For years materials chemists have been attempting to develop a new type of battery that is capable of storing solar or other light-sourced energy in chemical bonds instead of electrons, one that will discharge the energy on demand as heat rather than as electricity – addressing the necessity for long-standing, stable, efficient storage of solar energy.
Currently, a team of materials chemists at the University of Massachusetts Amherst led by Dhandapani Venkataraman, with Ph.D. student and first author Seung Pyo Jeong, Ph.D. students Larry Renna, Connor Boyle and others, state that they have solved one of the main obstacles in the field by designing a polymer-based system. It can produce energy storage density – the quantity of energy stored – more than two times higher than earlier polymer systems. Details of the research have been published in the current issue of Scientific Reports.
Venkataraman and Boyle say that earlier high energy storage density realized in a polymeric system was in the range of 200 Joules per gram, while their new system could attain an average of 510 Joules per gram, with a maximum of 690. Venkataraman says, “Theory says that we should be able to achieve 800 Joules per gram, but nobody could do it. This paper reports that we’ve reached one of the highest energy densities stored per gram in a polymeric system, and how we did it.”
The team says that as energy storage density progresses – and with their work, it is currently nearing the capacity of lithium batteries – applications for the new technology include such possibilities as solar pads that amass energy from the sun during the day, and then store it for heating living spaces, food, clothing, or blankets at night. Boyle notes that this method will be especially beneficial in areas where there is no access to a power grid.
Venkataraman says his team’s achievement would perhaps not have been possible without previous theoretical research by Jeffrey Grossman at MIT: “Without his paper and his thoughts on the theory, I don’t think we would have gotten where we are today.” Grossman had recommended that higher energy density might be realized if the frequently used compound, azobenzene molecules, were arranged along a stiff carbon nanotube. This frame would allow researchers to control the molecular interactions, which defines how much energy is taken up and discharged.
Venkataraman explains, “We understood the idea of controlling the arrangement, but we thought, What if we use a flexible polymer, not a rigid tube? Something like a string of Christmas lights, where the lights are the azobenzene molecules. Because what you cannot do with a carbon nanotube is reduce the distance between the molecules. We thought that the structure of a polymer chain would let the azobenzene groups get closer to each other and interact, which is when they gain energy and become more stable.”
Their idea worked, he adds, “but we didn’t understand why. The finding was unexpected, so we couldn’t stop there. Every time my students came to me with unexplained high numbers, I sent them back to do more control experiments to understand and validate the findings. We had to be skeptical, because we had an unusual result.”
Venkataraman says, “The twist in the story is that we thought that the distance between the lights in the string was the most important. It is important, but what is more important is the way that multiple strings and their lights are carefully arranged. It turns out that the processing solvent we used acts to arrange and regulate the architecture, so the azobenzene molecules attached to the polymer are arranged very neatly and compactly. It basically acts to ensure that there can be maximum packing density.”
They used the solvent tetrahydrofuran (THF) for this processing “simply because it’s good solvent for this polymer system,” Boyle says, not doubting that it would impact how much energy is stored and later released when they first began.
Venkataraman says, “This paper talks about how, on the molecular level, the THF affects the energy we see on the macro scale. It starts out with how the solvent molecule interacts with the polymer and it turns out that that is related to the molecular packing, how they are arranged in space. When the molecules are packed properly they can gain more energy. It took two years of work, but we finally were able to show that it’s true.”
He adds that a partnership with scientists at Schrödinger, Inc., a scientific software and solutions company located in New York, also played a crucial role in helping the UMass Amherst team to comprehend the origins of the detected high energy storage densities. Led by Shaun Kwak, a lead applications scientist at Schrödinger, with experts in force-field technology Ed Harder and Wolfgang Damm, the project got the required company support.
Kwak says, “Working directly with scientists with experimental background at the highest level gets always marked very high in value at Schrödinger.” He highlights the synergetic effect he witnessed first-hand during the collaboration. “It provides a great opportunity for us to showcase the power of computational chemistry on the verge of most innovative ideas, such as shown in this work.”
The materials chemists plan to follow up this finding with further research to solve certain practical difficulties related to charging the system, so they have not built a battery yet, but that will be done. This research was supported by UMass Amherst.