Solid Oxide Fuel Cell Materials Research Articles

Advancing Solid Oxide Fuel Cell Performance: Enhanced Electrochemical Properties of Pr1-xCaxBaFe2O5+δ Nanofiber Cathodes via Ca Doping.

The double perovskite oxide PrBaFe2O5+δ has great potential as a cathode material for solid oxide fuel cells (SOFCs). However, the electrochemical characteristics of Fe-based double perovskites are relatively inferior. To improve its electrochemical performance, Ca is investigated to partially replace Pr, forming Pr1-xCaxBaFe2O5+δ (PCBFx, x = 0.0-0.3) by an electrospinning technique. The PCBFx nanofibers exhibited a crystalline structure characterized by orthorhombic symmetry and space group P4/mmm. Furthermore, these PCBFx nanofibers displayed exceptional chemical compatibility with the Sm0.2Ce0.8O1.95 (SDC) electrolyte when sintered at a temperature of 900 °C for 5 h. The X-ray photoelectron spectroscopy (XPS) analysis reveals a progressive increase in the Fe4+ concentration as the Ca doping level rises. The polarization resistances (Rp) of the PCBF00, PCBF01, PCBF02, and PCBF03 nanofiber cathodes were 0.103, 0.079, 0.056, and 0.048 Ω cm2 at 750 °C. In the meantime, doping Ca increases the peak power density of the single cell by 46%, from 762.80 (PCBF00) to 1114.85 (PCBF03) mW cm-2 at 750 °C. The results demonstrate that PCBF03 double perovskite nanofibers exhibit great potential as cathode materials for SOFCs.

A medium-entropy engineered double perovskite oxide for efficient and CO2-tolerant cathode of solid oxide fuel cells

Developing highly active and robust cathodes remains challenging for solid oxide fuel cells (SOFCs). Therefore, we report an A-site medium-entropy double perovskite oxide PrBa0.5Sr0.25Ca0.2Ce0.05Co2O5+δ (PBSCCC) and its electrocatalytic activity and CO2 tolerance property are systematically probed. The PBSCCC cathode exhibits excellent oxygen reduction reaction (ORR) activity with a low polarization resistance of 0.027 Ω cm2 at 700 °C. More importantly, compared to PrBaCo2O5+δ (PBC) and PrBa0.8Ca0.2Co2O5+δ (PBCC) oxides, its CO2 resistance is prominently enhanced with significantly less BaCO3 and Co3O4 impurities formed. The superior ORR performance and structure stability are mainly attributed to its high electrical conductivity and significantly reduced surface cation segregations. Our results show that the potential of PBSCCC as a cathode material for SOFCs is enormous and also prove the rationality and feasibility of entropy engineering for cathode development.

Preparation and characterization of Pr1-xNdxBaFe2O5+δ nanofiber cathodes for solid oxide fuel cells

Double perovskite oxide Pr1-xNdxBaFe2O5+δ (PNBFx, x = 0.0–0.3) nanofiber were prepared using electrospinning and were evaluated as cathodes for solid oxide fuel cells (SOFCs). The crystalline structure, microstructure, and electrochemical performance of PNBFx were thoroughly investigated and discussed. The PNBFx nanofiber cathodes obtained after sintering at 800 °C crystallized in a tetragonal structure with the space group P4/mmm and had good chemical compatibility with SDC electrolytes at 900 °C for 4 h. Nd doping was found to increase the lattice parameter, while the average valence of praseodymium and iron content increased steadily in PNBFx. The Pr0.8Nd0.2BaFe2O5+δ (PNBF02) nanofiber cathode exhibited the best performance with a polarization resistance (Rp) of 0.072 Ω cm2 at 750 °C. When PNBF02 nanofiber was used as a single-cell cathode, the peak power density reached 1.52 W cm2 at 750 °C. All results indicate that double perovskite PNBF02 is a promising cathode material for SOFCs.

