Iodine doped composite with biomass carbon dots and reduced graphene oxide: A versatile bifunctional electrode for energy storage and oxygen reduction reaction

Article abstract of DOI:10.1039/c9ta05559b

We prepared carbon dots (CDs) via a sustainable, low-cost synthetic route employing eggplant biomass. A three-dimensionally porous composite consisting of eggplant-CDs and reduced graphene oxide (RGO) was first synthesized via one-pot hydrothermal process. The as-synthesized RGO/CD hybrid was further deoxygenated with HI acid to enhance ionic conductivity and introduce iodine dopants in the structure. The I-doped graphene/CD composite exhibited specific capacitances of 432 and 460 F g^-1 at 2 mV s^-1 and 1 A g^-1 with excellent capacitance retention of 68.4% and 76.5%, as the scan rate and current density increased from 2 to 1000 mV s^-1 and from 1 to 100 A g^-1, respectively. Superior cycling stability of 94.6% capacitance retention over 10000 cycles at 10 A g^-1 was measured. As an electrocatalyst for oxygen reduction reaction (ORR) in 0.1 M KOH, the composite demonstrated an onset potential of 0.93 V vs. RHE and a kinetic limiting current density of 19.8 mA cm^-2 at 0.1 V vs. RHE with an average electron transfer number of 3.57. The I-doped graphene/CD composite showed excellent ORR stability with current density retention of 97.7% under electrochemical stress (0.65 V vs. RHE) in comparison with 43.4% of the benchmark Pt/C under similar conditions. Thus, a sustainable process was developed to synthesize a multifunctional electrode based on agricultural wastes for energy storage and ORR.

Full text of DOI:10.1039/c9ta05559b

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Materials Chemistry A
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PAPER
Iodine doped composite with biomass carbon dots
and reduced graphene oxide: a versatile
bifunctional electrode for energy storage and
oxygen reduction reaction†
Cite this: J. Mater. Chem. A, 2019, 7,
22650
a
a
Khang Ngoc Dinhab and Vincent G. Gomes
*
Van Chinh Hoang,
We prepared carbon dots (CDs) via a sustainable, low-cost synthetic route employing eggplant biomass. A
three-dimensionally porous composite consisting of eggplant-CDs and reduced graphene oxide (RGO) was
first synthesized via one-pot hydrothermal process. The as-synthesized RGO/CD hybrid was further
deoxygenated with HI acid to enhance ionic conductivity and introduce iodine dopants in the structure.
The I-doped graphene/CD composite exhibited specific capacitances of 432 and 460 F gꢀ1 at 2 mV sꢀ1
and 1 A gꢀ1 with excellent capacitance retention of 68.4% and 76.5%, as the scan rate and current
density increased from 2 to 1000 mV sꢀ1 and from 1 to 100 A gꢀ1, respectively. Superior cycling stability
of 94.6% capacitance retention over 10 000 cycles at 10 A gꢀ1 was measured. As an electrocatalyst for
oxygen reduction reaction (ORR) in 0.1 M KOH, the composite demonstrated an onset potential of
0.93 V vs. RHE and a kinetic limiting current density of 19.8 mA cmꢀ2 at 0.1 V vs. RHE with an average
electron transfer number of 3.57. The I-doped graphene/CD composite showed excellent ORR stability
with current density retention of 97.7% under electrochemical stress (0.65 V vs. RHE) in comparison with
43.4% of the benchmark Pt/C under similar conditions. Thus, a sustainable process was developed to
synthesize a multifunctional electrode based on agricultural wastes for energy storage and ORR.
Received 25th May 2019
Accepted 20th September 2019
DOI: 10.1039/c9ta05559b
rsc.li/materials-a
Therefore, developing multifunctional electrode materials with
high capacitance and excellent ORR activity at low cost is highly
1. Introduction
desirable.
Typically, high conductivity materials with large surface
Increasing energy demands have spurred intense exploration of
green, sustainable and renewable energy conversion and
storage systems including metal–air batteries, fuel cells and
electrochemical capacitors. Supercapacitors, as electrochemical
energy storage devices, have demonstrated advantageous
features of high power density, fast charge/discharge rate and
long cycle stability,1 yet their widespread applications are
inhibited by the low energy density compared to the conven-
tional lithium ion batteries.2 Likewise, energy conversion
systems in terms of metal–air batteries and fuel cells have been
considered as alternative production technologies to replace the
use of fossil fuels. However, the commercialization of these
energy conversion devices is hindered by the prohibitive cost of
scarce noble metals such as Pt, Pd-based electrocatalysts and
sluggish reaction kinetics of the ORR at the cathode.3 Previous
studies have reported that the efficiency of supercapacitors and
ORR is based on the properties of electrode materials.4,5
area, abundant pore structures with high chemical and thermal
stabilities have proven to be ideal for electrochemical energy
conversion and storage.6,7 In this sense, carbon materials have
been intensively studied because of their abundant nature,
good electronic conductivity, large surface area, chemical
inertness, tunable morphology and functionality.8 Among
them, heteroatom-doped graphene has recently drawn atten-
tion as an electrode material in supercapacitors as well as
a metal-free ORR electrocatalyst.9,10 However, the low density of
carbon edge sites and low degree of practically achievable
electrochemically active surface area (ECSA) induced by adverse
aggregation/stacking nature of graphene nanosheets have
limited its commercial viability.11 To resolve this issue, the
incorporation of graphene with transition metal oxides could
provide 3D structures with increased ECSA and introduce
pseudocapacitance/catalytic active sites,4,5 yet this may seriously
sacrice both cyclability and rate capability of graphene12 or
limit the ORR kinetics due to insufficient electronic conduc-
tivity of these oxides.13 Recently, the combination of graphene
with other classes of carbon nanomaterials including carbon
nanotube, carbon nanober and fullerenes has shown an
aThe University of Sydney, School of Chemical and Biomolecular Engineering, NSW
2006, Australia. E-mail: vincent.gomes@sydney.edu.au
bEnergy Research Institute @ NTU (ERI@N), Interdisciplinary Graduate School,
Nanyang Technological University, Singapore 637553, Singapore
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c9ta05559b
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efficient way to enhance electrochemical properties.14 Never- a 3D porous RGO/CD hybrid via hydrothermal treatment, and
theless, it remains challenging to reduce the cost of these subsequently deoxygenated the hybrid by hydriodic acid to
additives for the industrial scale-up of the resultant composites. simultaneously improve its ionic conductivity and introduce
Carbon quantum dot, a relatively new allotrope of 0D carbon iodine dopants in its structure. The synthesized I-doped
nanomaterial, possesses high electrical conductivity, rapid graphene/CD composite was then applied as a bifunctional
electron transfer, chemical stability, low toxicity and ease of electrode material for testing its supercapacitance and ORR
surface functionalization, which points to bright promise of electrocatalysis performance.
