*3.3. Photo-Bioelectrocatalysis*

Photo-bioelectrocatalysis enables reducing a substrate with a lower formal potential by oxidizing a sacrificial reagent with a higher formal potential, which cannot spontaneously occur without solar energy. Electrons that photosensitizers accept from sacrificial reagents are excited by solar energy, and the electric potential is shifted in the negative direction, corresponding to the wavelength of the adsorbed light. The electrons are then donated to substrates via enzymes and mediators, as shown in Figure 6. Photosensitizers such as TiO<sup>2</sup> [80,81,160–162], PbS quantum dots [162], silver nanoclusters [80,81], and organic dyes [160,161] are incorporated in anodes of transparent electrode bases (ITO in general) with or without other catalysts. Particularly, in addition, the photosystem II (PSII) complex in the thylakoid membrane of cyanobacteria and higher plants is often used as a water-splitting anodic photo-bioelectrocatalyst [129,160–181]. Biosolar cells [163–173] and solar biosupercapacitors [129,164,168,174], using PSII/I, thylakoid membranes, or cyanobacteria in anodes, and BOD or laccase in cathodes, realized the conversion from solar to electric energy without any sacrificial reagents in total. On the other hand, photo-bioelectrosyntheses, also called artificial photosyntheses, are reported. H<sup>2</sup> is generated by H2ase [80,160], and CO<sup>2</sup> is fixed by FoDH and CODH [81,161]. These cathodic reactions proceed at quite low potentials. Thus, in order to improve the Faradaic efficiency of these photo-bioelectrosyntheses, it is essential to reduce electron leakage to dissolved oxygen as much as possible, especially when oxygen-generating photosynthetic proteins or organisms are used as anodic photo-bioelectrocatalysts. – – –

**Figure 6.** Potential profile in a simple photo-bioelectrocatalytic system.
