Showing papers in "Fuel Cells in 2004"

PBI‐Based Polymer Membranes for High Temperature Fuel Cells – Preparation, Characterization and Fuel Cell Demonstration

Abstract: Proton exchange membrane fuel cell (PEMFC) technology based on perfluorosulfonic acid (PFSA) polymer membranes is briefly reviewed. The newest development in alternative polymer electrolytes for operation above 100 °C is summarized and discussed. As one of the successful approaches to high operational temperatures, the development and evaluation of acid doped polybenzimidazole (PBI) membranes are reviewed, covering polymer synthesis, membrane casting, acid doping, physicochemical characterization and fuel cell testing. A high temperature PEMFC system, operational at up to 200 °C based on phosphoric acid-doped PBI membranes, is demonstrated. It requires little or no gas humidification and has a CO tolerance of up to several percent. The direct use of reformed hydrogen from a simple methanol reformer, without the need for any further CO removal, has been demonstrated. A lifetime of continuous operation, for over 5000 h at 150 °C, and shutdown-restart thermal cycle testing for 47 cycles has been achieved. Other issues such as cooling, heat recovery, possible integration with fuel processing units, associated problems and further development are discussed.

Direct Formic Acid Fuel Cells with 600 mA cm–2 at 0.4 V and 22 °C

Abstract: A demonstration of direct formic acid fuel cells (DFAFCs) generating very high power density at ambient temperature is reported. In particular, the performance of the Pd black as an anode catalyst for DFAFCs with different formic acid feed concentrations at different operating temperatures has been evaluated. The Pd black based DFAFCs with dry air and zero backpressure can generate a maximum power density of 248 and 271 mW cm–2 at 22 °C and 30 °C respectively. The open cell potential is 0.90 V. These results show that DFAFCs are potentially excellent alternative power sources for small portable electronic devices.

Effects of Deep Temperature Cycling on Nafion® 112 Membranes and Membrane Electrode Assemblies

Abstract: A study was conducted to understand the physical and chemical changes in fuel cell membranes that result from Freeze/Thaw (F/T) cycling which might occur in electric vehicles. Nafion™ membranes and membrane electrode assemblies (MEA) were subjected to 385 temperature cycles between +80 °C and –40 °C over a period of three months to examine the effects on key properties. These studies were done on both compressed and uncompressed materials in the un-humidified state. Although no catastrophic failures were seen, the analytical results shed some light on the relationship of temperature cycling to membrane structure, water management, ionic conductivity, gas permeability and mechanical strength. Changes in water swelling behavior and dry densities were noted and the effect on ionic conductivity and cell performance was examined. The impact on catalyst activity and structural integrity of MEAs was evaluated electrochemically.

Structural and Physical Properties of GDL and GDL/BPP Combinations and their Influence on PEMFC Performance

Abstract: The change in the structural and physical properties of the components assembled in a fuel cell stack, when being compressed, is important for performance evaluation. The physical properties of Gas Diffusion Layer (GDL) materials, such as thickness, through plane resistivity and gas permeability and pore size data are presented as a function of compressive force. The data obtained are correlated with fuel cell performance data. Beyond the materials and components specific properties the behaviour of combinations of BPP and GDL materials, which are manufactured by SGL Technologies GmbH, are evaluated and presented. Through plane resistance of a GDL-BPP-GDL sandwich is evaluated for varied compression forces and materials permutations.

A Review of Mathematical Models for Hydrogen and Direct Methanol Polymer Electrolyte Membrane Fuel Cells

Abstract: This paper presents a review of the mathematical modeling of two types of polymer electrolyte membrane fuel cells: hydrogen fuel cells and direct methanol fuel cells. Models of single cells are described as well as models of entire fuel cell stacks. Methods for obtaining model parameters are briefly summarized, as well as the numerical techniques used to solve the model equations. Effective models have been developed to describe the fundamental electrochemical and transport phenomena occurring in the diffusion layers, catalyst layers, and membrane. More research is required to develop models that are validated using experimental data, and models that can account for complex two-phase flows of liquids and gases.

