Effect of the Side-Chain Structure of Perfluoro-Sulfonic Acid Ionomers

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Research Article Cite This: ACS Catal. 2018, 8, 694−700

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Effect of the Side-Chain Structure of Perfluoro-Sulfonic Acid Ionomers on the Oxygen Reduction Reaction on the Surface of Pt Kensaku Kodama,*,† Kenta Motobayashi,‡,§ Akihiro Shinohara,† Naoki Hasegawa,† Kenji Kudo,† Ryosuke Jinnouchi,† Masatoshi Osawa,‡ and Yu Morimoto† †

Toyota Central R&D Laboratories, Inc., 41-1 Yokomichi, Nagakute, Aichi 480-1192, Japan Institute for Catalysis, Hokkaido University, Sapporo 001-0021, Japan

‡

S Supporting Information *

ABSTRACT: The effect of side-chain structures in perfluoro-sulfonic acid ionomers on the adsorption of the terminal sulfonate moiety on the surface of Pt is investigated with voltammetry and surface-enhanced infrared absorption spectroscopy (SEIRAS). Analyses with lowmolecular-weight model anions with and without an ether group in the perfluoro-alkyl chain indicate that the anions are adsorbed on Pt through one or two oxygen atom(s) of the terminal sulfonate group and that the oxygen atom of the ether group also interacts with the Pt surface, leading to stronger adsorption of the anions with an ether group. On the basis of the results obtained with the model anions, the adsorption of the terminal sulfonate moieties in perfluorinated sulfonic acid ionomers and its effect on oxygen reduction reaction (ORR) is discussed. It is shown that the ionomers having longer side chains more strongly block ORR due to the flexibility of the side chains. KEYWORDS: ionomer, side chain, catalyst poisoning, surface-enhanced infrared absorption spectroscopy (SEIRAS), Pt (111), oxygen reduction reaction

1. INTRODUCTION In catalyst layers of polymer electrolyte fuel cells (PEFCs), a solid polymer electrolyte, called the “ionomer” to distinguish it from the electrolyte membrane, is generally used to improve proton conductivity.1 Although ionomer-free nanostructured thin film (NSTF) catalyst layers were invented by the 3M company,2 their performance is still lower than that of ionomercontained catalyst layers under dry conditions due to the serious deterioration of proton conductivity.3 Perfluorinated sulfonic acid polymers (e.g., Nafion) have almost exclusively been used as ionomers, as well as electrolyte membranes, owing to their high chemical stability and acidity. The effect of sulfonate anions of the polymer on the kinetics of the oxygen reduction reaction (ORR) has been discussed,4−12 but inconsistent experimental results, e.g., suppression and acceleration of ORR, have been reported. Recently, by using Pt single-crystal electrodes coated with perfluorinated sulfonic acid ionomer films in a liquid electrolyte, Subbaraman et al.13,14 observed voltammetric features signaling the adsorptions of sulfonate moieties in the ionomers on the Pt surfaces and showed that ORR kinetics is slowed down. More recently, using a solid-state electrochemical cell, Kodama et al.15 found increased adsorptivity of sulfonate moieties on the Pt(111) surface with drying of the ionomer, and this result suggests that ORR activities of Pt catalysts are seriously deteriorated under dry conditions in practical PEFC operations. The ionomer © 2017 American Chemical Society

