Electrochemical performance of l-tryptophanium picrate as an efficient electrode material for supercapacitor application

Article abstract of DOI:10.1039/c9cp02536g

A new, prominently stable organic material, l-tryptophanium picrate, was synthesized via a one-step process. The as-prepared sample was characterized by UV-visible, FT-IR, Raman, TGA-DSC, XRD, FE-SEM, EDX, ¹H-NMR, ¹³C-NMR and GC-MS.

Full text of DOI:10.1039/c9cp02536g

View Article Online
View Journal
PCCP
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: S. Rajkumar, E.
Elaiyappillai, G. K. S, B. Antony Paulraj, A. Sathiyan, M. B and P. M. Johnson, Phys. Chem. Chem. Phys.,
2019, DOI: 10.1039/C9CP02536G.
Volume 18 Number
1
7
January 2016 Pages 1–636
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
PCCP
Physical Chemistry Chemical Physics
www.rsc.org/pccp
Accepted Manuscripts are published online shortly after
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
author guidelines.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the ethical guidelines, outlined
in our author and reviewer resource centre, still apply. In no
event shall the Royal Society of Chemistry be held responsible
for any errors or omissions in this Accepted Manuscript or any
consequences arising from the use of any information it contains.
ISSN 1463-9076
PERSPECTIVE
Darya Radziuk and Helmuth Möhwald
Ultrasonically treated liquid interfaces for progress in cleaning and
separation processes
rsc.li/pccp

Page 1 of 22
Physical Chemistry Chemical Physics
View Article Online
Electrochemical Performance of L-Tryptophanium picrate as an efficient electrode material
DOI: 10.1039/C9CP02536G
for Supercapacitor Application
a
a
b,*
a
a
Rajkumar Srinivasan , Elanthamilan Elaiyappillai , S. Gowri , A. Bella , A. Sathiyan ,
a
a,
B. Meenatchi , Johnson Princy Merlin *
a
Department of Chemistry, Bishop Heber College, Tiruchirappalli-620 017, Tamil Nadu, India.
b
Department of Physics, Cauvery College for Women, Tiruchirappalli-620 018, Tamil Nadu
India.
Corresponding Authors:
1
2
. J. P. Merlin
. S. Gowri
E-mail: pmej_68@yahoo.co.in
E-mail: gow.1976@gmail.com
Abstract
A new, prominently stable organic material, L-Tryptophanium picrate was synthesized via
a one-step process. The as-prepared sample was characterized by UV-visible, FT-IR, Raman, TGA-
1
13
DSC, XRD, FE-SEM, EDX, H-NMR, C-NMR and GC-MS. The significant capacitive behaviour
of L-Tryptophanium picrate was investigated by Cyclic Voltammetric studies (CV), Galvanostatic
Charge-Discharge test and Impedance Spectroscopy techniques in 1 M aqueous KOH. The newly
fabricated L-Tryptophanium picrate electrode material exhibited excellent specific capacitance of
-1
-1
2
9
63 F g at current density of 1 A g . Furthermore, it portrayed the capacitance retention of about
2% even after 2000 consecutive charge/discharge cycles. Moreover, the as-prepared electrode
material illustrated electrochemically reversible behaviour. In addition, the synthesized material
showed good electrochemical performance under the selected potential window. Thus, the
fabricated electrode was proved to be a promising alternate electrode for high performance energy
storage applications.
Keywords:
Cyclic Voltammetry, Supercapacitor, Charge-Discharge, Specific capacitance, L-Tryptophanium
picrate
1

Physical Chemistry Chemical Physics
Page 2 of 22
View Article Online
DOI: 10.1039/C9CP02536G
1
. Introduction
The global climate change and the slow deterioration of natural resources have accelerated
the development of technology associated with renewable energy conversion and storage1, 2
.
Developing improved energy storage technology is highly significant with a view to extend the
performance of portable electronics or to harvest renewable energy resources with many economic
and environmental benefits. Among various energy storage devices, batteries and capacitors are
typically regarded as the suitable choices to store energy by transforming chemical energy into
electrical energy. Therefore, the development of energy-storage devices, primarily Supercapacitors
(SC), have received great attention. These electrochemical capacitors play vital role in our day-to-
day life, due to their unique chemical and physical properties such as power density, thermal
stability, low-cost, longer cycle life and fast charge/discharge ability, compared to modern energy
2
storage devices . They have been widely used in industrial power devices, digital communication
devices, mobile phones, peak power sources, energy management etc.,3.
Generally, the energy storage mechanism of SC is classified into two types- Electric double
layer capacitor (EDLCs), due to adsorption/separation of nanoscopic charges at the electrode-
electrolyte interface and Pseudocapacitor (PCs), owing to fast-reversible Faradic charge transfer
4
reactions . Hybrid supercapacitors synergise these two-charge storage mechanisms, namely Faradic
and non-Faradic reactions, resulting in improved device characteristics. The nature of the electrode
material alters the storage mechanism. Recently researchers have explored the properties of several
supercapacitor electrode materials and primarily focused on creating electrode materials with more
ion-accessible surface sites, such as metal oxides, metal sulfides, metal hydroxides, porous carbon
materials, conductive polymers, and their composites. But challenge lies in achieving high energy
5
-7
density materials .
Indole compounds exist abundantly in living cells and function in various biologically
important roles, primarily as tryptophan in proteins and indolealkylamines in serotonin and
tryptamine etc. Because of the typical nature of indole ring, indole compounds tend to form the
charge-transfer complexes with many aromatic acceptors such as pyridine, flavin and thiamine
coenzymes and with nucleic acid bases8.
From 1969 onwards, organic materials were explored as electrodes for Electrochemical
energy storage systems (EESSs), with the first report of an organic cathode material using
dichloroisocyanuric acid. Thereafter, multiple research groups investigated a variety of organic
small molecules such as quinones, dianhydrides, phthalocyanines and poly(acetylene) as cathode
2

