In PEM fuel cells

Darling RM, Meyers JP. 2005. Mathematical model of platinum movement in PEM fuel cells. J Electrochem Soc 152 A242-A247. 

Healy J, Hayden C, Xie T, Olson K, Waldo R, Brundage A, Gasteiger H, Abbott J. 2005. Aspects of the chemical degradation of PPSA ionomers used in PEM fuel cells. Fuel Cells 5 302-308. 

Liu P, Nprskov JK. 2001a. Kinetics of the anode processes in PEM fuel cells— The promoting effect of Ru in PtRu anodes. Fuel Cells 1 192. 

Polymers have served roles in PEM fuel cell cathodes such as modifiers to macrocycle-based electrodes to improve conductivity and stability,165 composite materials with heteropolyacids,166 and as precursors to pyrolyzed catalysts.38,112,132,133 However, as discussed in the previous section, the activity of nitrogen-containing carbon raises the possibility of non-metal electrodes functioning in a cathode environment. Likewise, researchers have noted ORR activity for various conducting polymers containing nitrogen, and recently studies on their potential use in PEM fuel cell cathodes have been reported. 

Carbides have been studied for ORR in acidic media.219,220 Tungsten carbide was shown to be promising for ORR in acidic media,219 though WC has a corrosion problem in acidic systems.221 To increase the stability of the catalyst in PEM fuel cell conditions, tantalum was added to tungsten carbide.220 The Ta-WC catalyst was tested under fuel cell conditions and compared to WC. The corrosion resistance was markedly improved as well as the activity for ORR. It is thought that a Ta-W alloy acted as a stabilizer for the catalyst while WC remained the active site for ORR.220... 

Research on alternative catalysts for the ORR for use in PEM fuel cell cathodes is an exciting and growing field of research. Several classes of materials show potential for replacing precious metal cathodes, especially for automotive power applications and direct methanol systems. Increasing the understanding of active sites in alternative catalysts, the mechanisms for oxygen reduction, and optimization of full fuel cell preparation using alternative materials, will lead to further improvements in performance. 

Mukerjee, S. and Urian, R.C., Bifunctionality in Pt alloy nanocluster electrocatalysts for enhanced methanol oxidation and CO tolerance in PEM fuel cells electrochemical and in situ synchrotron spectroscopy, Electrochim. Acta, 47, 3219, 2002. 

The effect of annealing temperatures (65 - 250 °C) and blend composition of Nafion 117, solution-cast Nafion , poly(vinyl alcohol) (PVA) and Nafion /PVAblend membranes for application to the direct methanol fuel cell is reported in [148], These authors have found that a Nafion /PVAblend membrane at 5 wt% PVA (annealed at 230 °C) show a similar proton conductivity of that found to Nafion 117, but with a three times lower methanol permeability compared to Nafion 117. They also found that for Nafion /PVA (50 wt% PVA) blend membranes, the methanol permeability decreases by approximately one order of magnitude, whilst the proton conductivity remained relatively constant, with increasing annealing temperature. The Nafion /PVA blend membrane at 5 wt% PVA and 230 °C annealing temperature had a similar proton conductivity, but three times lower methanol permeability compared to unannealed Nafion 117 (benchmark in PEM fuel cells). 

The properties and composition of the CL in PEM fuel cells play a key role in determining the electrochemical reaction rate and power output of the system. Other factors, such as the preparation and treatment methods, can also affect catalyst layer performance. Therefore, optimization of the catalyst layer with respect to all these factors is a major goal in fuel cell development. For example, an optimal catalyst layer design is required to improve catalyst... 

Two main types of catalyst layers are used in PEM fuel cells polyfefrafluo-roethylene (PTFE)-bound catalyst layers and thin-film catalyst layers [3]. The PTFE-bound CL is the earlier version, used mainly before 1990. If confains two components hydrophobic PTFE and Pt black catalyst or carbon-supported Pt catalyst. The PTFE acts as a binder holding the catalyst together to form a hydrophobic and structured porous matrix catalyst layer. This porous structure can simultaneously provide passages for reacfanf gas fransport to the catalyst surface and for wafer removal from fhe cafalysf layer. In fhe CL, the catalyst acts as both the reaction site and a medium for electron conduction. In the case of carbon-supported Pt catalysts, both carbon support and catalyst can act as electron conductors, but only Pt acts as the reaction site. 

To overcome these disadvantages, a thin-film CL technique was invented, which remains the most commonly used method in PEM fuel cells. Thin-film catalyst layers were initially used in the early 1990s by Los Alamos National Laboratory [6], Ballard, and Johnson-Matthey [7,8]. A thin-film catalyst layer is prepared from catalyst ink, consisting of uniformly distributed ionomer and catalyst. In these thin-film catalyst layers, the binding material is not PTFE but rather hydrophilic Nafion ionomer, which also provides proton conductive paths for the electrochemical reactions. It has been found that the presence of hydrophobic PTFE in thin catalyst layers was not beneficial to fuel cell performance [9]. 

Antoine et al. [28] inveshgated the gradient across the CL and found that the Pt utilization was dependent on the CL porosity. In a nonporous CL, catalyst utilization was increased through the preferential locahon of Pt close to the gas diffusion layer in a porous CL, catalyst utilization efficiency was increased through the preferential location of Pt close to the polymer electrolyte membrane. In PEM fuel cells, fhe CL has a porous structure, and better performance is expected if higher Pf loading is used af preferential locahons close to the membrane/catalyst layer interface. 

