Massively parallel aligned nanofibers-based
nanogenerator deposited via in situ,
oriented poled near-field electrospinning
Yiin-Kuen Fuha,*, Shao-Yu Chena, Jia-Cheng Yeb
a
Department of Mechanical Engineering, National Central University,
Jhongli City, Taoyuan County, Taiwan, R.O.C
b Graduate Institute of Energy Engineering, National Central University,
Jhongli City, Taoyuan County, Taiwan, R.O.C
MATERIALS AND METHODS
1. Polymer solution
The polymer solutions were made from polyvinylidene fluoride (PVDF) powders
(Mw = 534 000, Sigma-Aldrich, USA), Dimethylformamide (DMF) (Tedia, USA),
acetone (Tedia, USA), and Capstone® FS-66 (Sigma-Aldrich, USA). Prior to
generation of the solutions, all reagents were not further purified or modified. It is
noted that Capstone® FS-66 is an anionic phosphate fluorosurfactant that was
provided without solvents.
2. PVDF solution preparation
The PVDF solutions with DMF/acetone solvents were prepared using the following
steps:
a. 1.6 g PVDF powder was dispersed in 4 g acetone for 30 minutes using a magnetic
stirrer.
b. The complete PVDF solution was prepared by mixing 6 g DMF with the
PVDF/acetone solution. Simultaneously, 0.4 g Capstone® FS-66 was added to the
mixture and stirred for a minimum of one hour to reach sufficient homogeneity.
c. This surfactant is magnetically stirred for 60 minutes to maintain a minimal
foaming.
All solutions were prepared and stored at room temperature in a well-ventilated
laboratory environment. All containers were sealed with Parafilm to minimize
evaporation. Specific content of the solutions is listed in Table S1 below. The
concentration of all substances is calculated in weight percentage (wt %) of the
solvent.
Table S1. Experimental data of PVDF solution.
PVDF concentration
Solvent ratio
(DMF : acetone)
Fluorosurfactant
concentration
16 wt %
6:4
3 wt %
3. Preparation of direct-write PVDF nanofibers on plastic substrate
The in-situ poled PVDF nanogenerator has dimensions of 6 cm × 2cm ×1.5mm
(length × width × thickness) that suspended multiple fibers across eleven copper
contact electrodes made of 55 μm-thick copper foils, was applied on the top surface of
the plastic polyvinyl chloride (PVC) substrate for piezoelectricity measurements.
NFES IN-SITU POLING PROCESS AND CHARACTERIZATION
Figure S1. Near-field electrospinning (NFES) is capable of combining direct-write,
mechanical stretching, and in situ electrical poling to precisely deposit piezoelectric
nanogenerators onto a flexible or rigid substrate such as plastic or silicon wafers
(redrawn after [13])
The in situ mechanical stretching and electrical poling during direct-write technique
by means of near-field electrospinning (NFES) can be schematically illustrated in
Figure S1 such that the strong electric fields (greater than 106 V/m) and stretching
forces from the electrospinning process naturally align dipoles in the nanofiber crystal.
It is experimentally validated by FTIR that the nonpolar α-phase (random orientation
of dipoles) is transformed into polar β-phase, resulting the aligned polarity of the
electrospun nanofiber.
Figure S2.FTIR spectra of the NFES electrospun nanofibers (needle-to-collector
distance is 1 mm with applied voltages 1.2 kV) and conventional electrospinning with
whipping instability.
Figure S2 shows FTIR spectra of both NFES and conventional electrospinning
samples. It is observed that FTIR spectra of NFES samples exhibit the α-phase related
bands at 614, 760,795 and 975 cm-1 and the β-phase related bands at 840 and 1278
cm-1, respectively. Prior data [1] also showed the similar trend such that the
α-phase-related bands locate at 614, 765, 795 and 975 cm-1,the β-phase-related bands
at 840 and 1278 cm-1 andγ-phase at 1225 cm-1, respectively In comparison, the
conventional electrospinning fibres share the similar characteristics in α-, β- and γ
-phase-related bands. However, the NFES nanofibers exhibit weaker and broad
diffraction peaks, which may be attributed to the small size of the crystallites and the
formation mechanism of β-phase can be found in [2]. Both electrospinning processes
show the effect of Columbic force promoted β-phase conformational changes to
straighter TTTT conformation In particular, the NFES samples show a prominent
β-phase related band around 840, indicating the piezoelectric polarity polar β phase
through aligning in-situ dipoles inside the crystal of electrospun NFs.[13]
VALIDATION OF PVDF SPINNABILITY FOR CONTINUOUS AND
LARGE-AREA DEPOSITION
Figure S3. Plots showing the spinnability and dependence of NFES nanofiber
diameters on various processing parameters (a) A 16 wt% PVDF and
needle-to-collector distance 1 mm. (b) Needle-to-collector distance. Other parameters
are maintained as the followings: 16 wt% PVDF applied voltage 1 kV
Figure S3 shows the spinnability and dependence of NFES nanofiber diameters on
various processing parameters. Though the diameter of nanofibers can be adjusted
through applied voltage and needle-tocollector distance, however, the continuous
spinnability in large area cannot (shown as red circle). In order to achieve the
continuous and large-area deposition, the NFES spinning window is experimentally
identified for PVDF solution (shown as green box), which is relatively narrow
compared to other commonly used solution such as PEO [12].
