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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e7
Available online at www.sciencedirect.com
ScienceDirect
journal homepage: www.elsevier.com/locate/he
Nitrogen-doped carbon supported platinum
catalyst via direct soft nitriding for
high-performance polymer electrolyte membrane
fuel cell
Dong-Jun Seo a,b,1, Myeong-Ri Kim a,b,1, Seung Yong Yang a,b,
Won-Young Choi a,c, Hyunguk Choi a,c, Seo-Won Choi a,c,
Myeong-Hwa Lee a,c, Young-Gi Yoon a, Min-Ho Seo a, Hansung Kim b,***,
Chi-Young Jung a,**, Tae-Young Kim a,*
a
Buan Fuel Cell Center, Korea Institute of Energy Research (KIER), Jeollabuk-do, 56332, Republic of Korea
Department of Chemical and Biomolecular Engineering, Yonsei University, Seoul, 03772, Republic of Korea
c
Department of Energy Storage and Conversion Engineering, Chonbuk National University, Jellabuk-do, 54596,
Republic of Korea
b
article info
abstract
Article history:
Control of doping levels of nitrogen to carbon support plays a key role to enhance the
Received 21 March 2018
catalytic activity of the Pt/C catalyst toward oxygen reduction reaction. Mass-production of
Received in revised form
such materials is still challenging issue for the practical use. Here, we demonstrate a facile
17 July 2018
approach for fabrication of the nitrogen-doped Pt/C catalysts via direct soft nitriding of the
Accepted 28 July 2018
Pt/C catalyst. The commercial 40 wt% Pt/C is first physically mixed with urea and then
Available online xxx
heat-treated at 300 C, which allowed a massive production of the 6.6 atom% nitrogendoped Pt/C catalysts without sacrificing the Pt catalysts. The specific activity increases
Keywords:
by 46.9% after the thermal treatment, while the particle size and crystallinity of Pt remain
Soft nitriding
similar to those before the thermal treatment. As a result, the fuel cell test showed a
Nitrogen doping
notable increase in the current density by 100% and 18.5% at 0.8 V and 0.5 V, respectively,
Electrocatalyst
for the membrane electrode assembly employing urea treated Pt/C catalyst. Hence, the soft
ORR
nitriding by urea offers great promise as a simple, energy-efficient and eco-friendly way in
PEMFC
manufacturing the nitrogen-doped Pt/C catalyst for the polymer electrolyte membrane fuel
cell applications.
© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author.
** Corresponding author.
*** Corresponding author.
E-mail addresses: elchem@yonsei.ac.kr (H. Kim), cyjung@kier.re.kr (C.-Y. Jung), kty@kier.re.kr (T.-Y. Kim).
1
D. Seo and M. Kim contributed equally to this work.
https://doi.org/10.1016/j.ijhydene.2018.07.173
0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Seo D-J, et al., Nitrogen-doped carbon supported platinum catalyst via direct soft nitriding for highperformance polymer electrolyte membrane fuel cell, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/
j.ijhydene.2018.07.173
2
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e7
Introduction
Polymer electrolyte membrane fuel cells (PEMFCs) are highly
efficient and green energy-conversion device, regarded as one
of the most promising next-generation energy technologies
[1]. However, PEMFCs still face several technological challenges, despite enduring periods of research. In particular, the
sluggish oxygen reduction reaction (ORR) of the carbon supported Pt (Pt/C) catalyst in the cathode plays an important role
in impeding the fuel cell performance [2]. Hence, the development of electrocatalyst with high catalytic activity towards
ORR has been one of the major issues that limit the
commercialization.
