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Tuning electrode surfaces to optimize solar fuel production

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Tuning electrode surfaces to optimize solar fuel production

Scientists have demonstrated that modifying the topmost layer of atoms on the surface of electrodes can have a remarkable impact on the activity of solar water splitting. As they reported in Nature Energy on Feb. 18, bismuth vanadate electrodes with more bismuth on the surface (relative to vanadium) generate higher amounts of electrical current when they absorb energy from sunlight.

This photocurrent drives the chemical reactions that split water into oxygen and hydrogen. The hydrogen can be stored for later use as a clean fuel. Producing only water when it recombines with oxygen to generate electricity in fuel cells, hydrogen could help us achieve a clean and sustainable energy future.

“The surface termination modifies the system’s interfacial energetics, or how the top layer interacts with the bulk,” said co-corresponding author Mingzhao Liu, a staff scientist in the Interface Science and Catalysis Group of the Center for Functional Nanomaterials (CFN), a U.S. Department of Energy (DOE) Office of Science User Facility at Brookhaven National Laboratory. “A bismuth-terminated surface exhibits a photocurrent that is 50-percent higher than a vanadium-terminated one.”

“”Studying the effects of surface modification with an atomic-level understanding of their origins is extremely challenging, and it requires tightly integrated experimental and theoretical investigations,” said co-corresponding author Giulia Galli from the University of Chicago and DOE’s Argonne National Laboratory.

“It also requires the preparation of high-quality samples with well-defined surfaces and methods to probe the surfaces independently from the bulk,” added co-corresponding author Kyoung-Shin Choi from the University of Wisconsin-Madison.

Choi and Galli, experimental and theoretical leaders in the field of solar fuels, respectively, have been collaborating for several years to design and optimize photoelectrodes for producing solar fuels. Recently, they set out to design strategies to illuminate the effects of electrode surface composition, and, as CFN users, they teamed up with Liu.

“The combination of expertise from the Choi Group in photoelectrochemistry, the Galli Group in theory and computation, and the CFN in material synthesis and characterization was vital to the study’s success,” commented Liu.

Bismuth vanadate is a promising electrode material for solar water splitting because it strongly absorbs sunlight across a range of wavelengths and remains relatively stable in water. Over the past few years, Liu has perfected a method for precisely growing single-crystalline thin films of this material. High-energy laser pulses strike the surface of polycrystalline bismuth vanadate inside a vacuum chamber. The heat from the laser causes the atoms to evaporate and land on the surface of a base material (substrate) to form a thin film.

“To see how different surface terminations affect photoelectrochemical activity, you need to be able to prepare crystalline electrodes with the same orientation and bulk composition,” explained co-author Chenyu Zhou, a graduate researcher from Stony Brook University working with Liu. “You want to compare apples to apples.”

As grown, bismuth vanadate has an almost one-to-one ratio of bismuth to vanadium on the surface, with slightly more vanadium. To create a bismuth-rich surface, the scientists placed one sample in a solution of sodium hydroxide, a strong base.

“Vanadium atoms have a high tendency to be stripped from the surface by this basic solution,” said first author Dongho Lee, a graduate researcher working with Choi. “We optimized the base concentration and sample immersion time to remove only the surface vanadium atoms.”

To confirm that this chemical treatment changed the composition of the top surface layer, the scientists turned to low-energy ion scattering spectroscopy (LEIS) and scanning tunneling microscopy (STM) at the CFN.

In LEIS, electrically charged atoms with low energy – in this case, helium – are directed at the sample. When the helium ions hit the sample surface, they become scattered in a characteristic pattern depending on which atoms are present at the very top. According to the team’s LEIS analysis, the treated surface contained almost entirely bismuth, with an 80-to-20 ratio of bismuth to vanadium.

“Other techniques such as x-ray photoelectron spectroscopy can also tell you what atoms are on the surface, but the signals come from several layers of the surface,” explained Liu. “That’s why LEIS was so critical in this study – it allowed us to probe only the first layer of surface atoms.”

In STM, an electrically conductive tip is scanned very close to the sample surface while the tunneling current flowing between the tip and sample is measured. By combining these measurements, scientists can map the electron density – how electrons are arranged in space – of surface atoms. Comparing the STM images before and after treatment, the team found a clear difference in the patterns of atomic arrangements corresponding to vanadium- and bismuth-rich surfaces, respectively.

“Combining STM and LEIS allowed us to identify the atomic structure and chemical elements on the topmost surface layer of this photoelectrode material,” said co-author Xiao Tong, a staff scientist in the CFN Interface Science and Catalysis Group and manager of the multiprobe surface analysis system used in the experiments. “These experiments demonstrate the power of this system for exploring surface-dominated structure-property relationships in fundamental research applications.”

