Nitrogen-doped porous carbon: Highly efficient trifunctional electrocatalyst for oxygen reversible catalysis and nitrogen reduction reaction

Article abstract of DOI:10.1039/c8ta01078a

Simple yet efficient design of an outstanding multifunctional electrocatalyst is crucial for advanced energy conversion and storage devices. Herein, we report the synthesis of an N-enriched hierarchically porous carbon electrocatalyst, which demonstrated

Full text of DOI:10.1039/c8ta01078a

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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
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DOI: 10.1039/C8TA01078A
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Nitrogen-Doped Porous Carbon: Highly Efficient Trifunctional
Electrocatalyst for Oxygen Reversible Catalysis and Nitrogen
Reduction Reaction
Received 00th January 20xx,
Accepted 00th January 20xx
Xiaoxuan Yang, Ke Li, Dongming Cheng, Wei-Lin Pang, Jiaqi Lv, Xinyu Chen, Hong-
Ying Zang*, Xing-long Wu*, Hua-Qiao Tan, Yong-Hui Wang and Yang-Guang Li*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Simple yet efficient design of an outstanding multifunctional electrocatalyst is significant for
advanced energy conversion and storage devices. Herein, we report the synthesis of a N-enriched
hierarchically porous carbon electrocatalyst, which demonstrated excellent overall oxygen electrode
activity (ΔE= EOER, 10
－ EORR, 1/2) of 0.730 V and great durability in 0.1 M KOH. The activity of our
material integrate a high BET surface area (1547.13 m2 g−1), accessible N dopant and suitable porous
architectures by mixing the cicada sloughs with ZnCl2, as the inorganic pore-fabricating agent, with
ball milling and followed by annealing treatment. Unexpectedly, nitrogen reduction reaction (NRR)
with great production yield (NH3: 15.7 μg h−1 mg−1 cat., Faradaic efficiency: 1.45%) and selectivity is
achieved at −0.2 V vs. RHE under ambient conditions. The present trifunctional catalytic activities are
markedly better than leading results reported in recent literature. These results highlight the
significance of deliberate structural engineering in the preparation of multifunctional electrocatalysts
for versatile electrochemical reactions.
Introduction
material. It is of industrial interest to use electricity to convert
water instead of hydrogen source and either N2 or air into NH3
Energy crisis and environmental pollution are serious global under atmospheric pressure and low temperature, which
problems stimulating an urgent demand to discover efficient proposes an eco-friendly NH3 synthesis method without the
and robust electrochemical energy conversion and storage emission of CO2.15
systems/devices to achieve a sustainable future. For example,
As these three reactions (ORR, OER, and NRR) involve
the oxygen evolution reaction (OER) and oxygen reduction rather complicated reaction pathways and sluggish kinetics, to
reaction (ORR)1–3 are decisive steps in efficiency the best of our knowledge, it is generally difficult to find one
improvements of energy conversion storage techniques such as catalyst that is effective for all these three catalytic types
overall water splitting, rechargeable metal–air batteries and fuel simultaneously, which complicates the device fabrication and
cells.4–8 Similarly, because of the integration of multiple operation as well as increases the production costs.16–17 For
advantages such as high energy density, ease of preparation, the instance, in metal–air batteries, Pt-based nanomaterials have
rechargeable metal–air batteries represent a viable energy shown remarkable catalytic activity for ORR but not for
technology,9–11 where the operation entails ORR when OER.18–20 In water splitting systems, whereas nanostructured
discharging and OER when charging at the same electrode. In Ru or Ir oxides are efficient catalysts for OER, their catalytic
addition, the reduction of atmospheric nitrogen gas with an activities for ORR are rather limited.21 Moreover, an effective
electrochemical process, namely, nitrogen reduction reaction electrocatalyst for NRR is still under investigation. In a recent
(NRR) is one of the essential processes for both human beings report, a single Mo atom supported on defective BN monolayer
and earth’s ecosystem,12–14 since the produced ammonia (NH3) opened up a new avenue of NH3 production. However, due to
provides the basis of nutrition for a large portion of the the non-conductivity of BN, it cannot be applied to ORR or
population on earth, and it is also a novel hydrogen storage OER. Therefore, the development of one catalyst that
integrating high activities toward ORR, OER and NRR has
been attracting particular interest from the viewpoint of
fundamental research and industrial applications.22–24
Key Laboratory of Polyoxometalate Science of the Ministry of Education, Faculty of
Chemistry, Northeast Normal University, Changchun, 130024, China. *E-mail:
zanghy100@nenu.edu.cn
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DOI: 10.1039/C8TA01078A
Scheme 1 Schematic illustration of the synthesis of the N-doped
carbon foam.
