Modeling of a Tubular‐SOFC: The Effect of the Thermal Radiation of Fuel Components and CO Participating in the Electrochemical Process

A mathematical model based on first principles is developed to study the effect of heat and electrochemical phenomena on a tubul solid oxide fuel cell (SOFC). The model accounts fordiffusion, inherent impedance, transport (momentum, heat and mass transfer) processes, internal reforming/shifting reaction, electrochemical processes, and potential losses (activation, concentration, and ohmic losses). Thermal radiation of fuel gaseous components is considered in detail in this work in contrast to other reported work in the literature. The effect of thermal radiation on SOFC performance is shown by comparing with a model without this factor. Simulation results indicate that at higher inlet fuel flow pressures and also larger SOFC lengths the effect of thermal radiation on SOFC temperature becomes more significant. In this study, the H₂ and CO oxidation is also studied and the effect of CO oxidation on SOFC performance is reported. The results show that the model which accounts for the electrochemical reaction ofCO results in better SOFC performance than other reported models. This work also reveals that at low inlet fuel flow pressures the CO and H₂ electrochemical reactions are competitive and significantly dependent on the CO/H₂ ratio inside the triple phase boundary.     

Experimental Investigations and Modeling of Direct Internal Reforming of Biogases in Tubular Solid Oxide Fuel Cells

Biogas‐fed Solid Oxide Fuel Cell (SOFC) systems can be considered as interesting integrated systems in the framework of distributed power generation. In particular, bio‐methane and bio‐hydrogen produced from anaerobic digestion of organic wastes represent renewable carbon‐neutral fuels for high efficiency electrochemical generators. With such non‐conventional mixtures fed to the anode of the SOFC, the interest lies in understanding the multi‐physics phenomena there occurring and optimizing the geometric and operation parameters of the SOFC, while avoiding operating and fuel conditions that can lead to or accelerate degradation processes. In this study, an anode‐supported (Ni‐YSZ) tubular SOFC was considered; the tubular geometry enables a relatively easy separation of the air and fuel reactants and it allows one to evaluate the temperature field of the fuel gas inside the tube, which is strictly related to the electrochemical and heterogeneous chemical reactions occurring within the anode volume. The experiments have been designed to analyze the behavior of the cell under different load and fuel utilization (FU) conditions, providing efficiency maps for both fuels. The experimental results were used to validate a multi‐physics model of the tubular cell. The model showed to be in good agreement with the experimental data, and was used to study the sensitive of some selected geometrical parameters modification over the cell performances.     

IT-SOFC阳极表面催化反应机理与传递过程的数值模拟与分析

固体氧化物燃料电池(SOFC)具有效率高、污染低、对燃料适应性好、功率大等特点。其性能与工作状态受发生在多孔阳极的化学反应与多种传递过程耦合的影响。基于流体力学方程组和多步基元化学反应模型,建立了描述上述耦合特性的三维数学模型,并自编程序求解分析。结果显示:重整反应主要发生在靠近通道进口的多孔阳极,表面成分Ni_s的覆盖率占70%～80%,其他主要表面成分为CO_s占20%～25%,H_s占6%,O_s占1.5%; Ni_s随工作温度升高而增加;加强吸附基元反应会提高燃料利用率和工作温度;渗透率增加会提高反应气体在多孔介质内的传递效果,但催化反应会因接触不充分而减弱。通过考虑基元反应机理研究表明,在微观层面,催化剂Ni利用率不高,催化反应受温度、化学反应速率常数、孔隙率等参数影响较大。     

The Effect of H2S on the Performance of SOFCs using Methane Containing Fuel

In recent years, the interest for using biogas derived from biomass as fuel in solid oxide fuel cells (SOFCs) has increased. To maximise the biogas to electrical energy output, it is important to study the effects of the main biogas components (CH₄ and CO₂), minor ones and traces (e.g. H₂S) on performance and durability of the SOFC. Single anode‐supported SOFCs with Ni–Yttria‐Stabilised‐Zirconia (YSZ) anodes, YSZ electrolytes and lanthanum‐strontium‐manganite (LSM)–YSZ cathodes have been tested with a CH₄–H₂O–H₂ fuel mixture at open circuit voltage (OCV) and 1 A cm^–2 current load (850 °C). The cell performance was monitored with electric measurements and impedance spectroscopy. At OCV 2–24 ppm H₂S were added to the fuel in 24 h intervals. The reforming activity of the Ni‐containing anode decreased rapidly when H₂S was added to the fuel. This ultimately resulted in a lower production of fuel (H₂ and CO) from CH₄. Applying 1 A cm^–2 current load, a maximum concentration of 7 ppm H₂S was acceptable for a 24 h period.     

