Ni-N Doped CNT Electrocatalyst for Hydrogen Evolution Reaction

Applied Surface Science 562 (2021) 150161
Available online 23 May 2021
0169-4332/© 2021 Elsevier B.V. All rights reserved.
Full Length Article
Controllable fabrication of abundant nickel-nitrogen doped CNT
electrocatalyst for robust hydrogen evolution reaction
Chidinma Judith Oluigbo
a
, Yuanguo Xu
a , *
, Hitler Louis
c
, Adegbemiga Bashir Yusuf
a
,
Waleed Yaseen
a
, Nabi Ullah
a
, Kumar Jagadeeh Alagarasan
a
, Meng Xie
b , *
,
Edison Esther Ekpenyong
c
, Jimin Xie
a
a
School of Chemistry & Chemical Engineering, Jiangsu University, Zhenjiang 212013, PR China
b
School of Pharmacy, Jiangsu University, Zhenjiang 212013, PR China
c
Computational and Bio-Simulations Research Group, Department of Pure and Applied Chemistry, Faculty of Physical Sciences, University of Calabar, P.O Box 540221,
Calabar, Cross River, Nigeria
ARTICLE INFO
Keywords:
Hydrogen evolution reaction
Electrocatalysis
Carbon nanotubes
Nitrogen
Alkaline medium
Density functional theory
ABSTRACT
Designing highly effective and durable non-noble metal based electrocatalysts for hydrogen evolution reaction
(HER) is cardinal for the sustainability of clean energy technologies. This present work explores the fabrication of
nickel-nitrogen doped CNT through a simplistic approach. The as-fabricated Ni-NCNT
8.0
showcases a robust HER
performance amongst the other catalysts (Ni-NCNT
7.5
, Ni-NCNT
8.5
, Ni-x, and Glu-x) with a small overpotential of
147 mV and Tafel slope of 57.6 mV dec
-1
to approach a current density of 10 mA cm
−2
in alkaline medium. It also
exhibits an excellent stability for 20 h. The high HER performance originates from the abundant and well-formed
N-doped CNT that serves as conductive support and the synergy between the nickel nanoparticles which sufce
as the catalyst active sites and the N-doped CNT. Additionally, the optimized electronic structure of the nickel
nitrogen-doped CNT provided by density functional theory calculations, revealed a low HOMO-LUMO energy
gap and high ionization potential that enhanced the stability of the electrocatalyst and conveys an effective HER.
1. Introduction
The escalating environmental pollution, energy challenges and
depleting fossil fuel sources are stipulating researchers globally to
explore modern and efcient energy resources, carriers and technologies
[1–6]. Hydrogen, having a high gravimetric density of 142 MJ/Kg, and
multifarious sources of production is a promising green energy carrier to
replace fossil fuels [7–9]. Water electrolysis propelled by electricity
among other production routes, has been identied as the most effective
and sustainable strategy for producing pure and clean hydrogen given its
zero-carbon emission coupled with the abundance of water resources
[10–15]. Hydrogen evolution reaction (HER) asides from oxygen evo-
lution reaction (OER) and oxygen reduction reaction (ORR) is an
essential electrochemical reaction important for fuel cells [16–19].
Although, Pt-composite catalysts are the representative bench mark
catalysts for HER from water through an electrochemical route due to
their small reduction overpotentials and good kinetics. However, their
outrageously exorbitant price and limited obtainability impairs their
commercialization [20–24]. Hence the continuous search for more
available, cheap, active and stable non precious metal electrocatalysts
that would mimic or even surpass the activities of the noble metals
cannot be overemphasized.
Nickel is one of the most promising non precious metals which is
believed to serve as an alternative to Pt as a result of its good catalytic
activity towards HER, availability and cheapness [3]. However, the
catalytic activity of bulk nickel is still far less that of the Pt catalyst.
Therefore, several approaches are being explored to enhance the elec-
trocatalytic activity of bulk nickel, some of which include nano-
structuring that improves the surface area of the electrode, heteroatoms
like nitrogen, boron, phosphorous and sulphur doping, and the incor-
poration of nanostructures of carbon like carbon nanotubes, graphene,
mesoporous carbon and carbon nanosheets [25,26].
