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.     

Impact of ‘redox’ cycles on performances of solid oxide fuel cells: Case of the electrolyte supported cells

Nickel with yttria stabilised zirconia (Ni-YSZ) is usually used as anode material for the solid oxide fuel cells (SOFCs) technology. Upon cyclic reduction and oxidation of Ni-YSZ ceramic-metal (cermet), the induced cell degradation constitutes one of the main limitations to the SOFCs lifetime. In this study, the effect of anode reduction and oxidation cycling on typical electrolyte supported cell (ESC) has been investigated. The electrochemical degradation has been followed by impedance spectroscopy. The cell damage has been characterised after testing with scanning electron microscopy (SEM). It has been shown that cells can withstand several ‘redox’ cycles without major decrease in electrochemical performances. The ‘redox’ cycling induces only a slight degradation rate estimated to ∼70 mΩ cm² per cycle. The microstructural observations are found to be consistent with the impedance diagram analysis. Both reveal that the damage is restricted to bulk cermet microstructure change, whereas no cracking is initiated at the anode/electrolyte interface.     

Modeling of SOFC running on partially pre-reformed gas mixture

A two dimensional model is developed to study the transport and reaction processes in solid oxide fuel cells (SOFCs) fueled by partially pre-reformed gas mixture, considering the direct internal reforming (DIR) of methane and water gas shift (WGS) reaction in the porous anode of SOFC. Electrochemical oxidations of H₂ and CO fuels are both considered. The model consists of an electrochemical, a chemical model, and a computational fluid dynamics (CFD) model. Two chemical models are compared to examine their effects on SOFC modeling results. Different from the previous studies on hydrogen fueled SOFC, higher gas velocity is found to slightly decrease the performance of SOFC running on pre-reformed gas mixture, due to suppressed gas composition variation at a higher gas velocity. The current density distribution along the gas channels at an inlet temperature of 1173K is quite different from that at 1073K, as DIR reaction is facilitated at a higher temperature. It is also found that neglecting the electrochemical oxidation of CO can considerably underestimate the total current density of SOFC running on pre-reformed hydrocarbon fuels. An alternative method is proposed to numerically determine the open-circuit potential of SOFC running on hydrocarbon fuels. Electrochemical reactions are observed at open-circuit potentials.     

Effect of operating conditions and gas flow patterns on the system performances of IIR-SOFC fueled by methanol

Mathematical models of an indirect internal reforming solid oxide fuel cells (IIR-SOFC) fueled by methanol were developed to analyze the thermal coupling of the internal endothermic steam reforming with exothermic electrochemical reactions and predict the system performance. The simulations indicated that IIR-SOFC fueled by methanol can be well performed as autothermal operation, although slight temperature gradient occurred at the entrance of the reformer chamber. Sensitivity analysis of five important parameters (i.e. operating voltage, reforming catalyst reactivity, inlet steam to carbon ratio, operating pressure and flow direction) was then performed. The increase of operating voltage lowered the average temperature along the reformer chamber and improved the electrical efficiency, but it oppositely reduced the average current density. Greater temperature profile along the system can be obtained by applying the catalyst with lower reforming reactivity; nevertheless, the current density and electrical efficiency slightly decreased. By using high inlet steam to carbon ratio, the cooling spot at the entrance of the reformer can be reduced but both current density and electrical efficiency were decreased. Lastly, with increasing operating pressure, the system efficiency increased and the temperature dropping at the reformer chamber was minimized.     

Solid Oxide Fuel Cells damage mechanisms due to Ni-YSZ re-oxidation: Case of the Anode Supported Cell

The effects of Ni-YSZ cermet re-oxidation in anode supported Solid Oxide Fuel Cells (SOFCs) have been investigated. Damage mechanisms have been studied in both cases of direct oxidation in air (i.e., fuel shutdown) or by an ionic current (i.e., fuel starvation).Direct oxidation tests show that the electrolyte cracks for a conversion degree of Ni into NiO ranging between ∼58 and ∼71%. This failure mode has been modelled considering both the bulk expansion of the cermet induced by the transformation of the Ni phase and the change of mechanical stresses in the multilayered cell.In the case of fuel starvation, a thin layer of the cermet was electrochemically re-oxidised at 800 °C and then reduced under a hydrogen stream. This ‘redox’ cycle was repeated until the degradation of the cell. The evolution of the impedance diagrams recorded after each cycle suggests that the cermet damages in an area close to anode/electrolyte interface. The mechanical modelling states that a delamination can occur along the interface between the Anode Functional Layer (AFL) and the Anode Current Collector (ACC) substrate. This theoretical result confirms the experimental trends observed by impedance spectroscopy.     

