Solid Oxide Research Articles

An extended finite element method for the Nernst-Planck-Poisson equations

The Nernst-Planck-Poisson (NPP) system of equations is used to study the ion diffusion mechanism in materials, commonly used in storage batteries and solid oxide fuel cells. In particular, NPP equations are used to predict the concentration of charged defects and electric potential. Depending on the size of the computational domain, the distributions exhibit steep gradients near the boundaries. The traditional finite element method, when employed, requires extremely refined mesh to capture the steep gradient as they employ simple polynomials. To alleviate the mesh dependence, in this work, we propose to augment the traditional finite element approximation space with a suitable ansatz to capture the steep gradient within the framework of the extended finite element method. The robustness and the accuracy of the proposed framework is demonstrated by comparing it with an overkill finite element solution.

Enhanced electrochemical performance of (PrBa)xFe1.9Nb0.1O5+δ symmetrical electrodes with A-site deficient for symmetrical solid oxide fuel cells

Symmetrical solid oxide fuel cells (SSOFCs) with symmetrical electrodes have many desirable characteristics compared with conventional SOFCs. However, the lack of electrode materials simultaneously satisfied the high electrocatalytic activity in oxidizing and reducing atmospheres. Herein, a novel perovskite oxide electrode (PrBa)xFe1.9Nb0.1O5+δ ((PB)xFN, x = 1, 0.97, 0.95 and 0.93) was reported as a promising symmetrical electrode material for SSOFCs. XRD results reveal that the (PB)0.95FN sample has a tetragonal structure and good chemical compatibility with Gd0.1Ce0.9O2-δ electrolyte. Among all the samples, (PB)0.95FN has the lowest polarization resistance (Rp). At 800 °C, compared with primary PBFN, the cathode Rp of (PB)0.95FN is reduced from 0.109 Ω cm2 to 0.063 Ω cm2 in air, whereas the anode Rp is decreased from 0.3808 Ω cm2 to 0.3109 Ω cm2 in H2. In addition, an electrolyte-supported single cell with (PB)0.95FN symmetrical electrodes achieves a maximum peak power density at 800 °C, which is enhanced by 14.67% compared with that of PBFN. A-site deficiency on PBFN perovskite shows better electrochemical performance, which is attributed to an increase in oxygen vacancy concentration. The results indicate that (PB)0.95FN is a potential electrode material for SSOFCs.

Structural and Electrochemical Performance of Sr0.9La0.1WO4 Electrolyte Materials for Solid Oxide Fuel Cells

A possible component of power generation technologies, solid oxide fuel cells (SOFCs) offer a high fuel-to-power conversion efficiency and no negative environmental impact. The solid-state sintering process was used to synthesize the scheelite-structured Sr0.9La0.1WO4 (SLW1) electrolyte material, and phase stability and ionic conductivity were evaluated for their technological applicability for SOFC applications. The resulting mixture, which was a single phase in a tetragonal crystal system, produced a scheelite structure of the space group I41/a. Its symmetry, space group, and structural characteristics are confirmed by X-ray diffraction (XRD) at room temperature and the Rietveld analysis that follows. A highly dense crystal structure was revealed by SEM examination. The ionic conductivity of the SLW1 sample is higher than SBW materials and lower than the conventional BCZY perovskite structure. At 900°C in both the wet Ar and dry Ar conditions, the SLW1 sample’s ionic conductivity was 2.41 × 10−5 S cm-1 and 1.76 × 10−5 Scm−1. This scheelite-like compound showed a thick microstructure and substantial conductivity, making it a potential mixed ion-conducting electrolyte for SOFC.

