ELECTROCHEMICAL METHOD FOR ENHANCING ELECTROCATALYTIC PERFORMANCE OF METAL DEPOSITION IN UNCONVENTIONAL-PHASE TRANSITION METAL DICHALCOGENIDES

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the priorities from the U.S. provisional patent application Ser. No. 63/549,570 filed Feb. 4, 2024, and the disclosure of which is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present invention pertains to the fields of electrochemistry and materials science.

BACKGROUND OF THE INVENTION

Ammonia production is a cornerstone of the global fertilizer industry and has potential as a hydrogen carrier and carbon-free fuel. However, current ammonia synthesis relies on the energy-intensive Haber-Bosch process, which consumes significant energy (approximately 5.5 exajoules annually) and results in high CO₂ emissions, contributing to environmental concerns. In addition, the process operates under extreme conditions of high temperature (300° C.-500° C.) and pressure (150-300 atm) and requires substantial fossil fuel consumption to break the stable N≡N bond in nitrogen molecules. Additionally, the Haber-Bosch process inadvertently contributes to environmental degradation through nitrate contamination in water bodies, promoting eutrophication. For this reason, there is a growing focus on the development of clean and efficient technologies for ammonia synthesis.

An emerging alternative to the Haber-Bosch process is the electrochemical nitrate reduction reaction (NO₃RR), which can convert surplus nitrates into ammonia. This approach offers a more sustainable route to ammonia synthesis and addresses the global nitrogen cycle imbalance caused by excessive fertilizer use. However, the NO₃RR process faces significant challenges, particularly sluggish reaction kinetics due to the eight-electron transfer required for the reaction. The critical bottleneck of NO₃RR lies in developing efficient catalysts, predominantly due to a limited understanding of the catalytic mechanism and the structure-activity relationship of catalysts.

Atomically dispersed catalysts (SACs) have garnered attention for their ability to maximize metal utilization and exhibit high selectivity and catalytic activity. However, SACs face major limitations due to the instability of the metal atoms on the support. Supported metal atoms often exhibit weak binding to the substrate, making them prone to migration and aggregation into larger particles under reaction conditions. This instability diminishes the advantages of SACs, particularly at high metal loadings.

Transition metal dichalcogenides (TMDs) have emerged as promising support materials due to their large surface areas and high catalytic potential. However, conventional TMD phases, such as 2H, often exhibit weak interactions with metal atoms, limiting their utility as supports for SACs. Unconventional-phase TMDs, such as the 1T′ phase, have demonstrated improved catalytic properties, particularly for reactions like NO₃RR. Nevertheless, precise control over the size, distribution, and stability of metal species on these unconventional-phase TMDs remains a significant challenge. Furthermore, the metastable nature of unconventional-phase TMDs complicates the synthesis of stable, well-defined metal/TMD interfaces that could unlock their full catalytic potential.

U.S. Ser. No. 11/652,206B2¹ discloses the use of two-dimensional (2D) TMDs and alloys as catalysts for the cathodes in lithium-sulfur (Li—S) batteries. The patent highlights the application of 2H (semiconducting)-1T (metallic) mixed-phase TMDs to suppress polysulfide shuttling by catalyzing polysulfide reactions. While this work demonstrates the use of TMD materials such as MoS₂ and MoSe₂ for sulfur cathodes, a key limitation is its restricted ability to control the morphology of the metal species on the TMD support. Additionally, the patent's approach is not adaptable to unconventional-phase TMDs, which could limit the versatility and applicability of the catalyst in various electrochemical processes.

US20210080419A1² discloses the phase transition of 2H to 1T phase-based TMD films for detecting strong electron donor chemical vapors. This approach is mainly intended for sensor applications and offers reversible phase transitions upon exposure to such vapors. However, the focus on sensor technology restricts its utility for electrocatalytic reactions. Furthermore, the work is limited to film structures, which may not provide the same performance benefits as nanostructured materials. This difference in application scope, alongside the inherent limitations of using thin films for catalysis, reduces the versatility of this technology compared to other methods in the field.

