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Nitrogen doped vertical graphene as metal-free electrocatalyst for hydrogen
evolution reaction
Article  in  Materials Research Bulletin · February 2021
DOI: 10.1016/j.materresbull.2020.111094
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Materials Research Bulletin 134 (2021) 111094
Available online 6 October 2020
0025-5408/© 2020 Published by Elsevier Ltd.
Nitrogen doped vertical graphene as metal-free electrocatalyst for hydrogen
evolution reaction
Yahao Li a
, Changzhi Ai b
, Shengjue Deng a
, Yadong Wang c
, Xili Tong d
, Xiuli Wang d
,
Xinhui Xia a
, Jiangping Tu a
a
State Key Laboratory of Silicon Materials, Key Laboratory of Advanced Materials and Applications for Batteries of Zhejiang Province, Department of Materials Science
and Engineering, Zhejiang University, Hangzhou, PR China
b
State Key Laboratory of Marine Resource Utilization in South China Sea, Hainan University, Haikou, PR China
c
School of Engineering, Nanyang Polytechnic, 569830, Singapore
d
State Key Laboratory of Coal Conversation, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, PR China
A R T I C L E I N F O
Keywords:
Metal-free catalyst
Vertical graphene
Nitrogen doping
Hydrogen evolution reaction
A B S T R A C T
Building up efficient water electrolysis system relays on the development of highly active, cost-effective, and
stable electrocatalysts. Carbon-based metal-free catalysts are one of the promising candidates for this purpose.
Herein, we report a facile strategy to synthesize nitrogen doped vertical graphene (N-VG) array for catalyzing
hydrogen evolution reaction (HER) in acidic electrolyte. The resultant N-VG metal-free catalyst exhibits
enhanced HER performance with an overpotential of 290 mV at 10 mA cm− 2
and good cycling stability. The
enhanced performance is due to the enlarged surface area and optimization of electronic structure induced by the
introduction of N heteroatom. Moreover, two possible enhancement mechanisms of N-VG are revealed with first-
principle calculations. Our work further demonstrates the effectiveness of N doping on enhancement of HER.
1. Introduction
Producing hydrogen via electrocatalytic hydrogen evolution is one of
the most efficient methods to achieve large-scale applications [1–3].
Nowadays, hydrogen evolution reaction (HER) is normally catalyzed by
noble-metal-based catalysts [4,5]. However, for the well-known reasons
(high expense, low reserves, poor stability, etc.), developing cheap,
stable, and highly active electrocatalysts to act as the alternatives for
noble metals are eagerly wanted. Building noble metal alloys [6–8],
searching suitable transition-metal-based compounds [9–11] and
developing single atom catalysts [12–14] are some of the mature stra
tegies and many excellent results were achieved. Despite these
achievements, drawbacks such as weak stability under harsh environ
ments, complicate fabrication procedures, and difficulty in large-scale
productions are hard to avoid with metal-based materials [15–17].
Metal-free catalysts are thus the kind of materials researchers are
looking for.
Owing to their high conductivity, abundant morphologies, and easy
fabrication/ functionalization, carbonaceous materials have attracted
great attention [18,19]. Graphene, carbon nanotubes, and activate
(porous) carbon are often applied as the conductive substrate for
developing composite catalysts [20–22]. However, pure carbon mate
rials exhibit poor electrocatalytic activity towards HER. Meanwhile,
nitrogen doping is verified as one of the effective strategies to func
tionalize the carbon materials and enhance its HER activity [23–25]. For
example, Zhang and coworkers prepared N-doped hollow carbon
nanoflowers with 5.3 at. % of N content, achieving a low overpotential
of 243 mV at 10 mA cm− 2
in acidic electrolyte [26]. Yang and coauthors
further decreased the overpotential to 143 mV with 3D porous nitrogen
doped carbon derived from cigarette butts [27]. Li’s group reported an
extremely low overpotential of 90 mV at 10 mA cm− 2
realized with N
doped ultrathin carbon nanosheets [28], whose value was even lower
than most reported transition-metal-based catalysts for HER.
Recently, free-standing vertical graphene array (VG) on the carbon
cloth have been widely explored and show advantages of high conduc
tivity, large surface area and robust stability. It has been used as sub
strates for electrode materials of Li-ion [29], Na-ion [30,31], Li-S [32,
33] batteries and electrocatalysts for HER/OER [21,34]. However, there
is no work on the N-doped VG for HER. Herein, we report the nitrogen
doping of VG via a simple NH3 treatment. The morphology of VG is
well-remained and three types of nitrogen are introduced. The obtained
N-VG exhibits a good HER performance with an overpotential of 290 mV
E-mail address: tujp@zju.edu.cn (J. Tu).
