Hierarchically Organized NiCo2O4 Microflowers Anchored on Multiwalled Carbon Nanotubes: Efficient Bifunctional Electrocatalysts for Oxygen and Hydrogen Evolution Reactions

Article abstract of DOI:10.1002/cplu.201900603

Hierarchical flower-like NiCo₂O₄ anchored on multiwalled carbon nanotubes (NiCo₂O₄-MCNTs) act as robust bifunctional electrocatalysts towards O₂ (OER) and H₂ (HER) evolution reactions. The NiCo₂O₄-MCNTs hybrid is synthesized in a facile surfactant-free hydrothermal process, and its flower-like morphology was shown to consist of 1D nanowires. The MCNT support is beneficial for enhanced reactive sites, efficient charge transfer, and durability of the composite. The material shows low overpotentials of 350 mV and 209 mV for OER and HER, respectively at 10 mA cm^?2. Such values are comparable to contemporary noble-metal catalysts and even lower than reported non-precious catalysts. Moreover, this hybrid shows high current density, lower Tafel slope, and excellent stability. The synergistic coupling effects between the MCNTs and NiCo₂O₄ are key factors for the superior catalytic activity of these hybrid materials.

Full text of DOI:10.1002/cplu.201900603

Accepted Article
Title: Hierarchically Organized NiCo2O4 Microflowers Anchored
on Multiwalled Carbon Nanotubes: Efficient Bifunctional
Electrocatalysts for Oxygen and Hydrogen Evolution Reactions
Authors: Umeshbabu Ediga, Hari Krishna Charan P, Justin Ponniah,
and Ranga Rao Gangavarapu
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: ChemPlusChem 10.1002/cplu.201900603
Link to VoR: http://dx.doi.org/10.1002/cplu.201900603
A Journal of
www.chempluschem.org

10.1002/cplu.201900603
ChemPlusChem
Hierarchically Organized NiCo2O4 Microflowers Anchored on
Multiwalled Carbon Nanotubes: Efficient Bifunctional
Electrocatalysts for Oxygen and Hydrogen Evolution Reactions
Ediga Umeshbabu,[a] P. Hari Krishna Charan,[b] Ponniah Justin[c] and G. Ranga Rao*[a]
Abstract: We have fabricated hierarchical flower-like NiCo2O4
anchored on multiwalled carbon nanotubes (NiCo2O4-MCNTs) as
robust bifunctional electrocatalyst towards O2 and H2 evolution
reactions in a facile surfactant free hydrothermal method. The results
show that hierarchical flower-like morphology of NiCo2O4-MCNTs are
assembled by 1D-nanowires. The supported MCNTs are beneficial for
enhanced reactive sites, efficient charge transfer and durability of the
composite. The NiCo2O4-MCNTs hybrid shows low overpotentials of
350 mV and 209 mV for OER and HER, respectively at 10 mA cm-2.
Such values are comparable to contemporary noble metal catalysts
and even smaller than reported non-precious catalysts. Moreover, this
hybrid shows high current density, lower Tafel slope and excellent
stability. The synergetic coupling effects between MCNTs and oxide
featured with flower-like morphology and improved microtexture
properties are key factors for the superior catalytic activity of NiCo2O4-
MCNTs hybrid.
poisoning and subsequently hindered the further hydrogen
utilization processes. On the other hand, electrochemical water
splitting reaction is one of the techniques that have been
proposed a decade ago to generate H2 and O2 from water via
oxygen evolution reaction at the anode and hydrogen evolution
reaction at the cathode, respectively.[2-4] Using this approach one
can effectively use hydrogen as an alternative fuel source
because it has high energy density. In other words, water splitting
reaction represents a way of storing energy in the form of H2 and
O2, which got attention due to its high energy efficiency, trivial
environmental pollution and other potentially wide range of
applications.[3-7] However, the overpotentials (η) greater than 1.80
V are required instead of ∼1.23 V theoretical value for both half-
cell reactions of water electrolysis (HER and OER) to achieve
expected reaction rates.[6,7] Moreover, both HER and OER of the
electrochemical water splitting are kinetically sluggish and thus
need catalysts to produce high purity hydrogen as economically
viable.
Introduction
The increasing consumption of fossil fuels gradually releases
greenhouse gases leading to climate change and global warming.
Besides polluting the environment, these non-renewable energy
resources are gradually being depleted.[1,2] One of the most
promising and sustainable approaches to prevail this issue is by
using alternative energy resources that are renewable. The
production of hydrogen from water-gas shift reaction is
contaminated with CO impurity, which may cause the catalyst
At present, effective electrochemical water oxidation at highly
reduced overpotentials is achievable only with noble metal based
catalysts such as RuO2/IrO2 for OER, and platinum or its alloys
for HER.[8,9] However, high-priced and scarcity of these noble
metals/metal oxides limit their widespread usage in practical
applications. Therefore, in the last few years, researchers have
made an enormous effort to downsize the noble metal content, to
tune the intrinsic activity and to increase the stability of the
catalytic component.[10-14] Altogether, a major challenge in this
field is to develop ideal bifunctional electrocatalyst with high
efficient, cost-effective and with a durable robust nature, which
can be integrated with fuel cell for the commercial application of
electric vehicles.
[a]
Dr. Ediga Umeshbabu, Prof. G. Ranga Rao
Department of Chemistry and DST-IITM Solar Energy Harnessing
Centre (DSEHC)
Indian Institute of Technology Madras
Chennai - 600036, India
Tel.: +91 44 2257 4226; Fax: +91 44 2257 0545.
E-mail: grrao@iitm.ac.in
To surmount the aforementioned bottlenecks in practical
applications several materials, for instance, metal oxides,[12,15]
hydroxides,[14,16] chalcogenides[13,17] and phosphides[18] were
largely scrutinized for electricity-driven water splitting catalysts.
Among the contenders, metal oxides particularly mixed transition
metal spinel oxides of type AB2O4 (A, B = 3d-transition metals)
are known to be highly active and efficient catalysts for water
oxidation and suggested as inexpensive and alternatives to
precious noble metal catalysts.[19-24] For instance, nickel cobaltite
[b]
[c]
Dr. P. Hari Krishna Charan
Department of Chemistry
Aditya Institute of Technology and Management
Tekkali - 532201, Andhra Pradesh, India
Dr. Ponniah Justin
Department of Chemistry
Rajiv Gandhi University of Knowledge Technologies
RK Valley, Kadapa - 516330, Andhra Pradesh, India
Supporting information: XRD pattern of hybrid precursor, Raman
spectra, and EDS mapping of composite and MCNTs are presented.
The comparison Table for OER activity of NiCo2O4-MCNTs catalyst with
literature reports, and HER activity of the bare GCE with Pt foil and
1
This article is protected by copyright. All rights reserved.

