Design concept for electrocatalysts

Abstract

Metal-based electrocatalysts with different sizes (single atoms, nanoclusters, and nanoparticles) show different catalytic behaviors for various electrocatalytic reactions. Regulating the coordination environment of active sites with precision to rationally design an efficient electrocatalyst is of great significance for boosting electrocatalytic reactions. This review summarizes the recent process of heterogeneous supported single atoms, nanoclusters, and nanoparticles catalysts in electrocatalytic reactions, respectively, and figures out the construct strategies and design concepts based on their strengths and weaknesses. Specifically, four key factors for enhancing electrocatalytic performance, including electronic structure, coordination environment, support property, and interfacial interactions are proposed to provide an overall comprehension to readers in this field. Finally, some insights into the current challenges and future opportunities of the heterogeneous supported electrocatalysts are provided.

Full text

Review Article
Design concept for electrocatalysts
YaoWang§,XiaoboZheng§,andDingshengWang(
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)
Department of Chemistry, Tsinghua University, Beijing 100084, China
§ YaoWang, and XiaoboZheng contributed equally to this work.
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©Tsinghua University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2021
Received: 25 June 2021 / Revised: 1 August 2021 / Accepted: 3 August 2021
ABSTRACT
Metal-based electrocatalysts with different sizes (single atoms, nanoclusters, and nanoparticles) show different catalytic
behaviorsforvariouselectrocatalyticreactions.Regulatingthecoordinationenvironmentofactivesiteswithprecisiontorationally
design an efficient electrocatalyst is of great significance for boosting electrocatalytic reactions. This review summarizes the
recentprocessofheterogeneoussupportedsingleatoms,nanoclusters,andnanoparticlescatalystsinelectrocatalyticreactions,
respectively,andfiguresouttheconstructstrategiesanddesignconceptsbasedontheirstrengthsandweaknesses.Specifically,
four key factors for enhancing electrocatalytic performance, including electronic structure, coordination environment, support
property, and interfacial interactions are proposed to provide an overall comprehension to readers in this field. Finally, some
insightsintothecurrentchallengesandfutureopportunitiesoftheheterogeneoussupportedelectrocatalystsareprovided.
KEYWORDS
electrocatalysts,electronicstructure,coordinationenvironment,supporteffect,interfacialinteractions
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1Introduction
The increasing seriousness of environmental pollution, energy
shortage, and climate warming has become major concerns for
humans. The excessive consumption of fossil fuels is the main
culprit triggering the above problems [1–3]. Developing
sustainable, fossil-free pathways to produce fuels and chemicals of
global importance could play a major role in reducing carbon
dioxide emissions while providing the feedstocks needed to make
the products we use on a daily basis. To meet the requirement, a
prospective tactic is to take full advantage of the existing, low-cost,
recyclable, and eco-friendly resources, such as water, carbon
dioxide and nitrogen, to replace traditional fossil fuel with
electrochemical conversion processes [4–9]. For the past few years,
electrocatalysis plays an important role in new energy conversion
and storage [10–17]. Although enormous breakthroughs have
been achieved, many bottlenecks have not been resolved well. For
example, as the one-half-cell reaction of water electrolyzer, oxygen
evolution reaction (OER) at the anode has much slower kinetics
than that of hydrogen evolution reaction (HER) at the cathode,
due to the sluggish four-electron transfer process of OER [18–22],
which greatly limits the hydrolysis rate. Proton exchange
membrane fuel cells (PEMFCs) are regarded as a promising
energy transformation to solve the energy issues around power
sources of automobiles, while the sluggish kinetics of oxygen
reduction reaction (ORR) restricts the whole reaction process of
PEMFCs [5, 23–27]. Nitrogen reduction reaction (NRR) and
carbon dioxide reduction reaction (CO2RR) are troubled by the
low Faraday efficiency [28–30]. Thus, the key point to solve the
bottleneck problem is to rationally design advanced electrocatalyst
with high activity, selectivity, and durability.
Over the past decade, substantial progress has been made in
understanding the intrinsic reaction mechanism of electrocatalysis,
to construct high-performance electrocatalysts enlightened from
the structure–performance relationship. Nanocatalysts including
single-atom site catalyst (SACs), nanocluster, and nanoparticle
(NP), show excellent catalytic performance in different catalytic
reactions [31]. From the view of fundamental theory, the catalytic
sites are usually enslaved to certain single or double active centers.
Thus, accurate regulation and optimization of the active moiety is
the main strategy to improve the catalytic performance [32–35]. In
general, increasing the number of active sites and enhancing the
intrinsic activity of active sites are effective strategies to enhance
the catalytic activity and stability. Because low-coordinated metal
atoms often function as catalytically active sites, the mass activity
increases considerably when decreasing the size of particles
[36–38]. Practically supported metal catalysts are inhomogeneous
and usually consist of a mixture of sizes from nanoparticles to
nanoclusters. Such heterogeneity not only reduces the metal atom
efficiency, but also frequently leads to undesired side reactions. It
also makes it extremely difficult, if not impossible, to uniquely
identify and control the active sites of interest. The ultimate small-
size limit for metal particles is the single atom, which can be
anchored on the support consisting of single-atom site catalysts
[39–41].
The complexity of catalyst preparation and complication of
catalytic mechanism put the rational design and precise regulation
of targeted electrocatalysts to the most important position in the
whole catalytic process. However, due to the difference in the
physical structure of SACs, nanoclusters, and nanoparticles, the
effect factors and design concepts for them are different. SACs
feature atomically dispersed metal atoms on the support, owning
isolated active sites, which can greatly increase the atomic
utilization [42–44]. Theoretical understanding of intrinsic
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ISSN 1998-0124 CN 11-5974/O4
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Address correspondence to wangdingsheng@mail.tsinghua.edu.cn

