High Current Density Region Research Articles

Reduced mass transport resistance in polymer electrolyte membrane fuel cell by polyethylene glycol addition to catalyst ink

Abstract Effects of Polyethylene glycol (PEG) addition to cathode catalyst ink were investigated by changing the addition amount of PEG. Performance of the polymer electrolyte membrane fuel cells (PEMFCs) increased and then decreased at the higher current density than 1.5 A/cm2 as the amount of PEG addition increased. However, durability was not changed by the addition of PEG to the catalyst ink. Three different molecular weights of PEG were compared for PEG additives to cathode catalyst ink. Performance at high current density region increased and then decreased as PEG molecular weight increases from 200 to 10000. Increased performance by addition of PEG was attributed from reduced mass transport resistance. However, addition of large molecular weight PEG to catalyst ink reduced the performance because it lowered ionomer conductivity in the catalyst layer and then reduced proton transport resistance. Increased pore size in the catalyst layer and increased hydrophilicity on the electrode were also analyzed by addition of PEG to catalyst ink.

Effect of Hydrophobicity Gradients within MPL and MEA on PEM fuel cell performance

The water inventory of PEMFC components strongly influences the performance under two-phase operating conditions. Within this work, the influence of hydrophobicity variation within electrode and microporous layer (MPL) on cell performance will be presented. Different weight percentages of PTFE is used in the catalyst and MPL inks as an additive in order to increase the hydrophobicity. The fabrication method for the membrane electrode assembly (MEA) is based on spraying the catalyst ink onto the membrane and coating MPL ink onto the GDL substrate using a doctor blade method. Fuel cell tests are carried out with a 25 cm² active area single cell at different relative humidities (RH) and oxygen flow rates to investigate the effect of PTFE on the ability of MEA and MPL on water management. Electrochemical impedance spectroscopy is performed as a diagnostic tool to investigate the reaction kinetics and mass transport. In order to assess the wettability of MEAs contact angle measurements were carried out. The materials are characterized by mercury porosimetry and scanning electron microscopy. Figure 1 shows the performance curve for cell 1 (conventional MEA +Conventional MPL), cell 2( MEA with 5 wt.% PTFE +Conventional MPL) and cell 3 (Conventional MEA+ MPL with 40 wt.% PTFE ) at different relative humidities. At dry conditions (RH=70%) cell 2 has a higher limiting current density compared to the other cells, which could be the indication that the hydrophobic electrode keeps the membrane material in the catalyst layer better hydrated. At over humidified conditions (RH=120%) the limiting current density value has decreased for cell 1 and 2 compared to the standard conditions (RH=100%) as a result of mass transport limitations. On contrary, Cell 3 shows an improved performance in high current density region at humid conditions, illustrating the ability of hydrophobic MPL to prevent the accumulation of liquid water at the interphase of MPL and catalyst layer at cathode side. In the presentation of the ongoing work, additional results, including HFR data will be reported. Figure 1

MgO-Templated Mesoporous Carbon as a Catalyst Support for Polymer Electrolyte Fuel Cells

An MgO-templated mesoporous carbon, CNovel®, was employed as a catalyst support for the cathode of polymer electrolyte fuel cells (PEFCs) after modifying its dimensional, crystalline, surface and porous structures and the electrochemical oxygen reduction reaction (ORR) activities were examined by the thin-film rotating disk electrode (RDE) method and as well as the membrane electrode assembly (MEA) method. Although the catalytic activity of Pt on CNovel® was comparable with that on a non-porous carbon, Vulcan®, in the RDE configuration without Nafion®, Pt/CNovel showed a considerably higher activity than Pt/Vulcan in the MEA condition with Nafion®. The mechanism inducing this difference was discussed from the results of electrochemical surface area and sulfonic coverage measurements which suggested that Pt particles on inside pores of CNovel® are not covered with Nafion® ionomer while protons can still reach those Pt particles through water network. The MEA performance in the middle and high current-density regions was drastically improved by heat-treatment in air, which modified the pore structure to through-pored ones.

