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The study investigates the freezing behavior of supercooled water in gas diffusion layers (GDLs) of polymer electrolyte fuel cells (PEFCs). Using an innovative advanced in situ differential scanning calorimetry (advanced DSC) technique, we analyze phase transitions within GDLs at a single-cell scale, excluding impacts of catalyst layers and membranes. This method overcomes the limitations of traditional DSC, allowing variable water saturation and distribution. We find that freezing-thawing and drying-saturating cycles contribute to noticeable variations in the freezing temperature (T o n s e t ) of supercooled water, ranging from −6 °C to −15 °C, attributed to the heterogeneous surface of GDL and water redistribution. Repeated drying-saturating cycles remove nucleation seeds, lowering the freezing point of water − a "cleaning effect." The freezing probability reduces at lower saturation levels, influenced by waterproof treatment uniformity on GDLs. Residual ice can be detrimental to the cold-start capability of PEFCs, as it promotes freezing of supercooled water around −1 °C. Our findings enhance understanding of water behavior in GDLs and its impact on PEFC cold-start performance. Applying this technique to other cell components, such as the catalyst layer, could further elucidate water freezing dynamics in PEFCs and guide the innovation of more reliable cell components for PEFC applications in broader climates. • In situ differential calorimetric observation of water freezing behavior in GDLs. • Noticeable inter-cycle variations explains the stochastic cold-start operation time. • Freezing temperature decreases with lower saturation level. • Understanding of how GDL property influences crystallization. • Discovery of the presence "0 °C freezing" and "cleaning effect" in GDL. [ABSTRACT FROM AUTHOR]

Proper water management plays an essential role in the performance and durability of Polymer Electrolyte Fuel Cells (PEFCs), but it is challenged by the variety of water transport phenomena that take place in these devices. Previous experimental work has shown the existence of fluctuations between low and high current density levels in PEFCs operated with wet hydrogen and dry air feed. The alternation between both performance states is accompanied by strong changes in the high frequency resistance, suggesting a cyclic hydration and dehydration of the membrane. This peculiar scenario is examined here considering liquid water distributions from neutron imaging and predictions from a 3D two-phase non-isothermal model. The results show that the hydration-dehydration cycles are triggered by the periodic condensation and shedding of liquid water at the anode inlet. The input of liquid water humidifies the anode channel and offsets the membrane dry-out induced by the dry air stream, thus leading to the high-performance state. When liquid water is flushed out of the anode channel, the dehydration process takes over, and the cell comes back to the low-performance state. The predicted amplitude of the current oscillations grows with decreasing hydrogen and increasing air flow rates, in agreement with previous experimental data. [ABSTRACT FROM AUTHOR]

The water level in a polymer electrolyte membrane fuel cell (PEMFC) affects the durability as is seen from the degradation processes during operation a PEMFC with fully- and nonhumidified gas streams as analyzed using an in-situ segmented cell for local current density measurements during a 300 h test operating under constant conditions and using ex situ SEM/EDX and XPS post-test analysis of specific regions. The impact of the RH on spatial distribution of the degradation process results from different water distribution giving different chemical environments. Under nonhumidified gas streams, the cathode inlet region exhibits increased degradation, whereas with fully humidified gases the bottom of the cell had the higher performance losses. The degradation and the degree of reversibility produced by Pt dissolution, PTFE defluorination, and contaminants such as silicon (Si) and nickel (Ni) were locally evaluated. [ABSTRACT FROM AUTHOR]

An accurate method for three-dimensional reconstruction of microstructures is developed to analyse mass transport phenomena in microporous layers (MPLs). The proposed method involves the use of focused ion beam–scanning electron microscope tomography. Numerical calculations of the structural and mass transport properties of MPLs are conducted using the reconstruction results and these results are found to be in good agreement with experimental results, demonstrating the accuracy of the new method. [ABSTRACT FROM AUTHOR]

Evaporative cooling has the potential to reduce stack and system volume of polymer electrolyte fuel cells by up to 30% and to cut costs significantly by simplifying the multi-layered structure of bipolar plates and eliminating the need for external humidification. This study provides an experimental proof of concept by analyzing the evaporation behavior, cooling power, internal humidification, electrochemical performance and operational stability of evaporative cooling under technical cell boundary conditions. Isothermal in situ as well as operando studies are carried out in an evaporatively cooled fuel cell (15 cm2 active area, 80 °C, ambient pressure). Relative humidity boundary conditions imitate the inlet, center and outlet of a technical cell. Main findings show that the evaporation rate is saturation limited at low gas velocities, whereas it is transport limited at higher gas speed. Operando measurements prove that evaporative cooling works at a technical cell level with multiple water supply lines. The entire waste heat is removed and sufficient membrane humidification is achieved when dry inlet gases are used. Furthermore, it is shown that optimal performance is achieved in a counter-flow arrangement. The observed electrochemical performance is comparable to conventional cooling with humidified gases at inlet, center and outlet of the cell. • Operando evaporative cooling analysis in a technical single cell. • Evaporation is saturation limited at low and transport limited at high gas speed. • Counter-flow is identified as optimal arrangement for evaporative cooling. • Sufficient membrane humidification is provided by evaporative cooling. • The performance is comparable to conventional cooling with humidified gases. [ABSTRACT FROM AUTHOR]

It is important to reduce the oxygen diffusion resistance through PEFC porous electrode, because it is the key to reduce the PEFC cost. However, the gas diffusion coefficient of CL is lower than MPL in spite of framework consisted of same carbon blacks. In this study, in order to understand the reasons of the lower gas diffusion performance of CL, the relationship between a carbon black agglomerate structure and ionomer adhesion condition is evaluated by a numerical analysis with an actual reconstructed structure and a simulated structure. As a result, the gas diffusion property of CL strongly depends on the ionomer adhesion shape. In the case of adhesion shape with the same curvature of ionomer interface, each pore can not be connected enough. So the pore tortuosity increases. Moreover, in the case of existence of inefficient large pores formed by carbon black agglomerate and ununiformly coated ionomer, the gas diffusion performance decrease rapidly. As the measurement values in actual CL are almost equal to that with model structure with inefficient large pores. These characteristics can be confirmed by actual cross-section image obtained by FIB-SEM. [ABSTRACT FROM AUTHOR]

