Anode-supported Planar Solid Oxide Fuel Cells Research Articles

Three-dimensional computational fluid dynamics modeling of anode-supported planar SOFC

Modeling plays a very important role in the development of fuel cells and fuel cell systems. The aim of this work is to investigate the electrochemical processes of a Solid Oxide Fuel Cell (SOFC) and to evaluate the performance of the proposed SOFC design. For this aim a three-dimensional Computational Fluid Dynamics (CFD) model has been developed for an anode-supported planar SOFC with corrugated bipolar plates serving as gas channels and current collector. The conservation of mass, momentum, energy and species is solved by using the commercial CFD code FLUENT in the developed model. The add-on FLUENT SOFC module is implemented for modeling the electrochemical reactions, loss mechanisms and related electric parameters throughout the cell. The distributions of temperature, flow velocity, pressure and gaseous (fuel and air) concentrations through the cell structure and gas channels is investigated. The relevant fuel cell variables such as the potential and current distribution over the cell and fuel utilization are calculated and studied. The modeling results indicate that, for the proposed SOFC design, reasonably uniform distributions of current density over the active cell area can be achieved. The geometry of the cathode gas channel has a substantial effect on the oxygen distribution and thus the overall cell performance. Methods for arriving at improved cell designs are discussed.

Determination of global and local residual stresses in SOFC by X-ray diffraction

Solid Oxide Fuel Cell (SOFC) is a high-performance electrochemical device for energy conversion. A single cell is composed of five layers made of different ceramic materials: anode support, anode functional layer, electrolyte, cathode functional layer and cathode. The mechanical integrity of the cell is a major issue during its lifetime, especially for the electrolyte layer. Damage of the cells is mainly due to the high operating temperature, the “redox” behaviour of the anode and the brittleness of the involved materials. Since residual stresses are known to play a significant role in the damage evolution, it is important to determine them. For this purpose, residual stresses in an anode-supported planar SOFC were measured by X-ray diffraction. Firstly, macroscopic stresses in each phase of each layer were studied using the sin 2 ψ method on a laboratory X-ray goniometer at room temperature. This technique enables the calculation of residual stress of the material from the measurement of the crystal lattice deformation. The electrolyte has been found under bi-axial compressive stress of −920 MPa. Secondly, X-ray measurements controlling depth penetration were made in the electrolyte using grazing incidence method. The results show that the stress is not homogenous in the layer. The first five micrometers of the electrolyte have been found less constrained (−750 MPa) than the complete layer, suggesting a gradient of deformation in the electrolyte from the interface with the Anode Functional Layer to the free surface. Finally, local stress measurements were made on the electrolyte layer by X-ray synchrotron radiation that allows high accuracy measurement on the (sub-) micrometer scale. Polychromatic and monochromatic beams are used to determine the complete strain tensor from grain to grain in the electrolyte. First results confirm the macroscopic stress trend of the electrolyte. These X-ray techniques at different scales will contribute to a better understanding of the residual stress in the electrolyte layer and thus to the involved damage mechanisms.

The effect of H 2S on the performance of Ni–YSZ anodes in solid oxide fuel cells

Biomass-derived fuel, e.g. biogas, is a potential fuel for solid oxide fuel cells (SOFCs). At operating temperature (∼850 °C) reforming of the carbon-containing biogas takes place over the Ni-containing anode. However, impurities in the biogas, e.g. H 2S, can poison both the reforming and the electrochemical activity of the anode. Tests of single anode-supported planar SOFCs were carried out in the presence of H 2S under current load at 850 °C. The cell voltage dropped as we periodically added 2–100 ppm H 2S to an H 2-containing fuel in 24 h intervals, but it regenerated to the initial value after we turned off the H 2S. Evaluation of the changes of the cell voltage suggests that saturation coverage was reached at approximately 40 ppm H 2S. A front-like movement of S-poisoning over the anode was seen by monitoring the in-plane voltage in the anode. Furthermore, impedance spectra showed that mainly the polarization resistance increased when adding H 2S. These changes in resistance were found to happen at 1212 Hz, which is related to reactions at the anode–electrolyte interface. These findings can be used to identify S-related effects on the performance, when an SOFC is fuelled with biogas or other fuels with H 2S impurities and thus help in the development of more sulfur tolerant SOFCs.

