in-plane pressure drop for the SGL GDLs is a result of the properties of the paper composition and manufacturing processes. Because of the much higher flow resistance in these materials, more of the gas is forced along the channels themselves, thereby enhancing the convective removal of liquid water. The use of low in-plane permeability GDL to augment water transport in the bipolar plate has been recommended in a published patent application [19]. In Table 1, it is evident that a difference exists in throughplane thermal resistance between the Toray and SGL materials. This parameter effectively controls the increase in temperature of the MEA above that of the flow field plate, which in these experiments was where the control thermocouple was located. Based on the known waste heat flux and geometry of the flow field plates, a two-dimensional conduction model was used to estimate the difference in temperature between the membrane and flow field lands. Similar calculations over the channels are complicated because the actual compression force, contact resistances, etc. are not well characterized. As shown in Fig. 13, there is very little difference in the values of T membrane T plate for the three GDL materials. Although the waste heat flux for the Toray material is generally higher that for SGL (due to the lower cell voltage performance at the same current density, Fig. 4), this is counteracted by a much lower thermal conductivity. From this simple analysis, it would be concluded that the differences in land-on-land water accumulation observed for the three different GDL materials is primarily a result of variations in the in-plane gas flow, and not thermal effects Effects of flow field channel properties on water accumulation Based on the measurements described above using different GDLs, it was decided that further experiments were warranted to understand the effects of channel properties on water accumulation in the cathode flow field. For this phase of the work, Toray 060 GDL was used exclusively, as this material was observed to consistently retain more water than either of the SGL materials with MPL. The baseline gold-plated aluminum flow field surface (hereafter referred to as uncoated ) had an average static contact angle of 40, as measured using a Krüss Model DSA 10 Drop Shape Analysis System. This surface is contrasted by PTFE coated gold ( coated ) with an average static contact angle of 95. A comparison of water accumulation in each flow field cross-sectional geometry (rectangular and triangular) was made for the test points summarized in Table 2. In all cases, the anode flow field retained the baseline characteristics of rectangular geometry with no PTFE coating. Throughout this part of the experimental program, a consistent trend was observed, attributed to the effect of water slug geometry upon increasing the static contact angle of the channel surface. The PTFE coated cathode flow channels generally formed smaller, more distributed water slugs throughout the channel compared to the uncoated flow fields. It is clear that the water slugs in the uncoated cathode channels block a large fraction of the channel cross-section in the two-dimensional

10 4498 J.P. Owejan et al. / International Journal of Hydrogen Energy 32 (2007) Fig. 12. Water thickness profiles across cathode lands. Fig. 13. Computed temperature differences between membrane and flow field plate. area captured by the radiograph (Fig. 14). The average water mass plot in Fig. 15 demonstrates that the PTFE flow field configuration retains more liquid water at a given current density. Over an averaged period of time this is concurrent with the behavior of a water slug, as the larger channel blocking slugs will be periodically purged out of the channel by the pressure drop they induce. In contrast, the smaller, more distributed slugs will remain in the flow field the entire period of time, because by not obstructing a large fraction of the channel, the pressure gradient required to remove these small water slugs will not be generated. In general, the triangular cross-section channels retained less water than the rectangular cross-section at the same current density (see Fig. 15). This is consistent with the distinct differences in slug shape observed in the two channel geometries. Generally smaller water slugs were retained in the triangular channels at the corners adjacent to the GDL; this was a result of the surface tension acting to force water to the corners encompassed by smaller angles. This observation is consistent with published results of phase distributions in air-water flows through small, non-circular channels (e.g., [20]), where the water tends to be transported in the corners with the air flowing in the high-velocity core. Figs. 16 and 17 illustrate the contrast in water slug shape with two radiographs taken at 0.5A/cm 2 for uncoated rectangular and triangular channels. It appears that in the rectangular case, the water slugs fill most of the channel cross-section and are affected by gravity, as many of these large slugs are retained at the lower channel edge. Between the large slugs are many smaller satellite droplets that are also stationary over the course of the neutron imaging sequence. Conversely, in the triangular channels, the slugs are usually formed in pairs and do not seem to be influenced by gravity to as great a degree, but are retained in the 43 angles adjacent to the GDL (see Fig. 2). A comparison of uncoated and coated triangular channels is illustrated in Fig. 18. As observed in the rectangular channel comparison in Fig. 12, with PTFE coating the water slugs are generally smaller although they are still formed at many locations as pairs retained at the channel angles nearest the GDL. It is expected that other channel cross-sectional geometries can be used to control the location of liquid water accumulation, preferably away from the GDL to minimize its influence on reactant mass transport [21]. The observed effect of gravity described above was initially somewhat unexpected, because it is widely reported that interfacial forces should dominate in two-phase flow through small

