Low-Coordinate Step Atoms via Plasma-Assisted Calcinations to Enhance Electrochemical Reduction of Nitrogen to Ammonia

Article abstract of DOI:10.1002/smll.202000421

The electrochemical N₂ reduction reaction (NRR) is emerging as a promising alternative to the industrial Haber–Bosch process for distributed and modular production of NH₃. Nevertheless, developing high-efficiency catalysts to simultaneously realize both high activity and selectivity for the development of a sustainable NRR is very critical but extremely challenging. Here, a unique plasma-assisted strategy is developed to synthesize iridium diphosphide nanocrystals with abundant surface step atoms anchored in P,N-codoped porous carbon nanofilms (IrP₂@PNPC-NF), where the edges of the IrP₂ nanocrystals are extremely irregular, and the ultrathin PNPC-NF possesses a honeycomb-like macroporous structure. These characteristics ensure that IrP₂@PNPC-NF delivers superior NRR performance with an NH₃ yield rate of 94.0 μg h^?1 mg^?1_cat. and a faradaic efficiency (FE) of 17.8percent. Density functional theory calculations reveal that the unique NRR performance originates from the low-coordinate step atoms on the edges of IrP₂ nanocrystals, which can lower the reaction barrier to improve the NRR activity and simultaneously inhibit hydrogen evolution to achieve a high FE for NH₃ formation. More importantly, such a plasma-assisted strategy is general and can be extended to the synthesis of other high-melting-point noble-metal phosphides (OsP₂@PNPC-NF, Re₃P₄@PNPC-NF, etc.) with abundant step atoms at lower temperatures.

Full text of DOI:10.1002/smll.202000421

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Low-Coordinate Step Atoms via Plasma-Assisted
Calcinations to Enhance Electrochemical Reduction
of Nitrogen to Ammonia
Xiaohui Yang, Faling Ling, Xiangrong Zi, Yanwei Wang, Han Zhang, Huijuan Zhang,
Miao Zhou,* Zaiping Guo,* and Yu Wang*
history because of its important role in
The electrochemical N reduction reaction (NRR) is emerging as a
promising alternative to the industrial Haber–Bosch process for distributed
facilitating the production of food and
maintaining the growth of the human
2
[
1,2]
population.
Benefiting from its high
and modular production of NH . Nevertheless, developing high-efficiency
3
energy density, zero-carbon emissions,
and facile storage and transportation,
catalysts to simultaneously realize both high activity and selectivity for the
development of a sustainable NRR is very critical but extremely challenging.
Here, a unique plasma-assisted strategy is developed to synthesize iridium
diphosphide nanocrystals with abundant surface step atoms anchored in
ammonia (NH ) is also being consid-
3
ered as a high-efficiency hydrogen fuel
[
3–5]
carrier.
Relative to the traditional
P,N-codoped porous carbon nanofilms (IrP @PNPC-NF), where the edges
energy- and capital-intensive HBP, the
electrochemical N2 reduction reaction
2
of the IrP nanocrystals are extremely irregular, and the ultrathin PNPC-NF
2
(
NRR) powered by renewable electricity
possesses a honeycomb-like macroporous structure. These characteristics
occurs under ambient conditions using
ensure that IrP @PNPC-NF delivers superior NRR performance with
2
the H O and N2 as feedstock materials
2
−
1
−1
an NH yield rate of 94.0 µg h mg
and a faradaic efficiency (FE) of
cat.
and is an environment-friendly process for
3
[
6–9]
1
7.8%. Density functional theory calculations reveal that the unique NRR
distributed, modular synthesis of NH3.
Therefore, electrochemical reduction of N2
performance originates from the low-coordinate step atoms on the edges of
to NH has been considered as a poten-
3
IrP nanocrystals, which can lower the reaction barrier to improve the NRR
2
tially promising alternative to HBP and
activity and simultaneously inhibit hydrogen evolution to achieve a high FE
[10–12]
has recently received great interest.
for NH formation. More importantly, such a plasma-assisted strategy is
3
The challenge for the development of
the electrochemical NRR in practice is that
general and can be extended to the synthesis of other high-melting-point
noble-metal phosphides (OsP @PNPC-NF, Re P @PNPC-NF, etc.) with
N molecule has low polarizability and is
2
2
3 4
highly stable and inert (with strong bond
abundant step atoms at lower temperatures.
−
1
energy of 940.95 kJ mol ), which makes
it difficult to cleaved and reduce to NH by
3
[
13,14]
hydrogenation.
To this end, numerous
1
. Introduction
efforts, both theoretical and experimental, have been devoted
to the exploration of high-efficiency NRR electrocatalysts.
