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Single Atom Catalysts for Fuel Cells and Rechargeable Batteries: Principles, Advances, and Opportunities

November 2021
ACS Nano

November 2021

Authors:

University of Jinan (Jinan, China)

Owing to the energy crisis and environmental pollution, developing efficient and robust electrochemical energy storage (or conversion) systems is urgently needed but still very challenging. Next-generation electrochemical energy storage and conversion devices, mainly including fuel cells, metal-air batteries, metal-sulfur batteries, and metal-ion batteries, have been viewed as promising candidates for future large-scale energy applications. All these systems are operated through one type of chemical conversion mechanism, which is currently limited by poor reaction kinetics. Single atom catalysts (SACs) perform maximum atom efficiency and well-defined active sites. They have been employed as electrode components to enhance the redox kinetics and adjust the interactions at the reaction interface, boosting device performance. In this Review, we briefly summarize the related background knowledge, motivation and working principle toward next-generation electrochemical energy storage (or conversion) devices, including fuel cells, Zn-air batteries, Al-air batteries, Li-air batteries, Li-CO 2 batteries, Li-S batteries, and Na-S batteries. While pointing out the remaining challenges in each system, we clarify the importance of SACs to solve these development bottlenecks. Then, we further explore the working principle and current progress of SACs in various device systems. Finally, future opportunities and perspectives of SACs in next-generation electrochemical energy storage and conversion devices are discussed.

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Single Atom Catalysts for Fuel Cells and

Rechargeable Batteries: Principles, Advances,

and Opportunities

Yuchao Wang,

Fulu Chu,

Jian Zeng, Qijun Wang, Tuoya Naren, Yueyang Li, Yi Cheng,

Yongpeng Lei,*and Feixiang Wu*

Cite This: ACS Nano 2021, 15, 210−239

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ABSTRACT: Owing to the energy crisis and environmental

pollution, developing eﬃcient and robust electrochemical

energy storage (or conversion) systems is urgently needed but

still very challenging. Next-generation electrochemical energy

storage and conversion devices, mainly including fuel cells,

metal-air batteries, metal-sulfur batteries, and metal-ion

batteries, have been viewed as promising candidates for future

large-scale energy applications. All these systems are operated

through one type of chemical conversion mechanism, which is

currently limited by poor reaction kinetics. Single atom

catalysts (SACs) perform maximum atom eﬃciency and well-deﬁned active sites. They have been employed as electrode

components to enhance the redox kinetics and adjust the interactions at the reaction interface, boosting device performance.

In this Review, we brieﬂy summarize the related background knowledge, motivation and working principle toward next-

generation electrochemical energy storage (or conversion) devices, including fuel cells, Zn-air batteries, Al-air batteries, Li-air

batteries, Li-CO2batteries, Li-S batteries, and Na-S batteries. While pointing out the remaining challenges in each system, we

clarify the importance of SACs to solve these development bottlenecks. Then, we further explore the working principle and

current progress of SACs in various device systems. Finally, future opportunities and perspectives of SACs in next-generation

electrochemical energy storage and conversion devices are discussed.

KEYWORDS: single atom catalysts, fuel cell, rechargeable battery, electrode, conversion reaction, coordination conﬁguration,

energy storage and conversion, high energy density

INTRODUCTION

Lithium-ion (Li-ion) batteries have been widely used in civil

applications, including mobile communication, transportation,

power storage, aerospace, as well as navigation. However, the

energy densities of commercial Li-ion batteries using Ni/Co-

based cathodes and graphite anodes meet their bottlenecks due

to the limited speciﬁc capacities of electrode active materials.

1,2

In the meantime, the battery market is still expanding and

demands lighter and thinner batteries for smart applications. In

recent years, the fuel cells with ultrahigh energy densities have

been viewed as promising candidates for future energy

conversion systems. Especially for electric vehicles (EVs), the

combination employment of fuel cells and Li-ion batteries can

realize the long-lasting and high-power performance. In

addition, rechargeable batteries with potentially high energy

densities have also attracted lots of research eﬀorts.

With the increase of energy consumption and the

appearance of the energy crisis, boosting promising energy

storage and conversion systems with excellent performance

and stability is essential and pressing. Thus, next-generation

energy systems including fuel cells, metal-gas batteries, metal-

sulfur batteries, and metal-ion batteries are widely desirable.

3,4

Nevertheless, these energy storage (or conversion) systems

based on multiple-electron transfer reactions have remained

challenges that hinder their developments and eventual

commercialization.

5,6

For example, both the oxygen reduction

Received: October 16, 2020

Accepted: December 30, 2020

Published: January 6, 2021

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reaction (ORR) and oxygen evolution reaction (OER)

undoubtedly suﬀer from the sluggish reaction kinetics and

large voltage hysteresis that severely cause rather poor energy

eﬃciency and low capacity utilization.

7,8

Regarding metal-

sulfur batteries, the shuttle eﬀects of dissolved polysulﬁdes

worsen the reaction interface and cause the loss of active

materials, resulting in short cycle life.

9,10

More importantly, the

conversion reactions among alkali metal sulﬁdes are limited by

poor reaction kinetics, which leads to low capacity utilization

of sulfur, large voltage hysteresis, and unsatisfactory energy

densities.

As is known, catalysts provide alternative pathways with

lower activation energy barriers to promote the chemical

reactions.

12-15

So far, homogeneous catalysts and heteroge-

neous catalysts have been employed to accelerate the redox

kinetics in energy storage and conversion systems.

16,17

Owing

to the high price and resource scarcity of precious metals, their

large-scale applications (Pt-based catalysts for ORR and Ru/Ir-

based catalysts for OER) are obstructed. Nonprecious metal

materials serve as the hopeful candidates of precious metals,

but facing low catalytic capability.

18-21

To reduce the usage of

precious metals and further increase the intrinsic activity of

metal atoms, single atom catalysts (SACs) have emerged and

received extensive attention.

22,23

Compared to catalysts in the

nanoscale, the SACs separate their active sites in the form of

single atoms (SAs) without forming nanoparticles or clusters.

Theoretically, SACs ensure the 100% atomic utilization

eﬃciency, which is extremely valuable for saving metal

resources, especially for precious metals. Through the strong

metal−support interactions, the SAs are anchored on supports

via chemical coordination.

Such strong chemical bonding not

only maintains the stability of central atoms, but also acts as a

bridge for charge transfer.

25−27

Moreover, due to their similar

chemical environment and quantized orbital distribution, SACs

show improved reactive selectivity.

The applications of SACs

in energy-related devices eﬀectively reduce the metal

consumption and cost and simultaneously keep very small

amounts of catalyst in electrodes while maintaining the

required electrode active mass loadings for achieving high

energy output.

29−31

As shown in Figure 1, various SACs have been successfully

elaborated in fuel cells, metal-air batteries, metal-CO2batteries,

and metal-sulfur batteries, promoting their development. In the

present contribution, we introduce the working principles of

various electrochemical energy storage (or conversion)

systems as well as their remaining limitations that could be

solved via SACs. In turn, the synthesis, tailored conﬁgurations,

function designs, and characterizations of SACs have been

presented to guide the future construction of SACs. Moreover,

according to available literature, the reported working

principles of SACs in advanced fuel cells and batteries are

discussed in detail. In the conclusion section, we compare the

Figure 1. SACs and their potential functions in next-generation electrochemical energy storage or conversion devices.

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output properties of reported SAC-based cells/batteries to

acquire a general understanding of the research level. Finally,

the opportunities and perspectives of future eﬀorts that mainly

concentrate on electrode materials and battery structures are

proposed.

ADVANCED FUEL CELLS AND RECHARGEABLE

BATTERIES

As next-generation electrochemical energy storage and

conversion systems, fuel cells and rechargeable batteries

(metal-air batteries, metal-CO2batteries, and metal-sulfur

batteries) display the advantages of large theoretical energy

density, low cost, sustainability, and environmental friend-

liness.

32,33

Fuel cells, being substitutes for internal combustion

engines, point out the direction of electric vehicle energy.

Moreover,wehavesummarizedthetheoreticalenergy

densities (in both gravimetric and volumetric) of diﬀerent

batteries (Figure 2). Zn-air batteries not only oﬀer the largest

volumetric energy density, but also demonstrate advantages

such as rechargeability and direct utilization of air.

The large

volumetric energy density is of great signiﬁcance to wearable

devices. As for other metal-gas batteries, the direct utilization

of high-capacity alkali metals (Li) is beneﬁcial to reduce the

overall weight of the batteries and increase gravimetric energy

density.

The charge and discharge processes of metal-sulfur

batteries depend on the conversion between elemental sulfur

and metal sulﬁdes.

36−39

During the discharge process, multiple

discharge plateaus will appear, corresponding to metal sulﬁdes

with diﬀerent stoichiometric ratios.

40−43

To fully understand

the basic mechanism and intrinsic challenges of metal-sulfur

batteries, the Li-S and Na-S batteries are chosen for detailed

Figure 2. Theoretical gravimetric/volumetric energy density of various batteries. The data were obtained from refs 34,39,62−64, and 86.

Figure 3. Operation principles of advanced fuel cells and rechargeable batteries.

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discussions, while other metal-sulfur batteries are yet in their

infancy.

44−50

Here, detailed descriptions about operating principles and

features of the above-mentioned batteries will be discussed

separately. Especially, the challenges that can be eﬀectively

solved by catalysts in each battery system are analyzed in detail

in this section.

Fuel Cells. Owing to the advantages of high energy

eﬃciency, large power density, and quick start, the acidic

polymer electrolyte membrane fuel cells (PEMFCs) have been

widely applied since the 1980s.

As the commonly used

PEMFCs, hydrogen oxygen (H2−O2) fuel cells can be seen as

atomically equivalent to the “reverse”devices for water

electrolysis.

The unit cell is composed of an anode, a

cathode, and a proton exchange membrane. The H2molecules

are oxidized at the anode side, while the cathode provides a

place in which O2molecules are reduced (Figure 3a). The

ORR process could be divided into two distinct pathways:

eﬃcient 4e−transfer (O2+4H

++4e

−→2H2O) and

ineﬃcient 2e−transfer (O2+2H

++2e

−→H2O2).

Brieﬂy,

the 4e−process represents that O2gets 4e−directly then

generates H2O. The 2e−pathway refers to the process in which

O2obtains 2e−to generate H2O2. It is worth noting that the

peroxide ions play harmful eﬀects on ORR which lead to low

eﬃciency.

Hence, the high selectivity for ORR catalysts

toward 4e−transfer pathway is critical on practical occasions.

Compared to the oxidation of H2, the cathodic ORR is the

rate-determining step to control the energy eﬃciency of fuel

cells. The robust and eﬃcient catalysts are required to catalyze

the cathode reaction. For other fuel cells that use organic fuels

as anode active materials, the demands for catalyzing anode

conversion are also urgent. Thus, bifunctional catalysts, which

can simultaneously achieve the reduction of O2and the

oxidation of anode fuels, are also required.

Zn-Air Batteries. Rechargeable Zn-air batteries possessing

high speciﬁc energy and volumetric energy density have a great

promise in portable equipment.

55−58

Owing to the employ-

ment of aqueous or solid electrolytes, Zn-air batteries have

advantages in terms of safety and portability. As shown in

Figure 3b, metal Zn serves as the negative electrode active

substance while O2acts as the positive electrode active

material. During the discharge process, electrons are released

from the Zn electrode to the air electrode and the Zn atoms

are oxidized into Zn2+ ions. Meanwhile, O2diﬀuses into the air

electrode and is reduced into OH−ions via the ORR route (O2

+2H

2O+4e

−→4OH−).

59,60

Then, the OH−ions migrate

around the Zn electrode and combine with Zn2+ ions to form

zincate ions (Zn(OH)42−) that would be decomposed into

ZnO under supersaturated concentrations (Zn + 2OH−→

ZnO + H2O+2e

−).

During the charge process, Zn-air

batteries convert electrical energy into chemical energy

through OER. The OH−ions are oxidized to generate O2

(4OH−→O2+2H

2O+4e

−).

62,63

The overall charge−

discharge processes can be summarized into the reversible

reaction (2Zn + O2⇔2ZnO) (Figure 3b).

64,65

However, Zn-

air batteries are suﬀering from short operating lifetimes and

Wiley-VCH.

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low energy conversion eﬃciency that are mainly due to the

inherently slow kinetics of the ORR/OER and the poor

stability of air electrodes in corrosive electrolytes.

66−68

Therefore, eﬀorts are being made to develop durable and

nonprecious metal ORR/OER catalysts.

Al-Air Batteries. Al-air batteries are made of high-purity Al

or Al alloy as anodes, O2(air) electrode as cathodes, and alkalis

or salts as electrolytes. During the discharge process, the anode

materials dissolve and the O2molecules are reduced to release

electrical energy, which is similar to the working principle of

Zn-air batteries. The overall reaction can be described as

follow: Al + 3/4O2+ 3/2H2O→Al(OH)3.

However,

thermodynamic limitation makes it hard to process the

reduction of Al(OH)3during charging. The deposition of Al

electrode requires very negative potentials in an aqueous

system, which cause intense H2evolution prior to Al

deposition.

Therefore, the rechargeability of Al-air batteries

is challenging. The catalytic sites with high intrinsic ORR

activity are expected to enable the fast kinetics and mass

transfer at the three-phase interface.

Li-Air Batteries. Li-air batteries employ metal Li as

negative electrodes, O2as positive electrode reactants and

nonaqueous solutions as electrolytes (Li-air batteries based on

alkali aqueous electrolytes are omitted in this Review). During

the discharge process, the ORR occurs at the cathode. The

metal Li anode dissolves and passes through the electrolytes to

react with O2, forming Li2O2or Li2O. For the charge process,

the Li2O2(or Li2O) decomposes into metal Li and O2, which

is the OER process (Figure 3c). The entire cycle can be

described as 2Li + O2⇔Li2O2or 4Li + O2⇔2Li2O.

72−74

In theory, the speciﬁc energy of Li-air batteries is higher than

those of other metal-air batteries.

But the kinetic problems in

Li-air batteries are serious. On one hand, the reactivity of metal

Li places severe requirements on the design of battery

structures. The direct contact between metal Li and H2O/air

should be prohibited. On the other hand, the slow kinetics

issues coming from the generation and decomposition of

Li2O2/Li2O reduce the round-trip eﬃciency (Figure 4a). In

particular, the decomposition of solid-state discharge products

is rather diﬃcult, ultimately causing the large overpotential,

low rate capability and poor cycling eﬃciency of Li-air

batteries.

