Electrochemical Polarization Dependence of the Elastic and Electrostatic Driving Forces to Aliovalent Dopant Segregation on LaMnO3

A Self-Assembled Multiphasic Thin Film as an Oxygen Electrode for Enhanced Durability in Reversible Solid Oxide Cells

The implementation of nanocomposite materials as electrode layers represents a potential turning point for next-generation of solid oxide cells in order to reduce the use of critical raw materials. However, the substitution of bulk electrode materials by thin films is still under debate especially due to the uncertainty about their performance and stability under operando conditions, which restricts their use in real applications. In this work, we propose a multiphase nanocomposite characterized by a highly disordered microstructure and high cationic intermixing as a result from thin-film self-assembly of a perovskite-based mixed ionic-electronic conductor (lanthanum strontium cobaltite) and a fluorite-based pure ionic conductor (samarium-doped ceria) as an oxygen electrode for reversible solid oxide cells. Electrochemical characterization shows remarkable oxygen reduction reaction (fuel cell mode) and oxygen evolution activity (electrolysis mode) in comparison with state-of-the-art bulk electrodes, combined with outstanding long-term stability at operational temperatures of 700 °C. The disordered nanostructure was implemented as a standalone oxygen electrode on commercial anode-supported cells, resulting in high electrical output in fuel cell and electrolysis mode for active layer thicknesses of only 200 nm (>95% decrease in critical raw materials with respect to conventional cathodes). The cell was operated for over 300 h in fuel cell mode displaying excellent stability. Our findings unlock the hidden potential of advanced thin-film technologies for obtaining high-performance disordered electrodes based on nanocomposite self-assembly combining long durability and minimized use of critical raw materials.

Improvement of oxygen reduction activity and stability on a perovskite oxide surface by electrochemical potential

The instability of the surface chemistry in transition metal oxide perovskites is the main factor hindering the long-term durability of oxygen electrodes in solid oxide electrochemical cells. The instability of surface chemistry is mainly due to the segregation of A-site dopants from the lattice to the surface. Here we report that cathodic potential can remarkably improve the stability in oxygen reduction reaction and electrochemical activity, by decomposing the near-surface region of the perovskite phase in a porous electrode made of La1-xSrxCo1-xFexO3 mixed with Sm0.2Ce0.8O1.9. Our approach combines X-ray photoelectron spectroscopy and secondary ion mass spectrometry for surface and sub-surface analysis. Formation of Ruddlesden-Popper phase is accompanied by suppression of the A-site dopant segregation, and exsolution of catalytically active Co particles onto the surface. These findings reveal the chemical and structural elements that maintain an active surface for oxygen reduction, and the cathodic potential is one way to generate these desirable chemistries.

Interface Diffusion and Compatibility of (Ba,La)FeO3-δ Perovskite Electrodes in Contact with Barium Zirconate and Ceria

Ba1-xLaxFeO3-δ perovskites (BLF) capable of conducting electrons, protons, and oxygen ions are promising oxygen electrodes for efficient solid oxide cells (fuel cells or electrolyzers), an integral part of prospected large-scale power-to-gas energy storage systems. We investigated the compatibility of BLF with lanthanum content between 5 and 50%, in contact with oxide-ion-conducting Ce0.8Gd0.2O2-δ and proton-conducting BaZr0.825Y0.175O3-δ electrolytes, annealing the electrode-electrolyte bilayers at high temperature to simulate thermal stresses of fabrication and prolonged operation. By employing both bulk X-ray diffraction and synchrotron X-ray microspectroscopy, we present a space-resolved picture of the interaction between electrode and electrolyte as what concerns cation interdiffusion, exsolution, and phase stability. We found that the phase stability of BLF in contact with other phases is correlated with the Goldschmidt tolerance factor, in turn determined by the La/Ba ratio, and appropriate doping strategies with oversized cations (Zn2+, Y3+) could improve structural stability. While extensive reactivity and/or interdiffusion was often observed, we put forward that most products of interfacial reactions, including proton-conducting Ba(Ce,Gd)O3-δ and mixed-conducting (Ba,La)(Fe,Zr,Y)O3-δ, may not be very detrimental for practical cell operation.

