PEM fuel cell

AbstractÐThe electrochemical compression of hydrogen is well known since years. But new developments of the polymer electrolyte membrane (PEM) fuel cells make it possible to realize the electrochemical compression in a PEM-cell with high eciency. A new cell design of PEM-cells for operation with a high pressure dierence between anode side and cathode side was developed. The puri®cation of hydrogen from carbon monoxide of the reformer gas can be integrated in the hydrogen compressor cell. Operation parameters are presented. #

The kinetics of oxygen reduction was investigated in acid solutions on Pt monolayers deposited on modified carbon-supported PdIr nanoparticles using the rotating diskelectrode technique. Iridium is introduced into the Pd substrate in order to fine-tune the Pt-Pd interactions and to improve Pd stability under operating conditions of the fuel cell. The kinetics of the oxygen reduction reaction shows enhancement with the Pt monolayer on the PdIr nanoparticle surfaces in comparison with the reaction on Pt/

A two-phase, one-dimensional steady model is developed to analyze the coupled phenomena of cathode flooding and mass-transport limiting for the porous cathode electrode of a proton exchange membrane fuel cell. In the model, the catalyst layer is treated not as an interface between the membrane and gas diffusion layer, but as a separate computational domain with finite thickness and pseudo-homogenous structure. Furthermore, the liquid water transport across the porous electrode is driven by the capillary force based on Darcy's law. And the gas transport is driven by the concentration gradient based on Fick's law. Additionally, through Tafel kinetics, the transport processes of gas and liquid water are coupled. From the numerical results, it is found that although the catalyst layer is thin, it is very crucial to better understand and more correctly predict the concurrent phenomena inside the electrode, particularly, the flooding phenomena. More importantly, the saturation jump at the interface of the gas diffusion layer and catalyst layers is captured, when the continuity of the capillary pressure is imposed on the interface. Elsewise, the results show further that the flooding phenomenon in the CL is much more serious than that in the GDL, which has a significant influence on the mass transport of the reactants. Moreover, the saturation level inside the cathode is determined, to a great extent, by the surface overpotential, the absolute permeability of the porous electrode, and the boundary value of saturation at the gas diffusion layer-gas channel interface. In order to prevent effectively flooding, it should remove firstly the liquid water accumulating inside the CL and keep the boundary value of liquid saturation as low as possible.

One of the current challenges for application of PEM fuel cell is to find corrosion resistant, electrically conductive, light weight, cost competitive bipolar plate material. Low temperature carburization (LTC) of stainless steels is a novel, patented process by Swagelok Company. This paper addresses the corrosion resistance characteristics of LTC SS 316 for polymer electrolyte membrane fuel cell (PEMFC) bipolar plate applications. Corrosion properties of this material were studied using potentiodynamic and potentiostatic tests in simulated (1 M H 2 SO 4 + 2 ppm HF, 0.5 M H 2 SO 4 , pH: 4.0, and 5% HCl + 5% Na 2 SO 4) PEMFC conditions. LTC SS 316 showed excellent corrosion resistance in these conditions compared to SS 316. The mechanism of anodic dissolution and general corrosion of LTC SS 316 was observed to be similar to SS 316 however the extent of LTC SS 316 corrosion was less. LTC SS 316 showed corrosion currents well below 16 A cm −2 in anodic and cathodic atmospheres under potentiostatic conditions. The potentiostatic current rapidly falls to ∼4.0 and ∼1.5 A cm −2 under anodic and cathodic conditions, respectively. LTC SS 316 was observed to form a thinner oxide layer as compared to SS 316 after 24 h of potentiostatic testing. Moreover LTC SS 316 lowered the interfacial contact resistance by approximately 24% as compared to SS 316 after corrosion testing. Hence this study clearly states the performance advantage of using LTC SS 316 as bipolar plate material as compared to conventional materials.