Exploring porous cathode catalysts to regulate the

growth and decomposition of solid-state discharge products is

of practical signiﬁcance.

Li-CO2Batteries. Metal-CO2batteries are not only eﬃcient

energy supply systems but also helpful to achieve CO2ﬁxation

and carbon neutral cycle.

Importantly, the research and

application of metal-CO2batteries will play a role in Mars

explorations due to the extremely high CO2content (over

95%) in the Mars atmosphere. Among the known metal-CO2

batteries, Li-CO2batteries demonstrate the largest theoretical

energy density. They adopt metal Li as anode materials, while

the CO2molecules are chosen as cathode reactants and energy

carriers.

The formation and decomposition of Li2CO3(4Li +

3CO2⇔2Li2CO3+ C) during the discharge/charge process

happens. In the discharge process, Li ions are released to the

gas electrode and react with CO2to produce Li2CO3

precipitates (Figure 3d). The large surface area of gas electrode

is required to accommodate these discharge products. In order

to achieve the decomposition of Li2CO3, the charging process

often requires an overpotential above 4 V. Such a high charging

voltage not only results in the loss of energy eﬃciency, but also

possibly causes the decomposition of organic electrolytes.

79,80

Therefore, the slow formation and decomposition of metal

carbonates restricts the development of metal-CO2batteries

(Figure 4b).

Gas electrode catalysts that promote the

decomposition of Li2CO3are currently the main research

content in Li-CO2batteries.

Li-S Batteries. Lithium-sulfur (Li-S) batteries using high-

capacity sulfur and Li as electrode active materials can deliver a

high theoretical energy density (Figure 3e). Moreover,

considering the lower economic cost and environmental

friendliness of earth-abundant sulfur, Li-S batteries have been

viewed as a promising candidate in the ﬁeld of future

batteries.

As it is well-known, the conversion reaction in

Li-S batteries can be summarized by the equation 16Li + S8→

8Li2S.

83−85

However, the discharge process is complicated and

involves multiple steps as presented in Figure 4c. It mainly

includes two steps (two visible plateaus in the discharge curve

in Figure 4c) according to solid-to-liquid (S8→Li2S8→Li2S6

→Li2S4) and liquid-to-solid (Li2S4→Li2S2→Li2S) phase

transformations. More precisely, the most challenging issue in

Li-S batteries is the shuttling eﬀect that stems from the former

intermediates: highly soluble long-chain polysulﬁdes (Li2S8,

Li2S6, and Li2S4) in most organic liquid electrolytes. This

unwanted shuttle between the Li metal and sulfur cathode

during the discharge−charge process worsens the reaction

interface, leading to increased internal resistance and capacity

loss with cycles. Moreover, as the discharge products, both

Li2S2and Li2S are natural insulators, causing the poor reaction

kinetics of the conversion reaction between Li2S2and Li2S.

This limited solid-to-solid conversion reaction largely reduces

the capacity utilization of sulﬁde cathode, rate performance,

energy eﬃciency, and the ﬁnal practical energy densities. In the

charge process, the activation of Li2S needs a visible charge-

overpotential that is the high energy barrier of Li extraction to

overcome. By feat of strategies based on physical or chemical

methodologies, the shuttle eﬀect of polysulﬁdes in Li-S

batteries has been partly alleviated so far.

Physical conﬁne-

ment availing of carbonaceous materials, such as graphene,

mesoporous carbon,

and metal−organic frameworks,

can

wrap up polysulﬁdes in pores or block them by interlayer

spaces. Meanwhile, chemical bonding with polysulﬁdes by

eﬀective heteroatom dopants in a substrate can strengthen the

interaction to further eliminate the detrimental shuttle

eﬀects.

90,91

In addition, the chemical interactions can

compensate for the unavoidable out-diﬀuse of polysulﬁdes

during extended cycles, as the binding force between nonpolar

carbon and polarized polysulﬁdes is so insuﬃcient.

92,93

The metallic Li anodes display severe challenges of side

reactions caused by high reactivity of Li.

As a result, various

components in the electrolyte system could induce the

interfacial layer on Li anodes with ion conduction but no

electron conduction, the so-called solid electrolyte interphase

(SEI).

However, the repeated Li plating/stripping behavior

can lead to the undesired fracture and growth of the SEI layer,

which causes rather thick SEI layer, continuous consumption

of both active lithium and electrolyte in parasitic reactions, as

well as low Coulombic eﬃciency (CE). Furthermore, the

irregular Li deposition and preferred orientation growth of

microcrystal easily generated lithium dendrites, resulting in

early failure and security problems.

When meeting tougher

conditions (to achieve practical high energy densities in Li-S

batteries), such as high sulfur loading, high rate discharging, or

lean electrolyte condition (low electrolyte/sulfur ratio), the

above issues in Li-S chemistry are becoming even worse. For

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example, the conversion reaction in rather thick S electrodes

will suﬀer from more sluggish kinetics, resulting in lower

capacity utilization.

97,98

Therefore, designing eﬃcient catalysts

would be an eﬀective and indispensable strategy to regulate the

performance of Li-S batteries via promoting fast conversion of

sulfur species, eﬃcient trapping of polysulﬁdes, and uniform

lithium nucleation.

Na-S batteries. Considering the large abundance and low

cost of Na, Na-S batteries with potentially high energy density

are capable of the adaptation of large-scale energy storage and

even EVs in the future.

99,100

Since the 1960s, according to the

history of Na-S batteries, high-temperature sodium-sulfur (Na-

S@HT) batteries took the lead in realizing commercialization

in the stationary energy storage ﬁeld, in which molten sulfur

coupled with sodium beta-alumina (β-NaAl11O17) served as an

eﬃcient solid-state Na-ion conductor.

101,102

However, owing

to the rather high operating temperatures at 300−350 °C, the

potential security risks and extra maintenance expenditure have

made conventional Na-S@HT batteries impossible to be

employed for EVs. Since 2012, plenty of research eﬀorts have

been conducted to develop room-temperature sodium-sulfur

(Na-S@RT) batteries for future practical applications.

103,104

Compared to Li-S conversion chemistry, Na-S@RT batteries

display more serious issues caused by polysulﬁdes shuttle

eﬀects, sluggish reaction kinetics, and Na dendrites. As a

consequence, the low eﬃciency, low capacity utilization and

fast capacity degradation during cycling indeed impeded

commercial development of Na-S@RT batteries, which

currently cannot compete with the cycling lifespan (2500

cycles) achieved in Na-S@HT batteries.

105,106

According to

the available literature,

103,104

the conversion reaction chemistry

in Na-S@RT batteries can be summarized in the following

equations (Figure 3f):

American Chemical Society. ALD method. Adapted with permission under a Creative Commons CC BY License from ref 112. Copyright

2020 Nature Publishing Group. Electrochemical etching method. Adapted with permission under a Creative Commons CC BY License from

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During the discharge process in Na-S@RT batteries, the

coexistence and interconversion of various polysulﬁde

substances (Na2Sn,4≤n≤8) will persistently occur in the

most complicated region II (Figure 4d). What is worse,

subsequent electrochemical reactions are usually not complete,

especially in region IV as the deﬁcient conductive property of

solid Na2S2and Na2S.

107

The slow reaction kinetics and high

polarization signiﬁcantly aﬀect the overall discharge perform-

ance. Thus, parts of the discharge reaction are disappeared. In

the opposite process, the utilization of active S-species would

be further decreased during the charge process. Meanwhile,

similar to the case of the metallic Li anode, the uneven Na

deposition, unsubstantial interfacial layer, and further dendritic

growth during cycling require special attention to be controlled

as well. Hence, to realize successful applications of Na-S@RT

battery, more signiﬁcant progress should be made to enhance

the reaction kinetics and cycle stability in Na-S battery, as well

as to comprehensively understand the Na-S conversion

reaction mechanism. Only then would Na-S@RT batteries

be an absolute option on account of low cost and reasonable

energy density.

Obviously, toward diverse battery systems, although

potential progress has been made, the research bottlenecks

of critical catalytic reactions still seriously exist. Such

challenges make their output performance far from the

theoretical level. As a promising part of battery construction,

applying SACs in electrode materials represents a research

direction which could narrow the gaps between practical

applications and theoretical level to a certain extent.

SYNTHESIS AND CHARACTERIZATION OF SACs

In order to meet the demand for highly active catalysts in

various catalytic situations, many synthesis strategies toward

SACs have been developed. In general, the “bottom-up”and

“top-down”methods are common strategies (Figure 5).

108

For

the former, the wet chemical methods consisting of wet-

impregnation and coprecipitation perform the advantages of

simple operation and scale preparation.

109,110

However, the

subsequent reduction treatments, such as H2reduction and

photochemical reduction, are necessary. The produced

catalysts usually demonstrate shortcomings of low metal

loading and agglomeration.

111

Requiring complicated equip-

ment, atomic layer deposition (ALD) method further improves

the productivity and controllability, while the precise control of

ﬁlm thickness can be achieved.

112

In addition, electrochemical

deposition and chemical vapor deposition have been reported

to prepare SACs.

113,114

As one of the top-down methods, the ball-milling is

relatively simple to realize large-scale preparation.

115

The

high-temperature atomic trapping,

116

solid phase thermal

diﬀusion

117

and the pyrolysis of metal organic complexes

also receive attention. The pyrolysis of metal organic

complexes typically employs various precursors with features

of molecular conﬁnement and organic ligand coordination.

And the pyrolysis procedure greatly improves the stability of

metal SAs, which is conducive to increasing metal loading and

enhancing catalytic durability.

118

It is worth noting that these

complex precursor-based pyrolysis strategies produced uni-

formly dispersed M−N−C sites, which are regarded as the

most hopeful nonprecious metal-based ORR catalysts.

119

Thus,

although consuming organic reagents, the top-down methods

display obvious advantages over those bottom-up methods.

Very recently, some meaningful strategies were also reported.

For instance, a shockwave synthesis method anchored SAs on

carbon supports, which could remain stable under a high

temperature.

120

Also, a universal precursor-dilution strategy

utilized the chelation between porphyrin molecules and metal

cations to prepare 24 diﬀerent SACs.

121

Diversiﬁed synthetic

strategies bring more opportunities for coordination conﬁg-

uration regulation and catalytic activity promotion.

In addition to the advancement of synthetic methods, the

development of SACs is also closely related to the progress of

characterization technologies. The aberration-corrected high-

angle annular dark-ﬁeld scanning transmission electron

microscopy (AC HAADF-STEM) and synchrotron-based X-

ray absorption spectroscopy (XAS) are most eﬀective

characterization tools in connection with SACs. At present,

the HAADF-STEM is the only tool that can directly observe

SAs via the brightness diﬀerence among diﬀerent elements.

The local information and spatial dispersion of atoms are

presented.

122

The XAS can be divided into X-ray absorption

near-edge structure (XANES) spectra and extended X-ray

absorption ﬁne structure (EXAFS).

123

The XANES is sensitive

to the oxidation states of atoms, while the EXAFS is sensitive

to the coordination distances, coordination species and

coordination numbers. Therefore, the local electronic structure

and chemical environment of the central atoms can be realized

at atomic scale.

124

On the basis of experimental information,

the reasonable structural model can be established for

theoretical study. As a special advantage of SACs, the

homogeneously dispersed and separated active sites provide

a great convenience for density functional theory (DFT)

calculations. It is worth mentioning that some of recent articles

have pointed out the reconstruction of catalysts during

reaction process. Hence, to ﬁgure out the real active species

and clarify the reaction mechanism, in situ technologies

Figure 6. Development history of SACs in fuel cells and batteries.

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(Raman spectrum, XANES, EXAFS, infrared spectrum, etc.)

are required.

125

Beneﬁting from the improvements in synthesis methods and

advancements in characterization technologies, the modulation

and recognition of coordination structure toward SACs

achieved signiﬁcant progress, which pushed their applications

in the energy ﬁeld. Here, we set our sights on SAC-based

electrode materials for advanced fuel cells and batteries. The

related parameters to evaluate device performance (cycle life,

energy density, etc.) are also presented (Figure 5).

ADVANCES OF SACs IN FUEL CELLS AND

RECHARGEABLE BATTERIES

Dispersed M−N−C active centers have received great

attention.

126

Among diﬀerent transition metals, Fe-based

SACs have been extensively studied due to their high ORR

intrinsic activity and no toxicity. The timeline of SACs in fuel

cells and batteries is shown in Figure 6. SACs have been

employed to develop Zn-air batteries since 2017. Then, they

were also chosen in PEMFCs, Li-S, and Na-S chemistries to

drive the energy devices toward high output, great portability,

environmental friendliness, and low cost. Very recently, SACs

have been used in Al-air, Li-CO2, and Li-air batteries.

SACs in PEMFCs. Since Pt-based catalysts have been

largely used in PEMFC cathode, improving the Pt utilization

and developing non-Pt catalysts is urgent and necessary.

127

et al.

128

simultaneously anchored Pt SAs and nanoparticles on

defective carbon supports. The Pt1.1/BPdefect-based PEMFCs

achieved a power density of 520 mW cm−2,which

corresponded to the Pt utilization of up to 0.09 gPt kW−1.

Shui et al.

129

prepared a series of precious metal SAs (Pt, Pd, Ir,

Rh) on ZIF-derived carbon. They proposed that the

electronegativity of coordinated anions should match with

the OH*absorbance ability of precious metal atoms to

enhance ORR activity. The atomically dispersed metal atoms

anchored on carbons are potential to replace their particle/

cluster counterparts.

130

Figure 7. SACs in PEMFCs. (a) Discharge polarization curves and power density plots. Reprinted with permission from ref 132. Copyright

American Chemical Society. (e) Polarization and power density curves. (f) Correlation between active site densities and power density.

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217

Under real operating conditions, the slow mass transfer

process would cause the accumulation of products and

insuﬃcient supply of reactants, which subsequently aﬀects

the output performance of fuel cells.

