S-Doped three-dimensional graphene (S-3DG): A metal-free electrocatalyst for the electrochemical synthesis of ammonia under ambient conditions

Article abstract of DOI:10.1039/c9dt04827h

In this study, we report sulfur-doped three-dimensional graphene (S-3DG) as a metal-free electrocatalyst for N₂ reduction reaction (NRR) under ambient conditions. Due to the high electron transport capacity and stable physicochemical properties

Full text of DOI:10.1039/c9dt04827h

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S-Doped three-dimensional graphene (S-3DG):
a metal-free electrocatalyst for the
electrochemical synthesis of ammonia under
ambient conditions†
Cite this: DOI: 10.1039/c9dt04827h
b
Jin Wang,a Shuang Wang
*
a,b and Jinping Li
*
In this study, we report sulfur-doped three-dimensional graphene (S-3DG) as a metal-free electrocatalyst
for N2 reduction reaction (NRR) under ambient conditions. Due to the high electron transport capacity
and stable physicochemical properties of 3DG, it was utilized to improve the NRR catalytic performance
Received 20th December 2019,
Accepted 17th January 2020
dramatically. Hence, in 0.05 M H2SO4 the S-3DG achieved a remarkably large NH3 yield of 38.81 µgNH
3
mgcat−1 h−1 and a high faradaic efficiency of 7.72% at −0.6 V versus a reversible hydrogen electrode (RHE),
which is superior to most non-metal catalysts. Notably, it shows such outstanding selectivity that no
hydrazine by-products were detected and the electrochemical stability passed a long-term durability test.
DOI: 10.1039/c9dt04827h
rsc.li/dalton
have been made, for example Ziqiang Wang et al. reported
flower-like Au as efficient electrocatalysts for the NRR and
Introduction
Ammonia is a very important chemical raw material, which is gained a high performance, in which the NH3 yield rate was
mainly used in chemical fertilizer production, chemical indus- 25.57 µgNH mgcat h−1 and the FE was 6.05% at −0.2 V in 0.1
−1
3
try, medicine, and other fields.1–3 The Haber–Bosch process, M HCl.13–18 However, limited by costs and scarce resources,
as the main method of the industrial production of ammonia, scientists have turned their attention to non-precious metals
produces over 150 million metric tons of ammonia each year such as Mn, Co, and Gr; for instance, the CoS2/NS-G hybrid
while consuming 1–2% of the global energy supply and emits shows a high yield rate and superior faradaic efficiency for
over 450 million metric tons of CO2 annually.4,5 Hence, it is NH3 production.19 The highest faradaic efficiency of 25.9%
urgent to find a sustainable way to replace the conventional was achieved at −0.05 V vs. RHE, while the highest yield of
−1
process. Compared to the harsh production conditions of the 25.0 µgNH mgcat h−1 was obtained at −0.2 V vs. RHE.20
3
traditional methods, electrocatalysis is promising as it is However, the metal ions that are released by the metal catalyst
driven by electric energy and thus, it can make the reaction of are unmanageable apart from the high cost.21–23 X. Sun et al.
the ammonia synthesis free from or less restricted by thermo- proved that the multishelled hollow Cr2O3 microspheres achieved
dynamic equilibrium.6,7
a high faradaic efficiency (6.78%) and a large NH3 yield
−1
Electrocatalysis is the core and key of technological pro- (25.3 µgNH mgcat h−1) at −0.9 V but the degradation of Cr3+
3
gress, energy saving, and emission reducing in an electro- takes more time and energy.24 Therefore, scientists urgently need
chemical ammonia synthesis. At present, the research on elec- to find a sustainable material to replace it, such as non-metals.
