A study of Ni + 8YSZ/8YSZ/La0.6Sr0.4CoO3−δ ITSOFC fabricated by atmospheric plasma spraying

An intermediate temperature solid oxide fuel cell (ITSOFC) based on 8YSZ electrolyte, La_0.6Sr_0.4CoO_3−δ (LSCo) cathode, and Ni − 8YSZ anode coatings were consecutively deposited onto a porous Ni-plate substrate by atmospheric plasma spraying (APS). The spray parameters including current, argon and hydrogen flow rate, and powder feed rate were investigated by an orthogonal experiment to fabricate a thin gas-tight 8YSZ electrolyte coating (80 μm). By proper selection of the spray parameters to decrease the particles velocity and temperature, the sprayed NiO + 8YSZ coating after reducing with hydrogen shows a good electrocatalytic activity for H₂ oxidation. With the same treatment, 100–170 μm dimensions LSCo particle could keep phase structure after spraying. And the deposited LSCo cathode shows a good cathode performance and chemical compatibility with 8YSZ electrolyte after operating at 800 °C for 50 h. Output power density of the sprayed cell achieved 410 mW cm⁻² at 850 °C and 260 mW cm⁻² at 800 °C. Electrochemical characterization indicated that IR drop of 8YSZ electrolyte, cathodic polarization, and the contact resistance at LSCo/8YSZ interface were the main factors restricting the cell performance. The results suggested that the use of APS cell allowed the reduction of the operating temperature of the SOFC to below 850 °C with lower production costs.     

Behavior of lanthanum-doped ceria and Sr-, Mg-doped LaGaO3 electrolytes in an anode-supported solid oxide fuel cell with a La0.6Sr0.4CoO3 cathode

Anode-supported solid oxide fuel cells (SOFCs) with lanthanum-doped ceria (LDC)/Sr-, Mg-doped LaGaO₃ (LSGM) bilayered or LDC/LSGM/LDC trilayered electrolyte films were fabricated with a pure La_0.6Sr_0.4CoO₃ (LSC) cathode. The behaviors of the two electrolytes in cells were investigated by using scanning electron microscopy, impedance spectroscopy and cell performance measurements. The reactions between LSGM and anode material can be suppressed by applying a ca. 15 μm LDC film. Due to the Co diffusion from the LSC cathode to the LSGM electrolyte during high temperature sintering, the electronic conductivity of the LDC electrolyte cannot be completely blocked with an LSGM layer below 50 μm, which leads to open-circuit potentials of these cells of ca. 0.988 V at 800 °C. The electrical conductivities of LDC and LSGM electrolytes in the cells under operation conditions are obtained from the dependence of the cell ohmic resistance on the electrolyte thickness. The electrical conductivity of LDC electrolyte is ca. 0.117 S cm⁻¹ at 800 °C on the bilayered electrolyte cells with a 50 μm LSGM layer. The bilayer electrolyte cells with a 25 μm LDC layer at 800 °C, had a cell ohmic resistance two-stage linear dependence on the LSGM layer thickness, which showed the electrical conductivity of ca. 1.9 S cm⁻¹ for the LSGM layer below 50 μm and 0.22 S cm⁻¹ for the LSGM layer above 100 μm. With a LDC/LSGM/LDC trilayered electrolyte film for the anode-supported cell, an open-circuit potential of 1.043 V was achieved.     

Anode-supported solid oxide fuel cell based on dense electrolyte membrane fabricated by filter-coating

A simple and cost-effective technique, filter-coating, has been developed to fabricate dense electrolyte membranes. Eight mole percent yttria-stabilized zirconia (YSZ) electrolyte membrane as thin as 7 μm was prepared by filter-coating on a porous substrate. The thickness of the YSZ film was uniform, and could be readily controlled by the concentration of the YSZ suspension and the rate of the suspension deposition. The YSZ electrolyte film was dense and was well bonded to the Ni-YSZ anode substrate. An anode-supported solid oxide fuel cell (SOFC) with a YSZ electrolyte film and a La_0.85Sr_0.15MnO₃ (LSM) + YSZ cathode was fabricated and its performance was evaluated between 700 and 850 °C with humidified hydrogen as the fuel and ambient air as the oxidant. An open circuit voltage (OCV) of 1.09 V was observed at 800 °C, which was close to the theoretical value, and the maximum power density measured was 1050 mW cm⁻². The results demonstrate that the dense YSZ film fabricated by filter-coating is suitable for application to SOFCs.     

