Silica-Assisted Fabrication of N-doped Porous Carbon for Efficient Electrocatalytic Nitrogen Fixation

Article abstract of DOI:10.1002/cctc.202000423

Here we demonstrate a silica-assisted strategy for the synthesis of N-doped porous carbon nanoparticles from zeolitic imidazolate framework precursors. As a metal-free electrocatalyst for N₂ reduction to NH₃ at ambient conditions, such porous carbon shows an improved catalytic performance compared with the counterpart without the assistance of silica, delivering a higher NH₃ formation rate of 7.22 μg h^?1 mg_cat^?1 and a Faradic efficiency of 7.42 % in 0.1 M HCl solution. The mesopores involved in the carbon catalyst are supposed to be responsible for the efficient electroreduction of N₂.

Full text of DOI:10.1002/cctc.202000423

The European Society Journal for Catalysis
Supported by
Accepted Article
Title: Silica-assisted fabrication of N-doped porous carbon for efficient
electrocatalytic nitrogen fixation
Authors: Chao Hu, Sucen Liang, Silin Bai, Juan Yang, Xu Zhang, and
Jieshan Qiu
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
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published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
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To be cited as: ChemCatChem 10.1002/cctc.202000423
Link to VoR: https://doi.org/10.1002/cctc.202000423
0
1/2020

ChemCatChem
10.1002/cctc.202000423
COMMUNICATION
Silica-assisted fabrication of N-doped porous carbon for efficient
electrocatalytic nitrogen fixation
[a]
[a]
[a]
[a]
[b]
[c, d]
Chao Hu,* Sucen Liang, Silin Bai, Juan Yang, Xu Zhang, and Jieshan Qiu*
[
a]
Prof. Chao Hu, Sucen Liang, Silin Bai, Dr. Juan Yang
School of Chemical Engineering and Technology
Xi'an Jiaotong University
Xi'an 710049, China
E-mail: huchao@xjtu.edu.cn
[
[
[
b]
c]
d]
Dr. Xu Zhang
School of Petroleum and Chemical Engineering
Dalian University of Technology
Dagong Road, Panjin, LN 124221, China
Prof. Jieshan Qiu
Liaoning Key Lab for Energy Materials and Chemical Engineering, School of Chemical Engineering, State Key Lab of Fine Chemicals
Dalian University of Technology
Dalian 116024, China
Prof. Jieshan Qiu
College of Chemical Engineering
Beijing University of Chemical Technology
Beijing 100029, China
E-mail: qiujs@mail.buct.edu.cn
Abstract: Here we demonstrate a silica-assisted strategy for the
synthesis of N-doped porous carbon nanoparticles from zeolitic
imidazolate framework precursors. As a metal-free electrocatalyst for
NRR.[5] Theoretical calculations have also revealed that pyridinic
defects involved within the carbon matrix may be responsible for
the NRR activity.[
5b, 6]
However, the ZIF precursor tends to fuse
N
2
reduction to NH
an improved catalytic performance compared with the counterpart
without the assistance of silica, delivering a higher NH formation rate
3
at ambient conditions, such porous carbon shows
and aggregate together at high carbonization temperature,
resulting in larger particles with a declined specific surface area
and decreased active sites accessible to reactants.
3
−
1
−1
of 7.22 μg h mgcat and a Faradic efficiency of 7.42% in 0.1 M HCl
In this research, we synthesize a N-doped porous carbon
electrocatalyst by coating a silica layer on the surface of ZIFs,
followed by carbonization and etching to remove the silica. With
this silica-assisted synthetic strategy, the resultant ZIF-derived N-
doped porous carbon exhibited a higher specific surface area and
more exposed active sites compared with the contrast sample
without silica coating, and showed an improved performance for
electrocatalytic NRR.
solution. The mesopores involved in the carbon catalyst are supposed
2
to be responsible for the efficient electroreduction of N .
3 2
The Haber–Bosch process for the formation of NH from N and
H
2
has been considered a revolutionary technology, because it
th
enabled an exponential explosion of population through the 20
century by affording NH
process is not particularly efficient, accounting for 1–3% of the
world’s energy consumption and releasing large amounts of CO
annually. Electrocatalytic N reduction reaction (NRR) driven by
renewable electricity has been proposed as promising
alternative for NH synthesis since it can be energy-efficiently
operated under ambient conditions.[2] The challenge for the
electrocatalytic NRR is the slow kinetics of N adsorption and the
subsequent N≡N triple bond cleavage. Besides, the side reaction,
i.e. the hydrogen evolution reaction (HER), should be suppressed
because of its comparable standard reduction potential (0 V vs.
standard hydrogen electrode, SHE) with that of NRR (0.057 V vs.
