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

Zengming Qin , Zhongxu Wang and Jingxiang Zhao *
Key Laboratory for Photonic and Electronic Bandgap Materials, Ministry of Education, School of Physics and Electronic Engineering, Harbin Normal University, Harbin, 150025, P. R. China. E-mail:

Received 26th March 2022 , Accepted 2nd April 2022

First published on 4th April 2022


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.

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 performance4–7 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 ORR11–13 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 (Ebind), as computed by: Ebind = ETm@VS2EVS2ETM, where ETm@VS2, EVS2, and ETM 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 + ΔGU + ΔGpH, 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. ΔGU = −nU, where n denotes the number of transferred electrons, while U is the applied electrode potential. ΔGpH is the correction of the pH of the electrolyte, which can be determined by ΔGpH = kBT[thin space (1/6-em)]ln[thin space (1/6-em)]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 (GO2) due to its high-spin state, H2 and H2O were chosen as references: GO2 = GH2O − 2GH2 + 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 (ηORR) can be determined by: ηORR = 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 (ηOER) can be obtained by ηOER = max{−ΔG1, −ΔG2, −ΔG3, and −ΔG4}/e − 1.23. Based on the above definitions, a catalyst with lower ηORR and ηOER 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.
image file: d2nr01671k-f1.tif
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 (Ebind) and their difference with the cohesive energy (EbindEcoh) 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 (Eclus = EbindEcoh) to assess the aggregation possibility of these anchored TM atoms on the VS2 monolayer, where Ecoh denotes the cohesive energy of TM atoms. According to this definition, a negative Eclus 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 Eclus < 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 (ΔGOH*, ΔGO*, and ΔGOOH*) 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 ΔGOOH*vs. ΔGOH* and ΔGO*vs. ΔGOH*, which can be expressed as ΔGOOH* = 0.83ΔGOH* + 3.24 (R2 = 0.96) and ΔGO* = 1.46ΔGOH* + 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.
image file: d2nr01671k-f2.tif
Fig. 2 Scaling relations between the adsorption energies of intermediates (ΔGO*vs. ΔGOH* and ΔGOOH*vs. ΔGOH*) 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.

image file: d2nr01671k-f3.tif
Fig. 3 The elementary reaction steps of the ORR and OER processes on TM@VS2 catalysts.

image file: d2nr01671k-f4.tif
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 ηORR (0.45 V) and ηOER (0.31 V) of Ni@VS2 are comparable (even lower) to those of traditional noble metal catalysts such as Pt (ηORR = 0.45 V)63 and RuO2 (ηOER = 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 ηORR of 0.36 V, suggesting their better catalytic performance for the ORR. However, as for the OER on these TM@VS2 candidates, the computed ηOER 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 Ebind 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 NiNx/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 ηORR values are decreased in all the cases to different degrees, while the ηORR 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 (ηORR = 0.28 V and ηOER = 0.31 V) followed by NiN2 (ηORR = 0.33 V and ηOER = 0.44 V).

image file: d2nr01671k-f5.tif
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 ηORR value of 0.28 V is unchanged with the solvation effect, while the ηOER 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 ΔGOH* and ΔGO* − ΔGOH* were chosen as the descriptors of the ORR and OER activity of various TM@VS2 catalysts, respectively. Based on this, the variation of the ηORR and ηOER values with ΔGOH* and ΔGO* − ΔGOH* are established in Fig. 6. The results showed that the best ORR catalyst possesses a ΔGOH* value of about 1.00 eV, and the ΔGO* − ΔGOH* 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.
image file: d2nr01671k-f6.tif
Fig. 6 The volcano plot of (a) −ηORRvs. ΔGOH* and (b) −ηOERvs. ΔGO* − ΔGOH* 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 (Nd) 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: image file: d2nr01671k-t1.tif, where ENaver 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: ΔGO* = 0.90φ − 4.28 eV, ΔGOH* = 0.62φ − 3.68 eV, and ΔGOOH* = 0.49φ − 0.24 eV with R2 values of 0.82, 0.91, and 0.85, respectively, in which the ΔGOH* value of the best electrocatalyst is about 1.00 eV. Interestingly, the ΔGOH* 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.

image file: d2nr01671k-f7.tif
Fig. 7 (a) ΔGO*, (b) ΔGOH*, and (c) ΔGOOH*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.


