Computational screening of single-atom catalysts supported by VS₂ monolayers for electrocatalytic oxygen reduction/evolution reactions

Abstract

The development of highly efficient bifunctional electrocatalysts to boost oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is highly desirable for energy conversion and storage devices. Herein, by means of comprehensive first-principles computations, we systematically explored the catalytic activities of a series of single transition metal atoms anchored on two-dimensional VS₂ monolayers (TM@VS₂) for ORR/OER. Our results revealed that Ni@VS₂ exhibits low overpotentials for both ORR (0.45 V) and OER (0.31 V), suggesting its great potential as a bifunctional catalyst, which is mainly induced by its moderate interaction with oxygenated intermediates according to the established scaling relationship and volcano plot. Interestingly, the substituted doping of nitrogen heteroatoms into the VS₂ substrate can further effectively improve the ORR/OER activity of the active metal atom to achieve more eligible ORR/OER bifunctional catalysts. Our results not only propose a new class of potential bifunctional oxygen catalysts but also offer a feasible strategy for further tuning their catalytic activity.

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1. Introduction

With the rapid development of human society, energy shortage and atmospheric pollution have aroused widespread concern. Fuel cells,¹ metal–air batteries,² and water electrolysis devices³ are recognized as promising substitutes for traditional energy sources due to their high energy conversion efficiency and environmental friendliness. As two critical processes in these energy conversion devices, oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) essentially determine their overall performance^4–7 as the two reactions involve a four-electron process with complicated mechanisms, leading to their sluggish kinetics and high overpotentials, which thus require highly active catalysts to accelerate them.^8–10 So far, precious metal electrocatalysts are the state-of-art catalysts for the two critical reactions: Pt-based materials are the best electrocatalysts for ORR^11–13 and Ru-based catalysts exhibit outstanding OER activities.^14–16 However, the OER catalytic performance of Pt-based catalysts and the ORR activity of Ru-based materials are far from satisfactory.¹⁷ In particular, the high cost, limited reserves, and low stabilities of these noble-metal catalysts greatly hinder their large-scale commercial applications in these renewable energy technologies. Therefore, the development of stable, inexpensive, and efficient bifunctional electrocatalysts for both ORR and OER is a hot topic, but still remains a huge challenge.

Single-atom catalysts (SACs), with isolated single metal atoms dispersed on a specific substrate, have emerged as a hot topic in heterogeneous catalysis due to their maximized atomic utilization of catalytically active metals and excellent catalytic performance, superior to their nanocluster and nanoparticle counterparts.^18–22 In particular, their intrinsic activity can be effectively tuned by altering the substrate microenvironment.^23–25 Therefore, in recent years, tremendous efforts have been made to explore the applications of SACs in a variety of electrochemical processes, such as hydrogen evolution reaction (HER), ORR/OER, CO₂ reduction reaction (CO₂RR), and nitrogen reduction reaction (NRR).^26–30 However, the reduced size of the metal particles easily increases the surface energy of the SACs, making them easily aggregate into larger metal clusters.²² Thus, choosing a suitable substrate to immobilize SACs plays a pivotal role in their practical applications.

In this regard, two-dimensional (2D) materials hold great promise as a class of promising substrates to support SACs due to their high specific surface areas and excellent physical and chemical properties.^31–40 Notably, among various 2D materials, transition metal dichalcogenide nanosheets (TMD), such as MoS₂, WS₂, MoSe₂, and WSe₂, have received considerable attention due to their unique structures and excellent properties, endowing them with wide applications as SACs substrates.^41,42 For example, Cao's group reported that a single Ru atom supported on a MoS₂ nanosheet can serve as a highly efficient HER electrocatalyst,⁴³ while Li et al. experimentally showed that a single Fe atom supported on a MoS₂ nanosheet exhibits high catalytic activity for the nitrate reduction reaction with a maximum Faradaic efficiency of 98%.⁴⁴ Besides, Meza et al. demonstrated that their synthesized single Ni atom anchored on WS₂ nanosheet displays good catalytic activity for OER.⁴⁵ Theoretically, Tian et al. found that single Ni or Co atom anchored on TiS₂, ZrS₂, TaS₂, and NbS₂ can perform as eligible ORR electrocatalysts,⁴⁶ and Ling et al. predicted the excellent catalytic activity of single Cu and Pd atom stabilized by 1T′-MoS₂ toward ORR.⁴⁷

