Unveiling the Electrooxidation of Urea: Intramolecular Coupling of the N?N Bond

Article abstract of DOI:10.1002/anie.202015773

The nitrogenous nucleophile electrooxidation reaction (NOR) plays a vital role in the degradation and transformation of available nitrogen. Focusing on the NOR mediated by the β-Ni(OH)₂ electrode, we decipher the transformation mechanism of the nitrogenous nucleophile. For the two-step NOR, proton-coupled electron transfer (PCET) is the bridge between electrocatalytic dehydrogenation from β-Ni(OH)₂ to β-Ni(OH)O, and the spontaneous nucleophile dehydrogenative oxidation reaction. This theory can give a good explanation for hydrazine and primary amine oxidation reactions, but is insufficient for the urea oxidation reaction (UOR). Through operando tracing of bond rupture and formation processes during the UOR, as well as theoretical calculations, we propose a possible UOR mechanism whereby intramolecular coupling of the N?N bond, accompanied by PCET, hydration and rearrangement processes, results in high performance and ca. 100 % N₂ selectivity. These discoveries clarify the evolution of nitrogenous molecules during the NOR, and they elucidate fundamental aspects of electrocatalysis involving nitrogen-containing species.

Full text of DOI:10.1002/anie.202015773

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How to cite: Angew. Chem. Int. Ed. 2021, 60, 7297–7307
International Edition: doi.org/10.1002/anie.202015773
German Edition: doi.org/10.1002/ange.202015773
Electroorganic Synthesis Very Important Paper
Unveiling the Electrooxidation of Urea: Intramolecular Coupling of
À
the N N Bond
+
+
Wei Chen , Leitao Xu , Xiaorong Zhu, Yu-Cheng Huang, Wang Zhou, Dongdong Wang,
Yangyang Zhou, Shiqian Du, Qiling Li, Chao Xie, Li Tao, Chung-Li Dong, Jilei Liu,
Yanyong Wang, Ru Chen, Hui Su, Chen Chen,* Yuqin Zou,* Yafei Li, Qinghua Liu, and
Shuangyin Wang*
Abstract: The nitrogenous nucleophile electrooxidation reac-
tion (NOR) plays a vital role in the degradation and trans-
formation of available nitrogen. Focusing on the NOR
nitrogen cycle, accompanied by severe environmental prob-
lems such as eutrophication and acidification of water bodies
and emissions of air pollutants, should also be taken into
[3]
mediated by the b-Ni(OH) electrode, we decipher the trans-
account. Both the production and the usage of available
nitrogen are significant. Meanwhile, the degradation of
excessive available nitrogen is imperative for the nitrogen
balance. Until now, traditional thermocatalytic reactions, such
as the Haber–Bosch process, urea and nitric acid synthesis,
2
formation mechanism of the nitrogenous nucleophile. For the
two-step NOR, proton-coupled electron transfer (PCET) is the
bridge between electrocatalytic dehydrogenation from
b-Ni(OH) to b-Ni(OH)O, and the spontaneous nucleophile
2
[4]
dehydrogenative oxidation reaction. This theory can give
a good explanation for hydrazine and primary amine oxida-
tion reactions, but is insufficient for the urea oxidation reaction
dominate the artificial nitrogen cycle. By comparison,
electrochemical reactions can be carried out at ambient
temperature and pressure; hence, nitrogen cycle electro-
catalysis, including nitrogen fixation, and degradation and
transformation of available nitrogen, has attracted consider-
(
UOR). Through operando tracing of bond rupture and
formation processes during the UOR, as well as theoretical
calculations, we propose a possible UOR mechanism whereby
intramolecular coupling of the NÀN bond, accompanied by
[1a,5]
able attention in recent years.
The nucleophile electro-
oxidation reaction (NOR) over nickel-based electrocatalysts,
with nitrogenous nucleophiles such as urea, ammonia, and
amines as substrate, is an important part of the nitrogen cycle
electrocatalysis, especially for the degradation and trans-
PCET, hydration and rearrangement processes, results in high
performance and ca. 100% N selectivity. These discoveries
2
clarify the evolution of nitrogenous molecules during the NOR,
and they elucidate fundamental aspects of electrocatalysis
involving nitrogen-containing species.
[
6]
formation of available nitrogen. The NOR—instead of the
oxygen evolution reaction (OER) with sluggish kinetics—can
couple with the hydrogen evolution reaction (HER) to realize
[
7]
Introduction
energy-saving hydrogen production. The urea oxidation
reaction is a modern urea-based energy conversion technol-
ogy, and it includes electrocatalytic and photoelectrochemical
urea splitting for hydrogen production and direct urea fuel
[1]
Nitrogen is an essential component for all living bionts.
Only a fraction of atmospheric nitrogen gas is utilized
naturally by lightning and dinitrogenase-containing organ-
isms; therefore, development of artificial nitrogen fixation
[
8]
cells.