Carbon nanotubes are normally used as electrocatalysts support due
to their good thermal and electronic conductivity, chemical inertness
and good mechanical properties [27]. On the other hand, heteroatom
such as N doping can increase the HER performance of the
* Corresponding authors.
E-mail addresses: [email protected] (Y. Xu), [email protected] (M. Xie).
Contents lists available at ScienceDirect
Applied Surface Science
journal homepage: www.elsevier.com/locate/apsusc
https://doi.org/10.1016/j.apsusc.2021.150161
Received 13 January 2021; Received in revised form 16 April 2021; Accepted 17 May 2021

Applied Surface Science 562 (2021) 150161
2
electrocatalyst by polarizing the C atoms given its better electronegative
value than carbon and its provision of more active sites [28], in addition
to the easy formation of the C
–
N bond by reason of the close proximity
of the N radius to that of C [29]. Previous reports has conrmed that
nitrogen doping on carbon nanostructures lowers the anti-bonding
density of states (DOS) of the adsorbed hydrogen, this in turn, reduces
the HER activation energy [30]. Thus metal nanoparticles or their alloys
encapsulated in nitrogen-doped carbon nanotubes exhibits an overall
improvement in their electrocatalytic activity [31,32]. Owning to the
advantages of N-doping on nickel encapsulated carbon nanotubes for
HER, researchers such as Oluigbo et al., fabricated NiNCNT with an
overpotential of 244 mV and Tafel slope of 93.3 mV dec
-1
at 10 mA cm
−2
in 1 M KOH solution [11]. Wang et al., had synthesized Ni@NCNT for
HER that afforded an overpotential of 256 mV and Tafel slope of 131.8
mV dec
-1
at 10 mA cm
−2
in 1 M KOH solution [33]. In addition, Ni/N-
CNT HER electrocatalyst prepared by Jia et al., in 1 M alkaline solu-
tion obtained
η
10
greater than 200 mV and a Tafel slope of 141 mV dec
-1
[34]. While the N-CNT/Ni HER electrocatalyst fabricated by Wang et al.,
in 1 M KOH electrolyte at 10 mA cm
−2
, afforded a value of 308 mV as
overpotential and a Tafel slope of 140.2 mV dec
-1
[35]. However,
preparation of the CNT requires CVD method which is expensive with
complex design of reactors. Therefore, in-situ fabrication of N-doped
NiCNT provides the advantages of a facile, eco-friendly, cheap and less
time-consuming synthetic pathway for efcient, highly conducting and
stable HER electrocatalysts.
In this work, a simplistic and in-situ route is employed in the fabri-
cation of nitrogen and nickel carbon nanotubes. In addition, the cata-
lysts are realized in a small time of 2 h and requires no rigorous post
treatments. The synthesis also involves melamine as a nitrogen and
reducing agent, nickel acetate and glucose as nickel and carbon sources
respectively. As the annealing process progressed, nickel ions (Ni
2+
)
were transmuted to metallic nickel by the reducing species from mel-
amine which then catalysed the formation of bamboo-like structured
CNT. The fabricated electrocatalyst portrayed outstanding performance
towards HER. Furthermore, the DFT calculations of the HOMO-LUMO
energy gap, ionization potential and adsorption energies furnished
more insight on the electrochemical performance of the electrocatalysts
towards HER.
2. Experimental section
2.1. Chemicals
Potassium hydroxide (KOH) (≥85.0%), glucose D (C
6
H
12
O
6
⋅H
2
O)
(≥98.0%), nickel (II) acetate tetrahydrate (NiC
4
H
6
O
4
⋅4H
2
O) (≥98.0%),
absolute ethanol (C
2
H
5
OH) (≥99.5%) and melamine (C
3
H
6
N
6
)
(≥99.0%) obtained from Sinopharm chemical reagent Co., Ltd. The
continuous carbon/formvar support lm was purchased from Zhong-
jingkeyi (Beijing) Film Technology Co., Ltd.
