(Digital Presentation) Sm0.5Sr0.5CoO3- δ -Ce0.9Sm0.1O1.95 Nanocomposites for Reversible Solid Oxide Cells

Abstract

Electricity production through environmentally friendly energy sources has been one of the main challenges of our society in the last decades. The rapid growth of renewable energy has a large influence in the transport sector and industrial processes. Nevertheless, the irregular and seasonal disposition of renewable energy requires advanced devices for energy storage and conversion into electrical power. In this sense, reversible solid oxide cells appear as a promising alternative to store and produce electrical energy in a more efficient and eco-friendly way than conventional batteries and generators, respectively [1].An important aspect for increasing the efficiency of reversible solid oxide cells is to improve the performance of the air electrode. Although the intrinsic properties of the ceramic material are of great importance, such as electrical conductivity and electrochemical activity, the real performance of the air electrode strongly depends on its microstructure: porosity, particle size, surface area and electrode-electrolyte adherence [2].It is reported that the efficiency of air electrodes may be improved by adding a second phase with high ionic conductivity, such as doped-CeO2, to obtain a composite electrode with more conducting paths for oxide-ions. In addition, composite electrodes have generally higher efficiency than single-phase ones due to the increased electrochemically active area in whole electrode volume [2]. Moreover, they are known to reduce the mechanical stress originated by the different thermal expansion coefficients between the electrode and the electrolyte layers, thus improving the mechanical stability and durability of the cell.Traditionally, composite electrodes are prepared by mechanical mixing of the pristine materials, requiring multiple steps and time-consuming methods. Moreover, it is difficult to control the phase distribution and electrode morphology with this method. In order to overcome this drawback, nanocomposite powders may be synthesized by different co-synthesis methods, showing a more uniform distribution of nanosized crystals with high thermal stability and improved electrochemical properties [3].In this context, Sm0.5Sr0.5CoO3- δ exhibits high mixed ionic and electronic conductivity but relatively high thermal expansion coefficients, ~21.5·10-6 K-1, to be physically compatible with standard electrolyte materials, i.e. 12·10-6 K-1 for doped-CeO2. In this work, Sm0.5Sr0.5CoO3- δ-Ce0.9Sm0.1O1.95 (SSC-CSO) nanocomposite cathodes are successfully prepared in a single process by using the freeze-drying precursor method from a precursor solution containing all cations in stoichiometric amounts. In this way, SSC and CSO phases are formed simultaneously at low sintering temperature, reducing drastically the preparation time. SSC-CSO composite electrodes with different SSC content, ranging from 50 to 100%, are investigated by different structural, microstructural and electrochemical techniques.The XRD patterns show that the composite electrodes at 800 ºC are a mixture of SSC and SCO phases with perovskite and fluorite-type structure, respectively. No secondary phases are detected for any of the samples (Fig. 1a). Moreover, the Rietveld analysis shows a phase fraction and lattice cell parameters similar to the theoretical ones, suggesting minor cation exchange during the co-sintering process.The sintered pellets for electrical characterization show a homogeneous distribution of SSC and CSO phases (Fig. 1b,c). Interestingly, the addition of CGO suppresses partially the grain growth due to cation diffusion limitations during the sintering process. Thus, the grain size decreases from 0.53 µm for 100SSC to 0.32 µm for 60SSC.The lowest values of polarization resistance are found for 50SSC with a value of 0.35 Ω cm2 at 600 °C compared to 0.75 for 100SSC (Fig. 1d). The results suggest that this is a promising strategy to achieve highly efficient ceramic electrodes for reversible solid oxide cells with a notably simplified fabrication process compared to traditional methods.