Anode-supported Yttria-stabilized Zirconia Research Articles

Nb and Pd co-doped La0.57Sr0.38Co0.19Fe0.665Nb0.095Pd0.05O3-δ as a stable, high performance electrode for barrier-layer-free Y2O3-ZrO2 electrolyte of solid oxide fuel cells

La0.6Sr0.2Co0.2Fe0.8O3-δ (LSCF) is the most intensively investigated high performance cathode for intermediate temperature solid oxide fuel cells (IT-SOFCs), but strontium segregation and migration at the electrode/electrolyte interface is a critical issue limiting the electrocatalytic activity and stability of LSCF based cathodes. Herein, we report a Nb and Pd co-doped LSCF (La0.57Sr0.38Co0.19Fe0.665Nb0.095Pd0.05O3-δ, LSCFNPd) perovskite as stable and active cathode on a barrier-layer-free anode-supported yttria-stabilized zirconia (YSZ) electrolyte cell using direct assembly method without pre-sintering at high temperatures. The cell exhibits a peak power density of 1.3 W cm−2 at 750 °C and excellent stability with no degradation during polarization at 500 mA cm−2 and 750 °C for 175 h. Microscopic and spectroscopic analysis show that the electrochemical polarization promotes the formation of electrode/electrolyte interface in operando and exsolution of Pd/PdO nanoparticles. The Nb doping in the B-site of LSCF significantly reduces the Sr surface segregation, enhancing the stability of the cathode, while the exsoluted Pd/PdO nanoparticles increases the electrocatalytic activity for the oxygen reduction reaction. The present study opens up a new route for the development of cobaltite-based perovskite cathodes with high activity and stability for barrier-layer-free YSZ electrolyte based IT-SOFCs.

Significant Promotion Effect of Bi2O3 on the Activity and Stability of Directly Assembled Lanthanum Manganite Based Cathodes of Solid Oxide Fuel Cells

Bismuth oxide exhibits extraordinary ionic conductivity and high surface exchange ability, but the low melting temperature and instability in reducing environment limits its wide application in intermediate temperature solid oxide fuel cells (IT-SOFCs). Herein, we incorporate yttria-stabilized bismuth oxide (YSB) as a promoter to enhance the electrocatalytic activity of directly assembled La0.76Sr0.19Mn0.95Pt0.05O3+δ (LSMPt) cathodes on yttria-stabilized zirconia (YSZ) electrolyte. The presence of YSB enhances the electrocatalytic activity of LSMPt for the oxygen reduction reaction (ORR) with significantly reduced activation energy of 127 kJ mol−1 for ORR, as compared to 171 kJ mol−1 for the reaction on pristine LSMPt cathode. An anode-supported YSZ film cell with the directly assembled LSMPt-YSB electrode exhibits a peak power density of 1.36 and 0.35 W cm−2 at 750 and 600°C, respectively and is stable with no degradation during the operation for over 200 h. Synchrotron near edge X-ray absorption fine structure spectroscopy (NEXAFS) and energy dispersive spectroscopy (EDS) reveal that the excellent performance and stability is owing to in situ formation of ionic conducting Mn-Bi containing particles or phase at the electrode/electrolyte interface under the influence of electrochemical polarization. The present study opens up a new avenue to apply the active bismuth oxide for the development of highly active and durable cathodes of IT-SOFCs.

Effect of flow geometry on anode-supported single chamber SOFCs arrayed as V-shape

The flow geometry of a single chamber solid oxide fuel cell was studied in order to reduce the effect of cell distance in the anode-facing-cathode stack. V-shape and converse V-shape configuration micro stacks consisting of two anode-supported yttria-stabilized zirconia membrane fuel cells were fabricated. The flow geometry reduced the effect of cell distance and improved the uniformity of performance of single cells in both stacks. The converse V-shape configuration was beneficial for maintaining the heat and the gases, in particular at low flow rates; the performance of the converse V-shape stack was better than that of the V-shape stack.

