Low-temperature constrained sintering of YSZ electrolyte with Bi2O3 sintering sacrificial layer for anode-supported solid oxide fuel cells

Solid oxide fuel cells (SOFCs) have strong potential for next-generation energy conversion systems. However, their high processing temperature due to multi-layer ceramic components has been a major challenge for commercialization. In particular, the constrained sintering effect due to the rigid substrate in the fabrication process is a main reason to increase the sintering temperature of ceramic electrolyte. Herein, we develop a bi-layer sintering method composed of a Bi₂O₃ sintering sacrificial layer and YSZ main electrolyte layer to effectively lower the sintering temperature of the YSZ electrolyte even under the constrained sintering conditions. The Bi₂O₃ sintering functional layer applied on the YSZ electrolyte is designed to facilitate the densification of YSZ electrolyte at the significantly lowered sintering temperature and is removed after the sintering process to prevent the detrimental effects of residual sintering aids. Subsequent sublimation of Bi₂O₃ was confirmed after the sintering process and a dense YSZ monolayer was formed as a result of the sintering functional layer-assisted sintering process. The sintering behavior of the Bi₂O₃/YSZ bi-layer system was systematically analyzed, and material properties including the microstructure, crystallinity, and ionic conductivity were analyzed. The developed bi-layer sintered YSZ electrolyte was employed to fabricate anode-supported SOFCs, and a cell performance comparable to a conventional high temperature sintered (1400 °C) YSZ electrolyte was successfully demonstrated with significantly reduced sintering temperature (<1200 °C).     

Atomic layer deposited YSZ overlayer on Ru for direct methane utilization in solid oxide fuel cell

We report on the fabrication of thin yttria-stabilized zirconia (YSZ) overlayers via atomic layer deposition (ALD) conformally coated on anodes of methane-fueled low-temperature solid oxide fuel cells. Nano-granular ALD YSZ overlayers that are broken into porous films at elevated temperature improved not only the initial electrochemical performance (35% increase in the maximum power density) but also the thermal stability of the porous Ru anodes by suppressing coarsening.     

Electrophoretically deposited thin film electrolyte for solid oxide fuel cell

Abstract

Thin films of 8 mol% yttria stabilised zirconia (YSZ) electrolyte have been deposited on non-conducting porous NiO–YSZ anode substrates using electrophoretic deposition (EPD) technique. Deposition of such oxide particulates on non-conducting substrates is made possible by placing a conducting steel plate on the reverse side of the presintered porous substrates. Thickness of the substrates, onto which the deposition has been carried out, varied in the range 0·5–2·0 mm. Dense and uniform YSZ thin films (thickness: 5–20 μm) are obtained after being cofired at 1400°C for 6 h. The thickness of the deposited films is seemed to be increased with increasing porous substrate thickness. Solid oxide fuel cell (SOFC) performance is measured at 800°C using coupon cells with various anode thicknesses. While a peak power density of 1·41 W cm^?2 for the cells with minimum anode thickness of 0·5 mm is achieved, the cell performance decreases with anode thickness.     

Tailoring the properties of yttria-stabilized zirconia powders prepared by the sol-gel method for potential use in solid oxide fuel cells

Yttria-stabilized zirconia (YSZ) powders have been prepared by the sol-gel method, following two alternative procedures: a series of powders was obtained by drying the sol-gel solutions in air at 100 °C until dry residue, and another series of powders was obtained by scratching the thin films deposited on cylindrical wide flat glassy surfaces after evaporating to dryness in air at 100 °C for 2 h. Samples were characterized by Scanning Electron Microscopy (SEM), nitrogen adsorption at −196 °C and Fourier Transform Infrared (FT-IR) spectroscopy. In general, a noticeable contraction of the pores is observed as the molecular size of the alcohols used grows. Powders prepared by conventional drying of sol-gel solutions at 100 °C exhibit remarkably high values of specific surface area (up to 148 m² g^− 1). On the contrary, samples prepared by scratching of the deposited thin films show a noticeable decrease in their specific surface area. Values of fractal dimension follow the same trend and indicate that, in general, the texture of the samples is mainly microporous for the first series of samples and more ordered for the second one. Finally, in order to investigate the effect of the calcination temperature on the morphological and textural properties of 3 mol% yttria-stabilized zirconia powders, once the 3YSZ powders were dried at 100 °C they were subjected to calcination at different temperatures. The experimental results suggest that the removal of residual water and alcohol occluded within the powder particles as well as the elimination of gases produced during the calcination stage play a very important role in the development of the porosity and surface area of the samples.     

