Effect of the Second-Shell Coordination Environment on the Performance of P-Block Metal Single-Atom Catalysts for the Electrosynthesis of Hydrogen Peroxide

Abstract

Hydrogen peroxide (H₂O₂) is an important chemical with a diverse range of industrial applications in chemical synthesis and medical disinfection. The traditional anthraquinone oxidation process, with high energy consumption and complexity, is being replaced by cost-effective and environmentally friendly alternatives. In order to explore suitable catalysts for the electrocatalytic synthesis of H₂O₂, the stability of B,N-doped graphene loaded with various p-block metal (PM) single atoms (i.e., PM-N_xB_y: x and y represent the number of atoms of N and B, respectively) and the effects of different numbers and positions of B dopants in the second coordination shell on the catalytic performance were studied by density functional theory (DFT) calculations. The results show that Ga-N₄B₆ and Sb-N₄B₆ exhibit enhanced stability and 2e⁻ oxygen reduction reaction (ORR) activity and selectivity. Their thermodynamic overpotential η values are 0.01 V, 0.03 V for Ga-N₄B₆’s two configurations and 0.02 V, 0 V for Sb-N₄B₆’s two configurations. Electronic structure calculations indicate that the PM single atom adsorbs OOH* intermediates and transfers electrons into them, resulting in the activation of the O-O bond, which facilitates the subsequent hydrogenation reaction. In summary, Sb-N₄B₆ and Ga-N₄B₆ exhibit extraordinary 2e⁻ ORR performance, and their predicted activities are comparable to those of known outstanding catalysts (such as PtHg₄ alloy). We propose effective strategies on how to enhance the 2e⁻ ORR activities of carbon materials, elucidate the origin of the activity of potential catalysts, and provide insights for the design and development of electrocatalysts that can be used for H₂O₂ production.

1. Introduction

Hydrogen peroxide (H₂O₂) is a chemical oxidant that is employed in a wide range of industrial processes. The most environmentally friendly aspect of H₂O₂ is that when it interacts with other substances, it breaks down into H₂O and O₂ without creating other impurities. In particular, the demand for water treatment and environmental remediation requires increased production of H₂O₂. However, the high cost of synthesizing H₂O₂ via the energy-intensive industrial anthraquinone oxidation process [1], as well as the associated costs of storage and transport, represent significant obstacles to further development [2]. Furthermore, this process has a significant negative impact on sustainability, as it generates large amounts of harmful effluents. Consequently, in order for hydrogen peroxide to be more widely and effectively utilized as a green and non-polluting product, the issues surrounding on-site preparation, storage and transportation must be resolved. A method for synthesizing H₂O₂ directly from O₂ and H₂ has been proposed. However, its practical application in factories is limited due to the risk of explosion in H₂/O₂ gas mixtures [3]. Therefore, new methods for H₂O₂ production, including the photocatalytic oxidation of water and the electrocatalytic reduction of oxygen [4,5], have been proposed and have received extensive attention.

The electrocatalytic reduction of oxygen allows the production of H₂O₂ under mild conditions using electrocatalysts. The development of highly active, selective and stable electrocatalysts is the key to the production of H₂O₂ [6]. Currently, platinum (Pt)-based catalysts are the most commonly used catalysts. However, the scarcity and high cost of the starting feedstock Pt have become the factors in limiting its widespread use [7]. To date, the field has concentrated on the investigation of more suitable catalysts, including transition metal oxides [8,9], carbides [10,11,12], nitrides [13,14,15], sulfides [16,17,18] and carbon materials [19,20,21]. Since the development of Pt₁/FeO_x by Qiao et al. in 2011, single-atom catalysts (SACs) have received considerable attention and have shown encouraging performance in oxygen reduction reactions (ORRs) [22,23], nitrogen reduction reactions (NRRs) [24,25], α,β-unsaturated aldehydes hydrogenation reactions [26], and hydrogen spillover [27,28]. SACs represent an ideal replacement for Pt-based metal catalysts, benefiting from their high atomic utilization, high activity and low cost of the component elements [29,30,31].

Previous studies have shown that the M-N₄ unit of metal–nitrogen–carbon (M-N-C) catalysts exhibits excellent electrocatalytic activity [32,33]. In recent studies, researchers have introduced a large number of other non-metallic elements (O, B, S, P, etc.) in addition to N. M-N-C was prepared by adjusting the type, number and doping position of heteroatoms (first, second and even higher coordination shells), which artificially modify the partial coordination environment of the metal single atom, thus providing the possibility to improve the electrocatalytic reaction activity [34,35,36].