In response, the suggested noble metals, such as Ir, Pt, Ru,
and Rh, possess relatively optimized nitrogen binding for
Artificial synthetic ammonia synthesized by the Haber–Bosch
process (HBP) is one of the greatest inventions in human
X. H. Yang, X. R. Zi, Y. W. Wang, Dr. H. Zhang, Dr. H. J. Zhang,
Prof. Y. Wang
School of Chemistry and Chemical Engineering
State Key Laboratory of Power Transmission Equipment &
System Security and New Technology
Chongqing University
Dr. F. L. Ling
College of Sciences
Chongqing University of Posts and Telecommunications
Chongqing 400065, P. R. China
Dr. F. L. Ling, Prof. M. Zhou
Key Laboratory of Optoelectronic Technology and System of Ministry
of Education
College of Optoelectronic Engineering
Chongqing University
1
74 Shazheng Street, Shapingba District, Chongqing 400044, P. R. China
E-mail: wangy@cqu.edu.cn
X. H. Yang, Prof. Z. P. Guo
Institute for Superconducting and Electronic Materials
Australian Institute for Innovative Materials
University of Wollongong
Innovation Campus, North Wollongong, NSW 2500, Australia
E-mail: zguo@uow.edu.au
174 Shazheng Street, Shapingba District, Chongqing 400044, P. R. China
E-mail: mzhou@cqu.edu.cn
Prof. Y. Wang
School of Electrical Engineering
Chongqing University
174 Shazheng Street, Shapingba District, Chongqing 400044, P. R. China
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/smll.202000421.
DOI: 10.1002/smll.202000421
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adsorption and activation of N2 molecules, so that they are
ultrathin P,N-codoped porous carbon nanofilms (denoted as
IrP @PNPC-NF). When it served as an electrocatalyst, IrP @
[2,15]
considered as the ideal candidates.
Unfortunately, their
2
2
electrocatalytic performance is not adequately presented for
PNPC-NF showed the following advantages: 1) Low-coordinate
+
NRR so far due to the competitive adsorption of H over N on
step atoms on the edges of IrP nanocrystals will intrinsically
2
2
these metals.
enhance the NRR activity and simultaneously weaken the com-
petitive HER; 2) 3D honeycomb-like macroporous structures
will favor intimate contact between the electrode and electrolyte
To address this critical but thorny problem, stepped facets
and doping modification may be the two effective ways to
intrinsically enhance their catalytic activity.[
16–21]
On the one
and promote rapid mass/charge transports; 3) IrP nanocrystals
2
hand, stepped facets usually have low coordination number
CN) of surface atoms, where these unsaturated sites can more
evenly anchored in P,N-codoped porous carbon nanofilms can
prevent shedding, thus ensuring the stability of the catalyst.
(
favorably adsorb and activate N molecule compared with the
As expected, based on the above advantages, IrP @PNPC-NF
2
2
flat facets.[ For example, Zhang’s group reported tetrahexahe-
dral gold nanorods enclosed by stepped {730} facets that exhibit
unexpectedly electrocatalytic NRR performance, indicating low-
coordinate step atoms are the favorable active sites in optimizing
the activity of the metal-based catalysts.[23] On the other hand,
doping modification can not only tune the electronic structure
of the catalysts, resulting in a massive change of the interac-
tion between the intermediates and catalyst, but also decrease
the number of noble metals.[ In particular, phosphorus (P)
atoms adjusted to form various phosphides have been proved
to represent an effective and appropriate pattern for the opti-
22]
exhibits superior NRR catalytic performances under ambient
−
1
−1
conditions with an excellent NH yield of 94.0 µg h mg
3
cat.
at −0.2 V relative to reversible hydrogen electrode (RHE), a
high faradaic efficiency (FE) of 17.8% at −0.1 V versus RHE, and
outstanding cycling stability. Moreover, this plasma-assisted
strategy was also applied to the synthesis of other high-melting-
point metal phosphides with abundant step atoms at a lower
temperature (OsP @PNPC-NF, Re P @PNPC-NF, where Re is
2
3 4
24]
a rhenium atom, etc.).
[
25]
mization of electrocatalytic performance, including the NRR,
2
. Results and Discussion
[26]
the hydrogen evolution reaction (HER), the oxygen evolution
reaction (OER),[ lithium storage, etc. Enlightened by these
reports, both stepped facets and doping modification can be
used to regulate the catalytic behaviors of nanomaterials, and
combining them together may achieve theoretically expected
high NRR performance over the ideal candidates with relatively
optimized nitrogen binding. However, a feasible approach to
acquire noble-metal phosphides with coordinatively unsatu-
rated active sites under mild conditions is lacking but extremely
desired.
27]
[28]
The formation process for IrP @PNPC-NF is illustrated in
2
Scheme 1. Typically, a mixture of iridium trichloride, pro-
pylphosphonic anhydride, chitosan, and preprepared silica
spheres about 500 nm in the diameter (Figure S1, Supporting
Information) was first prepared via a solvent evaporation pro-
cess. Next, this mixture was treated in a tube furnace equipped
with a plasma generator, where P was in situ doped into crystal
lattices of Ir to form IrP nanocrystals and meanwhile chitosan
2
was pyrolyzed to form a carbon film tightly attached to the
surfaces of the silica spheres (Figure S2, Supporting Informa-
Herein, taking Ir as a probe of noble metals with relatively
optimized nitrogen binding, we propose a novel, facile, and
versatile plasma-assisted strategy for the synthesis of IrP2
nanocrystals with abundant step atoms evenly anchored in
tion). Then, the honeycomb-like IrP @PNPC-NF was obtained
2
by leaching the silica spheres with hydrofluoric acid (HF).
