Graphdiyne Interface Engineering: Highly Active and Selective Ammonia Synthesis

Article abstract of DOI:10.1002/anie.202004213

A freestanding 3D graphdiyne–cobalt nitride (GDY/Co₂N) with a highly active and selective interface is fabricated for the electrochemical nitrogen reduction reaction (ECNRR). Density function theory calculations reveal that the interface-bonded GDY contributes an unique p-electronic character to optimally modify the Co-N compound surface bonding, which generates as-observed superior electronic activity for NRR catalysis at the interface region. Experimentally, at atmospheric pressure and room temperature, the electrocatalyst creates a new record of ammonia yield rate (Y (Formula presented.)) and Faradaic efficiency (FE) of 219.72 μg h^?1 mg_cat.^?1 and 58.60 percent, respectively, in acidic conditions, higher than reported electrocatalysts. Such a catalyst is promising to generate new concepts, new knowledge, and new phenomena in electrocatalytic research, driving rapid development in the field of electrocatalysis.

Full text of DOI:10.1002/anie.202004213

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
Chemie
International Edition
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Accepted Article
Title: Graphdiyne interface engineering: highly active and selective
ammonia synthesis
Authors: Yuliang Li, Yan Fang, Yurui Xue, Yongjun Li, Huidi Yu, Lan
Hui, Yuxin Liu, Chengyu Xing, Chao Zhang, Danyan Zhang,
Zhongqiang Wang, Xi Chen, Yang Gao, and Bolong Huang
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202004213
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10.1002/anie.202004213
Angewandte Chemie International Edition
RESEARCH ARTICLE
Graphdiyne interface engineering: highly active and selective
ammonia synthesis
Yan Fang,[a] Yurui Xue,*[a] Yongjun Li,*[a] Huidi Yu,[a] Lan Hui,[a] Yuxin Liu,[a] Chengyu Xing,[a] Chao
Zhang,[a] Danyan Zhang,[a] Zhongqiang Wang,[a] Xi Chen,[a] Yang Gao,[a] Bolong Huang,*[b] and Yuliang
Li*[a,c]
[a]
Y. Fang, Prof. Y. Xue, Prof. Y. Li, H. Yu, Dr. L. Hui, Y. Liu, C. Xing, C. Zhang, D. Zhang, Z. Wang, X. Chen, Dr. Y. Gao, Prof. Y. Li
Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, P. R. China
E-mail: xueyurui@iccas.ac.cn; liyj@iccas.ac.cn; ylli@iccas.ac.cn
[b]
Dr. B. Huang,
Department of Applied Biology and Chemical Technology, the Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong SAR, 999077, P. R.
China
E-mail: bhuang@polyu.edu.hk
[c]
Prof. Y. Li
University of Chinese Academy of Sciences, Beijing 100049, P. R. China
Supporting information for this article is given via a link at the end of the document.
Abstract:
A
freestanding 3D graphdiyne-cobalt nitrides
electrocatalysts are still low and the application potential is very
deficient.[5-13] Homogeneous catalysts are considered to have
great potential for selective NRR under ambient conditions,[14,15]
but their low stability and yield are not suitable for high-efficiency
electrocatalysis. On the other hand, heterogeneous catalysts,
which usually have high stability toward ECNRR under ambient
conditions and can be easily equipped with energy-conversion
devices, substaintially suffer from the competition between
ECNRR and hydrogen evolution reaction (HER), which causes
serious selectivity loss.[10] Based on the above analysis, catalysts
with high selectivity, high efficiency, high YNH3 and high stability
were not found so far. In recent years, the high-speed
development of science and technology, and the unique glamour
of high-end instruments and equipment in the characterization of
catalyst, lead to the best period of catalyst development. Various
strategies for constructing electrocatalyst used for energy
conversion have been proposed, including defect engineering
and surface/interface engineering, etc.,[6-8] providing inspiration
for ECNRR catalysts design.
(GDY/Co2N) with highly active and selective interface is fabricated for
electrochemical nitrogen reduction reaction (ECNRR). Density
function theory (DFT) calculations reveal that the interface bonded
GDY contributes an unique p-electronic character to optimally modify
the Co-N compound surface bonding, which generates as-observed
superior electronic activity for NRR catalysis at the interface region.