Selectivity is crucial for the electrooxidation of nitrogen-
containing compounds, which is closely related to the
substrate molecular structure. The NOR product is tightly
[
2]
methods is needed. The explosive growth of available
nitrogen (nitrogen with nonzero state) and the unbalanced
[
+]
[+]
[
*] W. Chen, L. Xu, D. Wang, Y. Zhou, S. Du, Q. Li, C. Xie, L. Tao,
Y. Wang, R. Chen, C. Chen, Y. Zou, Prof. S. Wang
State Key Laboratory of Chem/Bio-Sensing and Chemometrics,
College of Chemistry and Chemical Engineering, Hunan University
Changsha, Hunan, 410082 (P. R. China)
W. Zhou, J. Liu
College of Materials Science and Engineering, Hunan University
Changsha, Hunan, 410082 (P. R. China)
H. Su, Q. Liu
National Synchrotron Radiation Laboratory, University of Science and
Technology of China
Hefei, Anhui, 230029 (P. R. China)
E-mail: 1498710471@qq.com
yuqin_zou@hnu.edu.cn
shuangyinwang@hnu.edu.cn
Prof. S. Wang
X. Zhu, Prof. Y. Li
The National Supercomputing Center in Changsha, Hunan University
Jiangsu Collaborative Innovation Centre of Biomedical Functional
Materials, Jiangsu Key Laboratory of New Power Batteries, School of
Chemistry and Materials Science, Nanjing Normal University
Nanjing, Jiangsu, 210023 (P. R. China)
Changsha, Hunan, 410082 (P. R. China)
+
[
] These authors contributed equally to this work.
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.202015773.
Y. Huang, Prof. C. Dong
Research Center for X-ray Science & Department of Physics,
Tamkang University
1
51 Yingzhuan Rd., New Taipei City, 25137 (Taiwan)
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related to the substrate molecular structure. For the electro-
after 1.55, 1.65, and 1.75 V, respectively, according to the ring
current of a rotating ring-disc electrode (RRDE) (Figure 1a).
In theory, during electrocatalytic dehydrogenation
oxidative reaction of hydrazine with an NÀN skeleton, ca.
1
00% N selectivity is ascribed to intrinsic structural proper-
2
[
9]
À
À
ties. In contrast, for electrooxidation of nitrogenous nucle-
(b-Ni(OH)2 + OH = b-Ni(OH)O + H O + e circuit), the
2
ophiles without a NÀN bond (such as ammonia and amine),
b-Ni(OH)O intermediate is accumulated, thus forming com-
plex Ni O H , including NiOOH and NiO . In the OER
x y x
2+d
[14]
N selectivity is limited; amine substrate does not produce N
2
2
[6a,b, 10]
at all.
However, the urea oxidation reaction (UOR) is
system, drastic electrocatalytic oxidation turned blue–green
2+d
a particular case with ca. 100% N selectivity on nickel-based
b-Ni(OH) into black Ni O H ; however, for the UOR the
2 x y
2
[6c,11]
electrocatalysts.
Previous studies have reported on the
b-Ni(OH)2 electrode was protected from oxidation and
NOR mechanism by electrocatalysis; however, the mecha-
nism by which the transformation of nitrogenous molecules
takes place remains in doubt—especially for urea. Little
information is available as to the degradation path from urea
remained blue–green b-Ni(OH) (Figure S6). The evolution
2
of surface species on the b-Ni(OH) electrode was carried out
2
[12]
so as to analyze the mechanism of the reaction. In situ Raman
spectroscopy was used to monitor the bending and stretching
3
+
À1
to N during the UOR.
vibrations of Ni -O at 473 and 553 cm ; in 1 M KOH with 0
+
2
3
Herein, we focus on nitrogenous nucleophile electro-
oxidation over a b-Ni(OH)2 electrode. We decipher the
complex synergy between electrode function, molecular
structure, and substrate transformation mechanism. By com-
bining periodic sampling and in situ characterizations of the
electrode and products, it has been ascertained that NOR is
a two-step reaction that includes electrocatalytic dehydrogen-
ation from b-Ni(OH)2 to b-Ni(OH)O and spontaneous
nucleophile dehydrogenative oxidation. Proton-coupled elec-
tron transfer (PCET) is a pivotal link between two steps of
NOR: electrons and protons of the nucleophile are seq-
uestered by b-Ni(OH)O spontaneously, thus generating
and 50 mM urea, Ni -O species are accumulated after 1.40
and 1.65 V, respectively. Furthermore, in 1 M KOH with
3+
0.5 M urea, Ni -O species cannot be accumulated within
[14b,15]
1.65 V (Figure 1b).
Quasi in situ XPS measurements
were carried out to analyze the surface chemical states of Ni
and O on the b-Ni(OH) electrode (Figures S7–S10). Com-
2
2+d
pared with b-Ni(OH) , complex Ni O H species (including
2
x
y
NiOOH and NiO ) were identified by the higher binding
x
2+d
energy of Ni species at Ni 2p and the lower bonding energy
[
6a,16]
of the NiÀO bond at O 1s.
The peak shift of the bonding
energy, and its correlation with different (fitted) chemical
states, is shown in Figure 1c. The in situ Raman results
correspond with the potential of generation of Ni O H and
[13]
2+d
b-Ni(OH) and oxidation product.
This theory can give
2
x
y
a good explanation for hydrazine and primary amine oxida-
tion reactions, whose final oxidation products are nitrogen
the urea concentration in 1 M KOH.
Electrocatalytic oxidation from b-Ni(OH) to Ni O H
y
2+d
2
x
(
N ) and nitrile (R-CN), respectively. For the UOR from urea
can also be observed by transmission electron microscopy
2
[6b,9,11a]
to N and CO , the PCET theory is inadequate.
Isotope-
(TEM). For the b-Ni(OH) electrode in 1 M KOH with 0 and
2
2
2
labeled in situ differential electrochemical mass spectrometry
50 mM urea, the collapse of morphology, accompanied by
lattice distortion, appears after 1.45 and 1.65 V, respectively.
