Nanoscale Triboelectric-Effect-Enabled Energy Conversion for
Sustainably Powering Portable Electronics
Sihong Wang,
†,§
Long Lin,
†,§
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: Harvesting energy from our living environment is an
effective approach for sustainable, maintenance-free, and green power
source for wireless, portable, or implanted electronics. Mechanical
energy scavenging based on triboelectric effect has been proven to be
simple, cost-effective, and robust. However, its output is still
insufficient for sustainably driving electronic devices/systems. Here,
we demonstrated a rationally designed arch-shaped triboelectric
nanogenerator (TENG) by utilizing the contact electrification
between a polymer thin film and a metal thin foil. The working
mechanism of the TENG was studied by finite element simulation.
The output voltage, current density, and energy volume density
reached 230 V, 15.5 μA/cm
2
, and 128 mW/cm
3
, respectively, and an
energy conversion efficienc y as high as 10− 39% has been
demonstrated. The TENG was systematically studied and demonstrated as a sustainable power source that can not only
drive instantaneous operation of light-emitting diodes (LEDs) but also charge a lithium ion battery as a regulated power module
for powering a wireless sensor system and a commercial cell phone, which is the first demonstration of the nanogenerator for
driving personal mobile electronics, opening the chapter of impacting general people’s life by nanogenerators.
KEYWORDS: Energy harvesting, triboelectric nanogenerator, self-powered system, lithium ion battery
A
rapid expansion of electronic devices
1−4
toward wireless,
portability, and multifunction desperately needs the
development of independent and maintenance-free power
sources.
5−7
The emerging technologies for mechanical energy
harvesting
8−10
are effective and promising approaches for
building self-powered systems because of a great abundance of
mechanical energy existing in our living environment and
human body. Since 2006, piezoelectric nanogenerators
(PNGs)
11−14
have been developed to efficiently convert tiny-
scale mechanical energy into electricity. Recently, another
creative invention is the cost-effective and robust triboelectric
nanogenerators (TENGs)
15−17
based on the universally known
contact electrification effect.
18,19
TENG harvests mechanical
energy through a periodic contact and separation of two
polymer plates. However, in order to realize sustainable driving
of electronic devices/systems, the output of TENG must be
significantly improved through a rational design.
The two different types of nanogenerators presented above
have a similar underlying physical process
12,17
for producing
electricity: generation of immobile charges (ionic charges for
PNG or electrostatic charges on insulators for TENG), and a
periodic separation and contact of the oppositely charged
surfaces to change the induced potential across the electrodes,
which will drive the flow of free electrons through an external
load. The electrical output and efficiency are radically
determined by the effectiveness of the above two processes.
As for the charge generation in TENG, maximizing the
generation of electrostatic charges on opposite sides is critical,
which can be achieved by selecting the materials with the
largest difference in the ability to attract electrons
20
and the
modification of surface morphology.
16,17
In another aspect, the
effective periodic switching between separation and intimate
contact of the two charged plates is vitally important to
determine the electrostatic potential across the two electrodes,
which is the driving force for the free electrons. For the plate-
structured TENG, it is not easy to achieve both complete
contact and separation of the two oppositely charged plates
upon pressing and releasing, especially under the electrostatic
attraction between them. A firmly attached contact of the two
oppositely charged surfaces is unfavorable for electricity
generation. If we simply use a spacer between the two plates,
17
it will then hinder the complete contact of the two plates. In
this paper, we completely solved this problem through an arch-
shaped TENG with a steady gap between the plates at strain-
free conditions. This was achieved through ingeniously
introducing surface thermal stress during thin film deposi-
tion.
21,22
On the basis of this unique structure, the voltage and
current reached 230 V and 0.13 mA with an instantaneous
Received: September 25, 2012
Revised: November 1, 2012
Letter
pubs.acs.org/NanoLett
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maximum power density of 3.56 mW/cm
2
and 128 mW/cm
3
(note the latter was calculated using the entire volume of the
device here and afterward unless specified). The power of a
single device is high enough to continuously drive a LED. With
the first realization of charging a lithium ion battery (LIB) to
full capacity, the TENGs were combined with LIBs to form
power modules for driving a wireless sensor system and a
commercial cell phone. This is an unprecedented progress in
rational optimization of the TENG design and its first
application for self-powered systems, unambiguously demon-
strating its feasibility for powering portable electronics, sensors
for health care, environmental and infrastructure monitoring,
and security.
