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Well-defined hierarchical teddy bear sunflower-like

NiCo2O4 electrocatalyst for superior water
Open Access Article. Published on 09 November 2022. Downloaded on 9/16/2025 7:35:32 AM.

Cite this: Sustainable Energy Fuels,

2022, 6, 5491 oxidation†
Pradnya M. Bodhankar, ab Dattatray S. Dhawale, *c Sarbjit Giddey,c Ravi Kumarde
and Pradip B. Sarawade*ab

The development of a robust and efficient electrocatalyst for water oxidation is challenging due to the large
overpotential requirement to transfer four electrons. Herein, a novel spinel-type hierarchical teddy bear
sunflower-like NiCo2O4 electrocatalyst was synthesized through the facile solvothermal process and
evaluated for the challenging and demanding oxygen evolution reaction (OER) in the water electrolysis
process. The teddy bear sunflower-like NiCo2O4 supported on nickel foam (NF) delivers a current
density of 50 mA cm−2 at a small water oxidation overpotential (h50 = 319 mV) which is significantly
lower than that of the corresponding spherical NiO/NF (h50 = 338 mV), and sea-urchin like Co3O4/NF
(h50 = 357 mV). A large specific and electroactive surface area, as well as a high TOF value exhibited by
the hierarchical teddy bear sunflower-like NiCo2O4 electrocatalyst, demonstrates the potential of
NiCo2O4 to catalyze the water oxidation reaction efficiently. The impact of the near-Fermi-level d-
Received 13th August 2022
Accepted 3rd November 2022
orbital states in NiCo2O4 electrocatalyst for boosting OER activity was unveiled by the density functional
theory calculation. The stable performance even after 16 h and high catalyst utilization of the hierarchical
DOI: 10.1039/d2se01111e
teddy bear sunflower-like NiCo2O4 through the OER indicates that the catalyst is highly suitable for the
rsc.li/sustainable-energy large-scale water oxidation process.

and iridium (IrO2) are the most preferred catalysts for OER due
1. Introduction to their low overpotential for OER.4,5 However, the global low
At present, hydrogen generation via water splitting has been the reserves and high costs of these metals limit their use on an
subject of research for the reason that green energy plays industrial scale. To overcome these issues, scientists have come
a crucial role in environmental fortication.1 The hydrogen up with low-cost transition metal hydroxides/oxides, phos-
evolution reaction (HER) and oxygen evolution reaction (OER) phides, and suldes which have shown better performance in
occur at the cathode and anode, respectively and are the water electrolysis.6–9 More specically, the transition metals,
processes involved in electrochemical water-splitting. In this particularly Co, Ni, Fe, and Mn-based materials, are being
mechanism, the OER is of particular interest, as it is a four- considered owing to their superior OER performance for
electron transport reaction and demands high overpotential.2,3 potentially outperforming catalysts based on noble metals.10,11
In this context, it is imperative to develop highly efficient and To be practically advantageous, the catalyst needs to remain
stable water oxidation electrocatalysts that will reduce the stable while approaching high current densities (up to 200 mA
overpotential value and accelerate the process. To date, noble cm−2) by incurring low overpotentials. Thus, it is essential to
metal-based catalysts such as the oxides of ruthenium (RuO2) investigate the electrocatalyst, which will operate at high
current densities and need very little overpotential.
a
Amongst various transition metal compounds, nickel/cobalt-
National Centre for Nanosciences and Nanotechnology, University of Mumbai, Kalina,
based hydroxide and oxides have been extensively studied for
Mumbai-400098, India. E-mail: pradipsarawade@yahoo.co.in
b
Department of Physics, University of Mumbai, Kalina, Mumbai-400098, India
electrochemical water splitting owing to their multiple oxida-
c
CSIRO Energy, Private Bag 10, Clayton South 319, Victoria, Australia. E-mail: tion states, high active sites, high stability, spinel structure,
dattatray.dhawale@csiro.au high abundance, and low cost.12,13 However, their intrinsic
d
Academy of Scientic and Innovative Research, CSIR-Human Resource Development insulating nature limits their practical use.14 Thus, various
Center (CSIR-HRDC) Campus, Postal Staff College Area, Ghaziabad, Uttar Pradesh, supports, such as Ni-foam, carbon cloth, carbon black, etc., have
201002, India
e
been used to increase the conductivity of the material.15,16 These
Electronic Structure Theory Group, Physical and Materials Chemistry Division, CSIR-
kinds of supports provide electric pathways to nanoparticles,
National Chemical Laboratory, Pune, 411008, India
† Electronic supplementary information (ESI) available. See DOI:
prevent agglomeration during the electrolysis of water, and
https://doi.org/10.1039/d2se01111e increase the efficiency of catalyst utilization.17,18 Additionally, to