CDs in energy storage and conversion applications.15,16 In
contrast, the main obstacle for commercializing CDs are
synthesis of high quality and environmentally friendly CDs at
a low cost. In an alternative approach, price neutral and
2. Experimental
2.1. Fabrication of eggplant-CDs
sustainable CDs could be obtained from abundant biomass
resources, for instance eggs, soy milk, vegetables and plant
leaves.14,17 Recent reports have documented the design and
synthesis of graphene and CD composites as electrode mate-
rials for supercapacitors14,18–20 and fuel cells.18,21,22 For instance,
our group reported the supercapacitor performance of RGO/
biomass-derived CDs with a CD : GO mass ratio of 1 : 2, which
delivered a maximum capacitance of 278 F gꢀ1 at 0.2 A gꢀ1 in
1 M H2SO4 electrolyte with an excellent capacitance retention of
60% as the current density rose to 100 A gꢀ1 and superior cycle
All chemicals were purchased from Sigma Aldrich (Australia)
and used without further purication. Milli-Q (MQ) water with
18.2 MU cm resistivity was utilized throughout the study.
Eggplant obtained from a local supermarket was washed using
MQ water and then chopped into little patches, which were
added with MQ water into a 200 mL Teon-lined autoclave and
held at 220 ꢁC for 8 h. The resultant product was thoroughly
ltered with a 0.45 mm nylon membrane to remove large parti-
cles. Finally, the CD solution (6.4 mg mLꢀ1) was collected by
another ltration through a 0.22 mm nylon membrane and
stored in a refrigerator for further utilization.
life with 96.1% capacitance retention over 10 000 cycles at
a current density of 20 A gꢀ1
.
In another case, N,S co-doped
14
RGO/CD hybrid demonstrated to be electrochemical activity
towards ORR with an O2 reduction peak at ꢀ0.17 V (vs. Ag/AgCl),
an onset potential of 0.02 V (vs. Ag/AgCl) and an electron
transfer number in the range of 3.5–3.75.18 The enhanced
capacitance and electrocatalytic activity might derive from the
reduced graphene restacking/aggregation and increased active
sites due to numerous edge/surface defects on the surface/
interface of RGO/CDs.18,22 However, modest capacitance values
and limited ORR performance may be assigned to the high
oxygen content on RGO/CD composites, triggering low electrical
conductivity.19 Therefore, an effective deoxygenation is needed
to increase their electrical conductivity for practical
applications.
Generally, deoxygenating GO to produce high-quality gra-
phene could be obtained by thermal annealing and chemical
reduction.23 Compared with thermal reduction method, which
is time consuming and requires high temperatures (over 450
ꢁC), chemical reduction is advantageous for rapid mass
production and requires a lower temperature.23 Chemical
2.2. Fabrication of I-doped graphene/CD hybrid
GO was prepared via the modied Hummer's method.25 The
RGO/CDs were fabricated based on our previous study.14 In
a typical procedure, appropriate amount of CD solution was
added into 10 mL GO solution 2 mg mLꢀ1 to achieve a nal CD/
GO mass ratio of 1 : 2, followed by ultra-sonication for 30 min.
The resulting mixture was then sealed in a 200 mL Teon-lined
stainless steel autoclave, and kept at 200 ꢁC for 10 h. The solid
product was ltered with a nylon membrane, washed with MQ
ꢁ
water for several times and then freeze-drying at ꢀ40 C over-
night. Subsequently, the composite was placed above 57 wt% HI
solution at 100 ꢁC for 10 min, labeled as HI-RGO/CDs. The
obtained material was further washed several times with MQ
water and dried at 70 ꢁC for 10 h. For comparison, RGO was also
ꢁ
reduced with HI solution at 100 C for 10 min, denoted as HI-
RGO.