Experimentally, at the atmospheric pressure and room temperature,
the electrocatalyst creates a new record of ammonia yield rate (YNH3
)
and Faradaic Efficiency (FE) of 219.72 μg h1 mgcat. and 58.60 %,
respectively, in acidic conditions, higher than reported
electrocatalysts. Such catalyst is promising to generate new concepts,
new knowledge, and new phenomena in electrocatalytic research,
driving rapid development in the field of electrocatalysis.
1
Introduction
The emergence of synthetic ammonia has supplied the food of
billions of people in the world. Therefore, the research on the
synthesis of ammonia, especially at room temperature and
atmospheric pressure, has been the focus of great attention of
chemists. The electrically reducing nitrogen to the carbon-free
ammonia (N2 + 6H+ + 6e– → 2NH3) under ambient conditions is a
promising alternative to the energy- and carbon-intensive Haber-
Bosch process (N2 + H2 → 2NH3) that is widely used in industrial
NH3 production at high pressures and temperatures (200 atm, 400
– 450 °С).[1-4] In order to avoid the harsh synthetic conditions and
serious pollution caused by the above synthesized ammonia
route, the most critical of the synthetic ammonia process is the
synthesis of highly selective and efficient catalysts that can
effectively activate the inert NN bond of N2 to form NH3 at
ambient conditions. 3d-transition-metals (TM) have shown the
advantages for N2 activation via the “acceptance-donation”
interaction mechanism between the TM and N2, which is
beneficial to the ECNRR process. However, until now, there is still
no widespread concern on the study of 3d-transition-metal-based
materials as efficient ECNRR catalysts. And catalytic
performances (e.g., YNH3 and FE) of the reported ECNRR
In the light of the above, we propose electrocatalytic reactions
to have more advantages in kinetics and thermodynamics, which
are beneficial to new electrocatalytic systems. Incorporating metal
catalysts with carbon materials to form elaborately engineered
interface have been demosntrated to be an important approach
for improving the catalytic performances.[16-21] Graphdiyne (GDY),
a novel 2D carbon allotrope possessing unique porous structure,
uneven-distributed surface charge, (electro)chemical stability and
high conductivity, has received increasing interests on its versatile
applications in catalysis, energy conversion and storage, etc.[12,22-
32]
Remarkably, GDY can be grown on arbitrary substrates,[33-39]
providing engineered interfaces with improved charge transfer,
and increased active site numbers, which could substantially
enhance the overall catalytic activities and stabilities in the
electrocatalysis. In this work, we report the facile and scalable
synthesis of self-standing cathode by in-situ growing graphdiyne
nanosheet arrays (GDY NAs) on the surface of cobalt-nitride
nanowires (Co2N NWs) grown on carbon cloth (GDY/Co2N NWs,
Figure1a). With interface bonding modification, the interface Co-
sites indicate an electron-pumping like character for depleting the
electrons from Co2N towards GDY layer. We found that the 3D
1
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RESEARCH ARTICLE
self-standing GDY/Co2N cathode features the high selectivity and
efficiency for NH3 synthesis at ambient pressures and room
temperatures in both acidic and neutral conditions. Especially, in
0.1 M HCl (pH = 1) condition, it can achieve the highest YNH3 and
FE of 219.72 μg h1 mgcat.1 and 58.60% at a low potential of 0.2
V, outperforming previously reported ECNRR catalysts. Besides,
it also exhibits high YNH3 and FE. Moreover, such excellent
catalytic performances can be maintained over 114 and 144 h in
acidic and neutral conditions, respectively. Our results
demonstrate that the superior electronic activity of ECNRR
catalysis at the interface region arises from the unique GDY
electronic character. DFT calculation results suggest that the
anomalous decreasing of d-to-pσ* state transition minimizes the
barrier of N2-fixation for reaching the optimal ECNRR initiation.
These are the key promotion for high electronic activities and
ECNRR performance simultaneously.
could be clearly seen from the TEM (Figure 1g), STEM (Figure
1h) and HRTEM (Figure 1i) results. Energy-dispersive X-ray
spectroscopy (EDX) elemental mapping results indicate the Co
and N are homogeneously distributed only in the core region of
the nanowire, while the C element distributed over the whole width
of the nanowire. HRTEM image (Figure 1i) of GDY/Co2N
nanowire clearly showed that the Co2N has lattice spacing of
0.213 and 0.207 nm corresponding to the (002) and (111) facets
which in accordance with X-ray diffraction (XRD) results (Figure
S3a), covered by an amorphous GDY film (fringe spacing: 0.369
nm; the average thickness: ~ 6.0 nm). The resuls of BET show
that the average size of the pores of GDY/Co2N is around 5.4 nm
(Figure 1l), and the surface area is 24.99 m2 g–1 (GDY/Co2N,
Figure 1l) which is larger than pure Co2N NWs (18.08 m2 g–1,
Figure S2).
