Electrocatalytic reduction of N2 and nitrogen-incorporation process on dopant-free defect graphene

Article abstract of DOI:10.1039/c9ta10071g

The electrochemical reduction of N₂ to NH₃ under ambient conditions is a promising N₂ fixation method, which provides a new technical solution to remedy the limitations of the Haber-Bosch process. Defect engineering is considered an inspiring strategy for strong N₂ activation. Herein, we demonstrate the reduction of N₂ on dopant-free defect graphene, which was prepared via the molten salt method. Systematic experiments and density functional theory calculation revealed that the defect sites are the unique active sites for nitrogen adsorption and activation. The phenomenon of N incorporation into graphene using the product NH₃ from the NRR as the N source has never been reported before. This was thoroughly studied in this study, and thus serves as a unique perspective to illustrate the significance of defect sites in activating N₂

Full text of DOI:10.1039/c9ta10071g

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Electrocatalytic reduction of N2 and nitrogen-
incorporation process on dopant-free defect
Cite this: J. Mater. Chem. A, 2020, 8, 55
graphene†
a
Yanqiu Du,a Cheng Jiang,a Wei Xia,a Li Song,a Peng Li,
Bin Gao,a Chao Wu,a
Received 12th September 2019
Accepted 12th November 2019
b
a
a
Lei Sheng,a Jinhua Ye,
Tao Wang
and Jianping He
*
*
DOI: 10.1039/c9ta10071g
rsc.li/materials-a
The electrochemical reduction of N2 to NH3 under ambient conditions hydrogen evolution reaction.3 Consequently, the development
is a promising N2 fixation method, which provides a new technical of high-performance electrocatalysts with the ability to signi-
solution to remedy the limitations of the Haber–Bosch process. Defect cantly reduce the N2 activation energy is essential for the NRR.
engineering is considered an inspiring strategy for strong N2 activa-
Recently, numerous electrocatalysts based on noble metals
tion. Herein, we demonstrate the reduction of N2 on dopant-free and transition metals showed high activity, such as Ru4 and
defect graphene, which was prepared via the molten salt method. MoS2.5 The unsaturated d orbitals, which synergistically accept
Systematic experiments and density functional theory calculation electron density from and back donate to N2,6 endow transition
revealed that the defect sites are the unique active sites for nitrogen metal catalysts with prominent performances. Notably, many
adsorption and activation. The phenomenon of N incorporation into transition metal catalysts are anchored on carbon nano-
graphene using the product NH3 from the NRR as the N source has materials, such as porous carbon7 and CNT.8 However, the
never been reported before. This was thoroughly studied in this study, signication contribution of dopant-free carbon substrates for
and thus serves as a unique perspective to illustrate the significance of the NRR has rarely been investigated.
defect sites in activating N2.
Graphene, as an ideal platform to load electrocatalysts,9 is
always modied via the introduction of heteroatoms such as
nitrogen and boron to improve its application. It has been long
accepted that graphene can be activated by lone-pair electrons
from electron-rich dopants or vacant orbitals from electron-
decient dopants by breaking the integrity of p conjuga-
tion.10,11 This activation process is usually advantageous for
improving catalytic activity. However, intrinsic carbon defects
can produce the same effects, breaking the original electron
arrangement and improving the electron transfer from the
electrocatalyst to chemisorbed molecules.12 Fortunately,
a previous study proved that the edge-rich graphene is more
active than the basal plane in the oxygen reduction reaction.13
Besides, theoretical calculations and experimental works
conrmed that defective graphene is also effective for other
catalytic reactions such as the OER and HER.14 This inspired us
to consider whether the defects in graphene are functional for
the N2 reduction reaction.
