A novel fabrication of yttria-stabilized-zirconia dense electrolyte for solid oxide fuel cells by 3D printing technique

Three-dimensional (3D) printing technique represents a revolutionary advancement in the manufacturing sector due to its unique capabilities to process the shape complexity. This work is focusing on dense 8 mol.% yttria-stabilized-zirconia (8YSZ) electrolyte fabrication via digital light processing (DLP)-stereolithography-based 3D printing technique. Multiple 8YSZ electrolyte green bodies are printed simultaneously in a batch using ceramic-resin suspension made of 30 vol% 8YSZ powder loading in a photo-curable resin. Together with an optimized debinding and sintering procedure, the 8YSZ green body changes into a dense electrolyte, and the density of the sintered electrolyte was measured as 99.96% by Archimedes' water displacement method. The symmetric cell fabricated of silver-Ce_0.8Gd_0.2O_1.9 (Ag-GDC) as cathode/anode and dense 8YSZ electrolyte printed by DLP-stereolithography delivers a high open circuit voltage of approximately 1.04 V and a peak power density up to 176 mW·cm⁻² at 850 °C by using hydrogen as the fuel and air as the oxidant. The electrochemical performance of the symmetric cell Ag-GDC|YSZ|Ag-GDC with 8YSZ electrolyte fabricated via DLP-stereolithography is comparable to that of the same cell with 8YSZ electrolyte fabricated by conventional dry pressing method. This 3D printing technique provides a novel method to prepare dense electrolytes for solid oxide fuel cell (SOFC) with good performance, suggesting a potential application for one-step fabrication of complex structure SOFC stack.     

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.     

Production,performance and cost analysis of anode-supported NiO-YSZ micro-tubular SOFCs

In the present study, the anode-supported micro-tubular solid oxide fuel cells (MT-SOFCs) with an electrolyte thin interlayer were manufactured. The anode support tubes consisting of 56 wt% nickel oxide and 44 wt% YSZ (8 mol% yttria (Y₂O₃) stabilized zirconia (ZrO₂)) were produced by using the thermo-extrusion method, whereas the electrolyte and cathode layers were manufactured using the dip-coating method. The half-cells consisting of anode and electrolyte were manufactured by using two different methods. In the first method, the anode-support tubes were pre-sintered at 1200 °C, then covered with the electrolyte layer by using the dip-coating method and then exposed to second sintering at 1400 °C. In the second method, the anode and electrolyte layers were sintered together at 1400 °C (co-sintering) in order to produce the half-cells. The half-cells that were produced and then coated with cathode solutions by using the dip-coating method and the final cells were successfully produced at the end of the sintering at 1150 °C. The porosity and shrinkage percentage values of these MT-SOFCs differed from each other. The power densities of these cells were tested at 700 °C, 750 °C, and 800 °C by using H₂ gas as fuel and the results of the microstructural and cost analyses were compared.     

Microwave sintering of high-performance BaZr0.1Ce0.7Y0.1Yb0.1O3-δ (BZCYYb) electrolytes for intermediate-temperature solid oxide fuel cells

As a promising electrolyte material for solid oxide fuel cells (SOFCs), BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3-δ (BZCYYb) often surfers from its high sintering temperature, which causes Ba evaporation and sluggish grain growth, thus reducing the electrical conductivity. In this work, densified BZCYYb electrolytes were fabricated at temperatures as low as 1400 °C using the microwave sintering technique. Comparing with the conventional sintered ones, a temperature decrease of 150 °C is achieved. The Ba evaporation is effectively suppressed, and large grain sizes of ～4 μm are obtained. The total conductivity for microwave sintered symmetric cell measured in wet air at 700 °C is 3.8 × 10^?2 S cm^?1, benefiting from both enhanced bulk conductivities by 1–2 times and grain boundary conductivities by 50 times. With the microwave sintered BZCYYb as electrolyte, an anode-supported cell reaches a maximum power density of 0.64 W cm^?2 at 700 °C.     

A cost-effective process for fabrication of metal-supported solid oxide fuel cells

Metal-supported SOFC cells with Y₂O₃ stabilized ZrO₂ as the electrolyte were prepared by a low cost and simple process involving tape casting, screen printing and co-firing. The interfaces were well bonded after the reduction of NiO to Ni in the support and the anode. AC impedance was employed to estimate the cell polarizations under open circuit conditions. It was found that the electrode polarization resistance was high at low temperatures and became equivalent to the ohmic resistance at higher temperatures near 800°°C. The cell performance was evaluated with H₂ as the fuel and air as the oxidant, and maximum power density between 0.23 and 0.80  W/cm² was achieved in the temperature range of 650–800°C, which confirms the applicability of the cost-effective process in fabrication of metal-supported SOFC cells.     