The arch-shaped TENG is based on the contact
electrification between patterned polydimethylsiloxane
(PDMS)
16
as the top plate and patterned Al foil as the bottom
plate (Figure 1a). According to the triboelectric series
20,23
(Figure S1 in Supporting Information), the purposely chosen
PDMS and Al are almost at the two ends with very large
differences in ability to attract and retain electrons. The unique
arch-shaped structure of the TENG from the naturally bent top
plate, which helps to carry out the action of effective charge
separation and contact using the elasticity of the film, is
achieved by the following innovative fabrication process (Figure
1b,c): The top part starts from a piece of flat Kapton film
(Figure 1b <i>). A layer of 500 nm SiO
2
film is deposited using
plasma-enhanced chemical vapor deposition (PECVD) at 250
°C (Figure 1b <ii>). Upon cooling to room temperature, the
Kapton will shrink to a much larger extent than the SiO
2
film
because of the large difference in thermal expansion
coefficients, so that thermal stress across the interface will
make the plate bent naturally toward the SiO
2
side (the
curvature is calculated in supplementary discussion S1). Then,
the prefabricated PDMS film with pyramid patterns (Figure S2)
is glued to the inner surface through a thin PDMS bonding
layer (Figure 1b <iii>). Finally, the electrode is deposited on
top (Figure 1b <iv>). As for the bottom plate, an aluminum foil
(Figure 1c <i>) is patterned with a typical photolithography
process: defining the photoresist to the array of square windows
(Figure 1c <ii>); depositing a layer of aluminum on top (Figure
1c <iii>); and finally lift-off, leaving the patterned Al cubes on
the foil (Figure 1c <iv>). At last, the two as-fabricated plates of
the same size are attached face-to-face and sealed at the two
ends. The soft Al plate will be forced to bend outward under
the contraction from the other plate, so that a gap will form
naturally between. The patterned surfaces of PDMS fi lm
(Figure 1d) and Al foil (Figure 1e) are fabricated to enhance
the triboelectric charging and are characterized using scanning
electron microscopy (SEM). Both arrays are uniform and
regular across a very large area.
The operation of the arch-shaped TENG is realized by
applying a cycled compressive force onto the whole area of the
Figure 1. Structure and fabrication process of the arch-shaped triboelectric nanogenerator (TENG). (a) Schematic diagram showing the structural
design of the arch-shaped TENG; the inset is the photograph of a typical arch-shaped TENG device. (b,c) Fabrication flowchart for (b) the top plate
and (c) bottom plate of the TENG. (d,e) Top view SEM images of (d) the PDMS surface with pyramid patterns and (e) Al surface with cubic
patterns; the insets are high-magnification images at a tilted angle.
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device, so that the bending plates will be periodically pressed to
flatten and contact closely with each other. Once released, the
two plates will separate apart due to the stored elastic energy
and revert instantaneously back to their original arch shape due
to resilience. The working principle of the polymer−metal
TENG is schematically depicted in Figure 2 using the
numerically simulated electrostatic potential distribution arising
from triboelectric charges (using COMSOL package). These
semiquantitative results show that a cycled generation of the
potential difference between the two electrodes drives the flow
of electrons through an external load. At the original state
before the contact of the triboelectric films (Figure 2a), there is
no charge transferred, thus no electric potential. Upon the
pressing of the two films toward each other, they will be
brought into full surface contact and possibly relative sliding
would occur, which results in electron transfer from a material
in the positive side of the triboelectric series to the one in the
negative side in the series (Figure S1). Accordingly, electrons
will be injected from Al to the PDMS surface, leaving positive
charges on the Al foil. Previous theoretical study on
triboelectricity reveals that such a charge transfer process will
continue in the first few hundreds of cycles until the
accumulated charges reach a saturation and equilibrium; the
negative charges will be preserved on the PDMS surface due to
the nature of the insulator.
