Toshiaki Konomi

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.

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,

Effect of Flow Field Pattern and Microporous Layer on Gas Purge of a Polymer Electrolyte Fuel Cell

The effect of flow field pattern and microporous layer (MPL) on the exhaust of the residual water at the flow channel, gas diffusion layer (GDL), and catalyst layer (CL) in the cathode of a polymer electrolyte fuel cell during gas purge after shutdown is separately studied for preventing freeze damage and efficient cold start below freezing point. The time variations of the high frequency and charge transfer resistances are obtained from electrochemical impedance spectroscopy to detect the drying of each component separately. This can be used to stop the purging appropriately to minimize energy consumption. An interdigitated type flow field drys the GDL substrate faster than a serpentine flow field. In addition, MPL drying process and its moisture retention effect on the drying of the CL and electrolyte membrane are observed. The serpentine flow field is shown to dry the MPL faster than the interdigitated type flow field.

Development of a PEFC with Serpentine-Interdigitated Hybrid Pattern Gas Channels

Serpentine-interdigitated hybrid pattern gas channels were developed for polymer electrolyte fuel cells (PEFCs). The performance of conventional (triple-serpentine channel), interdigitated and serpentine-interdigitated hybrid pattern gas channels were compared. The performance of the interdigitated channel is significantly higher than that of the conventional channel. However, the interdigitated channel causes flooding under high air utilization. The hybrid pattern channels have the highest performance of the three patterns. A through-pore ratio (A/A0) was also developed. This ratio indicates the level of flooding in the gas diffusion layer (GDL) under the rib. According to calculated values of this ratio, flooding is suppressed when using the hybrid pattern channels.

Estimation of Water Layer Thickness at the Cathode Catalyst Layer Surface of Polymer Electrolyte Fuel Cells from Diffusion Impedance Analysis

We have analysed the diffusion impedance of a polymer electrolyte fuel cell (PEFC) by electrochemical impedance spectroscopy (EIS). As a result, we derive oxygen diffusion distance δ adjacent to the cathode electrode (catalyst layer) assuming planar oxygen diffusion in a thin water layer unlike the conventional flooded-agglomerate model and gas phase diffusion model in a gas diffusion layer. The experimental results and their analyses show that δ agrees with a value reported from our previous ex-situ measurement. Moreover, δ gives reasonable concentration overpotentials. Increasing current density and gas humidification brings about the growth of δ. It supports that δ corresponds to the thickness of the accumulated product water in liquid state at the cathode electrode. Thus EIS analysis of the diffusion impedance can be used to diagnose the water accumulation at the cathode electrode. This result enables the prediction of the flooding and drying-up of PEFCs and assures their stable operation.

Research on PEFC Overvoltage Analysis Method by Impedance Technique (2nd Report, Separation of Concentration OV from Cathode OV)

A new analytical method which divides the cathodic overvoltage (OV) in PEFC into the activation and concentration OVs has been developed. In this method, PEFC is simulated with an equivalent electric circuit consisting of resistances and capacitances, whose characteristic values are estimated by FFT (Fast Fourier Transform) impedance method. When oxygen gas is supplied into the cathode, the activation OV can be regarded as dominant since the cathodic OV obeys the Butler-Volmer equation. This is due to the negligibly small concentration OV. On the other hand, when air is supplied into the cathode, the cathodic concentration OV can be calculated by subtracting the cathodic activation OV from the cathodic OV. This concentration OV is larger than that measured with conventional method in high and limiting current density region. In addition, unknown OV which is obtained by subtracting the cell voltage, cathodic, anodic and ohomic OVs from the Nernstian potential (E) is investigated. Although the unknown OV is almost constant for the current density and in good agreement with the difference between E and OCV (Open Circuit Voltage) in the case of the oxygen operation, it decreases in large current density region in the case of the air operation. This suggests that the unknown OV decreases with decrease in side reactions such as H2O2 formation, which may be due to the smaller oxgen concentration in the supplied gas.