Hybridizing Energy Conversion and Storage in a Mechanical-to-Electrochemical Process for Self-Charging Power Cell

School of Materials Science and Engineering, Georgia Institute of Technology , Atlanta, Georgia 30332-0245, United States.
Nano Letters (Impact Factor: 13.59). 08/2012; 12(9):5048-54. DOI: 10.1021/nl302879t
Source: PubMed
ABSTRACT
Energy generation and energy storage are two distinct processes that are usually accomplished using two separated units designed on the basis of different physical principles, such as piezoelectric nanogenerator and Li-ion battery; the former converts mechanical energy into electricity, and the latter stores electric energy as chemical energy. Here, we introduce a fundamental mechanism that directly hybridizes the two processes into one, in which the mechanical energy is directly converted and simultaneously stored as chemical energy without going through the intermediate step of first converting into electricity. By replacing the polyethylene (PE) separator as for conventional Li battery with a piezoelectric poly(vinylidene fluoride) (PVDF) film, the piezoelectric potential from the PVDF film as created by mechanical straining acts as a charge pump to drive Li ions to migrate from the cathode to the anode accompanying charging reactions at electrodes. This new approach can be applied to fabricating a self-charging power cell (SCPC) for sustainable driving micro/nanosystems and personal electronics.

Full-text

Available from: Xinyu Xue, Jul 02, 2015
Hybridizing Energy Conversion and Storage in a Mechanical-to-
Electrochemical Process for Self-Charging Power Cell
Xinyu Xue,
,§
Sihong Wang,
,§
Wenxi Guo,
Yan Zhang,
and Zhong Lin Wang*
,,
School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245, United States
Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing, China
*
S
Supporting Information
ABSTRACT: Energy generation and energy storage are two distinct processes that are usually accomplished using two separated
units designed on the basis of dierent physical principles, such as piezoelectric nanogenerator and Li-ion battery; the former
converts mechanical energy into electricity, and the latter stores electric energy as chemical energy. Here, we introduce a
fundamental mechanism that directly hybridizes the two processes into one, in which the mechanical energy is directly converted
and simultaneously stored as chemical energy without going through the intermediate step of rst converting into electricity. By
replacing the polyethylene (PE) separator as for conventional Li battery with a piezoelectric poly(vinylidene uoride) (PVDF)
lm, the piezoelectric potential from the PVDF lm as created by mechanical straining acts as a charge pump to drive Li ions to
migrate from the cathode to the anode accompanying charging reactions at electrodes. This new approach can be applied to
fabricating a self-charging power cell (SCPC) for sustainable driving micro/nanosystems and personal electronics.
KEYWORDS: Self-charging power cell, mechanical energy, piezoelectricity, lithium ion battery, electrochemistry
E
nergy conversion and storage
13
are the two most
important technologies in todays green and renewable
energy science, which are usually separated units designed on
the basis of vastly dierent approaches. As for energy
harvesting, depending on the nature of energy sources, such
as solar,
47
thermal,
8
chemical,
9
and mechanical,
10,11
various
mechanisms have been developed for eectively converting
them into electricity. Take smaller scale mechanical energy as
an example, piezoelectric nanogenerators (NGs) have been
developed into a powerful approach for converting low-
frequency, biomechanical energy into electricity.
1214
The
mechanism of the NG relies on the piezoelectric potential
created by an externally applied strain in the piezoelectric
material for driving the ow of electrons in the external load.
12
As for energy storage, Li-ion battery
1520
is one of the most
eective approaches, in which the electric energy is stored as
chemical energy through the migration of Li ions under the
driving of an externally applied voltage source and the follow up
electrochemical reactions occurring at the anode and cathode.
21
In general, electricity generation and energy storage are two
distinct processes that are accomplished through two dierent
and separated physical units achieving the conversions from
mechanical energy to electricity and then from electric energy
to chemical energy, respectively. Here, we introduce a
fundamental mechanism that directly hybridizes the two
processes into one, through which the mechanical energy is
directly convert ed and simultaneously stored as chemical
energy, so that the nanogenerato r and the battery are
hybridized as a single unit. Such an integrated self-charging
power cell (SCPC), which can be charged up by mechanical
deformation and vibration from the environment, provides an
innovative approach for developing a new mobile power source
for both self-powered systems
22
and portable and personal
electronics.
The experimental design of a self-charging process is based
on the characteristics of both piezoelectric and electrochemical
properties, as schematically shown in Figure 1a. The device is
based on a sealed stainless-steel 2016-coin-type cell, as shown
in the highlight of Figure 1b. The SCPC is composed of three
major components: anode, separator, and cathode. The anode
is aligned TiO
2
nanotube (NT) arrays that are directly grown
on a Ti foil. Instead of using the polyethylene (PE) separator
23
Received: August 2, 2012
Published: August 9, 2012
Letter
pubs.acs.org/NanoLett
© 2012 American Chemical Society 5048 dx.doi.org/10.1021/nl302879t | Nano Lett. 2012, 12, 50485054
Page 1
as for traditional lithium ion battery, a layer of polarized
poly(vinylidene uoride) (PVDF) lm (Measurement Special-
ties, Inc., U.S.A.) is located above the TiO
2
nanotube arrays as
the separator. This PVDF lm can establish a piezoelectric
potential across its thickness under externally applied stress,
which not only converts mechanical energy into electricity
(Figures S1 and S2, Supporting Information) but also serves as
the driving force for the migration of Li ions. The cathodes are
LiCoO
2
/conductive carbon/binder mixtures on aluminum foils.
