Energy & Fuels 2009, 23, 397 402 397 Water Transport through a Proton-Exchange Membrane (PEM) Fuel Cell Operating near Ambient Conditions: Experimental and Modeling Studies D. S. Falcão, C. M. Rangel, C. Pinho, and A. M. F. R. Pinto*, Departamento de Engenharia Química, Centro de Estudos de Fenómenos de Transporte, Faculdade de Engenharia da UniVersidade do Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal, and Unidade de Electroquímica de Materiais, Instituto Nacional de Engenharia, Tecnologia e InoVação (INETI), Paço do Lumiar, 22 1649-038 Lisboa, Portugal ReceiVed June 22, 2008. ReVised Manuscript ReceiVed October 7, 2008 In the present work, an experimental study on the performance of an in-house -developed proton-exchange membrane (PEM) fuel cell with 25 cm 2 of active membrane area is described. The membrane/electrodes assembly (MEA), from Paxitech, has seven layers [membrane/catalyst layers/gas diffusion layers (GDLs)/gaskets]. The catalytic layers have a load of 70% Pt/C and 0.5 mg of Pt/cm 2 on both sides, and the membrane is made of Nafion 112. A multiserpentine configuration for the anode and cathode flow channels is used. Experiments were carried out under different anode and cathode relative humidities (RHs) and flow rates. Predictions from a previously developed one-dimensional model, coupling mass- and heat-transfer effects, are compared to experimental polarization curves. The influence of the anode and cathode relative humidification level on the cell performance is explained under the light of the predicted water content across the membrane. Under the operating conditions studied, the net water flux of water is toward the anode. Accordingly, the influence of the anode humidification is not significant, and the influence of the cathode humidification has a high impact in fuel cell performance. Results show that fuel cell performance is better for experiments where higher water content values were obtained. In comparison to the anode feed flow rate influence, the influence of the cathode feed flow rate has a major impact in fuel cell performance. 1. Introduction Fuel cells are an innovative alternative to current power sources, with potential to achieve higher efficiencies with renewable fuels with minimal environmental impact. In particular, the proton-exchange membrane (PEM) fuel cells (FCs) are today in the focus of interest as one of the most promising developments in power generation, with a wide range of applications in transportation and portable electronics. Although prototypes of fuel cell vehicles and residential fuel cell systems have already been introduced, their cost must be reduced and their efficiencies enhanced. Several coupled fluid flow, heat and mass transport processes occur in a fuel cell in conjunction with the electrochemical reactions. Generally, PEMFCs operate bellow 80 C. Anodic oxidation of hydrogen produces protons that are transported through the membrane to the cathode where the reduction of oxygen generates water. One of the most important operational issues of PEMFCs is the water management in the cell. 1,2 To achieve optimal fuel cell performance, it is critical to have an adequate water balance to ensure that the membrane remains * To whom correspondence should be addressed. Telephone: +351225081675. E-mail: apinto@fe.up.pt. Faculdade de Engenharia da Universidade do Porto. Instituto Nacional de Engenharia, Tecnologia e Inovação (INETI). (1) Eikerling, M.; Kharkats, Yu. I.; Kornyshev., A. A.; Volfkovrch, Yu. M. Phenomenological theory of electro-osmotic effect and water management in polymer electrolyte proton-conducting membranes. J. Electrochem. Soc. 1998, 145, 2684 2699. (2) Eikerling, M.; Kornyshev, A. A.; Kucerhak, A. R. Water in polymer electrolyte fuel cells: Friend or foe? Phys. Today 2006, 59, 38. hydrated for sufficient proton conductivity, while cathode flooding and anode dehydration are avoided. 3-5 The water content of the membrane is determined by the balance between water production and three water-transport processes: electro-osmotic drag of water (EOD), associated with proton migration through the membrane, back diffusion from the cathode, and diffusion of water to/from the oxidant/fuel gas streams. Understanding the water transport in the PEM is a guide for materials optimization and development of new membrane/ electrodes assemblies (MEAs). Recent studies 6 reported the influence of various operating conditions on fuel cell performance, such as temperatures, pressures, and humidity of reactant gases. On the basis of these investigations, the optimum conditions are operation at higher pressure and elevated temperature with the humidified reactant gases. Yan et al. 7 also studied the influence of various operating conditions, including the cathode flow rate, cathode inlet humidification temperature, and cell temperature on the per- (3) Baschuk, J. J.; Li, X. Modeling of polymer electrolyte membrane fuel cells with variable degrees of water flooding. J. Power Sources 2000, 86, 181 195. (4) Biyikoglu, A. Review of proton exchange fuel cell models. Int. J. Hydrogen Energy 2005, 30, 1185 1212. (5) Chang, H.; Kim, J. R.; Cho, S. Y.; Kim, H. K.; Choi, K. H. Materials and processes for small fuel cells. Solid State Ionics 2002, 8312. (6) Amirinejad, M.; Rowshanzamir, S.; Eikani, M. H. Effects of operating parameters on performance of a proton exchange membrane fuel cell. J. Power Sources 2006, 161 (2), 872 875. (7) Yan, W. M.; Chen, C. Y.; Mei, S. C.; Soong, C. Y.; Chen, F. Effects of operating conditions on cell performance of PEM fuel cells with conventional or interdigitated flow field. J. Power Sources 2006, 162 (2), 1157 1164. 