Recent progress in theoretical and computational investigations of structural stability and activity of single-atom electrocatalysts

Progress in Natural Science: Materials International 29 (2019) 256–264

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Review

Recent progress in theoretical and computational investigations of structural stability and activity of single-atom electrocatalysts

T

Youwei Wanga, Erhong Songa, Wujie Qiua, Xiaolin Zhaoa, Yao Zhoua, Jianjun Liua,∗, Wenqing Zhangb a

State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China b Department of Physics and Shenzhen Institute for Quantum Science & Technology, Southern University of Science and Technology, Shenzhen, Guangdong 518055, China

ABSTRACT

This review has been made an attempt to seek some observed descriptors to comprehensively optimize structural stability and catalytic activity of single-atom catalysts. Electronic structure analysis can reveal the underlying mechanisms of atomic phy-chemical properties and local coordination structures on binding strength. The free electrons around Fermi level play an important role to determine the binding strength, which can be further influenced by electronegativity difference between single atom and nearby support atoms. This investigation can contribute to understanding the underlying mechanisms for the design of stable and active catalysts, and further provide a deep insight to the potential pathways in the research field of single-atom catalysts.

1. Introduction Precious-metal-group (PMG) materials, Pt, Au, RuO2, and IrO2, are commonly utilized as state-of-the-art electrocatalysts in a wide range of electrochemical reactions, such as, hydrogen evolution reaction (HER), oxygen reduction reaction (ORR), and oxygen evolution reaction (OER) in energy conversion and storage devices [1–4]. However, the high costs and scarcity of PMG materials significantly prevent wide-spread applications. One way to deal this problem is to alloy with cheaper metal elements or develop the metal-free catalysts. Alternatively, an effective strategy is to disperse precious metals in ultra-small forms (ultrafine nanoparticles, clusters, single atoms). This approach can improve efficiency and catalytic activity due to providing a large number of active sites and cost-saving as well. Single-atom catalyst (SAC) represents the maximum utilization for metal catalysts, and it has also attracted tremendous attentions as a new research frontier of catalysis field [5–7]. The structural stability can be represented by the binding strength between single atom and supports, and it can contribute to the resistance to sintering and long-lasting of catalysts. However, the strong binding interaction may lead to a low catalytic activity. Therefore, tuning binding strength between single atom and supports is a big challenge because the suitable interaction is required to not only prevent single atom agglomeration into clusters, but also to perform high catalytic activity and selectivity for a specific electrochemical reaction [8–10]. Since the binding strength directly influences the structural

∗

stability and catalytic activity of SACs, the computational and experimental efforts have been made to reveal interaction mechanism and trends across different metals and supports in order to screen and design novel SACs [5,6]. To achieve the optimal catalysts, several noticeable structural characterizations via high-resolution and in-situ spectrums have been performed to capture single-atom migrations during high-temperature formation of SAC, and further to examine local coordination structure of catalytic sites [11,12]. These studies are beneficial to identify the characteristics of the active sites and formation mechanism of SAC. Density functional theory (DFT) has also been proven to serve as a powerful tool on exploring correlation of structural stability and catalytic activity with binding strength. Furthermore, DFT-based electron structure analysis exhibits a strong advantage to reveal the electron interaction mechanism between single atom and supports, and between single atom and catalyzed molecule. Zeng et al. obtained a universal principle formulated by atomic electronegativity and local structure coupled with electrocatalytic activity in ORR, OER, and HER by highthroughput calculations [13]. This pioneering work provides a deep insight into catalytic activity of SACs. Since several theoretical models have been developed to correlate the catalytic activity or structure stability with the adsorption strength of reaction intermediate, d-band center of transition metals, and eg filling number, and charge transfer ability [14], the simultaneous correlation of structural stability and catalytic activity with a specific parameter is not reported so far. Observed or tabulated parameters are

Corresponding author. E-mail address: [email protected] (J. Liu).

https://doi.org/10.1016/j.pnsc.2019.04.004 Received 18 March 2019; Received in revised form 3 April 2019; Accepted 4 April 2019 Available online 20 April 2019 1002-0071/ © 2019 Published by Elsevier B.V. on behalf of Chinese Materials Research Society This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).

