Roles of Bulk and Surface Chemistry in the Oxygen Exchange Kinetics and Related Properties of Mixed Conducting Perovskite Oxide Electrodes

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.

Advancing Electrode Properties through Functionalization for Solid Oxide Cells Application: A Review

Hydrogen energy has emerged as the only renewable which is capable of sustaining the prevalent energy crisis in conjunction with other intermittent sources. In this connection, solid oxide cell (SOC) is the most sustainable solid-state devices capable of recycling and reproducing green hydrogen fuel. It is operable in reversible modes viz, fuel cell (FC) and electrolysis cell (EC). SOC is capable of engaging multiple fuels thereby promoting carbon neutral planet. The all-solid design further augments the optimization of cost, efficiency, durability and endurance at higher temperature. Electrodes are therefore, an important component which is responsible for electrocatalytic processing of fuel and oxidant for higher recyclability of cell/stack. The present review article embarks a detailed overview on the past and present status of electrode composition, heterointerface engineering applicable for SOC's. Recent trends in electrode engineering and the possibilities for advancement in SOC is also reviewed with respect to both experimental and computational aspects.

A Review of X-ray Photoelectron Spectroscopy Technique to Analyze the Stability and Degradation Mechanism of Solid Oxide Fuel Cell Cathode Materials

Nondestructive characterization of solid oxide fuel cell (SOFC) materials has drawn attention owing to the advances in instrumentation that enable in situ characterization during high-temperature cell operation. X-ray photoelectron spectroscopy (XPS) is widely used to investigate the surface of SOFC cathode materials because of its excellent chemical specificity and surface sensitivity. The XPS can be used to analyze the elemental composition and oxidation state of cathode layers from the surface to a depth of approximately 5–10 nm. Any change in the chemical state of the SOFC cathode at the surface affects the migration of oxygen ions to the cathode/electrolyte interface via the cathode layer and causes performance degradation. The objective of this article is to provide a comprehensive review of the adoption of XPS for the characterization of SOFC cathode materials to understand its degradation mechanism in absolute terms. The use of XPS to confirm the chemical stability at the interface and the enrichment of cations on the surface is reviewed. Finally, the strategies adopted to improve the structural stability and electrochemical performance of the LSCF cathode are also discussed.

Surface potentials of acceptor- and donor-doped CeO2 thin films and their relation to oxygen surface exchange

Surface Fermi level positions, ionisation potentials, and work functions of acceptor-, donor-, and nominally undoped CeO₂ have been determined by means of in situ photoelectron spectroscopy on films grown with different surface orientation and preparation conditions. The Fermi energy varies in accordance with the doping and oxygen activity. The ionisation potentials are largely unaffected by the preparation conditions and surface orientation. They are comparable for nominally undoped, 1% donor-doped, and 1% acceptor-doped films. The majority of the 10% Gd-doped films exhibit a 0.5 eV lower ionisation potential, which might be related to the presence of a surface space-charge region. The lower ionisation potential of the 10% Gd-doped films compensates for their lower Fermi energies and eventually results in work functions being largely independent on doping. Oxygen surface exchange coefficients determined using secondary ion mass spectrometry and conductivity relaxation experiments reveal similar magnitudes and are not strongly affected by doping type, concentration, and surface orientation. The results indicate that surface space-charge regions are crucial for oxygen surface exchange but do not allow to finally identify the rate determining step for oxygen incorporation into CeO₂-based materials.

Layered Oxygen-Deficient Double Perovskites as Promising Cathode Materials for Solid Oxide Fuel Cells

Development of new functional materials with improved characteristics for solid oxide fuel cells (SOFCs) and solid oxide electrolysis cells (SOECs) is one of the most important tasks of modern materials science. High electrocatalytic activity in oxygen reduction reactions (ORR), chemical and thermomechanical compatibility with solid electrolytes, as well as stability at elevated temperatures are the most important requirements for cathode materials utilized in SOFCs. Layered oxygen-deficient double perovskites possess the complex of the above-mentioned properties, being one of the most promising cathode materials operating at intermediate temperatures. The present review summarizes the data available in the literature concerning crystal structure, thermal, electrotransport-related, and other functional properties (including electrochemical performance in ORR) of these materials. The main emphasis is placed on the state-of-art approaches to improving the functional characteristics of these complex oxides.

