Siddheshwar D Raut1, Nanasaheb M Shinde2, Yogesh T Nakate3, Balaji G Ghule1, Shyam K Gore4, Shoyebmohamad F Shaikh5, James J Pak2, Abdullah M Al-Enizi5, Rajaram S Mane1. 1. School of Physical Sciences, Swami Ramanand Teerth Marathwada University, Nanded 431501, Maharashtra, India. 2. School of Electrical Engineering, Korea University, Seoul 02841, Republic of Korea. 3. Department of Electronics, Kavayitri Bahinabai Chaudhari North Maharashtra University, Jalgaon 425001, Maharashtra, India. 4. Dnyanopasak Shikshan Mandal's Arts, Commerce and Science College, Jintur 431509, India. 5. Department of Chemistry, College of Science, King Saud University, Riyadh 11451, Saudi Arabia.
Abstract
The organic and eco-friendly materials are extended to prevail over the worldwide energy crisis where bio-inspired carbonaceous electrode materials are being prepared from biogenic items and wastes. Here, coconut water is sprayed over three-dimensional (3D) nickel foam for obtaining a carbonaceous electrode material, i.e., C@Ni-F. The as-prepared C@Ni-F electrode has been used for structural elucidation and morphology evolution studies. Field emission scanning electron microscopy analysis confirms the vertically grown nanosheets of the C@Ni-F electrode, which is further employed in the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), where excellent OER and HER performances with small overpotentials of 219 and 122 mV and with stumpy Tafel slopes, i.e., 27 and 53 mV dec-1, are respectively obtained, suggesting a bifunctional potential of the sprayed electrode material. Moreover, sustainable bifunctional performance of C@Ni-F proves considerable chemical stability and moderate mechanical robustness against long-term operation, suggesting that, in addition to being a healthy drink to mankind, coconut water can also be used for water splitting applications.
The organin class="Chemical">c anpan>d epan> class="Chemical">co-friendly materials are extended to prevail over the worldwide energy crisis where bio-inspired carbonaceous electrode materials are being prepared from biogenic items and wastes. Here, coconut water is sprayed over three-dimensional (3D) nickel foam for obtaining a carbonaceous electrode material, i.e., C@Ni-F. The as-prepared C@Ni-F electrode has been used for structural elucidation and morphology evolution studies. Field emission scanning electron microscopy analysis confirms the vertically grown nanosheets of the C@Ni-F electrode, which is further employed in the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), where excellent OER and HER performances with small overpotentials of 219 and 122 mV and with stumpy Tafel slopes, i.e., 27 and 53 mV dec-1, are respectively obtained, suggesting a bifunctional potential of the sprayed electrode material. Moreover, sustainable bifunctional performance of C@Ni-F proves considerable chemical stability and moderate mechanical robustness against long-term operation, suggesting that, in addition to being a healthy drink to mankind, coconut watercan also be used for water splitting applications.
Due to escalapan> class="Chemical">ting demand
of the clean and renewable energy caused
by backdrop calamity and environmental pollution, the development
of promising energy storage and conversion devices, i.e., catalysts,
became obligatory.[1−3] Hydrogen is a promising option for clean energy,
which can be produced by splitting water either in an electrocatalyzer
or by photocatalysis. Electrochemical water splitting involves the
hydrogen evolution reaction (HER) and oxygen evolution reaction (OER).