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

Notes and references

  1. L. Carrette, K. A. Friedrich and U. Stimming, ChemPhysChem, 2000, 1, 162–193 CrossRef CAS PubMed.
  2. F. Cheng and J. Chen, Chem. Soc. Rev., 2012, 41, 2172–2192 RSC.
  3. F. Safari and I. Dincer, Energy Convers. Manage., 2020, 205, 112182 CrossRef CAS.
  4. X. Wu, C. Tang, Y. Cheng, X. Min, S. P. Jiang and S. Wang, Chem. – Eur. J., 2020, 26, 3906–3929 CrossRef CAS PubMed.
  5. V. Rai, K. P. Lee, D. Safanama, S. Adams and D. J. Blackwood, ACS Appl. Energy Mater., 2020, 3, 9417–9427 CrossRef.
  6. M. Tahir, L. Pan, F. Idrees, X. Zhang, L. Wang, J.-J. Zou and Z. L. Wang, Nano Energy, 2017, 37, 136–157 CrossRef CAS.
  7. J. Stacy, Y. N. Regmi, B. Leonard and M. Fan, Renewable Sustainable Energy Rev., 2017, 69, 401–414 CrossRef CAS.
  8. L. Du, L. Xing, G. Zhang, M. Dubois and S. Sun, Small Methods, 2020, 4, 2000016 CrossRef CAS.
  9. H. Osgood, S. V. Devaguptapu, H. Xu, J. Cho and G. Wu, Nano Today, 2016, 11, 601–625 CrossRef CAS.
  10. Z.-F. Huang, J. Wang, Y. Peng, C.-Y. Jung, A. Fisher and X. Wang, Adv. Energy Mater., 2017, 7, 1700544 CrossRef.
  11. Y. Nie, L. Li and Z. Wei, Chem. Soc. Rev., 2015, 44, 2168–2201 RSC.
  12. S. Hussain, H. Erikson, N. Kongi, A. Sarapuu, J. Solla-Gullón, G. Maia, A. M. Kannan, N. Alonso-Vante and K. Tammeveski, Int. J. Hydrogen Energy, 2020, 45, 31775–31797 CrossRef CAS.
  13. A. Mahata, A. S. Nair and B. Pathak, Catal. Sci. Technol., 2019, 9, 4835–4863 RSC.
  14. T. Reier, M. Oezaslan and P. Strasser, ACS Catal., 2012, 2, 1765–1772 CrossRef CAS.
  15. N.-T. Suen, S.-F. Hung, Q. Quan, N. Zhang, Y.-J. Xu and H. M. Chen, Chem. Soc. Rev., 2017, 46, 337–365 RSC.
  16. T. Reier, Z. Pawolek, S. Cherevko, M. Bruns, T. Jones, D. Teschner, S. Selve, A. Bergmann, H. N. Nong, R. Schlögl, K. J. J. Mayrhofer and P. Strasser, J. Am. Chem. Soc., 2015, 137, 13031–13040 CrossRef CAS PubMed.
  17. Z. Xue, X. Zhang, J. Qin and R. Liu, J. Energy Chem., 2021, 55, 437–443 CrossRef.
  18. H. Zhang, G. Liu, L. Shi and J. Ye, Adv. Energy Mater., 2018, 8, 1701343 CrossRef.
  19. A. Wang, J. Li and T. Zhang, Nat. Rev. Chem., 2018, 2, 65–81 CrossRef CAS.
  20. S. K. Kaiser, Z. Chen, D. Faust Akl, S. Mitchell and J. Pérez-Ramírez, Chem. Rev., 2020, 120, 11703–11809 CrossRef CAS PubMed.
  21. T. Zhang, A. G. Walsh, J. Yu and P. Zhang, Chem. Soc. Rev., 2021, 50, 569–588 RSC.
  22. X.-F. Yang, A. Wang, B. Qiao, J. Li, J. Liu and T. Zhang, Acc. Chem. Res., 2013, 46, 1740–1748 CrossRef CAS PubMed.
