International Journal of Hydrogen Energy 33 (2008) 165 – 170 www.elsevier.com/locate/ijhydene Effects of membrane electrode assembly components on proton exchange membrane fuel cell performance Ayşe Bayrakçeken a,∗ , Serdar Erkan a , Lemi Türker b , İnci Eroğlu a a Department of Chemical Engineering, Middle East Technical University, 06531 Ankara, Turkey b Department of Chemistry, Middle East Technical University, 06531 Ankara, Turkey Received 25 March 2007; received in revised form 7 August 2007; accepted 24 August 2007 Available online 10 October 2007 Abstract The objective of this study is to determine the effects of various factors on the performance of proton exchange membrane (PEM) fuel cell. These factors are membrane thickness, hot-pressing conditions of the gas diffusion layer (GDL) either onto the membrane or membrane electrode assembly (MEA) and Teflon:carbon ratio in the GDL on PEM fuel cell performance. Homemade five-layer and commercial threelayer MEAs were used in the experiments. Nafion䉸 112 and 115 which have nominal thicknesses of 50 and 125 m, respectively, were used as membranes. It was observed that fuel cell performance is inversely proportional to membrane thickness. In the case of five-layer MEAs, optimum hot-pressing conditions of catalyst-coated GDLs onto the membrane were found as 172 N cm−2 . However, the maximum performance for three-layer MEAs was obtained with no press conditions. Also, by increasing Teflon:carbon ratio in the GDLs, PEM fuel cell performance increases up to a certain value, but further increase of this ratio worsen the performance. 䉷 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. Keywords: PEM fuel cell; Gas diffusion layer; Membrane electrode assembly 1. Introduction Fuel cells are considered to be the most efficient alternative energy conversion devices of the 21st century [1]. Because of their higher efficiency, simple design and environmental friendly operations they have a wide range of applications. There are several types of fuel cells, but proton exchange membrane (PEM) fuel cells are the most suitable ones for transportation and portable applications because of their low operation temperature, high energy density and efficiency [2,3]. The performance of PEM fuel cells is known to be influenced by many parameters depending on either operating conditions or construction materials which influence the activation, ohmic and mass transport losses [4,5]. Obviously, the performance of a PEM fuel cell can be improved by decreasing these losses. ∗ Corresponding author. Tel.: +90 312 2104372; fax: +90 312 2102600. E-mail address: aybayrak@metu.edu.tr (A. Bayrakçeken). Also, water management is very critical for PEM fuel cell performance. If the membrane is not perfectly humidified, the membrane resistance increases. Consequently, ohmic losses built up which causes some performance loss. To provide the required humidification condition, reactant gases are to be humidified. In the cell, water can be transported in two distinct ways: electro-osmotic flow from anode to cathode and back diffusion from cathode to anode. The excess water accumulated at the cathode side is due to electro-osmotic flow (water can be transported with the protons) and the oxygen reduction reaction. If there is a lack of humidity at the anode side, the concentration gradient develops and back diffusion of water from cathode to anode occurs [6,7]. Hence, hydrophobicity of the gas diffusion layers (GDLs) and also membrane thickness are parameters that have to be arranged to obtain higher performances. The role of the GDLs in a PEM fuel cell is to transfer the electrons between the electrodes and the bipolar plates and to distribute reactant gases which helps the complete utilization of the electrode area. The carbon content in these layers helps to 0360-3199/$ - see front matter 䉷 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2007.08.021 166 A. Bayrakçeken et al. / International Journal of Hydrogen Energy 33 (2008) 165 – 170 MEA Preparation Techniques Application of catalyst to membrane Application of catalyst to GDL Mode 1 Preparation of catalyst solution Mode 2 Preparation of catalyst GDL Preparation of catalyst solution Spraying Spraying Catalysed Teflon blanks Catalysed GDL Membrane Teflon blanks Membrane MEA Pressing Pressing MEA1 MEA2 Fig. 1. MEA preparation techniques. collect current by transporting the electrons whereas hydrophobic polytetrafluoroethylene (PTFE) content supports the water management [8,9]. The performance may also be affected by the changes in the structure of GDL (porosity, Teflon:carbon ratio etc.), the electrical contact resistance, and the excluded water at the membrane [10]. It has been shown that if the porosity of the GDL increases then the amount of oxygen