Raman measurements further provide evidences for the
formation of new defects in samples by quantifying the relative
intensity of D and G bands.[40,41] As shown in Figure 2a,
GDY/Co2N has three dominant peaks at 477.16, 516.53, and
676.05 cm–1 arising from the Co2N (Figure S5), and four peaks at
1373.21 (D band), 1586.28 (G band), 1954.92 and 2185.68 cm–1
(vibrations of the conjugated diyne links) corresponding to the
GDY species (Figure S5a).[22] The Raman results clearly show the
changes of the intensity ratio of the D and G band (ID/IG) of the
samples. GDY exhibits a low ID/IG ratio of 0.75, indicating the high
structural regularity; while the ID/IG ratio of GDY/Co2N increases
to 0.92, suggesting the hexagonal structure disruption and the
formation of some new defect sites. These clearly demonstrates
the generation of new defects after the incorporation of GDY with
Co2N.
The electronic states of chemical components of the catalysts
were further studied by using X-ray photoelectron spectroscopy
(XPS), X-ray absorption near edge structure (XANES) and
extended X-ray absorption fine structure (EXAFS). The C 1s XPS
spectrum of GDY/Co2N (Figure 2b) exhibits five peaks
corresponding to sp2-C (284.4 eV), sp-C (285.1 eV), C-O (286.7
eV), C=O (288.2 eV) and π-π* transition (290 eV), respectively.
[12,22,39]
The presence of π-π* transition peak verifies the
interaction between GDY and Co2N. As shown in Figure 2c, the
Co 2p spectrum of GDY/Co2N showed two spin-orbit triplets at
780.43, 782.14 and 778.62 eV and at 796.02, 797.71 and 793.72
eV as well as two shakeup satellite at 786.34 and 802.90 eV,
respectively. The triplets have been assigned to Co3+, Co2+ and
Co0 species, respectively. Compared with the pristine Co2N, the
Co 2p spectrum of GDY/Co2N shifts to higher binding energy by
0.43 eV (Figure 2c) while the N 1s spectra shifts to lower binding
energy by -0.45 eV (Figure 2d), revealing the obvious electron
charge transfer between Co2N and GDY. More Co3+ species were
generated after GDY growth, suggesting Co atoms shift to a
higher valence state in the interface. Besides, the N 1 spectra can
be deconvoluted into two typical peaks (Figure S5b)
corresponding to the nitrogen bonded to metal species (Co-N)
and adsorbed nitrogen, respectively. It was clearly that there were
no any oxide and oxynitride species on the surface of as-prepared
Co2N and Co2N/GDY, in accordance with the XRD and XPS
results. The O 1s XPS spectra of Co2N (Figure S5c) and
Co2N/GDY (Figure S5d) show two peaks centered at 531.0 and
532.8 eV, corresponding to the adsorbed oxygen and water
molecules, respectively. These findings solidly demonstrate that
there is no any oxide and oxynitride species in the prepared
catalysts. The XANES (Figures 2e and 2f) and EXAFS (Figure
Figure 1. Morphological characterizations. (a) Schematic illustration of the
preparation of self-standing GDY/Co2N. The scanning electron microscopy
(SEM) images of (b) CoLDH, (c) Co2N and (d) Co2N/GDY. The transmission
electron microscopy (TEM) images of (e) CoLDH, (f) Co2N, and (g) GDY/Co2N.
(h) Scanning TEM image and EDX elemental mapping of Co, N, and C in
GDY/Co2N. (i) HRTEM image of Co2N/GDY. (l) Adsorption and desorption
isotherm of GDY/Co2N (inset: pore size distribution of GDY/Co2N).
