Lorenzo Malavasi

Oxide-ion and proton conducting electrolyte materials for clean energy applications: structural and mechanistic features

This critical review presents an overview of the various classes of oxide materials exhibiting fast oxide-ion or proton conductivity for use as solid electrolytes in clean energy applications such as solid oxide fuel cells. Emphasis is placed on the relationship between structural and mechanistic features of the crystalline materials and their ion conduction properties. After describing well-established classes such as fluorite- and perovskite-based oxides, new materials and structure-types are presented. These include a variety of molybdate, gallate, apatite silicate/germanate and niobate systems, many of which contain flexible structural networks, and exhibit different defect properties and transport mechanisms to the conventional materials. It is concluded that the rich chemistry of these important systems provides diverse possibilities for developing superior ionic conductors for use as solid electrolytes in fuel cells and related applications. In most cases, a greater atomic-level understanding of the structures, defects and conduction mechanisms is achieved through a combination of experimental and computational techniques (217 references).

Evidence of a pressure-induced metallization process in monoclinic VO2

Raman and combined infrared transmission and reflectivity measurements were carried out at room temperature (RT) on monoclinic VO2 over the 0-19 GPa and 0-14 GPa pressure ranges. Both lattice dynamics and optical gap show a remarkable stability up to P* approximately 10 GPa whereas subtle modifications of V ion arrangements within the monoclinic lattice, together with the onset of a metallization process via band gap filling, are observed for P >P*. Differently from P=0, where the VO2 metallic phase is found only in conjunction with the rutile structure above 340 K, a new RT metallic phase within a monoclinic structure appears accessible in the high pressure regime.

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.

Raman spectroscopy of AMn2O4(A = Mn, Mg and Zn) spinels

First order Raman spectra in the region 200–800 cm−1 have been collected for AMn2O4 (A = Mn, Mg and Zn) tetragonal spinels. A possible correlation between Raman phonons and AO4 and MnO6 unit vibrations is proposed. Structural changes have been analyzed following the evolution of the Raman spectra with x for the Mg1−xMnxMn2O4 solid solution; the effect of the spinel inversion has been also studied on MgMn2O4 quenched samples from high temperatures. The results, taking into account also the X-ray diffraction and electron paramagnetic resonance data, show the influence of the Jahn–Teller effect on the Raman scattering for this class of materials.

Role of oxygen content on the transport and magnetic properties of La1−xCaxMnO3+δ manganites

Abstract In this paper we report about the synthesis and X-ray powder diffraction characterization of La 1− x Ca x MnO 3+ δ samples with x =0.1, 0.3 and −0.025≤ δ ≤0.054. Transport and magnetization measurements on these samples were performed as a function of the oxygen content both in over and under-stoichiometric regimes. We point out the role of the cation and anion vacancies and the obtained results suggest taking in relevant account the role of oxygen content when dealing with this kind of materials.

Effect of alkaline-doping on the properties of La2Mo2O9 fast oxygen ion conductor

Synthesis by means of a modified Pechini method of pure and alkaline-doped (Na, K and Rb) La2Mo2O9 (LAMOX) is presented. The electrical conductivity of LAMOX prepared by this method is completely analogous to that of the solid state prepared material. Doping with K and Rb hinders the α → β phase transition found in the pure and Na-doped material around 580 °C. Electrical measurements as a function of pO2 shows that LAMOX has a mainly ionic conductivity down to 10−18 atm. The electrical conductivity of the doped samples is close to that of the pure molybdate for temperatures higher than about 600 °C, while for K- and Rb-doped samples the conductivity is higher for lower temperatures, as a consequence of the absence of the phase transition. Finally, a non-Arrhenius behaviour is observed for all the investigated samples and correlated, at a first approximation, to the structural peculiarities of the LAMOX family.

Enhancement of room temperature ferromagnetism in N-doped TiO2−x rutile: Correlation with the local electronic properties

The magnetic and electronic properties of ferromagnetic undoped and N-doped TiO2−x rutile have been probed by soft x-ray spectroscopies. Upon N doping, a fivefold enhancement of the saturation magnetization is observed. Apparently, this enhancement is not related to an increase in oxygen vacancies, rather to additional in-gap states, arising from the replacement of O with N atoms in the rutile structure that can provide more favorable conditions for the onset of ferromagnetic ordering.The magnetic and electronic properties of ferromagnetic undoped and N-doped TiO2−x rutile have been probed by soft x-ray spectroscopies. Upon N doping, a fivefold enhancement of the saturation magnetization is observed. Apparently, this enhancement is not related to an increase in oxygen vacancies, rather to additional in-gap states, arising from the replacement of O with N atoms in the rutile structure that can provide more favorable conditions for the onset of ferromagnetic ordering.

High-pressure behavior of methylammonium lead iodide (MAPbI3) hybrid perovskite

In this paper we provide an accurate high-pressure structural and optical study of MAPbI3 hybrid perovskite. Structural data show the presence of a phase transition towards an orthorhombic structure around 0.3 GPa followed by full amorphization of the system above 3 GPa. After releasing pressure the systems keeps the high-pressure orthorhombic phase. The occurrence of these structural transitions is further confirmed by pressure induced variations of the photoluminescence signal at high pressure. These variations clearly indicate that the bandgap value and the electronic structure of MAPI change across the phase transition.

Role of defect chemistry in the properties of perovskite manganites

This Feature Article provides some representative examples of the role of defect chemistry in the properties of perovskite manganites. It will be shown that a precise control of their defect chemistry is essential in order to give a reliable and meaningful correlation between physical properties and composition.

Electrochemical open circuit voltage (OCV) characterization of SOFC materials

In this paper, we are reporting an extensive characterization, by means of open circuit voltage measurements, of Ce0.8Gd0.2O2, La0.9Sr0.1Ga0.8Mg0.2O3, and La2Mo0.6W1.4O9 oxide-ions and BaCe0.8Y0.2O3 and BaCe0.55Zr0.3Y0.15O3 proton-conducting electrolyte materials for solid oxide fuel cell (SOFC) applications. This simple and common technique, well known for a long time in the electrochemical study of solid oxide fuel cells, has been here proposed for the electrical characterization of these ceramic materials, in order to define their ionic transport numbers, the maximum voltage performances, the thermal and chemical stability, and also to suggest the ideal temperature range for different applications, as in the electrochemical devices, sensors, and SOFC field. In the paper, controlled and reproducible working conditions have been applied in a wide range of temperature, by means of ultrapure gas (H2 and O2), under operational conditions found in real SOFC devices and, mainly, without the usual problems related to the chemical compatibility, the depolarization efficiency, and the high current density required to the electrode materials in the design of a more efficient SOFC device.