Performance model of molten carbonate fuel cell

A performance model of a molten carbonate fuel cell (MCFC), an electrochemical energy conversion device for electric power generation, is discussed. The presumptive ability of the MCFC model is improved and the impact of MCFC characteristics in fuel cell system simulations is investigated. Basic data are obtained experimentally by single-cell tests. A correlation formula based on the experimental data is derived for the cell voltage and the oxygen and carbon dioxide partial pressures. Three types of MCFC systems are compared. With regard to fuel utilization, system characteristics using the proposed correlation are very similar to those obtained using a previous model. However, the amount of decrease predicted by the proposed model with respect to system efficiency is larger than that obtained by the previous model at high air utilization     

熔融碳酸盐燃料电池的电气特性研究

为了研究熔融碳酸盐燃料电池的电气特性,分析了熔融碳酸盐燃料电池单元的电化学过程机理,建立了基于电化学反应的熔融碳酸盐燃料电池电气模型,推导了熔融碳酸盐燃料电池平均电流密度与燃气利用率的关系,给出了采用电化学方程的熔融碳酸盐燃料电池电气特性的模型结构和算法,并进行了仿真研究和试验.试验结果表明:该模型结构简单、准确度高,可获得千瓦级熔融碳酸盐燃料电池的电气特性曲线.     

An algorithm for estimation of membrane water content in PEM fuel cells

Available humidity sensing techniques are often intrusive, and of limited practical interest for real-time control applications due to their cost, size, and inadequate response time and accuracy. In this study, we present a novel method for estimation of PEM fuel cell humidity by exploiting its effect on cell resistive voltage drop. This voltage loss is discerned from mass transport, concentration, activation losses and open circuit voltage by a well-known fuel cell voltage model. The proposed scheme makes use of measurements of voltage, current, temperature, and total pressure values in the anode and cathode. It also incorporates dynamic estimators for hydrogen and oxygen partial pressures, adapted from [M. Arcak, H. Gorgun, L.M. Pedersen, S. Varigonda, A nonlinear observer design for fuel cell hydrogen estimation, IEEE Trans. Control Syst. Technol. 12 (1) (2004) 101–110]. The membrane resistance thus obtained is then used to estimate membrane water content following functional characterizations presented in [T.E. Springer, T.A. Zawodzinski, S. Gottesfeld, Polymer electrolyte fuel cell model, J. Electrochem. Soc. 138 (8) (1991) 2334–2342]. Experiments with this estimation technique, performed at the Connecticut Global Fuel Cell Center, are presented and discussed.     

Electrochemical performances of NANOCOFC in MCFC environments

The electrochemical performances of fuel cells using nano-ceria-salt composites electrolyte (NANOCOFC) have been investigated at different temperatures in molten carbonate fuel cell (MCFC) environment. The maximum output power density increased with the temperature, and reached 140 mW/cm² at 650 °C. After operating for 200 h, the open circuit voltage (OCV) can keep the same value and the output power density only deceased 0.08%. It demonstrated that the NANOCOFC possessed the perfect stability of electrochemical performance in the MCFC environment. However, it was found that the output power density of the fuel cell in MCFC environment was much lower than that of fuel cell in SOFC environment. It was implied that the carbonate transfer would hinder the conduction of both proton and oxygen ion, which result in the poor output power density of fuel cells.     

Water equilibria and management using a two-volume model of a polymer electrolyte fuel cell

In this paper, we introduce a modified interpretation of the water activity presented in Springer et al. [T.E. Springer, T.A. Zawodzinski, S. Gottesfeld, Polymer electrolyte fuel cell model, J. Electrochem. Soc. 138 (8) (1991) 2334–2342]. The modification directly affects the membrane water transport between the anode and the cathode (two electrodes) of the polymer electrolyte membrane (PEM) fuel cell in the presence of liquid water inside the stack. The modification permits calibration of a zero-dimensional isothermal model to predict the flooding and drying conditions in the two electrodes observed at various current levels [D. Spernjak, S. Advani, A.K. Prasad, Experimental investigation of liquid water formation and transport in a transparent single-serpentine PEM fuel cell, in: Proceedings of the Fourth International Conference on Fuel Cell Science, Engineering and Technology (FUELCELL2006-97271), June 2006]. Using this model the equilibria of the lumped water mass in the two electrodes are analyzed at various flow conditions of the stack to determine stable and unstable (liquid water growth) operating conditions. Two case studies of water management through modification of cathode inlet humidification and anode water removal are then evaluated using this model. The desired anode water removal and the desired cathode inlet humidification are specified based upon (i) the water balance requirements, (ii) the desired conditions in the electrodes, and (iii) the maximum membrane transport at those conditions.     

