Improvement of the internal reforming of metal-supported SOFC at low temperatures

Automotive Solid oxide fuel cells (SOFCs) require improvements in mechanical robustness, power generation at low temperatures, and system compactness. To address these issues, we attempt to improve the internal reformation of metal-supported SOFCs (MS-SOFCs) via catalyst infiltration. After introducing nickel/gadolinium-doped ceria (Ni/GDC) nanoparticles, power densities of 1.16 Wcm⁻² with hydrogen (3%H₂O) and 0.85 Wcm⁻² with methane (Steam-to-Carbon ratio, S/C = 1.0) are obtained at 600 °C, 0.7 V. This is the highest performance achieved in previous studies on MS-SOFCs. Internal reforming with various hydrocarbon is also demonstrated. In particular 0.64 Wcm⁻² at 600 °C, 0.7 V is obtained when the fuel is iso-octane. We develop a numerical model to separately analyze reforming and electrochemical reaction. Catalyst infiltration dramatically increases the number of active sites for steam reforming. In addition, ruthenium/gadolinium-doped ceria (Ru/GDC) should be suitable as a catalyst metal at low temperatures because of the lower activation energy of steam reforming.     

Durability test and degradation behavior of a 2.5 kW SOFC stack with internal reforming of LNG

Forschungszentrum Jülich has demonstrated SOFC stacks and systems ranging from 50 W to 20 kW. Previous studies have shown the reproducible stable long-term performance of the F10-design short stacks developed in Forschungszentrum Jülich. Within this work, a 2.5 kW F20-stack consisting of eighteen cells was assembled, and tested at a furnace temperature of 700 °C mainly with the simulated reformate gas, which corresponds to 10% pre-reforming of liquefied natural gas (LNG). The current density and fuel utilization were mostly kept at 0.5 A cm⁻² and 70%, respectively. The purpose was to investigate the behavior of the stack in the kW-range for at least 5000 h with internal reforming of LNG or methane at a fuel utilization of at least 60%. A voltage degradation rate of around 0.3%/1000 h was obtained during the operation with pre-reformed LNG. The stack performance under normal working conditions and an unplanned redox cycle, as well as the results from post mortem analysis are discussed.     

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.     

Modeling of IT-SOFC with indirect internal reforming operation fueled by methane: Effect of oxygen adding as autothermal reforming

Mathematical models of an Intermediate Temperature Solid Oxide Fuel Cell (IT-SOFC) with indirect internal reforming operation (IIR-SOFC) fueled by methane were developed. The models were based on a steady-state heterogeneous two-dimensional tubular-design SOFC. The benefit in adding oxygen to methane and steam as the feed for autothermal reforming reaction on the thermal behavior and SOFC performance was simulated. The results indicated that smoother temperature gradient with lower local cooling at the entrance of the reformer channel can be achieved by adding a small amount of oxygen. However, the electrical efficiency noticeably decreased when too high oxygen content was added due to the loss of hydrogen generation from the oxidation reaction; hence, the inlet oxygen to carbon (O/C) molar ratio must be carefully controlled. Another benefit of adding oxygen is the reduction of excess steam requirement, which could reduce the quantity of heat required to generate the steam and eventually increases the overall system performance. It was also found that the operating temperature strongly affects the electrical efficiency achievement and temperature distribution along the SOFC system. By increasing the operating temperature, the system efficiency increases but a significant temperature gradient is also detected. The system with a counter-flow pattern was compared to that with a co-flow pattern. The co-flow pattern provided smoother temperature gradient along the system due to better matching between the heat supplied from the electrochemical reaction and the heat required for the steam reforming reaction. However, the electrical efficiency of the co-flow pattern is lower due to the higher cell polarization at a lower system temperature.     

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.     

