Felix N. Büchi

Operating Proton Exchange Membrane Fuel Cells Without External Humidification of the Reactant Gases Fundamental Aspects

Operation of proton exchange membrane fuel cells (PEMFC) without external humidification of the reactant gases is advantageous for the PEMFC system, because it eliminates the need of a gas-humidification subsystem. The gas-humidification subsystem is a burden in the fuel cell system with respect to weight, complexity, cost, and parasitic power. A model for the operation of PEMFC with internal humidification of the gases is presented and the range of operating conditions for a PEMFC using dry H 2 /air was investigated. The model predicts that dry air, entering at the cathode, can be fully internally humidified by the water produced by the electrochemical reaction at temperatures up to 70°C. This model was experimentally verified for cell temperatures up to 60°C by long-term operation of a PEMFC with dry gases for up to 1800 h. The current densities, obtained at 0.6 V, were 20 to 40% lower than those measured when both gases were humidified. The water distribution in the cell, while operating with dry gases, was investigated by measuring the amount of product water on the anode and cathode sides. It was found that the back-diffusion of product water to the anode is the dominant process for water management in the cell over a wide range of operating conditions. The dominating water back-diffusion also allows internal humidification of the hydrogen reactant and prevents drying out of the anode.

Study of radiation-grafted FEP-G-polystyrene membranes as polymer electrolytes in fuel cells

Proton exchange membranes for fuel cell applications were synthesized by pre-irradiation grafting of styrene/divinylbenzene mixtures into poly(fluoroethylene-co-hexafluoropropylene) films and subsequent sulfonation. Grafting of pre-existing films overcomes the problem of shaping the grafted polymer into thin membranes and makes this process a potentially cheap and easy technique for the preparation of solid polymer electrolytes. The grafted membranes were characterized by measuring their ion exchange capacity, swelling, specific resistivity and area resistance. Due to their thickness in the range 67–211 μm, some of the membranes have a considerably lower resistance than the most widely used membrane Nafion® 117 (DuPont). The short-term and long-term performance of these membranes was investigated in H2O2 fuel cells. Thin (< 100 μm), highly crosslinked (12% divinylbenzene) membranes show the best performance in the fuel cells. Tests for periods of up to 1400 h were performed to examine membrane stability and the degradation of grafted membranes.

Polymer electrolyte fuel cell durability

Stack Components.- Dissolution and Stabilization of Platinum in Oxygen Cathodes.- Carbon-Support Requirements for Highly Durable Fuel Cell Operation.- Chemical Degradation of Perfluorinated Sulfonic Acid Membranes.- Chemical Degradation: Correlations Between Electrolyzer and Fuel Cell Findings.- Improvement of Membrane and Membrane Electrode Assembly Durability.- Durability of Radiation-Grafted Fuel Cell Membranes.- Durability Aspects of Gas-Diffusion and Microporous Layers.- High-Temperature Polymer Electrolyte Fuel Cells: Durability Insights.- Direct Methanol Fuel Cell Durability.- Influence of Metallic Bipolar Plates on the Durability of Polymer Electrolyte Fuel Cells.- Durability of Graphite Composite Bipolar Plates.- Gaskets: Important Durability Issues.- Cells and Stack Operation.- Air Impurities.- Impurity Effects on Electrode Reactions in Fuel Cells.- Performance and Durability of a Polymer Electrolyte Fuel Cell Operating with Reformate: Effects of CO, CO2, and Other Trace Impurities.- Subfreezing Phenomena in Polymer Electrolyte Fuel Cells.- Application of Accelerated Testing and Statistical Lifetime Modeling to Membrane Electrode Assembly Development.- Operating Requirements for Durable Polymer-Electrolyte Fuel Cell Stacks.- Design Requirements for Bipolar Plates and Stack Hardware for Durable Operation.- Heterogeneous Cell Ageing in Polymer Electrolyte Fuel Cell Stacks.- System Perspectives.- Degradation Factors of Polymer Electrolyte Fuel Cells in Residential Cogeneration Systems.- Fuel Cell Stack Durability for Vehicle Application.- R&D Status.- Durability Targets for Stationary and Automotive Applications in Japan.

Investigation of the Transversal Water Profile in Nafion Membranes in Polymer Electrolyte Fuel Cells

The in situ resistance of Nafion membranes with different thickness was measured in one-dimensional fuel cells as a function of current density. Except for the thin Nation I 12 membrane, an increase of the ionic resistance with current density (in the range 0 to I A/cm 2 ) was found. The thicker the membrane, the stronger the increase in the same current density interval. The resistance distribution across the thickness of membranes was determined by using membranes composed from several thin sheets with interlying thin gold wires as potential probes. It was found that the increase of the resistance is always confined to the membrane sheet contacting the anode electrode. These measurements, combined with the results from experiments with membranes of different water content, lead to the conclusion that the resistance increase at the anode side is due to the insufficient compensation of the electro-osmotic drag by the hack transport of water to the anode. Based on a solution diffusion mechanism of the water motion in the membrane, the experimental results may he explained by a mechanism whereby the electro-osmotic drag coefficient is independent of the local membrane hydration and the water diffusion coefficient D H2O , is a strong function of the local membrane water content. The experimental data would, qualitatively, also he in line with a model proposing hack transport of water to the anode by convection of water in the submicropores of the membrane.

