We are a solar powered planet. The vast majority of the energy needed for life on Earth comes from the sun—and much of it, including food and fossil fuels, is the result of plant photosynthesis—the conversion of sunlight, water, and carbon dioxide into oxygen and sugars. The first chemical step in photosynthesis occurs in the chlorophyll that gives leaves their green color – and this step is instrumental in the water splitting process that breaks down H2O to oxygen, which is released into the air (thanks to plants), and positively charged hydrogen ions, which drive the rest of the process and ultimately allow plants to store energy in the form of carbohydrates.
Evolution has provided an extraordinary gift in photosynthesis, and as humanity works to free itself from the harmful side effects of fossil fuels, researchers are working to replicate and even improve this first step, hoping to develop artificial photosynthesis technologies that some predict will eventually be the cheapest way to produce hydrogen. Green, for use as a means of energy storage.
“Ultimately, we believe that artificial photosynthesis devices will be much more efficient than natural photosynthesis, which will provide a path toward carbon neutrality,” says Zetian Mei, a University of Michigan professor of electrical and computer engineering.
Mei and his team just published a paper in nature On what they see as a major leap in artificial photosynthesis. The team demonstrated a new photovoltaic water-splitting semiconductor that harnesses a broad spectrum of sunlight, including the infrared spectrum, to split water with a solid 9% efficiency—nearly ten times that of other devices of this type—which is very small. A relatively affordable device that gets better rather than deteriorated over time.
“We reduced the size of semiconductors by more than 100 times compared to some semiconductors that only operate at low light intensity,” said Peng Zhou, researcher in electrical and computer engineering and first author of the study. “The hydrogen produced by our technology could be very cheap.”
The new technology uses concentrated sunlight—an option not available to many other artificial photosynthesizers, because high-intensity light and high temperatures tend to cause them to break down. But UMich’s semiconductors—which a separate team announced last year and are made of indium gallium nitride nanostructures grown on a silicon surface—not only tolerate light and heat extremely well, they actually improve hydrogen production efficiency over time.
Where other systems aim to avoid heat, this device relies on it. The semiconductor absorbs high-frequency wavelengths of light to power the water separation process, and is placed in a chamber with water running over it. Low-frequency infrared light is used to heat the chamber to about 70 °C (158 °F), which speeds up the water-splitting reaction, while also suppressing the tendency of hydrogen and oxygen molecules to recombine into water molecules before they can be collected separately.
The device achieved 9% efficiency in exemplary lab tests using purified water. Moving on to tap water, it’s around 7%. And in an outdoor test simulating a large-scale photocatalytic water splitting system powered by widely varying natural sunlight, it returned an efficiency of 6.2%.
These photocatalytic efficiency numbers lag behind some of the photoelectrochemical devices we have reported, such as the ANU cell by 17.6% or the Monash University device by a record 22%. But these devices appear to be inherently more expensive, using photovoltaics to electrochemically split water; The final U.S. Department of Energy technical targets for hydrogen production are 25% efficiency from photovoltaic systems and 10% for two-bed photocatalytic systems—both representing a competitive hydrogen cost of about $2.10 per kilogram (2.2 lb), as calculated in 2011.
Perhaps most exciting is the fact that the UMich’s efficiency figure of 7% for tap water was also true for split seawater. Fresh water is far from an infinite resource; It is already in short supply in many areas, and it is widely expected to become even more rare and expensive in the coming decades. So a photocatalyst that can pull hydrogen from seawater without requiring any external energy input other than sunlight could be a real game-changer in the era of decarbonisation.
The team says it’s working to improve efficiency with more research, as well as the purity of the hydrogen that comes out, but parts of the intellectual property developed here have already been licensed to UMich Spinout NS Nanotech and NX Fuels.
“The materials we use, gallium nitride and silicon, can also be produced on a large scale, and we can take advantage of the existing infrastructure to generate low-cost green hydrogen in the future,” says Mi.
As always, commercial viability will determine the fate of this device. Green hydrogen must be cost-competitive, not only against dirty hydrogen produced using methane, but with cheap fossil fuels themselves if it works on a large scale. This method depends on some rare metals, in terms of gallium and indium, but the cost reached here is greatly reduced due to the small size of semiconductors required. We look forward to seeing how it stacks up to industrial use.
Publication of the research in the journal nature.
Check out the video below.
A more efficient way to harvest hydrogen
Source: University of Michigan