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catalyze the OER process, it is important to design the catalyst studies. The developed highly efficient OER catalyst by facile
with a very high specic and active surface area and optimal solvothermal route holds great potential for large-scale indus-
electrical conductivity. Various approaches have emerged to trial applications.
improve the catalytic activity, such as tuning the morphology of
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the material, elemental doping, and interfacial engineering.19–23

2. Experimental
Hierarchical structures generally offer higher surface area and
active sites and provide better catalytic performance.24 Materials 2.1 Materials
with hierarchical architectures have been extensively utilized Nickel(II) nitrate hexahydrate (Ni(NO3)2$6H2O), cobalt(II) nitrate
for electrochemical applications due to the fast diffusion of ions hexahydrate (Co(NO3)2$6H2O), urea (CO(NH)2), cetyl-
and acceleration of surface reaction during electrolysis.25 trimethylammonium bromide (CH3(CH2)15N(CH3)3Br) (CTAB),
Open Access Article. Published on 09 November 2022. Downloaded on 9/16/2025 7:35:32 AM.

Moreover, 3D architectures composed of 1D nanostructures are cyclohexane (C6H12), and 5 wt% Naon dispersion D520 were
more benecial in water-splitting reactions. Such complex purchased from Sigma-Aldrich. All materials used in the
structures promote the release of gas bubbles and display synthesis and electrochemical testing were of analytical grade
enhanced catalytic performance. Thus, it is of great interest to and used as received without further purication.
design such 3D hierarchical assemblies which consist of 1D
nanostructures, thus making the water oxidation process more 2.2 Synthesis of NiCo2O4, NiO, and Co3O4 catalysts
efficient.
For the synthesis of the NiCo2O4 catalyst, Co(NO3)2$6H2O and
Spinel AB2O4 has been considered a promising, efficient, and
Ni(NO3)2$6H2O metal precursors were used in a solvothermal
stable candidate for promoting water oxidation in alkaline
method. At rst, CO(NH)2, 0.66 M, and CTAB, 0.20 M were
solution. In this spinel structure, A and B occupy a combination
mixed in cyclohexane (C6H12, 35 mL). In 35 mL of distilled
of tetrahedral and octahedral sites with different oxidation
water, the metal precursors having 0.177 M of Co(NO3)2$6H2O
states.26 A spinel NiCo2O4 has shown eye-catching potential in
and 0.190 M Ni(NO3)2$6H2O, were dissolved. A homogeneous
energy storage and energy conversion applications due to the
mixture is obtained by adding precursor solution into cyclo-
existence of rich multiple oxidations states wherein, Ni2+
hexane by continuous stirring at 400 rpm for about 30 min. The
occupies the tetrahedral (Td) sites with eg4 t2g4 electronic
solvothermal reaction was conducted in a hot-air oven at 120 °C
conguration, and Co3+ occupies the octahedral (Oh) sites with
for 4 h. The gained products in the powder form were then
eg0 t2g6 electronic conguration.27,28 The partially lled 3d
washed and dried in the air in an oven at 60 °C for 8 h followed
orbitals in the spinel structure play a key role in generating
by annealing at 450 °C for 4 h in the air to get the NiCo2O4
outstanding electrochemical properties by facilitating the elec-
phase. The NiO and Co3O4 samples are prepared by the previ-
tron transfer process. Experimental observations and theoret-
ously reported method.34,35 The Co3O4 and NiO catalysts were
ical analysis have been performed to identify the active centers
solvothermal synthesized using Co(Cl)2$6H2O, 0.177 M and
and role of contributing orbital.29,30 For instance, Baglio et al.
Ni(NO3)2$6H2O, 0.177 M metal precursors, respectively. Both
suggested that the Co3+ and Ni3+ were accountable for better
reactions were carried out by keeping all other preparative
OER performance of NiCo2O4.31 Shanmugam and coworkers
parameters invariant as that of the NiCo2O4 catalyst. The
reported that the high concentration of Ni3+ in the NiCo2O4
prepared catalysts were then used for further structural and
electrode material enhanced the electrochemical perfor-
surface analysis, followed by water oxidation performance
mance.32 Liu et al. established improved OER performance by
investigations.
engineering NiO with Ni3+ ions.33 Thus, the determination of
surface-active species with their oxidation states that are
responsible for enhancing the catalytic activity becomes 2.3 Characterization details
a signicant interest. By employing physicochemical charac- The prepared samples were analyzed by employing various
terization tools such as X-ray photoelectron spectroscopy and structural and surface characterization techniques. The
cyclic voltammetry with low scan rates, the oxidation states of morphology of the samples was studied with scanning electron
the surface-active species could be determined. microscopy (FE-SEM, JSM-7600F, JEOL). Transmission electron
To develop a low-cost, efficient and stable OER electro- microscopy (TEM, JEM 2100, 200 kV) was used to determine the
catalyst, we describe a facile approach to prepare hierarchical microstructure of the samples. The lattice spacings in the
teddy bear sunower-like NiCo2O4, spherical NiO, and sea- crystal structure were determined by high-resolution trans-
urchin-like Co3O4 using the surfactant-assisted solvothermal mission electron microscopy (HR-TEM, JEM 2100, 200 kV). The
procedure. The prepared mesoporous hierarchical teddy bear selected area electron diffraction (SAED) patterns were acquired
sunower-like NiCo2O4/NF catalyst displayed excellent water using the same HR-TEM machine. EDX and elemental mapping
oxidation performance even at high current densities. To drive were acquired by STEM-EDX JEM-F200(URP). The crystallinity
the current densities of 10 mA cm−2, 50 mA cm−2, and 200 mA and phase of the samples were determined using a powder X-ray
cm−2, the NiCo2O4/NF catalyst requires low overpotential of diffractometer (XRD) (Rigaku Ultima IV) with Cu-ka irradiation.
290 mV, 319 mV and 330 mV, respectively. Furthermore, the X-ray photoelectron spectroscopy (XPS, K ALPHA Thermo
electronic structure and charge transport mechanism of all Fisher, Al Ka) was utilized to investigate the surface chemistry of
three structures were systematically investigated by assessing the catalysts. With the help of Avantage soware, the concen-
density functional theory (DFT) and supported by experimental tration of surface atoms of the prepared samples was estimated.