On the validation and applicability of multiphysics models for hydrogen SOFC

Solid oxide fuel cells (SOFC) are a viable alternative for environmentally-friendly conversion of hydrogen into energy and multiphysics simulation can be used to diminish the experimental effort to improve their efficiency. However, an appropriate model of the involved processes and their parameters must be chosen. This paper studies the effects of choice between Maxwell–Stefan and Fick’s law models, and uncertainty of electrode ionic conductivity σion and anodic reference exchange current density i0,ref,f, on cell performance as implemented in the COMSOL Multiphysics® software. In the case of Maxwell–Stefan, peak average power output increased by 21.9% as σion varies from 10−3 to 10−1 S/cm, while the model based on Fick’s law shows an increase of 55.2%. The Maxwell–Stefan model exhibits an increase in peak power of 6% as i0,ref,f ranges from 0.4 to 0.8 A/cm2, and the Fick’s law model an increase of 8.2%. The dependence of the Maxwell–Stefan model on σion is characterized as logarithmic in the studied range. The Maxwell–Stefan model is deemed preferable because its lower sensitivity to the studied parameters helps mitigate uncertainty. It is concluded that despite its limitations, multiphysics modeling is a useful tool for directing research on SOFC materials owing to its descriptive potential.

Nanostructured Materials for Enhanced Performance of Solid Oxide Fuel Cells: A Comprehensive Review

Solid oxide fuel cells (SOFCs) have emerged as promising candidates for efficient and environmentally friendly energy conversion technologies. Their high energy conversion efficiency and fuel flexibility make them particularly attractive for various applications, ranging from stationary power generation to portable electronic devices. Recently, research has focused on utilizing nanostructured materials to enhance the performance of SOFCs. This comprehensive review summarizes the latest advancements in the design, fabrication, and characterization of nanostructured materials integrated in SOFC. The review begins by elucidating the fundamental principles underlying SOFC operation, emphasizing the critical role of electrode materials, electrolytes, and interfacial interactions in overall cell performance, and the importance of nanostructured materials in addressing key challenges. It provides an in-depth analysis of various types of nanostructures, highlighting their roles in improving the electrochemical performance, stability, and durability of SOFCs. Furthermore, this review delves into the fabrication techniques that enable precise control over nanostructure morphology, composition, and architecture. The influence of nanoscale effects on ionic and electronic transport within the electrolyte and electrodes is thoroughly explored, shedding light on the mechanisms behind enhanced performance. By providing a comprehensive overview of the current state of research on nanostructured materials for SOFCs, this review aims to guide researchers, engineers, and policymakers toward the development of high-performance, cost-effective, and sustainable energy conversion systems.

Ni and Ru bimetal nanoparticles-anchored porous Mo2C nanorods used as an internal reforming catalyst layer for hydrocarbon-fueled solid oxide fuel cells

Solid oxide fuel cells (SOFCs) are considered the most efficient technology for the direct conversion of hydrocarbons to electrical energy. Although the commonly used Ni/YSZ cermet materials in SOFCs show excellent electrocatalytic properties in H2 fuel, they suffer from carbon deposition problem when hydrocarbon fuels are used. In this work, we study the use of Ni0·05Ru0.05-Mo2C with a porous nanorod-shaped morphology as an internal reforming catalyst layer for methane-fueled SOFCs to solve the degradation of cell performance due to carbon deposition. The electrochemical performance and coking tolerance of two types of cells with and without the catalyst layer were systematically studied using methane and hydrogen as fuels. With the catalyst layer, the single cell using methane as fuel exhibits an improvement in peak power density by 24%, increasing from 297 to 367 mW cm−2 and there was no diffusion barrier for fuel gases within the cell. Furthermore, the cell can stably run for 100 h in methane at 700 °C. The improvement of electrochemical performance is attributed to the synergistic effect of Ni and Ru sites anchored on the porous Mo2C nanorods catalyst based on strong metal-support interactions, promoting the effective catalytic conversion of CH4 to CO and H2. This study demonstrates that the integration of a catalyst layer for internal reforming is an effective strategy for the direct use of methane in SOFCs.

