Cathode materials for solid oxide fuel cells: a review

Electrocatalysis in Solid Oxide Fuel Cells and Electrolyzers

Interest in energy-to-X and X-to-energy (where X represents green hydrogen, carbon-based fuels, or ammonia) technologies has expanded the field of electrochemical conversion and storage. Solid oxide electrochemical cells (SOCs) are among the most promising technologies for these processes. Their unmatched conversion efficiencies result from favorable thermodynamics and kinetics at elevated operating temperatures (400-900 °C). These solid-state electrochemical systems exhibit flexibility in reversible operation between fuel cell and electrolysis modes and can efficiently utilize a variety of fuels. However, electrocatalytic materials at SOC electrodes remain nonoptimal for facilitating reversible operation and fuel flexibility. In this Review, we explore the diverse range of electrocatalytic materials utilized in oxygen-ion-conducting SOCs (O-SOCs) and proton-conducting SOCs (H-SOCs). We examine their electrochemical activity as a function of composition and structure across different electrochemical reactions to highlight characteristics that lead to optimal catalytic performance. Catalyst deactivation mechanisms under different operating conditions are discussed to assess the bottlenecks in performance. We conclude by providing guidelines for evaluating the electrochemical performance of electrode catalysts in SOCs and for designing effective catalysts to achieve flexibility in fuel usage and mode of operation.

A-Site Engineering of the High-Entropy Perovskite Pr0.4La0.4Ba0.4Sr0.4Ca0.4Fe2O5+δ Cathode for Intermediate-Temperature SOFCs

Mixed-oxygen ionic and electronic conduction is crucial for the cathode materials of solid oxide fuel cells, ensuring high efficiency and low-temperature operation. However, the electronic and oxygen ionic conductivity of traditional Fe-based layered perovskite cathode materials is low, resulting in insufficient oxygen reduction reactivity. Herein, a type of high-entropy perovskite oxide consisting of five equimolar metals, Pr0.4La0.4Ba0.4Sr0.4Ca0.4Fe2O5+δ (PLBSCF), a high-performance cobalt-free cathode derived from the PrBaFe2O5+δ (PBF), is proposed. Such A-site engineering could not only increase the oxygen vacancy concentration of PLBSCF but also give higher conductivity than PBF, thus significantly reducing the polarization impedance of the symmetric cell to only 0.052 Ω·cm2 at 750 °C. The good output performance of a single cell is also realized. The peak power density of the single cell with PLBSCF-Ce0.9Gd0.1O2-δ (GDC) as the cathode at 750 °C was 0.853 W·cm-2. Additionally, the single cell with the PLBSCF cathode exhibits a good durable performance of 100 h at 750 °C. Combining the distribution of relaxation time analysis, it can be seen that the enhancement of the oxygen reduction reaction is due to the reduction of intermediate-frequency and low-frequency resistance, indicating an improvement in the charge transfer process and adsorption/dissociation process of molecular oxygen.

Novel Anion-Doped Cathode Material SrFe1-x Si x O3-δF y for Intermediate-Temperature Solid Oxide Fuel Cells

SrFe1-x Si x O3-δF y cathode materials (x = 0.05, 0.1, 0.15; y = 0, 0.1, 0.5) were prepared via a solid-state method. X-ray diffraction results show that the synthesized F doping samples were perovskite structure. X-ray photoelectron spectroscopy findings show that F- anions were doped into SrFe1-x Si x O3-δ. Transmission electron microscopy and energy-dispersive spectroscopy were performed to analyze the microstructure and element distribution in the materials, respectively. Double-layer composite cathode symmetric cells were prepared through a screen printing method. Scanning electron microscopy images revealed that the double-layer composite cathode adhered well to the electrolyte. The doping with F- can increase the coefficient of thermal expansion of SrFe1-x Si x O3-δ. The electrochemical impedance spectroscopy results indicate that the oxygen transport capacity of the SrFe0.95Si0.05O3-δ material can be improved by doping with F-, but such a method can decrease the oxygen transport capacity of SrFe0.9Si0.1O3-δ. At 800 °C, the peak power density of the single cell supported by an anode and SrFe0.9Si0.1O3-δF0.1 as the cathode reached 388.91 mW/cm2. Thus, the incorporation of F- into SrFe1-x Si x O3-δ cathode materials can improve their electrochemical performance and enable their application as cathode materials for solid-oxide fuel cells.

Development of Nanostructured Lanthanum Strontium Cobalt Ferrite/Gadolinian-Doped Ceria Composite Electrodes of Solid Oxide Cells Formed by In Situ Polarization

In the development of nanoscale oxygen electrodes of high-temperature solid oxide cells (SOCs), the interface formed between the nanoelectrode particles and the electrolyte or electrolyte scaffolds is the most critical. In this work, a new synthesis technique for the fabrication of nanostructured electrodes via in situ electrochemical polarization treatment is reported. The lanthanum strontium cobalt ferrite (LSCF) precursor solution is infiltrated into a gadolinia-doped ceria (GDC) scaffold presintered on a yttria-stabilized zirconia (YSZ) electrolyte, followed by in situ polarization current treatment at SOC operation temperatures. Electrode ohmic and polarization resistances decrease with an increase in the polarization current treatment. Detailed microstructure analysis indicates the formation of a convex-shaped interface between the LSCF nanoparticles (NPs) and the GDC scaffold, very different from the flat contact between LSCF and GDC observed after heating at 800 °C with no polarization current treatment. The embedded LSCF NPs on the GDC scaffold contribute to the superior stability under both fuel cell and electrolysis operation conditions at 750 °C and a high peak power density of 1.58 W cm-2 at 750 °C. This work highlights a novel and facile route to in situ construct a stable and high-performing nanostructured electrode for SOCs.

