Influences of feedstock and plasma spraying parameters on the fabrication of tubular solid oxide fuel cell anodes

This study developed a tubular solid oxide fuel cell (SOFC) anode support layer via atmospheric plasma spraying, which is considered one of the most promising methods for producing SOFCs because of its faster deposition rate and lower cost compared with other film formation processes. Plasma spraying can replace the traditional use of extrusion technology to manufacture the anode base tube, eliminating the need for high-temperature sintering steps. In this study, commercially available powders were used to make the anode of a tubular SOFC from NiO/yttria-stabilized zirconia (YSZ) powder, and Na₂CO₃ and polymethyl methacrylate were tested as pore-forming agents. The anode composite powder was sprayed on the graphite base pipe, and the final product was changed by altering the spraying parameters and anode powder ratio. The direct current (DC) resistance measurements showed that the conductivity of the Ni/YSZ tubular anode formed with higher power plasma spraying could reach 428.55?S/cm at 800?°C. The experimental results showed that the power and parameters of atmospheric plasma spraying could affect the porosity and electron conductivity of tubular SOFC anodes.     

Characterizations of impedance responses in an anode-supported solid oxide fuel cell with an air blowing system

Effects of operation parameters on impedance responses are characterized to study electrochemical reactions of an anode-supported solid oxide fuel cell (SOFC) in an air blowing operation. The anode-supported SOFC, which consists of Ni-yttrium stabilized zirconia (YSZ) support/Ni-YSZ anode functional layer/YSZ electrolyte/gadolinium doped ceria (GDC) interlayer/La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ -GDC cathode, is fabricated by a tape casting and co-firing process. To investigate the electrochemical response on impedances, an equivalent circuit is modeled with five elements and fitted by the complex nonlinear least square (CNLS) method. Based on the impedance spectra with the operation parameters, two among five elements are clarified to be concerned with anodic reactions and another two concerned with gas diffusion reactions in electrodes. It is difficult to clarify one among five elements with the results here. The clarified elements may be used to study the effects of materials and processes for SOFC with impedance responses, which will be helpful to improve the performance and reliability.     

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.     

NI/NI‐YSZ Current Collector/Anode Dual Layer Hollow Fibers for Micro‐Tubular Solid Oxide Fuel Cells

A co‐extrusion technique was employed to fabricate a novel dual layer NiO/NiO‐YSZ hollow fiber (HF) precursor which was then co‐sintered at 1,400 °C and reduced at 700 °C to form, respectively, a meshed porous inner Ni current collector and outer Ni‐YSZ anode layers for SOFC applications. The inner thin and highly porous “mesh‐like” pure Ni layer of approximately 50 μm in thickness functions as a current collector in micro‐tubular solid oxide fuel cell (SOFC), aiming at highly efficient current collection with low fuel diffusion resistance, while the thicker outer Ni‐YSZ layer of 260 μm acts as an anode, providing also major mechanical strength to the dual‐layer HF. Achieved morphology consisted of short finger‐like voids originating from the inner lumen of the HF, and a sponge‐like structure filling most of the Ni‐YSZ anode layer, which is considered to be suitable macrostructure for anode SOFC system. The electrical conductivity of the meshed porous inner Ni layer is measured to be 77.5 × 10⁵ S m^–1. This result is significantly higher than previous reported results on single layer Ni‐YSZ HFs, which performs not only as a catalyst for the oxidation reaction, but also as a current collector. These results highlight the advantages of this novel dual‐layer HF design as a new and highly efficient way of collecting current from the lumen of micro‐tubular SOFC.     

Fabrication of YSZ electrolyte using electrostatic spray deposition (ESD):I – a comprehensive parametric study

Electrostatic spray deposition (ESD) was applied to fabricate a thin-layer (3 m thickness) yttria-stabilized zirconia (YSZ) electrolyte on a solid oxide fuel cell (SOFC) anode substrate consisting of nickel-YSZ cermet. Reducing the thickness of a state-of-the-art electrolyte, and thereby reducing the cell internal IR drop, is a promising strategy to make the intermediate temperature SOFC (ITSOFC) operating at 600–800 °C possible. About 8 mol% YSZ colloidal solution in ethanol was sprayed onto the substrate anode surface at 250–300 °C by ESD. After sintering the deposited layer at 1250–1400 °C for 17–6 h, the cathode layer, consisting of lanthanum strontium manganate (LSM), was sprayed or brush coated onto the electrolyte layer. Performance tests on the cell were carried out at 800 °C to evaluate the electrolyte layer formed by ESD. With a 97 H₂/3 H₂O mixture and air as fuel and oxidant gas, respectively, open circuit voltage (OCV) was found to be close to the theoretical value.     

