Effect of water transport properties on a PEM fuel cell operating with dry hydrogen

Transcription

1 Electrochimica Acta 51 (2006) Effect of water transport properties on a PEM fuel cell operating with dry hydrogen Yinghua Cai a,b, Jun Hu a, Haipeng Ma a,b, Baolian Yi a,, Huamin Zhang a a Fuel cell R&D Center, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian , PR China b Graduate School of Chinese Academy of Sciences, Beijing , PR China Received 28 February 2006; received in revised form 13 April 2006; accepted 15 April 2006 Available online 5 June 2006 Abstract In this work, membrane resistance measurement and water balance experiment were implemented to investigate the feasibility for a PEM fuel cell operating with dry hydrogen. The results showed that when a thin membrane was used in a cell the performance and the membrane resistance changed a little while the anode humidity changed from saturated to dry. Comparing with the anode humidity, the influence of the cathode humidity was serious on the cell performance. The water balance experiments showed that the net water transport coefficient was negative even the anode was humidified and liquid water existed not only in the cathode but also in the anode. High cathode humidity was disadvantage for the removal of water both in the anode and the cathode Elsevier Ltd. All rights reserved. Keywords: Proton exchange membrane fuel cells; Water transport; Net water transport coefficient; Membrane resistance 1. Introduction Comparing with other non-conventional power sources, the proton exchange membrane fuel cell (PEMFC) is considered to be the most promising alternative for automobiles, stationeries and portable applications for its low-temperature operating, zero-emission and high-power density [1 4]. However, in order to make large-scale application possible, its durability and stability still need to be further enhanced and the weight of the system needs to be reduced. One of the major subjects for obtaining high power density PEMFC system is the optimization of the water management in the anode and the cathode. It is known that the proton conductibility in polymer electrolyte membranes (PEMs) such as Nafion (Du Pont) depends on the water content and the water plays an important role for the performance of PEMFCs. Though, external humidification of feed gas is a conventional method to ensure retention of water in a PEM, it requires a large installation space and heat supply for the humidifier, which are burdens for a PEMFC system. Thus minimal use of an external humidifier and increase the Corresponding author. Tel.: ; fax: address: blyi@dicp.ac.cn (B. Yi). efficiency of internal humidification is desirable for a PEMFC system. As it is known that water transport in a PEM is dominated by three transport mechanisms, namely, electro-osmotic drag, back diffusion and convection generated by pressure gradient. The convection flow could be ignored if there is no pressure difference between the anode and the cathode [5,6]. Electro-osmotic drag results from the migration of proton from the anode to the cathode. Back diffusion is caused by the water concentration gradient between the cathode and the anode, and the transport direction is from the cathode to the anode. So it is possible for a PEMFC operated without anode external humidification. In this case, the back transport of water is a dominant process. In the past decades, the water transport process in a PEMFC has been the subject of several modeling studies [7 15]. The back flow of water in the membrane has been treated either by a solution water diffusion model [8 11] or a convective water transport model [12,13]. Weber and Newman [14 16] developed a combination model to simulate the water content distribution and water transport across the membrane. Recently, two-phase CFD models based on the commercial CFD package are prevailing [17,18]. It is often considered that the net amount of the water transport is from the anode to the cathode while it is not always the case. Some experimental studies on the water /$ see front matter 2006 Elsevier Ltd. All rights reserved. doi: /j.electacta