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Fig. 1. Illustration of competitive relationship between (a) catalytic activity and (b) structural stability of SACs.

still lacking to design and screen high-activity, while stable catalysts for practical application despite of much importance. In this review, an attempt has been made to obtain a rational description for structural stability and catalytic activity of SACs. Partial electrons (in purple) under the same orbital symmetry are applied to chemically bind with substrate atoms, therefore losing its catalytic ability, as shown in Fig. 1. Changing relative positions between active atom and substrate may simultaneously tune distribution amount of free electrons near Fermi level for stability and catalysis. The factors associated with competitive distribution of free electrons near Fermi level should be considered during simultaneously combined optimization of structural stability and catalytic activity. In Section 1, the relationship between the stability of SACs and the amount of free electrons near Fermi level has been addressed using the parameter of binding strength. In Section 2, the relationship between catalytic activity and the densities of free electrons was considered to optimize the chemisorption. The outlook has finally been provided based on the issues of stability and catalytic activity of SACs.

reducible surface, a metal atom with low oxygen affinity has still weak binding with the supports and thereby results in low stability of SACs. Compared the states density of strong-binding Ir@CeO2 and weakbinding Ag@CeO2, there are many overlapping states of Ir2+ with Ce3+ and O2− than those of Ag+ below Fermi level. It means that the covalent bond between metal atoms and oxide support is more stable than ionic bond. In Fig. 2, where blue and green denote depletion and accumulation of electron density, respectively, Ir@CeO2 causes significant charge transfer from Ir atom to the neighboring Ce and O atoms, which leads to the formation of Ir2+ states and strong binding strength between Ir atom and the surface. Ag@CeO2 has less charge transfer from Ag atom to the surface, whereas Ir@MgO, even worse, has much charge transfer from surface to Ir atom, which is less stable than Ir@CeO2. Based on the interaction between metal atom and supports, Connor et al. therefore chose the binding energy as a descriptor to screen metal-support combinations and thereby to produce stable SACs. In addition to the binding strength between metal and supports, the binding strength between metal and metal is another factor to overcome for the stable SACs.Wei et al. reported that noble metals nanoparticles (NPs), such as Pd, Pt and Au-NPs, can be transformed into thermally stable single atoms (Pd, Pt and Au-SAs) to produce stable SACs [16]. Using in-situ environmental transmission electron microscopy, dynamic process was recorded to show the competing sintering and atomizing processes during NP-to-SA conversion. Driven by the formation of the more thermodynamically stable Pd-N4 structure, when mobile Pd atoms were captured in the defects of nitrogen-doped carbon, Pd-NPs undergo thermal motion with CN and down sized by the coordination of surface Pd with N defects before final atomization. This becomes the major driven force of NP-to-SA transformation. The atomization and sintering depend on the large values of binding strength between metal-support (Pd-N) and metal-metal, (Pd-Pd). Based on DFT calculations, Wei showed the Pd transformation from Pd-NPs to Pd coordinated with two pyrrolic N and two pyridinic N at the defects of graphene. Though there is a kinetic barrier of 1.47 eV, the formation of a single-Pd-atom in the N4 defect releases a large exothermicity of 3.96 eV. As a result, a high temperature (∼900–1000 °C) is requested to overcome the kinetic barrier of 1.47 eV thereby to complete the atomization process. Wei et al. established a convenient top-down route to obtain single-atom catalysts from metal nanoparticles, and emphasize the effect of the competitive bonding strength between metal-support and metal-metal on the stability of SACs [16].

2. Structural stability of SACs SACs offer the maximal efficiency of expensive and active metal components. However, one of the main problems to prevent their utilization is to know whether metal atoms and the supports form SACs or agglomerate into clusters. Therefore, the interaction trends between metal atoms and supports become critical to produce sinter-resistant and high-activity SACs. Connor et al. recently investigated the effects of interaction between the metals and supports on the structural stability of SACs [15]. The degree of a metal anchored to a support is governed by the binding energy, i.e., metal atoms that exhibit strong exothermic binding to a support are less likely to diffuse across the support and/or agglomerate into clusters [9]. They found that there is a linear trend between the adsorption energy of a metal on an oxide surface and the formation enthalpy adsorbed of a metal oxide, and that the slopes of different surfaces are linearly correlated with the surface oxygen vacancy formation energies. This linear correlation over a large range of oxygen vacancy formation energies indicates that the reducibility of each supports reflects its capacity to strongly adsorb metal atoms, i.e., the most reducible surface TbO2 (111) can strongly adsorb a metal atom than the most irreducible surface MgO (100). Furthermore, in a 257

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Fig. 2. Metal–support interactions on CeO2 (111) and MgO (100) substrates. Isostructural charge density difference plots of (a) Ir/CeO2 (111), (b) Ag/CeO2 (111) and (c) Ir/MgO (100). (d) DOS plot of Ir/CeO2 (111) and an orbital density image of the peak, indicated with an arrow. Reproduced with permission from Ref. [15]. Copyright: 2018 Nature Publish Group.