As a typical layered TMD material, 2D vanadium disulfide (VS₂) monolayer is of particular interest due to its metallic character, earth-abundant composition, and abundant catalytic sites distributed on both the basal planes and edges.^48–50 To the best of our knowledge, however, there are few studies on the electrocatalytic performance of SACs anchored on VS₂ monolayers.^51,52 In particular, no SAC supported by VS₂ monolayers has been reported for ORR/OER to date, and such a study is essential not only to further enrich its application fields but also to develop novel electrocatalysts for energy-related devices.

Inspired by the excellent properties of VS₂ monolayers, in this work, we systematically explored the catalytic activities of 22 single TM atoms anchored on the VS₂ monolayer (TM@VS₂, TM = Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Hf, Ta, W, Re, Os, Ir and Pt) toward ORR/OER by performing comprehensive density functional theory (DFT) computations. Based on the computed free energy changes in all elementary steps during ORR/OER, we found that Ni@VS₂ exhibits good catalytic performance for both ORR and OER with low overpotentials (0.45 and 0.31 V), suggesting its great promise as an eligible bifunctional electrocatalyst for ORR/OER. More interestingly, its ORR/OER catalytic activity can be further enhanced by introducing a certain number of N dopants into the VS₂ substrate. Furthermore, the excellent ORR/OER activity origin of Ni@VS₂ was rationalized by exploring the scaling relationship of various oxygenated intermediates and their intrinsic properties involving the local structural environments. Our findings not only highlight a new family of promising bifunctional catalysts for ORR/OER but also provide cost-effective opportunities to rapidly screen out novel and highly efficient SAC-based advanced electrocatalysts.

2. Computational details

2.1. DFT computations

Spin-polarized density functional theory (DFT) computations were carried out using the Vienna ab initio simulation package (VASP).^53,54 The interactions of ions with electrons were treated by the projector augmented wave (PAW)^55,56 method with a cutoff energy of 500 eV. The generalized gradient approximation (GGA) in the form of Perdew–Burke–Ernzerhof (PBE)⁵⁷ exchange–correlation functional was used to describe the electron exchange–correlation. It is noteworthy that the GGA with the correction of Hubbard U (GGA + U) term was usually adopted to describe the partially filled d and f electronic states of transition metal elements.⁵⁸ However, our aim in this work is to screen the ideal ORR/OER catalysts from various candidates, and we are particularly interested in the catalytic tendency of these candidates for ORR/OER. According to previous studies,^59,60 the GGA + U approach could reveal the quantitative changes of the computed adsorption energies yet without challenging the identified scaling relations of the reaction intermediates on various electrocatalysts nor any of the conclusions taken from it, which was also consistent with our test (Table S1†). Thus, GGA + U was not further considered in this work to avoid the huge computational resources to compute all the possible pathways of all the considered catalysts. To accurately treat the possible van der Waals (vdW) interactions of oxygenated intermediates in the ORR/OER process with catalysts, the dispersion-corrected DFT + D3 method in the Grimme scheme⁶¹ was employed. All atomic structures were fully relaxed until the force and energy were smaller than 0.02 eV Å⁻¹ and 10⁻⁵ eV, respectively, and the Brillouin zones were sampled with 3 × 3 × 1 Monkhorst–Pack meshes.

A hexagonal 4 × 4 × 1 supercell of VS₂ monolayer, including 16 V atoms and 32 S atoms, was constructed as the substrate for anchoring these considered single TM atoms. The vacuum thickness was set to be 15 Å to minimize the interlayer interactions. Bader charge analysis was performed to describe the charge variation quantitatively.⁶² The thermodynamic stability of single TM atoms on the VS₂ monolayer was assessed by the binding energies (E_bind), as computed by: E_bind = E_Tm@VS2 − E_VS2 − E_TM, where E_Tm@VS2, E_VS2, and E_TM represent the total energies of TM@VS₂, pristine VS₂ monolayer, and a single TM atom, respectively. Notably, the energy of the single TM atom was computed in a vacuum by placing the TM atom into a 15 × 15 × 15 cubic box, including single Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Hf, Ta, W, Re, Os, Ir, and Pt atoms.

