The Crucial Role of Charge Accumulation and Spin Polarization in Activating Carbon-Based Catalysts for Electrocatalytic Nitrogen Reduction

Article abstract of DOI:10.1002/anie.201915001

Cost-effective carbon-based catalysts are promising for catalyzing the electrochemical N₂ reduction reaction (NRR). However, the activity origin of carbon-based catalysts towards NRR remains unclear, and regularities and rules for the rational design of carbon-based NRR electrocatalysts are still lacking. Based on a combination of theoretical calculations and experimental observations, chalcogen/oxygen group element (O, S, Se, Te) doped carbon materials were systematically evaluated as potential NRR catalysts. Heteroatom-doping-induced charge accumulation facilitates N₂ adsorption on carbon atoms and spin polarization boosts the potential-determining step of the first protonation to form *NNH. Te-doped and Se-doped C catalysts exhibited high intrinsic NRR activity that is superior to most metal-based catalysts. Establishing the correlation between the electronic structure and NRR performance for carbon-based materials paves the pathway for their NRR application.

Full text of DOI:10.1002/anie.201915001

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: The crucial role of charge accumulation and spin polarization in
activating carbon-based catalysts for electrocatalytic nitrogen
reduction
Authors: Tao Ling, Yuanyuan Yang, Lifu Zhang, Zhenpeng Hu, Yao
Zheng, Cheng Tang, Ping Chen, Ruguang Wang, Kangwen
Qiu, Jing Mao, and Shi-Zhang Qiao
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201915001
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Angewandte Chemie International Edition
10.1002/anie.201915001
RESEARCH ARTICLE
The crucial role of charge accumulation and spin polarization in
activating carbon-based catalysts for electrocatalytic nitrogen
reduction
[a]+
[b]+
[b]
[c]
[c]
[d]
Yuanyuan Yang, Lifu Zhang, Zhenpeng Hu, Yao Zheng, Cheng Tang, Ping Chen, Ruguang
[a]
[a]
[a]
[a]
[a,c]
Wang, Kangwen Qiu, Jing Mao, Tao Ling,* and Shi-Zhang Qiao*
[
5]
[6]
[7]
[8]
carbides /nitrides /sulphide /oxides
potential NRR active materials. However, the electrocatalytic
conversion of N to NH is still limited by the poor production rate
were evaluated as
2
3
Abstract: Cost-effective carbon-based catalysts are promising for
catalyzing electrochemical N reduction reaction (NRR). However, the
and unsatisfactory conversion efficiency due to the critical
challenge of ultra-stable N≡N bond and competing hydrogen
evolution reaction (HER).[3] The rational design of efficient and
durable electrocatalysts for NRR is still far from satisfactory.
In the past decade, carbon-based electrocatalysts have
shown their great potentials in catalysing the oxygen reduction
reaction (ORR),[9] oxygen evolution reaction (OER)[10] and HER[11]
for energy conversion and storage. Recently, heteroatom-doped
carbon materials have been proved to be electroactive to reduce
2
activity origin of carbon-based catalysts towards NRR remains
unclear, and regularities and rules in rational design carbon-based
NRR electrocatalysts are still lacking. Here, based on a combination
of theoretical calculations and experimental observations, we
systematically evaluated the chalcogen/oxygen group elements (O, S,
Se, and Te) doped carbon materials as potential NRR catalysts. We
revealed that heteroatom-doping induced charge accumulation
2
facilitates N adsorption on carbon atom and spin polarization boosts
as well.[12] The development of carbon-based materials with
N
2
the potential-determining step of the first protonation to form *NNH.
Te and Se-doped C catalysts exhibited high intrinsic NRR activity,
superior to most of metal-based catalysts. The establishing of the
correlation between the electronic structure and NRR performance for
carbon-based materials paves the pathway for their NRR application.
significantly improved electrocatalytic NRR activity is highly
desirable due to their low-cost, environment-amity, and promising
performance. However, there are still several issues unsolved: (i)
the available reports on carbon-based materials for NRR are
case-by-case studies and lack systematically theoretical and
experimental investigations; (ii) the catalytic nature and activity
origin of carbon-based catalysts towards NRR remain unclear; (iii)
regularities and rules in designing efficient carbon-based NRR
electrocatalysts are still not established.