2 6362 Y. Cai et al. / Electrochimica Acta 51 (2006) transport process and its influence on the cell performance have been also reported. Compared with other methods, such as in situ neutron imaging technique, [19 21] and transparent PEMFC method [22,23] the results gotten by water balance experiment [24 30] could give a quantitative analysis on the water transport across the MEA. In the present work, membrane resistance and water balance experiment were compared under different humidity condition to investigate the feasibility and the performance of a fuel cell operating with dry hydrogen. The change of ohmic drop over the membrane and the net water transport coefficient were discussed. The water transport properties and the performance of a PEMFC under different humidification were studied as well. 2. Experimental details Two types of membrane, Nafion 112 and Nafion 115 were pretreated according follows: (1) in 3% H 2 O 2 kept boiling for 1 h and (2) in 1 M H 2 SO 4 at 353 K for 1 h. Before and after each step they were washed with de-ion water for 30 min. The anode and the cathode electrodes used in this study were homemade, consisting of a backing layer (SGL TECHNIK, PE704), a micro-porous layer and a catalyst layer. The PTFE content in the backing layer was 30 wt.% to form hydrophobic channels for gas. The Pt loading in the catalyst layer was 0.3 mg cm 2 (30 wt.% Pt/C from TANAKA) and 0.7 mg cm 2 (45.5 wt.% Pt/C from TANAKA) for the anode and the cathode, respectively. The pretreated membrane was sandwiched between the anode and the cathode, hot pressed at 10 MPa, 413 K for 1 min to get the membrane electrode-assembly (MEA). The MEA was matched with flexible graphite flow field with 48 straight channels, the geometry parameter of the channel was 140 mm 0.8 mm 0.5 mm. The active area of the MEA was 128 cm 2. Hot water with constant temperature was injected into the cooling plate to regulate the cell temperature. The inlet velocity of the hot water was controlled to maintain the temperature difference between the inlet and the outlet was less than 0.5 K. Feed gas was humidified through bubbling in humidifier. The relative humidity of the inlet gas was controlled through changing the temperature of the humidifier. The temperature was measured by platinum resistance thermometer with an accuracy of ± 0.5 K. The cell temperature was kept at 333 K. The pipes between the humidifier and the fuel cell were heated to 353 K to avoid water vapor condensation. The fuel cell was supplied with pure hydrogen and air at 2 atm (absolutely). The stoichiometry of hydrogen and air was 1.1 and 2.5, respectively. The flow mode was co-flow. The membrane resistance was measured with the Arbin instrument. In order to collect the water in the exhaust, every outlet was connected to a stainless-steel case kept in ice bath to condense water vapor. Then the exhaust passed through a vessel containing silica gel to absorb the water vapor. The case and the vessel were weighed before and after the experiment. The relationship of the gas flow rate, the temperature of the humidifier and the relative humidity of the gas had been tested before the water balance experiment. The amount of water carried out by the humidified gas was measured to verify the humidity capability of the humidifier. In order to make sure that the fuel cell was operated stably the whole time, a protocol of equilibration as followed was prior to each experiment: the current and the humidity were set at the operating value, and the fuel cell was operated for 2 h under this condition with continuous monitoring of the cell voltage. After the fuel cell got an equilibration status, the fuel cell was operated at the constant state for 2 h and the water was collected only during this period from both compartments. 3. Results and discussion 3.1. Membrane resistance measurement The cell performance and the resistance under different conditions are cited in Table 1. During the test the cathode relative humidity was The results show that there was an obvious change in the resistance and the cell performance when Nafion 115 membrane was used in a cell. When dry hydrogen was fed, the average output of the cell was V at the load of 500 ma cm 2, the resistance was cm 2, and the ohmic loss was 246 mv. When saturated hydrogen was fed, the cell output increased to mv, the resistance decreased to cm 2, and the ohmic loss decreased to 172 mv. Comparing the data, it is shown that the increase of the cell voltage was 106 mv when RH a increased from 0 to 1, and the decrease of the ohmic loss was 74 mv, correspondingly. That is to say near to 70% of the cell voltage change was contributed by the change of the ohmic loss at the load of 500 ma cm 2. And this value was 67% at the load of 300 ma cm 2. When Nafion 112 membrane was used in a cell, the resistance under all conditions was near to 0.22 cm 2, and the cell output changed a little along with the change of anode humidity condition. The results indicate that anode humidification had little influence on the membrane resistance and the cell performance when a thin Table 1 The cell voltage and the resistance under different anode humidification and current density Membrane type Current density (ma cm 2 ) Dry hydrogen Saturated hydrogen Voltage (V) Resistance ( cm 2 ) Voltage (V) Resistance ( cm 2 ) Nafion Nafion