Oxygen Dissociation and Interfacial Transfer Rate on Performance of SOFCs with Metal‐Added (LaSr)(CoFe)O3‐(Ce,Gd)O2 – δ Cathodes

A solid oxide fuel cell (SOFC) unit is constructed with Ni‐Ce_0.9Gd_0.1O_{2 – δ} (GDC) as the anode, yttria‐stabilised zirconia (YSZ) as the electrolyte and Pt, Ag or Cu‐added La_0.58Sr_0.4Co_0.2Fe_0.8O_{3 – δ} (LSCF)–GDC as the cathode. The current–voltage measurements are performed at 800 °C. Cu addition leads to best SOFC performance. LSCF–GDC–Cu is better than LSCF–GDC and much better than GDC as the material of the cathode interlayer. Cu content of 2 wt.‐% leads to best SOFC performance. A cathode functional layer calcined at 800 °C is better than that calcined at higher temperature. Metal addition increases the O₂ dissociation reactivity but results in an interfacial resistance for O transfer. A balance between the rates of O₂ dissociation and interfacial O transfer is needed for best SOFC performance.     

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.     

Optimization of anode structure for intermediate temperature solid oxide fuel cell via phase‐inversion cotape casting

A functional layer and a porous support that together constitute an anode for a solid oxide fuel cell were simultaneously formed by the phase‐inversion tape casting method. Two slurries, one composed of NiO and yttria‐stabilized zirconia (YSZ) powders and the other of NiO, YSZ, and graphite were cocasted and solidified by immersion in a water bath via the phase‐inversion mechanism. The as‐formed green tape consisted of a sponge‐like thin layer and a fingerlike thick porous layer, derived from the first slurry and the second slurry, respectively. The former acted as the anode functional layer (AFL), while the latter was used as the anode substrate. The AFL thickness was varied between 20 and 60 μm by adjusting the blade gap for the tape casting. Single cells based on such NiO‐YSZ anodes were prepared with thin YSZ electrolytes and YSZ‐(La_0.8Sr_0.2)_0.95MnO_3?δ (LSM) cathodes, and their electrochemical performance was measured using air as oxidant and hydrogen as fuel. The maximum power densities obtained at 750°C were 720, 821, and 988 mW cm^?2 with the AFL thickness at 60, 40, and 20 μm, respectively. The satisfactory electrochemical performance was attributed to the dual‐layer structure of the anode, where the sponge‐like AFL layer provided plenty of triple‐phase boundaries for hydrogen oxidation, and the fingerlike thick porous substrate allowed for facile fuel transport. The phase‐inversion tape casting developed in this study is applicable to the preparation of other planar ceramic electrodes with dual‐layer asymmetric structure.     

Evaluation of a Cu/YSZ and Ni/YSZ Bilayer Anode for the Direct Utilization of Methane in a Solid‐Oxide Fuel Cell

Carbon deposition is an issue when operating solid oxide fuel cells (SOFC) on fuels other than hydrogen, and so a variety of strategies have been used to prevent carbon accumulation on the anodes. In this paper, we describe a bilayer anode that contains a functional layer consisting of Ni/YSZ and a conduction layer consisting of Cu/YSZ. The anode‐supported button cells were fabricated using a uni‐axially pressing technique to produce the anode, followed by impregnation with Cu. The cells were tested at 1,023 K in dry CH₄ and their performance compared to that of a typical Ni/YSZ anode. The Cu does not catalyze the cracking of methane and as such less carbon deposits in the conduction layer resulting in anode stability for over 100 h. The limitation with using Cu in the anode is the temperature of operation.     

Characterization of yttria-stabilized-zirconia/stainless steel joint interfaces with gold-based interlayers for solid oxide fuel cell applications

YSZ/stainless steel joints using two commercial gold-based interlayers, 96.4Au–3Ni–0.6Ti and 97.5Au–0.75Ni–1.75V (in wt.%), were fabricated and analyzed for the microstructure of interlayer matrix, interlayer/steel interface, and interlayer/YSZ interface by SEM and TEM/EDS. At the 96.4Au–3Ni–0.6Ti/steel interface, a plate-like Au-rich phase precipitated in the steel and a dense Au-rich layer formed between steel and interlayer. No such Au-rich layer formed at the 97.5Au–0.75Ni–1.75V/steel interface. Needle-like Fe-rich phase precipitates within the Au-rich matrix were observed in both gold-based interlayers. At the 96.4Au–3Ni–0.6Ti/YSZ interface, a Ti₂O₃ layer and a small amount of Ni₃Si₂ were detected. However, at the 97.5Au–0.75Ni–1.75V/YSZ interface, no interfacial layer was observed, except dispersed micro-cavities and a large amount of Au in the grain boundaries of zirconia. These observations are used to propose a scheme for the development of joint microstructure. The 96.4Au–3Ni–0.6Ti interlayer appears to be a better choice for joining of YSZ to steel in SOFC applications.     

