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.     

Design and numerical analysis of a planar anode-supported SOFC stack

In the present study, numerical simulations are conducted to examine the flow characteristics and attributes of electrochemical reactions in the stack through three-dimensional analysis using finite volume approach prior to the fabrication of the SOFC stack. The stack flow uniformity index is employed to investigate the flow uniformity whereas in the case of electrochemical modeling, different mathematical models are adopted to predict the characteristics of activation and ohmic overpotentials that occur during electrochemical reactions in the cell. The normalized mass flow rate is found almost same in each cell with flow uniformity index of 0.999. The calculated voltage and power curves under different average current densities are compared with experimental results for the model validation. The changes in the voltage and power of the SOFC stack, current density, temperature, over potential and reactants distributions in relation to varying amounts of reactants flow are also examined. The current density distribution in each cell is observed to vary along the anode flow direction. The temperature difference in each cell is almost same along the flow direction of reactants, and the irreversible resistance showed an opposite trend with a temperature distribution in each cell.     

Engineered anode structure for enhanced electrochemical performance of anode-supported planar solid oxide fuel cell

A novel approach of fabricating SOFC anode comprising graded compositions in constituent phases having layer wise microstructural variation is reported. Such anode encompasses conventional NiO–YSZ (40 vol% Ni) with higher porosity at the fuel inlet side and Ni–YSZ electroless cermet (28–32 vol% Ni) with less porosity toward the electrolyte. Microstructures and thicknesses of the bilayer anodes (BLA) are varied sequentially from 50 to 250 μm for better thermal compatibility and cell performance. Significant augmentation in performance (3.5 A cm⁻² at 800 °C, 0.7 V) is obtained with engineered trilayer anode (TLA) having conventional anode support in conjunction with layers of electroless cermet each of 50 μm having 28 and 32 vol% Ni. Engineered TLA accounts for substantial reduction both in cell polarization (ohmic ASR: 78 mΩ cm² versus 2835 mΩ cm²; cell impedance: 0.35 Ω cm² versus 0.9 Ω cm²) and degradation rate (76 μV h⁻¹ versus 219 μV h⁻¹) compared to cells fabricated with conventional cermet.     

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.     

Improving single-chamber performance of an anode-supported SOFC by impregnating anode with active nickel catalyst

In order to improve the single-chamber performance of a traditional anode-supported single-chamber solid-oxide fuel cell with NiO–ScSZ anode and ScSZ electrolyte, the modification of the anode with a fine nickel catalyst by impregnation method was exploited. Catalytic test demonstrated the nickel catalyst had higher catalytic activity than the severe sintered nickel–cermet anode between 700 and 900 °C, especially at the lower temperature range. SEM examination demonstrated the nickel catalyst impregnation increased the roughness of the nickel grains within the anode. Furthermore, some surfaces of the ScSZ grains are also covered with very fine nickel catalyst. By operating on a methane-air mixture gas with methane to oxygen ratio of 1.3:1, the cell with its anode impregnated with the nickel catalyst showed an open circuit voltage and peak power density of 0.954 V and 119 mW cm⁻² at a furnace temperature of 750 °C, respectively, as a comparison of 0.893 V and 79 mW cm⁻² for the cell without the nickel catalyst. The improved cell performance was attributed to the higher cell temperature and increased anode catalytic activity for methane partial oxidation.     

Characteristics and performance of lanthanum gallate electrolyte-supported SOFC under ethanol steam and hydrogen

This study is focused on the electrochemical performance of perovskite-type materials based on doped LaGaO₃. La_0.8Sr_0.2Ga_0.8Mg_0.2O_3−δ (LSGM) and La_0.8Sr_0.2Ga_0.8Mg_0.115Co_0.085O_3−δ (LSGMC) were used as electrolytes and (Pr_0.7Ca_0.3)_0.9MnO₃ (PCM) and La_0.75Sr_0.25Cr_0.5Mn_0.5O_3−δ (LSCM) as cathode and anode material, respectively. LSGM and LSGMC electrolytes were prepared by tape casting with a thickness of about 600 μm. The performance of LSCM/LSGMC/PCM was slightly superior to that obtained on LSCM/LSGM/PCM at different temperatures in both humidified hydrogen and ethanol steam atmospheres, good values of power output in LSCM/LSGMC/PCM were 182 and 169 mW cm⁻² using humidified hydrogen and ethanol steam as fuel, respectively, and oxygen as oxidant at 850 °C. Cell stability tests indicate no significant degradation in performance after 60 h of cell testing when LSCM anode was exposed to ethanol steam at 750 °C. Almost no carbon deposits were detected after testing in ethanol steam at 750 °C for >60 h on the LSCM anodes, suggesting that carbon deposition was limited during cell operation.     

