Systematic Evaluation of Polymer Electrolyte Fuel Cell Electrodes with

ARTICLE pubs.acs.org/JPCC

Systematic Evaluation of Polymer Electrolyte Fuel Cell Electrodes with Hydrocarbon Polyelectrolytes by Considering the Polymer Properties Tatsuya Nakajima, Takanori Tamaki, Hidenori Ohashi, and Takeo Yamaguchi* Chemical Resources Laboratory, Tokyo Institute of Technology, Nagatsuta 4259, Midori-ku, Yokohama, Kanagawa 226-8503, Japan ABSTRACT: When using a hydrocarbon polyelectrolyte, such as sulfonated poly(arylene ether sulfone) (SPES), in a catalyst layer of a polymer electrolyte fuel cell, significantly decreased performance was observed. We clarified the main reason for this decrease from a systematic investigation into the humidity dependence of the polymer properties, such as the swelling ratio and oxygen permeability, and by comparison with the performance of a membrane electrode assembly (MEA) with SPES electrodes. The swelling ratio of SPES increased markedly at >90% relative humidity (RH), which led to blocking of the catalyst layer pores. This tendency semiquantitatively corresponds to the decreased performance of MEA at >90% RH, considering the water generated by the reaction. Therefore, the main cause of the decrease is the blocking of oxygen diffusion in the pores from the swelling of SPES in the catalyst layer rather than oxygen permeation through the SPES layer, as previously suggested.

1. INTRODUCTION Polymer electrolyte fuel cells (PEFCs) are promising energy devices for residential, portable, and automotive applications because of their high energy conversion efficiency, even at low temperatures. In an ideal PEFC, even if much work (i.e., electrical current) is taken out, a high energy conversion efficiency (i.e., high voltage) can be maintained. However, in practice, the potential decreases with current flow because there is a resistance caused by the chemical reactions and mass transport in the PEFC. These reactions are the hydrogen oxidation reaction at the anode and the oxygen reduction reaction at the cathode. The mass transport is the transport of gases, protons, and electrons. The reaction and transport at the cathode are slower than those at the anode. Thus, the processes at the cathode were the focus of this study. A proton-conducting polyelectrolyte is used not only as the membrane but also as the catalyst layer of the electrode, as an ionomer to increase the reaction area and the proton transport.1,2 The membrane and the electrode are combined to form a membrane electrode assembly (MEA); this is the core of a PEFC system, and MEA greatly influences the system performance. A perfluorosulfonic acid (PFSA), such as Nafion, is chosen as the polyelectrolyte. As an alternative to PFSA, hydrocarbon polyelectrolytes, such as sulfonated poly(arylene ether sulfone)s (SPESs),312 sulfonated poly(ether ether ketone)s,5 sulfonated poly(arylene ether sulfone ketone)s,13 sulfonated poly(imide)s,1416 and sulfonated poly(benzimidazole)s,17 have been developed in recent years because they have several advantages, such as their low cost and low environmental burden. A novel polyelectrolyte with superior characteristics can be synthesized easily by copolymerizing oligomers or polymers that have different characteristics. Furthermore, some hydrocarbon polyelectrolytes show comparable proton conductivity to PFSA.7 r 2011 American Chemical Society

