Biologically inspired energy

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.

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.

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-separate state, but the vehicle that carries the charge-separate state is wobbling all the time, then it's not very reliable," Cheung said.

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."

A version of this story first appeared on the TACC website.