Electrophoretic Deposition of Zirconia Thin Film on Nonconducting Substrate for Solid Oxide Fuel Cell Application

Electrophoretic deposition (EPD) of 8 mol% yttria‐stabilized zirconia (YSZ) electrolyte thin film has been carried out onto nonconducting porous NiO‐YSZ cermet anode substrate using a fugitive and electrically conducting polymer interlayer for solid oxide fuel cell (SOFC) application. Such polymer interlayer burnt out during the high‐temperature sintering process (1400°C for 6 h) leaving behind a well adhered, dense, and uniform ceramic YSZ electrolyte film on the top of the porous anode substrate. The EPD kinetics have been studied in depth. It is found that homogeneous and uniform film could be obtained onto the polymer‐coated substrate at an applied voltage of 15 V for 1 min. After the half‐cell (anode + electrolyte) is co‐fired at 1400°C, a suitable cathode composition (La_0.65Sr_0.3MnO₃) thick film paste is screen printed on the top of the sintered YSZ electrolyte. A second stage of sintering of such cathode thick film at 1100°C for 2 h finally yield a single cell SOFC. Such single cell produced a power output of 0.91 W/cm² at 0.7 V when measured at 800°C using hydrogen and oxygen as fuel and oxidant, respectively.     

Development and Fabrication of a New Concept Planar‐tubular Solid Oxide Fuel Cell (PT‐SOFC)

The paper reports a new concept of planar‐tubular solid oxide fuel cell (PT‐SOFC). Emphasis is on the fabrication of the required complex configuration of Ni‐yttria‐stabilised zirconia (YSZ) porous anode support by tert‐butyl alcohol (TBA) based gelcasting, particularly the effects of solid loading, amounts of monomers and dispersant on the rheological behaviour of suspension, the shrinkage of a wet gelcast green body upon drying, and the properties of final sample after sintering at 1350 °C and reduction from NiO‐YSZ to Ni‐YSZ. The results show that the gelcasting is a powerful method for preparation of the required complex configuration anode support. The anode support resulted from an optimised suspension with the solid loading of 25 vol% has uniform microstructure with 37% porosity, bending strength of 44 MPa and conductivity of 300 S cm^—1 at 700 °C, meeting the requirements for an anode support of SOFC. Based on the as‐prepared anode support, PT‐SOFC single cell of Ni‐YSZ/YSZ/LSCF has been fabricated by slurry coating and co‐sintering technique. The cell peak power density reaches 63, 106 and 141 mW cm^—2 at 700, 750 and 800 °C, respectively, using hydrogen as fuel and ambient air as oxidant.     

Tailoring Electrochemical Property of Layered Perovskite Cathode by Cu‐doping for Proton‐Conducting IT‐SOFCs

Layered perovskite oxide YBaCuCoO_5+x (YBCC) was synthesized by an EDTA‐citrate complexation process and was investigated as a novel cathode for proton‐conducting intermediate temperature solid oxide fuel cells (IT‐SOFCs). The thermal expansion coefficient (TEC) of YBCC was 15.3 × 10⁻⁶ K⁻¹ and the electrical conductivity presented a semiconductor‐like behavior with the maximum value of 93.03 Scm⁻¹ at 800 °C. Based on the defect chemistry analysis, the electrical conductivity gradually decreases by the introduction of Cu into Co sites of YBaCo₂O_5+x and the conductor mechanism can transform from the metallic‐like behavior to the semiconductor‐like behavior. Thin proton‐conducting (BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3–δ) BZCYYb electrolyte and NiO–BZCYYb anode functional layer were prepared over porous anode substrates composed of NiO–BZCYYb by a one‐step dry‐pressing/co‐firing process. Laboratory‐sized quad‐layer cells of NiO‐BZCYYb / NiO‐BZCYYb / BZCYYb / YBCC with a 20 μm‐thick BZCYYb electrolyte membrane exhibited the maximum power density as high as 435 mW cm⁻² with an open‐circuit potential (OCV) of 0.99 V and a low interfacial polarization resistance of 0.151 Ωcm² at 700 °C. The experimental results have indicated that the layered perovskite oxide YBCC can be a cathode candidate for utilization as proton‐conducting IT‐SOFCs.     

