Phosphoric Acid Transfer along a Porous Carbon Plate due to Temperature Gradient

Abstract

Acid management is necessary in a phosphoric acid fuel cell to prevent an overall or localized shortages of the acid. We found that the transfer of phosphoric acid occurred when a temperature gradient was applied to the porous carbon plate in which the acid was stored. This is an important phenomenon because various temperature distributions may occur in a cell under actual operating conditions. The effect of temperature gradients was evaluated using 50 cm porous carbon plates with acid fill levels of 40%, 50%, or 60%. The temperature of one half of the specimen was maintained at 180°C, and the other at 150°C, 100°C or 50°C for 72 hours. Under these conditions, the quantity of phosphoric acid in the high temperature side of the cell measured by neutralization titration increased, and that in the cold temperature side decreased. The rate of phosphoric acid transfer increased with increases in the temperature gradient and fill level. Two phenomena were examined as potential causes of this transfer mechanism. Evaporated water from the high temperature section is considered to condense in the low temperature section, and the volume of phosphoric acid in the low temperature section increases, whereas that in the high temperature section decreases. Phosphoric acid transfer is therefore considered to occur in order to maintain proportionality of the fill level in the cell. It was found that the calculated values based on this assumption were approximately equal to measured transfer rates. It is shown that temperature dependency on the interfacial tension is not a dominant factor on the rate of acid transfer.