1

Defective Structures in Metal Compounds for Energy-Related Electrocatalysis

Xuecheng Yan, Yi Jia, Xiangdong Yao*

Dr. X. Yan, Dr. Y. Jia, Prof. X. Yao

Queensland Micro- and Nanotechnology Centre

Griffith University, Nathan Campus, QLD 4111, Australia

E-mail: x.yao@griffith.edu.au

Keywords: defect, anion vacancy, cation vacancy, metal compound, electrocatalysis, surface

engineering, electronic structure

Catalysts play a critical role in accelerating the electrochemical reactions. Engineering the

electronic structures and surface properties of electrocatalysts is a feasible approach to improve

their catalytic performance. Introducing defects such as anion and cation vacancies into

transition-metal-based materials to enhance their activity for various energy related

electrocatalytic reactions has been receiving increasingly attentions in recent years. In this

review, a systematic summary of the anion and cation defects on different electrochemical

reactions will be presented. In particular, the commonly used methods to produce anion

vacancies (such as oxygen, sulfur and phosphorus defects) and cation vacancies (such as Fe,

Co and Ni defects) will be discussed. The control of the defect density and refilling heteroatoms

to stabilize the anion defects and simultaneously to further enhance their catalytic performance

are also summarized. In addition, recent work on creating multivacancies in transition-metal-

based electrocatalysts for maximizing their catalytic activities is reviewed as well. Finally,

future research on defect engineering in metal compounds for electrocatalysis are proposed.

This review will guide the further development of various energy related electrocatalytic

reactions promoted by defective structures in metal composites.

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1. Introduction

The ever-increasing energy demand and the deteriorating global environment caused by using

fossil-based fuels urge us to seek clean and sustainable energy sources. Energy related

electrocatalysis plays a deciding role in accelerating the electrode reactions such as in

hydrogen-based proton exchange membrane fuel cells (PEMFCs), overall water splitting,

electrochemical nitrogen and carbon dioxide fixation, and lithium-ion batteries (LIBs).[1-7]

Extensive research has been devoted to exploiting low-cost and efficient materials to replace

the expensive noble metal-based electrocatalysts such as Pt, Pd, RuO2 and IrO2. Transition

metal compounds and metal-free carbon materials as the alternatives are widely studied.

Different strategies are utilized to improve the electrocatalytic performance of non-noble metal-

based composites from the aspects of increasing their reactivity and number of active sites,

including phase and morphology control,[8] defect engineering,[9-11] heteroatoms doping,[12] and

surface strain.[13, 14] Therein, defect engineering is considered as a promising approach to

increase the intrinsic activity of electrocatalysts at atomic levels.

It is known that all solid materials contain defects such as intrinsic vacancies as a result of the

dislocation of atoms. The studies show that engineering defects is a viable and effective strategy

to regulate the electronic structures and surface properties of the electrocatalysts for improved

catalytic activity. Transition-metal-based materials such as Fe, Co, Ni, Mn and Mo

oxides/hydroxides and their composites are promising catalysts for various electrochemical

reactions, and defects such as anion (O, S and P) and cation (Fe, Co and Ni) vacancies could

serve as the active sites to promote the electrocatalysis.[15-17] It is revealed that defects in metal

complexes could improve the electrical conductivity, charge redistribution, and optimize their

adsorption energy and band structures,[18-20] thus favorable for various electrochemical

reactions. As a booming area, researches on defective transition-metal-based catalysts have

received extensive attentions. Although many excellent reviews have summarized the influence

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of defects in metal-based materials on electrocatalysis,[15, 21-23] they merely discussed

anion/cation defects briefly or only focused on specific applications such as for water splitting,

fuel cells or nitrogen reduction. Since this area is rapidly developing, it is indispensable to

review recent important works on anion and cation defects in metal compounds for different

energy related electrocatalysis and outlook the future advancement directions.

In this review, we will summarize recent development on controllable synthesis of defects in

various metal-based materials for the electrochemical reactions. Specifically, we will focus on

the most common anion and cation vacancies in metal complexes. As illustrated in Figure 1,

both single type of anion and cation vacancies and multivacancies will be discussed to show

their influence on electrocatalysis, emphasizing the controllable synthesis, in-depth

characterizations, catalytic performance-structure relationship and the applications in energy

related electrocatalysis. Herein, the generally used methods for the controllable fabrication of

anion defects such as oxygen, sulfur and phosphorus vacancies will be firstly discussed.

Meanwhile, studies on controlling the concentration of the most widely investigated oxygen

defects through different synthetic techniques are presented. In addition, recent work on

refilling anion defects with heteroatoms or metal species to improve their stability and at the

same time to further enhance their catalytic performance will be outlined. Afterward, the

creation and influence of cation defects include Fe, Co and Ni vacancies on electrochemical

reactions will be reviewed. The joint effect of anion and cation defects, as well as the

multivacancies on particular electrocatalysis will be discussed at last. This review will provide

a comprehensive and latest update on the newly developed anion and cation defects in transition

metal composites for energy related electrocatalysis.

2. Anion Defects in Transition Metal Compounds for Electrocatalysis

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Defects almost exist in all transition-metal-based composites as it is very hard to produce a

perfect material without any defects. The introduction of anion vacancies into metal compounds

is a feasible way of modulating the electronic properties and enhancing the intrinsic activity of

active sites.[20, 24, 25] Among the anion defects such as oxygen, sulfur and phosphorus vacancies,

oxygen vacancies are the most popular and extensively investigated defects due to their low

formation energy. In this section, detailed discussions will be presented on producing anion

defects in transition-metal-based electrocatalysts by different synthetic methods for improved

catalytic performance. The generally used methods to produce anion vacancies in metal

compounds and the corresponding defects characterization techniques are summarized in Table

1.

2.1 Oxygen Vacancy

The generally used methods to create oxygen vacancies in metal complexes include NaBH4 and

hydrogen reduction, plasma irradiation, and thermal annealing under argon or air conditions.[19,

26, 27] The NaBH4 reduction is a simple method and does not need high-temperature treatment,

which provides an efficient way to control the structures of electrocatalysts at an atomic level.

For example, Zhuang and co-workers used NaBH4 as a fast reducing agent fabricated iron-

cobalt oxide nanosheets (Fe1Co1-ONS) with abundant oxygen vacancies (Figure 2a), which

show excellent oxygen evolution reaction (OER) performance due to the increased active

sites.[18] For comparison, they also synthesized a FeCo nanoparticle (Fe1Co1-ONP) with less

oxygen vacancies using hydrazine hydrate as a slow reductant. It is shown in Figure 2b that the

Fe1Co1-ONS exhibits much higher OER activity than that of the Fe1Co1-ONP, which is even

superior to that of the commercial RuO2. The corresponding X-ray photoelectron spectroscopy

(XPS) O 1s spectra in Figure 2c and d confirmed that the FeCo nanosheets sample has higher

oxygen defect concentration (denoted O2) than that of the FeCo nanoparticle. It is revealed that

the oxygen vacancies could enhance the electrical conductivity (Figure 2e) of the

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electrocatalysts and promote the adsorption of H2O onto the active sites, thus play a crucial role

in improving the OER activity. The nanosheet structures of the Fe1Co1-ONS also contributed

to its superb OER performance by providing rich surface active sites.[18] Similarly, Zheng et al.

utilized NaBH4 to treat mesoporous Co3O4 nanowires at a room temperature to create oxygen

vacancies for improved electrochemical performance (Figure 2f).[28] It is suggested that the

high surface area mesoporous feature of the 1D Co3O4 nanowires facilitated the efficient

production of oxygen vacancies for outstanding electrocatalysis. Compared to the untreated

Co3O4 nanowires, the reduced Co3O4 nanowires enrich oxygen defects show much higher OER

current density (Figure 2g) and supercapacitance. The density functional theory (DFT)

simulations uncover that the improved electrocatalytic performance of the reduced Co3O4

nanowires can be attributed to the existence of the oxygen vacancies. It is indicated that the

oxygen defects could improve the electrical conductivity of the electrocatalysts because the two

electrons on the oxygen vacancy can be excited into the conduction band with low formation

energy.[28] The investigations also show that oxygen defects could reduce the energy barrier for

forming the reaction intermediates toward improving the water splitting performance of

