Symmetric solid oxide fuel cells (SSOFCs) based on the Co-free La0.5Sr0.5Fe0.9Nb0.1O3-delta (LSFNb) perovskite oxide are prepared by the sol-gel method. The structural stability and catalytic activity of LSFNb as an electrode for both the hydrogen oxidation reaction and oxygen reduction reaction are studied. We show that Nb substitution in La0.5Sr0.5FeO3.delta greatly improves its chemical and phase stability under reducing atmosphere. At 800 degrees C, the area specific polarization resistances (ASR(p)) of La0.5Sr0.5Fe0.9Nb0.1O3-delta symmetric cells are as low as 0.06 and 0.24 Omega cm(2) in air and wet H-2 (3% H2O), respectively. LSFNb based electrolyte-supported SSOFCs deliver peak power densities of 1000 mW cm(-2) at 850 degrees C in wet H-2/air. In addition to high performance, the symmetric cell shows stable power output under load (measured for 140 h at a current density of 520 mA cm(-2), 800 degrees C) under air/humidified H-2 operation, indicating that LSFNb may be a particularly promising new symmetric electrode material for solid oxide fuel cells.