Nano Res.
Electronic Supplementary Material
Dipole-moment-induced effect on contact electrification
for triboelectric nanogenerators
Peng Bai1,2,§, Guang Zhu1,§, Yu Sheng Zhou1,§, Sihong Wang1, Jusheng Ma2, Gong Zhang2, and Zhong Lin
Wang1,3 ()
1
School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245, USA
Department of Mechanical Engineering, Tsinghua University, Beijing 100084, China
3
Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing, China
§
Authors with equal contribution and order of authors determined by coin toss.
2
Supporting information to DOI 10.1007/s12274-014-0461-8
1
Illustration of the force-dependence contact area
The output power of TENGs is mainly determined by the density of triboelectric surface charges which can be
affected by the choice of materials and effective contact area. The main purpose of the nanopore-based
modification on the aluminum foil is to increase the effective contact area when the PVDF film and aluminum
are in contact. Similar surface modification methods have been applied by previous workers (Refs. [17−19, 24]
in the main paper), in which the influence of nanowires, patterns and nanoparticles on the performance of
TENGs have been investigated. In our work, we utilize nanopores to enhance the TENG’s performance. As
demonstrated in Fig. S1, PVDF can deform and fill the vacant nanopores under a periodic compressive force,
thus leading to enhanced contact area. However, as we mentioned in the manuscript, the dependence of the
electrical output on contact force has a linear behavior under small contact force (15 to 63 N in the manuscript).
In such a small range, only a fraction of nanopores can be partially filled by PVDF, thus the effective contact
area does not reach its maximum value. To investigate the influence of nanopores on the performance of
TENGs, the open-circuit voltage (Voc) of TENGs fabricated using nonpolarized PVDF thin films and aluminum
foil without nanopores was measured under the same conditions as mentioned in the paper. A Voc around 60 V
can be achieved, corresponding to an approximate 28% decrease when compared with TENGs fabricated using
nonpolarized PVDF thin films and aluminum foil with uniformly distributed nanopores. Therefore, we
conclude that the nanopore structure on aluminum foil can enhance the effective contact area, and about 16%
of the nanopores can be fully filled by the PVDF film on average, leading to a significantly enhanced surface
charge density and thus a higher electrical output.
Address correspondence to [email protected]
Nano Res.
Figure S1 (a) Original position and (b) contact position with small contact force. (c) Larger force leads to larger contact area. (d) All
vacant nanopores are filled.
2
Measurement of the contact force
The force applied by an electric shaker at a frequency of 4 Hz was measured by a force plate as shown in Fig. S2.
Figure S2 The measured contact force is around 50 N.
3
Properties and structure of β-PVDF
According to the triboelectric series—that is, a list of materials based on their tendency to gain or lose charges—
aluminum tends to lose electrons while contact with polymers. It is also known that fluorine, which is present
in PVDF, has a large electronegativity and thus it can be expected that PVDF has a larger ability to gain electrons
compared with other macromoleclar polymer materials which can be polarized, such as polymethyl methacrylate
(PMMA). Therefore, the larger difference between aluminum and PVDF in triboelectric polarities can significantly
improve the output power of TENGs. In addition, PVDF has better hydrophobicity compared with other
macromolecular polymer materials, which can make triboelectric charges remain on its surface much longer
during the periodic contact process. Besides, PVDF has good chemical resistance, abrasion performance, and
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flexibility which offers a possibility for applications on curved surfaces, low cost and high mechanical strength,
which make it a good choice for TENGs.
Each PVDF chain has a coupling of positive and negative charges referred to as a dipole. The negatively
charged fluorine atoms are coupled with the positively charged hydrogen atoms (or protons). The dipoles are
rigidly attached to the carbon backbone and their orientation depends on the polymer crystal structure. As
sketched in Fig. S3, the β phase of PVDF forms a planar zigzag structure. This zigzag structure creates a very
organized crystal, and also allows higher packing density. In addition, the β phase has a highly polar
arrangement of the hydrogen and fluorine atoms. The charged hydrogen and fluorine atoms are aligned in the
β phase, which produces a net polarization of the unit cell. The dipoles along the carbon backbone align
themselves to maximize spontaneous polarization within the unit cell. The bond charges of PVDF which we
believe cause the effect discussed in this paper (shown in Fig. S3) come from the spontaneous polarization.