2.3. Material characterization
reduction has entailed use of reducing agents including The surface morphology was investigated by a scanning (SEM,
hydrazine or NaBH4 which reduce GO in the solution phase only Ultra-Plus, Zeiss) and a transmission (TEM, JEM-1400F, JEOL)
and increase of RGO sheet resistance, hence empeding their electronic microscopes. High resolution TEM measurements
widespread adoption for the deoxygenation of solid mate- were carried out on a transmission electron microscope (JEM-
rials.24,25 Recently, hydriodic acid (HI) has been demonstrated to 2200FS, JEOL) at 200 kV. The elemental compositions of the
efficiently reduce GO lms into high quality RGO with much materials were analyzed by energy-dispersive X-ray spectroscopy
increased electrical conductivity, higher graphitization degree (EDX) incorporated with SEM and X-ray photoelectron spec-
and iodine doping within 2–15 min.24,26 The iodine-doping not troscopy (XPS, Thermo Scientic K-Alpha, UK) adopting 12 kV Al
only increased pseudocapacitance of graphene,25 but also dis- Ka (1486.6 eV) radiation. The crystal structures of the samples
rupted the electroneutrality of graphitic carbon frameworks of were investigated by X-ray diffractometry (XRD, PANalytical
graphene, polarizing the adjacent carbon atoms for O2 X'Pert powder) with 45 kV and 40 mA Cu Ka radiation (l ¼
˚
adsorption, and favoring ORR by weakening the O–O bond 1.5406 A). The N2 adsorption–desorption isotherms was exam-
strength.27,28
ined by Quanta Chrome, Autosorb-1 at 77 K. Their surface areas
In this study, we rst synthesized environmentally-friendly were estimated by the conventional Brunauer–Emmet–Teller
CDs from eggplant – a natural biomass precursor, assembled (BET) technique while their pore volume and pore size
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distribution were determined by the density functional theory rotation speeds with a scan rate of 10 mV sꢀ1. All polarization
(DFT). The Fourier transform infrared (FTIR) spectra were ob- LSV curves were 95% iR-compensated. For comparison, the
tained using a Thermo Scientic Nicolet 6700 spectrometer. benchmark 20 wt% Pt/C was employed to prepare a reference
Raman microscope (InVia, Renishaw) was used to record electrode at the same mass loading.
Raman spectra with a 532 nm laser. A UV-Vis Spectrophotom-
eter (Cary 60, Agilent Technologies) and a photoluminescence
(PL) spectrouorometer (Horiba FluoroMax-4, USA) were used
to investigate the optical properties of the materials.
3. Results and discussion
3.1. Morphology and composition
Eggplant has an interconnected 3D microstructure and
contains abundant C moiety that enables excellent carboniza-
2.4. Electrochemical measurements
tion (Fig. S1, ESI†). Eggplant – a natural source, has high
The supercapacitor behavior was performed on a CHI760E
hydrocarbon content and constituent parts of cellulose and
potentiostat (CHI Instrument) employing a three-electrode cell
protein.29 The CDs were obtained through one-pot hydro-
in 1 M H2SO4 and 1 M KOH electrolytes with active materials
thermal treatment of the eggplant precursor. Under hydro-
loaded on glassy carbon electrode (GC, 5 mm in diameter), Pt
thermal treatment, these polymers and molecules undergo
plate and Ag/AgCl as the working electrode, counter electrode
dehydration and further carbonization to form CDs.30 TEM
and reference electrode, respectively. The ink was obtained by
image reveals that the eggplant-derived CDs appear as spherical
ultra-sonicating 5 wt% Naon and active materials in iso-
particles with their diameters distributed between 2 and 5 nm
(Fig. 1a). The HRTEM result reveals that the synthesized CDs
possess crystalline structures with well-resolved lattice spacing
propanol (IPA) to prepare 5 mg mLꢀ1 slurry. Aerwards, 20 mL
suspension was drop-casted on GC electrode, followed by drying
in air.
of 0.241 nm, which is close to the (100) facet of graphitic
Cyclic voltammetry (CV) and galvanostatic charge discharge
carbon18 (Fig. 1a). Fig. 1b depicts the FTIR spectrum of the CDs,
(GCD) plots were conducted in a potential window between
presenting several surface functional groups. For instance, an
0 and 1 V (vs. Ag/AgCl) in 1 M H2SO4 or ꢀ1 and 0 V (vs. Ag/AgCl)
in 1 M KOH. An AC voltage with 5 mV perturbation amplitude in
106 to 0.01 Hz was applied to record the electrochemical
impedance spectroscopy (EIS). The specic capacitance of the
materials was estimated by:
absorption peak at 2990–3680 cmꢀ1, clear modes at 1770 cmꢀ1
and 1085 cmꢀ1 are attributed to O–H, C]O and C–O stretching
vibration, respectively.10 The peak at 1400 cmꢀ1 is assigned to
C–H deformation of methyl/methylene groups, whereas the
band at 1585 cmꢀ1 results from C]C stretching.17 The existence
of these oxygen functional groups renders hydrophilicity to the
CDs. The FTIR analysis matches well with the EDX data which
reveal high C and O weightage with some impurities (Fig. S2,
ESI†).
Csp ¼ Celectrode/m
(1)
where Celectrode and m are the calculated capacitance and mass
of the active materials, respectively.
The optical properties of the CDs were investigated by means
of UV-vis and PL spectroscopy. The absorption spectrum shows
three clear peaks at 250, 280 and 320 nm with a tail extension
into the visible range (Fig. 2a). The absorption bands at 250 and
280 nm are attributed to p–p* transition of the C]C bond of
the sp2 carbon domains,17 whereas the peak at 320 nm is
ascribed to the n–p* transition of C]O groups.31 The as-
prepared CDs presented a light yellow solution under natural
light, while emitting a bright blue emission under 365 nm laser
Celectrode was estimated as follows:
ð
V
þ
1
Celectrode
¼
iðVÞdV
(2)
2Vn
V
ꢀ
where i(V) and n are the CV scan current and scan rate,
respectively; V (V ¼ V+ ꢀ V ) exhibits the potential scan.
ꢀ
Celectrode ¼ i/(dV/dt)
(3)
where, i and dV/dt are the discharge current and slope, (Fig. 2a inset). This provided a visual conrmation of the uo-
respectively.
rescence properties of the CDs.