The reduction of oxygen transfer resistance through porous components consisting of a gas diffusion layer (GDL), microporous layer (MPL), and catalyst layer (CL) is very important to reduce the cost and improve the performance of a PEFC system. This study involves a systematic examination of the relationship between the oxygen transfer resistance of the actual porous components and their three-dimensional structure by direct measurement with FIB-SEM and X-ray CT. Numerical simulations were carried out to model the properties of oxygen transport. Moreover, based on the model structure and theoretical equations, an approach to the design of new structures is proposed. In the case of the GDL, the binder was found to obstruct gas diffusion with a negative effect on performance. The relative diffusion coefficient of the MPL is almost equal to that of the model structure of particle packing. However, that of CL is an order of magnitude less than those of the other two components. Furthermore, an equation expressing the relative diffusion coefficient of each component can be obtained with the function of porosity. The electrical conductivity of MPL, which is lower than that of the carbon black packing, is considered to depend on the contact resistance. [ABSTRACT FROM AUTHOR]

In this work, we have used an efficient approach to prepare nitrogen-doped graphene supported Pt nanoparticles (Pt/N-rGO). The nitrogen-doped graphene nanocomposites (N-rGO) were derived from pyrolysis of graphene oxide/polyaniline composites in nitrogen atmosphere. X-ray powder diffraction, FTIR spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy and Transmission and Scanning electron microscopy (TEM&SEM) were used to characterize the morphology and microstructure of the prepared catalysts. The TEM and elemental mapping images indicate that metal nanoparticles are more uniformly dispersed on the surface of N-doped graphene than other supports, and Pt nanoparticles dispersed without any aggregation. The catalytic activity and durability of the catalysts was evaluated by various electrochemical techniques. Compared to undoped Pt/rGO and commercial Pt/C catalysts, an enhanced electrocatalytic activity was obtained in the case of the Pt/N-rGO with optimized composition and nanostructure. The maximum power density of MEA for Pt/N-rGO was 1.4 times more than that of MEA fabricated by commercial Pt/C 20%. [ABSTRACT FROM AUTHOR]

Evaporative cooling is a promising concept to reduce the fuel cell system volume and mass significantly. This paper investigates the interactions between the fuel cell stack and the balance of plant in an evaporatively cooled polymer electrolyte fuel cell system (PEFCS). For this, a zero-dimensional PEFCS model, comprising the fuel cell stack, air compressor, charge air cooler, humidifier, hydrogen recirculation blower, condensing radiator and water separator has been developed and analyzed. Two evaporative cooling system architectures are compared to conventional, liquid cooling. Optimal operating conditions are determined by a numerical optimization of the net system power output. Main results show that evaporative cooling works on the system level over a wide range of operating conditions. The optimum system power and highest efficiencies are achieved at high temperatures (80–90 °C), low pressure (125–150 kPa) and a corresponding cathode stoichiometry between 1.5 and 3, allowing for a closed water loop at the same time. The air compressor shows an increased power demand, compared to conventional cooling and the exhaust gas condenser is identified as the one critical component for evaporative cooling. Its performance is key to an efficient operation and closed water loop at all ambient conditions. • Interactions between evaporatively cooled PEFC stack and auxiliaries are analyzed. • Optimized operating conditions for evaporatively cooled systems are quantified. • A closed water loop is possible over a wide range of operating conditions. • An efficiency map for evaporative cooling is proposed. • The condenser is identified as the one critical component for evaporative cooling. [ABSTRACT FROM AUTHOR]

Conventional polymer electrolyte fuel cells (PEFCs) require a means of placing the series of laminar components that make up cells under mechanical compression so as to ensure effective electrical conduction, mass transport and gas-tight operation. This review describes the effect of mechanical compression and dimensional change on the components of PEFCs and reviews the range of methods used to achieve desired stack compression. The case is made for improved understanding of the mechanisms of fuel cell component compression and greater attention to the development of technological approaches for stack compression. [ABSTRACT FROM AUTHOR]

The influence of artificial starvation effects during automotive-related operating conditions is investigated within a polymer electrolyte fuel cell (PEFC) using non-dispersive infrared sensors and a current scan shunt. Driving cycles (DC) and single load change experiments are performed with specific fuel and oxidant starvation conditions. Within the DC experiments, a maximal CO 2 amount of 4.67 μmol per cycle is detected in the cathode and 0.97 μmol per cycle in the anode exhaust without reaching fuel starvation conditions during the DC. Massive cell reversal conditions occur within the single load change experiments as a result of anodic fuel starvation. As soon as a fuel starvation appears, the emitted CO 2 increases exponentially in the anode and cathode exhaust. A maximal CO 2 amount of 143.8 μmol CO 2 on the anode side and 5.8 μmol CO 2 on the cathode side is detected in the exhaust gases. The critical cell reversal conditions only occur by using hydrogen reformate as anode reactant. The influence of the starvation effects on the PEFC performance is investigated via polarization curves, cyclic and linear sweep voltammetry as well as electrochemical impedance spectroscopy. The PEFC performance is reduced by 47% as a consequence of the dynamic operation. [ABSTRACT FROM AUTHOR]

A potentiostatic polarization method is used to evaluate the corrosion behavior of SS316L in simulated anode and cathode environments of polymer electrolyte fuel cells. A passive barrier oxide film is observed to form and reach steady state within ∼10 h of polarization, after which time the total ion release rates are low and nearly constant at ∼0.4 μg cm −2 h −1 for all potentials investigated. The equilibrium film thickness, however, is a function of the applied potential. The main ionic species dissolved in the liquid are predominately Fe followed by Ni, that account for >90% of the steady-state corrosion current. The dissolution rate of Cr is low but increases systematically at potentials higher than 0.8 V. The experimental ion release rates can be correlated with a point defect model using a single set of parameters over a broad range of potentials (0.2–1 V) on the cathode side. The interfacial contact resistance measured after 48 h of polarization is observed to increase with increase in applied potential and can be empirically correlated with applied load and oxide film thickness. The oxide film is substantially thicker at 1.5 V possibly because of alteration in film composition to Fe-rich as indicated by XPS data. [ABSTRACT FROM AUTHOR]

