In-Situ Analysis of the in-Plane Current Distribution Difference between Electrolyte-Supported and Anode-Supported Planar Solid Oxide Fuel Cells by Segmented Electrodes

Abstract

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 (interconnector) 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 validating in-situ current distributions to reveal the impact of the separator structure have been reported. We have therefore addressed measurements of in-plane spatial current variations of electrolyte-supported planar SOFCs having segmented cathodes under the rib and the flow channels so far1,2. In the present study, we extend the elucidation of the effect of the rib width on the current distribution to that of an anode-supported planar cell to develop generalized numerical model validated with the in-situ acquired distributions in the electrolyte-supported and anode-supported cells. 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-YSZ anode-support, YSZ electrolyte, and LSCF cathode (H.C. Starck). 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 (I-V) measurements were carried out under voltage control using three electric loads to reproduce the electrode equipotential of a single cell3. 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. As a result, the larger rib width gives smaller current density under the rib as the case of the electrolyte-supported cell2. However, fuel consumption under the rib depending on the rib width shows significant impact on the current under the channel, whereas the channel current shows small variation with the change of the rib width in the electrolyte-supported cell2. We then 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. 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).Tatsuhiro Ochiai, Hironori Nakajima, Takahiro Karimata, Tatsumi Kitahara, Kohei Ito, Yusuke Ogura, Jun Shimano, In-situ Analysis of the In-plane Current Distributions in an Electrolyte-Supported Planar Solid Oxide Fuel Cell by Segmented Electrodes, ECS Trans., Vol. 75, 52, 91-98 (2017).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).