Activity Origins and Design Principles of Nickel-Based Catalysts for Nucleophile Electrooxidation

Full text of DOI:10.1016/j.chempr.2020.07.022

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Article
Activity Origins and Design Principles of
Nickel-Based Catalysts for Nucleophile
Electrooxidation
Wei Chen, Chao Xie, Yanyong
Wang, ..., Bo Zhou, Jianmin Ma,
Shuangyin Wang
shuangyinwang@hnu.edu.cn
HIGHLIGHTS
NOR activity origins of b-Ni(OH)2
and NiO are b-Ni(OH)O and
NiO(OH)ads, respectively
First step for NOR of b-Ni(OH)2 is
the electrogenerated catalyst
dehydrogenation reaction
Second step for NOR of b-Ni(OH)2
is the spontaneous nucleophile
dehydrogenation reaction
Explicit electrocatalyst design
principle of b-Ni(OH)2 for NOR is
proposed
Owing tolow energy consumptionand high-value-addedproducts, the nucleophile oxidation reaction
(NOR), as a rising half-reaction to replace the sluggish oxygen evolution reaction (OER) in overall water
splitting, has been researched widely based on nickel-based electrocatalysts; however, the
indeterminate nature of the NOR mechanism limits the development of this emerging field. Here,
focusing on model catalysts, including b-Ni(OH)2 and NiO, we discover that their NOR activity origins
are b-Ni(OH)O containing electrophilic lattice oxygen and NiO(OH)ads containing electrophilic
adsorption oxygen, respectively. For b-Ni(OH)2, NOR is a two-step, one-electron reaction including an
electrogenerated catalyst dehydrogenation reaction from lattice hydroxyl to electron-deficient lattice
oxygen (b-Ni(OH)2 + OHÀ = b-Ni(OH)O + H2O + eÀ) and a spontaneous nucleophile dehydrogenation
reaction (b-Ni(OH)O + HNu + eÀNu = b-Ni(OH)2). Hence, we report a general design principle of
electrocatalysts for NOR that involves tuning of the lattice oxygen ligand environment to effectively
change the dehydrogenation potential of b-Ni(OH)2, thus regulating NOR performance.
Chen et al., Chem 6, 1–20
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Activity Origins and Design
Principles of Nickel-Based Catalysts
for Nucleophile Electrooxidation
Wei Chen,1,5 Chao Xie,1,5 Yanyong Wang,1,5 Yuqin Zou,1,5 Chung-Li Dong,2,5 Yu-Cheng Huang,2,5
Zhaohui Xiao,1 Zengxi Wei,3 Shiqian Du,1 Chen Chen,1 Bo Zhou,1 Jianmin Ma,3
and Shuangyin Wang1,4,6,
*
SUMMARY
The Bigger Picture
Nucleophile oxidation reaction
(NOR) is an emerging field to
replace oxygen evolution reaction
(OER) in overall water splitting.
For b-Ni(OH)2, NOR is a two-step,
one-electron reaction, including
an electrogenerated catalyst
dehydrogenation reaction from
lattice hydroxyl to electrophilic
Understanding how electrocatalysts function in a nucleophile oxida-
tion reaction (NOR) on anode, replacing the oxygen evolution reac-
tion (OER) of water splitting, is vital to the development of hydrogen
generation and organic electrosynthesis. Here, we propose that
for b-Ni(OH)2 and NiO, the NOR activity origins are b-Ni(OH)O
containing electrophilic lattice oxygen and NiO(OH)ads containing
electrophilic adsorption oxygen, respectively. For b-Ni(OH)2, NOR
is a two-step, one-electron reaction, including an electrogener-
ated catalyst dehydrogenation reaction and a spontaneous nucleo-
phile dehydrogenation reaction. Therefore, the NOR activity of
b-Ni(OH)2 can be markedly regulated by tuning the lattice oxygen
ligand environment. For b-Co0.1Ni0.9(OH)2, the onset potential of
NOR with different nucleophiles is ꢀ1.29 V (1 M KOH), which breaks
the bottleneck of ꢀ1.35 V for most nickel-based catalysts. Overall,
we identify the activity origins and propose the design principles
of nickel-based catalysts for NOR. These provide theoretical guid-
ance for the development of NOR and organic electrosynthesis in
practical industrial applications.
lattice oxygen (b-Ni(OH)2 + OHÀ
b-Ni(OH)O + H2O + eÀ) and a
spontaneous nucleophile
=
dehydrogenation reaction
(b-Ni(OH)O + HNu + eÀ
=
Nu
b-Ni(OH)2). Based on this detailed
mechanism, we propose the
design principle of
electrocatalysts for NOR such that
the dehydrogenation potential of
lattice hydroxyl can be regulated
by tuning the lattice oxygen
ligand environment of b-Ni(OH)2,
thereby resulting in a steerable
NOR performance.
INTRODUCTION
Hydrogen energy, with the highest gravimetric energy density, is proposed as a
promising candidate for clean energy storage and conversion.1,2 Water electrolysis,
including the hydrogen evolution reaction (HER) and oxygen evolution reaction
(OER), as an ideal hydrogen production method, has been researched widely.3–5
However, the industrial application of water splitting is limited by its high energy
consumption because of OER with sluggish kinetics.4,6 Various kinds of alternative
oxidation reactions (such as alcohols, aldehydes, amines, urea) have emerged based
on nickel-based electrocatalysts since the biomass oxidation reaction was proposed
to replace OER.7–13 However, so far, there is no systematic induction and summary
for this emerging field, especially for precise definition. For these different organic
substrates, essentially, a nucleophilic group with an active hydrogen is the reaction
site (such as hydroxyl, aldehyde, and amino groups) during the biomass oxidation
reaction; hence, the term nucleophile oxidation reaction (NOR) has been used to
define these biomass oxidation reactions in this work. NOR coupled with HER has
exhibited huge potential because of low energy consumption and high-value-added
products. Compared with a conventional thermocatalysis method, NOR displays a
higher conversion and selectivity.14,15 Recently, nickel-based electrocatalysts have
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been utilized extensively on NOR.9,10,16–18 However, there is much debate in the
absence of any explicit conclusion regarding the mechanism of NOR.19–23 In addi-
tion, there is a lack of systematic research on design of electrocatalysts for NOR.
Therefore, identifying and clarifying the NOR mechanism is essential to further guide
electrocatalyst design.
With the absence of anode electrons during an electrooxidation reaction, electro-
philic intermediates are formed, which makes it possible to react with a nucleo-
phile.24–28 In essence, NOR is a dehydrogenation reaction that allows a nucleophile
to lose protons and electrons.29,30 NOR is usually considered to be closely related to
Ni2+/Ni3+ redox for nickel-based catalysts.9,16,17,22,30 For example, Sun’s group indi-
cated that electrogenerated Ni3+ species with Ni3+–O bonds might be NOR-active
intermediates.31 However, the surface species transformation of nickel-based non-
(hydro)oxide electrocatalysts (e.g., nickel sulfide, nickel phosphide, nitride nickel)
is complex and indeterminate during electrooxidation, which is not conducive to
further research of the NOR mechanism.9,16,17,30,31 Another hypothesis of the
NOR mechanism has been reported, which states that the nucleophile can react
with an electrophilic oxygen in OHads preferentially. So, Liu’s group proposed a
strategy with a nucleophile as an OHads probe to detect the number of OHads during
OER.24 Nevertheless, direct detection of OHads for verifying this hypothesis is
extremely difficult. In theory, the only receptor seizing both electrons and protons
during NOR is an electrophilic oxygen rather than an electrophilic nickel (Ni2+d).24
After seizing electrons and protons, the electrophilic oxygen is converted to a
hydroxyl group with an eight-electron oxygen; nevertheless, Ni2+d cannot seize
protons directly. Due to a lack of compelling evidence, the controversy consistently
remains as to which electron-deficient groups (Ni3+–O or OHads) with an electrophilic
oxygen is the activity origin of NOR.
