The dissolution process of the NiO cathodes for molten carbonate fuel cells: state-of-the-art

In this review article the analysis of the main problems related to the use of NiO as material for cathodes in Molten Carbonate Fuel Cells (MCFC) is reported. Thus, the most significant evidences of the mechanism of NiO dissolution have been reported as well as its correlation with the basicity of the carbonate melt, composition of the reactants gases and temperature. Some hypotheses described here have been also verified experimentally and the results of this validation are reported. In the final section, we have described the most promising alternative solutions to this problem and the advantages and shortcomings of these alternatives. © 1998 John Wiley & Sons, Ltd.     

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.     

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.     

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.     

Process analysis of molten carbonate fuel cells in carbon capture applications

Recently, Molten Carbonate Fuel Cells (MCFCs) are being increasingly investigated for carbon capture applications. The wet and low CO₂ cathode feeds of such applications can substantially affect the electrochemistry of the cell. A dual-anion mechanism has been introduced to model this electrochemical regime characterized by the parallel migration of carbonate and hydroxide ions. A model based on this mechanism has been implemented in an in-house-developed Fortran code that has been now integrated into Aspen Plus. The model is able to calculate the main performance parameters on the plane of a cell when geometry as well as feed flow rates, compositions, temperature, pressure, and current density are provided as input data. In the present work, the application of the simulation tool is presented in a process analysis aimed to optimize the formulation of the electrochemical module, further evaluate the controlling factors of the dual-anion mechanism, and discuss possible technological optimizations.     

Mechanical strength improvement of aluminum foam-reinforced matrix for molten carbonate fuel cells

During the cell operation of molten carbonate fuel cells (MCFCs), matrix cracks caused by poor mechanical strength accelerate cell performance degradation. Therefore, for a stable long-term cell operation, the improvement of mechanical properties of matrix is highly required. In this study, aluminum foam was used to enhance the mechanical strength of the matrix as a 3D (three dimensional) support structure. Unlikely conventional matrix (pure α-LiAlO₂ matrix) which has paste-like structure at the MCFC operating temperature, Al foam-reinforced α-LiAlO₂ matrix has significantly strong mechanical strength because the 3D network structure of Al foam can form the harden alumina skin layer during a cell operation. As a result, the mechanical strength of the Al foam-reinforced α-LiAlO₂ matrix was enhanced by 9 times higher than the pure α-LiAlO₂ matrix in a 3-point bending test. In addition, thermal cycle test showed notable cell stability due to strong mechanical strength of Al foam-reinforced α-LiAlO₂ matrix. The Al foam-reinforced α-LiAlO₂ matrix shows appropriate microstructure to preserve the liquid electrolyte when performing the mercury porosimeter analysis and differential pressure test between anode and cathode. Moreover, evaluation of stability and durability for a long-term cell operation were demonstrated by single cell test for 1,000 h.     

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.     

Effect of LiNiO2-coated cathode on cell performance in molten carbonate fuel cells

A double-layered electrode by coating a layer of nano-sized LiNiO₂ particles on a conventional electrode was fabricated to improve the performance of the cathode of the molten carbonate fuel cell. The layer consisting of nano-sized LiNiO₂ particles has a larger surface area than that of the conventional electrode, which is a lithiated NiO cathode. Therefore, it can provide numerous reaction sites and has higher electrical conductivity than the lithiated NiO electrode. Consequently, the cell performance can be improved at lower operating temperatures (600 °C or below). The performance of the nano LiNiO₂-coated cathode was examined in various ways such as by single-cell operation and electrochemical impedance spectroscopy (EIS). The improvement in performance was demonstrated by high cell voltage of over 0.87 V at 600 °C and current density of 150 mA cm⁻². This result was better than 0.81 V generated by the uncoated cathode cell. In the EIS analysis, the nano LiNiO₂ layer coating tended to significantly decrease the charge transfer resistance and increase the mass transfer resistance but caused an overall decrease in the cathode polarization. Thus, high performance was observed at low temperatures.     

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.     

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.     

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.     

