One of EPFL’s chemical engineering departments has created a new approach to artificial photosynthesis, which produces hydrogen from water as a clean fuel.
“Artificial photosynthesis is the holy grail of all chemists,” says Astrid Olaya, a chemical engineer at EPFL’s Institute of Chemical Sciences and Engineering (ISIC). “The goal is to capture sunlight, on the one hand to oxidize water to generate oxygen and protons, and on the other to reduce either protons to hydrogen or CO2 to chemicals and fuels. This is the essence of a circular chemical industry.”
Global demand for energy is increasing, which necessitates the development of environmentally friendly alternatives to fossil fuels. In simple fuel cells, hydrogen can be used as a source of energy and only water is wasted in the process.
The process of “water splitting,” in which water molecules are broken down into molecular hydrogen and oxygen, is one way to produce hydrogen. Light is absorbed in artificial photosynthesis to generate the energy required to break down water molecules.
There are only a few components in a traditional artificial photosynthesis device. These components include an antenna, a semiconductor to separate the electrical charges (anode and cathode), and the electrocatalyst that powers the water reduction-oxidation reaction.
However, the process remains too slow. Water oxidation with visible light (e.g. sunlight) is still a bottleneck for artificial photosynthesis, hindering large-scale development despite more than half a century of research. “The problem is that it’s hard to find electrode materials with high chemical stability, suitable optoelectronic properties, and high catalytic efficiency,” says Olaya.
A new approach to artificial photosynthesis has been developed by Olaya, an EPFL PhD student working in Hubert Girault’s EPFL lab. The Journal of the American Chemical Society Gold published the research (JACS Au).
“In this study, we photo-oxidized water with a simple organic molecule, namely tetrathiafulvalene (TTF),” says Olaya. “It has been shown that a salt version of TTF can self-assemble into microrods that act as antennas to capture the visible light and as electron pumps to oxidize water to oxygen.” Usually, this is a slow, multistep reaction but the stack of TTF salt molecules can capture the four electrons needed to oxidize a molecule of water.
The researchers also used water in an oil emulsion. “The TTF antenna can reside in the oil phase close to the water phase, where the protons produced from water oxidation are extracted,” says Olaya. “As in natural photosynthesis, the biphasic system allows an efficient separation of the reactants and products.”
Carbon, sulfur, and hydrogen atoms are the only components of TTF, all of which are readily available. Since platinum or iridium ions aren’t needed, the new method is both affordable and environmentally friendly. A new approach to artificial photosynthesis has been developed using just a few simple organic molecules, says Olaya, who led the research.
