Electrochemical Reduction of N2 into NH3 by Donor-Acceptor Couples of Ni and Au Nanoparticles with a 67.8% Faradaic Efficiency

Article abstract of DOI:10.1021/jacs.9b07963

The traditional NH₃ production method (Haber-Bosch process) is currently complemented by electrochemical synthesis at ambient conditions, but the rather low selectivity (as indicated by the Faradaic efficiency) for the electrochemical reduction of molecular N₂ into NH₃ impedes the progress. Here, we present a powerful method to significantly boost the Faradaic efficiency of Au electrocatalysts to 67.8% for the nitrogen reduction reaction (NRR) by increasing their electron density through the construction of inorganic donor-acceptor couples of Ni and Au nanoparticles. The unique role of the electron-rich Au centers in facilitating the fixation and activation of N₂ was also investigated via theoretical simulation methods and then confirmed by experimental results. The highly coupled Au and Ni nanoparticles supported on nitrogen-doped carbon are stable for reuse and long-term performance of the NRR, making the electrochemical process more sustainable for practical application.

Full text of DOI:10.1021/jacs.9b07963

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Communication
Electrochemical Reduction of N2 into NH3 by Donor-Acceptor
Couples of Ni and Au Nanoparticles with a 67.8% Faradaic Efficiency
Zhong-Hua Xue, Shi-Nan Zhang, Yun-Xiao Lin, Hui Su, Guang-Yao Zhai, Jing-
Tan Han, Qiu-Ying Yu, Xin-Hao Li, Markus Antonietti, and Jie-Sheng Chen
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b07963 • Publication Date (Web): 15 Sep 2019
Downloaded from pubs.acs.org on September 15, 2019
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Electrochemical Reduction of N into NH by Donor-Acceptor
Couples of Ni and Au Nanoparticles with a 67.8% Faradaic
Efficiency
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Zhong-Hua Xue, Shi-Nan Zhang, Yun-Xiao Lin, Hui Su, Guang-Yao Zhai, Jing-Tan Han ,
†
*†
‡
†
Qiu-Ying Yu , Xin-Hao Li , Markus Antonietti, and Jie-Sheng Chen
†
School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, P. R. China
Department of Colloid Chemistry, Max-Planck Institute of Colloids and Interfaces, Wissenschaftspark Golm, 14424
‡
Potsdam, Germany
Supporting Information Placeholder
To
date,
Au-based
electrocatalysts,
including
3
ABSTRACT: The traditional NH production method (Haber-
Bosch process) is currently complemented by electrochemical
synthesis at ambient conditions, but the rather low selectivity
nanostructured Au particles and metal-oxide-supported
amorphous Au, are promising candidates for the electro-
synthesis of NH
nanostructure engineering to boost the reaction rate for the
synthesis of ammonia, while a state-of-the-art Faradaic
efficiency of 35% is still unsatisfactory for practical
1
7‒21
3
from N
2
.
Much effort has been focused on
(
as indicated by the Faradaic efficiency) for the
electrochemical reduction of molecular N into NH impedes
2
3
the progress. Here, we present a powerful method to
significantly boost the Faradaic efficiency of Au
electrocatalysts to 67.8% for the nitrogen reduction reaction
22‒25
applications.
Again, developing efficient strategies to
promote the selective absorption and activation of N
molecules on Au-based NRR electrocatalysts lies at the
frontier of research on the ambient production of ammonia.
2
(
NRR) by increasing their electron density through the
construction of inorganic donor-acceptor couples of Ni and
Au nanoparticles. The unique role of the electron-rich Au
centres in facilitating the fixation and activation of N
2
was also
Here, we show that the rational arrangement of separated
Au and Ni nanoparticles on/in nitrogen-doped carbon
supports could generate an unusual case of inorganic donor-
acceptor couples, significantly boosting the selectivity to
ammonia in an aqueous electrolyte. The electron enrichment
of Au nanoparticles was promoted by accepting electrons
from the Ni nanoparticles due to the different work functions
of the Au and Ni metals, resulting in facilitating the
been investigated via theoretical simulation methods and
then confirmed by experimental results. The highly coupled
Au and Ni nanoparticles supported on nitrogen-doped carbon
are stable for reuse and long-term performance of the NRR,
making the electrochemical process more sustainable for
practical application.
