A Flexible Composite Mechanical Energy Harvester from a Ferroelectric Organoamino Phosphonium Salt

Article abstract of DOI:10.1002/anie.201805479

A new binary organic salt diphenyl diisopropylamino phosphonium hexaflurophosphate (DPDP?PF₆) was shown to exhibit a good ferroelectric response and employed for mechanical energy harvesting application. The phosphonium salt crystallizes in the

Full text of DOI:10.1002/anie.201805479

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Accepted Article
Title: A Flexible Composite Mechanical Energy Harvester from a
Ferroelectric Organo-amino Phosphonium Salt
Authors: Thangavel Vijayakanth, Anant Kumar Srivastava, Farsa
Ram, Priyangi Kulkarni, Kadhiravan Shanmuganathan, B.
Praveenkumar, and Ramamoorthy Boomishankar
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201805479
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10.1002/anie.201805479
Angewandte Chemie International Edition
COMMUNICATION
A Flexible Composite Mechanical Energy Harvester from a
Ferroelectric Organo-amino Phosphonium Salt
Thangavel Vijayakanth,[a] Anant Kumar Srivastava,[a] Farsa Ram,[c,d] Priyangi Kulkarni,[e] Kadhiravan
Shanmuganathan,*[c,d] B. Praveenkumar,*[e] Ramamoorthy Boomishankar*[a,b]
Abstract: A new binary organic salt diphenyl diisopropylamino
phosphonium hexaflurophosphate (DPDP·PF6) was shown to exhibit
a good ferroelectric response and employed as a mechanical energy
harvester. The phosphonium salt crystallizes in the monoclinic
noncentrosymmetric space group Cc and exhibits an H-bonded 1D-
chain structure due to N-H…F interactions. Ferroelectric
measurements on the single crystals of DPDP·PF6 gave a well
saturated rectangular hysteresis loop with a remnant (Pr) polarization
value ~6 μCcm‒2. Further, composite devices based on
nanogenerators based on ZnO nanowires, several piezoelectric
and ferroelectric materials including inorganic perovskite
ceramic materials such as barium titanate, lead titanate, lithium
niobate, sodium niobate and lead zirconate titanate have been
employed for energy harvesting applications.5 Ferroelectrics are
the subgroup of piezoelectric materials in which the electric
dipoles can be reoriented or flipped by the application of an
external electric field.6 Since the piezoelectric coefficient is
directly proportional to the electric polarization (d33 α εPr),
polydimethylsiloxane (PDMS) films for various weight percentages (3, ferroelectric materials with intrinsic permanent dipole moment,
5, 7, 10 and 20 wt %) of DPDP·PF6 were prepared and examined for
power generation by using a force-measurement setup. A maximum
output peak to peak voltage (VPP) of 8.5 V and an output peak to
peak current (IPP) of 0.5 μA was obtained for the non-poled
composite film with 10 wt% of DPDP·PF6. These results promise the
efficacy of organic ferroelectric substances as potential mechanical
energy harvesters.
high spontaneous polarization, and piezoelectric response is
expected to increase the performance of the nanogenerator.7,8
In this context, it is desirable to prepare non-ceramic and heavy-
metal free power generators for addressing certain issues
pertaining to heavier elements such as high-temperature
processing, high-voltage poling, toxicity and to impart improved
flexibility for the devices.9 Although polyvinylidene fluoride
(PVDF) and its copolymers based composite materials have
attracted greatly as energy harvesters due to their high
piezoelectric response, achieving polar ferroelectric and
piezoelectric order in these materials are still challenging.10
Moreover, PVDF requires mechanical stretching, high-
temperature annealing or poling and requires external additives
to obtain polar ferroelectric β phase.11 Alternatively, polymer
composites made up of ferro- and piezoelectric organic
compounds offer an attractive approach for this application.
Herein, we report a flexible mechanical energy harvester derived
from a simple ferroelectric phosphorus based organic salt
Ph2(iPrNH)2P]·PF6 (DPDP·PF6) embedded in a PDMS matrix.
The organo-aminophosphonium salt DPDP·PF6 retains its polar
symmetry for a wide range of temperatures from 100 to 463 K.