When interdigitated gas distributors are used in a PEM fuel cell, fluids entering the fuel cell are forced to flow through the electrodes porous layers. This characteristic increases transport rates of the reactants and products to and from the catalyst layers and reduces the amount of liquid water entrapped in the porous electrodes thereby minimizing electrode flooding. To investigate the effects of the gas and liquid water hydrodynamics on the performance of an air cathode of a PEM fuel cell employing an interdigitated gas distributor, a 2-D, two-phase, multicomponent transport model was developed. Darcy's law was used to describe the transport of the gas phase. The transport of liquid water through the porous electrode is driven by the shear force of gas flow and capillary force. An equation accounting for both forces was derived for the liquid phase transport in the porous gas electrode. Higher differential pressures between inlet and outlet channels yield higher electrode performance, because the oxygen transport rates are higher and liquid water removal is more effective. The electrode thickness needs to be optimized to get optimal performance because thinner electrode may reduce gas-flow rate and thicker electrode may increase the diffusion layer thickness. For a fixed-size electrode, more channels and shorter shoulder widths are preferred.

An investigation of the electrochemical activity of human white blood cells (WBC) for biofuel cell (BFC) applications is described. WBCs isolated from whole human blood were suspended in PBS and introduced into the anode compartment of a proton exchange membrane (PEM) fuel cell. The cathode compartment contained a 50 mM potassium ferricyanide solution. Average current densities between 0.9 and 1.6 μA cm-2 and open circuit potentials (Voc) between 83 and 102 mV were obtained, which were both higher than control values. Cyclic voltammetry was used to investigate the electrochemical activity of the activated WBCs in an attempt to elucidate the mechanism of electron transfer between the cells and electrode. Voltammograms were obtained for the WBCs, including peripheral blood mononuclear cells (PBMCs - a lymphocyte-monocyte mixture isolated on a Ficoll gradient), a B lymphoblastoid cell line (BLCL), and two leukemia cell lines, namely K562 and Jurkat. An oxidation peak at about 363 mV vs...

The notion of dominant designs deals with dominance in the market and the dominant design is thought to be dominant because of market selection forces. The notion thus ignores the possible selection that takes place in pre-market R&D stages of technological trajectories. In this paper we ask the question whether pre-market selection takes place and if this can lead to

Metal-oxide-recast Nafion composite membranes were studied for operation in hydrogen/oxygen protonexchange membrane fuel cells (PEMFC) from 80 to 130°C and at relative humidities ranging from 75 to 100%. Membranes of nominal 125 µm thickness were prepared by suspending a variety of metal oxide particles (SiO 2 , TiO 2 , Al 2 O 3 , and ZrO 2 ) in solubilized Nafion. The composite membranes were characterized using electrochemical, X-ray scattering, spectroscopic, mechanical, and thermal analysis techniques. Membrane characteristics were compared to fuel cell performance. These studies indicated a specific chemical interaction between polymer sulfonate groups and the metal oxide surface for systems that provide a good elevated-temperature (i.e., fuel-cell operation above 120°C) performance. Composite systems that incorporate either a TiO 2 or a SiO 2 phase produced superior elevated-temperature, lowhumidity behavior compared to that of a simple Nafion-based fuel cell. Improved temperature tolerance permits the introduction of at least 500 ppm CO contaminant in the H 2 fuel stream without cell failure, in contrast to standard Nafion-based cells, which fail below 50 ppm of carbon monoxide.

A 3D numerical study was carried out to analyze flow, heat and mass transfer first in a single half-cell cathode channel of proton exchange membrane (PEM) fuel cell. From practical point of view, it is necessary to put the appropriate number of cells in a stack. Hence, the above study on a single half-cell is extended to a stack of channels. Due to stacking, the assumption of uniform flow distribution would no longer hold true. Therefore, the channel flow-maldistribution is considered. The water formed at the active surface due to the electrochemical reaction diffuses through the porous layer and eventually enters the gas flow duct. The higher gas velocities in the duct result in faster water vapour removal which leads to a lower value of water vapour into the duct and hence a lower Nusselt number.

