RedOx study of anode-supported solid oxide fuel cell

The common technology for solid oxide fuel cells (SOFC) is based on a cermet (ceramic-metal composite) anode of nickel with yttria stabilized zirconia (YSZ), often used as the supporting structure. One of the main limitations of this technology is the tolerance of the anode towards reduction and oxidation (“RedOx”) cycles.In this study, two techniques are used to quantify the anode expansion after a RedOx cycle of the nickel at different temperatures. The first method considers the anode expansion above the electrolyte fracture limit by measuring the crack width in the electrolyte layer. In the second method, the anode porosity is measured using scanning electron microscopy (SEM) image quantification. The same measurement techniques are used to quantify anode expansion after consecutive RedOx cycles at constant temperature.The quantification technique is then applied to cells tested in real stack conditions. The cell corners can undergo several RedOx cycles depending on stack design and fuel utilization. The study of such zones allows estimating the number of cycles that the anode experienced locally.     

Transient modeling of anode-supported solid oxide fuel cells

An isothermal 2-D transient model is developed for an anode-supported solid oxide fuel cell. The model takes into account the transient effects of both charge migration and species transport in PEN assembly. Due to the lack of transient experimental data, the transient model, under steady state operating conditions, is validated using experimental results from open literature. Numerical results show that the cell can obtain very quick transient current response when subjected to a step voltage change, followed by a slow current transient period due to species diffusion effects within porous electrodes. It is also found that the transient response of the cell current is sensitive to oxygen concentration change at cathode/channel interface, whereas the current response is slow when step change of hydrogen concentration is applied at anode/channel interface. The cell transient performance can be improved by increasing porosity or decreasing tortuosity of electrodes.     

Experimental optimization of the fabrication parameters for anode-supported micro-tubular solid oxide fuel cells

A systematic optimization of several parameters significant in the fabrication of anode-supported micro-tubular solid oxide fuel cell via extrusion and dip coating is presented in this study. Co-sintering temperature of anode-support and electrolyte, the vehicle type and solid powder content used in electrolyte dip-coating slurry, electrolyte submersion time, cathode sintering temperature, powder ratio in the cathode functional layer, submersion time for the cathode functional layer and, submersion time and coating number of the anode functional layer are studied in this respect and optimized in the given order according to the performance tests and microstructural analyses. The performance of the micro-tubular cell is significantly improved to 0.49 Wcm⁻² at 800 °C after the optimizations, while that of the base cell is only 0.136 Wcm⁻². 12-cell micro-tubular stack is also constructed with the optimized cells and the stack is tested. Each cell in the stack is found to show very close performance to the single-cell performance and the stack with a maximum power of ~26 W at an operating temperature of 800 °C is therefore evaluated to be successful.     

Processing of high-performance anode-supported planar solid oxide fuel cell

Anode-supported planar single cells of dimensions 5 cm × 5 cm × 1.5 mm and 10 cm × 10 cm × 1.5 mm have been successfully fabricated using inexpensive and simple processing techniques. The process involves room temperature lamination of several porous layers of tapecast NiO–YSZ together with a dense layer of YSZ electrolyte followed by cofiring. The half-cell fabrication is optimized while the electrolyte thickness is lowered down from 40 to 10 μm. The fabricated single cells with screen printed La(Sr)MnO₃ (LSM)–YSZ as cathode active layer and LSM as current collector layer shows very high electrochemical performance although no separate active layer is used on the anode side of the fabricated cells. The single cells are tested with hydrogen on the anode and oxygen on the cathode sides. The current density and power density of a typical coupon cell of diameter 16 mm is found to be ≈1.7 A/cm² and ≈1.2 W/cm², respectively, at a cell voltage of 0.7 V measured at 800 °C. Area specific resistance (ASR) value, evaluated from the current–voltage plot, is as low as 0.205 Ω cm² at 800 °C. The performances of these cells are found to be almost size independent having excellent repeatability.     

Oxidation failure modes of anode-supported solid oxide fuel cells

Investigations on anode-supported solid oxide fuel cells (SOFCs) using Ni-based anode supports are presented aiming at understanding how much oxidation such a cell can tolerate before incurring irreversible mechanical damage. The cells were oxidised both directly in air and electrochemically. The different oxidation procedures performed exhibited different damage modes. For free-standing cells oxidised in air, the main damage mode was electrolyte cracking after oxidation of approximately 50% of the Ni in the substrate. However, cells oxidised electrochemically failed by substrate cracking after only ca. 5% of the Ni was oxidised, mainly due to the non-uniform nature of oxidation in the SOFC. Models of the stress generation and fracture processes were developed for interpretation of the results.     

