Anode-Supported Planar Solid Oxide Fuel Research Articles

Three-Dimensional Structure Research of Anode-Supported Planar Solid Oxide Fuel Cells

A three-dimensional model of anode-supported planar solid oxide fuel cells (SOFCs) was developed. The output performance, temperature distribution, and concentration distribution of a SOFC stack were studied when the pitch width and rib width on the anode side of the SOFC model were different from those on the cathode side. The results showed that under the same voltage, SOFC performance can be improved by using a smaller connector rib width on the cathode side than on the anode side, and the output power can be increased by 24%. It was also found that under the same conditions, the larger pitch width of the connector on the anode side and the smaller pitch width on the cathode side could improve the output performance of SOFCs. The research provides a reference for the design optimization of SOFC stacks.

Optimization of Cathode Structure for Anode-Supported Planar Solid Oxide Fuel Cell

Abstract It is always an important research direction to improve the performance of solid oxide fuel cells (SOFCs) through structural optimization. For anode-supported SOFC, the thin cathode thickness results in an uneven distribution of oxygen concentration in the porous electrode, which limits the output performance. In this paper, a three-dimensional model of the anode-supported planar SOFC was established, and the thickness of the cathode diffusion layer in the area covered by the connector was increased to improve the uniformity of oxygen distribution in the cathode. The results show that increasing the thickness of the cathode diffusion layer under the connector can significantly improve the overall output performance of SOFC. The uniformity of oxygen concentration distribution in the cathode is improved, the local current density under the connector is increased, and the maximum output power density of the cell is increased by 29.14%. The results show that the output performance of the SOFC and the uniformity of gas distribution in the porous electrode can be improved by increasing the thickness of the cathode diffusion layer under the connector in the structure design and practical application of the SOFC. The research of this paper provides a reference for the optimization design of SOFC.

Numerical Study on Mass Transfer and Electrical Performance of Anode-Supported Planar Solid Oxide Fuel Cells With Gradient Porosity Anode

Abstract Microstructure modification of thick anode is an effective way to enhance cell performance of the anode-supported planar solid oxide fuel cells (SOFCs). In this work, the influence of multilayer anode microstructure with gradient porosity on cell mass transfer and electrical performance is numerically investigated. The coupled phenomena of fluid flow, multicomponent mass transfer, charge transport, and electrochemical reactions of SOFC, in three-dimensions (3D), are simulated by using the finite element computational fluid dynamics approach. Quantitative analyses of hydrogen concentration and anodic overpotentials are conducted to better understand the effect mechanism of the gradient porosity anode on the cell performance. The effect of gradient porosity distribution on the cell performance is also systematically discussed. It is found that the gradient porosity anode can significantly enhance the cell mass transfer performance to reduce the anodic concentration overpotential. The combined effects of activation, concentration, and ohmic overpotentials can effectively improve the cell electrical performance. For the cases studied, porosity gradient and porosity of anode functional layer 2 (AFL2) both range from 0.1 to 0.3. Results indicate that increasing the porosity gradient or porosity of AFL2 can enhance the cell mass transfer performance. As the porosity of AFL2 is higher than 0.2, the gradient porosity anode design is beneficial to improve the cell electrical performance.

Electrochemical Impedance Spectroscopy Analysis of Carbon Deposition in an Anode-Supported Planar Solid Oxide Fuel Cell By Segmented Electrodes

Solid oxide fuel cells (SOFCs) have been of practical concern for high efficiency power generation and fuel flexibility. In the planar SOFCs, current and temperature distributions due to fuel consumption and steam generation occur from the upstream to the downstream part along the anode flow channels, causing problems such as lower efficiency and durability. Furthermore, durability to carbon deposition is a common problem with hydrocarbon fuels derived from natural gas and biogas. Identification of degradation factors and locations leads to development of diagnosis method and optimized operation control for prevention of failure and accident. In-situ electrochemical data by the degradation give useful parameters for health diagnosis of SOFCs. In this study, we aim to clarify the spatial degradation behavior using an anode-supported planar cell with a cathode layer segmented into three parts. We fed dry methane gas to the anode as accelerated aging tests and measured the current distribution directly among the upstream, midstream and downstream parts along the anode flow channels. Electrochemical impedance spectroscopy analysis was carried out for each segment as well. For the obtained impedance spectra, curve fitting by nonlinear least squares method was carried out to separate each resistance for the analyses. Upstream part of the cell exhibited larger resistance with more enhanced carbon deposition than the other parts with the dry methane fuel.

