Boosting Electrocatalytic Ammonia Production through Mimicking “π Back-Donation”

Article abstract of DOI:10.1016/j.chempr.2020.07.006

Electrocatalytic dinitrogen reduction reaction (N₂RR) is an emerging route for ammonia synthesis at ambient conditions. Albeit the “π back-donation” process enables N₂RR activity on transition metals with empty d-orbitals, given its dilemma in overcoming hydrogen evolution reaction (HER) competition, exploring p-block-element-based catalysts with relatively inferior HER activity is achievable for high selectivity. The challenge lies in designing rational structure to improve N₂RR activity. Here, we synergistically integrate oxygen vacancy (V_O) with hydroxyl on Bi₄O₅I₂ (V_O-Bi₄O₅I₂-OH), which render this p-block-element-based material active to mimic “π back-donation” behavior because of sufficient vacant orbitals. In neutral media, the electrocatalytic N₂RR performance of V_O-Bi₄O₅I₂-OH in terms of splendid faradic efficiency (32.4%) is superior to most of the prior reports using p-block-element-based catalysts. Our findings show a new strategy toward standout N₂RR activity, which holds great potential in exploiting other p-block-element-based electrocatalysts. Producing ammonia through an economical, sustainable approach has a profound impact on underpinning global agriculture. Electrocatalytic N₂ reduction reaction (N₂RR) is emerging as an alternative technology for synthesizing NH₃ at ambient conditions. The poor hydrogen evolution ability of p-block-element catalysts emerge as promising candidates to produce ammonia with high selectivity (faradic efficiency [FE]). Due to the absence of d-orbitals, nevertheless, N₂ activation and protonation are difficult on p-block-element catalysts. Employing rational surface electronic structure modulation can induce sufficient vacant orbitals, which promote N₂ reduction through mimicking the “π back-donation” process. A standout electrocatalytic N₂RR performance in terms of NH₃ rate (20.44 μg h^?1 mg^?1_cat.) and high FE of 32.4% is achieved in neutral electrolyte. This work motivates in-depth study to shed light on the new chemistry of ambient N₂RR not relying on Haber-Bosch. Mimicking “π back-donation” is proposed as a facile, feasible, and generalizable approach to boost electrocatalytic ammonia production from dinitrogen on p-block-element catalysts, which lack d-orbitals. Such behavior is realized by providing sufficient empty orbitals on the surface of Bi₄O₅I₂. The integrated modification of oxygen vacancy with hydroxyl achieves the generation of empty orbitals to reduce the energy barrier for N₂ protonation. The intriguing strategy that takes advantage of the electronic structure modulation endows this p-block-element catalyst with a relatively high ammonia synthesis of 20.44 μg h^?1 mg^?1_cat. in neutral media at a high faradic efficiency of 32.4%.

Full text of DOI:10.1016/j.chempr.2020.07.006

ll
Article
Boosting Electrocatalytic Ammonia
Production through Mimicking ‘‘p Back-
Donation’’
Chade Lv, Lixiang Zhong, Yao
Yao, ..., Shuzhou Li, Qingyu Yan,
Guihua Yu
alexyan@ntu.edu.sg (Q.Y.)
ghyu@austin.utexas.edu (G.Y.)
HIGHLIGHTS
Mimicking ‘‘p back-donation’’
surface sites are established on p-
block-element catalysts
Rational electronic structure
modulation endows catalyst with
plenty of empty orbitals
Energy barrier for the protonation
of N
2
is reduced to boost the
conversion into NH
3
2
Superior electrocatalytic N RR
activity is achieved with high
ammonia yield and FE
Mimicking ‘‘p back-donation’’ is proposed as a facile, feasible, and generalizable
approach to boost electrocatalytic ammonia production from dinitrogen on p-
block-element catalysts, which lack d-orbitals. Such behavior is realized by
providing sufficient empty orbitals on the surface of Bi
modification of oxygen vacancy with hydroxyl achieves the generation of empty
orbitals to reduce the energy barrier for N protonation. The intriguing strategy
4 5 2
O I . The integrated
2
that takes advantage of the electronic structure modulation endows this p-block-
À1
À1cat
element catalyst with a relatively high ammonia synthesis of 20.44 mg h mg
.
in neutral media at a high faradic efficiency of 32.4%.
Lv et al., Chem 6, 1–13
October 8, 2020 ª 2020 Published by Elsevier
Inc.
https://doi.org/10.1016/j.chempr.2020.07.006

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Article
Boosting Electrocatalytic Ammonia Production
through Mimicking ‘‘p Back-Donation’’
1
1
3
4
5
6
2
4
Chade Lv, Lixiang Zhong, Yao Yao, Daobin Liu, Yi Kong, Xiaoli Jin, Zhiwei Fang, Wenjie Xu,
1
1
3
4
5
1
Chunshuang Yan, Khang Ngoc Dinh, Minhua Shao, Li Song, Gang Chen, Shuzhou Li,
Qingyu Yan, * and Guihua Yu2,7,*
1,
SUMMARY
The Bigger Picture
Producing ammonia through an
Electrocatalytic dinitrogen reduction reaction (N RR) is an emerging
2
route for ammonia synthesis at ambient conditions. Albeit the ‘‘p
economical, sustainable approach
has a profound impact on
back-donation’’ process enables N RR activity on transition metals
2
with empty d-orbitals, given its dilemma in overcoming hydrogen
evolution reaction (HER) competition, exploring p-block-element-
based catalysts with relatively inferior HER activity is achievable
for high selectivity. The challenge lies in designing rational structure
underpinning global agriculture.