131

To alleviate this

problem, Wang and co-workers

132

constructed hierarchical

ordered porous carbon as a skeleton to accommodate Fe−N4

active sites. The resulting PEMFCs exhibited an open circuit

voltage (OCV) of 0.98 V and maximum power density of 0.42

Wcm

−2under 1 bar H2/air (Figure 7a). This hierarchical

ordered porous structure had a signiﬁcant improvement in the

mass transfer process, which was also proved by the work of

Liao et al.

133

Shui et al.

134

manufactured concave-morphology

Fe−N−C SACs (TPI@Z8(SiO2)-650-C) with 10 nm thin

layer to promote mass transfer. The SiO2coating with ZIF-8

played crucial roles in the formation of concave shape. The

proportional relationship between current density and active

site density proved the contribution of Fe−N−C sites to

catalytic activity. Thus, increasing active site density is a

reliable measurement to promote performance.

Aiming to resist the corrosion of electrode materials under

acidic conditions, Cheng et al.

135

developed FeSA-G with high

metal loading up to 7.7 wt % to meet the high temperature

operation and phosphate corrosion environments. In high

temperature phosphoric acid/polybenzimidazole (PA/PBI)

composite membrane fuel cells using FeSA-G as the cathode,

a power density of 325 mW cm−2was obtained at 230 °C,

which exceeded that of its counterparts using Pt/C (313 mW

cm−2). Under a cell voltage of 0.6 V, the polarization curves of

PEMFCs were stable at 230 °C for 100 h. No obvious

structural change was observed (Figure 7b). Wu and co-

workers

136

prepared 1.5Fe-ZIF and explored the reasons for

their performance degradation in acidic conditions. The 1.5Fe-

ZIF based PEMFCs were subjected to constant-potential

testing at 0.85 V in 0.5 M H2SO4until 100 h (Figure 7c).

Obviously, the catalytic current density was reduced to 30% of

the initial value. Due to the carbon corrosion, the N−C and

Fe−N bonding became weak and broken while the Fe clusters

appeared. Hence, the durability of Fe−N−C sites in acidic

conditions was still an extremely hard challenge.

The multisite structure could promote the formation of O−

O bridge models, which is beneﬁcial to boost the O−O bond

breakage and 4e−reduction reaction. Therefore, Sun et al.

137

introduced Ni−Nxinto Fe−N−C species to reduce the

adsorption of H2O2. Quantitative XANES ﬁtting indicated

that a Fe−N4conﬁguration combined with a Ni−N4

conﬁguration to form a FeNi−N6motif rather than M−N3

sites (Figure 7d). Compared to single Fe−N4sites, FeNi−N6

displayed better ORR activity, turnover frequency (TOF), 4e−

selectivity, stability, and methanol resistance. In PEMFCs using

FeNi−N6as a cathode, the initial maximum power density

reached 216 mW cm−2under the H2/O2condition. Due to the

coexistence of Ce3+ and Ce4+, ceria can release or store O well,

thus promoting the O2adsorption and occurrence of the ORR

process. Shao and co-workers

138

developed active Fe−N−C

sites (Ce/Fe-NCNW), in which the Fe SAs coordinated not

only with 4 N atoms but also with the 1 O atom of ceria.

Under the H2/O2condition with a pressure of 30 psi, the

power density of PEMFCs attained 496 mW cm−2. So, the

advantage of special oxides over carbon-based supports cannot

be ignored.

The Fenton reaction, during which Fe2+ mixes with H2O2to

oxidize organic compounds, eventually causes damage to the

polymer membrane and aﬀects the stability of fuel cells.

139

Although Fe−N−C species show great ORR property, it also

makes sense to develop other SACs to avoid the harmful

Fenton reaction. Shui et al.

140

utilized an electrochemical

method to control the density of CoN4. As the weight ratio of

cobalt acetate in precursor increased to 1.6, the 1.6%CoNC-

ArNH3catalyzed PEMFCs emerged the largest power density

of 826 mW cm−2under the pressure of 2.5 bar H2/O2(Figure

7e). Further increase of Co content caused the formation of

nanoparticles, which was not conducive to the occurrence of

ORR (Figure 7f). In addition, the utilization of Mn−N−Cin

fuel cells also eﬀectively avoided the Fenton reaction. Wu et

al.

141

discovered that the doped Mn in carbon mixture

probably enhanced the degree of graphitization, which was

beneﬁcial to improving the corrosion resistance of carbon

materials. In 0.5 M H2SO4, the 20Mn-NC-second endured

3000 cyclic voltammetry (CV) cycles with minor loss of half-

wave potential (E1/2). Then, at a constant potential of 0.8 V,

the 20Mn-NC-second possessed smaller current density decay

than 20Fe-NC-second, indicating the predominant carbon

corrosion resistance toward Mn-based active sites.

Table 1. Summary of SACs in PEMFCs

catalysts conﬁguration power density (mW cm−2) open circuit voltage (V) stability refs

Ir1-N/C Ir−N4870(O2)/380(air) 0.95(O2)/0.93(air) 50 h 129

Co@SACo-N-C Co−Nx420(O2)/230(air) 0.91(O2)/0.91(air) 10 h 131

FeN4/HOPC-c-1000 Fe−N4660(O2)/420(air) 0.99(O2)/0.91(air) 100 h 132

C-FeHZ8@g-C3N4-950 Fe−N4628(O2) 0.98(O2)8h133

TPI@Z8(SiO2)-650-C Fe−N4420(air) 0.91(air) 134

FeSA-G Fe−N4325(O2)

0.9(O2)

100 h 135

1.5Fe-ZIF Fe−N4660(O2)/360(air) 0.98(O2)/0.95(air) 100 h 136

FeNi-N6FeNi-N6216(O2) 0.82(O2)137

Ce/Fe-NCNW Fe−N4-O 496(O2)

1.02(O2)

138

20Co-NC-1100 Co−N4560(O2)/280(air) 0.95(O2)/0.89(air) 100 h 139

CoNC-ArNH3Co−N4440(O2)/221(air)

0.88(air)

20 h 140

20Mn-NC-second Mn−N4460(O2) 0.95(O2) 100 h 141

Fe−C−N950 680(O2) 0.80(O2)143

Fe SAs/N−CFe−N4680(O2)/350(air) 0.83(O2)/0.89(air) 145

Fe-SAs/NPS-HC Fe−N4333(air) 0.96(air) 148

SA-Rh/CN Rh−N/O 522.1 mW mg−10.77 50 000 s 142

Test under 230 °C.

Test under 30 psi.

Test under the pressure of 0.5 bar.

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Considering that the PEMFCs have high operating temper-

atures (up to 200 °C) and acidic working conditions, the

serious corrosion to carbon-based supports will bring

challenges to the practice of SACs. Table 1 summarizes the

recent PEMFCs using SACs as cathodes. Under H2/O2and

H2/air conditions, the power density and OCVs of PEMFCs

are obviously diﬀerent. Such declining performance under H2/

air atmosphere suggests that suﬃcient mass transfer inﬂuences

integral output performance. Apart from the intrinsic ORR

property of catalysts, developing diverse metal centers,

optimizing morphology structure to accelerate mass transfer

and designing reasonable component to enhance durability

should also be considered.

Although achievements of SACs in H2−O2fuel cells have

been made, the works toward other PEMFCs, such as direct

formic acid fuel cells (DFAFCs) are rarely reported. Wang and

co-workers

142

employed the SA-Rh/CN for DFAFCs anode

catalysts in which Rh atoms coordinated with 4 N atoms and 2

O atoms. When used for formic acid oxidation, SA-Rh/CN

displayed a mass activity of 16.1 A mg−1and a TOF of 8.55 s−1,

both exceeding those of commercial Pd/C. In DFAFCs, SA-

Rh/CN exhibited mass power densities of 522.1 mW mg−1

(8.8 times of Pd/C) at 30 °C and 1241.9 mW mg−1at 80 °C.

Additionally, the CO adsorption on the SA-Rh/CN surface

was weakened and the dehydration of formic acid was

suppressed, which was also conﬁrmed by DFT calculations.

The high CO production barrier and relatively unfavorable

binding between SA-Rh/CN and CO molecules resulted in

excellent CO tolerance.

SACs in Zn-Air Batteries. Toward ORR for Zn-air

batteries, Fe-based SACs have also attracted a lot of research

eﬀorts. Chen and co-workers

144

developed single Fe atoms

(metal loading of 1.96 wt %) on N-doped porous carbon

(NPC) by an in situ anchor strategy. Carrying a valence

between +2 and +3, each Fe atom coordinated with 4 N atoms

to form Fe−N4active sites that demonstrated O2activation

capacity (Figure 8a). The prepared Fe−N4SAs/NPC

displayed an onset potential (Eonset) of 0.972 V and an E1/2

of 0.885 V in 0.1 M KOH. The corresponding Zn-air batteries

showed a power density of 232 mW cm−2(Figure 8b) without

evident voltage changes after 108 cycles, which were better

than those of the Pt/C+Ir/C-based counterparts. To boost the

power density, Fe SAs (Fe−N4) with metal loading up to 3.8

wt % on crumpled N-doped carbon nanosheets (Fe SAs/N−

C) were developed, yielding an E1/2 of 0.91 V in 0.1 M KOH

(Figure 8c).

145

Correspondingly, the Zn-air batteries achieved

a power density of 225 mW cm−2. DFT calculations showed

that the limiting potential of Fe SAs/N−C reached up to 0.76

V, which was close to that of Pt (0.79 V).

Heteroatom doping in SACs has been widely employed to

improve the conductivity of the carbon matrix and modify the

coordination environment of central atoms.

146

As reported, the

codoped N and S atoms could cause the charge redistribution

in the carbon framework and boost bifunctional ORR/OER

activity of atomically dispersed Fe−Nxsites.

147

Wang et al.

148

synthesized eﬀective Fe SAs anchored on N, P, S codoped

hollow carbon (Fe-SAs/NPS-HC) via the so-called MOF@

polymer strategy. The E1/2 values of 0.912 and 0.791 V were

obtained in 0.1 M KOH and 0.5 M H2SO4, respectively. As

primary active sites, the isolated Fe−N4species activated and

reduced O2. The homogeneously dispersed P and S atoms did

not coordinate with Fe atoms directly but modulated the

electrical states via long-range interactions, which weakened

the binding toward OH*intermediates to release OH−(Figure

Figure 8. Fe-based SACs in Zn-air batteries. (a) EXAFS result. (b) Charge−discharge polarization curves and power density plots. Reprinted

charge. (e) Discharge polarization curves and power density plots. Inset: Schematic model of Fe-SAs/NPS-HC. Reprinted with permission

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8d). The Zn-air batteries displayed an OCV of 1.45 V and a

power density of 195.0 mW cm−2at a current density up to

375 mA cm−2(Figure 8e). Furthermore, the N, P co-

coordinated Fe−N3P conﬁguration showed the lower free

energy of primitive steps than traditional Fe−N4sites, which

was more conducive to the adsorption of O2and the

occurrence of rate-determining step (OH*to OH−)(Figure

8f).

149

Xiang and co-workers

150

coupled the graphene matrix

with covalent organic frameworks (COFs) containing Fe−N4

active species via intermolecular interactions. Apart from the N

atoms in COFs, Fe SAs simultaneously coordinated with C

atoms from graphene. The assembled Zn-air batteries delivered

an OCV of 1.41 V, a power density of 123.43 mW cm−2, and a

long-term stability of over 300 h with voltage attenuated less

than 0.1%. Apart from these, the oxygen dangling bonds on the

graphene oxide (GO) surface could trap metal (Fe, Co, Ni,

Cu) atoms and construct corresponding M−O4sites at room

temperature.

151

The mild and economically friendly synthesis

strategies operated under low temperature eﬀectively reduce

the energy consumption, deserving our attention.

Recently, the Co−N−C sites with good bifunctional ORR/

OER performance have been widely used in rechargeable Zn-

air batteries. Zhang’s group

152

investigated Co-POC SACs

using π−πcongregated porphyrin and graphene as pyrolysis

precursors. In 0.1 M KOH, Co-POC demonstrated an E1/2 of

0.83 V for ORR and a η10 of 470 mV for OER. A low ΔE(the

value between E1/2 and η10) of 0.87 V also proved the

bifunctional ORR/OER performance. They further designed

Co−Nx−C active moieties with the Co loading of 1.23 wt %

for ﬂexible Zn-air batteries (Figure 9a).

153

Applying alkaline

poly(vinyl alcohol) (PVA) gel as an electrolyte, the Zn-air

batteries showed a high OCV of 1.439 V. They also endured

long-term charge/discharge cycles and bending at diﬀerent

angles without obvious performance changes. In order to

enhance the ﬂexibility and wearability of Zn-air batteries, Guo

et al.

154

reported a bifunctional Co−N4moiety (2.05 wt %) on

electrospun ﬁbers (Co SA@NCF/CNF) as binder-free air

electrodes. An E1/2 of 0.88 V and a η10 of 400 mV were

achieved in 1 M KOH (Figure 9b). The wearable Zn-air

batteries displayed an OCV of 1.41 V and a capacity of 530.17

mAh gZn

−1. They showed higher discharge voltages and more

stable discharge platforms than those of their Pt/C+Ir/C-based

counterparts (Figure 9c). After frequent folds, the batteries can

still work well, displaying the resistance to external pressure

(Figure 9d). The supports possessing rough surfaces, rich

channels, and plentiful pores are beneﬁcial to mass transfer at

the electrochemical interface.

155

Lu et al.

156

combined the

coordination environment regulation and porosity/conductiv-

ity decoration in Co-based SACs. The constructed Co-N3C1-

based Zn-air batteries achieved a peak power density of 255

mW cm−2. The importance of combined electronic state

regulation and morphology optimization was highlighted.

Toward ﬂexible Zn-air batteries, other Co-based SACs were

also constructed.

157−159

Development of central atoms, synergy between multimetal

sites, and design of supports are the strategies currently

involved. Wu and co-workers

160

reported a high-temperature

gas-transport strategy to construct single Cu atoms on N-

doped carbon with rich defects. The Cu SAs, with a valency

between 0 and +2 (Figure 10a), coordinated with 3 N atoms

(Figure 10b). The Cu ISAS/NC-based Zn-air batteries

achieved a power density of 280 mW cm−2and a speciﬁc

Figure 9. Co-based SACs in ﬂexible Zn-air batteries. (a) Schematic illustration of the ﬂexible Zn-air batteries. Reprinted with permission

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capacity of 736 mAh g−1(Figure 10c). In addition, the Cu SAs

(Cu−N3) have been successfully ﬁxed on hollow N-deﬁcient

carbon nitride nanospheroids (CuSA@HNCNx). Under the

same synthesis conditions, the ORR/OER activities of CuSA@

HNCNxeven surpassed those of FeSA@HNCNxand CoSA@

HNCNx, illustrating the feasibility of Cu-based SACs for air

electrodes (Figure 10d).