The process of converting free nitrogen into nitrogenous
compounds is known as N2 xation, which provides a crucial
element for the survival of living organisms. As the most widely
used process for N2 xation, the Haber–Bosch method can be
performed only under high temperature and high pressure,1
which is extremely energy-dependent and responsible for
massive CO2 emissions. Considering the urgent demand for
more sustainable alternatives, many efforts have been devoted
to nding cleaner ways to remedy the defects of traditional
processes. The N2 reduction reaction (NRR) is the most prom-
ising candidate, during which N2 xation is driven by renewable
electricity.2 Unfortunately, the low efficiency of the NRR is still
far from satisfactory for its widespread use because it is limited
by the ultrahigh stability of the N^N bond and the competitive
Herein, we designed dopant-free few-layer graphene with
rich defects, which was obtained by controlling the synthetic
conditions, and investigated its NRR activity. The synthesized
ultrathin porous graphene achieved a faradaic efficiency (FE) of
8.51% at À0.4 V vs. RHE for NH3 electrosynthesis. This
remarkable performance was enabled by the enhancement in
defect density, higher surface area and larger pore volume. This
study not only shows the feasibility of pure carbon
aCollege of Materials Science and Technology, Jiangsu Key Laboratory of
Electrochemical Energy Storage Technologies, Nanjing University of Aeronautics and
Astronautics, 29 Yudao Street, Nanjing, 210016, P. R. China. E-mail:
wangtao0729@nuaa.edu.cn; jianph@nuaa.edu.cn; Fax: +86 25 52112626; Tel: +86
25 52112900
bInternational Center for Materials Nanoarchitectonics (WPI-MANA), National
Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c9ta10071g
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nanomaterials as highly efficient NRR catalysts, but also serves a thickness of 3 nm (Fig. 1c). Furthermore, numerous nano-
as a model system to investigate the origin of NRR activity in pores with a size in the range of 1–5 nm were observed on the
carbon-based electrocatalysts.
surface of DG-800 in the TEM images (Fig. 1d and e) and
The process to create defects is illustrated in Fig. 1a, where aberration-corrected HAADF-STEM image (Fig. S4†).18
freeze-dried oxidized graphite was annealed in molten salts. Concomitant with numerous nanopores, a higher edge expo-
Molten salt-based exfoliation has been successfully applied to sure will offer more active sites for N2 adsorption and activation.
synthesize thin BN nanosheets from bulk materials.15,16 Contrary to that treated with molten salt, obvious defects were
Inspired by this, we adopted this method to synthesize few-layer not found in the pristine graphene (denoted as PG), and its
graphene from bulk oxidized graphite (Fig. S1†). The scanning graphene sheet was multi-layer (Fig. S5†) and unbroken (Fig. 1f
electron microscopy (SEM) images reveal that the morphology and g). According to the N2-physisorption measurements, the
of graphene aer annealing at 800 ꢀC, labelled as DG-800, is Brunauer–Emmett–Teller (BET) specic surface area of DG-800
wrinkled and crimped (Fig. 1b and S2†), suggesting that the is 420.20 m2 gÀ1, which is much higher than that of PG (78.33
exfoliation process is based on its attachment to salts and m2 gÀ1). The inset pore size distribution curves indicate that the
subsequent peeling off (Fig. S3†).17 The morphology of gra- size of the mesopores is centered at 3.4 nm and the mesopore
phene is sensitively dependent on the ꢀannealing temperature. volume was signicantly boosted, which is mainly attributed to
When the temperature was up to 900 C, the product DG-900 the etching effect of the molten salt (Fig. 2a and Table S1†).
with distinct stacking was not favorable for effective utiliza- With the aid of the molten salt, the multi-layer bulk graphene
tion of its active area (Fig. S2e†). The graphene sheet was was exfoliated into few-layer ultrathin porous nanosheets with
investigated by atomic force microscopy (AFM), showing abundant mesopores and exposed edges. These features
Fig. 1 (a) Schematic illustration of the synthesis of defect graphene. (b) SEM image of defect graphene DG-800, where DG-800 is the sample
treated with molten salt at 800 ꢀC. (c) AFM image of DG-800. (d and e) TEM images of DG-800 at different magnifications. (f and g) TEM images
of PG at different magnifications, where PG is the sample treated without molten salt.
