Generating Defect-Rich Bismuth for Enhancing the Rate of Nitrogen Electroreduction to Ammonia

Article abstract of DOI:10.1002/anie.201903969

The electrochemical N₂ fixation, which is far from practical application in aqueous solution under ambient conditions, is extremely challenging and requires a rational design of electrocatalytic centers. We observed that bismuth (Bi) might be a promising candidate for this task because of its weak binding with H adatoms, which increases the selectivity and production rate. Furthermore, we successfully synthesized defect-rich Bi nanoplates as an efficient noble-metal-free N₂ reduction electrocatalyst via a low-temperature plasma bombardment approach. When exclusively using ¹H NMR measurements with N₂ gas as a quantitative testing method, the defect-rich Bi(110) nanoplates achieved a ¹⁵NH₃ production rate of 5.453 μg mg_Bi^?1 h^?1 and a Faradaic efficiency of 11.68 % at ?0.6 V vs. RHE in aqueous solution at ambient conditions.

Full text of DOI:10.1002/anie.201903969

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
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Accepted Article
Title: Generating Defect-Rich Bismuth for Enhancing Rate of Nitrogen
Electroreduction to Ammonia
Authors: Yue Wang, Miao-miao Shi, Di Bao, Fan-lu Meng, Qi Zhang,
Yi-tong Zhou, Kai-hua Liu, Yan Zhang, Jia-zhi Wang, Zhi-wen
Chen, Da-peng Liu, Zheng Jiang, Mi Luo, Lin Gu, Qing-hua
Zhang, Xing-zhong Cao, Yao Yao, Min-hua Shao, Yu Zhang,
Xin-Bo Zhang, Jingguang G. Chen, Jun-min Yan, and Qing
Jiang
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201903969
Angew. Chem. 10.1002/ange.201903969
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http://dx.doi.org/10.1002/ange.201903969

10.1002/anie.201903969
Angewandte Chemie International Edition
Generating Defect-Rich Bismuth for Enhancing Rate of Nitrogen
Electroreduction to Ammonia
Yue Wanga,†, Miao-miao Shia,†, Di Baob,†, Fan-lu Menga,†, Qi Zhangb, Yi-tong Zhoua, Kai-hua Liua, Yan Zhanga,
Jia-zhi Wangb, Zhi-wen Chena, Da-peng Liuc, Zheng Jiange, Mi Luoe, Lin Guf, Qing-hua Zhangf, Xing-zhong Caog,
Yao Yaoh, Min-hua Shaoh, Yu Zhang,c Xin-Bo Zhang,b Jingguang G. Chen,d, *Jun-min Yan,a,* Qing Jianga
Abstract: The electrochemical N2 fixation, which is far from practical
application in aqueous solutions under ambient conditions, is
extremely challenging and requires rational design of electrocatalytic
centers. Herein, we observed that semimetal bismuth (Bi) might be a
promising candidate because of its poor binding with H-adatoms for
increasing the selectivity and production rate, and successfully
synthesized defect-rich Bi nanoplates as an efficient noble metal-
free N2 reduction electrocatalyst via low-temperature plasma
bombardment technology. When exclusively using 1H NMR
measurement with N2 gas as a quantitative test method, the defect-
rich Bi(110) nanoplates achieved a 15NH3 production rate of 5.453 μg
mg-1 h-1 and a Faradaic efficiency (11.68% at -0.6 V vs. RHE) in
aqueous solutions at ambient conditions.
activation of inert N2.[2]
Recently, amounts of electrocatalysts (e.g., Au, Pd, Rh,
etc.)[3] have been shown to occur under ambient conditions in
aqueous media, but the electrochemical performances are still
very poor because most transition metals favour competitive
adsorption of H+ over N2.[4] Aside from this, the scarcity and
subsequent high cost lead to that their long-term usability is
problematic. The design principle for N2RR catalysts that can
tune
a
complex, multi-step, 6-electron process remains
unclear.[5] Thus developing of highly efficient and selective
electrocatalyst to obtain key products of N2 reduction for
reducing the high over-potential is extremely challenging.
