In conventional solid oxide fuel cell (SOFC) anodes, yttria-stabilized zirconia (YSZ), Ni and pore phases are randomly distributed, where oxide ion, electron, H₂ and H₂O gases diffuse. These species electrochemically react at the triple phase boundaries (TPBs). From the viewpoint of tortuosity, aligned YSZ and Ni structure in the thickness direction is desirable, since it can reduce transport resistance. In our previous work [1], it was demonstrated that Ni particles can be aligned by applying the magnetic field during the drying process after screen-printing of Ni/YSZ paste. In this study, the Ni-YSZ volume ratio was varied to investigate the effect of the volume ratio on the polarization resistance of anodes fabricated with and without the magnetic field. In the conventional random structure, volume ratio of Ni and YSZ is generally kept at 50:50 to maintain active TPBs. Here, ionic conductivity of YSZ is much smaller than the electronic conductivity of Ni. Therefore, if Ni alignment can be improved by applying the magnetic field, YSZ volume fraction can possibly be increased, alternatively volume fraction of Ni can be reduced. The trade-off between the decrease in TPB length and the increase in ionic conductivity must be carefully investigated.

The volume ratio of Ni and YSZ was varied as 50:50, 33:67, and 20:80 in the present study. The Ni/YSZ paste was screen-printed on both sides of 200-mm-thick YSZ electrolyte substrate. During the drying process, magnetic field was applied along the thickness direction. After the anodes were sintered at 1400˚C in air, they were reduced in the furnace before the electrochemical measurement. The polarization resistance was measured by electrochemical impedance spectroscopy at 800˚C in 5%H₂O, 95%H₂ gas. Figure 1 shows the Nyquist plots of the symmetrical cells (a) without and (b) with the magnetic field. Without the magnetic field, the polarization resistance of the cell increased as the Ni volume fraction is reduced, which is reasonable from the percolation theory. On the other hand, with the magnetic field, the polarization resistances remained nearly unchanged. It is considered that the decrease in TPB length cancelled out the increase in ionic conductivity of the YSZ path. On the other hand, it is expected that the reduction of Ni volume fraction will lead to the improvement of durability of the cell, since strengthened YSZ skeleton may supress Ni agglomeration and the mismatch in thermal expansion between electrolyte and anode can be improved.

[1] K. Nagato, N. Shikazono, A. Weber, D. Klotz, M. Nakao, E. Ivers-Tiffée, ECS Trans. 57, 1307-1311 (2013).