Notably, with the unique assistance of plasma, the temperature
Scheme 1. Schematic illustration of the synthesis of honeycomb-like macroporous IrP @PNPC-NF with abundant step atoms by a new strategy of
2
plasma-assisted calcination and its application for the electrochemical reduction of N to NH .
2
3
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Figure 1. Morphology and structure characterization of IrP @PNPC-NF. a) SEM image. b) TEM image, with the size distribution of the nanocrystal
2
structures in the inset. c) Aberration-corrected (AC)-STEM image. d) Magnified AC-STEM image based on image (c), showing a high density of surface
steps. Corresponding high-index facets of {202}, {212}, and {313} are marked based on the simulated STEM image along the 〈101〉 zone axis. e) HAADF-
STEM image with a purple line showing the line scanning path. HAADF-STEM-EDX elemental mapping images showing the element distributions of
f) Ir and P, g) Ir, h) P, i) C, and j) N based on image (e); scale bars: 5 nm. k) Compositional line-scanning profiles of the line marked by the purple
line in (e).
for the formation of IrP was reduced compared with previous
(Figure 1c). The irregular nature of the nanocrystals was further
confirmed, and the lattice fringe spacings of 3.69, 2.89, 2.67,
and 1.98 Å are obvious, corresponding to the (111), (020), (200),
2
reports, and more interestingly the edges of obtained IrP2
nanocrystals presented abundant step atoms, which were
expected to be highly efficient NRR active sites.[ Moreover,
the introduction of silica spheres can effectively protect the IrP2
nanocrystals from agglomeration during the pyrolysis process
due to the confinement effect.
24]
and (211) facets of monoclinic IrP (JCPDS Card No. 89-3985).
2
Figure 1d shows a magnified aberration-corrected (AC)-STEM
image, where the crystal orientation of the structure is calcu-
lated to lie along the 〈101〉 zone axis, based on (111) and (020)
facets. By combining the theory of crystal structure analysis and
simulation of the HAADF-STEM image (see detail in Figure S3
in the Supporting Information), it can be accurately deduced
that a large density of high-index facets, such as {202}, {212},
and {313}, frequently emerge around the irregular surfaces
of IrP2 nanocrystals, strongly verifying the existence of low-
Figure 1 shows the morphology and structure of IrP @
2
PNPC-NF as captured by scanning electron microscopy (SEM),
transmission electron microscopy (TEM), and high-angle
annular dark-field scanning transmission electron micros-
copy (HAADF-STEM). Obviously, after removing the silica
spheres, IrP @PNPC-NF presents a honeycomb-like macropo-
2
[
16,20,30]
rous structure, consisting of interconnected 3D spherical
voids and PNPC-NF as the supporting framework (Figure 1a).
It is noteworthy that the porous structure could facilitate
mass transport and the accessibility of active sites in the
electrocatalytic process.[ From the TEM image in Figure 1b,
we can see that PNPC-NF is decorated with plenty of small
nanocrystals, which are obviously irregular, indicating the
exposure of abundant low-coordinate unsaturated step atoms,
and the inset image shows that their size is about 5 nm. In
order to investigate the character of these nanocrystal struc-
tures, spherical-aberration-corrected STEM was carried out
coordinate atomic steps.
To corroborate the distributions
of the Ir, P, C, and N elements in IrP @PNPC-NF, mapping
2
analysis and line scans were performed in scanning transmis-
sion electron microscopy energy-dispersive X-ray spectroscopy
(STEM-EDX) (Figure 1e). As shown in Figure 1f–j, it is clear
that the C and N elements are homogeneously distributed
over the whole selected area, while Ir and P are mainly concen-
trated in the bright portion shown in Figure 1e, thus further
29]
implying the formation of IrP nanocrystals. Note here that a
2
small part of the P element also covers the whole selected area,
indicating that some of the P atoms have probably been doped
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into the ultrathin carbon film. The line-scan profiles along the
deconvoluted into four species of carbon states, and specifi-
cally, the peaks at 284.5, 285.2, 285.7, and 286.6 eV are assigned
to the binding energies (BEs) of CC, CN/CP, CO, and
purple line in Figure 1e confirm that the IrP nanocrystals were
2
formed and anchored in the ultrathin PNPC-NF (Figure 1k).
X-ray powder diffraction (XRD), Raman spectroscopy, and
X-ray photoelectron spectroscopy (XPS) were further performed
to study the crystal phases and detailed surface chemical states.
As shown in Figure 2a, all the diffraction peaks are well-indexed
[
31,32]
NCO, respectively (Figure 2c).
The XPS spectrum of Ir
4f presents two peaks at 61.2 eV for Ir 4f and at 64.2 eV for Ir
7
/2
[
24]
4f5/2 (Figure 2d). With regard to the P 2p spectrum, the peaks
at 129.1 and 129.9, 131.7, and 133.3 eV are consistent with IrP,
[
33]
to monoclinic IrP in JCPDS Card No. 89-3985, in good agree-
PC, and PO bonds, respectively (Figure 2e). The peak at
400.5 eV for the N 1s spectrum was derived from graphitic N,
and the pyrrolic N and pyridinic N correspond to the 399.5 and
2
ment with the characteristic planes in the AC-STEM image. The
Raman spectrum of the carbon component in Figure 2b shows
−
1
[34,35]
two typical characteristic peaks, the D-band at about 1354 cm
398.2 eV peaks, respectively (Figure 2f).