The ferroelectric measurements on the single crystals of this
compound gave a remnant polarization (Pr) of 6.32 μCcm‒2. The
fabricated composites with polymeric PDMS films were shown to
harvest electrical energy with a maximum output voltage of 8.5 V
and output current of 0.5 μA (at 15 N impact force and 10 Hz
frequency) for the composite with 10 wt% of DPDP·PF6. To the
best of our knowledge, this is the first report for a flexible energy
harvester based on a binary organic ferroelectric salt.
Ferroelectric materials derived from organic and organic-
inorganic hybrid assemblies have been the topic of intense
research in the past decade as alternate choice of materials for
high-technique applications in the areas of energy and
electronics.1 In addition to their traditional utility as ferroelectric
random access memories, capacitors, sensors, actuators and
electromechanical transducers, they exhibit applications in the
domains of photovoltaics and mechanical energy harvesters.2
Harvesting energy from sustainable and renewable sources
such as solar, thermal, vibrational and mechanical energy have
attracted immense attention to meet the global demands of
energy consumption in everyday life.3 Among various
approaches, converting mechanical energy into electrical energy
is of tremendous interest because of the simplicity of its
generation via the forces derived from pressure, bending, folding
and stretching motions.4 Inspired from the earlier discovery of
[a]
[b]
T. Vijayakanth, Dr. A. K. Srivastava and Prof. Dr. R. Boomishankar*
Department of Chemistry, Indian Institute of Science Education and
Research, Pune, Dr. Homi Bhabha Road, Pune – 411008, India.
Prof. Dr. R. Boomishankar*
Centre of Energy Science, Indian Institute of Science Education and
Research, Pune, Dr. Homi Bhabha Road, Pune – 411008, India.
E-mail: boomi@iiserpune.ac.in
[c]
[d]
F. Ram and Dr. K. Shanmuganathan*
Polymer Science and Engineering Division, CSIR-National Chemical
Laboratory, Dr. Homi Bhabha Road, Pune – 411008, India.
F. Ram and Dr. K. Shanmuganathan*
Academy of Scientific and Innovative Research, CSIR- Human
Resource Development Centre, Ghaziabad – 201002, India.
E-mail: k.shanmuganathan@ncl.res.in
Scheme 1. Synthesis of the phosphonium salts DPDP.Br and DPDP.PF6.
[e]
Priyangi Kulkarni and Dr. B. Praveenkumar
The
organoamino-phosphonium
salt
[Ph2(iPrNH)2P]·PF6,
PZT Centre, Armament Research and Development Establishment,
Pune, Dr. Homi Bhabha Road, Pune – 411021, India.
E-mail: praveenkumar@arde.drdo.in
Supporting information for this article is given via a link at the end of
the document. CCDC 1842608-1842612 contains the
crystallographic data.
DPDP·PF6, was synthesized from diphenylchlorophosphine
(Ph2PCl) as shown in Scheme 1 (Figures S1-S14, Supporting
Information). Crystals of DPDP·PF6 were directly obtained by the
anion exchange reaction of DPDP·Br with KPF6 and were solved
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10.1002/anie.201805479
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in the acentric monoclinic space group Cc (Table S1, Supporting
Information). The asymmetric unit consists of a phosphonium
cation and a non-coordinating hexafluorophosphate anion while
the unit-cell contains four DPDP·PF6 motifs (Figures 1a and 1b).
The coordination environment around the cationic phosphorus
center is distorted tetrahedral. The measured distances of
1.6210(18) and 1.6242(19) Å for P1-N1 and P1-N2, respectively,
were in the expected range typically observed for amino-P(V)
scaffolds. The N-H protons of the two amino groups are pointing
opposite to each other and involved in an intermolecular H-
bonding with the PF6‒ anions. As a result, the N1-P-N2 angle is
wider (avg. 120.76 (10)°) than the ideal tetrahedral angle.
Concomitantly, other angles were marginally distorted ranging
from 104.93(10) to 108.34(10)° within the cationic segment
(Tables S2 and S3, Supporting Information). Each amino proton
is hydrogen bonded to two F-atoms of PF6‒ anion in a chelating
fashion (with one strong and one weak interaction) and resulting
in the formation of a 1D-chain like structure (Figure 1c, Figure
S15, Supporting Information).
thermogravimetric analysis coupled with differential thermal
studies indicated that there are two heat anomalies, one for the
melting at 471
K and a subsequent one at 603 K for
decomposition (Figure S21, Supporting Information).
