Performance of an anode-supported solid oxide fuel cell with direct-internal reforming of ethanol

A theoretical study of a solid oxide fuel cell (SOFC) fed by ethanol is presented in this study. The previous studies mostly investigated the performance of ethanol-fuelled fuel cells based on a thermodynamic analysis and neglected the presence of actual losses encountered in a real SOFC operation. Therefore, the real performance of an anode-supported SOFC with direct-internal reforming operation is investigated here using a one-dimensional isothermal model coupled with a detailed electrochemical model for computing ohmic, activation, and concentration overpotentials. Effects of design and operating parameters, i.e., anode thickness, temperature, pressure, and degree of ethanol pre-reforming, on fuel cell performance are analyzed. The simulation results show that when SOFC is operated at the standard conditions (V = 0.65 V, T = 1023 K, and P = 1 atm), the average power density of 0.51 W cm⁻² is obtained and the activation overpotentials represent a major loss in the fuel cell, followed by the ohmic and concentration losses. An increase in the thickness of anode decreases fuel cell efficiency due to increased anode concentration overpotential. The performance of the anode-supported SOFC fuelled by ethanol can be improved by either increasing temperature, pressure, degree of pre-reforming of ethanol, and steam to ethanol molar ratio or decreasing the anode thickness and fuel flow rate at inlet. It is suggested that the anode thickness and operating conditions should be carefully determined to optimize fuel cell efficiency and fuel utilization.     

Dependence of polarization in anode-supported solid oxide fuel cells on various cell parameters

The performance of solid oxide fuel cells (SOFCs) is affected by various polarization losses, namely, ohmic polarization, activation polarization and concentration polarization. Under given operating conditions, these polarization losses are largely dependent on cell materials, electrode microstructures, and cell geometric parameters. Solid oxide fuel cells (SOFC) with yttria-stabilized zirconia (YSZ) electrolyte, Ni–YSZ anode support, Ni–YSZ anode interlayer, strontium doped lanthanum manganate (LSM)–YSZ cathode interlayer, and LSM current collector, were fabricated. The effect of various parameters on cell performance was evaluated. The parameters investigated were: (1) YSZ electrolyte thickness, (2) cathode interlayer thickness, (3) anode support thickness, and (4) anode support porosity. Cells were tested over a range of temperatures between 600 and 800 °C with hydrogen as fuel, and air as oxidant. Ohmic contribution was determined using the current interruption technique. The effect of these cell parameters on ohmic polarization and on cell performance was experimentally measured. Dependence of cell performance on various parameters was rationalized on the basis of a simple analytical model. Based on the results of the cell parameter study, a cell with optimized parameters was fabricated and tested. The corresponding maximum power density at 800 °C was ∼1.8 W cm⁻².     

High performance of direct methane-fuelled solid oxide fuel cell with samarium modified nickel-based anode

Natural gas is one of the most attractive fuels for solid oxide fuel cell (SOFC), while the anode activity for methane fuel has a great influence on the performance and stability of SOFC. Samarium is a good catalyst promoter for methane reforming. In this work, samarium is used to modify nickel catalyst, which results in small nickel oxide particles. The SmNi-YSZ (yttria-stabilized zirconia) anode has smaller particles and better interfacial contact between nickel and YSZ compared with conventional Ni-YSZ anode. The fine structure of SmNi-YSZ anode results in high activity for electrochemical oxidation of hydrogen and low polarization resistance of the cell. The performance of SmNi-YSZ anode cell with humidified methane as fuel is greatly improved, which is similar to that with hydrogen as fuel. The maximum power densities of SmNi-YSZ anode cell are 1.56 W cm⁻² for humidified hydrogen fuel and 1.54 W cm⁻² for humidified methane fuel at 800 °C. The maximum power density is increased by 221% when samarium is used to modify Ni-YSZ anode for humidified methane fuel at 650 °C. High cell performance results in good stability of SmNi-YSZ anode cell and the cell runs stably for more than 600 min for humidified methane fuel.     

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.     