161

Mo-based bifunctional catalysts

from the size of nanoparticles to SAs were reported.

162

The

Mo1N1C2active sites achieved the lowest ORR and OER

overpotentials compared to those of the nanoparticle and

cluster counterparts (Figure 10e). Theoretically, Kulkarni and

co-workers

163

selected Cu-modiﬁed covalent triazine frame-

works as model SACs (Cu/CTF) and carried out calculations.

Based on the electronic structure analysis and solvation eﬀects,

the appropriate descriptions to evaluate ORR activities were

proposed, which not only discovered the origin of ORR

activity toward Cu/CTF but also can be extended to other

SACs.

By simultaneously introducing multiple metal SAs on

carbon-based supports, the interactions between diﬀerent

metal atoms may enhance the catalytic property. Jia et al.

164

prepared Fe, Co SA coexisting catalysts with the proximate

conﬁguration of M−N4. According to the free energy path in

theoretical calculations, Fe and Co atoms aﬀected the

electronic structure of each other, which was conducive to

the occurrence of ORR process. In Zn-air batteries, FeCo-IA/

Figure 10. Non-Fe, Co-based SACs in Zn-air batteries. (a) Cu K-edge XANES and (b) EXAFS results. (c) Discharge polarization curves and

Elsevier. (f) Discharge polarization curves and power density plots. Inset: SA-PtCoF model. Reprinted with permission from ref 168.

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NC drove an OCV of 1.472 V and a power density of 115.6

mW cm−2. Ma and co-workers

165

constructed hollow graphene

nanospheres with Fe−N4sites in outer layers and Ni−N4sites

in inner layers. The excellent ORR and OER properties, which,

respectively, originated from Fe−N4and Ni−N4species,

endowed the Zn-air batteries with high energy eﬃciency and

durability. The metal−support interactions not only stabilize

metal SAs on the supports but also regulate their geometric

environment and electronic structure to enhance the intrinsic

activity.

166

Xu and co-workers

167

deposited Co SAs on Co3O4

particles and N-doped carbon. The interactions among Co

SAs, Co3O4and N-doped carbon enhanced the ORR/OER

activity. Yang’s group

168

built PtCo alloy nanosheets with

interstitial F doping (SA-PtCoF). The F atoms caused the

lattice disorder and weakened the Pt−Co bonds, eventually

generating stable Pt SAs on nanosheet edges. The electrons

transferred from Co to adjacent Pt atoms, pulling down the d-

band center of Pt and adjusting the adsorption behavior of

intermediates. Correspondingly, the electronic structure of Co

was also aﬀected by adjacent Pt atoms, which could promote

the OH−adsorption during OER process. The SA-PtCoF-

based Zn-air batteries gained a power density of 125 mW cm−2

(Figure 10f). Here, the Co supports also acted as active

species, while the Pt SAs served as cocatalysts to a certain

extent.

As a short summary, the Fe−N−C and Co−N−C sites are

the most commonly used active species for cathode materials

of Zn-air batteries (Table 2). Although Fe-based SACs

displayed good ORR performance, the Co-based SACs showed

evident advantages in rechargeable batteries due to their

bifunctional activity. The strategies such as heteroatom doping,

Table 2. Summary of SACs in Zn-Air Batteries

catalysts conﬁguration power density

(mW cm−2)speciﬁc capacity

(mAh g−1)open circuit

voltage (V) electrolytes charge/discharge

cycles refs

Fe−Nx−CFe−Nx96.4 641 1.51 6.0 M KOH + 0.2 M

Zn(Ac)2

300 h 126

1.49 hybrid polymer ﬁlm 120 h

Fe−N4SAs/NPC Fe−N4232 1.5 6.0 M KOH + 0.2 M

ZnCl2

108 (36 h) 144

Fe SAs/N−CFe−N4225 636 1.5 6.0 M KOH 260 h 145

CoNG Co−N4300 785 1.40 6.0 M KOH 15 h

146

S,N−Fe/N/CCNT Fe−Nx102.7 1.35 KOH + Zn(Ac)2100 147

Fe-SAs/NPS-HC Fe−N4195 1.45 6.0 M KOH + 0.2 M

Zn(Ac)2

500 (200 000 s) 148

Fe−N/P-C Fe−N3P 133.2 723.6 1.42 6.0 M KOH 40 h 149

pfSAC-Fe Fe−N−C 123.43 732 1.41 8.0 M KOH + 0.5 M

ZnO 300 h 150

Fe SAs/N-G Fe−N4275 738 1.24 120 h 151

Co-POC Co−Nx-C 78 1.5 6.0 M KOH + 0.2 M

ZnCl2

237 (79 h) 152

NGM-Co Co−Nx-C 152 750 1.3 0.2 M ZnCl2+ 6.0 M

KOH 60 h 153

29 1.439 PVA 18

CoSA@NCF/CNF Co−N4530.17 1.41 PVA 90 154

A-Co@CMK-3-D Co−C 162 765 1.25 6.0 M KOH 45 h 155

Co−N3C1@GC Co−N3C1255 1.40 10 h 156

NC-Co SA Co−Nx20.9 mW cm−31.411 Solid-state electrolyte 125 (2500 min) 157

Co-SAs@NC Co−N 105.3 897.1 1.46 6.0 M KOH + 0.2 M

ZnCl2

158

1.40 KOH-PVA 1000 min

CoN4/NG Co−N4115 730 1.51 6.0 M KOH + 0.2 M

ZnO 100 h 159

28 KOH + Zn(COOH)2+

PVA 5h

Cu ISAS/NC Cu−N3280 736 1.43 6.0 M KOH 45 h

160

CuSA@HNCNxCu−N3212 806 1.51 6.0 M KOH + 0.2 M

Zn(Ac)2

1800 (300 h) 161

202 793 1.51 biocellulose membrane 1500 (250 h)

Mo SACs/N-C Mo1N1C278 750 1.47 6.0 M KOH + 0.1 M

Zn(Ac)2

360 (120 h) 162

FeCo-IA/NC Fe−N4and

Co−N4

115.6 635.3 1.472 6.0 M KOH + 0.2 M

ZnCl2

100 164

FeCo(a)-ACM Fe−N4159.92 775.91 1.39 6.0 M KOH + 0.2 M

ZnCl2

5400 (900 h) 166

Co−Co3O4@NAC 164 721 1.449 6.0 M KOH + 0.2 M

Zn(Ac)2

195 h 167

SA-PtCoF Pt−Co 125 808 1.28 6.0 M KOH + 0.2 M

Zn(Ac)2

240 h 168

Co SANC-850 Co−N4860.95 1.48 6.0 M KOH + 0.2 M

Zn(Ac)2

44 h 169

Stability test at constant discharge current.

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multisite synergy, support design, as well as increasing metal

loading are eﬀective.

SACs in Al-Air Batteries. At the gas−solid−liquid three-

phase interface, only the metal atoms exposed on catalyst

surface can serve as the adsorption/desorption sites for

reaction intermediates. As known, reducing catalyst dimen-

sions to achieve more exposed active sites and promote mass

transfer is a feasible strategy to enhance ORR activity as well as

battery performance. Li et al.

169

synthesized Co−N4active

centers on biomass-derived 3D ultrathin porous carbon (Co

SANC-850) with high speciﬁc surface area and rich pore

volume, which could accommodate more active sites and boost

mass transfer. In 0.1 M KOH, the catalysts attached the

relatively positive Eonset (0.995 V), E1/2 (0.863 V) and low

Tafel slope (68.3 mV dec−1), which suggested the great ORR

property and fast kinetic process. The DFT calculations

displayed the smaller energy barriers on Co−N4sites than

those on graphitic N and pyridinic N models. The Al-air

batteries possessed an OCV of 1.8 V and a peak power density

of 494 mW cm−2(Figure 11a). And the voltage did not decay

obviously after constant current density discharge for 10 h,

which demonstrated the great stability (Figure 11b). Similarly,

3D carbon aerogels containing abundant Fe−N4sites

(NCALR/Fe) were prepared for Al-air batteries.

170

SACs in Li-Air Batteries. During the discharge process of

Li-air batteries, the Li2OorLi

2O2continuously accumulated in

air electrode. The catalysts for promoting the formation/

decomposition of discharge products is pivotal.

171

Qian and

co-workers

172

used Pt SAs on ultrathin g-C3N4nanosheets (Pt-

CNHS) as cathode catalysts. Through Pt−CorPt−N bonds

(Figure 11c), the electrons transferred from Pt atoms to g-

C3N4nanosheets. The Pt-CNHS-based Li-air batteries showed

the initial discharge capacity of 17059.5 mAh g−1at the current

density of 100 mA g−1. Also, Xu and co-workers

173

utilized a

combined polymer encapsulation/template replication strategy

to stabilize CoN4moiety on N-doped carbon sphere. The

assembled Li-air batteries displayed a high discharge capacity

of 14777 mA h g−1at 100 mA g−1and higher current densities

under various discharge voltages than Pt/C-based counterparts

(Figure 11d). During the discharge process, CoN4served as

active seeds to form Li2O2nanoparticles and then promoted

the growth of Li2O2nanosheets. The LiO2species were easier

to detach from the N-HP-Co SAC surface and diﬀuse into the

electrolytes during the charge process (Figure 11e). Therefore,

Figure 11. SACs in Al-air, Li-air and Li-CO2batteries. (a) Discharge polarization and power density plots. (b) Long-time discharge curves of

VC.

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the N-HP-Co SAC promoted the decomposition of Li2O2

through a one-electron process to accelerate the ORR/OER

kinetics. The N-HP-Co SAC electrode endured 261 discharge/

charge cycles at 100 mA g−1and the monodispersed CoN4

conﬁgurations were maintained (Figure 11f). This work reveals

the promotion of SACs in the formation/decomposition of

Li2O2, which was instructive for the clariﬁcation of reaction

paths and the design of electrode materials.

SACs in Li-CO2Batteries. Similar to Li-air batteries, the

accumulation of solid discharge products also hinders the

progress of Li-CO2batteries. To accelerate the formation and

decomposition of Li2CO3, Dai and co-workers

174

prepared Fe

SAs (Fe−N4) on N, S-doped graphene (Fe-ISA/N, S-HG). In

Li-CO2coin cells, the discharge capacity of 23174 mAh g−1

and charge capacity of 21520 mAh g−1were achieved at the

current density of 100 mA g−1(Figure 11g). At 1.0 A g−1, the

batteries could withstand discharge/charge cycles over 100

times. The addition of N and S caused the charge and spin

redistribution, which accompanied with the formation of Fe−

N4conﬁguration to boost CO2reduction and evolution. DFT

calculations indicated that the Fe−N4moieties displayed a

large output potential, which was consistent with experimental

data. The robust cycle stability and excellent catalytic

performance of catalysts provided a guarantee for improving

the application potential of metal-CO2batteries. To conclude,

the decomposition of discharge products in Li-CO2batteries

should be given great attention. The SACs provide more

opportunities toward the accelerated battery reactions.

Table 3. Summary of SAC-Based Metal-Sulfur Batteries

catalysts conﬁguration initial capacity

(mAh g−1)at(x) rate capacity (mAh g−1) after (n)

cycles at (x) rate electrolyte maximum loading

(mg cm−2) refs

Fe-PNC Fe(II)N41138.6 (0.1) 427.1 (300) (0.1) 1 M LiTFSI in DOL/DME (1:1, v:v)

+ 1 wt % LiNO3

S 1.3 185

557.4 (300) (0.5)

NC:SAFe Fe−N 1052 (1) 989 (0.5); 589 (12) 1 M LiTFSI in DME/DOL + 1 wt %

LiNO3

Li2S 2.0∼2.3 186

790 (200) (1)

490 (1000) (2)

(3350 mA g−1)

315 (1000) (5)

(8375 mA g−1)

Co−N−CCo−N−C 1247 (0.2) 1035 (2) 1 M LiTFSI in DOL/DME (1:1, v:v)

+ 2 wt % LiNO3

S 11.3 187

1161 (0.5) 850 (300) (0.5)

Co@C3N4∼1400 @ 1.6 mA cm−2∼800 @ 12.8 mA cm−21 M LiTFSI and 0.5 M LiNO3in

DOL/DME (1:1, v:v) S 4.0 188

∼980 (200) (1)

(3.2 mA cm−2)

Fe1/NG Fe−N complex ∼1200 (0.2) 891.6 (750) (0.5) 1 M LiTFSI in DOL/DME (1:1, v:v)

+ 0.1 M LiNO3

S 4.5 189

∼673 (5)

Ni@NG Ni−N41598 (0.1) 612 (10); 826.2 (500) (1) 1 M LiTFSI in DOL/DME (1:1, v:v)

+ 1 wt % LiNO3

S 6.0 190

428.4 (500) (10)

pouch cell

965.8 (200) (1)

Co-N/G Co−N4−C 1210 (0.2) 618 (4) 1 M LiTFSI in DOL/DME (1:1, v:v)

+ 2 wt % LiNO3

S 6.0 92

5.1 mAh cm−2

(100) (0.2)

681 (500) (1)

2D MOF-

Co Co−O41138 (0.1) 703 (200) (0.5) 1 M LiTFSI in DOL/DME (1:1, v:v)

+ 1 wt % LiNO3

S 7.8 191

∼450 (600) (1)

NC@SA-

Co Co−Nx1160 (0.1) 582 (5) 1 M LiTFSI in DOL/DME (1:1, v:v)

+ 1 wt % LiNO3

S56wt% 192

∼450 (700) (2) 7.2

CoPcCl Co−N41256.3 (0.1) 785 (2) 1 M LiTFSI in DOL/DME (1:2, v:v)

+ 0.1 M LiNO3

S 1.16 195

1124.1 (0.3) 830.9 (200) (0.1)

SAV@NG V−N41230 (0.2) 770 (100) (0.2) 1 M LiCF3SO3in DOL/DME (1:1,

v:v) + 0.1 M LiNO3

S5 196

780 (0.5) ∼551 (400) (0.5)

Co-HC single atomic Co and

Co clusters 1081 @ 100 mA g−1508 @ 100 mA g−1(600) 1 M NaClO4in PC:EC (1:1, v:v) + 5

wt % FEC S47wt% 193

220.3 @ 5 A g−15.0

Ni−O5Ni−O51040.9 @ 50 mA g−1608 @ 100 mA g−10.8 M KPF6in EC: PC (1:1, v:v) S 35 wt % 194

330.6 @ 1 A g−1(500) 1.2

1 C = 1675 mA h g−1based on S weight.

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SACs in Li-S Batteries. As we discussed in the previous

section, the major problem in Li-S chemistry should be the

unmanageable dissolution of polysulﬁde intermediates and

their further back-and-forth diﬀusion (so-called shuttle eﬀect).

