Defect‐Derived Catalysis Mechanism of Electrochemical Reactions in Two‐Dimensional Carbon Materials

In the past decades, remarkable progress has been achieved in the exploration of electrocatalysts with high activity, long durability, and low cost. Among these, defective graphene (DG)‐based catalysts are considered as one of the most potential substitutes for precious metal‐based electrocatalysts. DG‐based catalysts contain abundant active centers with different configurations resulting from their extraordinary high‐structural tunability. Herein, an overview on recent advancements in developing four kinds of DG‐based catalysts is presented: 1) heteroatoms‐doped graphene; 2) intrinsic DG (vacancy and topological defect); 3) nonmetal atoms or/and metal species‐modified intrinsic DG (heterogeneous species and intrinsic defects co‐tuned DG); and 4) DG‐based van der Waals‐type multilayered heterostructures. In particular, the synergistic effects between various defects are discussed, and the origin of catalytic activity is reviewed. Meanwhile, the established defect‐derived catalytic mechanism is summarized, which is beneficial for the rational design and fabrication of high‐performance electrocatalysts for practical energy‐related applications. Finally, challenges and future research directions on defect engineering in noble metal‐free materials for electrocatalysis are proposed.


Introduction
The ever-increasing environmental pollution and energy crisis caused by the excessive use of fossil fuels force us to develop green and sustainable energy sources. [1]Up to now, great efforts have been devoted to investigating the production and practical application of renewable clean energy.Hydrogen has been considered as one of the most promising alternatives for depleting fossil fuels. [2]Meanwhile, carbon capture reactions such as carbon dioxide reduction, can effectively alleviate the deteriorating greenhouse effect.Hence, developing energy-related electrochemical reactions, such as hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) for water splitting, oxygen reduction reaction (ORR) for fuel cell, and carbon dioxide reduction reaction (CO 2 RR), plays a decisive role in resolving these severe environmental concerns. [3]Nevertheless, noble-metal-based catalysts such as platinum and ruthenium are still irreplaceable in electrocatalysis, where the high cost and scarcity of them seriously hinder their widespread applications. [4]In recent years, increasing attention has been devoted to exploiting low cost and efficient alternatives, including 1) single-atom catalysts (SACs), which maximize the atomic efficiency by downsizing the Pt and Ru particles; [5] 2) reducing the utilization of noble metals by alloying them with non-noble metals or completely replacing them by non-noble metal-based catalysts; [6] and 3) developing metal-free catalysts. [7]Among them, carbon-based materials have been regarded as one of the most promising candidates on boosting the scalable applications of electrocatalytic energy conversion techniques. [8]wo-dimensional (2D) nanomaterials are believed to be excellent electrocatalysts because of their unique structural In the past decades, remarkable progress has been achieved in the exploration of electrocatalysts with high activity, long durability, and low cost.Among these, defective graphene (DG)-based catalysts are considered as one of the most potential substitutes for precious metal-based electrocatalysts.DG-based catalysts contain abundant active centers with different configurations resulting from their extraordinary high-structural tunability.Herein, an overview on recent advancements in developing four kinds of DG-based catalysts is presented: 1) heteroatoms-doped graphene; 2) intrinsic DG (vacancy and topological defect); 3) nonmetal atoms or/and metal species-modified intrinsic DG (heterogeneous species and intrinsic defects co-tuned DG); and 4) DG-based van der Waals-type multilayered heterostructures.In particular, the synergistic effects between various defects are discussed, and the origin of catalytic activity is reviewed.Meanwhile, the established defect-derived catalytic mechanism is summarized, which is beneficial for the rational design and fabrication of high-performance electrocatalysts for practical energy-related applications.Finally, challenges and future research directions on defect engineering in noble metal-free materials for electrocatalysis are proposed.characteristics, rich surface chemistry, and tunable electronic properties. [9]2D transition metal dichalcogenides (TMDs), [10] layered metal hydroxides (LMHs), [11] layered metal oxides (LMOs), [12] MXenes, [13] 2D metal-organic frameworks (MOFs), [14] 2D covalent organic frameworks, [15] and graphenebased electrocatalysts [16] have been successfully synthesized and applied to promote the catalytic activity of water splitting, fuel cells, and CO 2 RR.Compared to their bulk counterparts, 2D materials possess an extremely high lateral-area-to-thickness ratio, which can enhance their electrocatalytic activity through three aspects.First, an exotic electronic state, which is resulted from the loss of neighboring layers.This characteristic also means that the electrocatalytic properties of 2D electrocatalysts can be tailored during synthesis processes. [17]Second, abundant active bonding sites, which are chemically active and can directly interact with reactant intermediates to enhance the electrocatalytic activity. [18]At last, lattice distortion-induced lattice strain and abundant surface defects, altering the local chemical environment can change the electronic states of a material. [19]articularly, defects have been widely regarded as the origins of electrocatalytic activity, so defective 2D nanomaterials have attracted tremendous attention and great achievements have been attained in the field of electrocatalysis.
During the past decade, remarkable research work has been concentrated on developing defective graphene (DG)-based electrocatalysts because of their unique properties such as tunable molecular structures, spatial confinement and surface effect, abundance and excellent oxidation, and corrosion resistance. [20]Initially, it was reported that heteroatom doping such as N, B, and S can improve the catalytic performance of graphene. [21]However, the origin of the electrocatalytic activity in heteroatom-modified graphene is still controversial.For example, some researchers stated that the introduced heteroatoms were the active sites in the doped graphene structures. [22]However, others claimed that the defect structures caused by the incorporated foreign elements were the real active centers, and heteroatoms were mainly served as a channel to redistribute the electronic environment of pristine graphene. [23]20a,20b] Thus, a defect-derived catalytic mechanism has been established through a series of studies, which will be summarized in this review.Subsequently, Yao et al. proposed a concept of complex defects, [16a] taking the advantage of synergistic effect between heteroatoms/isolated metal species and defects, the electrocatalytic activity of intrinsic defective graphene (IDG), and the stability of heteroatoms (such as atomic metal atoms) were highly boosted.Simultaneously, IDG can also serve as an ideal substrate to fabricate efficient electrocatalysts.With strong interlayer electron coupling, the DG-based van der Waals-type multilayered electrocatalysts shed light on more efficient electrocatalytic reactions.
7a,9,16,20,24] In comparison, this review inherits the classification method of defective graphene, and four general structures of DG-based electrocatalysts will be summarized, including heteroatoms-doped graphene, intrinsic defective graphene, complex defective graphene, and multilayered DG-based electrocatalysts (interfacial effect).The representative configurations of the four kinds of defective carbon-based catalysts are shown in Figure 1.First, the typical heteroatom-modified graphene, such as N-, S-, B-, P-, and F-induced graphene will be presented briefly.Second, the investigations of IDG electrocatalysts will be summarized (Figure 2).Third, complex defects such as heteroatoms-integrated IDG and metal clusters-coupled IDG will be introduced.Finally, for the first time, DG-based interfacial structures will be included into complex defects, and interfacial synergistic effect, a new strategy to further regulate the electronic distribution of carbon atoms and boost IDG's electrocatalytic activity will be introduced.Moreover, the defect-derived catalytic mechanism is exclusively reviewed, directing the method of establishing a new comprehensive theoretical system.Importantly, this review will focus on uncovering the relationship between defective structures and catalytic performance by addressing the unique electrochemical properties of different defective structures.The most recent development of DG-based catalysts will be summarized as well.All in all, this review will provide an overview of this newly developed defect electrocatalysis, and the key challenges and future opportunities in this field will be outlooked.

Heteroatoms-Doped Graphene as Metal-Free Electrocatalysts
Carbon materials such as graphene can be easily modified by heteroatoms because of their unique physical and chemical Reproduced with permission. [60]Copyright 2020, Elsevier.Reproduced with permission. [54]Copyright 2016, John Wiley and Sons.Reproduced with permission. [100]Copyright 2018, John Wiley and Sons.Reproduced with permission. [101]Copyright 2018, John Wiley and Sons.Reproduced with permission. [85]Copyright 2022, American Chemical Society.Reproduced with permission. [98]Copyright 2022, American Chemical Society.
properties.For example, the ORR activity of different heteroatoms (such as N, O, P, B, and S) doped graphene catalysts has been systematically studied by integrating experiments and density functional theory (DFT) computations. [25]It is demonstrated that graphene-based metal-free catalysts have the potential to outperform the ORR performance of the benchmark Pt catalyst.