Polarization properties of La0.6Sr0.4Co0.2Fe0.8O3-based double layer-type oxygen electrodes for reversible SOFCs

We have developed double layer-type (catalyst layer/current collecting layer) oxygen electrodes (DLE) for reversible SOFCs. As the catalyst layer (cathode for SOFC and anode for steam electrolysis) interfaced with a samaria-doped ceria [(CeO₂)_0.8(SmO_1.5)_0.2, SDC] interlayer/YSZ solid electrolyte, mixed conducting La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF) and SDC particles were employed. The current collecting porous LSCF layer was formed on the catalyst layer. By controlling the SDC content, as well as the thickness and porosity of the catalyst layer, the gas diffusion rate and the conduction networks for electrons and oxide ions were optimized, resulting in a marked reduction of the overpotential. The LSCF + SDC/LSCF DLE exhibited higher performance than single-layer electrodes of LSCF + SDC or LSCF; the IR-free anode potential vs. an air reference electrode was 0.12 V (corresponding to an overpotential of 0.08 V) at 0.5 A cm⁻² and 900 °C under an atmosphere of O₂ (1 atm).     

Electrochemical Performance of a Ni and YSZ Composite Synthesised by Ultrasonic Spray Pyrolysis as an Anode for SOFCs

The electrochemical performance of an anode material for a solid oxide fuel cell (SOFC) depends highly on microstructure in addition to composition. In this study, a NiO–yttria‐stabilised zirconia (NiO–YSZ) composite with a highly dispersed microstructure and large pore volume/surface area has been synthesised by ultrasonic spray pyrolysis (USP) and its electrochemical characteristics has been investigated. For comparison, the electrochemical performance of a conventional NiO–YSZ is also evaluated. The power density of the zirconia electrolyte‐supported SOFC with the synthesised anode is ∼392 mW cm^–2 at 900 °C and that of the SOFC with the conventional NiO–YSZ anode is ∼315 mW cm^–2. The improvement is ∼24%. This result demonstrates that the synthesised NiO–YSZ is a potential alternative anode material for SOFCs fabricated with a zirconia solid electrolyte.     

Development of a Novel Ceramic Support Layer for Planar Solid Oxide Cells

The conventional solid oxide cell is based on a Ni–YSZ support layer, placed on the fuel side of the cell, also known as the anode supported SOFC. An alternative design, based on a support of porous 3YSZ (3 mol.% Y₂O₃–doped ZrO₂), placed on the oxygen electrode side of the cell, is proposed. Electronic conductivity in the 3YSZ support is obtained post sintering by infiltrating LSC (La_0.6Sr_0.4Co_1.05O₃). The potential advantages of the proposed design is a strong cell, due to the base of a strong ceramic material (3YSZ is a partially stabilized zirconia), and that the LSC infiltration of the support can be done simultaneously with forming the oxygen electrode, since some of the best performing oxygen electrodes are based on infiltrated LSC. The potential of the proposed structure was investigated by testing the mechanical and electrical properties of the support layer. Comparable strength properties to the conventional Ni/YSZ support were seen, and sufficient and fairly stable conductivity of LSC infiltrated 3YSZ was observed. The conductivity of 8–15 S cm^–1 at 850 °C seen for over 600 h, corresponds to a serial resistance of less than 3.5 m Ω cm² of a 300 μm thick support layer.     

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.     

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.     

Ni-Fe bimetallic anode as an active anode for intermediate temperature SOFC using LaGaO3 based electrolyte film

Effects of additives to Ni anode were studied and it was found that the anodic overpotential can be suppressed by addition of Fe. La_0.9Sr_0.1Ga_0.8Mg_0.2O₃ (LSGM) film was deposited on the dense anode substrate consisting of NiO(Fe₃O₄)-Sm doped CeO₂. After in-situ reduction of NiO and Fe₃O₄ in the dense substrate, the substrate turned to be porous, however, change in size was not large by mixing with SDC. As a result, LSGM dense film with few micrometer thickness was successfully obtained on the porous Ni based anode substrate. By optimizing the thickness of the LSGM film and application of SDC interlayer, the high power density of SOFC single cell using LSGM/SDC bi-layer film as electrolyte at decreased temperature was fabricated and the electrical power generating property was measured as a function of temperature. The high maximum power density could be achieved to a value of 2 W/cm² at 873 K. Even at 673 K, the maximum power density of ca. 80 mW/cm² is exhibited and this high power density was a result of the low electrolyte resistance and the small anodic overpotential of Ni-Fe bimetallic anode.     

