Antonia
Kagkoura
*a,
Shuangying
Wei
a,
Lunjie
Zeng
b,
Eva
Olsson
b,
Filipa M.
Oliveira
a,
Jan
Luxa
a and
Zdeněk
Sofer
*a
aDepartment of Inorganic Chemistry, Faculty of Chemical Technology, University of Chemistry and Technology Prague, Technická 5, 166 28 Prague 6, Czech Republic. E-mail: kagkourn@vscht.cz; soferz@vscht.cz
bDepartment of Physics, Chalmers University of Technology, 41296, Göteborg, Sweden
First published on 15th November 2024
The ongoing energy crisis has made it imperative to develop low-cost, easily fabricated, yet efficient materials. It is highly desirable for these nanomaterials to function effectively in multiple applications. Among transition metal dichalcogenides, tungsten diselenide (WSe2) shows great promise but remains understudied. In this work, we doped WSe2 with Mn using a simple hydrothermal method. The resulting material exhibited excellent electrocatalytic activity for the hydrogen evolution reaction, achieving a low overpotential of −0.28 V vs. RHE at −10 mA cm−2, enhanced conductivity, and high stability and durability. Moreover, as an anode material in lithium-ion batteries, the Mn-doped WSe2 outperformed pristine WSe2, reaching initial discharge capacities of 1223 and 922 mA h g−1, respectively. Additionally, the Mn-doped material maintained a significantly higher discharge capacity of 201 mA h g−1 compared to intact WSe2, which had 68 mA h g−1 after 150 cycles. This work offers novel insights into designing efficient bifunctional nanomaterials using transition metal dichalcogenides.
Electrochemical water splitting is another promising green process for hydrogen production. However, high cost and availability of current platinum-based electrocatalysts remain bottlenecks to large-scale commercialization.4,5 Besides this, lithium-ion batteries (LIBs) are considered the ideal candidates for storing electrical energy in portable and electrical vehicles considering their lightweight frame, high energy density, compact size, negligible memory effects, long cycle lifetime, and minimal environmental impact.6 Up until today, a major driving force for further improvement in capacity is finding an alternative to graphite as an active anode material.7–9 The specific capacity of graphite during discharging is inherently restricted to 372 mA h g−1 and thus restricts the overall capacity of LIBs.10 Since the energy density for batteries is increasing day by day, the said limitation becomes highly crucial and hence demands the development of some alternative anode materials that shall provide higher capacity and improve the performance of next-generation energy storage systems.
Transition metal dichalcogenides (TMDs) have emerged as an attractive alternative in electrocatalysis, with particular emphasis on hydrogen evolution reaction (HER), based on their natural abundance and inexpensiveness.11,12 Consequently, bottom-up approaches based on hydrothermal or solvothermal reactions between metallic and chalcogen molecular precursors enable the synthesis of high yields of both polymorphs of TMDs.13,14 Abundant defects and exposed edges are generated by such an approach, which improves the electrocatalytic activity of the material for HER. More interestingly, the TMDs are strong candidates as anode materials for LIBs owing to their outstanding chemical stability, low cost and high theoretical capacities.15,16 Their big interlayer spacing also promotes the reversible lithium ion transport and insertion/extraction reaction process.15,16
Among the TMDs members, WSe2 has a layered structure with a large interlayer distance as high as 0.65 nm, enabling highly efficient lithium-ion diffusion. In addition, its density is as high as 9.32 g cm−3; hence, WSe2 has great promise for high volumetric energy densities – a crucial parameter for demanding applications such as electric vehicles.17,18 However, nanostructured WSe2 anodes are barely explored yet. In the case of electrocatalysis as well, WSe2 has received less attention from the TMD family compared to molybdenum disulfide, although WSe2 has presented very good activity in the HER.13,19 Till now, the electrocatalytic HER activity of TMDs has been improved by tuning several parameters such as phase engineering and heteroatom doping.20,21 Indeed, metallic TMDs of the 1T type have intrinsic reduced charge-transfer resistance compared to semiconducting varieties of 2H type, favorable in electrocatalysis.20,21 Most importantly, the introduction of heteroatoms into TMDs is one of the most powerful ways to enhance their electrocatalytic activity.22,23 Particularly, element doping causes crystal distortion, thereby enhancing electronic structure. Besides that, due to the doped elements, some new adsorption sites are created increasing the total available adsorption sites in number.22 Moreover, element incorporation into the host structure is able to significantly improve the intrinsic conductivity of the material by changing the chemical bond or the local lattice, which is rather vital in the performance of LIBs.24 Manganese (Mn) is a non-precious and thus inexpensive metal that has also proved to be an efficient dopant in TMDs for HER.25,26 However, its potential in WSe2 with respect to HER and LIB remains unexplored up to now. Simpler doping processes remain scarce, and the processes at lower temperatures are few.22 Furthermore, WSe2-based materials are not widely explored in LIBs,17,18 and manganese-doped WSe2 remains to be studied.
