Ni-Sm2O3 cermet anodes for intermediate-temperature solid oxide fuel cells with stabilized zirconia electrolytes

Ni-LnO_x cermets (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd), in which LnO_x is not an oxygen ion conductor, have shown high performance as the anodes for low-temperature solid oxide fuel cells (SOFCs) with doped ceria electrolytes. In this work, Ni-Sm₂O₃ cermets are primarily investigated as the anodes for intermediate-temperature SOFCs with scandia stabilized zirconia (ScSZ) electrolytes. The electrochemical performances of the Ni-Sm₂O₃ anodes are characterized using single cells with ScSZ electrolytes and LSM-YSB composite cathodes. The Ni-Sm₂O₃ anodes exhibit relatively lower performance, compared with that reported Ni-SDC (samaria doped ceria) and Ni-YSZ (yttria stabilized zirconia) anodes, the state-of-the-art electrodes for SOFCs based on zirconia electrolytes. The relatively low performance is possibly due to the solid-state reaction between Sm₂O₃ and ScSZ in fuel cell fabrication processes. By depositing a thin interlayer between the Ni-Sm₂O₃ anode and the ScSZ electrolyte, the performance is substantially improved. Single cells with a Ni-SDC interlayer show stable open circuit voltage, generate peak power density of 410 mW cm⁻² at 700 °C, and the interfacial polarization is about 0.7 Ω cm².     

Fabrication and evaluation of anode and thin Y2O3-stabilized ZrO2 film by co-tape casting and co-firing technique

Co-tape casting and co-firing of supporting electrode and electrolyte layers could drastically increase productivity and reduce fabrication cost. In this study, Ni-YSZ anode supporting electrode and the YSZ electrolyte with the size of 6.5 cm × 6.5 cm have been successfully fabricated by co-tape casting and co-firing technique. The cell with 1.5 mm anode and 10 μm electrolyte is flat without warping, cracks or delaminations. The power density reaches 661, 856, 1085 mW cm⁻² at 0.7 V and 750, 800 and 850 °C, respectively. The EIS results demonstrate that the cathodic electrochemical resistance is 0.0680 Ω cm², about twice of the anode's which is 0.0359 Ω cm². SEM images show the dense YSZ film had a crack free of surface morphology. The anode and cathode layers are well-adhered to the YSZ electrolyte layer. The La_0.8 Sr_0.2 MnO_3−δ particles do not form a continuous network. Optimization of finer cathodic microstructure and anodic porosity are underway.     

Nanostructured La0.6Sr0.4Co0.8Fe0.2O3/Y0.08Zr0.92O1.96/La0.6Sr0.4Co0.8Fe0.2O3 (LSCF/YSZ/LSCF) symmetric thin film solid oxide fuel cells

Functional all-oxide thin film micro-solid oxide fuel cells (μSOFCs) that are free of platinum (Pt) are discussed in this report. The μSOFCs, with widths of 160 μm, consist of thin film La_0.6Sr_0.4Co_0.8Fe_0.2O₃ (LSCF) as both the anode and cathode and Y_0.08Zr_0.92O_1.96 (YSZ) as the electrolyte. Open circuit voltage and peak power density at 545 °C are 0.18 V and 210 μW cm⁻², respectively. The LSCF anodes show good lattice and microstructure stability and do not form reaction products with YSZ. The all-oxide μSOFCs endure long-term stability testing at 500 °C for over 100 h, as manifested by stable membrane morphology and crack-free microstructure.     