Novel Polymer Electrolyte Membranes for Automotive Applications – Requirements and Benefits

Abstract: During the past few years, the feasibility of using polymer electrolyte fuel cells in automotive power trains at an impressive performance level has been proven repeatedly. However, current fuel cell stacks are still largely based on decade-old polymer electrolyte membrane technology thus limiting performance, durability, reliability, and cost of the fuel cell systems. The major challenge for membrane R&D constitutes the demand for polymer electrolytes that allow for system operation at higher temperatures and lower water management requirements without increased conduction losses. None the less, demanding automotive requirements will not compromise on other properties such as mechanical and chemical stability and gas permeability.

Performance and Durability of Membrane Electrode Assemblies Based on Radiation‐Grafted FEP‐g‐Polystyrene Membranes

Abstract: Membrane electrode assemblies (MEAs) based on radiation-grafted proton exchange membranes developed at PSI have shown encouraging performance in the past in hydrogen and methanol fuelled polymer electrolyte fuel cells. In this study, the effect of the pre-treatment of crosslinked radiation-grafted FEP membranes prior to lamination with the electrodes on the performance of the MEAs was investigated. Two approaches were assessed separately and in combination: (1) the impregnation of the radiation-grafted membranes with solubilised Nafion®, and (2) the use of a swollen vs. dry membrane. It is found that the combination of coating the membrane with Nafion® ionomer and hot-pressing the MEA with the membrane in the wet state produce the best single cell performance. In the second part of the study, the durability of an MEA, based on a radiation-grafted FEP membrane, was investigated. The performance was stable for 4,000 h at a cell temperature of 80 °C. Then, a notable degradation of the membrane, as well as the electrode material, started to occur as a consequence of either controlled or uncontrolled start-stop cycles of the cell. It is assumed that particular conditions, to which the cell is subjected during such an event, strongly accelerate materials degradation, which leads to the premature failure of the MEA.

Cross‐Linked Polyaryl Blend Membranes for Polymer Electrolyte Fuel Cells

Abstract: The preparation and characterization of different types of proton-conducting polymer blend membranes are presented in this paper. The investigations are focused on the determination of the thermal stability of the membranes, because thermal stability is one of the important parameters for the application of the membranes in polymer electrolyte fuel cells. In addition to the thermal stability, chemical stability, proton conductivity and mechanical strength are required. The characterization of the membranes was performed by thermogravimetry (TGA), a combined TGA and FTIR analysis where infrared spectroscopy is used for the determination of decomposition products, and experiments on the dependence of water uptake (swelling) of the membranes on temperature. Ionically cross-linked membranes have been investigated. The TGA-FTIR coupling experiments showed clearly, that the decomposition of the membranes starts at ≈ 230 °C, and that in some cases the principal membrane component which is sulfonated polyaryletherketone splits off SO2 at slightly lower temperature in the membrane than in the pure substance. Most of the other polymeric components of the blend membranes were slightly more stable in the membrane than in the pure form. The ionically cross-linked membranes were tested in direct methanol fuel cells as well as in a H2/air fuel cell and exhibit a performance which compares favorably with standard membranes.

How good are the Electrodes we use in PEFC

Abstract: Basically, companies and laboratories implement production methods for their electrodes on the basis of experience, technical capabilities and commercial preferences. But how does one know whether they have ended up with the best possible electrode for the components used? What should be the (i) optimal thickness of the catalyst layer? (ii) relative amounts of electronically conducting component (catalyst, with support – if used), electrolyte and pores? (iii) “particle size distributions” in these mesophases? We may be pleased with our MEAs, but could we make them better? The details of excellently working MEA structures are typically not a subject of open discussion, also hardly anyone in the fuel cell business would like to admit that their electrodes could have been made much better. Therefore, we only rarely find (far from systematic) experimental reports on this most important issue. The message of this paper is to illustrate how strongly the MEA morphology could affect the performance and to pave the way for the development of the theory. Full analysis should address the performance at different current densities, which is possible and is partially shown in this paper, but vital trends can be demonstrated on the linear polarization resistance, the signature of electrode performance. The latter is expressed through the minimum number of key parameters characterizing the processes taking place in the MEA. Model expressions of the percolation theory can then be used to approximate the dependence on these parameters. The effects revealed are dramatic. Of course, the corresponding curves will not be reproduced literally in experiments, since these illustrations use crude expressions inspired by the theory of percolation on a regular lattice, whereas the actual mesoscopic architecture of MEA is much more complicated. However, they give us a flavour of reserves that might be released by smart MEA design.