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Fig. 2 (a) Nitrogen adsorption–desorption isotherms and the inset corresponding pore size distribution of the catalyst DG-800 and PG. (b) XRD
spectra of DG-800 and PG. (c) Raman spectra of PG and DG-800. (d) Full XPS spectra of DG-800 and PG. (e) N 1s XPS spectrum of DG-800. (f) C
1s XPS spectra of DG-800 and PG.
revealed by microscopy are benecial for the maximum deconvoluted into ve peaks, where the peaks at 284.75 and
inherent activity.
285.9 eV correspond to sp2 and sp3 carbon,22,23 respectively
The defect-induced alteration in local structure was further (Fig. 2f). The ratio of sp3 to sp2 carbons in DG-800 was deter-
characterized. Primarily, the X-ray diffraction (XRD) character- mined to be approximately 0.23, which is higher than that in PG
ization of PG and representative defect graphene is presented in and another quantitative description of more defects in DG-
Fig. 2b. The diffraction peak at 26.6ꢀ changes from a sharp peak 800.13 Thus, the above spectroscopic characterization fully
to a broader peak due to the destruction of the long-range order conrms the successful preparation of defect graphene.
in the atomic arrangements aer annealing.19 The ID/IG ratio of
The NRR activity of defect graphene was preliminarily pro-
PG and DG-800 in the Raman spectra, which is a powerful tool bed via the noticeable difference between Ar- and N2-saturated
to quantify disorder, increased from 1.65 to 1.98, implying electrolytes in LSV tests (Fig. 3a). Considering the increase in
a higher defect concentration and coinciding well with the XRD current density and the existence of N2, the N2 reduction reac-
measurements and TEM images (Fig. 2c).20 X-ray photoelectron tion is the most convincing reason for the above difference.
spectroscopy (XPS) (Fig. 2d) indicated that only carbon and Inspired by this, further NRR tests were performed to conrm
oxygen were present in the sample. Since no peaks for K+ and Li+ the electrocatalytic activity of the defect graphene. All the NRR
were initially detected by XPS (as shown in Fig. 2d), we then electrochemical tests were performed in an H-type cell with
proceeded to use ICP, a sensitive technique suitable for trace a continuous N2 ow, which was separated by a Naon
elements, to detect the metallic impurities of K+ and Li+. In DG- membrane. To clean ammonia from the membrane in between
800, the amount of K and Li was found to be 0.048 wt% and tests, the Naon membrane was retreated in 3% H2O2 and 0.5 M
ꢀ
0.0086 wt%, respectively, which are too low to be electroactive. H2SO4 for 1 h at 80 C aer the test. The NH3 produced in the
Considering permanganates are used in the Hummers' method, chronoamperometry tests were analyzed by Nessler's reagent
the analysis of residual Mn was also conducted, which was and indophenol blue together, thus ensuring the accuracy of
determined to be only 0.043 wt%. More specically, no obvious the results (the calibration curves are shown in Fig. S6 and S7†).
peak can be detected on the N 1s XPS spectra (Fig. 2e). Thus, the Furthermore, carefully controlling the conditions ensured that
+
problem of nitrogen species causing false positive results can be the calculated NH4 had an acceptable error, with a standard
avoided.21 The asymmetric high-resolution C 1s spectra were deviation of <2% (Table S2†). The absorbance of the electrolyte
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Fig. 3 (a) Linear sweep voltammetry curves of DG-800 in Ar- and N2-saturated electrolyte. (b) UV-vis spectra of the electrolytes after NRR at
À0.4 V vs. RHE for 2 h, where pristine electrolyte with indophenol indicator was used as a reference (yellow line). (c) NH3 yield rate at various
potentials. (d) Faradaic efficiency at various potentials. (e) UV-vis absorption spectra of the electrolytes in the control experiment stained with
indophenol indicator after 2 h electrolysis. (f) Stability test of the DG-800 catalyst in N2-saturated 0.01 M H2SO4 at À0.4 V vs. RHE under long-
time electrolysis.