A novel design of anode-supported solid oxide fuel cells with Y2O3-doped Bi2O3, LaGaO3 and La-doped CeO2 trilayer electrolyte

Anode-supported solid oxide fuel cells (SOFCs) with a trilayered yttria-doped bismuth oxide (YDB), strontium- and magnesium-doped lanthanum gallate (LSGM) and lanthanum-doped ceria (LDC) composite electrolyte film are developed. The cell with a YDB (18 μm)/LSGM (19 μm)/LDC (13 μm) composite electrolyte film (designated as cell-A) shows the open-circuit voltages (OCVs) slightly higher than that of a cell with an LSGM (31 μm)/LDC (17 μm) electrolyte film (designated as cell-B) in the operating temperature range of 500-700 °C. The cell-A using Ag-YDB composition as cathode exhibits lower polarization resistance and ohmic resistance than those of a cell-B at 700 °C. The results show that the introduction of YDB to an anode-supported SOFC with a LSGM/LDC composite electrolyte film can effectively block electronic transport through the cell and thus increased the OCVs, and can help the cell to achieve higher power output.     

Sr2Fe4/3Mo2/3O6 as anodes for solid oxide fuel cells

Sr₂Fe_4/3Mo_2/3O₆ has been synthesized by a combustion method in air. It shows a single cubic perovskite structure after being reduced in wet H₂ at 800 °C and demonstrates a metallic conducting behavior in reducing atmospheres at mediate temperatures. Its conductivity value at 800 °C in wet H₂ (3% H₂O) is about 16 S cm⁻¹. This material exhibits remarkable electrochemical activity and stability in H₂. Without a ceria interlayer, maximum power density (P_max) of 547 mW cm⁻² is achieved at 800 °C with wet H₂ (3% H₂O) as fuel and ambient air as oxidant in the single cell with the configuration of Sr₂Fe_4/3Mo_2/3O₆|La_0.8Sr_0.2Ga_0.83Mg_0.17O₃ (LSGM)| La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF). The P_max even increases to 595 mW cm⁻² when the cell is operated at a constant current load at 800 °C for additional 15 h. This anode material also shows carbon resistance and sulfur tolerance. The P_max is about 130 mW cm⁻² in wet CH₄ (3% H₂O) and 472 mW cm⁻² in H₂ with 100 ppm H₂S. The cell performance can be effectively recovered after changing the fuel gas back to H₂.     

Direct CH4 fuel cell using Sr2FeMoO6 as an anode material

A double-perovskite Sr₂FeMoO₆ (SFMO) has been synthesized with a combined citrate-EDTA complexing method. The material shows a double-perovskite structure after reduction in 5% H₂/Ar at 1100 °C for 20 h. A single fuel cell using this material as anode is constructed with the configuration of SFMO?La_0.8Sr_0.2Ga_0.83Mg_0.17O₃?Ba_0.5Sr_0.5Co_0.8Fe_0.2O₃. The cell exhibits a remarkable electrochemical activity in both H₂ and dry CH₄, respectively. With Oxygen as oxidant, the maximum power density is 863.7 mW cm⁻² with H₂ as the fuel and 604.8 mW cm⁻² with dry CH₄ as the fuel at 850 °C, respectively. SFMO has an almost linear thermal expansion coefficient from 30 to 900 °C and is very close to that of La_0.8Sr_0.2Ga_0.83Mg_0.17O₃. A durability test of the single cell indicates that SFMO is stable in dry CH₄ operation. Therefore SFMO can be recommended as a promising anode material for LaGaO₃-based solid oxide fuel cells operating with both H₂ and dry CH₄.     

La2O3-Al2O3-B2O3-SiO2 glasses for solid oxide fuel cell applications

La₂O₃-Al₂O₃-B₂O₃-SiO₂ glasses free of alkaline earth metals were prepared in this study for SOFC applications to relieve the poison caused by BaCrO₄ or SrCrO₄ formation. The apparent densities, coefficients of thermal expansion (CTE), and softening points of the La₂O₃-Al₂O₃-SiO₂-B₂O₃ glasses prepared in this study ranged respectively from 3.24 to 4.54 g/cm³, 4.1 to 8.1 ppm/°C, and 912 to 937 °C, depending on the glass composition. The CTE value dropped with the rise in SiO₂ content and escalated with increase in La₂O₃ content. Crystallization of La_9.51(SiO_1.0404)₆O₂ and La_4.67(SiO₄)₃O was observed in part of the glasses after soaking at 800 °C. Two CTE modifiers, MgO and SDC, effectively increased the CTE of La₂O₃-Al₂O₃-SiO₂-B₂O₃ glasses. The composites of selected La₂O₃-Al₂O₃-B₂O₃-SiO₂ glasses and SDC additive on the YSZ substrate were evaluated for use as sealing materials of solid oxide fuel cells (SOFCs). Results indicated that the leakage rates for the composites of A07 glass and 60-70 vol% SDC on the YSZ plate read less than 0.02 (sccm/cm)(kg/cm²) per min at 800 °C. This property seems highly promising for ensuring long-term stability of the sealing materials for SOFC applications.     

