Liquid water transport in a mixed-wet gas diffusion layer of a polymer electrolyte fuel cell

After PTFE treatment, a gas diffusion layer (GDL) of a polymer electrolyte fuel cell (PEFC) features mixed wettability, which substantially impacts liquid water transport and associated mass transport losses. A pore-network model is developed in this work to delineate the effect of GDL wettability distribution on pore-scale liquid water transport in a GDL under fuel cell operating conditions. It is found that in a mixed-wet GDL liquid water preferentially flows through connected GDL hydrophilic network, and thereby suppresses the finger-like morphology observed in a wholly hydrophobic GDL. The effect of GDL hydrophilic fraction distribution is investigated, and the existence of an optimum hydrophilic fraction that leads to the least mass transport losses is established. The need for controlled PTFE treatment is stressed, and a wettability-tailored GDL is proposed.     

Pore-network analysis of two-phase water transport in gas diffusion layers of polymer electrolyte membrane fuel cells

A pore-network model was developed to study the water transport in hydrophobic gas diffusion layers (GDLs) of polymer electrolyte membrane fuel cells (PEMFCs). The pore structure of GDL materials was modeled as a regular cubic network of pores connected by throats. The governing equations for the two-phase flow in the pore-network were obtained by considering the capillary pressure in the pores, and the entry pressure and viscous pressure drop through the throats. Numerical results showed that the saturation distribution in GDLs maintained a concave shape, indicating the water transport in GDLs was strongly influenced by capillary processes. Parametric studies were also conducted to examine the effects of several geometrical and capillary properties of GDLs on the water transport behavior and the saturation distribution. The proper inlet boundary condition for the liquid water entering GDLs was discussed along with its effects on the saturation distribution.     

Investigation of liquid water in gas diffusion layers of polymer electrolyte fuel cells using X-ray tomographic microscopy

In polymer electrolyte fuel cells (PEFCs), condensation of water within the pore network of the gas diffusion layers (GDL) can influence the gas transport properties and thus reduce the electrochemical conversion rates. The use of X-ray tomographic microscopy (XTM), which allows for a resolution in the order of one micrometer is investigated for studying ex situ the local saturation in GDL's. The strength of XTM is the high spatial resolution with simultaneous contrast for water and carbon, allowing for non-destructive 3D-imaging of the solid and the contained water. The application of this method for imaging the ex situ water intrusion into the porous network of GDLs is explored using absorption and phase contrast methods. It is shown that the inhomogeneous filling behavior of GDL materials can indeed be visualized with sufficient resolution. For Toray paper TGP-H-060 the local saturation was measured as function of the water pressure. The results, evaluated in 1D, 2D and 3D show a liquid water retention effect at the denser layers near the surface. A comparison with established capillary pressure functions is presented. Altogether, the results show the potential of the XTM-method as a tool for studying the liquid water behavior in PEFC on a microscopic scale.     

Two-phase transport and the role of micro-porous layer in polymer electrolyte fuel cells

Two-phase transport of reactants and products constitutes an important limit in performance of polymer electrolyte fuel cells (PEFC). Particularly, at high current densities and/or low gas flow rates, product water condenses in open pores of the cathode gas diffusion layer (GDL) and limits the effective oxygen transport to the active catalyst sites. Furthermore, liquid water covers some of the active catalytic surface, rendering them inactive for electrochemical reaction. Traditionally, these two-phase transport processes in the GDL are modeled using so-called unsaturated flow theory (UFT), in which a uniform gas-phase pressure is assumed across the entire porous layer, thereby ignoring the gas-phase flow counter to capillarity-induced liquid motion. In this work, using multi-phase mixture (M²) formalism, the constant gas pressure assumption is relaxed and the effects of counter gas-flow are studied and found to be a new oxygen transport mechanism. Further, we analyze the multi-layer diffusion media, composed of two or more layers of porous materials having different pore sizes and/or wetting characteristics. Particularly, the effects of porosity, thickness and wettability of a micro-porous layer (MPL) on the two-phase transport in PEFC are elucidated.     

