Corrosion-resistant coating for cathode current collector and wet-seal area of molten carbonate fuel cells

High-temperature corrosion of metallic bipolar plates is a main problem to limit system reliability of molten carbonate fuel cells (MCFCs). In particular, cathode current collector (CCC) and wet-seal area in the bipolar plates suffer severe corrosion during MCFCs operation. Herein, we are trying to explore facile and cost-effective coating materials and methods to enhance corrosion resistance of CCC and wet-seal area of MCFCs. Cobalt (Co) layer was coated by using electrodeposition method for CCC, and aluminum (Al) layer was coated by using mechanical cladding method for wet-seal area. Co and Al layers were transformed into in-situ lithiated oxides, which are LiCoO₂ and LiAlO₂, by reacting with electrolytes without detachment. These in-situ formed oxides efficiently impede formation and growth of corrosion scales by preventing permeation of electrolytes. We believe that Co electrodeposition and Al cladding would be suitable and practical methods for making protective layers to enhance corrosion resistance of CCC and wet-seal area in the MCFCs bipolar plates.     

Corrosion of anode current collectors in molten carbonate fuel cells

Corrosion of metallic parts is one of the life-time limiting factors in the molten carbonate fuel cell. In the reducing environment at the anode side of the cell, the corrosion agent is water. As anode current collector, a widely used material is nickel clad on stainless steel since nickel is stable in anode environment, but a cheaper material is desired to reduce the cost of the fuel cell stack. When using the material as current collector one important factor is a low resistance of the oxide layer formed between the electrode and the current collector in order not to decrease the cell efficiency. In this study, some candidates for anode current collectors have been tested in single cell molten carbonate fuel cells and the resistance of the oxide layer has been measured. Afterwards, the current collector was analysed in scanning electron microscope (SEM) equipped with energy dispersive spectrometer (EDS). The results show that the resistances of the formed oxide layers give a small potential drop compared to that of the cathode current collector.     

The control strategy for a molten carbonate fuel cell hybrid system

Based on mathematical modelling and numerical simulations, the control strategy for a molten carbonate fuel cell hybrid system (MCFC-HS) is presented. Adequate maps of performances with three independent parameters are shown. The independent parameters are as follows: stack current, fuel mass flow and compressor outlet pressure. Those parameters can be controlled by external load, fuel valve and turbine–compressor shaft speed, respectively.     

Modeling of molten carbonate fuel cell based on the volume-resistance characteristics and experimental analysis

The real-time dynamic simulation of MCFC is still difficult up to now. This work presents a one-dimensional mathematical model for MCFC considering the variation of local gas properties, and the experimental analysis for the validation of model. The volume-resistance (V-R) characteristic modeling method has been introduced. Using the V-R modeling method and the modular modeling idea, the partial differential equations for cell mass, energy and momentum balance can be modified in order to develop a model for quick simulation. Experiments have been carried out at Shanghai Jiaotong University Fuel Cell Research Institute. The experiments have been done under different operating pressures, and the results are used to validate the model. A good agreement between simulation and experimental results has been observed. Steady- and dynamic-state simulation results are analyzed. The results indicate that the V-R characteristic modeling method is feasible and valuable. The model can be used in the real-time dynamic simulation.     

A basic model for analysis of molten carbonate fuel cell behavior

A simple mathematical model, based on the basic chemical reactions and mass transfer, was developed to predict some important characteristics of molten carbonate fuel cells (MCFC) with LiNaCO₃ and LiKCO₃ electrolytes for steady state operating conditions. The parallel and cross gas flow patterns were analyzed. Model simulates polarization characteristics, the effect of temperature, pressure and electrolyte type on the cell performance, various losses in the cell and gas flow rate changes through cell. The effect of fuel utilization on the cell potential and efficiency was also analyzed. Model predicts a better performance for the MCFC with LiNaCO₃ electrolyte and the cross flow pattern, in general. Results show a strong influence of the operating temperature on the cell potential at temperatures below 625 °C, where cell potential increases rapidly with increasing temperature. Above this temperature, however, the cell potential has almost a steady asymptotic profile. The model predicts cell efficiency steadily improving with increase in fuel utilization. The cell potential decreases almost linearly with increase in the fuel utilization percentage for both electrolytes. Models results show a stronger dependency of the cell potential on the operating pressure than that described by the Nerst equation which is in line with fact that the real variations in the cell potential can be higher due to decreased various losses.     

Dual ionic conductive membrane for molten carbonate fuel cell

Within this study, the electrochemically inert, molten carbonate fuel cell (MCFC) $\text{γ}$-LiAlO₂ matrix is replaced by oxygen ion conducting ceramics, typical for solid oxide fuel cell (SOFC) application. Such solution leads to synergistic ion transport both by molten carbonate mix (CO₃^2-) and yttria-stabilized zirconia (YSZ) or samaria-doped ceria (SDC) matrix (O^2-).Single unit cell tests confirm that application of hybrid ionic membrane increases the performance (power density) of the MCFC over pure $\text{γ}$-LiAlO₂ for a wide range of operating temperatures (600 °C–750 °C). Cell power density with SDC and YSZ matrices is 2% and 13% higher, respectively, compared to the $\text{γ}$-LiAlO₂ at typical 650 °C operating temperature of MCFC.     

