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Redox-stable anodes are developed for zirconia-based electrolyte-supported SOFCs in order to improve the durability against fuel supply interruption and for higher fuel utilization, as an alternative to the conventional Ni-YSZ cermet. GDC (Ce 0.9 Gd 0.1 O 2 ) is utilized as a mixed ionic-electronic conductor (MIEC), and combined with LST (Sr 0.9 La 0.1 TiO 3 ) as an electronic conductor. Ni catalyst nanoparticles are incorporated via impregnation. The electrochemical characteristics of SOFC single cells using these anode materials are investigated in humidified H 2 at 800 • C. The stability against redox cycling and under high fuel utilization is analyzed and discussed. Ni-impregnated anodes with dispersed Ni catalyst nanoparticles on conducting oxide LST-GDC backbones exhibit lower anode non-ohmic overvoltage, and improve I-V performance. These anodes also show better redox stability compared to conventional anodes because of the isolation of Ni catalysts, preventing their agglomeration. Moreover, the co-impregnation of Ni catalysts and GDC nanoparticles further improves electrochemical characteristics due to a decrease in anode ohmic (IR) loss and non-ohmic overvoltage. This anode shows comparable I-V performance to conventional anodes for typical humidified hydrogen fuels, and is a promising redox-stable alternative for application at high fuel utilization. Solid oxide fuel cells (SOFCs) are promising electrochemical energy conversion systems that can directly produce electricity from chemical fuels, without combustion. The operating temperature is generally around 800• C, leading to several advantages including high electric conversion efficiency, fuel flexibility, and noble-metal-free fabrication. For example, in Japan, the commercialization of SOFCs as residential co-generation systems started in 2011. The electric efficiency reached a lower heating value (LHV) of 52% in 2016. Development of these systems for industrial applications is also in progress, and combining them with micro gas turbines and/or steam turbines can achieve even higher electric efficiency. 1The porous Ni yttrium-stabilized-zirconia (YSZ) cermet has been used as a conventional SOFC anode material for decades. However, the electron conducting pathways through the Ni metal phase (which also acts as the electrocatalyst) can be easily destroyed by redox reactions during potential cycling, where Ni redox reactions result in significant changes in volume. This leads to crack formation in the ionic conducting YSZ phase, and deterioration of the electrochemical performance, especially during e.g. system shutdown, fuel supply interruption, and at high fuel utilization.2-8 Therefore, extra hydrogen (or hydrogen-containing gas) is often required during system shutdown in order to maintain reducing conditions on the anode side of the device at all times. The fuel utilization in practical systems must also be kept low for the same reason. Higher efficiency is required for wide-spread commercialization of SOFCs in the future, and therefore tolerance...

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In situ transmission electron microscopy (TEM) observations of a Ni(O)-Sc2O3-stabilized ZrO2 (ScSZ; 10 mol% Sc2O3, 1 mol% CeO2, 89 mol% ZrO2) anode in a solid oxide fuel cell (SOFC) have been performed at high temperatures under a hydrogen/oxygen gas atmosphere using an environmental transmission electron microscope (ETEM); the specimens were removed from cross-sections of the real SOFC by focused ion beam milling and lifting. When heating the NiO-ScSZ anode under a hydrogen atmosphere of 3 mbar in ETEM, nano-pores were formed at the grain boundaries and on the surface of NiO particles at around 400°C due to the volume shrinkage accompanying the reduction of NiO to Ni. Moreover, densification of Ni occurred when increasing the temperature from 600 to 700°C. High-magnification TEM images obtained in the early stages of NiO reduction revealed that the (111) planes of Ni grew almost parallel to the (111) planes of NiO. In the case of heating Ni-ScSZ under an oxygen atmosphere of 3 mbar in ETEM, oxidation of Ni starting from the surface of the particles occurred above 300°C. All Ni particles became polycrystalline NiO after the temperature was increased to 800°C. Volume expansion/contraction by mass transfer to the outside/inside of the Ni particles in the anode during repeated oxidation/reduction seems to result in the agglomeration of Ni catalysts during long-term SOFC operation. We emphasize that our in situ TEM observations will be applied to observe electrochemical reactions in SOFCs under applied electric fields.

Repeated reduction and oxidation of metallic nickel in the anodes of solid oxide fuel cell (SOFC) causes volume changes and agglomeration. This disrupts the electron conducting network, resulting in deterioration of the electrochemical performance. It is therefore desirable to develop more robust anodes with high redox stability. Here, new cermet anodes are developed, based on nickel alloyed with Co, Fe, and/or Cr. The stable phases of these different alloys are calculated for oxidizing and reducing conditions, and their electrochemical characteristics are evaluated. Whilst alloying causes a slight decrease in power generation efficiency, the Ni-alloy based anodes have significantly improved redox cycle durability. Microstructural observation reveals that alloying results in the formation of a dense oxide film on the surface of the catalyst particle (e.g. Co-oxide or a complex Fe–Ni–Cr oxide). These oxide layers help suppress oxidation of the underlying nickel catalyst particles, preventing oxidation-induced volume changes/agglomeration, and thereby preserving the electron conducting pathways. As such, the use of these alternative Ni-alloy based cermets significantly improves the redox stability of SOFC anodes.