Figure 1c is a cross-sectional scanning electron microscopy
(SEM) image of the sandwich structure of the device. The TiO
2
nanotube arrays with anatase crystal structure (Figure S3,
Supporting Information) were fabricated on Ti substrate using
an anodization method with a post-annealing process in air
24
(Method Summary). The height and diameter of the nanotubes
are about 20 μm and 100 nm, respectively, as shown in Figure
1d. A commercial piezoelectric PVDF lm with a thickness of
110 μm is predominantly composed of β phase, which
generates the strong piezoelectric eect, and with a small
fraction of α phase
25
(Figure S4, Supporting Information). The
PVDF lm has been prior poled before assembly into the
battery. After placing LiCoO
2
cathode with a thickness of 20
μm on the other side, the system was lled with electrolyte (1
M LiPF
6
in 1:1 ethylene carbonate:dimethyl carbonate) and
nally sealed for measurements. The galvanostatic charge
discharge measurements, with comparison to traditional Li-ion
batteries using PE as separators, proved that the power cells act
also as a battery system (Figures S5 and S6, Supporting
Information). Periodic deformations were applied onto the
device in order to charge it up (Figure 1b), and the voltage and
current were monitored simultaneously in both charging and
discharging processes.
The working mechanism of the self-charging power cell is an
electrochemical process driven by deformation created piezo-
electric potential (Figure 2). At the very beginning, the device is
at a discharged state, with LiCoO
2
as the positive electrode
(cathode) material and TiO
2
NTs as the negative electrode
(anode), which is the originally fabricated structure of the
device, and the LiPF
6
electrolyte is evenly distributed across the
entire space, as shown in Figure 2a. A PVDF lm, which has
intimate contacts with both electrodes, serves as the separator
and it has the smallest Youngs modules among all of the
components in the device [PVDF in electrolyte solvent: 1.2
GPa (see section E in the Supporting Information); TiO
2
: 100
GPa (Y
a
) and 266 GPa (Y
c
); LiCoO
2
: 70 GPa; Ti foil: 100
110 GPa; Al foil: 69 GPa]; thus, it suers from the most severe
compressive strain when a compressive stress is applied onto
the device, as shown in Figure 2b. We purposely use the PVDF
lm with the polarity that results in a positive piezoelectric
potential (piezopotential) at the cathode (LiCoO
2
) side and
negative piezopotential at the anode (TiO
2
) under compressive
strain for separating the charges (see section F and Figure S7 in
the Supporting Information). Under the driving of the
piezoelectric eld with a direction from the cathode to the
anode, the Li ions in the electrolyte will migrate along the
direction through the ionic conduction paths present in the
PVDF lm separator for ion conduction in order to screen the
piezoelectric eld, and nally reach the anode, as shown in
Figure 2c (note: a PVDF lm is an ionic conductor for Li
+
,
which is why PVDF is used as the base for polymer
electrolyte
26
and also the binder
27
for electrodes in Li-ion
batteries). The decreased concentration of Li
+
at the cathode
will break the chemical equilibrium of the cathode electrode
reaction (LiCoO
2
Li
1x
CoO
2
+ xLi
+
+ xe
),
28
so that Li
+
will
deintercalate from LiCoO
2
, turning it into Li
1x
CoO
2
and
leaving free electrons at the current collector (Al foil) of the
cathode electrode. This process is driven by the tendency of
establishing new chemical equilibrium (see section G in the
Supporting Information). In the meanwhile, under the elevated
concentration of Li
+
at the anode, the reaction at the other
electrode (TiO
2
+ xLi
+
+ xe
Li
x
TiO
2
)
29
will move to the
forward direction for the same reason, enabling Li
+
to react
with TiO
2
so that Li
x
TiO
2
will be produced at the anode
electrode, leaving the positive charges at the Ti foil as the
current collector. During this process, Li
+
will continuously
migrate from the cathode to the anode and the device is
charged up a little bit owing to the large volume of the device.
