Proton-conducting materials

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... 

Zhang, Y, Litt, M., SavineU, R. R, Wainright, J. S. and Vendramini, J. 2000. Molecular design of polyimides toward high proton conducting materials. Abstracts of Papers of the American Chemical Society 220 11307-11307. 

The last comprehensive review covering proton conductivity and proton conducting materials was written by one of the authors (dating back to 1996) since then, there have been several other review articles of similar scope (e.g., see Colomban ). There are also many reviews available on separator materials used for fuel cells (see articles in refs 3 and 4 and references therein, recent review-type articles, " and a literature survey ), which, more or less, address all properties that are relevant for their functioning in a fuel cell. The transport properties are usually described in these articles however, the treatments are frequently restricted to macroscopic approaches and handwaving arguments about the transport mechanisms. The purpose of the present review is to combine a few recently published results in the context of a discussion of transport phenomena in proton-conducting separator materials, which have some relevance in fuel cell applications (for a more complete list of the comprehensive literature in the field, the interested reader is referred to the aforementioned references). 

Transport data have been reported for a huge number of proton-conducting materials, some of which are defined rather poorly. They have been recorded by a variety of methods under diverse conditions including ill-defined transient or steady-state situations (e.g., in a running fuel cell). Consequently, reproducibility and comparison of data from different laboratories is problematic. For this reason, we have only compiled the transport data that have... 

Recent Approaches toward New Proton-Conducting Materials for Fuel-Cell Applications... 

The suitability of proton-conducting materials as separators in a particular fuel-cell application is essentially dependent on its transport properties, durability, and reactivity. Thus far, this review has focused on the transport properties only, but any approach toward new separator materials must consider all relevant aspects, which makes the development of new competitive materials a complex and challenging task. 

A fullerene network of proton conductive materials, (II), was prepared by Nuber et al. (3) and used in fuel cell applications as illustrated in Eq. (1). 

In the previous section, /1-aluminas were discussed. If one replaces Na+ by H30+ or NH4 in these oxide compounds by, for example, electrolysis, then proton conducting materials can be obtained. Their ionic conductivity, however, is relatively low (10—5 Q-1 cm-1) because of the strong interactions between the small H+ ions and O2- ions in the conduction plane (OH ). The electrical conductivity is markedly higher in crystals of NH4 -/ -Ga203. 

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... 

Kreuer, K.D. Proton conductivity materials and applications. Chem. Mater. 1996, 8 (3), 610-641. 

Kreuer, K.D. (1997) On the development of proton conducting materials for technological applications. Solid State Ionics, 97, 1-15. 

D.J. Jones J. Roziere (1993). Solid State Ionics, 61, 13-22. Complementarity of optical and incoherent inelastic neutron scattering spectroscopies in the study of proton conducting materials. 

In a fiirther series of experiments, phosphotungstic acid (PWA) was impregnated onto silica particles and the resulting material was loaded in the recast Nafion. As is well known, heteropolyacids (HPAs) have demonstrated suitable characteristics to be used as proton conductive materials in fuel cells [4-6]. Due to dissolution phenomena in water, however, previous experiments using solid heteropolyacid did not result in stable fuel cell performance [5]. To overcome this problem, resulting in a short lifetime of the fuel cell, experiments of blocking the HPA in a host material were carried out [7-9]. Thus, in this work the phosphotungstic acid-modified membrane was compared, in terms of performance, with the bare silica-recast Nafion membrane in direct methanol fuel cell at 145°C. 

Ostrovskii and coworkers have also reported the application of PVDF-SPS proton conducting materials in Hj/Og polymer electrolyte fuel cells (PEFC). Proton conductivities as high as 0.13 S cm at room temperature were measured. The PVDF-SPS materials were fabricated by electron irradiation followed by grafting and sulfonation. It is well known that y rays, electrons, or... 

These results suggest that when the electrocatalytic zone is completely immersed in the perfluorocarbon fluid the diffusion of the reactants and/or products to the catalyst site is impeded by the blocking effect of the additive, which can be described as a flooded system. It is possible that the fluid also interacts with the ionomeric material in the electrocatalytic mixture, decreasing the ability to function as an proton conducting material owing to immiscibility of water with the perfluorocarbon compounds. However, the facility of reaction greatly increases as more of perfluorocarbon exits the system, as shown in Fig. 1.28. These results indicate that the presence of perfluorocarbon fluids have some beneficial effects upon the kinetics of oxygen reduction at Nafion -H-... 

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