A group of materials chemists at the University of Massachusetts Amherst led by Dhandapani Venkataraman, report that they have solved one of the major hurdles regarding the creation of a new type of battery that can store solar or other light-sourced energy in chemical bonds rather than electrons by developing a polymer-based system. It can yield energy storage density – the amount of energy stored – more than two times higher than previous polymer systems. The researchers say that previous high energy storage density achieved in a polymeric system was in the range of 200 Joules per gram, while their new system is able to reach an average of 510 Joules per gram, with a maximum of 690.
The authors say that as energy storage density improves – and with their work it is now approaching the capacity of lithium batteries – applications for the new technology include such possibilities as solar pads that collect energy from the sun by day, then store it for heating food, living spaces, clothing. Boyle notes that this approach will be especially valuable in areas where there is no access to a power grid.

The team follow the earlier theoretical work by Jeffrey Grossman at MIT: Grossman had suggested that higher energy density might be achieved if the commonly used compound, azobenzene molecules, were arranged along a rigid carbon nanotube. 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.”
The materials chemists plan to follow up this discovery with work to solve some practical problems related to charging the system, so they have not made a battery yet, but that is coming.