Journal of Mechanical Science and Technology 26 (6) (2012) 1957~1962

www.springerlink.com/content/1738-494x DOI 10.1007/s12206-012-0403-x

Supercooling release of micro-size water droplets on microporous surfaces

with cooling† ChunWan Park1 and Chaedong Kang2,*

1Graduate School, Chonbuk National University, Jeonju 561-756, Korea 2Department of Mechanical Engineering, Chonbuk National University, Geothermal Energy Technology Research Center, Jeonju 561-756, Korea

(Manuscript Received May 19, 2011; Revised December 19, 2011; Accepted February 29, 2012)

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Abstract The gas diffusion layer (GDL) of polymer electrolyte membrane fuel cells plays a key role in controlling moisture in these cells. When

the GDL is exposed to a cold environment, the water droplets or water nets in the GDL freeze. This work observed the supercooling and freezing behaviors of water droplets under low temperature. A GDL made of carbon fiber was coated with a waterproof material with 0%, 40%, and 60% PTFE (polytetrafluoroethylene) contents. The cooling process was investigated according to temperature, and the water droplets on the GDL were supercooled and frozen. Delay in the supercooling release was correlated with the size of water droplets on the GDL and the coating rate of the layer. Moreover, the supercooling degree of the droplets decreased as the number of freeze–thaw cycles in the GDL increased.

Keywords: Freezing; Gas diffusion layer (GDL); PEMFC; Supercooling; Water droplet ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 1. Introduction

Water is the main product of fuel reaction in polymer elec-trolyte membrane fuel cells (PEMFCs), which are frequently used as power supplies in vehicles. Moisture inside the fuel cell helps the PEMFC produce stable power outputs. The gas diffusion layer (GDL), a porous carbon fiber layer measuring only 0.5 mm or less in thickness, plays a key role as an electri-cal conductor and a good mass transport layer of reactant gases (hydrogen, air, oxygen) in the PEMFC. In addition, moisture is controlled by the GDL in the electrolyte mem-brane and maintained at an appropriate level by dispersing and discharging the water produced from the cell stack between the membrane electrode assembly and the separator plate [1].

However, when excessive moisture is generated in the cath-ode, a fully or partially blocking state (i.e., flooding) occurs in the GDL or the flow channel. The flooding leads to lower power outputs because of the reduced amount of oxygen transported to the cathode [2, 3]. Studies have demonstrated that cell performance is improved when the moisture in the GDL and that in the membrane are purged [3-5]. Other studies maintained the moisture inside the stack at an appropriate level either by spreading a microporous layer on the GDL

surface [6] or by coating it with PTFE (polytetrafluoroethyl-ene) [7]. Research has also shown that system startup is de-layed by the degradation of power outputs caused by residual moisture freezing resulting from an abnormal discharge in the stack under a low-temperature environment [2, 8, 9]. More-over, purging has been suggested to delay the freezing of water in the GDL and the gas flow path [8, 10, 11]. Delay in system startup [2], degradation of power outputs due to blocked flow channels [3], and deformation of the mi-crostructure [8, 12, 13] reportedly occur when water droplets are retained in the GDL because moisture discharge inside the stack freezes at low temperature. Lim et al. [12] found that the damage induced by repeated freeze–thaw cycles to the PTFE coating on the GDL surface or GDL structure reduces system performance. Other researchers [2-5, 8-13] have focused mainly on two issues in investigating water management in stacks: the flooding due to delayed water discharge and the power response of the system startup with frozen water in the GDL in a low-temperature environment [2-6, 14]. In their study on the cooling of micro-size water droplets, Takahashi and Tiller [15] found that supercooling is dependent on droplet size. Taken together, these data indicate that the freez-ing behavior of moisture in stacks under low temperature war-rants analysis to optimize the use of PEMFCs in obtaining reliable power outputs.