  23. X. Li, L. Liu, X. Ren, J. Gao, Y. Huang and B. Liu, Sci. Adv., 2020, 6, eabb6833 CrossRef CAS PubMed.
  24. X. Ma, H. Liu, W. Yang, G. Mao, L. Zheng and H.-L. Jiang, J. Am. Chem. Soc., 2021, 143, 12220–12229 CrossRef CAS PubMed.
  25. J. Zhang, H. Yang and B. Liu, Adv. Energy Mater., 2021, 11, 2002473 CrossRef CAS.
  26. Q. Zhang and J. Guan, Adv. Funct. Mater., 2020, 30, 2000768 CrossRef CAS.
  27. C. Zhu, S. Fu, Q. Shi, D. Du and Y. Lin, Angew. Chem., Int. Ed., 2017, 56, 13944–13960 CrossRef CAS PubMed.
  28. K. Kamiya, Chem. Sci., 2020, 11, 8339–8349 RSC.
  29. X. Zheng, P. Li, S. Dou, W. Sun, H. Pan, D. Wang and Y. Li, Energy Environ. Sci., 2021, 14, 2809–2858 RSC.
  30. L. Peng, L. Shang, T. Zhang and G. I. N. Waterhouse, Adv. Energy Mater., 2020, 10, 2003018 CrossRef CAS.
  31. A. Alarawi, V. Ramalingam and J.-H. He, Mater. Today Energy, 2019, 11, 1–23 CrossRef CAS.
  32. Q. Xu, J. Zhang, D. Wang and Y. Li, Chin. Chem. Lett., 2021 DOI:10.1016/j.cclet.2021.05.032.
  33. Y. Wang, J. Mao, X. Meng, L. Yu, D. Deng and X. Bao, Chem. Rev., 2019, 119, 1806–1854 CrossRef CAS PubMed.
  34. R. Gusmão, M. Veselý and Z. Sofer, ACS Catal., 2020, 10, 9634–9648 CrossRef.
  35. H.-Y. Zhuo, X. Zhang, J.-X. Liang, Q. Yu, H. Xiao and J. Li, Chem. Rev., 2020, 120, 12315–12341 CrossRef CAS PubMed.
  36. H. Niu, X. Wan, X. Wang, C. Shao, J. Robertson, Z. Zhang and Y. Guo, ACS Sustainable Chem. Eng., 2021, 9, 3590–3599 CrossRef CAS.
  37. H. Niu, X. Wang, C. Shao, Z. Zhang and Y. Guo, ACS Sustainable Chem. Eng., 2020, 8, 13749–13758 CrossRef CAS.
  38. H. Niu, Z. Zhang, X. Wang, X. Wan, C. Shao and Y. Guo, Adv. Funct. Mater., 2021, 31, 2008533 CrossRef CAS.
  39. L. Kong, Z. Chen, Q. Cai, L. Yin and J. Zhao, Chin. Chem. Lett., 2021 DOI:10.1016/j.cclet.2021.09.010.
  40. L. X. Chen, Z. W. Chen, M. Jiang, Z. Lu, C. Gao, G. Cai and C. V. Singh, J. Mater. Chem. A, 2021, 9, 2018–2042 RSC.
  41. M. Chhowalla, Z. Liu and H. Zhang, Chem. Soc. Rev., 2015, 44, 2584–2586 RSC.
  42. M. Chhowalla, H. S. Shin, G. Eda, L.-J. Li, K. P. Loh and H. Zhang, Nat. Chem., 2013, 5, 263–275 CrossRef PubMed.
  43. J. Zhang, X. Xu, L. Yang, D. Cheng and D. Cao, Small Methods, 2019, 3, 1900653 CrossRef CAS.
  44. J. Li, Y. Zhang, C. Liu, L. Zheng, E. Petit, K. Qi, Y. Zhang, H. Wu, W. Wang, A. Tiberj, X. Wang, M. Chhowalla, L. Lajaunie, R. Yu and D. Voiry, Adv. Funct. Mater., 2021, 2108316 CrossRef.
  45. E. Meza, R. E. Diaz and C. W. Li, ACS Nano, 2020, 14, 2238–2247 CrossRef CAS PubMed.
  46. S. Tian and Q. Tang, J. Mater. Chem. C, 2021, 9, 6040–6050 RSC.