With the in-depth research efforts in the development of
excellent electrocatalysts, carbon-based nanomaterials are
currently considered to be potential alternatives due to their
wide potential window and brilliant electrical conductivity as
well as structural diversity.25 Qiao’s Group synthesized a
variety of carbon-based materials, for instance, C3N4, graphene,
MXene, et. al, moreover, in depth explored the reaction
mechanism combining theory with experiment.26–28 Lou’s
Group used metal-organic frameworks (MOFs) materials as
precursors to prepare various novel carbon-based nanocatalysts,
deeply discussed the influence of the coordination function on
the properties and components in the structure construction
process.29–32
Recently, biomass-derived carbons have been achieving
significant attention as the raw materials are extensively
accessible, inexpensive and renewable natural resources,
providing alternative promising electrocatalysts and more
chances. In addition, some biomass contains other elements
besides carbon, such as nitrogen, enhancing the performance of
ORR. Recently, heteroatom (e.g., N, P, S, B, etc.) doped
carbons and their composites with nonprecious metals have
been identified to possess multifunctional catalytic activity.33–34
Yet, to the best of our knowledge, reports have been scarce on
the synthesis of multifunctional catalysts with remarkable
activity concurrently toward ORR, OER, and NRR. Therefore,
it has remained a challenge to develop cost-effective, high-
performance multifunctional electrocatalysts.22
Fig. 1 (a) SEM; (b) TEM of the NCF-900; (c) Corresponding
elemental mapping of C, N and O; (d) N2 adsorption/desorption
isotherm and (inset) the corresponding pore size distribution of
NCF-900; (e) Raman spectra of different samples and the
relative ID/IG ratios; (f) N 1s XPS spectra of NCF-900.
Scanning electron microscopy (SEM) and transmission
electron microscopy (TEM) characterizations were performed
to observe the morphology and structure of the obtained
materials. The random stacking of NCF-900 material leads to
the formation of a solid with a 3D porous structure, as shown in
Fig. 1a. Ball milling was used to grind bulk materials, which
not only facilitates the integration between ZnCl2 and cicada
sloughs, but also promotes ZnCl2 embedding of the bulk cicada
sloughs. In addition, the surface of NCF-900 material displayed
a loose and spongy structure containing micro- and mesopores
obviously (Fig. 1a and b). The results show that activation with
ZnCl2 can effectively fabricate a number of micropores as well
as mesopores followed by annealing treatment. Moreover, ball
milling and activation with ZnCl2 also efficiently enlarged the
surface area because of the destruction of interlayers and the
formation of porous structures. The elemental mapping images
in Fig. 1c manifest that C, N and O were well distributed on the
surface of materials. Overall, ball milling with ZnCl2 greatly
changed the morphology of the material surface by generating
micro- and mesoporous carbon foam.
Herein, we report a green and efficient synthesis to obtain
N-doped hierarchical porous carbon foams (NCF) via mixing
the cicada sloughs, which are shells after the eclosion of the
cicadidaes. Cicada sloughs are cheap, available and contain a
large number of amino acids, proteins and trace elements. The
obtained N-doped carbon material showed remarkable
electrocatalytic activities toward all three reactions of ORR,
OER, and NRR in the corresponding concentration of
electrolyte solution.
