Solid oxide fuel cells with Sm0.2Ce0.8O2−δ electrolyte film deposited by novel aerosol deposition method

In this study, dense electrolyte ceramic Sm_0.2Ce_0.8O_2−δ (SDC) thin films are successfully deposited on NiO-SDC anode substrate by aerosol deposition (AD) with oxygen as the carrier gas at the substrate temperature ranging from room temperature to 300 °C. To remove the effect of humidity on the starting powders, this study found that, in depositing SDC films, having the starting powders preheat-treated at 200 °C helped generate a smooth and dense layer, though a lower deposition rate was achieved. At a deposition time of 22 min, SDC films with a uniform thickness of 1.5 μm and grain sizes of ≈67 nm are obtained. SOFC single cells are then built by screen printing a LSCF cathode on the anode-supported substrates with SDC electrolyte. The cross-sectional SEM micrographs exhibit highly dense, granular, and crack-free microstructures. The open circuit voltages (OCV) of the single cells decrease with the rise in temperature, dropping from 0.81 V at 500 °C to 0.59 V at 700 °C. Maximum power densities (MPD) decline with decreasing operating temperature from 0.34 to 0.01 W cm⁻² due to the increase of the R₀ and R_P of the single cells. The electrochemical results testify to the fine quality of SDC films as well as illustrate the electrolyte thickness effect and the effect of mixed ionic and electronic conduction of the SDC electrolyte in the reducing atmosphere.     

Low-temperature solid oxide fuel cells with novel La0.6Sr0.4Co0.8Cu0.2O3−δ perovskite cathode and functional graded anode

The perovskite La_0.6Sr_0.4Co_0.8Cu_0.2O_3−δ (LSCCu) oxide is synthesized by a modified Pechini method and examined as a novel cathode material for low-temperature solid oxide fuel cells (LT-SOFCs) based upon functional graded anode. The perovskite LSCCu exhibits excellent ionic and electronic conductivities in the intermediate-to-low-temperature range (400-800 °C). Thin Sm_0.2Ce_0.8O_1.9 (SDC) electrolyte and NiO-SDC anode functional layer are prepared over macroporous anode substrates composed of NiO-SDC by a one-step dry-pressing/co-firing process. A single cell with 20 μm thick SDC electrolyte on a porous anode support and LSCCu-SDC cathode shows peak power densities of only 583.2 mW cm⁻² at 650 °C and 309.4 mW cm⁻² for 550 °C. While a cell with 20 μm thick SDC electrolyte and an anode functional layer on the macroporous anode substrate shows peak power densities of 867.3 and 490.3 mW cm⁻² at 650 and 550 °C, respectively. The dramatic improvement of cell performance is attributed to the much improved anode microstructure that is confirmed by both SEM observation and impedance spectroscopy. The results indicate that LSCCu is a very promising cathode material for LT-SOFCs and the one-step dry-pressing/co-firing process is a suitable technique to fabricate high performance SOFCs.     

Fabrication of bilayered YSZ/SDC electrolyte film by electrophoretic deposition for reduced-temperature operating anode-supported SOFC

Bilayered Y₂O₃-stabilized ZrO₂ (YSZ)/Sm₂O₃-doped CeO₂ (SDC) electrolyte films were successfully fabricated on porous NiO–YSZ composite substrates by electrophoretic deposition (EPD) based on electrophoretic filtration followed by co-firing with the substrates. In EPD, positively charged YSZ and SDC powders were deposited directly on the substrates, layer by layer from ethanol-based suspensions. Delamination between YSZ and SDC films was avoided by reducing the SDC films’ thickness to ca. 1 μm. A single cell was constructed on the bilayered electrolyte films composed of ca. 4 μm-thick YSZ and ca. 1 μm-thick SDC films. As a cathode in the cell, La_0.6Sr_0.4Co_0.2Fe_0.8O_3−x (LSCF) was used. Maximum output power densities greater than 0.6 W cm⁻² were obtained at 700 °C for the bilayered YSZ/SDC electrolyte cells thus constructed.     