Influence of Carbon Deposition on the Current Distribution in an Anode-Supported Planar Solid Oxide Fuel Cell In-Situ Assessed by Segmented Electrodes

For solid oxide fuel cells (SOFCs), durability to carbon deposition is a common problem to use hydrocarbon fuels from the city gas and the biogas. Identification of degradation factors and locations leads to development of diagnosis method and optimized operation control for prevention of failure and accident. Electrochemical data by the degradation give useful parameters for health diagnosis. In this study, we are aiming to understand spatial degradation, so that we prepared an anode-supported planar cell in which the cathode layer was segmented to three parts. We fed dry methane gas to the anode as accelerated aging tests and measured the current distribution directly among the upstream, midstream and downstream parts along the anode flow channels. Electrochemical impedance spectroscopy analysis was carried out in each segment as well. Upstream part of the cell exhibited larger overpotentials with more enhanced carbon deposition than the other parts with the dry methane fuel.

Segmented Electrode Analysis of an Anode-Supported Planar Solid Oxide Fuel Cell for the Diagnosis of Marine Power Applications

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 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).

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

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).

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

In the planar solid oxide fuel cell (SOFC), the fuel/oxidant distributions cause current and temperature distributions over the electrodes under the separator (interconnector) ribs and flow channels. Optimized designs of the separator to improve the output power and chemical/thermo-mechanical durabilities of practical stacks require numerical models validated by insitu current distributions measured. We have therefore clarified the difference of in-situ measurements inplane spatial current variations between an anode-supported planar SOFC and electrolyte-supported by segmented cathodes opposing the anode rib and flow channels. We focus on the effect of the rib width on the spatial current distribution. We model the current and hydrogen partial pressure distributions by finite element modeling so that the model agrees with the insitu measurements by the segmented cathodes with determining the exchange current densities, electrode porosities, electrolyte ion conductivities, and electrode ion/electron conductivities.

Segmented Electrode Analysis of an Anode-Supported Planar Solid Oxide Fuel Cell for the Diagnosis of Marine Power Applications

In planar solid oxide fuel cells (SOFCs), the fuel/oxidant distributions cause current and temperature distributions over the electrodes, giving chemical and thermo-mechanical cell degradations as well as decays in the total cell performance and efficiency. We 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. We therefore address in-situ measurements of inplane spatial current and impedance distributions along the flow channels of an anode-supported planar SOFC by segmented cathodes opposing to up, mid, and downstream parts of the anode.

Biogas from the organic fraction of municipal solid waste: Dealing with contaminants for a solid oxide fuel cell energy generator

The present work investigates electricity production using a high efficiency electrochemical generator that employs as fuel a biogas from the dry anaerobic digestion of the organic fraction of municipal solid waste (OFMSW).The as-produced biogas contains several contaminants (sulfur, halogen, organic silicon and aromatic compounds) that can be harmful for the fuel cell: these were monitored via an innovative mass spectrometry technique that enables for in-line and real-time quantification.A cleaning trap with activated carbons for the removal of sulfur and other VOCs contained in the biogas was also tested and monitored by observing the different breakthrough times of studied contaminants.The electrochemical generator was a commercial Ni anode-supported planar Solid Oxide Fuel Cell (SOFC), tested for more than 300h with a simulated biogas mixture (CH4 60vol.%, CO2 40vol.%), directly fed to the anode electrode. Air was added to promote the direct internal conversion of CH4 to H2 and CO via partial oxidation (POx).The initial breakthrough of H2S from the cleaning section was also simulated and tested by adding ∼1ppm(v) of sulfur in the anode feed; a full recovery of the fuel cell performance after 24h of sulfur exposure (∼1ppm(v)) was observed upon its removal, indicating the reliable time of anode exposure to sulfur in case of exhausted guard bed.

Fuel Flexibility of Anode-Supported Planar Solid Oxide Fuel Cell Evaluated With Developed Simulated-Reformate-Gas Generator

Background: In general, solid oxide fuel cell (SOFC) systems are said to be flexible to various kind of fuels such as natural gas and petroleum gas. The fuels are reformed at a reformer and supplied to anode. Cell testing for 10 to 100 W-class SOFCs needs steady supply of a real reformate gas or a simulated reformate gas. However, it is difficult to reform heavier hydrocarbons without know-how and to evaporate small flow-rate of water. In addition, cell performance comparison with reformate gases of various fuels has been scarcely reported. Method of approach: A new testing system, what we call “simulated-reformate-gas generator” was developed to simulate reformate gases from H2, O2, and CO2 stably and safely without dealing with toxic CO. An anode-supported planar Ni-YSZ/YSZ/LSCF cell (100 cm2) was subjected to voltage-current density (V-J) characteristic to discuss validity of the generator and to evaluate fuel flexibility with practical size of the cell. Results: It was clarified that equilibrium compositions at steam reforming of hydrocarbons and oxygen-containing biodiesel (C17H33COOCH3) can be simulated with ±1.0 mol.% precision by the generator. It was found that anode gas conditions can change quickly to shorten voltage stabilizing time at testing. Furthermore, it was elucidated that V-J characteristics hardly changed for simulated reformate of CH4, C3H8, kerosene (C12H24), and biodiesel at S/C = 3.0. DC electrical efficiency was estimated for the fuels as 54.2, 52.9, 52.5, 52.3% (LHV), respectively.Conclusions: The developed simulated-reformate-gas generator is so precise and useful for cell testing, making it easy to change anode gas conditions. As long as fuels for SOFC systems are reformed to thermodynamic equilibrium, cell performance and electrical efficiency will be comparable.