consumed also increases, thus higher current densities and a better performance is obtained. Therefore, if the cathode is flooded by excess water, because of lower effective porosity, the performance of the PEM fuel cell decreases [11]. There are several ways to prepare MEAs which is the most important part of the fuel cell where diffusion of the reactant gases, half cell reactions and current collection take place. The active layer of MEA can be obtained by applying catalyst ink directly onto the membrane (three-layer MEA) or onto the GDL (five-layer MEA) [12,13]. The procedures for MEA preparation are given in Fig. 1. In the present work, firstly the effects of anode and cathode humidification temperatures have been investigated and then the fuel cell has been operated by using these data. Then, fivelayer MEAs were prepared with the membranes which had different thicknesses and GDLs having different Teflon:carbon ratios. Also, the effects of hot-pressing conditions of the catalyst-coated GDLs onto the membrane and uncoated GDLs onto the commercial three-layer MEA have been investigated. For that purpose, a commercially available PEM fuel cell hardware was used and the tests were performed by means of a homemade fuel cell test station. 2. Experimental The membrane electrode assemblies (MEAs) were prepared by spraying catalyst ink onto the GDLs. GDLs (GDL 31 BA and GDL 31 BC) obtained from SGL Carbon (Germany) were used. The untreated GDL 31 BA type gas diffusion layer was treated with Teflon and carbon both dispersed in n-heptane. The platinum loading was 0.4 mg Pt cm−2 , whereas Nafion loading was set to 1.2 mg Nafion cm−2 in catalyst ink. After spraying the catalyst ink onto the GDL a five-layer MEA was prepared by pressing these GDLs onto the treated membrane at 130 ◦ C, 688 N cm−2 for 3 min. As three-layer MEA, a commercially available OMG C-10 MEA was used. In the case of hot-pressing condition experiments, the pressure of the hot press was varied between 688 and 172 N cm−2 . A torque wrench was used to adjust the torque about 1.7 Nm diagonally on each bolt. As gasket, a 0.2 mm silicone was used. Nafion䉸 112 and 115, each of which has a nominal thickness of 50 and 125 m, respectively, were used as the membrane. In the experiments, a commercially available PEM fuel cell hardware (Electrochem, FC05-01 SP REF) was used where electrode active area was 5 cm2 . The fuel cell tests were performed in a homemade fuel cell test station which is shown in Fig. 2. In order to access the required power, reactant gas flow rates were adjusted with mass flow controllers (Aalborg GFC 171). Prior to entering the fuel cell, these gases were humidified by passing them through the water columns contained in stainless steel A. Bayrakçeken et al. / International Journal of Hydrogen Energy 33 (2008) 165 – 170 167 Fig. 2. Schematic representation of the homemade PEM fuel cell test station. cylinders and heated with resistance heaters. The stainless steel gas lines were heated to prevent the condensation of the water in the lines. The temperature in the humidifiers, gas lines and fuel cell were controlled by temperature controllers. The purge gases left through the water columns placed at the exit of the fuel cell. A multimeter (Brymen BM 857) connected to the computer was used to measure and log the current. The voltage was measured by the help of an AD converter (Advantech PCL-711) also connected to the fuel cell and a computer. A variable resistor was used to alter the electrical load across the fuel cell to obtain the performance curves at the steady state. Polarization curves were obtained after reaching the steady state by using a certain protocol. Due to this protocol, the fuel cell was operated for 8 h at 0.5 V. Then the data were collected at time intervals of 15 min until no more change was observed in the performance. Hydrogen and oxygen gases were used as reactant gases in the performance tests where flow rates were set to 0.1 slm for both gases. Cell temperature was set to 70 ◦ C in all the experiments. The cathode humidification temperature was changed between 50 and 80 ◦ C while keeping the cell and anode humidification temperatures at 70 ◦ C. In the anode humidification temperature experiments, the temperature was varied between 60 and 80 ◦ C, also keeping the cell temperature at 70 ◦ C and the cathode humidification temperature at 30 ◦ C. 3. Results and discussion 3.1. Effect of anode and cathode humidification temperatures on PEM fuel cell performance As can be seen from Fig. 3, for lower current densities, the anode humidification temperature has no significant effect on the PEM fuel cell performance. But for higher current densities, the performance is significantly affected. Although the anode humidification temperature was