Ethanol and Acetaldehyde Adsorption on a Carbon-Supported Pt Catalyst: A Comparative DEMS Study

Abstract: The adsorption of ethanol and acetaldehyde on carbon Vulcan supported Pt fuel cell catalyst and the electrochemical desorption of the adsorption products were studied, using electrochemical measurements and differential electrochemical mass spectrosmetry (DEMS), under continuous flow conditions. Faradaic current adsorption transients at different constant adsorption potentials, which also include CO adsorption for comparison, show pronounced effects of the adsorption potential and the nature of the reactant molecule. Acetaldehyde adsorption is much faster than ethanol adsorption at all potentials. Pronounced Had induced blocking effects for ethanol adsorption are observed at very cathodic adsorption potentials, 0.31 V; for acetaldehyde adsorption, increasing bulk reduction is found at low potentials. Based on the electron yield per CO2 molecule formed and on the similarity with the CO stripping characteristics the dominant stable adsorbate is CO, coadsorbed with smaller amounts of (partly oxidized) hydrocarbon decomposition fragments, which are also oxidized at higher potentials (> 0.85 V) and which can be reductively desorbed as methane or, to a very small extent, as ethane. The presence of small amounts of adsorbed C2 species and the oxidative dissociation of these species in the main CO oxidation potential range is clearly demonstrated by increased methane desorption after a potential shift to 0.85 V. The data demonstrate that the Pt/Vulcan catalyst is very reactive for C-C bond breaking upon adsorption of these reactants.

A Two‐Phase Non‐Isothermal PEFC Model: Theory and Validation

Abstract: A two-dimensional, non-isothermal, two-phase model of a polymer electrolyte fuel cell (PEFC) is presented. The model is developed for conditions where variations in the streamwise direction are negligible. In addition, experiments were conducted with a segmented cell comprised of net flow fields. The, experimentally obtained, current distributions were used to validate the PEFC model developed. The PEFC model includes species transport and the phase change of water, coupled with conservation of momentum and mass, in the porous backing of the cathode, and conservation of charge and heat throughout the fuel cell. The current density in the active layer at the cathode is modelled with an agglomerate model, and the contact resistance for heat transfer over the material boundaries is taken into account. Good agreement was obtained between the modelled and experimental polarization curves. A temperature difference of 6 °C between the bipolar plate and active layer on the cathode, and a liquid saturation of 6% at the active layer in the cathode were observed at 1 A cm–2.

PEFC Stack Operating in Anodic Dead End Mode

Abstract: The polymer electrolyte fuel cell (PEFC) system is considered as an alternative power source and is particularly promising for mobile applications. This paper reports some experimental results performed on a PEFC stack, operating in anodic dead end mode. The flush frequency of the anode is investigated. In order to reduce hydrogen consumption in an embedded application, the flush frequency could be calculated from system modelling, according to the operating conditions. The first step of such a model is presented. It simulates the water exchanges between the anode and cathode channels in the anode dead end case. It has been implemented in a MATLAB® environment.