effects were also examined for Pt nanoparticles under wellcontrolled experimental conditions,16−18 and their ORR kinetics were shown to be suppressed by the ionomers. As mentioned above, terminal sulfonate moieties on the side chains in Nafion are believed to adsorb on Pt,13−15,19−22 and the decrease in active surface area of the catalysts caused by the adsorption of terminal sulfonate moieties is now widely recognized as one of the primary factors limiting the efficiency of PEFCs.23 Nevertheless, Berná et al.24 have shown that trifluoromethanesulfonic acid (TFMSA), CF3SO3H, is not adsorbed on low-index Pt single crystal electrodes. This observation suggests the presence of some other factors controlling the adsorptivity of sulfonate moiety and electrocatalytic activity on the surface of Pt. The aim of the present work is to elucidate the effect of sidechain structures of ionomers on the adsorption of the terminal sulfonate moiety and to clarify its relation to ORR. First, we examine the adsorption of low-molecular-weight perfluorinated sulfonic acids, nonafluorobutanesulfonic acid (NFBSA, C4F9SO3H), and perfluoro-(2-ethoxyethane) sulfonic acid (PESA, C2F5OC2F4SO3H), on (111)-oriented single-crystal and poly-oriented Pt electrodes by cyclic voltammetry and Received: October 19, 2017 Revised: December 1, 2017 Published: December 7, 2017 694

DOI: 10.1021/acscatal.7b03571 ACS Catal. 2018, 8, 694−700

Research Article

ACS Catalysis surface enhanced infrared absorption spectroscopy (SEIRAS).25 Those simple compounds were used instead of perfluorinated sulfonic acid polymers in the SEIRAS analysis because strong CF2 stretching vibrations of perfluoro-alkyl backbones of ionomers mask the spectral range of the S−O stretching bands of the terminal sulfonate and prevent the elucidation of their adsorbed structure. The two model acids have similar sizes but are differentiated by the absence and presence of an ether group on the perfluoro-alkyl chain (Figure 1a). The

2.1.3. Preparation of Ionomer Thin Films. The Pt(111) surface prepared as described in section 2.1.1 was coated with a thin film of an ionomer by dropping 20 μL of 0.005 wt % ionomer solution (20 wt % dimethylformamide (DMF) aqueous solution) followed by evaporation of the solvent on the surface in the stream of Ar, and finally heating the film at 420 K in the stream of Ar to improve the physicomechanical stability of the film. Further details were described elsewhere.21 The tested ionomers are Nafion, Aquivion and two 3M PFSA ionomers, which are termed respectively as “long side chain (LC) polymer”, “short side chain (SC) polymer”, and “long ether-SO3H distance (LD) polymer” as shown in Figure 1b. Their equivalent weights (EW; the weight of polymer in the units of grams per the amount of sulfonic acid group in the units of moles) and y/x ratios (the number ratios of tetrafluoroethylene (TFE) entities to side chain entities; see Figure 1) are shown in Table 1. The amounts of the ionomers Table 1. Tested Ionomers and Their Characteristics Nafion Aquivion 3M PFSA

Figure 1. (a) Molecular structures of nonafluorobutanesulfonic acid (NFBSA) and perfluoro-(2-ethoxyethane) sulfonic acid (PESA). (b)Those of Nafion, Aquivion, and 3 M PFSA ionomer, which are termed as “long side chain (LC) polymer”, “short side chain (SC) polymer”, and “long ether-SO3H distance (LD) polymer”, respectively.

