Hironori Nakajima

Electrochemical impedance spectroscopy analysis of an anode-supported microtubular solid oxide fuel cell

An impedance separation analysis of the anode and cathode of a practical solid oxide fuel cell (SOFC) is conducted. Electrochemical impedance spectroscopy with a two-electrode setup is applied to an anode-supported intermediate temperature microtubular SOFC composed of a Ni/(ZrO 2 ) 0.9 (Y 2 O 3 ) 0.1 cermet anode, a La 0.8 Sr 0.2 Ga 0.8 Mg 0.2 O 2.8 electrolyte, and a (La 0.6 Sr 0.4 ) (Co 0.2 Fe 0.8 )O 3 cathode. Measurements are carried out for the cell operated at 700°C with varying flow rates and compositions of the H 2 /N 2 mixture gas fed into the anode and the O 2 /N 2 mixture gas fed into the cathode. The anode and cathode impedances are thereby separately assigned to low and high frequency impedance spectra, respectively. An equivalent circuit model is applied to the spectra to acquire the polarization resistances and associated capacitances for the charge and mass transfer processes at the anode and cathode and the cell ohmic resistance. The variation in these circuit parameters are then obtained in accordance with current densities and anode gas-feed conditions. In addition, the hydrogen diffusion length correlated with the Nernst loss in the axial direction of the anode substrate tube is estimated. These parameters obtained separately and simultaneously for each part of the cell are informative for a detailed analysis and diagnosis of practical SOFCs under operation.

Electrochemical Impedance Parameters for the Diagnosis of a Polymer Electrolyte Fuel Cell Poisoned by Carbon Monoxide in Reformed Hydrogen Fuel

We have investigated the behavior of an operating polymer electrolyte fuel cell (PEFC) with supplying a mixture of carbon monoxide (CO) and hydrogen (H 2 ) gases into the anode to develop the PEFC diagnosis method for anode CO poisoning by reformed hydrogen fuel. We analyze the characteristics of the CO poisoned anode of the PEFC at 80°C including CO adsorption and electro-oxidation behaviors by current-voltage (I-V) measurement and electrochemical impedance spectroscopy (EIS) to find parameters useful for the diagnosis. I-V curves show the dependence of the output voltage on the CO adsorption and electro-oxidation. EIS analyses are performed with an equivalent circuit model consisting of several resistances and capacitances attributed to the activation, diffusion, and adsorption/desorption processes. As the result, those resistances and capacitances are shown to change with current density and anode overpotential depending on the CO adsorption and electro-oxidation. The characteristic changes of those parameters show that they can be used for the diagnosis of the CO poisoning.

The surface adsorption of hydride ions and hydrogen atoms on Zn studied by electrochemical impedance spectroscopy with a non-equilibrium thermodynamic formulation

Abstract We show that non-equilibrium thermodynamics theory for surfaces combined with electrochemical impedance spectroscopy can be used to derive the excess surface concentrations of reactants and products of an electrochemical reaction at an electrode. We predict the equivalent circuit for a postulated reaction using this theory, and derive expressions for the excess surface concentrations. The method is illustrated with experimental data for the following hydride reaction to hydrogen at a Zn anode in a molten eutectic mixture of LiCl and KCl at 673 K: The results support a two-step mechanism for hydrogen evolution via the hydrogen atom. We calculate the excess surface concentrations of the hydride ions and the hydrogen atoms at the metal surface, and find that the hydride ions cover a fraction of the surface while the hydrogen atoms are present in large excess. The excess surface concentration of the hydride ions varies largely with the polarized state of the surface, and so does its mean activity coefficient at the surface. The results contribute to a better understanding of the system in question. The method is general and is expected to give similar information for other electrodes.

Influence of Hydrophilic and Hydrophobic Double MPL Coated GDL on PEFC Performance without Cathode Humidification

Gas diffusion layers (GDLs) coated with a hydrophobic microporous layer (MPL) have been commonly used to improve the water management properties of polymer electrolyte fuel cells (PEFCs). In the present study, a new hydrophilic and hydrophobic double MPL coated GDL was developed to achieve further enhancement of PEFC performance without cathode humidification. A hydrophobic binder of polytetrafluoroethylene (PTFE) and a hydrophilic binder of polyvinyl alcohol (PVA) were used to coat the MPL on the carbon paper substrate. The hydrophilic layer is effective for conserving humidity at the catalyst layer, while the hydrophobic intermediate layer between the hydrophilic layer and the substrate prevents the removal of water in the hydrophilic layer via dry air in the substrate. Reducing the hydrophilic layer thickness to 5 μm was effective for enhancing PEFC performance. Appropriate enhancement of hydrophilicity by increasing the PVA content to 5 mass% was also effective for enhancing PEFC performance.