Usually, main-group metals are not considered to be catalytically active due to their electron-filled d orbitals (d¹⁰ orbitals) [37,38]. However, many studies reveal that the main-group metal single atoms can also be activated through local coordination environment engineering of the M-N_x unit, thereby demonstrating that main-group metals may also possess excellent catalytic properties [39,40]. Although p-block metal SACs with activated p-orbital electrons have been shown to be efficient and stable ORR electrocatalysts, previous studies have typically been focused on 4e⁻ ORR [41,42], with only a few studies on 2e⁻ ORR. For example, Zhang et al. [43] uncovered that the selectivity and activity of indium (In) SACs for 2e⁻ ORR could be improved through modulating the first coordination shell layer. Additionally, Gu et al. [44] reported that the introduction of oxygen-functional groups in the second-shell coordination environment could enhance the 2e⁻ and 4e⁻ ORR activity and selectivity of antimony (Sb) SACs (Sb-N₄). These studies suggest that the p-electrons of p-block metals can be activated to catalyze ORR by modulating the local coordination environment.

In this paper, we investigate the catalytic 2e⁻ ORR process on nitrogen-doped graphene (NG) substrates anchored with single atoms of different p-block metals (M = Al, Ga and Sb) using density functional theory (DFT) calculations. The second-shell coordination environment of the M-N₄ site is modified by the introduction of the heteroatom boron (B) to modulate the electronic structure of M. The results demonstrate that B doping optimizes the binding strength of the central metals (Sb and Ga) to the intermediate OOH* during the ORR process, resulting in a neither too strong nor too weak adsorption energy of OOH*. Encouragingly, the predicted activities of Sb-N₄B₆ and Ga-N₄B₆ are comparable to those of known commercial catalysts. The theoretical result demonstrates that the electrocatalytic activity of NG loaded with Sb and Ga single atoms can be enhanced via the introduction of heteroatom B in the second-shell coordination environment. Furthermore, they provide insights into the rational design of efficient catalysts for 2e⁻ ORR.

2. Results and Discussion

2.1. Structures and Stability

To select catalysts that are suitable for the electrochemical synthesis of H₂O₂, three screening criteria are considered: Firstly, SACs must have good stability. Secondly, SACs must have a low overpotential. Thirdly, SACs must be selective for the 2e⁻ ORR compared with the competitive 4e⁻ ORR. A 6 × 6 × 1 supercell of NG was constructed as a model. In order to reduce interactions between adjacent layers in periodically repeated cells, vacuum thicknesses were set to 15 Å. A metal (Al, Ga and Sb) atom was incorporated into the model to form the central site M-N₄. The heteroatom B was introduced to replace the C atoms in the second coordination shell, thereby forming M-N₄B_X (x = 1–6). The M-N₄B_X model and the locations of B atoms are shown in Figure 1a. The first blue circle in the graphene structure represents the first coordination shell, and the C atoms in the second red circle outside represent the second coordination shell. The coordination regulation of the second shell, as illustrated in Figure S1, allows for the formation of 39 distinct M-N₄B_X structures, each containing varying amounts of B atoms at different sites. This results in a total of 117 catalyst models.

Since the stability of the catalyst is the most fundamental prerequisite for its application, the thermodynamic stability of the catalyst was first evaluated by calculating the binding energy (E_b) between the PM atoms and the adjacent N atoms and the cohesion energy (E_c) of the PM atoms. E_b and E_c are calculated as follows:

$${\mathrm{E}}_{\mathrm{b}}{=\mathrm{E}}_{{\mathrm{M}-\mathrm{N}}_4{\mathrm{B}}_{\mathrm{X}}\mathrm{C}}-{\mathrm{E}}_{\mathrm{M}}-{\mathrm{E}}_{{\mathrm{N}}_4{\mathrm{B}}_{\mathrm{X}}\mathrm{C}}$$

(1)

where ${\mathrm{E}}_{{\mathrm{M}-\mathrm{N}}_4{\mathrm{B}}_{\mathrm{X}}\mathrm{C}}$, E_M and ${\mathrm{E}}_{{\mathrm{N}}_4{\mathrm{B}}_{\mathrm{X}}\mathrm{C}}$ represent the total energy of the catalyst, the energy of the isolated single metal atom and the energy of the catalyst without loading the metal single atom, respectively.

$${\mathrm{E}}_{\mathrm{c}}=\frac{{\mathrm{E}}_{\mathrm{bulk}}}{\mathrm{n}}-{\mathrm{E}}_{\mathrm{M}}$$

(2)

where E_bulk is the total energy of the metal bulk material and n represents the number of metal atoms in the bulk material.