(
DEMS) provided direct evidence for intramolecular NÀN
bond coupling (the two nitrogen atoms in each of the N2
molecules come from the same urea molecule). Based on the
proposed reaction simulation system, including VASP and
Gaussian modules, the detailed urea degradation path,
including PCET, intramolecular coupling of the NÀN bond,
The morphology and crystal structure of b-Ni(OH) nano-
2
sheets remained unchanged within 1.65 V in 1 M KOH with
0.5 M urea (Figures S11–S13). The aforementioned character-
ization methods reveal, in detail, evolution of species on the
b-Ni(OH) electrode during the OER and UOR (Figure 1d).
2
hydration, and rearrangement processes have been deci-
phered.
During electrocatalytic oxidation, as protons and electrons
leave the Ni(OH) octahedra, an isolated b-Ni(OH)O inter-
6
mediate with a single hydrogen defect is formed. The isolated
b-Ni(OH)O intermediate was difficult to identify by XPS or
2+d
Results and Discussion
Raman spectra unless complex Ni O H was formed by
x y
accumulative b-Ni(OH)O intermediates. The UOR mecha-
nism inhibits accumulation of b-Ni(OH)O intermediate so
Mechanism of the UOR over the b-Ni(OH) electrode.
2
2+d
b-Ni(OH) electrocatalyst was synthesized by a hydrothermal
that b-Ni(OH)2 remains unchanged without Ni O H
2
x
y
method, composed of hexagonal nanosheets (30 to 70 nm,
width by length; 7–10 nm thickness; Supporting Information,
Figures S1–S4). According to prior work, 50 mM was selected
as the substrate concentration for monitoring of the electro-
species.
To analyze the structure–activity relationship between the
evolution of electrode surface species and different reactions,
operando electrochemical impedance spectroscopy (EIS), as
an effective reaction interface identification method, was
utilized.
equivalent circuit models were employed to match the
different potentials during the OER or UOR, and the
optimum fit parameters for EIS data were determined
(Supporting Information, Figures S14–S16, Tables S1 and
S2). Based on the potential ranges of electrocatalytic
oxidation (1.40 to 1.75 V) and the OER (1.55 to 1.75 V) in
1 M KOH, interface reactions with a specific time constant
[12]
chemical performance.
OER and the electroreduction of Ni species are suppressed
Figure 1a; Figure S5). In 1 M potassium hydroxide (KOH)
As urea concentration increases,
3+
[17]
According to Nyquist and Bode plots, two
(
with 0.5 M urea, both the electrocatalytic reduction reaction
and OER barely exist within 1.70 V versus a reversible
hydrogen electrode (RHE). Hence, 1 M KOH with 0.5 M
urea electrolyte was adopted in operando measurements for
analysis of the electrode function. In 1 M KOH with 0, 50, and
5
00 mM urea, for the b-Ni(OH) electrode, OER only occurs
2
7
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Figure 1. In situ tracking of surface chemical states and crystal structure of the b-Ni(OH) electrode during the OER or UOR. a) LSV curves
2
À1
(
5 mVs ) with RRDE, b) in situ Raman spectra, and c) quasi in situ XPS spectra for the b-Ni(OH) electrode in 1 M KOH with 0, 50, and 500 mM
urea. d) A representation of the crystal structure transformation among Ni(OH) octahedra, b-Ni(OH)O intermediate, and Ni O H complex.
2
2
+d
6
x
y
were identified by operando EIS (Figures 1a and 2a). When
applied potential exceeded 1.40 V in 1 M KOH, the diameter
of the first semicircle and the phase angle of the high-
frequency region became smaller, representing electrocata-
lytic oxidation. Beyond 1.55 V of applied potential, the
diameter of the second semicircle and the phase angle of
the low-frequency region reduced, representing the OER
urea, the diameter of the first semicircle and the phase angle
of the high-frequency region became smaller but without
a low-frequency interface reaction, which indicates that the
electrons and protons of urea can be sequestered by the
À
b-Ni(OH)O intermediate (b-Ni(OH)O + Hurea + e
=
urea
b-Ni(OH) ), so that b-Ni(OH) remains unchanged without
2
2
2+d
Ni O H species (Figure 2d–f; Figure S15). At 1.50 V in 1 M
x
y
(
Figure 2a–c; Figure S14). Operando EIS, quasi in situ XPS,
KOH with 0.5 M urea, in addition to the first semicircle and
high-frequency interface reaction, both the second semicircle
and low-frequency interface reaction appear, which proves
that two different UOR mechanisms work at high- and low-
frequency interfaces, respectively. Based on a RRDE ring
current close to zero, oxygen is not the end product of OER
intermediates, which indicates that the OER intermediate,
formed at the low-frequency interface, is one of the origins of
and in situ Raman spectra are consistent with respect to
species evolution. The interface reaction proves that the high-
frequency interface (first semicircle) where electrocatalytic
oxidation occurs is between b-Ni(OH) and the diffuse double
layer (DDL), and the low-frequency interface (second semi-
2
2+d
circle) where the OER occurs is between Ni O H and the
DDL.
x
y
[12,14b,15]
From 1.35 to 1.45 V in 1 M KOH with 0.5 M
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Figure 2. Operando analysis of the intricate internal relationships among surface species, interface, and reaction for the b-Ni(OH) electrode
2
during the OER or UOR. a) Nyquist and b) Bode plots, c) correlation of the equivalent resistances (R and R ), time constants (t and t ), and
1
2
1
2
potentials for the b-Ni(OH) electrode in 1 M KOH. d) Nyquist and e) Bode plots, f) correlation of the equivalent resistances (R and R ), time
2
1
2
constants (t and t ) and potentials for the b-Ni(OH) electrode in 1 M KOH with 0.5 M urea. g) A representation of the OER and UOR
1
2
2
mechanisms for the b-Ni(OH) electrode.