definitions of metal dispersion degree, uniform active sites and
low coordination environment of active sites is necessary for site-
engineering of the active centers of SACs [45, 46]. For
nanoclusters, most of the metal atoms are exposed as much as
possible and are available for the reactant molecules. Therefore,
supported atomic clusters have much more atom utilization
efficiency in catalytic reactions than nanoparticles. Simultaneously,
in comparison with SACs, supported atomic clusters with three or
more atoms can provide enough sites for the adsorption and
activation of multiple substrates [47], which may induce a new
catalytic pathway, decreasing the reaction barrier and improving
the catalytic activity. In terms of nanoparticles, amounts of
influential factors make it the most complex in structural design
and synthesis strategy, such as morphology effect, crystal effect,
composition effect, strain effect, interfacial effect, size effect, and so
on [48–51].
Considering the above discussions, this review aims to
summarize the recent progress on the supported metal catalysts
based on the design concept of electrocatalysts, as shown in Fig. 1.
Meanwhile, we elaborate their electrocatalytic applications in
HER, OER, ORR, NRR, and CO2RR fields. Finally, we also
provide a concise conclusion and discuss the prospects and
challenges for the synthesis, characterizations, applications, and
mechanism exploration of SACs, nanoclusters, and nanoparticles.
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2Design strategy for SACs
SACs, which feature maximum atom-utilization efficiency, high
mass activity, and excellent selectivity, have enabled a range of
emerging research interests [52, 53]. In addition, SACs own
homogeneous under-coordinated active cites and quantum effects,
endowing an excellent platform to unravel interaction of
structure–activity and offering a good opportunity for theoretical
modeling and simulation via density functional theory (DFT)
calculations [54]. Due to these advantages, SACs have attracted
numerous attention [55–58]. They could be applied to reduce the
large usage of precious metal, such as Pt and Ir, in large-scale
energy-related applications, which is of great significance for
decreasing the overall cost and improving the economic benefits
[59–63]. Therefore, diversified SACs, including carbon and non-
carbon based SACs, have been well-designed and synthesized for a
wide range of applications, such as electrocatalysis [64–68],
thermocatalysis [69–71], photocatalysis [72–74], nano-enzyme
[42], and batteries [75, 76]. Herein, we focus on the electrocatalytic
applications such as HER, OER, hydrogen oxidation reaction
(HOR), ORR, CO2RR, and NRR, which have been considered as
green and sustainable energy conversion systems, to uncover the
great role of SACs in electrocatalysis. And then we will decipher
the underlying interaction between the electrocatalytic activity and
structure based on their electronic structural interactions, support
effects, and coordination environments.
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2.1ElectronicstructuremodulationforSACs
Understanding the electronic metal-support interaction (EMSI) is
of great significance for the rational design of SACs and
deciphering the underlying catalytic mechanisms [77–79]. The
electronic interaction involves the electron transfer between metal
and support, which might have profound effects on the
perturbations of their electronic properties, such as the shift of d-
band center and the change of charge distribution [80]. For
instance, Li and co-workers constructed some Ru/PtxCu4−x (x = 0,
1, 2, 3, and 4) models to investigate the electronic interaction
between Ru and PtxCu4−x substrates [81]. A linear scaling
relationship between the ΔGOOH and ΔGOH was observed, and
Ru/Pt3Cu SACs exhibited the best OER performance.
Additionally, the projected density of states (PDOS) suggested that
the d-band center of Ru species was shifted towards the Fermi
level owing to the presence of compressive strain accompanied by
the increased Pt atoms. Moreover, Bader charge analysis results
indicated that the Pt-Cu alloy can transfer electrons to
intermediates and boost the corrosion-resistant feature of Ru
species, evidencing the strong electronic interaction between Ru
and Pt-Cu alloy.
The electrochemical NRR, which could directly reduce N2 to
NH3 at room temperature, has received considerable attention in
recent years [82, 83]. SACs have been a kind of efficient catalyst
for catalyzing N2 due to their unique advantages [84, 85]. Tuning
the electronic interaction between single metal sites and substrates
might facilitate the enhancement of NRR performance. For
instance, Sun et al. obtained a Ru@ZrO2/NC SAC via a
coordination-assisted strategy for NRR [86]. The catalyst exhibited
good NRR activity with a high NH3 rate of about 3.7 mgNH3·h−1 per
milligram of Ru. Experimental observations and DFT calculations
suggested the synergistic electronic effects between Ru atoms and
ZrO2 with O defects, which enable the stabilization of *NNH and
weak adsorption of *H, contribute to the boosted NRR
performance.
Recently, Thomas et al. deployed the transition metal carbides
as the substrate to anchor bi-atomic Fe and Ni metal species for
OER. Combined with a precipitation reaction and pyrolysis
process, they obtained atomically dispersed Fe/Ni atoms confined
WCx crystallites (FeNi-WCx) supported by the carbon sheet
(Fig. 3(a)). Microscopic characterizations indicated that Fe/Ni
atoms distribute homogeneously and randomly on or near the
surface of WCx (Figs. 3(b)–3(e)). When measured in 1 M KOH,
the FeNi-WCx exhibited excellent OER activity with a low
overpotential with 237 mV (at 10 mA·cm−2) and remarkable
durability with the negligible decline (45 mV) after 1,000 h
operation (Figs. 3(f) and 3(g)). The turnover frequency (TOF) can
reach 4.96 s−1 at an overpotential of 300 mV, and the overpotential
reaching 10 mA·cm−2 can be reduced to 211 mV when the amount
of FeNi was enhanced (Figs. 3(h) and 3(i)). Benefiting from the
synergistic electronic effects between FeNi and tungsten carbides,
the FeNi-WCx possessed moderate oxygen-binding strength and
optimized adsorption energy with OER intermediates, thus
leading to the enhanced OER activity with excellent durability.
Zhou et al. designed a Pt1-Fe/Fe2O3(012) catalyst via anchoring
atomically dispersed Pt species on α-Fe2O3(012) facets [87]. The
obtained Pt1-Fe/Fe2O3(012) delivered outstanding ORR
performance, with the onset and half-wave potentials of 1.15 and
1.05 V, as well as the high mass activity of 14.9 A·mg−1Pt at 0.95 V
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Figure 1  Overview of the topics covered in this review.
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(Figs. 2(a) and 2(b)). In addition, the Pt1-Fe/Fe2O3(012) also
showed remarkable durability without any apparent decay
(Fig. 2(c)). DFT calculation results demonstrated that the strong
electronic coupling between Pt and Fe induced partially occupied
orbitals along with Pt–Fe pair sites, which is beneficial for
adsorbing and dissociating of oxygen molecular (Figs. 2(d)–2(h)).
The enhanced ORR performance can be attributed to the presence
of single-site Pt–Fe pair, which can cooperatively adsorb O2 as a
side-on model, beneficial for the formation of OOH* and the
dissociation of O=O bond, as well as the facile OH* dissociation
on single Pt sites (Figs. 2(i)–2(l)). Consequently, the Pt1-
Fe/Fe2O3(012) exhibited superior performance to the benchmark
of Pt/C in a zinc-air battery (ZAB) and H2-O2 anion-exchange
membrane fuel cell (AEMFC). Specifically, the power density and
specific capacity of Pt1-Fe/Fe2O3(012) are 182 mW·cm−2 and 778
mAh·gZn−1, better than Pt/C. Importantly, the H2-O2 AEMFC of
Pt1-Fe/Fe2O3(012) exhibited outstanding stability with no obvious
degradation after 180-h operation at 0.6 V.
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2.2CoordinationenvironmentregulationforSACs
It is well-known that the locally coordinated atoms surrounding
the active centers have a profound effect on the activity, stability,
and selectivity of the single-atom site catalysts [89–91].
Coordination engineering is an extremely effective strategy to
synthesize SACs with tailored function, especially for carbon-
supported SACs [92–96]. In this catalytic system, the ligand
species, such as N, P, and S, which feature lone pairs of electrons,
not only can be used for capturing isolated metal centers but also
for modulating their electronic structure [97–103]. Therefore, fine-
tuning the species, number, and the location of the coordination
shell of ligand atoms plays a critical role in determining the
activity, stability, and selectivity of SACs. For instance, He and co-
workers fabricated Ru/N-S-Ti3C2Tx SACs with Ti3C2Tx confined
atomically dispersed Ru atoms with the aid of N and S ligand
species [104]. The Ru/N-S-Ti3C2Tx showed superior HER activity
in 0.5 M H2SO4 solution to that of Ru/Ti3C2Tx and N-S/Ti3C2Tx,
reaching the current density of 10 mA·cm−2 with an overpotential
of only 76 mV. Additionally, DFT calculations indicated that
Ru/N-S-Ti3C2Tx owned an optimal Gibbs Hydrogen adsorption
free energy of 0.08 eV, suggesting the accelerated H adsorption
kinetics.
In general, coordination engineering can be achieved by precise
tuning the single metal active centers and coordinated atoms. It
should be noted that precise control of the coordination
environment of single metal active centers in M-Nx-C (M =
transition metal, N = non-metal) catalyst is extremely challenging
but effective in enhancing the catalytic activity and selectivity
[105]. For example, through finely tuning the number of N
coordination atoms, efficient electroreduction of CO2 can be
successfully achieved. In 2017, Li et al. designed a ZIF-assisted
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Figure 2  (a) Schematic illustration of WCx-FeNi catalyst, with Fe and Ni atoms anchored on WCx supported by carbon sheets. (b) High-angle annular dark-field
scanning transmission electron microscopy (HAADF-STEM) images of WCx. (c) STEM energy-dispersive X-ray spectroscopy (STEM-EDS) mapping, indicating the
distribution of W, Fe, and Ni atoms. (d) HAADF-STEM images of Fe and Ni single atoms confined WCx (c). (e) The corresponding HAADF intensity line profiles as
indicated in (d). (f) iR-corrected polarization curves of WCx, WCx-Fe, WCx-Ni, and WCx-FeNi. (g) The corresponding overpotential at 10 mA·cm−2. (h)TOFs curves of
all catalysts. (i) Overpotentials of WCx-FeNi with different FeNi contents, and the corresponding activity trend. (j) The schematic illustration of reaction pathway for
WCx-FeNi. (k) The Gibbs free energy of the WCx, WCx-Fe, WCx-Ni, and WCx-FeNi catalyst. (l) Volcano plot for all WCx-based catalysts and the corresponding
models. Reproduced with permission from Ref. [88], © The Author(s), under exclusive licence to Springer Nature Limited 2021.
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route to synthesize Ni single atoms anchored nitrogen-doped
carbon (Ni/N-C SACs) for CO2RR (Fig. 4(a)) [106]. Microscopic
characterizations demonstrated the presence of uniformly
distributed Ni single atoms on N-C support (Figs. 4(b)–4(g)). The
Fourier transform (FT) k3-weighted Ni k edge extended X-ray
absorption fine structure (EXAFS) spectra for Ni/N-C SACs
indicated the active center is NiN3 moiety with Ni atom
coordinated by three N atoms (Fig. 4(h)). Electrochemical
measurements demonstrated that the Ni-N3-C SACs showed a
high current density of 10.48 mA·cm−2 (at an overpotential of 0.89
V) and high TOF (5,272 h−1) with near 72% Faraday efficiency
(FE) for CO (Fig. 4(i)). DFT calculations suggested that the under-
coordinated N atom surrounding isolated Ni species might induce
the strong interaction with CO2, which enhances the overall
CO2RR performance. Thereafter, they fabricated Co-N5-C SACs
with atomically isolated Co atoms confined on polymer-derived
hollow N-doped porous carbon spheres [107]. The obtained Co-
N5-C SACs with abundantly coordinated N atoms showed near
100% CO selectivity from −0.73 to −0.79 V and outstanding
durability. Recently, Jiang et al. fabricated a series of Ni/Nx-C via a
post-synthetic metal substitution strategy [108]. They found that
the under-coordinated Ni-N3-C SACs can facilitate the formation
of COOH* intermediate and therefore enhance the reaction
kinetics of CO2RR. In addition, Ni-N3-C SACs also exhibited
superior performance in a Zn-CO2 battery.
Introducing the heteroatoms like S, O, and P into coordinated
atoms for singe metal center can be another effective strategy to
engineer the coordination environment [92]. Regulating the
number and species of these heteroatoms in the coordination layer
plays a vital role in enhancing the activity and selectivity for SACs.
This strategy has been widely applied in various catalytic reactions,
especially for ORR and HER. For example, Zhang et al. fabricated
N and S coordinated Cu (S-Cu/SNC) SAC via atomic interface
engineering for ORR [109]. X-ray adsorption characterizations
results unraveled that the moiety configuration in S-Cu/SNC
is the unsymmetrically arranged Cu-S1N3 (Figs. 4(k)–4(m)).
Surprisingly, the S-Cu/SNC delivered outstanding ORR
performance, with a high half-wave potential of 0.918 V, much
better than other counterpart catalysts and Pt/C benchmark (0.84
V). Additionally, the catalyst showed excellent durability with no
obvious decay after 5,000 cycles (Fig. 4(n)). Inspired by the
superior ORR performance, the assembled Zn-air battery
displayed good activity and excellent durability (Fig. 4(o)). In situ
X-ray absorption spectroscopy (XAS) revealed the low-valent Cu
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Figure 3  (a) Polarization curves of Pt1-Fe/Fe2O3(012), PtNp/Fe2O3(012), Pt1-O/Fe2O3(001), Fe2O3(012), 20% Pt/C in 0.1 M KOH solution. (b) The mass activity and
specific activity at 0.95 V versus reversible hydrogen electrode (RHE) for Pt1-Fe/Fe2O3(012), PtNp/Fe2O3(012) and 20% Pt/C. (c) The mass activity and specific activity of
Pt1-Fe/Fe2O3(012) before and after cycling. (d) Differential charge density. (e) The PDOS for the Pt d orbitals of Pt1-Fe/Fe2O3(012) and PtNp/Fe2O3(012), Pt1-
O/Fe2O3(001). (f) The corresponding detailed PDOS for the platinum d orbital. (g) The schematic illustration of Pt–Fe electronic coupling effect for O2 activation. (h)
Top view of the Pt–Fe orbital configuration. (i) Comparison of onset potential versus RHE between DFT calculations and experimental observations for different
catalysts. (j) The proposed ORR mechanism for Pt1-Fe/Fe2O3(012) and Pt1-O/Fe2O3(001). (K) The corresponding Gibbs free energy profiles. (l) The value of ΔGH2O −
ΔGH2O2 for 4e− and 2e− process. Reproduced with permission from Ref. [87], © The Author(s), under exclusive licence to Springer Nature Limited 2021.
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dx2
−y2
(+1) might act as the real active sites during ORR (Fig. 4(p)).
Theoretical study unraveled the extra π bonds derived from Cu
enhanced the adsorption strength with intermediates,
leading to the enhanced ORR activity (Fig. 4(q)). In another case,
the phosphorus element was introduced into the carbon matrix to
boost the hydrogen reaction kinetics. Specifically, Wang et al.
reported a Co-SA/P-in situ SAC with Co1-P1N3 interfacial
structure via in situ phosphatizing of triphenylphosphine
encapsulated within zeolitic imidazolate framework (ZIF)
(Fig. 5(a)) [110]. The Co1-P1N3 configuration was verified by the
EXAFS fitting, evidencing the presence of isolated Co atoms
surrounded by three nitrogen atoms and one phosphorus atom
(Fig. 5(b)). When tested in 0.5 M H2SO4 media, the Co-SA/P-in
situ showed a great enhancement of HER activity, with an
overpotential of 98 mV reaching 10 mA·cm−2, and decent stability
without appreciable evidence of degradation (Fig. 5(c)).
On other hand, the active centers, which directly interact with
reaction intermediates and intrinsically depend on the activity and
selectivity to a large degree, can be modulated by choosing suitable
metal species [67, 98]. The MN4 (M = transition metal) moiety is
one of the commonest active units in the M-N-C SACs, and
different active M centers can be tailored towards different
catalytic reactions [95]. For example, Wu and co-workers reported
an Mn-N-C catalyst featuring MnN4C12 moiety for ORR
synthesized by sequential two-step incorporation and adsorption
route (Fig. 5(d)) [111]. The obtained catalysts delivered a decent
activity with a half-wave potential of 0.8 V, and outstanding
durability with only 17 mV decay in E1/2 after 30,000 cycles in 0.5
M H2SO4 media (Fig. 5(e)). Theoretical investigations and XAS
analysis results revealed that MnN4C12 sites might be the most
optimal active centers, which is thermodynamically and kinetically
favorable for oxygen reduction reaction with 4e− process
(Figs. 5(f)–5(h)). Similarly, in another case, Li et al. reported a
facile and practical route to obtain the CuN4 SACs through
emitting and trapping strategy which can directly transfer the bulk
copper into single copper sites (Figs. 5(i) and 5(j)) [112]. As
indicated in the HAADF-STEM image (Fig. 5(k)), abundant single
Cu atoms were observed on the carbon matrix, revealing the
successful transformation of bulk Cu into atomically dispersed Cu
sites. The CuN4 moiety was uncovered by the Cu k edge EXAFS
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Figure 4  (a) Schematic diagram of the synthesis process of Ni/N-C SACs. (b) TEM and (c) HAADF-STEM images. (d) SAED pattern. (e) and (f) HAADF-STEM
images of Ni SAs/N-C. The Ni single atoms are marked with red circles. (g) The corresponding EDS mapping for C, N, and Ni elements. (h) The EXAFS fitting curves
for Ni/N-C SACs with the proposed Ni−N3 model. (i) Polarization curves of Ni/N-C and Ni NPs/N-C in N2 and CO2 saturated 0.5 M KHCO3 electrolyte. (j) Proposed
CO2RR mechanism by Ni/N-C SACs. Reproduced with permission from Ref. [106], © American Chemical Society 2017. (k) FT k3-weighted Cu K-edge EXAFS spectra
of Cu/S1N3C, Cu foil, CuS, and CuPc. (l) Schematic model of Cu/S1N3C. (m) Wavelet transform (WT)-EXAFS plots of Cu/S1N3C, CuS, and CuPc. (n) Linear sweep
voltammetry (LSV) curves of Cu/S1N3C before and after 5,000 cycles. (o) Schematic illustration of Zn-air battery. (p) In-situ Cu K-edge XANES spectra of Cu/S1N3C at
different bias voltages. (q) Molecular orbitals of O* adsorbed on Cu-S1N3. Reproduced with permission from Ref. [109], © Shang, H. S. et al. 2020.
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fitting, suggesting the active center consists of one Cu atom
surrounded by four N atoms (Fig. 5(l)). The Cu-N4-C SACs
exhibited good ORR activity, with the E1/2 of 0.895 V, superior to
Pt/C (0.87 V) and other counterparts (Fig. 5(m)). The
Koutecky–Levich plots demonstrated the Cu-N4-C SACs followed
a 4e− ORR route (Fig. 5(n)). More importantly, this strategy can be
a general route to get other M-N4-C SACs.
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2.3Supporting effect and interfacial interactions for
SACs
The strong metal–support interactions have a significant effect on
the activity, selectivity, and stability of electrocatalysts, which could
be achieved by precisely tuning the locally coordinated atoms of
active centers, further modulating the electronic and geometric
structure [36, 80, 113–117]. Further, the morphology, size,
composition, and structure of the support can be engineered to
achieve targeted reactions for SACs [118–120]. For instance, Datye
and co-workers found that polyhedral ceria with much more step
edges shows unique advantage for binding Pt atoms [115].
Carbides and nitrides, due to their good electrical conductivity,
hydrophilicity, and corrosion resistance, are usually deployed as
supports to capture single metal atoms. More importantly, the
electrocatalytic activity, selectivity, and durability can be
modulated by fine tuning these supports. For example, Lee et al.
compared the activity difference between Pt/TiC and Pt/TN SACs
in ORR [121]. Interestingly, the authors found that Pt/TiC could
deliver high activity, selectivity, and stability than Pt/TiN to
generate H2O2 via a 2e− process. DFT calculations demonstrated
that Pt/TiC possesses weak bonding energy with oxygen-related
intermediates compared with Pt/TiN, resulting in the unfavorable
2e− process. Similarly, Ma and co-workers deployed α-MoC and
MoN supports to capture Pt single atoms for ORR [122]. The
experimental evidence and DFT calculations demonstrated that
single dispersed Pt atoms were coordinated with Mo atoms in α-
MoC and N atoms in MoN. The electrochemical tests indicated
that Pt/MoN SACs delivered better activity than Pt/α-MoC, which
could be associated with the optimized binding energy of Pt-OH
due to the presence of surrounded N species. Wang et al.
fabricated single Pt atoms confined Mo2TiC2Tx for HER via an
electrochemical exfoliation strategy [123]. The isolated Pt species
can be captured by the Mo vacancies formed during the
exfoliation process (Fig. 6(a)). The obtained Pt/Mo2TiC2Tx