2
adsorption and dissociation of molecular N over the electron-
rich Au active sites and thus a remarkably high Faradaic
efficiency for the NRR.
3
Over the past century, ammonia (NH ) synthesis was
exclusively synthesized by the heterogeneously catalysed
Haber-Bosch process, which annually consumes 1-2% of global
To rationally construct the donor-acceptor couples of Ni
and Au nanoparticles, with a strong electronic connection
between Au and Ni, for a highly selective NRR, we developed
a straightforward Galvanic replacement method to deposit Au
nanoparticles adjacent to Ni nanoparticles, as presented in
Figure 1a. The atomic ratios of the Au-Ni couples in the Au/Ni
samples (Figure 1b) were tuned by gradually increasing the
energy production and simultaneously results in >1% of CO
emissions worldwide. Concerning the availability of excess
sustainable electricity, it is highly desirable for chemists to
2
1
‒5
find alternative approaches to produce NH
especially also locally and in a decentral fashion. However, the
progress in N fixation is challenged by the high activation
barrier of the N molecule, which is composed of a strong N-
N triple bond, and the low selective absorption of the N
molecule, which has an extremely low polarizability, onto
3 2
via N fixation,
2
3
+
amount of Au with a fixed amount of Ni nanoparticles during
the replacement process, while the chemical structure of the
carbon support remained stable according to the XPS results
2
2
(
Figure S1). It should be noted that the Au content reaches a
6,7
catalysts . Recently, the electrochemical reduction of N
NH has shown great potential for activating molecular
nitrogen under ambient conditions and at smaller scales.
2
into
limiting value in the Au40/Ni sample (Table S1) and no longer
3
3
+
increases with the addition of more Au because the Ni
nanoparticles embedded inside the carbon matrix were not
exposed to the solution-based replacement reaction. SEM and
TEM observation (Figures S2‒S3) further indicated similar
foam-like structures for the carbon supports decorated with
metal nanoparticles (mean size: ~ 17 nm) for the different
8
‒12
The core focus of this ongoing research is the rational design
of efficient and, most importantly, selective electrocatalysts to
catalyse the proton-coupled 6-electron nitrogen reduction
reaction (NRR) while retarding the competing 2-electron
1
3‒16
hydrogen evolution reaction (HER).
x
Au /Ni samples. The XRD patterns directly identify the
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presence of metallic Ni and Au nanoparticles (Figure 1c),26‒28
experiments using an Ar-saturated electrolyte or a bare carbon
cloth (Figures S13‒S14) or blank reactions without current
input (Figure S15) did not generate a detectable amount of
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and indicate a clear trend of increasing Au content from the
Ni to Au40/Ni samples. Further, HAADF-TEM and the
corresponding energy dispersive X-ray spectroscopy (EDX)
elemental mapping images (Figure 1d‒e and Figure S4)
demonstrated the successful deposition of Au nanoparticles
adjacent to the Ni nanoparticles on the nitrogen-doped
carbon support.
1
5
NH
formation of NH
Au /Ni electrode under standard conditions.
of the Au /Ni catalyst remained stable after prolonged NRR
operation (Figure S16). All these results confirmed the
efficient and stable NRR over the Au /Ni catalyst.
3
. A N isotopic labelling experiment further confirmed the
1
5
+
4
(inset of Figure 2a and Figure S14) over the
1
0,29
6
The structure
6
6
Care was taken to exclude the possibility of the formation
of Au-Ni bimetallic nanoparticles by analysing the EDX
mapping results (Figures S4‒S5). The high-resolution
HAADF-STEM image (Figure 1f and Figure S6) of the Au
nanoparticles with typical lattice fringes of 0.21 nm and 0.14
nm, which could be attributed to the (200) and (220) planes
of metallic Au, respectively, further confirmed the formation
of pristine Au nanostructures from the replacement of Ni
nanoparticles.
