Effects of anode and electrolyte microstructures on performance of solid oxide fuel cells

To improve the performance of anode-supported solid oxide fuel cells (SOFCs), various types of single cells are manufactured using a thin-film electrolyte of Yttria stabilized zirconia (YSZ) and an anode functional layer composed of a NiO–YSZ nano-composite powder. Microstructural/electrochemical analyses are conducted. Single-cell performances are highly dependent on electrolyte thickness, to the degree that the maximum power density increases from 0.74 to 1.12 W cm⁻² according to a decrease in electrolyte thickness from 10.5 to 6.5 μm at 800 °C. The anode functional layer reduced the polarization resistance of a single cell from 1.07 to 0.48 Ω cm² at 800 °C. This is attributed to the provision by the anode layer of a highly reactive and uniform electrode microstructure. It is concluded that optimization of the thickness and homogeneity of component microstructure improves single-cell performances.     

Thick-film electrolyte (thickness <20 μm)-supported solid oxide fuel cells

A novel design of solid oxide fuel cell (SOFC) which utilizes a thick film (<20 μm) as an electrolyte support is developed and tested. The sintered 16 μm-thick yttria-stabilized zirconia (YSZ) electrolyte film is mounted on a 1-mm thick YSZ ring by sintering the two pieces together. With this new configuration, it is possible to fabricate a thick (<20 μm) electrolyte-supported SOFC and measure the power density of the unit cell. With LSCF (La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ) as a cathode and Ni–YSZ as a composite anode, the cell with a 16 μm-thick YSZ electrolyte achieves a high performance, i.e., a maximum power density of 590 mW cm⁻² at 800 °C. This value is comparable with that of most anode-supported SOFCs using YSZ electrolytes.     

A novel design and performance of cone-shaped tubular anode-supported segmented-in-series solid oxide fuel cell stack

A novel design of cone-shaped tubular segmented-in-series solid oxide fuel cell (SOFC) stack is presented in this paper. The cone-shaped tubular anode substrates are fabricated by slip casting technique and the yttria-stabilized zirconia (YSZ) electrolyte films are deposited onto the anode tubes by dip coating method. After sintering at 1400 °C for 4 h, a dense and crack-free YSZ film with a thickness of about 7 μm is successfully obtained. The single cell, NiO-YSZ/YSZ (7 μm)/LSM-YSZ, provides a maximum power density of 1.78 W cm⁻² at 800 °C, using moist hydrogen (75 ml min⁻¹) as fuel and ambient air as oxidant.A two-cell-stack based on the above-mentioned cone-shaped tubular anode-supported SOFC is fabricated. Its typical operating characteristics are investigated, particularly with respect to the thermal cycling test. The results show that the two-cell-stack has good thermo-mechanical properties and that the developed segmented-in-series SOFC stack is highly promising for portable applications.     

Fabrication,microstructure and properties of a YSZ electrolyte for SOFCs

Yttria stabilized zirconia (YSZ) has widely been used as an electrolyte in solid oxide fuel cell (SOFC) stacks. The microstructure and properties of YSZ related to the fabrication process are discussed in this paper. For the named two-step sintering process, uniform and hexagonal grains with a size of 1–4 μm were obtained from the adobe following tape calendaring (TCL). Elliptical and hexagonal grains with a size of 0.4–3 μm were obtained from the adobe of tape casting (TCS) using the three-step process. The electrical conductivities of YSZ with different grain sizes were measured via the four-probe DC technique and grain conductivities and grain boundary conductivities of YSZ were investigated by impedance spectroscopy. YSZ electrolytes with a grain size of 0.1–0.4 μm had the highest electrical conductivity in the range of 500–1000 °C, especially at medium and low temperatures 550–800 °C. As the YSZ grain size becomes small, the thickness of the intergranular region decreased greatly. The YSZ electrolytes with sub-micrometer grain sizes, high ion conductivity and low sintering temperatures are important to the electrode-supported SOFC, on which the dense YSZ electrolyte films are optimized at 10 μm.     