10.1021/ef8004948 CCC: $40.75 2009 American Chemical Society Published on Web 12/09/2008
398 Energy & Fuels, Vol. 23, 2009 Falcão et al. Figure 1. Schematic representation of the experimental setup. formance of a PEMFC. Experimental results showed that cell performance is enhanced with increases in cathode inlet gas flow rate, cathode humidification temperature, and cell temperature. There are few studies on PEMFCs with a multiserpentine flow channel configuration. Li et al. 8 indicated that this design ensures adequate water removal by the gas flow through the channel and no stagnant area formation at the cathode surface as a result of water accumulation. Watkins et al. 9 reported that, under the same experimental conditions, the output power from the cell could be increased by almost 50% with this type of flow-field plate. In this work, the effect of cathode and anode flow rates and relative humidity on the performance and power of a PEMFC with multiserpentine channels is studied and some results are explained and compared to the predictions of a recently developed 1D model. 10 2. Experimental Section 2.1. Apparatus. A schematic drawing of the experimental apparatus used in this work is shown in Figure 1. Pure hydrogen (humidified or dry) as fuel and air (humidified or dry) as an oxidant are used. The pressure of the gases is controlled by pressure regulators (air, Norgreen 11400; H 2, Europneumaq mod. 44-2262- 241) and flow rates controlled by flow meters (KDG, Mobrey). The reactants humidity and temperatures are monitored by adequate humidity and temperature probes (air, Testo; H 2, Vaisala). (8) Li, X.; Sabir, I. Review of bipolar plates in PEM fuel cells: Flowfield designs. Int. J. Hydrogen Energy 2005, 30, 359 371. (9) Watkins, D. S.; Dircks, K. W.; Epp, D. G. U.S. Patent 5,108,849, 1992. (10) Falcão, D. S.; Oliveira, V. B.; Rangel, C. M.; Pinho, C.; Pinto, A. M. F. R. Water transport through a PEM fuel cell: A one-dimensional model with heat-transfer effects. Chem. Eng. Sci., manuscript submitted. Figure 2. Flow channel configuration and dimensions. The humidification of air and hydrogen gases is conducted in Erlenmeyer flasks by a simple bubbling process. To control the humidification temperature, each Erlenmeyer flask is thermally isolated and surrounded with an electrical resistance (50 W/m) activated by a Osaka OK 31 digital temperature controller. The same procedure is applied along the connecting pipes from the humidification point up to the entrance of the fuel cells to guarantee the temperature stabilization of each reacting gas flow as well as to control the operating temperature of the fuel cell. For the measurement and control of the cell electrical output, an electric load reference LD300 300W DC electronic load from TTI is used. This device could work with five different operating modes: (1) constant current, two possibilities were available, 0-8 A (with 1 ma resolution) and 0-80 A (10 ma resolution), with a precision of (0.2% + 20 ma; (2) constant voltage, two possibilities were available, V min upto8v(1maresolution) and V min up to 80
Water Transport through a PEM Fuel Cell Energy & Fuels, Vol. 23, 2009 399 Figure 3. Voltage versus current density for the base condition, experimental results and model predictions. Table 1. Set of Conditions Used in This Work cell temperature (K) 298 anode flow temperature (K) 313 anode relative humidity (%) 70 cathode flow temperature (K) 313 cathode relative humidity (%) 70 anode pressure (atm) 1.2 cathode pressure (atm) 2 anode flow rate (slpm) 0.15 cathode flow rate (slpm) 0.7 Table 2. Inlet Water Concentrations anode inlet water concentration (mol/cm 3 ) experience 1 (base condition) 2.0 10-6 2 (anode T ) 298 K; 9.8 10-7 RH ) 76%) 3 (anode T ) 298 K; 9.0 10-8 RH ) 7%) Table 3. Inlet Water Concentrations cathode inlet water experience concentration (mol/cm 3 ) 1 (base condition) 2.0 10-6 2 (cathode T ) 298 K; 1.2 10-6 RH ) 94%) 3 (cathode T ) 298 K; 1.3 10-8 RH ) 1%) cathode inlet water concentration (mol/cm 3 ) 2.0 10-6 anode inlet water concentration (mol/cm 3 ) 2.0 10-6 V [10 ma resolution (where V min is 10 mv for a low-power situation and 2 V for 80 A), with a precision of (0.2% + 2 digits; (3) constant power, the available power range goes from 0 to 320 W, with a precision of 0.5% + 2 W; (4) constant conductance, operating range from 0.01 up to 1 A/V (1 A/V resolution) and from 0.2 up Figure 5. Membrane temperature versus current density for different cell temperatures, model predictions. to 40 A/V (resolution of 0.01 A/V), with a precision of 0.5% + 2 digits; and (5) constant resistance, operating range from 0.04 up to 10 Ω (0.01 Ω resolution) and from 2 to 40 Ω (with 0.1 Ω resolution), with a precision of 0.5% + 2 digits. This load was connected to a data acquisition system composed by Measurement Computing boards installed in a desktop computer. The used data acquisition software was DASYLab. 