Toward Improved Alkaline Membrane Fuel Cell Performance Using

Mar 15, 2018 - Toward Improved Alkaline Membrane Fuel Cell Performance Using Quaternized Aryl-Ether Free Polyaromatics. Sandip Maurya† , Cy H. Fujim...
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Cite This: Chem. Mater. 2018, 30, 2188−2192

Toward Improved Alkaline Membrane Fuel Cell Performance Using Quaternized Aryl-Ether Free Polyaromatics Sandip Maurya,† Cy H. Fujimoto,‡ Michael R. Hibbs,‡ Claudia Narvaez Villarrubia,† and Yu Seung Kim*,† †

MPA-11: Materials Synthesis & Integrated Devices, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States ‡ Materials Science and Engineering Center, Sandia National Laboratories, Albuquerque, New Mexico 87185, United States S Supporting Information *

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hydrogen oxidation activity of Pt by measuring HOR voltammograms of Pt in contact with various alkaline ionomers. Second, the H2/O2 AMFC performance of membrane electrode assemblies (MEAs) employing Diels−Alder poly(phenylene) ionomers is evaluated. Commercial Pt/C and Pt− Ru/C alloy catalysts that have different phenyl adsorbing characteristics are used. Third, improved H2/O2 fuel cell performance of MEAs using less-phenyl group containing ionomers is demonstrated. Lastly, the H2 mass transport issue with the polyaromatic electrolytes is briefly discussed. Table S1 displays the physico-electrochemical properties and phenyl group contents of the ionomers used in this study. Figure 1a compares the HOR voltammograms of a Pt/C in 0.1 M tetramethylammonium hydroxide (TMAOH) and benzyltrimethylammonium hydroxide (BTMAOH). The HOR current density of Pt/C in 0.1 M BTMAOH drops as the electrode potential increases from 0.0 to 0.06 V vs RHE (reversible hydrogen electrode), then gradually recovers as the cell potential further increases to ∼0.3 V. This is due to the phenyl group adsorption in the orientation where the phenyl group is parallel to the Pt catalyst surface.30 Figure 1b compares the HOR voltammograms of a Pt microelectrode in contact with three alkaline ionomers, perfluorinated phenylpentamethyl guanidinium (PF-PMG),31 PVBA,32 and benzyltrimethylammonium functionalized poly(phenylene) (PP-BTMA). Lower HOR current densities of Pt were obtained with more phenyl group containing ionomers. The potential range of the suppressed HOR current density for PP-BTMA is 0.0−0.6 V, which is greater than that observed with Pt/C in 0.1 M BTMAOH (0.0−0.3 V). This is attributed to the adsorbed phenyl groups in the BTMAOH solution that starts to desorb at >0.06 V as the molecular configuration is rearranged by cationic group adsorption, which does not occur with nonionic group containg phenyl groups found in PP-BTMA. Phenyl group adsorption and its adverse impact on HOR activity can be reduced with Pt−Ru alloy catalysts. Figure 2a compares the HOR voltammograms of commercial Pt/C and Pt−Ru/C catalysts in 0.1 M BTMAOH. As shown, the suppression of HOR current observed with the Pt/C catalyst between 0.0 and 0.3 V disappeared with the Pt−Ru/C,

lkaline membrane fuel cells (AMFCs) offer an attractive alternative to acidic polymer electrolyte membrane fuel cells (PEMFCs) because inexpensive, nonprecious metal catalysts can potentially be employed.1 Quaternized polyaromatics are one of the promising polymer electrolytes for AMFCs as these materials have good hydroxide conductivity, mechanical properties, and processability. In addition, good thermo-oxidative stability of quaternized polyaromatics enables AMFCs to operate at >60 °C in which greater catalytic activity and lower cell resistance can be acquired in combination with less carbonation problems. Typical polyaromatic electrolytes are quaternized poly(aryl ether sulfone)s prepared via the polycondensation reaction between aromatic dihalides and dihydroxy monomers followed by quaternization.2−7 With this synthetic route, the formation of aryl-ether linkages (C−O−C bonds) is unavoidable. The aryl-ether group in the quaternized polyaromatics was found to be chemically unstable under high pH conditions.8−12 In order to avoid such degradation, aryl-ether free quaternized polyaromatics have been prepared via Diels−Alder reaction,13 acid-catalyzed Friedel−Craft polycondensation,14,15 metal-catalyzed coupling reactions,16−19 and cyclo-polycondensation.20 These aryl-ether free quaternized polyaromatics exhibited excellent alkaline stability. AMFC performance employing polyaromatic ionomers is, however, relatively poor. Though the best performing H2/O2 AMFC employing aryl-ether containing polyaromatic ionomers exhibited the peak power density of 1000 mW cm−2,21 more recent AMFCs employing other aryl-ether containing polyaromatics exhibited the peak power density of 300−380 mW cm−2.22−25 The AMFC performance using aryl-ether free polyaromatics was even lower, ca. the peak power density of 0.92 V, a similar performance between the Pt−Ru/C and Pt/C anode catalyzed MEAs was observed. This is consistent with the RDE results (Figure 2a), indicating that the intrinsic activity of both catalysts is similar. As the cell voltage decreases, however, the 2189