The effect of porosity gradient in a Nickel/Yttria Stabilized Zirconia anode for an anode-supported planar solid oxide fuel cell

In this paper, a graded Ni/YSZ cermet anode, an 8 mol.%YSZ electrolyte, and a lanthanum strontium manganite (LSM) cathode were used to fabricate a solid oxide fuel cell (SOFC) unit. An anode-supported cell was prepared using a tape casting technique followed by hot pressing lamination and a single step co-firing process, allowing for the creation of a thin layer of dense electrolyte on a porous anode support. To reduce activation and concentration overpotential in the unit cell, a porosity gradient was developed in the anode using different percentages of pore former to a number of different tape-slurries, followed by tape casting and lamination of the tapes. The unit cell demonstrated that a concentration distribution of porosity in the anode increases the power in the unit cell from 76 mW cm⁻² to 101 mW cm⁻² at 600 °C in humidified hydrogen. Although the results have not been optimized for good performance, the effect of the porosity gradient is quite apparent and has potential in developing superior anode systems.     

Fabrication and characterization of an anode-supported hollow fiber SOFC

In this study, an anode-supported hollow-fiber solid oxide fuel cell (SOFC) of diameter 1.7 mm has been successfully fabricated using the phase inversion and vacuum assisted coating techniques. The cell has a special structure consisting of a 12-μm-thick yttria-stabilized zirconia (YSZ) electrolyte film and a Ni-YSZ anode layer which has large finger-like pores on both sides of the hollow-fiber membrane. The hollow-fiber SOFC has an active electrode area of 0.63 cm² and generates maximum power densities of 124, 287 and 377 mW cm⁻² at 600, 700 and 800 °C, respectively, indicating that its use in applications requiring high power density is promising.     

Microstructural factors of electrodes affecting the performance of anode-supported thin film yttria-stabilized zirconia electrolyte (∼1 μm) solid oxide fuel cells

The effects of the microstructural factors of electrodes, such as the porosity and pore size of anode supports and the thickness of cathodes, on the performance of an anode-supported thin film solid oxide fuel cell (TF-SOFC) are investigated. The performance of the TF-SOFC with a 1 μm-thick yttria-stabilized zirconia (YSZ) electrolyte is significantly improved by employing anode supports with increased porosity and pore size. The maximum power density of the TF-SOFCs increases from 370 mW cm⁻² to 624 mW cm⁻² and then to over 900 mW cm⁻² at 600 °C with increasing gas transport at the anode support. Thicker cathodes also improve cell performance by increasing the active reaction sites. The maximum power density of the cell increases from 624 mW cm⁻² to over 830 mW cm⁻² at 600 °C by changing the thickness of the lanthanum strontium cobaltite (LSC) cathode from 1 to 2-3 μm.     

Experimental study on effect of compaction pressure on performance of SOFC anodes

NiO/yttria-stabilized zirconia (YSZ) anode substrates were fabricated at two compaction pressures of 200 and 1000 MPa, the particle size distributions of NiO and YSZ were investigated with powders treated under different conditions using a laser scattering technique (Mastersizer 2000, Malvern Instruments) and the effect of compaction pressure on the performance of solid oxide fuel cell (SOFC) anodes was investigated by studying the effect of compaction pressure on compaction density, sintered density, sintering shrinkage behavior, electronic and ionic conductivities. The results of investigation indicated that the SOFC with the anode compacted at a higher pressure exhibited a superior output performance, for example, a single cell with hydrogen as fuel and oxygen as oxidant exhibited excellent maximum power densities of 2.77 and 0.90 W cm⁻² at 800 and 650 °C, respectively, which suggested the development of an intermediate temperature SOFC through optimization of anode fabrication parameters.     

Microstructure and electrochemical characterization of solid oxide fuel cells fabricated by co-tape casting

