Ionic conductors with protons

Although the motion of protons does not lead to electrical conduction in the case of benzoic acid, electronic and even ionic conductivity can be found in other molecular crystals. A well-studied example of ionic conduction is a film of polyethylene oxide (PEO) which forms complex structures if one adds alkaline halides (AX). Its ionic conductivity compares with that of normal inorganic ionic conductors (log [cr (Q cm)] -2.5). Other polymers with EO-units show a similar behavior when they are doped with salts. Lithium batteries have been built with this type of... 

A prime need for a solid ionic conductor arises in the design of electrochemical fuel cells (Alberti and Casciola, 2001). Perhaps the most important type is the hydrogen fuel cell, shown diagrammatically in Fig. 8.5. Here the membrane which separates the two electrodes must be able to transfer protons efficiently. A material which combines the flexibility and toughness of a plastic with high protonic conductivity would be an ideal candidate. Prototype cells were successfully operated with membranes made from poly(styrenesulphonate),... 

The KSbOs structure contains pairs of edge-shared SbOe octahedra that share comers to form the cnbic network with K+ ions in three-dimensionally connected channels. The stmcture was originally investigated as a candidate for fast ion conductivity (see Ionic Conductors). The K atoms are readily exchanged in excess molten nitrates to form the phases MSb03 (M = Li, Na, Rb, Tl, and Ag). The hydronium phase HSb03 -H20 has been synthesized by acid exchange and found to have lower proton condnctivity than the same composition with the pyrochlore stmcture. ... 

As with VT, identifying conduction pathways with bond valence maps provides accurate predictions for ionic conductors (both crystalline and amorphous) with a percolation mechanism of conductivity, but is less successful when modeling proton conductors. 

Nowick AS, Du Y. High temperature protonic conductors with perovskite-related structures. Solid State Ionics. 1995 77 137. 

State-of-the-art proton conductors comprise acceptor-substituted perovskites, such as the barium-based ones (BaCe03, BaZr03, etc.) which exhibit proton conductivities in excess of 0.01 S cm i [51-55] and strontium-based ones (SrCe03) with somewhat lower conductivities. Both BaCe03 and BaZr03 are almost pure ionic conductors, and the electronic conductivity would, as such, rate limit the H2 flux across membranes of these materials [56]. 

Cermets for hydrogen separation consist of a ceramic and a metaUic phase contiguous in a dense matrix. In a cermet, one may combine one state-of-the-art pure proton conductor with a highly electron-conducting metaUic phase, and thereby circumvent the problem of having both electronic and ionic conduction in one and the same oxide. 

The Hall effect is well-known for electronic conductors. Although it forms the basis of the best method to determine the sign and effective number of electronically conducting species, only a few attempts have been made to use it with ionic conductors and these have been limited to non-protonic conductors . 

There are a number of review articles summarizing and critically examining the NMR studies of FlCs, and many of them contain some discussion of fast protonic conductors. Many features of the NMR studies are similar for protonic and other fast ionic conductors. However, there are also aspects peculiar to FPCs, an important one being the influence of local dynamics. In this article, we shall be specifically concerned with NMR studies of local motions in FPCs in contrast to the long range diffusion which is being discussed elsewhere in this book (see p. 412). 

The discovery of proton conduction in SrCeOa doped with rare earths such as trivalent Yb, when treated in hydrogen and/or water vapor (Iwahara et al. 1981a), opened up a new class of ionic conductors in which protons, present only as a minor constituent, can migrate by a simple hopping mechanism, in contrast with the low-temperature protonic conductors in which hydrogen is a major constituent and its transport involves complex mechanisms. 

Fig. 31. Comparison of a protonic conductor with an oxide ionic conductor in the case of hydrogen and ethane fuel cells. (Reprinted from Colomban 1992 by permission of the publisher, Cambridge University Press.)...

As mentioned before, high ionic conductivity is an important requirement for electrolyte materials to be used for ammonia synthesis devices. The results from many experiments confirm that proton conductors with adequate ionic conductivity can be used as electrolytes for a membrane reactor. Some examples of materials investigated as a membrane reactor are described below. 

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. 

A variant of the enhanced reaction zone concept is to utilize as catalyst support various porous three-dimensional electrodes with thickness between 200 to 2,000 pm. Thus, the electric contact resistance between the individual layers is eliminated. The three-dimensional matrix (such as various graphite felts, reticulated vitreous carbon, metal mesh, felt, and foam) supporting uniformly dispersed electrocatalysts (nanoparticles or thin mesoporous coating) could assure an extended reaction zone for fuel (methanol, ethanol, and formie aeid) electrooxidation, providing an ionic conductor network is established to link the catalytically active sites and the proton exchange membrane. The patent by Wilkinson et al. also suggests such electrode configurations (e.g., carbon foam, expended metal and reticulated metal) but experimental results were not provided [303]. 

The CL should keep the phase equilibrium with other PEM materials [13], The CLs applied onto both sides of the PEM are highly intercoimected with the membrane by their content of proton-conducting PFSA ionomer. The typical MEA structures show strains of PFSA ionomer running through the CLs, connecting catalyst particles to the membrane on the ionic conductor level. These strains form an ionic connection to the membrane not only for mobile protons, but also for all species which can enter the pores within these materials. In the normal case, the phase equilibrium at the interface between the CLs and the PEM material is always assumed to be established. 

A wide range of soUd and liquid electrolytes has been tested in various electrochromic devices and the results of these studies have been extensively discussed 4). The main dectrolyte groups are aqueous acidic dectrolytes, nonaqueous lidiium dectn tes, ceramic ionic conductms (P-alumina, Nasicon etc.), polyelectrolytes, polyrnCT solid dectrolytes (complexes of polyetfaers or polyimines with alkali metal salts and proton donors) and plasticized polymor ionic conductors. 

PAFCs are very efficient fuel cells, generating electricity at more than 50 % efficiency [13], About 85 % of the steam produced by the PAFC is used for cogeneration. This efficiency may be compared to about 35 % for the utility power grid in the United States. As with the PEMFC Pt or Pt alloys are used as catalysts at both electrodes [76]. The electrolyte is inorganic acid, concentrated phosphoric acid (100 %) which will conduct protons [77-79]. Operating temperatures are in the range of 150-220 °C. At lower temperatures, PAFC is a poor ionic conductor, and carbon monoxide (CO) poisoning of the platinum catalyst in the anode can become severe [76, 80,81]. 

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