Since the characteristics of a polyelectrolyte required for the membrane and the catalyst layer are substantially different, the catalyst layer needs to be optimized separately from the membrane. For example, the membrane must block the permeation of reactant gases. These gases are the fuel and so must diffuse rapidly through the ionomer layer that covers the catalystsupported carbon materials. Considering the advantages of hydrocarbon polyelectrolytes over PFSA, it is desirable to change the polyelectrolyte used for the electrode as well as for the membrane from PFSA to a type of hydrocarbon. However, few hydrocarbon polyelectrolytes have been used as ionomers in the catalyst layer.1823 The performance of the catalyst layer has been optimized by much trial and error when using PFSA as the ionomer. However, in the case of hydrocarbon ionomers, optimization of the performance by trial and error is almost impossible because a large number of ionomers have been synthesized. Furthermore, the performance of MEAs containing hydrocarbon ionomers in the catalyst layer has been reported to decrease at comparatively low current densities, 90%, with δS = 168% at RH = 98%. This value is 1.5 times higher than that for Nafion at RH = 98%, which was δS = 113%. A high value of δS in SPES has been reported in some random hydrocarbon polyelectrolytes because these have a weak microphase-separated structure caused by the low interaction between the hydrophilic and hydrophobic domains.30,31 The oxygen permeabilities of the SPES and Nafion are shown in Figure 4b. The values of PO2 for the SPES and Nafion at RH = 98% were 2.0 109 and 5.8 109 cm3 cm/(cm2 cmHg s), respectively, with the ratio SPES/Nafion = 0.34. Furthermore, the humidity dependence of PO2 for the Nafion was considerably lower than that for the SPES. This tendency and the observed values of PO2 are similar to those in previous reports that used different measurement methods.3234 Figure 4c shows the proton conductivities of the SPES and Nafion at various RH values from 50% to 98%. The conductivity of the SPES was comparable to that of Nafion for RH > 90%, while the conductivity of the SPES decreased more than that of the Nafion at low RH values. Electrode Characterization. The electrode structure in the dry state was investigated using FE-SEM, and the results are 1424

dx.doi.org/10.1021/jp206552k |J. Phys. Chem. C 2012, 116, 1422–1428

The Journal of Physical Chemistry C

ARTICLE

the secondary pores or voids of the catalyst layer, and so oxygen diffusion through the pores (Figure 1a) would be inhibited when using a hydrocarbon ionomer. Regarding the RH dependence, this blocking may be reduced in the low-RH region because of the lower degree of swelling in the SPES shown in Figure 4a. On the other hand, the oxygen permeation through the SPES ionomer layer around the secondary particles (Figure 1b) may also be inhibited because of the low oxygen permeation shown in Figure 4b and the thick ionomer layer of the SPES. The proton conductivity (Figure 1c) can also affect MEA performance, since a humidity dependence was observed, as shown in Figure 4c. The indices are used to explain the resistance relating to oxygen diffusion in the pores and oxygen permeation through the ionomer layer to make these properties easier to discuss. In contrast, the resistance related to the proton conduction was simply expressed as the inverse value of σ because the unit of siemens is the inverse of the unit of resistance, ohm. Effect of Ionomer Swelling on the Porosity of the Catalyst Layer. To discuss the effect of RH on the blocking of secondary pores by ionomer swelling, the relative porosity of the catalyst layer compared with the porosity without an ionomer, ϕ (%), was estimated using a simple calculation. The ionomer was assumed not to penetrate into the agglomerated carbon black (A-CB), which consisted of secondary particles of catalyst-supported carbon and primary pores (VPore). Then, the porosity and the volume of ionomer were calculated using the equations ! WðPt=CÞ ð1 mÞ VACB ¼ ð4Þ þ VPore FðKBÞ Figure 4. Humidity dependence of the polymer properties for a Nafion NR-212 membrane (solid circle) and an SPES cast membrane (solid square). The swelling ratio (a), oxygen permeability coefficient (b), and proton conductivity (c) were evaluated at 60 °C.

VCL ¼ Al Vionomer ¼

ϕ¼

Figure 5. SEM cross-sectional view of an SPES electrode: (a) BSE image and (b) SE image.

shown in Figure 5. Figure 5a shows a backscattered electron (BSE) image, and Figure 5b shows a secondary electron (SE) image of the SPES electrode. Since the intensity of the BSE image is influenced significantly by the atomic number, platinum particles present on the carbon support can be clearly observed, regardless of the layer of the SPES ionomer, as shown in Figure 5a. In contrast, the SE image shown in Figure 5b shows the surface of the sample directly, irrespective of atomic number, and it shows that the thick ionomer layer of the SPES covers the secondary particles. Consideration of the Effect of the Polymer Properties on the IV Performance. The main causes of the decrease in performance of the electrodes containing hydrocarbon ionomers can be understood by considering the polymer properties and the electrode structure described above. The polymer swelling (Figure 4a) and the thick ionomer layer (Figure 5b) could block