Both reactions require a proper catalyst to boost the reaction kinetics.[4−6] So far, precious metals (Pt, Ru, and Pd) and noble metal oxides
(IrO2) showed the best performance toward both OER and
HER. However, owing to the high cost and scarcity, they are obstructed.[7] Researchers have made several efforts to develop
nonprecious metal-based catalysts for water splitting such as from
sulfides,[8] selenides,[9] oxides,[10] etc. Among the known
catalysts, carbon/carbonaceous electrode materials have demonstrated
an excellent chemical and mechanical stability and porosity, high
surface area, fine-controlled pore coordination, i.e., pore-size distribution
and connectivity, good conductivity, low cost, and stability.[11] A variety of carbon materials with diverse morphologies
and functionalities such as carbon nanotubes (CNTs),[12] fullerenes,[13] and graphene[14] were prepared by several physical and chemical
methods. Most of the carbon/carbonaceous nanomaterials are reliant
on fossil-based precursors, viz., pitch, coal, phenol, etc., which
are expensive and detrimental to environmental health. Biomass, a
rich, environmentally friendly, low-cost, renewable, novel, and abundant
resource in nature, is an excellent source to prepare carbonaceous
materials for scientific and practical benefits, reducing the environmental
pollution greatly.[15] The animal- and plant-based
materials are usually considered as biomass, which can be derived
from nature.[16] Coconut is a preeminent
biomass resource to prepare biocarbon as every part of coconut can
be used to prepare biocarbon. Many researchers have prepared biocarbon
from various parts of coconut, viz., shell,[17−19] leaves,[20] husk waste,[21] kernel,[22] etc. The biocarbon prepared using such biomass
resources offers (i) relatively low internal resistance, (ii) superior
electrical conductivity, (iii) outstanding cyclic stability, (iv)
admirable chemical stability, and (v) excellent reaction kinetics.[17−22] Numerous synthesis techniques, i.e., hydrothermal technique, chemical
bath deposition (CBD), electrodeposition, successive ionic layer adsorption
and reaction (SILAR), and spray pyrolysis, were reported to prepare
various biocarbon nanostructures in the form of a film/powder. Among
them, spraying a direct solution over a hot substrate for obtaining
a film-type electrode is a cost-effective, safe, and uncomplicated
synthesis technique to operate, by which the structure, phase, and
morphology can be easily controlled by regulating various parameters
such as, spray rate, operating temperature, flow rate, deposition
time, etc.[23,24]
Prabu et al. reported the
catalytipan> class="Chemical">c activity of activated carbon
sheets prepared from bakery food waste toward OER and HER. The activated
carbon-sheet electrode demonstrated both OER and HER activities with
low overpotentials of 340 mV (at 10 mA cm–2) and
380 mV (at −10 mA cm–2), with corresponding
Tafel slopes of 43 and 85 mV dec–1.[25] Sathiskumar et al. prepared nitrogen-doped porous carbon
(N-PC) from biomass, i.e., golden shower pod biomass (GSB), via a
solvent-free strategy, which was further tested for electrolysis applications.
The as-prepared N-PC electrode revealed admirable performance toward
OER and HER with small overpotentials of 314 and 179 mV @ 10 mA cm–2 and low Tafel slopes of 132 and 98 mV dec–1, respectively, in KOH electrolyte solution.[26] The catalytic performance of the bio-based carbon electrode can
be determined through a three-dimensional porous structure, oxygen
vacancies, and carbon defects. However, with such promising performances,
still there is a lot of scope for the pioneering biogenic electrode
materials with enhanced catalytic activities.
Herein, in n class="Chemical">conpan> class="Chemical">tinuation
of our work[27] reporting a cost-effective,
earth-abundant, and bio-inspired carbonaceous
noble-metal-free electrode for clean and renewable energy, we have
reported the use of sprayed coconut water over three-dimensional (3D)
nickel foam, i.e., C@Ni-F, toward both OER and HER. The results obtained
in this study were compared with those reported for the carbonaceous
electrode derived from various biomasses for better understanding.
Instead of other coconut parts, here, we have used sprayed coconut
water for the first time for performing OER and HER.