Nitrogen-and Boron-Doped Graphene
Nitrogen is an electron-rich and electronegativity-strong element (3.04, compared with carbon: 2.55), nitrogen doping could greatly boost the electrocatalytic activity of graphene.This is because N can alter its electronic distribution and crystal structure, and enhance the chemical stability, surface polarity, electric conductivity, and electron-donor properties. [26]Previous reviews have summarized the excellent works on N-doped graphene, [7a,16b,27] so only a brief introduction is given here.As shown in Figure 3a, the four nitrogen species are pyridinic N, quaternary N, pyrrolic N, and pyridine-N-oxide.In 2010, Dai et al. fabricated NG by a chemical vapor deposition method using methane and ammonia as the reactants.22b] Figure 3b shows a digital photo of the flexible and transparent NG film, which contains only one or a few layers of graphite sheet.Figure 3c shows the atomic force microscopic (AFM) scanning image of the NG film, exhibiting a smooth surface with wrinkles because of its pliability.The electrochemical test results demonstrate that the NG electrode reduced oxygen in a direct four-electron pathway, and the highest steady catalytic current density is three times more than that of the Pt/C electrode over a wide potential range.Meanwhile, the NG electrode is more stable than the commercial Pt/C in alkaline fuel cells.
Besides heteroatom N, boron (B) atoms are also widely used to modify the electronic distribution and structure of graphene to enhance its electrocatalytic performance.B has a lower electron negativity (2.04) than that of C (2.55), which can cause charge redistribution around carbon atoms and facilitate the break of the stable sp 2 and π bonds of the aromatic carbons like N atoms. [28]For example, Valentin et al. discussed Langmuir-Hinschelwood and Eley-Rideal mechanisms during the ORR process on B-doped graphene (BG) catalysts by DFT calculations. [29]It is indicated that the intermediates and transition states (TS) are through the possible end-on and side-on reaction pathways.Additionally, the end-on dioxygen species, whereas in the case of pure graphene cannot be conceived, produced by the adsorption of molecular oxygen on the positively charged doped-B atom is the most important intermediate and reaction step.Besides, BG can also serve as an efficient catalyst for nitrogen reduction reaction (NRR). [30,31]The B site has an advantageous feature: a combination of reactive lone pair electrons and empty orbitals.Benefiting from these two types of orbitals, electron donation and back-donation can take place between the B site and N 2 , leading to the effective activation of the N≡N triple bond. [32]For example, by thermal anneal graphene oxide and boric acid, Zhang et al. successfully developed a kind of BG catalysts, containing doped-B configurations such as BC 3 , BC 2 O, BCO 2 . [30]Among them, BC 3 bond type was regarded as the key to boost the NRR activity of BG, mainly because the adsorption of N 2 is greatly enhanced by the positively charged B dopant (þ0.59 e, Figure 3d) with a bonding energy of 0.01 eV.Furthermore, in BC 2 O and BCO 2 , due to the higher electronegativity of oxygen than that of carbon and boron, the B atoms become more positive, enabling stronger B-N interaction with lower binding energies (À0.40 and À0.44 eV).However, the stronger N 2 binding capability is not beneficial to the release of the second NH 3 and thus increases the overpotential of NRR. Figure 3e shows the reaction pathway calculated by DFT for pure graphene and three BG structures, during which the BC 3 possesses the lowest overpotential (0.43 eV), indicating its best NRR performance.

Oxygen-Doped Graphene
As discussed above on the excellent work reported by Zhang et al., the doped oxygen atoms can strongly tune the charge distribution around carbon atoms due to their higher electronegativity (3.44) than those of carbon (2.55), boron (2.04), and nitrogen (3.04).Therefore, oxygen dopants are excellent electron acceptors, enabling positively charged carbon atoms as active sites.In recent years, the oxygen-doped carbon catalysts have gained extensive research attentions because of their high H 2 O 2 selectivity during the ORR. [33]However, as the general oxygen-doping methods will form multiple oxygen functional groups, such as C═O, C-OH, and COOH, the actual active sites for 2e À ORR are hard to confirm.Liu et al. synthesized an oxygen-doped carbon nanosheet (OCNS 900 ), which exhibits excellent 2e À ORR performance with an onset potential of 0.825 V (vs RHE), mass activity of 14.5 A g À1 at 0.75 V (vs RHE), and H 2 O 2 production rate of 770 mmol g À1 h À1 in a flow cell. [34]urthermore, the chemical titration strategy was performed to decipher the OCNS 900 to uncover the most active sites among the C═O, C-OH, and COOH.As shown in Figure 4a, PH, BA, and 2-BrPE were used as the titrants to facilitate the reaction with the C═O, C-OH, and COOH groups, respectively.Figure 4b presents the ORR performance of the titrated samples.Apparently, compared with OCNS 900 -BA and OCNS 900 -BrPE, the mass activity of OCNS 900 -PH shows a greater decrease from 14.5 to 2.7 A g À1 , and IR current also exhibits a larger reduction of 32%.At the same time, substantial increase of the charge transfer resistance (R ct ) was observed on the titrated C═O group, rather than C-OH, and COOH groups.Accordingly, the C═O, C-OH, and COOH groups all contribute to the production of H 2 O 2 but the C═O groups are the most active sites.
In addition, oxygen-doped graphene is also regarded as a promising candidate to effectively promote ambient electroreduction of N 2 to NH 3 , and remarkable achievements have been made. [35]35a] A large NH 3 yield of 21.3 μg h À1 mg cat À1 and a high Faradaic efficiency (FE) of 12.6% at À0.55 and À0.45 V (vs RHE) can be achieved.The C-O, C═O, and O═C-O functional groups were found in the experimentally synthesized catalyst.DFT calculations were further performed to investigate the adsorption structures for N 2 and related reaction mechanisms in three models (A, B, and C) based on the C-O, C═O, and O═C-O functional groups.There will be electrons transfer from carbon atoms to oxygen atom in all three models, leading to positively charged carbon and negatively charged oxygen atoms.However, model B (Figure 4d) and C (Figure 4e) possess lower overpotential (0.34 V) in the mixed mechanisms than c) AFM image of the N-graphene film.22b] Copyright 2010, American Chemical Society.d) Schematic illustration of NRR for BG and the atomic orbital of BC 3 for binding N 2 .e) Reaction pathways and the corresponding energy changes of NRR on BC 3 , BC 2 O, BCO 2 , and C, respectively.The dotted rectangular box indicates the steps that cannot take place.Reproduced with permission. [30]Copyright 2018, Elsevier.
that of model A in the alternative mechanism (Figure 4c).In general, all oxygen functional groups contribute to the NRR, but the C═O and O═C-O groups contribute the most to the excellent electrocatalytic activity in 2e-ORR and NRR, due to the formed charge enrichment carbon active sites by oxygen dopant in graphene.Reproduced with permission. [34]Copyright 2021, John Wiley and Sons.Slab models used to represent c) C-O (model A), d) C═O (model B), and e) O═C-O (model C) and related NRR mechanisms.Also shown are charge differences upon N 2 adsorption.35a] Copyright 2019, The Royal Society of Chemistry.

Other Heteroatom-Doped Graphene
21c,36] Huang et al. successfully fabricated SG by directly annealing graphene oxide and benzyl disulfide (BDS) in argon at a temperature range from 600 to 1050 °C. Figure 5a schematically illustrates the preparation of SG. [37] The LSV curves of different SG and commercial Pt/C can be seen in Figure 5b.Obviously, SG synthesized at 1050 °C (S-graphene-1050) shows superior ORR performance with more positive onset potential and higher limiting current density.Meanwhile, S-graphene-1050 exhibits high crossover effects tolerance, shown in Figure 5c.Notably, unlike N or B, which contain larger or smaller electronegativity than carbon, S hold a similar electronegativity with carbon (2.55 vs 2.58).However, doped-S atoms still can increase the ORR activity of graphene, suggesting alternative mechanisms need to be established to explain the origin of the catalytic activity.Accordingly, Xia et al. found that carbon atoms in SG are the real active sites for four-electron transfer ORR process through DFT calculations. [38]By investigating the four types of sulfur-doping structures, they confirmed that the spin and charge density of S or C atoms highly depend on their local position on the graphene, edge located S atoms with the highest charge density follow a two-electron transfer ORR pathway (Figure 5d) whereas the edge located carbon atoms with high spin and charge density serve as the real active sites to process a four-electron transfer reaction route (Figure 5e).Apparently, edge plays an irreplaceably important role in electrocatalysis.
Apart from N, B, and S heteroatoms, phosphorus (P) and halogens (i.e., fluorine (F), chlorine (Cl), bromine (Br), and iodine (I)) doped graphene samples also show improved electrocatalytic activity.For example, Wang et al. reported a Cl and F codoped graphene, fabricated by burning magnesium metal with Reproduced with permission. [37]Copyright 2012, American Chemical Society.ORR products when d) edge-doped S atom serves as active center, and e) edge-C atom serves as active center.Reproduced with permission. [38]Copyright 2014, American Chemical Society.
CCl 4 or CCl 3 F vapor, as efficient ORR catalysts. [39]The characterizations suggest that heteroatoms Cl and F located at the edge or defects of graphene are the key to boost the ORR performance.21b] The studies show that the improved catalytic activity is derived from the change in surface morphology and defective structures resulted from the incorporation of heteroatoms.
Many straightforward methods can be used to fabricate heteroatoms-doped graphene, including CVD method, [26c,40] plasma irradiation, [41] thermal annealing, [42] solvothermal synthesis, [43] and ball-milling, [21c,44] making heteroatoms doping a facile strategy to tune the electronic structures of graphene.Meanwhile, as a dopant, nitrogen can highly boost the 4e À ORR activity of graphene with a half-wave potential of 0.875 V (vs RHE, ND-GLC) in alkaline media, [45] and O dopants are more suitable for the 2e À ORR process. [33]Benefitting from its unique electronic structure, boron-doped graphene exhibits superior NRR activity. [30,31]Nonmetal heteroatoms can modulate the electronic and geometric properties of two-dimensional carbon materials, providing more active sites and improving the interaction between the graphene frameworks and active sites.The redistributed electronic environment around carbon and dopant atoms can lead to very strong adsorption bonding of O 2 , CO 2 , and N 2 molecules on the catalytic active sites, beneficial for their activation.Surprisingly, sulfur dopant in graphene can also cause strong structural deformation and there will be significant charge transfer although sulfur has a similar electronegativity with carbon.

