Proton conductivity/conductor

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 (

Thus, measurement of the total conductivity together with the cell voltage allows the transport numbers of the ions to be determined (Fig. 8.17). The results show that at lower temperatures proton conductivity is of greatest importance, at middle temperatures oxygen ion conductivity becomes dominant, and at high temperatures the material is predominantly a hole conductor. Between these temperatures, at approximately 350°C the solid is a mixed H+ and O2- conductor while at approximately 650°C it is a mixed hole and O2- conductor. 

A number of factors must be taken into account when the diagrammatic representation of mixed proton conductivity is attempted. The behavior of the solid depends upon the temperature, the dopant concentration, the partial pressure of oxygen, and the partial pressure of hydrogen or water vapor. Schematic representation of defect concentrations in mixed proton conductors on a Brouwer diagram therefore requires a four-dimensional depiction. A three-dimensional plot can be constructed if two variables, often temperature and dopant concentration, are fixed (Fig. 8.18a). It is often clearer to use two-dimensional sections of such a plot, constructed with three variables fixed (Fig. 8.18h-8.18<7). 

Typically, Nation ionomer is the predominant additive in the catalyst layer. However, other types of CLs with various hygroscopic or proton conductor additives have also been developed for fuel cells operafed xmder low relative humidity (RH) and/or at elevated temperatures. Many studies have reported the use of hygroscopic y-Al203 [52] and silica [53,54] in the CE to improve the water retention capacity and make such CEs viable for operation af lower relative humidity and/or elevated temperature. Alternatively, proton conducting materials such as ZrP [55] or heteropoly acid HEA [56] have also been added... 

Proton conduction at high temperatures occurs in certain perovskites such as doped strontium cerate, Sr Ceo.95Ybo.o503 t. In air, this material is primarily an electronic conductor due to the mixed valence of Ce. In the presence of moisture, water is absorbed by the reaction with positive holes to generate protons ... 

In a subject with the breadth of solid state electrochemistry, and with the inevitable constraints on the length of a textbook, some topics have been omitted. In particular, less emphasis on proton conduction has been given than on the transport of other ions, in part because of the existence of a recent book on this topic in the same series. Proton Conductors, Ed. Philippe Colomban, Cambridge University Press (1992). Also the important topic of intercalation into graphite has been largely omitted. Several excellent texts on this subject are already available. 

Interest in new solid polymer electrolytes has driven some research groups to investigate other materials containing proton conducting moieties aside from sulfonic acid. Polymers and copolymers from monomers containing phosphonic-based proton conductors have been reported. Phosphonic and/or phosphinic acid containing polymers have not been well studied because of the rather limited synthetic procedures available for their preparation, compared with sulfonic acid derivatives. Miyatake and Hay... 

Figure 25. Proton conductivity of various oxides, as calculated from data on proton concentrations and mobilities, according to Norby and Larring (the type of dopant is not indicated see ref 187 for source data). The conductivity of oxides with a perovskite-type structure are shown by bold lines, and the conductivity of the oxide ion conductor YSZ (yttria-stabilized zirconia) is shown for comparison, (reproduced with the kind permission of Annual Reviews, http //www.AnnualReviews.org).

Considering their possible applications in fuel cells, hydrogen sensors, electro-chromic displays, and other industrial devices, there has been an intensive search for proton conducting crystals. In principle, this type of conduction may be achieved in two ways a) by substituting protons for other positively charged mobile structure elements of a particular crystal and b) by growing crystals which contain a sufficient amount of protons as regular structure elements. Diffusional motion (e.g., by a vacancy mechanism) then leads to proton conduction. Both sorts of proton conductors are known [P. Colomban (1992)]. 

It has been reported (4,5) that solid electrolyte sensors using stabilized zirconia can detect reducible gases in ambient atmosphere by making use of an anomalous EMF which is unusually larger than is expected from the Nernst equation. However, these sensors should be operated in a temperature range above ca. 300°C mainly because the ionic conductivity of stabilized zirconia is too small at lower temperatures. On the other hand, solid state proton conductors such as antimonic acid (6,1), zirconium phosphate (8), and dodecamolybdo-phosphoric acid (9) are known to exhibit relatively high protonic conductivities at room temperature. We recently found that the electrochemical cell using these proton conductors could detect... 

To conclude this section on proton transfer, we have examined an alternative mechanism to that of von Grotthuss for proton conduction [105-110], We carried out B3LYP/6-31+G and PW91 DFT calculations on model compounds (1,2,3,4-tetrasubstituted benzenes, e.g., 99) showing that these compounds could play the role of proton conductors [104],... 

More recently, H. Iwahara et al. [20] reported that some compounds having the perovskite structure (see Section 2.7.3) become proton conductors if hydrogen is introduced into the crystal, and the solubility of water and proton mobility in perovskites are now actively researched topics [21]. The perovskites which can be tailored to exhibit high protonic conductivity have compositions of the type... 

The defects in doped BaCe03 perovskites, which are good proton conductors, are described here [32,33,48-54]. It has been recognized that temperature and atmosphere significantly affect the transport properties of the majority of protonic conductors [33], Certainly, under a definite temperature range and specific atmosphere, doped BaCe03 has significant protonic conductivity [48-54],... 

Proton-conducting materials [38-47], analogous to oxygen conductors but with stationary oxygen anions, can show mixed protonic-electronic conductivity, without considerable oxygen transport in hydrogen or water atmospheres [40,41], These materials have not been widely studied in comparison... 

Abstract. Nanopowders of nonstoichiometric tungsten oxides were synthesized by method of electric explosion of conductors (EEC). Their electronic and atomic structures were explored by XPS and TEM methods. It was determined that mean size of nanoparticles is d=10-35 nm, their composition corresponds to protonated nonstoichiometric hydrous tungsten oxide W02.9i (OH)o.o9, there is crystalline hydrate phase on the nanoparticles surface. After anneal a content of OH-groups on the surface of nonstoichiometric samples is higher than on the stoichiometric ones. High sensitivity of the hydrogen sensor based on WO2.9r(OH)0.09 at 293 K can be connected with forming of proton conductivity mechanism. 

Kreuer K-D, Wohlfarth A, de Araujo CC, Fuchs A, Maier J. Single alkaline-ion (Li+, Na+) conductors by ion exchange of proton-conducting ionomers and polyelectrolytes. Chem-PhysChem. 2011 12(14) 2558-60. 

A second commercially available electrolyzer technology is the solid polymer electrolyte membrane (PEM). PEM electrolysis (PEME) is also referred to as solid polymer electrolyte (SPE) or polymer electrolyte membrane (also, PEM), but all represent a system that incorporates a solid proton-conducting membrane which is not electrically conductive. The membrane serves a dual purpose, as the gas separation device and ion (proton) conductor. High-purity deionized (DI) water is required in PEM-based electrolysis, and PEM electrolyzer manufacturer regularly recommend a minimum of 1 MQ-cm resistive water to extend stack life. 

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),... 

---