Nickel supported on nitrogen-doped carbon nanotubes as hydrogen oxidation reaction catalyst in alkaline electrolyte

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The development of a low-cost, high-performance platinum-group-metal-free hydroxide exchange membrane fuel cell is hindered by the lack of a hydrogen oxidation reaction catalyst at the anode. Here we report that a composite catalyst, nickel nanoparticles supported on nitrogen-doped carbon nanotubes, has hydrogen oxidation activity similar to platinum-group metals in alkaline electrolyte. Although nitrogen-doped carbon nanotubes are a very poor hydrogen oxidation catalyst, as a support, it increases the catalytic performance of nickel nanoparticles by a factor of 33 (mass activity) or 21 (exchange current density) relative to unsupported nickel nanoparticles. Density functional theory calculations indicate that the nitrogen-doped support stabilizes the nanoparticle against reconstruction, while nitrogen located at the edge of the nanoparticle tunes local adsorption sites by affecting the d-orbitals of nickel. Owing to its high activity and low cost, our catalyst shows significant potential for use in low-cost, high-performance fuel cells.

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### ARTICLE

Received 12 Jul 2015 | Accepted 8 Nov 2015 | Published 14 Jan 2016 DOI: 10.1038/ncomms10141 OPEN
## Nickel supported on nitrogen-doped carbon nanotubes as hydrogen oxidation reaction catalyst in alkaline electrolyte

Zhongbin Zhuang 1,2,3, Stephen A. Giles 2,4, Jie Zheng 2,3, Glen R. Jenness 2,4, Stavros Caratzoulas 2,4,
Dionisios G. Vlachos 2,4 & Yushan Yan 2,3

The development of a low-cost, high-performance platinum-group-metal-free hydroxide

exchange membrane fuel cell is hindered by the lack of a hydrogen oxidation reaction catalyst

at the anode. Here we report that a composite catalyst, nickel nanoparticles supported on

nitrogen-doped carbon nanotubes, has hydrogen oxidation activity similar to platinum-group

metals in alkaline electrolyte. Although nitrogen-doped carbon nanotubes are a very poor

hydrogen oxidation catalyst, as a support, it increases the catalytic performance of nickel

nanoparticles by a factor of 33 (mass activity) or 21 (exchange current density) relative to

unsupported nickel nanoparticles. Density functional theory calculations indicate that the

nitrogen-doped support stabilizes the nanoparticle against reconstruction, while nitrogen

located at the edge of the nanoparticle tunes local adsorption sites by affecting the d-orbitals

of nickel. Owing to its high activity and low cost, our catalyst shows significant potential for

use in low-cost, high-performance fuel cells.

1 State Key Lab of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China. 2 Department of Chemical and
Biomolecular Engineering, University of Delaware, Newark, Delaware 19716, USA. 3 Center for Catalytic Science and Technology, University of Delaware,
Newark, Delaware 19716, USA. 4 Catalysis Center for Energy Innovation, University of Delaware, Newark, Delaware 19716, USA. Correspondence and
requests for materials should be addressed to D.G.V. (email: [vlachos@udel.edu)](mailto:vlachos@udel.edu) or to Y.S.Y. (email: [yanys@udel.edu).](mailto:yanys@udel.edu)

he hydrogen economy provides an efficient and
environmentally friendly pathway to store and consume
energy 1,2 . Fuel cells, especially the recently developed
# T
polymer electrolyte membrane fuel cells, are considered as the
most promising device to convert the chemical energy of
hydrogen to electricity 3 . The hydrogen fuel cells are based on
two half-cell reactions: hydrogen oxidation reaction (HOR) at the
anode and oxygen reduction reaction (ORR) at the cathode. To
date, Pt is the most active catalyst for both HOR and ORR 4 .
However, the commercialization of fuel cells is hindered by the
high price of Pt (B$50 g � 1 ). Although the total content of
platinum-group metals (PGMs) in the state-of-the-art proton
exchange membrane fuel cell (PEMFC) stacks has decreased
significantly in the past decades, more than 0.137 gPt kW � 1 is
still needed 5 . One promising approach to reduce the cost of
fuel cells is to switch the operating environment from an acidic
to a basic one (that is, a hydroxide exchange membrane fuel
cell, HEMFC), thus opening up the possibility of using PGM-free
catalysts and other cheaper components 6 . For the cathode of
the HEMFC, some PGM-free and metal-free ORR catalysts have
been developed that show comparable activity to Pt in alkaline
media 7–10 . However, for the anode side, only a few PGMs (for
example, Pt, Ir and Pd) show adequate activity 11,12 . The HOR
catalysed by Pt is very fast in acidic conditions so that a very low
loading of the Pt catalyst could be used relative to the cathode
side in PEMFCs. However, the HOR activities of PGMs are B100
times slower in alkaline solutions 11,13 . As a result, a much higher
loading of the HOR catalyst is required (0.4 mgPt cm � 2 in a
HEMFC compared with 0.03 mgPt cm � 2 in a PEMFC) to achieve
similar performance 5,14 . Thus, it is highly desirable to develop
PGM-free anode catalysts for the HOR in alkaline electrolyte.

Unlike its reverse reaction (hydrogen evolution reaction,
HER) 15–17, only a few PGM-free HOR catalysts have been
reported. One possibility is to use Raney Ni as the HOR catalyst
in liquid alkaline fuel cells 18–20 . However, it is functional only
under very high alkalinity (6 M KOH) while the activity remains
low. It is not catalytically active for a HEMFC, which can be
mimicked as 0.1–1 M KOH 21 . Efforts have been made to improve
the HOR activity of the Ni-based catalyst in the last decade. Ni
alloys, such as NiMo and NiTi, have been shown to enhance the
HOR activity 20 . Our recent work has also shown that
electrochemically deposited NiCoMo on an Au substrate has a
high HOR activity 22 . Zhuang and co-workers decorated
Ni particles with CrOx to weaken the Ni–O bond and stabilize
the Ni catalysts. A HEMFC incorporating this PGM-free catalyst
has been fabricated, and it exhibits a peak power
density of 50 mW cm � 2 (ref. 21) Although the power density is
still low (compared with the peak power density of more than
1,000 mW cm � 2 for PEMFCs), it demonstrates the possibility to
fabricate low-cost PGM-free fuel cells. However, their activities
are still incomparable with PGM-based catalysts.

