Junji Niikura

Proton Conductive Properties of Gadolinium-Doped Barium Cerates at High Temperatures

Abstract Proton conductive properties of Gd-doped BaCeO 3 ceramics were investigated by electrochemical methods (hydrogen permeation test and hydrogen-oxygen fuel cell test) in the temperature range of 600°C to 1000°C. As a result of electrochemical hydrogen- permeation tests, it was verified that the proton was the only conductive species in BaCe 1− x Gdi x O 3−α in the absence of oxygen at 600°C–1000°C. Next the relationship between conductivity and Gd-substitution value of x BaCe 1− x Gd x O 3−α (0.5 x 0.8 Gd 0.2 O 3-α ceramics with a thickness of 0.5 mm as a solid electrolyte was constructed. The short-circuit currents of this cell were 1.2AA cm -2 at 1000°C, and 0.8 A cm −2 at 800°C, which were the best performances in the fuel cell using perovskite-type electrolyte instead of YSZ. The transference numbers of mobile ions (protons and oxide ions) were determined from the evolution rate of water vapor at each electrode when the fuel cells were discharged. Regarding the temperature dependence on the transport numbers of proton and oxide ions, it was found that protonic conduction was fully dominant at 600°C. On the other hand, oxide ions were the main charge carriers at 1000°C and mixed conduction was observed in the temperature range of 600–1000°C.

The Exchange Current Density of Oxide Cathodes in Molten Carbonates

The exchange current density (i 0 ) for oxygen reduction has been measured by using a potential step technique on nonporous oxide electrodes in an Li/K(62/38 mole percent) carbonate eutectic melt at 923 K. Nonporous oxides were fabricated by a hot isostatic pressing method to be as dense as possible to study the kinetic parameters of the oxygen reduction on smooth oxide electrodes. However, the nonporous oxides had slightly rough surfaces. Accordingly, to allow for electrode roughness, the area of the nonporous oxide electrodes were corrected from the double-layer capacity values to make a comparison between our nonporous oxide electrodes and those used by other researchers

Fabrication and properties of combined electrode/electrolyte tape for molten carbonate fuel cells

A process which would simplify the fabrication procedure of a Molten Carbonate Fuel Cell (MCFC) is described. Green tapes of the combined electrode/electrolyte type were fabricated by a double doctor blade method. Then the combined tapes were burnt-out in-cell by elevating the temperature to 650°C. During the burn-out process, under stack compression (0.15 MPa), the electrolyte composites became dense (2.24 g cm−3) and gas-tight. On the other hand, the anode tape and the cathode tape were sintered orin situ oxidized to form porous plaques. Single cells made by this process exhibited performance characteristics of 0.80 V at 150 mA cm−2 (H2/CO2/H2O 66/16/18Uf=65%, air/CO2 70/30U0=61%). Though there remain some problems, such as thickness decrease of the tape and cross-leak during the burn-out process, this new process may be attractive for MCFCs.