Hence, encapsulating sulfur in a rational cathode host and

speeding up the redox kinetics of the polysulﬁde intermediates

appear to be vital. In 2014, inherently polar Ti4O7with the

Magneli phase was reported by the Nazar group, and this

metallic oxide can eﬀectively modify interface-mediated redox

chemistry to trigger the conversion of polysulﬁdes.

175

Inspired

by this work, various catalysts like metal nanoparticles (Pt, Co,

Pd),

176−179

metal nitrides,

180,181

metal oxides,

182

and metal

sulﬁdes

183,184

have been employed in Li-S batteries to boost

the conversion kinetics among both soluble polysulﬁdes and

solid sulﬁdes, which indeed prolonged cycling lifespan and

improved sulfur utilization. Recently, SACs show advantages as

the incomparable atom utilization eﬃciency (up to 100%). In

addition, since SACs are monodispersed in solid substrates,

and take up less volume and weight in a sulfur cathode,

employment of them is favorable to high-energy lithium-sulfur

batteries. Therefore, the selection of suitable SACs may

catalyze the liquid-to-solid and solid-to-solid reactions by

reducing the energy barrier, restrain the polysulﬁde shuttle

eﬀects, and guide the lithium deposition in Li-S batteries.

Recently, many reported works have successfully synthesized

various SACs to apply in Li-S batteries, demonstrating

excellent performances for polysulﬁdes conversion (see Table

3). For example, via a combination of a simple nanocasting

method and hard template method, a single Fe atom catalyst

homogeneously dispersed in porous nitrogen-doped carbon

Figure 12. SACs in sulfur cathodes for Li-S batteries. (a) UV−vis absorption spectra of the polysulﬁde solution. (c) Tafel polarization curves.

(e) Thermogravimetric curves. CV diagrams of the symmetric cells based on (b) Fe-PNC, (d) Co−N−C, and (f) Co−N/G electrodes and

corresponding reference samples. (g) Charge curves with diﬀerent overpotentials. (h) Calculations of free energy for sulfur species in

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matrix (Fe-PNC) was prepared for Li-S batteries.

185

The

produced Fe-PNC showed stronger interaction to trap Li2S6

according to the results based on UV−vis spectra (Figure 12a).

Furthermore, cyclic voltammetry (CV) measurements in

Figure 12b based on symmetric cells with Li2S6electrolyte

clearly revealed the catalytic eﬀect of diversiﬁed catalysts on

the redox conversion of polysulﬁdes. As a result, Fe-PNC/S

composites during electrochemical cycles presented higher

speciﬁc capacity (1138.6 mAh g−1at 0.1 C), smaller

polarization, and higher rate performance than those of

benchmark cells. Post-mortem analysis of the discharge

product showed well-retained Li2S nanospheres in the Fe-

PNC/S cathode. All these results indicated the kinetic

acceleration in the polysulﬁde conversion process by Fe-

SACs, which improved the reversible performance and

weakened the shuttle eﬀect. In addition, Fe-SACs have

successfully reduced the decomposition energy barrier of

Li2S and improved the reaction kinetics in the Li2S delithiation

reaction. As a result, the produced Li2S@NC:SAFe nano-

composites demonstrated a discharge capacity of 589 mAh g−1

at an ultrahigh rate of 12 C as well as a long-term cycling

lifespan at 5 C over 1000 cycles.

186

In addition, Li et al.

187

developed a Co−N−C nano-

composite in which adequate atomic-scale Co active sites

coupled with porphyrin as framework conductive substrates to

facilitate stepwise redox reaction of sulfur species. Especially,

the current density of cells using Co−N−C electrocatalyst

consistently outperformed the control one, which means the

high eﬃciency of the Co−N−C electrocatalyst in chain-

breaking reaction of polysulﬁdes and powerful activation of

initial Li2S. The deeply improved reaction kinetics can also be

obtained from Tafel plots in Figure 12c. According to CV

measurements (Figure 12d), Co−N−C catalyzed the con-

version reactions among polysulﬁdes showing sharper redox

peaks with higher currents. The long-term cycle with 0.1%

cyclic decay was achieved at 0.5 C, from an initial 1161 mAh

g−1to a maintained capacity of 850 mAh g−1. Besides Co−N−

C, single Co atoms dispersed in hierarchical carbon nitride

(C3N4) have been reported for lithium-polysulﬁde (Li-PS)

batteries.

188

Du and co-workers

proposed another cobalt-

based catalyst (Co−N/G) with atomic Co singly dispersed in a

Figure 13. SACs in separators for Li-S batteries. (a) Schematic illustration of the functional separator. (b) Voltammetry curves of Li-S cells at

spectra of Li2S6solution and corresponding digital photographs (inset). (d) Voltammetry curves of symmetric cells. (e) Proposed

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nitrogen-doped graphene matrix. This single-atom catalyst

eﬃciently promoted the phase transformation among poly-

sulﬁdes to increase S utilization for Li−S batteries. The mass

ratio of sulfur in the S@Co−N/G composite can achieve a

high value of about 90 wt % in a thermogravimetric experiment

(Figure 12e). The symmetric cell with the Co−N/G electrode

demonstrated a maximum current density and relatively lower

voltage hysteresis during the multistep redox process, as shown

by CV curves (Figure 12f). The best electrochemical kinetics

was apparently ascribed to the Co−N/G that eﬀectively

catalyzed polysulﬁde conversion. The concentration variation

curves of intermediate polysulﬁdes and ﬁnal Li2S species

during cycling also suggested the improved kinetics in a

complete phase evolution from polysulﬁdes to Li2S. And the

lowest overpotentials in the charge curves for S@Co−N/G

further proved the decreased energy barriers for Li2S

decomposition (Figure 12g). The corresponding theoretical

studies proved the lower Gibbs free energy for Co−N/G in the

whole discharge process, especially in the rate-limiting step of

Li2S2/Li2Sconversion(Figure 12h). In the subsequent

electrochemical studies, the S@Co-N/G cathode could deliver

a capacity of 1210 mAh g−1at 0.2 C with sulfur loading of 2.0

mg cm−2and improved rate capabilities. Surprisingly, with an

ultrahigh mass ratio (90 wt %) and practical S loading of 6.0

mg cm−2, the S@Co−N/G cathode still showed superior

performance with a 0.029% decay rate after 100 cycles. This

Figure 14. SACs in separators/interlayers for Li-S batteries. (a) Schematic illustration of the assembled Li-S pouch cell with a B/2D MOF-

Wiley-VCH. (c) Working mechanism of multifunctional interlayer in Li-S batteries. (d) Catalytic action of NC@SA-Co for polysulﬁdes

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work has signiﬁcant implications for devising advanced sulfur-

based cathode with single atom catalysts, as well as provides an

approach to realize high-performance Li-S batteries under

more practical conditions for further industrialization.

Author: Please verify that the changes made to improve the

English still retain your original meaning.Recently, SACs have

been employed to construct functional separators or interlayers

for Li-S batteries. Zhang et al.

189

proposed the multifunctional

separator with the coating of N-doped graphene foam (NG) in

which single-atom catalysts such as Fe, Co, or Ni atom were

fully impregnated (Figure 13a). The sample with Fe1/NG

showed the strongest bonding to polysulﬁdes. As a multifunc-

tional separator, the additional catalytic action of SACs was

demonstrated in further electrochemical measurements. Cyclic

voltammetry exhibited the improvements of polysulﬁdes redox

chemistry for Fe1/NG-modiﬁed separators, such as the obvious

reduction of the voltage gap and higher current response

(Figure 13b). After long-term cycling over 750 cycles, the Li-S

batteries still delivered superior discharge capacity with 891.6

mAh g−1at 0.5 C. Moreover, the modiﬁed separator resulted in

good rate capabilities from 0.2 to 5 C while it delivered a

discharge capacity of 673 mAh g−1at 5 C. Zhang et al.

190

fabricated a modiﬁed separator coated by Ni@NG composite

that are single Ni atoms distributed in nitrogen doped

graphene (NG). The restraint to polysulﬁde shuttling by

Ni@NG was intuitively presented in Li2S6adsorption experi-

ments and UV-vis spectra, resulting in the lightest color in

Figure 13c. Two pairs of obvious redox peaks and high

response current in CV curves also proved the improved

kinetics in polysulﬁdes conversion process (Figure 13d). The

calculated exchange current density derived from Tafel plots

also veriﬁed the enhanced catalytic activity in Li-S batteries.

Figure 15. SACs for Na-S and K-S batteries. (a) Schematic illustration of S@Con-HC. Schematic illustration of the reaction mechanism in (b)

S@Con-HC and (c) S@HC cathodes. (d) Long cycle performance RT-Na/S batteries. Reprinted with permission under a Creative Commons

reduction. (f) Cycling performance for potassium storage at 1000 mA g−1. (g) Schematic illustration of synthesis process of the NiSA/S/C

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Good rate performance conﬁrmed the kinetic conversion of

polysulﬁdes was visibly improved as well, delivering a satisfying

capacity of 612 mAh g−1at 10 C. Further DFT calculation was

adopted to clarify the working mechanism of Ni@NG,

revealing the great reduction of the Li2S decomposition energy

barrier from 1.98 to 1.23 eV. Therefore, the kinetics during the

delithiation reaction of Li2S was more favorable with the

presence of single Ni atoms on NG. The catalytic mechanism

was clearly identiﬁed with XPS spectra, Ni K edge XANES

spectra, and Raman spectra, as shown in Figure 13e. The peak

in XPS assigned to the bonding between Sx2−and Ni−N could

be used as an explanation that this strong interaction can

eﬃciently capture polysulﬁde from their dissolution as well as

catalyze the conversion process. Moreover, the low decom-

position energy barrier of Li2S and reduced free energy of

conversion reaction among LiPS would facilitate the reversible

electrochemical process even more to render excellent

performance. This work illustrates the signiﬁcant sights and

feasible applications of single Ni atoms incorporation with the

separator in Li-S batteries.

Moreover, the research ﬁndings by Li and co-workers

191

reported that a single Co atom array mimic could ensure safe

and long life Li-S batteries. They prepared a bifunctional

separator (B/2D MOF-Co), combining with bacterial cellulose

(BC) and two-dimensional ultrathin metal organic framework

nanosheets with loading of Co atoms. Introducing this

separator into the Li-S system would not only induce uniform

Li plating with strong interaction between Li and surface O

atoms to eliminate the dendrite growth, but also eﬀectively

restrain the shuttle eﬀect via Lewis acid−base interaction to

trap soluble polysulﬁdes. Beyond coin cells, the practicability of

this separator was further investigated with a ﬂexible pouch cell

(Figure 14a) at diﬀerent bending angles. Furthermore, the

Li2S6captured by 2D MOF-Co was determined by UV−vis

spectra, showing its stronger adsorption over other reported

materials (Figure 14b). Thus, Li-S cells based on the

bifunctional separator with single Co atoms showed safe and

stable cycling performance. Interlayers, as an additional

component in Li-S cells shown in Figure 14c, can play a

very valid role in governing the transport behavior of

polysulﬁdes. Hence, ﬁnding an ideal interlayer would

eﬀectively push forward the practical applications of Li-S

batteries. For example, a multifunctional interlayer (denoted as

C−C−N−Co) containing single-atom cobalt was successfully

developed and applied in Li-S batteries by Guo and co-

workers.

192

An abundant Co−N4structure was revealed by

various microstructures and spectroscopic characterizations

and served as an active site to enhance the redox conversion of

polysulﬁdes during electrochemical cycling (Figure 14d). With

the CV test of symmetric cells, the C−C−N−Co electrodes

showed the highest peak current and smallest overpotential,

demonstrating the ability to accelerate the conversion reaction

(Figure 14e). Therefore, the Li-S battery with this interlayer

can exhibit improved rate performance and long cycle life

(more than 700 cycles at 2 C).

Overall, those works have demonstrated the powerful roles

of various SACs in tackling series of challenges in Li-S

batteries, including the eﬃcient restrictions on dissolved

polysulﬁdes and obvious enhancement for their corresponding

redox conversion within the cathode matrix or modiﬁed

separators. Thus, with the minimization of the shuttle eﬀect

and the maximization of the polysulﬁdes redox kinetics, the

overall performances of high-energy Li-S batteries have

improved signiﬁcantly. However, considering that the work-

place for SACs functioning in cathodes and functional

separators mainly locates at the host/matrix surface, decent

host/matrix structures with appropriate loading of active SACs

should be further designed and developed. Additionally, more

atomic-level mechanistic insights via advanced experimental

characterization techniques and theoretical calculation are

quite advisable to guide the eﬃcient design of SACs.

SACs in Na-S Batteries. As for Na-S@RT batteries, the

relevant exploration and application of SACs in this ﬁeld are

still comparatively rare (see Table 3). Correlative research was

carried out by Zhang et al.,

193

where they used hollow carbon

(HC) nanospheres as the sulfur host with atomic Co

embedded in a carbon shell. This conﬁguration of a composite

cathode noted as S@Con-HC (Figure 15a) maximized the

electrocatalytic activity of atomic Co, eﬀectively improving the

overall performance of Na-S@RT batteries. According to their

studies, the dispersion of Co species including both Co clusters

and single atomic Co in C shells were revealed by HADDF-

STEM and XPS results. The sulfur content in S@Con-HC was

identiﬁed at 47 wt %. Then, the catalytic eﬀect of atomic Co

was further evidenced with cyclic voltammograms for S@Con-

HC, showing two obvious cathodic peaks at around 1.68 and

1.04 V during the initial scan. This clear and concise reaction

process demonstrated the fast kinetics of sodiation without

unnecessary peaks of various polysulﬁdes. The deep reaction

mechanism was further revealed by in situ XRD and Raman

spectroscopy. The ﬁnal reaction product was conﬁrmed as

Na2S, accompanied by the fast transition of Na2S4to Na2S.

