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A new method for measuring membrane degradation in polymer electrolyte fuel cells (PEFCs) is proposed. The method is based on the detection of fluoride ions in effluent water from the cathode- and anode outlet of the PEFC using photometry (PM). The fluoride emission rate (FER) is an indicator of the membrane's state of health (SoH) and can be used to measure the chemical membrane degradation. Commercial catalyst-coated membranes (CCMs) have been tested at 80 °C and 90 °C at 30% relative humidity (RH) to investigate the reliability of the developed method for fuel cell effluent samples. To verify the measurement, a mean-difference plot was created by measuring the same data with a fluorine selective electrode. The average difference was at ±0.13 nmol h−1 cm−2, which indicates good agreement between the two methods. These new findings imply that PM is a promising method for quick and simple assessment of membrane degradation in PEM technology. [ABSTRACT FROM AUTHOR]

Nanocellulose is a promising new membrane material for fuel cells, with much lower cost and environmental impact compared with Nafion or Aquivion. It is mechanically strong, is an excellent hydrogen barrier and has reasonable proton conductivity. Here, sulfonation of cellulose nanofibers is performed to enhance the conductivity (up to 2 × 10− 3 S cm− 1) without compromising the membrane integrity, and fuel cells are fabricated with 30 µm-thick "paper" membranes. The hydrogen crossover current is two orders of magnitude lower than for Nafion fuel cells with equivalent thickness, but the power density is rather low. Spray-coating is used to deposit 8 µm-thick membranes directly onto the electrocatalyst layer, in a process analogous to 3D printing or additive manufacturing. The resulting paper fuel cell has high current density (> 0.8 A cm− 2) and power density (156 mW cm− 2) under standard measurement conditions (H2/air; 80°C; 95% RH; 0.1 MPa), attributed to decreased membrane resistance. The cost of the spray-painted cellulose membranes is calculated to be ~ 50 $ m− 2, which is much lower than that of Nafion, even without taking into consideration economies of scale. This new concept in electrochemical energy conversion paves the way for the mass production of affordable, recyclable fuel cells. [ABSTRACT FROM AUTHOR]

The growing energy demand and the impact of polluting gases lead to the necessity of alternative energy sources and conversion energy devices. Fuel cells (FCs) appears as a suitable solution for facing the mentioned issues. Predicting the behavior of a polymer electrolyte fuel cell (PEFC) under different conditions represents a proper initial step to solve the several issues, e.g., aging water balance problems, which occur inside the cell during the energy conversion process. Understanding microstructural impacts of the diffusion media, water management issues of FCs or the impacts of the inlet reactant gases to the cell represent some of the processes that have to be analyzed to improve the efficiency and behavior of FCs. The current study aims, based on experimentally collected data, to propose empirical correlations that describe and predict the performance of a PEFC. The single cell considered in this study corresponds to a single PEFC with a Nafion® 112 membrane as electrolyte and with an effective area of 25 cm2. Relative humidity as a function of the reactive inlet gas temperature, as well as the power and the current density as a function of the cell/reactant gas temperature gradient are analyzed. In addition, correlations for power and current density as a function of the relative humidity (RH) have been proposed. Our correlations are obtained for an operating voltage of 0.6 V. It was shown a strong correlation between the power and current densities with the RH since the membrane conductivity depends mainly on the water content. The PEFC behavior was evaluated at different RHs. The results show big losses of operating power and current densities, as well as an increment of the resistance of the membrane when it operates at low RH. • An experimental study of a single PEFC with 25 cm2 of effective area is carried out. • The impact of the relative humidity on the performance of a PEFC has been analyzed. • Correlations for current and power density as a function of RHs have been proposed. • Predictions for current, power and RH related to the temperature have been proposed. [ABSTRACT FROM AUTHOR]