Electrocatalytic N
reaction (N RR) is emerging as an
alternative technology for
synthesizing NH at ambient
2
reduction
2
to improve N RR activity. Here, we synergistically integrate oxygen
3
2
vacancy (V ) with hydroxyl on Bi O I (V -Bi O I -OH), which render
conditions. The poor hydrogen
evolution ability of p-block-
element catalysts emerge as
promising candidates to produce
ammonia with high selectivity
(faradic efficiency [FE]). Due to the
absence of d-orbitals,
O
4
5 2
O
4
5 2
this p-block-element-based material active to mimic ‘‘p back-dona-
tion’’ behavior because of sufficient vacant orbitals. In neutral me-
dia, the electrocatalytic N RR performance of V -Bi O I -OH in
2
O
4
5 2
terms of splendid faradic efficiency (32.4%) is superior to most of
the prior reports using p-block-element-based catalysts. Our find-
ings show a new strategy toward standout N RR activity, which
2
holds great potential in exploiting other p-block-element-based
electrocatalysts.
2
nevertheless, N activation and
protonation are difficult on p-
block-element catalysts.
Employing rational surface
electronic structure modulation
can induce sufficient vacant
INTRODUCTION
The industrial synthesis of ammonia (NH
3
), a key feedstock for synthetic fertilizer, re-
lies on the traditional Haber-Bosch approach under extremely harsh conditions
orbitals, which promote N
reduction through mimicking the
‘p back-donation’’ process. A
standout electrocatalytic N RR
performance in terms of NH
2
ꢀ
ꢀ
1
(
100–200 bar and 400 C–500 C). Nitrogen fixation via electrocatalytic dinitrogen
reduction reaction (N RR) provides an intriguing picture for the production of NH
at mild conditions, potentially realizing a sustainable approach when driven by
2
3
‘
2
2
–7
renewable electricity (wind power, hydroelectric power, etc.).
Attributed to the
3
rate
availability of d-orbital electrons, advances in the chemistry of electrocatalytic
À1
À1cat
(
20.44 mg h mg
.) and high
N
2
RR have indicated transition-metal-based catalysts can produce NH
3
through
FE of 32.4% is achieved in neutral
electrolyte. This work motivates
in-depth study to shed light on the
8
–14
the ‘‘p back-donation’’ process to alleviate the kinetic barrier of N
2
activation.
Nevertheless, because of the adverse hydrogen evolution reaction (HER), it remains
1
5,16
a grand challenge in improving the selectivity (faradic efficiency [FE]).
new chemistry of ambient N
not relying on Haber-Bosch.
2
RR
Alternatively, p-block elements might serve as promising electrocatalysts for N
2
RR
owing to their unique electronic structure and poor HER activity. Group-VA-
1
7
element-based materials, such as black phosphorus, bismuth nanocrystals or nano-
1
8,19
20–23
sheets,
and bismuth/antimony-based compounds,
2
behave as active N RR
electrocatalysts with high FE. Another protocol is developing carbon-based cata-
lysts because of their mechanical flexibility and excellent electrical conductivity,
2
4
for instance, nitrogen-doped carbon nanospikes, chalcogen-elements-doped car-
2
5
26,27
bon, polymeric carbon nitride,
etc. Drawing inspiration from L e´ gar e´ and
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1

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Article
Figure 1. Schematic Illustration of Engineering Surface Active Site and the Mimicking ‘‘p Back-
Donation’’ Process
(
(
A) The electronic structure after the introduction of a V
B) The mimicking ‘‘p back-donation’’ process on Bi
O
and hydroxyl on Bi
4 5 2
O I .
4
O
5
I
2
modified by V and hydroxyl
O
simultaneously.
Braunschweig’s report that highlight the ‘‘p back-donation’’ process on boron (bor-
2
28
ylene) with empty sp orbital and occupied p orbital for effective N
current efforts to exploit p-block-element N RR electrocatalysts mainly center on
investigating boron-based materials, including boron nanosheet,
2
activation,
2
2
9,30
boron car-
3
1
32–34
35
36
bide, boron nitride,
boron phosphide, metal boride, boron-doped gra-
3
7,38
39
40
phene,
less, most p-block elements lack similar electronic structure, such as boron with
unoccupied orbitals, which handicaps the N RR activity on other p-block elements
boron-rich covalent organic frameworks, B/g-C
3
N
4
,
etc. Nonethe-
1School of Materials Science and Engineering,
Nanyang Technological University, Singapore
2
6
39798, Singapore
through mimicking the ‘‘p back-donation’’ process. Enlightened by the introduction
2Materials Science and Engineering Program,
Texas Materials Institute, The University of Texas
at Austin, Austin, TX 78712, USA
of anion defects to endow adjacent metal atom with empty orbitals as Lewis acid,
engineering oxygen vacancy (V
element-based materials. Due to the introduction of numerous unpaired electrons
accompanied with V rather than sufficient unoccupied orbitals, it remains difficult
to mimic ‘‘p back-donation’’ behavior. Taking this into account, it is highly desider-
ated to further modify the V -engineered p-block-element-based materials, which
can precisely modulate the electronic structure for N RR.
