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Article

CrS2 Supported Transition Metal Single Atoms as Efficient Bifunctional Electrocatalysts: A Density Functional Theory Study

School of Physics, Huazhong University of Science and Technology, Wuhan 430074, China
ChemEngineering 2025, 9(3), 43; https://doi.org/10.3390/chemengineering9030043
Submission received: 28 January 2025 / Revised: 10 April 2025 / Accepted: 14 April 2025 / Published: 23 April 2025

Abstract

Transition metal dichalcogenides (TMDs) are recognized for their exceptional energy storage capabilities and electrochemical potential, stemming from their unique electronic structures and physicochemical properties. In this study, we focus on chromium disulfide (CrS2) as the primary research subject and employ a combination of density functional theory (DFT) and first-principle calculations to investigate the effects of incorporating transition metal elements onto the surface of CrS2. This approach aims to develop a class of bifunctional single-atom catalysts with high efficiency and to analyze their catalytic performance in detail. Theoretical calculations reveal that the Au@CrS2 single-atom catalyst demonstrates outstanding catalytic activity, with a low overpotential of 0.34 V for the oxygen evolution reaction (OER) and 0.37 V for the oxygen reduction reaction (ORR). These results establish Au@CrS2 as a highly effective bifunctional catalyst. Moreover, the catalytic performance of Au@CrS2 surpasses that of traditional commercial catalysts, such as Pt (0.45 V) and IrO2 (0.56 V), suggesting its potential to replace these materials in fuel cells and other energy applications. This study provides a novel approach to the design and development of advanced transition metal-based catalytic materials.

1. Introduction

Energy is an important cornerstone for the development of human society. In recent years, fuel cells and metal-air batteries have stood out as representatives of the new generation of energy storage technologies by virtue of their highly efficient energy conversion and environmentally friendly and non-polluting characteristics. They show great potential for mitigation in response to the current energy crisis. The fundamental components of these technologies are oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) catalysts. Traditional implementations typically employ Pt(111) for the ORR, and IrO2 derivatives have historically been preferred for OER applications [1,2,3]. While these conventional commercial catalysts exhibit high efficiency and facilitate energy conversion processes in electrochemical devices, they are hindered by several critical challenges, including high cost, limited resource availability, poor long-term stability, and single-function catalytic performance. These limitations severely restrict their large-scale application and commercial viability [4,5]. The development of advanced catalytic architectures with enhanced cost-effectiveness and multifunctional capabilities represents a crucial research direction, offering dual benefits for both technological innovation and sustainable development objectives. Such technological breakthroughs are critical not only for advancing scalable production of electrocatalytic systems but also for addressing global energy sustainability challenges and mitigating environmental impacts.
Electrocatalysis utilizes conventional electrical energy to initiate chemical reactions and typically relies on metal electrocatalysts to enhance rapid electron transfer between reactants and electrodes. Excellent electrocatalytic performance requires high conductivity, carrier mobility, and active site density [6]. Due to their excellent physical and chemical properties, two-dimensional (2D) layered materials serve as ideal platforms for stabilizing monoatomic layers [7]. Transition metal dichalcogenides (TMDs) constitute a class of compounds with an MX2-type structure, where M represents transition metal elements (e.g., Mo, W, Re) and X represents chalcogen elements (S, Se, Te). These materials exhibit similarities in chemical properties and crystal structures. This isostructural nature facilitates comparative studies, allowing research on Se- and Te-containing compounds to inform the understanding of S-containing compounds. TMDs possess a distinctive two-dimensional layered structure, with intralayer atoms bound by strong covalent bonds and interlayer interactions governed by weaker van der Waals forces. In recent years, 2D-TMD materials have unique physicochemical properties and applications and are promising for applications in electrocatalyst materials [8,9,10,11]. Recent studies have highlighted the superior activity of 2D-TMDs in electrocatalysis. For example, Naiwrit Karmodak et al. demonstrated that 2D-TMDs exhibit excellent electrocatalytic activity in oxygen reduction reaction (ORR), with the best-performing one, MoTe2, having an ORR overpotential of 0.29 V [12]. A single Pd atom catalyst supported by a single 1T-MoSe2 monolayer with an ORR overpotential of 0.32 V was reported by Qin et al. [13].
The size of the single-atom catalyst particles is small, they are able to be mono-dispersed on the surface of the carrier and act as catalysts. Due to the localized chemical environment in which the metal atoms are located, the material can theoretically achieve 100% atom utilization [14,15,16]. On this basis, a novel catalytic system of SACs is proposed in this paper and used in new energy conversion devices [17]. How to prevent the agglomeration of single atoms on the carrier, and thus enhance the stability of the catalyst and maintain its long-lasting and high-efficiency catalytic performance, is the key to realizing the design of SACs [18,19].
Based on this, this article aims to systematically investigate the catalytic properties of transition metals on different crystalline surfaces using DFT in combination with first-principle calculations, with these transition metals as the primary focus of the study [20,21,22]. Au@CrS2 has good ORR and OER catalytic activity and can be used as an excellent bifunctional ORR/OER catalyst [23,24,25]. On this basis, the catalytic performance of the loaded single molecular sieve catalysts was further investigated to reveal their efficient catalytic mechanism [26,27]. The implementation of this article will provide new ideas for the development of inexpensive and efficient oxygen reduction/oxygen precipitation catalysts and lay the theoretical foundation for their application in the field of catalysis.

