Photogalvanic cells

Figure 5.58 presents a general scheme of photogalvanic cells based on reductive (a) or oxidative (b) quenching of S. These cells are regenerative if D and A are two forms of the same redox couple (D+ =A and D =A ), otherwise the products D+ and A accumulate in the electrolyte and the cell is photoelectrosynthetic. 

Fig. 5.58 Scheme of a photogalvanic cell. The homogeneous photoredox process takes place in the vicinity of the optically transparent anode (a) or cathode (b)... 

A regenerative photogalvanic cell with oxidative quenching (Fig. 5.58b) is based, for example, on the Fe3+-Ru(bpy)2+ system. In contrast to the iron-thionine cell, the homogeneous photoredox process takes place near the (optically transparent) cathode. The photoexcited Ru(bpy)2+ ion reduces Fe3+ and the formed Ru(bpy)3+ and Fe2+ are converted at the opposite electrodes to the initial state. 

More complicated photogalvanic cells may employ two light absorbing systems one in the cathodic process and the other in the anodic process. An... 

The efficiency of the photocurrent generation in practical photogalvanic cells is generally low, since it is limited by short lifetimes of the excited dyes, parasitic electron transfer reactions, etc. 

The electrochemical cell can again be of the regenerative or electrosynthetic type, as with the photogalvanic cells described above. In the regenerative photovoltaic cell, the electron donor (D) and acceptor (A) (see Fig. 5.62) are two redox forms of one reversible redox couple, e.g. Fe(CN)6-/4 , I2/I , Br2/Br , S2 /S2, etc. the cell reaction is cyclic (AG = 0, cf. Eq. (5.10.24) since =A and D = A ). On the other hand, in the electrosynthetic cell, the half-cell reactions are irreversible and the products (D+ and A ) accumulate in the electrolyte. The most carefully studied reaction of this type is photoelectrolysis of water (D+ = 02 and A = H2)- Other photoelectrosynthetic studies include the preparation of S2O8-, the reduction of C02 to formic acid, N2 to NH3, etc. 

When a photochemical process in solution gives a photoresponse at the electrode, the system can form a photogalvanic cell. The photoinduced redox reaction is typical for photogalvanic cells. The most well known photoredox system is thionine (TH+ 23) and ferrous ion88). The excited TH+ is reduced by Fe2+ to give TH and Fe3+ (Eq. (25)). 

Fig. 24. Photogalvanic cell composed of light and dark chambers...

Figure 9.11 Primitive iron-thionine photogalvanic cell.

Electrochemiluminescence typically involves the electrochemical generation of species that undergo homogeneous reactions to generate electronically excited dissolved compounds. In certain cases, emission can then be observed as these molecular species relax to the ground state. As such, ECL in some sense represents the opposite process of that observed in a photogalvanic cell. 

Another approach has been to use a photogalvanic cell with a rotating optically transparent Sn02 electrode. Light is used to drive a redox reaction in solution, e.g. equation (48) 42... 

Direct conversion of solar energy into electrical energy can be obtained by means of photogalvanic cells, in which a photochemical reaction causes a change in the concentration of the electroactive species. The principles and applications of photogalvanic cells have been recently reviewed107-110). 

Photoelectrolytic and photogalvanic cells. These cells involve the conversion of radiant energy in chemical energy for converting substances but unlike the situation in (2), AG>0. The potential applied to the electrode helps the conversion. 

The semiconductor electrode most studied in photoelectrolytic cells has been n-Ti02, and in photogalvanic cells n-Sn02. Because the bandgap energies are 3.0 eV and 3.5 eV respectively, they are not optimum semiconductors as they only make use of about 5 per cent of the solar energy. For this reason there has been research into other semiconductors, for example cadmium sulphide. In all cases the efficiency is fairly low. 

Photogalvanic cell An electrochemical ceU in which current or voltage changes result from photochemically generated changes in the relative concentrations of reactants in a solution phase oxidation-reduction couple. Compare photovoltaic cell. 

Photovoltaic cell A solid state device, usually a semiconductor, such as silicon, which absorbs photons with energies higher than or equal to the handgap energy and simultaneously produces electric power. Compare photogalvanic cell. 

The addition of electron donors or acceptors to the external electrolyte has allowed sustained photocurrents to be measured at sensitized metal interfaces, but the mechanism(s) often remain speculative. A photocurrent can be generated by excited state interfacial electron transfer like that shown in Figure 5, or by inter-molecular excited state electron transfer followed by dark redox reactions at the electrode. It can be experimentally difficult to distinguish between these distinct mechanisms and strong evidence exists only for the latter pathway which forms the basis of the photogalvanic cell. 

Figure 6. A regenerative photogalvanic cell where excited state electron transfer yields charge separated pair, D+ and A , that are collected at dark electrodes.

The first attempts to understand the mechanism of operation of photogalvanic cells was performed on the iron-thionine system [22, 23], Here we summarize the main observations. Thionine (Th) is a cationic purple dye extremely soluble as the chloride salt. Two other forms of the compound, Leukothionine (L) and Semithionine (S ), are also important in the sensitization process and their structures are shown in Figure 9. According to Albery, the light driven redox reactions that lead to a photocurrent are as follows ... 

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