In a recent study of Pt-MoS2, DFT calculations showed that Pt can be stabilized by bonding within S vacancies, and the corresponding energy barrier is much higher than that of S vacancies migration. Therefore, there is no preference for S vacancies to migrate next to Pt atoms in MoS2 [11]. The Pt atoms filling in the S vacancies have a large effect on the surface catalytic activity for HER, since the Pt-d electrons can shift up to the gap due to p-d hybridization, resulting in the relatively low binding energy. In the Mo-top configuration, hybrid electrons fill into the unoccupied orbits to stabilize the whole structure. Either an individual S vacancy or Pt fillings can active the catalytic site, however both of them would suppress the HER activity [11]. The reason is that additional lone pair electrons near Fermi level can be introduced to enhance the catalytic activity, while these unbound states normally reduce the stability of the localized defective structure. The Pt adsorption on the surface of organic semiconductor g-C3N4 can also exhibit single-atom catalytic activity [17]. Since the bandgap between the highest occupied molecular orbitals (LUMO) and lowest unoccupied molecular orbitals (HOMO) is determined to 2.7 eV in pure g-C3N4, the bandgap of Pt2+ doped g-C3N4 can reduced to 1.8 eV according to DFT calculations. The doped Pt leads to new occupied orbitals above the original HOMO state, which is the result of metal-toligand charge transfer (MLCT) between Pt2+ and g-C3N4 aromatic rings. Therefore, Pt2+ is stabilized by a strong p-d coupling between Pt atom and supports. The compound with the local characteristics can therefore be synthesized experimentally. MLCT also locally works in the N conjugated aromatic units. Since Pt2+ can provide an extra electron state to push the HOMO to a higher energy level, the local structure exhibits strong catalytic activity compared to that of pure g-C3N4. According to this strategy, g-C3N4-Cu+ similar to g-C3N4-Pt2+, can also achieve the high chemical stability and good catalytic performance. In short, the competitive binding strength between metal-support and metal-metal has a great effect on the stability of SACs. In the metalsupport, the strong binding strength leads to stable SACs, whereas in the metal-metal, the strong binding strength results in stable NPs and

unstable SACs. Since the binding strength depends on the p-d coupling between metal and support, the stable SACs request strong p-d coupling and deep states, while the unbound states near Fermi level may weaken the stability. 3. Catalytic activity for HER, ORR and OER In catalytic activity, Zeng et al. developed a coordination number and electronegativity principle to evaluate the HER, OER and ORR activity of graphene-based SACs [13]. In Fig. 3, a descriptor, E + × (nN × EN + nC × EC ) = d× M , where EN, EC, EO/H and EM represent the EO / H electronegativity of N, C, O/H and metal elements, respectively; nN and nC represent the number of nearest-neighbor N and C atoms; θd is the occupied d electron number of the metal element and α is the correction coefficient, shows a linear relationship with the adsorption energy of OH*, ΔGOH*, and a volcano trend with the adsorption energy of H*, ΔGH*. As the descriptor φ stands for the electronegativity of single metal atom at the surface, the relationship between φ and OH*/H* characterizes the binding strength between the free electron of single metal atom and adsorbates. The identified descriptor φ suggests that the intrinsic abilities of gaining and losing electron of single metal atoms play a key role in the catalytic activity of SACs on graphene. For example, the binding strength between single metal atoms and OH adsorbates become stronger and stronger as the free electron increase. This change offers a great effect on the catalytic activity. Therefore, the control of free electron of single metal atoms becomes significant to enhance the catalytic activity. 3.1. SACs for HER HER is a typical two-electron-transfer reaction, which can be divided into Volmer and Heyrovsky or Tafel reactions: Volmer step: H+ + e + 258

H

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Fig. 3. Schematic of SAC-like metal–macrocycle complexes (a) single vacancy with three carbon atoms (SV-C3-m), (b) double vacancy with four carbon atoms (DVC4-m), (c) four pyridine nitrogen atoms (pyridine-N4-m) and (d) four pyrrole nitrogen atoms (pyrrole-N4-m). And scatter of adsorption free energies of (e) OH* and (f) H* versus the descriptor φ, for all single TM atoms supported on macrocyclic molecules. Reproduced with permission from Ref. [13]. Copyright: 2018 Nature Publish Group.

