Contact resistance between gas diffusion layer and catalyst layer of PEM fuel cell

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Electrochemistry Communications 10 (2008) 47–51 www.elsevier.com/locate/elecom

Contact resistance between gas diffusion layer and catalyst layer of PEM fuel cell Iwao Nitta a

a,*

, Olli Himanen b, Mikko Mikkola

a

Helsinki University of Technology, Laboratory of Advanced Energy Systems, P.O. Box 2200, 02015 TKK, Finland b VTT Technical Research Center of Finland, Fuel Cells, P.O. Box 1000, 02044 VTT, Finland Received 10 September 2007; received in revised form 25 October 2007; accepted 27 October 2007 Available online 4 November 2007

Abstract In this study, the electrical contact resistance between gas diffusion layer (GDL) and catalyst layer (CL) on an electrolyte membrane was experimentally evaluated as a function of compression. The contact resistances between the GDL and CL decreased nonlinearly as the GDL thickness decreased due to the compression pressure. The values of the contact resistance between the GDL and CL were found to be more than one order of magnitude larger than the contact resistance between the GDL and graphite, and even comparable to the ionic resistance of the membrane. Because of the large value and variation in contact resistance between the GDL and CL, severe current distribution may be created inside the cell. The results reported here should be highly useful in providing a more accurate picture of the transport phenomena in a fuel cell. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Inhomogeneous compression; Contact resistance; Gas diffusion layer; Catalyst layer; PEM Fuel cell

1. Introduction Polymer electrolyte membrane (PEM) fuel cell has gained attention as a highly promising power source for a wide range of applications because of its high efficiency, high energy density, quick start capability and environmentally friendly operation [1–4]. One of the most important technological hurdles to overcome is the relatively poor life time of the fuel cell stack. Of all the factors limiting the lifetime of PEM fuel cells, damage in membrane is of particular importance. This is especially the case when membranes are becoming thinner, which is necessary for performance improvement and cost reasons. Even minute damages in such membranes, caused e.g. by local pressure or current distributions and consequent local heat spots, could become potential failure sites after a long operation period. Therefore, detailed experimental assessments of the physi-

*

Corresponding author. Tel.: +358 9 451 3209; fax: +358 9 451 3195. E-mail address: iwao.nitta@tkk.fi (I. Nitta).

1388-2481/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2007.10.029

cal properties of fuel cell components, such as contact resistance, must be conducted and subsequently, modeling mass, charge and heat transport in a fuel cell is necessary to investigate the local phenomena and to predict overall consequences. Because of computational limitations and a lack of experimental data, the geometric and modeling parameters and operating conditions in the models are typically simplified or based on rough assumptions. These simplifications or assumptions may lead into underestimation of actual phenomena occurring in the fuel cell, and as a consequence, the modeled results may not reflect real fuel cell conditions. To the authors’ knowledge, only few experimental studies on contact resistance between the gas diffusion layer (GDL) and catalyst layer (CL) have been carried out [5] and it has been typically estimated roughly or simply overlooked in modeling studies [6,7]. This parameter, however, may be a key factor to determine the current and temperature distribution inside the cell and needs to be investigated in sufficient detail. This is firstly because the value itself can be substantially larger than the contact resistance at other

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I. Nitta et al. / Electrochemistry Communications 10 (2008) 47–51

interfaces since the CL contains electrically non-conductive material and the roughness of GDL and CL are very different. And secondarily, significant variation in the contact resistance values over the cell area can exist because of the rib/channel structure of the neighboring bipolar plate as discussed in authors’ previous paper [8]. The purpose of this paper, therefore, is to present a method for determining the contact resistance between the GDL and CL as a function of compressed GDL thickness and to provide reliable data for modelers. 2. Experimental Experiments were based on AC impedance spectroscopy in a symmetrical H2/H2 cell. The H2/H2 cell can be used to study phenomena occurring in a PEM fuel cell, see e.g. [9– 14]. In H2/H2 cell hydrogen is fed into both the anode and cathode compartments. The anode and cathode reactions of a H2/H2 cell are: H2 ! 2Hþ þ 2e ðanodeÞ 2Hþ þ 2e ! H2 ðcathodeÞ

