- Email: [email protected]
A novel polymer electrolyte fuel cell for laboratory investigations and in-situ contact resistance measurements
Electrochimica Acta 46 (2001) 2899– 2911 www.elsevier.nl/locate/electacta
A novel polymer electrolyte fuel cell for laboratory investigations and in-situ contact resistance measurements Jari Ihonen a, Fre´de´ric Jaouen a, Go¨ran Lindbergh a,*,1, Go¨ran Sundholm b,1 a
Department of Chemical Engineering and Technology, Applied Electrochemistry, The Royal Institute of Technology, SE-10044 Stockholm, Sweden b Department of Chemical Technology, Laboratory of Physical Chemistry and Electrochemistry, Helsinki Uni6ersity of Technology, PO Box 6100, FIN-02015 Hut, Finland Received 12 July 2000; received in revised form 29 March 2001
Abstract A novel polymer electrolyte membrane fuel cell and assembly was developed for laboratory investigations. In this cell a simultaneous measurement of clamping pressure and contact resistances is possible. In the study presented this paper, the cell was utilised in in-situ contact resistance measurements of unplated and plated stainless steel (type 316). These contact resistances were studied in situ as a function of time, clamping pressure, gas pressure and current density. Ex-situ measurements were used to validate the in-situ contact resistance measurements. The validity and error sources of the applied in-situ measurement method were studied using both computer simulations and experiments. © 2001 Elsevier Science Ltd. All rights reserved. Keywords: Fuel cell; Contact resistance; Clamping pressure; Stainless steel; Platinum plating
1. Introduction The development of polymer electrolyte membrane fuel cells (PEMFC) during the last decade has brought these devices on the fringe of large-scale commercialisation. The progress made in catalyst and membrane research has enabled very high power densities (\ 1 W cm − 2) at acceptable cell efficiencies for the fuel cell ( \40%). The catalyst loading of electrodes has been reduced to 0.1 mgPt cm − 2 without significant reduction in performance. New types of ionomers suitable for use in the PEMFC are also under investigation [1].
* Corresponding author. Tel.: + 46-8-7908143; fax: + 46-8108087. E-mail address: [email protected] (G. Lindbergh). 1 ISE member.
In industrial and university research both commercial and in-house fuel cell hardware has been applied. Most commercial hardware utilises graphite-based fuel cells, supplied by Globe Tech Inc. (nowadays a part of Electrochem. Inc.), Electrochem Inc. or Fuel Cell Technologies Inc. A number of in-house cell constructions have also been developed in different laboratories [2 – 9]. In these in-house cells, both conventional serpentine gas channel structures [3,4] as well as interdigitated structures [5] have been used. In some of these cells, the flow channels have been replaced by a porous flow field material and radial construction has been employed to avoid short-circuiting of flow around the periphery of the flow field [6]. The main construction materials of these in-house cells have been graphite [3,4], plated and unplated stainless steel, and plated and unplated titanium [6 – 9]. This commercial and in-house hardware have been satisfactory for many measurements. A serious draw-
0013-4686/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 4 6 8 6 ( 0 1 ) 0 0 5 1 0 - 2
2900
J. Ihonen et al. / Electrochimica Acta 46 (2001) 2899–2911
back of these cells is the insufficient control of clamping pressure, i.e. the mechanical pressure to clamp current collectors against gas backings. Only in the work of Møller-Holst [8] this clamping pressure has been controlled in a reliable way. Furthermore, measuring contact resistance between current collector and gas backing is very difficult in most of these cells. This contact resistance can constitute a significant part of the total cell resistance, especially when stainless steel titanium or moulded graphite is chosen as construction material [8 – 17]. The contact resistance has previously been measured in situ only by Makkus et al. [12] and Miachon et al. [18]. However, both these papers lack a detailed cell description, which makes it difficult to estimate the magnitude of the actual contact pressure between gas diffusion layer and current collector in their measurements. In these papers only the total compaction pressure for sealing the cell is given, which is not the real clamping pressure, since part of the compaction force goes to compression of the sealings. In addition, the error induced from the measurement geometry cannot be estimated, since the precise placement of the potential probe was not given. The contact resistance in the cell changes during operation, which has been shown in both in-situ [12] and ex-situ measurements [10,13]. It would be very beneficial to measure these changes in membrane studies, where the level of membrane degradation and membrane humidification are monitored from the small changes of the membrane resistance [7,19]. Since clamping pressure also affects the water and gas transport in the porous gas backings and in the membrane, it has an influence on the cell performance. In previous studies these effects have been studied without simultaneous measurement of clamping pressure and contact resistances [8,9,20]. Therefore, the development of a cell in which these parameters can be controlled would open up new possibilities in mass transport studies in PEMFC. This paper reports on the in-situ and ex-situ contact resistance measurements which were obtained using our new fuel cell fixture. The contact resistances of the stainless steel current collectors were studied as a function of operating time, gas pressure and clamping pressure. The novel fuel cell hardware is first described in detail and then the results of the contact resistance studies are reported. In addition, the magnitude of error in in-situ measurements, due to the placement of the potential probe in the measurement system, is studied both by simulations and experimentally.
2. Experimental Membrane electrode assemblies (MEA) were prepared using the thin film preparation technique devel-
oped by Wilson et al. [21,22]. Nafion® membranes (E.I. du Pont de Nemours and Company) of different thickness and equivalent weight (115, 112, 1035) were used as the proton conducting membranes. The ink used in MEA fabrication contained a 5% Nafion® solution (1100 equiv. weight) and 20% Pt on a Vulcan XC-72R catalyst (E-TEK Inc.), with a dry weight ratio of 2:3, and 1.0 M TBAOH (Aldrich Chemical Co.) in methanol to ion exchange the Nafion®. In order to achieve good painting properties, Milli-Q® water (conductivity B0.05 S cm − 1) was used as well as glycerol (Merck Kebo Lab), which was later on replaced by isopropanol (Merck Kebo Lab). The catalyst loading of the thin film electrodes was between 0.15 and 0.3 mg Pt cm − 2. The loading was controlled by measuring the weight of the membranes before and after the painting procedure. Before weighing the membranes they were kept at constant temperature and humidity until their weight did not change. The gas diffusion backing material used was commercial double-sided ELAT® backing (E-TEK Inc.). The dc current through the fuel cell was controlled by an electronic load (Powerbox 3310) interfaced to a computer, and data was collected using programs written with LabView (National Instruments). Voltage differences were measured using the Hewlett Packard HP 34401A, HP 3478A or the Fluke 7040 multimeters. In all polarisation experiments, pure ( \99.999%, AGA AB) oxygen and hydrogen gases were used. Sufficient excessive stoichiometric flows (usually at least ten times) were used to eliminate depletion effects. In the commercial fuel cell the flow rates were 30 cm3 min − 1 for H2 and 60 cm3 s − 1 for O2. In the in-house cell the flow rates were 60 cm3 s − 1 for both O2 and H2. The corresponding linear velocities of the gases (Darcy’s velocity) in gas channels were such that they are also encountered in full size cells (0.5 –2 m s − 1). The oxygen flow was humidified to 85 –90% relative humidity (RH). Hydrogen flow was humidified to 100% RH or even slightly (5 – 10%) over that. Gas pressures in the result section are given as absolute pressure. Temperatures of the fuel cell and gas pipes, gas flow rates, humidification of the gases and gas pressure, were all controlled by fuel cell test stations. In part of the experiments, a commercial (Globe Tech Inc.) fuel cell substation was used, whereas in further experiments an in-house build substation (humidifiers bottles supplied by Fuel Cell Technologies Inc.) was utilised. Both commercial and in-house substations were calibrated for different temperatures and gas flow rates. These calibration results were used to calculate relative humidities of the gases. Calibration was performed using a Vaisala HMP42 humidity probe and a HMI41 humidity indicator, equipped with PC data collection abilities.
