Nanoporous Intermetallic Pd3Bi for Efficient Electrochemical Nitrogen Reduction

Article abstract of DOI:10.1002/adma.202007733

Electrocatalytic nitrogen reduction at ambient temperature is a green technology for artificial nitrogen fixation but greatly challenging with low yield and poor selectivity. Here, a nanoporous ordered intermetallic Pd₃Bi prepared by converting chemically etched nanoporous PdBi₂ exhibits efficient electrocatalytic nitrogen reduction under ambient conditions. The resulting nanoporous intermetallic Pd₃Bi can achieve high activity and selectivity with an NH₃ yield rate of 59.05?± 2.27?μg h^?1 mg_cat^?1 and a Faradaic efficiency of 21.52?± 0.71% at ?0.2?V versus the reversible hydrogen electrode in 0.05 m H₂SO₄ electrolyte, outperforming most of the reported catalysts in electrochemical nitrogen reduction reaction (NRR). Operando X-ray absorption spectroscopy studies combined with density functional theory calculations reveal that strong coupling between the Pd–Bi sites bridges the electron-transfer channel of intermetallic Pd₃Bi, in which the Bi sites can absorb N₂ molecules and lower the energy barrier of *N₂ for N₂ adsorption and activation. Meanwhile, the intermetallic Pd₃Bi with bicontinuous nanoporous structure can accelerate the electron transport during the NRR process, thus improving the NRR performance.

Full text of DOI:10.1002/adma.202007733

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Nanoporous Intermetallic Pd Bi for Efficient Electrochemical
3
Nitrogen Reduction
Xuejing Wang, Min Luo, Jiao Lan, Ming Peng,* and Yongwen Tan*
efficiencies due to the extremely weak N2
Electrocatalytic nitrogen reduction at ambient temperature is a green tech-
nology for artificial nitrogen fixation but greatly challenging with low yield and
adsorption and the sluggish cleavage of
[
4]
the intrinsic inactive N N bond. Var-
ious kinds of heterogeneous electrocata-
poor selectivity. Here, a nanoporous ordered intermetallic Pd Bi prepared by
3
[
5]
[6]
lysts, including metals, metal oxides,
converting chemically etched nanoporous PdBi exhibits efficient electrocata-
2
[
7]
[8]
sulfides, carbides, nonmetallic mate-
rials, etc., have already demonstrated
lytic nitrogen reduction under ambient conditions. The resulting nanoporous
[9]
intermetallic Pd Bi can achieve high activity and selectivity with an NH yield
3
3
to improve nitrogen adsorption and alle-
viate reaction barriers in NRR, although
the overall conversion efficiency remains
−1
−1
rate of 59.05 ± 2.27 µg h mg
and a Faradaic efficiency of 21.52 ± 0.71%
cat
at −0.2 V versus the reversible hydrogen electrode in 0.05 ꢀ H SO electro-
2
4
[
10]
unsatisfactory.
From the thermody-
lyte, outperforming most of the reported catalysts in electrochemical nitrogen
reduction reaction (NRR). Operando X-ray absorption spectroscopy studies
combined with density functional theory calculations reveal that strong cou-
pling between the Pd–Bi sites bridges the electron-transfer channel of interme-
tallic Pd Bi, in which the Bi sites can absorb N molecules and lower the energy
namic view, most electrons and protons on
catalyst contribute to hydrogen evolving at
relatively large applied potentials during
the electrochemical NRR process, leading
to the very low selectivity toward elec-
trosynthesis of NH . To increase the NH
3
2
barrier of *N for N adsorption and activation. Meanwhile, the intermetallic
3
3
2
2
selectivity, it would be desirable to design
an electrocatalytic system that could lower
Pd Bi with bicontinuous nanoporous structure can accelerate the electron
3
transport during the NRR process, thus improving the NRR performance.
the free energy barrier of *N to compete
2
with H for active sites and thus promote
[
11]
the NRR at low overpotentials.
