Yuichiro Tabuchi

A Multiphase Model for Cold Start of Polymer Electrolyte Fuel Cells

A multiphase and transient model is presented to describe transport and electrochemical processes with ice formation during startup of polymer electrolyte fuel cells (PEFCs) from subzero temperatures. The model accounts for ice/frost precipitation and growth in the cathode catalyst layer (CL) and gas diffusion layer, water transport at very low temperatures, heat transfer with phase transition, oxygen transport, electrochemical kinetics, and their mutual interactions. The governing equations of mass, momentum, species, heat, and charge transport under cold-start conditions are developed in a single-domain framework and solved by a finite-volume-based computational fluid-dynamics technique. Validated by extensive experimental data, this computational model is used to predict PEFC cold-start performance as well as to reveal 3D distributions of current density, temperature, membrane water content and ice fraction in the CL. Effects of startup current density and membrane thickness are numerically explored to illustrate the utility of the model.

Durability of Membrane Electrode Assemblies under Polymer Electrolyte Fuel Cell Cold-Start Cycling

Electrochemical and microstructural measurements of membrane electrode assemblies (MEAs) cycled under cold-start conditions are reported. An experimental protocol using a single-cell fixture was developed for MEA durability tests under cold-start cycling. Electrochemical diagnostics using high-frequency resistance and pure O 2 found that MEA no. 1 cycled under 100 mA/cm 2 from -30°C does not show any degradation after 100 cycles, MEA no. 2 cycled under 300 mA/cm 2 from -30°C exhibits mild degradation after 150 cycles, and MEA no. 3 cycled under 500 mA/cm 2 from -20°C suffers severe degradation after 110 cycles. Transmission electron microscopy and X-ray diffraction using cross-sectional samples of the aged MEAs further revealed three primary degradation mechanisms: (i) interfacial delamination between the cathode catalyst layer (CL) and membrane, (ii) cathode CL pore collapse and densification upon melting of a fully ice-filled CL, and (iii) Pt particle coarsening and Pt dissolution in perfluorosulfonic acid ionomer. The interfacial delamination and CL densification appear to be closely related to each other, and the key parameter to affect both is the ice volume fraction in the cathode CL after each cold-start step. Eliminating or minimizing these two degradation processes could improve the MEA cold-start durability by 280%. Mitigation strategies, such as improved gas purge prior to cold start, better MEA design, low startup current density, and low cell thermal mass, are proposed.

Calibrating the X-ray attenuation of liquid water and correcting sample movement artefacts during in operando synchrotron X-ray radiographic imaging of polymer electrolyte membrane fuel cells

Synchrotron X-ray radiography, due to its high temporal and spatial resolutions, provides a valuable means for understanding the in operando water transport behaviour in polymer electrolyte membrane fuel cells. The purpose of this study is to address the specific artefact of imaging sample movement, which poses a significant challenge to synchrotron-based imaging for fuel cell diagnostics. Specifically, the impact of the micrometer-scale movement of the sample was determined, and a correction methodology was developed. At a photon energy level of 20 keV, a maximum movement of 7.5 µm resulted in a false water thickness of 0.93 cm (9% higher than the maximum amount of water that the experimental apparatus could physically contain). This artefact was corrected by image translations based on the relationship between the false water thickness value and the distance moved by the sample. The implementation of this correction method led to a significant reduction in false water thickness (to ∼0.04 cm). Furthermore, to account for inaccuracies in pixel intensities due to the scattering effect and higher harmonics, a calibration technique was introduced for the liquid water X-ray attenuation coefficient, which was found to be 0.657 ± 0.023 cm(-1) at 20 keV. The work presented in this paper provides valuable tools for artefact compensation and accuracy improvements for dynamic synchrotron X-ray imaging of fuel cells.

Water Transport Across a Polymer Electrolyte Membrane under Thermal Gradients

A fundamental experimental and numerical study of the water transport across a perfluorosulfonic acid membrane under a temperature gradient is presented. The water transport phenomenon was experimentally investigated through water flux measurement and neutron radiography. The experimental observations found that water is transported in the direction from the high temperature side to the low temperature side, when both sides of the membrane are sufficiently humidified, and suggest that the transport mechanism is concentration gradient driven. The neutron radiography measurements detected the presence of water content gradient across the membrane and higher water content is seen at a larger thermal gradient. A numerical model was developed to investigate the experimental results. Water transport predictions agreed qualitatively but more accurate material and transport property characterizations are needed for further improvement.

Analysis of State of Water in Polymer Electrolyte Membrane by Raman Spectroscopy and DFT

Proton and water transport in polymer electrolyte membrane (PEM) are key transport properties for the improvement in PEMFCs performance under high current density operation which is desired in PEMFCs for automobiles. In this study, state of water in PEM is investigated by using confocal micro-Raman spectroscopy and density functional theory (DFT). The results showed that OH vibration peak shape indicated the state of water in the membrane. In particular, OH vibration peak around 3500cm-1 was strongly related to water transport through the membrane, while a peak around 3200cm-1 was assigned by formation of hydrogen-bonded water molecule network around sulfonic acid by DFT. DFT calculation also indicated that a hydronium ion around sulfonic acid contributes to the formation of water network.

