Composite of Ca-Cobaltite with Pr-Doped Ceria as Oxygen Electrode for Solid Oxide Electrochemical Cells
Authors: Araujo Allan Jedson Menezes , Loureiro Francisco José Almeida , Holz Laura Isabel Vilas , Grilo João Paulo Freitas , Macedo Daniel Araújo , Paskocimas Carlos Alberto , Fagg Duncan Paul 
Published: July 2021

A growing interest in the development of solid oxide electrochemical systems, including solid oxide fuel cell (SOFC) and solid oxide electrolyser cells (SOEC) has increased the demand for new electrode materials that can offer superior electrochemical performance. The misfit-layered Ca cobaltite ([Ca2CoO3-δ]0.62[CoO2], C349) has emerged as a potential oxygen electrode for solid oxide cells (SOCs), in recent years, due to its optimal electrical conductivity (~100 S cm-1, from room temperature to 800 °C), excellent thermal compatibility with typical electrolyte materials, its low cost and its avoidance of Sr surface segregation issues that can deplete the electrochemical performance in more common oxygen electrodes 1,2. Its structure consists of two monoclinic subsystems of layers of Ca2CoO3 (three-layered rock salt-type block, CaO-CoO-CaO) and CoO2 (triangular lattice), stacked along the c-axis 3. This compound offers mixed ionic and electronic conduction (MIEC), where the first subsystem is responsible for the ionic conductivity, due to its higher oxygen deficiency, while the second subsystem is responsible for the electronic behaviour. Nonetheless, the level of ionic conductivity of C349 is, comparatively, low, and this factor has prevented it from attaining a competitive electrode performance 4, to date when compared to peak electrode materials, such as La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF). For this reason, we overcome this limitation in this study by the addition of a predominately ion-conducting phase with a minor p-type electron-conductivity, Ce0.8Pr0.2O1.9 (CPO) to the C349 material. This new composite electrode is compared against the standard, high performing, electrode material LSCF in terms of microstructure, film thickness, and subsequent electrochemical properties.
Symmetrical cell assemblies were fabricated by a cheap and simple screen-printing technique using Ce0.9Gd0.1O1.95 electrolytes and repeated depositions were performed to investigate the electrode thickness effect.
A minimum R
p value can be found for the C349-based electrodes with 5 depositions (70 – 85 μm). By comparison to the state-of-the-art LSCF electrode, C349 has been shown to require a higher solid volume fraction close to the electrolyte/electrode interface for continuing to maximise the penetration of the ionic current into the electrode bulk and for extending the electrochemically active region for oxygen exchange at the gas/electrode interface. The presence of a higher solid fraction of material benefits the oxygen exchange/reaction processes, with additional surface diffusion. Furthermore, the addition of CPO to the C349 electrode is shown to dramatically lower its polarisation resistance, allowing this composite to achieve a competitive electrochemical performance to that of the state-of-the-art LSCF electrode (0.37 and 0.52 Ω·cm2, at 700 °C in oxygen atmosphere, respectively). Overall, a Gerischer contribution can explain the experimental R
p of the electrodes (except in the case of the C349 sample with just one deposition). These results are further supported by a detailed mechanistic study, based on a Distribution Function of Relaxation Times (DFRT) analysis, to understand the origin of this improvement as a function of electrode morphology and composition.
The successful outcome of this work is extremely important as it provides not only a highly competitive new electrode composition, but also valuable insight into future optimisation methods for similar new mixed ionic-electronic conducting electrodes that intrinsically suffer from low levels of oxygen diffusion.
Acknowledments
This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES) - Finance Code 001. Allan J. M. Araújo acknowledges the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq/Brazil, reference number 200439/2019-7). João P. F. Grilo, Carlos A. Paskocimas and Daniel A. Macedo also thank CNPq/Brazil (482473/2010-0, 446126/2014-4, 308548/2014-0, 307236/2018-8, 431428/2018-2 and 309430/2019-4). Laura I. V. Holz acknowledges Fundação para a Ciência e Tecnologia (FCT) for the PhD grant PD/BDE/142837/2018. The authors also acknowledge the projects, PTDC/CTM-CTM/2156/2020, PTDC/QUI-ELT/3681/2020, POCI-01-0247-FEDER-039926, POCI-01-0145-FEDER-032241, UIDB/00481/2020 and UIDP/00481/2020 and CENTRO-01-0145-FEDER-022083 - Centro Portugal Regional Operational Programme (Centro2020), under the PORTUGAL 2020 Partnership Agreement, through the European Regional Development Fund (ERDF).
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