Simple, Effective Molecular Strategy for the Design of Fuel Cell

Jul 2, 2016 - Proton-conducting membranes are key materials in polymer electrolyte fuel cells. In addition to high proton conductivity and durability,...
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Simple, Effective Molecular Strategy for the Design of Fuel Cell Membranes: Combination of Perfluoroalkyl and Sulfonated Phenylene Groups Takashi Mochizuki,† Makoto Uchida,‡ and Kenji Miyatake*,‡,§ †

Interdisciplinary Graduate School of Medicine and Engineering, ‡Fuel Cell Nanomaterials Center, and §Clean Energy Research Center, University of Yamanashi, 4 Takeda, Kofu, Yamanashi 400-8510, Japan S Supporting Information *

ABSTRACT: Proton-conducting membranes are key materials in polymer electrolyte fuel cells. In addition to high proton conductivity and durability, a membrane must also support good electrocatalytic performance of the catalyst layer at the membrane−electrode interface. We herein propose an effective molecular approach to the design of high-performance proton-conducting membranes designed for fuel cell applications. Our new copolymer (SPAF) is a simple combination of perfluoroalkylene and sulfonated phenylene groups. Because this ionomer membrane exhibits a well-controlled finely phase-separated morphology, based on the distinct hydrophilic−hydrophobic differences along with the polymer chain, it functions well in an operating fuel cell with good durability under practical conditions. The advantages of this ionomer, unlike typical perfluorosulfonic acid ionomers (e.g., Nafion), include easy synthesis and versatility in molecular structure, enabling the fine-tuning of membrane properties.

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Introducing pendant perfluorosulfonic acid groups onto the hydrocarbon polymers has been examined in the past.10−13 Some are claimed to show proton conductivity and fuel cell performance comparable to or higher than those of PFSA ionomers. In this Letter, we propose a simple but effective molecular approach for the design of better performing fuel cell membranes. Our new copolymer is the combination of perfluoroalkylene and sulfonated phenylene groups in order for the main chain to consist solely of C−C bonds (without heteroatom linkages). The copolymer membrane exhibits high proton conductivity, low gas permeability, good compatibility with the catalyst layers, and reasonable durability in accelerated durability testing in operating fuel cells. Furthermore, the copolymers can be prepared in a simple synthetic procedure and thus offer a large number of options to modify the polymer structure and the copolymer composition to further optimize the properties. The title sulfonated copolymer, SPAF, was synthesized according to Scheme 1. The copolymerization and the

olymer electrolyte fuel cells (PEFCs) are promising power-generating electrochemical devices for stationary, automotive, and portable applications. Perfluorosulfonic acid (PFSA) ionomers such as Nafion (Du Pont) have been most used as benchmark proton exchange membranes in PEFCs because of their high proton conductivity as well as good chemical and mechanical stability. However, PFSA ionomers still suffer from several drawbacks such as high cost, low thermal stability, and high gas permeability, which would impede the widespread dissemination of PEFCs.1−3 Because PFSA ionomers provide little molecular design flexilibity, possible approaches toward the improvement of properties are limited. Therefore, there has been a significant effort to develop alternative ionomer materials. Sulfonated hydrocarbon (aromatic) ionomers have probably been the most intensively investigated; however, none of these has fulfilled the required properties, in particular, proton conductivity and chemical and mechanical stability. In addition, incompatibility of the hydrocarbon ionomer membrane and the PFSA-based catalyst layers often lowers fuel cell performance.4−6 It was reported in the literature that polyphenylene ionomers exhibited high proton conductivity and chemical stability; however, the interfacial issues have not been well-addressed.7−9 © 2016 American Chemical Society

Received: June 10, 2016 Accepted: July 2, 2016 Published: July 2, 2016 348

DOI: 10.1021/acsenergylett.6b00198 ACS Energy Lett. 2016, 1, 348−352

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http://pubs.acs.org/journal/aelccp

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ACS Energy Letters Scheme 1. Synthesis of SPAF Copolymer