Nanoporous nickel thin film anode optimization for low-temperature solid oxide fuel cells

Thin film solid oxide fuel cells (TF–SOFCs) having anode–substrate nanostructure that was optimized for the low-temperature operation were fabricated. Nickel thin film anodes with four different anode thicknesses were deposited on anodic aluminum oxide templates, nanoporous substrates having two different pore sizes, by the sputtering method. Subsequently, a yttria-stabilized zirconia (YSZ) electrolyte and platinum cathode were deposited on them, which completed the entire fuel cell structure. The anode nanostructure of fuel cells in six combinations was analyzed by the cross-sectional view, surface microscopy method, and three-dimensional morphology observation. Those investigations enabled the anode nanostructure to be identified, such as the anode porosity and the roughness of the interface between anodes and electrolytes. Then, the six TF–SOFCs were electrochemically characterized in a 500 °C operating environment. The maximum power densities were obtained through the i–V–P curves, and the highest performance of 294.1 mW/cm² was measured in the cell having a combination of 200 nm–sized porous aluminum anodic oxide (AAO) and 1200 nm–thick Ni anode. This showed up to 20.1% improvement over the other cells. EIS analysis showed that the optimized ohmic and faradaic resistance originated from each part of the unique TF–SOFC structure.     

Performance analysis of hollow fibre-based micro-tubular solid oxide fuel cell utilising methane fuel

Solid oxide fuel cell (SOFC) has been studied as one of the most amazing development in energy production that could work directly with hydrocarbon fuel without reforming procedure. This study was conducted to analyse the micro-tubular solid oxide fuel cell (MT-SOFC) in terms of its performance by utilising methane as the fuel, subsequently compared with hydrogen. MT-SOFC that was investigated in this work consisted of thin cathode layer, coated onto co-extruded anode/electrolyte dual-layer hollow fibre (HF); in which its anode was made of nickel (Ni), coupled with cerium-gadolinium oxide (CGO) as an electrolyte, whereas the cathode was lanthanum strontium cobalt ferrite (LSCF) and CGO. The physical analyses carried out were three-point bending test and scanning electron microscopy (SEM). X-ray diffraction (XRD) analysis was further conducted to examine the carbon deposition in HFs. In evaluating the performance of HFs, current-voltage (IV) measurement, as well as impedance analysis of various temperatures range from 500 °C to 700 °C were performed. Based on the results, the OCV, maximum power density and ohmic ASR of MT-SOFC exposed to methane fuel, were at 0.79 V, 0.22 W cm⁻² and 0.31 Ω cm²; compared to the other that was exposed to hydrogen fuel, recorded at 0.89 V, 0.67 W cm⁻² and 0.19 Ω cm² respectively. This indicates that there was a significant reduction in cell performance when methane was used as the fuel, due to the carbon deposition as proven by SEM, three-point bending and XRD.     

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.     

A direct flame solid oxide fuel cell for potential combined heat and power generation

A sealant-free solid oxide fuel cell (SOFC) micro-stack was successfully operated inside a liquefied petroleum gas (LPG) flame during cooking. This micro-stack consisted of 4 single cells with infiltrated La_0.75Sr_0.25Cr_0.5Mn_0.5O_3-δ (LSCM) based composite anodes, achieving an open circuit voltage of 0.92 V and a peak power density of 348 mW cm⁻². This performance is significantly better than that of stack with its cathode operation outside flame. The results confirmed that the perovskite oxide anode showed good properties of carbon-free, redox-stability, quick-start (less than 1 min) and successful operation under a wide range of oxygen partial pressure. For comparison, the conventional Ni/yttria-stabilized zirconia (Ni/YSZ) anode was prepared and tested under the same conditions, showing an open circuit voltage of 0.915 V and a peak power density of 366 mW cm⁻², but obvious carbon deposition, poor stability and slow/difficult-start. The direct flame SOFC (DFFC) with a new configuration and design has a potential for combined heat and power generation for many applications.     