J. Ihonen et al. / Electrochimica Acta 46 (2001) 2899–2911
Our calibration work showed that the nominal humidifying temperatures, based on internal thermocouple levels, are not the real dew points of the gases in commercial humidifiers as also the study of Cleghorn et al. [23] has revealed. One reason for this deviation is that the connection from the humidifier bottle to the gas pipe (CN, in Fig. 1) can not be heated properly in commercial humidifiers, since this connection is located inside the instrument chassis. Our calibration work has shown that this connection should be heated to a higher temperature than that of the humidifier bottle, to achieve a sufficient and stable humidification level. Another reason for the deviation is an inaccurate placement of a thermocouple (TC1, Fig. 1) in the humidifying bottle. The temperature should be measured at the coldest point with which the humidified gas has contact inside the humidifier. Furthermore, the inaccurate placement of thermocouple makes the humidification level dependent on the water level in the humidifying bottle. A stable humidification level also requires controlled heating of the gas pipes leading from the humidifiers to the cell (H, Fig. 1). The temperatures of both gases were measured just before they entered the cell (TC2, Fig. 1) and were used for regulating the pipe heating. This control of incoming gas temperature is important especially when excessive stoichiometric gas flows are used, since overheated gas would have an effect on the cell performance. Contrary to the observation of Cleghorn et al. [23], we did not notice any significant dependency of humidification on gas flow rate. However, in our calibration measurements the maximum gas flow rate was 10
2901
ml s − 1 whereas Cleghorn et al. have used flow rates higher than 80 ml s − 1. This is an issue which is clearly dependent on the design and volume of the humidifier. The commercial fuel cell hardware was a 5 cm2 PEM fuel cell (FC05-01SP-REF) (Electrochem, Inc.) with double serpentine flow patterns (0.75 mm ×0.75 mm channels and 0.75 mm ribs) and graphite current collectors. To enable contact resistance measurements a very thin and narrow (20 mm × 2 mm) platinum strip of foil was placed between the sealings. This strip was insulated except for the end part, which was in contact with the gas backing surface, thus probing the potential. Potential probes were also attached to both current collectors, so that the potential losses over the interfaces could be measured and corresponding contact resistances could then be calculated. This measurement technique has previously been applied by Makkus et al. [12] and by Miachon et al. [18]. A detailed description of the in-house cell design and method to probe the gas backing potential is presented in Section 3. Platinum plating of stainless steel current collectors was achieved using 1 M HCl or 1M H2SO4 containing H2PtCl6. After mechanical removal of the oxide layer from the surface of the current collectors, they were placed in the solution where plating took place spontaneously. Plating was continued until vigorous hydrogen evolution was detected, indicating formation of a platinum layer on top of the stainless steel. After plating, the current collectors were boiled in 1 M NaOH 1h to remove impurities and to stabilise the surface. Contact electrical resistances were measured ex-situ on small samples (1 –2 cm2) by pressing them between current collectors, similarly to the measurements done by Barbir et al. [24]. In these experiments the same fuel cell fixture and clamping pressure control as in in-situ measurements were used. Bulk electronic conductivity of gas backing materials was determined using the 4-point probe method.
3. The new cell construction
Fig. 1. Schematic presentation of the humidification system. HB= humidification bottle, volume 1 dm3 (stainless steel 316). FC=fuel cell. G1 = gas inlet to HB. TC1 =thermocouple for controlling heating of the HB. H = Heated gas pipe from HB to FC. CN=connector between HB and gas pipe. TC2 =thermocouple for controlling heating of the gas pipe.
We have designed a single cell hardware where the individual resistances can be measured, while the clamping pressure can be precisely controlled during the measurement. This fuel cell framework is presented in Figs. 2 and 3a –b. The construction material of the cell body is polyether–etherketone (PEEK), which has high chemical and mechanical stability. Use of this electrically insulating host material makes the installation of potential probes and reference electrode an easy task. The potential probe E1 has a contact with the gas backing, while a similar probe (REF) is installed for the reference electrode. Another potential probe is applied to the current collectors, to avoid contact and wire resis-
2902
J. Ihonen et al. / Electrochimica Acta 46 (2001) 2899–2911
Fig. 2. Schematic presentation of the in-house fuel cell hardware. TC = a hole for thermocouple. CC =stainless steel current collector. COOL = cooling hole. E1 = a place potential probe for gas backing. Ref = a place for reference electrode. Gas in =a path of incoming gas. Gas out=a path of outgoing gas. H = holes for heating elements. o-ring =a place for o-rings. Dimensions in mm.
tances from the current collectors to the electronic load (or potentiostat). The current collectors (CC in Fig. 2) are placed in cylindrical holes and can be moved and pressed together freely by applying an external pressure. This pressure was controlled in the measurements of this paper using a spring screw with known spring constant (Eugen Wiberger AB, Sweden). For more accurate pressure control, pneumatic or hydraulic systems can be applied. A rather similar control of the clamping pressure system has been developed by Møller-Holst [8]. However, in our design the counter-force can be applied to the other current collector. This avoids shear stress on the membrane, which may not only break the membrane but can also cause a difference in clamping pressure on the different sides of the membrane. The clamping pressure was calculated by dividing the clamping force by the current collector area contacting gas backing (1.2 cm2), excluding the area of gas channels (0.8 cm2). The current collectors are sealed using O-rings (O, in Fig. 2). This type of sealing is sufficient to keep the cell
gas-tight to at least 6 bar absolute pressure and still allow the movement of current collectors. In this type of cell, the material and track field of current collectors can be easily changed. That facilitates the study of different materials and plating procedures. For the first material, we chose stainless steel of type 316, which is also one of the main candidate materials for commercial fuel cell stacks [25]. Some of the stainless steel current collectors were plated with platinum to decrease and stabilise the contact resistances. We would like to point out that this noble-metal plating concept is completely contrary to classic corrosion theory and therefore a risky method. If this chromium or titanium content of a plated alloy is sufficiently high it will stay in the passive regime and the corrosion rate is even reduced [26]. However, if this content is too low, a severe localised corrosion can occur. The critical chromium or titanium content is dependent on temperature and gas composition, which determine the corrosion potential. According to Weng et al., a porous noble metal plating does not cause localised corrosion with titanium but does cause it with
J. Ihonen et al. / Electrochimica Acta 46 (2001) 2899–2911
stainless steel (SS 316) [15]. In some of our experiments we have also detected visible corrosion evidence on the anode side, but none at the cathode side, where the contact resistances are higher. The gas channels in the current collectors are 0.5 mm deep and 1 mm wide. The ribs separating the gas channels are 1 mm wide. This possibility of changing rib/channel ratio and gas backing material is important if the effect of gas backing compression on the cell performance is to be studied.
2903
The area of the current collector is 2 cm2, which is also the upper limit for electrodes that can be studied in this fuel cell. The use of the small electrode area, together with high excessive stoichiometric flows effectively creates one-dimensional system, which is a necessity in electrode studies. The small electrode area also enables the use of potentiostats, which cannot in general operate with higher currents than 1 A. In this cell construction the produced heat is dissipated mostly through the metallic current collector. Therefore, the size of this component must be chosen carefully. With the dimensions of our present construction, heat production of about 2 W can be tolerated, but the minimum operating temperature of the cell is then 50°C. When subsidiary cooling is applied by blowing air to an additional hole (COOL in Fig. 2), a slightly higher heat production (up to 4 W) can be tolerated in the cell. The thermocouple (TC in Fig. 2), used for regulating the cell heating, is located only 1 mm away from the current collector surface to ensure that the measured temperature corresponds as well as possible to the real cell temperature. Fuel and oxidant gases are passed through channels in the polymer body. Incoming gases are passed in to the cell through a hole in the centre of the current collector and out from the hole from the side. This arrangement avoids possible difficulties with short-circuiting of the flow. Passing the incoming gas through the centre of the metallic current collector also minimises the temperature difference between the electrode and the incoming gas. However, the temperature gradient in the current collector should not be too high to prevent the gas condensing. The cell is heated by four cylindrical heating elements (Watlow EB Catridge heaters) placed in 10 mm holes (H in Fig. 2). The nominal power of 600 W per element is reduced to 30 W, by transforming the voltage from 230 V AC to 24 V DC. This is sufficient to heat the cell up to 130°C. The low DC voltage of the heaters makes it possible to use a single thermocouple for measuring and regulating the cell temperature. Furthermore, the noise coming from the mains is reduced, which is beneficial in transient measurements.
4. Results and discussion Fig. 3. (a) Photograph of the in-house single cell fixture. N= spring screw. TC =Thermocouple. E1 and Ref = potential probes for gas backings and reference electrode. H = holes for heating elements. (b) Photograph of one half of the in-house single cell fixture. Ref = probe for reference electrode E1 = potential probe for gas backing. MEA = membrane electrode assembly. GB = gas backing. CC =stainless steel current collector. STRIP = a strip of gas backing where the potential is measured. MEA and GB removed from their proper place to show the CC.
4.1. Measurements with commercial fuel cell The first in-situ contact resistance measurements were made using the commercial fuel cell. As can be seen from the results in Fig. 4 the contact resistance between the graphite current collector and gas backing can be significant. These results also show that the contact resistances are slightly dependent on current density
2904
J. Ihonen et al. / Electrochimica Acta 46 (2001) 2899–2911
Fig. 4. Cell voltage vs. current density measured with a commercial 5 cm2 fuel cell. Galvanostatic experiment. A delay between the measured points 1 – 2 min. Cell voltage measured from gas backings (GB) and current collectors (CC). Gas pressures 200 kPa. Description of MEA: Catalyst loading: 0.22 mg Pt cm − 2 for the cathode, 0.15 mg Pt cm − 2 for the anode. Membrane: Nafion® 115. Area of the electrode: 4 cm2.
and temperature. The anomaly in the measurement, at 80°C at current density 0.2 A cm − 2 can be considered to be an infrequent random error. The in-situ measured contact resistances, obtained by using the commercial test cell showed considerable scattering between the runs (of the order of 30%) even if the bolts of this fuel cell were tightened using the nominal momentum of dynamometric wrench recommended by the fuel cell manufacturer (3.4 Nm). This lack of repeatability is a consequence of poor control of the actual pressure over the MEA in this cell. The clamping pressure, controlled via the bolts sitting at the edges of
the cell body, is distributed over several components in the cell. The thickness and flexibility of these components will determine real pressure between the gas backing and the current collector.