Effects of anode surface modification on the performance of low temperature SOFCs

An anode functional layer (AFL, ∼5 μm) for improving the cell performance was fabricated by the slurry spin coating method on the porous surface of an anode substrate. The effects of the AFL on the anode/electrolyte interfacial morphology and the Sm_0.2Ce_0.8O_1.9 (SDC) film deposition process were evaluated. And the electrochemical characteristics of the cells with and without the AFL were tested for comparison. With the AFL layer, the cell performance was greatly improved and the maximum power density was increased from 0.733 to 0.884 W cm⁻² at 600 °C and from 1.085 to 1.213 W cm⁻² at 650 °C. The systematical analysis indicated that the AFL could effectively reduce the anode polarization loss by increasing the three-phase boundary (TPB) length.     

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_0.3MnO₃ (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.     

Effects of bi-layer La0.6Sr0.4Co0.2Fe0.8O3−δ-based cathodes on characteristics of intermediate temperature solid oxide fuel cells

In this study, anode-supported planar IT-SOFCs, with a thin Sm_0.2Ce_0.8O_2−δ (SDC) electrolyte film and a bi-layer cathode, are fabricated using tape-casting and screen-printing processes. The bi-layer cathode consists of a current collector La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF) layer and a functional LSCF-SDC composite layer in various thicknesses. Microstructure studies reveal that the interfaces among various layers show good adhesion, except for Cell A equipped with a cathode of pure LSCF. Cell A reports the lowest ohmic (R₀) and polarization (R_P) resistances. R_P, which increases with the thickness of the LSCF-SDC composite layer in the cathode, rises rapidly as the temperature drops, particularly at temperatures ≤550 °C. This indicates the high electrical conductivity of the cathode as a major contribution to the decrease of R_P at 500 °C. The best cell performances are observed at 650 °C for all cases, in which Cell A shows a maximum power density of 1.51 W cm⁻² and an open circuit voltage of 0.80 V. Considering both of the electrical and the mechanical integrity of the single cell, insertion of the composite layer is required to guarantee a good adhesion of cathode layer to electrolyte layer. However, the thickness of the composite layer should be retained as thin as possible to minimize the R₀ and R_P and maximize the cell performance.     

Anode-supported ScSZ-electrolyte SOFC with whole cell materials from combined EDTA–citrate complexing synthesis process

The potential application of combined EDTA–citrate complexing process (ECCP) in intermediate-temperature solid-oxide fuel cells (IT-SOFCs) processing was investigated. ECCP-derived scandia-stabilized-zirconia (ScSZ) powder displayed low packing density, high surface area and nano-crystalline, which was ideal material for thin-film electrolyte fabrication based on dual dry pressing. A co-synthesis of NiO + ScSZ anode based on ECCP was developed, which showed reduced NiO(Ni) and ScSZ grain sizes and improved homogeneity of the particle size distribution, as compared with the mechanically mixed NiO + ScSZ anode. Anode-supported ScSZ electrolyte fuel cell with the whole cell materials synthesized from ECCP was successfully prepared. The porous anode and cathode exhibited excellent adhesion to the electrolyte layer. Fuel cell with 30 μm thick ScSZ electrolyte and La_0.8Sr_0.2MnO₃ cathode showed a promising maximum peak power density of 350 mW cm⁻² at 800 °C.     

Internal shorting and fuel loss of a low temperature solid oxide fuel cell with SDC electrolyte

A solid oxide fuel cell with Sm_0.2Ce_0.8O_1.9 (SDC) electrolyte of 10 μm in thickness and Ni–SDC anode of 15 μm in thickness on a 0.8 mm thick Ni–YSZ cermet substrate was fabricated by tape casting, screen printing and co-firing. A composite cathode, 75 wt.% Sm_0.5Sr_0.5CoO₃ (SSCo) + 25 wt.% SDC, approximately 50 μm in thickness, was printed on the co-fired half-cell, and sintered at 950 °C. The cell showed a high electrochemical performance at temperatures ranging from 500 to 650 °C. Peak power density of 545 mW cm⁻² at 600 °C was obtained. However, the cell exhibited severe internal shorting due to the mixed conductivity of the SDC electrolyte. Both the amount of water collected from the anode outlet and the open circuit voltage (OCV) indicated that the internal shorting current could reach 0.85 A cm⁻² or more at 600 °C. Zr content inclusions were found at the surface and in the cross-section of the SDC electrolyte, which could be one of the reasons for reduced OCV and oxygen ionic conductivity. Fuel loss due to internal shorting of the thin SDC electrolyte cell becomes a significant concern when it is used in applications requiring high fuel utilization and electrical efficiency.     

