The catalytic activity and mechanism of oxygen reduction reaction on P-doped MoS₂

Abstract

With the approaching commercialization of proton exchange membrane fuel cell technology, developing active, non-precious metal oxygen reduction reaction (ORR) catalyst materials to replace currently used Pt-based catalysts is a necessary and essential requirement in order to reduce the overall system cost. Here, we report a single-atom doped molybdenum disulfide sheet (short as X-MoS₂) catalyst for the ORR using a dispersion-corrected density functional theory method. Of all the eleven X-MoS₂ (X = B, C, N, O; Al, Si, P; Ga, Ge, As, and Se) systems, only the phosphorus atom doped molybdenum disulfide (P-MoS₂) has an O₂ adsorption energy close to that of a Pt(111) surface. We have further explored the detailed ORR mechanism of P-MoS₂. Along the four-electron reaction pathway, the reduction of OH to H₂O is the rate-limiting step with the largest diffusion barrier of 0.79 eV.

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1. Introduction

In a proton exchange membrane fuel cell (PEMFC), the rate of the oxygen reduction reaction (ORR) at the cathode is much lower than that of the anode reaction,^1,2 and many studies have focused on improving the ORR activities using various catalysts. At present, platinum based materials are well known as the most efficient catalysts for the ORR.^3–6 However, as a noble metal, the high price and rarity of the platinum element limit the commercialization of PEMFCs.⁷ Recently, 2D materials with a large surface area, including graphene^8–11 and MoS₂ based materials,^12–23 have received much attention as alternative catalysts. MoS₂ is a transition metal chalcogenide that displays a layered structure similar to graphene, with an extended network of alternating Mo and S atoms. The MoS₂ nanosheets can be fabricated with controlled sizes, doped with heteroatoms, and surface-modified with other materials for unique crystal structures and high anisotropy. Moreover, the electronic structure of MoS₂ is highly dependent on the coordination of the Mo metal and its d-electrons, making it an important candidate with many potential uses in the field of catalysis. Furthermore, the edges of MoS₂ nanosheets have unsaturated coordination and dangling bonds that play an important role in surface-active applications. Consequently, these unexpected properties of MoS₂ nanosheets offer opportunities for new fundamental and technological research in a large variety of fields.^24–30

Wang et al.¹² demonstrated that the MoS₂ particles with the smallest size (∼2 nm) showed outstanding activity for the oxygen evolution reaction (OER) and ORR, in contrast to the inert bulk MoS₂. They concluded that the Mo edges on the extremely small MoS₂ nanoparticles should contribute to the four-electron reaction process of the ORR on the modified electrode. The incorporation of a heterogenous atom into MoS₂ may be a useful method to enhance the ORR performance. Controllable engineering of high electronegativity oxygen heteroatoms into MoS₂ ultrathin nanosheets was realized via a facile post-modification process by Huang et al.¹⁶ The incorporated oxygen atoms impart a dramatically enhanced ORR activity to the pristine nanosheets, with a 7.8-fold current increase as well as 180 mV and 160 mV positive shifts in both onset and half-wave potentials that are almost comparable to the commercial Pt/C catalyst. The same research group also achieved ultrathin phosphorus-doped MoS₂ nanosheets,¹⁵ which exhibit dramatically enhanced catalytic activity for the ORR. Both the onset and half-wave potentials are 160 mV more positive than the pristine form and the current density increased by 7-fold. Particularly, a change in the ORR selectivity from the two-electron to a four-electron route was observed. In addition, the proposed material demonstrates an improved stability and extra methanol tolerance compared to commercial Pt/C. The lower electronegativity of phosphorus atoms in the plane of MoS₂ nanosheets leads to an abundance of active sites, which may prefer to absorb oxygen molecules in the electrolyte and further accelerate the subsequent reduction processes. Xiao and coworkers¹⁷ predicted that Co or Ni doped MoS₂ can be used as suitable electrocatalysts for the ORR using a density functional theory (DFT) method because the doped MoS₂ can effectively adsorb O₂. Zhao et al.³¹ investigated the ORR activity of N- and P-doped MoS₂ monolayers in acidic medium by means of systematical DFT computations. Their results revealed that N doping provides the MoS₂ monolayer with better catalytic performance than P doping due to its moderate binding strength with the ORR species. Especially, the energy barrier and overpotential of the ORR on the N-doped MoS₂ monolayer are even lower than those on the Pt-based catalysts. The improved catalytic activity of the MoS₂ monolayer mainly originates from the induced suitable spin density due to N and P doping. Although attention has been devoted to understanding the ORR mechanism on pristine or doped 2D MoS₂ at the molecular level, the reason for enhancing or limiting the ORR activity by dopants is still inconclusive. More effort is needed to further increase the ORR activity of these catalysts.

