Thickness effect of thin-film barrier layers for enhanced long-term operation of solid oxide fuel cells

Abstract

Highly efficient solid oxide cells are one of the most promising technologies for a sustainable future based on renewable hydrogen. The diffusion barrier layer employed between zirconia-based electrolytes and state-of-the-art oxygen electrodes aims to limit the formation of electrically insulating secondary phases that dramatically reduce the cells’ performance. Conventional barrier layers manufactured by screen-printing technology lead to porous microstructures that enable the formation of insulating SrZrO3, partially blocking the active area of the cells. Opposite, homogeneous and dense barrier layers have proven to be the ultimate solution to limit interdiffusion, substantially improving the cells’ performance. Despite the relevance of this solution, the impact of the barrier layer thickness on the final performance of the cells is still unknown. In this work, gadolinia-doped ceria barrier layers with thicknesses between 200 and 800 nm made by pulsed laser deposition were studied in button cells. Excellent electrochemical performance was obtained for all the cells, improving 45% of the power output of the reference counterparts. Moreover, durability tests performed on the cell with the thinnest layer (200 nm) did not show any measurable degradation for 3500 h of continuous operation under high current densities of 0.77 A cm−2 (∼0.87 V) at 750 °C. Post-mortem characterization by synchrotron nano-x-ray fluorescence of a pristine cell and the aged cell allowed us to observe that some spots of SrZrO3 were present at the cathode/electrolyte interface since the cell manufacturing step without increasing during long-term operation. Indeed, the discontinuity of this insulating phase seems not to be critical for cell operation.