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Brunauer–Emmett–Teller (BET) surface area analyzer (Micro- before the redox reaction occurs to estimate the double-layer
meritics ASAP 2000) was utilized to obtain the N2 adsorption– capacitance (Cdl) and the electrochemically active surface area
desorption isotherm and physical surface area. (ECSA). The obtained potentials were calculated with respect to
the reversible hydrogen electrode (RHE) by using the Nernst
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2.4 DFT calculations equation: ERHE = E0 + (0.059 × pH).

The ab initio modeling is performed using Quantum ESPRESSO

(QE).36,37 The projector augmented wave method (PAW) is used
for calculations to describe the ion–electron interaction. A
3. Results and discussion
primitive cell of normal spinel of Ni2Co4O8 (contains two Fig. 1 depicts the fabrication method of the NiCo2O4 sample.
Open Access Article. Published on 09 November 2022. Downloaded on 9/16/2025 7:35:32 AM.

formula units) is used with a wave function cut-off of 90 Ry to The NiCo2O4 is synthesized with the facile solvothermal method
truncate the plane wave expansion. The energy cut-off was set to in which cationic surfactant (CTAB) plays an important role. As
10−6 Ry, and the cut-off for forces was set to 10−4 Ry bohr−1. given in the Experimental section, a 1 : 1 ratio of cobalt nitrate
Spin-polarized density functional theory (DFT) calculations are and nickel nitrate was used for the synthesis of NiCo2O4
performed using generalized gradient approximation in which composition. Initially, with increasing temperature, urea
Perdew–Burke–Ernzerhof (PBE) functional describes the decomposes and produces OH− ion. The Co2+ and Ni2+ ions
exchange and correlation.38 Dudarev et al. proposed the use of react with hydroxide ions which are liberated from urea and
Hubbard U correction to account for the correlation effect generate a Ni–Co hydroxide composite. Next, the Ni–Co
between 3d electrons of transition metals.39 Thus, we rst hydroxide composite gets arranged in a teddy bear sunower-
benchmark the value of Hubbard U correction for Ni and Co. like spherical structure wherein several nano spines grown
We get an energy minimum for U values of 3.0 and 1.6 for Co from the center could be observed.41 Lastly, aer the annealing
and Ni, respectively. The k-mesh of 7 × 7 × 7 is used for process, hydroxides of metal decompose completely and form
geometry optimization of NiCo2O4, and 14 × 14 × 14 is used for
density of state (DOS) and partial-DOS (pDOS) calculations. The
high symmetry points are generated based on Aow.40

2.5 Electrochemical study

The electrochemical water oxidation performance was investi-
gated using linear scale and cyclic voltammetry (CV) techniques.
Three-electrode system attached to the Autolab electrochemical
workstation was used for electrochemical measurements.
Initially, catalyst powder (5 mg) was dissolved in a mixture of
Naon (25 mL) and ethanol (1 ml), and the solution was soni-
cated for 10 min. Then 20 mL of prepared ink was drop cast onto
the 1 cm2 surface of the nickel foam electrode (NF) with a mass
loading of about 0.1 mg cm−2 and dried under the infra-red
lamp and used as a working electrode. The Pt wire and Ag/
AgCl lled with 3 M KCl electrolyte were used as counter and
reference electrodes, respectively. The linear sweep voltamme-
try (LSV) scans were acquired at a scan rate of 1 mV s−1 in the
potential window of 0 V to 0.8 V vs. Ag/AgCl. The CV curves at the
various scan rates (40, 60, 80, 100, and 120 mV s−1) were Fig. 2Indexed powder XRD patterns of NiCo2O4, NiO and Co3O4
recorded in the potential window of −0.01 to 0.05 V vs. Ag/AgCl samples.