2. Materials and Methods

All calculations are performed using the density functional method (DFT) for spin polarization in the Vienna Ab initio Simulation Package (VASP) [28,29]. The spin-polarized GGA of PBE was employed to describe exchange–correlation effects [30]. The van der Waals (vdW) interactions are calculated using the DFT-D3 method in Grimme’s scheme [31,32,33]. The charge transfer was analyzed by Bader charge analysis [34]. This article uses a 7 × 7 × 1 Monkhorst–Pack k-point mesh and an energy cutoff of 520 eV for structural optimizations with an ionic energy convergence threshold of 10−6 eV [35,36]. This article studies the energy band structure of a denser K-point mesh. In the whole calculation process, both spin polarization and dipole correction are considered. The spatial distance is set to 20 Å under vacuum conditions.
Eb and Ecoh refer to the binding and cohesion energies of the transition metal atoms loaded on the CrS2 surface, respectively:
Eb = ETM@CrS2 − ECrS2 − ETM
Ecoh = Ebulk/n − ETM
ETM@CrS2 represents the electron energy of TM@CrS2. ECrS2 represents pristine CrS2. ETM represents a single TM atom. Ebulk represents the electronic energy of the metal bulk. n represents the number of atoms in the crystal.
The mechanism of the oxygen reduction reaction (ORR) is usually expressed in the thermodynamic form of four H+/e transfer reactions. The Gibbs free energy (ΔG1, ΔG2, ΔG3, ΔG4) for the reactions are calculated according to the computational SHE approach [37,38].
The ORR overpotential (ηORR) calculation formula:
ηORR = max(−ΔG1, −ΔG2, −ΔG3, −ΔG4)/e + 1.23
where the free energy change at each step is ΔGn (n = 1–4). The OER overpotential (ηOER) can be expressed by Equation (4), which is the reverse reaction of the ORR. The lower the ηOER value, the higher the OER activity.
ηOER = max(ΔG1, ΔG2, ΔG3, ΔG4)/e − 1.23

