Adoption of the liquefied natural gas (LNG) fueled solid oxide fuel cell (SOFC) to the marine power supplies and motive powers is expected to reduce fuel consumption and toxic air pollutant (NOx, SOx, Particulate Matter (PM), CO2) emissions compared with the conventional marine diesel engine. In the near future, SOFC-diesel hybrid motive power would be promising as the marine use where the SOFC power units operate as auxiliary power unit (APU) for the steady power demand, while the diesel engine responds to the load change[1]. SOFC-micro gas turbine combined units for the marine power systems can be developed by scaling-up a combined system under demonstration[2].

We thus focus on the development of real-time abnormal-diagnosis method to improve the reliability and durability required for the long-term safety and stable operation of the marine SOFC. Significant accident due to the breakdown of the cell should also be prevented. We therefore apply electrochemical impedance spectroscopy[1,3] to the diagnosis which enables safety precautions, operating condition modification, and emergency shutdown by prior abnormality detection of the SOFC through an elucidation of the degradation factor accompanying the marine operation.

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. Although there have been a number of numerical analyses, very few experimental investigations confirming in-situ current distributions have been reported. We have therefore addressed measurements of spatial current variations of an anode-supported microtublar[4] and an electrolyte-supported planar SOFCs[5] having segmented cathodes so far. In the present study, we investigate longitudinal current distribution and electrochemical impedance variation along the anode flow channels.

We prepared a planar cell having three longitudinally segmented cathodes assembled with segmented cathode separators for electrical insulation. The cell was composed of NiO-YSZ anode-support, YSZ electrolyte, GDC interlayer, and LSC cathode (ASC-10B, Elcogen, Estonia).

Each cathode segment had an area of 2.25 cm² (1.5 x 1.5 cm) while the anode had 19.5 cm²(6.5 x 3.0 cm). The anode and cathode separators made of stainless steel (Crofer 22 APU, VDM Metals GmbH, Germany) had flow channels with a width of 1 mm and a depth of 1 mm (MAGNEX Co., Ltd., Japan). The anode separator had 8 parallel flow channels having a length of 4.9 cm. Silver mesh was employed as current collector.

Current voltage (I-V) 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 with mixtures of H₂/N₂ and dried air at constant flow rates, respectively, in a cross-flow configuration. The cell was maintained at 650 °C by a tubular electric furnace at open circuit voltage (OCV).

As a result, fuel starvation in the downstream segment showed decays in cell performance, giving rise to longitudinal current distribution and impedance variation. These in-situ acquired distribution data are useful as a basis to develop the diagnosis of SOFCs including marine power applications fueled with reformed LNG.

References

1. Hironori Nakajima and Tatsumi Kitahara, Real-Time Electrochemical Impedance Spectroscopy Diagnosis of the Marine Solid Oxide Fuel Cell, J. Phys.: Conf. Ser., Vol. 745, 032149 (2016).
2. Mitsubishi to develop, SOFC-turbine triple combined cycle system, Fuel Cells Bulletin, Vol. 2012, 7, 5-6 (2012).
3. Hironori Nakajima, Toshiaki Konomi, Tatsumi Kitahara, and Hideaki Tachibana, Electrochemical Impedance Parameters for the Diagnosis of a Polymer Electrolyte Fuel Cell Poisoned by Carbon Monoxide in Reformed Hydrogen Fuel, J. Fuel Cell Sci. Technol. Vol. 5 041013 (2008).
4. 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).
5. 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).