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Research Article  |  Open Access  |  23 Jun 2024

Atomic modulation and phase engineering of MoS2 for boosting N2 reduction

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Microstructures 2024;4:2024038.
10.20517/microstructures.2023.95 |  © The Author(s) 2024.
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Abstract

Electrochemical nitrogen reduction reaction (ENRR) has emerged as a potential alternative to the conventional Haber-Bosch process for ammonia production. However, ENRR technology is still restricted by the limited Faradaic efficiency due to the hard-to-break N-N triple bond. Herein, inspired by the biomimetic catalyst, we developed a Fe-modulated MoS2 catalyst (named Fe@MoS2) as an efficient ENRR catalyst. Raman spectra, coupled with the X-ray absorption spectroscopy, demonstrate the introduction of Fe into the MoS2 lattice and achieve partial 2H to 1T phase conversion. The presence of S-vacancies on MoS2 substrates was observed on scanning transmission electron microscopy images. Operando infrared absorption spectroscopy confirms that the constructed catalytic site significantly reduces barriers to nitrogen activation. The synthesized Fe@MoS2, with its superior geometric and electronic structures, exhibits a remarkable Faradaic efficiency of 19.7 ± 5.5% at -0.2 V vs. Reversible Hydrogen Electrode and a high yield rate of 20.2 ± 5.3 μg h-1 mg-1 at -0.8 V vs. Reversible Hydrogen Electrode. Therefore, this work provides a fresh direction for designing novel catalysts, eventually boosting the nitrogen reduction reaction kinetics and accelerating the ENRR application.

Keywords

Nitrogen reduction, heteroatom doping, molybdenum disulfide, phase engineering

INTRODUCTION

Ammonia (NH3) ranks among the most demanded chemical products in the world, playing an essential role in producing fertilizers, plastics, and medicines[1,2]. Furthermore, it is also an important carbon-free energy carrier, benefiting from its high hydrogen content (17.6 wt%) and high gravimetric energy density (3 kWh kg-1)[3,4]. The lower liquefaction difficulty makes it more convenient for storage and transportation[5]. Nowadays, the artificial synthesis of NH3 heavily depends on the Haber-Bosch (H-B) method. The N2 and H2 are transformed into NH3 on the catalyst surface under severe reaction conditions (400-600 °C, ~60 bar)[6-8]. However, the H-B process is energy-intensive and has high carbon emissions and high cost of raw materials (H2)[9,10]. Therefore, it is necessary to search for a green NH3 synthesis technology. Recently, an electrochemical nitrogen reduction reaction (ENRR) technology has been developed and can realize the N2 to NH3 conversion at ambient temperature, with H2O as the hydrogen source[11,12]. The total cost and carbon footprint of NH3 synthesis will be effectively reduced by driving ENRR using renewable energy sources[13]. Moreover, the simple process equipment makes flexible and decentralized NH3 preparation possible. Nevertheless, the ENRR process is still restricted by low yield rates and Faradaic efficiency (FE). Thus, the pursuit of highly efficient electrocatalysts for ENRR is greatly desirable but challenging.

Mo-based nitrogenase enables N2 fixation under ambient conditions[14,15], which inspired researchers to investigate biomimetic catalysts with similar elemental compositions. MoS2, as a layered two-dimensional (2D) semiconductor material, possesses a large surface area and crystal phase[16-19]. Multi-variable interlayer stacking forms and tunable electronic structure make it a promising candidate in the field of electrocatalytic synthesis of NH3[20-22]. Due to the difficulty of activating the non-polar N-N triple bond and the high adsorption barrier of N2, the NH3 yield rate and energy transfer efficiency in ENRR are limited. Moreover, the hydrogen evolution reaction (HER) has a close reaction equilibrium potential to that of ENRR, affecting the selectivity of ENRR in the aqueous phase and leading to unsatisfactory FE[23-25]. Therefore, several strategies have been reported to optimize the MoS2-based catalysts for improving ENRR performance, including vacancy engineering[26,27], heteroatom doping[28,29], phase engineering[30,31], etc.