Structural and electrical properties of Ni-YSZ cermet materials

Ceramic-metal composites (cermets) containing yttria-stabilized zirconia, YSZ, and Ni particles are commonly used as anode materials in solid oxide fuel cells. The long-term performance of fuel cells is strictly related to both the structural and electrical properties of anode materials. In order to achieve high mixed electrical conductivity and high activity of electrochemical reactions and hydrocarbon fuel reforming, it is necessary to select an appropriate chemical composition and a suitable method of preparation. Materials containing 8 mol% yttria-stabilized zirconia and Ni were prepared by means of two methods: co-precipitation and impregnation. The structure of the materials was characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) and porosity studies. The thermal expansion coefficient (TEC) was determined using the dilathometric method. Electrochemical impedance spectroscopy (EIS) and the Wagner polarization method were used to determine electrical conductivity and the electron transference numbers, respectively.     

Effect of PH3 poisoning on a Ni-YSZ anode-supported solid oxide fuel cell under various operating conditions

The Ni-YSZ anode-supported solid oxide fuel cell (SOFC) can generate electrical power by using coal-derived syngas as the fuel. However, trace contamination of phosphine (PH₃) in the syngas can cause irreversible degradation in cell performance. A series of tests at 10 ppm PH₃ in the fuel gas was carried out under a variety of operating conditions, viz, with/without electrochemical reaction in syngas and with/without H₂O in H₂ fuel at 750 °C, 800 °C and 850 °C. The poisoning effects were evaluated by both electrochemical methods and chemical analyses. The post-mortem analyses of the SOFC anode were performed by means of XRD, SEM/EDS, and XPS. The results show that the degradation rate is larger at the higher cell working temperature using syngas with PH₃ in a 200 h test though PH₃ is more reactive with Ni in the anode at lower working temperature and produces a secondary nickel phosphide (Ni_xP_y) phase. The dominant compositions of Ni_xP_y on the cell anode are Ni₅P₂ with the presence of H₂O, and Ni₁₂P₅ without the presence of H₂O. The production of Ni_xP_y can be generated on the cell anode using syngas or dry H₂ fuel with 10 ppm PH₃ contaminant. Further, the appearance of Ni_xP_y phases is independent of the electrochemical reactions in the cell.     

Thermal field under the effect of the chemical reaction of a direct internal reforming solid oxide fuel cell DIR-SOFC

The effects of direct internal reforming in a fuel cell solid oxide (SOFC) on thermal fields are studied by mathematical modeling. This study presents the thermal fields of a standard fuel cell (Ni-YSZ/YSZ/LSM) anode supported. This study is also made in the perpendicular plane at the flow of gases. The fuel cell is powered by air and fuel, CH₄, H₂, CO₂, CO and H₂O hence the birth of the phenomenon of direct internal reforming (DIR-SOFC). It is based on reforming chemical reactions, steam reforming reaction and water–gas shift reaction. The main purpose of this work is the visualization of temperature fields under the influence of global chemical reactions and the confirmation of the thermal behavior of this chemical reaction. The thermal fields are obtained by a computer program (FORTRAN).     

System design and process layout for a SOFC micro-CHP unit with reduced operating temperatures

Anode-supported cells (ASC) are considered as a promising SOFC technology for achieving higher power densities at significantly reduced operating temperatures. Thereby it is commonly expected to enhance both the profitability and durability of fuel cell systems in real world applications. In the collaborative project LOTUS, a micro-CHP system prototype will be developed and tested based on a novel ASC technology with an operating temperature of 650 °C. Extended system design considerations are required in the first place to establish a basis for the system development process. This paper reveals the initial system design investigations for the LOTUS system concept, thus addressing modeling approach and concluding design decisions.     

High-entropy alloy anode for direct internal steam reforming of methane in SOFC

High-entropy alloy (HEA) anode and reforming catalyst, supported on gadolinium-doped ceria (GDC), have been synthesized and evaluated for the steam reforming of methane under SOFC operating conditions using a conventional fixed-bed catalytic reactor. As-synthesized HEA catalysts were subjected to various characterization techniques including N₂ adsorption/desorption analysis, SEM, XRD, TPR, TPO and TPD. The catalytic performance was evaluated in a quartz tube reactor over a temperature range of 700–800 °C, pressure of 1 atm, gas hourly space velocity (GHSV) of 45,000 h^?1 and steam-to-carbon (S/C) ratio of 2. The conversion and H₂ yield were calculated and compared. HEA/GDC exhibited a lower conversion rate than those of Ni/YSZ and Ni/GDC at 700 °C, but showed superior stability without any sign of carbon deposition unlike Ni base catalyst. HEA/GDC was further evaluated as an anode in a SOFC test, which showed high electrochemical stability with a comparable current density obtained on Ni electrode. The SOFC reported low and stable electrode polarization. Post-test analysis of the cell showed the absence of carbon at and within the electrode. It is suggested that HEA/GDC exhibits inherent robustness, good carbon tolerance and stable catalytic activity,` which makes it a potential anode candidate for direct utilization of hydrocarbon fuels in SOFC applications.     