A comparison of temperature distribution in PEMFC with single-phase water cooling and two-phase HFE-7100 cooling methods by numerical study

Thermal management has been considered as one of the critical issues in proton exchange membrane fuel cell (PEMFC). Key roles of thermal management system are maintaining optimal operating temperature of PEMFC and diminishing temperature difference over a single fuel cell and stack. Severe temperature difference causes degradation of performance and deterioration of durability, so understanding temperature distribution inside a single fuel cell and stack is crucial. In this paper, two-phase HFE-7100 cooling method is suggested for PEMFC thermal management and investigated regarding temperature change inside a fuel cell. Also, the results are compared to single-phase water cooling method. Numerical study of temperature distribution inside a single PEMFC is conducted under various conditions for the two different cooling methods. Fuel cell model considering mass transfer, electrochemical reaction and heat transfer is developed.The result indicates that two-phase HFE-7100 cooling method has an advantage in temperature maintenance and temperature uniformity than single-phase water cooling method, especially in high current density region. It is also revealed that the cell temperature is less dependent on system load change with two-phase cooling method. It indicates that the fuel cell system with two-phase cooling method has high thermal stability. In addition, the effect of coolant flow rate and coolant inlet pressure in two-phase HFE-7100 cooling method are discussed. As a result, two-phase cooling method showed reliable cooling performance even with low coolant flow rate and the system temperature increased as coolant pressure rose.

Identifying the Major Source of Oxygen Transport Resistance By Modeling Studies

The interface between a perfluorosufonic acid ionomer like Nafion and platinum, the electrocatalyst, has been attracting more and more attention in the studies on PEFCs recently because experimental and theoretical studies have been revealing that the interface has the dominant impact on the PEFC performance in both the high and low current density regions. This tutorial lecture reviews those studies especially focusing on the theoretical and modeling studies and suggests key areas for further improvement.

Graphene based catalyst supports for high temperature PEM fuel cell application

In this study, the effect of graphene nanoplatelet (GNP) and graphene oxide (GO) based carbon supports on polybenzimidazole (PBI) based high temperature proton exchange membrane fuel cells (HT-PEMFCs) performances were investigated. Pt/GNP and Pt/GO catalysts were synthesized by microwave assisted chemical reduction support. X-ray diffraction (XRD), Thermogravimetric analysis (TGA), Brauner, Emmet and Teller (BET) analysis and high resolution transmission electron microscopy (HRTEM) were used to investigate the microstructure and morphology of the as-prepared catalysts. The electrochemical surface area (ESA) was studied by cyclic voltammetry (CV). The results showed deposition of smaller Pt nanoparticles with uniform distribution and higher ECSA for Pt/GNP compared to Pt/GO. The Pt/GNP and Pt/GO catalysts were tested in 25 cm2 active area single HT-PEMFC with H2/air at 160 °C without humidification. Performance evaluation in HT-PEMFC shows current densities of 0.28, 0.17 and 0.22 A/cm2 for the Pt/GNP, Pt/C and Pt/GO catalysts based MEAs at 160 °C, respectively. The maximum power density was obtained for MEA prepared by Pt/GNP catalyst with H2/Air dry reactant gases as 0.34, 0.40 and 0.46 W/cm2 at 160 °C, 175 °C and 190 °C, respectively. Graphene based catalyst supports exhibits an enhanced HT-PEMFC performance in both low and high current density regions. The results indicate the graphene catalyst support could be utilized as the catalyst support for HT-PEMFC application.

Experimental Study of Oxygen Transport Mechanisms in PEMFC Interdigitated Flow Field

Proton exchange membrane fuel cells for automotive application need to provide high performance at higher current density region where the oxygen transport is the limiting factor. Because of convective mass transport mechanism, interdigitated flow field has a potential to improve the oxygen transport limitation. In this paper, the mass transport mechanisms in an interdigitated flow field are experimentally studied using a segmented cell with 350 μm resolution in land-channel direction. At high current density operation, in general, the region under the land has higher values of local current density than regions under the channel which is opposite to a typical profile for conventional flow field. The local oxygen transport resistances measured at limiting current density conditions show the nearly constant values at lower oxygen concentration operation and then sudden increase at higher concentration. This indicates the presence of liquid water in the oxygen transport path when operated with high oxygen concentration. Convective-dominant and diffusive-dominant oxygen transport regions in gas diffusion layer (GDL) are identified from the oxygen transport resistance distribution, and the change of effective oxygen diffusivity in GDL due to liquid water is also estimated.