trocatalysts for N2 fixation can be divided into the following Subsequently, papers have been published progressively using P,
three categories: precious metal, non-precious metal, and non- B, and S as the catalytic center for NRR.19,25 X. Sun et al. pub-
metal.8–10 The research on the electrocatalytic properties of lished boron as the active center that gave an ideal result
−1
−1
precious metals as electrocatalysts was carried out earlier, such (13.22 µgNH mgcat h−1, 4.04% and 14.4 µgNH mgcat h−1
,
3
3
as Au, Ru, and Rh.11,12 Until now, there have been in-depth 3.4%) at −0.8 V in 0.1 M Na2SO4.26,27 Nevertheless, the perform-
researches on precious metal catalysts and some achievements ance of most non-metals catalyts is unsatisfactory because they
are natural electrical insulators, which greatly obstructs the elec-
tron mass transfer and directly leads to the sharp drop in the
aCollege of Environmental Science and Engineering, Taiyuan University of
nitrogen conversion yield and electron utilization rate.28–32 The
Technology, Jinzhong 030600, Shanxi, P.R. China. E-mail: wangshuang@tyut.edu.cn
best and most direct way to improve this situation is to use con-
bShanxi Key Laboratory of Gas Energy Efficient and Clean Utilization, Taiyuan
ductive substrates, such as carbon material.33–37
University of Technology, Taiyuan 030024, Shanxi, P.R. China
Graphene, as a material that has high electron transport
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
capacity, high specific surface area, and high physicochemical
c9dt04827h
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stability, stands out amongst many carbon substrates and is round-bottom flask containing flake graphite powder and the
usually selected to act as an optimal dopant apply to load the mixture was magnetically stirred in an ice water bath. Then,
non-metal for NRR to improve the electron conduction ability the same was magnetically stirred in an oil bath at 35 °C for
of the non-metallic catalyst.19,20,31,32,38–40 In 2012, Shi Zhang 2 h. After this, 92 mL of ultra-pure water was added, followed
Qiao et al. demonstrated that the synergistic enhancement of by stirring at 98 °C for 15 minutes. Then, 280 mL ultra-pure
thio-nitrogen co-doped mesoporous graphene electrocatalysts water and 5 mL hydrogen peroxide were added into the mixed
in oxygen reduction results from the redistribution of spin and solution successively after the solution was naturally cooled to
charge densities brought about by the dual doping of S and N room temperature. After that, the mixture was pumped and fil-
atoms, which leads to a large number of carbon atom active tered by a circulating water pump; meanwhile, 500 mL 10%
sites over graphene.41 Zhixiong You et al. reported Ru nano- hydrochloric acid solution was slowly added. The product was
crystallites on graphene substrate, which enhanced the NRR transferred to a centrifuge tube, diluted with ultrapure water,
by the formation of ruthenium–carbon bonds between gra- and centrifuged at 10 000 rpm for 20 min until the super-
phene and ruthenium coordination compounds, thus showing natant was neutral. The diluted sediment was put into a dialy-
that graphene has a promising future in ammonia synthetic sis bag and dialysis was carried out for 7 days. The ultrasonic
catalysts.42 3D-graphene does not only have excellent pro- dispersion of the suspended solution after dialysis was con-
perties of graphene but also the unique three-dimensional ducted at 120 W power by an ultrasonic cell grinder for
network structure, which provides more channels for electron 30 minutes, followed by centrifugation at 10 000 rpm; the
transport, more specific surface areas spread over the catalyst upper liquid obtained was the graphene oxide-water suspen-
evenly, and makes the physicochemical properties more stable, sion, which was diluted with water to a concentration of 5 mg
thus improving the nitrogen reduction performance.43
Herein, we have reported graphene with a unique three-
dimensional structure43–47 as the substrate of the supported
mL−1
.
2.3 3D-graphene preparation
catalyst, which loaded sulfur (S-3DG) at low temperature that The concentration of 5 mg mL−1 of graphene oxide in 5 mL
acts as an efficient NRR catalyst for electrochemical nitrogen fix- water suspension kept in 20 mL glass samples in the bottle
ation at ambient conditions and provided a satisfactory yield was measured and 100 µL hydrazine hydrate and 17.5 µL
and FE, which were 38.81 µgNH mgcat−1 h−1 and 7.72%, respect- ammonia were added; the samples of glass bottles were then
3
ively, in 0.05 M H2SO4 at −0.6 V. Moreover, the catalyst demon- put in a 100 mL PTFE autoclave tank with a sealed stainless
strates such superior selectivity that hydrazine was not detected steel shell and heated to 180 °C in the drum wind drying oven
in the product and long-term durability for the reaction.
temperature reaction for 3 h to obtain quick black cylindrical
3D-graphene hydrogels.
