Effects of the Diffusion Layer Characteristics on the Performance of Polymer Electrolyte Fuel Cell Electrodes

Several carbon blacks and graphite were investigated as candidates for diffusion layer preparation in polymer electrolyte fuel cell electrodes (PEFC). Single cell electrochemical characterizations under different working cell conditions were carried out on the electrodes by varying the kind of carbon in the diffusion layer. An improvement in cell performance was found by using Shawinigan Acetylene Black (SAB) as carbon, resulting in a measured power density of about 360 mW cm^–2 in H₂/air operation at 70°C and 1/1 bar. Pore size distribution and scanning electron microscopy analyses were carried out to help the understanding of the different behaviour of the investigated carbon diffusion layers.     

Effects of the cathode gas diffusion layer characteristics on the performance of polymer electrolyte fuel cells

Polymer electrolyte fuel cell (PEFC) electrodes were prepared by applying different porous gas diffusion half-layers (GDHLs) onto each face of a carbon cloth support, followed by the deposition of a catalyst layer onto one of these half-layers. The performance of PEFCs in H₂/air operation using cathodes with GDHLs presenting different characteristics were compared. The best result was obtained using cathodes with GDHLs having polytetrafluorethylene (PTFE) contents of 30 wt % in the gas side and 15 wt % in the catalyst side. This behaviour was explained in terms of a better water management within the cell.     

Iron(III) tetramethoxyphenylporphyrin(FeTMPP) as methanol tolerant electrocatalyst for oxygen reduction in direct methanol fuel cells

Carbon supported iron (III) tetramethoxyphenylporphyrin (FeTMPP) heat treated at 800°C under argon atmosphere was used as catalysts for the electroreduction of oxygen in direct methanol polybenzimidazole (PBI) polymer electrolyte fuel cells that were operated at 150°C. The electrode structure was optimized in terms of the composition of PTFE, polymer electrolyte and carbon-supported FeTMPP catalyst loading. The effect of methanol permeation from anode to cathode on performance of the FeTMPP electrodes was examined using spectroscopic techniques, such as on line mass spectroscopy (MS), on line Fourier transform infrared (FTIR) spectroscopy and conventional polarization curve measurements under fuel cell operating condition. The results show that carbon supported FeTMPP heat treated at 800°C is methanol tolerant and active catalyst for the oxygen reduction in a direct methanol PBI fuel cell. The best cathode performance under optimal condition corresponded to a potent ial reached of 0.6V vs RHE at a current density of 900 mAcm–2.     

Improvement in the diffusion characteristics of low Pt-loaded electrodes for PEFCs

Low Pt loading electrodes have been obtained by the direct mixing of electrocatalyst and Nafion® ionomer (for catalyst layer) and by the introduction of an intermediate hydrophobic carbon layer to optimize gas distribution. The influence of Teflon® content in the carbon layer has been studied and an optimum content of 20 wt% has been found. The behaviour of the improved electrodes as a function of temperature (70–95 °C) and gas (H2 and air) pressure (1–5 bar) has been evaluated in a 50 cm2 single cell. In air operation at 5 bar absolute pressure and 95 °C a maximum in the power density of about 450 mW cm–2 has been obtained.     

Modeling liquid water transport in gas diffusion layers by topologically equivalent pore network

A topologically equivalent pore network (TEPN) model is developed for the first time to extract pore networks directly from gas diffusion layer (GDL) microstructures and thus account for all structural features of a GDL material. A generic framework of TEPN modeling is presented to design GDL structures that enable improved water management. With TEPNs used as input to a two-phase flow simulator, constitutive relations and steady-state liquid saturation profiles for carbon paper and carbon cloth are obtained and reported in this work. The results indicate a strong influence of the GDL morphology on water transport characteristics, which helps unravel the structure-performance relationship for GDLs.     