O 2
) might boost the N RR performance on p-block-
3Department of Chemical and Biological
Engineering, The Hong Kong University of
Science and Technology, Hong Kong, China
O
4National Synchrotron Radiation Laboratory, CAS
Center for Excellence in Nanoscience, University
of Science and Technology of China, Hefei, Anhui
O
2
230029, China
5MIIT Key Laboratory of Critical Materials
Technology for New Energy Conversion and
Storage, School of Chemistry and Chemical
Engineering, Harbin Institute of Technology,
Harbin, Heilongjiang 150001, China
Here, as a proof-of-concept demonstration, we simultaneously introduce V
droxyl on the surface of Bi (V -Bi -OH), which endows this p-block-
element-based material with occupied orbitals as well as vacant orbitals at Bi atoms
Figure 1). In sharp contrast with pristine Bi and V -Bi without hydroxyl,
O
and hy-
4
O
5
I
2
O
4 5 2
O I
(
4
O
5
I
2
O
4 5 2
O I
6Engineering Technology Research Center of
Henan Province for Solar Catalysis, College of
Chemistry and Pharmaceutical Engineering,
Nanyang Normal University, Nanyang, Henan
the unique electronic structure of such surface sites plays an important role
in mimicking ‘‘p back-donation’’ behavior. In light of in situ Fourier transform
infrared (FTIR) measurements and density functional theory (DFT) calculations, as-
473061, China
designed configuration can decrease the energy barrier for N
sequent NH production by means of associative pathways. When serving as an
RR electrocatalyst, V -Bi -OH shows an average NH yield as high as
. in 0.1 M Na SO with an outstanding FE of 32.4% under ambient
2
RR and achieve sub-
7Lead Contact
3
*Correspondence: alexyan@ntu.edu.sg (Q.Y.),
N
2
O
4
O
5
I
2
3
ghyu@austin.utexas.edu (G.Y.)
À1cat
2
4
2
0.44 mg h mg
https://doi.org/10.1016/j.chempr.2020.07.006
2
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Article
conditions, outperforming most p-block-element-based electrocatalysts in neutral
electrolyte. This elegant strategy not only contributes to effective electrocatalytic
2 4 5 2
N RR activity on Bi O I , but also guides the construction of surface sites to mimic
the ‘‘p back-donation’’ process on other p-block-element-based electrocatalysts.
RESULTS
Structural Characterization
We crafted V
O 4 5 2
-Bi O I -OH through a one-step solvothermal approach (for experi-
mental details, please see the Supplemental Experimental Procedures). As
evidenced by X-ray diffraction (XRD), the p-block-element material of Bi
4
O
5
4
I
2
1
(
ICSD-412590) was obtained without any detectable impurity phase (Figure 2A).
According to the scanning electron microscopy (SEM), transmission electron micro-
scopy (TEM), and atomic force microscopy (AFM) images (Figures S1A, S1B, S1E,
O 4 5 2
and S1F), V -Bi O I -OH possesses a flower-like architecture (ca. 1 mm in size)
assembled by nanosheets (ca. 6 nm in thickness). High-resolution TEM (HRTEM) im-
ages (Figures 1C and 1D; Supplemental Information) from side view and top view of
˚
the nanosheets explicitly show lattice spacings of 9.7, 2.7, and 2.84 A, which corre-
spond to the (101), (303), and (020) crystal planes, respectively. It indicates that the
nanosheets of V
ure S2. To accurately identify the as-designed ingenious structure, we synthesized
control samples of Bi and V -Bi derived from V -Bi -OH (for experi-
mental details, please see the Supplemental Experimental Procedures). The Bi
and V -Bi displayed no significant change in crystal phase, morphology, or sur-
face area (Figures S3–S5).
O 4 5 2
-Bi O I -OH expose the (101) crystal facet as illustrated in Fig-
4
O
5
I
2
O
4
O
5
I
2
O
4 5 2
O I
4
5 2
O I
O
4 5 2
O I
To probe the V
O
, electron paramagnetic resonance (EPR) spectroscopy (Figure 2B)
was conducted at room temperature. There are three charge states for oxygen va-
+
2+ 42
cancies: neutral (V
O
), singly ionized (V
O
), and doubly ionized (V
O
). Among
+
them, only the V
of a spin S = 1/2, which will be reflected in the EPR signal.
signal emerges at g = 2.001 for V -Bi -OH, representing unpaired electrons
X-ray photoelectron spectroscopy (XPS) was performed
to further elucidate the incorporation of V in V -Bi -OH. Apart from lattice O at
29. 5 eV, the fitting peaks at 530.7 eV originating from V appeared in the de-con-
With the aim at capturing in-depth infor-
O
charge state occupied by one electron is paramagnetic because
4
3,44
Apparently, an EPR
O
4 5 2
O I
4
5,46
O
stemmed from V sites.