Abstract: The robustness of the cathode catalysts used in polymer electrolyte fuel cells (PEFCs) is one of the major factors that determines their durability. In this work, a new class of corrosion-resistant catalyst, Pt–Ti alloy nanoparticles deposited on nano-sized sub-stoichiometric titanium oxide (TiOx), was prepared, and the durability of a Pt–Ti/TiOx cathode under conditions of fuel cell operation was evaluated. Cell performance under a constant current density for >900 h was examined to demonstrate the practical stability of the Pt–Ti/TiOx MEA under PEFC operating conditions. To investigate the effect of high potentials on cathode catalyst activity, a potential cycling test between 1.0 V and 1.5 V vs. a hydrogen anode was applied to the MEA. The results indicated that the electrochemical surface area (ECA) of the Pt–Ti/TiOx MEA is much more stable than that of a conventional Pt/XC72 MEA, and there is almost no loss of ECA even after 10,000 potential cycles. In addition, there was almost no change in the internal resistance of the MEA. TEM analyses of the potential-cycled MEA clearly revealed the excellent stability of Pt nanoparticles supported on TiOx particles. [Copyright &y& Elsevier]

Abstract: The influences of the heat-treatment of Ketjen Black EC300J in the temperature range 1000–2000 °C on the catalytic activity of loaded Pt for oxygen reduction reaction (ORR) were studied. A maximum enhancement in the specific ORR activities (SOA) was observed for the carbon heat-treatment at 1500 °C. The heat-treatment of carbon induced decreases in porosity and the development of graphitic structures; however, no direct correlations were observed between these properties and the SOA. Transmission electron microscopy and X-ray photoelectron spectroscopy, respectively, showed enhancements in the uniformity of the Pt particle size distribution and of the extent of surface reduction of Pt with increasing heat-treatment temperature (HTT). Cyclic voltammetry in 0.5 M H2SO4 aqueous solution detected changes in the hydrogen adsorption at 0.12 V vs. a reversible hydrogen electrode, depending on the HTT of the carbon support, and this was ascribed to hydrogen adsorption on the Pt(110) surface, the most active crystal face of Pt for ORR. The fraction of Pt atoms belonging to the (110) surface out of the total surface Pt atoms showed an excellent correlation with the SOA. Heat-treatment of the carbon support was therefore concluded to be an effective treatment for enhancing the SOA of Pt. [Copyright &y& Elsevier]

Abstract: This paper describes the use of an in situ analytical technique based on simultaneous displacement and resistance measurement of gas diffusion layers (GDLs) used in polymer electrolyte fuel cells (PEFCs), when exposed to varying compaction pressure. In terms of the losses within fuel cells, the ohmic loss makes up a significant portion. Of this loss, the contact resistance between the GDL and the bipolar plate (BPP) is an important constituent. By analysing the change in thickness and ohmic resistance of GDLs under compression, important mechanical and electrical properties are obtained. Derived parameters such as the ‘displacement factor’ are used to characterise a representative range of commercial GDLs. Increasing compaction pressure leads to a non-linear decrease in resistance for all GDLs. For Toray paper, compaction becomes more irreversible with pressure with no elastic region observed. Different GDLs have different intrinsic resistance; however, all GDLs of the same class share a common compaction profile (change in resistance with pressure). Cyclic compression of Toray GDL leads to progressive improvement in resistance and reduction in thickness that stabilises after ∼10 cycles. [Copyright &y& Elsevier]

Abstract: The effect of chemical and mechanical membrane degradation on the gas separation is investigated in the early stage. The aim is to identify the trigger processes which lead to a nonuniform degradation. Chemical and mechanical degradation are investigated in accelerated stress tests at OCV, relative humidity cycling and a combination of both. Using a tracer gas concept, gas permeation in perfluorosulfonic acid membranes (Nafion) is analyzed locally and online in terms of diffusive and convective gas transport. Local phenomena, such as humidity fluctuations, cracks in the micro porous layer and carbon fibers, are found to initiate the degradation of the gas separation. Pinhole formation, induced by mechanical degradation processes, is crucial for the onset of a synergetic effect. Combined chemical and mechanical degradation accelerates pinhole growth, which is the major source of the increasing gas permeation in Nafion membranes with stabilized end-groups. The spatial nonuniformities of the relative humidity and humidity fluctuations are the main source of inhomogeneous membrane degradation. [Copyright &y& Elsevier]

Abstract: Proper water management (sufficient membrane hydration and effective liquid-water removal from catalyst layers, gas-diffusion layers, and channels) is key to achieving and maintaining the high performance of polymer electrolyte fuel cells (PEFCs). Specifically, liquid droplet formation and removal is an important issue in water management for PEFCs. In the first part of this series, explicit analytical solutions were derived by approximating the droplet shape as spherical and the Navier–Stokes equations were numerically solved to compute the forces over a droplet. In this second part of the series, results from numerical simulation using the Volume-of-Fluid (VOF) method and experiments are reported and they are compared with the analytical solutions derived previously. More specifically, experimental visualization of droplet dynamics in a micro channel is carried out. A high resolution CCD camera is employed to capture the droplet shape-change and detachment. Extensive numerical simulations via VOF are also performed to investigate droplet dynamics. Reasonably good agreements between analytical solution and VOF numerical simulation and experiment are obtained. [Copyright &y& Elsevier]

Abstract: Water management is widely investigated because it affects both the performance and the lifetime of polymer electrolyte fuel cells. Membrane hydration is necessary to ensure the high proton conductivity, but too much water can cause flooding and pore obstruction within the cathode gas diffusion layer and the electrode. Experimental studies prove that the characteristics of the diffusion layer have great influence on water transport; the introduction of a micro-porous layer between the gas diffusion layer and the electrode reduces flooding and stabilizes the performance of the fuel cell, although the reason is not fully explained. A quantitative method to characterize water transport through the diffusion layers was proposed in our previous work, and the present work aims to further understand the flooding phenomenon and the role of the micro-porous layer. The improved experimental setup and methodology allow an accurate and reliable evaluation of water transport through the diffusion layer in a wide range of operating conditions. The proposed 1D+1D model faithfully reproduces the experimental data adopting effective diffusivity values in agreement with literature. The presented experimental and modelling analysis allows us to evaluate the influence of pore obstruction on the effective diffusivity, the overall transport coefficient and water flow through the diffusion layer, elucidating the effect of the micro-porous layer on fuel cell performance and operation stability. [Copyright &y& Elsevier]