3 Y. Cai et al. / Electrochimica Acta 51 (2006) membrane was used. But when a thick membrane was used, the membrane resistance and the cell performance changed obviously while the anode humidification changed. That is to say the back transport of water in a thick membrane was not enough to make the membrane full hydrated, and high membrane resistance was an obstacle for a cell to get high performance when the anode was non-humidified. Table 1 also shows the influence of current density on the membrane resistance. For a cell with Nafion 112 membrane, when the current density increased from 300 to 500 ma cm 2, the change value of membrane resistance was cm 2 under dry hydrogen condition and cm 2 under fully humidified hydrogen condition. For a cell with Nafion 115 membrane, when the current density increased from 300 to 500 ma cm 2, the change in value of the membrane resistance was cm 2 under dry hydrogen feed and cm 2 under saturated hydrogen feed. The results show that the resistance of Nafion 115 membrane was very sensitive to the current density when the anode gas was dry. It indicates that the membrane resistance had a close relationship with the amount of water produced in the cathode side when a thick membrane was used under dry hydrogen condition. But the resistance of Nafion 112 was near to a constant while the produced water increased under non-humidified anode condition. This indicates that it was easy for water transport across a thin membrane from the cathode to the anode and it was easy to be hydrated for this type of membrane. Considering the dry hydrogen condition, the cell with Nafion 115 membrane had a less performance than that of Nafion 112 membrane. This was due to a marked increase in the membrane resistance. When Nafion 112 membrane was used, the dependence of the membrane resistance on current and the anode humidification was much less. So a thin membrane was more suitable for a fuel cell operated with dry hydrogen. Fig. 1 shows the influence of the cathode humidification on the performance of the cell operated with dry hydrogen. Nafion 112 membrane was used and the cell was loaded at 500 ma cm 2. When RH c changed from 0.35 to 0.56, the cell Fig. 1. The performance of the cell with Nafion 112 membrane under different cathode humidity conditions at the load of 500 ma cm 2. Fig. 2. Comparisons of W cath with W ele, W ele + W hc and W vc in water balance experiments. voltage average output increased from to V, the increment was 41 mv, and the membrane resistance decreased from to cm 2. The change of the cathode humidity had a great influence on the cell performance. In Table 1, itis shown that when RH a increased from 0 to 1, there had a little change on the membrane resistance and the cell performance. This indicates that the effect of cathode humidification was more than that of the anode humidification. That is to say, if the cathode of the cell was well humidified, it is possible for a PEMFC to be operated with dry hydrogen when a thin membrane is used Water balance in a thin membrane The results of water balance experiments and the water transport property in the Nafion 112 membrane were discussed as follows. The cell temperature was 333 K, RH c was 0.56, and RH a was 0.75 if the anode was humidified. Fig. 2 presents a comparison of W cath with W ele and W ele + W hc in different anode humidity conditions, in which W cath was the amount of water collected from the cathode exhaust, W ele was the water produced in the electrochemical process, calculated according to Faraday s law, W hc was the amount of the inlet water vapor from the cathode humidifier, which was calculated according Eq. (1) W hc = P s(t cell )RH c ξ c ja Mt (1) P P s (T cell )RH c x O2 4F and W ele + W hc was the sum of water enters into the cathode. The figure shows that either the anode gas was humidified or not W cath was less than the summation of W ele and W hc. This indicates that there was a net water transport across the membrane directed from the cathode to the anode even when the anode inlet gas was humidified. Fig. 2 also contains a comparison of W cath with W vc, which was the amount of saturated vapor removed by the cathode exhaust. W vc was calculated as a function of T cell and j according