Contents lists available at ScienceDirect
Materials Research Bulletin
journal homepage: www.elsevier.com/locate/matresbu
https://doi.org/10.1016/j.materresbull.2020.111094
Received 12 August 2020; Received in revised form 14 September 2020; Accepted 30 September 2020

2
at 10 mA cm− 2
, a Tafel slope of 121 mV dec-1
, and a long-term stability.
Moreover, we also reveal the possible enhancing mechanisms of
N-doping for HER with the help of DFT calculations. The results indicate
that introducing new active sites (pyridinic N) and enhancing existing
sites by optimizing electronic structures (pyrrolic N and graphitic N) can
both improve the overall HER activity of N-VG.
2. Experimental
2.1. Materials synthesis
Caution! The following synthesis involves the usage of high-power
plasma and pure NH3 gas and should handle with caution. Vertical
graphene was synthesized via a facile plasma enhanced chemical vapor
deposition (PECVD). A sheet of carbon cloth (2 × 3 cm2
) was cleaned
with deionized water and absolute ethanol for several times before
transferred into the tube furnace. A mix flow of H2, argon and CH4 with a
ratio of 2: 10: 1 was then introduced and the pressure was controlled to
be ~3 × 10− 2
torr. The carbon cloth was heated to 800 ◦
C in an hour and
the plasma generator was turned on with a power of 500 W and kept for
5 min to finish the reaction. After cooling to room temperature under
argon protection, the vertical graphene was obtained.
The N-VG was prepared by a simple NH3 treatment on VG. The as-
obtained VG was transferred into a tube furnace, heated to 300 ◦
C
with a heating rate of 5 ◦
C min− 1
under a NH3 flow and kept for 2 h.
After cooling in an argon flow, the nitrogen doped vertical graphene (N-
VG) was obtained.
2.2. Characterizations
Scanning electron microscopy (SEM, Hitachi SU8010) and trans
mission electron microscopy (TEM, JEOL 22100F) were carried out to
investigate the morphology and microstructure of as-obtained samples.
The XRD patterns were collected using an X-ray diffractometer (Rigaku
D/Max-2550) with Cu Kα radiation. Raman spectra were obtained using
a Renishaw inVia Raman microscopy under 514 nm laser excitation.
2.3. Electrochemical measurements
All tests were carried out with a three-electrode system using an
electrochemical workstation (CH Instrument 660E). VG or N-CG were
used as the working electrode directly (with a geometry surface area of 1
× 1 cm2
), while Pt sheet, saturated calomel electrode (SCE), and 0.5 M
H2SO4 were used as the counter electrode, reference electrode and
electrolyte, respectively. All obtained E vs. SCE were converted to E vs.
reversible hydrogen electrode (RHE) by adding 0.272 V. Linear sweep
voltammetry (LSV) was obtained at a scan rate of 5 mV s− 1
. Tafel plots
were converted from LSV curves obtained at 5 mV s− 1
. The stability tests
were carried out using chronopotentiometry with a cathodic current
density of 10 mA cm-2
.
2.4. Computational details
All density functional theory (DFT) calculations in this work were
conducted with Vienna ab initio simulation package (VASP). The
generalized gradient approximation (GGA) exchange-correlation in
teractions with the Perdew-Burke-Ernzerhof functional type were uti
lized. A 4 × 5×2 k-mesh was applied to integrate the Brillouin-zone, and
the plane-wave cutoff energy of 500 eV was used. Vacuum slab of 15 Å in
z direction was set to avoid strong interactions between neighboring
layers. The convergence criteria of energy and force in the structural
relaxation processes were set to 10− 4
eV/atom and 0.01 eV/Å, respec
tively. The Gibbs free energies of samples were calculated to describe the
activity of hydrogen evolution as follows:
ΔG(H) = E(Sub+H) - E(Sub) -1/2E(H2) + ΔEZPE - TΔS
Fig. 1. Typical SEM images of (a) VG and (c) N-VG; High magnification SEM images of (b) VG and (d) N-VG.
Y. Li et al.

3
where E(Sub+H), E(Sub) and E(H2) were the total energy of a substrate and
adsorbed H atom system, the substrate and a H2 molecule, respectively.
ΔEZPE represented the change of zero-point energy, and TΔS in this work
was estimated to be about 0.2 eV at room temperature.
Fig. 2. TEM images of (a, c) VG and (b, d) N-VG.