2.2. Fuel Cell Design. In the present work, all of the components of the PEMFC were in house -designed, with exception of the MEA. A Paxitech seven-layer MEA (Nafion 112) with 25 cm 2 active surface area is used. The channel configuration used for the anode and cathode flow channels is represented in Figure 2. The channel depth is 0.6 mm for the hydrogen flow and 1.5 mm for the air flow. 2.3. Experimental Conditions. In this work, a set of conditions was used as the base condition. Using this set of conditions and changing one variable, it is possible to evaluate the influence of this parameter on the cell temperature. The studied operating conditions were the cell temperature, anode humidification, cathode humidification, anode flow rate, and cathode flow rate. The base conditions are summarized in Table 1. Two experiments were performed at two different cell temperatures, 298 and 313 K. To study the influence of the anode/cathode humidification, dry hydrogen/air was introduced (to achieve lower humidity levels) and hydrogen/air was introduced at room temperature (to achieve intermediate humidity levels). In another set of experiments, the anode and cathode flow rates were set to double the base values. For each one of the studied conditions, the inlet water concentration at both sides of the cell was accurately determined using the relative humidity and inlet temperature values. 3. Results and Discussion In a previous work, Falcão et al. 10 developed a semi-analytical one-dimensional model considering the effects of coupled heat and mass transfer, along with the electrochemical reactions occurring in a PEMFC. The model can be used to predict the Figure 4. (a) Voltage versus current density and (b) power density versus current density for two different cell temperatures.
400 Energy & Fuels, Vol. 23, 2009 Falcão et al. Figure 6. (a) Voltage versus current density and (b) power density versus current density for different anode humidifications. Figure 7. Water content (λ) along the membrane for different values of anode humidification (current density of 0.1 A/cm 2 ), model predictions. hydrogen, oxygen, and water concentration profiles in the anode, cathode, and membrane as well as to estimate membrane water contents and the temperature profile across the cell. In this work, the developed model is used to predict the polarization curve for the base-operating conditions (Table 1). The model predictions and experimental results are compared in Figure 3. For low current densities, the model predicts very well the experimental results. For higher densities, the model predictions are higher than experimental results. This discrepancy is a common feature of single-phase models because the effect of reduced oxygen transport because of water flooding at the cathode at high current density is not accounted for. Model predictions are also useful to better understand experimental results. The membrane water content is a good indicator of membrane humidification and is easily calculated using this simple one-dimensional model. In this work, model Figure 9. Water content (λ) along the membrane for different values of cathode humidification (current density of 0.1 A/cm 2 ), model predictions. predictions of the membrane water content and temperatures are used to explain some experimental results. 3.1. Fuel Cell Temperature. In Figure 4, the polarization and power curves obtained in two experiments with different cell temperatures (298 and 313 K) are presented. In this range of low cell temperature operation, the influence on fuel cell performance is minimal. This range of temperatures was selected, bearing in mind the portable applications (excluding the use of heating equipment). In Figure 5, the predicted variation of membrane temperature with current density is presented for both fuel cell temperatures. As expected, the cell temperature increases with current density because of the cathode exothermic reaction (higher currents correspond to higher amounts of heat released). Although the cell temperature is different for the two experiments (15 K variation), the temperature profile through the membrane is quite similar (differences of 3 K). According to Figure 8. (a) Voltage versus current density and (b) power density versus current density for different values of cathode humidification.