−1
and the G-band at about 1585 cm , in IrP @PNPC-NF and the
Based on the above-mentioned thorough analysis and char-
2
intensity ratio of the peaks (I /I ) is 1.11. The XPS full scan
acterization, the NRR performance of IrP @PNPC-NF was
D
G
2
spectrum in Figure S4 in the Supporting Information confirms
measured to evaluate its electrocatalytic N2 reduction ability
using a typical two-compartment cell separated by a proton
exchange membrane. Before the NRR tests, the safety of the
test environment was first performed by the blank experi-
that IrP @PNPC-NF is composed of Ir, P, C, N, and O, sug-
2
gesting that the silica spheres were smoothly removed during
the leaching process. Figure 2c–f presents the high-resolution
XPS spectra of IrP @PNPC-NF. The C 1s spectrum can be
ments, including the fresh 0.05 ꢀ H SO4 solution and the
2
2
Figure 2. a) XRD pattern of IrP @PNPC-NF. b) Raman spectrum of IrP @PNPC-NF. High-resolution XPS spectra of c) C 1s, d) Ir 4f, e) P 2p, and
2
2
f) N 1s for IrP @PNPC-NF.
2
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Figure 3. Electrocatalytic NRR performance of IrP @PNPC-NF. a) Linear sweep voltammetry curves in Ar- and N -saturated 0.05 ꢀ H SO solution.
2
2
2
4
b) The average NH yields and faradaic efficiencies (FEs) at each given potential. c) Comparison of the performance with the recently well-developed
3
NRR electrocatalysts in 0.1 ꢀ acid solution at room temperature and under atmosphere pressure. d) Long-term electrocatalytic test in N -saturated
2
0
.05 ꢀ H SO solution at a potential of −0.2 V versus RHE. Inset shows eight consecutive cycling tests. e) Comparison with NH yields of common
2
4
3
catalysts at −0.2 V versus RHE. f) ECSA-normalized NH yields of IrP @PNPC-NF and IrP @PNC-NP at each given potential.
3
2
2
stale 0.05 ꢀ H SO electrolyte treated by N gas bubbling for
1
no absorption peaks are detected at a wavelength of 655 nm,
indicating that there are no possible adventitious contamina-
performed in Ar- and N -saturated 0.05 ꢀ H SO solution.
2 2 4
2
4
2
h.[ As shown in Figure S7 in the Supporting Information,
36]
A higher reduction current density is observed under the N2
atmosphere than under the Ar atmosphere (Figure 3a), sug-
gesting an additional contribution from the electrocatalytic
tions from the water medium and the high purity feeding N
reduction of N2 to NH . Chronoamperometry tests at each
given potential were then carried out (Figure S8, Supporting
Information). The NH3 production rates and corresponding
2
3
gas, respectively.[
37,38]
To preliminarily probe the NRR activity
of IrP @PNPC-NF, the linear sweep voltammetry (LSV) was
2
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FEs were assessed based on the ultraviolet–visible (UV–vis)
absorption spectra of the obtained electrolytes colored with
the indophenol indicator (Figure S9, Supporting Information).
that IrP @PNC-NP achieves the highest NH yield at −0.2 V
versus RHE. Their electrochemical active surface areas (ECSAs)
were then determined by estimating their electrochemical
2
3
As shown in Figure 3b, the NH yield first increases and then
double-layer capacitances (C ) (Figure S18, Supporting Infor-
3
dl
decreases, as the potential shifts from −0.1 to −0.5 V versus
mation). Here, we quote the concept of the roughness factors
(RFs) reported by previous papers, where the inherent activity
can be obtained by normalizing the ECSA surface area.
−1
−1
RHE. A maximum NH yield of 94.0 µg h mg
is achieved
3
cat.
[
36,39–41]
at −0.2 V versus RHE, and no byproduct N H is detected at
2
4
each given potential (Figure S10, Supporting Information). The
Supposing that IrP @PNC-NP has the RF value of 1, the RF of
2
highest FE value of 17.8% for NH production, by contrast, is
IrP @PNPC-NF can be calculated in the light of C processed
3
2
dl
reached at −0.1 V versus RHE. When the potential becomes
by cyclic voltammetry (CV) data (see the Experimental Section
more negative, the FE decreases dramatically due to the pro-
in detail). As shown in Figure 3f, the normalized NH yield of
3
−
1
−1
motion of HER activity. To the best of our knowledge, IrP @
IrP @PNPC-NF (46.3 µg h mg ) still has an about threefold
2
2
PNPC-NF with such outstanding electrocatalytic performance
is one of the best advanced NRR electrocatalysts recently
reported to work in the acidic electrolytes under ambient condi-
tions (Figure 3c; Table S1, Supporting Information). To verify
improvement compared with that of its counterpart, indicating
an increase of the intrinsic activity.
