Proton ceramic fuel cells (PCFCs) are an emerging clean energy technology; however, a key challenge persists in improving the electrolyte proton conductivity, e.g., around 10−3 −10−2 S cm−1 at 600 °C for the well-known BaZr0.8Y0.2O3 (BZY), that is far below the required 0.1 S cm−1 . Herein, we report an approach for tuning BZY from low bulk to high interfacial conduction by introducing a semiconductor CeO2−δ forming a semiconductor−ionic heterostructure CeO2−δ/BZY. The interfacial conduction was identified by a significantly higher conductivity obtained from the BZY grain boundary than that of the bulk and a further improvement from the CeO2−δ/BZY which achieved a remarkably high proton conductivity of 0.23 S cm−1 . This enabled a high peak power of 845 mW cm−2 at 520 °C from a PCFC using the CeO2−δ/BZY as the electrolyte, in strong contrast to the BZY bulk conduction electrolyte with only 229 mW cm−2 . Furthermore, the CeO2−δ/BZY fuel cell was operated under water electrolysis mode, exhibiting a very high current density output of 3.2 A cm−2 corresponding to a high H2 production rate, under 2.0 V at 520 °C. The band structure and a built-in-field-assisted proton transport mechanism have been proposed and explained. This work demonstrates an efficient way of tuning the electrolyte from low bulk to high interfacial proton conduction to attain sufficient conductivity required for PCFCs, electrolyzers, and other advanced electrochemical energy technologies.