DFT calculations were done to further support the proposed

reaction mechanism without and with Co6cluster on carbon

substrate. The stronger adsorption energy of sodium

polysulﬁdes with Co6cluster conﬁrmed the fast conversion

kinetics. Therefore, as shown in Figure 15b and c, the atomic

Co in C shells could restrain the dissolution of polysulﬁdes via

polar−polar attraction. Moreover, multifunctional atomic Co

could catalyze the remaining polysulﬁdes in C shells to rapidly

switch into Na2S. Thus, the shuttle eﬀect was validly relieved

and redox reactions could be more thorough. As a result, the

enhanced cycling stability could be obtained from the S@Con-

HC cathode as shown in Figure 15d, demonstrating a

discharge capacity of 508 mA h g−1at 100 mA g−1after 600

cycles.

SACs in K-S Batteries. Beyond those electrodes for Li/Na

storage, a recent research by Lin and co-workers

194

suggested

that single-atom design could also boost the performance of

potassium storage (K-storage) via enhancing conversion

kinetics in the K−S chemistry (Figure 15g and Table 3).

They developed the Ni/S/C ternary composite nanotubes

comprising the small-molecule sulfurs as active materials for

potassium ion batteries (PIBs). The physical structure of the

active Ni site was further identiﬁed by XANES and EXAFS,

showing the clearly isolated Ni−O5structure. Moreover, DFT

calculations corroborated the advantage of the atomically

dispersed Ni in the isolated Ni−O5structure. The Gibbs free

energy of the rate-limiting reaction, that is, the reversible

conversion between K2S2to K2S, was obviously decreased

(Figure 15e). The results indicate the reversibly electro-

chemical kinetics of the potassium sulfate (K2Sx,x≤3) during

charge and discharge was sharply raised. Various electro-

chemical measurements including GITT voltage proﬁles, Tafel

plots, and CV curves also exhibited the lower overpotential of

the NiSA/S/C nanotube during the potassiation and depot-

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assiation process. In consequence, the NiSA/S/C nanotubes

can deliver an outstanding capacity for high K-storage of 608

mAh g−1(calculation based on the mass of the composite)

under a current of 100 mA g−1(Figure 15f).

With the method of surface sulfurization involving single

metal atoms, both the conductivity and utilization of active

sulfur can be improved. Moreover, exploring more various

metal centers in the optimized coordination environment

might exhibit higher catalytic activity and tighter aﬃnity for

polysulﬁdes. Hence, more stable cycling and higher S

utilization in Li/Na/K-S batteries can reasonably expect to

be realized by suitable SACs. At the same time, attention will

also need to be given to the key challenges on the anode side,

because the uneven SEI layer and the uncontrollable metal

Figure 16. SACs for Li and Na metal anodes. (a) Schematic illustration of the induced dendrite-free morphology. (b) Comparison of Li

behavior and corresponding energy distribution mappings of Li adsorption on (c) pristine graphene (PG) and (d) SA metal@nitrogen-

American Chemical Society.

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dendrite growth would cause premature failure of metal-sulfur

batteries.

SACs in Li/Na Metal Anodes. Li metal batteries have

been viewed as promising candidates for next-generation high-

energy rechargeable batteries.

197

Therefore, high-performance

Li metal anodes are rather important. However, the Li dendrite

issue is always a big hindrance. Some recent works shed

substantial light on the controllable lithium nucleation via

homogeneously distributed single atoms. The representative

work involving single atoms to guide lithium deposits was

demonstrated by Zhang and colleagues.

198

They explored

carbon substrate with atomically Co and N atoms doping

(CoNC) to serve as lithium host. The uniform distribution of

Co was achieved with a Co−N coordination link, while the

local electronic structure was simultaneously tailored to

promote the adsorption and even nucleation of lithium (Figure

16a). Moreover, this hierarchical framework with suﬃcient

interior spaces could facilitate rapid diﬀusion of Li ions and

endure the inﬁnite volume changes during the repeated

electroplating/striping cycles (Figure 16b). Accordingly, the

lithium deposition showed smooth morphology, and the

CoNC-Li composite anodes aﬀord superb performances such

as a high CE of 98.2% even at 10.0 mA cm−2. Using a similar

host, nitrogen-doped graphene (NG), Zhai et al.

199

prepared

the composite anodes with the doping of diﬀerent single-atom

metals (Ni, Pt, Cu). The introduction of metal atoms showed

an eﬀective increase in the Li adsorption energy by DFT

calculations, which contributed to the experimentally uniform

Li deposition. In addition, the formation of a metal−N−C

coordination structure would be beneﬁcial for maintaining the

atomic structural stability (Figure 16c, d). Thus, high-

performance Li anodes without dendrites were achieved,

exhibiting a high CE of 98.45% over 250 cycles and a stable Li

plating/stripping performance at 4.0 mA cm−2. Also, single

iron atoms dispersed in a N-doped carbon matrix could act as

lithiophilic sites and minimize the Li nucleation overpotential

(as low as 0.8 mV) (Figure 16e).

200

Further molecular

dynamics (MD) simulations proved the enhanced aﬃnity

between the atomic Fe−N−C sites and Li ions. This strong

interaction could ensure the stable and uniform Li plating/

stripping behaviors and, more importantly, the obvious

suppression of dendrite growth. When FeSA−N−C/Li matched

with LiCoO2, the full cells showed long-life cycling over 200

cycles. When choosing MXene layers as the substrate, the

presence of single zinc atoms showed the ability of inducing

dendrite-free deposition.

201

DFT simulations revealed the low

energy barrier during Li migration and high surface binding

energy on the Zn single-atom (ZnSA) surface. In addition,

microscopic results validated the homogeneous Li nucleation

near the Zn atoms via forming a bowl-like morphology. As a

result, Li-ZnSAs anodes demonstrated a low overpotential for

Li deposition, long lifespan, and stable cycling performance

under various rates.

Moreover, to address the problems of Na metal anodes, Yan

and co-workers

202

proposed the combination of carbon

substrates and single Zn atoms (namely, ZnSA−N−C) to

achieve the even Na deposition. The homogeneous distribu-

tion of Zn atoms in substrates induced adequate sodium-philic

sites, where the Na electroplating presented good selectivity to

nucleate, refraining from the dendrite formation (Figure 16f,

g). In addition, the strong chemical aﬃnity between ZnSA−N−

C and Na ions proved by DFT calculations and near-zero

nucleation overpotential further elucidated the reduced

nucleation barriers and controlled the nucleation behavior.

Figure 17. Performance of reported SAC-based (a) Zn-air batteries and (b) PEMFCs.

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Consequently, superior Na electroplating/stripping perform-

ances were exhibited such as high CE of 99.8% upon 350

cycles and excellent cycle life over 1000 h in symmetric cells. In

addition, a full-cell conﬁguration with Na3V2(PO4)3and Na−

ZnSA−N−C electrode delivered good cycling stability over

1000 cycles with a CE of 100%. Hence, the SAC-modiﬁed

hosts can promote the uniform Li/Na deposition and show the

validity of the protection of metal anodes. Nevertheless, more

related works on safeguarding metal anodes remain to be

further developed, which is not only conﬁned to the even Li/

Na deposition but also the complete suppression of the

polysulﬁdes corrosion onto the active metal.

CONCLUSIONS AND PERSPECTIVES

In summary, we have concluded diﬀerent cases of SACs

applied in various energy storage (or conversion) devices,

including PEMFCs, metal-gas batteries, metal−sulfur batteries

and metal-ion batteries. When it comes to ORR, M−N−C

sites are currently the promising ORR catalysts to replace the

precious metal Pt. Plenty of experimental data show that the

homogeneously distributed metal SACs display exceeded ORR

performances compared to cluster or nanoparticle counter-

parts. The coordination unsaturated environment provides

suitable adsorption sites of reactant molecules, while the

metal−support interactions regulate the electronic structure of

central metal SAs and enhance their intrinsic activity.

At present, Fe−N−CandCo−N−Carethemost

commonly studied ORR active sites, consistent with the

activity trend predicted by Cheng’s work.

203

Figure 17 shows

the performance level of current SAC-based PEMFCs and Zn-

air batteries. On the whole, the signiﬁcant diﬀerences in power

density of PEMFCs under H2/O2or H2/air conditions are

observed. And the Fe SAC-based Zn-air batteries demonstrate

relatively higher power density than the others. For metal−

sulfur batteries, the introduction of SACs eﬃciently promotes

the overall performances because of the activated adsorption

and high activity in sulfur conversion. The meaningful works in

the future should focus on how to choose the adaptive SACs

with scientiﬁc theoretical guidance. Despite the impressive

achievements that have been made, the pace of progress is not

enough. The future perspectives are put forward as follows:

Diversiﬁcation of Active Sites. The design of SACs is not

limited to the known Fe−N−C and Co−N−C sites. To

prepare SAs on carbon-based supports, the choice of metal

sources and organic ligands will aﬀect the microscopic

coordination environment. The related explorations could

direct the modiﬁcation on active sites, which are signiﬁcant but

scarce. Also, construction of the multiple metal centers, even

the nonmetal centers, as well as the various coordination

elements from diﬀerent supports (oxides, sulﬁdes, selenides,

etc.) or heteroatom doping are important approaches. The

coexistence of multimetal sites in SACs provides more

opportunities for the bifunctionality of catalysts, such as

ORR/OER for rechargeable Zn-air batteries.

High-Throughput Synthesis and Performance Pre-

diction. The routine experimental process in the laboratory

wastes a lot of chemical resources and consumes valuable time

and eﬀort. Rapid property screening through high-throughput

design and automated operation will undoubtedly enhance

production eﬃciency and reduce meaningless eﬀorts. Fur-

thermore, theoretical simulation to predict catalytic perform-

ance serves as a powerful tool which improves experiment

eﬃciency. Such high-throughput synthesis and theoretical

simulation are also critical ways to achieve the large-scale

applications of SACs in energy-related devices.

The Adaptability of SACs in Electrolytes. Changing the

application environment of SACs from alkaline solution to

other medium, including the acidic environment in PEMFCs,

organic electrolyte systems in Li-air batteries, etc., poses

challenges for the applicability and durability of those catalysts.

As supports of M−N−C species, N-doped carbon materials are

susceptible to erosion by acidic or organic environments.

Considering the requirements for battery durability, this

problem must be optimized.

In-Depth Mechanistic Investigation. Due to the micro-

level constraints, to directly and accurately realize an in-depth

mechanism is diﬃcult. Some recent studies indicated that the

reconstruction of SACs under potentials could happen. Hence,

various in situ techniques will provide signiﬁcant eﬀects to

ﬁgure out the true active species and uncover the reaction

process. Theoretical calculations could construct catalytic

models and assess reaction barriers. The alliance between in

situ techniques and theoretical calculations will present the

profound catalytic mechanisms and structure−activity relation-

ships, which is meaningful to guide the material designs.

Design for Metal-Sulfur Batteries. In order to

suﬃciently catalyze the polysulﬁdes conversion, abundant

SACs are always required on the surface of the cathode host.

Hence, with ever-increasing sulfur loading in the cathode to

satisfy the practical application, higher loading of durable SACs

in the cathode host is necessarily imminent. Though the sulfur

electrode grows thicker, the mass transportation and phase

transition should be steady. The cell separator is also a wide

application space of SACs. With the bidimensional architecture

for functional separators and their specialized capability to

convert the dissociative polysulﬁdes, the lower amount in using

SACs could be possible and their eﬀects could be achieved to

the utmost degree. Thus, from the future commercialization

perspective, high areal sulfur loading with a suitable amount of

SACs embedded in lightweight substrates is an optimal

direction. While for designing the functional separators toward

practical metal-S batteries, it is better to serve as a highly

eﬃcient barrier and catalytic converter for polysulﬁdes with

lower amounts of SACs. Just as important, the works of SACs

in enhancing metal anodes should be studied, which would be

a promising strategy for future metal-sulfur batteries.

Toward Practical Application. The actual operating

environment brings additional demands toward SAC-based

electrode materials. Reasonable modiﬁcation of the electrode

interface and regulation of its hydrophilic/hydrophobic

properties are helpful to optimize the adsorption/desorption

at the reaction interface, therefore improving the output

capacity. Since the agglomeration of metal atoms restricts the

activity of SACs, some synthesis strategies are further needed

to keep exploring to maintain the good dispersion of SAs. As

the synthesis technology improves, large-scale production of

SACs with suﬃcient active sites would strongly spur on the

practical applications of the battery ﬁelds.

In conclusion, opportunities and challenges of SACs exist

side by side. Maintaining the activity and stability of metal SAs

while increasing the loading as much as possible is a conﬂict

that must be solved before practical application. Also, the high

selectivity of SACs displays disadvantages in bifunctional/

multifunctional catalysis, remaining to be resolved. In addition

to the energy systems discussed above, SACs could also be

widely applied in more electrochemical systems.

