Among various types of solid oxide electrochemical cells, protonic ceramic fuel cells (PCFCs) have been expected to reduce the operating temperature below 600 °C with their significantly high ionic conductivity and activation energy. For example, recently developed proton conductor (BaZr_0.4Ce_0.4Y_0.1Yb_0.1O_3-δ, BZCYYb) ideally has ~12-fold higher ionic conductivity (2.810^-2 S/cm at 600 °C) and low activation energy (E_a < 0.4 eV) than those of conventional oxide ion conductor (Y_0.08Zr0_.92O_2-δ; 2.810^-3 S/cm at 600 °C and E_a ~0.87 eV). However, the low chemical and physical stabilities of proton conducting oxides during fabrication processes, primarily due to the high mobility and volatility of Barium (Ba), induce the substantially lower electrochemical performance than their predictions, limiting their utilization and application. Here, we present the chemically and physically stable BZCYYb electrolyte with the desired stoichiometry and the average grain size of ~10 µm by controlling the chemical potential of the A-site cation, Ba, near the BZCYYb electrolyte surface during the sintering process. A stoichiometric BZCYYb-based PCFC in an anode-supported configuration exhibits 1.90/cm² and 1.01 W/cm² with an extremely low ohmic resistance of ~0.060 ohm·cm² at 650 °C and ~0.082 ohm·cm² at 550 °C, respectively, surpassing the values of all previously reported PCFCs without complicated engineering in materials and structures of other cell components.