2.2. Fabrication of nickel-nitrogen doped CNT
In a conventional procedure, Nickel (II) acetate tetrahydrate (2
mmol) and Glucose (2 mmol) were added to 20 mL aqueous solution of
melamine (40 mmol) which was kept stirring for 2 h at 80 ◦C. The
mixture, dried for 48 h at 80 ◦C was comminuted with an agate mortar
and pyrolyzed for 2 h at temperatures of 750, 800 and 850 ◦C and ramp
rate of 2◦min
−1
under a stream of N
2
. The samples obtained after being
thoroughly rinsed with water and absolute ethanol were placed in an
electric oven at 70 ◦C to dry. The samples were then labelled as Ni-
NCNT
7.5
, Ni-NCNT
8.0
and Ni-NCNT
8.5
in correspondence with the py-
rolysis temperatures.
2.3. Physical characterization
To investigate the crystal structure and phase purity of the samples
by X-ray diffraction (XRD), a D8 Advance X-ray diffractometer [Bruker
AXS Company, Germany] was employed. A DXR laser Raman spec-
trometer [Thermo Fisher Corporation, USA] using a 532 nm laser source
was employed in obtaining the Raman spectra at room temperature. X-
ray photoelectron spectrometer (PHI-500 Al K
α radiation) was
employed to record the chemical states of the elements present. For the
purpose of probing the morphology, composition and particle size of the
samples, scanning electron images (SEM), energy dispersive X-ray
spectroscopy (EDX) and elemental mapping images were captured with
a [Hitachi S-4800 II, Japan] microscope operating at 15.0 kV. Trans-
mission electron (TEM) and high-resolution transmission electron (HR-
TEM) images were measured on a [JEOL-JEM-2010 JEOL, Japan]. The
pore structure was probed with a surface analyser (Micrometrics, ASAP
2020 HD88) by determining the Brunauer-Emette-Teller (BET) specic
surface area and pore size distribution from the BET and Barrett-Joyner-
Halenda (BJH) models. The nickel content was determined by induc-
tively coupled plasma-optical emission spectroscopy (ICP-OES) with a
Thermo Scientic iCAP 7000.
2.4. Electrochemical characterization
All the electrochemical measurement was investigated on a 660D
electrochemical analyser with the traditional three-electrode arrange-
ment at 25 ◦C in a 1 M potassium hydroxide N
2
-saturated solution.
Carbon rod, Hg/HgO and modied glassy carbon electrode (GCE 3 mm
diameter) were used as the counter, reference and working electrodes,
respectively. The GCE was modied by dropping 5 µL of a homogenous
catalyst ink on its polished surface and left to dry at an ambient atmo-
sphere. The catalyst ink was readied by a 40 min ultrasonication of the
catalyst (5 mg), naon (30 µL) (5 wt%), absolute ethanol (250 µL) and
deionised water (720 µL) mixture. Consequently, the loaded mass of the
catalyst on the GCE was 0.357 mg cm
−2
and was activated by per-
forming 50 CV cycles. A 5 mV s
−1
scan rate and a potential limit of 0.124
to −0.476 V (vs RHE) were employed to record the polarization (LSV)
curves. With similar conditions, commercial Pt/C (10 wt%) was applied
for comparison. All the potentials were IR-corrected and translated to
the reversible hydrogen electrode (RHE) by the formula: E
RHE
=E
vs Hg/
HgO
+E
ϴHg/HgO
+0.059 pH. The electrochemical impedance spectros-
copy (EIS) was recorded by open circuit potentials with an amplitude of
5 mV in the frequency limit of 1.0 to 10
5
Hz. A potential value of −0.376
V vs RHE was applied in the chronoamperometry (i.t) experiment for a
20 h stability. For the electrochemically active surface area (ECSA),
double layer capacitances (C
dl
) were recorded from CV scans using scan
rates from 40 to 160 mV s
−1
. The C
dl
values were obtained from the
slopes of the graph of Δj at 0.1 V vs RHE against the scan rates.
