mechanism, so that the entire device will be slightly discharged
after many cycles of mechanical deformation. The response of
the device fabricated using PVDF ﬁlm with opposite polar-
ization was measured under the same deformation conditions,
as shown in Figure 4b. The voltage slightly decreased from 330
to 315 mV during the ∼8000 cycles of deformation that lasted
for ∼1.1 h. Rather than self-charging, the self-discharging has
been accelerated in such a device simply because the
piezopotential drove Li ions to migrate in the opposite
direction of the charging process. This could be more clearly
demonstrated on fully charged devices. As shown in Figure S9b
(Supporting Information), the piezopotential from periodic
deformations can help the device to self-discharge all the full
capacity in about 1 day’s time, which normally takes several
months. This further conﬁrmed the working principle of the
SCPC, as proposed in Figure 2, can continuously drive the
progress of electrochemical reactions for the energy storage.
In summary, a new mechanical-to-electrochemical process is
proposed by integrating piezoelectric material with an electro-
chemical system, in which an approach for fabricating a self-
charging power cell is demonstrated for converting and
simultaneously storing mechanical energy directly as chemical
energy, with a signiﬁcantly higher overall eﬃciency than the
traditional charging method composed of two separated units.
By replacing the PE separator as for conventional Li battery
with a piezoelectric PVDF ﬁlm, the piezoelectric potential from
the PVDF ﬁlm created under straining acts as a charge pump to
drive Li ions migrating from the cathode to the anode
accompanying charging reactions at electrodes, which can be
deﬁned as a piezo-electrochemical process. Using the
mechanism demonstrated here, we have hybridized a generator
with a battery for the ﬁrst time as a sustainable power source. It
provides an innovative approach for developing new energy
technology for driving personal electronics and self-powered
nanotube arrays were directly grown on Ti
foils (0.05 mm thick, 99.6% purity; Alfa Aesar) by electro-
chemical anodizing in ethylene glycol solution containing 0.3
wt % NH
F and 2 vol % H
O, with Pt as counter electrode.
Prior to growth, all Ti foils were ultrasonically cleaned in
acetone, water, and ethanol consecutively, and then dried in air.
A thin layer of PMMA was spin-coated on one side of the foil
to protect it from the etching solution. The prepared Ti foil was
anodized at 50 V for 5 h, and then treated by ultrasonication in
acetone for a few seconds, leaving hexagon-like footprints on
the surface of Ti foil. A second anodization was then performed
under the same condition for 2 h to produce well-aligned TiO
Finally, the two-step anodized nanotubes were annealed at 450
°C for 2 h in the air to form an anatase crystalline phase and
remove PMMA on the back of Ti foils.
Additional discussions and ﬁgures about properties of the
polarized PVDF ﬁlms, crystal structure of as-synthesized TiO
nanotubes, viability of the self-charging power cell as a battery
system, and the mechanism of the self-charging power cell. This
material is available free of charge via the Internet at http://
These authors contributed equally.
The authors declare no competing ﬁnancial interest.
This research was supported by DARPA (HR0011-09-C-0142),
Airforce, U.S. Department of Energy, Oﬃce of Basic Energy
Sciences under Award DE-FG02-07ER46394, NSF (CMMI
0403671), and the Knowledge Innovation Program of the
Chinese Academy of Sciences (Grant No. KJCX2-YW-M13).
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