Synthesis and characterisation of macroporous Yttria Stabilised Zirconia (YSZ) using polystyrene spheres as templates

Yttria Stabilised Zirconia (YSZ) is commonly used as an oxygen sensor and an oxygen pump in automotive and industrial applications, and is a choice electrolyte for Solid Oxide Fuel Cell (SOFC) technology. YSZ is also a major component of the SOFC electrodes, and is commonly mixed with 50% volume NiO to create a Ni/YSZ cermet anode. In both the adsorption and fuel cell applications homogeneous control of the porosity of YZS is important. Templating methods provide well ordered macroporous structures and have been used to prepare ordered, macroporous YSZ from metal nitrate precursors using polystyrene spheres of 1 μm as templates. Ordered three-dimensional structures were synthesised and the effects of sintering temperatures of 650–1400 °C on pore size, particle size and pore wall thickness were examined. Ordered porosity was maintained at all temperatures, though some structural degradation and sintering was observed at 1400 °C. This study demonstrated that templated porosity is maintained well above the conventional sintering temperature of the electrodes, and higher than previous studies reported. The stability of these structures at high temperatures makes this fabrication technique a promising alternative to conventional methods of synthesising porous materials.     

Pore orientation of the gadolinia‐doped ceria cathode interlayer for a tubular SOFC using dip‐coating

An active region of cathode interlayer in a tubular solid oxide fuel cell (SOFC) is structurally analyzed using a dual‐beam focused ion beam/scanning electron microscope (FIB/SEM). The GDC (10 mol% gadolinia‐doped ceria) cathode interlayer (about 1 μm in thickness) is dip‐coated, and then sintered on YSZ (8 mol% yttria‐stabilized zirconia) electrolyte. At 1150°C sintering temperature, the pores oriented more along the axial direction than the radial direction. The anisotropy of pore shape is accounted for the withdrawal force during the dip‐coating of the GDC interlayer.     

Efficiently enhance the proton conductivity of YSZ-based electrolyte for low temperature solid oxide fuel cell

Yttrium stabilized zirconia (YSZ) as a typical oxygen ionic conductor has been widely used as the electrolyte for solid oxide fuel cell (SOFC) at the temperature higher than 1000 °C, but its poor ionic conductivity at lower temperature (500–800 °C) limits SOFC commercialization. Compared with oxide ionic transport, protons conduction are more transportable at low temperatures due to lower activation energy, which delivered enormous potential in the low-temperature SOFC application. In order to increase the proton conductivity of YSZ-based electrolyte, we introduced semiconductor ZnO into YSZ electrolyte layer to construct heterointerface between semiconductor and ionic conductor. Study results revealed that the heterointerface between ZnO and YSZ provided a large number of oxygen vacancies. When the mass ratio of YSZ to ZnO was 5:5, the fuel cell achieved the best performance. The maximum power density (P_max) of this fuel cell achieved 721 mW cm^?2 at 550 °C, whereas the P_max of the fuel cell with pure YSZ electrolyte was only 290 mW cm^?2. Further investigation revealed that this composite electrolyte possessed poor O^2? conductivity but good proton conductivity of 0.047 S cm^?1 at 550 °C. The ionic conduction activation energy of 5YSZ-5ZnO composite in fuel cell atmosphere was only 0.62 eV. This work provides an alternative way to improve the ionic conductivity of YSZ-based electrolytes at low operating temperatures.     

