Artificial photosynthesis

Artificial photosynthesis generally involves light absorption, charge separation, and catalytic reactions to drive fuel-forming reactions. The most studied reaction is water splitting, which produces hydrogen and oxygen using sunlight. Another approach is CO2 reduction, which converts carbon dioxide and water into hydrocarbons or alcohols, such as methanol or methane.

Key components include a light-harvesting material (e.g., a semiconductor or molecular dye), a catalyst for the oxidation and reduction half-reactions, and a membrane or junction to separate products and prevent back reactions. Systems can be photoelectrochemical cells, where light-absorbing electrodes are immersed in an electrolyte, or particulate photocatalysts, where tiny particles suspended in water perform the reactions.

The concept dates to 1912 when Giacomo Ciamician proposed using photochemistry to produce fuels. The first experimental demonstration of photoelectrochemical water splitting was by Akira Fujishima and Kenichi Honda in 1972, using a titanium dioxide (TiO2) electrode under ultraviolet light. This sparked intense research into semiconductor-based systems.

In the 1980s and 1990s, molecular catalysts and dye-sensitized systems advanced, and in 1998, the first molecular system for visible-light water splitting was reported. Recent decades have seen improvements in efficiency and stability, with some systems achieving solar-to-hydrogen efficiencies above 10% under concentrated light.

Metal oxides like TiO2, WO3, and BiVO4 are common photoanodes, while III-V semiconductors such as GaAs and InP offer high efficiency but are costly. Earth-abundant materials, including iron, nickel, and cobalt-based catalysts, are being developed to replace rare and expensive platinum group metals.

Molecular catalysts, such as those based on cobalt or nickel complexes, can be attached to electrodes or used in solution. Recent research has explored metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) as tunable platforms for integrating light absorbers and catalytic sites.

Major challenges include achieving high efficiency, long-term stability, and scalability. Many systems degrade under prolonged illumination or in aqueous environments. Researchers are investigating protective coatings, novel electrolytes, and tandem configurations to overcome these issues.

Another focus is on CO2 reduction, which is more complex due to the multiple electron and proton transfers required. Selectivity for desired products remains a hurdle. Recent studies have demonstrated selective CO2 reduction to ethylene and ethanol using copper-based catalysts, but efficiencies are still low.

Beyond fuel production, artificial photosynthesis can be used to produce ammonia from nitrogen and water, a process that could replace the energy-intensive Haber-Bosch process. This is an emerging area with potential for sustainable fertilizer production.

Another niche application is the production of hydrogen peroxide, a valuable disinfectant and oxidant, directly from water and oxygen using sunlight. Some systems also explore the use of photoelectrochemical cells for wastewater treatment, where organic pollutants are degraded while generating hydrogen.

Historically, the term 'artificial photosynthesis' was also used in the 1970s to describe attempts to mimic the photosynthetic electron transport chain for solar energy storage, but the modern focus is on fuel production.

Photoelectrochemical cell

A device that uses light to drive electrochemical reactions, typically splitting water or reducing CO2.

Water splitting

The decomposition of water into hydrogen and oxygen, often using a catalyst and energy input.

CO2 reduction

The conversion of carbon dioxide into fuels or chemicals using electrons and protons.

Photocatalyst

A material that accelerates a chemical reaction when exposed to light.