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.     

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.     

Mechanical properties of cBN-Al composite materials

The relationship between microstructure and mechanical properties for a wide range of composite materials based on polycrystalline cubic boron nitride and aluminium as a binder phase (PcBN-Al) has been examined. The PcBN-Al composites were made using high-pressure, high-temperature (HPHT) sintering methods, yielding materials with grain sizes of cBN between 2 and 20 μm and an initial amount of Al binder between 15 and 25 vol.%. Hardness ranged between 15 and 40 GPa, while fracture toughness and strength were between 6.4-8.0 MPa m^1/2 and 355-454 MPa, respectively. Fractography was employed to investigate the large scatter in fracture strengths and correlate fracture strength with fracture toughness through the size of the fracture origins.     

Micro-scale evolution of mechanical properties of glass-ceramic sealant for solid oxide fuel/electrolysis cells

The structural integrity of the sealant is critical for the reliability of solid oxide cells (SOCs) stacks. In this study, elastic modulus (E), hardness (H) and fracture toughness (K_IC) of a rapid crystallizing glass of BaO–CaO–SiO₂ system termed “sealant G” are reported as determined using an indentation test method at room temperature. A wide range of indentation loads (1 mN–10 N) was used to investigate the load-dependency of these mechanical properties. Values of 95 ± 12 GPa, 5.8 ± 0.2 GPa and 1.15 ± 0.07 MPa m^0.5 were derived for E, H and K_IC using the most suitable indentation loads. An application relevant annealing treatment of 500 h at 800 °C does not lead to a significant change of the mechanical properties. Potential self-healing behavior of the sealant has also been studied by electron microscopy, based on heat treatment of samples with indentation-induced cracks for 70 h at 850 °C. Although the sealant G is considered to be fully crystallized, evidence indicates that its cracks can be healed even in the absence of a dead load.     

Mechanical behaviour of multi-layer half-cells of microtubular solid oxide fuel cells fabricated by the co-extrusion process

Multi-layer micro-tubes consisting of four anode layers of NiO and YSZ mixture, and an electrolyte layer, YSZ, were fabricated by co-extrusion. The die designed in this study is able to extrude 5 layers around a sacrificial core, which eliminates the difficulties from the use of mandrel in processing tubes. Scanning electron microscopy (SEM) results revealed that this technique can be used for the successful fabrication of multi-layer microtubes with good bonding between the layers. 3-point bending was used to evaluate the mechanical properties of these co-extruded multi-layer samples. Moreover, thermal shock resistance of the tubes was investigated by water quenching from an elevated temperature. The results were compared with those obtained for conventionally extruded single layer samples. The co-extruded samples were found to have the highest average strength and also the highest weibull modulus and reliability. It was also found that multi-layer anode can significantly improve thermal shock resistance.     

Durability and performance of CGO barriers and LSCF cathode deposited by spray-pyrolysis

Ce_0.9Gd_0.1O_1.95 (CGO) protective layers are prepared by two different methods to prevent the reaction between the Zr_0.84Y_0.16O_1.92 (YSZ) electrolyte and the La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF) cathode. In the first method, the CGO layers are deposited by an airbrushing technique from an ink containing CGO particles without and with cobalt as sintering aids. The second strategy consists in preparing both a dense CGO barrier layer and a porous LSCF cathode by spray-pyrolysis deposition, in order to further reduce the fabrication temperature and minimize the reaction between the cell components. The samples prepared by spray-pyrolysis exhibit better performance and durability than those obtained by conventional sintering methods. The results suggest that the interfacial reactivity between YSZ and LSCF as well as the Sr-enrichment at the cathode surface can be avoided by using low-temperature fabrication methods and by operating at temperatures lower than 650?°C.     

Effects of manganese substitution at the B-site of lanthanum-rich strontium titanate anodes on fuel cell performance and catalytic activity

Sr_0.4La_0.6Ti_1−xMn_xO_3−δ with rhombohedral structure has been investigated in terms of their electrochemical performance, redox stability, and electro-catalytic properties for solid oxide fuel cell anodes. The performance of Sr_0.4La_0.6Ti_1−xMn_xO_3−δ anodes for solid oxide fuel cells strongly depends on the Mn substitution at the B-site of the perovskites. Electrical conductivity of Sr_0.4La_0.6Ti_1−xMn_xO_3−δ increases with increasing Mn content. X-ray photoelectron spectroscopy analysis reveals that the amount of Mn³⁺ and Ti³⁺, which is an electronic charge carrier, increases with Mn doping. The reduced anode powders with high Mn/Ti ratio show oxygen storage capability and a low carbon deposition rate. Linear thermal expansion coefficients of Sr_0.4La_0.6Ti_1−xMn_xO_3−δ anodes range from 9.46×10⁻⁶ K⁻¹ to 11.3×10⁻⁶ K⁻¹. The maximum power densities of the single cell with the Sr_0.4La_0.6Ti_0.2Mn_0.8O_3−δ anode in humidified H₂ and CH₄ at 800 °C are 0.29 W cm⁻² and 0.24 W cm⁻², respectively.     

