Enhanced hydrogen evolution reaction performance of manganese-doped vanadium disulfide nanosheet across full pH spectrum and in simulated seawater

INTRODUCTION

The quest for sustainable and clean energy solutions has intensified global focus on hydrogen production via water electrolysis, where hydrogen evolution reaction (HER) plays a pivotal role^[1-5]. This fundamental water-splitting reaction necessitates catalysts that are both efficient and robust^[6,7], capable of operating in a range of aqueous environments^[8]. There is a particular emphasis on developing catalysts that can function effectively across the entire pH spectrum and in simulated seawater conditions^[9-11], to address the specific challenges these settings present. Despite significant advances in the field, crafting catalysts that are cost-effective, highly active, and durable remains a substantial challenge, especially for applications in such varied aqueous conditions^[12-14].

As research intensifies in the development of sustainable catalysts for HER, the exceptional catalytic efficiency of platinum (Pt) group metals serves as a critical reference point^[15-17]. However, due to the high cost and limited availability of these noble metals, there is a pressing need to explore more accessible alternatives^[18,19]. This search has steered scientific interest towards non-noble metal catalysts, particularly transition metal dichalcogenides (TMDs), known for their promising electrocatalytic properties^[20-22]. Among TMDs, materials such as MoS₂, WS₂, and vanadium disulfide (VS₂) have garnered significant attention due to their outstanding catalytic efficiencies and structural flexibility. The cost-effectiveness and abundance of TMDs make them an attractive choice for sustainable energy applications, as their unique two-dimensional structures provide numerous active sites that enhance catalytic performance. Recent advancements, such as the development of 2D/2D NiAl-based layered double hydroxide (LDH)/CoNi-based metal-organic framework (MOF) nanohybrid catalysts for elevated symmetric supercapacitor performance and H₂O₂ production under simulated solar light, further underscore the potential of layered 2D materials in catalysis^[23]. These innovative approaches demonstrate the continuous evolution in electrocatalyst design, where hybrid structures and material engineering play pivotal roles in boosting catalytic performance, positioning TMDs such as VS₂ as highly promising candidates for sustainable energy solutions.

VS₂ has emerged as a particularly promising candidate within TMDs due to its unique structural and electronic features^[24-28]. Its graphene-like layered structure not only facilitates high electronic conductivity, as suggested by its metallic nature in theoretical studies^[29,30], but also hosts numerous edge sites that are active for catalysis^[31,32], making it a compelling alternative to traditional noble metal catalysts in electrocatalytic applications. Recent studies have demonstrated that VS₂ can significantly enhance HER when combined with other materials such as MoS₂ or when defect-rich structures are used, improving both efficiency and multifunctionality. Additionally, VS₂ self-assembled into layered microstructures has shown promise for water and urea decomposition reactions, further proving its versatility in electrocatalysis^[33,34]. Particularly, the monolayer form of VS₂ exemplifies its potential by offering an abundance of active sites and exceptional in-plane conductivity, enhancing its suitability for HER^[35,36]. However, the potential of VS₂ has not been fully realized due to inherent challenges related to its vertical conductivity in multi-layer or bulk forms, which limits the overall catalytic efficiency. Addressing these challenges involves structural optimization to enhance the exposure of catalytically active sites and improve electrical conductivity, both of which are critical for efficient catalytic reactions^[37,38]. While several TMD materials have demonstrated catalytic promise, VS₂ is underexplored compared to MoS₂ and WS₂, particularly in the context of seawater electrolysis. This research aims to address that gap.

To address the intrinsic limitations of VS₂ in HER applications, researchers have turned to doping as a strategy to enhance its electrochemical properties^[39]. Doping introduces foreign atoms into the crystal lattice of VS₂, which can significantly alter its electronic structure and catalytic activity^[40,41]. Mn is specifically chosen for doping due to its ability to introduce new electronic states within the band structure of VS₂. These new states can facilitate improved electron transfer processes, crucial for the dynamics of HER. Additionally, Mn incorporation into VS₂ leads to structural distortions that are beneficial for catalysis. These distortions can create more favorable sites for hydrogen adsorption and desorption, which are essential steps in the catalytic cycle of HER^[42]. By adjusting the electronic properties and geometric configuration of VS₂, Mn doping enhances both the reactivity and stability of the catalyst, thereby addressing some of the key challenges faced by the pristine material in electrocatalytic applications^[43-45]. Unlike traditional doping strategies used for MoS₂ and WS₂, Mn doping in VS₂ significantly modifies the sulfur vacancy formation energy and enhances its stability in seawater conditions, which sets this study apart from previous works.