Figure 1. TEM images of (a) Nafion, (b) SPP, and (c) SPAF membranes. The samples were stained with lead (Pb2+) ions prior to the observation. Images were taken at the acceleration voltage of 200 kV.

ether sulfone ketone)].14 In the images, the dark areas represent hydrophilic domains composed of stained sulfonic acid groups and their aggregates, and the bright areas represent hydrophobic domains. It is well-known that PFSA ionomer membranes exhibit a phase-separated morphology based on their hydrophilic−hydrophobic differences.1 The SPP membrane also showed a phase-separated morphology, which resulted from the sequenced hydrophobic units; however, the interfaces between the hydrophilic and hydrophobic domains were less pronounced.14 In contrast, the SPAF membrane, composed of perfluoroalkylene and sulfonated phenylene groups, showed a more distinct morphology, similar to that of Nafion. The domain sizes were uniform (ca. 2 nm in diameter for both hydrophilic and hydrophobic domains) and smaller than those of Nafion (ca. 3−6 nm). The rigid, compact sulfonated phenylene units and their random distribution along the polymer main chains would be responsible for such small domain sizes. Despite the random copolymer structure, the hydrophilic−hydrophobic interfaces of SPAF were more distinct than those of SPP, possibly because of the large hydrophilic−hydrophobic differences between the perfluoroalkyl and sulfonated phenylene groups. The morphological changes with humidity were then investigated using the small-angle X-ray scattering (SAXS) technique for the three ionomer membranes with both temperature and humidity control. The SAXS profiles obtained at 30−90% relative humidity (RH) and 80 °C are shown in Figure 2. The data shown were taken from high to low humidity and were revsersible when they were taken from low to high humidity. Nafion exhibited two well-known diagnostic peaks (ionomer peak at higher q and matrix knee at lower q).15 Briefly, the ionic domains develop with increasing humidity, while the hydrophobic domains are insensitive to humidity. The SPAF membrane exhibited a single small peak at q = ca. 2−3 nm−1 assignable to the ionic domains. The d spacing of the ionic domains was ca. 2−4 nm, in good accordance with the domain sizes observed in the TEM image mentioned above. The peak intensity was much smaller than that of Nafion probably because the ionic domains were less developed and their size distribution was larger stemming from rigid sulfophenylene structure. The ionic peak developed with increasing humidity; however, the development was also less pronounced than that of Nafion. No hydrophobic peak was observed, suggesting hydrophobic domains that are more poorly ordered than those of Nafion. The SPP membrane exhibited a much larger ionomer peak, which became smaller with increasing humidity, due to the randomization of the ionic domains. This behavior is typical for sequenced aromatic ionomer membranes.16 SPAF utilized absorbed water more efficiently in the development of ionic domains, probably

subsequent sulfonation reactions proceeded well in homogeneous solutions. The chemical structure of the SPAF was analyzed by 1H and 19F NMR spectra (Figure S1). Compared to that of its precursor copolymer (PAF), the 1H NMR spectrum of SPAF was more complicated, and detailed peak assignment was not possible. Major aromatic proton peaks appeared at a lower magnetic field than those of PAF. The 19F NMR spectrum of SPAF was much less complicated and showed two sets of fluorine peaks, both of which appeared at a lower magnetic field than those of PAF. These NMR spectra suggest that the phenylene groups (including those attached to −CF2− groups) in the polymer main chain were sulfonated and that the sulfonation reaction was not regioselective. The molecular weight of the SPAF was estimated from GPC analysis to be Mn = 96.1 kDa, and Mw = 856 kDa (Table S1). Compared to its precursor PAF, the apparent molecular weights and the polydispersity index (Mw/Mn) value were larger for SPAF because of the electrostatic repulsion of the sulfonic acid groups along with the polymer main chain. Casting from dimethylsulfoxide solution provided thin (ca. 28 μm), bendable membranes of SPAF. The ion exchange capacity (IEC) of the SPAF membrane estimated from a back-titration was 1.59 mequiv g−1, which corresponded to a degree of sulfonation per phenylene group of 0.32. Despite a relatively low degree of sulfonation, all proton and fluorine peaks appeared at lower magnetic fields in the NMR spectra than those of the precursor PAF, presumably because of the electronic conjugation effect of the neighboring phenylene segments in the polymer main chain. Figure 1 shows cross-sectional transmision electron microscopy (TEM) images of Nafion NRE211 and SPAF membranes. For comparison, a TEM image of our previous aromatic hydrocarbon ionomer (SPP, see Figure S1 for the chemical structure) is also included, because it contains a similar sulfophenylene structure as the hydrophilic component but a different hydrophobic component [sequenced oligo(phenylene 349