Abstract: The Japan Aerospace Exploration Agency (JAXA) is developing polymer electrolyte fuel cell (PEFC) systems that can be operated under isolated low-gravity and closed environments. In the present study, we combine the PEFC with an electrolyzer in order to realize a regenerative fuel cell. Ideally, if a single cell can be operated as a fuel cell and the cell can be made reversible through the electrolysis reaction, then compact, lightweight regenerative fuel cell systems can be realized. A unitized regenerative fuel cell was prepared, and its operability was demonstrated. During 100-W class operations, a stable fuel cell and electrolysis reaction was observed. [Copyright &y& Elsevier]

Abstract: A new and practical testing technique was developed for measuring the flow rate distribution between cells in a stack that did not contain any internal sensors. The flow rate distribution is obtained by measuring the hydrogen limiting current of each cell in the stack while a mixed gas of hydrogen and dimethyl ether is supplied to the anode and hydrogen to the cathode. In order to measure large flow rate deviations between cells, it is necessary to decrease the flow rate of the anode hydrogen and to sufficiently humidify the cells. The faster the increasing rate of the current, the more the apparent hydrogen limiting current increases than the theoretical electrochemical equivalent current. However, the relative flow rate deviations between cells can be obtained by a practical accuracy using the ratio of the apparent hydrogen limiting current. Humidification of the cell is indispensable for the measurement and a method using dry anode gas and humidified cathode gas is recommended. The preferred test conditions for measuring the flow rate distribution between cells in a PEFC stack are proposed. [Copyright &y& Elsevier]

Abstract: Changes in the electronic states of platinum–cobalt (Pt–Co) alloy catalysts through potential cycling between 0.6 and 1.0V were investigated by X-ray photoemission spectroscopy (XPS) using synchrotron radiation. The electrochemical surface area loss and the particle size growth of the Pt catalyst were larger than those of the Pt–Co alloy catalyst. Pt 4f XPS spectra of the Pt–Co alloy catalyst do not show any change through the potential cycling, indicating that most part of Pt is stable during the potential cycling. Larger amount of Pt(OH)2 existed in the initial MEA of the Pt catalyst than the Pt–Co alloy catalyst, indicating that the Pt catalyst has a tendency to be oxidized. The Pt(OH)2 decreased and metallic platinum increased in the cycle-tested MEA, suggesting that the Pt(OH)2 dissolved and re-deposited as metallic states. The oxidation tendency explains the less durability of the Pt catalyst than the Pt–Co alloy catalyst. Co 2p XPS spectra imply that cobalt is absent on the surface of the catalyst particles and the Pt skin layer is thicker than 1.4nm (4 mono-layers). The absence of the cobalt oxide in the cycle-tested MEA demonstrates that the Pt–Co core under the Pt skin layer is stable during the potential cycling. [Copyright &y& Elsevier]

Abstract: The fuel cell transmogrified from a single cell that was the research object to stack is used in various fields such as cars, portable power sources, and fuel–cell cogeneration systems. It is preferable in the stack for the flow rate distribution between cells to be uniform because of the performance gain in power generation efficiency and longevity. As for the flow rate distribution between cells, the method for measuring using smoke in the measuring method and making visible the heat distribution in the stack is reported. However, a research stack was used with these measures, not a stack for practical use. In this report, a method for measuring the flow rate distribution between cells which can also be used for a cell that uses hydrogen limiting current in practical use was examined. [Copyright &y& Elsevier]

Abstract: Since the post-casting treatments influence the water entrapped in polymeric matrix and consequently its proton conductivity, an evaluation of annealing at 200°C and acid treatments was conducted on previously developed composite s-PEEK (1.55mequiv.g−1) membranes, containing a commercial aminopropyl-functionalised silica. DSC, WAXS, SEM–EDX and laser microscope measurements carried out on membranes swollen at different temperatures highlighted different membrane properties depending on post-casting treatments. It was found that composite membranes have different structural and morphological characteristics than pristine polymer membranes. The silica distribution was modified when different treatments are used. The state of water changed when silica was inserted into the membranes. Actually, contrary to the pristine membranes the presence of freezable water was revealed at temperature lower than 80°C. The proton conductivity was also affected by the presence and the amount of water trapped into the membranes and was particularly influenced by the post-casting treatments. The silica introduction reduced the swelling effect and improved the robustness of the membranes even if a higher water content in the freezable state was observed. Acid treatment leads to significant improvement in membrane properties, but the present work shows that annealing before acid treatment can affect the membrane morphology more strongly than other treatments resulting in a much better fuel cell performance. [Copyright &y& Elsevier]

Abstract: In this study, the support Pt catalyst was supported by a novel spherical carbon using a convenient technique. Two different preparation methods utilizing a nanocolloidal solution method without heat treatment were developed (methods 1 and 2). The scanning electron microscope (SEM) and transmission electron microscope (TEM) observations showed that the Pt nanoparticles (particle size) were supported, with higher dispersion being achieved with method 2 than method 1. The peak of the Pt metal was confirmed from the X-ray diffraction (XRD) measurement. Based on the inductively coupled plasma mass spectrometry (ICP-MS) measurements, Pt loading was 19.5wt.% in method 1 and approximately 50wt.% in method 2. The Pt specific surface area of the Pt/novel spherical carbon catalyst calculated from the cyclic voltammetry (CV) measurement result was larger than that of the commercially available Pt/Ketjen catalyst. These results indicated that the Pt nanoparticles were supported in high dispersion without heat treatment using novel spherical carbon as a carbon support. [Copyright &y& Elsevier]

