The present invention relates to rhomboidal phase bismuth oxide that maintains electric conductivity of at least about 110.sup.2 S/cm at temperature of about 500 C. for at least about 100 hours. In particular, the bismuth oxides of the invention have stable conductivity at a temperature range from about 500 C. to about 550 C.

A metal-solid oxide composite includes a plurality of nanowire portions including a solid oxide electrolyte material, a metal portion at one end of each of the plurality of nanowire portions and including a metal, and a center portion connected to the other ends of the nanowire portions.

A solid oxide fuel cell has a reinforced membrane-electrode assembly. The solid oxide fuel cell includes a first electrode layer, a second electrode layer, and an electrolyte membrane disposed between the first and second electrode layers. The solid oxide fuel cell further includes a gas-permeable structure adjacent to one or both of the electrode layers, for mechanical stabilization.

Disclosed is solid oxide cell that includes a unit cell including a solid oxide electrolyte including a first solid oxide electrolyte layer and a second solid oxide electrolyte layer, and a fuel electrode and an air electrode, and a frame having a first manifold surrounding a side of the unit cell and through which air supplied to the first solid oxide electrolyte layer flows.