Gas‐diffusion media (also known as gas diffusers and gas‐diffusion backings) are required in most polymer electrolyte fuel cell (PEFC) designs. Their function is to provide uniform reactant (H 2 , O 2 , and electrons) access to and product (H 2 O) removal from the electrodes, efficient heat removal from the membrane electrode assembly (MEA), and mechanical support to the MEA. The vast majority of gas‐diffusion media are based on carbon‐fiber materials; a variety of forms are used, with carbon‐fiber paper and carbon cloth receiving widest application. This chapter describes the production and properties of currently available and emerging materials. Commonly employed treatments and coatings used to tailor the wicking and hydrophobic properties of diffusion media for efficient water removal are discussed. Finally, ex‐situ and in‐situ methods for characterizing diffusion media are described.
Performance and durability of polymer electrolyte fuel cells are closely related to the water management. In gas diffusion layers (GDL), the presence of liquid water is associated with mass transport losses. For optimization of the materials, mechanisms and parameters influencing the water saturation have to be understood. Ex-situ water injection and withdrawal experiments, allowing for well-defined boundary conditions, have been performed with three different GDL materials, using X-ray tomographic microscopy to image the liquid water phase on the pore scale of the materials. The liquid saturation in the GDLs has been imaged as function of the capillary pressure. The results reveal that, due to the anisotropic structure of the GDLs, transport of water occurs mainly in the through-plane direction via parallel water paths. When the GDL is coated with a microporous layer (MPL), liquid saturation requires higher capillary pressure to overcome the MPL/GDL mixed region where pore and throat sizes are reduced and the water paths are restricted to the crack regions of the MPL.
Polymer electrolyte fuel cells (PEFC) require a sophisticated water management to operate efficiently, especially at high current densities which are needed to reach system cost targets. The description of the complicated two-phase water transport remains a challenge in PEFC models and requires experimental validation on various length scales. In this work, operando X-ray tomographic microscopy (XTM) with scan times of 10 s was used to depict the liquid water at defined conditions at a technically relevant cell temperature of 80 • C. Cells with Toray TGP-H-060 gas diffusion layer (GDL) with microporous layer (MPL) and different rib width were operated with different feed gas humidifications (under-and oversaturated) and current densities between 0.75 to 3.0 A/cm 2 . Based on the quantification of the local and average saturation, the distribution of water cluster size is analyzed. Different categories of the water cluster connectivity are defined and quantified. The analysis is complemented with numerical simulations of the permeability in the liquid phase of the GDL that is correlated to saturation for the different GDL domains. The numerical simulations of the pressure drop of liquid water flow from the catalyst layer toward the gas channels in channel-rib repetition units allows for conclusions on cluster growth mechanisms. In the past two decades, large developments have lead hydrogen fed polymer electrolyte fuel cell (PEFC) technology to the brink of commercialization, e.g. in the stationary sector with more than 100000 deployments in the Enefarm activity 1 as well as in the mobile sector where major car manufacturers have presented first commercial vehicles 2,3 and niche-market commercialization for logistic vehicles. 4While for the automotive market also hydrogen infrastructure is a major barrier for widespread application, in both fields of application, cost of the fuel cell system remains the main hindrance for extensive spread of PEFC technology. Cost is closely tied to materials and manufacturing processes, such as more effective electrocatalyst and more durable membrane materials. These developments are underway and have made significant progress. 5 Cost however is also closely tied to power density of the fuel cell stack. It is obvious that with higher power density less cells or an accordingly smaller cell area with the related reduced material use, is leading to a cost reduction if the high power density materials are of comparable cost.Automotive fuel cells with state of the art materials and cell structures reach today more than 1 W/cm 2 with current densities up to 3 A/cm 2 .5 At such high current densities water management becomes more and more important and has to be properly designed on various scales from the system level to nanoscale structures, including all cell components as flow field plates, gas diffusion layers (GDL), the polymer membrane and the catalyst layer (CL). In order to minimize the ohmic losses, the polymer membrane needs to be well humidified, 6 however at high current den...
During operation of polymer electrolyte fuel cells, water condenses in the porous structure of the gas diffusion layer (GDL). The condensed water limits efficiency and durability of the fuel cell. For optimization of the porous materials, understanding of the structure and characteristic length scale of the liquid water distribution is of crucial interest. X-ray tomographic microscopy was employed to image in situ the condensed water in GDLs of the type Toray TGP-H-060. It was found that the local water distribution pattern, created by the electrochemical reactions in the fuel cell, is mainly driven by the substrate structure on the micrometer scale, as repeatedly generated water patterns in the same structure have a local correlation. The concept of the representative equivalent area (REA) was employed to characterize the dry GDL structure and to identify the characteristic length scale of the liquid water phase. The dry fiber structure was found to have a representative area of 0.50 mm 2 . A similar area of 0.35−0.60 mm 2 is necessary for representing the water distribution characteristics with an error of 10% in a GDL with a liquid saturation of 42−49%. However, at a lower liquid saturation of 22− 25% the area increases to 1.35−1.60 mm 2 which indicates that the REA of the liquid saturation cannot be derived from the dry structure only.
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