Sun, Yifan, et al.³ focuses on the deposition of noble metals like Au and Ag on TMD nanostructures through a colloidal interface-mediated approach. While this technique achieves metal deposition on TMDs, it is primarily limited to noble metals, and there is no detailed control over the size or distribution of the metal species. As a result, the method may not offer the same level of flexibility and precision needed to optimize metal-TMD interactions. Furthermore, the narrow focus on noble metals limits the applicability of this approach for a broader range of electrocatalytic applications compared to methods capable of working with diverse metal species.

The lack of effective methods to control these interfaces and optimize metal-support interactions has hindered the development of high-performance electrocatalysts for ammonia synthesis and other reactions. Therefore, there is a need in the art for developing clean and efficient technologies for ammonia synthesis.

SUMMARY OF THE INVENTION

The precise control of metal-support interactions (MSI) using unconventional-phase TMD materials has not been extensively studied. Therefore, this invention seeks to carefully adjust the various forms of copper in the unconventional-phase Cu/1T′ WS₂ nanostructures, in order to optimize the electronic interaction between the metal and the support.

In a first aspect, the present invention provides an electrochemical method for enhancing electrocatalytic performance of metal deposition in unconventional-phase transition metal dichalcogenides. The method includes preparing a working electrode comprising copper (Cu)-deposited unconventional-phase transition metal dichalcogenides nanostructures as a catalyst; placing the working electrode in an electrochemical cell containing an aqueous electrolyte having nitrate ions; and applying a potential ranging from −0.6 V to −1.0 V relative to a reference hydrogen electrode and conducting electrocatalytic reduction of nitrate in the aqueous electrolyte to reduce nitrate ions to ammonia. The copper sites stabilize adsorbed nitrate intermediates, and the copper-deposited unconventional-phase transition metal dichalcogenides nanostructures provide activated hydrogen species to promote surface hydrogenation, leading to the formation of ammonia. The Cu-deposited unconventional-phase transition metal dichalcogenides nanostructures is performed under mild conditions of room temperature and atmospheric pressure, enabling large-scale production by amplifying the electrode area.

In one embodiment, the copper-deposited unconventional-phase transition metal dichalcogenides nanostructures are synthesized by the following steps: preparing one or more exfoliated transition metal dichalcogenides nanosheets via electrochemical exfoliation; preparing an electrolyte solution of 0.1 M H₂SO₄ comprising Cu salts of CuSO₄; and performing an electrochemical deposition of Cu onto the one or more exfoliated transition metal dichalcogenides nanosheets to obtain the copper-deposited unconventional-phase transition metal dichalcogenides nanostructures. The electrochemical deposition is conducted under an inert gas atmosphere.

In one embodiment, the Cu is deposited in an amount ranging from 4 wt % to 12 wt %.

In one embodiment, the electrochemical exfoliation involves an electrolyte solution containing tetraheptylammonium bromide dissolved in acetonitrile.

In one embodiment, the one or more exfoliated transition metal dichalcogenides nanosheets are defect-free and exhibit electrophilic properties.

In one embodiment, the unconventional-phase transition metal dichalcogenides nanostructures include 1T′ WS₂ nanosheet substrate, 1T′ MoS₂ nanosheet substrate, 1T′ MoSe₂ nanosheet substrate, or 1T′ WSe₂ nanosheet substrate. The 1T′ WS₂ nanosheet substrate, 1T′ MoS₂ nanosheet substrate, 1T′ MoSe₂ nanosheet substrate, or 1T′ WSe₂ nanosheet substrate has a thickness ranging from 0.6 nm to 5 nm.

In another embodiment, the method further includes controlling electrochemical deposition conditions to produce single-atomically dispersed Cu, amorphous Cu nanoclusters, or crystalline Cu nanoparticles on the 1T′ WS₂ nanosheet substrate, 1T′ MoS₂ nanosheet substrate, 1T′ MoSe₂ nanosheet substrate, or 1T′ WSe₂ nanosheet substrate.

In particular, the electrochemical deposition conditions include applied voltage, deposition time, electrolyte composition, and presence of ligands.

In one embodiment, the single-atomically dispersed Cu is formed by applying an underpotential deposition (UPD) within a potential range of 0.1 V to 0.6 V.

In another embodiment, the amorphous Cu nanoclusters is formed by applying a ligand-mediated deposition method within a potential range of −0.1 V to 0.6 V.