4.2. Measurements with in-house cell A few of the stainless steel current collectors for our in-house cell were plated by platinum to reach low and reproducible contact resistances. The effect of the plating procedure was first verified by ex-situ measurements. Fig. 5 gives the results of three experiments
Fig. 5. Contact resistance of unplated and plated stainless steel (type 316) and ELAT® gas backing as a function of clamping pressure. Measured ex situ.
J. Ihonen et al. / Electrochimica Acta 46 (2001) 2899–2911
where contact resistances of two different plated current collectors and one unplated current collector were studied. All the current collectors were boiled in 1 M NaOH one hour before the measurements to form oxidised surfaces. As can be seen from Fig. 5, the plated collectors have contact resistances order of magnitude lower than the unplated current collector. The difference between the results obtained by the plated current collectors is a consequence of different plating time and solution. The plated current collector with lower contact resistances was used at the anode side in the in-situ experiments of this paper.
2905
The low contact resistances of plated current collectors were also verified by in-situ measurements. Fig. 6a,b show the difference between contact resistances measured in-situ using plated and unplated current collectors. The cell voltages measured from gas backings are rather close, while the cell voltages measures from current collectors differ significantly due to the large difference in contact resistances. The result shown in Fig. 6b was obtained 20 h after installation of polished unplated current collector. As revealed by Fig. 6a,b the contact resistances are clearly dependent on current. This dependency is fur-
Fig. 6. (a) Cell voltage vs. current density measured with plated SS 316 current collectors. Galvanostatic experiment, sweep rate 2.5 mA cm − 2 s − 1. Cell voltage measured from gas backings (GB) and current collectors (CC). Cell temperature 80°C. Gas pressures 200 kPa. Clamping pressure 0.4 MPa. Description of MEA: Catalyst loading: 0.2 mg Pt cm − 2 for the cathode, 0.1 mg Pt cm − 2 for the anode. Membrane: Nafion® 112. Area of the electrode: 2 cm2. (b) Cell voltage vs. current density measured with unplated SS 316 current collectors. Conditions as in (a).
2906
J. Ihonen et al. / Electrochimica Acta 46 (2001) 2899–2911
Fig. 7. Contact resistances as a function of current density. Data extracted from the measurement presented in Fig. 6a,b.
ther illustrated in Fig. 7, which shows the contact resistances of Fig. 6a,b separated for the anode and cathode side. This current dependency was verified by other in-situ and also by ex-situ measurements, which also showed that high heat production at the interface decreases the contact resistance, especially when unplated current collectors are used. The dependency of contact resistance on current density was discovered earlier by Miachon et al. [18]. They measured a few percents increase on contact resistances in their graphite based system over the 0 –1 A cm − 2 range. It seems that
the magnitude and direction of the change in contact resistances are dependent on the contacting materials. Even if the dependency of contact resistances on current density is not large, it may have some importance in measurements where small changes of membrane resistance are studied as a function of current density [7]. The contact resistances are also dependent on total gas pressure and the gas pressure difference over the membrane as Fig. 8 shows. The graph in Fig. 8 is divided into sections, in which the gases had different
Fig. 8. Contact resistances as a function of gas pressures. Current collector: unplated SS 316. Cell voltage measured from gas backings. Galvanostatic experiment. Current density 0.375 A cm − 2. Clamping pressure 0.4 MPa. Conditions otherwise as in Fig. 6a.
J. Ihonen et al. / Electrochimica Acta 46 (2001) 2899–2911
2907
Fig. 9. Contact resistances as a function of clamping pressure. Current collector: unplated SS 316. Cell voltage measured from gas backings. Galvanostatic experiment. Current density 0.375 A cm − 2. Gas pressures 0.2 MPa. Conditions otherwise as in Fig. 6a.
pressures. As can be seen from the curves in Fig. 8, the gas pressures affect not only the electrochemical performance of the cell but also contact resistances. When the gas pressure on one side of the cell was increased, the corresponding contact resistance at that side also increased. When the pressure at both sides was increased from 0.1 to 0.2 MPa the sum of contact resistances increased by about 15% (10 mV cm − 2). As revealed by the graph in Fig. 8 the contact resistances increased when anode gas pressure was changed from 0.2 to 0.1 MPa and back to 0.2 MPa. This is a typical phenomenon for unplated stainless steel with oxide surfaces. Every time gas or clamping pressure is changed, the contact between gas backing and current collector is changed and more surface is exposed to oxide growth, which causes the increase in contact resistances. The increase of contact resistances, as a consequence of changing contact, is revealed further by the graph in Fig. 9. The graph is divided into sections of different clamping pressure. When the clamping pressure was first increased (0.42 –0.54–0.62–0.79–0.92 MPa) the contact resistances decreased, as can be expected. After these changes the clamping pressure was completely released and the current collectors were dragged apart in the cell. When the current collectors were clamped together again with the same pressures as before (0.42 and 0.92 MPa), the sum of contact resistances was increased dramatically (50%). These irreversible changes of contact resistance by changing clamping pressure have also been discussed by Makkus et al. [12], who also noticed that different pre-treatments greatly influence on contact resistances of different stainless steel and other alloys.
The drastic increase of contact resistance does not directly affect cell performance. Fig. 10 shows the results of three overnight constant current experiments, which were made at the different points of the experiment series, in which the effects of clamping and gas pressure were studied. The electrode was the same and the experimental conditions were identical. The only difference is that between these overnight experiments clamping and gas pressures had been increased and decreased many times in other measurements. It should be noted that in this experiment the current density and corresponding heat production at the interface were not very high. At higher current densities, the large heat production on the interface may affect cell performance. The results in Fig. 10 also indicate that stainless steel current collectors have fairly stable contact resistances as long as there are no changes in those experimental parameters (temperature, gas pressure, clamping pressure) that can effect the contact between gas backing and current collector. The contact resistances of the first two experiments, given in Fig. 11, confirm this. As can be seen in Fig. 11 there is some relaxation time when there has been changes in experimental conditions. After this the resistances level off to very stable level. In the first experiment this levelling off took more time, since this experiment was performed after installing freshly polished current collectors into the cell. The kink in the second experiment at 800 min is due to water addition to humidifiers. In the literature it has been proposed that the surface treatment permanently changes the state of the surface, which could be used to achieve lower contact resistances [12,16]. We consider this conclusion premature, since our results have shown that the low contact
2908
J. Ihonen et al. / Electrochimica Acta 46 (2001) 2899–2911
Fig. 10. Results of three overnight measurements at different points in the experimental series. The contacts between current collector and gas backing were altered by changing gas pressure and clamping pressure during the series. Current collector: unplated SS 316. Cell voltage measured from gas backings (GB) and current collectors (CC). DE: a voltage difference between GB and CC. Current density 0.375 A cm − 2. Gas pressures 0.2 MPa. Conditions otherwise as in Fig. 6a.
resistances can be maintained only if the contact between current collector and gas backing is not disturbed. In practical applications, the operating fuel cell stacks will be exposed in thermal and mechanical cycling, which inevitably changes the contact between bipolar plate and gas backing. Therefore, to verify the effect of surface treatment the contact resistances should preferably be studied in dynamic conditions, either using a similar in-situ set-up to that presented in this paper or using dynamic ex-situ methods, such as the CER technique [27].
4.3. Measurement errors in in-situ contact resistance measurements Some measurement error is always induced when the contact resistance is measured in-situ. If the potential probe is applied between the active layer and current collector, as done in this study with commercial cell and also by Makkus et al. [12] and Miachon et al. [18], the current path as well as local contact pressure around the potential probe are disturbed. The error induced in measurements is probably small, if thickness and width
Fig. 11. Relaxation of contact resistances. Data extracted from the measurement presented in Fig. 10. The lower curves: Overnight experiment after installation of CC. The upper curves: overnight experiment after the first changes in gas and clamping pressure.
J. Ihonen et al. / Electrochimica Acta 46 (2001) 2899–2911
of the probe are small compared with the thickness of gas backing. However, if the installation of the probe increases the local contact pressure so much that the surface oxide is broken, a substantially larger error can be induced in measurements. An inevitable error is also induced if the potential is probed on the gas backing far from the electrode. The
2909
magnitude of this error coming from the measurement geometry is studied further here. In this small study it is assumed that current density on the electrode is constant and electrode potential as well as bulk conductivity of the gas backing are constants, too. In our present configuration, the potential of the gas backing is measured from a part of the gas backing
Fig. 12. (a) Geometry of the simulated system. P1 is a gas backing with a thickness of 0.35 mm. R1 is a poorly conducting film between the gas backing and current collector with a thickness of 0.001 mm. R2 is the beginning of the current collector. B1 is the boundary of that side of the gas backing, which touches the electrode. B2 is an equipotential surface in the current collector. E1 and E2: Places of the potential probe in measurement. (b) Results of the simulation with the following parameters: conductivity of gas backing (P1) 50 S cm − 1, conductivity of film (R1) 0.001 S cm − 1, conductivity of current collector (R2) 10000 S cm − 1, area ratio electrode/current collector (B1/B2) 0.4.
