Effects of Nitride Formation on Anode Catalytic Activity in Ammonia-Fueled SOFCs

We have so far developed Ni-Fe and Ni-Mo anodes with high catalytic activity for ammonia(NH₃)-fueled SOFCs [1], and have proposed that the performance of NH₃ -fueled SOFCs was determined by a balance between NH₃ adsorption (Fe, Mo) and nitrogen(N₂) desorption (Ni) capability [2]. Furthermore, we also found that both Ni-Fe and Ni-Mo anodes react with NH₃ to form metal alloy nitrides (FeN_x, γ-Ni₂Fe₂N and Ni_0.2Mo_0.8N) and metal Ni under the operation condition, while pure Ni anode does not form nitrides. These results suggested that the nitrides enhance NH₃ adsorption at the anode and led to the high anode activity for NH₃ oxidation.

In the present study, we prepared three kinds of anodes, Ni-W, Ni-Ta and Ni-Nb and evaluated their performance as anode in NH₃-fueled SOFCs to investigate the correlation between the nitride formation and the anode activity. These anodes were synthesized by impregnating W, Ta and Nb precursor solutions into a pre-sintered Ni-SDC anode cermet. The addition of W and Ta enhanced the anode activity for NH₃ oxidation, but Nb did not.  XRD measurements of metal oxides (WO₃, Ta₂O₅ and Nb₂O₅) after annealing in NH₃ indicated that W and Ta in the anode reacted with NH₃ to form nitrides, but Nb did not under the operation condition (i.e. at 973-1173 K in NH₃). These results suggest that the easiness of the nitride formation is closely related to the high activity anode for NH₃ oxidation.

Acknowledge

This work was supported by the Kyoto Environmental Nanotechnology Cluster from MEXT, Japan.

Reference

[1] W. Akimoto et al., Solid State Ionics, 256, 1–4 (2014).

[2] W. Akimoto et al., ECS Trans., 57 (1), 1639-1645 (2013).