Counter transport, metals through

The various steps that characterize the transport of metal species through SLMs can be described with the help of Figure 31.4. Step 1 The metal species, after diffusing to the source-membrane interface, react with the metal carrier, ions are simultaneously released into the feed solution (counter-transport, acidic carrier) or ions accompany the metal ions into the membrane (cotransport, neutral, or basic carriers). Step 2 The metal-carrier complex diffuses across the membrane because its concentration gradient is negative. Step 3 At the membrane-receiver interface the metal-carrier complex releases metal ions into the aqueous phase, ions replace M ions into the membrane (counter-transport) or X ions are simultaneously released together with M ions into the strip solution (cotransport). Step 4 The uncomplexed carrier diffuses back across the membrane. 

The use of Nafion membrane as a proton conductor has been mentioned in a three-compartment electrodialysis set-up for converting sodium salicylate to salicylic acid . Use in Donnan analysis for the recovery of Cu ions from dilute solutions has been described . These metal ions are transferred through the membrane. The driving force is the proton-motive force resulting from the proton concentration difference between the two sides of the membrane. The diffusion flux of protons is coupled to a counter-transport of metal ions. 

Dye-sensitized solar cells (DSSCs) are photoelectrochemical solar devices, currently subject of intense research in the framework of renewable energies as a low-cost photovoltaic device. DSSCs are based upon the sensitization of mesoporous nanocrystalline metal oxide films to visible light by the adsorption of molecular dyes.5"7 Photoinduced electron injection from the sensitizer dye (D) into the metal oxide conduction band initiates charge separation. Subsequently, the injected electrons are transported through the metal oxide film to a transparent electrode, while a redox-active electrolyte, such as I /I , is employed to reduce the dye cation and transport the resulting positive charge to a counter electrode (Fig. 17.4). 

PPy films containing polymeric counter ions can be used to control the transport of metal ions through the film (Davey et al., 2001). A solution, containing Na, K, Ca and Mg cations, is separated from pure water by an electro-polymerised film which in the oxidised state is impermeable to the cations. Applying a square wave potential that switches the films from the oxidised to the reduced state incorporates cations from the solution and releases some of them into the water, so that the film is rendered permeable. Permeation rates scale with cation size. This process is distinct from the permeation of neutral species described in Section 10.3.7(c). 

Importantly, any process that results in lateral coverage gradients of either the additive or metal adatom will be countered by surface diffusion. Such surface transport is known to be strongly influenced by both potential and electrolyte composition through associated impact on the structure and composition of the surface. For example, anions are known to lead to substantial enhancement of metal adatom transport with diffusion coefficients ranging from 2 1CT16 up to 8 10-13 cm2 s 1 [137, 138],... 

Figure 8.1 Schematic of a liquid electrolyte dye-sensitised solar cell. Photoexcitation of the sensitiser dye is followed by electron injection into the conduction band of the mesoporous oxide semiconductor, and electron transport through the metal oxide film to the TCO-coated glass working electrode. The dye molecule is regenerated by the redox system, which is itself regenerated at the platinised counter electrode...

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