DOI: 10.1021/acs.chemmater.8b00358 Chem. Mater. 2018, 30, 2188−2192

Communication

Chemistry of Materials

Figure 3. (a) iR-corrected polarization curves of the MEAs employing PP-HTMA, PPA-HTMA; membrane: PP-HTMA, anode: Pt−Ru/C (0.5 mgPt cm−2), cathode: Pt/C (0.6 mgPt cm−2); Pabs = 78 kPa. (b) HOR voltammograms of Pt−Ru/C in contact with PPA-HTMA measured at 25 °C, 100% RH. Scan rate: 5 mV s−1, normalized the current density by the limiting current. (c) H2/O2 AMFC performance improvement of MEAs; Pabs of 285 kPa; membrane: PP-HTMA, anode: Pt/C (0.6 mgPt cm−2) or Pt−Ru/C (0.5 mgPt cm−2), cathode: Pt/C (0.6 mgPt cm−2). Flow rate: 500 sccm for H2 and 300 sccm for O2 for all MEAs except the high flow one (2000 sccm from H2 and 1000 sccm for O2).

Figure 3c exhibits the progressive AMFC performance improvement of MEAs employing quaternized Diels−Alder poly(phenylene)s by implementing mitigation strategies adopted in this study. The peak power density of the MEA employing PP-BTMA and Pt/C anode catalyst is 220 mW cm−2, which is comparable to the reported AMFC performance employing other aryl-ether free polyaromatics.8,15,26 Using PPA-HTMA with Pt−Ru/C anode catalyst, the peak power density of the MEA jumped to 460 mW cm−2, and further improvement to a peak power density of 650 mW cm−2 was achieved by employing PPA-HTMA and the Pt−Ru/C anode catalyst. When compared with the performance of acidic Nafionbased MEA, which exhibited 1600 mW cm−2 peak power density (Figure S2), the AMFC performance of the MEA employing PPA-HTMA and Pt−Ru anode catalyst is still significantly lower. Considering that the cell resistance difference between the AMFC and PEMFC MEAs was small ( ∼0.88 V, the AMFC performance of MEA employing PP-HTMA was similar to that of MEA using PP-BTMA. However, as the cell voltage decreases, the AMFC performance employing PPHTMA becomes lower. The inferior performance at the lower cell potentials is probably due to the less efficient desorption of the phenyl group connected to the alkylammonium side chain. It has previously been shown that directly attaching ammonium group to phenyl group (BTMA) helps desorbing the phenyl group from the catalyst surface30 and this effect is likely lessened when the cation is tethered by a flexible spacer (HTMA). Further structural change of PP-HTMA was made with the hexamethylene-trimethylammonium functionalized poly(phenylene alkylene) (PPA-HTMA), which has less phenyl groups in the polymer backbone and side chain. Figure 3a shows that the performance of the MEA employing PPAHTMA is superior to that of the other two MEAs. The kinetic performance of the MEA employing PPA-HTMA is even greater than that of the acidic Nafion-based MEA, ca. 24 mA cm−2 vs 13 mA cm−2 at 0.9 V, suggesting that kinetic overpotential in the AMFC employing PPA-HTMA was less than that in the Nafion-based PEMFC. The microelectrode experiment also shows that the HOR activity of Pt−Ru/C in contact with PPA-HTMA is superior to that of Pt−Ru/C in contact with PP-BTMA at the potential range of 0.0−0.4 V vs RHE (Figure 3b). This confirms that the improved the AMFC performance of MEA using PPA-HTMA in Figure 3a is due to the better HOR activity of Pt−Ru/C with the PPA-HTMA ionomer, which has fewer phenyl groups in the ionomer backbone. 2190

