Oxidation redox couples

Figure 11 Illustration of the interfacial CT processes in a nanocrystalline dye-sensitized solar cell. S / S+/S represent the sensitizer in the ground, oxidized and excited state, respectively. Visible light absorption by the sensitizer (1) leads to an excited state, followed by electron injection (2) onto the conduction band of Ti02. The oxidized sensitizer (3) is reduced by the I-/I3 redox couple (4) The injected electrons into the conduction band may react either with the oxidized redox couple (5) or with an oxidized dye molecule (6).

In addition to ferrocene, the oxidative redox couple that has received the most attention in supramolecular chemistry is tetrathiofulvalene (TTF), 35. This compound undergoes two reversible one-electron oxidations, first to a radical cation and then to a dication (Eq. 1.21). TTF first came to prominence in the 1970s when it was discovered that the charge transfer complex between it and 7,7,8,8-tetracyanoquinonedimethane (TCNQ) shows metallic conductivity. As a result, a large variety of different TTF derivatives have been prepared and characterized. This rich synthetic chemistry, coupled with the electroactivity, has intrigued supramolecular chemists for some time, with the result that the TTF unit has been incorporated into a wide variety of... 

Scheme 2 Pictorial representation of blocking of the oxidized redox couple reaching onto surface of TiO2 for conduction band electrons using hydrophobic sensitizers, which forms an aliphatic net work.

The rate of electron transport in dye-sensitized solar cells is a major element of the overall efficiency of the cells. The injected electrons into the conduction band, from optically excited dye, can traverse the Ti(>2 network and can be collected at the transparent conducting glass or can react either with oxidized dye molecule or with the oxidized redox couple (recombination). The reaction of injected electrons into the conduction band with the oxidized redox mediator gives undesirable dark currents, reducing significantly the charge-collection efficiency, and thereby decreasing the total efficiency of the cell (Fig. 7). 

Making the reasonable assumption that the regeneration of the oxidized redox couple at the counter electrode is faster than its recombination with the TiC>2 surface (7c6[e-] electron collection in the external circuit, r, the last term in the equation given above, can be simplified as follows ... 

One possible way of improving the performance of the cathode in a fuel cell using oxygen as the oxidant is by the use of redox systems 105). The nitric acid/nitric oxide redox couple has been... 

Smaller shifts were observed for the porphyrin oxidation redox couple. 

The oxidized redox couple is then cathodicly reduced at the electrode surface (reaction 3). 

Consider the electron-transfer reaction between a metal electrode and a reduc-tion/oxidation (redox) couple O and R in solution ... 

Although simple oxides such as LiCo02, LiNi02, and LiMn204 with highly oxidized redox couples respectively) were able to offer high ceU... 

Some undesirable reactions resulting in losses in the cell efficiency occur. They are the recombination of the injected electrons either with oxidized sensitizer (Eq. 5.5) or with the oxidized redox couple at the TiOa surface (Eq. 5.6). 

Free radicals can be produced by catalytic decomposition of a peroxide or, as illustrated below, a reducing agent can produce a Reduction-Oxidation (Redox) couple with the peroxide leading to free radical formation. 

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