Heyrovsky step: H+ + e + H Tafel step: H

1 H 2 2

H2 +

+

Correspondingly, the free energy changes are given by:

GVolmer = EH GHeyrovsky = E H2 + =

EHads

1

GTafel = 2 E H2 + =

EHads

1 E 2 H2

+ EZPE

EH

1 E 2 H2

( EZPE

T SH = EHads + EZPE ( EZPE

T SH

T SH )

T SH )

EH

( EZPE

( EZPE

T SH )

T SH ) (1)

The optimal value of ΔGH* is equal to 0 eV, where adsorbed atomic hydrogen in a thermo neutral state can perform efficient proton/electron-transferred and hydrogen release [18–20]. It is well known that Ptbased nanomaterials are the best-performing catalyst for the HER applications [21,22]. However, the need is to search for earth-abundant catalysts comparable to Pt catalysts due to the scarcity and high cost of Pt-based catalyst [23,24]. Herein, it is reported that a series of single transition metals-doped graphene (TM-Graphene) materials are promising alternatives as efficient HER electro-catalysts using combing experimentally measurements and density functional [13,25–29]. In Fig. 4, the relationship between the surface adsorption ability of TM-Graphene and its free electron around Fermi level can be explained by the underlying scheme for bond formation. When a proton from the electrolyte and an electron from the electrode combine on the TM-Graphene surface to form a H*, the free electronic states of the active TM-Graphene are interacting with that of hydrogen, and consequently their hybridized energy levels split into two groups. One is the anti-bonding states (σ*) that normally goes across the Fermi level, and another is the bonding orbital (σ) that is positioned under the Fermi level. Generally, the difference in the binding strength comes from the anti-bonding states, which is, with a higher location of the active center Ed, the anti-bonding states move higher with a lower occupancy. This state leads to a stronger binding

Fig. 4. Energy level diagram showing orbital hybridization of active sites and hydrogen adsorbate. EF is the Fermi level of the substrate.

strength between adsorbate and catalyst surface. Therefore, the peak of free electronic states of the active TM-Graphene closer to the Fermi level is determined to tune the catalytic activity, which achieve a stronger H* adsorption and consequently a lower value of ΔGH*. Cheng et al. constructed individual Pt atoms and clusters on the surface of N-graphene nanosheets (NGNs) via the atomic layer deposition (ALD) technique (see Fig. 5a) [25]. As ALD cycles control the size, dispersion and amount of Pt clusters, the uniformly dispersed single Pt atoms on the surface of NGNs were obtained through reducing the number of ALD cycles. Compared with the conventional Pt NP catalysts, the Pt single atoms and clusters on the surface of NGNs showed much high activity. In order to illuminate catalytic activity, electronic structure analyses of Pt-NGNs were carried out in Fig. 5b and c, where the upper, the middle and the lower part of the panels show the projected density of states (PDOS) of the graphene, the N atom and the d orbital of Pt, respectively. The 5d-oribtals of the single Pt atoms are hybridized with the N-2p orbitals around the Fermi level. In this case, the single Pt atoms on the N-doped graphene contain many unoccupied 5d densities of states above the Fermi level. In H adsorbed on Pt-NGNs, the 5d 259

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Fig. 5. (a) Deposition of Pt on NGNs via ALD method. (b) PDOS of non-H and (c) two H atoms adsorbed on a single Pt atom of ALDPt/NGNs. Reproduced with permission from Ref. [25]. Copyright: 2016 Nature Publish Group.

orbitals of the Pt atoms interact strongly with the 1s orbital of the H atoms, leading to electron pairing and hydride formation (Fig. 5c). Therefore, the catalytic activity of Pt atoms on NGNs are correlated with the 5d orbitals of the Pt atoms around the Fermi level, which determines the amount of electron transfer from the Pt to the N atoms. Tavakkoli et al. also reported that the single Pt atoms or sub-nanoclusters can be anchored on the surface of single-walled carbon nanotubes (SWNTs) using facile electroplating deposition [26]. In structural stability of SACs, the axial site is more stable for Pt adsorption at 0.14 eV. Furthermore, the bind strength on SWNT sites is stronger than that on the considered graphene, which demonstrates that the structural stability of Pt SWNT is more stable. DFT calculations confirmed that the Pt-modified SWNTs have showed a HER catalytic activity (ΔGH* = −0.03 eV) comparable to that of Pt/C. In the reaction mechanism, it is revealed that the Volmer-Heyrovsky path is the primary HER path, and the Heyrovsky reaction is the rate-determining step with a 0.92 eV activation energy. In summary, Pt atoms are stabilized by p-d coupling between SWNTs and Pt atom. This strategy can efficiently immobilize Pt atoms to achieve high catalytic activity. In the nonprecious SACs, Fei et al. fabricated atomically dispersed Co atoms on the surface of N-doped graphene by subjecting graphene oxide and cobalt salts to pyrolysis in an NH3 atmosphere [27]. Compared with N-doped graphene, the introduction of low-loaded Co atoms into N-doped graphene remarkably enhances the HER catalytic activity. The CoNx moieties consisting of active sites are responsible for the HER catalytic activity with a low overpotential. Fan et al. synthesized single Ni atom on graphitic carbon with Ni-containing MOFs as precursors using pyrolysis, acid etching, and subsequent electrochemical activation [28]. Similarly, the obtained Ni-C catalysts performed an exceptional HER activity, high exchange current density and satisfactory stability. The strong p-d orbital coupling between the single Ni atoms and graphitized carbon can enable a faster electron transfer to improve