side chain character

EW

y/x

notation

long short long distance between ether/SO3H

1100 830 825 1000

6.6 5.5 4.5 6.2

LC SC LD825 LD1000

deposited on the Pt(111) surface are equal on a weight basis (corresponding to the thickness of ca. 35 nm). The uniformities of the ionomer films were checked using scanning electron microscopy (SEM) after the electrochemical measurements as carried out in the previous study,21 and it was confirmed that the whole Pt(111) surface was kept covered by the ionomer throughout the electrochemical measurements for all the ionomers (Figure S1 in the Supporting Information). 2.2. Electrochemical Measurements. 2.2.1. Measurements with Low-Molecular-Weight Perfluorinated Sulfonic Acids. The Pt (111) surface prepared as described in section 2.1.1 was moved to N2-purged glovebox and then immersed in 0.1 M NFBSA or 0.1 M PESA electrolytes vacuum-deaerated. Afterward, the cyclic voltammograms (CVs) at a scan rate of 50 mV s−1 were acquired at room temperature. The deaeration by bubbling the solution with inert gas was not suitable in the present case because the solutions of the perfluorinated sulfonic acids are foamy. The vacuum deaeration combined with the electrochemical purification greatly improved the quality of the CVs (Figure S2 in the Supporting Information). 2.2.2. Measurements with Ionomer Thin Films. The Pt(111) surface coated with the ionomer thin films prepared in section 2.1.3 were immersed in 0.1 M HClO4 in the configuration of hanging meniscus rotating disk electrode (HM-RDE) and then were conditioned as described in a previous paper.21 Cyclic and linear-sweep voltammetries were carried out respectively under inert and oxygen-saturated conditions at a scan rate of 50 mV s−1. The temperature was 303 K. The electrode was rotated at 1600 rpm in the ORR linear-sweep voltammetry. 2.3. SEIRAS. The SEIRAS measurements were carried out with the Kretschmann attenuated total reflection (ATR) technique, where the infrared (IR) radiation was introduced from the behind of the working electrode (WE), a polycrystalline Pt film deposited on an Si prism. The Pt film was prepared on the total-reflecting plane of the Si prism with a chemical deposition technique using a commercially available plating

comparison of the two model compounds enables us to discuss the role of the ether group in the sulfonic acid adsorption. Then, we examine adsorption behaviors of several ionomers with difference in side-chain length and in the locations of the ether group on Pt (111) with voltammetry. The adsorption of the ionomers is discussed based on the results obtained with the low-molecular-weight compounds by focusing on the effect of the side-chain length and the role of the ether group. Those studies are expected to provide useful information for developing desirable ionomers.

2. EXPERIMENTAL SECTION 2.1. Materials. 2.1.1. Preparation of Pt(111). A (111)oriented Pt single crystal disk (99.99%, 0.196 cm2, MaTecK) was annealed using electromagnetic inductive heating for 10 min at 1400−1650 K in the flow of a mixture of H2 and Ar (3% H2, Taiyo Nippon Sanso, the purity of each gas: H2 99.99999%, Ar 99.999%). The annealed specimen was slowly cooled to room temperature in the flow of the same mixed gas and then, the Pt (111) surface was covered by a droplet of ultrapure water (Milli-Q, 18.2 MΩ). 2.1.2. Preparations of Electrolytes of Low-MolecularWeight Sulfonic Acids. The aqueous solutions of NFBSA and PESA with the concentration of 0.1 M were prepared from the chemicals of 98.0% NFBSA (Tokyo Chemical Industry) and 97% PESA (SynQest), respectively, and then purified with a conventional electrochemical method:26 organic impurities were collected with a platinized Pt gauze at the controlled potential of 0.44 V vs reversible hydrogen electrode (RHE). Afterward, the solutions were dosed with small amounts of Ba(OH)2 (to be 2 mM) and then kept at least overnight to precipitate residual sulfate in these chemicals ( SC, while those for the potentials of the adsorption and desorption peaks are in the order of LD825 < LC < LD1000 < SC. A clear correlation is not found between the amount and strength. Figure 6 shows the ORR voltammograms in the positive scans and Tafel plots of the ORR kinetic currents for bare and ionomer-coated Pt(111) surfaces. In the Tafel analysis, the usual mass-transport correction40 was performed. The ORR activity is significantly (70−80%) suppressed by all the ionomer coatings. Interestingly, however, the coverage of the sulfonate moieties adsorbed on the Pt surface is estimated to be only 0.07−0.09 monolayer (ML) from the CVs (Figure 5 and Table 2) by assuming that the adsorption is a one-electron-transfer process (−SO3− + Pt → −SO3 − Pt + e−). We will discuss later why ORR is suppressed so significantly by such a small amount of adsorbed sulfonate moiety. The degrees of the suppression are in the order of LC > LD825 > LD1000 ≥ SC, and this order is the same as the order for the amounts of sulfonate adsorption. This correlation is further confirmed quantitatively by Figure 7a, in which the ORR currents at 0.82 V (RHE) are plotted as a function of the peak charge. The linear relationship between the ORR current and the peak charge demonstrates that the adsorption of sulfonate moieties suppresses ORR by blocking active sites. In contrast, such a clear correlation is not found in the ORR