Results and Discussion
As shown in Figure 1a, the GDY/Co2N was synthesized through
a two-step strategy including the nitridation of the freestanding
cobalt layered double hydroxide nanowires (CoLDH) and the in-
situ growth of graphdiyne film on Co2N (GDY/Co2N). the Co2N
nanowires (Co2N) were first prepared by nitriding the cobalt
layered double hydroxide nanowires (CoLDH) through a facile
hydrothermal treatment. As a result of the nitridation, the surface
of the CoLDH (Figure 1b and e) becomes porous (Figure 1c and
f). The obtained Co2N was then used as a substrate for the growth
of graphdiyne. As shown in Figures 1d and 1g, a film of
graphdiyne nanosheets were successfully grown on Co2N surface
(GDY/Co2N). The intimate interface between the Co2N and GDY
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S6) were further performed to determine the chemical states of
the samples. The Co K-edge XANES spectrum (Figure 2e) of
GDY/Co2N show the increases of the edge and main peak energy
compared to Co2N. The E0 peak in the first-derivative curve of
XANES of the GDY/Co2N (Figure 2f) also located at higher
binding energies than that of Co2N and Co(NO3)2, but lower than
that of Co2O3. These indicate that the proportion of the Co atoms
with higher valence in GDY/Co2N is higher than that of Co2N. As
previously reported,[42,43] the Co atoms at higher valence are
proposed to be more catalytically active in electrocatalysis.
(pσ*), facilitating the N2-stabilization (Figure 3b). We compared
the 2p-orbital behavior between the interface bonded and pristine
GDY layers, where the C-2p orbital overall indicates an energetic
downshift of 2.5 eV. Such
a trend enhances the N≡N
electronegativity for capturing H+ instead of the H+-electron-
exchange (Figure 3c). Meanwhile, for those N sites bonding with
Co atoms, N-2p orbitals have been gradually pushed towards the
lower position from the surface sites to the interfacial region and
become more electron-rich. This is highly consistent with the
lower binding energy in experimental XPS results (Figure S7e).
Figure 3. Electronic activities and energetic pathways of GDY||Co2N for
ECNRR catalytic activity. (a) Site-dependent PDOSs of Co-3d band variation
from in-depth Co2N towards interface region, with compared to Co-metal state.
(b) Relative position alignment of p-d band between active interfacial Co-sites
and adsorbing N2. (c) PDOSs Comparison for GDY between cases of free-
standing and interface bonded. (d) Volcano-like correlation between electron-
transfer gap and N2-adsorption energy. (e) Work function comparison among
different interface and surface systems. (f) Schematic electron-transfer path
driven by the work-function differences and p-d orbital correlation. (g) Reaction
pathway of acidic NRR catalysis at the potential of U=0 V, U=-0.071 V, and U=-
0.252 V. (h) The local structural configurations for demonstrating the ECNRR
(acidic) process within the GDY||Co2N surface system.
Figure 2. Structural characterizations. (a) Raman spectroscopy of GDY/Co2N.
(b) The C 1s XPS spectra of GDY/Co2N and pristine GDY. (c) The Co 2p XPS
spectra of GDY/Co2N and Co2N. (d) The N 1s XPS spectra of GDY/Co2N and
Co2N. (e) The normalized Co K-edge XANES spectra of samples. (f) First-
derivative curves of XANES of samples.
According to the above discussed electron-transfer enhanced
N2-fixation, we further correlate the p-d coupling and the N2-
adsorption. There has been a volcano-like correlation discovered.
We reasoned the high ECNRR performance from DFT
calculations. The GDY shows less surface mismatching effect
with Co2N (111) compared with other directions. However, certain
strain effects have been induced by the interface (IF) bonding
between GDY and Co2N surface, which induces surface
reconstructions and various lattice distortions. This implies the
Co-sites play an active role for bonding with GDY layer (Figure
S7a-c). The bonding and anti-bonding orbitals near Fermi level
(EF) have been demonstrated. The GDY layer has been clearly
shown an electron-rich character, which indicates an electron-
transfer trend from Co2N towards GDY layer (Figure S7d). Site-
dependent Co-3d band center variation trend exhibits a near-
linear trend of uprising towards EF, from EV-1.9 eV for the bulk Co-
metal towards EV-1.0 eV for the Co-site at the IF, respectively.