Material Characterizations
The stn class="Chemical">rupan> class="Chemical">ctural elucidation and morphological evolution studies
of the C@Ni-F electrode were attempted using X-ray diffraction (XRD,
D8-Discovery Bruker, 40 kV, 40 mA, Cu Kα, λ = 1.5406 Å)
and field emission scanning electron microscopy (FE-SEM, Hitachi,
S-4800, 15 kV) with energy-dispersive X-ray spectroscopy (EDS) images,
respectively. The surface chemical composition and the oxidation conditions
were confirmed from X-ray photoelectron spectroscopy (XPS, VG Scientifics
ESCALAB250) spectra. The functional groups present over the electrode
surface were identified using a Fourier transform infrared spectroscopy
(FTIR) plot. The Raman spectrophotometry (Xper Ram 200, Nano Base,
South Korea) measurement was carried out to confirm the presence of
the vibrational modes.
Results and Discussion
Surface Appearance, Chemical
Configuration Analysis, and Structural
Elucidation
The n class="Chemical">FE-SEM image with EDS surfapan> class="Chemical">ce elemental composition
is shown in Figure . The upright standing interconnected nanosheets were observed over
plane Ni-F (discussed later) (Figure a). These interwoven-type nanosheets were 300–900
nm in length and 20–30 nm in thickness. There were several
crevices between these nanosheets. The two side surfaces of these
nanosheets were open, which would help to improve the reaction active
area during the interaction with the electrolyte followed by proficient
charge transportation for better performance. Moreover, as evidenced,
bulky voids would defend the volume change, if any, while cycling.[29,30] The FE-SEM images of C@Ni-F and pristine Ni-F with different magnifications
are shown in Figure S1. The elemental mapping
images of as-prepared C@Ni-F are given in Figure b–f. The surface elemental composition
of the C@Ni-F electrode as shown in Figure g showed carbon (C) and oxygen (O) as the
major elements and phosphorous (P), potassium (K), and sodium (Na)
in insignificant quantity due to which these elements were not detected
during structural analysis. The FE-SEM image of the bare Ni-F as shown
in Figure h revealed
the presence of only Ni and O elements (Figure i,j). The presence of oxygencould be due
to a trace amount of environmental oxygen adsorbed over Ni-F.
Figure 1
(a) FE-SEM
image and (b–f) mapping images for C, O, P, K,
and Na elements; (g) elements and their proportions obtained from
the EDS spectrum of C@Ni-F; (h) FE-SEM image and (i, j) elemental
mapping images of Ni and O; and (k) elemental composition of pristine
Ni-F for knowing substrate specifications.
(a) n class="Chemical">FE-SEM
image anpan>d (b–f) mapping images for pan> class="Chemical">C, O, P, K,
and Na elements; (g) elements and their proportions obtained from
the EDS spectrum of C@Ni-F; (h) FE-SEM image and (i, j) elemental
mapping images of Ni and O; and (k) elemental composition of pristine
Ni-F for knowing substrate specifications.
The XRD pattern of the as-prepared n class="Chemical">C@pan> class="Chemical">Ni-F electrode is shown in Figure a, where two major
peaks at 24.15 and 43.16° corresponding to the (002) and (101)
reflection planes (JCPDS 41-1487) with interplanar spacings of 0.38
and 0.28 Å were, respectively, obtained, confirming the presence
of carbon as the majority entity in the electrode material. The absence
of peaks of other elements was due to their trace amount and low atomic
numbers, which was also observed in XPS and EDAX analyses. The chemical
structure and the oxidation conditions were obtained from the XPS
spectra recorded for P, C, K, O, and Na elements (Figure b–g). Figure c presents deconvoluted P 2p
peaks of P–C and P–O bonds at, respectively, 133.3 and
134.4 eV binding energies.[31−33] The deconvoluted C 1s spectrum
with four Gaussian curves as shown in Figure d revealed the sp2 carbon, sp3 carbon, C–O, and C=O bondings at 284.55, 285.51,
286.6, and 287.77 eV, respectively.[34−36] The as-observed peaks
were of hydroxyl (C=C/C–C) and epoxy groups (C–O),
suggesting the presence of oxygen-containing groups in the form of
epoxy and hydroxyl groups, which is in good agreement with the Lerf–Klinowski
model of carbonaceous electrodes.[37,38]Figure e shows the K 2p spectrum with
spin energy partition of 2.8 eV, where two major peaks are evidenced
at 292.7 and 295.5 eV for the K 2p3/2 and K 2p1/2 levels, respectively.[39] The peak observed
at 531.01 eV in O 1s spectra was for C–O–C[40] (Figure f). A single broad deconvoluted peak of Na 1s is also observed
in Figure g at 271.5
eV, suggesting the presence of Na+.[41]
Figure 2
(a) XRD pattern; (b) survey and enlarged spectra for (c) P 2p,
(d) C 1s, (e) K 2p, (f) O 1s, and (g) Na 1s; and (h) FTIR and (i)
Raman spectra of C@Ni-F.
