STUDY ON AMMONIUM POLYPHOSPHATE PROTON CONDUCTORS AS ELECTROLYTES FOR NOVEL INTERMEDIATE TEMPERATURE FUEL CELLS

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Ammonium polyphosphates (NH_4PO_3) have high proton conductivity, end are promising materials as electrolytes in intermediate temperature fuel cells operated at around 300℃. In the present study, thermal stability of the ammonium polyphosphates was improved by adding suitable matrix materials and by partial substitution of ammonium ion.(NH_4)_2SiP_4O_<13> was added as a matrix to form NH_4PO_3/(NH_4)_2SiP_4O<13> composites. The composite (4:1) gave a high proton conductivity of 5.4 mS cm^<-1> at 300℃ with improved thermal stability. The proton conductivity increased with an increase in the content of the matrix. However, NH_4PO_3/(NH_4)_2SiP_4O_<13> composites with molar ratio greater than 8:1 were not stable at 300℃, especially under humidified atmosphere. Hence, the composites with molar ratio lower than 8:1 are preferable in practical use.(NH_4)_<1-x>K_xPO_3 solid solutions were then prepared to improve thermal stability. (NH_4)_<0.2>K_<0.8>PO_3 kept a solid state even at 400℃ and gave a high proton conductivity. In these solid solutions, NH_3 was eliminated during heating and phase separation into a crystalline phase of KPO_3 and an amorphous phase of HPO_3 occurred. The former phase has a high thermal stability and works as a matrix. The latter phase plays an vital role in proton conduction.