electrochemical catalytic efficiency. Nanoporous graphene with single Ni atom was also obtained using the acid etching of the Ni foam which is coated with a CVD-derived graphene nanolayer [29]. The resultant catalyst with substitutional Ni dopants is geometrically and electronically favorable for the HER activity due to the strong interplay between single Ni atoms and the surrounding carbon atoms. In short, a large variety of SACs for HER have emerged in recent studies. It is shown that the single metal atoms can serve as the active sites. The activity of catalyst is attributed to the free electron around Fermi level through p-d coupling between the metal centers and carbon supports to produce additional catalytic active centers. 3.2. SACs for OER SACs provides an atomic-scale way to maximize the atom efficiency and enhance the OER activity. Since OER is a four-electron oxidation and the kinetically sluggish process for electrolysis of water, the reactions and Gibbs free energy changes of OER in water electrolysis are given by:

OH + H+ + e

H2 O (l) + G1 = GOH

+ 1 2 H2

ads G H2 O (l) = EOH + EOH +

1 EH 2 2

eU

E H2 O (l) (2)

OH

O +

G2 = GO

H+

+ 1 2 H2

+e

GOH = (EOads

ads EOH ) + EO +

1 EH 2 2

eU

EOH (3)

O + H2 O (l) 260

OOH +

H+

+e

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G3 = GOOH EO OOH

+H2 O

ads = (EOOH

EOads ) + EOOH +

1 EH 2 2

2 H2

H+

than that of Ir/C (82 mV/dec). The faster charge transfers in aNi@DG during the OER process results in a faradaic efficiency of 98.6% and a small overpotential of 270 mV to reach the current density of 10 mA/ cm2. In Fig. 7, aNi@DG significantly tunes the local electronic structures near Fermi level and serves as an active site for electrocatalytic reactions. Based on d-band center theory [37], single transition-metal atom with the d-band states near Fermi level will strongly binding with HOMO and LUMO of the adsorbates. A Ni@Di-vacancy, in which single Ni atom has a high density of states near Fermi level, has a strong bonding strength between the adsorbates and SAC and therefore performs the high catalytic activity for OER. The OER-activity of single-metal-atom catalysts depends on the states of single metal atom near Fermi level. After binding with the supports, the residual free electron number of single metal atom becomes the dominating factor for catalytic activity. Less free electron of single metal atom leads to weaker interaction with reactants, while more free electron results in stronger interaction. Therefore, a single metal atom with the opportune free electron, which leads to the adsorption energy difference between *O and *OH around 1.5 eV, i.e., ΔG*O-ΔG*OH ≈ 1.5 eV, will perform high OER-activity.

eU (4)

E H2 O

O 2 (g ) +

G4 = GO2+ 1

GO

+ 1 2 H2

+e +

GOOH = EO2 +

1 EH 2 2

eU

ads EOOH

EOOH

(5)

and the overpotential (η) can be calculated as the maximal Gibbs free energy difference between the step reactions and the intrinsic water splitting reaction:

= max( G1, G2, G3, G4 )/e

1.23[V ]

(6)