10.1002/cplu.201900603
ChemPlusChem
(NiCo2O4) stands out as a promising electrocatalyst for water
oxidation due to its overwhelming benefits such as abundant,
huge specific surface area, high conductivity, strong corrosion
resistance, low overpotential, more active sites and rich redox
chemistry originated from the co-existence of nickel and cobalt
ions.[25-27] As a result of multiple oxidation states in NiCo2O4 spinel
where cobalt reside in both voids (octahedral and tetrahedral)
while nickel reside only on tetrahedral voids, both redox couples
(Co3+/Co2+ and Ni3+/Ni2+) safeguard an enhanced electrocatalytic
activity. Moreover, the hybrid nanostructured materials fabricated
by hybridizing NiCo2O4 with conductive carbon supports (e.g., 2D
graphene and 1D carbon nanotubes) demonstrated an attractive
electrocatalytic activity and good stability compared to the solitary
NiCo2O4.[11,28-32] Such hybridization can empower adaptable and
appropriate properties with performances far beyond than those
of the individual materials.
be ideal candidates for a large-scale commercial production of
hydrogen and oxygen.
Herein we report porous hierarchical flower-like nickel cobalt
spinel anchored on conductive MCNTs (NiCo2O4-MCNTs) by a
facile surfactant free hydrothermal process followed by thermally
driven conversion treatment. The combined physicochemical and
electrochemical measurements were carried out on hybrid
NiCo2O4-MCNTs and pristine NiCo2O4 samples in order to check
the role of conductive MCNTs. The NiCo2O4-MCNTs composite
required an extremely small overpotentials of 350 and 209 mV,
respectively, for OER and HER to drive a benchmark current
density of 10 mA cm-2 which are strikingly lower than those of pure
NiCo2O4 and MCNTs. By taking the advantage of the improved
porosity, surface area and conductivity of nickel cobalt spinel in
hybrid, which are critical factors in improving mass transport and
number of active sites, and thereby promoting as the bifunctional
electrocatalyst for the overall water splitting reaction in an alkaline
electrolyte.
Till date, various NiCo2O4/carbon composites have been
developed by different methods and are effectively utilized for
OER applications.[32-36] For instance, surfactant-assisted NiCo2O4
nanoplatelets anchored on graphene exhibits good catalytic
activity in oxygen reduction and evolution reactions.[28] N-doped
graphene-NiCo2O4 hybrid thin film unveils a remarkable OER
activity with enhanced reaction kinetics and stability.[33] Further,
NiCo2O4 supported on graphene-MnO2 3D frame work prepared
by a self-assembly approach shows prominent activity and
stability in OER compared to NiCo2O4-graphene and benchmark
RuO2.[34] The NiCo2O4 nanosheet-MCNTs composite produced by
one-pot solution process using polyvinylpyrrolidone surfactant
demonstrates improved OER activity and stability compared to
NiCo2O4/activated carbon and NiCo2O4.[35] Of late, we have also
reported solvent dependent on surface morphology and textural
properties of the NiCo2O4/rGO hybrid composite for anodic OER
applications.[36] However, most of the literature pertinent to the
NiCo2O4 hybrids including NiCo2O4/MCNTs have focused mainly
on OER applications[37] and there are no reports on bifunctional
catalysts which are effective for both HER and OER catalysis in
the same electrolyte environment. Moreover, many reported
synthesis approaches of the hybrid composites in the literature
are complicated, expensive and time-consuming processes. It is,
therefore, a challenging and desirable to develop an efficient
bifunctional electrocatalyst for overall water oxidation. We have
taken up this problem to develop highly active and stable
bifunctional catalyst by simple and cost-effective hydrothermal
method without using structural directing agents such as
surfactants and/or chelating agents. Such a bifunctional catalyst
can be more active and stable for electrochemical water splitting
process in alkaline medium. They can also reduce the costs and
Results and Discussion
Morphology and structural analysis
The phase purity and crystallographic structure of the
hydrothermally prepared hybrid precursor sample are obtained
first by X-ray diffraction (XRD) analysis, as shown in Figure S1.
The diffraction peaks in XRD detected at respective positions can
be attributed to the nickel cobalt hydroxyl carbonate, NCHC;
(Ni,Co)2CO3 (OH)2 (JCPDS # 35-0501 and 48-0083).[38] The
formation of NCHC precursor and its further decomposition to
corresponding NiCo2O4 spinel has been reported in the
literature.[38,39a] The thermal weight loss of the hybrid and pristine
NCHC precursor samples were determined by thermogravimetric
(TG) and differential thermal gravimetric (DTG) analysis, as
shown in Figure 1. Both precursor samples display two kinds of
weight loss in their TG profiles, the initial sharp weight loss
observed at around 313 °C for the NCHC-MCNTs hybrid and
312 °C for pristine NCHC samples are ascribed to decomposition
of NCHC precursors into corresponding NiCo2O4 spinel by
releasing H2O and CO2 gases. The weight loss at about 800 °C
can be ascribed to the decomposition of spinel nickel cobaltite into
parent simple nickel oxide and cobalt oxides.[39a] As confirmed in
Figure 1A, additional weight losses observed only in the hybrid
precursor at about 420 °C are attributed to the combustion of
carbon skeleton in MWCNTs.[36] Compared to NCHC precursor,
the hybrid NCHC-MCNTs precursor exhibits higher total weight
loss of 25% due to removal of additional oxygenated functional
groups from the MCNTs.
2
This article is protected by copyright. All rights reserved.

10.1002/cplu.201900603
ChemPlusChem
Figure S3 compares the Raman spectra of NiCo2O4-MCNTs
hybrid and pure MCNTs samples measured at room temperature.
Both of the samples display two characteristic bands of carbon
materials including D and G bands, respectively at about 1334
and 1586 cm-1.[31,36] The D band attributes to disorder/defective
nature of carbon atoms and the G band represents the graphitic
structure due to E2g phonon of sp2 carbon atoms in the graphite
sheets.[32,36] From the conductivity measurements, as shown in
Figure S4 (detailed information can be found in the experimental
methods), the hybrid NiCo2O4-MCNTs composite (∼7.83 S/cm)
shows six times higher conductivity compared to the pristine
NiCo2O4 (∼1.32 S/cm). Moreover, the obtained conductivity value
of our NiCo2O4-MCNTs hybrid is better than the recently reported
NiCo2O4-rGO nanocomposite (∼6.68 S/cm)[40a] and NiCo2O4
nanoparticles (∼4 S/cm)[40b]. Such high conductivity of the hybrid
NiCo2O4-MWCNTs could endow sufficient electroactive sites,
minimizing ion transport pathways and enhancing the electron
collection efficiency.
Figure 1. TG and DTG profiles of (A) nickel cobalt hydroxy carbonate-MCNTs
hybrid and (B) pristine nickel cobalt hydroxy carbonate precursor samples.
The XPS full-scan survey spectrum of the hybrid composite
(Figure 3A) illustrates the existence of prominent elements Ni, Co,
C, and O on the near-surface of NiCo2O4-MWCNTs, which is in
good agreement with the literature reports.[41] The deconvolution
of Co 2p, Ni 2p, O 1s and C 1s spectral regions are displayed in
Figure 3B-E. The Ni 2p spectrum, as shown in Figure 3B, best
After sintering at 350 °C in air, the NCHC-MCNTs hybrid and
pristine NCHC precursors are completely transformed into
NiCo2O4-MWCNTs and NiCo2O4 spinel as evidenced by powder
XRD in Figure 2. Both hybrid and pristine NiCo2O4 samples reveal
well diffraction peaks owing to the formation of face centered
cubic spinel structure (JCPDS # 73-1702) without any impurity
phase. In addition, the XRD pattern of pristine MCNTs (Figure S2)
shows diffraction peak at 25.2° and 42.6° correspond to the (100)
and (110) planes of hexagonal graphitic carbon. However, such
diffraction peaks are not observed in the NiCo2O4-MCNTs hybrid
on account of relatively low amount of carbon in the composite.[39b]
The computed mean crystallite size of the hybrid NiCo2O4-MCNTs
composite is much smaller (∼12 nm) than that of pristine NiCo2O4
(∼21 nm). Likewise, the lattice parameter for NiCo2O4-MCNTs is
0.895 nm and for NiCo2O4 is 0.891 nm.
fitted by considering two spin-orbit doublets (Ni 2p1/2 and Ni 2p3/2
)
and two shake-up satellites (marked as “Sat.”).[37,41a] Specifically,
the fitting peaks at binding energy (BE) of 856.5 and 874.0 eV
ascribed to Ni3+, while the other peaks at 854.7 and 872.2 eV are
assigned to Ni2+. The satellite peaks are recognized at 861.6 and
879.3 eV. Likewise, the core level Co 2p spectrum (Figure 3C)
shows two spin-orbit doublets (Co 2p1/2 and Co 2p3/2) and two
shakeup satellites.[42] The peaks at BE of 779.7 and 795.0 eV,
corresponds to Co3+ while the peaks at 782.3 and 798.2 eV due
to Co2+. The shakeup satellite peaks evidenced at 788.2 and
804.1 eV. The higher valances are expected to assist fast charge
carrier transport across the electrode/electrolyte interface. In
Figure 3D, the core level spectrum of O 1s deconvoluted into four
different oxygen species located at BE of 529.1, 530.3, 531.2 and
532.4 eV which have been represented as O1, O2, O3 and O4,
respectively.[23,] Among four, the O1 component is the most
intense one and is attributed to the oxygen bonding with metal (M-
O-M), indicates most of the oxygen is present in the lattice.[26,28]
The second O2 component arises as a result of large number of
defect sites primarily due to existence of a variety of oxygenated
species such as hydroxyl, chemisorbed oxygen with minimum
oxygen coordination.[23,25] The component O3 is corresponding to
water bound on surface of sample.[40a] The final O4 component is
Figure 2. Powder XRD pattern of NiCo2O4-MCNTs hybrid and pristine NiCo2O4
samples.
3
This article is protected by copyright. All rights reserved.