Revealing the Correlation of OER with Magnetism: A New Descriptor of Curie/Neel Temperature for Magnetic Electrocatalysts

Developing accurate descriptors for oxygen evolution reaction (OER) is of great significance yet challenging, which roots in and also boosts the understanding of its intrinsic mechanisms. Despite various descriptors are reported, it still has limitations in the facile prediction, given that complicated analytical techniques as well as time-consuming modeling and calculations are indispensable. In the present work, strong correlation of magnetic property with OER performance is revealed by in-depth investigations on the crystal and electronic structures. A facile descriptor of Curie/Neel temperature (T_C/N ) is developed for La_{2- x} Sr_x Co₂ O_{6- δ} perovskite oxides, based on the inference that both magnetism and OER are rooted in the electron exchange interaction. Specifically, both the T_C/N and OER activity are proportional to the degree of p-d orbital hybridization, which increases with enlarged bond angle of Co─O─Co and/or increased oxidation of Co. This finding reveals that T_C/N from magnetic characterizations is an effective descriptor in designing novel OER electrocatalysts, and interdisciplinary researches are advantageous for revealing the controversial mechanisms of OER process.

Surface Segregation in Solid Oxide Cell Oxygen Electrodes: Phenomena, Mitigation Strategies and Electrochemical Properties

Abstract

Solid oxide cells (SOCs) are highly efficient and environmentally benign devices that can be used to store renewable electrical energy in the form of fuels such as hydrogen in the solid oxide electrolysis cell mode and regenerate electrical power using stored fuels in the solid oxide fuel cell mode. Despite this, insufficient long-term durability over 5–10 years in terms of lifespan remains a critical issue in the development of reliable SOC technologies in which the surface segregation of cations, particularly strontium (Sr) on oxygen electrodes, plays a critical role in the surface chemistry of oxygen electrodes and is integral to the overall performance and durability of SOCs. Due to this, this review will provide a critical overview of the surface segregation phenomenon, including influential factors, driving forces, reactivity with volatile impurities such as chromium, boron, sulphur and carbon dioxide, interactions at electrode/electrolyte interfaces and influences on the electrochemical performance and stability of SOCs with an emphasis on Sr segregation in widely investigated (La,Sr)MnO3 and (La,Sr)(Co,Fe)O3−δ. In addition, this review will present strategies for the mitigation of Sr surface segregation.

Graphic Abstract

Lanthanum Ferrites-Based Exsolved Perovskites as Fuel-Flexible Anode for Solid Oxide Fuel Cells

Exsolved perovskites can be obtained from lanthanum ferrites, such as La_0.6Sr_0.4Fe_0.8Co_0.2O₃, as result of Ni doping and thermal treatments. Ni can be simply added to the perovskite by an incipient wetness method. Thermal treatments that favor the exsolution process include calcination in air (e.g., 500 °C) and subsequent reduction in diluted H₂ at 800 °C. These processes allow producing a two-phase material consisting of a Ruddlesden–Popper-type structure and a solid oxide solution e.g., α-Fe_100-y-zCo_yNi_zO_x oxide. The formed electrocatalyst shows sufficient electronic conductivity under reducing environment at the Solid Oxide Fuel Cell (SOFC) anode. Outstanding catalytic properties are observed for the direct oxidation of dry fuels in SOFCs, including H₂, methane, syngas, methanol, glycerol, and propane. This anode electrocatalyst can be combined with a full density electrolyte based on Gadolinia-doped ceria or with La_0.8Sr_0.2Ga_0.8Mg_0.2O₃ (LSGM) or BaCe_0.9Y_0.1O_3-δ (BYCO) to form a complete perovskite structure-based cell. Moreover, the exsolved perovskite can be used as a coating layer or catalytic pre-layer of a conventional Ni-YSZ anode. Beside the excellent catalytic activity, this material also shows proper durability and tolerance to sulfur poisoning. Research challenges and future directions are discussed. A new approach combining an exsolved perovskite and an NiCu alloy to further enhance the fuel flexibility of the composite catalyst is also considered. In this review, the preparation methods, physicochemical characteristics, and surface properties of exsoluted fine nanoparticles encapsulated on the metal-depleted perovskite, electrochemical properties for the direct oxidation of dry fuels, and related electrooxidation mechanisms are examined and discussed.

