Showing papers in "Fuel Cells in 2005"

Prospects for Alkaline Anion-Exchange Membranes in Low Temperature Fuel Cells†

Abstract: This article introduces the radical approach of applying alkaline anion-exchange membranes (AAEMs) to meet the current challenges with regards to direct methanol fuel cells (DMFCs). A review of the literature is presented with regards to the testing of fuel cells with alkaline membranes (fuelled with hydrogen or methanol) and also to candidate alkaline anion-exchange membranes for such an application. A brief review of the directly related patent literature is also included. Current and future research challenges are identified along with potential strategies to overcome them. Finally, the advantages and challenges with the direct electrochemical oxidation of alternative fuels are discussed, along with how the application of alkaline membranes in such fuel cells may assist in improving performance and fuel efficiency.

Aspects of the Chemical Degradation of PFSA Ionomers used in PEM Fuel Cells

Abstract: The chemical degradation of perfluorosulfonic acid (PFSA) membranes was studied both in-situ (during fuel cell operation) and ex-situ (by Fenton's test). During fuel cell operation, the degradation rate was quantified by monitoring the rate of fluoride release. The rate of degradation was found to be strongly dependent on operating conditions. Nuclear magnetic resonance (NMR) and mass spectrometry (MS) were used to identify degradation products other than fluoride generated during fuel cell operation. Strong similarities were found between the organic fragments generated from both the in-situ (fuel cell operation) and ex-situ (Fenton's test) degradation processes. The chemical structure of the fragment is consistent with that of the side chain on the PFSA ionomer used in the experiments. The implications of the existence of this product for the chemical degradation mechanism are discussed.

The Chemical and Structural Nature of Proton Exchange Membrane Fuel Cell Properties

Abstract: The chemical and structural (morphological) features of proton exchange membranes are directly tied to their fuel cell relevant transport properties. A large body of research has focused on characterizing the structure or investigating the properties of Nafion® and other proton exchange membranes, but few studies have linked chemical composition to membrane morphology, and resulting transport properties. This paper systematically discusses the key chemical and structural features of proton exchange membranes that impact properties critical for fuel cell applications. We focus our discussion on the fuel cell relevant transport properties of proton conductivity, methanol permeability, water diffusion coefficient, and electro-osmotic drag coefficient, using evidence from our work and from the literature to illustrate the connection between structure and properties in these materials. It is hoped that this document will serve as a guide to the rational, systematic design of new proton exchange membrane materials with improved properties.

Blended and Cross-Linked Ionomer Membranes for Application in Membrane Fuel Cells†

Abstract: Differently cross-linked blend membranes were prepared from commercial arylene main-chain polymers of the poly(etherketone) and poly(ethersulfone) classes, modified with sulfonate groups, sulfinate cross-linking groups, and basic N-groups. The following membrane types have been prepared: (i) Van-der Waals/dipole-dipole blends by mixing a polysulfonate with unmodified PSU. This membrane type showed a heterogeneous morphology, leading to extreme swelling and even dissolution of the sulfonated component at elevated temperatures. (ii) Hydrogen bridge blends by mixing a polysulfonate with a polyamide or a polyetherimide. This membrane type showed a partially heterogeneous morphology, also leading to extreme swelling/dissolution of the sulfonated blend component at elevated temperatures. (iii) Acid-base blends by mixing a polysulfonate with a polymeric N-base (in-house developed/commercial). A wide range of properties could be achieved with this membrane type by variation of the different parameters. Membranes showing excellent stability and good fuel cell performance up to 100 °C (PEFC) and 130 °C (DMFC) were obtained. (iv) Covalently cross-linked (blend) membranes by either mixing a polysulfonate with a polysulfinate or by preparing a polysulfinatesulfonate, followed by reaction of the sulfinate groups in solution with a dihalogeno compound under S-alkylation. The membranes prepared showed effective suppression of swelling without a loss in the H+-conductivity. The membranes showed good PEFC (up to 100 °C) and DMFC (up to 130 °C) performance. (v) Covalent-ionically cross-linked blend membranes by mixing polysulfonates with polysulfinates and polybases or by mixing a polysulfonate with a polymer carrying both sulfinate and basic N-groups. The covalent-ionically cross-linked membranes were tested in a DMFC up to 110 °C and demonstrated good performance. (vi) Differently cross-linked organic-inorganic blend composite membranes via various procedures. The best results were obtained with blend membranes having a layered zirconium phosphate “ZrP” phase: they were transparent, and showed good H+-conductivity and stability. The application of one of these composite membranes in a PEFC yielded good performance up to T = 115 °C.

