Proton exchange membrane relationships

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. 

Peckham, T. J., Schmeisser, J., Rodgers, M. and Holdcroft, S. 2007. Main-chain, statistically sulfonated proton exchange membranes The relationships of acid concentration and proton mobility to water content and their effect upon proton conductivity. Journal of Materials Chemistry 17 3255-3268. 

Michael Hickner received his B.S. in Chemical Engineering from Michigan Tech in 1999 and his Ph.D. in Chemical Engineering in 2003 under the direction of James McGrath. Michael s research in Dr. McGrath s lab focused on the transport properties of proton exchange membranes and their structure-property relationships. He has spent time at Los Alamos National Laboratory studying novel membranes in direct methanol fuel cells and is currently a postdoc at Sandia National Laboratories in Albuquerque, NM. 

Proton exchange membranes, whether operating in electrolysis mode or fuel cell mode, have the property of higher efficiency at lower current density. There is a 1 1 relationship in electrolysis between the rate of hydrogen production and current applied to the system. 

Polymeric proton exchange membrane needs to be maintained properly humidified to guarantee a satisfactory ion conductivity during stack operation (see Sect. 3.2). In fact it exists a strong relationship between proton conductivity and water content of Nafion material used as membrane in PEMFC [24, 25]. Unfortunately the water produced at cathode side and the air moisture could be not sufficient to maintain properly wet the membranes in all working conditions, because of complex phenomena involving water within MEA [26] (Fig. 4.7). 

Hickner, M., Wang, F., Kim, Y. et al. (2001) Chemistry-morphology-property relationships of novel proton exchange membranes for direct methanol fuel cells, ACS Fuel (Part 1), Vol. 222, August 26—30, Chicago, p. 51. 

Kim, J., Lee, J., and Tak, Y. (2009) Relationship between carbon corrosion and positive electrode potential in a proton-exchange membrane fuel cell during start/stop operation. J. Power Sources, 192, 674-678. 

Figure 22.1. Relationship of Pt surface area to particle size, based on spherical geometry [3]. (Reproduced from Thompsett D. Catalysts for the proton exchange membrane fuel cell. In Hoogers G, editor. Fuel cell technology handbook. Boca Raton CRC Press, 2003. With permission from CRC.)...

Liang Y, Zhang H, Tian Z, Zhu X, Wang X, Yi B. Synthesis and structure-activity relationship exploration of carbon-supported PtRuNi nanocomposite as a CO-tolerant electrocatalyst for proton exchange membrane fuel cells. J Phys Chem B 2006 110(15) 7828-34. 

The methanol permeability through the proton exchange membranes was proportional to the proton conductivities, as shown in Fig. 4.16. That is, the proton conductivity has a trade-off in its relationship with the methanol permeability. Target membrane would be located in the upper left-hand comer, of which the fluorenyl copolymers show the same tendency. Series of sulfonated poly(aryl ether ketone)s membranes obtained by direct copolymerization using various sulfonated monomers are listed in Table 4.2. The water content and proton conductivities of these membranes are shown in Fig. 4.17. 

St-Pierre, J. and Jia, N. (2002) Successful demonstration of Ballard PEMFCs for space shuttle applications. J. New Mater. Electrochem. Syst. 5, 263-271 St-Pierre, J., Jia, N. and Rahmani, R. (2008) Proton exchange membrane fuel cell contamination model - Competitive adsorption demonstrated with NO J. Electrochem. Soc. 155, B315-B320 St-Pierre, J., WrUdnson, D. P., Knights, S. and Bos, M. (2(XX)) Relationships between water management, contamination and Ufetime degradation in PEFC. J. New Mater. Electrochem. Syst. 3,99-106... 

Referring to the description of Nebrand s ID model of proton exchange membrane, the ion-exchange capacity X and the density of proton exchange membrane in a wet state can be expressed as a function of water content X ,. The relationship is expressed as follows ... 

Figure 16 Upper bound relationship for proton exchange membranes. Reprinted with permission from Elsevier Robeson, L. M. Hwu, H. H. McGrath, J. E. J. Membr. Sci. 2007, 302,70V ...

Zawodzinski et al. [58] have reported NMR relaxation measurements on water in Nafion membranes. In contrast with proton NMR relaxation studies, which are difficult to interpret because of various inseparable contributions to the observed relaxation rates, a direct relationship often exists between the observed relaxation rate and rotational dynamics of the deuteron-bearing species. The time scale probed by such measurements is in the pico- to nanosecond range, and thus very short-range motions are probed. In a membrane equilibrated with saturated water vapor, a Ti on the order of 0.2s was observed. This relaxation rate for D2O in the membrane is only higher by a factor of two than that in liquid D2O, indicating a bulk water-like mobility within the pore at high membrane hydration levels. The relaxation rate increases (i.e., local water motion in the membrane becomes slower) as the water to ion-exchange site ratio decreases. 

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