Please cite this article in press as: A.S. Bondarenko, et al., J. Power Sources (2009), doi:10.1016/j.jpowsour.2009.06.001 ARTICLE IN PRESS G Model POWER-12037; No. of Pages 4 Journal of Power Sources xxx (2009) xxx–xxx Contents lists available at ScienceDirect Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour Short communication Superprotonic KH(PO 3 H)–SiO 2 composite electrolyte for intermediate temperature fuel cells Alexander S. Bondarenko, Weihua Zhou, Henny J.M. Bouwmeester ∗ Inorganic Membranes, Faculty of Science and Technology & MESA+ Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands article info Article history: Received 5 February 2009 Received in revised form 25 May 2009 Accepted 2 June 2009 Available online xxx Keywords: Fuel cells Electrolyte Composite Solid acid Proton conductor abstract Novel thin film composite electrolyte membranes, prepared by dispersion of nano-sized SiO 2 particles in the solid acid compound KH(PO 3 H), can be operated under both oxidizing and reducing conditions. Long-term stable proton conductivity is observed at ∼140 ◦ C, i.e., slightly above the superprotonic phase transition temperature of KH(PO 3 H), under conditions of relatively low humidification (pH 2 O ≈ 0.02 atm). © 2009 Elsevier B.V. All rights reserved. 1. Introduction Inorganic solid acids like CsHSO 4 exhibiting high proton con- ductivity have potential for use as electrolyte in fuel cells [1–6]. The higher operating temperatures relative to polymer electrolytes, typically in the range 100–250 ◦ C, contribute to improved elec- trode kinetics and tolerance of known electrode catalysts to CO. Unlike hydrated sulphonated polymers such as Nafion ® , no water molecules are required to facilitate proton transport in the solid acids, eliminating the need for continuous humidification of reac- tant gases. Haile and co-workers have showed the use of solid acid proton conductors both in H 2 /O 2 and direct methanol fuel cells [1,2]. Using supported thin CsH 2 PO 4 electrolyte membranes on porous stainless steel gas-diffusion electrodes, peak power densities as high as 415 mW cm -2 were obtained [3]. Proton conductivity in the solid acid compounds (e.g., sulphates, selenates and phosphates) arises upon a structural phase transfor- mation at elevated temperature. The transition, often referred to as superprotonic phase transition [1,2], creates dynamical disorder in the H-bonded XO 4 network (where X = S, Se, P), enabling fast transport of protons mediated by rapid reorientations of the XO 4 tetrahedra (Grotthuss mechanism)[1,7]. The proton conductivity at the superprotonic phase transition increases abruptly by 2–3 orders of magnitude and may reach values up to 10 -3 to 10 -2 S cm -1 [1]. To date, however, implementation of superprotonic solid acids in fuel ∗ Corresponding author. Tel.: +31 53 489 4611. E-mail address: h.j.m.bouwmeester@tnw.utwente.nl (H.J.M. Bouwmeester). cells is hindered by a poor chemical and mechanical stability [1,7]. The alkali–metal hydrogen sulphates and selenates decompose in hydrogen containing atmospheres [1,8], whereas their dihydrogen phosphate counterparts need significant levels of humidification, for example, up to a water vapour pressure of 0.30 atm for CsH 2 PO 4 [2,9], to keep their superprotonic properties. The best superpro- tonic solid acids known to date are prepared from costly caesium. These facts prompted us in previous research to explore the proton conducting properties of alkali–metal acid phosphites MH(PO 3 H) (M = Li + , Na + ,K + , Rb + , Cs + , NH 4 + ), in view of their good stability under hydrogen atmospheres [10]. Identification was made of a superprotonic phase transition in potassium dihydrogen phosphite, KH(PO 3 H), at an onset temperature of 132 ◦ C, reaching a proton con- ductivity of 4.2 × 10 -3  -1 cm -1 (at 140 ◦ C). The compound adopts a monoclinic structure at room temperature, and transforms to cubic in the superprotonic phase. In this study, we assess the opera- tional stability window of KH(PO 3 H), and demonstrate its viability in thin film KH(PO 3 H)–SiO 2 composite membranes for use as elec- trolyte in solid acid fuel cells. 2. Experimental Powders of KH(PO 3 H) were prepared by slow evaporation of an aqueous solution obtained by mixing of H 3 PO 3 (99%, Aldrich) and KOH (99.5%, Merck) in molar ratio 1:1. The powders were dried in an oven at ∼105 ◦ C for 20 h, ground in an agate mortar and stored in a desiccator due to hygroscopicity of the pure salt. Powders of KH(PO 3 H)–SiO 2 composites were prepared by dispersing of SiO 2 powder (fumed silica, Aldrich, particle size 14nm) in an aqueous 0378-7753/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jpowsour.2009.06.001