First-principles methods have been proven to be extremely successful in the rational design of solid catalysts.³² In this work, we have done calculations with a dispersion-corrected DFT (DFT-D) method to understand the ORR activity on X-MoS₂, and further to identify the mechanism and the kinetics of the involved ORR elementary reaction. The aim of this article is to explore the origins of the enhancement and limitation of dopants and provide a reference for designing better catalysts in the future.³³

2. Computational method

The spin-polarized DFT calculations were performed using the DMol3 code embedded in Materials Studio (Accelrys, San Diego, CA)³⁴ with a long-range dispersion correction via Grimme's scheme.³⁵ The generalized gradient approximation with the Perdew–Burke–Ernzerhof (PBE) functional is employed to describe exchange and correlation effects.³⁶ The DFT semi-core pseudopods core treatment method is implemented for relativistic effects, which replace core electrons by a single effective potential and introduce some degree of relativistic correction into the core.³⁷ The double numerical atomic orbital augmented by a polarization function was chosen as the basis set.³⁸ During geometrical optimization, the basis set cut-off was chosen to be 4.9 Å. The convergence tolerances for the geometry optimization were set to 10⁻⁵ Ha for the energy, 0.002 Ha Å⁻¹ for the force, and 0.005 Å for the displacement. The electronic SCF tolerance was set to 10⁻⁶ Ha. A Fermi smearing parameter of 0.005 Ha was used in the calculations. The reciprocal space was sampled with a 4 × 4 × 1 k-point grid generated automatically using the Monkhorst–Pack method³⁹ for the relaxation calculations and a (12 × 12 × 1) k-point grid was used for electronic structure computations. Complete linear synchronous transit/quadratic synchronous transit (LST/QST) calculations were performed to locate transition states (TSs). Moreover, transition states were identified by the number of imaginary frequencies (NIMG) with NIMG = 1, the vibrational modes, and the ‘‘TS conformation’’ implemented in DMol3. Our DFT calculated lattice parameter for MoS₂ is 3.20 Å, which is consistent with the experimental value⁴⁰ and other computational results.^17,41,42 A more than 15 Å thick vacuum was added to avoid the artificial interactions between MoS₂ and its images. The metastable 1T phase persists after exfoliation and coexists with the 2H phase. However, since the metastable 1T phase can be diminished by heating or aging,⁴³ the monolayer MoS₂ with the stable 2H phase was adopted and modeled as a periodically repeated (4 × 4) supercell. In all of the structure optimization calculations, all the atoms are fully relaxed.

The adsorption energies (ΔE_ads) of the adsorbates are calculated through

ΔE_ads = E_ads/X-MoS2 − E_X-MoS2 − E_ads

where E_ads/X-MoS2, E_X-MoS2 and E_ads are the total energies of the adsorption systems, the pristine or doped MoS₂ monolayer and an isolated adsorbate species, respectively; “ads” denotes single adsorption of O₂, O, OH, OOH, and H₂O, and co-adsorption species O&O, O₂&H, O&OH, OH&OH, H₂O&O, H₂O&OH and OH&H. ΔE_ads < 0 corresponds to an exothermic adsorption process.

3. Results

3.1 The adsorption structures of O₂ on X-MoS₂

We have firstly investigated the adsorption of O₂ on the X-MoS₂ (X = B, C, N, O, Al, Si, P, Ga, Ge, As, and Se) nanosheets. There are four kinds of possible adsorption geometry for the pristine MoS₂ and doped MoS₂, which are the end-on site, side-on site, top site and hollow site on X atoms, as shown in Fig. S1 (ESI†). The most stable adsorption geometries, adsorption energies, Hirshfeld charges and p band centers are listed in Table 1. For the dopants in the same group, we find that the O₂ adsorption strength has a decreasing trend when the doping elements move from left to right: B > C > N–O (Al > Si > P > S; Ga > Ge > As > Se). The p band center of X atoms decreases in the same order: B > C > N > O (Al > Si > P > S; Ga > Ge > As > Se). This means that the more active the X atom (p band center close to the Fermi level), the stronger the binding energies of O₂. However, the Hirshfeld charges of X atoms do not have obvious trends. Furthermore, it is well known that the O₂ molecule possesses triplet states in which two single electrons occupy 2p_x* and 2p_y*. The empty bands of X atoms offer an orbital for their anti-orbital electrons and this induces a stronger binding between O₂ and X-MoS₂.