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(11) Mohanty, A. D.; Tignor, S. E.; Krause, J. A.; Choe, Y. K.; Bae, C. Systematic Alkaline Stability Study of Polymer Backbones for Anion Exchange Membrane Applications. Macromolecules 2016, 49, 3361− 3372. (12) Yamaguchi, S. M. T.; Miyanishi, S. Ether Cleavage-Triggered Degradation of Benzyl Alkylammonium Cations for Polyethersulfone Anion Exchange Membranes. Phys. Chem. Chem. Phys. 2016, 18, 12009−12023. (13) Hibbs, M. R.; Fujimoto, C. H.; Cornelius, C. J. Synthesis and Characterization of Poly(Phenylene)-Based Anion Exchange Membranes for Alkaline Fuel Cells. Macromolecules 2009, 42, 8316−8321. (14) Lee, W. H.; Kim, Y. S.; Bae, C. Robust Hydroxide Ion Conducting Poly(Biphenyl Alkylene)s for Alkaline Fuel Cell Membranes. ACS Macro Lett. 2015, 4, 814−818. (15) Lee, W. H.; Park, E. J.; Han, J.; Shin, D. W.; Kim, Y. S.; Bae, C. Poly(Terphenylene) Anion Exchange Membranes: The Effect of Backbone Structure on Morphology and Membrane Property. ACS Macro Lett. 2017, 6, 566−570. (16) Lee, W. H.; Mohanty, A. D.; Bae, C. Fluorene-Based Hydroxide Ion Conducting Polymers for Chemically Stable Anion Exchange Membrane Fuel Cells. ACS Macro Lett. 2015, 4, 453−457. (17) Qiu, X.; Ueda, M.; Fang, Y.; Chen, S. W.; Hu, Z. X.; Zhang, X.; Wang, L. J. Alkaline Stable Anion Exchange Membranes Based on Poly(Phenylene-co-Arylene Ether Ketone) Backbones. Polym. Chem. 2016, 7, 5988−5995. (18) Ono, H.; Miyake, J.; Shimada, S.; Uchida, M.; Miyatake, K. Anion Exchange Membranes Composed of Perfluoroalkylene Chains and Ammonium-Functionalized Oligophenylenes. J. Mater. Chem. A 2015, 3, 21779−21788. (19) Dong, X.; Xue, B. X.; Qian, H. D.; Zheng, J. F.; Li, S. H.; Zhang, S. B. Novel Quaternary Ammonium Microblock Poly(p-Phenylene-coAryl Ether Ketone)s as Anion Exchange Membranes for Alkaline Fuel Cells. J. Power Sources 2017, 342, 605−615. (20) Pham, T. H.; Olsson, J. S.; Jannasch, P. N-Spirocyclic Quaternary Ammonium Ionenes for Anion-Exchange Membranes. J. Am. Chem. Soc. 2017, 139, 2888−2891. (21) Wang, Y.; Wang, G.; Li, G.; Huang, B.; Pan, J.; Liu, Q.; Han, J.; Xiao, L.; Lu, J.; Zhuang, L. Pt-Ru Catalyzed Hydrogen Oxidation in Alkaline Media: Oxophilic Effect or Electronic Effect? Energy Environ. Sci. 2015, 8, 177−181. (22) Zhu, L.; Zimudzi, T. J.; Wang, Y.; Yu, X. D.; Pan, J.; Han, J. J.; Kushner, D. I.; Zhuang, L.; Hickner, M. A. Mechanically Robust Anion Exchange Membranes via Long Hydrophilic Cross-linkers. Macromolecules 2017, 50, 2329−2337. (23) Park, A. M.; Wycisk, R. J.; Ren, X. M.; Turley, F. E.; Pintauro, P. N. Crosslinked Poly(Phenylene Oxide)-Based Nanofiber Composite Membranes for Alkaline Fuel Cells. J. Mater. Chem. A 2016, 4, 132− 141. (24) Akiyama, R.; Yokota, N.; Nishino, E.; Asazawa, K.; Miyatake, K. Anion Conductive Aromatic Copolymers from Dimethylaminomethylated Monomers: Synthesis, Properties, and Applications in Alkaline Fuel Cells. Macromolecules 2016, 49, 4480−4489. (25) Zhu, L.; Pan, J.; Wang, Y.; Han, J. J.; Zhuang, L.; Hickner, M. A. Multication Side Chain Anion Exchange Membranes. Macromolecules 2016, 49, 815−824. (26) Mahmoud, A. M. A.; Elsaghier, A. M. M.; Otsuji, K.; Miyatake, K. High Hydroxide Ion Conductivity with Enhanced Alkaline Stability of Partially Fluorinated and Quaternized Aromatic Copolymers as Anion Exchange Membranes. Macromolecules 2017, 50, 4256−4266. (27) Mamlouk, M.; Horsfall, J. A.; Williams, C.; Scott, K. Radiation Grafted Membranes for Superior Anion Exchange Polymer Membrane Fuel Cells Performance. Int. J. Hydrogen Energy 2012, 37, 11912− 11920. (28) Wang, L.; Magliocca, E.; Cunningham, E. L.; Mustain, W. E.; Poynton, S. D.; Escudero-Cid, R.; Nasef, M. M.; Ponce-Gonzalez, J.; Bance-Souahli, R.; Slade, R. C. T.; Whelligan, D. K.; Varcoe, J. R. An Optimized Synthesis of High Performance Radiation-Grafted AnionExchange Membranes. Green Chem. 2017, 19, 831−843.

stable aryl-ether free polyaromatic ionomers in use of AMFCs. H2 mass transport issue in the high current density region remains to be resolved for further improvement of AMFC performance and durability.

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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/acs.chemmater.8b00358. Synthesis and characterization of polymers and microelectrode data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Y. S. Kim. E-mail: [email protected]. ORCID

Sandip Maurya: 0000-0002-7600-2008 Yu Seung Kim: 0000-0002-5446-3890 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We cordially thank Dr. K. S. Lee and Dr. E. J. Park for supplying PVBAs for this study. This work was supported by the US DOE, Office of Energy Efficiency and Renewable Energy (EERE), Fuel Cell Technologies Office (FCTO) under contract no. DE-AC52-06NA25396 (Los Alamos National Laboratory) and DE-AC04-94AL85000 (Sandia National Laboratories).

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REFERENCES

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