Effect of Fe doped Co–Ni protective coatings on the oxidation behavior and electrical properties of SOFC alloy interconnect materials

In order to enhance the oxidation resistance and electrical properties of interconnects for solid oxide fuel cells (SOFCs), three Co–Ni alloy coatings with varying contents of Fe doping were prepared onto SUS 430 substrate using electroplating method and oxidized in air at 800 °C for 1000 h. The experimental results demonstrated that the alloy coating significantly enhanced both the oxidation resistance and electrical properties of the interconnect substrate. The multi-layer oxidation structure blocked Cr from diffusing outward. Notably, the (Co,Fe)3O4/(Ni,Co)O in the oxidized layer of 5% Fe doped Co–Ni coated steel completely transformed into (Fe,Co,Ni)3O4 spinel layer after long term oxidation, which had the lowest activation energy and the strongest ability of restricting oxygen diffusion inward. Thus, the 5% Fe doped Co–Ni coated steel showed superior oxidation resistance (kp = 8.76 × 10−14 g2 cm−4 s−1) and excellent electrical properties (ASR = 13.19 mΩ cm2) following oxidation for 1000 h.

Thermal dynamic analysis and multi-objective optimization of a 10 kW dual preheating solid oxide fuel cell system

The increasingly serious environmental problems caused by the excessive development of fossil fuels necessitate the promoting of novel and effective energy technologies. As an environmentally friendly and effective promising solution in terms of its low emission, and fuel adaptability, the solid oxide fuel cell (SOFC) system has attracted widespread attention. To better promote the progress of SOFC, the SOFC testing system (SOFC TS) is very important. In this paper, we have established a dual preheating SOFC TS configuration where the fuel is first preheated by exhaust gas, and then preheated by the electric heater to the setting temperature. A comprehensive thermodynamic analysis is conducted to investigate the characteristic of this SOFC TS. Building upon this designed testing system, a multi-objective optimization approach is adopted to enhance the performance of the thermal management subsystem. The results showed that the SOFC stack is the main exergy component in the SOFC TS, the selected point of multi-objective optimization can reduce the thermal management costs by 60$ and improve system efficiency by 4.5%.

The performance of NiO/NiFe2O4 coated pre-oxidized SUS 430 interconnect with different NiO thickness for solid oxide fuel cell

Ni/NiFe2 coatings with different thickness of Ni were deposited on 50-h pre-oxidized SUS 430 steel by magnetron sputtering for solid oxide fuel cell (SOFC) interconnects application, which were subsequently exposed to 800 °C air for durations of up to 5040 h. Ni/NiFe2 coating was thermally converted to an oxide structure comprising inner NiO layer and outer NiFe2O4 spinel layer. The effects of different NiO diffusion barrier thickness on the oxidation behavior, electrical performance and long-term working stability of NiO/NiFe2O4 coated steel were investigated. The chromium vaporization from bare and coated steels was also evaluated at 800 °C in air-12 % H2O atmosphere. Results showed that scale delamination and the spike of area specific resistance (ASR) were achieved for the sample with thicker NiO layer after 2520 h thermal exposure. Moreover, NiO gradually migrated outward through the solid-state reaction between NiO and NiFe2O4 spinel and eventually aggregated near the outer surface. Breakaway oxidation of the bare steel occurred during the chromium vaporization test and undulating Fe oxide nodules were strewn on the steel surface. The chromium vaporization rate was effectively suppressed by the application of NiO/NiFe2O4 coating.

Enhancement of Performance and Sulfur Tolerance of Ceria-Based Fuel Electrodes in Low Temperature SOFC

The trend of operating the solid oxide fuel cell at significantly lower operation temperatures enables the application of electrodes with finer microstructure or even nanostructured electrodes with increased active surface and enhanced performance. To maintain the high performance in hydrocarbon fuels commonly impurified with sulfur compounds, a required sulfur tolerance has to be maintained. In this study we compare performance and H2S-poisoning of four ceria-based electrodes: conventional Ni/Ce0.9Gd0.1O2−δ cermets and sub-μm scaled Ce0.8Gd0.2O2−δ -electrodes with and without infiltrated nickel. Symmetrical cells were operated in a hydrogen/steam/nitrogen gas mixture with and without minor amounts of H2S at 600 °C. The performance is analyzed by electrochemical impedance spectroscopy. The distribution of relaxation times is applied to deconvolute the electrochemical processes followed by a complex nonlinear least square fit to quantify the loss processes and the impact of sulfur. Whereas two different Ni/Ce0.9Gd0.1O2−δ cermet electrodes exhibit polarization resistances at 600 °C without/with 0.1 ppm H2S of 2.89/5.56 Ωcm2 and 2.15/2.75 Ωcm2, the single phase Ce0.8Gd0.2O2−δ electrode reaches 0.98/2.37 Ωcm2. With an infiltration of Ni-nitrate forming nickel nanoparticles on the gadolinia-doped ceria-surfaces, the ASR could be drastically reduced to 0.32/0.37 Ωcm2.

