Researchers around the world seek to use photosynthesis — the sunlight-driven chemical reaction that green plants and algae use to convert carbon dioxide (CO2) into cellular fuel— to generate the kinds of fuel that can power homes and vehicles. If the necessary technology could be refined past theoretical models and lab-scale prototypes, this idea, known as artificial photosynthesis, has the potential to generate large sources of completely renewable energy using the surplus CO2 in the atmosphere.
Scientists have now developed an artificial photosynthesis system, made of nano-sized tubes, that appears capable of performing the key steps of the fuel-generating reaction. The design allows for the rapid flow of protons from the interior space of the tube, where they are generated from splitting water molecules, to the outside, where they combine with CO2 and electrons to form the fuel. That fuel is currently carbon monoxide but the team is working toward making methanol. Fast proton flow, which is essential for efficiently harnessing sunlight energy to form a fuel, has been a thorn in the side of past artificial photosynthesis systems.
Now that the team has showcased how the tubes can perform all the photosynthetic tasks individually, they are ready to test the complete system. The individual unit of the system will be small, square, “solar fuel tiles” (several inches on a side) containing billions of the nano-scale tubes sandwiched between a floor and ceiling of thin, slightly flexible silicate, with the tube openings piercing through these covers. Building a solar fuel farm out of many individual tiles could proceed quickly.
Each tiny (about 0.5-micrometer-wide) hollow tube inside the tile is made of three layers: an inner layer of cobalt oxide, a middle layer of silica, and an outer layer of titanium dioxide. In the inner layer of the tube, energy from sunlight delivered to the cobalt oxide splits water (in the form of moist air that flows through the inside of each tube), producing free protons and oxygen. The protons easily flow through to the outer layer, where they combine with carbon dioxide to form carbon monoxide - and methanol in a future step - in a process enabled by a catalyst supported by the titanium dioxide layer. The fuel gathers in the space between tubes and can be easily drained out for collection.
The middle layer of the tube wall keeps the oxygen produced from water oxidation in the interior of the tube and blocks the carbon dioxide and the evolving fuel molecules on the outside from permeating into the interior, thereby separating the two very incompatible chemical reaction zones.
This design mimics actual living photosynthetic cells, which separate oxidation and reduction reactions with organic membrane compartments inside the chloroplast. In line with nature’s original blueprint, the team’s membrane tubes allow the photosynthetic reaction to occur over a very short distance, minimizing the energy loss that occurs as ions travel and preventing unintended chemical reactions that would also lower the system’s efficiency.