The semimetal bismuth (Bi) might be a promising candidate
in N2RR due to its poor binding with H-adatoms. However, as a
VA group element, its stable valence electronic structure is
unfavourable for N2 adsorption and activation. Fortunately, the
electronic properties of Bi is modulated through specific
synthetic method, Bi(110) nanoplates with a high fraction of
isolated Bi vacancies are fabricated using a low-temperature
plasma bombardment technology on Bi2O3 nanoplates (Figure
1A). These defect-rich Bi(110) nanoplates show a high faradaic
efficiency of 11.68% at -0.6 V vs. RHE, and a 15NH3 production
rate of 5.453 μg mg-1 h-1 (2.400 μg h-1 cm-2 normalized by
roughness factor) at -0.9 V vs. RHE under ambient conditions in
Electroreduction of N2 to NH3 under ambient conditions, especially
powered by renewable sources, provides a ‘clean’ strategy to
replace the power-consuming Haber-Bosch process for ‘green’ NH3
synthesis with the aim to replace fossil fuel feedstocks and feed
more of the global population for sustainable development.[1] The N2
reduction reaction (N2RR), theoretically treated under ambient
conditions and using water as a hydrogen source, has been
proposed as a sustainable alternative for NH3 production to alleviate
the problems of environmental pollution and reduce greenhouse gas
emissions. The critical bottleneck lies in the adsorption and
1
aqueous solution. Rigidly speaking, it is the first time using H
NMR measurement with 15N2 or 14N2 gas as a quantitative test
method for all data in aqueous electrolytes.
[a]
Yue Wang, Miao-miao Shi, Fan-lu Meng, Yi-tong Zhou, Kai-hua-Liu,
Yan Zhang, Zhi-wen Chen, Prof. Jun-min Yan, Prof. Qing Jiang
Key Laboratory of Automobile Materials, Ministry of Education and
Department of Materials Science and Engineering
Jilin University
The X-ray diffraction (XRD) pattern (Figure 1B), scanning
electron microscopy (SEM), transmission electron microscopy
(TEM), atomic force microscopy (AFM) image and X-ray
photoelectron spectroscopy (XPS) demonstrated that Bi2O3
nanoplates precursor with a thickness of 5-6 nm (Figure S1) was
successfully fabricated. And its high-resolution transmission
electron microscopy (HRTEM) image shows a planar spacing of
0.319 nm, corresponding to the (111) facet of Bi2O3.
Changchun 130012, Jilin, China.
E-mail: junminyan@jlu.edu.cn
[b]
[c]
[d]
[e]
Di Bao, Qi Zhang, Jia-zhi Wang, Xin-bo Zhang
State Key Laboratory of Rare Earth Resource Utilization
Changchun Institute of Applied Chemistry, Chinese Academy of
Sciences
Changchun 130022, Jilin, China.
Da-peng Liu, Yu Zhang
Key Laboratory of Bio-Inspired Smart Interfacial Science and
Technology of Ministry of Education School of Chemistry
Beihang University
Changchun 130012, Jilin, China. Beijing 100191, China.
Jingguang G. Chen
Department of Chemical Engineering
Columbia University
New York, NY 10027, USA
E-mail: jgchen@columbia.edu
Zheng Jiang, Mi Luo
Shanghai Institute of Applied Physics, Chinese Academy of
Sciences
Beijing 100080, China.
[f]
Lin Gu, Qing-hua Zhang
Institute of Physics, Chinese Academy of Sciences
Beijing 100080, China.
[g]
[h]
Xing-zhong Cao
Institute of High Energy Physics, Chinese Academy of Sciences
Beijing 100049, China.
Figure 1. Characterization of Bi2O3 and Plasma R-O-Bi nanoplates. A)
Schematic illustration of the synthesis of defect-rich Bi nanoplates and
application for N2RR. B) XRD patterns of Bi2O3 and Plasma R-O-Bi. C) TEM
image, D) AFM image and E) HRTEM image and the corresponding atomic
configuration models (inset) of Plasma R-O-Bi.