Figure 5  (a) Schematic illustration of the synthesis process of Co-SA/P-in situ. (b) k3-weight FT-EXAFS Co K-edge fitting curves of Co-SA/P-in situ. (c)
Overpotentials of CoSA/P-in situ and other catalysts. Reproduced with permission from Ref. [110], © American Chemical Society 2020. (d) Schematic diagram of the
fabrication of atomically dispersed MnN4 site catalyst. (e) LSV curves before and after cycling of 20MnNC-second catalyst. (f) Atomistic structure of MnN4C12 active
site. (g) The corresponding Gibbs free energy diagram. (h) The schematic illustration of the evolution of MnN4C12-OOH. Reproduced with permission from Ref. [111],
© Li, J. Z. et al. 2018. (i) The schematic of apparatus diagram for the fabrication of Cu/N-C SACs. (j) Proposed reaction mechanism. (k) HAADF-STEM image of
Cu/N-C SACs. (l) ORR polarization curves of pyrolyzed ZIF-8, Cu/N-C SACs, and Pt/C. (m) ORR polarization curves of Cu/N-C SACs at different rotation rates.
Inset is the Koutecky–Levich plots, indicating the electron transfer number of Cu/N-C SACs. (n) Stability test of Cu/N-C SACs. Reproduced with permission from Ref.
[112], © Qu, Y. T. et al. 2018.
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exhibited superior HER performance with only 77 mV to reach
100 mA·cm−2, and rapid HER reaction kinetics with low Tafel
slope (Figs. 6(b) and 6(c)). Theoretical calculations suggested that
the introduction of Pt into Mo2TiC2O2 induced the charge
delocalization from Pt to coordinated atoms (Figs. 6(d)–6(h)). The
enhanced occupied states near the Fermi level and the optimized
hydrogen adsorption energy of Pt/Mo2TiC2Tx promoted the faster
charge transfer and formation of H2.
Metals (alloys) are another kind of effective supports for
immobilizing single metal atoms, and the resultant catalysts could
be called single atom alloys (SAAs). The high-performance SAA
can be approached by regulating the metals (alloys) substrates to
achieve higher activity and good selectivity. Chen et al. fabricated
an atom-pair catalyst (APC) which consists of Cu10–Cu1x+ pair
structure maintained by surface Te vacancies on Pd10Te3 alloy
nanowires (NWs) towards CO2 reduction reaction (Fig. 6(i))
[124]. EDS mapping demonstrated the homogenous distribution
of isolated Cu species on the Pd10Te3 (Fig. 6(j)). When tested in 0.2
M NaHCO3 solution, the APC catalyst with 0.10 wt.% Cu
delivered superior CO2RR activity, reaching a current density of
18.74 mA·cm−2 at −0.98 V vs. RHE, and high CO selectivity (92%)
(Figs. 6 (k) and 6(l)). The enhanced CO2RR performance can be
attributed to the unique Cu10–Cu1x+ pair configuration, in which
Cu1x+ facilitates the adsorption of water molecules and CO2 can be
anchored by the Cu10 sites, hence reducing the energy barrier and
accelerating the reduction of carbon dioxide. Duan and co-
workers synthesized a single atom alloy catalyst featuring
atomically dispersed Ni species immobilizing Pt nanowires for
HER, methanol oxidation reaction (MOR) and ethanol oxidation
reaction (EOR) [125]. The atomically isolated Ni atoms
distributed homogeneously on the Pt nanowires, as evidenced by
the electron energy loss spectroscopy (EELS) elemental mapping
and Ni k edge EXAFS fitting (Figs. 6(m)–6(o)). The Ni/Pt SAA
showed high electrochemically active surface area (ECSA) and
HER activity, with a mass activity of 11.8 ± 0.43 A·mgPt−1 (at an
overpotential of 70 mV vs. RHE), much superior to Pt/C