Results and discussion
As can be seen from Fig. 1d and Table 1, the NCF-900
shows a Brunauer–Emmet–Teller (BET) surface area of
1547.13 m2 g-1 on account of a remarkable amount of
micropores around 1.92 nm and abundant mesopores around
25.3 nm. Moreover, a strong adsorption in the low pressure
region and an obvious hysteresis loop in the medium pressure
The structure of NCF material was performed by mixing the
cicada sloughs with ZnCl2, as a chemical activating agent and
an inorganic pore-fabricating agent, embedded into the carbon
precursor together with ball milling, followed by direct
pyrolysis in furnace (Scheme 1).
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Table 1. Specific surface area, pore volume and elemental composition of catalysts.
ARTICLE
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BET Surface
Area (m2/g)
Pore Volume
(cm3/g)
Ca
Na
pyrDidONI: a10.1039/C8TA01078A
Oa
Sample
Pore Size (nm)
(at%)
(at%)
(at%)
(at%)
1112.24
1332.64
1547.13
1468.58
3.1
3.5
4.2
3.7
0.19
0.26
0.33
0.31
86.17
87.72
89.24
90.44
2.20
2.04
1.96
1.73
0.96
1.03
1.25
1.17
11.63
10.24
8.80
NCF-800
NCF-850
NCF-900
NCF-950
7.83
a) The data from XPS.
region are ascribed to micro- and mesopores, respectively. The indicated that no peak was observed under N2-saturated
pores here are likely caused by the activation with ZnCl2, which electrolyte while there was a distinct peak in O2-saturated
contributes to the development of porous activated carbon.35 electrolyte solution. In addition, the onset potential of NCF-900
The abundant hierarchically porous structures are favorable for was 1.05 V with a peak potential of 0.89 V in O2-saturated
the electrolyte entering and shortening the transport distance electrolyte solution. Linear sweep voltammetry (LSV) in Fig.
between the electrolyte and electrode as well as the transport of 2a conforms that the NCF-900 material possesses improved
guest molecules.36 In summary, we use an easy synthetic ORR performance in alkaline solution. For instance, our
fabrication of N-doped, porous carbon material with a large material achieved the Eonset of 1.05 V, E1/2 of 0.890 V (vs. RHE)
surface area.
and current density of –8.66 mA cm−2, which values are higher
The Raman spectrum is used to determine defects and than those of the Pt/C catalyst (Eonset of 0.989 V, E1/2 of 0.845
disordered of materials. In Fig. 1e, the absorption peak at 1345 V,
cm–1 and 1590 cm–1 indicated the existence of the limiting current density of –6.17 mA cm−1, respectively). From
defective/disordered carbon (D band) and the crystallized Fig. 2a, we can see that our NCF-900 material stood out as the
graphitic carbon (G band), respectively.37–38 The relative best electrocatalyst by comparing with the Ej, Eonset and current
intensity ratios between D- and G-bands (ID/IG) increases from densities of the NCF-X materials. This prominent performance
0.948 to 1.43 upon increasing the temperature from 800°C to may be attributed to the unique nitrogen dopants enhancing the
900°C, also confirms the large number of defects in the NCF- first electron transfer of oxygen reduction. It has been reported
900 skeleton at higher temperature. However, when the that the pyridinic N is the significant active site for ORR and
temperature was increased to 950°C, ID/IG value was decreased indeed, the NCF-900 material contains the highest proportion
to 1.28, indicating fewer defects in NCF-950.