In this study, b–Ni(OH)2 and NiO were selected as model catalysts due to their
relatively unambiguous structural evolutions during OER. The structural evolutions
during OER and NOR were characterized via transmission electron microscopy
(TEM) and X-ray photoelectron spectroscopy (XPS). The NOR mechanism was iden-
tified by operando electrochemical impedance spectroscopy (EIS), in situ Raman
spectroscopy, and in situ X-ray absorption spectroscopy (XAS). Time-of-flight sec-
ondary ion mass spectrometry (TOF-SIMS) was carried out to trace hydrogen isotope
labeled in the nucleophile during NOR. Combining verification experiments and
density functional theory (DFT) calculations, we propose different NOR pathways
for b-Ni(OH)2 and NiO; moreover, NOR activity origins are b-Ni(OH)O with electro-
philic lattice oxygen and NiO(OH)ads with electrophilic adsorption oxygen. Based on
the unique NOR pathway of b-Ni(OH)2 involving lattice oxygen, we report a definite
modification strategy to enhance NOR activity through tuning of the lattice oxygen
ligand environment. In brief, for NOR, we identified the activity origins and pro-
posed an electrocatalyst design principle, which provides theoretical guidance for
the development of NOR and industrial application of organic electrosynthesis.
1State Key Laboratory of Chem/Bio-Sensing and
Chemometrics, College of Chemistry and
Chemical Engineering, Hunan University,
Changsha, Hunan 410082, PR China
2Research Center for X-Ray Science &
Department of Physics, Tamkang University, 151
Yingzhuan Rd., New Taipei City 25137, Taiwan
3School of Physics and Electronics, Hunan
University, Changsha 410082, China
RESULTS AND DISCUSSION
4The National Supercomputing Center in
Changsha, Hunan University, Changsha 410006,
China
Nucleophile Oxidation Reaction Pathways for b-Ni(OH)2 and NiO
b-Ni(OH)2 was synthesized via the hydrothermal method. The detailed characteriza-
tions are shown in Figures S1 and S2. In the NOR system, ethanol was adopted as the
main research object. For the activity test, 50 mM ethanol was selected because it
exhibited a preferable NOR activity; however, for in/ex situ characterization mea-
surements, a higher alcohol concentration (0.5 M) was selected to eliminate the
5These authors contributed equally
6Lead Contact
*Correspondence: shuangyinwang@hnu.edu.cn
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Figure 1. Surface States and Interface Reaction Analysis for b-Ni(OH)2 during OER and NOR
(A) Photograph of b-Ni(OH)2, b-Ni(OH)2 after NOR, and b-Ni(OH)2 after OER. These suspensions
were obtained with ultrasonic dispersion of catalysts on glassy carbon plate electrode.
(B) The area ratio of oxygen with different ligand environments (H2O, M-OH, and M-O) of
b-Ni(OH)2, b-Ni(OH)2 after NOR, and b-Ni(OH)2 after OER. It was derived from the O 1s XPS spectra
(Figure S7).
(C) Two equivalent circuit models for different potentials during OER and NOR (Figure S9).
(D) LSV (5 mV sÀ1, without iR-correction) for b-Ni(OH)2 on glassy carbon electrode in 1 M KOH with/
without 0.5 M ethanol (keeping the same condition as in situ tests).
(E) Bode plots of b-Ni(OH)2 electrode for OER and NOR in different potentials (Figure S11).
(F) Correlation of the equivalent resistances (R1 and R2) and potentials for b-Ni(OH)2 during OER or
NOR (Tables S1–S3).
influence of OER (Figure S3). Both electrooxidation of b-Ni(OH)2 and OER were in-
hibited during NOR; hence, the possible NOR activity origins were divided into
two categories: (1) b-Ni(OH)O intermediate and oxidation products (NiOOH,
NiOx, and NiOxHy) during catalyst electrooxidation, and (2) intermediates (OH*,
O*, and OOH*) during OER (Figures S3 and S4).32–37
After OER, the color of the b-Ni(OH)2 electrode changed from blue-green to black
because black NiOOH and NiOx were formed (Figure 1A).13,28,38,39 The b-Ni(OH)
O is an important intermediate, and b-NiOOH, g-NiOOH, and NiOx were formed
with an accumulation of b-Ni(OH)O (Figure S4A).30,31,40,41 Surprisingly, after NOR,
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b-Ni(OH)2 remained blue-green, which was a hint that phase transition might not
occur (Figure 1A). The Ni 2p XPS spectra showed that after OER, the binding energy
of Ni 2p peak was raised obviously due to Ni2+d species; however, after NOR, Ni 2p
peaks had no obvious change without Ni2+d species (Figures S5 and S6).20,30,31,42
Meanwhile, abundant Ni–O bonds, which were attributed to NiOx, were formed af-
ter OER; however, there was no apparent Ni–O bond signal after NOR (Figures 1B
and S7).43,44 As shown in the TEM images, b-Ni(OH)2 structures consisted of regular
hexagonal nanosheets with lateral sizes ranging from 30 to 60 nm (Figure S8). After
OER, the hexagonal nanosheets collapsed, and the NiO phase was found, attributed
to the spontaneous reduction of NiOx. However, after NOR, the morphological and
crystal structures of b-Ni(OH)2 maintained unchanged without the NiOx phase. Look-
ing at the XPS spectra and TEM images, it seems incredible that after NOR, the crys-
tal structure, morphology, and surface electronic states of b-Ni(OH)2 were not
changed, which implies that the special NOR mechanism enables b-Ni(OH)2 to retain
structure and avoid phase transition. In consideration of the electrochemical dehy-
drogenation of b-Ni(OH)2, most probably protons of the nucleophile can fill in the
hydrogen defect of b-Ni(OH)O intermediate, thereby avoiding the phase transition
(Figure S4A).