Thin film solid oxide fuel cells with copper cermet anodes

Thin film solid oxide fuel cells (SOFCs), composed of thin coatings of 8 mol% Y₂O₃-stabilized ZrO₂ (YSZ) and thick substrates of (La_0.8Sr_0.2)_0.98MnO₃ (LSM)-YSZ cathodes, are fabricated using the conventional tape casting and tape lamination techniques. Densification of YSZ electrolyte thin films is achieved at 1275 °C by adjusting the cathode tape formulation and sintering characteristics. Two types of copper cermets, CuO-YSZ-ceria and CuO-SDC (Ce_0.85Sm_0.15O_1.925)-ceria, are compared in terms of the anodic performance in hydrogen and propane. Maximum power densities for hydrogen and propane at 800 °C are 0.26 W cm⁻² and 0.17 W cm⁻² for CuO-YSZ-ceria anodes and 0.35 W cm⁻² and 0.22 W cm⁻² for CuO-SDC-ceria anodes, respectively. Electrochemical impedance analysis suggests that CuO-SDC-ceria exhibits a much lower anodic polarization resistance than CuO-YSZ-ceria, which could be explained by the intrinsic mixed oxygen ionic and electronic conductivities for SDC in the reducing atmosphere.     

Performance improvement of ceria-based solid oxide fuel cells with yttria-stabilized zirconia as an electronic blocking layer by pulsed laser deposition

A yttria stabilized zirconia (YSZ) layer (∼2 μm) is fabricated by pulsed laser deposition (PLD) technique on an Ce_0.8Sm_0.2O_2−δ (SDC) electrolyte film which is prepared by a co-pressing process on a NiO-SDC anode substrate. La_0.8Sr₀₂MnO_3−δ-YSZ (LSM-YSZ, 70:30 wt.%) cathode is applied onto the SDC/YSZ bilayer electrolytes to form a single cell. The open circuit voltages of the cell increase significantly compared with that of the SDC single electrolyte cell. Preparation of an SDC buffer layer (∼500 nm) on the bilayer electrolytes by PLD method is also studied for reducing the cathode polarization losses with a Sm_0.5Sr_0.5CoO_3−δ-SDC (SSC-SDC, 70:30 wt.%) cathode and preventing the interfacial chemical reaction between YSZ and SSC. The YSZ thin film blocks electrical current leakage in the SDC layer, whereas the SDC buffer layer with the SSC-SDC cathode decreases the cathode polarization losses, which results in the overall enhanced performance.     

Electrochemical performance of PrBaCo2O5+δ layered perovskite as an intermediate-temperature solid oxide fuel cell cathode

PrBaCo₂O_5+δ (PBCO) powder was prepared by a combined EDTA and citrate complexing method. The electrochemical performance of PBCO as a cathode for intermediate-temperature solid oxide fuel cells (IT-SOFCs) was evaluated. A porous layer of PBCO was deposited on a 42 μm thick electrolyte consisting of Ce_0.8Sm_0.2O_1.9 (SDC), prepared by a dry-pressing process. A fuel cell with a structure PBCO/SDC/Ni-SDC provides a maximum power density of 866, 583, 313 and 115 mW cm⁻² at 650, 600, 550 and 500 °C, respectively, using hydrogen as the fuel and stationary air as the oxidant. The total resistance of the cell was about 0.41, 0.51, 0.57 and 0.77 Ω cm², respectively. This encouraging data identifies PBCO as a potential cathode material for IT-SOFCs.     

A composite oxygen-reduction electrode composed of SrSc0.2Co0.8O3−δ perovskite and Sm0.2Ce0.8O1.9 for an intermediate-temperature solid-oxide fuel cell

SSC (70 wt.% SrSc_0.2Co_0.8O_3−δ)–SDC (30 wt.% Sm_0.2Ce_0.8O_1.9) composite was evaluated as cathode for intermediate-temperature solid-oxide fuel cells. The effect of firing temperature on the chemical interaction between SSC and SDC was characterized by oxygen-temperature programmed desorption (O₂-TPD) techniques. Certain type of phase reactions occurred between SSC and SDC at calcination temperatures higher than 950 °C. The conductivity of the composite was measured by a four-prober direct current technique. The electro-catalytic activity of the composite electrode for oxygen reduction was measured by electrochemical impedance spectroscopy (EIS) in a symmetric cell configuration. The electrode fired at 950 °C showed the best performance. By applying the SSC + SDC-composite electrode, a cell with a ∼20-μm thick SDC electrolyte delivered a peak power density of 760 mW cm⁻² at 600 °C. This suggested that an SSC + SDC-composite electrode may be a promising cathode for intermediate-temperature solid-oxide fuel cells.     