of pyridinic nitrogen via XPS measurements. Furthermore, the
To detect the components of the as-prepared NCF catalyst, highest surface area of NCF-900 material was the most
X-ray photoelectron spectroscopy (XPS) investigations were favorable for mass transfer during the reaction. Furthermore,
conducted. The XPS characterization demonstrates the presence the ORR performance was evaluated by the rotating disk
of N 1s, O 1s, and C 1s (Fig. S2) and two types of nitrogen electrode (RDE) voltammograms at various rotation speeds. As
species, quaternary N (401.3 eV) and pyridinic N (398.7 eV) can be seen in Fig. 2b and Fig. S4†, the NCF-900 material
were present in the NCF-900 material (Fig. 1f). Compared to provides larger current densities at various speeds compared
the other NCF-X materials, the NCF-900 has the highest with ones calcined at other heat-treatment temperatures, which
content of pyridinic N (1.96 at.%) (Table 1), which has been also significantly exceed the benchmark Pt/C.
generally considered to be directly responsible for the great The Koutecky–Levich (K–L) plots of our NCF-900 material
ORR performance.39
have a great linearity (Fig. 2c), indicating first-order ORR
In order to find out thoroughly whether there is a possible kinetics with respect to O2.43–44 The Tafel slope recorded from
trace amount of metal such as Fe element which might promote our NCF-900 material was determined to be 79.8 mV dec–1,
electrocatalysis, ICP-MS was used in this work and it was no slightly better than the slope of Pt/C catalyst (80.5 mV dec–1,
accident that the value of Fe contents in our NCF-900 material Fig. S5†). As a demonstration, RDE measurements on the
was 0.032 wt.%. In addition, because of the synergistic effect, NCF-900 electrode at different rotation rates were employed to
the complex active site highly improved ORR activity measure the electron transfer number (n) per oxygen molecule
compared with a single N- or Fe-doped active site.40–41 In a during ORR according to the K–L equation (Fig. 2b, c). The n
recent report, Xu et al. have synthesized a nitrogen-doped value was calculated to be ~4.0, suggesting that the NCF-900
carbon black containing 0.05 wt.% trace Fe material which undergoes the 4e- pathway with large current density. The
indicated a higher ORR activity than other materials with much rotating ring disk electrode (RRDE) measurements (Fig. 2d and
Fe contents.42
Fig. S6†) further reveal that our NCF-900 material can perform
Cycle voltammetry (CV) was initially carried out to the ORR with ca. 4.0 electron transfer pathway and H2O2 yield
investigate their electrochemical response in O2-free/saturated below 5.52%. The measured performance parameters indicate
0.1 M KOH solution (Fig. S3†). The CV curves of NCF-900
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the feasibility of our easily-synthesized material as a promising
carbon electrocatalyst towards ORR.
The above NCF-900 material was also efficient for OER in
DOI: 10.1039/C8TA01078A
alkaline solution, which is highly desired for rechargeable
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Fig. 3 (a) LSV curves of the NCF-900 and IrO2/C for the OER
at 1600 rpm in 1 M KOH solution; (b) the corresponding Tafel
plots from the LSV curves; (c) LSV curves of NCF-900 before
and after CCT in 1 M KOH solution and (d) LSV curves
showing the bi-functional ORR/OER activities of different
catalysts in 0.1 M KOH solution.
metal–air batteries.47 Polarization curves indicate that the NCF-
900 catalyst can improve the OER performance with an
overpotential of 340 mV at a current density of 10 mA cm2 in 1
M KOH solution, which exceeds the IrO2/C catalyst (1.614 V)
in Fig. 3a. In the corresponding Tafel plots shown in Fig. 3b, a
slope of 66 mV dec−1 was identified for the NCF-900 sample.
Note that the Tafel slope is only 16 mV dec−1 lower than the
IrO2/C (50 mV dec−1).
