Journal of Alloys and Compounds 486 (2009) 380–385 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom Conductivity of a new pyrophosphate Sn 0.9 Sc 0.1 (P 2 O 7 ) 1-ı prepared by an aqueous solution method Rong Lan a , Shanwen Tao a,b,∗ a Department of Chemistry, Heriot-Watt University, Edinburgh EH14 4AS, UK b School of Chemistry, University of St. Andrews, Fife KY16 9ST, Scotland, UK article info Article history: Received 16 March 2009 Received in revised form 23 June 2009 Accepted 25 June 2009 Available online 16 July 2009 Keywords: Conductivity Pyrophosphate Structure Sn0.9Sc0.1(P2O7) 1-ı abstract New pyrophosphate Sn 0.9 Sc 0.1 (P 2 O 7 ) 1-ı was prepared by an aqueous solution method. The structure and conductivity of Sn 0.9 Sc 0.1 (P 2 O 7 ) 1-ı have been investigated. XRD analysis indicates that Sn 0.9 Sc 0.1 (P 2 O 7 ) 1-ı exhibits a 3 × 3 × 3 super structure. It was found that Sn 0.9 Sc 0.1 (P 2 O 7 ) 1-ı prepared by an aqueous method is not conductive. The total conductivity of Sn 0.9 Sc 0.1 (P 2 O 7 ) 1-ı in open air is 2.35 × 10 -6 and 2.82 × 10 -9 S/cm at 900 and 400 ◦ C respectively. In wet air, the total conductivity is about two orders of magnitude higher (8.1 × 10 -7 S/cm at 400 ◦ C) than in open air indicating some proton conduction. SnP 2 O 7 and Sn 0.92 In 0.08 (P 2 O 7 ) 1-ı prepared by an acidic method were reported fairly conductive but prepared by sim- ilar solution methods are not conductive. Therefore, the conductivity of SnP 2 O 7 -based materials might be related to the synthetic history. The possible conduction mechanism of SnP 2 O 7 -based materials has been discussed in detail. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Fuel cells are electrochemical devices which can directly con- vert chemical energy to electricity with high efficiency. Fuel cell cars fuelled with hydrogen are very important from the points of both hydrogen economy and climate change due to CO 2 emission. Proton exchange membrane fuel cells (PEMFCs) based on Nafion electrolyte and Pt-based catalysts are used for transport applica- tions. However, the complicated water management, CO poisoning of Pt catalyst limit their large scale applications. Ultra-pure hydro- gen with CO level <10ppm has to be used for PEMFCs. The CO poisoning would be less significant if a fuel cell may be operated at a temperature above 150 ◦ C. For a normal PEMFC, hydrogen contain- ing up to 3%CO may be used for a PEMFC operating at 200 ◦ C [1,2]. Therefore new electrolyte materials with a proton conductivity >10 -2 S/cm at ∼200 ◦ C are in high demand to replace the conven- tional polymer electrolyte. Less pure hydrogen may be directly used for the intermediate temperature fuel cells and the Pt-based elec- trode materials could be replaced by cheaper materials as well due to increased catalytic activity at evaluated temperatures. It is well known that inorganic proton conductors, such as heteropolyacids show high protonic conductivity [3–6], the con- ductivity normally starts to decrease at a temperature above 100 ◦ C ∗ Corresponding author at: Department of Chemistry, Heriot-Watt University, Edinburgh EH14 4AS, UK. Tel.: +44 0131 451 4299. E-mail address: s.tao@hw.ac.uk (S. Tao). therefore difficult to be used as electrolytes for intermediate temperature fuel cells. There are a lot of reports regarding high tem- perature proton-conducting materials but the proton conductivity at the temperature range 200–300 ◦ C is not high enough for practi- cal application [7–11]. It has been reported that In-doped SnP 2 O 7 , Sn 0.9 In 0.1 (P 2 O 7 ) 1-ı exhibits a conductivity of 1.95 × 10 -1 S/cm at 250 ◦ C without extra humidity. A power density of 264mW/cm 2 was achieved at 250 ◦ C with an electrolyte thickness 0.35 mm when hydrogen was used as the fuel [12,13]. The fuel cell performance is not affected in the presence of 10% CO although Pt-based electrodes were used as both anode and cathode. This material is a promising electrolyte for intermediate temperature fuel cells. However, the ionic transfer number is ∼0.9 for Sn 0.9 In 0.1 (P 2 O 7 ) 1-ı . Normally the electronic conduction is from the d electrons of multi-valence tran- sition elements such as indium. The ionic transfer number could be further improved if the multi-valent indium could be replaced by elements such as Mg, Ca, Sr, Sc, Ga, Al which exhibits fixed valence in ionic states. In a previous report, we investigated the structure and conductivity of SnP 2 O 7 and In-doped SnP 2 O 7 prepared by an aqueous method [14]. In this paper, the structure and conductivity of a new pyrophosphate Sn 0.9 Sc 0.1 (P 2 O 7 ) 1-ı are presented. The for- mula of Sc-doped SnP 2 O 7 is written as Sn 0.9 Sc 0.1 (P 2 O 7 ) 1-ı since the charge is likely compensated by P 2 O 7 vacancies as described below. 2. Experimental 2.1. Materials syntheses Two methods have been applied for the preparation of Sn0.9Sc0.1(P2O7) 1-ı . For the first method, commercial SnO2, Sc2O3 and H3PO4 with P/(Sn + Sc) molar ratio 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.06.203