2910
J. Ihonen et al. / Electrochimica Acta 46 (2001) 2899–2911
which is outside of the active electrode area (STRIP in Fig. 3b). This is a very convenient way to measure the gas backing potential, since it is easy to install a potential probe and contact pressure between current collector and gas backing is not disturbed. However, when the potential is measured at a point which is not between the current collector and electrode, an additional error occurs due to potential distribution in gas backing. This error is small when the current collector, gas backing and active electrode all have the same area. However, when the area of the electrode is much smaller than the area of the gas backing and current collector, the corresponding error is considerably larger. If the bulk resistance of the gas backing is high and contact resistance is low, this error is sufficiently large to make quantitative in-situ contact resistant measurements invalid. This error was first studied using the pde-toolbox simulations of MATLAB (The MathWorks, Inc.). For simulations, the bulk electronic conductivity of ELAT® (50 S cm − 1) was measured by a four-point probe method. The dimensions and geometry of the simulated system are presented in Fig. 12a. P1 is the gas backing, which has a thickness of 0.35 mm, R1 is a 1 mm thin, poorly conducting film, between the gas backing and current collector. This film simulates the contact resistance. R2 is the beginning of the current collector. B1 is the boundary of that side of the gas backing, which contacts the electrode. At the boundary B1, the potential was set to 0 V. The width of boundary B1 was set to 9 mm, corresponding to an area ratio of 0.4 between the electrode and current collector. The left side of the
boundary is at the same vertical level as the current collector, simulating the situation, when the components have the same size. The right boundary is located 5 mm away from the vertical edge of the current collector, simulating the situation, when electrode area is much smaller than the current collector area. B2 is an equipotential surface in the current collector. The width of this boundary was set to 14 mm and the potential to 0.01 V. Thus the total voltage drop due to contact and bulk resistance is 10 mV. On the other boundaries, Neumann boundary condition n·(|9V)= 0 is applied, corresponding to insulated surfaces. The points E1 and E2 in Fig. 12a indicate the places in the strip of gas backing where potential could be probed in an actual measurement in our in-house cell, see Figs. 2 and 3. As can be seen from the results in Fig. 12b, the potential at the point E2 does not give the correct potential in the gas backing between the electrode and current collector. When potential from this point is used, the calculated contact resistance is underestimated by about 50%. On the other hand, potential at the point E1 gives the correct value. In this simulation, a very low value (10 mV cm2) for contact resistance was used. However, such low resistances do occur, when plated current collectors are used and therefore the possibility of this source of error can not be neglected. The geometrical error was verified experimentally by placing a MEA in the cell in such a way that the geometry was similar to the situation in the simulation. The potential probes at the cathode and anode side had positions like E1 and E2 in the simulation. The results of this experiment, given in Fig. 13, validate the results
Fig. 13. Cell voltage vs. current density measured with plated SS 316 current collectors. Description of MEA: catalyst loading: 0.2 mg Pt cm − 2 for the cathode, 0.07 mg Pt cm − 2 for the anode. Membrane: Nafion® 1035. Area of the electrode: 0.75 cm2. Conditions otherwise as in Fig. 6a.
J. Ihonen et al. / Electrochimica Acta 46 (2001) 2899–2911
of the simulation. A comparison of results from Figs. 7 and 13 shows that the measured contact resistance at the anode side was almost 50% smaller, when a smaller electrode was used. On the other hand, at the cathode side the contact resistances are almost identical. It can be concluded that if contact resistances are very low (under 10 mV cm2), the potential probe should be placed between the active electrode and current collector to reach quantitative contact resistance results. 5. Conclusions The results of this paper, obtained using our novel fuel cell hardware, give several important insights into contact resistances, which are a difficult subject in experimental polymer fuel cell research and stack development. The results show that the contact resistances are dependent on clamping pressure, gas pressure, current density and temperature. It has also been shown that when stainless steel is used as a collector material, the contact resistances are not stable, but relax to a new level after every change in operating parameters. Therefore, dynamic methods should be used in contact resistance studies. Eventually, the contact resistances of stainless steel seem to grow to unacceptable level, when cell is exposed to thermal and mechanical cycling. The contact resistances of stainless steel could be drastically reduced by platinum plating, applying a simple plating procedure. This plating seems to solve most of the problems caused by contact resistances in laboratory research, since the contact resistances of plated stainless steel surfaces are not only much lower, but also much more stable than those of unplated stainless steel surfaces. We also demonstrate by simulations and experiments that an error can appear in in-situ contact resistance measurements if the probe is inserted in the gas backing at a point outside of the actual main current path. This error can, in the worst case, make the quantitative measurements completely invalid. The stability and good reproducibility of measurements of our in-house construction has made it appropriate to now utilise it for the study of the influence of different fabrication parameters on the performance of the MEA. The possibility to distinguish the compression effect on contact resistances from other effects will also be utilised in further research. Acknowledgements The financial support of the Swedish Foundation for Strategic Environmental Research, MISTRA, is gratefully acknowledged. The work was done within the framework of the Jungner Center.
2911
References [1] O. Savadogo, J. New Mater. Electrochem. Syst. 1 (1998) 47. [2] A. Parthasarathy, S. Srinivasan, A.J. Appelby, C.R. Martin, J. Electroanal. Chem. 339 (1992) 101. [3] M. Uchida, Y. Aoyama, N. Eda, A. Ohta, J. Electrochem. Soc 142 (1995) 463. [4] M. Hogarth, P. Christensen, A. Hamnett, A. Shukla, J. Power Sources 69 (1997) 125. [5] T.V. Nguyen, J. Electrochem. Soc 143 (1996) L103. [6] M.S. Wilson, T.E. Springer, J.R. Davey, S. Gottesfeld, in: S. Gottesfeld, G. Halbert, A. Landgrebe (Eds.), Proton Conducting Membrane Fuel Cells I, The Electrochemical Society Proceedings, PV 95-23, The Electrochemical Society, Inc., Pennington, New Jersey, 1995, p. 115. [7] F.N. Bu¨ chi, G.G. Scherer, J. Electroanal. Chem. 404 (1996) 37. [8] S. Møller-Holst, PhD Thesis, NTNU, Trondheim, Norway, 1996, p. 34. [9] P.L. Hentall, J.B. Lakeman, G.O. Mepsted, P.L. Adcock, J.M. Moore, J. Power Sources 80 (1999) 235. [10] D.P. Davies, P.L. Adcock, M. Turpin, S.J. Rowen, J. Appl. Electrochem. 30 (2000) 101. [11] J.-T. Wang, J.R.F. Savinell, J. Wainright, M. Litt, H. Yu, Electrochim. Acta 41 (1996) 193. [12] R.C. Makkus, A.H.H. Janssen, F.A. de Bruijn, R.K.A.M. Mallant, J. Power Sources 86 (2000) 274. [13] D.P. Davies, P.L. Adcock, M. Turpin, S.J. Rowen, J. Power Sources 86 (2000) 237. [14] F. Mahlendorf, O. Niemzig, C. Kreutz, in: Seminar Abstracts of Fuel Cells-Powering the 21st century, The Fuel Cell Organizing Committee (United States), Portland, Oregon, 2000, p. 138. [15] D. Weng, G. Woodcock, T. Rehg, Z. Iqbal, J. Guiheen, J. Matrunich, in: Seminar Abstracts of 1998 Fuel Cells-Powering the 21st Century, The Fuel Cell Organizing Committee (United States), Portland, Oregon, 2000, p. 106. [16] C.E. Reid, W.R. Merida, G. Mclean, in: Seminar Abstracts of 1998 Fuel Cell Seminar, The Fuel Cell Organizing Committee (United States), Palm Springs, California, 1998, abstract 1174. [17] T.M. Besmann, J.W. Klett, J.J. Henry Jr, E. Lara-Curzio, J. Electrochem. Soc. 147 (2000) 4083. [18] S. Miachon, P. Aldebert, J. Power Sources 56 (1995) 31. [19] T.E. Springer, T.A. Zawodzinski, M.S. Wilson, S. Gottesfeld, J. Electrochem. Soc. 143 (1996) 587. [20] W.-K. Lee, C.-H. Ho, J.W. Van Zee, M. Murthy, J. Power Sources 84 (1999) 45. [21] M.S. Wilson, US patents no. 5 211 984 and no. 5 234 777. [22] M.S. Wilson, J.A. Valerio, S. Gottesfeld, Electrochim. Acta 40 (1995) 355. [23] S.J.C. Cleghorn, C.R. Derouin, M.S. Wilson, S. Gottesfeld, J. Appl. Electrochem. 28 (1998) 663. [24] F. Barbir, J. Braun, J. Neutzler, J. New Mater. Electrochem. Syst. 2 (1999) 197. [25] C. Zawodzinski, M.S. Wilson, S. Gottesfeld, in: Seminar Abstracts of 1998 Fuel Cell Seminar, The Fuel Cell Organizing Committee (United States), Palm Springs, California, 1998, abstract 1191. [26] M.G. Fontana, Corrosion Enginering, 3rd edn., McGrawHill, Singapore, 1987, pp. 497 – 499. [27] T. Saario, J. Piippo, Mater. Sci. Forum 185-188 (1995) 621.