Fabrication and characterization of a YSZ/YDC composite electrolyte by a sol–gel coating method

The compatibility of a composite electrolyte composed of a yttria stabilized zirconia (YSZ) film and a yttria-doped ceria (YDC) substrate in a solid oxide fuel cell (SOFC) that can be operated under 800 °C was evaluated. The YSZ film coated on a YDC substrate was derived from a polymeric YSZ sol using a sol–gel spin coating method followed by heat-treatment at 1400 °C for 2 h. The SEM and XRD analysis indicated that there were no cracks, pinholes, or byproducts. The composite electrolyte comprising a YSZ film of 2 μm thickness and a YDC substrate of 1.6 mm thickness was used in a single cell performance test. A 0.5 V higher value of open circuit voltage (OCV) was found for the composite electrolyte single cell compared with an uncoated YDC single cell between 700 and 1050 °C and confirmed that the YSZ film was an electron blocking layer. The maximum power density of the composite electrolyte single cell at 800 °C, 122 mW/cm² at 285 mA/cm², is comparable with that of a YSZ single cell with the same thickness at 1000 °C, namely 144 mW/cm² at 330 mA/cm². The hypothetical oxygen partial pressure at the interface between the YSZ film and the YDC substrate for the composite electrolyte with the same thickness ratio at 800 °C is 5.58×10⁻¹⁸ atm which is two orders of magnitude higher than the equilibrium oxygen partial pressure of Ce₂O₃/CeO₂, 2.5×10⁻²⁰ atm, at the same temperature.     

Improvement of anode performance by surface modification for solid oxide fuel cell running on hydrocarbon fuel

A Ni/ yttria-stabilized zirconia (YSZ) cermet anode was modified by coating with samaria-doped ceria (SDC, Sm_0.2Ce_0.8O₂) sol within the pores of the anode for a solid oxide fuel cell (SOFC) running on hydrocarbon fuel. The surface modification of Ni/YSZ anode resulted in an increase of structural stability and enlargement of the triple phase boundary (TPB), which can serve as a catalytic reaction site for oxidation of carbon or carbon monoxide. Consequently, the SDC coating on the pores of anode made it possible to have good stability for long-term operation due to low carbon deposition and nickel sintering.The maximum power density of an anode-supported cell (electrolyte; 8 mol% YSZ and thickness of 30 μm, and cathode; La_0.85Sr_0.15MnO₃) with the modified anode was about 0.3 W/cm² at 700 °C in the mixture of methane (25%) and air (75%) as the fuel and air as the oxidant. The cell was operated for 500 h without significant degradation of cell performance.     

Yttria-stabilized zirconia thin films deposited on NiO–(Sm2O3)0.1(CeO2)0.8 substrates by chemical vapor infiltration

Fabrication of YSZ films deposited on NiO–samaria-doped ceria (SDC) substrate was studied by the chemical vapor infiltration method (CVI). A NiO–SDC substrate was used as oxygen source. The main mechanism of YSZ growth was electrochemical vapor deposition (EVD), while the contribution of oxygen in the carrier gas increased with increasing NiO content of the substrate above 60.6 mol%. The YSZ film on SDC used as the anode proved effective in obtaining high cell performance. In particular, a YSZ film thickness of 1 μm yielded the highest cell performance in the temperature range from 973 to 1073 K. The CVI method was useful for preparing a dense and strong YSZ film on the complex-shaped NiO–SDC substrate.     

Thin yttria-stabilized zirconia electrolyte and transition layers fabricated by particle suspension spray

In order to develop high performance intermediate temperature (<800 °C) solid oxide fuel cells (SOFCs) with a lower fabrication cost, a pressurized spray process of ceramic suspensions has been established to prepare both dense yttria-stabilized zirconia (YSZ) electrolyte membranes and transition anode layers on NiO + YSZ anode supports. A single cell with 10 μm thick YSZ electrolyte on a porous anode support and ∼20 μm thick cathode layer showed peak power densities of only 212 mW cm⁻² at 700 °C and 407 mW cm⁻² for 800 °C. While a cell with 10 μm thick YSZ electrolyte and a transition layer on the porous anode support using a ultra-fine NiO + YSZ powder showed peak power densities of 346 and 837 mW cm⁻² at 700 and 800 °C, respectively. The dramatic improvement of cell performance was attributed to the much improved anode microstructure that was confirmed by both scanning electron microscopes (SEM) observation and impedance spectroscopy. The results have demonstrated that a pressurized spray coating is a suitable technique to fabricate high performance SOFCs and at lower cost.     