Operando EIS was used to study the potential-dependent interface behavior during
OER or NOR.45–48 Two equivalent circuit models were employed to match the
different potentials during OER or NOR according to the Nyquist and Bode plots
(Figures 1C and S9–S11).49–51 Optimum fit parameters for EIS data are shown in
Tables S1–S3. Regardless of whether it was in the OER or NOR system, only the
high-frequency interface with extremely weak charge transfer existed at 1.15 to
1.35 V; therefore, the high-frequency interface was between the diffuse double layer
(DDL) and b-Ni(OH)2 (Figures S10 and S11).52,53 Based on potential consistency
between the linear sweep voltammetry (LSV) curve and operando EIS during OER
at 1.35 to 1.65 V, there were two different interface reactions, including the catalyst
electrooxidation occurring at the high-frequency interface and OER occurring at the
low-frequency interface (Figures 1D, S10A, and S11A).38,39 For the NOR system at
1.35 to 1.45 V, only the high-frequency interface reaction took place with high cur-
rent density, which proves that NOR occurs at the high-frequency interface between
DDL and b-Ni(OH)2 (Figures 1D, 1E, and S11B). After 1.50 V during NOR, the low-
frequency-interface reaction occurred; after 1.60 V, according to the second half
semi-circle that appeared in the second quadrant of Nyquist plots, catalyst surface
passivation appeared (Figures S10B and S11B).48 It also conformed to the LSV curve
of b-Ni(OH)2, which shows an obvious passivation trend after 1.50 V during NOR
(Figure 1D). Therefore, for b-Ni(OH)2, NOR mainly occurs at the high-frequency
interface between b-Ni(OH)2 and DDL. The NOR activity of low-frequency interface
was much less than that of the high-frequency interface and resulted in passivation
after 1.50 V during NOR. The variation trend of high- and low-frequency-interface
reactions under different potentials for b-Ni(OH)2 is presented clearly via combining
equivalent resistances and potentials (Figure 1F; Tables S1–S3). For the OER system,
at 1.15 to 1.30 V, R1 of the high-frequency-interface reaction was huge because of
extremely weak charge transfer. As the electrooxidation of b-Ni(OH)2 happened at
1.35 V, the low-frequency interface (R2) occurred immediately. After 1.35 V, R1
decreased swiftly, representing increasing electrooxidation rate of b-Ni(OH)2, and
R2 decreased with increasing OER current density. For the NOR system, at 1.15 to
1.30 V, R1 was gigantic, similar to the OER system. At 1.35 to 1.45 V during NOR,
R1 decreased swiftly with rapidly increasing NOR current density, but there was no
low-frequency interface and R2, which proves that NOR occurs at the high-frequency
interface. After 1.50 V during NOR, R2 appeared and decreased. After 1.60 V during
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Figure 2. Further Exploration of the Relationship between Surface Species, Interfaces, and
Reactions Based on In Situ Raman Spectroscopy
(A and B) In situ Raman spectroscopy of b-Ni(OH)2 electrode for (A) OER (1 M KOH) and (B) NOR
(1 M KOH with 0.5 M ethanol).
(C–F) Schematic illustrations of the relationship between surface species, interfaces, and reactions:
(C) R(QR) model in both OER and NOR systems from 1.15 to 1.30 V; (D) R(QR)(QR) model in OER
system from 1.35 to 1.65 V; (E) R(QR) model in NOR system from 1.35 to 1.45 V; and (F) R(QR)(QR)
model in NOR system from 1.50 to 1.65 V.
NOR, negative R2 represented the passivation reaction occurring at the low-fre-
quency interface. It showed the interface reaction evolutions on b-Ni(OH)2 electrode
for the OER and NOR system (Figure 1F).
For the EIS analysis, the high-frequency interface between b-Ni(OH)2 and DDL was
more explicit than the low-frequency interface. In situ Raman spectroscopy was car-
ried out to further identify the low-frequency interface. In the Raman spectra, two
peaks at 473 and 553 cmÀ1 corresponded to Ni3+-O bending and stretching, respec-
tively.17,30 After 1.35 V, the electrooxidation potential of b-Ni(OH)2, Ni3+-O of the
oxide layer (Ni2+dOxHy) can be detected distinctly during OER (Figures 1D and
2A). Nevertheless, Ni3+-O species only accumulated after 1.60 V during NOR
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(Figure 2B). Combining in situ Raman spectroscopy and operando EIS, we discov-
ered that Ni3+-O species were synchronized with the low-frequency interface, which
proves that the low-frequency interface is between the oxide layer (Ni2+dOxHy) and
DDL (Figures 2A, 2B, and S9–S11). Based on the above evidence, schematic illustra-
tions of the relationship between surface species, interfaces, and reactions during
OER and NOR are shown in Figures 2C–2F. Regardless of whether it was in the
OER or NOR system, at 1.15 to 1.30 V, only the high-frequency interface between
b-Ni(OH)2 and DDL existed without an electrochemical reaction, which suits the
R(QR) model (Figure 2C). For the OER system, after 1.35 V, b-Ni(OH)2 was electro-
chemically oxidized to Ni2+dOxHy at the high frequency between Ni2+dOxHy and
b-Ni(OH)2; meanwhile, OER intermediates were adsorbed and left as oxygen on
the Ni2+d site at the low frequency between Ni2+dOxHy and DDL (Figure 2D). For
the NOR system, at 1.35 to 1.45 V, NOR occurred at the high-frequency interface be-
tween b-Ni(OH)2 and DDL; thereinto, electrons and protons of excess nucleophile
can fill the hydrogen defect of b-Ni(OH)O promptly and completely (b-Ni(OH)O +
HNu + eÀ
= b-Ni(OH)2), so that the morphology and electronic structure of
Nu
b-Ni(OH)2 can remain unchanged without phase transition (Figure 2E). After
1.50 V in the NOR system, the electron transfer rate of the catalyst dehydrogenation
reaction (b-Ni(OH)2 + OHÀ = b-Ni(OH)O + H2O + eÀ) was faster than that of the
nucleophile dehydrogenation reaction (b-Ni(OH)O + HNu + eÀNu = b-Ni(OH)2), which
resulted in local phase transition and passivation (Figures 1D and 2F). These results
strongly demonstrate that, for b-Ni(OH)2, the NOR activity origin is a b-Ni(OH)O
intermediate rather than Ni2+dOxHy or OER intermediates.
TOF-SIMS was carried out to trace the hydrogen isotope, deuterium, labeled in
nucleophile during NOR (Figure S12). TOF-SIMS spectra of b-Ni(OH)2 electrode
before and after NOR in 1 M KOH with 0.5 M CD3OD prove that D of CD3OD
does replace hydrogen atom of b-Ni(OH)2 during NOR (b-Ni(OH)O + DNu
+
eÀ
= b-Ni(OH)(OD)) (Figures 3A, 3B, and S12). For the NOR mechanism of
Nu
b-Ni(OH)2, the catalyst dehydrogenation reaction was expressly revealed above;
however, there is some debate as to whether nucleophile dehydrogenation is a
spontaneous reaction. Although the electron that b-Ni(OH)O robbed from the
nucleophile cannot be quantified directly (b-Ni(OH)O + HNu + eÀ = b-Ni(OH)2),
Nu
we can deduce it via the measuring charge transferred from the circuit (b-Ni(OH)
O + H2O + eÀ
= b-Ni(OH)2 + OHÀ) (Figures 3C and S13). Hence, first, the
circuit
high potential was set (1.45, 1.50, and 1.6 V) for the b-Ni(OH)2 electrode to enrich
the b-Ni(OH)O intermediate, then the nucleophile was injected during the open
circuit state, so that b-Ni(OH)O could react with the nucleophile adequately; finally,
the potential was switched to 1.095 V (the open circuit potential) to identify the num-
ber of surplus b-Ni(OH)O (Figures 3D and S14–S16). As a result, b-Ni(OH)O could not
be probed at 1.095 V, either when NOR occurred directly or the nucleophile was
added half-way, which proves that for b-Ni(OH)2, the nucleophile dehydrogenation
reaction is spontaneous. Hence, we hold that for b-Ni(OH)2, the NOR mechanism is a
two-step, one-electron reaction, including an electrogenerated catalyst dehydroge-
nation reaction and a spontaneous nucleophile dehydrogenation reaction. First, a
hydroxide ion (OHÀ) combines with protons from b-Ni(OH)2 and leaves as H2O;
meanwhile, electrons are transferred to the circuit, which results in the formation
of b-Ni(OH)O with an electrophilic oxygen. Second, the electrophilic oxygen of
b-Ni(OH)O takes electrons and protons from the nucleophile spontaneously and
promptly, which results in b-Ni(OH)O reverting to b-Ni(OH)2 (Figure 3E). Remark-
ably, when the reaction rate of the second step is much faster than that of the first
step, b-Ni(OH)2 is theoretically unchanged, because the b-Ni(OH)O intermediate
cannot be accumulated, avoiding the phase transition. Hence, with excessive
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Figure 3. NOR Mechanism Including Electrogenerated Catalyst Dehydrogenation Reaction and
Spontaneous Nucleophile Dehydrogenation Reaction for b-Ni(OH)2
(A) Schematic illustration of isotope labeling experience during NOR.