Fig. 1 Schematic representation for the synthesis of hierarchical teddy bear sunflower-like NiCo2O4 catalyst.

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crystalline NiCo2O4 metal oxide. It can be seen from Fig. 1 that a hydrophobic tail giving rise to the spherical micellar structure.
a well-dened hierarchical teddy bear sunower structure of During the reaction, the inorganic anions from the cobalt salt
NiCo2O4 catalyst is obtained. The surfactant (CTAB) plays and nickel salt intrinsically adsorbed on the surfactant and thus
a signicant role in the growth mechanism of such spherical directed the nal morphology. The nal shape of the catalyst
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structures along with reaction parameters.42–44 The cationic depends on the exact conditions of the synthesis. Consequently,
surfactant CTAB contains a hydrophilic polar head and a combination of different metal precursors and CTAB
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Fig. 3 Low and high magnification FE-SEM images of hierarchical teddy bear sunflower-like NiCo2O4 (a–c, respectively). Low and high
magnification TEM images of NiCo2O4 (d–f, respectively). HR-TEM (g and h), and STEM image (i), respectively. STEM-elemental mapping of Co,
Ni, and O with an overlay of all three elements (j). EDX of NiCo2O4 (k).

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formulates diverse morphologies. The NiO and Co3O4 have SEM image. The nano spines in the NiCo2O4 are made up of
followed a similar mechanism in forming spherical and sea- numerous nanoparticles with diameters of 14–18 nm and
urchin-like structures, respectively. resemble teddy bear sunowers, as revealed by high-
The phase identication and the structure investigation of magnication TEM images (Fig. 3(e) and (f)). These nano-
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the prepared NiCo2O4, NiO, and Co3O4 samples were carried out particles interconnect and form the porous teddy bear
using X-ray powder diffraction (XRD) analysis (Fig. 2). The sunower-like structure that exhibits a high specic surface
prepared NiCo2O4 and Co3O4 samples exhibit FCC cubic spinel area. Such porous structures may adsorb enough oxygen and
structure. The observed diffraction peaks match well with the offer more active sites.48 Fig. 3(g) displays the HRTEM image of
JCPDS le no. 20-0781 for NiCo2O4 and JCPDS, no. 42-1467 for encircled region of Fig. 3(f). The enlarged view (Fig. 3(h)) reveals
Co3O4.30,45 The smallest average crystallite size was obtained for the lattice plane with a spacing of 0.25 nm attributed to the (3 1
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NiCo2O4 (5.4 nm), which was determined by the Debye–Scherrer 1) plane of the NiCo2O4 phase.49 The inset of Fig. 3(h) depicts
equation (Table S1†). The XRD pattern of the NiO sample show the SAED pattern of the hierarchical teddy bear sunower-like
peaks indexed to the pure phase of FCC NiO (JCPDS le no. 78- NiCo2O4 catalyst, which indicates the polycrystalline nature of
0643).46 The cubic lattice constants for NiCo2O4, Co3O4, and NiO the prepared catalyst and could be indexed to the spinel phase
samples (Table S1†) are obtained from high-intensity peaks and of NiCo2O4. The STEM-EDS spectra and mapping of the
are in accordance with the theoretical values indicating high NiCo2O4 shows the even distribution of the Ni, Co, and O
phase purity of all the prepared samples.47 elements on the surface, shown in Fig. 3(i) and (j), respectively.
The surface morphology of the NiCo2O4 catalyst was inves- FE-SEM images of NiO and Co3O4 samples exhibit spherical and
tigated by FE-SEM, displayed in Fig. 3(a)–(c). From low- sea-urchin-like morphology, which are displayed in Fig. S1(a)
magnication FE-SEM images (Fig. 3(a) and (b)), we can and (b).†
observe that the prepared NiCo2O4 sample displays uniform The oxidation states and elemental composition of hierar-
microspheres (4 mm diameter). The high-magnication FE-SEM chical teddy bear sunower-like NiCo2O4, spherical NiO, and
image (Fig. 3(c)) depicts that these microspheres are composed sea-urchin-like Co3O4 samples were investigated using the X-ray
of several nano spines with a length of 1 mm. The TEM image of photoelectron spectroscopy technique (Fig. 4 and S2(a)–(f)†)
the NiCo2O4 sample (Fig. 3(d)) agrees well with the obtained FE- and the analysis data is shown in Table S2.† The presence of

Fig. 4 XPS survey scan of hierarchical teddy bear sunflower NiCo2O4 catalyst (a), high-resolution XPS spectra for Co 2p, Ni 2p, and O 1s states in
NiCo2O4 (b–d, respectively).