The catalytic activity and stability of MoS2 with different phases vary greatly in ENRR. Lin et al.[30] compared the ENRR performance of the metastable phase (1T'- and 1T'''-MoS2) and the stable phase (2H-MoS2). Thanks to the high localized electron density around Mo-Mo, the NH3 yield and FE of 1T'''- MoS2 are approximately nine and 12 times higher than those of 2H-MoS2, respectively. Besides, constructing defects on the MoS2 basal plane can expose more active sites for N2 adsorption. You et al.[32] successfully introduced S vacancies in natural molybdenite through a one-step annealing method. The electron structure at the Mo edge was altered, which optimized the free energy barrier of the rate-limiting step. The prepared rich S-vacancy MoS2 exhibited significantly improved ENRR performance compared to the raw material. In addition, benefiting from the synergistic effect among different elements, superior ENRR activity can be achieved by doping heteroatoms in MoS2. Zhao et al. synthesized Fe-doped MoS2 nanosheets (Fe-MoS2/CC) using a chemical reduction method[33]. The introduction of Fe induced the redistribution of charges on the MoS2 basal plane and generated new active sites, significantly improving the NH3 synthesis performance. The NH3 yield was 12.5 μg-1 h-1 cm-2 with FE of 10.8% at -0.1 V vs. Reversible Hydrogen Electrode (RHE). Moreover, the average size and chemical valence state of Fe nanodots did not change significantly after long-term electrolysis, which demonstrated the excellent stability of Fe-MoS2/CC.

In this work, we developed a synergetic optimization strategy to enhance the ENRR activity of MoS2, coupling vacancy engineering, heteroatom doping, and phase engineering. Biomimetic Fe@MoS2 electrocatalysts were prepared using a one-pot hydrothermal method. The partial MoS2 converts from 2H to 1T phase after Fe atom doping. Furthermore, the unique hydrangea-like morphology and defects on the MoS2 basal plane promote the accessibility of the active site. In-situ and electrochemical characterizations demonstrated that Fe atoms had been inserted into the lattice of MoS2, and its unique coordination structure was analyzed which accelerates the reaction kinetics of N2 to NH3, thus achieving a promising ENRR performance. The maximum FE of 19.7% ± 5.5% and the highest NH3 yield of 20.2 ± 5.3 μg h-1 mg-1 were achieved by Fe@MoS2 at -0.2 V vs. RHE and -0.8 V vs. RHE, respectively.

MATERIALS AND METHODS

Synthesis of materials

One-step hydrothermal synthesis was used to prepare MoS2 [Figure 1A]. Briefly, the mixed solution after sonication (30 mL deionized water (D.I water)) with 0.4 mmol Na2MoO4·6H2O and 0.82 mmol thioacetamide) was transferred into a stainless steel autoclave (50 mL). The heating parameter was set to 190 °C for 20 h. After cooling, filtration, washing and drying at room temperature, MoS2 was collected. The synthesis process of Fe@MoS2 was the same as that of MoS2, except for adding 0.004 mmol FeCl3.

Atomic modulation and phase engineering of MoS<sub>2</sub> for boosting N<sub>2</sub> reduction

Figure 1. Schematic illustration of (A) synthesis process and (B) atomic structure; (C) XRD patterns of MoS2 and Fe@MoS2. (D) SEM image of Fe@MoS2, (E) TEM image of Fe@MoS2, (F) Raman spectra of MoS2 and Fe@MoS2.

Electrochemical measurement

ENRR electrochemical performance was evaluated using a three-electrode system (0.25 M LiClO4) under an ambient environment (25 °C, 1 atm). The working, reference, and counter electrodes are ENRR catalyst, Ag/AgCl (saturated KCl solution) electrode, and Pt wire, respectively. Firstly, 2 mg of catalyst was dispersed into 1 mL Nafion/ISO/D.I water solution (with the volume ratio of 1:9:40) with the sonication for 30 min. Then, 100 μL catalyst inks (MoS2 and Fe@MoS2) were drop-casted on carbon paper (1 cm2) with 0.2 mg cm-2 and dried in the ambient environment. Before the ENRR test, the electrolyte was bubbled with N2 gas (purity, 99.99%) for 20 min. During the ENRR test, the N2 flow was continuously inputted with a flow rate of 20 sccm.