catalyzed by DG-800 was overwhelmingly enhanced compared the high selectivity of the electrocatalyst. Considering the
À
with that of PG, indicating the better performance of DG-800 possible existence of NOx residue such as NO3 in the electro-
toward NH3 synthesis (Fig. 3b and S8†). The introduction of lyte aer the chronoamperometry tests at À0.4 V for 2 h, DG-800
mesoporosity and accessible defect sites in DG-800, acting as as the catalyst was analyzed using an ion chromatography (IC)
channels for the reaction medium,24 greatly favored the elec- to verify the presence of nitraÀte. The IC spectra of the electrolyte
trocatalytic activity. By contrast, PG was a poor catalyst towards aer being tested and NO3 at different concentrations are
the NRR. More specic performances are presented in Fig. 3c provided in Fig. S13.† The peak arÀea of the electrolyte was barely
and d, where it can be seen that the faradaic efficiency (FE) and visibly, indicating negligible NO3 was present, thus excluding
NH3 yield both increased with an increase in potential up to NH3 oxidation and the inuence of NO3À on the ammonia yield.
À0.4 V vs. RHE and both have peak values, and then decreased To unambiguously conrm the source of ammonia, a series of
for potentials <À0.4 V vs. RHE. The degradation of the perfor- control experiments were implemented. Blank carbon paper
mances at a more negative potential can be explained by the was used as the electrode and the N2 ow was replaced by an Ar
competing hydrogen evolution reaction (HER), in which more ow, and the absorption spectra showed no obvious difference
+
protons occupy the active sites.25 Subsequently, we studied the with the pristine electrolyte (Fig. 3e), that is, no NH4 was
effect of the defect density on NRR activity by comparing the generated. The former experiment made it clear that the
properties of the graphene synthesized at different tempera- synthesized graphene, rather than carbon paper, was the cata-
tures (Fig. S9†). DG-800 achieved the best performance, NH3 lyst for the NRR. In the latter case, we did not detect NH4+ in the
yield rate (4.31 mg hÀ1 mgcatÀ1) and FE (8.51%). Consistent with Ar-saturated electrolyte, which lacked a nitrogen source for
Fig. S10,† we found that the defect density positively affected synthesizing ammonia since the electrocatalyst was nitrogen-
the NRR process. Besides, we further performed the method free.
reported by Watt and Chrisp to detect the side product of N2H4
To evaluate the stability, a durability test consisting of three
(the calibration curve is shown in Fig. S11†). As illustrated in cycles of 10 h chronoamperometric tests at À0.4 V vs. RHE was
Fig. S12,† even a negligible gap between the pristine electrolyte conducted (Fig. 3f). The current density was well maintained at
and electrolytes aer NRR could not be detected. Therefore, it is around 0.1 mA cmÀ2 at the of end each cycle, showing little
reasonable to conclude that no N2H4 was produced, indicating degradation. The key performance metrics aer three cycles of
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FE (7.24%) and NH3 yield rate (1.84 mg hÀ1 mgcatÀ1) were still at 0 to 1.4%, providing solid spectroscopic evidence of the
a high level for a carbon-based material without any hetero- successful xation of N2 on DG-800. Further, the N 1s spectrum
atoms or transition metals. This remarkable performance was deconvoluted into three peaks (Fig. 4c) corresponding to
+
clearly demonstrates the superiority of DG-800 for the NRR.
NH4 (402.5 eV),28–30 graphitic N (401.2 eV),31,32 and pyrrolic N
The FT-IR spectra of the catalysts aer the NRR allowed us to (400.1 eV).33 Thus, the reduction of N2 to ammonia on DG-800
directly trace the protonation of N2 on the electrocatalyst was veried again by the peak at around 402.5 eV. The edges
surface (Fig. 4a). As reported in the literature, the two sharp and defect sites afford C atoms that are much more chemically
peaks at 2930 cmÀ1 and 2850 cmÀ1 are attributed to the char- reactive than that in the plane of perfect graphene for the
+
26
acteristic H–N–H stretching of NH4
.