(B) Negative TOF-SIMS spectra of OÀ, OHÀ, and ODÀ for b-Ni(OH)2, b-Ni(OH)2 soaking in 1 M KOH
with 0.5 M CD3OD, and b-Ni(OH)2 after NOR in 1 M KOH with 0.5 M CD3OD.
(C) Schematic illustrations of verification experiment on whether the proton-coupled electron
transfer reaction between nucleophile and b-Ni(OH)O intermediate is a spontaneous reaction.
(D) Multi-potential step curves of b-Ni(OH)2 electrode (including 2-s open circuit time).
(E) Proposed NOR mechanism scheme for b-Ni(OH)2.
nucleophile at the appropriate potential, the morphology, crystal structure, and sur-
face electronic state of b-Ni(OH)2 can be maintained during NOR.
Based on the above characterizations and analysis, for peculiar NOR mechanism of
b-Ni(OH)2, the reciprocal transformation between b-Ni(OH)2 and b-Ni(OH)O, as an
intermediary during NOR, is directly related to the crystal structure of hydroxide.
Spontaneously, the importance of lattice hydroxyl in b-Ni(OH)2 for NOR was system-
atically studied by comparing surface evolutions of b-Ni(OH)2 and NiO during NOR,
because NiO lacks hydroxyl and has octahedral structures (Ni2+O6), similar to
b-Ni(OH)2. The characterizations of NiO are shown in Figures S17 and S18.
The LSV curves, Nyquist plots, Bode plots, and two equivalent circuit models for NiO
in the OER and NOR are shown in Figures S19–S21. The optimum fit parameters for
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the EIS data are shown in Tables S4–S6. For NiO, NOR occurs at the low-frequency
interface similar to OER, which proves that the NOR activity origin may be one of the
adsorption OER intermediates (Figures 4A and S20). Electrophilic oxygen in OH*,
formed preferentially at the low-frequency interface, is more likely to be the NOR ac-
tivity origin of NiO.24 The variation trend of interface reactions for NiO during OER or
NOR is presented clearly (Figure 4B). Corresponding schematic illustrations of the
relationship between surface species, interfaces, and reactions during OER and
NOR are shown in Figures 4C–4F. In both OER and NOR systems, at 1.15 to
1.30 V, only the high-frequency interface between NiO and DDL exists without elec-
trochemical reaction, which suits the R(QR) model (Figure 4C). At 1.35 to 1.50 V in the
OER system, OH* can be adsorbed as NiO(OH)ads but cannot be further oxidized to
oxygen, which results in only the presence of high-frequency interface suiting the
R(QR) model (Figure 4D). After 1.55 V during OER, NiO is electrochemically oxidized
to Ni2+dO at the high frequency between NiO (inner layer) and Ni2+dO (oxide layer);
meanwhile, OER takes place at the low frequency between Ni2+dO and DDL (Fig-
ure 4E). After 1.35 V in the NOR system, NOR also occurs at the low-frequency inter-
face between Ni2+dO and DDL, similar to OER (Figure 4F). It is worth noting that, at
1.35 to 1.50 V, in the OER system, Ni2+dO(OH)ads cannot be further transformed to
oxygen; however, in the NOR system, Ni2+dO(OH)ads seizes protons and electrons of
the nucleophile (Ni2+dO(OH)ads + HNu + eÀNu = Ni2+dO + H2O) as the low-frequency-
interface reaction (Figures 4D, 4F, and S19). After 1.55 V in the OER system, R2 of the
low-frequency-interface reaction appears and reduces gradually, which is synchro-
nous with the OER current density (Figure 4B). At 1.35 to 1.45 V in the NOR system,
R2 appears and reduces gradually, which is synchronous with the NOR current den-
sity; however, R2 increases with voltage because of passivation after 1.50 V (Figures
4B, S19, S20B, and S20D). In view of passivation occurring at the low-frequency
interface (1.50 to 1.65 V), the NOR activity of Ni2+dO(O)ads or Ni2+dO(OOH)ads, which
is formed at a higher potential, is far lower than that of Ni2+dO(OH)ads formed
at ꢀ1.35 V. Hence, the NOR activity origin for NiO is Ni2+dO(OH)ads rather than
Ni2+dO(O)ads or Ni2+dO(OOH)ads. The NOR pathway for b-Ni(OH)2 has been
described in detail above (Figure 4G). Based on the above experiments, we propose
the NOR pathway for NiO: (1) Ni sites can lose electrons and adsorb OHÀ after
1.35 V, and therefore, Ni2+dO(OH)ads with electrophilic oxygen is generated; and
(2) Ni2+dO(OH)ads can seize protons and electrons from the nucleophile and leave
as H2O (Figure 4H).
The NOR activity origins for b-Ni(OH)2 and NiO are b-Ni(OH)O intermediate (which
reacts with the nucleophile at the high-frequency interface) and NiO(OH)ads (which
reacts with nucleophile at the low-frequency interface), respectively. The electro-
philic oxygens of activity origins for b-Ni(OH)2 and NiO derive from lattice oxygen
and adsorption oxygen, respectively. Above all, we explored the NOR activity ori-
gins, which provides theoretical guidance for the design principle of NOR
electrocatalysts.
Modification Strategy for b-Ni(OH)2 to Enhance the NOR Activity
For NOR of b-Ni(OH)2, the key aspect of the modification strategy is regulating the
production potential of b-Ni(OH)O because b-Ni(OH)O can rob protons and elec-
trons from the nucleophile spontaneously. In other words, the dehydrogenation
from lattice hydroxyl group with eight-electron oxygen of b-Ni(OH)2 to electrophilic
lattice oxygen of b-Ni(OH)O is directly related to NOR performance; hence, tuning
the lattice oxygen ligand environment can effectively change the dehydrogenation
potential of b-Ni(OH)2, thus regulating NOR performance. Previous studies have
proved that doping Co2+ or Mn2+ in nickel-based hydroxide can lower the onset
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Figure 4. Different NOR Mechanisms for b-Ni(OH)2 and NiO via Reaction Interface Analysis
(A) Bode plots of NiO electrode for OER and NOR at different potentials (Figure S20).
(B) Correlation of the charge transfer resistances (R1 and R2) and potentials for NiO during OER and NOR.