NiO–YSZ cermets supported low temperature solid oxide fuel cells

Solid oxide fuel cells with thin electrolyte of two types, Sm_0.2Ce_0.8O_1.9 (SDC) (15 μm) single-layer and 8 mol% Yttria stabilized zirconia (YSZ) (5 μm) + SDC (15 μm) bi-layer on NiO–YSZ cermet substrates were fabricated by screen printing and co-firing. A Sm_0.5Sr_0.5CoO₃ cathode was printed, and in situ sintered during a cell performance test. The SDC single-layer electrolyte cell showed high electrochemical performance at low temperature, with a 1180 mW cm⁻² peak power density at 650 °C. The YSZ + SDC bi-layer electrolyte cell generated 340 mW cm⁻² peak power density at 650 °C, and showed good performance at 700–800 °C, with an open circuit voltage close to theoretical value. Many high Zr-content micro-islands were found on the SDC electrolyte surface prior to the cathode preparation. The influence of co-firing temperature and thin film preparation methods on the Zr-islands’ appearance was investigated.     

Growth mechanism of thin films of yttria-stabilized zirconia by chemical vapor infiltration using NiO–ceria substrate as oxygen source

The deposition of yttria-stabilized zirconia films on a NiO–ceria substrate by chemical vapor infiltration (CVI) using ZrCl₄ and YCl₃ as metal sources and NiO–ceria as oxygen source was studied. The resultant films were cubic YSZ with a Y₂O₃ content of 3.7–4.2 mol%, and were transparent and strong. A NiO content of NiO–ceria above 60 mol% increases the growth rate of the YSZ film from about 5 to 25 μm over 2 h, indicating that chemical vapor deposition (CVD) occurred in addition to electrochemical vapor deposition (EVD), whereas NiO contents below 60 mol% does not affect the growth rate, indicating that only electrochemical vapor deposition occurred. The growth mechanism of the YSZ film is determined and a YSZ thin film is successfully fabricated on NiO–ceria to improve mechanical strength.     

Fabrication and characterization of micro tubular SOFCs for operation in the intermediate temperature

Tubular SOFC systems appear to be well-suited to accommodate repeated cycling under rapid changes in electrical load and in cell operating temperatures. Our goal is to develop innovative processing method to fabricate new micro tubular SOFCs with sub-millimeter diameter and its stack module which enable to generate high volumetric power density. In this study, micro tubular SOFCs under 1 mm diameter have been successfully fabricated and tested in the intermediate temperature region (550 °C or under). The cell consists of NiO–Gd doped ceria (GDC) as an anode (support tube), GDC as an electrolyte and (La, Sr)(Fe, Co)O₃ (LSCF)–GDC as a cathode. The single tubular cell with 0.8 mm diameter and 12 mm length generated over 70 mW at 550 °C with H₂ fuel, which indicates that the cell generated over 0.3 W cm⁻² at 550 °C.     

Screen-printed thin YSZ films used as electrolytes for solid oxide fuel cells

A thin yttria-stabilized zirconia (8 mol% YSZ) film was successfully fabricated on a NiO-YSZ anode substrate by a screen-printing technique. The scanning electron microscope (SEM) results suggested that the YSZ film thickness was about 31 μm after sintering at 1400 °C for 4 h in air. A 60 wt% La_0.7Sr_0.3MnO₃ + 40 wt% YSZ was screen-printed onto the YSZ film surface as cathode. A single cell was tested from 650 to 850 °C using hydrogen as fuel and ambient air as oxidant, which showed an open circuit voltage (OCV) of 1.02 V and a maximum power density of 1.30 W cm⁻² at 850 °C. The OCV was higher than 1.0 V, which suggested that the YSZ film was quite dense and that the fuel gas leakage through the YSZ film was negligible. Screen-printing can be a promising method for manufacturing YSZ films for solid oxide fuel cells (SOFCs).     