Among many, the nitrogen-doped Pt/C catalyst has been
widely accepted as one of the highly efficient electrocatalysts
towards ORR [3e6]. Although the active sites of nitrogendoped carbon materials are unclear, it is suggested that the
carbon atoms adjacent to pyridinic nitrogen may play a key
part in promoting ORR under acidic conditions, which may be
synergistic to the Pt catalysts [7,8]. In this regard, several efforts have been made to control the doped nitrogen content
and nitrogen atom configuration. Li et al. first reported that
the nitrogen doping starts at 300 C and reaches the highest
doping level of 5 atom% at 500 C when ammonia (NH3) is used
[9]. Luo et al. suggested that the surface area of the nitrogendoped carbons increases with higher temperature and
longer time while the nitrogen content decreases with
increasing temperature [10]. Similarly, Zhang et al. found that
the nitrogen content in the nitrogen-doped graphene varied at
different temperatures, with the highest nitrogen content
obtained at 500 C and the lowest at 800 C, and concluded that
the temperatures ranging from 500 to 600 C may be acceptable for stabilizing all the nitrogen-containing species
(pyrrolic, pyridinic and graphitic nitrogen) [11]. Later, Zhao
et al. reported N-doped carbon nanotubes and nanofibers
interacting with various metal catalysts for ORR [12].
Furthermore, the impact of different nitrogen-containing
precursors on the nitrogen contents and configurations has
been studied. For instance, Lai et al. revealed that the
annealing of graphene oxide with NH3 preferentially leads to
the formation of pyridinic and graphitic nitrogens [13]. Despite
these efforts, the current synthetic process of the nitrogendoped carbons, without exception, has suffered from at least
partially using toxic nitrogen sources, e.g. ammonia. In addition, mass-production of such materials is still critical challenge for the practical use.
Recently, Liu et al. have synthesized the nitrogen-doped
carbon supports via “soft nitriding” technique, which introduces nitrogen onto the carbon surfaces by employing NH3
and isocyanic acid (HCNO) obtained from thermal decomposition of urea [14]. After the heat treatment, the precious metal
can be loaded onto the nitrogen-doped carbon support. Their
results set a useful pathway to manufacture the nitrogendoped carbon supports for noble metal catalysts, yet have
been employed in the further applications with Pt/C.
In this work, we, for the first time, fabricated the nitrogendoped Pt/C catalyst via direct soft nitriding of the commercial
Pt/C catalyst for promoting the ORR. Unlike the previous work
[14], the commercial 40 wt% Pt/C is first physically mixed with
urea by vigorous grinding and then mildly heat-treated up to
300 C to facilitate the fabrication process, thus enabling efficient nitrogen doping in a large quantity. Neither high
annealing temperatures above 500 C [9,10] nor toxic gas such
as ammonia [11] is required for nitrogen doping. First, the
surface state of the nitrogen-doped carbon is investigated.
Subsequently, the particle diameter and crystallinity of Pt
catalyst are explored to examine the chemical stability during
Fig. 1 e A schematic illustration for soft nitriding of the commercial Pt/C into nitrogen-doped Pt/C.
Please cite this article in press as: Seo D-J, et al., Nitrogen-doped carbon supported platinum catalyst via direct soft nitriding for highperformance polymer electrolyte membrane fuel cell, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/
j.ijhydene.2018.07.173
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e7
the soft nitriding. Finally, the electrochemical performance of
the nitrogen-doped Pt/C catalyst and fuel cell performance are
evaluated in comparison to those of the as-received Pt/C.
Experimental
Soft nitriding by urea
Soft nitriding of the commercial Pt/C catalyst (HiSPEC 4000,
Johnson Matthey) was conducted through a mild reaction
between the carbon supports and ammonia/isocyanic acid
generated by thermal decomposition of urea (Fig. 1). The direct
soft nitriding may facilitate the catalyst preparation with
reduced fabrication steps: 3 g of the Pt/C catalyst and 4.5 g of
urea were physically grinded. Subsequently, the mixture was
heat-treated at 300 C for 1 h under nitrogen atmosphere, with
the temperature elevation rate of 5 C min1. After cool down,
the product was washed with water and ethanol for 3 times to
completely remove residual urea. Finally, the as-synthesized
nitrogen-doped Pt/C was dried in a vacuum oven at 60 C
over 24 h before further characterization.