During the progress of charging electrochemical reactions at
the two electrodes, extra free electrons will transfer from the
cathode to the anode, in order to maintain the charge neutrality
and the continuity of the charging reaction. There are generally
two ways for the electrons to transfer: either inside the battery
system in some manner or through the external circuitry. After
comparing the self-charging behavior of the SCPC with and
without an outer circuitry (that is the Electrochemical
Workstation connected between cathode and anode, to
monitor the change of voltage), we suggest that there should
probably be some internal mechanisms for the electrons to
transfer across the two electrodes (see section H and Figure S8
Figure 1. Structure design of a self-charging power cell by hybridizing
a piezoelectric nanogenerator and a Li-ion battery. (a) Schematic
diagram showing the design and structure of the self-charging power
cell. The anode is aligned TiO
2
nanotube arrays that are directly grown
on Ti foils; a layer of polarized PVDF lm performs as the separator;
the cathode is a LiCoO
2
mixture on aluminum foil. This structure is
sealed in stainless-steel 2016-coin-type cells, as shown in the inset. (b)
Sticking a power cell on the bottom of a shoe, the compressive energy
generated by walking can be converted and stored directly by SCPC.
(c) Cross-sectional SEM image of the self-charging power cell, which
is composed of aligned nanotubes as anode, piezoelectric polymer lm
as separator and cathode. (d) Enlarged view of the aligned TiO
2
nanotubes. The inset is a top view SEM image of the nanotubes.
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Page 2
in the Supporting Information), although this exact process is
still to be further investigated.
Under the mechanical deformation, the pie zopotenti al
continues to drive the migration of Li
+
ions until a point
when the chemical equilibriums of the two electrodes are re-
established and the distribution of the Li
+
can balance the
piezoelectric eld in the PVDF lm, with no Li
+
drifting
through PVDF (Figure 2d); that is to say, a new equilibrium is
achieved and the self-charging process will cease. This is the
process of converting mechanical energy directly into chemical
energy.
In the second step, when the applied force is released, the
piezoelectric eld of the PVDF disappears, which breaks the
electrostatic equilibrium, so that a portion of the Li ions diuse
back from the anode to the cathode (Figure 2e) and reach an
even distribution of Li
+
all over the space in the device again
(Figure 2f). Then, a cycle of charging is completed through an
electrochemical process of oxidizing a small amount of LiCoO
2
at the cathode to Li
1x
CoO
2
and reducing a bit of TiO
2
to
Li
x
TiO
2
at the anode. When the device is mechanically
deformed again, the process presented above is repeated,
resulting in another cycle of charging by converting mechanical
energy directly into chemical energy.
In this self-charging mechanism, the role played by the
piezoelectric material (PVDF) is similar to the DC power
supply used in the conventional charging process of a Li
Figure 2. The working mechanism of the self-charging power cell driven by compressive straining. (a) Schematic illustration of the self-charging
power cell in discharged state with LiCoO
2
as cathode and TiO
2
nanotubes as anode. (b) When a compressive stress is applied onto the device, the
piezoelectric separator layer (e.g., PVDF lm) creates a piezopotential, with the positive polarity at the cathode side and negative piezopotential at
the anode. (c) Under the driving of the piezoelectric eld, the Li ions from the cathode will migrate through the PVDF lm separator in the
electrolyte toward the anode, leading to the corresponding charging reactions at the two electrodes. The free electrons at the cathode and positive
charges at the anode will dissipate inside the device system. (d) The status where chemical equilibrium of the two electrodes is re-established and the
self-charging process ceases. (e) When the applied force is released, the piezoelectric eld of the PVDF disappears, which breaks the electrostatic
equilibrium, so that a portion of the Li ions will diuse back to the cathode. (f) This electrochemical system reaches a new equilibrium, and a cycle of
self-charging is completed.
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Page 3
battery. Both of them can be deemed as charge pumps, but the
specic mechanisms are dierent. As for the conventional
charging method, the DC power supply will pump the electrons
from the positive electrode to the negative electrode through
the external circuit and the Li ions will go in the same direction
but within the cell, in order to remain a neutral charge balance.
Thus, the electrochemical reactions on the two electrodes occur
and the battery is charged up. However, for our SCPC
proposed here, the piezoelectric material pumps the Li
+
ions,
rather than the electrons, from the positive to the negative
electrode, which also accomplishes the charging of the device.
This mechanism can also be explained by thermodynamics.
According to Nernsts theory,
30
the relative electrode potentials
of the two electrodes have the following relationships with Li
+
concentration:
φφ=
°
+
−−
RT
Fa
ln
1
[(Li)]
x
Li CoO /LiCoO Li CoO /LiCoO
c
xx12 2 12 2
φφ=
°
+
RT
Fa
ln
1
[(Li)]
x
TiO /Li TiO TiO /Li TiO
a
xx22 22
where φ
Li
1x
CoO
2
/LiCoO
2
and φ
TiO
2
/Li
x
TiO
2
are actual electrode
potentials of the cathode and anode, φ
Li
1x
CoO
2
/LiCoO
2
° and
φ
TiO
2
/Li
x
TiO
2
° are standard electrode potentials of these two
electrodes, and a
c
(Li
+
) and a
a
(Li
+
) are the activities of Li
+
around the cathode and anode, respectively, which can be
approximately equated to the concentrations, R is the gas
constant, T is the temperature, and F is the Faraday constant.
