In this study, the performance of anode-supported and electrolyte-supported cells was compared under similar operating conditions for direct ammonia-fueled solid oxide fuel cell (SOFC); the cell configuration of both types of cells was as follows, Ni‒YSZ|YSZ|GDC|LSCF. The anode-supported cell showed better performance as compared with that of the electrolyte-supported cell. A power density of ca. 0.358 W cm^-2 was achieved for the anode-supported cell at 700 ^oC, which was higher than that of the electrolyte-supported cell. Furthermore, the ohmic and polarization resistances were lower for anode-supported cell. This is because in the anode-supported cell, the thinner electrolyte layer is responsible for the reduced ohmic resistance and improved cell efficiency.

The effect of humidity on the performance of both types of cells using NH₃ and H₂ fuels was studied. The experimental results indicated that in the case of ammonia fuel, a small amount of humidity (1.67% H₂O) was beneficial for better performance. However, further increase in humidity caused cell performance degradation. The effect of humidity on the cell performance was further analyzed. The anode microstructure of both types of cells was investigated after 10 h of discharge operation at 0.2 A cm^-2 at 700 ^oC using NH₃ and H₂ fuels under various humidified conditions. For the anode-supported cell, the surface of Ni particles was covered with the small-sized particles after discharge operation; the Ni particles on the anode surface coarsened significantly in the ammonia fuel as compared with the hydrogen fuel. This is because the anode surface is exposed to the gaseous mixture containing large amount of ammonia which may cause the nitridation of Ni particles. On the other hand, at the anode/electrolyte interface the extent of coarsening of Ni particles was almost similar regardless of the fuel. This can be explained by the fact that most of the NH₃decomposes catalytically and thermally at the anode surface. Hence, the partial pressure of ammonia will be lower at the anode/electrolyte interface.

In this study, it was found that the anode-supported design is suitable for direct-ammonia fueled SOFCs. In the case of anode-supported cells, the improvement in the performance and the stability can be expected by optimizing the operating conditions, such as the operating temperature and the steam concentration in the inlet gas.

Acknowledgement

This work was supported by Council for Science, Technology and Innovation (CSTI), Cross-ministerial Strategic Innovation Promotion Program (SIP), “energy carrier” (Funding agency: JST).