SHE). To solve these issues, effective NRR catalysts with high
catalytic activity and selectivity are urgently required. Noble metal
materials have been proven to be active toward electrocatalytic
NRR; however, their commercial use is largely hindered by high
price and low abundance.[3] Carbon materials, especially doped
with heteroatoms (such as N, B, S and P), normally exhibit high-
performance catalytic activity toward various electrochemical
reactions.[4] Recently, reported studies have also demonstrated
that metal-free N-doped zeolite-imidazolate-framework (ZIF)
derived carbon materials have electrocatalytic activity toward
-based fertilization.[1] However, this
3
For a typical procedure, ZIFs were first synthesized using the
reported method.[5b] A silica shell was then coated onto the
2
surface of ZIFs through the alkaline catalyzed hydrolysis of
tetraethyl orthosilicate. After carbonization at 1100 oC in Ar
atmosphere and HF etching to remove the silica layer, the
resultant N-doped carbon nanoparticles (NCNPs) were finally
obtained. For comparison, N-doped carbon nanoparticles without
silica coating (NCNPs-w/o) were also fabricated under the
identical conditions. Scanning electron microscopy (SEM) image
in Figure 1a shows a cross-linked aggregated morphology for
NCNPs-w/o, while NCNPs have a better monodispersed particle
shape with an average diameter of ca. 200 nm (Figure 1d).
Transmission electron microscopy (TEM) images further reveal
that NCNPs have a morphology which is different from that of
NCNPs-w/o (Figure 1b, 1c, 1e and 1f). From the high-resolution
TEM image of NCNPs shown in Figure 1g, a lattice spacing of
2
a
3
2
0.37 nm could be evidently confirmed, which is indexed to the
(
002) crystal face of graphitic domains involved within an
amorphous carbon matrix. High-angle annular dark field scanning
TEM and the corresponding energy dispersive X-ray
spectroscopy images show that C and N atoms are
1
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Figure 1. (a-c) SEM and TEM images of NCNPs-w/o. (d-f) SEM and TEM images of NCNPs. (g) High-resolution TEM image of NCNPs. (h, i) HAADF-STEM
images of NCNPs (h) and NCNPs-w/o (i), and the corresponding EDS mappings for C and N elements.
homogeneously dispersed on the surface of NCNPs and NCNPs-
w/o (Figure 1h and 1i).
the pyridinic nitrogen being the highest in content. Based on the
above structure and composition analysis, we could deduce that
NCNPs and NCNPs-w/o have similar crystalline structures and
surface heteroatom-doped states.
To figure out the protective effect of the silica layer for NCNPs,
the surface area and pore structure of NCNPs and NCNPs-w/o
The characteristic structures of NCNPs and NCNPs-w/o
were also confirmed by X-Ray diffraction (XRD) patterns and
Raman spectroscopy. In the XRD patterns shown in Figure 2a,
o
o
broad peaks at 23 and 40 attributable to the (002) and (100)
planes of graphitic carbons, respectively, could be readily
observed in both NCNPs-w/o and NCNPs. The Raman spectra
were
further
analyzed
by
N
2
adsorption/desorption
measurements at 77 K. As shown in Figure 2e, NCNPs displays
−1
given in Figure 2b show two peaks located at 1350 and 1580 cm , the type IV isotherm, indicating that micropores and mesopores
3
indicating the existence of disordered sp carbon (D-band) and
graphitic sp carbon (G-band), respectively.[7] The ratio of the D
are both involved in the sample. In contrast, NCNPs-w/o shows
the type I isotherm, which is mainly governed by adsorption in
micropores at low relative pressures. As shown in Table 1, the
specific surface area and total pore volume of NCNPs are 1122.3
m2 g−1 and 0.456 m3 g , respectively, much higher than the
2
D G
and G band intensities (I /I ) of NCNPs was calculated to be 1.03,
which is similar to that of NCNPs-w/o (1.02). X-ray photoelectron
spectroscopy (XPS) was employed to probe the composition and
chemical states of the component elements on the surface of
samples. The nitrogen content of NCNPs was determined to be
−1
corresponding values for NCNPs-w/o (800.5 m g and 0.316 m3
2
−1
−
1
g ). The pore size distribution (Figure 2f) calculated for NCNPs
has a distinct micropore with the diameter of 1.2 nm, and a
broader mesopore with the average diameter of ca. 11.3 nm. For
NCNPs-w/o, only a micropore with the diameter of 1.2 nm is
present, indicating the material possesses far less mesoporosity
5.5 at%, which is comparable to that of NCNPs-w/o (5.4 at%)
based on the XPS analysis. High-resolution XPS N 1s spectra of
NCNPs further reveal the presence of pyridinic, pyrrolic, and
graphitic nitrogen atoms for both NCNPs and NCNPs-w/o, with
2
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Figure 2. (a) Raman spectra, (b) XRD patterns, (c) XPS spectra, (d) High-resolution XPS N1s spectra, (e) N
2
adsorption/desorption isotherms, and (f) pore size
distribution curves of NCNPs and NCNPs-w/o.