DOI: 10.1021/acsenergylett.6b00198 ACS Energy Lett. 2016, 1, 348−352

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ACS Energy Letters

Figure 3. (a) Water uptake and (b) proton conductivity of Nafion, SPP, and SPAF membranes at 80 °C as a function of relative humidity.

S4). The membrane exhibited high storage moduli (E′), ca. 1 GPa, at a wide range of humidity. In the loss moduli (E″) and tan δ (= E″/E′) curves, no peaks assignable to glass transition were detected, suggesting stable mechanical behavior under the tested conditions. A membrane−electrode assembly (MEA) was fabricated with an SPAF membrane and mounted in a PEFC single cell (SPAFcell) to compare the electrochemical properties with those of SPP- and Nafion-cells. The same electrodes (catalyst layers), consisting of Pt/CB catalyst and Nafion binder, were used for both anode and cathode in the three cells. Therefore, the three cells exhibited similar cyclic voltammograms (CVs) typical for polycrystalline Pt (Figure S5). Their electrochemically active surface area (ECSA) values, estimated from the hydrogen adsorption charges of the CVs, were comparable within acceptable errors among the three cells under a wide range of humidity conditions (Figure S6), indicating that the interfacial contact between the Pt catalyst and Nafion binder was similar in these catalyst layers. The CV of the Nafion-cell was shifted upward because of the oxidation current of hydrogen permeating through the Nafion membrane. To evaluate quantitatively the hydrogen permeability of the three membranes, linear sweep voltammograms (LSVs) were measured at 80 °C while supplying fully humidified hydrogen and nitrogen to the anode and cathode, respectively. The oxidation current density of the permeated hydrogen was 1.45 mA cm−2 for Nafion, 0.35 mA cm−2 for SPP, and 0.40 mA cm−2 for SPAF. It is noteworthy that the hydrogen permeability of SPAF was comparable to that of SPP and less than 30% that of Nafion. While the perfluorinated alkyl chain can be assumed to facilitate the dissolution and diffusion of small molecules (such as hydrogen) in ionomer membranes, the aromatic moieties could have counteracted this tendency in the SPAF membrane. The fuel cell performances were evaluated for the three cells at 80 °C. Figure 4 shows polarization curves (ohmic dropcorrected, IR-free) and ohmic resistances of the three cells under humidity conditions of 100% and 30% RH (see Figure

Figure 2. SAXS profiles for (a) Nafion, (b) SPP, and (c) SPAF membranes as a function of the scattering vector (q) value at relative humidity from 90 to 30% RH and 80 °C. The dashed arrows indicate increasing humidity.

because it is composed of perfluorinated alkyl chains, without heteroatom-based functional groups such as ether, ketone, and sulfone groups, to which water can be coordinated, as in the case of SPP. Water uptake and proton conductivity of the SPAF membrane were measured under the same conditions as those for SAXS and are plotted as a function of RH in Figure 3 with those for the Nafion and SPP membranes. Water uptake increased with increasing IEC value and humidity. The number of water molecules per acid group (often known as λ) for SPAF was slightly higher than that for Nafion and SPP (Figure S3). The proton conductivity also followed the same order as the IEC values for humidity greater than 80% RH. The proton conductivity of SPAF (0.19 S cm−1) was slightly higher than that (0.16 S cm−1) of Nafion at 95% RH. With decreasing humidity, SPAF exhibited a more serious drop in conductivity than the other two membranes. The proton conductivity of SPAF was 1.1 mS cm−1 at 20% RH, which was lower than those of SPP (7.3 mS cm−1) and Nafion (6.4 mS cm−1). The lower IEC of SPAF than that of SPP was responsible. The higher conductivity of Nafion despite the lower IEC can be ascribed to the superacidity of the perfluorosulfonic acid groups (pKa = ca. −6) compared to arylsulfonic acid groups (pKa = ca. −1).17 The mechanical properties of SPAF membrane were evaluated via dynamic mechanical analyses (DMA) under the controlled conditions (at 80 °C from 5% to 90% RH) (Figure 350

