Northwestern University scientists have developed a new liquid material that charges like a battery, transforms like a living organism and then resets itself in open air.
Traditionally, harvesting energy, storing it and using it require separate materials or devices. The new platform merges all three functions into a single material, opening the door for adaptive, clean, renewable systems that don't require plastics or metals.
To design the material, the researchers drew inspiration from the cytoskeleton - a cell's dynamic internal scaffold that enables it to maintain its shape, move and divide. Unlike animals' rigid skeletons, cytoskeletons constantly build, dismantle and rebuild themselves. Northwestern's new material behaves in a similar way, repeatedly assembling and disassembling as it stores and releases energy. But instead of running on biological fuels, it is powered by electrons harvested from sunlight, electricity, X-rays and other energy sources.
After absorbing energy, the yellow liquid material transforms into a black gel that can store energy for months. Then users can tap that stored energy to drive chemical reactions, much like drawing electricity from a battery. To reset the process, the material simply needs oxygen from open air, which causes the gel to dissolve back into liquid. Then, the material can be recharged and used again and again.
The study recently was published online and appears in the June 11 issue of the journal Chem. It marks the first report of a material that stores energy by physically rebuilding itself.
"Living systems are remarkably dynamic," said Northwestern's Samuel I. Stupp, the study's senior author. "They constantly build structures, break them apart and rebuild them. We wanted to create a synthetic material that behaves in a similar way while performing useful functions. Our material's ability to store and release energy on demand could make it useful for energy storage, environmental remediation and next-generation soft electronics."
A pioneer in self-assembling materials, Stupp is the Board of Trustees Professor of Materials Science and Engineering, Chemistry, Medicine and Biomedical Engineering at Northwestern, where he has appointments in the McCormick School of Engineering, Weinberg College of Arts and Sciences and Northwestern University Feinberg School of Medicine. He also directs the Center for Regenerative Nanomedicine. Stupp group members Tyler Jaynes (a recent Ph.D. graduate) and Luka Dordevic (a former postdoctoral researcher in the lab) are the co-first authors of the study.
One system, three functions
For decades, scientists have developed materials that can absorb energy from light, electricity and other sources. But in many cases, that captured energy must be used immediately or stored in separate devices such as batteries. As a result, the processes of harvesting, storing and using energy are often separated into different materials and devices.
To combine these roles into a single system, Stupp and his team turned to supramolecular materials, or molecules that self-organize into larger structures. Called ANI-MV, Stupp's custom-designed molecule combines two main components. An amino naphthalene aromatic unit (ANI) responds to light, and methyl viologen (MV) stores electrons.
When the ANI portion absorbs energy, it donates electrons to the MV portion. As the MV unit becomes electron-rich, neighboring molecules strongly attract one another, forming structures called pimers. The pimers then organize into semiconducting nanoscale ribbons with delocalized electrons, meaning the electrons can move freely across the structure rather than remaining confined. As the ribbons become entangled, they form a black gel that stores electrons throughout its molecular network. The ribbons mark the first example of a supramolecular polymeric pimer - a self-assembled material built from electron-storing molecular pairs.
Enabling 'dark photocatalysis'
In experiments, Stupp and his team demonstrated that the charged gel could transfer its stored energy to oxygen, creating highly reactive molecules that then powered chemical reactions in complete darkness.
The material also proved remarkably versatile. Whether powered by a chemical fuel, light, electricity or X-rays, the material consistently transformed from a yellow liquid into an energy-rich, conductive black gel. Exposure to open air reversed the process, returning the material to tiny, non-conductive clusters of molecules suspended in a yellow liquid. Because light can selectively trigger the transformation, scientists can use the material to create microscopic conductive patterns that later disappear when the material resets.
"Most light-driven materials stop working when the light source is gone," Stupp said. "Our material enables a form of 'dark photocatalysis.'"
Because the material can repeatedly harvest, store and release energy, the researchers envision potential applications ranging from clean-energy technologies and environmental remediation to adaptive soft electronics and programmable materials. Stupp estimates that just one gram of the material could hold enough power to charge a smartwatch or other wearable device.
"The world generates enormous amounts of solar energy, but it's challenging to store it until it's needed," Stupp said. "For energy storage, our material performs the same function as a battery. However, it runs entirely in water, requires no metal or plastics and can be recharged repeatedly. This kind of clean, flexible platform could open new doors for renewable energy."
The study, "Dynamic self-assembly mediated by stored and released electrons in pimer supramolecular polymers of chromophore amphiphiles," was supported as part of the Center for Bio-inspired Energy Science, an Energy Frontier Research Center funded by the U.S. Department of Energy (award number DE-SC0000989).