Lowering the co-sintering temperature of cathode–electrolyte bilayers for micro-tubular solid oxide fuel cells

To prevent undesirable reactions between the cathode and electrolyte materials in cathode-supported solid oxide fuel cells (SOFCs), the co-sintering temperature of these two layers must be lowered. In the present work, we employed different strategies to lower the co-sintering temperature of cathode–electrolyte bilayers for micro-tubular SOFCs by increasing the cathode sintering shrinkage and adding sintering aids to the electrolyte. Strontium-doped lanthanum manganite (LSM) and yttria-stabilized zirconia (YSZ) were used as the cathode and electrolyte materials, respectively. To facilitate densification of the electrolyte layer by controlling the shrinkage of the cathode support, the particle size of the LSM powder was reduced by high-energy ball milling and different amounts of micro-crystalline cellulose pore former were used. Sintering aids, namely NiO and Fe₂O₃, were also added to the YSZ electrolyte to further improve its low-temperature sintering. Our results indicate that with the improvement in the cathode support shrinkage and use of the small amounts of sintering aids, the cathode–electrolyte co-sintering temperature can be reduced to 1250–1300 °C. It was also observed that the presence of the sintering aids helps to reduce the reactivity between the LSM cathode and YSZ electrolyte.     

Boosting intermediate temperature performance of solid oxide fuel cells via a tri-layer ceria–zirconia–ceria electrolyte

Using cost-effective fabrication methods to manufacture a high-performance solid oxide fuel cell (SOFC) is helpful to enhance the commercial viability. Here, we report an anode-supported SOFC with a three-layer Gd_0.1Ce_0.9O_1.95 (gadolinia-doped-ceria [GDC])/Y_0.148Zr_0.852O_1.926 (8YSZ)/GDC electrolyte system. The first dense GDC electrolyte is fabricated by co-sintering a thin, screen-printed GDC layer with the anode support (NiO–8YSZ substrate and NiO–GDC anode) at 1400°C for 5 h. Subsequently, two electrolyte layers are deposited via physical vapor deposition. The total electrolyte thickness is less than 5 μm in an area of 5 × 5 cm², enabling an area-specific ohmic resistance as low as 0.125 Ω cm⁻² at 500°C (under open circuit voltage), and contributing to a power density as high as 1.2 W cm⁻² at 650°C (at an operating cell voltage of 0.7 V, using humidified [10 vol.% H₂O] H₂ as fuel and air as oxidant). This work provides an effective strategy and shows the great potential of using GDC as an electrolyte for high-performance SOFC at intermediate temperature.     

Suppression of Ni agglomeration in PLD fabricated Ni-YSZ composite for surface modification of SOFC anode

The suppression of Ni agglomeration in Ni-yttria stabilized zirconia (Ni-YSZ) nano-composite thin films deposited by pulsed laser deposition (PLD) has been investigated by varying post-annealing temperatures at a range of 800–1200 °C. Grain growth to a certain extent appears to be necessary to obtain a stable Ni-YSZ composite microstructure by suppressing massive Ni agglomeration. The microstructurally stable and uniform nano-porous Ni-YSZ thin film was obtained by 1200 °C post-annealing and reduction of the NiO–YSZ thin film, and it was applied as the surface modification layer of the bulk anode support used in conventional solid oxide fuel cells (SOFCs). By this approach, we were able to successfully realize a thin film electrolyte SOFC exhibiting the open cell voltage (OCV) higher than 1 V with a 1-μm thick film electrolyte on the porous anode support.     

Intermediate Temperature Anode‐Supported Fuel Cell Based on BaCe0.9Y0.1O3 Electrolyte with Novel Pr2NiO4 Cathode

A proton conducting ceramic fuel cell (PCFC) operating at intermediate temperature has been developed that incorporates electrolyte and electrode materials prepared by flash combustion (yttrium‐doped barium cerate) and auto‐ignition (praseodymium nickelate) methods. The fuel cell components were assembled using an anode‐support approach, with the anode and proton ceramic layers prepared by co‐pressing and co‐firing, and subsequent deposition of the cathode by screen‐printing onto the proton ceramic surface. When the fuel cell was fed with moist hydrogen and air, a high Open Circuit Voltage (OCV > 1.1 V) was observed at T > 550 °C, which was stable for 300 h (end of test), indicating excellent gas‐tightness of the proton ceramic layer. The power density of the fuel cell increased with temperature of operation, providing more than 130 mW cm^–2 at 650 °C. Symmetric cells incorporating Ni‐BCY10 cermet and BCY10 electrolyte on the one hand, and Pr₂NiO_{4 + δ} and BCY10 electrolyte on the other hand, were also characterised and area specific resistances of 0.06 Ω cm² for the anode material and 1–2 Ω cm² for the cathode material were obtained at 600 °C.     