Table 1 The oxygen molecular adsorption mode, adsorption energy ΔE_ads (eV), bond length of adsorbed oxygen molecule d_O–O (Å), Hirshfeld charge Δq (e) and p band center ε_p of bare X-MoS₂

Geometry structure	Adsorption mode	ΔEads	d O–O	Δq (Xbare)	ε p
B-MoS2	Side-on	−2.52	1.60	−0.17	−0.70
C-MoS2	End-on	−1.21	1.35	−0.26	−1.69
N-MoS2	Top	−0.09	1.23	−0.32	−2.83
O-MoS2	Top	−0.11	1.23	−0.29	−3.27
Al-MoS2	Side-on	−2.69	1.48	0.16	−0.33
Si-MoS2	Side-on	−2.57	1.59	−0.004	−0.80
P-MoS2	Side-on	−0.87	1.58	−0.14	−1.17
MoS2	Hollow	−0.02	1.23	−0.11	−2.01
Ga-MoS2	Side-on	−1.53	1.44	0.21	−0.20
Ge-MoS2	Side-on	−1.13	1.55	0.07	−0.47
As-MoS2	Side-on	0.27	1.55	−0.06	−0.90
Se-MoS2	Side-on	2.45	1.59	−0.04	−1.65

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Oxygen molecule adsorption is the first step in the ORR and the adsorption strength should be within a suitable range. If the adsorption is too strong, it is not conducive to product desorption. On the other hand, if the adsorption strength is too weak, the coverage of O₂ may be too low to have a high reaction rate and the O–O bond would be too strong to be broken. Previous works have shown that platinum demonstrates a high ORR activity. According to previous theoretical studies of O₂ adsorbed on a Pt surface, the adsorption energy is suggested to vary from −0.49 to −1.02 eV^44–46 based on different models and methods. Therefore, the adsorption energy on the Pt surface can be used as an important reference to evaluate ORR activities of other catalysts. In all doping systems, only the adsorption energy of O₂ on the P-MoS₂ sheet falls in the range [−0.49, −1.02] (eV). Therefore, in the further study, we only consider the ORR mechanism on the P-MoS₂ sheet. Our results are different from those of Zhao et al.³¹ due to the differences in calculation methods. In the experiment in ref. 31, the influence of the water environment was considered. The oxygen adsorption energy of the P-MoS₂ catalyst was −0.93 eV in ref. 31, slightly higher than −0.87 eV in our experiment. However, the difference in oxygen adsorption energy of the N-MoS₂ catalyst is more significant (−0.37 eV vs. −0.09 eV). The response to the water environment is different for different adsorption structures and this will be our next focus. In this experiment, we did not consider the ORR activity of the N-MoS₂ catalyst, given its very low oxygen adsorption energy. Our results give different activity explanations for the P-MoS₂ catalyst, which is the meaning of this article.

3.2 The adsorption of various ORR involved species on P-MoS₂

We have firstly investigated the adsorption of various ORR involved species on P-MoS₂, including single adsorption of O₂, O, OH, OOH, and H₂O, and co-adsorption of O&O, O₂&H, O&OH, OH&OH, H₂O&O, H₂O&OH, and OH&H. Different adsorption sites have been tested for various adsorption configurations. The optimized geometries and the calculated ΔE_ads values for the most stable configurations are summarized in Fig. 1 and Table 2.

Fig. 1 Atomic structures of the relaxed geometries for various ORR chemical species adsorbed on P-MoS₂. (a) O₂, (b) co-adsorption of two O atoms, (c) O₂ and H co-adsorption, (d) OOH species, (e) co-adsorption of an O atom and a OH species, (f) two OH species co-adsorption, (g) O and H₂O co-adsorption, (h) OH species and H₂O co-adsorption, (i) atomic O species, (j) OH species, (k) H atom and OH species co-adsorption, and (l) H₂O. The ΔE_ads values (in eV) for the related species are marked in the corresponding figures.