ð1Þ ð2Þ

Since the electrochemical kinetics of the reaction in Eq. (2) is substantially faster than that of the oxygen reduction reaction, the activation overpotential is small, which makes the AC impedance measurement easier and helps in obtaining accurate results. Furthermore, since the heat production by the reaction in this system is smaller than in PEM fuel cell systems, the H2/H2 cell can be assumed isothermal. This is important because membrane properties such as water uptake and proton conductivity depend on temperature [15,16]. Other advantage in the H2/H2 cell is that because there is no water production in this system the water content of the cell can be accurately controlled by reactant humidification conditions. When membrane parameters can be assumed constant the contact resistance between the GDL and CL can be calculated by subtracting the membrane resistance and other bulk and contact resistances from the measured total cell resistance.

a

The schematic of the measurement system and the H2/ H2 cell employed in this study are illustrated in Fig. 1a and b, respectively. The system was basically similar to the one developed by Himanen et al. [14] with slight modifications. The flow rates of hydrogen were kept at constant 20 ml min1 by mass flow controllers (Brooks 5850S). Hydrogen was humidified in a commercial humidification unit (Fuel Cell Technologies Inc.) before introduction into the cell. The H2/H2 cell consisted of the cell body and two cylindrical graphite current collectors (ISEM-3 grade, Tanso Ab), between which the MEA (Goreä PrimeaÒ Series 5510) and circular GDLs (SGL SIGRACETÒ GDL 10BA) with a diameter of 8.5 mm were placed. The current collectors could move in the cell frame, allowing separation of sealing pressure onto the gaskets and compression onto the active cell area. The compression pressure was controlled using a pneumatic cylinder and pressure controller. The same concept of cell structure is presented in more detail by Ihonen et al. [17]. To achieve uniform compression on the GDLs and MEA, the current collectors were not grooved, i.e. there were no gas channels. Hydrogen entered the cell from the inlet hole located in the center of the current collector, spread radially from the center and exited through the outlet located at its outer edge. The cell temperature was set to 40 °C and controlled using a West 6100 controller, four heating elements (Watlow EB) inside the cell frame and K-type thermocouples in the current collectors. The dew point temperature of hydrogen was varied from 45 to 55 °C to ensure that the membrane was fully hydrated. The temperature of the gas pipes between the humidifier and the H2/H2 cell was always set to 5 °C higher than the dew point temperature to avoid water accumulation inside the pipe. Potential probes were attached to the current collectors. Prior to the AC impedance measurement, the cell was run in fuel cell mode to ensure gas tightness, and subsequently flushed with humidified nitrogen for 3 h to remove oxygen and to achieve water equilibrium condition. The

b H2 tank

compression pressure

out-flow in-flow

cell body

GDL

PC

mass flow controller gasket

PC

humidification unit

impedance analyzer

MEA H2/H2 cell

potentiostat

in-flow

current collector

out-flow

exhaust

compression pressure

Fig. 1. Schematic of measurement system: (a) whole system, and (b) enlarged H2/H2 cell.

I. Nitta et al. / Electrochemistry Communications 10 (2008) 47–51

AC impedance spectra were recorded with an impedance spectrum analyzer (Zahner IM6 Electrochemical Workstation) and potentiostat (Zahner PP240 Power Potentiostat) by sweeping the frequency from 500 mHz to 20 kHz. The measurements were conducted in galvanostatic mode with 20 mA amplitude and 0 mA DC-level. After the AC impedance spectra were obtained, the compression pressure onto the active area was increased stepwise from 0.66 to 4.71 MPa and the same procedure was repeated. Recording the impedance spectrum was repeated five times for each compression pressure. 3. Results and discussion Fig. 2 shows typical examples of the spectra obtained from the AC impedance measurement with various compression pressures onto the active area. The high frequency arc is associated with the charge transfer across the catalyst interface and the low frequency arc is attributed to both adsorption and desorption of hydrogen on the electrode surface and water transport in the membrane [9–11]. The real axis intercept of the impedance spectrum at high frequency indicates the total cell resistance, Rcell. Only the Rcell, which includes the bulk resistances of MEA, GDL and graphite current collectors, and the contact resistances between the components, is needed here. The detailed analysis of impedance behavior of the cell is beyond the scope of this study and left for future work. The measured impedance spectrum, as expected, shifted to the left as higher compression pressure was applied, indicating a decrease of the Rcell. The measurements with different inlet hydrogen humidities showed that the membrane was fully humidified and the measurements were repeatable (see Fig. 2). The compression pressure applied to the active area was converted to the compressed GDL thickness, based on a

