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An ancient chemical process enabled Earth to become a lush place teeming with life. Now researchers are replicating this process in an attempt to slow global warming.

Every plant, animal, and person owes their life to one sequence of chemical reactions: photosynthesis. The process, which converts water and carbon听dioxide into food using sunlight, first evolved in cyanobacteria more than 2 billion years ago.听

That鈥檚 right. Plants weren鈥檛 the 铿乺st organisms to develop photosynthesis, though they are better known for it. Cyanobacteria are the ones that originally 铿乴led the atmosphere with photosynthesis鈥檚 gaseous by-product, oxygen (O₂), which set the stage for more diverse life on Earth.听听

As bene铿乧iaries of photosynthesis, humans depend on plants in a sort of carbon seesaw. Plants take in CO₂听and release O₂. They store that carbon as sugar. Hanging vines, grass, and trees all grow by pulling carbon atoms out of the air. We do the reverse, taking in O₂听and releasing CO₂. Finally, everything we eat completes the handoff: Human eats plant (or the animal who already did), human exhales, plant stores carbon, and the cycle continues.

This seesaw is part of the much broader carbon cycle that has affected the radiation balance of our planet. Cutting down huge swaths of forests and the burning of carbon-based fossil fuels causes the levels of CO₂, a major greenhouse gas, to rise. And plants on Earth along with other natural parts of the carbon cycle can鈥檛 restore the balance on their own.

But what if we could copy what plants do to grab some of that excess CO₂听to make fuels sustainably, instead of relying so heavily on fossilized carbon?听

鈥淎rti铿乧ial photosynthesis is a really attractive approach,鈥� says Jillian Dempsey, a chemist at the University of North Carolina, Chapel Hill. 鈥淵ou鈥檙e able to store the energy of the sun in the bonds of [molecules].鈥澨�

At a large enough scale, such sun-powered processes could give us enough energy-storing molecules to make fuels out of thin air and water like plants and cyanobacteria do.听

Plants Make it Look Easy

While it鈥檚 tempting to write photosynthesis off as a simple reaction鈥擟O₂ and water comes in, a leaf makes its food, and O₂听heads out鈥攖he chemistry occurring is surprisingly complex. It鈥檚 a dance of protons, electrons, and biological machinery that had millions of years to evolve.听

That machinery severs the strong bonds in H₂O and CO₂ to build more complex molecules, such as glucose (C₆H₁₂O₆).听

What the reaction equations don鈥檛 show is what spurs the reactions: the catalysts, which are substances that speed up speci铿乧 chemical reactions. With the right catalysts, researchers have succeeded in building small devices that not only mimic natural photosynthesis, but also beat it.

Photosynthesis 2.0

Let鈥檚 take a look at the role catalysis plays in photosynthesis. First, plants absorb sunlight in their chloroplasts, which are organelles found in plant and algae cells. Inside the chloroplasts are chlorophyll pigments that absorb mostly red and blue, but not green wavelengths of听light (thus their color). This absorbed energy powers the plant鈥檚 chemical reactions.听

Arti铿乧ial photosynthesis needs its own version of chlorophyll. To 铿乶d substances to act like chlorophyll in an arti铿乧ial leaf, scientists are testing natural pigments, synthetic dyes, or other materials that absorb visible light.

In plants, once light is absorbed, a complicated chain of biological catalysts called听enzymes听carry out reactions in a precise order to create fuels such as glucose. In an arti铿乧ial leaf, a similar thing must happen鈥攅xcept instead of glucose,听the leaf should produce a fuel, such as ethanol, that our energy-driven technologies can use. Alternatively, some arti铿乧ial leaf technologies don鈥檛 require CO₂听but rather take in water to produce hydrogen fuel (H₂).

Pigment Power!

To make their own food, green plants contain chlorophyll pigments that absorb energy to drive chemical reactions. Pigments absorb light with levels of energy that are just right to excite their electrons. The type and amount of a molecule鈥檚 bonds in铿倁ence what those absorption wavelengths actually are. Chlorophyll-a and chlorophyll-b are important pigments that enable photosynthesis in green plants. They absorb light in the violet-blue and orange-red parts of the electromagnetic spectrum.

The Simplest Fuel

Hydrogen, when consumed in a fuel cell, produces no carbon emissions鈥攊ts only emission is water. Today, most H₂ is produced from methane (CH₄) in a process that emits CO₂.听

If scientists could 铿乬ure out how to make H₂ using sunlight in an affordable way, hydrogen fuels would be more sustainable.听

To do this with an arti铿乧ial leaf, it would have to carry out the oxidation and reduction reactions required to make H₂ from H₂O, or 鈥渨ater splitting.鈥� Like photosynthesis, the reactions seem straightforward on paper.

