Quantifying the impact of processing conditions on the performance of infiltrated Solid Oxide Fuel Cell electrodes

The development of commercially viable Solid Oxide Fuel Cells has presented a series of complex materials challenges that need to be overcome. Most notably under the elevated operating temperature of SOFCs (T ≥ 800 °C) the fuel electrode catalyst sinters to reduce its surface energy leading to diminished stack performance. High surface area nanostructured infiltrated electrodes have been developed to accommodate for this performance loss. However, current infiltrated systems are also prone to catalyst coarsening. To ensure these materials can withstand the harsh reductive conditions at the anode extensive work needs to be done on characterising the catalyst sintering mechanism and mitigating it. In this thesis the impact of different processing steps on the early stage (t ≤ 50 h) degradation of infiltrated Ni-YSZ electrodes has been investigated. This included the characterisation of the first use of an inert N2 drying atmosphere on infiltrated Ni-YSZ symmetrical cells samples to control the particle size and dispersion of the catalyst. This included tracking the crystallite size changes across the first 3 h of ageing using the Debye-Scherrer equation. Drying under a Nitrogen atmosphere showed a 20 % reduction in the Ni crystallite size in comparison with air-dried cells leading to a 1.5 % increase in the percent dispersion for the N2 dried nanoparticles. The formation of these smaller particles was linked to an egg-shell drying model is presented from Scanning Electron Microscopy and Fourier Transform Infrared spectroscopy data. The impact of utilising different current collector materials (Ag, Au, NiO) on the electrochemical impedance spectroscopy of infiltrated symmetrical anodes was also investigated. Nickel oxide produced the lowest polarisation resistance over 50 h of ageing. In addition, Ptychographic X-ray Computed Tomography was used to quantify the microstructural changes of present in infiltrated Ni-YSZ pillars between 0-4 h of ageing. This entailed the first successful phase segmentation of the catalyst, ceramic and pore phases for infiltrated fuel electrode samples from an in ex-situ snapshot imaging procedure. This included tracking the nickel migration towards the triple phase boundary and pore blocking also imaged with FIB-SEM. This led to the identification of the infiltrated nickel coarsening mechanism as particle migration coalescence at ageing times after 2h. Overall, this work demonstrated the potential of utilising an inert drying step to control the dispersion and size of infiltrated fuel electrode catalysts. The PXCT dataset has highlighted the importance of phase quantification on the degradation characterisation of infiltrated SOFC anodes. Further work is needed to optimise these processes to inform future sinter resistant SOFC electrode designs.