Redox Durability of Ni-Co Alloy Cermet Anodes for SOFCs

Abstract

Introduction Nickel (Ni) is commonly used as the anode material of SOFCs. However, in the power generation, as the oxygen partial pressure in the downstream of the SOFC systems increases, the metallic Ni can be oxidized to nickel oxide (NiO), causing volume expansion (1) This volume expansion and shrinkage in redox cycles cause irreversible degradation, by breaking electrode microstructure. Highly durable SOFC anodes using Ni-based alloy are therefore desirable (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 redox durability. Experimental The cells were fabricated by screen-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. Ni-Co-GDC cermet anodes were prepared by mixing Ni-Co-based oxide powder by ammonia co-precipitation with Ce0.9Gd0.1O2 (GDC) in a specific ratio by weight. La1-xSrxMnO3 (LSM) powder was used as the cathode material. The electrochemical characteristics of the cells, with the Ni-Co cermet anodes, were evaluated by measuring I-V measurements coupled with the current interruption method under the condition at 800oC, where 3%-humidified hydrogen fuel was supplied to the anode. Furthermore, the redox stability was evaluated using the durability test protocol simulating the fuel interruption (3). Anode microstructure was observed by a focused-ion beam system coupled with a scanning electron microscope (FIB-SEM). Results and discussion Figure 1 shows the I-V characteristics and overvoltages of the Ni-Co-GDC anode with various Co concentrations at 800oC with 3%-humidified hydrogen fuel. With increasing the Co content, the electrochemical performance decreased due to an increase in overvoltages. However, when the Co content was as low as 5 mol%, performance decrease was negligible. Figure 2 shows the redox stability of the Ni-Co-GDC anode evaluated by using the durability test protocol simulating the fuel interruption. With increasing the Co content, the redox stability was improved, associated with relatively-low ohmic losses. The microstructure of Ni-Co-GDC anodes will be reported and discussed. References Q. Fang et al., Int. J. Hydrogen Energy, 40, 1128 (2015).Y. Ishibashi et al., J. Electrochem. Soc., 167, 124517 (2020).M. Hanasaki et al., J. Electrochem. Soc., 161 (9), F850-60 (2014). Figure 1