In the current study, we synthesize Ni nanoparticles supported
on N-doped carbon nanotubes (Ni/N-CNT) by a wet chemical
method, and the Ni/N-CNT shows a high HOR catalytic activity
in 0.1 M KOH solution. N-CNTs are not only the support for the
Ni nanoparticles, but also a promoter for the catalytic activity.
Owing to its special electronic properties, the N-doped carbon
structure has been employed to enhance the catalytic activity of
the methanol oxidation reaction and the ORR 23 . We use N-CNT
as a support for the HOR catalyst and demonstrate that,
relative to Ni nanoparticles, the mass activity and exchange
current density of Ni/N-CNT increases by a factor of 33 and 21,
respectively. To understand the interaction between the Ni
nanoparticle and the N-CNT support, density functional theory
(DFT) calculations are undertaken. The DFT calculations indicate
that, when nitrogen dopants are present at the edge of the

nanoparticle, the Ni nanoparticle is stabilized on the support and
locally activated for the HOR because of modulation of the Ni
d-orbitals. Using the volcano relationship between the HOR
activity and the hydrogen adsorption energy, predicted firstprinciples exchange current densities of the model systems are in
good agreement with the measured exchange current densities of
the experimental catalysts. Owing to its high activity and low cost,
Ni/N-CNT has great potential to be used as the anode in
HEMFCs, thereby finally bringing to fruition a high-performance
and low-cost PGM-free HEMFC.

Results
Catalyst synthesis and characterization. A two-step approach
was developed to synthesize the Ni/N-CNT hybrid catalysts. First,
Ni was selectively grown on mildly oxidized multiwalled CNTs by
reducing nickel salt in an aqueous solution. Second, ammonia
and hydrazine were added and then subjected to a hydrothermal
treatment at 150 �C. This step led to better crystallization of the
Ni nanoparticles, partial reduction of the oxidized CNTs and,
more importantly, doping of nitrogen into the CNTs.

The scanning electron microscopy (SEM, Fig. 1a) and
transmission electron microscopy (TEM, Fig. 1b) images clearly
show that the nanoparticles with an average size of 20 nm are
selectively grown on the surface of the CNT. The diffraction
rings in the selected area electron diffraction (inset of Fig. 1b)
pattern can be either indexed to Ni or CNT. The
energy-dispersive X-ray spectroscopy (EDS, Supplementary
Fig. 1) also indicated the presence of the Ni component.
The enlarged TEM image (Fig. 1c) shows that all the
nanoparticles are directly supported on the CNT. Moreover,
the high-resolution TEM (HRTEM, Fig. 1d) image clearly
indicates the layered structure of the CNT wall, with an
interplanar spacing of 0.35 nm, which is similar to the (002)
plane of graphite. The lattice fringe of the nanoparticle has an
interplanar spacing of 0.21 nm and is ascribed to the (111) plane
of Ni.

The X-ray diffraction pattern (Fig. 2a) further confirms the
mixed Ni and CNT composition of the catalyst. The loading of Ni
on carbon was calculated as B70 wt % by thermogravimetric
analysis (TGA, method shown in Supplementary Fig. 2). The N
doping was examined with X-ray photoelectron spectroscopy
(XPS, Fig. 2b). A peak at ca. 400 eV clearly exists in the
high-resolution N 1s XPS spectrum (inset of Fig. 2b), confirming
the N doping. It can be deconvoluted into two peaks, 399.0 and
400.3 eV, corresponding to pyridinic N and pyrrolic N in doped
CNT, respectively 24,25 . The N-doping amount to carbon was
calculated from the XPS spectrum as 2.4 at %.

Electrochemical HOR performance. The electrocatalytic activity
for the HOR was investigated by rotating disk electrode
measurements using a standard three-electrode system in 0.1 M
KOH. The catalyst was uniformly cast on a 5-mm glassy
carbon electrode with a Ni loading of 0.25 mgNi cm � 2 . Undoped
CNT supported Ni nanoparticles (represented as Ni/CNT) and
unsupported Ni nanoparticles (represented as Ni) catalysts
(corresponding TEM images and X-ray diffraction patterns are
shown in Supplementary Figs 3 and 4, respectively) with the same
loading were also studied for comparison.

Figure 3a shows the polarization curve of the catalysts in
H2-saturated electrolyte. The anodic current above 0 V (versus
the reversible hydrogen electrode, same hereafter) is assigned to
the oxidation of H2. The catalytic activity for HOR on the three
Ni-based catalysts follows the sequence of Ni/N-CNT4Ni/
CNT4Ni. The current density of Ni/N-CNT is much higher
than either Ni/CNT or Ni and shows an onset potential as low as

a b

c d

0.35 nm

(002)CNT

Ni
311

220
200
111

CNT

002

0.21 nm

(111)Ni

Figure 1 | Electron microscopy of the Ni/N-CNT. (a) SEM image. Scale bar, 100 nm. (b) TEM image. Scale bar, 100 nm. Inset is selected area electron
diffraction pattern. (c) A magnified TEM image. Scale bar, 20 nm. (d) HRTEM images of nickel particle and CNT, respectively. Scale bars, 2 nm.

O
O

N 1 _s_

Pyrrolic
Pyridinic

404 400 396

Binding energy (eV)

Ni
Ni

C (75–1,621) Ni (04–0850)

20 30 40 50 60 70 80

_2�_ (°)

1,000 800 600 400 200 0

Binding energy (eV)

C
N Ni

Figure 2 | X-ray diffraction pattern and XPS spectra of the Ni/N-CNT. (a) X-ray diffraction pattern. The standard pattern of Ni (JCPDS card No.
04-0850) and graphite (JCPDS card No. 75-1621) are shown beneath the plot. (b) XPS spectrum. The inset is the high-resolution N 1s XPS spectrum.