10.1002/cplu.201900603
ChemPlusChem
due to multiplicity of physi-/chemisorbed water at or within the
surface of the spinel.[23,35] The deconvoluted C 1s spectrum
(Figure 3E) reveals a divergent functional groups of carbon like
C=C/C−C, −C−OH and −C=O groups in the surface of oxidized
MCNTs,[33,35] which are strongly coupled with NiCo2O4 through
strong chemical bond like covalent bond and by weak bonds like
van der Walls forces and/or hydrogen bonding leading to direct
growth of the spinel NiCo2O4 on MCNTs surface. The XPS results
illustrate that the hybrid composite consists chemical composition
of Ni2+/Ni3+, Co2+/Co3+, C and O species. Therefore, it is expected
that the binary solid state redox couples such as Co3+/Co2+ and
Ni3+/Ni2+ in the composite can afford an adequate electroactive
sites in electrochemical water splitting.
Figure 3. (A) XPS survey spectrum and high-resolution XPS spectra of (B) Ni 2p, (C) Co 2p, (D) O 1s and (E) C 1s regions of the NiCo2O4-MCNTs hybrid composite.
4
This article is protected by copyright. All rights reserved.

10.1002/cplu.201900603
ChemPlusChem
To investigate the morphology of NiCo2O4-MWCNTs hybrid,
field emission scanning electron microscopy (FESEM) was
carried out and the corresponding images are shown in Figure 4.
The FESEM images of the NiCo2O4-MWCNTs composite in
Figure 4A, B display a large-scale, dense, porous hierarchical
flower-like morphology with diameter of about 4-5 µm and are
strongly anchored on the surface of MWCNTs. In comparison to
NiCo2O4-MCNTs hybrid, the pristine NiCo2O4 also exhibits similar
flower-like morphology (Figure 4D, E). The high-magnified
FESEM images of the hybrid and pristine samples are presented
in Figure 4C, F. As seen that hierarchical flower-like morphologies
are developed by dozens of nanowires interconnected to each
other through the center and the diameter of the nanowires
gradually decreases from centre to the tip.
contact sites between MCNTs, forming a better conductive
network. The high-resolution TEM images of the NiCo2O4-MCNTs
composite (Figure 5E, F) revealed a well-resolved crystal lattice
fringes with d-spacing of 0.284 and 0.463 nm can be assigned to
the (220) and (111) crystallographic plane of oxide spinel. The
selected area electron diffraction (SAED) pattern of the
corresponding sample shown in the inset of Figure 5F, which
reveals the polycrystalline nature of the nickel cobalt spinel. The
SAED pattern possesses well-defined diffraction rings, which are
well indexed to the (111), (220), (311), (400), (511) and (440)
crystallographic planes respectively of the nickel cobalt spinel
(JCPDS no # 73-1702). The EDS mapping images further confirm
the presence of elements Ni, Co, O, and C evenly dispersed in
the hybrid composite (Figure S6). The atomic ratio of Co/Ni in the
NiCo2O4-MCNTs composite is about 2:1, which is in good
consistence with experimental observations (Figure S7). Besides,
the morphology of surface oxidized MCNTs in Figure S8 exhibits
well-ordered tubes with an average diameter of 25 nm and their
lengths are about 10 μm.
The formation of hierarchical flower-like nickel cobalt
hydroxyl carbonate from nascent crystallite under hydrothermal
condition (mainly at high temperature and pressure) involves
multifarious contributing factors that encompass van der Waals
forces, crystal-face attraction, intermolecular interactions, steric
factors, hydrophobic attraction, hydrogen bonding, intrinsic crystal
contraction, electrostatic and dipolar fields, and Ostwald
ripening.[38,43a] Other factors such as kinetics and concentration of
the reactants as well as reaction conditions are also equally
contributed in determining the size and shape of inorganic
nanocrystals which further influenced the final morphology.[38,43b]
However, the growth of flower-like morphology primarily involve
the (i) nucleation of NCHC and grow up to nanosized particles, (ii)
Ostwald repining followed by self-assembling to nanowires, and
(iii) impetuous self-transformation to hierarchical flower-like
structure during the sintering at an elevated temperature.[38,43,44]
Moreover, in the course of the growth process, the oxidized
MCNTs render good anchoring sites for positively charged Ni2+
and Co2+ cations via electrostatic forces owing to the presence of
abundant oxygenated functional groups, for instance, −C−OH,
−C=O and −COOH on the surface of MCNTs.[40a] The schematic
possible formation of hierarchical flower-like NiCo2O4-MCNTs
hybrid is shown in Scheme 1.
Figure 4. FESEM images of (A-C) NiCo2O4-MCNTs hybrid and (D-F) pristine
NiCo2O4 samples at different magnifications.
To obtain more detailed information about morphology and
elemental distribution of the hybrid NiCo2O4-MCNTs material,
transmission electron microscopy (TEM) was performed and the
corresponding results are presented in Figure 5 and S6. The TEM
pictures in Figure 5A, B revealed that the flower-like morphology
of NiCo2O4 are constructed by numerous long nanowires which
epitaxially grown from the center. The diameter of the nanowires
is about 25-30 nm and the texture of nanowires is mesoporous in
nature (inset of Figure 5C). More prominently, the nanowires are
hybridized effectively over MWCNTs (Figure 5B-D) which affords
a much adequate synergetic effect between MWCNTs and
NiCo2O4 spinel. Figure S5 compares the different magnified TEM
images of the hybrid and pristine NiCo2O4 samples. It can be
found that when compared to pristine NiCo2O4 (Figures S5D-F),
the hybrid composite revealed anchoring of NiCo2O4 nanowires
on MCNTs (Figure S5A-C). Besides, the mutual contact between
nanowires and MCNTs indicates that the morphology of the oxide
material grows not only on individual MCNTs but also at the
5
This article is protected by copyright. All rights reserved.

10.1002/cplu.201900603
ChemPlusChem
Figure 5. (A-D) TEM and (E, F) high-resolution TEM images for the NiCo2O4-MCNTs composite; Insets in Figure C and F show the high-magnified TEM image of
nanowire, and the SAED pattern of the composite sample.
Scheme 1. Schematic illustration for the formation of hierarchical flower-like NiCo2O4-MCNTs hybrid under facile hydrothermal conditions
The surface textural characteristics of NiCo2O4-MCNTs hybrid
and pristine (NiCo2O4 and MCNTs) samples were evaluated by
using nitrogen adsorption-desorption isotherms (Figure 6). Both
hybrid and pristine NiCo2O4 samples show typical type-IV with H3
hysteresis loops and the capillary condensation step from P/P0 of
0.4 to 0.99, corresponds to mesoporous texture of the samples.[45]
The hybrid NiCo2O4-MCNTs (inset of Figure 6A) exhibits bi-modal
pore size distribution with pore maxima values 3.2 and 10.5 nm,
whereas in pristine NiCo2O4 (inset of Figure 6B) exhibit tri-modal
pore size distribution with 3.9, 8.5 and 15.8 nm located pore
maxima values. The calculated specific surface areas for the
hybrid NiCo2O4-MCNTs and pure NiCo2O4 samples are 103 and
64 m2 g-1 respectively. As seen from specific surface area values,
the incorporation of a small amount of MCNTs into NiCo2O4
significantly improves the surface area and porosity of the oxide.
However, the specific surface area of pure MCNTs is 198 m2 g-1
(Figure 6C), which is fairly higher than that of the hybrid NiCo2O4-
MCNTs. The downsized specific surface area in the composite
6
This article is protected by copyright. All rights reserved.