ORR measurements were conducted on CHI760E adopting
Fig. 2b exhibits the PL excitation (PLE) and PL spectra of the
a
three-electrode cell in 0.1
M
KOH solution with CDs with an intense blue emission and PLE peak maxima at
electrocatalyst-loaded GC rotating disk electrode (RDE, 5 mm ꢂ400 nm and 320 nm, respectively. The maximum PLE peak at
in diameter or surface area of 0.19625 cm2), Pt wire and Ag/ 320 nm matches well with the UV-vis results, indicating that the
AgCl served as the working electrode, counter electrode and electron transition of triplet carbene from the s and p orbitals
reference electrode, respectively. The slurry was prepared by (highest occupied molecular orbitals or HOMO) to the lowest
ultra-sonically dispersing 5 wt% Naon and catalysts and in unoccupied molecular orbitals (LUMO) of C]O groups.32,33 In
IPA to prepare 1 mg mLꢀ1 ink suspension. Aerwards, 20 mL addition, the CDs exhibited an excitation dependent down-
resultant ink was spread on RDE electrode, followed by drying conversion PL emission (Fig. 2c) whose strongest emission
in air.
peak was red-shied from 400 to 543 nm with a gradually
The potential range for CV was ꢀ1.0 to 0.2 V (vs. Ag/AgCl) at decreasing intensity as the excitation wavelength was increased
10 mV sꢀ1 scan rate. Linear sweep voltammetry (LSV) curves from 320 to 500 nm. The PL efficiency was attributed to the
were recorded in O2-saturated 0.1 M KOH in the potential surface passivation derived from oxygen functional groups,
window between 0.1 and ꢀ1.0 V (vs. Ag/AgCl) at different which effectively trap the excitons and hinder the non-radiative
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Fig. 1 (a) TEM image and (b) FTIR spectra of CDs. The inset in a presents the HRTEM of CDs.
recombination.17,34 This effect is also seen in the contour maps achieve a uniform dispersion. During this process, –OH
of the CDs (Fig. 2d). groups on the surface of CDs reacted with –COOH on GO,
To synthesize RGO/CDs, the solution containing CDs was along with the formation of hydrogen bonds between oxygen
added to GO in aqueous media, followed by ultra-sonication to functional groups, producing strong interactions between GO
Fig. 2 (a) UV-vis absorption spectra of the as-prepared CDs with the inset showing the image of the CDs in aqueous solution under 365 nm
excitation; (b) PL and PLE spectra of CDs; (c) PL spectra of CDs under variable excitation wavelength; (d) excitation and emission contour map of
as-prepared CDs.
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and CD. Moreover, the dehydration and condensation process electronegativity between I (2.66) and C (2.55), the charge
during the hydrothermal treatment resulted in the formation redistribution among carbon within I-doped carbon occurs via
of 3D structured RGO/CDs.20 Hydriodic acid was used to a charge-transfer process. As a result, the adjacent carbon atoms
simultaneously reduce the RGO/CD composite and introduce become positively charged.27 In addition, the band gap between
iodine dopants into the structure. The reduction mechanism the highest-occupied molecular orbital (HOMO) and the lowest-
of oxygen functional groups by HI vapor is proposed as unoccupied molecular orbital (LUMO) is reduced because the
follows:24,35
HOMO energy level is lied to a higher value, which leads to the
modication of the electronic properties of the carbon
network.36
The morphology of the materials was determined by SEM
and their elemental composition was examined by EDS. Fig. 3a
shows that RGO layers are densely packed whereas the RGO/
CDs composite possesses a highly porous network with
several mm-size pores (Fig. 3b), which may be ascribed to the
effective prevention of the p–p stacking of graphene nano-
sheets. The CD incorporation into graphene network increases
oxygen concentration and decreases carbon content (see Table 1
and Fig. S3 and S4, ESI†). The rich oxygen containing functional
groups impart the hydrophilicity to RGO and provide large
number of active redox sites from numerous edge/surface
defects on the surface/interface of RGO/CDs.18,22 Aer treat-
ment with HI acid vapor, HI-RGO/CDs still maintained a unique
3D structure, yet the pore sizes were reduced compared to RGO/
CDs as a consequence of the removal of oxygen containing
groups. A uniform distribution of iodine element was observed
in HI-RGO/CDs (Fig. S5, ESI†). Fig. 3e indicates that CDs are
uniformly decorated on graphene nanosheets, conrming
a prominent interaction of RGO and CDs. In addition, the
interlayer spacing of CDs in HI-RGO/CDs was found to be
0.209 nm (Fig. 3e), which is lower than that of the as-prepared
CDs, indicating that the CDs were efficiently reduced by the
HI vapor treatment and the HI vapor was effective for the
recovery of the p-conjugated framework of the CDs.24 The HI
(4)
(5)
(6)
Iodine incorporation into the sp2 carbon framework alters
the electron donor property. Due to the differences in size and
Fig. 3 SEM images of (a) RGO, (b) RGO/CDs and (c) HI-RGO/CDs. The scale bar is 2 mm; TEM images of (d) RGO and (e) HI-RGO/CDs. The inset in
(e) shows the HRTEM image of CDs in HI-RGO/CDs.
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Table 1 Elemental analysis and specific surface area of three investigated samples
Specic surface
Pore volume
Samples
C (wt%)
O (wt%)
I (wt%)
ID/IG
area (m2 gꢀ1
)
(cm3 gꢀ1
)
RGO
RGO/CDs
HI-RGO/CDs
89.4
84.7
82.7
10.6
15.3
12.6
—
—
4.7
0.94
1.00
1.11
191
400
250
0.96
2.84
0.75
vapor treatment poses a rise in iodine proportion and a decline micrographs shown in Fig. 3. The physisorption isotherms in
in both carbon and oxygen contents (Table 1). The iodine Fig. 4a show type IV isotherms with a hysteresis loop indicating
doping is not only benecial for the electronic conductivity of that these porous samples are mainly composed of mesopores.