In this study, manganese-doped vanadium disulfide (Mn-VS₂) nanosheets were synthesized via the colloidal chemistry method to rigorously evaluate their electrocatalytic HER activities across a comprehensive pH spectrum and in simulated seawater conditions. The synthesized catalyst demonstrated notably low overpotentials in varied electrolytes, confirming its superior performance. The integration of Mn substantially enhanced the HER performance of VS₂, as evidenced by the remarkably low overpotentials required to achieve a current density of -10 mA·cm^-2 in 1 M KOH, 1 M phosphate buffered saline (PBS), and 0.5 M H₂SO₄ solutions, which were 41, 55, and 37 mV, respectively. Particularly in simulated seawater, Mn-VS₂ demonstrated an exceptional capability, achieving the same current density at an overpotential of just 40 mV, and maintaining this performance with remarkable stability over 100 h, thereby underscoring its significant potential as an effective catalyst for direct seawater electrolysis. Theoretical and experimental investigations have demonstrated that the incorporation of Mn decreases the formation energy of sulfur vacancies in VS₂, enhancing their stability and catalytic efficacy. This unique stability and high performance in seawater conditions underscore the practical applicability of Mn-VS₂ for sustainable hydrogen production. This modification leads to increased hydrophilicity and refined control over the electronic properties at these active sites, significantly enhancing the overall catalytic activity and promoting hydrogen evolution. This study validates the critical influence of modified electronic structures in boosting the electrocatalytic performance of other TMDs. The performance of the Mn-VS₂ catalyst in seawater electrolysis demonstrates a clear advancement over previously reported TMD-based catalysts, highlighting a major step toward achieving cost-effective and scalable hydrogen production.

EXPERIMENTAL

The Mn-VS₂ catalyst was synthesized using a colloid chemistry method (experimental details are provided in the Supplementary Materials). In summary, sulfur powder was dissolved in oleylamine and heated under stirring. At the same time, VO(acac)₂ and MnCl₂ were dissolved in oleylamine, ensuring thorough mixing. The mixture was then heated under a nitrogen atmosphere; the sulfur solution was injected into the preheated VO(acac)₂ and MnCl₂ solutions, and the temperature was raised to 330 °C and maintained for 1.5 h. After cooling to ambient temperature, the resulting product was separated by centrifugation. The powder was then treated by heating at 500 °C for 1 h in a tube furnace under a nitrogen atmosphere.

RESULTS AND DISCUSSION

Mn-VS₂ nanosheets were synthesized using a colloidal chemistry approach, as depicted in Figure 1A. Initially, a precursor sol containing V, S, and Mn was prepared. This sol gradually transformed into a gel with a defined shape and structure. During the synthesis phase, to prevent oxidation, the material was heated to 330 °C under a nitrogen atmosphere and maintained at this temperature for 1.5 h to ensure complete conversion of the precursor into Mn-VS₂^[24,27,46].

Figure 1. (A) Synthetic process route for Mn-VS₂; (B) HRTEM image of Mn-VS₂; (C) IFFT analysis of Mn-VS₂ derived from (B); (D) 3D AOGF mappings from (B); (E) AFM image of Mn-VS₂; (F) EDS mapping spectra of various elements in Mn-VS₂. HRTEM: High-resolution transmission electron microscopy; IFFT: inverse fast fourier transform; AOGF: Atom-Overlapping Gaussian Function; AFM: atomic force microscopy; EDS: energy dispersive spectroscopy.