10.1002/cplu.201900603
ChemPlusChem
sample could be attributed to the introduction of nickel cobaltite
particles since they have large particles size and relatively high
density compared to the carbon materials such as MCNTs.
Nonetheless, the enriched texture properties of NiCo2O4-MCNTs
along with indispensable pore-size distribution provide a large
number of electroactive sites for an efficient mass transport during
the electrochemical process. Therefore, the hierarchical flower-
like NiCo2O4-MCNTs hybrid promotes fast electron transfer and
increased utilization of the active catalyst.
oxidation.[47] As one can see in Figure 7A, the intensity of
oxidation wave is more pronounced in case of NiCo2O4-MCNTs
compared to pristine NiCo2O4. The sharp increase in oxidation
current density beyond 1.48 V (vs. RHE) owing to the evolution of
oxygen from water oxidation reaction, OER: 4OH− → 2H2O + O2
+ 4e−.[23,36] Among electrode materials, NiCo2O4-MCNTs exhibits
excellent electrocatalytic activity with low OER onset potential of
1.48 V (vs. RHE) and maximum current density over 66 mA cm-2
at 2.0 V (vs. RHE). To the best of our knowledge, onset potential
of our hybrid NiCo2O4-MCNTs is much smaller than the onset
potentials of various reported nonprecious electrocatalysts such
as metal oxides, hydroxides, chalcogenides and its hybrids (Table
S1). The OER kinetics of resulting electrode materials is further
examined by Tafel plots (Figure 7B). A smaller Tafel slope of 54
mV dec-1 is witnessed for the hybrid NiCo2O4-MCNTs compared
to individual electrodes (i.e., NiCo2O4 is 67 mV dec-1 and MCNTs
is 141 mV dec-1) and benchmark RuO2 (86 mV dec-1), suggesting
efficient electron and mass transfer of the hybrid composite.
In order to precisely evaluate the OER catalytic activity further,
we computed overpotential necessary to achieve magnitude
current density of 10 mA cm-2.[20,46] The hybrid NiCo2O4-MCNTs
composite require as low as 350 mV of overpotential to reach the
current density of 10 mA cm-2 (η10), which is smaller than the
individual components (NiCo2O4 (∼430 mV) and MCNTs (∼660
mV)) and benchmark RuO2 (380 mV). It is to be noted that the
overpotential of our hybrid NiCo2O4-MCNTs composite reported
here is in good comparable or even much better than many non-
Figure 6. N2 adsorption-desorption isotherms of (A) NiCo2O4-MCNTs hybrid,
(B) NiCo2O4 and (C) MCNTs samples. Insets illustrate the BJH pore-size
distribution curves.
noble metal based OER electrocatalysts reported recently (Figure
48-53]
7C).[15,20,22,28,29,33,
The low overpotential, a large current
Electrochemical studies
density and smaller Tafel slope underscores the superiority of the
hybrid NiCo2O4-MCNTs towards OER activity. The improved
performances of the hybrid can be attributed to a synergetic
coupling between MCNTs and electrocatalytically active NiCo2O4.
Further we measured the electrochemical impedance
spectroscopy to assess the kinetics of the electrode reaction.[16]
The Nyquiest plots (Z′ vs. Z″) of the hybrid and pristine NiCo2O4
electrodes were recorded in 0.1 M KOH at an applied potential of
1.49 V (vs. RHE) and displayed in Figure S9 with data fitting
equivalent circuit. The circuit to illustrate the resulting impedance
data mainly consist of three components, namely the solution
resistance (Rs), charge transfer resistance (Rct) across the
The electrocatalytic activity towards anodic oxygen evolution
reaction (OER) process of the NiCo2O4-MCNTs hybrid is first
examined by linear sweeping voltammetry (LSV) using a three-
electrode configuration in 0.1 M KOH (detailed methods are given
in supporting information). For comparison, the OER activity of
the pristine NiCo2O4 and MCNTs, as well as benchmark RuO2
catalysts are also examined under identical conditions. Figure 7A
compares the OER polarization curves of four different electrodes
at a sweep rate of 5 mV s-1. It is evident that the pristine MCNTs
have shown poor catalytic activity towards OER. In contrast, the
polarization curves of NiCo2O4-MCNTs and NiCo2O4 show two
types of anodic peaks. The broad oxidation feature centered at
around 1.33 V (vs. RHE) is signature of the oxidation of metals
from lower to higher oxidation states (Ni2+ to Ni3+ and Co2+ to Co3+
through the following reaction in an alkaline medium, NiCo2O4 +
OH− + H2O ↔ NiOOH + 2CoOOH + e-).[46] Typically, NiOOH and
CoOOH have been considered as the active sites for water
electrode/solution interface and
a constant-phase element
(CPE).[15,47b] Among two electrodes, the NiCo2O4-MCNTs exhibits
a smaller diameter of the semicircle, indicating smaller Rct of 15.1
Ω than that of pristine NiCo2O4 (∼19.3 Ω), indicative of faster OER
kinetics and an efficient charge transport at the hybrid electrode
during electrochemical process.
7
This article is protected by copyright. All rights reserved.

10.1002/cplu.201900603
ChemPlusChem
Figure 7. (A) OER polarization curves and (B) Tafel plots of the NiCo2O4-MCNTs hybrid, NiCo2O4, RuO2 and MCNTs in 0.1 M KOH solution at a sweep rate of 5
mV s-1; (C) Comparison of the overpotentail for OER, ηOER (at a current density of 10 mA cm-2) for NiCo2O4-MCNTs hybrid with other reported electrodes in the
literature.
According to the widely accepted mechanism proposed by
Bockris and Otagawa for OER on metal oxide catalysts system
involves five consecutive steps, which can be described in the
eqns. S1 to S5 in supporting information.[54] Therein, the initial
step begins with an adsorption of hydroxyl anion (OH−) from
alkaline media on the surface active sites (S) of the catalyst to
produce adsorbed OH species with the liberation of e− (eqn. S1).
The consequent step is the reaction between hydroxyl anion
(OH−) and adsorbed OH species to generate surface oxide layer
(SO) with the liberation of H2O and e− (eqn. S2). In third step,
hydroxylation reaction between surface oxide layer and hydroxyl
anion (OH−) to form an active intermediate oxy-hydroxide layer
such as OOH species (eqn. S3). The intermediate oxy-hydroxide
layer further reacts with superfluous hydroxyl anion, to form
adsorbed O2 and H2O with liberation of e− (eqn. S4). The final step
implicates oxygen evolution from the surface of active sites (eqn.
S5).
the electrocatalytic activity of electrode.[31,46] Thus, we quantified
the ECSA of both hybrid and pristine electrodes from cyclic
voltammetry (CV) study. For this, the CV study were recorded at
varies sweep rates (2−20 mV s-1) in non-Faradaic potential range
of 1.0−1.1 V (vs. RHE), as depicted in Figure 8A, B. It has been
reported that the charging current, ic is equal to the product of the
sweep rate, ν and the electrochemical double layer capacitance,
Cdl which can be obtained from the following eqn.
.
[55] The ic values for both electrodes are measured
ic = ν×Cdl
at a potential of 1.05 V vs. RHE from CV sweeps in Figure 8A, B,
and plotted against sweep rate (from 2 to 20 mV s-1), which give
a linear relationship with a slope of Cdl (Figure 8C). The estimated
Cdl value for the NiCo2O4-MCNTs composite is 1.67 mF cm-2 while
that for pristine NiCo2O4 is 0.82 mF cm-2, which is only half of the
hybrid composite. The ECSA values are computed by the eqn.:
, where CS signifies the theoretical capacitance
ECSA = Cdl/CS
developed on oxide surface, typically considered as 60 µF cm-2.[16]
The electrochemically active surface area (ECSA) of an
electrode is also an important deciding factor that can influence
The relative magnitude of the respective roughness factor (Rf) is
8
This article is protected by copyright. All rights reserved.