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Figure 2. NRR activity over the Au
in an N -saturated and Ar-saturated 0.05 M H
The inset of (a) shows the H NMR spectra of the resultant
6
/Ni catalyst. (a) CV curves
2
2
SO electrolyte.
4
1
1
4
15
electrolyte fed with
N
2
and
N
2
. (b) Chronoamperometry
yield rate and Faradaic
results at different potentials. (c) NH
3
efficiency at the corresponding potentials and (d) during the
recycling test.
The pristine Ni catalyst could deliver negligible NH
production (Figure 3a and Figure S17), but the incorporation
of a small amount of Au triggers a significant NH yield of 1.6
3
Figure 1. Preparation and structure. (a) Synthesis of donor-
acceptor couples of Au and Ni nanoparticles. (b) Atomic
content of Au and Ni as analysed by ICP and XPS and (c) XRD
patterns of Ni and Au /Ni samples (x% represents the Au-to-
x
Ni molar ratio). A typical (d) HAADF-TEM image, the
corresponding (e) EDX mapping results and (f) HAADF-STEM
3
−1
−1
μg h mg cat with a Faradaic efficiency of 14.2% over the
Au0.5/Ni catalysts, which has the lowest Au content of 0.28
wt%. The turnover frequency (TOF) of NRR over Au catalysts
(
Figure 3b) increased with decreasing Au contents, matching
well with the trend of electron enrichment of the Au
components, as indicated by the gradual shift of the Au 4f XPS
peaks to lower binding energies (Figure 3d). It should be noted
that the nitrogen-doped carbon-supported Ni nanoparticles
have a relatively higher work function (6.86 eV) compared to
that (6.75 eV) of the supported Au nanoparticles according to
the ultraviolet photoelectron spectroscopy (UPS) results
6
image of Au /Ni. The inset in (f) shows the FFT pattern.
We initially evaluated the catalytic performance of the Au-
Ni couple-based NRR electrocatalysts in an H-type cell (Figure
S7). An obvious shift in the cyclic voltammetry curves of the
Au/Ni samples, taking Au
indicates a possible NRR in the electrolyte after saturation
with N gas to replace the Ar gas (Figure 2a). The yields of NH
6
/Ni sample to a lower potential
(
Figures S18‒S19), presumably due to the combination of the
2
3
3
0‒33
support effect (Figure S20) and size effect,
even though the
were demonstrated by a Nessler reagent colorimetric method
(Figure S8) and simultaneously confirmed by ion
chromatography (Figure S9). Under increased work potential
theoretical work functions of bulk Au and Ni are all
3
4
approximately 5.1 eV. The Ni nanoparticles thus act as an
electron donor here (Figure 3c), as further confirmed by an
obvious shift of the Ni 2p XPS peaks to higher binding energies
after the deposition of Au nanoparticles (Figure S21). All these
results indicated the electron-enrichment-dependent NRR
activity of the Au active centres. Indeed, the Au0.5/Ni catalyst
with the highest electron donor/acceptor ratio had the most
pronounced electron enrichment and provided the highest
and current density (Figure 2b) outputs, the Au
electrode, with an optimized catalyst loading of 2.0 mg cm
6
/Ni-based
−
2
−
1
−1
(
Figure S10), offered the highest NH
and a Faradaic efficiency of 67.8% at −0.14 V (Figure 2c and
Figure S11). Most importantly, the Au /Ni-based electrode also
provided a stable current density for the NRR (Figure S12) and
reproducible NH yields and Faradaic efficiencies over six
consecutive runs at −0.14 V vs. RHE (Figure 2d). Control
3
yield of 7.4 μg h mg cat
6
3
−
1
−1
3
TOF value (6.7 mol NH mol Au h ).