Intrinsic Defects of Graphene Boosted Electrochemical Reactions
Various defects can be created and stably exist in graphene due to their unique structure and electron distribution.Intrinsic defects can be classified into different types based on their structures, including vacancy defect, topologic defect, and grain boundary defect.In essence, the missing of carbon atoms in the hexagonal carbon lattice is what causes the vacancy defect.In addition, topological defect is generally composed of one or more nonhexagons rings resulting from the regular and rigid lack of carbon atoms (e.g., pentagon (C5), pentagon-octagon-pentagon (C5-8-5), pentagon-heptagon-heptagon-pentagon (C5-7-7-5), pentagonheptagon-pentagon-heptagon (C5-7-5-7), etc.).Remarkably, vacancy and topological defects with high electrocatalytic activity are normally located at the edges of graphene.Therefore, a brief summary of the specific properties of graphene edges will be presented in this section.Moreover, we will summarize the history of using IDG as high-performance electrocatalysts, and discuss a new catalytic mechanism motivated by intrinsic defects.

A Defect-Driven Electrocatalysis Mechanism
Although the incorporation of heteroatoms into graphene could improve the catalytic performance, many issues still need to be solved.For example, the relationship between the nitrogen concentration and the ORR performance of the NG is uncertain and controversial. [46]Similarly, whether graphitic N or pyridinic N is more active for the ORR is an ongoing debate. [47]Combining recent achievements in heteroatoms-doped graphene, the alternative electron distribution of carbon atoms may be the real origin of the increased catalytic activity.As shown in Figure 6a, Kotakoski et al. calculated the electron density of three kinds of defects by first-principles simulations. [48]Clearly, defects greatly influence the electronic configuration around carbon atoms, which may boost the electrocatalytic performance of graphene.
In recent years, the development of defective carbon-based electrocatalysts has progressed rapidly. [49,50]In 2015, Yao group for the first time systematically investigated defective carbonpromoted ORR theoretically and experimentally. [51]By removing two carbon atoms, a monolayer graphene with G585 defect (topologic defect contain two pentagons and one octagon) was built for the DFT modeling (Figure 6b).The calculation results show that comparing with pristine graphene and NG, G585 defect shows the lowest overpotential with the adsorption of oxygen as the rate-determining step (RDS).Subsequently, this conclusion was examined by experimentally synthesizing defective carbon from a N-enriched porous organic framework (PAF-40).From the XPS and Raman spectra shown in Figure 6c,d, it is clearly that the N content of the prepared samples was decreased while the density of defects was increased with the increase of the calcination temperature.It is worth noticing that the electrocatalytic performance shows a positive correlation with the heat treatment temperature (Figure 6e), which indicated that other than the doped N atoms, defects are the actual catalytic sites.Furthermore, by carbonizing a Zn metalorganic framework (Zn-MOF), in which Zn atoms were evaporated at high temperatures, a heteroatoms-free defect carbon catalyst for ORR was synthesized. [52]The resulting catalyst shows a similar onset potential with that of commercial Pt/C, indicating that the newly created defects in the sp 2 carbon matrix are effective for the ORR.In addition, Dai et al. developed an edge-rich and heteroatoms-free graphene by Ar plasma etching, and the prepared materials can catalyze the ORR through a one-step and four-electron reaction process with obviously improved activity compared to pure graphene. [53]Edge-rich graphene exhibits more positive onset potential and cathodic reduction peak (0.87 and 0.70 V vs RHE) than that of the pristine graphene (0.76 and 0.59 V vs RHE) for ORR in O 2 -saturated 0.1 M KOH, indicating the enhanced ORR electrocatalytic activity of edge-rich graphene.The DFT calculations found that higher charge density around the edge carbon atoms than that of basal plane, which can facilitate the adsorption of O 2 and boost the ORR activity.This research presents the key role of edge defects in IDG ORR catalysts.Meanwhile, Yao et al. developed a simple N-removal method to fabricate a defect-enriched graphene (DG) from a N-doped precursor (NG). [54]Figure 6f shows the high-angle annular darkfield scanning transmission electron microscopy (HAADF-STEM) image of DG, from which we can clearly see three kinds of intrinsic defects, i.e., pentagons, heptagons, and octagons located at the edge of graphene.Furthermore, the electrochemical test results indicate that the DG can efficiently boost ORR, OER, and HER with activity that is much higher than its N-doped counterpart (Figure 6g,h).Three edge-defective models were established (C5, C585, and C7557) to reveal the catalytic mechanism.According to the DFT calculations, C5 is responsible for the ORR, C585 and C5 both can effectively promote the OER.The most efficient active site for the HER is C7557.These results prove the versatility of the defect-driven catalysis mechanism for electrocatalysis.
Further investigations have been carried out to verify the defect-driven catalysis mechanism and clarify the real active centers of heteroatoms-doped graphene catalysts. [55]Kakimoto et al. performed DFT calculations to uncover the electronic structures of N-doped graphene with vacancy defect. [56]They found that the monovacancy in graphene acts as hole dopants, and the resulting electron redistribution is twice than that of one N dopant.Meanwhile, the role of N doping regarding the formation of free carriers in the bulk π bands is significantly changed by the interference between native point defects and N dopants.The computing results reveal that vacancy defect in graphene is more powerfully influencing the electronic structure of graphene than nitrogen atoms, and vacancy can enhance the electron modification brought by N-doping.Subsequently, Yao et al. chose pentagon rings (C5), formatting from monovacancy, and heteroatomsdoped C5 in carbon as electrocatalysts to investigate the ORR performance. [57]Figure 7a exhibits the total 14 configurations of single and dual heteroatom-tuned C5 defect, and DFT calculations on the free energies of the acidic ORR pathways were performed based on the proposed models.Experimentally, heteroatom-incorporated C5 defective graphene (DG) was fabricated by a facile heteroatom doping and removal method.Figure 7b demonstrates the experimental half-wave potentials well fitted with the theoretically calculated overpotentials of the prepared materials in acidic solutions.As can be observed, NS-G (N and S codoped graphene) and DG-NS (N and S codoped DG) possess the lowest and highest ORR half-wave potentials Figure 6.a) Three types of topological carbon defects and their electronic density of states.Reproduced with permission. [48]Copyright 2011, American Physical Society.b) Pictorial representation of G585 defects in graphene.c) High-resolution N 1s XPS spectra with deconvolutions.d) Raman spectra for the prepared catalysts C-700, 800, 900, and 1,000.e) Kinetic current densities ( J k ) and transferred electronic number based on the Koutecky-Levich plots for C-700, C-800, C-900, and C-1000.Reproduced with permission. [51]Copyright 2015, The Royal Society of Chemistry.f ) HAADF image of DG with an acceleration voltage of 80 kV.Hexagons, pentagons, heptagons, and octagons were labeled in orange, green, blue, and red, respectively.Linear sweeping voltammetry curves of the pristine graphene, NG, and DG in alkaline solution: g) Oxygen reduction reaction, and h) Oxygen evolution reaction.Reproduced with permission. [54]Copyright 2020, Elsevier.d) Synthetic scheme for the preparation of a D-HOPG sample.e) The N 1s highresolution XPS spectra of HOPG (purple), Ar-HOPG (red), N-HOPG (blue), and D-HOPG (black).The pink and green curves are the fitted characterized curves of the pyridinic N and graphitic N in the XPS spectra.f ) The Raman spectra of HOPG, Ar-HOPG, N-HOPG, and D-HOPG (colors as for (f ).g) The HAADF-STEM image of N-G.h) The HAADF-STEM image of the derived D-G.i) The LSV curves of Ar-HOPG (red), N-HOPG (blue), and D-HOPG (purple) (for the ORR in 0.1 M H 2 SO 4 solution).Inset: correlated onset potentials (V vs RHE at 0.05 mA cm À2 ).j) ORR activities of N-HOPG with etching times from 60 to 120 min.k) ORR activities of D-HOPG with etching times from 60 to 120 min.Reproduced with permission. [58]Copyright 2019, Springer Nature.
among the prepared samples, respectively.It means that it is C5 defect other than doped heteroatoms that take the key role in boosting ORR activity.The HAADF image of DG-NS in Figure 7c shows the plain structure of N and S atoms located in the edge-C5 defects, which strongly supports the defectspromoted catalytic mechanism.Furthermore, to probe the ORR activity origin of N-doped graphene and defect graphene, a pentagon-enriched graphene was further fabricated through a plasma and doped-N removal method. [58]Figure 7d shows the preparation process of the sample.Pristine highly oriented pyrolytic graphite (HOPG, multilayer graphene) was etched by argon plasma through a nickel mesh first (Ar-HOPG), and then washed with nitric acid.Afterward, the Ar-HOPG was heated at 700 °C in an ammonia flow for 3 h to form N-doped HOPG (N-HOPG).Finally, the annealing temperature was increased to 1150 °C for 2 h to remove the N atoms and obtain defect HOPG (D-HOPG).The XPS spectra in Figure 7e indicate that N-HOPG mainly contains pyridinic N with trace graphitic N, and D-HOPG is nitrogen-free.Figure 7f shows the Raman spectra of the prepared materials, indicating that D-HOPG maintains the highest disorder level (highest I D /I G ), resulting from the removal of N dopants.Figure 7g,h are the HAADF-STEM images of N-HOPG and D-HOPG, demonstrating the pyridinic N and defective pentagons clearly, which directly confirmed the N dopants have been removed after heat treatment.According to the LSV curves in Figure 7i, N-HOPG shows a similar onset potential with Ar-HOPG (0.76 V vs 0.73 V), implying that heteroatoms hardly boost the ORR activity.However, D-HOPG shows a much higher onset potential (0.81 V) than that of the N-HOPG.To further uncover the ORR origin of the N-HOPG and D-HOPG, the electrocatalytic performance of various N-HOPG and D-HOPG samples treated by plasma etching for different durations was evaluated and the results are shown in Figure 7j,k.We can see that the density of N dopants has little influence on ORR, nevertheless, the ORR activity of D-HOPG is increased with the increase of pentagon density.All in all, the produced edge pentagon defect is the major active site for the acidic ORR, superior to that of pyridinic N in N-HOPG.