A co-tape casting technique was applied to fabricate electrolyte/anode for solid oxide fuel cells. YSZ and NiO-YSZ powders are raw materials for electrolyte and anode, respectively. Through adjusting the Polyvinyl Butyral (PVB) amount in slurry, the co-sintering temperature for electrolyte/anode could be dropped. After being co-sintered at 1400 °C for 5 h, the half-cells with dense electrolytes and large three phase boundaries were obtained. The improved unit cell exhibited a maximum power density of 589 mW cm⁻² at 800 °C. At the voltage of 0.7 V, the current densities of the cell reached 667 mA cm⁻². When the electrolyte and the anode were cast within one step and sintered together at 1250 °C for 5 h and the thickness of electrolyte was controlled exactly at 20 μm, the open-circuit voltage (OCV) of the cell could reach 1.11 V at 800 °C and the maximum power densities were 739, 950 and 1222 mW cm⁻² at 750, 800 and 850 °C, respectively, with H₂ as the fuel under a flow rate of 50 sccm and the cathode exposed to the stationary air. Under the voltage of 0.7 V, the current densities of cell were 875, 1126 and 1501 mA cm⁻², respectively. These are attributed to the large anode three phase boundaries and uniform electrolyte obtained under the lower sintering temperature. The electrochemical characteristics of the cells were investigated and discussed.     

YSZ-based SOFC with modified electrode/electrolyte interfaces for operating at temperature lower than 650 °C

A NiO/Yttrium-stabilized zirconia (YSZ) transition layer and/or a SDC function layer were introduced into the anode/electrolyte and/or electrolyte/cathode interface to decrease the activation polarization resulted from the mass transfer at electrode/electrolyte interface. With a NiO/YSZ transition layer, the activation polarization simulated from I–V curves drops from 4.42 to 2.42 Ω cm² at 600 °C, about 45% less than that of cell I; with additional SDC function layer, no activation polarization is obviously observed. The cell performance was also remarkably improved with the introduction of both the transition layer and the SDC function layer. Peak power densities of 187 and 443 mW cm⁻² at 600 and 650 °C, respectively, were achieved for a single cell with both a transition layer and a function layer, with an increment of 87% and 95% compared to that of the cell without any structural improvement, and about 30% and 25% compared to that of the cell with only anode transition layer. The study by ac impedance spectroscopy technique also indicated that the interfacial polarization resistance, the main source of cell resistance, could be effectively reduced by interface improvement.     

Polarization measurements on single-step co-fired solid oxide fuel cells (SOFCs)

Anode-supported planar solid oxide fuel cells (SOFC) were successfully fabricated by a single step co-firing process. The cells comprised of a Ni + yttria-stabilized zirconia (YSZ) anode, a YSZ or scandia-stabilized zirconia (ScSZ) electrolyte, a (La_0.85Ca_0.15)_0.97MnO₃ (LCM) + YSZ cathode active layer, and an LCM cathode current collector layer. The fabrication process involved tape casting of the anode, screen printing of the electrolyte and the cathode, and single step co-firing of the green-state cells in the temperature range of 1300–1330 °C for 2 h. Cells were tested in the temperature range of 700–800 °C with humidified hydrogen as fuel and air as oxidant. Cell test results and polarization modeling showed that the polarization losses were dominated by the ohmic loss associated with the electrodes and the activation polarization of the cathode, and that the ohmic loss due to the ionic resistance of the electrolyte and the activation polarization of the anode were relatively insignificant. Ohmic resistance associated with the electrode was lowered by improving the electrical contact between the electrode and the current collector. Activation polarization of the cathode was reduced by the improvement of the microstructure of the cathode active layer and lowering the cell sintering temperature. The cell performance was further improved by increasing the porosity in the anode. As a result, the maximum power density of 1.5 W cm⁻² was achieved at 800 °C with humidified hydrogen and air.     

A study of solid oxide fuel cell stack failure by inducing abnormal behavior in a single cell test

It is well known that cell imbalance can lead to failure of batteries. Prior theoretical modeling has shown that similar failure can occur in solid oxide fuel cell (SOFC) stacks due to cell imbalance. Central to failure model for SOFC stacks is the abnormal operation of a cell with cell voltage becoming negative. For investigation of SOFC stack failure by simulating abnormal behavior in a single cell test, thin yttria-stabilized zirconia (YSZ) electrolyte, anode-supported cells were tested at 800 °C with hydrogen as fuel and air as oxidant with and without an applied DC bias. When under a DC bias with cell operating under a negative voltage, rapid degradation occurred characterized by increased cell resistance. Visual and microscopic examination revealed that delamination occurred along the electrolyte/anode interface. The present results show that anode-supported SOFC stacks with YSZ electrolyte are prone to catastrophic failure due to internal pressure buildup, provided cell imbalance occurs. The present results also suggest that the greater the number of cells in an SOFC stack, the greater is the propensity to catastrophic failure.     