CO Desorption Rate Dependence on CO Partial Pressure over Platinum Fuel Cell Catalysts

Abstract: Carbon monoxide adsorption on high area platinum fuel cell catalysts was investigated. Isotopic exchange experiments were performed to determine the exchange rate (k) of CO under different partial pressures of CO (p co) in argon. A linear dependence of In(k) with In(pco) was observed. This pressure dependence of the rate of exchange is explained by considering a change in surface coverage of CO with different CO pressures and a subsequent reduction in the CO binding energy as demonstrated by Density Functional Theory (DFT) calculations. High Pressure Scanning Tunneling Microscopy (HP STM) studies on the Pt(111) surface have also displayed a pressure dependency of the coverage consistent with this data. The relevance of these observations to the Polymer Electrolyte Membrane Fuel Cell (PEMFC) anode reaction is discussed. (Less)

Alkaline Fuel Cells

Abstract: Publisher Summary Alkaline Fuel Cells (AFCs) are easy to handle, have very high electrical efficiency and are very suitable for dynamic operating modes. They can be built into small compact systems as well as in large power plants. However, many groups in Europe have stopped working on this technology. As well as the advantages, this chapter describes the problems and disadvantages of AFCs and discusses some misconceptions that have appeared in the literature. The AFC has the highest electrical efficiency of all fuel cells, but it typically uses very pure gases. This is often considered a major constraint for most applications; however, the restrictions are probably not that important. The stack design of AFCs is different from the most common designs of polymer-electrolyte or solid oxide fuel cells. Alkaline fuel cells contain a liquid electrolyte in most cases; therefore, in principle, the cell has three chambers that are divided by electrodes or separators. There are two chambers for the reactants and one for the electrolyte. An AFC can be operated at up to 230 °C depending on the fuel cell model used. New developments foresee the integration of stable anion exchange membranes, which can be used like the proton exchange membrane in a Proton Exchange Fuel Cell (PEFC) and hence the design will be similar to the PEFC stack. The chapter describes various designs of AFCs.

Characterising PEM Fuel Cell Performance Using a Current Distribution Measurement in Comparison with a CFD Model

Abstract: The characterisation of a proton exchange membrane (PEM) fuel cell with a straight channel flow field design is performed. Spatially resolved current distribution measurements at different air flow rates are compared to numerical simulation results. The numerical model is validated by agreement of measured and simulated current distribution. The test cell is segmented. It is operated at steady state conditions and the gas flow rates and cell temperature are controlled. The numerical simulation is realised with a PEM fuel cell model based on the FLUENT T M computational fluid dynamics (CFD) software. It accounts for mass transport in the gaseous phase, heat transfer, electrical potential field and the electrochemical reaction. It provides three-dimensional distributions of, for example, current densities, reactant concentrations and temperature.

CO Tolerance of Commercial Pt and PtRu Gas Diffusion Electrodes in Polymer Electrolyte Fuel Cells

Abstract: The CO tolerance of commercial Pt and PtRu anode electrodes from different suppliers (E-Tek and Tanaka) has been examined in polymer electrolyte fuel cells (PEFC) using AC-impedance spectroscopy along steady-state current-voltage curves. A simple mathematical model has been derived in order to extract important kinetic parameters for CO poisoning on different anode electrodes. The Tanaka PtRu (40:60) electrode demonstrated the best CO tolerance under the selected operating conditions. Inductive behavior in the low frequency region of the impedance spectra for the E-Tek Pt and PtRu electrode proved to be characteristic for CO poisoning. However, the impedance spectra of the Tanaka PtRu electrode did not show any inductive behavior and its CO surface coverage, extracted by fitting the experimental data to the model, was lower than the surface CO coverage of the E-Tek electrodes.

Hydrogen Production via Autothermal Reforming of Diesel Fuel

Abstract: Hydrogen, for the operation of a polymer electrolyte fuel cell, can be produced by means of autothermal reforming of liquid hydrocarbons. Experiments, especially with ATR 4, which produces a molar hydrogen stream equivalent to an electrical power in the fuel cell of 3 kW, showed that the process should be preferably run in the temperature range between 700 ° and 850 °. This ensures complete hydrocarbon conversion and avoids the formation of considerable amounts of methane and organic compounds in the product water. Experiments with commercial diesel showed promising results but insufficient long-term stability. Experiments concerning the ignition of the catalytic reaction inside the reformer proved that within 60 s after the addition of water and hydrocarbons the reformer reached 95% of its maximum molar hydrogen flow. Measurements, with respect to reformer start-up, showed that it takes approximately 7 min. to heat up the monolith to a temperature of 340 ° using an external heating device. Modelling is performed, aimed at the modification of the mixing chamber of ATR Type 5, which will help to amend the homogeneous blending of diesel fuel with air and water in the mixing chamber.

