Experimental Investigation of the Porous Nickel Anode in the Molten Carbonate Fuel Cell

Abstract

The electrochemical performance of the porous nickel anode in a molten carbonate fuel cell was experimentally investigated in this study. The electrode structure was studied by scanning electron microscope and Hg porosimetry. The effect of electrolyte filling, on overpotential at a constant current density, was also examined. Kinetics for the hydrogen reaction was studied by obtaining stationary polarization curves, for porous nickel anodes at varying temperatures and anode gas compositions. The slopes of these polarization curves were analyzed at low overpotentials, with the assumption that the porous anode was under kinetic control, and the corresponding exchange current densities were determined using a simplified porous electrode model. The obtained partial pressure dependencies of the exchange current density were high, and therefore, difficult to explain by the generally assumed mechanisms.The molten carbonate fuel cell ͑MCFC͒ operates at high temperatures ͑600-700°C͒, which gives it the potential to reach high overall efficiency in an integrated system. It is in general operated using natural gas as its fuel, although high concentrations of carbon monoxide and carbon dioxide can also be a part of the feed. This makes it very suitable for use with biogas, gasified coal, or gasified biomass feeds although the two latter fuels will result in much lower concentrations of hydrogen gas. Operation with a lower concentration of hydrogen increases the losses at the anode, providing a reason for investigating and optimizing the performance of the anode under such conditions. The overvoltage losses at the anode are considered to be relatively small, which is partly due to the relatively low current densities that present MCFC systems operate at. A significant increase in current density, which is desirable in order to improve the power density, would result in the increase of losses at the anode.A general procedure for elucidating the mechanism of the anode reaction is to determine the partial pressure dependency for the components, hydrogen, carbon dioxide, and water, that partake in the overall electrochemical reactionSeveral studies have previously been made to find out the reaction mechanism at the anode, 1-18 Table I. Most experiments were conducted on flag electrodes, which are quite different from porous electrodes, when regarding the electrode surface and mass transfer conditions. Despite this research, the reaction mechanism is still not well defined. One problem when evaluating the partial pressure dependency is the actual gas composition inside the cell, since different reactions can take place at the anode. One such reaction is the electrochemical reaction of carbon monoxideAnother possible reaction is the shift reaction, which is sum of Reactions 1 and 2According to Borucka and Appleby, 19 the reaction rate for direct carbon monoxide oxidation is extremely slow at a surface of 99.99% pure gold. They claimed that the most likely way for carbon monoxide to be utilized is through the shift reaction as that ...

Electrode Kinetics of the Ni Porous Electrode for Hydrogen Production in a Molten Carbonate Electrolysis Cell (MCEC)

“…When compared to reaction orders for hydrogen oxidation mentioned in Table I, the experimentally obtained data for water electrolysis are still higher. But the partial pressure dependencies evaluated in this study are rather close to the result of Lindbergh et al, 13 0.5 for hydrogen and carbon dioxide, and 0.7 for water in fuel cell operation. Since similar porous electrode materials were used and analyzed by polarization data in both studies, it implies that hydrogen production in MCEC mode may be the reversed reaction of hydrogen oxidation in fuel cell operation.…”

Operating the nickel electrode with hydrogen-lean gases in the molten carbonate electrolysis cell (MCEC)

“…In addition, the effective conductivity of the Ni electrode is much higher than that of the electrolyte phase, then a simplified expression (Eq. 1) is obtained for the polarization resistance of the Ni electrode [21,22].…”

“…The effective conductivity of the electrolyte is set to be 5.0 S·m -1 at 650°C [22]. By using the Arrhenius equation [26], the values of 4.…”

“…However, the structure seems to consist of very small particles that are not easily distinguished in the SEM micrographs. Thus, to obtain particle sizes and pore size distribution more accurately, it would be necessary to perform, for example, mercury porosimetry measurements [22]. The iR-corrected polarization curves of the Ni electrode are measured in the temperature range of 600-650 °C for all gas compositions, see Fig.…”

Electrode Kinetics of the Ni Porous Electrode for Hydrogen Production in the Molten Carbonate Electrolysis Cell (MCEC)

“…This gave a higher dependence on the hydrogen partial pressure. The Ni porous electrode was also experimentally studied for hydrogen oxidation, 11,13 but the reaction mechanism is still not well defined.…”