ð5Þ WðionomerÞ δS FðionomerÞ 100

VCL VACB VPTFE Vionomer 100 VCL VACB VPTFE

ð6Þ

ð7Þ

where VACB, VCL, Vionomer, and VPTFE are the volumes of the ACB, the catalyst layer, the ionomer, and the PTFE, respectively. The terms W(Pt/C) and W(ionomer) are the actual introduced amounts of Pt-supported carbon and ionomer, respectively, and m is the mass ratio of Pt. The terms F(KB) and F(ionomer) are the densities of Ketjenblack (KB) and ionomer, respectively, A is the area of the catalyst layer, and l is its thickness. The value of l was determined from a comparison of the calculated porosity of the Nafion electrode with the reported value of ca. 40% and was found to be l = 7.5 μm. The value of VPore was obtained from the cumulative volume curve of the electrode without an ionomer below 40 nm,25 measured using a mercury porosimeter, and was found to be VPore = 2.7 104 cm3. The relative porosity of the SPES electrode dropped to 15% at RH = 98%, compared with the porosity without the ionomer, while that of the Nafion electrode dropped slightly to 66% at RH = 98%. These results indicate that the secondary pores are easily blocked when the SPES ionomer is used for the electrode. Effect of Oxygen Permeability around the Secondary Particle. To discuss the difficulty of oxygen permeation through the ionomer layer, the resistance to oxygen permeation (R O 2 ) was calculated by considering the permeability (PO2), the polymer swelling (δS), and the density (F). The value of RO2 includes the 1425



ARTICLE

effect of the thickness of the ionomer layer around the secondary particles: the resistance increases with thickness. Here, RO2 is a relative value compared with the Nafion electrode and was calculated using the equation R O2 ¼

δSðSPESÞ =δSðNafionÞ ðFðSPESÞ =FðNafionÞ ÞðPO2 ðSPESÞ =PO2 ðNafionÞ Þ

ð8Þ

The ratio of the densities of SPES and Nafion was 0.6. Therefore, the value of RO2 for the SPES electrode was RO2 = 7.2 at RH = 98%. In other words, the permeation of oxygen through the SPES ionomer layer was 7.2 times more difficult than that through the Nafion ionomer layer. This value required us to consider the resistance to oxygen permeation in the catalyst layer at comparatively low current densities when using the SPES ionomer in the catalyst layer, although the resistance is not serious when using the Nafion ionomer. Humidity Dependence of Each Effect on the Performance of the MEA. To compare the resistances of the processes, the values of 100 ϕ, RO2, and 1/σ for various RH values were evaluated, and these are summarized in Table 1 and Figure 6. The data in Table 1 and Figure 6 mean that the higher these values become, the larger the resistance becomes. The resistance to proton conduction was expressed as the inverse of σ. In addition, the difficulty of oxygen diffusion through the pores was expressed as 100 ϕ (%). The value of RO2 at each RH was calculated by comparison with the Nafion electrode at RH = 98%. Table 1. Summary of the Humidity Dependence on the Properties of Each Polymer That Affect the Performance of the MEA

The value of 100 ϕ increased with increasing RH, as shown in Figure 6a. This means that if the blocking of the secondary pores caused by the SPES swelling was the main influence behind the decrease in performance, then the performance would improve by decreasing the RH because the value of 100 ϕ would be lower. In addition, the value of 100 ϕ would increase at RH > 90%, since the swelling ratio of SPES increases significantly at RH > 90%. This phenomenon may also affect the MEA performance. In contrast, the values of 1/σ and RO2 decreased with increasing RH, as shown in Figures 6b and 6c, respectively. This means that if either the oxygen permeation through the ionomer layer or the limitation of proton conduction was the main cause of the decrease in performance, then the MEA performance would decrease with decreasing RH. IV Performance of the SPES Electrodes Compared with the Nafion Electrodes. The IV curves of MEAs consisting of the Nafion electrode (Nafion-MEA) and the SPES electrode (SPES-MEA) were compared first at 60 °C and RH = 90%, as shown in Figure 7. The SPES-MEA performance decreased markedly compared with that of the Nafion-MEA around 400 mA/cm2. It is difficult to interpret this phenomenon for the catalyst layer consisting of several materials using only the MEA performance. Based on a consideration of the polymer properties and the electrode structure, the main causes of the decrease in performance when using a hydrocarbon ionomer are the inhibition of oxygen diffusion by blocking the secondary pores or the resistance to oxygen permeation through the ionomer layer or to proton conduction. However, the reason for the decrease in performance at comparatively low current densities using a hydrocarbon ionomer in the electrode cannot be clarified from this result. To clarify the main causes of the decrease in performance, the dependence of the humidity on the IV