(C–F) Schematic illustrations of the relationship between surface species, interfaces, and reactions:
(C) R(QR) model in both OER and NOR systems from 1.15 to 1.30 V; (D) R(QR) model in OER system
from 1.35 to 1.50 V; (E) R(QR)(QR) model in OER system from 1.55 to 1.65 V; and (F) R(QR)(QR) model
in NOR system from 1.35 to 1.70 V.
(G and H) The NOR mechanism schemes for (G) b-Ni(OH)2 and (H) NiO.
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Figure 5. The Correlation of NOR Activity and the Oxygen Ligand Environments for b-Ni(OH)2
(A–D) LSV curves for b-CoxNi1Àx(OH)2 (x = 0, 0.05, 0.1, 0.15, and 0.2) in (A) 1 M KOH and (B) 1 M KOH
with 50 mM ethanol, b-M0.1Ni0.9(OH)x (M = Co, Mn, Fe, and Al) in (C) 1 M KOH and (D) 1 M KOH with
50 mM ethanol.
(E and F) Ni K-edge in situ XANES spectra for b-Co0.1Ni0.9(OH)2 during (E) OER (1 M KOH) and (F)
NOR (1 M KOH with 0.5 M ethanol). Insets show LSV curves.
potential from Ni(OH)2 to Ni(OH)O significantly; however, doping Fe3+ or Al3+ in-
hibits the transformation from Ni(OH)2 to Ni(OH)O and increases its onset poten-
tial.54–56 Hence, b-CoxNi1-x(OH)2, b-Mn0.1Ni0.9(OH)2, b-Fe0.1Ni0.9(OH)2.1
, and
b-Al0.1Ni0.9(OH)2.1 were synthesized; the corresponding characterizations are shown
in Figures S22–S26.
With the increase in doping Co, the onset potential of b-CoxNi1-x(OH)O gradually
moved toward lower (Figure 5A). With the increase in doping Co from 0% to 10%,
the NOR activity was enhanced significantly, which is consistent with the onset po-
tential of b-CoxNi1-x(OH)O, and the NOR onset potential obviously decreases
from 1.395 to 1.356 V (Figures 5B, S27A, and S27B). The NOR activity of
b-Co(OH)2 is very poor, and NOR does not occur within the oxidation potential
range of Co2+/Co3+ redox (Figures S27C and S27D). In order to identify the role
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of Co2+ in b-CoxNi1-x(OH)2 during NOR, crystal and electronic structures were char-
acterized (Figures S28–S31). Based on the TEM images and XPS spectra, after OER,
Co2+ and Ni2+ species were oxidized to Co3+ and Ni3+ species, respectively, accom-
panied by dilapidated morphology and NiCoOx phase with metal oxygen bonds.
However, after NOR, Ni2+ species, oxygen ligand structure, and electrocatalyst
morphology kept original status, and a small number of Co2+ species were still
oxidized to Co3+ species (Figures S28–S31). Owing to a special mechanism during
NOR, excessive nucleophile prevented Ni3+ species from forming and kept electro-
catalyst morphology; meanwhile, surface Co2+ species were oxidized to Co3+ spe-
cies because of lacking NOR activity for Co2+ species; however, unchanged
morphology prevented inner Co2+ species from being oxidized. These experiments
further indicate that for NOR of b-Co0.1Ni0.9(OH)2, doping Co2+ (Co-OH) plays a key
role in tuning a lattice oxygen (Ni-OH) ligand environment rather than providing
NOR performance directly. Although increasing doping Co can reduce the onset po-
tential of b-CoxNi1-x(OH)O, b-CoxNi1-x(OH)O with excessive doping Co cannot rob
electron and proton from nucleophile spontaneously in the second step of NOR,
which results in a degenerative NOR performance. When the doping content of
Co exceeds 10%, the NOR activity has a slight decrease (Figure 5B). To avoid the
influence of specific surface area, LSV polarization curves normalized by Brunauer,
Emmett and Teller (BET) surface area are shown in Figure S32, which indicates
that for NOR, the optimal doping content of Co in b-Ni(OH)2 is 10%.57 In addition,
NOR performance normalized by electrochemical activity surface area proves
once again that the NOR intrinsic activity of b-Ni(OH)2 can be drastically promoted
by tuning lattice oxygen ligand environment, such as doping Co (Figure S33).
Doping Mn2+ in b-Ni(OH)2 can also reduce onset potential significantly and improve
NOR activity; however, doping Fe3+ or Al3+ can increase the onset potential and
weaken NOR activity (Figures 5C and 5D). The above experimental results agree
with DFT calculations based on a special NOR mechanism for b-Ni(OH)2 (Figure S34).
Owing to the spontaneous nucleophile dehydrogenation reaction, the internal rela-
tionship between NOR performance and lattice oxygen ligand structure of
b-Ni(OH)2 is elaborated in Figure S35. In short, tuning the lattice oxygen ligand envi-
ronment of b-Ni(OH)2 by moderate doping Co2+ or Mn2+ can effectively reduce the
dehydrogenation potential from lattice hydroxyl group of b-M0.1Ni0.9(OH)2 to elec-
trophilic lattice oxygen of b-M0.1Ni0.9(OH)O, which can seize protons and electrons
from the nucleophile spontaneously, thus improving NOR activity. However, tuning
the lattice oxygen ligand environment of NiO by doping Co cannot reduce the onset
potential of NOR, which verifies that the NOR activity origin of NiO is adsorbed as
OH* rather than lattice oxygen (Figures S36–S38).
To further illustrate the NOR mechanism of b-Ni(OH)2 and b-Co0.1Ni0.9(OH)2, the sur-
face atomic structures of Ni and Co centers were characterized by in situ XAS during
OER or NOR (Figures S39–S41). The Ni valence state in b-Ni(OH)2 increased signif-
icantly after 1.35 V during OER because of the electrooxidation from b-Ni2+(OH)2 to
b-Ni2+d(OH)O (Figure S40A).58 At 1.35 to 1.45 V, the increase of Ni valence state in
b-Ni(OH)2 during NOR is smaller than that during OER, because b-Ni2+d(OH)O can
seize electrons and protons of nucleophile spontaneously and convert to
b-Ni2+(OH)2 (Figure S40). The FT k3c(k)-R space curves for b-Co0.1Ni0.9(OH)2, fitting
analysis of extended X-ray absorption fine structure (EXAFS), further prove that Ni-O
bonds are replaced by Co-O bonds in b-Ni(OH)2 (Figure S41).59 The Ni valence state
in b-Co0.1Ni0.9(OH)2 also increased significantly after 1.3 V during OER; however, at
1.25 to 1.45 V, the increase of Ni valence state during NOR was much smaller than
that during OER, because b-Co0.1Ni0.92+d(OH)O can seize electrons and protons of
nucleophile spontaneously and convert to b-Co0.1Ni0.92+(OH)2 (Figures 5E and 5F).
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The NOR mechanism for b-Ni(OH)2 is a two-step reaction; thereinto, in situ X-ray
near-edge structure (XANES) spectra during OER represent the result of the first
step in NOR, and in situ XANES spectra during NOR represent the result of second
step in NOR. The proton-coupled electron transfer of two-step reaction (NOR)
for b-Ni(OH)2 is revealed: at the first step, the Ni valence state increases because
b-Ni2+d(OH)O is generated. At the second step, compared with the first step, the
Ni valence state decreases because b-Ni2+d(OH)O converts into b-Ni2+(OH)2 by
seizing electrons and protons of the nucleophile spontaneously.