Determination of Material Properties of Gas Diffusion Layers: Experiments and Simulations Using Phase Contrast Tomographic Microscopy

Understanding the transport properties of porous materials plays an important role in the development and optimization of polymer electrolyte fuel cells (PEFCs). In this study numerical simulations of different transport properties are compared and validated with data obtained using recently developed experimental techniques. The study is based on a Toray TGP-H-060 carbon paper, a common gas diffusion layer (GDL) material in PEFC. Diffusivity, permeability, and electric conductivity of the anisotropic, porous material are measured experimentally under various levels of compression. A sample of the GDL is imaged with synchrotron-based X-ray tomography under three different compression levels. Based on these three-dimensional images, diffusivity, permeability, and conductivity are calculated numerically. Experimental and numerical results agree in general. Deviations are observed for the through-plane conductivity. An explanation for the discrepancy is presented and affirmed by numerical simulations on a virtually created structure model. This proves that numerical simulation based on tomography data is a versatile tool for the investigation and development of porous structures used in PEFCs.

In-situ resistance measurements of Nafion® 117 membranes in polymer electrolyte fuel cells

Abstract The resistance of the Nafion® 117 membrane in H 2 O 2 and H 2 air polymer electrolyte fuel cells (PEFCs) has been measured in situ using fast current pulses. The dependence of the membrane resistance on current density, temperature, pressure and flow-field design was investigated. It was found that, independent of other variations, the resistance increases with increasing current density. When the current density in the cell is increased from 0.2 to 0.7 A cm−2, the membrane resistance increases by up to 22%. Even on open circuit the resistance at 60°C is 15%–35% higher than that measured ex situ, indicating that the membrane is not fully hydrated under the fuel cell operating conditions. The resistance on open circuit also depends on the design of the flow field. In a design with forced gas convection the resistance at 60°C is substantially higher (210 mΩ cm2) than in a design without forced convection (186 mΩ cm2).

Fuel-Cell Hybrid Powertrain: Toward Minimization of Hydrogen Consumption

In this paper, the powertrain sizing of a fuel-cell hybrid vehicle (FCHV) is investigated. The goal is to determine the fuel-cell system (FCS) size, together with the energy storage system (ESS) size, which leads to the lowest hydrogen consumption. The power source (FCS + ESS) capabilities should also respect the vehicle driveability constraints. Batteries and supercapacitors are considered as ESSs. The power management strategy is a global optimization algorithm respecting charge sustaining of the ESS. The impacts of the driving cycle (urban, outer urban, and highway), ESS technology, and vehicle driveability constraints on hydrogen consumption are analyzed in detail.

Performance of Differently Cross‐Linked, Partially Fluorinated Proton Exchange Membranes in Polymer Electrolyte Fuel Cells

A series of differently cross-linked FEP-g-polystyrene proton exchange membranes has been synthesized by the preirradiation grafting method [FEP: poly(tetrafluoroethylene-co-hexafluoropropylene)]. Divinylbenzene (DVB) and/or triallyl cyanurate (TAC) were used as cross-linkers in the membranes. It was found that the physical properties of the membranes, such as water-uptake and specific resistance, are strongly influenced by the nature of the cross-linker. Generally it can be stated that DVB decreases water-uptake and increases specific resistance; on the other hand TAC increases swelling and decreases specific resistance to values as low as 5.0 {Omega} cm at 60 C. The membranes were tested in H{sub 2}/O{sub 2} fuel cells for stability and performance. It was found that thick (170 {micro}m) DVB cross-linked membranes showed stable operation for 1,400 h at temperatures up to 80 C. The highest power density in the fuel cell was found for the DVB and TAC double-cross-linked membrane; it exceeded the value of a cell with a Nafion{reg_sign} 117 membrane by more than 60%.

Progress in In Situ X-Ray Tomographic Microscopy of Liquid Water in Gas Diffusion Layers of PEFC

Water management is an important factor for optimizing polymer electrolyte fuel cells (PEFC) under high current density conditions as required for the automotive application. The characteristics of the local liquid saturation of the gas diffusion layer (GDL) is of particular interest. Here we report on the development of in-situ X-ray tomographic microscopy (XTM) with a pixel sizes in the order of 2 μm and sensitivity for carbon and liquid water for the quantitative analysis of liquid water in GDLs. In-situ XTM of PEFC is a major experimental challenge. A complete cell needs to be operated under realistic conditions in the constraint space of the small field of view on the beamline sample stage. Further phase segmentation of the images is required to successfully analyze the quantitative properties of the different phases. For this a workflow, applying differential images between dry and wet structures has been developed. Cells with Toray TGP-H-060 GDLs were analyzed in-situ. Droplets that appear on the GDL surface are connected to a significant water structure inside the GDL. Further the water cluster size distribution in the GDL shows that while small droplets (<100 pl) are numerous, most of the water is contained in few larger clusters.

Crosslinked ion exchange membranes by radiation grafting of styrene/divinylbenzene into FEP films

Abstract Ion exchange membranes were prepared by simultaneous radiation grafting of styrene into FEP films and subsequent sulfonation. Divinylbenzene was used as crosslinker in the grafting medium. The distribution of polystyrene grafts across the membrane matrix was determined by microprobe measurements. It was observed that for low levels of grafting such as 13%, crosslinked membranes had a better homogeneity in graft distribution across its matrix as compared to the non-crosslinked ones. The latter showed a much higher concentration of grafts at the surface than in the middle of the film. The membrane characteristics, such as swelling and specific resistivity as a function of the degree of grafting and crosslinking were evaluated. Membranes showed a sharp decrease in the resistivity up to ∼ 25% grafting, beyond which the decrease was not significant. The higher the crosslinker content, the higher was the ionic resistivity. A relation between the degree of grafting and membrane properties, such as degree of swelling and specific resistivity as a function of the crosslinker content was established.