Polymer electrolyte fuel cells functions

As can be seen from Eigure 11b, the output voltage of a fuel cell decreases as the electrical load is increased. The theoretical polarization voltage of 1.23 V/cell (at no load) is not actually realized owing to various losses. Typically, soHd polymer electrolyte fuel cells operate at 0.75 V/cell under peak load conditions or at about a 60% efficiency. The efficiency of a fuel cell is a function of such variables as catalyst material, operating temperature, reactant pressure, and current density. At low current densities efficiencies as high as 75% are achievable. 

Numerous demonstrations in recent years have shown that the level of performance of present-day polymer electrolyte fuel cells can compete with current energy conversion technologies in power densities and energy efficiencies. However, for large-scale commercialization in automobile and portable applications, the merit function of fuel cell systems—namely, the ratio of power density to cost—must be improved by a factor of 10 or more. Clever engineering and empirical optimization of cells and stacks alone cannot achieve such ambitious performance and cost targets. 

The second contribution spans an even larger range of length and times scales. Two benchmark examples illustrate the design approach polymer electrolyte fuel cells and hard disk drive (HDD) systems. In the current HDDs, the read/write head flies about 6.5 nm above the surface via the air bearing design. Multi-scale modeling tools include quantum mechanical (i.e., density functional theory (DFT)), atomistic (i.e., Monte Carlo (MC) and molecular dynamics (MD)), mesoscopic (i.e., dissipative particle dynamics (DPD) and lattice Boltzmann method (LBM)), and macroscopic (i.e., LBM, computational fluid mechanics, and system optimization) levels. 

Figure 5.30. Schematic of the catalyst layer geometry and its composition, exhibiting the different functional parts, a A sketch of the layer, used to construct a continuous model, b A one-dimensional transmission-line equivalent circuit where the elementary unit with protonic resistivity Rp, the charge transfer resistivity Rch and the double-layer capacitance Cj are highlighted [34], (Reprinted from Journal of Electroanalytical Chemistry, 475, Eikerling M, Komyshev AA. Electrochemical impedance of the cathode catalyst layer in polymer electrolyte fuel cells, 107-23, 1999, with permission from Elsevier.)...

Xie, Z. et ah. Functionally graded cathode catalyst layers for polymer electrolyte fuel cells, J. Electrochem. Soc., 152, A1171, 2005. 

Figure 3.52. Efficiency of a reversible PEM fuel cell as a function of the amount (at. % or mol %) of Ir in the form of IrOj relative to Pt in the positive electrode catalyst, for fuel cell electricity production (EC) or for water electrolysis (WE). Also the product of the two efficiencies relevant for storage cycles is shown. The catalyst is otherwise similar to that of Fig. 3.51, with PTFE and Nation channels. (From T. loroi, K. Ya-suda, Z. Siroma, N. Fujiwara, Y. Miyazaki (2002). Thin film electrocatalyst layer for unitized regenerative polymer electrolyte fuel cell. J. Power Sources 112, 583-587. Used by permission from Elsevier. See also loroi et al. (2004).

From all that has been said above, it can be concluded that polymer electrolyte membrane fuel cells, working at elevated temperatures, are highly promising. Many difficulties must still be overcome in order to develop models, which will function in a stable and reliable manner, and for extended periods of time. At present, about 90% of all publications on fuel cells are concerned precisely with the attempts to overcome these difficulties. Most of the publications deal with research into new varieties of membrane materials. Some results of these works are described in the reviews on elevated-temperature-polymer electrolyte fuel cells (Zhang et al., 2006 Shao et al., 2007). 

The three components of the fuel cell, anode, cathode, and electrolyte form a membrane-electrolyte assembly, as, by analogy with polymer electrolyte fuel cells, one may regard the thin layer of solid electrolyte as a membrane. Any one of the three membrane-electrode assembly components can be selected as the entire fuel cell s support and made relatively thick (up to 2 mm) in order to provide mechanical stability. The other two components are then applied to this support in a different way as thin layers (tenths of a millimeter). Accordingly, one has anode-supported, electrolyte-supported, and cathode-supported fuel cells. Sometimes though an independent metal or ceramic substrate is used to which, then, the three functional layers are applied. 

Platinum-Based Cathode Catalysts for Polymer Electrolyte Fuel Cells, Fig. 2 DFT activities as a function of the oxygen binding energy derived by density functional theory (DFT) calculations. The volcano is in good agreement with experiment, showing that Pt is the best catalysts for oxygen reduction (Ref. [16])... 

