Proton conduction complications

Water content affects many processes within a fuel cell and must be properly managed. Proton conductivity within the polymer electrolyte typically decreases dramatically with decreasing water content (especially for perfhiorinated membranes such as Nation ), while excessive liquid water in the catalyst layers (CLs) and gas diffusion layers (GDLs) results in flooding, which inhibits reactant access to the catalyst sites. Water management is complicated by several types of water transport, such as production of water from the cathode reaction, evaporation, and condensation at each electrode, osmotic drag of water molecules from anode to cathode by... 

In reality the conductivity of proton conducting phases (Section 5.3) is more complicated than described. Successfully doped materials are not electronic insulators, as the aforementioned equations imply, but are generally weak p-type semiconductors. This comes about because at high temperatures a small amount of oxygen can react with the defective perovskite to partially fill the vacancies and generate a population of holes ... 

In the first part of this short chapter, it is discussed where protons find energetically favored sites in oxides with the perovskite structure, before the mechanisms of proton diffusion via these sites are described in detail. While this discussion is restricted to structure and dynamics of protonic defects in ideal (cubic) perovskites, the following section addresses complications such as symmetry reduction and the effect of the presence of dopants, before, finally, a few implications for the development of proton-conducting electrolytes for fuel cell applications are discussed. 

DuPont s Nafion is the most advanced commercially available proton conducting polymer material, which is produced in membrane form with thickness between 25 and 250 pm. Nafion is the electrolyte against which other membranes are judged and is in a sense an Industrial Standard . It is a copolymer of tetrafluoroethylene (TFE), and perfluoro (4-methyl-3,6-dioxa-7-octene-l-sulfonyl fluoride) or vinyl ether , as shown in Fig. 3. The methods of creating and adding the side chains are highly complicated and the process involving many steps is proprietary. 

In the case of PEMs, the situation is more complicated because the sulfonate counter-ions (in the case of a PEM such as Nafion ) are bound to the polymer chain and are thus relatively immobile, in contrast to the free counter-ion in a small molecule acid such as sulfuric or acetic acid. Tethering of the sulfonate group can be considered to be an impediment to the mobility of the proton as it traverses the membrane. Proton mobility is also affected by the effective mean-free path of connectivity of the conduction pathway as shown in Figure 3.2. In situation (a), the increased number of dead ends and tortuosity of the aqueous domains through which proton transport occurs over the situation in (b) leads to lower overall mobility. This has been demonstrated by Kreuer and will be discussed later in this section. 

Cation—sulfonate interactions, as well as proton mobility, are also expressed in the electrical conductance behavior of these membranes. Many studies of this property have been reported, and there is no attempt in this review to cite and describe them all. Rather, a few notable examples are chosen. Most testing is done using alternating current of low voltage to avoid complications in the form of chemical... 

Dye sensitized electron injection in the empty conduction band of chloranil has been described by Michel-Beyerle and Brickl 92>. The principal events listed in points 1) to 3) for hole injection also apply in the corresponding formulation to sensitized electron injection. The detailed formulation of a reaction scheme, however, is more complicated for sensitized electron injection at chloranil since protons also can take part in the formation of the reduced hydrochinone form of the reduced chloranil molecule. 

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