High-Temperature Protonic Conduction in La2 NiO4+ δ -Based Ruddlesden-Popper Type Oxides: Correlation with Concentration of Interstitial Oxide Ions

Oxygen-excess La2 NiO4+ δ (LNO) conducts oxide ions, electron holes, and hydroxide ions simultaneously on exposing to wet oxygen, exhibiting the potential as a cathode material in protonic ceramic fuel cells. Since the incorporation of protons in oxygen-excess LNO is via the hydration reaction assisted by interstitial oxide ions, in this work, the concentration of interstitial oxide ions is reduced and increased by substituting Ni with Cu and Co, respectively. A higher concentration of interstitial oxide ions leads to a high proton concentration, indicating the predominant role of interstitial oxide ions in the hydration reaction, different from that in the oxygen-deficient oxides, where protons are introduced by dissociative absorption of water molecules by oxygen vacancies. The theoretical calculation indicates that protons in Co-doped LNO prefer to locate between the interstitial oxide ions and unshared apical oxide ions. A trapping effect is found between protons and the oxide ions near Cu, leading to decreased proton mobility. Protonic conductivity at 400-575 °C is then directly measured by a Hebb-Wagner direct current polarization method with La0.99 Ca0.01 NbO4- δ as the blocking electrode, enabling the observation that Co-doped LNO has the highest protonic conductivity among the samples studied in this work.

A High Temperature Harvestorer Based on a Photovoltaic Cell and an Oxygen Ion Battery

Hybrid devices for combined energy harvesting and storage, i.e., harvestorers, are attractive solutions for powering small autonomous devices (e.g., "smart appliances", Internet of things nodes), which are ever more prominent as the digitalization and technologization of our society progresses. A concept for a high temperature (HT) harvestorer is presented, and the operational characteristics of a prototype device are discussed. It is based on photovoltaic (PV) energy harvesting and HT electrochemical energy storage. The HT-PV cells employ SrTiO3/La0.9Sr0.1CrO3-δ heterojunctions for energy harvesting and produce photovoltages up to 1 V and photocurrents of several mA cm-2 upon UV illumination at 350 °C. Electrochemical energy storage is realized by oxygen ion battery (OIB), a device based on mixed ionic and electronic conducting oxide thin film electrodes and an yttria stabilized zirconia electrolyte. The OIB exhibits capacities of up to 11 mC cm-2 (3 μA h cm-2) at 0.6 V (350 °C). A prototype harvestorer device was fabricated by integrating an HT-PV and an OIB cell into one device. This harvestorer was operated over several cycles consisting of harvesting and storing energy under illumination, followed by retrieval of the stored energy without illumination. Up to 3.5 mJ cm-2 (1 μW h cm-2) was stored with energy efficiencies up to 67%. Approaches for further optimization are discussed.

The effect of temperature and oxygen partial pressure on the concentration of iron and manganese ions in La1/3Sr2/3Fe1-xMnxO3-δ

The oxygen content was measured in cubic perovskite-type La1/3Sr2/3Fe1-xMnxO3-δ (x = 0.1, 0.17, 0.25, and 1/3) in the range of oxygen partial pressure from 10-22 to 0.5 atm at 750-950 °C with a step of 50 °C by coulometric titration. Gradual removal of oxygen from the oxides during the measurements was carried out until the stability limit was achieved and the reductive decomposition began. An increase in manganese content was shown to lead to a decrease in the stability of La1/3Sr2/3Fe1-xMnxO3-δ under reducing conditions. The obtained data on oxygen content were used for defect chemistry modeling in the oxides. The enthalpy of the Fe3+ to Fe4+ and Mn3+ to Mn4+ oxidation reactions (ΔHox0) was determined to be -103.2 ± 0.3 and -250 ± 2 kJ mol-1, respectively, for the x = 0.1 composition, and increased slightly with increasing manganese content. The large difference in ΔHox0 determines a strong distinction between the behavior of iron and manganese in perovskite-type oxides. An increase in manganese content in La1/3Sr2/3Fe1-xMnxO3-δ was found to lead to a decrease in the concentration of Fe4+ ions, but did not affect the concentration of Fe2+ ions. The impact of La/Sr ratio was evaluated by comparison of the obtained data with that for La0.5Sr0.5Fe1-xMnxO3-δ, and found to be different for iron and manganese. An increase in lanthanum fraction causes a decrease in the concentration of Fe2+ ions and an increase in the concentration of Mn2+ under reducing conditions.

Evolution of the surface atomic structure of multielement oxide films: curse or blessing?

Atomically resolved scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) are used to gain atomic-scale insights into the heteroepitaxy of lanthanum-strontium manganite (LSMO, La1-xSrxMnO3-δ, x ≈ 0.2) on SrTiO3(110). LSMO is a perovskite oxide characterized by several composition-dependent surface reconstructions. The flexibility of the surface allows it to incorporate nonstoichiometries during growth, which causes the structure of the surface to evolve accordingly. This happens up to a critical point, where phase separation occurs, clusters rich in the excess cations form at the surface, and films show a rough morphology. To limit the nonstoichiometry introduced by non-optimal growth conditions, it proves useful to monitor the changes in surface atomic structures as a function of the PLD parameters and tune the latter accordingly.