Review of perovskite-structure related cathode materials for solid oxide fuel cells

In this article, different perovskite-structure related materials are reviewed, which could be potential candidates for cathode materials in solid oxide fuel cells. Solid oxide fuel cells provide an alternative, environmentally viable and efficient option to conventional electricity-producing devices. Different properties are required for the materials to qualify as a cathode for solid oxide fuel cells. Therefore, the analysis and review are done based on the process parameters and their effect on the electrical conductivity, electrochemical properties, the coefficient of thermal expansion and mechanical properties of different cathode materials. Fracture toughness and hardness have been the focus while analysing the mechanical properties. The selection of the initial composition, dopants and their valence plays a vital role in deciding the properties mentioned above of cathode materials. The prospective cathode materials classified as cobalt-based and cobalt-free are further bifurcated based on the A-site elements of the perovskite (ABO₃) structure. Also given in this article is the summary of the latest development on the cathode materials. As observed from the properties studied, cobalt-based materials tend to have higher conductivity than cobalt-free materials. While cobalt-free compositions are cost-effective and have a comparable coefficient of thermal expansion with other components of solid oxide fuel cells. The last section of the article gives the future scope of the research.     

Gas-tight yttria-doped barium zirconate thin film electrolyte via chemical solution deposition technique

A 500 nm thick yttria-doped barium zirconate (BZY) proton conducting electrolyte film, fabricated via a low-cost and high-throughput chemical solution deposition (CSD) technique, was sintered at a remarkably low temperature of 1000 °C, which is much lower than the typical solid state sintering temperature of minimum 1300 °C. Therefore, the detrimental issues, commonly encountered in solid state sintering, such as barium evaporation and phase separation, were not observed. Gas-tightness of the BZY film was confirmed by 8 h of stable open circuit voltage (OCV) at 1.08 V from a button fuel cell with NiO-BZY anode substrate and LSCF cathode. The application of the film is aimed at the electrolytes of intermediate to low temperature solid oxide fuel cells (SOFCs).     

Development of La(CrCoFeNi)O3 system perovskites as interconnect and cathode materials for solid oxide fuel cells

The purpose of this research is to develop interconnect and cathode materials for use in solid oxide fuel cells (SOFCs) which demonstrate desired properties of outstanding sintering properties, high electrical conductivity, and excellent chemical stability at high temperatures. Five different perovskite oxides of lanthanum in combination with chromium, iron, cobalt and nickel oxides powders, i.e. LaCr_0.7Co_0.1Fe_0.1Ni_0.1O₃(LCr7CFN), LaCo_0.7Cr_0.1Fe_0.1 Ni_0.1O₃(LCo7CFN), LaFe_0.7Cr_0.1Co_0.1Ni_0.1O₃(LFe7CCN), LaNi_0.7Cr_0.1Co_0.1Fe_0.1O₃(LNi7CCF), and LaCr_0.25Co_0.25Fe_0.25Ni_0.25O₃(LCCFN), were synthesized through the Pechini method. XRD results show that all materials are in single phase, either rhombohedral or orthorhombic crystal structure. The resulting powders were able to be sintered to a high relative density at a temperature of 1400 °C for 2 h in air. The electrical conductivity of the sintered sample was measured and evaluated from 300 °C to 800 °C. The LCCFN sample appears to have the best combination of sintering property (approximate 94% relative density) and electrical conductivity (88.13 Scm⁻¹ at 800 °C).     

Double Torsion testing of thin porous zirconia supports for energy applications: Toughness and slow crack growth assessment

Thin, porous zirconia-based ceramic components are of high interest in energy application devices where they are used as structural ceramics. Mechanical reliability of such devices is not only dependent on the fracture toughness of the ceramic components, but also on their sensitivity to slow crack growth (SCG). In this work, the fracture toughness and SCG behavior of porous (4.5–45.5%) and thin (∼ 0.25 mm) 3Y-TZP ceramics are investigated using the Double Torsion method. The analysis of the double torsion data, previously developed for dense materials, was here assessed and adapted. The compliance of the samples was observed to change linearly with crack length and the measured stress intensity factor was dependent on crack length, as for dense materials. This dependency decreased by increasing the sample porosity. For all materials, the ratio of the SCG threshold to fracture toughness was of 0.56 ± 0.06.     

