Photoelectrode reaction

Chapter 10 deals with photoelectrode reactions at semiconductor electrodes in which the concentration of minority carriers is increased by photoexcitation, thereby enabling the transfer of electrons to occur that can not proceed in the dark. The concept of quasi-Fermi level is introduced to account for photoenergy gain in semiconductor electrodes. Chapter 11 discusses the coupled electrode. mixed electrode) at which anodic and cathodic reactions occur at the same rate on a single electrode this concept is illustrated by corroding metal electrodes in aqueous solutions. 

Trigonal, metallic selenium has been investigated as photoelectrode for solar energy conversion, due to its semiconducting properties. The photoelectrochemistry of the element has been studied in some detail by Gissler [35], A photodecomposition reaction of Se into hydrogen selenide was observed in acidic solutions. Only redox couples with a relatively anodic standard potential could prevent dissolution of Se crystal. 

The photovoltaic effect is initiated by light absorption in the electrode material. This is practically important only with semiconductor electrodes, where the photogenerated, excited electrons or holes may, under certain conditions, react with electrolyte redox systems. The photoredox reaction at the illuminated semiconductor thus drives the complementary (dark) reaction at the counterelectrode, which again may (but need not) regenerate the reactant consumed at the photoelectrode. The regenerative mode of operation is, according to the IUPAC recommendation, denoted as photovoltaic cell and the second one as photoelectrolytic cell . Alternative classification and terms will be discussed below. 

The photopontential also approaches to zero when the semiconductor photoelectrode is short-circuited to a metal counterelectrode at which a fast reaction (injection of the majority carriers into the electrolyte) takes place. The corresponding photocurrent density is defined as a difference between the current densities under illumination, /light and in the dark, jDARK ... 

Figure 5. Design of a cell for photoassisted electrolysis of C02 under elevated pressures.97 (1) Photoelectrode (2) reference electrode (3) counter electrode (4) sampling port with septum (5) pressure regulator (6) pressure gauge (7) O-rings (8) reaction cell (9) separator (10) quartz window (11) insulated connection (12) bolts (13) connections to potentiostat.

In such devices the light-absorbing semiconductor electrode immersed in an electrolyte solution comprises a photosensitive interface where thermodynamically uphill redox processes can be driven with optical energy. Depending on the nature of the photoelectrode, either a reduction or an oxidation half-reaction can be light-driven with the counterelectrode being the site of the accompanying half-reaction. N-type semiconductors are photoanodes, p-type semiconductors are photocathodes, and... 

The conclusions from these considerations are that semiconductor photoelectrodes can be used to effect either reductions (p-type semiconductors) or oxidations (n-type semiconductors) in an uphill fashion. The extent to which reaction can be driven uphill, Ey, is no greater than Eg, but may be lower than Eg owing to surface states between Eqb and Eye or to an Inappropriate value of Ere(jox. Both Eg and Epg are properties that depend on the semiconductor bulk and surface properties. Interestingly, Ey can be independent of Ere(jox meaning that the choice of Ere(jox and the associated redox reagents can be made on the basis of factors other than theoretical efficiency, for a given semiconductor. Thus, the important reduction processes represented by the half-reactions (3)-(5) could, in principle, be effected with the same efficiency at a Fermi level pinned (or... 

Ruthenium bipyridyl complexes are suitable photosensitizers because then-excited states have a long lifetime and the oxidized Ru(III) center has a longterm chemical stability. Therefore, Ru bipyridyl complexes have been studied intensively as photosensitizers for homogeneous photocatalytic reactions and dye-sensitization systems. The Ru bipyridyl complex, bis(2,2 -bipyridine)(2,2 -bipyri-dine-4, 4,-dicarboxylate)ruthenium(II), having carboxyl groups as anchors to the semiconductor surface was synthesized and single-crystal Ti02 photoelectrodes sensitized by this Ru complex were studied in 1979 and 1980 [5,6]. 

Photoelectrochemical Storage Cells. These cells are secondary batteries which are recharged by light. A photochemical reaction produces some photoproduct(s) at the photoelectrode, this product then being stored ready for reaction at the dark electrode. The dark reverse reaction of these products... 

Gerischer(16), Bard and Wrighton(17) have recently discussed a simple model for the thermodynamic stability of a range of photoelectrodes. As has been discussed previously, except for the rare case where the anodic and cathodic decomposition potentials lie outside the band gap, the electrode will be intrinsically unstable anodically, cathodically, or both.(16) It is the relative overpotential of the redox reaction of interest compared to that of the appropriate decomposition potential which determines the relative kinetics and thus stability of the electrode as illustrated in Figure 4. The cathodic and anodic decomposition potentials may be roughly estimated by thermodynamic free-energy calculations but these numbers may not be truly representative due to the mediation of surface effects. 

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