Effect of sintering temperature on electrochemical performance of porous SrTi1-xFexO3-δ (x = 0.35, 0.5, 0.7) oxygen electrodes for solid oxide cells

This work evaluates the effects of the sintering temperature (800 °C, 900 °C, 1000 °C) of SrTi1-xFexO3-δ (x = 0.35, 0.5, 0.7) porous electrodes on their electrochemical performance as potential oxygen electrode materials of solid oxide cells. The materials were prepared by a solid-state reaction method and revealed the expected cubic perovskite structure. After milling, the powders were characterised by a sub-micrometre particle size with high sinter-activity. It was shown that the lowest area specific resistance was achieved after sintering SrTi0.65Fe0.35O3 electrodes at 1000 °C, and SrTi0.5Fe0.5O3 and SrTi0.30Fe0.70O3 electrodes at 800 °C, which can be considered to be a relatively low temperature. In general, EIS measurements showed that increasing the Fe content results in lowered electrode polarisation and a decrease of the series resistance. Even though the studied materials have much lower total conductivities than state-of-the-art electrode materials (e.g. (La,Sr)(Co,Fe)O3), the polarisation resistances obtained in this work can be considered low.

Preparation of Hydrogen Electrodes of Solid Oxide Cells by Infiltration: Effects of the Preparation Procedure on the Resulting Microstructure

In this work, the infiltration technique was used to produce hydrogen electrodes for solid oxide cells. Different infiltration methodologies were tested in order to try to shorten the infiltration cycle time. The porous scaffolds used for infiltration were based on highly porous yttria-stabilized zirconia (YSZ) obtained by etching the reduced nickel from the Ni-YSZ cermet in HNO₃ acid. The support had a complex structure which included a ~130 µm porous functional layer with small pores and a ~320 µm thick supporting layer with large pores. Infiltrations have been carried out using aqueous nickel nitrate solutions. Various infiltration procedures were used, differing in temperature/time profiles. The results show that slow evaporation is crucial for obtaining a homogeneous material distribution leading to high-quality samples. A longer evaporation time promotes the proper distribution of nickel throughout the porous scaffold. The shortening of the heat treatment procedure leads to blockage of the pores and not-uniform nickel distribution.

Compositional Engineering of a La1-xBaxCoO3-δ-(1-a) BaZr0.9Y0.1O2.95 (a = 0.6, 0.7, 0.8 and x = 0.5, 0.6, 0.7) Nanocomposite Cathodes for Protonic Ceramic Fuel Cells

Compositionally engineered a La1-xBaxCoO3-δ-(1-a) BaZr0.9Y0.1O2.95 (a = 0.6, 0.7, 0.8 and x = 0.5, 0.6, 0.7) (LBZ) nanocomposite cathodes were prepared by oxidation driven in situ exsolution of a single-phase material deposited on a BaZr0.9Y0.1O2.95 electrolyte. The processing procedure of the cathode was optimized by reducing the number of thermal treatments as the single-phase precursor was deposited directly on the electrolyte. The exsolution and firing of the cathodes occurred in one step. The electrochemical performance of symmetrical cells with the compositionally engineered cathodes was investigated by impedance spectroscopy in controlled atmospheres. The optimized materials processing gave web-like nanostructured cathodes with superior electrochemical performance for all compositions. The area specific resistances obtained were all below 12 Ω·cm2 at 400 °C and below 0.59 Ω·cm2 at 600 °C in 3% moist synthetic air. The resistances of the nominal 0.6 La0.5Ba0.5CoO3-δ-0.4 BaZr0.9Y0.1O2.95 and 0.8 La0.5Ba0.5CoO3-δ-0.2 BaZr0.9Y0.1O2.95 composite cathodes were among the lowest reported for protonic ceramic fuel cells cathodes in symmetrical cell configuration with ASR equal to 4.04 and 4.84 Ω·cm2 at 400 °C, and 0.21 and 0.27 Ω·cm2 at 600 °C, respectively.