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.     

Optimum start-up strategies for direct internal reforming molten carbonate fuel cell systems

The study of start-up performance for a direct internal reforming molten carbonate fuel cell (DIR-MCFC) system is presented. Since a kW-class stack is assembled with an additional preheating design, the improvement of start-up behavior is conducted to find the proper operating strategy. For a cold start-up fuel cell system, both start-up delay and inverse response are strictly detected. When the optimum operating strategy is determined by solving the steady-state optimization algorithm subject to stack temperature constraint, the rapid system start-up as well as the maximum power output can be achieved simultaneously.     

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.     

Analysis of a solid oxide fuel cell and a molten carbonate fuel cell integrated system with different configurations

A solid oxide fuel cell with internal reforming operation is run at partial fuel utilization; thus, the remaining fuel can be further used for producing additional power. In addition, the exhaust gas of a solid oxide fuel cell still contains carbon dioxide, which is the primary greenhouse gas, and identifying a way to utilize this carbon dioxide is important. Integrating the solid oxide fuel cell with the molten carbonate fuel cell is a potential solution for carbon dioxide utilization. In this study, the performance of the integrated fuel cell system is analyzed. The solid oxide fuel cell is the main power generator, and the molten carbonate fuel cell is regarded as a carbon dioxide concentrator that produces electricity as a by-product. Modeling of the solid oxide fuel cell and the molten carbonate fuel cell is based on one-dimensional mass balance, considering all cell voltage losses. Primary operating conditions of the integrated fuel cell system that affect the system efficiencies in terms of power generation and carbon dioxide utilization are studied, and the optimal operating parameters are identified based on these criteria. Various configurations of the integrated fuel cell system are proposed and compared to determine the suitable design of the integrated fuel cell system.     

Electrolyte effect on the catalytic performance of Ni-based catalysts for direct internal reforming molten carbonate fuel cell

An active and tolerant Ni-based catalyst for methane steam reforming in direct internal reforming molten carbonate fuel cells (DIR-MCFCs) was developed. Deactivation of reforming catalysts by alkali metals from the electrolyte composed of Li₂CO₃ and K₂CO₃ is one of the major obstacles to be overcome in commercialization of DIR-MCFCs. Newly developed Ni/MgSiO₃ and Ni/Mg₂SiO₄ reforming catalysts show activities of ca. 80% methane conversion. Subsequent to electrolyte addition to the catalyst, however, the activity of Ni/Mg₂SiO₄ decreases to ca. 50% of its initial value, whereas Ni/MgSiO₃ catalyst retains its initial activity. Results obtained from temperature-programmed reduction and X-ray photoelectron spectroscopy identify unreduced Ni³⁺ as a decisive factor in keeping catalytic activity from the electrolyte.     

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.     

Lifetime optimization of a molten carbonate fuel cell power system coupled with hydrogen production

In this paper, a biogas fuelled energy system for combined production of electricity and hydrogen is considered. The system is based on a molten carbonate fuel cell stack integrated with a micro gas turbine. Hydrogen is produced by a pressure swing absorption system. A multi-objective optimization is performed, considering the electrical efficiency and the unit cost of electricity as the objective functions.The system operation is affected by variations in fuel composition, ambient temperature and performance degradation of the components occurring during its lifetime. These effects are considered while defining the objective functions.     

Stirling based fuel cell hybrid systems: An alternative for molten carbonate fuel cells

This paper presents a new design for high temperature fuel cell and bottoming thermal engine hybrid systems. Now, instead of the commonly used gas turbine engine, an externally fired - Stirling - piston engine is used, showing outstanding performance when compared to previous designs.Firstly, a comparison between three thermal cycles potentially usable for recovering waste heat from the cell is presented, concluding the interest of the Stirling engine against other solutions used in the past.Secondly, the interest shown in the previous section is confirmed when the complete hybrid system is analyzed. Advantages are not only related to pure thermal and electrochemical parameters like specific power or overall efficiency. Additionally, further benefits can be obtained from the atmospheric operation of the fuel cell and the possibility to disconnect the bottoming engine from the cell to operate the latter on stand alone mode. This analysis includes on design and off design operation.     