Electrochemical study of a planar solid oxide fuel cell: Role of support structures

This paper presents a performance analysis of a planar solid oxide fuel cell (SOFC) with different support structures, i.e., electrode (anode and cathode) and electrolyte supports. An electrochemical model, taking into account structural and operational parameters and gas diffusion at the electrodes, is used to analyze the characteristics of the planar SOFC. Simulation results demonstrate that under cell operation at an intermediate temperature (1073 K), an anode-supported SOFC is superior to an electrolyte- and cathode-supported SOFC. Analysis of individual cell voltage loss indicates that ohmic loss dominates the performance of an electrolyte-supported SOFC whereas activation and ohmic overpotentials constitute the major loss in an electrode-supported counterpart. Sensitivity analyses of the anode-supported SOFC show that decreasing the electrolyte and anode thickness can improve cell performance. A decrease in operating temperature causes the cell to operate at a lower range of current density due to an increase in ohmic and activation overpotentials. Further, increasing the operating pressure and degree of pre-reforming reduces the concentration overpotential and thereby enhances cell performance.     

Electrochemical performance of a solid oxide fuel cell with an LSCF cathode under different oxygen concentrations

A new performance study has been performed on a commercially available anode supported planar SOFC containing an LSCF cathode. The SOFC cell is tested at different temperatures and different cathode gas compositions. The temperature and cathode gas dependence on the electrochemical performance is studied using voltage-current density curves and impedance spectroscopy at different cell voltages. The cell tested shows excellent performance at all temperatures and is not limited by diffusion losses for the tested conditions. This new study indicates that the cell impedance spectroscopy is comprised of at least four semicircles of which two are partially dependent on the cathode gas conditions. It was found that historical effects play a role in the impedance spectra, showing some scatter in the ohmic resistance as a function of applied voltage. The cell ohmic resistance decreases as the temperature increases and as the cathode gas conditions are switched from air to O₂-He mixture. However, the cell ohmic resistance under pure O₂ was found to be higher than the O₂-He mixture. In virtually all IS data, the cell ohmic resistance showed a maximum value around 0.8 V. The cell ohmic ASR shows that interfacial resistances are a significant portion of the total ohmic resistance. The total electrode polarization decreases as the temperature increases and as the cathode gas conditions are switched from air to O₂-He mixture and to pure O₂. Finally, the peak frequency of the largest semicircle observed at high frequency shows a linear dependence on the applied voltage in most cases. This behavior is related to the charge transfer that occurs in the high frequency range and indicates that the electrochemical reactions are occurring at faster rates as more current flows through the cell.     

A cost-effective process for fabrication of metal-supported solid oxide fuel cells

Metal-supported SOFC cells with Y₂O₃ stabilized ZrO₂ as the electrolyte were prepared by a low cost and simple process involving tape casting, screen printing and co-firing. The interfaces were well bonded after the reduction of NiO to Ni in the support and the anode. AC impedance was employed to estimate the cell polarizations under open circuit conditions. It was found that the electrode polarization resistance was high at low temperatures and became equivalent to the ohmic resistance at higher temperatures near 800°°C. The cell performance was evaluated with H₂ as the fuel and air as the oxidant, and maximum power density between 0.23 and 0.80  W/cm² was achieved in the temperature range of 650–800°C, which confirms the applicability of the cost-effective process in fabrication of metal-supported SOFC cells.     

Electrochemical behaviors of Y-doped La0.7Sr0.3CrO3−δ anode in sulfur-containing fuel SOFC

Fuel gas containing sulfur to feed solid oxide fuel cell is a challenge for extending the application of SOFC. Yttrium doped into La_xSr_1−xCrO₃ as potential anode tolerant to H₂S was investigated by XRD, XPS and electrochemical impedance spectra (EIS). Good sinter characteristic for (La,Y)_0.7Sr_0.3CrO_3−δ (LYSC) observed by SEM contributes to the low ohmic loss (high conductivity) in SOFC fueled by H₂(3%)–H₂S(1%). Maximum power density of 20 mW/cm² and open circuit voltage of 0.95 V for SOFC with LYSC can be obtained at 700 °C. The results by EIS indicate charge transfer loss in polarization resistance dominates in the total resistance, especially lower than 650 °C. Compared to ohmic loss, polarization resistance in LYSC is still the main cause to hinder the improvement of SOFC performance. Thus, LYSC with doped non-variant valence Y maintains good sulfur tolerance determined by XPS without improved electro-catalytic activity as EIS suggest.     