Electrochemical characterization of cobalt-encapsulated nickel as cathodes for MCFC

The stability of the NiO cathodes in molten carbonate fuel cell (MCFC) has been improved through microencapsulation of the NiO cathode with nanostructured Co. Cobalt was deposited on the NiO cathode using an electroless deposition process. The electrochemical oxidation behavior of the Co-coated electrodes is similar to that of the bare NiO cathode. The cobalt-coated electrodes have a lower solubility in the molten carbonate melt when compared to bare nickel oxide electrodes in the presence of cathode gas. The solubility decreased more than 50% due to microencapsulation with cobalt. The thermal oxidation rate was also lower in case of the cobalt-encapsulated electrode. Impedance data from the modified electrode indicate that the oxygen reduction reaction depended inversely on the CO₂ and directly on the oxygen partial pressures respectively suggesting a similar reaction mechanism to that of nickel oxide. The results indicated that cobalt-encapsulated NiO is a viable solution in the development of alternate cathodes for MCFC applications.     

Electrochemical performance of reversible molten carbonate fuel cells

The electrochemical performance of a state-of-the-art molten carbonate cell was investigated in both fuel cell (MCFC) and electrolysis cell (MCEC) modes by using polarization curves and electrochemical impedance spectroscopy (EIS). The results show that it is feasible to run a reversible molten carbonate fuel cell and that the cell actually exhibits lower polarization in the MCEC mode, at least for the short-term tests undertaken in this study. The Ni hydrogen electrode and the NiO oxygen electrode were also studied in fuel cell and electrolysis cell modes under different operating conditions, including temperatures and gas compositions. The polarization of the Ni hydrogen electrode turned out to be slightly higher in the electrolysis cell mode than in the fuel cell mode at all operating temperatures and water contents. This was probably due to the slightly larger mass-transfer polarization rather than to charge-transfer polarization according to the impedance results. The CO₂ content has an important effect on the Ni electrode in electrolysis cell mode. Increasing the CO₂ content the Ni electrode exhibits slightly lower polarization in the electrolysis cell mode. The NiO oxygen electrode shows lower polarization loss in the electrolysis cell mode than in the fuel cell mode in the temperature range of 600–675 °C. The impedance showed that both charge-transfer and mass-transfer polarization of the NiO electrode are lower in the electrolysis cell than in the fuel cell mode.     

Thermodynamic performance analysis of a molten carbonate fuel cell at very high current densities

This study is basically composed of two sections. In the first section, a CFD analysis is used to provide a better insight to molten carbonate fuel cell operation and performance characteristics at very high current densities. Therefore, a mathematical model is developed by employing mass and momentum conservation, electrochemical reaction mechanisms and electric charges. The model results are then compared with the available data for an MCFC unit, and a good agreement is observed. In addition, the model is applied to predict the unit cell behaviour at various operating pressures, temperatures, and cathode gas stoichiometric ratios. In the second section, a thermodynamic model is utilized to examine energy efficiency, exergy efficiency and entropy generation of the MCFC. At low current densities, no considerable difference in output voltage and power is observed; however, for greater values of current densities, the difference is not negligible. If the molten carbonate fuel cell is to operate at current densities smaller than 2500 A m⁻², there is no point to pressurize the system. If the fuel cell operates at pressures greater than atmospheric pressure, the unit cell cost could be minimized. In addition, various partial pressure ratios at the cathode side demonstrated nearly the same effect on the performance of the fuel cell. With a 60 K change in operating temperature, almost 10% improvement in energy and exergy efficiencies is obtained. Both efficiencies initially increase at lower current densities and then reach their maximum values and ultimately decrease with the increase of current density. By elevating the pressure, both energy and exergy efficiencies of the cell enhance. In addition, higher operating pressure and temperature decrease the unit cell entropy generation.     