Enhanced Oxygen Evolution Rate and Anti-interfacial Delamination Property of the SrCo0.9Ta0.1O3-δ@La0.6Sr0.4Co0.2Fe0.8O3-δ Oxygen-Electrode for Solid Oxide Electrolysis Cells

Interfacial delamination between the oxygen-electrode and electrolyte is a significant factor impacting the reliability of solid oxide electrolysis cells (SOECs) when operating at high voltages. The most effective method to mitigate this delamination is to decrease the interfacial oxygen partial pressure, which can be accomplished by amplifying the oxygen exsolution rate and the O2- transport rate of the oxygen-electrode. In this study, a SrCo0.9Ta0.1O3-δ (SCT) film with an outstanding oxygen surface exchange coefficient and an outstanding O2- conductivity was introduced onto the La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) surface by infiltration. This composite oxygen-electrode exhibited a notably high electrochemical catalytic activity primarily due to the significantly improved O2- transport and oxygen surface exchange rate. Single cells with a 15-LSCF oxygen-electrode achieved a peak power density of 1.33 W cm-2 at 700 °C and a current density of 1.25 A cm-2 at 1.3 V (60% H2O-H2) at 750 °C. Additionally, an electrolysis cell with a 15 wt % SCT-infiltrated LSCF oxygen-electrode demonstrated stable operation even at high current densities for over 330 h with no noticeable delamination. The remarkable durability of the 15-LSCF oxygen-electrode can be attributed to the boosted oxygen exsolution reaction (OER) activity and the suppression of Sr segregation due to SCT infiltration. The impressive OER activity and resistance to interfacial delamination make the 15-LSCF a promising candidate for a composite oxygen-electrode in SOECs.

Heuristic Approach to Predict the Performance Degradation of a Solid Oxide Fuel Cell Cathode

The present work aims to predict the degradation in the performance of a solid oxide fuel cell (SOFC) cathode owing to cation interdiffusion between the electrolyte and cathode and surface segregation. Cation migration in the (La0.60Sr0.40)0.95Co0.20Fe0.80O3-x (LSCF)-Gd0.10Ce0.90O1.95 (GDC) composite cathode is evaluated in relation to time up to 1000 h using scanning transmission electron microscopy (STEM)-energy-dispersive X-ray spectroscopy (EDXS). The resulting insulating phase formed within the GDC interlayer is quantified by means of the volume fraction using a two-dimensional (2D) image analysis technique. For the very first time, the amount of the insulating phase in the GDC interlayer is quantified, and the corresponding performance degradation of the LSCF cathode is predicted. Mathematical relationships are established for the estimation of degradation due to surface segregation of the cathode. The ohmic resistance between the cathode and the GDC interlayer/electrolyte interface and the polarization resistance of the cathode, characterized by electrochemical impedance spectroscopy (EIS), show an excellent match with the predicted results. The combined degradation analysis and modeling for the cathode lifetime prediction provide a systematic understanding of the time-dependent cation migration and segregation behavior.

Unveiling the Electrocatalytic Activity of the GdBa0.5Sr0.5Co2-xCuxO5+δ (x ≥ 1) Oxygen Electrodes for Solid Oxide Cells

The A-site cation-ordered GdBa0.5Sr0.5Co2-xCuxO5+δ (GBSCC) double perovskites are evaluated regarding the development of high-performance oxygen electrodes for reversible solid oxide cells (rSOCs). The aims are to maximally decrease the content of toxic and expensive cobalt by substitution with copper while at the same time improving or maintaining the required thermomechanical and electrocatalytic properties. Studies reveal that compositions with 1 ≤ x ≤ 1.15 are particularly interesting. Their thermal and chemical expansions are decreased, and sufficient transport properties are observed. Complementary density functional theory calculations give deeper insight into oxygen defect formation in the considered materials. Chemical compatibility with La0.8Sr0.2Ga0.8Mg0.2O3-δ (LSGM) and Ce0.9Gd0.1O2-δ (GDC) solid electrolytes is evaluated. It is documented that the GdBa0.5Sr0.5Co0.9Cu1.1O5+δ oxygen electrode enables obtaining very low electrode polarization resistance (Rp) values of 0.017 Ω cm2 at 850 °C as well as 0.111 Ω cm2 at 700 °C, which is lower in comparison to that of GdBa0.5Sr0.5CoCuO5+δ (respectively, 0.026 and 0.204 Ω cm2). Systematic distribution of relaxation times analyses allows studies of the electrocatalytic activity and distinguishing elementary steps of the electrochemical reaction at different temperatures. The rate-limiting process is found to be oxygen atom reduction, while the charge transfer at the electrode/electrolyte interface is significantly better with LSGM. The studies also allow elaborating on the catalytic role of the Ag current collector as compared with Pt. The electrodes manufactured using materials with x = 1 and 1.1 permit reaching high power outputs, exceeding 1240 mW cm-2 at 850 °C and 1060 mW cm-2 at 800 °C, for the LSGM-supported cells, which can also work in the electrolysis mode.

Cu-doped Nd0.6Sr0.4Co1-xCuxO3-δ (x = 0, 0.05, 0.1, 0.15, 0.2) as the cathode for intermediate-temperature solid oxide fuel cells

Nd_0.6Sr_0.4Co_1-xCu_xO_3-δ (x = 0, 0.05, 0.1, 0.15, 0.2) (NSCC_x) was prepared by replacing Co with Cu. Its chemical compatibility, electrical conductivity, and electrochemical properties were studied by X-ray powder diffractometry, scanning electron microscopy, and X-ray photoelectron spectroscopy. The conductivity, AC impedance spectra, and output power of the single cell were tested in an electrochemical workstation. Results showed that the thermal expansion coefficient (TEC) and electrical conductivity of the sample decreased with the increase in Cu content. The TEC of NSCC_0.1 decreased by 16.28% in the temperature range of 35 °C-800 °C, and its conductivity was 541 S cm^-1 at 800 °C. Furthermore, a single cell was constructed with NSCC_x as the cathode, NiO-GDC as the anode, and GDC as the electrolyte. The peak power of the cell at 800 °C was 444.87 mW·cm^-2, which was similar to that of the undoped sample. Compared with the undoped NSCC, NSCC_0.1 showed lower TEC while maintaining its output power. Therefore, this material can be used as a cathode for solid oxide fuel cells.