NiCo2O4 with necklace-like multishelled hollow structures.[26]

Apart from NaBH4 reduction, hydrogen thermal treatment is also a feasible approach to create

oxygen defects in metal compounds with improved electrochemical properties.[29-31] The

formation mechanism of oxygen vacancies in TiO2 by hydrogen reduction is shown as follows:

(1) when the reduction temperature is below 300 °C, hydrogen interacts with the surface oxygen

physically; (2) when the temperature is increased to above 300 °C, electrons are transferred

from the hydrogen atoms to the lattice oxygen atoms in TiO2. Subsequently, the lattice oxygen

atoms will be extracted from the TiO2 surface and form H2O with the hydrogen atoms, thus the

oxygen vacancies are produced.[32] For instance, Yang et al. used a hydrogen reduction

annealing method to controllably synthesize single phase Ca2Mn2O5 with oxygen vacancies for

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high-performance OER catalysis under mild reaction temperatures.[30] In comparison with the

pristine perovskite CaMnO3/C, the hydrogen reduced sample Ca2Mn2O5/C with rich oxygen

defects shows much higher OER activity (Figure 2h). Specifically, the OER mass activity of

the Ca2Mn2O5/C is around 4−6 times higher than that of the CaMnO3/C under all the given

potentials (Figure 2i). It can be deduced from the unit cell structures of CaMnO3 and Ca2Mn2O5

in Figure 2j that the Ca2Mn2O5 favors the readily transport of the OH− ion. This combines with

the high spin electron configuration of manganese cation and the oxygen vacancy assisted

bonding formation between Mn3+ and OH− promote the OER activity of the oxygen deficient

perovskite.[30]

Plasma is normally identified as a partially or fully ionized gas.[33] It is a unique medium that

can be used in materials fabrication, etching and surface modification.[34-40] Plasma is a

generally utilized method to create defects in various nanomaterials. As shown in Figure 3a,

by utilizing a convenient Ar/O2 plasma treatment method, Wang et al. successfully exfoliated

cobalt diselenide (CoSe2) and realized the cubic-to-orthorhombic phase transformation.[41] The

synthesized o-CoSe2-O ultrathin nanosheets (UNs) are rich in oxygen vacancies due to the

plasma irradiation, as confirmed by the XPS O 1s spectrum in Figure 3b. The X‐ray absorption

spectra at Co K-edge were acquired to gain the detailed local coordination structures of the

prepared samples. The X-ray absorption near edge structure (XANES) spectra of the Co K-

edge in Figure 3c show the surface oxidation state of the o-CoSe2-O UNs. Compared to the

pristine c-CoSe2/DETA (diethylenetriamine), the o-CoSe2-O UNs exhibits a slightly reduced

oscillation amplitude in the Co K-edge k3χ(k) oscillation curve (Figure 3d). This further proves

the re-constructions of the surface atoms in o-CoSe2-OUNs, possibly formed oxygen defects.

Remarkably, the oxygen vacancies enriched sample o-CoSe2-OUNs only requires an

overpotential of 251 mV to reach a current density of 10 mA cm–2 for the OER, which is much

smaller than that of the c-CoSe2/DETA (350 mV) and the commercial RuO2 (280 mV, Figure

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3e).[41] In view of the surface area and electronic states of electrocatalysts play an important

role in deciding their catalytic performance, Wang and co-workers used an Ar plasma to

engrave a Co3O4 nanosheets, aiming to increase the surface area and create oxygen defects to

provide more accessible active sites for the OER catalysis (Figure 3f).[42] The scanning electron

microscopy (SEM) images in Figure 3g and h show that compared to the pristine Co3O4

nanosheets, the plasma treated sample presents a discontinuous and porous morphology.

Accordingly, the Brunauer–Emmett–Teller (BET) surface area of the pristine Co3O4 is

increased from 95.27 m2/g to 160.26 m2/g after the plasma treatment. The fine-scanned XPS

Co 2p and O 1s spectra reveal the existence of the surface oxygen vacancies in the plasma-

engraved Co3O4. The linear sweep voltammetry (LSV) curves suggest that the Ar-plasma

engraved Co3O4 nanosheets sample is much better than the pristine Co3O4 nanosheets for

catalyzing the OER, in terms of onset potential and current density (Figure 3i). The

electrochemical impedance spectroscopy (EIS) test results in Figure 3j indicate that the plasma

treated Co3O4 sample has lower charge transfer resistance than that of the pristine Co3O4 for

the OER.[42] It means that plasma engraving is an efficient strategy to fabricate highly active

non-noble-metal-based catalysts for electrocatalysis.

The investigations show that for certain electrocatalysis, the adsorption of the reactant is a rate-

determining step, such as the adsorption of H2O molecules onto the active sites for the OER.[43]

For example, Xie et al. firstly used DFT simulations to uncover the influence of oxygen

vacancies on the adsorption of H2O during the OER process.[44] They established two models

using NiCo2O4 as an OER catalyst, one is perfect NiCo2O4 and the other is defective NiCo2O4

(with an oxygen vacancy). As can be seen from Figure 4a, the NiCo2O4 with one oxygen

vacancy is more favorable for the adsorption of H2O onto the Co3+ site. The DFT calculations

also reveal that the presence of oxygen vacancies in the NiCo2O4 could lower the adsorption

energy of H2O molecules, which will enhance the reactivity of active sites and accordingly

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promote the OER performance. Experimentally, they synthesized ultrathin NiCo2O4 nanosheets

rich (NiCo-r) or poor (NiCo-p) in oxygen vacancies by calcining the NiCo hydroxides in an air

or oxygen condition. From the XPS O 1s spectra in Figure 4b, it is obvious that the

concentration of the oxygen defect in the NiCo-r is higher than that of the NiCo-p and bulk

NiCo2O4 samples. The excellent OER activity of the NiCo-r sample is presented in Figure 4c,

which displays the smallest overpotential and the highest current density among the three

samples.[44] This result agrees very well with the theoretical predications. Similarly, they also

fabricated oxygen vacancy-rich (VO-rich) In2O3 porous sheets using a mesoscopic-assembly

fast-heating strategy (Figure 4d).[45] Apart from the XPS spectra, the oxygen vacancies of the

VO-rich In2O3 sample are also reflected by the higher intensity of the photoluminescence (PL)

spectra in Figure 4e. In addition, the high content of the oxygen defects in the VO-rich In2O3

sample is verified by the electron spin resonance (ESR) spectra in Figure 4f. The detected ESR

signal at g=2.004 suggests that electrons are trapped on oxygen vacancies.[46] The superiority

of the O-vacancies in catalyzing the solar water splitting is shown in Figure 4g. It is revealed

that the O-vacancies could narrow the bandgap of the In2O3 porous sheets and thus facilitated

the visible light harvesting.