O
O
4 5 2
O I
5
O
4
6–48
voluted O 1s XPS spectra (Figure 2C).
mation about defective structure and atomic coordination, extended X-ray absorp-
tion fine structure (EXAFS) measurements were conducted. Figure 2D displays the
2
k -weighted Fourier transforms (FT) curves at R-space of Bi LIII-edge EXAFS spectra
for Bi
foil serve as references. The low-intensity peak at 1.6 A assignable to Bi–O bond
in V -Bi -OH manifests a decrease in the Bi–O coordination number than that
of commercial Bi and Bi , which results from V -induced coordinatively un-
saturated Bi atoms (Figure 2D). As reflected in EPR, XPS spectra, and FT-EXAFS
Figures 2B–2D and S6), the V vanished in Bi and regenerated in V -Bi
after experiencing re-calcination in air and H /Ar atmosphere, respectively. In
contrast, no defect of iodine emerged during the above process as confirmed by I
d XPS spectra (Figure S7). Ultraviolet-visible (UV-vis) diffuse reflectance spectra
DRS) recorded on V -Bi -OH and V -Bi suggest the existence of V
because of the red shift relative to pristine Bi
Bi -OH and V -Bi possess the same absorption edge as shown in UV-vis
DRS, which is closely related to the band structure after introducing V
4 5 2 O 4 5 2 O 4 5 2 2 3
O I , V -Bi O I , and V -Bi O I -OH. In addition, commercial Bi O and Bi
˚
O
4 5 2
O I
2
O
3
4
O
5
I
2
O
4
9
(
O
4
O
5
I
2
O
4 5 2
O I
2
3
(
O
4
O
5
I
2
O
4
O
5
I
2
O
4
8
4
5 2 O
O I (Figure 2E). Besides, V -
4
O
5
I
2
O
4 5 2
O I
5
0
O
.
In
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Article
Figure 2. Structural Characterizations and Illustration
(
(
(
(
(
(
(
(
(
A) XRD pattern.
B) EPR spectra.
C) O 1s XPS spectra.
D) Bi LIII-edge FT-EXAFS spectra.
E) UV-vis diffuse reflectance spectra.
F) Bi 4f XPS spectra.
G) Bi LIII-edge XANES profiles.
H) FTIR spectra.
O 4 5 2
I) Surface structure of V -Bi O I -OH.
combination with the result of O 1s XPS measurements, V
O 4 5 2 O
-Bi O I -OH and V -
+
Bi are consistent in the ratios of V , while the former possesses less V
4
O
5
I
2
O
O
.
Generally, the presence of V
O
will endow the metal atoms in the vicinity of V
O
with
unpaired electrons to balance the charge at defective sites, which might lead to
5
1
3+
3+d
remarkable oxidation state variation. Apart from Bi , the V
O
gives rise to Bi
4
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3
Àd
52
and Bi
stark contrast to the pristine Bi
changed in V -Bi -OH (Figures 2F and S8). X-ray absorption near-edge struc-
ture (XANES) spectra (Figure 2G) collected on V -Bi -OH also suggest a nearly
consistent oxidation state in contrast to Bi and commercial Bi
-Bi as a reference, a weaker EPR signal owing to less unpaired electrons
) and unchanged oxidation state of Bi imply the V -induced coordinatively un-
saturated Bi atoms in V -Bi -OH might be partially re-bonded by some other
species. From the observation in O 1s XPS spectra (Figure 2C), the fitting peak at
31.6 eV in V -Bi -OH is assignable to surface hydroxyl, which might bond
states of Bi in V
O
-Bi
4
O
5
I
2
, as confirmed by Bi 4f XPS spectra (Figure 2F). In
4
5 2
O I
, the oxidation state of Bi atoms is barely
O
4 5 2
O I
O
4 5 2
O I
5
3,54
4
O
5
I
2
2
O
3
.
Taking
V
O
4 5 2
O I
+
(
V
O
O
O
4 5 2
O I
5
O
4 5 2
O I
4
7
with V
O
-induced coordinatively unsaturated Bi atoms. Lastly, the surface hydroxyl
À1
modification is verified by the peak of v(O–H) stretching mode (3,200–3,600 cm ) in
5
5
the FTIR spectra (Figure 2H). The above deduction can be explicitly validated by
the higher Bi–O peak of V -Bi -OH than that of V -Bi in FT-EXAFS spectra
Figure 2D), while the latter one possesses negligible surface hydroxyl (Figures 2C
and 2H). Based on a range of spectroscopic measurements, the integration of V
O
4
O
5
I
2
O
4 5 2
O I
(
O
O 4 5 2
with hydroxyl on V -Bi O I -OH was successfully engineered as depicted in
Figure 2I.