Studying Surface Chemistry of Mixed Conducting Perovskite Oxide Electrodes with Synchrotron-Based Soft X-rays

A fundamental understanding of the electrochemical reactions and surface chemistry at the solid-gas interface in situ and operando is critical for electrode materials applied in electrochemical and catalytic applications. Here, the surface reactions and surface composition of a model of mixed ionic and electronic conducting (MIEC) perovskite oxide, (La0.8Sr0.2)0.95Cr0.5Fe0.5O3-δ (LSCrF8255), were investigated in situ using synchrotron-based near-ambient pressure (AP) X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine-structure spectroscopy (NEXAFS). The measurements were conducted with a surface temperature of 500 °C under 1 mbar of dry oxygen and water vapor, to reflect the implementation of the materials for oxygen reduction/evolution and H2O electrolysis in the applications such as solid oxide fuel cell (SOFC) and electrolyzers. Our direct experimental results demonstrate that, rather than the transition metal (TM) cations, the surface lattice oxygen is the significant redox active species under both dry oxygen and water vapor environments. It was proven that the electron holes formed in dry oxygen have a strong oxygen character. Meanwhile, a relatively higher concentration of surface oxygen vacancies was observed on the sample measured in water vapor. We further showed that in water vapor, the adsorption and dissociation of H2O onto the perovskite surface were through forming hydroxyl groups. In addition, the concentration of Sr surface species was found to increase over time in dry oxygen due to Sr surface segregation, with the presence of oxygen holes on the surface serving as an additional driving force. Comparatively, less Sr contents were observed on the sample in water vapor, which could be due to the volatility of Sr(OH)2. A secondary phase was also observed, which exhibited an enrichment in B-site cations, particularly in Fe and relatively in Cr, and a deficiency in A-site cation, notably in La and relatively in Sr. The findings and methodology of this study allow for the quantification of surface defect chemistry and surface composition evolution, providing crucial understanding and design guidelines in the electrocatalytic activity and durability of electrodes for efficient conversions of energy and fuels.

Activation in the rate of oxygen release of Sr0.8Ca0.2FeO3-δ through removal of secondary surface species with thermal treatment in a CO2-free atmosphere

We elucidate the underlying cause of a commonly observed increase in the rate of oxygen release of an oxygen carrier with redox cycling (here specifically for the perovskite Sr_0.8Ca_0.2FeO_3-δ ) in chemical looping applications. This phenomenon is often referred to as activation. To this end we probe the evolution of the structure and surface elemental composition of the oxygen carrier with redox cycling by both textural and morphological characterization techniques (N₂ physisorption, microscopy, X-ray powder diffraction and X-ray absorption spectroscopy). We observe no appreciable changes in the surface area, pore volume and morphology of the sample during the activation period. X-ray powder diffraction and X-ray absorption spectroscopy analysis (at the Fe and Sr K-edges) of the material before and after redox cycles do not show significant differences, implying that the bulk (average and local) structure of the perovskite is largely unaltered upon cycling. The analysis of the surface of the perovskite via X-ray photoelectron and in situ Raman spectroscopy indicates the presence of surface carbonate species in the as-synthesized sample (due to its exposure to air). Yet, such surface carbonates are absent in the activated material, pointing to the removal of carbonates during cycling (in a CO₂-free atmosphere) as the underlying cause behind activation. Importantly, after activation and a re-exposure to CO₂, surface carbonates re-form and yield a deactivation of the perovskite oxygen carrier, which is often overlooked when using such materials at relatively low temperature (≤500 °C) in chemical looping.

Fast Surface Oxygen Release Kinetics Accelerate Nanoparticle Exsolution in Perovskite Oxides