Investigation of mass transport in gas diffusion layer at the air cathode of a PEMFC

In a polymer electrolyte membrane fuel cell (PEMFC), slow diffusion in the gas diffusion electrode may induce oxygen depletion when using air at the cathode. This work focuses on the behavior of a single PEMFC built with a Nafion^® based MEA and an E-TEK gas diffusion layer and fed at the cathode with nitrogen containing 5, 10 and 20% of oxygen and working at different cell temperatures and relative humidities. The purpose is to apply the experimental impedance technique to cells wherein transport limitations at the cathode are significant. In parallel, a model is proposed to interpret the polarization curves and the impedance diagrams of a single PEMFC. The model accounts for mass transport through the gas diffusion electrode. It allows us to qualitatively analyze the experimental polarization curves and the corresponding impedance spectra and highlights the intra-electrode processes and the influence of the gas diffusion layer.     

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.     

Effect of gas diffusion layer compression on the performance in a proton exchange membrane fuel cell

This study investigates the gas permeability, bulk density, thickness and conductivity of two types of gas diffusion layer (OC14, NC14) as a function of the compressed thickness. The compression of a gas diffusion layer reduces gas permeability and contact resistance. The performance is measured using a single proton exchange membrane fuel cell (PEMFC) with an active area of 25 cm². The results provide an optimum value of compression ratio that maximizes the cell performance. For OC14 the optimum compression ratio is about 64%, whereas for NC14 it is 59%. The best performances are 375 mA/cm² and 296 mA/cm² at 0.7 V, respectively. These results concerning the balance between compression and performance provide vital information for the fabrication of stacks and support for industrial applications.     

Modification and durability of carbon paper gas diffusion layer in proton exchange membrane fuel cell

To solve the durability problem caused by carbon corrosion in the porous carbon paper gas diffusion layer (GDL) of proton exchange membrane fuel cell (PEMFC), a Cr₇C₃ ceramic coating on porous carbon paper by molten salt method was prepared. The obtained Cr₇C₃ coating is dense and shows good adherent with the carbon paper. Based on the corrosion resistance testes, it is concluded that the Cr₇C₃ coated carbon paper can keep high chemical stability in the long-time acid immersion accelerated corrosion tests and simulated cathodic proton exchange membrane fuel cells environment. The carbon paper with Cr₇C₃ coating shows a small current density with a value of about 1.1 × 10^?5 A cm^?2 in the potentiostatic polarization test under 1 M H₂SO₄ + 2 ppm HF at 70 °C and1.4 V_SCE for 72 h. Meanwhile, the resistivity of carbon paper with Cr₇C₃ coating prepared is 30% lower than that of pure carbon paper, and the conductivity is 1.5 times that of pure carbon paper.     

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.     

On the modeling of water transport in polymer electrolyte membrane fuel cells

Water management is a critical issue in polymer electrolyte membrane (PEM) fuel cells, and water transport through the membrane, catalyst layer and gas diffusion layer has significant impact on the cell performance and durability. In this study, the mechanism of water transport processes in PEM fuel cells has been analyzed through 3-D unsteady non-isothermal simulations, along with a comprehensive examination of various modeling approaches in literature. It is shown that the finite rates of sorption/desorption of water in membrane affect the cell current output and the cell response time. Water dissolved in the membrane should be taken as the proper mechanism of water formation in the cathode of practical PEM fuel cells. Capillary pressure and relative permeability have significant impact on the distribution of liquid water saturation and transport, implying a need for their determination under specific PEM fuel cell conditions.     

Gas diffusion layer with titanium carbide for a unitized regenerative fuel cell

In this work, a gas diffusion layer (GDL) prepared with metallic ceramics TiC for a unitized regenerative fuel cell (URFC) was first investigated. By the measurements of morphological characteristic, electrical conductivity, absolute through-plane permeability and electrochemical stability, the characteristics of the novel GDLs and the conventional one were compared. A high corrosion-resistive and low-cost GDL with 80 wt.% TiC and 20 wt.% IrTiO_x was expected to enhance the cycle performance of URFC. And the total loading of Ir in the novel URFC was only 1.3 mg cm⁻². The URFC with the novel GDL exhibited the similar initial performance under both fuel cell and electrolysis modes as that using the conventional GDL. However, the life cycle testing over 60 h showed that the URFC with the novel GDL was more stable than the URFC with the traditional GDL, indicating that the GDL with TiC and IrTiO_x was beneficial to improve the cycle life of the URFC.     