high, the cathode humidification temperature could not be sufficient for membrane hydration. Also, flooding at the anode side was not observed at the anode humidification temperature of 80 ◦ C which was higher than fuel cell operating temperature. It could have happened because of two reasons: (i) the cathode humidification temperature was 30 ◦ C, so back diffusion did not occur, (ii) the dehydration of the anode side; because of the electro-osmotic drag was compensated due to the high anode humidification temperature. But at higher current densities, the performance was improved which could be attributed to higher water production at the cathode which compensated the lack of humidity at the cathode side. From Fig. 4, it was observed that cathode humidification temperature had a positive effect on the performance up to 70 ◦ C, which was equal to the fuel cell temperature. For lower current densities, the performance increased with increasing the 168 A. Bayrakçeken et al. / International Journal of Hydrogen Energy 33 (2008) 165 – 170 0.8 Potential, V 0.25 1.0 0.20 0.8 0.15 0.6 0.4 0.10 0.2 0.05 Nafion 115 Nafion 112 80°C Potential, V 70°C Power density, W cm-2 60°C 0.25 0.20 0.6 0.15 0.4 0.10 0.2 0.05 0.0 0 100 200 300 0 0.00 500 0.0 400 0.30 Power density, W cm-2 1.0 100 200 300 400 500 600 0.00 700 Current density, mA cm-2 Current density, mA cm-2 Fig. 3. Effect of anode humidification temperature on PEM fuel cell performance, Tcell = 70 ◦ C, Thumidifier,Cathode = 30 ◦ C. Fig. 5. Effect of membrane thickness on PEM fuel cell performance, Tcell = Thumidifier,Anode,Cathode = 70 ◦ C. 0.7 0.20 60°C 0.18 80°C Potential, V 0.14 0.12 0.6 0.10 0.08 0.4 0.06 Power density, W cm-2 0.16 0.8 0.04 0.2 0.02 0.00 0.0 0 100 200 300 400 Potential, V 50°C 70°C 517 N cm-2 172 N cm-2 1.0 344 N cm-2 No press 0.6 0.8 0.5 0.6 0.4 0.3 0.4 0.2 0.2 Power density, W cm-2 1.0 0.1 0.0 0 200 400 600 800 1000 0.0 1200 Current density, mA cm-2 Fig. 6. Effect of hot-pressing conditions of GDLs onto the commercial three-layer MEA on PEM fuel cell performance. Current density, mA cm-2 Fig. 4. Effect of cathode humidification temperature on PEM fuel cell performance, Tcell = 70 ◦ C, Thumidifier,Anode = 70 ◦ C. cathode humidification temperature. In this region, the dehydration of the anode side (because of electro-osmotic drag) was compensated by back diffusion [14]. But for higher current densities flooding occurs due to the higher water production and this prevents the mass transport of oxygen resulting in performance loss. This sharp decrease in the performance was also observed at current density of approximately 280 mA cm−2 . electro-osmotic drag at the anode side, the membrane dried. Since the anode and cathode humidification temperatures were identical, back diffusion occurs because of the water production at the cathode side. However, in thinner membranes, back diffusion of water was enough to counteract the anode drying effect [14]. In the literature, it was also reported that for Nafion 112 and 115 the average membrane resistances were approximately 65 and 150 m cm2 , respectively [15]. Because of these ohmic losses, at 0.6 V for Nafion 115 and 112 current densities were 220 and 280 mA cm−2 , respectively. 3.2. Effect of membrane thickness on PEM fuel cell performance 3.3. Effect of hot-pressing conditions of the GDL onto commercial three-layer MEA on PEM fuel cell performance It was observed from Fig. 5 that the thinner the membrane the better the performance was. In this case, because the resistance to mass transfer of the protons decreases, the flux increases. Moreover, water management may also get easier in thinner membranes. While experimenting, because of the In these experiments, commercial three-layer MEA (OMG C-10) was used. It was observed that, in case of three-layer MEA, pressing of GDL onto the MEA causes performance losses. From Fig. 6, it was seen that for three-layer MEA without pressing of GDL the maximum performance was achieved. 169 A. Bayrakçeken et al. / International Journal of Hydrogen Energy 33 (2008) 165 – 170 688 N cm-2 0.25 0.8 0.20 0.6 0.15 0.4 0.10 0.2 Electrode Teflon:carbon ratio Tafel slope (mV dec−1 ) 1 2 3 4 3:2 3:7 2:3 1:1 59 56 52 51 0.05 0.0 0 100 200 300 400 500 600 0.35 1.0 0.00 700 Current density, mA cm-2 2 4 1 3 0.8 0.30 Since, the sufficient contact between the catalyst and the membrane in three-layer MEA is provided, further pressing of the GDL may change the structure of the GDL [16]. This can affect the electrical conductivity of the GDL and may also cause the changes in hydrofobicity which results in poor water management. In these experiments, it was observed that if the GDLs were not pressed onto the membrane, at 0.6 V approximately 1200 mA cm−2 current density value can be obtained. 