Effect of sintering temperature on the microstructure,roughness and electrochemical impedance of electrophoretically deposited YSZ electrolyte for SOFCs

In the present work, the microstructures of YSZ electrolyte films, which were sintered at various temperatures in the range of 1300–1600 °C, were investigated. First, a suitable and uniform film was deposited on the surface of NiO–YSZ composite by EPD. After the consequence sintering, the surfaces of deposited YSZ films were observed by SEM. In addition, other characteristics of the YSZ electrolyte films such as surface roughness and morphology of the sintered films were investigated by AFM. The ability of ionic transfer and permeability of the YSZ electrolyte was examined by electrochemical impedance spectroscopy at different temperatures. It seems that the YSZ electrolyte sintered at 1400 °C was appropriate for SOFCs applications, because this film had the minimum impedance, minimum roughness and the maximum conductivity. Furthermore, the temperature of 1400 °C was the minimum temperature in which a dense film of YSZ was formed uniformly on the surface of anode and coated it completely.     

Effect of Al and Ce doping on the deformation upon sintering in sequential tape cast layers for solid oxide fuel cells

Water-based tape casting is an attractive production route for planar solid oxide fuel cells (SOFCs) due to its high productivity and reduced environmental issues. In this work planar anode supported SOFCs with thin electrolyte were prepared by water-based sequential tape casting and co-sintering. An in situ high temperature monitoring apparatus was assembled to allow the determination of free sintering shrinkage of thin green tape cast layers and to follow the curvature developed in multilayers during the entire sintering process.The instantaneous curvature developed upon co-sintering was studied as a function of the firing schedule and layer composition. It was found that by tailoring the electrode composition it is possible to reduce the shrinking rate difference between anode and electrolyte thus obtaining defect-free electrolyte, minimising the residual curvature of the half-cell and improving the electrochemical performances of the cell.     

Achieving high mechanical-strength CH4-based SOFCs by low-temperature sintering (1100 °C)

Despite much progress achieved in the past decades in the process of advancing the low-temperature sintering technologies for Solid oxide fuel cells (SOFCs), such as via the structure design of the electrode materials, the practical application of low-temperature sintered SOFCs (with disqualified mechanical strength) remains challenging. In this work, first, we demonstrate that the appropriate amount of CuO as sintering aids can successfully reduce the co-firing temperature of conventional micron size NiO-YSZ (yttrium-stabilized zirconia (Y₂O₃)_0.08–(ZrO₂)_0.92) anode from about 1400 °C to only 1100 °C. Second, the quantitative structure-activity relationship among the mechanical strength (low-temperature sintering ability) of anode cermets with the inclusion of CuO contents and the densification of YSZ electrolyte was synthetically evaluated, and the optimal Cu–NiO-YSZ anode composition demonstrates almost the equal mechanical strength when compared with the traditional NiO-YSZ anode (sintering at 1400 °C). At last, by comprehensive assessment, 8%Cu–52NiO-40YSZ (8%CuO–NiO-YSZ) shows excellent low-temperature sintering ability, high mechanical strength, optimal power output, and anti-carbon deposition when using as hydrocarbon-based anode for SOFCs.     

Anode supported micro-tubular SOFC fabricated with mixed particle size electrolyte via phase-inversion technique

