Ionic conductors Nafion

It is known that the catalyst layer is far from uniform, especially in the case of a gradient catalyst layer. Thus, profiling properties, such as conductivity, in the catalyst layer are important. Both an electronic conductor (carbon) and an ionic conductor (Nafion ) exist in the catalyst layer, which can be considered a conductive polymer. The conductive polymer electric circuit model has been applied to the catalyst layer, and an ionic conductivity profile was obtained [8], as shown in Figure 4.33. 

The total active area of the catalyst is typically much higher than the electrochemical surface area determined from cyclic voltammetry measurements. That means that not all catalyst particles are in contact with the ionic conductor Nafion and only a certain amount of it participates in electrochemical reaction. 

The first polymer ionic conductor Nafion was invented in the 1960s. Until that time electrochemistry had worked with liquid electrolytes. The polarization behaviour of catalyst layers with aqueous electrolyte had been studied in numerous works (see Striebel et al. (1995) and Perry et al. (1998) and the literature cited therein). 

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... 

The fuel-cell effect was discovered in 1838, and for more than 100 years after this event, low-temperature fuel cells have utilized liquid electrolyte as an ionic conductor between the anode and the cathode [1]. The situation changed with invention of Nafion, the first stable proton-conducting soUd polymer. Nafion has radically modified the design of low-temperature cells and stacks. A liquid electrolyte makes cells bulky and unsafe, and it requires sophisticated seahng. In Nafion-based cells, the electrodes are separated by a thin, 20-100 g.m thick polymer film. In addition, Nafion allowed cell electrodes that contain electrolyte to be much thinner. 

Ionic current between the anode and the cathode is transported through an ionic conductor. In low-temperature fuel cells, internal charge carriers are protons and the proton conductor is a solid polymer electroljde membrane (typically Nafion ). It took more than 10 years to clarify the mechanism of proton transport in this membrane. However, the membrane structure and its dependence on water content are still unclear. 

The main requirement of a good electrode is a three-phase boundary between the gas supply on the one hand and the catalyst particle and the ionic conductor on the other [85]. The particles must be in direct contact with an electronic conductor to ensure the electrons are supplied to or taken away from the reaction site. Electronic conductivity is usually provided by a carbon support onto which the catalyst particles are mounted. The three-phase boundary is made by impregnating the catalyst/support powder with an ionomeric binder (usually Nafion solution) before pressing the electrode onto the membrane. This ensures good contact of most catalyst particles with the ionomer material that has ionic contact with the membrane. 

The reason for Nafion LB-film fabrication was the wish to obtain the highly ordered systems from perfluorinated ion exchange polymer with multilayered structure, where the ionic layers (conductors) would alternate with fluorocarbon polymer layers (insulators), and to investigate the properties of such films.74 This polymer contains a hydrophobic fluorocarbon polymeric chain and hydrophilic ionic groups, so it is sufficiently amphiphilic it has a comblike structure that makes it a suitable polymer for LB-film deposition. 

So far, more than 70 different catalytic reactions (oxidations, hydrogenations, dehydrogenations, isomerizations, decompositions) have been electrochemically promoted on Pt, Pd, Rh, Ag, Au, Ni, IrC>2, Ru02 catalysts deposited on O2- (YSZ), Na+ (/i"-Al2Oi), H+ (Ca/,ro2)ln0. Oj a, Nafion), F (CaF2), aqueous, molten salt, and mixed ionic-electronic (TiC>2, CeC>2) conductors [23]. 

Summary. We have shown that ion transport in "Nafion" per-fluorinated membrane is controlled by percolation, which means that the connectivity of ion clusters is critical. This basically reflects the heterogeneous nature of a wet membrane. Although transport across a membrane is usually perceived as a one-dimensional process, our analysis suggests that it is distinctly three-dimensional in "Nafion". (Compare the experimental values of c and n with those listed in Table 7.) This is not totally unexpected since ion clusters are typically 5.0 nm, whereas a membrane is normally several mils thick. We have also uncovered an ionic insulator-to-conductor transition at 10 volume % of electrolyte uptake. Similar transitions are expected in other ion-containing polymers, and the Cluster-Network model may find useful application to ion transport in other ion containing polymers. Finally, our transport and current efficiency data are consistent with the Cluster-Network model, but not the conventional Donnan equilibrium. 

There are also mixed conductors with ionic and electronic conductance. The sulfides, selenides, and tellurides of silver and lead are examples of this. New plastic materials with ionic conductance have been discovered in polymer chemistry. Nafion is a polymer with ionic conductivity, and it is used in multigas analyzers in anesthesia, where the special high permeability to water is useful. 

As listed in Table 36.1, a good deal of research has been carried out so far on proton conductor-based gas sensors workable at ambient temperature. Various inorganic and organic ion exchangers, such as hydrogen uranyl phosphate (HUP), zirconium phosphate (ZrP), antimonic acid (AA), and NAFION membrane, have been utilized in the form of a disc, thick- or thin-film. The ionic conductivities of these proton conductors, in the range lO " 10 S cm are modest but seem to be still sufficient for chemical sensing devices. 

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