10.1002/cplu.201900603
ChemPlusChem
obtained by taking the estimated ECSA and dividing it by
electrode surface area. The estimated ECSA values for NiCo2O4-
MCNTs and NiCo2O4 are 27.8 and 13.6 cm2, and Rf values are
100 and 49, respectively. The large ECSA and Rf values further
strengthen the fact that the NiCo2O4-MCNTs composite provides
more exposed interior surfaces with enriched exploitation of
electroactive sites leading to higher OER activity. At a potential of
1.60 V (vs. RHE), the mass activity and specific activity for
composite shows excellent HER performance with lower onset
potential along with a rapid increment in current density in
compared to individual NiCo2O4 and MCNTs. Specifically, for
NiCo2O4-MCNTs hybrid needs overpotential as low as of 209 mV
to reach a magnitude current density of -10 mA cm-2, which is
smaller than those of NiCo2O4 (∼257 mV) and MCNTs (∼425 mV).
Notably, the benchmark Pt/C is still exhibits the best HER activity
with extremely small overpotential of 35 mV. It is worth noting that
the hybrid NiCo2O4-MCNTs composite performs as an efficient
electrocatalyst with a smaller overpotential than the reported
numerous non-noble metal HER catalysts, as listed in Figure 9C.
[12,13,32,50,57-62] Even though MCNTs itself have shown insignificant
HER activity, once it is effectively hybridized with NiCo2O4, as a
result, large current density along with downsized overpotential
from 257 to 209 mV is achieved. The enhanced HER activity of
the NiCo2O4-MCNTs composite is attributed to the synergistic
effect between MCNTs and metal oxide leading to enhanced
reactive sites and fast charge transport.
NiCo2O4-MCNTs hybrid are 48.33 A g-1 and 0.116 mA cm-2
,
ECSA
respectively, outperform the pristine NiCo2O4 are 22.37 A g-1 and
0.067 mA cm-2ECSA. The intrinsic activities of the resulting
electrodes are further resolved by turnover frequency (TOF),
presuming each metal atom to be catalytically active. As shown
in Figure 8D, the TOF value of the NiCo2O4-MCNTs is found to be
3.29×10−2 s−1 which is higher than pristine NiCo2O4 (1.35×10−2 s−1)
and reported NiCo2O4/NiO and NiO,[15] suggesting excellent OER
catalytic activity of the hybrid.
The linear fitting of Tafel plots of different electrode materials
were derived from the HER polarization curves. As shown in
Figure 9B, a smaller Tafel slope of 59 mV dec-1 is noticed for
NiCo2O4-MCNTs composite than those of individual constituents
of NiCo2O4 (∼72 mV dec-1) and MCNTs (∼156 mV dec-1), implying
a faster HER process with higher efficiency. The Tafel slope of 49
mV dec-1 is attributes to the benchmark Pt/C, which is coincides
well with previous reports.[12] It has been reported that the cathodic
HER process mainly follows two different pathways such as
Volmer-Heyrovsky and Volmer-Tafel in alkaline solutions.[12,13,27]
In both ways the initial step begins with adsorption of H2O
molecules. The consequent step involves two Hads combines
directly to produce H2 in the case of Volmer-Tafel mechanism
(eqn. S6, supporting information) or formation of H2 by combining
with another proton source and e− according to Volmer-Heyrovsky
mechanism (eqn. S7).[27,62,63] Depending upon rate determining
step, the Tafel slopes of 120, 40, and 30 mV dec-1 should be
observed for Volmer, Heyrovsky or Tafel component, respectively.
Therefore, in this study, the Tafel slope of 59 mV dec-1 is observed
for NiCo2O4-MCNTs composite (Figure 9B), implying that HER on
the hybrid composite electrode occurs via Volmer-Heyrovsky
mechanistic pathway and the Heyrovsky step is the rate-limiting
step for this reaction. Moreover, the Tafel slope of NiCo2O4-
MCNTs hybrid is comparatively smaller than the various reported
non-precious metal catalysts.[18,32,50,51,57]
Figure 8. CV curves of (A) NiCo2O4-MCNTs hybrid and (B) pristine NiCo2O4
electrodes recorded in 0.1 M KOH at scan rates of 2, 5, 8, 10, 15 and 20 mV s-
1; (C) Capacitive current (ic) measured at 1.05 V (vs. RHE) as a function of scan
rate for the two electrodes and (D) comparison of roughness factor (Rf) and
turnover frequency (TOF) of NiCo2O4-MCNTs and NiCo2O4 electrodes.
The catalytic capabilities of the resulting hybrid and pristine
electrode materials for HER were further evaluated in 0.1 M KOH
to inspect the possibility for overall water splitting (Figure 9).[12,16]
The HER measurements of NiCo2O4-MCNTs hybrid and pristine
NiCo2O4 electrode materials were carried out in a standard three-
electrode configuration at a sweep rate of 5 mV s-1. As the group
for comparison, commercial carbon-supported Pt catalyst (20
wt% Pt on Vulcan XC-72, Pt/C) was also evaluated under identical
conditions. Figure 9A compares the HER polarization curves of
the NiCo2O4-MCNTs, NiCo2O4, MCNTs and benchmark Pt/C at a
sweep rate of 5 mV s-1. It can be found that the NiCo2O4-MCNTs
9
This article is protected by copyright. All rights reserved.

10.1002/cplu.201900603
ChemPlusChem
Figure 9. (A) HER polarization curves and (B) Tafel plots of NiCo2O4-MCNTs hybrid, NiCo2O4, 20% Pt/C, and MCNTs in 0.1 M KOH solution at a sweep
rate of 5 mV s-1; (C) Comparison of the overpotentail for HER, ηHER (at a current density of -10 mA cm-2) for NiCo2O4-MCNTs hybrid with other reported
electrodes in the literature.
For energy conversion and storage systems, the long-time
electrocatalytic durability towards the complex electrochemical
reactions is essential factor to determine the quality of the
electrode material. Therefore, the long-term durability of the
representative NiCo2O4-MCNTs hybrid and pristine NiCo2O4
toward OER and HER are evaluated separately by using
chronoamperometry (CA) experiment in 0.1 M KOH solution. The
OER durability curves for both hybrid and pristine electrode
materials were acquired at a constant bias potential of 1.62 V vs.
RHE for consecutive 10 h (upper current-time (I–t) curves in
Figure 10). As seen, the current density of hybrid NiCo2O4-
MCNTs composite decreases rapidly in the beginning 30 min and
afterword it becomes constant even up to 10 h without any
significant current decay, whereas in case of pristine NiCo2O4 the
current gradually drops with time.
solitary NiCo2O4 (62% retention). Furthermore, HER polarization
curves of the resulting electrodes were recorded before and after
chronoamperometry (CA) stability test (Figure S10). The profiles
of NiCo2O4-MCNTs hybrid exhibits highly stable performance with
negligible change in overpotential (∼8 mV) even after long-term
durability for 10 h, whereas in pristine NiCo2O4
a large
enhancement of ∼55 mV can be found in overpotential at η10.
Altogether, the durability test confirmed that the NiCo2O4-MCNTs
composite have remarkable stability for both OER and HER under
an alkaline condition. The increased durability of the composite
electrode evokes from a strong synergistic interaction between
the nickel cobaltite and MCNTs. Moreover, the supportive MCNTs
could effectively prevent from the aggregation and dissolution of
oxide nanoparticles during the vigorous electrochemical process,
usually involves huge amount bubble formation on the surface of
electrodes.
The durability test for HER was also performed under similar
conditions at a constant bias potential of -0.16 V vs. RHE (lower
I–t curves in Figure 10). The NiCo2O4-MCNTs composite exhibits
highly stable performance with 80% catalytic current retention in
long-term continuous operation for 36000 s (10 h) than that of
In order to confirm whether the Pt counter electrode can
influence on HER performance, we performed LSV polarization
curves for HER in 0.1 M KOH solution using bare GCE (glassy
carbon electrode) as the working electrode and Pt foil and/or
10
This article is protected by copyright. All rights reserved.