Determination of NH3

The quantification of NH3 was determined using the salicylic acid-based colorimetric method[34]. To obtain the calibration curve, standard NH4Cl solutions with different concentrations of 0.0, 0.1, 0.2, 0.5, 1.0, and 2.0 g mL-1 in the electrolyte were first prepared. Then, 50 μL catalytic solution (10 mL D.I water with 0.1 g sodium nitroprusside dihydrate), 500 μL coloring solution (0.32 M NaOH and 0.4 M sodium salicylate), and 50 μL oxidation solution (NaClO solution (pCl = 4 - 4.9) with 0.75 M NaOH) were mixed with the 4 mL standard NH4Cl solution, and standing for 1h for color development. Next, the ultraviolet-visible (UV-Vis) spectrophotometer was used to measure the absorbance (λ = 655 nm) with different concentrations of NH4Cl. Finally, we got the calibration curve with a good linear relationship [Supplementary Figure 1]. The NH3 concentration in the reaction electrolyte is calculated based on the standard curve.

The NH3 formation rate can be calculated by:

$$ \begin{equation} \begin{aligned} R_{N H 3}\left(\mu g h^{-1} \mathrm{mg}^{-1}\right)=\frac{x(p p m) \times 10^{3}(\mu g / \mathrm{mg}) \times V(L)}{t(h) \times m(\mathrm{mg})} \end{aligned} \end{equation} $$

where:

x (ppm): calculated concentration of ammonia.

V (L): volume of reacted electrolyte.

t (h): reaction time in hours.

m (mg): mass of catalyst on the carbon paper.

RNH3 µg h-1 mg-1

The FE is calculated using:

$$ \begin{equation} \begin{aligned} F E_{N H 3}(\%)=\frac{3 \times R_{N H 3}\left(\mu g h^{-1} \mathrm{mg}^{-1}\right) \times t(h) \times 10^{-6}(\mathrm{~g} / \mu \mathrm{g}) \times m(\mathrm{mg}) \times F}{M r_{N H 4}(g / m o l) \times I(A) \times t(s)} \times 100 \% \end{aligned} \end{equation} $$

where:

x (ppm): calculated concentration of ammonia.

Mr (NH4+): 18 (g/mol).

V (L): the volume of reacted electrolyte.

I (A): the average current during the reaction.

F: the Faraday constant 96,485 mol-1.

Operando attenuated total reflection surface enhanced infrared absorption spectroscopy

Operando attenuated total reflection surface enhanced infrared absorption spectroscopy (ATR-SEIRAS) utilizes the same three-electrode system as the electrochemical test, and the electrolyte is 0.1 M Li2SO4(N2 saturated). Before each measurement, the background spectrum of the catalyst electrode needs to be collected at the open circuit potential. Afterward, the spectra at different potentials were collected. All collected spectra were given by the absorbance (-log(R/R0)) with a spectral resolution of 4 cm-1.

RESULTS AND DISCUSSION

Material characterization

The preparation process of Fe@MoS2 and the atomic structure are illustrated in Figure 1A and 1B. X-ray diffraction (XRD) patterns of MoS2 and Fe@MoS2 are shown in Figure 1C. The diffraction peaks at 32.5°, 35.8°, and 57.2° could be indexed to (100), (102), and (110) planes of the standard MoS2 phase (PDF #37-1492)[35-37], respectively. The diffraction peaks of Fe@MoS2 do not change significantly compared to MoS2, and no FeS or FeS2 phases were observed, which indicates that traces of Fe element did not form new sulfide. A scanning electron microscopy (SEM) image [Figure 1D] shows that the synthesized Fe@MoS2 is assembled from multiple 2D nanosheets and exhibits a hydrangea-like morphology. Moreover, Brunauer-Emmett-Teller (BET) test was carried out to characterize the specific surface area of catalysts. Supplementary Figure 2 shows the N2 adsorption-desorption isotherm curve of MoS2 and Fe@MoS2. Fe@MoS2 has a higher BET specific surface area of 54.034 m2 g-1 and a higher pore volume of 0.32 cm3 g-1 than MoS2 (22.63 m2 g-1 and 0.09 cm3 g-1, respectively). The catalytic activity was improved due to more exposed active sites [Supplementary Table 1]. The high specific surface area exposes more active sites, which benefits its catalytic activity. Electrochemically active surface area (ECSA) can be used to evaluate the number of active sites in a catalyst, which is proportional to the double-layer capacitance (Cdl). Supplementary Figure 3 presents the Cdl values obtained after linear fitting. Notably increased Cdl values were observed in Fe@MoS2 (17.85 mF cm-2), compared to MoS2 (0.35 mF cm-2), indicating the beneficial effect of Fe doping on the number of active sites.