Simulation studies on the fuel electrode of a H2---O2 polymer electrolyte fuel cell

The macro-homogeneous porous electrode theory is used to develop a model which describes the catalyst layer of the hydrogen electrode formed by catalyst particles that are bonded to the membrane. The water transport in the catalyst layer and polymer electrolyte membrane is considered. The effects of catalyst layer structure parameters such as polymer volume fraction, catalyst layer thickness, platinum loading and reactant gas humidity as well as CO poison on the hydrogen electrode behavior are examined. The results show that the catalyst layer thickness has a significant effect on the electrode performance. A thicker catalyst layer will result in a larger ohmic voltage loss and higher catalyst cost. The optimal polymer volume fraction and catalyst layer thickness are 0.5 and 1.5–4 μm, respectively, for this electrode. An optimal platinum surface coverage on carbon need not exceed 20% (20 wt% Pt/C). Larger platinum coverage will increase the cost, but only slightly enhance the electrode performance.     

Oxygen mass transport limitations at the cathode of polymer electrolyte membrane fuel cells

Oxygen transport across the cathode gas diffusion layer (GDL) in polymer electrolyte membrane (PEM) fuel cells was examined by varying the O₂/N₂ ratio and by varying the area of the GDL extending laterally from the gas flow channel under the bipolar plate (under the land). As the cathode is depleted of oxygen, the current density becomes limited by oxygen transport across the GDL. Oxygen depletion from O₂/N₂ mixtures limits catalyst utilization, especially under the land.The local current density with air fed PEM fuel cells falls to practically zero at lateral distances under the land more than 3 times the GDL thickness; on the other hand, catalyst utilization was not limited when the fuel cell cathode was fed with 100% oxygen. The ratio of GDL thickness to the extent of the land is thus critical to the effective utilization of the catalyst in an air fed PEM fuel cell. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Influence of hydrophilicity in micro-porous layer for polymer electrolyte membrane fuel cells

Water management is one of the most important factors for improving the performance in polymer electrolyte membrane fuel cells (PEMFCs). The micro-porous layers (MPLs) in the membrane-electrode assembly provide proper pores and paths for mass transport, thereby allowing for the control of the water balance. In this study, a copolymer containing hydrophilic functional groups is introduced into the binder materials of the MPL instead of a highly hydrophobic binder. When 10 wt.% of the binder is incorporated in the MPL on the cathode side, the best performance is exhibited and the ohmic resistance is decreased. Although the charge transfer resistance at low potential is higher than that of the hydrophobic treated MPL, due to the flooding effects, the charge transfer resistance at high potential becomes smaller. This indicates that excess liquid absorption from the catalyst layer to the hydrophilic MPL occurs more strongly than in the case of the hydrophobic MPL. This may bring about an increase in the accessibility of oxygen to the active sites, because the excess liquid near the catalyst agglomerates is expelled as fast as possible. Consequently, the hydrophilicity control in the MPL has a positive effect on the water management in PEMFCs.     

Characteristics of subzero startup and water/ice formation on the catalyst layer in a polymer electrolyte fuel cell

This work experimentally explores the fundamental characteristics of a polymer electrolyte fuel cell (PEFC) during subzero startup, which encompasses gas purge, cool down, startup from a subfreezing temperature, and finally warm up. In addition to the temperature, high-frequency resistance (HFR) and voltage measurements, direct observations of water or ice formation on the catalyst layer (CL) surface have been carried out for the key steps of cold start using carbon paper punched with microholes and a transparent cell fixture. It is found that purge time significantly influences water content of the membrane after purge and subsequently cold-start performance. Gas purge for less than 30 s appears to be insufficient, and that between 90 and 120 s is most useful. After gas purge, however, the cell HFR relaxation occurs for longer than 30 min due to water redistribution in the membrane-electrode assembly (MEA). Cold-start performance following gas purge and cool down strongly depends on the purge time and startup temperature. The cumulative product water measuring the isothermal cold-start performance increases dramatically with the startup temperature. The state of water on the CL surface has been studied during startup from ambient temperatures ranging from −20 to −1 °C. It is found that the freezing-point depression of water in the cathode CL is 1.0 ± 0.5 °C and its effect on PEFC cold start under automotive conditions is negligible.     

