Effect of hydration on electrical and electromechanical properties of lanthanum-cerium oxides

Or Ben Zion¹, Tahel Malka¹, David Ehre¹, Isaac Abrahams² and Igor Lubomirsky¹

¹Department of Molecular Chemistry and Materials Science, Weizmann Institute of Science, Rehovot 7610001, Israel
² Department of Chemistry, Queen Mary University of London, Mile End Road, London E1 4NS, UK.

Keywords: Lanthanum cerium oxide; Lanthanum hydroxide; Proton conductivity; Ceramics; Grain boundaries

In recent years, there has been a growing interest in the pursuit of efficient, clean, and environmentally friendly energy sources. Fuel cells have emerged as a promising technology, allowing the conversion of the chemical energy present in fuel gases into electrical work. Solid oxide fuel cells, utilizing ion-conducting ceramics as electrolytes, have been at the forefront of these developments. However, a significant drawback has been the high operational temperatures typically required, ranging from 300°C to 900°C, due to the thermally activated nature of ion conduction in ceramics.

All solid oxide fuel cells (SOFCs) and electrolyzers (SOEs) operating above 300 °C demonstrate rapid electrode kinetics, especially those based on proton-conducting systems, but are limited in their long-term stability due to thermal stress. Ceramic devices operating between 150-250 °C could enjoy rapid electrode kinetics, even without Pt-based catalysts, and would avoid large thermal stress. However, proton conducting ceramics showing appreciable conductivity in this temperature range have yet to be identified. Developing such a material could revolutionize the field of renewable energy.

Hydrated rare earth oxides, known for their chemical and thermal stability, have emerged as potential low-temperature proton conducting ceramics. Our study of La(OH)₃, a material insoluble in water and chemically stable in the presence of carbon dioxide, revealed that the dominant contribution to the conductivity of La(OH)₃ below 200 °C is protonic. However, it is not sufficiently high for practical applications. The grain boundaries in La(OH)₃ ceramics are non-blocking for proton transport, inspiring further investigation.

We have investigated the partial substitution of La³⁺ by a tetravalent cation, like Ce⁴⁺, to increase the oxygen to hydrogen ratio. We have found that ceramics of La_xCe_1-xO_2-x/2 (LCO) with x > 0.5 decompose to La(OH)₃ and CeO₂ upon contact with water or water vapor, limiting the search range to x ≤ 0.5.

We have developed a hydration protocol for LCO50 ceramics, leading to ≈7.5±1% of hydration with respect to the theoretical limit. The hydration results in a 0.17% lattice expansion due to water incorporation and a large increase in electrical conductivity (Figure 1). Further hydration results in the mechanical disintegration of the ceramic pellets, prompting the investigation of LCO with x < 0.5.

No evidence for water incorporation for x = 0.2 and x = 0.4 compositions was found under any conditions. Hydration of LCO45 at 200 °C for 48 hours in steam causes fragmentation of the ceramic pellet. However, as determined by thermogravimetric analysis, the replacement of steam by water-saturated nitrogen (25°C) results in 17% of the oxygen vacancies being filled up, expansion of the lattice by 0.18%, an increase in electrical conductivity, and a decrease in activation energy.

We expect that further work will lead to a viable proton conducting ceramics suitable for use a bulk and micro-fabrication-compatible thin films.