Cobalt-Free Double Perovskite Oxide as a Promising Cathode for Solid Oxide Fuel Cells

Double perovskite oxide PrBaFe₂O_5+δ is a potential cathode material for intermediate-temperature solid oxide fuel cells. To improve its electrochemical performance, the trivalent element Ga is investigated to partially replace Fe, forming PrBaFe_2-xGa_xO_5+δ (PBFGx, x = 0.05, 0.1, and 0.15). The doping effects on physicochemical properties and electrochemical properties are analyzed regarding the phase structures, element valence states, amount of oxygen vacancies, content of oxygen species, oxygen surface exchange coefficients (k_chem), electrochemical polarization resistance, and single-cell performance. Specifically, PBFG0.1 exhibits improved k_chem, such as a 19% improvement from 4.09 × 10^-4 to 4.86 × 10^-4 cm s^-1 at 750 °C, due to the increased concentration of reactive oxygen species and oxygen vacancies. Consequently, the interfacial polarization resistance is decreased by 28% from 0.057 to 0.041 Ω cm² at 800 °C. The subreaction steps of the oxygen reduction reaction in the PBFG0.1 cathode are further investigated, which suggests that the oxygen dissociation process is greatly enhanced by doping Ga. Meanwhile, doping Ga increases the peak power density of the anode-supported single cell by 36% from 629 to 856 mW cm^-2 at 800 °C. The single cell with the PBFG0.1 cathode also exhibits good stability in 100 h of long-term operation at 750 °C.

Peculiar Properties of the La0.25Ba0.25Sr0.5Co0.8Fe0.2O3-δ Perovskite as Oxygen Reduction Electrocatalyst

The electrochemical reduction of molecular oxygen is a fundamental process in Solid Oxide Fuel Cells and requires high efficiency cathode materials. Two La0.25Ba0.25Sr0.5Co0.8Fe0.2O3-δ-based perovskite compounds were prepared by solution combustion synthesis, and characterized for their structural, microstructural, surface, redox and electrochemical properties as potential cathodes in comparison with Ba0.5Sr0.5Co0.8Fe0.2O3-δ and La0.5Sr0.5Co0.8Fe0.2O3-δ perovskites. Results highlighted that calcination at 900 °C led to a "bi-perovskite heterostructure", where two different perovskite structures coexist, whereas at higher calcination temperatures a single-phase perovskite was formed. The results showed the effectiveness of the preparation procedures in co-doping the A-site of perovskites with barium and lanthanum as a strategy to optimize the cathode's properties. The formation of nanometric heterostructure co-doped in the A-site evidenced an improvement in oxygen vacancies' availability and in the redox properties, which promoted both processes: oxygen adsorption and oxygen ions drift, through the cathode material, to the electrolyte. A reduction in the total resistance was observed in the case of heterostructured material.

Tuning Surface Acidity of Mixed Conducting Electrodes: Recovery of Si-Induced Degradation of Oxygen Exchange Rate and Area Specific Resistance

Metal oxides are an important class of functional materials, and for many applications, ranging from solid oxide fuel/electrolysis cells, oxygen permeation membranes, and oxygen storage materials to gas sensors (semiconducting and electrolytic) and catalysts, the interaction between the surface and oxygen in the gas phase is central. Ubiquitous Si-impurities are known to impede this interaction, commonly attributed to the formation of glassy blocking layers on the surface. Here, the surface oxygen exchange coefficient (k_chem ) is examined for Pr_0.1 Ce_0.9 O_2-δ (PCO), a model mixed ionic electronic conductor, via electrical conductivity relaxation measurements, and the area-specific resistance (ASR) by electrochemical impedance spectroscopy. It is demonstrated that even low silica levels, introduced by infiltration, depress k_chem by a factor 4000, while the ASR increases 40-fold and we attribute this to its acidity relative to that of PCO. The ability to fully regenerate the poisoned surface by the subsequent addition of basic Ca- or Li-species is further shown. This ability to not only recover Si-poisoned surfaces by tuning the relative surface acidity of an oxide surface, but subsequently outperform the pre-poisoned response, promises to extend the operating life of materials and devices for which the catalytic oxygen/solid interface reaction is central.

Transition-Metal-Based Compounds for Electrochemical Energy Conversion Processes

The era of ever-growing worldwide energy requirements demands the development of new methods of energy conversion, where the design of novel materials and the improvement of the efficiency of existing ones are of great importance [...].

Evaluation of Structural and Functional Properties of La0.6Sr0.4MnO3 Perovskite Prepared by the Fast Solution Combustion Approach

A series of La0.6Sr0.4MnO3 (LSM) perovskite was made using the rapid solution combustion method, which was calcined by varying the temperatures. In order to determine how the calcination temperature affected the nanopowders produced and calcined at various temperatures, their microstructural, morphological, compositional, optical, and electrical properties were analyzed using corresponding characterization tools. The XRD results showed the coexistence of the rhombohedral polymorphs R-3c and Pm-3m for the perovskite phase under a calcination temperature of 1400 °C, which were eliminated with increased calcination temperature. The average grain size was found to increase with increasing calcination temperature. The EDS analysis showed better agreement of the stoichiometry with the theoretical composition. The apparent porosity decreased with increasing temperature due to the coalescence of sintering pores. The sample obtained after calcination at 1500 °C showed 10.3% porosity. The hardness also improved with increasing calcination temperature and reached a maximum value of 0.4 GPa, which matched the bulk density. A similar trend was observed in the resistivity studies as a function of temperature, and all the samples exhibited a low resistivity of ~1.4 Ω·cm in the temperature range of 500–600 °C. The optical characterization showed broad absorption at 560–660 nm and bandwidth values between 3.70 and 3.95 eV, according to the applied heat treatment.