Figure 6  (a) Schematic illustration of the fabrication mechanism for Pt/Mo2Ti2O2. (b) LSV curves of CP, Mo2TiC2Tx, Mo2TiC2Tx-VMo, Pt/Mo2TiC2Tx, and Pt/C. (c)
The corresponding Tafel slopes. Theoretical model of (d) Mo2Ti2O2 and (e) Pt/Mo2Ti2O2. (f) Differential charge density of Pt/Mo2Ti2O2. (g) Calculated PDOS of
Mo2Ti2O2 and Pt/Mo2Ti2O2. (h) Calculated Gibbs free energy diagrams. Reproduced with permission from Ref. [123], © Zhang, J. Q. et al. 2018. (i) Schematic diagram
of the fabrication process of atom-pair structured Cu anchored on Pd10Te3 nanowires. (j) Atomic-resolution EDS mapping. (k) Atomic structure of Cu atom pair
imbedded on Pd10Te3 nanowires. (l) LSV curves of different catalysts with various content of Cu atoms. Reproduced with permission from Ref. [124], © Jiao, J. Q. et al.
2019. (m) Schematic diagram for single Ni atoms decorated Pt nanowire. Calculation model for (n) Pt(111) surface, and (o) with the Ni atom immobilized in the
surface layer. (p) ECSA and specific activity and mass activity for Pt/C, Pt nanowires, and isolated Ni atoms decorated Pt nanowires. (q) The corresponding
polarization curves. Reproduced with permission from Ref. [125], © Li, M. F. et al. 2019.
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(Figs. 6 (p) and 6(q)). The DFT calculations suggested that the
atomically dispersed Ni atoms, coordinated by two OH groups,
could electronically enhance the neighboring Pt species and
greatly reduce the ΔGH, thereby increasing the HER activity and
stability. Additionally, Ni/Pt SAA also exhibited outstanding MOR
and EOR activity, with 3–6 and 2–6 times better than the reported
state-of-the-art catalysts, respectively.
Metal chalcogenides supported SACs have gained considerable
attention for HER and OER in recent years [126, 127]. For
instance, Zeng and co-workers reported a universal method to
synthesize single atom-site catalysts via electrochemical deposition
strategy [128]. Different SACs can be obtained via fine tuning the
metal species, supports, and electrodes. The authors synthesized a
series of Ir/M (M = MnO2, MoS2, Co0.8Fe0.2Se2, and N-C) SACs via
an anodic electrochemical deposition approach (Figs. 7(a) and
7(b)). A-Ir/Co0.8Fe0.2Se2 exhibited the best OER performance,
much better than A-Ir/Co(OH)2, with only 230 mV to achieve 10
mA·cm−2, suggesting that suitable support may play a significant
role in enhancing the electrocatalytic activity of catalysts.
(Fig. 7(c)) Additionally, A-Ir/Co0.8Fe0.2Se2@Ni foam showed
superior overall water splitting performance with a cell voltage of
1.39 V reaching 10 mA·cm−2. Furthermore, Co0.8Fe0.2Se2@Ni foam
exhibited long-term durability tests for over 100 h at 10, 100, and
500 mA·cm−2 (Fig. 7(d)). Similarly, Chen et al. deployed the site-
specific electro-deposition method to fabricate a series of
Pt/TMDs (TMDs = MoS2, WS2, MoSe2, and WSe2) to reveal the
effect of EMSI on the HER activity (Figs. 7(e) and 7(f)). As
indicated in the HAADF-STEM images, single Pt atoms can be
successfully anchored on the TMDs supports (Figs. 7(g)–(j)).
Among them, Pt/MoSe2 exhibited the best alkaline HER activity,
with an overpotential of ~ 29 mV at 10 mA·cm−2 and high mass
activity of 34.4 A·mg−1 (at an overpotential of 100 mV) (Figs. 7(k)

Figure 7  (a) Schematic of anodic deposition strategy of Ir species. (b) Anodic deposition of single Ir species on MnO2, MoS2, Co0.8Fe0.2Se2, and N doped C. (c) LSV
curves of IrO2 and anodically deposited A/B (A = Rh, Ag, and Ir; B = Co(OH)2 and Co0.8Fe0.2Se2) SACs. (d) Chronopotentiometric curves of Ir/Co0.8Fe0.2Se2@Ni foam
for full water splitting at different current densities. Reproduced with permission from Ref. [128], © Zhang, Z. R. et al. 2020. (e) Schematic illustration of electronic
metal–support interactions regulation of single Pt site for HER. (f) Schematic diagram of transition metal dichalcogenides (TMDs) supported Pt SACs. HAADF-STEM
images for (g) Pt/MoSe2, (h) Pt/WSe2, (i) Pt/MoS2, (j) Pt/WS2 SACs (scale bars: 5 nm) and the corresponding elemental mappings (right side, scale bars: 100 nm). (k)
LSV curves and (l) the corresponding required overpotential at 10 mA·cm−2 and mass activity for Pt/TMDs SACs. (m) Ultraviolet photoelectron spectroscopy (UPS)
valence-band spectra (VBS) of single-atom Pt relative to the Fermi level. (n) Schematic illustration suggesting the EMSI effect on the d-band center for single Pt atom,
and the interaction between Pt and adsorbed H atom. Reproduced with permission from Ref. [129], © Shi, Y. et al. 2021.
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and 7(l)). The authors ascribed this to the EMSI regulation
between TMDs supports and Pt single-atom, in which the d-band
center position of Pt in Pt/WSe2 exhibited a maximum upshift,
enabling the stronger Pt-H adsorption energy (Figs. 7(m) and
7n)).
Except for the above-mentioned substrates, other supports such
as metal oxides, metal hydroxides, and phosphides can also be
employed to anchor isolated metal species, finally achieving the
rational construction of SACs with high activity, selectivity, and
durability [130–132]. For detailed information regarding how to
select suitable support for anchoring single metal atoms towards
targeted electrochemical reactions, we refer the readers to our
recent review, in which all non-carbon supported SACs are
systematically introduced, including their underlying anchoring
mechanisms and electrocatalytic applications [133].

