Proton-Conductive Electrolyte Consisting of NH[sub 4]PO[sub 3]/TiP[sub 2]O[sub 7] for Intermediate-Temperature Fuel Cells

Abstract

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...

In Situ Formed Phosphoric Acid/Phosphosilicate Nanoclusters in the Exceptional Enhancement of Durability of Polybenzimidazole Membrane Fuel Cells at Elevated High Temperatures

“…This is probably due to the condensation and separation of PA from the phosphosilicate phase at elevated high temperatures. The condensation of short-chain phosphate would be irreversible in a dry atmosphere condition, resulting in a diminution in proton conductivity [36]. On the other hand, the in situ formed phosphosilicates or phosphosilicate/PA showed a high proton conductivity of 7.2  10 -2 S cm -1 at 250 o C, more than an order of magnitude higher than 3.6  10 -3 S cm -1 with the pure phosphosilicate phase.…”

Review of proton conductors for hydrogen separation

“…Proton conductivity will occur in H 2 -or water-rich atmospheres for neutral sulphates [52], phosphates [53,54], and nitrates [51]. [56].…”

Stability and conductivity study of NH4PO3–PTFE composites at intermediate temperatures

“…Recently, ammonium polyphosphate (APP, NH 4 PO 3 ) has been studied as a intermediate-temperature proton conducting electrolyte [8][9][10][11][12][13]. However, pure phase APP is not stable at the temperature of more than 250 • C. Considerable efforts have recently been devoted to the application of supporting matrix, which can enhance its stability, mechanic strength and ionic conductivity.…”