Vol 59, No 6 (2023)

In this work, a high-density ceramics Ln2Hf2O7 (Ln = La, Nd, Sm, Eu, Gd) were synthesized by mechanical activation followed by high-temperature synthesis at 1600°C (3–10 h) and their transport properties were compared with those of Ln2.1Hf1.9O6.95 (Ln = La, Nd, Sm, Eu) doped solid solutions. The total conductivity of ceramics was studied using impedance spectroscopy and dc four-probe method; for Ln2Hf2O7 (Ln = Sm, Eu), by determining the total conductivity as a function of oxygen partial pressure. The maximum oxygen-ion conductivity was observed for Gd2Hf2O7 (~1 × 10–3 S/cm at 700°C); it was shown to approach the conductivity of Gd2Zr2O7 (~2 × 10–3 S/cm at 700°C) for the first time. Thus, the gadolinium hafnate can be a promising material for further doping in order to obtain highly conductive electrolytes. Among pure rare-earth hafnates, the proton conductivity was reliably observed for Nd2Hf2O7 only; however, ac measurements detected low-temperature proton conductivity in the Gd2Hf2O7 up to 450°С as well. With a decrease in the lanthanide ionic radius, the oxygen-ion conductivity increased in the Ln2Hf2O7 (Ln = La, Nd, Sm, Gd) series. Although the conductivity of samarium hafnate is an order of magnitude lower than that of Gd2Hf2O7, it has a wide range of oxygen-ion conductivity (~10–18–1 atm at 700, 800°C); there is no contribution from hole conductivity in air, in contrast to Eu2Hf2O7. Among doped Ln2.1Hf1.9O6.95 pyrochlore solid solutions (Ln = La, Nd, Sm, Eu), the proton conductivity of ~8 × 10−5 S/cm at 700°C was shown in Ln2.1Hf1.9O6.95 (Ln = La, Nd). With a decrease in the lanthanide ionic radius, the proton conductivity disappeared; the oxygen-ion one, increased.

A symmetrical electrochemical cell Bi3Ru3O11−35 wt % Bi2O3 porous electrode|Bi2O3−0.2 wt % B2O3 solid-molten electrolyte|Bi3Ru3O11−35 wt % Bi2O3 porous electrode is developed. The values of the cell ohmic and polarization resistances, Faraday efficiency, and oxygen permeation flux of the cell were measured using impedance spectroscopy and Coulomb volumetric technique at 740°C. These values are 0.046 and 0.077 Ω cm2, 97%, and 5×10–7 mol cm–2 s–1, respectively. The effect of wetting of the porous electrode surface on the polarization resistance was analyzed. The Bi3Ru3O11−35 wt % Bi2O3 and solid-molten Bi2O3−0.2 wt % B2O3 composites have a great potential to be used as the electrode and electrolyte materials in electrochemical oxygen generators.