Mo-Doped LSCF as a Novel Coke-Resistant Anode for Biofuel-Fed SOFC.

Climate change and damage to the environment, as well as the limitations of fossil fuels, have pushed governments to explore infinite renewable energy options such as biofuels. Solid Oxide Fuel Cell (SOFC) is a sustainable energy device that transforms biofuels into power and heat. It is now being researched to function at intermediate temperatures (600-700 °C) in order to prevent material deterioration and improve system life span. However, one of the major disadvantages of reducing the temperature is that carbon deposition impairs the electrochemical performance of the cell with a Ni-YSZ traditional anode. Here, molybdenum was doped into La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCFMo) as an innovative anode material with higher coke resistance and better phase stability under reducing conditions. X-ray diffraction (XRD) analysis showed increasing phase stability by increasing the Mo dopant. Electrochemical measurements proved that the LSCFMo anode is an active catalyst towards the methanol oxidation even at low temperatures as 600 °C, with an open circuit voltage (OCV) of 0.55 V, while GDC10 (Ga0.9Ce0.1O1.95) is used as the electrolyte. As an insightful result, no trace of any carbon deposition was found on the anode side after the tests. The combination of phase composition, morphological, and electrochemical studies demonstrate that LSCFMo is a suitable anode material for SOFCs fed by biofuels.

Ce-doping enhanced ORR kinetics and CO2 tolerance of Nd1-xCexBaCoFeO5+δ (x = 0–0.2) cathodes for solid oxide fuel cells

Addressing challenges in oxygen reduction efficiency and CO2 endurance is essential for advancing the electrochemical energy conversion performance of solid oxide fuel cells (SOFCs). This investigation introduces an innovative strategy by employing Ce doping to enhance the oxygen reduction kinetics and CO2 resistance of NdBaCoFeO5+δ (NBCF) cathodes. Ce doping effectively regulates the oxygen vacancy content, the ratio of Co3+/Co4+ and Fe3+/Fe4+ in NBCF, leading to substantial improvements in oxygen exchange, lattice diffusion, and charge transfer processes. Consequently, a significant enhancement in the oxygen reduction reaction catalytic activity and CO2 endurance is achieved. At 800 °C, the introduction of 10 mol% Ce doping results in a 65.3% reduction in Rp, reaching 0.024 Ω cm2. Simultaneously, the power density experiences a 50.8% enhancement, reaching 1.0 W cm−2. The current investigation presents a novel avenue towards the advancement of next-generation high-efficiency and stable cathode materials for SOFCs.

Ta/Nb doping motivating cubic phase stability and CO2 tolerance of Sr2Co1.6Fe0.4O6-δ double perovskite for high-performing SOFC cathode

Developing cathodes that are both phase-stable and tolerant to carbon dioxide (CO2) is a crucial and challenging task for solid-oxide fuel cells (SOFCs) operating at reduced temperatures. The activity of oxygen reduction reaction (ORR) in SOFC cathodes is typically influenced by factors such as geometric features and oxygen vacancies. In this study, new and robust cathodes for SOFCs, Sr2Co1.5Fe0.4Ta0.1O5+δ (SCFT) and Sr2Co1.5Fe0.4Nb0.1O5+δ (SCFN) are developed by doping a small amount of niobium (Nb) and tantalum (Ta) into Sr2Co1.6Fe0.4O5+δ (SCF) using a simple solid-state reaction technique. The effects of Nb/Ta doping on the crystal structure, oxygen vacancies, electrical conductivity, and electrochemical properties of SCFT and SCFN are investigated. The results reveal that these materials possess a fully cubic structure in the Fm-3m space group, with improved oxygen vacancy and electrical conductivity. They also exhibit good chemical compatibility with electrolytes at 1100 °C for 8 h and remain thermally stable when exposed to air and CO2 at 700 °C, without any additional phase formation. The area-specific resistance (ASR) of the SCFT cathode is found to be only 0.06 Ω cm2, whereas the SCFN cathode has an ASR of 0.10 Ω cm2. The superior performance of SCFT can be attributed to the higher oxygen vacancy, which enhances oxygen surface exchange kinetics and diffusivity compared to SCFN. When tested in a single cell using humidified H2 as the fuel and ambient air as the oxidant, the SCFT cathode achieves a power density of 656 mW cm−2 at 700 °C, while the SCFN cathode reaches 559 mW cm−2, indicating the potential of SCFT as a promising cathode material for SOFCs. The excellent performance of these materials can be attributed to their mixed electronic-ionic conduction properties, unique structural features, and high observed oxygen vacancy, which contribute to their remarkable electronic conductivity and significant ionic mobility.