Co-sintering anode and Y2O3 stabilized ZrO2 thin electrolyte film for solid oxide fuel cell fabricated by co-tape casting

Fabricating a large-area unit cell is very important for the development of solid oxide fuel cell (SOFC) stack. In this study, details of sintering process of half cell with NiO-yttria stabilized zirconia (YSZ) anode-supported YSZ thin electrolyte film fabricated by co-tape casting have been discussed. The results demonstrates that the shrinkages and shrinking rates mismatches between the electrolyte and the anode can be controlled by the organic additive content in the anode slurry composition and heating rate. Low heating rate suppresses the cracks formation in the electrolyte films. A warp-free unit cell with size of 100 × 100 mm² and dense electrolyte has been successfully fabricated. A power of 22.2 W, with a power density of 0.27 W cm⁻² has been achieved at 0.7 V and 750 °C in O₂/humidified H₂ atmosphere. The area specific resistance of the cell is 1.20 Ω cm² at 0.7 V and 750 °C.     

Constrained sintering of Y2O3-stabilized ZrO2 electrolyte on anode substrate

Understanding the sintering processes extensively is critical in fabricating a flat cell for solid oxide fuel cell stacks, but few have reported the sintering process and stress development during the constrained sintering of the electrolyte layer on anode substrate. In this study, we show that the green tape of half cell fabricated by co-tape casting cracks into several pieces when it is heated directly to 1400 °C of profile I, while it remains flat and complete when the green tape is sintered with additional pre-sintered profile at 1300 °C of profile II. The strain rate characteristics indicate that the difference of 2.43 × 10⁻⁶ s⁻¹ between the electrolyte and the anode layer leads to the stress development in the directly sintered cell, while it reduces to 6.7 × 10⁻⁸ s⁻¹ for the pre-sintered cell, which is only 3% of that without pre-sintering. The stress based on continuum model calculated results in the sintered cell demonstrates that the stress increases from 0 at about 1000 °C to 2.60 MPa at 1300 °C, and increased from 2.60 MPa to 6.54 MPa in temperature range of 1300–1400 °C. But it was lower than half of the stress for the pre-sintered cell according to profile II. The SEM images, together with a circuit voltage of 2.22 V for two cell stack, indicate that the electrolyte of the unit cell is dense. The power is 41.7 W, with a power density of 0.26 W cm⁻² at 1.4 V and 750 °C for a two tells stacks sintered according to profile II. The ASR of the two cells stack is 2.50 Ω cm².     

A novel bilayered Sr0.6La0.4TiO3/La0.8Sr0.2MnO3 interconnector for anode-supported tubular solid oxide fuel cell via slurry-brushing and co-sintering process