Page 3 of 22
Physical Chemistry Chemical Physics
View Article Online
materials, followed by many other conjugated polymers such as polypyrrole, polythiophene,
DOI: 10.1039/C9CP02536G
9
polyaniline and their derivatives . Furthermore, research on organic electrode materials has been
revived due to the increased demand for low-cost high-performance energy storage system.
However, the reported organic based electrode materials have their limitations such as low electrical
conductivity, low specific capacitance low EDLC type capacitance and low mechanical/chemical
stability, which reduce their applications in supercapacitors9.
To overcome these issues of organic based electrode materials, the design of new type of
organic materials gained momentum, in which the redox pathway can additionally provide
pseudocapacitance with sufficiently large space as the diffusion channels of electrode ions. Zhuoxin
Liu group reported the design of a novel Soft Device-Level, Dynamically Super-Tough
1
0
Supercapacitor with an Energy-Dissipative Dual-Crosslinked Hydrogel Electrolyte . This super
tough supercapacitor was found to have great potential in wearable applications. R.S. Hastak et al.
successfully the synthesised activated carbon (AC) and phosphoric acid doped poly[2,5
1
1
benzimidazole] (ABPBI), with better electrochemical performance . Bidhan et al. fabricated a new
-1
type of benzimidazole based COF, with a high specific capacitance of 335 F g with 93% retention
1
2
after 1000 cycles . Yan Huang et al. fabricated a polypyrrole based supercapacitor with excellent
cyclic stability and superior pseudocapacitivity with specific capacitance of 214 F g-1 13
.
H. Jeong group successfully synthesized amino acid derived N-doped carbon material as
-1
-1
electrode material with the specific capacitance of 58 F g at current density of 0.1 A g with 2000
1
4
cycling stability . Very recently directly electrodeposited of imidazole modified poly(pyrrole)
copolymers were utilised as electrode material, which exhibited the specific capacitance 201 F g-1
at 10 mVs-1 15. Although the organic based electrode materials have good electrochemical
performance due to redox active species, the number of related reports highlighting the structural
characteristics is still limited. Therefore, in order to develop an organic based electrode material
with notable supercapacitive performance, an alternate organic material with a favourable structure
is designed in the present work, to ensure pseudocapacitive process with facile electrolyte diffusion
and fast electron transfer 9, 16
.
Recently several kinds of L-tryptophan derivatives and L-tryptophan based metal oxides
were used as functional materials in sensor applications 17, 18. The charge-transfer interactions were
shown to play an important part. In addition, L-tryptophan exhibits photochromism and
1
8
thermochromism, which elevate its scope and potential usage in optoelectronics and sensors . The
3

Physical Chemistry Chemical Physics
Page 4 of 22
View Article Online
literature scanning showed that few tryptophan-based materials had been tested as electrode
DOI: 10.1039/C9CP02536G
materials in energy storage applications.
Thus, in the present work, a new, remarkably stable tryptophan derivative namely, L-
Tryptophanium picrate was prepared by a simple, one-step route via two components acid-base
reaction between L-tryptophan and picric acid in high yield under ambient condition. The title
compound has not been examined as an electrode material in SC application so far. Herein, we
report, a systematic study of evaluation of L-Tryptophanium picrate as an effective, sustainable,
cheap and scalable electrode material for energy storage applications.
2
2
. Experimental Methods
.1.
Materials
L-Tryptophan, Picric acid, Carbon black, Polyvinylidine difluoride (PVdF), N-methyl-2-
pyrrolidone (NMP), nickel foil, KOH and Ethanol were purchased from Merck and used as
procured. All the reagents used were of analytical grade.
2
.2.
Preparation of L-Tryptophanium picrate
NH
O N
NO2
2
H
O N
NO2
2
N
+
NH3+
NH2
-O
HO
aq. C H OH
2
5
NO2
NO2
OH
O
O
OH
L-Tryptophanium picrate
picric acid
L-Tryptophan
Scheme 1: Synthesis of L-Tryptophanium picrate
The reddish needle shaped crystalline compound of L-Tryptophanium picrate was purified
with water-acetone mixture and then vacuum dried (Scheme 1).
2
.3.
Characterization techniques
IR and Raman spectra were performed by Bruker Vector 22 spectrophotometer and Brucker
RFS 27: Standalone FT-Raman spectrophotometer respectively. XRD analyses were done using
Shimadzu XRD 6000 (Japan), respectively. The UV-vis spectrum was recorded using Perkin-Elmer
Lambda 35 UV/Vis spectrometer. The FE-SEM and Energy Dispersive X-ray (EDX) were captured
1
13
by means of Hitachi SU6600. H- and C- NMR spectral analyses were done by Bruker AVANCE
4

Page 5 of 22
Physical Chemistry Chemical Physics
View Article Online
III 500 MHz (AV 500) multi nuclei solution NMR Spectrometer. TGA and DSC analyses were
DOI: 10.1039/C9CP02536G
carried out using TG analyzer NETZSCH STA 449 F3 Jupiter and DSC 6000 respectively. GC-MS
analysis was performed by SHIMADZU/ QP2020. Supercapacitor studies were performed using
Princeton Applied Research (VSP-1) Electrochemical work station.
2
.4.
Fabrication of Electrode Material
With a view to analyse the electrochemical performance of the fabricated electrode of L-
Tryptophanium picrate, CV, Galvanostatic Charge-Discharge and impedance techniques were
employed in aqueous 1 M potassium hydroxide (electrolyte), with the three-electrode assembly
of platinum wire (counter electrode), Ag/AgCl (reference electrode) and L-Tryptophanium
picrate coated Nickel foil (working electrode). The Cyclic voltamogram was recorded in the
-1
potential range of 0 to 0.64 V at different scan rates (5 to 100 mVs ). Subsequently,
galvanostatic charge/discharge studies were also achieved at different current densities (1 to 5
-1
A g ). Further, EIS was recorded in the frequency range from 100 kHz to 0.1 Hz with an
amplitude of 10 mV AC voltage using Ag/ AgCl/ KCl (sat’d) (reference electrode) in the
potential range of -1 V to +1 V.
In order to design the working electrode, the nickel foil was pre-treated sequentially with
acetone, 1 M HCl, absolute ethanol and deionized water for 20 mins to ensure the clean surface.
Then the slurry, prepared by mixing 80% L-Tryptophanium picrate (active material), 10%
carbon black and 10% poly(vinylidene)difluoride (PVDF), with N-methyl-2-pyrrolidone as
solvent was coated on nickel foil (1 cm × 1 cm). The L-Tryptophanium picrate coated electrode
was dried in a hot air oven for 6 hours and then the electrode weight was measured to determine
the amount of L-Tryptophanium picrate coated on the nickel foil. The characteristic mass of
the fabricated electrode material was found to be 0.002 g. The specific capacitance (C ) values
sp
were evaluated from CV and GCD measurements using equations (1) and (2), respectively
∫
풊풅풗
푪 =
풔풑
…………………… (1)
ퟐ × 푺 × ∆푽 × 풎
푰 × ∆풕
푪 =
풔풑
………………………… (2)
풎 × ∆푽
Where, ∫ idV indicates the integral area of one complete cycle of CV curve, ΔV potential
window (V), ‘m’, mass of the active material (mg), S, scan rate (mV/s), I (A), discharge current
and Δt (s), the discharge time.
5