Extension of the Results to Other Nucleophiles for Industrial Application
To verify the universality of b-Ni(OH)2 for NOR, the corresponding NOR perfor-
mances of n-propanol, n-butanol, n-amyl alcohol, and benzyl alcohol were also
tested (Figure S42). With the increase of steric hindrance effect of nucleophile, the
NOR activity of b-Ni(OH)2 or b-Co0.1Ni0.9(OH)2 on the glass carbon electrode (planar
electrode) decreases markedly, especially for n-pentyl alcohol and benzyl
alcohol.60,61 Therefore, in order to weaken the negative effect of steric hindrance
and improve the NOR current density, electrocatalysts were grown on nickel
foam (NF). The X-ray diffraction (XRD) pattern shows that b-Ni(OH)2/NF and
b-Co0.1Ni0.9(OH)2/NF were successfully synthesized (Figure S43). With the increase
of nucleophile steric hindrance, for b-Co0.1Ni0.9(OH)2/NF, NOR activity declines
slightly; meanwhile, b-Co0.1Ni0.9(OH)2/NF exhibits a better NOR activity with high
current density (Figures 6A and 6B). It has established two prerequisites for the
industrialization of NOR: (1) high current density, and (2) wide adjustability of sub-
strates without obvious deactivation.
The b-Co0.1Ni0.9(OH)2/NF exhibits excellent activity for ethanol oxidation reaction
(EOR) and shows a lower onset potential, which is down about 60 mV compared
with b-Ni(OH)2/NF (Figure 6C). At 1.35 V during EOR, b-Co0.1Ni0.9(OH)2/NF can
reach 98 mA cmÀ2; however, there is almost no current density (5 mA cmÀ2) for
b-Ni(OH)2/NF. For EOR, the oxidation product is acetic acid through gas chroma-
tography-mass spectrometry (GC-MS) and 1H NMR (Figures S44 and S45). Figure S46
shows the GC chromatograms of ethanol and oxidation product (acetic acid) before
and after EOR. The conversion of ethanol can reach ꢀ100%, the selectivity can be as
high as 98%, and its faradic efficiency is 95% for b-Co0.1Ni0.9(OH)2/NF (Figure S46).
Long-term test on b-Co0.1Ni0.9(OH)2/NF electrode and LSV curves before/after long-
term test in 1 M KOH with 50 mM ethanol are shown in Figure S47, which proves the
good stability of b-Co0.1Ni0.9(OH)2/NF during NOR.
CoPx/NF was synthesized as a cathodic electrocatalyst owing to its excellent HER ac-
tivity reaching 100 mA cmÀ2 at À0.125 V.62 The nucleophile has almost no negative
effect on HER in 1 M KOH with 50 mM ethanol (Figure 6D). When CoPx/NF is used as
a cathode and EOR is coupled with HER, the voltage cell with b-Co0.1Ni0.9(OH)2/NF
as an anode shows a decrease by ꢀ70 mV at 100 mA cmÀ2 compared with b-Ni(OH)2/
NF; in addition, the cell voltages are lowered to 1.46 and 1.49 V to achieve 100 and
200 mA cmÀ2, respectively (Figure 6D).
Phenylcarbinol, 5-Hydroxymethylfurfural (HMF), urea, and benzylamine were chosen
as substrates to extend nucleophiles in the NOR system. The oxidation products of
ethanol, phenylcarbinol, HMF, and benzylamine are acetic acid, benzoic acid, 2,5-
Furandicarboxylic acid (FDCA), and phenylnitrile, respectively (Figures S44 and
S45). The conversions of NOR with different nucleophiles are close to ꢀ100%, and
the selectivities are above 96%. Owing to low efficiency of electrolysis at ultra-low
concentration, faradic efficiencies are slightly lower but still above 93% (Figure S46;
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Figure 6. Electrocatalytic NOR Measurements for Industrial Application about Overcoming the Negative Effect of Steric Hindrance, Amplifying the
Current Density and Broadening Applicability
(A) LSV curves for b-Co0.1Ni0.9(OH)2/NF electrode in 1 M KOH with 50 mM different nucleophiles. Inset shows LSV curves in the same condition for
b-Ni(OH)2 on glassy carbon electrode.
(B) Current densities of b-Ni(OH)2, Co0.9Ni0.1(OH)2 on glassy carbon electrode, and b-Co0.1Ni0.9(OH)2/NF at different potentials during NOR with
different nucleophiles.
(C) LSV curves for b-Ni(OH)2/NF and b-Co0.1Ni0.9(OH)2/NF electrodes in 1 M KOH with 50 mM ethanol. Inset shows the conversion, selectivity, and
faradic efficiency of EOR for b-Co0.1Ni0.9(OH)2/NF.
(D) With CoPx/NF as cathode and b-Ni(OH)2/NF or b-Co0.1Ni0.9(OH)2/NF as anode, LSV curves for EOR coupled HER in a two-electrode electrochemical
cell setup. Inset shows LSV curves of CoPx/NF in 1 M KOH with/without 50 mM ethanol.
(E–H) LSV curves for b-Ni(OH)2/NF and b-Co0.1Ni0.9(OH)2/NF in 1 M KOH with 50 mM (E) phenylcarbinol, (F) HMF, (G) urea, and (H) benzylamine.
Table S7). The b-Co0.1Ni0.9(OH)2/NF exhibits excellent NOR activities for different
nucleophiles and shows a decrease in onset potential by ꢀ60 mV compared with
b-Ni(OH)2/NF (Figures 6E–6H). By tuning the lattice oxygen ligand environment of
b-Ni(OH)2, for b-Co0.1Ni0.9(OH)2/NF, the onset potentials of NOR overtly decreased
to ꢀ1.29 V, which breaks the bottleneck of ꢀ1.35 V for most nickel-based catalysts
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proposed in recent studies (Table S8). These provide theoretical guidance and tech-
nologic support for the development of NOR coupled with hydrogen generation
and organic electrosynthesis in industrial applications.
In summary, on the one hand, NOR, as a half-reaction to couple with HER, is a prom-
ising reaction for hydrogen energy and organic synthesis. Compared with overall
water splitting, it can reduce the energy consumption of hydrogen production and
also obtain highly pure organic products with added value. On the other hand,
the deficiency of a clear understanding of NOR mechanisms restricts the develop-
ment of this emerging reaction. Aiming at this issue, we discovered that for
b-Ni(OH)2 and NiO, the activity origin of NOR is b-Ni(OH)O (which is generated by
electrooxidation of b-Ni(OH)2 at a high-frequency interface between b-Ni(OH)2
and DDL) and NiO(OH)ads (which is one of the intermediates during OER at a low-fre-
quency interface between Ni2+dO and DDL), respectively, through comprehensive
analysis combining surface species, interfaces, and reactions during OER or NOR.
