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Tiny Tubes for Artificial Synthesis

Modern civilization is dependent upon fossil fuels. Fossil-fuel dependence, however, has serious consequences which concern energy security issues and greenhouse gas emissions. This could be avoided by fuel-producing artificial systems that emulate natural photosynthesis and directly convert solar energy into fuel. Researchers all over the world want to use this chemical reaction, which is initiated by sunlight and used by green plants and algae to transform carbon dioxide (CO2) into cellular fuel, to generate fuels that can power homes and vehicles. If scientists succeed in getting past theoretical models and lab-scale prototypes, artificial photosynthesis has great potential to generate large sources of carbon-neutral energy using the surplus CO2 in our atmosphere.

Now (2020), a new design has brought scientists one step closer to realising artificial photosynthesis. Scientists at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) have developed an artificial photosynthesis system, which is made of nanosized tubes, that is able to perform all the key steps of the fuel-generating reaction. This design enables the rapid flow of protons from the interior space of the tube, where they are created by splitting water molecules, to the outside, where they combine with CO2 and electrons to form the fuel. That fuel is currently carbon monoxide, which they hope can eventually be converted into methanol. The individual unit of the system consists of small square “solar fuel tiles” (several inches on a side) with billions of the nanoscale tubes located between a floor and ceiling of thin, flexible silicate, with the tube openings piercing through these covers.

Each tiny 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, which is delivered to the cobalt oxide, splits water and creates free protons and oxygen. The middle layer of the tube wall keeps the oxygen in the interior of the tube and keeps the carbon dioxide and the evolving fuel molecules on the outside from permeating into the interior.

Scientists have spent decades working towards building an artificial version of one of nature’s most elegant and effective machines - the leaf. In 2018, scientists from the University of Cambridge and the Ruhr University, Bochum, discovered a new technique that mimics the natural process of photosynthesis in plants, which could be used to produce hydrogen fuel. The team of scientists primarily used hydrogenase, an enzyme which has been dormant in algae for millions of years. They combined hydrogenase with synthetic pigments to provoke sunlight to split water into hydrogen and oxygen unassisted. They were trying to “establish a new line of research by combining the best of the natural and artificial worlds and take highly efficient and abundant biological catalysts, such as enzymes, and combine them with synthetic materials in solar devices for efficient solar fuel synthesis.”

In 2019, scientists developed a method to achieve artificial photosynthesis, producing high-energy hydrocarbons by leveraging electron-rich gold nanoparticles as a catalyst. The team found that tiny spherical gold particles, which measure only nanometres in size, could absorb visible green light and transfer photo-excited electrons and protons. In this study the scientists also succeeded in converting CO2 into complex hydrocarbon fuel molecules – including propane and methane – which were synthesised by combining green light with the gold nanoparticles in an ionic liquid. The method also made it possible to photosynthesise ethylene, acetylene, and propene - complex molecular arrangements that could one day enable viable energy storage in fuel cells.

The unique system developed at Berkeley Lab which uses nanosized tubes and rapid flow of protons for artificial photosynthesis has opened new horizons for fuel production; it emulates living photosynthetic cells, which separate oxidation and reduction reactions with organic membrane compartments inside the chloroplast. Similarly, the membrane tubes allow the photosynthetic reaction to occur over a very short distance, minimizing the energy loss that occurs as ions travel and prevent unintended chemical reactions that would also lower the system’s efficiency.

This work is part of scientists’ commitment to find solutions to the energy issues posed by climate change. The only task remaining is to deliver a photosynthesis system that works without any loss of performance. Engineers in industry can then take over, connect these tiles and create a device able power our lives.