Proton conductor

There is a class of nonporous materials called proton conductors which are made from mixed oxides and do not involve transport of molecular or ionic species (other than proton) through the membrane. Conduction of protons can enhance the reaction rate and selectivity of the reaction involved. Unlike oxygen conductors, proton conductors used in a fuel cell configuration have the advantage of avoiding dilution of the fuel with the reaction products [Iwahara ct al., 1986]. Furthermore, by eliminating direct contact of fuel with oxygen, safety concern is reduced and selectivity of the chemical products can be improved. The subject, however, will not be covered in this book. 

Intercalated or surface adsorbed species in the electron conductor. Protons and li ions, if present in the hole conductor, may be capable of intercalating into the oxide film, affecting both the potential energy for electron stabilization and the ease of electronic conduction. Surface absorbed cations influence electronic states near the surface. Adsorption of OH groups influences electronic states and proton transport. Water molecules, which are expected to be present around the oxide surface even in dry solvent, may be dissociatively adsorbed leaving adsorbed OH species and mobile protons, which influence electronic states and conductivity. 

In addition to halide ion conductors, proton conductors such as Nation membrane enabled the development of a unique amperometric oxygen sensor operating at ambient temperature. 

Polymer Electrolyte Fuel Cell. The electrolyte in a PEFC is an ion-exchange (qv) membrane, a fluorinated sulfonic acid polymer, which is a proton conductor (see Membrane technology). The only Hquid present in this fuel cell is the product water thus corrosion problems are minimal. Water management in the membrane is critical for efficient performance. The fuel cell must operate under conditions where the by-product water does not evaporate faster than it is produced because the membrane must be hydrated to maintain acceptable proton conductivity. Because of the limitation on the operating temperature, usually less than 120°C, H2-rich gas having Htde or no (

The Dissociation of a Molecule into Ions. The Removal of Ions from a Metal Surface. The Removal of Ions from the Surface of an Ionic Crystal. The Solvation Energy of an Ion. Work Done against Electrostatic Forces. Molecules and Molecular Ions Containing One or More Protons. Proton Transfers. The Quantities D, L, Y, and J. Two Spherical Conductors. 

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... 

Nevertheless there are some reactions which never change. Thus NO reduction on noble metals, a very important catalytic reaction, is in the vast majority of cases electrophilic, regardless of the type of solid electrolyte used (YSZ or P"-A1203). And practically all oxidations are electrophobic under fuel lean conditions, regardless of the type of solid electrolyte used (YSZ, p"-Al203, proton conductors, even alkaline aqueous solutions). 

M. Makri, A. Buekenhoudt, J. Luyten, and C.G. Vayenas, Non-Faradaic Electrochemical Modification of the Catalytic Activity of Pt using aCaZr09ln0 03.a Proton Conductor, Ionics 2, 282-288 (1996). 

Ammonia Synthesis on Fe Supported on a Proton (CaZro.9Ino.1O3., ) Conductor... 

A recent electrochemical promotion study ofNH3 synthesis43 utilized a commercial fully promoted Fe-based NH3 synthesis catalyst (BASF S6 -10RED) deposited on CaZro.9lno.i03 a, a proton conductor. 

This study is noteworthy because it was the first one showing that a proton conductor can be used for the electrochemical promotion of an oxidation reaction. It underlines that, contrary to chemisorbed H which is a... 

Proton conductors ammonia synthesis, 468 conductivity, 93 ethylene oxidation, 470 hydrogen oxidation, 457 list of electrochemically promoted reactions, 146... 

FIGURE 25.10 Conductivity of some proton conductors as a function of HT. The overlapping area between BaCeOj and SrCeOj indicates sohd solutions based on these compounds. (From Gelings and Bouwmeester, 1997, Fig. 6.17, with permission from CRC Press LLC via CCC.)... 

Bohnke, O., Applications of proton condnctors in electrochromic devices, in Proton Conductors Solids, Membranes and Gels—Materials and Devices, P. Colomban, Ed., Cambridge University Press, New York, 1992. 

Significant advances have been made in this decade in electrochemical H2 separation, mostly through the use of solid polymer electrolytes. Since the overpotentials for H2 reduction and oxidation are extremely low at properly constructed gas diffusion electrodes, very high current densities are achievable at low total polarization. Sedlak [13] plated thin layer of Pt directly on Nafion proton conductors 0.1-0.2cm in thickness, and obtained nearly 1200 mA/cm2 at less than 0.3 V. The... 

Guan, J., S.E. Dorris, U. Balachandran, and M. Liu, The effects of dopants and A B site nonstoichiometry on properties of perovskite-type proton conductors, /. Electrochem. Soc., 145,1780,1998. 

The coordination chemistry of the trichalcogenophosphonates is very undeveloped when compared to the analogous metal organophosphonates (RP032), which have been extensively studied owing to their potential and practical applications as ion exchangers, sorbents, sensors, proton conductors, nonlinear optical materials, photochemically active materials, catalysts and hosts for the intercalation of a broad spectrum of guests.145... 

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