A novel polymer electrolyte fuel cell for laboratory investigations and in-situ contact resistance measurements Jari Ihonen a, Fre´de´ric Jaouen a, Go¨ran Lindbergh a,*,1, Go¨ran Sundholm b,1 a
Department of Chemical Engineering and Technology, Applied Electrochemistry, The Royal Institute of Technology, SE-10044 Stockholm, Sweden b Department of Chemical Technology, Laboratory of Physical Chemistry and Electrochemistry, Helsinki Uni6ersity of Technology, PO Box 6100, FIN-02015 Hut, Finland Received 12 July 2000; received in revised form 29 March 2001
Abstract A novel polymer electrolyte membrane fuel cell and assembly was developed for laboratory investigations. In this cell a simultaneous measurement of clamping pressure and contact resistances is possible. In the study presented this paper, the cell was utilised in in-situ contact resistance measurements of unplated and plated stainless steel (type 316). These contact resistances were studied in situ as a function of time, clamping pressure, gas pressure and current density. Ex-situ measurements were used to validate the in-situ contact resistance measurements. The validity and error sources of the applied in-situ measurement method were studied using both computer simulations and experiments. © 2001 Elsevier Science Ltd. All rights reserved. Keywords: Fuel cell; Contact resistance; Clamping pressure; Stainless steel; Platinum plating
1. Introduction The development of polymer electrolyte membrane fuel cells (PEMFC) during the last decade has brought these devices on the fringe of large-scale commercialisation. The progress made in catalyst and membrane research has enabled very high power densities (\ 1 W cm − 2) at acceptable cell efficiencies for the fuel cell ( \40%). The catalyst loading of electrodes has been reduced to 0.1 mgPt cm − 2 without significant reduction in performance. New types of ionomers suitable for use in the PEMFC are also under investigation [1].
* Corresponding author. Tel.: + 46-8-7908143; fax: + 46-8108087. E-mail address: [email protected] (G. Lindbergh). 1 ISE member.
In industrial and university research both commercial and in-house fuel cell hardware has been applied. Most commercial hardware utilises graphite-based fuel cells, supplied by Globe Tech Inc. (nowadays a part of Electrochem. Inc.), Electrochem Inc. or Fuel Cell Technologies Inc. A number of in-house cell constructions have also been developed in different laboratories [2 – 9]. In these in-house cells, both conventional serpentine gas channel structures [3,4] as well as interdigitated structures [5] have been used. In some of these cells, the flow channels have been replaced by a porous flow field material and radial construction has been employed to avoid short-circuiting of flow around the periphery of the flow field [6]. The main construction materials of these in-house cells have been graphite [3,4], plated and unplated stainless steel, and plated and unplated titanium [6 – 9]. This commercial and in-house hardware have been satisfactory for many measurements. A serious draw-
0013-4686/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 4 6 8 6 ( 0 1 ) 0 0 5 1 0 - 2
2900
J. Ihonen et al. / Electrochimica Acta 46 (2001) 2899–2911
back of these cells is the insufficient control of clamping pressure, i.e. the mechanical pressure to clamp current collectors against gas backings. Only in the work of Møller-Holst [8] this clamping pressure has been controlled in a reliable way. Furthermore, measuring contact resistance between current collector and gas backing is very difficult in most of these cells. This contact resistance can constitute a significant part of the total cell resistance, especially when stainless steel titanium or moulded graphite is chosen as construction material [8 – 17]. The contact resistance has previously been measured in situ only by Makkus et al. [12] and Miachon et al. [18]. However, both these papers lack a detailed cell description, which makes it difficult to estimate the magnitude of the actual contact pressure between gas diffusion layer and current collector in their measurements. In these papers only the total compaction pressure for sealing the cell is given, which is not the real clamping pressure, since part of the compaction force goes to compression of the sealings. In addition, the error induced from the measurement geometry cannot be estimated, since the precise placement of the potential probe was not given. The contact resistance in the cell changes during operation, which has been shown in both in-situ [12] and ex-situ measurements [10,13]. It would be very beneficial to measure these changes in membrane studies, where the level of membrane degradation and membrane humidification are monitored from the small changes of the membrane resistance [7,19]. Since clamping pressure also affects the water and gas transport in the porous gas backings and in the membrane, it has an influence on the cell performance. In previous studies these effects have been studied without simultaneous measurement of clamping pressure and contact resistances [8,9,20]. Therefore, the development of a cell in which these parameters can be controlled would open up new possibilities in mass transport studies in PEMFC. This paper reports on the in-situ and ex-situ contact resistance measurements which were obtained using our new fuel cell fixture. The contact resistances of the stainless steel current collectors were studied as a function of operating time, gas pressure and clamping pressure. The novel fuel cell hardware is first described in detail and then the results of the contact resistance studies are reported. In addition, the magnitude of error in in-situ measurements, due to the placement of the potential probe in the measurement system, is studied both by simulations and experimentally.
2. Experimental Membrane electrode assemblies (MEA) were prepared using the thin film preparation technique devel-
oped by Wilson et al. [21,22]. Nafion® membranes (E.I. du Pont de Nemours and Company) of different thickness and equivalent weight (115, 112, 1035) were used as the proton conducting membranes. The ink used in MEA fabrication contained a 5% Nafion® solution (1100 equiv. weight) and 20% Pt on a Vulcan XC-72R catalyst (E-TEK Inc.), with a dry weight ratio of 2:3, and 1.0 M TBAOH (Aldrich Chemical Co.) in methanol to ion exchange the Nafion®. In order to achieve good painting properties, Milli-Q® water (conductivity B0.05 S cm − 1) was used as well as glycerol (Merck Kebo Lab), which was later on replaced by isopropanol (Merck Kebo Lab). The catalyst loading of the thin film electrodes was between 0.15 and 0.3 mg Pt cm − 2. The loading was controlled by measuring the weight of the membranes before and after the painting procedure. Before weighing the membranes they were kept at constant temperature and humidity until their weight did not change. The gas diffusion backing material used was commercial double-sided ELAT® backing (E-TEK Inc.). The dc current through the fuel cell was controlled by an electronic load (Powerbox 3310) interfaced to a computer, and data was collected using programs written with LabView (National Instruments). Voltage differences were measured using the Hewlett Packard HP 34401A, HP 3478A or the Fluke 7040 multimeters. In all polarisation experiments, pure ( \99.999%, AGA AB) oxygen and hydrogen gases were used. Sufficient excessive stoichiometric flows (usually at least ten times) were used to eliminate depletion effects. In the commercial fuel cell the flow rates were 30 cm3 min − 1 for H2 and 60 cm3 s − 1 for O2. In the in-house cell the flow rates were 60 cm3 s − 1 for both O2 and H2. The corresponding linear velocities of the gases (Darcy’s velocity) in gas channels were such that they are also encountered in full size cells (0.5 –2 m s − 1). The oxygen flow was humidified to 85 –90% relative humidity (RH). Hydrogen flow was humidified to 100% RH or even slightly (5 – 10%) over that. Gas pressures in the result section are given as absolute pressure. Temperatures of the fuel cell and gas pipes, gas flow rates, humidification of the gases and gas pressure, were all controlled by fuel cell test stations. In part of the experiments, a commercial (Globe Tech Inc.) fuel cell substation was used, whereas in further experiments an in-house build substation (humidifiers bottles supplied by Fuel Cell Technologies Inc.) was utilised. Both commercial and in-house substations were calibrated for different temperatures and gas flow rates. These calibration results were used to calculate relative humidities of the gases. Calibration was performed using a Vaisala HMP42 humidity probe and a HMI41 humidity indicator, equipped with PC data collection abilities.