Yao Yao, Min-hua Shao
Department of Chemical and Biological Engineering, Hong Kong
University of Science and Technology
Hong Kong, 999077, China.
[†]
These authors contributed equally to this work.
This article is protected by copyright. All rights reserved.

10.1002/anie.201903969
Angewandte Chemie International Edition
The Bi catalyst with preferentially exposed Bi(110) facets and
abundant lattice defects is prepared from oxide precursors with
the assistance of low-temperature plasma bombardment, as
illustrated from the scheme (Figure 1A). In order to shed light on
the microstructure of defect-rich Bi nanoplates (denoted as
Plasma R-O-Bi), some related characterization tests were also
carried out. The XRD pattern confirms hexagonal Bi (PDF No.
44-1246, Figure 1B) of Plasma R-O-Bi with a partially preserved
porous nanoplate structure indicated in the SEM and TEM
characterizations. (Figure 1C and Figure S2). The AFM
measurements verify a rough surface and thickness in the range
of approximately 10~15 nm for Plasma R-O-Bi (Figure 1D). The
Bi(110) square lattice is confirmed in the high-angle annular
dark-filed (HAADF) image (Figure 1E), with a lattice spacing of
0.227 nm. The Brunauer-Emmett-Teller (BET) surface area and
Barrett-Joyner-Halenda (BJH) pore-size distribution of Plasma
R-O-Bi are determined by N2 adsorption-desorption
measurements (Figure S3). The relevant isotherms indicate that
the specific surface area of Plasma R-O-Bi is smaller than that
of its parent Bi2O3 nanoplates (declining to 43.052 m3 g-1 from
63.660 m3 g-1), which can be attributed to the much higher
molecular weight of Bi than that of O and the increase in the
thickness despite the larger pore sizes and additional exposed
inner surface derived from efficient plasma engraving.
if the conductive substance of XC-72 will influence the accuracy
of NMR results, undoubtedly, there is no signal detected. Most
importantly, test in the absence of 14N2 or 15N2 (Argon instead)
(Figure 2E, Table S1) is conducted to demonstrate that the
N2RR actually occurs rather than impurities in the air causing the
results.[1d,6] A blank experiment on whether applying a potential
to nitrogen reduction is prerequisite or not is further implemented.
All the results show that the N2 and electrical energy are
necessary for N2RR. Then, in situ electrochemical ATR-FTIR
spectroscopy was respectively recorded in N2- and Ar-saturated
Na2SO4 solution via the cyclic voltammogram (CV) segments of
Plasma R-O-Bi catalyst with a scan rate of 2.5 mV s-1, and the
spectrum at -0.2 V was used as a reference. Indicated by the
observed spectrum (Figure S8), the upward IR bands at ~1450
cm-1 can be attributed to the H-N-H bending absorption on
Plasma R-O-Bi catalyst as the applied potentials being
decreased,[3d] demonstrating the realization of N2 protonation
reaction for Plasma R-O-Bi catalyst.
To further determine the electrocatalytic N2 reduction ability
of the defective Bi nanoplates, electrocatalytic N2RR
measurements are performed. The amount of NH3 generated in
the electrochemical process can be quantified from the presence
of 14NH4 and 15NH4 using H-NMR (Figure S4). Linear sweep
voltammetry (LSV) measurements are firstly performed under
N2- and Ar- saturated Na2SO4 solutions to confirm the onset
electrochemical potential of the N2RR (Figure 2A). The LSV
curves have the same shape but an increased current density
below -0.5 V vs. RHE under the N2 atmosphere, indicating the
reaction of N2 to NH3 occurred. And then chronoamperometry
tests are then carried out at potentials from -0.5 to -1.0 V vs.
RHE to explore the catalytic activity and optimized potential of
Plasma R-O-Bi catalyst for electrochemical N2RR (Figure S5).
