Comparison with Reactions at Metal Electrodes

Further studies on the mechanism of electron transfer from an excited dye to a nanocrystalline semiconductor are necessary for a full understanding of the intermediates and products involved in these processes. 

In some cases, however, an anodic sensitization current was observed if a reducing agent was added to the dye solution contacting the metal electrode. This has been found, for instance with rhodamine-B as a dye and allylthiourea as a reducing agent [58]. The reaction is given by 

The corresponding current value is mainly determined by the back-reaction between S and D . This kind of photocurrent is based on photochemical effects in solution and is also known as the photogalvanic effect, already reported in 1961 [59], Very large photocurrents have been obtained by using the leuco dye (SH2) as an electron donor. 

10 Electron Transfer Processes - Excited Molecules and Semiconductor Electrodes 

Here the radical is the only product of the reaction. Since the radicals may exhibit a large lifetime in 02-free solution, many of them reach the electrode which leads to a large current [58]. 

Since it was reported that a dye-sensitized solar cell with a conversion efficiency of about 7% was obtained [49], many researchers started to study the electron transfer processes which limit the solar conversion efficiency. This is discussed in detail in Section 11.1.1.2. 

The corresponding current value is mainly determined by the back-reaction between S and D . This kind of photocurrent is based on photochemical effects 

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PEMFC)/direct methanol fuel cell (DMFC) cathode limit the available sites for reduction of molecular oxygen. Alternatively, at the anode of a PEMFC or DMFC, the oxidation of water is necessary to produce hydroxyl or oxygen species that participate in oxidation of strongly bound carbon monoxide species. Taylor and co-workers [Taylor et ah, 2007b] have recently reported on a systematic study that examined the potential dependence of water redox reactions over a series of different metal electrode surfaces. For comparison purposes, we will start with a brief discussion of electronic structure studies of water activity with consideration of UHV model systems. 

Given that inner-sphere pathways are commonly encountered at metal-solution interfaces, as between reactants in homogeneous solution, a key question concerns the manner and extent to which the reactant-electrode interactions associated with such pathways lead to reactivity enhancements compared with weak-overlap pathways (Sect. 3.5.2). A useful tactic involves the comparison between the kinetics of structurally related reactions that occur via inner- and outer-sphere pathways. This presumes that the outer-sphere route yields kinetics which approximate that for the weak-overlap limit. For this purpose, it is desirable to estimate the work-corrected uni-molecular rate constant for the outer-sphere pathway at a particular electrode potential, k° , from the corresponding work-corrected measured value, kCOTr, using [cf. eqns. (10) and (13)]. 

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