3. Results

3.1. Structural and Electronic Properties of TM@CrS2

In this article, 15 different transition metals are proposed to be selected to construct a novel TM@CrS2 single-atom catalytic system by solidifying the CrS2 monolayer [39,40]. It has been found that the atomic energy levels of transition metals can form strong bonds with the substrate, which is a prerequisite for achieving long-term and efficient catalysis [41]. Due to the poor interfacial bonding, it is easy for the transition metals to migrate and agglomerate on the catalyst, thus affecting its catalytic performance. On this basis, CrS2 was taken as the object of study in this article, and three sites (Cr-top, S-top, and hollow) (see Figure 1a) [42] were considered to investigate its energy band structure and energy level density. The results show that the density of electronic states of CrS2 is close to the Fermi energy level, with the valence band at the top originating from the 3d electrons of Cr and the bottom of the electronic energy band of the 3p of S. CrS2 has a density of electronic states, similar to the Fermi energy level. In addition, by studying its energy band structure, we find that the electronic energy bands are not bound near the Fermi surface, while some of them are outside the Fermi surface [43]. The preliminary study reveals that CrS2 has metallic properties, which can promote electron transfer and facilitate the catalytic reaction. On this basis, the binding energies of 15 transition metals on the CrS2 substrate are calculated by (1), as shown in Figure 1c. Theoretically, a lower binding energy means a weaker interaction between the transition metal and the carrier, which in turn may increase the binding strength between the transition metal and the reaction substrate. This enhanced binding strength contributes to the stability of the catalyst and helps the catalytic reaction to proceed smoothly. As can be seen from this figure, the binding energies are all in the negative state, indicating a strong adsorption capacity of the transition metal on the CrS2 surface. In particular, the transition group elements examined in this article have the lowest binding energies at the S-top site and the hollow site, indicating that they are more easily anchored at these two sites of the CrS2 substrate. In view of this, this article proposes to investigate TM@CrS2 catalysts with transition metals anchored to the S-top position and the hollow site.
Next, we investigated the structure of the designed SACs. As shown in Table 1, in the initially selected stable structure, after the transition metals bond to the corresponding sites, the average bond lengths of the resulting transition metal bonds first decrease and then increase with the atomic number within the same period. Furthermore, the bonding trend remains generally consistent across different periods. Second, the risk of agglomeration of transition metal elements in transition metal oxides is investigated by calculating the binding energies of transition metal elements in the system by (2). The results show that the binding energies at the S-top and hollow sites on the CrS2 surface are larger for transition metal atoms than for the corresponding bulk TM atoms, suggesting that they are able to be anchored to the carriers as single atoms, which is favorable for catalytic reactions.
In addition to structural stability, electrical conductivity is also an important factor affecting its electrocatalytic activity. Good electrical conductivity is necessary to realize the effective charge transfer in the electrode, and this article proposes to use the Bader charge method calculation method to determine the electron transfer law between the CrS2 substrate and the transition metal ions (e.g., Table 1). The CrS2 monolayer loaded with a transition metal (TM@CrS2) exhibits significant charge transfer properties, with electron redistribution between dopant atoms ranging from 0.04 to 0.63 e. This phenomenon arises from a change in the coordination environment as well as from the fact that the transition metal atoms are more electronegative than the sulfur atoms. Comparative electronic structure analysis shows that while pristine CrS2 already shows metallic conductivity (as evidenced by its characteristic band structure), the doped TM induces a significant change. Specifically, the Fermi level shifts near the Fermi surface and the enhanced density of states indicates that the charge transport capacity of the TM@CrS2 system has been improved. These electronic modifications help accelerate the electron transfer between the catalytic site and the reaction intermediates, thus potentially enhancing the electrocatalytic activity by improving the reaction kinetics.