2
UOR activity, and the most likely effective intermediate is
is equal to the electrogenerated dehydrogenation rate from
[18]
OH . However, beyond 1.55 V the diameter of the second
b-Ni(OH)2 to b-Ni(OH)O; that is, the remaining crystal
ads
2+d
semicircle and the phase angle of the low-frequency region
increase, representing a passivation reaction, which proves
that the UOR activity of b-Ni(OH)O at the high-frequency
interface is well above that of the OER intermediate at the
structure and chemical state of b-Ni(OH) without Ni O H
2 x y
species. However, in the UOR system at high potential (after
1.55 V), due to the limiting mass transfer rate, the generation
rate of b-Ni(OH)O is greater than the supply rate of protons
and electrons from urea, so that the b-Ni(OH)O intermediate
is accumulated, which leads to a low-frequency interface
[19]
low-frequency interface. In 1 M KOH with 50 mM urea, the
CV curve shows a significant passivation at high potential (ca.
2
+d
1
.60 V) owing to the competition of two different UOR
between Ni O H and DDL where the OER intermediate is
x y
mechanisms; accordingly, the UOR activity origins at high-
generated. Although the OER intermediate can also seques-
ter protons and electrons belonging to urea—especially with
respect to OHads—its UOR activity is less than the UOR
activity of the b-Ni(OH)O intermediate, which results in
passivation at high potential.
Spontaneous urea dehydrogenative oxidation during the
UOR. The mechanism of dehydrogenative oxidation between
b-Ni(OH)O intermediate and urea is vague. It is crucial to
determine the driving force of urea dehydrogenative oxida-
tion and its relationship with applied voltage. Hence, periodic
sampling of the UOR mixture was carried out to separate
urea dehydrogenative oxidation from electrocatalytic dehy-
drogenation (Figure 3a). At high (1.55 V) and low applied
and low-frequency interfaces are b-Ni(OH)O with electro-
2
+d
philic lattice oxygen and Ni (OH)ads with electrophilic
[12]
adsorbed oxygen, respectively (Figure 2g; Figure S17).
Based on analysis of the reaction interface during OER
and UOR, composite OER and UOR mechanisms involving
multiple elementary reactions are proposed (Figure 2g). In
1
M KOH, electrocatalytic oxidation and the OER occur at
high- and low-frequency regions, respectively; however, for
2+d
the UOR, b-Ni(OH)O and Ni (OH)ads are the origins of
UOR activity at high- and low-frequency regions, respective-
ly. In the UOR system at low potential (1.35 to 1.45 V), the
supply rate of protons and electrons from urea to b-Ni(OH)O
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potentials (1.08 V), the positive and negative currents repre-
Fourier-transform infrared spectroscopy (SR-FTIR) and
in situ DEMS were carried out to trace bond rupture and
sent electrocatalytic oxidation from b-Ni(OH)2 to
b-Ni(OH)O and electroreduction from b-Ni(OH)O to
[5b,22]
formation during the UOR.
Before 1.40 V, and without
2+d
b-Ni(OH) , respectively. Electrogenerated Ni O H were
reaction, urea and electrolyte adsorbed on electrode were
2
x
y
identified by in situ Raman spectroscopy. Periodic sampling
of the UOR was divided into three stages (Figure 3b;
Figure S18): 1) 1.55 V (0 to 60 s), 2) open circuit (60 to
identified by operando SR-FTIR over the b-Ni(OH) elec-
2
trode during UOR. The bands at ca. 3442, ca. 1699, ca. 1449
À1
and ca. 2925 cm are assigned to the stretching vibrations of
1
20 s), and 3) 1.08 V (120 to 200 s). In 1 M KOH at the third
NÀH, C=O, and CÀN bonds in urea, and the OÀH bond in
À1
stage, the negative current represents electroreduction from
b-Ni(OH)O to b-Ni(OH) , which demonstrates that the
electrolyte, respectively. Bands at ca. 3291 and ca. 3172 cm
[5b,23]
represent the stretching vibrations of NH (Figure 4a,b).
2
2
accumulative b-Ni(OH)O intermediate formed in the first
stage remains stable at open circuit voltage. In 1 M KOH with
After 1.45 V, the vibration intensities of NH and NH2
reduced, whereas the vibration intensity of C-N only reduced
after 1.55 V. However, for the vibration intensities of the C=O
bond in urea and the OÀH bond in the electrolyte, there was
0
.5 M urea, although there is a large UOR current at the first
stage, no obvious reduction current signal appears at the third
stage, indicating that b-Ni(OH)O cannot be accumulated
during the UOR. During periodic sampling of the UOR,
b-Ni(OH)O is accumulated at 1.55 V in 1 M KOH; however,
after injecting urea at the open circuit stage, b-Ni(OH)O
cannot be probed at the third stage, which attests that the
reaction between b-Ni(OH)O and urea is spontaneous.
Synchronous in situ Raman spectra revealed the evolution
no noticeable attenuation, which indicated that the dehydro-
genation of the amino group occurs prior to CÀN bond
rupture during the UOR (Figure 4a–c). A difficulty then
remains in how to analyze paths of rearrangement and NÀN
bond coupling during the UOR.