Deposition Via Dip Coating Technique of Dense Yttrium Stabilized Zirconia Layers

A deposition of yttrium stabilized zirconia layer for its use as an electrolyte in solid oxide fuel cell was performed using dip coating technique. Two commercially available surfactant systems were evaluated; nonionic surfactant which stabilizes only by steric repulsion and anionic surfactant which provides both steric and electrostatic repulsion. Dip coating process was optimized to two step deposition process. Uniform 10–15 μm thick yttria stabilized zirconia (YSZ) electrolyte layer is obtained after the final sintering step at 1400°C. Impedance spectroscopy measurements showed that selected phosphate ester based surfactant has negligible effect on the performance of the YSZ material as electrolyte.     

Electrophoretic deposition for fabrication of YSZ electrolyte film on non-conducting porous NiO–YSZ composite substrate for intermediate temperature SOFC

Electrophoretic deposition (EPD) of YSZ electrolyte films onto porous NiO–YSZ composite substrates that had been pre-coated with graphite thin layers was carried out in the following two means for solid oxide fuel cell application: one was EPD based on electrophoretic filtration by which YSZ films were formed on the reverse sides without the graphite layers; the other was EPD on a graphite thin layer pre-coated on the substrates. Dense YSZ electrolyte thin films were successfully obtained in both means, although it was difficult to form YSZ films that were strongly adherent to the substrates using the latter means. The densification of YSZ films was assisted by shrinkage of the substrates during co-firing. A single cell was constructed on ca. 5 μm thick dense YSZ films fabricated using the EPD based on electrophoretic filtration. Maximum power densities over 0.06, 0.35, 1.10 and 2.01 W/cm² were attained, respectively, at 500, 600, 700 and 800 °C on the cell.     

Large-area anode-supported protonic ceramic fuel cells combining with multilayer-tape casting and hot-pressing lamination technology

Anode-supported protonic ceramic fuel cells (PCFCs) have many advantages such as the excellent performance and durability at lower temperatures. However, the fabrication of large-area PCFCs is still a big challenge. In this work, the anode supported 10 × 10 cm² PCFCs with BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3-δ (BZCYYb) electrolyte are fabricated by an optimized sintering process combining with multilayer-tape casting and hot-pressing lamination technology. Through optimizing the sintering shrinkage and thermal expansion behavior, anode supported large-area PCFCs with dense BZCYYb electrolyte are obtained with optimal sintering process. The maximum power density of the cell using H₂ as fuel reaches 400 mW/cm² at 700 ℃. The cell also shows a good thermal cycling performance and durability in 425 h test. Preliminary experimental results indicate that the fabrication technology with multilayer-tape casting and hot-pressing lamination is successful for large-area PCFCs, which can improve the cell fabrication efficiency and provide positive reference for large-area PCFCs.     

Electrochemical performance of intermediate temperature micro-tubular solid oxide fuel cells using porous ceria barrier layers