Performance analysis of new cathode materials for molten carbonate fuel cells

The slow dissolution of the lithiated nickel oxide cathode represents one of the main causes of performance degradation in molten carbonate fuel cells (MCFC). Two main approaches were studied in ENEA laboratories to overcome this problem: protecting the nickel cathode covering it by a thin layer of a material with a low solubility in molten carbonate and stabilizing the nickel cathode doping it with iron and magnesium.Among several materials, due to its low solubility and good conductivity, lithium cobaltite was chosen to cover the nickel cathode and slow down its dissolution. A nickel electrode covered with a thin layer of lithium cobaltite doped with magnesium, was fabricated by complex sol-gel process. To simplify electrode preparation, no thermal treatments were made after covering to produce lithium cobaltite, and during the cell start-up LiMg_0.05Co_0.95O₂ was obtained in situ.To stabilize the nickel cathode, metal oxides Fe₂O₃ and MgO were chosen as dopant additives to be mixed with NiO powder in a tape-casting process (Mg_0.05Fe_0.01Ni_0.94O).On the prepared materials TGA analysis, morphological analysis by scanning electron microscopy (SEM-EDS) and electrical conductivity measurements were carried out.A conventional nickel cathode, the nickel cathode covered by lithium cobaltite precursors and the nickel cathode stabilized by iron and magnesium oxides were each tested in a 100 cm² fuel cell.Polarization curves and internal resistance (iR) measurements were acquired during the cell lifetime (1000 h) and the effect of gas composition variation on the cell performance was studied.From a comparison with the conventional nickel cathode it can be observed that the new materials have similar performance and show a good potential stability during the cell operating time. From the post-test analysis both the nickel cathode covered by lithium cobaltite and the nickel cathode doped with iron and magnesium seem to succeed in reducing nickel dissolution.     

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.     

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.     

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.     

Improved performance of molten carbonate fuel cells with (Li/Na)2CO3 electrolytes by using BYS coated cathode at low operating temperatures

In order to improve the stack life time of MCFCs, it is necessary to reduce the operating temperature of MCFCs below 600 °C, because reduced operating temperature minimizes electrolyte loss due to evaporation and corrosion. However, at the low operating temperature below 600 °C, the cell performance of MCFCs with (Li/Na)₂CO₃ electrolyte is too low to operate the fuel cell stack and system. In this study, we have performed wettability control of the liquid molten carbonate electrolyte by coating NiO cathodes with poor wetting property of the mixed ionic and electronic conductor (MIEC) such as BYS (Bi_1.5Y_0.3Sm_0.3O_3-δ). From experiments with symmetrical cells, each polarization component with various temperatures and gas conditions were studied. To investigate effects of the BYS coated cathode on the performance of MCFCs, a 100 cm² single cell of MCFCs was employed. The performance of a 100 cm² single cell with BYS coated cathode was better than that with conventional cathode by a factor of 1.84, because BYS coated cathode reduces activation polarization and mass transfer resistance greatly.     

Control‐oriented nonlinear modelling of molten carbonate fuel cells

Performance and availability of molten carbonate fuel cells (MCFC) stack are greatly dependent on its operating temperature. Control of the operating temperature within a specified range and reduction of its temperature fluctuation are highly desirable. The models of MCFC stack existing are too complicated to be suitable for design of a controller because of its lack of clear input–output relations. In this paper, according to the demands of control design, a quantitative relations model of control‐oriented MCFC between the temperatures of the stack and flowrates of the input gases is developed, based on conservation laws. It is an affine nonlinear model with multi‐input and multi‐output, the flowrates of fuel and oxidant gases as the manipulated vector and the temperatures of MCFC electrode–electrolyte plates, separator plates as the controlled vector. The modelling and simulation procedures are given in detail. The simulation tests reveal that the model developed is accurate and it is suitable to be used as a model in designing a controller of MCFC stack. Copyright © 2004 John Wiley & Sons, Ltd.     

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.     

Molten carbonate fuel cells: A high temperature fuel cell on the edge to commercialization

The Molten Carbonate Fuel Cell (MCFC) technology has been developed in USA, Japan, Korea and Europe for many years. What has started about 30 years ago as an interesting laboratory object has now matured to a potential alternative to conventional power generation systems. Especially the combined heat and power (CHP) generation is an area, where MCFC power plants can be applied with great advantage, due to the high efficiencies which can be achieved. It was demonstrated by several manufacturers that in the sub-MW region MCFC power plants can reach electrical efficiencies of 47%. By making use of the heat generated by the system, total efficiencies of more than 80% can be achieved.

The present paper will discuss some aspects of the development work going on with a focus on the role of the molten carbonate contained in the cells. An outlook will be given for the future prospects of this young technology in a changing energy market.