Ammonia (NH ) is an important massive industrial product
Generally, N2 adsorption and activation are considered as
a critical step in the NRR. Under this process, strong interac-
3
of the fertilizer industry related to human population and eco-
[1]
nomic development. The Haber–Bosch approach has undis-
tion between the N molecules and the catalyst is an essential
2
putedly been the industrial method for NH manufacture under
prerequisite to efficiently catalyze NRR. Following this line of
thought, various catalysts have been developed, such as Pd-
based electrocatalysts, that have achieved considerable pro-
gress for the electrosynthesis of NH from N , while far from
3
harsh conditions, which operates at high pressure and tempera-
ture, with 1% global energy consumption and 300 million tons
[2]
of CO emission per year. Electrochemical nitrogen reduction
2
3
2
reaction (NRR) has recently been considered as an alternative
satisfactory for practical applications due to the weak interac-
approach to the Haber–Bosch process for efficient N fixation
tion with N and the preferred kinetics for hydrogen evolution
2
2
[
12]
due to its mild operation conditions at ambient temperature
and normal atmospheric pressure, which involves the poten-
tial coupling with green energy for the reaction without CO2
emission, and the utilization of abundant water instead of high-
purity hydrogen gas.[ Unfortunately, the electrochemical NRR
process usually suffers from low ammonia yields and Faradaic
reaction (HER). By contrast, less HER-active Bi of the main
group with occupied d orbitals has strong binding ability with
nitrogen-containing molecules, but the interaction can also
cause desorption difficulty of intermediate, resulting in high
3]
[13]
overpotentials and reduced reaction rates. From feasible and
representative perspectives, alloying Pd and Bi would be an
extremely important strategy for both adsorptive activation of
N and intermediate desorption from the catalyst surface. Com-
2
X. Wang, J. Lan, Prof. M. Peng, Prof. Y. W. Tan
College of Materials Science and Engineering
Hunan University
pared with the random alloy nanocatalysts, the ordered arrange-
ment based on well-defined stoichiometry of constituent atoms
and the intermetallic crystal nanostructures with intensified
ligand/strain effects have contributed to improve catalytic
Changsha, Hunan 410082, China
E-mail: pengming@hnu.edu.cn; tanyw@hnu.edu.cn
Prof. M. Luo
Department of Physics
Shanghai Polytechnic University
Shanghai 201209, China
[
14]
activity and durability. The synergistic effect of intermetallic
compounds could also regulate the adsorption and desorption
of the N species while suppressing competitive HER, resulting
in a superior performance for the lower reaction energy barrier
and high catalytic activity. Whereas, to design and synthesize
intermetallic Pd–Bi with certain stoichiometry and morphology
remains a great challenge.
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adma.202007733.
DOI: 10.1002/adma.202007733
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In this work, we successfully developed a nanoporous inter-
metallic Pd Bi (np-Pd Bi), which can be achieved by converting
Figure 1a illustrates the synthesis of nanoporous interme-
tallic Pd Bi by a two-step etching method. First, the Pd Bi
3
3
3
20 80
chemically etched nanoporous PdBi (np-PdBi ). Significantly,
precursor alloy was designed, which was composed of two
phases with different compositions and crystal structures: the
trigonal metallic Bi and the monoclinic intermetallic PdBi2
(Figure S1, Supporting Information), as identified by X-ray dif-
fraction (XRD) analysis (Figure S2a, Supporting Information).