Meanwhile, the eg-t2g splitting effect has been minimized where
the eg-component above EF downshifts and merges with t2g orbital
staying near EF (Figure 3a). We selectively demonstrate the 3d-
bands of interfacial Co-sites and N2-2p orbitals for stable N2-
adsorption. We confirm that the Co-sites are electronic active for
charge transfer from Co-3d to the empty-state of N2-2p orbital
This correlation tells
a trend that, the interface-bonding
modification promotes the electron-transfer from Co-3d band on
the Co2N surface towards the empty pσ*-orbitals of N2 for stable
chemical adsorption, for preventing early stage of N≡N bond
cleavage (Figure 3d). From the initial N2 adsorption to the final
desorption of NH3, we observe the two consecutive linear
correlations within the position of the dominant peak of N-2p
orbitals during ECNRR. The N-containing species gradually
becomes more electron-rich as the formation of ammonia. For the
generation of each NH3, the electronic linear correlation is the
pivotal factors for actualizing efficient ECNRR with the associative
mechanism (Figure S7f). In addition, the highly active inter-layer
bonding enhances the electron-transfer between Co2N and GDY
layers. From the work function comparison among different
systems, it reflects that stronger interface bonding between GDY
and Co2N exhibits a lower work function for transferring electrons
towards adsorbing N-species (Figure 3e). Schematic picture
shows that, the Co2N layer plays as an electron-pumping center
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for maintaining the electron-rich character for GDY at higher
electronegative level (Figure 3f).
verified the production of ammonia from nitrogen. These results
confirmed the 100%-selectivity of the GDY/Co2N towards the
ECNRR electrolysis.
Further viewing on energetic trend of ECNRR pathway, we find
the reaction-heat shows an overall energetic gain at -1.51 eV and
energetic favorable with most steps in downhill trend. The overall
energetic gain has been further transformed into the electrode
potential between -0.071 V and -0.252 V, which show a good
agreement with experimental measurement. The dominant barrier
at U=-0.252 V is denoted at the step of forming *N2H3+3(H++e-)
from *N2H2+4(H++e), which is only 0.13 eV at the U=0 V (Figure
3g). For the dissociation mechanism under the same applied
potential of U = -0.252 V, the reaction pathway still displays a
large barrier of 0.33 eV at the formation of the first ammonia,
indicating the low preference of the dissociation mechanism for
ECNRR. Meanwhile, the competitive HER process on Co2N/GDY
is not energetically favorable due to the high barrier of 0.539 eV
for chemisorption of H, which significantly suppresses the further
HER process. Moreover, the barrier of chemisorption of HER also
demonstrates the H overbinding effect has been alleviated,
supporting efficient hydrogenation within ECNRR (Figure S7g).
From local structural configurations (Figure 3h), we find all N-
species are energetically favorable adsorbing on the lower
coordinated Co-site stably bonded by GDY layer, while the nearby
C-sites with alkyne bond plays an enhanced electron-proton
exchange center for re-distributing the H-species representing a
rather high selectivity.
Stability is another important factor for ECNRR electrocatalysts.
As shown in Figure 4d, the GDY/Co2N exhibits high long-term
stability with 87.68% and 71.02% retention of the YNH3 and FE,
respectively, after 7 times recycling tests. And there is negligible
decrease in the current density during a 144-hour electrocatalysis
(Figure S11). XRD (Figure S12) and SEM (Figure S13)
measurements exhibited the well-retained crystalline structure
and morphology after 7 times ECNRR tests. XPS results after
cycling test also revealed the formation of cobalt oxides during
ECNRR (Figure S14).
The electrochemical measurements were firstly performed in
0.1 M Na2SO4 electrolyte in a two-compartment cell separated by
Nafion 117 membrane. Freshly-prepared samples, saturated
calomel electrode and graphite rod were used as the working
electrode, the reference electrode and the counter electrode,
respectively. The ECNRR was operated in the N2 atmosphere (1
atm) with N2 gas continuously being supplied at the cathode.
During the reduction reaction process, the protons transporting
from the electrolyte to the catalyst, and NH3 was then synthesized
on GDY/Co2N surface and protonated in the electrolytes forming
Figure 4. Neutral ECNRR performances. (a) YNH3 of GDY/Co2N at different
potentials in N2-and Ar-saturated 0.1 M Na2SO4 (bars represent standard
deviation). (b) FEs of GDY/Co2N at different potentials in N2-and Ar-saturated
0.1 M Na2SO4 (bars represent standard deviation). (c) YNH3 and FEs of
GDY/Co2N and some reported excellent electrocatalysts performed in neutral
conditions. (d) Cycling test of GDY/Co2N at 0.055 V versus RHE in 0.1 M
Na2SO4. NMR data from experiments of (e) the reference sample, and the
samples obtained from the (f) pure 15N2 and (g) 15N2-14N2 mixed gas
environments.