In this work, we adopt a simple hydrothermal method to dope Mn into WSe2, resulting in Mn-WSe2. Its electrocatalytic activity for HER is investigated; accordingly, it exhibits a low overpotential of 298 mV at −10 mA cm−2 and a low charge transfer resistance of 46 Ω with high stability and durability. In fact, the advantages of Mn doping were realized in the far higher performance of Mn-WSe2 when used, to the best of our knowledge, for the first time as an anode in lithium-ion batteries. Compared to pure WSe2, Mn-WSe2 exhibited much higher initial discharge and charge capacities. In addition, even after 150 cycles, Mn-WSe2 maintained much higher capacities of 201 mA h g−1 compared to 68 mA h g−1 for undoped WSe2.
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Fig. 2 Deconvoluted X-ray photoelectron spectra of (a) W 4f and (b) Se 3d for WSe2 for Mn-WSe2. (c) W 4f and (d) Se 3d for WSe2. |
The 1T phase is confirmed by the deconvoluted W 4f spectrum of Mn-WSe2, which shows a doublet of the W4+ peaks at 30.8 eV for W4+ 4f7/2 and 33.7 eV for W4+ 4f5/2, assigned to the 1T phase (Fig. 2a).14 The shifts to lower binding energy compared to pristine WSe2 (31.6 eV for W4+ 4f7/2 and 33.7 eV for W4+ 4f5/2), as seen in (Fig. 2c), strongly indicate successful Mn doping.30 Additionally, signals at 32.43 and 34.53 eV correspond to W4+ 4f7/2 and W4+ 4f5/2 of the 2H phase. Peaks at 34.9 and 37.8 eV are associated with the presence of W6+ species within WSe2, favorable in electrocatalytic reactions.31–33 Moreover, the Se 3d peak profile of Mn-WSe2 can be deconvoluted into two sets of doublet peaks: the higher energy peaks at 55.68 eV for Se 3d3/2 and 54.7 eV for Se 3d5/2 correspond to the 2H phase, while the lower energy peaks at 54.62 eV for Se 3d3/2 and 53.82 eV for Se 3d5/2 correspond to the 1T phase (Fig. 2b). Signals at higher energies are attributed to Mn 3p. For pristine WSe2, doublets for 2H and 1T phases, are located in higher energies, doublets at 55.61 eV for Se 3d3/2 and 54.74 eV for Se 3d5/2 (2H phase) and 54.78 eV for Se 3d3/2 and 53.86 eV for Se 3d5/2 (1T phase), as seen in Fig. 2d. All observed shifts support that Mn doping modifies the redox states of both W and Se.30 Additionally, the Mn 2p spectrum displays two spin–orbit doublets: Mn 2p3/2 at 641.28 eV and Mn 2p1/2 at 653.5 eV, corresponding to Mn2+ (Fig. S1†).34 Further evidence supporting the hybrid structure was obtained through powder X-ray diffraction (XRD). Fig. S2† shows the reflection signals corresponding to plane reflections of hexagonal WSe2 (reference JCPDF no. 38-1388).35
The morphological and elemental analysis of Mn-WSe2 along with the intact WSe2 was realized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SEM images in Fig. 3 reveal that both pristine and doped WSe2 nanostructures exhibit a similar morphology, with small, randomly oriented nanosheets interconnected to form intricate, flower-like formations. During the solvothermal process, these nanosheets self-assembled into larger, micro-scale “ball-flowers” with a hierarchical structure—indicating an organization that extends from the nanoscale sheets to complex, micro-scale formations. This morphology is common for TMDs deriving from solvothermal bottom-up approaches.13,14