3Design strategy for nanoclusters
Clusters with uniform metal sites and definite low-nuclearity are
the intermediate states between nanoparticles and nanoclusters.
Benefiting from the unique geometric and electronic structures,
the strong metal–metal bond can induce synergistic effects among
every metal atom, which contributes to achieving unique catalytic
properties different from SACs and nanoparticles. In addition,
metal atoms in the cluster are nearly exposed and possess high
atom utilization efficiency than traditional nanoparticles in the
catalytic reactions [134]. On the other hand, the coordination
environment of nanoparticle surface atoms is different at edge
sites, corner sites, and planes [135]. Thus, developing the desired
nanocluster to meet the requirement of fundamental research and
industrial applications is of great importance. Next, we summarize
several design strategies to demonstrate the recent process in
constructing advanced cluster catalysts.

3.1Electronicstructureregulationfornanocluster
In all catalyst systems, the study of the electronic structure of
active sites is of great significance for understanding the
underlying structure–performance relationships. Surface vacancies
can alter the local electronic structure, thus enabling a higher
intrinsic activity for catalyst. Anion vacancies such as sulfur and
oxygen defect are one of the most common types of vacancies in
inorganic materials, which is usually deployed to boost the
catalytic performance [136–138]. For example, it is reported that
oxygen vacancies in ultrathin InO2 nanosheets would lead to a
new donor level and increase the density of states, resulting in an
enhanced carrier separation efficiency and accelerated water
splitting [139]. On the other hand, the creating of metal vacancy is
recognized as another effective method to enhance catalytic
activity through electronic regulation. For example, Sun and co-
workers reported that Co vacancies can be atomically tuned to
have different cluster sizes through controlling the migration of Ir
single atoms, as illustrated in Fig. 8(a) [140]. They revealed that the
co-existence of Co vacancy clusters at the surface of IrCo alloy
would induce an increased d-band center position and optimize
the H adsorption, finally resulting in an improved HER
performance. Sample IrCo@NC-850 showed a low overpotential
of 82/302 mV for the HER/OER in 1 M KOH electrolyte and
50/315 mV for HER/OER in 0.5 M H2SO4 solution (Fig. 8(b)).
The further theoretical calculations revealed that the Co vacancy
cluster can modify the electronic structure of IrCo@NC-850 via
optimizing the interactions between catalysts and
oxygen/hydrogen intermediates (Fig. 8(c)).
In addition, adopting a foreign element not only regulates the
structure property, but also improves the catalytic performance,
including activity and stability. As we know, the size of
nanocluster is about 1 nm, far away from nanoparticles, and
therefore most of metal atoms at this size are accessible for
catalysis, keeping the high utilization efficiency. Today, the metal
nanocluster still suffers the challenges of thermal sintering and
consequent deactivation owing to the loss of metal surface areas
particularly in the applications of high-temperature reactions. Yin
and co-workers reported that introducing S into a carbon matrix
can stabilize ~ 1 nm metal (Pt, Ru, Rh, Os, and Ir) nanocluster at
high temperatures up to 700 °C (Fig. 8(d)) [141]. They found that
increasing the adhesion strength between metal clusters and S-
doped carbon materials is in favor of retarding the metal atom loss
and migration. X-ray photoelectron spectroscopy (XPS)
characterizations indicated that the dopant of S could further
induce the electronic transfer between metal cluster and carbon
support (Fig. 8(e)). The further theoretical calculations
demonstrated that the prepared Pt/S-C nanocluster catalysts with
interfacial electronic effects exhibited distinctly better activity and
selectivity than the state-of-the-art dehydrogenation catalysts, with
only a slight performance deactivation, as shown in Fig. 8(f).

3.2Coordinationenvironmentregulationfornanocluster
Coordination environment plays a dominated role in the
heterogeneous catalysis. The larger the particle size, the more
complex the coordination environment. For example,
nanoparticles possess different surface coordination environment
due to the presence of surface defects such as kink，corner, and
terrace. However, the coordination environment of cluster is
simpler than nanoparticles, due to small size and uniform
dispersion. On the one hand, size parameter can greatly influence
the coordination environment to some extent, thus affecting the
catalytic performance. Gihan Kwon and co-workers developed a
series of subnanometer metal and metal oxide clusters with
different sizes[142–145]. These materials showed outstanding
activity and specificity in certain reactions. In addition, they
revealed an efficiency and turnover rate for three Pd clusters with
different sizes (Pd4, Pd6, and Pd17) supported on the thin ultra-
nanocrystaline diamond (UNCD) for the OER [146]. The above
experiments disclosed a deep understanding of size effect on the
catalysis: (i) Clusters of this size are essentially single active sites;
(ii) Ideal structure–activity relationship could be constructed to
reveal catalytic mechanisms; (iii) Calculations can be done at
sufficient accuracy to allow guidance for improvement.
On the other hand, tuning the element composition to regulate
the coordination environment of cluster is another method. Zhao’
s group performed high-throughput DFT calculations to explore
the composition effect of nanoclusters towards HER [147]. They
selected a stable structures for Cu55-nMn (M = Co, Ni, Ru, and Rh,
and n ≤ 22) nanoclusters adopting the core–shell configurations
with Cu as the shell atoms. After the exploration, they draw
several conclusions as following: (i) In terms of stability, the
core–shell structures with M-core and Cu-shell are energetically
preferred for Cu-based nanoclusters of Cu55-nMn, which is the
most stable structure among them; (ii) In this system, the
core–shell structure shows the highest HER performance, and the
enhancement is attributed to the coordination structure of
heterogeneous metal atoms. Liu et al. proposed an active site of Fe3
nanocluster supported on the θ-Al2O3(010) for ammonia synthesis
[148], revealing that the large spin polarization of Fe3 is
responsible for N2 activation, and that the low oxidation state iron
atom works as an electron reservoir, regulating the charge
variation of the whole process. Thus, for all catalysts, including
nanoparticles, nanoclusters, and single atoms, regulating with high-
efficiency coordination environment of active sites plays an
important role in the optimization of catalytic performance.
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3.3Supporting effect and interfacial interactions for
nanocluster
In the homogeneous catalysis field, nanocluster catalysts usually
are supported on suitable substrates to stabilize their structure.
Seeking a suitable support is of great importance for the cluster in
comparison with nanoparticles because nanoparticles possess a
self-supporting effect. The structure and property of support are
more significant for clusters. Constructing desired support with
rich anchor site is a promising method to interact the metal
clusters with high rations of surface-exposed atoms. However,
fabrication of practical and stable subnanometric catalysts remains
a considerable challenge because, typically, such metal clusters are
spontaneously liable to aggregate and particle grows under realistic
reaction conditions [149]. In comparison with nanoparticle and
single-atom, supported nanoclusters not only possess enough
metal–metal interactions, but also have a strong metal–support
interaction between active component and support. Targeted
design for support is important to improve the catalytic activity.
Lu and co-workers reported that oxygen vacancies (Vo) in the
support can induce a unique dual effect (specifically, reversed
charge transfer and enhanced hydrogen spillover) in VO-rich
Pt/TiO2 (Figs. 9(a)–9(d)) [150]. The deep electron spin resonance
(ESR) characterization revealed that VO-rich Pt/TiO2 catalysts
possessed rich oxygen vacancies, as illustrated in Fig. 9(e).
Simultaneously, Ti 2p XPS results disclosed that the two subpeaks
in Ti 2p correspond to Ti3+ and Ti4+, respectively, and the intensity
ratios of Ti3+/Ti4+ are close to each other for TiO2, VO-deficient and
VO-rich Pt/TiO2. The HER results revealed that VO-rich Pt/TiO2
possessed 58.8 times and 16.7 times mass activity and specific
activity than state-of-the-art, respectively (Figs. 9(g) and 9(h)). The
finally theoretical calculations demonstrated that the formation of
oxygen vacancies in TiO2 support optimizes the ΔGH value on Pt
nanocrystals (NCs), and promotes the hydrogen spillover from Pt
NCs to TiO2 support, which synergistically boosts the
electrocatalytic HER activity. Gao et al. used the TiO2 and CeO2 as

(d)
(e) (f)
(a)
(b) (c)
Figure 8  (a) Schematic illustration and micrograph of the formation of IrCo@NC. (b) The overall overpotential at 10 mA·cm−2 under acidic and alkaline conditions.
(c) Schematic illustration of the formation mechanism of IrCo@NC-850. Reproduced with permission from Ref. [140], © © Wiley-VCH GmbH 2021 . (d) The S–C
supported metal nanocluster catalysts are resistant to sintering owing to the strong metal–sulfur interaction, whereas S-free-C supported catalysts easily aggregate into
larger particles in high-temperature reactions. (e) In-situ XPS of Pt 4f on Pt/S-C and Pt/S-free-C after H2 treatment at 200 °C for 30 min. (f) Energy barrier of Pt38
cluster desorption on S-Graphene and Graphene. Reproduced with permission from Ref. [141], © Yin, P. et al. 2021.
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support to explore the supporting effect for electrocatalysis [151].
The Pt-TiO2 or Pt-CeO2 catalysts after activation exhibited much
higher catalytic activity and stability as compared to the
commercial Pt/C catalyst.
For the cluster, structure construction is an effective way to
substantially adjust the chemical and physical properties of
materials. Chen’s group developed a molecular-level strategy to
anchor and stabilize the subnanometric Pt clusters through
engineering the co-existence of graphitic carbon nitride (g-C3N4)
and carbon nanotubes (CNT) [152]. Firstly, they displayed the
chemical integration of g-C3N4 on CNT is critical in optimizing
the electronic structures and catalytic properties of Pt catalysts via
theoretical calculations. The integrated co-existed g-C3N4 on CNT
is beneficial to Pt clusters in d-band position, and significantly
strengthens its adsorption behaviors for the key reaction
intermediates during the methanol electrooxidation process and
energetically decreases the energy barriers in the multistep
reaction pathways.