2.5. Computational method
The density functional theory (DFT) calculations was carried out to
optimize the geometry of carbon nano-tube (CNT) using DMol3 code in
the Material Studio Software [36]. The CNT was imported from material
studio structural data collection and was modelled using the build
nanotube interface. The electronic structures of CNT were studied with
generalized gradient approximation (GGA) and Perdew-Burke-
Ernzerhof (PBE) exchange–correlation function including dispersion
corrections (DFT-D) [36,37]. The calculations were spin restricted with
displacement and convergence maximum force at 0.005 Å and 0.004
Ha/ Å respectively. The armchair CNT was only considered because it
has enhanced charge transport features which corresponds with the
experimental ndings of our studies and has been widely used to
simulate CNT [38,39]. A total of 8 congurations of the arm-chair car-
bon nanotubes were optimized. The optimized armchair nanotube was
further doped with nitrogen and nickel atoms by replacing a carbon
atom with a nitrogen or nickel atom in the middle side wall of the carbon
nanotubes. The bond lengths, bond angles, and the HOMO-LUMO en-
ergy gap were calculated. The adsorption of water molecule on the
C.J. Oluigbo et al.

Applied Surface Science 562 (2021) 150161
3
optimized doped-carbon nanotubes was also performed.
3. Results and discussion
In the fabrication process as represented in Fig. 1, during the
annealing process, the reducing species from melamine reduced the
nickel ions (Ni
2+
) to metallic nickel.
The melamine further transmuted into graphitic carbon nitride (g-
C
3
N
4
). The g-C
3
N
4
then decomposed into bamboo-like structured carbon
nanotubes with the metallic nickel nanoparticles acting as catalysts as
temperature increased [40]. In addition, the glucose acting as the carbon
source increased the abundance as well as ensuring a proper
morphology of the carbon nanotubes formed. The structural information
and purity of the fabricated catalysts was examined with X-ray diffrac-
tion. Fig. 2a illustrates the XRD patterns at different annealing temper-
atures of 750, 800 and 850 ◦C. The similar patterns suggest the
uniformity of the components of the catalysts prepared. The peak at
26.3◦is assigned to the graphitic carbon (0 0 2) which is representative
of the CNT and indexed to JCPDS No. 75–1621. The peaks at 44.5◦, 51.8◦
and 76.4◦corresponds with the lattice planes (1 1 1), (2 0 0), and (2 2 0)
of metallic nickel with JCPDS No. 87–0712. The graphitization and
defects degree was further determined through Raman spectroscopy.
Fig. 2b shows the Raman spectra possessing the representative D, G and
2D bands typical of carbonaceous materials. The D-band corresponds to
the breathing pattern of the κ-point phonon of the disordered carbon or
defects and edges of sp3-carbon atoms, whereas, the G-band is assigned
to the tangential vibration of the corresponding ordered sp2-bonded
carbon atoms (Brillouin zone centre E
12g
mode) [41–43] and 2D-band
corresponds to the number of carbon layers [44].
The D bands appeared at about 1346, 1342 and 1334 cm
−1
whereas,
the G bands appeared at about 1573, 1573 and 1581 cm
−1
for Ni-
NCNT
7.5
, Ni-NCNT
8.0
and Ni-NCNT
8.5
, respectively and the 2D bands
occurred at 2680 cm
−1
. The intensity ratio of the D and G band (I
D
/I
G
)
reects the degree of carbon graphitization and density of defects [45].
The I
D
/I
G
intensity ratio was 0.89, 1.06 and 0.91 for the as fabricated
catalysts at 750, 800 and 850 ◦C, respectively. Thus, as the carbonization
temperature increases, the ratio increases as observed for Ni-NCNT
8.0
which is 1.06 (>1) as a result of enormous defects compared to the
samples (Ni-NCNT
7.5
and Ni-NCNT
8.5
)
.
