Energy conversion in natural and artificial photosynthesis

Abstract

Modern civilization is dependent upon fossil fuels, a nonrenewable energy source originally provided by the storage of solar energy. Fossil-fuel dependence has severe consequences, including energy security issues and greenhouse gas emissions. The consequences of fossil-fuel dependence could be avoided by fuel-producing artificial systems that mimic natural photosynthesis, directly converting solar energy to fuel. This review describes the three key components of solar energy conversion in photosynthesis: light harvesting, charge separation, and catalysis. These processes are compared in natural and in artificial systems. Such a comparison can assist in understanding the general principles of photosynthesis and in developing working devices, including photoelectrochemical cells, for solar energy conversion.

Figure 1

Solar spectrum utilization of natural and artificial pigments: (a) absorption spectra of chlorophylls a and b, plus bacteriochlorophylls a and b (in methanol or ethanol) are indicated in color (Blankenship, 2002); (b) absorption spectra of a TiO₂ thin film sensitized with a Ru-based Red Dye, NBI-Zn-Chlorin and Zn-Chlorin are indicated in color, where the Red Dye has the formula of RuL₂(NCS)₂•2TBA (L = 2,2′–bipyridyl–4,4′–dicarboxylic acid; TBA = tetrabutylammonium) and NBI represents naphthalene bisimide. The solar spectrum incident on the Earth’s surface (air mass 1.5, NREL) is indicated in grey in both panels. (The spectra of NBI-Zn-Chlorin and Zn-Chlorin are reproduced from (Röger et al., 2006).)

Figure 2

Exciton and electron transfer in bacterial photosynthesis. (a) a potential path of exciton energy transfer through the bacterial light harvesting system (blue arrow), entering as a photon at an LH2, and traveling though another LH2, then LH1 before arriving at the RC (also denoted with the blue arrow in part (b)). The Bchls of the P870 special pair (RC) and the rings of Bchls in B850 (LH2) and B875 (LH1) and are represented by white boxes with black outlines. (b) The path of electron transfer after a charge separation event at the bacterial reaction center (purple arrows). The electron travels from P870 via accessory Bchl (B_A) to Bpheo (H_A) to the ubiquinone (Q_A). A doubly reduced ubiquinone docked in the Q_B site leaves the reaction center by exchanging with the pool of oxidized quinone in the membrane.

Figure 3

Energy level diagram of purple bacterial, and higher plant (photosystems I and II) reaction centers in addition to a dye sensitized solar cell using N3 red dye (Blankenship, 2002).

Figure 4

Structures of biomimetic compounds (a) a carotenoid–porphyrin–quinone molecular triad, (b) a dinuclear di-μ-oxo Mn(III,IV) water-oxidation catalyst, and (c) a Ru–Mn(II,II) complex.

Figure 5

Photoinduced electron transfer in different artificial systems: (a) a molecular triad, (b) a DSSC, (c) a semiconductor photocatalyst, (d) a DSSC coupled to a water–oxidation catalyst, and (e) a tandem water–splitting cell. Energies are not to scale. Abbreviations: CB and VB (conduction band and valence band), D (electron donor), C (chromophore), C^* (photoexcited chromophore), A (electron acceptor), N3 and N3^* (ground state and excited state of the Ru–based N3 dye), Ox (water–oxidation catalyst), and CB′ and VB′ (conduction band and valence band of a narrow bandgap semiconductor).

Figure 6

Schematic representations of photoelectrochemical cells: (a) a DSSC, and (b) a DSSC coupled to a water–oxidation catalyst. See Figure 5(b and d) for corresponding energy diagrams. Abbreviations: N3 (ruthenium dye), C (chromophore), and Ox (water–oxidation catalyst).