The current density of the UOR is large with ca. 100% N2
selectivity; however, for the ammonia oxidation reaction
(AOR) the reaction current density is low and poor N2
2
+d
of Ni O H during periodic sampling of the electrochemical
x
y
reaction, which further verified that it is accurate to judge the
presence of b-Ni(OH)O on the basis of the current signal
selectivity is accompanied by NO byproducts (Figure 4d;
x
[11a,21a]
Figure S21 and S22).
As a consequence of the isolated
(
Figure 3c).
Based on periodic sampling of the UOR, b-Ni(OH)O can
nitrogen atom of ammonia molecule, in the AOR intermo-
lecular coupling of the NÀN bond is the only path to produce
sequester electrons and protons from urea spontaneously.
However, it remains in doubt as to whether urea can be
completely oxidized by b-Ni(OH)O. Hence, in situ DEMS
measurements were carried out to identify the oxidation
product of urea during periodic sampling of the UOR
N . Moreover, it is necessary for the isolated nitrogen
2
intermediate to adsorb adjacently to form an intermolecular
NÀN bond, which limits the current density and N selecti-
2
[20b,23b]
vity.
The current density of the AOR is far less than that
of the UOR, which indicates that NÀN bond coupling is not
[20]
(
Figure 3d). After enriching the b-Ni(OH)O intermediate,
a limiting step for the UOR (Figure 4d). The two amino
groups of the urea molecule raise the possibility of intra-
molecular coupling of the NÀN bond. Hence, through feeding
the applied voltage (1.55 V) was cut off and urea solution was
injected. N was detected immediately with a 28 m/z ratio
(
isotope-labeled CO( NH ) solution was injected; N with
a 30 m/z ratio was detected immediately, which demonstrates
that urea can be completely oxidized to N by b-Ni(OH)O
without applied potential (Figure 3 f).
2
14
15
Figure 3e). To eliminate contributions from N in air, an
of CO( NH ) and CO( NH ) in a 1:1 ratio, in situ DEMS
2 2 2 2
2
15
was used to measure the mass-to-charge ratio of the N2
product during the UOR, thus deciphering the coupling
mechanism of the NÀN bond. Assuming that the reaction
2
2
2
2
14
15
probabilities of CO( NH ) and CO( NH ) are identical,
2 2 2 2
On balance of the evidence obtained thus far, over the
different coupling mechanisms will lead to disparate mass-to-
b-Ni(OH) electrode the UOR is a two-step reaction includ-
charge ratio distributions of the N2 product. For the N
product of the UOR, 1:1:2 and 1:1:0 ratios of N N to
2
2
14
14
ing electrocatalytic oxidation from b-Ni(OH) to b-Ni(OH)O
2
1
5
15
14 15
and spontaneous dehydrogenative oxidation from urea to N2
N N to N N represent the intermolecular and intra-
molecular coupling mechanisms of the NÀN bond, respec-
(
Figure 3g). PCET is the bridge between electrocatalytic
14
14
dehydrogenation and spontaneous nucleophile dehydrogen-
tively. The in situ DEMS results indicate that N N (ca.
[12,13]
15 15
14 15
ative oxidation.
For NOR with hydrazine (H N-NH ) and
49%), N N (ca. 49%), and N N (ca. 2%) were obtained
during the UOR, providing compelling evidence to support
the dominance of the intramolecular NÀN bond coupling
2
2
primary amine (R-CH -NH ) as substrate, only the PCET
2
2
process, dehydrogenation of amide (-NH ) and alpha CÀH
2
15
bonds (a-CH -) occurs, without other complex rearrangement
mechanism (Figure 4e,f; Figure S23). CO( NH ) provides
2 2
2
15
14 15
and hydration reactions, thus forming N with NꢀN triple
98% of the available N. N N (ca. 2%) is derived from
CO( NH )( NH ) impurity, which follows the same intra-
2 2
2
1
5
14
bonds and nitrile (R-CN) with CꢀN triple bonds, respectively
[6b, 9]
(
Figures S19 and S20).
On the other hand, for ammonia
molecular coupling mechanism. Diagrams depicting the
AOR, NOR, and UOR are shown in Figure 4g. For the
AOR, the protons and electrons of ammonia are first
sequestered by b-Ni(OH)O, and subsequently two adjacent
electron-deficient nitrogen intermediates combine through an
intermolecular coupling mechanism of the NÀN bond to
with an isolated nitrogen atom and urea with multiple
nucleophilic groups, the oxidation products of NOR are
unconventional, such as NOx and N , which cannot be
2
[6a,11a,21]
explained by PCET theory.
The subsequent reaction processes associated with the
urea molecular structure should be clarified to explain ca.
produce N . There are two negative factors for the AOR:
2
[13]
1
00% N2 selectivity.
Operando synchrotron-radiation
1) electron-deficient nitrogen species need to be adsorbed
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Figure 3. Periodic sampling of the UOR to ascertain the extent of spontaneous urea dehydrogenative oxidation. a) Method for identification of
b-Ni(OH) and accumulated b-Ni(OH)O by ring current and in situ Raman spectroscopy. b) Periodic electrochemical measurement of the UOR
2
over the b-Ni(OH) electrode at 1.55 V (0 to 60 s), open circuit state (60 to 120 s), and at 1.08 V (120 to 200 s). c) In situ Raman spectra for the
2
b-Ni(OH) electrode and d) in situ DEMS during periodic UOR electrochemical measurements. In situ DEMS for periodic measurement of the
2
1
4
15
UOR with e) CO( NH ) and f) CO( NH ) as substrates. g) The UOR mechanism on the b-Ni(OH) electrode.