This can cause a modication in
the electronic structure and increase the active sites which can enhance
the HER activity [46]. However, further increase in temperature to
850 ◦C decreases the I
D
/I
G
intensity ratio to 0.91. In general, lower I
D
/I
G
values implies smaller degree of defects at 750 and 850 ◦C, which could
be due to low or high temperatures for the formation or destruction of
C
–
N related bonds respectively [47,48].Thus, the Raman results are
consistent with the XRD and SEM results. The XPS was carried out to
study the various electronic and binding states of the elements in the
representative sample (Ni-NCNT
8.0
). The survey spectra in Fig. S1a
portray Ni, C, N, and O to be present. The high-resolution spectrum in
Fig. 3a of Ni 2p reveals four peaks which are tted into two nickel
species Ni
2+
and Ni
3+
. The peaks at 856.8 and 874.5 eV with their sat-
ellites at 862.5 and 880.5 eV are assigned to Ni 2p
3/2
and Ni 2p
1/2
respectively [49]. No metallic nickel was detected because the XPS
analysis technique deals with the surface (1–10 nm) composition of the
nanomaterial. The C 1 s deconvoluted spectrum Fig. 3b exhibits three
peaks. The stronger peak at 284.5 eV reects the sp
2
C
–
C/C
–
–
C, while
the weaker peaks at 285.5 and 288.9 eV denotes the C
–
N and O
–
C
–
–
O
respectively [50,51]. The C
–
N peak shows the efcacious doping of
nitrogen onto the carbon nanotubes.
The N 1s high resolution spectrum Fig. 3c shows three prominent
peaks at 398.5, 400.3, and 401.2 eV that corresponds to the pyridinic-N,
pyrrolic-N and graphitic-N respectively [51]. Previous works has
established that the pyridinic and the graphitic nitrogen effectively
improves the electrochemical activity of the catalyst [52]. Furthermore,
the high-resolution spectrum in Fig. 3d of O 1s reveals peaks at 531.3,
532.3 and 534.0 eV assigned to C
–
–
O, C
–
OH and C
–
O respectively.
More insights into the composition of the samples were obtained from
the EDX measurements of Ni-NCNT
8.0
. The elements C, O, N and Ni as
revealed by the EDX spectra (Fig. S1b) is in accord with the XPS results.
The uniform distribution of the elements is shown by the mapping im-
ages (Fig. 4a–d).
The morphology and microstructure of the as-fabricated catalysts
were recorded using the SEM analysis. Fig. 5(a–d) reveals the SEM im-
ages for Ni-NCNT
7.5
, Ni-NCNT
8.0
, and Ni-NCNT
8.5
. The SEM image of Ni-
NCNT
7.5
in Fig. 5a shows a poor morphology which could be as a result
of a relatively low temperature leading to the partial formation of the
CNT. While, the SEM images of Ni-NCNT
8.0
, and Ni-NCNT
8.5
Fig. 5b, c
and d show clusters of well-structured and properly formed CNT
together with the nickel nanoparticles (NiNP).
Besides the pyrolysis temperature, the concentration of the nickel
and glucose also plays signicant roles in determining the morphology
of the as-synthesized samples. Therefore, the concentration of the nickel
was varied from 0.0, 0.5, 1.0, 1.5 … 3.0 mmol, while still maintaining
the pyrolysis temperature at 800 ◦C and glucose concentration at 2
mmol. The as-fabricated catalysts were labelled as Ni-x where ×stands
for the concentration of nickel. The SEM images are shown in Fig. S2
(a–g). It is observed that with no/low nickel concentrations such as 0.0,
0.5 and 1.0 mmol, there are no formation of carbon nanotubes only
T:\scanning\Elsevier\Journal\APSUSC\150161\
Fig. 1. Schematic illustration for the fabrication of nickel-nitrogen doped CNT.
C.J. Oluigbo et al.