24
However, the positive triboelectric
charges on the conductive Al foil would attract the electrons in
the opposite electrode to flow through an external load, which
is the observed current in this case. This process is different
from the mechanism proposed for the polymer−polymer based
TENG. After cycles of deformation, when the device is pressed
and the surfaces with charges are in close contact with each
other, all of the triboelectric charges will stay on the inner
surfaces with the same surface density (σ
0
) (Figure 2b). Thus,
these charges with opposite signs will be virtually in the same
plane, and there will be little potential difference across the two
layers due to the negligible charge separation. Once the
pressing force is released, the TENG will immediately rebound
back to its original arch shape due to the elasticity of the film so
that a gap will form again between the two plates. From the
numerical simulation result, if the charge transfer has not
happened at the moment, the electric field generated by the
separated surface charges will give rise to a much higher
potential on the Al foil side than the top electrode (TE)
(Figure 2c). Such a potential difference will drive the flow of
positive charges from Al foil to TE through the external load
until the potential difference is fully offset by the transferred
charges, rendering the TE with a surface charge density of Δσ,
while the Al is left with σ
0
− Δσ (Figure 2d). Subsequently,
when the TENG is pressed again to reach the close contact of
the two plates, these redistributed charges will inversely build a
positive potential on TE (Figure 2e), which will drive all of the
transferred charges (Δσ)toflow back to the inner surface of
Figure 2. Working principle of the polymer−metal based TENG. (a) Two-dimensional sketch showing the initial state of the TENG before any
deformation. (b−e) Finite element simulation of the periodic potential change between the two electrodes upon cyclic deformation, showing the
driving force for the back-and-forth charge flow generated by the TENG. A cycle is generally divided into four states: (b) device under pressing, (c)
deformation released, (d) charges transferred, and (e) device gets pressed again and final charge transferred again to cycle back to (b). (f)
Relationship of the ratio between transferred charge density (Δσ) and triboelectric charge density (σ
0
), with the planar separation distance, which
shows the importance of the effective charge/plate separation. The inset is the scale of the voltage for the simulation results presented in (b−e).
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the Al foil. Then a cycle is achieved, and the device will go back
to the equilibrium state depicted in Figure 2b. This is a full
cycle of electricity generation.
From the above simulation result, we can easily tell that both
the voltage and current outputs are related to the amount of
charges transferred (AΔσ, where A is surface area of the plate),
which is determined by the triboelectric charge density (σ
0
) and
the separation distance of the two plates. As σ
0
is regarded as a
constant, we did an analytical cal culation based on the
simplified model of quasi-infinite flat plates to give an idea
on the magnitude of the distance required for the optimum
output (supplementary discussion S2). The relationship of the
ratio between Δσ and σ
0
, with the planar separation distance, is
shown in Figure 2f. We can see that, when the separation
distance starts to increase from 0 to 0.7 mm, Δσ keeps a very
rapid increase from 0 to ∼90% of σ
0
. Then, the slope of this
curve starts to decrease. Thus, it can be concluded that both an
intimate contact and a subsequent separation of nearly 1 mm
are required for the phenomenal transferring of charges. This is
just the unique innovation and advantage of the purposely
designed arch-shaped structure, which introduces sufficient
resilience for separating the plates but without sacrificing the
intimate contact and electrification.