Figure S3 The zigzag structure of β-PVDF.
4
Dipole moments in the β phase of PVDF before and after polarization
The strong dipole formed by the zigzag structure in the crystalline unit cell creates a net charge in the β phase
of PVDF. It has been shown by different researchers (as listed in Refs. [33–36] in the main paper) that the net
charge is zero when β forms naturally because of the random arrangement of dipoles (Fig. S4(a)). As shown by
simplified models in Fig. S4(b), when a large electrical potential is applied across the materials during the
polarization, the dipoles align and produce a net positive charge. This net charge allows the polymer to respond
to electrical fields. Although the mechanisms for poling are not well understood, crystals within a polymer are
influenced with an electric field to create a net polarization. This net polarization of PVDF aligns the individual
crystals within the polymer causing them to collectively respond to changes in their surrounding electric field,
Figure S4 (a) Before the poling process, the center of positive charges and center of negative charges do no coincide in every molecule. (b)
After the poling process, all the dipole moments in the same direction will add up, resulting in bond charges on the surfaces.
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and thus resulting in bond charges. During polarization, the orientation of unit cells can be changed when a
sufficient electric field is applied, as shown in Fig. S4(b). For the forward-polarized PVDF, the bond charges are
positive while the ones on the surface of reverse-polarized PVDF are negative, as has been indicated in
Fig. S4(b) respectively.
5
Definition of equivalent direct current
Equivalent direct current density (equivalent Jsc) is defined as the average quantity of positive charges flowing
through the circuit per second after being rectified. It is can be calculated by
Jsc = Q/(t·S)
where Q is the accumulative charge generated per cycle as we discussed in the electricity generation process, t
is the time needed per cycle and S is the total projected area of contact surfaces. A full-wave bridge was
introduced to measure the accumulative charge, which will transform AC current signal to direct current. The
accumulative charge was measured by a programmable electrometer (Keithley model 6514).
6
Enlarged view of Jsc
Figure S5 Enlarged view of Jsc measured from TENGs fabricated using different types of PVDF films.
7
Detailed description of the electricity generation process for a TENG
For the piezoelectric performance of forward-polarized PVDF film, when a compressive strain is applied along
the poling direction, the distance between the positive charge center and negative center in each molecule is
changed, so that the bond charge density changes, resulting in a potential difference across the film. In the
demonstrated structure of forward-polarized TENGs, this potential difference will drive electrons to flow from
aluminum foil to the back electrode, which corresponds to a current flow in the direction that is opposite to the
current generated from triboelectric charges under the same condition as shown in Fig. S6(f). Likewise, when
the compressive strain is withdrawn, electrons will flow from the back electrode back to the aluminum foil due
to the change of potential difference across the forward-polarized PVDF thin film. Therefore, we can state that
the polarity of the piezoelectric output is opposite to that of the triboelectric output, and they cannot be added up.
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Figure S6 (a) Original position with a separation between the substrates. (b) External impact brings the PVDF thin film and the aluminum
foil into contact, generating positive triboelectric charges on the aluminum side and negative charges on the PVDF side. (c) When the
compressive force is withdrawn, the triboelectric charge still remains on the contacting surface. (d) Withdrawal of the force causes a
separation. Potential difference drives electrons from the back electrode to the contact electrode, corresponding to an instantaneous current
in the external circuit. (e) The two substrates revert to their original positions and positive triboelectric charges are completely screened.
(f) When a periodic compressive fore is applied again, a reverse potential will be established to drive the induced electrons flow back to
the back electrode, resulting in another instantaneous current in the reverse direction.
