Concentrated photosynthesis device promises cheap green hydrogen

Concentrated photosynthesis device promises cheap green hydrogen

Concentrated photosynthesis device promises cheap green hydrogen

We are a planet powered by solar energy; 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 actually a water splitting operation that breaks down H2O into oxygen, which is released into the air (thanks, 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 delivered an extraordinary gift in photosynthesis, and as humanity strives to free itself from the harmful side effects of fossil fuels, researchers are working to replicate, and even improve upon, this first step, in the hope of developing artificial photosynthesis techniques that some predict. will ultimately be the cheapest way to produce green hydrogen for use as an energy storage medium.

“Ultimately, we believe that artificial photosynthesis devices will be much more efficient than natural photosynthesis, which will pave the way to carbon neutrality,” says Zetian Mi, professor of electrical and computer engineering at the University of Michigan. .

Mi and his team just published an article in Nature on what they consider to be a major advance in artificial photosynthesis. The team demonstrated a new photocatalytic water-splitting semiconductor that harnesses a broad spectrum of sunlight, including the infrared spectrum, to split water with a solid efficiency of 9% – a almost ten times better than other devices of its kind – and it’s a tiny, relatively affordable device that gets better, not worse, over time.

The device was tested using a window-sized lens to focus sunlight
The device was tested using a window-sized lens to focus sunlight

Brenda Ahearn/University of Michigan

“We reduced the size of the semiconductor by more than 100 times compared to some semiconductors that only work at low light intensity,” said Peng Zhou, an electrical and computer engineering researcher and first author of the study. . “Hydrogen produced by our technology could be very cheap.”

The new technology uses concentrated sunlight – an option not available to many other artificial photosynthetic devices because high intensity light and high temperatures tend to cause them to fail. But the UMich semiconductor – announced by a separate team last year and made from indium gallium nitride nanostructures grown on a silicon surface – not only resists light and heat extremely well. , it actually improves its hydrogen production efficiency over time.

The photocatalyst, made from indium gallium nitride nanostructures grown on a silicon surface, exhibits self-healing properties and can withstand concentrated sunlight up to the equivalent of 160 suns.
The photocatalyst, made from indium gallium nitride nanostructures grown on a silicon surface, exhibits self-healing properties and can withstand concentrated sunlight up to the equivalent of 160 suns.

University of Michigan

Where other systems aim to avoid heat, this device depends on it. The semiconductor absorbs high frequency wavelengths of light to power its water splitting process, and it is placed in a chamber with water flowing over it. Low frequency infrared light is used to heat the chamber to approximately 70°C (158°F), which speeds up the water splitting reaction, while 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 idealized lab tests using purified water. Switching to tap water, it reached around 7%. And in an outdoor test simulating a large-scale photocatalytic water splitting system powered by widely varying natural sunlight, it yielded an efficiency of 6.2%.

These photocatalytic efficiency figures lag behind some photoelectrochemical devices we have reported, such as the ANU cell at 17.6% or the Monash University device with a record 22%. But these devices appear to be more expensive in nature, using photovoltaic cells to power electrochemical water separation; the U.S. Department of Energy’s ultimate technical goals for hydrogen production are 25% efficiency for photoelectrochemical systems and 10% for dual-bed photocatalytic systems – both representing a competitive hydrogen cost of approximately US$2.10 per kg (2.2 lb), as calculated in 2011.

The team says the device's unique semiconductor improves rather than degrades when exposed to intense sunlight and high temperatures
The team says the device’s unique semiconductor improves rather than degrades when exposed to intense sunlight and high temperatures

University of Michigan

Perhaps most exciting is the fact that the UMich device’s 7% efficiency figure for tap water also applies to separating seawater. to be an infinite resource; it is already in critical shortage in many areas, and is expected to become even rarer and more valuable in the decades to come. Thus, a photocatalytic device capable of extracting hydrogen from seawater without the need for an external energy input other than sunlight could be a game-changer in the era of decarbonization.

The team says they are working to improve the efficiency of future research, as well as the purity of the hydrogen that comes out of it, but some of the intellectual property developed here has already been licensed to spin-off companies. from UMich NS Nanotech and NX Fuels.

“The materials we use,” explains Mi, “gallium nitride and silicon, can also be produced on a large scale, and we can leverage the current infrastructure to generate green hydrogen in the future at low cost”.

As always, commercial viability will determine the fate of this device. Green hydrogen must be competitive not only with dirty hydrogen produced using methane gas, but also with cheap fossil fuels themselves if it is to operate at scale. This method relies on some rare metals, in terms of gallium and indium, but the cost hit here is greatly reduced by the small size of the semiconductors needed. We can’t wait to see how it performs in industrial use.

The research is published in the journal Nature.

Check out a video below.

A more efficient way to harvest hydrogen

Source: University of Michigan

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