Biodiesel formulations as fuel for internally reforming solid oxide fuel cell

Biodiesel (alkyl ester of rapeseed oil) is prepared using various, methyl, ethyl and butyl alcohols through the transesterification process. Sodium hydroxide and sulfuric acid are used as catalyst for methyl alcohol, ethyl alcohol and butyl alcohol respectively. Biodiesel-water formulations are formulated using water and emulsifiers like sodium lauryl sulphate (SLS) and SPAN 80 in a high shear mixer. The formulations are tested at 800 °C as fuel for internal reforming in solid oxide fuel cells (SOFCs). The formulations based on methyl and butyl esters require the use of emulsifiers to prepare stable emulsions, while ethyl esters are able to form stable emulsions without emulsifiers. The decrease in the biodiesel concentration of formulation does not have any effect on the power density of the ethyl ester formulation. Fuel cells fuelled with 20% formulations lasted longer than 50% formulations in all the formulations tested as result of increase in steam carbon ratio resulting in effective removal of carbon deposited on the anode surface. Butyl ester formulations exhibited the worst performance in both types of formulation tests. The best performance was exhibited by 20% ethyl formulation in terms of life of the cell but 50% methyl ester formulations exhibit the highest power density.     

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.     

Modeling the temperature field in the reforming anode of a button-shaped solid oxide fuel cell

A model predicting the temperature field in the porous reforming anode of a solid oxide fuel cell is presented herein. The model is based on mass, momentum, and heat balances of a chemically reacting mixture of gases within the porous matrix of the anode. The important novel characteristic of the model is the consideration of the both internal reforming and electrochemical reactions in the bulk of the porous anode. The electronic and ionic currents in the anodes are calculated utilizing the solution of the Poisson equations for the electric potentials in the porous medium. The transfer current density is described by the Butler–Volmer equation.The model is applied to investigate the temperature field and the reactive flow in button-shaped fuel cells with uniform and graded (multi-layer) anodes composed of Ni and YSZ particles with methane/water vapor mixture used as the fuel. The maximum temperature difference between the hot and cold spots of the anodes is found to reach up to 200 K. The results indicate that the generation of Joule heating caused by the current passing through the anode and the activation losses are the dominating heat sources compared to the gas-water shift and electrochemical reactions.     

Modelling of an indirect internal reforming solid oxide fuel cell

Creation of an autothermal system by coupling an endothermic to an exothermic reaction demands the matching of the thermal requirements of the two reactions. The application under study is a solid oxide fuel cell (SOFC) with indirect internal reforming (IIR) of methane, whereby the endothermic steam reforming reaction is thermally coupled to the exothermic oxidation reactions. A steady-state model of an IIR-SOFC has been developed to study the mismatch between the thermal load associated with the rate of steam reforming at typical SOFC temperatures and the local amount of heat available from the fuel cell reactions. Results have shown a local cooling effect, undesirable for ceramic fuel cells, close to the reformer entrance. The system behaviour towards changes in catalyst activity, fuel inlet temperature, current density, and operating pressure has been studied. Increasing the operating pressure is shown to be an effective way of reducing both the local cooling caused by the reforming reactions and the overall temperature increase across the cell. Simulations for both counter-flow and co-flow configurations have been performed and compared.     

平板式直接内重整固体氧化物燃料电池的一维动态建模与仿真

This article aims to investigate the transient behavior of a planar direct internal reforming solid oxide fuel cell (DIR-SOFC) comprehensively. A one-dimensional dynamic model of a planar DIR-SOFC is first developed based on mass and energy balances, and electrochemical principles. Further, a solution strategy is presented to solve the model, and the International Energy Agency (IEA) benchmark test is used to validate the model. Then, through model-based simulations, the steady-state performance of a co-flow planar DIR-SOFC under specified initial operating conditions and its dynamic response to introduced operating parameter disturbances are studied. The dynamic responses of important SOFC variables, such as cell temperature, current density, and cell voltage are all investigated when the SOFC is subjected to the step-changes in various operating parameters including both the load current and the inlet fuel and air flow rates. The results indicate that the rapid dynamics of the current density and the cell voltage are mainly influenced by the gas composition, particularly the H2 molar fraction in anode gas channels, while their slow dynamics are both dominated by the SOLID (including the PEN and interconnects) tem-perature. As the load current increases, the SOLID temperature and the maximum SOLID temperature gradient both increase, and thereby, the cell breakdown is apt to occur because of excessive thermal stresses. Changing the inlet fuel flow rate might lead to the change in the anode gas composition and the consequent change in the current den-sity distribution and cell voltage. The inlet air flow rate has a great impact on the cell temperature distribution along the cell, and thus, is a suitable manipulated variable to control the cell temperature.     