2.2 Control of Oxygen Defect Density

It can be concluded from the above discussions that oxygen vacancies (VO) are crucial to the

catalytic reactions. However, it is still a grand challenge to precisely control the density of the

oxygen vacancies in metal compounds. If the concentration of the defects can be tuned and

optimized, the catalytic activity of the defective electrocatalysts is supposed to be further

improved. Meanwhile, the controllable synthesis of defects is also beneficial for probing the

structure-performance relationships, thus facilitating the rational design and fabrication of more

efficient electrocatalysts for the target reactions.[47-49]

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A hydrogenation treatment method is used by Yao and co-workers to adjust the density of the

oxygen vacancies in Fe1Co1Ox nanosheets (Figure 5a).[49] Relatively low temperature is

utilized to avoid the phase change or separation of the binary-metal oxides. This will exclude

the influence of other factors except the concentration of VO on the electrocatalytic performance

of the obtained catalysts. A series of samples were synthesized by varying the hydrogenation

temperature (200 to 400 °C) and hydrogen pressure (0.5 MPa to 4.0 MPa). The high-resolution

transmission electron microscopy (TEM) image of the Fe1Co1Ox-200 °C-2.0 MPa sample in

Figure 5b reveals its disordered structures, suggesting the loss of the oxygen atoms during the

hydrogenation process. The enlarged image of the circled area in Figure 5b further schemes the

atomic structures and the possible location of the oxygen vacancy in the Fe1Co1Ox-200 °C-2.0

MPa sample (Figure 5c). At the hydrogenation temperature of 200 °C, the density of the oxygen

vacancies on the surface of the sample is increased linearly with the increase of the hydrogen

pressure, as confirmed by the XPS analysis (Figure 5d). The Co2+/Co3+ atomic ratio is another

indicator to show the concentration of oxygen defects in cobalt compounds with that higher

Co2+/Co3+ atomic ratio suggests higher oxygen vacancy density.[42, 44] The deconvoluted high-

resolution XPS Co 2p spectra in Figure 5e also indicate the increase of the oxygen defects, since

the Co2+/Co3+ atomic ratio shows a continuous increase when the hydrogen pressure is raised.

Furthermore, the trend of the oxygen defects change in the prepared samples is also revealed

by the photoluminescence spectra in Figure 5f, which is consistent with the XPS results.

However, the OER performance of the synthesized catalysts is not constantly increased at raised

oxygen vacancy levels (Figure 5g), implying that an optimized concentration of oxygen defects

will minimize the charge transfer resistance (Figure 5h), thus being beneficial for the OER

catalysis. The studies show that the excessive amount of oxygen defects may decrease the

electrical conductivity and result in the dissociation of the adsorbed intermediates, which are

not conducive to the OER.[49]

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In addition, the density of the oxygen defects can also be adjusted by controlling the calcination

parameters. For instance, a thermodynamical stable rutile-type β-MnO2 (Figure 5i) was selected

to probe the influence of oxygen vacancy concentration on the electrochemical oxygen

reduction reaction (ORR).[50] Specifically, MnO2 with different densities of oxygen vacancies

can be obtained by tuning the heat treatment temperature, time and atmosphere (air or argon).

At a high temperature of 500 °C treatment, a structural modification was observed (Figure 5j).

The structure reconstructions and phase changes are possibly caused by the generation of

oxygen vacancies.[51] The electrocatalytic test results show that the newly introduced oxygen

defects in the MnO2 have obvious influence on the ORR performance, and a modest content of

oxygen vacancies could optimize the ORR activity of the MnO2 catalyst. The DFT calculations

reveal that the Mn–O bond and the surface structures were altered after the creation of the

oxygen vacancies. It is indicted that the oxygen vacancies could facilitate the adsorption,

activation and dissociation of oxygen. Figure 5k shows the energy profiles of the reduction of

the adsorbed oxygen to peroxide. It also suggests that moderate oxygen defects (Path I with one

oxygen vacancy) will be thermodynamically feasible for the ORR, which shows a reduced

kinetic barrier.[50]

Recent studies show that heteroatom-doping is a promising strategy to uniformly and

controllably create oxygen vacancies in metal composites.[52-54] Li et al. fabricated mesoporous

Co3O4 nanosheets with tunable oxygen vacancies using a facile phosphorus-doping method

(Figure 5l).[52] The samples with different concentrations of oxygen vacancies were synthesized

by altering the NaH2PO2⸱H2O addition amount. The XPS Co 2p and O 1s spectra suggest that

the density of the oxygen vacancies is increased with the increase of the P doping level. As

presented in Figure 5m, the sample with a medium content of oxygen vacancies exhibits the

highest OER activity, which is even better than that of the commercial RuO2. The excellent

OER performance of the oxygen vacancies enriched sample could be owing to the mesoporous

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nanosheet structures and the appropriate oxygen defects that provide abundant active sites such

as cobalt oxyhydroxide (CoOOH) for the water oxidation.[52, 55, 56]

2.3 Sulfur Vacancy

Apart from oxygen defects, sulfur vacancy is another widely investigated anion defect. The

transition metal dichalcogenide-based composites such as MoS2[57-61] and CoS2

[62-64] are the

potential electrocatalysts to replace platinum (Pt) for the hydrogen evolution reaction (HER). It

is known that the edges of MoS2 are much more active than its basal plane, so further improving

the HER performance via activating the basal planes is a promising strategy.[65, 66] For example,

the inert MoS2 basal planes could become active for the HER by introducing S-vacancies to

lower the hydrogen adsorption energy.[66, 67] The S-vacancies can be in-situ formed during the

exfoliation process,[67] or through annealing,[68] plasma treatment,[69] electrochemical

desulfurization,[70] and chemical treatment.[71]

Luo and co-workers firstly used a hydrothermal method synthesized ammonia-intercalated

MoS2, which was subsequently being exfoliated to monolayer MoS2 and produced S-vacancies

through the ultrasonication (Figure 6a).[67] The atomic force microscopy (AFM) image

suggests that the synthesized sample is a single layer MoS2. It can be observed from the high-

resolution TEM image in Figure 6b that a high density of defects were formed in the monolayer

MoS2 (highlighted by red circles). The corresponding relaxed atomic structures in Figure 6b

suggest that both one S-vacancy (one S atom is absent) and three S-vacancies (three S atoms

are lost) were created in the monolayer MoS2.[72, 73] The XPS measurements further confirm

that S-vacancies were generated in the exfoliation process. The polarization curves measured

in 0.5 M H2SO4 show that the monolayer MoS2 rich in S-vacancies demonstrates the highest

HER activity among the prepared MoS2-based samples (Figure 6c). The small charge transfer

resistance of the fabricated single layer MoS2 is possible resulted from its monolayer structure

with abundant S vacancies (Figure 6d). The corresponding DFT simulations disclose that the

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basal planes of the MoS2 are activated by the S-vacancies, which brings localized donor states

into the bandgap. This could lower the hydrogen adsorption energy and finally promotes the

HER performance.

Through annealing the MoS2 nanosheets under hydrogen atmospheres at different temperatures,

a series of defective multilayered MoS2 with variable concentrations of S-vacancies were

fabricated.[68] It is shown that the concentration of the S-vacancies is increased when the

annealing temperature is raised. The produced S-vacancies in the ultrathin layer of defective

MoS2-7H can be clearly observed from the high-resolution TEM image in Figure 6e. A two-

stage phenomenon can be noticed from Figure 6f , showing the relationship between the S:Mo

ratio and the annealing temperature. Stage 1 refers to the point defects with S:Mo > 1.7 (below

600 °C, low content of S-vacancies), and Stage 2 corresponds to the S-stripping with S:Mo <

1.7 (above 600 °C, high content of S-vacancies). It reveals that the point defects could

contribute to the fast improvement of the HER performance, while the S-stripping is capable of

further promoting the HER at a relative slow rate.[68] Apparently, the combination of these two

defects is possible to maximize the HER activity of the MoS2 nanosheets.

In addition, a novel and scalable electrochemical desulfurization method is developed by Abild-

Pedersen et al. to controllably create S-vacancies on the MoS2 basal plane.[70] Interestingly, the

known inert sulfur atoms on the basal planes of MoS2 can be successfully activated and

removed as H2S gas to form the S-vacancies under the given potentials. The density of the S-

vacancies can be adjusted via altering the applied desulfurization potentials. The XPS S 2p

spectra of the desulfurized sample with S-vacancies (V-MoS2) and the pristine as-synthesized

catalyst (P-MoS2) in Figure 6g suggest that some S atoms were removed from the surface of

the MoS2 during the desulfurization treatment, as the S 2p peaks of the V-MoS2 show decreased

intensities compared to those of the P-MoS2. The measured LSV curves in Figure 6h clearly

indicate that the V-MoS2 sample is more advantageous than the P-MoS2 for catalyzing the HER.