Thus, under the driving of piezoelectric eld, because of
depletion of Li
+
concentration near the positive electrode, the
electrode potential φ
Li
1x
CoO
2
/LiCoO
2
will decrease; likewise, the
elevation of Li
+
concentration will result in an increase of
φ
TiO
2
/Li
x
TiO
2
at the negative electrode. For conventional Li-ion
battery, the electrode potential φ
Li
1x
CoO
2
/LiCoO
2
is larger than
φ
TiO
2
/Li
x
TiO
2
, so that the battery can discharge spontaneously
through the reduction of Li
1x
CoO
2
and oxidization of Li
x
TiO
2
.
However, for the self-charging process, because the change of
Li
+
concentration will possibly make φ
TiO
2
/Li
x
TiO
2
larger than
φ
Li
1x
CoO
2
/LiCoO
2
, the device is self-charged through the reduction
of TiO
2
and oxidization of LiCoO
2
.
By using a mechanical setup that can provide a periodic
compressive stress onto the device, we demonstrated the self-
charging process of the power cell. Figure 3a is a typical self-
charging and discharging cycle. Under the compressive force
Figure 3. Self-charging process of SCPC under periodic compressive straining and the corresponding discharging process. (a) A typical self-charging
process simply by applying cycled mechanical compressive strain to the device (green shadowed region), during which the voltage keeps raising but
the current owing through an external load connected between the cathode and anode remains almost zero, indicating that the charging process is
accomplished by the migration of the Li ions in the internal circuit rather than the ow of electrons in the external load. In the discharge process
(blue shadowed region), the stored power is released in the form of electron ow in the external load, as indicated by the measured current and drop
of voltage. (b) Self-charging and discharge cycles of SCPC under dierent forces and frequencies, respectively. Note that the force indicated in the
gure was the force applied to the entire device, most of which was consumed at the stainless shell of the cell; only a very small fraction of the force
reached the PVDF. The inset shows the operation of a commercial calculator using the SCPCs as the power source. (c) As a comparison of
eciency, the SCPC is separated into two individual units: a PVDF piezoelectric generator and a Li-ion battery by using PE as a separator. This plot
shows the voltage across the battery as being charged by the generator for 4 min under the same condition as for part a, followed by a discharge
process under the current of 1 μA. The inset is a schematic circuit of the traditional charging methods with separated generator and storage units
connected by a bridge rectier.
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applied to the SCPC at a frequency of 2.3 Hz, the voltage of the
device increased from 327 to 395 mV in 240 s. After the self-
charging process, the device was discharged back to its original
voltage of 327 mV under a discharge current of 1 μA, which
lasted for about 130 s. Thus, we proved that the proposed
power cell can be charged up under the repeated deformation
by directly converting mechanical energy to chemical energy. In
this experimental case, the stored electric capacity of the power
cell was about 0.036 μAh.
From th e theory of piezoelectricity, within the elastic
deformation regime, the magnitude of the piezopotential is
linearly proportional to the magnitude of strain, and thus also
to the magnitude of the compressive force (Figure S2,
Supporting Information). Figure 3b shows that, as we solely
increased the mechanical force applied to the device, the self-
charging eect would be enhanced. Thus, an increased
piezopotential can give an enhanced charging eect. In
addition, the self-charging eect is a lso aected by the
frequency at which the deformation is applied, as shown in
Figure 3b. Under a constant applied force of impact, a higher
frequency resulted in an increased charging voltage simply due
to higher input power. When the force and the frequency were
both kept constant, the rate at which the voltage was increased
was relatively stable. The above results are additional proof that
the self-charging process is due to the piezoelectric eect. We
have demonstrated that a series connection of several SCPCs
drove the operation of a commercial calculator for more than
10 min, as shown in the inset of Figure 3b.
The overall eciency of our proposed SCPC has two parts
the energy converting eciency of piezoelectric material and
the energy storage eciency of this mechanical-to-electro-
chemical process. We compared this with the eciency of the
traditional charging method, which is composed of a separated
generator and a storage unit connected though a bridge rectier
(inset of Figure 3c). The generator unit was fabricated by
sealing the PVDF lm in the same coin cell to create a similar
straining condition as SCPC. After being charged for 4 min via
cycled deformation of the separate PVDF generator, the voltage
of the battery has only increased by 10 mV (Figure 3c), which
is a lot lower than that of SCPC (65 mV) (Figure 3a).
Therefore, the single mechanical-to-chemical process for SCPC
is much more ecient than the mechanical-to-electric and then
electrical-to-chemical double-processes for charging a tradi-
tional battery. This is because our current study demonstrated a
new approach for directly converting mechanical energy into
chemical energy without going through the generation of
electricity as an intermediate state, which saves at least the
energy wasted on the outer circuitry, including the rectifying
component (Figure S11, Supporting Information). This is the
innovation of the power cell. Moreover, this self-charging eect
also works for SCPC with relatively higher voltage (1.5 V), as
shown in Figure S9a (Supporting Information). It is clearly
shown that the mechanical-to-ele ctrochemical process we
proposed here is e ective in a wide voltage range, which
means that the SCPC could be fully charged on the basis of the
mechanism we proposed. From the above experiments, we can
see that the SCPC we proposed here could be a powerful
device to simultaneously harvest and store mechanical energy,
after making several improvements on piezoelectric separators,
device structure, fabrication techniques, and development of
exible devices: rst, the rigid stainless steel coin cell, which
consumed a large portion of mechanical energy, is apparently
not an optimal way for the packaging of SCPC. Moreover, the
sealing provided by such a coin cell seems to be not good
enough under periodic deformation, so that the leakage of a
normal coin-cell-type battery will deteriorate when subjected to
deformation, which largely aected the performance of SCPC.