Microstructure Optimization Designs for Anode-Supported Planar Solid Oxide Fuel Cells

Suitable porous electrode design may play a significant role in the performance enhancement of solid oxide fuel cells (SOFCs). In this paper a genetic algorithm optimization method is employed to design electrodes based on a 2D planar SOFC model development. The objective is to find suitable porosities and particle sizes distributions for both anode and cathode electrodes so that the cell performance can be maximized. The results indicate that the optimized heterogeneous morphology may better improve SOFC performance than the homogeneous counterpart, particularly under relatively high current density conditions. The optimization results are dependent on the operating conditions. The effects of inlet mass flow rates and fuel compositions are investigated. The proposed approach provides a systematical method for electrode microstructure designs of high performance SOFCs.

Microscale Modeling of an Anode‐Supported Planar Solid Oxide Fuel Cell

AbstractA microscale model of a solid oxide fuel cell (SOFC) involving the mass transfer together with the electrochemical reaction, the transportation of electrons and ions through the respective spherical shaped electron conducting and ion conducting particles inside the electrodes was mathematically developed. Couples of useful parameters were introduced in order to represent the characteristics of the cell. The predicted cell performance was showed according to various operating and design conditions. The effects of microscale electrode geometry on the cell performance were also taken into account. Parametric study according to the volumetric fraction of ionic and electronic conducting particles was conducted in order to examine the effects of operating conditions on the cell overpotentials. The study results substantiate the fact that SOFC overpotential could be effectively decreased by increasing the operating temperature as well as operating pressure. This present study reveals the working mechanisms of SOFC at the microscale level, while demonstrating the use of microscale relations to enhance the SOFC performance. The accuracy of the presented model was validated by comparing with already existing experimental results from the available literatures.

Fabrication and Characterization of Anode-Supported Planar Solid Oxide Fuel Cell Manufactured by a Tape Casting Process

A planar anode-supported electrolyte was fabricated using a tape casting method that involved a single step cofiring process. A standard NiO∕8YSZ cermet anode, 8mol% YSZ electrolyte, and a lanthanum strontium manganite cathode were used for the solid oxide fuel cell unit cell. A pressurized cofiring technique allows the creation of a thin layer of dense electrolyte about 10μm without warpage. The open circuit voltage of the unit cell indicated negligible fuel leakage through the electrolyte film due to the dense and crack-free electrolyte layer. An electrochemical test of the unit cell showed a maximum power density up to 0.173W∕cm2 at 900°C. Approximated electrochemical properties, e.g., activation energy, Ohmic resistance, and exchange current density, indicated that the cell performance was significantly influenced by the electrode properties of the unit cell.

Modeling of Convective Heat and Mass Transfer Characteristics of Anode-Supported Planar Solid Oxide Fuel Cells

Convective heat and mass transfer in a planar, trilayer, solid oxide fuel cell (SOFC) module is considered for a uniform supply of volatile species (80%H2+20%H2O vapor) and oxidant (20%O2+80%N2) to the electrolyte surface with a uniform electrochemical reaction rate. The coupled heat and mass transfer is modeled by steady incompressible fully developed laminar flow in the interconnect ducts of rectangular cross sections for both the anode-side fuel and cathode-side oxidant flows. The governing three-dimensional mass, momentum, energy, species transfer, and electrochemical kinetics equations are solved computationally. The homogeneous porous-layer flow, which is in thermal equilibrium with the solid matrix, is coupled with the electrochemical reaction rate to properly account for the flow-duct and anode/cathode interface heat/mass transfer. Parametric effects of the rectangular flow-duct cross-sectional aspect ratio and anode porous-layer thickness on the variations in temperature and mass/species distributions, flow friction factor, and convective heat transfer coefficient are presented. The thermal and hydrodynamic behavior is characterized for effective convective cooling performance, and interconnect channels of cross-sectional aspect ratio of ∼2-3 along with relative anode porous-layer thickness of ∼0.5-1.5 are seen to provide optimal thermal management and species mass transport benefits in the SOFC module.