Dual Oxygen Defects in Layered La1.2Sr0.8-xBaxInO4+δ (x = 0.2, 0.3) Oxide-Ion Conductors: A Neutron Diffraction Study

The title compounds exhibit a K₂NiF₄-type layered perovskite structure; they are based on the La_1.2Sr_0.8InO_4+δ oxide, which was found to exhibit excellent features as fast oxide-ion conductor via an interstitial oxygen mechanism. These new Ba-containing materials were designed to present a more open framework to enhance oxygen conduction. The citrate-nitrate soft-chemistry technique was used to synthesize such structural perovskite-type materials, followed by annealing in air at moderate temperatures (1150 °C). The subtleties of their crystal structures were investigated from neutron powder diffraction (NPD) data. They crystallize in the orthorhombic Pbca space group. Interstitial O3 oxygen atoms were identified by difference Fourier maps in the NaCl layer of the K₂NiF₄ structure. At variance with the parent compound, conspicuous oxygen vacancies were found at the O2-type oxygen atoms for x = 0.2, corresponding to the axial positions of the InO₆ octahedra. The short O2–O3 distances and the absence of steric impediments suggest a dual oxygen-interstitial mechanism for oxide-ion conduction in these materials. Conductivity measurements show that the activation energy values are comparable to those typical of ionic conductors working by simple vacancy mechanisms (~1 eV). The increment of the total conductivity for x = 0.2 can be due to the mixed mechanism driving both oxygen vacancies and interstitials, which is original for these potential electrolytes for solid-oxide fuel cells.

Emergence of Rapid Oxygen Surface Exchange Kinetics during in Situ Crystallization of Mixed Conducting Thin Film Oxides

The oxygen surface exchange kinetics of mixed ionic and electronic conducting oxides (MIECs) play a critical role in the efficiency of intermediate-to-high-temperature electrochemical devices. Although there is increasing interest in low-temperature preparation of MIEC thin films, the impact of the resultant varied degrees of crystallinity on the surface exchange kinetics has not been widely investigated. Here, we probe the effect of crystallization on oxygen surface exchange kinetics in situ, by applying an optical transmission relaxation (OTR) approach during annealing of amorphous films. OTR enables contact-free, in situ, and continuous quantification of the oxygen surface exchange coefficient ( k_chem); we previously applied it to Pr _xCe_{1- x}O_2-δ and SrTi_{1- x}Fe _xO_3-δ thin films. In this work, the OTR approach was successfully extended to other mixed conducting thin film compositions for the first time (i.e., perovskite SrTi_0.65Co_0.35O_3-δ and Ruddlesden-Popper Sr₂Ti_0.65Fe_0.35O_4±δ), as well as to Pr_0.1Ce_0.9O_2-δ, enabling quantification of the k_chem of their native surfaces and comparison of the behavior of films with different final crystal structures. All thin films were prepared by pulsed laser deposition at 25 or 700-800 °C and subject to subsequent thermal treatments with simultaneous OTR monitoring of k_chem. The surface roughness, grain size, and crystallinity were evaluated by scanning probe microscopy, X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. Fluorite Pr_0.1Ce_0.9O_2-δ films grown at 25 °C did not exhibit an increase in k_chem after annealing, as they were already crystalline as grown at 25 °C. For all other compositions, OTR enabled in situ observation of both the crystallization process and the emergence of rapid surface exchange kinetics immediately upon crystallization. Perovskite SrTi_0.65Co_0.35O_3-δ and Ruddlesden-Popper Sr₂Ti_0.65Fe_0.35O_4±δ thin films grown at 25 °C exhibited at least 1-2 orders of magnitude enhanced k_chem after annealing compared with highly crystalline thin films grown at 800 °C, indicating the benefits of in situ crystallization.

