METALLIC OXIDES

In the field of metal oxides, the GREENER group aims to a three-fold target. On the one hand, we continue working in the framework of the synthesis under high pressures of novel metal oxides with electronic or magnetic properties, which we believe in having contributed with substantial work. On the other hand, we are also interested in certain families of metal oxides, involving mixed valence of the transition metals and good electronic conductivity, which can be successfully used as electrodes (cathodes or anodes) in solid-oxide fuel cells (SOFC). In the third term, we aim to continue with a recently-opened research line concerning the preparation and study of new metal hydrides, with interest in the field of hydrogen storage by reaction, under pressure, of simple hydrides; we have recently obtained promising results in this direction.

• Fuel Cells

A fruitful research line of our group corresponds to materials for energy-conversion devices, in particular SOFC, since many of the oxides that traditionally were studied from the magnetic or electronic point of view (superconducting, colossal magneto-resistant oxides…) find applications, with subtle modifications, as electrodes in these electrochemical devices. In particular, the vast family of transition-metal oxides with perovskite structure presents a rich panoply of polyvalent materials that fulfill most of the features required for mixed ionic-electronic conductor (MIEC) oxides. In the last years, we have developed innovative cathode and anode materials that have allowed us to achieve impressive performances in electrolyte-supported fuel cells. Some of the classic transition metal oxides with perovskite structure such as SrFeO3-d, LaNiO3, or SrCoO3-d, have been well-known since the fifties for their elevated electronic conductivity and oxygen diffusion properties; the introduction of selected doping elements into the structure has allowed us, during our research work in the last few years, to stabilize their crystal structures at the working temperature of the SOFC’s while preserving the required transport properties.

These investigations have resulted in the competitive cathode and anode materials that, once integrated into single fuel cells, surpass the power-density target for intermediate-temperature SOFCs using some selected double perovskites as anodes. Among the different materials designed, prepared, and tested in single SOFC cells, we can mention the examples of SrCo1-xSbxO3, SrCo1-xMoxO3, and Sr0.7Y0.3CoO3 as cathodes and SrMo1-xFexO3 or SrMo1-xCrx as anodes; all of them are MIEC electrodes that allow decreasing the working T of operation of the SOFCs.

• Metastable oxides

Many transition-metal oxides in unusual valence states, with exceptional electronic properties, are metastable under ambient conditions and require special synthesis conditions such as high pressure, or the use of strongly oxidizing or reducing requirements, or, in general, the utilization of moderate treatment temperatures. Concerning their preparative chemistry, our Laboratory of the “Instituto de Ciencia de Materiales de Madrid” has been pioneering, at a national level, in setting up and exploiting two different types of equipment of high-pressure synthesis, which have shown to be, so far, extraordinarily fruitful.

As a small overview, the high hydrostatic pressure favours the formation of the short and strongly covalent chemical bonds characterizing the high oxidation states; on the other hand, the high pressure applied by a reactive gas like oxygen provides the highly oxidizing conditions necessary for the stabilization of the mentioned valence states. Thus, we have access to certain materials which, given the difficulty of their preparation, had been little studied so far, despite the exciting properties they present. This gave rise to collaborations with very diverse Research Institutions interested in the materials produced by our group. Additionally, pressure prevents the decomposition of unstable reactants at the synthesis temperature (e.g., Tl2O3, CrO2); it helps increase the coordination numbers and favours the denser phases in perovskite-like materials. Finally, the reaction kinetics is substantially enhanced under pressure. High pressure is an effective tool in preparing new compounds with low stability or a metastable character for all these reasons.

• Metal hydrides

The utilization of hydrogen as an energy vector in the coming decades has boosted the research and improvement of hydrogen storage procedures. New classes of materials composed of much lighter constituents (Li, Be, B, C, N, O, Na, Mg, Al, Si, P, S) have shown a much higher hydrogen storage capacity per weight than the conventional LaNi5 materials. Especially interesting are the Mg-based hydrides, given their high mass-storage capacity; as a drawback, MgH2 is thermally stable, with decomposition temperatures above 450ºC, which prevents applications. The de-stabilization of MgH2 by doping with different metals has been an active research field in the last years. Recently, we have successfully prepared, for the first time, Mg2FeH6-d and other Mg2FeHx (M= Co, Ni) phases by direct reaction between the simple hydride MgH2 and the transition metals under high-pressure conditions in gold capsules at 2 GPa.

It is worth mentioning that Mg2FeH6 has one of the best H mass capacities ever described, almost 6%. These Mg2MHx phases are usually obtained by mechanochemical activation, ball-milling the precursor materials under H2, resulting in poorly crystallized samples. In our case, we obtained Mg2MH6-d with an excellent crystallinity, which would allow us to carry out a structural study by neutron diffraction and establish invaluable structure-properties relationships. This success in the preparative protocol has stimulated the design of a new line of research-based upon the direct reaction of simple hydrides under high-pressure conditions, which prevent the thermal decomposition of the reactants. Among them, we prepared new hydride perovskites of formula ABH3, or double perovskites, A2BB’H6, by reaction under the pressure of AH and BH2. This synthesis procedure has been rarely explored. In all these compounds, the localization of H atoms, the study of the tilting of the BH6 octahedra, the presence of H vacancies, etc., is also crucial knowledge to interpret the sorption/desorption kinetics. RMN and neutron diffraction techniques are powerful (and unique) tools to localize hydrogen in condensed matter, which we have successfully used in many metal hydrides.

Recent important contributions

• • Kyser, A. Muñoz, J.L. Martínez, F. Fauth, M.T. Fernández-Díaz, J.A. Alonso, Enhancing the Néel temperature in 3d/5d R2NiIrO6 (R=La, Pr and Nd) double perovskites by reducing the R3+ ionic radii, Acta Materialia. 207 (2021) 116684. https://doi.org/10.1016 /j.actamat.2021.116684.