J. Ihonen et al. / Electrochimica Acta 46 (2001) 2899–2911
Our calibration work showed that the nominal humidifying temperatures, based on internal thermocouple levels, are not the real dew points of the gases in commercial humidifiers as also the study of Cleghorn et al. [23] has revealed. One reason for this deviation is that the connection from the humidifier bottle to the gas pipe (CN, in Fig. 1) can not be heated properly in commercial humidifiers, since this connection is located inside the instrument chassis. Our calibration work has shown that this connection should be heated to a higher temperature than that of the humidifier bottle, to achieve a sufficient and stable humidification level. Another reason for the deviation is an inaccurate placement of a thermocouple (TC1, Fig. 1) in the humidifying bottle. The temperature should be measured at the coldest point with which the humidified gas has contact inside the humidifier. Furthermore, the inaccurate placement of thermocouple makes the humidification level dependent on the water level in the humidifying bottle. A stable humidification level also requires controlled heating of the gas pipes leading from the humidifiers to the cell (H, Fig. 1). The temperatures of both gases were measured just before they entered the cell (TC2, Fig. 1) and were used for regulating the pipe heating. This control of incoming gas temperature is important especially when excessive stoichiometric gas flows are used, since overheated gas would have an effect on the cell performance. Contrary to the observation of Cleghorn et al. [23], we did not notice any significant dependency of humidification on gas flow rate. However, in our calibration measurements the maximum gas flow rate was 10
2901
ml s − 1 whereas Cleghorn et al. have used flow rates higher than 80 ml s − 1. This is an issue which is clearly dependent on the design and volume of the humidifier. The commercial fuel cell hardware was a 5 cm2 PEM fuel cell (FC05-01SP-REF) (Electrochem, Inc.) with double serpentine flow patterns (0.75 mm ×0.75 mm channels and 0.75 mm ribs) and graphite current collectors. To enable contact resistance measurements a very thin and narrow (20 mm × 2 mm) platinum strip of foil was placed between the sealings. This strip was insulated except for the end part, which was in contact with the gas backing surface, thus probing the potential. Potential probes were also attached to both current collectors, so that the potential losses over the interfaces could be measured and corresponding contact resistances could then be calculated. This measurement technique has previously been applied by Makkus et al. [12] and by Miachon et al. [18]. A detailed description of the in-house cell design and method to probe the gas backing potential is presented in Section 3. Platinum plating of stainless steel current collectors was achieved using 1 M HCl or 1M H2SO4 containing H2PtCl6. After mechanical removal of the oxide layer from the surface of the current collectors, they were placed in the solution where plating took place spontaneously. Plating was continued until vigorous hydrogen evolution was detected, indicating formation of a platinum layer on top of the stainless steel. After plating, the current collectors were boiled in 1 M NaOH 1h to remove impurities and to stabilise the surface. Contact electrical resistances were measured ex-situ on small samples (1 –2 cm2) by pressing them between current collectors, similarly to the measurements done by Barbir et al. [24]. In these experiments the same fuel cell fixture and clamping pressure control as in in-situ measurements were used. Bulk electronic conductivity of gas backing materials was determined using the 4-point probe method.
3. The new cell construction
Fig. 1. Schematic presentation of the humidification system. HB= humidification bottle, volume 1 dm3 (stainless steel 316). FC=fuel cell. G1 = gas inlet to HB. TC1 =thermocouple for controlling heating of the HB. H = Heated gas pipe from HB to FC. CN=connector between HB and gas pipe. TC2 =thermocouple for controlling heating of the gas pipe.
We have designed a single cell hardware where the individual resistances can be measured, while the clamping pressure can be precisely controlled during the measurement. This fuel cell framework is presented in Figs. 2 and 3a –b. The construction material of the cell body is polyether–etherketone (PEEK), which has high chemical and mechanical stability. Use of this electrically insulating host material makes the installation of potential probes and reference electrode an easy task. The potential probe E1 has a contact with the gas backing, while a similar probe (REF) is installed for the reference electrode. Another potential probe is applied to the current collectors, to avoid contact and wire resis-
2902
J. Ihonen et al. / Electrochimica Acta 46 (2001) 2899–2911
Fig. 2. Schematic presentation of the in-house fuel cell hardware. TC = a hole for thermocouple. CC =stainless steel current collector. COOL = cooling hole. E1 = a place potential probe for gas backing. Ref = a place for reference electrode. Gas in =a path of incoming gas. Gas out=a path of outgoing gas. H = holes for heating elements. o-ring =a place for o-rings. Dimensions in mm.
tances from the current collectors to the electronic load (or potentiostat). The current collectors (CC in Fig. 2) are placed in cylindrical holes and can be moved and pressed together freely by applying an external pressure. This pressure was controlled in the measurements of this paper using a spring screw with known spring constant (Eugen Wiberger AB, Sweden). For more accurate pressure control, pneumatic or hydraulic systems can be applied. A rather similar control of the clamping pressure system has been developed by Møller-Holst [8]. However, in our design the counter-force can be applied to the other current collector. This avoids shear stress on the membrane, which may not only break the membrane but can also cause a difference in clamping pressure on the different sides of the membrane. The clamping pressure was calculated by dividing the clamping force by the current collector area contacting gas backing (1.2 cm2), excluding the area of gas channels (0.8 cm2). The current collectors are sealed using O-rings (O, in Fig. 2). This type of sealing is sufficient to keep the cell
gas-tight to at least 6 bar absolute pressure and still allow the movement of current collectors. In this type of cell, the material and track field of current collectors can be easily changed. That facilitates the study of different materials and plating procedures. For the first material, we chose stainless steel of type 316, which is also one of the main candidate materials for commercial fuel cell stacks [25]. Some of the stainless steel current collectors were plated with platinum to decrease and stabilise the contact resistances. We would like to point out that this noble-metal plating concept is completely contrary to classic corrosion theory and therefore a risky method. If this chromium or titanium content of a plated alloy is sufficiently high it will stay in the passive regime and the corrosion rate is even reduced [26]. However, if this content is too low, a severe localised corrosion can occur. The critical chromium or titanium content is dependent on temperature and gas composition, which determine the corrosion potential. According to Weng et al., a porous noble metal plating does not cause localised corrosion with titanium but does cause it with
J. Ihonen et al. / Electrochimica Acta 46 (2001) 2899–2911
stainless steel (SS 316) [15]. In some of our experiments we have also detected visible corrosion evidence on the anode side, but none at the cathode side, where the contact resistances are higher. The gas channels in the current collectors are 0.5 mm deep and 1 mm wide. The ribs separating the gas channels are 1 mm wide. This possibility of changing rib/channel ratio and gas backing material is important if the effect of gas backing compression on the cell performance is to be studied.
2903
The area of the current collector is 2 cm2, which is also the upper limit for electrodes that can be studied in this fuel cell. The use of the small electrode area, together with high excessive stoichiometric flows effectively creates one-dimensional system, which is a necessity in electrode studies. The small electrode area also enables the use of potentiostats, which cannot in general operate with higher currents than 1 A. In this cell construction the produced heat is dissipated mostly through the metallic current collector. Therefore, the size of this component must be chosen carefully. With the dimensions of our present construction, heat production of about 2 W can be tolerated, but the minimum operating temperature of the cell is then 50°C. When subsidiary cooling is applied by blowing air to an additional hole (COOL in Fig. 2), a slightly higher heat production (up to 4 W) can be tolerated in the cell. The thermocouple (TC in Fig. 2), used for regulating the cell heating, is located only 1 mm away from the current collector surface to ensure that the measured temperature corresponds as well as possible to the real cell temperature. Fuel and oxidant gases are passed through channels in the polymer body. Incoming gases are passed in to the cell through a hole in the centre of the current collector and out from the hole from the side. This arrangement avoids possible difficulties with short-circuiting of the flow. Passing the incoming gas through the centre of the metallic current collector also minimises the temperature difference between the electrode and the incoming gas. However, the temperature gradient in the current collector should not be too high to prevent the gas condensing. The cell is heated by four cylindrical heating elements (Watlow EB Catridge heaters) placed in 10 mm holes (H in Fig. 2). The nominal power of 600 W per element is reduced to 30 W, by transforming the voltage from 230 V AC to 24 V DC. This is sufficient to heat the cell up to 130°C. The low DC voltage of the heaters makes it possible to use a single thermocouple for measuring and regulating the cell temperature. Furthermore, the noise coming from the mains is reduced, which is beneficial in transient measurements.
4. Results and discussion Fig. 3. (a) Photograph of the in-house single cell fixture. N= spring screw. TC =Thermocouple. E1 and Ref = potential probes for gas backings and reference electrode. H = holes for heating elements. (b) Photograph of one half of the in-house single cell fixture. Ref = probe for reference electrode E1 = potential probe for gas backing. MEA = membrane electrode assembly. GB = gas backing. CC =stainless steel current collector. STRIP = a strip of gas backing where the potential is measured. MEA and GB removed from their proper place to show the CC.
4.1. Measurements with commercial fuel cell The first in-situ contact resistance measurements were made using the commercial fuel cell. As can be seen from the results in Fig. 4 the contact resistance between the graphite current collector and gas backing can be significant. These results also show that the contact resistances are slightly dependent on current density
2904
J. Ihonen et al. / Electrochimica Acta 46 (2001) 2899–2911
Fig. 4. Cell voltage vs. current density measured with a commercial 5 cm2 fuel cell. Galvanostatic experiment. A delay between the measured points 1 – 2 min. Cell voltage measured from gas backings (GB) and current collectors (CC). Gas pressures 200 kPa. Description of MEA: Catalyst loading: 0.22 mg Pt cm − 2 for the cathode, 0.15 mg Pt cm − 2 for the anode. Membrane: Nafion® 115. Area of the electrode: 4 cm2.
and temperature. The anomaly in the measurement, at 80°C at current density 0.2 A cm − 2 can be considered to be an infrequent random error. The in-situ measured contact resistances, obtained by using the commercial test cell showed considerable scattering between the runs (of the order of 30%) even if the bolts of this fuel cell were tightened using the nominal momentum of dynamometric wrench recommended by the fuel cell manufacturer (3.4 Nm). This lack of repeatability is a consequence of poor control of the actual pressure over the MEA in this cell. The clamping pressure, controlled via the bolts sitting at the edges of
the cell body, is distributed over several components in the cell. The thickness and flexibility of these components will determine real pressure between the gas backing and the current collector.