Understanding Structure-Activity Relationships in Sr1- xY xCoO3-δ through in Situ Neutron Diffraction and Electrochemical Measurements

In this work, we report a systematic study on temperature-dependent local structural evolution, oxygen stoichiometry, and electrochemical properties of an oxygen-deficient perovskite Sr_0.7Y_0.3CoO_3-δ (SYC30) for oxygen electrocatalysis. The obtained results are then closely compared with its analogue Sr_0.9Y_0.1CoO_3-δ (SYC10) of different crystal structures to establish structure-activity relationships. The comparison shows that both SYC30 and SYC10 consist of alternate layers of oxygen-deficient Co1-polyhedra and oxygen-saturated Co2-octahedra with Co1-polyhedra being responsible for V_o^•• migration. It is also found that the distribution and concentration of oxygen vacancies within the Co1-layer are, respectively, less symmetrical and lower in SYC30 than those in SYC10, making the former unfavorable for oxygen transport. A molecular orbital energy analysis reveals that the energy gap between Fermi level and O 2p level in the active Co1-polyhedra is larger in SYC30 than that in SYC10, further suggesting that SYC10 is a better oxide-ion conductor and thus a better electrocatalyst for oxygen reduction reaction, which is unambiguously confirmed by the subsequent electrochemical measurements.

Controlling cation segregation in perovskite-based electrodes for high electro-catalytic activity and durability

Solid oxide cell (SOC) based energy conversion systems have the potential to become the cleanest and most efficient systems for reversible conversion between electricity and chemical fuels due to their high efficiency, low emission, and excellent fuel flexibility. Broad implementation of this technology is however hindered by the lack of high-performance electrode materials. While many perovskite-based materials have shown remarkable promise as electrodes for SOCs, cation enrichment or segregation near the surface or interfaces is often observed, which greatly impacts not only electrode kinetics but also their durability and operational lifespan. Since the chemical and structural variations associated with surface enrichment or segregation are typically confined to the nanoscale, advanced experimental and computational tools are required to probe the detailed composition, structure, and nanostructure of these near-surface regions in real time with high spatial and temporal resolutions. In this review article, an overview of the recent progress made in this area is presented, highlighting the thermodynamic driving forces, kinetics, and various configurations of surface enrichment and segregation in several widely studied perovskite-based material systems. A profound understanding of the correlation between the surface nanostructure and the electro-catalytic activity and stability of the electrodes is then emphasized, which is vital to achieving the rational design of more efficient SOC electrode materials with excellent durability. Furthermore, the methodology and mechanistic understanding of the surface processes are applicable to other materials systems in a wide range of applications, including thermo-chemical photo-assisted splitting of H₂O/CO₂ and metal-air batteries.

Oxygen surface exchange kinetics measurement by simultaneous optical transmission relaxation and impedance spectroscopy: Sr(Ti,Fe)O3-x thin film case study

We compare approaches to measure oxygen surface exchange kinetics, by simultaneous optical transmission relaxation (OTR) and AC-impedance spectroscopy (AC-IS), on the same mixed conducting SrTi_0.65Fe_0.35O_3-x film. Surface exchange coefficients were evaluated as a function of oxygen activity in the film, controlled by gas partial pressure and/or DC bias applied across the ionically conducting yttria-stabilized zirconia substrate. Changes in measured light transmission through the film over time (relaxations) resulted from optical absorption changes in the film corresponding to changes in its oxygen and oxidized Fe (~Fe⁴⁺) concentrations; such relaxation profiles were successfully described by the equation for surface exchange-limited kinetics appropriate for the film geometry. The k_chem values obtained by OTR were significantly lower than the AC-IS derived k_chem values and k_q values multiplied by the thermodynamic factor (bulk or thin film), suggesting a possible enhancement in k by the metal current collectors (Pt, Au). Long-term degradation in k_chem and k_q values obtained by AC-IS was also attributed to deterioration of the porous Pt current collector, while no significant degradation was observed in the optically derived k_chem values. The results suggest that, while the current collector might influence measurements by AC-IS, the OTR method offers a continuous, in situ, and contact-free method to measure oxygen exchange kinetics at the native surfaces of thin films.