Journal of Power Sources 171 (2007) 483–488 Short communication Effect of pyrophosphates as supporting matrices on proton conductivity for NH 4 PO 3 composites at intermediate temperatures Toshiaki Matsui a , Naoto Kazusa a , Yukinari Kato a , Yasutoshi Iriyama a , Takeshi Abe a,∗ , Kenji Kikuchi b , Zempachi Ogumi a a Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan b Department of Materials Science, The University of Shiga Prefecture, Hikone, Shiga 522-0057, Japan Received 27 April 2007; received in revised form 11 June 2007; accepted 11 June 2007 Available online 23 June 2007 Abstract Composite electrolytes of NH 4 PO 3 /pyrophosphate (NH 4 PO 3 /ZrP 2 O 7 , NH 4 PO 3 /Sr 2 P 2 O 7 , and NH 4 PO 3 /TiP 2 O 7 ) with various molar ratios were fabricated, and their thermal and electrochemical properties were compared at intermediate temperatures. The XRD pattern of NH 4 PO 3 /Sr 2 P 2 O 7 composite was consistent with a mixed phase of crystalline NH 4 PO 3 and Sr 2 P 2 O 7 regardless of the composition ratio, whereas those of the other composites were identical to pyrophosphates. A significant difference in conductivity was observed depending on the supporting matrices of pyrophosphates although each composite contained almost the same molar concentration of NH 4 PO 3 . Among the composites, NH 4 PO 3 /ZrP 2 O 7 (molar ratio; 1:1) exhibited the highest proton conductivity, which was more than twice that of NH 4 PO 3 /TiP 2 O 7 (1:1). The conductivity of NH 4 PO 3 /Sr 2 P 2 O 7 (2:1) composite was 2–3 orders of magnitude lower than that of NH 4 PO 3 /ZrP 2 O 7 (1:1). These results suggest that the surface property of pyrophosphates strongly affects the electrochemical properties of composites. Furthermore, a fuel cell that used NH 4 PO 3 /ZrP 2 O 7 composite as an electrolyte was successfully demonstrated at 300 ◦ C. © 2007 Published by Elsevier B.V. Keywords: Proton conductor; Intermediate-temperature fuel cells; Pyrophosphate; Ammonium polyphosphate 1. Introduction Recently, considerable effort has been devoted to develop novel solid state proton conductors for intermediate-temperature applications (100–300 ◦ C) such as fuel cells, sensors and pumps [1–4]. At low temperatures (below 100 ◦ C), hydrated perfluo- rosulfonic acid membranes such as Nafion ® are widely used as electrolytes. However, these materials require water to main- tain high conductivity. Thus, complicated water management is one of the major obstacles for intermediate-temperature oper- ation since the relative humidity decreases with an increase in temperature between 100 and 300 ◦ C. Although many heat- resistant polymers with high water-holding capability also have been investigated extensively, usage of polymers derived mainly from organic substances limits the operating temperature below ∗ Corresponding author. Tel.: +81 75 383 2483; fax: +81 75 383 2488. E-mail address: abe@elech.kuic.kyoto-u.ac.jp (T. Abe). 150 ◦ C. On the other hand, inorganic materials are more ther- mally stable and several compounds have been reported with relatively high proton conductivity. Among them, oxoacid salts such as CsH 2 PO 4 are a class of promising electrolyte at inter- mediate temperatures [5–9]. Ammonium polyphosphate (NH 4 PO 3 , APP) is one of the oxoacid salts, which is used as fertilizers and flame retar- dants. This material shows proton conductivity accompanied with the partial decomposition of NH 4 PO 3 into HPO 3 at ca. 250 ◦ C even under a dry atmosphere, whereas the resultant compound does not stay in the solid state at intermediate temperatures. Thus, the heterogeneous system is preferable, and several groups have reported the electrochemical proper- ties of the composite electrolyte of NH 4 PO 3 /(NH 4 ) 2 SiP 4 O 13 [10,11]. Although the compound of (NH 4 ) 2 SiP 4 O 13 serves as a supporting matrix, composite effects have not been consid- ered carefully. In previous reports [12,13], we revealed that the structural change of supporting matrix of (NH 4 ) 2 MP 4 O 13 into MP 2 O 7 (M = Ti and Si) occurred at intermediate tempera- 0378-7753/$ – see front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.jpowsour.2007.06.060