Introduction
Ammonia production plays a vital role in human society as
fertilizers and chemicals.[1] Industrial-scale ammonia synthesis is
operated by the Haber-Bosch process at high temperature and
pressure, which is energy-intensive and releases massive
Moreover, previous theoretical and experimental efforts
have revealed that heteroatom-doping induced charge[9a] and
spin[13] redistribution facilitates chemisorption of
enhances ORR activity. According to Molecular Orbital theory,
the electronic configurations of ground-state O and N molecules
are entirely different – O is spin-triplet with two unpaired
electrons, whilst N is diamagnetic with all electrons pair up. This
2
O , thus
greenhouse gases.[2] In contrast, electrochemical N
reduction
2
2
2
reaction (NRR) via renewable energy to produce ammonia at
ambient temperature and pressure is highly promising and
attracts growing interests.[3] Recently, great advances have been
2
2
may lead to different heteroatom doping induced electronic effect
on NRR compared to ORR, which requires mechanistic
understanding and plays crucial roles in rational design of carbon-
based NRR catalysts.
achieved
in
NRR,
and
metal,[4]
metal
+
[
a]
Y. Yang , R. Wang, K. Qiu, Dr. J. Mao, Prof. T. Ling, Prof. S. -Z.
Qiao
Here, using metal-organic framework (MOF) derived nano
porous carbon as a design platform, we theoretically and
experimentally investigated the nonmetallic elements of the
chalcogen/oxygen group (O, S, Se, and Te) as dopants in carbon
catalysts to tune their NRR activity. We revealed that the
Key Laboratory for Advanced Ceramics and Machining Technology
of Ministry of Education, Tianjin Key Laboratory of Composite and
Functional Materials, School of Materials Science and Engineering,
Tianjin University, Tianjin, 300072, China
heteroatom-doping induced charge accumulation facilitates N
2
Email: lingt04@tju.edu.cn
adsorption on carbon atom and spin polarization boosts the
potential-determining step of the first protonation to form *NNH;
these two effects jointly contribute to overall NRR rate. With the
+
[
b]
c]
L. Zhang , Prof. Z. Hu
School of Physics, Nankai University, Tianjin 300071, China.
Dr. Y. Zheng, Dr. C. Tang, Prof. S. -Z. Qiao
School of Chemical Engineering, The University of Adelaide,
Adelaide, SA 5005, Australia
[
2
promotion of both N adsorption and *NNH formation, Se- and Te-
doped C catalysts exhibited the highest NRR activity among the
investigated ones. Our systematically combined computational
and experimental work uncovers the catalytic nature of carbon-
based materials towards NRR and paves the pathway for their
future application.
Email: s.qiao@adelaide.edu.au
[
d]
+]
Prof. P. Chen
School of Chemistry and Chemical Engineering, Anhui University,
Hefei 230000, China.
[
These two authors contribute equally to this work.
1
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Angewandte Chemie International Edition
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RESEARCH ARTICLE
Results and Discussion
S-doped C (-0.121 eV) < Se-doped C (-0.275 eV) < Te-doped C
(
-0.568 eV) (Figure 1a). To gain further insight, Bader charges of
pristine and doped C were analysed. As shown in Figure 1b, the
high electronegativity of O (χ = 3.5) results in an accumulation of
electrons from nearby carbon atoms (χ = 2.5), and introduces a
net positive charge on its neighbouring carbon atom. In contrast,
S (χ = 2.5), Se (χSe = 2.4) and Te (χTe = 2.1) create a net negative
charge on their adjacent carbon atoms. Our computational results
reveal that N molecule barely adsorbs on carbon atom of pristine
and O-doped C with ΔEN2 ≈ 0 eV. In contrast, the N adsorption is
In this work, chalcogen/oxygen group elements, O, S, Se, and Te,
were chosen to comprehensively explore the doping effect on the
NRR activity of carbon-based materials. Firstly, DFT calculations
were performed to evaluate the NRR activity of these catalysts.
We screened possible atom sites, and found that the carbon atom
adjacent to the dopant is the active site for NRR (Figures S1-S8
O
C
S
and Note S1). Primarily, the adsorption energy of N
catalysts (ΔEN2) was computed because the N adsorption is the
2
on these
2
2
2
greatly enhanced on S-, Se-, and Te-doped C with charge
first step to initialize the NRR, playing an important role in the
subsequent reaction pathway. ΔEN2 on these catalysts is in the
following order: pristine C (-0.009 eV) ≈ O-doped C (-0.002 eV) <
enriched carbon atom (Figure 1b). Therefore, charge
accumulation on carbon atom plays a significant role for N
2
adsorption.
