Deposition of Gadolinium-Doped Ceria Thin Films by Ultrasonic Spray Pyrolysis for High-Temperature Electrolysis

The lifetime, performance, and cost competitiveness of solid oxide electrolysis cells can be substantially enhanced by incorporating functional layers, such as gadolinium-doped ceria (CGO), which serve as a diffusion barrier between the solid electrolyte and the perovskite oxygen electrode. The thickness of CGO, composed of scarce and expensive metals, is typically within 5-20 mu m. Yet, it can be drastically reduced to several hundred nanometers by applying cost-effective and scalable thin film deposition methods, like ultrasonic spray pyrolysis (USP). However, achieving high-quality, structurally intact films that reliably cover larger substrate areas with USP remains challenging. The main challenges are the poor material utilization associated with the ventilation of low-velocity droplets in the spraying stream, as well as defect formation during film growth due to the presence of large droplets. This paper presents a systematic investigation of the USP parameters for CGO from nitrate precursor solution, addressing the above challenges. Specifically, the material utilization is optimized by increasing the shaping air pressure, leading to small, high-velocity droplets, which efficiently deposit on the substrate. By introducing an intermediate waiting step between spraying cycles, active cooling of the substrate after each spraying cycle, combined with an optimized postdeposition thermal treatment protocol, defect-poor, homogeneous films could be obtained. CGO films of 350 nm, deposited on scandia-stabilized zirconia solid electrolyte substrates, show surface roughness of 1 and 14 nm before and after thermal treatment, respectively, and defect density of <0.1% of the coated area. Finally, the ionic conductivity of the CGO film, deposited on an insulating alumina substrate, was characterized via electrochemical impedance spectroscopy in the relevant temperature window between 500 and 900 degrees C.