This study examined the effect of the supercooling degree of micro-size droplets on a microporous GDL on freezing

*Corresponding author. Tel.: +82 63 270 2318, Fax.: +82 63 270 2315 E-mail address: ckang@jbnu.ac.kr

† Recommended by Editor Yong Tae Kang © KSME & Springer 2012

1958 C. W. Park and C. Kang / Journal of Mechanical Science and Technology 26 (6) (2012) 1957~1962

when the droplets are exposed to a low-temperature environ-ment. To this end, we froze water droplets on a GDL surface to observe their freezing behavior according to water droplet size, surface characteristics, and repeated freeze–thaw cycles and subsequently measured the supercooling degree of the water droplets.

2. Experimental setup

The flooding and freezing of water droplets in the GDL of the PEMFC were observed using the experimental apparatus illustrated in Fig. 1. The experimental apparatus consists of a cooling/visualization part with a low-temperature bath circula-tor (Jeio Tech-RBC31, 0–45°C), a measurement part with a T-type thermocouple for the GDL (ϕ = 0.12 mm), and a data acquisition part with a stereoscopic microscope (Leica M165C ×120), a data logger (Agilent 34970A, 20 Ch), and Pentium 4 PC. The cooling/visualization part was insulated with polysty-rene resin foam and had a transparent acrylic resin window in the inner upper portion of the insulator to allow occasional observation by microscopy and ultimately to ensure minimal environmental interference on the cooling process. Five ther-mocouples were fixed to five water droplet set points using an acrylic spacer. The interval error between each set point was ±0.1 mm. Fig. 1(a) shows the mounted GDL and the fixed positions of the thermocouples on the cooling device. Ther-mocouples were installed at the inlet and outlet of the cooling plate, at five points on the cooling plate, and at one point in-side the insulation box to measure temperature.

The cooling/visualization part was cooled by brine introduced by a low-temperature bath circulator to simulate a low-temperature environment. The cooling temperature was controlled based on the measured temperatures at the inlet and outlet of the cooling plate. The cooling rate was kept constant using a bypass valve. The growth of ice crystals near the water droplets was observed through a stereoscopic microscope when supercooling was released and after freezing was com-pleted in the test section. The temperatures on the GDL sur-face were measured, and the data were then transferred to the

PC via data logging. The GDL was cut into a 5 cm × 5 cm section for fixation; the area of this section was the same as the cell area of an ordinary unit cell.

We used a micropipette (1–25 μL) to simulate the randomly sized water droplets generated on the GDL surface due to flooding. The water droplets (6, 15, and 30 μL in volume) were formed using distilled water, which was placed in con-tact with the thermocouple fastened on the GDL. We kept the distance between the water droplets to a maximum to prevent interference from the ice that was released among the super-cooled water droplets on the GDL surface. Furthermore, we ensured that the front end of the thermocouple that was in contact with the water droplets (to be exact, subsiding within the water droplets) was micro-welded to a spherical shape to reduce the influence of supercooling. A preliminary experi-ment was conducted to ensure that there was as little influence on the release of supercooling as possible. We used brine (ethylene glycol, 50 vol% solution) in the low-temperature bath circulator, which circulated the brine up to the cooling plate inside the insulation box through an insulation tube. The cooling heat transfer rate, temperature of the cooling surface, and relative humidity were maintained at approximately 275 W, −20°C, and 40%, respectively. The cooling heat transfer rate was calculated based on the difference in temperature between the inlet and the outlet of the cooling plate. In addi-tion, the cooling and freezing of the water droplets due to their contact with the GDL surface were examined using the tem-perature data collected every second and the images collected by microscopy every minute, and the data were sent to the PC.

The cooling and freezing characteristics of the water drop-lets on the GDL surface according to the GDL surface condi-tion were then examined. The conditions of this experiment and the water droplet sizes are given in Table 1. The cooling experiment in the water droplets was conducted 20 times for each condition. Although all GDLs (TGPH-060, Toray, Ja-pan) had the same basic properties, their PTFE coating rates varied according to the degree of water-repellent treatment on the surfaces. Table 2 shows the properties of the GDL [16].

Fig. 1. Experimental cooling apparatus.