Experimental Study of the Carbon Deposition from CH4 onto the Ni/YSZ Anode of SOFCs

A challenge in the operation of solid oxide fuel cells (SOFCs) with hydrocarbon fuels is the carbon deposition on the nickel/yttria‐stabilized zirconia (Ni/YSZ) anode. The Grabke‐type kinetic model has been proposed for the carbon formation based upon the assumption of elementary steps, which consist of a rate‐limiting dissociative chemisorption step and a stepwise dehydrogenation of the chemisorbed methyl group. This work experimentally studied the carbon formation on a SOFC Ni/YSZ anode exposed to CH₄+H₂ gas mixtures. Experiments were conducted with various gas compositions of CH₄/H₂ and temperatures in the range from 873 K to 1,123 K. The experimental results were used to determine a kinetic model that was applied to the SOFC operating environments. Based on the experimental data, the formula for the carbon formation rate that is dependent on the operating temperature and the gas compositions of CH₄/H₂ was established.     

Numerical Investigation into Thermofluidic and Electrochemical Characteristics of Tubular‐type SOFC

Three‐dimensional simulations based on a multi‐physics model are performed to examine the thermofluidic and electrochemical characteristics of a tubular, anode‐supported solid oxide fuel cell (SOFC). The simulations focus on the local transport characteristics of the cathode and anode gases and the distribution of the temperature field within the fuel cell. In addition, the electrochemical properties of the SOFC are systematically examined for a representative range of inlet gas temperatures and pressures. The validity of the numerical model is confirmed by comparing the results obtained for the correlation between the power density and the current density with the experimental results presented in the literature. Overall, the present results show that the performance of the tubular SOFC is significantly improved under pressurised conditions and a higher operating temperature.     

Spatial Distribution of Electrochemical Performance in a Segmented SOFC: A Combined Modeling and Experimental Study

Spatially inhomogeneous distribution of current density and temperature in solid oxide fuel cells (SOFC) contributes to accelerated electrode degradation, thermomechanical stresses, and reduced efficiency. This paper presents a combined experimental and modeling study of the distributed electrochemical performance of a planar SOFC. Experimental data were obtained using a segmented cell setup that allows the measurement of local current‐voltage characteristics, gas composition and temperature in 4 × 4 segments. Simulations were performed using a two‐dimensional elementary kinetic model that represents the experimental setup in a detailed way. Excellent agreement between model and experiment was obtained for both global and local performance over all investigated operating conditions under varying H₂/H₂O/N₂ compositions at the anode, O₂/N₂ compositions at the cathode, temperature, and fuel utilization. A strong variation of the electrochemical performance along the flow path was observed when the cell was operated at high fuel utilization. The simulations predict a considerable gradient of gas‐phase concentrations along the fuel channel and through the thickness of the porous anode, while the gradients are lower at the cathode side. The anode dominates polarization losses. The cell may operate locally in critical operating conditions (low H₂/H₂O ratios, low local segment voltage) without notably affecting globally observed electrochemical behavior.     

A Degradation Model for Solid Oxide Fuel Cell Anodes due to Impurities in Coal Syngas: Part I Theory and Validation

A new phenomenological one‐dimensional model is formulated to simulate the typical degradation patterns observed in solid oxide fuel cell (SOFC) anodes due to coal syngas contaminants such as arsenic (As) and phosphorous (P). The model includes gas phase diffusion and surface diffusion within the anode and the adsorption reactions on the surface of the Ni‐YSZ‐based anode. Model parameters such as reaction rate constants for the adsorption reactions are obtained through indirect calibration to match the degradation rates reported in the literature for arsine (AsH₃), phosphine (PH₃), hydrogen sulfide (H₂S), and hydrogen selenide (H₂Se) under accelerated testing conditions. Results from the model demonstrate that the deposition of the impurity on the Ni catalyst starts near the fuel channel/anode interface and slowly moves toward the active anode/electrolyte interface as observed in the experiments. Parametric studies performed at different impurity concentrations and operating temperatures show that the coverage rate increases with increasing temperature and impurity concentration, as expected. The calibrated model was then used for prediction of the performance curves at different impurity concentrations and operating temperatures. Good agreement is obtained between the predicted results and the experimental data reported in the literature.     