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only Ni, Co, and O elements in the survey spectrum (Fig. 4(a)) of catalyst.54 Thus, the existence of high valence states of the metal
the hierarchical teddy bear sunower-like NiCo2O4 sample is in cations as well as oxygen adsorption species endows the hier-
line with the STEM-EDX spectrum (Fig. 3(k)). The deconvoluted archical teddy bear sunower-like NiCo2O4 catalyst with greater
(Gaussian) Co 2p3/2, Ni 2p3/2, and O 1s XPS spectrum are shown electrical conductivity and rapid charge transport for water
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in Fig. 4(b)–(d), respectively. The high-resolution Co 2p3/2 splitting.

spectrum in Fig. 4(b) establishes that spin-doublets were Brunauer–Emmett–Teller (BET) study was performed for the
attributed to Co3+ and Co2+.50 These characteristic peaks are hierarchical teddy bear sunower-like NiCo2O4, spherical NiO,
also accompanied by Co 2p3/2 satellite peaks. Fig. 4(c) shows the and sea-urchin-like Co3O4 samples to examine the surface area,
high-resolution Ni 2p3/2 XPS spectrum. The binding energy shown in Fig. S3.† BET surface area of NiCo2O4 and NiO and
peaks attributed to the Ni3+ 2p3/2 and Ni2+ 2p3/2, respectively.50,51 Co3O4 catalysts were measured to be 100.79 m2 g−1, 68.53 m2
Open Access Article. Published on 09 November 2022. Downloaded on 9/16/2025 7:35:32 AM.

These oxidation states of Ni and Co in NiCo2O4 are compared g−1, and 59.76 m2 g−1, respectively. The marginal difference in
with the Ni 2p and Co 2p3/2 line shapes from NiO and Co3O4 surface areas and morphologies suggests the large variety of
samples (Fig. S2(b) and (e)†, respectively) and it is observed that surface areas contributes more to enhancing OER activities in
both Ni 2p3/2 and Co 2p3/2 in NiCo2O4 sample exhibit a small NiCo2O4 and NiO and Co3O4 catalysts discussed in the
upward shi of 0.3 eV and extra-large shi of 1.1 eV towards following. The prominent hysteresis of type IV H3 was observed
higher binding energy, respectively indicates the signicant for the hierarchical teddy bear sunower-like NiCo2O4 sample,
electron donors adjacent to Co. A remarkable increase in the demonstrated by BET adsorption–desorption. It is well estab-
intensity ratio for Ni3+/Ni2+ was observed in NiCo2O4 (Fig. 4(c)) lished that the larger specic surface area of the catalyst
as compared to NiO (Fig. S2(b)†). Whereas no such change is provides more electroactive sites for the faradaic reactions.55,56
detected in the case of Co oxidation states when compared to This high surface area of NiCo2O4 and well-dened nano-
Co3O4. This directs that a large amount of Ni2+ ions were particles can promote water oxidation performance effectively.
oxidized to Ni3+. The XPS spectrum for O 1s can be deconvo-
luted into three peaks which can be related to the lattice oxygen
(OI), surface absorbed hydroxyl (OII), and surface C–O(OIII), 4. DFT calculation for density of
respectively (Fig. 4(d)).52–54 The peak that appeared in the O 1s states (DOS)
state of NiCo2O4 at 530.98 eV was ascribed to the coordination
of oxygen–Ni2+, signifying the maximum Ni2+ content on the Before directly measuring the water oxidation performance of
surface of the hierarchical teddy bear sunower-like NiCo2O4 all three samples, we carried out a DFT calculation to gain the
DOS statistics at the surface of each sample. To understand the

Fig. 5 The partial and total projected density of states for NiCo2O4 (a), NiO (b), and Co3O4 (c), respectively, calculated using density functional
theory (DFT)+U, (d) schematic diagram of the spin-states in the Co3O4 in the left and NiCo2O4 in the right.

