Bulk and grain boundary conductivity in doped BaZrO3: Bulk contribution dominates at operating temperatures

This study evaluates the role of microstructure for the conductivity of BaZrO3-based protonic conductors. Bulk conductivity was found to be the limiting conductivity contribution in BaZr0.8Y0.2O3-δ (BZY20) and BaZr0.7Ce0.2Y0.1O3-δ (BZCY72) at operating temperatures. The effective grain boundary conductivity only dominates the total conductivity below a crossover point (200 °C for BZY20, 150 °C for BZCY72). The crossover point was determined by using a well-calibrated electrochemical impedance spectroscopy setup to measure from 600 °C to 50 °C with up to 10 MHz. As such, bulk and grain boundary could be fitted separately up to 350 °C instead of 200 °C, as often seen in literature. According to these results, the impact of grain boundaries on the total conductivity of BZY20 and BZCY72 at operating temperature might be less than assumed. Therefore, research should not only focus on increasing the conductivity of the grain boundary, but also on maximizing the bulk conductivity.