2
2
2
2
2
adjacently, which limits the rate of intermolecular coupling;
) the electron-deficient nitrogen species easily reacts with
OH , generating NOx byproduct. For the NOR with
ads
2
hydrazine and primary amine as substrate, N with a NꢀN
2
7
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Figure 4. The intermolecular and intramolecular coupling mechanisms of the NÀN bond for the AOR and UOR. Operando SR-FTIR spectroscopy
À1
À1
for the UOR over b-Ni(OH) electrode a) in the range of 2800 to 3500 cm and b) in the range of 1300 to 2800 cm . c) Diagram depicting urea
2
dehydrogenative oxidation with the order of rupture of NÀH and CÀN bonds. d) LSV curves for the b-Ni(OH) electrode in 1 M KOH, 1 M KOH
2
1
4
15
with 50 mM urea, and 50 mM ammonia. e) In situ DEMS for b-Ni(OH) electrode in 1 M KOH with 25 mM CO( NH ) and 25 mM CO( NH ) .
f) In situ DEMS of the UOR over the b-Ni(OH) electrode at different potentials with 25 mM CO( NH ) and 25 mM CO( NH ) as substrate.
2
2
2
2 2
1
4
15
2
2
2
2 2
g) Representations of the mechanisms of the AOR, hydrazine oxidation reaction, primary amine oxidation reaction, and the UOR over a
b-Ni(OH) electrode.
2
triple bond and nitrile (R-CN) with a CꢀN triple bond are
dehydrogenative oxidation because of its unique advantages
in molecular simulation. We confirmed the possibility of
b-Ni(OH)O (with hydrogen defect) formation on the basal
[25]
formed. For the UOR, the proton and electron of the amine
are gradually sequestered by b-Ni(OH)O while an intra-
molecular NÀN bond is formed from two electron-deficient
plane of b-Ni(OH) thermodynamically and dynamically; for
2
nitrogen atoms in the same urea molecule. Eventually, after
complex hydration, rearrangement and dehydrogenation
reactions, this intermediate with an intramolecular NÀN bond
electrocatalytic dehydrogenation, self-desorption from the
surface was energy demanding with an ultrahigh energy
barrier (2.60 eV) to overcome, thus generating the
b-Ni(OH)O intermediate (Figure 5a). As shown in Figure 5b,
b-Ni(OH)O can catch the electrons and protons of urea
spontaneously via the PCET process. In the case of PCET, the
bond dissociation free energy (BDFE) of XÀH bonds
is degraded to CO and N .
2
2
To examine the electrocatalytic dehydrogenation and
spontaneous urea oxidation in the UOR, density functional
theory (DFT) calculations were performed using the Vienna
ab initio simulation package (VASP) and Gaussian. VASP is
appropriate for electrocatalytic dehydrogenation, owing to
the matching between its own periodic system and the
provides an effective approach to reveal the UOR mecha-
[13]
nism. According to cyclic voltammetry (CV) curves for the
b-Ni(OH) electrode at multiple pH values, E8(b-Ni(OH)O/
2
[24]
periodic crystal structure of the electrocatalyst. In contrast,
Gaussian is more suitable for examining the spontaneous urea
b-Ni(OH) ) is generated at ca. 1.51 V. Through the conversion
2
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formula from E8(X/XH) to BDFE(X-H), as shown in
Figure S24). The hydration reaction is also critical, which
Equation (1):
provides two active hydrogen atoms in OÀH bonds for the
[26]
PCET process (PCET and PCET ).
Through transition
5
6
BDFEðX-HÞ ¼ 23:06 E ðpH 0Þ þ 57:6 kcal molÀ1
ꢁ
ð1Þ
state (TS) calculations, hydration of urea needs to overcome
À1
a high barrier of TS (51.8 kcalmol ), and the free energies of
À1
BDFE(b-Ni(OH) ) is about 92.42 kcalmol (Figure 5c,d).
À1
2
the final state (FS) is 29.8 kcalmol , which proves the slow
Hence, for PCETactuated by b-Ni(OH)O intermediate, those
XÀH bonds in substrates whose BDFEs are significantly
reaction rate of urea hydrolysis (Figure S25). In fact, at room
temperature urea hydrolysis is so sluggish that the swift UOR
rate cannot be matched, which indicates that hydration of
À1
[13b]
greater than 92.42 kcalmol are ineffective.
[27]
During spontaneous urea dehydrogenative oxidation, the
PCET process plays a significant part in transferal of electrons
and protons from urea/oxidation intermediate to b-Ni(OH)O,
urea is not the first step for the UOR. Namely, Path 3 is not
feasible. For the hydration reaction of CO(NN) intermediate
in Path 1, the free energies of TS and FS are 27.1 and
À1
and results in dehydrogenation product and b-Ni(OH) . For
À13.4 kcalmol , respectively; hence, the hydration reaction
2
a multistep PCET process between b-Ni(OH)O and urea with
four active hydrogen atoms, there are two paths available:
Path 1) the active hydrogen is sequestered from two amino
groups in urea by b-Ni(OH)O alternately. Path 2) Two active
hydrogen atoms in the same amino group are captured
preferentially, and the remaining two active hydrogen atoms
in the other amino group are seized subsequently (Figure 5e;
Figure S24). In either Path 1 or Path 2, the first dehydrogen-
from CO(NN) to C(OH) (NN) is spontaneous and extremely
2
fast, matching the swift PCET process so that Path 1 is offers
a reasonable mechanism (Figure S25). In brief, through
combining experimental evidence and DFT calculation, the
spontaneous urea dehydrogenative oxidation (including
PCET process, hydration, and rearrangement) has been
revealed. 1) Four active hydrogen atoms and four protons
are sequestered from two amino groups in urea by
b-Ni(OH)O alternately. As each amino group loses an active
ation reaction (PCET process) for urea with a BDFE of
1
À1
1
04.3 kcalmol is relatively difficult, which proves that those
hydrogen atom and a proton (PCET1 and PCET ), an
2
À1
XÀH bonds with BDFEs below 104.3 kcalmol can react
intramolecular NÀN bond is formed from two electron-
deficient nitrogen atoms. After further loss of two active
hydrogen atoms and two protons (PCET and PCET ), an
with b-Ni(OH)O spontaneously, and results in XC free radical
intermediates and b-Ni(OH) . The second dehydrogenation
2
3
4
reaction of the PCET process is the decisive factor in
intramolecular N=N bond is generated in a CO(NN) inter-
identifying the dehydrogenation path. Through molecular
mediate. 2) Hydration of the CO(NN) intermediate takes
À1
simulation, the BDFE of PCET in Path 1 (80.1 kcalmol ) is
place swiftly, thus generating C(OH) (NN) intermediate.