10.1002/cplu.201900603
ChemPlusChem
graphite rod as the counter electrode.[64,65] Figure S11A compares
the HER performance of GCE working electrode when graphite
rod or Pt foil is separately used as the counter electrode. No
significant change has been observed in current densities and/or
onset potentials while changing the counter electrode from Pt foil
to graphite rod. The HER activity is maintained stable with
increasing numbers of CV cycles (Figure S11B, C). Therefore, the
effect of counter electrode on HER is negligible in this work.
properties such as large specific surface area with indispensable
pore-size distribution, hierarchical flower-like morphology and
large conductivity, which are all essential in improving the
performance of electrochemical overall water splitting. The
electrochemical measurements demonstrated that NiCo2O4-
MCNTs hybrid showed an attractive OER and HER
electrocatalytic activity in terms of low overpotential, favorable
kinetics and durable stability in alkaline solution. Notably, very
small overpotentials, 350 and 209 mV are needed to achieve a
benchmark current density of 10 mA cm-2 for OER and HER. The
overpotential values of hybrid are much lower than that of the
individual NiCo2O4 and MCNTs and literature reported non-
precious catalysts. The improved electrocatalytic activity of the
hybrid is originating from the strong synergistic chemical coupling
between MCNTs and nickel cobaltite leading to dual-active-site
mechanisms. This work presents a versatile strategy for the
fabrication of new and tailored non-precious oxide based spinel
hybrid nanostructures as highly efficient, cost-effective and stable
electrocatalysts for overall water splitting (i.e., both anodic OER
and cathodic HER) under alkaline electrolyte.
Figure 10. Long-term stability (i-t) curves for OER and HER on NiCo2O4-MCNTs
hybrid and NiCo2O4 electrodes in 0.1 M KOH electrolyte; applied bias potentials
are 1.60 V (vs. RHE) for OER and -0.16 V (vs. RHE) for HER.
Experimental Section
Materials
Unless otherwise specified, all the chemicals employed in this
study were used as received without any further purification.
Ni(NO3)2.6H2O (99.9%, SD Fine), Co(NO3)2.6H2O (99.9%, SD
Fine), urea (Thomas-Baker), NH4F (Merck), H2SO4 (Merck),
HNO3 (Merck) and MCNTs (Sigma Aldrich). Deionized (DI) water
was used in all the experiments.
Based on the above results, the improved electrochemical
performance towards both OER and HER of the hybrid NiCo2O4-
MCNTs composite can be ascribed from following aspects:[29,66,67]
the (i) NiCo2O4-MCNTs with high surface area with indispensable
pore-size distribution greatly enhance contact area between
electrode and electrolyte interface and promote fast electron
transfer. (ii) Functionalized MCNTs consists several hydrophilic
functional groups which can intimately interact with NiCo2O4 via
electrostatic interactions and van der Walls forces, which could
provide a good dispersion and stability of NiCo2O4. (iii) When
combining the NiCo2O4 and MCNTs at a molecular scale, there
are strong synergistic interactions between them and promote
good structural and electrochemical properties compared to
individual NiCo2O4. (iv) Hierarchical flower-like morphology
assembled with 1D-nanowires alleviates the strain caused by the
volume expansion, leading to a remarkable stability.
Oxidation of multi-walled carbon nanotubes
For a covalent functionalization of MWCNTs, a 500 mg of as-
received MCNTs were well dispersed in the mixture of
concentrated H2SO4 and HNO3 (3:1 v/v) by an ultra-sonication
technique for 1 h. Then, the total reaction mixture was refluxed in
an oil bath at 80 °C for 10 h, so as to graft hydroxy functional
groups on surface of MCNTs. The resulting mixture underwent
centrifugation, decanting and purified by repeated washing using
alcohol and DI H2O, and subsequently vacuum-dried at 100 °C for
overnight.
Synthesis of NiCo2O4-MCNTs hybrid composite
Conclusions
Typically, 50 mg of oxidized MCNTs was uniformly dispersed into
100 mL of deionized water with the assistance of ultra-sonication
for at least 30 min. Then, Ni(NO3)2.6H2O (3 mmol) and
Co(NO3)2.6H2O (6 mmol) metal precursors (Ni and Co, molar ratio
In summary, we have adopted
a simple, cost-effective
hydrothermal technique to synthesize porous hierarchical flower-
like NiCo2O4-MCNTs hybrid with a strong synergistic effects and
rendering it as an efficient bifunctional electrode for both OER and
HER under alkaline conditions. The as-fabricated hybrid NiCo2O4-
MCNTs nanocomposite reaps remarkable surface textural
1:2) were added with constant intense stirring for
2 h.
Subsequently, 12 mmol of NH4F and 32 mmol of urea were added
11
This article is protected by copyright. All rights reserved.

10.1002/cplu.201900603
ChemPlusChem
under stirring. The resulting homogeneous transparent mixture
was transferred to a 150 mL capacity Teflon-lined autoclave,
sealed and heated for 12 h at 150 °C in an oven. After cool down
to room temperature, the sample was collected, thoroughly rinsed
with DI water and alcohol by centrifugation (5 min each) for
several times to remove residual reagents, if any. The as-obtained
nickel cobalt hydroxyl carbonate-MCNTs (NCHC-MCNTs)
precursor product was dried at 60 °C, and then sintered in air at
350 °C for 3 h with a temperature rising rate of 5 °C min-1 to result
in a well crystallized hybrid NiCo2O4-MCNTs composite. The
NiCo2O4 content in the resulting hybrid composite is 92 wt% and
MCNTs content is 8 wt%. For comparison, pristine NiCo2O4 was
prepared using a similar process without adding MCNTs.
polarizations (current, I vs. time, t curves) in Figure S4, using the
following equations:
V
Resistance (R) =
I
l
Conductivity (σ) =
R×a
where l and a represent the thickness (∼1.5 cm) and surface area
(∼0.785 cm2) of the pressed pellet, respectively. And I and V are
the current (in amperes) and potential (volts).
Electrochemical measurements
The electrocatalytic activity towards OER and HER
measurements of the hybrid and pristine electrodes were
performed via three electrode system employing CH instruments
electrochemical workstation (Modal: CHI6081C). The saturated
calomel electrode (SCE) and Pt foil (the results were verified with
graphite counter electrode too) were used as reference and
counter electrodes, respectively. The working electrodes were
fabricated as follows: a known amount of the catalyst powder was
well dispersed in water-ethanol mixture (2 mL, 3:2 v/v) and 5 wt%
Nafion solution (200 µL) with the aid of ultrasonication for 30 min,
in order to achieve a homogeneous catalyst ink. The resulting
catalyst ink was dropped onto the surface of a polished glassy
Materials characterization
Thermal analysis of the as-synthesized NCHC-MCNTs and
NCHC precursors were tested using thermogravimetric (TG) and
differential thermal gravimetric (DTG) analysis in O2 flow at a scan
speed of 20 °C min-1 on a TA Q500V20.10 Build 36 instrument.
X-ray diffraction (XRD) measurements (Cu Kα radiation source)
were performed on the products with a Bruker AXS D8
diffractometer to identify the crystallographic phases. Raman
spectrums were recorded using Horiba HR 800UV Raman
spectrometer with a laser excitation of 632.8 nm. Multi-point
specific surface area and pore-size distribution of the sintered
materials were studied by Brunauer-Emmett-Teller (BET) and
Barret-Joyner-Halenda (BJH) methods on Micromeritics ASAP
2020 physisorption tool. For the surface topography and
composition of the products, field emission scanning electron
microscopy (FESEM) was conducted at FEI, Quanta 400
microscope equipped with an energy dispersive X-ray (EDX,
AMETEK) spectrometry detector. TEM images were monitored
with a JEM 3010 instrument. Elemental analysis and their valence
states of NiCo2O4-MWCNTs composite were examined by X-ray
photoelectron spectrum (XPS) using Omicron Nanotechnology
instrument with Mg Kα X-ray radiation source. All spectra were
calibrated based on the C1s peak (284.50 eV).
carbon (GCE,
6 mm in diameter) electrode by using a
micropipette tip and left to dry at room temperature. The catalyst
(NiCo2O4-MCNTs hybrid or pristine NiCo2O4) loading in each of
the GCE is 0.24 mg/cm2. Commercial benchmark RuO2 and 20
wt% Pt/C catalysts were used as a reference for evaluating the
OER and HER performance of oxide composites.
Linear sweep voltammetry (LSV), cyclic voltammetry (CV)
and chronoamperometry (CA) measurements were carried out in
0.1 M KOH electrolyte. Electrochemical impedance spectroscopy
(EIS) were carried out at an applied potential of 1.49 V (vs. RHE)
using a perturbation amplitude of 10 mV. All the experiments were
performed at room temperature.
All the potentials scale in this study was calibrated with
respect to a reversible hydrogen electrode (RHE) by using the
Evs. RHE = Evs. SCE + 0.0591×pH + Eo
following equation:
where the value of
,
Conductivity measurements
vs. SCE
Eo
Electrical conductivity of the NiCo2O4-MCNTs hybrid and pristine
NiCo2O4 were conducted by direct current (dc) polarization test at
0.2 V, according to the method described in the literature.[68] Prior
to experiment, the hybrid and/or pristine NiCo2O4 powders were
pressed into pellets with a PTFE (polytetrafluoroethylene) cylinder
body having inner diameter of 10 mm, and then carbon foils as
blocking electrodes were laid on both sides of the pellet by
pressing at 200 MPa. The electrical conductivity (σ) of the
samples at room temperature were obtained from the dc
vs. SCE was 0.2412 (vs. RHE) and pH = 13
for 0.1 M KOH. Overpotential (η) was calculated by the equation:
η(V) = ERHE − 1.23 V. The Tafel plots were obtained from the LSV
polarization curves by using the Tafel relation as η = a + b log j,
where j and b indicates the current density and Tafel slope.
12
This article is protected by copyright. All rights reserved.