Zeng et al. grafted a Pt atom onto Fe center of Fe-N4 in Fe-N-C nonprecious metal catalyst through a bridging oxygen molecule, to create a new active moiety of Pt1-O2-Fe1-N4 as multifunctional electrocatalyst [30]. The Fe-N-C catalysts have been reported as high-activity ORR catalyst by Dodelet and co-works [31,32]. Though Pt onto Fe-N4 has no significant increase on the ORR activity of Fe-N-C, Pt@FeN-C improves the durability compared with the support Fe-N-C. In addition, Pt@Fe-N-C performs similar HER kinetics as Pt nanoparticles. The Tafel slope of Pt@Fe-N-C, 42 mV/dec suggests an electrochemical desorption mode as Heyrovsky reaction. The excellent HER activity can be ascribed to the hetero atom pairs, Pt1-O2-Fe1, which favors the cleavage of H-OH bond in H2O to improve the HER kinetics. Based on the Gibbs free energy of H adsorption (ΔGH*), Pt@Fe-N-C shows a low ΔGH* of 0.16 eV, which is relatively higher than 0.09 eV of Pt/C. Despite of the low OER activities of Pt@C and Fe-N-C, Pt@Fe-N-C shows high OER activity with a 310 mV overpotential and outperforms the RuO2 catalyst. Besides, the similar function to HER of active heteroatom moiety, the low coordinated dangling Pt4+ atoms can facilitate the H2O adsorption to promote the OER process. Since Pt metal and oxides have the optimal binding energies with H* (ΔGH*≈0.9 eV) and O*/OH* (ΔGO*-ΔGOH*≈1.6 eV), which make Pt metal and oxide become highactivity catalysts for HER and OER, respectively [33,34], the HER and OER activities of SACs attribute to single Pt atom. The superior HER/ OER/ORR multifunction of Pt@Fe-N-C, in which the HER and OER activity is mainly due to the intrinsic activity of Pt atoms, raises the possibility to combine the fuel cell and water splitting in one device. In addition to oxide support, Li et al. theoretically studied singleatom catalytic activity of five types of TMs (Pt, Pd, Co, Ni and Cu) for OER in graphitic carbon nitride (g-CN) and screened Co@CN and CoO@CN to be the high-activity catalysts for OER [35]. In TM atoms, there are two types of bonds, i.e., (i) the strong covalent bond between TM atoms and g-CH prohibits metal aggregations, which leads to high durability, and (ii) the TM-O bond between TM atoms and *O from adsorbate, which is the key factor for OER catalytic activity. In Fig. 6a, the strong covalent bond leads to an effective charge transfer of 0.92e from Co to g-CN, which has a strong binding strength of 4.68 eV. That strong binding strength prohibits metal aggregation to obtain high durability. Apart from metal atoms, Li et al. chose metal-oxygen as active sites and got positive charges on the TM atoms as single metal atoms (see Fig. 6b). In consistent with previous studies on other OER catalysts [34], the catalytic activities of TM@CN performs a volcano dependence on the energy difference between the Gibbs free energies for *O and *OH catalytic steps (see Fig. 6c). In Fig. 6d, the volcano trend of OER activity is ascribed to the polarized charge distributed on the active TM single-atom. The catalytic activity of TM-O@CN decreases as polarized charges increase. The Gibbs free energy differences and polarized charges revealed that the overpotential is always in the order of TM@CN < TM-O@CN, which means that the catalytic activities of single metal atoms are better than those of metal-oxygen groups. Zhang et al. trapped atomic Ni in graphene defects to form an integrity of a Ni atom at defective graphene (aNi@DG), which is experimentally identified by transmission electron microscopy (TEM) [36]. The Tafel slope of aNi@DG (47 mV/dec) is almost half smaller

3.3. SACs for ORR In efficient and reliable electrochemical energy devices, although polymer electrolyte membrane fuel cells (PEMFCs) are promising alternatives, the large-scale application of fuel cell technology requires a significant replacement of noble metal in catalysts [38]. In recent decades, though great progress in ORR electrocatalysts has been achieved, the further development of PEMFCs is severely limited by the high cost of materials, low power density and the kinetic sluggishness. To resolve these concern, electrocatalysts with low cost as well as metal-free electro-catalysts have been investigated widely for exploration as state of the art ORR catalysts [39–42]. The ORR contains numerous elementary steps, including either a four-electron pathway to convert O2 directly into H2O (O2 + 4H+ + 4e− = 2H2O) or a twoelectron pathway with the production of H2O2 (O2+ 2H++ 2e− = H2O2). In the four-electron pathway to form water, the reactions step and Gibbs free energies is given by:

O2 + * O2

G0 = GO2 (U ) O2 + H+ + e

OOH +

H+

GO

+ 1 2 H2

1 EH 2 2

ads EOOH )

EO2

(U ) = E H2 O +

1 EO 2 2

EOOH

eU

(9)

OH

GO

1 EH 2 2

eU

G4 = G H2 O (U )

) + EOOH EOads 2

(8)

GOOH

G3 = GOH (U )

OH + H+ + e

ads (U ) = (EOOH

H2 O + O

G2 = G H2 O + O (U )

O + H+ + e

1

2 + 2 H2

eU

+e

+ (EOads

(7)

OOH

G1 = GOOH (U ) 1 EH 2 2

EO2 = EOads 2

EO2 = EO2

+ 1 2 H2

ads (U ) = (EOH

EOads ) + EOH

EO (10)

H2 O +

GOH

+ 1 2 H2

(U ) = E H2 O

EOH

ads EOH

1 EH 2 2

eU (11)

While the four-electron pathway is preferred in the electrocatalytic 261

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Fig. 6. Computed bader charge differences before and after (a) a single Co atom and (b) a Co-O group bind to the g-CN. Computed OER overpotential η as (c) a function of ΔGO*-ΔGOH* and (d) a function of the polarized charge on the active centers. Reproduced with permission from Ref. [35]. Copyright: The Royal Society of Chemistry 2016.