The large-scale application of polymer electrolyte membrane fuel cells (PEMFCs) requires a reduction of the costs and an improvement in performance and stability. Particularly, the automotive market needs the PEMFC works under harsh conditions (high temperature, low relative humidity, etc.), maintaining a good performance level and low degradation. These operating conditions permit to mitigate the constraints concerning the thermal and water management allowing both a simplification and a volume reduction of the system inside the car with a strong impact on costs and reliability. However, PEMFCs require the development of the components such as catalysts, ionomers and membranes with a proper stability. In this work, PtCo/KB and Pt/KB electrocatalysts were investigated with the aim of verifying the performance and stability, coupling them with an innovative reinforced Aquivion® membrane. An optimization of the catalytic ink composition was carried out, according to the different ionomers characteristics to be used in the catalytic layer in order to reduce the degradation level. The stability is assessed by considering the variations of the performance and electrochemical parameters, such as electrochemical surface area (ECSA) and mass activity (j m), at the beginning and end of accelerated stress test procedures. • MEA with PtCo/KB catalyst shows superior durability compared to Pt/C. • CL with ionomer with EW830 has higher degradation resistance than with EW790. • Optimised MEA maintains higher performance and ECSA after AST. [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 reduction of platinum-loading on the cathode side of polymer-electrolyte fuel cells leads to a poorly understood increase in mass-transport resistance (MTR) at high current densities. This local resistance was measured using a facile hydrogen-pump technique with dilute active gases for membrane-electrode assemblies with catalyst layers of varying platinum-loading (0.03–0.40 mg Pt /cm 2 ). Furthermore, polarization curves in H 2 /air were measured and corrected for the overpotential caused by the increased MTR for low loadings on the air side due to the reduced concentration of reactant gas at the catalyst surface. The difference in performance after correction for all resistances including the MTR is minor, suggesting its origin to be diffusive in nature, and proving the meaningfulness of the facile hydrogen-pump technique for the characterization of the cathode catalyst layer under defined operation conditions. [ABSTRACT FROM AUTHOR]

Diagnosing faulty conditions of engineering systems is a highly desirable process within control structures, such that control systems may operate effectively and degrading operational states may be mitigated. The goal herein is to enhance lifetime performance and extend system availability. Difficulty arises in developing a mathematical model which can describe all working and failure modes of complex systems. However the expert's knowledge of correct and faulty operation is powerful for detecting degradation, and such knowledge can be represented through fuzzy logic. This paper presents a diagnostic system based on fuzzy logic and expert knowledge, attained from experts and experimental findings. The diagnosis is applied specifically to degradation modes in a polymer electrolyte fuel cell. The defined rules produced for the fuzzy logic model connect observed operational modes and symptoms to component degradation. The diagnosis is then tested against common automotive stress conditions to assess functionality. [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]

The present study reports the system configurations of a self-operated Polymer electrolyte fuel cell (PEFC) that is specially designed for dead-end operation of both pure hydrogen and oxygen gas lines, including its operation features and durability. The PEFC stack with 16 unit cells was constructed using homemade membrane electrode assemblies with 300 cm of apparent active area and bipolar plates. A self-operated PEFC system was developed using a homemade PEFC stack, balance of plant, and electric control unit for automatic fuel cell system operation. Performance parameters of the self-operated PEFC system, including fuel utilization, parasitic loss, released heat, purge loss, and efficiency of the PEMFC stack and system were evaluated during the automatic operation of the PEFC system. A Daily start-and-stop (DSS) test was also performed for one month to check the durability of the self-operated PEFC system. Electric efficiencies of the PEFC stack and system are approximately 60% and 49%, respectively, at an operating condition of 1.1 kW electric output; no performance degradation of the PEFC system was observed for a month during the DSS test. [ABSTRACT FROM AUTHOR]

This work deals with the theoretical and experimental analysis of fuel-cell-based auxiliary power units operated with reformate from diesel and kerosene reforming for trucks and aircraft. In the theoretical part, a PEFC and an HT-PEFC system were analyzed using process simulation software. In the experimental part, a fuel processor consisting of an autothermal reformer, a water-gas shift reactor and a catalytic burner with 28 kW thermal power was characterized using different diesel and kerosene fuels. These fuels included desulfurized Jet A-1 and Aral Ultimate diesel as petroleum-based fuels and GTL kerosene, GTL diesel (winter and summer grades) and BTL diesel as non-petroleum-based synthetic fuels. The PEFC system showed a calculated electrical net efficiency of 28.5%, whereas 22.3% was calculated for the HT-PEFC system. A high-quality reformate was produced using various diesel and kerosene fuel qualities in the reformer with a relevant technical power class for the APU application. Although a performance loss of the shift reactor was observed, it was kept at an acceptable level at the end of experiments. [ABSTRACT FROM AUTHOR]

