Impedance Studies of the Reduction Process in NiO-YSZ SOFC Anodes

The optimization of Ni-YSZ anodes is an important step in obtaining electrically efficient anode supported SOFCs. Despite the continuous studies concerning optimization of the anode reduction conditions, degradation phenomena, oxidation-reduction cycling behaviour and microstructural changes, the anode reduction continues to be an active field for deeper insight [1,2], since as a final stage of the cell preparation, it offers an opportunity for optimization in respect to mechanical stability, electronic conductivity, catalytic activity and gas permeation. This is additionally complicated due to the counter-flow of the fuel and exhaust water vapour.

This study combines two approaches – impedance spectroscopy and gas permeability studies that ensure experimental information about the main processes occurring during anode reduction: (i) formation of a new microstructure influencing the diffusive mass transfer and (ii) development of an electrically conductive network. The gas permeability studies [3] are carried out on anode pellets at room temperature before and after reduction performed at different conditions (temperature, reduction atmosphere). The observed changes are discussed and compared with SEM microstructural analysis. The impedance measurements are carried out directly during the reduction of button cells, segmented cells and NiO-YSZ anode pellets and thus the reduction process is studied during its evolution.  The influence of the reduction conditions, including the duration of the process, is discussed in respect to the formation of an optimal Ni network with good electrical connectivity and catalytic activity.

The presented figure shows two impedance diagrams measured on a segment from segmented fuel cell in different stages of the  reduction process .

Acknowledgement:The research leading to these results has received funding from the European Union's Seventh Framework Programme (FP7/2007-2013) Fuel Cells and Hydrogen Joint Undertaking (FCH-JU-2013-1) under grant agreement No 621207.

References

1. S.-W. Baek, J. Bae, Internat. J. Hydrogen energy 36 (2011) 689
2. B. Liu, Y. Zhang, B. Tu, Y. Dong, M. Cheng, J. Power Sources 165 (2007) 14
3. E. Mladenova,  D. Vladikova,  Z. Stoynov,  A. Chesnaud,  A. Thorel,  M. Krapchanska, Bulg. Chem. Communic.  3 (2013) 366