High-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) imaging shows the microscopic structure of the doped and pristine WSe2 at the nanometer and atomic scales (Fig. 4). STEM images clearly show that WSe2 nanocrystals (Fig. 4a and c) consist of randomly distributed WSe2 nanosheets (Fig. 4b and d). In the STEM images of both doped and pristine WSe2, there are fringes with an inter-fringe distance of ∼0.65 nm (Fig. 4b and d), which is the interlayer distance in WSe2. The fringes are from nanoscale WSe2 sheets, which are randomly orientated and distributed within the crystals. When the basal atomic planes in the nanosheets are parallel to the electron beam direction, fringes that show the stacking of basal planes are visible. The thickness of the WSe2 nanosheets is about 5 nm and the length of the nanosheets is usually larger than 10 nm. Furthermore, the elemental distribution of Mn in Mn-WSe2 was analyzed by STEM-energy dispersive spectroscopy (EDS) mapping. STEM-EDS maps and the corresponding STEM image of a doped WSe2 nanocrystal are shown in Fig. 4e–h. As evident from the elemental maps, Mn is homogeneously and randomly distributed within the WSe2 matrix at the nanometer and atomic scales. On average, the atomic concentration of Mn is ∼4%.
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Fig. 4 (a) and (b) HAADF STEM images of Mn-WSe2. (c) and (d) HAADF STEM images of pristine WSe2. (e)–(h) HAADF STEM image and the corresponding STEM-EDXS elemental maps of Mn, Se and W. |
The electrocatalytic performance of Mn-WSe2 in the HER was assessed using linear sweep voltammetry (LSV) in an aqueous 0.5 M H2SO4 electrolyte (Fig. 5a). Interestingly, Mn-WSe2 showed a low onset potential at −0.077 V vs. RHE, only slightly lower-by 71 mV – of that of Pt/C at −0.006 V versus RHE; onset potentials were recorded at j = −1 mA cm−2 for all materials.36 By contrast, pristine WSe2 showed a much higher onset overpotential of 193 mV than Mn-WSe2 at −0.27 V vs. RHE. Moreover, the electrocatalytic activity of HER was further determined at the current density of −10 mA cm−2. Interestingly, Mn-WSe2 exhibited an overpotential as low as 280 mV, 150 mV lower than that of pristine WSe2. Such improvement in catalytic performance arises from the preserved coordination vacancies due to the confined atoms acting as active catalytic sites. Strong electronic interaction between the incorporated Mn heteroatoms and WSe2 induces new electronic states that are favorable for electrocatalytic activity.
To investigate the mechanism of the reaction, Tafel slopes were obtained from LSV curves. Mn-WSe2 presented a smallest Tafel slope of 80 mV dec−1 in expectation, which means that the rate-determining reaction step is the desorption of absorbed hydrogen atoms from the electrode (Heyrovsky step). In sharp contrast, the pristine WSe2 showed a much higher Tafel slope value of 143 mV dec−1, which means that the rate-determining step corresponds to the initial proton adsorption onto the electrode surface through a reduction process (Volmer step). The easier reaction kinetics in the doped material can also be further evident from the EIS results depicted in Fig. 5c. Meanwhile, it also had a semicircle similar to Pt/C, which corresponded to the charge transfer resistance (Rct) at 46 Ω or close to that of Pt/C (42 Ω). Meanwhile, the Rct value for pure WSe2 was determined at around 299 Ω, much higher than that of Mn-WSe2, again underlining the positive effect of doping. The lower Tafel slope and Rct values obtained in the doped material attested to higher conductance and more facile electron transfer kinetics and were ascribed to the successful incorporation of Mn atoms into the selenium defective sites.