Thermodynamic analysis for a solid oxide fuel cell with direct internal reforming fueled by ethanol

In the present study, a detailed thermodynamic analysis is carried out to provide useful information for the operation of solid oxide fuel cells (SOFC) with direct internal reforming (DIR) fueled by ethanol. Equilibrium calculations are performed to find the ranges of inlet steam/ethanol (H₂O/EtOH) ratio where carbon formation is thermodynamically unfavorable in the temperature range of 500-1500 K. Two types of fuel cell electrolytes, i.e., oxygen-conducting, and hydrogen-conducting electrolytes, are considered. The key parameters determining the boundary of carbon formation are temperature, type of solid electrolyte and extent of the electrochemical reaction of hydrogen. The minimum H₂O/EtOH ratio for which the carbon formation is thermodynamically unfavored decreases with increasing temperature. The hydrogen-conducting electrolyte is found to be impractical for use, due to the tendency for carbon formation. With a higher extent of the electrochemical reaction of hydrogen, a higher value of the H₂O/EtOH ratio is required for the hydrogen-conducting electrolyte, whereas a smaller value is required for the oxygen-conducting electrolyte. This difference is due mainly to the water formed by the electrochemical reaction at the electrodes.     

Evaluation of a Single Cell and Candidate Materials with High Water Content Hydrogen in a Generic Solid Oxide Fuel Cell Stack Test Fixture,Part II: Materials and Interface Characterization

A generic solid oxide fuel cell (SOFC) test fixture was developed to evaluate candidate materials under realistic operating conditions. A commercial 50 mm × 50 mm NiO‐YSZ anode‐supported thin YSZ electrolyte cell with lanthanum strontium manganite (LSM)/YSZ cathode was tested to evaluate the stability of candidate materials. The cell was tested in two stages at 800°C: stage I with low (~3% H₂O) humidity and stage II with high (~30% H₂O) humidity hydrogen fuel in constant voltage or constant current mode. Part I of the work, published previously, provided information regarding the generic test fixture design, materials, cell performance, and optical post‐mortem analysis. In part II, detailed microstructure and interfacial characterizations are reported regarding the SOFC candidate materials: (Mn,Co)‐spinel conductive coating, alumina coating for sealing area, ferritic stainless steel interconnect, refractory sealing glass, and their interactions with each other. Overall, the (Mn,Co)‐spinel coating was very effective in minimizing Cr migration. No Cr was identified in the cathode after 1720 h at 800°C. Aluminization of metallic interconnects also proved to be chemically compatible with alkaline‐earth silicate sealing glass. The details of interfacial reaction and microstructure development are discussed.     

Electrochemical Performance of a Ni and YSZ Composite Synthesised by Ultrasonic Spray Pyrolysis as an Anode for SOFCs

The electrochemical performance of an anode material for a solid oxide fuel cell (SOFC) depends highly on microstructure in addition to composition. In this study, a NiO–yttria‐stabilised zirconia (NiO–YSZ) composite with a highly dispersed microstructure and large pore volume/surface area has been synthesised by ultrasonic spray pyrolysis (USP) and its electrochemical characteristics has been investigated. For comparison, the electrochemical performance of a conventional NiO–YSZ is also evaluated. The power density of the zirconia electrolyte‐supported SOFC with the synthesised anode is ∼392 mW cm^–2 at 900 °C and that of the SOFC with the conventional NiO–YSZ anode is ∼315 mW cm^–2. The improvement is ∼24%. This result demonstrates that the synthesised NiO–YSZ is a potential alternative anode material for SOFCs fabricated with a zirconia solid electrolyte.     

