Membrane lateral proton conduction

Thermally or photochemically induced proton transfers represent bistable switching processes and are of interest for information storage. A lateral transfer of information on the surface of biological membranes is thought to occur by fast proton conduction through protonic networks [8.234]. 

In a later work, both the CuCl/KCl molten salt Wacker oxidation system and a [Bu4N][SnCl3] system (melting point 60 °C) was applied to the electrocatalytic generation of acetaldehyde from ethanol by co-generation of electricity in a fuel cell [56]. In the cell set-up, porous carbon electrodes supported with an ionic liquid catalyst electrolyte were separated by a proton conducting membrane (Fig. 5.6-4), and current efficiency and product selectivity up to 87% and 83%, respectively, were reported at 90 °C. 

PEMFGs use a proton-conducting polymer membrane as electrolyte. The membrane is squeezed between two porous electrodes [catalyst layers (CLs)]. The electrodes consist of a network of carbon-supported catalyst for the electron transport (soHd matrix), partly filled with ionomer for the proton transport. This network, together with the reactants, forms a three-phase boundary where the reaction takes place. The unit of anode catalyst layer (ACL), membrane, and cathode catalyst layer (CCL) is called the membrane-electrode assembly (MEA). The MEA is sandwiched between porous, electrically conductive GDLs, typically made of carbon doth or carbon paper. The GDL provides a good lateral delivery of the reactants to the CL and removal of products towards the channel of the flow plates, which form the outer layers of a single cell. Single cells are connected in series to form a fuel-cell stack. The anode flow plate with structured channels is on one side and the cathode flow plate with structured channels is on the other side. This so-called bipolar plate... 

Some vinyl fluoride-based polymers with side chains of perfluorosulfonic acid (the Nation family) are important ion-exchange membrane materials used in practice for electrolysis of NaCl and in certain fuel cells. They show a proton conductivity of 0.01 S cm- at room temperature. However, such fast ionic transport occurs only when they are swollen with water. It is therefore not appropriate to call them solid electrolytes in the tme sense of the word. It was in 1970 that anionic conductivity, though not high, was reported for crown ether complexes such as dibenzo-18-crown-6 KSCN, in which cations are trapped by the ligand. " A few years later much higher cationic (instead of anionic) conduction was found in complexes of a chain-like polyether such as PEO or PPO with alkaline salts here, PEO stands for poly(ethyleneoxide), (CHjCHj-O), and PPO for poly(propyleneoxide)."2>"3 These were the flrst examples of tme polymer solid electrolytes and were followed by a great number of studies. Polymeric electrolytes are advantageous in practice because they are easily processed and formed into flexible Aims. 

Membranes based on Nation , Flemion, and Aciplex have been commercialized for brine electrolysis and they are used in the form of alkali metal salt. The technology was applied to PEMFC membranes with a thickness of over 50 pm later [42]. For the synthesis of this type of polymer, a fluorosulfonyl monomer is frequently copolymerized with tetrafluoroethylene (TFE). The synthetic scheme of this monomer is shown in Fig. 2.2 [51]. The lEC is about 0.9-1.1 mequiv./g dry polymer. As the lEC increases, water absorption increases, and the crystallinity based on successive sequences of the TFE monomer unit becomes smaller, which lowers the mechanical strength. On the other hand, when the lEC decreases, water absorption decreases, which lowers the proton conductivity. 

Teissie and coworkers detected rapid lateral movement of protons on a phospholipid monolayer-water interface by a number of measurements fluorescence from a pH indicator dye near the membrane surface, electrical surface conductance, and surface potentiaL These investigators found that the conduction of protons along the surface is considerably faster than proton conduction in the bulk phase (2 to 3 min versus 40 min for a comparable distance in their measurement setup). This novel conduction mechanism is proton-specific, as confirmed by a radioactive electrode measurement as well as by replacement with deuterated water. It is a consequence of cooperativity between neighboring phospholipid molecules the conduction mechanism disappears when phospholipid molecules are not in contact with each other. 

It has been proposed that Reaction II is caused by a lateral and transversal delocalization on inner membrane electric fields associated with the liberation of protons in inner membranes domains near the Fe-S cyt b-f protein complex (Westerhof et al., 1983). These domains might be connected via lateral H-conductive channels with other membranes domains i.e. the ATP synthetase. In this respect it is of interest to mention that, in confirmation with result of others (Schuurmans et al., 1981 Schreiber, Rienits, 1982), Reaction II can also be induced in the dark towards its saturation level by ATP driven reversed electron flow. The light-induced response of Reaction II, absent during ATP hydrolysis, reappears after the ATPase has become inactivated or, with broken chloroplasts in the presence of DTE, when ATP has been consumed (R.L.A. Peters, this s)niiposium). Whether the existence of the membrane domains and their highly efficient... 

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