0 V. The bare N-CNT shows almost no catalytic activity for HOR,
indicating that the enhanced catalytic activity for Ni/N-CNT
comes from the synergetic effect of the combined structure. The
Ni/N-CNT catalyst was also tested in an Ar-saturated electrolyte
(Supplementary Fig. 5) and showed no anodic current, confirming the catalytic reaction of HOR.

The polarization curves at different rotating speeds have also
been studied (Fig. 3b). The current density increases with

increasing rotating speed owing to improved mass transport.
The overall current density is under mixed kinetic-diffusion
control. The diffusion-limited current density (jd) for a rotating
disk electrode is described by the Levich equation as a function of
rotating speed (o):

jd ¼ Bc0o 1=2 ¼ 0:62nFD 2=3 u � 1=6 c0o 1=2 ð1Þ

where B is the Levich constant, c0 is the solubility of H2 in 0.1 M

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

0.1

0.01

1E–3

1E–4

1E–5

3,600 r.p.m.

2,500 r.p.m.

1,600 r.p.m.

900 r.p.m.

400 r.p.m.

Ni/N-CNT

Ni/CNT

N-CNT

H2-satruated 0.1 M KOH
Scan rate: 1 mV s –1

H2-saturated 0.1 M KOH
Scan rate: 1 mV s –1

Rotating at 2,500 r.p.m.

_E_ (V versus RHE) _E_ (V versus RHE)

0.43 cm2 mA–1

0.0

0.00 0.05 0.10 0.15 0.20

_�_ –1/2 (s1/2)

0.00 0.02 0.04 0.06 0.08 0.10

1.5

1.0

0.5

0.0

1.5

1.0

0.5

@ 50 mV

0.00 0.02 0.04 0.06 0.08

0.030

0.025

0.020

0.015

0.010

0.005

0.000

1E–6
–0.050 –0.025 0.000 0.025 0.050

_E_ (V versus RHE)

Figure 3 | HOR performances. (a) Polarization curves of Ni/N-CNT, Ni/CNT, Ni (all of the three catalysts with a loading of 0.25 mgNi cm � 2 ) and N-CNT
(0.1 mgC cm � 2 ) catalysts in H2-saturated 0.1 M KOH at a scan rate of 1 mVs � 1 and rotating speed of 2,500 r.p.m. (b) Polarization curves of Ni/N-CNT in
H2-saturated 0.1 M KOH at a scan rate of 1 mVs � 1 and various rotating speeds. Inset is the Koutecky–Levich plot at an overpotential of 50 mV. (c) HOR/
HER Tafel plots of the specific current density on Ni/N-CNT, Ni/CNTand Ni in H2-saturated 0.1 M KOH. The dashed lines indicate the Butler–Volmer fitting.
(d) Mass activity at 50 mV (unpatterned) and exchange current density (patterned) of the Ni/N-CNT, Ni/CNT and Ni, respectively.

KOH 26 . The Levich constant is a function of the diffusivity of
H2 (D), number of the electrons (n) transferred in the HOR and
kinematic viscosity of the electrolyte (n). The overall current
density (j) can be deconvoluted into kinetic (jk) and diffusional
(jd) components following the Koutecky–Levich equation:

1j ¼ j 1 k

þ 1

¼ 1

1
þ ð2Þ
Bc0o 1=2

nanoparticle, making Ni/N-CNT one of the most active
PGM-free catalysts for HOR (Supplementary Table 2 lists
the HOR mass activity of the PGM-free catalyst reported in the
literature).

To understand the intrinsic activity of the Ni/N-CNT catalyst,
we normalized the current by the electrochemical surface
area (ECSA). The ECSAs of each Ni-based catalyst were
measured in Ar-saturated 0.1 M KOH using a cyclic voltammetry
method and were calculated from the OH desorption
region of Ni subtracted by HER current using a charge
density of 514 mC cmNi � 2 for one monolayer of OH adsorption
on Ni (Supplementary Fig. 6) 33,34 . Figure 3c shows the
ECSA-normalized kinetic currents as a function of the
overpotential. The HOR kinetic current was obtained from
the Koutecky–Levich equation (equation 2). The exchange
current density (j0, shown in Fig. 3d and Supplementary
Table 1) was obtained by fitting the ECSA-normalized HOR/
HER kinetic current density to the Bulter–Volmer equation:

A linear relationship between the inverse of j at 50 mV and

- 1/2 is observed in the Koutecky–Levich plots (inset of Fig. 3b).
The calculated slope is 5.21 cm 2 mA � 1 s � 1/2, which is reasonably
close to the theoretical number (4.87 cm 2 mA � 1 s � 1/2, for the
two-electron transfer of HOR) 13 . The intercept of the
extrapolated line corresponds to the inverse of the purely
kinetic current density, and the jk for Ni/N-CNT is
2.33 mA cmdisk� 2 at 50 mV overpotential. Normalizing by the
metal loading of the catalyst, the mass activity of the Ni/NCNT is 9.3 mA mgNi � 1 at 50 mV overpotential, shown in Fig. 3d
and Supplementary Table 1 along with that of Ni/CNT and Ni.
The mass activity of our synthesized unsupported Ni
nanoparticles (0.28 mA mgNi � 1 ) is comparable to the activities of
Raney Ni reported in the literature 27–32 . By contrast, the mass
activity of the Ni/N-CNT is 33 times as high as the pure Ni

Here a is the charge transfer coefficient, F is Faraday’s constant,
R is the universal gas constant, T is the temperature and Z is the

�eRTaF ~~Z~~ - eða �RT1ÞFZ� ð3Þ

RTaF ~~Z~~ - e

jk ¼ j0 e

overpotential 13 . All the curves can be fitted with a value of a in
the range of 0.4B0.5, indicating a good symmetry for the HOR
and HER branches. The j0 values obtained from linear fitting of
micropolarization regions ( � 10–10 mV, see Supplementary
Fig. 7 and Supplementary Table 1) are consistent with the
values of j0 obtained from Bulter–Volmer fitting. The Ni/N-CNT
has an exchange current density of 0.028 mA cmNi � 2, which is
three times higher than Ni/CNT and 21 times higher than Ni.
The exchange current density for Ni/N-CNT is among the best
PGM-free HOR catalysts and is comparable to some PGM
catalysts, such as Pd (Supplementary Table 3) 11,35 . Although the
HOR activity of the Ni/N-CNT is still over one order of
magnitude less than the state-of-the-art Pt catalyst, the ultralow
price of Ni makes it promising for commercial applications.

