Mechanism and Activity of Ni-Based (Ni-M: M = Fe, Mo, W, Ta) Cermet Anodes for Ammonia Oxidation in SOFCs

Ammonia (NH₃) is attractive as a hydrogen (H₂) carrier and a suitable fuel for solid oxide fuel cells (SOFCs) because it has high hydrogen density (17.6 %) and can be easily liquefied at an ambient temperature. However, the activity of conventional Ni cermet anodes of SOFCs for NH₃ oxidation is much lower than that when H₂ is used as a fuel [1]. It is necessarily to improve the anode activity for NH₃ oxidation for realizing NH₃-fueled SOFCs.

 We have investigated the activity of anodes prepared with transition metals nickel(Ni), iron (Fe) and their alloy for NH₃-SOFCs[2,3]. The Fe/Sm-doped ceria (SDC) cermet anode was more active for NH₃ oxidation than Ni/SDC. We also found that the activity of the Ni₄₀-Fe_60//SDC for NH₃ oxidation was highest by optimizing the metal composition. These results suggested that the rate limiting steps in a direct NH₃ oxidation (or reforming NH₃) at the anode were (i) NH₃ adsorption and (ii) N₂ desorption. We concluded that Fe and Ni atoms had the roles of accelerating reactions (i) and (ii), respectively, and the synergetic effect of Fe and Ni resulted in the enhancement of the anode activity for NH₃oxidation.

 On the basis of the mechanism of NH₃ oxidation at the anode, we prepared the anode of Ni and Molybdenum (Mo) since Mo has the high ability of NH₃ adsorption, and evaluated their performance for NH₃-SOFCs. The Ni-Mo/SDC anodes were fabricated by impregnating a MoCl₅ aqueous solution into a Ni/SDC pre-sintered cermet on an electrolyte. It was revealed that the optimum addiction of Mo enhanced the anode activity for NH₃ oxidation. Our experimental results suggested that the anode activity for NH₃ oxidation was determined by a balance between abilities of NH₃ adsorption and N₂desorption.

Acknowledge

This work was supported by “Kyoto Environmental Nanotechnology Cluster” and “Kyoto Regional Scientific Innovation Hub” from MEXT in Japan.

Reference

[1] Q. Ma et al., J. Power Sources 161 (2006) 95.

[2] W. Akimoto et al., Solid State Ionics 256 (2013) 1.

[3] W. Akimoto et al., ECS Transactions 57 (2013) 1639.