Oxidation studies of anodes layer microstructures in metal supported solid oxide fuel cells

The effect of oxidation on the microstructural and mechanical stability of ceramic layers in metal supported solid oxide fuel cells is reported. Half-cells that are produced with a reduced nickel based anode are oxidized for different times and temperatures in order to assess stability limits. Samples are analyzed in terms of the effective cell curvature and microstructure, where further insight is obtained via the observation of microstructures before and after oxidization. The interpretation is aided by a comparison to the behavior of structures without electrolyte layer. Electrolyte cracking and anode delamination are observed after oxidation, where the latter is absent in case of oxidation experiments without electrolyte layer, highlighting the failure relevance of strain induced by electrolyte deposition.     

Enhancing the performance and stability of solid oxide fuel cells by adopting samarium-doped ceria buffer layer

In this work, the Sm_0.2Ce_0.8O_1.9 (SDC) buffer layer was used to replace the Gd_0.1Ce_0.9O_1.95 (GDC) buffer layer to improve the long-term stability and performance of the solid oxide fuel cells (SOFCs) in the intermediate temperature (550–750 °C). The buffer layer was prepared by screen printing method. The micromorphology of the SDC buffer layer and the cell structures was observed by scanning electron microscopy (SEM). The electrochemical impedance spectroscopy (EIS) results showed that the polarization resistance (R_P) of the cell with SDC buffer layer was smaller than that of the cell with GDC buffer layer, reducing the R_P values by 43.52% and 43.33%, respectively (SDC-cell: 0.12 Ω cm² at 650 °C and 0.27 Ω cm² at 600 °C). The maximum power density of the cell with SDC buffer layer is 560 mW cm⁻² at 650 °C, which was 25% higher than that with GDC buffer layer. The long-term durability of the cell with SDC buffer layer was better than that of the cell with GDC buffer layer. These provide an excellent prospect for utilizing SDC buffer layer.     

Synthesis and characterization of co-doped ceria-based electrolyte material for low temperature solid oxide fuel cell

Co-doped CeO₂ (Ba_0.10Ga_0.10Ce_0.80O_3–δ) was synthesized via a cost-effective co-precipitation technique, and the electrochemical properties of the solid oxide fuel cell were studied. The microstructural and surface morphological properties were investigated by XRD and SEM, respectively. The structure of the prepared material was found to be cubic fluorite with an average crystallite size of 36?nm. The ionic conductivity of the prepared BGC (Ba_0.10Ga_0.10Ce_0.80O_3–δ) electrolyte material was measured as 0.071?S?cm^?1. The activation energy was found to be 0.46?eV using an Arrhenius plot. The maximum power density and current density achieved were 375?mW?cm^?2 and 893?mA?cm^?2, respectively, at 650?°C with hydrogen as a fuel. This study shows that the prepared co-doped electrolyte material could be used as a potential electrolyte to lower the operating temperature of solid oxide fuel cells.     

An investigation of metal ion diffusion in ceria‐based solid oxide fuel cell with barium‐containing anode

Metal ion diffusion is an effective strategy to suppress the internal electronic short circuit in ceria‐based solid oxide fuel cells (SOFCs). This could be achieved by fabricating an electron‐blocking layer between the barium‐containing anode and ceria‐based electrolyte. In this paper, a 0.6NiO‐0.4BaZr_0.1Ce_0.7Y_0.2O_3‐δ (NiO‐BZCY) anode‐supported cell based on Gd_0.1Ce_0.9O_2‐δ (GDC) electrolyte was employed to evaluate the internal metal ion diffusion behavior. The high open circuit voltages of about 1 V obtained at 550‐700°C can be attributed to in situ formation of an electron‐blocking interlayer between NiO‐BZCY and GDC. Microstructural analyses of the interlayer grains obtained by traditional solid‐state reaction were carried out. Phase identification demonstrated that the electron‐blocking interlayer had a perovskite structure. SEM and TEM analyses indicated formation of a new compound in the interlayer, of which the composition was determined as Zr, Y, and Ni co‐doped BaCe_0.9Gd_0.1O₃ with orthorhombic structure.     

Electrospun composite nanofibers for intermediate-temperature solid oxide fuel cell electrodes

Considering the dominant loss associated with surface oxygen exchange reactions among other complex electrochemical processes, the design of an electrode structure for the reasonable operation of solid oxide fuel cells is challenging. The surface oxygen exchange reaction can be considerably facilitated using composite nanofibers containing the electrode, electrolyte, and catalyst. The composite nanofiber electrodes containing Pd show the smallest polarization resistance of 0.031 Ωcm² and the maximum power density of 0.7 W/cm² at 650 °C, which are 39.2% and 12.5% improved values compare to the catalyst-free composite nanofiber electrodes, respectively. These results provide an facile fabrication strategy for developing high-performance electrodes for use in solid oxide fuel cells.     

