Electrochimica Acta 51 (2006) 3719–3723 Composite effects of silicon pyrophosphate as a supporting matrix for CsH 5 (PO 4 ) 2 electrolytes at intermediate temperatures Toshiaki Matsui ∗ , Tomokazu Kukino, Ryuji Kikuchi, Koichi Eguchi ∗ Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan Received 2 September 2005; received in revised form 14 October 2005; accepted 18 October 2005 Available online 21 November 2005 Abstract The proton-conductive electrolytes of CsH 5 (PO 4 ) 2 /SiP 2 O 7 composites were synthesized, and composite effects of silicon pyrophosphate as a supporting matrix at intermediate temperatures were investigated by comparing the properties of CsH 5 (PO 4 ) 2 /SiO 2 composite. Although both composites showed similar thermal stability, the temperature dependence of the conductivity was quite different each other; the conductivity of the composite electrolyte of CsH 5 (PO 4 ) 2 /SiP 2 O 7 was about one-order magnitude higher at every temperature investigated and the maximum conductivity achieved was 116 mS cm -1 at 230 ◦ C. These results suggested that the interfacial interaction between the proton-conductor phase of CsH 5 (PO 4 ) 2 and the matrix of SiP 2 O 7 played an important role in the proton conduction mechanism. © 2005 Elsevier Ltd. All rights reserved. Keywords: Intermediate-temperature fuel cells; Proton conductor; Oxo-acid salt; Phosphate; Composite 1. Introduction Fuel cells are attractive energy conversion devices because of the high efficiencies and low emissions, and many studies have been conducted. Among them, solid state fuel cells opera- tive at 200–300 ◦ C are promising technology since they combine many advantages of high and low temperature fuel cells. How- ever, they have not been developed sufficiently because of the absence of good ionic-conductors with high thermal stability at intermediate temperatures. The proton-conductive solid acid salts, such as CsH 2 PO 4 and CsHSO 4 , are well known as one of the promising materials for intermediate temperature applications [1–7]. It is noted that these materials undergo the phase transition to the superionic phase and exhibit high conductivity of ∼10 -2 S cm -1 . However, their conductivity is somewhat lower than that requested for the prac- tical use and the operating temperature range is narrow because of the phase transition. The addition of oxides as matrices has been studied as an effective method for the improvement in the ionic conductivity [8–12], and it was clarified that the low con- ∗ Corresponding authors. Tel.: +81 75 383 2523; fax: +81 75 383 2520. E-mail addresses: matsui@elech.kuic.kyoto-u.ac.jp (T. Matsui), eguch@scl.kyoto-u.ac.jp (K. Eguchi). ductivity phase was remarkably affected by the surface area, pore sizes, and the molar ratio of oxides. However, the notable results at intermediate temperatures have not been achieved so far. Recently, good compatibility of pyrophosphates as the matrix for ammonium polyphosphate electrolytes has been reported [13]. The composite electrolytes of NH 4 PO 3 /TiP 2 O 7 showed high proton conductivity up to 300 ◦ C as a result of interac- tion at the proton-conductor phase/pyrophosphate interface. In the previous paper, we synthesized a novel proton-conductive electrolyte of CsH 2 PO 4 /SiP 2 O 7 -based composite and reported their electrochemical properties [14,15]. In this composite, dur- ing heat-treatment at intermediate temperatures, the composition of CsH 2 PO 4 chemically reacted with a part of SiP 2 O 7 matrix, and then the new ionic-conduction phase of CsH 5 (PO 4 ) 2 was formed. The resulting composite was in the solid states at elevated temperature despite the melting of CsH 5 (PO 4 ) 2 and showed high proton conductivity of 44 mS cm -1 at 266 ◦ C under 30% H 2 O/Ar atmosphere. However, the proton-conduction mechanism has not been elucidated sufficiently. Then, in this study, we aimed to investigate composite effects of silicon pyrophosphate as a supporting matrix for CsH 5 (PO 4 ) 2 elec- trolytes. The composite electrolytes of CsH 5 (PO 4 ) 2 /SiP 2 O 7 and CsH 5 (PO 4 ) 2 /SiO 2 were synthesized, and their thermal stability and electrochemical properties were compared at intermediate temperatures. 0013-4686/$ – see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2005.10.026