3.2. OER/ORR Activity and Catalytic Process of TM@CrS2

The oxygen reduction/oxygen evolution performance of each transition metal oxide@CrS2 system was investigated by determining its structural stability and conductivity. A prerequisite for a successful reaction is the effective adsorption of reaction intermediates by the catalyst, and there is a strong correlation between the adsorption capacity of the catalyst and its catalytic activity. Therefore, tuning the adsorption capacity of intermediate species can effectively enhance the catalytic efficiency of ORR/OER. Theoretical investigation reveals that oxygen-containing intermediates (OOH*, O*, OH*) critically determine the catalytic performance in oxygen reduction and evolution reactions. To address the limitations of native materials, this study proposes single-atom catalysts as potential solutions. Notably, monolayer CrS2 exhibits excessively strong adsorption toward both OH* and O* intermediates, which leads to a high overpotential for the oxygen reduction reaction. This fundamental limitation motivates our exploration of transition metal-doped CrS2 (TM@CrS2) systems. Following the Sabatier principle, which dictates that optimal catalysts should maintain moderate adsorption strength for reaction intermediates, we systematically evaluated the free energy profiles of key species. Figure 2 demonstrates strong linear correlations between adsorption energies: ΔGO* = 0.57ΔGOH* + 0.82 (R2 = 0.92) and ΔGOOH* = 3.46ΔGOH* + 0.45 (R2 = 0.98). These linear relationships suggest potential descriptor-based optimization strategies, as the free energies of reaction intermediates show significant interdependence. Particularly, the scaling relation between ΔGO* and ΔGOH* aligns with established electrocatalytic principles for oxygen-based reactions.
For the reaction process OER/ORR, we have performed theoretical simulations of the pathways of the systems found in previous studies that are capable of spontaneously generating this reaction [44,45,46]. Among them, Au@CrS2 is the most representative electrocatalyst, and the entire ORR/OER process shown in Figure 3 consists of four reaction steps [47,48].
Figure 4 shows the variation in the OER Gibbs free energy of TM@CrS2 with the four-electron migration pathway at different electrode voltages. In the 15 SACs systems, some atoms (Ni, Zn, Pd, Cd, Pt, Hg) could not be carried spontaneously due to the descending step, and the adsorption of these intermediates does not actually occur and is therefore disregarded. The free energy change ladder plots for the remaining nine catalysts show a stepwise increasing trend, indicating that the OER is an energy dissipative process, with the black arrows highlighting its potential decision step (PDS) [49]. We found that the PDS of Fe and Ir occurs in the second step (OH*-O*), and the PDS of Co, Cu, Ru, Rh, Ag, Os, and Au occurs in the fourth step (OOH*-O2). In general, the adsorption capacity of reaction intermediates is a key factor affecting the OER/ORR electrochemical activity of the catalysts. Strong adsorption of intermediates on the catalyst leads to blockage of the catalyst surface, which can severely inhibit the reaction. Conversely, too-weak adsorption does not provide sufficient energy for proton bonding, reducing the catalytic performance of the catalyst. The oxygen evolution reaction (OER) overpotentials (ηOER) of Au@CrS2, Cu@CrS2, Ru@CrS2, and Ag@CrS2 catalysts measure 0.34 V, 0.38 V, 0.44 V, and 0.48 V, respectively, demonstrating superior catalytic activity compared to the benchmark IrO2 catalyst (ηOER = 0.56 V). Under zero applied potential conditions, our computational analysis reveals that Au@CrS2 exhibits catalytic performance comparable to theoretically ideal catalysts. At the thermodynamic equilibrium potential of 1.23 V, the reaction pathway analysis shows the second elementary step (O* → OOH*) proceeds spontaneously (ΔG = −0.10 eV), while subsequent steps maintain relatively low activation barriers of 0.10 eV and 0.22 eV for the third and fourth steps, respectively. These findings suggest that transition metal-doped CrS2 systems, particularly Au@CrS2, show potential to supplement conventional platinum-group catalysts in alkaline water electrolysis applications.
To evaluate the bifunctional catalytic performance, we systematically analyzed the oxygen evolution reaction (OER) and its reverse process, the oxygen reduction reaction (ORR), through free energy calculations. At zero applied potential (U = 0 V), all reaction steps for ORR exhibit exothermic characteristics in these single-atom catalysts (SACs). Figure 5 identifies the potential-determining steps (PDSs) for each system, revealing ORR overpotentials (ηORR) of 0.37 V and 0.42 V for Au@CrS2 and Os@CrS2, respectively. Notably, these values are lower than the benchmark Pt-based catalyst (ηORR = 0.45 V) [23], demonstrating superior oxygen reduction performance. Contrary to the conventional Sabatier principle which posits that effective OER catalysts typically exhibit poor ORR activity, Au@CrS2 demonstrates exceptional bifunctionality. Remarkably, this catalyst achieves ηOER = 0.34 V while maintaining ηORR = 0.37 V. Such low overpotentials for both reactions (Δη = 0.03 V) suggest that Au@CrS2 represents a promising bifunctional catalyst, potentially surpassing current state-of-the-art Pt/IrO2 systems. This anomalous behavior may originate from the unique electronic structure modulation induced by Au single-atom doping, which warrants further investigation into the underlying mechanisms.