Structural Control of Carbon-Free Catalyst Layer Using Connected Platinum-Iron Nanoparticle Catalyst for Improved Mass-Transport in Polymer Electrolyte Fuel Cells

We have developed the connected platinum-iron (PtFe) nanoparticle catalyst with a porous hollow capsule structure as a carbon-free catalyst-layer material for polymer electrolyte fuel cells (PEFCs). [1,2] As shown in Fig. 1A, this catalyst consists of a nanosized beaded network by the fusion of PtFe nanoparticles with a crystallite size of 6~7 nm and an L10type chemically-ordered (face centered tetragonal) structure. The beaded metal network is electrically conductive, enabling the removal of carbon supports form a catalyst layer. The carbon-free cathode catalyst layer using a connected PtFe catalyst exhibits high durability against start-stop cycles, because of elimination of carbon corrosion problems. In addition, the specific activity of a connected PtFe catalyst for oxygen-reduction-reaction (ORR) is about 9 times enhanced, compared with that of a commercial Pt-nanoparticle catalyst supported on carbon black (Pt/C). On the other hand, inside a cathode catalyst layer, complex phenomena occur during a fuel-cell operation, such as ORR on catalyst surface, diffusion of oxygen molecules, proton conduction via ionomers, etc. Thus, a high performance of PEFC requires not only high ORR activity and stability but also lower resistance of oxygen mass transport. In this study, the following structural effects on fuel-cell performances for the carbon-free catalyst layers using connected PtFe catalysts were investigated; (i) inner-space in hollow capsule, and (ii) inter-space between capsules (Fig. 1B). The effect of inner-space in hollow capsule was discussed by the comparison of the connected PtFe catalysts with hollow structure or on solid (silica) support. The inter-space between capsules was controlled by the use of different capsule sizes (capsule size = 300 nm, or 500 nm in diameter). The electrochemical analyses of the prepared catalyst layers revealed that the hollow structure and the enlargement of inter-space between capsules improved the IV performance, especially at the high current density region, implying an improved oxygen mass-transport due to a sufficient space for oxygen diffusion in the catalyst layer and/or effective removal of liquid water from the catalyst layer. Moreover, in this study, the structural effect of gas-diffusion layer on fuel-cell performances was also investigated. It was found that the gas diffusion layer with higher gas permeability would facilitate the removal of liquid water from the capsule catalyst layer, resulting in a lower oxygen-transport resistance. In this presentation, by means of structural and electrochemical analyses of the capsule catalyst layers, the relationship between the catalyst-layer nanostructures and the fuel-cell performances will be discussed in details. The knowledge obtained in this study will provide new insights into the design of a carbon-free catalyst layer to achieve an enhanced PEFC performance. [1] T. Tamaki, H. Kuroki, S. Ogura, T. Fuchigami, Y. Kitamoto, and T. Yamaguchi, Energy Environ. Sci., 8, 3545-3549 (2015). [2] H. Kuroki, T. Tamaki, and T. Yamaguchi, J. Electrochem. Soc., 163(8), F927-F932 (2016). Figure 1

Luminescent Ir(III) complexes bearing benzothiazole or benzoxazole-based pincer ligand

Ir complexes bearing pincer ligands are expected to be efficient phosphorescent materials. This work investigated the solid-state structures and photophysical properties of Ir(III) complexes that contain different NCN pincer ligands (NCN = 1,3-bis(2-benzothiazolyl)benzene, 1,3-bis(2-benzoxazoyl)benzene, or 1,3-bis(2-benzimidazoyl)benzene derivatives), bidentate cyclometalating NC ligands (NC = 2-phenylpyridine or 2-phenylbenzothiazole), and anionic ancillary ligands. The structure of the NCN pincer ligands is a dominant factor for determining the emission wavelength, rather than the bidentate NC and ancillary ligands. The organic light emitting diodes (OLEDs) with an Ir(III) complex show electroluminescence with 14% external quantum efficiency at 1 mAcm−2. Modification of the device structure improved the efficiency at the high current density region.

PEMFC modeling based on characterization of effective diffusivity in simulated cathode catalyst layer

Oxygen diffusion in the cathode catalyst layer (CCL) is crucial to the high performance of polymer electrolyte membrane fuel cells (PEMFCs), especially in high current density or concentration loss regions. Recently, PEMFC performance has been reported to be enhanced by increasing CCL pore size and pore volume due to the reduction of diffusion resistance by capillary water equilibrium [Yim et al., Electrochimica Acta 56 (2011) 9064–9073]. Herein, we simulate these experimental results utilizing a new one-dimensional PEMFC model considering the effects of accumulated water film in CCL on oxygen diffusion. Two CCL microstructures were numerically generated based on agglomerate models to examine the experimental results obtained for two membrane electrode assembly (MEA) samples with different CCL porosity. The effective diffusivity of oxygen in the CCL was estimated by performing auxiliary simulations of oxygen concentration in CCL microstructures covered by a film of liquid water, with exponential correlation obtained between effective diffusivity and the thickness of the above film. Polarization curves predicted by the present model were in good agreement with experimental results. In agreement with the results of Yim et al., the present model predicts that the MEA featuring a CCL with smaller pores (which are more easily filled by liquid water) should exhibit a larger concentration loss at high current densities.