Effects of operating conditions on the performance of a micro-tubular solid oxide fuel cell (SOFC)

A parametric analysis is carried out to study the effects of the operating conditions on the performance and operation of a micro-tubular solid oxide fuel cell. The computational fluid dynamics model incorporates mass, momentum, species and energy balances along with ionic and electronic charge transfers. Effects of temperature, fuel flow rate, fuel composition, anode pressure and cathode pressure on fuel cell performance are investigated. Polarization curves are compared to allow an understanding of the effects of different operating conditions on the performance of the fuel cell. Effects of anode flow rate on fuel cell efficiency and fuel utilization are also investigated. Moreover, influence of operating temperature on the internal electronic current leaks is outlined. Temperature distributions, current density profiles and hydrogen mole fraction profiles are also utilized to have a better understanding of the spatial effects of operating parameters. It is predicted that at 550 °C, for an output current demand of 0.53 A cm⁻², fuel cell needs to generate 0.65 A cm⁻² ionic current density where the difference in these values is attributed to internal current leaks. On the other hand for temperatures lower than 500 °C, the effect of electronic leakage currents are not significant.     

Performance analysis of a SOFC under direct internal reforming conditions

This paper presents the performance analysis of a planar solid-oxide fuel cell (SOFC) under direct internal reforming conditions. A detailed solid-oxide fuel cell model is used to study the influences of various operating parameters on cell performance. Significant differences in efficiency and power density are observed for isothermal and adiabatic operational regimes. The influence of air number, specific catalyst area, anode thickness, steam to carbon (s/c) ratio of the inlet fuel, and extend of pre-reforming on cell performance is analyzed. In all cases except for the case of pre-reformed fuel, adiabatic operation results in lower performance compared to isothermal operation. It is further discussed that, though direct internal reforming may lead to cost reduction and increased efficiency by effective utilization of waste heat, the efficiency of the fuel cell itself is higher for pre-reformed fuel compared to non-reformed fuel. Furthermore, criteria for the choice of optimal operating conditions for cell stacks operating under direct internal reforming conditions are discussed.     

Ni-8YSZ cermet re-oxidation of anode supported solid oxide fuel cell: From kinetics measurements to mechanical damage prediction

The ‘redox’ tolerance of a typical anode supported cell was evaluated for three temperatures of re-oxidation. For this purpose, an experimental work has been coupled to a modelling approach to estimate the risk of electrolyte failure during re-oxidation. A special attention has been paid to take into account both (i) the heterogeneity of oxidation and (ii) the cermet visco-plasticity in operation.     

Investigation of praseodymium and samarium co-doped ceria as an anode catalyst for DIR-SOFC fueled by biogas

The Pr and Sm co-doped ceria (with up to 20 mol.% of dopants) compounds were examined as catalytic layers on the surface of SOFC anode directly fed by biogas to increase a lifetime and the efficiency of commercially available DIR-SOFC without the usage of an external reformer.The XRD, SEM and EDX methods were used to investigate the structural properties and the composition of fabricated materials. Furthermore, the electrical properties of SOFCs with catalytic layers deposited on the Ni-YSZ anode were examined by a current density-time and current density-voltage dependence measurements in hydrogen (24 h) and biogas (90 h). Composition of the outlet gasses was in situ analysed by the FTIR-based unit.It has been found out that Ce_0.9Sm_0.1O_2-δ and Ce_0.8Pr_0.05Sm_0.15O_2-δ catalytic layers show the highest stability over time and thus are the most attractive candidates as catalytic materials, in comparison with other investigated lanthanide-doped ceria, enhancing direct internal reforming of biogas in SOFCs.     

Structured catalyst supports and catalysts for the methane indirect internal steam reforming in the intermediate temperature SOFC

Open cell metal foams made from Ni, Fe–Cr steel and Ni–Al intermetallic were studied as candidate catalyst supports for the internal indirect methane steam reforming. All the samples exhibited good corrosive resistance during 500–1000 h testing in H₂–H₂O–Ar environment at 600 °C. NiO/8YSZ composite based catalysts doped with fluorite-like (Pr_0.3Ce_0.35Zr_0.35O₂) or perovskite-like (La_0.8Pr_0.2Mn_0.2Cr_0.8O₃) complex oxides with high lattice oxygen mobility and promoted with Pt or Ru were prepared and deposited on the foam-structure supports. Both good catalyst adhesion and stable catalyst performance were achieved in the case of the Ni–Al foam supported catalysts. The Fe–Cr support reacted with the catalytic active components resulting in fast catalyst deactivation. The foam supported catalyst performance was compared with the same catalyst prepared in a form of 0.25 mm fraction. Porous supports with different porosities were prepared by the metal foam deformation and the catalyst performance depending on the support porosity (75–95%) was studied.     

