Performance evolution analysis of a solid oxide cell operated in fuel-cell, electrolysis and cycle modes

Abstract

Solid oxide cells (SOCs) are especially important in the context of a boom in the intermittent renewable energy. However, the widespread commercialization of SOCs is still constrained by stability. To investigate the performance evolution mechanisms, fuel-cell, electrolysis, and reversible operations of an industrial-size (10 cm × 10 cm) SOC were conducted. The electrochemical impedance spectroscopy (EIS) measured under open-circuit/a small DC bias and operating current was analyzed employing the distribution of relaxation times (DRT) method and equivalent circuit model (ECM) fitting. Under the fuel-cell and electrolysis modes, the resistances corresponding to the electrode processes held different change trends with increasing DC biases. Compared with the fuel-cell mode, the proportion of the resistance related to the gas diffusion and conversion processes of the fuel electrode was higher in the electrolysis mode. Meanwhile, the resistances associated with the charge transfer reaction, gas diffusion and conversion processes of fuel electrode increased faster in the electrolysis mode. Besides, through the evolution of j-V curves and resistances of electrode processes, the whole operation process was divided into the initial stage (first activation and then rapid-degradation) and the stable stage. In the post-mortem analysis, Ni non-percolating, Ni coarsening and change of pore morphology in the fuel electrode were mainly observed. Combined with the detailed EIS analysis and microstructure changes, the dominant performance evolution mechanism in different stages of the overall operation process was proposed.