10.1002/cplu.201900603
ChemPlusChem
Calculation of electrochemical parameters
[9] E. Gottlieb, M. Kopec, M. Banerjee, J. Mohin, D. Yaron, K.
Matyjaszewski, T. Kowalewski, ACS Appl. Mater. Interfaces 2016,
8, 21531−21538.
The detailed calculations of the mass activity (A g-1), specific
activity (mA cm-2ECSA), and turnover frequency (s-1) of the resulting
electrode materials are as follows.
[10] a) M. I. Jamesh, J. Power Sources 2016, 333, 213−236; b) Y.
Jiang, Y. -P. Deng, R. Liang, J. Fu, D. Luo, G. Liu, J. Li, Z. Zhang,
Y. Hu, Z. Chen, Adv. Energy Mater. 2019, 9, 1900911.
[11] a) Y. Liang, Y. Li, H. Wang, H. Dai, J. Am. Chem. Soc. 2013,
135, 2013−2036; b) Z. Xing, Y. -P. Deng, S. Sy, G. Tan, A. Li, J.
Li, Y. Niu, N. Li, D. Su, J. Lu, Z. Chen, Nano Energy 2019, 65,
104051; c) J. Zhao, F. Rong, Y. Yao, W. Fan, M. Li, Q. Yang, J.
Energy Chem. 2018, 4, 1261−1267.
The mass activity of electrode materials can be obtained from the
mass of an electrocatalyst loaded and the measured current
density (mA cm-2geo) at 1.60 V (vs. RHE) which as follows.
current density at a fixed potential
Mass activity =
electrocatalyst loading
[12] R. Li, D. Zhou, J. Luo, W. Xu, J. Li, S. Li, P. Cheng, D. Yuan,
The specific activity was estimated by normalizing the current
attained at a specific potential of 1.60 V (vs. RHE) with
electrochemically active surface area, as given by following eqn.
J. Power Sources 2017, 341, 250−256.
[13] a) A. Sivanantham, P. Ganesan, S. Shanmugam, Adv. Funct.
Mater. 2016, 26, 4661–4672; b) H. Yu, D. Tang, Y. Huang, W.
Zhang, X. Sun, X. Yang, Z. -A. Qiao, J. Wang, Z. Zhao,
ChemPlusChem 2019, 84, 1604−1609.
observed current
Specific activity =
electrochemically active surface area
[14] L. Feng, A. Li, Y. Li, J. Liu, L. Wang, L. Huang, Y. Wang, X.
Ge, ChemPlusChem 2017, 82, 483−488.
The turnover frequency (TOF) typically denotes the number of
oxygen molecules generated per active site in one second. The
TOF value of the electrodes was estimated from the following eqn.
[15] C. Mahala, M. Basu, ACS Omeg 2017, 2, 7559−7567.
[16] T. Bhowmik, M. K. Kundu, S. Barman, ACS Appl. Energy
Mater. 2018, 1, 1200−1209.
J×A
Turnover frequency (TOF) =
4×F×n
[17] F. Li, D. Zhang, R.-C. Xu, W. -F. Fu, X. -J. Lv, ACS Appl.
Energy Mater. 2018, 1, 3929–3936.
where J (A cm-2) is current density corresponding to overpotential
of 370 mV, A is the surface area of the working electrode (∼0.28
cm2), F is the Faraday constant (96485.3 C mol-1), and n is the
number of moles of the metal atom on the working electrode.
[18] a) Y. Hua , Q. Xu , Y. Hu, H. Jiang, C. Li, J. Energy Chem.
2019, 37, 1−6; b) L. Y. Wang, C. D. Gu, X. Ge, J. L. Zhang, H. Y. Zhu,
J. P. Tu, Adv. Mater. Interfaces. 2017, 4, 1700481.
[19] V. D. Silva, L. S. Ferreira, T. A. Simões, E. S. Medeiros, D. A.
J. Colloid Interface Sci. 2019, 540, 59−65.
[20] X. Liu, J. Liu, Y. Li, Y. Li, X. Sun, ChemCatChem 2014, 6,
2501−2506.
[21] X. Liu, Z. Chang, L. Luo, T. Xu, X. Lei, J. Liu, X. Sun, Chem.
Acknowledgements
Mater. 2014, 26, 1889−1895.
DSEHC has been funded by the Department of Science and
Technology, Government of India, through grant No.
DST/TMD/SERI/HUB/1(C).
[22] P. W. Menezes, A. Indra, N. R. Sahraie, A. Bergmann, P.
Strasser, M. Driess, ChemSusChem 2014, 8, 164-171.
[23] H. Shi, G. Zhao, J. Phys. Chem. C 2014, 118, 25939−25946.
[24] T. Maiyalagan, K. R. Chemelewski, A. Manthiram, ACS Catal.
2014, 4, 421−425.
[25] a) C. Jin, F. Lu, X. Cao, Z. Yang, R. Yang, J. Mater. Chem.
A, 2013, 1, 12170−12177; b) R. Vadakkekara, R. Illathvalappil, S.
Kurungot, ChemElectroChem 2018, 5, 4000–4007.
[26] A. R. Jadhav, H. A. Bandal, A. A. Chaugule, H. Kim,
Electrochim. Acta 2017, 240, 277−287.
Keywords: electrocatalyst • hydrogen evolution reaction •
multiwalled carbon nanotubes • nickel cobalt spinels • oxygen
evolution reaction
References
[1] M. G. Walter, E. L. Warren, J. R. McKone, S. W. Boettcher, Q.
Mi, E. A. Santori, N. S. Lewis, Chem. Rev. 2010, 110, 6446–6473.
[2] M. S. Dresselhause, I. L. Thomas, Nature 2010, 414, 332−337.
[3] M. Carmo, D. L. Fritz, J. Mergel, D. Stolten, Int. J. Hydrogen
Energy 2013, 38, 4901−4934.
[27] X. Gao, H. Zhang, Q. Li, X. Yu, Z. Hong, X. Zhang, C. Liang,
Z. Lin, Angew. Chem. Int. Ed. 2016, 55, 6290−6294.
[28] D. U. Lee, B. J. Kim, Z. Chen, J. Mater. Chem. A 2013, 1,
4754−4762.
[29] Z. -Q. Liu, H. Cheng, N. Li, T. Y. Ma, Y. -Z. Su, Adv. Mater.
[4] J. D. Holladay, J. Hu, D. L. King, Y. Wang, Catal. Today 2009,
139, 244–260.
2016, 28, 3777−3784.
[30] Y. Zhang, G. Jia, H. Wang, B. Ouyang, R. S. Rawat, H. J.
Fan, Mater. Chem. Front. 2017, 1, 709−715.
[31] J. Liu, Y. Xie, Y. Nan, G. Gou, X. Li, Y. Fang, X. Wang, Y.
Tang, H. Yang, J. Ma, Electrochim. Acta 2017, 257, 233–242.
[32] S. Debata, S. Patra, S. Banerjee, R. Madhuri, P. K. Sharma,
Appl. Surf. Sci. 2018, 449, 203−212.
[5] a) J. A. Turner, Science 2004, 305, 972−974; b) Y. P. Deng,
Y. Jiang, D. Luo, J. Fu, R. Liang, S. Cheng, Z. Bai, Y. Liu, W.
Lei, L. Yang, ACS Energy Lett. 2017, 2, 2706−2712.
[6] Y. Jiao, Y. Zheng, M. Jaroniec, S. Z. Qiao, Chem. Soc. Rev.
2015, 44, 2060−2086.
[7] Y. Yan, B. Y. Xia, B. Zhao, X. Wang, J. Mater. Chem. A 2016,
4, 17587–17603.
[8] Y. Lee, J. Suntivich, K. J. May, E. E. Perry, Y. Shao-Horn, J.
Phys. Chem. Lett. 2012, 3, 399–404.
[33] S. Chen, S. -Z. Qiao, ACS Nano 2013, 7, 10190−10196.
[34] S. Chen, J. Duan, W. Han, S. Z. Qiao, Chem. Commun. 2014,
50, 207−209.
[35] H. Cheng, Y. -Z. Su, P. -Y. Kuang, G. F. Chen, Z. -Q. Liu, J.
Mater. Chem. A 2015, 3, 19314−19321.
13
This article is protected by copyright. All rights reserved.