8 Electricity generation process by the piezoelectric effect with polarized PVDF thin
films in an open-circuit condition
For the forward-polarized PVDF thin films, a layer of copper was deposited on the back of PVDF thin films as
the back electrode while a layer of aluminum was deposited as the other electrode for measuring the
piezoelectric output under the same contact force as the TENGs. In such a case, when a compressive force is
applied onto the top of the aluminum electrode, a potential difference will be developed with the negative
polarity at the aluminum side and the positive one at the copper electrode side, resulting in a negative voltage
signal. For the reverse-polarized PVDF thin films, the copper and aluminum electrodes were deposited on the
opposite sides of the polarized PVDF thin films compared with the forward-polarized ones, and will produce a
positive voltage signal under a compressive force.
Figure S7 Electricity-generation process by the piezoelectric effect using (a) forward-polarized and (b) reverse-polarized PVDF thin
films in an open-circuit.
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9
Enlarged view of electric signals from the triboelectric output and the piezoelectric output
The current signal of the TENG is generated when the two contacting materials start to separate from each
other, while the piezoelectric signal is generated when the pressure is withdrawn. Since the separation process
of two materials takes more time, the current signal of the TENG (0.046 s) lasts longer than the piezoelectric one
(0.030 s).
Figure S8 (a) Enlarged view of Jsc measured from the forward-polarized TENG and (b) the piezoelectric output of polarized PVDF
thin film.
10 Equivalent Jsc from different types of TENGs under different contact force
In Fig. 4(a) (in the main manuscript), we investigated the dependence of the electrical output on the contact
force for all three types of TENGs. It was found that the ratio between the forward-polarized TENGs and
nonpolarized TENGs remains the same for different contact forced. Such a phenomenon can be also found for
nonpolarized TENGs and reverse-polarized TENGs.
Table S1
Equivalent Jsc from different types of TENGs under different contact force
Equivalent Jsc (μA/cm2 )
Contact force (N)
11
Ratio
Forward-polarized
Nonpolarized
Reverse-polarized
Forward-polarized/
non-polarized
Non-polarized/
reverse-polarized
15.26
1.54
0.91
0.65
1.69
1.40
33.55
2.90
1.40
0.78
2.07
1.79
52.05
4.51
2.27
1.31
1.99
1.73
62.92
5.91
3.14
1.69
1.88
1.86
Technical datasheet for the polarized PVDF film
Table S2 Technical datasheet for the polarized PVDF film used in this work
PVDF piezoelectric film with 25 μm (±5%) thickness Technical datasheet
Piezo/pyroelectric properties (at 23 °C)
d33 (pC/N)
15 ± 20%
d31 (pC/N)
6 ± 20%
d32 (pC/N)
1 ± 20%
g33 (V·m/N) at 1kHz
0.14 ± 20%
2
p3 (μC/m K)
–25 ± 25%
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(Continued)
PVDF piezoelectric film with 25 μm (±5%) thickness Technical datasheet
Dielectric properties (at 23 °C)
εr
– at 0.1 kHz
11.5 ± 10%
– at 1 kHz
11.5 ± 10%
– at 10 kHz
11 ± 10%
tan δ
– at 0.1 kHz
0.010 ± 10%
– at1 kHz
0.015 ± 10%
– at 10 kHz
0.035 ± 10%
DC breakdown voltage
670 ± 30%
Mechanical properties (at 23 °C)
Young’s modulus (MPa)
– machine direction
3200 ± 20%
– transverse direction
3200 ± 20%
Tensile strength at break (MPa)
– machine direction
240 ± 15%
– transverse direction
60 ± 15%
Elongation at break (%)
– machine direction
20 ± 30%
– transverse direction
5 ± 30%
Thermal properties (at 23
Melting point (
°C)
Transverse direction
°C)
175 ± 5%
90–100
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