Perfluorinated sulphonic acid (PFSA) polymers such as Nafion ® are widely used for both electrolyte membrane and ionomer in the catalytic layer because of their high proton conductivity (σ H ) as well as chemical and thermal stability. When PFSA polymers alternative to Nafion ® with a different EW are used, it is necessary to optimize the composition of the catalytic ink, considering the different ionomer characteristics. Moreover, fuel cell operating conditions play an important role regarding the catalytic layer optimization. In this work, the influence of ionomer amount in the catalytic layer was evaluated. The electrochemical studies were carried out in a single cell in H 2 –O 2 at intermediate temperatures (80 °C–130 °C) by using a fumapem ® F-950 membrane as an electrolyte. A hydro-alcoholic dispersion of fumion ® F was also used as an ionomer in the electrode catalytic structure with an amount ranging from 30 to 60%. Under operating conditions finalised to automotive applications (intermediate temperature, low relative humidity and low pressure of gases) an improvement of performance in terms of maximum power density was obtained by using optimized ionomer content. [ABSTRACT FROM AUTHOR]

In this study, an impedance model based on electrochemical theory considering hydrogen peroxide formation during a two-step oxygen reduction reaction (ORR) in polymer electrolyte fuel cells (PEFCs) has been developed. To validate the theoretical treatment, electrochemical impedance spectroscopy (EIS) measurements were carried out in an open-cathode 16 cm 2 H 2 /air PEFC stack. The results show that inductive loops at low frequencies of the impedance spectra are attributed to mechanisms related to hydrogen peroxide formation during ORR. The results also demonstrate that the mechanisms during consumption of hydrogen peroxide to form water (second-step in ORR) can be the dominating process for losses in the PEFC compared to the mechanisms during oxygen consumption to form hydrogen peroxide (first-step in ORR). Oxygen transport limitations can be a result of hydrogen peroxide adsorbed onto the surface of the electrode which reduces the number of active sites in the cathode catalyst layer for oxygen to react. This study could support results from other experimental techniques to identify hydrogen peroxide formation during the ORR that limit the performance of PEFCs. [ABSTRACT FROM AUTHOR]

A computationally efficient model of a polymer electrolyte fuel cell (PEFC) is presented, based on a 2+1D FEM modelling approach. This approach is suitable to take the high aspect ratio between the in-plane and the through-plane dimensions of fuel cells into account, and to avoid expensive 3D calculations. The anode and cathode are described by 2D transport models. The coupling between the anode and cathode side is established by a nonlinear point-to-point 1D model representing the membrane electrode assembly (MEA). This 1D boundary value problem is formulated using the computer algebra software Mathematica. The approach is based on the symbolic weak form expressions of a nonlinear system of PDEs. The integrands of the tangential element stiffness matrix and the element residual vector of the coupled FEM problem are computed analytically. These integrands are converted to C code automatically. The model is applied to simulate a micro PEFC without gas diffusion layers (GDLs). The simulations reveal an inhomogeneous in-plane electric current density. Further, neutron radiography data obtained with the micro fuel cell is compared to the calculated water flux between the 1D MEA model and the 2D domains. The model is used to explain the locations where water condensation is found. [ABSTRACT FROM AUTHOR]

This paper reports a study on the understanding of the performance decrease mechanisms of polymer electrolyte fuel cells under critical operating conditions. In order to investigate the durability of perfluorosulfonate membranes at low humidification conditions, long-term fuel cell tests have been carried out. Results evidenced a strong effect of low relative humidity on the commercial polymer membrane lifetime. Prolonged dehydration of the membranes led to a decrease of the three-dimensional reaction zone due to the ionomer degradation in the catalytic layer and a continuous loss of material in the membrane evidenced by a thickness decrease. The last effect provoked a collapse of the electrode structure. [ABSTRACT FROM AUTHOR]

A study was performed to establish a method for measuring the position and amount of leak current increase during operation of a fuel cell. This measurement method was applied to load fluctuation testing at low humidification and the increase in the leak current was observed in the downstream zone of the cell. The tested cell was disassembled and cross sections were observed under the SEM. The SEM images revealed the formation of a Pt band near the cathode catalyst in the downstream zone of the cell. This may be the cause of the degradation of the membrane at the downstream. © 2010 Wiley Periodicals, Inc. Electr Eng Jpn, 174(1): 1-9, 2011; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/eej.21038 [ABSTRACT FROM AUTHOR]