The FE values and corresponding NH3 production rates at
different applied potentials for Plasma R-O-Bi catalyst are
calculated based on the 1H-NMR spectra under labelled 15N2 gas
(Figure S6). The total current density increases from
approximate 0.051 to more than 3.658 mA cm-2, as the potential
shifts from -0.5 to -1.0 V (Figure S5B). The FE value at the
applied potential of -0.5 V is 6.25% (Figure 2B), which may be
caused by the low production rate owing to the low current
density. With the applied potential shift to -0.6 V, the value
reaches the maxima of 11.68%. Unexpectedly, the FE values
decrease obviously as the applied potentials being more
negative from -0.6 V due to the increase in the competing
hydrogen evolution reaction (HER) (as detected by gas
chromatography, Figure S7). We then calculate the production
rates of NH3 (Figure 2C), the value over Plasma R-O-Bi
increases as the potential declines and reaches the highest
average value of 5.453 µg mg-1 h-1 at -0.9 V vs. RHE, and no
N2H4 by product is detected over the entire electrocatalytic
potential range.
+
+
1
Figure 2. Performance of electrocatalytic N2RR. A) Linear sweep voltammetric
curves of Plasma R-O-Bi catalyst in N2-saturated (red line) and Ar-saturated
(black line) 0.2 M Na2SO4 aqueous solution, respectively. The details at -0.6V
is amplified (inset) B) FE for different catalysts at each given potential in a
15N2-saturated 0.2 M Na2SO4 aqueous solution. C) The average NH3 yield for
different catalysts at each given potential. D) Comparison of the properties
reported recently referred to the NMR tests qualitatively and partly
quantitatively. E) 1H-NMR spectra of 15N2 and blank experiments in
concentrated Na2SO4 aqueous solution under various conditions. F) Chrono-
amperometry results for Plasma R-O-Bi at the potential of -0.6 V vs. RHE, the
inset is cycling stability results of Plasma R-O-Bi at the potential of -0.6 V vs.
RHE under consecutive recycling electrolysis in 0.2 M Na2SO4 aqueous
solution.
On the basis of the aforementioned results, blank
experiment for 0.2 M Na2SO4 electrolyte has been done firstly to
verify the safety of the test environment. No doubt, the NMR
spectrum result shows that the test environment is guaranteed.
Immediately, the tests without catalyst are executed to figure out
During long-term electrocatalytic tests at the applied potential
of -0.6 V vs. RHE, Plasma R-O-Bi shows an ignored current
density decay over 24 hr (Figure 2F). The initial current density
appears slightly stable after electrolyzing for about 15 min with a
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10.1002/anie.201903969
Angewandte Chemie International Edition
value of 0.058 mA/cm-2, and then gradually stabilized at 0.053
mA/cm-2 at about 25 min for a current density decay of 8.6%,
then keep stable after 24 hr. The stability is further confirmed by
XRD characterization (Figure S9), and no significant change in
peak position is occurred, indicating Plasma R-O-Bi catalyst is
still retained. To acquire the information of microstructure of
Plasma R-O-Bi after electrolysis, a TEM technique is implanted
(Figure S10), the nanoplates like morphology is preserved,
including the (110) facet.
yield rates (normalized RNH3) for all catalysts can be calculated.
As indicated from the calculated results (Figure S14), the
normalized RNH3 of the Plasma R-O-Bi catalyst (2.400 µg h-1 cm-
2) is still higher than other compared catalysts, specified an
increase of the intrinsic activity.