Figure 1. Physical origin of NRR on pristine and heteroatom-doped C. (a) Adsorption free energy of N
2
(ΔEN2) on pristine and heteroatom-doped C catalysts. (b)
Bader charge ΔQdopant and ΔQ for the circled dopant atom and the circled carbon atom (neighbouring the dopant), respectively, in various heteroatom-doped C
C
catalysts. The positive and negative charge values indicate the electron gain and loss, respectively. (c) Gibbs free energy for the formation of *NNH (ΔG*NNH) on
pristine and heteroatom-doped C catalysts. (d) Spin moment of the carbon atom adjacent to the dopant, which is circled in the inset. (e) Top and side views of
-
3
isosurface of the spin-resolved density pictures of Se- and Te-doped C. The isosurface value is set to be 0.0005 e Å . (f) The correlation between ΔG*NNH and the
spin moment of the active carbon atom.
The weak N
2
adsorption on pristine C leads to a high Gibbs
(Figure S9). However, this value dramatically increases to 0.057,
0.062, 0.107, and 0.113 µ after O, S, Se, and Te doping,
free energy for the first protonation to form NNH* (ΔG*NNH = 2.32
eV). As illustrated in Figure 1c, ΔG*NNH on these catalysts follows
the order: pristine C (2.32 eV) > O-doped C (1.47 eV) ≈ S-doped
C (1.53 eV) > Se-doped C (1.38 eV) > Te-doped C (1.29 eV).
Notably, O-doped and S-doped C with deficient and enriched
charge as compared to pristine C, respectively, exhibit very
similar ΔG*NNH, therefore charge accumulation does not
significantly affect *NNH formation. To uncover the underlying
mechanism, the spin distributions on pristine and doped C were
calculated (Figure 1d). It should be mentioned that the carbon
atoms in pristine C are not magnetic with negligible spin moment
B
respectively. To further clarify such spin polarization effect, the
top and side views of the spin-resolved density of doped
substrates are presented in Figure 1e and Figure S10. As shown,
O, S, Se and Te dopants induce the distributing of ‘spin clouds’ at
the vicinity of adjacent carbon atoms, which are responsible for
the facilitated *NNH formation.[14] It is found that there is a linear
correlation between the spin moment and ΔG*NNH (Figure 1f).
This gives a quantitative explanation of the role of spin
2
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Angewandte Chemie International Edition
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RESEARCH ARTICLE
that spin localization on carbon atoms can enhance the
adsorption of O
that ground-state O
electrons, thus spin polarization on carbon atom favours O
adsorption. In the case of NRR, N is diamagnetic molecule with
all electrons pair up. Therefore, spin polarization has no obvious
effect on N adsorption, but on *NNH formation. This was
2
,[13,15] thus boost ORR activity. It is reasonable
2
molecule is spin-triplet with two unpaired
2
2
2
supported by the evidence that O-doped C with polarized spin
density, however affords ΔEN2 ≈ 0 eV (Figure 1a).
Of particular interest is that compared with O- and S-doped
C, Se- and Te-doped C exhibit larger amounts of enriched charge
and polarized spin moments on the active centres, suggesting
their better activity towards NRR. To gain details about the
2 3
conversion of the activated N to NH on these two catalysts, their
free energy diagrams were computed. The NRR was considered
through an associative distal pathway with reaction intermediates
2 2
*NNH, *NNH , *N, *NH, and *NH (Figure S11). As noted before,
2
Figure 2. Free energy diagrams of the N electrochemical reduction with an
associative distal pathway on Se- and Te-doped C catalysts.
for pristine C, the first protonation step to form *NNH is the
potential-determining step with a high ΔG*NNH of 2.32 eV. In the
case of Se- and Te-doped C, although the first protonation step is
still endergonic and potential-determining, the value of ΔG*NNH is
significantly reduced to 1.38 and 1.29 eV, respectively (Figure 2).
polarization in determining the energy barrier of the first
protonation step. Noticing that previous works have demonstrated
Figure 3. Characterization of heteroatom-doped C. (a) Schematic illustration of the synthetic procedure for heteroatom-doped C. (b) and (c) Scanning electron
microscopic (SEM) and transmission electron microscopic (TEM) images of heteroatom-doped C, respectively. (d) HAADF-STEM image and corresponding EDS
mapping results of C, N, and doping elements. The catalyst characterized here is Te-doped C.
Undoubtedly, the remarkably decreased energy barrier in the
potential-determining step assures Se- and Te-doped C with
greatly enhanced NRR activity.
derived nanoporous carbon as a design platform (Figure 3a). As
shown in Figure S12, the derived carbon exhibits
rhombododecahedral structure with an average size of 100 nm.