In this research, the development of a bioethanol reforming system for fuel cells (FBSR: fuel cell with bioethanol steam reforming) using sunlight as a heat source was investigated. The system was investigated using the experimental result of catalyst performance, and numerical analysis. If ethanol purity is high, the production method of the bioethanol used for the proposal system will not be limited. The overall efficiency of the production of electricity and heat power of this system was determined by examining its thermal output characteristic. The FBSR was introduced into standard individual houses in Sapporo, Japan, for analysis. The amount of hydrogen production, the production-of-electricity characteristic, and the thermal output characteristic were examined using meteorological data on representative days in March and August. Compared with the representative day in March (28.0 MJ day-1), the solar radiation of the representative day in August (37.0 MJ day-1) is large. However, the amount of solar radiation fluctuation of the representative day in August in this analysis is large compared with the representative day in March. It depends for the overall efficiency of the system on the amount of solar radiation fluctuation rather than the amount of solar radiation. As a result, the overall efficiency of the system, defined as the rate of power and heat output compared with the amount of solar heat collected, was calculated to be 47.4 and 41.9% on the representative days in March and August, respectively. Copyright © 2009 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Abstract: A fuel cell microgrid with photovoltaics effectively reduces greenhouse gas emission. A system operation optimization technique with photovoltaics and unstable power is important. In this paper, the optimal operation algorithm of this compound microgrid is developed using numerical weather information (NWI) that is freely available. A GA (genetic algorithm) was developed to minimize system fuel consumption. Furthermore, the relation between the NWI error characteristics and the operation results of the system was clarified. As a result, the optimized operation algorithm using NWI reduced the energy cost of the system. [Copyright &y& Elsevier]

Abstract: CNR–ITAE is developing several hydrogen and fuel cell demonstration and research projects, each intended to be part of a larger strategy for hydrogen communities settling in small Sicilian islands. These projects involve vehicle design, hydrogen production from renewable energy sources and methane, as well as implementation strategies to develop a hydrogen and renewable energy economy. These zero emission lightweight vehicles feature regenerative braking and advanced power electronics to increase efficiency. Moreover, to achieve a very easy-to-use technology, a very simple interface between driver and the system is under development, including fault-recovery strategies and GPS positioning for car-rental fleets. Also marine applications have been included, with tests on PEFC applied on passenger ships and luxury yacht as power system for on-board loads. In marine application, it is under study also an electrolysis hydrogen generator system using seawater as hydrogen carrier. For stationary and automotive applications, the project includes a hydrogen refuelling station powered by renewable energy (wind or/and solar) and test on fuel processors fed with methane, in order to make the power generation self-sufficient, as well as to test the technology and increase public awareness toward clean energy sources. [Copyright &y& Elsevier]

In this paper, a quasi-potential approach along with conformal mapping is used to model the diffusion media of a polymer-electrolyte fuel cell. This method provides a series solution that is grid independent and only requires integration along a single boundary to solve the problem. The approach accounts for nonisothermal phenomena, two-phase flow, correct placement of the electronic potential boundary condition, and multilayer media. The method is applied to a cathode diffusion medium to explore the interplay between water and thermal management and performance, the impact of the rib-to-channel ratio, and the existence of diffusion under the rib and flooding phenomena. [ABSTRACT FROM AUTHOR]

When the effects of humidification temperature on air utilization properties of PEFCs were investigated: (1) in standard humidification conditions, cell voltage decreased with an increase in air utilization; (2) in low humidification conditions, cell voltage increased with an increase in air utilization; (3) in high humidification and high utilization conditions, cell voltage suddenly dropped with a slight increase in air utilization. The authors proved that based on experimentally obtained ohmic polarization, oxygen gain, and internal resistance free cell voltage during a pure oxygen test, by combining the following results they could provide an explanation for cell voltage behavior in the three cases: (1) calculation of water vapor pressure in the cell and change in activation polarization based on oxygen concentration decrease in the cell; (2) increase in relative humidity in the cell due to increase in air utilization, and consequent increase of concentration polarization; (3) change in the catalyst utilization rate with a change in humidification temperature, and resulting change in properties such as cell voltage. © 2008 Wiley Periodicals, Inc. Electr Eng Jpn, 166(3): 18– 26, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20850 [ABSTRACT FROM AUTHOR]