Preparation and Characterization of Anode-Supported YSZ Thin Film Electrolyte by Co-Tape Casting and Co-Sintering Process

In this study, a co-tape casting and co-sintering process has been developed to prepare yttria-stabilized zirconia (YSZ) electrolyte films supported on Ni-YSZ anode substrates in order to substantially reduce the fabrication cost of solid oxide fuel cells (SOFC). Through proper control of the process, the anode/electrolyte bilayer structures with a size of 7.8cm × 7.8cm were achieved with good flatness. Scanning electron microscopy (SEM) observation indicated that the YSZ electrolyte film was about 16 μm in thickness, highly dense, crack free and well-bonded to the anode support. The electrochemical properties of the prepared anode-supported electrolyte film was evaluated in a button cell mode incorporating a (LaSr)MnO3-YSZ composite cathode. With humidified hydrogen as the fuel and stationary air as the oxidant, the cell demonstrated an open-circuit voltage of 1.081 V and a maximum power density of 1.01 W/cm2 at 800°C. The obtained results represent the important progress in the development of anode-supported intermediate temperature SOFC with reduced fabrication cost.

An ion-plasma technique for formation of anode-supported thin electrolyte films for IT-SOFC applications

This paper describes a preparation method and structural and electrochemical properties of a thin bilayer anode-electrolyte structure for a solid oxide fuel cell operating at intermediate temperatures (IT-SOFC). Thin anode-supported yttria-stabilized zirconia electrolyte films were prepared by reactive magnetron sputtering of a Zr–Y target in an Ar–O 2 atmosphere. Porous anode surfaces of IT-SOFCs were modified by a pulsed low-energy high-current electron beam prior to film deposition; the influence of this pretreatment on the performance of both the deposited films and a single cell was investigated. The optimal conditions of the pulsed electron beam pretreatment were obtained. For the electrolyte thickness about 2.5 μm and the value of gas permeability of the anode/electrolyte structure 1.01 × 10 −7 mol m −2 s −1 Pa −1, the maximum power density achieved for a single cell at 800 °C and 650 °C was found to be 620 and 220 mW cm −2 in air, respectively.

Effect of co-sintering temperature on the performance of SOFC with YSZ electrolyte thin films fabricated by dip-coating method

Anode-supported yttria-stabilized zirconia (YSZ) electrolyte thin films were fabricated by the dip-coating method using the simple methylethylketone/ethanol-based YSZ suspension. Influences of the sintering temperature on the performance of solid oxide fuel cells were studied. Fully dense YSZ electrolyte thin films were obtained after being sintered at 1,300 °C as well as at 1,400 or 1,500 °C through SEM observation. The open circuit voltages of the cells all reached above 1.10 V at the testing temperatures. The testing results proved that both the anode performance and the electrolyte conductivity sintered at 1,300 °C were superior to those sintered at 1,400 or 1,500 °C. Investigation on the cell voltage drop under operation condition revealed, that the sintering temperature has more significant effect on the performance of the electrodes than on the electrolyte.

Fabrication of an anode-supported yttria-stabilized zirconia thin film for solid-oxide fuel cells via wet powder spraying

The wet powder spraying (WPS) technique was used for the deposition of dense and thin (Y 2O 3) 0.08(ZrO 2) 0.92 (YSZ-8) films on an anode substrate being used for fuel cell applications. Both agglomeration of the powder and the presence of organics in the substrate have a significant effect on the quality and densification of the thin electrolyte layer. High-energy ball milling effectively broke up the agglomerates and enhanced the packing density of the green layer. Pre-calcination of the substrate at ∼1000 °C enhanced the match of sintering shrinkage between the electrolyte layer and the substrate and improved the quality of the YSZ-8 thin film significantly. Crack-free dual-layer assembly with a highly densified YSZ-8 film as thin as 10 μm was successfully fabricated by optimizing the fabrication parameters. The cells with a La 0.8Sr 0.2MnO 3 cathode showed a high open circuit voltage of 1.071 V and a peak power density of 894 mW cm −2 at 850 °C operated with hydrogen fuel.

Fabrication characteristics of an anode-supported thin-film electrolyte fabricated by the tape casting method for IT-SOFC

Abstract An anode-supported yttria-stabilized zirconia (YSZ) electrolyte was fabricated using tape casting and co-sintering techniques for use in a solid oxide fuel cell (SOFC). Nickel/YSZ, lanthanum strontium manganite (La 0.85 Sr 0.15 MnO 3 ) and 8 mol% Y 2 O 3 –ZrO 2 were used for the anode, the cathode and the electrolyte, respectively. The anode-supported electrolyte was prepared with a tape casting technique followed by hot pressing lamination. Co-sintering between the anode and the electrolyte was applied at 1350 °C under pressurized conditions to fabricate the anode-supported electrolyte of the unit cell. Scanning electron microscopy (SEM) revealed the crack-free and dense structure of the electrolyte in the unit cell. The open circuit voltage (OCV) of the unit cell was 1.08 V at 750 °C, indicating negligible leakage of fuel through the electrolyte layer. The active layer was applied to the interface between the anode and the electrolyte layer to increase the three-phase boundary, the electrochemical reaction site of the anode. The unit cell having an active layer exhibits good performance due to the drastic reduction of polarization resistance.