Theoretical Calculations
To uncover the impact of V
corresponding N RR activity of Bi
tion details, please see the Supplemental Experimental Procedures). As observed in
Figures 3B and 3C, the generation of V leads to adjacent coordinatively unsatu-
rated Bi atoms in V -Bi . The density of states (DOS), stemmed from the Bi 6p
state, shows aligned peaks (red, À0.7 eV) below the Fermi level and indicates
weak bonding between two coordinatively unsaturated Bi atoms because of V (Fig-
ures 3D and 3E). When the coordinatively unsaturated Bi atom is modified by hy-
droxyl (Bi1) in V -Bi
-OH (Figure 3A), the aligned peaks at À0.7 eV disappear,
and the DOS becomes similar as the pristine Bi (Figure 3D, blue and black
lines). It indicates strong interaction between the Bi atom and the hydroxyl, as well
as the break of Bi–Bi bond in V -Bi -OH. For the other Bi atom without hydroxyl
modification (Bi2) in V -Bi -OH, the break of the Bi–Bi bond leads to the emer-
O
and surface hydroxyl on the electronic structure and the
, we conducted DFT calculations (for calcula-
2
4 5 2
O I
O
O
4 5 2
O I
O
O
4 5 2
O I
4
5 2
O I
O
4 5 2
O I
O
4 5 2
O I
gence of occupied orbitals (À0.3 eV) and empty orbitals (+0.3 eV) at the Bi2 atom as
disclosed by the DOS analysis (Figure 3E, blue). Further DFT calculations of charge
density difference unveils that the co-existence of V
nificant charge transfer from coordinatively unsaturated Bi atoms to the hydroxyl and
affirms the strong interaction between the hydroxyl and V , as depicted in Figure 3F.
Therefore, integrating V with hydroxyl modification endows the Bi2 atom with
vacant orbitals, which has the potential of serving as ‘‘p back-donation’’ sites
O
and hydroxyl can result in sig-
O
O
3
6
through accommodating the lone pair electrons of N
2
molecule. According to
molecule is difficult to be chemisorbed on the surfaces
-Bi -OH samples. The optimized configurations of
our DFT calculations, the N
of Bi , V -Bi
2
4
O
5
I
2
O
4
O
5
I
2
, or V
O
4 5 2
O I
N
2
adsorbed on these surfaces are all in physical adsorption states, in which the N
2
molecules are quite far away from the surfaces, and the binding energies are similar
and close to zero (Figure S9). The N
2
RR performance is determined by both N
2
5
6
adsorption and the protonation of N
mance for N adsorption, the V -Bi
into *NNH through a proton-coupled electron transfer process to mimic ‘‘p
back-donation’’. Confirmed by the charge density difference of V -Bi -OH
with *NNH, transfer of charges to N atoms from Bi2 atom on V -Bi -OH results
in the possible activation to dramatically decrease the N RR energy barrier (Fig-
ure S10). Figure S11 shows the optimized configurations of *NNH on V -Bi
2
.
Albeit these surfaces have similar perfor-
2
O
4 5 2
O I -OH might facilitate the protonation of
N
2
O
4 5 2
O I
O
4 5 2
O I
2
O
4 5 2
O I
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Figure 3. DFT Calculations
(
(
(
A–C) The atomic configurations of V
D and E) DOS for Bi1 (D) and Bi2 (E) atoms.
F) The charge density difference of the V -Bi
O
-Bi
4
O
5
I
2
-OH (A), Bi
À
O 4 5 2
-OH upon the adsorption of a hydroxyl on V -Bi O I . The yellow and cyan isosurfaces (G 0.002 e /
4 5 2 O 4 5 2
O I (B), and V -Bi O I (C).
O
4
O
5
I
2
3
Bhor ) represent charge accumulation and depletion in the real space, respectively.
G) Free energy diagram for N RR on V -Bi -OH and V -Bi at 0 V versus RHE.
(
2
O
4
O
5
I
2
O
4 5 2
O I
and V
the protonation step to form *NNH rules out further reduction reaction (Figure S12).
The *NNH on both V -Bi and V -Bi -OH bond to the surface by one N
atom. This indicates that the N RR prefers the distal pathway (N / *NNH /
NNH / *N / *NH / *NH ). To shed light on the effect of Bi
with different surface configurations on the N reduction performance, the free en-
ergy profiles of N RR intermediates on V -Bi -OH and V -Bi are summa-
rized in Figure 3G. When implanted with only a V , the free energy barrier from
to *NNH is still quite high (1.96 eV), which is the rate-determining step for the
overall N RR process. Importantly, V -Bi -OH possessing occupied and unoc-
cupied orbitals enables a much lower free energy barrier of 1.28 eV. The adsorption
free energies of *H are 2.052, 0.867, and À0.076 eV for Bi , V -Bi , and V
O 4 5 2 4 5 2
-Bi O I -OH. As for pristine Bi O I , the high free energy barrier (>2.5 eV) in
O
4
O
5
I
2
O
4 5 2
O I
2
2
1
*
2
2
/ NH
3
4 5 2
O I
2
2
O
4
O
5
I
2
O
4 5 2
O I
O
N
2
2
O
4 5 2
O I
4
O
5
I
2
O
4
O
5
I
2
O
-
6
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Bi
face of Bi
the protonation of N
tion of V with hydroxyl can provide surface sites that mimic the ‘‘p back-donation’’
process to boost N
4
O
5
I
2
-OH, respectively. It indicates that the H atom is difficult to absorb on the sur-
or V -Bi but easy for V -Bi -OH, which may also facilitate
. Suggested by the above theoretical illustration, the integra-
4
O
5
I
2
O
4
O
5
I
2
O
4 5 2
O I
2
O
2
RR activity.