Oxygen Permeation Measurement of LaCrO3-based Interconnect Embedded in Real SOFCs

A method for evaluating the oxygen permeation flux through LaCrO3-based interconnects embedded in real solid oxide fuel cells (SOFCs) is proposed. The interconnect of the cell was placed in a two-chamber setup (fuel | interconnect | air), and the oxygen permeation flux was successfully determined by quantifying the amount of water vapor generated at the fuel side due to the permeation of oxygen from the air side to the fuel side. Using the proposed method, the oxygen permeation fluxes through the interconnects of the cells before and after 20,000 h of continuous operation were compared to examine the long-term stability of the LaCrO3-based interconnects. No significant difference was observed in the oxygen permeation flux before and after continuous operation, indicating that the LaCrO3-based interconnects have excellent chemical and thermal stability. In addition, the effect of oxygen permeation on energy conversion efficiency was investigated by calculating the Nernst loss. The calculations showed that oxygen permeation has almost no effect on the energy conversion efficiency.

Improving the stability of La0.6Sr0.4Co0.2Fe0.8O3-δ electrode for solid oxide fuel cells by introducing sintering inhibitors

In this study, nano-sized Al2O3 and Gd0.1Ce0.9O1.95 (GDC) particles are added to La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) cathode material as sintering inhibitors for solid oxide fuel cells (SOFC) to improve the stability of the cell performance. Studies reveal that as the amount of the inhibitors increases, the sintering activation energy of the composite LSCF sample increases, resulting in retarded sintering kinetics indicated by the suppressed grain size of the sintered composite LSCF cathode. Long-term durability tests show that due to the enhanced stability of the microstructure of the composite LSCF cathode, the composite cathode exhibits improved performance stability. Furthermore, both the Al2O3 and GDC additions can improve the hardness and elastic modulus of the cathode, while the electrical conductivity of both cathodes exceed 100 S/cm and can still meet the requirements for the SOFC in the operation temperature range of 600–800 °C. In light of the boosted electrochemical durability of the Al2O3-added LSCF cathode and much lower cost of Al2O3 compared to the rare-earth element based GDC materials, LSCF cathode with a proper amount of the nano-sized Al2O3 inhibitors is a good solution to enhance the mechanical property, electrochemical durability of LSCF cathode for the SOFCs at low cost.

Electrochemical properties of B-site non-stoichiometric layered perovskite SmBa0.5Sr0.5CoxO5+d (x=1.9–2.1) cathodes for IT-SOFC

In this study, layered perovskites were synthesized using a non-stoichiometric synthesis strategy that allowed the variation of the amount of Co substitution in the basic composition of SmBa0.5Sr0.5Co2O5+d (SBSCO), used as a cathode material for Intermediate Temperature-operating Solid Oxide Fuel Cells (IT-SOFCs).SmBa0.5Sr0.5CoxO5+d (x = 1.9–2.1, SBSCO-1.9–2.1), B-site non-stoichiometric layered perovskites, were analyzed to characterize the electrical and electrochemical properties with respect to the Co substitution. Among them, SmBa0.5Sr0.5Co2.05O5+d (SBSCO-2.05) exhibited the best electrical and electrochemical properties with an ASR of 0.082 Ωcm2 and an electrical conductivity of 268.42 S/cm at 700 °C (measured in air within a series of measurements of increasing measuring temperature). Therefore, B-site non-stoichiometric perovskite compositions with increased Co substitution on their B sites represent a key strategy for achieving high performance cathodes compared to cathodes with stoichiometric compositions.