Metal-supported solid oxide fuel cell operated at 400–600 °C

A metal-supported SOFC with a samarium doped ceria (SDC)/scandia-stabilized zirconia (ScSZ) bilayer electrolyte was fabricated by a combination of pulsed laser deposition (PLD) and wet ceramic processes. The cell performance and aging characteristics during operation were analyzed by both AC impedance spectroscopy and current-voltage measurements in the temperature range from 400 °C to 600 °C. The power generation characteristics of this metal-supported SOFC at low temperatures should allow for rapid start-up and help to reduce the performance deterioration seen in high temperature SOFCs due material oxidation and instability. In this paper, our early research results are presented.     

Fabrication and characterization of Sm0.2Ce0.8O2−δ-Sm0.5Sr0.5CoO3−δ composite cathode for anode supported solid oxide fuel cell

The Sm_0.5Sr_0.5CoO_3−δ (SSC) with perovskite structure is synthesized by the glycine nitrate process (GNP). The phase evolution of SSC powder with different calcination temperatures is investigated by X-ray diffraction and thermogravimetric analyses. The XRD results show that the single perovskite phase of the SSC is completely formed above 1100 °C. The anode-supported single cell is constructed with a porous Ni-yttria-stabilized zirconia (YSZ) anode substrate, an airtight YSZ electrolyte, a Sm_0.2Ce_0.8O_2−δ (SDC) barrier layer, and a screen-printed SSC-SDC composite cathode. The SEM results show that the dense YSZ electrolyte layer exhibits the good interfacial contact with both the Ni-YSZ and the SDC barrier layer. The porous SSC-SDC cathode shows an excellent adhesion with the SDC barrier layer. For the performance test, the maximum power densities are 464, 351 and 243 mW cm⁻² at 800, 750 and 700 °C, respectively. According to the results of the electrochemical impedance spectroscopy (EIS), the charge-transfer resistances of the electrodes are 0.49 and 1.24 Ω cm², and the non charge-transfer resistances are 0.48 and 0.51 Ω cm² at 800 and 700 °C, respectively. The cathode material of SSC is compatible with the YSZ electrolyte via a delicate scheme employed in the fabrication process of unit cell.     

Solid oxide fuel cells with bi-layered electrolyte structure

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

Study of steam electrolysis using a microtubular ceramic reactor

Steam electrolysis was carried out using a microtubular ceramic reactor with the following cell configuration: La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF)–Ce_0.8Gd_0.2O_1.9 (CGO) electrode/CGO buffer layer/(ZrO₂)_0.89(Sc₂O₃)_0.1(CeO₂)_0.01 (ScSZ) electrolyte/Ni-ScSZ electrode supporting tube. 10% H₂/Ar gas was used as steam carrier gas, and 18% steam was supplied to the ceramic reactors. The cell performance was as follows: 1.43 V at 0.1 A cm⁻² and 650 °C (Area specific resistance: 4.7 Ω cm²) or 1.37 V at 0.1 A cm⁻² and 700 °C (4.3 Ω cm²). During steam electrolysis, hydrogen production proportionally increased with current density according to Faraday's law, and heat generation at a low current density was observed by an electrochemical technique. Voids and Zr diffusion from the ScSZ electrolyte were confirmed in the CGO buffer layer. Such factors near the surface probably influenced the increase in ohmic loss and electrode polarization.     

Thermal stability study of SDC/Na2CO3 nanocomposite electrolyte for low-temperature SOFCs

The novel core–shell nanostructured SDC/Na₂CO₃ composite has been demonstrated as a promising electrolyte material for low-temperature SOFCs. However, as a nanostructured material, stability might be doubted under elevated temperature due to their high surface energy. So in order to study the thermal stability of SDC/Na₂CO₃ nanocomposite, XRD, BET, SEM and TGA characterizations were carried on after annealing samples at various temperatures. Crystallite sizes, BET surface areas, and SEM results indicated that the SDC/Na₂CO₃ nanocomposite possesses better thermal stability on nanostructure than pure SDC till 700 °C. TGA analysis verified that Na₂CO₃ phase exists steadily in the SDC/Na₂CO₃ composite. The performance and durability of SOFCs based on SDC/Na₂CO₃ electrolyte were also investigated. The cell delivered a maximum power density of 0.78 W cm⁻² at 550 °C and a steady output of about 0.62 W cm⁻² over 12 h operation. The high performances together with notable thermal stability make the SDC/Na₂CO₃ nanocomposite as a potential electrolyte material for long-term SOFCs that operate at 500–600 °C.     