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The electron-enrichment-dependent NRR activity of the Au
(ΔG) of the rate-limiting N
a pristine Au catalyst to 1.13 eV, and the ΔG of the following
three hydrogenation steps via *NNH and *NNH to *NNH
The polarization of the adsorbed N molecules induced by the
electron-rich Au surface, which benefits the further addition
of a hydrogen atom to the electron-deficient N, was well
presented by the obvious differences in the electron densities
(Hirshfeld charge) of each N atom after changing the catalytic
surface from the Au to the electron-rich Au models (Figure 4b
and Figure S23). Such a strong polarizing effect also enhanced
2
dissociation step from 1.39 eV over
1
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catalysts was further confirmed by the positive correlation
between the ammonia yields and the total electron densities
(as indicated by the measured work functions) of the
supported Au nanoparticles. After achieving a balance
between the electron enrichment and the number of Au active
centres for a fixed total content of the Au and Ni metals
Figure 3c), the Au component in the Au /Ni sample accepted
6
the maximum total amount of electrons from the Ni and/or
support and thus exhibited the lowest work function among
2
3
4
.
2
(
all the Au
Figure S22). Accordingly, the Au
x
/Ni samples (Figure 3f) and improved conductivity
/Ni sample also performed
2
the pre-adsorption of N molecules onto the electron-rich Au
surface, as theoretically predicted by the much higher
adsorption energy, and then validated experimentally by the
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(
6
as the best-in-class NRR catalyst in this paper and was used
for the following characterizations and reactions.
stronger
programmed desorption (TPD) isotherm of the Au
(Figure 4d and Figure S24). It was also found that the
desorption energy of the NH molecule decreased on the
N
2
adsorption peak in the
N
2
temperature-
6
/Ni sample
3
electron-rich Au surface (Figure 4c), indicating a rapid release
of the active site for new reactions.
Figure 3. (a) NH
Ni and Au /Ni catalysts at -0.14 V vs. RHE. (b) TOF values of
the NH yield based on the Au content. (c) Electron transfer
model for the donor-acceptor couples of Au and Ni
nanoparticles. (d) Au 4f XPS spectra of the Au /Ni catalysts.
Insets of (d) show the corresponding electron state models of
Au. (e) UPS spectra in the onset (E ) energy regions and (f) the
3
yield rate and Faradaic efficiency using the
x
3
x
i
x
measured work functions (Φ) via UPS analysis of the Au /Ni
samples.
Figure 4. (a) Gibbs free energy diagram of the NRR for Au and
electron-rich Au. The insets show the corresponding
optimized absorption structures. (b) The calculated electron
Computationally, we further investigated the possible roles
of the electron-rich Au in accelerating the adsorption and
activation of the N
2
molecules. Based on density functional
density maps for N
electron-rich Au (Au+e ). (c) The calculated adsorption
energies of N and desorption energies of NH adsorbed onto
-TPD results of the Ni, Au /Ni and
2
adsorbed onto the surface of Au and
theory (DFT) calculations and experimental results in
-
17,35,36
pioneering works
and the calculated standard free energy
2
3
diagram for the NRR in this work (Figure 4a), we propose an
associative process on the Au, in which the dissociation of
-
Au and Au+e . (d) The N
2
6
Au40/Ni catalysts normalized by the Au content. The insets
show the corresponding structural models. (e) Faradaic
efficiencies for the NRR over various noble-metal-based
electrocatalysts.
adsorbed N
further splitting of *NNH
2
(*N
2
) into *NNH is the rate-limiting step, and the
into NH and *NH by the addition
4
3
2
of one proton proceeds automatically. Due to the similar
configurations for each step of the NRR, the electron-rich Au
-
catalyst (Au+e ) significantly reduces the Gibbs free-energy
3
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Taken together, a rational construction of electron-rich Au
nanoparticles with an optimal ratio of the Ni-Au donor-
acceptor pairs could promote the activity of the whole NRR
(5) Martín, A. J.; Shinagawa, T.; Pérez-Ramírez, J.
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process by enhancing the pre-adsorption and activation of N
and facilitating the desorption of ammonia. As a result, the
Au /Ni catalyst provided a high Faradaic efficiency of greater
than 67.8%, far outperforming the values of all reported
2
6
1
7‒25,37
38
39,40
Au-,
Ru-, or Pd-based
electrocatalysts (Figure 4e
and Table S2).
