Epitaxial Growth of Defect-Free Yttrium-Doped Barium Zirconate Films by Co-Sputtering for Improved Protonic Conductivity in Protonic Ceramic Fuel Cells

Protonic ceramic fuel cells (PCFCs) have emerged as a promising technology for the generation of clean energy for their low operating temperature (&lt; 600°C) and high efficiency. Among the various proton-conducting materials, yttrium-doped barium zirconate (BZY) is commonly utilized as an electrolyte material due to its high bulk conductivity and high chemical stability in H₂O and CO₂ environments. However, owing to the poor sinterability of protonic ceramic materials, conventional slurry-based fabrication methods require high-temperature sintering above 1500°C, resulting in detrimental effects such as the evaporation of constituent elements and the formation of secondary phases. To mitigate these issues, we investigated a novel approach for fabricating yttrium-doped barium zirconate (BZY) through the co-sputtering of BaCO₃ and yttrium-stabilized zirconia (YSZ) targets at a significantly lower temperature than that required for conventional wet processes. We explored various sputtering parameters and substrates to fabricate BZY films with different microstructures and compositions.<br/>The incorporation of carbon from the BaCO₃ target and undesirable carbonate formation in the film was prevented by investigating different sputtering parameters related to the thermodynamics and kinetics of carbonate deposition, including radio frequency (RF) power applied to the targets and oxygen partial pressure. Sputtering parameters that influence the growth kinetics, including deposition rate, deposition pressure, and substrate temperature, were controlled to minimize the structural defects that reduce the film's proton conductivity. Successful deposition of stoichiometric, defect-free epitaxial BZY film on a MgO substrate was confirmed by advanced characterization techniques, including transmission electron microscopy (TEM), depth-profile X-ray photoelectron spectroscopy (DP-XPS), X-ray energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), and field emission scanning electron microscopy (FE-SEM). The epitaxial BZY film had no grain boundaries, thereby minimizing the negative contribution of the grain boundaries to proton conductivity. Consequently, the in-plane proton conductivity of the epitaxial BZY film measured via the AC impedance method was significantly higher than that of wet-processed polycrystalline BZY. Our results demonstrate the potential of co-sputtering as a low-temperature fabrication approach for producing high-quality BZY electrolytes with improved protonic conductivity and reduced thickness for PCFC applications.<br/><br/>This work was supported by the Hyundai Motor Chung Mong-Koo Foundation.