Preferably, a hybrid material of the amorphous Cu nanoclusters on the 1T′ WS₂ nanosheet substrate achieves a Faradaic efficiency (FE) of at least 98% for ammonia production at −0.8 V relative to the reference hydrogen electrode.

In one embodiment, the electrochemical deposition is carried out in a three-electrode configuration.

In another embodiment, the method further includes recycling the catalyst at least 6 times without significant performance decay.

In another embodiment, the method further includes determining produced ammonia concentration using an indophenol blue spectrophotometric method.

In one embodiment, the aqueous electrolyte includes 0.5 M K₂SO₄ and 0.1 M KNO₃.

In another embodiment, the aqueous electrolyte further includes penicillamine as a surfactant.

In another aspect, the present invention provides a Cu/1T′ WS₂ hybrid material prepared by the aforementioned methods. The Cu is predominantly bound to the 1T′ WS2 nanosheets via Cu—S bonds. The Cu nanostructures exhibit uniform dispersion and strong metal-support interactions.

In one embodiment, the Cu is in the form of amorphous Cu nanoclusters with an average size of approximately 4 nm.

In one embodiment, the Cu is in the form of crystalline Cu nanoparticles.

In one embodiment, the Cu is in the form of single-atomically dispersed Cu.

In one embodiment, the Cu/1T′ WS₂ hybrid material exhibits high catalytic activity and stability in electrocatalytic applications.

The present invention provides unconventional-phase Cu/1T′ WS₂ composites, including single-atomically dispersed Cu (s-Cu), amorphous Cu (a-Cu) nanoclusters, and crystalline Cu (c-Cu) nanoparticles on 1T′ WS₂ nanosheets. The as-obtained a-Cu clusters/1T′WS₂ composites show the highest performance, including excellent catalytic activity, higher selectivity, and better long-term durability than commercial catalysts.

The produced a-Cu clusters/1T′ WS₂ composites can also be used for pilot-scale catalytic applications during this stage. Moreover, it allows for scalable production by increasing the electrode area, making it ideal for large-scale manufacturing of highly active metal/TMDs composites suitable for real-world applications.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention are described in more details hereinafter with reference to the drawings, in which:

FIG. 1 shows the proposed synthetic routes for the preparation of 1T′ WS₂ supported Cu nanostructures, and the proposed tandem catalytic mechanism for NO₃RR over the s-Cu/1T′ WS₂ catalysts;

FIG. 2A shows TEM images of the exfoliated 1T′ WS₂ nanosheets. FIG. 2B shows HRTEM images of the exfoliated 1T′ WS₂ nanosheets. FIG. 2C shows SAED patterns of the exfoliated 1T′ WS₂ nanosheets;

FIG. 3A shows TEM images of the s-Cu/1T′ WS₂ nanosheets. FIG. 3B shows HRTEM images of the s-Cu/1T′ WS₂ nanosheets. FIG. 3C shows SAED patterns of the s-Cu/1T′ WS₂ nanosheets. FIG. 3D shows atomic-resolution HAADF-STEM image of s-Cu/1T′-WS₂ showing the isolated Cu atomically dispersed on 1T′-WS₂ nanosheets. FIGS. 3E-3G show simulated atomic structures and the corresponding simulated STEM images of Cusub, Cuads-S and Cuads-W, respectively. The yellow, gray and red balls in e-g represent the S, W and Cu atoms, respectively. FIG. 3H shows the intensity profiles in are taken from the corresponding yellow rectangles in the STEM images d. FIG. 3I shows HAADF-STEM image and corresponding EDS element mapping images (Cu yellow, S green, W red) of s-Cu/1T′ WS₂ nanosheets;

FIG. 4 shows EDX spectra of s-Cu/1T′ WS₂ nanosheets;

FIG. 5A shows TEM images of the a-Cu clusters/1T′ WS₂ nanosheets. FIG. 5B shows HAADF-STEM images of the a-Cu clusters/1T′ WS₂ nanosheets. FIG. 5C shows enlarged HAADF-STEM images of the a-Cu clusters/1T′ WS₂ nanosheets. FIG. 5D shows HAADF-STEM images of the a-Cu clusters/1T′ WS₂ nanosheets. FIG. 5E shows simulation of the magnified area highlighted with the orange square in FIG. 5D for a-Cu clusters/1T′ WS₂ nanosheets. FIG. 5F shows line-scanning intensity profile obtained along the yellow lines 1 and 2 in FIG. 5D for a-Cu clusters/1T′ WS₂ nanosheets. FIG. 5G shows HAADF-STEM image and corresponding EDS element mapping images of the a-Cu clusters/1T′ WS₂ nanosheets;

