BaCe0.7–xZr0.2Y0.1FexO3–δ derived from proton-conducting electrolytes: A way of designing chemically compatible cathodes for solid oxide fuel cells

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Highlights

  • BaCe0.7–xZr0.2Y0.1FexO3–δ (BCZYFx) materials were prepared for the first time.

  • The BCZYFx materials are single-phase across the entire x range (0 = x ≤ 0.7).

  • Fe-doping leads to an increase in both ionic and electronic conductivities (at x ≥ 0.4).

  • At the same time, TECs values increase and chemical strain becomes more pronounced.

  • BCZYF0.6 with an optimal combination of properties was utilised as a SOFC cathode.

Abstract

The present study describes the rational engineering, preparation and characterisation of new mixed ionic-electronic conductors (MIECs) having great potential for application in solid oxide fuel cells (SOFCs) based on proton-conducting electrolytes. The developed MIECs are derived from promising Ba(Ce,Zr)O3-materials doped with iron as a transition element: BaCe0.7–xZr0.2Y0.1FexO3–δ (x = 0–0.7, Δx = 0.1). The comprehensive analysis of the functional properties (crystal structure, densification, microstructure, defect structure, thermal expansion behaviour, electrical conductivity) indicate that a moderate iron content is beneficial in order to achieve a certain compromise. In detail, the gradual Fe-doping results in an enhancement of transport properties (including, ionic-electronic conductivity level) and deterioration of mechanical characteristics (an increase of average thermal expansion coefficient values). The composition with x = 0.6, identified as more optimal, was used as a cathode for an intermediate-temperature SOFC; this cathode displays a satisfactory polarisation resistance level, 0.21 Ω cm2 at 700 °C, without the use of any additional electroactivation technique. The obtained results indicate that BaCe 0.7–xZr0.2Y0.1FexO3–δ MIECs having one of the highest chemical compatibility with state-of-the-art proton-conducting electrolytes can be considered as advanced electrodes for designing new solid oxide electrochemical devices with prolonged and stable operation mode.

Introduction

Although proton-conducting oxide (PCO) materials have already been extensively studied for around 40 years, increased interest at the present time is due to their suitability for use as the functional components of various electrochemical devices [[1], [2], [3], [4], [5]]: solid oxide fuel cells (SOFCs), electrolysis cells (SOECs), electrochemical sensors and membrane reactors. Owing to the high levels of proton conductivity that can be attained at 400–700 °C, the PCO-based devices yield promising efficiency and performance across an intermediate temperature range [[6], [7], [8], [9]]. However, the achievement of outstanding output characteristics as well as ensuring the stability of such devices over a long period of time remains a complex tasks consisting in designing not only more conductive and stable PCOs, but also electrochemically active electrodes that simultaneously demonstrate chemical and thermal compatibilities with the corresponding electrolytes.

Recently, a numerous number of materials has been developed as suitable electrodes for proton-conducting electrolytes [[10], [11], [12]]. These can be categorised in various ways depending on a common sign: e.g., basic element (ferrites, cobaltites, nickelites and etc.) or structure type (perovskite, rock-salt, scheelite etc.). Despite many promising results, some severe disadvantages inherent to PCO-based devices continue to impede their scaling, commercialisation and long-term operation [[13], [14], [15]]. Some of these drawbacks are due to chemical or mechanical incompatibilities between the electrodes and electrolyte at elevated temperatures, in particular, strong chemical interaction [16,17] or thermal expansion discrepancy [18,19]. In order to overcome these problems, a smart electrode material design approach is required.

From general viewpoint, chemical interaction in an electrolyte/electrode pair can be diminished in the case of a reduced number of uncommon cations. If the structure of both phases is the same, a slight cationic difference between functional materials can also be favourable in terms of thermomechanical functionality. In this regard, the partial substitution of a B-site cation of the conventional Ba(Ce,Zr)O3-based electrolytes with a transition element might be a useful doping strategy for the rational design of mixed ionic-electronic conductors used as electrodes of SOFCs & SOECs or ionic-permeable membranes [20].

