Ammonia (NH₃) has attracted much attention as a fuel for SOFCs. In the case of well-established Ni-based cermets anode, however, the cell performance is poor due to the low catalytic activity for NH₃ reforming. In this respect, it is necessary to develop the highly active anode for realizing direct NH₃-fueld SOFCs.

Previously, we prepared Ni-M (M : Mo, Ta, W and Nb)/ Sm-doped CeO₂(SDC) anodes by infiltration of metal solutions into pre-sintered Ni/SDC cermets and revealed the activity of them as an anode for direct NH₃-fueled SOFCs at temperature of 700 to 900 °C [1-2]. From these studies, we have proposed that the synergetic effect of Ni and added metals, which weakly and strongly bind to N atom, enhances the activity of the anodes for NH₃ oxidation(NH₃ reforming, followed by hydrogen oxidation). In addition, it was found that the enhancement of the activity for NH₃ oxidation strongly depended on the added metal. Among them, Mo was the most effective element as an additive. On the other hand, the obvious enhancement of the activity for NH₃ oxidation was not observed when Nb, Ta and W were added, which have the larger metal-nitrogen (M-N) binding energy than Mo. The activity of the Ni-M binary anode seemed to be determined by the interaction potential between catalytic surface and N atom, produced by Ni and added metals. Here, we focus chromium(Cr) and vanadium(V) as an additive, because Cr and V have the moderate M-N binding energy (ranked in the order of Mo < Cr < V < W < Nb < Ta) among the metals used in our previous studies.

The addition of Cr and V into the Ni/SDC cermets enhanced the activity of the anode for NH₃ oxidation at temperature of 700 to 900 °C. The formation of Ni metal and nitrides (CrN and VN) was confirmed in each anode at 700 °C. This suggests that the synergetic effect of Ni and Cr(V), which has weekly and strongly bind to N atom enhances the activity of the anode for NH₃ oxidation. Cr was more effective than Mo as an additive, but V was not. The relationship between the activity of the anode and the M-N binding energy of the added metal are discussed.

Acknowledge

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

 

Reference

[1] M. Hashinokuchi et al., Solid State Ionics, 285, (2016) 222

[2] R. Yokochi et al., ECS Trans. 68(2015) 2745.