Biologically Inspired Energy
Researchers use TACC's Ranger supercomputer to investigate photosynthetic materials
In the 1990s, Devens Gust at Arizona State University discovered a potentially world-shattering material able to convert sunlight into chemical energy by mimicking the processes plants use to derive sustenance from the sun. However, the artificial photosynthetic material he discovered — known as the carotenoid-porphyrin-C60 molecular triad — has proven difficult to commercialize.
"The drawback is that this molecule can only be controlled or contained in experimental labs," said Margaret Cheung, Assistant Professor of Physics at the University of Houston.
A triad molecule (left) is rendered in bond representation. Carbon atoms are in cyan; nitrogen in blue; hydrogen in white.
Cheung has studied the effect of confinement on materials for many years. However, this is the first project where she has applied those insights to artificial photosynthetic materials. She credits the new direction, in part, to the impact of the energy industry in her home city of Houston.
"Because of the energy crisis that we've experienced in recent decades, I've thought about how my research can contribute to the energy field," Cheung said. "Even though my training isn't in biology, I'm always thinking about this. And when you research something passionately, you get some ideas."
Solar power could transform the energy landscape in the United States, reducing the nation's reliance on coal and natural gas for electricity. Today, however, solar power remains more expensive on average than fossil fuels.
"You may think that the sun is abundant, but traditional photovoltaics require rare earth elements, and a lot of them are imported from areas that have wars or where it is difficult to extract, which raises the cost," said Cheung. "If we learn from plants, which use only common elements — hydrogen, nitrogen, carbon, oxygen and some others — then we will be able to bring the cost down. This is the reason why we look at bio-inspired materials as possible resources for solar energy."
To enable her research, Cheung uses the powerful Ranger supercomputer at the Texas Advanced Computing Center (TACC) to explore the role that confinement, temperature, and solvents play in the stability and energy efficiency of the light-harvesting triad. Her results provide a way to test, tailor, and engineer nano-capsules with embedded triads that, when combined in large numbers, could greatly increase the ability to produce clean energy.
(A) A 2-D chemical diagram of the triad. (B) A 3-D representation of the triad. (C) A snapshot of an all-atomistic representation of the triad solvated in an SC, which is defined by the half-length of a central cylinder, a, and the radius of a hemisphere, b. a is 3.65 nm and b is 1.61 nm in the figure. Atoms are colored by their types: hydrogen in white, carbon in cyan, nitrogen in blue, and oxygen in red. The carbon atoms of porphyrin are colored in purple for visual guidance. Click image to view larger version.
The project is funded by the Department of Energy and supported by advanced computers at TACC and the National Energy Research Scientific Computing Center (NERSC). Since 2011, the project has used more than 2.5 million computing hours on Ranger and 2 million hours at NERSC. The results of her studies were published in the Journal of Physical Chemistry B in February 2012.
"By using computation, we can understand the properties and the behavior of this molecule and gain insight into improving it," she said. "If we can capture the mechanism that converts solar energy into chemical fuel, it opens the door to many opportunities."
Unlike typical photovoltaic cells, which are made out of solid-state materials, the carotenoid-porphyrin-C60 molecular triad is a bioinorganic compound, combining biological and inorganic components. These hybrid molecules are more flexible, fragile and prone to breaking.
The light-harvesting molecular triad that Cheung investigates combines three components: a carotenoid (an organic pigment, similar to the chromophore in plants); a fullerene or buckyball (a carbon-based molecule that forms a hollow sphere); and a porphyrin (an organic compound that can bind ligands to metals, as with hemoglobin).
"When photons hit the triad, the molecule becomes excited," Cheung explained. "This excited state scatters the electrons, providing a driving force to move electrons into a polarized distribution, like a dipole."
This separation of positive and negative charges in the system becomes the stored chemical potential from which energy can be produced.
The problem is that bioinorganic compounds are flexible in nature and cannot remain in a fixed configuration over a short period of time. "If we want to harness this charge-separated state, but the vehicle that carries the charge-separated state is wobbling all the time, then it's not very reliable," Cheung said.
The Cheung Group at the University of Houston, 2012.
The wobbliness of the triad also challenged efforts to simulate its dynamics in the past. Cheung had to pioneer new methods that combine quantum chemistry approaches, molecular dynamics simulations, and statistical physics to take into account the microscopic landscape of the molecules and the many configurations that the triad might be in when photons hit the material.
Cheung and her team simulated the triad in solution at many different temperatures and confinement conditions to map the impact of these changes on the behavior of the molecule. They discovered that the triad conformation distribution could be manipulated by temperature fluctuations in the solvent. Furthermore, they concluded that when the presence of confinement is considered, the network of solvent molecules is disrupted, which dictates both the positions of the components in the confinement and its attraction to the wall.
Ultimately, the goal is to use information from the computer simulations to design a scalable system that maximizes the generation of chemical energy while maintaining the triad's stability.
"If we want to stabilize the triad there are many different strategies to do so," Cheung said. "Maybe this involves redesigning the molecules. Maybe it involves the design of different solvents or different capsule sizes. We don't know unless we try different conditions and probe its responses. By using computation, we can understand the properties and the behavior of this molecule, which can give us some insight into how to improve them."
May 9, 2012
The Texas Advanced Computing Center (TACC) at The University of Texas at Austin is one of the leading centers of computational excellence in the United States. The center's mission is to enable discoveries that advance science and society through the application of advanced computing technologies. To fulfill this mission, TACC identifies, evaluates, deploys, and supports powerful computing, visualization, and storage systems and software. TACC's staff experts help researchers and educators use these technologies effectively, and conduct research and development to make these technologies more powerful, more reliable, and easier to use. TACC staff also help encourage, educate, and train the next generation of researchers, empowering them to make discoveries that change the world.
- Inspired by plants, scientists have created a light-harvesting material that can turn sunlight into chemical energy. However, creating a stable form of the material for large-scale usage has proved difficult.
- Researchers from the University of Houston used computer simulations to determine how the size, temperature, and solvent in which the light-harvesting material is contained affect its ability to create energy.
- They found that the material's structure and energy-generating capacity could be manipulated by temperature fluctuations in the solvent. The results of the study were published in the Journal of Physical Chemistry B in February 2012.
Science and Technology Writer