Modeling the temperature field in the reforming anode of a button-shaped solid oxide fuel cell

A model predicting the temperature field in the porous reforming anode of a solid oxide fuel cell is presented herein. The model is based on mass, momentum, and heat balances of a chemically reacting mixture of gases within the porous matrix of the anode. The important novel characteristic of the model is the consideration of the both internal reforming and electrochemical reactions in the bulk of the porous anode. The electronic and ionic currents in the anodes are calculated utilizing the solution of the Poisson equations for the electric potentials in the porous medium. The transfer current density is described by the Butler–Volmer equation.The model is applied to investigate the temperature field and the reactive flow in button-shaped fuel cells with uniform and graded (multi-layer) anodes composed of Ni and YSZ particles with methane/water vapor mixture used as the fuel. The maximum temperature difference between the hot and cold spots of the anodes is found to reach up to 200 K. The results indicate that the generation of Joule heating caused by the current passing through the anode and the activation losses are the dominating heat sources compared to the gas-water shift and electrochemical reactions.     

Enhancing the performance of traditional La2NiO4+x cathode for proton-conducting solid oxide fuel cells with Zn-doping

A Zn-doping strategy was employed to modify the Ruddlesden-Popper (R–P) structure oxide La₂NiO_4+x to improve hydration and proton diffusion ability. First-principles calculations indicated the formation of interstitial oxygen instead of oxygen vacancy is favorable for Zn-modified and Zn-free La₂NiO_4+x. The doping of Zn significantly lowered the hydration energy for La₂NiO_4+x and decreased the proton migration barrier. The electrical conductivity relaxation confirmed that the Zn-modified La₂NiO_4+x sample had a higher proton diffusion rate than the Zn-free sample. Furthermore, the Zn-doping strategy did not alter the thermal expansion behavior of the material, and both Zn-modified and Zn-free La₂NiO_4+x samples showed a similar thermal expansion coefficient value, which was also close with the electrolyte materials. As a result, the Zn-modified La₂NiO_4+x exhibited suitability as the cathode for proton-conducting solid oxide fuel cells (H–SOFCs). An H–SOFC using the Zn-modified La₂NiO_4+x showed a relatively high peak power density of 1070 mW cm^-2 at 700 °C, significantly larger than that La₂NiO_4+x-based H–SOFCs reported previously.     

Nanostructured engineering of nickel cermet anode for solid oxide fuel cell using inkjet printing

A single-step wet-chemical synthesis of NiO-SDC (Sm³⁺ doped ceria) colloidal ink for the inkjet printing (IJP) of nanostructured anodic layers with enhanced catalytic activity for solid oxide fuel cells (SOFCs) is developed and characterized. Dynamic light scattering, scanning electron microscope, and Raman spectroscopy revealed stable nanoparticles with the main size of 11.85 nm within the ink solution. Rheology parameters were analyzed, and the anode was printed. Porous post-sintered Ni-cermet layer, with a thickness of 15?25 μm contained near-spherical nanoparticles of 40?80 nm, was obtained. X-ray diffraction confirmed the phase composition of the cermet layer. Electrical impedance spectroscopy demonstrated a significant reduction, by more than 80 %, in the area-specific resistance of the IJP half-cell in comparison with the Screen-printed half-cell. The microstructure engineering using IJP provides fabrication of the cermet NiO-SDC layer with a conjugated structure, which ultimately enhances the catalytic activity of the SOFC.     

Biodiesel formulations as fuel for internally reforming solid oxide fuel cell

Biodiesel (alkyl ester of rapeseed oil) is prepared using various, methyl, ethyl and butyl alcohols through the transesterification process. Sodium hydroxide and sulfuric acid are used as catalyst for methyl alcohol, ethyl alcohol and butyl alcohol respectively. Biodiesel-water formulations are formulated using water and emulsifiers like sodium lauryl sulphate (SLS) and SPAN 80 in a high shear mixer. The formulations are tested at 800 °C as fuel for internal reforming in solid oxide fuel cells (SOFCs). The formulations based on methyl and butyl esters require the use of emulsifiers to prepare stable emulsions, while ethyl esters are able to form stable emulsions without emulsifiers. The decrease in the biodiesel concentration of formulation does not have any effect on the power density of the ethyl ester formulation. Fuel cells fuelled with 20% formulations lasted longer than 50% formulations in all the formulations tested as result of increase in steam carbon ratio resulting in effective removal of carbon deposited on the anode surface. Butyl ester formulations exhibited the worst performance in both types of formulation tests. The best performance was exhibited by 20% ethyl formulation in terms of life of the cell but 50% methyl ester formulations exhibit the highest power density.     