RGO/CD hybrid but also provides higher pseudocapacitance as The corresponding pore-size-distribution plots (Fig. 4b) exhibit
well as polarizes the adjacent carbon atoms for O2 adsorption, that both RGO/CDs and RGO consist of micro/mesopore radii
favoring the ORR by weakening the O–O bonding between 0.8 and 18 nm while HI-RGO/CDs has micro/mesopore
strength.24,27,28,37,38
radii narrowing down between 1.1 and 5.8 nm. Such micro/
The porosity of the materials was investigated by N2 phys- mesoporous structure is conducive to rapid ion penetration
isorption isotherms and the results are illustrated in Fig. 4 and and fast electron transfer, enhancing the electrochemical
listed in Table 1. The CD combination has a remarkable inu- characteristics.
ence on the structure of RGO network with its specic surface
XRD analysis was used to characterize the graphitic structure
area increasing from 191 to 400 m2 gꢀ1 and pore volume rising of the materials. A typical broad peak at 2q ¼ 9.99ꢁ was observed
from 0.96 to 2.84 cm3 gꢀ1. Nevertheless, aer HI acid treatment, for GO, corresponding to the (002) diffraction39 (Fig. 5a).
HI-RGO/CDs display a decrease in these values to 250 m2 gꢀ1 However, upon successful hydrothermal process, the (002) peak
and 0.75 cm3 gꢀ1. These results are in accordance with the SEM showed a dramatic shi to higher position at 2q ¼ 24.30ꢁ in
˚
RGO with an interlayer spacing of 3.66 A, which is much lower
˚
than that of GO (8.86 A), demonstrating an effective reduction
of GO via the hydrothermal process. In addition, it further
shied to higher position at 25.14ꢁ in RGO/CDs with the
˚
˚
d spacing of 3.54 A, close to that of graphite (3.34 A), demon-
strating the benets of CD decoration for the restoration of p-
conjugated graphitic framework of RGO.14,19 Notably, aer
exposure to HI acid vapor, no obvious change was observed in
the (002) peak and interlayer spacing of HI-RGO/CDs, which
implies that the restacking/aggregation of RGO sheets in RGO/
CDs was effectively inhibited during the reduction procedure by
HI acid treatment.40
Fig. 5b shows the Raman spectra of the materials with a D
band at ꢂ1340 and a G band at ꢂ1587 cmꢀ1, respectively. The D
to G intensity ratio (ID/IG) reects the structural defects/
disorders of the carbon-based samples.41,42 The ID/IG increased
from 0.94 in RGO to a peak of 1.11 in HI-RGO/CDs, which could
be attributed to larger number of surface/edge disorders from
O/I heteroatoms.24,26,37,40 The Raman spectrum of RGO shows
two broad peaks at ꢂ2690 and 2910 cmꢀ1, corresponding to the
2D and 2G bands, respectively. The intensities of these bands
decreased signicantly for the RGO/CDs, indicating that RGO
consists of a substantial amount of graphite-like multilayer
graphene sheets because of aggregation41 compared with RGO/
CD composite. Insignicant variance in the intensities of 2D
and 2G peaks was observed aer HI acid treatment. These
results authenticated that CD decoration not only hinders gra-
phene restacking/aggregation, enlarging surface area, but also
generates more surface/edge defects. Notably, no peak of either
ꢀ
ꢀ
I3 (117 cmꢀ1) or I5 (165 cmꢀ1) polyiodide is detected in the
Raman spectrum of HI-RGO/CDs.25 Besides, the intensities of
C–O (1561 cmꢀ1) and C–O–C (1198 cmꢀ1) stretching peaks in
the FTIR spectrum become more pronounced in the RGO/CD
Fig. 4 (a) Nitrogen adsorption–desorption isotherms, and (b) pore
size distributions of three investigated samples.
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Fig. 5 (a) XRD patterns and (b) Raman spectra of three investigated samples. High resolution XPS C 1s (c) and I 3d (b) spectra HI-RGO/CDs.
composite due to the oxygen functional groups of CDs, yet network and form I2.25 Therefore, the XRD, Raman, FTIR and
weaken aer HI vapor treatment (Fig. S6, ESI†),24,26 demon- XPS results attest that RGO was effectively reduced to graphene
strating an efficient reduction of O-containing groups on RGO/ and I-doped graphene/CD composite was successfully
CD by hydriodic acid. A similar phenomenon was observed in fabricated.
HI-RGO (Fig. S7 and S8, ESI†). Interestingly, there is no
detectable peak of hydroxyl groups in these samples, revealing
an effective reduction of GO into RGO by hydrothermal
treatment.
3.2. Electrochemical analyses
The electrochemical behavior of the samples was rst exam-
ined by means of CV, GCD and EIS techniques in 1 M H2SO4
medium adopting a 3-electrode system, and the results are
plotted in Fig. 6. The CV plots of RGO and RGO/CDs (Fig. 6a
and S9, ESI†) exhibit reversible redox humps between 0.1 and
0.6 V, which derive from the pseudo-capacitive contribution
from oxygen-containing groups,10,19 yet these peaks are more
noticeable in the case of HI-RGO/CDs, which is ascribed to the
pseudo-capacitance caused by oxygen containing groups and
doped iodine heteroatoms.24,37 In addition, nearly symmetric
triangular shapes of their GCD curves without any obvious iR
The surface elemental composition of HI-RGO/CDs was
further analyzed via XPS. The high resolution C1s spectrum
(Fig. 5c) can be tted into four components arising from C]C/
C–C (284.6 eV, 41.6%), C–I (285.5 eV, 46.0%), C]O (288.6 eV,
6.1%) and O–C]O (290.5 eV, 6.3%).25 The existence of C–I
bond indicates that iodine atoms are doped into the carbon
framework. Besides, the C/O atomic ratio of HI-RC33-10 is 5.6,
implying an efficient reduction of GO by hydrothermal process
and restoration of p-conjugated graphitic carbon due to the
exposure to HI vapor. Fig. 5d depicts the chemical states of I
3d5/2 spectra, which have been deconvoluted into two peaks at
620.4 and 618.6 eV, corresponding to neutral iodine (I2, 71.4%)
and iodide anion (Iꢀ, 28.6%),37 respectively, which indicates
that a large proportion of aromatic iodides was further
donated their electrons to recover the p conjugated graphene
drop indicate
a high efficiency supercapacitor behavior
(Fig. 6b and S10, ESI†). Their specic capacitances estimated
from the corresponding CV and GCD plots are shown in Fig. 6c
and d. The HI-RGO/CDs has the highest specic capacitance of
432 F gꢀ1 at the scan rate of 2 mV sꢀ1 compared to those of
RGO/CDs (248 F gꢀ1) and RGO (139 F gꢀ1). Notably, with an
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Fig. 6 (a) CV curves at 50 mV sꢀ1 in 1 M H2SO4; (b) GCD curves at 1 A gꢀ1 of three samples in 1 M H2SO4; specific capacitances estimated from (c)
CV and (d) GCD curves.