Catalytic effect of aluminum on the structural, optical, and electrical properties of LaSrCrO3-δ anode

Solid oxide fuel cell is promising conversion technology due to fuel flexibility, higher efficiency, and environment friendly compared to traditional technology. To enhance the efficiency of the device, selection of anode material is important because it affects the electrochemical performance and operating temperature of the cell. In this work, La0.5Sr0.5AlxCr1-xO3-δ (x = 0, 0.1, 0.2, 0.3, 0.4) anode has been developed using sol-gel method. XRD analysis exhibited a two-phase structure of the prepared anode, one orthorhombic and second rhombohedral phase with crystalline size in the range 15–18 nm. The surface morphology of the prepared anode material shows nano-spheres, dumbbells, and platelets. The elemental analysis confirms the presence of Sr, Cr, La, Al, and O in the prepared material. The metal-oxide formation has been confirmed with Fourier Transform Infrared Spectroscopy. The absorption spectra revealed red shift in bandgap 3.65–2.85 eV by increasing the Al concentration. The maximum dc electrical conductivity of the composition anode La0.5Sr0.5Al0.4Cr0.6O3-δ is found to be 8.01 Scm−1 at 600 °C. The achieved power density of the anode La0.5Sr0.5Al0.4Cr0.6O3-δ is 69 mWcm−2 using hydrogen fuel at 600 °C.

Semiconductor ionic Cu doped CeO2 membrane fuel cells

Operation of low-temperature solid oxide fuel cells (LT-SOFCs) in the range of 300–500 °C has emerged as a promising technology and desired operational temperature for clean and efficient energy conversion. However, the lack of suitable electrolyte materials for these temperatures presents a significant challenge. In this article, a novel approach is proposed to address the hurdle of operational temperature by tailoring copper-doped cerium oxide, Cu0.1Ce0.9O2 as a semiconductor ionic membrane (SIM) to act as an electrolyte in a cell. The Cu0.1Ce0.9O2 formed a single-phase cubic fluorite structure. As a SIM, it shows enhanced ionic conductivity and reduced electrolyte/electrode polarizations, therefore achieved good fuel cell performance and electro-kinetics, reaching a power density of 515 mW/cm2 at 500 °C, with an ionic conductivity exceeding 0.1 S/cm. The detailed investigation shows dominant protonic conduction (H+) behavior in the Cu0.1Ce0.9O2 membrane. Interesting phenomenon of coupling effect elaborated through the distribution of relaxation time (DRT) analysis depicted to understand the romance of protons and electrons conduction mechanism. These advancements lead to Cu0.1Ce0.9O2 as a new type of SIM in LT-SOFCs, emphasizing the need for further research and development in this promising field to unlock the SIM and fuel cell potential.

Protonic ceramics Ba5In2–xYxAl2ZrO13 with the perovskite-related hexagonal structure for solid oxide fuel cells: Synthesis, optical band gap and transport properties

The solid solution Ba5In2–xYxAl2ZrO13 (0 ≤ х≤0.50) with hexagonal structure (space group P63/mmc) was prepared by the solid-state reaction method. The effects of isovalent Y3+-substitution on the structure, hydration, bandgap and transport properties have been investigated. The introduction of yttrium was accompanied by lattice expansion, which led to an increase in the concentration of protons during hydration. The doping did not lead to a significant increase in oxygen-ion conductivity since there was no change in oxygen stoichiometry. At the same time, doping led to an increase in ionic transport numbers due to a decrease in hole conductivity. Proton conductivity contribution and the values of proton conductivity increase with the increase in yttrium concentration. The phases with yttrium content x > 0.2 were predominant proton conductors at the temperature below 600°С under wet air.

BaCe 0.8Fe 0.1Ni 0.1O 3− δ -impregnated Ni–GDC by phase-inversion as an anode of solid oxide fuel cells with on-cell dry methane reforming

BaCe _0.8Fe _0.1Ni _0.1O _{3− <em>δ</em>}-impregnated Ni–GDC by phase-inversion as an anode of solid oxide fuel cells with on-cell dry methane reforming

Modeling and characterization of sintered YSZ/NiO porous electrode structural properties using coarse-graining molecular dynamics method