Poly(Arylene Ether Sulfone) Copolymers and Related Systems from Disulfonated Monomer Building Blocks: Synthesis, Characterization, and Performance – A Topical Review

Abstract: Research and development efforts have been focussed over the last five years towards the preparation of new, but potentially commercially viable low-cost copolymers for use as proton exchange membrane for fuel cell and other membrane applications. Our primary efforts centered on the direct synthesis of disulfonated copolymers via step-polycondensation methods. These novel series of disulfonated copolymers include optionally fluorinated poly(arylene ethers), poly(thioethers), polyimides, polybenzimidazoles, and polybenzoxazoles, as well as multiblock copolymer systems. The first generation of alternative proton exchange membranes (PEMs) has focused on wholly aromatic, disulfonated poly(arylene ether sulfone) random copolymers. Detailed herein are the development and current state of these disulfonated poly(arylene ether sulfone) copolymers and their fuel cell performance in both hydrogen-air PEMFCs and direct methanol fuel cells (DMFC).

Synthesis and Characterization of Pyridine-Based Polybenzimidazoles for High Temperature Polymer Electrolyte Membrane Fuel Cell Applications

Abstract: A series of polybenzimidazoles (PBIs) incorporating main chain pyridine groups were synthesized from the pyridine dicarboxylic acids (2,4-, 2,5-, 2,6- and 3,5-) and 3,3′,4,4′-tetraaminobiphenyl, using polyphosphoric acid (PPA) as both solvent and polycondensation reagent. A novel process, termed the PPA process, has been developed to prepare phosphoric acid (PA) doped PBI membranes by direct-casting of the PPA polymerization solution without isolation or re-dissolution of the polymers. The subsequent hydrolysis of PPA to PA by moisture absorbed from the atmosphere usually induced a transition from the solution state to a gel-like state and produced PA-doped PBI membranes with a desirable suite of physiochemical properties. The polymer structure characterization included inherent viscosity (I.V.) determination as a measurement of polymer molecular weight and thermal stability assessment via thermogravimetric analysis. Physiochemical properties of the doped membrane were studied by measurements of the PA doping level, ionic conductivity and mechanical properties. The resulting pyridine-based polybenzimidazole membranes displayed high PA doping levels, ranging from 15 to 25 mol of PA per PBI repeat unit, which contributed to their unprecedented high proton conductivities of 0.1 to 0.2 S cm–1 at 160 °C. The mechanical property measurements showed that the pyridine-based PBI membranes were thermally stable and maintained mechanical integrity even at high PA doping levels. Preliminary fuel cell tests demonstrated the feasibility of the novel pyridine-based PBI (PPBI) membranes from the PPA process for operating fuel cells at temperatures in excess of 120 °C without any external humidification.

About the Choice of the Protogenic Group in PEM Separator Materials for Intermediate Temperature, Low Humidity Operation: A Critical Comparison of Sulfonic Acid, Phosphonic Acid and Imidazole Functionalized Model Compounds

Abstract: Traditionally, sulfonated polymers are used as separator materials in PEM fuel cells. Based on recent experimental results on model compounds this paper critically discusses the potentials and limits of sulfonic acid and alternatively phosphonic acid and heterocycles (imidazole) as protogenic groups for PEM fuel cell electrolytes operating at intermediate temperatures (T > 100 °C) and low humidification. Apart from transport properties, the stability and reactivity of mono-functionalized model compounds (1-heptylsulfonic acid (S-C7), 1-heptylphosphonic acid (P-C7) and 2-heptyl-imidazole (I-C7)) and a few diphosphonic acids are examined under wet and dry conditions. These are characterized with respect to their proton conductivity (ac impedance spectroscopy), proton diffusion coefficient (pulsed-field gradient NMR), thermo-oxidative stability (TGA under air), electrochemical stability (cyclic voltammetry) and their hydration behavior (TGA under water vapor). The sulfonic acid functionalized compound shows reasonable properties only when a minimum hydration level is guaranteed, while phosphonic acid functionalized compounds combine satisfactory proton conductivity even in the water-free state at intermediate temperatures (T < 200 °C), comparatively high thermo-oxidative and electrochemical stability and electrochemical reactivity (hydrogen oxidation and oxygen reduction at platinum surfaces). The presence of water leads to moderate water uptake allowing for reasonable conductivities even at room temperature and prevents condensation reactions at higher temperature. The imidazole based system shows the largest electrochemical stability window, but its moderate proton conductivity and thermo-oxidative stability and the very high overpotential for oxygen reduction on platinum turn out to be severe disadvantages for the envisaged application.