2.2. Free energy changes

According to the widely-employed computational hydrogen electrode (CHE) model,^63,64 the Gibbs free-energy change (ΔG) of each elementary step during the ORR/OER was defined as: ΔG = ΔE + ΔZPE − TΔS + ΔG_U + ΔG_pH, in which ΔE is the reaction energy obtained from DFT computations. ΔZPE and ΔS denote the difference of zero-point energy and entropy, respectively, which can be derived from the computed vibrational frequencies for the adsorbed oxygenated intermediates, while those of gas-phase molecules (H₂, O₂, and H₂O) were derived from the thermodynamic NIST database. ΔG_U = −nU, where n denotes the number of transferred electrons, while U is the applied electrode potential. ΔG_pH is the correction of the pH of the electrolyte, which can be determined by ΔG_pH = k_BT ln 10 × pH. Based on previous theoretical studies,^65–69 the value of pH in this work was assumed to be zero for an acidic medium. To avoid the inaccurate computations of the traditional DFT methods on the free energy of isolated O₂ molecule (G_O2) due to its high-spin state, H₂ and H₂O were chosen as references: G_O2 = G_H2O − 2G_H2 + 4.92.

The four-electron pathway was adopted for the ORR due to its high efficiency for energy conversion, which can proceed along the following elementary steps in an acidic medium:

* + O₂ (g) + H⁺ + e⁻ → OOH* (1)

OOH* + H⁺ + e⁻ → O* + H₂O (l) (2)

O* + H⁺ + e⁻ → OH* (3)

OH* + H⁺ + e⁻ → H₂O (l) (4)

in which * represents the active sites within these TM@VS₂ catalysts. According to the CHE method, the ORR overpotential (η^ORR) can be determined by: η^ORR = max{ΔG₁, ΔG₂, ΔG₃, and ΔG₄}/e + 1.23. On the other hand, the OER is the reverse reaction of the ORR and its overpotential (η^OER) can be obtained by η^OER = max{−ΔG₁, −ΔG₂, −ΔG₃, and −ΔG₄}/e − 1.23. Based on the above definitions, a catalyst with lower η^ORR and η^OER will exhibit better catalytic activity toward both ORR and OER.

3. Results and discussion

3.1. Structures, stabilities, and electronic properties of TM@VS₂

First, we examined the adsorption structures of these single TM atoms on the VS₂ monolayer. To determine the most stable configurations, different possible surface sites, including the top and hollow sites, were considered for the binding of these TM atoms on the VS₂ monolayer (Fig. 1a). After full geometrical optimization, we found that some TM atoms, including Mn, Cu, Ru, Rh, Pd, Ag, and Pt, prefer to be adsorbed on the top of the V site by interacting with three surface S atoms, and the shortest lengths of the newly formed TM–S bonds are in the range of 2.21 to 2.54 Å. On the other hand, the Ti, V, Cr, Fe, Co, Ni, Zr, Nb, Mo, Hf, Ta, W, Re, Os, and Ir atoms are energetically favorable to be adsorbed over the hollow site, also forming three TM–S bonds with the lengths ranging from 2.08 to 2.32 Å.

Fig. 1 (a) The initial configurations of single metal atoms anchored on the VS₂ monolayer and the considered metal atoms as SAC candidates. (b) Binding energy (E_bind) and their difference with the cohesive energy (E_bind − E_coh) of the metal atom. (c) Local charge density difference of Ni@VS₂. The cyan and yellow areas represent charge depletion and charge accumulation, respectively. (d) The positive charges on the anchored metal atoms based on Bader charge analysis. (e) The computed band structure and projected density of states (PDOSs) of the Ni@VS₂ monolayer. The Fermi level was set to zero in dotted line.