Polarization measurements of anode-supported solid oxide fuel cells studied by incorporation of a reference electrode

A three-electrode system configuration was applied to an anode-supported solid oxide fuel cell where the anode to cathode surface area ratio was ∼7.9, and Ni/YSZ was used as the anode, LSM as the cathode, Pt as the reference electrode, and thin YSZ film as the electrolyte. The cell was polarized potentiostatically at −0.2, −0.4, −0.6 and −0.8 V versus open circuit voltage (OCV) and the potential change versus a reference electrode were recorded to ascertain the relative electrode polarization contributions. The results of these studies suggested that, while the anode contributions to cell polarization were less significant than that observed for the cathode, they were not negligible. Furthermore, the disparity in the relative electrode polarization contribution was observed to decrease with increasing temperature and polarization. Electrode polarization studies suggested that cathodic overvoltage decreased remarkably with increasing temperature whereas anodic overvoltage increased slightly with increasing temperature. Electrode kinetic parameters were extracted from these polarization experiments and the implications of these parameters to cell performance were discussed. Lastly, electrochemical impedance spectroscopy (EIS) data was presented to further elucidate the relative contributions of the anode and cathode impedances on button cell performance.     

On flow uniformity in various interconnects and its influence to cell performance of planar SOFC

This paper investigates flow uniformity in various interconnects and its influence to cell performance of a planar SOFC. A transparent hydraulic platform was used to measure flow uniformity in different rib-channel modules of interconnects. Several 3D numerical models implemented by CFD-RC packages were established to first simulate these hydraulic experiments and then used to evaluate the cell performance of a single-cell stack using different designs of interconnects with different flow uniformity over a wide range of a hydraulic Reynolds number (Re) based on a hydraulic diameter of rib-channels. Numerical flow data are found in good agreement with experimental results. It is proposed that a new design, using simple small guide vanes equally spaced around the feed header of the double-inlet/single-outlet module, can effectively improve the degree of flow uniformity in interconnects resulting in 11% increase of the peak power density (PPD) which can be further increased when applying a Ni-mesh on anode. Numerical analyses demonstrate a strong influence of Re on cell performance, of which appropriate ranges of Re in both anode and cathode sides are identified for achieving a reasonably good PPD while remaining an economic fuel utilization rate and having less temperature variations in the single-cell stack.     

Tape casting fabrication,co-sintering and optimisation of anode/electrolyte assemblies for SOFC based on BIT07-Ni/BIT07

BaIn_0.3Ti_0.7O_2.85 (BIT07) is an electrolyte material for SOFC due to its high ionic conductivity level and its compatibility with mixed ionic and electronic conductor (MIEC) cathode materials, such as LSCF and Nd₂NiO_4+δ. BIT07 is also compatible with NiO to form a cermet as anode material. In this study, the electrolyte material has been prepared by tape casting and characterised. The coupled influences of the powder grain size and the firing temperature have been investigated and optimised to yield a dense electrolyte at 1300 °C. Then anode-supported half-cells (electrolyte/anode), based on BIT07/BIT07-Ni have been prepared by tape casting and co-sintered. The composition and the microstructure of the cermet anode (BIT07 grain size, amount and nature of pore-forming agent) have been optimised to achieve an area surface resistance (ASR) value of about 0.15 Ω cm² at 700 °C under wet reducing atmosphere. The stability of the most performing anode has been followed during 500 h and the observed degradation seems to be due a loss of nickel percolation.     