As shown in Supplementary Figure 1, the as-synthesized Mn-VS₂ exhibits a sheet-like morphology, evident from the scanning electron microscopy (SEM) images taken at different magnifications. Large-scale transmission electron microscopy (TEM) images further confirm the nanosheet structure of Mn-VS₂, complementing the SEM observations [Supplementary Figure 2]. High-resolution TEM (HRTEM) images of Mn-VS₂ are shown in Figure 1B, displaying clear lattice fringes. The measured lattice spacing of 0.239 nm corresponds to the (001) planes of VS₂. Figure 1C, which is a magnified view of the area marked in red in Figure 1B, includes a fast fourier transform (FFT) analysis commonly utilized in HRTEM studies to highlight defects within the crystal structure. In these images, defects are circled in yellow, revealing the microstructural impacts of Mn doping, which are markedly distinct from the HRTEM images of undoped VS₂ [Supplementary Figure 3]. Such microstructural doping likely contributes to the enhanced electrocatalytic HER performance of Mn-VS₂. Additionally, Figure 1D presents the 3D Atom-Overlapping Gaussian Function (AOGF) mappings for Mn-VS₂, employing a Gaussian-function fitting to visualize the spatial distribution and intensity of atomic sites. Significant color variations in the AOGF plots highlight the presence of defects, as changes in atomic density and arrangement alter the Gaussian overlap, thereby indicating the presence of structural defects. Conversely, the AOGF plot for undoped VS₂, as shown in Supplementary Figure 4, exhibits a uniform color distribution, suggesting more intact structural integrity. To further investigate the structure of Mn-VS₂, atomic force microscopy (AFM) characterization was conducted, as depicted in Figure 1E. AFM is a sophisticated technique utilized for assessing surface roughness, morphology, and thickness of materials. In the image, varying color blocks represent different heights, effectively illustrating the surface topography and thickness variations of the sample. The inset, an AFM-derived height profile, reveals that the nanosheets possess a thickness of approximately 0.75 nm, indicating that the Mn-VS₂ exists in a monolayer structure. Monolayer structures are known for their high specific surface area, unique electronic properties, and high surface activity, all contributing to their exceptional catalytic performance in reactions. It is worth mentioning that the monolayer configuration of Mn-VS₂ offers a high density of exposed active sites, which maximizes the material’s surface area and facilitates efficient interaction with reactants. Additionally, the exceptional in-plane conductivity of the monolayer structure ensures rapid electron transport, a key factor for enhancing the efficiency of reactions such as hydrogen evolution. The reduced thickness of the monolayer minimizes electron scattering, further improving the material’s conductivity. However, in multi-layer or bulk configurations, the limited vertical conductivity between stacked layers can restrict electron flow, thereby reducing the overall catalytic efficiency. Figure 1F shows Energy-dispersive X-ray spectroscopy (EDS) mapping, which provides a clear visualization of the homogeneous distribution of Mn, V, and S elements within the composite material. This mapping is crucial for a detailed analysis of the elemental composition of the material, further supporting the understanding of its enhanced catalytic properties.

As shown in Figure 2A, the X-ray diffraction (XRD) patterns of both Mn-VS₂ and pristine VS₂ correspond to the hexagonal phase of VS₂ (JCPDF No. 89-1640), with primary reflections at (100) 32.10°, (011) 35.68°, and (012) 45.23°. Pristine VS₂ exhibits sharp and intense diffraction peaks, indicating a highly ordered structure and excellent crystallinity. In contrast, the Mn-VS₂ sample displays noticeably broadened peaks and slight shifts in peak positions, which reflect a reduction in grain size and an increase in lattice strain. These changes suggest that Mn doping introduces lattice distortions and local