Mónica Burriel

Advances in layered oxide cathodes for intermediate temperature solid oxide fuel cells

In the context of solid oxide fuel cells (SOFCs) applications, mixed-ionic electronic conductors offer significant advantages over conventional cathodes especially in the intermediate-to-low range of temperatures where the performance of the cathode is of fundamental importance. An increasing number of layered oxide materials have been found to present excellent properties as mixed ionic-electronic conductors. Therefore, considerable efforts have been recently devoted to better understand and evaluate layered ordered structures. This article highlights the most important advances in this topic concentrating on both structural aspects and impact in cathode performance for SOFCs applications.

Surface termination and subsurface restructuring of perovskite-based solid oxide electrode materials

We study the outer atomic surfaces of a series of perovskite-based ceramics using low energy ion scattering spectroscopy. After high temperature treatment, segregated A-site (or acceptor substituent) cations dominate the outer surfaces with no B-site cations detected. We also find evidence of an associated B-cation enriched region below the surface.

Absence of Ni on the outer surface of Sr doped La2NiO4 single crystals

A combination of surface sensitive techniques was used to determine the surface structure and chemistry of La2−xSrxNiO4+δ. These measurements unequivocally showed that Ni is not present in the outermost atomic layer, suggesting that the accepted model with the B-site cations exposed to the environment is incorrect.

New perspectives in the surface analysis of energy materials by combined time-of-flight secondary ion mass spectrometry (ToF-SIMS) and high sensitivity low-energy ion scattering (HS-LEIS)

Time-of-flight secondary ion mass spectrometry (ToF-SIMS) and low-energy ion scattering (LEIS) are recently attracting great interest in energy materials research due to their capabilities in terms of surface sensitivity and specificity, spatial resolution and their ability to analyse the isotopic chemical composition. This work shows the synergy provided by this powerful combination to further our understanding of the surface chemistry and structure that ultimately determines the electrochemical performance in advanced electro-ceramic materials for energy storage and energy conversion applications. In particular, this novel approach has been applied to the analysis of (Li3xLa2/3−x□1/3−2x)TiO3 perovskite materials used as the electrolyte in lithium batteries and (La, Sr)2CoO4+δ epitaxial thin films used as oxygen electrodes in solid oxide fuel cells and solid oxide electrolysers. The analysis of these two promising materials requires the development and optimisation of new analytical approaches that take advantage of the recent instrumental developments in order to characterise the outermost and near-surfaces at the atomic scale.

Anisotropic 18O tracer diffusion in epitaxial films of GdBaCo2O5+δ cathode material with different orientations

The layered structure of the orthorhombic GdBaCo2O5+δ (GBCO) double perovskite compound, currently considered as a promising cathode material in Solid Oxide Fuel Cells (SOFCs), is believed to induce a high degree of anisotropy in the oxygen diffusion coefficient, being maximum along the a–b plane in comparison to the diffusion along the c-axis direction. In this study we have deposited films with different orientation: pure c-axis and a-axis orientation on SrTiO3(001) and NdGaO3(110) single crystals, respectively. The oxygen diffusion was analysed by isotopic 18O exchange depth profiling (IEDP) and Time-of-flight Secondary Ion Mass Spectrometry (ToF-SIMS) in the films along the longitudinal and transverse directions at different exchange temperatures and exposure times. The magnitude of longitudinal D* at low temperatures shows a clear anisotropy. The oxygen diffusion along the a-axis shows comparable values to the bulk polycrystalline GBCO, while it is about one order of magnitude lower along the c-axis of the structure. The corresponding oxygen surface exchange rates k* do not show any anisotropy having comparable values for c-axis and a-axis orientation. These k* values are slightly larger than those reported for bulk material showing that thin film textured cathodes may have enhanced activity for oxygen reduction at low temperatures.

Design of La2−xPrxNiO4+δ SOFC cathodes: a compromise between electrochemical performance and thermodynamic stability

Architecturally designed La2−xPrxNiO4+δ (with x = 0, 0.5, 1 and 2) cathodes on the Ce0.9Gd0.1O2−δ (CGO) electrolyte have been prepared with a view to take advantage of the complimentary properties of the two extreme compositions La2NiO4+δ and Pr2NiO4+δ, i.e. the superior stability of La2NiO4+δ and the higher electronic conductivity of Pr2NiO4+δ. The design consists of stacking of two layers starting with a 3D tree-like microstructure (∼20 μm thick) over a thin dense base layer (∼100 nm) fabricated in one step by electrostatic spray deposition (ESD) and then topped by using a screen-printed (SP) current collecting layer of the same composition. X-ray diffraction confirms the formation of a complete solid solution crystallizing in a single phase orthorhombic structure with the Fmmm space group. The thermodynamic stability and polarisation resistance (Rpol) decrease by increasing the Pr content. Among the complete La2−xPrxNiO4+δ solid solutions, LaPrNiO4+δ shows the best compromise between electrochemical properties (the lowest Rpol value available in the literature for this composition, 0.12 Ω cm2 at 600 °C) and thermodynamic stability in air. Moreover, an anode supported single cell (Ni-3YSZ/Ni-8YSZ/8YSZ/CGO) including the LaPrNiO4+δ double layer electrode shows a maximum power density of 438 mW cm−2 at 700 °C.