Experimental and numerical analysis of a three-dimensional flow field for PEMFCs

It has been well recognized that both the performance and operation stability of proton exchange membrane fuel cells (PEMFCs) are closely associated with water transport and accumulation behaviors in the membrane electrode assembly. Therefore, an optimal water management is highly desired. Conventional serpentineflow field (CSFF) can effectively facilitate water removal and prevent water flooding. However, CSFF would cause a high pressure drop from the inlet to outlet, thus resulting in large parasitic power loss. In this study, a novel three-dimensional flow field (WSFF), patterned with waved serpentineflow channels, is designed and analyzed by combing the simulating method with experimental method. A three-dimensional, multi-phase, steady, isothermal, laminar simulation model is firstly established based on FLUENT PEM fuel cell module, and this model reveals that WSFF is overall better than CSFF in promoting oxygen transport though the diffusion layer and removing liquid water accumulated in microstructure. Its periodic waved structure introduces cyclical variation of local flow direction, local flow velocity and local pressure, thus leading to enhanced forced-convection. The superior performance of WSFF has also been experimentally verified, proving that WSFF not only enables a lower pressure drop over the entire current density range, but also improves the cell performance in comparison to CSFF at high current density region. Specifically, there is a 17.8% increment in the peak power density due to the use of WSFF.

Influence of the Gas Diffusion Layer Compression on the Oxygen Transport in PEM Fuel Cells at High Water Saturation Levels

The impact of the gas diffusion layer (GDL) compression on the oxygen transport is investigated in single cell assemblies at 50°C, RH = 77%, 200 kPaabs and under differential flow conditions. For this, the oxygen transport resistance at low and high current densities is determined by limiting current density measurements at various oxygen concentrations for GDLs with and without microporous layer (MPL). At small current densities (≤0.4 A cm−2), where no liquid water in the GDL/MPL is present, a linear increase of oxygen transport resistance with GDL compression is observed, with the GDL without MPL exhibiting a significantly lower transport resistance. For low compressions of ≈8%, we find that the oxygen transport resistance for the GDL with MPL is increasing disproportionately high in the high current density region (>1.5 A cm−2), where water condensation in the porous media takes place. A similar trend is observed for a GDL without MPL at a typical compression of 22%. Based on these results, we hypothesize that a developing liquid water film is hindering the oxygen diffusion at the interface between MPL and cathode, analogous to what is known to be formed on the cathode surface for GDLs without MPL.

Effect of Catalyst Layer Ionomer Content on Performance of Intermediate Temperature Proton Exchange Membrane Fuel Cells (IT-PEMFCs) under Reduced Humidity Conditions

For intermediate-temperature polymer electrolyte membrane fuel cells (IT-PEMFCs), the effects of the cathode gas flow rate and the ionomer content are experimentally examined at 120°C under conditions of low relative humidity (RH). The IT-PEMFC operation at low RH should be beneficial in developing compact systems with smaller humidifiers. First, analysis of the effect of gas flow rate at various current densities confirms that drying of the membrane electrode assembly (MEA) is an important factor in IT-PEMFC operation, whereas cathode flooding becomes significant in regions of high current density with low flow rates. Then, MEAs with various contents of Aquivion™ ionomer are fabricated, and the combined effect of drying and flooding is further investigated by IT-PEMFC tests at various RH conditions and current densities. The optimum ionomer content increases with decreasing current density at 20% RH or below, indicating that MEA drying becomes dominant over cathode flooding.

Metal based gas diffusion layers for enhanced fuel cell performance at high current densities

Abstract The gas diffusion layer strongly influences the performance and durability of polymer electrolyte fuel cells. A major drawback of current carbon fiber based GDLs is the non-controlled variation in porosity resulting in a random micro-structure. Moreover, when subjected to compression these materials show significant reduction in porosity and permeability leading to water management problems and mass transfer losses within the fuel cell. This study investigated the use of uniform perforated metal sheets as GDLs in conjunction with microchannel flowfields. A metal sheet design with a pitch of 110 μm and a hole diameter of 60 μm in combination with an MPL showed superior performance in the high current density region compared to a commercially available carbon paper based GDL in a single cell environment. Fuel cell testing with different oxidants (air, heliox and oxygen) indicate that the metal sheet offers both superior diffusion and reduced flooding in comparison to the carbon based GDL. The presence of the MPL has been found to be critical to the functionality of the metal sheet suggesting that the MPL design may represent an important optimisation parameter for further improvements in performance.