Theoretical investigation. On the basis of a Wulff construction,
fcc metals, such as Ni, are known to prefer a cuboctahedral
geometry, exposing the {111} and {100} facets 36 . To model the
nanoparticle, a 13-atom Ni nanocluster (Ni13) was chosen, as this
was the smallest of the cuboctahedral ‘magic numbers’ (a larger
nanoparticle, Ni37, was also studied and is discussed below) 37 .
Because of the relatively large size of both the Ni nanoparticles
and carbon nanotubes used experimentally, previous studies
suggest that the curvature of the CNT support has a minimal
effect 38 ; as such, the CNT support is approximated with a
graphene sheet to reduce the complexity of the model. This
results in a Ni13 nanoparticle being deposited on both graphene
(Ni/graphene) and nitrogen-doped graphene (Ni/N-graphene),
with both the Ni13 nanoparticle and the support being allowed to
relax. We have investigated the effect of the location of the
nitrogen dopant relative to the nanoparticle by considering
nitrogen at the centre (Nc) and nitrogen at the edge (Ne), as
shown in Supplementary Fig. 8.

In alkaline solutions, the HOR follows either the Tafel–Volmer
or the Heyrovsky–Volmer mechanism. The possible elementary
steps comprising the HOR are shown in Equations 4–6.

Tafel : H2 þ 2 � ! 2Had ð4Þ

Heyrovsky : H2 þ OH � þ � ! Had þ H2O þ e � ð5Þ

Volmer : Had þ OH � ! H2O þ e � þ � ð6Þ

The key intermediate is the adsorbed hydrogen (Had) on the
catalyst surface, which leads to the existence of a Sabatier volcano
relationship with the hydrogen-binding energy of the catalyst as
the descriptor 39 . Although mechanistically the Had written in the
elementary steps above is likely the overpotential deposited
hydrogen (HOPD), the HOR/HER activity seems to correlate well
to the electrochemical adsorption energy of the underpotential
deposited hydrogen (HUPD), which is shown to be similar to the
chemical adsorption energy of hydrogen 40–42 . In this study we
calculate chemical adsorption energy of hydrogen on Ni13
nanoclusters.

The volcano relationship involving adsorbed hydrogen has
been well established in acidic solutions 43 . While some debate
remains on the adequacy of using the hydrogen-binding energy as
a descriptor in alkaline media, there is significant evidence that
the hydrogen-binding energy remains the key descriptor of
activity 12,44,45 . This allows for the implementation of a hybrid
data-driven/first-principles model to evaluate the efficacy of
different catalysts and to estimate activity compared with single
metals. This hybrid approach was employed successfully in
propane total oxidation 46 . Details of the construction of the
hybrid activity model are given in the Supplementary Methods.

The heterogeneity of the hydrogen-binding sites in the
supported Ni nanocluster model system results in a distribution

where DEH rig is the hydrogen-binding energy on the rigid
nanocluster, and DEH rel is the hydrogen-binding energy on the
relaxed nanocluster. Here a larger Erelax implies a larger degree of
Ni reconstruction. Figure 4b shows Erelax for all Ni-binding sites
for the systems considered. The unsupported nanocluster is the
least stable during hydrogen binding. With the exception of a
fraction of binding sites, the nitrogen-doped supports exhibit
better stabilization of the Ni nanocluster. In agreement with its
lack of strongly binding sites (cf. Fig. 4a), Ni/Ne-graphene exhibits
local relaxation with minimal reconstruction. In addition, when
considering the rigid nanocluster, the difference in the average
hydrogen-binding energy of Ni/Nc-graphene and Ni/Ne-graphene
is less than 0.01 eV (versus 0.09 eV difference for the relaxed
nanocluster). Thus, we observe that the rigid nanocluster’s
binding characteristics are not greatly affected by the support
(Nc-graphene or Ne-graphene).

Next, we examine what fundamental electronic properties of the
Ni13 nanocluster are being affected by the support. A commonly
employed electronic descriptor for the binding energies on metal
surfaces is the energy of the d-band centre, ed, relative to the Fermi
level, eF (ref. 48). The d-band centre describes the relative filling of
the hybridized (d–s) and antibonding (d–s)* states of the Ni–H
system. For an adsorbate, such as atomic hydrogen with a
low-lying electronic state, the stabilization of the bonding states
between the metal d-states and the hydrogen s-states decreases
when the d-band of the metal is shifted up in energy (ed–eF is
more positive) because of the energy mismatch between the metal
d-band and the hydrogen s-state. As a result, the resulting
antibonding part of the band is pushed up just above the Fermi
level, which leads to stronger binding between the metal atoms and
the adsorbate. In Fig. 4c, we see an upshift in the d-band centre of
adjacent Ni-binding sites when an edge nitrogen atom is present.
Owing to these sites originally lying in the weak binding region of
Fig. 4a, the upshift in the d-band centre consequently activates
these sites adjacent to Ne for the HOR by strengthening the
binding energy by 0.1 eV. XPS valence band spectra, which are
related to the d-band centres, were also studied (Supplementary
Fig. 10) 49 . Thus, while the geometric effect of the support is
responsible for the decrease in strong binding sites, edge nitrogen
further tunes local binding sites by modulating the electronic
properties of Ni.