100 ϕa (%) RH

SPES electrode

Nafion electrode 30

RO2b

1/σc (cm/S)

0

51

50

60

40

60 70

60 61

29 15

79.5 34.1

80

62

12

18.7

90

65

33

8.6

9.04

98

85

34

7.2

6.12

163

ϕ is the relative porosity of the electrode with the SPES or Nafion ionomer compared with the electrode without the ionomer. b RO2 is the resistance to oxygen permeation of the SPES electrode compared with the Nafion electrode at 98% RH. c σ is the proton conductivity of the SPES membrane at 60 °C. a

Figure 7. IV curves of MEAs containing a Nafion electrode (solid circle) and an SPES electrode (solid square) at 60 °C and 90% RH. The flow rate was H2 = 100 mL/min at the anode and O2 = 500 mL/min at the cathode.

Figure 6. Humidity dependence of (a) 100 ϕ, (b) the resistance to oxygen permeation, and (c) the inverse of the proton conductivity of SPES at 60 °C. 1426



Figure 8. (a) Humidity dependence of the MEA performance for an SPES electrode at 60 °C. The flow rate was H2 = 100 mL/min at the anode and O2 = 500 mL/min at the cathode. The relative humidity was 90% (solid square), 80% (solid circle), 70% (solid triangle), 60% (open square), and 50% (open circle). (b) Reversible cycling at two different RH (from 90% to 50%, from 50% to 90%, and from 90% to 50%).

performance was measured, since reverse tendencies were observed in the inhibition of oxygen diffusion by such blocking (i.e., the value of 100 ϕ) and the resistance to oxygen permeation (i.e., the value of RO2) or 1/σ with decreasing RH, as discussed above (Table 1 and Figure 6). Dependence of IV Performance on the Relative Humidity and the Main Cause of Decreased Performance. The dependence of IV performance on the RH was evaluated using the SPES-MEA, as shown in Figure 8. To focus only on the decrease in performance at >400 mA/cm2, the SPES ionomer with a high ion exchange capacity (IEC) (x = 56 in Figure 2) was used for the SPES electrode because the high-IEC ionomer would minimize the decrease in performance at low current densities. Figure 8a shows that the IV performance of the SPES-MEA improved with decreasing RH at >400 mA/cm2. Additionally, the IV tests were carried out after cycling at two different RH values. The MEA performance was checked first at 50% RH and 90% RH and again reversed from at 90% RH to 50% RH. Almost similar performances were obtained in both the curves as shown in Figure 8b. Therefore, the delamination between the membrane and the electrode did not occur. Figure 8a and the data in Figure 6 show that the main cause of the decrease in performance is not the resistance to oxygen permeation or to proton conduction, but the blocking of the pores from ionomer swelling: only the value of 100 ϕ decreased with decreasing RH, as shown in Figure 6a. To discuss this tendency in more detail, the actual value of RH in the electrode was estimated by considering the amount of water generated by the oxygen reduction reaction at the cathode. The generated water was proportional to the current density. The current density, at which the actual value of the RH at the cathode reached 90%, was determined for input RH = 70%, 60%, and 50% to give values of