Fuel processing in direct propane solid oxide fuel cell and carbon dioxide reforming of propane over Ni-YSZ

This work is aimed at understanding the reaction mechanism of propane internal reforming in the solid oxide fuel cell (SOFC). This mechanism is important for the design and operation of SOFC internal processing of hydrocarbons. An anode-supported SOFC unit with Ni-YSZ anode operating at 800 °C was tested with direct feeding of 5% propane. CO₂ reforming of propane was carried out in a reactor with Ni-YSZ catalyst to simulate internal propane processing in SOFC. The performance of this direct propane SOFC is stable. The C specie formed over the anode functional layer of SOFC can be completely removed. The major gas products of SOFC are H₂, CO, CH₄, C₂H₄ and CO₂. Pseudo-steady-state internal processing of propane in the anode catalytic layer of SOFC is associated with a CO₂/C₃H₈ molar ratio of about 1.26 and basically CO₂ reforming of propane. CO₂ dissociation to produce the O species to oxidize the C species from dehydrogenation and dissociation of propane and its fragments should be the major reaction during CO₂ reforming of propane.     

Implications for Using Biogas as a Fuel Source for Solid Oxide Fuel Cells: Internal Dry Reforming in a Small Tubular Solid Oxide Fuel Cell

The feasibility of operating a solid oxide fuel cell (SOFC) on biogas has been studied over a wide compositional range of biogas, using a small tubular solid oxide fuel cell system operating at 850 °C. It is possible to run the SOFC on biogas, even at remarkably low levels of methane, at which conventional heat engines would not work. The power output varies with methane content, with maximum power production occurring at 45% methane, corresponding to maximal production of H₂ and CO through internal dry reforming. Direct electrocatalytic oxidation of methane does not contribute to the power output of the cell. At higher methane contents methane decomposition becomes significant, leading to increased H₂ production, and hence transiently higher power production, and deleterious carbon deposition and thus eventual cell deactivation.     

Electrochemical Analysis of a System Based on W and Ni Combined with CeO2 as Potential Sulfur‐tolerant SOFC Anode

A formulation of tungsten and nickel combined with CeO₂ (WNi‐Ce) was prepared and evaluated as sulfur‐tolerant anode for SOFC at intermediate temperature. Structural and morphological changes that take place in the system upon interactions with hydrogen sulfide were analyzed. The electrochemical performance was tested in a single cell, WNi‐Ce/LDC/LSGM/LSFC, varying H₂S concentration (0–500 ppm) at 750 °C using I–V curves, impedance spectroscopy and load demands. The highest cell performance was reached in H₂ and decrease with H₂S content increase in the fuel from 226 mW cm⁻² in pure H₂ to 108 mW cm⁻² in 500 ppm H₂S/H₂. Essentially, no decay in the cell performance was observed in the several short‐term load tests studied under several H₂S concentration (0–500 ppm) during 1h, and even in 500 ppm H₂S/H₂ during 70 h, indicating that this material could be a potential sulfur‐tolerant anode.     

Direct Applicability of La0.6Sr0.4CoO3 – δ Thin Film Cathode to Yttria Stabilised Zirconia Electrolytes at T ≤ 650 °C

For investigating the direct applicability of highly active cobalt containing cathodes on YSZ electrolytes at a lower processing and operating temperature range (T ≤ 650 °C), we fabricated a thin film lanthanum strontium cobalt oxide (LSC) cathode on an yttria stabilised zirconia (YSZ)‐based solid oxide fuel cell (SOFC) via pulsed laser deposition (PLD). Its electrochemical performance (5.9 mW cm^–2 at 0.7 V, 650 °C) was significantly inferior to that (595 mW cm^–2 at 0.7 V, 650 °C) of an SOFC with a thin (t ∼ 200 nm) gadolinium doped ceria (GDC) buffer layer in between the LSC thin film cathode and the YSZ electrolyte. It implies that even though the cathode processing and cell operating temperatures were strictly controlled not to exceed 650 °C, the direct application of LSC on YSZ should be avoided. The origin of the cell performance deterioration is thoroughly studied by glancing angle X‐ray diffraction (GAXRD) and transmission electron microscopy (TEM), and the decomposition of the cathode and diffusion of La and Sr into YSZ were observed when LSC directly contacted YSZ.     