10.1002/cplu.201900603
ChemPlusChem
[36] E. Umeshbabu, G. Rajeshkhanna, P. Justin, G. Ranga Rao,
J. Solid State Electrochem. 2016, 20, 2725–2736.
[37] Z. Ma, H. Fu, C. Gu, Y. Huang, S. Hu, Q. Li, H. Wang, RSC
Adv. 2018, 8, 28209−28215.
[59] Z. Chen, D. Cummins, B. N. Reinecke, E. Clark, M. K.
Sunkara, T. F. Jaramillo, Nano Lett. 2011, 11, 4168−4175.
[60] Y. Hou, M. R. Lohe, J. Zhang, S. Liu, X. Zhuang, X. Feng,
Energy Environ. Sci. 2016, 9, 478−483.
[38] a) Y. Zhao, H. Ma, S. Huang, X. Zhang, M. Xia, Y. Tang, Z.
F. Ma, ACS Appl. Mater. Interfaces 2016, 8, 22997−23005; b) J.
Wu, R. Mi, S. Li, P. Guo, J. Mei, H. Liu, W. -M. Lau, L. -M. Liu,
RSC Adv. 2015, 5, 25304–25311.
[39] a) E. Umeshbabu, G. Rajeshkhanna, G. Ranga Rao, Int. J.
Hydrogen Energy 2014, 39, 15627−15638; b) E. Umeshbabu, G.
Rajeshkhanna, G. Ranga Rao, J. Solid State Electrochem. 2016,
20, 1837−1844.
[61] B. G. Amin, A. T. Swesi, J. Masud, M. Nath, Chem. Commun.
2017, 53, 5412−5415.
[62] J. Liu, J. Wang, B. Zhang, Y. Ruan, H. Wan, X. Ji, K. Xu, D.
Zha, L. Miao, J. Jiang, J. Mater. Chem. A 2018, 6, 2067−2072.
[63] M. Arif, G. Yasin, M. Shakeel, M. Asim Mushtaq, W. Ye, X.
Fang, S. Ji, D. Yan, Mater. Chem. Front. 2019, 3, 520−531.
[64] M. B. Stevens, L. J. Enman, A. S. Batchellor, M. R. Cosby, A.
E. Vise, C. D. M. Trang, S. W. Boettcher, Chem. Mater. 2017, 29,
120−140.
[65] S. P. Narayanan, P. Thakur, A. P. Balan, A. A. Abraham, F.
Mathew, M. Yeddala, T. Subair, C. Tiwary, S. Thomas, T. N.
Narayanan, P. M. Ajayan, M. M. R. Anantharaman, Phys. Status
Solidi RRL 2019, 13, 1900025.
[40] a) E. Umeshbabu, G. Ranga Rao, Electrochim. Acta 2016,
213, 717–729; b) I. T. Papadas, A. Ioakeimidis, G. S. Armatas, S.
A. Choulis, Adv. Sci. 2018, 5, 1701029.
[41] a) J. Pu, J. Wang, X. Jin, F. Cui, E. Sheng, Z. Wang,
Electrochim. Acta 2013, 106, 226−234; b) J. Zhang, P. S.
Shewale, K.-S. Yun, Energies 2019, 12, 3127; c) L. Wang, C. Gu,
X. Ge, J. Zhang, H. Zhu, J. Tu, ChemNanoMat 2018, 4, 124 −131.
[42] a) J. F. Marco, J. R. Gancedo, M. Gracia, J. Solid State Chem.
2000, 153, 74−81; b) B. Cui, H. Lin, Y. Z. Liu, J. B. Li, P. Sun, X.
C. Zhao, C. J. Liu, J. Phys. Chem. C 2009, 113, 14083−14087; c)
J. Yang, C. Yu, S. X. Liang, S. F. Li, H. W. Huang, X. T. Han, C.
T. Zhao, X. D. Song, C. Hao, P. M. Ajayan, J. S. Qiu, Chem. Mater.
2016, 28, 5855.
[66] Z. Wang, H. Lei, R. Cao, M. Zhang, Electrochim. Acta 2015,
171, 81−88.
[67] J. Xiao, X. Bian, L. Liao, S. Zhang, C. Ji, B. Liu, ACS Appl.
Mater. Interfaces 2014, 6, 17654−17660.
[68] S. Wang, M. Yan, Y. Li, C. Vinado, J. Yang, J. Power Sources
2018, 393, 75–82.
[43] a) K. J. M. Bishop, C. E. Wilmer, S. Soh, B. A. Grzybowski,
Small, 2009, 5, 1600–1630; b) Y. -W. Jun, J. -S. Choi, J. Cheon,
Angew. Chem. Int. Ed. 2006, 45, 3414−3439.
[44] a) S. Bag, C. Retna Raj, J. Mater. Chem. A 2016, 4, 8384–
8394; b) E. Umeshbabu, G. Ranga Rao, ChemPhysChem 2016,
17, 3524–3534.
[45] F. H. Rouquerol, J. Rouquerol, K. Sing, Adsorption by
powders and porous solids, Academic press, London, 1999.
[46] C. Chang, L. Zhang, C. -W. Hsu, X. -F. Chuah, S. -Y. Lu,
ACS Appl. Mater. Interfaces 2018, 10, 417−426.
[47] a) N. Xu, G. Cao, Z. Chen, Q. Kang, H. Dai, P. Wang, J. Mater.
Chem. A 2017, 5, 12379−12384; b) L. Zhang, W. He, X. Xiang, Y.
Li, F. Li, RSC Adv. 2014, 4, 43357–43365.
[48] Q. Liu, J. Jin, J. Zhang, ACS Appl. Mater. Interfaces 2013, 5,
5002−5008.
[49] M. -R. Gao, Y. -F. Xu, J. Jiang, Y. -R. Zheng, S. -H. Yu, J.
Am. Chem.Soc. 2012, 134, 2930−2933.
[50] R. Liu, H. Zhang, X. Zhang, T. Wu, H. Zhao, G. Wang, RSC
Adv. 2017, 7, 19181−19188.
[51] Z. Zhao, H. Wu, H. He, X. Xu, Y. Jin, Adv. Funct. Mater. 2014,
24, 4698−4705.
[52] A. Aijaz, J. Masa, C. Rösler, W. Xia, P. Weide, A. J. R. Botz,
R. A. Fischer, W. Schuhmann, M. Muhler, Angew. Chem. Int. Ed.
2016, 55, 4087−4091.
[53] X. Ji, X. Ren, S. Hao, F. Xie, F. Qu, G. Du, A. M. Asirif, X.
Sun, Inorg. Chem. Front. 2017, 4, 1546−1550.
[54] J. O. Bockris, Otagawa, J. Phys. Chem. 1983, 87, 2960−2971.
[55] C. C. L. McCrory, S. Jung, J. C. Peters, T. F. Jaramillo, J.
Am. Chem. Soc. 2013, 135, 16977−16987.
[56] H. Jin, J. Wang, D. Su, Z. Wei, Z. Pang, Y. Wang, J. Am.
Chem. Soc. 2015, 137, 2688−2694.
[57] Z. Wang, S. Zeng, W. Liu, X. -W. Wang, Q. Li, Z. Zhao, F.
Geng, ACS Appl. Mater. Interfaces 2017, 9, 1488−1495.
[58] X. Sun, Q. Shao, Y. Pi, J. Guo, X. Huang, J. Mater. Chem. A
2017, 5, 7769−7775.
14
This article is protected by copyright. All rights reserved.

10.1002/cplu.201900603
ChemPlusChem
Entry for the Table of Contents
The hierarchical flower-like NiCo2O4
spinel material anchored on MCNTs by
facile surfactant free hydrothermal
process is an effective bifunctional
electrocatalyst for OER and HER in an
alkaline medium. The NiCo2O4-MCNTs
hybrid electrode delivers extremely low
overpotentials of 350 and 209 mV for
Ediga Umeshbabu, P. Hari Krishna
Charan, Ponniah Justin and G. Ranga
Rao*
Page No. – Page No.
Hierarchically Organized NiCo2O4
Microflowers Anchored on
Walled Carbon Nanotubes: Efficient
Bifunctional Electrocatalysts for
Multi-
OER and HER, respectively at
a
benchmark current density of 10 mA
cm-2. Moreover, it also has a smaller
Tafel slope and excellent durability
than the solitary NiCo2O4 and MCNTs.
Oxygen and Hydrogen Evolution
Reactions
15
This article is protected by copyright. All rights reserved.