While plants absorb a great deal of sunlight, they only store about 1% of the energy absorbed which is locked into the sugars and other organic molecules used for cell growth. While scientists have boosted this figure by a few percentage points with light-absorbing microbes and genetic engineering, researchers have taken an even bigger step with solar panels, creating a hybrid device that uses a combination of catalysts and microbes to convert 10% of the captured solar energy into liquid fuels and other commodity chemicals.
Renewable energy storage solved?
While this process shows that it is possible to attain high efficiency in solar chemical conversion, the new fuels could also solve another major issue and that is renewable energy storage. With solar and wind power generation escalating daily across the globe, researchers are searching for ways in which to store the excess energy that these natural resources generate.
Batteries are still considered to be too costly for storing more than nominal amounts but energy-rich chemicals, which can be piped around and kept in chemical tanks, could store much more cost effectively.
The research kicked off in 2011, when researchers led by Professor Daniel Nocera, a chemist at Harvard University, created an artificial leaf that used energy from sunlight to split water into oxygen and hydrogen gas (H2). H2 can then be run through a fuel cell to produce electricity. But because its energy density is so low because of its vapour state—any fuel produced requires massive storage tanks or high pressures to compress it into smaller, more manageable volumes.
Several research teams followed up by combining the H2 with the carbon in carbon dioxide (CO2) to produce energy-dense liquid hydrocarbons. Last year, for example, Nocera’s group reported that it developed a hybrid system that used bacteria and electricity to “stitch” together H2—generated from splitting water—and the carbon from CO2 into a liquid alcohol called isopropanol. But the setup was problematic because the catalyst used to split water was made from a nickel alloy that generated a form of highly reactive oxygen that killed the bacteria. The only solution then was to use an unusually high voltage of electricity, which produced fewer reactive oxygen molecules. It also sharply reduced the efficiency of converting the energy in the electricity to chemical bonds in the fuel. In the end, the system converted only 3.2% of the input energy into chemical fuel.
Now, Nocera and his colleagues have replaced the nickel catalyst with a new cobalt-phosphorous alloy version, which does not make reactive oxygen species. That allowed the team to lower the voltage, leading to a “dramatic increase in efficiency.” Their new hybrid setup can convert 10% of the energy in sunlight to a variety of chemicals and fuels, far exceeding the efficiency of plants.
Solar fuel to increase access
As tantalizing as it seems to produce fuel merely from the base ingredients of sunlight, water, and CO2 in the air, Nocera cautions that the solar fuel approach still has a long way to go before “dethroning oil as the king of fuels.” He says: “It’s very hard to make this competitive with digging [oil] out of the ground.”
However, he says that solar fuel setups may one day help provide fuels and chemicals to the billions of people in developing countries who lack access because of poor infrastructure.
His team is already testing the process in India, where he is negotiating with researchers to pass along the intellectual property for the new method.
This is not the first time we have written about Professor Daniel Nocera. His energy storage research and development company, Sun Catalytix, was snapped up by global security, aerospace, and information technology company, Lockheed Martin in 2014. [Lockheed Martin Snaps Up Energy Storage Firm.]