increase in the scan rate to 1000 mV sꢀ1, RGO/CD and HI-RGO/ a potential window between ꢀ0.1 and 0.9 V (vs. Ag/AgCl) in 1 M
CD composites retain ꢂ70% of their capacitances while RGO Na2SO4. The electrode still delivers specic capacitances of
capacitance retention declines to about 50%, which might 434 F gꢀ1 at 2 mV sꢀ1 and 436 F gꢀ1 at 0.2 A gꢀ1, respectively
come from the higher electrical conductivity and ionic (Fig. S15†). Note that the reduced rate capability is assigned to
migration due to both oxygen containing groups and iodine the restacked graphene at a high loading which triggers low
dopants.24,37 The enhanced capacitance induced by iodine electrochemical performance at high rates.10 The performance
heteroatoms was further authenticated by the electrochemical of HI-RGO/CD hybrid is comparable to recently reported
performance of HI-RGO (Fig. S11, ESI†), which delivers doped graphene based supercapacitors (Table S1, ESI†).
a maximum specic capacitance of 199 F gꢀ1 at 2 mV sꢀ1 and
Following that, the HI-RGO/CD hybrid was subjected to
still maintains 115 F gꢀ1 at 1000 mV sꢀ1, retaining 58% of its galvanostatic charge/discharge cycles at a current density of
initial capacitance. However, the electrochemical performance 10 A gꢀ1 (Fig. 7a). Aer 10 000 GCD cycles, the hybrid maintains
of HI-RGO is much lower than that of HI-RGO/CDs, demon- 94.6% its initial capacitance, demonstrating a superior dura-
strating the signicance of the CD-RGO sheet for the improved bility of the HI-RGO/CDs. SEM and EDX were employed to
electrochemical behavior. We also investigated the electro- characterize the HI-RGO/CD aer 10 000 GCD cycles. Insigni-
chemical properties of these materials in 1 M KOH electrolyte cant changes in the morphology with C, O and I distributing
and a similar tendency in the CV and GCD plots was observed uniformly (Fig. S16, ESI†) indicates excellent preservation in the
(Fig. S12–S14, ESI†). For the preparation of supercapacitor structure and morphology of the HI-RGO/CD electrode.
electrode with high loadings, the ink was obtained by ultra-
EIS characteristics was used to elucidate the impact of CD
sonicating 5 wt% Naon and HI-RGO/CDs in IPA to prepare addition and HI acid treatment on the electrochemical perfor-
15 mg mLꢀ1 slurry. Subsequently, the resulting suspension mance of the material. The Nyquist plots of three samples are
was dropped on a nickel foam (0.5 cm ꢃ 0.5 cm) with the total presented in Fig. 7b and c. The real axis intercept at the high
mass of 2 mg or an areal mass loading of 8 mg cmꢀ2, followed frequency region indicates the solution resistance (Rs) while the
by drying in air. The electrochemical tests were conducted in diameter of the semicircle at middle frequency region yields
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Fig. 7 (a) Cycling stability of HI-RGO/CDs at 10 A gꢀ1 over 10 000 GCD cycles (inset depicts the last ten GCD plots for 10 000 cycles); (b and c)
Nyquist plots and (d) Z0 versus uꢀ1/2 of various electrodes; (e) the relationship between gravimetric capacitance and inverse square root of scan
rate of various electrodes; (f) differentiation of surface/diffusion-controlled ratio at various scan rates of HI-RGO/CDs.
charge transfer resistance (Rct) at the electrode/electrolyte
The diffusion coefficients were estimated from the slope of
interface.18,25 Minimal changes were observed in Rs (ꢂ5.8 U) the diagram between Re(Z) and uꢀ1/2 (Fig. 7d), and summarized
and time constant so (inverse of frequency at 45ꢁ in Bode phase in Table 2. A continuous increase in the proton diffusion coef-
plots) (Fig. S17, ESI†) of the three investigated samples. The cient from 1.2 ꢃ 10ꢀ18 cm2 sꢀ1 for RGO to the peak of 85.5 ꢃ
values were in the range of 0.10–0.26 s, implying their similar 10ꢀ18 cm2 sꢀ1 for HI-RGO/CDs demonstrates that CDs and I-
internal resistance and wettability.43,44 The Rct values (Table 2) doping are conducive to fast diffusing electrolyte ions to the
strongly support the aforementioned results. RGO has a higher electrode, which results in excellent capacitance retention for
Rct of 13.8 U compared to RGO/CD electrode, which is due to RGO/CD and HI-RGO/CD samples compared to RGO. Similarly,
aggregated/stacked graphene layers in RGO, inducing its poor the DH value of HI-RGO is 12.6 ꢃ 10ꢀ18 cm2 sꢀ1, exceeding that
+
conductivity.9,10,43 Charge transfer resistance of HI-RGO/CD of pure RGO (Table S2, ESI†). The diffusion coefficients are in
sample is lower than that of RGO/CD, indicating that HI deox- accordance with the aforementioned electrochemical results,
ygenation is conducive to the ionic conductivity at the revealing that the enhanced electrochemical characteristics of
electrolyte/electrode interphase.