In solid oxide fuel cell (SOFC), sintering of YSZ and NiO powders into porous anode plays a crucial role in determining its overall performance. In this study, a coarse-graining (CG) molecular dynamics (MD) model is developed to capture sintered half-cell composed of porous functional layer with NiO/YSZ particles and dense YSZ ceramic electrolyte layer with applied in anode-supported SOFC. The sintering process is simulated in two stages, i.e., heating process followed by temperature-holding process. This study also explores effects of sintering parameters (e.g., temperature, powder size and material compositions) on structural characteristics, including distribution of sintered triple-phase boundary (TPB) length, porosity, and pore size. It is found that structural properties vary mainly during the heating stage of sintering, while a higher sintering temperature leads to a larger TPB length and a lower porosity, while increasing in diameter ratio of the NiO powders to YSZ powders is advantageous for increasing TPB length, porosity and size pores for promoting electrode electrochemical activity. This work provides a bottom-up approach for potential optimization of SOFC fabrication condition and parameters.

Towards highly dense electrolytes at lower sintering temperature (∼1200 °C): Optimization strategies for BaCe0.7Zr0.1CuxY0.2-xO3-δ in SOFCs

High-conducting and long-term stable proton-conducting electrolytes are of utmost importance. This study investigates the synthesis and characterization of BaCe0.7Zr0.1CuxY0.2-xO3-δ (BCZCuxY; x = 0, 0.005, 0.01, 0.02, 0.05, and 0.1) electrolytes tailored for proton-conducting solid oxide fuel cells (SOFCs). Through Cu2+ doping at the B-site of BaCe0.7Zr0.1Y0.2O3-δ (BCZY), a successful reduction of the sintering temperature to 1200 °C was achieved. BCZCu0.02Y exhibited a high relative density of approx. 98.1% at this temperature, but beyond 2 mol% Cu doping, the appearance of a BaCuO2 secondary phase adversely impacted conductivity. The electronic properties of BCZY and BCZCuxY are elucidated via partial density of states (PDOS) analysis, revealing optimized crystal structures and band gap reductions (from approx. 1.9 eV–1.1 eV) upon Cu doping. Notably, BCZCu0.02Y demonstrated a commendable conductivity, with values of 3.5 × 10−2 S. cm−1 in air and 4.8 × 10−2 S. cm−1 in a moist atmosphere at 750 °C. Remarkably, excellent electrochemical stability was observed in a moist hydrogen atmosphere for up to 450 h at 600 °C. Single cells incorporating BCZCu0.02Y electrolytes exhibited peak power densities of 380 mW/cm2 at 750 °C. The incorporation of 2 mol% Cu2+ in the BCZY lattice holds promise for achieving low-temperature sintering and high-performance proton-conducting SOFCs.

Reversible Operation of Metal Supported Solid Oxide Cells

Solid oxide cells (SOCs) can operate in fuel cell and electrolysis mode. This option allows for production of electricity and heat from a green fuel in fuel cell mode and for storage of electricity as gas or use as fuel in electrolysis mode. Demonstration of reversible SOCs has progressed over the last few years. Increase of lifetime and reduction of costs are major factors for successful commercialization. In metal supported SOCs (MSCs) the thickest layer in the cell, the support layer of a few hundred μm, uses metal instead of Ni/YSZ cermet as in state-of-the-art (SoA) fuel electrode supported cells, thereby enabling a significant cost reduction. The present study investigates SoA Ni/YSZ SOCs and MSCs, fabricated by tape casting, lamination, and screen-printing, in reversible operation at 650 °C in 50/50 steam/hydrogen. In the initial few hundred hours, the degradation rate in electrolysis mode is smaller on a MSC compared to a SoA Ni/YSZ cell, while they are comparable in fuel cell mode. According to electrochemical impedance evaluation, the degradation is due to a simultaneous increase of the serial and polarization resistances in the MSC, while it is mainly due to an increase of the polarization resistance in the SoA cell.