Figure 6. (a) ORR polarization curves in the positive scans at 50 mV s−1 with the electrode rotation of 1600 rpm and (b) Tafel plots of the ORR kinetic currents for Pt(111) surfaces covered by the ionomers and for the bare surface. 698

DOI: 10.1021/acscatal.7b03571 ACS Catal. 2018, 8, 694−700

Research Article

ACS Catalysis

Figure 8. Possible adsorbed structures of ionomers on the Pt surface: (a) LC, (b) SC, and (c) LD.

The difference in flexibility can explain the smaller amount of the sulfonate adsorption and the less significant ORR suppression for SC than for LC. The situation for LD is the same as for SC (Figure 8c), but its longer side chain and its higher flexibility can suppress ORR more strongly than SC. Accordingly, ORR suppression arising from the structural causes (chain length and flexibility) should be in the order of LC > LD > SC. Actually, this trend is consistent with experimental results with membrane electrode assemblies (MEAs) reported by Jomori et al.41 and by Park et al.42 Their experimental results showed improvements in MEA performance in a low current region by using short-side-chain ionomers and can be reasonably explained at molecular scale with the combination of voltammetry with single-crystals and SEIRAS in the present study. In addition, the consistent results with the MEA tests, where Pt nanoparticles were used for the catalysts, suggest that the proposed adsorption state also represents the situations on the practical catalysts. Actually, the size of PESA anion, ca. 1 nm in length, estimated from typical bond lengths in the molecule (ca. 0.15 nm) is equal to or smaller than that of facets on Pt nanoparticles, ca. 1−2 nm (5− 10 atoms) in width,43 and thus, the depictions in Figure 4 and 8 are applicable also to Pt nanoparticles. Discussion above also gives an answer to the question raised before why small amounts of sulfonate adsorption significantly suppress ORR: not only sulfonate moiety but also perfluoro-alkyl chain blocks the Pt sites for ORR. Finally, it should be noted that there is still a large gap between the activities on the bare and SC-coated surfaces (Figure 6). One strategy to further mitigate the ionomerinduced ORR suppression is to combine the two concepts of shortening side-chain and the structure modification of anionic group. The latter concept has been confirmed to have a definite effect for the purpose.21 Another strategy is to further reduce the interactions between the ether groups and Pt surface, and this topic will be discussed in near furture.

becomes less significant with the decrease in the length of side chain, presumably owing to lowered flexibilities of the side chains for being oriented to the state where ether groups interact with the Pt surface. Those findings can provide guidelines in designing the side chain structures of ionomers in the development of PEFCs.

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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/acscatal.7b03571. Confirmation of the uniformities of ionomer films on the Pt(111) surface, effects of deaeration method and purification on the quality of CV for Pt(111), and CVs in SEIRAS experiments (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Kensaku Kodama: 0000-0003-1120-6306 Kenta Motobayashi: 0000-0003-3090-2112 Yu Morimoto: 0000-0001-9140-1691 Present Address §

K.M.: Department of Physical Science and Engineering, Nagoya Institute of Technology, Nagoya 466-8555, Japan.

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The 3M ionomers were supplied with arrangements by Steven Hamrock, Kazuki Noda, and Michael Yandrasits from the 3M company.