(a) XRD pattern; (b) survey and enlarged spen class="Chemical">ctra for (pan> class="Chemical">c) P 2p,
(d) C 1s, (e) K 2p, (f) O 1s, and (g) Na 1s; and (h) FTIR and (i)
Raman spectra of C@Ni-F.
The existence of the
funpan> class="Chemical">ctional groups on the electrode surface
was confirmed using the FTIR spectrum (Figure h). The band observed at 1457.37 cm–2 was attributed to the plane deformation vibrations of the C–H
moiety in −CH3, −CH2–,
and −O–CH3.[42] The
peak at 1056.15 cm–2 was due to the saccharide structure
of cellulose and hemicelluloses, and the peak at 873.45 cm–2 was assigned to the bending vibrations of the aromaticcompounds.[43,44] The Raman spectrum as shown in Figure i confirmed two peaks at 1355.66 and 1582.85
cm–2 for D and G bands, respectively.[45] The observed D band indicated the disorder in
the carbon structure with A1g symmetry, while the G band
showed the C=C stretching vibration with E2g mode.
The ID/IG ratio
was found to be 0.86, providing information about the crystallite
dimension, plane defects, edge defects, and the nature of disorder
of the carbon derivative.[46] It is confirmed
that the presence of K, P, and Na in C@Ni-F could be responsible for
the enhancement in the formation of defects, which results in the
increasing intensity of the G band following incorporation of −OH
groups in the layered structures for better performance.[47,48]
Electrochemical Measurements
The electropan> class="Chemical">chemical properties
of the C@Ni-F electrode were studied using an IVIUM electrochemical
workstation by employing a three-electrode system: platinum as a counter
electrode, Ag/AgCl as a reference electrode, and deposited Ni-F as
a working electrode. The OER polarization curves were obtained in
a 1.0 M KOH electrolyte and compared with that of bare Ni-F for assessing
the performance of the sprayed carbonaceous electrode material only.
Oxygen and Hydrogen Evolution Study
The polarizationcurves of pan> class="Chemical">C@Ni-F showed a promising OER activity with a small overpotential
(η) of 219 mV, calculated by eq 2 in the Supporting Information, which is much inferior to the overpotential
of Ni-F, i.e., 330 mV (Figure a) at a current density of 10 mA cm–2. The
Tafel slopes (determined using eq 3 in the Supporting Information) of C@Ni-F and Ni-F electrodes, extracted from
the polarization curves, are shown in Figure b. The OER kinetics of the C@Ni-F electrode
can be understood using Tafel plots. The lower Tafel slope specifies
the good reaction kinetics. Herein, the C@Ni-F electrode presented
the smallest Tafel slope of 27 mV dec–1, while Ni-F
revealed 79 mV dec–1, which indicates the superior
reaction kinetics of the previous electrode toward OER over the latter
one.[49] The cyclic and chemical stability
of C@Ni-F was verified with continuous OER at a fixed potential and
is shown in Figure c. The as-prepared C@Ni-F electrode demonstrated excellent cyclic
and chemical stability. The C@Ni-F electrode revealed mostly stable
current density after 1000 cycles and 48 h for long-term cycling and
chemical stability (inset of Figure c). After 1000 cycles, a minute change of just 4 mV,
i.e., 98.20% retention, was observed in the overpotential of the C@Ni-F
electrode. The OER activity of the as-obtained C@Ni-F electrode was
comparable to the performance of previously reported carbonaceous
electrode materials, which is graphically shown in Figure d.