To decode the unique NRR performance for IrP nanocrys-
2
tals with high-index facets, we conducted density functional
theory (DFT) calculations for the free energetics of the NRR
1
5
the N source of the obtained NH , N isotopic labeling experi-
3
1
ments combined with H nuclear magnetic resonance (NMR)
technique were further carried out.
as the feeding gas, H NMR spectrum for the electrolyte after
and HER steps on the (111) and (313) facets of IrP as repre-
2
[6,9,18]
15
When N is chosen
sentative low- and high-index crystal planes. We first inves-
2
1
+
tigated the adsorption energies of N and H on the different
2
the electrocatalytic process at −0.2 V versus RHE shows two
facets. As shown in Figure S19 in the Supporting Informa-
tion, the H adsorption energy on the (111) and (313) facets is
1
5
+
+
resonance peaks, which nicely match the standard NH4 with
a doublet coupling (Figure S11, Supporting Information). This
result further confirms that the synthesized NH3 is derived
from the N electroreduction over IrP @PNPC-NF.
−0.44 and −0.55 eV, respectively, while the N adsorption energy
2
of the (313) facet is −0.63 eV, indicating that N molecules are
2
+
more easily adsorbed on the (313) surface compared with H .
2
2
The stability as a key criterion in estimating the value of a
catalyst was investigated by the long-term electrocatalytic test
and consecutive cycling tests at −0.2 V versus RHE. For the
consecutive eight cycles of electrolysis, no obvious changes
were observed in the current densities (Figure 3d) or in the
Note here that for the IrP (111), the surface is covered by the
P atoms (Figure S20, Supporting Information), which leads to
2
nonenergetic adsorption of the N molecules. In contrast, the
2
(313) surface of IrP was alternately terminated with P and Ir
2
atoms and plane steps appear, which provide potential sites
NH yield rates and FEs (Figure S12, Supporting Information).
for activating the N N bond of the N molecule (Figure S21,
3
2
The long-term electrocatalytic test results showed negligible
current density decay over 50 h (Figure 3d). Both indicate that
Supporting Information). These results indicate that the HER
is dominant for the (111) facet (Figure 4a) and the (313) facet
is favorable for NRR (Figure 4b). Then, we inspected all the
sites on the (313) stepped surface and found that the exposed
Ir atoms at the step are in the most advantageous position for
IrP @PNPC-NF possesses excellent chemical stability. On the
2
other hand, the SEM and STEM images collected after the catal-
ysis (Figure S13, Supporting Information) confirm the mor-
phological stability of IrP @PNPC-NF, while the XRD pattern
the adsorption of N molecules. In order to explore the possible
2
2
and XPS spectra (Figure S14, Supporting Information) also
demonstrate that there are no significant changes in the crystal
NRR mechanisms, a series of structural configurations of each
potential intermediate state over the (313) surface were estab-
lished and optimized toward the favorable electroreduction of
N to NH . The optimized results demonstrate that the NRR
structure or the surface chemical states, indicating that IrP @
2
PNPC-NF is extremely robust. The reason for the high stability
2
3
of IrP @PNPC-NF is that the unique structure guarantees that
follows the associative mechanism along with the alternating
pathway (Figure 4c). The free-energy changes (ΔG, eV) of each
elementary reaction is discussed in Figure S22 and Table S2 in
the Supporting Information, where the rate-determining step
(RDS) during the whole NRR is the first protonation process to
2
the active species, the IrP nanocrystals, are solidly anchored
2
in ultrathin PNPC-NF and effectively prevents their shedding
during the NRR electrocatalysis.
To clarify the cause of the superior catalytic performance
+
−
for IrP @PNPC-NF, comparable materials, including
form *NHN (*N + H + e → *NHN), in line with the previous
2
2
[
2,14,23]
IrP @PNC-NP without porous structure, pure IrP -NP, and
reports.
Notably, the change in the free-energy value (ΔG)
2
2
substrate PNPC, were prepared (Figures S15 and S16, Sup-
for the RDS is only 0.58 eV, which is much lower than those of
most reported advanced NRR electrocatalysts. Such a low reac-
tion barrier indicates that the electroreduction of N to NH
porting Information), and their N electroreduction capability
2
was measured at a potential of −0.2 V versus RHE. The NH
3
2
3
−1
−1
yield over IrP @PNPC-NF (94.0 µg h mg
) is much higher
over the high-index facet is thermodynamically more favorable.
In addition, to consider the competing HER during the NRR
process, we further studied the HER performance on (111) and
2
cat.
1
−
−1
than those over IrP @PNC-NP (16.6 µg h mg
), IrP -NP
2
cat.
−1
2
−
1
−1
−1
(
3.7 µg h mg cat.), and substrate PNC (0.94 µg h mg cat.),
indicating that IrP @PNPC-NF has the best electroreduc-
(313) surfaces of IrP . As shown in Figure S23 and Table S3 in
2
2
tion capability of converting N to NH (Figure 3e). To further
the Supporting Information, we can see that the free energy of
H on the (111) surface is −0.20 eV, much closer to zero than
that on the (313) surface (−0.31 eV), indicating that the occur-
rence of the HER on the high-index surface needs to overcome
2
3
attest the superiority of IrP @PNPC-NF, a deep comparison
2
via the similar of IrP @PNPC-NF and IrP @PNC-NP was
2
2
conducted. Figure S17 in the Supporting Information shows
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Figure 4. Diffusion adsorption of nitrogen molecules and protons on the surfaces of a) a low-index facet of IrP (111) and b) a high-index facet of IrP2
2
(
313). c) NRR mechanism over the high-index facet of IrP (313). d) Comparison of the absolute value of ΔG
(the rate-determining step) for the
2
RDS
NRR and HER on the different facets.
