Surface protonic conduction in porous alkaline earth zirconate perovskites CaZrO3, SrZrO3, and BaZrO3

Abstract

We herein report a first experimental account of surface protonic conductivity in chemisorbed and physisorbed water layers in porous perovskite alkaline earth zirconates AZrO₃, measured by impedance spectroscopy in wet atmospheres. We have studied the effects of temperature, p_H2O, H/D isotope exchange, A-site cation (A = Ca, Zr, Ba), sintering temperature, and cation non-stoichiometry, and supported conclusions by thermogravimetry (TG) and X-ray photoelectron spectroscopy (XPS) and infrared (IR) spectroscopy. The proton conduction in the chemisorbed layer dominates above 200 °C and exhibits small p_H2O-dependences and large pre-exponentials, activation energies (90 ± 10 kJ mol⁻¹), and H/D isotope effects, all consistent with protonic conduction by dissociated protons in a well-covering layer of mainly molecularly chemisorbed water. Differences between CaZrO₃, SrZrO₃, and BaZrO₃ supported by XPS results suggest that the higher acidity of smaller A-site cations and under-coordinated cations in the surface increase the proton conductivity. Below 200 °C, protonic conduction in the 1st solid-like physisorbed water layer exhibits positive p_H2O-dependences and strongly negative activation energies hence dominated by adsorption in a far-from-complete physisorbed layer. A vanishing isotope effect suggests that migration is not by free protons in the physisorbed layer, but a vehicular species such as hydroxide ions. At the highest relative humidities near room temperature, a sharp increase in conductivity is indicative of increasing mobilities as the thicker water layer goes liquid over a hydrophilic surface. If these results are transferrable to electron conducting perovskites used as positrode materials for proton ceramic fuel cells at, e.g., 600 °C, they suggest that the strong chemisorption of water may block oxygen adsorption, while the high activation energy limits and gives a strong temperature dependency to surface diffusion of protons.