La4Ni3O10−δ as an efficient solid oxide fuel cell cathode: electrochemical properties versus microstructure

The higher-order Ruddlesden–Popper phase La4Ni3O10−δ is prepared for the first time by electrostatic spray deposition (ESD) on a CGO (Ce0.9Gd0.1O2−δ) electrolyte and evaluated as an intermediate temperature solid oxide fuel cell cathode. Different and innovative microstructures are obtained by varying the deposition time, nozzle to substrate distance, substrate temperature, solution flow rate, concentration and solvents. Single phase La4Ni3O10−δ films crystallize in an orthorhombic layered Ruddlesden–Popper (n = 3) structure after calcination at 950 °C for 8 h in air and is maintained after further sintering at 1100 °C for 6 h in air. The surface morphology, observed by SEM-FEG, is strongly influenced by the solution concentration, the nature of the solvents and the deposition temperature. The electrochemical properties are found to be strongly dependent on the microstructure of the cathode films. The lowest polarization resistance values obtained for the double layer (a 3-D coral-type film by ESD topped by a screen-printed layer) cathode are 2.01, 0.30 and 0.05 Ω cm2 at 600 °C, 700 °C and 800 °C, respectively. To the best of our knowledge, this La4Ni3O10−δ cathode shows the best performance reported to date for this composition.

Design of interfaces in efficient Ln2NiO4+δ (Ln = La, Pr) cathodes for SOFC applications

In this work, a novel architecture of Ln2NiO4+δ (LnNO; Ln = La, Pr) cathodes is prepared on the Ce0.9Gd0.1O2−δ (CGO) electrolyte by sequentially using screen-printing (SP) and electrostatic spray deposition (ESD) techniques for the first time. Both LnNO samples crystallize into a single Fmmm orthorhombic layered Ruddlesden–Popper structure. The role of the electrode/air and electrode/electrolyte interfaces has been evaluated by impedance spectroscopy. A drastic reduction in polarization resistance (Rpol) from 3.33 to 0.42 Ω cm2 and from 0.83 to 0.08 Ω cm2 is obtained at 600 °C for LaNO and PrNO, respectively, when the ESD electrode (with a dense thin, ∼100 nm, LnNO sub-layer) is topped by a SP current collector. A further Rpol reduction down to 0.16 and 0.04 Ω cm2 is successfully obtained for LaNO and PrNO, respectively, when the LnNO sub-layer is replaced by a thicker (∼3 μm) porous CGO/LnNO composite sub-layer. This composite sub-layer plays a main role in obtaining the best electrochemical properties of nickelates available in the literature, to the best of our knowledge. Moreover, the values of polarization resistance for both electrodes are constant at 650 °C for 15 days in air, proving their suitability as SOFC cathodes at this intermediate temperature.

Interface-type resistive switching in perovskite materials

Resistive switching (RS) is currently one of the hot topics in the frontier between materials science and microelectronics, crosslinking both research communities. Among the different types of RS phenomena that have been reported, this review focuses particularly on interface-type RS, for which the change in resistance is related to a modification in the materials properties occurring at the interface over the entire electrode area. In particular we have summarized the most interesting reports on perovskite oxides, a versatile oxide crystal structure which presents a plethora of functional properties depending on its exact composition and structural symmetry. We present the most relevant mechanisms inducing RS, such as valence change, due to a combination of oxygen vacancy drift and redox reactions; electronic correlations; and ferroelectricity. For each case we explain the physico-chemical processes triggered by the application of an external voltage (or current), which ultimately lead to a change in resistance at the interface between the metal electrode and the oxide. Special attention is paid to the material aspects of interface-type switching, and in particular to how the RS characteristics can be improved or triggered by cation doping and oxygen off-stoichiometry, by the introduction of additional layers and by changing the nature of the electrodes. Recent progress in memristive devices based on perovskites is also reported and the figures of merit reached are compared to those obtained for state-of-the-art filamentary type RS binary oxides.

Unveiling the outstanding oxygen mass transport properties of Mn-rich perovskites in grain boundary-dominated La0.8Sr0.2(Mn1-xCox)0.85O3±δ nanostructures

Ion transport in solid-state devices is of great interest for current and future energy and information technologies. A superior enhancement of several orders of magnitude of the oxygen diffusivity has been recently reported for grain boundaries in lanthanum–strontium manganites. However, the significance and extent of this unique phenomenon are not yet established. Here, we fabricate a thin film continuous composition map of the La0.8Sr0.2(Mn1–xCox)0.85O3±δ family revealing a substantial enhancement of the grain boundary oxygen mass transport properties for the entire range of compositions. Through isotope-exchange depth profiling coupled with secondary ion mass spectroscopy, we show that this excellent performance is not directly linked to the bulk of the material but to the intrinsic nature of the grain boundary. In particular, the great increase of the oxygen diffusion in Mn-rich compositions unveils an unprecedented catalytic performance in the field of mixed ionic–electronic conductors. These results present grain boundaries engineering as a novel strategy for designing highly performing materials for solid-state ionics-based devices.