Stefano Boldrini

High conductivity and chemical stability of BaCe1−x−yZrxYyO3−δ proton conductors prepared by a sol–gel method

High-temperature proton conductors are promising as electrolytes for intermediate-temperature solid oxide fuel cells. Among them, BaCeO3-based materials have high proton conductivity but rather poor chemical stability. In contrast, barium zirconates are rather stable, but have poorly reproducible densities and conductivities. In this study, the investigation of BaCe1−x−yZrxYyO3−δ solid solutions (x = 0, 0.10, 0.20, 0.30, 0.40; y = 0.15, 0.20) was undertaken, with the final aim of finding a composition having both high conductivity and good stability. The influence of the modified sol–gel Pechini synthetic approach on the powder morphology, and of a barium excess on the densification were demonstrated. Single-phase perovskite powders were prepared and high density pellets were obtained at temperatures lower than those commonly employed. Stability tests demonstrated that the Zr introduction into doped barium cerate greatly enhanced the chemical stability, particularly for Zr ≥ 20%. The proton conductivities, measured in a humidified H2/Ar atmosphere by impedance spectroscopy, were only slightly influenced by the Zr amount. Overall, BaCe1−x−yZrxYyO3−δ solid solutions having Zr ≈ 20–40% and Y ≈ 15–20% showed good chemical stability and high conductivity.

Exceptional hydrogen permeation of all-ceramic composite robust membranes based on BaCe0.65Zr0.20Y0.15O3−δ and Y- or Gd-doped ceria

Mixed proton and electron conductor ceramic composites were examined as hydrogen separation membranes at moderate temperatures (higher than 500 °C). In particular, dense ceramic composites of BaCe0.65Zr0.20Y0.15O3−δ (BCZ20Y15) and Ce0.85M0.15O2−δ (M = Y and Gd, hereafter referred to as YDC15 and GDC15), as protonic and electronic conducting phases respectively, were successfully prepared and tested as hydrogen separation membranes. The mixture of these oxides improved both chemical and mechanical stability and increased the electronic conductivity in dual-phase ceramic membranes. The synthetic method and sintering conditions were optimized to obtain dense and crack free symmetric membranes. The addition of ZnO as a sintering aid allowed achieving robust and dense composites with homogeneous grain distribution. The chemical compatibility between the precursors and the influence of membrane composition on electrical properties and H2 permeability performances were thoroughly investigated. The highest permeation flux was attained for the 50 : 50 volume ratio BCZ20Y15–GDC15 membrane when the feed and the sweep sides of the membrane were hydrated, reaching values of 0.27 mL min−1 cm−2 at 755 °C on a 0.65 mm thick membrane sample, currently one of the highest H2 fluxes obtained for bulk mixed protonic–electronic membranes. Increasing the temperature to 1040 °C, increased the hydrogen flux up to 2.40 mL min−1 cm−2 when only the sweep side was hydrated. The H2 separation process is attributed to two cooperative mechanisms, i.e. proton transport through the membrane and H2 production via the water splitting reaction coupled with oxygen ion transport. Moreover, these composite systems demonstrated a very good chemical stability under a CO2-rich atmosphere such as catalytic reactors for hydrogen generation.

Eco-Friendly Screen-Printing Inks of Gadolinia Doped Ceria

Gadolina doped ceria (GDC) is a promising electrolyte for intermediate temperature solid oxide fuel cells (IT-SOFC). Dense layers of this material can be economically deposited by screen printing. However, the inks for this technique generally use organic compounds that can raise health and safety issues, as well as economical issues. In order to obtain a low-cost alternative to the generally accepted organic-based ink, four different blends of binders were considered to prepare water-based GDC inks. The systems were deposited onto green NiO/GDC anodes produced by tape casting and treated at 1673 K for 4 h. By choosing the right combination of solvents and binders, it was possible to obtain a dense crack-free film of GDC from a water-based system.

Structural, compositional and functional properties of Sb-doped Mg2Si synthesized in Al2O3-crucibles

Magnesium silicide (Mg2Si) is a promising candidate for thermoelectric energy conversion due to its low toxicity, the abundance of its raw constituents and its low density, allowing manufacturing of light, sustainable and relatively cheap devices. Mg2Si needs to be doped in order to increase its efficiency, making this material competitive among materials operating in the intermediate temperature range. In this work, a synthesis procedure based on melting of the raw elements in easily available and cheap Al2O3 crucibles was developed to obtain polycrystalline Sb doped Mg2Si materials in a wide range of compositions. Powders from the crushed lumps were consolidated via spark plasma sintering and then thermally annealed to obtain dense pellets of Sb:Mg2Si with Sb = 0.0, 0.1, 0.3, 0.5, 0.7, 1.0 and 1.5 at%. The effects of Sb doping and of the synthesis and sintering technique on composition, morphology and stability of n-type Mg2Si are discussed. Transport properties (Seebeck coefficient, electrical and thermal conductivity, charge carrier density) were evaluated in order to elucidate the composition–property relationship within this material system and find the optimal doping amount to optimize its thermoelectric properties.

Silica-Based Materials for Thermoelectric-Legs Embedding

Sol-gel chemistry was used to synthesize low-density SiO2 aerogel for matrix embedding of silicide-based (Mg2Si and MnSi(2−x)) thermoelectric legs. In thermoelectric (TE) modules, the heat conduction in air and the convective and radiative contribution to the heat transfer play an important role in the reduction of the efficiency of the module. Silica aerogels are known for the lowest thermal conductivity of any non-evacuated solid. With this in mind, silica-based aerogel materials were employed to fill the void spaces between the thermoelectric legs of a module. In order to do this, different synthesis procedures were taken into account to produce suitable silica materials. It is important that the silica can be easily cast into place, avoiding mechanical cracks of the matrix. Silica aerogel typically requires a supercritical drying step to remove the pore fluid from the SiO2 gel, avoiding the collapse of the pores. This procedure is not practical for TE-legs embedding and it is dangerous, expensive, and time-consuming. It is known that replacing the –OH groups with organic hydrophobic substituents in the SiO2 pores prevents the pore-shrinkage and the sintering of the matrix during solvent evaporation step. This allows synthesizing relatively light materials at low temperature and ambient pressure, with no need of supercritical drying of the gel. The obtained aerogels were characterized by thermogravimetric analysis and differential scanning calorimetry to evaluate the stability of the material and the chemical modification with increasing temperature. The thermal expansion of the silica was evaluated by means of dilatometry. Finally, the thermal diffusivity was measured with the laser flash method.

Finite Element Approach for the Evaluation and Optimization of Silicide-Based TEG

Numerical modelling represents an effective tool for designing and evaluating the performances of thermoelectric power generators (TEG). In particular, the finite element (FE) method allows performing multiphysics simulation, that is coupling different physical phenomena, such as heat transfer, thermoelectric effects, and Joule heating. In this work, FE modeling is at first used to reproduce the results of the open circuit voltage and output power measurements on an undoped Mg2Si TE-chip under large temperature differences. Furthermore, the conversion efficiency of a 16-chip TEG module has been calculated with different ratios of the cross sections of the n-type (Bi-doped Mg2Si) and the p-type (higher manganese silicide, HMS) legs. In both analyses, the thermal and electrical conductivities and Seebeck coefficient are given, as input, in function of temperature. The effects of thermal and electrical contact resistances were taken into account, by introducing thin thermally/electrically resistive layers in the numerical model.