Influence of Gas Diffusion Layers with Microporous Layer on the Performance of Polymer Electrolyte Fuel Cells

The gas diffusion layers (GDLs) coated with a microporous layer (MPL) in polymer electrolyte fuel cells (PEFCs) have been commonly used to avoid drying-out of the membrane electrode assembly (MEA) under lowhumidity conditions and also to reduce flooding under high-humidity conditions. Because the pore size, porosity, thickness and hydrophobicity of the MPL significantly influence the gas diffusion and water management characteristics, it is important to clarify appropriate design parameters of the MPL enhancing the PEFC performance. The present study was carried out to find out the influences of the MPL design parameters on the PEFC performance under highand low-humidity conditions. The MPLs, containing 20% PTFE and 80% carbon black, were coated on the substrate using a bar coating machine. The substrate was a GDL with hydrophobic treatment by 5% PTFE loading (SGL SIGRACET 24BA). Figures 1 and 2 show the surface and cross-sectional views of a GDL with the MPL. The mean flow pore diameter dm of the MPL varied between 1 and 10 m. The MPL thickness hMPL considering the penetration in the substrate varied between 90 and 240 m. The total thickness of the GDLs with the MPL was set at 250 m. The PEFC performance tests were carried out under highand low-humidity conditions. The relative humidity of the supplied gases at the cathode was set at either 100% or 0%, while maintaining the relative humidity of 100% at the anode. The cell temperature was set at 75°C. The hydrogen utilization was set at 70% and the air utilization was set at 60%. The active area of the MEA (PRIMEA 5580) was 4.2cm. Figure 3 shows the influence of the MPL mean flow pore diameter for the cathode GDL on the PEFC performance under high-humidity conditions. The 24BA GDL without the MPL was used at the anode. The performance obtained with the 24BA cathode GDL without the MPL is low due to flooding at high current densities. The MPL coating reduces flooding, enhancing the PEFC performance. The PEFC performance varies greatly depending on the MPL pore diameter. A decrease in the MPL pore diameter is effective for reducing flooding, enhancing the PEFC performance. However, when the pore diameter becomes too small, the PEFC performance tends to decrease. Figure 4 shows the influence of the MPL thickness on the PEFC performance under high-humidity conditions. Reducing the MPL thickness improves in-plane permeability. This also enhances the ability of the MPL to avoid flooding. Figure 5 shows the influence of the MPL mean flow pore diameter for the cathode GDL on the PEFC performance under low-humidity conditions. The MPL is effective for preventing drying-out of the MEA. A decrease in the MPL pore diameter reduces through-plane permeability, enhancing the ability of the MPL to prevent drying-out.

Estimation of Water Layer Thickness Adjacent to the Cathode Catalyst Layer of a PEFC (Analysis Using Electrochemical Impedance Spectroscopy)

PEFC (Polymer Electrolyte Fuel Cell) is one of the most appropriate energy devices for future vehicles and houses. Nevertheless, there are still many difficult problems to solve. In particular, water management including the flooding at GDL (Gas Diffusion Layer) and electrode (catalyst layer), plugging in channel, and drying-up of MEA (Membrane Electrode Assembly) are the representative problems. Concerning this, detecting these phenomena at an early stage offers more reliable and practical PEFC system. To date, we have researched the diagnosis of PEFCs using EIS (Electrochemical Impedance Spectroscopy) 1, . In the present study, to detect the flooding, plugging, and drying-up on ahead, we then research water accumulation at the cathode electrode (catalyst layer) from the analysis of impedance variation with time at constant current density by EIS. Assuming planar oxygen diffusion in a thin water layer (Fig. 1) at the cathode unconventionally, impedance for the finite length diffusion, Zdif, is expressed as below,

Infrared Spectroscopy of Molten LiCl-KCl-LiH

Infrared spectroscopy of molten LiCl-KCl-LiH at 673 K provided absorption band in the wavenumber region of for several ion concentrations. This absorption band is ascribed to the vibration of ion pair in the melt. The absorption band ascribed to the vibration of ion pair was not observed in the melt. The existence of the Li-H interaction shows a strong attractive force between ion and ion in the melt.

Real-Time Electrochemical Impedance Spectroscopy Diagnosis of the Marine Solid Oxide Fuel Cell

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.

Enhancement of fuel transfer in anode-supported honeycomb solid oxide fuel cells

An anode-supported honeycomb solid oxide fuel cell can achieve high volumetric power density and improve thermo-mechanical durability at high temperatures. We have so far shown the promising power densities and investigated the effect of flow channel configurations on the cell performance in terms of the hydrogen partial pressure distributions in the cell under operation. In the present study, current-voltage characteristics of the cell depending on thicknesses of the porous anode substrate and forced convection in the substrate are studied under different flow rates of fed hydrogen to clarify the effect of 3-dimensional fuel transport in the porous anode substrate on the cell performance.

Performance of an Anode-Supported Honeycomb Solid Oxide Fuel Cell

An anode-supported honeycomb solid oxide fuel cell can work with high power density and improve thermo-mechanical durability at high temperatures. We have thus fabricated the honeycomb cell with an electrolyte layer of 8YSZ on an anode honeycomb substrate of Ni/8YSZ. The cathode layer is LSM-YSZ composite. Current-voltage and current-power density characteristics of the cells having different anode and cathode flow channel configurations are measured under different hydrogen flow rates. We also evaluate the hydrogen mole fraction distributions in the honeycomb cell using finite element method, and discuss appropriate anode and cathode flow channel configurations. The present study is a starting point of developing an anode-supported honeycomb cell for cell stacks assembled with multiple and large scale honeycomb cells which can achieve high efficiency flow channel and current collecting configurations, and enhanced thermo-mechanical durability.