4Design strategy for NPs
Nanosized particle has been widely concerned by enormous
researchers due to their intrinsic activity from the perspectives of
both scientific research and industrial applications. The past
decades has witnessed an explosion in novel strategies for
synthesizing metal nanoparticles with controlled compositions,
sizes, shapes, and structures and for elegantly controlling the ways
in which these nanoparticles can be connected to and even
surrounded by support materials [9, 153–155]. The chemical and
physical properties of alloy NPs can be well tuned by varying the
size, composition, and atomic ordering. In general, alloy materials
have distinct binding properties with reactants in contrast to those
for monometallic metal catalysts. However, constructing a high-
performance metal-based nanoparticle still faces many challenges,
especially in ORR. In general, the surface environment of NPs is
complex, which makes the dispersion of active sites on NPs non-
uniform. The in-depth understanding of physico-chemical
property of nanoparticles is of great significance for constructing
intrinsic structure–performance relations between NPs and
electrocatalysis at the atom-level. In this section, we elaborated the
design concept for advanced electrocatalysts from the electronic
effect, coordination effect, supporting effect, and interfacial effect.


(a)
(e) (f)
(g) (h)
(b) (c) (d)
Figure 9  (a) Schematic illustration for the synthesis of VO-deficient and VO-rich Pt/TiO2. (b) TiO-CN, (c) porous TiO2, and (d) VO-rich Pt/TiO2 TEM images. (e) ESR
spectra of oxygen vacancies. (f) XPS profiles of Ti 2p for TiO2, VO-deficient and VO-rich Pt/TiO2. (g) The polarization curves for VO-rich Pt/TiO2, VO-deficient Pt/TiO2
and Pt/C in 0.5 M H2SO4 with a scan rate of 5 mV·s−1. (h) The polarization curves of VO-rich Pt/TiO2 at the initial stage and after 1,000 cycles with a scan rate of 5
mV·s−1. Reproduced with permission from Ref. [150], © Wiley-VCH GmbH 2021.
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4.1ElectronicstructuremodulationforNPs
Electronic effect usually occurs on the different surface atoms at
the NPs with dissimilar electronic structure (possessing different d-
band center) that makes for the electronic charge transfer between
them, which will cause an optimization to catalytic performance.
Generally, alloying with other transition metals is beneficial for the
electronic structure optimization through electron donation
[156–158]. Wang’s group reported a dendritic defect-rich Pd-Cu-
Co ternary nanoalloy and showed an excellent activity and
durability toward ORR and formic acid oxidation. They ascribed
the enhanced performance to a combination of effects, including
defects, a synergistic effect, change of d-band center of Pd, and
surface strain [159]. For example, Huang et al. [160] reported a
Mo doped PtNi octahedral nanostructure supported on carbon, as
show in Figs. 10(a) and 10(b). In comparison with other
transition metals, the Mo ‐Pt3Ni/C showed the best ORR
performance, with a specific activity of 10.3 mA·cm−2 and mass
activity of 6.98 A·mgPt−1, which are 81- and 73 ‐fold
enhancements compared with the commercial Pt/C catalyst
(0.127 mA·cm−2 and 0.096 A·mgPt−1). Theoretical calculations
suggested that Mo prefers subsurface positions near the particle
edges in vacuum and surface vertex/edge sites in oxidizing
conditions, where it enhances both the performance and the
stability of the Pt3Ni catalyst (Figs. 10(c)–10(f)). Apart from the
performance enhancement, the electronic effect also triggers the
additional impact on the nanoparticles. Intrinsically, the leaching
process would be slowed via suppressing the lattice atom diffusion,
which can be achieved by strengthening the interaction between
Pt and the alloyed metal atoms from the perspective of
thermodynamics [161, 162]. Alloying Pt with Ga, Gd, and Tb
simultaneously achieved the high activity and stability, which
results from the strong electronic interaction between the Pt and
transition metals [163–165]. Huang’s group reported that a novel
catalyst on the basis of PtGa alloy nanowires, which possesses an
enhanced p-d hybridization interaction deriving from the
electronic transfer [166]. The optimum Pt4.31Ga NWs catalyst
showed 10.5- and 12.1-fold improvement in the ORR mass
activity and specific activity than commercial Pt/C, respectively.
Similarly, Guo and co-workers reported a Ga-doped PtCo NWs
and showed an enhanced ORR performance [167]. The further
DFT calculations revealed that the Ga doping could optimize the
surface energy, because the formation of proper Ga−O bonding
can promote oxygen binding to approach an optimal value, which
results in an enhanced ORR activity.
Introducing another element with lower electronegativity
than O onto the catalyst surface to tune the electronic effect of
Pt atoms is a promising effective way in maintaining the
initial activity via strong electronic effect. Sung Jong Yoo's
group used N‐containing polymers as “active auxiliary” to tune
the electron structure of Pt, which is an efficient method to
stabilize the surface structure and improve the catalyticactivity
through electronic interactions [169]. Under this guidance, the S,
Se, and N are also selected to modulate the surface physico-
chemical property [170–172]. As we know, the water- dissociation-
related Volmer process is the rate-limiting step for alkaline HER
on Pt-based catalysts and is still unclear. Xie and co-workers

M-COCH3 + M-OH→M-CH3COOH + M(metal)
Figure 10  (a) and (b) TEM images of PtNi and Mo/PtNi catalysts. The average site occupancies of the second layer of (c) the Ni1175Pt3398 NC and (d) the
Mo73Ni1143Pt3357 NC. (e) The calculated binding energies for a single oxygen atom on all fcc and hcp sites on the (111) facet of the Mo6Ni41Pt178 NC, relative to the
lowest binding energy. (f) The change in binding energies when a Ni47Pt178 NC is transformed to a Mo6Ni41Pt178 NC by the substitution of Mo on its energetically
favored sites in the second layer below the vertices. Reproduced with permission from Ref. [160], © American Association for the Advancement of Science 2015. (g)
Scheme for improving catalytic performance by shortening Pd–Ni active site distance. (h) high-resolution region of Pd 3d of Pd38Ni45P17 and Pd40Ni43P17 NPs.
Reproduced with permission from Ref. [168], © Chen, L. et al. 2017.
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reported that the sluggish water-dissociation behavior could be
resolved via N-induced orbital steering [171]. This work exhibit
that N modified Pt–Ni nanowires deliver an ultralow
overpotential of 13 mV at 10 mA·cm−2, which represents a new
benchmark for alkaline HER catalysis. Additional experimental
and theoretical results reveal that N-doping can modulate the
electronic structure of Ni sites and cause empty dZ2 orbitals with
superior orientation for water dissociation and activation. Wang et
al. found that the electronic effect triggered from the non-metal
element not only modulates the electronic structure of
nanoparticles, but also induces the structure evolution of NPs.
They reported a nanosized (5 nm) Pd-Ni-P ternary NPs with
tunable Ni/Pd atomic ratio and controlled distance under the
impact of P through a two-step solvothermal strategy [168], as
illustrated in Fig. 10(g). By controlling the phosphorization
temperature and time, the Pd/Ni-P heterodimers could evaluate
into homogeneous alloy structure. The activity of ethanol
electrooxidation was improved up to 4.95 A per mgPd, which is
6.88 times higher than commercial Pd/C (0.72 A per mgPd). The
DFT calculations revealed that the enhanced performance was
attributed to the promoted free OH radicals. From the results of
Pd 3d XPS (Fig. 10(h)), the positive shift of Pd 3d5/2 centered at
335.6 eV indicates that the core-level of Pd shifts down with
respect to the Fermi level of Pd, corresponding to a down-shift of
the d-band center of Pd due to the strong electron interactions
involving Pd, Ni, and P [173, 174].

