A new proton-conductive electrolyte consisting of x NH 4 PO 3 /TiP 2 O 7 (x ϭ 0.2-1.0) was synthesized for use in intermediatetemperature fuel cells, and its electrochemical properties were studied in the temperature range 150-300°C. For each composite electrolyte, the X-ray diffraction pattern was identical to that for TiP 2 O 7 , and NH 4 PO 3 was not detected due to a change to an amorphous state at sintering temperature. The proton conductivity of the composite electrolyte was improved by increasing the molar ratio of NH 4 PO 3 under both dry and wet atmospheres. These results indicate that NH 4 PO 3 is responsible for high proton conductivity and that TiP 2 O 7 serves as a supporting matrix. However, under each atmosphere, the temperature dependence of proton conductivity showed non-Arrhenius-type behavior and was almost reversible. Higher proton conductivities were observed at 200-250°C and 150-200°C under dry and wet atmospheres, respectively. For x ϭ 1.0, the highest proton conductivity was determined to be 24. have not yet been developed. Fuel cells in this temperature region, in particular at around 300°C, offer many advantages: (i) metal and resin are available for the fabrication of cells, resulting in a drastic reduction in manufacturing cost; (ii) the efficiency of energy conversion is higher than in polymer electrolyte fuel cells ͑PEFCs͒; (iii) CO poisoning of the electrode is considerably suppressed; (iv) rapid heating and cooling are advantageous compared to solid oxide fuel cells ͑SOFCs͒, etc.Highly ion-conductive electrolytes of Nafion and yttria-stabilized zirconia ͑YSZ͒ are the major driving forces behind R&D on PEFCs and SOFCs. Therefore, highly ion-conductive electrolytes at intermediate temperature regions are required to develop intermediatetemperature fuel cells, although only a few studies have been conducted on electrolytes for use at 250-600°C. [5][6][7][8] Recently, an ammonium polyphosphate composite, NH 4 PO 3 /(NH 4 ) 2 SiP 4 O 13 , was reported to exhibit conductivities of 20 and 100 mS cm Ϫ1 at 300°C under dry and wet atmospheres, respectively.9 In this composite, NH 4 PO 3 is responsible for high ionic conductivity in a supporting matrix of (NH 4 ) 2 SiP 4 O 13 .8 The high ionic conductivity originates in the partial decomposition of NH 4 PO 3 to HPO 3 at around 250°C. Accordingly, the supporting matrix has to be stable and should contribute to ion conduction. In our previous paper, 10 we focused on the supporting matrix and reported a new electrolyte that consisted of a (NH 4 ) 2 TiP 4 O 13 -based material. We showed that the partial decomposition of (NH 4 ) 2 TiP 4 O 13 to TiP 2 O 7 should be responsible for high proton conductivity. Its proton conductivity was determined to be about 5 mS cm Ϫ1 at 300°C under a dry Ar atmosphere, which is acceptably high considering that the measurement was conducted under a dry atmosphere. Furthermore, to study its properties as a supporting matrix in an NH 4 PO 3 composite electrolyte, we previously synthesized NH 4 PO 3 /(NH 4 ) 2 TiP 4 O...

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