Preparation and performance characterization of the Fe–Ni/ScSZ cermet anode for oxidation of ethanol fuel in SOFCs

An anodic cermet of Fe–Ni alloy and scandia stabilized zirconia (ScSZ) has been investigated for a solid oxide fuel cell (SOFC) running on ethanol fuel. Composite anodes having alloy compositions of 0, 12.5, 25, 37.5, 50 and 100 wt.% Ni were exposed to ethanol stream at 700 °C for 12 h to demonstrate that carbon formation is greatly suppressed on the Fe–Ni alloys compared to that of pure Ni. Then the short-term stability for the cells with the Ni/ScSZ and Fe_0.5Ni_0.5/ScSZ anodes in ethanol stream at 700 °C was checked over a relative long period of operation. Open circuit voltages (OCVs) increased from 1.03 to 1.1 V, and power densities increased from 120 to 460 mW cm² as the operating temperature of a SOFC with Fe_0.5Ni_0.5/ScSZ anode was increased from 700 to 850 °C in ethanol stream. Electrochemical impedance spectra (EIS) illustrated that the cell with Ni/ScSZ anode exhibits slightly less total impedance than that observed for the cell with Fe_0.5Ni_0.5/ScSZ anode. The performance of a fuel cell made with the Ni/ScSZ and Fe_0.5Ni_0.5/ScSZ anodes was tested in ethanol stream for 48 h and showed a significant decrease in polarization resistance with time. Impedance spectra of similar fuel cells suggest that small carbon deposits are formed with time and that the decrease in polarization resistance is due to enhanced electronic conductivity in the anode.     

In-situ patterned intra-anode triple phase boundary in SOFC electroless anode: An enhancement of electrochemical performance

Co-existence of intra-anode and conventional triple phase boundary (TPB) is reported for the first time in core (YSZ)-shell (Ni) electroless anode cermet. A mathematical model is proposed for determination of intra-anode TPB length and has been validated experimentally. Retention of longer intra-anode TPB even after repeated redox cycling is responsible for relatively lower conductivity degradation of such cermets. Existence of intra-anode TPB in electroless anode results in significant improvement of electrochemical performance. Single cells with electroless anode exhibit a current density of 2.5 A cm⁻² compared to cells with conventional anode (1.7A cm⁻²) at 800 °C, 0.7 V.     

Anti-carbon deposition mechanism of H2O on nickel-based anode of SOFC: A ReaxFF molecular modelling

Carbon deposition occurs when Dimethyl ether (DME) fuel is used for SOFC, leading to battery degradation. In order to study the effect of water addition on carbon deposition, this work used reactive force field molecular dynamics (Reaxff MD) to simulate the process of carbon deposition with or without water addition, and analyze its anti-carbon deposition mechanism on nickel-based anode.It is found that the number of carbon atoms on nickel can be effectively reduced by mixing water with fuel. As the H₂O/DME ratio increases, there are fewer carbon atoms on the nickel anode. And there are two main ways for water molecules to resist carbon deposition. First is that the OH group generated by decomposition of water molecules at high temperature reacts with CH component to form aldehyde group, which reduces the formation of carbon deposition precursor. The other is that the increase of water molecules introduces more oxygen atoms into the system, and the carbon atoms formed by DME molecules combine with oxygen atoms to form CO, thus reducing carbon deposition. This study is helpful to promote the industrialization of DME as SOFC fuel.     

A comparative study of copper-cermet anode material synthesized by different technique

The present work is focused on the comparative analysis of electrochemical and structural properties of anode materials for solid oxide fuel cells (SOFCs) and the influence of factors affected on electrode performance. The Cu_0.5Ce_0.5O_2−δ was prepared by Citrate–Nitrate route (CNP) and its formation is confirmed by XRD. The crystallite size of anode materials decreases with change of synthesis route. The highest conductivity is found to be 3.7 × 10⁻² and 5.2 × 10⁻² S cm⁻² at 660 °C before and after reduction for CNP with suitable mechanical strength. The electrochemical performance of anode/electrolyte/anode interface of Cu_0.5Ce_0.5O_2−δ is studied after reduction in presence of gas mixture (10%H₂ + 90%N₂) using electrochemical impedance spectroscopy. The conductivity for the Cell-800 prepared by CNP in presence of gas (10%H₂ + 90%N₂) shows lowest activation energy 1.28 eV. Thus, CNP is most promising method for obtaining the suitable anode material for the application of SOFC than Urea–Nitrate Process (UNP) and Glycine–Nitrate Process (GNP).     

