Photoelectrolytic Cells

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. 

When a molecule in a solution absorbs light, an electron in the highest occupied molecular orbital (HOMO) is excited to the lowest unoccupied molecular orbital (LUMO) creating an electron vacancy, that is, a hole in the HOMO. The electron may be provided to a molecule in the solution to reduce it, whereas the hole in the HOMO may be provided to a molecule in the solution to oxidize it. This is similar to the reduction-oxidation process in the bulk semiconductor/electrolyte photoelectrolytic cell described earlier [13,17]. 

A typical photoelectrolytic cell decomposes water to produce gaseous hydrogen and o gen molecules, the overall reaction of which is expressed in Eqn. 10- 1 ... 

Fig. 10-28. Polarization curves for cell reactions of photoelectrolytic decomposition of water at a photoezcited n-type anode and at a metal cathode solid curve M = cathodic polarization curve of hydrogen evolution at metal cathode solid curve n-SC = anodic polarization curve of oxygen evolution at photoezcited n-type anode (Fermi level versus current curve) dashed curve p-SC = quasi-Fermi level of interfadal holes as a ftmction of anodic reaction current at photoezcited n-type anode (anodic polarization curve r re-sented by interfacial hole level) = electrode potential of two operating electrodes in a photoelectrolytic cell p. sc = inverse overvoltage of generation and transport ofphotoezcited holes in an n-type anode.

The photoelectrolytic cell can be composed of both a photoexcited n-type anode and a photoexcited p-type cathode here, the anode and cathode can be the same semiconductor or different semiconductors. The energy diagrams for a photo-electrol3 c cell of an n-type anode and a p-type cathode of different semiconductors are illustrated in Fig. 10-30. 

The photoelectrolytic cell consisting of n-lype and p-lype semiconductor electrodes provides an advantage over the cell consisting of semiconductor and metal electrodes a cell consisting of two semiconductor electrodes with their small band gaps adsorb the energy of solar photons more efficiently than the cell consisting of semiconductor and metal electrodes, in which the semiconductor electrode requires a relatively wide band gap for the decomposition of water. 

Fig. 10-31. Energy diagram for a photoelectrolytic cell of decomposition of water consisting of a p-type cathode of gallium phosphide and an n-type anode of titanium oxide.

It follows from Eqn. 10-58 that the affinity of the reaction of the photoelectrolytic cell, for which the polarization curves are shown in Fig. 10-32, is represented by the difference of the quasi-Fermi levels of interfacial minority charge carriers, between the anode and the cathode. Equation 10-58... 

Photoelectrolytic cells. Chemical compounds are converted irreversibly. Relevant examples of possible industrial importance are the decomposition of water or hydrogen sulphide into hydrogen and oxygen or hydrogen and sulphur respectively. 

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. 

Figure 13. Schematic of the photoelectrolytic cell designed for the generation of hydrogen using a light source (UV or visible). The anode is carbon-doped titania nanotubular arrays prepared by the sonoelectrochemical anodization technique and the cathode is platinum nanoparticles S3mthesized on undoped titania nanotubular arrays. (Redrawn from Misra et al. [220] with permission from publisher, American Chemical Society. License Number 2627061508363).

Interesting enough, it is the second type of device, namely a photoelectrolytic cell (Fig. 27b), that first caught the attention of a scientific and technological community in the 1970s that was searching... 

Photoelectrolytic cells (Optical energy stored as chemical energy in endorgic reactions e.g. H2O —> H2 -I-1/2 O2)... 

Note that the slurry-based approach can be used for water photosplitting purposes as well and has been a popular configuration by many researchers in Japan and elsewhere. In this scenario there is a crucial factor in a suspension-based vis-a-vis an electrode-based photoelectrolytic cell geometry that is worthy of attention. In the former a potential explosive mixture of H2 and O2 are photoegenrated in close proximity. Back-reactions involving the photoproducts (leading to a photostationary state) are also an issue in the former case. More simply one of the half-reactions (say the oxygen evolution reaction or OEC) is substituted with a (sacrificial) redox half-reaction such that the net reaction is thermodynamically down-hill and the reaction now becomes photocatalytic rather thanp/iofo-synthetic [20]. 

Figure IV.15 Photoelectrolytic cell with SrTi03 - electrode.

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