内部重整固体氧化物燃料电池发电系统的非线性预测控制

固体氧化物燃料电池（SOFC）发电系统运行除了电堆本体外还需要包含诸多其他辅助组件以期获得系统输出的最大效率,为了使SOFC电堆能够对纯氢以外的燃料具有更好的适用性,加入了燃料内部重整装置和燃烧室两个重要辅助组件。文中在对系统展开建模的基础上提出了采用非线性模型预测控制策略,能够更有效地使输出燃料气体的组分、温度、压力、浓度和流率满足燃料电池堆正常运行的需要,通过仿真分别论证了线性模型预测控制和非线性模型预测控制两种不同控制方案的有效性和适用性。     

Calculation of the metal-carbon bond strength of surface carbon deposited on solid oxide fuel cell nickel/zirconia fuel reforming anodes

The activation energy for the removal of surface carbon formed by methane decomposition following high-temperature reforming, from a nickel/zirconia solid oxide fuel cell (SOFC) anode has been calculated using two methods based on temperature-programmed oxidation. It is found that there is a fairly good agreement between the two methods. In addition, it was observed that the addition of small quantities of lithium to the anode resulted in a significant lowering of the activation energy for surface carbon removal by about 50 kJ mol^-1. This revised version was published online in July 2006 with corrections to the Cover Date.     

Electrophoretically deposited thin film electrolyte for solid oxide fuel cell

Abstract

Thin films of 8 mol% yttria stabilised zirconia (YSZ) electrolyte have been deposited on non-conducting porous NiO–YSZ anode substrates using electrophoretic deposition (EPD) technique. Deposition of such oxide particulates on non-conducting substrates is made possible by placing a conducting steel plate on the reverse side of the presintered porous substrates. Thickness of the substrates, onto which the deposition has been carried out, varied in the range 0·5–2·0 mm. Dense and uniform YSZ thin films (thickness: 5–20 μm) are obtained after being cofired at 1400°C for 6 h. The thickness of the deposited films is seemed to be increased with increasing porous substrate thickness. Solid oxide fuel cell (SOFC) performance is measured at 800°C using coupon cells with various anode thicknesses. While a peak power density of 1·41 W cm^?2 for the cells with minimum anode thickness of 0·5 mm is achieved, the cell performance decreases with anode thickness.     

Integrated methane decomposition and solid oxide fuel cell for efficient electrical power generation and carbon capture

This work proposes the application of methane decomposition (MD) as a fuel processor to replace methane steam reforming (MSR) for hydrogen production for a methane-fuelled solid oxide fuel cell (SOFC) system. In this work, comparison between the MD–SOFC and the MSR–SOFC was performed in terms of SOFC performances and economic analysis to demonstrate a benefit of using MD as a fuel processor. Energy analysis of SOFC system was evaluated based on thermally self-sufficient condition where no external energy is required for the system. Although the MD–SOFC system offers lower electrical efficiency than that of the MSR–SOFC as solid carbon is generated without being further combusted to generate energy; however, the MD–SOFC stack can be operated at higher power density due to high purity of hydrogen supplied to the fuel cell, resulting in smaller size of the system when compared to the MSR–SOFC. Moreover, the MD–SOFC system is less complicated than that of the MSR–SOFC as the CCS facility is not necessary to be included to reduce CO₂ emission. Economic analysis demonstrated that the SOFC system with MD is more competitive than the conventional system with MSR when considering the valuable by-products of solid carbon even with the low-valued carbon black. It is suggested that the success of this proposed SOFC system with MD relies on the technology development on cogeneration of hydrogen and valuable carbon products.     

使用丙烷燃料的便携式固体氧化物燃料电池研究进展

阐述了丙烷燃料应用于固体氧化物燃料电池(solid oxide fuel cell,SOFC)的工艺及其原理,其中包括重整、部分氧化;综述了使用丙烷燃料SOFC阳极材料研究进展,现有的研究工作主要围绕着如何阻止积炭进行,主要途径是改善阳极性能和选用合适的阳极催化剂等;介绍了当今世界上针对便携式应用的各式SOFC的研究发展现状,特别介绍了单气室SOFC;对便携式SOFC的发展前景进行了展望。     

Atomic layer deposited YSZ overlayer on Ru for direct methane utilization in solid oxide fuel cell

We report on the fabrication of thin yttria-stabilized zirconia (YSZ) overlayers via atomic layer deposition (ALD) conformally coated on anodes of methane-fueled low-temperature solid oxide fuel cells. Nano-granular ALD YSZ overlayers that are broken into porous films at elevated temperature improved not only the initial electrochemical performance (35% increase in the maximum power density) but also the thermal stability of the porous Ru anodes by suppressing coarsening.     

