Strategies and new developments in the field of molten carbonates and high-temperature fuel cells in the carbon cycle

This paper introduces the special issue of this Journal dedicated to the “International Workshop on Molten Carbonates and Related Topics”, IWMC2011. Molten carbonates are mostly known as electrolytes in molten carbonate fuel cells (MCFCs). Many advances have been reached in the last decade on the development of such electrochemical generators. MCFCs can be considered as a mature technology close to the real market. However, new materials and approaches are necessary to increase their performance and lower their costs. Lately, molten carbonates are also used for CO₂ capture with ambitious industrial projects and a growing interest in many countries, in particular in Europe. Furthermore, molten carbonates are also seriously considered in direct carbon fuel cells (DCFCs), directly as electrolytes or as media to feed solid oxide fuel cells with solid carbon fuel. Noteworthy is the astonishing research activity on composite fuel cells combining molten carbonates with solid oxide fuel cells. All these promising topics and others are briefly described emphasising their novelty.     

Effect of Dy on the corrosion of NiO/Ni in molten (0.62Li,0.38K)2CO3

The dissolution of NiO cathodes and the balance between the corrosion and contact ohmic resistance of current collector materials in molten carbonates are great obstacles to the commercialization of molten carbonate fuel cells (MCFC). Rare-earth element dysprosium was proposed to modify the NiO cathodes and to alloy nickel to explore its possible applications in MCFC. The measured solubility of NiO impregnated with 0.5–3 wt.% Dy in (0.62Li,0.38K)₂CO₃ at 650 °C in 60% CO₂–40% O₂ indicated that Dy addition increased noticeably the stability of NiO, while 1 wt.% Dy content produced better effectiveness. An investigation of the passive behavior of nickel and of Ni–Dy alloys containing 1–10 wt.% Dy in molten (Li,K)₂CO₃ with dynamic polarization measurements as well as X-ray diffraction and X-ray photoelectron spectroscopy indicated that the addition of Dy to nickel decreased its passive anodic current, and thus improved its corrosion resistance. The lithiation of NiO in the melt was a very significant reaction that could be promoted by the Dy additives to a certain extent, and increased the electrical conductivity of NiO.     

Electrochemical analysis on the growth of oxide formed on stainless steels in molten carbonate at 650 °C

The oxide growth on stainless steel (SS) 310S and 316L, used as a cathode current collector material of molten carbonate fuel cell (MCFC), were examined in the mixture of 62 mol% Li₂CO₃–38 mol% K₂CO₃ at 650 °C by measuring the change in corrosion potential and potentiodynamic response of the alloys and also in terms of impedance analysis on the alloy|oxide layer|electrolyte system. The corrosion potential of SS 316L was in an active region for 12 h-immersion, whereas that of SS 310S drastically increased after 6 h-immersion due to an active to passive transition. The corrosion rate of the two SSs decreased with immersion due to the growth of protective oxide. However, the decrease in the corrosion rate of SS 310S is much greater than that of SS 316L. The oxide formed on the two SSs was found to be duplex layer, composed of inner Cr enriched oxide and outer Fe enriched oxide. However, the inner Cr enriched layer of 310S is more clearly separated from the outer Fe enriched layer than that of SS 316L due primarily to the higher Cr content in SS 310S. The drastic increase in the corrosion potential of SS 310S after 6 h-immersion is closely associated with the growth of the inner Cr enriched oxide layer. The corrosion resistance of SS depends dominantly on the resistance of the inner Cr enriched oxide that is determined form the impedance analysis on the alloy|oxide layer|electrolyte system. The higher corrosion resistance of SS 310S compared with SS 316L results from the more protective inner Cr enriched oxide layer, as confirmed by its higher resistance associated with the higher Cr content in SS 310S.     

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.     