defects, thereby decreasing the overall crystallinity. Additionally, the incorporation of Mn not only alters the lattice constants of VS₂ but also induces local structural distortions and stress, increasing the density of defect sites and the active surface area. This structural modification is beneficial for enhancing the electrocatalytic hydrogen evolution performance. Figure 2B presents the Raman spectroscopy analysis of Mn-VS₂. The principal Raman peaks identified in the spectrum can be attributed to the characteristic vibrational modes of VS₂. The marked Mn-S Raman peaks correspond to vibrational modes between sulfur and manganese atoms within the VS₂ lattice, introduced by Mn doping. The peaks labeled S-S arise from interactions between sulfur atoms in VS₂. Additionally, the inset in Figure 2B shows the Raman spectrum in the low wavenumber region, where the A_g and E_g peaks are typical vibrational modes of VS₂. The Raman characteristics indicate that while the fundamental structure of VS₂ remains unchanged by the introduction of Mn, new local vibrational modes are introduced, suggesting successful doping of Mn into the VS₂ lattice and potential impacts on its electronic and vibrational properties. Figure 2C presents the electron paramagnetic resonance (EPR) spectra of the pristine VS₂ and Mn-VS₂ samples. EPR is a technique used to detect unpaired electrons in materials, which are typically closely related to the electronic structures and catalytic properties of the materials. The EPR spectrum of the pristine VS₂ sample primarily shows low-intensity signals induced by defects, which may be associated with a small number of point defects or boundary defects. In contrast, the Mn-VS₂ sample exhibits significantly enhanced and broadened EPR signals, indicating that Mn doping has introduced more unpaired electrons, primarily from the presence of Mn²⁺ ions. The incorporation of Mn²⁺ ions not only increases the number of unpaired electrons but also introduces lattice defects by substituting V atom sites, leading to local magnetic field inhomogeneity, manifested as broader EPR peaks. This indicates that Mn²⁺ ions, acting as paramagnetic centers, are widely present in Mn-VS₂, enhancing the paramagnetic characteristics of the material. These paramagnetic centers and the local magnetic field inhomogeneity introduced by doping work together to improve the electron transfer efficiency of the material, thereby enhancing its electrocatalytic hydrogen evolution performance. To further characterize the chemical composition and the electronic states of the elements, X-ray photoelectron spectroscopy (XPS) analyses were conducted for both undoped VS₂ and Mn-VS₂. The XPS survey spectrum for Mn-VS₂, depicted in Supplementary Figure 5, confirms the presence of V, S, and Mn, aligning with the elemental composition observed in EDS results. Figure 2D presents the XPS spectrum for Mn in Mn-VS₂, showing two principal peaks, Mn 2p_3/2 and Mn 2p_1/2, corresponding to the different electronic states of manganese atoms. The existence of these peaks confirms the presence of Mn and their splitting indicates the valence states of Mn, specifically Mn²⁺ and Mn⁴⁺. In Figure 2E, a shift toward higher binding energies for the V element suggests a decrease in electrons, indicative of a more positive oxidation state. This shift could result from electron transfer, particularly under the influence of Mn doping, which may alter the local electronic environment of the vanadium atoms. Conversely, as shown in Figure 2F, a shift toward lower binding energies for sulfur suggests an increase in its reduced state, indicating electron gain. The enhanced reduction state of sulfur on the catalyst surface could lead to increased surface affinity for protons, as the reduced sulfur provides a higher density of electrons that can stabilize protons, thus facilitating the conversion of H⁺ to H₂ during the HER process. This electron enrichment at the sulfur sites underlines the potential catalytic advantages introduced by Mn doping in enhancing the electrocatalytic efficiency of VS₂ for HER.