2
2
the np-Pd Bi catalyst achieved an NH yield rate of 59.05 ±
3
3
−
1
−1
2
0
.27 µg h mgcat and a Faradaic efficiency (FE) of 21.52 ±
.71% at −0.2 V versus reversible hydrogen electrode (RHE),
together with high selectivity and excellent stability for efficient
electrochemical NRR at ambient condition. Operando X-ray
absorption spectroscopy (XAS) and density functional theory
[
15]
Using a selective phase dissolution approach, the PdBi phase
2
with a nanoporous structure (Figure S3, Supporting Informa-
tion) could be created through selectively etching the active
(DFT) calculations reveal that strong orbital interaction between
Pd and neighboring Bi sites in the intermetallic Pd Bi induces
Bi phase. Subsequently, intermetallic Pd Bi with a uniform
3
3
a strong electrostatic interaction, which can stabilize the transi-
tion state and lower the barrier for efficient NRR. The optimal
nanoporous structure (Figure 1b) was prepared from the con-
version of np-PdBi through the electrochemical etching pro-
2
adsorption of both N and H* in acidic media guarantees the
superior consecutive NRR process.
cess to remove Bi at room temperature (Figure S4, Supporting
Information), which is also confirmed by XRD (Figure S2b,
2
[
16]
Figure 1. Preparation and characterizations of np-Pd Bi. a) Schematic illustration of the preparation procedure. b) SEM image of np-Pd Bi. c) HAADF–
3
3
STEM image of np-Pd Bi. Inset: FFT pattern. d) Crystal structure of Pd Bi. e) The STEM–EDS elemental mapping of np-Pd Bi. f,g) XPS spectra of np-
3
3
3
Pd Bi in Pd 3d (f) and Bi 4f (g) regions. Scale bars: (b) 200 nm, (c) 2 nm, (e) 20 nm.
3
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Supporting Information) and transmission electron micro-
scopy (TEM, Figure S5, Supporting Information). Figure 1c
shows the atomic-resolution high-angle annular dark-field
scanning transmission electron microscopy (HAADF–STEM)
np-Pd Bi show a significant positive shift originated from the
Bi–Pd intermetallic interaction. It is clearly displayed that the
3
majority of the Pd in np-Pd Bi is mainly metallic state. Com-
3
0
pared with np-PdBi , the Pd 3d binding energy of np-Pd Bi
2
3/2
3
image of one intermetallic Pd Bi ligament, along with the cor-
has a negative shift from 335.9 to 335.8 eV and 3d5/2 from 341.3
to 341.1 eV, which is ascribed to the weakened electronic inter-
action between Pd and Bi atoms as the Pd/Bi ratio increases.
The XPS spectra of Bi 4f reveal two distinct peaks of Bi 4f7/4
and Bi 4f5/4 orbit levels (Figure 1g), respectively. Compared with
3
responding fast Fourier transform (FFT) pattern. The clear lat-
tice fringes with d-spacings of 0.23 and 0.22 nm are highlighted
in Figure 1c, which correspond to the (400) and (012) crystal
planes of the orthorhombic Pd Bi (Figure 1d), respectively. The
3
0
STEM–energy-dispersive spectrometry (STEM–EDS) mapping
image (Figure 1e) reveals the uniform distribution of Pd and
Bi element throughout the whole nanoporous networks. These
results indicate the reconstruction of nanoporous intermetallic
Pd Bi after complete phase conversion from PdBi via the elec-
np-PdBi , the negative shift of Bi 4f orbit levels in np-Pd Bi
2
3
further confirms the weakened Bi–Pd coupling of intermetallic
3
+
interaction originated from the regulated Pd/Bi ratio. The Bi
peak at 159.1 and 164.4 eV indicates that the surface of np-Pd Bi
3
is slightly oxidized in the basic etching solution, which could
be reduced during the reduction electrolysis. The Pd/Bi atomic
3
2
trochemical etching process. The resulting np-Pd Bi shows
3
2
−1
a large Brunauer–Emmett–Teller surface area of 21.17 m g
ratio of np-Pd Bi obtained from the XPS analysis is 3.16:1 after
3
with an average nanopore size of ≈25 nm using the Barrett–
Joyner–Halenda method (Figure S6, Supporting Information),
in agreement with the TEM observation. The evolution of sur-
face chemical state of the Pd–Bi intermetallic compound before
and after the electrochemical etching was characterized by
X-ray photoelectron spectroscopy (XPS). As shown in Figure 1f,
it can be observed that the Pd 3d spectra show two distinct
peaks which are assigned to the Pd 3d5/2 and Pd 3d3/2 orbit
levels. Each peak can be further split into two doublets, which
complete conversion via the electrochemical etching, which
is well consistent with the result by the inductively coupled
plasma-optical emission spectrometer (Table S1, Supporting
Information).