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Plasma treatment is a commonly used method to create oxygen defects, it can also be applied

to generate S defects in sulfur compounds, including MoS2 and cobalt sulfide.[74-76] For instance,

Zhang and co-workers utilized a plasma-induced dry method exfoliated lamellar Co3S4/TETA

(triethylenetetramine) into ultrathin porous Co3S4 nanosheets rich in S-vacancies (Co3S4 PNSvac)

for the alkaline HER (Figure 7a).[76] The Ar plasma is an efficient approach to create pores and

defects in the resulting sample, as confirmed by the corresponding atomic level high-angle

annular dark-field scanning TEM (HAADF-STEM) image in Figure 7b. Notably, the superior

HER performance of the Co3S4 PNSvac with abundant S-vacancies can be reflected from the

mass activity comparisons shown in Figure 7c. The advantages of the defective sample for the

HER is more obvious at higher overpotentials, for example, the Co3S4 PNSvac is 14 times and

107 times higher than that of the Co3S4 nanosheets (NS) and Co3S4 nanoparticles (NP), and it

even outperforms the commercial Pt/C for catalyzing the alkaline HER at an overpotential of

200 mV (vs. RHE). The DFT modellings suggest that the Co3S4 with ultrathin structure and S-

vacancies is more favorable than the Co3S4 NS and Co3S4 NP to promote the adsorption of H2O

molecules during the HER process. The corresponding density of states (DOS) results in Figure

7d indicate that the Co3S4 PNSvac sample has more electrons than that of the Co3S4 NS and

Co3S4 NP near the Fermi level, implying the improved electrical conductivity.[77] The 2D

porous structures and the rich S-vacancies contribute to the abundant active sites, enhanced

intrinsic activity and electrical conductivity of the Co3S4 PNSvac, rendering it a promising

electrocatalyst for electrocatalytic alkaline hydrogen evolution.

Apart from creating S-vacancies in cobalt pyrite (CoS2), Ding et al. also introduced dopant

nitrogen atoms to synergically improve the HER performance.[20] They used a simple sintering

method prepared N-doped CoS2 catalysts with different N and S-vacancies contents under an

argon atmosphere. The quantitative XPS analysis shows that the N concentrations in the NCS-

1, NCS-2, and NCS-3 samples are 1.2, 3.3, and 5.5 at.%, respectively. The density of the S-

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vacancies decreases with the increase of the N content, which is confirmed by the electron spin

resonance (ESR) spectra in Figure 7e. The signal of the unpaired electrons detected at around

321 mT (g at 1.905−2.003) suggests the existence of the S-vacancies in the synthesized samples,

while a stronger intensity is an indicator of more vacancies.[78, 79] The LSV polarization curves

of the prepared samples obtained in a 0.5 M H2SO4 electrolyte in Figure 7f show that the NCS-

2 sample with moderate S-vacancies and N contents exhibits the best HER performance. The

potential reasons were probed via the theoretical calculations. The calculated hydrogen

adsorption free energy (ΔGH*) results of the Co site and the Ns site for the established models

are presented in Figure 7g and h, respectively. It indicates that the coexistence of N dopants and

S-vacancies could greatly improve the activity of the Co sites by decreasing the ΔGH* from 0.41

to 0.18 eV in N-substituted S in CoS2 with S defect (Ns-Vs-CoS2) (Figure 7g). In addition, it

can be observed that replacing a S atom with a N atom in CoS2 and simultaneously introducing

a S-vacancy could remarkably reduce the ΔGH* (from 1.31 to −0.11 eV), which is approaching

the value of Pt.[20] This further proves the crucial roles of S-vacancies and N dopant for the

HER.

2.4 Phosphorus Vacancy

The advantageous features of transition metal phosphides (such as Ni2P, CoP, FeP, Cu3P,

etc.),[80-84] including the superior electrical conductivity and rapid electron transfer at the

electrode-electrolyte interface render them suitable for high-performance electrocatalysis.[85]

Defect engineering such as introduce P vacancies into metal phosphides is a prospective

strategy to enhance the catalytic performance via improving their conductivity and electron

transport. Compared with O and S vacancies, phosphorus (P) vacancies are rarely reported due

to the higher formation energy. The commonly used methods to generate P defects in metal

compounds include NaBH4 reduction[81, 86] and plasma treatment.[87]

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Long et al. synthesized Ni2P nanosheets with abundant P vacancies on a carbon cloth (Vp-

Ni2P@CC) through a facile NaBH4 reduction method for promoting the ORR and OER in

lithium-oxygen batteries (LOBs) (Figure 8a).[81] The presence of P vacancies in the Vp-

Ni2P@CC sample is confirmed by the high-resolution P 2p spectra in Figure 8b, as evidenced

by the shift of the P 2p peak to a lower bonding energy in the Vp-Ni2P@CC as compared to the

Ni2P@CC.[86] The intensity of the photoluminescence (PL) spectrum of the Vp-Ni2P@CC is

much higher than that of the Ni2P@CC (Figure 8c), further suggesting the creation of P

vacancies in the Vp-Ni2P@CC sample. In addition, the lattice defects and distortions possibly

induced by the P vacancies were directly detected in the Vp-Ni2P@CC sample through the high-

resolution TEM, represented by the circled areas in red (Figure 8d). It is found that the rich P

vacancies in the Vp-Ni2P@CC sample are capable of tuning its surface electronic structures and

narrowing the bandgap, leading to the high catalytic performance for the LOBs.[81] Feng and

co-workers developed a general approach to introduce P vacancies into Co0.68Fe0.32P through

Ar plasma treatment (Figure 8e).[87] The XPS and synchrotron radiation soft X-ray absorption

spectroscopy test results indicate the presence of P vacancies in the Ar plasma treated sample.

The outstanding OER activity of the defective sample Co0.68Fe0.32P-60 (Figure 8f) demonstrates

the crucial role of P vacancies in optimizing the surface state of the electrocatalysts for

improved performance. The studies show that the introduction of P vacancies into Cu3P

nanosheets is a feasible strategy to promote the electrochemical nitrogen reduction reaction

(NRR).[84] Through a doped-oxygen-induced method, Zhang et al. successfully synthesized P

vacancies enriched Cu3P samples. The corresponding theoretical studies suggest that the

oxygen-induced P vacancies in the RO-Cu3P (partially reduced O-Cu3P) are the actual NRR

active sites, which could lower the free energy barrier of the rate-determining step, thus being

beneficial for catalyzing the NRR (Figure 8g).[84]

2.5 Modifying Anion Vacancies for Durable and Enhanced Electrocatalysis

16

The investigations indicate that the anion defects such as oxygen vacancies are not stable under

highly oxidizing conditions, for example, during the OER process,[31, 88] so stabilizing or

modifying the active anion defects to maintain their electrocatalytic activity is of pivot

importance. Refilling the anion vacancies with heteroatoms or atomic metal species is not only

a feasible and efficient method to improve the stability, but also a potential strategy to further

enhance the catalytic performance.[16, 89-91]

The normally used heteroatoms to improve the stability of the most popular oxygen defects are

S, P and N. For instance, Yao et al. refilled the created oxygen vacancies in iron-cobalt oxide

(FeCoOx-Vo) nanosheets with S atoms under Ar atmospheres using S power as the S source

(Figure 9a).[89] The successfully refilling of S and the formation of the Co(Fe)-S bond in the

resulting sample FeCoOx-Vo-S are confirmed by the XPS and X-ray absorption spectroscopy

(XAS). The corresponding DFT calculations indicate that the newly formed Co-S coordination

in the defective sample is favorable for the adsorption of H2O onto the Co active sites, which

will facilitate the OER. The excellent OER performance of the S-modified sample FeCoOx-Vo-

S is shown in Figure 9b. It only needs an overpotential of 260 mV to reach a high current density

of 200 mA cm−2 in a 1.0 M KOH solution, which is much smaller than that of the pure defective

sample FeCoOx-Vo (503 mV) and the commercial RuO2 (585 mV). The durability

measurement results suggest that the FeCoOx-Vo-S sample is very stable for the OER, implying

that the refilled S atoms could improve the OER activity and stability as well.[89] Meanwhile,

oxygen-vacancy-rich cobalt oxysulfide single crystals anchored on N-doped graphene

nanomeshes (CoO0.87S0.13/GN) were synthesized by treating the precursor CoOxS1.097/G under

an ammonia atmosphere. It is shown that the significantly improved ORR and OER durability

and activity of the resulting electrocatalyst is attributed to the formation of the active oxygen

defects and the Co–O hybridization caused by tuning the S doping degree.[92] In addition, P

was also used to stabilize the oxygen vacancies in the Co3O4 and simultaneously to make it