A novel fabrication technique for micro proton exchange membrane fuel cells (PEMFCs) based on carbon-MEMS (C-MEMS) was optimized to yield higher performance cells. Polymer manufacturing is relatively easy compared to directly patterning graphite as is typically done to make fuel cell bipolar plates. In a C-MEMS approach, fuel cell bipolar plates are fabricated by first patterning polymer Cirlex ® sheets. By subsequently pyrolyzing the machined polymer sheets at high temperature in an inert atmosphere, carbon bipolar plates with intricate groove structures to distribute the reactants are obtained. Using an improved assembly technique such as polishing the carbonized plates to minimize the contact resistance between gas diffusion layers (GDL) and bipolar plates, better pyrolysis temperature control and a better end plate design, a PEMFC with a 0.64 cm 2 active surface was fabricated using the newly developed bipolar plates. At 1 atm and 25 • C a maximum power density of ∼76 mW cm −2 was obtained, and at 2 atm and 25 • C ∼85 mW cm −2 was achieved. These data are comparable with data reported in the literature for PEMFCs and are a dramatic improvement over earlier results reported for the same C-MEMS based fuel cell. Electrochemical Impedance Spectroscopy (EIS) and cyclic voltammetry were carried out to characterize steady-state and transient characteristics of the novel C-MEMS fuel cell. (Y. Wang). 1 An exchange student from Brazil working at RERL of the UC Irvine. of electrically connecting the adjacent cells while at the same time distributing the reactant gases over the anode and cathode. Carbonbased substrates such as carbon paper and cloth constitute a typical GDL. The GDLs are placed on the membrane electrode assembly (MEA) and perform several functions: gas species transport, heat transport and last but not least the electrical connection between the bipolar plates and the catalyst layers on the proton-conductive membrane. The catalyst layers comprise the C/Pt or Pt alloy catalyst itself, an ionomer, and void space. The electrochemical reactions take place at the triple-phase boundary (TPB) of the catalyst layer, the gas phase and the ionically conductive membrane. The membrane, typically made of Nafion ® , fulfills a dual role, i.e. as the gas/electron separator and for conducting protons. The Nafion ® membrane is characterized by hydrophobic, fluorinated main chains with hydrophilic sulfonic acid pendant groups, allowing for protons to weakly interact with SO 3 − groups and to travel through the hydrated regions associated with those sulfonic groups. Different from the traditional power generation in heat engines, fuel cells are electrochemical devices and their efficiency is not limited by the Carnot heat cycle. Efficiencies of more than 50% have been achieved in many commercial units operating at low currents. Fuel cell efficiency is especially high at smaller scales and in this regard, fuel cells may be ideal for powering portable/micro electrical devices and may have the potential to replace batteries whose 0378-7753/$ -see front matter

Water removal from the gas diffusion layer (GDL) is crucial for the efficient operation of proton exchange membrane (PEM) fuel cell. Static pressure gradient caused by the fast reactant flow in the flow channel is one of the main mechanisms of water removal from GDL. Reactant can leak or cross directly to the neighboring channel via the porous GDL in the cells with serpentine flow channel and many of its modifications. Such cross flow plays an important role for the removal of liquid water accumulated in the GDL especially under land area. To investigate the characteristics of liquid water behavior in the GDL under pressure gradient, the fibrous porous structure of the carbon paper is modeled by three dimensional impermeable cylinders randomly distributed in the in-plane directions and unsteady two-phase simulations are conducted. It is shown that the permeability from the numerical model matches well the experimental measurements of the common GDLs in the literature. The contact angle and pressure gradient are the key parameters that determine the initiation and the process of liquid water transport in the GDL which is initially wet with stagnant liquid water. It has been observed that the larger contact angle results in faster water removal from the GDL. Numerical simulations are performed for a wide range of pressure gradient with different contact angles to determine the minimum pressure gradient that initiates the liquid water transport in the GDL. It is found that the amount of pressure gradient caused by the cross flow is sufficient and effective to get rid of the liquid water accumulated in the GDL. The simulation results are also compared with experimental data in literature showing a good agreement. The characteristics of liquid water discharging from the gas diffusion layer are also described.