Fabrication and properties of anode-supported tubular solid oxide fuel cells

Tubular solid oxide fuel cell (SOFC) systems have many desirable characteristics over their planar counter-parts. Anode-supported tubes provide an excellent platform for individual cells. They allow for a thin electrolyte layer to be applied to the outside of the tube, which helps to minimize polarization losses. This paper describes the fabrication of nickel–zirconia (Ni–YSZ)-based anode tubes via extrusion of a plastic mass through a die of the required dimensions. The anode tubes were then dried and fired. Tests were performed on the tubes to determine the effects of firing temperature on porosity to allow for a pinhole-free electrolyte coating to be applied. Thin layer coating techniques, including vacuum-assisted dip coating and painting, were compared. Ni–YSZ anode-supported tubular SOFCs with a gas-tight thin YSZ electrolyte layer were then realized. Microstructure of the anode support, electrolyte and cathode thin films was also examined.     

Redox-induced performance degradation of anode-supported tubular solid oxide fuel cells

Redox behavior of a Ni-Y₂O₃-stabilized ZrO₂ (YSZ) composite anode support and the performance degradation of an anode-supported tubular solid oxide fuel cell (SOFC) were studied under complete oxidation and reduction conditions (degrees of oxidation and reduction = 100%). Materials characterization studies showed that the exposure time in oxidizing and reducing atmospheres played a critical role in the degradation of the porous structures and the physical properties of the anode support. In particular, the redox cycling with an 8 h exposure time resulted in the cracking of YSZ network, leading to significant decay of the mechanical strength. The polarization experiments on the redox-cycled anode-supported tubular cell showed serious performance degradation as a result of the decreases of open-circuit potential and power density. The ac-impedance measurements combined with microstructural observations indicated that the performance degradation resulted mainly from (i) the degradation of anode support, (ii) microcracks across the whole cell, and (iii) interface delamination.     

Nickel foam anode-supported solid oxide fuel cells with composite electrolytes

In this paper, three composite electrolytes (Ce_0.8Sm_0.2O_1.9/Na₂CO₃) SDC, (Ce_0.8Gd_0.2O_1.9/Na₂CO₃) GDC and (La_0.8Sr_0.2Ga_0.8Mg_0.2O_3−δ/Na₂CO₃) LSGM have been studied to review the effect of Na₂CO₃, present as second phase, and effect of Ni support, sintering and pressure for dry pressing for fuel cell performance. The electrolytes were prepared through a co-precipitation method. Different techniques were used to characterize the prepared electrolytes. The crystal structures were determined using x-ray diffraction (XRD) and structural morphologies were analyzed through scanning electron microscopy (SEM). The four-probe dc conductivity and two probe AC impedance analysis were carried out in open air. The fuel cell performance was determined with and without Ni support on anode side which resulted in a considerable difference in obtained peak power densities. It is suggested that prepared electrolytes can find potential applications in low temperatures Solid Oxide Fuel Cells (LT-SOFCs).     

The effect of phosphine in syngas on Ni-YSZ anode-supported solid oxide fuel cells

Ni-YSZ cermet is commonly used as the anode of a solid oxide fuel cell (SOFC) because it has excellent electrochemical performance, not only in hydrogen fuel, but also in a clean blended synthetic coal syngas mixture (30% H₂, 26% H₂O, 23% CO, and 21% CO₂). However, trace impurities, such as phosphine (PH₃), in coal-derived syngas can cause degradation in cell performance [J.P. Trembly, R.S. Gemmen, D.J. Bayless, J. Power Sources 163 (2007) 986-996]. A commercial solid oxide fuel cell was exposed to a syngas with 10 ppm PH₃ under a constant current load at 800 °C and its performance was evaluated periodically using electrochemical methods. The central part of the anode was exposed directly to the syngas without an intervening current collector. Post-mortem analyses of the SOFC anode were performed using Raman spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). The results show that the impurity PH₃ caused a significant loss of the Ni-YSZ anode electrochemical performance and an irreversible Ni-YSZ structural modification. Ni₅P₂ was confirmed to be produced on the cell surface as the dominant nickel phosphorus phase.     

