Laurent Constantin’s research while affiliated with Swiss Federal Institute of Technology in Lausanne and other places

Recent progress in our development of anode supported electrolyte (ASE) SOFcells is reported. The cells are fabricated by cocasting of aqueous slurries of NiO/YSZ and 8YSZ, followed by cofiring. A gradient cathode is deposited by screenprinting successive layers of LSM/YSZ composite and lanthanum cobaltite, followed by a 2nd firing step. Full cells show a final thickness of ? 0.25 mm, high flexibility and reasonable warpage. Proper anode current collection is crucial and obtained by cofiring an additional outer layer together with the substrate. Power density can reach 0.2 W/cm2 at 600°C with active cathodes (LSC, short-lived) and >1 W/cm2 at 810°C with the LSM cathode. Stability was tested during 2900 h, giving an average degradation of 3%/1000 h at 0.5 A/cm2 current density. Degradation can be delayed by short-time polarisation at higher current density: e.g. “reactivation” at 0.85 A/cm2 (30 min.) delayed degradation by 300 h.

To increase power density at reduced temperature operation of SOFC, thin film 8YSZ electrolytes were deposited on Ni–YSZ anode support plates by tape casting and cofiring. Cathode behaviour, limiting cell output at lower temperature, was studied in more detail. Cathodes were deposited by screenprinting and firing. With consecutive layers of doped lanthanum manganite and cobaltite, power density of 0.5 W cm−2 at 750°C was obtained using hydrogen fuel. On cells of 100 cm2, no fuel diffusion limitation above 70% conversion occurred, and electrical efficiency of 35% was achieved. Actual cell temperature increases significantly above a current density of 0.3 A cm−2. This effect causes erroneous electrode and cell characteristics.

Ni-YSZ anode supports show a fundamentally different microstructure from screenprinted Ni-YSZ anodes on YSZ electrolyte supports. Of high density and composed of fine zirconia and nickel oxide to allow for cofiring with a thin zirconia electrolyte film (5 µm), the anode support shows advantages of adequate strength and increased electrochemical activity probably not only limited to the TPB, unlike screen printed porous anodes. Areas up to 100 cm2 have been fabricated. Electrochemical cell testing produced results of >80% fuel utilisation, 35% electrical efficiency and >20 W at 800°C using hydrogen fuel. Unusually high ohmic loss (*3 x theoretical) on supported cells is consistently observed. The smallest loss is measured when employing thin dense cathodes of a mixed conducting material. Overall cell performance (at *800 °C) is more determined by the cathode than by the anode support.