Fig. 7. (a) Structures and PDOS of three different types of catalytic active sites corresponding to a single Ni atom supported on perfect hexagons, D5775, and Divacancy. (b) Energy profiles of the three configurations for OER. Reproduced with permission from Ref. [36]. Copyright: 2017 Elsevier B.V.

reactions with Pt nanoparticle (NP) based catalysts, the reaction should be steered towards the two-electron pathway for the safe production of H2O2. The electrochemical properties are related well to the binding strength of intrinsic oxygen-containing intermediates (adsorbed species) on the catalyst surface. The difference in the binding strength comes from the anti-bonding states, i.e., the peak of free electronic states in the active SACs near Fermi level is determined to tune the catalytic activity, which leads to a stronger adsorption of OOH* and OH* intermediates. The selectable pathway towards H2O2 or H2O depends on the ablitiy of the catalyst to break the OeO bond. In order to achieve a high selectivity, the concept of SACs was proposed to offer a particularly promising means for the safe production of H2O2 [43,44]. In the high selective production of H2O2, the breaking of the OeO bond necessitates two adjacent active site to adsorb one O2 molecule. A rational design with individual Pd/Au atoms surrounded by host matrix have achieve the process [43]. Compared with Pt-NPs based catalyst, it is expected that Pt-based SACs will be the most promising catalysts due

to their individual atomic sites and high atomic utilization. Yang et al. reported that single Pt atoms supported on TiN/TiC NPs were synthesized, confirming the two-electron pathway for the selectivity of H2O2 with high efficiency [45,46]. Based on DFT calculations, the existence of activation barrier in the Pt@TiC system enable the formation of H2O2 more easily compared with the TiN support. The binding strength between the single Pt atom and the support plays a great role on the electrocatalytic activity of SACs, which serve as an catalytically anchoring site in the reaction. M-N-C (M = Fe, Co) nanostructures have attracted great interest as non-noble metal electro-catalysts due to their abundant sources, easy preparation, and high ORR performance [47–52]. Recently, experimental and theoretical calculations also reveal that the Fe-N moieties as active sites are comparable to Pt-based catalysts in ORR applications [53–55]. It is stimulated that single-atom M-N-C nanostructures allow a homogeneous dispersion of catalytic active sites with high activity and leads to high ORR performances. Contrary to Pt-based SACs for the twoelectron pathway, M-N-C catalysts perform high ORR activity via a 262

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Fig. 8. Calculation models of (a) nitrogen-doped graphene and (b) FeN4-embedded graphene. (c) Correlation between the adsorption energies of the OOH and OH intermediate species. (d) Reaction free energy diagram of reaction steps 1–4 in ORR at the various atomistic sites. (e) DOS plots of N-doped graphene and (f) FeN4 embedded graphene. Reproduced with permission from Ref. [55]. Copyright: 2017, American Chemical Society.