17

active for the HER.[88] Specifically, P was refilled into the defective Co3O4 using NaH2PO2 as

the P source during the production of oxygen vacancies by Ar plasma treatment. The Co K-

edge extended X-ray absorption fine structure (EXAFS) test results suggest that the P filled

sample P-Co3O4 has higher coordination numbers than those of the Vo-Co3O4 (pure oxygen

vacancy-rich Co3O4) in the first shell, indicating the successfully filling of P into the oxygen

vacancies. The Co K-edge X-ray absorption near edge structure (XANES) spectra in Figure 9c

suggest that electrons are more readily to be transferred to the Co 3d orbital of P-Co3O4 than to

that of VO-Co3O4, showing the lower charge transfer resistance of the P-Co3O4 for

electrocatalysis. The electrocatalytic performance measurement results show that the P-Co3O4

exhibits superior activities for both the OER and HER, which can be used for efficiently

catalyzing the overall water splitting. The calculated density of states (DOS) in Figure 9d reveal

that the P-Co3O4 displays the lowest bandgap among the three catalysts, implying its excellent

electrical conductivity. This could reduce the electron transfer resistance and accordingly

contribute to the improved HER and OER performance of the P filled defective Co3O4.[88]

Similarly, Wang and co-workers refilled the oxygen vacancies in ultrafine tungsten oxide (WOx)

clusters with N atoms to improve the lithium-ion storage performance.[90] As illustrated in

Figure 9e, the N dopant was introduced to the oxygen vacancies in WOx through an in-situ

annealing method. The reduced oxygen vacancies signal intensity in the measured electron

paramagnetic resonance (EPR) spectra in Figure 9f confirms the refilling of N atoms into the

oxygen vacancies in the WOx. The EXAFS spectra in Figure 9g further prove the incorporation

of N atoms into the WOx lattice by forming the W-N bond. It is revealed that the excellent

lithium-ion storage performance of the N-WOx is attributed to the coordinated nitrogen atoms

that serve as the extra sites for the lithium-ion storage while at the same time boost the charge

transfer, thus enhances the storage kinetics of lithium-ion.[90] In addition to the non-metal atoms,

atomic metal species such as Pt is also used to stabilize and improve the catalytic performance

18

of CoSe2 with selenium (Se) defects.[91] The Pt species were refilled to the Ar plasma produced

Se vacancies under ultraviolet irradiation conditions. The refilling of atomic Pt species into the

Se defects of CoSe2 and the formation of the Pt-Co-Se coordination (CoSe2-x-Pt) is proved by

the HAADF-STEM and XAS characterizations. The exceptional high OER performance of the

CoSe2-x-Pt sample is presented in Figure 9h, in terms of overpotential, mass activity and specific

activity. The theoretical simulations uncover that the highly asymmetrical electronic

distribution on the defective CoSe2 induced by the filling of the atomic Pt species contributes

to the outstanding OER activity of the CoSe2-x-Pt sample.

3. Cation Defects in Transition Metal Composites for Electrocatalysis

Engineering the surface electronic configurations of electrocatalysts to improve their catalytic

performance via introducing cation defects is a promising and effective strategy. Compared to

the creation of anion defects in transition-metal-based composites, the higher formation energy

makes it more challenging to controllably produce cation vacancies in metal compounds. The

methods used to create cation defects in metal complexes include in-situ formation,[93-96]

thermal calcination,[19, 97] chemical/electrochemical etching,[98-100] and plasma treatment.[75, 101,

102] In this section, the influence of single type of cation defects such as Co, Fe and Ni vacancies,

and multivacancies such as oxygen, Fe and Co vacancies on the electrochemical reactions will

be discussed. The commonly used methods to generate cation defects in transition metal-based

electrocatalysts and the characterization techniques of defects are summarized in Table 1.

3.1 Single Type of Cation Vacancies

Cation defects are suggested to be effective in improving the intrinsic electrocatalytic activity

of 3d transition metal (Co, Fe and Ni) compounds by modulating the electronic structures and

increasing the number of active sites. For example, Xie et al. introduced Co vacancies into 2D

ultrathin CoSe2 nanosheets during exfoliating the lamellar CoSe2/DETA (diethylenetriamine)

19

through the ultrasonic treatment (Figure 10a).[93] Positron annihilation spectrum (PAS) is a

powerful technique to study the type and relative concentration of defects in materials.[103, 104]

The positron lifetime spectra in Figure 10b and the corresponding X-ray absorption fine

structure spectroscopy of the synthesized samples clearly show the presence of Co vacancies in

the ultrathin CoSe2 nanosheets. The OER polarization curves in Figure 10c suggest that the

ultrathin CoSe2 nanosheets with Co vacancies exhibit much higher current density and lower

overpotential than those of the bulk CoS2 and CoSe2/DETA samples. The first-principles

calculations indicate that the created Co vacancies on the surface of the ultrathin CoSe2

nanosheets are the OER active sites that could increase the adsorption energy of H2O molecules,

thus contributes to the excellent OER performance of the defective CoS2 sample.[93] Meanwhile,

Co defects can also be produced in Co compounds such as Co3O4 by the thermal calcination

method (Figure 10d).[19] The Co defect containing samples (Co-300, Co-500 and Co-700) were

synthesized by treating the precursor glycerolatocobalt(II) (CoGly) at different temperatures.

The EXAFS studies show that the Co-300 sample has low Co coordination number, implying

its rich Co defects in the crystals. The high shorter lifetime (230.5−280.2 ps) of the Co-300

detected by the positron annihilation lifetime spectra also reflects that it has a large amount of

single metal vacancies. The calculated Co defect concentration for the Co-300 is 8.7%, which

is higher than that of the Co-500 (4.3%) and Co-700 (0.1%). It means that the higher treatment

temperature will lead to the reconstruction of the crystal lattice and reduce the density of the

Co defects.[105] The established crystal structures of the normal and defective Co3O4 crystals,

and the partial charge density in Figure 10d and e suggest that the Co vacancies cause distinct

electronic delocalization, leading to the faster carrier transport to participate in the

electrocatalytic reactions along the defective conducting channels. The experimental results

agree well with the theoretical predictions, showing that the Co-300 with abundant Co defects

exhibits the best OER performance among the fabricated electrocatalysts (Figure 10f).[19]

20

Transition metal-based layered double hydroxides (LDHs) have attracted extensive research

attentions for electrocatalysis owing to their special electronic structures and the adjustable

chemical component.[106] Further improving the instinct activities of LDHs by introducing

cation defects is proved to be feasible. For instance, the Fe and Ni defects were successfully

generated in the NiFeAl LDHs and NiZnFe LDHs nanosheets via a strong alkali electrolyte

etch method, the resulting defective samples are NiFe LDHs-VFe and NiFe LDHs-VNi,

respectively (Figure 11a).[99] The electrocatalytic performance test results in Figure 11b show

the crucial roles of Fe and Ni defects in prompting the OER of the NiFe LDHs, and Ni vacancies

are more efficient than Fe defects for catalyzing the OER. The calculated density of states (DOS)

in Figure 11c suggest that the introduction of Fe or Ni defects into the NiFe LDHs could narrow

the bandgap near the Fermi level, leading to the enhanced electrical conductivity. It also

indicates that the Fe and Ni vacancies could boost the adsorption of the reaction intermediates,

accordingly improves the OER performance.[99]

Similar to the anion defects, the control of the cation defects concentration is also an important

way of optimizing the catalytic performance of transition metal-based electrocatalysts. For

example, the synthesis of spinel-type FeNi2O4 (FNO) with adjustable density of Fe defects (VFe-

FNO) is realized by an electrochemical reduction etching method.[98] The defective FNO with

different concentrations of Fe vacancies can be fabricated through altering the cathodic

potential or tuning the etching time (Figure 11d). The produced reactive Fe species during the

cathodic reaction can be etched in 1.0 M KOH (Figure 11e)[107], thereby Fe defects are created

in the spinel iron nickel oxide. Both the scanning transmission electron microscopy (STEM)

and aberration-corrected TEM images confirm the presence of vacancies in the VFe-FNO. It is

shown that the FNO with a moderate concentration of Fe defects (synthesized at −0.58 V (vs.