Second, the straining of piezoelectric material in such device
structure is not very eective, which could be solved by
designing exible devices in the future.
As a comparison and control experiment, we measured the
response of a conventional Li-ion battery under the same
deformation conditions, which had the same structure as the
power cell except using PE lm as the separator instead of the
piezoelectric PVDF lm. As shown in Figure 4a, the voltage
remained the same at 325 mV during the 4000 cycles of
deformation that lasted for 0.5 h. Thus, the conventional Li-
ion battery cannot be charged up at all by applying cycled
mechanical deformation. This is because there is no piezo-
electric potential that drives the migration of Li ions. This
comparison experiment rules out the possible contribution
from the electrostatic noise or the measurement system to the
charging process presented in Figure 3a for SCPC.
Alternatively, if the piezoelectric polarization as presented in
Figure 2 is reversed, the piezoelectric eld in the PVDF lm is
pointing from anode to cathode, which will drive the migration
of Li ions in the opposite direction. As a result, the reverse
electrochemical process will take place under the similar
Figure 4. Response of devices of the similar structure as self-charging power cell but with dierent lms as separators. (a) For a conventional Li-ion
battery using PE as the separator, no charging eect is observed by applying cycled mechanical deformation, indicating the result presented in Figure
3a is due to the piezoelectric driven charging process. (b) For a device that has the same structure as a SCPC but with the PVDF lm having a
piezoelectric eld pointing from anode to cathode, there is no charging eect either, just as expected from the mechanism presented in Figure 2.
Nano Letters Letter
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mechanism, so that the entire device will be slightly discharged
after many cycles of mechanical deformation. The response of
the device fabricated using PVDF lm 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 days time, which normally takes several
months. This further conrmed 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 signicantly higher overall eciency than the
traditional charging method composed of two separated units.
By replacing the PE separator as for conventional Li battery
with a piezoelectric PVDF lm, the piezoelectric potential from
the PVDF lm 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
dened as a piezo-electrochemical process. Using the
mechanism demonstrated here, we have hybridized a generator
with a battery for the rst time as a sustainable power source. It
provides an innovative approach for developing new energy
technology for driving personal electronics and self-powered
systems.
Method. TiO
2
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
4
F and 2 vol % H
2
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
2
.
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.
ASSOCIATED CONTENT
*
S
Supporting Information
Additional discussions and gures about properties of the
polarized PVDF lms, crystal structure of as-synthesized TiO
2
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://
pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: zlwang@gatech.edu.
Author Contributions
§
These authors contributed equally.
Notes
The authors declare no competing nancial interest.
ACKNOWLEDGMENTS
This research was supported by DARPA (HR0011-09-C-0142),
Airforce, U.S. Department of Energy, Oce 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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Page 7
1
Supporting Information
Hybridizing Energy Conversion and Storage in a Mechanical-to-
Electrochemical Process for Self-Charging Power Cell
Xinyu Xue
, Sihong Wang
, Wenxi Guo, Yan Zhang, Zhong Lin Wang*
A. Piezoelectric output of the polarized PVDF films
Figure S1. Study of the piezoelectric performance of PVDF film as for nanogenerator prior
to fabrication of a self-charging power cell. The piezoelectric performance of PVDF separator
film was investigated by sealing Pt-PVDF-Pt sandwich structures in a stainless-steel 2016-coin-
Page 8
2
type cell without electrolyte injection, similar to the structure of piezoelectric nanogenerator. (a)
& (b) are the voltage output profiles of PVDF separator films with two opposite polarizations
under a periodic mechanical compressive straining. The output signal is reversed by reversing the
polarity of the PVDF film (b). When the PVDF film is subjected to a compressive strain, a
piezoelectric field is created inside the film, which results in a transient flow of free electrons in
the external load to screen the piezopotential. When the strain is released, the piezopotential
diminishes and the accumulated electrons are released. The applied stress drives the electrons to
flow back and forth in the external circuit, resulting in an alternating output. The difference in the
magnitude of the output voltage is due to the difference in straining rates of the PVDF film when
be compressed and released.
Figure S2. As we solely increase the mechanical force applied to PVDF film, the
piezopotential is increased because of the increased strain. This study is important to prove
that PVDF is effective for converting mechanical energy into electricity.
