Preprint of "Surface and Bulk Reduction Kinetics of Liquid Plasma-Sprayed NiO-YSZ Layers for Solid Oxide Cells"

Nickel oxide–yttria stabilized zirconia (NiO-YSZ) is the established electrode material for solid oxide cells (SOCs), where NiO reduction dictates final microstructure and electrochemical performance. 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 sacrificial 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 a 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 activation duration, with higher NiO loading extending reduction times.