Materials Science and Engineering B 173 (2010) 260–266 Contents lists available at ScienceDirect Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb Composite electrolytes composed of Cs-substituted phosphotungstic acid and sulfonated poly(ether–ether ketone) for fuel cell systems Song-Yul Oh a,∗ , Toshihiro Yoshida a , Go Kawamura a , Hiroyuki Muto b , Mototsugu Sakai a , Atsunori Matsuda a,∗ a Department of Materials Science, Toyohashi University of Technology, 1-1 Hibarigaoka, Tempaku-cho, Toyohashi, Aichi 441-8580, Japan b Department of Materials Science and Engineering, Kurume National College of Technology, 1-1-1 Komorino, Kurume, Fukuoka 830-8555, Japan article info Article history: Received 25 August 2009 Received in revised form 1 March 2010 Accepted 2 March 2010 Keywords: Fuel cell Cs-substituted phosphotungstic acid Sulfonated poly(ether–ether ketone) Advanced composite abstract Composite electrolytes composed of cesium hydrogen sulfate containing phosphotungstic acids (CsHSO 4 –H 3 PW 12 O 40 ) and sulfonated poly(ether–ether ketone) (SPEEK) were prepared by casting the corresponding precursor for application in fuel cells. Partially Cs-substituted phosphotungstic acids (Cs x H 3-x PW 12 O 40 ) were formed in the CsHSO 4 –H 3 PW 12 O 40 system by mechanochemical treatment. SPEEK was prepared from PEEK by sulfonation using concentrated sulfuric acid. Flexible composite elec- trolytes were obtained and their electrochemical properties were markedly improved with the addition of Cs x H 3-x PW 12 O 40 , into the SPEEK matrix. A maximum power density of 213 mW cm -2 was obtained from the single cell test for 50H 3 PW 12 O 40 –50CsHSO 4 in SPEEK (1/5 by weight) composite electrolyte at 80 ◦ C and at 80 RH%. Electrochemical properties and transmission electron microscopy (TEM) results suggest that three-dimensional cluster particles were formed and homogeneously distributed in the SPEEK matrix. The mechanochemically synthesized Cs x H 3-x PW 12 O 40 incorporated into the SPEEK matrix increased the number of protonate sites in the electrolyte. The composite electrolytes were successfully formed with Cs x H 3-x PW 12 O 40 , which consist of hydrogen bonding between surface of inorganic solid acids and not only –HSO 4 - dissociated from CsHSO 4 but also –SO 3 H groups in the SPEEK. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Perfluorinated and partially fluorinated electrolytes exhibit the best performance and highest longevity for some fuel cell sys- tem applications. However, they have a limitation for large-scale commercialization of electrolytes in fuel cell systems due to their high cost and/or their poor heat resistance [1]. Consequently, inorganic–organic composite electrolytes are attracting increas- ing interest as one of the key arguments for the development of new proton conductive electrolytes in fuel cell applications [2,3]. Recently, various aromatic polymers for use as electrolytes in fuel cell systems have been attempted. Indeed, they showed promis- ing performance as not only electrolyte membranes but also as matrix for inorganic–organic composite electrolytes. In particular, the high proton conductivity of poly(ether–ether ketone) (PEEK) can be attained via sulfonation, consequently, sulfonated PEEK (SPEEK) electrolyte has good performance as a polymer electrolyte in hydrogen fuel cells [4]. The degree of sulfonation (DS) of SPEEK ∗ Corresponding authors. Tel.: +81 532 44 6800; fax: +81 532 48 5833. E-mail addresses: ms089203@tutms.tut.ac.jp (S.-Y. Oh), matsuda@tutms.tut.ac.jp (A. Matsuda). can be controlled during the sulfonation via reaction conditions, such as time, temperature, etc. However, increasing DS leads to increased solubility in water and other solvents, and swelling. Heteropoly acids (HPAs) having Keggin structure are known as an inorganic solid acid that shows high proton conductivity and excellent catalytic property based on their strong acidity [5–7]. However, despite the good performances of HPAs, their chemical stabilities are unsatisfactory due to their hygroscopicity. Partially Cs + -substituted HPAs (Cs-HPAs) have attracted significant atten- tion due to their high catalytic activity in many acid-type reactions [8]. Such partially substituted solid acids are generally prepared by precipitation from aqueous solutions [9,10]. In contrast, we have shown that a solid state reaction involving mechanochemical treat- ment by using a high-energy ball mill is a promising way to improve the proton conductivity of inorganic solid acids and to synthe- size a new class of inorganic solid acid-based composites [11,12]. When mixtures of cesium hydrogen sulfate (CsHSO 4 ) and phospho- tungstic acid (H 3 PW 12 O 40 , WPA) were mechanochemically treated, Cs x H 3-x PW 12 O 40 was formed. On the other hand, we have also prepared proton conducting composite electrolytes from phos- phosilicate gel (P 2 O 5 –SiO 2 ) powders and thermally stable organic polymers, the application of which is for medium temperature range operation of the hydrogen fuel cells [13,14]. 0921-5107/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2010.03.002