Page 9
3
B. Crystal structure of as-synthesized TiO
2
nanotubes
Figure S3. XRD pattern of as-synthesized TiO
2
nanotubes to define their phase. All of the
main peaks can be indexed to anatase TiO
2
crystal (JCPDS File No. 21-1272).
C. Phase composition of PVDF film
Figure S4. FTIR spectrum of PVDF separator film. Fourier transform infrared spectroscopy
(FTIR) was used to characterize the phase of PVDF separator film. The mixture of the polar β
phase and the non-polar α phase is indexed.
Page 10
4
D. Viability of the self-charging power cell as a battery system
Figure S5. Galvanostatic charge-discharge voltage profiles to prove the viability of SCPC as
a battery system. Initial charging and discharging cycle of (a) Conventional lithium ion battery
with PE as the separator, and (b) SCPC with poled PVDF at C/10 rate (10 hours per half cycle).
SCPC possess almost the same capacity of 0.36 mAh/cm
2
as that of traditional Li-ion battery,
which confirms that the proposed structure of SCPC is fully eligible as a battery system. The only
thing needed to be noted is that the discharge plateau of SCPC is lower than that of conventional
lithium ion battery because the PVDF film is thicker and with less ionic conduction paths, thus
the internal resistance of SCPC is higher.
Page 11
5
Figure S6. Dependence of the reversible discharge capacity of a SCPC on number of
operation cycles at a C/10 rate if it is taken solely as a battery.
Page 12
6
E. The Young’s modulus of PVDF after immersing in electrolyte
It is generally known that the elastic modulus of PVDF may change after absorbing liquid. In
order to get an accurate value of the elastic modulus of the PVDF film for our calculation, we
experimentally compared the young’s modulus of the PVDF film before and after absorbed with
electrolyte. Our measurement was based on quantifying the maximum deflection at the tip of a
PVDF cantilever under a transverse deflection force of f
y
, which is given by:
xx
y
EI
lf
v
3
3
max
=
where the cantilever length is
l
and I
xx
is its moment of inertia related to the geometrical shape of
the cantilever. Therefore, the deflection is inversely proportional to the Young’s modulus. Since
the Young’s modulus of the dry PVDF film is known, which is 2 GPa, by measuring the relative
change in deflection distance, the Young’s modulus of the PVDF after absorbing the electrolyte
was determined to be 1.2±0.1 GPa. This is the value we used for quantifying the efficiency of the
SCPC.
Page 13
7
F. The origin of piezoelectricity and charge distribution in polarized PVDF films
The piezoelectricity of the PVDF originates from the spontaneous electric polarization in its
polar β phase, where the center of positive charges and center of negative charges do not
coincidence in every molecule. After the PVDF film is polarized, all of the electric dipole
moments align along the direction of the electric field during poling process. All of the dipole
moments in the same direction will add up, so that the two surface of PVDF film will have
bonded charges. For a strain free PVDF film in electric equilibrium, the bond charges are
compensated by the opposite space charges (Fig. S7a). When a compressive (or tensile) strain is
applied onto the PVDF film along the poling direction, the distance (d) between the positive
charge center and the negative charge center is changed, so that the dipole moment (p=q d) is
changed. This will result in the change of the bond charge density at the two surfaces, so that the
opposite signed space charge will no longer be in equilibrium to balance the bonded charges, thus
there will be a voltage drop across the surfaces, which is the piezo-potential referred in the text
(Fig. S7b).
The mechanism of the mechanical-to-electrochemical process we proposed in this paper
applies not only for all of the piezoelectric materials, such as PVDF and PZT and so on, but also
for other type of piezoelectric materials which do not have spontaneous electric polarization, like
wurtzite structure, such as ZnO and GaN. For simplicity and clarity, because the piezo-potential
works the same for all of the piezoelectric materials, we just present it using the very basic
definition of piezoelectricity in the mechanism as illustrated in Fig. 2.
Figure S7. The origin of electricity and charge distribution in polarized PVDF film . (a)
Strain-free state with zero macroscopic piezopotential. (b) Under compressive strain with
measurable macroscopic piezopotential.
Page 14
8
G. Intercalation and deintercalation of Li ions across the interfaces between electrolyte
and the two electrodes in the SCPC
During the self-charging process of SCPC, the Li ions will deintercalate from and intercalate
into the cathode and anode electrodes, respectively, in the same manner as in traditional Li ion
batteries, without being affected by the presence of piezopotential, which is mostly confined
inside of the PVDF film, because the positive piezo-charges at the top of the film and the negative
piezo-charges at its bottom act like a parallel plate capacitor” with little leakage of field outside
of the film, so that it will not repel the Li ions to move in an opposite direction for charging. In
addition, the intercalation/deintercalation process is the result of the shifting of the chemical
equilibriums around the two electrodes, which will not be influenced by the piezoelectric field.
The chemical equilibrium at the electrodes is likely to be broken by the change of the Li ion
concentration in the electrolyte (inside the pores of the PVDF film) around the two electrodes
under the drive of the piezoelectric field. In order to establish a new chemical equilibrium, the Li
ions will deintercalate from and intercalate into the cathode and anode, respectively. Finally, as
for the devices sealed in the coin cells, both of the electrodes have intimate contacts with the
PVDF separator, so that the electrodes will have equal potential with the PVDF in contact and the
ambient electrolyte.