Neutron and X-ray Scattering: Suitable Tools for Studying Ionomer Membranes†

Abstract: Neutrons and x-rays were extensively used to study the microstructure of the polymeric membranes for fuel cells. The small-angle x-ray and neutron scattering (SAXS and SANS resp.) data for Nafion® and alternative membranes present specific features. The different analyses and proposed models to interpret these data are reviewed. A special emphasis is devoted to the recent elongated polymer particle model which appears as the most efficient model for Nafion® to interpret not only the numerous scattering data but also the transport swelling and mechanical properties. Neutrons can also be used to study the water management in situ in an operating fuel cell either to visualise the water accumulation in the gas distributors or to determine the water concentration profiles through the membrane and during operation.

Recent Developments on Proton Conduc‐ting Poly(2,5‐benzimidazole) (ABPBI) Membranes for High Temperature Poly‐mer Electrolyte Membrane Fuel Cells

Abstract: Commercial polybenzimidazole (PBI) membranes impregnated with phosphoric acid have become a serious candidate as electrolytes for high temperature PEMFC. Much research effort has been devoted to the study of this proton conducting membrane.[1, 2] On the other hand, PBI is not the simplest nor the cheapest of this family and many other polybenzimidazoles would also be of great interest in this respect. Among them, poly(2,5-benzimidazole) (ABPBI) is one of the best choices [3]. ABPBI is easy to polymerize from a single monomer, even without previous purification, it absorbs more acid than PBI in the same bath concentration, and it has the same thermal stability and high proton conductivity at temperatures up to 200 °C. Here we review all the recent developments on the preparation of proton conducting ABPBI membranes, mainly by phosphoric acid impregnation, but also by sulfonation or doping with heteropolyacids to form hybrid organic-inorganic membranes.

Radiation Grafted Membranes for Polymer Electrolyte Fuel Cells

Abstract: The cost of polymer electrolyte fuel cell (PEFC) components is crucial to the commercial viability of the technology. Proton exchange membranes fabricated via the method of radiation grafting offer a cost-competitive option, because starting materials are inexpensive commodity products and the preparation procedure is based on established industrial processes. Radiation grafted membranes have been used with commercial success in membrane separation technology. This review focuses on the application of radiation grafted membranes in fuel cells, in particular the identification of fuel cell relevant membrane properties, aspects of membrane electrode assembly (MEA) fabrication, electrochemical performance and durability obtained in cell or stack tests, and investigation of failure modes and post mortem analysis. The application in hydrogen and methanol fuelled cells is treated separately. Optimized styrene / crosslinker grafted and sulfonated membranes show performance comparable to perfluorinated membranes. Some properties, such as methanol permeability, can be tailored to be superior. Durability of several thousand hours at practical operating conditions has been demonstrated. Alternative styrene derived monomers with higher chemical stability offer the prospect of enhanced durability or higher operating temperature.

Synthetic Strategies for Controlling the Morphology of Proton Conducting Polymer Membranes

Abstract: The nanostructure and morphology of proton conducting polymers is of considerable interest in the search for next generation materials and optimization of existing ones. Synthetic methodologies for tailoring molecular structures that promote nanoscopic phase separation of ionic and non-ionic domains, and the effect of phase separation on parameters such as proton conductivity, are considered. Rather than distinguish proton conducting polymers according to chemical class, they are categorized under sub-headings of random, block, and graft copolymers. The synthetic methodology available to access archetypal polymer structures is dependent on the nature of the monomers and restrictive compared to conventional non-ionic polymer systems. Irrespective of the methodology, ionic aggregation and phase separation are consistently found to play an important role in the proton conductivity of low ion exchange capacity (IEC) membranes, but less of a role in high IEC membranes. Significant research is required to further develop relationships between polymer architecture, morphology, and electrolytic properties.