The point group symmetry for the structure of DPDP·PF6 is Cs
which belongs to one of the ten polar point groups (C1, Cs, C2,
C2v, C3, C3v, C4, C4v, C6, and C6v) that are suitable for
ferroelectric measurements.13 The polarization (P) vs. electric
field (E) measurements were performed on the single crystals of
DPDP·PF6 by using a Sawyer-Tower circuit at an operating
frequency of 0.1 Hz (Figure 2 and Figure S22, Supporting
Information). These crystals gave a remnant (Pr) and saturation
(Ps) polarization values of 6.32 and 3.50 μCcm‒2, respectively
(Figure 2a). The coercive fields (Ec) obtained in these studies
were found to be 4.27 kVcm‒1.
Figure 2. (a) P-E hysteresis loop and the corresponding leakage current plot
of DPDP·PF6 and (b) Ferroelectric fatigue measurements up to 104 cycles and
P-E loop after the fatigue cycles. Temperature dependent (c) dielectric
constant (ε') and (d) dielectric loss (tan δ) for various frequencies.
Figure 1. (a) Molecular structure of DPDP·PF6 and (b) its packing diagram
along the b-axis. (c) The 1D-chain structure of DPDP·PF6 mediated by N-H…F
hydrogen bonding.
While the Pr for DPDP·PF6 is comparable with some of the well-
known molecular ferroelectrics, the obtained Ec value is much
lower than most of them.14 The observed Pr values are much
higher than certain other phosphonium stabilized ferroelectric
materials.15 Also, several metal-organic materials stabilized by
related N-donor functionalized amido-P(V) ligands have shown
to exhibit ferroelectric behavior.16 The leakage current density
measurements indicate low leakage behavior which are of the
order of 1.2 μAcm‒2 along with peaks associated with domain
switching typical for ferroelectric materials (Figure 2a). Further,
fatigue measurements exemplified the retention of ferroelectric
polarization for several switching cycles as 90% of its Pr as well
as the rectangular shape of the hysteresis loop is retained even
after 104 cycles (Figure 2b). The high and stable polarization in
this species can be attributed to its asymmetric ionic nature
coupled with its H-bond assisted 1D-chain like alignment of
bipolar phosphonium cations and PF6‒ anions along the b-axis
(which is the direction of measurements) with a net long-range
polar order.
A closer look at the structure of DPDP·PF6 reveals that the
organo- and amino groups around the central phosphonium
cation presumably has played
a key role for imparting
asymmetry in the system.12 To probe the presence of any
centrosymmetric phase in the structure of DPDP·PF6, single
crystal X-ray diffraction analysis was performed in the
temperature range between 100 and 463 K with an incremental
step of 25 K between the measurements. A detailed structural
analysis confirmed no change in symmetry and space group at
all the measured temperatures albeit with a marginal increase in
the lattice distances and cell volume (Figures S18 and S19,
Supporting Information). The powdered unsieved sample of this
salt shows a second harmonic generation (SHG) efficiency of
0.2, with respect to the reference KDP which confirms the
noncentrosymmetric nature of DPDP·PF6 at room temperature.
