Performance Recovery of Sulfur Poisoning Anode and Prevention of Sulfur Poisoning by Shifting Anode Potential for Solid Oxide Fuel Cell

A small and high-performance solid oxide fuel cell (SOFC) system has a potential as a power source for mobile use like automobile. To realize the small SOFC system, the direct use of liquid hydrocarbon fuel, which has high energy density and is easy to storage and supply, is desirable. In order to use the existing fuel supply infrastructure, the SOFC should be operated by gasoline. Gasoline sold in Japan contains H₂S up to 10ppm. Thus, automotive SOFC must be able to be stably operated at 10ppm H₂S-containing gasoline. A porous cermet of Ni and yttria-stabilized zirconia (YSZ), which has widely been studied as an anode electrode for the internal reforming operation SOFC, is one of promising anode electrode materials; however, performance degradation of anode electrode due to sulfur poisoning occurs by the impurity in the gasoline. Principal sulfur poisoning reaction is sulfide generation of Ni in the middle and low temperature region where the anode electrode is exposed at starting and stopping of the SOFC.

In this study, the effect of controlling the potential of the anode electrode to the stable region on the suppressing the generation of nickel sulfide is revealed. The possibility of performance recovery of the sulfurated anode by shifting the anode electrode potential to the stable region is also investigated. First of all, three kinds of reactions of H₂+0.5O₂＝H₂O, H₂S＝H₂＋0.5S₂, and S₂＋1.5Ni＝Ni₃S₂ are assumed to occur on the anode electrode and the stable electrode potential region for the various cell temperatures was theoretically estimated by using the Gibbs energy of the reactions calculated by the thermodynamic database MALT2. Then, experimental studies were conducted by using Ni model electrodes and Ni-YSZ cermet electrodes. Microstructure change of the electrode was analyzed by scanning electron microscopy and electron probe microanalysis. Electrode performances were investigated by electrochemical analysis (current-voltage measurement and ac-impedance method). The reduction of the sulfurated nickel electrode generated at 400 ^oC in 10ppm- H₂S containing hydrogen fuel to metallic Ni by shifting the electrode potential was investigated. Independently of open circuit voltage state or power generation state, all of the Ni surface and the Ni/YSZ interface formed a sulfur compound. When the electrode potential was shifted to -1.9 V vs reference electrode, the reduction reaction was preceded. Although the reduction rate decreases with decreasing the applied potential, in the range of more than -1.9 V it was independent of the applied potential. The decreased performance of the anode electrode by sulfurization of nickel was recovered by controlling the electrode potential. Although it takes a long time to reduce when the Ni particles is poisoned to the inside, it was recovered in a short period of time when the only the surface of Ni is poisoned. Effect of the holding the electrode potential in the stable region for the inhibition of the generation reaction of nickel sulfide was investigated. When the electrode potential was kept at -1.9 V vs reference electrode, the Ni-electrode was maintained the metal state even in the 10ppm-H₂S/H₂ atmosphere at 400 ^oC.