Based on the most stable structures abovementioned, we evaluated the thermodynamic stabilities of these single TM atoms on the VS₂ substrate, as good stability is a prerequisite for their practical applications as efficient ORR/OER catalysts, which can be guaranteed by the strong binding strengths between TM atoms and the substrate to prevent their aggregation. Our results demonstrated that their binding energies range from −2.27 to −7.64 eV (Fig. 1b), indicating that the binding of all TM atoms on the VS₂ monolayer is an exothermic process. In particular, among the TM atoms, the Ta atom exhibits the strongest interaction, whereas Ag displays the weakest binding. The strong metal affinity to the VS₂ monolayer could be ascribed to the high surface reactivity of the metallic VS₂ monolayer. Furthermore, we computed the cluster energies (E_clus = E_bind − E_coh) to assess the aggregation possibility of these anchored TM atoms on the VS₂ monolayer, where E_coh denotes the cohesive energy of TM atoms. According to this definition, a negative E_clus value indicates that TM atoms are energetically more favorable to dispersedly deposit rather than aggregate into clusters on the VS₂ monolayer. The results showed that only seven anchored TM atoms, including Ti, Mn, Fe, Co, Ni, Hf, and Zr, exhibited a stronger binding strength for TM–S bonds than that of TM–TM bonds in bulk, suggesting their excellent thermodynamic stabilities. Notably, even if E_clus < 0 cannot be achieved, the anchored single TM atom may be stable on the substrate when its diffusion barrier is high enough to prevent its aggregation, indicating its good kinetic stability.^70–72 In any case, all TM@VS₂ candidates will be still retained for comparison to obtain the comprehensive OER/ORR catalytic trend.

In general, both the ORR and OER processes occur in aqueous solutions. Thus, assessing the stability of catalysts in an aqueous environment is crucial for the applications of catalysts under realistic conditions. As a representative, we performed ab initio molecular dynamics (AIMD) simulations in an NVT ensemble on Ni@VS₂ at 500 K under the water environments using the Nosé–Hoover method.⁷³ The time evolutions of energy and temperature are plotted in Fig. S1.† Our results indicated that, after 10 ps, the atomic structure of Ni@VS₂ still remains intact in water without any noticeable deformation, and the deviations of Ni–S and V–S bond lengths are less than 0.03 Å with respect to the equilibrium structure under vacuum. Thus, the single metal atom immobilized by the VS₂ monolayer with excellent stability in aqueous solutions at ambient temperature may be utilized as a kind of robust catalyst for various electrochemical reactions.

To validate the intense interaction between TM atoms and VS₂ substrate, we computed the charge density difference of TM@VS₂. It can be observed from Fig. 1c and Fig. S2† that significant amounts of electrons are transferred from TM atoms to the VS₂ substrate. As a result, the TM atoms are positively charged (≈0.14–1.74e⁻, Fig. 1d) based on the Bader charge analysis, indicating that they can act as active sites to bind with the oxygenated intermediates during the ORR/OER. Furthermore, we computed the band structures and projected density of states (PDOSs) of these TM@VS₂ materials (Fig. 1e and Fig. S3†). Interestingly, we found that the metallic properties of the VS₂ monolayer are still preserved, and some new impurity levels were observed due to the introduction of the partially occupied d bands of the TM atoms. Meanwhile, the high peaks of the TM-d states around the Fermi level in their PDOSs suggest the high density of carriers, thus endowing TM@VS₂ with excellent electronic conductivity, which normally boosts their catalytic performance in electrochemical reactions.^74–76 In particular, there is obvious hybridization between the TM-d and S-3p orbitals, further suggesting the strong interactions of TM atoms with the VS₂ substrate.

