Journal of Membrane Science 212 (2003) 263–282 Fabrication and characterization of heteropolyacid (H 3 PW 12 O 40 )/directly polymerized sulfonated poly(arylene ether sulfone) copolymer composite membranes for higher temperature fuel cell applications Yu Seung Kim a , Feng Wang a,1 , Michael Hickner a , Thomas A. Zawodzinski b,2 , James E. McGrath a,∗ a Department of Chemistry, Materials Research Institute, Virginia Polytechnic Institute and State University, 2108 Hahn Hall (0344), Blacksburg, VA 24061, USA b Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, NM 87545, USA Received 4 February 2002; received in revised form 18 October 2002; accepted 22 October 2002 Abstract The feasibility of heteropolyacid (HPA)/sulfonated poly(arylene ether sulfone) composite membranes for use in proton ex- change membrane (PEM) fuel cells was investigated. Partially disulfonated poly(arylene ether sulfone)s (BPSH) copolymers were prepared by direct aromatic nucleophilic copolymerization and solution-blended with a commercial HPA, phospho- tungstic acid. Fourier transform infrared (FTIR) spectroscopy band shifts showed that sulfonic acid groups on the polymer backbone interact with both bridging tungstic oxide and terminal tungstic oxide in the phosphotungstic acid molecule, in- dicative of an intermolecular hydrogen bonding interaction between the copolymer and the HPA additive. The composite membranes generally exhibited a low HPA extraction after water vapor treatment, except for the 60 mol% disulfonated BPSH where significant HPA extraction from the composite membrane occurred because of excessive matrix swelling. The com- posite membrane not only had good thermal stability (decomposition temperature in nitrogen >300 ◦ C), but also showed improved mechanical strength and lower water uptake than the unfilled membranes possibly due to the specific interaction. The composite membranes displayed good proton conductivity especially at elevated temperatures (e.g. 130 ◦ C). For example, fully hydrated membranes consisting of 30 wt.% HPA and 70 wt.% BPSH with 40 mol% disulfonation had a conductivity of 0.08 S/cm at room temperature which linearly increased up to 0.15 S/cm at 130 ◦ C. In contrast, the pure copolymer had a proton conductivity of 0.07 S/cm at room temperature only reached a maximum conductivity of 0.09 S/cm, most probably due to dehydration at elevated temperatures. The dehydration process was monitored by dynamic infrared spectra by observing the intensity reduction of the sulfonate group and distinctive changes of shape in the hydroxyl vibrations as the sample was heated. Combining infrared results with dynamic thermogravimetric data showed that the composite membrane had much higher water retention from 100 to 280 ◦ C than the pure sulfonated copolymer. Those results suggested that the incorporation ∗ Corresponding author. Tel.: +1-540-2315976; fax: +1-540-2318517. E-mail address: jmcgrath@vt.edu (J.E. McGrath). 1 Present address: PPG Industries Inc., 440 College Park Drive, Monroeville, PA 15146, USA. 2 Present address: Department of Chemical Engineering, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, OH 44106, USA. 0376-7388/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII:S0376-7388(02)00507-0