Measurement of oxygen chemical potential in Gd2O3-doped ceria-Y2O3-stabilized zirconia bi-layer electrolyte, anode-supported solid oxide fuel cells

Solid oxide fuel cells (SOFC) were fabricated with gadolinia-doped ceria (GDC)-yttria stabilized zirconia (YSZ), thin bi-layer electrolytes supported on Ni + YSZ anodes. The GDC and YSZ layer thicknesses were 45 μm, and ∼5 μm, respectively. Two types of cells were made; YSZ layer between anode and GDC (GDC/YSZ) and YSZ layer between cathode and GDC (YSZ/GDC). Two platinum reference electrodes were embedded within the GDC layer. Cells were tested at 650 °C with hydrogen as fuel and air as oxidant. Electric potentials between embedded reference electrodes and anode and between cathode and anode were measured at open circuit, short circuit and under load. The electric potential was nearly constant through GDC in the cathode/YSZ/GDC/anode cells. By contrast, it varied monotonically through GDC in the cathode/GDC/YSZ/anode cells. Estimates of oxygen chemical potential, μO₂, variation through GDC were made. μO₂ within the GDC layer in the cathode/GDC/YSZ/anode cell decreased as the current was increased. By contrast, μO₂ within the GDC layer in the cathode/YSZ/GDC/anode cell increased as the current was increased. The cathode/YSZ/GDC/anode cell exhibited maximum power density of ∼0.52 W cm⁻² at 650 °C while the cathode/GDC/YSZ/anode cell exhibited maximum power density of ∼0.14 W cm⁻² for the same total electrolyte thickness.     

Performance of Ni/ScSZ cermet anode modified by coating with Gd0.2Ce0.8O2 for an SOFC running on methane fuel

A Ni/scandia-stabilized zirconia (ScSZ) cermet anode was modified by coating with nano-sized gadolinium-doped ceria (GDC, Gd_0.2Ce_0.8O₂) prepared using a simple combustion process within the pores of the anode for a solid oxide fuel cell (SOFC) running on methane fuel. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were employed in the anode characterizations. Then, the short-term stability for the cells with the Ni/ScSZ and 2.0 wt.%GDC-coated Ni/ScSZ anodes in 97%CH₄/3%H₂O at 700 °C was checked over a relative long period of operation. Open circuit voltages (OCVs) increased from 1.098 to 1.179 V, and power densities increased from 224 to 848 mW cm⁻², as the operating temperature of an SOFC with 2.0 wt.%GDC-coated Ni/ScSZ anode was increased from 700 to 850 °C in humidified methane. The coating of nano-sized Gd_0.2Ce_0.8O₂ particle within the pores of the porous Ni/ScSZ anode significantly improved the performance of anode supported cells. Electrochemical impedance spectra (EIS) illustrated that the cell with Ni/ScSZ anode exhibited far greater impedances than the cell with 2.0 wt.%GDC-coated Ni/ScSZ anode. Introduction of nano-sized GDC particles into the pores of porous Ni/ScSZ anode will result in a substantial increase in the ionic conductivity of the anode and increase the triple phase boundary region expanding the number of sites available for electrochemical activity. No significant degradation in performance has been observed after 84 h of cell testing when 2.0 wt.%GDC-coated Ni/ScSZ anode was exposed to 97%CH₄/3%H₂O at 700 °C. Very little carbon was detected on the anodes, suggesting that carbon deposition was limited during cell operation. Consequently, the GDC coating on the pores of anode made it possible to have good stability for long-term operation due to low carbon deposition.     

