Stability of NiO membranes on photostructurable glass substrates for micro solid oxide fuel cells

NiO thin films with thicknesses in the range of 100 to 900 nm were deposited by spray pyrolysis onto photostructurable glass substrates and, ultimately, free-standing membranes with diameters of 100, 200 and 300 µm were fabricated using these thin films. The membranes are intended to act as simplified anodes or anode current collectors in micro solid oxide fuel cells (µSOFCs) and their differential pressure and thermal stability were characterized. The membranes tolerated a differential pressure between 13,700 and 158,600 Pa. Smaller membranes showed more pressure tolerance than larger membranes. A membrane diameter of 100 µm and a film thickness of 400–500 nm turned out to be a promising geometry for µSOFC membranes. All membranes survived temperatures higher than the intended operating temperature of µSOFCs (350–600 °C). We attribute the good thermal stability to the match of the thermo-mechanical properties of the substrate and the NiO thin films for the lower temperature regime and the substrate softening at higher temperatures releasing stresses in the thin films. Furthermore, the thermal expansion of the substrate is close to thermal expansion of materials used in SOFCs and circular geometries can be realized using wet etching.