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4. CONCLUSIONS The adsorptivities of various perfluoro-sulfonic acids, including low-molecular-weight model compounds and ionomers, on Pt surface were investigated with voltammetry using Pt(111) surface and with SEIRAS on a Pt polycrystalline film. The cyclic voltammograms and SEIRA spectra with low-molecular-weight perfluoro-sulfonic acids suggested that the anion adsorptivity is higher for the compounds with an ether group than for those without it. The band assignments in SEIRA spectra suggested an important role of the interaction between the ether group and Pt surface in the adsorption process. Experiments with Pt (111) surfaces coated with ionomer films exhibited that the suppression of ORR by sulfonate moieties in ionomers

REFERENCES

(1) Wilson, M. S.; Gottesfeld, S. J. Appl. Electrochem. 1992, 22, 1−7. (2) Debe, M. K. Nature 2012, 486, 43−51. (3) Sinha, P. K.; Gu, W.; Kongkanand, A.; Thompson, E. J. Electrochem. Soc. 2011, 158, B831−B840. (4) Gottesfeld, S.; Raistrick, I. D.; Srinivasan, S. J. Electrochem. Soc. 1987, 134, 1455−1462. (5) Lawson, D. R.; Whiteley, L. D.; Martin, C. R.; Szentirmay, M. N.; Song, J. I. J. Electrochem. Soc. 1988, 135, 2247−2253. (6) Paik, W.; Springer, T. E.; Srinivasan, S. J. Electrochem. Soc. 1989, 136, 644−649. (7) Uribe, F. A.; Springer, T. E.; Gottesfeld, S. J. Electrochem. Soc. 1992, 139, 765−773. (8) Parthasarathy, A.; Srinivasan, S.; Appleby, A. J.; Martin, C. R. J. Electrochem. Soc. 1992, 139, 2530−2537. 699

