Shoji Yoshioka

Dependence of Entropy Change of Single Electrodes on Partial Pressure in Solid Oxide Fuel Cells

This paper reports on the dependence on hydrogen partial pressure of the entropy change of the hydrogen electrode reaction on platinum electrodes on yttria-stabilized zirconia that was estimated from measurements of the Seebeck coefficient of the yttria-stabilized zirconia. The entropy change of the oxygen reaction also was estimated by Seebeck coefficient measurements. The total entropy change of the reaction in a solid oxide fuel cell was calculated from the thermodynamic data. An empirical equation for the dependence of the entropy change of the hydrogen electrode reaction on hydrogen partial pressure was obtained from the observed entropy change of the oxygen electrode and the total entropy change. The observed dependence of the entropy change of the hydrogen reaction on hydrogen partial pressure was in good agreement with the empirical equation.

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.

Induction of In-plane Current at Start-and-Stop Operations on a PEFC

In-plane induction currents in electrodes and bipolar plates by reactant fed into a PEFC were evaluated using a 20-segmented cell. When hydrogen was fed into anode in the condition of anode (negative electrode) and cathode (positive electrode) exposed air, in-plane currents were detected in both electrodes in spite of the open-circuit condition. In-plane currents of anode and cathode in each segment had each other opposite direction and nearly equal quantity. Simultaneously, carbon dioxide and oxygen were also detected in the cathode exhaust.

Development of PEFC Electrode for Stable Running Under High Temperature and Low Humidity Condition

Introduction It is effective to operate proton exchange membrane fuel cells (PEFC) system at the high temperature (more than 85 oC) to utilize exhaust heat of the PEFC in a wide use. However, when PEFC is operated at a high temperature, it is necessary to prepare membrane-electrode assembly (MEA) corresponding to the low humidification condition of operation because there is the problem that relative humidity of the supply gas decreases, electrolyte in MEA dries and cell performance deteriorates. Ionomer in catalyst layer shows good proton conductivity when it is properly hydrated. In order to prepare the high water-holding catalyst layer showing good cell performance under low humidity condition, it is tested that quantity of water contained in the catalyst layer and behavior of the water detachment by estimating the weight change. Perfluorosulfonic acid-based ionomer and carbonsupported platinum catalyst was mixed, and the prepared catalyst layer includes various additive ratio of ionomer to catalyst layer weight and various ion exchange capacity (IEC) ionomer. Then the catalyst layer was maintained for two hours in the constant temperature and humidity bath which was kept 80 oC and 80%RH in order to adsorb water. After that, I measured a weight change by thermogravimetry analysis (TG) when it was heated in 4oC /min and dried and calculated moisture content from quantity of adsorbed water. In addition, the catalyst layer was applied to cathode electrode of a 25 cm sized cell and tested its characteristic. The cell voltage was measured when it was operated at 0.25A/cm consecutively more than three hours under constant humidity condition.