2.4 S-3DG preparation
2. Experimental section
A 3D-graphene hydrogel was immersed in sodium thiosulfate
solution (Na2S2O3, 2.5 M, 50 mL) at the ambient temperature
of 60 °C under magnetic stirring for 5 h. After the reaction, the
container was placed in the refrigerator and pre-freezed. Half
an hour later, the same pre-frozen mixture (HCl, 1 M, 6 mL)
was added to the solution drop by drop under magnetic stir-
ring at 500 rpm. The as-obtained sulfur–graphene hydrogels
were washed several times and then subjected to oven drying
at 150 °C for 12 h to produce sulfur–graphene gels.
2.1 Materials
Sodium gluconate (C6H11NaO7, 99.0%), sodium sulfate (Na2SO4,
99.0%), hydrochloric acid (HCl, 99.0%), ammonium chloride
(NH4Cl), salicylic acid (C7H6O3), sodium citrate dehydrate
(C6H5Na3O7·2H2O), p-dimethylamino-benzaldehyde (C9H11NO),
sodium nitroferricyanide dehydrate (C5FeN6Na2O·2H2O), and
sodium hypochlorite solution (NaClO) were purchased from
Aladdin Ltd (Shanghai, China). Nafion (5 wt%) solution was
purchased from Sigma-Aldrich Chemical Reagent Co., Ltd.
Hydrochloric acid, nitric acid, sulfuric acid, hydrogen peroxide,
hydrazine monohydrate (N2H4·H2O), and ethyl alcohol
(C2H5OH) were purchased from Sinopharm Chemical Reagent
Co., Ltd, China. The ultrapure water used throughout all experi-
ments was purified through a Millipore system. The high-purity
nitrogen and argon gases used in the experiment, as well as
other experimental gases, all came from Anxuhongyun
Technology Development Co. Ltd. All the reagents were analyti-
cal reagent grade and were used without further purification.
2.5 Electrode preparation
2 mg of the catalyst was dispersed in 960 µL (Vwater : Vethanol
=
1 : 2) mixed solution and 40 μL Nafion solution (5 wt%) was
sonicated for at least 40 min to form a homogeneous ink.
Then, 50 µL of the catalyst ink was drop-casted onto carbon
paper with a loading of 0.1 mg. The area of carbon paper elec-
trode was 1 × 1.5 cm2 and the practically immersed area in the
electrolyte was 1 × 1 cm2. It was allowed to dry naturally and
left under a baking lamp for 1 h, then dipped in the prepared
electrolyte for 1 h. The working electrode was ready to be used.
2.2 GO preparation
2.6 Characterizations
GO was prepared using the classic Hummers’ method, which
uses flake graphite powder as the carbon source. Simply, SEM images were collected from a tungsten lamp-equipped
46 mL concentrated sulfuric acid was poured into a 250 mL SU8010 scanning electron microscope at an accelerating
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voltage of 20 kV (HITACHI, Japan). The crystal structures of the
synthesized samples were analyzed on a Rigaku Mini Flex II
benchtop powder X-ray diffractometer (XRD) with Cu-Kα radi-
ation (40 kV, 40 mA, λ = 0.15418 nm) in the 2θ range of 5–80°
with a scanning rate of 8° min−1. The morphologies of the
samples were observed by scanning electron microscopy
(Hitachi, SU8010). Transmission electron microscopy (TEM)
was carried out on a JEM-2100F instrument. The X-ray photo-
electron spectroscopy (XPS) measurements were performed on
a Thermo Fisher Scientific K-Alpha spectrometer with Al Ka
(1486.6 eV) as the X-ray excitation source. Thermogravimetric
analysis was carried out with a heating rate of 5 K min−1 from
303 K to 1273 K in N2 on STA 449 F5 obtained from the
NETZSCH company. The absorbance data of the spectrophoto-
meter were measured on a PerkinElmer Lambda 650 ultra-
violet-visible (UV-Vis) spectrophotometer.
2.7 Electrochemical measurements
The electrochemical tests were carried out in a three electrode
system on an electrochemical workstation (VERSASTAT3) at room
temperature. The electrochemical reduction of N2 to ammonia
was performed in a two-compartment cell at room temperature
and the cells were separated by a Nafion 117 membrane. An Ag/
AgCl (saturated KCl solution) electrode was used as the reference
electrode and a platinum foil electrode was used as the counter
electrode. The electrolyte volume in the two parts of the H-cell
was 70 mL. For electrocatalytic N2 reduction, potentiostatic tests
were conducted in N2-saturated 0.05 M H2SO4 for 2 h, which was
purged with N2 for 30 min before the measurement. Pure N2 was
continuously fed into the cathodic compartment with a properly
positioned sparger during the experiments.
