Biomimetic approaches to artificial photosynthesis

Fossil fuels currently provide more than 80% of the primary energy needs of mankind. The sustainable development and well-being of our global society will require substantial increases in clean energy production over the next few decades. Solar energy is one of the few alternatives that can meet these future energy needs without increasing our fossil fuel dependence. More solar energy is received by our planet in an hour than our current consumption during a year. Direct conversion of solar energy into a fuel—a solar fuel—is of particular interest as it gives a storable product and balances out daily and yearly variations in solar irradiation. The solar fuel thus produced could, for example, be molecular hydrogen, or small alcohols and other carbon-based products from CO₂ reduction.

Natural photosynthesis is a major energy converter: every year, global photosynthesis converts solar energy into biomass corresponding to ca. 50 times the total yearly human energy consumption. Yet, the overall energy conversion efficiency of a plant is low, typically much less than 1%. The best energy crops and microorganisms can reach efficiencies of only a few percent under optimal conditions, which is a result of energy losses related to growth and other life processes. In contrast, the primary photosynthetic energy conversion processes: light capture, charge separation and water splitting, are very efficient. There is a strong interest, therefore, in understanding these processes and replicating them in more efficient, man-made systems, which defines the field of artificial photosynthesis.

Artificial photosynthesis builds on the natural photosynthetic principles of direct photon to solar fuel production: (1) efficient photon absorption to create an electronically excited state; (2) excited state electron transfer reactions that generate energy rich charge-separated states; and (3) storage of the energy captured in the separated charges by multi-electron catalysis to split water and fix CO₂.

Research on artificial photosynthesis is directed both towards fundamental studies—to understand the processes and principles of natural photosynthesis by use of models and biomimetic systems—and towards potential applications, where knowledge from the fundamental studies is implemented in novel designs. There is currently no solar fuel technology based on artificial photosynthesis. However, strong research efforts may lead to practical systems for direct solar fuel production. Direct solar fuel production that bypasses intermediate energy carriers, such as biomass and electricity, has the potential to be more efficient.

In this issue we have collected Perspectives and Reviews, as well as original Papers and Communications, which focus on biomimetic aspects of artificial photosynthesis. Together, they comprise all three points above, viz. light harvesting, charge separation and charge utilization for water splitting and solar fuels catalysis. They describe research on entirely synthetic molecular systems as well as hybrids composed of synthetic molecules, enzymes and conducting nanoparticles or electrodes. We believe this collection gives an updated overview on the state-of-the-art of biomimetic artificial photosynthesis, as well as indicating future research directions and challenges in this field.

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