Abstract: A novel sulfonated polyimide membrane containing triazole groups (SPI-8) was subjected to long-term fuel cell operation. Excellent durability of the SPI-8 membrane was confirmed by single cell operation for 5000h at 80°C. Open circuit voltage and hydrogen crossover through the membrane showed only minor changes during cell operation, indicating a lack of catastrophic damage for the SPI-8 membrane. It was found by post-test analyses of the membrane that the ion exchange capacity (IEC) decreased only slightly, but the molecular weight decreased to 1/10, resulting in a loss of mechanical strength. It was concluded that the major degradation mode of the sulfonated polyimide membrane involves the ring-opening of the imide linkages via hydrolysis, while a certain degree of side chain degradation occurs as a result of oxidative attack by radical species. [Copyright &y& Elsevier]

Abstract: A comparative, experimental diffusivity study of gas diffusion layer (GDL) materials for polymer electrolyte fuel cells (PEFC) is presented for the first time. The GDL plays an important role for electrochemical losses due to gas transport limitations at high current densities. Characterization and optimization of these layers is therefore essential to improve power density. A recently developed method which allows for fast diffusimetry is applied and data compared to the literature values. Measurements are made as a function of direction and compression and the effect of different binder structures and hydrophobic treatments on effective diffusivities are discussed. A better understanding of the results is gained by including novel GDL cross-section images and a meaningful unit cell model for the interpretation of the data. The diffusivity data is valuable for GDL manufacturers and future PEFC models. The study reveals that a binder–fiber ratio larger than 50% has a negative impact on the effective diffusion properties. The hydrophobic treatment which is necessary to improve the water management can impede diffusion and thus reduce the power density. Furthermore binder has an isotropic effect while compression pronounces the in-plane orientation of the fibers. [Copyright &y& Elsevier]

Abstract: For a fuel cell system using pure hydrogen as fuel, high hydrogen utilization operation is required because the exhausted hydrogen from the fuel cell is not used as a heat source for a reformer. While the hydrogen utilization is very high, any minor imbalance in the distributed flow causes a shortage of hydrogen gas. In order to achieve high hydrogen utilization operation, cells humidified at the cathode side, of counter flow type and with two-block divided stack where the hydrogen gas is fed to the first block with its exhaust hydrogen being fed to the second block have been developed. Five kilowatts class (100 cells) short stack systems fabricated to above specification showed stable operation at hydrogen utilizations up to 96%. [Copyright &y& Elsevier]

Abstract: Fe/N/C catalysts for the reduction of oxygen in PEM fuel cells were prepared by pyrolyzing three series of iron acetate-impregnated developmental carbon blacks at 950°C. The carbon supports used were derived from the N234, N330, and N650 commercial furnace grades. In this study, we tried to increase the performance of Fe/N/C-based cathode of PEM fuel cells by using the following two approaches: (1) increasing the number of catalytic sites on the carbon black either by optimizing the structural parameters of the pristine carbon supports or by increasing the initial metal content above 0.2wt% Fe on the carbon support; (2) increasing the catalyst loading in the cathodic layer of a PEM fuel cell. For (1), we show, on the one hand, that optimizing the structural parameters of the pristine carbon support, in order to increase the number of catalytic sites, has its limits and that these limits have been reached for the present synthesis method of Fe/N/C catalysts. On the other hand, increasing the initial metal content above 0.2wt% Fe leads to a decrease in catalytic activity. For (2), it is shown that increasing the catalyst loading per cm2 of cathode well improves the performance of a cathode based on Fe/N/C catalysts in the kinetic region of the polarization curve. At lower potentials, a large improvement in the performance of these non-precious metal cathodes would occur if the mass transport properties in these electrodes were significantly increased. [Copyright &y& Elsevier]

Abstract: The influence of CO2, CO and air bleed on current distribution was studied during transient operation, and the dynamic response of the fuel cell was evaluated. CO causes significant changes in the current distribution in a polymer electrolyte fuel cell. The current distribution reaches steady state after approximately 60min following addition of 10ppm CO to the anode fuel stream. Air bleed may recover the uneven current distribution caused by CO and also the drop in cell voltage due to CO and CO2 poisoning. The recovery of cell performance during air bleed occurs evenly over the electrode surface even when the O2 partial pressure is far too low to fully recover the CO poisoning. The O2 supplied to the anode reacts on the anode catalyst and no O2 was measured at the cell outlet for air bleed levels up to 2.5%. [Copyright &y& Elsevier]

