Electrochemical Characteristics of Ni-Co Alloy Cermet Anodes for SOFCs
Authors: Matsumoto Kohei , Tachikawa Yuya , Matsuda Junko , Taniguchi Shunsuke , Sasaki Kazunari 
Published: December 2020

Introduction

Nickel (Ni) is widely used as the anode material for SOFCs. However, metallic Ni is oxidized to nickel oxide (NiO) in an oxidizing atmosphere at SOFC operational temperatures. Ni anode degradation occurs when the hydrogen-containing fuel supply is reduced in high fuel utilization operation or interrupted during the shutdown [1]. Due to the redox cycles, Ni volume expansion and shrinkage cause irreversible degradation, by breaking electrically-conducting pathways. Highly durable SOFC anodes using e.g. Ni-based alloy are therefore under development [2]. Here, in this study, we focus on the Ni-Co alloy, by preparing a new Ni-Co-GDC cermet anode using the Ni-Co alloy, and evaluate the effects of Cobalt addition on their electrode performance and durability.

Experimental

In this study, we prepared the Ni-alloy cermet by mixing the powder and the binder, preparing and printing electrode paste on the electrolyte plate, followed by heat treatment. Scandia-Stabilized Zirconia (ScSZ, 10mol% Sc2O3-1mol% CeO2-89mol% ZrO2) was used as the solid electrolyte. The powder prepared by mixing Ni-Co-based oxide powder prepared by ammonia co-precipitation with Ce0.9Gd0.1O2 (GDC) in a specific ratio by weight, was used as the anode material. La1-xSrxMnO3 (LSM) powder was used as the cathode material. The electrochemical characteristics of the cells, with the Ni-alloy cermet anodes, were evaluated by I-V measurements and the current interruption method under the condition at 800oC, and 3%-humidified hydrogen fuel was supplied to the anode. Furthermore, the redox stability was evaluated using the durability test protocol simulating the fuel interruption. Anode microstructural changes before and after the redox cycling tests were observed by a focused-ion beam system coupled with a scanning electron microscope (FIB-SEM).

Results and discussion

Ni-alloy GDC anode with x mol% of Co added to Ni, is denoted as Ni(100 – x)Cox -GDC anode. The anodes of x = 0, 5, 10, 20, and 30 were fabricated. The I-V characteristics and overvoltages, under the condition where the operating temperature was 800oC and 3%-humidified hydrogen fuel was supplied, are shown in Fig. 1 for different Co contents in the alloy. With increasing the Co content, the electrochemical performance decreases due to a gradual increase in overvoltages. However, when the Co content is as low as 5 mol%, performance decrease is negligible. The microstructure of the Ni95Co5-GDC anode shown in Fig. 2 also shows that the distribution of Ni and Co is overlapped, confirming that a Ni-Co alloy is actually formed. Electrochemical properties and durability of the cells with the Ni-Co alloy anodes with various Co concentrations will be reported and discussed.

References

[1] Q. Fang, L. Blum, R. Peters, M. Peksen, P. Batfalsky, and D. Stolten, Int. J. Hydrogen Energy, 40, 1128 (2015).

[2] Y. Ishibashi, S. Futamura, Y. Tachikawa, J. Matsuda, Y. Shiratori, S. Taniguchi, and K. Sasaki, ECS Trans., 91 (1) 1889 (2019).

Figure 1