study of mechanical properties of the GDL [18]. The obtained Rcell as a function of compressed GDL thickness and compression pressure on the active area is illustrated in Fig. 3. Fig. 3 also includes the negligibly small contact resistance between the GDL and graphite current collector, Rc(GDL/GR), and the bulk GDL resistance, Rb(GDL), which were evaluated in a previous study [8], and the membrane resistances Rm calculated as described below. The Rcell decreased nonlinearly as the GDL was compressed. This was mainly due to the reduction of contact resistances between the GDL and CL, Rc(GDL/CL). The error estimate of Rcell was obtained by repeating the measurement and represents the 90% confidence limit when the applied compression pressure to the active area exceeded 1 MPa. This error was small enough to warrant a detailed analysis of the results. However, the error limits were larger at the lowest compression pressure of 0.66 MPa, because Rcell was found to be fairly susceptible to the assembling process when low compression pressures were applied. For the calculation of the Rm, the values of conductivity for membrane humidified with water vapor, such as reported by Cleghorn et al. [19], could not be used in this study. This is because the water uptake of the membrane from saturated water vapor and liquid water are significantly different, see e.g. [20,21]. Severe flooding was thought to occur in the cell since there was no flow field channel and humid hydrogen was used, and thus, the MEA was at least partially in contact with liquid water. Therefore, the membrane conductivity was calculated from the assumed water content of the membrane, k. k is defined as the number of water molecules per sulfonic acid group þ (k = mol H2O/mol SO 3 H ) and can be calculated from k¼

WU  EW Mw

ð3Þ

compression pressure on the active area (MPa) 4.71 3.93 3.32 2.82 2.27 1.72 1.11 0.66

-3

x10 4

0.16

o

* dew point temperature: 55 C o ** dew ponint temperature: 45 C

Pressure (MPa)

2

0.14

resistance ( Ω cm2)

1.72* 2.27* 2.82* 3.32* 3.93* 3.93**

3

Im(Z) (Ω cm2)

49

0.12

total cell resistance contact resistance (GDL/GR) [8] bulk resistance of GDL [8] membrane resistance (WU: 20%) membrane resistance (WU: 30%)

0.10 0.08 0.06 0.04

1

0.02 0

0 0.05

0.06

0.07

0.08

Re(Z) (Ω cm2) Fig. 2. Impedance curves measured with compression pressure of 1.72– 3.93 MPa on the active area.

150

200 250 300 compressed GDL thickness (μm)

Fig. 3. Resistance as a function of compressed GDL thickness and compression pressure on the active area.

I. Nitta et al. / Electrochemistry Communications 10 (2008) 47–51

compression pressure on the active area (MPa) 4.71 3.93 3.32 2.82 2.27 1.72 1.11 0.66 0.06

70

0.05

60

0.04 50

0.03 0.02

40

0.01

30

ratio of 2*Rc(GDL/CL) to Rcell (%)

where WU is the water uptake of the membrane defined as the mass of water in the membrane divided by the mass of dry membrane (WU = Kg H2O/Kg dry membrane), EW is the equivalent weight of the membrane defined as the weight of membrane per mole of sulfonic acid groups þ (EW = Kg membrane/mol SO 3 H ) and Mw is the molar mass of water. The published values of WU have considerable variation. The WU of 32% measured by Kolde et al. [22] may be too high compared to the case in this measurement, because their membrane was equilibrated with boiling water, which lead to higher WU values than those when the membrane was soaked in water at lower temperature as reported by Springer et al. [23]. Another possible reason for the high WU value measured in [22] is that they used the plain membrane and not the MEA as in this study. As observed by Matic et al. [12], water content in the membrane with CLs is highly uneven, which may yield substantially different WU values compared to the plain membrane. This is because the CL is usually hydrophobized to enhance the water removal, and therefore, the measurement may give a lower value of WU for a membrane with CL. On the other hand, the WU of 10% for the MEA measured by Himanen et al. [14] was so small that the calculated membrane resistance exceeded the total cell resistance in Fig. 3. This was most probably due to the inaccuracies in the measurement. Based on above review of literature, the value of WU was assumed to be between 20 and 30%. Using these values of WU and the value of EW given by the manufacturer, 1.1 kg mol1 [22], the k was calculated to be between 12.2 and 18.3. The membrane conductivity, rmem, which is a function of the water content k and the temperature T can be calculated with the empirical equation for a Nafion membrane [23] with a correlation for the Gore membrane [24],   1 1  Þ rmem ¼ 100 ð0:005139k  0:00326Þ exp 2222ð 303 T

contact resistance, Rc(GDL/CL) (Ω cm2)

50

0 150

200 250 compressed GDL thickness (μm)

300

Fig. 4. Contact resistance between the GDL and CL, and ratio of two of the contact resistance to total cell resistance.