+
NH4 , which could be determined by the spectrophotometry
method (Figures S8 and S15).[12] The possible by-product
hydrazine was detected by the Watt–Chrisp method (Figure
S9).[44] Linear sweep voltammetry (LSV) curves recorded in N2-
and Ar-saturated electrolytes clearly revealed the occurrence of
the efficient ECNRR (Figure S9a). The UV-vis adsorption curves
and the chronoamperometry curves recorded at different
potentials were shown in Figures S9b-c, respectively. As
expected, no N2H4 by-product could be detected at all applied
potentials in N2-saturated electrolytes (Figure S9d); and no NH3
could be detected in Ar-saturated 0.1 M Na2SO4 (Figure S10 and
Figure 4a-b). GDY/Co2N achieves the highest YNH3 and FE of
123.66 μg h1 mgcat.1 and 35.84%, respectively, at 0.055 V versus
The ECNRR performances of the samples were further studied
in acidic electrolyte (0.1 M HCl, Figures S16 and S17). Figure 5a-
b displays the average NH3 yield (YNH3, with standard deviation)
and corresponding FEs with standard deviation at different
potentials. At the applied potential of 0.101 V versus RHE,
GDY/Co2N exhibits the ECNRR performances with the YNH3 of
1
151.24 ± 38.50 μg h1 mgcat. and FE of 53.82 ± 4.57 % (Figure
5a-b and Table S3). The highest values of YNH3 and FE reach up
RHE (Figure 4a-c, Table S1), which are higher than Co2N (YNH3
=
1
to 219.72 μg h1 mgcat. and 58.60%, respectively, which are
13.22 μg h1 mgcat.1, FE = 5.11%) and the reported ones (Table
S2), such as Pd/C,[45] Mo2C/C,[46] DR MoS2,[47] and MHCMs,[48]
measured in neutral conditions, revealing the excellent catalytic
activity of the GDY/Co2N.
higher than previously reported ECNRR electrocatalysts (Figure
5c) such as B4C,[50] a-Au/CeOx-RGO,[11] FL-BP NSs[51] and VN[5]
(Table S2).
To verify that detected NH3 was originated from the reduction
of N2 by GDY/Co2N, the electrolysis was performed in Ar-
saturated electrolytes at all applied potentials. As shown in Figure
S17, no signals could be seen from the UV-Vis spectra, which
indicates that no NH3 was formed (Figure 5a-b). UV-vis spectra
for detection of N2H4 showed no peak in 455 nm (Figure S18).
These observations demonstrate that the high reaction selectivity
of our catalyst. As shown in Figure S19, the 1H NMR signal for
Isotope-labelled measurements were carried out to determine
the origin of the N source of the produced ammonia.[49] Two
resonance peaks for 15NH4+ could be observed from the 1H NMR
data for both the reference sample (Figure 4e) and that obtained
after ECNRR in 0.1 M Na2SO4 using isotopically labelled 15N2 as
the feeding gas (Figure 4f). An additional experiment using a 15N2-
14N2 mixed gas (containing about 2% 14N2, Figure 4g) again
4
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+
15NH4 was detected after ECNRR under 0.1 M HCl condition
of 7.17 mF cm2, approximately 1.65 times that of Co2N (4.35 mF
cm2), indicating that the former had larger ECSA. Results above
certificated that incorporation of GDY and Co2N could facilitate the
charge transfer behavior and offer more electrochemical active
sites, which resulted in great improvements of ECNRR catalytic
performances.
using 15N2 as the feeding gas. The spectrum showed that the
characteristic peak for 15NH4+ was split by the nuclear spin of 15
N
into a well-known doublet in the range of 6.85-7.15 ppm, in
consistent with the reported works.[45] These results confirm the
NH3 was really produced from the N2 reduction but not the N
element of Co2N species. We further investigated the origin of the
ECNRR abilities of GDY/Co2N. The CF substrate and Co2N
presented low YNH3 of only 0.08 and 14.83 μg h1 mgcat.1 (Figure
S20), which are much smaller than that of GDY/Co2N. According
to the above discussion, the incorporation of GDY and Co2N can
effectively enhance the ECNRR ability of an electrocatalyst
towards more efficient N2 reduction electrocatalysis. As shown in
Figure 5d, the YNH3 and FE of GDY/Co2N exhibited slight
decrease after 5-cycle ECNRR tests. Besides, the GDY/Co2N
shows almost no variation in the current density during the 114 h
continuous electrolysis (Figure S21). The XRD (Figure S22) and
SEM (Figure S23) measurements reveal that no obvious changes
in the crystalline and the morphologies of the GDY/Co2N obtained
after stability tests, demonstrating the morphological and
structural stability of our electrocatalyst. The Co 2p, N 1s, O 1s
and C 1s XPS spectra of GDY/Co2N suggested that more Co3+
species were generated and the surfaces of Co2N partially
transferred to oxide species during a long-time electrocatalysis
(Figure S24).