ARTICLE

430, 650, and 880 mA/cm2, respectively. Comparing these values with the results of the IV performance of SPES-MEA, the voltage fell markedly when the actual RH was >90%. The rapid decrease in performance at RH > 90% is in accord with the rapid increase in resistance from the blocking of the pores, 100 ϕ, for RH > 90%, as shown in Figure 6a. As mentioned above, the main cause of the decrease in performance with the hydrocarbon ionomer could be clarified by considering the MEA performance, the polyelectrolyte properties, and the structure of the catalyst layer. The main cause is not the limitation of oxygen permeation through the ionomer layer, as has been suggested before,24,26,27 or the limitation of proton conduction, but rather the limitation of oxygen diffusion in the secondary pores. The oxygen diffusion in the pores outside the agglomerated catalyst-supported carbon is closely correlated to the ionomer swelling. In other words, the MEA performance was greatly influenced by the ionomer swelling in the catalyst layer of the cathode. Accordingly, an optimized structure of the catalyst layer for the hydrocarbon polyelectrolyte would be fabricated using an ionomer with a low swelling ratio. In addition, this study has underscored the importance of a systematic consideration of both the polymer properties and the electrode structure when optimizing an electrode.

4. CONCLUSIONS We have evaluated the humidity dependence of polymer properties, such as the swelling ratio, the proton conductivity, and the oxygen permeability, and have investigated the electrode structure to clarify the main reasons for the decrease in performance of MEAs containing the SPES ionomer. To consider the effect of the polymer properties on MEA performance, the relative porosity and the resistance to oxygen permeation were estimated as indices of the resistance to oxygen diffusion in the pores and the resistance to oxygen permeation through the ionomer layer, respectively. With increasing RH, the resistance to oxygen diffusion in the pores, or the value of 100 ϕ, increased significantly for RH > 90%, while the resistance to oxygen permeation and proton conduction decreased gradually. The performance of MEAs with SPES electrodes was measured at various values of RH, and the performance of the SPES-MEA decreased at >400 mA/cm2 with increasing RH. The actual RH values, based on the water generated at the cathode, were calculated, showing that the decrease in performance of the SPESMEA occurred for RH > 90%. These results suggest that the main cause of the decrease in performance is the blocking of the secondary pores caused by the ionomer swelling in the catalyst layer when using a hydrocarbon ionomer. Therefore, the oxygen diffusion around the secondary pores needs to be improved by using a low-swelling polyelectrolyte when employing a hydrocarbon ionomer. ’ AUTHOR INFORMATION Corresponding Author

*Tel: +81-45-924-5254. Fax: +81-45-924-5253. E-mail: yamag@ res.titech.ac.jp.

’ REFERENCES (1) Wilson, M. S.; Gottesfeld, S. J. Appl. Electrochem. 1992, 22, 1–7. (2) Ticianelli, E. A.; Derouin, C. R.; Redondo, A.; Srinivasan, S. J. Electrochem. Soc. 1988, 135, 2209–2214. 1427