Furthermore, we also evaluated the long-term stability by
taking continuous CV cycling at a scan rate of 50 mV s−1 for
1000 times. The LSV before and after cycling are recorded with
the scan rate of 5 mV s−1. From Fig. 3c, the current density after
CV cycling has very little change compared to the LSV
collected before CV cycling. In addition, the i-t measurement
further reveals that the material can maintain good catalytic
Fig. 2 (a) LSV of various electrocatalysts on a rotating disk
electrode in O2-saturated 0.1 M KOH with a sweep rate of 5
mV s-1 at a rotation rate of 1600 rpm; (b) LSV curves of NCF-
900 electrode with a sweep rate of 5 mV s-1 at the different
rotation rates in O2-saturated 0.1 M KOH solution; (c) the
corresponding Koutecky–Levich plots at different electrode
potentials and (inset) the electron transfer number at various
potentials; (d) The hydrogen peroxide yield (%) (black) from
RRDE with regard to the total oxygen reduction products and
the electron-transfer number (n) (red) of NCF-900 in O2-
saturated 0.1 M KOH solution; (e) LSV curves of NCF-900 and
Pt/C before and after CCT in O2-saturated 0.1 M KOH solution
and (f) the relative retention of current vs. Time in O2-saturated
0.1 M KOH for NCF-900 and Pt/C.
activity for at least 10 h in alkaline solution (Fig. S8
exhibiting that the electrocatalyst is very stable.
To evaluate the ORR-OER bifunctional activities in practical
conditions, LSV were recorded in 0.1 M KOH aqueous
solution. From the voltammograms in Fig. 3d, the oxygen
† ),
Moreover, after 1000 continuous cycling tests (CCT) in the
O2-saturated alkaline condition, there was no evident change
for Eonset and a loss of only 10 mV in E1/2 after 1000 cycles (Fig.
2e). For comparison, Pt/C catalyst loses 50 mV after 1000
cycles, and the degradation is caused by the
dissolution/agglomeration of Pt nanoparticles on the supports.45
Lastly, the long-term operation durability of our NCF-900
material was tested. Compared with the benchmark Pt/C, our
catalyst exhibits improved methanol tolerance and better
durability (Fig. S7† and Fig. 2f). In a 100 h current–time (i–t)
chronoamperometric measurement, our NCF-900 material
preserved 71.2% while the benchmark Pt/C catalyst lost ~50%
under similar condition. The great durability possibly results
from the incorporated heteroatoms, which avoid the detachment
and collapse.46
electrocatalytic activity is evaluated by the voltage difference
48
(ΔE), namely, EOER, 10
－
EORR, 1/2
.
A smaller ΔE corresponds
to a lower efficiency loss and a better activity for oxygen
electrocatalysis when a catalyst is used as a reversible oxygen
electrode.49 The ΔE of the NCF-900 catalyst was only 0.730 V,
much lower than those of others in the series and the value is
better than those of reported carbon materials and non-noble
metal based materials (Table S2†).
Given the significant ORR and OER performance of our
material, we assembled it into
a classic two-electrode
rechargeable Zn–air battery. Notably, the battery shows an
4 | J. Name., 2012, 00, 1-3
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excellent charge-discharge durability at 2 mA cm−2 for 12 h and addition, no hydrazine has been detected, indicatViniegw AtrhticelegOonloinde
DOI: 10.1039/C8TA01078A
no evident changed was seen (Fig. S9a†). Three light emitting
selectivity of the catalyst.
Furthermore, NRR has been studied at different
temperatures under the same potential. As shown in Fig. 4b, the
N2 reduction rate is enhanced with the catalytic temperatures,
and the NH3 yield at 60°C is ≈ 3 times of that at 0°C. Under
high temperature, the mass transfer rate of reactants is faster
than that at low temperature. The electrocatalytic NRR of
different NCF-X are studied under the same catalytic potential
(−0.2 V vs. RHE), the NH3 yield and Faradaic efficiency are
plotted in Fig. 4c. The NCF-900 catalyst which has the largest
BET area shows the highest yield of NH3 and Faradaic
efficiency. Thus, the high BET area of N-doped carbon material
may be a key factor for its high electrocatalytic activity for NH3
formation from N2 reduction. To verify that the detected
reduction product is generated via the NRR of NCF material,
some comparison experiments are performed, where carbon
paper is used as the working electrode. The corresponding
UV−vis absorption spectra are presented in Fig. 4d. As a result,
there is no product NH3 detected in either case for the carbon
paper. Finally, the long-term stability of NCF-900 material was
investigated for NH3 production in Fig. S10 (†). As can be seen
in Fig. S10 (†), the NCF-900 material exhibits a good NRR
stability for the duration of 12 h.