A general electrolyte–electrode-assembly model for the performance characteristics of planar anode-supported solid oxide fuel cells

A general electrode–electrolyte-assembly (EEA) model has been developed, which is valid for different designs of solid oxide fuel cells (SOFCs) operating at different temperatures. In this study, it is applied to analyze the performance characteristics of planar anode-supported SOFCs. One of the novel features of the present model is its treatment of electrodes. An electrode in the present model is composed of two distinct layers referred to as the backing layer and the reaction zone layer. The other important feature of the present model is its flexibility in fuel, having taking into account the reforming and water–gas shift reactions in the anode. The coupled governing equations of species, charge and energy along with the constitutive equations in different layers of the cell are solved using finite volume method. The model can predict all forms of overpotentials and the predicted concentration overpotential is validated with measured data available in literature. It is found that in an anode-supported SOFC, the cathode overpotential is still the largest cell potential loss mechanism, followed by the anode overpotential at low current densities; however, the anode overpotential becomes dominant at high current densities. The cathode and electrolyte overpotentials are not negligible even though their thicknesses are negligible relative to the anode thickness. Even at low fuel utilizations, the anode concentration overpotential becomes significant when chemical reactions (reforming and water–gas shift) in the anode are not considered. A parametric study has also been carried out to examine the effect of various key operating and design parameters on the performance of an anode-supported planar SOFCs.

Operating characteristics of a 5 kW class anode-supported planar SOFC stack for a fuel cell/gas turbine hybrid system

We designed and constructed a pressurized 5 kW anode-supported planar solid oxide fuel cell (SOFC) power generation system with a pre-reformer for a fuel cell/gas turbine hybrid system. The 5 kW class SOFC stack, which was made by Forschungszentrum Jülich (FZJ) in Germany, was assembled and installed at KIER. We also investigated the stack's typical operating characteristics, particularly with respect to the thermal cycling test, the fuel change from H 2 to pre-reformed gas, and the pressurized operation at 3.5 atm (abs.) under hybrid operating conditions with a micro-gas turbine. In atmospheric pressure, the output of the SOFC stack was 8.1 kW for the H 2 and 4.7 kW for the pre-reformed gas. When the stack with an liquefied natural gas (LNG) pre-reformer was operated in combination with a micro-gas turbine, the stack showed an output of 5.1 kW at 3.5 atm (abs.). From the experimental results, we conclude that the anode-supported planar SOFC stack operates successfully in a pressurized hybrid system with a micro-gas turbine. We also established a unique procedure and a basic technology in which the anode-supported planar SOFC stack could be operated under pressurized conditions in combination with a pre-reformer. Moreover, we achieved successful operation of a hybrid system that combines an SOFC power system with a micro-gas turbine.

Multilayer tape-casting method for anode-supported planar SOFCs

We have developed a multilayer tape-casting and co-sintering process to fabricate a large-area, anode-supported electrolyte film, which is critical for planar-type, reduced-temperature solid oxide fuel cells (SOFCs). Nickel/yttria-stabilized zirconia (Ni/YSZ), nickel/scandia-stabilized zirconia (Ni/ScSZ) cermets, ScSZ, Ce0.8Gd0.2O1.9 (CGO), and La0.6Sr0.4Co0.2Fe0.8O3δ (LSCF)–CGO were used as materials for anode substrate, anode functional layer, electrolyte, interlayer and cathode, respectively. The powders of these functional layers were ball-milled with organic additives to form slurries, which were assembled together with the multilayer tape-casting procedure to get the green tapes. After drying, the green tape was co-sintered at 1400°C for 4 h in air to get the large-area, anode-supported electrolyte film (thickness 15 μm). For cell preparation, the LSCF–CGO composite cathode was deposited by the screen-printing method and sintered at 1100°C for 3 h. The area-specific resistance (ASR) of the obtained single cell was found to be 0.99 ωcm2 at 850°C with H2/O2 as the operating gases, and the maximum power density achieved was 0.63 W/cm2. The thickness of the (Zr,Ce)O2-based solid solution formed at the ScSZ/CGO interface during high-temperature sintering was investigated. The results illustrate that fabrication of an anode-supported electrolyte film for planar SOFCs with a CGO interlayer is possible by the multilayer tape-casting procedure, which is both cost-effective and feasible.