a larger reaction barrier than on the low-index surface. To
understand the superiority of the high-index facet for the NRR,
we investigated the absolute values of ΔGRDS (|ΔGRDS|) for the
NRR and HER on the different facets (Figure 4d). In theory, the
smaller the |ΔGRDS| is, the easier the reaction should be. Inter-
estingly, the |ΔGRDS| value of the NRR on the (313) surface is
close to that of its HER, which is larger than that of the HER
on the (111) surface. This result indicates that on the high-index
surface, the NRR is favorable and meanwhile the HER is inhib-
ited compared with the low-index surface. Therefore, we can
reasonably conclude that the root cause of the superior NRR
Card No. 34-0334) (Figure S24, Supporting Information) and
Re3P4 (JCPDS Card No. 89-2559) (Figure S25, Supporting
Information). Although the obtained phosphides are not all
diphosphides, they still present a honeycomb-like macroporous
structure like IrP @PNPC-NF (Figures S26 and S27, Supporting
2
Information). The mapping analysis further confirms the
synthesis of OsP @PNPC-NF and Re P @PNPC-NF. It is inter-
2
3 4
esting to note that the surfaces of these nanocrystals are equally
irregular, indicating the existence of abundant step atoms on the
edges. Therefore, the unique plasma-assisted method is quite
versatile and provides an opportunity to develop other phosphide
functional materials, including other high or low-melting-point
metals and/or alloys. Further work to synthesize other transition
metal compounds, such as sulfides, carbides, nitrides, etc., is
needed to determine the generality of the synthetic method.
performance of IrP @PNPC-NF is the high-index facets in the
2
IrP2 nanocrystals, where low-coordinate step atoms can not
only enhance the NH yield rate, while also improve the FE of
3
NH formation during the NRR process.
3
When the Ir precursor was replaced with those of other
high-melting-point noble metals (such as Os and Re), the cor-
responding phosphides were also successfully obtained using
the unique plasma-assisted method. The typical XRD results
3
. Conclusion
In summary, we have developed a superior NRR electrocata-
lyst with abundant step atoms on IrP2 nanocrystals evenly
show that the diffraction peaks are consistent with OsP (JCPDS
2
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anchored in ultrathin PNPC-NF, with a honeycomb-like
porous structure achieved via a plasma-assisted calcination
the upstream side of the furnace. The weight ratio of IrO2 to red
phosphorous was 1:3. Subsequently, the samples were heated at 980 °C
for 200 min in an argon atmosphere.
strategy. Impressively, IrP @PNPC-NF achieves an ultrahigh
2
Preparation of OsP @PNPC-NF and Re P @PNPC-NF: OsP @
−1
−1
2
3
4
2
NH yield of 94.0 µg h mg
(at −0.2 V vs RHE) and an
3
cat.
PNPC-NF and Re P @PNPC-NF were prepared following the same
3
4
excellent FE of 17.8% (at −0.1 V vs RHE), and it is one of the
most highly advanced NRR electrocatalysts recently reported
in acidic electrolyte under ambient conditions. Moreover, this
catalyst also exhibits outstanding stability without significant
loss of activity after eight consecutive cycles of testing and a
long-term electrocatalytic test lasting for 50 h. The combined
experimental and computational results demonstrate that the
procedure as for IrP @PNPC-NF, only replacing IrCl ·xH O with
2
3
2
OsCl ·xH O and NH ReO , respectively.
3
2
4
4
Physical Characterization: The morphology and structure of the
samples were determined by field emission scanning electron
microscopy (FESEM, JEOL JSM-7800F, 15 kV), TEM (FEI, Talos F200S,
2
00 kV) coupled with energy dispersive X-ray (EDX) analysis, HAADF-
STEM (FEI Titan Cubed Themis G2 300, 200 kV), and Brunauer–
Emmett–Teller surface area measurements (BET, Quantachrome
Autosorb-2MP surface area and pore size analyzer). XRD patterns were
collected on a Bruker D8 Advance diffractometer with Cu Kα radiation
(λ = 1.5418 Å). Raman measurements were performed with LabRAM
HR Evolution micro-Raman spectrometer from Jobin Yvon. XPS was
conducted using an ESCALAB 250Xi from Thermo Scientific using
monochromatic Al Kα radiation (225 W, 15 mA, 15 kV). The absorbance
data for the spectrophotometer were collected by UV-3600 UV–vis
spectrophotometer from SHIMADZU.
