Alessandra Sanson

Hydrogen tunneling in the perovskite ionic conductorBaCe1−xYxO3−δ

We present low-temperature anelastic and dielectric spectroscopy measurements on the perovskite ionic conductor BaCe(1-x)Y(x)O(3-x/2) in the protonated, deuterated and outgassed states. Three main relaxation processes are ascribed to proton migration, reorientation about an Y dopant and tunneling around a same O atom. An additional relaxation maximum appears only in the dielectric spectrum around 60 K, and does not involve H motion, but may be of electronic origin, e.g. small polaron hopping. The peak at the lowest temperature, assigned to H tunneling, has been fitted with a relaxation rate presenting crossovers from one-phonon transitions, nearly independent of temperature, to two-phonon processes, varying as T^7, to Arrhenius-like. Substituting H with D lowers the overall rate by 8 times. The corresponding peak in the dielectric loss has an intensity nearly 40 times smaller than expected from the classical reorientation of the electric dipole associated with the OH complex. This fact is discussed in terms of coherent tunneling states of H in a cubic and orthorhombically distorted lattice, possibly indicating that only H in the symmetric regions of twin boundaries exhibit tunneling, and in terms of reduction of the effective dipole due to lattice polarization.

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.

Stainless steel porous substrates produced by tape casting

In this work the technological issues related to the production of tape cast large-area porous stainless steel supports for Solid Oxide Fuel Cells (SOFC) applications were carefully investigated. The slurry formulation was optimized in terms of amount and nature of the organic components needed: rice starch and polymethyl metacrylate were found to be, respectively, the most suitable pore former and binder because easily eliminated during the thermal treatment in reducing atmosphere. The compatibility of the binder system chosen with the most widely used solvents for screen printing inks was also evaluated. Finally the influence of the sintering temperature and of the refractory supports to be used during the thermal treatments onto the production of porous stainless steel supports was discussed. The whole process optimization allows to produce flat, crack-free metallic substrate 900-1000 μm thick, dimensions up to 5×5 cm and with a tailored porosity of 40% suitable for SOFCs application.

Biomineralization of a titanium-modified hydroxyapatite semiconductor on conductive wool fibers

Metal ions are frequently incorporated into crystalline materials to improve their electrochemical properties and to confer new physicochemical properties. Naturally-occurring phosphate apatite, which is formed geologically and in biomineralization processes, has extensive potential applications and is therefore an attractive functional material. In this study, we generate a novel building block for flexible optoelectronics using bio-inspired methods to deposit a layer of photoactive titanium-modified hydroxyapatite (TiHA) nanoparticles (NPs) on conductive polypyrrole(PPy)-coated wool yarns. The titanium concentration in the reaction solution was varied between 8-50 mol% with respect to the phosphorous, which led to titanate ions replacing phosphate in the hydroxyapatite lattice at levels up to 17 mol%. PPy was separately deposited on wool yarns by oxidative polymerization, using two dopants: (i) anthraquinone-2,6-disulfonic acid to increase the conductivity of the PPy layer and (ii) pyroglutamic acid, to reduce the resistivity of the wool yarns and to promote the heterogeneous nucleation of the TiHA NPs. A specific titanium concentration (25 mol% wrt P) was used to endow the TiHA NPs on the PPy-coated fibers with a desirable band gap value of 3.68 eV, and a specific surface area of 146 m2 g-1. This is the first time that a thin film of a wide-band gap semiconductor has been deposited on natural fibers to create a fiber-based building block that can be used to manufacture flexible electronic devices.

Study of reversible SOFC/SOEC based on a mixed anionic-protonic conductor

This paper deals with the fabrication and electrochemical study of a high temperature solid electrolyte supporting cell operating as SOFC (Solid Oxide Fuel Cell) and SOEC (Solid Oxide Electrolysis Cell). The cell is based on a dual membrane (DM) electrolyte design, advantageously separating the cell into three different chambers: hydrogen side, oxygen side, and dual membrane (DM), where H2O production or splitting takes place in SOFC or SOEC mode respectively. The supporting electrolyte consists of a dense/porous/dense tri-layer, exclusively made of BaCe0.85Y0.15O3−δ (monolithic design), which is a mixed anionic-protonic conductor. The assembly was fabricated by tape casting, adding pore formers to control porosity. The cell was then electrochemically studied under different operating conditions of temperature, overpotentials and gas feeding, either in SOFC and SOEC mode. From the results presented here, it can be observed that, in spite of dense and thick electrolyte layers and platinum electrodes, the electrochemical study of the cell showed: (i) promising power density, (ii) interesting SOFC/SOEC operating mode reversibility, (iii) proved H2O production in the porous dual membrane when the cell operates as a fuel cell, and proved splitting of the H2O molecules contained in the porous dual membrane when the cell operates as an electrolyser. Investigations of cell performance degradation were also conducted.

Effect of O vacancies on the Young’s modulus of the BaCe1−xYxO3−δ perovskite

The effect of oxygen vacancies on the elastic properties of BaCe0.9Y0.1O3−δ is studied by measuring the complex Young’s modulus between 80 and 850 K varying the content of O vacancies from 0 to nearly 0.05. The Young’s modulus measured at a fixed temperature above 300 K may change by more than 20% but this is due to a shift of the rhombohedral-orthorhombic transition by 250 K and to proton and vacancy hopping. Below 100 K these effects are frozen and the filling of the O vacancies with OH ions increases the Young’s modulus by ∼1.3%.