Using the hybrid activity model for the HOR, we can now
predict the intrinsic exchange current density of each of the
model systems that were investigated (Supplementary Fig. 11 and

of binding energies shown in Fig. 4a. Here we see a large amount
of variability among the metal-support systems at high binding
energies, with an unsupported Ni13 nanocluster and Ni/graphene
having a large number of sites that adsorb hydrogen too strongly
to be active for HOR. On the other hand, Fig. 4a shows that a Ni
nanoparticle supported on Ne-graphene does not possess large
binding energies as in the unsupported Ni and the Ni/graphene
systems.

The differences in the binding energies between the
unsupported Ni13 nanocluster and the supported Ni13 arise
from either an electronic (that is, charge transfer) or a geometric
(that is, relaxation) origin 47 . For the strong binding sites, the
nanocluster undergoes reconstruction as shown in Supplementary
Fig. 9. The relative contributions of the electronic and the
geometric support effects are decoupled by considering a
nanocluster which is treated as rigid during hydrogen binding.
For a rigid nanocluster, any differences in the binding energies
result from the differences in the electronic structure of the Ni13
nanocluster because of the support. We define a relaxation
energy, Erelax, as

Erelax¼DEH rig

H rel ð7Þ

H rig � DE H rel

Ni Ni/graphene Ni/Nc-graphene Ni/Ne-graphene

Weak
adsorption

Optimal
for HOR

Nanoparticle
reconstruction

0
–1.0 –0.9 –0.8 –0.7 –0.6 –0.5 –0.4 –0.3 –0.2 –0.1

Δ _E_ H (eV)

–0.15

–0.20

–0.25

–0.30

–0.35

Ni/graphene
Ni/Nc-graphene
Ni/Ne-graphene

0.8

0.6

0.4

0.2

0.0

–0.2

(2,3,4)

(1,2,3)

(2,3,4) (1,2,3)

2
1

4
3

(2,3,4)

(1,2,3)

–0.40
–1.35 –1.30 –1.25 –1.20 –1.15 –1.10 –1.05

_�_ d _– �_ F (eV)

Figure 4 | Computational results. (a) Distribution of site-dependent hydrogen-binding energies for each model system. (b) Distribution of relaxation
energies for each model system on hydrogen-binding to each site. The ends of the dashed line represent the minimum and maximum values of Erelax for
each model structure. These ranges collectively represent the distribution of Erelax values for each hydrogen adsorption site on the Ni13 cluster. The bottom
of the box represents the first quartile (that is, splits the lowest 25% of relaxation energies from the highest 75%) and the top of the box represents the
third quartile (that is, splits the highest 25% of relaxation energies from the lowest 75%). Bold horizontal band is the median value. (c) Shifts in the d-band
centre with respect to the Fermi level and binding energy at adjacent Ni sites (1,2,3) and (2,3,4) for Ni/graphene, Ni/Nc-graphene and Ni/Ne-graphene.
Inset is graphical depiction of the sites (Ni/Ne-graphene as an example). The (1,2,3) site represents the hollow site in coordination with the #1, #2 and #3
Ni atoms. The (2,3,4) site represents the hollow site in coordination with the #2, #3 and #4 Ni atoms. Blue, grey and red spheres represent Ni, C and N
atoms, respectively.

Supplementary Note 1). In agreement with experimental
measurements, Fig. 5 shows that an order of magnitude increase
in the exchange current density is predicted for a graphenesupported nanocluster compared with an unsupported Ni
catalyst. Additional improvement occurs when doping the
graphene support with nitrogen, where having nitrogen at
the edge of nanocluster (that is, directly interacting with surface
Ni atoms) appears to be more effective than having nitrogen at
the centre of the nanoparticle. Relating this discovery to the
current experimental procedure, it is expected that edge nitrogen
is predominant in the experimental material. This is because
nitrogen doping is performed after the Ni nanoparticle was grown
from the CNT surface. Thus, it is expected that only sites at the
edge of the nanoparticle are available for nitrogen doping, and
that lattice diffusion is the only mechanism by which nitrogen
could be transported to the centre of the nanoparticle.

To confirm that the Ni13 is sufficiently large to capture the
metal-support interaction of the experimental system, we

performed calculations on a larger Ni37 nanoparticle
(Supplementary Note 2). The d-orbitals of the Ni13 cluster
correspond well to the larger Ni37 model (Supplementary Fig. 12).
The difference in the calculated d-band centre is only 0.01 eV for
the two nanoparticles, which indicates that the Ni13 is a good
approximation for larger nanoparticles both in terms of its
adsorption and electronic properties. The electronic effect of the
N-dopant was found localized to the adsorption sites of the
nearby Ni atoms (Supplementary Fig. 13 and Supplementary
Table 4). Our findings suggest that even higher HOR activity
could be obtained by making smaller Ni nanoparticles because of
a larger fraction of adsorption sites that could be affected
electronically by the N-dopant.

In summary, Ni/N-CNT as a highly active PGM-free HOR
catalyst in alkaline condition was reported in this paper. The
promoted HOR activity is attributed to the synergetic effect of the
edge N atom in the CNT and Ni. The theoretical investigation
indicated that the Ni nanoparticle has more sites with an optimal

1E–3

1E–4

1E–5

1E–6

1E–7

1E–8

Calculated

Measured

Ni Ni/graphene Ni/N-graphene

Figure 5 | Comparison of calculated exchange current densities to
measured values. Unpatterned bars are the calculated exchange current
densities and patterned bars are the measured values. The calculated
exchange current density of Ni/Ne-graphene is shown for Ni/N-graphene.
Error bars are 75% confidence intervals resulting from the regression of the
volcano relationship in Supplementary Equation 2.

hydrogen-binding energy because of both a geometric and
electronic interaction with the edge N atoms. Relative to Ni
nanoparticles, the mass activity and exchange current density of
Ni/N-CNT increases by a factor of 33 and 21, respectively. These
results show that Ni/N-CNT is promising to be applied as the
anode catalyst for HEMFCs.