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contribution of different orbitals in catalyzing the OER process, shown in Fig. 6(c), the hierarchical teddy bear sunower-like
we analyze the projected density of states (pDOS). A series of NiCo2O4/NF catalyst exhibited the ultralow Tafel slope value
pDOS plots with up and down spins for NiCo2O4, NiO, and of 37 mV dec−1, which is substantially smaller than the spher-
Co3O4 materials are shown in Fig. S4(a)–(d).† The partial and ical NiO/NF (52 mV dec−1) and the sea-urchin like Co3O4/NF (96
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total DOS for NiCo2O4, NiO, and Co3O4 materials are shown in mV dec−1). The low Tafel slope value in the case of the hierar-
Fig. 5(a)–(c). We observed that a major contribution to the up- chical teddy bear sunower-like NiCo2O4/NF catalyst indicates
channel comes from the p orbitals of the O atom followed by the facile charge transport at the interface.
the d-orbital of the Co atom in Co3O4 (Fig. S4(d)†). The contri- The electrochemical active surface area (ECSA) of electro-
bution of O-p up-spin orbitals is almost twice that of Co-d up- catalysts plays an imperative role in evaluating the activity of the
spin orbitals. In NiO no signicant contribution from the up- catalyst.58 The ECSA was evaluated by dividing the obtained
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channel was detected (Fig. S4(c)†). double-layer capacitance (Cdl) value by the specic capacitance
In NiCo2O4, the up spin and down spin of each atom (Cs) of the catalyst. To determine the Cdl of all three catalysts,
contribute equally to the entire system (Fig. 5(a)), indicating the CVs were obtained in the non-faradaic potential region at
composite effect in the electronic system. The 3d-orbitals from different scan rates (Fig. S6†). As shown in Table S4,† it illus-
Ni atoms show a higher density of states near the Fermi level (Ef) trates that the Cdl of the hierarchical teddy bear sunower-like
than Co-2d (Fig. S4(a)†) in NiCo2O4. Our calculation shows that NiCo2O4/NF, spherical NiO/NF, and sea-urchin-like Co3O4/NF
DOS around Ef is dominated by the Ni2+ cations occupying the catalysts are 1.68, 1.07, and 0.69 mF cm−2, respectively and
tetrahedral sites. Fig. 5(d) shows the predicted electron occu- presented in Fig. 6(d). The Cs value of 26 mF cm−2 and 25 mF
pation in d orbitals for the spinel Co3O4 and NiCo2O4 systems cm−2 and 27 mF cm−2 are used to calculate the ECSA of hier-
based on the oxidation states. The new electronic state is archical teddy bear sunower-like NiCo2O4/NF, spherical NiO/
present in the conduction band of NiCo2O4 (Fig. 5(d)), indi- NF, and sea-urchin Co3O4/NF catalysts.59 The higher ECSA of
cating that more electrons are available to participate in the 64.61 cm2 for hierarchical teddy bear sunower-like NiCo2O4
catalytic process. The electronic states prediction is compared catalyst compared to spherical NiO/NF (42.8 cm2), and sea-
with the literature, which matches our theoretical calculations urchin Co3O4/NF (25.55 cm2) catalysts conrms the close
and experimental results.47,57 The d-orbital occupancy inte- correlation between active sites and surface morphology. The
grated from the DOS results is presented in Table S3,† which is electrochemical impedance spectroscopy (EIS) was conducted
close to our prediction of electronic conguration. Hence, we at a potential of 1.536 V vs. RHE in the frequency range of 0.01–
postulate that the Ni atom has a major contribution to catalytic 100 kHz for all three catalysts to extract the charge transfer
activity, whereas the Co atom is responsible for conductivity in kinetics at the electrode–electrolyte interface. The Nyquist plots
NiCo2O4 material. and the tted equivalent circuit composed of a charge transfer
resistance (Rct), electrolyte resistance (Rs), and an electrical
5. Water oxidation performance double-layer capacitance (Cdl) are shown in Fig. 6(e) and inset of
Fig. 6(e), respectively. The capacitance and resistance values are
The fabricated hierarchical teddy bear sunower-like NiCo2O4, calculated and tabulated in Table S4.† The low Rct value of 1.15
spherical NiO, and sea-urchin-like Co3O4 catalysts supported on U cm2 for hierarchical teddy bear sunower-like NiCo2O4
nickel foam were subjected to electrochemical water oxidation compared to the other catalysts suggests a signicantly higher
performance through OER in 1 M KOH alkaline electrolyte as charge-transfer kinetics resulting in enhanced OER
shown in Fig. 6. The OER performances of the NiCo2O4/NF, NiO/ performance.
NF, and Co3O4/NF are evaluated by LSV polarization curves The redox reactions for the hierarchical teddy bear
(Fig. 6(a)) at a scan rate of 1 mV s−1 aer 15 cycles of CV sunower-like NiCo2O4/NF, spherical NiO/NF, and sea-urchin-
measurement. The superior OER performance was observed for like Co3O4/NF catalysts were investigated to understand the
the hierarchical teddy bear sunower-like NiCo2O4/NF catalyst surface reactivity by employing CVs between 1.036 to 1.836 V vs.
(h10 = 290 mV) as compared to the spherical NiO/NF (h10 = 300 RHE at a sweep rate of 2 mV s−1 in 1 M KOH (Fig. S7†). A pair of
mV), the sea-urchin-like Co3O4/NF (h10 = 322 mV), and redox peaks were noticed in all three catalysts. The redox peaks
commercial RuO2 (h10 = 335 mV) to achieve a current density of are principally ascribed to the formation of MO/MOOH (M
10 mA cm−2. The overpotential values with respect to different denotes Co/Ni/NiCo ions) reaction intermediate in the faradaic
current densities (10 mA cm−2, 50 mA cm−2, and 200 mA cm−2) process.60,61 The occurrence of a single anodic oxidation peak at
for NiCo2O4/NF, NiO/NF, and Co3O4/NF catalysts are plotted 1.38 V vs. RHE for both NiCo2O4/NF and NiO/NF catalysts
and presented in Fig. 6(b). The enhanced catalytic activity of attributed to the Ni2+/Ni3+ transition, conrming that a large
NiCo2O4/NF catalyst relative to NiO/NF and Co3O4/NF is attrib- amount of Ni2+ ions were oxidized to Ni3+ ions which is
uted to the porous hierarchical nanostructure exhibited by consistent with the XPS analysis.62 The sharp cathodic peaks at
NiCo2O4/NF, which could provide high surface area and abun- 1.29 V vs. RHE are observed for NiO/NF and NiCo2O4/NF cata-
dant active sites also facilitating the fast ion transport kinetics. lysts, respectively.63 Moreover, both the surface analysis tech-
Concerning the impinge of specic surface area on the intrinsic niques (XPS and CV) revealed that the Ni cations are present in
activity, BET surface area normalized LSV was plotted (Fig. S5†). large amounts on the surface of NiCo2O4/NF catalyst compared
The observed OER activity trend was similar to that of LSV to Co cations demonstrating that the Ni cations are mainly
concerning the geometrical surface area. Furthermore, as

