(a & b) Concept and prototype (c & d) Microscope images of material composite (via Georgia Tech & NANO Letters)
The idea of converting the energy used when walking to electricity has travelled through my mind, as I am sure it has through many of yours. The question is first, how to generate it, and then how can we store it? A group of researchers from the Georgia Institute of Technology in Atlanta Georgia have essentially answered both of those questions using one device: a piezoelectric generator/battery unit.
To achieve this feat of elegant simplicity, a regular lithium ion coin battery was modified. Instead of the regular polyethylene separator between the electrodes, the team put a thin film of polyvinylidene difluoride (PVDF) with piezoelectric properties. Piezoelectric materials create electrical potentials within them when a force or pressure is applied to them. These are very common in nature and even in organics. Bones and even dna have been found to be piezoelectric materials. The battery is composed of a TiO2 anode — composed of nanotubes of 200 nm in diameter — and a LiCoO2 cathode. The electrodes are separated by the thin PVDF film. Outside, the anode and cathode are covered by Ti foil and Al foil respectively.
At frequencies of 2.3 Hz or 2.3 compression cycles per second, the battery can be charged from 327mV to 385 mV in 4 minutes. Researchers showed these power cells could also self-charge at voltages closer to 1.5V. This makes them appetizing to use in conjunction with small portable electronic devices.
Placement of these units on the sole of a shoe, can deliver the force needed to charge these power cells. When continuously deformed, the PVDF film causes an ion migration process that moves positive Li ions from the cathode to the anode and chemically charge the battery. The power cell does not discharge on its own because after it is charged the Li ions form LiTiO with the anode.
The battery only discharges when a potential difference is required in a circuit. As the battery discharges, the cathode starts collecting the Li ions from the anode. Once the battery reaches equilibrium between the electrodes, it can be recharged.
Possible improvements will be attempted using different piezoelectric and making the battery casing flexible for more piezoelectric deformation. The GIT team’s results were published in a recent issue of Nano Letters. Stay tuned.
Cabe