Modelling and Analysis of Electro-chemical, Thermal, and Reactant Flow Dynamics for a PEM Fuel Cell System

Abstract: In this paper an approach for the dynamic modelling of polymer electrolyte membrane fuel cells is presented. A mathematical formulation based on empirical equations is discussed and several features, exhibiting dynamic phenomena, are investigated. A generalized steady state fuel cell model is extended for the development of a method for dynamic electrochemical analysis. Energy balance and reactant flow dynamics are also explained through physical and empirical relationships. A well-researched system (Ballard MK5-E stack based PGS-105B system) is considered in order to understand the operation of a practical fuel cell unit. Matlab-SIMULINKTM has been used in simulating the models. The proposed method appears to be relatively simple and consequently requires less computation time. Simulation results are compared with available experimental findings and a good match has been observed.

Mastering Sulfonation of Aromatic Polysulfones: Crucial for Membranes for Fuel Cell Application†

Abstract: The publication deals with the sulfonation of aromatic polysulfones that results in ionomers intended to be used as membranes for Proton Exchange Membrane Fuel Cells. The mechanical properties and the lifespan of the membranes depend, in particular, on the mode of synthesis of the ionomers and on chemical degradations which can occur and cut the polymeric chains. This article gives a progress report on the various methods giving access to polysulfone ionomers based on sulfonic acid functions. Among these methods, particular attention has been paid to electrophilic substitution from commercially available polysulfone. Although electrophilic sulfonation of aromatic molecules has been known for about one century, its application to the sulfonation of polymers still raises problems, e.g. homogeneity and degradations. A mechanism of chemical degradation is proposed to explain the chain breakings occurring in the course of reaction. The difficulty in passing from the laboratory scale to an industrial production is illustrated from reactions carried out at the pilot scale.

Comparison of Finite Volume SOFC Models for the Simulation of a Planar Cell Geometry

Abstract: This paper discusses the development of a finite volume model for a planar solid oxide fuel cell. Two different levels of detail for the definition of the basic cell elements are considered, the first with the assumption of isothermal behavior for a finite volume, defined by a portion of the cell PEN structure with pertinent air and fuel channels, and the second with a more refined element subdivision, capable of simulating temperature differences at a smaller scale. The model applies a detailed electrochemical and thermal analysis to a planar SOFC of a defined geometry (with co-flow, counter-flow or cross-flow configuration), material properties and input flows. Electrochemical modeling includes an evaluation of ohmic, activation and diffusion losses as well as a kinetic model of the hydrocarbon reactions involved. The model calculates internal profiles of temperature, flow composition, current density, and cell energy balances. Internal heat exchange coefficients are evaluated with a specific fluid-dynamic analysis. After a preliminary calibration of the model, a comparison of the simulation results generated by the two models is presented and a parametric analysis to investigate the effects of different assumptions on a selection of key parameters (heat losses, air stoichiometric ratio and inlet temperatures) is carried out. The results show that the refined model developed here could significantly help in the design of efficient fuel cell stack projects and in the careful consideration of the influence of heat losses, air ratio and the endothermic reforming reaction on cell temperature distribution and global performances.

Fuel cell membrane materials by chemical grafting of aromatic main-chain polymers

Abstract: An extensive world-wide pursuit for new efficient fuel cell membranes materials is currently motivating research on proton-conducting ionomers based on durable aromatic main-chain polymers. In this context, most ionomers. have been prepared either by direct sulfonation of polymers, using for example. fuming. sulfuric-acid, or by direct polymerizations using different sulfonated monomers, Far less exploited are chemical grafting reactions carried out to introduce sulfonic acid units, or alternative acidic units, directly on the polymer main-chain, or on side-chains to the polymer main-chain. This versatile method offers very interesting possibilities, not only to control the degree and the site of sulfonation, but also when it comes to manipulating the molecular mobility of the sulfonic acid units and their distance from the polymer main-chain. The length and nature of the grafted units have shown to have a large influence on for example the water-uptake characteristics and conductivity of,ionomer membranes, especially at temperatures above 100 degrees C. Grafting can also be used to introduce other useful functions to the polymers, or to crosslink membranes. This paper reviews various grafting reactions carried out on aromatic main-chain polymers, especially polybenzimidazoles and polysulfones, to prepare membrane materials, as well as the characteristics of these materials regarding their use in fuel cells. (Less)

