Solar Energy

Study reveals a reaction at the heart of many renewable energy technologies

Published

9 months ago

January 17, 2024

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Study reveals a reaction at the heart of many renewable energy technologies

by Anne Trafton for MIT News

Boston MA (SPX) Jan 17, 2024

A key chemical reaction – in which the movement of protons between the surface of an electrode and an electrolyte drives an electric current – is a critical step in many energy technologies, including fuel cells and the electrolyzers used to produce hydrogen gas.

For the first time, MIT chemists have mapped out in detail how these proton-coupled electron transfers happen at an electrode surface. Their results could help researchers design more efficient fuel cells, batteries, or other energy technologies.

“Our advance in this paper was studying and understanding the nature of how these electrons and protons couple at a surface site, which is relevant for catalytic reactions that are important in the context of energy conversion devices or catalytic reactions,” says Yogesh Surendranath, a professor of chemistry and chemical engineering at MIT and the senior author of the study.

Among their findings, the researchers were able to trace exactly how changes in the pH of the electrolyte solution surrounding an electrode affect the rate of proton motion and electron flow within the electrode.

MIT graduate student Noah Lewis is the lead author of the paper, which appears in Nature Chemistry. Ryan Bisbey, a former MIT postdoc; Karl Westendorff, an MIT graduate student; and Alexander Soudackov, a research scientist at Yale University, are also authors of the paper.

Passing protons

Proton-coupled electron transfer occurs when a molecule, often water or an acid, transfers a proton to another molecule or to an electrode surface, which stimulates the proton acceptor to also take up an electron. This kind of reaction has been harnessed for many energy applications.

“These proton-coupled electron transfer reactions are ubiquitous. They are often key steps in catalytic mechanisms, and are particularly important for energy conversion processes such as hydrogen generation or fuel cell catalysis,” Surendranath says.

In a hydrogen-generating electrolyzer, this approach is used to remove protons from water and add electrons to the protons to form hydrogen gas. In a fuel cell, electricity is generated when protons and electrons are removed from hydrogen gas and added to oxygen to form water.

Proton-coupled electron transfer is common in many other types of chemical reactions, for example, carbon dioxide reduction (the conversion of carbon dioxide into chemical fuels by adding electrons and protons). Scientists have learned a great deal about how these reactions occur when the proton acceptors are molecules, because they can precisely control the structure of each molecule and observe how electrons and protons pass between them. However, when proton-coupled electron transfer occurs at the surface of an electrode, the process is much more difficult to study because electrode surfaces are usually very heterogenous, with many different sites that a proton could potentially bind to.

To overcome that obstacle, the MIT team developed a way to design electrode surfaces that gives them much more precise control over the composition of the electrode surface. Their electrodes consist of sheets of graphene with organic, ring-containing compounds attached to the surface. At the end of each of these organic molecules is a negatively charged oxygen ion that can accept protons from the surrounding solution, which causes an electron to flow from the circuit into the graphitic surface.

“We can create an electrode that doesn’t consist of a wide diversity of sites but is a uniform array of a single type of very well-defined sites that can each bind a proton with the same affinity,” Surendranath says. “Since we have these very well-defined sites, what this allowed us to do was really unravel the kinetics of these processes.”

Using this system, the researchers were able to measure the flow of electrical current to the electrodes, which allowed them to calculate the rate of proton transfer to the oxygen ion at the surface at equilibrium – the state when the rates of proton donation to the surface and proton transfer back to solution from the surface are equal. They found that the pH of the surrounding solution has a significant effect on this rate: The highest rates occurred at the extreme ends of the pH scale – pH 0, the most acidic, and pH 14, the most basic.

To explain these results, researchers developed a model based on two possible reactions that can occur at the electrode. In the first, hydronium ions (H3O+), which are in high concentration in strongly acidic solutions, deliver protons to the surface oxygen ions, generating water. In the second, water delivers protons to the surface oxygen ions, generating hydroxide ions (OH-), which are in high concentration in strongly basic solutions.

However, the rate at pH 0 is about four times faster than the rate at pH 14, in part because hydronium gives up protons at a faster rate than water.

A reaction to reconsider

The researchers also discovered, to their surprise, that the two reactions have equal rates not at neutral pH 7, where hydronium and hydroxide concentrations are equal, but at pH 10, where the concentration of hydroxide ions is 1 million times that of hydronium. The model suggests this is because the forward reaction involving proton donation from hydronium or water contributes more to the overall rate than the backward reaction involving proton removal by water or hydroxide.

Existing models of how these reactions occur at electrode surfaces assume that the forward and backward reactions contribute equally to the overall rate, so the new findings suggest that those models may need to be reconsidered, the researchers say.

“That’s the default assumption, that the forward and reverse reactions contribute equally to the reaction rate,” Surendranath says. “Our finding is really eye-opening because it means that the assumption that people are using to analyze everything from fuel cell catalysis to hydrogen evolution may be something we need to revisit.”