Na0.6Co3O4- La/Pr co-Doped Ceria as Semiconductor-Ionic Heterostructure Material for Fuel Cell Application

Functional Sodium-doped cobalt oxide (Na0.6Co3O4, NCO) was incorporated to regulate and improve the electrochemical performance of La/Pr co-doped ceria (LCP) electrolytic materials with good operative stability, forming an p-n heterostructure electrolyte (LCP-NCO) for low-temperature solid oxide fuel cell (LTSOFC) application. LCP-NCO is a new potential semiconductor-ionic material, achieving a maximum power density of 1075 mW cm−2 along with a high open-circuit voltage of 1.061 V at 520 °C. Scanning electron microscopy combined with transmission electron microscopy unveiled the crystallographic microstructure of heterostructure interface between LCP and NCO. Raman spectra and Fourier transform infrared spectroscopy spectra were analyzed to distinguish the functional groups and the vibrational properties. Ultraviolet–visible absorption and ultraviolet photoelectron spectroscopy have determined the accurate band edge positions of LCP and NCO and p-n heterojunction nature. Built-in electric field in semiconductor heterostructure and more oxygen vacancies created through the variation of Co3+/Co2+ ratio in LCP-NCO during the fuel cell test, contributed to the enhanced ionic transport. Characteristic of competent conductivity of 0.26–0.42 S cm−1 at 400 °C–520 °C, and the improved cell duration, revealed that the LCP-NCO as a hybrid oxygen ion and protonic conductor would be a potential electrolyte for LTSOFC.

Unveiling the Structural and Optical Properties of Samarium Titanates Pyrochlore, Sm2Ti2O7

A magnetic system whose geometry prevents all pairwise spin interactions from being fulfilled simultaneously to minimize the energy is said to be frustrated. These systems are prevalent throughout all disciplines of physical and chemical study. This article reports a detailed structural and optical analysis of such frustrated Sm2Ti2O7 (STO), a pyrochlore structure. The ordered pyrochlore, STO of cubic symmetry, space group Fd3¯m [no. 227], has been synthesized using the standard solid-state reaction method. The Tauc’s plot reveals a direct band gap value of 3.8 eV from ultraviolet-visible reflectance data. Photoluminescence studies for STO using 280 nm excitation wavelength show the most intense emission peak at 395 nm, 468 nm, and other weak emissions at 368 nm, 412 nm, 430 nm, 449 nm, and 514 nm corresponding to the respective electronic transitions, indicates that the materials show photoluminescence properties. However, the unique characteristics of Sm2Ti2O7, including its wide band gap, photoluminescence, and high refractive index, make it a promising candidate for various magneto-optical, solid oxide fuel cell device fabrication applications.

Theoretical Understanding of Effects of Operating Modes on the Performance Durability of Solid Oxide Cells: A Comparison between Potentiostatic and Galvanostatic Operations

In solid oxide cells (SOCs), the choice between galvanostatic (constant current) and potentiostatic (constant voltage) modes does not significantly affect the performance of SOCs as long as the cell components remain unchanged and intact. However, the degradation of cell components, which leads to changes in electrochemical and physical properties of the cell, elevates the importance of the selected operating mode. This paper aims to investigate the effects of galvanostatic and potentiostatic operating modes on the evolving properties of SOCs and their subsequent influence on performance durability. Employing non-equilibrium thermodynamic analysis, a crucial approach for understanding the degradation phenomena within an active electrochemical system, this study aims to provide in-depth insights into how these operating modes affect the longevity and efficacy of SOCs. Key findings include: In cases where oxygen electrode (OE) degradation is accelerated by higher partial pressure of oxygen ( pO2 ), operating under constant voltage electrolysis can mitigate the high pO2 at the OE|electrolyte (OE|EL) interface. Conversely, if OE degradation occurs more rapidly under a lower pO2, constant current electrolysis is more effective in suppressing degradation by achieving a high pO2 at the OE|EL interface. For degradation of the fuel electrode (FE) due to higher pO2, constant current electrolysis is beneficial for more stable performance, which helps maintain low pO2 at the FE|EL interface. When FE degradation is accelerated by lower pO2, constant voltage electrolysis can avert low pO2 at the FE|EL interface. In practical scenarios, more complex degradation mechanisms come into play, especially when pO2 significantly deviates from initial conditions. Degradation in one electrode can influence pO2 in the other electrode, a phenomenon more pronounced in potentiostatic than in galvanostatic electrolysis.