High-Entropy Materials in SOFC Technology: Theoretical Foundations for Their Creation, Features of Synthesis, and Recent Achievements

In this review, recent achievements in the application of high-entropy alloys (HEAs) and high-entropy oxides (HEOs) in the technology of solid oxide fuel cells (SOFC) are discussed for the first time. The mechanisms of the stabilization of a high-entropy state in such materials, as well as the effect of structural and charge factors on the stability of the resulting homogeneous solid solution are performed. An introduction to the synthesis methods for HEAs and HEOs is given. The review highlights such advantages of high-entropy materials as high strength and the sluggish diffusion of components, which are promising for the use at the elevated temperatures, which are characteristic of SOFCs. Application of the medium- and high-entropy materials in the hydrocarbon-fueled SOFCs as protective layers for interconnectors and as anode components, caused by their high stability, are covered. High-entropy solid electrolytes are discussed in comparison with traditional electrolyte materials in terms of conductivity. High-entropy oxides are considered as prospective cathodes for SOFCs due to their superior electrochemical activity and long-term stability compared with the conventional perovskites. The present review also determines the prioritizing directions in the future development of high-entropy materials as electrolytes and electrodes for SOFCs operating in the intermediate and low temperature ranges.

Tuning Cu-Content La1-xSrxNi1-yCuyO3-δ with Strontium Doping as Cobalt-Free Cathode Materials for High-Performance Anode-Supported IT-SOFCs

Cu-content La_1-xSr_xNi_1-yCu_yO_3-δ perovskites with A-site strontium doping have been tuned as cobalt-free cathode materials for high-performance anode-supported SOFCs, working at an intermediate-temperature range. All obtained oxides belong to the R-3c trigonal system, and phase transitions from the R-3c space group to a Pm-3m simple perovskite have been observed by HT-XRD studies. The substitution of lanthanum with strontium lowers the phase transition temperature, while increasing the thermal expansion coefficient (TEC) and oxygen non-stoichiometry δ of the studied materials. The thermal expansion is anisotropic, and TEC values are similar to commonly used solid electrolytes (e.g., 14.1 × 10^-6 K^-1 for La_0.95Sr_0.05Ni_0.5Cu_0.5O_3-δ). The oxygen content of investigated compounds has been determined as a function of temperature. All studied materials are chemically compatible with GDC-10 but react with LSGM and 8YSZ electrolytes. The anode-supported SOFC with a La_0.95Sr_0.05Ni_0.5Cu_0.5O_3-δ cathode presents an excellent power density of 445 mW·cm^-2 at 650 °C in humidified H₂. The results indicate that La_1-xSr_xNi_1-yCu_yO_3-δ perovskites with strontium doping at the A-site can be qualified as promising cathode candidates for anode-supported SOFCs, yielding promising electrochemical performance in the intermediate-temperature range.

Nonstoichiometric Strontium Ferromolybdate as an Electrode Material for Solid Oxide Fuel Cells

This review is devoted to the application of Sr2FeMoO6−δ (SFM) and Sr2F1.5Mo0.5O6−δ (SF1.5M) in La1−xSrxGa1−yMgyO3−δ (LSGM)-based SOFCs. We consider the most relevant physical properties (crystal structure, thermodynamic stability, iron and molybdenum valence states, oxygen vacancy formation and oxygen non-stoichiometry, electrical conductivity), A- and B-site ion substitution, and the performance of SF1+xM SOFCs (polarization resistance, operation with hydrogen, operation with hydrocarbons and methanol). Their properties can be tailored to a particular application by the substitution of different metal cations into their lattices. SF1+xM materials are excellent catalysts in hydrocarbon oxidation and can prevent carbon deposition due to the ability to exchange lattice oxygen with the gaseous phase. Moreover, they are sulfur tolerant. This opens the way to direct hydrocarbon-fueled SOFCs, eliminating the need for external fuel reforming and sulfur removal components. Such SOFCs can be greatly simplified and operate with much higher overall efficiency, thus contributing to the solution to the lack of energy problem in our modern world.

Solid-State Electrochemistry and Solid Oxide Fuel Cells: Status and Future Prospects

Solid-state electrochemistry (SSE) is an interdisciplinary field bridging electrochemistry and solid-state ionics and deals primarily with the properties of solids that conduct ions in the case of ionic conducting solid electrolytes and electrons and/or electron holes in the case of mixed ionic and electronic conducting materials. However, in solid-state devices such as solid oxide fuel cells (SOFCs), there are unique electrochemical features due to the high operating temperature (600–1 000 °C) and solid electrolytes and electrodes. The solid-to-solid contact at the electrode/electrolyte interface is one of the most distinguished features of SOFCs and is one of the fundamental reasons for the occurance of most importance phenomena such as shift of the equipotential lines, the constriction effect, polarization-induced interface formation, etc. in SOFCs. The restriction in placing the reference electrode in solid electrolyte cells further complicates the SSE in SOFCs. In addition, the migration species at the solid electrode/electrolyte interface is oxygen ions, while in the case of the liquid electrolyte system, the migration species is electrons. The increased knowledge and understanding of SSE phenomena have guided the development of SOFC technologies in the last 30–40 years, but thus far, no up-to-date reviews on this important topic have appeared. The purpose of the current article is to review and update the progress and achievements in the SSE in SOFCs, largely based on the author’s past few decades of research and understanding in the field, and to serve as an introduction to the basics of the SSE in solid electrolyte devices such as SOFCs.