Moving on, the stability of both doped and undoped WSe2 was scrutinized by 10000 consecutive electrocatalytic cycles, as described in Fig. 5a. Indeed, Mn-WSe2 manifested a negligible potential loss of 40 mV, while WSe2 revealed an overpotential as high as 80 mV. Additionally, chronoamperometric analysis was also conducted at a constant applied potential of −2.16 V vs. RHE for 10
000 s, accompanied by a rotation speed of 1600 rpm (Fig. 5d), in further evaluation of the durability of Mn-WSe2. Astonishingly, Mn-WSe2 showed excellent stability, reflecting only a negligible decay in current density of about 9%.
The electrochemically active surface area (ECSA) determination could deliver information about Mn-WSe2 charge transport dynamics. ECSA values were calculated by using the equation ECSA = Cdl/Cs, wherein Cdl is the electrochemical double-layer capacitance and Cs is the specific capacitance of a flat surface taken as 40 μF cm−2 for the flat electrode having 1 cm2 of real surface area. The cyclic voltammograms of Mn-WSe2 and pristine WSe2 were recorded in a non-faradaic region at various scan rates ranging from 50 to 500 mV s−1, as shown in Fig. S3 of the ESI.† Hence, Mn-WSe2 gave the maximum ECSA value of about 21.95 cm2 compared to an equivalent value of about 1.85 cm2 for WSe2. Values of ECSA are generally higher, pointing to a larger functional surface area of catalytic active sites and enabling improved electrocatalytic performance. Such results are also in good agreement with the overall electrocatalytic results. The summary of detailed electrocatalytic HER parameters for Mn-WSe2 and other tested materials can be seen in Table S1.†
Next, the lithium storage capacities of Mn-WSe2 and WSe2 were evaluated using galvanostatic cycling measurements within a potential range of 0.001–3.0 V vs. Li+/Li. Fig. 6a and b illustrate the galvanostatic charge/discharge potential profiles for the Mn-WSe2 and WSe2 electrodes, respectively, which were recorded at a current density of 0.3 C (where 1C equals 314 mA g−1). The initial discharge and charge profiles of the WSe2 and Mn-WSe2 electrodes reveal few discernible kinks or plateaus. Interestingly, the Mn-WSe2 electrode exhibits a significantly higher initial discharge capacity of 1223 mA h g−1 compared to 922 mA h g−1 for pristine WSe2. It also shows a higher charge capacity, with Mn-WSe2 achieving 789 mA h g−1versus 673 mA h g−1 for undoped WSe2. These capacities are partially offset by the irreversible conversion process between WSe2 and lithium ions (Li+), as well as the formation of a solid–electrolyte interface (SEI) film during the first cycle. Interestingly, Mn-WSe2 exhibits higher charge and discharge capacities compared to other WSe2-based materials. For instance, WSe2 grown onto graphene has demonstrated charge and discharge capacities of 531 and 744 mA h g−1, respectively.18 Consequently, the initial coulombic efficiencies for Mn-WSe2 and intact WSe2 are 73.0% and 64.5%, respectively. Additionally, the subsequent cycles display consistent behavior, demonstrating the reversibility of the lithium storage reactions within Mn-WSe2. Remarkably, the electrochemical performance of Mn-WSe2 (200 mA h g−1 after 100 cycles) surpasses that of previously reported WSe2-based materials, such as few-layer thick WSe2 nanosheets in 1T′ crystal structures have shown a capacity of only 102 mA h g−1 at 0.2 C after 100 cycles.17Fig. 6c showcases the cycling stability over 150 cycles. The capacities maintain levels above 68 mA h g−1 for WSe2 and 201 mA h g−1 for Mn-WSe2 through the 150th cycle. The improved cycling performance of Mn-WSe2 can be attributed to the successful incorporation of Mn, which significantly enhances the material's conductivity and greatly promotes charge transfer during the electrochemical reaction.
Footnote |
† Electronic supplementary information (ESI) available: XPS spectra for Mn-WSe2, table for electrocatalytic HER parameters in for all tested materials and cyclic voltamographs of (a) Mn-WSe2 and (b) WSe2. See DOI: https://doi.org/10.1039/d4nr04348k |
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