X-ray powder diffraction (XRD) and high-resolution transmission
electron microscopy (HRTEM) characterizations indicate the
Guided by above theoretical predictions, we experimentally
attempted to fabricate O-, S-, Se- and Te-doped C, using MOF
3
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RESEARCH ARTICLE
crystalline nature of small-sized graphitic carbon in as-derived
carbon material (Figures S13 and S14). The broad D band in
Raman spectrum (Figure S15) reveals the existence of large
amounts of defects in as-derived carbon, which is beneficial for
heteroatom-doping.
of the derived carbon frameworks. The high-angle annular dark-
field scanning TEM (HAADF-STEM) image and corresponding
energy-dispersive X-ray spectroscopy (EDS) elemental mapping
present the homogeneously uniform distribution of the C, N, and
dopant elements over the whole rhombododecahedron (Figure
3d and Figure S17). Notably, in these carbon frameworks, the N
element is from the original MOF precursor, which is
experimentally and theoretically proved to have no significant
impact on the NRR performance of as-fabricated carbon-based
catalysts (Figure S18).
The incorporation of heteroatoms in as-derived carbon was
realized via high-temperature processing in dopant vapor (Figure
S16). The dopant concentration can be easily tuned by controlling
the processing temperature (Table S1). As shown in Figure 3b
and 3c, the doped C maintains the rhombododecahedral structure
2
Figure 4. The chemical environment of heteroatoms in carbon frameworks. (a) HAADF-STEM image of Te-doped C catalyst. (b)-(d) FTIR, XPS and N -TPD spectra
of O-, S-, Se-, and Te-doped C catalysts, respectively.
Furthermore, the chemical nature of heteroatom doping was
investigated by combination of HAADF-STEM, Fourier
confirm the successful incorporation of heteroatoms in carbon
frameworks.
As suggested by our calculations, the doping element
governs the catalytic NRRactivity of the catalysts (Figures 1 and
2). To experimentally verify the computational findings, the N
temperature-programmed desorption (N -TPD) for pristine and
a
transform infrared spectroscopy (FTIR), and X-ray photoelectron
spectroscopy (XPS) spectra. The atomic-level HAADF-STEM
image clearly presents the individual dispersion of heteroatoms in
the derived carbon (Figure 4a). In FTIR spectra, the characteristic
peaks attributed to the vibrations of C=O, S-C, Se-C, and Te-C in
O-, S-, Se-, and Te-doped catalysts, respectively, are clearly
observed (Figure 4b). Correspondingly, the signals belonging to
C=O, C-S-C, C-Se-C, and Te-C are revealed in XPS spectra for
O-, S-, Se-, and Te-doped C (Figure 4c), respectively. Besides,
S-O, Se-O, and Te-O signals are resolved in XPS spectra for S-,
Se- and Te-doped C (Figure 4c), indicating partial oxidation of
these dopants in carbon frameworks (Figure S19). These results
2
2
doped catalysts were conducted. As shown in Figure 4d, all
samples exhibit a peak in the range of 70-180 °C, which can be
attributed to the physical adsorption of N
2
. Meanwhile, the
desorption peak for chemical adsorbed N is in the range of 250-
2
400 °C, which is gradually shifted to higher temperature with
larger adsorption area from pristine, through O-, S-, and Se- to
2
Te-doped C. This indicates that the N binding strength on these
catalysts is in the following order: pristine C < O-doped C < S-
doped C < Se-doped C < Te-doped C. This N
2
-TPD result
4
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RESEARCH ARTICLE
Figure 5. Experimental evaluation of NRR performance of pristine and heteroatom-doped C catalysts. (a) Flow diagram of control experiments to confirm that NH
production over the investigated catalysts. (b) 1H NMR spectra (600 MHz) of standard samples of 14NH + and 15NH +
, and the electrolyte produced from the NRR
reaction using 14N and 15
as the isotopic N source. (c) and (d) Faradaic efficiency and NH yield rate of pristine and heteroatom-doped C catalysts at various
potentials, respectively. These experimental data were obtained using ammonia sensitive electrode method. (e) The correlation between ln (TOF) and ΔGN2
ΔG*NNH. The value of TOF is the maximum for each catalyst. (f) Faradaic efficiencies and NH yield rates of Se- and Te-doped C catalysts during five consecutive
-saturated 0.1 M KOH. The catalysts characterized here are 11.32% O-doped C, 3.09% S-doped C, 2.74% Se-doped C, and 1.88% Te-
3
4
4
2
N
2
2
3
+
3
recycling electrolysis in N
doped C.
2
coincides well with the computational trend in ΔEN2 for pristine and
doped C catalysts (Figure 1a).
isotope labelling were carried out (Figure 5a-b, Figure S24, and
Note S3).[17] Furthermore, the NH
generation from the
3
Afterwards, the electrocatalytic NRR activity of these
carbon-based catalysts was experimentally evaluated using
ammonia sensitive electrode[16] in a gas-tight measurement
system with a gas cleaning cell, a two-compartment cell (H-cell)
separated by a proton-exchange membrane, and a gas collection
decomposition of nitrogen containing MOF-derived carbon
catalysts was also carefully excluded (Figure S25).