No recognizable nanoparticles or clusters are observed in the transmission electron microscopy (TEM) image [Figure 1E]. This suggests that Fe was more likely to be inserted into the MoS2 plane rather than deposited on the surface, which requires further analysis in combination with spectra. Raman spectra [Figure 1F] were employed to further investigate the effect of Fe insertion on MoS2. The results suggest that synthesized MoS2 exhibits distinct 2H phase signals, with two peak positions at 376.5 and 403 cm-1 corresponding to the E12g vibrational peak and A1g vibrational peak of 2H-MoS2, respectively[35,38,39]. However, Fe@MoS2 is a mixed 1T/2H-MoS2 phase; two new 1T phase characteristic peaks of J1 (146 cm-1) and J3(334 cm-1) are also clearly observed, indicating that the doping of Fe atoms triggers a phase transition from 2H-MoS2 to 1T-MoS2[40-42].

A high-resolution TEM (HRTEM) image [Figure 2A] shows the same results as those obtained from TEM images, with no impurities or other Fe-based structures observed in Fe@MoS2. Further, with the help of dark field scanning TEM (STEM) characterization [Figure 2B and Supplementary Figure 4], it is seen that defects are present on the surface of the 2D structure. Atomic-level Fe (weak Z contrast intensity) replaces some Mo atoms and dopes in the panel of the 2D MoS2 plane. To explore the distribution of different elements in the catalysts, corresponding energy dispersive spectroscopy (EDS) elemental mapping was conducted to characterize Fe@MoS2. Figure 2C demonstrates that Mo, S, and Fe are uniformly distributed in Fe@MoS2.

Atomic modulation and phase engineering of MoS<sub>2</sub> for boosting N<sub>2</sub> reduction

Figure 2. Material Characterization. (A) HRTEM image of Fe@MoS2. (B) STEM image, and (C) the corresponding EDS elemental mappings of Fe@MoS2, (D)XANES spectra of the Fe K-edge in Fe foil, FeO, Fe2O3, and Fe@MoS2, (E) FT-EXAFS spectra of the Fe K-edge in Fe foil, FeO, and Fe@MoS2. (F) XAFS fitting result of Fe@MoS2 at R space. Inset: Atomic illustration of Fe@MoS2 (Mo atoms: purple; S atoms: yellow; Fe atoms: red).

X-ray absorption near edge structure (XANES) and Fourier transform-extended X-ray absorption fine structure (FT-EXAFS) spectra further revealed the coordination structure and coordination number of Fe atom in Fe@MoS2. As shown in Figure 2D, the absorption edge of Fe@MoS2 is close to the curve of FeO, indicating that Fe of Fe@MoS2 is dominated by a chemical state of +2. In the FT-EXAFS spectra [Figure 2E], a peak can be observed at 1.87 Å for Fe@MoS2, corresponding to the first coordination[43,44]. No Fe-O and Fe-Fe are observed in Fe@MoS2, indicating that the atomically dispersed Fe may dope within Mo-based vacancy and bond with S atoms. Fitting curves [Figure 2F, Supplementary Figure 5, Supplementary Table 2] of the Fe K-edge EXAFS spectra of Fe@MoS2 indicate that the Fe-S coordination number is approximately 5.5. This suggests that Fe primarily replaces the Mo atom and coordinates with five S atoms near the S vacancy in MoS2[45]. X-ray Photoelectron Spectroscopy (XPS) reveals the valence changes of the Mo and S elements after Fe doping. Figure 3A and B shows the S 2p and Mo 3d spectra of MoS2 and Fe@MoS2. Two characteristic peaks of 162.35 and 160.8 eV for MoS2 were assigned to S 2p1/2 and S 2p3/2, respectively[46,47]. The introduction of Fe resulted in a positive shift of the peak position with altered binding energies of 162.70 and 161.20 eV, which may be related to the Fe-S bond formation. In addition, the peaks with binding energies at 230.9 and 227.7 eV correspond to Mo 3d3/2 and Mo 3d5/2[48]. A positive shift of 1.1 eV was observed in Fe@MoS2, indicating an elevated valence state for Mo as well, which is well in line with the previously reported research[49,50].