Cerium-gadolinium oxide is a promising material for electrolytes of intermediate temperature solid oxide fuel cells (IT-SOFCs) due to its high electrical conductivity at relatively lower temperatures of 400–700 °C. However, a high sintering temperature of up to 1550 °C is typically required to produce dense CGO electrolyte, eventually leading to an interfacial interdiffusion between the electrolyte and electrode components as well as generate a highly resistive interface which reduces ionic conductivity. Lowering the sintering temperature of the electrolyte will greatly benefit the fabrication of SOFCs. This study examines the effectiveness of introducing nano size CGO particles as an approach to get dense CGO electrolyte at lower sintering temperature. A series of dope suspensions with 0–50% nano size loading were prepared to observe rheology and measure viscosity. Then, 30% loading was selected and casting into flat sheet via phase-inversion technique. The flat sheet was characterized by morphology, surface roughness and mechanical strength tests. The suspension was extruded into dual-layer hollow fiber (DLHF) as well. The electrolyte/anode dual-layer hollow fibers (DLHFs) half-cell of micro-tubular solid oxide fuel cells (MT-SOFCs) were prepared via phase inversion based co-extrusion/co-sintering technique. The developed half-cell was characterized by morphological and gas tightness tests which further compared them with fully micron ones. The results show that the incorporation of 30% nanoparticle yielded to dense and tight CGO layers sintered at temperature 1450 °C, which about 50 °C lower than those reported previously for 100% micron particles. The I–V measurements demonstrated the maximum power density of 0.66 Wcm^?2 at temperatures 500 °C using 100% H₂ as fuel. Therefore, this approach is able to reduce the energy cost for the microstructural control of the prepared fiber and thus is recommended for the fabrication of low-cost dual-layer hollow fiber micro tubular SOFCs.     

Aqueous tape casting technique for the fabrication of Sc0.1Ce0·01Zr0·89O2+Δ ceramic for electrolyte-supported solid oxide fuel cell

Despite some the advantages of the solid oxide fuel cell (SOFC), one of the greatest challenges that hinders the SOFC from rising to dominance in the field of power generation is its high fabrication cost. As a solution, the tape casting process has been widely used to fabricate low-cost, uniform and thin SOFC electrolytes. Compared to organic-based tape casting, aqueous-based tape casting is a much more environmentally friendly technique. In this work, a large-area electrolyte-supported solid oxide fuel cell was fabricated by this technique together with sintering. A 10 cm × 10 cm and 0.17 mm thick supported Sc_0.1Ce_0·01Zr_0·89O_2+△ (SSZ) electrolyte was obtained with good flatness, low ohmic resistance and high open-circuit voltage.     

Metal-supported solid oxide fuel cells with in-situ sintered (Bi2O3)0.7(Er2O3)0.3–Ag composite cathode

Metal-supported solid oxide fuel cells (SOFCs) containing porous 430L stainless steel support, Ni-YSZ anode and YSZ electrolyte were fabricated by tape casting, laminating and co-firing in a reduced atmosphere. (Bi₂O₃)_0.7(Er₂O₃)_0.3–Ag composite cathode was applied by screen printing and in-situ sintering. The polarization resistances of the composite cathode were 1.18, 0.48, 0.18, 0.09 Ω cm² at 600, 650, 700 and 750 °C, respectively. A promissing maximum power density of 568 mW cm⁻² at 750 °C was obtained of the single cell. Short-term stability was measured as well.     

Fabrication of cathode supported solid oxide fuel cell by multi-layer tape casting and co-firing method

La_0.3Sr_0.7FeO_3-δ (LSF)/CeO₂ cathode supported Ce_0.8Sm_0.2O_2-δ (SDC) electrolyte was prepared by a simple multilayer tape casting and co-firing method. SDC electrolyte slurry and LSF/CeO₂ cathode slurry were optimized and the green bi-layer tapes were co-fired at different temperature. Phase characterizations and microstructures of electrolyte and cathode were studied by X-ray diffraction (XRD) and Scan Electronic Microscopy (SEM). No additional phase peak line was observed in electrolyte and cathode support when the sintering temperature lower was than 1400 °C. The electrolytes were extremely dense with the thickness of about 20 μm. The cathode support was porous with electrical conductivity of about 4.21 S/cm at 750 °C. With Ni/SDC as anode, Open Current Voltage and maximum power density reached 0.61 V and 233 mW cm⁻² at 750 °C, respectively.     

Microstructure and electrochemical characterization of solid oxide fuel cells fabricated by co-tape casting