Zhang, H., Wang, X., Zhang, J., and Zhang, J. Conventional catalyst ink, catalyst layer, and MEA preparation. In PEM fuel cell electrocatalysts and catalyst layers Fundamentals and applications, ed. J. Zhang. London Springer, 2008. 

Bender, G., Zawodzinski, T. A., and Saab, A. P. Fabrication of high-precision PEFG membrane electrode assemblies. Journal of Power Sources 2003 124 114—117. Ihm, J. W., Ryu, H., Bae, J. S., Ghoo, W. K., and Ghoi, D. K. High performance of electrode with low Pt loading prepared by simplified direct screen printing process in PEM fuel cells. Journal of Materials Science 2004 39 4647--4649. 

Proton exchange membranes (PEMs) are a key component in PEM fuel cells (PEMECs) and an area of active research in commercial, government, and academic institutions. In this chapter, the review of PEM materials is divided into two sections. The first will cover the most important properties of a membrane in order for it to perform adequately within a PEMFC. The latter part of this chapter will then provide an overview of existing PEM materials from both academic and industrial research facilities. Wherever possible, the membranes will also be discussed with respect to known structure-property relationships. 

Liu, W., Ruth, K. and Rusch, G. 2001. Membrane durability in PEM fuel cells. Journal of New Materials for Electrochemical Systems 4 227-232. 

Savett, S. C., Atkins, J. R., Sides, G. R., Harris, J. L., Thomas, B. H., Greager, S. E., Pennington, W. T. and DesMarteau, D. D. 2002. A comparison of [(perfluoroalkyl) sulfonyl] imide ionomers and perfluorosulfonic acid ionics for applications in PEM fuel-cell technology. Journal of the Electrochemical Society 149 A1527-A1532. 

In direct liquid fuel cells, the use of MPLs is also very popular and most of the details explained earlier also apply to the liquid fuel cells. However, some of the parameters differ from those in PEM fuel cells because there are other mass transfer-based issues in DLFCs, especially on the anode side related to methanol crossover and CO2 production. 

Schmitz et al. [184] tested various carbon fiber papers with different thicknesses as cathode DLs in PEM fuel cells. It was observed that the cell resistance dropped when the thickness of the DL increased thus, thicker materials are desired in order to improve the electrical conductivity. It was also mentioned that the optimal thickness for the DL is usually between the thinnest and the thickest materials because the two extremes give the lowest performance. In fact, in thin DLs, the water produced can fill pores within the material, resulting in flooding and the blockage of available flow paths for the oxygen. Similarly, Lin and Nguyen [108] concluded that thinner DLs (without MPLs) were more prone to liquid water accumulation than thicker ones. 

In another, similar study, Mukundan et al. [260] performed 100 freeze-thaw cycles (from -40 to 80°C) with different types of CFPs and CCs. After 100 cycles, no obvious degradation was observed in the carbon cloth DL in fact, the performance of the fuel cell slightly improved. On the other hand, after 45 cycles, the CFPs showed significant breakage of the carbon fibers at the edges between the flow channels and the landing widths (or ribs). Thus, it was concluded that this breakage could potentially become a serious failure mechanism in PEM fuel cells when the system was started at subzero temperatures. 

M. Glora, M. Wiener, R. Petricevic, H. Probstle, and J. Fricke. Integration of carbon aerogels in PEM fuel cells. Journal of Noncrystalline Solids 285 (2001) 283-287. 

K. Jiao and B. Zhou, hmovative gas diffusion layers and their water removal characteristics in PEM fuel cell cathode. Journal of Power Sources 169 (2007) 296-314. 

S. Park, J. W. Lee, and B. N. Popov. Effect of PTFE content in microporous layer on water management in PEM fuel cells. Journal of Power Sources 177 (2008) 457-463. 

J. E. Owejan, P. T. Yu, and R. Makharia. Mitigation of carbon corrosion in microp-orous layers in PEM fuel cells. ECS Transactions 11 (2007) 1049-1057. 

K. T. Jeng, S. E. Lee, G. E. Tsai, and C. H. Wang. Oxygen mass transfer in PEM fuel cell gas diffusion layers. Journal of Power Sources 138 (2004) 41-50. 

X. Li and I. Sabir. Review of bipolar plates in PEM fuel cells Flow-field designs. International Journal of Hydrogen Energy 30 (2005) 359-371. 

B. Andreaus and M. Eikerling. Catalyst layer operation in PEM fuel cells From structural pictures to tractable models. In Device and materials modeling in PEM fuel cells, ed. K. Promislow and S. Paddison, Topics in applied physics 113, 41-90. New York Springer, 2009. 

In PEM fuel cells, catalyst activity and catalyst efficiency are still significant issues. Russell and Rose summarize fundamental work involving X-ray absorption spectroscopy on catalysts in low temperature fuel cell systems. These types of studies are very useful for developing a detailed understanding of the mechanisms of reactions at catalyst surfaces and could lead to the development of new improved efficient catalysts. Important in the development of fuel cell technology are mathematical models of engineering aspects of a fuel cell system. Wang writes about studies related to this topic. 

Lim, C. Wang, C. Y. Measurement of contact angles of liquid water in PEM fuel cell gas diffusion layer (GDL) by sessile drop and capillary rise methods. Penn State University Electrochemical Engine Center (ECEC) Technical Report no. 2001 03, Perm State University State College, PA, 2001. 

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