We describe the manufacture and electrochemical characterization of micro-tubular anode supported solid oxide fuel cells (mT-SOFC) operating at intermediate temperatures (IT) using porous gadolinium-doped ceria (GDC: Ce_0.9Gd_0.1O_2−δ) barrier layers. Rheological studies were performed to determine the deposition conditions by dip coating of the GDC and cathode layers. Two cell configurations (anode/electrolyte/barrier layer/cathode): single-layer cathode (Ni–YSZ/YSZ/GDC/LSCF) and double-layer cathode (Ni–YSZ/YSZ/GDC/LSCF–GDC/LSCF) were fabricated (YSZ: Zr_0.92Y_0.16O_2.08; LSCF: La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ). Effect of sintering conditions and microstructure features for the GDC layer and cathode layer in cell performance was studied. Current density–voltage (j–V) curves and impedance spectroscopy measurements were performed between 650–800 °C, using wet H₂ as fuel and air as oxidant. The double-cathode cells using a GDC layer sintered at 1400 °C with porosity about 50% and pores and grain sizes about 1 μm, showed the best electrochemical response, achieving maximum power densities of up to 160 mW cm⁻² at 650 °C and about 700 mW cm⁻² at 800 °C. In this case GDC electrical bridges between cathode and electrolyte are preserved free of insulating phases. A preliminary test under operation at 800 °C shows no degradation at least during the first 100 h. These results demonstrated that these cells could compete with standard IT-SOFC, and the presented fabrication method is applicable for industrial-scale.     

Effect of Ni diffusion into BaZr0.1Ce0.7Y0.1Yb0.1O3−δ electrolyte during high temperature co-sintering in anode-supported solid oxide fuel cells

Diffusion behavior of Ni during high temperature co-sintering was quantitatively investigated for anode-supported solid oxide fuel cells (SOFCs) that had BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3?δ (BZCYYb) proton-conducting electrolyte and NiO-BZCYYb anode. Although diffused Ni in such SOFCs effectively acts as a sintering aid to densify the BZCYYb electrolyte layer, it often negatively affects the electrolyte conductivity. In the present study, field emission electron probe microanalysis (with wavelength dispersive X-ray spectroscopy) clearly revealed that Ni diffused into the BZCYYb electrolyte layer, and that the amount of diffused Ni increased with increasing co-sintering temperature. In particular, relatively high Ni concentration within the electrolyte layer was observed near the electrolyte/anode interface, e.g., approximately 1.5 and 2.8 wt% at co-sintering temperature of 1300 and 1400 °C, respectively. Electrochemical measurements showed that, compared with the lower co-sintering temperatures (1300–1350 °C), the highest co-sintering temperature (1400 °C) led to the highest ohmic resistance because of lower electrolyte conductivity. These results suggest that high co-sintering temperature causes excessive Ni diffusion into the BZCYYb electrolyte layer, thus degrading the intrinsic electrolyte conductivity and consequently degrading the SOFC performance.     

BaZr0.1Co0.4Fe0.4Y0.1O3-SDC composite as quasi-symmetrical electrode for proton conducting solid oxide fuel cells

Symmetric solid oxide fuel cells (SSOFCs) with the identical anode and cathode electrocatalysts show promise to reduce material and system cost while increasing the cell lifespan. In this work, BaZr_0.1Co_0.4Fe_0.4Y_0.1O₃ (BZCFY) oxide perovskite is proposed as a symmetric electrode for SSOFCs based on proton conducting electrolyte, with targets of reducing temperature and high-performance application. Active oxygen ionic conductor and catalyst, SDC, is composited to improve the cell performance and electrode durability. Those materials show good chemical compatibility while BZCFY is decomposed to alloy and mixed oxide composite, which significantly affects electrode activity. SDC-BZCFY composite gives an electrode polarization resistance of 1.35–13.7 Ω cm² and 0.32–1.59 Ω cm² for hydrogen oxidation reaction and oxygen reduction reaction on the proton conducting electrolyte, BZCY, at the temperature range of 700–550 °C, respectively. Moreover, it displays an excellent oxygen reduction kinetics with an impressive activation energy of 0.91 eV. The polarization resistances are significantly reduced in the fuel cell condition owning to the electrochemical promotion effect under open-circuit condition. Quasi-SSOFCs with BZCY electrolyte in a thickness of 480 μm and electrode thickness of 25 μm give a peak power density of 114.8 and 74.3 mW cm⁻² at 650 and 600 °C, respectively. In addition, SSOFC also displays acceptable durability under constant voltage operational condition for 25 h. This work highlights alternative active electrode material for symmetric solid oxide fuel cells for low temperature operation.     