Considering that conventional lanthanum chromate (LaCrO₃) interconnector is hard to be co-sintered with green anode, we have fabricated a novel bilayered interconnector which consists of La-doped SrTiO₃ (Sr_0.6La_0.4TiO₃) and Sr-doped lanthanum manganite (La_0.8Sr_0.2MnO₃). Sr_0.6La_0.4TiO₃ is conductive and stable in reducing atmosphere, locating on the anode side; while La_0.8Sr_0.2MnO₃ is on the cathode side. A slurry-brushing and co-sintering method is applied: the Sr_0.6La_0.4TiO₃ and La_0.8Sr_0.2MnO₃ slurries are successively brushed onto green anode specimen, followed by co-firing course to form a dense bilayered Sr_0.6La_0.4TiO₃/La_0.8Sr_0.2MnO₃ interconnector. For operating with humidified hydrogen and oxygen at 900 °C, the ohmic resistances between anode and cathode/interconnector are 0.33 Ω cm² and 0.186 Ω cm², respectively. The maximum power density is 290 mW cm⁻² for a cell with interconnector, and 420 mW cm⁻² for a cell without it, which demonstrates that nearly 70% of the power output can be achieved using this bilayered Sr_0.6La_0.4TiO₃/La_0.8Sr_0.2MnO₃ interconnector.     

La-doped SrTiO3 interconnect materials for anode-supported flat-tubular solid oxide fuel cells

An interconnect layer in an anode-supported flat-tubular solid oxide fuel cell connects electrically unit cells and separates fuel from oxidant in the adjoining cells. Nano-sized La-doped SrTiO₃ for the interconnect is synthesized in this study by the Pechini method using citric acid. The materials with stoichiometric and Sr-deficient compositions are prepared and sintered in an oxidizing atmosphere. The synthesized fine powders exhibit high sinterability, leading to near-full densification. The Sr deficiency plays a crucial role in mechanical, electrical and thermal expansion properties. The interconnect is coated using the synthesized powder on a porous flat-tubular anode support by a screen printing process. The thin and dense layer is obtained after co-sintering in air, and the interconnect/anode interface remains intact upon reduction.     

Numerical modeling of an anode-supported SOFC button cell considering anodic surface diffusion

A two-dimensional isothermal mechanistic model of an anode-supported solid oxide fuel cell was developed based on button-cell geometry. The model coupled the intricate interdependency among the ionic conduction, electronic conduction, gas transport, and the electrochemical reaction processes. All forms of polarizations were included. The molecular diffusion, Knudsen diffusion, as well as the simplified competitive adsorption and surface diffusion were also considered. An electric analogue circuit was used to determine the effective hydrogen diffusivity. The model results showed good agreement with the published experimental data in different H₂–H₂O mixtures without any other calibrations after the parameter estimation according to the experimental data in baseline operating condition. The distributions of species concentration and current density were predicted and the effects of cathode area, gas components, and anode thickness on the cell performance were studied.     

Solution combustion synthesis of Ce0.6Mn0.3Fe0.1O2 for anode of SOFC using LaGaO3-based oxide electrolyte

This paper describes the potential of solution combustion synthesis (SCS) method for preparing Ce_0.6Mn_0.3Fe_0.1O₂ (CMF) as the anode material for solid oxide fuel cells (SOFC). The stability, crystallinity, morphology, and surface area of the products were depended on the fuel ratio used in SCS as investigated by TGA, XRD, SEM, and BET, which correspondingly influenced their electrochemical properties. The SCS-derived products were directly used for preparing anodes by sintering the screen-printed powders on the electrolyte membrane, and were evaluated from power generation performance, which were compared with the conventional solid-state-reaction (SSR) sample. Significantly, under configuration of the cell of CMF/La_0.8Sr_0.2Ga_0.8Mg_0.15Co_0.05O₃/Sm_0.5Sr_0.5CoO₃ using humidified hydrogen gas as a fuel and O₂ as an oxidizing agent, the maximum power densities obtained were 1.23 W/cm² at 1000 °C for the SCS product (CMF1) obtained at ? = 0.5. This value was higher than 1.09 W/cm² for the SSR-derived sample under the same evaluation conditions. The results appealed benefits of SCS method for preparing CMF as the anode material with high power generation performance for SOFC, due to its large surface area and nanosized grains, in which fuel ratio was a key parameter for its synthesis.     

