Conductivity of a new pyrophosphate Sn0.9Sc0.1(P2O7)1−δ prepared by an aqueous solution method
Introduction
Fuel cells are electrochemical devices which can directly convert 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 CO2 emission. Proton exchange membrane fuel cells (PEMFCs) based on Nafion electrolyte and Pt-based catalysts are used for transport applications. However, the complicated water management, CO poisoning of Pt catalyst limit their large scale applications. Ultra-pure hydrogen with CO level <10 ppm 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 containing 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 conventional polymer electrolyte. Less pure hydrogen may be directly used for the intermediate temperature fuel cells and the Pt-based electrode 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], [4], [5], [6], the conductivity normally starts to decrease at a temperature above 100 °C therefore difficult to be used as electrolytes for intermediate temperature fuel cells. There are a lot of reports regarding high temperature proton-conducting materials but the proton conductivity at the temperature range 200–300 °C is not high enough for practical application [7], [8], [9], [10], [11]. It has been reported that In-doped SnP2O7, Sn0.9In0.1(P2O7)1−δ exhibits a conductivity of 1.95 × 10−1 S/cm at 250 °C without extra humidity. A power density of 264 mW/cm2 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 Sn0.9In0.1(P2O7)1−δ. Normally the electronic conduction is from the d electrons of multi-valence transition 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 SnP2O7 and In-doped SnP2O7 prepared by an aqueous method [14]. In this paper, the structure and conductivity of a new pyrophosphate Sn0.9Sc0.1(P2O7)1−δ are presented. The formula of Sc-doped SnP2O7 is written as Sn0.9Sc0.1(P2O7)1−δ since the charge is likely compensated by P2O7 vacancies as described below.
Section snippets
Materials syntheses
Two methods have been applied for the preparation of Sn0.9Sc0.1(P2O7)1−δ. For the first method, commercial SnO2, Sc2O3 and H3PO4 with P/(Sn + Sc) molar ratio 2.6 and 3.0 were used (acid method). The SnO2 and Sc2O3 powders were dried at 500 and 700 °C respectively to remove absorbed water and gases before weighing. Single phase pyrophosphate was not obtained. As for the aqueous solution method, Sc2O3 was dried at 700 °C for a couple of hours first. A calculated amount of Sc2O3 was weighed and
Structure of Sn0.9Sc0.1P2O7
The reported Sc-doped SnP2O7 was first synthesized by an acidic method similar to that reported before [12] using SnO2, Sc2O3 and H3PO4 as precursors. In our experiments, some unreacted SnO2 was observed when calculated amounts of SnO2 and H3PO4 were used for the synthesis of SnP2O7. The possible reason is that the yielded SnP2O7 was coated onto the SnO2 particles which isolate SnO2 particles from H3PO4. The SnO2 phase still cannot be fully removed yet even the H3PO4/SnO2 molar ratio was
Conclusions
Sn0.9Sc0.1(P2O7)1−δ has been prepared by an aqueous solution method. The 3 × 3 × 3 superlattice peaks were observed in Sn0.9Sc0.1(P2O7)1−δ. The total conductivity of Sn0.9Sc0.1(P2O7)1−δ obtained at 1000 °C is 2.35 × 10−6 and 2.82 × 10−9 S/cm at 900 and 400 °C respectively. The conductivity of undoped SnP2O7 is even lower. The conductivity of Sn0.9Sc0.1(P2O7)1−δ in wet air is about two orders of magnitude higher than that in open air indicating some proton conduction but total conductivity in wet air is
Acknowledgements
The authors are indebted to EPSRC for funding. Tao thanks EaStCHEM for a fellowship. Thanks also go to the Royal Society of Edinburgh for an International Exchange Programme with China and to Prof. John Irvine at University of St Andrews for kind support.
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2012, Journal of Alloys and CompoundsCitation Excerpt :However, it should be kept in mind that samples in these previous studies were prepared by merely pressing SnP2O7-based powders into pellets and the heat-treated temperatures were all less than 700 °C. Until now, only a few papers dealt with the preparation of SnP2O7-based ceramic pellets through sintering procedure [20–22]. Tao [21] reported that Sn0.92In0.08P2O7 − δ ceramic pellet was prepared by an aqueous solution route and a single phase pyrophosphate was formed by sintering the green pellet at 1000 °C, giving a conductivity of 8 × 10−9 S cm−1 at 400 °C in air.