Experimental and modelling studies of CO poisoning in PEM fuel cells

This article is an examination of the CO poisoning and cleaning (stripping) phenomenon that occur in a PEM fuel cell operating on an impure hydrogen stream such as reformed hydrocarbons or alcohols. A range of experimental results including cell polarization curves, measurements of spontaneous and transient oscillations of the anode potential and current pulsing behaviour are presented. Detailed examination of the pulsing process has shown that optimization of both the pulse width and pulse initiation potential will have an important impact on the overall fuel cell efficiency. To optimize these processes, the development of a mathematical model to understand and control the poisoning and cleaning processes is going to be important. In this paper, we have extended the model of Zhang et al. [J. Zhang, Investigation of CO tolerance in proton exchange membrane fuel cells, PhD thesis, Worcester Polytechnic Institute, June, 2004; J. Zhang, R. Datta, J. Electrochem. Soc. 149 (2002) A1423; J. Zhang, J.D. Fehribach, R. Datta, J. Electrochem. Soc. 151 (2004) A689] to include mass transfer effects. It is shown that this new model gives results that are in reasonable agreement with our experimental data.     

Wetting of polarized materials by molten carbonate

This paper presents a summary of our recent work on wetting of polarized materials by molten carbonate, also termed “electrowetting”. Electrowetting is here defined as wetting of conducting materials by ionic liquids under conditions of Faradaic reaction, that is, current load. Our recent work has focused on three issues of practical importance: (1) the effect of alkaline-earth (Ca, Sr, Ba) additions to simple alkali carbonate eutectics with respect to the wetting characteristics of the melt; (2) the driving force behind the meniscus movement of molten carbonate at electrodes under polarization; (3) the effect of changes in wetting characteristics such as caused by alkaline-earth addition on molten carbonate fuel cell (MCFC) performance. These three issues are briefly summarized by means of a few key experimental and computational results, with reference to more detailed communications now in process of publication.     

Degradation mechanism of molten carbonate fuel cell based on long-term performance: Long-term operation by using bench-scale cell and post-test analysis of the cell

In order to introduce molten carbonate fuel cells (MCFCs) in commercial applications, the target lifetime of a MCFC has been set at 40,000 h. We have carried out long-term operation tests on several bench-scale MCFCs, which include a 66,000-h continuous operation, and clarified the question of voltage degradation in relation to operating time. We have also carried out post-test analyses on the long-term operated cell components including the electrodes, the electrolyte matrix and the current collectors. The results of the long-term operation and the post-test analyses are described in this paper. The degradation mechanisms of voltage and components are discussed.     

Improved molten carbonate fuel cell performance via reinforced thin anode

The effects of anode thickness on electrochemical performance and cell voltage stability of molten carbonate fuel cell (MCFC) were examined using single cell test. It was found that supported thin nickel-aluminum (Ni–Al) anode with small pore size enhanced cell performance by reducing its mass transfer resistance and crossover. The stability of cell voltage was also observed. This was achieved after 0.25 mm thick anode was reinforced with Ni 60 mesh. Unsupported 0.3 mm thick anode yielded poor performance due to deformation and cracks after a long thermal exposure. The performance was improved significantly after all the anodes were reinforced with Ni mesh.     

The effect of reservoirs and fuel dilution on the dynamic behavior of a PEMFC

Data are presented to characterize the effects of reservoir size and hydrogen dilution on the dynamic behavior of a proton exchange membrane fuel cell (PEMFC) subjected to rapid changes in the voltage when the flowrates are constant. The data consist of the responses of the current density during low fuel stoichiometries in an effort to expand an understanding of the previously observed overshoot/undershoot behavior. That is, recent studies of the dynamic behavior of a PEMFC have shown pseudo-second-order dynamics of the current response to a change in voltage [J. Power Sources (2004); J. Electrochem. Soc. (2004)]. The data reported here lend further evidence that under fuel starved conditions, rapid changes in the cell voltage between 0.7 and 0.5 V yield pressure differences sufficient to create a “vacuum” effect. This vacuum effect may cause fuel to be drawn from the manifold in a stack or cause ambient air to enter a laboratory scale cell. The vacuum effect explained in our previous work [J. Power Sources (2004)] is shown here to depend on diameter and volume of fuel reservoirs and on the concentration of hydrogen in the fuel.     