Influence of Cs and Rb additions in LiK and LiNa molten carbonates on the behaviour of MCFC commercial porous Ni cathode

Improvement of the molten carbonate fuel cell electrolyte is a key parameter to increase the performance of such electrochemical generator. One of the main challenges is to enhance the global oxygen reduction at the state-of-the-art porous nickel cathode. In this study, the effect of adding 5 mol% of caesium in LiK and LiNa molten carbonate eutectics or 5 mol% of rubidium in LiK melt was analysed with respect to the behaviour of nickel cathode towards oxygen reduction. Evolution of the open-circuit potential and electrochemical impedance spectroscopy measurements were carried out over time. In LiK melt, both Cs and Rb additives induced an improved cathode behaviour: more rapid equilibration reaching more rapidly the equilibrium potential, and significantly lower total resistance (9 times less with Cs and 3 times less with Rb), in particular mass transport, with respect to the pristine electrolyte. Moreover, charge transfer resistance was significantly lower with Rb and nearly the same with Cs versus pristine LiK. Both additions are significantly positive for enhancing oxygen reduction at the porous electrode, which seems to be particularly the case for Cs addition. However, addition of Cs to LiNa did not show an important effect. Changing the composition of LiK with the mentioned additives could be an important step towards a more performing MCFC, but more insight on oxygen reduction kinetics with Rb addition and single cell tests with both cases are compulsory.     

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.     

Corrosion of metallic stack components in molten carbonates: Critical issues and recent findings

The influence of stainless steel hardware corrosion on molten carbonate fuel cell (MCFC) cell performance decay modes is briefly reviewed. Emphasis has been given to the cathode-side corrosion of the separator plate, which is the most critical performance-limiting factor due to the growth of thick oxide scales with a poor electrical conductivity causing relevant cell voltage losses on prolonged operations. Voltage decay is related to loss of electrolyte by reaction with the growing oxide scale and to the onset of an ohmic resistance at the point of contact between the corroded separator plate and the cathode. The increase of ohmic resistance over the time is the major cause of voltage decay with the currently used austenitic 316L and 310S stainless steels. A short literature survey is presented in the second part of this paper reporting on the most promising alternative corrosion-resistant alloys or protective coatings suitable for fabrication of long-term stable cathode-side MCFC separator plates.     

熔融碳酸盐燃料电池（MCFC）性能研究

简要叙述了MCFC微观工作过程,然后分别详细讨论了压力,温度,反应气体的组分和利用率,电流密度,电解质板结构和电解质的成分,杂质和运动时间对MCFC性能和寿命的曩,并结合文献和实验数据对其机理进行了阐述,最后得出了为提高电池性能和瞎长其寿命的几点结论和建议。     

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.     

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.     

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.     

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.     

Electrical conductivity and related properties of molten carbonates coexisting with ceria-based oxide powder for hybrid electrolyte

The electrical, thermal and structural properties of composite electrolyte containing Ce_0.9Gd_0.1O_1.95 (GDC) powder and (Li_0.52Na_0.48)₂CO₃ eutectics are investigated by AC impedance, differential thermal analysis and polarized Raman scattering spectroscopy. The system shows a dependence of the electrical conductivity upon the temperature. The transition point varies with the apparent average thickness of the liquid phase, while the activation energy, ΔE_a, remains constant at any distance from the solid phase. Higher electrical conductivity was obtained for the GDC/(Li_0.52Na_0.48)₂CO₃ composite than that for α-Al₂O₃/(Li_0.52Na_0.48)₂CO₃. Even in the N₂ or Air gas flow, the weight loss caused by decomposition of CO₃²⁻ ion based on Lux–Flood equilibrium was rarely observed. The symmetric stretching mode of the polarized Raman spectra shows that carbonate ion maintains its D_3h symmetry in the presence of ceria. A constant value of the depolarization ratio of the ν₁(A′₁) mode with regard to the apparent average thickness confirms that the symmetry of carbonate ions in the molten state is not altered by the presence of ceria powder. It was more stable than that for the system containing α-Al₂O₃ as a reference sample. These findings contribute to the understanding of the properties of ceria-based carbonate electrolyte for intermediate temperature solid oxide fuel cells.     

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 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.     

Lifetime Expectancy of molten carbonate fuel cells: Part I. Effect of temperature on the voltage and electrolyte reduction rates

Electrolyte depletion is a significant setback in the operation of molten carbonate fuel cells (MCFCs). The electrolyte loss mostly occurs as a result of the high operating temperatures of over 873 K. The effect of temperature on MCFCs was studied using several 7 cm² coin-type MCFCs operated at 873, 973 and 1073 K. Lithium-potassium carbonate (Li/K) was used as an electrolyte in this study. A decrease in cell performance with time was observed at all temperatures. The performance degradation was found to be more severe at 1073 K than at 973 K and 873 K. The electrolyte loss rate was observed by chemical means to have increased with increasing temperature. A more severe electrolyte loss rate was observed in cells operated at 1073 K, such that the electrolyte amount reduced by half after 250 h of cell operation. In this research work, a factor, F_WV, which correlates the electrolyte loss rate, voltage reduction rate, and cell life, is introduced. Its dependence on the cell electrode area and operating temperature make it a suitable parameter for simulating MCFC's lifetime.     