Abstract: The gas diffusion layers (GDLs) of a membrane electrode assembly (MEA) serve as link between flow field and porous electrode within a polymer electrolyte fuel cell. Beside ensuring sufficient electrical and thermal contact between the whole electrode area and the flow field, these typically 200–400μm thick porous structures enable the access of educts to the electrode area which would be occluded by the flow field lands if the flow field is directly attached to the electrode. Hence, the characterisation of properties pertaining to mass transport of educts and products through these structures is indispensable whilst examining the contribution of the GDLs to the overall electrochemical characteristics of a MEA. A fast and cost effective method to measure the effective diffusivity of a GDL is presented. Electrochemical impedance spectroscopy is applied to measure the effective ionic conductivity of an electrolyte-soaked GDL. Taking advantage of the analogy between Ficks and Ohms law, this provides a measure for the effective diffusivity. The method is described in detail, including experimental as well as theoretical aspects, and selected results, highlighting the anisotropy and dependence on the degree of compression, are shown. Moreover, a two-dimensional model consisting of regularly spaced ellipses is developed to represent the porous structure of the GDL, and by using conformal maps, the agreement between this model and experiment with respect to the sensitivity of the effective diffusivity towards compression is shown. [Copyright &y& Elsevier]

Abstract: Tungsten and nickel tungsten carbides were evaluated as the anode catalysts of a polymer electrolyte fuel cell (PEFC). These catalysts were prepared by the temperature-programmed carburization of tungsten and nickel tungsten oxides from 573 to 873–1073K in a stream of 20% CH4/H2 and kept at temperature for 3h. The 30% tungsten and nickel tungsten carbides mixed with Ketjen carbon (KC) were evaluated by cyclic voltammetry and linear sweep voltammetry using a rotating disk electrode and electrocatalytic activity (I–V performance) using a single cell. The W1023/KC catalyst achieved a power density of 6.4mW/cm2 (current density: 15.2mA/cm2) which corresponded to 5.7% of that achieved by a commercial 20% Pt/C catalyst in a single cell (20% Pt/C: 111.7mW/cm2) using our setup. From the XRD data, α-W2C together with a small amount of WC was active during the anodic oxidation. The maximum power density of the 30wt% 873K-carburized NiW/KC was 8.2mW/cm2 at the current density of 19.0mA/cm2 which was 7.3% of the 20wt% Pt/C. [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: This paper deals with a two-dimensional model of a preferential oxidation (PROX) reactor to be used in a beta 5kWe hydrogen generator, named HYGen II, to integrate with polymer electrolyte fuel cells (PEFCs) for residential applications. The reactor geometrical configuration developed is a single-stage multi-tube configuration, in which a cocurrent air flow in the interspace is aimed at improving heat transfer and consequently controlling the temperature of the reactor. The aim of the model is to investigate the process performance of the reactor in order to enhance optimization and control of the PROX unit. The model concerns chemical kinetics and heat and mass transfer phenomena in the reactor. The model plays a key role in overcoming the issues of system design, by evaluating the temperature and the gas concentration profiles in the reactor. The CO removal from simulated reformate was examined with varying inlet temperature. Simulation results showed the strong dependence of the overall performance upon the reactor geometrical configuration. [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: The cathode catalyst layer (CL), due to sluggish oxygen reduction reaction and several transport losses therein, plays an important role in the overall performance of polymer electrolyte fuel cells (PEFCs). The relative volume fractions of the constituent phases, i.e. the electronic, electrolyte and void phases, of the cathode CL need to be selected appropriately in order to achieve an optimal balance between oxygen diffusion and proton conduction. In this work, the influence of electrolyte and void phase fractions of the cathode CL on the cell performance is investigated based on a pore-level description of species and charge transport through a random CL microstructure via the direct numerical simulation (DNS) model. Additionally, the effects of inlet relative humidity and net water transport from the anode on the cathode performance have been studied which indicate the interdependence between the CL composition and the cell operating conditions. The results indicate that the low humidity operation benefits the performance by enhancing the oxygen transport especially under high current densities. Finally, the DNS model predicts the volume fractions of 0.4 and 0.26 for the void and electrolyte phases, respectively, as the optimal composition of the catalyst layer for the best performance. [Copyright &y& Elsevier]

