Imporved characteristics of optimized YTTRIA-stabilized zirconia thin film grown using sol-gel dip-coating technique

Abstract

Clean and sustainable energy production has become the priority of the 21st century to save Earth’s environment from ever-increasing pollution and climate change. Solid Oxide Fuel Cell (SOFC) is one of the emergent technologies that which can produce a vast amount of clean energy using water; thus, considered to be renewable, sustainable, and pollution-free. All previous studies have used high-temperature SOFC electrolyte preparation methods which were expensive and cumbersome. Based on these factors, in this study, good quality yttria-stabilized zirconia (YSZ) electrolyte thin film was synthesizing using soda lime glass and sapphire as a substrate by sol-gel dip-coating method which use low temperature preparation method for YSZ thin film. Several solvents were tested to obtain homogeneous suspension of YSZ such as water, acetone and alcohol and it was found out that alcohol is the suitable one. The samples then were characterized by diverse analytical techniques to determine their potency for the SOFC applications. As-deposited thin films were characterized using X-ray diffractometer (XRD) analyses, atomic force microscopy (AFM), Raman spectroscopy, field emission scanning electron microscopy (FESEM) and Four-point probe techniques. The impact of various substrates, sintering temperatures and layer-count on the structural, morphological and electrical properties of these electrolyte thin films was evaluated. Furthermore, the optimum thin film was chosen to demonstrate its viability as an effective SOFC electrolyte. XRD analysis showed that the YSZ thin films deposited on soda-lime glass and sapphire were highly amorphous and crystalline in nature, respectively. Hence, the sapphire substrate was selected for further deposition of thin films. To improve the overall characteristics of the YSZ films, two types of heat treatments like sintering and normalizing, were performed. The film which had been sintered at 1300 °C revealed the formation of crystallites with lower density than the normalized film. Conversely, the film which had been sintered above 1300 °C showed much better morphology compared to those obtained using the normalized heat treatment. The single-layer YSZ film which had been sintered at 1300 °C exhibited excellent crystallinity, dense morphology with fewer cracks, very low porosity and high electrical conductivity (2.905 S/m). The double-layered YSZ film sintered at 1400 °C displayed the highest conductivity (3.552 S/m) and best crystalline density (5.976 g/cm3). An increase in the number of layers from three to ten was found to degrade the crystallinity, density and morphology of the grown films. The YSZ electrolyte films deposited with up to three layers were not suitable for practical use in the SOFC. Therefore, the double-layered film which had been sintered at 1400 °C was chosen as the optimum film. The growth of the proposed films was explained using various mechanisms to provide new insight into the preparation of the YSZ electrolyte thin films with tailored properties via the low-cost and simple method at low temperatures. This study focused only on two types of substrates, a few sintering temperatures and one preparation method. However, another synthesis method, substrate and characterization technique such as spin-coating, porous anodic alumina and UV-Visible absorption or emission spectroscopy, respectively, may be useful for better sample optimization. The use of lower temperature for the deposition of the thin films can yield higher electrical conductivity of the electrolyte useful for room temperature operation of SOFC. Based on the results it can be concluded that the interplay of the type of substrate, sintering temperature and the number of coating layers plays a vital role in obtaining high-quality electrolyte thin film, showing that synthesis of the YSZ thin film is essential thus the best sample is established.