Thermo-Economic Modelling and Optimisation of Fuel Cell Systems

Abstract: If process modelling is a well known approach to compute the performances of fuel cell systems, the optimal design of systems where the process configuration is not fixed a priori remains an issue. The goal of the presented methodology for integrating and optimising fuel cell systems is to help in this preliminary design step. The thermo-economic model developed includes three parts : 1) the energy flow model that represent the thermodynamic performances of the chemical and energy conversions in the physical units operation considered in the system ; 2) the model of the heat transfer system that is based on process integration thechniques [1] and 3) an economic module that computes a cost estimation of each of the devices considered in the system. The method uses a superstructure that includes the major options to be considered for the energy conversion in the system. A multi-objective optimisation approach based on evolutionary algorithms is used to extract from the superstructure the most promising configurations and compute for each of them the best operating conditions. The objectives considered are specific cost of electricity production and the system efficiency. The proposed method is generic and may be applied to diffrent types of fuel cell systems. Here, the results of the thermo-economic optimization are given for a PEM (Proton Exchange Membrane) fuel cell system.

New Preparation Methods for Composite Membranes for Medium Temperature Fuel Cells Based on Precursor Solutions of Insoluble Inorganic Compounds

Abstract: Current PEMFCs, using perfluorinated membranes, can only operate at temperatures no higher than 70–80 °C, since their performance is dramatically reduced at higher temperatures. Encouraging results, for avoiding the reduction in performance, can be obtained by filling Nafion® membranes with inorganic nano-particles (composite membranes) and/or by adding inorganic nano-particles at the membrane/electrode interfaces. However, the efficient insertion of nano-particles, other than metal oxides, inside a polymeric matrix is often very complicated or not possible. Therefore, it was of interest to find simpler methods for the insertion. In this paper, the preparation of organic precursor solutions of the insoluble compound Zr(O3P-OH)(O3P-C6H4SO3H), a proton conductor exhibiting very high conductivity (10–1 S cm–1 at 100 °C and 90 %RH) is reported. It was found that these precursor solutions are more stable the higher the Kb of the solvent and the lower the temperature. Precursor solutions, stable for many days at room temperature, were found by using proton acceptor solvents, such as alkanols, N,N-dimethyl formamide (DMF), and N-methyl pyrrolidone (NMP). The particularity of these solutions is that insoluble phosphonate particles are formed when the solvent is evaporated. They are therefore very suitable for filling porous membranes, for the preparation of composite proton conducting membranes, and for the preparation of composite electrodes. Some examples of preparation and use are reported and discussed.

Heat‐Integrated Reactor Concepts for Hydrogen Production by Methane Steam Reforming

Abstract: Steam reforming of hydrocarbons is the major source of hydrogen on an industrial scale. Conventional, large scale, processes for hydrogen production are not optimal for the decentralized, stand-alone supply of hydrogen for fuel cell systems. Their major drawback is limited thermal efficiency due to restricted heat recovery from the reformer effluents. A promising approach to overcome these limitations is the utilization of multifunctional reactor concepts, integrating the major reaction steps and process heat management. Coupling endothermic and exothermic reactions into heat-integrated processes can be realized in different ways. One possible design is based on micro-structured devices employing recuperative heat exchange between the process streams. A comparable specific performance and functionality can be achieved with adiabatic fixed-bed reactors, operating in a transient mode. A major issue for both alternatives is the proper axial distribution of the process heat. Novel design solutions, developed in our group, are reviewed in this paper. Detailed simulation studies, as well as proof-of-concept experiments, confirm their feasibility and potential to significantly enhance the efficiency of the process.

