Proton-Conducting Polymeric Membranes

A typical PEFC, shown schematically in Fig. 1, consists of the anode and cathode compartments, separated by a proton conducting polymeric membrane. The anode and cathode sides each comprises of gas channel, gas diffusion layer (GDL) and catalyst layer (CL). Despite tremendous recent progress in enhancing the overall cell performance, a pivotal performance/durability limitation in PEFCs centers on liquid water transport and resulting flooding in the constituent components.1,2 Liquid water blocks the porous pathways in the CL and GDL thus causing hindered oxygen transport to the... 

The proton-conducting polymeric membrane is the unique element of the PEFC, as is indeed reflected by any of the three names used for this... 

Chemical Structure and Chemical Stability of Proton-conducting Polymeric Membranes... 

Today, the term solid electrolyte or fast ionic conductor or, sometimes, superionic conductor is used to describe solid materials whose conductivity is wholly due to ionic displacement. Mixed conductors exhibit both ionic and electronic conductivity. Solid electrolytes range from hard, refractory materials, such as 8 mol% Y2C>3-stabilized Zr02(YSZ) or sodium fT-AbCb (NaAluOn), to soft proton-exchange polymeric membranes such as Du Pont s Nafion and include compounds that are stoichiometric (Agl), non-stoichiometric (sodium J3"-A12C>3) or doped (YSZ). The preparation, properties, and some applications of solid electrolytes have been discussed in a number of books2 5 and reviews.6,7 The main commercial application of solid electrolytes is in gas sensors.8,9 Another emerging application is in solid oxide fuel cells.4,5,1, n... 

Proton exchange membranes (PEM) fuel cells (or polymer electrolyte fuel cells - PEFCs), with H -conducting polymeric membranes, transports hydrogen (fuel) cations, generated at the anode, to an ambient air exposed cathode, where they are electro-oxidised to water at low temperatures. 

The development of new polymeric materials for polymer electrolyte fuel cell is one of the most active research areas, aiming at the new energy sources for electric cars and other devices. The mainstream of the material research for fuel cell is perfluoroalkyl sulfonic acid membranes such as Nafion, Acipex, and Flemion. The most well-known one is Nafion of Du Pont, which is derived from copolymers of tetrafluoro-ethylene and perfluorovinyl ether terminated by a sulfonic acid group.Protons, when dissociated from the sulfonic acid groups in aqueous environment, become mobile and the membrane becomes a proton conducting electrolyte membrane. 

PEM (proton exchange membrane) electrolysis the electrodes are separated by a proton-conducting polymeric solid electrolyte (membrane)... 

In general, the ionic transport in linear or crosslinked swollen polymers containing a low molecular weight polar or ion-chelating additive mainly occurs in the solvent phase [118, 120]. This concept has been applied to develop proton conducting polymeric gel or hydrogel membranes [121-123] which reach conductivity values around 10 S cm at room temperature and are not destroyed or dissolved even at high humidity levels. 

Most conventional electrolysers use a Nafion membrane (Ito et al, 2011). The proton-conducting polymeric film is immersed in distilled water. Under a sufficiently high potential, protons migrate through the membrane and are reduced at the cathode. The general reactions are ... 

There is a clear need for the synthesis and more complete characterization of new PEM polymeric materials. Polymers bearing functional moieties for proton conduction might also be designed to serve as a host for inorganic compounds to afford a proton conducting component in a blend, as well as a standalone PEM. The two current hurdles for polymeric membranes are the high protonic conductivity at low water contents (e.g. under conditions of 120 °C and... 

Many different kinds of fuel cells are presently known, most of them suitable for high-temperature applications— for details see Ref. [101]. The polymeric proton-conducting membranes (polymer electrolyte membranes PEM) are however suitable for low temperamre operations (<100°C) and have the advantage of low weight. 

Alberti G, Casciola M, Pica M, and De Cesare G. Preparation of nano-stmctured polymeric proton conducting membranes for use in fuel cells. In Li NN, DrioU E, Ho WSW, and Lipscomb GG, eds. Annals of the New York Academy of Sciences vol. 984 Advanced Membrane Technology. New York The New York Academy of Sciences, 2003, pp. 208-225. 

G. Alberti, M. Casciola, L. Massinelli, and B. Bauer. Polymeric proton conducting membranes for medium temperature fuel cells (110-160 degrees C). Journal of Membrane Science 185, 73-81 2001. 

S. Hohnberg, P. Holmlund, C.E. Wilen, T. KaUio, G. Sundhohn, and F. Sundhohn. Synthesis of proton-conducting membranes by the utilization of preirradiation grafting and atom transfer radical polymerization techniques. Journal of Polymer Science Part A Polymer Chemistry 40, 591-600 2002. 

The protonic conductivity of a polymeric membrane is strongly dependent on membrane structure and membrane water content. A central challenge in the evaluation of ionomeric membranes for fuel cell applications has thus been the analysis of combined structural and water uptake characteristics required to achieve the highest protonic conductivity in an operating PEFC. Section 5.3.1 will address water uptake by ionomeric membranes employed in PEFCs, the state of water in such membranes and the resulting protonic conductivities obtained. 

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