SrTiO3/LaAlO3 heterostructures were constructed, which achieved a high ionic conductivity of 0.24 S cm-1 and fuel cell power output of 675 mW cm-2 at 520 °C. Theoretical calculations and experimental determinations found that the resultant built-in electric field plays an important role in fast ionic conduction through the interface with the electron accumulation layer in the interface region.