The artificial leaf: clean air, clean energy
by Antonia Alalitei
The beauty and potential that lays within bioengineering comes also from the fact that this field is not only related to the medical field. It comprises a wider range of applications related to biotechnologies and other creative forms of bringing the natural world phenomena, humans and technology together, to come up with original solutions to prevalent problems.
Which is the reason why today we’ve chosen a topic at the edge between the natural world and technology, with tremendous potential applications, as well as helping with the long-term goal of a carbon-neutral living.
Climate change is demanding creative solutions
Man-made industrial products have been producing CO2 emissions for decades. It is expected that quite soon, alterations to Earth’s climate will reach a point of irreversibility. It is crucial to realize that, although the shift towards carbon-neutral energy production is rising and CO2 emissions are massively cut down worldwide, we still have a way to go in terms of implementation and shift of habits.
Therefore, an ingenuous engineering solution for this transition period towards clean energy would require the production of clean fuels that would allow us more time to transition towards carbon-neutral living, while still relying on fuel-powered automobiles, since there are still many dependent industries: heavy transport, shipping, air travel etc.
Not only this, but an efficient solution would, most importantly, need to be able to capture that CO2 from the air that has already been released for such a long time and has not been removed. Can nature provide any inspiration to scientists? As always, whenever in a deadlock, just a walk in the park might provide unexpected ideas.
What is the artificial leaf?
The artificial leaf is a relatively new type of device, its first successful implementation having happened less than 10 years ago, by the American chemist Daniel Nocera and his colleagues MIT. With a silicon-based chip at its core, it attempts to mimic the simple, yet delicate process of photosynthesis in nature. Just like in the natural process, the reactants are water and carbon dioxide, which are both pumped within a transparent container that holds the “leaf”. In the presence of sunlight and a catalyst that speed up the reaction process, the device splits the water into oxygen and some other hydrogen-based compound.
In nature, this hydrogen-containing compound happens to be glucose, for the simple reason that plants need their own source of fuel to function. The catalyst that plants need for glucose production is the chlorophyll pigment within their leaves. The green colour of this pigment is not random either, since this colour can capture Sun light in multiple wavelengths, thus being able to power the chemical reactions.
Of course, humans also use forms of glucose as fuel, but the purpose of this artificial application would be to produce clean energy for industrial purposes. Therefore, the particularization of the process to our needs comes from a change in catalyst. Nocera’s device uses a semiconductor solar cell, with two distinct and separated sides. On one of them, a cobalt catalyst is used, which releases oxygen from water, while the other side is coated in a nickel alloy catalyst that releases hydrogen gas from water, which can be directed to a fuel cell for later electricity production.
For years after the first prototype, teams of scientists have been working to improve the efficiency of the artificial leaf, as well as its applicability outside laboratory conditions. During 2019, different teams have made their improvements public. The team led by Yimin A. Wu, a researcher with the Center for Nanoscale Materials at the Argonne National Laboratory (ANL) in Illinois and a professor of engineering from the Waterloo Institute for Nanotechnology (WIN) created a highly reflective powder from a special reaction, which then was used as a catalyzer in order to obtain methanol as the focus fuel.
Only a couple of weeks earlier, Professor Erwin Reisner from Cambridge’s Department of Chemistry and his colleagues announced their own approach to the artificial leaf, using cobalt-based catalystic light absorbers to produce oxygen and syngas, another compound based on hydrogen and carbon monoxide. Both of these fuels are widely used in industries, and their non-polluting production could provide a great alternative to standard methods.
A very important aspect of this engineering solution was tackled earlier in 2019 by Meenesh Singh and his colleague Aditya Prajapati, which dealt with the mechanism of real-life absorption of CO2 within the artificial leaf. In nature, leaves are equipped with stomata, special openings that rely on the difference in concentration of the two gases in order to make the exchange possible. Therefore, they coated their artificial leaf within a permeable membrane, that would allow taking into CO2 directly from air, rather than special pressurized tanks.
What’s next? Artificial forests?
The “artificial tree” concept was developed by Klaus Lackner, director of the Lenfest Center for Sustainable Energy at Columbia University. A while ago already, Lackner proposed a method where artificial trees working with mechanisms described above could remove CO2 from air at a much higher rate than natural trees. The leaves would be left to soak on carbon dioxide until saturation, when they would be placed in water to create biofuels.
In a dystopian view, where humanity doesn’t have enough time, the future may rely on fast production of trees that would produce both oxygen and “food” for humans. But in the meantime, let’s keep our planet truly green by planting the one type of wonderful natural tree we all know so well.