3.2. ORR/OER on TM@VS₂

After confirming the geometric structures, stabilities, and electronic properties of these TM@VS₂ candidates, we investigated their catalytic performance towards ORR/OER. According to a summary of previous studies,^77–80 the adsorption Gibbs free energies of the oxygenated intermediates (ΔG_OH*, ΔG_O*, and ΔG_OOH*) on catalysts generally determine their intrinsic catalytic activity toward the ORR/OER, and their scaling relationship between each other can be not only useful to rapidly screen out highly active catalysts but also to deeply unravel their catalytic activity origin. Our results showed that all of the oxygenated intermediates are adsorbed on top of TM atoms. Furthermore, we computed the adsorption Gibbs free energies of OH*, O*, and OOH* on TM@VS₂ catalysts (Table S2†), and the scaling relationships between each other are plotted in Fig. 2. Clearly, there exists a linear relationship between ΔG_OOH*vs. ΔG_OH* and ΔG_O*vs. ΔG_OH*, which can be expressed as ΔG_OOH* = 0.83ΔG_OH* + 3.24 (R² = 0.96) and ΔG_O* = 1.46ΔG_OH* + 1.11 (R² = 0.88). Thus, the adsorption Gibbs free energies of the oxygenated intermediates can be utilized as the rational descriptors to describe the catalytic activities of these TM@VS₂ materials, which will be further explored in the following discussions.

Fig. 2 Scaling relations between the adsorption energies of intermediates (ΔG_O*vs. ΔG_OH* and ΔG_OOH*vs. ΔG_OH*) on various TM/VS₂ candidates.

As we mentioned above, the ORR along a four-electron (4e⁻) pathway can be expressed as O₂ + * → OOH* → O* → OH* → H₂O + * (Fig. 3). Based on the computed ΔG values of OH*, O*, and OOH* on these TM@VS₂ catalysts, the free energy diagrams of ORR were computed (Fig. 4a and Fig. S4†). Taking Ni@VS₂, as an example, during the ORR process, all elementary steps are downhill at U = 0 V, indicating that the ORR can spontaneously take place on Ni@VS₂. Notably, we also considered the possibility of H₂O₂ formation along the two-electron (2e⁻) pathway. The results demonstrated that the free energy from OOH* to H₂O₂ is −0.56 eV, which is much less than that of O* formation (−1.54 eV), indicating the greater feasibility of the 4e⁻ pathway than the 2e⁻ pathway for the ORR using this catalyst.

Fig. 3 The elementary reaction steps of the ORR and OER processes on TM@VS₂ catalysts.

Fig. 4 (a) ORR and (b) OER free energy profiles of the Ni@VS₂ catalyst at various applied voltages. (c) The computed ORR and OER overpotentials of various TM/VS₂ candidates.

Moreover, we also studied the effect of electrode potential on the ORR processes, which is rather critical for electrochemical systems. The results showed that, although all elementary ORR steps on Ni@VS₂ catalyst are downhill in the free energy profile at zero potential, some of the intermediate steps become uphill with the increase in potential. In detail, in this pathway, the maximum value of U at which all reactions are still exothermic (namely, limiting potential) is 0.78 V; when U is larger than 0.78 V, the OOH* formation will be endothermic. At the equilibrium potential of U = 1.23 V, the ORR on Ni@VS₂ will be completely hampered, as the first reduction step of O₂ to OOH* is uphill in the free energy profile by 0.45 eV, while the other steps still remain downhill. Thus, OOH* formation can be considered as the potential-determining step (PDS) for ORR on Ni@VS₂, corresponding to the overpotential of 0.45 V.

On the other hand, the OER is the reverse reaction of the ORR. It is noted that when the equilibrium potential of 1.23 V was applied, the formation of OOH* from O* during OER on Ni@VS₂ requires to overcome a maximum energy barrier of 0.31 eV among all the elementary steps (Fig. 4b and Fig. S4†), thus inducing an overpotential of 0.31 V. Interestingly, the values of η^ORR (0.45 V) and η^OER (0.31 V) of Ni@VS₂ are comparable (even lower) to those of traditional noble metal catalysts such as Pt (η^ORR = 0.45 V)⁶³ and RuO₂ (η^OER = 0.42 V),⁸¹ indicating that Ni@VS₂ can serve as a promising bifunctional electrocatalyst with a rather high efficiency for ORR and OER.