Kinetics of (reversible) internal reforming of methane in solid oxide fuel cells under stationary and APU conditions

The internal reforming of methane in a solid oxide fuel cell (SOFC) is investigated and modeled for flow conditions relevant to operation. To this end, measurements are performed on anode-supported cells (ASC), thereby varying gas composition (y_CO = 4–15%, y_H2=5−17%, y_CO2=6−18%, y_H2O=2−30%, y_CH4=0.1−20%) and temperature (600–850 °C). In this way, operating conditions for both stationary applications (methane-rich pre-reformate) as well as for auxiliary power unit (APU) applications (diesel-POX reformate) are represented. The reforming reaction is monitored in five different positions alongside the anodic gas channel by means of gas chromatography. It is shown that methane is converted in the flow field for methane-rich gas compositions, whereas under operation with diesel reformate the direction of the reaction is reversed for temperatures below 675 °C, i.e. (exothermic) methanation occurs along the anode. Using a reaction model, a rate equation for reforming could be derived which is also valid in the case of methanation. By introducing this equation into the reaction model the methane conversion along a catalytically active Ni-YSZ cermet SOFC anode can be simulated for the operating conditions specified above.     

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.     

Numerical study of thermoelectric characteristics of a planar solid oxide fuel cell with direct internal reforming of methane

A three-dimensional mathematical thermo-fluid model coupling the electrochemical kinetics with fluid dynamics was developed to simulate the heat and mass transfer in planar anode-supported solid oxide fuel cell (SOFC). The internal reforming reactions and electrochemical reactions of carbon monoxide and hydrogen in the porous anode layer were analyzed. The temperature, species mole fraction, current density, overpotential loss and other performance parameters of the single cell unit were obtained by a commercial CFD code (Fluent) and external sub-routine. Results show that the current density produced by electrochemical reactions of carbon monoxide cannot be ignored, the cathode overpotential loss is the biggest one among the three overpotential losses, and that the proper decrease of the operating voltage leads to the increase of the current density, PEN structure temperature, fuel utilization factor, fuel efficiency and power output of the SOFC.     

Direct methane solid oxide fuel cell working by gradual internal steam reforming: Analysis of operation

A solid oxide fuel cell was designed to be operated with pure hydrocarbons, without additive or carrier gas, in order to bring technological simplifications, cost reductions and to extend the fuel flexibility limits. The cell was built-up from a conventional cell (LSM/YSZ/Ni-YSZ), to which was added a Ir-CeO₂ catalyst layer at the anode side and an original current collecting system. The cell was first operated with steam in gradual internal reforming (GIR) conditions (R = [H₂O]/[CH₄] < 1) with carrier gas at the anode. The optimal operating parameters were determined in terms of flow rates, cell potential, and fuel utilisation. The cell was finally operated with pure dry methane at 900 °C at 0.6 V yielding current density of about 0.1 A cm⁻² at max power for 120 h. Small but abrupt deterioration of the performances was observed, but no carbon deposition. Electrical and chemical analysis of this degradation are provided.At total, the fuel cell was operated for more than 200 h in pure dry methane, demonstrating that gradual internal reforming actually occurred efficiently in the anode compartment, which make possible operation without reforming agent such as H₂O or CO₂ for other hydrocarbon fuels.     

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).     

Thermodynamic analysis and heat integration for hydrogen production from bio-butanol for SOFC application: Steam reforming vs. autothermal reforming

Thermodynamic analysis of hydrogen production by steam reforming and autothermal reforming of bio-butanol was investigated for solid oxide fuel cell applications. The effects of reformer operating conditions, e.g., reformer temperature, steam to carbon molar ratio, and oxygen to carbon molar ratio, were investigated with the objective to maximize hydrogen production and to reduce utility requirements of the process and based on which favorable conditions of reformer were proposed. Process flow diagram for steam reforming and autothermal reforming integrated with solid oxide fuel cell was developed. Heat integration with pinch analysis method was carried out for both the processes at favorable reformer conditions. Power generation, electrical efficiency, useful energy for co-generation application, and utility requirements for both the processes were compared.     

