Here, for the first time, we present data on proton conductivity of high-entropy, single-phase perovskites. The BaZr0.2Sn0.2­Ti0.2Hf0.2Ce0.2O3−δ, BaZr0.2Sn0.2­Ti0.2Hf0.2­Y0.2O3−δ, BaZr1/7Sn1/7­Ti1/7Hf1/7­Ce1/7Nb1/7Y1/7O3−δ, and BaZr0.15Sn0.15­Ti0.15Hf0.15­Ce0.15Nb0.15­Y0.10O3−δ single-phase perovskites were synthesized. Before electrical measurements, materials were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and thermogravimetric analysis (TGA). The following experimental results demonstrated that studied high-entropy perovskites are proton conductors: (1) The observed mass increase upon the switch from dry to wet atmosphere confirmed the water incorporation into materials structure. (2) The electrochemical impedance spectroscopy (EIS) revealed that the total conductivity increased while its activation energy decreased in the presence of water vapor in the atmosphere. (3) The conductivity in atmosphere humidified with H2O and D2O differed one from another, showing typical of proton conductors isotope effect in high-entropy oxides.

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Belonging to the not fully explored REBaCo2‑x Mn x O5+δ system, a series of REBaCoMnO5+δ (RE: selected rare earth elements) oxides having perovskite-type structure is synthesized and studied in terms of their structural properties, oxygen content, stability, thermal expansion, and transport properties. Impact of RE3+ on physicochemical properties of the compounds is derived, with smaller cations causing a decrease of the unit cell volume, lowering of the total oxygen content and thermal expansion, but also suppressing electrical conductivity. It is shown that a proper chemical modification enables to successfully utilize REBaCoMnO5+δ in applications, in which redox processes associated with oxygen reduction/oxidation and transport determine the effectiveness of the working material. In particular, NdBaCoMnO5+δ (with larger Nd3+) shows good chemical stability in relation to Ce0.8Gd0.2O2−δ and La0.8Sr0.2Ga0.8Mg0.2O3‑δ solid electrolytes and moderate thermal expansion, 20.04(4)·10–6 K–1 in 300–900 °C. In symmetrical configuration with La0.8Sr0.2Ga0.8Mg0.2O3‑δ electrolyte its cathodic polarization resistance is found to be only 0.036 Ω cm2 at 900 °C, making it an excellent candidate cathode for solid oxide fuel cells. At the same time, YBaCoMnO5+δ (with small and cheap Y3+) delivers reversible oxygen storage capacity surpassing 3.4 wt % during the oxygen partial pressure swing process between air and 5 vol % H2 in Ar at 500 °C.

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BaxSr1−xTi1−yFeyO3−δ-based perovskite materials with different barium and iron contents are reported as triple conducting oxides (TCOs), which may conduct three charge carriers: oxygen ions, protons and electrons/holes.