Electrocatalytic N
2
RR Performance
Intrigued by the prediction based on theoretical calculation, as-engineered surface
sites can mimic ‘‘p back-donation’’ behavior on V -Bi -OH to enable effective
nitrogen reduction and ammonia synthesis. The electrocatalytic N RR activity was
fully examined by using chrono-amperometry (CA) at fixed potentials in 0.1 M
Na SO and 0.05 M H SO saturated with N bubbling under ambient conditions.
O
4 5 2
O I
2
2
4
2
4
2
In the first place, a low overpotential initiating from À0.05 to À0.2 V versus reversible
hydrogen electrode (RHE) was conducted to determine the optimized potential for
V
O
-Bi
NH yield using 0.1 M Na
method (Figures S15 and S16; details are presented in Supplemental Experimental
4
O
5
I
2
-OH. The CA results are displayed in Figures S13 and S14. The average
3
2
SO , quantitatively detected by the indophenol blue
4
À1
À1cat
Procedures), reaches the peak value of 20.44 mg h mg
tial increases until À0.1 V versus RHE (Figure 4A). The ammonium ion-selective elec-
trode was also employed to quantify the NH production, which is in great accor-
. as the negative poten-
3
5
7
dance with the indophenol blue method (Figures S17 and S18). Strikingly, an
ammonia-producing FE of 32.4% was realized at this potential, which surpasses
the current state-of-the-art p-block-element-based N
neutral electrolyte as summarized in Table S1. No side reaction toward N
emerged during the electrocatalysis process, declaring excellent selectivity toward
nitrogen fixation product (Figures S19 and S20). When 0.05 M H SO serves as elec-
. at À0.05 V versus RHE is
2
RR electrocatalysts using
2
H
4
2
4
À1
À1cat
trolyte, higher average NH
3
yield of 21.92 mg h mg
SO , which could be due to abundant H for the pro-
+
achieved than that in 0.1 M Na
2
4
5
6
ton-coupled electron transfer process (Figures 4B, S21, and S22). Unlike neutral
electrolyte, the highest FE in acidic electrolyte is only 14.6% at À0.05 V versus
2
RHE. As the adverse HER participates more, the N RR performance, in terms of
NH
3
yield as well as FE, is cast into shade when increasing the negative potential
for both neutral and acidic electrolytes.
For evidencing the deduction that the as-detected NH
the electroreduction of N on V -Bi -OH, a series of control measurements were
performed as below. Primarily, as confirmed by N 1s XPS spectrum of V -Bi
3
originates exclusively from
2
O
4 5 2
O I
O
4 5 2
O I -
OH, no residual nitrogen species on the surface excluded the potential nitrogen
source from catalyst (Figure S23). In the absence of catalyst loading, only carbon pa-
per itself showed no N
When performed at open circuit potential (OCP) or replacing N
gas, no NH was probed as reflected in UV-vis spectra (Figures S24B and S24C).
Hence, above results reveal that as-detected ammonia was indeed derived from
the electrocatalytic N RR process rather than the contamination from catalyst, car-
bon paper, electrolyte, or feed gas.
2
RR activity under the exact same conditions (Figure S24A).
2
by Ar as inlet
3
2
5
8,59
To further qualitatively verify the origin of
ammonia, the N isotopic labeling experiment was carried out on V -Bi
(98 atom % N), only a doublet coupling (~72 Hz) assign-
1
5
O
4 5 2
O I -
6
0
15
15
OH. When fed by
N
2
1
5
+
1
able to NH
4
was detected as observed in the H nuclear magnetic resonance
6
1
(
NMR) spectra (Figure 4C). By contrast, a triplet coupling (~ 52 Hz) appeared after
1
4
14
+
62
employing
N
2
as feed gas, which represents the signal of NH
yield and corresponding FE are comparable with the data using
Figure 4D).
4
(Figure 4C).
1
5
14
The NH
3
N
2
(
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Figure 4. N
A and B) NH
SO (B).
C) H NMR spectra of the electrolyte fed by
catalyst.
D) Comparison of the NH
À0.1 V versus RHE.
E and F) NH yield rates and corresponding FEs of Bi
À0.1 V versus RHE in 0.1 M Na SO (E) and À0.05 V versus RHE in 0.05 M H
2
RR Tests
(
3
yield rates and corresponding FEs of V
O
4 5 2 2 4
-Bi O I -OH in 0.1 M Na SO (A) and 0.05 M
H
2
4
1
15
14
(
N
2
and
2 2 O 4 5 2
N after N RR tests for V -Bi O I -OH
1
5
14
(
3
yield rates and corresponding FEs using
N
2
and
N
2
for N
2
2
RR tests at
-OH at
(
3
4
O
5
I
2
, V
O
-Bi
4
O
5
I
2
, and V
O
-Bi
4
O
5
I
2
4
2
SO
4
(F).