than NCNPs. These results reveal the pivotal role of the silica
layer, which prevents the aggregation and fusion of ZIF
nanoparticles during the high-temperature pyrolysis and
carbonization process.
compartment was collected and the as-obtained products, i.e.
NH and N , were determined by the indophenol blue method
and the Watt and Chrisp method, respectively.
analysis showed that only NH could be detected. Isotope labeling
3
2 4
H
[
3b, 8]
The initial
3
All the electrocatalytic NRR experiments were carried out in
a two-compartment cell (H-cell) separated by a Nafion membrane
at the ambient temperature and atmospheric pressure. Ag/AgCl
experiments were performed to verify the nitrogen source of the
. As shown in Figure 3a, 15NH
+
and 14NH +
-saturated
produced NH
be detected when NRR occurred in 15N
electrolytes, respectively. We also performed a series of control
- and Ar-saturated
electrolytes. Compared with electrolysis in Ar-saturated
electrolyte, NCNPs showed much higher NH formation rates and
Faradaic efficiencies (FEs) in N -saturated electrolyte. The NCNP
sample in N -saturated electrolyte without applied potentials was
3
4
4
could
- and 14N
2
2
electrode (filled with saturated KCl electrolyte) and Pt plate
2
(
1ꢀ×ꢀ1ꢀcm ) were used as the reference and the counter electrodes, experiments using NCNPs in alternating N
2
respectively. The working electrode was prepared by dispersing
catalyst inks onto a carbon cloth. N was continuously fed into the
cathodic electrolyte (0.1 M HCl solution) with a flow rate of 20 mL
2
3
2
−
1
min . After reacting for one hour, the electrolyte from the cathodic
2
3
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Figure 3. (a) H NMR spectra of the cathodic electrolytes after 6 h of electrochemical NRR at −0.1 V (vs. RHE) using 15N
1
and 14
N
as the feeding gas. (b) NH
formation
2
2
3
formation rates and FEs of NCNPs at different potentials in N
2
-saturated 0.1 M HCl solution. (c) Recycling test of NCNPs at −0.1 V (vs. RHE). (d) NH
-saturated 0.1 M HCl solution.
3
rates and FEs of NCNPs-w/o at different potentials in N
2
Table 1. Surface areas and pore volumes of NCNPs and NCNPs-w/o.
BET surface area
External surface
area (m /g)
Micropore area
Total pore volume
Micropore volume
3 [b]
Sample
2
2
2
3
[a]
(m /g)
(m /g)
(m /g)
(m /g)
NCNPs-w/o
NCNPs
800.5
143.6
673.7
656.9
448.7
0.316
0.456
0.255
0.185
1122.4
[
o
a] Total pore volume was calculated at P/P =0.142; [b] Micropore volume was calculated using the t-plot method.
−1
also tested, just showing the same amount of NH
saturated electrolyte. These results suggest that most of the
produced NH is derived from electrocatalytic NRR. The trace of
3
tested in Ar-
the highest NH
3
formation rate of 6.06 μg h−1 mgcat and highest
FE of 2.56% were obtained at −0.3 and −0.1 V (vs. RHE),
respectively. Nevertheless, the performance of NCNPs-w/o was
inferior to that of NCNPs under the tested potentials.
