Protonic conductivity

Figure 4.1 Schematic of the atomic structure of the active three-phase interface between the metal particle that catalyzes the reaction, the carbon support necessary to conduct electrons, and the polymer electrolyte and solution necessary to conduct protons for electrocatalytic systems.

Figure 2.1 shows a schematic structure of the fuel cell membrane electrode assembly (MEA), including both anode and cathode sides. Each side includes a catalyst layer and a gas diffusion layer. Between the two sides is a proton exchange membrane (PEM) conducting protons from the anode to the cathode. 

Porphin derivatives are unique since they conduct proton ejection as well as electron transfer (24). 

PANI is unique in that its most oxidized state, the pernigraniline form (which can be accessed reversibly), is not conducting. In fact, it is the intermediately oxidized emeraldine base that exhibits the highest electrical conductivity. Protonic Acid Doping is the most general means by which to obtain this partially pro-tonated form of PANI [301]. Exposure of the emeraldine salt to alkali solutions reverses this process and brings a return to the insulating state. 

Solid Polymer Electrolyte Fuel Cell Here, there is no apparent liquid solution, or high-temperature ionic conductor. The usual ionic solution between the electrodes is replaced by a well-humidified membrane made of a perfluorosulfonic acid polymer that conducts protons. 

The main disadvantage of Nafion is that the inverse micelles in the structure must contain water if the polymer is to conduct protons or sodium ions, and dehydration of the membrane at higher temperatures leads to failure. A second disadvantage, particularly serious in... 

In a H2/air fuel cell, the protons produced at the anode side need to be transferred to the cathode side to react with 02. This requires a proton transport electrolyte. Nafion membranes, composed of a perfluorosulfonated polymer, are the most commonly used polymer electrolyte membranes to conduct protons. The structure of the Nafion membrane is shown in Figure 1.5. Nafion can take on a... 

Key Words Dipolar glasses, Ferroelectric relaxors, Conducting polymers, NMR line shape, Disorder, Local polarization related to the line shape, Symmetric/asymmetric quadrupole-perturbed NMR, H-bonded systems, Spin-lattice relaxation, Edwards-Anderson order parameter, Dimensionality of conduction, Proton, Deuteron tunnelling. 

Although the secrets of maximal rates of proton conduction are well illustrated in gA, multifunctional proteins that couple H+ conduction to other events do not exhibit well-formed, proton-conducting hydrogen bond networks. Indeed, in the bacterial reaction center the putative active path is poorly connected by hydrogen bonds detectable in the best current X-ray structures (2.2 A resolution Stowell et al., 1997). Paddock et al. (1999) have shown that chemical blockage or a simple mutational lesion of this active path diminishes proton transfer rates by at least 1000-fold. Thus, the several well-connected (but not quite continuous) files of water that are seen in the X-ray structures, reaching toward the Qg site from the cytoplasmic side, do not conduct protons at significant rates. 

Glipa, X. et al.. Synthesis and characterisation of sulfonated polybenzimidazole a highly conducting proton exchange polymer. Solid State Ionics, 97, 323, 1997. 

Fj-Fq complex of mitochondria (Section 18.4.1). CFq conducts protons across the thylakoid membrane, whereas CF] catalyzes the formation of ATP from ADP and Pj. 

The most important fuel cells that are in use nowadays are the polymer electrolyte membrane fuel ceU (PEMFC), the molten carbonate fuel cell (MCFC), and the solid oxide fuel cell (SOFC). In a PEMFC, the electrolyte is a polymer membrane that conducts protons, in an MCFC the electrolyte is a carbonate melt in which oxygen is conducted in the form of carbonate ions, CO , and in an SOFC the electrolyte is a solid oxide that conducts oxygen ions, While a PEMFC can be operated at low temperatures of about 80 °C, an MCFC works at intermediate temperatures of about 650 °C, and an SOFC needs relatively high temperatures of 800-1000 °C (see next sections). 

The state-of-the-art proton-conducting polymer membranes use water networks to conduct protons. Thus, they require sufficient membrane hydration for functioning, which limits operation of the present-day cells to temperatures lower... 

The proton-motive force generated by the light reactions is converted into ATP by the ATP synthase of chloroplasts, also called the CFi-CF complex (C stands for chloroplast and F for factor). CF -CF,) ATP synthase closely resembles the F —Fo complex of mitochondria (p. 522). CFo conducts protons across the thylakoid membrane, whereas CF catalyi es the formation of ATP from ADP and Pj. 

NASICON can also be modified so that it conducts protons (see Ref. [69] and Chapter 7). However, alkaline-earth cerates and zirconates forming the perovskite structure are more commonly used as high-temperature proton conductors [70-78]. 

An ion, on the other hand, is of the same general size as parts of the structure to which it migrates. The proton is perhaps a limiting case in that there may be a number of sites to which it can migrate, as the facility with which aqueous systems conduct protons shows [75,76], Nevertheless, a structure, a membrane surface for example, that admits ion transfer across the boundary at particular points can indeed reveal sites that are of the size of the probe. The interpretation of the... 

Hydroxide ions also have abnormally high conductivities, and the explanation in their case is similar to that for hydrogen-ion conductivity. Protons are transferred from water molecules to hydroxide ions ... 

Uncoupling proteins (UCPs) form channels through the inner mitochondrial membrane that are able to conduct protons from the intermembrane space to the matrix, thereby short-circuiting ATP synthase. 

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