Energy storage usually brings to mind batteries, capacitors, tanks of hydrogen, or maybe some giant gravity system hauling blocks up a tower. Northwestern University researchers have now demonstrated something a lot stranger: a yellow liquid that “charges” by rebuilding itself into a black gel, stores electrons for months, then releases them on demand to drive chemical reactions.

Their technology is a cell-inspired chemical system that stores energy as electrons inside a self-assembled molecular structure. When the stored energy is needed, the material releases those electrons to oxygen, generating reactive oxygen species that can carry out oxidation reactions even in the dark.

Now, the researchers' description, “a material that stores energy,” may sound more familiar than the actual science. The material is not a battery in the everyday “lithium-ion cell” sense, and you are not going to plug your phone into a vial of black goo any time soon.

What Northwestern has built is a soft molecular platform that integrates energy capture, energy storage, structural change, and catalysis into a single material.

In its discharged state, the material is a yellow liquid composed of small globular molecular aggregates. When exposed to visible light, electricity, chemical fuels, or X-rays, its molecules accept electrons. That electronic change triggers the molecules to stack and bind together through π-π interactions and radical “pimer” formation, causing the loose liquid to reorganize into long supramolecular polymer fibers. The visible result is a dramatic transformation of the yellow liquid into a black, electrically conductive hydrogel.

An interesting aspect is that the material’s charged state is also its assembled state. The molecules do not merely sit there holding electrons like ions in a conventional battery electrode. They physically reorganize around those electrons, forming a new soft structure that stabilizes the stored charge. Under oxygen-free conditions, the researchers say the gel can hold that stored energy for months.

To release the energy, oxygen is introduced. Oxygen accepts the stored electrons from the gel, producing reactive oxygen species. These highly reactive oxygen-containing molecules can then oxidize organic substrates. In other words, the stored energy comes back out as chemical redox activity, not as a neat stream of electrical current through a circuit.

That is why the researchers describe the system as a model for “dark photocatalysis.” In ordinary photocatalysis, light is required during the reaction. Here, the material can be charged earlier by light or another energy source, stored in the dark, and then used later to power a chemical reaction after the original energy input has been expended.

The work is being presented as the first report of a material that stores energy by physically rebuilding itself. This is another true innovation here. Fixed materials inside engineered devices normally handle energy storage and release – for example, electrodes in a battery or semiconductors in a solar cell. This material blurs those roles. It captures energy, stores electrons, changes its structure, and later uses the stored electrons to drive chemical reactions.

To be absolutely clear once again, what the material stores is chemical redox energy, specifically in the form of extra electrons held inside the charged gel. When the gel is exposed to oxygen, it pulls electrons from the gel, generating reactive oxygen species that can drive oxidation reactions. So the useful work is chemical work, not electrical work. The material can power reactions such as oxidizing organic molecules, potentially degrading pollutants, sterilizing surfaces, or enabling photocatalytic chemistry to continue later in the dark.

Long-term storage depends on keeping oxygen away. Once the gel is exposed to open air, the stored electrons begin doing their job, resetting the material back toward its original liquid state. This reset is part of the appeal. Once oxygen dissolves the charged gel back into the yellow liquid state, the system can be charged again and reused.

The team's research is published in the journal Chem.

Source: Northwestern University