Physical Chemistry Chemical Physics
Page 6 of 22
View Article Online
DOI: 10.1039/C9CP02536G
3
3
. Results and Discussion
.1. FT-IR Analysis
Fig. 1: a) FT-IR and b) Raman spectrum of L-Tryptophanium picrate
-1
Fig.1a depicted a series of bands from 4000 to 400 cm . The absorption frequency at 3430
-1
cm was labelled to N–H stretching of indole group. Moreover, the wider band at about 3300–2700
-1
+
cm was probably due to ν
of NH groups, ν
of aromatic ring and νC-H of CH aliphatic
C-H 2
N-H
3
-1
groups. Thus, the carbon–hydrogen stretching vibrations displayed bands around 3000–3100 cm .
-1
+
The broadband at 1634 cm was assigned to the asymmetric NH deformation mode. The
3
−
1
stretching frequency observed at 1554 cm was the signature of C–C bond. As expected, C–H in-
−
1
plane bending vibration was observed between 1000–1300 cm . The strong intense band at 1271
−
1
cm was owing to in-plane C–H bending vibration. The C–H out of plane vibrations were noted in
−
1
−1
the region 675–1000 cm . The stretching frequency at 638 cm was assigned to C–C–C
1
9
deformation band of the phenyl ring .
-1
The characteristic frequencies of 1634 and 1595 cm were assigned as the signature peaks
+
of asymmetric stretching of carboxylate group and asymmetric deformation of the NH group.
3
Significantly, it is noticed that they are mainly due to ring vibrations of the picrate anion and 2,4,6-
trinitrophenol20, 21. Hence the IR spectrum of L-Tryptophanium picrate provided substantial
structural conformation.
3
.2.
Raman spectral analysis
The characteristic moderate band in Raman spectrum of L-Tryptophanium picrate (Fig. 1b) at
-1
3
158 cm was ascribed to the N-H stretching of indole group. It is noticed that these lines are
-1
superimposed on the broad bands between 3300–2500 cm , which were attributed to stretching of
+
21
-1
N-H bonds of NH group . The prominent band at 3102 cm was the signature band of stretching
3
6

Page 7 of 22
Physical Chemistry Chemical Physics
-1
View Article Online
DOI: 10.1039/C9CP02536G
mode of C-H bond of pyrrole ring. The weak band at 2934 cm was owing to O-H stretching
-1
-1
vibration. The bands observed at 1442 cm and 1428 cm were suggestive of the deformation
-1
scissoring vibration of CH group. The band at 1416 cm was labelled as the symmetric stretching
2
-
-1
of COO (carboxylate) group. Additionally, the very strong band at 3060 cm due to C-H stretching
-1
and the bands at 1600, 1024, 1044 cm due to ν
of benzene ring were supportive of L-
C─ C
Tryptophanium picrate structure 21, 22
.
3
.3. UV-Vis Spectral Study
Wavelength (nm)
Fig. 2: a) UV-vis and b) Tauc plot of L-Tryptophanium picrate
The UV-vis spectrum of the L-Tryptophanium picrate (Fig.2a) showed the absorption
maximum at 280 nm owing to its charge transfer intermolecular π→π* transitions and a shoulder
observed at 327 nm was assigned to the transitions taking place in tryptophan and picric acid moiety
23, 24.
The experimental energy gap (E ) of L-Tryptophanium picrate has been calculated using Tauc
g
plot (Fig.2b) and E , was calculated as 2.08 eV. All these evidences are supportive of the suggested
g
structure and the characteristic features of L-Tryptophanium picrate.
7

Physical Chemistry Chemical Physics
Page 8 of 22
View Article Online
DOI: 10.1039/C9CP02536G
3
.4.
Electrochemical Properties
(a)
(b)
E vs (Ag/AgCl)/ V
-
1
Fig. 3: a) CV of L-Tryptophanium picrate in DMSO (0.5 mM) at 50 mVs
-1
b) CV of standard Ferrocene (0.1mM) at 50 mVs
The corresponding HOMO levels were calculated using Eox (onset) obtained from cyclic
voltammogram of L-Tryptophanium picrate in DMSO (Fig.3a) using the empirical relation EHOMO
(E - E +4.8] eV. Ferrocene was used as external standard while recording CV. The CV
=
[
ox
1/2(ferrocene)
of L-Tryptophanium picrate exhibited two anodic peaks at 0.65 V and 0.5 V (Fig.3b). E1/2 of
ferrocene (0.59 V) was used to calculate EHOMO and EHOMO was found to be -5.41 eV. Further, using
the calculated HOMO value and the ힴonset value from UV absorption spectrum, the LUMO energy
was evaluated by the empirical relation ELUMO= (1240/ힴonset)- EHOMO. Thus, ELUMO was calculated
2
5
as -3.20 eV and the calculated energy gap (E ) of the L-Tryptophanium picrate was 2.21 eV .
g
3
.5.
DFT-TDDFT study
a
b
HOMO
2
.49 eV
LUMO
L-Tryptophanium Picrate
8