In summary, inorganic donor-acceptor couples of Ni and Au
nanoparticles have been designed to improve the highly
selective nitrogen reduction reaction. Both the experimental
and theoretical results demonstrated the key role of the as-
formed electron-rich Au nanoparticles, which accept
electrons from the Ni nanoparticles, in facilitating the
(
8) Guo, C.; Ran, J.; Vasileff, A.; Qiao, S.-Z. Rational design of
electrocatalysts and photo(electro)catalysts for nitrogen
reduction to ammonia (NH ) under ambient conditions.
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Energy Environ. Sci. 2018, 11, 45–56.
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reduction of dinitrogen to ammonia under ambient
conditions. Adv. Energy Mater. 2018, 8, 1800369.
2
adsorption and activation of N molecules and producing a
high Faradaic efficiency of 67.8% at -0.14 V vs. RHE in an acidic
electrolyte. This work offers a guideline for the fabrication of
electrocatalysts using metal-metal electron transfer to achieve
high selectivity for electrochemical nitrogen fixation in
combination with novel reactors for practical uses and
provides a direct and intriguing method to develop highly
efficient catalysts for the challenging activation of small
molecules.
(
10) Suryanto, B. H. R.; Du, H.-L.; Wang, D.; Chen, J.; Simonovꢀ,
A. N.; MacFarlane, D. R. Challenges and prospects in the
catalysis of electroreduction of nitrogen to ammonia. Nat.
Catal. 2019, 2, 290–296.
(
11) Song, Y.; Johnson, D.; Peng, R.; Hensley, D. K.; Bonnesen,
P. V.; Liang, L.; Huang, J.; Yang, F.; Zhang, F.; Qiao, R.; Baddorf,
A. P.; Tschaplinski, T. J.; Engle, N. L.; Hatzell, M. C.; Wu, Z.;
Cullen, D. A.; Meyer III, H. M.; Sumpter, B. G.; Rondinone, A.
J. A physical catalyst for the electrolysis of nitrogen to
ammonia. Sci. Adv. 2018, 4, e1700336.
ASSOCIATED CONTENT
Supporting Information
(
12) Qiu, W.; Xie, X.-Y.; Qiu, J.; Fang, W.-H.; Liang, R.; Ren, X.;
Experimental details, more characterization results, and
detailed discussions. This material is available free of charge
via the Internet at http://pubs.acs.org
Ji, X.; Cui, G.; Asiri, A. M.; Cui, G.; Tang, B.; Sun, X. High-
performance artificial nitrogen fixation at ambient conditions
using a metal-free electrocatalyst. Nat. Commun. 2018, 9, 3485.
(
13) Foster, S. L.; Bakovic, S. I. P.; Duda, R. D.; Maheshwari, S.;
AUTHOR INFORMATION
Milton, R. D.; Minteer, S. D.; Janik, M. J.; Renner, J. N.;
Greenlee, L. F. Catalysts for nitrogen reduction to ammonia.
Nat. Catal. 2018, 1, 490–500.
Corresponding Author
xinhaoli@sjtu.edu.cn (XHL)
(
14) Wang, M.; Liu, S.; Qian, T.; Liu, J.; Zhou, J.; Ji, H.; Xiong,
J.; Zhong, J.; Yan, C. Over 56.55% Faradaic efficiency of
ambient ammonia synthesis enabled by positively shifting the
reaction potential. Nat. Commun. 2019, 10, 341.
Notes
The authors declare no competing financial interests.
(
15) Hao, Y.-C.; Guo, Y.; Chen, L.-W.; Shu, M.; Wang, X.-Y.; Bu,
ACKNOWLEDGMENT
T.-A.; Gao, W.-Y.; Zhang, N.; Su, X.; Feng, X.; Zhou, J.-W.;
Wang, B.; Hu, C.-W.; Yin, A.-X.; Si, R.; Zhang, Y.-W.; Yan, C.-
H. Promoting nitrogen electroreduction to ammonia with
bismuth nanocrystals and potassium cations in water. Nat.
Catal. 2019, 2, 448–456.
This work was supported by the National Natural Science
Foundation of China (21722103, 21720102002, and 21673140),
the Shanghai Basic Research Program (16JC1401600), and the
SJTU-MPI partner group.
(
16) Xue, Z.-H.; Su, H.; Yu, Q.-Y.; Zhang, B.; Wang, H.-H.; Li,
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