Magnetic Dissipation in Relativistic Jets

The most promising mechanisms for producing and accelerating relativistic jets, and maintaining collimated structure of relativistic jets involve magnetohydrodynamical (MHD) processes. We have investigated the magnetic dissipation mechanism in relativistic jets via relativistic MHD simulations. We found that the relativistic jets involving a helical magnetic field are unstable for the current-driven kink instability, which leads to helically distorted structure in relativistic jets. We identified the regions of high current density in filamentary current sheets, indicative of magnetic reconnection, which are associated to the kink unstable regions and correlated to the converted regions of magnetic to kinetic energies of the jets. We also found that an over-pressured relativistic jet leads to the generation of a series of stationary recollimation shocks and rarefaction structures by the nonlinear interaction of shocks and rarefaction waves. The differences in the recollimation shock structure due to the difference of the magnetic field topologies and strengths may be observable through mm-VLBI observations and space-VLBI mission.

Optimization of Cathode Catalyst Layer of PEFC Using Silk-Derived Activated Carbon By 2-Step Mixing Method Introduction

IntroductionFuel cell is expected to be used as one of the major power sources in the near future, from the view point of high energy efficient and low environmental impact. Among the fuel cells, polymer electrolyte fuel cell (PEFC) is beginning to be put into practical use as residential and transportation power generation system. However, there are still some problems to be overcome for the spread of PEFC. One of them is the high cost, due to the use of platinum (Pt). Pt is used as cathode catalyst for reduction of overpotential in most PEFCs. Since Pt is a metal of low abundance and high cost, alternative catalyst is required. Many oxides, nitride and carbon materials have been investigated as non-precious metal catalysts. However, most of them show low oxygen reduction reaction (ORR) activity, low conductivity or low durability. We have reported the performance of silk derived activated carbon (SAC) as a non-precious metal catalyst for the PEFC cathode1,2). Compared with Pt, SAC has lower ORR activity but it can be fabricated cheaper. Therefore, the optimum structure of the catalyst layer using SAC is expected to be different from that using Pt. In this study, we prepared cathode catalyst ink of SAC using different solvent, and investigated the effect of mixing ratio of the solvent and the effect of 2-step mixing to the performance, which is necessary to obtain information to optimize the electrode structure for non-Pt cathode for PEFC. ExperimentalSAC was prepared using silk fibroin as starting material after removing sericin from silk fiber. Silk fibroin was primary carbonized under nitrogen atmosphere at 500 ˚C. The obtained carbon was ball milled and then acid washed. Then it was heated under nitrogen atmosphere for secondary carbonization at 900 ˚C.Membrane electrode assembly (MEA) was prepared using SAC as cathode catalyst. Cathode catalyst layer was prepared by dropping the dispersion ink on the carbon paper. Butyl acetate (BA) and/or n-propanol (NP) were used as solvent. Dispersion ink was prepared by two methods, 1-step mixing and 2-step mixing. In 1-step mixing, carbon black (CB) and SAC were mixed in the solvent simultaneously. On the other hand, in 2-step mixing, CB and SAC were mixed separately. Catalyst layer was observed by scanning electron microscope (SEM) and energy dispersive X-ray spectrometer (EDX). Terminal voltage vs. current density was measured at 80 ˚C under ambient pressure, using hydrogen and oxygen for anode and cathode, respectively. Both gases were humidified by a vaporizer. Results and discussionFigure 1 shows polarization curves of MEA prepared by two types of solvent. As can be seen in the figure, in the low current density region, the cell performance of MEA prepared by using mixture of BA and NP as the solvent showed higher performance than that prepared by using only BA. On the other hand, the effect of the solvent to the performance was opposite in the high current density region. It is reported that different solvent affects the structure of the ionomer, therefore, the different effect observed in different current density region is assumed to be caused by the structure of the ionomer in the electrode.Figure 2 shows polarization curves of MEA prepared by1-step mixing and 2-step mixing. The maximum power density increased by 2-step mixing. The improvement performance by 2-step mixing is assumed to be caused by the optimized electrode structure, which was achieved by coating SAC with the ionomer during the first step, and by building up the continuous network of ionomer during the second step.1) T. Iwazaki, R. Obinata, W. Sugimoto and Y. Takasu, Electrochem. Comm., 11, 376 (2009). 2) H. Fukunaga, R. Shishido, T. Takatsuka and Y. Takasu, Innovative Materials for Processes in Energy Systems 2010. B. B. Saha et al., eds. Research Publishing, 142 (2011). Figure 1