The H+ diffusion coefficients (DH ) of the electrodes were the surface of the material.
calculated by: To explore the mechanism of charge storage in the hybrids,
the HI-RGO/CD hybrid are due to the quick ionic diffusion to
+
their corresponding CV sweep curves were analyzed to quanti-
tatively differentiate the contribution of surface-controlled and
pseudo-capacitive reactions (Fig. S9, ESI†) as follows:14,25,46
2
2
2
+
DH ¼ 1/2[RT/Sn F Csw]
(7)
where R is the universal gas constant, T is absolute temperature,
C is H+ concentration, F is Faraday constant, S is electrode surface
ꢀ
ꢁ
q*ðnÞ ¼ q*s þ qd* ¼ q*ðn/NÞ þ const nꢀ1=2
(8)
area, n is electron transfer number, and sw is Warburg prefactor.
where q*(n) is the total charge, q*s is the capacitive contribution,
q*d is the pseudo-capacitive contribution and n is the scan rate.
The surface-controlled contribution and pseudo-capacity are
computed from the slopes of the diagrams between the gravi-
metric capacitance and nꢀ1/2 (Fig. 7e), and the results are pre-
sented in Table S3.† Both surface-controlled and diffusion-
controlled contributions increase from RGO to those for RGO/
CDs and HI-RGO/CDs. The surface capacitance of HI-RGO/
CDs which accounts for 78.1% of the total capacitance at
2 mV sꢀ1 increases to 98.6% at 1000 mV sꢀ1 (Fig. 7f), indicating
a surface-dominant process.
The sw value was determined from Re(Z) ꢂ sw(2pf)ꢀ1/2
.
25,45
+
Table 2 Rct Warburg prefactor and DH values of electrodes
RGO
RGO/CDs
HI-RGO/CDs
Rct (U)
13.8
861.6
1.2
7.9
213.8
20.1
2.3
103.7
85.5
sw (U sꢀ1/2
)
DH (ꢃ10ꢀ18 cm2 sꢀ1
)
+
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The ORR electrocatalytic activity of the samples was inves- of a reduced diffusion length of O2 diffusion to the catalyst
tigated in a 0.1 M KOH electrolyte. The CV curves of HI-RGO/ surface.47,48 The electron transferred number per O2 involved in
CDs in Fig. 8a show an obvious peak at 0.84 V vs. RHE in the ORR was determined by Koutecky–Levich (K–L) equations:47,48
O2-saturated 0.1 M KOH electrolyte whereas no peak is observed
1
1
1
1
1
¼
þ
¼ þ
jk Bu1=2
(9)
in N2-saturated KOH solution, demonstrating its ORR catalytic
activity. Next, LSVs curves recorded at an electrode rotation rate
of 1600 rpm and a scan rate of 10 mV sꢀ1 in O2-saturated media
to evaluate the effect of CD and I doping on ORR activities with
the reference Pt/C electrocatalyst (Fig. 8b). The onset potential
and the half-wave potentials increase from 0.80 and 0.65 V
(RGO) to 0.85 and 0.69 V (RGO/CDs) and 0.93 and 0.77 V (HI-
RGO/CDs), respectively. Moreover, the diffusion-limited
j
jk jL
B ¼ 0.2nFC0D02/3nꢀ1/6
(10)
where j is the experimentally measured current density, jk is the
kinetic current density, jL is diffusion-limiting current density, B
is the Levich constant, u is the electrode rotating speed in rpm,
n is the electron transferred number, F is the Faraday constant
(96 485 C molꢀ1), C0 is the oxygen bulk concentration (1.2 ꢃ
10ꢀ3 mol Lꢀ1), D0 is the oxygen diffusion coefficient in 0.1 M
KOH electrolyte (1.9 ꢃ 10ꢀ5 cm2 sꢀ1), and y is the electrolyte
kinematic viscosity (0.01 cm2 sꢀ1).
Aer calculating the slopes of the K–L plots of various elec-
trodes at potential of 0.1–0.6 V vs. RHE (Fig. S19, ESI†), we
computed the electron transferred number n and the results are
plotted in Fig. 8d. The average numbers of electron transfer (n)
current density (jL) of HI-RGO/CD hybrid is 4.62 mA cmꢀ2
,
which is superior to those for RGO (3.53 mA cmꢀ2) and RGO/
CDs (4.04 mA cmꢀ2). Thus, the ORR activity of the HI-RGO/CD
composite is close to that of the benchmark Pt/C (onset
potential and jL of 0.95 V and 5.69 mA cmꢀ2, respectively).
The ORR kinetics of the materials were further investigated
via its LSV curves at various rotation rates ranging between 400
and 2500 rpm. Fig. 8c and S18 (ESI†) show that the jL values
signicantly increase with a rise in rotation frequency because
Fig. 8 (a) CV curves of HI-RGO/CDs at 10 mV sꢀ1 in N2 and O2-saturated 0.1 M KOH. (b) LSV curves of materials and commercial Pt/C at
1600 rpm and 10 mV sꢀ1 in O2-saturated 0.1 M KOH. (c) LSV curves of HI-RGO/CDs at different electrode rotation rates in O2-saturated 0.1 M
KOH. (d) Electron transfer numbers as a function of the potential.
