Electrical properties of BaZr0.5Ce0.3Y0.1Yb0.1O3-δ proton conductor for reversible proton-conducting solid oxide electrochemical cells

Abstract

Proton-conducting oxide-based reversible solid oxide electrochemical cells (protonic-SOEC, P-SOEC) have a great potential to be the most efficient transition technology operating at intermediate temperatures. Here, we have introduced a perovskite-type proton conductor BaZr0.5Ce0.3Y0.1Yb0.1O3-δ (BZCYYb5311) and evaluated its electrochemical and transport properties. The chemical stability of BZCYYb5311 was confirmed by thermo-gravimetric analysis (TGA) under pure CO2 conditions and X-ray diffraction analysis. The total and partial conductivities were obtained using DC four-probe conductivity measurements under various pO2 and pH2O conditions. The ratio of proton conductivity to ionic conductivity including the oxygen-ion conductivity of this material was 98% at 600 °C, indicating that it is a relatively pure proton conductor with most of the ionic conduction almost exclusively carried out by protons. Chemical diffusivities were calculated from Fick's 2nd law by non-linear least squares fitting during oxidation/reduction and hydration/dehydration processes and were faster than other proton conductor BZCY (Y-doped BaCeO3-BaZrO3) series, which have a similar zirconium contents. In addition, the theoretical performance and efficiency of the reversible P-SOEC were determined from the spatial distribution of oxygen and hydrogen partial pressure in BZCYYb5311, calculating the maximum power density (4.5 Wcm−2 at 600 °C, 3% of water vapor pressure) for the fuel cell mode and current density (12.3 Acm−2 at 1.5 V, 600 °C, and 20% of water vapor pressure) for the electrolysis mode. The fuel consumption efficiency is high (99.1% at 0.6 V at 600 °C), while the electrolysis efficiency is low (72.3% at 1.6 V) due to the dominant p-type regime in the electrolyte during electrolysis. This low electrolysis efficiency can be enhanced by recovering the electrolyte regime by increasing pH2O or decreasing pO2 in the air electrode; however, it will still have low efficiency. Consequently, BZCYYb5311 is not an efficient electrolyte material for electrolyzers, but is promising for fuel cells because of its superior chemical stability, exceptional pure proton conductivity, and better bulk diffusion than those of the BZCY system.