four-electron pathway [49–52]. Based on theoretical studies, the highly coordinate unsaturated Fe center behaves as the active site. The single Fe atom confined in the graphene catalyst by 4N atoms can be conducive to their electro-catalytic stability [55,56]. Chen et al. reported that the Fe center of FeN4 exhibited a much higher activity than carbon atoms of N-doped graphene (Fig. 8a and b). The ORR activity has been limited by two reactions, the reduction of free oxygen into adsorbed oxygen (Eq. (6)) and the transition from adsorbed oxygen into adsorbed hydroxyl (Eq. (9)) [53–55]. The binding strength of the catalytic active sites to oxygen species cannot be too strong or too weak. In this mechanism model, the overall reaction will be optimized with a smallest reaction free energy of the limiting step when ΔG1 = ΔG4. As ΔG2+ΔG3 = ΔEOH*–ΔEOOH*(where ΔG is reaction free energy and ΔE is adsorption energy), and based on the Faraday's law, the total energy ΔG = ΣΔGi = −4E0 = −4.92 eV with E0 being the formal potential (+1.23 V) of oxygen reduction. Therefore, the lowest possible free energy is ΔGave = ΔG1 = ΔG4 = 1/ 2(ΔG1+ΔG4) = 1/2[ΔG–(ΔEOH*–ΔEOOH*)]. Because there is a linear relationship between ΔEOOH* and ΔEOH*, ΔEOOH*–ΔEOH* is a constant, which is defined as b, and the optimal potential is obtained as U = −ΔGave/2e = −(−4.92 + b)/2. Fig. 8c, where the two data points at the bottom are for the Fe sites in Normal FeN4 and StoneWales FeN4, shows the linear relationship between the calculated binding energies of OOH and OH intermediates at the various carbon active sites, including carbon atoms in the FeN4 structures. Notably, the normal FeN4 structures have a more optimal potential than the carbon sites, corresponding the optimal potentials can be estimated to be 0.96 V. The binding strength of intermediates between the Fe centers and support determines the ORR catalytic activity. In Fig. 8d, the binding of oxygen for FeN4 moiety is more favorable than other carbon atoms. The binding of oxygen is more favorable when the amount of free electronic states near Fermi level (see Fig. 8e and f). In the FeN4 graphene system (Fig. 8e), FeN4 has one order of magnitude higher density of state below Fermi level composed predominantly of the Fe 3d orbitals than neighboring nitrogen and carbon atoms. The Fe center favorable to the adsorption of O2 may donate electrons to reduce O2. Due to the low concentrations of complex nitrogen species (the low density of MNx active sites) in conventional M-N-C nanocatalysts, it is necessary to search for other promising supports alternative for novel ORR catalysts with high density of MNx. Recently, graphitic carbon nitride (g-C3N4) contains a large amount of pyridine-like nitrogen

atoms, which offer numerous electron lone pairs to capture metal ions [57–61]. Although the electronic conductivity of g-C3N4 is poor, the incorporation of g-C3N4 into carbon supports (such as graphene and CNTs) could improve the charge transfer. Qiao and co-workers have designed novel Co-C3N4/CNTs for ORR catalysis [58]. The SACs with highly dispersed atomic Co can be used as an excellent platform to investigate the ORR performance and further identify the active sites. In many cases, SACs are not only catalytically active, but provide the structural stability. The strong binding between the single metal atoms and the corresponding anchoring sites on the support surfaces stabilize SACs during catalytic reactions. At the level of electronic structure, the free electron states near Fermi level from SACs effectively modify the binding strength between the adsorbate and catalyst, leading to a similar activity origin. Consequently, the densities of free electrons extremely optimize the chemisorption of intermediates and the subsequent electron transfer for bond breaking. Different configurations of dopants are thought to contribute discriminatively to the electron structure deformation, and thereby distinct catalytic activities. Therefore, the specific SACs-doping is the most effective strategy to alter electron structures and to achieve optimal ORR activities in electro-catalysts. 4. Outlook This review has provided an overview to the SACs combined with structural stability and catalytic activity. The extraordinary catalytic activity of SACs with structural stability is associated with its unique electronic structure, which can be represented by the binding strength to depict the universal characteristics. There should be orbital hybridization between single metal atoms and supports, and p-d hybridization or charge transfer can induce orbital splits. The fully occupied bonding orbitals correspond to the stability between the single atom and the supports, while the free electrons near Fermi level caused by the unsaturated coordination can be responsible for the catalytic activity. Compounds without such free electrons may not display excellent catalytic activity. Therefore, the single atom from SACs should be strongly coupled to the supports, and the localized structural coordination is unsaturated to form a dangling bond near Fermi level. Here, combining the conventional screening criteria of stability (formation energy) and catalytic activity (adsorption energy) can provide a new insight into understanding and developing new catalyst with good 263

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structural stability and superior activity. Since the high-throughput prediction and big data analysis have gradually become one of the important methods for material science, it is still necessary to establish a set of material-performance criteria that can be clearly defined and calculated to screen data and predict new materials. Taking full advantage of novel theoretical modeling, impeccable synthetic strategies and advanced characterization technologies will play important role in advancing the development of SACs from fundamentals to industrial applications. The current development of SACs as tunable catalysts provides a unique opportunity to optimize the catalytic activity and stability of heterogeneous catalysts. However, the single-metal content is still very low (< 1 wt%) in the current SACs, which may reduce the overall electrochemical performances. Therefore, it is essential to design desirable materials for high catalytic activity and thermally stable SACs via judicious tuning the synergy between the single atoms and appropriate support and then perform the maximum utilization of metal atoms. Furthermore, we suspected that, after highactivity and stable SACs are synthesized, the following researches of dual-/multi-atom catalysts would attract more researchers' interests for the reason that it usually includes more than one rate determining step in one electrochemical reaction, which will be a frontier field of electrocatalytic sciences.

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