Effect of magnetron sputtered anode functional layer on the anode-supported solid oxide fuel cell performance

Nickel oxide-yttria stabilized zirconia (NiO-YSZ) thin films were reactively sputter-deposited by pulsed direct current magnetron sputtering from the Ni and ZrY targets onto heated commercial NiO-YSZ substrates. The microstructure and composition of the deposited films were investigated with regard to application as thin anode functional layers (AFLs) for solid oxide fuel cells (SOFCs). The pore size, microstructure and phase composition of both as-deposited and annealed at 1200 °C for 2 h AFLs were studied by scanning electron microscopy and X-ray diffractometry and controlled by changing the deposition process parameters. The results show that annealing in air at 1200 °C is required to improve structural homogeneity of the films. NiO-YSZ films have pores and grains of several hundred nanometers in size after reduction in hydrogen. Adhesion of deposited films was evaluated by scratch test. Anode-supported solid oxide fuel cells with the magnetron sputtered anode functional layer, YSZ electrolyte and La_0.6Sr_0.4Co_0.2Fe_0.8O₃/Ce_0.9Gd_0.1O₂ (LSCF/CGO) cathode were fabricated and tested. Influence of thin anode functional layer on performance of anode-supported SOFCs was studied. It was shown that electrochemical properties of the single fuel cells depend on the NiO volume content in the NiO-YSZ anode functional layer. Microstructural changes of NiO-YSZ layers after nickel reduction-oxidation (redox) cycling were studied. After nine redox cycles at 750 °C in partial oxidation conditions, the cell with the anode NiO-YSZ layer showed stable open circuit voltage values with the power density decrease by 11% only.     

Numerical analysis of electrochemical characteristics and heat/species transport for planar porous-electrode-supported SOFC

In this work, a fully three-dimensional mathematical model for planar porous-electrode-supported (PES) solid oxide fuel cell (SOFC) has been constructed to simulate the steady state electrochemical characteristics and multi-species/heat transport. The variation of chemical species concentrations, temperature, potential, current and current density for two types of PES-SOFC developed by central research institute of electric power industry (CRIEPI) of Japan are studied in the co-flow pattern. In the numerical computation, the governing equations for continuity, momentum, mass, energy and electrical charge conservation are solved simultaneously using the finite volume method. Activation, ohmic, and concentration polarizations are considered as the main sources of irreversibility. The Butler–Volmer equation, Ohm's law, and Darcy's gas model with constant porosity and permeability are used to determine the polarization over-potential, respectively. The output voltages measured in experiments and calculated using the above models agree well. For the cell using the same material and manufacturing process, the results show the type-II PES-SOFC is with better performance. However, the electrolyte of type-II PES-SOFC should be with higher maximum ionic conductivity. Furthermore, these results will be used to evaluate the overall performance of a PES-SOFC stack, and to significantly help optimize their design and operation in practical applications.     

Two-dimensional numerical study of temperature field in an anode supported planar SOFC: Effect of the chemical reaction

In the present work the effect of the chemical reaction on the temperature field in an anode supported planar SOFC is numerically studied by the aid of a two-dimensional mathematical model. For the model development the mass transport phenomena, the energy conservation, the species flow governed by Darcy’s law and the electrochemistry are coupled. The finite difference method is used to solve numerically the system of the equations.The temperature field within each component of the SOFC (interconnection, cathode, anode and electrolyte) is calculated via the mathematical model which is implemented in FORTRAN language. The model results demonstrate the effect of different expressions of the chemical heat source, expressed in terms of enthalpy and entropy, on the temperature field and the location of the higher temperatures that occur within the SOFC during its operation.     

