Moisture-Triggered Motions Of New Material Harnessed To Generate Electrical Power

Scientists have long struggled to design sustainable batteries and sensors that power themselves via natural stimuli such as light, heat, and even the motion of walking. Now, researchers have developed a material that harvests energy from tumbling motions triggered by a ubiquitous environmental source: water vapor (Science, DOI: 10.1126/science.1230262).

The polymeric material requires only a tiny amount of water vapor to generate power, says postdoc Mingming Ma, a member of the MIT team led by Robert S. Langer that developed the substance. Even the moisture on your skin is sufficient, Ma says. “You can put it on your hand, and it will work.”

The MIT researchers didn’t initially set out to develop an energy-harvesting substance, Ma tells C&EN. Instead, they were working toward a strong, flexible polymer for use in organic electrodes. “But when we saw how water-responsive it was,” Ma explains, “we switched directions.”

The substance is a composite made of a rigid polypyrrole matrix and a soft polyol-borate gel. When the material absorbs moisture, water molecules disrupt hydrogen bonds between its polypyrrole and polyol components, and the composite swells.

That swelling comes in handy when the material is placed in a water-vapor gradient like the one on a moist surface: A 30-µm-thick sheet of the composite rapidly curls up and then flips over continually like it’s performing a gymnastics routine. When the sheet flips over, it expels water molecules from the side exposed to the ambient air and sucks them in from the side lying against the water-covered surface, repeating the tumbling cycle.

Rather than making a material that responds to time-varying stimuli, as has been done in the past, “Langer’s team has devised a simple and elegant means to harness a constant environmental condition to drive motion,” says polymer scientist Ryan C. Hayward of the University of Massachusetts, Amherst.

To convert the mechanical energy generated by the material’s rapid movements into power, the MIT team stuck a piezoelectric film—which transforms mechanical force into electrical charge—to one side of the polymer sheet. The researchers then attached the whole assembly to an electrical circuit.

A 6- by 3-cm patch of the material generated some 5.6 nW of electrical power on average. That’s orders of magnitude less than the power density created by a cell-phone battery, Ma says.

In a commentary associated with the new report in Science, engineers Hyoki Kim and Sunghoon Kwon of Seoul National University say that this paltry power output might limit the material’s usefulness. But, they add, “it should be possible to increase the power output in a straightforward way by stacking multiple devices.”

Ma agrees, adding that he and the MIT team are also now developing a more efficient piezoelectric material to harness the composite’s mechanical energy. The researchers have submitted a patent application for the new substance.