Anode supported solid oxide fuel cells (SOFC) by electrophoretic deposition

Solid oxide fuel cells (SOFC), with its ability to use hydrocarbon fuels and capability to offer highest efficiency, have attracted great attention in India in recent years as an alternative energy generation system for future. But a great deal of problems associated with SOFC is needed to be solved before it can find commercial application. The relatively high operating temperature of 800-1000 °C of SOFC imposes a stringent requirement on materials that significantly increases the cost of SOFC technology. Reducing the operating temperature of an SOFC to below 800 °C can reduce degradation of cell components, improve flexibility in cell design, and lower the material and manufacturing cost by the use of cheap and readily available materials such as ferritic stainless steel. The operating temperature can be reduced by two possible approaches: (i) developing alternative electrolyte materials with high ionic conductivity at lower temperature, and (ii) developing much thinner and denser electrolyte layer such that the ohmic losses are minimised.In this work we report the use of inexpensive Electrophoretic deposition (EPD) technique in making about 10 micron thin and dense YSZ electrolyte on NiO-YSZ substrate. The effect of different operating parameters such as applied voltage, deposition time etc have been optimised during deposition from YSZ suspension in acetylacetone. The YSZ/NiO-YSZ bi-layers were then co-sintered at 1450 °C for 5 h. The single SOFC cells were then fabricated by brush painting LSM:YSZ (50:50) paste on the electrolyte layer followed by sintering at 1200 °C for 2 h. The single SOFC cell when tested using H₂ as fuel and ambient air as oxidant exhibited an open circuit voltage (OCV) of 1.03 V and the peak power density of about 624 mW/cm² at 800 °C.     

Comparison of efficiencies of low, mean and high temperature fuel cell Systems

Fuel cell systems are always said to show high electrical efficiency. The results achieved up to now, however, differ considerably, especially between the various fuel cell types all using natural gas as fuel. With the presented study the reasons for the different results and general potentials for fuel cell systems are highlighted. For that purpose several system lay-out concepts were elaborated for PEFC, PAFC and SOFC.The performed energy balance calculations for eight different plant concepts (three PEFC, two PAFC and three SOFC) for steady state operation with methane revealed that because of external reforming PEFC and PAFC systems are limited to about 67% and 70%, respectively, for the fuel utilisation. High temperature fuel cells can achieve at least 80% because of the possibility of internal reforming, or even over 90% in case of anode off-gas recycling. In combination with a cell voltage which is about 100 mV lower than that of MCFC and planar SOFC, PEFC can only achieve 38% of electrical net efficiency, PAFC 42% and tubular SOFC 54%. The latter is similar to MCFC, which is operated at higher cell voltage but lower fuel utilisation. The highest efficiency with up to 63% can be achieved with planar SOFC systems, because this concept allows high fuel utilisation together with high cell voltages.     

Two-dimensional transient model of a cascaded micro-tubular solid oxide fuel cell fed with methane

A two-dimensional transient simulation model for a cascaded (10 cascades) micro-tubular solid oxide fuel cell (SOFC) and a common micro-tubular SOFC with a preheater tub was developed to calculate the distribution of gas species, the local current, voltage and temperature. The goal was to compare these two different SOFC designs under steady state and load change conditions with respect to power density. Steady state simulation results have shown that the cascaded cell concept is able to operate at higher average cell voltages in principle. Compared to the cascaded cell concept the average ohmic anodic and cathodic resistance of the common cell has to be 84 times lower to reach the same average power density at an average voltage of 0.7 V. The cascaded cell concept gives the opportunity to operate at a higher average voltage than the lowest Nernst voltage offers. This could be interesting to realize high fuel utilization (>85%). Transient simulation results have shown that both cell concepts respond within 10 s on an electrical load change. The cascaded cell concept has shown a lower temporary voltage drop than the common cell concept. This paper describes the theory, solution techniques and results.     