Simulated STM images based on surface structural models derived from first-principle calculations (those based on the fundamental laws of physics) closely matched the experimental results.

“Our first-principle calculations provided a wealth of information, including the electronic properties of the surface and the exact positions of the atoms,” said co-author and Galli Group postdoctoral fellow Wennie Wang. “This information was critical to interpreting the experimental results.”

After proving that the chemical treatment successfully altered the first layer of atoms, the team compared the light-induced electrochemical behavior of the treated and nontreated samples.

“Our experimental and computational results both indicated that the bismuth-rich surfaces lead to more favorable surface energetics and improved photoelectrochemical properties for water splitting,” said Choi. “Moreover, these surfaces pushed the photovoltage to a higher value.”

Many times, particles of light (photons) do not provide enough energy for water splitting, so an external voltage is needed to help perform the chemistry. From an energy-efficiency perspective, you want to apply as little additional electricity as possible.

“When bismuth vanadate absorbs light, it generates electrons and electron vacancies called holes,” said Liu. “Both of these charge carriers need to have enough energy to do the necessary chemistry for the water-splitting reaction: holes to oxidize water into oxygen gas, and electrons to reduce water into hydrogen gas. While the holes have more than enough energy, the electrons don’t. What we found is that the bismuth-terminated surface lifts the electrons to higher energy, making the reaction easier.”

Because holes can easily recombine with electrons instead of being transferred to water, the team did additional experiments to understand the direct effect of surface terminations on photoelectrochemical properties. They measured the photocurrent of both samples for sulfite oxidation. Sulfite, a compound of sulfur and oxygen, is a “hole scavenger,” meaning it quickly accepts holes before they have a chance to recombine with electrons. In these experiments, the bismuth-terminated surfaces also increased the amount of generated photocurrent.

“It’s important that electrode surfaces perform this chemistry as quickly as possible,” said Liu. “Next, we’ll be exploring how co-catalysts applied on top of the bismuth-rich surfaces can help expedite the delivery of holes to water.”

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Argentina starts removing solar panels from Chilean border

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Argentina starts removing solar panels from Chilean border


Argentina starts removing solar panels from Chilean border

by AFP Staff Writers

Santiago (AFP) June 17, 2024






Argentina on Monday began removing solar panels that were installed by accident on the wrong side of its shared border with Chile, after a complaint from Chilean President Gabriel Boric.

In late April, the Argentine Navy inaugurated a maritime surveillance post on the border with Chile, in the Patagonia region of South America.

But the solar panels, which provide energy to that military unit, were set up on the Chilean side of the frontier.

In a statement, the Argentine Navy acknowledged the mistake and said it had “transferred personnel and means to begin the removal of a solar panel installed in the territory of the sister republic of Chile, north of the Island of Tierra del Fuego.”

Earlier in the day, Boric demanded that the panels be removed or Chile itself would do it.

“Borders are not something that can be ambiguous. It is a basic principle of respect between countries and therefore they must remove those solar panels as soon as possible or we are going to do it,” Boric told reporters during a visit to Paris.

Chile and Argentina share a border of about 5,000 kilometers (more than 3,000 miles).

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Chinese Premier Li targets clean energy in Australia visit

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Chinese Premier Li targets clean energy in Australia visit


Chinese Premier Li targets clean energy in Australia visit

by AFP Staff Writers

Sydney (AFP) June 18, 2024






Premier Li Qiang toured a Chinese-controlled lithium refiner in Perth on Tuesday, a sign of his country’s vast appetite for Australian “critical minerals” required for clean energy technologies.

Li ended his four-day visit to Australia with a tour of the low-carbon energy industry in resource-rich Western Australia.

His first stop was Tianqi Lithium Energy Australia, a 51-percent Chinese-owned venture comprising a mine for hard rock lithium ore, and a lithium refinery.

Along with at least a dozen other officials, China’s second most powerful man donned a white helmet during a rainy visit to the facility south of Perth.

The Chinese premier will also view a private research facility for clean energy-produced “green hydrogen” — touted as a fuel of the future to power heavy-duty items such as trucks and blast furnaces.

Australia extracts 52 percent of the world’s lithium, the vast majority of it exported as an ore to China for eventual refining and use in batteries, notably in China’s world-dominant electric vehicle industry.

But despite being a huge Australian customer, China’s involvement in the country’s critical mineral industry is sensitive because of its dominance of global supply chains.

Australia has only recently begun refining lithium rather than exporting the ore.

And the government has announced a strategic plan to develop new supply chains with friendly countries for critical minerals such as lithium, nickel and so-called rare earths.

Earlier this year, the government ordered five China-linked shareholders to sell off a combined 10 percent stake in Northern Minerals, a producer of the rare earth dysprosium.

Such foreign ownership was against Australia’s “national interests”, Treasurer Jim Chalmers said.