4.2CoordinationenvironmentregulationforNPs
The catalytic reaction usually occurs on the surface/interface of
catalysts. The coordination environment of active sites plays an
important role in governing the activity, while it is also affected by
other factors, such as coordination number, facet structure, and so
on. The crystal facet effect on the catalytic performance originates
from the difference of the surface atomic arrangement and the
electronic properties of nanocrystals, which influences the
adsorption and desorption energy [175]. In 2007, Stamenkovic’s
group reported that the Pt3Ni(111) surface is 10 times more active
towards ORR than Pt(111) surface and 9 times more active than
commercial Pt/C [176]. This is because the Pt3Ni(111) surface
possesses an unusual electronic structure and arrangement of
surface atoms in the near‐surface region. Herein, it is noted
that the facet effect is another non ‐negligible factor
influencing the ORR catalytic performance. It is reported that
exposed Pt3Ni(111) facets show an enhanced ORR activity than
(110) and (100) facets. Crystal facets, vertices, and edges govern
the energy landscape of metal surfaces and thus the chemical
interactions on the surface [177, 178]. Noah J. J. Johnson et al.
revealed a different hydrogen absorption behavior on the
nanocrystal containing exclusively (111) or (100) facets, as shown
in Figs. 11(a)–11(c) [179]. The deeply experimental results
disclosed that the rate of hydrogen absorption is higher for those
nanocrystals containing a higher number of vertices, consistent
with hydrogen absorption occurring quickly after β-phase
nucleation at lattice-strained vertices [180, 181]. In terms of ORR,
the activities on single-crystalline Pt surfaces have been extensively
studied. The ORR activity on low index Pt surface has been
demonstrated to be in the order of Pt(100) < Pt(111) < Pt(110) in
perchloric acid (HClO4) [182], while in sulfuric acid (H2SO4), it
follows the order of Pt(111) < Pt(110) < Pt(100). The difference is
attributed to the stronger adsorption of SO42− on Pt(111) than on
Pt(100).
Apart from the crystal facets, the coordination number of active
sites is of great importance for electrocatalysis. Researchers usually
construct a unique site structure through endowing active center
with a defect to decrease the coordination number. Wu and co-
workers [183] reported a mesoporous, highly excavated octahedral
PtCu3 nanostructures prepared by a facile one-pot synthesis with
mutually perpendicular interlaced mesoporous nanosheets with a
thickness of ≈ 4.5 nm. The porous structure makes these
octahedral PtCu3 nanostructures with enormous defects for active
centers. The low coordination number is beneficial for
electrocatalytic methanol oxidation reaction. Zhang’ group [184]
reported a hierarchical Rh nanostructure (Rh NSs) composed of
ultrathin nanosheets, composed of hexagonal close-packed
structure embedded with nanodomains that adopt a vacated
Barlow packing (VBP) with ordered vacancies through a wet-
chemical synthesis (Figs. 11(d)–11(i)). Directly creating vacancy
sites in metallic nanostructures is very challenging due to the large
energy deviation of vacated nanostructures from those densest-
packed ground-state geometries. The obtained Rh NSs exhibit
remarkably enhanced electrocatalytic activity and stability toward
the HER in alkaline media, as compared with the Rh/C,
commercial Pt/C, and most reported electrocatalysts.
In addition, as typical low-coordination crystal facets with high
surface free energy and amounts of undercoordinated surface
atoms, the high-index facets possess a higher catalytic
performance compared with low-index facets [34, 185, 186]. It is
reported that the undercoordinated surface active atom can greatly
decrease the reaction energy barriers of the rate-determining step,
resulting in an improved electrocatalytic activity, because their
coordination numbers are usually lower than typical crystal
surfaces with coordination numbers of 8 or 9 for (100) or (111)
facets, respectively [187]. A pioneering work was reported by Sun’
s group that tetrahexahedral Pt nanocrystals covered with high-
index facets were fabricated by electrochemical synthesis [188]. In
addition, wang and co-workers [189] revealed internal relations
between coordination number and electrocatalytic HER
performance through theoretical calculations, as illustrated in
Figs. 11(j) and 11(k). They revealed that: (i) In terms of surface Pt
atoms, the undercoordinated Pt atoms have a benefit on higher
HER performance, because of the lower energy barrier for water
dissociation; (ii) The HER performance in basic solution can be
further enhanced at the interface of undercoordinated Pt atoms
and defective Ni(OH)2 by reducing water dissociation barrier. And
this enhancement will be further increased with the decrease of
coordination number of Pt surface atoms.

4.3SupportingeffectforNPs
Modulating the underlying support could greatly affect the
associated electronic properties of surface catalysts, which in turn,
lead to significant effects on catalytic performance [190–192].
Supporting effect has been widely used in heterogeneous catalytic
reactions such as water–gas shift reaction [193], hydrogenation
reaction [194], methane oxidation [195] and so on. Recently,
regulating the metal–support interactions became an efficient
strategy for enhancing the catalytic reactions through optimizing
the bonding properties of catalyst and reactant, thus resulting in a
high performance [196, 197]. In the year of 2006, Thomas F.
Jaramillo’s group reported a significant enhancement of the OER
activity of electrodeposited NiOx films resulting from the
combined effects of using cerium as a dopant and gold as a metal
support [198]. The obtained NiCeOx-Au catalyst showed an
enhanced OER activity in basic media. The further theoretical
calculations and experimental results revealed that highly active
under-coordinated sites at the oxide support interface modified by
the local chemical binding environment and by doping the host
Ni oxide with Ce are the dominant factors towards improved
OER.
As we know, bimetal-based nanocrystals are usually
thermodynamically unstable which can easily cause surface
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restruction and rearrangement in electrolytes because of the highly
active unsaturated atoms and the high surface energy [185, 186,
199]. The easy-dissolution of transition metals usually accelerates
the transition metal atoms with respect to migration on the
catalyst surface and aggregation to each other [200, 201], resulting
in a rapid performance loss. Thus, seeking a suitable support is
pressing for stabilizing the nanocrystals structure and maintaining
the original activity. Sun’s group introduced WO2.72 nanorods as
support to stabilize the Cu component in the PdCu nanocrystals,
as illustrated in Figs. 12(a) and 12(b) [202]. The CuPd/WO2.72 can
stabilize Cu in 0.1 M HClO4 solution and, as a result, they show
Cu, Pd composition dependent activity for the electrochemical
oxidation of formic acid in 0.1 M HClO4 + 0.1 M HCOOH
(Fig. 12(c)). Due to the excellent acid resistance and good electron
conductivity of WO2.72, they disclosed that the excellent
performance derived from the strong interfacial interactions
between PdCu nanoparticles and WO2.72. Today, most concepts
achieve their high ORR activity by increasing the Pt specific
activity at the expense of a lower ECSA to meet the US
Department of Energy targets. Matthias Arenz’s group reported a
self-supported Pt-CoO networks that combine a high specific
activity with a high ECSA (Figs. 12(d) and 12(e)) [203]. The high
ECSA was achieved by a Pt-CoO bone nanostructure that exhibits
unprecedentedly high mass activity for self-supported ORR

Figure 11  (a)–(c) Schematic representations of the three palladium nanocrystals, octahedral (Pdoct), cubic (Pdcube), and truncated cubic (Pdmix), showing the respective
facets positioned above the corresponding high-resolution TEM (HRTEM) images. Reproduced with permission from Ref. [179], © Johnson, N. J. J. et al. 2019. (d) and
(e) HAADF-STEM images and (g)–(i) HRSTEM images of Rh NSs. (f) Projected crystallographic structural models and unit cells of diverse Barlow packings (FCC,
HCP, and 4H) and VBPs (VBP-1 and VBP-2) perpendicular to the stacking direction. Reproduced with permission from Ref. [184], © Zhang, Z. C. et al. 2021. (j)
Reaction energy diagram of water dissociation on Pt(111) (black) and Pt(111)-step (red). (k) Reaction energy diagram of water dissociation on Pt4/Ni(OH)2 (blue),
Pt4/vac-Ni(OH)2 (green), and Pt18/vac-Ni(OH)2 (purple) surface. Reproduced with permission from Ref. [189], © Elsevier Ltd. 2018.
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catalysts. Sun and coworkers designed a multifunctional
heterostructured Pt/LiCoO2 electrocatalyst towards overall water
splitting [80]. The Pt NPs were anchored on the exfoliated two-
dimensional (2D) LiCoO2 nanosheets by a wet-chemical
procedure, and the successful synthesis of Pt/LiCoO2
heterostructure can be verified by the corresponding microscopic
characterizations (Figs. 12(f) and 12(g)). The obtained
electrocatalyst showed superior electrocatalytic activity, with low
overpotential of 61 mV and 285 mV for HER and OER
(Figs. 12(j) and 12(k)). DFT calculations and XAS analysis results
indicated that the unique heterostructured Pt/LiCoO2 architecture
induced the electronic, defect and coordination effects play a vital
role in promoting the reaction kinetics (Figs. 12(h), 12(i), and
12(l)–12(n)). Interestingly, in this catalytic system, the active
center can be transferred between LiCoO2 support and Pt NPs.
When HER occurs, the Pt NPs are the active centers and LiCoO2
serves as the co-catalyst (Fig. 12(o)). The LiCoO2 can facilitate the
dissociation of H2O, optimize Pt–Had affinity and offer a platform
to hamper the aggregation of Pt species. When the reaction
transfers into OER, the active center turns into LiCoO2 and Pt
NPs act as the co-catalyst for OER. The Pt species can facilitate the
reduction of Co and the generation of O defects, accelerating the
OER kinetics and enhancing the activity of LiCoO2.

