Yoshihiro Mugikura

Performance analysis of molten carbonate fuel cell using a Li/Na electrolyte

Several years ago, Li/Na carbonate (Li2CO3/Na2CO3) was developed as the electrolyte of molten carbonate fuel cells (MCFCs) in place of the usual Li/K carbonate (Li2CO3/K2CO3) to the advantage of a higher ionic conductivity and lower rate of cathode NiO dissolution. To estimate the potential of Li/Na carbonate as the MCFC electrolyte, the dependence of the cell performance on the operating conditions and the behavior during long-term performance was investigated in several bench-scale cell operations. The obtained data on the performance of Li/Na cells was analyzed to estimate the impact of voltage losses by using a performance model and discussed in comparison with the data of conventional Li/K cell performance.

Effects of NH3 and NOx on the performance of MCFCs

Abstract To evaluate the effect NH 3 and NO x have on the performance of molten carbonate fuel cells (MCFCs), bench-scale cell tests and half-cell experiments have been performed with fuel gas containing NH 3 , or with oxidant gases containing NO x . Most of the added NH 3 is discharged from the anode, and does not affect the cell voltage. The NO x does harm to the cell voltage during earlier operating stages, but the harm tends to decrease with increasing operating time. The main cause for the cell voltage drop is the increase of internal resistance. As a result of the analyses regarding the electrolyte composition in the operated cells, the gas composition and the cyclic voltammograms, the behavior of NO x in the cell is found to be as follows: NO x reacts with the carbonate and dissolves in the electrolyte to make NO 2 − and NO 3 − . These ions react with the hydrogen in the fuel gas and lead to the production of N 2 and a small amount of NH 3 . Consequently, NO 2 − and NO 3 − are not accumulated in the electrolyte, and the effect of NO x on the cell life expectancy is slight.

Model of Cathode Reaction Resistance in Molten Carbonate Fuel Cells

A model of the performance of a molten carbonate fuel cell (MCFC) is required to estimate the efficiency of an MCFC power plant or to simulate the internal state of a stack. The model should provide an accurate representation of the performance under various operating conditions. However, the performance estimated by previous models has been found to deviate from the measured performance under low oxygen and carbon dioxide cathode partial pressures. To solve this problem, the authors carried out a systematic analysis of the performance of several bench-scale cells operated under various cathode gas conditions and investigated a model of cathode polarization according to the oxygen reduction mechanism in molten carbonate. As a result, it has been clarified that the behavior of cathode polarization under various conditions is described well by the dependence of mixed diffusion of superoxide ion O{sub 2}{sup {minus}} and CO{sub 2} in the melt on the assumed partial pressures at each total operating pressure.

An Electrolyte Distribution Model in Consideration of the Electrode Wetting in the Molten Carbonate Fuel Cell

In the molten carbonate fuel cell, the electrolyte distribution in the electrode is one of the major factors affecting cell performance. An electrolyte distribution model was developed in consideration of the electrodes wetting properties and the pore size distribution within the electrode. Because wettability data, e.g., contact angles, are required for model calculations, the meniscus heights of (Li/K)CO{sub 3} and (Li/Na)CO{sub 3} on Ni were measured under various anode gas conditions, and contact angles were derived.

The effects of H{sub 2}S on electrolyte distribution and cell performance in the molten carbonate fuel cell

To evaluate the effects of H{sub 2}S on the performance of molten carbonate fuel cells, bench-scale cell tests were performed and the meniscus heights of the electrolyte on Ni were measured with fuel gases containing various amounts of H{sub 2}S. In bench-scale cell tests, H{sub 2}S in the fuel gas had a large effect on cell voltage in the early operating stages, but this effect showed a tendency to decrease with operating time. Basic wetting property measurements revealed that Ni becomes better wetted at higher H{sub 2}S concentrations. In calculations of the electrolyte distributions, the electrolyte fill of the anode with {sub 2}S was found to be higher than that without H{sub 2}S. This study simulates the electrolyte distributions taking into account the effects of H{sub 2}S levels, the electrolyte loss and the change in pore size distributions of the electrodes, and discusses the relation between electrolyte distribution and cell performance.

Meniscus Behavior of Metals and Oxides in Molten Carbonate under Oxidant and Reducing Atmospheres I. Contact Angle and Electrolyte Displacement

The wetting of metals and oxides by molten carbonate is an important factor affecting the performance of a molten carbonate fuel cell (MCFC). The distribution of the electrolyte among electrodes and matrix in the MCFC is dominated by the pore characteristics and wetting properties of these components. However, data on wetting, especially under load (current passage), are limited. In this study, the behavior of the meniscus at a metal is used to obtain information on wetting and electrochemical reactions. Meniscus height and current were measured under varioius atmopsheres. The contact angle was calculated from the meniscus height. The electrolyte distribution in the MCFC was estimated using contact angles thus obtained in oxidant and reducing atmospheres. The results suggest that upon applicatioin of load the electrolyte moves from the anode to the cathode and that capillary effects can worsen the performance of a cell, especially if it is in an unbalanced state of electrolyte filling.