Page 9 of 22
Physical Chemistry Chemical Physics
View Article Online
DOI: 10.1039/C9CP02536G
Fig.4: a) DFT-Frontier Molecular Orbitals (FMO) and Energy Level Diagram of
L-Tryptophanium picrate b) Optimized Geometry of L-Tryptophanium picrate
To understand the relationship between geometric configuration and electrochemical
performance of L-Tryptophanium picrate, Density Functional theory (DFT) simulations were
performed. The ground state (S ) geometry of L-Tryptophanium picrate was optimized with
0
Gaussian 09 package at B3LYP/6-31+G (d,p) level27-33. The Calculated vibrational frequency
analysis confirms that the optimized geometry corresponds to minima on the potential energy
surface. The ground state geometry and frontier orbital distribution of L-Tryptophanium picrate are
presented in Fig.4a-b.
26
The HOMO-LUMO energy gap calculated by DFT analysis for L-Tryptophanium picrate
was found to be 2.49 eV, which is observed to be much closer with the experimentally derived band
gap value of 2.21 eV through CV. Moreover, the analysis predicts the HOMO energy level of L-
Tryptophanium picrate as -6.01 eV and LUMO as -3.52 eV. It is worth mentioning that the electron
charge distribution of HOMO orbitals is more on the indole and picrate group whereas LUMO’s
are mainly spread over the picrate group. Hence, it sufficiently explains the fact that conduction
electrons are originated mainly from Tryptophan moiety with lesser contribution from picrate
moiety. The UV-visible spectrum of L-Tryptophanium picrate calculated by Time Dependent-DFT
3
4
(
TD-DFT) method with the suitable cam-B3LYP/6-31+G (d,p) level of basis set was observed to
be 324 nm which is close to the experimentally obtained UV-vis spectrum of L-Tryptophanium
picrate (327 nm) Fig. S1.
IR stretching calculated for L-Tryptophanium picrate is similar to experimentally obtained
IR spectrum of L-Tryptophanium picrate in Fig.S2. Furthermore, IR stretching frequency of L-
_
_
-1
Tryptophanium picrate with N H ----O H stretching band obtained at 1637 cm , closely
4
0
41
31
42
-1
+
matches to the experimentally obtained value of 1634 cm , thereby supports that the NH in
3
tryptophan is in close proximity with the picrate moiety.
9

Physical Chemistry Chemical Physics
Page 10 of 22
View Article Online
DOI: 10.1039/C9CP02536G
3
.6.
Thermal and XRD Analysis
2
θ (degree)
Fig.5: a) TGA- DSC Curve of L-Tryptophanium picrate b) XRD pattern of
L-Tryptophanium picrate
Fig.5a shows the TGA-DSC curve of L-Tryptophanium picrate. The pattern noticed in TGA-
DSC curve provided the vital information about the thermal stability of L-Tryptophanium picrate.
The observations were made in nitrogen atmosphere (at a heating rate of 20°C from 30–800°C)
(Fig.5a). The initial mass of L-Tryptophanium picrate was 2.79 mg and the last mass left out after
the experiment was 2.01 % from the initial mass at 800°C, suggesting the major decomposition of
the sample. Moreover, the curve portrayed a two-step decomposition, suggesting that the melting
was followed by decomposition, between 200°C to 800°C. The first stage decomposition with the
weight loss of 56.88 % occurred at 197°C. Further, it indicates that there is no phase transition
1
9
before melting .
Fig.5a indicates the melting point (T ) as 220°C. There is a gradual and significant weight-loss
m
as the temperature was increased above the melting point. It is seen that at different stages, various
gaseous fractions like CO, CO , NH etc., were liberated and the total decomposition of the
2
3
compound took place at about 800°C. The subsequent decomposition pathway of L-Tryptophanium
picrate clearly indicates its high structural stability and therefore suggests that it can withstand high
3
5
temperature during device operation .
The XRD pattern of L-Tryptophanium picrate is presented in Fig.5b. The peaks are located at
2
θ = 7.95°, 15.94°, 32.24°, 37.11° and 45.97°, which matched with the peaks observed in the case
of L-tryptophan (JCPDS cards file no. 25-1960) and picric acid (JCPDS cards file no. 03-0484).
These values agreed well with the reported values and hence this observation supports the clear
signature of indole moiety in L-Tryptophanium picrate as reported by Gowri et al 18, 19
.
1
0

Page 11 of 22
Physical Chemistry Chemical Physics
View Article Online
DOI: 10.1039/C9CP02536G
3
.7.
FE-SEM and EDX Analysis
(a)
(b)
(c)
2
0 µm
20 µm
2 µm
Element
Wt %
At %
(e)
(f)
C
49.40
36.39
14.21
100.00
51.64
34.39
13.97
(d)
O
N
Total
2
µm
1 µm
Fig. 6: a-e) FE-SEM Images of L-Tryptophanium picrate at different magnifications
f) EDX analysis of L-Tryptophanium picrate
The surface morphological images of the freshly prepared L-Tryptophanium picrate (Fig.6a-
e), showed disassembly of nanolayers like morphology. The observed L-Tryptophanium picrate
nanolayers could provide the prominent electro-active sites for charge-transfer processes, at the
electrode-electrolyte interfaces. In Fig. 6f, the recorded EDX spectrum exhibited elemental peaks
only for C, N, O and thus substantiating the presence of the important elements in the as-synthesized
L-Tryptophanium picrate sample.
1
1

