The idea of a human-made device that can process solar energy to make usable fuels has been tantalizing researchers since the 1970s. There being no such thing as a free lunch, it is not so easy to engineer a device that mimics photosynthesis, which Mother Nature perfected a long time ago. Nevertheless, researchers at the Department of Energy’s Lawrence Berkeley Lab in California appear to have solved an important piece of the “artificial leaf” challenge.
Solar Energy & The Artificial Leaf Of The Future
The concept of the artificial leaf first crossed the CleanTechnica radar in the form of a card-sized photoelectrochemical cell, back in 2011. Instead of converting sunlight into electricity, the cell acts as a catalyst that deploys solar energy to break water into oxygen and hydrogen.
Initial versions of the device required purified water. By 2013, researchers figured out how to prevent a film of impurities from coating the solar catalyst, enabling it to be used in contaminated water.
Around the same time, additional improvements on the artificial leaf concept popped up, one main area of research being to lower the cost of the solar catalyst. New iterations also included an artificial leaf that uses sunlight to convert carbon dioxide into carbon monoxide, a ubiquitous chemical building block.
By 2014 researchers at Berkeley Lab were working on a “bionic” version of the artificial leaf concept. They zeroed in on hydrogen production through solar energy, based on the premise that the energy storage value of hydrogen would help create a cost-effective pathway for a commercial version of the artificial leaf.
Just one year later, researchers at Lund University announced a “supersonic” artificial leaf, based on engineered molecules that can collect solar energy and act as a catalyst. This version, too, was aimed at hydrogen production, with the potential for adding methane to the list.
Why Bother With Synthetic Photosynthesis When We Have Wind & Solar Energy?
Now that wind and solar energy are so inexpensive, it’s fair to ask why researchers are still so hot to make synthetic photosynthesis happen. If the aim is to produce hydrogen from water, that can be done by deploying solar energy to run an electrolysis system, which uses an electrical current to push hydrogen gas out of water.
For the answer, let’s turn to Purdue University, which ran an article last June on the advantages of mimicking photosynthesis over generating electricity from wind turbines and solar panels.
“The closest process to artificial photosynthesis humans have today is photovoltaic technology, where a solar cell converts the sun’s energy into electricity. That process is famously inefficient, able to capture only about 20% of the sun’s energy,” Purdue noted. “Photosynthesis, on the other hand, is radically more efficient; it is capable of storing 60% of the sun’s energy as chemical energy in associated biomolecules.”
Sad, but true. Researchers keep coming up with new solar cells that keep breaking solar energy conversion records. The most efficient ones have topped the 20% mark by a wide margin, but they don’t come anywhere close to 60%.
Purdue cited biophysicist and solar energy researcher Yulia Pushkar, who emphasized the artificial leaf advantage.
“With artificial photosynthesis, there are not fundamental physical limitations,” she said. “You can very easily imagine a system that is 60% efficient because we already have a precedent in natural photosynthesis. And if we get very ambitious, we could even envision a system of up to 80% efficiency.”
You Can’t Fool Mother Nature (Unless You Try Really Hard)
When a plant processes solar energy, it grows. Unfortunately, when scientists try to do the same thing with an artificial leaf, it falls apart. That just shows how much of a genius Mother Nature is.
That brings us to latest development. Earlier this month Berkeley Lab noted that a research team headed up by staff scientist Francesca Toma recently achieved a breakthrough. Their contribution to the artificial photosynthesis field focuses on the question of durability.
The weakness in a typical artificial leaf system is the result of using a crystalline form of copper called cuprous oxide. Cuprous oxide is a preferred material in photoelectrochemical cells because of its high reactivity to light, but it breaks down after a few minutes when exposed to light.
So, why stick with a losing proposition?
“Despite its instability, cuprous oxide is one of the best candidate materials for artificial photosynthesis because it is relatively affordable and has suitable characteristics for absorbing visible light,” Berkeley Lab explains.
With that motivation, Toma and her team took a fresh look at the photoelectrochemical reaction. On closer examination, the team realized that the culprit might not lie within the cell itself. Instead, it might be something in the water — literally, the water electrolyte used in artificial leaf systems.
“We knew it was unstable – but we were surprised to learn just how unstable it really is,” said Toma. “When we began this study, we wondered, maybe the key to a better solar fuels device isn’t in the material by itself but in the overall environment of the reaction, including the electrolyte.”
Solar Energy & The “Z Scheme”
The team concluded that hydroxides in the water contribute to corrosion. The study also yielded a potential workaround, consisting of a photoelectrochemical cell protected by a layer of silver on top, and a gold/iron oxide on the bottom.
“This ‘Z scheme,’ which is inspired by the electron transfer that takes place in natural photosynthesis, should create a ‘funnel’ that sends holes from cuprous oxide to the gold/iron oxide ‘sink,” explains Berkeley Lab.
Got all that? Good! For all the juicy details, check out the study “Investigation and mitigation of degradation mechanisms in Cu2O photoelectrodes for CO2 reduction to ethylene” in the journal Nature Energy.
“The resulting photocathode exhibits a stable photocurrent for CO2 reduction with ~60% Faradaic efficiency for ethylene with a balance of hydrogen for hours, whereas bare Cu2O degrades within minutes,” Toma and her team conclude.
Watch Out, Fossil Fuels — The Liquid Sunlight Alliance Is Coming For You
If you caught that thing about producing ethylene, that’s another potential advantage over electrolysis systems. Here, let’s have our friends over at BMC Biology explain it:
“The simple hydrocarbon ethylene (C2H4) is a tiny gaseous molecule of great significance. In addition to being the most widely produced organic compound in the world (used in manufacturing numerous products such as rubber, plastics, paints, detergents and toys), ethylene is a major hormone in plant biology.”
The ability to scale up the non-fossil production of ethylene could be a game changer. Fossil industry stakeholders have been banking on hydrogen, ethylene, and other petrochemicals to stay afloat as the global economy moves away from fossil fuels, but now it looks like solar energy could have those routes blocked off as well.
With that in mind, let’s take a quick look at the Liquid Sunshine Alliance, which is the umbrella organization supporting Toma’s team among others. LiSA was founded in 2020 in the waning days of the Trump administration, as one of two projects funded by the US Department of Energy through the Fuels from Sunlight Energy Innovation Hub, which goes back to the Obama administration.
“The Liquid Sunlight Alliance is developing the science principles by which durable coupled microenvironments can be co-designed to efficiently and selectively generate liquid fuels from sunlight, water, carbon dioxide, and nitrogen,” LiSA explains, and that’s not all.
The LiSA mission statement positions the the solar fuels field as one that advances “diversity, equity and inclusion, (DEI) aspects that are essential and foundational to expanding knowledge in science and engineering and developing the scientific workforce of the future,” which is somewhat ironic considering that former President Trump attempted to stymie federal DEI programs in the closing months of his losing candidacy for re-election in 2020.
For the record, the other program funded through Fuels From Sunlight in 2020 is CHASE, the Center for Hybrid Approaches in Solar Energy to Liquid Fuels. CHASE is on a mission to develop “hybrid photoelectrodes for fuel production that combine semiconductors for light absorption with molecular catalysts for conversion and fuel production.”
In terms of the role of solar energy in rapid global decarbonization, it seems you ain’t seen nothing yet, so hold onto your hats.
Follow me on Twitter @TinaMCasey.
Photo: “A model solar fuels device called a photoelectrochemical cell. A research team led by Francesca Toma, a staff scientist at the Liquid Sunlight Alliance in Berkeley Lab’s Chemical Sciences Division, designed the model” (credit: Thor Swift/Berkeley Lab).
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