Graphical abstract

BaFe0.875Re0.125O3−δ and BaFe0.75Ta0.25O3−δ as potential cathodes for solid-oxide fuel-cells: a structural study from neutron diffraction data

In this work, two new perovskites of composition BaFe0.875Re0.125O3−δ and BaFe0.75Ta0.25O3−δ , designed from ab-initio calculations to fulfill different requisites of cathode materials for solid-oxide fuel cells (SOFC), were prepared and studied from the structural point of view from neutron powder diffraction (NPD) data. They are both derivatives of BaFeO3 hexagonal perovskite (space group P6 3 /mmc), typified as the 6H polytype, stabilized when the perovskite tolerance factor slightly overpasses the unity. Whereas BaFe0.875Re0.125O3−δ keeps this structural type, as demonstrated in this crystallographic study from NPD data at 295 and 4 K, with unit-cell parameters a = 5.70177(7); c = 14.0334(2) Å at 295 K, the second material, BaFe0.75Ta0.25O3−δ , is cubic and can be defined in the Pm-3m space group, corresponding of the perovskite arystotype, with a = 4.05876(3) Å. A conspicuous oxygen deficiency is observed, with a refined stoichiometry of 2.86(3) per formula unit. The anisotropic displacement factors for oxygen atoms in this last material are flattened disks perpendicular to the (Fe,Ta)-O-(Fe,Ta) direction, suggesting a dynamic tilting of the octahedra that could be related to the oxygen motion via oxygen vacancies across the structure. This is a pre-requisite for functional mixed-ionic-electronic (MIEC) materials performing as cathodes in SOFC.

BaCO3 Nanoparticles-Modified Composite Cathode with Improved Electrochemical Oxygen Reduction Kinetics for High-Performing Ceramic Fuel Cells

The effects of the electrochemical oxygen reduction reaction (ORR) on the surface of single-phase perovskite cathodes are well understood, but its potential for use in a complex system consisting of different material types is unexplored. Herein, we report how BaCO3 nanoparticles-modified La0.6Sr0.4Co0.2Fe0.8O3-δ-Gd0.2Ce0.8O2-δ (LSCF–GDC)-composite cathodes improved the electrochemical oxygen reduction kinetics for high-performing ceramic fuel cells. Both X-ray diffraction (XRD) and thermogravimetric analysis (TGA) studies reveal that BaCO3 is stable, and that it does not show any solid-state reaction with LSCF–GDC at SOFCs’ required operating temperature. The electrochemical conductivity relaxation (ECR) study reveals that during the infiltration of BaCO3 nanoparticles into LSCF–GDC, the surface exchange kinetics (Kchem) are enhanced up to a factor of 26.73. The maximum power density of the NiO-YSZ anode-support cell is increased from 1.08 to 1.48 W/cm2 via surface modification at 750 °C. The modified cathode also shows an ultralow polarization resistance (Rp) of 0.027 Ω.cm2, which is ~4.4 times lower than that of the bare cathode (~0.12 Ω.cm2) at 750 °C. Such enhancement can be attributed to the accelerated oxygen surface exchange process, possibly through promoting the dissociation of oxygen molecules via the infiltration of BaCO3 nanoparticles. The density functional theory (DFT) illustrates the interaction mechanism between oxygen molecules and the BaCO3 surface.

Synthesis and Evaluation of LaBaCo2-xMoxO5+δ Cathode for Intermediate-Temperature Solid Oxide Fuel Cells

LaBaCo_2-xMo_xO_5+δ (LBCMx, x = 0-0.08) cathodes synthesized by a sol-gel method were evaluated for intermediate-temperature solid oxide fuel cells. The limit of the solid solubility of Mo in LBCMx was lower than 0.08. As the content of Mo increased gradually from 0 to 0.06, the thermal expansion coefficient decreased from 20.87 × 10^-6 K^-1 to 18.47 × 10^-6 K^-1. The introduction of Mo could increase the conductivity of LBCMx, which varied from 464 S cm^-1 to 621 S cm^-1 at 800 °C. The polarization resistance of the optimal cathode LBCM0.04 in air at 800 °C was 0.036 Ω cm², reduced by a factor of 1.67 when compared with the undoped Mo cathode. The corresponding maximum power density of a single cell based on a YSZ electrolyte improved from 165 mW cm^-2 to 248 mW cm^-2 at 800 °C.

Ge-Containing Oxide-Ion Conductors with CaEu2Ge3O10-Type Structure Discovered by the Bond-Valence Method and Experiments

In the present work, we have discovered the first example of a CaEu₂Ge₃O₁₀-type oxide-ion conductor, Ca_1.05Sm_1.95Ge₃O_9.975. The CaEu₂Ge₃O₁₀-type structure was selected by screening 624 Ge-containing materials by the bond-valence-based-energy calculations. CaEu₂Ge₃O₁₀-type CaEu₂Ge₃O₁₀, CaGd₂Ge₃O₁₀, and a new material CaSm₂Ge₃O₁₀ were synthesized. CaSm₂Ge₃O₁₀ showed the highest electrical conductivity among these three materials. Ca_1+xSm_2-xGe₃O_10-x/2 (x = 0.05, 0.1, and 0.2) were also synthesized, and we found that Ca_1.05Sm_1.95Ge₃O_9.975 exhibited the highest conductivity of 1.2 × 10^-5 S cm^-1 at 1373 K. Oxygen transport numbers in Ca_1.05Sm_1.95Ge₃O_9.975 were determined to be 0.64(5) at 1073 K and 0.65(8) at 1123 K, which indicates that the major carrier is the oxide ion. Therefore, CaEu₂Ge₃O₁₀-type Ca_1.05Sm_1.95Ge₃O_9.975 is a new structure family of oxide-ion conductors. The crystal structures of the new materials CaSm₂Ge₃O₁₀ and Ca_1.05Sm_1.95Ge₃O_9.975 were successfully analyzed by the CaEu₂Ge₃O₁₀-type structure (space group P2₁/c) using the single-crystal X-ray diffraction data. The bond-valence-based-energy calculation for the refined crystal structure of Ca_1.05Sm_1.95Ge₃O_9.975 suggested that oxide ions migrate along the [2 0 1], [0 1 0], and [12.88 6.43 1] directions with energy barriers of 0.88, 0.92, and 1.1 eV, respectively, which indicates three-dimensional oxide-ion diffusion in Ca_1.05Sm_1.95Ge₃O_9.975.