The researchers are now using their experimental setup to study how adding different types of ions to the electrolyte solution surrounding the electrode may speed up or slow down the rate of proton-coupled electron flow.

“With our system, we know that our sites are constant and not affecting each other, so we can read out what the change in the solution is doing to the reaction at the surface,” Lewis says.

The research was funded by the U.S. Department of Energy Office of Basic Energy Sciences.

Research Report:“A molecular-level mechanistic framework for interfacial proton-coupled electron transfer kinetics”

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Solar Energy

Sandia evaluates heat shields for Mars Sample Return and Titan missions

Published

2 days ago

October 16, 2024

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Sandia evaluates heat shields for Mars Sample Return and Titan missions

by Clarence Oxford

Los Angeles CA (SPX) Oct 16, 2024

Sandia National Laboratories’ National Solar Thermal Test Facility is utilizing solar energy to simulate the intense heat experienced during atmospheric reentry and hypersonic flight. The latest tests aim to support NASA missions, including the Mars Sample Return campaign, a joint effort with the European Space Agency to bring Martian rock samples to Earth for analysis. These samples could reveal evidence of ancient life and aid preparations for future human missions to Mars.

As part of the Mars Sample Return mission, a Sample Retrieval Lander would carry the heaviest payload ever sent to Mars, along with a rocket for launching the collected samples into Martian orbit. Heat shield materials for the lander recently underwent testing at Sandia, said Sandia engineer and test director Ken Armijo. “This would be the first mission to return rocks from Mars to Earth; it’s got a bigger payload,” Armijo explained. “The heavier the payload and the bigger the entry vehicle, the hotter the vehicle gets during atmospheric entry, and the better the heat shield needs to be.”

Sandia’s solar testing facility uses hundreds of heliostat mirrors to focus sunlight on samples up to three feet wide, simulating atmospheric conditions on different planets. Unlike arc jets and lasers, which consume significant power, this approach saves between 15,000 to 60,000 kilowatts per test, equivalent to running 5,000 to 20,000 clothes dryers simultaneously.

The facility’s solar power tower, 200 feet tall and equipped with 212 heliostats, provides a unique environment for testing materials under high solar flux and heat. “We have high flux and high flux distribution on the Solar Tower,” said Armijo. The facility can simulate hypersonic flight conditions and accommodate large test samples, including full aircraft sections. Sunlight is concentrated up to 3,500 times its normal intensity, allowing precise control over heat exposure.

Compared to the $100,000-per-day cost of arc jet testing and the $150,000-per-day cost of laser testing, solar testing costs about $25,000 per day, Armijo noted. The intensity of sunlight can be adjusted by varying the number of heliostats focused on the sample, mimicking different reentry conditions. NASA’s lead engineer for the Sample Retrieval Lander’s heat shield, Brandon Smith, commented, “Sandia’s ability to test at this size nicely complements our other test facilities.”

Testing also supports NASA’s Dragonfly mission to Titan, Saturn’s largest moon, with heat shield materials made from Phenolic Impregnated Carbon Ablator, developed at NASA’s Ames Research Center. Previously used in missions like Stardust and Mars 2020, the material was tested on two-foot-wide samples at Sandia. The facility’s capability to test larger samples allowed NASA to simulate the stress and strain experienced during atmospheric entry.

Dragonfly, a rotorcraft designed to explore Titan’s methane-rich atmosphere, faces unique challenges due to Titan’s dense atmosphere, which is four times thicker than Earth’s. To recreate the thermal conditions of Martian and Titan atmospheric entry, nitrogen gas is blown over the heat shield samples during tests. A newly installed gas line running from the base to the top of the power tower ensures adequate gas flow, said Armijo.

Daniel Ray, a mechanical technologist at Sandia, was responsible for setting up the gas line and addressing issues during tests. “My role on every project is to make it work,” Ray said. He resolved an issue with the carbon felt catching fire by designing ceramic shields to protect the system.

In 2022, Sandia also supported the Applied Physics Laboratory’s tests on a heat exchanger prototype intended for future spacecraft. The prototype endured light levels equivalent to 2,000 suns, reaching temperatures of 3,100 degrees Fahrenheit, demonstrating its ability to withstand the intense heat of a close solar flyby.

The facility’s history includes various aerospace projects, such as testing radar protection domes and evaluating materials for space shuttles and military aircraft. Sandia has refined its solar testing methods over the years, said Armijo. “Because we can dial-in the profiles, we have more confidence that it’s going to survive and function well during a mission. Having confidence that it will make it to Mars, land and pick up the rocks safely is important.”