Fig. 4 Electrocatalytic NRR of the NCF-900. (a) Yield of NH3
(red) and Faradaic efficiency (blue) at each given potential; (b)
Yield of NH3 (red), Hydrazine (blue) and Faradaic efficiency
(black) against catalytic temperature under atmospheric
pressure at −0.2 V (vs. RHE); (c) Yield of NH3 (black) and
Faradaic efficiency (blue) with different catalysts at −0.2 V (vs.
RHE) at room temperature and atmospheric pressure and (d)
UV-vis absorption spectra of the electrolyte stained with
indophenol indicator after charging at −0.2 V (vs. RHE) for 2 h
under different electrochemical conditions.
Conclusion
Taken together, a green and efficient synthetic method was
developed for preparing micro- and mesoporous, N-enriched
carbon material. The NCF-900 material showed outstanding
performance in multifunctional electrocatalysis, including ORR,
OER and NRR. The outstanding performance was attributed to
the high BET surface area (1547.13 m2 g-1), abundant N dopant and
hierarchically porous architectures obtained by mixing ZnCl2, as the
inorganic pore-fabricating agent, with ball milling and followed by
annealing treatment. The N-enriched porous carbon material was
considered as a bi-functional ORR/OER catalyst, and ΔE value
was only 0.730 V in 0.1 M KOH solution. Moreover, our NCF-900
material lead to the effective and stable electrochemical NRR with
great enhancement in NH3 yield (15.7 μg h−1 mg−1 cat.), Faradaic
efficiency (1.45%), as well as the selectivity (no formation of N2H4)
at ambient conditions, and the promising electrochemical
performance should spur interest toward using a more efficient
catalyst for the electrochemical reduction of N2. Overall, this work
provides an idea to develop a highly efficient multifunctional
electrocatalyst in a sample and effective way toward energy
storage and conversion applications.
diodes (LED) were lighted using two batteries in series (Fig.
S9b†), indicating that such a device could be utilized for actual
energy storage and conversion applications.
The effect of heteroatoms was further studied as
probableactive sites. The larger electronegativity of enriched N
could lead to the redistribution of the charge density, which is
generally considered to be effective for ORR activity.50–51 More
specifically, as the electron-donating atom, the pyridinic N is
the important active site for ORR.39 The trace Fe dopant may
form new complex active sites, which can activate the ORR
process. Moreover, previous papers reported that the electron-
withdrawing pyridinic N can make the adjacent C atoms
positively charged, thereby promoting the adsorption of the
intermediates during OER process.52–54
As electrocatalytic NRR is also a typical hydrogenation
reduction process, we suggest that NRR is indeed possible at
room temperature and atmospheric pressure by using our
material in 0.1
M
HCl solution. During the NRR
electrocatalytic process, using the NCF material as the cathodic
catalyst, NRR is initiated under N2 saturation at room
temperature. As shown in Fig. 4a, the NRR rate and Faradaic
efficiency rise with the negative potential increasing until −0.2
V (vs. RHE), where the NH3 yield and Faradaic efficiency can
reach the best values of 15.7 μg h−1 mg−1 cat. and 1.45%,
respectively. Beyond this negative potential, the obvious
decrease is ascribed to the competitive adsorption of N2 and H2.
