A concentrated photosynthesis device promises cheap, green hydrogen

A concentrated photosynthesis device promises cheap, green hydrogen

A concentrated photosynthesis device promises cheap, green hydrogen

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, comes from plant photosynthesis – converting sunlight, water and carbon dioxide into oxygen and sugars. The first chemical step in photosynthesis occurs in chlorophyll, which gives leaves their green color – and this step is actually a water splitting operation that breaks H2O in oxygen that is released into the air (thanks to plants) and positively charged hydrogen ions that drive the rest of the process and ultimately allow plants to store energy in the form of carbohydrates.

Evolution has provided a remarkable gift for photosynthesis, and as humanity seeks to rid itself of the harmful side effects of fossil fuels, scientists are working to replicate and even improve on that first step, hoping to develop artificial photosynthesis techniques that some predict will ultimately be the cheapest way to produce green hydrogen for use as an energy carrier.

“Ultimately, we believe that artificial photosynthesis devices will be much more efficient than natural photosynthesis, paving the way to carbon neutrality,” says Zetian Mi, University of Michigan Professor of Electrical and Computer Engineering.

Mi and his team just published an article in Nature on what they consider a major leap in artificial photosynthesis. The team demonstrated a new photocatalytic water-splitting semiconductor that uses the broad spectrum of sunlight, including the infrared spectrum, to split water with a whopping 9 percent efficiency – a nearly ten-fold improvement over other devices of its kind – and is a small and relatively inexpensive device. that gets better over time, not worse.

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 operating only at low light intensity,” said Peng Zhou, a research fellow in electrical and computer engineering and first author of the study. “Hydrogen produced with our technology can be very cheap.”

The new technology uses concentrated sunlight – an option not available for many other artificial photosynthesis devices, as high-intensity light and high temperatures damage them. But the UMich semiconductor – announced by a separate team last year and made of indium and gallium nitride nanostructures grown on a silicon surface – not only resists light and heat very well, but improves hydrogen production efficiency over time.

The photocatalyst, made of indium and 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 of indium and 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 are designed to avoid heat, this device depends on it. The semiconductor absorbs higher frequency light waves to power the water splitting process and is placed in a chamber where the water flows. Lower frequency infrared light is used to heat the chamber to approximately 70°C (158°F), which accelerates the water splitting reaction while inhibiting the tendency of hydrogen and oxygen molecules to recombine into water molecules before they can be collected separately.

The device achieved an efficiency of 9% in idealized laboratory tests using purified water. Turning to tap water, it reached about 7%. And in an outdoor test simulating a large-scale photocatalytic water splitting system powered by widely varying natural sunlight, it achieved an efficiency of 6.2%.

These photocatalytic efficiency figures lag behind some of the photoelectrochemical devices we’ve written about, such as the ANU cell at 17.6% or the Monash University device at a record 22%. But these devices tend to be inherently more expensive because they use photovoltaic cells to power electrochemical water splitting; the U.S. Department of Energy’s final technical targets for hydrogen production are 25% efficiency for photoelectrochemical systems and 10% for twin-bed photocatalytic systems – both representing a competitive hydrogen cost of around $2.10 per kg (2.2 lb) as calculated in 2011 .

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

University of Michigan

Perhaps most excitingly, UMich’s 7% efficiency rate for tap water also applies to seawater separation. Fresh water is not an infinite resource; it is already critically scarce in many areas and is widely expected to become even rarer and more valuable in the coming decades. So a photocatalytic device that can extract hydrogen from seawater without the need for an external energy input other than sunlight could be a real breakthrough in the era of decarbonization.

The team says it’s working to improve efficiency in further research, as well as the purity of the hydrogen it produces, but some of the intellectual property developed here has already been licensed to spinout companies UMich, NS Nanotech and NX Fuels.

“The materials we use,” says Mi, “gallium nitride and silicon can also be produced on a large scale, and we can use the current infrastructure to produce cheap, green hydrogen in the future.”

As always, commercial viability will decide the fate of this device. Green hydrogen must be cost-competitive not only with dirty hydrogen produced using methane, but also with cheap fossil fuels alone if it is to be operated on a large scale. This method relies on some rare metals such as gallium and indium, but its cost is drastically reduced due to the small size of the semiconductors required. We are looking forward to seeing how it will perform in industrial applications.

The subject is published in a journal Nature.

Watch the video below.

A more efficient method of obtaining hydrogen

Source: University of Michigan

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