The Evolution of Solid Oxide Fuel Cell Materials

Solid oxide fuel cells (SOFCs) are a key component of the future energy landscape. Although there is considerable research on the physical properties and technology of classic oxide materials for electrode and electrolytes in SOFCs, the field is very active as new experimental and theoretical techniques are now available that can improve these systems. In the present review, we consider key systems such as perovskite-related materials, the impact of strain and interfaces and advanced concepts that can improve the properties of SOFC materials. In particular, we consider the oxygen diffusion properties of perovskite-related materials and focus on La2NiO4+δ and the double perovskites such as GdBaCo2O5.5. Then, we review the importance of interfaces and strain as a way to engineer defect processes. Finally, we consider advanced concepts to form designed structures that explore the effect of local high entropy on lattice stabilization.

Boosting the electrochemical performance of oxygen electrodes via the formation of LSCF-BaCe0.9–xMoxY0.1O3–δ triple conducting composite for solid oxide fuel cells: Part II

This research is the continuation of our previous work, in which we introduced novel proton-conducting electrolytes BaCe0.9–xMoxY0.1O3–δ (BCMxY; x = 0.025, 0.05). In this study, we explore the potential of the proton-conducting BCM0.025Y electrolyte by creating a composite with La0.6Sr0.4Co0.2Fe0.8O3–δ (LSCF) to form triple conducting electrodes for solid oxide fuel cells (SOFC). The formation of the LSCF-BCM0.025Y composite enhances both the three-phase reaction interface length and the concentration of oxygen vacancies, contributing to improved dissociation rates and enhanced oxygen adsorption. The desired characteristics, including density, structure, composition, electrochemical performance, and thermal stability, have been confirmed through a comprehensive set of analyses including scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDS), Fourier-transform infrared spectroscopy (FTIR), electrochemical impedance spectroscopy (EIS), and thermogravimetric analysis (TGA) coupled with differential scanning calorimetry (DSC), respectively. The cell configuration of Ni-YSZ | BCZY | LSCF-BCM0.025Y exhibited a remarkable maximum power density (MPD) of 418.7 mW cm−2, which is approximately 29 % higher than that achieved with a typical LSCF cathode (325.6 mW cm−2) at an operating temperature of 600 °C. The outstanding performance and enduring stability of the LSCF-BCM0.025Y composite over a 500 h period demonstrate its potential as a promising cathode material for intermediate-temperature SOFCs.

Study on the composite structure of cobalt-free oxides as cathodes for intermediate temperature solid oxide fuel cells (IT-SOFCs)