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Solar Energy

Computer simulations offer new insights into enhancing solar cell materials

Published

2 days ago

October 16, 2024

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Computer simulations offer new insights into enhancing solar cell materials

by Robert Schreiber

Berlin, Germany (SPX) Oct 16, 2024

Researchers from Chalmers University of Technology in Sweden have made progress in understanding halide perovskites, a promising class of materials for solar cells. These materials could serve as an efficient and cost-effective alternative to traditional silicon-based cells, but they face challenges with stability. The new insights are expected to aid the development of more reliable and efficient solar cells, key components in the transition to sustainable energy.

Halide perovskites refer to a group of materials recognized for their potential in flexible, lightweight solar cells and various optical applications, such as LEDs. They exhibit high efficiency in light absorption and emission, making them suitable for next-generation solar technologies. However, understanding the causes of rapid degradation remains a hurdle in optimizing these materials.

Advanced computer simulations reveal material behavior

The research team employed advanced computer simulations and machine learning to study 2D perovskite materials, which are typically more stable than their 3D counterparts. The findings, published in *ACS Energy Letters*, provide new insights into the factors that influence the materials’ properties.

“By mapping out the material in computer simulations and subjecting it to different scenarios, we can draw conclusions about how the atoms in the material react when exposed to heat, light, and so on,” explained Professor Paul Erhart from the research team. “We now have a microscopic description of the material that is independent of what experiments have shown, but which we can show to lead to the same behavior as the experiments.”

Simulations allow researchers to analyze material behaviors at a detailed level, offering a unique view that complements experimental data. This approach has made it possible to observe what leads to specific outcomes in experiments, deepening the understanding of 2D perovskites’ functionality.

Machine learning enables broader and deeper analysis

The integration of machine learning techniques allowed the researchers to study larger systems over longer durations than was previously feasible.

“This has given us both a much broader overview than before, but also the ability to study materials in much more detail,” said Associate Professor Julia Wiktor. “We can see that in these very thin layers of material, each layer behaves differently, and that’s something that is very difficult to detect experimentally.”

The composition and interaction of layers in 2D perovskites

2D perovskites consist of inorganic layers separated by organic molecules, which play a crucial role in determining the material’s stability and optical properties. Understanding the atomic movements within these layers and their connection to the organic linkers is essential for designing efficient devices.

“In 2D perovskites, you have perovskite layers linked with organic molecules. What we have discovered is that you can directly control how atoms in the surface layers move through the choice of the organic linkers,” noted Erhart. “This movement is crucial to the optical properties, creating a domino effect that extends deep inside the material.”

Future research directions

The study’s results pave the way for developing more stable and efficient optoelectronic devices by identifying which molecular configurations could enhance performance. The researchers aim to extend their work to more complex systems, focusing on interfaces that are essential for device functionality.

“Our next step is to move to even more complex systems and in particular interfaces that are fundamental for the function of devices,” Wiktor added.

Research Report:Impact of Organic Spacers and Dimensionality on Templating of Halide Perovskites

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Solar Energy

Bright future for solar panels and screens with new nanocrystal research

Published

2 days ago

October 16, 2024

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Bright future for solar panels and screens with new nanocrystal research

by Simon Mansfield

Sydney, Australia (SPX) Oct 16, 2024

Curtin University researchers have made a significant discovery that could enhance everyday technology, from TV screens to solar panels and medical diagnostics. The study, led by Associate Professor Guohua Jia, revealed how to increase the number of molecules, known as ligands, that adhere to zinc sulfide nanocrystals by manipulating the shape of these tiny particles.

Associate Professor Jia from Curtin’s School of Molecular and Life Sciences explained that ligands are essential for influencing the behavior and performance of zinc sulfide nanocrystals across various applications. “Ligands play an important role in controlling the behaviour and performance of zinc sulfide nanocrystals in various important technologies,” Jia said.

The research found that flatter, more uniform particles, termed nanoplatelets, can accommodate a greater number of tightly bound ligands compared to other shapes such as nanodots and nanorods. “In a discovery that could open new possibilities for developing smarter, more advanced devices, our study found flatter, more even particles called nanoplatelets allow more ligands to attach tightly, compared to other shapes like nanodots and nanorods,” Jia explained.

By tailoring the shapes of these nanocrystals, the researchers were able to enhance their interactions with surrounding environments, boosting the efficiency of a wide range of applications. Jia highlighted that these findings could potentially transform the efficiency and performance of products such as LED lights, screens, solar panels, and medical imaging devices.

The discovery also holds promise for advancing optoelectronic devices, which either generate light or utilize it to perform various functions. “Optoelectronics are important in many modern technologies, including telecommunications, medical devices and energy production,” Jia noted. The ability to control the manipulation of light and electricity is vital for the development of faster, more efficient, and compact electronic systems.

The applications include LEDs used in light bulbs and TV screens, solar cells that convert sunlight into electrical power, photodetectors in cameras and sensors, and laser diodes in fiber-optic communication systems.

Research Report:Deciphering surface ligand density of colloidal semiconductor nanocrystals: Shape matters