About 99 percent of the world’s dysprosium — used in high-performance magnets — is currently produced in China.

China has invested in critical minerals in Latin America, Africa and Australia over the past 10-20 years, said Marina Zhang, associate professor at the University of Technology Sydney’s Australia-China Relations Institute.

Developing supply chains independent of China is “fine and dandy” but unlikely to be achieved even in the short to medium term, she said.

“We are facing a very time-pressing issue that is fighting against climate change — so that issue should be at the centre of the discourse,” Zhang said.

“But unfortunately the Western allies are taking the approach that China’s dominance across the supply chains of critical minerals is imposing national security threats,” she said.

China’s narrative, however, was that it was investing and making a contribution to sustainability and environmental protection, the analyst said.

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Rice Lab Reports Significant Advances in Perovskite Solar Cell Stability

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Rice Lab Reports Significant Advances in Perovskite Solar Cell Stability


Rice Lab Reports Significant Advances in Perovskite Solar Cell Stability

by Clarence Oxford

Los Angeles CA (SPX) Jun 18, 2024






Solar power is growing rapidly as an energy technology, recognized for its cost-effectiveness and its role in reducing greenhouse gas emissions.

A Rice University study published in Science details a method for synthesizing formamidinium lead iodide (FAPbI3) into stable, high-quality photovoltaic films. The efficiency of these FAPbI3 solar cells declined by less than 3% over more than 1,000 hours of operation at 85 degrees Celsius (185 Fahrenheit).



“Right now, we think that this is state of the art in terms of stability,” said Rice engineer Aditya Mohite. “Perovskite solar cells have the potential to revolutionize energy production, but achieving long-duration stability has been a significant challenge.”



This breakthrough represents a major step towards making perovskite photovoltaics commercially viable. The researchers added specially designed two-dimensional (2D) perovskites to the FAPbI3 precursor solution, which served as a template to enhance the stability of the crystal lattice structure.



“Perovskite crystals get broken in two ways: chemically – destroying the molecules that make up the crystal – and structurally – reordering the molecules to form a different crystal,” explained Isaac Metcalf, a Rice graduate student and a lead author on the study. “Of the various crystals that we use in solar cells, the most chemically stable are also the least structurally stable and vice versa. FAPbI3 is on the structurally unstable end of that spectrum.”



The researchers found that while 2D perovskites are more stable, they are less effective at harvesting light. By using 2D perovskites as templates, they improved the stability and efficiency of FAPbI3 films. The addition of well-matched 2D crystals facilitated the formation of high-quality FAPbI3 films, showing less internal disorder and better illumination response.



The study showed that solar cells with 2D templates retained their efficiency and durability significantly better than those without. Encapsulation layers further enhanced the stability of these solar cells, extending their operational life to timescales relevant for commercial applications.



“Perovskites are soluble in solution, so you can take an ink of a perovskite precursor and spread it across a piece of glass, then heat it up and you have the absorber layer for a solar cell,” Metcalf said. “Since you don’t need very high temperatures – perovskite films can be processed at temperatures below 150 Celsius (302 Fahrenheit) – in theory that also means perovskite solar panels can be made on plastic or even flexible substrates, which could further reduce costs.”



Silicon, the most commonly used semiconductor in photovoltaic cells, requires more resource-intensive manufacturing processes than perovskites, which have seen efficiency improvements from 3.9% in 2009 to over 26% currently.



“It should be much cheaper and less energy-intensive to make high-quality perovskite solar panels compared to high-quality silicon panels, because the processing is so much easier,” Metcalf said.



“We need to urgently transition our global energy system to an emissions-free alternative,” he added, referring to UN estimates that highlight the importance of solar energy in replacing fossil fuels.



Mohite emphasized that advancements in solar energy technologies are crucial for meeting the 2030 greenhouse gas emissions target and preventing a 1.5 degrees Celsius rise in global temperatures, essential for achieving net zero carbon emissions by 2050.



“If solar electricity doesn’t happen, none of the other processes that rely on green electrons from the grid, such as thermochemical or electrochemical processes for chemical manufacturing, will happen,” Mohite said. “Photovoltaics are absolutely critical.”



Mohite holds the title of William M. Rice Trustee Professor at Rice, is a professor of chemical and biomolecular engineering, and directs the Rice Engineering Initiative for Energy Transition and Sustainability. The study’s lead authors also include Siraj Sidhik, a Rice doctoral alumnus.



“I would like to give a lot of credit to Siraj, who started this project based on a theoretical idea by Professor Jacky Even at the University of Rennes,” Mohite said. “I would also like to thank our collaborators at the national labs and at several universities in the U.S. and abroad whose help was instrumental to this work.”



Research Report:Two-dimensional perovskite templates for durable, efficient formamidinium perovskite solar cells


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