The Ligand Effect: CO Desorption from Pt/Ru Catalysts

Abstract: Density Functional Theory (DFT) calculations of the adsorption energy of CO, for a platinum overlayer on Ru(0001), have been performed. For all coverages a significant reduction in the binding energy of up to 0.5 eV has been observed compared to that obtained on Pt(111). In addition, a Steady-State Isotopic Transient Kinetic Analysis (SSITKA) study has been performed to determine the desorption rate dependence on the partial pressure of CO over commercial Pt/Ru electrocatalysts. As expected, no significant difference in the rate of exchange of CO at any given pressure is observed on going from Pt to Pt/Ru electrocatalysts when the diluted gas used was argon since the CO states will be filled to the same desorption energy for the two catalysts. However on changing the diluent gas to hydrogen, a reduction in the exchange rate for CO is observed clearly reflecting the lower CO binding energy and the increased competition for sites at the surface of the catalyst. The reduction efficiency of the Pt/Ru electrocatalyst was also studied and found to be highly dependent on whether CO or hydrogen was used. These results will be discussed with reference to the anode catalysis of the Polymer Electrolyte Membrane Fuel Cell (PEMFC).

Advances in Mixed‐Reactant Fuel Cells

Abstract: The mixed-reactant fuel cell (MRFC) is a new concept, in which a mixture of aqueous fuel and gaseous oxygen (or air) flows directly through a porous anode-electrolyte-cathode structure or through a strip-cell with an anode-electrolyte-cathode configuration. These structures can be single cells or parallel stacks of cells and may be in a planar, tubular or any other geometry. Selectivity in the electrocatalysts for MRFCs is mandatory to minimize mixed-potential at the electrodes, which otherwise would reduce the available cell voltage and compromise the fuel efficiency. MRFC offers a cost effective solution in fuel cell design, since there is no need for gas-tight structure within the stack and, as a consequence, considerable reduction in sealing, manifolding and reactants delivery structure is possible. In recent years, significant advances have been made in MRFCs, using methanol as a fuel. This paper reviews the status of mixed reactant fuel cells and reports some recent experimental data for methanol fuel cell systems.

Modelling of Pressurised Hybrid Systems Based on Integrated Planar Solid Oxide Fuel Cell (IP-SOFC) Technology

Abstract: This work describes different models, developed by the Thermochemical Power Group at the University of Genoa (Italy), for the simulation of solid oxide fuel cell and gas turbine hybrid systems. The paper focuses on both “cores” of the system: the fuel cell stack on the one hand and the turbomachinery and the auxiliaries on the other hand. Therefore, in the first part of the paper the models developed for the analysis of the Rolls-Royce Integrated Planar SOFC cells are presented; the results are compared to experimental data, and carefully analysed and discussed. In the second part of the paper, design and off design models of IP-SOFC pressurised hybrid systems in the range 250 kW–20 MW are presented; the hybrid performance results are presented and discussed, also taking ambient condition effects and a possible control strategy into account. Finally, using an in-house general purpose transient system analysis code (TRANSEO code), where chemical composition, heat transfer, and fluid dynamic influences vs. time are considered in detail, a preliminary time dependent investigation of a pressurised hybrid system behaviour is presented.

Dynamic Modelling and Simulation of a Fuel Cell Generator

Abstract: In this paper the dynamics of a polymer electrolyte membrane (PEM) fuel cell system and its associated power electronics are modelled and simulated. The fuel cell system model includes the dynamics of reactant flow, membrane resistance and charge, double layer capacitance as well as steady state equations. The DC Output of the 5 kW fuel cell stack is converted to 120 V, 60 Hz AC by a pulse width modulated inverter. The inverter output is held constant by a PID controller. Matlab-Simulink™ and Power System Blockset (PSB) are used for the modelling and simulation of the fuel cell generator. The effects of load variation on output voltage, current, and fuel cell reactant flows are investigated. Simulation results indicate that variations in the systems electrical and physical parameters are within acceptable limits. Such a fuel cell generator could be used in grid connected and stand-alone applications.

Characterization of sulfonated poly(ether ether ketone)/polytetrafluoroethylene composite membranes for fuel cell applications

Abstract: The sulfonated poly(ether ether ketone) (SPEEK) / polytetrafluoroethylene (PTFE) composite membranes were produced to aid the swelling properties. The PTFE porous films were used as reinforcing material for the SPEEK/PTFE composite membranes. Scanning electron micrographs showed that SPEEK resin was distributed uniformly in the composite membrane and completely plugged the micropores; there is a continuous thin SPEEK film present on the PTFE surface. The dimensional stability and mechanical strength of the composite membranes were apparently improved. The composite membrane had a smaller oxygen permeability than the Nafion (R) membrane despite it being larger than that of the SPEEK homogenous membrane. The performance and stability of the PEMFC with SPEEK/PTFE composite membranes were also tested. Results showed that the fuel cell had a larger resistance than the SPEEK homogenous membrane or the Nafion (R) membrane, but these disadvantages are compensated for with the thinner composite membrane, because of its low gas permeability and high mechanical strength. Stability testing showed that the cell performance could be kept steady for more than 150 hours. The structure of the composite membrane didn't undergo any change and a tight bond remained between the composite membrane and the catalyst layers of electrode after more than 300 hours of intermittent operation.