As the potential moves below −0.2 V (vs. RHE), the hydrogen
evolution reaction (HER) becomes the primary reaction.55 In
Experimental
Synthesis of N-doped carbon foams
Typically, the cicada sloughs were rinsed off by deionized
water to remove the dust on the surface and followed by drying
at 80°C overnight. Afterwards, the pretreated cicada sloughs
were mechanically mixed with zinc chloride with a weight ratio
of 1:4 and the obtained mixture was agitated (planetary ball-
milling at 500 rpm for 10 h), and then subjected to heat
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treatment at 800~950°C for 6 h under nitrogen atmosphere. is the electron-transfer rate constant. For the RRDE measures,
View Article Online
DOI: 10.1039/C8TA01078A
After cooling to ambient temperature, the products were then the curves were collected at a rotation rate of 1600 rpm. The
washed with 0.5 M HNO3, 1 M HCl and deionized water potential of the ring was set to 1.36 V (vs. RHE). The collecting
thoroughly, and then dried at 80°C overnight to obtain the N- efficiency of the RRDE (N) was 0.37. The H2O2% and the
doped carbon foams, designated as NCF-X, where X denoted electron transfer number (n) were calculated as follows:
I
/ N
the heating temperature (800, 850, 900 and 950°C).
Material characterization
r
H O %  200
(4)
2
2
I
 I / N
d r
The morphology and structure of the materials was
characterized by scanning electron microscopy (SEM, Hitachi,
SU-70), transmission electron microscopy (TEM, JEOL, JEM-
2100F), X-ray diffraction (XRD, Siemens, D5005, Cu-Kα
radiation), X-ray photoelectron spectroscopy (XPS, VG
ESCALAB MKII, Al-Kα radiation) and Raman spectra with a
LabRAM HR high-resolution Raman spectrometer (Horiba-
Jobin Yvon). The surface area, pore size, and pore-size
distribution of the materials were determined by Brunauer–
Emmett–Teller (BET) nitrogen adsorption–desorption and
Barrett–Joyner–Halenda (BJH) methods (ASAP 2020 M). The
final Fe contents in the catalysts were obtained from ICP-MS
(ICAP-6000, Thermo Fisher Scientific).
I
d
n  4
(5)
I
 I / N
d
r
where Id is the disk current and Ir is the ring current.
Electrochemical NRR measurements
Typically, the N2 (99.99%) reduction was tested in a two-
compartment cell which was separated by Nafion 115
membrane. Before NRR measures, the Nafion membrane was
pretreated by heating in H2O2 (5%) aqueous solution for 1 h, 1
M H2SO4 solution for 1 h and deionized water for another 1 h at
80°C,
respectively.
Moreover,
the
electrochemical
measurements were performed on Princeton Electrochemical
Workstation. The NCF material was the working electrodes, a
carbon rod served as the counter, an Ag/AgCl reference
electrodes served as the reference electrodes, respectively. For
N2 reduction experiments and potentiostatic test were
conducted in N2 saturated 0.1 M HCl solution (50 mL) (the
electrolyte was purged with N2 for 30 min before the
measurement).
To prepare the working electrode, 4.0 mg catalyst was
ultrasonically dispersed in 100 μL of ethanol solution
containing 0.05 wt.% Nafion for 30 min. Then, the dispersion
of homogeneous ink was loaded onto a carbon paper electrode
with area of 1×1 cm2 and dried under ambient condition.
Concentration of produced NH3 was spectrophotometrically
determined by the indophenol blue method. In detail, 2 mL of
the corresponding solution was removed from the
electrochemical reaction vessel. Then, 2 mL of 1 M NaOH
solution containing 5 wt.% salicylic acid and 5 wt.% sodium
citrate, followed by addition of 1 mL of 0.05 M NaClO and 0.2
mL of 1 wt.% sodium nitroferricyanide. After 2 h at room
temperature, the absorption spectrum was measured using an
ultraviolet-visible spectrophotometer. The formation of
indophenol blue was determined using the absorbance at a
wavelength of 655 nm. The concentration-absorbance curves
were calibrated by using standard ammonia chloride solutions,
which contained the same concentrations of HCl as used in the
electrolysis experiments.