Numerical studies of a separator for stack temperature control in a molten carbonate fuel cell

The use of a separator to control stack temperature in a molten carbonate fuel cell was studied by numerical simulation using a computational fluid dynamics code. The stack model assumed steady-state and constant-load operation of a co-flow stack with an external reformer at atmospheric pressure. Representing a conventional cell type, separators with two flow paths, one each for the anode and cathode gas, were simulated under conditions in which the cathode gas was composed of either air and carbon dioxide (case I) or oxygen and carbon dioxide (case II). The results showed that the average cell potential in case II was higher than that in case I due to the higher partial pressures of oxygen and carbon dioxide in the cathode gas. This result indicates that the amount of heat released during the electrochemical reactions was less for case II than for case I under the same load. However, simulated results showed that the maximum stack temperature in case I was lower than that in case II due to a reduction in the total flow rate of the cathode gas. To control the stack temperature and retain a high cell potential, we proposed the use of a separator with three flow paths (case III); two flow paths for the electrodes and a path in the center of the separator for the flow of nitrogen for cooling. The simulated results for case III showed that the average cell potential was similar to that in case II, indicating that the amount of heat released in the stack was similar to that in case II, and that the maximum stack temperature was the lowest of the three cases due to the nitrogen gas flow in the center of the separator. In summary, the simulated results showed that the use of a separator with three flow paths enabled temperature control in a co-flow stack with an external reformer at atmospheric pressure.     

Energy and exergy analyses of a hybrid molten carbonate fuel cell system

This study deals with the energy and exergy analysis of a molten carbonate fuel cell hybrid system to determine the efficiencies, irreversibilities and performance of the system. The analysis includes the operation of each component of the system by mass, energy and exergy balance equations. A parametric study is performed to examine the effect of varying operating pressure, temperature and current density on the performance of the system. Furthermore, thermodynamic irreversibilities in each component of the system are determined. An overall energy efficiency of 57.4%, exergy efficiency of 56.2%, bottoming cycle energy efficiency of 24.7% and stack energy efficiency of 43.4% are achieved. The results demonstrate that increasing the stack pressure decreases the overpotential losses and, therefore, increases the stack efficiency. However, this increase is limited by the remaining operating conditions and the material selection of the stack. The fuel cell and the other components in which chemical reactions occur, show the highest exergy destruction in this system. The compressor and turbine on the other hand, have the lowest entropy generation and, thus, the lowest exergy destruction.     

Overpotential analysis with various anode gas compositions in a molten carbonate fuel cell

The effect of anode gas composition on the overpotential in a 100 cm² class molten carbonate fuel cell is investigated. A total of five different gas compositions are used. They are applied to cross-check the effect of flow rate and composition, e.g., a given composition with different gas flow rates and a total flow rate with different gas compositions. The overpotential at the anode is analyzed via steady-state polarization, inert gas step addition and reactant gas addition methods. The analyses reveal that the anodic overpotential depends on the flow rate of the reactant species rather than the composition. Two identical gas compositions show less overpotential when a larger flow rate is applied. Compositions with large flow rates of CO₂ and H₂O also yield less overpotential due to the gas species. Overpotential analyses show that the three measurements have complementary relationships.     

Integration of molten carbonate fuel cell and looped multi-stage thermoacoustically-driven cryocooler for electricity and cooling cogeneration

Thermal management is essential for high-temperature molten carbonate fuel cell (MCFC) because the accumulated waste heat may degrade the durability. In this paper, looped multi-stage thermoacoustically-driven cryocooler (LMTC) is proposed to reuse the waste heat from MCFC for cooling production, which not only can tackle with the thermal management issue but also can provide additional usages. Accounting various irreversible dissipation, the models of MCFC, LMTC and MCFC-LMTC hybrid system are analytically formulated. Performance features of MCFC-LMTC hybrid system are revealed and the advantages are expounded via calculation examples. Calculations indicate that the maximum power density and corresponding efficiency of the hybrid system are 1688.9 W m^?2 and 39.7%, which are 11.4% and 1.3% bigger than that of the sole MCFC system, respectively. By comparing with other available systems, the superiority of using LMTC to recover MCFC waste heat for refrigeration is clearly demonstrated. Considerable parametric studies show that the heat-transfer coefficient of hot heat exchange for LMTC is not suggested to be greater than 2.5 × 10^?3 W m^?2 K^?1. In addition, an increase in the working temperature, working pressure of MCFC, reactant concentration or engine stage number of LMTC positively benefits the hybrid system performance, while an increase in the thermodynamic loss coefficient worsens the hybrid system performance. The obtained results may offer new insights into improving the performance of MCFCs through thermal management approaches.     

Silver coated cathode for molten carbonate fuel cells

Within this study, a layered cathode for use in a Molten Carbonate Fuel Cell (MCFC) has been developed. The substrate layer and reference MCFC cathode made of porous nickel was covered by a porous silver film with defined porosity and pore size. Both layers were fabricated using the tape casting method and further fired in a reductive atmosphere. The new cathode was assembled with other reference cell components to form a single MCFC, which was subjected to performance and durability tests. Scanning electron microscopy was used to analyze the microstructure of the materials before and after tests. The reference cathode was also studied for the comparison.The results show that the porous silver layer was able to enhance the electron transport between the cathode and current collector. It was also found that oxygen reduction is enhanced due to the presence of silver in the gas supply. As a result, the power density of the cell was increased by 50%. On the other hand, due to the separation from electrolyte by the NiO layer, no significant degradation of the silver layer, identified by SEM or electrochemical tests, was found after 1000 h.     

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.