DOI: 10.1021/acsenergylett.6b00198 ACS Energy Lett. 2016, 1, 348−352

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ACS Energy Letters

The mass activity (MA) of Pt catalysts at 0.85 V was estimated from the polarization curves in Figure 4 and plotted as a function of RH in Figure 5. The SPAF-cell achieved MA

Figure 5. Mass activity (MA) at 0.85 V for the Nafion-, SPP-, and SPAF-cells at 80 °C as a function of relative humidity.

values comparable to or even better than that of the NRE211cell at all humidities investigated. The MA of the SPAF-cell was ca. 2 times higher than that of the SPP-cell at any humidity investigated. These results indicate that the SPAF membrane, having a small-scale phase-separated morphology, is highly suitable for practical fuel cell operating (dynamic) conditions, more so than expected based on the static conditions of ex-situ proton conductivity measurements, in terms of electrode performance and ohmic resistance. Nevertheless, the effectiveness (EfPt)18 of Pt catalysts was only 7% for SPAF- and Nafioncells even at 100% RH and thus needs further improvement. We then operated the SPAF-cell under more severe conditions (100 °C and 30% RH). IR-free I−V performance similar to that for the Nafion-cell was obtained up to 300 mA cm−2, implying good thermal stability of the SPAF membrane (Figure S8). The durability of the SPAF membrane was further investigated in an open-circuit voltage (OCV) hold test (Figure S9). It is well-known that membrane degradation is accelerated at OCV under low humidity conditions because of the increased formation of hydrogen peroxide and the resulting presence of oxidizing radical species.19 The OCV of the SPAFcell was initially 1.02 V and decreased slightly to 0.97 V after 1000 h. This oxidative stability of the SPAF-cell was much higher than that of the Nafion-cell; the OCV of the Nafion-cell dropped after 140 h under the same conditions.20 The ohmic resistance maintained the initial value of ca. 0.09 Ω cm2 during the OCV hold test for the SPAF-cell. Such high oxidative stability can be ascribed to the chemical robustness of the perfluoroalkylene and sulfonated phenylene groups and also to the low gas permeability of the SPAF membrane. Hydrocarbon ionomer membranes often fail in the mechanical durability in humidity cycling because of their rigidity and high swellability (or dimensional instability).21−23 SPAF membrane absorbs less water than that of our aromatic SPP membranes (see Figure 3a) and thus is expected to exhibit better mechanical durability. The humidity cycling test and other stability evaluation of SPAF membranes are in our future agenda and will be reported elsewhere. In conclusion, we have successfully designed and synthesized a new copolymer (SPAF) composed of perfluoroalkylene and sulfonated phenylene groups. The SPAF ionomer membrane exhibited a much smaller-scale phase-separated morphology than that of Nafion, as confirmed by TEM and SAXS analyses.

Figure 4. IR-corrected polarization curves and ohmic resistances for the Nafion-, SPP-, and SPAF-cells at 80 °C under humidity conditions of (a) 100% and (b) 30% RH. The Pt loading amounts were 0.5 mg cm−2 for both electrodes. Pure hydrogen and air were supplied to the anode and cathode, respectively, without back pressure. The gas utilizations at the anode and the cathode were 70% and 40%, respectively.