Influence of the Teflon loading in the gas diffusion layer of PBI-based PEM fuel cells

The influence of the PTFE content in commercial Toray graphite paper gas diffusion layer (GDL) on the performance of a PBI-based polymer electrolyte membrane fuel cell (PEMFC) has been studied. These materials have been characterised by evaluating the porosity, pore size distribution, SEM micrographs, hydrophobicity, air permeability and electrical resistance. Fuel cell results show that the lower the Teflon content, the better the cell performance and the lower the losses when oxygen was replaced by air. These results led to non-Teflonized carbon paper to be postulated as the most suitable candidate, provided that its mechanical integrity can be maintained throughout the whole process of preparation and testing of the MEA. However, some practical problems with this type of commercial non-Teflonized carbon paper were experienced in this work and led to damage of the support. The detrimental effects are described and discussed. As conclusion, the use of a minimally PTFE-loaded (10%) carbon paper is suggested because the inclusion of this level of Teflon improved properly the mechanical properties of the carbon support and only caused a very small drop in the performance.     

Identification of liquid water constraints in micro polymer electrolyte fuel cells without gas diffusion layers

A simplified, miniaturized polymer electrolyte fuel cell without gas diffusion layers was investigated under operation by neutron radiography. By visualizing liquid water, it was possible to identify limiting effects, which are directly related to the simplified construction principle. Depending on the operation conditions, undesired water accumulation either in particular micro-channels or on the cathode catalyst layer as well as drying of the anode catalyst layer was observed. As a consequence, the design of a fuel cell without gas diffusion layers must take into account these limitations visualized by neutron radiography.     

Influence of the PTFE content in the diffusion layer of low-Pt loading electrodes for polymer electrolyte fuel cells

The influence of the structure and composition of the diffusion layer on polymer electrolyte fuel cell (PEFC) cathode performance was investigated. Electrodes were prepared with different poly-tetrafluoroethylene (PTFE) content in the diffusion layer and maintaining a constant composition for the catalytic layer with a low-Pt loading (0.11 mg cm⁻²). Electrodes were characterized by Hg-intrusion porosimetry, scanning electron microscopy and electrochemical techniques (cyclic voltammetry, galvanostatic polarization and ac-impedance spectroscopy).     

Effect of carbon paper substrate of the gas diffusion layer on the performance of proton exchange membrane fuel cell

Gas diffusion layers (GDLs) were fabricated using non-woven carbon paper substrates with various thicknesses developed by Hollingsworth & Vose Co. Highly consistent carbon slurry containing Pureblack carbon and vapor grown carbon fiber (3:1 ratio) with 25 wt.% Teflon was prepared by using a dispersion agent, Novec-7300 in isopropyl alcohol. Micro-porous layer was coated by using a fully automated Coatema coating tool with a uniform carbon loading of 2.6-3 mg cm⁻² using carbon slurry. The surface morphology, contact angle and pore size distribution of the GDLs were examined using SEM, Goniometer and Hg Porosimeter, respectively. Various cathode GDLs assembled into MEAs were evaluated in a single cell PEMFC under various operating relative humidity (RH) conditions using H₂/O₂ and H₂/air as reactants. The peak power density of the single cell using the optimum carbon paper substrate thickness was about 1400 and 700 mW cm⁻² with H₂/O₂ and H₂/air at 60% RH, respectively. It was found that the pore diameter as well as the corresponding pore volumes of the GDLs played a key role in exhibiting the optimum fuel cell performance.