Abstract: Carbonized silk fibroin (CS), which is free of metallic elements, showed high catalytic activity for oxygen-reduction reaction (ORR). The catalytic activity of CS for ORR was greatly enhanced by steam activation forming silk-derived activated carbon (CS-AC). The surface morphology, surface area, pore structure and remaining nitrogen species of the CSs were compared with those of the CS-ACs. The open-circuit potential and the power density of a polymer electrolyte fuel cell using a CS900-AC, which was heat-treated at 900°C prior to the steam activation, and a platinum/C (C: carbon black) anode under pure oxygen and hydrogen gases, respectively, both at 0.2MPa, were 0.92V and 142mWcm−2 at 80°C. The ORR on the activated carbon, CS900-AC, proceeded with a 3.5-electron reaction at 0.6V (vs. RHE); however, this was improved to a 3.9-electron reaction with the addition of zirconium oxide at 20wt% to CS900-AC. [Copyright &y& Elsevier]

Abstract: A numerical approach is developed to determine the real-time Net Water Transport Coefficient (NWTC) based on the experimental water vapor pressure for the cathode and anode outlet obtained by the optical humidity sensors with Tunable Diode Laser Absorption Spectroscopy (TDLAS). The results show that there are sharp vibrations for NWTC in the process of start-up and shut-down. And the time needed for the water transport balance increases with the increase in the current. The balanced NWTC ranges from −0.2 to 0.2, and it increases with the increase in the operation current in the present research. In the view of flooding prevention, it is reasonable to humidify the anode inlet gas with the lower temperature than that of cathode side by decreasing the osmotic-drag water from anode to cathode. [Copyright &y& Elsevier]

Abstract: Optimization of water management in polymer electrolyte membrane fuel cells (PEMFC) and in direct methanol fuel cells (DMFC) is a very important factor for the achievement of high performances and long lifetime. A good hydration of the electrolyte membrane is essential for high proton conductivity; on the contrary water in excess may lead to electrode flooding and severe reduction in performances. Many studies on water transport across the gas diffusion layer (GDL) have been carried out to improve these components; anyway efforts in this field are affected by lack of effective experimental methods. The present work reports an experimental investigation with the purpose to determine the global coefficient of water transport across different diffusion layers under real operating conditions. An appropriate and accurate experimental apparatus has been designed and built to test the single GDL under a wide range of operating conditions. Data analysis has allowed quantification of both the water vapor transport across different diffusion layers, and the effects of micro-porous layers; furthermore flooding onset and its consequences on the mass transport coefficient have been characterized by means of suitably defined parameters. [Copyright &y& Elsevier]

Abstract: The electrode of polymer electrolyte fuel cell (PEFC) consists of the porous catalyst layer and gas diffusion layer (GDL). Quantitative evaluation of the influence of these porous layers’ degradation on the cell performance was attempted. The cell was assembled by using the catalyst layer or GDL, which had been corroded ex situ, as the cathode and the cell performance was characterized. The oxygen diffusion polarizations of the catalyst layer and that of the GDL were evaluated from the polarization curves. The polarization curves before and after a long-term operation were also analyzed by the same way, and the influences of the degradation of catalyst layer and GDL were evaluated. The increase of the gaseous diffusion loss in the catalyst layer was found to cause the cell performance loss mainly from the analysis of the simulated corrosion test and the long-term operation cell. [Copyright &y& Elsevier]

Abstract: Comprehension of gas phase transport processes and species distribution in operating polymer electrolyte fuel cells (PEFC) is an important task for understanding and optimizing the related coupled processes. Few efforts for in-situ gas analysis in fuel cells are described in the literature, and previous work has focused mainly on integral species concentration or investigation in small laboratory scale cells under comparably dry conditions only. In this work, a methodology for flexible local in-situ gas analysis in polymer electrolyte fuel cells of technical size is presented. This includes conceptual description of measurement modes during fuel cell operation in particular for local membrane and gas diffusion layer characterization. Beside characterization of the analysis system, the potential of the method is demonstrated by accurate local concentration measurement of gas species including water vapor up to saturated conditions. Accurate measurement of local membrane tracer gas permeation under open circuit and load conditions emphazises the flexibility of the developed methodology. In particular, the ability to determine local membrane permeation characteristics is of great value for membrane degradation investigation. [Copyright &y& Elsevier]

Abstract: The knowledge of physical properties, such as the thermal conductivity, plays an important role in the management of the heat transfer in fibrous materials like PEFC gas diffusion layers. Measurement of thermal conductivity by experimental means is not easy (due to the anisotropy and the high porosity), therefore the availability of experimental data is rather limited. In this paper, the numerical determination of the effective thermal conductivity of fibrous materials is investigated using a three-dimensional approach. Two different fiber geometries were studied with randomly generated fiber structures with overlapping and non-overlapping fibers. The corresponding anisotropic thermal conductivities are computed through the solution of the energy transport equation. The results were validated through a comparison with existing experimental data and the influence of different parameters such as fiber orientation, fiber diameter and binding material were investigated. [Copyright &y& Elsevier]

Abstract: The effect of water on polymer electrolyte fuel cell degradation was examined with humidity as a parameter. Polymer electrolyte fuel cells were subjected to long-term operation of 10000h to examine the relation between decline in cell voltage and degradation of the catalyst layers or gas diffusion layers. The diffusion overpotential increased during long-term operation at relatively high humidification of 81% RH, but only in the catalyst layer and not in the gas diffusion layer. At low humidification of 52% RH, the increase in diffusion overpotential was small, indicating that the increase was more likely to occur under high humidification. Post-analysis of the catalyst layer revealed that the membrane electrode assembly had increased diffusion overpotential during operation under high humidification, as a result of the sharp decline in porosity. The increase of diffusion overpotential in the catalyst layer was also investigated by the observation of the degradation due to the oxidation of the Pt–carbon supports. However, it was found that the oxidation of carbon support which had increased diffusion overpotential was small. [Copyright &y& Elsevier]