Methods
Synthesis of nickel-based composite catalyst. In a typical synthesis of
Ni/N-CNT, mildly oxidized MWCNTs (25 mg, 10–20 nm in diameter, Shenzhen
Nanotech Port Co. Ltd., mildly oxidized through the modified Hummers
method 50 ) were dispersed in deionized water (10 ml) assisted by sonication for
30 min. After that, NiSO4 � 6H2O (0.262 g, 1 mmol) was added to the MWCNT
suspension and sonicated for another 30 min. The suspension was transferred to a
flask placed in an ice-water bath. Then ice-cold NaBH4 aqueous solution (1 wt %,
10 ml, B2.5 mmol) was added dropwise. After reaction at 0 �C for 2 h, the
suspension was transferred to a 45-ml Teflon-lined stainless steel autoclave.
Ammonium hydroxide solution (28–30% NH3 basis, 5 ml) and hydrazine hydrate
solution (78–82%, 2 ml) were added, and then the autoclave was sealed and
subjected to a hydrothermal treatment at 150 �C for 4 h. The final product was
collected by centrifugation at 8,000 r.p.m. for 10 min, washed with water and then
redispersed in water (10 ml).

Ni and Ni/CNT were synthesized by similar procedures. Ni was synthesized
without adding oxidized MWCNT and the pH was adjusted to 12 by ammonium
hydroxide before the hydrothermal treatment. Ni/CNT was synthesized without
adding ammonium hydroxide and hydrazine.

Physical characterization. The SEM was performed on a JEOL JSM-7400F SEM
working at an acceleration voltage of 3 kV. The TEM, HRTEM and EDS were
performed on a JEOL JEM-3010 TEM operating at 300 kV equipped with EDS
system. The X-ray diffraction patterns were collected on a Bruker D8 Discovery
diffractometer with Cu Ka radiation (l ¼ 0.15418 nm) operating at 40 kV and
40 mA. The TGA was performed on a Mettler Toledo TGA/DSA 1 STAR e System
under air flow (60 ml min � 1 ) with a heating rate of 5 �C min � 1 . The XPS spectra
were recorded on Thermo Fisher ESCALAB 250Xi XPS system with a
monochromatic Al Ka X-ray source. The survey scans were conducted at a pass
energy of 100 eV using a step size of 1 eV, and the high-revolution scans were
conducted at a pass energy of 50 eV using a step size of 0.05 eV.

Electrochemical tests. The electrodes were prepared by casting catalyst inks on
glassy carbon electrodes (5 mm in diameter, Pine Instruments, polished to a
mirror-finishing with 0.05 mm alumina). The catalyst inks were prepared by
dispersing catalyst in water with 0.05 wt % Nafion at a concentration of
2.5 mgNi ml � 1 . The catalyst ink (20 ml) was deposited on a glassy carbon electrode
and dried in vacuum at room temperature, resulted in a metal loading of
0.25 mgNi cm � 2 . Electrochemical studies were carried out in a standard
three-electrode system controlled with a multichannel potentiostat (Princeton
Applied Research). As-prepared thin film electrode, which served as the working

electrode, was mounted on a rotator and was immersed into 0.1 M KOH solution.
An Ag/AgCl electrode with double junctions was used as the reference electrode. A
Pt wire was used as the counterelectrode, which has an outer glass tube with a
ceramic fritz to prevent the contamination of Pt. All potentials reported in this
paper are referenced to the reversible hydrogen electrode potential. All the
polarization curves were corrected for solution resistance, which was measured
using AC-impedance spectroscopy from 200 kHz to 100 mHz and a voltage
perturbation of 10 mV.

Computational methodology. Periodic DFT calculations were performed using the
Vienna ab initio software package (VASP, version 5.3.2) 51 . The optPBE-vdW
exchange-correlation functional 52 was selected because of its qualitatively accurate
representation of the potential energy surface of a Ni monolayer and graphene
system 53 . The core electrons were represented with the projector-augmented
wavefunction (PAW) method 54,55, and a plane-wave cutoff of 400 eV was used for
the valence electrons. The Methfessel–Paxton method of electron smearing was used
with a smearing width of 0.1eV (ref. 56). All geometry optimizations were
performed using the conjugate gradient algorithm 57 as implemented in VASP. The
forces and energies were converged to 0.05 eV Å � 1 and 10 � 4 eV, respectively. All
calculations were spin-polarized because of the magnetic moment of Ni. For the Ni
nanocluster systems, all calculations were performed at the G-point. A 5 � 5
graphene unit cell was used as the support. In the z direction, a vacuum layer of 15Å
was included. This unit cell was chosen to minimize interaction of the Ni13
nanocluster with its periodic image while still being computationally tractable. For
the single metal surfaces used to establish the hybrid activity model, a 4 � 4 unit cell
was used with a 3 � 3 � 1 Monkhorst–Pack k-point sampling of the Brillouin zone 58 .