Effect of nanostructured anode functional layer thickness on the solid-oxide fuel cell performance in the intermediate temperature

Effect of anode functional layer thickness on the performance of solid-oxide fuel cells (SOFCs) has been investigated in the intermediate temperatures of 600–650 °C. Three types of cells with different thickness (0, 4, 10 micron) of nanostructured anode functional layer (AFL) consisting of Ni-ScSZ (Scandia stabilized zirconia) are prepared. The SOFCs consist of Ni-3YSZ (3 mol% yttria stabilized zirconia) anode tube support with the AFL, ScSZ electrolyte, and LSCF (lanthanum strontium cobalt ferrite) and GDC (gadolinium doped ceria) mixture cathode. It is shown that the performance of the cell is improved as the thickness of the anode functional layer increases. Power densities of the cell with 10 micron thick AFL at 600 and 650 °C are shown to be 0.22 and 0.27 W/cm² at 0.75 V, respectively. According to impedance spectroscopy, improvement of both ohmic and polarization resistances has been observed by increasing the thickness of the AFL, suggesting that the AFL also acts as a better contact layer between the electrolyte and the anode support, and the effectiveness of the AFL by optimizing the thickness.     

Influence of anode diffusion layer on the performance of a liquid feed direct methanol fuel cell by AC impedance spectroscopy

Effect of anode diffusion layer over the performance of the liquid feed direct methanol fuel cell has been studied by AC impedance spectroscopy. The anode employed comprises of the catalyst layer and diffusion layer. The latter comprises of backing layer and catalyst‐supporting layer. The supporting layer is present in between the backing layer and the catalyst layer. The composition of the supporting layer is optimized based on the information obtained from polarization and AC impedance measurements. Among the three types of carbons (Black pearl 2000, Vulcan XC‐72, Shawinigan acetylene black), Black pearls 2000 is found to be the ideal type of carbon used in the supporting layer. The optimized loading compositions of carbon, Nafion and PTFE in the supporting layer are reported to be 3 mg cm^?2, 10 wt%, and 0 wt%, respectively. These values are rationalized on the basis of the transport of methanol and carbon dioxide and the crossover of methanol from the anode to the cathode. Copyright © 2006 John Wiley & Sons, Ltd.     

The effect of heavy tars (toluene and naphthalene) on the electrochemical performance of an anode-supported SOFC running on bio-syngas

The effect of heavy tar compounds on the performance of a Ni-YSZ anode supported solid oxide fuel cell was investigated. Both toluene and naphthalene were chosen as model compounds and tested separately with a simulated bio-syngas. Notably, the effect of naphthalene is almost negligible with pure H₂ feed to the SOFC, whereas a severe degradation is observed when using a bio-syngas with an H₂:CO = 1. The tar compound showed to have a remarkable effect on the inhibition of the WGS shift-reaction, possibly also on the CO direct electro-oxidation at the three-phase-boundary. An interaction through adsorption of naphthalene on nickel catalytic and electrocatalytic active sites is a plausible explanation for observed degradation and strong performance loss. Different sites seem to be involved for H₂ and CO electro-oxidation and also with regard to catalytic water gas shift reaction. Finally, heavy tars (C ≥ 10) must be regarded as a poison more than a fuel for SOFC applications, contrarily to lighter compounds such benzene or toluene that can directly reformed within the anode electrode. The presence of naphthalene strongly increases the risk of anode re-oxidation in a syngas stream as CO conversion to H₂ is inhibited and also CH₄ conversion is blocked.     

Viable image analyzing method to characterize the microstructure and the properties of the Ni/YSZ cermet anode of SOFC

The quantitative analysis of the microstructure and related physicochemical properties of the anode substrate are essential for the identification of the optimum fabrication conditions for the best performance of the SOFC unit cell. However it is not easy to characterize the correlation of the microstructure and the property of the Ni/YSZ cermet anode due to its very complicated structural features. Moreover, it is not a simple matter to differentiate all of the phases in the Ni/YSZ anode via simple microscopic observations. In this study, we developed an image analyzing method to differentiate each constituent phase of the Ni/YSZ cermet, enabling us to investigate the microstructural evolution of the Ni/YSZ anode and the characterization of the correlation between the microstructure and the properties of the Ni/YSZ anode.