Summary: Solid Oxide Fuel Cells (SOFCs) represent a promising technology for efficient and clean energy conversion. This study focuses on the design and development of a novel cathode material for SOFCs, specifically exploring the use of a free-cobalt cathode within a composite structure. Investigating the structure of cobalt-free composites of Ba0.5Sr0.5FeO3-δ (BSF) and Ba0.5Sr0.5Fe0.8B0.2O3-δ (B=Cu, Zn) (BSFB) have been prepared and evaluated as cathodes for IT-SOFCs. The solid-state reaction was employed for generating and modifying the composite structure of the model system. The decomposition reaction and the formation of perovskite structure have been observed using thermal gravimetric analysis and X-ray diffraction. The study encompasses experimental procedures, data processing using GSAS software, and the application of the Rietveld method to achieve precise analysis results. Rietveld analysis outcomes offer intricate details about the crystal structure, encompassing crystal unit parameters, and scale factors. The solid-state reaction led to the reduction in the weight of the composite during the heating process. The decomposition reaction of oxides was generated between 650 to 950 oC. The average of total weight loss during the period treatment was achieved up to 18 % and the perovskite phase formed in the composite structure. A single-phase perovskite from a cubic structure with space group Pm-3m was demonstrated by all composite’s models. The lattice parameter and a unit cell volume were obtained from the model system of BSF, BSFC and BSFZ, respectively. This study explores the potential development of SOFCs technology with the hope of making a positive contribution to advancing clean and sustainable energy solutions in the future

Real-time deformation and stress response of the planar SOFC during sintering

The thermal-mechanical response of the anode-supported planar solid oxide fuel cell (SOFC) during sintering is important for its manufacturing and in-service operation. The integrated micro/nano-indentation and finite element (FE) modeling method was used to characterize the Young’s modulus, yield stress and creeping for the NiO-3YSZ anode and 8YSZ electrolyte at room temperature. Based on the real-time full-field deformation of the half-cell SOFC which was measured using the 3D digital image correlation (DIC), the FE simulation was used to estimate the stress response in a manner of sensitivity study. It shows that the half-cell SOFC warps towards the anode substrate during heating and points in the opposite direction during cooling. The warping deformation of FE simulation matches well with the DIC measurement. In addition, the maximum tensile stress in the electrolyte coat of the half-cell SOFC plate is about 207 MPa if assuming temperature-independence of creeping for the SOFC materials.

Defect chemistry and transportation within La1.7Ba1.3In2O6.85 mixed ionic-electronic conducting membrane: Towards the application of solid oxide fuel cells

La1.7Ba1.3In2O6.85, as a representative material belonging to La2BaIn2O7-based family, is a novel and promising electrolyte material for solid oxide fuel cells (SOFCs). However, due to the co-existence of various mobile charged defects including oxygen vacancy, proton, and electron-hole, the conduction properties and behaviors of La1.7Ba1.3In2O6.85 are complex and not fully understood. In this work, we systematically explore La1.7Ba1.3In2O6.85 with a focus on defect chemistry and transportation. A theoretical model is built to analyze the experimentally measured conductivities of La1.7Ba1.3In2O6.85 at varying temperatures and oxygen partial pressures. Subsequently, the conduction properties and behaviors of La1.7Ba1.3In2O6.85 membrane at open circuit conditions and operation conditions are investigated thoroughly for the first time. Our findings reveal that in reducing and humidified atmospheres, La1.7Ba1.3In2O6.85 behaves as a dual-ion conducting oxide, with oxygen-ionic transport numbers exceeding 0.7. Our work can provide a new and overall understanding of the La2BaIn2O7-based material for SOFCs’ application.

Development of high-performance perovskite La(Mg2/3Nb1/3)O3 electrolyte with hybrid protonic/oxide ion conduction for low-temperature solid oxide fuel cells

Perovskite structures material such as the doped barium cerate-zirconates possessing high concentration and mobility of proton defects have drawn substantial attention. However, the low thermodynamic stability of alkaline earth cations hinders it further application in solid oxide fuel cells (SOFCs). From this point of view, lanthanum-based perovskites without alkaline earth metals can be considered as suitable electrolyte material for SOFC. In this study, a new perovskite material La(Mg2/3Nb1/3)O3 (LMN) is introduced as a high performing electrolyte with considerable proton conduction conductivity of 0.0023S cm−1 at 550 °C for low temperature solid oxide fuel cells (LT-SOFCs). LMN electrolyte is characterized in terms of phase structure, morphological feature and surface properties. It is found that LMN sample is stable under fuel cell condition and deliver promising performance with a maximum output power density of 742 mW cm−2 at 550 °C. Moreover, concentration cells experiment confirmed that the LMN possessed hybrid proton/oxide conduction and was dominated by proton conduction surface properties. Our work thus vouch that LMN can be an efficient alternative electrolyte for low temperature fuel cell applications.