ACS Nano www.acsnano.org Review

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ACS Nano 2021, 15, 210−239

232

AUTHOR INFORMATION

Corresponding Authors

Feixiang Wu −School of Metallurgy and Environment,

Engineering Research Center of the Ministry of Education for

Advanced Battery Materials, Central South University,

Changsha 410083, P. R. China; orcid.org/0000-0002-

9688-2428; Email: feixiang.wu@csu.edu.cn

Yongpeng Lei −State Key Laboratory of Powder Metallurgy,

Hunan Provincial Key Laboratory of Chemical Power

Sources, College of Chemistry and Chemical Engineering,

Central South University, Changsha 410083, P. R. China;

Email: leiyongpeng@csu.edu.cn,lypkd@163.com

Authors

Yuchao Wang −State Key Laboratory of Powder Metallurgy,

Hunan Provincial Key Laboratory of Chemical Power

Sources, College of Chemistry and Chemical Engineering,

Central South University, Changsha 410083, P. R. China

Fulu Chu −School of Metallurgy and Environment,

Engineering Research Center of the Ministry of Education for

Advanced Battery Materials, Central South University,

Changsha 410083, P. R. China

Jian Zeng −State Key Laboratory of Powder Metallurgy,

Hunan Provincial Key Laboratory of Chemical Power

Sources, College of Chemistry and Chemical Engineering,

Central South University, Changsha 410083, P. R. China

Qijun Wang −State Key Laboratory of Powder Metallurgy,

Hunan Provincial Key Laboratory of Chemical Power

Sources, College of Chemistry and Chemical Engineering,

Central South University, Changsha 410083, P. R. China

Tuoya Naren −State Key Laboratory of Powder Metallurgy,

Hunan Provincial Key Laboratory of Chemical Power

Sources, College of Chemistry and Chemical Engineering,

Central South University, Changsha 410083, P. R. China

Yueyang Li −State Key Laboratory of Powder Metallurgy,

Hunan Provincial Key Laboratory of Chemical Power

Sources, College of Chemistry and Chemical Engineering,

Central South University, Changsha 410083, P. R. China

Yi Cheng −School of Metallurgy and Environment,

Engineering Research Center of the Ministry of Education for

Advanced Battery Materials, Central South University,

Changsha 410083, P. R. China

Complete contact information is available at:

https://pubs.acs.org/10.1021/acsnano.0c08652

Author Contributions

Y.W. and F.C. contributed equally.

Notes

The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS

This work was ﬁnancially supported by the Innovation-Driven

Project of Central South University (No. 2019CX033), the

National Natural Science Foundation of China (No:

51904344), and the Hunan Provincial Science and Technology

Plan Project (No. 2017TP1001, 2020JJ4710)

VOCABULARY

Voltage hysteresis, voltage diﬀerence between the charge and

the discharge ﬂats; multiple-electron transfer reaction, reaction

with more than one electron transfer per redox center; SEI,

ion-conductive solid electrolyte interphase (a layer) formed on

electrode surfaces from decomposition products of electro-

lytes; ALD, atomic layer deposition, a method for plating

materials layer by layer on substrate surface in the form of

single atomic ﬁlm; GITT, galvanostatic intermittent titration

technique (GITT), widely applied in battery research to study

the diﬀusion of lithium in electrode materials

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Confined cobalt single‐atom catalysts with strong electronic metal‐support interactions based on a biomimetic self‐assembly strategy

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Full-text available

Apr 2024

Designing high‐performance and low‐cost electrocatalysts for oxygen evolution reaction (OER) is critical for the conversion and storage of sustainable energy technologies. Inspired by the biomineralization process, we utilized the phosphorylation sites of collagen molecules to combine with cobalt‐based mononuclear precursors at the molecular level and built a three‐dimensional (3D) porous hierarchical material through a bottom‐up biomimetic self‐assembly strategy to obtain single‐atom catalysts confined on carbonized biomimetic self‐assembled carriers (Co SACs/cBSC) after subsequent high‐temperature annealing. In this strategy, the biomolecule improved the anchoring efficiency of the metal precursor through precise functional groups; meanwhile, the binding‐then‐assembling strategy also effectively suppressed the nonspecific adsorption of metal ions, ultimately preventing atomic agglomeration and achieving strong electronic metal‐support interactions (EMSIs). Experimental characterizations confirm that binding forms between cobalt metal and carbonized self‐assembled substrate (Co–O 4 –P). Theoretical calculations disclose that the local environment changes significantly tailored the Co d‐band center, and optimized the binding energy of oxygenated intermediates and the energy barrier of oxygen release. As a result, the obtained Co SACs/cBSC catalyst can achieve remarkable OER activity and 24 h durability in 1 M KOH ( η 10 at 288 mV; Tafel slope of 44 mV dec ⁻¹ ), better than other transition metal‐based catalysts and commercial IrO 2 . Overall, we presented a self‐assembly strategy to prepare transition metal SACs with strong EMSIs, providing a new avenue for the preparation of efficient catalysts with fine atomic structures.

In situ synthesis of manganese oxide/iron oxide/polyaniline composite catalyst for oxygen evolution reaction

Article

Jan 2024

Non-noble-metal-based electrocatalysts possessing high inherent activity toward the oxygen evolution reaction (OER) are highly desired. Considering the synergetic effect of different metal sites and the high conductivity of polyaniline (PANI), here we report polymer-based bimetallic ternary nanocomposite (Fe–Mn@PANI), which is synthesized by in situ polymerization method. Fe–Mn@PANI/NF has shown the best OER activity with lower onset potential (1.46 V vs RHE), low overpotential of 353 mV at 10 mA cm−2, and Tafel slope of 68 mV dec−1. The remarkable outcomes can be attributed to the synergistic effect of the two distinct metals and PANI, which particularly has a unique π-conjugated structure and abundant nitrogen species with lone pairs of electrons providing a consistent flow of ions across entire surface of composite and also improving OH− adsorption capacity which makes it a highly efficient, cost-effective, and stable catalyst with improved performance of the OER.

A simple strategy for synthesis of side-chain sulfonated poly(arylene ether ketone sulfone) constructing hydrophilic/ hydrophobic phase separation structure

Preprint

Full-text available

Oct 2023

A series of poly(aryl ether ketone sulfone) polymers (C-PAEKS-DBS) were synthesized from 4-carboxyphenyl hydroquinone, allyl bisphenol S, and 4,4-difluorobenzophenone by nucleophilic polycondensation. And further a series of side chain sulfonated poly(aryl ether ketone sulfone) proton exchange membranes (CPDA-x) were prepared by simply introducing hydrophilic long alkyl side chains (AMPS) into the polymers by means of double bond cross-linking. The structure of the prepared CPDA-x were characterized by FT-IR, ¹ H NMR and SEM. This series of composite membranes showed excellent dimensional stability which including moderate water absorption (ranging from 27.91–33.33% at 80 o C) and low swelling ratio (ranging from 10.26–16.67% at 80 o C). In addition, the prepared films also exhibited high oxidative stability and mechanical property. This simple strategy for the producing of proton exchange membranes (PEMs) has certain potential and may be applied to the producing of fuel cell membranes with hydrophilic and hydrophobic phase separation structures.

O, N Coordination-Mediated Nickel Single-Atom Catalysts for High-Efficiency Generation of H2O2

Article

Jul 2023
ACS APPL MATER INTER

Electrochemical selective two-electron oxygen reduction shows great potential for on-site electrochemical production of hydrogen peroxide (H2O2). Herein, we demonstrated the synthesis of Ni single-atom sites coordinated by three oxygen atoms and one nitrogen atom (i.e., Ni-N1O3) supported by oxidized carbon black (OCB) by pyrolyzing nickel-(pyridine-2,5-dicarboxylate) coordination complexes. Aberration-corrected scanning transmission electron microscopy (AC-STEM) combined with X-ray absorption spectroscopy (XAS) proves the presence of atomically dispersed Ni atoms attached on OCB (labeled as Ni-SACs@OCB), in which Ni single atoms are stabilized by a N, O-mediated coordination configuration. This Ni-SACs@OCB catalyst shows high H2O2 selectivity (95%) in a range of 0.2-0.7 V undergoing a two-electron oxygen reduction process, with a kinetic current density of 2.8 mA cm-2 and a mass activity of 24 A gcat.-1 at 0.65 V (vs RHE). In practice, H-cells with Ni-SACs@OCB as catalysts displayed a high H2O2 production rate of 98.5 mmol gcat.-1 h-1 with negligible current loss during testing, suggesting the high H2O2 generation efficiency and robust stability. DFT theoretical calculations revealed that Ni single-atom sites coordinated by O, N coordination exhibit advantages in oxygen adsorption and increased reactivity toward the intermediate species, *OOH, which is beneficial to high selectivity for H2O2 production. This work provides a novel N, O-mediated four-coordinate nickel single-atom catalyst as a promising candidate for practical decentralized production of H2O2.

ORR Catalysts Derived from Biopolymers

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Full-text available

Dec 2022

Due to the limited reaction rate of the oxygen reduction reaction (ORR), it is considered as a limiting factor in the performance of fuel cells and metal-air batteries. Platinum is considered the benchmark catalyst for ORR; however, the scarcity of platinum, its high price, the drift phenomenon, its insufficient durability, and its susceptibility to gas poisoning are the reasons for the constant search for new ORR catalysts. Carbon-based catalysts show exceptional promise in this respect considering economic profitability and activity, and, in addition, they have favorable conductivity and often a large specific surface area. The use of chitin, cellulose, lignin, coconut shell particles, shrimp shells, and even hair for this purpose was reported, as they had similar electrochemical activity regarding Pt. Alginate, a natural polymer and a constituent of brown algae, can be successfully used to obtain carbon materials that catalyze ORR. In addition, metal atomic-level catalysts and metal N-doped porous carbon materials, obtained from sodium alginate as a precursor, have been proposed as efficient electrocatalysts for ORR. Except for alginate, other biopolymers have been reported to play an important role in the preparation of ORR catalysts. In this review, recent advances regarding biopolymer-derived ORR catalysts are summarized, with a focus on alginate as a source.

Single-Atom Catalysts for Metal-Sulfur Batteries

Chapter

Apr 2024

Lithium-sulfur batteries (LSBs) are recognized as a prospective contender for future-generation electrochemical energy storage technologies due to their high theoretical energy density, affordable price, and ecological sustainability. However, challenges such as the slow redox kinetics of sulfur species and the shuttle effect cause a significant amount of capacity loss and polarization. These problems have been addressed using a variety of methodologies, such as physical barriers, chemical adsorption techniques, and electrocatalysts, which have improved the rate capability and cycle performance of sulfur electrodes. Recently, the integration of single-atom catalysts (SACs) with high catalytic efficiency has been introduced in LSBs to expedite sulfur conversion kinetics, aiming to boost their conversion rates. This chapter provides a concise overview of recent advancements in enhancing the electrochemical performance of LSB cathodes through the incorporation of various SACs. It delves into the catalytic mechanisms employed by SACs and explores synthesis methods, including the spatial confinement approach and coordination design strategy. This chapter also discusses challenges in designing high-performance sulfur electrodes and proposes potential solutions.

Metal-organic framework-based single-atom electro-/ photocatalysts: Synthesis, energy applications, and opportunities

Article

Full-text available

Jul 2023

Single-atom catalysts (SACs) have gained substantial attention because of their exceptional catalytic properties. However, the high surface energy limits their synthesis, thus creating significant challenges for further development. In the last few years, metal-organic frameworks (MOFs) have received significant consideration as ideal candidates for synthesizing SACs due to their tailorable chemistry, tunable morphologies, high porosity, and chemical/ thermal stability. From this perspective, this review thoroughly summarizes the previously reported methods and possible future approaches for constructing MOF-based (MOF-derived-supported and MOF-supported) SACs. Then, MOF-based SAC's identification techniques are briefly assessed to understand their coordination environments, local electronic structures, spatial distributions, and catalytic/electrochemical reaction mechanisms. This review systematically highlights several photocatalytic and electrocatalytic applications of MOF-based SACs for energy conversion and storage, including hydrogen evolution reactions, oxygen evolution reactions, O 2 /CO 2 /N 2 reduction reactions, fuel cells, and rechargeable batteries. Some light is also shed on Carbon Energy. 2023;e382. wileyonlinelibrary.com/journal/cey2 |

Overview of emerging catalytic materials for electrochemical green ammonia synthesis and process

Article

Full-text available

May 2023

The concept of “green‐ammonia‐zero‐carbon emission” is an emerging research topic in the global community and many countries driving toward decarbonizing a diversity of applications dependent on fossil fuels. In light of this, electrochemical nitrogen reduction reaction (ENRR) received great attention at ambient conditions. The low efficiency (%) and ammonia (NH 3 ) production rates are two major challenges in making a sustainable future. Besides, hydrogen evolution reaction is another crucial factor for realizing this NH 3 synthesis to meet the large‐scale commercial demand. Herein, the (i) importance of NH 3 as an energy carrier for the next future, (ii) discussion with ENRR theory and the fundamental mechanism, (iii) device configuration and types of electrolytic systems for NH 3 synthesis including key metrics, (iv) then moving into rising electrocatalysts for ENRR such as single‐atom catalysts (SACs), MXenes, and metal–organic frameworks that were scientifically summarized, and (v) finally, the current technical contests and future perceptions are discussed. Hence, this review aims to give insightful direction and a fresh motivation toward ENRR and the development of advanced electrocatalysts in terms of cost, efficiency, and technologically large scale for the synthesis of green NH 3 .

Recent Advancements in the Preparation and Application of Copper Single-Atom Catalysts

Article

Mar 2023

Single-atom catalysts (SACs) have developed as a potential advanced variety of catalysts wherein copper SACs (Cu SACs) have appeared as a rising star in catalysis in which promising catalytic activity is achieved by a well-adjusted d-band center. Herein, the most recent advancements in the synthesis of copper SACs are highlighted. Various electrocatalytic applications such as carbon dioxide reduction reaction, oxygen reduction reaction, nitrogen reduction reaction, biotherapeutic applications, coupling reactions, hydroxylation, hydrogenation, photocatalytic reactions, and other miscellaneous reactions of Cu SACs coordinated into different supports have been thoroughly reviewed, and the stability and electrocatalytic performance of Cu SACs in fuel cells, zinc-CO2 batteries, and zinc-air batteries have been discussed. This review summarizes the study of most recent developments in the synthesis and applications of Cu SACs.