Control of Intrinsic Graphene Defect Types, Density, and Specific Catalytic Activity
The diversity of carbon defects provides various active sites with different electronic structures, making carbon-based catalysts promising for electrochemical reactions.As previously mentioned, a wide range of methods have been developed to fabricate defects in carbon materials, such as in situ doping and postmodification techniques.For example, topological defects can be produced using heteroatom removal and carbon thermal reduction methods, while vacancy defects and edge-based defects can be created using plasma etching, and electron beam etching.However, it is generally known that when utilizing these strategies to synthesis DGs, various types of defects are coexisted, which hinders the discovery of the real active centers.The precise synthesis of the target defect configuration and the establishment of activity-structure relationship are grand challenges and have attracted extensive attention from theoretical and experimental chemists.
In 2016, Dai et al. obtained an edge-rich graphene by plasma etching in Ar atmosphere and found that graphene edge is responsible for the ORR activity. [53]Meanwhile, Nakamura et al. designed a plasma etching, N þ implantation, and NH 3 modification method to controllably synthesize pyridinic-N enriched graphene based on HOPG. [59]Four types of catalysts have been fabricated, including pyridinic N-dominated HOPG, graphitic N-dominated HOPG, edge-enriched HOPG, and clean HOPG.The actual ORR active sites in N-doped carbon materials are carbon atoms with Lewis basicity, which were created by the pyridinic N. Inspiration by this work, Yao group developed a pyridinic N doping and removal method and applied for generating edge pentagon defect precisely on HOPG, which has been identified as the actual active site for the acidic ORR. [58]In addition, the synthesis of topological defect in carbon materials was systematically investigated. [60] Combining theoretical calculations and experimental investigations, Ren et al. found that the heterogeneous electron transfer (HFT) rate of graphene, which is crucial for its electrochemical activity, would be significantly influenced by the density of vacancy defects. [61]With the increase of defect density, the density of states (DOS) of graphene increases, while the conductivity would decrease.Therefore, except for the intrinsic activity of the active center, the density of active sites is also a key factor that influences the electrocatalytic performance of catalysts.Herein, a brief overview of the synthesis of carbon-based materials with high defect density will be provided, and the related catalytic mechanisms will be discussed as well.
For methods such as electron beam etching, oxidation etching, and plasma etching, extending treatment time or increasing output power of energy sources is the simplest approach to enhance the defect density.For example, Ye et al. fabricated graphene with amorphous heterophase from small molecules by infrared laser irradiation. [49]Through tuning the laser power from 3% to 14%, graphene samples contain different ratios of amorphous carbon were obtained to serve as metal-free  [60] Copyright 2020, Elsevier.
electrocatalysts for direct nitrate-to-ammonia reduction.The catalyst synthesized with the most defective structures shows the highest NH 3 FE (83.7%) and yield (2456.8 μg h À1 cm À2 ).Furthermore, DFT calculations further reveal that the adsorption of NO 2 À is the potential determining step, and G-57 (adjacent pentagon and heptagon carbon rings) is the catalytic active site with the lowest energy barrier of 1.17 eV.Arie et al. synthesized graphene with different densities of defects through adjusting the plasma power and treatment time. [62]Hasegawa et al. developed an ultraviolet irradiation method to controllably generate graphene defects in an oxygen atmosphere. [63]As shown in Figure 9a, the oxygen concentration, rather than the power of mercury lamp, is the key factor that influences the density of defects.Meanwhile, metal-organic frameworks (MOFs), polymers, and small organic molecules are also the common precursors to form defective carbon materials, and many ingenious strategies have been designed to tune the defect density.For example, Yamauchi et al. selected a 2D crystal-and shapemodified zeolite imidazolate framework (ZIF) to fabricate an edge-enriched N-doped graphene nanomesh (NGM) with high porosity. [64]The Raman spectra of NGM annealed at different temperatures demonstrate that a much higher percentage of carbon atoms at the edges of graphene layer with the increase of treatment temperature, resulting in the excellent ORR activity comparable with the commercial Pt/C catalyst.Besides, a precursor with core-shell structures was designed to obtain carbon electrocatalysts with high defect density and excellent ORR performance. [65]Carbon thermal reduction reaction is a practical way to reconstruct carbon atoms, and the forms of carbon atoms after thermal reduction are CO 2 and CO.Thus, it is clearly that a  [65] Copyright 2022, Elsevier.
higher ratio of CO in the formed gas, a higher removal efficiency of carbon atoms could be achieved.Finite element method (FEM) simulations were conducted to predict the free diffusion state and transformation degree (from CO 2 to CO) of CO 2 gas within carbon shells by different opening angles and flowing speeds of the carrier gas. Figure 9b shows the relevant redox reaction and process to form carbon defects inside the carbon shells.The FEM simulation results and Raman spectra in Figure 9c,d indicate that the larger open degree of carbon shell and gas flow speed would result in lower defect density.Based on the simulation results, two extreme cases with fully closed and open structures were selected for experimental studies, corresponding to the higher and lower defect density porous carbon (donated to HDPC and LDPC), respectively.Meanwhile, from the HRTEM image of HDPC shown in Figure 9e, pentagon defects are located at carbon edges prove the high density of defects.The HDPC materials show higher ORR activity in alkaline solutions than that of the commercial Pt/C catalyst with a half-wave potential of 0.90 V (vs RHE, Figure 9f ).As confirmed by the DFT calculations, the potential determining step of all models with different pentagon defect densities is the reductive protonation of O 2 to OOH*, and the theoretical overpotential of these models has been decreased from 0.46 to 0.42 eV, which is consistent with experimental results.With the increase of defect density, more electrons are transferred between C and the adsorbed OOH groups, leading to the stronger C-O bonding and lower Gibbs free energy.The gradient "proximity effect" between defects is responsible for the high electrocatalytic activity.
According to the experimental and theoretical results, pentagon defects (C5) show the highest ORR catalytic activity and are considered as an alternative to the Pt/C catalyst. [65]hile C5 and C585 both exhibit excellent OER catalytic activity, C585 and C5775 are more suitable to catalyze the HER. [54,60]he facile synthesis methods and remarkable high activity indicate that the creation of defects in graphene is a viable strategy to enhance its catalytic performance.Furthermore, the combination of topological defects and graphene edges can present much superior catalytic activity because of the terminated edges in graphene.Edge electrons participate in heterogeneous charge transfer and exhibit 4 orders of magnitude higher specific capacitance than that of graphene basal plane.In addition, edges also exhibit higher electrocatalytic ORR activity than that of the graphene basal plane. [66]Interestingly, O, N, H atoms, NH 3 , and water molecular prefer to be adsorbed at the edges, [67] thus could promote the electrochemical reactions.Two kinds of edge structures named zigzag edge and armchair edge have different electronic properties.For example, graphene zigzag nanoribbons are metallic while armchair ones can be metallic or semiconducting, resulting in different electrocatalytic activities among armchair and zigzag edges. [24,68]For instance, zigzag edge shows much better ORR performance than that of the armchair edge. [69]Moreover, heteroatoms can also significantly change the conductivity of the graphene edges due to different effects of the localized edge states, thus could further modulate the electrocatalytic performance of graphene edges.The combination of heteroatoms and edges is one type of complex defect, which will be discussed in detail in the following sections.

Complex Defects Promoted Electrochemical Reactions
One of the most notable advantages of carbon defects is the high compatibility with other species, which derived a new kind of defect known as complex defects.Currently, the two primary methods for creating complicated defects are integrating nonmetal heteroatoms (N, P, S, O, etc.) or isolating metal species (single atoms or clusters) on the carbon defect sites.As discussed above, heteroatoms could significantly rebuild the electronic structures of carbon rings.Therefore, the introduction of heteroatoms or metal species into carbon defects would be a feasible strategy to further improve the electrocatalytic performance of IDG.