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Fig. 6 (a) Linear polarization curves at a scan rate of 1 mV s−1 NiCo2O4/NF, NiO/NF, Co3O4/NF, RuO2, and bare NF catalysts for OER. (b)
Overpotential vs. current density plot of NiCo2O4/NF, NiO/NF, and Co3O4/NF catalysts. (c) Tafel plots of NiCo2O4/NF, NiO/NF, and Co3O4/NF. (d)
Plot representing the relationship between the difference in the anodic and cathodic current densities and scan rate. The slope of the fitted line is
the double-layer capacitance (Cdl) of NiCo2O4/NF, NiO/NF, and Co3O4/NF catalysts. (e) Electrochemical impedance spectra (Nyquist curves) of
NiCo2O4/NF, NiO/NF, and Co3O4/NF catalysts; the inset shows the equivalent circuit for the data. (f) Schematic of the four-step reaction pathway
for OER in alkaline electrolyte. (g) Comparison of the overpotential (h10) and Tafel slopes for the catalyst prepared in this work and recently
reported NiCo2O4.

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responsible for enhanced OER performance by forming NiOOH trigger the formation of NiOOH which was critical for
as a reaction intermediate and provide more active sites.50,54 enhancing OER. Certainly, the above-analyzed DFT results
Another inuential parameter for catalyst performance manifest that the morphology of the material has a consider-
evaluation is mass activity (MA). To eradicate the impact of able effect on the electronic structure near the Ef of the elec-
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mass loading, MA was determined by normalizing the current trocatalysts. Co3O4 has a normal spinel structure, where Co2+ is
density with the mass loading of the material on the surface of located at tetrahedral (Td) sites and Co3+ at (Oh) sites; however,
the electrode.64 The MA values of 140 A g−1, 95 A g−1, and 44 A NiCo2O4 has an inverse spinel structure, where preferably Ni2+
g−1 are obtained at a potential of 1.53 V vs. RHE for the hier- occupies Oh sites and Co3+ occupies both Oh and Td sites.
archical teddy bear sunower-like NiCo2O4/NF, spherical NiO/ Nevertheless, Co3+ at Td sites is thermodynamically unstable
NF, and sea-urchin-like Co3O4/NF catalyst, respectively. Mani- and likely to convert to a more stable Co2+ state. This results in
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festly, the trend is in good agreement with that of overpotential the oxidation of a similar amount of Ni2+ to Ni3+ to sustain
values for all three catalysts. Higher specic and mass activities charge neutrality. Therefore, the electronic state induced by
of the hierarchical teddy bear sunower-like NiCo2O4/NF cata- morphology is the key to describing enhanced OER activity in
lyst reect the higher electrocatalytic activity as compared to the NiCo2O4. According to the Gerischer–Marcus model of charge
other two catalysts (NiO/NF and Co3O4/NF). To further evaluate transfer, orbital overlapping occurs due to the high DOS near
the efficiency of prepared catalysts, turnover frequency (TOF) the Ef which then could adsorb OH− species and reduce the
values which is the measure of intrinsic activity were deter- energy to accept or donate electrons at the adsorbate–catalyst
mined.65 Considering that all of the available surface metal interface reaction.67 As mentioned above, 3D nanostructure
atoms are catalytically active, TOF at a xed potential of 1.53 V morphology in NiCo2O4 exhibits far more active sites and could
vs. RHE was determined as 0.61 s−1, 0.26 s−1, and 0.18 s−1 for adsorb higher amounts of OH− species, also providing a suffi-
the NiCo2O4/NF, NiO/NF, and Co3O4/NF catalysts, respectively, cient conduction path to prevent serious ohmic losses demon-
further suggesting that the hierarchical teddy bear sunower- strated by EIS investigations and adsorption isotherms. We,
like NiCo2O4/NF is indeed an efficient and potent catalyst. therefore, plot the ECSA-corrected LSV curves for all three
electrocatalysts (Fig. S8†). It is illustrated that porous nano-
5.1 Correlation of the OER activity with electronic states in structure fairly enhances the intrinsic catalytic activity of
different structures NiCo2O4/NF. Moreover, the higher intrinsic activity with large
Herein, the theoretical calculations and experimental study electron transfer has been witnessed by catalyst utilization
complement each other for the study of OER mechanisms from measurement. Catalyst utilization is as the ECSA of the active
the perspective of thermodynamics and kinetics. The water catalyst divided by the BET surface area of the catalyst (see ESI
oxidation reaction in an alkaline solution is expressed as; eqn (2)†). The high efficiency of catalyst utilization (64.28%) in
NiCo2O4/NF is attributed to the higher adsorption of OH− ions
4OH− / O2(g) + 2H2O(l) + 4e− (1) and better charge conductivity at the electrode interface (Table
S5†).
This specic water oxidation process of OER can be Of note, all the aforementioned OER activity evaluation
described in terms of the adsorbate evolution mechanism parameters, including electroactive surface area, charge trans-
(AEM) with four elementary steps as follows (Fig. 6(f));66 fer resistance, electrical conductivity, catalyst utilization, and
TOF (Table 1) values suggest that the NiCo2O4/NF exhibiting
M + OH− / MOH + e− (2) teddy-bear sunower-like morphology is highly active for the
water oxidation process compared to spherical NiO/NF and sea-
MOH + OH− / MO + H2O + e− (3) urchin like Co3O4/NF. Such hierarchical nanostructure unveiled
by NiCo2O4/NF achieves high current density at low over-
MO + OH− / MOOH + e− (4)
potential and delivers high intrinsic activity owing to facile
MOOH + OH− / MO2 + H2O + e− (5) electrolyte penetration along with the prominently high density
of Ni 3d states near the Fermi level. Fig. 6(g) and Table S6† show
where MOH, MO and MOOH represent adsorbed reaction the comparison of overpotential values at a current density of 10
intermediates with metal surface site M. Notably, the XPS and mA cm−2 and Tafel slope for recently reported nickel and
slow scan CV investigation provide evidence for the existence of cobalt-based bimetallic catalysts.68–74 It is noteworthy that, the
more active Ni3+ cations than Co on the surface of NiCo2O4 to fabricated hierarchical teddy-bear sunower-like NiCo2O4/NF in