XAS was further performed to analyze the valence state and
local coordination environment of the np-Pd Bi. X-ray absorp-
3
tion near edge spectra (XANES) at the Pd K-edge (Figure 2a)
and the Bi L -edge (Figure 2b) show the intrinsic valance
3
state of Pd and Bi in np-Pd Bi. The overall peak trend of Pd
3
0
2+
0
are assigned to Pd and Pd chemical valence states. Com-
in np-Pd Bi is similar with metallic Pd, indicating that Pd is
3
pared with the 3d5/2 and Pd 3d orbit levels of the metallic
the main composition in np-Pd Bi. But, the Pd K-edge of the
3/2
3
Pd (334.9 and 340.2 eV),[ the Pd peaks of np-PdBi2 and
17]
np-Pd Bi shows a shift of 2.2 eV toward higher energy than
3
Figure 2. X-ray absorption spectroscopy (XAS) of np-Pd Bi. a) Normalized XANES at the Pd K-edge of np-Pd Bi, Pd metal, and PdO. b) Normalized
3
3
XANES at the Bi L -edge of np-Pd Bi, Bi metal, and Bi O . c) The corresponding FT-EXAFS spectra of the Pd K-edge derived from (a). d) The corre-
3
3
2
3
sponding FT-EXAFS spectra of the Bi K-edge derived from (b). e) The whole contour plots of wavelet transform (WT) of Pd metal, Pd@np-Pd Bi, Bi
3
metal, and Bi@np-Pd Bi (from top to bottom).
3
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Figure 3. Electrocatalytic NRR performance of the catalysts. a) LSV curves of np-Pd Bi, np-PdBi , and commercial Pd/C in Ar- and N -saturated 0.05 ꢀ
3
2
2
H SO . The inset shows the LSV curves of Pd/C. b) The mass-normalized NH yield rates at applied potential of np-Pd Bi, np-PdBi , and commercial
2
4
3
3
2
Pd/C. c) NRR Faradaic efficiency at each applied potential catalyzed by np-Pd Bi, np-PdBi , and commercial Pd/C. d) H selectivity of np-Pd Bi and
3
2
2
3
1
15
14
15
14
commercial Pd/C at given applied potentials. e) H NMR spectra of N and N catalyzed by np-Pd Bi using N and N as the isotopic source.
3
2
2
f) NH yield rates (V ) and FEs of np-Pd Bi in the consecutive NRR recycles.
3
NH3
3
that of Pd reference, implying an increase in the Pd valence
state of the np-Pd Bi. The Bi L -edge of np-Pd Bi also shows a
respectively. The np-Pd Bi electrode exhibits a significantly
3
enhanced current response in the N -saturated electrolyte than
3
3
3
2
slight positive shift of ≈0.8 eV as compared with Bi reference.
in the Ar-saturated electrolyte, confirming the electrochemical
conversion of N on the catalyst. Moreover, the np-Pd Bi elec-
These results imply the strong Pd–Bi coupling in np-Pd Bi
3
2
3
as compared with single metal component. The interactions
trode shows an appropriate current response as compared with
between Pd and Bi in np-Pd Bi is further confirmed by the
np-PdBi and Pd/C, revealing the intrinsic difference of catalytic
3
2
corresponding FT-EXAFS spectra in Figure 2c,d. The corre-
sponding FT-EXAFS spectra in Figure 2c exhibit a prominent
peak at 2.7 Å, which is different from the Pd–Pd characteristic
peak (2.6 Å) of Pd metal. The main peak in FT-EXAFS spectra
property. The chronoamperometry spectra at each applied poten-
tial and UV–vis absorption spectra of the electrolytes after NRR
for 2 h stained with indophenol indicator were collected to quan-
tify the produced NH on np-Pd Bi, np-PdBi , and Pd/C catalyst
3
3
2
of Bi L -edge displays a shorter bond length than that in pure
(Figures S8 and S9, Supporting Information). The average NH3
yield rate and the corresponding FE at a series of potentials of np-
Pd Bi, np-PdBi , and Pd/C catalyst were measured (Figure 3b,c).