Synergistic Effect between Doped Heteroatoms and Intrinsic Graphene Defects
The removal of heteroatoms such as nitrogen atoms and the pyrolysis of MOFs and polymers are the commonly used methods to create carbon defects.Unavoidably, foreign atoms would be maintained in the crystal of defective carbons as impurities.Recent studies show that heteroatoms in the defective carbons also contribute to the electrocatalysis.In 2018, Yang et al. designed a N-S-C coordination-structured active site by integrating edged thiophene S, graphitic N, and pentagon defects. [70]entagon S defect and metaposition graphitic N were confirmed to remarkably increase the ORR activity in acidic environment.Yao et al. demonstrated that N and S atoms codoped C5 defect exhibits the best ORR performance among the 14 configurations of single and dual heteroatom tuned C5 defect, both theoretically and experimentally. [57]Meanwhile, Dai et al. fabricated an edgeenriched holey graphene (hG) by annealing graphene oxide, and edge N was doped by calcining hG in an Ar/NH 3 atmosphere. [71]he excellent ORR performance of the resulted catalyst that is close to that of Pt/C is attributed to the high specific surface area and high edge N-content as well as abundant edge defects.Except ORR, heteroatoms-doped DG also has the potential to boost other electrochemical reactions.For example, Du et al. investigated  Si-doped graphene (Si@G) as a metal-free catalyst for CO 2 reduction with high selectivity and activity. [72]Based on the DFT calculations, the single Si-doped armchair edge can highly promote the formation of CH 3 OH with a limiting potential of À0.49V, and for C 2 productions on Si chain doped zigzag edge, the generation of C 2 H 5 OH has the highest activity with a limiting potential of only À0.60 V. Sun et al. designed a series of transition metal (TM) atoms doped on N-graphene catalysts for N 2 reduction.Through DFT calculations, they found that the bonded Mn-Fe catalyst can efficient activate N 2 with an extreme low theoretical overpotential (0.08 V). [73]

Coordination of Embedded Metal Species and Intrinsic Graphene Defects
As mentioned above, the adsorption of O 2 , N 2 , CO 2 , etc. is the first important and speed-limiting step.However, for nonmetaldoped graphene, gas molecules are usually physiosorbed with small adsorption energy and long distance from graphene plane, resulting in high overpotentials.Therefore, non-noble metal active centers are introduced to graphene to build chemisorption between reactants and graphene. [74]Various kinds of electrocatalysts composed by metal clusters or single atom embedded on suitable substrates (SACs) have been designed and fabricated due to their increased exposed active sites and higher specific surface area.However, one of the fatal issues of those catalysts is the aggregation of metal species, which limits their loading amount and electrocatalytic performance.Carbon materials have large surface area, high electron conductivity, and chemical stability, rendering them to be ideal substrates to stabilize metal species.Besides, DG can trap more metal species than pristine carbon by its edge, vacancy, and topologic defects, and the strong interactions between metal species and defects can also effectively avoid the aggregation.Meanwhile, similar to heteroatoms, the embedded metal clusters or atoms could tune the electronic structures of DG, which could be the underlying reason for the enhanced catalytic performance of metal atoms loaded DG catalysts (MA@DGs).
TM-N 4-x C x (x = 0-4) is a common configuration in TM and N atoms codoped defective carbon materials. [75,76]Atomic Fe-N 4 species-doped carbon-based catalyst (aFe@AC) was successfully synthesized by a wetness impregnation method and obtained highly active for ORR in both acidic and alkaline electrolytes.76b] The combination of the DFT calculations and LSV test results reveals that the configuration of Co and N 4 exhibits the highest ORR activity, which is comparable with that of Pt/C both theoretically and experimentally.The CoC 4 atomic species in defect carbon nanofiber (DCNF) also show good catalytic performance and low theoretical overpotential, implying that defect-Co configurations other than N atoms could be the actual ORR active sites.76c] Theoretically, the calculated highest-occupied molecular orbital (HOMO) and lowestunoccupied molecular orbital (LUMO) of center and edge FeN 4 configurations (c-ND-Fe and e-ND-Fe) suggested a higher electron density around e-ND-Fe, leading to a greater electron donor capacity than c-ND-Fe, thus exhibiting a better reactivity.Besides, a narrower HOMO-LUMO energy gap of e-ND-Fe sites than that of c-ND-Fe facilitates electron transfer in the catalytic processes (Figure 10a).Experimentally, the electrochemical measurements reveal that the ORR performance is positively related to the ratio of e-ND-Fe motifs, matching the theoretical predictions very well.Recently, Guan et al. fabricated a dual-atom Co-Fe catalyst based on N-doped graphene with FeN 3 -CoN 3 active sites. [77]Remarkable activity and stability for the ORR have been achieved with a half-wave potential of 0.952 V (Figure 10b) and 250 h long lifetime at 5 mA cm À2 .Based on the XPS and EXAFS spectrum, atomic Fe and Co species both show oxidation valence state close to þ2.Combined with DFT calculations, CoFe-NG model shows higher ORR activity than that of the FeN 4 and CoN 4 sites, with a lower theoretical overpotential (Figure 10c).Figure 10c also illustrates that the rate-limiting step for CoFe-NG is *OH to H 2 O (0.38 eV), representing a stronger adsorption than single metal catalysts, and atomic Fe species are the real active sites.As discussed above, graphene-based FeN 4 sites exhibit different ORR activities in acidic and alkaline media.To uncover the underlying mechanisms, Li et al. established a pH-dependent regulation mechanism in the Fe-N-C materials by DFT calculations with explicit pH, solvation, and electrode potential conditions. [78]The results show that under high potentials, as well as typical operation potentials, the active center Fe atom was covered by intrinsic intermediates *OH and *O at pH = 1 and 13, respectively.Meanwhile, under low potentials, FeN 4 centers are uncovered.Figure 10d shows the models and Bader charge analysis of these three kinds of active sites.Combined with the PDOS in Figure 10e, the Fe atoms of FeN 4 -O exhibit higher valence state and downshifted d-band center than that of FeN 4 -OH, leading to weaker interactions onto adsorbates and better catalytic activity for ORR.Therefore, the Fe-N-C materials exhibit superior activity in alkaline media, and the reaction mechanism under different pH values is schematically summarized in Figure 10f.
41a] First, defects in graphene were created by highenergy atom/ion bombardment.Then, metal atoms such as Pt, Co, and In were deposited by electron beam or conventional sputtering technique.Recently, MA@DGs have been widely applied in electrocatalytic conversion. [79,80]For example, Du et al. investigated the synergistic effect between a series of TM single atoms (TM = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ag, Au, Pt) and 558 grain boundaries (TM@558 GB) in graphene through DFT calculations. [81]The TM@558 GB (TM = V, Fe, Co, Ni) shows superior HER activity than that of 558 GB, and even close to or better than that of the Pt and MOS 2 catalysts, resulting from the charge transfer between TM atoms and the adsorbed H atoms.Meanwhile, Yao et al. selected a low-cost and easy scale-up activated carbon as the substrate to synthesize Mn-Co nanoparticles anchored defective activated carbon by a simple solvothermal method, which shows superior ORR performance and durability. [82]In addition, a non-noble FeCo nanoparticlesembedded DG catalyst (DG@FeCo) was successfully fabricated using an impregnation method for alkaline ORR. [83]As shown in the TEM image of DG@FeCo (Figure 11a), the metal species were homogeneously dispersed on the defective graphene, and the inset histogram shows that the diameter of loaded metal nanoparticles is mainly around 10 nm.The LSV curves in Figure 11b reveal that DG@FeCo exhibits a significant ORR activity similar to that of Pt/C.Furthermore, a bifunctional electrocatalyst whereby graphene defects trap atomic Ni species (aNi@DG) was synthesized that exhibits extraordinary catalytic activity for OER and HER, e.g., a HER overpotential of 70 mV at 10 mA cm À2 that is comparable to the commercial Pt/C and an OER overpotential of 270 mV at 10 mA cm À2 that is much lower than that of the IrO 2 . [84]The HAADF-STEM image in Figure 11c clearly shows the Ni species encapsulated in the divacancy defect.Through the extended X-ray absorption fine structure (EXAFS) curves of A-Ni@DG, Ni@DG, and the Ni foil (Figure 11d), it is obviously that A-Ni@DG contains no appreciable Ni-Ni peak, confirming that Ni is predominantly present as atomic state on DG substrate.Through DFT calculations, four carbon atoms coordinated with Ni atom are identified to be the OER active site, and the very high DOS of A-Ni@DG near the Fermi level resulted in stronger interactions between the reaction intermediates and substrates.Moreover, a highly active DG-based catalyst for HER with atomic Pt as the active sites (Pt@DG) was fabricated as well. [85]As schematically shown in Figure 11e, the Pt-C 3 configuration composed with single vacancy of graphene and trapped Pt atom was obtained via the photoreduction of H 2 PtCl 6 on DG.From the HAADF-STEM in Figure 11f, the evenly distributed bright dots can be observed on the graphene, indicating the successful formation of atomic Pt.According to the X-Ray adsorption near-edge structure (XANES) spectra of Pt@DG in Figure 11g, it is suggested that Pt atoms in Pt@DG exhibit a positively charged state about þ2.3.The EXAFS spectra of Pt@DG and the comparison samples such as Pt foil and PtO 2 in Figure 11h show the absence of Pt-O and Pt-Pt coordination in Pt@DG, confirming the existence of atomic Pt in Pt@DG.Remarkably, the acidic HER performance of synthesized Pt@DG in this work is among the best of the state-of-the-art catalysts with an onset potential of  (down).Reproduced with permission. [77]Copyright 2023, Elsevier.d) Charge density difference of the pyridinic-type FeN 4 , FeN 4 -OH, and FeN 4 -O centers.The yellow and green areas denote electron accumulation and depletion, respectively.e) Partial density of states (PDOS) of the Fe centers and their d-band center.f ) Scheme of the ORR on FeÀN-C catalysts at pH = 1 and pH = 13.Reproduced with permission. [78]Copyright 2023, American Chemical Society.
30 mV at 10 mA cm À2 (Figure 11i).Meanwhile, DFT calculations found that atomic Pt loaded on graphene exhibits too strong interaction between Pt and protons, hampering the desorption process by the high energy barrier.However, Pt-C 3 active site contains a lower d band center than that of the atomic Pt, resulting in weaker adsorption bonding and higher HER activity.