Electrochemical behaviour of propane-fed solid oxide fuel cells based on low Ni content anode catalysts

Two different low Ni content (10 wt.%) anode catalysts were investigated for intermediate temperature (800 °C) operation in solid oxide fuel cells fed with dry propane. Both catalysts were prepared by the impregnation of a Ni-precursor on different oxide supports, i.e. gadolinia doped ceria (CGO) and La_0.6Sr_0.4Fe_0.8Co_0.2O₃ perovskite, and thermal treated at 1100 °C for 2 h. The Ni-modified perovskite catalyst was mixed with a CGO powder and deposited on a CGO electrolyte to form a composite catalytic layer with a proper triple-phase boundary. Anode reduction was carried out in-situ in H₂ at 800 °C for 2 h during cell conditioning. Electrochemical performance was recorded at different times during 100 h operation in dry propane. The Ni-modified perovskite showed significantly better performance than the Ni/CGO anode. A power density of about 300 mW cm⁻² was obtained for the electrolyte supported SOFC in dry propane at 800 °C. Structural investigation of the composite anode layer after SOFC operation indicated a modification of the perovskite structure and the occurrence of a La₂NiO₄ phase. The occurrence of metallic Ni in the Ni/CGO system caused catalyst deactivation due to the formation of carbon deposits.     

Robust and reliable solid oxide fuel cells with modified thin film YSZ electrolyte

Reduce electrolyte thickness can improve solid oxide fuel cell (SOFC) performance. However, thinner electrolyte often contains prominent defects and flaws, which may decrease the yield and increase operation risk. This work proposes a method to modify the thin film YSZ electrolyte, to improve cell reliability and durability. The as-sintered anode supported half-cell with screen printed YSZ electrolyte was immersed in precursor solution of Y(NO₃)₃·6H₂O and Zr(NO₃)₄·5H₂O, and being treated under hydrothermal condition of 150°C for 12 h. As a result, the modified cells show slight increase in the OCV values. Furthermore, the hydrothermal modification effectively promotes interface sintering between YSZ electrolyte and GDC barrier layer, yielding a smaller ohmic resistance of .142 Ω·cm² (a decrease of ∼11%) and a higher peak power density of .964 W/cm² (an increase of ∼18%) at 750°C, than pristine cell. Moreover, the modified cell operates stably over 300 h, while the pristine cell presents large and irregular voltage fluctuations. This work suggests that the hydrothermal modification is an effective and promisingly industrial applicable method for thin film electrolyte recovery in SOFCs.     

Development of LSM/YSZ composite cathode for anode-supported solid oxide fuel cells

This paper presents the effect of (La,Sr)MnO₃ (LSM) stoichiometry on the polarization behaviour of LSM/Y₂O₃-ZrO₂ (YSZ) composite cathodes. The composite cathode made of A-site deficient (La_0.85Sr_0.15)_0.9MnO₃ (LSM-B) showed much lower electrode interfacial resistance and overpotential losses than that made of stoichiometric (La_0.85Sr_0.15)_1.0MnO₃ (LSM-A). The much poorer performance of the latter is believed to be due to the formation of resistive substances such as La₂Zr₂O₇/SrZrO₃ between LSM and YSZ phases in the composite electrode. A slight A-site deficiency (∼0.1) was effective in inhibiting the formation of these resistive substances. A power density of ∼1 W cm⁻² at 800 °C was achieved with an anode-supported cell using an LSM-B/YSZ composite cathode. In addition, the effects of cathodic current treatment and electrolyte surface grinding on the performance of composite cathodes were also studied.     

Fabrication of electrolyte materials for solid oxide fuel cells by tape-casting

In this research, solid oxide fuel cell electrolytes were fabricated by aqueous tape-casting technique. The basic compositions for SOFC electrolyte systems were focused on yttria-stabilized zirconia (YSZ) system. The powders used in this study were from different sources. ZrO₂-based system doped with 3, 8, and 10 mol% of Y₂O₃, and 8YSZ electrolyte tape illustrated the desirable properties. The grain size of the sintered electrolyte tapes was in the range of 0.5–1 μm with 98–99% of theoretical density. Phase and crystal structure showed the pure cubic fluorite structure for 8–10 mol% YSZ and tetragonal phase for 3 mol% doped. The electrolyte tapes sintered at 1450 °C for 4 h had the highest ionic conductivity of 30.11 × 10⁻³ S/cm which was measured at 600 °C. The flexural strengths were in the range of 100–180 MPa for 8–10 mol% YSZ, and 400–680 MPa for 3 mol% YSZ.     