3
Bi due to the Bi–Bi/–Pd contribution (Figure 2d). Continuous
Cauchy wavelet transform analysis with high resolution in
both k and R spaces of Bi L -edge and Pd K-edge of np-Pd Bi
3
2
Remarkably, the np-Pd Bi group achieves a highest NH yield rate
3
3
3
3
−1
−1
−1
−1
EXAFS is shown in Figure 2e. The Pd K-edge of np-Pd Bi
of 60.75 µg h mgcat (average: 59.05 ± 2.27 µg h mgcat ) and a
largest FE of 22.23% (average: 21.52 ± 0.71%) at −0.2 V versus RHE,
3
−
1
displays only one intensity maximum about 10.1 Å from
Pd–Pd/Bi contributions, which is higher than Pd. But the
main peak in Bi L -edge of np-Pd Bi is lower than Bi, which
−
1
−1
which are much higher than those of np-PdBi (6.50 µg h mgcat
,
2
−1
−1
18.56%) and Pd/C (8.70 µg h mgcat , 0.14%) with the same
3
3
matches well with the nearest Bi-Pd coordination in the cell
structure. The corresponding specific structural parameters
summarized by the fitting results are shown in Table S2 (Sup-
porting Information).
condition, even surpassing the most reported electrocatalysts
[18]
including Pd- and Bi-based (Table S3, Supporting Information).
The FEs of HER by np-Pd Bi electrode are quantitatively deter-
3
mined to be complementary with those of NRR (Figure 3d),
The NRR performance of the different catalysts in
demonstrating that np-Pd Bi catalyst has a preferable selectivity
3
0
.05 ꢀ H SO was evaluated in an H-cell separated by a proton
toward N reduction and an effective suppression of hydrogen
2
4
2
exchanged membrane (Nafion 117) at room temperature and
atmospheric pressure (Figure S7, Supporting Information).
evolution. Furthermore, only NH was obviously observed under
3
all applied potentials, and no N H by-product was detected in
2
4
Ultrahigh purity of N gas (99.999%) was continuously purged
the experiment (Figures S10 and S11, Supporting Information),
confirming a high selectivity of np-Pd Bi for N fixation toward
2
into the cathodic chamber during electrochemical NRR process.
Figure 3a shows the linear sweep voltammetry (LSV) curves of
the catalysts in Ar- and N -saturated 0.05 ꢀ H SO solution,
3
2
NH . In addition, the sample solutions catalyzed by np-Pd Bi
3
3
at the open circuit potential and in the Ar atmosphere do not
2
2
4
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show the presence of NH (Figure S12, Supporting Informa-
tion), as the case of blank glass carbon electrode, revealing that
were performed in N -saturated H SO solution with a well-
designed cell. Figure 4a,b shows the normalized operando
3
2
2
4
[
19]
the NH product is produced by the voltage-driven reduction
Pd K-edge and Bi L -edge XANES spectra at different applied
3
3
of N catalyzed by np-Pd Bi, demonstrating the intrinsic NRR
catalytic property. N (99 at% N) isotope-labeling experiments
were conducted to further confirm the N source of the obtained
potentials, respectively. With the increase of the operated bias
2
3
15
15
voltages from the open-circuit voltage to −0.05 V versus RHE,
the edges of both Pd K-edge and Bi L -edge show a downshift,
3
NH . As shown in Figure 3e, only doublet peaks corresponding
suggesting the reduction of surface oxides and a lower unoc-
cupied electronic state at the Fermi level. Furthermore, with
increase of the operated bias voltages from −0.05 to −0.1 V
versus RHE, the absorption onset of the Pd K-edge shows no
distinct change (Figure 4a), which is caused by its preferable
H absorption, resulting in less affected electronic structure
3
5
1
+
1
to NH4 were observed in the H nuclear magnetic resonance
14
+
(
NMR) spectra, while no triple signal representing NH4 was
detected in the electrolyte after NRR at −0.2 V versus RHE using
15
N as the feeding gas, definitely confirming that the produced
2