Exsolution is a recent advancement for fabricating oxide-supported metal nanoparticle catalysts via phase precipitation out of a host oxide. A fundamental understanding and control of the exsolution kinetics are needed to engineer exsolved nanoparticles to obtain higher catalytic activity toward clean energy and fuel conversion. Since oxygen release via oxygen vacancy formation in the host oxide is behind oxide reduction and metal exsolution, we hypothesize that the kinetics of metal exsolution should depend on the kinetics of oxygen release, in addition to the kinetics of metal cation diffusion. Here, we probe the surface exsolution kinetics both experimentally and theoretically using thin-film perovskite SrTi_0.65Fe_0.35O₃ (STF) as a model system. We quantitatively demonstrated that in this system the surface oxygen release governs the metal nanoparticle exsolution kinetics. As a result, by increasing the oxygen release rate in STF, either by reducing the sample thickness or by increasing the surface reactivity, one can effectively accelerate the Fe⁰ exsolution kinetics. Fast oxygen release kinetics in STF not only shortened the prereduction time prior to the exsolution onset, but also increased the total quantity of exsolved Fe⁰ over time, which agrees well with the predictions from our analytical kinetic modeling. The consistency between the results obtained from in situ experiments and analytical modeling provides a predictive capability for tailoring exsolution, and highlights the importance of engineering host oxide surface oxygen release kinetics in designing exsolved nanocatalysts.

Controlling the Size of Au Nanoparticles on Reducible Oxides with the Electrochemical Potential

Controlling the size of Au nanoparticles (NPs) and their interaction with the oxide support is important for their catalytic performance in chemical reactions, such as CO oxidation and water-gas shift. It is known that the oxygen vacancies at the surface of support oxides form strong chemical bonding with the Au NPs and inhibit their coarsening and deactivation. The resulting Au/oxygen vacancy interface also acts as an active site for oxidation reactions. Hence, small Au NPs are needed to increase the density of the Au/oxide interface. A dynamic way to control the size of the Au NPs on an oxide support is desirable but has been missing in the field. Here, we demonstrate an electrochemical method to control the size of the Au NPs by controlling the surface oxygen vacancy concentration of the support oxide. Oxides with different reducibilities, La_0.8Ca_0.2MnO_3±δ and Pr_0.1Ce_0.9O_2-δ, are used as model support oxides. By applying the electrochemical potential, we achieve a wide range of effective oxygen pressures, pO₂ (10^-37-10¹⁴ atm), in the support oxides. Applying the cathodic potential creates a high concentration of oxygen vacancies and forms finely distributed Au NPs with sizes of 7-13 nm at 700-770 °C in 10 min, while the anodic potential oxidizes the surface and increases the size of the Au NPs. The onset cathodic potential required to create small Au NPs depends strongly on the reducibility of the support oxide. The Au NPs did not undergo sintering even at 700-770 °C under the cathodic potential and also were stable in catalytically relevant conditions without potential.

Studies of the Structure and Optical Properties of BaSrMgWO6 Thin Films Deposited by a Spin-Coating Method

Highly transparent thin films with the chemical formula BaSrMgWO₆ were deposited by spin coating using a solution of nitrates of Ba, Sr, and Mg and ammonium paratungstate in dimethylformamide with a Ba:Sr:Mg:W ratio = 1:1:1:1. XRD, SEM, EDX, and XPS investigations evidenced that annealing at 800 °C for 1 h results in an amorphous structure having a precipitate on its surface, and that supplementary annealing at 850 °C for 45 min forms a nanocrystalline structure and dissolves a portion of the precipitates. A textured double perovskite cubic structure (61.9%) was found, decorated with tetragonal and cubic impurity phases (12.7%), such as BaO₂, SrO₂, and MgO, and an under-stoichiometric phase (24.4%) with the chemical formula Ba_2-(x+y) Sr_xMg_yWO₅. From transmittance measurements, the values of the optical band gap were estimated for the amorphous (E_gdir = 5.21 eV, E_gind = 3.85 eV) and nanocrystalline (E_gdir = 4.69 eV, E_gind = 3.77 eV) phases. The presence of a lattice disorder was indicated by the high Urbach energy values and weak absorption tail energies. A decrease in their values was observed and attributed to the crystallization process, lattice strain diminution, and cation redistribution.