Abstract: In this contribution a simplified, isothermal, two-phase, one-dimensional model for the calculation of the cathodic gas flow along the flow field channels of a polymer electrolyte fuel cell (PEFC) is presented. The composition of the humidified oxidant gas, average gas velocity, pressure drop, and other quantities can be calculated for any gas distributor structures with one channel. Thereby, the model requires several input parameters which have to be determined solely by experiment and pre-defined operation conditions, e.g. the water content of the feed gas, local current densities, and gas flow rates. In contrast to other models, the cross-section reduction has been taken into account which results from the penetration of the gas diffusion layer into the flow field channels due to the mounting pressure. Beyond this, the model needs no fit-parameters for further adjustment. For close examination of the factors limiting the performance of a PEFC, the DLR has developed several techniques for measuring the current density distribution with spatial resolution. In order to investigate the origin of the corresponding effects, one of these techniques has been improved by implementing the model of the cathodic gas flow as an on-line feature. The combination of a spatially resolved measurement technique with a real-time simulation gives a better understanding of the local processes within the cell and represents a helpful tool for the development of fuel cell components as well as for the optimization of the operating conditions. Exemplarily, the presentation the results for a 25cm2 serpentine flow field at different operation modes are shown in this paper. [Copyright &y& Elsevier]

Abstract: Most designers of Polymer Electrolyte Fuel Cells (PEFCs) supply the PEFC with humidified gas to prevent its membrane from drying. Because the steam generated by the electrochemical reaction is added to a humidified supply gas, the steam partial pressure in the cathode channel forces a supersaturated state. Therefore, the PEFC has water management issues, such as flooding and plugging. Many researchers have studied these issues in the cathode side using a visualization technique, and have introduced water repellency processing to the gas channel and GDL (gas diffusion layer) as a solution. However, the flooding/plugging phenomena in the cathode do not occur alone, and are influenced by the flooding/plugging phenomena in the anode channel through the membrane. Moreover, the water transport phenomenon through the membrane is affected by the locations of the flooding/plugging phenomena in each gas channel. Therefore, we aim to examine the water transport phenomenon through the membrane by the two-pole simultaneous image measurement, and to propose an optimum water management method. This work shows that the flooding/plugging phenomena on the anode side are clearly related to water transportation from the cathode side through the membrane. [Copyright &y& Elsevier]

Abstract: Liquid water transport in a polymer electrolyte fuel cell (PEFC) is a major issue for automotive applications. Mist flow with tiny droplets suspended in gas has been commonly assumed for channel flow while two-phase flow has been modeled in other cell components. However, experimental studies have found that two-phase flow in the channels has a profound effect on PEFC performance, stability and durability. Therefore, a complete two-phase flow model is developed in this work for PEFC including two-phase flow in both anode and cathode channels. The model is validated against experimental data of the wetted area ratio and pressure drop in the cathode side. Due to the intrusion of soft gas diffusion layer (GDL) material in the channels, flow resistance is higher in some channels than in others. The resulting flow maldistribution among PEFC channels is of great concern because non-uniform distributions of fuel and oxidizer result in non-uniform reaction rates and thus adversely affect PEFC performance and durability. The two-phase flow maldistribution among the parallel channels in an operating PEFC is explored in detail. [Copyright &y& Elsevier]

Abstract: We have investigated cell voltage characteristics and AC impedance characteristics of polymer electrolyte fuel cells (PEFCs) at various humidity temperatures for H2/O2 and H2/air test conditions (current density: 200, 400, and 600mAcm−2, cell temperature: 80°C, humidity temperature at respective electrodes: 40, 50, 60, and 70°C). The diffusion overpotential increases with decreasing humidity in the low humidity temperature region such as 40 and 50°C and the Nyquist plots obtained from AC impedance measurements show a small arc superposed on an elliptic arc in the low frequency region. The diameter of this small arc increases with decreasing humidity temperature from 50 to 40°C or with increasing current density. These results suggest that oxygen transport across the ionomer film in the catalyst layer is significantly reduced in the low humidity condition, which causes a decrease in cell voltage, increase in diffusion overpotential, the appearance of overlapping arcs (two separate arcs) in the lower frequency region on the Nyquist plots, and the increase of mass transport resistance from Nyquist plots. [Copyright &y& Elsevier]

Abstract: The carbon support effect of Pt–Ni(mqph) electrocatalysts on the performance of CO tolerant anode catalysts for polymer electrolyte fuel cells (PEFCs) was investigated using carbon black and multi-walled carbon nanotubes (MWCNTs), with and without defect preparation. 20%Pt–Ni(mqph)/defect-free CNTs showed a very high CO tolerance (75% compared to the CO-free H2 case) under 100ppm CO level in the half-cell system of the hydrogen oxidation reaction. On the other hand, the hydrogen oxidation current on Pt–Ni(mqph)/defective CNTs, Pt–Ni(mqph)/VulcanXC-72R and Pt–Ru/VulcanXC-72R significantly decreased with increasing concentration of CO up to 100ppm (25–47% compared to the CO-free H2 case). It is thus considered that the carbon support materials strongly affect the CO tolerance of anode catalysts. This is ascribed to a change in the electronic structure of the Pt particles due to the interaction with the graphene surface, leading to a reduction in the adsorption energy of CO. Ni(mqph) also mitigates CO poisoning due to its ability of CO coordination on Ni metal center. [Copyright &y& Elsevier]

Abstract: In polymer electrolyte fuel cell (PEFC), it is important to understand the behavior of liquid water in gas diffusion layer (GDL) which is one of the constructional elements so as to improve the output performance and the durability. As this behavior of liquid water is attributed to not only the hydrophilicity but also inhomogeneous structure, it is needed to examine in consideration of an actual GDL structure. In this study, as the basic examination of two-phase flow analysis in an actual GDL, a simulated GDL was made by numerical analysis considering the fiber placement. Furthermore, the prediction methods for pore size distribution, permeability and tortuosity of this simulated GDL were developed with the numerical analysis. These parameters of flow and mass transfer were compared with other studies, and the validity of this simulated GDL was confirmed. In addition, effective diffusion coefficient was calculated from tortuosity in simulated GDL, and PEFC output performance was evaluated by a simple model. Moreover, the optimal GDL was examined in consideration of the effect of porosity and fiber diameter at the fiber level. [Copyright &y& Elsevier]