RF-Sputtering Deposition of Gadolinia Doped Ceria Films for IT-SOFCs Applications

Solid oxide fuel cell technology offers substantial advantages for clean and efficient power generation. However, the high working temperatures impose severe restrictions on the materials selection. The key to prolong the stack lifetime, widen the selection of materials and reduce the costs lies in lowering the working temperature. Two main approaches have been applied for this purpose: testing materials based on Gd doped CeO2, having higher ionic conductivity at lower temperatures, and decreasing the electrolyte thickness. At present, different methods have been studied for the deposition of fully dense thin coatings. Among them, the radio frequency magnetron sputtering technique is a powerful and versatile tool to deposit mixed oxide thin films in a wide thickness range (100 nm÷10 μm). In this report, the results about the deposition by RF magnetron sputtering of 1÷5 μm GDC films on anodic substrates will be presented.

All-ceramic asymmetric membranes with superior hydrogen permeation

BaCe0.65Zr0.20Y0.15O3−δ–Gd0.2Ce0.8O2−δ (BCZY–GDC) composite with a planar asymmetrical architecture was investigated for the first time as a hydrogen separation membrane in the 600 °C to 750 °C temperature range. Asymmetrical membranes comprising a 20 μm thick dense BCZY–GDC layer supported on a 600 μm thick porous substrate with the same composition were successfully produced through a tape casting process. The influence of the dense layer composition on hydrogen separation and the catalytic activation of the dense membrane layer and porous support were thoroughly studied. The hydrogen permeation process was further characterized by varying the environment humidification degree and temperature and by shifting the sweep and feed gas compositions. This work highlights the importance of preserving the dual proton (BCZY) and electron (GDC) conductor phases during the sintering step of the membrane to enhance H2 permeation; a value of 0.47 mL min−1 cm−2 at 750 °C was obtained, which is currently one of the highest H2 fluxes obtained for all-ceramic proton-conducting membranes. In addition, a strong improvement is achieved when the feed/sweep sides of the membrane are reversed, i.e. H2 feeding in the support side leads to a flux increase up to 0.68 mL min−1 cm−2 at 750 °C when only the sweep side is hydrated. These results demonstrate that these asymmetric dual phase BCZY–GDC membranes are the most promising candidates for H2 separation applications at high temperatures.

Electrochemical Performances of a Reversible High Temperature Fuel Cell Based on a Mixed Anionic-Protonic Conductor

This paper deals with the fabrication and study of a high temperature solid electrolyte supporting electrochemical cell operating as SOFC and SOEC. The cell is based on the idea of a dual membrane electrolyte, which has the advantage to separate the cell in three different chambers: hydrogen side, oxygen side and dual membrane (D.M.), where water production-SOFC or splitting-SOEC takes place. The tape casted supporting electrolyte is constituted by a dense/porous/dense tri-layer, made of BaCe0.85Y0.15O3-δ (BCY), which is a mixed ionic conductor. The electrolyte sandwich was formed by in-situ sintering of stacked green tapes with or without pore former. The process optimization (slurry formulation, lamination and thermal treatments) led to flat, crack-free, 580 µm thick BCY discs with a well-controlled microstructure. Platinum electrodes were deposited on both sides of the sandwich-structured electrolyte. The cell was then electrochemically studied under different operating conditions of temperature, overpotentials and gas feeding, either in SOFC and SOEC mode. From the presented results it can be highlighted that despite the dense electrolyte layers were kept thick and the electrodes were made of platinum, the electrochemical study of the cell show: i) a promising power density, ii) an interesting reversibility SOFC/SOEC, iii) a proved splitting of the water contained in the porous central membrane when the cell operates as an electrolyser (SOEC). Degradation of the cell performance is also focused.

Influence of Doping on the Structural Transformations of the Proton Conducting Perovskite BaCe1xYxO3-D

From neutron diffraction it is known that the BaCeO3 perovskite undergoes a sequence of phase transformations from high temperature cubic C to rhombohedral R, to orthorhombic O1 (Imma) and to orthorhombic O2 (Pnma). Doping Y3+ on the Ce4+ site introduces charge compensating O vacancies (VO) that may be partially filled with OH complexes with exposition to H2O, so making the material an ionic conductor. Anelastic relaxation experiments have been carried out on samples doped with 2%Y and 10%Y; the real part s’(T) of the complex elastic compliance presents softenings at the transitions, and the loss s’’/s’ curves allow the content of VO and H to be monitored. Doping has a strong effect on the temperature of the Pnma/R transition: with 10%Y in the fully hydrated state TO1-R increases up to 750 K while after full outgassing falls to 500 K, meaning that the introduction of ~5% VO shifts the transition of 250 K. While the effect of cation substitution on the transitions temperature is easily explained in terms of simple arguments usually valid for perovskites based on bond length considerations, the remarkable stabilization of the R phase by VO requires to take into account the anomalous sequence of phase transitions of undoped BaCeO3, where the R structure transforms into orthorhombic Pnma on cooling with the loss of an octahedral tilt system.