Understanding Temperature Stability of Surface Cation Dopants on LSCF Electrodes

The widespread application of solid oxide fuel cells (SOFCs) is primarily inhibited by degradation issues impacting the stability and oxygen exchange kinetics of the oxygen electrode. State-of-the-art SOFC materials such as La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) boast fast oxygen ion and electron conduction as well as high oxygen reduction activity. Furthermore, materials such as LSCF provide a potential pathway to reduce the operating temperature of SOFCs from 800-100 ° C to 400-800 ° C. But degradation issues continue to impede LSCF, primarily through strontium segregation to the electrode surface and subsequent passivating poisoning reactions due to chromium- and sulfur-containing species.In this work, we investigate a solution infiltration technique to introduce cation dopants to the surface of LSCF porous electrodes on symmetrical cells to improve the stability and performance through suppression of Sr segregation. We have previously demonstrated that introducing more oxidizable transition metal cations on the LSC electrode surface improves both the stability and oxygen exchange kinetics. Others have demonstrated that lanthanides can electrochemically stabilize the LSCF surface as well (Yu Chen, et al. Energy Environ. Sci., 2017, 10, 964-971). However, the temperature dependence of these cation dopants on electrode surface composition and electrochemical stability is not well understood. Thus, we are interested in exploring more oxidizable transition metal and lanthanide cations as infiltrated dopants on the porous LSCF electrode as a function of temperature.Preliminary experiments demonstrate that both transition metal and lanthanide cation dopants diffuse into the LSCF lattice at SOFC operating temperatures (above 700 ° C) and Sr segregates to the surface with increasing temperature. We do also observe some fundamental differences between the transition metal and lanthanide cations, such as the temperature dependence of A:B site ratio. Our initial electrochemical measurements indicate that these cation dopants are found to improve the stability of LSCF electrodes when tested at SOFC operating temperatures (800 ° C in ambient air) despite having largely diffused into the lattice at these temperatures.These preliminary results indicate a potential electrode surface reconstruction as a function of cation type and temperature. We aim to elucidate 1) the temperature stability of these cations at the surface, 2) the impact of these cations on the electrode surface composition and cell electrochemical stability as a function of temperature, and 3) the mechanism for electrochemical influence of these sub-surface cations.We utilize electrochemical impedance spectroscopy and chronoamperometric stability studies to characterize the impact of infiltrated cations on electrode activity and stability. We investigate the stability of cation dopants and reasons for observed electrochemical changes through x-ray photoelectron spectroscopy and x-ray diffraction studies. This work aims to elucidate the impact of transition metal and lanthanide cation dopants on electrode stability and performance as well as the stability of the dopants at the surface at different operating conditions, with a focus on developing a mechanistic understanding of sub-surface dopant influence on electrochemical performance.

Ion Beam as an External and Dynamic Metal Reservoir to Induce Nanoparticle Exsolution in Oxides