Figure 3
(a) OER polarization
curves, (b) Tafel plots, (c) cyclic stability
(inset shows chemical stability), and (d) comparison graph of OER
activity of C@Ni-F with former data (the red circle indicates the
overpotential and Tafel slope of the current work).
(a) OER polarizationcurves, (b) Tapan> class="Chemical">fel plots, (c) cyclic stability
(inset shows chemical stability), and (d) comparison graph of OER
activity of C@Ni-F with former data (the red circle indicates the
overpotential and Tafel slope of the current work).
The HER activity of the pan> class="Chemical">C@Ni-F electrode as shown in Figure was carried out
under analogous
conditions to OER. The polarization curve of C@Ni-F showed admirable
activity toward HER with petite overpotential of 122 mV, whereas Ni-F
revealed an overpotential of 220 mV at a current density of −10
mA cm–2 (Figure a). The corresponding Tafel slopes, an intrinsic property
of electrocatalysts, extracted from HER polarization curves of C@Ni-F
and pristine Ni-F are shown in Figure b. A smaller Tafel slope indicates a higher HER rate,
and a Tafel slope as small as 53 mV dec–1 signifies
that the HER obeys the Volmer–Heyrovsky mechanism on the electrode
surface.[50] The cyclic and chemical stability
of the C@Ni-F electrode is shown in Figure c. After 1000 cycling operations, the C@Ni-F
electrode showed an excellent stability (94.57% retention) with insignificant
variation in overpotential, showing almost stable performance even
after 48 h (inset of Figure c). The comparative performance of the C@Ni-F electrode toward
HER has been graphically represented in Figure d and is given in Table .[51−65]
Figure 4
(a)
HER polarization curves, (b) Tafel plots, (c) cyclic stability
(inset shows chemical stability), and (d) performance comparison of
the HER activity of C@Ni-F with survey data (the red circle indicates
the overpotential and Tafel slope of the current work).
Table 1
Comparison of Electrochemical Properties
of C@Ni-F with Carbonaceous Electrodes
η (mV)/Tafel slope (mV dec–1)
catalyst
electrolyte
OER
HER
J (mA cm–2)
ref
NMWN
1.0 M NaOH
320/68
340/68
10
(51)
Co@N-CNTF
1.0 M KOH
350/61.4
226/149.9
10
(52)
NCNs
1.0 M KOH/0.5 M H2SO4
410/142
90/43
10
(53)
ZIF-8-C6
0.1 M KOH/0.5 M H2SO4
528/91.9
155/54.7
10
(54)
PNC/Co
1.0 M KOH
370/76
270/131
10
(55)
NiFe@C
1.0 M KOH
274/57
195/111
10
(56)
Co@N-C
1.0 M KOH
400/–
200/100
10
(57)
Co@NC-G
1.0 M KOH
322/73.7
140/62
10
(58)
PO-Ni/Ni-N-CNFs
1.0 M KOH
420/113.10
262/97.42
10
(59)
Ni@C
1.0 M KOH
300/145
150/143
10
(60)
Ni3C
1.0 M KOH
275/62
292/41.3
10
(61)
Co/CNFs
1.0 M KOH
320/79
190/66
10
(62)
Fe3C-Co/NC
1.0 M KOH/0.5 M H2SO4
340/100
238/108.8
10
(63)
CoP/NCS
1.0 M KOH
254/57
71/109
10
(64)
CoP@PNC/C
1.0 M KOH
330/64
120/67
10
(65)
C@Ni-F
1.0 M KOH
219/27
122/53
10
present work
(a)
HER polarizationcurves, (b) Tapan> class="Chemical">fel plots, (c) cyclic stability
(inset shows chemical stability), and (d) performance comparison of
the HER activity of C@Ni-F with survey data (the red circle indicates
the overpotential and Tafel slope of the current work).