Plasma‐Polymerised Proton Conductive Membranes for a Miniaturised PEMFC

Abstract: Thin films, containing fluorocarbons and sulphonic acid groups, were prepared by plasma polymerisation of 1,3-butadiene/CF3SO3H and styrene/CF3SO3H mixtures using two different types of plasma discharges The morphology, density, and chemical composition of the synthesised plasma polymers were examined by SEM, small-angle X-ray reflectometry, and FTIR and XPS analyses, respectively Proton conductivity was measured with electrochemical impedance spectroscopy The prepared films are relatively uniform and free from defects Plasma materials exhibiting higher sulphonic acid group contents and lower densities are those formed, from the styrene/CF3SO3H mixture, in the after glow discharge configuration; which proves that the kind of discharge and the nature of the monomer mixture have a large influence on the microstructure of plasma materials Consequently, plasma films, prepared in the after glow discharge from the styrene/CF3SO3H mixture, show proton conductivities (up to 98 × 10–2 mS cm–1) about one order of magnitude higher than films prepared in the glow discharge (up to 22 × 10–3 mS cm–1 and 25 × 10–3 mS cm–1 for films prepared from 1,3-butadiene/CF3SO3H and styrene/CF3SO3H mixtures, respectively) When compared to the commercially available Nafion®, the reference electrolyte polymer in the PEMFC, our most competitive plasma polymers are intrinsically 100 times less conductive Nevertheless, due to their thinness (about 1 μm), these plasma films show specific resistances (about 10 Ω cm2), which are lower than that of the Nafion® membrane (19 Ω cm2 specific resistance) This explains their potential significance as polymer electrolyte membranes in a miniaturised PEMFC

Chemical Modification of Perfluorosulfo-nated Membranes with Pyrrole for Fuel Cell Application: Preparation, Characteri-sation and Methanol Transport

Abstract: In situ chemical polymerisation of pyrrole in Nafion®-115 membranes produces a composite material that shows a decrease in methanol crossover compared with the non-modified material, and a good protonic conduction (area resistance of 0.65 Ω cm2). Pyrrole is impregnated from an appropriate solvent, and the choice of oxidising agent (hydrogen peroxide, ammonium peroxodisulfate) provides two types of modified membrane. Membrane characterisation using infrared spectroscopy shows the presence of pyrrole oligomers and polymer, but the proton conductivity is lower than that of the Nafion®-115 membrane by only a factor 2–3. Methanol transport measurements in 2 mol dm–3 methanol with and without application of an electric field show ca. 30% lower crossover for the pyrrole-modified membranes. In single cell hydrogen – oxygen fuel cell tests, the pyrrole-modified Nafion® prepared using hydrogen peroxide oxidant gave higher open circuit voltage and current density, in particular at > 80 °C, than non-modified Nafion®-115.

Process Simulation for Molten Carbonate Fuel Cells

Abstract: In the framework of electricity production from molten carbonate fuel cells (MCFC), this work presents the results obtained from steady state process simulation, coupled with a detailed dynamic model for the cell. The derivation of the model is briefly outlined, and detailed results of the simulation are reported. Macroscopic state variables of the overall system process are investigated by means of sensitivity analysis, yielding clear directions for process improvement and optimisation. The detailed steady state and dynamic simulation of the MCFC for a bi-dimensional cross flow configuration showed the importance of the state variable distribution in the cell. Both models were based on data from real plants, and the simulation conditions were selected according to real operating conditions. The process studied was a 500 kW MCFC power system based on Ansaldo fuel cell (AFC) technology. The steady state simulation revealed the main interactions among the different devices involved in the process, and the subsequent sensitivity analysis showed that there is room for improvement in electrical efficiency by increasing: i) the steam to methane ratio, ii) the pressure, and iii) the air feed ratio. The dynamic simulation yielded the quantitative response of the system to several, different, perturbations and proved to be a valid tool for determining temperature, current, and voltage profiles as a function of time, within the fuel cells.

