Interfacial conduction mechanism of cesium hydrogen phosphate and silicon pyrophosphate composite electrolytes for intermediate-temperature fuel cells

a r t i c l e i n f o Keywords: Intermediate-temperature fuel cells Cesium hydrogen phosphate Silicon pyrophosphate Proton conduction Effective medium approximation As an electrolyte for intermediate-temperature fuel cells (ITFCs), which are operative at 200 °C to 300 °C, a composite electrolyte CsH 5 (PO 4) 2 /SiP 2 O 7 has been investigated. The CsH 5 (PO 4) 2 /SiP 2 O 7 composite is reported to exhibit higher conductivity than pure CsH 5 (PO 4) 2 , possibly by forming a highly-conductive new phase at the interface between CsH 5 (PO 4) 2 and SiP 2 O 7. In this study, we have prepared several SiP 2 O 7 matrices of different surface properties, and studied the effect of the surface properties on the total conductivity as well as on the formation mechanism of the interfacial conductive phase. An effective medium approximation method was applied to the measured conductivity to analyze the interfacial conductive phase. Several SiP 2 O 7 matrices of different acid properties and crystalline properties were fabricated, and the conductivity of the CsH 5 (PO 4) 2 /SiP 2 O 7 composite electrolyte was measured by AC impedance method. The acidity and crystallinity of the SiP 2 O 7 matrices were found to be important properties for the formation of the interfacial conductive phase. The highest conductivity of the interfacial conductive phase was estimated to be 500 mS cm −1 , which is almost three times larger than that of the pure CsH 5 (PO 4) 2 , 160 mS cm −1. Fuel cells convert chemical energy directly to electrical energy with high efficiency and at low pollution levels. High energy conversion efficiency can be expected for fuel cells, since the efficiency is not confined by Carnot cycle as in the case of internal combustion engines. Fuel cells are classified according to the types of electrolytes and operating temperatures. Low-temperature types operated below 200 °C and high-temperature types operated above 600 °C have been actively investigated. Particularly, polymer electrolyte fuel cells (PEFCs) and solid oxide fuel cells (SOFCs) have already been commercially available in Japan. On the other hand, intermediate-temperature fuel cells (ITFCs) operative at 200–600 °C are not fully developed because ionic conductors used as the electrolytes for ITFCs are under investigation to improve conductivity as well as thermal stability [1,2]. ITFCs are attractive energy conversion systems since they offer various …