1. Piezoelectric PVDF and non-piezoelectric polyethylene oxide nanofibers
Figure S4. (a) Voltage output of a PVDF nanogenerator subject to continuous stretch
and release. (b)Voltage output of electrospun PEO nanofibers subject to continuous
stretch and release
Figure S4 shows the comparison of the electrical outputs generated from
piezoelectric PVDF and non-piezoelectric poly(ethylene oxide) (PEO) nanofibers,
respectively. Only noise with no visible peaks is observed in the output voltage
(Figure S4b), indicating the strikingly different behaviors between PEO nanofibers
and PVDF nanofibers (Fig. S4a). Therefore, the possibility of any artifact such as
residual charges, friction, or contact potential as the cause of the observed electric
outputs can be excluded for the PVDF nanofibers.
2. Polarity-oriented PVDF nanofibers
Electrospun PVDF nanofibers are deposited in two types (A&B), mainly due to the
polarity accumulated in the similar or opposite directions (Figure S5a,b, S6a,b). In
type A deposition, the PVDF nanofiber arrays basically have the opposite polarity and
were cancelling out one another such that the overall electric outputs were relatively
small (Figure S5c.d). Red arrows illustrate the poling direction. Since the diameters of
PVDF nanofibers are in the range of 0.9 μm -2.5μm, which may explained why the
overall electric output is not zero. For the same-direction-poled alignment of type B,
the enhancement of both output voltage and current is prominent, indicating the
importance on polarity alignment during nanofibers deposition..
(b)
Figure S5. Type A electrospun NFs for opposite-poled alignment (a) Optical image
of PVDF nanofibers deposited between two metal electrodes. Red arrows illustrate the
poling direction. (b) Schematic electrode pattern and deposited method of Type A
electrospun NFs (c)Voltage output subject to continuous stretch and release. (d)
Current output subject to continuous stretch and release. NFES conditions are: voltage
is 1.0 kV and needle-to-collector distance is 1 mm, flow rate is 0.005ml/hr.
(a)
(b)
Figure S6. Type B electrospun NFs for the same direction-poled alignment. (a)
Optical image of PVDF nanofibers deposited between two metal electrodes. (b)
Schematic electrode pattern and deposited method of Type A electrospun NFs.(c)
Voltage output of the PVDF nanofibers subject to continuous stretch and release. (d)
Current output of the PVDF nanofibers subject to continuous stretch and release.
NFES conditions are: voltage is 1.0 kV and needle-to-collector distance is 1 mm ,flow
rate is 0.005ml/hr.
3. Enhancement of electrical outputs
Superposition of enhanced voltage and current outputs were demonstrated by making
serial and parallel connections of nanogenerators as show in Fig. S7.
(a)
9
90
(b)
(c)
(d)
9
90
(e)
(f)
Figure S7. Two nanogenerators were superimposed to enhance the output voltages
which (a) nanogenerator #1 and (b) nanogenerator #2 subject to continuous stretch
and release. (c) Constructively, output voltages were basically added when two
nanogenerators are in serial connection. Output currents of (d) nanogenerator #1 and
(e) nanogenerator #2 subject to continuous stretch and release. (f) When two
nanogenerators are in parallel connection, output currents add constructively. All
measurement data are performed when the two nanogenerators operated in the same
strain, strain rate, and frequency.
References and Notes
1. Yee W.A., Kotaki M., Liu Y., Lu X.: ‘Morphology, polymorphism behavior and
molecular orientation of electrospun poly (vinylidene fluoride) fibers’, Polymer,
2007,48, pp. 512–521
2. Yiin-Kuen Fuh, Li-Chih Lien, Jason S.C. Jang. Micro & Nano Letters, 2012, Vol. 7,
Iss. 4, pp. 376–379