Figure 2. (A) XRD patterns of VS₂ and Mn-VS₂; (B) Raman spectrum of Mn-VS₂; (C) EPR spectra of VS₂ and Mn-VS₂; XPS spectrum of (D) Mn 2p in Mn-VS₂ (E) V 2p, (F) S 2p in VS₂ and Mn-VS₂. XRD: X-ray diffraction; EPR: electron paramagnetic resonance; XPS: X-ray photoelectron spectroscopy.

To realize the commercial potential of HER catalysts, the development of highly active and enduringly stable catalysts has become imperative. Achieving this is challenging, particularly under alkaline or neutral conditions where the paucity of protons leads to slower water dissociation steps, thus impeding the rate of hydrogen evolution. Despite the expansion of HER research to encompass the entire pH spectrum in recent years, developing superior electrocatalysts that perform efficiently across all pH conditions remains a formidable task.

Electrochemical tests conducted on VS₂, Mn-VS₂ and Pt/C with varying doping levels, using a three-electrode setup across different pH environments, have undergone 95% iR compensation. The polarization curves derived [Figure 3A-C] indicate that Mn-VS₂ exhibits the lowest overpotentials required to achieve a current density of -10 mA·cm^-2 in 1 M KOH, 1 M PBS, and 0.5 M H₂SO₄ solutions at 41, 55, and 37 mV, respectively, significantly outperforming pure VS₂ (202 mV in 1 M KOH, 306 mV in 1 M PBS, and 190 mV in 0.5 M H₂SO₄). The Tafel slopes, derived from the linear regions of the polarization curves [Figure 3D-F], reveal that Mn-VS₂ exhibits significantly improved HER kinetics compared to pristine VS₂ in all tested electrolytes: specifically, Mn-VS₂ shows slopes of 74.69, 35.01, and 35.5 mV·dec^-1 in 1 M KOH, 1 M PBS, and 0.5 M H₂SO₄, respectively, whereas VS₂ displays relatively higher slopes of 129, 220.31, and 171.1 mV·dec^-1 under the same conditions. These results indicate that Mn doping substantially enhances the catalytic performance of VS₂, reducing the kinetic barriers associated with proton adsorption, hydrogen intermediate formation, and subsequent H₂ evolution steps. In acidic media (0.5 M H₂SO₄), although free protons are abundant, pristine VS₂ struggles to facilitate efficient formation and desorption of hydrogen intermediates, resulting in a higher Tafel slope. By contrast, Mn-VS₂ optimizes the surface electronic structure and active sites, lowering the activation energy and thus achieving a substantially reduced Tafel slope (35.5 mV·dec^-1vs. 171.1 mV·dec^-1). In neutral and alkaline electrolytes, where proton availability relies on water dissociation, the HER pathway on VS₂ is further challenged, leading to even higher Tafel slopes (220.31 and 129 mV·dec^-1, respectively). However, Mn doping again proves beneficial, as the Mn-VS₂ catalyst exhibits significantly lower slopes (35.01 mV·dec^-1 in PBS and 74.69 mV·dec^-1 in KOH) by effectively stabilizing intermediates and proton transfer processes. Thus, the incorporation of Mn into VS₂ not only aids in overcoming the thermodynamic and kinetic limitations associated with different pH environments but also ensures a more favorable and consistent HER pathway, ultimately improving catalytic performance across acidic, neutral, and alkaline conditions. Analysis across the entire pH range indicates superior HER performance of Mn-VS₂ [Figure 3G-I]. As the doping level of Mn increases, the electrocatalytic HER performance of Mn-VS₂ initially improves and then diminishes [Supplementary Figures 6-8], possibly due to the optimal doping level enhancing the microstructural properties such as crystal orientation, increasing lattice defects, and improving electronic structure, thereby enhancing the electrocatalytic performance. Excessive doping of Mn may lead to significant lattice distortions, which can be detrimental to the electrocatalytic process, such as excessive lattice strain that complicates the charge transfer pathways. Furthermore, long-term stability tests conducted over 100 h show that Mn-VS₂ maintains excellent electrochemical stability across different pH conditions [Figure 3J], as demonstrated by the comparison of polarization curves before and after stability testing [Supplementary Figures 9-11]. Additionally, XRD patterns collected before and after the stability tests [Supplementary Figure 12] reveal no significant changes in the crystal structure, indicating that Mn-VS₂ retains its layered structure without the formation of secondary phases or noticeable degradation, further confirming its structural robustness under prolonged operating conditions. In contrast, VS₂ exhibited noticeable performance degradation under the same conditions, with significant changes in the polarization curves observed after stability testing [Supplementary Figure 13]. Compared to current advanced vanadium-based catalysts, Mn-VS₂ offers distinct advantages across various pH values [Figure 3K and Supplementary Table 1]^[47-55].

Figure 3. HER performance of VS₂ and Mn-VS₂ with varying doping levels (iR compensation at 95%). (A-C) HER polarization curves; (D-F) Tafel slopes; (G-I) Overpotentials at current densities of -10 mA·cm^-2 and -100 mA·cm^-2; (J) Stability at a current density of -10 mA·cm^-2. Measurements were conducted in 0.5 M H₂SO₄ (A, D, G), 1 M PBS (B, E, H), and 1 M KOH (C, F, I) aqueous solutions; (K) Comparison of the overpotential (current density of -10 mA·cm^-2) of Mn-VS₂ with other recently reported vanadium-based electrocatalysts in 0.5 M H₂SO₄, 1 M PBS, and 1 M KOH. HER: Hydrogen evolution reaction.

Electrochemical assessments were conducted in an alkaline environment, where the electrochemical C_dl was derived from CV at different scan rates, as illustrated in Supplementary Figure 14. Mn-VS₂ exhibited a notably higher C_dl compared to undoped VS₂, indicating a larger electrochemically active surface area. This enhancement in active surface area is instrumental for improving the kinetics of HER. Further electrochemical impedance spectroscopy (EIS), as shown in Supplementary Figure 15, demonstrated that Mn-VS₂ exhibits a significantly reduced charge transfer resistance (R_ct) compared to pristine VS₂. The lower R_ct of Mn-VS₂ highlights the critical role of manganese doping in enhancing charge transport efficiency. Doping introduces additional electronic states into the band structure of VS₂, facilitating faster electron transfer and reducing the energy barriers for charge transfer at the electrode-electrolyte interface. This reduction in R_ct is essential for minimizing voltage losses at high current densities, thereby accelerating the HER process^[56,57]. Moreover, the wetting properties of Mn-VS₂ were evaluated through contact angle measurements, as depicted in Supplementary Figure 16. Mn-VS₂ exhibited improved wettability with a lower contact angle relative to VS₂. This enhancement in hydrophilicity can be attributed to the lattice defects induced by Mn doping. Such defects often create unsaturated chemical bonds or regions of increased chemical activity, which can attract and retain water molecules, thereby augmenting the hydrophilicity of the material. The increased wettability of Mn-VS₂, likely contributing to its superior electrocatalytic performance in HER, underscores the importance of surface properties in catalytic efficiency.