Propane conversion over a Ru/CGO catalyst and its application in intermediate temperature solid oxide fuel cells

A Ru/CGO catalyst was investigated in combination with a Cu current collector for the direct electro-oxidation and internal reforming of propane in a solid oxide fuel cell. The electrochemical power densities for the direct oxidation were larger than in the internal reforming process at 750 °C. The electrochemical performance in the presence of propane was significantly affected by the polarization resistance which was about three times larger than that obtained for the SOFC fed with hydrogen at 750 °C. However, out-of-cell steam reforming tests showed a C₃H₈ conversion to syngas approaching 90% at 800 °C. Thus, significant enhancements may be achieved by properly optimizing the anode structure. No formation of carbon deposits was observed both upon operation of the anode in the direct oxidation and internal reforming processes at 750 °C.     

Alternative concept for SOFC with direct internal reforming operation: Benefits from inserting catalyst rod

Mathematical models of direct internal reforming solid oxide fuel cell (DIR‐SOFC) fueled by methane are developed using COMSOL® software. The benefits of inserting Ni‐catalyst rod in the middle of tubular‐SOFC are simulated and compared to conventional DIR‐SOFC. It reveals that DIR‐SOFC with inserted catalyst provides smoother temperature gradient along the system and gains higher power density and electrochemical efficiency with less carbon deposition. Sensitivity analyses are performed. By increasing inlet fuel flow rate, the temperature gradient and power density improve, but less electrical efficiency with higher carbon deposition is predicted. The feed with low inlet steam/carbon ratio enhances good system performances but also results in high potential for carbon formation; this gains great benefit of DIR‐SOFC with inserted catalyst because the rate of carbon deposition is remarkably low. Compared between counter‐ and co‐flow patterns, the latter provides smoother temperature distribution with higher efficiency; thus, it is the better option for practical applications. © 2009 American Institute of Chemical Engineers AIChE J, 2010     

Co‐Generation of Electric Power and Carbon Nanotubes from Dimethyl Ether (DME)

Coking occurs easily and would significantly degrade the electrochemical performance for hydrocarbon‐fueled solid oxide fuel cells (SOFCs). Here, we report an integrated device combining a SOFC and a stainless steel tubing as catalyst for hydrocarbon pyrolysis in the upstream of the fuel cell. Considerable carbon nanotubes (CNTs) are grown around the outside of the stainless steel tubing when dimethyl ether (DME) is fed to the device, which dramatically reduces the C:O ratio in the fuel reaching the cell anode. Correspondingly, the carbon‐removal reforming significantly prolongs the performance stability of the fuel cell compared to that directly fueled by DME. The present results suggest that hydrocarbons can be utilized more efficiently and economically by combining a SOFC and a fix‐bed reactor containing catalysts for CNTs formation, accompanying with the co‐generation of electric power and CNTs.     

Efficiently enhance the proton conductivity of YSZ-based electrolyte for low temperature solid oxide fuel cell

Yttrium stabilized zirconia (YSZ) as a typical oxygen ionic conductor has been widely used as the electrolyte for solid oxide fuel cell (SOFC) at the temperature higher than 1000 °C, but its poor ionic conductivity at lower temperature (500–800 °C) limits SOFC commercialization. Compared with oxide ionic transport, protons conduction are more transportable at low temperatures due to lower activation energy, which delivered enormous potential in the low-temperature SOFC application. In order to increase the proton conductivity of YSZ-based electrolyte, we introduced semiconductor ZnO into YSZ electrolyte layer to construct heterointerface between semiconductor and ionic conductor. Study results revealed that the heterointerface between ZnO and YSZ provided a large number of oxygen vacancies. When the mass ratio of YSZ to ZnO was 5:5, the fuel cell achieved the best performance. The maximum power density (P_max) of this fuel cell achieved 721 mW cm^?2 at 550 °C, whereas the P_max of the fuel cell with pure YSZ electrolyte was only 290 mW cm^?2. Further investigation revealed that this composite electrolyte possessed poor O^2? conductivity but good proton conductivity of 0.047 S cm^?1 at 550 °C. The ionic conduction activation energy of 5YSZ-5ZnO composite in fuel cell atmosphere was only 0.62 eV. This work provides an alternative way to improve the ionic conductivity of YSZ-based electrolytes at low operating temperatures.