Analysis of the literature data has shown that barium zirconates simultaneously doped with cobalt and iron (BaCo0.4Fe0.4Zr0.1Y0.1O3–δ, BaCo0.4Fe0.4Zr0.2O3–δ) are the state-of-the-art electrode materials for PCO-based SOFCs and SOECs [[21], [22], [23], [24]]. These materials may demonstrate triple (electron, oxygen-ion and proton) conducting behaviour, thus greatly improving the electrochemical activity of the corresponding electrodes [24,25]. However, Co-containing oxides are very reactive [26], as well as exhibit high thermal expansion coefficients [27], that impede a solution of the incompatibility issues. The gradual substitution of cobalt with another transition element (for example, iron) allows a compromise between chemical, mechanical and electrochemical properties to be achieved [28]. In this context, many efforts have been made to develop Co-free Fe-based electrodes [[29], [30], [31], [32], [33], [34]]; however, such electrodes were designed on the basis of the BaCeO3 [[29], [30], [31]] or BaZrO3 [[32], [33], [34]], which can result in the preservation of some disadvantages of the parent phases, such as the low chemical stability of cerates [35,36] and insufficient ionic-electronic conductivity of zirconates [37,38].

Therefore, in the present work, in order to evaluate the Fe-doping effect on the main functional properties of the designed materials, we develop stable and highly conductive oxides based on Fe-doped cerate-zirconates (BaCe0.7–xZr0.2Y0.1FexO3–δ) and provide physicochemical and electrochemical characterisation to confirm their applied prospects for intermediate-temperature SOFCs.

Section snippets

Preparation of powder and ceramic samples

The powders of BaCe0.7–xZr0.2Y0.1FexO3–δ (x = 0–0.7, Δx = 0.1; hereinafter referred as BCZYFx) were prepared by means of a citrate-nitrate synthesis method. High-purity nitrates of barium, cerium, zirconium, yttrium and iron were used as starting materials. Following the dissolution of stoichiometric amounts of these powders in distilled water, citric acid was added to serve as a complexing agent. The molar ratio of citric acid to total metal ions was adjusted to about 1.5. The obtained

Crystal structure

According to the XRD data (Fig. 1 a), the formation of single pure perovskite-type structures without any impurity phase is confirmed for all the studied BCZYFx. Depending on the iron concentration (x), different perovskite structures are detected at room temperature; the Rietveld refinement results (Fig. S1, Table S1) confirm an orthorhombic structure (space group Imma) for BCZYF0, a rhombohedral structure (space group R3¯c) for BCZYF0.1 and a cubic one (space group Pm3¯m) for other

Conclusions

Complex oxides of BaCe0.7–xZr0.2Y0.1FexO3–δ (BCZYFx, x = 0–0.7, Δx = 0.1) were successfully designed and prepared for use as suitable cathodes for solid oxide fuel cells (SOFCs) based on proton-conducting electrolytes. The single-phase materials were achieved for all studied compositions. Perovskite structures having different symmetries were formed at room temperature: orthorhombic, rhombohedral and cubic variants for x = 0, x = 0.1 and 0.2 ≤ x ≤ 0.7, respectively. Fe-doping has a beneficial

CRediT authorship contribution statement

Liana R. Tarutina: Investigation, Visualization, Resources. Gennady K. Vdovin: Software, Methodology. Julia G. Lyagaeva: Conceptualization, Data curation, Funding acquisition. Dmitry A. Medvedev: Writing - original draft, Validation, Project administration, Writing - review & editing, Supervision, Formal analysis.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was partly supported by the grant [no. МК-1654.2019.3] and scholarship [no. СП-161.2018.1] of the President of the Russian Federation for young scientists.

The characterisation of powder and ceramic materials was carried out at the Shared Access Centre “Composition of Compounds” of Institute of High-Temperature Electrochemistry (Yekaterinburg, Russia).

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