Artificial Photosynthesis - Current Research

Current Research

In energy terms, natural photosynthesis can be divided in three steps:

• Light-harvesting complexes in bacteria and plants capture photons and transduce them into electrons, injecting them into the photosynthetic chain.
• Proton-coupled electron transfer along several cofactors of the photosynthetic chain, causing local, spatial charge separation.
• Redox catalysis, which uses the aforementioned transferred electrons, to oxidize water to dioxygen and protons; these protons can in some species be utilized for dihydrogen production.

Using biomimetic approaches, artificial photosynthesis tries to construct systems doing the same type of processes. Ideally, a triad assembly could oxidize water with one catalyst, reduce protons with another and have a photosensitizer molecule to power the whole system. One of the simplest designs is where the photosensitizer is linked in tandem between a water oxidation catalyst and a hydrogen evolving catalyst:

• The photosensitizer transfers electrons to the hydrogen catalyst when hit by light, becoming oxidized in the process.
• This drives the water splitting catalyst to donate electrons to the photosensitizer. In a triad assembly, such a catalyst is often referred to as a donor. The oxidized donor is able to perform water oxidation.

The state of the triad with one catalyst oxidized on one end and the second one reduced on the other end of the triad is referred to as a charge separation, and is a driving force for further electron transfer, and consequently catalysis, to occur. The different components may be assembled in diverse ways, such as supramolecular complexes, compartmentalized cells, or linearly, covalently linked molecules.

Research into finding catalysts that can convert water, carbon dioxide, and sunlight to carbohydrates or hydrogen is a current, active field. By studying the natural oxygen-evolving complex, researchers have developed catalysts such as the "blue dimer" to mimic its function. Photoelectrochemical cells that reduce carbon dioxide into carbon monoxide (CO), formic acid (HCOOH) and methanol (CH₃OH) are under development. However, these catalysts are still very inefficient.