An anode-supported honeycomb SOFC gives high volumetric power density and improve thermo-mechanical durability at high temperatures. We have so far fabricated and tested the honeycomb cell with a cathode layer of LSM and an electrolyte layer of 8YSZ on a porous anode honeycomb substrate of Ni/8YSZ. The anode-supported honeycomb cell exhibited promising volumetric power densities. In the present study, current-voltage and current-power density characteristics of the cells having different flow channel arrangement are measured under various inlet gas flow rates of fed hydrogen. We measure ohmic resistances of the honeycomb cells by current interrupt method, and indicate the impact of Ni re-oxidation resulting in high ohmic resistances. Fuel depletion in the cell causes Ni re-oxidation, and deteriorates the performance of the honeycomb cell.
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 cm 2 (1.5 x 1.5 cm) while the anode had 19.5 cm 2 (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 cell 2 . The anode and cathode were electrically connected with the four-terminal method. The anode NiO was reduced to Ni by feeding H 2 /N 2 mixture gas for 2 hours prior to measurements. During measurements, anode and cathode were fed with mixtures of H 2 /N 2 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 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). Mitsubishi to develop, SOFC-turbine triple combined cycle system, Fuel Cells Bulletin , Vol. 2012, 7, 5-6 (2012). 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). 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). 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).
We have investigated the behavior of an operating solid oxide fuel cell (SOFC) with supplying a simulated syngas to develop SOFC diagnosis method for marine SOFC units fueled with liquefied natural gas. We analyse the characteristics of syngas fueled anode of an intermediate temperature microtubular SOFC at 500 °C as a model case by electrochemical impedance spectroscopy (EIS) to find parameters useful for the diagnosis. EIS analyses are performed with an equivalent circuit model consisting of several resistances and capacitances attributed to the anode and cathode processes. The characteristic changes of those circuit parameters by internal reforming and anode degradation show that they can be used for the real-time diagnosis of operating SOFCs.
While generating power by an SOFC, reactants hydrogen and oxygen are consumed; simultaneously, hydrogen is diluted with the product water-vapor. Namely, concentrations of the reactants and product vary over the electrochemical active area along the respective flow fields. The chemical potentials in the anode and cathode therefore change along the flow fields, giving rise to the reversible Nernst-loss. When the concentration variations along the anode flow field are too large, i.e., the oxygen pressure is much higher than that of hydrogen, which is likely to occur in the downstream provided that the air is supplied at sufficient rates, nickel particles, the conventional catalysts in the anode, tend to re-oxidize. As a result, the length of the three-phase boundary would shorten, limiting the electrochemical performance. Besides, the anode microstructure would expand due to the larger volume of nickel-oxide, resulting in stresses and micro-cracks [1]. To prevent the nickel re-oxidation, concentration variations are desired to be identified and mitigated. Spatial concentration variations give also rise to spatial current and temperature variations. Current variations result in performance degradation, reducing the electric efficiency of the power generation. Given that the overpotentials are released as the waste heat, temperature variations develop in relation with the involving heat transport processes, e.g., convective and radiant heat transfer processes. The temperature variations induce thermal stresses into the cell components, and they affect the current variations through the overpotentials as well. It was shown that the concentration and temperature variations couple in the counter-flow configuration, resulting in larger variations in comparison with the co-flow configuration [2]. Spatial current and temperature variations are hence of great importance from both the energy conversion efficiency and mechanical durability aspects. Spatial characterization of concentration, current and temperature variations is rather challenging. The high operation temperature (773-1273 K) of SOFCs makes the spatial characterization more difficult. Vibrational Raman Spectroscopy [3] and IR Thermography [4] can be employed for diagnosing the spatial concentration and temperature, respectively; however, both of them are quite expensive and they require transparent materials for the gas distribution plate. Although the segmentation method is easy to implement on tubular-SOFCs [2], it is quite laborious to apply on planar-SOFCs [5]. These challenges can be