1991 Volume 111 Issue 3 Pages 324-331
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.