Assessment of the reactor network approach for integrated modelling of an SOFC system

Incorporation of Solid-oxide fuel cells (SOFC) into hybrid systems with CHP capabilities is an attractive option for clean and efficient decentralised electricity generation. SOFC system operation on practical liquid fuels requires an efficient preparation system for the formation of a homogeneous reformate mixture. This can be accomplished with the use of a stabilized cool flame vapouriser (SCFV) combined with a thermal partial oxidation (T-POX) reformer, and such systems are already under development. The successful and efficient thermochemical operation of an SOFC system requires an accurate determination of the optimum conditions for each constituent component (e.g. fuel processing unit, fuel cell stack, off-gas burner) and for the integrated system. The present work demonstrates a computational methodology for the thermochemical assessment of a novel SOFC system operated on liquid fuels. Simulations have been performed, both at component and system levels, using a reactor network approach, involving a simplified flow and mixing representation, while retaining full detailed chemistry. Computations are performed at a component level with reactor networks specially formulated for the SCFV and the T-POX reactors, derived on the basis of CFD calculations, coupled with detailed kinetic mechanisms for n-heptane, a reasonable diesel fuel surrogate. Model predictions are compared against experimental data, wherever possible. The individual components are integrated at a system level and parametric analyses are performed so as to determine optimum conditions for efficient and clean operation.     

Modelling of solid oxide fuel cells with internal glycerol steam reforming

The direct application of glycerol in solid oxide fuel cell (SOFC) for power generation has been demonstrated experimentally but the detailed mechanisms are not well understood due to the lack of comprehensive modeling study. In this paper, a numerical model is developed to study the glycerol-fueled SOFC. After model validation, the simulated SOFC demonstrates a performance of 7827 A m^?2 at 0.6 V, with a glycerol conversion rate of 49% at 1073 K. Then, parametric analyses are conducted to understand the effects of operation conditions on cell performance. It is found that the SOFC performance increases with decreasing operating voltage or increasing inlet temperature. However, increasing either the fuel flow rate or steam to glycerol ratio could decrease the cell performance. It is also interesting to find out that the contribution of H₂ and CO to the total current density is significantly different under various operating conditions, even sometimes CO dominates while H₂ plays a negative role. This is different from our conventional understanding that usually H₂ contributes more significantly to current generation. In addition, cooling measures are needed to ensure the long-term stability of the cell when operating at a high current density.     

Internal reforming SOFC running on biogas

Direct feeding of biogas to SOFC, which is derived from municipal organic wastes, has been investigated as a carbon-neutral renewable energy system. CH₄/CO₂ ratio in the actual biogas fluctuated between 1.4 and 1.9 indicating biogas composition is strongly affected by the kinds of organic wastes and the operational conditions of methane fermentation. Using anode-supported button cells, stable operation of biogas-fueled SOFC was achieved with the internal reforming mode at 800 °C. Cell voltage above 0.8 V was recorded over 800 h at 200 mA cm⁻². It has been revealed that air addition to actual biogas reduced the risk of carbon formation and led to more stable operation without compromising cell voltage due to the lowering of anodic overvoltage.     

Numerical analysis of an SOFC stack under loss of oxidant related fault conditions using a dynamic non-adiabatic model

This paper presents a numerical analysis of a 1000 W-class solid oxide fuel cell stack. The study includes simulation of dynamic operation of the unit under conditions which are qualified as faults. The simulation tool was developed to address the effects of oxidant-related faults on the operating parameters of the stack. Additionally, a control system was proposed in order to mitigate the effects of the sudden reduction in the flow of oxidant and passivation of the cells inside a 60-cell stack. In the current study, those occurrences were related to the loss of tightness of the sealants in the stack of planar cells. The model of an adiabatic-stack was used to generate the temperature profiles and was used in two reference cases. In the first case, the control system was activated in order to maintain the key parameters within the safe range, in the second case the simulations with deactivated controls enabled prediction of the temperature, voltage and power in the stack which continues operation without counteractions oriented toward minimizing the negative impacts on the performance due to exceeding the given limiting values of parameters. In the current study, two scenarios were analyzed: partial loss of oxidant and partial failure of stack modules resulting in decrease of the generated electric power. The results of both cases are presented, with and without the fault prevention control modules considered. Adjustment of the operating parameters can effectively limit the rapid increase in thermal gradients inside the stack. To complement the discussion, a classification of the typical faults of SOFC stack is presented.     