Table 2 Summary of adsorption energies (ΔE_ads in eV) and Hirshfeld charges (Δq in e) for the various adsorbed species

Structures	Species	ΔEads	Δq (ads)	Δq (P)	Δq (P-Mo3)	Δq (P-Mo3S6)
Bare MoS2	—	—	—	−0.14	0.49	−0.16
Fig. 1(a)	O2	−0.87	−0.25	0.06	0.66	0.07
Fig. 1(b)	2O	−3.40	−0.54	0.20	0.88	0.35
Fig. 1(c)	O2&H	−0.41	−0.30, 0.10	0.03	0.61	0.10
Fig. 1(d)	OOH	−2.31	−0.05	−0.001	0.62	−0.05
Fig. 1(e)	O&OH	−4.30	−0.26, −0.08	0.16	0.82	0.22
Fig. 1(f)	2OH	−5.70	−0.06, −0.14	0.16	0.75	0.09
Fig. 1(g)	O	−5.53	−0.29	0.04	0.65	0.08
Fig. 1(h)	OH	−4.01	−0.03	0.01	0.63	−0.07
Fig. 1(i)	OH&H	0.18	−0.11, −0.02	0.08	0.68	0.01
Fig. 1(j)	H2O	−0.23	−0.10	−0.03	0.53	−0.12
Fig. 1(k)	H2O&O	−5.83	−0.05, −0.28	0.04	0.65	0.09
Fig. 1(l)	H2O&OH	−4.84	0.09, −0.11	0.02	0.64	−0.05

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3.2.1 Single molecular adsorption. According to our results, the P atom is the active center for the O₂ molecule adsorption. The obtained most stable configurations with two O atoms binding with P are presented in Fig. 1(a), which show that the O₂ molecule is stably anchored on the P atom with the ΔE_ads value of −0.87 eV. The O₂ molecule is negatively charged by 0.25 e, and the two O atoms are negatively charged by 0.12 and 0.13 e, respectively. The P atom is positively charged, and loses 0.20 e compared with that on the bare substrate. Each of the three Mo atoms around P gains around 0.01 e and consequently a total of 0.03 e is transferred into Mo₃ compared with that on the bare substrate. Furthermore, each of the six S atoms around P loses 0.01 e and consequently a total of 0.06 e is transferred away from S₆ compared with that on the bare substrate. Upon O₂ adsorption, the change of charge state of the fragment P-Mo₃S₆ in the system is about 0.23 e, which is comparable to the Hirshfeld charge of adsorbed O₂ (0.25 e). Charge transfer occurs mainly between O₂ and P-Mo₃S₆ moieties, which are considered to be the active center.

After the hydrogenation of oxygen molecules, the formed OOH species in Fig. 1(d) is negatively charged by 0.05 e and the adsorption energy is −2.31 eV. Upon the OOH adsorption, the O atom without H binds with P, the O–O bond points to the Mo top site and the charge transfer number of the fragment P-Mo₃S₆ in this system is about 0.11 e. Fig. 1(g) and (h) present the most stable configurations resulting from the O and OH adsorption, respectively. It is found that O and OH both prefer to be bound strongly on the P top sites, with the ΔE_ads values of −5.53 and −4.01 eV, respectively. The adsorbed atomic O and OH are negatively charged by 0.29 and 0.03 e, respectively, observed from the Hirshfeld charge analysis. For the H₂O adsorption in Fig. 1(j), the ΔE_ads value is −0.23 eV. The weak interactions indicate that the water can be easily released as the final product of the ORR.