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3. Results and discussion

3.1 Influence of water droplet size on supercooling

Fig. 2 is a graph of the temperature history of the water droplets obtained from the freezing experiment using 6 μL water droplets.

The water droplets and the GDL were cooled by a cooling plate maintained at approximately −20°C when its initial tem-perature was 18°C. They remained in the supercooling state under freezing temperature before supercooling was released, as shown in Fig. 3 (time point tsc). The environment inside the cover block (Fig. 1) ranged from nearly −1.5 to −3.2°C during cooling, whereas the temperature of the water droplets in-creased up to the freezing temperature before being frozen. In this case, we defined the interval from the freezing point (Tfp = 0°C) to the supercooling release temperature (Tr) as the super-cooling degree (ΔTsc = Tfp − Tr).

Fig. 3 shows the freezing process and the temperature his-tory of the water droplets (15 μL) as they were cooled. When

the supercooling degree of the water droplets was small, the freezing spread slowly throughout the whole volume of each water droplet after supercooling was released. On the contrary, when the supercooling degree was large, the freezing spread quickly after supercooling was released. In addition, super-cooling was released more rapidly in uncoated surfaces than in coated ones.

We next classified the supercooling degree based on its magnitude and frequency. Fig. 4 is a graph showing the distri-bution of the supercooling degree with an interval of 1 K dur-ing the experiment, which was repeated more than 20 times, for each condition to three sizes of water droplets on the GDL surface with 40% PTFE content. The range of the supercool-ing degree was observed broadly to be 3–12 K. Although the variation in the supercooling degree was distributed over a wide range, each droplet size exhibited a dominant supercooling degree. Therefore, all supercooling degrees had to be averaged to obtain the representative value for each droplet size and coating rate. The frequency of the supercool-ing release was directly influenced by the average supercool-ing degree. We applied the basic data on the supercooling degree collected during the cooling and freezing of the water droplets to the following Eq. (1) to calculate the average su-percooling degree of the water droplets:

sc,avg sc,

m m

i i ii i

T T n nΔ = Δ ×∑ ∑ (1)

Table 1. Experimental conditions.

PTFE content (Wet proofing) Droplet size (μL)

GDL (TGPH-060, Toray, 5×5cm2) 0%

40% 60%

6 (×5 Point) 15 (×2 Point) 30 (×1 Point)

Relative humidity 40%RH (cooling/visualization part) Table 2. Thermal properties of GDL.

Properties Unit TGPH-060

Thickness mm 0.19

Bulk density g/cm3 0.44

Porosity % 78

Surface roughness μm 8

Gas permeability mL·mm/(cm2·hr·mmAq) 1900 Electrical resistivity

-through plane -in plane

mΩ·cm mΩ·cm

80 5.8

Fig. 2. Temperature history of droplets.

(a)

(b) Fig. 3. Cooling and freezing of water droplets: (a) enlargement of time variation in temperature; (b) snapshot of droplet freezing related toeach point of (a).

1960 C. W. Park and C. Kang / Journal of Mechanical Science and Technology 26 (6) (2012) 1957~1962

where ni is the number of cooling cycles for every droplet. Fig. 5 is a graph showing the variation in the average super-

cooling degree according to water droplet size and the different surface conditions of the GDL. The average supercooling de-gree decreased as the water droplet size increased from 6 μL to 15 and 30 μL on the GDL. The water droplet size is hypothe-sized to be proportional to the sizes of the surface and the con-tact area, such that the release of a water droplet in the super-cooling state is readily influenced by its neighbor, the cooling-induced frost adjacent to the droplet. This implies that the aver-age supercooling degree is expected to decrease because water droplets on a relatively large surface are more likely to experi-ence supercooling release earlier than those on a small surface during cooling. In addition, the weighted mean of the average supercooling degree, ΔTsc,avg, for each PTFE content increased from 6.5 K to 7.1 and 9.1 K as the PTFE content of the GDL increased from 0% to 40% and 60%, respectively. Although a deviation of ΔTsc,avg was detected in the GDL with 60% PTFE content, the relationship between supercooling degree and drop-let size is in approximate agreement with the logarithmic pattern proposed by Takahashi and Tiller [15].