Abstract: Fuel cells allow an environmentally friendly and highly efficiently conversion of chemical energy to electricity and heat. Therefore, they have a high potential to become important components of an energy-efficient and sustainable economy. The main challenges in the development of fuel cells are cost reduction and long-term durability. Whereas the cost can be significantly reduced by innovative mass production, the knowledge to enhance the lifetime sufficiently is not available. Surface science analysis methods used for the characterization of the new and used electrodes can be use to determine the alterations in the fuel cell components and in this way to identify the degradation processes, but they do not allow to quantify the influence of the alterations in the electrodes on the electrochemical performance. For this purpose electrochemical methods are necessary; especially the electrochemical impedance spectroscopy (EIS) allows to separate the performance losses individually and to assign them to different components and processes of the cell via a model, whereas the choice of the right model can be problematic. Two important and distinct structural degradation processes were identified by surface analysis of the electrodes before and after fuel cell operation: first, the decomposition of poly tetra-fluoro-ethylene (PTFE) which is used as an organic binder and as a hydrophobic agent in the electrodes and second, a change of the structure of the catalysts. The observed decomposition of the PTFE is associated with a decrease of the hydrophobicity of the electrode. A loss of hydrophobicity influences drastically the required operation conditions and leads to a more critical water management of the fuel cell. In contrast, the alteration of the catalysts structure in the electrodes causes an irreversible decrease of the electrochemical performance. In polymer electrolyte fuel cells (PEFCs) a particle agglomeration of the platinum catalysts at the cathodes is detected. With EIS the effect of two different degradation processes in the membrane-electrode-assembly was quantified. During continuous operation the degradation of the PTFE induces an approximately two times higher performance loss compared with the performance loss related to the agglomeration of the platinum catalyst. [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: 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: A novel measurement principle for measuring the current distribution in polymer electrolyte fuel cells (PEFCs) is introduced. It allows, in contrast to all other known techniques, for the first time for a resolution smaller than the channel/rib scale of the flow field in PEFCs. The current density is obtained by considering the electron conductors in the cell as a two-dimensional resistor with the voltage drop caused by the current. The method was applied to a cell operated on oxygen up to high current densities. The results show that the ohmic resistances govern the current distribution in the low current regime, whereas mass transport limitations hamper the current production under the land at high loads. [Copyright &y& Elsevier]

Abstract: A novel proton conducting polymer electrolyte membrane, poly(arylene ether) ionomer containing sulfofluorenyl groups (SPE-1), was subjected to long term fuel cell operation. High durability of the SPE-1 membrane was confirmed by single cell test at 80°C and at a constant current density of 0.2A/cm2. Under 90% RH operation, superior hydrolytic stability of the SPE-1 membrane was demonstrated even after 5000h with no noticeable degradations with respect to cell performances, ion exchange capacity, thickness, and molecular weight. Under 60% RH operation, some change in the molecular weight was observed after 2350h presumably due to the oxidative degradation caused by the increased cross-over of reactant gases through the membrane. [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: In order to apply a numerical analysis to design an actual scale separator in polymer electrolyte fuel cell (PEFC), it is needed to enlarge the calculation size. In this study, mass transfer and flow in gas diffusion layer were calculated, and gas diffusion layer (GDL) mass transfer approximate model based on the theoretical model was developed. Next, with this model, PEFC reaction and thermal flow analysis model, which enabled us to calculate the case of an actual scale cell, was made. Furthermore, the effects of separator channel depth on output performance and current density distribution were examined with this numerical analysis. As a result, in the case of shallow channels, the oxygen transfer rate to electrode increased because of gas flow in GDL. However, current density distribution and pressure drop increased, too. This calculation model can help us to design the optimal separator shape from the comprehensive viewpoint. [Copyright &y& Elsevier]