Model development for a SOFC button cell using H2S as fuel

In this paper we present a hierarchy of models built to describe the overall performance of a single H₂S fuelled button cell solid oxide fuel cell (SOFC). The cell, used in the experimental studies of Liu et al. [M. Liu, G. Wei, J. Luo, A.R. Sanger, K.T. Chuang, Use of metal sulfides as anode catalysts in H₂S–air SOFCs, J. Electrochem. Soc. 150 (2003) 1025–1029], was a planar cell with a circular disc-like electrode assembly and the fuel and air flowing through a concentric cylindrical tube assembly. The goal is to model the electrochemical reaction coupled with mass transfer, fluid flow and current/voltage distribution in an yttria stabilized zirconia electrolyte fuel cell assembly operated between 750 and 850 °C. The models built range in complexity from an algebraic system of equations that calculates the activation, concentration and ohmic losses, to a two-dimensional finite element model that solves all the physics in the SOFC simultaneously. Kinetic parameters in these (progressively more comprehensive) models have been estimated and compared, leading hopefully to more accurate estimates for these parameters.     

Analytical modeling of polarizations in a solid oxide fuel cell using biomass syngas product as fuel

An analytical model is developed to study fuel type effect on polarizations and performance of SOFC. We consider especially two types of fuel: pure hydrogen and syngas (mixture of H₂ and CO) produced by biomass gasification. The proposed model is based on simultaneous direct oxidation of H₂ and CO at the anode side and uses the dusty-gas model with appropriate diffusion coefficient (binary or mixture coefficient in porous material) to evaluate the concentration polarization and the Butler–Volmer equation to calculate the activation polarization when ohmic polarization is expressed by the well known Ohm’s law. Results analysis show that a fraction of CO of about 24% in syngas improves the performance of SOFC by 23% compared to that obtained by pure H₂.     

Microstructure and polarization characteristics of anode supported tubular solid oxide fuel cell with co-precipitated and mechanically mixed Ni-YSZ anodes

An anode support tubular solid oxide fuel cell (SOFC) is fabricated and the dependence of its polarization resistance on anode microstructural parameters is investigated by means of stereology and concept of contiguity (c-c) theory. Nickel yttria-stabilized zirconia (Ni-YSZ) anode supported cell with YSZ electrolyte, lanthanum-strontium-manganite (LSM)-YSZ composite cathode, and LSM cathode layers is fabricated by dip coating. Submicrometer resolution images of anode microstructure are successfully obtained by low voltage SEM-EDX and quantified by stereological analysis. Cell voltage measurements and impedance spectroscopy are performed at temperatures of 650 and 750 °C with hydrogen and nitrogen mixture gas as a fuel. A quantitative relationship between polarization resistance and microstructural parameters such as circularity, three-phase boundary length, contiguity, etc. is investigated using the concept of contiguity (c-c) theory. The effectiveness of correlating polarization resistance of anode supported tubular SOFC using stereology and c-c theory is evaluated.     

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.     

The impact of steam and current density on carbon formation from biomass gasification tar on Ni/YSZ, and Ni/CGO solid oxide fuel cell anodes

The combination of solid oxide fuel cells (SOFCs) and biomass gasification has the potential to become an attractive technology for the production of clean renewable energy. However the impact of tars, formed during biomass gasification, on the performance and durability of SOFC anodes has not been well established experimentally. This paper reports an experimental study on the mitigation of carbon formation arising from the exposure of the commonly used Ni/YSZ (yttria stabilized zirconia) and Ni/CGO (gadolinium-doped ceria) SOFC anodes to biomass gasification tars. Carbon formation and cell degradation was reduced through means of steam reforming of the tar over the nickel anode, and partial oxidation of benzene model tar via the transport of oxygen ions to the anode while operating the fuel cell under load. Thermodynamic calculations suggest that a threshold current density of 365 mA cm⁻² was required to suppress carbon formation in dry conditions, which was consistent with the results of experiments conducted in this study. The importance of both anode microstructure and composition towards carbon deposition was seen in the comparison of Ni/YSZ and Ni/CGO anodes exposed to the biomass gasification tar. Under steam concentrations greater than the thermodynamic threshold for carbon deposition, Ni/YSZ anodes still exhibited cell degradation, as shown by increased polarization resistances, and carbon formation was seen using SEM imaging. Ni/CGO anodes were found to be more resilient to carbon formation than Ni/YSZ anodes, and displayed increased performance after each subsequent exposure to tar, likely due to continued reforming of condensed tar on the anode.