Advance fuel cells using Al2O3-nNaAlO2 composite as ion-conducting membrane

In present work, we reported an novel oxide-salt Al₂O₃NaAlO₂ composite, which was prepared by mixing Al₂O₃ and Na₂CO₃ two phase materials in different weight ratio, and then sintering at 1100 °C. The X-ray diffraction pattern, scanning-electron microscope and impedance spectra are applied to characterize the crystal structure, morphology and electrical properties of the Al₂O₃NaAlO₂ composite. The Al₂O₃NaAlO₂ composite as electrolyte membrane was sandwiched by two pieces of Ni_0.8Co_0.15Al_0.05Li-oxide (NCAL) electrode layer to construct advanced fuel cell. Optimizing the weight ratio of Al₂O₃ and NaAlO₂, such cell delivered an highest power density of 789 mW/cm² and an open circuit voltage (V_oc) of 1.13 V at 575 °C. The superior performance is mainly due to the excellent ion-conducting of Al₂O₃NaAlO₂ composites and the outstanding catalysis activity of the NCAL eletrodes. The EIS results revealed that the Al₂O₃NaAlO₂ composite possessed superior ionic conductivity of 0.121 S/cm at 575 °C. The interfacial effects between oxide-salt two phase including space-charge and structural misfit at the interface region dominated the ion transport for Al₂O₃NaAlO₂ composite.     

Effect of anode thickness on impedance response of anode-supported solid oxide fuel cells

This study examines effects of the anode functional layer thickness on the performance of anode-supported solid oxide fuel cells (SOFCs). The SOFCs with different AFL thicknesses (8 μm, 19 μm, and 24 μm) exhibit similar power densities at the measured current density range (0–2 A cm⁻²), but show different impedance responses. Further investigation on the spectra using the CNLS fitting method based on DRT-based equivalent circuit model helps us pinpoint two electrochemical processes directly affected by the AFL thickness changes, the charge transfer reaction in the AFL as well as the diffusion-coupled charge transfer reaction in the AFL. The combined effects of these two electrochemical processes probably forged a minimal impact on the overall fuel cell performance by offsetting each other, which offers a reasonable explanation of the seemingly little influence of the AFL thickness on the SOFC performance.     

Catalytic behavior of Co3O4 in electroreduction of H2O2

Spinel structure Co₃O₄ nanoparticles with an average diameter of around 17 nm were prepared and evaluated as electrocatalysts for H₂O₂ reduction. Results revealed that Co₃O₄ exhibits considerable activity and good stability for electrocatalytic reduction of H₂O₂ in 3 M NaOH solution. The reduction occurs mainly via the direct pathway when H₂O₂ concentration is lower than 0.5 M. An Al-H₂O₂ semi fuel cell using Co₃O₄ as cathode catalyst was constructed and tested at room temperature. The fuel cell displayed an open circuit voltage of 1.45 V and a peak power density of 190 mW cm⁻² at a current density of 255 mA cm⁻² operating with a catholyte containing 1.5 M H₂O₂. This study demonstrated that Co₃O₄ nanoparticles are promising cathode catalysts, in place of precious metals, for fuel cells using H₂O₂ as oxidant.     

Effect of additives on the thermal properties and sealing characteristic of BaO-Al2O3-B2O3-SiO2 glass-ceramic for solid oxide fuel cell application

The apparent densities, coefficients of thermal expansion (CTE), and softening points of the BaO-Al₂O₃-SiO₂-B₂O₃ glasses prepared in this study range from 2.61 to 3.92 g/cm³, 4.92 to 10.98 ppm/°C, and 656 to 854 °C, respectively, depending on the glass composition. The softening point decreases with increasing BaO or B₂O₃ content in the glass. The feasibility of using MgO, KAlSi₂O₆, and KAlSiO₄ to tune the CTE of BaO-Al₂O₃-SiO₂-B₂O₃ glass is investigated. Composites containing MgO additive show little change in CTE at high temperature and report a high structural stability with the change in time. Wetting angle of the glass on the YSZ substrate depends to a great extent on the soaking temperature and the BaO content in the glass. Composites of selected BaO-Al₂O₃-SiO₂-B₂O₃ glasses and MgO additive on the YSZ substrate are evaluated for use as sealing material of solid oxide fuel cells (SOFCs). Results indicate that leakage rates for the composites of L06-20 vol% MgO, L08-30 vol% MgO, and L09-40 vol% MgO are lower than the detectable limit in this study.     