The Bruggeman approximation has widely been used for estimating the effective conductivity and diffusivity of both the catalyst and gas diffusion layers of polymer electrolyte membrane (PEM) fuel cells. This approximation is based on the Bruggeman's Effective Medium Theory [Bruggeman D. Berechnung verschiedener physikalischer konstanten von heterogenen substanzen. Ann Phys (Leipzig) 1935;24:636-79], which provides empirical correlation for the effective properties of a composite system. Since it is an empirical correlation, a unique correlation based on the Bruggeman approximation does not always hold for the PEM fuel cell effective properties. Rather, the Bruggeman correlation is a cell specific and experiment dependent correlation that depends on structure, phase composition, water saturation, experimental parameters, etc. Further, this correlation needs to be combined with other correlations to estimate the effective diffusivities. In this article, a set of mathematical formulations has been proposed for the effective transport properties in both the catalyst and gas diffusion layers of a PEM fuel cell. The effective conductivity and diffusivity expressions are derived from the mathematical formulations of the Hashin Coated Sphere model [Hashin Z. The elastic moduli of heterogeneous materials. J Appl Mech 1962;29:143-50], which provides an identical mathematical foundation for each of these effective properties rather than an empirical correlation and avoid to use of multiple correlations together. The present model formulations agree well with the results available in literature for the limiting case. Hence, the proposed formulations for the effective transport properties will be a useful estimating tool in the numerical modeling of PEM fuel cells.

A numerical study of channel-to-channel flow cross-over through the gas diffusion layer in a PEM-fuel-cell flow system using a serpentine channel with a trapezoidal cross-sectional shape ✩ Abstract A numerical study of pressure distribution and flow cross-over through the gas diffusion layer (GDL) in a PEMFC flow plate using a serpentine channel system has been undertaken for the case where the channel has a trapezoidal cross-sectional shape. The flow has been assumed to be 3-D, steady, incompressible and single-phase. The flow through the porous diffusion layer has been described using the Darcy model. The governing equations have been written in dimensionless form and solved by using the commercial CFD solver, FIDAP. The results obtained indicate that:

This paper presents a dynamic model of a fuel cell system for residential power generation. The models proposed include a fuel cell stack model, reformer model and DC/AC inverter model. Furthermore a fuzzy logic (FLC) controller is used to control the active power of PEM fuel cell system. The controller modifies the hydrogen flow feedback from the terminal load. Simulation results confirmed the high performance capability of the fuzzy logic controller to control power generation. K e y w o r d s: polymer-electrolyte fuel cell, dynamic model, residential power, fuzzy controller

An analytical method has been developed to differentiate the electrical and thermal resistance of the PEM fuel cell assembly in the fuel cell operating conditions. The usefulness of this method lies in the determination of the electrical resistance based on the polarization curve and the thermal resistance from the mass balance. This method also paves way for the evaluation of cogeneration from a PEMFC power plant. Based on this approach, the increase in current and resistance due to unit change in temperature at a particular current density has been evaluated. It was observed that the internal resistance of the cell is dependent on the electrode fabrication process, which also play a major role in the thermal management of the fuel cell stack.

This paper presents the results of an optimization study using a comprehensive three-dimensional, multi-phase, non-isothermal model of a PEM fuel cell that incorporates the significant physical processes and the key parameters affecting fuel cell performance. The model accounts for both the gas and liquid phase in the same computational domain and, thus, allows for the implementation of phase change inside the gas diffusion layers. The model includes the transport of gaseous species, liquid water, protons, energy and water dissolved in the ion conducting polymer. Water is assumed to be exchanged among three phases; liquid, vapor and dissolved, and equilibrium among these phases is assumed. The model features an algorithm that allows a more realistic representation of the local activation overpotentials, which leads to improved prediction of the local current density distribution. This model also takes into account convection and diffusion of different species in the channels as well as in the porous gas diffusion layer, heat transfer in the solids as well as in the gases and electrochemical reactions. The results showed that the present multi-phase model is capable of identifying important parameters for the wetting behavior of the gas diffusion layers and can be used to identify conditions that might lead to the onset of pore plugging, which has a detrimental effect on the fuel cell performance. This model is used to study the effects of several operating, design and material parameters on fuel cell performance. Detailed analyses of the fuel cell performance under various operating conditions have been conducted and examined.