The variable-temperature powder X-ray diffraction (VT-PXRD)
profile of DPDP·PF6 did not exhibit any phase change or the
systematic absence of peaks upon increasing the temperatures
from 298 to 463 K (Figure S20, Supporting Information). The
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Further, we performed temperature dependent dielectric
permittivity studies for DPDP·PF6 on its compacted
polycrystalline sample. The real part of the dielectric constant (ε')
initially increases slowly with an onset of anomalous behaviour
at 390 K (Figure 2c). However, no dielectric peak maxima were
attained up to the temperature close to its melting point. These
results are consistent with the crystallographic (both VT-SCXRD
and VT-PXRD) data and with certain recent reports pertaining to
molecular ferroelectrics.17 The observed ε' values were found to
vary between 7 and 27 upon sweeping the temperature from
200 to 463 K. The sharp rise in the ε' values can be attributed to
its ionic structure that shows increase in charge carriers and ion
mobility at higher temperatures. A similar trend has also been
observed in the corresponding dielectric loss (tan δ) plot as well
(Figure 2d). The frequency dependent dielectric measurements
gave higher ε' values at lower frequencies indicating the
involvement of all the polarization (interface, dipolar, ionic and
images show a random distribution of the DPDP·PF6 particles in
the PDMS matrix (Figure 3d and Figures S27 and S28,
Supporting Information). A comparative analysis revealed a
much higher particle distribution and density for the 20 wt%
composite than the 10 wt% composite. Furthermore, the
presence of ferroelectric particles and the crystallite purity of
DPDP·PF6 in the composites was identified by using Raman
spectroscopy. Pure PDMS matrix has characteristic peaks at
487 and 708 cm-1 for the symmetrical Si-O-Si and Si-C
stretching modes, respectively.20 For DPDP·PF6, major peaks
are found for aromatic, and aliphatic Raman active modes at
1590, 1316, 1117, 998 cm‒1. These peaks were present in the
DPDP·PF6/PDMS (10 wt %) composite as well in addition to the
peaks due to Si-O-Si and Si-C bonds in PDMS (Figure S29,
Supporting Information).
atomic)
mechanisms
(Figure
S23-S25,
Supporting
Information).18 A preliminary piezo measurements on the single
crystals of DPDP·PF6 gave a piezoelectric coefficients (d33) of 8
pC/N under the applied force of 0.20 N. The observed piezo-
response may be attributed to the oriented domain
configurations in the single crystals.19 These values are
consistent with some of the known molecular and polymeric
ferro- and piezoelectric materials (Table S4, Supporting
Information).
Figure 4. The mechanical generator output voltages (a) and currents (c) for all
the DPDP·PF6/PDMS devices. The output voltage (b) and current (d) obtained
in a one cycle of measurement for the 10 wt % device.
Figure 3. Images of (a) neat PDMS, (b) DPDP·PF6/PDMS (10 wt%) and (c)
the corresponding device. (d) The 3D-Xray tomography images of
DPDP·PF6/PDMS (10 wt%; grid scale: 50μm).
The performances of the devices (with electrodes) have been
tested by using an in-house fabricated impact machine and an
oscilloscope. The output voltage and output currents for non-
poled DPDP·PF6/PDMS materials with different ratios (3%, 5%,
7%, 10% and 20 wt%) of ferroelectric DPDP·PF6 fillers are
generated from a periodically applied mechanical force. The
performance of the DPDP·PF6/PDMS devices improved with
increase in concentration of DPDP·PF6 upto 10 wt% and
decreased for the 20 wt% composite. The obtained maximum
output peak to peak voltage (VPP) and output peak to peak
current (IPP) of DPDP·PF6/PDMS were found to be 8.5 V and 0.5
μA, respectively, for the 10 wt% composite under an optimized
15 N compressive force and 10 Hz frequency (Figures 4a and 4c,
Figures S30 and S31, Supporting Information). These
observations are consistent with a few ceramic oxide-polymer
composite nanogenerators which show particle agglomeration at
higher oxide concentrations with a net reduction in the dipole
moments.21 The 3D-Xray tomography images support this claim,
where the higher 20 wt% composite shows some aggregations
Owing to a greater attention for flexible mechanical energy
harvesters and nanogenerators based on piezo- and
ferroelectric materials, we examined the power generator
response of DPDP·PF6 in its composite form with the
polydimethylsiloxane (PDMS) polymer (Figures 3a, 3b and 3c).
The composite devices with 3, 5, 7, 10 and 20 wt% of DPDP·PF6
in PDMS (DPDP·PF6/PDMS) were prepared by the dispersion of
appropriate quantities of DPDP·PF6 into
a homogeneous
solution of PDMS precursors (Scheme S3 and Table S5,
Supporting Information). All the obtained films unveiled
exceptional flexibility as demonstrated by its ease for twisting,
two-fold bending and rolling operations (Figure S26, Supporting
Information). The morphologies of all the DPDP·PF6/PDMS
composites were investigated by X-ray 3D-tomography analysis.
