Sputter-deposited medium-temperature solid oxide fuel cells with multi-layer electrolytes

The deposition, interfacial impedances, and characteristics of solid oxide fluel cells (SOFC) with thin-film multi-layer electrolytes are described. The cell layers - a 1 μm thick AgYSZ cermet cathode, a 15 to 20 μm thick NiYSZ anode - were deposited on porous alumina by reactive magnetron co-sputtering of metal tartgets in ArO₂ mixtures. The effect of adding Y-stabilized Bi₂O₃ (YSB) and Y-doped CeO₂ (YDC) layers at the ytria-stabilized-zirconia (YSZ) electrolyte surfaces was investigated. The open circuit voltage of the H₂/H₂O (3%), NiYSZ / electrolyte / AgYSZ, air fuel cells tested at 750°C was 0.78-0.85 V, less than expected theoretically, indicating some porosity in the electrolyte layers. The cell resistance was 4.5 Ω cm² for a YSZ electrolyte, due mainly to the electrode interfacial resistances, and the maximum power density was 35 mW / cm². Adding a 60 nm-thick YSB layer at the YSZ/AgYSZ interface reduced the air electrode resistance from ≈1.4 to 0.45 Ω cm², leading to an increase in the maximum power density to ≈50 mV/cm². Adding a 100 nm-thick YDC layer at the NiYSZ/ YSZ interface further increased the maximum power density to 110 mW/cm² at a cell resistance of 1.6 μ cm². The three-layer YSB/YSZ/YDC electrolyte thus resulted in a factor-of-three increase in power density over a YSZ electrolyte.