The road to high-entropy: sequential doping study of Ba2In2O5

Giuditta Perversi^a, Niels Schreiner^a^,^b, Angela Angelodimou^a

^aMaastricht University, P.O. Box 616, Maastricht, 6200, Netherlands

^bDepartment of Materials Science and Engineering, University of Sheffield, Sheffield, UK

Proceedings of 24th International Conference on Solid State Ionics (SSI24)

Emerging Materials for High-Performance Devices

London, United Kingdom, 2024 July 14th - 19th

Organizers: John Kilner and Stephen Skinner

Oral, Giuditta Perversi, presentation 109
Publication date: 10th April 2024

For a sustainable energy future, solid-oxide fuel cells (SOFC) play an essential role: they generate electricity from industrial waste gases, with water as their only byproduct. The cornerstone of this technology are materials capable of splitting gaseous oxygen (O₂) to conduct it as oxygen anions (O^2-) in the device.¹ Commercial SOFC currently rely on strontium-doped lanthanum perovskites (La_1-xSr_xMO_3-d with M=Co, Mn, Fe) as a cathode. The high-performance of this perovskite comes from three key factors: the atomic arrangement is cubic, it is inherently oxygen deficient, and contains multivalent metals that allow for mixed ionic-electronic conductivity. However, the activation of the oxygen-ion conductivity requires high temperature (T > 800°C), and lanthanum is a rare-earth element with extreme supply issues.^2,3

The quest for new materials with equivalent (or improved) performances and a more attractive elements composition starts with the need to stabilise a cubic perovskite arrangement, optimal for ionic and electronic conductivity. However, with the same ideal composition (ABO₃, where A = non-metal, B = metal) the cubic structure might be unachievable or unstable.
The established approach in materials chemistry is to mix the components in B with one element at a time (A(B’B’’)O₃).⁴

A well-known, rare-earth free material for fuel cells is BaInO_2.5 ⁵, whose ionic conductivity (normally active at temperatures above 1040°C) can be lowered with a variety of doping of the B-site (V, Mo, W, Cu, Ga, Y, Zr, Sn, Ti, Al, Fe, Co, Mn, etc) to yield a cubic perovskite ^6-13. This compositional flexibility makes it a great candidate for a systematic study on the synthesis of high-entropy perovskites (HEPs): a cubic perovskite in which the B position is equally shared by five or more atoms, where the disorder of a multi-element system creates an entropic advantage. Once synthesised, HEPs are often reported to be better performing their they low-doping equivalent, with lower activation temperature and higer conductivity. ¹⁴

In this study we perform a systematic study on sequential on multi-elemental doping of BaInO_2.5, attempting a rational incremental approach of 2->3->4->5 elements sharing the B position, taking from a pool of 10 elements (In, Sn, Ti, V, Cr, Mn, Fe, Co, Ni, Zr) that exclude rare-earth materials. All the syntheses were performed with solid state ceramic methods, with maximum reaction temperature fixed at 1100°C.

We will highlight 8 new stable compositions in the BaIn_xM_1-xO_3-d (M = mixed metal) compositional space, of which three medium entropy and one high entropy. The structural results in this system highlight a complex interplay between charge, size and orbital configuration and the capability (or not) to form a multi-doped system. The thermogravimetric characterisation provides insight on the protonic and electronic conductivity performance, in terms of amount of mobile vacancies and the temperature of activation for their mobility.

This systematic study opens up to answer the question: is high-entropy stabilisation always achievable, and always beneficial, for a functional material?

References:

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Acknowledgements:

The authors acknowledge the Maastricht Science Programme, the Universiteitfunds Limburg and the Brightlands Campus for the experimental laboratory facilities and the financial support to the work. We also acknowledge Professor Werner Paulus and Dr Monica Ceretti for additional characterisation performed in support of the results.

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