The most significant difference between two different NOR paths for b-Ni(OH)2
and NiO is that their electrophilic oxygens come from lattice oxygen of b-Ni(OH)O
and adsorbed oxygen of NiO(OH)ads, respectively. Moreover, we proved that for
b-Ni(OH)2, NOR is a two-step, one-electron reaction, including an electrogenerated
catalyst dehydrogenation reaction and a spontaneous nucleophile dehydrogenation
reaction. Based on the unique activity origin of b-Ni(OH)2 involving lattice oxygen
and special two-step mechanism for NOR, we propose a general strategy to regulate
NOR activity for b-Ni(OH)2 by tuning the lattice oxygen ligand environment. These
may provide theoretical guidance and technologic support for the development
of NOR and organic electrosynthesis in industrial applications.
EXPERIMENTAL PROCEDURES
Resource Availability
Lead Contact
Information and requests for resources and reagents should be directed to and will
be fulfilled by the Lead Contact, Shuangyin Wang (shuangyinwang@hnu.edu.cn).
Materials Availability
This study did not generate new unique reagents.
Data and Code Availability
The published article includes all datasets generated or analyzed during this study.
Synthesis of b-Ni(OH)2 and b-MxNi1-x(OH)y (M = Co, Mn, Fe)
All chemical reagents were produced by Sinopharm Chemical Reagent Co., Ltd. In a
typical procedure, 12 mmol Ni(NO3)2$6H2O was dissolved in 40-mL deionized water
and stirred for 15 min. The aqueous 0.6 M NaOH solution was added to the
Ni(NO3)2$6H2O (0.3M) solution until the final pH of the mixed suspension was
greater than 13. After stirring for 30 min, the Ni(OH)2 suspension was transferred
to a 100-mL Teflon-lined stainless-steel autoclave and maintained at 160ꢁC for 6
h. The autoclave was cooled down naturally to room temperature. The b-Ni(OH)2
was washed with deionized water and anhydrous ethanol three times and dried at
60ꢁC for 10 h. Finally, the b-Ni(OH)2 was obtained. In addition, the b-MxNi1-x(OH)y
(M = Co, Mn, Fe) was synthesized using the same method.
Synthesis of b-Ni0.9Al0.1(OH)2
0.27 mmol Ni(NO3)2$6H2O, 0.03 mmol Al(NO3)3$9H2O, 1.5 mmol urea, and 1 mmol
NH4F were dissolved in 60-mL deionized water and stirred for 15 min. The
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suspension was transferred to a 100-mL Teflon-lined stainless-steel autoclave and
maintained at 130ꢁC for 6 h. The autoclave was cooled down naturally to room tem-
perature. The b-Ni0.9Al0.1(OH)2 was washed with deionized water and anhydrous
ethanol three times and dried at 60ꢁC for 10 h.
Synthesis of b-Co(OH)2
0.5 g CoCl2$6H2O was dissolved with 25-mL deionized water and stirred for 20 min.
The aqueous 50-mL NaOH solution (1 M) was added to the CoCl2$6H2O solution and
stirred for 30 min. After stirring, the Co(OH)2 suspension was transferred into a
100-mL Teflon-lined stainless-steel autoclave and maintained at 100ꢁC for 8 h.
The autoclave was cooled down naturally to room temperature. The b-Co(OH)2
was washed with deionized water and anhydrous ethanol three times and dried at
60ꢁC for 10 h, and then b-Co(OH)2 was obtained.
Synthesis of b-NixCo1-x(OH)2/NF (x = 1, 0.9)
10 mmol Ni(NO3)2 and 5 mmol NH4NO3 were dissolved with 32-mL deionized water
and stirred for 15 min. 18-mL ammonia (28 wt %) was dribbled into the mixed solu-
tion with vigorous stirring. After stirring for 15 min, the mixed solution was poured
into a culture dish. The culture dish with solution was preheated at 90ꢁC for 2 h.
Then the clean NF was placed in the mixed solution and maintained at 90ꢁC for
12 h. The b-Ni(OH)2/NF was washed with deionized water and anhydrous ethanol
three times and dried at 60ꢁC for 10 h. Finally, the b-Ni(OH)2/NF was obtained. In
addition, the b-Co0.1Ni0.9(OH)2/NF was synthesized using the same method.
Synthesis of NiO and Co0.1Ni0.9
O
Under Ar atmosphere, the b-Ni(OH)2 and b-Co0.1Ni0.9(OH)2 were heated at 300ꢁC
for 2 h in a tube furnace. Both heating and cooling rates were maintained at 5ꢁC/min.
Finally, the NiO and the Co0.1Ni0.9O were obtained.
Material Characterization
The structure and morphology of all catalysts were characterized by scanning elec-
tron microscope (SEM, Hitachi, S-4800), TEM (FEI Tecnai G20), and XRD (Bruker D8
Advance diffractometer, Cu Ka1). The BET surface area was measured by JWGB JW-
BK200C. XPS was tested by AXIS Supra (Axis Supra, Kratos, England). All XPS
spectra were adjusted by C 1s (284.8 eV). TOF-SIMS was carried out by TOF-SIMS
5 instrument (ION-TOF GmbH, Munster, Germany).
¨
Electrochemical Measurements
All of the electrochemical tests (excluding operando EIS) were measured with the
CHI 760D electrochemical workstation. The deionized water was produced by EDI
touch-Q/S (HHitech, China, Shanghai). Rotating ring-disc electrode was carried
out by Compact Pine Rotator+WaveNow (PINE, America). For powdered catalysts,
the catalysts (4 mg) were mixed with deionized water (950 mL) with ultrasonic disper-
sion for 20 min. The suspensions were added with 50 mL 5 wt % Nafion solution, and
the mixture was sonicated for 20 min. 10 mL of the catalyst suspension (0.2 mg cmÀ2
)
was dropped on a glassy carbon electrode (5 mm in diameter). The electrochemical
tests were carried out with the three-electrode configuration (excluding measure-
ments for undivided cell reaction). During the measurements of half-cell reactions
(HER, OER, and NOR), the reference electrode was saturate calomel electrode
(SCE) with a double-salt bridge (217-01, Leici, Shanghai, China). During the mea-
surements of undivided cell reaction, the electrochemical tests were carried out
with the two-electrode configuration. The counter electrode was platinum gauze
for HER. The SCE reference electrode was corrected by testing the reversible
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hydrogen electrode (RHE) potential with platinum gauze electrode in 1 M KOH
hydrogen-saturated solution. For LSV, the scan rate was set at 5 mV sÀ1 in order
to minimize the effect of capacitance. All electrochemical data were recorded
until the test results kept a stable state.63 The LSV (excluding b-Ni(OH)2/NF
and b-Ni0.1Co0.9(OH)2/NF) curves were adjusted by eliminating iR (100%) drop
of the solution ohmic resistance. The LSV curves for b-Ni(OH)2/NF and
b-Ni0.1Co0.9(OH)2/NF were adjusted by eliminating iR (90%) drop of the solution
ohmic resistance. Operando EIS was carried out by Autolab PGSTAT302N (Eco
Chemie, Utrecht, the Netherlands) in the three-electrode system under different po-
tentials over the frequency range from 1 MHz to 0.1 Hz in 1 M KOH with/without
0.5 M ethanol. All experiments were carried out at room temperature (25ꢁC).
In Situ XAS Measurements
Synchrotron-based XAS is a potential characterization tool to research the detailed
coordination structure of materials and the element valence. XAS technology has
been widely used in the structural studies of catalytic material.64 The complete
XAS spectrum can be divided into two parts: XANES, which refers to the portion
of the spectrum that absorbs energy in the energy range of about 20–40 eV, and
EXAFS, which refers to the extension from the beginning of the absorption edge
40–800 eV. In situ XAS reveals the redistribution of dynamic electron charge, the
movement of the conduction band edge, the change of unoccupied state density,
and the improvement of electrocatalytic activity; so, this has been extensively
studied.