14–60 times larger than the Rc(GDL/GR) depending on the compression. This is most probably due to the presence of non-conductive material in the CL. Compared to a value of Rc(GDL/CL) reported by Makharia et al. [5], the values obtained here are more than 10 times larger. This is most probably because they used a GDL coated with an MPL, which reduces the Rc(GDL/CL), see e.g. [25]. Fig. 4 also includes the ratio of two of Rc(GDL/CL) to Rcell. The Rc(GDL/CL) is the dominant part of the Rcell and quite comparable to the Rm, especially when the small compression pressure was applied. Even with the very high compression pressure which compressed the GDL to 40% of the initial thickness, the Rc(GDL/CL) accounts for 35% of the Rcell. These results indicate that the Rc(GDL/CL) cannot be neglected. 4. Summary and conclusion

ð4Þ Eq. (4) yields the rmem to be 7.5 and 11.5 S m1 with the WU value of 20 and 30%, respectively. The area specific resistance of membrane was then calculated with fixed thickness of membrane, 25 lm. The membrane swelling due to water uptake was not considered here. Furthermore, the Rm was assumed to independent of the compression pressure. The area specific resistance of bulk CL and graphite current collector was ascertained small enough [8] and neglected. The results for Rc(GDL/CL) which were obtained by subtraction of the Rc(GDL/GR), Rb(GDL) and Rm from the Rcell are shown in Fig. 4. The error bars of the Rc(GDL/CL) were calculated based on the error estimates of the measured total cell resistances and the variation in the membrane resistance calculated with two different WU values. The Rc(GDL/CL) nonlinearly decreased as the GDL was compressed. The values of the Rc(GDL/CL) were found to be

The aim of this study was to provide an accurate modeling parameter for the contact resistance between the GDL and CL, which has typically been ignored in previous modeling studies. Using the H2/H2 cell and applying the AC impedance method, the contact resistance between the GDL and CL was evaluated as a function of compressed GDL thickness. The evaluated values of contact resistance between the GDL and CL changed nonlinearly from 0.044 to 0.0078 X cm2 when the GDL was compressed from 300 to 140 lm thickness, respectively. The important finding of this study was that the contact resistance between the GDL and CL was relatively large, an order of magnitude larger than the contact resistance between the GDL and graphite current collector and even comparable to the membrane resistance. The contact resistance between the GDL and CL, and membrane resistance form the dominant part of the cell resistance.

I. Nitta et al. / Electrochemistry Communications 10 (2008) 47–51

The high value of the contact resistance between the GDL and CL and its significant variation with compression suggest that uneven compression pressure on the active area of a fuel cell due to the rib/channel structure of the flow field plate may give rise to a notable current density distribution. A modeling study which takes into account the uneven compression pressure and pressuredependent material parameters may provide insight into how local variation of compression affects the local transport phenomena and cell operation. Furthermore, measurements with an MPL coated GDL will be carried out in the future. Acknowledgement The author gratefully acknowledges the financial support from the Fortum foundation. References [1] R.V. Helmolt, U. Eberle, J. Power Sources 165 (2007) 833–843. [2] A. Kundu, J.H. Jang, J.H. Gil, C.R. Jung, H.R. Lee, S.H. Kim, B. Ku, Y.S. Oh, J. Power Sources 170 (2007) 67–78. [3] A.S. Patil, T.G. Dubois, N. Sifer, E. Bostic, K. Gardner, M. Quah, C. Bolton, J. Power Sources 136 (2004) 220–225. [4] M. Uzunoglu, O.C. Onar, M.S. Alam, J. Power Sources 168 (2007) 240–250. [5] R. Makharia, M.F. Mathias, D.R. Baker, J. Electrochem. Soc. 152 (5) (2005) A970–A977. [6] P. Zhou, C.W. Wu, J. Power Sources 170 (2007) 93–100. [7] T. Hottinen, O. Himanen, S. Karvonen, I. Nitta, J. Power Sources 171 (2007) 113–121. [8] I. Nitta, T. Hottinen, O. Himanen, M. Mikkola, J. Power Sources 171 (2007) 26–36. [9] M. Ciureanu, S.D. Mikhailenko, S. Kaliaguine, Catal. Today 82 (2003) 195–206.

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