Physical Chemistry Chemical Physics
Page 12 of 22
View Article Online
DOI: 10.1039/C9CP02536G
3
.8.
NMR & GC-MS Analysis
(a)
(b)
NH
O2N
NO2
NH3+
-O
NO2
O
OH
δ (ppm)
δ (ppm)
(c)
NH
O2N
NO2
M+
NH3+
-O
NO2
O
OH
m/z
1
13
Fig.7: a) H-NMR and b) C-NMR c) GC-MS spectrum of L-Tryptophanium picrate
1
13
The H and C- NMR spectrum of L-Tryptophanium picrate are shown in Fig.7 (a) and (b)
respectively. The chemical shift values (δ) of the protons of L-Tryptophanium picrate are plotted
on the X-axis and the intensity is plotted on the Y-axis (Fig.7a). The multiplets appeared between
δ 7-7.2 ppm were attributed to the aromatic protons of tryptophan moiety. The doublets observed
at δ 7.55 and 7.42 ppm were owing to the similar kind of protons in picric acid moiety, whereas in
bare picric acid, the same appeared at δ 8.53 ppm. This upfield shift of frequency accounts for the
increase of electron density, owing to charge transfer in the synthesized material. The appearance
+
of new chemical shift value at δ 8.75 ppm was ascribed to [NH –H] proton. Moreover, it confirms
2
3
6
the transfer of –OH proton to the nitrogen atom of tryptophan .
1
3
In C-NMR spectrum of L-Tryptophanium picrate (Fig.7b), the signal at δ 179.05 ppm was
assigned to aromatic carbon present in the picrate ion. The series of peaks at δ 124.89, 126.53, 113.3
and 135.87 ppm were ascribed to carbon in aromatic rings. The solvent peak appeared at 40 ppm.
1
13
Thus, NMR ( H and C) spectral analysis provided the useful information about the various carbon
and hydrogen environments prevailing in the molecular structure of L-Tryptophanium picrate 37, 38
.
The GC-MS spectrum (Fig.7c) also confirms the formation of L-Tryptophanium picrate, by
exhibiting the intense peak at m/z = 433.00 (calculated m/z for L-Tryptophanium picrate= 433.09).
1
2

Page 13 of 22
Physical Chemistry Chemical Physics
View Article Online
DOI: 10.1039/C9CP02536G
3
3
.9.
Electrochemical capacitance measurements
.9.1. Capacitance measurement using CV
(a)
(b)
(c)
(d)
Fig. 8: a) CV curves of L-Tryptophanium picrate at various scan rates in 1M aqueous KOH
b) Specific capacitance of L-Tryptophanium picrate as a function of Scan rate
c) GCD curves of L-Tryptophanium picrate at different current density
d) Specific Capacitance of L-Tryptophanium picrate as a function of Current density
Fig.8a shows the CV curve of the L-Tryptophanium picrate electrode recorded at different
scan rates in the potential range of 0 to 0.64 V. The calculated specific capacitance values were 253,
-1
-1
2
31, 205, 191, 174, 163 and 156 F g at 5, 10, 20, 25, 50, 75 and 100 mVs scan rate respectively,
Fig.8a). A pronounced redox peak, characteristic of the pseudocapacitive behaviour was noted for
(
3
9
L-Tryptophanium picrate electrode material, due to the redox reaction . The probable redox
pathway of L-Tryptophanium picrate electrode accounting for its supercapacitive behaviour is
displayed in Scheme 2. Moreover, redox peaks are nearly symmetrical in nature, revealing the
predominant reversibility of the redox electrode.
1
3

Physical Chemistry Chemical Physics
Page 14 of 22
View Article Online
DOI: 10.1039/C9CP02536G
N
O N
NO
2
NH
O N
NO2
2
2
-
OH-
+ H O + e
2
+
NH3+
NH3+
-O
-O
NO2
NO2
O
OH
O
OH
Scheme 2: Redox pathway of synthesized material as modified electrode
Fig.8b, displays the effect of scan rate on the specific capacitance (C ) value. A noticeable
sp
decrease in specific capacitance with scan rate, was observed. Only a very slight change in shape
of the plot was identified and hence it is further suggestive of good kinetic reversibility of the
synthesized electrode material. From this plot, it is observed that increase of scan rate decreased the
specific capacitance value, mainly due to ion-exchange storage mechanism 39-41. This type of
behaviour reflects the fact that an electrochemical process might easily occur at low scan rates and
furthermore it confirms that the charge storage mechanism is a diffusion controlled redox reaction.
With the increase of scanrate, the anodic peak potential (E ) and cathodic peak potential (E ) were
pa
pc
found to be slightly shifted towards positive and negative potentials respectively, attributed to the
polarization effect in addition to ion diffusion. The excellent capacitive behaviour of L-
Tryptophanium picrate originates from the pseudocapacitive behavior, owing to the more redox
sites in the synthesized material.
3
.9.2. Capacitance Measurement using GCD
The typical GCD curves of L-Tryptophanium picrate electrode recorded at five different
current densities in the potential range of 0 to 0.64 V are shown in Fig.8c, which further indicate the
pseudocapacitive electrochemical performance. Apparently, the electro-active L-Tryptophanium
-1
picrate electrode delivered C of 263, 211, 170,147 and 127 F g at current density of 1, 2, 3, 4 and
sp
-1
5
A g respectively (Fig. 8c). The obtained charge-discharge curves of L-Tryptophanium picrate
4
2
electrode showed dependence on applied current density . The recorded charge/ discharge plot
exhibited a distinct plateau in the charging and discharging processes and a few voltage drops
indicated the Faradic behaviour of the reaction and also the superior rate capability. Fig.8d reveals
that the C value is indirectly related with current density, thereby following the same trend as noted
sp
in the case of CV analysis 43, 44. In view of the above observations, it is noticed that the electrochemical
performance of L-Tryptophanium picrate in terms of specific capacitance is significantly better than
that of other organic based materials described in the literature (Table 1).
1
4