2
2
À1
significantly lower than that in Path 2 (94.5 kcalmol ), which
3) C(OH) (NN) intermediate undergoes PCET and PCET
6
2
5
indicates that, during the multistep PCET process (PCET1–
processes, losing two active hydrogen atoms and two protons,
which results in C(OO)(NN) intermediate possessing two
PCET ), the two amino groups in the same urea molecule are
4
dehydrogenated alternately, following Path 1 (Figure 5e;
Figure 5. The spontaneous urea dehydrogenative oxidation path. a) DFT calculation depicting the reaction pathway from b-Ni(OH) to b-Ni(OH)O
2
À1
intermediate, carried out by using VASP. b) A depiction of the UOR mechanism. c) CV curves (20 mVs ) for the b-Ni(OH) electrode at multiple
2
pH values. d) Plot of E1/2 vs. pH for redox feature from b-Ni(OH) to b-Ni(OH)O intermediate, showing the corresponding BDFE. e) Urea
2
dehydrogenative oxidation reaction path including PCET, hydration, and rearrangement processes based on molecular simulations carried out
using Gaussian.
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electron-deficient oxygen atoms
and an intramolecular N=N bond.
This species finally rearranges to
N and CO .
2
2
Deciphering a high-efficiency
electrocatalyst design strategy for
the UOR. Based on the above
UOR mechanism, including elec-
trocatalytic dehydrogenation and
spontaneous urea oxidation, the
key to enhance the UOR activity
is to reduce the generation poten-
tial of b-Ni(OH)O, which is the
species from which the UOR ac-
tivity originates. Namely, the UOR
performance depends on electro-
generated hydrogen-defect forma-
tion processes from the lattice
hydroxy group of b-Ni(OH)2 to
the electrophilic lattice oxygen of
b-Ni(OH)O. In theory, the gener-
ation potential of electrophilic lat-
tice oxygen in b-Ni(OH)O can be
optimized by tuning the oxygen
ligand environment of b-Ni(OH)2.
Doping with cobalt (Co) can ach-
[
12,28]
ieve this outcome.
DFT calcu-
lations on the electrocatalytic de-
hydrogenation reaction indicated
that Co doping can change the
electron density distribution and
is an effective method to modulate
the properties of b-Ni(OH) so that
2
the energy barrier for hydrogen
defect formation on b-Co Ni -
0.1
0.9
(
OH) can be reduced to 2.16 eV,
2
Figure 6. The UOR electrocatalyst design strategy through tuning of the lattice oxygen ligand
thus leading to a lower generation
potential for b-Ni(OH)O and ex-
cellent UOR activity (Figure 6a,b).
b-Co Ni (OH) electrocatalyst,
environment. a) DFT calculations for the reaction pathways from b-Ni(OH) to b-Ni(OH)O and from
2
b-Co0.1Ni0.9(OH) to b-Co0.1Ni0.9(OH)O. b) Differential charge densities of b-Co0.1Ni0.9(OH) and
2
2
a diagram depicting the electrocatalyst design strategy. c) LSV curves for b-Ni(OH) and b-Co0.1Ni0.9
-
2
(OH) electrodes in 1 M KOH without/with 50 mM urea. d) The structure-function relationship
0
.1
0.9
2
2
composed of hexagonal nano-
sheets, was synthesized by a hydro-
thermal method (Figures S26–
S29).
between the lattice oxygen ligand environment of b-Ni(OH)
and the UOR activity. e) LSV curves for
2
b-Ni(OH) /NF and b-Co0.1Ni0.9(OH) /NF electrodes in 1 M KOH with 50 mM urea. f) Three consec-
2
2
utive long-term tests of b-Co0.1Ni0.9(OH) /NF in 1 M KOH with 50 mM urea.