Applied Surface Science 562 (2021) 150161
4
masses of porous carbon. However, with an increase in the nickel con-
centration to 1.5 mmol, carbon nanotubes start to form but further in-
crease to 2.5 and 3.0 mmol, causes agglomeration and increase in size of
the nickel particles leading to very poor morphology and low formation
of the carbon nanotubes. Similarly, the concentration of glucose was also
varied from 0.0, 0.5, 1.0, 1.5 … 3.0 mmol, while maintaining the py-
rolysis temperature at 800 ◦C and nickel concentration at 2 mmol. The
as-fabricated catalysts were labelled as Glu-x where ×denotes the
glucose concentration. The SEM images are shown in Fig. S3(a–g). The
presence of CNT is observed in all the as prepared catalyst. However,
their morphology differs as the catalysts with 1.5 and 2.0 mmol of
glucose presents better and well-formed CNT than the other concen-
trations. The CNT prepared without any glucose presents more of fatter,
coiled and irregular morphology. With an increase in glucose concen-
tration from 2.5 to 3.0 mmol, the CNT are formed together with carbon
Fig. 2. (a) XRD patterns and (b) Raman spectra of Ni-NCNT
7.5
, Ni-NCNT
8.0
and Ni-NCNT
8.5.
Fig. 3. High resolution XPS spectra of Ni-NCNT
8.0
(a) Ni 2p (b) C 1s (c) N 1s (d) O 1s.
Fig. 4. Mapping images of (a) C (b) O (c) N and (d) Ni for Ni-NCNT
8.0.
C.J. Oluigbo et al.

Applied Surface Science 562 (2021) 150161
5
sheet-like structures thus, reducing the concentration of the formed
CNT. Further information about the morphology and particle sizes was
obtained from the TEM and HRTEM analysis. The TEM images of Ni-
NCNT
8.0
(Fig. 5e) displays the jointed structure of bamboo-like CNT
representative of nitrogen doped carbon nanotubes. The TEM images,
reveal that the diameter of the CNT varied from 21.13 to 48.46 nm with
the average diameter around 32.14 nm while the NiNP ranged from
18.39 to 71.96 nm with the mean size around 31.96 nm. Fig. 5f reveals
the high resolution TEM (HRTEM) image of Ni-NCNT
8.0
and the nickel
nanoparticle shelled by graphitic carbon layers with lattice fringe of
about 0.34 nm assigned to (0 0 2) plane of graphitic carbon. The nickel
nanoparticle exhibited a lattice spacing of 0.20 nm in correspondence to
the (1 1 1) lattice plane of nickel, the HRTEM results obtained is in
conformance to the XRD results. The nitrogen-sorption was executed to
unravel the specic surface area and pore size distribution of the as-
synthesized Ni-NCNT. As shown in Fig. S4(a–b), the isotherm exhibits
feature of type IV isotherm with hysteresis loop at (P/Po ≈0.4–1.0)
revealing the presence of mesopores. The results revealed BET specic
surface areas of 71.93, 63.47 and 35.15 m
2
g
−1
for Ni-NCNT
7.5
, Ni-
NCNT
8.0
and Ni-NCNT
8.5
, respectively. Furthermore, the pore volumes
obtained from the BJH analysis reveals values of 0.20, 0.17 and 0.13
cm
3
g
−1
for Ni-NCNT
7.5
, Ni-NCNT
8.0
and Ni-NCNT
8.5
, respectively. It is
observed that as temperature increases, the specic surface area reduces
this is because a higher temperature treatment can alter the pore
structure thus affecting the catalytic activity [53]. From the ICP-OES,
the nickel content for the samples prepared at 750, 800 and 850 ◦C
were 44, 44.2 and 46.5 ppm respectively.
The HER electrocatalytic activity of the samples was studied in a 1 M
KOH alkaline solution with a three-electrode set-up. Fig. 6a illustrates
the LSV plots with IR-compensation reveals that Pt/C as expected,
1 µm 1 µm 100 nm
1 µm
a b c
de f
Fig. 5. SEM images of (a) Ni-NCNT
7.5
(b and c) Ni-NCNT
8.0
and (d) Ni-NCNT
8.5
(e) TEM and (f) HRTEM images of Ni-NCNT
8.0.
Fig. 6. (a) LSV curves and (b) Tafel plots for the HER of Ni-NCNT
7.5
, Ni-NCNT
8.0
, Ni-NCNT
8.5
and Pt/C (c) Nyquist plots for Ni-NCNT
7.5
, Ni-NCNT
8.0
and Ni-NCNT
8.5
(d) Chronoamperometric (i-t) curve for Ni-NCNT
8.0
(inset LSV curves before and after i-t).
C.J. Oluigbo et al.