To highlight the importance of surface sites on V
O
-Bi
4
O
5
I
2
-OH in promoting the
and V
SO and
N
2
RR activity, the electrocatalysis tests for the counterparts of Bi
4
O
5
I
2
O
-
Bi
.05 M H
tine Bi
4
O
5
I
2
were executed at À0.1 and À0.05 V versus RHE in 0.1 M Na
2
4
0
2
SO
only exhibited extremely low NH
. in 0.1 M Na SO (Figure 4E) and 0.05 M H
of which the FEs only attained 4.3% and 2.1%. In comparison, V
4
, respectively. With the absence of V
O
or surface hydroxyl, the pris-
À1
4
O
5
I
2
3
yields of 3.54 and 3.60 mg h
SO
À1cat
2
4
2
4
mg
(Figure 4F), respectively,
-Bi nearly
. and 10.0% (4.2%)
), respectively (Figures 4E and 4F). The relevant
UV-vis absorption spectra and the quantification by ammonium ion-selective elec-
trode method are illustrated in Figures S25–S27. Since electrocatalytic N RR per-
formance of V -Bi is better than that of Bi , it suggests V can facilitate
RR kinetics, as highlighted in previous reports. Nevertheless, without
O
4 5 2
O I
À1
À1cat
tripled the NH
in 0.1 M Na SO
3
yields and FEs to 9.35 (12.03) mg h mg
(0.05 M H SO
2
4
2
4
2
O
4
O
5
I
2
4
O
5
I
2
O
6
3,64
N
2
8
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Figure 5. Cycling N
2
RR Tests, XPS, and FTIR Spectra
RR tests at À0.1 V versus RHE in 0.1 M Na
C) FTIR spectra before and after N RR tests in 0.1 M Na SO and 0.05 M H
D and E) O 1s XPS spectra after N RR tests in 0.1 M Na SO (D) and 0.05 M H
F) In situ electrochemical ATR-FTIR spectra of V -Bi -OH in N with 0.1 M Na
(
(
(
(
A and B) Cycling N
2
2
SO
4
(A) and at À0.05 V versus RHE in 0.05 M H
SO
SO
2 4
SO (B).
2
2
4
2
4
.
2
2
4
2
4
2
(E).
SO
O
4
O
5
I
2
2
4
electrolyte (from positive to negative potential).
surface hydroxyl, V
donation’’ behavior as evidenced by the DFT calculations. Note that 32.4% (14.6%)
of FEs were achieved toward N RR for V -Bi -OH in 0.1 M Na SO (0.05 M
SO ), a remarkable promotion relative to 4.3% (2.1%) for Bi and 10.0%
4.2%) for V -Bi . Taken together, a large contributor to this dramatic N RR
with surface
O 4 5 2
-Bi O I lacks sufficient vacant orbitals for mimicking ‘‘p back-
2
O
4
O
5
I
2
2
4
H
2
4
4 5 2
O I
(
O
4
O
5
I
2
2
performance difference is probably the integrated modification of V
O
hydroxyl.
Further, to confirm the significance of surface hydroxyl modification on V
OH, the cycling N RR tests were conducted. As attested by long-term (12 h) and
repeated eight runs of CA measurements À0.1 V versus RHE in 0.1 M Na SO
no obvious fluctuation in the current density is exhibited in Figures S28 and S29.
The RR performance basically stabilized after three runs and delivered
O 4 5 2
-Bi O I -
2
2
4
,
N
2
À1
À1cat
1
7.42 mg h mg
. of NH
RR activity was also stable in 0.1 M KOH electrolyte after eight cycles
yield
. at À0.05 V versus RHE after four
successive cycles and stabilized for the rest four cycles (Figures 5B and S33).
XRD patterns and SEM images of V -Bi -OH after N RR tests demonstrate
3
yield after eight runs (Figures 5A and S30). The elec-
trocatalytic N
2
(
Figures S31 and S32). Unexpectedly, for acidic electrolyte, the NH
3
À1
À1cat
decreased from 23.91 to 10.11 mg h mg
O
4
O
5
I
2
2
relatively good stability in phase structure and morphology (Figures S34
and S35). Thus, the maintenance of such surface sites might be the key factor
for the cycling N
2
RR performance. Indicated by FTIR spectra recorded on
RR tests for 2 h, the v(OÀH) stretching mode (3,200–
V
O
-Bi -OH after N
4
O
5
I
2
2
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À1
3
0
,600 cm ) was basically retained for the catalyst examined in 0.1 M Na
2
SO
4
and
.1 M KOH (Figures 5C and S36). However, these two peaks originating from sur-
face hydroxyl almost vanished when employing acidic electrolyte (Figure 5C). As
clearly elucidated in the O 1s XPS spectra (Figures 5D and 5E), V -Bi -OH still
possessed the peaks assignable to V with consistent intensities after the N RR
tests for both 0.1 M Na SO and also 0.05 M H SO . It is worthwhile noting that
the relative intensity of surface hydroxyl dropped slightly for 0.1 M Na SO , while
O
4 5 2
O I
O
2
2
4
2
4
2
4
it decreased drastically in the acidic electrolyte (Figures 5C–5E). In combination
with the above FTIR results, these manifest that the surface hydroxyl is not prefer-
entially reserved in acidic media, which results in poor cycling stability in 0.05 M
H
2
SO
4
electrolyte. To sum up, the surface hydroxyl plays a key role in establishing
to mimic ‘‘p back-donation’’ behavior.
active sites with V
O
Mechanism Analysis
To gain a greater understanding of the N
attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy
measurements were employed on V -Bi -OH (for details, please see the Sup-
2
RR mechanism, in situ electrochemical
O
4 5 2
O I
plemental Experimental Procedures). Evidenced by the Figure 5F, during the
negative scan from 0.1 to À0.2 V, the infrared (IR) bands of H–NÀH bending vi-
À1
bration at ~1,442 cm started to appear at ~ À0.1 V, which should stem from
6
5,66
the NH
x
species.