DG-Based Van Der Waals-Type Multilayered Heterostructure for Electrocatalysis
As a new class of catalysts, the strong electron interaction at the heterointerface between graphene and other 2D materials or crystalline substrate is expected to greatly alter the chemical stability and reactivity of the electrocatalysts. [86]Recent theoretical works have shown that graphene-based heterostructures can substantially improve electrocatalytic reaction activities. [87]For example, the reaction pathways of g-C 3 N 4 with and without carbon substrate were theoretically investigated. [88]As shown in Figure 12a, g-C 3 N 4 with conductive support such as carbon boosted the ORR in an efficient 4e À transition pathway with lower free energy than 0e À and 2e À pathways of the pristine g-C 3 N 4 , confirming the enhanced electron transfer ability that can significantly facilitate the electrocatalytic reactions.A multilayer catalyst composed of graphene coupling with 2D Co 3 O 4 nanosheets was fabricated by Zhu et al., the coupling effects between graphene and 2D metal oxides were investigated. [89]igure 12b shows the microstructures of Co 3 O 4 sheet/graphene (Co-S/G-3), and the interface of metal oxides nanosheet and graphene plane was highlighted by red arrows.The LSV curves in Figure 12c indicate that Co-S/G-3 exhibits a superior ORR performance than that of Co 3 N 4 nanoparticle/graphene (Co-P/G) and close to that of commercial Pt/C in alkaline solutions.DFT calculations were performed to further reveal the reaction mechanism.Figure 12d shows the charge density difference of the interfaces between graphene/N-doped graphene and the (111) surface of type-B Co 3 O 4 (half of oxygen atoms are 2-foldcoordinate).Graphene sheets were bent obviously due to the strong electron coupling at the interface, and significant charge transfer from graphene to Co 3 O 4 , which consequently improves the electron transfer speed across the whole structure and enhances the ORR activity.Subsequently, Du et al. implemented theoretical investigations to reveal the origin of electrocatalytic activity of DG and metal surface heterostructures. [90]The fully relaxed structure of di-vacancy graphene (DV-G) supported by Ni(111) surface is shown in Figure 12e.It can be seen that the Ni atoms of the first layer just below the double vacancy are lifted and rebound with four surrounding C atoms.However, monovacancy cannot be filled with metal atoms because of its limited space.The projected DOS of DV-G on Co(0001) and Ni(111) in Figure 12f shows that the Reproduced with permission. [83]Copyright 2017, The Royal Society of Chemistry.c) The HAADF-STEM image of the defect area of A-Ni@DG.d) the K 2 -weighted Fourier transform spectra of Ni@DG, A-Ni@DG, and the Ni foil reference samples.Reproduced with permission. [84]Copyright 2017, Elsevier.e) Schematic illustration of the preparation of Pt@DG.f ) HAADF-STEM images of Pt@DG.g) XANES spectra for Pt@DG, Pt foil, and PtO 2 at Pt L 3 -edge.h) EXAFS spectra of Pt@DG, Pt foil, and PtO 2 .i) TOF value of Pt@DG and other state-of-the-art electrocatalysts in 0.5 M H 2 SO 4 .Reproduced with permission. [85]Copyright 2022, American Chemical Society.
heterostructures exhibit metallic behavior and the graphene sheet is found to strongly hybridize to the metal substrates, which destructs the π-conjugations of the carbon rings.Combining the charge density difference plots in Figure 12g and Bader charge analysis, each Co atom in the top layer loses 0.25 e to the C atoms, and that of Ni atom is 0.28 e.Meanwhile, TM atoms in the divacancy are in the charge depletion area, and the surrounding carbon atoms are in the charge accumulation area.Above all, the charge transfer between the interface and the electron structure modified around the vacancy defects is the origin of the low ORR and OER overpotentials (0.36 and 0.39 V, respectively).
Figure 12. a) Free energy plots of ORR and optimized configurations of adsorbed species on the g-C 3 N 4 surface with zero, two, and four electron participation.And schemes of ORR's pathway on pristine g-C 3 N 4 without electron participation, pristine g-C 3 N 4 with 2e À participation, and g-C 3 N 4 and conductive support composite with 4e À participation, respectively (red areas represent the active sites facilitating ORR).Reproduced with permission. [88]Copyright 2011, American Chemical Society.Reproduced with permission. [89]Copyright 2015, American Chemical Society.e) Top and side views of the optimized configurations for DV-G on Ni(111).Green and grey atoms represent Ni and C, respectively.f ) Projected DOS for the d-orbitals of a TM atom in double vacancy of graphene and total DOS for DV-G on Co(0001) and Ni(111).The vertical red-dashed lines represent the location of the d-band center of the doped TM atoms.g) Charge density difference plots of DV-G/Co(0001) and DV-G/ Ni(111).The yellow and cyan colors represent the charge accumulation and depletion area, respectively.Ni, Co, and C atoms are in green, pink, and grey, respectively.Reproduced with permission. [90]Copyright 2019, American Chemical Society.
Apart from the theoretical investigations, many graphenebased heterostructure catalysts have been fabricated experimentally. [91,92]11c] The obtained catalyst exhibits excellent activity for overall water splitting with a current density of 20 mA cm À2 at 1.5 V (Figure 13b and inset).The Ni-Fe LDH-NS@DG catalyst significantly outperforms other nonnoble metal bifunctional catalysts for overall water splitting, as shown in Figure 13c.A solar power-assisted water-splitting device shown in Figure 13d was built and the evolution of oxygen and hydrogen bubbles could be clearly observed.DFT calculations were conducted to further probe the functions of interface and defects in boosting electrocatalytic performance (Figure 13e,f ).It is obvious that charge density transferred from LDH to graphene, producing a large number of holes on Fe-Ni LDH that facilitate the OER, and the defect sites were charge accumulation areas, which is supposed to improve HER performance.Figure 13g schematically shows the theoretical model with electrons and holes for HER and OER, respectively.Furthermore, the synergistic effect between adjacent graphitic layers in the multilayered metal atoms embedded carbon catalysts was also studied. [93]A Co and Pt codoped carbon (A-CoPt-NC) catalyst was synthesized by electrochemical activation method and it shows superior HER activity than the commercial Pt/C in a wide range of pH values (overpotential at 10 mA cm À2 are 27 and 50 mV in acidic and alkaline electrolytes, respectively).The metal species can be seen evenly dispersed on the defective sites of the N-doped carbon sample A-CoPt-NC (Figure 13h).Meanwhile, two Co and Pt trapped single-layer and double-layer defective graphene models were built to theoretically investigate the interaction between graphene layers.As shown in Figure 13i,j, the plots of charge density difference of two models show the atomic metal species located at inner layer could induce an apparent new charge polarization by altering the electron structure of the outer layer, which is believed to promote the HER.
For preparing DG and DG-based electrocatalysts, many postfunctionalized and in situ synthesis methods can be used.For example, both ball milling [94] and plasma technology [49,58] are efficient and cost-effective postfunctionalized approaches to create defects in DG.Through a bottom-up pathway, in situ synthetic strategies such as template-based carbonization, [95] dopant-removal annealing, [54] and pyrolysis of Zn-contained metal-organic frameworks [52,60,65] can synthesize defectsenriched graphene in one step.Moreover, many remarkable electrocatalysts based on DG have been discussed experimentally and theoretically in this section.Benefiting from the widely used facile synthesis strategies, the design and use of DG-based materials is an effective approach to modulate the electronic environment around the complex defects and thus utilize the synergistic effect between heterogeneous phases and DG.Adsorption energy between active sites and reaction intermediates plays a key role in evaluating the activity of catalysts and is highly influenced by the charge transfer.It is difficult for catalysts with physical or weak chemical adsorption to accomplish the activation of reactants such as O 2 , N 2 , and CO 2 .More importantly, their ratelimiting step is the first step of the reaction in metal-free defect graphene catalysts.While there are scaling relationships between the adsorption energy of reaction intermediates, [96] thus catalysts with strong chemical adsorption will face another significant problem, desorption of the products, and the required high desorption energy will impact their catalytic activity.76b,77] Combined with the metal and nonmetal active sites and their mixed electronic properties, complex defects composed by defective graphene and heterogeneous species such as nonmetal and metal atoms, clusters, and other materials plane would show mild adsorption bonding between reaction intermediates and catalysts, thus exhibit extraordinary catalytic activity.For example, Fe-N-C active sites exhibit more superior ORR activity than that of the commercial Pt/C catalyst. [77,97]The interaction between nickel and carbon can either increase OER activity or strengthen the durability of atomic nickel materials. [84,90]Meanwhile, the adsorption energy between Pt and proton can be tuned by the surrounding carbon atoms, leading to the lower overpotential for HER. [85]Atomic cooper or cooper clusters trapped by DG can also be excellent catalysts for CO 2 RR. [98,99]Additional advantages of defect-based electrocatalysts including high atom utilization, high electronic conductivity, long durability, and high chemical tolerance make them to be remarkable electrocatalysts.However, the controllable synthesis of the target configurations and evenly dispersed heterogeneous species are still a grand challenge.The synergistic effect between defects themselves, heterogeneous species and defects is still unclear.More systemically and purposeful investigation should be conducted theoretically and experimentally.

