Poly(p-phenylene sulfone)s with High Ion Exchange Capacity: Ionomers with Unique Microstructural and Transport Features C. C. de Araujo 1 , K.-D. Kreuer 1 , M. Schuster 2 , V. Atanasov 1 , G. Portale 3 , J. Maier 1 1 Max Planck Institute for Solid State Research, Heisenbergstr. 1, D-70569 Stuttgart, Germany 2 FuMATech, Am Grubenstollen 11, D-66386 St. Ingbert, Germany 3 DUBBLE, BM26 at ESRF, 6 rue Jules Horowitz, BP220, F-38043 Grenoble, Cedex, France e-mail: c.araujo@fkf.mpg.de Acknowledgements The authors thank A. Fuchs, U. Klock and U. Traub for technical assistance. Financial support from the BMBF (project 03SF0323C, HiPEM) and the Stiftung Energieforschung Baden-Württemberg (project FKZ A 233 05) is also acknowledged. Preparation Abstract Poly(phenylene) ionomers which contain merely sulfone units (-SO 2 -) connecting the phenyl rings and in which each phenyl ring is sulfonated (-SO 3 H) have been characterized with respect to their hydrolytic stability, microstructural and transport properties. The high degree of sulfonation leads to the development of a microstructure characterized by very narrow hydrated, hydrophilic domains which are well connected on larger scales. These features together with high absolute water uptakes at given relative humidities and the high charge carrier concentration corresponding to the high ion exchange capacity (IEC ~ 4.5 meq./g) result in very high proton conductivities but also low water transport coefficients (water diffusion and presumably also electroosmotic drag and permeation). Compared to the transport properties of Nafion, these trends increase with increasing water content and temperature. Under the same conditions sulfonated poly(p-phenylene sulfone)s show very high hydrolytic stability. Transport SO 2 F F SO 3 Na NaO 3 S SO 2 S SO 3 Na NaO 3 S SO 2 HO 3 S Na 2 S / NaOAc / NMP H 2 O 2 n n 2n 10000 100000 0.0 0.5 1.0 1.5 2.0 2.5 W (log M) Molecular Weight (g/mol) M w = 61 000 g/mol Gel Permeation Chromatography (GPC) IEC = 4.5 mmol/g EW = 220 g/mol 0 20 40 60 80 0 2 4 6 8 10 time (h) [H 2 O] / [-SO 3 H] 100 110 120 130 140 150 160 170 180 190 Temperature (°C) Water uptake and hydrolytic stability, p(H 2 O) = 10 5 Pa The narrow spacing within S-220 (~1.44 nm) is almost two times smaller than that of Nafion (~3.3 nm) Increasing degree of sulfonation: decrease in the characteristic separation length development of well ordered hydrated structure Increasing water content:  increase of the typical separation length Proton Mobility (D  ) and Water Self-Diffusion Mobility (D H2O ) 0 5 10 15 20 25 30 0.01 0.1  = [H 2 O]/[-SO 3 H]  Scm -1 ) reversible weight changes during cooling and heating runs: high hydrolytic stability Microstructure (SAXS) ppm (t1) 7.00 7.50 8.00 8.50 9.00 8.464 8.438 8.396 8.081 8.053 H 1 H, liquid-state NMR Stability HO 3 S H H 2 SO 4 + + H 2 O References [1] M. Schuster, K.-D. Kreuer, H. T. Andersen, J. Maier, Macromolecules, 40, 598-607 (2007). [2] M. Schuster, C. C. de Araujo, V. Atanasov, H. T. Andersen, K.-D. Kreuer, J. Maier, Macromolecules, 42, 3129 (2009). [3] C. C. de Araujo, K. D. Kreuer, M. Schuster, G. Portale , H. Mendil-Jakani, G. Gebel, and J. Maier, PCCP, 11, 3305 (2009). Conductivity Conductivity, T = 298 K S-360 2.7 mmol/g Nafion 1100 0.9 mmol/g 2.20 2.25 2.30 2.35 2.40 2.45 2.50 2.55 2.60 2.65 1E-3 0.01 0.1  Scm -1 ) 1000/T (K -1 ) S-1070 0.9 mmol/g S-360 2.7 mmol/g S-220 4.5 mmol/g Nafion 0.9 mmol/g RH = 25% RH = 50% RH = 16% Conductivity at high temperature p(H 2 O) = 1 atm (10 5 Pa) 140 °C 25% RH Conductivity increase at high temperature and low humidification is more pronounced: 7x higher than Nafion! At low water content S-220 and Nafion have similar proton conductivity. At high water content (~10) S-220 conductivity is about 3 times higher. The reasons for higher conductivity are higher absolute water content and higher concentration of protonic charge carriers water volume fraction  0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 D / cm 2 s -1 10 -7 10 -6 10 -5 Nafion 0.9 mmol/g S-220 4.5 mmol/g D  D  D H 2 O D H 2 O SO 2 H O 3 S n S-220 4.5 mmol/g S-1070 0.9 mmol/g S-360 2.7 mmol/g S-220 4.5 mmol/g Nafion 0.9 mmol/g SO 2 H O 3 S n Nafion, RH = 75%,  = 7,  = 0.2 S-220, RH = 75%,  = 7,  = 0.5 S-220, RH = 40%,  = 5,  = 0.4 S-220 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 0.01 0.1  =3.1 160 °C 135 °C 100% 85% 50%  = 11.6  =3.2  = 4.9 1000/T / K -1  Scm -1  = 8.7 RH = 30% 100 °C x7 T / °C Conclusions and Outlook Highly sulfonated poly(p-phenylene sulfone) are shown to combine high hydrolytic stability with very high proton conductivity. This is the result of the very high charge carrier concentration (IEC up to 4.5 meq/g) and the very high water uptakes for a given relative humidity leading to the development of a well connected aqueous channel like structure facilitating the long range transport of water and hydrated species. Locally, it Nafion S-220 consists of very narrow structures retarding the water diffusion. However, the polymer with the highest IEC is water soluble and brittle in the dry state. The preparation of membranes with highly sulfonated poly(p-phenylene sulfone) polymer as main constituent (e.g. as part of blends, interpenetrating networks, complexes, copolymers) is in progress in our group. At room temperature and high water content D  hardly exceeds the water diffusion coefficient, meaning that there is no significant conductivity contribution from structure diffusion (Grotthuss mechanism) Low water diffusion coefficient of the sulfonated poly(p- phenylene sulfone)s even at high degrees of hydration as a consequence of the very narrow water structures At room temperature S-220 has proton conductivity in the range of the conductivity of Nafion Under conditions similar to those relevant for PEM fuel cell, even the half-sulfonated poly(p-phenylene sulfone), S-360, has conductivity higher than Nafion Room Temperature Less stable complexes are obtained when they are ortho- and para- electron acceptor substituted.