A new impact-based design turns gentle everyday motion into bursts of electricity, greatly boosting the power output of flexible piezoelectric devices for real-world use.
(Nanowerk Spotlight) A person walking down a sidewalk produces dozens of tiny mechanical events: shoes bend, fabric flexes, and bones compress slightly under load. Bridges do the same when cars pass over them, and so do building walls when wind pushes against them. Each of these motions is small, but they are constant and scattered throughout the environments where electronics increasingly reside. Devices that could directly draw energy from these everyday movements would not need batteries that wear out or cables that limit placement. The idea is conceptually simple: convert motion into electricity.
Piezoelectric materials are among the most promising ways to do this. When they are compressed, stretched, or bent, the arrangement of positive and negative charges within their structure changes. That shift produces a voltage, which can drive current through a circuit. Piezoelectric technology powers sensors, actuators, and ultrasound devices, and it has already been used to harvest energy from vibration.
The difficulty comes from how the materials respond to motion. They generate the most useful electrical signals when the deformation happens quickly, at frequencies near the natural resonance of the material. Many piezoelectric films operate most efficiently at hundreds or thousands of hertz. The slow, irregular motions present in human movement or environmental vibration fall below 10 Hz, so the electrical response becomes weak.
Researchers have tried to work around this by improving the materials themselves. Ceramic crystals have been doped to enhance their internal polarization. Polymer films have been stretched, electrically aligned, or combined with ceramic particles to strengthen their dipoles. Device structures have been modified with porous layers and stacked electrodes to increase output. These approaches can boost performance, but they still rely on slow external motion. The input frequency remains the limiting factor.
A different approach looks at the timing of the mechanical stress rather than its magnitude. A sharp impact applies the same overall force in a concentrated moment. Instead of compressing a piezoelectric material gradually over several milliseconds, an impact compresses it over a fraction of that time. The separation of charges occurs faster, and the resulting electrical pulse becomes stronger. The underlying motion can remain slow, but the material experiences it as a rapid event. That change in how deformation is delivered has significant consequences for energy generation.
The researchers used electrospinning to form nanofibers about 200–400 nm in diameter. During this process, the polymer is both mechanically stretched and exposed to a high electric field, which encourages the β phase to form.
Barium titanate strengthens this effect. The nanoparticles serve as nucleation points that help transform the polymer from the nonpolar α phase to the polar β phase. Their high permittivity also increases the local electric field within the spinning jet. Structural measurements show the change clearly. The β phase fraction rises from 58.8% in pure polyvinylidene fluoride to 78.9% at 20 wt% barium titanate. Measurements of the piezoelectric charge coefficient d₃₃, which quantifies how much charge is produced per unit force, increase from 13.1 pC N⁻¹ in the neat polymer to 17.3 pC N⁻¹ at 30 wt% filler. The finished composite film is 20–30 µm thick.
Piezoelectric output performance under impact mode. (a) Schematic diagram of the device system and mechanical loading. (b) The applied force curve under impact mode and compression mode (Inset graph) with different driving frequencies. (c) The enlarged force and output current curve within one cycle. (d) Output current of BaTiO3/PVDF composite film with different BT content. (e) Output current varied with the external impedance. (f) Open-circuit voltage Voc fitted by the current-impedance relationship curve. (Image: Reproduced with permission from Wiley-VCH Verlag) (click on image to enlarge)
To build a working nanogenerator, the authors placed 25 µm aluminum electrodes on each side of the film and sealed it within a protective plastic layer. The active area of the device was 2 × 2 cm². They first tested it using linear compression. A controlled motor pressed and released the device at frequencies from 2 Hz to 10 Hz. The peak short circuit current rose with frequency. Pure polymer produced 0.016 mA at 2 Hz and 0.116 mA at 10 Hz. The stronger composite films increased this further. The explanation was timing. The duration of each force pulse, measured at its full width half maximum, dropped from 17.2 ms at 2 Hz to 4.3 ms at 10 Hz. Short circuit current equals transferred charge divided by time. Compressing the same material faster increases the current.
The researchers then replaced gradual compression with impact. They added a spring between the motor and the device. The motor compressed the spring, then released it to drive a 53 g mass block into the nanogenerator. The impact created a sharp primary force pulse of about 128 N. Unlike linear compression, which deformed the device over several milliseconds, the impact pulse lasted about 0.5 ms and was followed by a series of smaller decaying pulses.
The change in timing transformed output. Under impact, the pristine polyvinylidene fluoride nanogenerator produced a peak short-circuit current of 2.6 mA. The composite with 30 wt% barium titanate reached 4.16 mA. For a direct comparison, the device with 20 wt% filler increased its current from about 0.1 mA under linear compression to 3.7 mA under impact at the same driving frequency. To capture the total charge per cycle, the authors integrated the current over time. Impact produced 0.48 µC of transferred charge, while compression produced 0.054 µC.
The mechanism is rooted in how dipoles respond to deformation. In polyvinylidene fluoride, each CH₂–CF₂ unit has an inherent dipole because fluorine attracts electrons more strongly than hydrogen. When the polymer is compressed slowly, the electron cloud can shift with the atomic structure. When the material is compressed rapidly, the chain backbone moves before the electron density can fully follow. This lag increases the separation between positive and negative charge centers and strengthens the dipole moment. A stronger moment produces more piezoelectric charge.
Voltage followed the same pattern. The authors connected resistors ranging from 0.1 kΩ to 6.8 MΩ and used the relation I = U / (R + r) to estimate open circuit voltage as external resistance approached infinity. Under impact, pure polymer reached 906.5 V. The 30 wt% composite reached 1 154.0 V. Internal resistances were around 2.6–2.8 × 10⁵ Ω. These values approach the breakdown voltage of the nanofiber membrane, showing how force concentration drives polarization close to its material limit.
The method also applies to other polymers. Devices made from polyacrylonitrile increased current from 0.02 mA in compression to 1.85 mA in impact. Devices made from P(VDF TrFE) increased current from 0.04 mA to 2.71 mA. The improvement is therefore not tied to a single material system. It arises from the mechanical delivery of stress.
Power output is central to practical use. By varying load resistance, the 30 wt% composite reached a peak power density of 322.2 mW cm⁻² at an optimal load of 330 kΩ in impact mode. In compression mode, the same composite delivered 0.16 mW cm⁻² at 600 kΩ. This represents a 2013.7-fold increase in peak power density. Because the current pulses are brief, their duty cycle is low, so average power is smaller. Integrating I²R over one cycle produced an average of 4.9 W m⁻² at the optimal load.
The device sustained 63 530 impact cycles over 180 min without performance loss. A 16 cm² version illuminated two thousand light emitting diodes and nine bulbs rated at 1.5 W. In a separate test, the nanogenerator charged a 1 mF capacitor used to power a wireless temperature and humidity sensor. Once the capacitor reached the 2.5 V startup threshold, the sensor transmitted data via Bluetooth every 30 s. The nanogenerator recharged the capacitor between transmissions and maintained steady operation.
Piezoelectric nanogenerators often struggle because slow environmental motion is mismatched to the speeds at which their dipoles respond most efficiently. The work in Advanced Energy Materials focuses on the duration of force rather than its size. By compressing the same energy into a shorter moment, it increases charge separation and significantly raises electrical output. The approach uses simple mechanical elements and applies to multiple nanofiber materials. It offers a practical route toward powering sensors and embedded devices in environments where high frequency vibration is scarce.
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