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for RGO and RGO/CDs are calculated to be 2.79 and 3.25, detect the generated H2O2. The number of electron transfer (n)
respectively while the n value for HI-RGO/CDs is 3.57, which is was calculated by:
close to 4.08 of the commercial Pt/C, suggesting that the HI-
4id
n ¼
(11)
RGO/CDs enable a near four-electron pathway. Moreover, the
calculated jk value of HI-RGO/CDs is 19.8 mA cmꢀ2 at 0.1 V vs.
RHE, larger than those of RGO (12.4 mA cmꢀ2) and RGO/CDs
(12.0 mA cmꢀ2). The superior ORR activity of HI-RGO/CDs
may be ascribed to the enhanced oxygen adsorption caused by
I-doping28 and synergistic effect between 3D RGO network and
well-dispersed CDs. The improved electrocatalytic activity due
to I doping is further substantiated in HI-RGO (Fig. S20, ESI†),
which exhibits an onset potential of 0.83 V, a half-wave potential
of 0.68 V, jL and jk values of 3.54 and 10.6 mA cmꢀ2, respectively
with an average n of 3.05. However, the ORR activity of HI-RGO
is far lower than that of HI-RGO/CDs, indicating the signi-
cance of the combination of CDs with RGO for the improved
ORR catalytic performance. The ORR activity of the HI-RGO/CDs
is comparable to that for the recently reported metal-free carbon
based ORR electrocatalysts (Table S4, ESI†).
ir
id þ
N
The H2O2 yield was calculated as follows:
200ir
ꢀ
ꢂ
ꢃ
% HO2
¼
(12)
ir
N
id þ
N
where id is the disk current, ir is the ring current; and N is the
current collection efficiency of the Pt ring and is 0.38.
The decent ORR electrocatalytic activity of HI-RGO/CDs
catalyst is further highlighted by the RRDE test, which indi-
cated close to four electron transfer number (3.56–3.80) and low
H2O2 yield (10.6–20.0%) at 0.0–0.86 V (vs. RHE) potential range
(Fig. 9a and b). These results are in agreement with the values
obtained from the K–L plots. We further enhanced the ORR
stability of catalysts by their crossover tolerance to methanol.
Fig. S21 (ESI†) shows that HI-RGO/CDs exhibits an improved
crossover tolerance to methanol than the benchmark Pt/C
electrode.
The rotating ring-disk electrode (RRDE) test was conducted
with a Pt ring surrounded by a 5.5 mm diameter glassy carbon
electrode. The Pt ring electrode was set at 1.3 V (vs. RHE) to
Fig. 9 (a) RRDE voltammograms of HI-RGO/CDs catalyst in O2-saturated 0.1 M KOH at 1600 rpm, (b) number of electron transfer (n) and H2O2
yield derived from the RRDE measurements. (c) Current transient of HI-RGO/CDs and Pt/C at 0.65 V vs. RHE for 22 h. (d) LSV curves of HI-RGO/
CDs and Pt/C before and after the electrochemical durability test at 0.65 V vs. RHE for 22 h.
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The durability of responses from HI-RGO/CDs and the an average electron transfer number of 3.57. The I-doped
benchmark Pt/C were assessed by chronoamperometric graphene/CD composite shows excellent ORR stability with
measurements at 0.65 V vs. RHE at 1600 rpm for 22 h in O2- current density retention of 97.7% under the electrochemical
saturated 0.1 KOH. Fig. 9c illustrates that the current density of stress (0.65 V vs. RHE) in comparison with 43.4% obtained for
commercial Pt/C drops to ꢂ43.4% whereas HI-RGO/CDs the benchmark Pt/C under the same conditions. Thus, a low-
demonstrate outstanding stability with current density reten- cost sustainable electrode material was developed, based on
tion of 97.7%, indicating its superior durability under electro- agricultural waste material for both supercapacitor and ORR
chemical stress (0.65 V vs. RHE). Aer the ORR durability test, applications.
the LSV polarization of HI-RGO/CDs remains almost unchanged
while the Pt/C reveals a downshi of 50 and 100 mV in onset
potential and the value of E1/2, respectively (Fig. 9d), suggesting
Conflicts of interest
the excellent relative durability of HI-RGO/CDs. Furthermore,
the HI-RGO/CDs aer 22 h treatment was also characterized by
There are no conicts of interest to declare.
SEM and EDX. Minor change is observed in the catalyst
morphology with C, O and I distributing uniformly in the hybrid Acknowledgements
(Fig. S22, ESI†), demonstrating excellent preservation in the
V. C. Hoang is grateful to the University of Sydney for an
structure and morphology of the HI-RGO/CD electrode.
International Postgraduate Research Scholarship. We also wish
To explore the possible role of CD and I-doping on the ORR
to acknowledge the experimental assistance from Dr Shengli
catalytic behavior of the materials, we measured the ECSA of the
Zhai at Nanyang Technological University, Singapore, Dr Xuncai
catalysts from their electrical double layer capacitance (Cdl),
Chen and Mr Long H. Nguyen at The University of Sydney and
the ACMM (Australian Centre for Microscopy and Microanal-
ysis) for access to instruments for morphological measure-
ments and Sydney Analytical, the University of Sydney.
recorded in the non-faradaic range of ꢀ0.1 to 0 V (vs. Ag/AgCl) at
various scan rates from 5 to 100 mV sꢀ1 (Fig. S23a–d, ESI†). Aer
normalized by carbon's specic capacitance, the ECSA results
are depicted in Fig. S23e,† which agree well with the ORR
catalytic activities. The RGO/CD hybrid has an ECSA of 345 cm2
gꢀ1, higher than that of RGO (130 cm2 gꢀ1), which is due to the
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activity of the materials is related to the alterations in their
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