MCFC versus other fuel cells—Characteristics, technologies and prospects

Characteristics of molten carbonate fuel cell (MCFC) were critically compared to these of polymer electrolyte membrane fuel cell (PEMFC), alkaline fuel cell (AFC), phosphoric acid fuel cell (PAFC) and solid oxide fuel cell (SOFC). In comparison to the other fuel cells, the MCFC operates with the lowest current densities due to limited zones of effective electrode reactions and low solubilities of oxygen and hydrogen in molten carbonates; also it has a thickest electrodes–electrolyte assembly. In consequence, the applications of MCFC are almost limited to stationary power generators. Although the MCFC stationary power generators have now approached high technological level of precommercialization, in the future they may face a serious contest from SOFC and PEMFC, for which improvement of operational parameters is believed to be achieved easier.     

Molten carbonate fuel cell (MCFC)-based hybrid propulsion systems for a liquefied hydrogen tanker

This study proposes a molten carbonate fuel cell (MCFC)-based hybrid propulsion system for a liquefied hydrogen tanker. This system consists of a molten carbonate fuel cell and a bottoming cycle. Gas turbine and steam turbine systems are considered for recovering heat from fuel cell exhaust gases. The MCFC generates a considerable propulsion power, and the turbomachinery generates the remainder of the power. The hybrid systems are evaluated regarding system efficiency, economic feasibility, and exhaust emissions. The MCFC with a gas turbine has higher system efficiency than that with a steam turbine. The air compressor consumes substantial power and should be mechanically connected to the gas turbine. Although fuel cell-based systems are less economical than other propulsion systems, they may satisfy the environmental regulations. When the ship is at berth, the MCFC systems can be utilized as distributed generation that is connected to the onshore-power grid.     

Demonstration of coal gas run molten carbonate fuel cell concept

Integrated gasifier‐molten carbonate fuel cell (IG‐MCFC) offers a clean and efficient route for power generation from coal. A molten carbonate fuel cell (MCFC) was assembled and its performance was tested with simulated coal gas. The output and the stability was found to be comparable to that with conventional feed gas. It was also observed that switching from one type of feed gas to another had only a marginal effect on the cell performance. Copyright © 1999 John Wiley & Sons, Ltd.     

基于加权残值法的高温燃料电池温度分布特性的数值分析

加权残值法是一种可以直接从偏微分方程中求得近似解的数学方法。通过对熔融碳酸盐燃料电池(MCFC)内部传热传质过程的热力学性能分析,在质量守恒和能量守恒的基础上建立了MCFC温度动态分布的数学模型,并采用加权残值法对其进行求解分析。确定了满足模型边界条件的试函数,以三次正交多项式为基函数,利用加权残值法中的迦辽金法,结合Matlab工具得到MCFC温度的动态分布特性曲线。分析结果表明,燃料电池内部各点温度在空间分布上有很大差异;当供给燃料电池的燃料流量和氧化剂流量变化时,所引起的温度动态特性是复杂的。图3参5。     

Experimental determination of effective surface area and conductivities in the porous anode of molten carbonate fuel cell

Stationary polarization curves and electrochemical impedance spectroscopy of a porous nickel anode in a molten carbonate fuel cell were obtained in order to determine the active surface area and conductivities with varying degree of electrolyte filling for two anode feed-gas compositions, one simulating operation with steam reformed natural gas and the other one gasified coal. The active surface area for coal gas is reduced by around 70–80% compared to the standard gas composition in the case of Li/Na carbonate. Moreover, an optimal degree of electrolyte filling was shifted toward higher filling degree in the case of operation with coal gas.In order to evaluate the experimental data a one-dimensional model was used. The reaction rate at the matrix/electrode interface is about five times higher than the average reaction rate in the whole electrode in case of 10% electrolyte filling. This result suggests that the lower limit of the filling degree of the anode should be around 15% in order to avoid non-uniform distribution of the reaction in the electrode. Therefore, in the case of applying Li/Na carbonate in the MCFC, an electrolyte distribution model taking into account the wetting properties of the electrode is required in order to set an optimal electrolyte filling degree in the electrode.