The Influence of the Liquid Water Behavior at the GDL-Channel Interface on Cell Performance

Effective diffusivity of gaseous species in the gas diffusion layers (GDLs) and/or gas channels is one of the most important properties in reducing mass transport losses in polymer electrolyte fuel cell (PEFC) operation and it is strongly influenced by liquid water behavior and its distribution. Especially, liquid water in the gas channels is known to have a significant impact on cell performance (1, . So far, many researches, such as visualization techniques or numerical calculations, have been carried out to elucidate the liquid water behavior in the gas channel, however the relationship between the liquid water behavior in the gas channel and the cell performance have not been fully understood (3, . In this study, coupled cell performance evaluation, liquid water visualization and numerical modeling were performed to investigate the role of the surface properties of the GDL and separator on the liquid water transport, and to provide refined insights to the mass transport in the PEFC operation. Cell performances were evaluated with two types of separators. One is hydrophobic carbon separator and the other is hydrophilic gold coating separator. The hydrophobic GDL (PTFE treated) is utilized through these analyses. Operating conditions are summarized in Table 1. Figure 1 represents the comparison of the IR corrected cell voltages. Hydrophilic separator showed better cell performance than hydrophobic separator under high current density condition. Ex-situ optical visualization was performed for evaluating the liquid water behavior on the surface of the GDL. Liquid water was injected from the bottom of the GDL at the rate of 0.5 l/m, which corresponded to the current density of 2.0 A/cm. And then the liquid water behavior at the GDL-channel interface was observed from the opposite direction to the gas flow. Figure 2 shows the images of the liquid water behavior at the GDL-channel interface. In case of hydrophobic separator, the emerged liquid water grew on the surface of the GDL. On the other hand, the liquid water moved up along the wall of the separator when hydrophilic separator was used. In addition, due to the difference of the liquid water behavior in the gas channel, the surface area of the GDL covered with liquid water became large in the hydrophobic separator compared to the hydrophobic separator. The experimental observations were examined by numerical model. Moving particle semi-implicit method (MPS) was utilized to express two-phase and track the interface between liquid and gas phase in the gas channel. Figure 3 shows the snapshots of the liquid water behavior at the GDL-channel interface. The numerical calculations were in good agreement with experimental results. That is, the liquid water in the gas channel easily spread along the wall of the hydrophilic separator by capillary motion, while it became large on the surface of the GDL in case of hydrophobic separator due to the little difference of the capillary pressure between separator and GDL. These visualization and numerical results suggested that the difference of the liquid water behavior in the gas channel caused the cell voltage loss under high current density conditions since oxygen transport was hindered due to the reduced surface area of the GDL. Through these analyses, it was found that the control of the liquid water behavior at the GDL-channel interface was one of key factors to improve the cell performance under high current density and the robustness of PEFC.

Surface Structure Analysis of Polymer Electrolyte Membrane under Water Permeation Process by Tapping-Mode Atomic Force Microscopy

Water transport in polymer electrolyte membranes (PEMs) is of great interest for water management of PEM fuel cells. Reportedly, the net water flux across a membrane varies with operating conditions of the fuel cells. Relative humidity and its associated differences of feed gases supplied to the anode and the cathode considerably affect water movement across the membrane. These observations suggest that water sorption, desorption, and diffusion processes are important for water management of operational PEMFCs. Recent experimental investigations show that interfacial transport processes across the membrane-gas interface affect water transport across the membrane and are candidates as a rate-determining process [1, 2]. It is well known that Nafion membrane is segregated into nano-scaled hydrophilic and hydrophobic regions. Recently, Takimoto et al. performed AFM investigation on various types of polymer electrolyte membranes and showed variation of the cluster size with relative humidity in Nafion membrane [3]. In their measurement, the membrane was placed in a humidity controlled chamber and the membrane in the equilibrium state was observed by ACmode AFM. In practical operating conditions in PEMFCs, relative humidity differences in the anode and the cathode induces water movement across the membrane. Therefore, understanding on cluster structure of the membrane under such a water permeation process is of great importance to clarify interfacial mass transport processes across the membrane-gas interface. In this study, we investigated variation of cluster structure of a Nafion surface under water permeation process by AC-mode AFM. We focused our attention on effect of RH condition induced to the one side of the membrane on cluster structure on the other side of the membrane.

