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For practical utilization of proton-conducting ceramic fuel cells and electrolyzers, it is essential to lower the sintering temperature and processing time of BaZrO₃-based proton conductors. We investigated the effect of sintering temperature and time on the structural and electrochemical properties of dense BaZr_0.8Y_0.2O_{3−δ} (BZY) prepared by a solid-state reactive sintering process, using NiO as a sintering aid. The sintered BZY prepared from the micronized precursor powder exhibited a density higher than 93%, and an average grain size in the range of 0.6 to 1.4 μm. The orthorhombic BaY₂NiO₅ phase was also observed in the sintered BZY from the combined conventional and synchrotron X-ray diffraction measurements. Electrochemical impedance spectroscopy showed that the total proton conductivities of BZY can be modulated by sintering temperature in a wet reducing atmosphere. The maximum total ion transport number achieved was 0.89 at 600 °C, and the maximum power density of the symmetric BZY electrolyte supported cell with Pt electrodes was 5.24 mW·cm^−2 at 900 °C.