4.2. Measurements with in-house cell A few of the stainless steel current collectors for our in-house cell were plated by platinum to reach low and reproducible contact resistances. The effect of the plating procedure was first verified by ex-situ measurements. Fig. 5 gives the results of three experiments
Fig. 5. Contact resistance of unplated and plated stainless steel (type 316) and ELAT® gas backing as a function of clamping pressure. Measured ex situ.
J. Ihonen et al. / Electrochimica Acta 46 (2001) 2899–2911
where contact resistances of two different plated current collectors and one unplated current collector were studied. All the current collectors were boiled in 1 M NaOH one hour before the measurements to form oxidised surfaces. As can be seen from Fig. 5, the plated collectors have contact resistances order of magnitude lower than the unplated current collector. The difference between the results obtained by the plated current collectors is a consequence of different plating time and solution. The plated current collector with lower contact resistances was used at the anode side in the in-situ experiments of this paper.
2905
The low contact resistances of plated current collectors were also verified by in-situ measurements. Fig. 6a,b show the difference between contact resistances measured in-situ using plated and unplated current collectors. The cell voltages measured from gas backings are rather close, while the cell voltages measures from current collectors differ significantly due to the large difference in contact resistances. The result shown in Fig. 6b was obtained 20 h after installation of polished unplated current collector. As revealed by Fig. 6a,b the contact resistances are clearly dependent on current. This dependency is fur-
Fig. 6. (a) Cell voltage vs. current density measured with plated SS 316 current collectors. Galvanostatic experiment, sweep rate 2.5 mA cm − 2 s − 1. Cell voltage measured from gas backings (GB) and current collectors (CC). Cell temperature 80°C. Gas pressures 200 kPa. Clamping pressure 0.4 MPa. Description of MEA: Catalyst loading: 0.2 mg Pt cm − 2 for the cathode, 0.1 mg Pt cm − 2 for the anode. Membrane: Nafion® 112. Area of the electrode: 2 cm2. (b) Cell voltage vs. current density measured with unplated SS 316 current collectors. Conditions as in (a).
2906
J. Ihonen et al. / Electrochimica Acta 46 (2001) 2899–2911
Fig. 7. Contact resistances as a function of current density. Data extracted from the measurement presented in Fig. 6a,b.
ther illustrated in Fig. 7, which shows the contact resistances of Fig. 6a,b separated for the anode and cathode side. This current dependency was verified by other in-situ and also by ex-situ measurements, which also showed that high heat production at the interface decreases the contact resistance, especially when unplated current collectors are used. The dependency of contact resistance on current density was discovered earlier by Miachon et al. [18]. They measured a few percents increase on contact resistances in their graphite based system over the 0 –1 A cm − 2 range. It seems that
the magnitude and direction of the change in contact resistances are dependent on the contacting materials. Even if the dependency of contact resistances on current density is not large, it may have some importance in measurements where small changes of membrane resistance are studied as a function of current density [7]. The contact resistances are also dependent on total gas pressure and the gas pressure difference over the membrane as Fig. 8 shows. The graph in Fig. 8 is divided into sections, in which the gases had different
Fig. 8. Contact resistances as a function of gas pressures. Current collector: unplated SS 316. Cell voltage measured from gas backings. Galvanostatic experiment. Current density 0.375 A cm − 2. Clamping pressure 0.4 MPa. Conditions otherwise as in Fig. 6a.
J. Ihonen et al. / Electrochimica Acta 46 (2001) 2899–2911
2907
Fig. 9. Contact resistances as a function of clamping pressure. Current collector: unplated SS 316. Cell voltage measured from gas backings. Galvanostatic experiment. Current density 0.375 A cm − 2. Gas pressures 0.2 MPa. Conditions otherwise as in Fig. 6a.
pressures. As can be seen from the curves in Fig. 8, the gas pressures affect not only the electrochemical performance of the cell but also contact resistances. When the gas pressure on one side of the cell was increased, the corresponding contact resistance at that side also increased. When the pressure at both sides was increased from 0.1 to 0.2 MPa the sum of contact resistances increased by about 15% (10 mV cm − 2). As revealed by the graph in Fig. 8 the contact resistances increased when anode gas pressure was changed from 0.2 to 0.1 MPa and back to 0.2 MPa. This is a typical phenomenon for unplated stainless steel with oxide surfaces. Every time gas or clamping pressure is changed, the contact between gas backing and current collector is changed and more surface is exposed to oxide growth, which causes the increase in contact resistances. The increase of contact resistances, as a consequence of changing contact, is revealed further by the graph in Fig. 9. The graph is divided into sections of different clamping pressure. When the clamping pressure was first increased (0.42 –0.54–0.62–0.79–0.92 MPa) the contact resistances decreased, as can be expected. After these changes the clamping pressure was completely released and the current collectors were dragged apart in the cell. When the current collectors were clamped together again with the same pressures as before (0.42 and 0.92 MPa), the sum of contact resistances was increased dramatically (50%). These irreversible changes of contact resistance by changing clamping pressure have also been discussed by Makkus et al. [12], who also noticed that different pre-treatments greatly influence on contact resistances of different stainless steel and other alloys.
The drastic increase of contact resistance does not directly affect cell performance. Fig. 10 shows the results of three overnight constant current experiments, which were made at the different points of the experiment series, in which the effects of clamping and gas pressure were studied. The electrode was the same and the experimental conditions were identical. The only difference is that between these overnight experiments clamping and gas pressures had been increased and decreased many times in other measurements. It should be noted that in this experiment the current density and corresponding heat production at the interface were not very high. At higher current densities, the large heat production on the interface may affect cell performance. The results in Fig. 10 also indicate that stainless steel current collectors have fairly stable contact resistances as long as there are no changes in those experimental parameters (temperature, gas pressure, clamping pressure) that can effect the contact between gas backing and current collector. The contact resistances of the first two experiments, given in Fig. 11, confirm this. As can be seen in Fig. 11 there is some relaxation time when there has been changes in experimental conditions. After this the resistances level off to very stable level. In the first experiment this levelling off took more time, since this experiment was performed after installing freshly polished current collectors into the cell. The kink in the second experiment at 800 min is due to water addition to humidifiers. In the literature it has been proposed that the surface treatment permanently changes the state of the surface, which could be used to achieve lower contact resistances [12,16]. We consider this conclusion premature, since our results have shown that the low contact
2908
J. Ihonen et al. / Electrochimica Acta 46 (2001) 2899–2911
Fig. 10. Results of three overnight measurements at different points in the experimental series. The contacts between current collector and gas backing were altered by changing gas pressure and clamping pressure during the series. Current collector: unplated SS 316. Cell voltage measured from gas backings (GB) and current collectors (CC). DE: a voltage difference between GB and CC. Current density 0.375 A cm − 2. Gas pressures 0.2 MPa. Conditions otherwise as in Fig. 6a.
resistances can be maintained only if the contact between current collector and gas backing is not disturbed. In practical applications, the operating fuel cell stacks will be exposed in thermal and mechanical cycling, which inevitably changes the contact between bipolar plate and gas backing. Therefore, to verify the effect of surface treatment the contact resistances should preferably be studied in dynamic conditions, either using a similar in-situ set-up to that presented in this paper or using dynamic ex-situ methods, such as the CER technique [27].
4.3. Measurement errors in in-situ contact resistance measurements Some measurement error is always induced when the contact resistance is measured in-situ. If the potential probe is applied between the active layer and current collector, as done in this study with commercial cell and also by Makkus et al. [12] and Miachon et al. [18], the current path as well as local contact pressure around the potential probe are disturbed. The error induced in measurements is probably small, if thickness and width
Fig. 11. Relaxation of contact resistances. Data extracted from the measurement presented in Fig. 10. The lower curves: Overnight experiment after installation of CC. The upper curves: overnight experiment after the first changes in gas and clamping pressure.
J. Ihonen et al. / Electrochimica Acta 46 (2001) 2899–2911
of the probe are small compared with the thickness of gas backing. However, if the installation of the probe increases the local contact pressure so much that the surface oxide is broken, a substantially larger error can be induced in measurements. An inevitable error is also induced if the potential is probed on the gas backing far from the electrode. The
2909
magnitude of this error coming from the measurement geometry is studied further here. In this small study it is assumed that current density on the electrode is constant and electrode potential as well as bulk conductivity of the gas backing are constants, too. In our present configuration, the potential of the gas backing is measured from a part of the gas backing
Fig. 12. (a) Geometry of the simulated system. P1 is a gas backing with a thickness of 0.35 mm. R1 is a poorly conducting film between the gas backing and current collector with a thickness of 0.001 mm. R2 is the beginning of the current collector. B1 is the boundary of that side of the gas backing, which touches the electrode. B2 is an equipotential surface in the current collector. E1 and E2: Places of the potential probe in measurement. (b) Results of the simulation with the following parameters: conductivity of gas backing (P1) 50 S cm − 1, conductivity of film (R1) 0.001 S cm − 1, conductivity of current collector (R2) 10000 S cm − 1, area ratio electrode/current collector (B1/B2) 0.4.