Atomic modulation and phase engineering of MoS<sub>2</sub> for boosting N<sub>2</sub> reduction

Figure 3. (A) S 2p and (B) Mo 3d XPS of MoS2 and Fe@MoS2.

Electrochemical measurements

The ENRR performance of the Fe@MoS2 is evaluated in an H-type cell equipped with a three-electrode system [Figure 4A]. The linear scan voltammetry (LSV) was first performed to compare the response current density under Ar- or N2-saturated electrolyte of Fe@MoS2. As shown in Figure 4B, higher current density in N2 indicates the possibility of the Fe@MoS2 towards N2 reduction. The subsequent ENRR performance was evaluated using a chronoamperometry method in N2-saturated electrolyte for 3 h, and the response current densities at different potentials were recorded [Figure 4]. The reacted electrolyte was collected and analyzed for the NH3 concentration[34]. The relevant equations have been mentioned above.

Atomic modulation and phase engineering of MoS<sub>2</sub> for boosting N<sub>2</sub> reduction

Figure 4. Electrochemical performance evaluation. (A) Schematic illustration of the reactor for electrochemical N2 reduction. (B) LSV curves of Fe@MoS2 in Ar and N2-saturated 0.25 M LiClO4 at a scan rate of 10 mV s-1. (C) Chronoamperometric curves of Fe@MoS2 at different applied potentials. NH3 yield rate and FE of (D) MoS2 and (E) Fe@MoS2. The picture of the reacted solution of (F) MoS2 and (G) Fe@MoS2.

The yield rate and FE of NH3 over the electrocatalyst Fe@MoS2 are higher than those of the pristine MoS2 electrocatalyst, which indicates the optimization of the introduction of Fe element in the pristine MoS2 plane. As shown in Figure 4D, the MoS2 can achieve the yield rate of NH3 less than 10 μg h-1 mg-1 between the potential of -0.2 to -0.8 V vs. RHE, while the Fe@MoS2 can achieve the yield rate of NH3 larger than 13.9 μg h-1 mg-1. The highest yield rate of 20.2 ± 5.3 μg h-1 mg-1 is obtained at -0.8 V vs. RHE, about two times higher than that of MoS2 [Figure 4E]. As shown in Figure 4F and G, the colors of the reacted electrolytes of Fe@MoS2 and MoS2 are green and yellow, respectively, suggesting that more NH3 is generated with Fe@MoS2 than that of MoS2. The highest FE of ~19.7% ± 5.5% is obtained at -0.2 V vs. RHE. The FE dramatically decreases as the applied potentials increase, suggesting that the reaction is dominated by side-reaction HER. Compared with previously reported MoS2-based catalysts, Supplementary Table 3 shows that Fe@MoS2 exhibits a relatively high FE and yield rate of NH3. In addition, Fe@MoS2 demonstrates reliable stability in long-term (10 h) stability tests. Supplementary Figure 6 illustrates no significant deterioration of the current density of Fe@MoS2.

Operando ATR-SEIRAS was performed to characterize the reaction intermediates and study the reaction mechanism. Figure 5 recorded the ATR-SEIRAS spectra of MoS2 and Fe@MoS2 under different potentials during the ENRR process. As shown in Figure 5A, no obvious peak can be observed for pristine MoS2, indicating no intermediates generated. In contrast, several peaks gradually increase with the increasing potentials for Fe@MoS2 [Figure 5B]. Some peaks attributed to N-H stretching are located at 3,361 and 3,286 cm-1[51,52]. The peaks at 3,112 and 1,650 cm-1 correspond to the O-H stretching and bending mode of H2O, respectively[53,54]. The bending vibration absorption peak of -N-H at 1,552 cm-1 gradually strengthened[55], indicating a strong N2 reduction reaction over the Fe@MoS2. These results demonstrate that introducing Fe can accelerate the ENRR kinetics.