3.3. Reaction Mechanism Analysis of TM@CrS2

In this article, we will investigate the density of states of these novel single-particle systems in order to reveal the mechanism and law of high activity. Our analysis reveals optimal performance in Cu@CrS2 and Au@CrS2 systems, where their distinctive electronic configurations enable efficient intermediate activation and desorption. Figure 6 demonstrates that the electron concentration at the Fermi level (EF) remains predominantly governed by Cr-3d orbitals in both pristine CrS2 and TM@CrS2 systems. This similarity in electron distribution masks significant differences in their catalytic performance, as evidenced by substantial electronic structure enhancements observed in TM@CrS2 single-atom catalysts compared to monolayer CrS2. Based on their d-electron configurations, these catalysts can be categorized into two distinct groups. Ni, Zn, Pd, Cd, Pt, and Hg@CrS2 exhibit relatively low DOS at EF, which is not sufficient to provide adequate electron transfer for reaction intermediates. Surprisingly, Fe, Co, Ru, Rh, Ag, Os, Ir, and Au@CrS2 demonstrate excessive d-electron populations at metal ions that significantly exceed reaction requirements, leading to strong catalyst-intermediate interactions that impede reaction kinetics. Notably, Cu@CrS2 and Au@CrS2 achieve an optimal balance in DOS characteristics at EF, effectively activating intermediates while promoting their timely desorption, thereby exhibiting superior catalytic performance.
We further analyzed the charge transfer dynamics in the Au@CrS2 single-atom catalyst using Bader charge analysis throughout the catalytic cycle. As shown in Figure 7, Au atoms exhibited bidirectional charge transfer (donation and back-donation) during intermediate adsorption at active sites. This analysis reveals a three-component system comprising adsorbed intermediates, Au atoms, and the CrS2 substrate. Notably, both reaction intermediates and the CrS2 substrate displayed significant charge fluctuations (Δq = ±0.5–0.8e), while the Au atom maintained stable charge states (±0.05e). These observations suggest that CrS2 functions as an electron reservoir, dynamically supplying electrons during catalytic processes. Transition metal atoms mediate electron transfer between intermediates and the CrS2 substrate through their modified electronic structures. Our calculations demonstrate that embedding different transition metals alters interfacial electronic interactions. This structural adaptability enables precise tuning of intermediate binding energies, ultimately governing the ORR/OER catalytic efficiency. The preserved metallic character of the Au atom combined with the CrS2 electron buffering capacity explains the exceptional activity of TM@CrS2 catalysts.

4. Conclusions

In this study, we systematically investigated a series of transition metal single-atom catalysts (TM@CrS2) supported on chromium disulfide monolayers for bifunctional oxygen reduction and oxygen evolution (ORR/OER) applications. Our analysis reveals optimal performance in Au@CrS2 systems, where their distinctive electronic configurations enable superior catalytic activity. Through density functional theory calculations, we investigated the effect of three different adsorption sites on the stability of TM@CrS2 catalysts using 15 different transition metals. The results show that Au@CrS2 has improved electron transfer and charge storage properties as a result of its ideal density of states at the Fermi level and for the fast intermediate activation and desorption. The research goes further to identify potential-determining steps (PDSs) in the reaction pathways, indicating that adsorption strength has a central role in determining catalytic activity. This work demonstrates the feasibility of TM@CrS2 systems and, in particular, Au@CrS2, as efficient bifunctional catalysts for energy conversion with high sustainability. These computationally optimized systems provide a robust framework for experimental validation and scalable catalyst design, offering critical insights into the governing principles of transition metal-based electrocatalysts for sustainable energy conversion applications.

Funding

This research received no external funding.

Data Availability Statement

Data Availability Statements are available in section “MDPI Research Data Policies” at https://www.mdpi.com/ethics.

Conflicts of Interest

The author declares no conflicts of interest.

