Operando diagnosis, optimization and degradation of solid oxide fuel cell technology

Accelerating the energy transition toward systems based on low-cost renewable feedstocks and diverse energy sources is imperative. Solid oxide fuel cell (SOFC) technology represent a promising solution, offering high conversion efficiency, fuel flexibility, and combined heat and power capabilities. While material development has reached maturity, the field still lacks systematic and comprehensive investigation across multiple scales: from fundamental electrochemical processes in single cells to system-level challenges including control optimization and degradation criteria in compact CHP units. This thesis systematically investigates the electrochemical performance of Ni-GDC electrolyte supported cells (ESCs) from single-cell to system levels, based on which the control optimization algorithm and the degradation criteria are developed. Chapter 1 introduces SOFC technology and its comparative advantages. Chapter 2 employs electrochemical impedance spectroscopy (EIS) with distribution of relaxation times, equivalent circuit model , and complex nonlinear least square fit to deconvolute single-cell processes, identifying dominant resistances in gas conversion (0.1-1 Hz), O$^{2-}$ surface exchange in GDC (0.1-10 Hz), coupled gas diffusion/surface exchange (1-50 Hz), and charge transfer (>50 Hz). Chapter 3 examines degradation mechanisms under biogas reformate conditions with and without sulfur poisoning. EIS analysis indicated that major degradation originated from ohmic resistance and electrode charge transfer resistances. Sulfur poisoning tests demonstrated H$_2$S causes irreversible degradation, while degradation caused by dimethyl sulfide were fully reversible. Active negatrode GDC surface reaction was identified as the main contributor of better sulfur resistance than the Ni-YSZ anode supported cell. At the system level (Chapter 4), a Ό-CHP SOFC system was characterized under operational extremes. EIS and total harmonic distortion analysis identified fuel starvation thresholds (safe utilization factor