Table 1 Summary of catalytic performances for the prepared NiCo2O4/NF, NiO/NF, and Co3O4/NF electrocatalysts

Overpotential (mV) Tafel slope mV BET surface Mass activity

Electrocatalyst at 10 mA cm−2 dec−1 ECSA m2 g−1 area m2 g−1 (mA mg−1) TOF s−1

NiCo2O4/NF 290 37 64.61 100.79 140 0.61

NiO/NF 300 48 42.40 68.53 95 0.26
Co3O4/NF 322 54 25.55 59.76 44 0.18

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Fig. 7 The chronoamperometric curve at 1.55 V vs. RHE of NiCo2O4/NF catalyst (a), LSV curves of NiCo2O4/NF catalyst before and after stability
for OER in 1 M KOH at a scan rate of 1 mV s−1 (b).

this work is the potential low-cost candidate as an OER catalyst electrochemical performance and sustained morphology, even
for practical applicability in the water-splitting process. aer prolonged cycling, indicate that the catalyst is highly
In addition to catalytic efficiency, stability is another suitable for the large-scale water oxidation reaction. This work
important parameter for an OER catalyst. To conrm the provides a systematic analysis of the structural and electro-
stability of the hierarchical teddy bear sunower-like NiCo2O4/ chemical performance supported by DFT calculation of the
NF catalyst, chronoamperometry measurement was carried out prepared catalyst toward their OER counterparts.
at 1.55 V vs. RHE for 16 h, shown in Fig. 7(a). It clearly indicates
that the hierarchical teddy bear sunower-like NiCo2O4/NF
catalyst is highly stable in 1 M KOH. The LSV at 1 mV s−1 for the
Conflicts of interest
catalyst was recorded aer stability testing and compared with There are no conicts of interest to declare.
the initial LSV (before stability), shown in Fig. 7(b). The hier-
archical teddy bear sunower-like NiCo2O4/NF catalyst dis-
played similar activity even aer 16 h (Fig. 7(b)). The Acknowledgements
morphology of the hierarchical teddy bear sunower-like
This work was supported by the Department of Science and
NiCo2O4/NF catalyst aer the OER operation was shown in
Technology-WOS-A scheme (SR/WOS-A/PM-9/2018(G)).
Fig. S9(a)–(d).† The characteristic teddy bear sunower-like
structure was preserved even aer 16 h, demonstrating the
practical electrocatalytic durability of the product. References
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