Abstract: A study is performed to find the optimal operating conditions of hydrogen polymer electrolyte fuel cells using an efficient optimization approach based on validated multi-resolution fuel cell simulation tool developed in house. Through the design of experiment method, a set of designed simulation runs were carried out using the fuel cell simulation tool. Based on the simulation results, an analytic metamodel was then constructed using the radial basis function approach. A feasible sequential quadratic programming scheme was then employed to optimize the metamodel to achieve the global optimal solutions. To illustrate the optimization approach, four control parameters including cell temperature, cathode stoichiometry, cathode pressure, and cathode relative humidity were considered. The optimization objective is defined as the maximization of the overall efficiency of the fuel cell system under ideal or realistic system assumptions. The study shows that different optimal solutions exist for different system assumptions, as well as different current loading levels, classified into small, medium, and large current densities. The approach adopted in this study is generic and can be readily applied to a larger number of control parameters and further to the fuel cell design optimizations. [Copyright &y& Elsevier]

Abstract: This paper presents a study of a polymer electrolyte fuel cell (PEFC) power generator based on a multi-stack association dedicated to transportation applications. First, a dynamic model of the fuel cell for high frequencies, which can be used in association with the power converter is presented. In a second hand, an original power converter architecture has been studied which authorizes the electrical association of two fuel cell stacks. Such a configuration is well adapted for the testing of fuel cell in normal or degraded mode which corresponds to real operating conditions encountered on-board a vehicle. Finally, simulation results of the complete twin-stacks power system are presented and discussed. [Copyright &y& Elsevier]

Abstract: Different types of commercial stainless steels, Ni-based alloys and nitride-coated steels were evaluated as metallic bipolar plate in terms of interfacial contact resistance (ICR) and corrosion resistance in conditions typical of PEFC anode and cathode environments. Results show that stainless steel have a high ICR and undergo corrosion in both anode and cathode. Moreover, although Ni-based alloys showed an ICR comparable with graphite, their behaviour was not satisfactory in corrosive acidic medium. Only nitride-coated stainless steel demonstrated to have low ICR and very good corrosion resistance. [Copyright &y& Elsevier]

Abstract: Voltage decreases of polymer electrolyte fuel cells (PEFC) are affected by the relative humidity of the reaction gas inside the cells. A study was conducted to establish a method for measuring relative humidity and current distribution inside PEFC cells in order to identify the factors affecting the voltage decay of such fuel cells. The humidity distribution was measured using a humidity sensor for directly monitoring the relative humidity of the reaction gas flowing through the air flow channel of the cathode separator. The current distribution was measured directly by attaching a current sensor to the rib of the cathode separator. Typical results of relative humidity and current distribution are described and interpretations are discussed. [Copyright &y& Elsevier]

Abstract: A dynamic model of a (portable) polymer electrolyte fuel cell system including the heat transfer between stack and periphery – here stack and metal hydride storage – has been developed within MATLAB-Simulink. The implemented equations describe the steady-state as well as the dynamic operation of the PEFC system with sufficient accuracy, although considerable simplifications have been made to keep model complexity and computing time low. As simulations are performed in 100-fold real time, different operating conditions and control strategies for PEFCs can be analyzed and evaluated in short-time. Here, emphasis is given to the operation limits of the PEFC system, i.e. the conditions for trouble-free operation at different loads and high or low ambient temperature are established. The model has been validated by experiments on a home-made portable system. [Copyright &y& Elsevier]

Abstract: This paper explores the behaviour of a dynamically operated fuel cell system regarding to an automotive application, examining the air supply and their interaction with the fuel cell stacks. The dynamic limits of stack operation are also discussed. Finally, the paper provides a description of the test facility used in these investigations. The research of dynamically operated fuel cell stacks shows that in order to achieve high energy efficiency, a power-dependent modulation of the pressure and flow rate of the air supply is necessary. A test facility designed for energy management and power train research (up to 42kW) was used for the experiment. A number of 11.5kW fuel cell stacks was examined experimentally with respect to performance in interaction with the air supply in stationary and dynamic operation. The stacks were tested individually and in parallel. All of them were operated “dead-end” on the hydrogen side. Experimental results varying the parameters and load curves applied to the air supply system are given in the paper. The results show different dynamic behaviours between the stacks and a substantial difference in efficiency and dynamic response of the fuel cell system operated with different strategies. The results of different operating strategies for fuel cell systems, with respect to the interaction between the fuel cell stack and the air supply and in their dependency on air mass flow and pressure level are presented. The examinations were done with the test facility using real current demand profile of the experimental car HyLite® from zero to full load and the New European Drive Cycle (NEDC). [Copyright &y& Elsevier]