The intensities of these formation of nitrogen-containing groups.34
To further conrm that defect sites are responsible for the
two bands increased with time, suggesting an increase in the
production of NH3.26 Contrary to the blank carbon paper, a new enhanced performance, the XPS results for PG and DG-800 were
band at 1459 cmÀ1 was observed, which is related to the compared. As presented in Fig. S16,† only a small amount of N
bending vibration of H–N–H in the intermediates and NH3 was doped in PG, mostly in the form of pyrrolic N. Combined
species.27 The appearance of H–N–H bending and stretching with the nearly perfect basal plane of PG, it is credible that N
bands implies that proton–electron pairs transferred to the was predominantly incorporated into the edges of PG, an
adsorbed N2 and reaction intermediates. When PG was used as unavoidable defective domain in graphene. Considering the
the electrocatalyst, the characteristic stretching bands of NRR, the basal plane was found to be inert and N2 molecules
H–N–H at 2930 cmÀ1 and 2850 cmÀ1 on the electrode surface cannot be effectively absorbed.35 This comparison highlights
were not evident until the reaction time was 10 h (Fig. S14†). the necessity of creating a favorable environment, such as defect
This result is due to the low NRR activity of PG.
sites, for molecular chemisorption. In addition, DFT calcula-
XPS measurements, which give fresh insight into N2 xation, tions were employed to describe the N2 adsorption and activa-
were conducted on the carbon paper with DG-800 aer the NRR tion on PG and defect sites of DG, in which both side-on and
test, and as a control, the same lm sample before the NRR test. end-on congurations were considered. N2 adsorption on DG
As shown in Fig. 4b, the XPS spectra on the lm sample before is stronger than that on PG due to the more negative energy
the test revealed C, O, F, and S signals without N, where F and S value of DG,36 which manifests that the NRR activity benets
originate from the Naon binder (C9HF17O5S). Apparently, from the defects in DG (Fig. 4d and e). The N2 molecules on the
a clear N signal on the sample aer the NRR test was detected, edge of DG are the most active, where the N–N bond is elon-
˚
whereas there was no N signal on the control sample. The XPS gated to 1.116 A, resulting in more efficient charge transfer.
data for nitrogen content (Table S3†) distinctly changed from
Fig. 4 (a) FT-IR spectra of blank electrode and DG-800 electrode after different reaction times. (b) Full XPS spectra and N 1s spectra (the inset) of
the DG-800 electrode before and after the NRR. (c) N 1s spectra of the DG-800 electrode after different reaction times. (d) Adsorption energies
(Eads, eV) of N2 molecule with the side-on configuration. (e) Adsorption energies (Eads, eV) of the N2 molecule with the end-on configuration.
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Next, the evolution of nitrogen species during the reaction
was monitored to study the reaction mechanism (Fig. 4c). As
illustrated in Fig. S17,† we speculated that the possible cong-
urations to form pyrrolic N and graphitic N are armchair edges
and point defects, respectively. Obviously, the possible defect
sites for N-dopants are much more than the two mentioned
above, which are still challenging to exactly identify.37 Thus, we
focused on the edge sites and point defects to study the process
concomitant with the NRR. On one hand, the peak of
graphitic N appeared with nearly a stable area, which is asso-
ciated with a decrease in the amount of point defects available
for the incorporation of N atoms as the reaction proceeds
(Fig. 4c). This implies that point defects are active in the
primary stage. Then, once the point defects are “saturated”
with N atoms, these sites are not active anymore. A previous
study also reported that graphitic nitrogen is NRR-inactive since
it is difficult for N2 to adsorb on graphitic N-doped carbon.38 On
the other hand, pyrrolic N shows constant improvement with
time (Fig. 4c and Table S4†). Mostly originating from rich edges,
it can be predicted that more and more pyrrolic nitrogen will be
detected as electrolysis continues.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
The authors express their appreciations for the nancial
support from the National Natural Science Foundation of China
(11575084, 51602153), the Natural Science Foundation of
Jiangsu Province (BK20160795), the Fundamental Research
Funds for the Central Universities (No. NE2018104), and
a project funded by the Priority Academic Program Develop-
ment of Jiangsu Higher Education Institutions (PAPD).
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