low-coordinate step atoms on the edges of IrP @PNPC-NF are
2
the key to enhancing the NRR catalytic performance, which can
simultaneously facilitate the electroreduction of N to NH by
2
3
lowering the free energy barrier of RDS and improve the FE
of NH formation by hindering the competitive HER. Such a
3
unique plasma-assisted strategy is also appropriate to synthe-
size other high-melting-point noble-metal phosphides with
abundant step atoms, including OsP @PNPC-NF, Re P @
2
3
4
Electrochemical Measurements: All electrochemical measurements
were undertaken at room temperature using a typical H-cell arrangement
separated by a Nafion-1035 proton exchange membrane with a typical
three-electrode system. Carbon paper coated with the as-obtained
catalysts as the working electrode and Ag/AgCl as the reference
electrode were placed in the cathode compartment, while Pt foil as
the counter electrode was fixed in the anode compartment. During the
progress of the experiment, high-purity N2 was continuously fed into
PNPC-NF, etc., at a lower temperature. We believe that the
construction of low-coordinate step atoms on the surfaces of
nanocrystals for enhanced electrocatalytic NRR performance
is also easily extended to other challenging electrocatalytic
reactions for renewable energy conversion.
the cathode compartment, in which the N reacted with protons of the
2
electrode on the catalyst surface to form NH . Before the NRR tests, the
3
4
. Experimental Section
Nafion-1035 membrane was first boiled to remove organic impurities in
3% H2O2 solution for 30 min and then protonated by boiling in 0.05 ꢀ
H2SO4 for 1 h. After rinsing with deionized water several times, the
prepared Nafion membrane was stored in deionized water for later use.
For working electrode preparation, 5 mg of the as-obtained samples
were added respectively into the mixed solution containing 950 µL of
ethanol and 50 µL of Nafion (5 wt%), and sonicated for 30 min to form
a homogeneous ink. Then, 80 µL of the ink was loaded onto a carbon
Reagents: IrCl ·xH O, propylphosphonic anhydride solution (50 wt%
3
2
in ethyl acetate), iridium oxide (IrO ), OsCl ·xH O, and NH ReO were
2
3
2
4
4
purchased from Aladdin Reagents Ltd. Chitosan, tetraethyl orthosilicate
TEOS), HF, ammonium hydroxide (NH ·H O), salicylic acid,
(
3
2
sodium nitroferricyanide(III) dihydrate (C FeN Na O·2H O), sodium
5
6
2
2
hypochlorite (NaClO), 4-dimethylaminobenzaldehyde (C H NO),
9
11
sodium hydroxide (NaOH), and ethyl alcohol were supplied by Chengdu
Kelong Chemical Ltd. All chemicals were used without any purification
in this work.
2
paper electrode with an area of 1 × 1 cm and dried under ambient
−1
condition (loading: 0.4 mg cm ). The electrochemical experiments were
carried out on an electrochemical workstation (CHI 660D). All potentials
reported in this work were converted to the RHE reference scale via
calibration by the following equation: E (vs RHE) = E (vs Ag/AgCl) +
0.21 + 0.0591 × pH (in 3 ꢀ aqueous KCl corresponding to the standard
potential of Ag/AgCl, E°Ag/AgCl = 0.21 V). The presented current density
was normalized to the geometric surface area. Before the measurements,
N2 (99.999%) was purged into the electrolyte for at least 30 min to
remove residual air from the cathode compartment. The potentiostatic
tests were conducted at different potentials, including −0.1, −0.2, −0.3,
−0.4, and −0.5 V versus RHE, in N2-saturated 0.05 ꢀ H2SO4 solution
for 1 h. Linear sweep voltammetry was conducted at a scan rate of
Preparation of SiO Spheres: SiO spheres about 500 nm in diameter
2
2
[
42,43]
were prepared by the classical Stöber method.
ethanol, 25 mL of deionized (DI) water, and 10 mL of 25% NH ·H O
were mixed and stirred at room temperature. Then, 5 mL of TEOS was
quickly added into the mixture. After powerful stirring for 2 h, the SiO2
spheres were isolated by centrifugation. Finally, the white precipitate was
washed with ethanol five times and dried at 60 °C.
60 mL of absolute
3
2
Preparation of IrP @PNPC-NF: The synthesis of iridium diphosphide
2
nanocrystals anchored in P,N-codoped porous carbon nanofilms
[
34,44]
(
IrP @PNPC-NF) was based on previous reports
with appropriate
2
modification. The above SiO spheres (200 mg) were evenly distributed
2
−1
in DI water (5 mL) at room temperature, followed by the addition of
chitosan, propylphosphonic anhydride, and IrCl ·xH O. The mixture
was stirred and evaporated at 100 °C to form a homogeneous powder.
Then, the obtained solids were annealed in Ar atmosphere at 650 °C for
100 mV s . After the electrochemical reduction, the electrolyte was
analyzed by chromogenic reactions for qualitative measurements.
Quantification of Ammonia: The amount of produced NH3 was
3
2
[45,46]
spectrophotometrically detected by the indophenol blue method.
As
2
00 min, accompanied by Ar plasma with power of 100 W. The sample
shown in Figure S5 in the Supporting Information, the standard curve
was constructed by measuring a series of absorbances for reference
was collected and leached in hydrofluoric acid to remove the SiO2
spheres, followed by washing several times with DI water and ethanol
and drying at 60 °C.
+
1
solutions with different NH4 concentrations (0, 0.02, 0.04, 0.08 , 0.16,
−1
0.32, 0.64, 1.28, and 2.56 µg mL ) in 0.05 ꢀ H2SO4 solution. The
background was corrected with a blank solution. In detail, 2 mL of the
electrolyte from the cathode compartment after the chronoamperometry
measurement was placed in the colorimetric tube, followed by the
addition of the 2 mL of 1 ꢀ NaOH solution containing 5% salicylic acid
and 5% sodium citrate. Then, 1 mL of 0.05 ꢀ NaClO and 0.2 mL of 1%
C5FeN6Na2O·2H2O were successively added into the above solution.