Porous conducting carbon paper has been identified as the most suitable material to be used as a backing material for the fuel cell electrode. The surface of carbon fiber, the major constituent of the carbon paper was modified by: (1) removing the functional groups by heat cleaning process and (2) coating the non-functionalized carbon fiber with multi-walled carbon nanotubes (MWCNTs). This has a marked influence on the fiber-matrix interactions during later stages of processing of carbon paper that helped in controlling its various characteristic properties. Using the carbon paper formed with CNT coated carbon fiber as electrode, the maximum power density achieved from a unit fuel cell was found to be 783 mW/cm 2 as compared to 630 mW/cm 2 when the paper was formed with normal fiber.

Modelling and simulation of PEM fuel cell stack operation is developed in Simulink Ò environment and validated through experimental data. The present work is the starting point for the development of a user friendly and versatile tool aimed at controlling and optimizing the operation of a PEMFC stack; in addition, it could be of help in stack and BOP components design, for instance feeding and humidification systems, cooling circuit, temperature control logic and electrical interface. The constitutive equations used to model the FC stack operation are the fundamental equations of electrochemistry. First, the model is used to describe the behaviour of a single cell under steady-state conditions upon varying variables such as temperature, pressure and relative humidity of reactants; then, it is applied to simulate the operation of a stack configuration, including also fluid-dynamics aspects, thermal and kinetic behaviour of feed systems. In particular, thermal control modelling is based on a simplified approach where different heat removal mechanisms are accounted for in a separate way. In its present state, the simulation tool so developed allows a feasible investigation of some process variables influence on the FC stack performances. The stack modelling is tested against benchmark results obtained from a 300W 20-cell air-cooled stack under variable operative conditions. MEAs based on Nafion 112 and Carbon cloth GDLs developed ad hoc are assembled into each cell of the stack. Although the model is quite simple, these preliminary results point out that it may be an adequate tool to set design targets and support further steps of optimization.

This paper reviews over 120 papers regarding the effect of heat treatment on the catalytic activity and stability of proton exchange membrane (PEM) fuel cell catalysts. These catalysts include primarily unsupported and carbon-supported platinum (Pt), Pt alloys, non-Pt alloys, and transition metal macrocycles. The heat treatment can induce changes in catalyst properties such as particle size, morphology, dispersion of the metal on the support, alloying degree, active site formation, catalytic activity, and catalytic stability. The optimum heat-treatment temperature and time period are strongly dependent on the individual catalyst. With respect to Pt-based catalysts, heat treatment can induce particle-size growth, better alloying degree, and changes in the catalyst surface morphology from amorphous to more ordered states, all of which have a remarkable effect on oxygen reduction reaction (ORR) activity and stability. However, heat treatment of the catalyst carbon supports can also significantly affect the ORR catalytic activity of the supported catalyst. Regarding non-noble catalysts, in particular transition metal macrocycles, heat treatment is also important in ORR activity and stability improvement. In fact, heat treatment is a necessary step for introducing more active catalytic sites. For metal chalcogenide catalysts, it seems that heat treatment may not be necessary for catalytic activity and stability improvement. More research is necessary to improve our fundamental understanding and to develop a new strategy that includes innovative heat-treatment processes for enhancing fuel cell catalyst activity and stability.

In this paper, the performance and durability of hybrid PEM fuel cell vehicles are investigated. To that end, a hybrid predictive controller is proposed to improve battery performance and to avoid fuel cell and battery degradation. Such controller deals with this complex control problem by handling binary and continuous variables, piecewise affine models and constraints. Moreover, the control strategy is to track motor power demand and keep batteries close to a desired battery state of charge which is appropriately chosen to minimize hydrogen consumption. It is important to highlight the consideration of constraints which are directly related to the goals of this paper, such as minimum fuel cell power threshold and time limitation between fuel cell startups and shutdowns. Furthermore, different models have been elaborated and particularized for a vehicle prototype. These models include few innovations such as a reference governor which smooths fuel cell power demand during sharp power profiles, forcing batteries to supply such peaks and resulting a longer fuel cell lifetime. Battery thermal dynamics are also taken into account in these models in order to analyze the effect of battery temperature on its degradation. Finally, this paper studies the feasibility of the real implementation, presenting an explicit formulation as a solution to reduce execution time. This explicit controller exhibits the same performance as the hybrid predictive controller does with a reduced computational effort. All the results have been validated in several simulations.