Page 15
9
H. The transfer of electrons across the electrodes during the self-charging process.
The migration of Li
+
under the driving of piezopotential will break the local chemical
equilibium around the two electrodes, so that cathode will release Li
+
and electrons while the
anode will combine Li
+
with electrons, leaving positive charges. The redundancy of electrons at
the cathode and the lack of electrons at anode will give them the tendency to transfer from
cathode to anode.
There are generally two ways for the electrons to transfer from cathode to anode: either inside
the battery system in some manner, or through the leakage of the external circuitry (that is the
electrochemical workstation connected across the two electrodes of the device for the monitor of
open-circuit voltage during the self-charging process). If the electrons is mainly transferred
through the leakage current of the electrochemical workstation, when we disconnect the device
with the electrochemical workstation, which means that there will be no external circuitry, then
either of these two cases will happen: both the extra electrons at the cathode and the positive
charges at the anode will preserve at the current collectors during the electrochemical reactions,
or the self-charging process cannot move forward continuously. Out of this consideration, we
compared the self-charging behavior of SCPC with and without the parallel connection of the
electrochemical workstation. After the periodic deformation for 1 hour in both cases, we
connected the devices across the two ends of a resistor (~30kΩ) with a current amplifier in series,
to measure the discharge current. As shown in Fig. S8a for self-charging without connection to
the electrochemical workstation and Fig. S8b with the connection, the discharge behavior is
similar: at the very moment that the device was connected to the resistor, the discharge current
abruptly went to several µA, then started to decay gradually in a relaxation manner (the reason
that the initial discharge current is different is because the resistors used are different). From this
comparison, we can see that the SCPC can be self-charged independently under the repeated
deformation no matter there is an out circuitry or not, and charges are transferred from one
electrode to the other inside the device, so that the energy is successfully stored through
harvesting the mechanical energy.
Thus, we intend to suggest that the electrons move inside the battery system during the self-
charging process. Although theoretically the medium—electrolyte and PVDF separator—between
the two electrodes are electronic insulators, there still exists some leakage of electrons insides the
battery, which is the reason for the self-discharge phenomena universally presented in all the
batteries
31, 32
. We can name a few possible leakage mechanisms according to previous
investigation on the self-discharge, such as the resistive leakage of electrons due to the actual
Page 16
10
finite resistance of electrolyte, and some side redox reactions with the participation of electrolyte,
which can transfer the electrons indirectly from one electrode to another. Upon the subjection of
periodic piezoelectric field, these approaches for electron leakage could be possibly enhanced.
Although so far it is still difficult to nail down to certain exact electronic conducting mechanisms,
it is very likely that the charges are transferred and stored through the electrochemical reactions
under the effect of piezoelectric field.
Figure S8. The discharge current of SCPC after self-charging in two conditions through
cycled deformation: (a) without and (b) with the connection to an electrochemical
workstation (symbolized by a volt meter in the figure).
Page 17
11
I. Self-charging effect of SCPC in relatively high voltage range.
In Fig.3 and Fig.4, the self-charging behavior of SCPC under the effect of piezoelectric field
are demonstrated from the as-fabricated initial voltage of the battery structure without any
previous electrical charging or discharging, which is in the low voltage regime. In order to
validate the mechanism we proposed in wider voltage range, and to show the potential of fully
charging the SCPC under the mechanism, we performed similar experiments in realtive higher
voltage range on the SCPC, the similar structure with PVDF separator in opposite poling
direction, and the one with unpolarized PVDF separator. We firstly charged the battery-type
storage units to higher voltage. After the charging process was over and enough time for the
voltage to stablize, we started applying the periodic deformation (the same magnitude and
frequency with Fig. 3a) to examine the change of the open-circuit voltage.
As shown in Fig. S9a, the SCPC still can be self-charged starting from 1.445V with the
voltage increased by 55mV in 7000s. Although the self-charing process is much slower at higher
voltage for a number of reasons, the mechamism still works. Thus, through the future
improvement of device structure and fabrication techniques, the SCPC could be fully charged
based on the as-proposed mechanical-to-electrochemical process.
The device with the PVDF film in the opposite poling direction is firstly fully charged. Upon
periodic deformation, the voltage decreased from 2.1V to 0.4V in 1 day’s time, passing by the
discharge plateau (Fig. S9b), which normally takes months of time for ordinary battery. Thus, we
can see that the self-discharging rate is largely increased by the piezopotential in the opposite
direction. Moreover, this fully discharge process confirms that the mechanical-to-electrochemical
process we proposed can continuously drive the progress of electrochemical reactions to
accomplish the entire charging or discharging process.