(Invited) Impedance Analysis Using Equivalent Circuits with Transmission Line Model for Reaction Distribution in Polymer Electrolyte Fuel Cell and Li-Ion Battery

The development of environmentally friendly vehicles such as fuel cell electric vehicles (FCEVs) and battery electric vehicles (BEVs) for effective fuel utilization is one of the major areas upon which efforts are focused to counteract global warming. Among the various types of fuel cells, polymer electrolyte fuel cells (PEFCs) are the most attractive option for development because of their capability to be operated at a low temperature and their highly efficient generation of power without emissions. Meanwhile, energy storages, such as lithium ion batteries (LIBs), are also attractive because of their indispensableness for the application of FCEVs and BEVs. These energy devices are evaluated by electrochemical impedance spectroscopy (EIS) for development of their electrode materials and structures. The electrodes of these energy devices are designed to increase reaction site for higher power density and energy density. Therefore, the electrodes of fuel cells and batteries have a three-dimensional complicated structure. Generally, basic electrochemical systems are evaluated by a simple equivalent circuit, such as Randles-type equivalent circuit, which treats the electrode as a homogeneous system. On the other hand, electrochemical reactions occur inhomogeneously in those practical energy devices due to ionic resistance in the electrode under actual high-load operation. In the present paper, to count the reaction distribution in the electrodes, EIS analysis by using equivalent circuit with transmission line model (TLM) is introduced. For PEFC, a TLM represents only the ionic resistance in the catalyst layer, that is electrode, and the overall distribution of the catalyst. However, because the carbon-supported catalysts form agglomerates in the actual catalyst layers, the inner and outer catalysts of these agglomerates should be considered separately (Figure 1 (a)). Herein, a novel TLM, which counts the distribution of the oxygen reduction reaction (ORR) in primary and secondary pores in a catalyst layer of a MEA, was designed for the evaluation of a degraded cathode catalyst layer in PEFCs (Figure 1 (b)). The equivalent circuit was applicable from low current density region to high current density region, showing excellent match between experimental data and calculated data. The EIS with low current density region enable to understand a degree of degradation of cathode catalyst layer.1 The EIS with high current density region enable to grasp the flooding effect in the microstructured cathode catalyst layer of PEFCs.2 For LIBs, if there is a degradation of the electrodes in LIBs, it is possible that it will change the ionic pass in the electrode layer due to the accumulation of decomposed products inside the electrode layer and due to the collapse of the active materials and the micro pores (Figure 1 (c)). The impedance response of LIBs is composed of overlapped elemental steps such as charge transfer for chemical reaction and ion migration of cathode and anode. Therefore, the LIB impedance response was separated into the cathode and anode impedance response through the use of the LiAl micro reference electrode. The separated impedance responses were analyzed using the equivalent circuit with the TLM to count the ionic resistance in the electrodes (Figure 1 (d)). The equivalent circuit with a TLM was enabled to fit the impedance responses simulated by the equivalent circuit with those measured at various depths of discharge, as well as the Randles type equivalent circuit fit them. The present paper demonstrates the potential of the EIS with an equivalent circuit coupled with a TLM for diagnosis of LIBs in power applications.3 At the presentation, EIS analysis for energy devices by using equivalent circuit with TLM will be discussed in detail. Acknowledgement These work were partly supported by “Research & Development Initiative for Scientific Innovation of New Generation Batteries (RISING)” and “R & D Basic research of cell degradation factors and analysis of MEA durability” from New Energy and Industrial Technology Development Organization (NEDO) of Japan, the Grant-in-Aid for Specially Promoted Research “Establishment of Electrochemical Device Engineering”, and the Global COE program “Practical Chemical Wisdom” from the Ministry of Education, Culture, Sports, Science and Technology, Japan.

