Development of a Novel Layered Barium Cobaltite Electrode for Protonic Ceramic Cells

Vollständiger Abstract

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The escalating global demand for clean and sustainable energy necessitates the development of efficient and environmentally friendly energy conversion technologies. Protonic ceramic cells (PCCs), operating at intermediate temperatures (400-700°C), have emerged as a promising alternative to traditional fuel cells due to their ability to utilise a wide range of fuels, including hydrogen and hydrocarbons, with high efficiency and minimal pollutant emissions [1]. A critical challenge for realising the full potential of PCCs lies in enhancing the oxygen reduction reaction (ORR) at the cathode, which significantly impacts the overall cell performance. The ORR involves a complex multi-step process involving oxygen adsorption, dissociation, and charge transfer, and its sluggish kinetics often limit the fuel cell efficiency. Therefore, the development of highly active and durable oxygen electrode materials is crucial for advancing PCC technology [2]. While significant progress has been made in developing high-performance oxygen electrode materials for solid oxide cells (SOCs) operating at higher temperatures, the direct translation of these materials to the lower-temperature regime of protonic ceramic fuel cells (PCCs) remains a significant challenge [3]. One of the main problems is that many promising oxygen electrode materials developed for traditional SOCs react adversely with the state-of-the-art barium-zirconate/cerate-based proton-conducting electrolytes commonly employed in PCCs. This incompatibility, often leading to the formation of undesired phases or interfacial degradation, necessitates the exploration and development of novel electrode compositions specifically tailored for PCFC applications. Furthermore, even when chemically compatible, some SOC oxygen electrode materials exhibit insufficient activity for the water splitting/formation reactions crucial to the PCC mechanism or demonstrate instability under the high steam concentrations prevalent in PCC operation [3]. Therefore, searching for effective PCFC oxygen electrodes requires high electrocatalytic activity, robust chemical compatibility with proton-conducting electrolytes, and stability under humid atmospheres. This work focuses on Ba 2 Co 9 O 14 (BCO), a layered cobaltite, as a potential oxygen electrode material for PCFCs. BCO possesses a unique layered intergrowth structure, distinct from the state-of-the-art perovskite structure, which may offer advantages for the ORR process [4]. The layered structure can facilitate the diffusion of oxygen ions along the layers, while the mixed oxidation states of cobalt (Co 2+ /Co 3+ ) can provide active sites for the ORR [4]. Furthermore, BCO may be able to mitigate undesirable cation diffusion and minimise the concentration disparity of the alkaline-earth element within the electrolyte. Therefore, this study aims to investigate the structural, electrochemical, and electrocatalytic properties of BCO and evaluate its potential as a high-performance oxygen electrode for PCFCs. The X-ray diffraction (XRD) pattern of the synthesised BCO powder confirmed the formation of a single-phase material with a layered intergrowth structure. All the diffraction peaks could be indexed to the reported crystallographic data for Ba 2 Co 9 O 14 , indicating the absence of any impurity phases. The bond valence sum (BVS) calculation of the Co-ions indicated a mixed Co 3+ /Co 2+ charge ordering at the different Co-site positions, with an average oxidation state of +2.75. This value is close to the nominal oxidation state of +2.67 for Ba 2 Co 3 2+ Co 6 3+ O 14 [5]. Scanning electron microscopy (SEM) images revealed a platelet-like morphology of the BCO particles. The platelet-like structure can provide a large surface area for oxygen adsorption and facilitate the diffusion of oxygen species along the layers [6]. X-ray photoelectron spectroscopy (XPS) analysis confirmed the presence of Co 2+ /Co 3+ in BCO. The presence of this redox pair is believed to be crucial for enhancing the electrocatalytic activity of the material. XPS also revealed a significant concentration of oxygen vacancies on the surface of BCO. Oxygen vacancies act as active sites for oxygen reaction and diffusion at the surface and play a crucial role in the ORR [7–9]. Thermogravimetric analysis (TGA) showed excellent thermal stability of BCO in O 2 up to 800 °C, with negligible weight loss. This indicates that BCO is stable under typical PCFC operating conditions and does not undergo significant oxygen loss or phase decomposition. Interestingly, unlike some perovskite oxides that exhibit proton uptake in humid atmospheres [10], BCO did not show any significant weight gain in wet conditions, suggesting that proton incorporation into the BCO structure is minimal. The TEC values determined by dilatometry in air were in the range ~23-24 x 10 -6 °C -1 above ~350°C. This range of TEC values is typical of other Co-based oxide systems [11,12]. Electrochemical impedance spectroscopy (EIS) measurements showed that the BCO electrode exhibited a polarisation resistance ( R p ) comparable to, and in some cases lower than, state-of-the-art oxygen electrodes under similar operating conditions. The electronic leakage current through the BZY15 electrolyte plays a crucial role in accurately interpreting the electrode kinetics. The measured impedance spectra were carefully analysed to deconvolute the electrode processes from the electrolyte contribution. An appropriate equivalent circuit model was developed to account for the electronic leakage current, and the true electrode polarisation resistance was determined [13,14]. The analysis of the impedance spectra as a function of p O2 revealed that the rate-limiting step for the ORR was identified as surface oxygen diffusion towards the triple-phase boundary (TPB), where the gas phase, the electrode material, and the electrolyte meet [15]. Due to the relatively low bulk ionic conductivity of the BCO material [16], oxygen transport is primarily governed by surface diffusion along with the layered structure (Fig. 1) [6]. This highlights the importance of the platelet-like morphology observed in the SEM images, as it provides a large surface area for oxygen diffusion. Overall, this study demonstrates the potential of Ba 2 Co 9 O 14 (BCO) as a promising novel oxygen electrode material for PCFCs. The results of this study suggest that BCO is a promising candidate for replacing or complementing conventional perovskite oxygen electrodes in PCFCs. This work contributes to the development of high-performance PCFCs and paves the way for the utilisation of novel layered cobaltites for electrochemical energy conversion. Fig. 1 - Schematic representation of the oxygen reaction mechanism in BCO electrodes. Adapted from [6]. Acknowledgements The authors acknowledge the following grants/projects: 2020.02797.CEECIND/CP1589/CT0030 ( https://doi.org/10.54499/2020.02797.CEECIND/CP1589/CT0030 ), 2022.02498.PTDC ( https://doi.org/10.54499/2022.02498.PTDC ), UIDB/00481/2020 ( https://doi.org/10.54499/UIDB/00481/2020 ), and UIDP/00481/2020 ( https://doi.org/10.54499/UIDP/00481/2020 ) from Fundação para a Ciência e a Tecnologia (FCT); and CENTRO-01-0145-FEDER-022083 from Centro Portugal Regional Operational Programme (Centro2020), under the PORTUGAL 2020 Partnership Agreement, through the European Regional Development Fund (ERDF). Rafael A. Raimundo and Daniel A. Macedo also acknowledge the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq/Brazil, 309430/2019-4 and 151879/2022-2). This study was also financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – Brasil (CAPES) – Finance Code 001. References [1] C. Duan, R.J. Kee, H. Zhu, C. Karakaya, Y. Chen, S. Ricote, A. 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