The overpotentials of ORR and OER on other TM@VS₂ are summarized in Fig. 4c. Except for Ni@VS₂, the anchored Cu and Pd atoms on the VS₂ substrate even exhibit a lower η^ORR of 0.36 V, suggesting their better catalytic performance for the ORR. However, as for the OER on these TM@VS₂ candidates, the computed η^OER values (0.58–2.36 V) were all larger than that of state-of-the-art RuO₂ (0.42 V), implying that their OER catalytic activities are unsatisfied.

3.3. Enhancing the ORR/OER activity of Ni@VS₂ by nitrogen doping

Controlling the chemical environments of the active metal atom, such as coordination number and local composition, was recently revealed as an effective strategy to improve the whole catalytic performance of SACs.^23–25 Then, an interesting question naturally arises: can we search for an effective method to further enhance the catalytic activity of Ni@VS₂ material toward the ORR/OER? Interestingly, we noted that many studies recently synthesized N-substitutional doped 2D TMD monolayers for various applications, such as N-doped MoS₂ and WS₂ nanosheets.^82–85 Inspired by these pioneering studies, we thus explored the effects of N doping within the VS₂ substrate on the ORR/OER activity of the anchored single Ni atom.

To this end, one, two, and three N heteroatoms were adopted to substitute the corresponding number of S atoms of the VS₂ monolayer and to further anchor the single Ni atom, resulting in the formation of NiN₁, NiN₂, and NiN₃ moieties on the VS₂ monolayer (Fig. S5†). Interestingly, we found that the E_bind values of the single Ni atom on the three kinds of N-doped VS₂ monolayers are computed to be −5.35, −5.62, and −5.69 eV, respectively, which are larger than that on pristine VS₂ substrate (−4.84 eV), suggesting the enhanced thermodynamic stability for single Ni atom anchored on the VS₂ substrate after the introduction of N dopants. Furthermore, due to the larger electronegativity of the N dopant than its S counterpart (3.04 vs. 2.58), the N-doped VS₂ monolayer can loot more electrons from the Ni atom, thus making it carry a larger positive charge (0.46, 0.57, and 0.65e⁻) than the perfect VS₂ monolayer (0.31e⁻), which will intrinsically affect its catalytic performance for the ORR/OER.

Based on the above results, we mainly focused on the ORR/OER catalytic activity of NiN_x/VS₂ (x = 1, 2, and 3) catalysts, and the computed free energy profiles are presented in Fig. 5. Amazingly, we found that the overpotentials for ORR on NiN₁, NiN₂, and NiN₃ catalysts are decreased to 0.37, 0.33, and 0.28 V, respectively, as compared with the Ni@VS₂ catalyst (0.45 V). In particular, the ORR process on NiN₂ and NiN₃ materials has changed into the final step (OH* → H₂O). In contrast, the η values for OER are increased from 0.31 V of Ni@VS₂ to 0.58 on NiN₁ and 0.44 V on NiN₂, respectively, whereas that of NiN₃ is unchanged (Fig. 5). Notably, the PDS during OER on the three catalysts becomes the step of OH* → O*. Apparently, the η^ORR values are decreased in all the cases to different degrees, while the η^ORR values are increased or unchanged after introducing N dopants. More importantly, we found that the NiN₃ material exhibits the best catalytic performance for bifunctional ORR/OER activity due to its lowest overpotentials (η^ORR = 0.28 V and η^OER = 0.31 V) followed by NiN₂ (η^ORR = 0.33 V and η^OER = 0.44 V).

Fig. 5 The computed free energy profiles for the ORR and OER on single Ni atom anchored on three kinds of N-doped VS₂ monolayers at the equilibrium potential of 1.23 V.

As above discussed, the ORR/OER generally proceeds under solvent conditions, the solvation effect on the catalytic activity of the two electrochemical reactions should not be neglected. To this end, using the implicit solvation model as implemented in VASPsol,⁸⁶ we took the best electrocatalyst, i.e., NiN₃@VS₂, as a representative to assess the involved solvation effect. As shown in Fig. S6,† the η^ORR value of 0.28 V is unchanged with the solvation effect, while the η^OER value is slightly increased by 0.10 V, suggesting that the solvation has a small effect on the ORR/OER catalytic activity of single atom catalysts supported by VS₂-based nanomaterials. Overall, by carefully tuning the numbers of the introduced N-dopants within the VS₂ monolayer, the stability and catalytic performance of the anchored Ni atom for the ORR/OER can be effectively enhanced to achieve more eligible bifunctional catalysts for both the ORR and OER.