Developing a Novel Design for a Tubular Solid Oxide Fuel Cell Current Collector

This study presents a novel current collector design for a tubular SOFC and numerically investigates its performance. The new current collector design has a flow channel with a trapezoidal shape. Several channels, such as four, eight, and twelve, are investigated, and their effects on cell performance are reported and compared. Additionally, a traditional tubular SOFC and the newly developed design are presented. The equations of mass conservation, momentum, charge transport, and energy were considered in the numerical model, and the ANSYS Fluent SOFC module was used to solve the numerical model. The results show that the developed design performed better than the traditional design. The new design with twelve channels collected 0.384 A, higher than the other designs. Although the design with twelve channels gave a high concentration of hydrogen at the outlet compared to the designs with four and eight channels, it gave higher performance than the designs with four and eight channels. Increasing the number of channels in the developed design enhanced the cell performance significantly due to the increased contact area, leading to the efficient collection of the generated current.

Defect engineering of oxide surfaces: dream or reality?

In this brief perspective we analyze the present status of the field of defect engineering of oxide surfaces. In particular we discuss the tools and techniques available to generate, identify, quantify, and characterize point defects at oxide surfaces and the main areas where these centers play a role in practical applications.

Tuning the Phase Transition of SrFeO3-δ by Mn toward Enhanced Catalytic Activity and CO2 Resistance for the Oxygen Reduction Reaction

Developing high-performance cathodes with sufficient stability against CO₂ rooting in ambient atmosphere is crucial to realizing the practical application of solid-oxide fuel cells. Herein, the Mn dopant is investigated to regulate the phase structure and cathode performance of SrFeO_3-δ perovskites through partially replacing the B-site Fe. Compared with parent SrFeO_3-δ, Mn-doped materials, SrFe_1-xMn_xO_3-δ (x = 0.05 and 0.1), show stabilized cubic perovskites at room temperature. Meanwhile, doping Mn accelerates the oxygen reduction reaction process, showing a reduced polarization resistance of 0.155 Ω·cm² at 700 °C for SrFe_0.95Mn_0.05O_3-δ, which is less than 30% of SrFeO_3-δ. In addition, the Mn dopant improves the chemical oxygen surface exchange and bulk diffusion coefficients. Furthermore, Mn enhances the tolerance toward CO₂ corrosion in various CO₂ atmospheres. Density functional theory calculations also reveal that Mn can strengthen the structural stability and increase the activity for the oxygen reduction reaction.

Multi-Physical and Electrochemical Coupling Model for the Protonic Ceramic Fuel Cells with H+/e−/O2− Mixed Conducting Cathodes

A protonic ceramic fuel cell (PCFC) has great potential for medium temperature power generation. Its working process, however, is complicated and quite different from the traditional oxygen ionic solid oxide fuel cell (O2−-SOFC) and proton exchange membrane fuel cell (PEMFC). In this paper, a multi-physical model for the PCFC with H+/e−/O2− mixed conducting cathode is established, in which the fuel- and oxidant-diffusing processes; electron-, oxygen ion-, and proton-conducting processes; three electrochemical reactions; and their coupling working details are carefully considered. Taking Ni-BZCY/BZCY/BZCY-LSCF PCFC as an example, the validation of the model is well verified by good agreements with the experiment iop-Vop curves at different temperatures. The result shows that the cathodic electrochemical reactions will be concentrated to a small thickness near the electrolyte because of the greatly decreased ionic conductivity compared with the high electronic conductivity at an intermediate temperature. O2− within the PCFC cathode is only an intermediate transform substance between the electrons and protons. Thus, there is a peak oxygen ion current distribution within the composite cathode of PCFC. The cathodic oxygen reduction half reaction is found to be a key factor to dominate the total PCFC voltage loss at the intermediate temperature zone. The concentration polarization of anode-supported PCFC is small, due to the vapors that are generated in the cathode side instead of anode side.

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.

Protective Coatings for Ferritic Stainless Steel Interconnect Materials in High Temperature Solid Oxide Electrolyser Atmospheres

Stainless steel interconnect materials used in solid oxide fuel cells and electrolysers need to be coated to improve oxidation resistance and to mitigate Cr-vaporization. This work aimed to explore the optimal steel/coating combinations suitable for use in reversible solid oxide stacks and evaluated (Co,Mn)3O4 spinel, LaFeO3 perovskite, Ce/Co and Y-based coatings, on AISI441 and Crofer 22 APU steels. The coatings were evaluated based on measurements of mass gain and oxide scale thickness after exposure at 700 and 800 °C to fuel side (90 vol.% H2O/10 vol.% H2) and air/oxygen side (pure O2) atmospheres. In pure O2, the most efficient coatings for limiting oxide scale formation and Cr evaporation, compared to the bare steel, were (Co,Mn)3O4 and CeCo on Crofer 22 APU. In 90 vol.% H2O/10 vol.% H2, the Y-based coating showed the largest improvement in oxidation resistance.