The structural and compositional characterizations including
HRTEM and XRD indicate the conversion of Chemical R-O-Bi
from Bi2O3, and the morphology and phase of the commercial Bi
remained nearly the same by plasma engraving (Figure S15). To
quest the inherent and underlying mechanism, Positron
annihilation spectrometry[8] and X-ray absorption fine structure
(XAFS)[9] measurements are performed to characterize the
defect level and types, as well as the induced change in
electronic properties. Plasma R-O-Bi has a higher proportion of
the short positron annihilation lifetime (τ1) than the other
samples, and the increase in τ1 is assigned to the lattice-
isolated Bi vacancy defect (Figure 3C). An increase in τ1 is also
observed for Plasma C-Bi, indicating more lattice vacancies than
those in C-Bi and the key role of the plasma treatment in
creating the lattice vacancies. In contrast, the other samples
have a higher proportion of the longer lifetime component (τ2)
due to large defect clusters and the longest lifetime component
(τ3) due to large voids (nanoscale). The X-ray absorption near
edge structure (XANES) data at the Bi LIII-edge of Plasma R-O-
Bi, C-Bi and Bi2O3 nanosheet precursor are recorded at room
temperature in the fluorescent mode, and the absorption edge
position is defined as the energy of the inflexion point in the
rising edge. The absorption edge position of Plasma R-O-Bi is
closely to c-Bi, and the XANES features of Plasma R-O-Bi
remain similar before and after N2RR (Figure 3D). To ascertain
quantitative structural parameters for the atoms surrounding of
the central Bi atoms, we have fitted the main peaks from 1.15 to
3.6 Å for all of the samples. The structural parameters are
summarized (Table S2), and the extended X-ray absorption fine
structure (EXAFS) fitting curves in R-space (the fine data are
displayed in Figure S16). The distance between absorber and
Figure 3. Performance of comparative catalysts. A) FE for different catalysts
at each given potential in a 14N2-saturated 0.2 M Na2SO4 aqueous solution. B)
The average NH3 yield for different catalysts at each given potential. C)
Positron annihilation data for C-Bi, Plasma C-Bi, Chemical R-O-Bi and Plasma
R-O-Bi. D) Bi LIII-edge XANES spectra of Plasma R-O-Bi before (green curve)
and after (pink curve) N2RR, commercial Bi standard (red dashed curve) and
Bi2O3 standard (blue dashed curve).
To clarify the cause of the catalytic activity and selectivity of
Plasma R-O-Bi, comparative samples, including pristine Bi2O3
nanoplates chemically reduced by NaBH4 to Bi metal,
commercial Bi particles and their derivatives treated with plasma
etching, are also prepared and characterized as Chemical R-O-
Bi, C-Bi, and Plasma C-Bi, respectively. We further test the
performance of different catalysts (Figure S11), the FE values of
NH3 formation over Plasma R-O-Bi (11.68% at -0.6 V vs. RHE)
is much higher than that obtained with Chemical R-O-Bi
(3.729%), Plasma C-Bi (2.649%) and C-Bi (2.565%), indicating
that Plasma R-O-Bi has the best selectivity for N2RR (as shown
in Figure 3A). In addition, to compare the ammonia yield rate in
a most advantageous expression, the geometric area is divided.
Through the analysis of the results (Figure S12), a maxima of
2.727 µg h-1 cm-2 at -0.9 V vs. RHE is obtained for Plasma R-O-
Bi, higher than Chemical R-O-Bi (1.864 µg h-1 cm-2), Plasma C-
Bi (1.324 µg h-1 cm-2) and C-Bi (1.282 µg h-1 cm-2).
To further decode the origin of the increased N2RR activity
for Plasma R-O-Bi catalyst, electrochemical active surface areas
are measured for the samples via electrochemical double-layer
capacitance (Cdl), here we refer to Feng’s paper about the
concept of roughness factors (RF). In which the inherent activity
(surface area normalized) can thus be acquired to exclude the
effects of surface roughness.[7] Supposed C-Bi catalyst
possessed the RF of 1, we can calculate the approximate RF
value of other catalysts in the light of Cdl processed by CV data
(Figure S13). Therefore, active surface-area normalized NH3
backscatter atoms
R of all the paths are fixed to their
corresponding values as references, which produced
a
satisfactory fit with the percentage misfit (R-factor) being in
1.29% and 0.91%, respectively. To decrease the number of
fitting parameters, we commonly fix the Debye-Waller factor 2
to the theoretical values (Table S2). The Bi-Bi bond coordination
number of Plasma R-O-Bi is smaller than that of Bi standard,
further confirming the existence of the lattice-isolated Bi vacancy
defect.
