Direct Current Distribution Measurement of an Electrolyte-Supported Planar Solid Oxide Fuel Cell under the Rib and Channel by Segmented Electrodes

Solid oxide fuel cells have been of practical concern as a high efficiency power generation device. Problems of the SOFCs include current distribution that decays the total cell performance and efficiency, and causes electrode degradation chemically and thermo-mechanically.

In the case of the planar SOFCs, the fuel/oxidant distributions and current collecting resistance cause current and temperature distributions over the electrodes under the separator ribs and flow channels. Optimized design of the separator is hence required to improve the power generation characteristics and durability of practical fuel cell stacks.

Although there have been a number of numerical analyses, very few experimental investigations confirming in-situ current distributions to reveal the influence of the separator structure have been reported. The aim of the present study is thus to measure in-plane spatial current variations of an electrolyte-supported planar SOFC with segmented cathodes under the rib and the flow channel.

We used the planar cell having three segmented cathodes (Fig. 1) assembled with segmented cathode separators for electrical insulation. The segmented cathodes were placed opposing to a rib and a set of parallel flow channels of the anode separator. The cell was composed of NiO-10Sc1CeSZ anode, 10Sc1CeSZ electrolyte, GDC interlayer, and LSCF-GDC cathode. The electrode area was 1.4 cm²(2.8 x 0.5 cm) each. The anode and cathode separator made of SUS430 had the flow channels with a width of 3 mm, a depth of 1 mm, and a length of 2.8 cm. Silver mesh was employed for the current collection of both sides.

Current voltage (I-V) measurements were carried out under voltage control using three electric loads to reproduce the electrode potential of a single cell[1]. The anode and cathode were electrically connected with the four-terminal method. The anode NiO was reduced to Ni by feeding H₂/N₂ mixture gas for 2 hours prior to measurements. During measurements, anode and cathode were fed upward with mixtures of H₂/N₂and dried air at constant flow rates, respectively. The cell was maintained at 800 °C by an electric furnace at open circuit voltage (OCV)

We prepared two types of the cathode separators. One of them had the same flow channel configuration as the anode separator where the rib and flow channels faces each other. The other had additional flow channels opposing to the anode rib. With the former cathode separator, the overpotential at the electrodes under the anode rib was significantly large compared with the flow channel part, whereas the latter case exhibited smaller overpotentials under the rib. This indicates that oxygen transport becomes poor under the wider rib in the cathode side. The latter case also indicated that the electrode under the rib exhibited smaller overpotential than the flow channel part when fuel is sufficient, owing to the smaller contact resistance under the rib by the compression. We also calculated current and hydrogen/oxygen partial pressure distributions by finite element simulation (COMSOL Multiphysics). The current distributions can be calculated so that they agree with the in-situ current distributions derived by the segmented cathodes from setting the exchange current densities, electrode porosities, electrolyte ion conductivities, and electrode ion/electron conductivities. Thereby oxygen starvation under the wider cathode rib and larger electron conduction resistance for the flow channel part were numerically evaluated to explain the in-situ currents for the segmented cathodes. We will present the results for the case of H₂ starvation.

[1] Ö. Aydin, T. Koshiyama, H. Nakajima, T. Kitahara, In-situ Diagnosis and Assessment of Longitudinal Current Variation by Electrode-Segmentation Method in Anode-Supported Microtubular Solid Oxide Fuel Cells, J. Power Sources, Vol. 279, 218–223 (2015).

Fig. 1 Segmented cathodes on the planar SOFC.