Proton conductivities

Proton conducting perovskite oxides are the subject of numerous current studies, as these materials have a potential use in a number of electrochemical applications, including fuel cells, electrochromic displays and hydrogen sensors. Perovskite 

BaTlOj, BaCeOj and BaZrOj and the A Bp perovskite derivative Ba hijOj. 

The oxide Ba Inp adopts the brownmillerite structure at lower temperatures (Section 2.4). The structure begins to disorder at approximately 900°C and forms a cubic perovskite phase with a high concentration of oxygen vacancies somewhere above this temperature. Proton conduction in the solid is brought about by the incorporation of water molecules. Water vapour can react directly with the oxide at higher temperatures, following disproportionation at the oxide surface  

The perovskites BaCeOj and BaZrOj are insulating solids when prepared in air. These are converted to oxygen-deficient phases by doping the B-sites with fixed valence trivalent ions, such as In . Charge balance is not possible by valence induction and instead is achieved by the introduction of a population of oxygen vacancies. The presence of two substituents is then balanced by the formation of one oxygen vacancy. The formulae of the products are BaCej MPj or BaZrj M 03 2 example, with defect populations of 2In VJJ. 

The hydration process in all proton conducting perovskites can be formally written as a defect equation  

The bulk proton conductivity of a piece of PEM can be measured by the impedance technique. The membrane is placed between two electrodes, one working electrode and one counter electrode. An alternating current (AC) with a magnitude typically smaller than 10 mV is applied to the electrodes at a frequency of 1000 Hz or higher. The real impedance Z is the resistance of the PEM. If the AC voltage is applied for a frequency range (e.g., 1000 Hz to 1 Hz), the real impedance at the highest frequency is the resistance of the 

PEM soaked in liquid water has higher conductivities than PEM equilibrated with water vapor. Also, the conductivities increase with the RH. 

Proton conductivity measurement of PEM by the two-probe impedance technique at 25°C and 100% RH. Courtesy of Dalian Institute of Chemical Physics, Chinese Academy of Sciences. 

In addition to oxygen ion conduction, perovskite oxides have attracted considerahle attention as high-temperature proton conductors, with promising apphcations in fuel cells, hydrogen sensors, and steam electrolysers [116]. 

Some perovskites can absorb water from the atmosphere below 400 °C via the reaction 

Relevant samples which exhibit high H + ion conductivity in water vapor, and also in an atmosphere containing hydrogen, are substituted per- 

Most attention has focused on A B O3 perovskites such as ACe03 (A = Sr, Ba) [120,121]andAZr03 (A = Ca, Sr) [122], with very few studies having been conducted with A + B + 03 oxides [123]. More recently, a high proton conductivity was discovered in complex systems of the type A3B B 209 (Ba3Cai igNbi.8209). 

The current state of proton conductivity in perovskites has been discussed recently in detail by Wakamura [128]. 

Freiherr Christian Johann Dietrich Theodor von Grotthuss (1785-1822) was a Lithuania-born German chemist who introduced a number of new theories including the first theory of electrolysis known as the Grotthuss mechanism of proton conductance via hopping process of electrolytic conductivity. 

Inzelt, and F. Scholz, Electrochemical Dictionary, Springer, Berlin, Germany, 2008. 

FIGURE 3.21 Hopping (Grotthuss) mechanism of conductivity in a H+(aq)-containing solution. 

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The concept of the reversed fuel cell, as shown schematically, consists of two parts. One is the already discussed direct oxidation fuel cell. The other consists of an electrochemical cell consisting of a membrane electrode assembly where the anode comprises Pt/C (or related) catalysts and the cathode, various metal catalysts on carbon. The membrane used is the new proton-conducting PEM-type membrane we developed, which minimizes crossover. 

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 (

Ren, X. Springer, T. E. and Gottesfeld, S. (1998). Direct Methanol Fuel Cell Transport Properties of the Polymer Electrolyte Membrane and Cell Performance. Vol. 98-27. Proc. 2nd International Symposium on Proton Conducting Membrane Euel Cells. Pennington, NJ Electrochemical Society. 

Jacobs et al. [59,925,926] (Fig. 17). While this scheme conveniently summarizes many features of the observed behaviour, a number of variations or modifications of the mechanisms indicated have been proposed. Maycock and Pai Vemeker [924,933] emphasize the possible role of point defects and suggest, on the evidence of conductivity measurements, that the initial step may be the transfer of either a proton or an electron. Boldyrev et al. [46] suggest that proton conduction permits rapid migration of HC104 within the reactant and this undergoes preferential decomposition in distorted regions. More recently, the ease of proton transfer and the mobilities of other species in or on AP crystals have been investigated by a.c. [360] and d.c. [934] conductivity measurements. Owen et al. [934] could detect no surface proton conductivity and concluded that electron transfer was the initial step in decomposition. At the present time, these inconsistencies remain unresolved. 