CNTs Grown on Al2O3 Spheres As an Efficient Catalyst Support for High Performance MEA in PEM Fuel Cells

Polymer electrolyte membrane fuel cells (PEMFCs) have been widely accepted as a clean energy device because of its capability of generating high energy density and only water as a product via direct converting of the chemical energy into the electrical energy. However, durability problems originating from the carbon corrosion and Pt agglomeration have been one of the major issues for practical applications. The high potentials formed in a cell during the fuel cell operations lead to serious carbon corrosion in catalyst layers, which causes the decrease of cell performance. To resolve the carbon corrosion, highly durable carbon materials such as carbon nanotubes (CNTs) have been intensively studied. Although the corrosion-resistant characteristics of CNTs were efficient to utilize as an electrocatalyst support for PEMFCs, the difficulty in processing the CNTs mainly due to poor dispersibility and easy bundle formation in a catalyst ink limited their practical applications. In this respect, vertically aligned carbon nanotubes (VACNTs) have been tried and showed promising results in a PEMFC MEA. However, VACNTs need complex process to make MEAs, remaining the issues of mass production. In this work, we report CNTs grown on Al2O3 (Al2O3-CNT) as an efficient support material for PEMFC MEAs. The catalyst support was designed to utilize durable CNT properties while resolving the CNT bundling problems. The catalyst was characterized by transmission electron microscope (TEM), field emission scanning spectroscopy (FESEM), and X-ray photoelectron spectroscopy (XPS) in order to investigate structural and electronic properties. The kinetics of O2 reduction is examined with the rotating disk electrode (RDE) technique. A potential cycling test was applied to evaluate durability of the catalyst. The electrochemical surface area (ECSA) of Pt/Al2O3-CNT slightly decreased (17 %), while Pt/C loses nearly 46 % of its ECSA of its initial value over repeated cycling test under carbon corrosion potential range, representing more durable properties of CNTs. The single cell test was also performed with Pt/Al2O3-CNT and Pt/C after fabricating the catalysts to MEAs. The performance of the Pt/Al2O3-CNT was comparable to that of Pt/C and showed better performance at high current density regions. Furthermore, after the accelerated stress test (AST), the Pt/Al2O3-CNT also showed approximately 2 times higher stability than Pt/C in a single cell test.

Effects of High Oxygen Permeability Ionomers on the Cathode Performance of Polymer Electrolyte Fuel Cells

For the large-scale commercialization of polymer electrolyte fuel cells (PEFCs), it is very important to reduce the amount of Pt by developing highly efficient cathode catalyst layers (CLs) due to the high cost and limited availability of Pt resources. In previous work, we demonstrated the importance of two factors for the design of high performance cathode CLs. The first is higher effective Pt surface area (S(e)Pt), which practically contributes to the oxygen reduction reaction (ORR) [1]; the n-Pt/AB250 catalyst, for which all of the Pt particles exist only on the AB250 exterior surface, is the most attractive in order to generate the large current densities required by actual fuel cell operation [2]. The second is the optimized distribution of ionomer on the surfaces both of Pt particles and carbon particles; short-side-chain (SSC) perfluorosulfonic acid ionomers with high ion-exchange capacity (IEC) showed better continuity and uniformity on the Pt and graphitized carbon black (GCB) particles than the conventional ionomer, which might have led to the improvement of both the mass transport and the proton-conducting network in CLs [3].In this study, we investigate the effects of high oxygen permeability ionomers on the cathode performance of PEFCs. The high oxygen permeability ionomers are expected to increase the flux of oxygen near the Pt surface of the three-phase boundary, in the case of extremely low Pt loadings [4]. The low Pt loading cathode CLs (ca. 0.05 ± 0.003 mg-Pt cm-2) were prepared from the AB250-supported Pt catalyst, with a higher effective Pt surface area (n-Pt/AB250: S(e)Pt = 107 m2 gPt -1, specific surface area of AB250: 219 m2 g-1, Denka Co., Ltd.) and ionomers developed by Asahi Glass Co., Ltd. (AGC); AGC PFSA (IEC = 1.13 meq g-1), AGC sample A (IEC = 1.17 meq g-1, O2 permeability 1.5 times higher than AGC PFSA) and AGC sample B (IEC = 1.50 meq g-1, O2 permeability 1.5 times higher than AGC PFSA). Figure 1 (a) shows the resistance-corrected (IR-free) polarization curves of the high oxygen permeability ionomers for the ORR at 80 oC fed with ambient pressure air humidified at 80% RH. The mass activity (MA) at 0.85 V and the mass power values, calculated from the maximum outputs of the IR-free polarization curves, are plotted as a function of RH in Figure 1 (b) and (c), respectively. The cathode cell performance was significantly increased by using the high oxygen permeability ionomers, i.e., AGC sample A and AGC sample B. However, the ionomers showed different performance behavior at low and high current densities. In the high potential regions (low current densities), in which there is a smaller effect of mass transport, the AGC sample B with the higher IEC of 1.50 exhibited higher cell performance than the other ionomers, as can be confirmed in the MA results in Figure 1 (b). In contrast, the AGC sample A with the lower IEC of 1.17 led to a significant improvement of the cathode performance in the high current density region, which led to the achievement of very high performance, greater than 22 W mgPt -1at 80-100% RH, as presented in the mass power results in Figure 1 (c). These results show that the improved oxygen permeability of the ionomers could result in highly efficient cathode CLs for PEFCs. Acknowledgment This work was supported by funds for the “Superlative, Stable, and Scalable Performance Fuel Cell” (SPer-FC) project from the New Energy and Industrial Technology Development Organization (NEDO) of Japan. The authors are grateful to Asahi Glass Co., LTD. for kindly providing the experimental ionomers. Also, the authors are grateful to Denka Co., Ltd. for kindly providing the experimental AB supports.