This paper presents a dynamic nonlinear model of Polymer Electrolyte Membrane Fuel Cells (PEMFC) for the purpose of constructing a nonlinear control for PEMFC by using exact linearization. Since it has been known that large deviation between hydrogen and oxygen partial pressures can cause severe membrane damages, exact linearization is applied in this paper to achieve the control objective for PEMFC such that the deviation between hydrogen and oxygen partial pressures can be kept as small as possible during disturbances or load variations. PEMFC is modeled as a nonlinear multiple-input multiple-output (MIMO) system so that the exact linearization can be directly utilized. During the control design, hydrogen and oxygen inlet flow rates are defined as the control input variables, and pressures of hydrogen and oxygen are also appropriately defined as state variables. The design of the control scheme is described in details. Simulation results show that PEMFC equipped with the proposed nonlinear control has better transient performances than linear control.

The achievement of a proper and uniform pressure distribution between the membrane electrode assembly (MEA) and the bipolar plates of a proton exchange membrane fuel cell (PEMFC) is a key factor of stack design and assembly. Contact pressure levels are usually controlled by selecting an appropriate external clamping pressure on the endplates. Very few studies have been focused on the measurement of the contact pressure distribution within the fuel cell and its correlation with the applied external clamping torque. This study explores the possibility of using matrix-based piezoresistive thin-film sensors, to be placed between the MEA and the monopolar plate of a PEMFC, to investigate this correlation. Before embedding the sensor array into the fuel cell, it was validated for accuracy and repeatability by designing a pneumatic calibration device which allows to apply uniform static pressure levels over the whole sensor area. Preliminary results reported in this study showed that, as the clamping torque on the endplates is increased, the average pressure on the MEA remains almost constant but its distribution changes. The core area of the electrode becomes progressively more unloaded while average stresses on the gasket rise up, with significant stress concentration around the edge corners.

A fuel cell clearly is a multidisciplinary system. The authors have chosen an original energy approach that they have applied via the Bond Graph formalism. A PEM fuel cell Bond Graph model is proposed and detailed. Its potentialities are then illustrated by studies on the fuel cell dynamic behavior (current harmonic effects, etc.) and system simulations (electricity generating unit based on a PEM fuel cell, etc.).

A dynamic model of a discrete reversible fuel cell (RFC) system has been developed in a Matlab Simulink ® environment. The model incorporates first principles dynamic component models of a proton exchange membrane (PEM) fuel cell, a PEM electrolyzer, a metal hydride hydrogen storage tank, and a cooling system radiator, as well as empirical models of balance of plant components. Dynamic simulations show unique charging and discharging control issues and highlight factors contributing to overall system efficiency.

The results of a numerical simulation of the current distribution of a three-dimensional, tubular shaped, proton exchange membrane fuel cell model are presented. An integrated flow and current density model to predict current density distributions in two dimensions along the membrane has been developed. The numerical model has a cylindrical geometry that includes diffusion layers on the anode and cathode side, the anode being the inner most electrode, and solves the same primary flow related variables along the channels and the diffusion layers. The simulation was performed with commercial flow solver software where a control volume approach was used and source term equations that characterize the electrochemical aspects of the fuel cell have been added.

This paper presents an experimental assessment of fuel cell hybrid propulsion systems for scooters based on a modular 1.2 kW PEM fuel cell. The tests of the hybrid system are carried out using a programmable electronic load. Different configurations of the fuel cell/battery and the fuel cell/supercapacitor hybrid systems are explored. Both systems demonstrate their ability to deliver the requested load satisfactorily. The distributions of the fuel cell power delivery, although different between the two systems, are within the region where the fuel cell efficiency is approximately constant. As a result, the rates of fuel consumption show no discernable difference between the two systems for all three driving cycles considered. In addition to the fuel consumption, considerations including bus voltage, cost and packaging issues suggest that the supercapacitor has advantages over the battery for the use as secondary energy storage in fuel cell hybrid propulsion system for scooters.