Polymer Electrolyte Fuel Cells, Mass Transport, Fig. 3 Effective relative diffusivity e/x as a function of the porosity of plain Toray 060 GDL (without PTFE) for different compressions (expressed in porosity), ip in-plane, tp through-plane. The gray lines indicate iso-tortuosity levels... 

Zh. Xie, T. Navessin, K. Shi, R. Chow, Q. Wang, D. Song, B. Andreaus, M. Eikerling, Zh. Liu, and S. Holdcroft. Functionally graded cathode catalyst layers for polymer electrolyte fuel cells. J. Electrochem. Soc., 152 A1171—A1179, 2005. 

Maruyama J, Abe I (2006) Carbonized hemoglobin functioning as a cathode catalyst for polymer electrolyte fuel cells. Chem Mater 18 1303-1311... 

Sambandam, S., Ramani, V. 2007. SPEEK/functionalized silica composite membranes for polymer electrolyte fuel cells. Journal of Power Sources, 170,259-267. 

FIGURE 1.1 Layout of a polymer electrolyte fuel cell, showing functional components and processes. FF, DM, and CL abbreviate flow field, diffusion media, and catalyst layer, respectively. Note that throughout this book, DM will also be referred to as a GDL (gas diffusion layer). 

FIG U RE 4.10 Stability diagram showing the stable state of operation of the CCL as a function of the pore volume fraction of secondary pores, Xm, and jq. (Reprinted from Electmchim. Acta, 53(13), Liu, J., and Eikerling, M. Model of cathode catalyst layers for polymer electrolyte fuel cells The role of porous structure and water accumulation. 4435 1446. Copyright (2008), Elsevier. With permission.)... 

Kannan R, Kakade BA, Pillai VK (2008) Polymer electrolyte fuel cells using Nafion-based composite membranes with functionalized carbon nanotubes. Angew Chem Int Ed 47 (14) 2653-2656... 

For most polymer electrolyte fuel cell (PEFC) systems, unit cells are combined into a PEFC stack to achieve the voltage and power output level required for the appUcation. The unit cells normally use a membrane electrode assembly, consisting of a sohd polymer electrolyte with an associated gas-diffusion layer sandwiched between two bipolar plates. The bipolar plates make connections over the entire surface of one cathode and the anode of the next cell. These bipolar plates serve several functions simultaneously (1) they create channels for fuel, air, and water, (2) they separate the unit cells in the stack, (3) they carry current away from the cell, and (4) they support the membrane electrode assembly (Lee et al. 2006 Shao et al. 2007). 

Polymer electrolyte fuel cell (PEFC) components generally have to fulfill the following four technical requirements to allow for an effective long-term fuel cell operation functionality, which is mostly determined by the product design service life which is influenced by the material as well as the design reliability, essentially determined by the manageability and process stability and sustainability, which is predominantly influenced by function integration and series production. 

Abstract This chapter describes the behavior and stability of metallic bipolar plates in polymer electrolyte fuel ceU application. Fundamental aspects of metallic bipolar plate materials in relation to suitability, performance and cell degradation in polymer electrolyte fuel cells are presented. Comparing their intrinsic functional properties with those of carbon composite bipolar plates, we discuss different degradation modes and causes. Furthermore, the influence and possible improvement of the materials used in bipolar plate manufacturing are described. 

Fig. 2.13 Comparison in efficiency among sohd oxide fuel cells, polymer electrolyte fuel cells and gas engines as a function of size of generators [Courtesy of Osaka Gas]...

Xie Z, Navessin T, Shi K, Chow R, Wang Q, Song D, Andreaus B, Eikerling M, Liu Z and Holdcroft S (2005) Functionally Graded Cathode Catalyst Layers for Polymer Electrolyte Fuel Cells II. Experimental Study of the Effect of Nafion Distribution, J. Electrochem. Soc., 152, pp. A1171-A1179. 

Umeda, J Suzuki, M Kato, M Moriya, M. Sakamoto, W. Yogo, T. (2010). Proton conductive inorganic-organic hybrid membranes functionalized with phosphonic acid for polymer electrolyte fuel cell. /. Power Sources, 195, 5882-5888, ISSN 0378-7753. 

A polymer electrolyte must contain sufficient water for the fuel cell to function but how much water is sufficient Water is a product of the fuel cell reaction. Can the fuel cell make enough water to keep the fuel cell functioning If the feed... 

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