Direct-methane solid oxide fuel cell (SOFC) with Ni-SDC anode-supported cell

The performance of a Ni-SDC anode-supported cell operating with a dry CH₄ feed stream and the effectiveness of exposing the anode to H₂ as a method of removing carbon deposits are evaluated. This has involved the continuous monitoring of the outlet gas composition during CH₄ operation and H₂ exposure. A degradation rate in the cell voltage (～1.33 mV h^?1) is observed during 100 h operation with dry CH₄. Carbon is detected in the Ni-SDC anode after the stability test but only in the portion of the anode closest to the fuel channel. No carbon is detected at the electrolyte-anode interface, which is the likely reason that the cell performance remains relatively stable. The information obtained from SEM and gas outlet composition analyses can be explained by a process whereby most of the CH₄ that reacts decomposes into H₂ and C in the Ni-SDC anode near the fuel channel. H₂ then makes its way to the anode-electrolyte interface where it is electrochemically oxidized to H₂O which can also react with any C that may have formed, leaving behind C primarily at the fuel channel. When an aged cell is exposed to H₂, carbon-containing gases (CO, CH₄ and CO₂) are released, indicating that some carbon has been removed from the anode. Examination of the anode after the test shows that some carbon still remains after this treatment.     

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.     

Analysis of heat and mass transport characteristics in anode-supported solid oxide fuel cells at various operating conditions

In this study, the heat and mass transport characteristics of anode-supported solid oxide fuel cells (SOFCs) are numerically investigated by using a two-dimensional model. The mathematical model is validated by comparing the numerical results with experimental data found in the open literature. The species and temperature distributions of SOFCs at different cell voltages are presented and compared. Effects of operating temperature, flow direction arrangement, and flow velocity on the overall cell performance and local temperature distribution are also analyzed. It is concluded that the local temperature is increased with decreasing operating cell voltage, increasing operating temperature, and decreasing cathode flow velocity. The temperature distribution is significantly changed when counter-flow arrangement is used instead of coflow arrangement. In addition, the effect of anode flow velocity on temperature distribution is negligible.     

The effect of HCl in syngas on Ni-YSZ anode-supported solid oxide fuel cells

The Ni-YSZ cermet anode of the solid oxide fuel cell (SOFC) has excellent electrochemical performance in a clean blended synthetic coal syngas mixture. However, chloride, one of the major contaminants existing in coal-derived syngas, may poison the Ni-YSZ cermet and cause degradation in cell performance. Both hydrogen chloride (HCl) and chlorine (Cl₂) have been reported to attack the Ni in the anode when using electrolyte-supported SOFCs. In this paper, a commercial anode-supported SOFC was exposed to syngas with a concentration of 100 ppm HCl under a constant current load at 800 °C for 300 h and 850 °C for 100 h. The cell performance was evaluated periodically using electrochemical methods. A unique feature of this experiment is that the active central part of the anode was exposed directly to the fuel without an intervening current collector. Post-mortem analyses of the SOFC anode were performed using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). The results show that the 100 ppm concentration of HCl causes about 3% loss of performance for the Ni-YSZ anode-supported cell during the 400 h test. Permanent changes were noted in the surface microstructure of the nickel particles in the cell anode.     

Effect of nickel-phosphorus interactions on structural integrity of anode-supported solid oxide fuel cells

An integrated experimental/modeling approach was utilized to assess the structural integrity of Ni-yttria-stabilized zirconia (YSZ) porous anode supports during the solid oxide fuel cell (SOFC) operation on coal gas containing trace amounts of phosphorus impurities. Phosphorus was chosen as a typical impurity exhibiting strong interactions with the nickel followed by second phase formation. Tests were performed using Ni-YSZ anode-supported button cells exposed to 0.5-10 ppm of phosphine in synthetic coal gas at 700-800 °C. The extent of Ni-P interactions was determined by a post-test scanning electron microscopy (SEM) analysis. Severe damage to the anode support due to nickel phosphide phase formation and extensive crystal coalescence was revealed, resulting in electric percolation loss. The subsequent finite element stress analyses were conducted using the actual anode support microstructures to assist in degradation mechanism explanation. Volume expansion induced by the Ni phase alteration was found to produce high stress levels such that local failure of the Ni-YSZ anode became possible under the operating conditions.     

Effect of anode thickness on impedance response of anode-supported solid oxide fuel cells

This study examines effects of the anode functional layer thickness on the performance of anode-supported solid oxide fuel cells (SOFCs). The SOFCs with different AFL thicknesses (8 μm, 19 μm, and 24 μm) exhibit similar power densities at the measured current density range (0–2 A cm⁻²), but show different impedance responses. Further investigation on the spectra using the CNLS fitting method based on DRT-based equivalent circuit model helps us pinpoint two electrochemical processes directly affected by the AFL thickness changes, the charge transfer reaction in the AFL as well as the diffusion-coupled charge transfer reaction in the AFL. The combined effects of these two electrochemical processes probably forged a minimal impact on the overall fuel cell performance by offsetting each other, which offers a reasonable explanation of the seemingly little influence of the AFL thickness on the SOFC performance.     