Evaluating the Band Gaps of Semiconductors by Cataluminescence

A rapid and efficient methodology for the evaluation of band gaps of semiconductors is highly desirable to analyze and assess the intrinsic properties and extending application scopes of semiconductor materials. Here, the negative correlation of the cataluminescence (CTL) signal in the presence of H₂S and the band gap of Aurivillius-type perovskite oxide Bi_4+nFe_nTi₃O_12+3n (n = 1-4) was confirmed, where the H₂S-induced CTL signal acts as a probe to evaluate the band gaps of semiconductor materials. The related mechanism shows that the thermal energy obtained by heating makes the electrons in the valence band more easily excite into the conduction band of a narrower band gap material and further promotes electron transfer between the gaseous compounds and semiconductor materials, causing acceleration of the catalytic oxide process. In addition, the extensibility was further verified by exploring the layered perovskite containing other insertion structures, including Bi_4+nCo_nTi₃O_12+3n (n = 1-4), Bi₅NiTi₃O₁₅, and Bi₅MnTi₃O₁₅, which was also consistent with the results characterized by UV diffuse reflectance spectroscopy. The established CTL probe for band gap evaluation shows rapid response, is simple to operate, and is of low cost, which is expected to become an innovative alternative to the conventional band gap assessment approach.

Enhancing the Intrinsic Activity and Stability of Perovskite Cobaltite at Elevated Temperature Through Surface Stress

Perovskite-based oxides attract great attention as catalysts for energy and environmental devices. Nanostructure engineering is demonstrated as an effective approach for improving the catalytic activity of the materials. The mechanism for the enhancement, nevertheless, is still not fully understood. In this study, it is demonstrated that compressive strain can be introduced into freestanding perovskite cobaltite La_0.8 Sr_0.2 CoO_{3- δ} (LSC) nanofibers with sufficient small size. Crystal structure analysis suggests that the LSC fiber is characterized by compressive strain along the ab plane and less distorted CoO₆ octahedron compared to the bulk powder sample. Accompanied by such structural changes, the nanofiber shows significantly higher oxygen reduction reaction (ORR) activity and better stability at elevated temperature, which is attributed to the higher oxygen vacancy concentration and suppressed Sr segregation in the LSC nanofibers. First-principle calculations further suggest that the compressive strain in LSC nanofibers effectively shortens the distance between the Co 3d and O 2p band center and lowers the oxygen vacancy formation energy. The results clarify the critical role of surface stress in determining the intrinsic activity of perovskite oxide nanomaterials. The results of this work can help guide the design of highly active and durable perovskite catalysts via nanostructure engineering.

A high-entropy manganite in an ordered nanocomposite for long-term application in solid oxide cells

The implementation of nano-engineered composite oxides opens up the way towards the development of a novel class of functional materials with enhanced electrochemical properties. Here we report on the realization of vertically aligned nanocomposites of lanthanum strontium manganite and doped ceria with straight applicability as functional layers in high-temperature energy conversion devices. By a detailed analysis using complementary state-of-the-art techniques, which include atom-probe tomography combined with oxygen isotopic exchange, we assess the local structural and electrochemical functionalities and we allow direct observation of local fast oxygen diffusion pathways. The resulting ordered mesostructure, which is characterized by a coherent, dense array of vertical interfaces, shows high electrochemically activity and suppressed dopant segregation. The latter is ascribed to spontaneous cationic intermixing enabling lattice stabilization, according to density functional theory calculations. This work highlights the relevance of local disorder and long-range arrangements for functional oxides nano-engineering and introduces an advanced method for the local analysis of mass transport phenomena.

Hf Deposition Stabilizes the Surface Chemistry of Perovskite Manganite Oxide

Stable composition and catalytic activity of surfaces are among the key requirements for materials employed in energy storage and conversion devices, such as solid oxide fuel cells (SOFCs). Perovskite oxides that serve as cathode in SOFCs suffer from segregation of the aliovalent substitutional cations and the formation of an inert, non-conductive phase at the surface. Here, we demonstrate that the surface of the state-of-the-art SOFC cathode material La_0.8Sr_0.2MnO₃ (LSM) is stabilized against the segregation of Sr at high temperature by submonolayer coverages of Hf. The Hf is vapor-deposited onto the LSM thin film surface by e-beam evaporation. Using in situ near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS), we analyze the surface composition of LSM thin films. Half the LSM surface was kept as-prepared, and half was Hf-modified, for a direct comparison of untreated and Hf-treated regions on the same sample. The formation of a binary SrO_x surface species is quantified as descriptor for surface degradation. The onset of Sr segregation is observed at 450 °C on the bare LSM, followed by a substantial advance at 550 °C. Hf-treated regions of the same LSM surface exhibit significantly less Sr surface segregation at 450-550 °C. We interpret this stabilization imparted by Hf to arise from the suppression of the electrostatic attraction of Sr²⁺ cations to surface oxygen vacancies. Doping the surface layer with Hf, that has a higher affinity to oxygen, reduces this attraction by decreasing the surface oxygen vacancy concentration. In doing so, the use of physical vapor deposition highlights the direct role of the metal species in this system and excludes artifacts that could be introduced via chemical routes. The present work demonstrates this stabilizing effect of Hf on the surface of LSM, broadening the relevance of our prior findings on surface metal doping of other perovskite oxides.