3.2.2 Co-adsorption. The most stable adsorption configuration of the two O atoms is presented in Fig. 1(b), where the two O atoms are negatively charged by 0.25 e (for the atomic O on the P–Mo bridge site) and 0.30 e (for the atomic O binding with P only), respectively. The dissociative atomic O configuration is significantly thermodynamically more favorable than the molecular O₂ adsorption configurations. As another ORR reactant pathway (O₂ + H-OOH), the H could bind favorably to the S neighboring the adsorbed O₂ molecule, as shown in Fig. 1(c). The adsorbed atomic H is positively charged by 0.10 e, as observed from the Hirshfeld charge analysis, confirming that the adsorbed H could be taken as H⁺ in the calculations. Fig. 1(e) presents the most stable configuration resulting from the co-adsorption of the O atom and OH fragment (dissociation species from the adsorbed OOH fragment). The dissociative fragment O&OH configuration is significantly thermodynamically more favorable than the molecular OOH and O₂&H adsorption configurations, in which we adopt the same reference energy. As shown in Fig. 1(f), we also give the adsorption structure and ΔE_ads (−5.70 eV) of two OH fragments in their co-adsorption configuration. The adsorbed atomic OH is negatively charged by 0.06 e (pointing to the top site of Mo) and 0.14 e (pointing to the hollow site), respectively, observed from the Hirshfeld charge analysis. For the atomic O adsorption with or without H₂O co-adsorption (Fig. 1(g) and (k)), the ΔE_ads values are −5.53 and −5.83 eV, respectively. The difference of the ΔE_ads values (0.30 eV) is comparable to the ΔE_ads value of single H₂O (−0.23 eV), which indicates the weak interaction between the atomic O and the water molecule. The weak binding energy of a single H₂O means the easy realization of water molecules as the final product of the ORR. The situation of OH adsorption with or without H₂O co-adsorption is different; the corresponding ΔE_ads values are −4.84 and −4.01 eV, respectively. The difference of ΔE_ads (0.83 eV) is larger than the ΔE_ads of single H₂O (−0.23 eV), which indicates the enhancement effect of H₂O in the adsorption of fragment OH. In summary, all of the considered ORR involved species prefer to be bound to the center of the P-Mo₃S₆ complex.

3.3 The ORR element reactions on the P-MoS₂ nanosheet

As mentioned above, an O₂ molecule can be adsorbed on the P atom, and the chemisorption of the O₂ molecule is believed to be the first necessary step to initiate the ORR on the catalyst.^47–51 Upon the chemisorption of the O₂ molecule, there are two possible reaction pathways for the adsorbed O₂ molecule: dissociation into two atomic O and hydrogenation into OOH species.^48,50,52 Then, the dissociation product (2O) and the hydrogenation product (OOH) can be further hydrogenated and dissociated respectively to form the adsorbed O and OH intermediates. Similar to the OOH species, O and OH intermediates can undergo further hydrogenation to form 2(OH) species or O and H₂O species, respectively. Thus, the four-electron pathways in the ORR on the P-MoS₂ nanosheet can proceed through several possible reaction pathways, as shown in Fig. 2.

Fig. 2 The possible reaction pathways for the ORR on the P-MoS₂ catalyst. In the parentheses, the numbers on the left are the energy barrier, E_b, while those on the right are the reaction energy (in eV), E_r. The symbol * denotes that the ORR species is adsorbed on the catalyst surface.

From the calculation of the transition states (TSs), the reaction energy and reaction barrier are obtained. The reaction energy, E_r, which is defined as the change in total energies between initial states (ISs) and final states (FSs), can be used to evaluate the feasibility of the reaction. Furthermore, the reaction barrier, E_b, is defined as the change in total energies between transition states and final states and it can be used to assess the ease of the reaction.

3.3.1 O₂ dissociation. In the first step, the chemisorbed O₂ undergoes the O–O bond cleavage to form two separated O atoms. In the final state, two O atoms are adsorbed on the P atom and P–Mo bridge site, respectively. The O₂ dissociation process on P-MoS₂ is presented in Fig. S2 (ESI†). The calculated reaction energy and energy barrier are −2.52 and 0.08 eV, respectively. After the dissociation, the lengths of the P–O bond are shortened to 1.49 and 1.54 Å compared with 1.66 and 1.68 Å before dissociation. This suggested that the interaction of separated O atoms with the catalyst is stronger. In the following reaction, the two adsorbed O atoms will proceed with two sequential hydrogenation reactions to form either OH&OH or O&H₂O.

3.3.2 Formation of OH&OH. The hydrogenation of the co-adsorbed O&O (the O atoms adsorb on the P top site) to form O&OH is an exothermic reaction (−1.37 eV) with a high reaction barrier of 0.74 eV (Fig. S3, ESI†). For the second step, the hydrogenation of the remaining O to form OH&OH has a medium energy barrier of 0.42 eV, as shown in Fig. S4 (ESI†). The length of the P–O bond is elongated after O is attached to the H atom. For instance, the length of the P–O bond is 1.49 Å before H is attached, while it is 1.65 Å after H is attached.