T.I. Politova, V.A. Sobyanin, and V.D. Belyaev, Ethylene hydrogenation in electrochemical cell with solid proton-conducting electrolyte, Reaction Kinetics and Catalysis Letters 41(2), 321-326 (1990). 

N. Kurita, N. Fukatsu, K. Ito, and T. Ohashi, Protonic Conduction Domain of Indium-Doped Calcium Zirconate,/. Electrochem. Soc. 142(5), 1552-1559 (1995). 

Figure 9.32. Experimental set-up (a) Machinable ceramic holders and two proton conducting pellets showing the location of catalyst, counter and reference electrodes, (b) Twenty four pellet unit, (c) High-pressure reactor, gas feed and analysis unit.43 Reprinted with permission from the American Chemical Society.

For last few years, extensive studies have been carried out on proton conducting inorganic/organic hybrid membranes prepared by sol-gel process for PEMFC operating with either hydrogen or methanol as a fuel [23]. A major motivation for this intense interest on hybrid membranes is high cost, limitation in cell operation temperature, and methanol cross-... 

But when the contents of Nafion ionomer was increased from 30 to 45 % to find out the better electrode structures, the Pt-Ru/SRaw, which had showed the lowest single cell performance, became the best electro-catalyst. By this result one can conclude that as long as the structure of the electrode can be optimized for the each of new electro-catalysts, the active metal size is a more important design parameter rather than inter-metal distances. Furthermore, when the electro-catalysts are designed, the principal parameters should be determined in the consideration of the electrode structures which affect on the electron conduction, gas permeability, proton conductivity, and so on. 

Figure 15. Extent of methanol crossover through different ETFE proton-conducting membranes. Comparison with the behavior ofNafion 117 ( ).

Apart from the problems of low electrocatalytic activity of the methanol electrode and poisoning of the electrocatalyst by adsorbed intermediates, an overwhelming problem is the migration of the methanol from the anode to the cathode via the proton-conducting membrane. The perfluoro-sulfonic acid membrane contains about 30% of water by weight, which is essential for achieving the desired conductivity. The proton conduction occurs by a mechanism (proton hopping process) similar to what occurs... 

S. R. Narayanan, A. Kindler, B. Jeffries-Nakamura, W. Chun, H. Frank, M. Smart, S. Surampudi, and G. Halpert, in Proc. of the First International Symposium on Proton Conducting Membrane Fuel Cells, Ed. by S. Gottesfield, G. Halpert, and A. R. Landgrebe, The Electrochemical Society, Pennington, NJ, PV 95-23, 1995, pp. 261-266. 

A considerable decrease in platinum consumption without performance loss was attained when a certain amount (30 to 40% by mass) of the proton-conducting polymer was introduced into the catalytically active layer of the electrode. To this end a mixture of platinized carbon black and a solution of (low-equivalent-weight ionomeric ) Nafion is homogenized by ultrasonic treatment, applied to the diffusion layer, and freed of its solvent by exposure to a temperature of about 100°C. The part of the catalyst s surface area that is in contact with the electrolyte (which in the case of solid electrolytes is always quite small) increases considerably, due to the ionomer present in the active layer. 

Inzelt, G., M. Pineri, 1. W. Schultze, and M. A. Vorotyntsev, Electron and proton conducting polymers recent developments and prospects, Electrochim. Acta, 45, 2403 (2000). 

Electrolytes for Electrochromic Devices Liquids are generally used as electrolytes in electrochemical research, but they are not well suited for practical devices (such as electrochromic displays, fuel cells, etc.) because of problems with evaporation and leakage. For this reason, solid electrolytes with single-ion conductivity are commonly used (e.g., Nafion membranes with proton conductivity. In contrast to fuel cells in electrochromic devices, current densities are much lower, so for the latter application, a high conductivity value is not a necessary requirement for the electrolyte. 

Implementation of Pt/C catalysts in PEFC technology using recast Nafion as a proton conducting and bonding agent [Raistrick, 1986 Wilson and Gottesfeld, 1992]. 

Nagle, J. F., Theory of passive proton conductance in lipid bilayers, J. Bioenerg. Biomem. 19, 413—426 (1987). 

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