Novel CVD Techniques for Micro- and IT-SOFC Fabrication

Abstract: At present, there are intensive research efforts towards intermediate temperature solid oxide fuel cells (IT-SOFCs) with a thin film electrolyte and electrode. In view of the nature of molecular level reaction and the great potential of CVD processes for IT-SOFCs and even Micro-SOFC fabrication, various novel CVD techniques have been developed to prepare multi-component oxide films and multi-layers related to the SOFC. In this paper, novel CVD techniques are described and reviewed for possible applications in thin film electrolytes and electrodes for SOFCs, mainly based on the research activities of the author's laboratory. The techniques include metal-organic CVD (MOCVD) with metal β-diketonate chelates as precursors, single mixed source MOCVD, and aerosol assisted CVD (AACVD). AACVD can be operated with spray AAMOCVD, plasma-AACVD, or oxy-acetylene combustion AACVD. The results show their possible application in the fabrication of electrolyte and electrode thin films for SOFCs.

Nonlinear model reduction of a two-dimensional MCFC model with internal reforming

Abstract: A reduced nonlinear model of a planar molten carbonate fuel cell is presented. The model is derived from a spatially distributed dynamic model of the cell by applying the Karhunen Loeve Galerkin procedure. The reduced model is of considerably lower order than the original one and requires much less computation time. The comparison between the two models shows that the reduced model can describe the dynamics of the temperature field with sufficient accuracy and has good extrapolation qualities with respect to changes in the model parameters.

Characterisation of Fuel Cell Membranes as a Function of Drying by Means of Contact Angle Measurements

Abstract: In another paper in this volume, it is demonstrated that the electrochemical interface in MEAs, and thus the polarization performance of the resulting fuel cells, can be improved by optimising the hot-pressing procedure in the MEA preparation. In particular, the extent of drying of the membrane during MEA preparation was shown to be critical. In the present investigation, the effect of the drying process, and thus water content, on the hydrophilicity, wetting, and surface energies of some fuel cell membranes is examined. Wetting and surface energies are well known to influence the bonding behaviour of materials. Conclusions about how membrane drying and changes in water content influence membrane bonding and the relative importance of these surface effects are drawn.

Modeling the Effects of Methanol Crossover on the DMFC

Abstract: A mathematic model is established to simulate the effects of methanol crossover on the DMFC. The transport and reactions of both oxygen and methanol at the cathode are described and the theory of “parallel electrode reactions” is applied to calculate the cathode over-potential caused by methanol crossover. The influence of methanol concentration, fuel cell temperature, oxygen pressure, and membrane properties on the cathode over-potential is evaluated. Simulation results show that methanol crossover considerably increases the cathode over-potential at low current density, but its effect is significantly reduced when the current density is increased to reasonable values. It also shows that of the two parameters characterizing a polymer electrolyte membrane, proton conductivity and methanol permeability, the former has more impact on the performance of a DMFC.

Fuel Cells for People

Abstract: Portable power promises to be the first truly commercial market for fuel cell technology. In this keynote presentation, which is accompanied by an up-to-date survey published on 1 July 2003 at www.fuelcelltoday.com/surveys, we review the activity that has so far taken place worldwide in this exciting sector of the fuel cell industry. Features include a review of the number of systems so far built, the different fuel cell types under development, fuel choice and region of development. In addition, this article will also discuss the various potential markets for portable fuel cells, and the some of the issues that are associated with them.