Reducing the operation temperature of a solid oxide fuel cell using a conventional nickel-based cermet anode on dimethyl ether fuel through internal partial oxidation

Dimethyl ether (DME)-oxygen mixture as the fuel of an anode-supported SOFC with a conventional nickel-cermet anode for operating at reduced temperatures is systematically investigated. The results of the catalytic tests indicate that sintered Ni-YSZ has high activity for DME partial oxidation, and DME conversion exceeds 90% at temperatures higher than 700 °C. Maximum methane selectivity is reached at 700 °C. Cell performance is observed between 600 and 800 °C. Peak power densities of approximately 400 and 1400 mW cm⁻² at 600 and 800 °C, respectively, are reached for the cell operating on DME-O₂ mixture. These values are comparable to those obtained using hydrogen as a fuel, and cell performance is reasonably stable at 700 °C for a test period of 340 min. SEM results demonstrate that the cell maintains good geometric integrity without any delimitation of respective layer after the stability test, and EDX results show that carbon deposition occurrs only at the outer surface of the anode. O₂-TPO analysis shows that carbon deposition over the Ni-YSZ operating on DME is greatly suppressed in the presence of oxygen. Internal partial oxidation may be a practical way to achieve high cell performance at intermediate-temperatures for SOFCs operating on DME fuel.     

High performance solid oxide fuel cells based on tri-layer yttria-stabilized zirconia by low temperature sintering process

Performance of solid oxide fuel cells (SOFCs) depends critically on the composition and microstructure of the electrodes. It is fabricated a dense yttria-stabilized zirconia (YSZ) electrolyte layer sandwiched between two porous YSZ layers at low temperature. The advantages of this structure include excellent structural stability and unique flexibility for evaluation of new electrode materials for SOFC applications, which would be difficult or impossible to be evaluated using conventional cell fabrication techniques because of incompatibility with YSZ under processing conditions. The porosity of porous YSZ increases from 65.8% to 68.6% as the firing temperature decreased from 1350 to 1200 °C. The open cell voltages of the cells based on the tri-layers of YSZ, co-fired using a two-step sintering at 1200 °C, are above 1.0 V at 700-800 °C, and the peak power densities of cells infiltrated LSCF and Pd-SDC electrodes are about 525, 733, and 935 mW cm⁻² at 700, 750, and 800 °C, respectively.     

Novel fabrication technique of hollow fibre support for micro-tubular solid oxide fuel cells

In this work, a cerium-gadolinium oxide (CGO)/nickel (Ni)-CGO hollow fibre (HF) for micro-tubular solid oxide fuel cells (SOFCs), which consists of a fully gas-tight outer electrolyte layer supported on a porous inner composite anode layer, has been developed via a novel single-step co-extrusion/co-sintering technique, followed by an easy reduction process. After depositing a multi-layers cathode layer and applying current collectors on both anode and cathode, a micro-tubular SOFC is developed with the maximum power densities of 440-1000 W m⁻² at 450-580 °C. Efforts have been made in enhancing the performance of the cell by reducing the co-sintering temperature and improving the cathode layer and current collection from inner (anode) wall. The improved cell produces maximum power densities of 3400-6800 W m⁻² at 550-600 °C, almost fivefold higher than the previous cell. Further improvement has been carried out by reducing thickness of the electrolyte layer. Uniform and defect-free outer electrolyte layer as thin as 10 μm can be achieved when the extrusion rate of the outer layer is controlled. The highest power output of 11,100 W m⁻² is obtained for the cell of 10 μm electrolyte layer at 600 °C. This result further highlights the potential of co-extrusion technique in producing high quality dual-layer HF support for micro-tubular SOFC.     