  47. F. Ling, W. Xia, L. Li, X. Zhou, X. Luo, Q. Bu, J. Huang, X. Liu, W. Kang and M. Zhou, ACS Appl. Mater. Interfaces, 2021, 13, 17412–17419 CrossRef CAS PubMed.
  48. Y. Liu, J. Wu, K. P. Hackenberg, J. Zhang, Y. M. Wang, Y. Yang, K. Keyshar, J. Gu, T. Ogitsu, R. Vajtai, J. Lou, P. M. Ajayan, B. C. Wood and B. I. Yakobson, Nat. Energy, 2017, 2, 17127 CrossRef CAS.
  49. J. Feng, X. Sun, C. Wu, L. Peng, C. Lin, S. Hu, J. Yang and Y. Xie, J. Am. Chem. Soc., 2011, 133, 17832–17838 CrossRef CAS PubMed.
  50. L. Cai, Q. Zhang, J. P. Mwizerwa, H. Wan, X. Yang, X. Xu and X. Yao, ACS Appl. Mater. Interfaces, 2018, 10, 10053–10063 CrossRef CAS PubMed.
  51. H. Li, Y. Liu, K. Chen, J. T. Margraf, Y. Li and K. Reuter, ACS Catal., 2021, 11, 7906–7914 CrossRef CAS.
  52. J. Zhu, L. Cai, X. Yin, Z. Wang, L. Zhang, H. Ma, Y. Ke, Y. Du, S. Xi, A. T. S. Wee, Y. Chai and W. Zhang, ACS Nano, 2020, 14, 5600–5608 CrossRef CAS PubMed.
  53. G. Kresse and J. Hafner, Phys. Rev. B: Condens. Matter Mater. Phys., 1993, 47, 558–561 CrossRef CAS PubMed.
  54. G. Kresse and J. Furthmüller, Phys. Rev. B: Condens. Matter Mater. Phys., 1996, 54, 11169–11186 CrossRef CAS PubMed.
  55. P. E. Blochl, Phys. Rev. B: Condens. Matter Mater. Phys., 1994, 50, 17953–17979 CrossRef PubMed.
  56. G. Kresse and D. Joubert, Phys. Rev. B: Condens. Matter Mater. Phys., 1999, 59, 1758–1775 CrossRef CAS.
  57. J. P. Perdew, K. Burke and M. Ernzerhof, Phys. Rev. Lett., 1996, 77, 3865–3868 CrossRef CAS PubMed.
  58. M. Cococcioni and S. de Gironcoli, Phys. Rev. B: Condens. Matter Mater. Phys., 2005, 71, 035105 CrossRef.
  59. H. Li, Y. Liu, K. Chen, J. T. Margraf, Y. Li and K. Reuter, ACS Catal., 2021, 11, 7906–7914 CrossRef CAS.
  60. M. H. N. Assadi and H. Katayama-Yoshida, J. Phys.: Condens. Matter, 2015, 27, 175504 CrossRef CAS PubMed.
  61. S. Grimme, J. Comput. Chem., 2006, 27, 1787–1799 CrossRef CAS PubMed.
  62. W. Tang, E. Sanville and G. Henkelman, J. Phys.: Condens. Matter, 2009, 21, 084204 CrossRef CAS PubMed.
  63. J. K. Nørskov, J. Rossmeisl, A. Logadottir, L. Lindqvist, J. R. Kitchin, T. Bligaard and H. Jonsson, J. Phys. Chem. B, 2004, 108, 17886–17892 CrossRef.
  64. A. A. Peterson, F. Abild-Pedersen, F. Studt, J. Rossmeisl and J. K. Nørskov, Energy Environ. Sci., 2010, 3, 1311–1315 RSC.
  65. Z. Fu, C. Ling and J. Wang, J. Mater. Chem. A, 2020, 8, 7801–7807 RSC.
  66. J.-H. Yuan, L.-H. Li, W. Zhang, K.-H. Xue, C. Wang, J. Wang, X.-S. Miao and X. C. Zeng, ACS Appl. Mater. Interfaces, 2020, 12, 13896–13903 CrossRef CAS PubMed.
