La/Ba-based cobaltites as IT-SOFC cathodes: a discussion about the effect of crystal structure and microstructure on the O2-reduction reaction
Introduction
The Solid Oxide Fuel Cells (SOFCs) are promising devices designed to improve efficiency and to develop a cleaner energy generation [1], [2]. These cells can produce electricity when they are feeded with hydrocarbons or bio combustibles, can produce renewable fuels (H2, CO) by reverse work in the electrolysis mode [3], [4] and also can store electricity by switching between the fuel cell and the electrolysis modes [5]. Despite the good perspectives of this technology, the irruption of these devices in the market is limited by the high costs of production and operation and degradation issues, mainly due to the high working temperatures (T > 800 °C) which limit the span of operation life. Thus, recent research efforts have been focused on finding new materials able to operate with better performances in an intermediate temperature range (500–700 °C, IT-SOFC) [6].
Regarding the device performance, the highest loss of efficiency of an IT-SOFC is originated in the cathode side, due to the high activation energy of the O2 electrode reaction [7]. Two complementary strategies can be adopted to reduce the cathode polarization resistance (RC,P): first, the use of new oxides with mixed ionic and electronic conductivity (MIEC) increasing the reaction zone beyond the triple phase boundary, and second, the increase of the electrode’s specific surface by adopting nanostructured materials.
Cobaltites with perovskite structure, and mainly the Strontium-Barium cobaltites (BSCF) [8], [9], are one of the most promising cathode materials for IT-SOFCs. In these perovskites, the Ba plays a key role since its large cation radii distorts the cubic crystal structure promoting the oxygen vacancy formation and migration [10]. In addition, Ba atoms also assist the O2 reduction kinetics and reduce the Rc,p due to the improvement of the O-surface exchange and the O-ion diffusion. However, the same structural distortion, also induces a slow segregation of an hexagonal perovskite phase [11], [12], [13], which deteriorates the O2 reduction kinetics with time [14].
Beside the BSCF cobaltites, other Barium-based perovskites such as La1-xBaxCoO3-δ(LBC) with x = 0.4-0.7 [15], [16], [17], [18] and La1−xBaxCo1−yFeyO3−δ(LBCF) [19] also exhibit low RC,P. As in the case of BSCF perovskites, the cubic phase is metastable below 1000 °C, affecting the long term stability [15]. In this context, the LnBaCo2O6-δ layered perovskites (Ln = La, Pr, Nd, Sm, Gd) became attractive cathode materials. These oxides not only present RC,P values as low as 0.02-0.5 Ωcm2 at 700 °C (i.e LaBaCo2O6-δ [20] PrBaCo2O6-δ [21] NdBaCo2O6-δ [22–25], SmBaCo2O6-δ [26], GdBaCo2O6-δ [22], [27], [28], [29]), high rates of oxygen surface exchange and O-ion diffusivity [30], [31], [32], [33], [34] and electrical conductivity [21], [35], but also show a huge advantage over the other Ba-based cobaltites: the formation of the hexagonal phase has not been observed in the ordered tetragonal phase.
The cationic order in LnBaCo2O6-δ layered perovskites oxides takes place due to the large difference between the ionic radii of Ba2+ and Ln3+ ions. As a consequence of this, the ionic and electronic transport are affected [36]. However, it remains unclear wether the cationic ordering modifies the mechanism of the O2-reduction reaction, decreasing the polarization resistance, or not. With the aim of answering this topic, the electrochemical response of two materials, which differ only in their crystal structure (the cubic La0.5Ba0.5CoO3-δ and the tetragonal LaBaCo2O6-δ) wasevaluated.
As mentioned above, it is well known that the cathode polarization resistance can be appreciably decreasedbyimproving theelectrodés microstructure. Indeed, the polarization resistance decreases orders of magnitude for (La,Sr)CoO3-δ [37], [38] (La,Sr)(Co,Fe)O3-δ [39], [40], [41], [42], [43] and (Sm,Sr)CoO3-δ [44] perovskites with particle size below 100–200 nm. Therefore, this work also explores a sol-gel route in order to obtain LaBaCo2O6-δ with two different particle sizes and evaluates the changes in theirperformancewhen working as electrodes.
Section snippets
Synthesis
Powder samples, with nominal La0.5Ba0.5CoO3-δ, (LBC) composition were obtained by two different methods: the conventional Solid State Reaction method (SSR) and a Soft Chemical Route (SCR) involving a gel formation by polymerization of Acetyl-Acetone (AcAc) and Hexamethylenetetramine(HMTA).
The SSR method was previously used to obtain cubic La0.5Ba0.5CoO3-δ and tetragonal LaBaCo2O6-δ from La2O3 (99.99%, Alfa Aesar), Co3O4 (99.7%, Alfa Aesar) and BaCO3 (99.95%, Alfa Aesar) by applying different
Synthesis and Characterization
The minimum temperature to obtain both LBC phases by the soft chemical reaction method was determined by comparison of the XRD patterns of SCR-T-Air and SCR-T-Ar (see Table 1) with those synthesized by solid state reaction (the cubic La0.5Ba0.5CoO3-δ SSR-1100-Air and the tetragonal LaBaCo2O6-δ SSR-1150-Ar). Fig. 1 shows the resultant XRD patterns and Fig. 2 shows the FEG-SEM images of the as-synthesized powders.
The treatment in air allows to obtain a single cubic phase justat 1100 °C,
The ALS model and its range of validtiy
The ALS model has been recently used to describe the response of porous MIEC electrodes based on LSC [49], LSCF [50] and LSF [51] perovskites and NNO Ruddlesden-Popper phases [52]. In those cases where the O-diffusion follows a bulk transport path, is weakly pO2-dependent and if the O-surface exchange is controlled by the dissociative adsorption of O2, then (is the O-surface exchange coefficient). The ALS model works well under those conditions where surface exchange and bulk
Conclusions
The effect of the crystal structure and microstructure on the electrode polarization resistance was studied for samples with the same chemical composition. LBC materials were selected as systems of study, because cationic ordered or disordered phases with different crystal structures and microstructures can be obtained by slightly modification on the synthesis routes. Thus, two comparisons were performed:
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La0.5Ba0.5CoO3-δ, vs LaBaCo2O6-δ, which exhibit similar microstructures (dp ∼5–10 μm) but
Acknowledgment
This work was supported by CNEA (Argentinean National Commission of Atomic Energy), CONICET (Argentinean National Council of Scientific and Technical Research), UNCuyo (National University of Cuyo), ANPCyT (National Agency for Science and Technology Promotion).
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