Fig. 3. (a) XRD and (b) Raman spectra of VG and N-VG; (c) N 1s and (d) C 1s spectra of N-VG.
Y. Li et al.

4
3. Results and discussion
Nitrogen doped vertical graphene was synthesized via a simple NH3
treatment process. The typical SEM images of VG and N-VG are shown in
Fig. 1a and c, where graphene sheets with thickness of 5–10 nm are
vertically aligned and uniformly distributed on the surface of carbon
cloth substrate. These aligned graphene sheets form a conductive
framework with numerous marcopores, which will largely enhance the
specific surface area and provide more active sites. The high-resolution
SEM image of VG and N-VG (Fig. 1b and d) demonstrates the similar
micromorphology of them, suggesting that the nitrogen doping achieved
by NH3 treatment maintains the morphology integrity of VG.
TEM images of nanosheets obtained from VG and N-VG are shown in
Fig. 2. Graphene sheets are clearly observed from Fig. 2a and b, sug
gesting the successful synthesis of vertical graphene on the carbon cloth
substrate. Similar to SEM result above, there is no obvious difference
found between VG and N-VG. In the high-resolution TEM results, clear
lattice fringes can be observed, suggesting that the graphene are few-
layer graphene. Moreover, some defects can be found on N-VG
(Fig. 2d), which may be ascribed to the introduction of N into the lattice
of graphene.
XRD patterns of VG and N-VG are shown in Fig. 3a, all diffraction
peaks of carbon appear and are well indexed with graphite (JCPDS
01− 0640) but with broader peaks, which may be due to the few-layer
nature of VG. No new diffraction peak is observed in the pattern of N-
VG, suggesting that no impurity is introduced during the nitrogen
doping and the crystal structure of VG is well remained. Raman spectra
before and after nitrogen doping are shown in Fig. 3b with only two
peaks around 1350 and 1580 cm− 1
, ascribing to the characteristic of D
band and G band of carbon material. The intensity ratios between the D
band and G band (ID/IG) of VG and N-VG are calculated to be 0.23 and
0.28, respectively, indicating the high degree of graphitization of both
VG materials. Moreover, the increase of ID/IG for N-VG is ascribed to the
doping of nitrogen. In addition, defects are introduced during the NH3
treatment, causing the decrease of graphitization of N-VG, consistent
with the observation in TEM. The doping of nitrogen is further
confirmed with XPS. As shown in Fig. 3c, N peaks can be found in the
binding energy around 400 eV of the XPS spectrum of N-VG, suggesting
the existence of N in N-VG. Moreover, the total N content of N-VG is
measured to be 3.7 at.%. The deconvolution of the N 1s peak is carried
out and the result shows that N on N-VG has three structures, namely
pyridinic N (1.8 at.%), pyrrolic N (0.5 at.%) and graphitic N (1.4 at.%).
Fig. 4. (a) LSV curves at a scan rate of 5 mV s− 1
, (b) Tafel plots, and (c) ECSA of VG and N-VG.
Fig. 5. (a) Stability test result for N-VG. (b) LSV curves of N-VG before and after stability test. (c) SEM images of N-VG after stability test.
Y. Li et al.

5
Moreover, the deconvolution of C 1s peak of N-VG consists of C–
–C,
C–N and C–O again confirmthe successful doping of N.
The hydrogen evolution performances of VG and N-VG were inves
tigated using a three-electrode system. The LSV curves of VG and N-VG
are shown in Fig. 4a. It is obvious that the HER activity of N-VG is largely
enhanced with the help of nitrogen doping, expressing an overpotential
of 290 mV at 10 mA cm− 2
. Meanwhile, the VG can barely reach 10 mA
cm− 2
within the potential range monitored. The Tafel plots are illus
trated in Fig. 4b. The Tafel slopes of VG and N-VG are measured to be
158 and 121 mV Dec-1
, respectively, suggesting that both VG and N-VG
are catalyzing HER via Volmer-Heyrovsky mechanism [35]. Meanwhile,
the Volmer reaction, namely the H+
adsorption on the catalytic sites, is
the rate-determining step [35]. Moreover, the smaller Tafel slope of
N-VG again confirms the ability of nitrogen doping to improve the HER
performance of carbon materials. The electrochemically active surface
areas (ECSA) of VG and N-VG are tested and the results are plotted in
Fig. 4c. N-VG shows higher ECSA than VG, owing to the introduction of
defects and active sites induced by the N doping. Thanks to the larger
ECSA and better HER kinetics, N-VG expresses better HER properties.