Stabilisation of composite LSFCO–CGO based anodes for methane oxidation in solid oxide fuel cells

A La_0.6Sr_0.4Fe_0.8Co_0.2O₃–Ce_0.8Gd_0.2O_1.9 (LSFCO–CGO) composite anode material was investigated for the direct electrochemical oxidation of methane in intermediate temperature solid oxide fuel cells (IT-SOFCs). A maximum power density of 0.17 W cm⁻² at 800 °C was obtained with a methane-fed ceria electrolyte-supported SOFC. A progressive increase of performance was recorded during 140 h operation with dry methane. The anode did not show any structure degradation after the electrochemical testing. Furthermore, no formation of carbon deposits was detected by electron microscopy and elemental analysis. Alternatively, this perovskite material showed significant chemical and structural modifications after high temperature treatment in a dry methane stream in a packed-bed reactor. It is derived that the continuous supply of mobile oxygen anions from the electrolyte to the LSFCO anode, promoted by the mixed conductivity of CGO electrolyte at 800 °C, stabilises the perovskite structure near the surface under SOFC operation and open circuit conditions.     

Intermediate temperature fuel cells based on doped ceria–LiCl–SrCl2 composite electrolyte

A new type of oxide-salt composite electrolyte, gadolinium-doped ceria (GDC)–LiCl–SrCl₂, was developed and demonstrated its promising use for intermediate temperature (400–700 °C) fuel cells (ITFCs). The dc electrical conductivity of this composite electrolyte (0.09–0.13 S cm⁻¹ at 500–650 °C) was 3–10 times higher than that of the pure GDC electrolyte, indicating remarkable proton or oxygen ion conduction existing in the LiCl–SrCl₂ chloride salts or at the interface between GDC and the chloride salts. Using this composite electrolyte, peak power densities of 260 and 510 mW cm⁻², with current densities of 650 and 1250 mA cm⁻² were achieved at 550 and 625 °C, respectively. This makes the new material a good candidate electrolyte for future low-cost ITFCs.     

Dependence of polarization in anode-supported solid oxide fuel cells on various cell parameters

The performance of solid oxide fuel cells (SOFCs) is affected by various polarization losses, namely, ohmic polarization, activation polarization and concentration polarization. Under given operating conditions, these polarization losses are largely dependent on cell materials, electrode microstructures, and cell geometric parameters. Solid oxide fuel cells (SOFC) with yttria-stabilized zirconia (YSZ) electrolyte, Ni–YSZ anode support, Ni–YSZ anode interlayer, strontium doped lanthanum manganate (LSM)–YSZ cathode interlayer, and LSM current collector, were fabricated. The effect of various parameters on cell performance was evaluated. The parameters investigated were: (1) YSZ electrolyte thickness, (2) cathode interlayer thickness, (3) anode support thickness, and (4) anode support porosity. Cells were tested over a range of temperatures between 600 and 800 °C with hydrogen as fuel, and air as oxidant. Ohmic contribution was determined using the current interruption technique. The effect of these cell parameters on ohmic polarization and on cell performance was experimentally measured. Dependence of cell performance on various parameters was rationalized on the basis of a simple analytical model. Based on the results of the cell parameter study, a cell with optimized parameters was fabricated and tested. The corresponding maximum power density at 800 °C was ∼1.8 W cm⁻².     

Fabrication and performance of gadolinia-doped ceria-based intermediate-temperature solid oxide fuel cells

Anode-supported solid oxide fuel cells (SOFC) based on gadolinia-doped ceria (GDC) are developed in this study. A carbonate co-precipitation method is used to synthesize the nano-sized GDC powders. A dense GDC electrolyte thin film supported by a Ni–GDC porous anode is fabricated by dry-pressing and spin-coating processes, respectively. In comparison with dry pressing, it is easy to prepare a thinner electrolyte film by the novel spin-coating method. Cell performance is examined using humidified (3% H₂O) hydrogen as fuel and air as oxidant in the temperature range of 500–700 °C. Cell performance is strongly dependent on the electrolyte thickness. With a porous Ni–GDC anode, a dense 19-μm GDC electrolyte film and a porous La_0.6Sr_0.4Co_0.2Fe_0.8O₃–GDC cathode, the cell exhibits maximum power densities of 130, 253, 386 and 492 mW cm⁻² at 500, 550, 600 and 650 °C, respectively. It is also found that at the low operating temperature about 500 °C, the cell resistance is significantly dominated by the electrode polarization resistance.     