NH origins from the feeding N stream during the process of
3
2
15
[20]
NRR. Notably, the measured NH yield rate and FE and those
around Pd. Meanwhile, the intensity of the white line peaks
3
1
4
of NH determined by the NMR signals (Figures S13 and S14,
of the Bi L -edge XANES spectra (Figure 4b) shows obvious
3
3
Supporting Information) match well with the above coloration
enhancement, which is attributed to the delocalization of the
unpaired electron from Bi due to the interaction with N spe-
method, indicating that NH was entirely produced from the
3
[
21]
electroreduction of N catalyzed by the catalysts. Remarkably, the
cies during NRR. According to molecular orbital theory, the
interaction between the Bi atomic orbital and the N 2p orbital
of the same main group would lead to the formation of a NN
2
NH product is produced by the voltage-driven reduction of N2
3
catalyzed by np-Pd Bi, demonstrating the intrinsic efficient cata-
3
[
22]
lytic active sites in the intermetallic nanostructure. The stability
π antibonding orbital and a decrease of the bond order. With
of the np-Pd Bi electrode was evaluated by consecutive recycling
voltage bias, the voltage-driven electron transfer from the Bi
3
δ−
electrolysis and long-term chronoamperometry test at −0.2 V
versus RHE. As shown in Figure 3f, no obvious changes in
atom to the N 2p orbital leads to the formation of N2 adsorp-
tion species, and the electron backdonation could effectively
the NH yield rate and FE are observed during five consecutive
weaken the N N molecular bond, resulting in the activation of
2
3
[
23]
recycling electrolysis, indicating the excellent repeatability of np-
Pd Bi for electrosynthesis of N to NH at ambient conditions.
N and formation of NH species. These results are further
confirmed by the corresponding FT-EXAFS spectra, as shown
in Figure 4c,d. The FT-EXAFS spectra of Pd K-edge exhibit a
very slight radial extension since EXAFS is not sensitive to light
3
2
3
Furthermore, the NRR process by np-Pd Bi at −0.2 V versus RHE
3
shows a very stable in current response under continuous N gas
2
[
24]
steam for 35 h (Figure S15, Supporting Information). Moreover,
atoms like absorbed H (Figure 4c). Correspondingly, the FT-
no obvious variation of the NH yield rate and FE is observed
EXAFS spectra of the Bi L -edge show a significant extension
3
3
after long-term chronoamperometry test, indicating the excel-
radial extension of the Bi–Pd scattering as the applied poten-
tials become more negative (Figure 4d), which is originated
from the electron delocalization of Pd–Bi bonds to the absorbed
N species during the NRR process. This viewpoint can be con-
firmed by DFT calculations. As shown in Figure 4e, the pro-
jected density of states (PDOS) of the Pd and Bi 3d orbitals of
np-Pd Bi and the N 2p orbital of the N bonded to the catalytic
lent electrochemical durability of np-Pd Bi toward ambient NH3
3
synthesis. Significantly, the morphology and chemical compo-
sition could be well maintained after the long-term NRR elec-
trolysis, as indicated in the scanning electron microscopy (SEM)
image and XPS (Figure S16, Supporting Information), respec-
tively. Interestingly, N signal is generated on np-Pd Bi after the
3
3
2
NRR, confirming the interaction between the active electrode
and N species.
sites exhibit a significant hybridization near the Fermi level,
and the N 2p orbital shows a vast majority overlap with the
Bi 3d orbital according to the charge density difference calcu-
lation (Figure 4f). This observation confirms the role of Bi in
Considering the superior catalytic performance of np-
Pd Bi, the electrochemical double-layer capacitance (C ) of the
3
dl
catalysts was performed to investigate the specific active area.
