Towards Artificial Photosynthesis

Abstract

Artificial photosynthesis in the broadest sense encompasses human attempts to convert sunlight, water and carbon dioxide into carbohydrates, other organic compounds and oxygen, to split water into hydrogen and oxygen using sunlight, or to convert sunlight directly into electricity. Although largely still in its infancy, artificial photosynthesis is being researched on various fronts. Moderately-high-efficiency silicon photovoltaic generators are the most established and durable devices, but their efficiencies are still very much below thermodynamic limitation; thus, further improvements are feasible. Of great concern is how to lower the costs of manufacturing silicon photovoltaic devices. Dye-sensitized photovoltaic cells, when designed to absorb a broad spectrum of solar radiation, seem to offer great promise, especially in terms of lower manufacturing costs. The field of photon-harvesting for non-biological hydrogen production has advanced steadily, ever since the discovery of light-induced splitting of water into hydrogen and oxygen at a semiconductor electrode (Fujishima A, Honda K, Nature 238:37-38, 1972). A deficiency that needs to be addressed, however, is the limited absorption of the solar spectrum by a semiconductor. A tandem device with two semiconductor electrodes, absorbing complementary portions of the solar spectrum and working in series, partially overcomes the poor absorption, in a way that is analogous to the "Z Scheme" of natural photosynthesis in which two photosystems operate in series. Photoconversion efficiency, however, has to be further improved. Artificial reaction centers capable of light-induced charge separation have been constructed, based on porphyrin-fullerene complexes, ruthenium complexes or synthetic proteins holding redox cofactors. The longer-term application of such artificial reaction centers is their coupling to chemical/biochemical reactions that produce useful products/devices. Another spin-off from research on artificial reaction centers is the basic understanding of the mechanisms of forward and back charge transfers that can be gained from a comparison of natural and artificial reaction centers. Perhaps the most ambitious goal in artificial photosynthesis is the in vitro making of carbon-based end products, especially foodstuffs. Such a supplementation of the natural process may be necessitated by a gradual loss of agricultural land, a shortage of fresh water and a need to feed an increasing world population. Artificial photosynthesis for food production may present one of the greatest challenges to future scientists and technologists.