4.4InterfaceregulationforNPs
Because of the complexity of hybrid electrocatalysts, the
fundamental investigations on the structure–performance
relationship became challenging. This complexity also brings
diverse structural configurations that can be favorably regulated.
For example, it is significant to select a suitable component
material, which not only affects the structure and coupling
domains of hybrid materials, but also influences corresponding
electrocatalytic performance [204]. Interface effect usually triggers
an unpredictable surface structure due to the different strong
bonding or electronic interactions between two different active
components. Thus, the reconstructed active enters of interface can
induce an optimized activity and stability [205]. The HER and
OER kinetics are intrinsically dominated by the chemisorption of

Figure 12  (a) HAADF-STEM image of C-Cu48Pd52/WO2.72 and (b) STEM-EELS elemental mapping of a coupled C-Cu48Pd52/WO2.72. (c) FAOR mass activity for C-
Cu49Pd51, C-Cu48Pd52/WO2.72, and C-(Cu49Pd51+WO2.72). Reprinted with permission from Ref. [202], © American Chemical Society 2017. (d) SEM image and (e)
elemental distributions of Pt–CoO. Reproduced with permission from Ref. [203], © Sievers, G. W. et al. 2021. (f) Schematic illustration of the fabrication procedure of
Pt/LiCoO2. (g) The corresponding microscopic characterization. (h) Pt L3-edge EXAFS spectra. (i) Co K-edge EXAFS spectra. LSV curves of all the catalysts for (j) OER
and (k) HER. (l) Differential charge density of Pt/LiCoO2. (m) WT plots for the Pt L3-edge k3-weighted EXAFS signal for Pt NPs and 30% Pt/LiCoO2 heterostructures.
(n) WT plots for the Co K-edge k3-weighted EXAFS for LiCoO2 and 30% Pt/LiCoO2 heterostructures. (o) Schematic illustration of the active-center-transfer
mechanism for Pt/LiCoO2. Reproduced with permission from Ref. [80], © Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 2020.
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precursor molecules (H2O, OH−, or H+) and reaction
intermediates (e.g., *H, *OH, *O, etc.) [206, 207]. The interface
regulation provides a promising method to improve the
electrocatalytic performance through optimizing the
chemisorption of reactant molecules and intermediates.
The electrochemical NRR is a very efficient method for
sustainable NH3 production, but it requires effective catalysts to
expedite the NRR kinetics and inhibit the concomitant HER. Chu
and co-workers reported a 2D/2D MoS2/C3N4 heterostructure as
an active and stable NRR catalyst [210]. MoS2/C3N4 exhibited a
conspicuously improved NRR performance with an NH3 yield of
18.5 μg·h−1·mg−1 and a high Faradaic efficiency of 17.8% at −0.3 V,
far better than those of the individual MoS2 or C3N4 component.
The theoretical calculations revealed that the interfacial charge
transport from C3N4 to MoS2 could enhance the NRR activity of
MoS2/C3N4 by promoting the stabilization of the key intermediate
*N2H on Mo edge sites of MoS2 and concurrently decreasing the
reaction energy barrier. Zheng’s group reported a hydrothermal
synthesis method for fabricating M-OH-Pt interfaces (M-OH,
where M is Fe and Ni) by partially covering the surface of
monodisperse Pt NCs (which are < 5 nm in diameter) with
atomically thick M-OH layers, as illustrated in Figs. 13(a) and
13(b) [208]. The oxide-supported PtFeNi nanocatalyst rapidly and
fully removed CO from humid air without decay in activity for 1
month, which is attributed to the promoting effect of Fe3+-OH-Pt
interfaces.
Apart from the nanoparticles, the interface effect on the
nanowires and nanosheets also shows a promoting effect for
electrocatalytic reaction. Given the fundamental mechanism of the
interfacial interaction between Pt and oxophilic components,
engineering a moderate oxygen-friendly surface environment of
Pt-based catalysts is beneficial for endowing Pt with high catalytic
activity for HER. Wang et al. prepared ultralong jagged
Pt85Mo15–S nanowires with rich interfacial active sites to improve

Figure 13  (a) Cartoon showing the structural difference between the core–shell overgrowth Pt/FeNi(OH)x catalyst and the interwoven PtFeNi catalyst, and the alloy-
assisted strategy for the synthesis of the PtFeNi catalyst. (b) Representative HAADF-STEM image of PtFeNi NPs. Reproduced with permission from Ref. [208], ©
American Association for the Advancement of Science 2014. (c) HAADF-STEM image and (d) HRTEM image of Pt85Mo15–S NWs. (e) Schematic illustration of water
dissociation on Pt85Mo15–S NWs under the electronic effect and synergistic effect. Reproduced with permission from Ref. [170], © The Royal Society of Chemistry
2019. (f) Large-area TEM image, (g) aberration-corrected HAADF-STEM images, and (h) structural model of RuOx-on-Pd NSs. (i) FT k2-weighted X(k)-function of
the EXAFS spectra. (j) ORR polarization curves in 0.1 M KOH. (k) Comparison of the mass and specific activities of the stated catalysts at 0.9 V vs. RHE. (l) Histogram
of normalized mass activity changes during the accelerated durability tests (ADTs). Reproduced with permission from Ref. [209], © 2021 Wiley-VCH GmbH 2021.
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the HER performance, as shown in Figs. 13(c) and 13(d) [170].
This catalyst shows an excellent HER activity with 3.62 times the
specific current density and 4.03 times the mass current density of
commercial Pt/C as well as excellent stability. The enhanced HER
performance was attributed to the interfacial electronic effect and
synergistic effect (Fig. 13(e)). Xie and co-workers reported a 2D
inverse nanocatalyst, RuOx on-Pd nanosheets, by in situ creating
atomically dispersed RuOx/Pd interfaces densely on ultrathin Pd
nanosheets via a one-pot synthesis (Figs. 13(f)–13(i)) [209]. The as-
prepared RuOx/Pd shows a high ORR performance with 8.0- and
22.4-fold enhancement in mass activity compared to the state-of-
the-art Pt/C and Pd/C. The theoretical calculations disclosed that
the improved ORR performance is ascribed to the partial charge
transfer at the interface.

5Summary and perspective
The development of green energy storage and conversion systems
is of huge significance to our human society. Electrochemical
storage and conversion devices featuring green and sustainable
merits have attracted considerable attention in recent years. The
ration design of highly efficient heterogeneous catalysts plays a
critical role in the advancement of the development of these
devices. Various heterogeneous catalysts from supported
nanoparticles, to clusters and single-atom, have been well-
developed towards different electrocatalytic applications and
achieved exciting advancements. Herein, we comprehensively
outline the encouraging process regarding heterogeneous catalysts
from supported nanoparticles, to clusters and single-atom.
Importantly, to better understand the relationships of
structure–activity and unravel the underlying electrocatalytic
mechanisms, four engineering effects including electronic,
coordination, supporting, and interfacial effect are proposed based
on their electrocatalytic applications such as HER, OER, ORR,
NRR, and CO2RR. Although the exciting achievements have been
made in the electrocatalytic applications using the heterogeneous
supported catalysts, numerous challenges need to be solved in
both fundamental investigations and practical applications
(Fig. 14).
(1) Bridge the gap in the nanoparticle, cluster, and single-atom
Cluster with limited metal atoms features different coordinated
structures and under-coordinated surfaces, which can endow
them with many novel properties. Compared with single-atom
and nanoparticle catalysis, the study of cluster catalysis is emerging
as a new research frontier, especially in the electrocatalysis field.
Therefore, more efforts should be dedicated to the rational
construction of metal clusters with tailored size and electronic
structure, finally achieving the targeted application in
electrocatalysis.
(2) Catalyst screening
High-throughput DFT calculations and active machine-
learning techniques should be deployed to accelerate the discovery
of heterogeneous supported catalysts [211–214]. With the aid of
advanced techniques, the development of efficient catalysts with
high activity, selectivity, and durability will be highly accelerated.
Also, the cost of fabricating the target catalyst will be hugely
reduced.
(3) Controllable synthesis
It should be noted that there is still a great challenge to achieve
the commercialization of advanced heterogeneous supported
catalysts due to the difficulty in controllable synthesis. Therefore,
the rational design of large-scale synthesis methods and universal
fabrication routes are of great significance [215]. Universal
fabrication strategies provide the possibility of facile synthesis of
various heterogeneous supported catalysts via the same method.
Currently, some efficient general synthesis methods like
host–guest strategy and electrochemical deposition are well-
developed to acquire different heterogeneous supported catalysts.
[97, 128]. For example, Li et al. designed a universal host–guest
strategy to obtain different M1/CN (M = Pt, Ir, Pd, Ru, Mo, Ga,
Cu, Ni, and Mn) SACs.
(4) Mechanism exploration
Currently, systematic study of the relationship of
structure–property from single-atom, cluster, to nanoparticle has
been rarely reported, which plays a vital role in understanding the
corresponding underlying catalytic mechanisms. A detailed study
to reveal the dynamic evolution process from single atom to
cluster and nanoparticle using diverse in-situ techniques to
monitor the dynamic metal-atom arrangement and electronic
structure evolution process, such as in situ/operando XAS, and
STEM, is of great significance during the synthesis and reaction
process.
(5) Broader applications
The applications of heterogeneous supported catalysts need to
be widened to different energy-related fields and beyond, such as
rechargeable battery systems, biomedicine, and nano-enzymes. In
particular, the applications of cluster catalysts have been relatively
limited, and thus efficient cluster catalysts should be well-designed
and developed for various applications.

Acknowledgements
This work was supported by the National Key R&D Program of
China (No. 2018YFA0702003), the National Natural Science
Foundation of China (Nos. 21890383 and 21871159), the science
and Technology Key Project of Guangdong Province of China
(No. 2020B010188002), and the China Postdoctoral Science
Foundation (Nos. 2021M691757, 2021M690086, and
2021TQ0170).
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