A co-tape casting technique was applied to fabricate electrolyte/anode for solid oxide fuel cells. YSZ and NiO-YSZ powders are raw materials for electrolyte and anode, respectively. Through adjusting the Polyvinyl Butyral (PVB) amount in slurry, the co-sintering temperature for electrolyte/anode could be dropped. After being co-sintered at 1400 °C for 5 h, the half-cells with dense electrolytes and large three phase boundaries were obtained. The improved unit cell exhibited a maximum power density of 589 mW cm⁻² at 800 °C. At the voltage of 0.7 V, the current densities of the cell reached 667 mA cm⁻². When the electrolyte and the anode were cast within one step and sintered together at 1250 °C for 5 h and the thickness of electrolyte was controlled exactly at 20 μm, the open-circuit voltage (OCV) of the cell could reach 1.11 V at 800 °C and the maximum power densities were 739, 950 and 1222 mW cm⁻² at 750, 800 and 850 °C, respectively, with H₂ as the fuel under a flow rate of 50 sccm and the cathode exposed to the stationary air. Under the voltage of 0.7 V, the current densities of cell were 875, 1126 and 1501 mA cm⁻², respectively. These are attributed to the large anode three phase boundaries and uniform electrolyte obtained under the lower sintering temperature. The electrochemical characteristics of the cells were investigated and discussed.     

Multilayer tape casting of large-scale anode-supported thin-film electrolyte solid oxide fuel cells

Tape casting is conventionally used to prepare individual, relatively thick components (i.e., the anode or electrolyte supporting layer) for solid oxide fuel cells (SOFCs). In this research, a multilayer ceramic structure is prepared by sequentially tape casting ceramic slurries of different compositions onto a Mylar carrier followed by co-sintering at 1400 °C. The resulting half-cells contains a 300 μm thick NiO–yttria-stabilized zirconia (YSZ) anode support, a 20 μm NiO–YSZ anode functional layer, and an 8 μm YSZ electrolyte membrane. Complete SOFCs are obtained after applying a Gd_0.1Ce_0.9O₂ (GDC) barrier layer and a Sm_0.5Sr_0.5CoO₃ (SSC) -GDC cathode by using a wet-slurry spray method. The 50 mm × 50 mm SOFCs produce peak power densities of 337, 554, 772, and 923 mW/cm² at 600, 650, 700, and 750 °C, respectively, on hydrogen fuel. A short stack including four 100 mm × 150 mm cells is assembled and tested. Each stack repeat unit (one cell and one interconnect) generates around 28.5 W of electrical power at a 300 mA/cm² current density and 700 °C.     

Polarization measurements on single-step co-fired solid oxide fuel cells (SOFCs)

Anode-supported planar solid oxide fuel cells (SOFC) were successfully fabricated by a single step co-firing process. The cells comprised of a Ni + yttria-stabilized zirconia (YSZ) anode, a YSZ or scandia-stabilized zirconia (ScSZ) electrolyte, a (La_0.85Ca_0.15)_0.97MnO₃ (LCM) + YSZ cathode active layer, and an LCM cathode current collector layer. The fabrication process involved tape casting of the anode, screen printing of the electrolyte and the cathode, and single step co-firing of the green-state cells in the temperature range of 1300–1330 °C for 2 h. Cells were tested in the temperature range of 700–800 °C with humidified hydrogen as fuel and air as oxidant. Cell test results and polarization modeling showed that the polarization losses were dominated by the ohmic loss associated with the electrodes and the activation polarization of the cathode, and that the ohmic loss due to the ionic resistance of the electrolyte and the activation polarization of the anode were relatively insignificant. Ohmic resistance associated with the electrode was lowered by improving the electrical contact between the electrode and the current collector. Activation polarization of the cathode was reduced by the improvement of the microstructure of the cathode active layer and lowering the cell sintering temperature. The cell performance was further improved by increasing the porosity in the anode. As a result, the maximum power density of 1.5 W cm⁻² was achieved at 800 °C with humidified hydrogen and air.     

Solid oxide fuel cells with bi-layered electrolyte structure

In this work, we have developed solid oxide fuel cells with a bi-layered electrolyte of 2 μm SSZ and 4 μm SDC using tape casting, screen printing, and co-firing processes. The cell reached power densities of 0.54 W cm⁻² at 650 °C and 0.85 W cm⁻² at 700 °C, with open circuit voltage (OCV) values larger than 1.02 V. The electrical leaking between anode and cathode through an SDC electrolyte has been blocked in the bi-layered electrolyte structure. However, both the electrolyte resistance (R_el) and electrode polarization resistance (R_p,a+c) increased in comparison to cells with single-layered SDC electrolytes. The formation of a solid solution of (Ce, Zr)O_2−x during sintering process and the flaws in the bi-layered electrolyte structure seem to be the main causes for the increase in the R_el value (0.32 Ω cm²) at 650 °C, which is almost one order of magnitude higher than the calculated value.     

