The effect of current density and temperature on the degradation of nickel cermet electrodes by carbon monoxide in solid oxide fuel cells

Abstract

The oxidation of dry carbon monoxide (CO) in intermediate temperature solid oxide fuel cells (IT-SOFCs) has been studied using a three electrode assembly. Ni/CGO:CGO:LSCF/CGO three electrode pellet cells at 500, 550 and 600 °C were exposed to dry carbon monoxide for fixed periods of time, at open circuit and under load at 50 and 100 mA cm −2, in an aggressive test designed to accelerate electrode degradation. It is shown that if the anode is kept under load during exposure to dry CO, degradation in anode performance can be minimised, and that under most conditions the anode showed significant irreversible degradation in performance after subsequent load cycling on dry H 2. Only at 500 °C and at 100 mA cm −2 was the degradation in performance after operation on dry CO and subsequent load cycling on dry H 2 within the background degradation rates measured. Where anode performance was compromised, this appeared to be caused by a reduction in the exchange current density for hydrogen oxidation, and the relatively large degradation after load cycling on dry H 2 was primarily caused by an increase in the series resistance of the anode. It is suggested that this increase in series resistance is associated with the removal of carbon deposited in the non-electrochemically active region of the electrode during operation on dry CO, and that operation under load inhibits carbon deposition in the active region.