A central theme in solid oxide fuel cells (SOFC) and electrolyzers (SOEC) is to design stable and catalytically active solid/gas interfaces toward desired reactions. A recent advance in this regard is to synthesize nanoparticles decorated electrodes in a process termed “exsolution”. Exsolution generates stable and catalytically active metal nanoparticles via phase precipitation out of a host oxide. Unlike traditional nanoparticle infiltration techniques, the nanoparticle catalysts from exsolution are anchored in the parent oxide. This strong metal-oxide interaction makes the exsolved nanoparticles more resistant against particle agglomeration as compared to the infiltrated ones. In addition, the exsolved particles also open up the possibility of regeneration of catalysts. To date, the concept of exsolution has been successfully applied to a number of applications including SOFC/SOEC, ceramic membrane reactors, chemical looping combustion, and (electro)catalysis.As the exsolution process starts with a solid-solution that contains the to-be-exsolved transition metal ions, this method is often limited by the intrinsic solubility limit of the host oxide. Here, we propose that ion beam irradiation to be a unique tool to promote exsolution as it can introduce defects and dopants into the host oxides and at concentrations higher than thermodynamic limits. Moreover, due to the surface sputtering effect, ion beam irradiation can modulate the surface morphology, creating novel nanostructures.To demonstrate this approach, we chose thin-film perovskite SrTi0.65Fe0.35O3 (STF) and fluorite Ce0.5Zr0.5O2 (CZO) as model systems, both of which are promising materials in SOFC and SOEC. We modulated metal exsolution in both materials with in-situ 10-150 keV Ni irradiation at 800 °C in vacuum. Ni irradiation on STF controllably changed the exsolved particle composition from unitary Fe to bimetallic Fe-Ni. Moreover, it also reduced the particle size down to sub-2 nm, which outperforms other tuning methods thus far in the literature. As a result, the irradiation-modified STF demonstrated superior catalytic activity toward room-temperature oxygen evolution reactions (OER) than the conventional thermally exsolved STF. Regarding CZO, Ni irradiation also decorated the surface with well-dispersed nanoparticles, leading to enhanced high-temperature H2O splitting kinetics. The effective size and composition control over exsolution highlights the utility of metal ion beams in promoting and tailoring exsolved nanocatalysts for a broad range of applications in clean energy and fuel conversion. Figure 1

Vanadium-Doped Ni/YSZ Anode Functional Layers for Solid Oxide Fuel Cells Produced via Magnetron Sputtering

The electrode performance of solid oxide fuel cells (SOFC) strongly depends on its microstructural characteristics, such as the porosity, percolated paths of ionically and electronically conductive phases and in particular, the grain size1. Deploying alternative manufacturing techniques that deposit nanostructured materials results in finer particle sizes and thus, increases the quantity of triple-phase boundaries. For that reason, the magnetron sputtering technique, which offers elemental distribution at the nanoscale, high deposition rates, reproducibility, scalability and excellent uniformity over large-area substrates has been chosen as the deposition method to fabricate anode functional layers (AFLs) for SOFCs.Nanostructured NiO-YSZ thin films have been previously produced by reactive pulsed DC magnetron co-sputtering of metallic targets of zirconium-yttrium and nickel, defining the optimal deposition parameters to create state-of the-art AFLs. Former studies reported limitations of the process control during reactive magnetron sputtering, regarding the film’s composition 1,2. Since the composition strongly relies on the amount of reactive gas, i.e., oxygen, present during deposition, a feedback control system is required to prevent oxygen built-up leading to target poisoning and lower deposition rates.Herein, we present a reactive feedback control system, that manages quantities of oxygen introduced to the sputtering chamber during deposition based on the oxygen partial pressure. This facilitates stable operation conditions, prevents target poisoning, and maintains the coating characteristics, i.e., its microstructure and desired composition when varying process parameters, such as the deposition pressure, target-substrate distance, or the deposition angle.Based on recent studies 3–5, the future of oxide-based anode materials for SOFCs will greatly focus on reducing the Ni catalyst content through alternative non-precious metal doping and increasing the cell performance by tailoring the microstructure of the AFL. This would allow Ni coarsening to be mitigated and maintain the nanostructure over the lifetime of the cell. Therefore, in this study complex transition metal oxides, such as vanadium, tantalum or manganese oxides, were doped into SOFC anodes to study their influence on the structural and morphological properties of magnetron sputtered AFL. The effect of dopant concentration on the properties of Ni-YSZ films in as-deposited, pre-annealed and reduced state was analysed using SEM, EDS, XRD and XPS. To characterise the electrochemical performance of the deposited films, polarisation curves were obtained from SOFC single stack assemblies under hydrogen and air flows for anode and cathode, respectively, at operating temperatures of 750, 800 and 850 ℃.References Lim, Y., Lee, H., Hong, S. & Kim, Y. B. Co-sputtered nanocomposite nickel cermet anode for high-performance low-temperature solid oxide fuel cells. J. Power Sources 412, 160–169 (2019).Ionov, I. V. et al. Reactive co-sputter deposition of nanostructured cermet anodes for solid oxide fuel cells. Jpn. J. Appl. Phys. 57, 30–34 (2018).Atkinson, A. et al. Advanced anodes for high-temperature fuel cells. Nat. Mater. 3, 17–27 (2004).Van Overmeere, Q. & Ramanathan, S. Thin film fuel cells with vanadium oxide anodes: Strain and stoichiometry effects. Electrochim. Acta 150, 83–88 (2014).Garcia-Garcia, F. J., Beltran, A. M., Yubero, F., Gonzalez-Elipe, A. R. & Lambert, R. M. High performance novel gadolinium doped ceria / yttria stabilized zirconia / nickel layered and hybrid thin film anodes for application in solid oxide fuel cells. J. Power Sources 363, 251–259 (2017). Figure 1