Direct observation of the oxidation nickel in molten carbonate

Abstract Polarization of the nickel oxide (NiO) cathode limits the performance of the state-of-the-art MCFC. It is therefore important to clarify the phenomena which occur when, as is usually the case, the NiO cathode is formed in situ in the MCFC. This occurs by chemical or electrochemical reactions between nickel, which is the base material of the cathode, and molten carbonate (usually Li2CO3:K2CO3=62:38 mol%), which is the electrolyte. To clarify these formation phenomena, a direct observation method involving a telescope and CCD (charge coupled device) camera, in combination with potential measurements, is applied to the oxidation of a nickel sheet which is partially immersed in molten carbonate. In an atmosphere of pure CO2, a partially immersed nickel sheet is relatively stable, as is a gold foil even in oxidant gas. In the case of nickel exposed to oxidant gas, however, the area exposed directly to the oxidant gas is rapidly covered by an electrolyte film, and undergoes intensive chemical or electrochemical reactions with CO2 gas generation during oxidation and lithiation. As a consequence, a progressively rougher NiO surface develops over the entire sheet. After oxidation and lithiation, the non-immersed part of the sheet remains covered with electrolyte. Although the oxygen reduction current at the in situ lithiated NiO is over one order of magnitude higher than that at a gold electrode at the same applied potential, the extended meniscus region is the dominant reaction site for oxygen reduction. The same is true for the much more limited meniscus region of the gold electrode.

NiO Dissolution in Molten Carbonate Fuel Cells: Effect on Performance and Life

The short-circuit phenomenon caused by dissolution of the NiO cathode in the molten carbonate fuel cell was experimentally investigated. Monitoring CO{sub 2} concentration in the anode exhaust gas can be an effective way to detect cell short circuit. The effects of matrix thickness and cathode CO{sub 2} partial pressure on shorting were elucidated. The time-to-initial-short-circuit (shorting time) is approximately proportional to the second power of the matrix thickness and the reciprocal of the cathode CO{sub 2} partial pressure. This can be explained by the relationship between conductance of the short circuit and the Ni content of the matrix. A simple model to correlate the conductance with the shorting time was developed. It is concluded that part of the deposited Ni exists as lithiated NiO.

Performance and life of 10-kW molten-carbonate fuel cell stack using Li/K and Li/Na carbonates as the electrolyte

NiO cathode dissolution is a serious problem with molten carbonate fuel cells (MCFC). The target life-time of such cells is 40,000 h, but shorting by NiO cathode dissolution markedly decreases cell performance. NiO cathode dissolution depends on the composition of the molten carbonate electrolyte. The electrolyte generally comprises a mixture of lithium carbonate and potassium carbonate. Since the solubility of NiO in a mixture of lithium carbonate and sodium carbonate is lower than in lithium and potassium carbonate, it is expected that shorting by NiO cathode dissolution will take longer in a mixture of lithium and sodium. Therefore, a mixture of lithium carbonate and sodium carbonate is a strong candidate electrolyte. A unique 10-kW class stack, which uses a mixture of lithium and potassium carbonate and mixture of lithium and sodium carbonate as the electrolyte, has been developed and tested. The basic performance and life time of both electrolyte cells of the stack are reported. In particular, the change in cathode polarization caused by NiO cathode dissolution is evaluated quantitatively.

NiO Cathode Dissolution and Ni Precipitation in Li/Na Molten Carbonate Fuel Cells: Distribution of Ni Particles in the Matrix

Due to the dissolution of the lithiated nickel oxide cathode, the life expectancy of a molten carbonate fuel cell is reduced. The use of a Li/Na carbonate electrolyte is expected to lead to a higher voltage and a longer life expectancy due to its higher ionic conductivity and its lower nickel oxide cathode solubility. Using the Li/Na electrolyte, single cells have been tested to evaluate their performance and their life expectancy. Empirical equations for these cells have been presented to determine the temperature, the CO 2 partial pressure in the cathode gas, and the matrix thickness. The results prove that the life expectancy of LilNa cells is reduced by nickel short-circuiting in comparison to Li/K cells, for which the life expectancy is many times longer. The dependence of the nickel-containing particle distribution in the matrix on the temperature has been evaluated using an image processing method. At 973 K, most of the particle distribution moves toward the anode more rapidly than at 873 K, because the rate of particle growth is lower at the higher temperature, and the particles move toward the anode due to the convection of the molten carbonate in the matrix. The initiation time for nickel short-circuiting was derived from the results of this study to explain the relationship between the shorting conductance and the volume of nickel-containing materials in the matrix porosity. Moreover, the results show that the predominant element contributing to short-circuiting is the nickel oxide, and not the metal.