Cobalt-free composite cathode for SOFCs: Brownmillerite-type calcium ferrite and gadolinium-doped ceria

A cobalt-free composite Ca2Fe2O5 (CFO) – Ce0.9Gd0.1O1.95 (GDC) is investigated as a new cathode material for intermediate-temperature solid oxide fuel cells (IT-SOFCs) based on a Gd0.1Ce0.9O1.95 (GDC) electrolyte. The cathodes had brownmillerite structure with x wt.% Gd0.1Ce0.9O1.95 (GDC) – (100−x) wt.% Ca2Fe2O5 (CFO), where x = 0, 10, 20, 30 and 40. The effect of GDC incorporation on the thermal expansion coefficient (TEC), electrochemical properties and thermal stability of the CFO–GDC composites is investigated. The composite cathode of 30 wt.% GDC – 70 wt.% CFO (CG30) coated on Gd0.1Ce0.9O1.95 electrolyte showed the lowest area specific resistance (ASR), 0.294 Ω cm2 at 700 °C and 0.122 Ω cm2 at 750 °C. The TEC of the CG30 cathode was 13.1 × 10−6 °C−1 up to 900 °C, which is a lower value than for CFO alone (13.8 × 10−6 °C−1). Long-term thermal stability and thermal cycle testing of CG30 cathodes were performed. Stable ARS values were observed during both tests without delamination at the cathode–electrolyte interface. An electrolyte-supported single cell with a 300-μm-thick GDC electrolyte and an anode-supported single cell with ∼10-μm-thick yttria-stabilized zirconia (YSZ) with a GDC buffer layer attained maximum power densities of 395 mW cm−2 at 750 °C and 842 mW cm−2 at 800 °C, respectively. The unique composite composition of CG30 demonstrates enhanced electrochemical performance and good thermal stability for IT-SOFCs.