Given the global scarcity of freshwater resources, harnessing the abundant seawater resources on Earth is crucial for advancing the commercialization of water electrolysis technologies and meeting the escalating energy demands. However, the complex composition of natural seawater poses additional challenges to direct seawater electrolysis. For instance, chloride ions can compete with the oxygen evolution reaction (OER) at the anode, thereby reducing the overall efficiency of seawater electrolysis. Furthermore, catalyst degradation due to chloride-induced corrosion and poisoning by insoluble precipitates (such as dust, colloids, and bacteria) can diminish both the catalytic activity and stability of the catalyst.

In this study, the electrocatalytic water-splitting performance of Mn-VS₂ was evaluated in an alkaline simulated seawater environment (1 M KOH + 0.5 M NaCl). For comparative analysis, HER activities of undoped VS₂ and Mn-VS₂ with different doping ratios were also tested under the same conditions. Mn-VS₂ exhibited superior electrochemical performance, as demonstrated by its low overpotential of 40 mV at a current density of -10 mA·cm^-2 [Supplementary Figure 17A], which is comparable to its catalytic performance in pure alkaline solutions. Additionally, as demonstrated in Supplementary Figure 17B, the lower Tafel slope of Mn-VS₂, recorded at 39.34 mV·dec^-1, further confirms its accelerated HER kinetics in simulated seawater. This value is comparable to other advanced electrocatalysts designed for HER in simulated seawater, such as Pt-based catalysts (41 mV·dec^-1)^[46] and Ru-doped Ni_0.85Se nanosheets (48 mV·dec^-1)^[58]. Furthermore, Mn-VS₂ exhibits a lower overpotential at -10 mA·cm^-2 compared to commercial Pt/C under simulated seawater conditions, making it a cost-effective and high-performance alternative for HER applications in challenging environments. The superior performance of Mn-VS₂ highlights its improved charge transfer properties and optimized electronic structure due to Mn doping, making it highly competitive for seawater HER applications.

The long-term stability of the catalysts in simulated seawater was assessed using chronoamperometric methods, and as depicted in Supplementary Figure 17D, after 100 h of stability testing, the electrocatalytic performance of Mn-VS₂ showed negligible degradation, demonstrating exceptional stability. Collectively, these experimental results confirm that Mn-VS₂ not only exhibits excellent catalytic performance across the entire pH spectrum and in simulated seawater but also maintains outstanding stability, highlighting its significant potential for practical applications. To further elucidate the role of sulfur in Mn-VS₂, the material was subjected to calcination in an air atmosphere for 5 min to partially remove sulfur, yielding a product referred to as Mn-VS₂-Ox-5. This modified sample underwent a series of characterizations and electrochemical tests. The XRD peaks presented in Supplementary Figure 18 align with the standard diffraction data for VS₂ (JCPDF No. 89-1640) and V₂O₅ (JCPDF No. 89-0612), indicating that while the VS₂ structure was retained post-calcination, partial transformation to V₂O₅ occurred.

XPS full spectrum analysis of Mn-VS₂-Ox-5, shown in Supplementary Figure 19A, confirms the presence of Mn, V, S, and O. Supplementary Figure 19B displays the XPS spectra of vanadium in both Mn-VS₂ and Mn-VS₂-Ox-5. For Mn-VS₂-Ox-5, the V 2p peaks predominantly suggest a V⁴⁺ oxidation state, indicating an increase in the oxidative state of vanadium due to the calcination process, which typically involves reactions between vanadium and atmospheric oxygen, reducing the divalent vanadium content relative to Mn-VS₂. The XPS spectrum of Mn in Supplementary Figure 19C reveals the Mn 2p_3/2 peak split into two parts, representing the valence states of Mn²⁺ and Mn⁴⁺. In Mn-VS₂-Ox-5, these peaks also display the Mn²⁺ and Mn⁴⁺ states, but with changes in relative intensity and a shift of approximately 0.56 eV towards higher binding energy for Mn²⁺. Such a shift typically indicates a change in the valence state or chemical environment, likely due to partial oxidation of manganese during the calcination. Supplementary Figure 19D illustrates the XPS spectra of sulfur in Mn-VS₂ and Mn-VS₂-Ox-5. In Mn-VS₂-Ox-5, the binding energy shift towards higher values indicates electron loss by sulfur atoms, i.e., partial oxidation, as sulfur atoms give up electrons to oxygen during the oxidation process, leading to an increase in binding energy reflecting the oxidized state of sulfur. Additionally, the analysis of the S^2- area in the XPS spectra shows a decrease in the relative proportion of S^2- from 20.4% in Mn-VS₂ to 15.9% in Mn-VS₂-Ox-5, suggesting a reduction in the sulfide content relative to other components after calcination. Supplementary Figure 19E presents the XPS spectra for oxygen in Mn-VS₂-Ox-5, where peaks for oxides (M-O) and hydroxyl/water groups (H-O) are evident, indicating the presence of oxygen primarily in oxide form and as adsorbed water or hydroxyl groups after calcination. The dominance of oxide peaks suggests significant oxidation of the material surface or structure during calcination, aligning with the observed changes in the oxidation state of sulfur in the S 2p spectra. The presence of H-O peaks might indicate adsorption of water molecules or hydroxyl groups on the surface, a common factor considered in analyzing surface properties of materials. Through XRD and XPS characterizations, it has been confirmed that the original Mn-VS₂, upon calcination in air, has partially transformed into an oxide form.