S7 for IR-included polarization curves). The ohmic resistance of the cells was measured as a high-frequency resistance, which was mainly dominated by the proton transport resistance of the membranes. At 100% RH, the three cells exhibited similarly low ohmic resistance (ca. 0.04 Ω cm2) because of their high proton conductivity (>0.1 S cm−1). At 30% RH, the SPAF-cell exhibited the highest ohmic resistance (ca. 0.33 Ω cm2 at current densities above 0.1 A cm−2), reflecting its low proton conductivity (Figure 3). Interestingly, the ohmic resistance of the SPAF-cell was lower than that expected from the proton conductivity of SPAF (0.59 Ω cm2 at 30% RH), while the experimental ohmic resistances of the Nafion-cell (ca. 0.15 Ω cm2) and SPP-cell (ca. 0.18 Ω cm2) were comparable with those (0.20 and 0.18 Ω cm2, respectively) calculated from their conductivities. The ohmic resistance lower than that calculated from the proton conductivity for the SPAF-cell is an indication that the product water in the cathode catalyst layer was effectively utilized by the membrane (so-called back-diffusion). The IR-free I−V performance mostly reflects the electrode properties. Nevertheless, the three cells exhibited different performances, in the order SPAF ≈ Nafion > SPP. In our previous study, we have discovered the effect of membrane morphology (in particular, at the interface with the catalyst layer) on the cathode performance.6 Membranes with large phase-separated morphology encounter problems with interfacial proton and water transport, causing lower catalytic activity of the cathode. This was the case for the SPP-cell. In contrast, the SPAF membrane, having small ionic domains, was able to promote interfacial proton transport, resulting in improved electrode performance. It is considered that such interfacial compatibility of SPAF and the catalyst layers contributes to the back-diffusion of water from the cathode to the membrane mentioned above. 351