RHE) and 1500 s) exhibits the best OER performance. The optimized defective sample VFe-

FNO displays much higher OER activity than that of the defect-free FNO and the commercial

21

RuO2 (Figure 11f). The excellent electrical conductivity (Figure 11g), increased number of

active sites and the optimized surface electronic structure induced by the Fe vacancies together

facilitate the OER of the spinel iron nickel oxide.[98]

3.2 Multivacancies

In view of both anion and cation defects could modulate the surface electronic configurations

of transition metal compounds, creating multivacancies may further optimize the electronic

structures and increase the active sites density of the electrocatalysts with unexpected high

catalytic performance. In this regard, Wang et al. fabricated ultrathin CoFe LDHs nanosheets

with multivacancies using water-plasma-enabled exfoliation (Figure 12a).[102] The water

plasma not only efficiently exfoliated the stacked LDHs, but also simultaneously produced

different defects such as O, Fe and Co vacancies in the resulting CoFe LDHs nanosheets. The

high-resolution TEM image of the exfoliated LDHs shows atom-sized pores, which should be

the water plasma induced atomic vacancies. The lower Co−OOH coordination number of the

ultrathin CoFe LDHs nanosheets than that of the pristine CoFe LDHs shown in the Co K-edge

Fourier-transformed EXAFS suggests the presence of the oxygen vacancies in the exfoliated

sample. Besides, the smaller coordination number of the Co−M (M=Co or Fe) in the exfoliated

LDHs than that of the original LDHs indicates the formation of metal vacancies (Co and Fe

defects), and the defective LDHs have more metal vacancies than the oxygen vacancies (Figure

12b). The improved OER performance of the exfoliated LDHs nanosheets shows the

advantages of the multivacancies in boosting the OER from the aspects of regulating the

electronic structures and increasing the number of active sites.[108, 109]

Furthermore, the introduction of two kinds of cation defects (Fe and Co vacancies) into

Co0.5Fe0.5OOH was achieved via a fast reduction and in-situ phase transformation method

(Figure 12d).[110] The normalized Co K-edge XANES in Figure 12e suggest the incorporation

of the Fe resulted in the disordered octahedral coordination near the Co sites in the fabricated

22

nanometer-sized ultrathin Co0.5Fe0.5OOH platelets (Co0.5Fe0.5OOH-NSUPs). In addition, the

co-existence of the Fe and Co defects in the Co0.5Fe0.5OOH-NSUPs sample is proved by the

corresponding Fourier transform EXAFS results. The superior OER performance of the

defective-rich sample is reflected by the high current density and smaller overpotential (Figure

12f). The calculated free energy change profiles of the intermediates and product in Figure 12g

suggest that the presence of the cation defects in the Co0.5Fe0.5OOH-NSUPs could greatly

reduce the energy barriers of the reaction intermediates and facilitate the release of the produced

oxygen, thus improving its intrinsic OER activity.[110]

4. Summary and Perspectives

In this review, we have systematically summarized the controllable synthesis of anion and

cation defects in transition metal compounds for energy related electrocatalysis. The generally

used methods for creating anion defects (such as O, S and P vacancies) and cation defects (such

as Fe, Co and Ni vacancies) are discussed in detail, including NaBH4 and hydrogen reduction,

plasma irradiation, thermal annealing, and chemical/electrochemical etching. Recent studies on

controlling the density of the defects in metal composites are summarized as well, showing that

a moderate concentration of defects could lower the charge transfer resistance and reduce the

kinetic barrier, which will be beneficial for the electrocatalytic reactions. In view of the anion

defects are not stable under strong oxidizing environment, related investigations on stabilizing

anion vacancies, for example, through refilling heteroatoms (such as S, P and N) or atomic

metal species (such as Pt) are presented. The refilling is also an effective strategy to further

enhance the catalytic performance of the defective catalysts. Apart from the single type of

defects, the influence of multivacancies, including both anion and cation defects and multiple

cation vacancies on electrocatalysis are reviewed as well. The techniques used to characterize

defects include X-ray photoelectron spectroscopy (XPS), high-resolution TEM, X-ray

23

absorption spectroscopy (XAS), photoluminescence (PL) spectra, electron spin resonance (ESR)

and positron annihilation spectrum (PAS). These advanced characterization tools are beneficial

in uncovering the structure-property relationships of the electrocatalysts. Both experimental and

theoretical studies confirm that the introduction of anion and cation defects into transition

metal-based composites could modulate their electronic structures and surface properties,

which is also capable of increasing the number of active sites, thus could optimize the catalytic

performance. For instance, defect engineering can be used to promote the oxygen evolution

reaction (OER), hydrogen evolution reaction (HER), oxygen reduction reaction (ORR),

electrochemical nitrogen reduction reaction (NRR), and lithium-ion storage performance.

Although defect engineering in transition metal-based materials has been robustly developed in

the past decade, further work should be focused on the following aspects.

First, create more types of defects through simple and cost-effective methods. At present, only

a few anion and cation defects have been produced in metal complexes, so it is of great

significance to generate anion and cation vacancies in a variety of materials to broaden their

applications in different electrocatalytic reactions. In addition, the creation of anion and cation

defects in noble metal-based electrocatalysts is rarely reported. If the desired defects can be

produced in noble metals such as in Pt, Pd, Ru and Ir to improve the intrinsic activity, the usage

amount can be reduced, which is also a feasible method to lower the catalysts cost without

compromising the catalytic performance.

Second, precisely control the defects density and distributions. Although recent studies have

been carried out to control the concentrations of both anion and cation vacancies in metal

compounds, it is still a great challenge to accurately synthesize defective electrocatalysts with

the designed density of defects. In addition, the distributions of the generated defects cannot be

controlled by the currently developed synthetic methods. It is therefore highly demand to

exploit novel methods to precisely control the concentrations and distributions of defects in a

24

wide range of materials, which will be beneficial to quantitively probe the correlation between

the defects and the electrocatalytic performance.

Third, advanced in-situ characterizations and machine learning. The in-situ characterizations

such as in-situ TEM, Raman and XAS are capable of revealing the changes of defects during

the electrocatalytic process, which will be helpful for gaining in-depth understandings on the

reaction mechanisms. Furthermore, machine learning as a powerful technique can be used to

guide the rational design of defective electrocatalysts by predicting the optimal compositions.

For example, the combination of different defects (such as integrating anion/cation vacancies

with low-coordinated defects or atomic-metal-species defects) in multi-metal electrocatalysts

to optimize their catalytic performance can be predicted through comprehensive machine

learning algorithms. In addition, machine learning can also be applied to acquire fundamental

understandings on the formation and stabilization mechanisms of defects in metal compounds.

Finally, fabrication of defective metal-based electrocatalysts at large scales for practical

applications. Currently, the synthesis of defects promoted electrocatalysts is limited to lab

scales. It is crucial to exploit cost-effective and scalable methods to prepare highly active

defective electrocatalysts to accelerate their applications in real energy-related devices.

Conflict of Interest

The authors declare no conflict of interest.

Acknowledgements

The authors would like to thank Australian Research Council (ARC DP200103043) for

financial support during the course of this work.

Received: ((will be filled in by the editorial staff))

Revised: ((will be filled in by the editorial staff))

Published online: ((will be filled in by the editorial staff))

25

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31

Figures

Figure 1. Schematic illustration of the anion and cation defects in metal compound.