N adsorption and further activation via electronic donation.
2
The Cdl of np-Pd Bi is obviously higher than that of np-PdBi2
According to the above analysis, we illustrate the NRR process
on the np-Pd Bi active sites in Figure 4g. The N molecule is
3
(Figure S17, Supporting Information), indicating more acces-
3
2
sible active sites constructed on np-Pd Bi catalyst. In addition,
preferentially adsorbed on Bi sites, while H is adsorbed on the
3
CO stripping test was further handled to determine the elec-
trochemical active area of the catalysts (Figure S18, Supporting
Information), confirming the higher gas accessible sites of
np-Pd Bi than np-PdBi . Furthermore, N temperature-pro-
surrounding Pd sites. Simultaneously, N is activated via elec-
2
tron delocalization from the Bi 3d orbitals to the N 2p orbitals,
followed by hydrogenation with H on the Pd sites at the applied
potentials.
3
2
2
gramed desorption of np-Pd Bi and np-PdBi was measured
DFT calculations were further performed to understand
the electrochemical NRR mechanism on the catalysts (see
Computational Section in the Supporting Information for
3
2
(Figure S19, Supporting Information). The chemical desorption
of N occurs in the same temperature range on np-Pd Bi and
2
3
[
25]
np-PdBi . The enhanced peak intensity of np-Pd Bi suggests
more details). It is known that adequate N adsorption and
2
3
2
that the larger specific surface area may promote the chem-
activation on catalyst is the essential prerequisite for an effi-
isorption ability and capability of N . Above all, we believe that
cient NRR process. Thus, N adsorption on different catalytic
2
2
the strong adsorption and activation of N contribute to the
sites on catalysts were evaluated via the theoretical adsorp-
tion energy (Figure S20, Supporting Information). As shown
2
enhanced NRR performance of np-Pd Bi.
3
To identify the origin of the intrinsic catalytic activity of
np-Pd Bi, the operando XAS measurements of the np-Pd Bi
in Figure 5a, the Bi sites are more preferable for N absorp-
2
tion than Pd sites. More importantly, the Bi sites of Pd Bi
3
3
3
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Figure 4. Operando X-ray absorption spectroscopy for the active site of NRR. a,b) Normalized operando Pd K-edge (a) and Bi L -edge (b) XANES
3
spectra for np-Pd Bi under different applied voltages versus RHE in 0.05 ꢀ H SO solution. c,d) The corresponding FT-EXAFS spectra of Pd K-edge (c)
3
2
4
and Bi L -edge (d) derived from (a) and (b), respectively. e) The PDOS for N absorbed on Pd Bi (400), and f) the charge density differences calcula-
3
2
3
−
3
tion of N bonded to Pd Bi (400) (isosurface level = 0.002 e Å ; negative charge: blue; positive charge: yellow.). g) The schematic illustration of the
NRR process on np-Pd Bi.
2
3
3
(
Bi@Pd Bi) has an efficient N adsorption energy of −0.143 eV,
(400) crystal face of Pd Bi, (311) crystal face of PdBi , and (111)
3 2
crystal face of Pd (Figure S22, Supporting Information). As
shown in Figure 5c, the formation of *NNH is a determining
3
2
while the Pd sites of Pd Bi (Pd@Pd Bi) display an inferior N
adsorption energy of −0.127 eV, suggesting more feasible N2
adsorption on Bi sites than Pd sites in Pd Bi, which is in good
3
3
2
step during the formation of the first NH molecule, which
3
3
agreement with the operando XAS analysis. Besides, the Gibbs
free energies of the HER process on the catalysts were further
calculated (Figure 5b). The free energy value is −0.893 eV for
Pd Bi catalyst, far away from the optimal value (∆G = 0 eV),
even lower than that of pure Pd catalyst, indicating that the
introduction of Bi could suppress the hydrogen evolution.