Conclusions
and Perspectives
In this work, we sun class="Chemical">cpan> class="Chemical">cessfully
prepared vertically grown C@Ni-F nanosheet electrodes using the facile
spray pyrolysis synthesis method over a Ni-F substrate. The XRD patterns
confirmed the formation of the C@Ni-F electrode, while the elements
present on the electrode surface were verified using XPS. The surface
bands and functional groups were confirmed by Raman and FTIR spectroscopies,
which confirmed the formation of C@Ni-F. The as-prepared C@Ni-F electrode
on employing in electrochemical measurements for OER and HER water
splitting applications showed lower overpotentials and smaller Tafel
slopes. The OER overpotential of the C@Ni-F electrode was 219 mV,
while for HER, it was 122 mV. The C@Ni-F electrode, with the small
overpotential toward both OER (27 mV dec–1) and
HER (53 mV dec–1), showed good reaction kinetics.
This admirable performance of the C@Ni-F electrode is attributing
to a high surface area, upright standing platelet surface morphology
that facilitates east and easy charge transfer processing, and availability
of bounteous active sites sowing to interconnected porous nanosheets
network. A C@Ni-F electrode was obtained at one side of the substrate
surface only, forming a bisprayed surface with metal oxides, polymers,
and carbonaceous materials on the other side would open a new research
avenue in energy storage and water splitting devices for manufacturing
various technologies.
Experimental Section
Materials
Fresh
young n class="Chemical">copan> class="Chemical">conuts from Partur, Maharashtra,
India, and deionized (DI) water and Ni-F-110 with 320 g m–2 mass density from Artenano Company Limited, Hong Kong, were purchased
and used after cleaning.
Spraying of Coconut Water
The n class="Chemical">copan> class="Chemical">conut-water-mediated
carbonaceous electrode was prepared using the spray pyrolysis method
with the protocol reported earlier.[28] Coconut
water was collected from fresh kernel-free green coconut, which was
further filtered using Whatman filter paper to avoid any solid impurity.
Ni-F (1 cm × 4 cm × 0.1 mm) was used as a substrate, which
was cleaned by 1.0 M HCL solution, DI water, and ethanol for 15 min
each with ultrasonication to avoid impurity additives. Afterward,
the filtered coconut water was poured into the solution dispenser
of the spray pyrolysis instrument (HOLMARC HO-TH-04, India), which
was connected to and operated by a personal computer. A cleaned 3D
Ni-F substrate was kept on a hot plate by adjusting the X–Y coordinates appropriately. The temperature
of the hot plate was optimized to 150 °C using a controller.
Furthermore, spraying was n class="Chemical">condupan> class="Chemical">cted with a flow rate of 2 mL min–1 and air-flow rate of 15 L min–1 in a closed chamber for 14 min. After deposition, the substrate
temperature was allowed to return to room temperature naturally to
avoid the quenching effect. Finally, the prepared deposited electrode
Ni-F was annealed at 400 °C before applying for further electrochemical
measurements. The schematic of the experimental setup used to synthesize
C@Ni-F is shown in Scheme .
Scheme 1
Schematic of Obtaining a Carbon Film with a Platelet
Architecture
on Spraying Coconut Water on Ni-F
Authors: K S Novoselov; A K Geim; S V Morozov; D Jiang; Y Zhang; S V Dubonos; I V Grigorieva; A A Firsov Journal: Science Date: 2004-10-22 Impact factor: 47.728
Authors: Hong-Zhang Geng; Ki Kang Kim; Kang Pyo So; Young Sil Lee; Youngkyu Chang; Young Hee Lee Journal: J Am Chem Soc Date: 2007-05-31 Impact factor: 15.419
Figure 1
Figure 2
Figure 3
Figure 4
Scheme 1