Review of factors affecting the performance degradation of Ni-YSZ fuel electrodes in solid oxide electrolyzer cells

Solid oxide electrolysis cells (SOECs) are an attractive technology for addressing the global environmental and energy problems caused by the heavy use of fossil fuels. The commercialization of SOECs is important for achieving decentralized hydrogen energy systems. SOECs have the advantages of high efficiency and a high conversion rate for the produced hydrogen compared to competing technologies. However, SOEC systems suffer from performance degradation after long-term operation, which mainly originates from the fuel electrode side, e. g., the Ni/yttria-stabilized zirconia (YSZ) electrode. The most common degradation phenomena are Ni agglomeration, Ni separation from the YSZ electrolyte, Ni re-oxidation, carbon deposition, and poisoning by gas impurities. These phenomena are mainly related to Ni migration occurring in the Ni/YSZ cathode owing to factors such as high water vapor concentration, high applied voltage or current density, high temperature, and the presence of gas impurities. In this review, the most important factors affecting Ni migration and other related degradation phenomena are summarized to provide insights into the degradation mechanisms of Ni/YSZ fuel electrodes. Based on these insights, strategies for improving the lifetime of Ni/YSZ fuel electrodes are discussed.