3
Characterization
Morphology of the nitrogen-doped Pt/C was observed by
transmission electron microscopy (TEM, JEOL, 2010F). X-ray
diffraction (XRD, Panalytical, PW1825) patterns were collected
using Cu Ka radiation (l ¼ 0.154 nm), with 2q ranging from 10
to 100 . In addition, surface composition was analyzed by Xray photoelectron spectroscopy (XPS, Kratos, Axis Ultra DLD)
with Mg Ka radiation (1253.6 eV), where the spectrum was
calibrated to give the C1s signal at 284.5 eV. Thermogravimetric analysis (TGA) was also conducted by heating the
samples in air at a constant rate of 10 C min1 up to 700 C.
For the electrochemical characterization, cyclic voltammetry (CV) and linear sweeping voltammetry (LSV) were performed as follows. 0.1 M HClO4 was used as electrolyte in the
presence of N2 to obtain CV curves, where potential scan ranges
from 0 to 1.2 V at scan rate of 20 mV s1. In the LSV experiments, 0.1 M HClO4 was used as electrolyte in the presence of
O2, and potential scan range was from 0.05 to 1.2 V at scan rate
of 20 mV s1 and rotating speeds of 900, 1600 and 2400 rpm. The
kinetic current (Ik) is calculated by the KouteckyeLevich
equation that can be expressed as follows [15]:
Fig. 2 e Material stability of the Pt nanoparticles in the Pt/C catalysts during soft nitriding. A: TEM image before soft
nitriding, B: TEM image after soft nitriding, C: XRD patterns and D: TG curves of the Pt/C catalysts before and after soft
nitriding.
Please cite this article in press as: Seo D-J, et al., Nitrogen-doped carbon supported platinum catalyst via direct soft nitriding for highperformance polymer electrolyte membrane fuel cell, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/
j.ijhydene.2018.07.173
4
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e7
1 1 1
¼ þ
I Ik Id
(1)
where the I and Id are the measured current and diffusionlimited current, respectively. The Id can be defined as [15]:
1
1
=
=
2
=
Id ¼ 0:62nFAD 3 n 6 u 2 CO2
(2)
where n, F, A, D, n, u and CO2 are the number of electrons
transferred, Faraday's constant (96,485 C mol1), area of the
working electrode, diffusivity of the oxygen in the 0.1 M HClO4
electrolyte (1.93 105 cm2 s1), kinematic viscosity
(1.01 102 cm2 s1), angular frequency of rotation and molar
concentration of oxygen in the 0.1 M HClO4 electrolyte
(1.26 106 mol cm3), respectively [15]. Subsequently, the CV
and LSV curves were also obtained after 10,000 triangular
potential cycles between 0.6 and 1.0 V at a scan rate of
50 mV s1 to evaluate the stability. The Pt/C catalysts before
and after the potential cycling test are denoted as beginning of
life (BOL) and end of life (EOL), respectively.
For the fuel cell test, the catalyst slurry was prepared by
mixing 0.7 g Pt/C (anode) or urea-treated Pt/C (cathode), 2.57 g
of DI water, 0.51 g of isopropyl alcohol, and 5 wt.% Nafion
ionomer solution (DE521, Ion Power). The membrane electrode assembly (MEA) was fabricated by the decal method, as
shown in our previous work [16]. All single cell experiments
were conducted at 70 C, 100% RH, and atmospheric pressure.
The temperature of the gas lines to the anode and the cathode
were set 10 C above the temperature of the humidifier to
avoid water condensation. H2 and air gases were used as
anode and cathode reactants, respectively. The stoichiometric
quantities of H2 and air were 1.5 and 2.0, respectively.
Results and discussion
Fig. 2 shows that the Pt/C catalyst maintains the average
particle size and its distribution of Pt after the thermal treatment with urea. The TEM images are carefully examined as
the thermal annealing may result in the formation of Pt aggregates to a certain extent [17]. The average diameter of the Pt
catalyst was initially 3.84 nm (Fig. 2A) and only slightly
increased to 3.98 nm after urea treatment (Fig. 2B). It is thus
suggested that there may be only negligible thermal and
chemical influence on the Pt catalyst during the process. The
size of the platinum catalysts can be further supported by
XRD, as presented in Fig. 2C. The peak intensities at ca. 40 ,
46 , 68 , 81 and 86 representing for (111), (200), (220), (311)
and (222) planes, respectively, have been negligibly reduced,
and the mean diameter of the Pt catalyst estimated from the
(200) diffraction data by using Scherrer's equation remained at
around 3.8 nm that is consistent with the TEM images [18].