The stability of molten carbonate fuel cell electrodes: A review of recent improvements

The electrode stability is a key issue for the development of conventional hydrogen fuelled and direct internal reforming (DIR) molten carbonate fuel cells (MCFCs). While for conventional MCFC anodes the stability problem has been addressed by the addition of Al or Cr to Ni, the problems of the dissolution of the NiO cathode and of the deactivation of DIR-MCFC anodes have not been fully resolved too. This review reports recent improvements in the chemical and physicochemical stability of cathode and anode materials in MCFCs and DIR-MCFCs, respectively.     

The solubility of Ni in molten Li2CO3–Na2CO3 (52/48) in H2/H2O/CO2 atmosphere

In this work the solubility of a Ni–Al anode for MCFC has been studied at atmospheric pressure and two different temperatures using various gas compositions containing H₂/H₂O/CO₂. It is well known that nickel is dissolved at cathode conditions in an MCFC. However, the results in this study show that nickel can be dissolved also at the anode, indicating that the solubility increases with increasing CO₂ partial pressure of the inlet gas and decreasing with increasing temperature. This agrees with the results found by other authors concerning the solubility of NiO at cathode conditions. The dissolution of Ni into the melt can proceed in two ways, either by the reduction of water or by the reduction of carbon dioxide.     

A highly active Ni-Al2O3 catalyst prepared by homogeneous precipitation using urea for internal reforming in a molten carbonate fuel cell (MCFC): Effect of the synthesis temperature

Ni-Al₂O₃ catalysts for use in internal reforming in a molten carbonate fuel cell (MCFC) were prepared by homogeneous precipitation method at various synthesis temperatures. The effects of synthesis temperature on physicochemical properties and catalytic activities of the Ni-Al₂O₃ catalysts were investigated. XRD measurements exhibited that the peak intensity of NiAl₂O₄ in the calcined catalysts increased with higher synthesis temperatures. TPR measurements demonstrated that reduction peaks appeared around 670–680 °C for every synthesis temperature, indicating that the Ni particles interacted strongly with the support. Hydrogen chemisorption results showed that nickel dispersion and nickel surface area decreased in the order: K52_80C > K52_85C > K52_90C > K52_95C > K52_100C. TEM images of the reduced Ni-Al₂O₃ catalysts revealed that the average sizes of Ni particles were 13.1, 13.4 and 15.9 nm for K52_80C, K52_90C and K52_100C, respectively, which means that a higher synthesis temperature yielded a larger Ni particle. The performance of the catalysts in methane steam reforming showed that catalysts prepared at the lowest synthesis temperature (80 °C) exhibited the highest reaction rate. These results suggest that a lower synthesis temperature is favorable to prepare highly active Ni-Al₂O₃ catalysts by the homogeneous precipitation method.     

The preparation and properties of multi-component molten salts

This paper was focused on thermal stability of molten salts and their thermo-physical properties at high temperature. In this experiment, multi-component molten salts composed of potassium nitrate, sodium nitrite and sodium nitrate with 5% additive A of the chlorides were prepared by statical mixing method. The experiments found molten salt with 5% additive A had higher thermal stability and its best operating temperature would be increased to 550 °C from 500 °C when comparing to ternary nitrate salt. Meanwhile, thermal stability and thermal cycling analysis showed molten salts with 5% additive A had lower freezing point and loss of nitrite content and deterioration time of molten salts were reduced at the same time. DSC tests also indicated loss of latent heat in molten salts with 5% additive A was decreased. Besides, thermo-physical properties measured showed molten salt with 5% additive A had a heat capacity of 2.32 kJ/kg °C, lower than 4.19 kJ/kg °C for water between 0 °C and 100 °C and a low viscosity range from 3.0 to 1.4 cp between 150 °C and 500 °C, analogous with 1.8–0.3 cp for water between 0 °C and 100 °C. Other thermo-physical properties, such as thermal conductivity, density and linear thermal expansion, were also determined here.