Fig. 1 SEM of (a) 3D-graphene and (b) S-3DG and TEM of (c) 3D-gra-
phene and (d) S-3DG (e–g) SEM and EDS elemental mapping images of
C and S for S-3DG.
3.2 Raman analysis
By comparing the Raman spectra of 3D-graphene (Fig. S5†)
and S-3DG (Fig. 2), we can calculate that the ID/IG of S-3DG is
higher than that for 3D-graphene, which demonstrates that
S-3DG has more defects due to sulfur doping where the D
characteristic peak represents the defects in graphene and the
G characteristic peak indicate that sp2 carbon atoms vibrate in
the plane. The increase in the defect sites is more conducive
for nitrogen adsorption and dissociation.
3. Results and discussion
3.1 Structural and morphological analysis
3.3 XRD analysis
The X-ray diffraction (XRD) patterns of 3D-graphene and
S-3DG are displayed in Fig. 3, the diffraction peaks at 23.7°
corresponding to the (002) plane of 3D-graphene and the
bulge confirmed the amorphous nature of 3D-graphene.
S-3DG was prepared by the following reaction: Na2S2O3 + 2HCl
→ S↓ + 2NaCl + H2O + SO2↑. Note that sulfur was loaded on
the surface of 3D-graphene in an ice bath to undergo inter-
facial growth. Then, a certain number of carbon–sulfur bonds
were formed between them at a high temperature maintained
at for 10 hours. The scanning electron microscopy (SEM)
images show that the S-3DG samples (Fig. 1b) maintain the
intrinsic interlinked porous network of 3D-graphene (Fig. 1a)
well with even more fold morphologies, which are indicative of
unique characteristics including large folded morphology,
which results in higher specific surface areas and morphology
stability. The distribution of C and S elements is shown in
Fig. 1f and g, respectively. The distribution mapping shows
that sulfur was evenly distributed across the graphene sheets;
thus, the uniformity of doping is proved. The TEM images of
3D-graphene (Fig. 1c) and S-3DG (Fig. 1d) were measured with
the same magnification. Obviously, in S-3DG, there were large
black shaded areas because of sulfur doping in contrast with
Fig. 1c. The significant doping of sulfur in the 3D graphene
was successfully and uniformly demonstrated.
Fig. 2 Raman spectrum of S-3DG.
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Fig. 4 XPS spectra for S-3DG in the (a) C 1s and (b) S 2p regions.
plate electrode as reference electrode and counter electrode,
respectively. In this paper, unless otherwise specified, all the
reported potentials were converted to values versus RHE. The
electrocatalytic NRR activity of S-3DG was tested using con-
trolled potential electrolysis in N2-saturated electrolyte for 2 h.
Fig. 5a shows the linear sweep voltammetric (LSV) curves for
S-3DG/CP in Ar- and N2-saturated 0.05 M H2SO4. It obviously
shows that S-3DG/CP display a higher current density in N2-
saturated solution than in Ar-saturated solution, which indi-
cated the high activity towards NRR. For electrocatalysts, stabi-
lity is an important index; the result of the long-term durability
measurement test of S-3DG/CP for 20 h is shown in Fig. 5b,
which proved the expected stability of S-3DG/CP. Also, Fig. 5c
displays the chronoamperometric curves of S-3DG/CP at
different potentials from −0.8 to −0.4 V for 2 h. To compare,
the same loading quantity of 3DG/CP was also tested for
electrochemical reaction under the same conditions, as dis-
played in Fig. 5d. Meanwhile, a comparison diagram of the
yield under argon and open circuit voltage is also shown in
Fig. 5d. According to a report in the literature, in order to elim-
inate the experimental error due to the repeated use of the
membrane for the deposition of ammonia released into the
Fig. 3 XRD patterns of 3D-graphene and S-3DG.