Atomic modulation and phase engineering of MoS<sub>2</sub> for boosting N<sub>2</sub> reduction

Figure 5. Mechanism study. Operando ATR-SEIRAS of (A) MoS2 and (B) Fe@MoS2.

CONCLUSIONS

In conclusion, an inspired biomimetic electrocatalyst of Fe@MoS2 is designed and successfully prepared using a one-step hydrothermal method. The introduction of atomic-level Fe achieves the geometric and electronic structure modulation of MoS2 and triggers the phase transition of the 2H-phase MoS2 into 1T-phase MoS2. Thanks to the improved atomic structure and increased electrical conductivity of Fe@MoS2, the kinetics are greatly accelerated, thus enhancing ENRR performance. Compared to the pristine 2H MoS2, the prepared Fe@MoS2 boosts the ENRR kinetics and exhibits superior ENRR performance with a yield rate of 20.2 ± 5.3 μg h-1 mg-1 at -0.8 V vs. RHE and a FE of ~19.7% ± 5.5% at -0.2 V vs. RHE. This work provides a new insight for designing efficient ENRR catalysts by synergistic doping and phase engineering.

DECLARATIONS

Authors’ contributions

Synthesis and testing of materials, data collection, and original manuscript writing: Jia Y, Shao G, Li Y, Yang R

Validation and original manuscript revision: Huang M, Huang H, Liu M, Huang G, Lu Q, Gu C

Data collection: Jia Y, Shao G, Li Y, Yang R

Data analysis, writing, review, and editing: Jia Y, Li Y, Shao G

Review and discussion: Jia Y, Li Y, Shao G, Huang H, Huang G, Gu C

Supervision, funding acquisition: Li Y, Huang M

All authors have read and agreed to the published version of the manuscript.

Availability of data and materials

According to reasonable requirements, all of the data examined in this research can be obtained from the correspondents.

Financial support and sponsorship

This work is supported by the National Natural Science Foundation of China (No. 22308322 and No. 52373223), the Science Foundation of Donghai Laboratory (Grant No. DH-2022ZY0010), R&D Project of State Grid Corporation of China (No. 5108-202218280A-2-439-XG), and Sichuan Science and Technology Program (No. 2023NSFSC0434).

Conflicts of interest

Liu M is affiliated with “State Grid Zhejiang Electric Power CO., LTD Research Institute, Hangzhou, China”. While the other authors have declared that they have no conflicts of interest.

Ethical approval and consent to participate

Not applicable.

Consent for publication

Not applicable.

Copyright

© The Author(s) 2024.

Supplementary Materials

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OAE Style

Jia Y, Shao G, Li Y, Yang R, Huang M, Huang H, Liu M, Huang G, Lu Q, Gu C. Atomic modulation and phase engineering of MoS2 for boosting N2 reduction. Microstructures 2024;4:2024038. http://dx.doi.org/10.20517/microstructures.2023.95

AMA Style

Jia Y, Shao G, Li Y, Yang R, Huang M, Huang H, Liu M, Huang G, Lu Q, Gu C. Atomic modulation and phase engineering of MoS2 for boosting N2 reduction. Microstructures. 2024; 4(3): 2024038. http://dx.doi.org/10.20517/microstructures.2023.95

Chicago/Turabian Style

Yansong Jia, Guining Shao, Yang Li, Ruizhe Yang, Ming Huang, Hua Huang, Min Liu, Gai Huang, Qunjie Lu, Chaohua Gu. 2024. "Atomic modulation and phase engineering of MoS2 for boosting N2 reduction" Microstructures. 4, no.3: 2024038. http://dx.doi.org/10.20517/microstructures.2023.95

ACS Style

Jia, Y.; Shao G.; Li Y.; Yang R.; Huang M.; Huang H.; Liu M.; Huang G.; Lu Q.; Gu C. Atomic modulation and phase engineering of MoS2 for boosting N2 reduction. Microstructures. 2024, 4, 2024038. http://dx.doi.org/10.20517/microstructures.2023.95

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