Page 15 of 22
Physical Chemistry Chemical Physics
View Article Online
DOI: 10.1039/C9CP02536G
Table 1: Comparison of the Electrochemical Performance of Organic based Electrode
Materials
Electrode
Materials
Electrolyte
Specific
Capacitance
Cycle
Number
Capacitance Current
Ref. No
retention
density
His-NCMs
ANEG
6 M KOH
1 M KOH
Ionic liquid
1 M Na SO
58 F g-1
115 F g-1
204 F g-1
203.1 F g-1
48 F g-1
2000
1000
32500
3000
5000
-
0.1 A g-1
4 A g-1
14
45
46
47
48
93 %
-
N-MCNF
0.25 A g-1
1 A g-1
CNT@Mn-MOF
88 %
85 %
2
4
β-ketoenamine-
1 M H SO
0.1 A g-1
2
4
linked COF
β-ketoenamine-linked 1 M H SO
252 F g-1
10000
-
1 A g-1
49
2
4
Conjugated micro
Porous Polymer
Ni-pPDA/GC
Mn-pPDA/GC
TpPa-COF@PANI
NiBDZ
1 M KOH
1 M KOH
184.7 F g-1
109.9 F g-1
95 F g-1
5000
5000
30000
-
80 %
97 %
83 %
-
1 A g-1
1 A g-1
0.2 A g-1
-
50
50
51
52
1 M H SO
2
4
1 M H PO
127.87 F g-1
88.6 F g-1
230 F g-1
214 F g-1
256 F g-1
263 F g-1
3
4
BiBDZ
1 M H PO
5000
1000
10000
2000
2000
93.61 %
-
0.5 A g-1
5 A g-1
52
53
13
54
3
4
BNC capsules
PPY-based
0.1 M KOH
1 M H PO
98 %
98 %
92 %
0.5 A g-1
0.5 A g-1
1 A g-1
3
4
TPT-DAHQ
1 M KOH
L-Tryptophanium
Picrate
1 M KOH
This
work
1
5

Physical Chemistry Chemical Physics
Page 16 of 22
View Article Online
DOI: 10.1039/C9CP02536G
3
.9.3. Electrochemical Impedance Spectroscopic Analysis
9
2 %
(b)
(c)
(a)
Fig. 9: a) Nyquist Plot of L-Tryptophanium picrate at 10mV AC voltage
b) Magnified Nyquist plot of L-Tryptophanium picrate
c) Cyclic stability of L- Tryptophanium picrate
The EIS plot of the fabricated electrode material represented the typical Nyquist plots at the
open circuit potential (Fig.9a). Generally, the diameter of the semi-circular region denotes the
charge transfer resistance. The curve consists of the electrolyte resistance (R ) and charge
s
transfer resistance (R ). In Fig.9b shows the magnified view of Nyquist plot of L-
ct
Tryptophanium picrate, which possesses a small semicircle produced by the polarization effect.
At lower frequency, a straight line was detected, which might be due to the mass transfer effect.
From Fig.9a, it is found that L-Tryptophanium picrate exhibited small Equivalent Series
Resistance (ESR) value of 2.49 Ω, which promotes electrolyte diffusion into the active electrode
surface. The appearance of straight line at low frequency portion of the plot suggests that an
extraordinary capacitive behavior of synthesized electrode material. Thus, the EIS analysis is
also evident of the supercapacitive nature of as-synthesized L-Tryptophanium picrate 27, 55, 56
.
3
.10. Stability Measurement
The stability measurement of L-Tryptophanium picrate electrode was examined by 2000
-1
consecutive GCD cycles at 1 A g current density. The C value of L-Tryptophanium picrate shows
sp
9
2% retention even after 2000 GCD cycles (Fig.9c). The effective ion diffusion between L-
Tryptophanium picrate and KOH interfaces is predominantly accountable for the enhanced cycling
stability. Hence, the electrochemical studies (CV, GCD and EIS) proved that L-Tryptophanium
picrate has promising behaviour as electrode material for supercapacitor applications. This
1
6

Page 17 of 22
Physical Chemistry Chemical Physics
View Article Online
capacitive behaviour of L-Tryptophanium picrate is mainly attributed to its effective and abundant
DOI: 10.1039/C9CP02536G
redox sites with high thermal stability.
However, the as-fabricated L-Tryptophanium picrate electrode material exhibited the
comparable C value with the reported works. Thus, the electrochemical performance of the L-
sp
Tryptophanium picrate electrode can be enhanced by increasing the surface area, porosity,
morphology, when the dopants adding such as conducting polymers based, metal/ metal oxides
based and carbon based materials50, 54, 57-66
.
4
. Conclusion
In summary, the new, cheap and easily scalable electrode material, L-Tryptophanium picrate
synthesized and investigated for its applicability as an electrode material in supercapacitors. The L-
-1
-1
Tryptophanium picrate showed high capacitance of 263 F g at the current density of 1 A g in 1M
aqueous KOH and it retained 92 % of its initial specific capacitance even after 2000 GCD cycles at
-1
1
A g . Moreover, the fabricated electrode material exhibited high specific capacitance with
outstanding long-term cyclic performance at ambient conditions. Additionally, DFT calculations
were performed to view the optimized HOMO-LUMO energy level of L-Tryptophanium picrate
and to calculate the theoretical energy gap. Thus, the comparison of experimental and computational
data proves that the notable L-Tryptophanium picrate has the optimal band gap, suitable for
sustainable energy storage devices. These findings have proved that the as-synthesized L-
Tryptophanium picrate is a better electrode material for supercapacitor applications.
Acknowledgement
The authors thank the Management of Bishop Heber College (Autonomous),
Tiruchirappalli-620 017, Tamil Nadu, India for the support and facilities provided through DST-
FIST Instrumentation Centre at Bishop Heber College (HAIF). Mrs. A. Bella sincerely
acknowledges DST for the financial support under DST-WOSA Scheme- File. No. SR/ WOS-A /
CS-22/ 2016.
Conflicts of Interest
The authors have no conflicts to declare.
1
7