Compared with 3D-graphene, the diffraction peaks of S-3DG
clearly corresponded to the PDF card (PDF 52-0976 red and
PDF 42-1278 black), which is consistent with the carbon–
sulfur compound and pristine S, respectively. Hence, the result
of XRD also qualitatively demonstrates the successful doping
of sulfur in 3D-graphene. The quantitative measurement of
sulfur doped was done by thermogravimetry (TG) and the
results in Fig. S6† show that the sulfur content of S-3DG is up
to 68.8%, in which most of it is in the form of elemental sulfur
and few of it is in the form of carbon–sulfide compound,
which combines the result of EDS (Fig. S7†) after the TG
testing is completed (Table S1†).
3.4 XPS analysis
X-ray photoelectron spectra (XPS) for S-3DG for C 1s and S 2p
is shown in Fig. 4a and b, respectively. In the C 1s region
(Fig. 4a), the peaks at the binding energies (BEs) of 284.7,
285.3, and 289.2 eV are assigned to C–C/CvC, C–S/C–O, and
OvC–O carboxyl groups, respectively. The S 2p spectrum
(Fig. 4b) can be resolved into three peaks at BEs of 164.0,
165.2, and 168.8 eV, which are assigned to the –C–S–C–, –C–S–,
and –C–SOx (x = 2, 3, and 4) species, respectively. The XPS for
3DG, as a comparison data, is shown in Fig. S9† to prove that
there was no sulfur in raw 3DG. Moreover, we did low concen-
tration comparisons to get the suitable sulfur source concen-
tration and the XPS spectra for S-3DG (1 M Na2S2O3) for C 1s
and S 2p is shown in Fig. S10,† the characteristic peaks of the
C–S bond can be clearly seen from the map. The XPS data also
fully demonstrates that sulfur was doped in 3D-Graphene in
the form of elemental sulfur and carbon–sulfide compound
successfully.
3.5 Electrocatalytic performance for ammonia synthesis
The experiment used a classical H-type electrolytic cell, which
was separated by a Nafion membrane in 0.05 M H2SO4 and
with three-electrode configuration at the ambient temperature.
S-3DG was dispersed in a mixture of ultrapure water and
ethanol in the ratio of 1 : 2 (vol%) and the ink was doped on
Fig. 5 (a) LSV curves of S-3DG/CP in Ar and N2 saturated 0.05 M
H2SO4, respectively. (b) Long-term durability measurement of S-3DG/CP
at −0.6 V for 20 h. (c) Chronoamperometric curves of S-3DG/CP at
different selected potentials for 2 h. (d) NH3 yields of after 2 h electroly-
sis under different conditions: −0.6 V in N2, −0.6 V in Ar, 3DG/CP and
carbon paper as the working electrode, Ag/AgCl and platinum open circuit in N2.
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paring the catalyst in large quantities at low temperature.
Three-dimensional graphene as a substrate supported non-
metal catalyst has successfully improved the conductivity of
the non-metal and enhanced the stability of non-metal electro-
chemistry. This strongly proves the application prospect of
carbon-based materials in catalyst substrates. Therefore, we
are still further studying other applications of three-dimen-
sional graphene as a substrate material in nitrogen reduction.
Conflicts of interest
There are no conflicts to declare.
Fig. 6 (a) UV/vis absorption spectra of the electrolytes after electrolysis
stained with indophenol indicator for 2 h. (b) Average NH3 yield and fara-
daic efficiency for S-3DG/CP at the corresponding potential. (c) UV/vis
absorption spectra of the different standard electrolytes after electroly-
sis stained with indophenol indicator for 2 h. (d) Faradaic efficiency and
NH3 yield during the recycling test.
Acknowledgements
The authors acknowledge the financial support of National
Natural Science Foundation of China (Grant No. 21671147)
and Shanxi Province Natural Science Foundation for Youths
(201901D211117).
electrolyte in the reaction process, we tried to use three new
membranes for the electrochemical reaction under the same
test conditions and satisfactory results were obtained, as
shown in Fig. S11,† the experimental results of the new mem-
branes is almost the same as the old.48 It also proves that the
repeatability is good. After multiple detection tests, it was
proved that there was no N2H4 in the electrolyte after reaction.
Fig. 6a shows the UV/Vis absorption spectra of the electro-
lytes after electrolysis in the potential range from −0.8 to −0.4
V with indophenol indicator for 2 h. The yield of NH3 and the
corresponding FEs of the catalyst at various potentials are
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