Transport phenomena in a steam-methanol reforming microreactor with internal heating

An experimental and theoretical study of steam reforming of methanol is carried out in a packed-bed microreactor with internal heating. Experimental results of the methanol conversion and carbon monoxide concentration in an internally heated reformer are compared with those of an externally heated reformer. Higher methanol conversion and carbon monoxide concentration are obtained for internal heating at the same conditions. The results show the conversion efficiency of methanol and CO concentration increase with increasing internal heating rate over the range of operating conditions. A correlation for the conversion efficiency of methanol has been obtained as a function of the internal heating rate and a dimensionless time parameter which represents the ratio of the characteristic time of the methanol flow to the time for chemical reaction.     

Ethanol internal steam reforming in intermediate temperature solid oxide fuel cell

This study investigates the performance of a standard Ni-YSZ anode supported cell under ethanol steam reforming operating conditions. Therefore, the fuel cell was directly operated with a steam/ethanol mixture (3 to 1 molar). Other gas mixtures were also used for comparison to check the conversion of ethanol and of reformate gases (H₂, CO) in the fuel cell. The electrochemical properties of the fuel cell fed with four different fuel compositions were characterized between 710 and 860 °C by I-V and EIS measurements at OCV and under polarization. In order to elucidate the limiting processes, impedance spectra obtained with different gas compositions were compared using the derivative of the real part of the impedance with respect of the natural logarithm of the frequency.Results show that internal steam reforming of ethanol takes place significantly on Ni-YSZ anode only above 760 °C. Comparisons of results obtained with reformate gas showed that the electrochemical cell performance is dominated by the conversion of hydrogen. The conversion of CO also occurs either directly or indirectly through the water-gas shift reaction but has a significant impact on the electrochemical performance only above 760 °C.     

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.     

Advanced tubular solid oxide fuel cells with high efficiency for internal reforming of hydrocarbon fuels

Solid oxide fuel cells (SOFCs) constitute an attractive power-generation technology that converts chemical energy directly into electricity while causing little pollution. NanoDynamics Energy (NDE) Inc. has developed micro-tubular SOFC-based portable power generation systems that run on both gaseous and liquid fuels. In this paper, we present our next generation solid oxide fuel cells that exhibit total efficiencies in excess of 60% running on hydrogen fuel and 40+% running on readily available gaseous hydrocarbon fuels such as propane, butane etc. The advanced fuel cell design enables power generation at very high power densities and efficiencies (lower heating value-based) while reforming different hydrocarbon fuels directly inside the tubular SOFC without the aid of fuel pre-processing/reforming. The integrated catalytic layered SOFC demonstrated stable performance for >1000 h at high efficiency while running on propane fuel at sub-stoichiometric oxygen-to-fuel ratios. This technology will facilitate the introduction of highly efficient, reliable, fuel flexible, and lightweight portable power generation systems.     

Thermodynamic analysis of direct internal reforming of methane and butane in proton and oxygen conducting fuel cells

We present results of a thermodynamic analysis of direct internal reforming fuel cells, based on either a proton conducting fuel cell (FC-H⁺) or an oxygen ion conducting fuel cell (FC-O²⁻). We analyze the option of methane as fuel as well as butane. The model self-consistently combines all chemical equilibria in both the anode and cathode compartments with the proton or oxygen transfer rates through the membrane without predefining fuel utilization.