Optimum processing parameters to improve sealing performance in solid oxide fuel cells

Glass–ceramic composites are among the favorable candidates as a sealing material for solid oxide fuel cells (SOFC). In order to obtain a reliable, robust and hermetic sealing, the glass–ceramics must chemically bond to both the metallic interconnector and the ceramic electrolyte. A high-bonding strength and good wetting, which strongly depend on the thermal treatment, are always preferred to ensure gas-tight sealing. The thermal treatment involves three stages: binder burnout (stage-I), sintering (stage-II), and cooling (stage-III). This study investigates effects of various parameters on the sealing quality at the sintering stage. The effects of sintering temperature, clamping pressure and sealant thickness are considered. The glass–ceramic laminates are produced employing a tape casting method. The sealing quality is evaluated by measuring leakage and final macro-structure of the sealing region. It is suggested that a 900–930 °C sintering temperature and 1.5–7.6 N cm⁻² clamping pressure ranges are better for successful sealing. The initial thickness of glass–ceramic laminates is also desired to be between 0.25–0.5 mm thickness range for both a cost-effective and reliable sealing.     

Catalytic aspects of the steam reforming of hydrocarbons in internal reforming fuel cells

Steam reforming of hydrocarbons such as natural gas is an attractive method of producing the hydrogen fuel gas required by fuel cells. It may be carried out external to the fuel cell or internally. The two types of fuel cell in which internal reforming is most appropriate are the molten carbonate (MCFC), operating at ca. 650°C and the solid oxide (SOFC) which currently operates above 800°C. At such temperatures, the heat liberated by the electrochemical reactions within the cell can be utilised by the endothermic steam reforming reaction. This paper reviews some of the catalytic aspects of internal reforming in these two types of cell. In the MCFC the major catalyst issue is that of long term activity in the presence of a corrosive alkaline environment produced by the cell's electrolyte. In Europe, this is being addressed by British Gas and others, in a programme part-funded by the European Commission. In this programme, potential catalysts for the direct internal reforming MCFC were evaluated in ‘out-of-cell’ tests. This has led to the demonstration of a 1 kW proof-of-concept DIR-MCFC stack and the start of a European ‘Advanced DIR-MCFC’ project. For the SOFC, it has been shown that state-of-the-art nickel cermet anodes can provide sufficient activity for steam reforming without the need for additional catalyst. However, anode degradation may occur when steam reforming is carried out for long periods. New anode materials could therefore offer significant benefits.     

The influence of activated carbon as an additive in anode materials for low temperature solid oxide fuel cells

The effects of activated carbon (AC) as an additive in multi-oxide nano composite LiNiCuZn–O for application as anode in solid oxide fuel cell (SOFC) is reported. The composite was synthesized using solid state reactions method with varying content of AC in range 0.1%–0.9% for use as anode in the cell. The cell was composed of the synthesized composite as anode, LiNiCuZn–O as cathode and Samaria doped ceria (SDC) as electrolyte. The prepared composites were characterized for morphology and crystal structure by scanning electron microscope (SEM) and x-ray diffraction (XRD) respectively. Furthermore, the crystallite sizes of LiNiCuZn–O and LiNiCuZn–O with AC as an additive have been found in the range from 50 nm to 70 nm. The prepared composite materials were observed porous and the porosity of the sample having 0.5% additive was found highest. The conductivity and power density of the SOFC were studied at temperature of 600 °C. The maximum value of conductivity was found as 4.79 S/cm for the composite containing 0.5% AC as measured by using 4-probe method. The maximum value of power density of the fuel cell with anode comprising of 0.5% AC along with the mentioned cathode and the electrolyte was 455 mW/cm². Therefore, out of the compositions studied, the composite comprising of LiNiCuZn–O with 0.5% AC offered best performance for anode in the cell. This oxide composite is reported as a potential candidate for use as anode in low temperature SOFCs.     