Proton-Conducting Polymer Electrolyte Membranes Based on Fluoropolymers Incorporating Perfluorovinyl Ether Sulfonic Acids and Fluoroalkenes: Synthesis and Characterization†

Abstract: This paper presents the synthesis of new polymer electrolyte membranes based on fluoropolymers incorporating aromatic perfluorovinyl ether sulfonic acids. A novel synthetic route describing the preparation of perfluorovinyl ether monomer containing sulfonic functionalities, 4-[(α,β,β-trifluorovinyl)-oxy]benzene sulfonic acid (TFVOBSA), is reported. The radical (co) and terpolymerisation of 4-[(α,β,β-trifluorovinyl)oxy]benzene sulfonyl chloride (TFVOBSC) with 1,1-difluoroethylene (or vinylidene fluoride, VDF), hexafluoropropene (HFP), and perfluoromethyl vinyl ether (PMVE) is described. The terpolymers of TFVOBSC with VDF and HFP, or VDF and PMVE, were hydrolysed and also led to new fluorinated terpolymers bearing sulfonic acid aromatic side-groups. The terpolymers were characterized by 1H and 19F NMR spectroscopies, SEC, DSC, and TGA. Membranes incorporating these functional fluoropolymers were prepared and the electrochemical properties (IEC, proton conductivity, swelling rates) were studied and discussed.

Multidimensional Modelling of Polymer Electrolyte Fuel Cells under a Current Density Boundary Condition

Abstract: A three-dimensional numerical model of the polymer electrolyte fuel cell (PEFC) is applied to study current distribution and cell performance under a current density boundary condition. Since the electronic resistance in the along-channel direction in the current collector plate is much larger than in the other two directions, i.e., 50 mΩ cm2vs. 0.5 mΩ cm2, it significantly affects current flow, and current and cell voltage distributions in a PEFC. An identical polarization curve results with two different boundary conditions, constant cell voltage and constant current density, however, the current density profiles in the along-channel direction differ significantly; it is much flatter for the constant current boundary condition. Increasing the electronic conductivity of the bipolar plate diminishes the difference in the current density distribution under the two boundary conditions. The results also point out that an experimental validation of a PEFC model based on the polarization curve alone is insufficient, and that detailed current density distribution data in the along-channel direction is essential.

SOFC System Operating Strategies for Mobile Applications

Abstract: SOFC systems, working at high temperatures of about 800 °C, have recently attracted significant interest for application as automotive and stationary power supply systems, based on gasoline or diesel heating. For periodically changing operation, the thermal management of the system, including start-up and load following capability, is considered a crucial point. In this work, the thermal behaviour during start-up, operation, shut down, and restart of a simple SOFC system for mobile applications is studied. Different operating strategies, such as keeping the system at operating temperature and thermal cycling, are investigated and compared with respect to heat management, energy consumption, and start-up performance. The characteristics depend on the system size and weight. For a 50 kWel system, with suitable insulation, immediate restart of the vehicle should be achievable for up to three days, hence no external heat is needed within that period of time. Smaller systems, e.g., Auxiliary Power supply Units (APU), replacing the conventional alternator in vehicles, cool down more rapidly. If an immediate restart is desirable ‘keeping the system at temperature’ would be the more favourable strategy.

Multilayer Sulfonated Polyaromatic PEMFC Membranes

Abstract: Sulfonated polyetheretherketone (sPEEK) membranes have been developed in which a hierarchical organisation into multilayers has been favoured during preparation using solvent casting. Bilayer membranes comprising sPEEK of different degrees of sulfonation (having ion exchange capacities between 1.1 and 1.6 meq g–1) have been characterised for their water uptake properties and proton conductivity. The composite membranes show no tendency to delaminate, even under prolonged operation in a hydrogen – oxygen fuel cell. By associating sPEEK of different water uptake characteristics, water back diffusion in an operating fuel cell is favoured, leading to a degree of control over the direction of water production at the anode or the cathode. Such a bilayer membrane has been operated at 110 °C without reactant gas hydration for over 900 h.