Abstract:

A tubular electrolyte-supporting solid oxide fuel cell (SOFC) was fabricated by a slip casting technique. Yttrium stabilized zirconia (YSZ) was used as the electrolyte and silver was used as both of the anode and cathode materials. Activated carbon was directly used to fuel the cell without any gas feeding. The cell, with an effective area of 2.5 cm2, gave a maximum power of 16 mW at 800 ℃. The relationship of the open circuit voltage vs temperature was consistent with the theoretical expectation. Operating the cell continuously and stably for 37 h at a constant current of 30 mA resulted in more than 42% (w) of the carbon fuel consumed, demonstrating that the cell was self-sustainable. Compared with the SOFCs with graphite fuel, the operation stability has been improved significantly and this is attributed to the greater microporosity and larger surface area of the activated carbon. The performance degraded rapidly after 37 h. A decrease in the active surface area of the carbon fuel, arising from carbon sintering and a reduction in the amount of fuel during the operation was assumed to be the main reason for the degradation. An electrochemical impedance measurement revealed that polarization resistance dominated the total loss of the cell. By analyzing the mechanisms of the cell reaction, we recognized that the cycle of the two reactions, i.e., the electrochemical oxidation of CO on the anode/electrolyte interface and the Boudouard reaction on the surface of the carbon fuel, maintained the cell operation. We suggest that a significant improvement in cell performance can be expected by using catalysts that promote these two reactions.

Key words: Solid oxide fuel cell, Carbon, CO, Boudouard reaction, Electrochemical oxidation, Stability test