ARTICLE

(3) Cho, C. G.; Kim, Y. S.; Yu, X.; Hill, M.; McGrath, J. E. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 6007–6014. (4) Kim, Y. S.; Dong, L. M.; Hickner, M. A.; Glass, T. E.; Webb, V.; McGrath, J. E. Macromolecules 2003, 36, 6281–6285. (5) Kim, Y. S.; Wang, F.; Hickner, M.; McCartney, S.; Hong, Y. T.; Harrison, W.; Zawodzinski, T. A.; McGrath, J. E. J. Polym. Sci., Part B: Polym. Phys. 2003, 41, 2816–2828. (6) Lee, H. S.; Roy, A.; Lane, O.; Dunn, S.; McGrath, J. E. Polymer 2008, 49, 715–723. (7) Li, Y. X.; Roy, A.; Badami, A. S.; Hill, M.; Yang, J.; Dunn, S.; McGrath, J. E. J. Power Sources 2007, 172, 30–38. (8) Li, Y. X.; Wang, F.; Yang, J.; Liu, D.; Roy, A.; Case, S.; Lesko, J.; McGrath, J. E. Polymer 2006, 47, 4210–4217. (9) Liu, D.; Kyriakides, S.; Case, S. W.; Lesko, J. J.; Li, Y. X.; McGrath, J. E. J. Polym. Sci., Part B: Polym. Phys. 2006, 44, 1453–1465. (10) Robeson, L. M.; Hwu, H. H.; McGrath, J. E. J. Membr. Sci. 2007, 302, 70–77. (11) Wang, F.; Hickner, M.; Kim, Y. S.; Zawodzinski, T. A.; McGrath, J. E. J. Membr. Sci. 2002, 197, 231–242. (12) Wiles, K. B.; Wang, F.; McGrath, I. E. J. Polym. Sci., Part A: Polym. Chem. 2005, 43, 2964–2976. (13) Miyatake, K.; Bae, B.; Yoda, T.; Uchida, H.; Watanabe, M. Angew. Chem., Int. Ed. 2010, 49, 317–320. (14) Asano, N.; Aoki, M.; Suzuki, S.; Miyatake, K.; Uchida, H.; Watanabe, M. J. Am. Chem. Soc. 2006, 128, 1762–1769. (15) Fang, J. H.; Guo, X. X.; Harada, S.; Watari, T.; Tanaka, K.; Kita, H.; Okamoto, K. Macromolecules 2002, 35, 9022–9028. (16) Genies, C.; Mercier, R.; Sillion, B.; Cornet, N.; Gebel, G.; Pineri, M. Polymer 2001, 42, 359–373. (17) Bae, J. M.; Honma, I.; Murata, M.; Yamamoto, T.; Rikukawa, M.; Ogata, N. Solid State Ionics 2002, 147, 189–194. (18) Jung, H. Y.; Cho, K. Y.; Sung, K. A.; Kim, W. K.; Kurkuri, M.; Park, J. K. Electrochim. Acta 2007, 52, 4916–4921. (19) Ramani, V.; Swier, S.; Shaw, M. T.; Weiss, R. A.; Kunz, H. R.; Fenton, J. M. J. Electrochem. Soc. 2008, 155, B532–B537. (20) Selvarani, G.; Sahu, A. K.; Choudhury, N. A.; Sridhar, P.; Pitchumani, S.; Shukla, A. K. Electrochim. Acta 2007, 52, 4871–4877. (21) Sung, K. A.; Jung, H. Y.; Kim, W. K.; Cho, K. Y.; Park, J. K. J. Power Sources 2007, 169, 271–275. (22) von Kraemer, S.; Lindbergh, G.; Lafitte, B.; Puchner, M.; Jannasch, P. J. Electrochem. Soc. 2008, 155, B1001–B1007. (23) Yoda, T.; Shimura, T.; Bae, B.; Miyatake, K.; Uchida, M.; Uchida, H.; Watanabe, M. Electrochim. Acta 2009, 54, 4328–4333. (24) Astill, T.; Xie, Z.; Shi, Z. Q.; Navessin, T.; Holdcroft, S. J. Electrochem. Soc. 2009, 156, B499–B508. (25) Beleke, A. B.; Miyatake, K.; Uchida, H.; Watanabe, M. Electrochim. Acta 2007, 53, 1972–1978. (26) Easton, E. B.; Astill, T. D.; Holdcroft, S. J. Electrochem. Soc. 2005, 152, A752–A758. (27) Sambandam, S.; Ramani, V. Electrochim. Acta 2008, 53, 6328–6336. (28) Larminie, J.; Dicks, A. Fuel Cell Systems Explained, 2nd ed.; John Wiley & Sons: West Sussex, UK, 2003. (29) Broka, K.; Ekdunge, P. J. Appl. Electrochem. 1997, 27, 117–123. (30) Pang, J. H.; Zhang, H. B.; Li, X. F.; Jiang, Z. H. Macromolecules 2007, 40, 9435–9442. (31) Pang, J. H.; Zhang, H. B.; Li, X. F.; Ren, D. F.; Jiang, Z. H. Macromol. Rapid Commun. 2007, 28, 2332–2338. (32) Mohamed, H. F. M.; Kobayashi, Y.; Kuroda, C. S.; Takimoto, N.; Ohira, A. J. Membr. Sci. 2010, 360, 84–89. (33) Zhang, L.; Hampel, C.; Mukerjee, S. J. Electrochem. Soc. 2005, 152, A1208–A1216. (34) Zhang, L.; Ma, C. S.; Mukerjee, S. Electrochim. Acta 2003, 48, 1845–1859.

1428


Systematic Evaluation of Polymer Electrolyte Fuel Cell Electrodes with

Recommend Documents