Though the similar experiment on the device with unpolarized PVDF film as the separator
(Fig. S9c), which is just an conventional battery, we can find that the voltage will slightly
decrease in the higher voltage range (from 1.46V) when the battery is subjected to periodic
deformation. Thus, with the current device structure and packaging technique using the coin cell,
the battery will suffer a more severe leakage problem because of deformation, which hinders the
higher efficiency and stability of the current self-charging power cell.
Page 18
12
Figure S9. Reponse of devices to perodic deformation at relative high voltage range. (a) Self-
charging behavior of SCPC starting from 1.445V. (b) Accelerated self-discharging for the device
of the similar structure but with the PVDF film of the opposite poling direction. (c) Response of a
conventional Li-ion battery with unpolarized PVDF film as separator.
Page 19
13
J. The approximate determination of the self-charging current
When connecting the SCPC with a resistor parallely, it will start to discharge through the
resistor, and the discharge current will slow decay, because the open circuit voltage gradually
decreases with the discharging process. As the device discharged to only a small capacity left and
a very small discharging current (~60nA), we began to apply the cycled deformation onto the
device, which will start the self-charging process, along with the discharge (Fig. S10). As we can
find from Fig. S10, the discharge current started to increase, because the energy storage rate is
higher can energy dissipation rate. The discharge current will gradully saturate when these two
rates got equal, where the discharge current became appoximately the same with the self-charging
current. Thus, through Fig. S10 we can find that the self-charging current is appoximately 120nA.
Figure S10. The discharge current when SCPC was connected to a resistor. In the green
shadow region, the cycled deformation was applied onto the device when the discharge process
went on.
Page 20
14
K. In the traditional charging methodology, the voltage output of the generator unit
before and after the rectificatioin.
Figure S11. Output of the generator unit and rectification in traditional charging method.
(a) Open circuit voltage of the generator unit fabricated by sealing a PVDF film in a stainless
coin cell so that it will be subjected to the same straining condition as the one in the self-charging
power cell. (b) Open circuit voltage of the generator unit after rectification, which shows that the
positive signals are preserved with the amplitude a little bit smaller, while the negative ones are
almost cut-off. So we can tell that a substation portion of energy is lost after rectification.
Page 21
15
31 Blyr, A., Sigala, C., Amatucci, G., Guyamard, D., Chabre, Y., & Tarascon, J-M. Self-discharge of
LiMn
2
O
4
/C Li-ion cells in their discharged state. J. Electrochem. Soc. 145, 194-209 (1998).
32 Sloop, S. E., Kerr, J. B., Kinoshita, K. The role of Li-ion battery electrolyte reactivity in
performance decline and self-discharge. J. Power Sources 119, 330-337 (2003)
Page 22
  • Source
    • "47 and 48 at an ambient temperature of 4 °C.Figure 1 shows that the battery capacity does not always decrease monotonically, but instead experience a sudden increase during the cycle. As an example, Figure1a shows the capacity of different batteries increasing rapidly during the 90th cycle, resulting from self-charging [31] during the rest period. The explanation for this effect is that some chemical reactions occur during battery use, and some chemical products appear near the two electrodes, which retard the internal chemical reactions. "
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    Preview · Article · Oct 2014 · Energies
  • [Show abstract] [Hide abstract] ABSTRACT: Photoelectrochemical (PEC) processes are fundamental for photon water splitting and energy storages. The key to the PEC efficiency is dictated by the charge generation and separation processes. In this chapter, we present the piezoelectric on PEC, in which a consistent enhancement or reduction of photocurrent was observed when tensile or compressive strains were applied to the ZnO anode, respectively. The photocurrent variation is attributed to a change in barrier height at the ZnO/electrolyte interface. We also introduce a fundamental mechanism that directly hybridizes the two processes into one, using which the mechanical energy is directly converted and simultaneously stored as chemical energy without going through the intermediate step of first converting into electricity. By replacing the polyethylene (PE) separator as for conventional Li battery with a piezoelectric poly(vinylidene fluoride) (PVDF) film, the piezoelectric potential from the PVDF film as created by mechanical straining acts as a charge pump to drive Li ions to migrate from cathode to the anode accompanying with charging reactions at electrodes. This new approach can be applied to fabricating a self-charging power cell (SCPC) for sustainable driving micro/nano-systems and personal electronics.
    No preview · Chapter · Jan 2012
  • [Show abstract] [Hide abstract] ABSTRACT: We demonstrate a pyroelectric nanogenerator (PENG) based on a lead zirconate titanate (PZT) film, which has a pyroelectric coefficient of about -80 nC/cm(2)K. For a temperature change of 45 K at a rate of 0.2 K/s, the output open-circuit voltage and short-circuit current density of the PENG reached 22 V and 171 nA/cm(2), respectively, corresponding to a maximum power density of 0.215 mW/cm(3). A detailed theory was developed for understanding the high output voltage of PENG. A single electrical output pulse can directly drive a liquid crystal display (LCD) for longer than 60 s. A Li-ion battery was charged by the PENG at different working frequencies, which was used to drive a green light-emitting diode (LED). The demonstrated PENG shows potential applications in wireless sensors.
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