Correlations between structure, microstructure and ionic conductivity in (Gd,Sm)-doped ceria.

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.

Inkjet Printing Functionalization of SOFC LSCF Cathodes.

An important segment of the future renewable energy economy is the implementation of novel energy generation systems. Such electrochemical systems are solid oxide fuel cells, which have the advantage of direct conversion of the chemical energy stored in the fuel to electrical energy with high efficiency. Improving the performance and lowering the cost of solid oxide fuel cells (SOFCs) are strongly dependent on finding commercially viable methods for nano-functionalization of their electrodes via infiltration. Inkjet printing technology was proven to be a feasible method providing scalability and high-resolution ink delivery. LaxSr1-xCoyFe1-yO3-δ cathodes were modified using inkjet printing for infiltration with two different materials: Gd-doped ceria (CGO) commonly used as ion-conductor and La0.6Sr0.4CoO3-δ (LCO) commonly used as a mixed ionic electronic conductor. As-modified surface structures promoted the extension of the three-phase boundary (TPB) and enhanced the mechanisms of the oxygen reduction reaction. Electrochemical impedance measurements revealed significantly lowered polarization resistances (between 2.7 and 3.7 times) and maximum power output enhancement of 24% for CGO infiltrated electrodes and 40% for LCO infiltrated electrodes.

Infiltration of Metal Substrates with Nanostructured CeO2 by a Room-Temperature Wet Process.

A room-temperature infiltration procedure for the deposition of CeO2 nanopowders on Ni-based foams employing stable CeO2 suspensions in water has been developed. It consists of a two-steps dipping process, the first in nanopowder suspension at pH 6.5 followed by further dipping into a NH3OH solution at pH 12. The pH shift represents a key factor to improve the homogeneity and dispersion of infiltrated powder by avoiding coalescence during the drying step. Water-based suspensions have been prepared starting from a commercial nanostructured CeO2. Powder was characterized by X-ray diffraction, particle size and specific surface area measurements, transmission electron microscopy. Stability of suspensions was studied by zeta potential measurements at low concentration, while sedimentation tests were carried out on highly concentrated suspensions as a function of pH, CeO2 amount and surfactant presence. Effect of CeO2 concentration, surfactant addition, pH value, substrate composition and microstructure were taken in account. Under best conditions, very homogeneous infiltrations could be obtained without any preferential orientation or agglomerates. Thermal stability of the composites infiltrated materials was also tested. The technique seems to be very promising in advanced nanostructured decorations and coating preparation.

Ferroelectric hollow particles obtained by solid-state reaction.

Hollow particles of barium titanate were obtained by a two-step process combining colloidal chemistry and solid-state reaction. BaCO(3) crystals (size ≈1 µm) suspended in a peroxy-Ti(IV) aqueous solution were coated with an amorphous TiO(2) shell using a precipitation process. Calcination of the BaCO(3)@TiO(2) core-shell particles at 700 °C resulted in the formation of BaTiO(3) hollow particles (shell thickness of ≈70 nm) which retain the morphology of the BaCO(3) crystals. Formation of the cavity occurs because out-diffusion of the core phase is much faster than in-diffusion of the shell material. X-ray diffraction (XRD) and Raman spectroscopy indicate that the hollow particles possess a tetragonal ferroelectric structure with axial ratio c/a = 1.005. Piezoresponse force microscopy has shown strong piezoactivity and 180° ferroelectric domains. The process described provides a general route to fabricate hollow ferroelectric structures of several compounds.