Precipitation of dopants on acceptor-doped LaMnO3±δ revealed by defect chemistry from first principles

Perovskite oxides degrade at elevated temperatures while precipitating dopant-rich particles on the surface. A knowledge-based improvement of surface stability requires a fundamental and quantitative understanding of the dopant precipitation mechanism on these materials. We propose that dopant precipitation is a consequence of the variation of dopant solubility between calcination and operating conditions in solid oxide fuel cells (SOFCs) and electrolyzer cells (SOECs). To study dopant precipitation, we use 20% (D = Ca, Sr, Ba)-doped LaMnO_3+δ (LDM20) as a model system. We employ a defect model taking input from density functional theory calculations. The defect model considers the equilibration of LDM20 with a reservoir consisting of dopant oxide (DO), peroxide (DO₂), and O₂ in the gas phase. The equilibrated non-stoichiometry of the A-site and B-site as a function of temperature, T, and oxygen partial pressure, p(O₂), reveals three regimes for LDM20: A-site deficient (oxidizing conditions), A-site rich (atmospheric conditions), and near-stoichiometric (reducing conditions). Assuming an initial A/B non-stoichiometry, we compute the dopant precipitation boundaries in a p-T phase diagram. Our model predicts precipitation both under reducing (DO) and under highly oxidizing conditions (DO₂). We found precipitation under anodic, SOEC conditions to be promoted by large dopant size, while under cathodic, SOFC conditions precipitation is promoted by initial A-site excess. The main driving forces for precipitation are oxygen uptake by the condensed phase under oxidizing conditions and oxygen release assisted by B-site vacancies under reducing conditions. Possible strategies for mitigating dopant precipitation under in electrolytic and fuel cell conditions are discussed.

Ca-doped rare earth perovskite materials for tailored exsolution of metal nanoparticles

Perovskite-type oxide materials (nominal composition ABO₃) are a very versatile class of materials, and their properties are tuneable by varying and doping A- and B-site cations. When the B-site contains easily reducible cations (e.g. Fe, Co or Ni), these can exsolve under reducing conditions and form metallic nanoparticles on the surface. This process is very interesting as a novel route for the preparation of catalysts, since oxide surfaces decorated with finely dispersed catalytically active (often metallic) nanoparticles are a key requirement for excellent catalyst performance. Five doped perovskites, namely, La_0.9Ca_0.1FeO_3-δ, La_0.6Ca_0.4FeO_3-δ, Nd_0.9Ca_0.1FeO_3-δ, Nd_0.6Ca_0.4FeO_3-δ and Nd_0.6Ca_0.4Fe_0.9Co_0.1O_3-δ, have been synthesized and characterized by experimental and theoretical methods with respect to their crystal structures, electronic properties, morphology and exsolution behaviour. All are capable of exsolving Fe and/or Co. Special emphasis has been placed on the influence of the A-site elemental composition on structure and exsolution capability. Using Nd instead of La increased structural distortions and, at the same time, hindered exsolution. Increasing the amount of Ca doping also increased distortions and additionally changed the Fe oxidation states, resulting in exsolution being shifted to higher temperatures as well. Using the easily reducible element Co as the B-site dopant significantly facilitated the exsolution process and led to much smaller and homogeneously distributed exsolved particles. Therefore, the Co-doped perovskite is a promising material for applications in catalysis, even more so as Co is catalytically a highly active element. The results show that fine-tuning of the perovskite composition will allow tailored exsolution of nanoparticles, which can be used for highly sophisticated catalyst design.