Figure 1b shows the discrepancy between the binding energy and the cohesive energy (E_b − E_c). If the difference is less than zero (E_b − E_c < 0), the bulk of the metal is more likely to be isolated by M-N₄B_X-C during the reaction rather than agglomerating to form nanoparticles, and thus the structure is stable. The value of (E_b − E_c) for all the structures in the figure is less than 0, which means that the three main groups of metals are available to be stably embedded in the hole of NG through the nitrogen–metal (N-M) bonds.

2.2. Activity and Selectivity of the Catalysts

The 4e⁻ ORR reaction pathway is as follows:

$${\mathrm{O}}_2{+*+\mathrm{H}}^{+}{+\mathrm{e}}^{-}\to \mathrm{OOH}*,$$

(3)

$${\mathrm{OOH}*+\mathrm{H}}^{+}{+\mathrm{e}}^{-}\to {\mathrm{H}}_2\mathrm{O}+\mathrm{O}*,$$

(4)

$${\mathrm{O}*+\mathrm{H}}^{+}{+\mathrm{e}}^{-}\to \mathrm{OH}*,$$

(5)

$${\mathrm{OH}*+\mathrm{H}}^{+}{+\mathrm{e}}^{-}\to {\mathrm{H}}_2\mathrm{O}+*,$$

(6)

The complete reaction can be expressed as

$${\mathrm{O}}_2{+4\mathrm{H}}^{+}{+4\mathrm{e}}^{-}\to {2\mathrm{H}}_2\mathrm{O}(E_0=1.23\mathrm{V} \text{vs. RHE}),$$

(7)

The 2e⁻ ORR reaction pathway is as follows:

$${\mathrm{O}}_2{+*+\mathrm{H}}^{+}{+\mathrm{e}}^{-}\to \mathrm{OOH}*,$$

(8)

$${\mathrm{OOH}*+\mathrm{H}}^{+}{+\mathrm{e}}^{-}\to {\mathrm{H}}_2{\mathrm{O}}_2+*,$$

(9)

The complete reaction can be expressed as

$${\mathrm{O}}_2{+2\mathrm{H}}^{+}{+2\mathrm{e}}^{-}\to {\mathrm{H}}_2{\mathrm{O}}_2(E_0=0.7\mathrm{V} \text{vs. RHE}),$$

(10)

The ORR process is shown in Figure 2, it can be understood as follows: O₂ adsorbs at the active site, gaining an electron (e⁻) and a proton (H⁺) from the aqueous solution to produce an OOH* intermediate. OOH* is then reduced by (H⁺ + e⁻), further generating either H₂O₂ (2e⁻ ORR) or H₂O and O* intermediates (4e⁻ ORR). The key factor in the synthesis of H₂O₂ is the breaking or non-breaking of the O-O bond. In both the 4e⁻ and 2e⁻ pathways, the intermediate OOH* is formed in the first reaction step of oxygen adsorption and hydrogenation. Therefore, we calculated the free energy of OOH* (G_OOH*) first, as shown in Tables S1–S3. Subsequently, the ORR performance of the M-N₄ site and the M-N₄B_X under acidic conditions (pH = 0) was investigated.

The synthesis of H₂O₂ is dependent upon the moderate adsorption strength of the OOH* intermediate on the SACs, which is a critical factor in determining the catalytic performance of the process. The adsorption energy must be neither too strong nor too weak, as this would result in the O-O bond being broken or the subsequent reaction steps being unable to proceed. From Equation 10, it can be seen that the ΔG of each electron step in 2e⁻ ORR is 0.70 eV (G_OOH* = 4.22 eV). Therefore, the ΔG for the entire reaction is 1.40 eV (${\mathrm{G}}_{{\mathrm{H}}_2{\mathrm{O}}_2}$ = 3.52 eV). Theoretically, the closer the U_L is to the equilibrium potential (0.70 V vs. RHE), the closer the G_OOH* is to 4.22 eV, and the more likely the free energy of all reactions is to be zero, indicating that the catalyst is highly active in the 2e⁻ ORR process. As illustrated in the Tables S1–S4, the differing numbers of B in the second coordination shell and the distinct central atoms exhibit varying adsorption strengths for the intermediate of OOH*.