2
3+
X-ray absorption spectroscopy (XAS) of b-Co Ni -
OH) revealed that doping with Co does replace the Ni
2
fairly unchanged, except for generation of Co species. The
coordination number of lattice oxygen is four for b-Ni(OH)2,
including three metalÀoxygen bonds and an HÀO bond. XPS
0.1
0.9
(
atom in Ni(OH) octahedra of b-Ni(OH) , thus regulating the
6
2
lattice oxygen ligand environment with CoÀO bonds (Fig-
ure S30). CoÀO bond doping reduces the b-Ni(OH)O for-
mation potential from ca. 1.40 to ca. 1.35 V, but also promotes
the UOR activity synchronously (Figure 6c). To research the
effect of Co doping on the UOR activity, detailed character-
izations was carried out (Figures S31–S33). After the OER,
the morphology of b-Co Ni (OH) hexagonal nanosheets
of b-Co Ni (OH) provided a Co 2p spectrum that may be
0.1
0.9
2
interpreted with the following probabilities: two NiÀO and
one CoÀO bonds (81%), one NiÀO and two CoÀO bonds
(18%), three CoÀO bonds (1%). Co atom doping in the
lattice oxygen ligand environment can reduce the formation
potential of b-Co Ni (OH)O; however, for the electrophilic
x
1Àx
lattice oxygen of b-Co Ni (OH)O, those oxygen atoms that
0.1
0.9
2
0.1
0.9
3+
3+
collapsed, accompanied by generation of Ni /Co species
have two or three CoÀO bonds cannot promote urea
3+
and metalÀoxygen bonds and color transformation of the
dehydrogenation, and they produce a small amount of Co
species after the UOR (Figure 6d).
electrocatalyst from pale green to black. However, after the
UOR the color, morphology, and chemical properties (XPS
spectra at Ni 2p and O 1s) of b-Co Ni (OH) remained
The promotional effect of tuning the lattice oxygen ligand
environment upon the UOR activity is more prominent for
0.1
0.9
2
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b-Co Ni (OH) grown in nickel foam (b-Co Ni (OH) /
2) a urea dehydrogenation intermediate without active hydro-
gen, CO(NN), is hydrolyzed to an intermediate with dual
hydroxy and intramolecular coupling of the NÀN bond to
0
.1
0.9
2
0.1
0.9
2
NF). Compared with b-Ni(OH) /NF, the UOR onset potential
2
of b-Co Ni (OH) /NF is reduced from ca. 1.36 to ca. 1.30 V,
0
.1
0.9
2
which breaks the bottleneck of ca. 1.35 V for most nickel-
based catalysts proposed in recent studies (Figure 6e;
Table S3). Specifically, at 1.36 V, the UOR activity of
produce C(OH) (NN); 3) C(OH) (NN) intermediate is dehy-
2
2
drogenated, thus generating N and CO . Based on theoretical
2
2
calculations and mechanistic analysis, an effective electro-
catalyst design strategy is proposed whereby tuning of the
lattice oxygen ligand environment of b-Ni(OH)2 with Co
reduces the energy barrier for b-Ni(OH)O formation, thus
leading to excellent activity in the UOR. This work clarifies
the nitrogenous nucleophile electrooxidation mechanism
À2
b-Co Ni (OH) /NF (192.3 mAcm ) is far beyond that of
0
.1
0.9
2
À2
b-Ni(OH) /NF (4.6 mAcm ). In addition, periodic sampling
2
of the UOR over the b-Co Ni (OH) electrode was carried
0.1
0.9
2
out to ascertain the extent of spontaneous urea dehydrogen-
ative oxidation (Figure S34). After introducing urea to the
b-Co Ni (OH) electrode with abundant electrophilic in-
over b-Ni(OH) electrode—especially for urea—and it pro-
0
.1
0.9
2
2
termediates, b-Co Ni (OH)O intermediate was not identi-
fied by its circular current, which proves that the UOR
vides fundamental information regarding transformation of
nitrogen species during electrocatalysis.
0
.1
0.9
mechanism of the b-Co Ni (OH) electrode is the same as
0.1
0.9
2
that of the b-Ni(OH) electrode. This optimization of electro-
2
catalyst acts on the electrocatalytic dehydrogenation reac- Acknowledgements
tion; however, the spontaneous non-electrocatalytic process,
including PCET, intramolecular coupling of the NÀN bond,
and hydration, cannot be affected, thus showing excellent
performance in the UOR. In three consecutive urea electro-
oxidation processes during a long-term test, the peak current
density hardly decayed, which proves the stability of the UOR
on the b-Co Ni (OH) /NF electrode (Figure 6 f; Fig-
The authors acknowledge support from the National Key
R&D Program of China (2020YFA0710000), the National
Natural Science Foundation of China (Grant No. 21573066,
21902047, 21825201, and U1932212), the Fundamental Re-
search Funds for the Central Universities (Grant Nos.
531118010127), the China Postdoctoral Science Foundation
(Grant No. 2020M682542), and the Provincial Natural
Science Foundation of Hunan (2020JJ5045). Most of the
computational work was performed on the supercomputers of
the National Supercomputer Centers in Changsha.
0
.1
0.9
2
ure S35). Based on a detailed mechanistic analysis, this
modified strategy also applied to hydrazine and benzylamine
oxidation reactions of the Ni(OH) /NF electrode, which show
2
a decrease in onset potential by 60 mV, from ca. 1.36 to ca.
1
.30 V (Figure S36). This electrocatalyst design strategy not
only significantly reduces energy consumption, but also
enhances electrolytic efficiency with a large current density, Conflict of interest
which provides theoretical guidance and technological sup-
port for the degradation and transformation of available
nitrogen in industrial applications.
The authors declare no conflict of interest.
Keywords: coupling reactions · dehydrogenation ·
electrocatalysis · nickel · urea
Conclusion
Nitrogen cycle electrocatalysis has attracted considerable
attention in recent years, and still presents mechanistic
ambiguity. The focus of this work was to illustrate the
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Manuscript received: November 26, 2020
Accepted manuscript online: December 24, 2020
Version of record online: February 22, 2021
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