Researchers make greener fuel with O

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Image: The calculations for the project were performed at the High Performance Computing Research Center in Princeton.
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Image credit: Photo by Dennis Applewhite, Princeton University Communications Office

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Researchers at Princeton and Rice universities have combined iron, copper, and a simple LED to demonstrate a low-cost technology that could be key to distributing hydrogen, a fuel that packs loads of energy without carbon pollution.

The researchers used advanced experiments and calculations to develop a technique that uses nanotechnology to separate hydrogen from liquid ammonia, a process that until now was costly and energy-intensive.

In an article published online Nov. 24 in the journal SciencesIn this article, the researchers describe how they used light from a standard LED to break down ammonia without the high temperatures or expensive elements that such chemistry requires. This technology overcomes a critical hurdle toward realizing hydrogen’s potential as a clean, low-emissions fuel that can help meet energy demands without exacerbating climate change.

“We hear a lot about hydrogen as the ultimate clean fuel, if only it was less expensive and easier to store and retrieve for use,” said Naomi Halas, a professor at Rice University and one of the study’s lead authors. “This result demonstrates that we are moving rapidly toward this goal, with a novel and simplified method for on-demand hydrogen release from a practical hydrogen storage medium using Earth-abundant materials and the technology breakthrough of solid-state lighting.”

Hydrogen offers many advantages as an environmentally friendly fuel including high energy density and zero carbon pollution. It is also used everywhere in industry, for example in the manufacture of fertilizers, food and minerals. But pure hydrogen pressure is expensive to transport and difficult to store for long periods. In recent years, scientists have sought to use chemical intermediates to transport and store hydrogen. Ammonia (NH3), and is composed of three hydrogen atoms and one nitrogen atom. Unlike pure hydrogen gas (H2), liquid ammonia, while dangerous, has systems in place for safe transportation and storage.

“This discovery paves the way for sustainable, low-cost hydrogen that can be produced locally rather than in huge centralized plants,” said Peter Nordlander, a professor at Rice and another lead author.

One persistent problem for advocates was that breaking down ammonia into hydrogen and nitrogen often required high temperatures to drive the reaction. Conversion systems can require temperatures higher than 400°C (732°F). This requires a lot of energy to convert the ammonia, as well as special equipment to handle the process.

Researchers led by Halas and Nordlander at Rice University, and Emily Carter, Gerhard R. Andlinger Professor of Energy and Environment and Professor of Mechanical and Aerospace Engineering and Applied and Computational Mathematics at Princeton University, wanted to transform the splitting process to make ammonia a more sustainable and economically viable carrier material for hydrogen fuel. The use of ammonia as a hydrogen carrier has attracted significant research interest due to its potential to drive hydrogen economy, a recent review by the American Chemical Society showed.

Industrial processes often break down ammonia at high temperatures using a variety of substances as catalysts, which are substances that speed up a chemical reaction without being altered by the reaction. Previous research has demonstrated that it is possible to lower the reaction temperature using a ruthenium catalyst. But ruthenium, a metal in the platinum group, is very expensive. The researchers think they can use nanotechnology to allow cheaper elements such as copper and iron to be used as a catalyst instead.

The researchers also wanted to address the energy cost of cracking ammonia. Current methods use a lot of heat to break the chemical bonds that hold ammonia molecules together. The researchers thought they could harness light to cut chemical bonds like a scalpel, rather than using heat to break them down like a hammer. To do this, they turned to nanotechnology, along with a much cheaper catalyst that contained iron and copper.

The combination of fine metallic structures in nanotechnology and light is a relatively new field called plasmonics. By highlighting structures smaller than a single wavelength of light, engineers can manipulate light waves in unusual and specific ways. In this case, the Rice team wanted to use this engineering light to excite electrons in metallic nanoparticles as a way to split ammonia into its hydrogen and nitrogen components without the need for intense heat. Because plasmonics requires certain types of metals, such as copper, silver, or gold, researchers added iron to copper before creating the microstructures. When finished, the copper structures act as antennas to manipulate the light from the LED to excite the electrons to higher energies, while the iron atoms embedded in the copper act as catalysts to speed up the reaction carried out by the excited electrons.

The researchers created the structures and ran the experiments in the labs at Rice. They were able to adjust many variables about the interaction such as pressure, light intensity, and light wavelength. But calibrating the exact parameters was tedious. To explore how these variables affect the interaction, the researchers worked with lead author Carter, who specializes in detailed investigations of interactions at the molecular level. Using Princeton’s high-performance computing system, Terascale Infrastructure for Pioneering Research in Engineering and Science (TIGRESS), Carter and her postdoctoral colleague, Junwei Lucas Bao, ran the interactions through a specialized quantum mechanics simulator uniquely capable of studying excited electronic stimulation. The molecular interactions of such interactions are incredibly complex, but Carter and her fellow researchers were able to use the simulator to understand which variables need to be modified to increase the interaction.

“By simulating quantum mechanics, we can determine the steps of a rate-limiting reaction,” said Carter, who also holds appointments at the Princeton Center for Energy and the Environment, in Applied and Computational Mathematics, and at the Princeton Laboratory of Plasma Physics. “These are the obstacles.”

By optimizing the process, taking advantage of the atomic-scale understanding that Carter and her team provided, Rice’s team has been able to consistently extract hydrogen from ammonia using only light from energy-efficient LEDs at room temperature without additional heating. The researchers say the process is scalable. In further research, they plan to investigate other potential catalysts with a focus on increasing process efficiency and reducing cost.

Carter, who also currently chairs the National Academies’ Committee on Carbon Use, said the critical next step will be to cut costs and carbon pollution from ammonia production that starts the transport cycle. Currently, most ammonia is created at high temperatures and pressures using fossil fuels. This process is energy intensive and polluting. Carter said many researchers are working to develop green technologies for ammonia production as well.

“Hydrogen is used everywhere in industry and will increasingly be used as a fuel as the world seeks to decarbonize its energy sources,” she said. “However, today it is made unsustainably from natural gas – which leads to CO2 emissions – and is difficult to transport and store. Hydrogen needs to be sustainably manufactured and transported when needed. If carbon-neutral ammonia can be produced, for example By electrolytic reduction of nitrogen using decarbonized electricity, it can be transported, stored, and possibly serve as an on-demand source of green hydrogen using iron-copper photocatalysts with LED lighting reported here.”

Article, Earth-abundant photocatalyst for the generation of H2 from NH3 with light-emitting diode illumination, It was published in the November 25 issue of Sciences. Besides Carter, Halas, and Nordlander, co-authors include Hossein Rubatjazi, who received his Ph.D. from Rice University and is now Senior Scientist at Syzygy Plasmonics. Jonoy Lucas Pao, now a professor at Boston College; Yigao Yuan, Jingyi Zhou, Aaron Bales, Lin Yuan, Minghe Lou, and Minhan Lou from Rice University; Linan Zhou of Rice University and South China University of Technology, and Suman Khatiwada of Syzygy Plasmonics. Halas and Nordlander are co-founders of Syzygy and own shares in the company.

The research was supported in part by the Welch Foundation, the Air Force Office of Scientific Research, Syzygy Plasmonics, and the Department of Defense.

Rice University’s Office of Public Affairs contributed to this article.

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