Physical Chemistry Chemical Physics
Page 18 of 22
View Article Online
DOI: 10.1039/C9CP02536G
References
1
2
.
.
L. Dai, D. W. Chang, J. B. Baek and W. Lu, Small, 2012, 8, 1130-1166.
A. S. Arico, P. Bruce, B. Scrosati, J.-M. Tarascon and W. Van Schalkwijk, Mater. Sustain.
Energy, 2011, 4, 148-159.
3
4
5
.
.
.
P.-C. Chen, G. Shen, Y. Shi, H. Chen and C. Zhou, ACS Nano, 2010, 4, 4403-4411.
J. R. Miller and P. Simon, Science Magazine, 2008, 321, 651-652.
W. Wei, L. Mi, S. Cui, B. Wang and W. Chen, ACS Sustain. Chem. Eng., 2015, 3, 2777-
2
785.
6
7
8
9
.
.
.
.
M. Winter and R. J. Brodd, Chem. Rev., 2004, 104, 4245–4270.
K. Chen and D. Xue, ACS Appl. Mater. Interfaces, 2016, 8, 29522-29528.
A. R. Fiorucci and É. T. G. Cavalheiro, J. Pharm. Biomed. Anal., 2002, 28, 909-915.
T. B. Schon, B. T. McAllister, P.-F. Li and D. S. Seferos, Chem. Soc. Rev., 2016, 45, 6345-
6
404.
10.
11.
12.
13.
14.
15.
Z. Liu, G. Liang, Y. Zhan, H. Li, Z. Wang, L. Ma, Y. Wang, X. Niu and C. Zhi, Nano
Energy, 2019, 58, 732-742.
R. Hastak, P. Sivaraman, D. Potphode, K. Shashidhara and A. Samui, Electrochim. Acta,
2
012, 59, 296-303.
B. C. Patra, S. Khilari, L. Satyanarayana, D. Pradhan and A. Bhaumik, Chem. Commun.,
016, 52, 7592-7595.
Y. Huang, J. Tao, W. Meng, M. Zhu, Y. Huang, Y. Fu, Y. Gao and C. Zhi, Nano Energy,
015, 11, 518-525.
2
2
H. Jeong, H. J. Kim, Y. J. Lee, J. Y. Hwang, O.-K. Park, J.-H. Wee, C.-M. Yang, B.-C. Ku
and J. K. Lee, Mater. Lett., 2015, 145, 273-278.
F. Wolfart, B. M. Hryniewicz, L. F. Marchesi, E. S. Orth, D. P. Dubal, P. Gomez-Romero
and M. Vidotti, Electrochim. Acta, 2017, 243, 260-269.
1
1
6.
7.
Z. Song and H. Zhou, Energy Environ. Sci., 2013, 6, 2280-2301.
Ö. A. Yokuş, F. Kardaş, O. Akyıldırım, T. Eren, N. Atar and M. L. Yola, Sens. Actuators B
Chem., 2016, 233, 47-54.
1
1
8.
9.
S. Gowri, T. U. Devi, D. Sajan, S. Bheeter and N. Lawrence, Spectrochim. Acta A Mol.
Biomol. Spectrosc., 2011, 81, 257-260.
A. Petrosyan, M. Fleck and V. Ghazaryan, Spectrochim. Acta A Mol. Biomol. Spectrosc.,
2
013, 104, 486-491.
1
8

Page 19 of 22
Physical Chemistry Chemical Physics
View Article Online
2
0.
V. Ghazaryan, M. Fleck and A. Petrosyan, Spectrochim. Acta A Mol. Biomol. Spectrosc.,
DOI: 10.1039/C9CP02536G
2
015, 136, 743-750.
2
1.
2.
V. Ghazaryan, G. Giester, M. Fleck and A. Petrosyan, J. Mol. Struct., 2015, 1101, 219-227.
D. Joshi, D. Kumar, A. K. Maini and R. C. Sharma, Spectrochim. Acta A Mol. Biomol.
Spectrosc., 2013, 112, 446-456.
2
2
2
2
2
2
2
3.
4.
5.
6.
7.
8.
P. Vishnoi, S. Sen, G. N. Patwari and R. Murugavel, New J. Chem., 2015, 39, 886-892.
A. Divyashree and G. Hegde, RSC Adv., 2015, 5, 88339-88352.
A. Shafiee, M. M. Salleh and M. Yahaya, Sains Malays., 2011, 40, 173-176.
M. Frisch, J. Chem. Phys., 1999, 110, 2822.
C. Lee, W. Yang and R. G. Parr, Physical Review B, 1988, 37, 785.
T. Ishida, H. Nagata, M. Doi, M. Inoue, M. W. Extine and A. Wakahara, Chem. Pharm.
Bull., 1993, 41, 433-438.
2
3
9.
0.
R. Zarrougui, R. Hachicha, R. Rjab and O. Ghodbane, J. Mol. Liq., 2018, 249, 795-804.
A. Jänes, J. Eskusson, T. Thomberg, T. Romann and E. Lust, J. Energy Chem., 2016, 25,
6
09-614.
R. Zarrougui, R. Hachicha, R. Rjab, S. Messaoudi and O. Ghodbane, RSC Adv., 2018, 8,
1213-31223.
B. T. McAllister, T. B. Schon, P. M. DiCarmine and D. S. Seferos, Polym. Chem., 2017, 8,
194-5202.
3
1.
2.
3
3
5
3
3
3.
4.
S. Paranthaman, S. Sampathkumar and N. Murugasenapathi, J. Chem. Sci, 2018, 130, 164.
A. Szemik-Hojniak, K. Oberda, I. Deperasińska, Y. P. Nizhnik and L. Jerzykiewicz,
Polyhedron, 2015, 88, 190-198.
35.
36.
37.
38.
R. M. Silverstein, F. X. Webster, D. J. Kiemle and D. L Bryce, Spectrometric Identification
of Organic Compounds, John Wiley & Sons, Hoboken, 2014.
T. Dhanabal, V. Tharanitharan, G. Amirthaganesan and M. Dhandapani, , Spectrochim. Acta
A Mol. Biomol. Spectrosc., 2014, 128, 694-700.
D. Arthi, E. Ilango, M. Mercina, D. Jayaraman and V. Joseph, J. Mol. Struct., 2017, 1127,
1
56-163.
A. González, E. Goikolea, J. A. Barrena and R. Mysyk, Renew. Sustain. Energ Rev., 2016,
8, 1189-1206.
5
3
4
4
9.
0.
1.
X. Liu, Y. Gao and G. Yang, Nanoscale, 2016, 8, 4227-4235.
M. Liao, Y. Liu, Z. Hu and Q. Yu, J. Alloys Compd., 2013, 562, 106-110.
F. Zhang, Y. Bao, S. Ma, L. Liu and X. Shi, J. Mater. Chem. A Mater., 2017, 5, 7474-7481.
1
9