Multilayer tape casting of large-scale anode-supported thin-film electrolyte solid oxide fuel cells

Tape casting is conventionally used to prepare individual, relatively thick components (i.e., the anode or electrolyte supporting layer) for solid oxide fuel cells (SOFCs). In this research, a multilayer ceramic structure is prepared by sequentially tape casting ceramic slurries of different compositions onto a Mylar carrier followed by co-sintering at 1400 °C. The resulting half-cells contains a 300 μm thick NiO–yttria-stabilized zirconia (YSZ) anode support, a 20 μm NiO–YSZ anode functional layer, and an 8 μm YSZ electrolyte membrane. Complete SOFCs are obtained after applying a Gd_0.1Ce_0.9O₂ (GDC) barrier layer and a Sm_0.5Sr_0.5CoO₃ (SSC) -GDC cathode by using a wet-slurry spray method. The 50 mm × 50 mm SOFCs produce peak power densities of 337, 554, 772, and 923 mW/cm² at 600, 650, 700, and 750 °C, respectively, on hydrogen fuel. A short stack including four 100 mm × 150 mm cells is assembled and tested. Each stack repeat unit (one cell and one interconnect) generates around 28.5 W of electrical power at a 300 mA/cm² current density and 700 °C.     

Degradation of cathode current-collecting materials for anode-supported flat-tube solid oxide fuel cell

Different types of cathode current-collecting material for anode-supported flat-tube solid oxide fuel cells are fabricated and their electrochemical properties are characterized. Current collection for the cathode is achieved by winding Ag wire and by painting different conductive pastes of Ag–Pd, Pt, La_0.6Sr_0.4CoO₃ (LSCo), and La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF) on the wire. Cell performance at the initial operation time is in the order of Pt > LSCo > LSCF > Ag–Pd. On the other hand, the performance degradation rate is in the order of LSCo < LSCF < Pt < Ag–Pd. LSCo paste as a cathode current-collector shows the most stable long-term performance of 0.8 V, 300 mA cm⁻² at 750 °C, even under a thermal cycle condition with heating and cooling rates of 150 °C h⁻¹. The performance degradation of the Ag–Pd and Pt pastes is caused by increased polarization resistance due to metal particle sintering. From these results, it is concluded that a cathode current-collector composed of wound silver wire with LSCo paste is useful for anode-supported flat-tube cells as it does not experience any significant degradation during a long operation time.     

Three-dimensional thermo-fluid and electrochemical modeling of anode-supported planar solid oxide fuel cell

This paper presents a three-dimensional model of an anode-supported planar solid oxide fuel cell with corrugated bipolar plates serving as gas channels and current collector above the active area of the cell. Conservation equations of mass, momentum, energy and species are solved incorporating the electrochemical reactions. Heat transfer due to conduction, convection and radiation is included. An empirical equation for cell resistance with measured values for different parameters is used for the calculations. Distribution of temperature and gas concentrations in the PEN (positive electrode/electrolyte/negative electrode) structure and gas channels are investigated. Variation of current density over the cell is studied. Furthermore, the effect of radiation on the temperature distribution is studied and discussed. Modeling results show that the relatively uniform current density is achieved at given conditions for the proposed design and the inclusion of thermal radiation is required for accurate prediction of temperature field in the single cell unit.     

Low temperature anode-supported solid oxide fuel cells based on gadolinium doped ceria electrolytes

The utilization of anode-supported electrolytes is a useful strategy to increase the electrical properties of the solid oxide fuel cells, because it is possible to decrease considerably the thickness of the electrolytes. We have successfully prepared single-chamber fuel cells of gadolinium doped ceria electrolytes Ce_1−xGd_xO_2−y (CGO) supported on an anode formed by a cermet of NiO/CGO. Mixtures of precursor powders of NiO and gadolinium doped ceria with different particle sizes and compositions were analysed to obtain optimal bulk porous anodes to be used as anode-supported fuel cells. Doped ceria electrolytes were prepared by sol–gel related techniques. Then, ceria-based electrolytes were deposited by dip coating at different thicknesses (15–30 μm) using an ink prepared with nanometric powders of electrolytes dispersed in a liquid polymer. Cathodes of La_1−xSr_xCoO₃ (LSCO) were also prepared by sol–gel related techniques and were deposited on the electrolyte thick films. Finally, electrical properties were determined in a single-chamber reactor where propane, as fuel, was mixed with synthetic air below the direct combustion limit. Stable density currents were obtained in these experimental conditions. Flux rate values of the carrier gas and propane partial pressure were determinants for the optimization of the electrical properties of the fuel cells.