This indicates that the Pt catalyst remained similar lattice
structure with single-crystalline phase after urea treatment.
Fig. 2D also shows that the change in Pt mass loading during
the thermal treatment was less than 1 wt.%.
The surface composition of the Pt/C catalyst after urea
treatment is further investigated by XPS. As shown in Fig. 3A,
the survey spectra displays the distinctive peaks corresponding to O1s, N1s, C1s and Pt4f, where the amount of
Fig. 3 e XPS spectra of Pt/C catalysts before and after soft
nitriding. A: Survey, B: Core-level N1s and C: Core-level
Pt4f.
nitrogen incorporated into the catalyst was elevated from ca.
0 to 6.6 at.% after the treatment. Relative atomic percentage of
each element including four different kinds of nitrogen is
provided in Table 1. The peak centered at 400 eV represents
the N1s region. To gain further insights on the nitrogen
doping, the high-resolution N1s spectra was deconvoluted
into four Gaussian-Lorenzian peaks and presented in Fig. 3B.
Peaks centered at 398.1 eV, 399.7 eV, 401.6 eV and 404.4 eV can
be attributed to the pyridinic, pyrrolic, graphitic and oxidized
nitrogen species, respectively [19]. As seen in Table 1, the
pyridinic and pyrrolic nitrogen species are the major sources
of nitrogen. Among various types of nitrogen atoms, the pyridinic nitrogen may provide a shift of electrons towards the
nitrogen and carbon adjacent to nitrogen, resulting in strong
Please cite this article in press as: Seo D-J, et al., Nitrogen-doped carbon supported platinum catalyst via direct soft nitriding for highperformance polymer electrolyte membrane fuel cell, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/
j.ijhydene.2018.07.173
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e7
Table 1 e Atomic percentage of the elements in the Pt/C
catalysts after soft nitriding.
C1s
O1s Pt4f
N1s
Pyrrolic Pyridinic Graphitic Oxidized
N
N
N
N
86.21
4.28
2.87
3.16
2.85
0.34
0.27
interaction between the adjacent carbon and Pt and thus
generating larger number of electro-catalytically active sites
[20]. This can be further assisted by the core-level Pt4f spectra
(Fig. 3C), where the Pt4f peaks are shifted to lower binding
energy by 0.38 eV [21]. It is widely accepted that the thermal
treatment under pure nitrogen enlarge the size of Pt catalyst
5
with poor size distribution with an increasing temperature
[22]. However, our results suggest that the nitrogen doping of
commercial carbon-supported Pt catalyst can be achieved
without sacrificing the size and crystallinity of Pt via the mild
thermal treatment employing urea.
More importantly, the electrocatalytic activity of nitrogendoped Pt/C catalyst after urea treatment was determined by
ORR in acidic media. The electrochemically available surface
area (ECSA) was first evaluated by CV and the results are
presented in Fig. 4A,B. In consistent to TEM and XRD observations, the CV curve at BOL remained similar and ECSA was
only slightly decreased from 41.61 to 36.86 m2 g1
Pt after urea
treatment, indicating that there is no obvious change in the Pt
catalyst. To further investigate the ORR activity, we performed
the rotating disk electrode (RDE) measurements in 0.1 M HClO4
Fig. 4 e Electrochemical properties - Cyclic voltammetry of Pt/C (A) before and (B) after soft nitriding, linear sweeping
voltammetry of Pt/C (D) before and (D) after soft nitriding, and the Koutecky-Levich plot for Pt/C (E) before and (F) after soft
nitriding.