The ΔG of the entire 2e⁻ ORR process is known to be 1.40 eV. If the free energy of G_OOH* becomes too small, the free energy change of the first electron step will be greater than 1.40 eV (ΔG₁ < −1.40 eV) and G_OOH* < 3.52 eV, which causes the ΔG₂ to increase during the thermodynamic reaction (ΔG₂ > 0), and this is thermodynamically unfavorable. Initially, it is found that the G_OOH* of M-N₄ (M = Sb, Ga, and Al) was less than 4.22 eV, with a significant difference (Table S1). This is not conducive to 2e⁻ ORR, and thus M-N₄ is not suitable to serve as the catalytic site of 2e⁻ ORR.

On the other hand, an examination of Tables S2 and S3 reveals that the G_OOH* of Al-N₄B_X and Ga-N₄B_X is becoming larger in the process of increasing heteroatom B atoms; in addition, the G_OOH* of Al-N₄B_X (X = 1–4) is almost less than 3.52 eV. Consistently, the G_OOH* for all structures is observed to be almost less than 3.52 eV in Ga-N₄B_X. This indicates that in these structures, the intermediate OOH* will be strongly adsorbed at the reaction site, resulting in the O-O bond breaking during the subsequent reaction process, which is not conducive to the synthesis of H₂O₂. Therefore, these structures are unsuitable as catalysts for the 2e⁻ ORR. Furthermore, the G_OOH* of Sb-N₄B_X in Table S4 displays an initial increase and subsequent decrease with the addition of B heteroatoms (G_OOH* rises at X = 1–4 and then declines at X = 5–6). Among the structures, the G_OOH* of Sb-N₄B₄ is almost all greater than 4.92 eV, indicating that Sb-N₄B₄ has difficulty adsorbing OOH* intermediates and activating the O-O bond of OOH*, which is also not conducive to the formation of H₂O₂ by the 2e⁻ ORR process. Therefore, the above structures were screened out via the calculation of the adsorption energy of the OOH* intermediate (Al-N₄B_X, X = 1–4; Ga-N₄B_X, X = 1–3; Sb-N₄B_X, X = 4).

In the remaining models, the U_L values during the ORR were calculated. Figure 3 shows the calculated U_L value for the Al-N₄B_X (X = 5–6), Ga-N₄B_X (X = 4–6) and Sb-N₄B_X (X = 1–3, 5–6) systems. Notably, Ga-N₄B₆ (0.69 V and 0.67 V), Sb-N₄B₁ (0.63 V and 0.59 V), Sb-N₄B₂ (0.49 V and 0.67 V) and Sb-N₄B₆ (0.68 V and 0.70 V) exhibited U_L values that are very close to 0.70 V, indicating that Ga-N₄B₆ and Sb-N₄B_X (X = 1–2, 6) have high activity in the synthesis of H₂O₂. Of these, Ga-N₄B₆ and Sb-N₄B₆ have the highest U_L values and had the highest activity. In addition, the U_L values of Al-N₄B_X (X = 5–6), Ga-N₄B_X (X = 4–5) and Sb-N₄B_X (X = 3, 5) are too small, implying that the activity of these catalysts is quite poor. Therefore, these catalysts will not be further considered.

The adsorption strength of the intermediate at the active center on the surface of the catalyst plays an equally essential role in determining the activity and selectivity of the catalytic reaction. Thus, the free energies of ORR at each basic reaction step on Ga-N₄B₆ and Sb-N₄B₆ were calculated, allowing the determination of the η and potential-limiting steps (PDSs). This, in turn, enables the assessment of the ORR selectivity of Ga-N₄B₆ and Sb-N₄B₆. The reaction free-energy profiles of the 2e⁻ ORR and 4e⁻ ORR on Ga-N₄B₆ and Sb-N₄B₆ are shown in Figure 4.