Investigation of Water-Molecule Network in Hydrated Polymer Electrolyte Membranes by Raman Spectroscopy: Effect of Polymer Structure and Cation Impregnation

Low relative humidity (RH) and high temperature operation possibly gives a large impact on cost reduction of polymer electrolyte membrane fuel cell (PEMFC) systems for automobile applications. However, these severe operating conditions causes lower proton conductivity in the membrane, resulting in intensive deterioration of the cell performance. Perfluorinated sulfonic acid (PFSA) membranes have been used in PEMFCs to their higher proton conductivity and durability. More recently, hydrocarbon (HC) membranes have been developed as targeted to further cost reduction of the membrane, but their proton conductivity is still insufficient especially in low RH condition. Higher proton conductivity in proton exchange membranes used in PEMFCs is generally regarded as a result of effective proton-hopping process via formation of hydrogen bonding network of water molecules in hydrated membranes [1]. It has been demonstrated that Raman spectroscopy is a powerful tool to investigate watermolecule network (water cluster) in a Nation® membrane [2,3]. Density function theory (DFT) analysis confirmed that lower frequency shift of OH vibration peaks in Raman spectra corresponded to formation of watermolecule network. In this study, we applied Raman spectroscopy to investigate water-molecule network in both PFSA and HC membranes under different RH conditions in order to obtain a fundamental insight in formation of watermolecule network that was possibly affected by polymer structure. We also performed measurement of Raman spectra on PFSA membrane in which two different cations, Sodium ion (Na) or potassium ion (K), was impregnated to examine an influence of physicochemical properties of cations on formation of water-molecule network in the membrane. Figure 1 and 2 show Raman spectra of a PFSA membrane and a HC membrane at different relative humidity. In all measurements, we used an in-house flow cell designed for Raman measurement [2, 3] to control relative humidity surrounding a membrane. We used a Nafion115 membrane as a PFSA membrane and a HC membrane. As reported in our previous study, OH vibration peak intensity (3000 to 3800 cm) increased with relative humidity in both membranes. At RH=30% and 60%, OH vibration peaks (A’, B’ and C’) in the HC membrane was separately identified. This suggests that water-molecule network in the HC membrane under lower RH condition was not well established, but was more isolated than those in the PFSA membrane in which the OH vibration peaks (A, B and C) was likely to overlap each other. With increase in RH up to 95% and more, the HC membrane showed broad OH vibration peaks as observed in the PFSA membrane. We also confirmed that under these RH conditions, the HC membrane showed better proton conductivity in the same level as the PFSA membrane. This suggests that formation of water-molecule network is correlated with proton conductivity and that poor formation of watermolecule network in the HC membrane at low RH condition results in less proton conductivity. We also examined effect of cation on watermolecule network in the PFSA membrane. Figure 3 showed Raman spectra in H-, Kand Na-form PFSA membrane under the same hydration. Kand Na-form PFSA membranes were prepared by soaking the membrane with 1M aqueous solution. The K-form and the Na-form membrane showed less formation of watermolecule network than the H-form. This indicates that dissociation of cation from sulfonic acid in the Na-form and the K-form membrane was deteriorated, because sodium and potassium are more interacted with the sulfonic acid. It was also observed that Na-form membrane showed less water-molecule network than the K-form. This is possibly related with ionic radius of cations where ionic radius of sodium ion is slightly different with hydronium ion and thus hindered hydrogen bonding network in the membrane. References [1] K.D. Kreuer, S.J. Paddison, E. Spohr, M. Schuster, Chem. Rev. 104 (2004) 4637. [2] Y. Tabuchi et al., J.Power Sources,196(2011),652. [3] Y. Tabuchi et al., ECS Trans., 33(2010), 1045.

Analysis of Mass Transport Loss in PEMFC Induced by Rib/Channel Geometry

Key challenges to the acceptance of Polymer Electrolyte Membrane Fuel Cells (PEMFCs) for automobiles are the cost reduction and its improvement in power density for compactness. In order to get the solutions for these issues, further improvement in the cell performance is required with high current density operation. In this study, the impacts of heat and water transport on the cell performance under high current density were investigated by experimental evaluation of liquid water distribution and numerical validation. Liquid water distribution in-plane direction was evaluated by neutron radiography. Furthermore, electrochemical reaction distribution was also evaluated by using inserted metal wires at anode, and then the experimental results were qualitatively validated by the numerical model. The experimental and numerical validation results revealed that significant increase in mass and ohmic loss was induced by temperature, liquid water, and electrochemical reaction distribution in-plane direction.Copyright

RIB/Channel Effect on Cell Performance Under High Current Density Operation

Heat and water transport in polymer electrolyte membrane fuel cell (PEMFC) has considerable impacts on cell performance under high current density which is desired in PEMFC for automobiles. In this study, the impact of rib/channel, heat and water transport on cell performance under high current density was investigated by experimental evaluation of liquid water distribution and numerical validation. Liquid water distribution between rib and channel is evaluated by Neutron Radiography. In order to neglect the effect of liquid water in channel and the distribution of oxygen and hydrogen concentration distribution along with channel length, the differential cell was used in this study. Experimental results show that liquid water under channel was dramatically changed with Rib/Channel width. From numerical study, it is found that the change of liquid water distribution was strongly affected by temperature distribution between rib and channel. In addition, not only heat transport but also water transport through membrane also significantly affected cell performance under high current density operation. From numerical validation, it is concluded that this effect on cell performance under high current density could be due to the enhancement of back-diffusion of water through membrane.Copyright