Flexible self-supporting interconnected cobalt sulfide nanosheets enable high-loading and long-cycling Li-S batteries with high areal capacity

Preprint

Full-text available

Feb 2023

Lithium-sulfur batteries (LSB) with high theoretical specific capacity/energy density still face some practical challenges, for instance shuttle effect, sluggish redox kinetics and corrosion of Li anode, which leads to rapid capacity decay. To overcome these challenges, herein, a porous and flexible sulfur host composed of interconnected Co 9 S 8 nanosheets in-situ grown on carbon cloth surface was constructed by a one-pot solvothermal method and applied as binder-free self-supporting electrode of LSB. The interconnected carbon fiber skeleton and highly conductive Co 9 S 8 nanosheets can provide abundant electron-transport channels to ensure excellent electric conductibility for electrode. Meanwhile, the abundant adsorption and catalytic sites provided by Co 9 S 8 nanosheets can effectively inhibit dissolution of polysulfides and improve conversion kinetics of polysulfides, effectively suppressing “shuttle effect” and protecting Li anode. The interconnected Co 9 S 8 nanosheets can also offer adequate void to facilitate penetration for Li 2 S 6 solution/electrolyte, accelerate lithium-ion diffusion and accommodate volume expansion of sulfur, thus ensuring high sulfur utilization and remarkable cycle stability of electrode. The Co 9 S 8 -CC/Li 2 S 6 electrode achieves impressive lithium-storage performance, including high discharge capacity (1315.1 mA h g − 1 , 0.1 C), excellent rate capability (872.4 mA h g − 1 , 2 C) and outstanding cyclic stability (decay of 0.02%/cycle over 1500 cycles, 2 C). Under a high sulfur-loading of 6.2 mg cm − 2 , the Co 9 S 8 -CC/Li 2 S 6 electrode still delivers high discharge capacity (1115.1 mAh g − 1 , 0.1 C) and good cycling stability (decay of 0.129%/cycle during 200 cycles, 0.5 C). This study offers insights for rational designing and structure engineering of self-supporting metal sulfide based composite host for high-performance LSB application.

Electrospun Inorganic Nanofibers for Oxygen Electrocatalysis: Design, Fabrication, and Progress

Article

Full-text available

Oct 2020
Adv. Energy Mater.

Electrospinning has received tremendous attention in terms of the design and fabrication of 1D nanofibers for oxygen electrocatalysis in fuel cells and metal‐air batteries, owing to their high surface area, short charge transport path, superior chemical stability, etc. In this review, a brief introduction is given for fuel cells, metal‐air batteries, and the operation principle of electrospinning. After that, the recent progress of the noble‐metal‐free electrocatalysts prepared through electrospinning, mainly containing heteroatoms‐doped electrospun carbon nanofibers (ECNFs), ECNFs functionalized with transition metal nanoparticles and their compounds (alloy, oxides, carbides, nitrides, sulfides, phosphides), and unique nanostructured transition metal‐nitrogen‐ECNFs (including single‐atom catalysts) are summarized. In this section, electrospun electrodes with self‐standing peculiarity are emphasized. Next, other kinds of electrospun nanofibers based on oxides, nitrides, and carbides as electrocatalysts or their hybrid materials are also discussed. Finally, the prospects and possible future research direction of electrospun NFs‐based oxygen electrocatalysts are also presented. This comprehensive review is anticipated to be valuable and helpful in the context of understanding the design and fabrication of advanced energy conversion and storage devices.

Activation of subnanometric Pt on Cu-modified CeO2 via redox-coupled atomic layer deposition for CO oxidation

Article

Full-text available

Aug 2020

Improving the low-temperature activity (below 100 °C) and noble-metal efficiency of automotive exhaust catalysts has been a continuous effort to eliminate cold-start emissions, yet great challenges remain. Here we report a strategy to activate the low-temperature performance of Pt catalysts on Cu-modified CeO2 supports based on redox-coupled atomic layer deposition. The interfacial reducibility and structure of composite catalysts have been precisely tuned by oxide doping and accurate control of Pt size. Cu-modified CeO2-supported Pt sub-nanoclusters demonstrate a remarkable performance with an onset of CO oxidation reactivity below room temperature, which is one order of magnitude more active than atomically-dispersed Pt catalysts. The Cu-O-Ce site with activated lattice oxygen anchors deposited Pt sub-nanoclusters, leading to a moderate CO adsorption strength at the interface that facilitates the low-temperature CO oxidation performance. Improving low-temperature activity and noble-metal efficiency remains a challenge for next generation exhaust catalysts. Here, the authors achieve the activation of subnanometric Pt on Cu-modified CeO2 for low-temperature CO oxidation with an onset below room temperature.

Single-atom site catalysts for environmental catalysis

Article

Full-text available

Aug 2020

In recent decades, the environmental protection and long-term sustainability have become the focus of attention due to the increasing pollution generated by the intense industrialization. To overcome these issues, environmental catalysis has increasingly been used to solve the negative impact of pollutants emission on the global environment and human health. Supported platinum-metal-group (PGM) materials are commonly utilized as the state-of-the-art catalysts to eliminate gaseous pollutants but large quantities of PGMs are required. By comparison, single-atom site catalysts (SACs) have attracted much attention in catalysis owing to their 100% atom efficiency and unique catalytic performances towards various reactions. Over the past decade, we have witnessed burgeoning interests of SACs in heterogeneous catalysis. However, to the best of our knowledge, the systematic summary and analysis of SACs in catalytic elimination of environmental pollutants has not yet been reported. In this paper, we summarize and discuss the environmental catalysis applications of SACs. Particular focus was paid to automotive and stationary emission control, including model reaction (CO oxidation, NO reduction and hydrocarbon oxidation), overall reaction (three-way catalytic and diesel oxidation reaction), elimination of volatile organic compounds (formaldehyde, benzene, and toluene), and removal/decomposition of other pollutants (Hg0 and SO3). Perspectives related to further challenges, directions and design strategies of single-atom site catalysts in environmental catalysis were also provided.

Gram‐Scale Synthesis of High‐Loading Single‐Atomic‐Site Fe Catalysts for Effective Epoxidation of Styrene

Article

Full-text available

Jul 2020
ADV MATER

Single‐atomic‐site (SAS) catalysts, a new frontier of catalysts, always show extremely high atom efficiency and unexpected catalytic properties. Herein, a pyrolyzing coordinated polymer (PCP) strategy is developed, which is facile and widely applicable in the synthesis of a series of SAS catalysts including SAS‐Fe, SAS‐Ni, SAS‐Cu, SAS‐Zn, SAS‐Ru, SAS‐Rh, SAS‐Pd, SAS‐Pt, and SAS‐Ir. The as‐obtained SAS catalysts can be easily synthesized at gram scale and the metal loading of SAS‐Fe catalysts achieves a record value of 30 wt%, which meets the requirement of practical applications. Moreover, it is discovered that SAS‐Fe catalysts show unprecedented catalytic performance for epoxidation of styrene using O2 as the only oxidant (yield: 64%; selectivity: 89%), while Fe nanoparticles and ironporphyrin are inactive. This discovery is believed to pave the way for exploiting the unparalleled properties of SAS catalysts and promoting their industrial applications. A facile and general PCP strategy is developed to synthesize SAS catalysts. This PCP strategy can be expended to synthesize a series of SAS catalysts including SAS‐Fe, SAS‐Ni, SAS‐Cu, SAS‐Zn, SAS‐Ru, SAS‐Rh, SAS‐Pd, SAS‐Pt, and SAS‐Ir. Moreover, SAS‐Fe shows excellent catalytic performance in the epoxidation of styrene using 1 atm O2 as the only oxidant.

Dual Single-Atomic Ni-N 4 and Fe-N 4 Sites Constructing Janus Hollow Graphene for Selective Oxygen Electrocatalysis

Article

Full-text available

Jun 2020
ADV MATER

Nitrogen‐coordinated metal single atoms in carbon have aroused extensive interest recently and have been growing as an active research frontier in a wide range of key renewable energy reactions and devices. Herein, a step‐by‐step self‐assembly strategy is developed to allocate nickel (Ni) and iron (Fe) single atoms respectively on the inner and outer walls of graphene hollow nanospheres (GHSs), realizing separate‐sided different single‐atom functionalization of hollow graphene. The Ni or Fe single atom is demonstrated to be coordinated with four N atoms via the formation of a Ni‐N4 or Fe‐N4 planar configuration. The developed Ni‐N4/GHSs/Fe‐N4 Janus material exhibits excellent bifunctional electrocatalytic performance, in which the outer Fe‐N4 clusters dominantly contribute to high activity toward the oxygen reduction reaction (ORR), while the inner Ni‐N4 clusters are responsible for excellent activity toward the oxygen evolution reaction (OER). Density functional theory calculations demonstrate the structures and reactivities of Fe‐N4 and Ni‐N4 for the ORR and OER. The Ni‐N4/GHSs/Fe‐N4 endows a rechargeable Zn–air battery with excellent energy efficiency and cycling stability as an air‐cathode, outperforming that of the benchmark Pt/C+RuO2 air‐cathode. The current work paves a new avenue for precise control of single‐atom sites on carbon surface for the high‐performance and selective electrocatalysts.

Potential active sites of Mo single atoms for electrocatalytic reduction of N2

Article

Nov 2020
CHINESE CHEM LETT

Single atom catalysts (SACs) with isolated metal atoms dispersed on supports exhibit distinctive performances for electrocatalysis reactions. The designable realization of well-dispersed single metal atoms is still a great challenge owing to their ease of aggregation. Here, Mo single atomic sites (Mo-N3C) combined with some ultrasmall Mo2C/MoN clusters (Mo-SA/Mo2C-MoN-Cs, mean diameter < 2 nm) on nitrogen-doped porous carbon were synthesized via a simple pyrolysis of bimetallic Zn/Mo metal-organic frameworks. X-ray absorption near edge spectra (XANES) in combination with various characterizations show that most of Mo species in sample exist in the form of single sites and the exact structure is Mo-N3C. Density functional theory (DFT) calculation further shows that as the number of N-coordination in the Mo-NxC moieties increases, the positive charge of Mo atoms increases. The single Mo atoms in Mo-N3C have the best capability of N2 adsorption, which may serve as main active sites for further electrochemical N2 reduction.

Regulating the coordination structure of metal single atoms for efficient electrocatalytic CO 2 reduction

Article

Oct 2020

The electrochemical CO2 reduction reaction (CO2RR) to eliminate excess CO2 and produce fuels/chemicals under mild conditions provides a sustainable way to maintain carbon balance and alleviate the energy shortage. However, the low current density, competitive H2 evolution reaction and uncontrollable product distribution are urgent to be solved. Benefiting from the well-defined active centers in single atom catalysts (SACs), the coordination bond, bond length/angle, etc. can be modulated to affect the selectivity/activity towards the CO2RR, which relies on electron transfer, adsorption states and so on. Hence, an in-depth summary about the regulation of the coordination structure for the CO2RR is significant and pressing. This review concentrates on the manipulation of central atoms and coordination atoms on carbon and other supports in SACs for the CO2RR. As a prototype, a definitive correlation between the structure and XAFS spectra characteristics was properly decoded. Then, the influence of the coordination atoms and coordination number on the current density, reduction products, reaction path, etc., is highlighted. Finally, prospects such as identification of the coordination structure, machine learning, fine adjustment of the coordination atoms and control of the product distribution were put forward. This Review contributes profound insights for the rational design of the coordination structure towards highly efficient and even guided electrocatalytic CO2 reduction.

Bifunctional Atomically Dispersed Mo-N2/C Nanosheets Boost Lithium Sulfide Deposition/Decomposition for Stable Lithium-Sulfur Batteries

Article

Jul 2020

The sluggish kinetics of lithium polysulfides (LiPS) transformation is recognized as the main obstacle against the practical applications of Lithium sulfur (Li-S) battery. Inspired by molybdoenzymes in biological catalysis with stable Mo-S bonds, porous Mo-N-C nanosheets with atomically dispersed Mo-N2/C sites are developed as a cathode to boost the LiPS adsorption and conversion for Li-S battery. Thanks to its high intrinsic activity and unique Mo-N coordination structure, the rate capability and cycling stability of S/Mo-N-C are greatly improved compared with S/N-C due to the accelerated kinetics and suppressed shuttle effect. The S/Mo-N-C delivers a high reversible capacity of 743.9 mAh g-1 at 5 C rate and an extremely low capacity decay rate of 0.018% per cycle after 550 cycles at 2 C rate, outperforming most of the reported electrodes. Density functional theory (DFT) calculations suggest that the Mo-N2/C sites can bifunctionally lower the activation energy for Li2S4 to Li2S conversion and the decomposition barrier of Li2S, accounting for its inherently high activity towards LiPS transformation.

Clean utilization of palm kernel shell: sustainable and naturally heteroatom-doped porous activated carbon for lithium–sulfur batteries

Article

Jun 2020

Lithium–sulfur batteries (LSBs) have received much concern as emerging high-power energy storage system. Nevertheless, the low conductivity of sulfur and polysulfide shuttle results in low rate capability and rapid capacity decay, which seriously limit its commercial application. Here, facile, sustainable and cost-effective strategy for preparing heteroatom-doped porous activated carbon (PAC) derived from biomass palm kernel shell (PKS) was developed for high-performance LSB applications. The presence of N, P and S heteroatoms with modification of the surface polarity brings about large amounts of active sites and improved adsorption property compared to those of common carbon materials. The PAC sample possesses desirable specific surface area (SSA) (2760 m2·g−1) as well as pore volume (1.6 cm3·g−1). Besides, the good electrical conductivity of PAC endows the material with excellent rate performance. The PAC-S electrode with a 60% of sulfur loading has a desirable first discharge capacity (1045 mAh·g1, 200 mA·g−1) with superb discharge capacity (869.8 mAh·g−1, 100th cycle) and cyclability (312.6 mAh·g−1, 800 mA·g−1, 1000th cycle), which can be mainly ascribed to its unique porous properties and the good conductivity of PAC.

Bifunctional iron-phtalocyanine metal–organic framework catalyst for ORR, OER and rechargeable zinc–air battery

Article

Jun 2020

Exploring non-noble metal and high-activity electrocatalysts through a simple and controllable protocol remains a great challenge for oxygen reduction reaction (ORR) and zinc–air batteries. Herein, we developed a melt polymerization strategy to synthesize iron-polyphthalocyanine (FePPc) metallic–organic frameworks (MOFs) over the carbon black matrix (FePPc@CB). Through non-covalent π–π interactions, FePPc molecules can anchor on carbon matrix, thus facilitating the electron transfer process and stabilizing the systems. Owing to abundant free electrons and atomically MN4 catalytic sites in the macrocycle structure, FePPc@CB exhibits excellent oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) electrocatalytic activity. The FePPc@CB also delivers excellent performances for liquid and flexible all-solid-state batteries compared to that of commercial Pt/C, making it a promising ORR/OER electrocatalyst.

Single Atom Catalysts for Fuel Cells and Rechargeable Batteries: Principles, Advances, and Opportunities

Abstract

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