As shown in Figure 4a,c, for the 2e⁻ pathway on Sb-N₄B₆, the minimum ΔG value at U = 0.00 V is 0.68 eV and 0.70 eV. However, for the structure of Sb-N₄B₆-1, the hydrogenation of the O₂ adsorbed at the central site on the catalyst surface to form OOH* intermediate represents a PDS of the entire 2e⁻ ORR process. Nevertheless, for Sb-N₄B₆-2, any step during the reaction can be a PDS. This allows the 2e⁻ ORR process to be carried out spontaneously on Sb-N₄B₆ at U = 0.68 V and 0.70 V, without the need for additional energy input. According to Formula (3), the η of Sb-N₄B₆-1 and Sb-N₄B₆-2 can be calculated to be 0.02 V and 0 V, respectively. For the 4e⁻ ORR process (Figure 4b,d), the minimum ΔG was calculated at U = 0.00 V to be 0.67 eV and 0.66 eV, which is the last step of the 4e⁻ ORR process. This indicates that the hydrogenation of OH* to form H₂O is a PDS for Sb-N₄B₆-1 and Sb-N₄B₆-2. All of the reaction steps occurred spontaneously at 0.67 V and 0.66 V, respectively. Consequently, the η value of the 4e⁻ ORR (0.56 V and 0.57 V) is greater than that of the 2e⁻ ORR (0.02V and 0 V). The calculation results indicate that Sb-N₄B₆ is more likely to form H₂O₂ via the 2e⁻ ORR pathway than to produce H₂O via the 4e⁻ ORR pathway. This indicates that Sb-N₄B₆ exhibits high selectivity for 2e⁻ ORR. Similarly, for Ga-N₄B₆, during the 2e⁻ ORR process, the PDSs are the first and second steps, with a value of ΔG to be 0.69 eV and 0.67 eV, respectively. The corresponding η is 0.01 and 0.03 V, respectively. During the 4e⁻ ORR, the PDSs are the first and fourth steps, with ΔG values of 0.69 eV and 0.59 eV, respectively. The corresponding η is 0.54 V and 0.65 V, respectively. The η of the 4e⁻ ORR obtained by Ga-N₄B₆ (0.54 and 0.65 V) was larger than that of 2e⁻ ORR (0.01 and 0.03 V), and Ga-N₄B₆ also demonstrated a preference for the 2e⁻ ORR pathway over the 4e⁻ ORR pathway, resulting in the formation of H₂O₂ rather than H₂O. In general, Sb-N₄B₆ and Ga-N₄B₆ are more selective for 2e⁻ ORR than 4e⁻ ORR, and thus they are ideal catalysts for H₂O₂ synthesis. Moreover, the performance of Sb-N₄B₆ was slightly superior to that of Ga-N₄B₆. More importantly, the predicted activity of Sb-N₄B₆ and Ga-N₄B₆ is comparable to that of known outstanding electrocatalysts for H₂O₂ production, for example, the PtHg₄ alloy [6].

2.3. ORR Activity Origin

Essentially, the properties of catalysts, such as activity, stability and selectivity, are typically influenced by the combined effects of geometry and electronic structure. In the previous section, the effect of the geometry on the 2e⁻ ORR activity and selectivity was elucidated in detail. To gain a deeper understanding of the nature of the catalyst activity, the origin of the catalyst activity is discussed through an analysis of the electronic structure. In this process, a bridge is needed to connect the activity and electronic information. In the preceding discussion of thermodynamics, the adsorption free energies of the reaction and intermediates play a crucial role in the activity and selectivity, corresponding with the micro-changes (e.g., the number of valence electrons, d-band center, p-band center, electron spin and others) of the active metals.

The key factors responsible for the excellent activity and selectivity of Sb-N₄B₆ and Ga-N₄B₆ SACs were identified through an electronic structure analysis. A Bader charge and charge differential density (CDD) analysis were conducted to quantitatively determine the charge transfer that occurs upon adsorption of the reaction intermediate. As illustrated in Figure 5, the results indicate that there is a significant electron transfer between both Sb and Ga with N atoms of Sb-N₄B₆ and Ga-N₄B₆. The Bader charge analysis revealed that about 1.53 e⁻ transferred to the substrate N atoms from Sb in Sb-N₄B₆-1 and Sb-N₄B₆-2, respectively. Furthermore, the CDD plot (Figure 5a,b) showed that there was a significant charge transfer between Sb and OOH* when OOH* adsorbed to Sb-N4B₆-1 and Sb-N₄B₆-2, with about 0.53 and 0.54 e⁻ from Sb to the adsorbed OOH, respectively. This leads to an elongation of the O-O bond length of the OOH* from 1.21 Å to 1.475 Å and 1.48 Å (

Table 1. Bader charges before the center of the metal adsorption OOH (q_M (e⁻)) after (Q_M (e⁻)) lost charge; OOH* intermediates of charge are (q_OOH (e⁻)) and O-O bond length.