References
1. Bockris, J. O. M. Hydrogen economy in the future. Int. J. Hydrogen Energy 24,
1–15 (1999).
2. Bockris, J. O. M. The hydrogen economy: its history. Int. J. Hydrogen Energy
38, 2579–2588 (2013).
3. Gasteiger, H. A. & Markovic, N. M. Just a dream-or future reality? Science 324,
48–49 (2009).
4. Gasteiger, H. A., Kocha, S. S., Sompalli, B. & Wagner, F. T. Activity
benchmarks and requirements for Pt, Pt-alloy, and non-Pt oxygen reduction
catalysts for PEMFCs. Appl. Catal. B 56, 9–35 (2005).
5. Steinbach, A. DOE Hydrogen and Fuel Cells Program 2015 Annual Progress
Report. Report no. FC104 (US Department of Energy, Washington, DC, USA,
2015).
6. Varcoe, J. R. & Slade, R. C. T. Prospects for alkaline anion-exchange
membranes in low temperature fuel cells. Fuel Cells 5, 187–200 (2005).
7. Chung, H. T., Won, J. H. & Zelenay, P. Active and stable carbon
nanotube/nanoparticle composite electrocatalyst for oxygen reduction. Nat.
Commun. 4, 1922 (2013).
8. Wu, G. & Zelenay, P. Nanostructured Nonprecious Metal Catalysts for Oxygen
Reduction Reaction. Acc. Chem. Res. 46, 1878–1889 (2013).
9. Piana, M. et al. H-2/air alkaline membrane fuel cell performance and durability,
using novel ionomer and non-platinum group metal cathode catalyst. J. Power
Sources 195, 5875–5881 (2010).
10. Sa, Y. J. et al. Carbon nanotubes/heteroatom-doped carbon core-sheath
nanostructures as highly active, metal-free oxygen reduction electrocatalysts for
alkaline fuel cells. Angew. Chem. Int. Ed. Engl. 53, 4102–4106 (2014).
11. Durst, J. et al. New insights into the electrochemical hydrogen oxidation and
evolution reaction mechanism. Energy Environ. Sci. 7, 2255–2260 (2014).
12. Strmcnik, D. et al. Improving the hydrogen oxidation reaction rate by
promotion of hydroxyl adsorption. Nat. Chem. 5, 300–306 (2013).
13. Sheng, W., Gasteiger, H. A. & Shao-Horn, Y. Hydrogen oxidation and
evolution reaction kinetics on platinum: acid vs alkaline electrolytes. J.
Electrochem. Soc. 157, B1529–B1536 (2010).
14. Wang, Y. et al. Pt-Ru catalyzed hydrogen oxidation in alkaline media: oxophilic
effect or electronic effect? Energy Environ. Sci. 8, 177–181 (2015).
15. Nocera, D. G. The artificial leaf. Acc. Chem. Res. 45, 767–776 (2012).
16. Gong, M. et al. Nanoscale nickel oxide/nickel heterostructures for active
hydrogen evolution electrocatalysis. Nat. Commun. 5, 4695 (2014).
17. McCrory, C. C. L. et al. Benchmarking hydrogen evolving reaction and oxygen
evolving reaction electrocatalysts for solar water splitting devices. J. Am. Chem.
Soc. 137, 4347–4357 (2015).
18. Kenjo, T. Chromium-doped Raney-nickel catalyst for hydrogen electrodes in
alkaline fuel-cells. J. Electrochem. Soc. 132, 383–386 (1985).
19. Kiros, Y. & Schwartz, S. Long-term hydrogen oxidation catalysts in alkaline fuel
cells. J. Power Sources 87, 101–105 (2000).
20. Mund, K., Richter, G. & Sturm, F. V. Titanium-containing Raney-nickel
catalyst for hydrogen electrodes in alkaline fuel-cell systems. J. Electrochem. Soc.
124, 1–6 (1977).
21. Lu, S., Pan, J., Huang, A., Zhuang, L. & Lu, J. Alkaline polymer electrolyte fuel
cells completely free from noble metal catalysts. Proc. Natl Acad. Sci. USA 105,
20611–20614 (2008).

22. Sheng, W. et al. Non-precious metal electrocatalysts with high activity for
hydrogen oxidation reaction in alkaline electrolytes. Energy Environ. Sci. 7,
1719–1724 (2014).
23. Wood, K. N., O’Hayre, R. & Pylypenko, S. Recent progress on nitrogen/carbon
structures designed for use in energy and sustainability applications. Energy
Environ. Sci. 7, 1212–1249 (2014).
24. Yang, Z. X., Xia, Y. D. & Mokaya, R. Aligned N-doped carbon nanotube
bundles prepared via CVD using zeolite substrates. Chem. Mater. 17,
4502–4508 (2005).
25. Wang, H., Maiyalagan, T. & Wang, X. Review on recent progress in nitrogendoped graphene: synthesis, characterization, and its potential applications. ACS
Catal. 2, 781–794 (2012).
26. Markovic, N. M., Sarraf, S. T., Gasteiger, H. A. & Ross, P. N. Hydrogen
electrochemistry on platinum low-index single-crystal surfaces in alkaline
solution. J. Chem. Soc. Faraday Trans. 92, 3719–3725 (1996).
27. Ewe, H., Justi, E. & Schmitt, A. Structure and properties of Raney-nickel catalysts
with alloying for alkaline fuel-cells. Electrochim. Acta 19, 799–808 (1974).
28. Ewe, H., Justi, E. & Schmitt, A. Studies of preservation of Raney-nickel-mixed
catalysts. Energy Convers. 14, 35–41 (1975).
29. Jenseit, W., Khalil, A. & Wendt, H. Meterial properties and processing in the
production of fuel-cell components. 1. Hydrogen anodes from Raney-nickel for
lighweight alkaline fuel-cells. J. Appl. Electrochem. 20, 893–900 (1990).
30. AlSaleh, M. A., Gultekin, S., AlZakri, A. S. & Khan, A. A. A. Steady state
performance of copper impregnated Ni/PTFE gas diffusion electrode in alkaline
fuel cell. Int. J. Hydrogen Energy 21, 657–661 (1996).
31. SleemUrRahman, M. A., AlSaleh, M. A., AlZakri, A. S. & Gultekin, S.
Preparation of Raney-Ni gas diffusion electrode by filtration method for
alkaline fuel cells. J. Appl. Electrochem. 27, 215–220 (1997).
32. Al-Saleh, M. A., Sleem, Ur, R., Kareemuddin, S. & Al-Zakri, A. S. Novel
methods of stabilization of Raney-Nickel catalyst for fuel-cell electrodes. J.
Power Sources 72, 159–164 (1998).
33. Machado, S. A. S. & Avaca, L. A. The hydrogen evolution reaction on nickel
surfaces stabilized by H-absorption. Electrochim. Acta 39, 1385–1391 (1994).
34. Grden, M., Alsabet, M. & Jerkiewicz, G. Surface science and electrochemical
analysis of nickel foams. ACS Appl. Mater. Interfaces 4, 3012–3021 (2012).
35. Henning, S., Herranz, J. & Gasteiger, H. A. Bulk-palladium and palladium-ongold electrocatalysts for the oxidation of hydrogen in alkaline electrolyte.
J. Electrochem. Soc. 162, F178–F189 (2015).
36. Wulff, G. On the question of speed of growth and dissolution of crystal
surfaces. Z. Krystallogr. Mineral. 34, 449–530 (1901).
37. Miehle, W., Kandler, O., Leisner, T. & Echt, O. Mass-spectrometric evidence for
icosahedral structrue in large rare-gas clusters -Ar, Kr, Xe. J. Chem. Phys. 91,
5940–5952 (1989).
38. Popov, V. N. Curvature effects on the structural, electronic and optical
properties of isolated single-walled carbon nanotubes within a symmetryadapted non-orthogonal tight-binding model. New J. Phys. 6, 17 (2004).
39. Trasatti, S. Work Function, electronegativity, and electrochemical behavior of
metals. 3. Electrolytic hydrogen evolution in acid solutions. J. Electroanal.
Chem. 39, 163–184 (1972).
40. Jerkiewicz, G. & Zolfaghari, A. Determination of the energy of the
metal � underpotential-deposited hydrogen bond for rhodium electrodes.
J. Phys. Chem. 100, 8454–8461 (1996).
41. Jerkiewicz, G. Hydrogen sorption AT/IN electrodes. Prog. Surf. Sci. 57, 95–186
(1998).
42. Conway, B. E. & Tilak, B. V. Interfacial processes involving electrocatalytic
evolution and oxidation of H-2, and the role of chemisorbed H. Electrochim.
Acta 47, 3571–3594 (2002).
43. Norskov, J. K. et al. Trends in the exchange current for hydrogen evolution.
J. Electrochem. Soc. 152, J23–J26 (2005).
44. Sheng, W., Myint, M., Chen, J. G. & Yan, Y. Correlating the hydrogen
evolution reaction activity in alkaline electrolytes with the hydrogen
binding energy on monometallic surfaces. Energy Environ. Sci. 6, 1509–1512
(2013).
45. Sheng, W. et al. Correlating hydrogen oxidation and evolution activity on
platinum at different pH with measured hydrogen binding energy. Nat.
Commun. 6, 5848 (2015).
46. Peela, N. R., Zheng, W., Lee, I. C., Karim, A. M. & Vlachos, D. G. Core-shell
nanocatalyst design by combining high-throughput experiments and firstprinciples simulations. Chemcatchem 5, 3712–3718 (2013).

