It’s Like a Leaf, But Better
Artificial photosynthetic systems for integrated carbon capture and conversion may put leaves out of business.
Not to be melodramatic, but when you break it down, our global population thrives best when it’s busy destroying the only home we have in this solar system.
The invention of fossil fuels has allowed our population to explode. When we’re not burning fossil fuels to heat our homes or to run our cars, we’re using fossil fuels to make our lives more simple by transporting our exotic foods long distances or manufacturing products made solely from petrochemicals.
However, because we as a species tend to postpone responsibility when we can (thank you to Shin Jie Yong for pointing this out so eloquently in a comment on my last article), innovation to combat the fast-approaching global-temperature targets set by the Paris Agreement has brought us remarkable carbon capture technology that is more effective than carbon-capturing leaves from nature.
Biomimicry is to thank for many innovations in the scientific world and the partnership between science and nature is far from over. If anything, it’s just beginning.
In November, the Argonne National Laboratory and SLAC National Accelerator Laboratory announced that they will be developing artificial photosynthesis methods that will be used to conduct direct air capture of carbon dioxide. From there, they will look to expand sources of energy by converting the captured carbon dioxide into fuels and other chemicals to be used by industry.
Carbon capture involves trapping carbon dioxide (along with any other gases in the vicinity), transporting it to a storage location, and then isolating the carbon dioxide gas specifically. Argonne and SLAC in partnership will be looking to develop the photochemical methods necessary to capture the carbon dioxide directly from the air (thereby removing several steps in the old process) and then will look to combine this with photochemical conversion technology that will begin transforming the carbon dioxide into fuels and chemicals.
According to Argonne, the molecular photoreactor that will house the entire carbon capture transformation process will be comprised of what they call “molecular Lego pieces”. Each piece will be designed to fulfill a specific function. For instance, Argonne describes the different pieces that will make up the photoreactor, including chromophores that will absorb sunlight, molecules that will capture the carbon dioxide from the atmosphere, and catalysts that will help in the conversion of carbon dioxide into chemicals useable by industry.
While carbon capture technology and artificial photosynthesis exist on their own, Argonne and SLAC are hoping to combine the two processes into a methodology termed “photoreactive capture”. One is great, but two are better, and nothing demonstrates that more accurately than combining technology that scrubs carbon dioxide from the atmosphere and then turns it back into a type of renewable energy that can then be used as chemicals or as fuel.
The field of chemistry has already had prior success in engineering artificial photosynthetic systems that can be used in tandem with carbon capture technology to turn carbon dioxide into usable fuels.
The first photosynthetic biohybrid system was produced in 2015. The system developed at UC Berkeley used semiconductors and live bacteria to produce a photosynthetic reaction that would use solar energy to produce liquid fuels using carbon dioxide and water. At the time, it was predicted that if the technology improved, it may become the future of energy. The process involves catalysts harvesting solar energy to generate charge which is then transferred to bacteria. The bacteria use the electric charge from the catalysts to instigate a chemical reduction reaction that turns carbon dioxide into liquid fuel. Butanol, acetate, polymers, and pharmaceutical precursors were produced as a result of this experiment. Furthermore, it was found that this first artificial photosynthetic system had a solar-to-chemical conversion efficiency of 0.38%, which is comparable to the efficiency of a natural leaf.
The innovation didn’t stop there though.
A year later at McGill University in Montreal, Canada, the chemical transformation of carbon dioxide into fuels was achieved using metal-nitride nanowires as a catalyst to overcome the eventual efficiency bottleneck that can occur when using conventional photocatalytic technologies. While the typical conversion process of carbon dioxide into fuel requires high temperatures, high pressures, and/or extremely reactive reagents, the metal-nitride nanowire photocatalyst developed allows the process to be completed more stably and efficiently, while also occurring at room temperature. The result: solar fuels generated using an efficient artificial photosynthetic system that can be used to pull carbon dioxide out of the atmosphere, bringing society closer to a net-zero future.
UC Berkeley and McGill University aren’t alone in developing this groundbreaking technology. Just a few years later, another breakthrough came in the world of artificial photosynthetic systems.
In a 2019 report published in the American Chemical Society journal of Sustainable Chemistry and Engineering, the solar-to-fuel efficiency of an artificial photosynthetic (AP) system was evaluated. The study looked directly at how the AP system can capture carbon dioxide directly from the atmosphere and can then convert it into fuel using sunlight. The report concludes that a modern, fully integrated AP system could reduce the carbon dioxide levels in the surrounding air by 10% during steady-state operation, which makes it fourteen times more efficient than natural leaves.
This report was analyzing a study conducted by the Department of Chemical Engineering from the University of Illinois, who had successfully produced fuels using water, visible light, and carbon dioxide using artificial photosynthesis. Scientists conducting the study developed an artificial photosynthesis process that used the same visible green light portion of the electromagnetic spectrum used by plants during natural photosynthesis to convert carbon dioxide and water into fuel. The process used a plasmonic excitation of gold nanoparticles (used as a catalyst) to produce a rich, electrically-charged environment at the gold particle-carbon dioxide solution interface which is conducive to carbon dioxide activation. From there, an ionic liquid stabilizes the charged intermediates that formed on the surface of the interface which begins to facilitate a multi-step chemical reduction reaction coupled with carbon coupling. Multi-carbon chains can be produced, resulting in the formation of methane, ethylene, acetylene, propane, and propene.
The University of Illinois chemists who conducted the study wanted to focus on creating liquid fuels because they are easier, safer, and more economical to store and transport than regular gasoline. Furthermore, liquid fuels are comprised of long-chain molecules containing more bonds than regular gasoline, giving them a higher energy density. While the goal of the study was to learn how to use catalysts to increase the efficiency of the chemical reactions undergone during the carbon dioxide-to-fuel conversion experiment, the chemists of the study agree that the hard work will begin when they start determining how they can scale up the process.
When it comes to real-world applications, the solar fuel produced from artificial photosynthesis is more efficient and usable than the energy produced by solar panels. While solar panels convert solar energy into electricity, the solar energy used in artificial photosynthesis is converted and stored in the carbon-carbon or carbon-hydrogen bonds of liquid fuel. While solar energy has come a long way, it still is not ready to power entire electrical grids servicing entire cities. This liquid fuel may be used to run electrical power plants in a supporting role as the conversion to energy grids powered by renewable energy occurs.
The efficiency of solar fuels doesn’t stop at running electrical power grids. While personal vehicles can be powered using electricity, it would be highly impossible to power large transport trucks, ships, or planes with batteries. Imagine a plane loaded down with enough batteries to power it through a trans-oceanic flight. That would probably break physics. However, with the higher energy density of liquid fuels as pointed out by the chemists at the University of Illinois, large freight will still get transported, just with less of an impact on the planet.
While nothing beats planting millions of trees in the fight against future climate change, the rise of artificial photosynthesis and its pairing with carbon capture technology may be just the helping hand the forests need in cleaning up the atmosphere.
It may be wishful thinking to hope that the future of the planet can be rewritten just through people coming to their senses and changing their habits. However, in the face of adversity, great technologies imitating one of the greatest natural processes known to man, offer the opportunity for a brighter future that uses our current situation to our advantage.
Will artificial photosynthetic systems put leaves out of business? Probably not. But a natural and artificial leaf partnership looks to bring society the beginning of a solution for combatting climate change while also delivering energy-dense fuel to satiate an energy-hungry population.