Figure 4 displays the electrocatalytic HER performance of Mn-VS₂ and its calcined derivatives, Mn-VS₂-Ox-5 and Mn-VS₂-Ox-60, in various electrolytes. The polarization curves [Figure 4A-D] were measured in 1 M KOH, 1 M PBS, 0.5 M H₂SO₄, and simulated seawater with 95% iR compensation applied. These graphs reveal that under all test conditions, Mn-VS₂-Ox-5 requires a higher overpotential to achieve the same current density compared to Mn-VS₂, indicating a decrease in electrochemical activity following the partial removal of sulfur. Moreover, as the calcination time increases, leading to further removal of sulfur, the electrochemical performance deteriorates progressively. In Figure 4E, the data for overpotentials are visually represented in a color-coded heat map, clearly illustrating the performance differences between Mn-VS₂ and Mn-VS₂-Ox at current densities of -10 mA·cm^-2 and -100 mA·cm^-2. Across all environments, whether at lower or higher current densities, the overpotentials for Mn-VS₂-Ox variants are consistently higher than those for Mn-VS₂. This observation supports the hypothesis that sulfur likely serves as an active site for HER. Typically, in TMD catalysts, sulfur participates as an active site involved in the adsorption/desorption of protons, and its presence is crucial for providing an appropriate catalytic environment. Consequently, the partial removal of sulfur might lead to a reduction in the number of active catalytic sites, altering the chemical environment and thereby diminishing the catalytic efficiency.

Figure 4. Polarization curves of Mn-VS₂, Mn-VS₂-Ox-5 and Mn-VS₂-Ox-60 measured in (A) 0.5 M H₂SO₄, (B) 1 M PBS, (C) 1 M KOH, and (D) simulated seawater with iR compensation at 95%; (E) Overpotentials of Mn-VS₂, Mn-VS₂-Ox-5 and Mn-VS₂-Ox-60 at current densities of -10 and -100 mA·cm^-2 in different environments.

To further investigate the mechanism of the HER in Mn-VS₂, in situ Raman spectroscopy was performed, as shown in Figure 5A. The tests covered a potential range from 0 to -600 mV relative to the Ag/AgCl reference electrode, encompassing both non-Faradaic and HER-active regions. Notably, at a potential of -300 mV, a new Raman peak at 934 cm^-1 was observed, which intensified with further increases in cathodic potential. Previous studies suggest that this Raman peak likely originates from the stretching vibration of the S–H bond^[59]. The enhancement of this S–H vibrational mode indicates the possible formation of an intermediate during the electrocatalytic process, where the increased potential facilitates the adsorption of hydrogen atoms or protons at the electrode surface, subsequently forming S–H bonds with sulfur atoms. The presence of S–H bonds indicates that sulfur atoms act as active sites in the HER catalyzed by Mn-VS₂. To better understand the type of defects, calculations were performed on the formation energy of sulfur vacancies. As displayed in Figure 5B, the energy required to form a sulfur vacancy in VS₂ is 3.35 eV; however, with the introduction of Mn, the formation energy significantly decreases to 2.13 eV. This result implies that Mn doping substantially reduces the formation energy of sulfur vacancies, making their formation more feasible. This finding is consistent with the results from EPR studies shown in Figure 2C, further illustrating that Mn doping promotes the generation of sulfur vacancies.

