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Introduction A Nickel (Ni) porous metal is used for anode current collector in solid oxide fuel cells (SOFCs)[1]. However, it is difficult to be applied as cathode current collector due to its large oxidation resistance at high temperature. In this work, we investigated characteristics and applicability of a new Ni-Sn porous alloy as the cathode current collector for SOFCs. Experimental The Ni porous alloy was prepared by electroplating process. Sn was deposited continuously by electroplating onto the surface of the Ni porous metal. Then, heat treatment was performed under hydrogen atmosphere to obtain the Ni-Sn porous alloy [2]. Figure 1(A) shows a SEM image of three-dimensional structure of the as-prepared sample. Oxidation resistances were evaluated by measuring weight increase due to the heat treatment, and the area-specific resistance at high temperature. The crystalline phases of the specimens before and after heat treatment were identified by XRD. Mechanically polished cross-sections of the specimens were observed via SEM-EPMA. Finally, the prototype SOFCs were fabricated using the Ni-Sn porous alloy as the cathode current collector. Results and Discussion For Ni-Sn porous alloys with various Sn contents, figure 1(B) shows the weight increase due to the heat treatments in air at 800 °C for 1,000 hours and at 600 °C for 3,000 hours. In the case of the heat treatment at 800 °C for 1,000 hours, the weight of Ni-Sn porous alloys increased regardless of the Sn content. And the weight increased more than 1.7 mg/cm2, revealing a noticeable level of oxidation. In contrast, for Ni-Sn porous alloy with Sn content of 5 wt% or more, the oxidation was noticeably retarded in spite of a long-term heat treatment at 600 °C for 3,000 hours. Figure 1(C) shows the distribution of oxygen by using EPMA-EDS to analyze the cross-section area of the Ni-10wt%Sn porous alloy before and after heat treatment at 600 °C in air for 1,000 hours. After heat treatment, one can see a clear existence of oxygen in the depth about 1 μm from the surface, indicating the oxidation of this region. The IV relationship and the power output characteristics of SOFCs with the cathode current collectors made of Pt mesh or the Ni-10wt%Sn porous alloy. Both the behavior of the IV relationship and the output power of the SOFC with the Ni-10wt%Sn porous alloy were almost equivalent to that using the Pt mesh. Conclusions The Ni-Sn porous alloy has relatively low electrical resistance after the heat-treatment in oxygen, and also high electric conductivity at 600 °C. Thus, this new Ni porous alloy has promising perspective to be applied as the cathode current collector for SOFCs operating in an intermediate temperature range. And such demand is expected to grow in near future. [1] W. Guan, et al., Fuel Cells, 12(2012), 1085-1094 [2] K. Okuno, et al., SEI Technical Review, 75(2012), 137-140 Figure 1 |