Reproduced with permission.[22] Copyright 2018, Wiley-VCH.

32

Figure 2. a) Schematic diagram of the preparation of Fe1Co1-ONS and Fe1Co1-ONP. b) LSV

curves of Fe1Co1-ONS, Fe1Co1-ONP and RuO2 measured in a 0.1 M KOH solution. c) Fitted

XPS O 1s spectrum for Fe1Co1-ONS. d) Fitted XPS O 1s spectrum for Fe1Co1-ONP. e) Nyquist

plots of OER on Fe1Co1-ONS and Fe1Co1-ONP. Reproduced with permission.[18] Copyright

2017, Wiley-VCH. f) Schematic of the NaBH4 reduction for in-situ creation of oxygen

vacancies in Co3O4 NWs for efficient catalysis of oxygen evolution reaction and enhanced

supercapacitor capacitance. g) Water oxidation currents of the reduced Co3O4 NWs (red curve),

pristine Co3O4 NWs (blue curve), IrOx (brown curve) and Pt/C (black curve) at 5 mV s−1.

Reproduced with permission.[28] Copyright 2014, Wiley-VCH. h) iR-corrected data of

Ca2Mn2O5/C, CaMnO3/C, and Vulcan carbon XC-72. i) Mass activities at various applied

potentials. j) Unit cell structures of CaMnO3 (left) and Ca2Mn2O5 (right). Reproduced with

permission.[30] Copyright 2014, American Chemical Society.

33

Figure 3. a) Illustration of the preparation of o-CoSe2-O UNs from c-CoSe2/DETA by Ar/O2

plasma. b) High-resolution XPS O 1s spectra of c-CoSe2/DETA and o-CoSe2-O UNs. c) Co K-

edge XANES spectra for c-CoSe2/DETA, o-CoSe2-O UNs, and CoO. CoO was used as a

reference (right: the magnified images). d) Co K-edge extended XANES oscillation functions

k3χ(k). e) LSV curves of the c-CoSe2/DETA, c-Ar-CoSe2 UNs, o-CoSe2-O UNs, and the

commercial RuO2. Reproduced with permission.[41] Copyright 2018, Wiley-VCH. f) Illustration

of the preparation of the Ar-plasma-engraved Co3O4 with oxygen vacancies and high surface

area. g) SEM image of the pristine Co3O4. h) SEM image of the Ar-plasma engraved Co3O4. i)

Polarization curves of OER on pristine Co3O4 (0 s) and the plasma engraved Co3O4 (120 s). j)

Nyquist plots of OER on pristine Co3O4 and the plasma engraved Co3O4. Reproduced with

permission.[42] Copyright 2016, Wiley-VCH.

34

Figure 4. a) Schematic illustration of the adsorption of H2O molecules onto the spinel structure

and the partial charge density of NiCo2O4 with oxygen vacancies. b) XPS O 1s spectra of the

samples. c) Polarization curves of the various NiCo2O4 samples. Inset: enlargement of the

region near the onset. Reproduced with permission.[44] Copyright 2015, Wiley-VCH. d)

Schematic illustration for the formation of VO-rich/poor atomically thin In2O3 porous sheets at

different atmosphere. e) Room-temperature photoluminescence spectra of the samples. f)

Electron spin resonance spectra of the samples. g) Photocurrent vs applied potential curves

under 300 W Xe lamp irradiation (λ > 420 nm). Reproduced with permission.[45] Copyright

2014, American Chemical Society.

35

Figure 5. a) Schematic illustration of the hydrogenation process for the creation of oxygen

vacancies in Fe1Co1Ox-origin. b) High-resolution TEM image of Fe1Co1Ox-200 °C-2.0 MPa; c)

Magnified image of the defect region in (b). d) and e) Fitted XPS O 1s and Co 2p spectra of

Fe1Co1Ox-origin and Fe1Co1Ox-200 °C-z samples; (f) Room-temperature PL spectra of

Fe1Co1Ox-origin and Fe1Co1Ox-200 °C-z samples. g) OER linear sweeping voltammetry curves

of Fe1Co1Ox-origin and Fe1Co1Ox-200 °C-z samples in 1.0 M KOH. h) Nyquist plots for OER

on Fe1Co1Ox-origin and Fe1Co1Ox-200 °C-z samples. Reproduced with permission.[49]

Copyright 2018, Springer Nature. i) Structure of rutile-type MnO2 with oxygen vacancies. j)

HRTEM image of the β-MnO2 heated in Ar (inset shows the FFT patterns). k) Energy profiles

and configurations of ORR on β-MnO2 surfaces with one (Path I) and two (Path II) oxygen

vacancies. Asterisks denote adsorbed species. TS=transition state. Reproduced with

permission.[50] Copyright 2013, Wiley-VCH. l) Schematic plot of controllably introducing Ov

into mesoporous Co3O4 NSs aggregation by P-doping strategy. m) Linear polarization curves

of the samples. Reproduced with permission.[52] Copyright 2018, Elsevier.

36

Figure 6. a) Schematic illustration of monolayer MoS2 synthesis. b) HRTEM image of the

monolayer MoS2 (S vacancies are highlighted by red circles. Right B: Relaxed atomic model

of the top views of monolayer MoS2 with one S-vacancy; C: Relaxed atomic model of the top

views of monolayer MoS2 with three S-vacancies). Blue and yellow balls indicate Mo and S

atoms, respectively. c) Polarization curves of different MoS2 samples tested in 0.5 M H2SO4

without iR compensation. d) Nyquist plots of different MoS2 samples. Reproduced with

permission.[67] Copyright 2016, The Royal Society of Chemistry. e) High-resolution TEM

image of an ultrathin layer of defected MoS2-7H. The hexagonal symmetry of 2H MoS2 can be

identified. Examples of sulfur vacancies are highlighted by red circles. f) Evolution of the S:Mo

and Mo(UC):Mo(IV) ratios as a function of the annealing temperature of the electrodes. The

two domains of point defects and S stripping are shaded in orange and red, respectively. Inset:

Evolution of the Mo(UC):Mo(IV) as a function of the S:Mo ratio. Reproduced with

permission.[68] Copyright 2019, American Chemical Society. g) XPS S 2p peaks of pristine

MoS2 (P-MoS2, upper curves) and MoS2 with S-vacancies (V-MoS2, lower curves). The filled

symbols are measured data, and the solid lines are the parametric fits. h) LSV curves of

monolayer MoS2 before and after desulfurization, respectively. Reproduced with permission.[70]

Copyright 2017, Springer Nature.

37

Figure 7. a) Scheme for the preparation of Co3S4 PNSvac. b) HAADF image and the associated

simulative structure (inset in b) of Co3S4 PNSvac. c) Mass activity of different samples as a

function of overpotential. d) Density of states for Co3S4 PNSvac, Co3S4 NS, and Co3S4 NP (EF,

Fermi level). Reproduced with permission.[76] Copyright 2018, American Chemical Society. e)

ESR spectra for N-doped CoS2 catalysts. f) LSV polarization curves of pure CoS2, N-doped

CoS2, and platinum in 0.5 M H2SO4. g) HER free energy diagram for different Co sites. h) HER

free energy diagram for different Ns sites. The insets are the Ns-Vs-CoS2 structures with H

adsorption on the Co site and Ns site, respectively. Reproduced with permission.[20] Copyright

2017, American Chemical Society.

38

Figure 8. a) Fabricating procedure and structure diagram of Ni2P@CC and Vp-Ni2P@CC

nanosheets. b) P 2p XPS spectrum of Vp-Ni2P@CC and Ni2P@CC. c) Photoluminescence (PL)

spectra of Vp-Ni2P@CC and Ni2P@CC. d) HRTEM image of Vp-Ni2P@CC. Reproduced with

permission.[81] Copyright 2020, Elsevier. e) Illustration of the process of phosphorus vacancy

generation and surface reaction of the Ar-plasma-treated Co0.68Fe0.32P. f) Polarization curves of

the samples tested in 1.0 M KOH solution. Reproduced with permission.[87] Copyright 2020,

The Royal Society of Chemistry. g) Free-energy diagrams of nitrogen reduction reaction on the

surface vacancies of RO-Cu3P catalysts with different concentrations of doped oxygen.

