Thin-film solid oxide fuel cell (SOFC) structures containing electrolyte membranes 50 - 150 nm thick were fabricated with the help of sputtering, lithography, and etching. The submicrometer SOFCs were made of yttria-stabilized zirconia (YSZ) or YSZ/gadolinium-doped ceria composites electrolyte and 80 nm porous Pt as cathode and anode. The peak power densities were 200 and 400 mW/cm(2) at 350 and 400 degrees C, respectively. The high power densities achieved are not only due to the reduction of electrolyte thickness but also to the high charge-transfer reaction rates at the interfaces between the nanoporous electrodes (cathode and/or anode) and the nanocrystalline thin electrolyte. (c) 2006 The Electrochemical Society.