DOI: 10.1021/acsenergylett.6b00198 ACS Energy Lett. 2016, 1, 348−352

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Synthesis and Physical Properties of a Novel Polyelectrolyte. Macromolecules 2005, 38, 5010−5016. (8) Hickner, M. A.; Fujimoto, C. H.; Cornelius, C. J. Transport in Sulfonated Poly(phenylene)s: Proton Conductivity, Permeability, and the State of Water. Polymer 2006, 47, 4238−4244. (9) Skalski, T. J.; Britton, B.; Peckham, T. J.; Holdcroft, S. Structurally-Defined, Sulfo-Phenylated, Oligophenylenes and Polyphenylenes. J. Am. Chem. Soc. 2015, 137, 12223−12226. (10) Chang, Y.; Mohanty, A. D.; Smedley, S. B.; Abu-Hakmeh, K.; Lee, Y. H.; Morgan, J. E.; Hickner, M. A.; Jang, S. S.; Ryu, C. Y.; Bae, C. Effect of Superacidic Side Chain Structures on High Conductivity Aromatic Polymer Fuel Cell Membranes. Macromolecules 2015, 48, 7117−7126. (11) Nakagawa, T.; Nakabayashi, K.; Higashihara, T.; Ueda, M. Polymer Electrolyte Membrane Based on Poly(ether sulfone) Containing Binaphthyl Units with Pendant Perfluoroalkyl Sulfonic Acids. J. Polym. Sci., Part A: Polym. Chem. 2011, 49, 2997−3003. (12) Xu, K.; Oh, H.; Hickner, M. A.; Wang, Q. Highly Conductive Aromatic Ionomers with Perfluorosulfonic Acid Side Chains for Elevated Temperature Fuel Cells. Macromolecules 2011, 44, 4605− 4609. (13) Ghassemi, H.; Schiraldi, D. A.; Zawodzinski, T. A.; Hamrock, S. Poly(arylene ether) s with Pendant Perfluoroalkyl Sulfonic Acid Groups as Proton- Exchange Membrane Materials. Macromol. Chem. Phys. 2011, 212, 673−678. (14) Miyake, J.; Mochizuki, T.; Miyatake, K. Effect of the Hydrophilic Component in Aromatic Ionomers: Simple Structure Provides Improved Properties as Fuel Cell Membranes. ACS Macro Lett. 2015, 4, 750−754. (15) Gebel, G.; Lambard, J. Small-Angle Scattering Study of WaterSwollen Perfluorinated Ionomer Membranes. Macromolecules 1997, 30, 7914−7920. (16) Mochizuki, T.; Kakinuma, K.; Uchida, M.; Deki, S.; Watanabe, M.; Miyatake, K. Temperature- and Humidity-Controlled SAXS Analysis of Proton-Conductive Ionomer Membranes for Fuel Cells. ChemSusChem 2014, 7, 729−733. (17) Kreuer, K. D. On the Development of Proton Conducting Polymer Membranes for Hydrogen and Methanol Fuel Cells. J. Membr. Sci. 2001, 185, 29−39. (18) Lee, M.; Uchida, M.; Yano, H.; Tryk, D. A.; Uchida, H.; Watanabe, M. New Evaluation Method for the Effectiveness of Platinum/Carbon Electrocatalysts under Operating Conditions. Electrochim. Acta 2010, 55, 8504−8512. (19) Inaba, M.; Kinumoto, T.; Kiriake, M.; Umebayashi, R.; Tasaka, A.; Ogumi, Z. Gas Crossover and Membrane Degradation in Polymer Electrolyte Fuel Cells. Electrochim. Acta 2006, 51, 5746−5753. (20) Miyahara, T.; Hayano, T.; Matsuno, S.; Watanabe, M.; Miyatake, K. Sulfonated Polybenzophenone/Poly(arylene ether) Block Copolymer Membranes for Fuel Cell Applications. ACS Appl. Mater. Interfaces 2012, 4, 2881−2884. (21) Sethuraman, V. A.; Weidner, J. W.; Haug, A. T.; Protsailo, L. V. Durability of Perfluorosulfonic Acid and Hydrocarbon Membranes: Effect of Humidity and Temperature. J. Electrochem. Soc. 2008, 155, B119−B124. (22) Lai, Y. H.; Mittelsteadt, C. K.; Gittleman, C. S.; Dillard, D. A. Viscoelastic Stress Analysis of Constrained Proton Exchange Membranes Under Humidity Cycling. J. Fuel Cell Sci. Technol. 2009, 6, 021002−021013. (23) Miyatake, K.; Furuya, H.; Tanaka, M.; Watanabe, M. Durability of Sulfonated Polyimide Membrane in Humidity Cycling for Fuel Cell Applications. J. Power Sources 2012, 204, 74−78.

The proton conductivity of the SPAF membrane was more dependent on humidity and was lower compared to Nafion at low humidities because of the lower acidity of the aromatic sulfonic acid groups in SPAF. Nevertheless, the ohmic resistance of the fuel cell using the SPAF membrane was lower than that calculated from the proton conductivity. The SPAF-cell showed fuel cell performance, in particular, cathode catalytic performance, comparable to or even higher than those for the Nafion- and SPP (our previous hydrocarbon ionomer)cells under all humidity conditions. These results indicate that the SPAF ionomer membrane, with its well-controlled smallscale phase-separated morphology, was effective in improving the interfacial contact in terms of the mass transport (water and protons) between the membrane and the cathode catalyst layers. While fuel cells can be operated under a wide variety of configurations and conditions, SPAF offers an advantage in its ability to be fine-tuned in membrane properties for each application, by adjusting the polymer characteristics such as copolymer composition, perfluoroalkyl length, sulfonation degree, and comonomer sequence.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.6b00198. Experimental details, structural and properties data, and fuel cell performance of SPAF membrane (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partly supported by the New Energy and Industrial Technology Development Organization (NEDO) through the SPer-FC Project and the Ministry of Education, Culture, Sports, Science and Technology (MEXT) Japan through a Grant-in-Aid for Scientific Research (26289254).

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REFERENCES

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DOI: 10.1021/acsenergylett.6b00198 ACS Energy Lett. 2016, 1, 348−352