After standing in the dark for 2 h, the absorption spectrum was measured
using an UV–vis spectrophotometer at a wavelength of 655 nm.
Preparation of IrP @PNC-NP: IrP @PNC-NP, where PNC is P,N
2
2
codoped carbon, was prepared following the same procedure as for
IrP @PNPC-NF without the SiO spheres.
Preparation of PNPC: PNPC was also prepared following the same
procedure as for IrP @PNPC-NF without IrCl ·xH O.
2
2
2
3
2
Preparation of Pure IrP -Nanoparticles (NPs): In order to synthesize
2
IrP -NPs, commercial IrO and red phosphorus were placed in two
separate positions in a graphite boat with the red phosphorus on
2
2
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Isotope Labeling Experiments: In order to identify the source of N in the
where ΔE, ΔEZPE, and ΔS are the changes in the reaction energy, zero-
point energy, and entropy during the reaction. These values of each
adsorbed state were obtained from DFT calculations, whereas those of
the gas molecules were from standard tables.
NH produced from electrochemical NRR, isotope labeling experiments
3
1
15
combined with the H NMR technology were carried out using N2 as
the feed gas. After the electrolytic test at −0.2V versus RHE for 24 h, the
obtained electrolyte was concentrated to 0.5 mL and mixed with 0.5 mL
of dimethyl sulfoxide (DMSO). Then, the mixture solution was evaluated
1
by using the H NMR spectroscopy.
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Quantification of Hydrazine: The concentration of a possible byproduct
[2,47,48]
N H was examined by the method of Watt and Chrisp.
Similarly,
2
4
the standard curve was constructed by measuring a series of absorbances
for the reference solutions with different N H concentrations (0, 0.2,
2
4
−
1
0
.4, 0.6, 0.8, 1.0, and 2.0 µg mL ) in 0.05 ꢀ H SO solution (Figure S6,
2 4
Supporting Information). The color reagent was prepared by mixing
.99 g C H NO, 30 mL HCl, and 300 mL ethanol. Typically, 2 mL of the
electrolyte from the cathode compartment after the chronoamperometry
measurement was placed in the colorimetric tube and mixed with
5
9
11
Acknowledgements
X.H.Y. and F.L.L. contributed equally to this work. This work was
financially supported by the Fundamental Research Funds for the
Central Universities (0301005202017, 2018CDQYFXCS0017, and
106112017CDJXSYY0001), the Thousand Young Talents Program of
the Chinese Central Government (Grant No. 0220002102003), the
National Natural Science Foundation of China (NSFC, Grant Nos.
U19A20100, 21971027, 21373280, and 21403019), Beijing National
Laboratory for Molecular Sciences (BNLMS) and Hundred Talents
Program at Chongqing University (Grant No. 0903005203205),
and the State Key Laboratory of Mechanical Transmissions Project
(SKLMT-ZZKT-2017M11).
5
mL color reagent. After stirring for 20 min at room temperature, the
absorbance of the resulting solution was measured using an UV–vis
spectrophotometer at a wavelength of 458 nm.
Electrochemical Surface Area Measurement: The ECSAs of IrP @
2
PNPC-NF and IrP @PNC-NP catalysts were measured by estimating
their electrochemical double-layer capacitances (C ). Here, a potential
window of −0.05 to 0.05
processes occur, was selected to test the cyclic voltammetry at various
rates (20, 40, 60, 80, 100, 120, 140, and 160 mV s ). As shown in
Figure S18 in the Supporting Information, the Cdl value is the slope
of the linear regression, which is fitted by plotting the current density
variation (Δj = (j − j )/2, at 0 V vs Ag/AgCl) against scan rate. Based on
the previous reports,
by dividing its capacitance by that of IrP @PNC-NP (supposing an RF
2
dl
V versus Ag/AgCl, where non-faradaic
−
1
a
c
[
36,41,49]
RF of IrP @PNPC-NF was further calculated
2
Conflict of Interest
The authors declare no conflict of interest.
2
value of 1 for IrP @PNC-NP).
2
Calculation of the Yield Rate and the faradaic Efficiency: The NH yield
3
rate (νNH3) was calculated using the following equation
νNH3
=
(
cNH3 × V
)
/
(
t × mcat.
)
(1)
Keywords
The FE was estimated from the ratio of the charge consumed for NH3
generation to the total charge passed through the electrode
iridium diphosphide, nanocrystals, nitrogen reduction, plasma-assisted
calcinations, step atoms
FE =
(
3×F × cNH3 ×V
)
/
(
17 ×Q
)
(2)
Received: January 21, 2020
Revised: February 18, 2020
Published online:
where cNH3 is the measured NH mass concentration; V is the volume
of the electrolyte from the cathode compartment; t is the reduction
time (1 h); m is the catalyst loading mass; F is the Faraday constant
3
cat.
−
1
(96 485 C mol ); and Q is the total charge passed through the electrode.
The normalized NH yield rate can be obtained by the equation
3
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