Development of LSM-based cathodes for solid oxide fuel cells based on YSZ films

In an attempt to achieve desirable cell performance, the effects of La 0.7Sr 0.3MnO 3 (LSM)-based cathodes on the anode-supported solid oxide fuel cells (SOFCs) were investigated in the present study. Three types of cathodes were fabricated on the anode-supported yttria-stabilized zirconia (YSZ) thin films to constitute several single cells, i.e., pure LSM cathode, LSM/YSZ composite by solid mixing, LSM/Sm 0.2Ce 0.8O 1.9 (SDC) composite by the ion-impregnation process. Among the three single cells, the highest cell output performance 1.25 W cm −2 at 800 °C, was achieved by the cell using LSM/SDC cathode when the cathode was exposed to the stationary air. Whereas, the most considerable cell performance of 2.32 W cm −2 was derived from the cell with LSM/YSZ cathode, using 100 ml min −1 oxygen flow as the oxidant. At reduced temperatures down to 700 °C, the LSM/SDC cathode was the most suitable cathode for zirconia-based electrolyte SOFC in the present study. The variation in the cell performances was attributed to the mutual effects between the gas diffusing rate and three-phase boundary length of the cathode.

Effect of SDC-impregnated LSM cathodes on the performance of anode-supported YSZ films for SOFCs

Sm 0.2Ce 0.8O 1.9 (SDC)-impregnated La 0.7Sr 0.3MnO 3 (LSM) composite cathodes were fabricated on anode-supported yttria-stabilized zirconia (YSZ) thin films. Electrochemical performances of the solid oxide fuel cells (SOFCs) were investigated in the present study. Four single cells, i.e., Cell-1, Cell-2, Cell-3 and Cell-4 were obtained after the fabrication of four different cathodes, i.e., pure LSM and SDC/LSM composites in the weight ratios of 25/75, 36/64 and 42/58, respectively. Impedance spectra under open-circuit conditions showed that the cathode performance was gradually improved with the increasing SDC loading. Similarly, the maximum power densities (MPD) of the four cells were increased with the SDC amount below 700 °C. Whereas, the cell performance of Cell-4 was lower than that of Cell-3 at 800 °C, arising from the increased concentration polarization at high current densities. This was caused by the lowered porosity with the impregnation cycle. This disadvantage could be suppressed by lowering the operating temperature or by increasing the oxygen concentration at the cathode side. The ratio of electrode polarization loss in the total voltage drop versus current density showed that the cell performance was primarily determined by the electrode polarization. The contribution of the ohmic resistance was increased when the operating temperature was lowered. When a 100 ml min −1 oxygen flow was introduced to the cathode side, Cell-3 produced MPDs of 1905, 1587 and 1179 mW cm −2 at 800, 750 and 700 °C, respectively. The high cell outputs demonstrated the merits of the novel and effective SDC-impregnated LSM cathodes.

Fabrication and performance of anode-supported YSZ films by slurry spin coating

Yttria-stabilized zirconia (YSZ) thin films were fabricated by slurry spin coating on NiO-YSZ anode substrates. The effect of processing factors, such as the baking temperature and spinning number was investigated and discussed. According to the thermogravimetric analysis, the organic additives could be removed by baking at 400 °C for 10 min. Under this heating condition for each coating, the defects in the electrolyte film could be repaired by repetitive coating. Dense and crack-free electrolyte films were consequently obtained. When dry hydrogen was used as the fuel and the stationary air as the oxidant, a single cell based on a 14-μm-thick YSZ film produced open-circuit voltages (OCVs) of > 1.10 V in all of the operating temperatures (500–700 °C). The cell exhibited maximum power densities of 634, 493 and 316 mW/cm 2 at 700, 650 and 600 °C, respectively. By using pure oxygen at a flow rate of 100 mL/min, the maximum power density increased to 658 mW/cm 2 at 650 °C.

