Fuel battery cell and fuel battery module

A fuel cell comprising:

• a cathode electrode;
• an anode electrode;
• a solid electrolyte layer disposed between the cathode electrode and the anode electrode and comprising polycrystalline zirconia or polycrystalline ceria doped with divalent or trivalent positive ions and having proton conductivity;
• and
• an atmosphere containing a variable concentration of hydrogen, wherein electrical conductivity of the solid electrolyte increases by increasing the hydrogen concentration and decreases by decreasing the hydrogen concentration,
• wherein the cathode electrode and the solid electrolyte layer are stacked with a first oxygen ion blocking layer interposed therebetween,
• wherein the anode electrode and the solid electrolyte layer are stacked with a second oxygen ion blocking layer interposed therebetween and the first oxygen-ion-blocking layer is a polycrystalline titanium-oxide film having a film thickness greater than 50 nm and less than 5 μm.