circumvented by numerical tools. In principle, numerical tools are obliged to be verified by benchmark experimental data for assuring the reliability of investigations. For verifying SOFC models, we need to consider in situ measurable properties, such as voltage, current, and temperature. Among these properties, I-V (current-voltage) validation and temperature validation appear to be the most practical options, which are to ensure the computation-reliability of concentration. Even though the conventional I-V curves provide a good basis for the model-validation, they may not ensure the accurate computation of the spatial variations. It is a fact that an I-V validated model might predict a number of distinct temperature fields depending on the incorporated heat transfer processes. Thereby, the computation-accuracy of the electrochemical performance is expected to be highly affected by the inaccurate temperature fields. This study is hence devoted to investigating the role of temperature variations on the reliability of the numerical tools for computing the associated properties. Herein we present the spatial variations in the characteristic properties of a microtubular-SOFC, firstly calculated by the model validated with only the conventional I-V curve, and secondly by the model verified with temperature variations, in addition to validating with the conventional I-V curve. For these evaluations, we exploit the experimentally and numerically obtained spatial current and temperature variations in a microtubublar-SOFC. We in situ acquired the experimental data by applying the segmentation method on a microtubular-SOFC, whereas we computed the numerical data by a two-dimensional model developed for the respective experimental conditions. References [1] A. Faes et al. Membranes, vol.2, yy.2012, pp.585 [2] Ö.Aydın et al., J. Electrochem. Soc. , vol.163, yy.2016, pp.F216 [3] G. Schiller et al., Appl. Phys. B , vol.111, yy.2013, pp.29 [4] Y. Takahashi et al., Solid State Ionics , vol.225, yy.2012, pp.113 [5] P. Metzger et al., vol.177, yy.2006, pp.2045
The present paper refers to experimental studies on tribological characteristics of lubricated ceramics for cylinder liners and piston rings. The experiments have been carried out under the large sliding velocity condition close to that of practical engines with pin-on-disk-type reciprocative slide test equipment. The major results are: (i) For the materials, it is desirable that the ring surface get a moderate running-in under the more elastic contact condition, and that the liner and the ring be superior in thermal diffusibility and in heat resistance, respectively; (ii) The respective influences of roughness and velocity on the scuffing are so large that the roughness should be kept smaller at the faster slide; and (iii) The effects of additives in mineral oil on the lubricity and the scuffing resistance disappear, and used oil has worse influences on the two, so it is necessary to select the proper lubricants.
In reciprocating internal combustion engines, one of the most important issues is to clarify the frictional characteristics between the piston ring and the cylinder liner because of the demand for reduction in frictional loss as well as the solution to problems such as scuffing and wear with the advances in engine performance achieved in recent years. In the present study, the floating liner method in which the cylinder liner is supported by means of hydrostatic bearings was developed to measure accurately the frictional force of the piston assembly as a function of the crank angle during firing operation. The influences of engine operating conditions (engine speed, cylinder wall temperature, gas pressure in the cylinder), lubricating oil viscosity, polymer-containing multigrade oil and friction modifier on the frictional characteristics were evaluated on the basis of the experimental results obtained with this equipment.
Microporous layers (MPLs) applied to the catalyst layer (CL) side of the gas diffusion layer (GDL) of polymer electrolyte fuel cells have been developed to mitigate liquid water accumulation in the CL for oxygen transport to the cathode CL. A three-dimensional porous structure of our in-house hydrophobic MPL is numerically modeled with a pore network model (PNM). The convective air permeability and oxygen diffusivity, which depend on liquid saturation, are evaluated. To construct the PNM, focused ion beam scanning electron microscopy (FIB-SEM) is used to derive the pore size distribution. The model is ex-situ validated through air permeability and oxygen diffusivity tests with controlled saturation of non-volatile wetting liquid that is stable in the hydrophobic MPL. Oxygen diffusivity of the MPL is obtained by identifying the diffusion resistances of the concentration boundary layers and GDL substrate in the tests. The model predicts the effects of liquid water saturation in the MPL on the air and liquid water permeations, and the oxygen diffusion.