The hydrazine was assessed by the way of Watt and
Chrisp.56 The color reagent was prepared by mixing 5.99 g
para-(dimethylamino) benzaldehyde, 30 mL concentrated HCl
and 300 mL ethanol. Calibration curve was plotted as follow:
First, preparing a series of reference solutions, by pipetting
suitable volumes in colorimetric tubes; Second, making up to 5
mL with 0.1 M HCl solution; Third, adding 5 mL above
prepared color reagent and stirring 10 min at room temperature;
Fourth, the absorbance of the solution was tested at 455 nm.
Absolute calibration was achieved by using hydrazine
monohydrate solutions of known concentration as standards.
Electrochemical ORR/OER measurements
All the electrochemical measurements were performed on
Princeton Electrochemical Workstation (PMC CHS08A) at
25°C using a standard three-electrode cell. The rotating-disk
glassy carbon electrode (RD-GCE) was the working electrodes,
a carbon rod served as the counter, a Hg/HgO in alkaline media
served as the reference electrodes, respectively. A rotating disk
electrode (RDE) with a glassy disk (5.0 mm diameter) and a
rotating ring-disk electrode (RRDE) with a glassy carbon disk
(5.61 mm diameter) and a Pt ring (6.25 mm inner-diameter and
7.92 mm outer-diameter) served as the substrate for the
working electrode for evaluating the ORR performance.
To prepare the working electrode, 3.0 mg of the catalysts
(including reference catalysts: 20 wt.% Pt/C) were
ultrasonically dispersed in 1.0 mL of ethanol solution
containing 0.05 wt.% Nafion for 30 min and then 10 uL of
catalyst ink was then pipetted onto the surface of a glassy
carbon disk. All measures were performed in at the scan rate of
5 mV s-1.
For the RDE tests, the LSV curves were obtained at
predefined rotation rates (100, 400, 900, 1600, and 2500 rpm).
The electron transfer numbers (n) were calculated based on
Koutecky-Levich equations:
1
1
1
1
1




1/ 2
(1)
(2)
(3)
J
J
J
J
K
B
L
K
2/3 1/6
B  0.2nFC (D )
v
0
0
1
J

k
nkFC
0
where J is the measured current density, JL and JK are the
diffusion- and kinetic-limiting current densities, ω is the
angular velocity, F is the Faraday constant (F = 96500 C/mol),
C0 is the bulk concentration of O2 in 0.1 M KOH solution (C0 =
1.2 × 10-6 mol/cm3), D0 is the diffusion coefficient of O2 in 0.1
M KOH solution (D0 = 1.9 × 10-5 cm2/s),46–47 respectively, v is
the kinematic viscosity of the electrolyte (v = 0.01 cm2/s) and k
6 | J. Name., 2012, 00, 1-3
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Journal of Materials Chemistry A
Journal Name
ARTICLE
Assuming three electrons were needed to produce one NH3 15. M. A. Shipman and M. D. Symes, Catal. Today, 2017, 286, 57–
View Article Online
DOI: 10.1039/C8TA01078A
molecule, the Faradaic efficiency can be calculated as follows:
68.
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Chem. Int. Ed., 2016, 128, 2270–2274.
V
Faradaic efficiency  3F c

(6)
NH
3
17Q
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Park, A. Casimir and G. Wu, Adv. Funct. Mater., 2015, 25, 5799–
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where F is the Faraday constant and V is the volume of
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Conflict of Interest
The authors declare no conflict of interest.
Acknowledgements
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The authors gratefully acknowledge the financial support from the
National Natural Science Foundation of China (No. 21471028 and
No. 21671036), National Key Basic Research Program of China
(No. 2013CB834802), Changbai Mountain Scholarship, Natural
Science Foundation of Jilin Province (No. 20150101064JC),
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8 | J. Name., 2012, 00, 1-3
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Page 9 of 9
Journal of Materials Chemistry A
View Article Online
DOI: 10.1039/C8TA01078A
A N-enriched hierarchically porous carbon trifunctional electrocatalyst with high activities for
oxygen reversible catalysis and nitrogen reduction reaction was synthesized.