High activity and stability of cobalt-free SmBa0.5Sr0.5Fe2O5+δ perovskite oxide as cathode material for solid oxide fuel cells

It is very important to develop cathode material with high stability and strong catalytic activity to realize the commercial application of solid oxide fuel cells (SOFCs). Here, we prepared a cobalt-free cathode material which partial substitution of Sr for Ba in SmBa0.5Sr0.5Fe2O5+δ (SBSF) sample and systematically investigated its electrochemical performance. Density functional theory (DFT) results showed that the oxygen vacancy formation energy of SBSF was lower than that of SmBaFe2O5+δ (SBF). The local density of state (LDOS) results showed that bandwidth center energy of SBSF between O-2p and Fe-3d obviously overlapped compared with parent SBF. The XRD spectra show that the SBSF formed a perovskite structure and had good chemical compatibility with La0.9Sr0.1Ga0.83Mg0.17O3-δ (LSGM) and CeO2-based electrolyte, respectively. At 800 °C, the polarization resistance and maximum power density of SBSF cathode on LSGM electrolyte were 0.023 Ω cm2 and 874 mWcm−2, respectively. In addition, the polarization resistance of SBSF electrode was stable during the 100 h test. The preliminary results show that SBSF oxide is a promising cathode material for SOFCs.

A review on new cobalt-free cathode materials for reversible solid oxide fuel cells

The exponential growth in the requirement of fuel cells and batteries leads to increased demand for cobalt due to its common use in high-performance Li-ion batteries and high-temperature fuel cells/electrolyzers. This sharp increment in demand raises concern about the availability of limited reserves of cobalt which can impact the price of cobalt. Moreover, the geographic limitations of cobalt resources may endanger the whole supply chain. In addition to all those, huge moral issues of cobalt mining are also another problem. Hence, leading battery, fuel cells and electrolyzer manufacturers are looking for sustainable alternatives to reduce cobalt dependency. A more specific limitation is shown in Solid Oxide Fuel Cells (SOFCs) cathode materials that contain cobalt. Incompatibilities have already been observed between the cathode materials containing cobalt and the electrolytes in terms of the thermal expansion coefficient mismatch during the transition of the operating temperature from high to low. An advantage of low operating temperatures is the reduction of material costs compared to high temperature. Increasing the electrochemical performance of the cell and eliminating thermal expansion coefficient difference problems are in concert aimed at the development of cobalt-free cathode materials. Therefore, cobalt-free cathode materials are vital for the sustainability of SOFCs and green transition of the energy sector since they can be used as cathode and anode material in symmetrical SOFCs which is also known as reversible SOFC (RSOFC). In this review, we comprehensively summarize the recent advances of cobalt-free perovskite cathode materials for intermediate temperature RSOFCs.