Clearly, the silica coating plays a critical role for NCNPs in
improving the electrocatalytic performance toward NRR, because
of its function of preventing the catalyst from fusion and
aggregation and creating more mesopores for N to reach active
2
sites. The electrocatalytic activity toward NRR is supposed to be
associated with the pyridinic N atoms involved in NCNPs and
3
NH
or N
3
detected in Ar-saturated electrolyte under tested potentials
-saturated electrolyte at an open circuit voltage may originate
2
from the NH
nitrogen involved in catalysts may also contribute the NH
formation.[6] Therefore, the accurate NH
formation rate and FE
should be determined by subtracting the calculated values in Ar-
saturated electrolyte from the ones in N -saturated electrolyte.
The NH formation rates and FE values for NCNPs at different
3
species wafting through the air, and the doped
3
3
2
NCNPs-w/o,[
5b, 6]
while the differences in activity and selectivity
3
applied potentials were then measured and shown in Figure 3b,
in which the highest FE of 7.42% could be obtained at −0.1 V (vs.
may be attributed to morphological factors and especially the pore
structure of the two carbon catalysts.[9] The N
molecule can be
2
reversible hydrogen electrode, RHE). The highest NH
3
formation
adsorbed and activated on the active site consisting of three
pyridinic N atoms and one adjacent carbon vacancy.[ The
hydrogenation process for ammonia synthesis on NCNPs is as
5b]
rate was achieved at −0.3 V (vs. RHE), with the value reaching as
high as 7.22 μg h−1 mgcat−1. As the potential negatively shifted, the
formation rate and FE significantly decreased because of the
competitive HER. The NCNP sample showed a negligible decline
in formation rate and FE after five cycles (Figure 3c), indicating
good stability toward electrocatalytic NRR. The catalytic
performance of NCNPs-w/o under the identical test conditions
was also investigated for comparison. As depicted in Figure 3d,
2 3 3
follows: *N–N → *N–NH → *N–NH → *N+NH → *NH+NH
→*NH +NH → 2NH . As shown in Figure 2e and Table 1, NCNPs
2
3
3
are rich in mesopores and the external surface area is as high as
673.66 m2 g , which is much larger than that of NCNPs-w/o
−1
2
−1
(143.64 m g ). The presence of mesopores maybe beneficial for
NCNPs in achieving high NH selectivity and formation rate, just
3
4
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nanoparticles were collected by centrifugation and washed with
ethanol. The obtained ZIF@SiO
h under N , followed by cooling to room temperature. To remove
the SiO shell, the pyrolyzed sample was immersed in aqueous
2
was pyrolyzed at 1100 °C for 1
2
2
HF (10 wt.%), followed by centrifugation and washing with
deionized water and ethanol. For comparison, N-doped carbon
nanoparticles without SiO
prepared by directly pyrolyzing the ZIF precursors at 1100 C for
2
protection (NCNPs-w/o) were
o
1
2
h under N .
Characterization: The X-ray diffraction (XRD) patterns of the
samples were recorded on a Shimadzu XRD-6100 diffractometer
with Cu Kα X-rays operating at 30 mA and 40 kV, using Cu Kα as
the radiation source (λ=0.15418 nm). The scanning electron
microscopy (SEM) images of the samples were taken on a
TESCAN MAIA3 at a working voltage of 5 kV. Transmission
electron microscopy (TEM) images were obtained using a JEOL
JEM-2100Plus transmission electron microscope. The FT-IR
spectra were recorded within the 6000-400 cm-1 region on a
Nicolet iS50 spectrometer using KBr pellets. Raman
spectroscopy was performed using HORIBA Laser Raman
Spectrometer at 532 nm excitation. X-ray photoelectron
spectroscopy (XPS) was conducted using a Thermo Fisher
ESCALAB 250Xi+ spectrometer with monochromatized Al Kα
radiation operating at a power of 300 W. The specific surface area
Figure 4. Tafel plots for NCNPs and NCNPs-w/o.