Fabrication,microstructure and properties of a YSZ electrolyte for SOFCs

Yttria stabilized zirconia (YSZ) has widely been used as an electrolyte in solid oxide fuel cell (SOFC) stacks. The microstructure and properties of YSZ related to the fabrication process are discussed in this paper. For the named two-step sintering process, uniform and hexagonal grains with a size of 1–4 μm were obtained from the adobe following tape calendaring (TCL). Elliptical and hexagonal grains with a size of 0.4–3 μm were obtained from the adobe of tape casting (TCS) using the three-step process. The electrical conductivities of YSZ with different grain sizes were measured via the four-probe DC technique and grain conductivities and grain boundary conductivities of YSZ were investigated by impedance spectroscopy. YSZ electrolytes with a grain size of 0.1–0.4 μm had the highest electrical conductivity in the range of 500–1000 °C, especially at medium and low temperatures 550–800 °C. As the YSZ grain size becomes small, the thickness of the intergranular region decreased greatly. The YSZ electrolytes with sub-micrometer grain sizes, high ion conductivity and low sintering temperatures are important to the electrode-supported SOFC, on which the dense YSZ electrolyte films are optimized at 10 μm.     

Experimental optimization of the fabrication parameters for anode-supported micro-tubular solid oxide fuel cells

A systematic optimization of several parameters significant in the fabrication of anode-supported micro-tubular solid oxide fuel cell via extrusion and dip coating is presented in this study. Co-sintering temperature of anode-support and electrolyte, the vehicle type and solid powder content used in electrolyte dip-coating slurry, electrolyte submersion time, cathode sintering temperature, powder ratio in the cathode functional layer, submersion time for the cathode functional layer and, submersion time and coating number of the anode functional layer are studied in this respect and optimized in the given order according to the performance tests and microstructural analyses. The performance of the micro-tubular cell is significantly improved to 0.49 Wcm⁻² at 800 °C after the optimizations, while that of the base cell is only 0.136 Wcm⁻². 12-cell micro-tubular stack is also constructed with the optimized cells and the stack is tested. Each cell in the stack is found to show very close performance to the single-cell performance and the stack with a maximum power of ~26 W at an operating temperature of 800 °C is therefore evaluated to be successful.     

Development of functionally graded anode supports for microtubular solid oxide fuel cells by tape casting and isostatic pressing

This paper suggests an alternative method to manufacture functionally graded anode supports for microtubular solid oxide fuel cells by employing tape casting and isostatic pressing for the first time in the literature. In this regard, six different anode support strips with various pore former contents are produced by tape casting. Besides the anode supports made from uniform tapes, three-layered anode supports composed of various combination of these tapes are also fabricated by wrapping the corresponding tape(s) of the same total length on a metallic rod followed by isostatic pressing. Microtubular cells are then built on these anode supports by dip coating the other layers and evaluated by microstructural investigations and electrochemical performance tests performed under the same conditions. Porosity measurements of the homogeneous anode supports are also carried out. Microstructural examinations reveal that not only the homogeneous anode supports but porosity graded anode supports can be also successfully manufactured by the suggested method. Electrochemical tests indicate that the performance of the cells with a uniform anode support tends to increase with the anode support porosity up to ∼26% porosity then shows a decreasing trend. The highest maximum performance of 0.645 Wcm⁻² at 800 °C under 0.3 NLmin⁻¹ hydrogen and stationary air, on the other hand, is obtained from the cell with a porosity graded anode support.     

Performance of an anode support solid oxide fuel cell manufactured by microwave sintering

An effective and facile method has been developed to manufacture anode support solid oxide fuel cells in a multimode domestic microwave oven with selective susceptors. Anode support substrate pellets are prepared by an uniaxial pressing method, and then a thin YSZ electrolyte film is coated by a spray coating method. The electrolyte thickness is kept less than 10 m. The anode supported electrolyte is co-sintered being sandwiched by two spacers and two susceptors in the microwave oven. A cathode is then screen-printed onto the sintered dense electrolyte film and sintered again in the microwave oven with only one spacer and one susceptor. The whole solid oxide fuel cell is sintered at lower temperatures compared to conventional thermal sintering temperature. The performance of the present solid oxide fuel cell is measured in an intermediate temperature range of 650–800 °C. The maximum power densities of 0.09, 0.12, 0.2 and 0.26 W cm⁻² are obtained at operating temperatures of 650, 700, 750 and 800 °C, respectively.