Subsequently, the NRR activities of pristine and
heteroatom-doped
concentration (Figures S26 and S27) were measured in N
saturated 0.1 M KOH at various potentials. Their Faradaic
efficiencies, NH production rates and turnover frequencies (TOF,
Table S1, and Note S4) are summarized in Figure 5c-e. Overall,
C
catalysts with optimized dopant
2
-
cell (Figures S20-S23 and Note S2). To verify that NH
production was indeed produced through electrochemical N
reduction process over these catalysts rather than contamination,
meticulous eight-step control experiments together with 15N
3
2
3
2
the maximum Faradaic efficiency, NH
3
production rate, and TOF
5
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RESEARCH ARTICLE
of these catalysts follow a similar trend: pristine C < O-doped C <
S-doped C < Se-doped C < Te-doped C. This experimental
observation is in excellent agreement with our computationally
predicted trend in NRR activity for different dopant-incorporated
carbon catalysts (Figure 1a-b).
Foundation of Tianjin City (19JCJQJC61900), the Fok Ying Tung
Education Foundation (151008), and the Fundamental Research
Funds for the Central Universities Nankai University (63196010).
Calculations were performed on TianHe-1A at the National
Supercomputer Center, Tianjin.
As expected, Te- and Se-doped C achieve much better
NRR performance than pristine, O-, and S-doped C catalysts. The
Faradaic efficiency of Te- and Se-doped C catalysts reaches 4.67%
Keywords: nitrogen reduction reaction • electrocatalysis •
carbon materials •
@
–0.50 VRHE and 3.92% @ –0.45 VRHE (Figure 5c), which was
.3 and 5.3 times, respectively, greater than the maximum value
of pristine C (0.74% @ –0.55 VRHE). Meanwhile, Te- and Se-
doped C catalysts achieve the maximum average NH production
rates of 1.91 µg h–1 cm–2 @ –0.50 VRHE and 1.14 µg h–1 cm–2 @ –
.45 VRHE, respectively (Figure 5d). Besides, Te- and Se-doped
exhibit excellent intrinsic activity, and their TOFs are
6
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1]
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0
C
determined to be 9.67 × 10–5 and 3.90 × 10–5 s–1, respectively
(
Table S2 and Note S4). These values are comparable or better
[
[
2]
3]
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than those of previously reported NRR catalysts (Table S2).
To correlate the intrinsic NRR activity with the nature of
heteroatom-doped C catalysts, the experimental TOFs are plotted
as a function of their corresponding sum of Gibbs free energy for
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2
adsorbed N and energy barrier for *NNH formation. As shown in
Figure 5e, a clear linear relation is observed between ln (TOF)
and ΔGN2 + ΔG*NNH. Apparently, even more active carbon-based
catalysts can be expected by further lowering ΔGN2 + ΔG*NNH
,
which can be achieved by multiple-element doping and/or defect
creating.
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3
the NH yield rate (Figure 5f). Moreover, no noticeable variation
of dopant content was observed after durability tests (Table S3).
These collective results demonstrate the good stability of Te- and
Se-doped C catalysts in NRR electrocatalysis.
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Conclusion
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In summary, we have systematically evaluated O-, S-, Se-, and
Te-doped C as potential NRR catalysts based on combined
theoretical and experimental investigations. We have
demonstrated that the heteroatom-doping induced charge
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2
accumulation facilitates N adsorption on carbon atom and spin
polarization boosts the potential-determining step of the first
protonation to form *NNH. We found that Se and Te dopants
remarkably promote the electrocatalytic NRR activity. Our work
establishes the correlation between the electronic structure and
NRR performance for carbon-based materials. We believe it is
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This article is protected by copyright. All rights reserved.

Angewandte Chemie International Edition
10.1002/anie.201915001
RESEARCH ARTICLE
RESEARCH ARTICLE
Yuanyuan Yang, Lifu Zhang,+
Zhenpeng Hu, Yao Zheng, Cheng
Tang, Ping Chen, Ruguang Wang,
Kangwen Qiu, Jing Mao, Tao Ling,*
and Shi-Zhang Qiao*
+
This systematically combined
computational and experimental
work uncovers the catalytic
nature of carbon-based materials
towards electrocatalytic nitrogen
reduction reaction
The crucial role of charge
accumulation and spin
polarization in activating carbon-
based catalysts for
electrocatalytic nitrogen
reduction
8
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