3.4. ORR/OER activity origin

As mentioned above, the free adsorption energies of OH* exhibit a scaling relationship with those of the OOH* and O* species. Thus, as proposed by previous theoretical studies,^77–80 ΔG_OH* and ΔG_O* − ΔG_OH* were chosen as the descriptors of the ORR and OER activity of various TM@VS₂ catalysts, respectively. Based on this, the variation of the η^ORR and η^OER values with ΔG_OH* and ΔG_O* − ΔG_OH* are established in Fig. 6. The results showed that the best ORR catalyst possesses a ΔG_OH* value of about 1.00 eV, and the ΔG_O* − ΔG_OH* value of about 1.50 eV is required for the best OER catalyst. Clearly, the single Ni atom anchored on the pristine and doped VS₂ monolayers by two and three N atoms located at the peak of the ORR and OER volcano plots clarifies its superior catalytic performance for both the ORR and ORR and thus suggests its great potential as bifunctional ORR/OER electrocatalysts.

Fig. 6 The volcano plot of (a) −η^ORRvs. ΔG_OH* and (b) −η^OERvs. ΔG_O* − ΔG_OH* for the ORR and OER on various TM/VS₂ candidates.

To better understand the remarkable difference in the catalytic activities of various TM@VS₂ toward the ORR/OER, we examined the intrinsic features of the active sites by considering the number of d orbital electrons (N_d) and the electronegative (EN) of TM atoms. Motivated by the recent reports,^87,88 a descriptor (φ) based on these intrinsic properties can be determined by: , where EN_aver represents the average electronegativity of the anchored TM atom and its nearest neighboring atoms. According to these computed descriptors, we examined the relationship between φ and the adsorption free energies of oxygenated species on various TM@VS₂ (Fig. 7). Clearly, the adsorption free energies of these intermediates highly correlate and φ via the following equations: ΔG_O* = 0.90φ − 4.28 eV, ΔG_OH* = 0.62φ − 3.68 eV, and ΔG_OOH* = 0.49φ − 0.24 eV with R² values of 0.82, 0.91, and 0.85, respectively, in which the ΔG_OH* value of the best electrocatalyst is about 1.00 eV. Interestingly, the ΔG_OH* values of the single Ni atom supported by the pristine and N-doped VS₂ catalysts exactly fall in this area with the φ values of 7.00 to 8.00. Thus, the φ value involving the local environment of the active site can be utilized as a good descriptor of the ORR/OER catalytic activity, which provides a cost-effective solution to rapidly screen highly efficient TM@VS₂-based bifunctional catalyst for the ORR and OER even without complicated DFT computations.

Fig. 7 (a) ΔG_O*, (b) ΔG_OH*, and (c) ΔG_OOH*vs. coordination descriptor φ on TM anchored on the VS₂ monolayer.

4. Conclusions

In summary, by performing comprehensive density functional theory (DFT) computations, we systematically investigated the structures, stabilities, and the ORR/OER catalytic activities of a series of single TM atoms supported by VS₂ monolayers. Based on the computed free energy profiles, Ni@VS₂ was identified as a highly efficient bifunctional catalyst for the ORR/OER with comparable (even lower) overpotentials to the widely employed Pt- and Ru-based catalysts. More interestingly, introducing a certain number of N dopants into VS₂ substrates can effectively enhance the catalytic activity of the anchored Ni atom toward the ORR/OER. Furthermore, the intrinsic properties of these TM@VS₂ candidates involving coordination environments were analyzed to gain a deeper insight into their ORR/OER activity origin. Our findings not only provide a promising strategy for the design of novel SAC-based bifunctional electrocatalysts for the ORR/OER but also further enrich the potential applications of 2D VS₂-based materials in electrocatalysis.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

This work was financially supported by the Natural Science Funds (NSF) for Distinguished Young Scholar of Heilongjiang Province (no. JC2018004).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2nr01671k

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