Effect of unsintered gadolinium-doped ceria buffer layer on performance of metal-supported solid oxide fuel cells using unsintered barium strontium cobalt ferrite cathode

In this study, a Gd_0.1Ce_0.9O_1.95 (GDC) buffer layer and a Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) cathode, fabricated without pre-sintering, are investigated (unsintered GDC and unsintered BSCF). The effect of the unsintered GDC buffer layer, including the thickness of the layer, on the performance of solid oxide fuel cells (SOFCs) using an unsintered BSCF cathode is studied. The maximum power density of the metal-supported SOFC using an unsintered BSCF cathode without a buffer layer is 0.81 W cm⁻², which is measured after 2 h of operation (97% H₂ and 3% H₂O at the anode and ambient air at the cathode), and it significantly decreases to 0.63 W cm⁻² after 50 h. At a relatively low temperature of 800 °C, SrZrO₃ and BaZrO₃, arising from interaction between BSCF and yttria-stabilized zirconia (YSZ), are detected after 50 h. Introducing a GDC interlayer between the cathode and electrolyte significantly increases the durability of the cell performance, supporting over 1000 h of cell usage with an unsintered GDC buffer layer. Comparable performance is obtained from the anode-supported cell when using an unsintered BSCF cathode with an unsintered GDC buffer layer (0.75 W cm⁻²) and sintered GDC buffer layer (0.82 W cm⁻²). When a sintered BSCF cathode is used, however, the performance increases to 1.23 W cm⁻². The adhesion between the BSCF cathode and the cell can be enhanced by an unsintered GDC buffer layer, but an increase in the layer thickness (1-6 μm) increases the area specific resistance (ASR) of the cell, and the overly thick buffer layer causes delamination of the BSCF cathode. Finally, the maximum power densities of the metal-supported SOFC using an unsintered BSCF cathode and unsintered GDC buffer layer are 0.78, 0.64, 0.45 and 0.31 W cm⁻² at 850, 800, 750 and 700 °C, respectively.     

Performance of anode-supported solid oxide fuel cell using novel ceria electrolyte

The potential of a novel co-doped ceria material Sm_0.075Nd_0.075Ce_0.85O_2−δ as an electrolyte was investigated under fuel cell operating conditions. Conventional colloidal processing was used to deposit a dense layer of Sm_0.075Nd_0.075Ce_0.85O_2−δ (thickness 10 μm) over a porous Ni-gadolinia doped ceria anode. The current-voltage performance of the cell was measured at intermediate temperatures with 90 cm³ min⁻¹ of air and wet hydrogen flowing on cathode and anode sides, respectively. At 650 °C, the maximum power density of the cell reached an exceptionally high value of 1.43 W cm⁻², with an area specific resistance of 0.105 Ω cm². Impedance measurements show that the power density decrease with decrease in temperature is mainly due to the increase in electrode resistance. The results confirm that Sm_0.075Nd_0.075Ce_0.85O_2−δ is a promising alternative electrolyte for intermediate temperature solid oxide fuel cells.     

Micro-tubular solid oxide fuel cells with graded anodes fabricated with a phase inversion method

Micro-tubular proton-conducting solid oxide fuel cells (SOFCs) are developed with thin film BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3−δ (BZCYYb) electrolytes supported on Ni-BZCYYb anodes. The substrates, NiO-BZCYYb hollow fibers, are prepared by an immersion induced phase inversion technique. The resulted fibers have a special asymmetrical structure consisting of a sponge-like layer and a finger-like porous layer, which is propitious to serving as the anode supports for micro-tubular SOFCs. The fibers are characterized in terms of porosity, mechanical strength, and electrical conductivity regarding their sintering temperatures. To make a single cell, a dense BZCYYb electrolyte membrane about 20 μm thick is deposited on the hollow fiber by a suspension-coating process and a porous Sm_0.5Sr_0.5CoO₃ (SSC)-BZCYYb cathode is subsequently fabricated by a slurry coating technique. The micro-tubular proton-conducting SOFC generates a peak power density of 254 mW cm⁻² at 650 °C when humidified hydrogen is used as the fuel and ambient air as the oxidant.