11 J.P. Owejan et al. / International Journal of Hydrogen Energy 32 (2007) Fig. 14. Uncoated and PTFE coated rectangular channels at 0.5A/cm 2. channels. The Bond number characterizes the relative influence of gravity and capillary forces and is defined as Bo Δρgl2, (1) σ where g is gravitational acceleration, Δρ is the difference between liquid and gas densities, l is the characteristic length scale and σ is the surface tension. For capillary forces to dominate, the channel size needs to be selected such that the condition Bo>1 is satisfied. For example, for Bo = 0.1 in an air water system at 80 C, this dictates that the characteristic length scale must be less than 0.8 mm. This value is well less than the width of the rectangular channels used in the current study (1.37 mm) which may be the characteristic length scale, because this is the dimension aligned with gravity. However, the channel hydraulic diameter (0.59 mm) is less than this critical channel size based on the Bond number criterion. Therefore, at least for the rectangular geometry, it is reasonable to expect that gravity will play some role in the morphology of the water slug distribution. In Figs. 16 and 17, it is apparent that many of the largest water volumes reside on the lower channel edge, and are clearly influenced by the action of gravity. All four flow field configurations performed similarly in regard to their respective polarization curves (Fig. 19). Every configuration displayed significant mass transport losses at 1.0A/cm 2 and higher loads. The flow fields were chosen to exaggerate losses in the mass transport region; hence voltage losses were expected at high current densities. Again, a performance correlation is ascertained relating accumulated water mass and performance. It is evident in Fig. 19 that smaller water slugs adjacent to the GDL, produced by altering channel surface energy and geometry, can improve performance. The channel water slug size and distribution are also important Fig. 15. Measured water mass profiles for channel property study. considerations for reasons other than fuel cell power performance alone. For example, in a triangular channel, the smaller accumulated water slugs left by the operation of a PEMFC after shut-down have more space in the channel to expand under a freeze condition than larger slugs accumulated in rectangular channels that could potentially damage the brittle GDL. 4. Conclusions The neutron radiography method has been applied to operating 50 cm 2 PEM fuel cells to assess the effects of GDL and

12 4500 J.P. Owejan et al. / International Journal of Hydrogen Energy 32 (2007) Fig. 16. Uncoated rectangular and triangular channels at 0.5A/cm 2. Fig. 17. Enlarged view of slug formation in rectangular (top) and triangular (bottom) channels. Fig. 18. Uncoated and PTFE coated triangular channels at 0.5A/cm 2.

13 J.P. Owejan et al. / International Journal of Hydrogen Energy 32 (2007) expected to impact freeze operation and long-term material durability. Acknowledgments This work was supported by the US Department of Commerce, the NIST Ionizing Radiation Division, the Director s office of NIST, the NIST Center for Neutron Research, and the Department of Energy interagency agreement No. DE\_AI01-01EE Lee Whitehead is acknowledged for his calculation of the fuel cell temperature gradients for the various GDL materials. References Fig. 19. Fuel cell performance comparison for channel variations. flow field channel properties on liquid water accumulation. The test apparatus featured anode and cathode flow fields, which were arranged orthogonally, to enable separate analysis of the water content on either side of the fuel cell. The GDLs manufactured by Toray and SGL varied most significantly in their in-plane gas permeability and through-plane thermal conductivity, which control the convective flow through the material and the effective temperature gradient between the membrane and the flow field plate. It was determined that the relatively low in-plane gas permeability of the Toray material accounts for the greater volume of retained water under the flow field lands. Despite the wide differences in thermal conductivity between the Toray and SGL samples, a simple two-dimensional thermal model indicated that the temperature gradient from the membrane to the flow field was within 0.5 C over the entire range of current densities. It was observed that channel geometry and surface property both have appreciable effects on the volume of accumulated water and on the morphology of water droplets retained in the flow field channels. For both a rectangular and triangular channel with the same cross-sectional area, channel-level water accumulation was reduced by use of a PTFE coating, which provided a static contact angle of 95. For a given flow field surface energy (either PTFE or uncoated gold-plated aluminum) the triangular channels retained less water. Moreover, the water morphology was generally characterized by pairs of droplets captured in the channel angles between the diffusion media and the flow field plate. In the rectangular channels, the water droplets were larger and dispersed individually in the direction of flow with smaller satellite droplets between. It was also apparent that gravitational forces influenced the water accumulation profile, at least for the rectangular channels. 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