The available power generated from a fuel cell (FC) power plant may not be sufficient to meet sustained load demands, especially during peak demand or transient events encountered in stationary power plant applications. An ultracapacitor (UC) bank can supply a large burst of power, but it cannot store a significant amount of energy. The combined use of FC and UC has the potential for better energy efficiency, reducing the cost of FC technology, and improved fuel usage. In this paper, we present an FC that operates in parallel with a UC bank. A new dynamic model and design methodology for an FC-and UCbased energy source for stand-alone residential applications has been developed. Simulation results are presented using MATLAB, Simulink, and SimPowerSystems environments based on the mathematical and dynamic electrical models developed for the proposed system.

A thorough analysis of a polymer electrolyte membrane (PEM) fuel cell system is presented. A generalized performance model of a single PEM fuel cell is developed and applied in a semi-empirical form for a Ballard Mark V 35-cell, 5 kW PEM fuel cell. A thermodynamic and economic analysis of the components and of the whole system is performed. The purpose of this study is to explore the intrinsic relations among various fuel cell system performance and cost indicators in order to provide insights for new cost effective and high performance designs. Optimization techniques have been applied in order to determine the optimal design and operation mode of the system. An application of the system onboard merchant ships has been considered as an example.

A simple method is described to synthesize carbon nanotubes (CNTs) by the thermal decomposition of toluene at 750°C over a thin catalyst film deposited on Al powder. This method allows the bulk metal surface to act as both the catalyst and support for CNT growth. The catalyst film on Al was prepared from an ethanol solution of iron nitrate. Under the growth conditions, iron nitrate formed an amorphous iron oxide layer that transform into crystalline Fe 2 O 3 , which was further reduced to Fe 3 O 4 and Fe 3 C. It is believed that the growth of CNTs took place on iron carbide nanoparticles that were formed from FeO. The characterization of CNTs was mainly carried out by powder X-ray diffraction and scanning electron microscopy, X-ray fluorescence and thermogravimatric analysis. The CNTs were found to be highly dispersed in Al powder. This composite powder could be further used for the fabrication of Al matrix composites using powder metallurgy process in which the powder were first cold pressed at 500-550 MPa followed by sintering at 620°C for 2 h under a vacuum of 10 -2 torr. The mechanical properties of the sintered composites were measured using a microhardness tester and a Universal testing Instron machine.

In designing and controlling fuel cell systems, it is advantageous to have models which predict fuel cell behaviour in steady-state as well as in dynamic operation. This work examines the use of electro-chemical impedance spectroscopy (EIS) for characterising and developing an impedance model for a high temperature PEM (HT-PEM) fuel cell stack. A Labview virtual instrument has been developed to perform the signal generation and data acquisition which is needed to perform EIS. The typical output of an EIS measurement on a fuel cell is a Nyquist plot, which shows the imaginary and real parts of the impedance of the measured system. The full stack impedance depends on the impedance of each of the single cells of the stack. Equivalent circuit models for each single cell can be used to predict the stack impedance at different temperature profiles of the stack. The information available in such models can be used to predict the fuel cell stack performance, e.g. in systems where different electronic components introduce current harmonics.

We have analysed membrane electrode assemblies (MEAs) involving fabricated and commercially available electrodes using a scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS) and developed simple mathematical models to simulate the best performance and design conditions. The analysis showed that a MEA surface with the catalyst layer consisting of 10 wt% Pt/C and 30 wt% Teflon â (PTFE, designated E2) loaded with 0.38 mg Pt/cm 2 showed good localisation of the platinum particles. The SEM image of the E2 electrode showed the existence of a diffusion layer, while the crosssection of electrode E3 (without diffusion layer) showed only the backing layer of the carbon cloth. It was seen that good adhesion of the catalyst on the membrane was obtained as a result of the hot press used in fabrication. XPS analysis showed that the electrode surfaces consisted of C, O, F, Si and Pt, whose binding energies for the PTFE/C layer were C 1s, O 1s, F 1s and Si 2p states and were 285.0, 532.7, 689.5 and 103.0 eV, respectively. While for the catalyst layer, the binding energies for the elements, C 1s, O 1s, F 1s, Si 2p and Pt 4f states, were 284.3, 532.4, 689.3, 102.9 and 74.1 eV, respectively. Similar observations were made for a commercial E-TEK electrode. The mathematical and simulation investigations supported the hypothesis made in an earlier study in terms of optimum PEM fuel cell performance determination and design simulation. The calculated values of the voltage operational limit V opl cal: agreed quite well with the experimental data V opl exp: reported earlier. Other works from the open literature were also correlated using the mathematical model, and it was found that the V opl values were comparable. Hydrogen usage thus calculated was best with the E2 electrode compared to E1, E3 and the commercially available E-TEK electrode.