The synchrotron X-ray spectroscopies were performed at the National Synchrotron
Radiation Research Center (NSRRC), Taiwan. The Co K-edge and Ni K-edge XAS
were recorded in transmission mode via the Quick-EXAFS recorder system at Taiwan
Photon Source (TPS) 44A beamline at NSRRC.65–67 Pure cobalt and nickel metal foils
were used for energy calibration. All spectra were carried out with 1 Hz oscillating
frequency for 5 min. This means we have 600 spectra to average and increase the
S/N ration and normalize to unit step height in the absorption coefficient from
well below to well above the edges. Raw X-ray absorption data were analyzed
according to standard procedures, including front- and back-edge background sub-
traction, normalization for edge hopping, Fourier transform, and nonlinear least
squares curve fitting. A detailed description of the process can be found else-
where.68 All computer programs are implemented in the package UWXAFS 3.0.69
The backscatter amplitude and phase shift of a particular atom pair are calculated
from the beginning by using the FEFF8 code.70
In Situ Raman Spectrum
In situ Raman spectrum was carried out on the confocal Raman microscope
(Alpha300R, WETEC, Germany) under different potentials by CHI 760D electro-
chemical workstation. The electrolytic cell for in situ Raman spectrum comprises a
Teflon shell, quartz glass plate, and glassy carbon electrode. For powdered cata-
lysts, the catalysts (4 mg) were mixed with deionized water (1 mL) with ultrasonic
dispersion for 20 min. 6.4 mL of the catalyst suspension (0.2 mg cmÀ2) was dropped
on a glassy carbon electrode (4 mm in diameter). All of the electrochemical tests
were carried out with the three-electrode configuration. During the measurements,
the reference electrode was SCE. The counter electrode was a platinum wire for HER.
The SCE reference electrode was corrected by testing the RHE potential with a plat-
inum gauze electrode in 1 M KOH hydrogen-saturated solution. For the OER system,
the electrolyte is 1 M KOH. For the NOR system, electrolyte is 1 M KOH with 0.5 M
ethanol.
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DFT Calculations
Here, we performed spin-polarized DFT by using the Vienna Ab initio Simulation
Package (VASP).71 The DFT+U approach was employed to improve the errors
from the overdelocalization of electrons of transition metals. The U-J correction of
5.5, 5.2, 5, and 5 eV were added for Ni 3d, Mn 3d, Fe 3d, and Co 3d, respectively.
The generalized gradient approximation with the Perdew-Burke-Ernzerhof method
was used.72 The frozen-core projector-augmented wave method was used.73 The
˚
convergence criterion for the atomic force and energy was set to 0.02 eV/A and
10À5 eV, respectively. The kinetic energy cutoff was 520 eV for this work, with gamma
point centered Monkhorst-Pack k-point meshes of 63636 for NiO, 83834 for
NiOOH, and 93935 for b-Ni(OH)2, as well as 33331 for the (001) lattice plane of
b-Ni(OH)2 and (200) lattice plane of NiO, respectively. The DFT-D3 method was
introduced in this work.
Product Detection
Nucleophiles and the products of nucleophiles were identified by 1H NMR (Bruker
400 MHz) and GC-MS (Shimadzu GCMS-QP2010). Nucleophiles and the products
of nucleophiles (excluding HMF and FDCA) were quantified by Shimadzu GC-2010
gas chromatography instrument (Shimadzu, Japan). HMF and FDCA were quanti-
fied by HPLC instrument (LC 20A, Shimadzu, Japan) because of poor thermal
stability of HMF and FDCA. All samples (excluding benzylamine and phenylnitrile)
were mixed with the same volume of 0.5 M H2SO4 solution in order to protect
instruments. Benzylamine and phenylnitrile were extracted with ethyl acetate
(6 3 30 mL). The theoretic total charge of NOR (excluding HMF and FDCA) is
ꢀ579 C in 1 M KOH solution (30 mL) with 50 Mm nucleophile. The theoretic total
charge of HMF electrooxidation is ꢀ724 C in 1 M KOH solution (25 mL) with 50 mM
HMF.
The theoretic total charge of NOR (excluding HMF and FDCA) is as follows:
4 3 (0.03 L) 3 (0.05 mol LÀ1) 3 (6.02 3 1023 molÀ1) 3 (1.6 3 10À19 C) = ꢀ579 C
The theoretic total charge of HMF electrooxidation is as follows:
6 3 (0.025 L) 3 (0.05 mol LÀ1) 3 (6.02 3 1023 molÀ1) 3 (1.6 3 10À19 C) = ꢀ724 C
The conversion rate, selectivity, and faradic efficiency for NOR are as follows:
Conversion rate (%) = (mol of nucleophile after electrolysis)/(mol of initial primary
nucleophile) 3 100%
Selectivity (%) = (mol of product after electrolysis)/(mol of consumed primary nucleo-
phile) 3 100%
Faradic efficiency (%) = (mol of product after electrolysis) 3 n 3 F/(total passed charge)
3 100%
n = 4 for ethanol, phenylcarbinol, and benzylamine
n = 6 for HMF
F is the Faraday constant (96,485 C molÀ1).
Chem 6, 1–20, November 5, 2020
17

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ll
Article
SUPPLEMENTAL INFORMATION
Supplemental Information can be found online at https://doi.org/10.1016/j.chempr.
2020.07.022.
ACKNOWLEDGMENTS
The authors acknowledge the support received from the National Natural Science
Foundation of China (grant nos. 51402100, 21573066, 21905088, 21825201, and
21522305), the Provincial Natural Science Foundation of Hunan (grant nos.
2016JJ1006, 2016TP1009, and 2020JJ5039), and the China Postdoctoral Science
Foundation (grant no. 10 2019M662766). Most of the computation was performed
on supercomputers of the National Supercomputer Centers in Changsha. The au-
thors would also like to thank the Ministry of Science and Technology, Taiwan, for
financially supporting this research under contracts MoST107-2112-M-032-004-
MY3 and MoST 108-2218-E-032-003-MY3.
AUTHOR CONTRIBUTIONS
S.W. conceived the idea and supervised the work. W.C. performed most experi-
ments and wrote the manuscript. Y.W. directed the research and analyzed and inter-
preted the data in the upper part of manuscript about NOR mechanism. Y.Z.
directed the research and analyzed and interpreted the data in the lower part of
manuscript about extending substrates. C.-L.D. and Y.-C.H. performed in situ XAS
and analyzed corresponding data. C.X. and S.D. performed operando EIS and
analyzed corresponding data. Z.W. and J.M. performed the DFT calculations. Z.X.
and B.Z. performed in situ Raman spectrum. C.C. performed the TEM characteriza-
tion. All of the authors have read the manuscript and agree with its content.
DECLARATION OF INTERESTS
The authors declare no competing interests.
Received: March 23, 2020
Revised: May 12, 2020
Accepted: July 21, 2020
Published: August 18, 2020
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