The magnified cross-sectional views of 3D-Xray tomography
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10.1002/anie.201805479
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COMMUNICATION
(Figure S27 and S28, Supporting Information). A closer view of
Keywords: binary phosphonium salts • H-bonding • polar order •
ferroelectricity • polymer composites • energy harvesters
the output voltage and current plots confirm
a single
piezoelectric response generated from each press and release
cycle of the composite film (Figures 4b and 4d). The maximum
current density (CD) and power density (PD) values of 0.28
μAcm‒2 and 1.74 μWcm‒3, respectively, were observed for the
10% composite (Figure S32, Supporting Information). The
performance of the DPDP·PF6/PDMS system is fairly
comparable to several composites derived from ferroelectric
(organic-inorganic and inorganic)-nonpiezoelectric polymer
materials.8a,21c,23b, 22
The observed room temperature ε' values for the neat PDMS,
3%, 5%, 10% and 20% DPDP·PF6/PDMS were found to be 5.6,
5.8, 6.9, 7.3 and 7.5, respectively, at 1 kHz (Figure S33,
Supporting Information). A linear increase in ε' values from 3 to
20 wt % indicates the contribution from the ferroelectric particles
for the overall dipole moment of the composite films.23 Similarly,
a marginal increase in the piezoelectric coefficient (d33) values
from 3 to 5 pC/N has been observed upon increasing the wt% of
the ferroelectric component from 3% to 20% (Figure S34,
Supporting Information). This clearly indicates that the organic-
polymer matrix interface plays a vital role in determining the
dielectric and piezoelectric properties of the DPDP·PF6/PDMS
composites. Furthermore, it is to be noted that the effective
energy harvesting performance of these materials depends on
the layer thickness, strength of the mechanical force applied,
structural morphologies and the weight ratio between polymer
and piezoelectric filler.24
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In summary, we have reported the synthesis of
a new
[5]
[6]
ferroelectric polar organic binary salt of DPDP·PF6 containing
organo- and amino functionalities. Ferroelectric measurements
on the single crystals of this salt gave a Pr value ~6 μCcm‒2. The
‒
1D-arrangement of organo aminophosphonium cations and PF6
anions aided by the presence of N-H...F intermolecular
hydrogen bond in its asymmetric lattice helps in the effective
alignment of dipoles responsible for ferroelectric behaviour. Also,
it exhibits high thermal and phase stability and sharp polarization
response. In combination with polymeric PDMS films, device
structures with excellent mechanical flexibility for various wt% (3,
5, 7, 10, 20%) of DPDP·PF6 were prepared. The mechanical
energy harvesting performance of these devices show the
current density of 0.28 μAcm‒2, and power density of 1.74
μWcm‒3 for the 10 wt% DPDP·PF6/PDMS composite. These
results demonstrate the effectiveness of organic ferroelectric
materials for energy harvesting applications and promise their
potential utility in future wearable electronics.
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Acknowledgements
This work was supported by ARMREB, DRDO, India via Grant
No. ARMREB/MAA/2017/189 (R. B.), and SERB, India via Grant
No. EMR/2016/000614 (R.B.) and Nanomission Project, DST,
India via Grant No. SR/NM/TP-13/2016. T. V. thanks the UGC,
India for the fellowship. We thank A. Torris for X-ray tomography
experiments.
[12] The related [Ph3(iPrNH)P]·PF6 (TPAP·PF6), and [Ph(iPrNH)3P]·PF6
(PTAP·PF6) were found to crystallize in the centrosymmetric monoclinic
space groups C2/c and P21/n, respectively (Figures S16 and S17,
supporting Information).
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This article is protected by copyright. All rights reserved.

10.1002/anie.201805479
Angewandte Chemie International Edition
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
A noncentrosymmetric binary organic
salt Ph2(iPrNH)2P]·PF6 was
synthesized and found to exhibit
excellent thermal and phase
stabilities. Ferroelectric
Thangavel Vijayakanth, Anant Kumar
Srivastava, Farsa Ram, Priyangi
Kulkarni, Kadhiravan Shanmuganathan*,
B. Praveenkumar*, Ramamoorthy
Boomishankar*
measurements on this salt gave
remnant polarization value ~6 μCcm‒2.
Further in combination with PDMS
polymer it was utilized as a composite
mechanical energy harvester that
shows a highest output voltage of 8.5
V.
Page No. – Page No.
A Flexible Composite Mechanical
Energy Harvester from a Ferroelectric
Organo-amino Phosphonium Salt
This article is protected by copyright. All rights reserved.