The Gibbs free energy △G on perfect Bi and defective Bi for
N2RR is obtained from density functional theory (DFT)
calculations and analysed as an effective descriptor for
investigating catalytic activities and possible mechanism of
N2RR.[10] In the entire reaction network, the protonation process
to form N2H* is the rate-determining step for converting N2 to
NH3. As indicated by the Gibbs free energy diagrams (Figure 4A
and Figure S17, Bi defect-(110) catalyse N2 to form N2H* under
mild conditions due to its lower △GN2H* value than that of perfect
Bi(110). And the corresponding binding strength between
catalyst and N2H* intermediate can also be inferred from the
Gibbs free energy diagram. Then volcano plot of the potential-
limiting steps and △GN2H* are constructed (Figure 4B),[11] and the
Gibbs free energy of N2H* on Bi defect-(110) is 0.89 eV, which is
closer to the peak of the volcano than that of other metals such
as Pt, Pd, and Cu. Here, considering the hydrogen evolution as
the main side reaction of N2RR, we also compare the hydrogen
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10.1002/anie.201903969
Angewandte Chemie International Edition
evolution performance of different catalysts, and Bi shows the
poor activity due to its poor binding with H-adatoms (Figure
4C).[4b,12] The DFT calculations with CI-NEB method (Figure 4D)
is further conducted and revealed that the active barrier of the
defect Bi(110) facet is only 0.56 eV, which is much lower than
that of perfect Bi(110), indicating the favorable kinetic
characteristics. Overall, our results suggest that the defect
Bi(110) facet enables the surface active sites to adsorb and
activate the N2 to form adsorbed N2H* species with favourable
N2RR energetics, which is responsible for the superior N2RR
performance.
Research Program of the Chinese Academy of Sciences
(XDA21010210), Program for JLU Science and Technology
Innovative Research Team (2017TD-09), and the United States
Department of Energy (DE-FG02-13ER16381).
Keywords: bismuth nanosheet • defects rich • nitrogen
reduction • NMR quantitative test • non-noble metal
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transform a nearly non-catalytic N2RR material into a very robust
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This work was financially supported by the National Natural
Science Foundation of China (51522101 and 51631004), the
National Key Research and Development Program of China
(2017YFA0206700 and 2016YFB0100100), the Strategic Priority
[12] J. Greeley, T. F. Jaramillo, J. Bonde, I. Chorkendorff, J. K. Nørskov,
Nat. Mater. 2006, 5, 909-913.
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10.1002/anie.201903969
Angewandte Chemie International Edition
COMMUNICATION
Defect-rich Semimetal bismuth (Bi)
Yue Wang, Miao-miao Shi, Di Bao, Fan-
lu Meng, Qi Zhang, Yi-tong Zhou, Kai-
hua Liu, Yan Zhang, Jia-zhi Wang, Zhi-
wen Chen, Da-peng Liu, Zheng Jiang,
Mi Luo, Lin Gu, Qing-hua Zhang, Xing-
zhong Cao, Yao Yao, Min-hua Shao, Yu
Zhang, Xin-Bo Zhang, Jingguang G.
Chen, Jun-min Yan, Qing Jiang
nanoplates
production rate of 5.453 μg mg-1 h-1
and higher Faradaic efficiency
(11.68% at -0.6 vs. reversible
achieved
a
15NH3
a
V
hydrogen electrode) in aqueous
solutions at ambient conditions
because of its poor binding with H-
adatoms for increasing the selectivity
and production rate. In addition, we
Generating Defect-Rich Bismuth for
Enhancing
Rate
of
Nitrogen
1
firstly use H NMR measurement with
Electroreduction to Ammonia
N2 gas as a quantitative test method
for all data in aqueous electrolytes.
This article is protected by copyright. All rights reserved.