4 6364 Y. Cai et al. / Electrochimica Acta 51 (2006) Fig. 3. Comparisons of W a with W ha and W va in water balance experiments. to the formula: W vc = P s(t cell ) P P s (T cell ) ( ) ξc ja 1 Mt (2) x O2 4F Fig. 2 shows that the lines of W cath were higher than that of W vc. This indicates that only part of the produced water was removed as water vapor form and liquid water existed in the cathode side. If the liquid water is not removed by the exhaust in time the cathode catalyst layer and diffusion layer are likely to be flooding. However, a steady state had been obtained throughout the experiment, this means that the cathode outlet stream removed not only saturated vapor, but also liquid droplets. Fig. 3 shows the evidence for the water transport across the membrane under different anode humidity conditions. A comparison of W a, W ha and W va is shown, in which W ha and W va calculated as follows: W ha = P s(t cell )RH a ja ξ a Mt (3) P P s (T cell )RH a 2F W va = P s(t cell ) P P s (T cell ) (ξ a 1) ja Mt (4) 2F The comparison shows the lines of W a were higher than those of W ha whether anode was humidified or not, this indicates that the water collected from the anode was more than the water carried by feed gas. That is to say, a great deal of water transported from the cathode to the anode across the membrane. In Fig. 4, the parameter f was compared under different anode humidity conditions, in which f described the proportion of the net amount of the transported water to the produced water. f = W a W ha (5) W ele The figure shows that more than 20% of produced water discharged from the anode at the load of 300 ma cm 2 whether the anode was humidified or not, and the ratio increased with the increase of current density. But it seems that the ratio would go down if the current density higher than 600 ma cm 2. This indi- Fig. 4. The proportion of produced water transports from cathode to anode as a function of current density under different anode humidity. cates that at j < 500 ma cm 2 the back transport increased faster with j than the electro-osmotic drag, whereas from 500 ma cm 2 onwards the reverse seemed to be the case. It is known that when a current is drawn from the cell, protons migrate from the anode to the cathode and carry water molecules with them. The average number of water molecules transported per proton is called the electro-osmotic drag coefficient. In Nafion membranes, the value of this coefficient is between 1 (gas vapor equilibrated membranes) and 2.5 (liquid water equilibrated membranes) [27]. A net water transport coefficient n ed was defined to express the net amount of the water transport from the anode to the cathode per proton. Here n ed was an average value throughout the active area. The net water transport coefficient was obtained by the experimental values of W a, W ha and operating current density, calculated as: n ed = W ha W a jamt/f Fig. 5 is the net water transport coefficient of Nafion 112 membrane. The figure shows when the RH a of the anode gas was 0.75, the net water transport coefficient was 0.03 to When the anode gas was dry, the net water transport coefficient was to It was more negative than the value gotten under the former condition. This is because the anode gas could absorb more water before being saturated under this condition. The figure also shows that the absolute value of the net water transport coefficient increased while the current density increased. Compared with Fig. 3, the back transport of water was sufficient for a saturated anode outlet gas whether anode was humidified or not. This explains that why the membrane resistance and the performance of the fuel cell are non-sensitive to the anode humidity in Nafion 112 membrane. The water transport properties under different cathode humidity conditions were given in Fig. 6. W cath /W vc and W a /W va are defined to describe the flooding degree of the electrode. The figure shows that all the values of W cath /W vc and W a /W va were (6)

5 Y. Cai et al. / Electrochimica Acta 51 (2006) Conclusions Fig. 5. The net water transport coefficient as a function of current density and anode humidification. above 1, this indicates liquid water occurred not only in the cathode but also in the anode even the cell was under a low humidification both in the anode and the cathode. And W cath /W vc and W a /W va increased along with the increase of RH c. When RH c changed from 0.35 to 1, W cath /W vc increased from 1.52 to 2.22 as well as W a /W va changed from 4.44 to The flooding degree in the cathode was increased while the cathode humidity increased. Since the oxygen reductive reaction is sensitive to the oxygen concentration and oxygen diffusion in the cathode, it is disadvantage for a fuel cell operated with high W cath /W vc. Furthermore, the removal of the liquid water in the anode was more difficult because of the flow gas flow rate. The disadvantage of the liquid water accumulation in the anode was observed during the fuel cell stack operation. The accumulation caused the anode block in several cells and led to a lapse of the whole stack. The optimization of W a /W va and W cath /W vc for the cell performance hoped to be investigated in further. Fig. 6. Comparisons of W cath /W vc and W a /W va under different cathode humidified condition. Membrane resistance measurement