In-Situ Analysis of the in-Plane Current Distributions in an Electrolyte-Supported Planar Solid Oxide Fuel Cell By Segmented Electrodes

Abstract

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 planar SOFCs, the fuel and oxidant distributions 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. We have therefore addressed measurements of in­-plane spatial current variations of an electrolyte-­supported planar SOFC having segmented cathodes under the rib and the flow channels so far1. In the present study, we investigate the effect of the rib width on the current distribution. We used the planar cell having three segmented cathodes assembled with segmented cathode separators for electrical insulation. The segmented cathodes were employed 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 for the channel was 1.3 cm2 (2.5 x 0.5 cm) each while those for the ribs were 0.63 cm2 (2.5 x 0.25 cm), 1.3 cm2 (2.5 x 0.5 cm), 1.9 cm2 (2.5 x 0.75 cm). 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 2.5 cm. Silver mesh was employed for the current collection of both sides. Current voltage (I­V) measurements were carried out under voltage control using three electric loads to reproduce the electrode potential of a single cell2. 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/N2 and dried air at constant flow rates, respectively. The cell was maintained at 800 °C by a tubular electric furnace at open circuit voltage (OCV). We prepared cathode separators with flow channels facing those of the anode. Several flow channels were added opposing to the anode rib to minimize the cathode overpotential by poor oxygen transport under the cathode rib indicated in our previous report1 since we focus on the current distributions ascribed to the anode overpotential. Larger rib width results in larger anodic overpotentials under the anode rib compared with the anode flow channel according to a decrease in the inlet hydrogen flow rate. We also model the current and hydrogen partial pressure distributions by finite element simulation (COMSOL Multiphysics) so that the model agrees 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. This modeling is useful to design separators to improve the performance and durability of practical stacks. 1. Takahiro Koshiyama, Hironori Nakajima, Takahiro Karimata, Tatsumi Kitahara, Kohei Ito, Soichiro Masuda, Yusuke Ogura, Jun Shimano, Direct Current Distribution Measurement of an Electrolyte-Supported Planar Solid Oxide Fuel Cell under the Rib and Channel by Segmented Electrodes, ECS Trans., Vol. 68, 1, 2217-2226 (2015). 2. Özgür Aydin, Takahiro Koshiyama, Hironori Nakajima, Tatsumi 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).