Long-term stability is an important parameter of electrocatalysts for
practical applications. Thus, the stability of N-VG was tested with
chronopotentiometry at a cathodic current of 10 mA cm− 2
and operated
for 10 h. As shown in Fig. 5a, no obvious change can be observed for the
overpotential of N-VG over 10 h, suggesting its high stability. This result
is supported by the almost overlapped LSV curves and the unchanged
morphologies of N-VG before and after stability test, as shown in Fig. 5b
and 5c.
As mentioned above, N doping can largely enhance the HER activity
of VG. Meanwhile, three types of doped N are found in the VG. We thus
referred to DFT calculations to evaluate the contribution of each type of
N towards HER. The simulated models for hydrogen adsorption on C,
Graphitic N, Pyrrolic N, and Pyridinic N are shown in Fig. 6a-d. Mean
while, since the electronegativity of N is different from C, the doping will
change the electronic structure of carbon atoms adjacent to the N dop
ants [28]. Thus, the models for hydrogen adsorption on carbon sites
adjacent to each type N dopants are also built (Fig. 6e-g). The corre
sponding Gibbs free energies are shown in Fig. 7. It can be seen that
pyridinic N exhibits much smaller ΔG(H) of -0.74 eV compared to bare
carbon (1.74 eV), while the ΔG(H) for both pyrrolic N (-1.79 eV) and
graphitic N (1.87 eV) are larger than it of bare carbon, suggesting that
the pyridinic N is the most favorable for hydrogen adsorption. But when
it comes to the carbon sites adjacent to N dopants, the situation is
different. For pyridinic N-C site (Fig. 6g), the ΔG(H) is slightly improved
to 1.65 eV compared to bare carbon. While the ΔG(H) are 0.59 and 0.17
eV for graphitic N-C and pyrrolic N-C sites, respectively, even smaller
than that of pyridinic N site. This result suggests that the three types of N
dopants all have positive contributions to the enhanced HER activity of
the N-VG via different mechanisms: (i) The introduction of pyridinic N
provides more adsorption sites for hydrogen; (ii) The electronic struc
ture modulation by N doping optimizes the ΔG(H) of carbon sites. The
DFT simulation results we obtained here confirm the aforementioned
experimental observation that N-doping can largely enhance the
hydrogen evolution activity of the carbon materials. In addition, since
N-doping introduces various kinds of active sites, different surface
electronic structures, and matrix defects onto VG, the resultant N-VG
will also be an interesting substrate for developing highly active
electrocatalysts.
4. Conclusion
To sum up, we have successfully synthesized nitrogen doped vertical
graphene as a metal-free electrocatalyst for hydrogen evolution reac
tion. Enhanced HER performances with an overpotential of 290 mV at
10 mA cm− 2
, a Tafel slope of 121 mV dec-1
, and a long-term stability are
realized by N-VG. N-doping not only introduces three types of N dopants
into the VG substrate, but also introduce defects on the carbon matrix,
which lead to the enlargement of ECSA and optimization of electronic
structures. Moreover, two possible enhancement mechanisms of N
Fig. 6. Models of (a) C-H, (b) graphitic N-H, (c) pyrrolic N-H, (d) pyridinic N-H, (e) graphitic N-C-H, (f) pyrrolic N-C-H, and (g) pyridinic N-C-H. (Gray ball: carbon;
white ball: hydrogen; blue ball: nitrogen.).
Fig. 7. The Gibbs free energy of H+
adsorption on different sites.
Y. Li et al.

6
doping are also revealed and demonstrated by DFT calculations. Our
research may provide an excellent substrate material for developing
highly efficient and cost-effective electrocatalyst for hydrogen
evolution.
CRediT authorship contribution statement
Yahao Li: Methodology, Investigation, Formal analysis, Visualiza
tion, Writing - original draft. Changzhi Ai: Methodology, Formal anal
ysis, Investigation. Shengjue Deng: Formal analysis. Yadong Wang:
Resources. Xili Tong: Funding acquisition. Xiuli Wang: Writing - re
view & editing. Xinhui Xia: Writing - review & editing, Supervision.
Jiangping Tu: Supervision.
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Acknowledgements
This work is supported by National Natural Science Foundation of
China (Grant No. 51772272), Natural Science Funds for Distinguished
Young Scholar of Zhejiang Province (Grant No. LR20E020001), sup
ported by the Foundation of State Key Laboratory of Coal Conversion
(Grant No. J20-21-909), and supported by China Postdoctoral Science
Foundation (Grant No. 2020M671713). S. J. D. acknowledges the sup
port by the 2019 Zhejiang UniversityAcademic Award for Outstanding
Doctoral Candidates.
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