Fuel cell (FC) technologies are expected to become a viable solution for vehicular applications because they use alternative fuel converters and are environmentally friendly. However, a stand alone FC system may not be sufficient to satisfy the load demands, especially during cold start, peak demand periods or transient events, for vehicular applications. In addition, the FC system is not capable of being reversed for regenerative energy. An ultra-capacitor (UC) bank can supply a large burst of power but cannot store much energy. By operating the FC and UC in parallel, both steady state and peak power demands can be satisfied. Use of a FC/UC hybrid model provides a potential solution for better energy efficiency while reducing the cost of FC power technology. This paper describes a new modeling and design methodology for FC/UC hybrid vehicular power systems. A feasible design and a dynamic model have been presented for the proposed technique. Simulation results are presented, using the MATLAB Ò , Simulink Ò and SimPowerSystems Ò environments, based on the mathematical and electrical models of the proposed system.

The compression of gases in conventional compressors is often combined with low ef®ciency and the contamination of the compressed gases. In this paper, a system is described that uses an electrochemical cell for the compression of hydrogen. This electrochemical hydrogen compressor operates at lower hydrogen¯ux than mechanical hydrogen compressors. Further development of polymer electrolyte membrane (PEM) fuel cells afford a good prospect of realizing the electrochemical compression on a competitive basis. A specialized gas diffusion layer established the possibility to pressurize the cathode volume up to 54 bar. The ohmic resistance and pressure stability have been rescued by an improved membrane electrode assembly (MEA). A three cell stack on a laboratory scale has been designed. Data of single cell and stack experiments will be discussed in this paper. The advantages of the electrochemical system, apart from the ef®ciency, are: noiseless operation, puri®ed hydrogen and simplicity of the system cooling. # 2002 Published by Elsevier Science B.V.

Today's increasingly competitive industrial environment demands shorter product development lead-times, lower costs, and higher quality products. These requirements produce more complex design problems that are characterized by multiple design objectives as well as complex design objective and constraint relations. The optimization of these computationally intensive design problems leads to new technical challenges. This work applies a new global optimization search scheme, the Adaptive Response Surface Method (ARSM), to the optimal design of a complex mechanical system --the radiator stack PEM fuel cell system. The design optimum manifests a significant increase in system performance and decrease in cost. The proposed method can also be applied to the solution of other complex design optimization problems.

In recent years, hybrid photovoltaicefuel cell energy systems have been popular as energy production systems for different applications. A typical solar-hydrogen system can be modeled the electricity supplied by PV panels is used to meet the demand directly to the maximum extent possible. If there is any surplus PV power over demand, and capacity left in the tank for accommodating additional hydrogen, this surplus power is supplied to the electrolyser to produce hydrogen for storage. When the output of the PV array is not sufficient to supply the demand, the fuel cell draws on hydrogen from storage and produces electricity to meet the supply deficit.

The water balance inside a fuel cell was analysed and several equations were introduced as functions of fuel cell gas-stream inlet and outlet pressures, inlet relative humidities (RHs), temperature, pressure drops across flow channels, and reactant partial pressures. The effect of RH on PEM fuel cell performance was studied at elevated temperatures under ambient backpressure using Nafion ® -based MEAs. The results showed that fuel cell performance could be depressed significantly by decreasing RH from 100 to 25%. AC impedance and cyclic voltammetry techniques were employed to diagnose the RH effect on fuel cell reaction kinetics. Reducing RH can result in slower electrode kinetics, including electrode reaction and mass diffusion rates, and higher membrane resistance.