Solar Energy

Tuning electrode surfaces to optimize solar fuel production

Published

4 years ago

February 21, 2021

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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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Solar Energy

Existing EV batteries may last significantly longer under real-world conditions

Published

7 hours ago

December 10, 2024

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Existing EV batteries may last significantly longer under real-world conditions

by Clarence Oxford

Los Angeles CA (SPX) Dec 10, 2024

Electric vehicle (EV) batteries subjected to typical real-world driving scenarios-such as heavy traffic, urban commutes, and long highway trips-could last up to 40% longer than previously projected, according to new research from the SLAC-Stanford Battery Center, a collaboration between Stanford University’s Precourt Institute for Energy and SLAC National Accelerator Laboratory. This finding suggests EV owners may delay the costly replacement of battery packs or the purchase of new vehicles for several more years than expected.

Traditionally, battery scientists have tested EV batteries in labs using a constant charge-discharge cycle. While effective for quick evaluations of new designs, this method does not accurately reflect the varied usage patterns of everyday drivers, the study published in *Nature Energy* on Dec. 9 reveals.

Although battery costs have fallen by approximately 90% over the past 15 years, they still represent about one-third of an EV’s price. This research could provide reassurance to current and prospective EV owners about the longevity of their vehicle’s batteries.

“We’ve not been testing EV batteries the right way,” said Simona Onori, the study’s senior author and an associate professor at Stanford’s Doerr School of Sustainability. “To our surprise, real driving with frequent acceleration, braking, stopping for errands, and extended rest periods helps batteries last longer than previously thought based on industry-standard tests.”

Real-World Driving Profiles Improve Battery Lifespan

The researchers developed four distinct EV discharge profiles, ranging from constant discharge to dynamic patterns based on actual driving data. Testing 92 commercial lithium-ion batteries over two years, they found that batteries subjected to realistic driving scenarios demonstrated significantly improved longevity.

Machine learning algorithms were crucial in analyzing the extensive data, revealing that certain driving behaviors, like sharp accelerations, slowed battery degradation. This contradicted prior assumptions that acceleration peaks harm EV batteries. “Pressing the pedal hard does not speed up aging. If anything, it slows it down,” explained Alexis Geslin, one of the study’s lead authors and a PhD candidate in materials science and computer science at Stanford.

Aging from Use vs. Time

The study differentiated between battery aging caused by charge-discharge cycles and aging from time alone. While frequent cycling dominates battery aging for commercial vehicles like buses or delivery vans, time-induced aging becomes a larger factor for personal EVs, which are often parked and idle.

“We battery engineers have assumed that cycle aging is much more important than time-induced aging,” said Geslin. “For consumers using their EVs for daily errands but leaving them unused most of the time, time becomes the predominant aging factor.”

The researchers identified an optimal discharge rate balancing both time and cycle aging for the batteries tested, which aligns with typical consumer driving habits. Manufacturers could update battery management software to incorporate these findings, potentially extending battery lifespan under normal conditions.

Implications for the Future

Evaluating new battery chemistries and designs under realistic conditions is critical for future advancements, said Le Xu, a postdoctoral scholar in energy science and engineering. “Researchers can now revisit presumed aging mechanisms at the chemistry, materials, and cell levels to deepen their understanding,” Xu added.

The study’s principles could apply beyond EV batteries to other energy storage systems, plastics, solar cells, and biomaterials where aging is a key concern. “This work highlights the power of integrating multiple areas of expertise-from materials science and modeling to machine learning-to drive innovation,” Onori concluded.

Research Report:Dynamic cycling enhances battery lifetime

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Solar Energy

So you want to build a solar or wind farm? Here’s how to decide where

Published

2 days ago

December 8, 2024

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So you want to build a solar or wind farm? Here’s how to decide where

by David L. Chandler | MIT News

Boston MA (SPX) Dec 08, 2024

Deciding where to build new solar or wind installations is often left up to individual developers or utilities, with limited overall coordination. But a new study shows that regional-level planning using fine-grained weather data, information about energy use, and energy system modeling can make a big difference in the design of such renewable power installations. This also leads to more efficient and economically viable operations.

The findings show the benefits of coordinating the siting of solar farms, wind farms, and storage systems, taking into account local and temporal variations in wind, sunlight, and energy demand to maximize the utilization of renewable resources. This approach can reduce the need for sizable investments in storage, and thus the total system cost, while maximizing availability of clean power when it’s needed, the researchers found.

The study, appearing in the journal Cell Reports Sustainability, was co-authored by Liying Qiu and Rahman Khorramfar, postdocs in MIT’s Department of Civil and Environmental Engineering, and professors Saurabh Amin and Michael Howland.

Qiu, the lead author, says that with the team’s new approach, “we can harness the resource complementarity, which means that renewable resources of different types, such as wind and solar, or different locations can compensate for each other in time and space. This potential for spatial complementarity to improve system design has not been emphasized and quantified in existing large-scale planning.”

Such complementarity will become ever more important as variable renewable energy sources account for a greater proportion of power entering the grid, she says. By coordinating the peaks and valleys of production and demand more smoothly, she says, “we are actually trying to use the natural variability itself to address the variability.”