Physical Chemistry Chemical Physics
Page 20 of 22
View Article Online
4
2.
D. Hulicova, J. Yamashita, Y. Soneda, H. Hatori and M. Kodama, Chem. Mater., 2005, 17,
DOI: 10.1039/C9CP02536G
1
241-1247.
4
4
3.
4.
M. Zhi, C. Xiang, J. Li, M. Li and N. Wu, Nanoscale, 2013, 5, 72-88.
S. Bose, T. Kuila, A. K. Mishra, R. Rajasekar, N. H. Kim and J. H. Lee, J. Mater. Chem.,
2
012, 22, 767-784.
T. Kuila, P. Khanra, N. H. Kim, S. K. Choi, H. J. Yun and J. H. Lee, Nanotechnology, 2013,
4, 365706.
4
5.
2
4
4
6.
7.
P. Ramakrishnan and S. Shanmugam, J Power Sources, 2016, 316, 60-71.
Y. Zhang, B. Lin, Y. Sun, X. Zhang, H. Yang and J. Wang, RSC Adv., 2015, 5, 58100-
5
8106.
4
4
8.
9.
C. R. DeBlase, K. E. Silberstein, T.-T. Truong, H. D. Abruña and W. R. Dichtel, , J. Am.
Chem. Soc., 2013, 135, 16821-16824.
S. Chai, N. Hu, Y. Han, X. Zhang, Z. Yang, L. Wei, L. Wang and H. Wei, RSC Adv., 2016,
6
, 49425-49428.
5
5
0.
1.
Y. Y. Kannangara, U. A. Rathnayake and J.-K. Song, Chem. Eng J., 2019, 361, 1235-1244.
S. Liu, L. Yao, Y. Lu, X. Hua, J. Liu, Z. Yang, H. Wei and Y. Mai, Mater. Lett., 2019, 236,
3
54-357.
5
5
5
2.
3.
4.
A. Roy, S. Mondal, A. Halder, A. Banerjee, D. Ghoshal, A. Paul and S. Malik, Eur. Polym.
J., 2017, 93, 448-457.
Z. Zhou, X. Zhang, L. Xing, J. Liu, A. Kong and Y. Shan, Electrochim. Acta, 2019, 298,
2
10-218.
N. S. Punde, C. R. Rawool, A. S. Rajpurohit, S. P. Karna and A. K. Srivastava, Chemistry
Select., 2018, 3, 11368-11380.
5
5
5.
6.
A. Zolfaghari, H. R. Naderi and H. R. Mortaheb, , J. Electroanal. Chem., 2013, 697, 60-67.
Y. Ren, Q. Xu, J. Zhang, H. Yang, B. Wang, D. Yang, J. Hu and Z. Liu, ACS Appl. Mater.
Interfaces, 2014, 6, 9689-9697.
5
7.
N. Li, Z. Xu, Y. Liu and Z. Hu, J. Alloys Compd., 2019, 789, 613-621.
5
5
8.
9.
N. Li, Z. Xu, Y. Liu and Z. Hu, J. Power Sources, 2019, 426, 1-10.
Y. Y. Kannangara, U. A. Rathnayake and J.-K. Song, Electrochim. Acta, 2019, 297, 145-
1
54
6
0.
D. Xue, D. Zhu, W. Xiong, T. Cao, Z. Wang, Y. Lv, L. Li, M. Liu, L. Gan, ACS Sustain.
Chem. Eng., 2019, 7, 7024-7034.
2
0

Page 21 of 22
Physical Chemistry Chemical Physics
View Article Online
DOI: 10.1039/C9CP02536G
6
6
1.
2.
L. Miao, X. Qian, D. Zhu, T. Chen, G. Ping, Y. Lv, W. Xiong, Y. Liu, L. Gan, M. Liu,
Chinese Chem. Lett., DOI: 10.1016/j.cclet.2019.03.010..
L. Hu, L. Ma, Q. Zhu, L. Yu, Q. Wu, C. Hu, N. Qiao and B. Xu, New J. Chem., 2017, 41,
1
3611-13618.
6
6
3.
4.
Y. Wang, L. Zhao, H. Peng, X. Dai, X. Liu, G. Ma and Z. Lei, Ionics, 2019, 1-9.
K. R. Reddy, B. Hemavathi, G. R. Balakrishna, A. Raghu, S. Naveen and M. Shankar,
Polymer Composites with Functionalized Nanoparticles, Krzysztof Pielichowski and
Tomasz M. Majka, Elsevier, Netherlands, First Edition, 2019, 357-379.
6
6
5.
6.
S. Huo, M. Liu, L. Wu, M. Liu, M. Xu, W. Ni and Y.-M. Yan, J Power Sources., 2019, 414,
3
83-392.
B.-H. Cheng, F.-X. Zeng, W.-J. Chen, H.-Y. Cheng, R. J. Zeng and H. Jiang, iScience, 2019,
2, 204-215.
1
2
1

Physical Chemistry Chemical Physics
Page 22 of 22
View Article Online
DOI: 10.1039/C9CP02536G
L-Tryptophanium picrate was synthesized and evaluated for its supercapacitor
-1
behavior and 263 F g of specific capacitance was achieved.