Development of Fuel-Flexible SOFC

Efficient utilization of low-grade biomass resources (bio-wastes) such as sewage sludge, agricultural residues, livestock manures and garbage in a sustainable way must bring about a promising solution against global environmental issues. Biogas, mixture of CH4 and CO2, produced by anaerobic fermentation of bio-wastes is a useful fuel for power generation in a community abundant in biomass resources. For the biomass power generation, generally gas engine (GE) systems are applied, but then their low electrical efficiency of 20-30 % attributed to Carnot limitation and energy losses caused by multiple energy conversions can become critical problems for small-sized distributed power generation. Solid oxide fuel cell (SOFC), highly efficient energy conversion device, is one of the most promising candidates as a distributed power generator in the next generation, because fuel processing can be proceeded within the cell to produce syngas which will be directly converted to electricity without combustion process, in other words not through thermal energy. This operation mode is called direct internal reforming (DIR) operation. Hydrocarbon fuels including biogas can be converted into electrical energy with 2-3 times higher efficiency in comparison to GEs. In this study, for the global dissemination of SOFC, simplification of fuel processing was studied for further enhancement of energy conversion efficiency and creation of compact system, focusing on biogas as a fuel. With the above motivation, flexible paper-structured catalyst (PSC) has been developed. Especially, hydrotalcite (HT)-dispersed PSC (HT-PSC) exhibits high tolerance for sulfur poisoning in the methane dry reforming, and therefore it was applied to biogas-fuelled SOFC as a reaction field of the internal reforming or pre-reformer. In this study, planar-shaped compact reformer in which HT-PSC was embedded (PSC-reformer) was developed. The PSC reformer was applied as a pre-reformer to a stack testing fuelled by simulated biogas (equimolar mixture of CH4 and CO2). The PSC-reformer exhibited stable CH4 conversion leading to stable stack operation with power density nearly equal to that for pure H2 under the same load current during which residual CO2 can contribute to the reduction of anodic activation loss in the low current density region. In the high current density region, H2 amount is boosted up through water gas shift (WGS) reaction between CO and H2O produced by reforming and electrochemical oxidation of H2, respectively, which has positive impact on the stack operation. For further enhancement of the catalytic activity of PSC to suppress carbon formation during dry reforming, core-shell structure catalyst (Ni/CeO2) was prepared and dispersed in the fiber network. The reformate composition with CO/H2 > 1 suggests that Ni particles with the CeO2shell can promote reverse Boudouard reaction, resulting in the considerable suppression of carbon deposition. Moreover, in comparison with a conventional PSC, Ni loading amount can be suppressed to get a certain methane conversion probably due to the oxygen storage capacity of ceria. Technology of the fuel-flexible SOFC is now studied in the Vietnam-Japan International joint research project, “Project for Sustainable Development of Rural Area by Effective Utilization of Bio-wastes with Highly Efficient Fuel Cell Technology”, under the SATREPS program supported by Japan Science and Technology Agency (JST) and Japan International Cooperation Agency (JICA). Purpose of this joint research is to develop and demonstrate an SOFC-integrated energy circulation system for the environmental conservation and the stable energy supply in Mekong Delta region. Sludge accumulated at the bottom of a shrimp pond will be pumped out and fed to methane fermentation reactor to produce biogas. This biogas is supplied to SOFC system to generate electricity which will be used for the shrimp culture. In this system, PSC technology will be applied to increase energy conversion efficiency.