Water Transport through a PEM Fuel Cell Energy & Fuels, Vol. 23, 2009 401 Figure 10. (a) Voltage versus current density and (b) power density versus current density for different anode feed flow rates. Figure 11. (a) Voltage versus current density and (b) power density versus current density for different cathode feed flow rates. these results, it is predictable that the two conditions lead to similar fuel cell performances, as shown in Figure 4. 3.2. Anode Humidification. Experiments with different anode relative humidification levels were performed (Table 2). The corresponding polarization and power curves are plotted in Figure 6. For the used MEA, the manufacturer indicates that there is no need to humidify the anode stream. As is evident from the plots of Figure 6, the influence of the anode humidification on the performance of the cell is not significant. These results are in agreement with the MEA manufacturer specifications. Such cell behavior could be useful for portable applications because the use of a humidifier for the anode stream could be avoided. The model predictions of the water content in the membrane trough parameter λ (the ratio of the number of water molecules to the number of charged SO 3 - H + sites) are presented in Figure 7 for the same three experiences. For the conditions studied, the net flow of water is toward the anode. For these conditions, the amount of water is higher for the cathode side because of the importance of water transport by electro-osmotic drag and water generation by the reaction. As can also be seen from the plots, the water content near the anode catalyst layer is lower for the three curves, in particular for the less humidified anode. These results are in accordance with experiments. The water content is similar, and consequently, the cell performance is similar too. 3.3. Cathode Humidification. To analyze the cathode humidification influence, two experiments were performed and compared to the results obtained with the base condition (Figure 8). The different values of the inlet water concentrations determined are presented in Table 3. In contrast to the case analyzed previously, the cathode humidification level has a significant impact on the fuel cell performance. As indicated above, the water management is a critical issue. Water acts like a proton shuttle in the membrane and catalyst layers because excessive water amounts filling the pores inhibit the access to active sites and block the transport of gaseous reactants and products. On the contrary, dehydration of anodic regions because of electro-osmotic drag can cause a breakdown of proton conductivity and even a structural degradation of the PEM. The membrane must therefore have an ideal humidification level to achieve optimal performances. The plot of the predicted values of the water content across the membrane for the same three experiences (corresponding to a current density of 0.1 A/cm 2 ) is shown in Figure 9 and contributes to a better explanation of the results shown in Figure 8. These values are in agreement with experiments since the intermediate water content value (experiment 2) leads to the best performance (Figure 8). For this condition, the mean water content through the cell is higher corresponding to an enhanced proton conductivity and consequently a better performance. No predicted anode dehydration occurs for all of the studied conditions. 3.4. Anode Feed Flow Rate Influence. For the base operation condition, the hydrogen flow rate used corresponds to a stoichiometric ratio of 1, at 1 A/cm 2 (a hydrogen flow rate sufficient even for current densities up to 25 A). An experiment with a stoichiometric ratio of 2 was also performed. The results for both conditions are represented in Figure 10. As shown in Figure 10, the hydrogen flow rate increase has no significant influence on the fuel cell performance. These results are expected since, for both conditions, the hydrogen flow rate is largely in excess, even for high values of the current density. 3.4. Cathode Feed Flow Rate Influence. For the base condition, the air flow rate corresponds to a stoichiometric ratio of 3, at 1 A/cm 2. An experiment with a cathode stoichiometric ratio of 6 was performed to check the influence of increasing the air flow rate on the cell performance. The results are presented in Figure 11.
402 Energy & Fuels, Vol. 23, 2009 Falcão et al. As expected, the air flow rate increase improves fuel cell performance probably to an enhanced water removal. The improvement in the fuel cell performance is more significant for higher current densities because of the more pronounced formation of water at these conditions. Consequently, it is advantageous to work with higher air flow rates when using humidified cathode feeds, namely, for high current densities. 4. Conclusions In the present study, an experimental study on the performance of an in-house -developed PEM fuel cell with 25 cm 2 of active membrane area is described. A multiserpentine configuration for the anode and cathode flow channels was used. Experiments were carried out under different anode and cathode RHs and flow rates. The influence of the anode and cathode relative humidification level on the cell performance is explained under the light of the predictions of water content across the membrane from a recently developed model. Under the operating conditions studied, the net water flux of water is toward the anode and, accordingly, the influence of the anode humidification is not significant. These results are in accordance with the specifications of the manufacturer. The cathode humidification has a more important impact on the cell performance probably because of a more significant effect on the proton conductivity. An enhanced performance was obtained for the condition where a higher water content was obtained probably because of a better proton conductivity. The influence of the anode and cathode feed flow rates was also studied. This work is the starting point for a more detailed study, aiming at the setup of optimized and tailored MEAs adequate for different applications (namely, low-humidity operation). Acknowledgment. The partial support of Fundação para a Ciência e TecnologiasPortugal through project POCI/EME/55497/ 2004 is gratefully acknowledged. POCTI (FEDER) also supported this work via CEFT. EF8004948