Reproduced with permission.[84] Copyright 2020, The Royal Society of Chemistry.

39

Figure 9. a) Schematic illustration of the VO creation and S atoms modification processes on

FeCoOx nanosheets. b) OER polarization curves of the samples in 1.0 M KOH to reach a high

OER current density. Reproduced with permission.[89] Copyright 2020, Wiley-VCH. c) Co K-

edge XANES spectra of pristine, VO-Co3O4, and P-Co3O4. Top inset shows the magnification

of the main peak region. Bottom insets show the magnification of the pre-peak region. d)

Computed density of states of pristine Co3O4, VO-Co3O4, and P-Co3O4 systems. The green

dashed lines denote Fermi level. Reproduced with permission.[88] Copyright 2017, The Royal

Society of Chemistry. e) Schematic illustration of the synthesis process for WOx and N-WOx

from the TALP/graphene precursor. f) Electron paramagnetic resonance spectra of WOx, N-

WOx, and WO3. g) Comparison of EXAFS data at W L3-edge in R-space of as-prepared WOx,

N-WOx, and WO3. Reproduced with permission.[90] Copyright 2019, Wiley-VCH. h)

Comparisons of overpotentials to reach a current density of 10 mA cm−2, Tafel slope, EIS at a

potential of 0.65 V versus Ag/AgCl, ECSA, mass activity, and specific activity at an

overpotential of 300 mV among CoSe2-origin, CoSe2−x, and CoSe2−x-Pt, and the inset shows

the radar chart comparing the activity between CoSe2−x-Pt and RuO2 in 1.0 m KOH.

Reproduced with permission.[91] Copyright 2019, Wiley-VCH.

40

Figure 10. a) Schematic of the formation of Co vacancies in CoSe2 ultrathin nanosheets. b)

Positron lifetime spectra of ultrathin CoSe2 nanosheets and bulk CoSe2. c) Linear sweep

voltammetry curves of the synthesized samples tested in 0.1 M KOH. Reproduced with

permission.[93] Copyright 2014, American Chemical Society. d) Schematic formation of Co

vacancies in Co3−xO4 via the thermal calcination of CoGly precursor and the established cell

structures. e) Optimized cell structures and the corresponding charge density mapping of Co-

defected Co3−xO4. f) OER polarization curves of the samples. Reproduced with permission.[19]

Copyright 2018, American Chemical Society.

41

Figure 11. a) The synthesis of NiFe LDHs-VFe and NiFe LDHs-VNi by strong alkali etching

LDHs. b) LSV curves of NiFe LDHs, NiFe LDHs-VFe, and NiFe LDHs-VNi nanosheets. c)

Density of states of the constructed three structures. Reproduced with permission.[99] Copyright

2018, Wiley-VCH. d) Schematic illustration of the synthesis of Fe-vacant FeNi2O4 (VFe-FNO).

Working, counter, and reference electrodes are abbreviated as WE, CE, and RE, respectively.

(e) Voltammetry of FNO in 1.0 M KOH at a scanning rate of 5 mV s−1. f) OER polarization

curves of FNO, VFe-FNO, RuO2, CP, and CP-R tested in 1.0 M KOH with iR-correction. g)

Nyquist plots obtained at applied potential of 1.52 V versus RHE. Reproduced with

permission.[98] Copyright 2020, Chinese Chemical Society.

42

Figure 12. a) Schematic illustration of the water-plasma-enabled exfoliation of CoFe LDH

nanosheets. The dielectric barrier discharge (DBD) plasma reactor is designed with the plate-

to-plate electrode at 50 V powered by the AC high voltage generator. b) Co K-edge FT-EXAFS

for pristine and water-plasma exfoliated CoFe LDH nanosheets with homologous curve-fitting

results. c) LSV curves for OER on pristine CoFe LDHs and the water-plasma exfoliated CoFe

LDH nanosheets. Reproduced with permission.[102] Copyright 2017, Wiley-VCH. d) Scheme

for the fabrication of Co0.5Fe0.5OOH-NSUPs. e) XANES of Co K-edge in CoOOH-MSUPs and

Co0.5Fe0.5OOH-NSUPs. f) LSV curves of various samples after 90% iR-correction. g) Free

energy change profiles of intermediates and product on CoOOH and Co0.5Fe0.5OOH (001) slab.

Reproduced with permission.[110] Copyright 2019, American Chemical Society.

43

Table 1. Summary of the creation methods and characterization techniques of defects in metal

compounds.

Defect Creation Methods Electrocatalysts Defect Type Defect Characterization Techniques

References

Reduction Treatment FeCo oxide nanosheets O vacancies XPS, PL spectra [18]

Mesoporous Co3O4 nanowires

O vacancies XPS [28]

Ni2P nanosheets P vacancies XPS, PL spectra, EPR spectra [81]

Ultrathin Co0.5Fe0.5OOH

Fe and Co vacancies

XPS, XAS [110]

Plasma Irradiation CoSe2 ultrathin nanosheets

O vacancies XPS, XAS [41]

Co3O4 nanosheets O vacancies XPS [42]

Co3S4 nanosheets S vacancies HAADF-STEM, EPR spectra, XAS, XPS

[76]

Co0.68Fe0.32P-60 P vacancies XPS, XAS [87]

Thermal Annealing NiCo2O4 O vacancies XPS [44]

VO-rich In2O3 O vacancies XPS, PL spectra, ESR spectra [45]

MoS2 nanosheets S vacancies HRTEM, EPR spectra, XPS [68]

N-doped CoS2 S vacancies XAS, ESR spectra [20]

Co3−xO4 Co vacancies XAS, PAS [19]

Exfoliation Monolayer MoS2 S vacancies HRTEM, XPS [67]

CoSe2 nanosheets Co vacancies PAS, XAS [93]

CoFe LDHs nanosheets

O, Fe and Co vacancies

HRTEM, XAS [102]

Chemical/Electrochemical Etching

MoS2 S vacancies XPS [70]

NiFe LDHs-VFe Fe vacancies XPS [99]

NiFe LDHs-VNi Ni vacancies XPS [99]

FeNi2O4 Fe vacancies STEM, XAS, EPR spectra [98]

44

Photographs and Biographies of the Authors

(1) Xuecheng Yan

Xuecheng Yan was awarded his PhD degree in Materials Science at Queensland Micro- and

Nanotechnology Centre (QMNC) of Griffith University (Australia) in 2016, under the

supervision of Prof. Xiangdong Yao. Afterward, he worked as a Research Assistant from March

2016 to March 2017 at QMNC. Currently, he is a postdoc Research Fellow in Prof. Yao’ group.

His research interests focus on the design and fabrication of efficient and economical catalysts

for electrocatalysis, such as synthesizing defective electrocatalysts for fuel cell and water

splitting applications.

(2) Xiangdong Yao

Xiangdong Yao received his PhD degree in Materials Engineering at the University of

Queensland (UQ, Australia) in 2005. He joined the ARC Centre of Excellence for Functional

Nanomaterials at UQ since 2003, working on nanomaterials for hydrogen storage. He joined

Griffith University in 2009 as an Associate Professor and then was promoted to a full Professor

in 2013. He is the group leader of advanced energy materials at QMNC of Griffith University.

Prof. Yao has made a number of significant contributions to the area of energy materials and

he has been published over 200 articles in peer-reviewed journals.

45

The Table of Contents Entry

Defect engineering such as introduce anion and cation vacancies into transition metal-based

materials is a feasible and effective strategy to regulate their electronic structures and surface

properties. This review summarizes recent studies on controllably creating the anion (such as

O, S and P) and cation (such as Fe, Co and Ni) vacancies for optimizing the electrocatalytic

performance of metal-based compounds.

X. Yan, Y. Jia, X. Yao*

Defective Structures in Metal Compounds for Energy-Related Electrocatalysis

ToC figure