Transition metal oxides have been used as sintering aids for proton-conducting barium cerate-zirconates, which are promising electrolyte materials for low-temperature solid oxide fuel cells (SOFCs) and high-performance electrochemical membrane reactors. However, the effects of the additives on properties other than the density of the electrolytes have been ignored. Here, we report our findings that transition metal additives also affect the electrical properties, stability, and even catalytic activity of proton-conducting ABO-type perovskites. BaCeZrYO (BCZY) is selected as the basic material, and 2 mol % of NiFe ( range: from 0 to 1.0) oxides and 4 mol % of FeO are, respectively, added into BCZY to prepare electrolytes of anode-supported SOFCs. All of the electrolytes with additives can be densified after sintering at 1400 °C for 5 h, while BCZY without additive is porous. X-ray diffraction (XRD) spectra show that Ni and Fe are doped into the lattice of BCZY. For the first time, we find a positive function of Fe additive in BCZY that it not only acts as a good sintering aid but also improves the electrical performance and stability of the BCZY electrolyte in CO and HO at reduced temperatures. The cell with the 2 mol % NiFe-doped BCZY electrolyte, with an unoptimized cathode, gives a power density of 973 mW cm at 700 °C, 120 mW cm at 450 °C, and 45 mW cm at 350 °C. It operates under a constant current of 800 mA cm at 650 °C for over 200 h, during which the voltage decreases from 0.73 to 0.71 V. A newly discovered densified layer, formed in the cathode during the SOFC operation, may cause the degradation.