Figure 5. (A) In situ Raman spectra of Mn-VS₂ at various potentials in 1 M KOH solution; (B) Formation energy of sulfur vacancies in VS₂ and Mn-VS₂; (C) Differential charge density maps of VS₂ and Mn-VS₂; (D) Free energy diagrams of VS₂ and Mn-VS₂ during the HER reaction; (E) DOS diagram of VS₂ and (F) Mn-VS₂. DOS: Density of states; HER: hydrogen evolution reaction.

Figure 5C displays the differential charge density maps for VS₂ and Mn-VS₂. In these maps, regions of increased charge density are depicted in yellow, while areas of decreased charge density appear in blue. In the VS₂ model shown on the left, the charge distribution is relatively uniform, with no significant areas of charge accumulation or loss, indicating an even electron distribution among the atoms with slight charge accumulation at sulfur vacancies. In contrast, the Mn-VS₂ model on the right exhibits distinct yellow regions around sulfur vacancies, indicating substantial charge accumulation due to the introduction of Mn. Such electron rearrangement is often associated with electrocatalytic activity, as HER typically requires electron density to facilitate the adsorption of reactants. This charge accumulation at sulfur vacancies suggests enhanced adsorption capabilities and catalytic activity at these sites. Combined with electrochemical test results and in situ Raman spectroscopy, these observations further suggest that sulfur acts as an active center for HER. The redistribution of the electron cloud around sulfur due to sulfur vacancies likely increases the electron affinity of sulfur for adsorbates such as water molecules or hydrogen atoms, facilitating the catalytic process. In alkaline conditions, the water dissociation process, or Volmer step, often presents a high reaction barrier and is the rate-determining step in HER. Analysis from Figure 5D indicates that the introduction of Mn significantly lowers the water dissociation barrier in Mn-VS₂ compared to undoped VS₂. This suggests that Mn addition promotes the Volmer step by reducing the dissociation barrier, making water adsorption and decomposition more efficient on the Mn-VS₂ surface. Moreover, the free energy diagram reveals that the subsequent conversion step of adsorbed hydrogen atoms (H^*) also requires lower free energy on Mn-VS₂, indicating a higher catalytic activity of Mn-VS₂ in HER compared to VS₂, which exhibits larger changes in free energy for the same steps, indicating a higher reaction barrier to overcome during HER. Therefore, it can be concluded that Mn-VS₂ not only exhibits superior catalytic performance in the Volmer step but also demonstrates excellent intrinsic activity throughout the entire HER process.

The impact of Mn doping on the electronic properties of sulfur vacancies in VS₂ was further explored through changes in the density of states. As revealed in Figure 5E and F, the introduction of Mn leads to an accumulation of electrons at sulfur vacancies, which in turn restructures the electronic configuration of the material, particularly near the Fermi level, where a noticeable change in the density of states is observed. Adjusting the electronic structure is crucial for enhancing the material’s catalytic activity, thus making sulfur vacancies a key factor in optimizing electron transport. These sulfur vacancies not only reduce electron density in adjacent regions but may also enhance the material’s adsorption capabilities for electrons and protons.

CONCLUSIONS

In conclusion, this study has successfully synthesized Mn-VS₂ using a colloidal chemistry approach, systematically examining their HER performance across the full pH spectrum and in simulated seawater environments. This research thoroughly elucidated the catalytic mechanisms at sulfur sites, highlighting them as active sites for HER. The uniform doping of Mn within the VS₂ nanostructures was achieved by finely tuning the colloidal chemistry reaction conditions. The Mn-VS₂ catalysts demonstrated remarkably low overpotentials of 41, 55, and 37 mV to reach a current density of -10 mA·cm^-2 in 1 M KOH, 1 M PBS, and 0.5 M H₂SO₄ solutions, respectively. Particularly noteworthy is the performance of the catalyst in simulated seawater, where it achieved the same current density at an overpotential of just 40 mV, showcasing its potential as an effective catalyst for direct seawater electrolysis. In situ Raman spectroscopy and analyses assessing the impact of partial sulfur removal have confirmed the pivotal role of sulfur in the HER process. The introduction of Mn was found to increase the number of sulfur vacancies, thereby affecting the electron cloud density around the active sulfur sites and enhancing the adsorption of reaction intermediates. This study provides substantial evidence supporting the development of Mn-VS₂ as an effective catalyst for HER, offering valuable insights for future strategies aimed at designing and improving HER electrocatalysts.