Ba 0.5Sr 0.5Co 0.8Fe 0.2O 3− δ as a cathode for IT-SOFCs with a GDC interlayer

The chemical compatibility of the Ba 0.5Sr 0.5Co 0.8Fe 0.2O 3− δ (BSCF) with the yttria-stabilized zirconia (YSZ) electrolyte or Gd-doped ceria electrolyte (GDC), as well as that of the GDC with the YSZ electrolyte were examined. It was found that BSCF had a good compatibility with the GDC electrolyte but a poor chemical compatibility with the YSZ electrolyte. The BSCF cathode was adopted for anode-supported YSZ electrolyte cells with and without the application of a 1 μm thick GDC buffering layer between the cathode and the YSZ electrolyte. The interfacial reactions of the BSCF with the YSZ electrolyte surface and the GDC coated YSZ surfaces were investigated. The single cells were evaluated by using I– V curve measurements and AC impedance spectroscopy. The results depicted a great improvement in cell performance and a significant decrease in polarization resistance after adding the GDC buffer layer. The optimum firing temperature of the GDC film onto the YSZ film was around 1250 °C, which led to the maximum power density of 1.56 W cm −2 at 800 °C using air as oxidant and hydrogen as fuel.

Direct EPD of YSZ Electrolyte Film onto Porous NiO-YSZ Composite Substrate for Reduced-Temperature Operating Anode-Supported SOFC

Electrophoretic deposition (EPD) was used for the fabrication of anode-supported yttria-stabilized zirconia (YSZ) electrolyte films. For the EPD, thin layers of graphite were pre-coated on the surface of a nonconducting porous NiO-YSZ composite anode substrate. Uniform YSZ green films were formed on the reverse sides, which did not have the graphite layers. The specimens were transformed into dense bodies ∼5 to 10 μm thick after being co-fired with the substrates. The cell performance of the ∼5 μm thick dense YSZ films supported on the anode substrates was tested using a La(Sr)Co(Fe)O 3 cathode. Maximum output power densities of ∼0.19, ∼0.61, and ∼1.02 W/cm 2 were attained at 600, 700, and 800°C, respectively.

Fabrication and characterization of anode-supported electrolyte thin films for intermediate temperature solid oxide fuel cells

Anode-supported electrolyte thin films for improving the solid oxide fuel cell (SOFC) performance at intermediate temperature (IT) have been manufactured by a wet-chemical process, and their microstructures, gas permeabilities, and electrical performances have been investigated. NiO–YSZ anode supports of a flat tube type were prepared by the extrusion method, and their surfaces were modified via slurry coating of fine NiO–YSZ particles for controlling the surface roughness and the pore size. An anode-supported yttria-stabilized zirconia (YSZ) electrolyte was fabricated by dip-coating YSZ slurry (viscosity 4.5 cP, solid contents 2.7 vol.%) onto the modified anode support, then it was coated with YSZ sol (viscosity 2.5 cP), and sintered at 1400 °C. The cathode consisted of three consecutive layers of LSM–YSZ composite, strontium-doped lanthanum manganite (LSM), and La 0.6Sr 0.4Co 0.2Fe 0.8O 3−δ (LSCF). After each successive slurry layer was applied it was co-fired at 1200 °C. The thickness of YSZ electrolyte layers could be controlled below 15 μm, and the YSZ layers’ acceptability as an electrolyte film for an SOFC was estimated from the result of the gas impermeability ranging below 2 bar. The unit cells fabricated in this work showed a good electrical performance of 550 mW cm −2 at 850 °C. This is attributed to the reduced resistance through the thin YSZ electrolyte.

Interaction between La(Sr)FeO 3 SOFC cathode and YSZ electrolyte

Anode-supported yttria-stabilized zirconia (YSZ) solid oxide fuel cells utilizing a Sr-doped LaFeO 3 (LSF) cathode show improved performance with the incorporation of a Sm-doped CeO 2 layer between the cathode and YSZ electrolyte. Detailed X-ray diffraction (XRD) analysis of LSF–YSZ reaction mixtures indicates no strontium or lanthanum zirconate formation between these materials even at 1400 °C. However, a significant shift in the LSF diffraction peaks is readily apparent and corresponds to a unit cell volume expansion. The most likely scenario for the change in volume is considered to be the incorporation of Zr 4+ cations in the perovskite structure. The presence of the Zr cations subsequently results in reduced electrical conductivity of the cathode, potentially explaining the need for the aforementioned ceria interlayer.