Effect of the adding ferrum in nickel/GDC anode-supported solid-oxide fuel cell in the intermediate temperature

NiO-Fe₂O₃/Gadolinium-doped CeO₂ (GDC), NiO/GDC anode-supported fuel cells were compared at intermediate temperature to investigate the effect of ferrum (Fe) on anode microstructure and the cell performance. The porosity of Fe₂O₃-containing anode tubes are 36.3% (those sintered at 1350 °C) and 31.8% (those sintered at 1400 °C), which are respectively 1.1% and 5.9% higher than that of the tubes without Fe₂O₃. The effect of Fe on microstructure of anode has also been indicated by analyzing the volume fraction of the different-size pores. At the operating temperature of 600 °C, the maximum power density increased nearly 100% using Fe-Ni (after reducing) bimetal anode. From the impedance spectra, it is deduced that the improvement in power density is due to the decrease of overpotential resistance (low frequency part) related to gas transportation.     

Performance of an anode-supported solid oxide fuel cell with direct-internal reforming of ethanol

A theoretical study of a solid oxide fuel cell (SOFC) fed by ethanol is presented in this study. The previous studies mostly investigated the performance of ethanol-fuelled fuel cells based on a thermodynamic analysis and neglected the presence of actual losses encountered in a real SOFC operation. Therefore, the real performance of an anode-supported SOFC with direct-internal reforming operation is investigated here using a one-dimensional isothermal model coupled with a detailed electrochemical model for computing ohmic, activation, and concentration overpotentials. Effects of design and operating parameters, i.e., anode thickness, temperature, pressure, and degree of ethanol pre-reforming, on fuel cell performance are analyzed. The simulation results show that when SOFC is operated at the standard conditions (V = 0.65 V, T = 1023 K, and P = 1 atm), the average power density of 0.51 W cm⁻² is obtained and the activation overpotentials represent a major loss in the fuel cell, followed by the ohmic and concentration losses. An increase in the thickness of anode decreases fuel cell efficiency due to increased anode concentration overpotential. The performance of the anode-supported SOFC fuelled by ethanol can be improved by either increasing temperature, pressure, degree of pre-reforming of ethanol, and steam to ethanol molar ratio or decreasing the anode thickness and fuel flow rate at inlet. It is suggested that the anode thickness and operating conditions should be carefully determined to optimize fuel cell efficiency and fuel utilization.     

Investigation of layered Ni0.8Co0.15Al0.05LiO2 in electrode for low-temperature solid oxide fuel cells

A symmetrical cell composed of Ce_0.9Gd_0.1O_2?δ electrolyte is constructed with 0.5 mm thickness and Ni_0.8Co_0.15Al_0.05LiO₂ (NCAL)-foam Ni composite electrodes. Electrochemical performance of the cell and electrochemical impedance spectra (EIS) are measured using the three-electrode method. The maximum power densities of the cell are 93.6 and 159.7 mW cm^?2 at 500 and 550 °C, respectively. The polarization resistances of the cathode are 0.393 and 0.729 Ω cm^?2 at 550 and 500 °C, indicating that NCAL has good ORR activity. HT-XRD results for NCAL do not show phase transitions or any additional new phases at elevated temperatures, indicating that NCAL has a stable phase structure. The surface characteristics of the NCAL powders are studied by XPS and FTIR. The results reveal that Li₂CO₃ and the cation-disordered “NiO-like” phase are formed on the surface of the layered NCAL structure due to prolonged exposure to air and contain a large number of oxygen vacancies. The cation-disordered “NiO-like” phase and Li₂CO₃ composite in the melt and partial melt states in the high temperature region are considered to possess very high ionic conductivity and lower activation energy for oxygen reduction reactions.