as the mesopore-rich carbon-based catalysts have performed in
other electrocatalytic reactions.[10] The mass transport
requirements for HER and NRR are different, while the
competitive HER reaction could be severely restrained within a
mesoporous catalyst.[11] Furthermore, the wetting properties of
porous carbon materials are largely dependent on their
morphology and pore structure, and the pore-rich carbon shows
a relatively more hydrophobic surface with a lower surface
We infer that the change in surface energy of NCNPs
with the hierarchical porous structure may be beneficial for
affording a solid-liquid-gas three-phase reaction interface that
facilitates the NRR.[12] In addition, the Tafel plots derived from the
partial current density of N reduction versus cathode potential
2
(
S
BET
)
measurements
were
carried
out
using
N
2
energy.[
10a]
adsorption/desorption at 77 K on a Micromeritics ASAP 2460
apparatus. Samples were degassed at 200 °C for 5 h under
vacuum prior to nitrogen physisorption measurements. The
absorbance data of spectrophotometer were measured on
SHIMADZU
UV-2600
ultraviolet-visible
(UV-Vis)
spectrophotometer.
show that the NCNPs electrode has a smaller Tafel slope than the
NCNPs-w/o electrode (100.9 mVꢀdec−1 for NCNPs versus 132.1
mVꢀdec−1 for NCNPs-w/o), suggesting that the NCNPs electrode
exhibits faster kinetics than the NCNPs-w/o electrode (Figure 4).
In summary, we have developed a silica-assisted strategy for
the fabrication of N-doped porous carbon electrocatalyst for NRR.
Owing to its mesopore-rich structure with an enlarged specific
surface area and increased number of active sites, the obtained
catalyst shows an improved electrocatalytic performance
compared with the micropore-rich counterpart. More importantly,
these results highlight the observed selectivity for NRR cannot be
exclusively related to intrinsic active sites on the surface of
catalysts, but also the morphological factors should be taken into
consideration when defining the experimental selectivity. This
research would shed new light to boost the NRR performance by
only morphological design of NRR electrocatalysts.
Electrochemical NRR measurements: All electrochemical
measurements were performed on a CHI 660E electrochemical
workstation (CH Instrumental Corporation, Shanghai, China)
using a two-compartment cell, which was separated by Nafion
211 membrane. Different catalyst inks were deposited on a
carbon cloth used as the working electrode. The Ag/AgCl
electrode (saturated with KCl electrolyte) was used as the
reference electrode and a Pt plate was used as the counter
electrode. The polarization curves were measured with a scan
rate of 5.0 mV s-1 at room temperature. For N
(NRR) experiments, the potentiostatic test was conducted for 1 h
in N -saturated 0.1 M HCl solution (30 mL) by continuously
supplying N into the electrolyte under ambient conditions. For
reduction reaction
2
2
2
every NRR test, the HCl electrolyte was bubbled with N
min before the measurement.
2
for 30
Experimental Section
Acknowledgements
3 2 2
Catalyst preparation: Zn(NO ) ·6H O (5.9498 g) and 2-
methylimidazole (6.5680 g) were dissolved separately in 200 mL
of methanol. The solution of 2-methylimidazole was gradually
added to the metal salt solution and stirred for 1 h, then aged at
ambient temperature for 24 h. The solid product (ZIFs) was
collected by centrifugation and washed with methanol, and dried
This research was supported by the Fundamental Research
Funds for the Central Universities (xjj2017083) and the National
Natural Science Foundation of China (51702254). The authors
also thank Jiao Li at the Instrument Analysis Center of Xi'an
Jiaotong University for their assistance with TEM analysis.
at 60 °C for 6 h. ZIFs (600 mg) were dispersed in 240 mL of H
2
O,
and then 6 mL of an aqueous cetyltrimethylammonium bromide
-1
solution (25 mg mL ) and 9.6 mL of an aqueous NaOH solution
Conflict of interest
-1
(
6 mg mL ) were added. Tetraethyl orthosilicate (1.2 mL in 6 mL
of methanol) was injected into the above solution, and the
resulting dispersion was stirred for 0.5 h. The resulting ZIF@SiO
The authors declare no conflict of interest.
2
5
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Keywords: Ammonia • Nitrogen reduction reaction • Porous
carbon • Faradaic efficiency • Electrocatalysis
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10.1002/cctc.202000423
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This paper revealed that the morphological and pore structural changes in the N-doped ZIF-derived carbon catalyst have a
particularly significant impact on its performance toward electrochemical N reduction to NH .
2 3
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This article is protected by copyright. All rights reserved.