Investigating oxygen reduction pathways on pristine SOFC cathode surfaces by in situ PLD impedance spectroscopy

The oxygen exchange reaction mechanism on truly pristine surfaces of SOFC cathode materials (La0.6Sr0.4CoO3-δ = LSC, La0.6Sr0.4FeO3-δ = LSF, (La0.6Sr0.4)0.98Pt0.02FeO3-δ = Pt:LSF, SrTi0.3Fe0.7O3-δ = STF, Pr0.1Ce0.9O2-δ = PCO and La0.6Sr0.4MnO3-δ = LSM) was investigated employing in situ impedance spectroscopy during pulsed laser deposition (i-PLD) over a wide temperature and p(O2) range. Besides demonstrating the often astonishing catalytic capabilities of the materials, it is possible to discuss the oxygen exchange reaction mechanism based on experiments on clean surfaces unaltered by external degradation processes. All investigated materials with at least moderate ionic conductivity (i.e. all except LSM) exhibit polarization resistances with very similar p(O2)- and T-dependences, mostly differing only in absolute value. In combination with non-equilibrium measurements under polarization and defect chemical model calculations, these results elucidate several aspects of the oxygen exchange reaction mechanism and refine the understanding of the role oxygen vacancies and electronic charge carriers play in the oxygen exchange reaction. It was found that a major part of the effective activation energy of the surface exchange reaction, which is observed during equilibrium measurements, originates from thermally activated charge carrier concentrations. Electrode polarization was therefore used to control defect concentrations and to extract concentration amended activation energies, which prove to be drastically different for oxygen incorporation and evolution (0.26 vs. 2.05 eV for LSF).

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.

Colloidal Processing of Y0.08Zr0.92O2/La0.80Sr0.20MnO3 Semi-Cells Using a Sr-Doped Lanthanum Manganite Synthesized by a Citrate Route

In this paper, the interface between yttria stabilized zirconia (Y_0.08Zr_0.92O₂, YSZ) electrolyte and Sr-doped lanthanum manganite (La_0.80Sr_0.20MnO₃, LSM) cathode for solid oxide fuel cells (SOFCs) is studied. For such a purpose, the combination of a suitable synthesis route for obtaining fine powders and simple aqueous colloidal shaping routes is proposed. The synthesis of nanosized particles of La_0.80Sr_0.20MnO₃ by a citrate route and their full characterization, including the colloidal stability and the densification and phase development determined by X-ray diffraction and electron microscopy at different temperatures, is reported. In a second step, YSZ tapes were obtained by aqueous tape casting and used as substrates for the preparation of LSM coatings by dip-coating using aqueous slurries. YSZ tapes were used either in the green state or after a pre-sintering treatment. Co-sintering at 1350 °C led to a sharp interface with excellent adhesion, also achieved when coating pre-sintered tapes. In both cases, the substrates are dense and the coatings are porous, with thicknesses of 85 and 60 μm for green and pre-sintered tapes, respectively. No diffusion of Zr and Y occurs at the LSM layer, but some diffusion of La and Mn towards the YSZ layer takes place.

Electronic excitation spectra of cerium oxides: from ab initio dielectric response functions to Monte Carlo electron transport simulations

Nanomaterials made of cerium oxides CeO₂ and Ce₂O₃ have a broad range of applications, from catalysts in automotive, industrial or energy operations to promising materials to enhance hadrontherapy effectiveness in oncological treatments. To elucidate the physico-chemical mechanisms involved in these processes, it is of paramount importance to know the electronic excitation spectra of these oxides, which are obtained here through high-accuracy linear-response time-dependent density functional theory calculations. In particular, the macroscopic dielectric response functions  of both bulk CeO₂ and Ce₂O₃ are derived, which compare remarkably well with the available experimental data. These results stress the importance of appropriately accounting for local field effects to model the dielectric function of metal oxides. Furthermore, we reckon the energy loss functions Im(-1/) of the materials, including the accurate evaluation of the momentum transfer dispersion from first-principles calculations. In this respect, by using Mermin-type parametrization we are able to model the contribution of different electronic excitations to the dielectric loss function. Finally, from the knowledge of the electron inelastic mean free path, together with the elastic mean free path provided by the relativistic Mott theory, we carry out statistical Monte Carlo (MC) electron transport simulations to reproduce the major features of the reported experimental reflection electron energy loss (REEL) spectra of cerium oxides. The good agreement with REEL experimental data strongly supports our approach based on MC modelling, whose main inputs were obtained using ab initio calculated electronic excitation spectra in a broad range of momentum and energy transfers.

Structural and Magnetic Properties of BaFeO2.667 Synthesized by Oxidizing BaFeO2.5 Obtained via Nebulized Spray Pyrolysis

A vacancy-ordered perovskite-type compound Ba₃Fe₃O₈ (BaFeO_2.667) was prepared by oxidizing BaFeO_2.5 (P2₁/c) with the latter compound obtained by a spray pyrolysis technique. The structure of Ba₃Fe₃O₈ was found to be isotypic to Ba₃Fe₃O₇F (P2₁/m) and can be written as Ba₃Fe³⁺₂Fe⁴⁺₁O₈. Mössbauer spectroscopy and ab initio calculations were used to confirm mixed iron oxidation states, showing allocation of the tetravalent iron species on the tetrahedral site, and octahedral as well as square pyramidal coordination for the trivalent species within a G-type antiferromagnetic ordering. The uptake and release of oxygen were investigated over a broad temperature range from room temperature to 1100 °C under pure oxygen and ambient atmosphere via a combination of DTA/TG and variable temperature diffraction measurements. The compound exhibited a strong lattice enthalpy driven reduction to monoclinic and cubic BaFeO_2.5 at elevated temperatures.