Electrochemical characteristics of electrospun La0.6Sr0.4Co0.2Fe0.8O3−δ-Gd0.1Ce0.9O1.95 cathode

The poor activity of cathode materials for electrochemical reduction of oxygen in intermediate and low temperature regime (<700 °C) is a key obstacle to reduced-temperature operation of solid oxide fuel cells (SOFCs). In our previous work, the direct methane fuel cell exhibits approximately 1 W cm⁻² at 650 °C in hydrogen atmosphere without any functional layers when the electrospun LSCF–GDC cathode was applied into the La₂Sn₂O₇–Ni–GDC anode-supported cell, which is approximately two times higher performance than 0.45 W cm⁻² of the cell with the conventional LSCF–GDC cathode. For detailed analysis of the fibrous cathode, the symmetrical cells with the electrospun and conventional LSCF–GDC cathode are fabricated, and then their electrochemical characteristics are measured by using electrochemical impedance spectroscopy (EIS). Each resistance contribution is determined by equivalent circuit consisting of a series resistance (R_s) and three arcs to describe the polarization resistance of the cathode. Total polarization resistance of the electrospun LSCF–GDC cathode is approximately two times lower than that of the conventional LSCF–GDC cathode at 650 °C, which is attributed to fibrous microstructures and large amount of pores in 100–200 nm. The results correspond to the difference in the cell performances obtained from our previous work.     

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.     

The influence of activated carbon as an additive in anode materials for low temperature solid oxide fuel cells

The effects of activated carbon (AC) as an additive in multi-oxide nano composite LiNiCuZn–O for application as anode in solid oxide fuel cell (SOFC) is reported. The composite was synthesized using solid state reactions method with varying content of AC in range 0.1%–0.9% for use as anode in the cell. The cell was composed of the synthesized composite as anode, LiNiCuZn–O as cathode and Samaria doped ceria (SDC) as electrolyte. The prepared composites were characterized for morphology and crystal structure by scanning electron microscope (SEM) and x-ray diffraction (XRD) respectively. Furthermore, the crystallite sizes of LiNiCuZn–O and LiNiCuZn–O with AC as an additive have been found in the range from 50 nm to 70 nm. The prepared composite materials were observed porous and the porosity of the sample having 0.5% additive was found highest. The conductivity and power density of the SOFC were studied at temperature of 600 °C. The maximum value of conductivity was found as 4.79 S/cm for the composite containing 0.5% AC as measured by using 4-probe method. The maximum value of power density of the fuel cell with anode comprising of 0.5% AC along with the mentioned cathode and the electrolyte was 455 mW/cm². Therefore, out of the compositions studied, the composite comprising of LiNiCuZn–O with 0.5% AC offered best performance for anode in the cell. This oxide composite is reported as a potential candidate for use as anode in low temperature SOFCs.     

Fabrication and characterization of a dual layer ceramic interconnect on a porous NiO-YSZ anode support

An La_0.8Sr_0.2MnO₃ (LSM)/Sr_0.88Y_0.08TiO₃ (SYT) dual layer interconnect was coated on an NiO-YSZ porous support via screen printing and co-firing processes. SYT and LSM single phase powders were synthesized by solid state reaction and Pechini method, respectively. The thickness of the dual layer was approximately 200 μm. The LSM/SYT dual layer showed a good gas-tightness and had a uniform microstructure without cracks, delamination or warpage. No atomic inter-diffusion between the dual layer and the NiO-YSZ support was detected, suggesting that SYT is phase compatible with NiO-YSZ. The area specific resistance (ASR) of the LSM/SYT/Ni-YSZ sample was 0.19 Ω cm² at 800 °C under oxidizing/reducing atmospheres; this value was lower than that of the SYT/Ni-YSZ sample.     