On the Stability of Selected Monomeric and Polymeric Aryl Sulfonic Acids on Heating in Water (Part 1)

Abstract: A number of low molecular weight sulfonated aromatic compounds: carboxamides, imides, sulfonamides, as well as sulfonated polymers; aramides and poly(styrene-p-sulfonic acid), were tested for their stability on heating in water and dilute acid to temperatures ranging from 130–200 °C. Desulfonation was observed with some compounds. Sulfonamide linkages were found to be stable, while carboxamide linkages and phthalimides were not.

Recent Advances to the Development of Low‐Temperature Solid Oxide Fuel Cells

Abstract: While existing solid oxide fuel cell (SOFC) technology has demonstrated much higher energy efficiency with minimal pollutant emission than the conventional technologies, the costs of the current SOFC systems are still prohibitive for broad commercialization. To be economically competitive, both the cost of materials and the cost of fabrication must be dramatically reduced. One effective approach to cost reduction is to reduce the operating temperature; many cell components, including interconnect, heat exchangers, and other structural or balance-of-plant components, can be fabricated from much less expensive materials when the operating temperature is sufficiently low. However, the interfacial polarization resistances increase rapidly as the operating temperature is reduced. This paper will address the critical issues in creating electrodes and interfaces of minimal polarization resistance in order to minimize internal electrochemical losses at low temperatures. In particular, design, fabrication, and characterization of composite electrodes consisting of silver and bismuth oxide will be discussed. Another approach to cost reduction is to reduce the cost of fabrication. Dense electrolyte films supported on porous anodes fabricated by screen-printing and dry press will be presented.

Relationship Between Methanol Permeability and Structure of Different Radiation‐Grafted Membranes

Abstract: Styrene grafted and sulfonated poly(vinylidene fluoride) and poly(vinylidene fluoride-co-hexafluoropropylene) films are candidates as electrolytes in direct methanol fuel cells. Their behaviour in water, 1 and 3 mol dm–3 aqueous methanol, and pure methanol were studied. According to SAXS results, water and methanol-water solutions have similar effects on the membranes, i.e., the lamellar period increases and the ionic domains enlarge. Furthermore, differences in the ionic domain structures in pure methanol and water were observed. These structural changes together with dissimilar liquid uptakes in water and in methanol are reflected as changes in the conductivities. An increase in the SAXS intensity and changes in the Bragg distance of the ionic peak were observed in methanol compared to aqueous solutions. This may be related to the hydrophobicity of the CH3 group on methanol. Dissimilarities in methanol permeability through the radiation-grafted membrane can be related to structural differences in membranes observed with SAXS. Permeabilities were observed to be lower for the radiation-grafted membranes compared to Nafion® 115, which compensates for the higher area resistance of the experimental membranes and thus improves their performance in a fuel cell.

Chloride free Pt‐ and PtRu‐ Nanoparticles Stabilised by “Armand's Ligand” as Precursors for Fuel Cell Catalysts

Abstract: Chloride residues on the surface of fuel cell catalysts are known to decrease the catalytic activity, especially for O2 reduction. Using Armand's ligand, which contains the chloride free DCTA anion, for the colloidal stabilisation of nanoscopic Pt and PtRu catalysts precursors (< 2 nm size) leads to PEMFC and DMFC catalysts with improved activity compared to commercial E-TEK catalysts as evidenced by both methanol oxidation and CO-stripping voltametric studies.

Experimental Investigation of Effect of Flow Bed Design on Performance of Liquid Feed Direct Methanol Fuel Cells

Abstract: The effect of flow bed design on the performance of a liquid feed direct methanol fuel cell was studied experimentally. The flow beds were designed and manufactured with different channel densities, channel depths, and ridge widths. The flow beds were machined on stainless steel polar plates. The experimental results showed that ridge width affects the slope of the ohmic polarization curve. A ridge width of 3.30 mm provided the highest peak power. When the contact surface area between the flow bed and electrode was constant, the effect of channel density on cell performance was minimal, under the experimental conditions of this paper. The channel depth had a considerable influence on cell performance with a 2 mm deep channel achieving the best performance. The mechanism of the experimental phenomena was analyzed in this paper.

