Massimo Viviani

The structural, microstructural, Raman and ionic conductivity properties of (Gd,Sm)-doped ceria were studied and compared to the ones of similar ceria systems with the aim of deepening the comprehension of the correlations between defect chemistry and movement of oxygen vacancies in such materials, which are ideal candidates as electrolytes in solid oxide cells. The system was chosen as it combines the advantages of using the most effective doping ions for ceria, namely Sm³⁺ and Gd³⁺, and the expected positive effects of multiple doping. The main effect of double doping on the structure is the enlargement of the compositional region where ionic conductivity takes place, due to the entrance of the smaller doping ions into defect clusters, mainly trimers and dimers (RE ≡ rare earth). On the other hand, the formation of such clusters also affects ionic conductivity, as it causes the occurrence of a double activation energy with a temperature threshold located at ∼770 K. The dissociation of trimers above this temperature induces the appearance of a high temperature activation energy which is lower than the one observed in singly-doped systems, such as Sm- and Nd-doped ceria, showing the unique value of this parameter.

Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ perovskite-type compounds are well-known mixed ionic-electronic conductors for oxygen electrocatalytic applications, although their performance is strictly dependent on the selected preparation methodology and processing parameters. The reducers-to-oxidizers ratio (Φ) is a very important parameter in the solution combustion synthesis of mixed ionic-electronic conductors. Selection of Φ is not trivial and it strongly depends on the type of fuel used, the chemical composition and the specific application of the material. This work clarifies the role of Φ in the solution combustion synthesis of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ for application as oxygen electrocatalysts. Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ powders were synthesized by solution combustion synthesis using sucrose-polyethylene glycol fuel mixtures with reducers-to-oxidizers ratio values between 1 (stoichiometric) and 3 (over-stoichiometric). Chemical-physical properties were studied by X-ray diffraction, scanning electron microscopy, N₂ adsorption at −196 °C, H₂-temperature programmed reduction and thermogravimetric analysis. The results evidenced the direct role of Φ on the intensity and redox environment of the combustion process, and its indirect influence on the Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ electrode materials properties. Taking into account the general picture, the highly over-stoichiometric Φ was selected as the optimal one and the electrochemical activity of the corresponding powder was tested by electrochemical impedance spectroscopy on electrolyte-supported half-cells employing a Ce_0.8Sm_0.2O_2-x electrolyte.

In the last decades, several works have been carried out on solid oxide fuel cell (SOFC) and solid oxide electrolysis cell (SOEC) technologies, as they are powerful and efficient devices for energy conversion and electrochemical storage. By increasing use of renewable sources, a discontinuous amount of electricity is indeed released, and reliable storage systems represent the key feature in such a future energy scenario. In this context, systems based on reversible solid oxide cells (rSOCs) are gaining increasing attention. An rSOC is an electrochemical device that can operate sequentially between discharging (SOFC mode) and charging (SOEC mode); then, it is essential the electrodes are able to guarantee high catalytic activity, both in oxidation and reduction conditions. Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF) has been widely recognized as one of the most promising electrode catalysts for the oxygen reduction reaction (ORR) in SOFC technology because of its astonishing content of oxygen vacancies, even at room temperature. The purpose of this study is the development of BSCF to be used as anode material in electrolysis mode, maintaining enhanced energy and power density. Impregnation with a La_0.8Sr_0.2MnO₃ (LSM) discrete nanolayer is applied to pursue structural stability, resulting in a long lifetime reliability. Impedance spectroscopy measurements under anodic overpotential conditions are run to test BSCF and LSM-BSCF activity as the electrode in oxidation mode. The observed results suggest that BSCF is a very promising candidate as an oxygen electrode in rSOC systems.

Transport and structural properties of heavily doped ceria can reveal subtle details of the interplay between conductivity and defects aggregation in this material, widely studied as solid electrolyte in solid oxide fuel cells. The ionic conductivity of heavily Gd-doped ceria samples (Ce_1−xGd_xO_2−x/2 with x ranging between 0.31 and 0.49) was investigated by impedance spectroscopy in the 600–1000 K temperature range. A slope change was found in the Arrhenius plot at ~723 K for samples with x = 0.31 and 0.34, namely close to the compositional boundary of the CeO₂-based solid solution. The described discontinuity, giving rise to two different activation energies, points at the existence of a threshold temperature, below which oxygen vacancies are blocked, and above which they become free to move through the lattice. This conclusion is well supported by Raman spectroscopy, due to the discontinuity revealed in the Raman shift trend versus temperature of the signal related to defects aggregates which hinder the vacancies movement. This evidence, observable in samples with x = 0.31 and 0.34 above ~750 K, accounts for a weakening of Gd–O bonds within blocking microdomains, which is compatible with the existence of a lower activation energy above the threshold temperature.

The gelcasting method was used to form gastight Ba_0.95Ca_0.05Ce_0.9Y_0.1O₃samples proposed for use as proton-conducting electrolytes in solid oxide fuel cells. Methylcellulose was used as an environmentally friendly shaping agent for Ba_0.95Ca_0.05Ce_0.9Y_0.1O₃powder in an ethanol solution. Samples of Ba_0.95Ca_0.05Ce_0.9Y_0.1O₃were also prepared from the same powder via traditional isostatic pressing, as a reference for cast samples, and sintered in the same conditions. Comparative studies of the physicochemical properties of Ba_0.95Ca_0.05Ce_0.9Y_0.1O₃electrolytes, formed by means of these two methods and then sintered at 1550 °C for 2.5 h, were presented and discussed. Using the X-ray diffraction method, only the pure orthorhombic phase of BaCe_0.9Y_0.1O₃was detected in the Ba_0.95Ca_0.05Ce_0.9Y_0.1O₃powder, as well as in the Ba_0.95Ca_0.05Ce_0.9Y_0.1O₃sintered pellets formed via both gelcasting (A) and isostatic pressing (B). Thermal effects occurring during heating of methylcellulose, as well as ceramic Ba_0.95Ca_0.05Ce_0.9Y_0.1O₃powder, dried cast samples obtained from the prepared slurry, and sintered Ba_0.95Ca_0.05Ce_0.9Y_0.1O₃samples, were examined by differential scanning calorimetry, differential thermal analysis, thermogravimetric analysis, and evolved gas analysis of volatile products using a quadrupole mass spectrometer. The measurements were performed within the temperature range of 20–1200 °C in air. Based on dilatometric tests, it was found that the Ba_0.95Ca_0.05Ce_0.9Y_0.1O₃cast samples exhibited slightly higher degree of sinterability than the 5CBCY samples obtained by isostatic pressing. In comparison with pressed pellets, higher values of total electrical conductivity in air or in a gas mixture of 5% H₂in Ar were also attained for Ba_0.95Ca_0.05Ce_0.9Y_0.1O₃cast samples. The Ba_0.95Ca_0.05Ce_0.9Y_0.1O₃samples were used to construct oxygen–hydrogen electrolytes for solid oxide fuel cells. The results of the electrochemical performance of solid oxide fuel cells with Ba_0.95Ca_0.05Ce_0.9Y_0.1O₃electrolytes were comparable to the data in the literature on BaCe_0.9Y_0.1O₃electrolytes. An electrochemical study of a Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ|Ba_0.95Ca_0.05Ce_0.9Y_0.1O₃interface was also performed. Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δappears to be a suitable cathode material for a Ba_0.95Ca_0.05Ce_0.9Y_0.1O₃electrolyte.