“Though human ingenuity may make various inventions which, by the help of various machines answering the same end, it will never devise any inventions more beautiful, nor simpler, nor more to the purpose than Nature does; because in her inventions nothing is wanting, and nothing is superfluous.”
Quote from The Notebooks of Leonardo Da Vinci
Imitating nature was always Leonardo da Vinci’s goal, and inspired him to dream up some of his most fabulous machines. Following his example, in the 20th century human technology finally managed to imitate the flight of birds to produce the first airplanes, or the structure of some sticky seeds to invent Velcro. But what we have not yet managed to replicate is the functioning of something that seems so basic: the leaf of a plant. Achieving artificial photosynthesis is the Holy Grail of sustainability, and while there are still major obstacles ahead, the last decade has seen huge advances.
In 2019, chemists at the University of Illinois presented their new method of artificial photosynthesis capable of producing fuels (such as propane and methane) from sustainable resources: “Scientists often look to plants for insight into methods for turning sunlight, carbon dioxide and water into fuels,” says Prashant Jain, one of the authors of the study published in Nature Communications. The researchers used gold nanoparticles as substitutes for chlorophyll, which acts as a catalyst in natural photosynthesis. In Mother Nature’s method, energy from the sun is captured via the chlorophyll pigment and transferred to water and CO2 molecules, so that they react with each other to produce the plants’ basic fuel (sugars).
The best kept secret of plants
The idea of artificial photosynthesis dates back more than a century. It was in 1912 when Italian chemist Giacomo Ciamician posed the question in Science magazine as to whether we could use “the best kept secret of plants” to capture solar energy (using photochemical devices) instead of burning coal and oil to produce energy. In his visionary article, Ciamician imagined smoke-free industrial colonies and energy farms in the form of plains covered with glass tubes. He insisted on the need to seek alternatives to fossil fuels: “And if in a distant future the supply of coal becomes completely exhausted, civilization will not be checked by that, for life and civilization will continue as long as the sun shines!”
Nowadays, gazing across fields covered with solar panels, one might think that Giacomo Ciamician’s futuristic dream has already become a reality in less than a century. Solar energy is experiencing a boom. When the giant Ivanpah Park in the Mojave Desert (California, USA) opened in 2014, it became the largest in the world. “The facility has the capacity to generate 392 megawatts (MW) of clean electricity —enough to power 94,400 average American homes,” says the U.S. Department of Energy. And taking into account all the production costs, the new large-scale solar farms are the cheapest way to produce energy (along with wind) and are much less expensive than thermal and nuclear power plants, according to the 2019 Lazard annual report.
The cost of solar energy has fallen tenfold over the past decade, the Lazard report notes. And from a technical perspective, in 2016 a group at the University of New South Wales in Australia managed to break the efficiency record of a photovoltaic cell: 34% of the solar energy received was converted into electricity. Outside the laboratory, for solar panels in commercial use the conversion is around 20%, and in nature, most plants are no more than 1% efficient in terms of energy conversion.
The age of solar fuels
However, conventional solar energy is still far from being able to imitate plants. We are not yet able to store —efficiently and for months— the enormous amount of energy provided by solar farms, which produce it not when we need it, but when it is sunny. “In the coming world of sustainable energy we’re going to have a serious storage issue. Batteries will be part of the solution, but imagine you’re in Norway or Sweden where they have 10 times as much sun in summer as winter. You won’t be able to store energy for six months in a battery,” researcher Harry Atwater recently told New Scientist magazine. Atwater heads the Joint Center for Artificial Photosynthesis (JCAP), a collaboration of California’s top universities founded in 2010 to achieve artificial photosynthesis —and thus produce solar fuels— to solve the storage problem.
The idea is to imitate nature twice over. On the one hand there are fossil fuels, such as oil and coal, which have stored solar energy for millions of years. “We use them every day because they’re amazingly energy-dense, storing them is inexpensive and they’re portable. Chemicals represent the ultimate form of energy storage,” Atwater reminds us. On the other hand, plants are tremendously efficient at storing solar energy, which they store in the sugars produced by photosynthesis.
One of the great milestones in this race to replicate photosynthesis were the artificial leaves unveiled in 2011 by a team from Harvard University led by David Nocera. These are thin silicon wafers, in which artificial photosynthesis uses solar energy to break down water molecules, producing oxygen and hydrogen. Hydrogen is the simplest fuel possible, and can be stored in so-called fuel cells, which are already the core of a new generation of electric vehicles.
From hydrogen fuel cells to semi-artificial photosynthesis
But so far, hydrogen has not succeeded in being the complete solution to replace gasoline and diesel. Our transport infrastructure works with fuels like these, which are liquid hydrocarbons, and hydrogen is a gas that does not fit that model: it would require replacing all the cars and service stations. Furthermore, it is complicated to produce and use hydrogen on a large scale. All these processes require the use of catalysts made from metals that are too expensive and scarce, such as platinum.
Catalysts are now also key in the race for artificial photosynthesis. They are the main characters of all the most recent (and modest) improvements that are constantly made—some of them have been announced as if we were close to the finish line. Unfortunately, that’s not the case; they are mere laboratory demonstrations that cannot be converted into large-scale commercially-viable systems. At JCAP in California, one of the world’s leading research centres in this field, Harry Atwater’s team is working on the most ambitious form of artificial photosynthesis: producing hydrocarbons using the excess CO2 floating in the atmosphere, which exacerbates the greenhouse effect.
That would really square the circle of sustainability. Although it may not seem ideal to continue burning fuels, the emissions are offset if these fuels are generated by capturing CO2 from the atmosphere. So far, Atwater’s lab has managed to convert atmospheric CO2 into a mixture of ethanol (fuel), ethylene (used to make plastics) and hydrogen, but the process is still rather inefficient. Improving it depends on finding new catalysts that replicate the role of chlorophyll in natural photosynthesis. Some research groups are seeking such a revolution by using natural enzymes as catalysts; others employ microbes to optimize the process. These are the two ways to achieve a sort of hybrid, semi-artificial photosynthesis, a creative alternative to reach the goal taking another path —rather than going with traditional metallic catalysts. It should be noted that in both cases, once again, the idea is to copy Mother Nature.
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