Under the above-described periodic deformation scenario,
the electric output measurement was performed on an arch-
shaped TENG device at a size of 3 cm × 2.8 cm, with the
triggering frequency of 6 Hz and controlled amplitude. Since
the accumulation of the triboelectric charges increases and
reaches equilibrium in a certain period of time after multiple
cycles, the output will gradually go up in the first stage upon
deformation (Figure S3). Then, the open-circuit voltage (V
OC
)
will stabilize at 230 V (Figure 3a), measured by an electrometer
with infinite input resistance. From the inset of Figure 3a, when
the bottom Al is connected to the positive probe of the
electrometer, upon the release of the pressing force, a positive
voltage is generated because of the immediate c harge
separation. Since in an open-circuit condition the electrons
cannot flow to screen the induced potential difference between
the two electrodes, the voltage will hold at a plateau until the
subsequent pressing deformation in the second half cycle. As
shown in Figure 3b, the peak value of the short-circuit current
(I
SC
) reaches 94 μ A, corresponding to the half cycle of pressing
that is at a higher straining rate than releasing. The integration
Figure 3. Characterizing the performance of the arch-shaped TENG. (a) Open-circuit voltage and (b) short-circuit current of the TENG under the
deformation frequency of 6 Hz. These are studies under both the forward connection (Al connected to the positive probe) shown at the left-hand
side and reverse connection shown at the right-hand side. The insets are the magnified output curve in one cycle and the sketch of the corresponding
connection polarity. (c,d) Influence of the deformation frequency on (c) the open-circuit voltage and (d) the short-circuit current. The inset in (d) is
the integration of a single current peak from each frequency, which gives the total charges transferred in a half cycle, showing that the total contact
charges generated are almost independent of the deformation frequency.
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of each current peak gives the total charges transferred in a half
cycle of deformation. Moreover, when the connection polarity
to the electrometer is switched, both the voltage and current
signal will be completely reversed. The gap from the arch-
shaped structure is a key factor for the enhanced output
because, without the SiO
2
-film-introduced bending, there will
be much smaller and unstable electrical output (Figure S4).
Besides the triboelectric charge density and the separation
distance of the two plates, another factor that could influence
the output of the TENG is the deformation frequency, which
affects the straining rate. Since the mechanical energy from the
environment is always irregular and varies in frequencies, it is
necessary to study the dependence of TENG’s output on the
frequency. Thus, we tested the TENG device in a series of nine
different frequencies, from 2 to 10 Hz, with the amplitude of
the triggering motor remaining constant. As shown in Figure
3c, V
OC
almost remains the same at different frequencies. The
probable reason is that, at the open-circuit condition, it does
not involve the dynamic process of charge transfer. The voltage
is only determined by the triboelectric charge density and the
plate separation at any given time. But for the short-circuit
current, it presents a very clear increasing trend with the
increase of frequency, from 35 μA at 2 Hz to 130 μAat10Hz
(Figure 3d), because the deformation rate increases with
deformation frequency, which leads to a higher flow rate of
charges, that is, higher current (Figure S5), but the total
amount of charges transferred is constant at given triboelectric
condition and separation distance. This is confirmed by the
integration of every single current peak from each of the nine
different frequencies (inset of Figure 3d). Therefore, the
instantaneous power output increases with the increase of
frequency, so that it will be more eligible to drive electronic
devices with larger power consumption. From the above results,
when the frequency reaches 10 Hz, the instantaneous power
output reaches up to 3.56 mW/cm
2
and 128 mW/cm
3
.
The goal of the development of nanogenerators is to drive
electronic devices by harvesting small-scale mechanical energy,
so that the self-powered system can be realized. As for the
energy conversion devices such as TENGs, there are generally
two methodologies for them to be used as a power source. The
first choice is to power devices directly using the pulses
generated by TENGs, such as a chemical sensor
25
and a LCD.
26
In practice, the output power to the load depends on the match
between the load and the power source. When the working
TENG (under a frequency of 6 Hz) is directly connected to the
loads of different resistances, we can find that the current
through the load will generally decrease from I
SC
when the
resistance increases from 0 (Figure 4a), but the voltage across
Figure 4. Arch-shaped TENG as a direct power source to drive electronic devices. (a,b) When the TENG driving a load without rectification, the
dependence of (a) the output voltage, current, and (b) instantaneous power on the load resistance. (c,d) When the TENG driving a load with
rectification, the dependence of (c) the output voltage, current, and (d) instantaneous power on the resistance of the load. All of the dots are
measured values, and the solid lines are fitted curves. (e) Current through a LED driven by a rectified TENG under 8 Hz. The inset is the
characteristic I−V curve of the LED. (f) Snapshots of the TENG-driven flashing LED, corresponding to the magni fied current peaks.
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