The average supercooling degree increased as the PTFE coating rate increased from 0 to 40°C with 60% (Δ) PTFE content. The porous layer is hypothesized to become more

water repellent, i.e., a hydrophobic surface with difficulty in dispersing and absorbing water, decreasing and simplifying the surface area for the same amount of water, as the coating rate of the GDL increases, thereby making it difficult for wa-ter to penetrate the GDL and reducing the contact area of the water droplets. Consequently, the supercooling release factor can have less effect on a GDL surface with a complex surface structure. This reduced effect can increase the average super-cooling degree.

3.2 Changes in supercooling degree due to repeated freezing

The GDLs used in this study were sheet shaped and meas-ured 0.19 mm in thickness. For normal operation of a PEMFC, it is important for the GDL to not only discharge water and maintain moisture at an appropriate level but also be durable. Therefore, the average supercooling degree according to water droplet size for a coated GDL was found to vary in the same cooling and freezing experiments that were conducted for the non-coated GDL.

Fig. 6 shows the variation in the average supercooling de-gree based the measured temperature of a droplet on the GDL surface with 40% and 60% PTFE contents from repeated cool-ing experiments. The average supercooling degree decreased with increasing frequency of the cooling cycle even though the data were scattered. The data fitted the first-order line by the least-squares method. The decreasing rate of the average supercooling degree for the GDL surface with 60% PTFE content was greater than that for the GDL surface with 40% PTFE content. However, the average supercooling degree was scattered but almost constant at nearly 8.7 K for the bare GDL (without PTFE content). Although the hydrophobicity of the GDL was improved by PTFE coating, the repetition of the freezing experiment on the water droplets on the GDL partly exfoliated the PTFE coating on the carbon fiber. This partial exfoliation reduced the hydrophobicity of the GDL and in-creased the contact area between the GDL and the water drop-lets and increased the penetration of the water droplets into the

Fig. 6. Relationship between the number of experiments and the super-cooling degree.

Fig. 4. Distribution of average supercooling degree to water dropletson the GDL with 40% PTFE coating.

Fig. 5. Variation in the average supercooling degree against water droplet size.

C. W. Park and C. Kang / Journal of Mechanical Science and Technology 26 (6) (2012) 1957~1962 1961

GDL. Although the partial exfoliation resulted in an increased supercooling degree, the average supercooling degree stabi-lized to higher than 6 K as the frequency of the freeze–thaw cycle increased.

Fig. 7 shows the SEM images of some parts of the GDL sur-face with water droplets before and after the repeated experi-ment obtained using SN-3000 (Hitachi, Japan). Panel (a) shows enlarged images of the GDL surface before the freezing ex-periment. The PTFE layer is spread evenly around the carbon fiber surface. Panel (b) shows the images of the GDL surface after the experiment was repeated 25 times. Although the posi-tions of the exfoliation between panels (a) and (b) do not coin-cide exactly, all the images show the surface contacting the water droplet. The figure shows that exfoliation of the PTFE coating occurred partially. Droplets at five locations where the droplets had undergone freeze–thaw repetition were measured using a contact angle meter (SEO, Phoenix 300, ±0.1°) to inves-tigate the effect of the exfoliation on PTFE coating. The contact angle was obtained from image processing.

Fig. 8 displays the decrease in the measured contact angle with freeze–thaw repetition. The contact angle of the water droplet was approximately 100° on the GDL surface without PTFE coating. The contact angle was larger for the GDL with 60% PTFE content than that with 40%. On the GDL with PTFE coating, the contact angle decreased from 129°–130° to 108°–112° with 40% PTFE content and from 133°–136° to 117°–121° with 60% PTFE content after the experiment. The decrease in the contact angle was affected by the increase in wetting due to the carbon fiber on the GDL. The uncertainty of the contact angle, however, for the coating rate of 40% PTFE increased with the number of freeze–thaw repetitions.