Conclusion and Outlook
In summary, defects-enriched graphene-based electrocatalysts exhibit huge potential to substitute the expensive and rare noble metal catalysts because of their excellent activity and low cost.In this review, both theoretical and experimental works on defective graphene-based electrocatalysts have been summarized, including heteroatoms-doped graphene, intrinsic DG, heteroatoms or metal species-modified DG, and DG-based multilayered heterostructures.Electrocatalysts with different combinations of heterogeneous species and intrinsic graphene defects show different catalytic activities.Pentagon defects located at edges, nitrogen dopants, and iron dopants demonstrate great the potential to achieve remarkable ORR activity.Atomic Pt coordinated with carbon and edge C585 defects can be excellent HER active site.B dopants can substantially boost the NRR, and Cu species loaded on DG can be remarkable CO 2 RR catalysts with high selectivity.We can find that defects at the edges can greatly promote the ORR activity of graphene.However, metal dopants such as Fe and Co atoms can boost the ORR through tuning the electronic environment and bonding state between active sites and adsorptions.Furthermore, many dopants, such as B, O, Pt, and Cu can make graphene-based materials to have great potential for catalyzing more electrochemical reactions.
Remarkably, a defect-derived catalytic mechanism is highlighted, showing that any method that could alter the charge environment of the graphene is capable of improving the  [93] Copyright 2019, John Wiley and Sons.electrocatalytic performance.It opens a new avenue for catalyst design and fabrication.However, more depth investigations should be carried out in defect-promoted electrocatalysis, particularly on the controllable design and fabrication of electrocatalysts with specific active sites for different applications, which need combined efforts from experimental and theoretical chemists.The future development of defects-based electrocatalysis should be mainly focused on the following six major aspects: 1) Probing the relationship between defective structures and specific electrocatalytic activity.Different structures of graphene defects provide a tremendous "gene pool" for boosting various electrochemical reactions.However, it is difficult to combine specific defective structures with target functions (such as ORR, HER, and CO 2 RR catalytic activity) via traditional trial-and-error method.Therefore, theoretical predictions should be used in electrocatalyst design and synthesis.In addition, machine learning and high-throughput calculations can also be used to facilitate the screening of defect structures with specific and excellent performance.This will assist the design and synthesis of electrocatalysts for broad applications; 2) Synthesizing specific defect structures.For the conventional synthetic methods, it is very hard to create single type of defects in carbon materials.Typically, carbon defects with different configurations are coexisted.On the one hand, defective structures in graphene are closely related to their specific catalytic performance, therefore, synthesizing DG with specific defective structures is beneficial to achieving the target catalytic activity.Coexisted defective structures, on the other hand, make it difficult to study the reaction mechanisms and pinpoint the precise active sites.Therefore, it is promising to develop synthetic strategies to fabricate single-type long-range ordered defective carbon-based materials for a specific reaction; 3) Investigating the interaction between defects and adjacent 2D planes.The synergistic effect normally promotes the catalytic reactions by modifying the electronic structures surrounding the active sites.However, not all modifications have positive influence on the activity of catalysts.More depth research needs to be conducted to build appropriate descriptors, such as defect density, charge transfer, and the gap between surfaces, to targetedly promote the catalytic performance of defective electrocatalysts; 4) Developing advanced electrochemical in situ characterization techniques.With the development of advanced characterization methods, including synchrotron atomic X-ray absorption spectrum (XAS), synchrotron X-ray diffraction, and X-Ray photoelectron spectroscopy (XPS), thermogravimetry-mass spectrum (TG-MS), and Raman, it is possible to observe the electrocatalytic process dynamically, and identify the actual active centers, and accordingly reveal the catalytic mechanisms via operando characterizations; 5) Developing long-lifetime DG-based catalysts and investigating the relationship between structures and stability.Apart from the catalytic activity, stability is another key parameter to evaluate the performance of electrocatalysts.Nowadays, high electrocatalytic activity has been achieved by DG-based materials.However, the short lifetime of defective catalysts significantly hampers the practical use of DG-based catalysts, especially under oxidation potentials.Therefore, the design of ultra-stable defective electrocatalysts is an important step toward their wide application.Multilayered heterostructure has the potential to exhibit both high activity and long-term stability because of the electronic interaction between adjacent layers; and 6) Developing more accurate theoretical modeling methods.DFT calculations have irreplaceable functions, for example, confirming the electronic structures, predicting reaction pathways, and searching transition states.However, the requirement of evaluating the kinetics and reaction barriers of fundamental steps under realistic reaction environments cannot be achieved by existing theoretical techniques or functionals characterizing interface charges.Multiscale modeling can assist the understanding of the transport implications for interfacial catalytic processes.

Figure 3 .
Figure3.a) Types of nitrogen species that can be incorporated into graphene, where silver, brown, red represent N, C, and O atoms, respectively.b) A digital photo of a transparent N-graphene film floating on water after removal of the nickel layer by dissolving in an aqueous acid solution.c) AFM image of the N-graphene film.Reproduced with permission.[22b]Copyright 2010, American Chemical Society.d) Schematic illustration of NRR for BG and the atomic orbital of BC 3 for binding N 2 .e) Reaction pathways and the corresponding energy changes of NRR on BC 3 , BC 2 O, BCO 2 , and C, respectively.The dotted rectangular box indicates the steps that cannot take place.Reproduced with permission.[30]Copyright 2018, Elsevier.

Figure 4 .
Figure 4. a) Illustration of the chemical titration for the three kinds of oxidation groups.b) Mass activity at 0.75 V (vs RHE) and relative I R at 0.55 V (vs RHE) current for different catalysts in 0.1 M KOH.Reproduced with permission.[34]Copyright 2021, John Wiley and Sons.Slab models used to represent c) C-O (model A), d) C═O (model B), and e) O═C-O (model C) and related NRR mechanisms.Also shown are charge differences upon N 2 adsorption.Reproduced with permission.[35a]Copyright 2019, The Royal Society of Chemistry.

Figure 5 .
Figure 5. a) Schematic illustration of S-graphene preparation.b) LSV curves of various graphene and a Pt/C catalyst on a glass carbon rotating disk electrode saturated in O 2 at a rotation rate of 1600 rpm.c) Chronoamperometric responses of S-graphene-1050 and Pt/C-modified GC electrodes.Reproduced with permission.[37]Copyright 2012, American Chemical Society.ORR products when d) edge-doped S atom serves as active center, and e) edge-C atom serves as active center.Reproduced with permission.[38]Copyright 2014, American Chemical Society.

Figure 7 .
Figure 7. a) Schematic summary of the proposed 14 configurations of the single and dual heteroatom-tuned C5 defect.brown, blue, green, pink, and yellow represent C, N, B, P, and S atoms, respectively.b) Correlations between the experimental half-wave potentials and the theoretically calculated overpotentials of the samples in acidic media.c) The fast Fourier transformation-filtered HAADF image of DG-NS measured with an acceleration voltage of 80 kV.Reproduced with permission.[57]Copyright 2020, Elsevier.d) Synthetic scheme for the preparation of a D-HOPG sample.e) The N 1s highresolution XPS spectra of HOPG (purple), Ar-HOPG (red), N-HOPG (blue), and D-HOPG (black).The pink and green curves are the fitted characterized curves of the pyridinic N and graphitic N in the XPS spectra.f ) The Raman spectra of HOPG, Ar-HOPG, N-HOPG, and D-HOPG (colors as for (f ).g) The HAADF-STEM image of N-G.h) The HAADF-STEM image of the derived D-G.i) The LSV curves of Ar-HOPG (red), N-HOPG (blue), and D-HOPG (purple) (for the ORR in 0.1 M H 2 SO 4 solution).Inset: correlated onset potentials (V vs RHE at 0.05 mA cm À2 ).j) ORR activities of N-HOPG with etching times from 60 to 120 min.k) ORR activities of D-HOPG with etching times from 60 to 120 min.Reproduced with permission.[58]Copyright 2019, Springer Nature.
Figure8a-cschematically shows the configurations of the possible topological defects and the controllable synthesis processes via a N doping and removal method.The formation energy was calculated by DFT modeling.Briefly, 1) doped N in perfect carbon matrix prefers to be graphitic-N with an energy increase of 0.76 eV.Removing one graphitic-N can generate a vacancy, while single vacancies tend to migrate and merge into divacancies (C585); 2) In edge-rich carbon, N atoms are more easily located at zig-zag edge with an energy decrease of 2.48 eV and producing pentagon-rich carbon (S-C5) by removing pyridinic-N; and 3) The removal of two types of pyrrolic-N atoms in pentagon-edge-rich carbon would generate regular hexagonal rings or adjacent pentagon defects (A-C5) on carbon edge.Based on the theoretical study, a fabrication strategy of directional synthesis of topological defects was designed.By regulating the amount of Zn species, three samples contain different defective structures were fabricated, which are D-CNF-1 with C585 defect, D-CNF-2 with S-C5 defect, and D-CM with A-C5 defect.Figure8d,e show the structures of C585 and A-C5 defects, as revealed by the AC-HAADF-STEM images of D-CNF-1 and D-CM after fast Fourier transformation filtering.All the possible defect structures are schematically demonstrated in Figure8f.Based on the electrochemical performance tests shown in Figure8g,h, D-CM (A-C5) and D-CNF-1 (C585) exhibit excellent electrocatalytic activity for the ORR and HER, respectively.Furthermore, DFT calculations were performed to predict the electrocatalytic activity of all the possible defective structures, and the theoretical overpotentials are shown in Figure8i.A-C5 and C585-2 (edge C585) exhibit the highest ORR and HER activity, respectively, which are highly consistent with the experimental studies.This work provided a general method to synthesize target topological defects by controlling specific N-doping and removal mode.Combining DFT calculations with the experimental works, specific electrocatalytic activity of different defective structures has been revealed, beneficial for the target design of efficient electrocatalysts.