Effects of hydrophobic polymer content in GDL on power performance of a PEM fuel cell

Effects of hydrophobic polymer content within a carbon paper, used as the cathode gas diffusion layer (GDL), on power performance of a H₂/air proton exchange membrane fuel cell (PEMFC) have been studied. Electrochemical methods are used in conjunction with morphology and wetting property characterization. Surface contact angle of wet-proof-treated GDL as a function of temperature is measured by a novel capillary rise method. It is shown that the contact angle generally decreases with the temperature, and that there is insignificant difference in contact angle on carbon papers treated with different contents of fluorinated ethylene propylene (FEP) ranging from 10 to 40 wt.%. Under all humidification conditions in this study, a membrane-electrode assembly (MEA) consisting of 10 wt.% FEP-impregnated GDL shows higher power densities than 30 wt.% FEP-impregnated one. Surface morphology of the hydrophobic polymer-treated carbon paper has been analyzed by scanning electron microscopy (SEM) and is identified as playing a crucial role in affecting the power performance of such treated GDL in the PEM fuel cell.     

Influence of the structure in low-Pt loading electrodes for polymer electrolyte fuel cells

The influence of the structure and composition of the electrodes on a polymer electrolyte fuel cell (PEFC) performance has been investigated. Electrodes have been prepared by varying the composition of diffusion and/or catalyst layer. Improvements have been obtained by introducing a hydrophobic carbon layer between the carbon paper and the catalyst layer for the gas diffusion backing. High performance has been achieved with low Pt-loading electrodes (0.15 mg/cm²) by including the ionomer Nafion in the catalyst ink. Electrodes have been characterized by SEM-EDX analyses and electrochemical tests in a 50 cm² single cell.     

Characterization of membrane electrode assemblies in polymer electrolyte fuel cells using a.c. impedance spectroscopy

The most common methods used to characterize the electrochemical performance of fuel cells are to record current–voltage U(i) curves. However, separation of electrochemical and ohmic contributions to the U(i) characteristics requires additional experimental techniques. The application of electrochemical impedance spectra (EIS) is an approach to determine parameters which have proved to be indispensable for the development of fuel cell electrodes and membrane electrode assemblies (MEAs). This paper proves that it is possible to split the cell impedance into electrode impedances and electrolyte resistance by varying the operating conditions of the fuel cell (current load) and by simulation of the measured EIS with an equivalent circuit. Furthermore, integration in the current density domain of the individual impedance elements enables the calculation of the individual overpotentials in the fuel cell and the determination of the voltage loss fractions.     

Model of cathode catalyst layers for polymer electrolyte fuel cells: The role of porous structure and water accumulation

The cathode catalyst layer (CCL) is the major competitive ground for reactant transport, electrochemical reaction, and water management in a polymer electrolyte fuel cell (PEFC). Our model, presented here, accounts for the full coupling of random porous morphology, transport properties, and electrochemical conversion in CCLs. It relates spatial distributions of water, oxygen, electrostatic potential, and reaction rates to the effectiveness of catalyst utilization, water handling capabilities, and voltage efficiency. A feedback mechanism, involving the non-linear coupling between liquid water accumulation and oxygen depletion is responsible for the transition from a state of low partial saturation with high voltage efficiency to a state with excessive water accumulation that corresponds to highly non-uniform reaction rate distributions and large voltage losses. The transition between these states could be monotonous or it could involve bistability in the transition region. We introduce stability diagrams as a convenient tool for assessing CCL performance in dependence of composition, porous structure, wetting properties, and operating conditions.     