Direct ceramic inkjet printing of yttria-stabilized zirconia electrolyte layers for anode-supported solid oxide fuel cells

Electromagnetic drop-on-demand direct ceramic inkjet printing (EM/DCIJP) was employed to fabricate dense yttria-stabilized zirconia (YSZ) electrolyte layers on a porous NiO-YSZ anode support from ceramic suspensions. Printing parameters including pressure, nozzle opening time and droplet overlapping were studied in order to optimize the surface quality of the YSZ coating. It was found that moderate overlapping and multiple coatings produce the desired membrane quality. A single fuel cell with a NiO-YSZ/YSZ (∼6 μm)/LSM + YSZ/LSM architecture was successfully prepared. The cell was tested using humidified hydrogen as the fuel and ambient air as the oxidant. The cell provided a power density of 170 mW cm⁻² at 800 °C. Scanning electron microscopy (SEM) revealed a highly coherent dense YSZ electrolyte layer with no open porosity. These results suggest that the EM/DCIJP inkjet printing technique can be successfully implemented to fabricate electrolyte coatings for SOFC thinner than 10 μm and comparable in quality to those fabricated by more conventional ceramic processing methods.     

Fabrication and evaluation of Ni-GDC composite anode prepared by aqueous-based tape casting method for low-temperature solid oxide fuel cell

Large-size, 8 cm × 8 cm, NiO-Gd_0.1Ce_0.9O_1.95 (Ni-GDC) composite anodes have been successfully fabricated by aqueous-based tape casting method for anode-supported solid oxide fuel cell (SOFC). The pre-sintered anode green tape was coated with a GDC electrolyte film by spray coating method and then co-sintered together to obtain electrolyte/anode bi-layer. The cathode, which is made of La_0.8Sr_0.2Co_0.2Fe_0.8O₃-GDC (LSCF-GDC) was screen printed onto the electrolyte film and sintered to form a complete anode-supported SOFC. The performance of the cell was evaluated on an in-house developed test station between 500 and 650 °C. Due to the limitation of the test station for large-cell testing, small-size samples with dimensions of 2.5 cm × 2.5 cm were cut out from the large-cell. For the single cell with humidified hydrogen as fuel and air as oxidant, the maximum power density achieved 909, 623, 335 and 168 mW cm⁻² at 650, 600, 550 and 500 °C, respectively. Impedance analysis confirmed that the performance of single cells below 600 °C was retarded primarily due to the slow interfacial reaction kinetics at reduced temperatures. Development of catalytically active electrode materials, especially the cathode material and improvement of the electrode microstructure are thus crucial for achieving a high performance low-temperature SOFC.     

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.     

Applications of nano-composite materials for improving the performance of anode-supported electrolytes of SOFCs

In order to improve the performance of the anode-supported electrolyte of solid oxide fuel cells (SOFCs), the anode electrode is modified by inserting an anode functional layer of nano-composite powders between a Ni–YSZ electrode and YSZ electrolyte. The NiO–YSZ nano-composite powders are fabricated by coating nano-sized Ni and YSZ particles on the YSZ core particle by the Pechini process. The reduction of the polarization resistance of a single cell that is applied to the anode functional layer is attributed to the increasing reaction of three-phase boundaries (TPBs) within the layer and the micro-structured uniformity in the electrode. Two methods were used, namely tape-casting/dip-coating and tape-casting/co-firing, for studying the performance. It can be concluded that the cell with an anode functional layer thickness (15–20 μm) and a microstructure of NiO–YSZ nano-composite materials which was fabricated by the tape-casting/dip-coating method improved the output power (to 1.3 W cm⁻²) at 800 °C using hydrogen as fuel and air as an oxidant.     

GdBaCo2O5+x layered perovskite as an intermediate temperature solid oxide fuel cell cathode

GdBaCo₂O_5+x (GBCO) was evaluated as a cathode for intermediate-temperature solid oxide fuel cells. A porous layer of GBCO was deposited on an anode-supported fuel cell consisting of a 15 μm thick electrolyte of yttria-stabilized zirconia (YSZ) prepared by dense screen-printing and a Ni–YSZ cermet as an anode (Ni–YSZ/YSZ/GBCO). Values of power density of 150 mW cm⁻² at 700 °C and ca. 250 mW cm⁻² at 800 °C are reported for this standard configuration using 5% of H₂ in nitrogen as fuel. An intermediate porous layer of YSZ was introduced between the electrolyte and the cathode improving the performance of the cell. Values for power density of 300 mW cm⁻² at 700 °C and ca. 500 mW cm⁻² at 800 °C in this configuration were achieved.