  67. Y. Zhou, G. Gao, J. Kang, W. Chu and L.-W. Wang, Nanoscale, 2019, 11, 18169–18175 RSC.
  68. Y. Zhou, G. Gao, J. Kang, W. Chu and L.-W. Wang, Nanoscale Adv., 2021, 2, 710–716 RSC.
  69. Y. Qin, M. Yang, C. Deng, W. Shen, R. He and M. Li, Nanoscale, 2021, 13, 5800–5808 RSC.
  70. C. Liu, Q. Li, J. Zhang, Y. Jin, D. R. MacFarlane and C. Sun, J. Mater. Chem. A, 2019, 7, 4771–4776 RSC.
  71. J.-C. Liu, Y. Tang, Y.-G. Wang, T. Zhang and J. Li, Natl. Sci. Rev., 2018, 5, 638–641 CrossRef CAS.
  72. X. Lv, W. Wei, H. Wang, B. Huang and Y. Dai, Appl. Catal., B, 2020, 264, 118521 CrossRef CAS.
  73. G. J. Martyna, M. L. Klein and M. Tuckerman, J. Chem. Phys., 1992, 97, 2635–2643 CrossRef.
  74. X. Zhang, A. Chen, Z. Zhang, M. Jiao and Z. Zhou, J. Mater. Chem. A, 2018, 6, 11446–11452 RSC.
  75. Y. Sun, S. Wang, D. Jiao, F. Li, S. Qiu, Z. Wang, Q. Cai, J. Zhao and C. Sun, Chin. Chem. Lett., 2021 DOI:10.1016/j.cclet.2021.11.034.
  76. Z. Qin and J. Zhao, J. Colloid Interface Sci., 2022, 605, 155–162 CrossRef CAS PubMed.
  77. X. Li, Z. Su, Z. Zhao, Q. Cai, Y. Li and J. Zhao, J. Colloid Interface Sci., 2022, 607, 1005–1013 CrossRef CAS PubMed.
  78. Y. Wang, N. Zhou and Y. Li, Chem. Eng. J., 2021, 425, 130631 CrossRef CAS.
  79. M. Hu, S. Li, S. Zheng, X. Liang, J. Zheng and F. Pan, J. Phys. Chem. C, 2020, 124, 13168–13176 CrossRef CAS.
  80. J. Zhang, Z. Zhou, F. Wang, Y. Li and Y. Jing, ACS Sustainable Chem. Eng., 2020, 8, 7472–7479 CrossRef CAS.
  81. I. C. Man, H.-Y. Su, F. Calle-Vallejo, H. A. Hansen, J. I. Martínez, N. G. Inoglu, J. Kitchin, T. F. Jaramillo, J. K. Nørskov and J. Rossmeisl, ChemCatChem, 2011, 3, 1159–1165 CrossRef CAS.
  82. K. Lv, W. Suo, M. Shao, Y. Zhu, X. Wang, J. Feng, M. Fang and Y. Zhu, Nano Energy, 2019, 63, 103834 CrossRef CAS.
  83. W. Xiao, P. Liu, J. Zhang, W. Song, Y. P. Feng, D. Gao and J. Ding, Adv. Energy Mater., 2017, 7, 1602086 CrossRef.
  84. C. Sun, J. Zhang, J. Ma, P. Liu, D. Gao, K. Tao and D. Xue, J. Mater. Chem. A, 2016, 4, 11234–11238 RSC.
  85. H. Wang, Z. Xu, Z. Zhang, S. Hu, M. Ma, Z. Zhang, W. Zhou and H. Liu, Nanoscale, 2020, 12, 22541–22550 RSC.
  86. K. Mathew, R. Sundararaman, L. Weaver, K. Arias and T. Hennig, J. Chem. Phys., 2014, 140, 084106 CrossRef PubMed.
  87. H. Xu, D. Cheng, D. Cao and X. C. Zeng, Nat. Catal., 2018, 1, 339–348 CrossRef CAS.
  88. W. Gao, Y. Chen, B. Li, S.-P. Liu, X. Liu and Q. Jiang, Nat. Commun., 2020, 11, 1196 CrossRef CAS PubMed.


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