M-N4B6	qM (e−)	QM (e−)	qOOH (e−)	Bond Length (Å)
Ga-N4B6-1	−1.48	−1.57	0.43	1.44
Ga-N4B6-2	−1.45	−1.57	0.47	1.45
Sb-N4B6-1	−1.53	−2.30	0.53	1.475
Sb-N4B6-2	−1.53	−2.32	0.54	1.48

). The calculation result implies that the Sb single atom effectively activates the O-O bond, thereby facilitating the subsequent hydrogenation of OOH* to generate the final H₂O₂ product. There are approximately 0.43 and 0.47 e⁻ transferred from Ga to adsorbed OOH* in the same Ga-N₄B₆-1 and Ga-N₄B₆-2 (Figure 5c,d), respectively, resulting in the O-O bond lengths of OOH* being elongated to 1.44 Å and 1.45 Å (Table 1). Thus, the analysis demonstrated that both Sb-N₄B₆ and Ga-N₄B₆ exhibit good ORR activity and predominantly followed the 2e⁻ pathway during the reaction. The understanding of the activity origin can provide insights at the molecular level into the catalytic process, thereby enabling the design of catalysts with optimal performance. It is noteworthy that the curvature of graphene can influence the catalytic performance, as reported in previous studies [45,46], and it may be interesting to investigate the "curvature effect" of electrocatalysis in our future work.

3. Computational Methods

All the spin-polarized DFT calculations were performed using the Vienna ab initio simulation package (VASP.5.4.4) [47]. The projector-augmented-wave (PAW) potential was employed for modeling the interactions between ions and electrons, with a plane wave energy cutoff set to 450 eV [48,49]. The exchange correlation potential was treated by the Perdew–Burke–Ernzerhof (PBE) version of generalized gradient approximation (GGA) [50]. The k-point grid was set to 3 × 3 × 1 for structural optimization and also for sampling of the Brillouin region in electronic structure calculations. The zero-damping DFT-D3 method [51] proposed by Grimme et al. [52] was used to describe the van der Waals interactions. The convergence standard of energy was set at 10⁻⁴ eV, and all structures were relaxed until the forces on each ion were less than 0.05 eV/Å.

Based on Nørskov’s computational hydrogen electrode model [53,54], we calculated the Gibbs free energy change (∆G) for each elementary step as follows:

$$\Delta \mathrm{G}=\Delta \mathrm{E}+\Delta {\mathrm{E}}_{\mathrm{zpe}}-\mathrm{T}\Delta \mathrm{S}+\mathrm{eU},$$

(11)

where ΔE is the energy difference directly calculated by DFT before and after each elementary reaction step; ΔE_zpe and TΔS are the differences in the zero-point energies and entropy at 298.15 K, respectively; and the value of eU is determined by the applied potential (U). In addition, VASPKIT [55] was used to perform vibrational frequency analysis to obtain ΔEzpe and ΔS values.

In addition, we defined a descriptor of limiting potential (U_L) to describe the activity of the electrocatalytic ORR with the free energy change of the potential-determining step (PDS):

$${\mathrm{U}}_{\mathrm{L}}=-\Delta {\mathrm{G}}_{\mathrm{PDS}}/\mathrm{e},$$

(12)

where the ∆G_PDS is the Gibbs free energy change of the PDS.

The overpotential (η) was calculated as

$${\mathsf{\eta }=\mathrm{U}}_{\mathrm{equilibrium}}-{\mathrm{U}}_{\mathrm{L}}$$

(13)

where the equilibrium potentials (U_equilibrium) are 1.23 V and 0.70 V for the 4e⁻ ORR and 2e⁻ ORR, respectively.

4. Conclusions

In this work, the 2e⁻ ORR performance of a series of PM-N₄B_X/C (PM = Al, Ga and Sb) catalysts in an acidic environment is investigated in terms of catalytic activity and product selectivity by DFT calculations. The results show that Sb-N₄B₆ and Ga-N₄B₆ have good stability and high activity and selectivity for 2e⁻ ORR, and Al-N_xB_y catalysts are all screened out due to their poor activity. In order to ascertain the origin of the catalytic activity, the electronic structure of the OOH* intermediates adsorbed on the two catalysts was calculated. The results revealed that electrons are transferred from the central atom into OOH*, which activates the O-O bond and facilitates continued hydrogenation. These findings elucidate the origin of the exceptional 2e⁻ ORR electrocatalytic activity of Sb-N₄B₆ and Ga-N₄B₆. This work offers theoretical insights into the rational design of efficient catalysts for H₂O₂ synthesis.