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.     

Thin film yttria-stabilized zirconia electrolyte for intermediate-temperature solid oxide fuel cells (IT-SOFCs) by chemical solution deposition

A 500 nm thick thin film YSZ (yttria-stabilized zirconia) electrolyte was successfully fabricated on a conventionally processed anode substrate by spin coating of chemical solution containing slow-sintering YSZ nanoparticles with the particle size of 20 nm and subsequent sintering at 1100 °C. Incorporation of YSZ nanoparticles was effective for suppressing the differential densification of ultrafine precursor powder by mitigating the prevailing bi-axial constraining stress of the rigid substrate with numerous local multi-axial stress fields around them. In particular, adding 5 vol% YSZ nanoparticles resulted in a dense and uniform thin film electrolyte with narrow grain size distribution, and fine residual pores in isolated state. The thin film YSZ electrolyte placed on a rigid anode substrate with the GDC (gadolinia-doped ceria) and LSC (La_0.6Sr_0.4CoO_3?δ) layers deposited by PLD (pulsed laser deposition) processes revealed that it had fairly good gas tightness relevant to a SOFC (solid oxide fuel cell) electrolyte and maintained its structural integrity during fabrication and operation processes. In fact, the open circuit voltage was 1.07 V and maximum power density was 425 mW/cm² at 600 °C, which demonstrates that the chemical solution route can be a viable means for reducing electrolyte thickness for low- to intermediate-temperature SOFCs.     

Fabrication of YSZ electrolyte for intermediate temperature solid oxide fuel cell using electrostatic spray deposition: II – Cell performance

Electrostatic spray deposition (ESD) was applied to fabricate a thin-layer of yttria-stabilized zirconia (YSZ) electrolyte on a solid oxide fuel cell (SOFC) anode substrate consisting of nickel-YSZ cermet. A colloidal solution of 8 mol% YSZ in ethanol was sprayed onto the substrate anode surface at 250–300 °C by ESD. After sintering the deposited layer at 1250–1400 °C for 1–2 h depending on temperature, the cathode layer, consisting of lanthanum strontium manganate (LSM), was sprayed or brush coated onto the electrolyte layer. Performance tests and AC impedance measurements of the complete cell were carried out at 800 °C to evaluate the density and conductance of the electrolyte layer formed by ESD. With a 97% H₂/3% H₂O mixture and air as fuel and oxidant gas, respectively, the open-circuit voltage (OCV) was close to theoretical and electrolyte impedance was about 0.23Ω cm². A power density of 0.45 W cm⁻² at 0.62 V was obtained. No abnormal degradation was observed after 170 h operation. The electrolyte sintering temperature and time did not significantly affect the electrolyte impedance. on leave from     

Characterization of a Cu‐La0.75Sr0.25Cr0.5Mn0.5O3 ‐CeO2/La0.75Sr0.25Cr0.5Mn0.5O3‐YSZ/Ni‐ScSZ Three‐Layer Structure Anode in Thin Film Solid Oxide Fuel Cell Running on Methane Fuel

Anodes for Solid Oxide Fuel Cell that is capable of directly using hydrocarbon without external reforming have been of great interest recently. In this paper, a three‐layer structure anode running on methane is fabricated by tape casting and screen printing method. The slurry of catalyst layer Cu‐LSCM‐CeO₂ (with weight ratios of 1.5:7.0:1.5, 2.0:7.0:1.0, 2.15:7.0:0.85 and 2.25:7.0:0.75, weight ratios of Cu/CeO₂ is 1:1, 2:1, 2.5:1 and 3:1, respectively) is screen‐printed on LSCM‐YSZ support layer and Ni‐ScSZ active layer. Thus, LSCM‐YSZ/Ni‐ScSZ anodes with Cu‐LSCM‐CeO₂ catalyst layer (denoted as LSCM‐YSZ1010, LSCM‐YSZ2010, LSCM‐YSZ2510 and LSCM‐YSZ3010, respectively) are obtained. Single cells with three‐layer structure anode are also fabricated and measured, of which the maximum power density reaches 491 and 670 mW cm⁻² for the cell with LSCM‐YSZ2510 anode running on methane at 750 °C and 800 °C, respectively. No significant degradation in performance has been observed after 240h of cell testing when LSCM‐YSZ2510 anode is exposed to methane at 750 °C. Very little carbon deposition is detected on the anode, suggesting that carbon deposition is limited during cell operation. Consequently, Cu‐LSCM‐CeO₂ catalyst layer on the surface of LSCM‐YSZ support layer makes it possible to have good stability for long‐term operation in methane due to very little carbon deposition.