The same IR bands were also observed when performing
the positive scan in reverse, as displayed in Figure S37. In addition, ex situ elec-
trochemical ATR-FTIR spectra of V -Bi -OH elucidated the emergence of
NÀN stretching attributed to the adsorbed N species after N RR test (Fig-
ure S38). The above-mentioned FTIR results justify an associative mechanism
on V -Bi -OH, which can be well correlated with the theoretical models, as
displayed in Figure 3G. To further validate the N RR mechanism on as-con-
O
4 5 2
O I
2
H
y
2
6
5
O
4 5 2
O I
2
structed surface sites, a complementary experiment of in situ photocatalytic
FTIR spectroscopy measurement was also carried out. Despite the fact that the
driven force is light instead of electric energy, the IR bands of s(N–H) bending
3
mode, v(N–H) stretching mode, and absorbed NH also suggest the associative
6
7–70
mechanism on V
O
-Bi
4
O
5
I
2
-OH (Figure S39).
Moreover, apart from IR bands
À1
of physisorbed N
2
at the region of 2,320–2,380 cm , the peaks observed at
À1
2
,273 cm are assignable to the N–N stretching mode of chemisorbed activated
7
0,71
N
2
.
O 4 5 2
On the basis of above results, V -Bi O I -OH can successfully mimic ‘‘p
back-donation’’ behavior, which is beneficial to the whole electrocatalytic N
performance under ambient conditions.
2
RR
DISCUSSION
To summarize, we have engineered surface sites to endow a p-block-element-
based electrocatalyst (Bi ) with the ability of mimicking ‘‘p back-donation’’.
This is realized via integrated modification of V with hydroxyl on V -Bi
yield of 20.44 mg
4
5 2
O I
O
O
4 5 2
O I -
OH, which reaches a maximum FE of ca. 32.4% and an NH
3
À1cat
h mg
. at À0.1 V versus RHE in neutral electrolyte at ambient conditions. Su-
, the occupied orbitals and empty orbitals of Bi
-OH can decrease the energy barrier for protonation step
and facilitate the whole N RR process, as elucidated by the DFT calculations.
perior to the pristine Bi
atoms in V -Bi
4 5 2
O I
O
4 5 2
O I
2
In combination with the in situ FTIR measurements, the promoted ammonia pro-
duction mainly originates from an associative reaction pathway. This work pro-
vides a viable strategy for engineering active sites and may inspire further for
the development of new p-block-element-based electrocatalysts toward efficient
2
N RR.
10
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EXPERIMENTAL PROCEDURES
Resource Availability
Lead Contact
Further information and requests for resources and reagents should be directed to
and will be fulfilled by the Lead Contact, Professor Guihua Yu (ghyu@austin.
utexas.edu).
Materials Availability
This study did not generate new reagent nor materials. All materials used in this work
are commercially available.
Data and Code Availability
The published article includes all datasets generated or analyzed during this
study.
All experimental details are provided in the Supplemental Experimental Procedures.
SUPPLEMENTAL INFORMATION
Supplemental Information can be found online at https://doi.org/10.1016/j.chempr.
2
020.07.006.
ACKNOWLEDGMENTS
Q.Y. acknowledges the funding support from Singapore MOE AcRF Tier 1 grant nos.
RG113/15 and 2016-T1-002-065, and Tier 2 grant nos. 2017-T2-2-069 and 2018-T2-
0
1-010. G.Y. acknowledges the funding support from the Welch Foundation F-1861.
The authors greatly thank the Facility for Analysis, Characterization, Testing and
Simulation (FACTS) of Nanyang Technological University, Singapore, for using their
TEM, SEM, and XRD equipment.
AUTHOR CONTRIBUTIONS
Q.Y. and G.Y. conceived the research and designed the project; C.L. performed
most of the experiments; S.L. and L.Z. performed the simulations; L.S., D.L., and
W.X. contributed to the XAS measurements; M.S. and Y.Y. contributed to the in
situ electrochemical ATR-FTIR measurements; G.C. and Y.K. contributed to BET
and EPR characterizations; K.N.D. contributed to XPS measurements; X.J. contrib-
uted to the in situ photocatalytic FTIR measurements and TEM characterizations;
C.L., L.Z., Y.Y., D.L., C.Y., G.Y., and Q.Y. analyzed the data and co-wrote the paper.
All authors discussed the results on the manuscript.
DECLARATION OF INTERESTS
The authors declare no competing interests.
Received: April 13, 2020
Revised: May 26, 2020
Accepted: July 9, 2020
Published: July 31, 2020
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