2H₂O听鈫� O₂ + 4H⁺ + 4e^-

听 听 听4H⁺ + 4e^-听鈫� 2H₂

But in practice, water splitting is an electrochemical process that relies on electricity to power the oxidation and reduction reactions.听

鈥淚f I just put a cup of water out in the听sunshine, it鈥檚 not going to spontaneously turn into hydrogen and oxygen,鈥� says Dempsey, who designs catalysts for solar-fuel production.听

H₂O doesn鈥檛 easily part with its electrons and protons, or hydrogen ions (H⁺). Electrolysis needs a steady flow of external electrical energy, which can be supplied by a solar-powered arti铿乧ial leaf. A current helps separate the oxygen atom from the hydrogen ions by pulling electrons away from the water molecule (oxidation). Those electrons flow from one part of the electrochemical cell, known as the听anode, to another, known as the听cathode. There, H⁺听ions pair up with electrons (reduction) to form H₂听gas, which forms bubbles that can then be captured for use.听听

Catalysts, such as iron oxide and platinum, speed up the forming and breaking of chemical bonds.听

鈥淐atalysis gives us access to seemingly impossible chemical transformations,鈥� Dempsey says.

Making Carbon Fuels

Ethanol, (C₂H₅OH), typically makes up 10% of the gas you pump into most cars. Some flex-fuel cars can handle up to 85% C₂H₅OH.听

Today, most ethanol in the United States comes from fermenting crops, which rely on a lot of land, water, and energy from fossil fuels to grow. This is where arti铿乧ial-leaf researchers come in. They are engineering catalysts to produce ethanol and butanol听

(C₄H₉OH) in a more sustainable way. Butanol has similar uses to ethanol.

鈥淭o build carbohydrates or ethanol or butanol,鈥� says Dempsey, 鈥渨e need to carefully choreograph how the protons and the electrons are being reassembled.鈥�

Turning a couple of CO₂ molecules into C₂H₅OH requires shuffling around 12 protons and 12 electrons.听

2CO₂ + 12H⁺ + 12e^- 鈫� C₂H₅OH + 3H₂O

Scientists design new catalysts for arti铿乧ial photosynthesis to make the process as ef铿乧ient and sustainable as possible.听

鈥淲e have gotten to the point where scientists have done enough preliminary studies to know that what we are proposing is achievable,鈥� Dempsey says.

Natural Inspiration

What鈥檚 extraordinary is that these chemical choreographies work. Light, CO₂, and water go in; fuel comes out. And in some ways, chemists have designed catalysts to make these fuels more ef铿乧iently than plants produce sugars.

鈥淚t turns out that natural photosynthesis isn鈥檛 actually that ef铿乧ient of a process,鈥� Dempsey says. Only about 1% of the solar energy that hits a plant turns into fuel energy. The ef铿乧iency of arti铿乧ial leaf technologies can exceed 20%.

This doesn鈥檛 mean arti铿乧ial photosynthesis copies natural photosynthesis in all aspects though. Plants are less ef铿乧ient, but they make more complex fuels with greater precision than arti铿乧ial devices. 鈥淪ome of these systems that are showing promise make a whole slew of stuff鈥�12 different products mixed together,鈥� Dempsey says.听

A major current challenge in catalyst research involves designing materials that speed up speci铿乧 reactions and make only the products we want.

Making an Artificial Leaf

So, what would an arti铿乧ial leaf device actually look like?听

鈥淚t basically looks like this sandwich structure鈥攖he catalyst layers are sandwiching the photo-absorber,鈥� says Peter Agbo, a staff scientist at Lawrence Berkeley National Lab who works with the Joint Center for Arti铿乧ial Photosynthesis.

Existing devices are also small. A hydrogen device with 12.6% ef铿乧iency that Agbo recently built was less than one inch across. For arti铿乧ial photo-synthesis to become practical, it needs to produce fuels at a large scale to compete with the world鈥檚 existing energy supply of relatively inexpensive and abundant fossil fuels.听

Scaling up arti铿乧ial photosynthesis is still far off, but it鈥檚 moving along.听

鈥淲e鈥檝e identi铿乪d the fundamental scienti铿乧 challenges,鈥� Dempsey says. 鈥淲e鈥檙e learning how to put the different pieces together.鈥澨�

What might ultimately get the technology across the 铿乶ish line is another turn back to nature鈥攂ut this time, instead of just copying it, scientists want to use it. Engineers from the University of California at Berkeley, for example, recently combined nanoparticles with living nonphotosynthetic bacteria.听

Their microbe of choice, Moorella thermoacetica, naturally reduces CO₂ to make a small amount of acetic acid (CH₃COOH). When the researchers fed the bacteria tiny clusters of gold atoms and exposed them to sunlight, the gold clusters were able to pull electrons from the amino acid cysteine and send them to the microbe鈥檚 enzymes. The electrons interacted with the bug鈥檚 enzymes, which spurred the bacteria to make much more acetic acid from CO₂ than they normally would have. The acetic acid can then be used to make fuels and other valuable chemicals.听

It will take a lot of time and money before arti铿乧ial photosynthesis can compete with fossil fuels. But the needed investment won鈥檛 come close to听the societal cost of climate change. A recent survey of more than 2,000 economists projected the economic damages from climate change will reach $1.7 trillion per year by 2025 and roughly $30 trillion per year by 2075.听听

听Arti铿乧ial photosynthesis could inch us back toward a better balance on the planet鈥檚 carbon seesaw.听