Preparation of LaGaO3-based perovskite oxide film by a pulsed-laser ablation method and application as a solid oxide fuel cell electrolyte

LaGaO₃-based perovskite oxide films are deposited on a dense substrate consisting of NiO, Fe₃O₄, and Sm-doped CeO₂ (SDC). After in situ reduction, NiO and Fe₃O₄ are reduced to form an alloy and during reduction, the substrate becomes porous, and therefore can be used as a porous electrode substrate in a solid oxide fuel cell (SOFC). Since the reaction between NiO and LaGaO₃-based oxide is known, an interlayer of SDC is introduced between the LaGaO₃ film and the substrate. The LaGaO₃/SDC bilayer film exhibits electrical conductivity close to that of a bulk one. A single fuel cell using the LaGaO₃/SDC bilayer film shows an open-circuit potential of 1.1 V, which is close to the theoretical value. A quite large power density of 0.6 W cm⁻² is achieved at 773 K with a LaGaO₃ film of 5 μm in thickness. The effects of LaGaO₃ film thickness on power generation are further studied. Although the open-circuit potential increases, the maximum power density decreases with increasing thickness. On the other hand, the open-circuit potential becomes lower with thicknesess below 5 μm. This suggests that the reaction between NiO and the LaGaO₃ film occurs with an excessively thin film. Therefore, the largest power density is obtained with a film thickness of 5 μm. The effects of interlayer compound are also examined. The application of La(Sr)Ga(Fe)O₃ (LSGF) is also effective in obtaining high power density, but the maximum value is less than that achieved with a SDC interlayer. A two-cell stack is successfully demonstrated.     

Optimisation of processing and microstructural parameters of LSM cathodes to improve the electrochemical performance of anode-supported SOFCs

To improve the electrochemical performance of LSM-based anode-supported single cells, a systematic approach was taken for optimising processing and materials parameters. Four parameters were investigated in more detail: (1) the LSM/YSZ mass ratio of the cathode functional layer, (2) the grain size of LSM powder for the cathode current collector layer, (3) the thickness of the cathode functional layer and the cathode current collector layer, and (4) the influence of calcination of YSZ powder used for the cathode functional layer.Results from electrochemical measurements performed between 700 and 900 °C with H₂ (3 vol.% H₂O) as fuel gas and air as the oxidant showed that the performance was the highest using an LSM/YSZ mass ratio of 50/50. A further increase of the electrochemical performance was obtained by increasing the grain size of the outer cathode current collector layer: the highest performance was achieved with non-ground LSM powder. In addition, it was found that the thickness of the cathode functional layer and cathode current collector layer also affects the electrochemical performance, whereas no obvious detrimental effects occurred with the different qualities of YSZ powder for the cathode functional layer. The highest performance, i.e. 1.50 ± 0.05 A cm⁻² at 800 °C and 700 mV, was obtained with a cathode functional layer, characterised by an LSM/YSZ mass ratio of 50/50, a d₉₀ of the LSM powder of 1.0 μm, non-calcined YSZ powder, and a thickness of about 30 μm, and a cathode current collector layer, characterised by d₉₀ of the LSM powder of 26.0 μm (non-ground), and a thickness of 50–60 μm. Also interesting to note is that the use of non-ground LSM for the cathode current collector layer and non-calcined YSZ powder for the cathode functional layer obviously simplifies the production route of this type of fuel cell.     

Ni–YSZ-supported tubular solid oxide fuel cells with GDC interlayer between YSZ electrolyte and LSCF cathode

Highly sinterable gadolinia doped ceria (GDC) powders are prepared by carbonate coprecipitation and applied to the GDC interlayer in Ni–YSZ (yttria stabilized zirconia)-supported tubular solid oxide fuel cell in order to prevent the reaction between YSZ electrolyte and LSCF (La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ) cathode materials. The formation of highly resistive phase at the YSZ/LSCF interface was effectively blocked by the low-temperature densification of GDC interlayer using carbonate-derived active GDC powders and the suppression of Sr diffusion toward YSZ electrolyte via GDC interlayer by tuning the heat-treatment temperature for cathode fabrication. The power density of the cell with the configuration of Ni–YSZ/YSZ/GDC/LSCF–GDC/LSCF was as high as 906 mW cm⁻², which was 2.0 times higher than that (455 mW cm⁻²) of the cell with the configuration of Ni–YSZ/YSZ/LSM(La_0.8Sr_0.2MnO_3−δ)–YSZ/LSM at 750 °C.