DOI: 10.1021/acscatal.7b03571 ACS Catal. 2018, 8, 694−700

Research Article

ACS Catalysis (9) Zecevic, S. K.; Wainright, J. S.; Litt, M. H.; Gojkovic, S. L.; Savinell, R. F. J. Electrochem. Soc. 1997, 144, 2973−2982. (10) Ayad, A.; Naimi, Y.; Bouet, J.; Fauvarque, J. F. J. Power Sources 2004, 130, 50−55. (11) Yano, H.; Higuchi, E.; Uchida, H.; Watanabe, M. J. Phys. Chem. B 2006, 110, 16544−16549. (12) Ohma, A.; Fushinobu, K.; Okazaki, K. Electrochim. Acta 2010, 55, 8829−8838. (13) Subbaraman, R.; Strmcnik, D.; Stamenkovic, V.; Markovic, N. M. J. Phys. Chem. C 2010, 114, 8414−8422. (14) Subbaraman, R.; Strmcnik, D.; Paulikas, A. P.; Stamenkovic, V. R.; Markovic, N. M. ChemPhysChem 2010, 11, 2825−2833. (15) Kodama, K.; Jinnouchi, R.; Suzuki, T.; Murata, H.; Hatanaka, T.; Morimoto, Y. Electrochem. Commun. 2013, 36, 26−28. (16) Shinozaki, K.; Zack, J. W.; Pylypenko, S.; Pivovar, B. S.; Kocha, S. S. J. Electrochem. Soc. 2015, 162, F1384−F1396. (17) Garrick, T. R.; Moylan, T. E.; Yarlagadda, V.; Kongkanand, A. J. Electrochem. Soc. 2017, 164, F60−F64. (18) Shinozaki, K.; Morimoto, Y.; Pivovar, B. S.; Kocha, S. S. J. Power Sources 2016, 325, 745−751. (19) Gomez-Marin, A. M.; Berna, A.; Feliu, J. M. J. Phys. Chem. C 2010, 114, 20130−20140. (20) Ahmed, M.; Morgan, D.; Attard, G. A.; Wright, E.; Thompsett, D.; Sharman, J. J. Phys. Chem. C 2011, 115, 17020−17027. (21) Kodama, K.; Shinohara, A.; Hasegawa, N.; Shinozaki, K.; Jinnouchi, R.; Suzuki, T.; Hatanaka, T.; Morimoto, Y. J. Electrochem. Soc. 2014, 161, F649−F652. (22) Tymoczko, J.; Calle-Vallejo, F.; Colic, V.; Koper, M. T. M.; Schuhmann, W.; Bandarenka, A. S. ACS Catal. 2014, 4, 3772−3778. (23) Kongkanand, A.; Mathias, M. F. J. Phys. Chem. Lett. 2016, 7, 1127−1137. (24) Berná, A.; Feliu, J. M.; Gancs, L.; Mukerjee, S. Electrochem. Commun. 2008, 10, 1695−1698. (25) Osawa, M. Bull. Chem. Soc. Jpn. 1997, 70, 2861−2880. (26) Damjanovic, A.; Genshaw, M. A.; Bockris, J. O. M. J. Electrochem. Soc. 1967, 114, 466−472. (27) Miki, A.; Ye, S.; Osawa, M. Chem. Commun. 2002, 1500−1501. (28) Wakisaka, M.; Suzuki, H.; Mitsui, S.; Uchida, H.; Watanabe, M. Langmuir 2009, 25, 1897−1900. (29) Bondarenko, A. S.; Stephens, I. E. L.; Hansen, H. A.; PerezAlonso, F. J.; Tripkovic, V.; Johansson, T. P.; Rossmeisl, J.; Norskov, J. K.; Chorkendorff, I. Langmuir 2011, 27, 2058−2066. (30) Strmcnik, D.; Escudero-Escribano, M.; Kodama, K.; Stamenkovic, V. R.; Cuesta, A.; Markovic, N. M. Nat. Chem. 2010, 2, 880−885. (31) Warren, D. S.; McQuillan, A. J. J. Phys. Chem. B 2008, 112, 10535−10543. (32) Chu, D.; Gervasio, D.; Razaq, M.; Yeager, E. B. J. Appl. Electrochem. 1990, 20, 157−162. (33) Gejji, S. P.; Hermansson, K.; Lindgren, J. J. Phys. Chem. 1993, 97, 3712−3715. (34) Danilczuk, M.; Lin, L.; Schlick, S.; Hamrock, S. J.; Schaberg, M. S. J. Power Sources 2011, 196, 8216−8224. (35) Yamaguchi, M.; Ohira, A. J. Phys. Chem. A 2012, 116, 10850− 10863. (36) Osawa, M.; Ataka, K.; Yoshii, K.; Nishikawa, Y. Appl. Spectrosc. 1993, 47, 1497−1502. (37) Jinnouchi, R.; Hatanaka, T.; Morimoto, Y.; Osawa, M. Phys. Chem. Chem. Phys. 2012, 14, 3208−3218. (38) Osawa, M.; Tsushima, M.; Mogami, H.; Samjeske, G.; Yamakata, A. J. Phys. Chem. C 2008, 112, 4248−4256. (39) Jinnouchi, R.; Kudo, K.; Kitano, N.; Morimoto, Y. Electrochim. Acta 2016, 188, 767−776. (40) Bard, A. J.; Faulkner, L. R. In Electrochemical methods, Fundamentals and Applications, 2nd ed.; John Wiley & Sons: Hoboken, NJ, 2001; pp 339−341. (41) Jomori, S.; Komatsubara, K.; Nonoyama, N.; Kato, M.; Yoshida, T. J. Electrochem. Soc. 2013, 160, F1067−F1073.

(42) Park, Y. C.; Kakinuma, K.; Uchida, H.; Watanabe, M.; Uchida, M. J. Power Sources 2015, 275, 384−391. (43) Aarons, J.; Jones, L.; Varambhia, A.; MacArthur, K. E.; Ozkaya, D.; Sarwar, M.; Skylaris, C. K.; Nellist, P. D. Nano Lett. 2017, 17, 4003−4012.

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DOI: 10.1021/acscatal.7b03571 ACS Catal. 2018, 8, 694−700