Electrochemical behaviour of propane-fed solid oxide fuel cells based on low Ni content anode catalysts

Two different low Ni content (10 wt.%) anode catalysts were investigated for intermediate temperature (800 °C) operation in solid oxide fuel cells fed with dry propane. Both catalysts were prepared by the impregnation of a Ni-precursor on different oxide supports, i.e. gadolinia doped ceria (CGO) and La_0.6Sr_0.4Fe_0.8Co_0.2O₃ perovskite, and thermal treated at 1100 °C for 2 h. The Ni-modified perovskite catalyst was mixed with a CGO powder and deposited on a CGO electrolyte to form a composite catalytic layer with a proper triple-phase boundary. Anode reduction was carried out in-situ in H₂ at 800 °C for 2 h during cell conditioning. Electrochemical performance was recorded at different times during 100 h operation in dry propane. The Ni-modified perovskite showed significantly better performance than the Ni/CGO anode. A power density of about 300 mW cm⁻² was obtained for the electrolyte supported SOFC in dry propane at 800 °C. Structural investigation of the composite anode layer after SOFC operation indicated a modification of the perovskite structure and the occurrence of a La₂NiO₄ phase. The occurrence of metallic Ni in the Ni/CGO system caused catalyst deactivation due to the formation of carbon deposits.     

Operation viability and performance of solid oxide fuel cell fuelled by different feeds

The performances of solid oxide fuel cells (SOFCs) fed by different types of feed, i.e. biogas, biogas-reformed feed, methane-reformed feed and pure hydrogen, are simulated in this work. Maximum temperature gradient and maximum cell temperature are regarded as indicators for operation viability investigation whereas power density and electrical efficiency are considered as performance indicators. The change in operating parameters, i.e. excess air, fuel feed rate and operating voltage, affects both the performance and operation viability of SOFC, and therefore, these operating parameters should be carefully selected to obtain best possible power density and reasonable temperature and temperature gradient. Pure hydrogen feed offers the highest SOFC performance among the other feeds. Extremely high excess air is required for SOFC fed by biogas to become operation viable and, in addition, its power density is much lower than those of SOFCs fed by the other feeds. Methane-reformed feed offers higher power density than biogas-reformed feed since H₂ concentration of the former one is higher.     

Dehydrogenation of light alkanes over oxidized diamond-supported catalysts in the presence of carbon dioxide

Oxidized diamond demonstrated excellent support for the dehydrogenation of light alkanes to alkenes in the presence of CO₂. Oxidized diamond-supported Cr₂O₃ and V₂O₅ catalysts exhibited comparatively higher catalytic activities in the dehydrogenation of lower alkanes in the presence of CO₂. In the dehydrogenation of propane, the oxidized diamond-supported Cr₂O₃ and V₂O₅ catalysts in the presence of CO₂ afforded nearly twofold higher activities than that in the absence of CO₂. The activity of the oxidized diamond-supported V₂O₅ catalyst in the dehydrogenation of propane increased with increasing reaction temperatures. Furthermore, in the dehydrogenation of n-butane and iso-butane, a promoting effect of CO₂ on butane conversion and butenes yields was observed over the oxidized diamond-supported Cr₂O₃ and V₂O₅ catalysts, though the promotion effect was small.

UV-Vis analyses of the fresh and the reacted catalysts in the presence and absence of CO₂ revealed that CO₂ kept the surface V₂O₅ and Cr₂O₃ in a state of oxidation slightly higher than that in the absence of CO₂.     

Hydrocarbon steam reforming on Ni alloys at solid oxide fuel cell operating conditions

We demonstrate that supported Sn/Ni alloy catalyst is more resistant to deactivation via carbon deposition than supported monometallic Ni catalyst in steam reforming of isooctane at moderate steam to carbon ratios, irrespective of the average size of metal particles and the metal loading. The experiments were performed for average diameters of catalytic particles ranging from 30 to 500 nm and for the loading of active material ranging from 15 to 44 wt% with respect to the total mass of catalyst. The steam reforming reactions were performed at conditions that are consistent with typical solid oxide fuel cell (SOFC) operating conditions. DFT calculations show that the reasons for the enhanced carbon-tolerance of Sn/Ni compared to monometallic Ni are high propensity of Sn/Ni to oxidize carbon and lower driving force to form carbon deposits on low-coordinated metal sites.     

Oscillation of electrical current during direct methane oxidation over Ni-added LSCF–GDC anode of solid oxide fuel cells

Solid oxide fuel cells are studied under direct methane feeding with 10–70% CH₄. When either La_0.58Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF)–Ce_0.9Gd_0.1O_1.95 (GDC) or Ni-added LSCF–GDC composite is used as the anode, the oscillations of the electrical current and the formation rates of CO and CO₂ occur. The oscillation of the electrical current can be explained by a mechanism of periodic oxidation–reduction of the bulk lattice of the anode, with the determining factor being the build-up of the concentration of the oxygen vacancies to a certain extent. As the methane concentration increases, the current density increases and becomes larger with Ni addition. Higher methane concentration leads to higher possibility to induce the oscillation, to start it earlier, and to result in a larger amplitude. Ni addition inhibites the occurrence of the oscillation of the electrical current but promotes that of the CO₂ formation rate.