Low-cost artificial leaf can produce hydrogen for weeks

A new artificial leaf device made of plentiful materials produces hydrogen from water and sunlight for weeks, shattering previous records, researchers report in a new study. The finding could provide a sustainable, and affordable path to produce green hydrogen for fuel.

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Hydrogen packs an energy punch as a fuel, holding three times more energy per weight than gasoline or diesel, and far more than lithium-ion batteries. Plus, hydrogen fuel cell vehicles, which run on battery-like devices that are fueled by hydrogen, only produce water at their tailpipes as a by-product. Hydrogen is an especially promising fuel for long-distance transport such as aviation and shipping.

But to decarbonize transportation, hydrogen will have to be greener. Right now, it is mostly made by splitting methane, which releases carbon dioxide. Green hydrogen would be produced by using renewable electricity to split water.

Another electricity-free way to make green hydrogen are devices that can harvest sunlight to split water in a method akin to photosynthesis in plants. But artificial leaf devices made from low-cost, earth-abundant materials such as perovskites don&#;t fare well when submerged in water, typically lasting a few days.

In the new Nature Materials paper, a team of chemists and materials scientists in the UK report devices made from a non-toxic, readily available light-absorbing material called bismuth oxyiodide. &#;We have been working on this material for some time, due to its wide-ranging potential applications, as well as its simplicity of fabrication, low toxicity and good stability,&#; said Judith Driscoll, a professor of materials science and metallurgy at the University of Cambridge in a press release.

To further increase the stability of the bismuth oxyiodide, the researchers sandwiched it between two other metal oxide layers. Then they coated the device with a water-repellant graphite paste. This increased the stability of the light-absorbing layer from a few minutes to a few weeks.

In an innovative design inspired by solar cells, the team then made devices composed of smaller light-absorbing areas, or pixels, on glass surfaces. Tests showed that these multi-pixel devices performed better than conventional devices with a single larger pixel of the same area. That&#;s because of some pixel areas are faulty, they do not affect the performance of others.

The materials and the pixel design concept could pave the way for large-scale artificial leaf systems for sustainable hydrogen production, the team says.

Source: Virgil Andrei et al, Long-term solar water and CO2 splitting with photoelectrochemical BiOI&#;BiVO4 tandems, Nature Materials, .

Image: Pixabay

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UIC student and study co-author Rohan Sartape with an artificial leaf developed in the laboratory of Meenesh Singh, assistant professor of chemical engineering at the College of Engineering. (Photo: Jim Young/UIC)

Engineers at the University of Illinois Chicago have built a cost-effective artificial leaf that can capture carbon dioxide at rates 100 times better than current systems. Unlike other carbon capture systems, which work in labs with pure carbon dioxide from pressurized tanks, this artificial leaf works in the real world. It captures carbon dioxide from more diluted sources, like air and flue gas produced by coal-fired power plants, and releases it for use as fuel and other materials. 

&#;Our artificial leaf system can be deployed outside the lab, where it has the potential to play a significant role in reducing greenhouse gases in the atmosphere thanks to its high rate of carbon capture, relatively low cost and moderate energy, even when compared to the best lab-based systems,&#; said Meenesh Singh, assistant professor of chemical engineering in the UIC College of Engineering and corresponding author on the paper. 

Using a&#;previously reported theoretical concept, the scientists modified a standard artificial leaf system with inexpensive materials to include a water gradient &#; a dry side and a wet side &#; across an electrically charged membrane. 

On the dry side, an organic solvent attaches to available carbon dioxide to produce a concentration of bicarbonate, or baking soda, on the membrane. As bicarbonate builds, these negatively charged ions are pulled across the membrane toward a positively charged electrode in a water-based solution on the membrane&#;s wet side. The liquid solution dissolves the bicarbonate back into carbon dioxide, so it can be released and harnessed for fuel or other uses. 

The electrical charge is used to speed up the transfer of bicarbonate across the membrane. 

Illustration of a carbon capture process designed by UIC College of Engineering scientists. Carbon dioxide from air or flue gas is absorbed by a dry organic solution to form bicarbonate ions, which migrate across a membrane and are dissolved in a liquid solution to concentrated CO2. Carbon atoms are shown in red, oxygen atoms are shown in blue and hydrogen atoms are shown in white. (Credit: Aditya Prajapati/UIC)

When they tested the system, which is small enough to fit in a backpack, the UIC scientists found that it had a very high flux &#; a rate of carbon capture compared with the surface area required for the reactions &#; of 3.3 millimoles per hour per 4 square centimeters. This is more than 100 times better than other systems, even though only a moderate amount of electricity (0.4 KJ/hour) was needed to power the reaction, less than the amount of energy needed for a 1 watt LED lightbulb. They calculated the cost at $145 per ton of carbon dioxide, which is in line with recommendations from the Department of Energy that cost should not exceed around $200 per ton. 

&#;It&#;s particularly exciting that this real-world application of an electrodialysis-driven artificial leaf had a high flux with a small, modular surface area,&#; Singh said. &#;This means that it has the potential to be stackable, the modules can be added or subtracted to more perfectly fit the need and affordably used in homes and classrooms, not just among profitable industrial organizations. A small module of the size of a home humidifier can remove greater than 1 kilogram of CO2 per day, and four industrial electrodialysis stacks can capture greater than 300 kilograms of CO2 per hour from flue gas.&#; 

The UIC scientists report on the design of their artificial leaf and the results of their experiments in &#;Migration-assisted, moisture gradient process for ultrafast, continuous CO2 capture from dilute sources at ambient conditions,&#; which is published in Energy & Environmental Science. &#; 

The research is funded by a grant (DE-SC-) from the U.S. Department of Energy. A patent application titled &#;Artificial photosynthetic systems for integrated carbon capture and conversion&#; has been filed by the Office of Technology Management at UIC. 

Co-authors of the paper from UIC, Argonne National Laboratory, Oklahoma State University and Braskem are Aditya Prajapati, Rohan Sartape, Tomas Rojas, Naveen Dandu, Pratik Dhakal, Amey Thorat,&#;Jiahan Xie, Ivan Bessa, Miguel Galante,&#;Marcio Andrade, Robert Somich, Marcio Rebouças, Gus Hutras, Nathalia Diniz, Anh Ngo and Jindal Shah. 

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