Impact of carbon monoxide on PEFC catalyst carbon support degradation under current-cycled operating conditions

It is well known that, even at ppm levels, the presence of CO in a PEFC anode feed stream has a significant impact on the MEA performance. Numerous work on short-term CO impact on PEFC performance under steady-state current demands has been carried out. However, to the best of our knowledge, the impact of long-term (i.e., >600 h) CO contamination on intrinsic Pt and C support aging (Pt oxidation/dissolution/ripening, C oxidation, …) under current-cycled operating conditions has never been explored. In this paper, on the basis of a combined theoretical and experimental approach, we investigate the long-term CO effect on PEFC performance and degradation. Firstly, on the basis of our previously published PEFC materials degradation models, we suggest that anodic CO poisoning could be used to mitigate the cathodic carbon catalyst-support corrosion phenomena and thus to enhance the MEA durability. Secondly, endurance experiments are performed on single fuel cells with current-cycled protocols representative of transport applications. The impact of CO on electrochemical transient response shows a reasonable agreement with simulated behaviors, and it is experimentally demonstrated that the impact of CO on the cell potential degradation rate is strongly dependent on the current-cycle mode.     

Polymer membranes for high temperature proton exchange membrane fuel cell: Recent advances and challenges

Proton-exchange membrane fuel cells (PEMFCs) are considered to be a promising technology for efficient power generation in the 21st century. Currently, high temperature proton exchange membrane fuel cells (HT-PEMFC) offer several advantages, such as high proton conductivity, low permeability to fuel, low electro-osmotic drag coefficient, good chemical/thermal stability, good mechanical properties and low cost. Owing to the aforementioned features, high temperature proton exchange membrane fuel cells have been utilized more widely compared to low temperature proton exchange membrane fuel cells, which contain certain limitations, such as carbon monoxide poisoning, heat management, water leaching, etc. This review examines the inspiration for HT-PEMFC development, the technological constraints, and recent advances. Various classes of polymers, such as sulfonated hydrocarbon polymers, acid-base polymers and blend polymers, have been analyzed to fulfill the key requirements of high temperature operation of proton exchange membrane fuel cells (PEMFC). The effect of inorganic additives on the performance of HT-PEMFC has been scrutinized. A detailed discussion of the synthesis of polymer, membrane fabrication and physicochemical characterizations is provided. The proton conductivity and cell performance of the polymeric membranes can be improved by high temperature treatment. The mechanical and water retention properties have shown significant improvement., However, there is scope for further research from the perspective of achieving improvements in certain areas, such as optimizing the thermal and chemical stability of the polymer, acid management, and the integral interface between the electrode and membrane.     

Organic-inorganic nanocomposite polymer electrolyte membranes for fuel cell applications

Organic-inorganic nanocomposite polymer electrolyte membrane (PEM) contains nano-sized inorganic building blocks in organic polymer by molecular level of hybridization. This architecture has opened the possibility to combine in a single solid both the attractive properties of a mechanically and thermally stable inorganic backbone and the specific chemical reactivity, dielectric, ductility, flexibility, and processability of the organic polymer. The state-of-the-art of polymer electrolyte membrane fuel cell technology is based on perfluoro sulfonic acid membranes, which have some key issues and shortcomings such as: water management, CO poisoning, hydrogen reformate and fuel crossover. Organic-inorganic nanocomposite PEM show excellent potential for solving these problems and have attracted a lot of attention during the last ten years. Disparate characteristics (e.g., solubility and thermal stability) of the two components, provide potential barriers towards convenient membrane preparation strategies, but recent research demonstrates relatively simple processes for developing highly efficient nanocomposite PEMs. Objectives for the development of organic-inorganic nanocomposite PEM reported in the literature include several modifications: (1) improving the self-humidification of the membrane; (2) reducing the electro-osmotic drag and fuel crossover; (3) improving the mechanical and thermal strengths without deteriorating proton conductivity; (4) enhancing the proton conductivity by introducing solid inorganic proton conductors; and (5) achieving slow drying PEMs with high water retention capability. Research carried out during the last decade on this topic can be divided into four categories: (i) doping inorganic proton conductors in PEMs; (ii) nanocomposites by sol-gel method; (iii) covalently bonded inorganic segments with organic polymer chains; and (iv) acid-base PEM nanocomposites. The purpose here is to summarize the state-of-the-art in the development of organic-inorganic nanocomposite PEMs for fuel cell applications.