2910
J. Ihonen et al. / Electrochimica Acta 46 (2001) 2899–2911
which is outside of the active electrode area (STRIP in Fig. 3b). This is a very convenient way to measure the gas backing potential, since it is easy to install a potential probe and contact pressure between current collector and gas backing is not disturbed. However, when the potential is measured at a point which is not between the current collector and electrode, an additional error occurs due to potential distribution in gas backing. This error is small when the current collector, gas backing and active electrode all have the same area. However, when the area of the electrode is much smaller than the area of the gas backing and current collector, the corresponding error is considerably larger. If the bulk resistance of the gas backing is high and contact resistance is low, this error is sufficiently large to make quantitative in-situ contact resistant measurements invalid. This error was first studied using the pde-toolbox simulations of MATLAB (The MathWorks, Inc.). For simulations, the bulk electronic conductivity of ELAT® (50 S cm − 1) was measured by a four-point probe method. The dimensions and geometry of the simulated system are presented in Fig. 12a. P1 is the gas backing, which has a thickness of 0.35 mm, R1 is a 1 mm thin, poorly conducting film, between the gas backing and current collector. This film simulates the contact resistance. R2 is the beginning of the current collector. B1 is the boundary of that side of the gas backing, which contacts the electrode. At the boundary B1, the potential was set to 0 V. The width of boundary B1 was set to 9 mm, corresponding to an area ratio of 0.4 between the electrode and current collector. The left side of the
boundary is at the same vertical level as the current collector, simulating the situation, when the components have the same size. The right boundary is located 5 mm away from the vertical edge of the current collector, simulating the situation, when electrode area is much smaller than the current collector area. B2 is an equipotential surface in the current collector. The width of this boundary was set to 14 mm and the potential to 0.01 V. Thus the total voltage drop due to contact and bulk resistance is 10 mV. On the other boundaries, Neumann boundary condition n·(|9V)= 0 is applied, corresponding to insulated surfaces. The points E1 and E2 in Fig. 12a indicate the places in the strip of gas backing where potential could be probed in an actual measurement in our in-house cell, see Figs. 2 and 3. As can be seen from the results in Fig. 12b, the potential at the point E2 does not give the correct potential in the gas backing between the electrode and current collector. When potential from this point is used, the calculated contact resistance is underestimated by about 50%. On the other hand, potential at the point E1 gives the correct value. In this simulation, a very low value (10 mV cm2) for contact resistance was used. However, such low resistances do occur, when plated current collectors are used and therefore the possibility of this source of error can not be neglected. The geometrical error was verified experimentally by placing a MEA in the cell in such a way that the geometry was similar to the situation in the simulation. The potential probes at the cathode and anode side had positions like E1 and E2 in the simulation. The results of this experiment, given in Fig. 13, validate the results
Fig. 13. Cell voltage vs. current density measured with plated SS 316 current collectors. Description of MEA: catalyst loading: 0.2 mg Pt cm − 2 for the cathode, 0.07 mg Pt cm − 2 for the anode. Membrane: Nafion® 1035. Area of the electrode: 0.75 cm2. Conditions otherwise as in Fig. 6a.
J. Ihonen et al. / Electrochimica Acta 46 (2001) 2899–2911
of the simulation. A comparison of results from Figs. 7 and 13 shows that the measured contact resistance at the anode side was almost 50% smaller, when a smaller electrode was used. On the other hand, at the cathode side the contact resistances are almost identical. It can be concluded that if contact resistances are very low (under 10 mV cm2), the potential probe should be placed between the active electrode and current collector to reach quantitative contact resistance results. 5. Conclusions The results of this paper, obtained using our novel fuel cell hardware, give several important insights into contact resistances, which are a difficult subject in experimental polymer fuel cell research and stack development. The results show that the contact resistances are dependent on clamping pressure, gas pressure, current density and temperature. It has also been shown that when stainless steel is used as a collector material, the contact resistances are not stable, but relax to a new level after every change in operating parameters. Therefore, dynamic methods should be used in contact resistance studies. Eventually, the contact resistances of stainless steel seem to grow to unacceptable level, when cell is exposed to thermal and mechanical cycling. The contact resistances of stainless steel could be drastically reduced by platinum plating, applying a simple plating procedure. This plating seems to solve most of the problems caused by contact resistances in laboratory research, since the contact resistances of plated stainless steel surfaces are not only much lower, but also much more stable than those of unplated stainless steel surfaces. We also demonstrate by simulations and experiments that an error can appear in in-situ contact resistance measurements if the probe is inserted in the gas backing at a point outside of the actual main current path. This error can, in the worst case, make the quantitative measurements completely invalid. The stability and good reproducibility of measurements of our in-house construction has made it appropriate to now utilise it for the study of the influence of different fabrication parameters on the performance of the MEA. The possibility to distinguish the compression effect on contact resistances from other effects will also be utilised in further research. Acknowledgements The financial support of the Swedish Foundation for Strategic Environmental Research, MISTRA, is gratefully acknowledged. The work was done within the framework of the Jungner Center.
2911
References [1] O. Savadogo, J. New Mater. Electrochem. Syst. 1 (1998) 47. [2] A. Parthasarathy, S. Srinivasan, A.J. Appelby, C.R. Martin, J. Electroanal. Chem. 339 (1992) 101. [3] M. Uchida, Y. Aoyama, N. Eda, A. Ohta, J. Electrochem. Soc 142 (1995) 463. [4] M. Hogarth, P. Christensen, A. Hamnett, A. Shukla, J. Power Sources 69 (1997) 125. [5] T.V. Nguyen, J. Electrochem. Soc 143 (1996) L103. [6] M.S. Wilson, T.E. Springer, J.R. Davey, S. Gottesfeld, in: S. Gottesfeld, G. Halbert, A. Landgrebe (Eds.), Proton Conducting Membrane Fuel Cells I, The Electrochemical Society Proceedings, PV 95-23, The Electrochemical Society, Inc., Pennington, New Jersey, 1995, p. 115. [7] F.N. Bu¨ chi, G.G. Scherer, J. Electroanal. Chem. 404 (1996) 37. [8] S. Møller-Holst, PhD Thesis, NTNU, Trondheim, Norway, 1996, p. 34. [9] P.L. Hentall, J.B. Lakeman, G.O. Mepsted, P.L. Adcock, J.M. Moore, J. Power Sources 80 (1999) 235. [10] D.P. Davies, P.L. Adcock, M. Turpin, S.J. Rowen, J. Appl. Electrochem. 30 (2000) 101. [11] J.-T. Wang, J.R.F. Savinell, J. Wainright, M. Litt, H. Yu, Electrochim. Acta 41 (1996) 193. [12] R.C. Makkus, A.H.H. Janssen, F.A. de Bruijn, R.K.A.M. Mallant, J. Power Sources 86 (2000) 274. [13] D.P. Davies, P.L. Adcock, M. Turpin, S.J. Rowen, J. Power Sources 86 (2000) 237. [14] F. Mahlendorf, O. Niemzig, C. Kreutz, in: Seminar Abstracts of Fuel Cells-Powering the 21st century, The Fuel Cell Organizing Committee (United States), Portland, Oregon, 2000, p. 138. [15] D. Weng, G. Woodcock, T. Rehg, Z. Iqbal, J. Guiheen, J. Matrunich, in: Seminar Abstracts of 1998 Fuel Cells-Powering the 21st Century, The Fuel Cell Organizing Committee (United States), Portland, Oregon, 2000, p. 106. [16] C.E. Reid, W.R. Merida, G. Mclean, in: Seminar Abstracts of 1998 Fuel Cell Seminar, The Fuel Cell Organizing Committee (United States), Palm Springs, California, 1998, abstract 1174. [17] T.M. Besmann, J.W. Klett, J.J. Henry Jr, E. Lara-Curzio, J. Electrochem. Soc. 147 (2000) 4083. [18] S. Miachon, P. Aldebert, J. Power Sources 56 (1995) 31. [19] T.E. Springer, T.A. Zawodzinski, M.S. Wilson, S. Gottesfeld, J. Electrochem. Soc. 143 (1996) 587. [20] W.-K. Lee, C.-H. Ho, J.W. Van Zee, M. Murthy, J. Power Sources 84 (1999) 45. [21] M.S. Wilson, US patents no. 5 211 984 and no. 5 234 777. [22] M.S. Wilson, J.A. Valerio, S. Gottesfeld, Electrochim. Acta 40 (1995) 355. [23] S.J.C. Cleghorn, C.R. Derouin, M.S. Wilson, S. Gottesfeld, J. Appl. Electrochem. 28 (1998) 663. [24] F. Barbir, J. Braun, J. Neutzler, J. New Mater. Electrochem. Syst. 2 (1999) 197. [25] C. Zawodzinski, M.S. Wilson, S. Gottesfeld, in: Seminar Abstracts of 1998 Fuel Cell Seminar, The Fuel Cell Organizing Committee (United States), Palm Springs, California, 1998, abstract 1191. [26] M.G. Fontana, Corrosion Enginering, 3rd edn., McGrawHill, Singapore, 1987, pp. 497 – 499. [27] T. Saario, J. Piippo, Mater. Sci. Forum 185-188 (1995) 621.