3.3.3 Formation of OH&H₂O. One of the two OH would proceed via a hydrogenation reaction with a reaction barrier of 0.68 eV and a reaction energy of −2.51 eV, forming a H₂O molecule, which would be released easily due to the weak interaction between water and the support. The distance between H₂O and P is elongated from 1.65 Å to 3.61 Å after the OH hydrogenation, as shown in Fig. S5 (ESI†).

3.3.4 Formation of O&H₂O. The hydrogenations of the O and OH co-adsorption structures are more complicated, as shown in Fig. S6 (ESI†). In the hydrogenation process, H will diffuse from the P–Mo bridge site to the S atom next to the P atom with a high diffusion barrier of 0.78 eV to form an intermediate state (endothermic). The subsequent hydrogenation reaction occurs with a relatively low reaction barrier (0.10 eV) and a high reaction energy (−2.18 eV). Due to the much higher reaction energy (−2.18 eV) for the OH hydrogenation and the slightly disadvantageous reaction barrier (0.78 eV) compared to the hydrogenation of the atomic O, we believe that the hydrogenation processes of the atomic O and OH are both possible under the fuel cell working conditions. When the reaction proceeds with atomic OH hydrogenation, there are atomic O and H₂O formed, which would be released easily due to the weak interaction between water and the support.

3.3.5 O hydrogenation. After the release of the H₂O molecule, the atomic O on the top site of the P atom will be reduced to OH through a hydrogenation reaction with a small reaction barrier of 0.07 eV and a reaction energy of −1.47 eV, as shown in Fig. S7 (ESI†).

3.3.6 OH hydrogenation. Similar to the process of forming O&H₂O, the hydrogenation of OH on the top site of the P atom proceeds via H atom diffusion and the sequential hydrogenation reaction to form a H₂O molecule. In the OH hydrogenation process, H will diffuse from the P–Mo bridge site to the S atom next to the P atom with a high diffusion barrier of 0.79 eV to form an intermediate state (endothermic). The subsequent hydrogenation reaction has a reaction barrier of 0.59 eV and a reaction energy of −0.74 eV. Due to the weak interaction between water and the support (−0.23 eV), the formed H₂O is easily released, resulting in the recovery of the P–MoS₂ support, as shown in Fig. 3.

Fig. 3 The diffusion of the H atom from the P–Mo bridge site to the S atom near to P and the formation of H₂O from the hydrogenation of OH, and the release of H₂O.

3.3.7 O₂ hydrogenation. Fig. S8 (ESI†) presents the hydrogenation process of the adsorbed oxygen molecule. The hydrogenation of O₂ is an exothermic reaction (−1.90 eV) with an energy barrier of 0.16 eV, slightly higher than the barrier of O₂ dissociation (0.08 eV). Thus, the O₂ dissociation is more advantageous for the ORR on the P-MoS₂ catalyst in terms of thermodynamics and kinetics. However, the low energy barrier of O₂ hydrogenation (0.16 eV) also means the possibility of O₂ hydrogenation. During the hydrogenation process, the length of the P–O bond is elongated from 1.68 Å without H attachment in the initial state to 2.62 Å with H attachment in the final state. After the formation of OOH, there are three possible pathways, i.e., dissociation of OOH to form O&OH, and direct hydrogenation of OOH to form OH&OH and O&H₂O.

3.3.8 Dissociation of OOH. For the OOH dissociation process, the O–O bond of OOH is split, producing an O atom adsorbed on the P–Mo bridge site and an OH species adsorbed on the top of the P atom pointing to a hollow site. The OOH species dissociates with a relatively high barrier (0.50 eV) and exothermic reaction energy (−1.99 eV) (Fig. S9, ESI†). After the formation of O&OH, the further hydrogenation gives either OH&OH or O&H₂O, the same as O₂ dissociation.

3.3.9 Direct hydrogenation of OOH. The direct hydrogenation of OOH gives possibly OH&OH and O&H₂O. However, in this article, this situation is different. After the optimization of H&OOH co-adsorption, the conformation will form O&H₂O spontaneously.

According to the reaction stages presented above, five possible paths for the ORR on the P-MoS₂ support are proposed in the following and summarized in Fig. 2, which give an overall picture of the entire ORR on the P-MoS₂ support.