Typically, in planning large-scale renewable energy installations, Qiu says, “some work on a country level, for example saying that 30 percent of energy should be wind and 20 percent solar. That’s very general.” For this study, the team looked at both weather data and energy system planning modeling on a scale of less than 10-kilometer (about 6-mile) resolution. “It’s a way of determining where should we, exactly, build each renewable energy plant, rather than just saying this city should have this many wind or solar farms,” she explains.

To compile their data and enable high-resolution planning, the researchers relied on a variety of sources that had not previously been integrated. They used high-resolution meteorological data from the National Renewable Energy Laboratory, which is publicly available at 2-kilometer resolution but rarely used in a planning model at such a fine scale. These data were combined with an energy system model they developed to optimize siting at a sub-10-kilometer resolution. To get a sense of how the fine-scale data and model made a difference in different regions, they focused on three U.S. regions – New England, Texas, and California – analyzing up to 138,271 possible siting locations simultaneously for a single region.

By comparing the results of siting based on a typical method vs. their high-resolution approach, the team showed that “resource complementarity really helps us reduce the system cost by aligning renewable power generation with demand,” which should translate directly to real-world decision-making, Qiu says. “If an individual developer wants to build a wind or solar farm and just goes to where there is the most wind or solar resource on average, it may not necessarily guarantee the best fit into a decarbonized energy system.”

That’s because of the complex interactions between production and demand for electricity, as both vary hour by hour, and month by month as seasons change. “What we are trying to do is minimize the difference between the energy supply and demand rather than simply supplying as much renewable energy as possible,” Qiu says. “Sometimes your generation cannot be utilized by the system, while at other times, you don’t have enough to match the demand.”

In New England, for example, the new analysis shows there should be more wind farms in locations where there is a strong wind resource during the night, when solar energy is unavailable. Some locations tend to be windier at night, while others tend to have more wind during the day.

These insights were revealed through the integration of high-resolution weather data and energy system optimization used by the researchers. When planning with lower resolution weather data, which was generated at a 30-kilometer resolution globally and is more commonly used in energy system planning, there was much less complementarity among renewable power plants. Consequently, the total system cost was much higher. The complementarity between wind and solar farms was enhanced by the high-resolution modeling due to improved representation of renewable resource variability.

The researchers say their framework is very flexible and can be easily adapted to any region to account for the local geophysical and other conditions. In Texas, for example, peak winds in the west occur in the morning, while along the south coast they occur in the afternoon, so the two naturally complement each other.

Khorramfar says that this work “highlights the importance of data-driven decision making in energy planning.” The work shows that using such high-resolution data coupled with carefully formulated energy planning model “can drive the system cost down, and ultimately offer more cost-effective pathways for energy transition.”

One thing that was surprising about the findings, says Amin, who is a principal investigator in the MIT Laboratory of Information and Data Systems, is how significant the gains were from analyzing relatively short-term variations in inputs and outputs that take place in a 24-hour period. “The kind of cost-saving potential by trying to harness complementarity within a day was not something that one would have expected before this study,” he says.

In addition, Amin says, it was also surprising how much this kind of modeling could reduce the need for storage as part of these energy systems. “This study shows that there is actually a hidden cost-saving potential in exploiting local patterns in weather, that can result in a monetary reduction in storage cost.”

The system-level analysis and planning suggested by this study, Howland says, “changes how we think about where we site renewable power plants and how we design those renewable plants, so that they maximally serve the energy grid. It has to go beyond just driving down the cost of energy of individual wind or solar farms. And these new insights can only be realized if we continue collaborating across traditional research boundaries, by integrating expertise in fluid dynamics, atmospheric science, and energy engineering.”

Research Report:Decarbonized energy system planning with high-resolution spatial representation of renewables lowers cost

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Solar Energy

China to send batteries to Europe via route bypassing Russia: Kazakhstan

Published

2 days ago

December 8, 2024

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China to send batteries to Europe via route bypassing Russia: Kazakhstan

by AFP Staff Writers

Almaty, Kazakhstan (AFP) Dec 6, 2024

China will soon send lithium-ion batteries to Europe via Kazakhstan on a trade route that bypasses sanctions-hit Russia, the Central Asian country said Friday.

Trade via the Trans-Caspian International Transport Route (TITR) that crosses the Caspian Sea has jumped since Moscow invaded Ukraine in 2022, as European countries seek to avoid imports that transit Russia.

Kazakhstan has agreed to “jointly develop” the route with Beijing, launching a “trial run for the transportation of lithium-ion batteries from China” in December, Kazakhstan’s transport ministry said Friday.

China is the world’s largest producer of lithium-ion batteries and among the top miners of the metal, which is used to power phones and electric vehicles.

“The volume of transportation from China along the TITR (in the direction of China to Europe) has exceeded the equivalent of 27,000 20-foot containers, which is 25 times more than in the same period last year,” the ministry said.

The ministry also noted an increase in goods transported between China and Kazakhstan, with both sides discussing the idea of opening new transport routes across their shared border.

Europe has looked to Central Asia as a key trading partner since Moscow launched its Ukraine offensive, triggering a barrage of Western sanctions on Moscow.

Beijing has also invested billions of dollars in building rail and road routes that traverse Central Asia, as it seeks to turn the region into a trading hub for its “New Silk Road”.

Construction is underway to build a China-Kyrgyzstan-Uzbekistan railroad that will shorten transport times between China and Europe.

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