3.4. Effect of hot-pressing conditions of the catalyst coated GDL onto the membrane on PEM fuel cell performance The effects of hot-pressing conditions on the performance of homemade five-layer MEAs were also investigated. From Fig. 7, it can be seen that there is a significant increase in current density when the hot-pressing condition was decreased from 688 to 172 N cm−2 . When GDLs were hot-pressed onto the membrane over an optimum value, performance decreased sharply because of an increase in ohmic resistance. The current densities obtained at 0.6 V were approximately 500 and 280 mA cm−2 for 172 and 688 N cm−2 hot-pressing conditions, respectively. For three-layer MEA, maximum performance was obtained when the GDLs were not pressed onto the membrane. But in the case of five-layer MEA, with no press, the required contact between the membrane and the electrodes could not be provided, so the performance was worse than the case of hotpressing conditions of 172 N cm−2 . From these experiments, it was found that to achieve the maximum performance either in the three- or five-layer MEAs the hot-pressing conditions were very crucial and had to be optimized. 3.5. Effect of Teflon:carbon ratio in the GDL on PEM fuel cell performance In this part of experimentation, the GDLs (GDL 31 BA) were treated with an admixture of Teflon, carbon and n-heptane. This mixture was sprayed on the GDLs and then heat-treated. Potential, V 0.25 Fig. 7. Effect of hot-pressing conditions of catalyst coated GDLs onto the membrane on PEM fuel cell performance. 0.6 0.20 0.15 0.4 0.10 0.2 Power density, W cm-2 Potential, V 0.30 No press 1.0 Table 1 Teflon:carbon ratios and Tafel slopes (at high potentials) of the prepared electrodes 0.35 172 N cm-2 Power density, W cm-2 1.2 0.05 0.0 0 100 200 300 400 500 600 0.00 700 Current density, mA cm-2 Fig. 8. Effect of Teflon:carbon ratio in the GDL on PEM fuel cell performance, Tcell = Thumidifier,Anode,Cathode = 70 ◦ C. Table 1 shows the Tafel slopes and Teflon:carbon ratios of the electrodes. From Fig. 8, was seen that at lower current densities there is not a significant performance difference between the electrodes 1, 2, 3 and 4. But it was observed that for higher current densities the difference becomes significant. The performance decreases sharply for both the lower and higher current density regions for electrode-1 which has the highest Teflon:carbon ratio in the GDL. As the Teflon:carbon ratio increases, the ohmic resistance increases because the amount of non-conducting Teflon content increases which causes the internal resistance to raise up as can be seen from Table 1. Also the Teflon:carbon ratio affects the porosity of the GDLs which results in the performance losses due to the diffusion limitations. The Tafel slopes obtained for high potential regions range between 50 and 60 mV dec−1 . It was reported that at high potentials and high coverages, the Tafel slope was 60 mV dec−1 and reaction order is 23 with respect to H+ activity [17]. This behaviour is a result of intermediate coverage of oxygenated species (–O and –OH and –O2 H) derived from water oxidation. In this region, the coverage of these species is linearly potential and pH dependent, and can be described by Temkin adsorption [17]. Since the same catalyst was used at the anode and cathode electrodes of the prepared GDLs, the Tafel slopes obtained for high potential region gave similar results. The mechanism of oxygen reduction where the reaction involves an initial fast charge transfer step followed by a chemical step corresponds to a Tafel slope of 60 mV dec−1 170 A. Bayrakçeken et al. / International Journal of Hydrogen Energy 33 (2008) 165 – 170 and higher values of Tafel slopes are assumed for low oxygen concentration, due to a mixed activation/mass transport control [18]. 4. Conclusion The experiments have indicated that membrane thickness, hot-pressing conditions of GDL onto either three-layer MEA or membrane, and the Teflon:carbon ratio in the GDL are significant parameters to provide a good performance in the cell. In the experiments, it was observed that Nafion 112 gave better performance results than Nafion 115 which was thicker than Nafion 112. It was also found that, hot-pressing of GDL onto the three-layer MEA decreases the fuel cell performance and the best performance was achieved when the GDLs were not pressed onto the MEA. But in the case of five-layer MEA, there was an optimum value for the hot-pressing conditions, which was 172 N cm−2 , and gave the best performance result. Also, polarization curves indicate that by increasing the Teflon:carbon ratio in the GDLs, PEM fuel cell performance increases up to a certain value, but further increase of this ratio worsen the performance. 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