Figure 8 .
Figure 8. a) Schematic and formation energy calculation of transformation from edge-deficient carbon to GN-dominated carbon and then to divacancyrich carbon.b) Schematic and formation energy calculation of transformation from edge-rich carbon to PDN-dominated carbon and then to pentagonrich carbon.c) Schematic and formation energy calculation of transformation from pentagon-edge-rich carbon to PON-dominated carbon and then to special carbon reconstruction.AC-HAADF-STEM images of d) D-CNF-1 (C585) and e) D-CM (A-C5) after fast Fourier transformation filtering.f ) The schematic of all defect types.g) ORR performance of PON-CM and D-CM (A-C5) in 0.1 M KOH.h) HER LSV curves of three defective samples (D-CNF-1, D-CNF-2, and D-CM) in 0.5 M H 2 SO 4 .i) Comparison of calculated overpotential for the different carbon defect models in ORR and HER.Reproduced with permission.[60]Copyright 2020, Elsevier.

Figure 9 .
Figure 9. a) Conceptual illustration of UV irradiation for each oxygen concentration.Reproduced with permission. [63]Copyright 2021, Elsevier.b) Diagram of CO 2 gas diffusion on carbon cavity and relevant redox reaction at pyrolytic process.c) The variation tendency of concentration of CO and CO 2 gas on the carbon cavity under different open angles.d) Comparison of I D /I G values (based on Raman spectrum) for catalysts with different pyrolysis conditions.e) HRTEM of HDPC with fast Fourier transformation (FFT) filtering.f ) LSV curves of LDPC, HDPC, and 20% Pt/C in O 2 -saturated 0.1 M KOH at 1,600 rpm.g) Free energy diagram SC-5, C-56 665, C-5665, and C-565 models at potential of 0 V. h) The electron density corresponding to adsorbed OOH* on SC-5, C-56 665, C-5665, and C-565 sites.Reproduced with permission.[65]Copyright 2022, Elsevier.

Figure 10 .
Figure 10.a) Highest-occupied molecular orbital (HOMO) and lowest-unoccupied molecular orbital (LUMO) distributions of the c-ND-Fe (left) and e-ND-Fe (right) models.Reproduced with permission. [76c] Copyright 2020, John Wiley and Sons.b) LSV curves of synthesized TM-N-G catalysts.c) Free energy diagram for the ORR of Fe-NG, Co-NG, and CoFe-NG at the potential of 1.23 V (upper), and schematical diagram of ORR process on CoFe-NG(down).Reproduced with permission.[77]Copyright 2023, Elsevier.d) Charge density difference of the pyridinic-type FeN 4 , FeN 4 -OH, and FeN 4 -O centers.The yellow and green areas denote electron accumulation and depletion, respectively.e) Partial density of states (PDOS) of the Fe centers and their d-band center.f ) Scheme of the ORR on FeÀN-C catalysts at pH = 1 and pH = 13.Reproduced with permission.[78]Copyright 2023, American Chemical Society.

Figure 11 .
Figure 11.a) TEM image of the prepared DG@FeCo sample (inset: a histogram shows the metal particle size distribution).b) Polarization curves of the prepared samples and Pt/C measured in an O 2 -saturated 0.1 M KOH solution.Reproduced with permission.[83]Copyright 2017, The Royal Society of Chemistry.c) The HAADF-STEM image of the defect area of A-Ni@DG.d) the K 2 -weighted Fourier transform spectra of Ni@DG, A-Ni@DG, and the Ni foil reference samples.Reproduced with permission.[84]Copyright 2017, Elsevier.e) Schematic illustration of the preparation of Pt@DG.f ) HAADF-STEM images of Pt@DG.g) XANES spectra for Pt@DG, Pt foil, and PtO 2 at Pt L 3 -edge.h) EXAFS spectra of Pt@DG, Pt foil, and PtO 2 .i) TOF value of Pt@DG and other state-of-the-art electrocatalysts in 0.5 M H 2 SO 4 .Reproduced with permission.[85]Copyright 2022, American Chemical Society.
Figure12.a) Free energy plots of ORR and optimized configurations of adsorbed species on the g-C 3 N 4 surface with zero, two, and four electron participation.And schemes of ORR's pathway on pristine g-C 3 N 4 without electron participation, pristine g-C 3 N 4 with 2e À participation, and g-C 3 N 4 and conductive support composite with 4e À participation, respectively (red areas represent the active sites facilitating ORR).Reproduced with permission.[88]Copyright 2011, American Chemical Society.b) TEM image of Co 3 O 4 sheet/graphene (CoÀS/G) composites.c) LSVs of CoÀS/G-3, CoÀP/G, and Pt/C at 1600 rpm in O 2 -saturated 0.1 M KOH.d) The side view of 3D charge density difference plot for the interface between (left) a graphene sheet and a Co 3 O 4 layer and a (right) N-doped graphene sheet and a Co 3 O 4 layer.Yellow and cyan isosurfaces represent charge accumulation and depletion in the 3D space with an isosurface value of 0.004 e Å 3 .Brown, blue, red, and green balls represent C, Co, O, and N atoms, respectively.Reproduced with permission.[89]Copyright 2015, American Chemical Society.e) Top and side views of the optimized configurations for DV-G onNi(111).Green and grey atoms represent Ni and C, respectively.f ) Projected DOS for the d-orbitals of a TM atom in double vacancy of graphene and total DOS for DV-G on Co(0001) andNi(111).The vertical red-dashed lines represent the location of the d-band center of the doped TM atoms.g) Charge density difference plots of DV-G/Co(0001) and DV-G/Ni(111).The yellow and cyan colors represent the charge accumulation and depletion area, respectively.Ni, Co, and C atoms are in green, pink, and grey, respectively.Reproduced with permission.[90]Copyright 2019, American Chemical Society.

Figure 13 .
Figure 13.a) Schematic illustration of the preparation of NiFe LDH-NS@DG nanocomposite.b) Linear sweeping voltammetry curve of NiFe LDH-NS@DG10 as OER and HER bifunctional catalyst in 1 M KOH for overall water splitting, with the inset showing comparison of different catalysts including (i) NiFe LDH-NS@DG10 with 2 mg cm À2 loading; (ii) NiFe LDH-NS@DG10 with 1 mg cm À2 loading; (iii) NiFe LDH-NS@NG10 with 2 mg cm À2 loading; (iv) NiFe LDH-NS@ G10 with 2 mg cm À2 loading; (v) bare Ni foam electrode).c) Comparison of the required voltage at a current density of 20 mA cm À2 for the NiFe LDH-NS@DG catalyst with other state-of-the-art noble metal-free bifunctional catalysts.d) Demonstration of a solar power-assisted water-splitting device with a voltage of 1.5 V. e) The top views of optimized Ni-Fe LDH-NS@DG (DG-5, DG-585, or DG-5775) based composite interfaces.f ) The side views of 3D charge density difference plot for the interfaces between a defective graphene sheet (DG-5, DG-585, or DG-5775) and a Ni-Fe LDH-NS layer are demonstrated.g) The schematic of the probable electrocatalytic mechanism of Ni-Fe LDH-NS@DG for HER and OER is presented based on the DFT calculation results.The pink and purple spheres represent electrons and holes, respectively.Reproduced with permission. [11c] Copyright 2017, John Wiley and Sons.h) HAADF image of A-CoPt-NC after a fast Fourier transformation (FFT) filtering.The bright red spots are metal atoms, and the cyan spots are carbon/nitrogen atoms.i) Side view and top view of the charge distribution of single-layered model.j) Side view and top view of the charge distribution of double-layered model.C brown, N green, Pt white, Co blue.Yellow and cyan isosurface represent electron accumulation and electron depletion.Reproduced with permission.[93]Copyright 2019, John Wiley and Sons.