FIG. 6 shows EDX spectra of a-Cu/1T′ WS₂ nanosheets;

FIG. 7A shows TEM images of the c-Cu/1T′ WS₂ nanosheets. FIG. 7B shows HRTEM images of the c-Cu/1T′ WS₂ nanosheets. FIG. 7C shows SAED patterns of the c-Cu/1T′ WS₂ nanosheets. FIG. 7D shows EDS image of the c-Cu/1T′ WS₂ nanosheets;

FIG. 8 shows EDX spectra of c-Cu/1T′ WS₂ nanosheets;

FIG. 9 shows XRD spectra of various Cu/1T′ WS₂ nanosheets;

FIG. 10A shows Raman spectra of s-Cu/1T′ WS₂, a-Cu/1T′ WS₂, and c-Cu/1T′ WS₂ nanosheets. FIG. 10B shows Cu 2p XPS spectra of s-Cu/1T′ WS₂, a-Cu/1T′ WS₂, and c-Cu/1T′ WS₂ nanosheets;

FIG. 11 shows Cu LMM Auger spectra of various Cu/1T′ WS₂ nanosheets;

FIG. 12A shows high-resolution XPS spectra of W 4d on various Cu/1T′ WS₂ nanosheets. FIG. 12B shows high-resolution XPS spectra of S 2p on various Cu/1T′ WS₂ nanosheets;

FIG. 13A shows XANES spectra of s-Cu/1T′ WS2, a-Cu/1T′ WS₂, c-Cu/1T′ WS₂, and reference materials at Cu K-edge. FIG. 13B shows FT-EXAFS curves of s-Cu/1T′ WS₂, a-Cu/1T′ WS₂, c-Cu/1T′ WS₂, and reference materials at Cu K-edge. FIG. 13C show first-shell fitting of EXAFS spectra of s-Cu/1T′ WS₂, a-Cu/1T′ WS₂, and c-Cu/1T′ WS₂ nanosheets;

FIG. 14 shows linear sweep voltammetry curves of 1T′ WS₂ and a-Cu/1T′ WS₂ nanosheets in 0.1 M K₂SO₄ electrolyte with and without NO₃;

FIG. 15A shows the UV-vis curves for detecting NH₄⁺ based on the indophenol blue spectrophotometry. FIG. 15B shows the standard curves with different NH₄⁺ concentrations;

FIG. 16A shows potential-dependent FE for ammonia and over pure 1T′ WS₂, s-Cu/1T′ WS₂, a-Cu/1T′ WS₂, and c-Cu/1T′ WS₂ nanosheets. FIG. 16B shows yield rate for ammonia and over pure 1T′ WS₂, s-Cu/1T′ WS₂, a-Cu/1T′ WS₂, and c-Cu/1T′ WS₂ nanosheets;

FIG. 17 shows the consecutive recycling test at −0.7 V over a-Cu/1T′ WS₂ catalysts;

FIG. 18 shows the plot of current density and FE for ammonia against time during a 10 h test over the a-Cu/1T′ WS₂ catalysts;

FIGS. 19A-19B show the structural characterization of a-Cu/1T′ WS₂ catalysts after a 10 h stability test. (FIG. 19A) TEM and (FIG. 19B) HRTEM;

FIG. 20 shows 1H NMR spectra of a-Cu/1T′ WS₂ catalysts at −0.6 V vs RHE in blank K₂SO₄ solutions without the NO₃⁻;

FIG. 21A shows 1H NMR spectra of the electrolyte after electrocatalysis using 15NO₃⁻ and 14NO₃⁻ as the nitrogen source. FIG. 21B shows the standard curve of integral area (¹⁵NH₄⁺-¹⁵N/C₄H₄O₄) against ¹⁵NH₄⁺-¹⁵N concentration and integral area (¹⁴NH₄⁺-¹⁴N/C₄H₄O₄) against ¹⁴NH₄⁺-¹⁴N concentration. FIG. 21C shows a comparison of the FE for ammonia when detecting with NMR method and UV-vis method;

