Study on Carbon Dioxide Evolution Phenomena at Start-up Processes of Phosphoric Acid Fuel Cells

Abstract

This paper presents experimental studies and theoritical considerations on CO₂ evolution phenomena from phosphoric acid fuel cells at start-up processes of the plant.
When hydrogen is supplied to cells under an air already supplied condition, a CO₂ pulse is detected in the exhaused air. Rise time of the CO₂ pulse depends on the flow rate of the hydrogen and coincides with rise time of the cell voltage and the duration of the negative current in a cell.
The negative current in the cell increase in its value with time and suddenly change to a positive pulse in regular sequence from the upper stream to the lower stream of the hydrogen and the time lag of their sequence depend on the flow rate of the hydrogen. Measured CO₂ volume coincides approximately with an estimated CO₂ volume from an electro-chemical equivalent value of the negative current.
All these results suggest that the electromotive force of the upper stream portion in the cell causes corrosive current on the lower stream portion of the cell.
High flow rate of the hydrogen or low hydrogen content in the fuel gas are not enough to extinguish the CO₂ evolution, though increasing flow rate of the hydrogen are effective to decrease the CO₂ volume. When the air is supplied to cells under the hydrogen already supplied condition, any trace of the CO₂ gas can not be detected.
In conclusion, the fuel gas should be supplied prior to the air at start-up processes of phosphoric acid fuel cells.