Furthermore, the PDOS calculations on the various catalytic
sites in Figure S21 (Supporting Information) show that the
Pd signal gradually shifts down away from the Fermi level as
the Bi amount increases. Since Pd sites are preferable for H
intermediate adsorption, the HER performance is well regu-
lated by Bi. The d orbital with a suitable energy level relative
to the Fermi level is regarded as a key factor for the electron
takes 0.197 and 0.878 eV for Pd Bi and Pd, respectively, indi-
3
cating that Pd Bi provides the coordination path with a lower
3
energy gap for NRR. For the formation of the second NH mole-
3
cule, the formation of *NH is the determining step for Pd Bi,
3
*H
2
3
while the desorption of *NH is the determining step for Pd,
3
confirming the different catalytic property of the two electrocat-
alysts. In addition, PdBi follows similar reaction pathway with
2
Pd Bi but higher energy gap, indicating the significance of a
3
suitable Pd/Bi ratio. The above results suggest that the NRR
process is the most favorable to occur on the Bi sites of Pd Bi
3
and the intermetallic interaction between Pd and Bi atoms of
Pd Bi not only reduces the free energy of transition state, but
3
also optimizes the free energy of H intermediate to a confined
HER catalytic activity. The excellent NRR performance ori-
interaction with N and activated N species. Significantly, the
2
modulated electronic structure of Bi–Pd contributes to higher
NRR activity and selectivity. In addition, the Gibbs free energy
of NRR transition states of the distal path are calculated on the
gins from the facilitated N adsorption and activation and the
reduced energy barrier of the rate limiting step after the opti-
mization of Pd/Bi component.
2
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Figure 5. The NRR mechanism of the catalyst during the NRR process. a) The adsorption energy of N on various catalyst active sites. b) Free energy
2
diagrams for hydrogen evolution reaction on the surface of Pd Bi, PdBi , and Pd catalysts. c) Reaction pathways of Pd Bi, PdBi , and Pd for NRR.
3
2
3
2
5
1901076), the Youth 1000 Talent Program of China, the Outstanding
In summary, we have presented that the nanoporous inter-
Youth Scientist Foundation of Hunan Province (Grant No. 2020JJ2006),
the Natural Science Foundation of Hunan Province (Grant No.
metallic Pd Bi is stable and active for the electrocatalytic reduc-
3
tion of N to NH under ambient conditions. Significantly, the
2
3
2
019JJ50051), and the Fundamental Research Funds for the Central
nanoporous intermetallic Pd Bi exhibits a high NH yield rate
3
3
Universities. The authors thank Dr. Ying-Rui Lu and Dr. Ting-Shan Chan
for the synchrotron measurement at the National Synchrotron Radiation
Research Center (NSRRC, BL01C1).
−
1
−1
of 59.05 ± 2.27 µg h mgcat and a high FE of 21.52 ± 0.71% at
0.2 V versus RHE. Operando XAS characterization and DFT
−
calculations reveal that strong interaction between the Pd–Bi
sites in the intermetallic Pd Bi induces a strong electrostatic
3
interaction, which can stabilize the transition state and lower Conflict of Interest
the barrier for efficient NRR. This work highlights the potential
The authors declare no conflict of interest.
of intermetallic compounds for catalytic NRR and provides new
insight for producing nanoporous intermetallic compounds for
energy conversion applications.
Data Availability Statement
Research data are not shared.
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Keywords
electrocatalytic nitrogen reduction, intermetallics, nanoporous alloys,
Pd Bi
3
Acknowledgements
X.W. and M.L. contributed equally to the work. This work was supported
by the Natural Science Foundation of China (Grant Nos. 51771072 and
Received: November 13, 2020
Revised: February 23, 2021
Published online: April 1, 2021
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