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Figure 1. (a) Geometry of the initial CrS2 and the TM atoms at the three surface positions. (b) Band structure of the initial CrS2 with the Fermi scale (dashed line) defined as 0 and DOS of each element. (c) Binding energies at three positions on the surface of the CrS2 monolayer. (d) Binding energies of TM atoms at corresponding positions and cohesion energies of individual TM atoms.
Figure 1. (a) Geometry of the initial CrS2 and the TM atoms at the three surface positions. (b) Band structure of the initial CrS2 with the Fermi scale (dashed line) defined as 0 and DOS of each element. (c) Binding energies at three positions on the surface of the CrS2 monolayer. (d) Binding energies of TM atoms at corresponding positions and cohesion energies of individual TM atoms.
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Figure 2. A linear correlation between the free energy of adsorption of OH* and O*, OOH* molecules on the catalyst surface.
Figure 2. A linear correlation between the free energy of adsorption of OH* and O*, OOH* molecules on the catalyst surface.
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Figure 3. ORR/OER pathway on TM@CrS2 material.
Figure 3. ORR/OER pathway on TM@CrS2 material.
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Figure 4. The Gibbs free energy change diagrams for the OERs of Fe, Co, Cu, Ru, Rh, Ag, Os, Ir and Au @CrS2 materials under the four-electron pathways.
Figure 4. The Gibbs free energy change diagrams for the OERs of Fe, Co, Cu, Ru, Rh, Ag, Os, Ir and Au @CrS2 materials under the four-electron pathways.
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Figure 5. The Gibbs free energy change diagrams for the ORRs of Fe, Co, Cu, Ru, Rh, Ag, Os, Ir, and Au @CrS2 materials under the four-electron pathways.
Figure 5. The Gibbs free energy change diagrams for the ORRs of Fe, Co, Cu, Ru, Rh, Ag, Os, Ir, and Au @CrS2 materials under the four-electron pathways.
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Figure 6. The electronic density of states of 15 recombinant single-atom catalysts with different structures was investigated. Electronic density of states of 15 kinds of designed recombinant monoatomic catalysts.
Figure 6. The electronic density of states of 15 recombinant single-atom catalysts with different structures was investigated. Electronic density of states of 15 kinds of designed recombinant monoatomic catalysts.
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Figure 7. Charge variation of CrS2, Au, and reaction intermediates (ads) during ORR/OER.
Figure 7. Charge variation of CrS2, Au, and reaction intermediates (ads) during ORR/OER.
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Table 1. Calculate the binding energies (eV), cohesion energies (eV), average bond lengths between metal atoms and neighboring S atoms (Å), Bader charge (e), and the binding sites of transition metal atoms anchored to CrS2 clusters for the X@CrS2 material.
Table 1. Calculate the binding energies (eV), cohesion energies (eV), average bond lengths between metal atoms and neighboring S atoms (Å), Bader charge (e), and the binding sites of transition metal atoms anchored to CrS2 clusters for the X@CrS2 material.
PristineEAverage
Bond
Length (Å)
ΔQ
(e)
Binding Site
Eb (eV)Ecoh (eV)
Fe−6.42−2.932.270.04H
Co−5.65−3.292.180.21H
Ni−5.40−4.232.120.34TS
Cu−4.14−4.472.180.45H
Zn−2.94−4.922.240.23TS
Ru−7.00−3.112.230.37TS
Rh−6.05−3.572.320.46H
Pd−4.17−3.482.29−0.06H
Ag−0.22−2.022.500.56TCr
Cd−1.87−4.762.460.63H
Os−7.79−2.752.300.31TS
Ir−6.97−3.622.300.57H
Pt−5.41−4.062.270.54TS
Au−2.72−4.082.370.36H
Hg−0.64−4.632.870.28TS
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Wang, Y. CrS2 Supported Transition Metal Single Atoms as Efficient Bifunctional Electrocatalysts: A Density Functional Theory Study. ChemEngineering 2025, 9, 43. https://doi.org/10.3390/chemengineering9030043

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Wang Y. CrS2 Supported Transition Metal Single Atoms as Efficient Bifunctional Electrocatalysts: A Density Functional Theory Study. ChemEngineering. 2025; 9(3):43. https://doi.org/10.3390/chemengineering9030043

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Wang, Ying. 2025. "CrS2 Supported Transition Metal Single Atoms as Efficient Bifunctional Electrocatalysts: A Density Functional Theory Study" ChemEngineering 9, no. 3: 43. https://doi.org/10.3390/chemengineering9030043

APA Style

Wang, Y. (2025). CrS2 Supported Transition Metal Single Atoms as Efficient Bifunctional Electrocatalysts: A Density Functional Theory Study. ChemEngineering, 9(3), 43. https://doi.org/10.3390/chemengineering9030043

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