Development of Solid Oxide Fuel Cells by Applying DC and RF Plasma Deposition Technologies

Abstract: Based on advanced plasma deposition technology with both DC and RF plasmas DLR Stuttgart has developed a concept of a planar SOFC with consecutive deposition of all layers of a thin-film cell onto a porous metallic substrate support. This concept is an alternative approach to conventionally used sintering techniques for SOFC fabrication without needing any sintering steps or other thermal post-treatment. Furthermore, is has the potential to be developed into an automated continous production process. For both stationary and mobile applications, adequate stack designs and stack technologies have been developed. Future development work will focus on light-weight stacks to be applied as an Auxillary Power Unit (APU) for on-board electricity supply in passenger cars and airplanes. This paper describes the plasma deposition technologies used for cell fabrication and the DLR spray concept including the resulting stack designs. The current status of development and recent progress with respect to materials development and electrochemical characterization of single cells and short-stacks is presented.

Modelling Approach for Planar Self-Breathing PEMFC and Comparison with Experimental Results

Abstract: This paper presents a model-based analysis of a proton exchange membrane fuel cell (PEMFC) with a planar design as the power supply for portable applications. The cell is operated with hydrogen and consists of an open cathode side allowing for passive, self-breathing, operation. This planar fuel cell is fabricated using printed circuit board (PCB) technology. Long-term stability of this type of fuel cell has been demonstrated. A stationary, two-dimensional, isothermal, mathematical model of the planar fuel cell is developed. Fickian diffusion of the gaseous components (O2, H2, H2O) in the gas diffusion layers and the catalyst layers is accounted for. The transport of water is considered in the gaseous phase only. The electrochemical reactions are described by the Tafel equation. The potential and current balance equations are solved separately for protons and electrons. The resulting system of partial differential equations is solved by a finite element method using FEMLAB (COMSOL Inc.) software. Three different cathode opening ratios are realized and the corresponding polarization curves are measured. The measurements are compared to numerical simulation results. The model reproduces the shape of the measured polarization curves and comparable limiting current density values, due to mass transport limitation, are obtained. The simulated distribution of gaseous water shows that an increase of the water concentration under the rib occurs. It is concluded that liquid water may condense under the rib leading to a reduction of the open pore space accessible for gas transport. Thus, a broad rib not only hinders the oxygen supply itself, but may also cause additional mass transport problems due to the condensation of water.

PEM – Fuel Cell System for Residential Applications†

Abstract: Viessmann is developing a PEM fuel cell system for residential applications. The uncharged PEM fuel cell system has a 2 kW electrical and 3 kW thermal power output. The Viessmann Fuel Processor is characterized by a steam-reformer/burner combination in which the burner supplies the required heat to the steam reformer unit and the burner exhaust gas is used to heat water. Natural gas is used as fuel, which is fed into the reforming reactor after passing an integrated desulphurisation unit. The low temperature (600 °C) fuel processor is designed on the basis of steam reforming technology. For carbon monoxide removal, a single shift reactor and selective methanisation is used with noble metal catalysts on monoliths. In the shift reactor, carbon monoxide is converted into hydrogen by the water gas shift reaction. The low level of carbon monoxide at the outlet of the shift reactor is further reduced, to approximately 20 ppm, downstream in the methanisation reactor, to meet PEM fuel cell requirements. Since both catalysts work at the same temperature (240 °C), there is no requirement for an additional heat exchanger in the fuel processor. Start up time is less than 30 min. In addition, Viessmann has developed a 2 kW class PEFC stack, without humidification. Reformate and dry air are fed straight to the stack. Due to the dry operation, water produced by the cell reaction rapidly diffuses through the electrolyte membrane. This was achieved by optimising the MEA, the gas flow pattern and the operating conditions. The cathode is operated by an air blower.