Abstract: Stainless steel is quite attractive as bipolar plate material for polymer electrolyte fuel cells (PEFCs). Passive film on stainless steel protects the bulk of it from corrosion. However, passive film is composed of mixed metal oxides and causes a decrease in the interfacial contact resistance (ICR) between the bipolar plate and gas diffusion layer. Low ICR and high corrosion resistance are both required. In order to impart low ICR to stainless steel (SUS304), carbon-coating was prepared by using plasma-assisted chemical vapor deposition. Carbon-coated SUS304 was characterized by Raman spectroscopy and atomic force microscopy. Anodic polarization behavior under PEFC operating conditions (H2SO4 solution bubbled with H2 (anode)/O2 (cathode) containing 2ppm HF at 80°C) was examined. Based on the results of the ICR evaluated before and after anodic polarization, the potential for using carbon-coated SUS304 as bipolar plate material for PEFC was discussed. [Copyright &y& Elsevier]

Abstract: In the past various techniques to measure the current density distribution in PEFC were developed at the DLR Institute in Stuttgart. These techniques are used successfully to advance and to support the development of fuel cell stacks. Furthermore, the current density distribution measurements can also be used as a feedback control system to optimize the operation conditions, to avoid unfavourable conditions and to detect problems and defects at an early state. For this purpose three different techniques for determination of current densities were developed. In all techniques at least one of the flow fields is segmented. In an early design the individual segments are connected together with external resistances which are used for the measurement of the current density. This design is only suitable for the current density measurements in end plates of stacks or in single cells. Second, hall sensors are used to determine the current inside the segments. A recent development uses a design in which the segments as well as the resistances are realized in a printed circuit board (PCB) which can be used as a bipolar plate in stacks as well as in single cells. Results can be used to validate models and to optimise components of a stack, e.g. flow field and manifold design, as well as the operating conditions of fuel cells. By applying segmented bipolar plates as sensor plates for system control integrated in stack systems high performance, safe operation and long life cycles can be achieved. The main challenge in the development of control strategies based on current density measurements is their interpretation. Therefore, a simple and fast simulation of measured current density distribution has been developed taking into account the major influences on fuel cell performance (mainly the water management). As a result the actual local partial pressures, e.g. of oxygen and of water can be determined in short intervals and as a consequence a malfunction of the stack can be detected and avoided. [Copyright &y& Elsevier]

Abstract: A transient, one-dimensional thermal model for a generic polymer electrolyte fuel cell (PEFC) stack is developed to investigate the cold-start ability and the corresponding energy requirement over different operating and ambient conditions. The model is constructed by applying the conservation of energy on each stack component and connecting the component''s relevant boundaries to form a continuous thermal model. The phase change of ice and re-circulation of coolant flow are included in the analytical framework and their contribution to the stack thermal mass and temperature distribution of the components is also explored. A parametric study was conducted to determine the governing parameters, relative impact of the thermal mass of each stack component and ice, and anticipated temperature distribution in the stack at start-up for various operating conditions. Results indicate that 20 cells were sufficient to accurately experimentally and computationally simulate the full size stack behavior. It was observed that an optimum range of operating current density exists for a chosen stack design, in which rapid start-up of the stack from sub-zero condition can be achieved. Thermal isolation of the stack at the end plates is recommended to reduce the start-up time. Additionally, an end plate thickness exceeding a threshold value has no added effect on the stack cold-start ability. Effect of various internal and external heating mechanisms on the stack start-up were also investigated, and flow of heated coolant above 0°C was found to be the most effective way to achieve the rapid start-up. [Copyright &y& Elsevier]

Abstract: Internally humidified, edge-air-cooled PEFC stacks are promising for portable systems in terms of specific power and specific cost. However, their main drawbacks are thermal power limitations due to limited heat removal from inside the stack. The aim of this work is to minimize the cooling limitation with a simultaneous cost and weight reduction by optimization of the stack geometry. A steady-state, thermal FE-model was developed and validated against experimental temperature distributions. The model includes anisotropic heat conduction and heat convection by the cooling air. Cell voltage, liquid water fraction and limiting temperature were experimentally determined for improved accuracy. Complex flowfield structures were approximated with the numerical volume averaging method to reduce computational cost. As a result of the optimization study specific power was improved by +86% with simultaneous reduction of specific cost by %. [Copyright &y& Elsevier]

Abstract: Fuel cells and hydrogen storages, eventually integrated in hybrid power systems with hydrogen production from renewables, represent an interesting option for small stationary applications such as power generation in remote sites beyond the grid or back up power for telecom stations. This paper deals with the CESI RICERCA experiences on a polymer electrolyte fuel cell (PEFC) power system fuelled with the hydrogen supplied by a metal hydride storage. The power system consists of three ReliOn Independence 1000 PEFC units, a battery bank and a 3.3kWe DC–AC converter (inverter). The hydrogen storage is made of LaNi5 type powders and can supply more than 6Nm3 of hydrogen per discharge cycle. The PEFC units, the inverter and the hydrogen storage performances were characterised. These subsystems were integrated into an automated power generation system and connected to a local grid including other power generators, power quality analysers, energy storage systems and electrical loads. The main features of the integrated system are analysed herein. In particular the overall system stability upon cycling, the heat transfer issues and the possibility of recovering the fuel cell waste heat to extract hydrogen from the metal hydrides are discussed. Finally, during grid-connected operations, the power quality indexes were measured and found in agreement with the EN 50160 standard. [Copyright &y& Elsevier]

Abstract: In actual size of polymer electrolyte fuel cell stack with heat management of cooling water, relative humidity distribution was calculated under various kinds of operating conditions and various shapes of gas channel with numerical analysis. And the optimal separator shape and the optimal flow pattern of gas and cooling water that make the relative humidity higher and more uniform and that lead to the improvement of cell durability were examined under each operating condition. As a result, the effects of humidify temperature, the temperature of cooling water at an outlet and the average current density on humidity distribution which was affected by vapor concentration and gas temperature were examined and it was found that the optimal combination of flow pattern of gas and cooling water was the same under each operating condition. As regards the operating condition in this paper, the relative humidity is the highest and the most uniform in the following cases: gas flow pattern is counter, the cooling water is synchronized with cathode gas flow and the ordinary serpentine separator with 1.0mm depth channels is used in cathode and anode sides. However, it is found that it is possible to occur the flooding in such cases. [Copyright &y& Elsevier]