Microstructural characterization of spray-dried NiO-8YSZ particles as plasma sprayable anode materials for metal-supported solid oxide fuel cell

The plasma spray method is widely used to produce NiO-8YSZ (composed of nickel oxide (NiO) and 8 mol% yttria-stabilized zirconia) anode layers in metal-supported solid oxide fuel cell (SOFC). Flowability control of microsized particles is important for achieving consistent performance of the SOFC anode layer. When microsized particles are fabricated via spray drying and sintering, the most significant factors that influence flowability are their sizes, distribution, and surface conditions. Thus, the aim of this study is to analyze the fabrication conditions for microsized NiO-8YSZ cermet particles made from a nanoscale, sinterable NiO-8YSZ dispersion solution by using an appropriate spray-drying and sintering process. The characteristics of the as-sprayed and sintered NiO-8YSZ composite particles (such as size, distribution, roughness, and nanostructure) were analyzed via field emission scanning electron microscope (FE-SEM), energy dispersive spectroscopy (EDS), particle size distribution (PSD), Brunauer–Emmett–Teller (BET) surface area, and atomic force microscopy (AFM). The as-sprayed microsized NiO-8YSZ particles became smaller and more uniformly distributed as the rotational speed used for spray drying increased. As a result of sintering, the extent of shrinkage of as-sprayed microsized NiO-8YSZ particles generated at high RPMs was lower than that of particles formed at low RPMs. No significant difference was observed in the distribution of the nanosized NiO and 8YSZ particles at different rotational speeds. Furthermore, the highest BET surface areas were observed for particles generated at 8000 RPM before sintering at 13.74 m²/g. After sintering, the highest BET surface area was 0.94 m²/g for particles generated at 16,000 RPM. Differences in nanostructure and surface roughness between as-sprayed and sintered microsized NiO-8YSZ particles were identified via AFM. This study is expected to provide important fundamental information useful for optimizing SOFC efficiency by promoting flowability control during the production of SOFC anodes via plasma spraying.     

Influence of doctor blade gap on the properties of tape cast NiO/YSZ anode supports for solid oxide fuel cells

In this study, various tape cast NiO/YSZ anode support layers with similar geometric properties are fabricated by varying the doctor blade from 100?µm to 200?µm with an increment of 25?µm. The mechanical properties of the anode support layers are investigated by three point bending tests of 30 samples for each doctor blade gap. The reliability curves of the flexural strength data are also obtained via two-parameter Weibull distribution method. The effects of the doctor blade gap on the microstructure and the electrochemical performance of the anode support layers are determined via SEM investigations and single cell performance-impedance tests, respectively. The apparent porosities of the samples are also measured by Archimedes’ principle. The results indicate that the doctor blade gap or the resultant tape thickness influences the microstructure of tape cast NiO/YSZ anode supports significantly, yielding different mechanical and electrochemical characteristics. At a reliability level of 70%, the highest flexural strength of 110.20?MPa is obtained from the anode support layer with a doctor blade gap of 175?µm and the 16?cm² active area cell with this anode support layer also exhibits the highest peak performance of 0.483?W/cm² at an operating temperature of 800?°C. Thus, a doctor blade gap of 175?µm is found to have such a microstructure that provides not only better mechanical strength but also higher electrochemical performance.     

Fabrication of dense and defect-free diffusion barrier layer via constrained sintering for solid oxide fuel cells

To enable the development of next-generation solid oxide fuel cells (SOFCs), the fabrication of dense and defect-free diffusion barrier layers via constrained sintering has been a significant challenge. Here, we present a double layer technique that enables complete densification of a defect-free gadolinia-doped ceria diffusion barrier layer. In this approach, top and bottom layers were individually designed to perform unique functions based on systematic analysis of constrained sintering. The top layer, which contains 1 wt% CuO as a sintering aid, provides sufficient sintering driving force via liquid-phase sintering to allow complete densification of the film, while the bottom layer without a sintering aid prevents detrimental chemical reactions and regulates the global sintering rate to eliminate macro-defects. Such fabrication of dense diffusion barrier layers via a standard ceramic processing route would allow the use of novel cathode materials in practical SOFC manufacturing. Furthermore, the strategy presented in this study could be exploited in various multi-layer ceramic applications involving constrained sintering.