Please cite this article in press as: Seo D-J, et al., Nitrogen-doped carbon supported platinum catalyst via direct soft nitriding for highperformance polymer electrolyte membrane fuel cell, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/
j.ijhydene.2018.07.173
6
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e7
doped carbon, which will be further investigated in our future
work.
Conclusions
Fig. 5 e Fuel cell performance of the MEA employing Pt/C
catalysts before and after soft nitriding.
solution saturated with oxygen. In the LSV curves at BOL, the
half-wave potential is increased by 39 mV after the treatment
at 1600 rpm, where the mass activity at 0.9 V improved from
0.16 to 0.235 A mg1
Pt (Fig. 4C,D). This indicates that the pyridinic nitrogen in a synergistic relation with Pt may play a
dominant role in enhancing the electrocatalytic activity towards ORR, as previously stated by Zhao et al. [23] and Duan
et al. [24]. After the potential cycling test, the nitrogen-doped
Pt/C and commercial Pt/C exhibited similar ECSA values of
33.36 and 34.01 m2 g1
Pt , respectively. Furthermore, the changes
in onset potential and half-wave potential values were even
smaller than that of the commercial Pt/C catalyst, demonstrating that the nitrogen-doped Pt/C catalyst after urea
treatment is reasonably stable when benchmarked with the
commercial Pt/C catalyst. As shown in Fig. 4E,F, ORR on the Pt/
C catalysts before and after soft nitriding follow 4-electron
process with the n values of 4 and 3.89, respectively. It is
widely accepted that graphitic nitrogen species reduce oxygen
into hydrogen peroxide from eOOH intermediates by twoelectron pathway while the pyridinic and pyrrolic nitrogen
species reduce oxygen via a four-electron process [25,26],
where the pyrrolic and pyridinic nitrogen species are absolutely dominating nitrogen species in the Pt/C catalyst after
soft nitriding (Table 1).
Finally, we conducted the fuel cell performance test with
the MEAs employing Pt/C catalysts before and after urea
treatment, as shown in Fig. 5. The MEA after the treatment
showed a dramatic increase in the current density at 0.8 V and
0.5 V by 100 mA cm2 and 270 mA cm2 as compared to that of
the MEA before the treatment. The current density recorded at
0.5 V was 1659 mA cm2, larger than that of the state-of-theart Gore commercial MEA (Gore PriMEA MESGA, Cathode: 0.2
mgPt cm2) with 1587 mA cm2 at 0.5 V [27]. Meanwhile, the
high-frequency resistance remained similar to 50 mOhm cm2
at 0e1600 mA cm2, for the both cases, indicating that there
may be no significant difference in the ohmic resistance.
Hence, it can be concluded that the MEA employing the urea
treated Pt/C not only exhibits facilitated electrode kinetics but
also displays an enhanced transportation of O2. The latter can
be ascribed to the different surface nature of the nitrogen
We synthesized the nitrogen-doped Pt/C catalysts via soft
nitriding technique. The nitrogen species were directly introduced onto the carbon surfaces of the Pt/C catalyst by thermal
decomposition of urea at a relatively lower temperature of
300 C as compared to the conventional method proceeded
with ammonia gas at 900 C. As a result, the electro-catalytic
performance has been notably improved with the half-wave
potential increase in the RDE test reaching approximately
39 mV in the acidic environment, after urea treatment,
without significantly sacrificing the mass loading and size of
Pt. This is also supported by the 100% and 18.5% increases in
the current density at 0.8 V and 0.5 V, respectively, in the cell
performance. Therefore, the results demonstrated not only
simple but also energy-efficient and eco-friendly approach to
mass-produce the nitrogen doped Pt/C catalyst for the PEMFC
applications.
Acknowledgements
This work was conducted under the framework of the
research and development program of the Korea Institute of
Energy Research (B8-2423).
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j.ijhydene.2018.07.173
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Please cite this article in press as: Seo D-J, et al., Nitrogen-doped carbon supported platinum catalyst via direct soft nitriding for highperformance polymer electrolyte membrane fuel cell, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/
j.ijhydene.2018.07.173
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