Performance and stability of nanostructured solid oxide fuel cell cermet electrodes

Abstract

Nickel-ceramic cermets are the most industrial relevant materials in solid oxide fuel cell (SOFC) anode and solid oxide electrolyser cell (SOEC) cathode today. Nano-structure engineering of the electrode provides the benefits of high electrochemical activity, but the electrode’s stability has been questionable. For commercial applications, SOFC cells should sustain good performance for 40,000 hours of high temperature operations. Nickel coarsening between 500 °C to 1000 °C for high temperature SOFCs leads to both a reduction in the triple phase boundary (TPB), which is the electrochemical reaction site and a reduction in network connectivity, which is essential for current collection. In this work, particular attention is paid to the evolution of nickel nanoparticles in the cermet and its structural relationship with the ceramic phase. Impregnation is one of the most popular techniques to incorporate nanoparticles in SOFC electrodes. In this study, nickel-impregnated scandia stabilised zirconia (ScSZ) electrodes were annealed isothermally at 650, 800 and 950 °C in 3% H2O-5% H2-N2. An initial degradation of the electrodes was observed during the first 17 h of annealing. At 800 °C, the in-plane conductivity degraded from 1000 S cm-1 to 700 S cm-1 and the area specific resistance (ASR) for electrochemical reaction from 0.17 to 1.15 Ω cm2 seen in electrochemical impedance spectra (EIS). Post-mortem scanning electron microscopy (SEM) images revealed an increase in particle size of the impregnated nickel nanoparticles, which caused a reduction in both active electrode area and TPB density. The nickel-impregnated electrode showed good performance initially but degraded in a relatively short time as a result of nickel coarsening. Alternatively, the bottom-up manufacturing which uses nanoparticles from the beginning of powder mixing has great industrial potential. Composite nanoparticles of nickel oxide-yttria stabilised zirconia (YSZ) fabricated by continuous hydrothermal flow synthesis (CHFC) were processed into solid oxide cell (SOC) electrodes. The nanocomposite powders have a high surface area of 189 m2 g-1 and form 1 – 10 µm agglomerates after freeze-drying and ball milling. Focused ion beam (FIB)-SEM images and 3-D reconstruction revealed the electrode had a dual porosity microstructure with micron-size agglomerates, containing inter-penetrating networks of Ni (180 nm), YSZ (170 nm) and pores (310 nm), separated by micron-sized pores. The electrode has a high TPB density of 11.1 µm-2. The electrodes aged at 800 °C in 3% H2O-5% H2-N2 for 600 h showed excellent stability. The electrochemical reaction resistance was stable at 0.15 Ω cm2. FIB-SEM 3-D reconstruction suggested that after aging, the TPB density was still high at 10.0 µm -2 and the particle size of Ni and YSZ remained approximately 100 nm. Similarly, nanoscale nickel gadolinia-doped ceria (CGO) electrodes were fabricated from similar nanocomposite powders. The electrodes have high TPB density of 8.4 µm-2 and double phase boundary (DPB) density of 1.91 µm-1. During isothermal aging at 800 °C in 3% H2O-5% H2-N2, the total polarisation resistance was stable at 0.20 Ω cm2. During the aging, the TPB density decreased but the DPB density remained relatively constant. In the Ni-CGO electrodes, the electrochemical reactions primarily take place on the surface of CGO and the nickel network mainly serves as current collector. The excellent stability of nanostructured electrodes made by hydrothermal synthesis, in comparison to nickel impregnation, indicated that the microstructural relationship between the nickel and the ceramic phase is important to its stability. The ceramic phase can effectively inhibit the coarsening of nickel, especially when their particle sizes are comparable and when they form networks intertwined with each other.