Conclusion
In summary, self-standing GDY/Co2N was successfully
synthesized through a facile and scalable method. It has been
demonstrated to be a promising catalyst for ECNRR with 100%
selectivity and high ammonia yield rate and Faradaic efficiency,
which compared favorably to previously reported electrocatalysts.
Our results showed that the GDY layer at the interface region play
a significant role for promoting electron-transfer towards N2.
Meanwhile, the GDY layer uniquely modifies the Co2N surface
states for acting as electron-pumping center, which drives the
interface Co-sites effectively transfer electrons from Co2N
towards GDY layer. And the introduction of GDY could generate
more active sites and promote the charge transfer between metal
species and GDY, which are beneficial to the improvement of the
ECNRR performances. The overall interface exhibits an important
p-d coupling for N2-fixation, which reflects a subtle volcano-plot
showing a positive correlation between catalytic and electronic
activities. Such interplay suggests an innovative strategy and path
towards
performance.
improving electrochemical
Ultimately, such new
based N-reduction
graphdiyne-based
electrocatalyst supplies an attractive prospect for the carbon-free
ammonia production.
Acknowledgements
This work was supported by the National Nature Science
Foundation of China (21790050, 21790051 and 21771156), the
National Key Research and Development Project of China
(2016YFA0200104 and 2018YFA0703501), the Key Program of
the Chinese Academy of Sciences (QYZDY-SSW-SLH015), and
the Early Career Scheme (ECS) fund (Grant No.: PolyU
253026/16P) from the Research Grant Council (RGC) in Hong
Kong. We would like to thank the XAFS station (beam line 1W1B)
of the Beijing Synchrotron Radiation Facility for the XAS
measurements.
Figure 5. Acidic ECNRR performances. (a) YNH3 of GDY/Co2N at different
potentials in N2-and Ar-saturated 0.1 M HCl (bars represent standard deviation).
(b) FEs of GDY/Co2N at different potentials in N2-and Ar-saturated 0.1 M HCl
(bars represent standard deviation). (c) YNH3 and FEs of GDY/Co2N and some
reported excellent electrocatalysts performed in acid conditions. (d) Cycling test
of GDY/Co2N at 0.101 V versus RHE in 0.1 M HCl.
Keywords: graphdiyne • 2D carbon allotrope • heterostructure •
self-supporting electrocatalyst • ammonia synthesis
In order to gain deeper understanding on the origin of the
ECNRR performance of GDY/Co2N, the electrochemical
impedance spectroscopy (EIS, Figure S25) was measured and
fitted by using an equivalent circuit model (R(QR)(QR)) containing
solution resistance (Rs), charge transfer resistance (Rct) and
adsorption resistance (Rad). Compared with all reference samples,
the GDY/Co2N possessed the lowest Rs and Rct values in both 0.1
M HCl (Rs = 10.07 Ω, Rct = 0.75 Ω) and 0.1 M Na2SO4 (Rs = 8.69
Ω, Rct = 2.5 Ω), indicating the most favorable charge transfer
kinetics of GDY/Co2N during the ECNRR process, which is
beneficial to the improvement of the catalytic activity. We further
measured the electrochemically active surface area (ECSA) of
the samples (Figures S26 and S27). Electrochemical
measurements showed that GDY/Co2N had a largest capacitance
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Entry for the Table of Contents
Insert graphic for Table of Contents here.
Graphdiyne-cobalt nitride electrocatalyst (GDY/Co2N) with rationally designed and constructed interface shows high selectivity and
efficiency for ammonia synthesis at ambient pressures and room temperatures in both acidic and neutral conditions. Our studies show
that the unique GDY electronic character plays a critical role for promoting the superior electronic activity and ECNRR performance
simultaneously.
7
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