Planar Solid Oxide Fuel Cells Using PSZ,Processed by Sequential Aqueous Tape Casting and Constrained Sintering

A process for obtaining planar anode‐supported solid oxide fuel cells was developed. Aqueous‐based slurries were prepared and sequentially deposited via tape casting to form half cell tapes consisting of the electrolyte, functional, and structural anode. Sintering of the three‐layered tapes was done in two stages: presintering circular samples of 25 mm diameter in free conditions first, and then sintering them using zirconium disks as light loads (90 Pa), to obtain half cells having 20 mm and 3.8 m^?1 in diameter and curvature, respectively. Active materials for the electrolyte, anode, and cathode were partially stabilized zirconia (PSZ), Ni and LSM, respectively. Finally, thicknesses of complete cells were 400, 30, 30, and 80 μm for the structural anode, functional anode, electrolyte, and cathode, respectively. The cells were tested in a no‐chamber (direct‐flame) setup evaluating electrochemical performance and shock thermal resistance. Open circuit voltage was 830 mV at 560°C using methanol as fuel in a burner with porous media to modify the shape of the flame. Cells were also strong enough to resist the rapid temperature changes during several no‐chamber tests.     

Fabrication and evaluation of NiO/Y2O3-stabilized-ZrO2 hollow fibers for anode-supported micro-tubular solid oxide fuel cells

In this work NiO/3 mol% Y₂O₃–ZrO₂ (3YSZ) and NiO/8 mol% Y₂O₃–ZrO₂ (8YSZ) hollow fibers were prepared by phase-inversion. The effect of different kinds of YSZ (3YSZ and 8YSZ) on the porosity, electrical conductivity, shrinkage and flexural strength of the hollow fibers were systematically evaluated. When compared with Ni–8YSZ the porosity and shrinkage of Ni–3YSZ hollow fibers increases while the electrical conductivity decreases, while at the same time also exhibiting enhanced flexural strength. Single cells with Ni–3YSZ and Ni–8YSZ hollow fibers as the supported anode were successfully fabricated showing maximum power densities of 0.53 and 0.67 W cm⁻² at 800 °C, respectively. Furthermore, in order to improve the cell performance, a Ni–8YSZ anode functional layer was added between the electrolyte and Ni–YSZ hollow fiber. Here enhanced peak power densities of 0.79 and 0.73 W cm⁻² were achieved at 800 °C for single cells with Ni–3YSZ and Ni–8YSZ hollow fibers, respectively.     

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).     

Fabrication and Characterization of Anode‐Supported Low‐Temperature SOFC Based on Gd‐Doped Ceria Electrolyte

Large‐size, 9.5 cm × 9.5 cm, Ni‐Gd_0.1Ce_0.9O_1.95 (Ni‐GDC) anode‐supported solid oxide fuel cell (SOFC) has been successfully fabricated with NiO‐GDC anode substrate prepared by tape casting method and thin‐film GDC electrolyte fabricated by screen‐printing method. Influence of the sintering shrinkage behavior of NiO‐GDC anode substrate on the densification of thin GDC electrolyte film and on the flatness of the co‐sintered electrolyte/anode bi‐layer was studied. The increase in the pore‐former content in the anode substrate improved the densification of GDC electrolyte film. Pre‐sintering temperature of the anode substrate was optimized to obtain a homogeneous electrolyte film, significantly reducing the mismatch between the electrolyte and anode substrate and improving the electrolyte quality. Dense GDC electrolyte film and flat electrolyte/anode bi‐layer can be fabricated by adding 10 wt.% of pore‐former into the composite anode and pre‐sintering it at 1,100 °C for 2 h. Composite cathode, La_0.6Sr_0.4Fe_0.8Co_0.2O₃, and GDC (LSCF‐GDC), was screen‐printed on the as‐prepared electrolyte surface and sintered to form a complete single cell. The maximum power density of the single cell reached 497 mW cm^–2 at 600 °C and 953 mW cm^–2 at 650 °C with hydrogen as fuel and air as oxidant.