47. Jenness, G. R. & Schmidt, J. R. Unraveling the role of metal-support
interactions in heterogeneous catalysis: oxygenate selectivity in fischer-tropsch
synthesis. ACS Catal. 3, 2881–2890 (2013).
48. Hammer, B. & Norskov, J.K. in Advances in Catalysis, Vol. 45. (eds Gates, B. C.
& Knozinger, H.) 71–129 (Elsevier, 2000).
49. Gorzkowski, M. T. & Lewera, A. Probing the limits of d-band center theory:
electronic and electrocatalytic properties of Pd-shell-Pt-core nanoparticles. J.
Phys. Chem. C 119, 18389–18395 (2015).
50. Liang, Y. et al. Oxygen reduction electrocatalyst based on strongly coupled
cobalt oxide nanocrystals and carbon nanotubes. J. Am. Chem. Soc. 134,
15849–15857 (2012).
51. Kresse, G. & Furthmuller, J. Efficiency of ab-initio total energy calculations for
metals and semiconductors using a plane-wave basis set. Comput. Mater. Sci. 6,
15–50 (1996).
52. Klimes, J., Bowler, D. R. & Michaelides, A. Chemical accuracy for the van der
Waals density functional. J. Phys. Condens. Matter 22, 022201 (2010).
53. Wellendorff, J. et al. Density functionals for surface science: exchangecorrelation model development with Bayesian error estimation. Phys. Rev. B 85,
235149 (2012).
54. Blochl, P. E. Projector augmented-wave method. Phys. Rev. B 50, 17953–17979
(1994).
55. Kresse, G. & Joubert, D. From ultrasoft pseudopotentials to the projector
augmented-wave method. Phys. Rev. B 59, 1758–1775 (1999).
56. Methfessel, M. & Paxton, A. T. High-precision sampling for Brillouin-zone
intergration in metals. Phys. Rev. B 40, 3616–3621 (1989).
57. Teter, M. P., Payne, M. C. & Allan, D. C. Solution of Schrodinger-equation for
large systems. Phys. Rev. B 40, 12255–12263 (1989).
58. Monkhorst, H. J. & Pack, J. D. Special points for Brillouin-Zone integrations.
Phys. Rev. B 13, 5188–5192 (1976).

Acknowledgements
The experimental work was partially supported by the ARPA-E programme of the US
Department of Energy under the Award Number DE-AR0000009. The computational
work was financially supported from the Catalysis Center for Energy Innovation, an
Energy Frontier Research Center funded by the US Department of Energy, Office of
Science, Office of Basic Energy Sciences under Award Number DE-SC0001004. S.A.G.
acknowledges a UDEI fellowship. This research used resources of the National Energy
Research Scientific Computing Center, a DOE Office of Science User Facility
supported by the Office of Science of the US Department of Energy under Contract No.
DE-AC02–05CH11231.

Author contributions
Z.Z. and Y.Y. conceived the idea. Z.Z. performed the catalyst synthesis, characterization
and electrochemical test. J.Z. performed SEM observations. S.A.G., G.R.J., S.C. and
D.G.V. performed the DFT calculations and analysed the computational data. Z.Z.,
S.A.G., D.G.V. and Y.Y. wrote the manuscript. All authors discussed the results and
commented on the manuscript.

Additional information
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How to cite this article: Zhuang, Z. et al. Nickel supported on nitrogen-doped carbon
nanotubes as hydrogen oxidation reaction catalyst in alkaline electrolyte. Nat. Commun.
7:10141 doi: 10.1038/ncomms10141 (2016).

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