and water balance experiments have been implemented in this paper to investigate the influence of water transport properties on a PEM fuel cell performance when dry hydrogen was fed to the anode. It proves that when Nafion 115 membrane was used in a cell, the cell performance and the membrane resistance had a close relationship with the current density if the cell was operated with dry hydrogen. The high membrane resistance was the obstacle for the cell to get high performance. When Nafion 112 was used in a cell, the net amount of water transport across the membrane was from the cathode to the anode whether anode was humidified or not. The membrane resistance and the cell performance changed a little while the anode humidification was changed. Comparing with the humidity of hydrogen, the performance of the fuel cell was much more dependent on the cathode gas humidity. Thus, the present study reveals that it is possible for a PEMFC operated with dry hydrogen when a thin membrane was used. Under this condition, the cell performance is sensitive to the cathode humidification. Liquid water presented not only in the cathode but also in the anode. Then, high cathode humidity was disadvantage for the water removal both in the anode and the cathode. The optimization of the cathode humidity condition is advantage for a PEMFC improving the performance when dry hydrogen was used. Acknowledgments The authors gratefully acknowledge the financial support from the National Natural Science Key Foundation of China (No ) and the State Key Development Program for basic Research of China (No. G ). Appendix A. Nomenclature A avtive area (cm 2 ) F Faraday constant (96,487 C mol 1 ) j current density (ma cm 2 ) M molar weight of water (g mol 1 ) n ed net water transport coefficient P operating pressure (Pa) P s saturated pressure (Pa) RH a relative humidity of the anode gas RH c relative humidity of the cathode gas t duration of the experiment (s) T cell operating temperature of cell (K) W a the amount of water collected in the anode (g) W cath the amount of water collected in the cathode (g) W ha the amount of water fed to the cell by humidification of the inlet anode gas (g) W hc the amount of water fed to the cell by humidification of the inlet cathode gas (g) the amount of water that can evaporate in anode (g) W va W vc W ele the amount of water that can evaporate in cathode (g) the amount of water produced through the electrochemical reaction (g)

6 6366 Y. Cai et al. / Electrochimica Acta 51 (2006) x O2 ξ a ξ c References percentage of oxygen in the cathode inlet gas stoichiometry of anode gas stoichiometry of cathode gas [1] S. Gamburzev, A.J. Appleby, J. Power Sources 107 (2000) 5. [2] R. Shinnar, Technol. Soc. 5 (2003) 455. [3] K.A. Adamson, Int. J. Hydrogen Energy 28 (2003) 771. [4] J. Linssen, Th. Grube, B. Hoehlein, M. Walbeck, Int. J. Hydrogen Energy 28 (2003) 735. [5] J.S. Yi, T.V. Nguyen, J. Electrochem. Soc. 145 (1998) [6] S.H. Ge, B.L. Yi, J. Power Sources 124 (2003) 1. [7] C.Y. Wang, Chem. Rev. 104 (2004) [8] T.E. Springer, T.A. Zawodzinski, S. Gottesfeld, J. Electrochem. Soc. 138 (1991) [9] T.E. Springer, M.S. Wilson, S. Gottesfeld, J. Electrochem. Soc. 140 (1993) [10] T. Okada, J. Electroanal. Chem. 465 (1999) 1. [11] T. Okada, J. Electroanal. Chem. 465 (1999) 18. [12] M. Eikerling, Y.I. Kharkats, A.A. Kornyshev, Y.M. Volfkovich, J. Electrochem. Soc. 145 (1998) [13] F. Meier, G. Eigenberger, Electrochim. Acta 49 (2004) [14] A.Z. Weber, J. Newman, J. Electrochem. Soc. 150 (2003) A1008. [15] A.Z. Weber, J. Newman, J. Electrochem. Soc. 151 (2004) A326. [16] A.Z. Weber, J. Newman, J. Electrochem. Soc. 151 (2004) A311. [17] U. Pasaogullari, C.Y. Wang, J. Electrochem. Soc. 151 (2004) A399. [18] U. Pasaogullari, C.Y. Wang, J. Electrochem. Soc. 152 (2005) A380. [19] R. Satija, D.L. Jacobson, M. Arif, S.A. Werner, J. Power Sources 129 (2004) 238. [20] A.B. Geiger, A. Tsukada, E. Lehmann, Fuel Cells 2 (2003) 92. [21] D. Kramer, J.B. Zhang, R. Shimoi, Electrochim. Acta 50 (2005) [22] K. Tüber, D. Pócza, C. Hebling, J. Power Sources 124 (2003) 403. [23] X.G. Yang, F.Y. Zhang, A.L. Lubawy, C.Y. Wang, Electrochem. Solid-State Lett. 7 (2004) A408. [24] X.G. Yang, N.A. Burke, C.Y. Wang, K. Tajiri, K. Shinohara, J. Electrochem. Soc. 152 (2005) A759. [25] T.A. Zawodzinski, J. Davey, J.A. Valerio, S. Gottesfeld, Electrochim. Acta 40 (1995) 297. [26] K.-H. Choi, D.-H. Peck, C.S. Kim, D.-R. Shin, T.-H. Lee, J. Power Sources 86 (2000) 197. [27] G.J.M. Janssen, M.L.J. Overvelde, J. Power Sources 101 (2001) 117. [28] M. Ciureanu, M. Badita, J. New Mater. Electrochem. Syst. 6 (2003) 163. [29] T. Abe, H. Shima, K. Watanabe, Y. Ito, J. Electrochem. Soc. 151 (2004) A101. [30] M. Ise, K.D. Kreuer, J. Marier, Solid State Ionics 125 (1999) 213.