A reduced 1D dynamic model of a planar direct internal reforming solid oxide fuel cell for system research

A reduced 1D dynamic model of a planar direct internal reforming SOFC (DIR-SOFC) is presented in this paper for system research by introducing two simplifications. The two simplification strategies employed are called Integration and Average, respectively. The present model is evaluated with a detailed 1D SOFC model, which does not introduce the two simplifications, and a lumped parameter (i.e., 0D) SOFC model. Results show that under the operating conditions investigated the accuracy of the reduced model is not significantly compromised by the two simplifications in prediction of the outlet gas flow rates and molar fractions, the outlet temperatures, and the cell voltage, while its computational time is significantly decreased by them. Moreover, it is quite simple in form. Therefore, the reduced SOFC model is attractive for system research. Compared with the lumped model, the reduced SOFC model is an improvement with regard to accuracy because it takes into account the spatially distributed nature of SOFCs to a certain extent. The discretized node number for solving the reduced model can be taken as an adjustable parameter in modeling, and is determined according to specific modeling requirements.     

Impact of pore microstructure evolution on polarization resistance of Ni-Yttria-stabilized zirconia fuel cell anodes

Temperature induced degradation in Solid Oxide Fuel Cell (SOFC) Ni-YSZ anodes was studied using both impedance spectroscopy and three-dimensional tomography via Focused Ion Beam-Scanning Electron Microscopy. A 100 h anneal at 1100 °C caused a 90% increase in cell polarization resistance, which correlated with the observed factor of ∼2 reduction in the electrochemically active three-phase boundary (TPB) density. The TPB decrease was caused by a significant decrease in pore percolation, and a reduction in pore interfacial area due to pores becoming larger and more equiaxed. The anneal caused no measurable change in average Ni particle size; Ni coarsening was apparently highly constrained in these anodes due to the relatively large YSZ volume fraction and low pore volume.     

Direct utilization of carbonaceous fuels in multifunctional SOFC anodes for the electrosynthesis of chemicals or the generation of electricity

The difficulty associated with the direct utilization of anhydrous carbonaceous fuels in a solid oxide fuel cell (SOFC) was overcome with the development of new electrocatalysts for multifunctional anodes. Two approaches are herein proposed. In one case the SOFC anode was designed for the electrosynthesis of C₂-type hydrocarbons, co-generating thermal and electric energies from the electrochemical oxidative coupling of methane. It was composed of a new ceramic material, based on lanthanum aluminate, LaAl_0.50Mn_0.50O₃, produced by impregnation of nitrates through a porous electrolyte material skeleton. In the other case a SOFC anode composed of Cu–Zr_1-xCe_xO_2-δ–Al₂O₃ was designed for the direct utilization of anhydrous ethanol to promote the electrochemical oxidation of ethanol and/or of the products therefrom formed during its thermal decomposition. Both anode materials presented convenient microstructure, high mechanical stability, selective electrocatalytic activity for the electrosynthesis of chemicals or the generation of electricity and resistance to carbon coking and clogging.     

Catalytic steam reforming of methane,methanol, and ethanol over Ni/YSZ: The possible use of these fuels in internal reforming SOFC

This study investigated the possible use of methane, methanol, and ethanol with steam as a direct feed to Ni/YSZ anode of a direct internal reforming Solid Oxide Fuel Cell (DIR-SOFC). It was found that methane with appropriate steam content can be directly fed to Ni/YSZ anode without the problem of carbon formation, while methanol can also be introduced at a temperature as high as 1000 °C. In contrast, ethanol cannot be used as the direct fuel for DIR-SOFC operation even at high steam content and high operating temperature due to the easy degradation of Ni/YSZ by carbon deposition. From the steam reforming of ethanol over Ni/YSZ, significant amounts of ethane and ethylene were present in the product gas due to the incomplete reforming of ethanol. These formations are the major reason for the high rate of carbon formation as these components act as very strong promoters for carbon formation.     

Study on paper-structured catalyst for direct internal reforming SOFC fueled by the mixture of CH4 and CO2

Inorganic fiber network including YSZ fiber which acts as catalyst support was created by the simple paper-making process, and novel Ni-loaded paper-structured catalysts (PSCs) with excellent catalytic activity for the dry reforming of methane were designed and developed. The PSCs exhibited high fuel conversion comparable to the conventional powdered catalysts with less than one-tenth catalyst weights. The significant advantages of the PSCs are their high mechanical flexibility and material workability. So far, a functionally-graded catalytic reaction field which leads to uniform temperature distribution during biogas reforming resulting in stable operation of planar SOFC was successfully developed by the PSC array based on the kinetic simulation model built in this research.