3D modeling of anode-supported planar SOFC with internal reforming of methane

Deposition of carbon on conventional anode catalysts and formation of large temperature gradients along the cell are the main barriers for implementing internal reforming in solid oxide fuel cell (SOFC) systems. Mathematical modeling is an essential tool to evaluate the effectiveness of the strategies to overcome these problems. In the present work, a three-dimensional model for a planar internal reforming SOFC is developed. A co-flow system with no pre-reforming, methane fuel utilization of 75%, voltage of 0.7 V and current density of 0.65 A cm −2 was used as the base case. The distributions of both temperature and gas composition through the gas channels and PEN (positive electrode/electrolyte/negative electrode) structure were studied using the developed model. The results identified the most susceptible areas for carbon formation and thermal stress according to the methane to steam ratio and temperature gradients, respectively. The effects of changing the inlet gas composition through recycling were also investigated. Recycling of the anode exhaust gas, at an optimum level of 60% for the conditions studied, has the potential to significantly decrease the temperature gradients and reduce the carbon formation at the anode, while maintaining a high current density.

An efficient ceramic-based anode for solid oxide fuel cells

A new ceramic-based multi-component material, containing one electronic conductor (Y-substituted SrTiO 3, SYT), one ionic conductor (YSZ) and a small amount (∼5 vol.%) of Ni catalyst, was investigated as an alternative anode material for solid oxide fuel cells (SOFCs). The ceramic framework SYT/YSZ shows good dimensional stability upon redox cycling and has an electrical conductivity of ∼10 S cm −1 under typical anode conditions. Owing to the substantial electrocatalytic activity of the fine and well-dispersed Ni particles on the surface of the ceramic framework, the electrode polarization resistance of 5 vol.% Ni-impregnated SYT/YSZ anode reached 0.21 Ω cm 2 at 800 °C in wet Ar/5%H 2. Based on these results, a redox-stable anode-supported planar SOFC is expected using this anode material, thus offering great advantages over the current generation of Ni/YSZ-based SOFCs.

Evaluation of Sr- and Mn-substituted LaAlO 3 as potential SOFC anode materials

Lanthanum–aluminate-based oxides, (La 0.8Sr 0.2) 1− y Al 1− x Mn x O 3− δ ( x = 0, 0.3, 0.5; y = 0 or 0.06) (LSAM), were synthesized and evaluated in detail as potential anode materials for solid oxide fuel cells (SOFCs). The electrical conductivity of LSAM (Mn ≥ 30 mol%) is dominated by p-type electronic conduction and can be treated as a diluted system of lanthanum manganites, (La,Sr)MnO 3. At 810 °C, the electrical conductivity of (La 0.8Sr 0.2) 0.94Al 0.5Mn 0.5O 3− δ (LSAM8255b) reaches 12 S/cm in air and 2.7 S/cm in humidified Ar/4% H 2 ( p(O 2) ≈ 10 − 18 bar). The thermal expansion coefficients of LSAM8255a and LSAM8255b match YSZ very well and no chemical reaction was observed between these two perovskite materials and YSZ up to at least 1400 °C. Fairly good electrochemical performance was observed for an LSAM8255b–YSZ composite anode. At 850 °C, the polarization resistances are only 0.34 and 0.50 Ω cm 2 in wet (∼3% H 2O) Ar/20% H 2 and wet Ar/20% CH 4, respectively. In addition, an exposure to Ar/20% CH 4/3% H 2O for 35 h did not cause any apparent carbon deposition on the electrode. However, the chemical stability of LSAM8255a and LSAM8255b in a typical anode environment under open circuit conditions does not seem sufficient, leading to performance degradation with time in wet Ar/20% H 2 or wet Ar/20% CH 4. Furthermore, relatively large chemical expansion (0.3–0.5%) was observed when the atmosphere was switched from air to wet Ar/4% H 2, which might cause intolerable stress on the thin film electrolyte layer for a large-area anode-supported planar SOFC, but which might be tolerable for small geometries or electrolyte-supported SOFCs.

Development of interconnect materials for solid oxide fuel cells

The state of the art development of interconnect materials for solid oxide fuel cells (SOFCs) is reviewed. The specific interconnect requirements from a materials design point of view are described in detail. Doped-LaCrO 3 ceramics considered as interconnects for electrolyte-supported planar SOFC suffer from their extreme sensitivity to oxygen partial pressure and high manufacturing cost. Advent of anode-supported planar SOFC allows lower operating temperatures and great efforts have been directed towards the development of metallic interconnects. Chromia formed on the surface of chromium-based alloys are intrinsically too volatile to be viable as interconnects for SOFCs operating above 800 °C. Some intermediate phases occurring between iron-based interconnect and cathode significantly increase the contact resistance. Metallic materials that are capable of developing stable oxide scales with acceptable growth rate and reasonably high electrical conductivity over the expected SOFC lifetime need to be developed.