Path I: Step0 + StepA1 + StepA2 + StepA3 + StepA4 + Step5;

Path II: Step0 + StepA1 + StepA2 + StepB3 + StepB4 + Step5;

Path III: Step0 + StepB1 + StepB2 + StepB3+ StepB4 + Step5;

Path IV: Step0 + StepB1 + StepB2 + StepA3 + StepA4 + Step5;

Path V: Step0 + StepB1 + StepC2 + StepB4 + Step5.

In Path I, the progression of the ORR on P-MoS₂ starts from O₂ dissociation. Our TS calculation shows that the dissociation of O₂ is kinetically favorable, with a small reaction barrier. With two continuous hydrogenation reactions, the two atomic O will convert into two OH fragments, and finally form a H₂O molecule. The rate-limiting step in Path I is Step5 with a reaction barrier of 0.79 eV, corresponding to the diffusion of the H atom in the OH hydrogenation reaction. In Path II, following the O₂ dissociation, the two atomic O will be sequentially hydrogenated to form H₂O. The rate-limiting steps in Path I and Path II are the same. Path III, Path IV and Path V all start from the hydrogenation of the adsorbed O₂ molecules to form OOH. Upon dissociation of the OOH species, the formed O and OH would be hydrogenated into two OH (for Path III) and O&H₂O (for Path IV), respectively. In addition to the direct hydrogenation reaction of OOH, H&OOH will spontaneously form O&H₂O (for Path V). Upon the release of H₂O, P-MoS₂ is recovered. The rate-limiting step of Path III, Path IV and Path V is also Step5 with the reaction barrier of 0.79 eV, corresponding to the diffusion of the H atom in the OH hydrogenation reaction.

For all the elementary reactions, the energy barrier for the hydrogenation reaction is particularly high, especially if the H-atom is localized at the P–Mo bridge site. This is the main factor limiting the ORR activity of P-MoS₂ nanosheets. In order to explore the origin of this phenomenon, we calculated the density of states (DOS) and orbital information, as shown in Fig. 4. It was found that the orbitals of the P atom and the Mo atom are hybridized to form strong bonds, especially near the highest occupied orbital. The calculations show that the density of electrons is mainly localized at the P–Mo bridge site, and these rich electrons have an anchor effect on the H atom. In addition, we also calculated the deformation charge density map, and found that the P–Mo bridge site is a negative charge accumulation zone, which has a strong interaction with the H atom.

Fig. 4 The electronic structures of the P-MoS₂ nanosheet: (a) DOS of P, S and Mo atoms and orbitals at a specific energy level; (b) deformation charge density and Hirshfeld charge.

The activation energy of the rate-determining step of the ORR is 0.79 eV on Pt(111),⁵¹ 0.80 eV on Pt(100),⁵³ and 0.56 eV on FeN4-gra.⁵⁴ The current DFT calculations show that P-MoS₂ could promote the ORR with an energy barrier of 0.79 eV for the rate-determining step of the most preferred pathway, which suggests that the catalytic activity of P-MoS₂ could be comparable to those of the Pt and FeN4-gra catalysts. All of the elementary steps of the five pathways are exothermic, suggesting that P-MoS₂ could promote the ORR via five kinds of pathways with catalytic activity comparable to that of the Pt catalyst.

4. Conclusions

The detailed kinetic behaviors of the ORR on P-MoS₂ are studied via a DFT-D method. It is found that the dopant P plays a key role in initiating the ORR and the P-Mo₃S₆ moiety is the active center for all possible elementary steps. Summarizing our calculation results of activation energies, it is concluded that V is the most feasible reaction pathway. The largest reaction barrier is 0.79 eV and comparable to the Pt surface. However, the P–Mo bridge site restricts the diffusion of H atoms and this limits the ORR activity of the P-MoS₂ nanosheet. To break this limitation, reducing the P–H–TM binding energy or enhancing the binding strength between H and the neighboring site S (or change to X) via a co-doping method is a potential solution.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Yu S. S. is grateful for the financial support from Innovation Foundation of the 46th Research Institute of China Electronics Technology Group Corporation Program (Grant No. CJ20160902) and the National Natural Science Foundation of China (Grant No. 51602305). The DFT calculations utilized resources of High Performance Computing Center, Jilin University.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8cp01294f

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