Surface and bulk reduction kinetics of liquid plasma-sprayed NiO-YSZ layers

Nickel oxide–yttria stabilized zirconia (NiO-YSZ) is the established material for Solid Oxide Fuel Cell (SOFC) and Solid Oxide electrolysis Cell (SOEC) electrodes, where NiO reduction dictates the final microstructure and subsequent properties. In this study, NiO-YSZ layers were fabricated via liquid plasma spraying using a hybrid water–argon stabilized plasma torch (WSP-H), to create porous layers without pore-forming additives. To evaluate the influence of feedstock chemistry on reduction behavior, four coatings with varying NiO:YSZ ratios were prepared from ethanol-based solution-suspensions, alongside one from water-based solution. The surface and bulk reduction kinetics were systematically characterized through a combination of near-ambient pressure X-ray photoelectron spectroscopy (NAP–XPS), thermogravimetric analysis (TGA), and X-ray diffraction (XRD). Layers derived from the ethanol-based feedstock exhibited higher porosity (up to ∼ 58%) and finer phase intermixing compared to the water-based counterpart. The reduction behavior of the NiO-YSZ layers strongly correlated with the feedstock solvent and nickel content. Surface reduction initiated above 300 °C with porous ethanol-based layers showing a 60 °C kinetic advantage. Conversely, bulk reduction was inhibited by mass-transfer limitations, requiring temperatures near 750 °C for completion. Finally, total nickel content was identified as the primary factor controlling the volumetric reduction duration, with higher NiO loading extending reduction times.