Abstract: Polymer electrolyte fuel cell (PEFC) needs to be examined relative humidity distribution and current density distribution to improve the durability. In this study, the PEFC reaction and thermal flow analysis model including the effect of cooling water was developed. Furthermore, the effects of the shape of separator channels and the flow pattern on current density distribution and relative humidity distribution were examined by this numerical analysis. As a result, it was found that the uniform current density distribution did not directly relate to the uniform humidity distribution, because the humidity distribution was complexly affected by the generated water, the water transfer between the anode and the cathode and the gas temperature. Moreover, it was confirmed that the optimal flow pattern of gas and cooling water could make the relative humidity higher and more uniform. This calculation model can help us to design the optimal separator shape and to determine the optimal operating conditions which can narrow the low humidity area and improve the cell durability. [Copyright &y& Elsevier]

Abstract: This study proposes a novel anode catalyst layer structure to improve the CO tolerance and cell performance. A layered structure is composed of an outer and an inner catalyst layer. The outer catalyst layer acts as a CO barrier and consists of a nano Ru/Pt layer (0.06mgcm−2) deposited by magnetron sputtering and a Pt50–Ru50 layer (0.10mgcm−2) applied by screen-printing to the GDL. The inner catalyst layer is a pure Pt layer (0.07mgcm−2) produced by a direct-printing method onto the PEM. The roles of outer and inner catalyst layers which improve the CO tolerance and cell performance are investigated in this paper. SEM, X-ray, EDS and EPMA analyses were used to characterize microstructures, phases, chemical composition and distributions of the obtained electrocatalyst layers. Hydrogen containing 50ppm CO is continuously fed to the anode side to investigate the relationships. The MEAs consist of a Nafion 117 membrane and a commercial electrocatalyst (20% Pt/C from E-TEK) on the cathode side. The results demonstrate that this proposed anode catalyst layer structure exhibits a superior performance and CO tolerance capability in pure hydrogen and CO containing hydrogen fuels. The improved CO tolerance capability is attributed to the filtering effect of the outer catalyst layer. [Copyright &y& Elsevier]

Abstract: Because the parasitic load on most polymer electrolyte fuel cell systems is dominated by pressure losses across the flow fields, understanding the flow channel dynamics has garnered significant recent attention. The current work is aimed at enabling advanced design of reactant flow channels by elucidating the details of the velocity fields within various reactant flow channel geometries. Gas-phase measurement is critical to ensuring that the results are valid for real fuel cell systems, and for application of this technique to the study of flow channels with two-phase obstruction phenomena. In the current research, gas-phase velocity fields have been measured using particle image velocimetry (PIV) in representative geometries, including a straight channel (for technique validation) and a 180° switchback with channel-to-land ratio of 1:1. The accuracy of the measurement depends on the ability of the seed particles to follow the flow. Numerical simulations have been performed to develop a criterion for identifying appropriate seed particles. The research presented is a first step toward measuring velocity fields in novel flow reactant channels in operational cells. [Copyright &y& Elsevier]

Abstract: The paper describes the research activities carried out at the CNR Institute for Advanced Energy Technologies “Nicola Giordano”, aimed to develop, assemble and test a hydrogen generator, named HYGen I, to integrate with a PEFC (polymer electrolyte fuel cell) for residential applications. The unit, capable to convert light hydrocarbons (methane, propane, LPG, butane), with a nominal hydrogen production of 2Nm3 h−1 and a maximum hydrogen production of 5Nm3 h−1, can be considered the only prototype of this size in Italy. Preliminary experimental results, with a proprietary Pt/CeO2 catalyst for the autothermal reforming (ATR) and commercial catalysts for intermediate water gas shift (ITWS) and preferential oxidation (PROX) have been reported. [Copyright &y& Elsevier]

Abstract: The Japan Aerospace Exploration Agency (JAXA) is developing polymer-electrolyte fuel cell (PEFC) systems that can operate under isolated low-gravity and closed environments. Subsystems and operating methods such as the closed gas operation subsystem, the working gases counter-flow method, and a dehydrator using wicking material were developed to simplify assembly of the fuel cell system. We combined these subsystems with the 1kW-class fuel cell stack and simulated the operations of a spacecraft in Earth orbit. The system performed stably for 1100h under various operational conditions. [Copyright &y& Elsevier]

Abstract: PEFC cannot be operated for extended periods because it has some problems. However, if the deterioration factors can identified on time, PEFC can be operated for extended periods by overcoming these problems. Therefore, performance diagnostics needs to be developed to judge the PEFC deterioration factors instantaneously and simply. MCFC performance diagnostic has now been developed by the current-pulse method. Therefore, a thesis was examined relating to the applicability of the diagnostic to a PEFC characteristic diagnosis. The diagnostic evaluates changes in the seven parameters of the fitting equation led from PEFC''s transient response between 100ms obtained by the current-pulse method. The relationship between these parameters and each deterioration factor is evaluated using a PEFC single cell whose electrode area is 25cm2. In this case, deterioration factors include cell temperature, the flooding phenomenon and the gas composition/flow rate. As a result, each of seven parameters in the transient response of PEFC corresponds to an ohmic potential drop, anode/cathode gas diffusion resistance, reactive resistance, three-phase interfacial resistance and electrolyte properties, respectively. When the flooding phenomenon is caused in the cathode channel, the transient response of the PEFC obviously differs from the standard value. The change in each parameter is examined by gradually raising the humidity of the cathode supply gas (flooding gradually being caused in the cathode channel). The influence of the flooding phenomenon clearly appeared in three parameters, and in the same way, the temperature dependency of each parameter is confirmed. Therefore, the proposed performance diagnostic will be able to specify the PEFC deterioration factors while driven, easily and on time. [Copyright &y& Elsevier]