Durability of Ni anodes during reoxidation cycles

Anodes manufactured from NiO- and yttria-stabilized zirconia (Y2O3 doped ZrO2, YSZ) powders are today's state of the art for solid oxide fuel cells (SOFCs) because they are easy to manufacture and have high performance in both anode-supported and electrolyte-supported cells. However, such cells can show significant degradation or fail completely if nickel is reoxidized during high-temperature operation even though it can be reduced again. Tests with stacks and systems have shown that system shutdown procedures, accidental air entry due to component failure or controlled air feed to the anode side as a result of operational necessities may occur and result in the reoxidation of the metallic nickel. This reoxidation is not only associated with a volume expansion, but also with significant structural changes in the anode microstructure, generating stresses in the anode itself, as well as in the electrolyte. These stresses can exceed the stability of the components, potentially promoting crack growth, which leads to degradation of the SOFC or complete failure.This problem has been addressed by a number of contributions in the literature over the last decade, but interest is increasing, particularly because SOFC systems are being discussed for transport and mobile applications requiring new system specifications. The most critical problem to be overcome is the tolerance of a large number of intentional redox cycles due to system requirements during operating lifetime.This article gives an overview of the various approaches to the redox problem by summarizing many of the contributions, starting with a basic understanding of the underlying physicochemical processes of Ni reduction and oxidation and ending at stack-level results, leading finally to their combination with recent findings. It aims to elaborate reliable results and open questions on this topic considering the mechanical and electrochemical aspects of the problem.