For the same repetition, there was a random increase in the bare part (carbon fiber) at 40% PTFE rather than at 60% PTFE. In addition, the error of the coating rate was larger at 40% PTFE than at 60% PTFE. Therefore, the water droplet would come in contact with a larger portion of bare part close to the natural carbon fiber, which is more hydrophilic than the PTFE surface fiber. Moreover, the decrease in the contact angle according to the increase in the number of freeze–thaw repetitions for the cooling experiment was quantitatively con-sistent with the results shown in Fig. 5. This decrease was correlated with the increase in the wettability of the GDL sur-face due to the partial exfoliation of the PTFE coating.

4. Conclusions

This study analyzed the cooling and freezing of water drop-lets on a GDL surface to examine how water droplet size, surface characteristics, and freeze–thaw repetition, which was considered problematic for PEMFC activity at low tempera-ture, affected the supercooling degree. We present the follow-ing conclusions:

(1) The average supercooling degrees of water droplets with varying sizes decreased as the size of water droplets increased from 6 μL to 15 and 30 μL on the hydrophobic GDL surface.

(2) The average supercooling degrees of the water droplets on the surface increased from 6.9 K to 7.5 and 10.1 K as the PTFE coating rate of the GDL increased from 0% to 40% and 60% PTFE contents, respectively.

(3) The repetition of the water droplet freeze–thaw cycle on the GDL with an established coating rate demonstrated exfoliation of the PTFE coating on the GDL surface. This exfoliation reduced the contact angle and the supercooling degree gradually.

(4) The increased number of freeze–thaw repetitions con-firmed the partial exfoliation of the PTFE coating inside the GDL and the gradual decrease in the contact angle of the wa-ter droplets on the GDL surface. Contact angle measurement showed that the hydrophobicity of the GDL surface decreased

(a)

(b) Fig. 7. SEM images of carbon fiber in GDL: (a) initial state of PTFE coating; (b) damaged PTFE coating after cycle test.

Fig. 8. Relationship between the number of experiments and contact angle.

1962 C. W. Park and C. Kang / Journal of Mechanical Science and Technology 26 (6) (2012) 1957~1962

as repetitions of the freeze–thaw cycle increased. With respect to supercooling behavior, the average supercooling degree decreased as the number of freeze–thaw cycles increased. Moreover, the decreasing rate of the average supercooling degree in the GDL with 40% PTFE content was smaller than that detected in the GDL with 60% PTFE content.

Acknowledgements

This study was supported by the Ministry of Education, Science, and Technology (2009) and the National Research Foundation of Korea (2009-0077782). We thank the staff of the Center for University-Wide Research Facilities in Chon-buk National University for helping us with SEM imaging.

Nomenclature------------------------------------------------------------------------

GDL : Gas diffusion layer m : Total number of supercooling release for each drop-

let size ni : The number of supercooling release by ΔTsc,i for

each droplet size PEMFC : Polymer electrolyte membrane fuel cell PTFE : Polytetrafluoroethylene tsc : Time for release of supercooling [sec] ΔTsc : Supercooling degree [K] ΔTsc,i : Round-off supercooling degree by supercooling

release for each droplet size ΔTsc,avg : Average supercooling degree [K] θ : Contact angle [deg]

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Chaedong Kang received a B.S. degree in mechanical engineering from Kyung-hee University in 1985 and an M.S. degree in mechanical engineering from KAIST in 1989. He then went on to earn his Dr.Eng. degree from the Tokyo Institute of Technology in 1997. Dr. Kang is currently an Associate Professor

of the Department of Mechanical Engineering at Chonbuk National University in Jeonju, Korea. His research interests are in the areas of refrigeration, building HVACs, ice storage systems, and molecular simulation.

Chun Wan Park received his B.S. and M.S. degrees in mechanical engineering from Chonbuk National University, Jeonju, Korea, in 2009 and 2011, re-spectively. Currently, he is a Ph.D. can-didate in Mechanical Engineering at the same university. Park’s research inter-ests are in the areas of refrigeration,

building HVACs, as well as new and renewable energy.