Ni–LnOx (Ln = Dy, Ho, Er, Yb and Tb) cermet anodes for intermediate-temperature solid oxide fuel cells

Ni–LnO_x (Ln = Dy, Ho, Er, Yb and Tb) cermets are investigated as the anodes of intermediate-temperature solid oxide fuel cells using ceria-based electrolyte to seek insights into the properties and electrocatalytic activity of these lanthanide oxides, whose oxygen ion conductivity is negligible. They have displayed similar electrochemical activity which is comparable to, if not higher than, those of the commonly Ni-doped ceria cermets. The anode performance has been found to depend strongly on cermet composition and porosity. Temperature programmed reduction study and EIS analysis under different hydrogen partial pressure suggest that the catalytic activity of the Ni–LnO_x cermets might be originated from the hydrogen adsorption ability on the LnO_x surface, promoting hydrogen spillover process, and consequently enhancing the electrochemical oxidation of the fuel.     

Carbon contents design and characterization of gradient cemented carbonitrides with nano-TiN addition

Gradient cemented carbonitrides with brittle cubic phase depleted in the surface layer were prepared in the paper. This gradient material is believed to be a promising composite used as the substrate of a coated insert for machining operation. The formation of gradient layers stems from nitrogen decomposition. In this paper, nano-TiN was introduced as a nitrogen supplier, and besides it functioned as a grain growth inhibitor of (Ti,W)C cubic phase. The microstructure, fracture morphology, mechanical and magnetic properties of the gradient cemented carbonitrides with carbon contents from 6.04wt% to 6.34 wt% were investigated systematically. The results show that lattice parameters of (Ti,W)C in the transit zone increase due to more Ti solid solution when the gradient layer thickens. (Ti,W)C grains in the inner bulk are refined effectively by nano-TiN. Intergranular fracture along WC grain, transgranular fracture of (Ti,W)C grains, and the tearing of binder are found in the bulk of the gradient cemented carbonitride. When cracks encounter the gradient layer, they grow from new origins and a macroscopic boundary is formed between the gradient layer and the bulk. The transverse rupture strength is promoted with the carbon contents increased, and its stability is also increased. Additionally, the microhardness of the gradient cemented carbonitride is correlated closely with the Ti and Co distribution.     

Tailoring cathode composite boosts the performance of proton-conducting SOFCs fabricated by a one-step co-firing method

A strategy of tailoring the ceramic cathode composite is presented to improve the performance of proton-conducting solid oxide fuel cells (SOFCs) prepared by a one-step co-firing process. Comparing to the conventional way of using BaCe_0.7Zr_0.1Y_0.2O_3-δ (BCZY) in the composite cathode for BCZY-electrolyte based cells, the replacement of BCZY by BaZr_0.8Y_0.2O_3-δ (BZY) mitigates the reaction between the two ceramic phases in the composite cathode during the co-firing process and also keeps the cathode with sufficient porosity for ample gas diffusion which could assist in adequate cathode reactions. As a result, the BCZY-electrolyte based cell with Sm_0.5Sr_0.5CoO_3-δ (SSC)-BZY composite cathode shows a power output of 300?mW?cm^?2 at 600?°C, which is the largest ever reported for proton-conducting SOFCs prepared by a one-step co-firing process. The strategy of tailoring the composite cathode offers both small ohmic resistance and polarization resistance, providing a promising way to develop single-step co-fired proton-conducting SOFCs.     

Synthesis of nanocrystalline samarium-doped CeO2 (SDC) powders as a solid electrolyte by using a simple solvothermal route

Samarium-doped CeO₂ is a leading electrolyte for applications in solid oxide fuel cells (SOFCs), which requires a typical sintering temperature of 1400–1600 °C. In this work, fully dense CeO₂ ceramics doped with 10–20 at.% samarium have been fabricated by a simple polyol process. The XRD and SEM results show that a complete solid solution between CeO₂ and samarium was obtained at the sintering temperature of 1300 °C. And also the densification temperature is significantly lower than those (1400–1600 °C) reported for the SDC powders processed by modified sol–gel process and hydrothermal treatment. The resultant ceramics show the sizes of ultrafine grain are lower than 1 μm.     

Combining coal gasification, natural gas reforming, and solid oxide fuel cells for efficient polygeneration with CO2 capture and sequestration

Several polygeneration process systems are presented which convert natural gas and coal to gasoline, diesel, methanol, and electricity. By using solid oxide fuel cells as the primary electricity generator, the presented systems improve upon a recently introduced concept by which natural gas is reformed inside the radiant cooler of a gasifier. Simulations and techno-economic analyses performed for a wide range of process configurations and market conditions show that this strategy results in significant efficiency and profitability improvements when CO₂ capture and sequestration are employed. Market considerations for this analysis include variations in purchase prices of the coal and natural gas, sale prices of the products, and CO₂ emission tax rates.