Issue 39, 2024

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, pH2O, 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 pH2O-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 pH2O-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.

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

Supplementary files

Article information

Article type
Paper
Submitted
26 jun 2024
Accepted
04 set 2024
First published
05 set 2024

J. Mater. Chem. A, 2024,12, 26555-26567

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

J. Gu, X. Sun, L. Jiang, Z. Zhang, T. Norby and D. Han, J. Mater. Chem. A, 2024, 12, 26555 DOI: 10.1039/D4TA04438J

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