The impact of redox cycling on nickel cermet anodes for solid oxide fuel cells

Abstract

Fuel Cells comprise a broad range of technologies that offer more efficient ways of converting fuel into electricity. The solid oxide fuel cell (SOFC) is a promising energy conversion device, which offers a high operating efficiency alternative to combustion technology for electricity production. Nickel-Yttria-stabilized zirconia (Ni-YSZ) cermets are a commonly used anode material with relatively low cost and good performance. In order to understand the interaction between microstructural, mechanical properties and electrochemical changes of Ni-YSZ electrodes after redox cycling, the impact of redox cycling on electrode degradation has been studied, including: conventional NI-YSZ, Ni infiltrated YSZ scaffolds, and Ni infiltrated YSZ fibre scaffolds in Chapters 4, 5 and 6. It has been found that, due to the nickel volume change during redox cycling, the YSZ framework breaks down in conventional Ni-YSZ electrodes, inhibiting ionic transport and leading to significant electrochemical degradation; in order to enhance the YSZ framework stability, the sintering temperature impact on YSZ scaffold has been studied based on balancing the mechanical properties and porosity. Then the selected YSZ scaffold was infiltrated with nickel to test its redox cycling stability. It was found that the YSZ scaffold kept a stable structure, while the infiltrated nickel migrated out to the electrode top surface, leading to a significant nickel content loss within the electrode. This indicates that the nickel dewetting behaviour plays an important role during anode degradation. A multi-porosity layered electrode, comprising a top fibre layer and scaffold base layer, was introduced to eliminate the triple phase boundary loss when the nickel moved to the electrode top surface. In this structure, the scaffold layer contributes to a high electrochemical performance at a lower temperature compared to conventional analogues. The fibre layer improved electrochemical stability due to its highly porous and larger pore size compared to the YSZ scaffold layer. In Chapter 7, Ni dewetting behaviour was also studied utilizing a new methodology developed in this work, based on 2D Ni films on YSZ electrolytes. By in-situ observation, it was found that both electrochemical impedance spectroscopy measurement (EIS), electrode simulation, and experimental Environmental-SEM (E-SEM) images consistently identified a maximum in active triple phase boundary (aTPB) length before the start of Ni de-percolation. Results reveal that neither evaporation-condensation nor surface diffusion of Ni are the main mechanisms of dewetting over the temperature range 560-800 °C. Rather, the energy barrier for pore nucleation within the dense Ni film appears to be the most important factor. The proposed methodology allows the prediction of Ni dewetting behaviour and its impact on the electrochemical degradation of SOFC anodes under a variety of conditions, unparalleled to current methods.