FIGS. 22A-22B show LSV curves of a-Cu/1T′ WS₂ and pure 1T′ WS₂ in H₂O and D₂O;

FIG. 23A shows in situ Raman spectra of 1T′ WS₂ nanosheets for NO₃RR at different overpotentials. FIG. 23B shows in situ Raman spectra of a-Cu/1T′ WS₂ nanosheets for NO₃RR at different overpotentials;

FIG. 24 shows on-line DEMS measurements of nitrate reduction over a-Cu/1T′ WS₂ nanosheets; and

FIG. 25 shows the proposed tandem catalysis mechanism for NO₃RR based on the a-Cu/1T′ WS₂ catalysts.

DETAILED DESCRIPTION

In the following description, unconventional-phase Cu/1T′ WS₂ composites and the likes and facile electrochemical methods are set forth as preferred examples. It will be apparent to those skilled in the art that modifications, including additions and/or substitutions may be made without departing from the scope and spirit of the invention. Specific details may be omitted so as not to obscure the invention; however, the disclosure is written to enable one skilled in the art to practice the teachings herein without undue experimentation.

Despite its potential, the NO₃RR process faces significant challenges, such as sluggish reaction kinetics and the need for highly efficient catalysts. Currently, various materials have been explored as potential substrates for preparing atomically dispersed catalysts. For example, the SACs have shown promise in addressing these challenges, but their instability, especially due to weak metal-support interactions, limits their performance. Metal oxides with abundant surface defects, unsaturated sites at boundaries, have been widely employed as substrates to stabilize single metal atoms, while their limited specific surface areas restrict the loading density of metal atoms (<5 wt %). Additionally, while the TMDs have emerged as promising support materials, their conventional phases, such as 2H, exhibit weak interactions with metal atoms, reducing their efficacy. The unconventional-phase TMDs, like the 1T′ phase, show improved catalytic properties, but precise control over the metal species' size, distribution, and stability on these substrates remains a major hurdle. Therefore, effective methods to control metal/TMD interfaces and optimize metal-support interactions are essential for the development of high-performance electrocatalysts for ammonia synthesis and other important reactions.

Recently 2D materials such as graphene, hexagonal boron nitrogen, graphitic carbon nitride (g-C₃N₄), and the TMDs have emerged as promising candidates. Due to their ultrahigh specific surface areas, 2D materials provide a suitable platform for supporting high-density single metal atoms. Unfortunately, the surfaces of these 2D materials are typically chemically inert, resulting in weak interaction with the anchored metal atoms. Therefore, it is of high significance to develop 2D material that can stabilize single-atomically dispersed metals without introducing additional defects or heteroatoms.

Accordingly, the present invention provides a facile electrochemical method to control the growth of metallic nanostructures (e.g., Cu) on unconventional-phase TMDs (e.g., 1T′ WS₂). This approach allows the formation of various Cu nanostructures on 1T′ WS₂ nanosheets, including single-atomically dispersed Cu (s-Cu), amorphous Cu (a-Cu) nanoclusters, and crystalline Cu (c-Cu) nanoparticles. Also, the technique can be extended to other unconventional-phase TMDs, including 1T′ MoS₂, 1T′ MoSe₂, and 1T′ WSe₂. The synthesis of Cu nanostructures on other 1T′ TMDs follows the aforementioned typical protocol except for changing the 1T′ WS₂ templates to the corresponding 1T′ MoS₂, 1T′ MoSe₂, and 1T′ WSe₂.

The Cu/1T′ TMDs structures produced using this method exhibit excellent performance in various catalytic applications. The defect-free WS₂ nanosheets in the unconventional phase (1T′) can effectively immobilize Cu owing to its unique electrophilic property as compared to the conventional 2H phase. For the s-Cu/1T′ WS₂ hybrids and a-Cu/1T′ WS₂ hybrids, Cu are mainly in the form of Cu—S bonds, corresponding to the strong metal-support interaction, which is different from c-Cu nanoparticles/1T′ WS₂ with more pronounced obstruction from metal-metal interactions.

The as-prepared a-Cu clusters/1T′ WS₂ composites exhibit superior activity and selectivity for the electrocatalytic nitrate reduction reactions to produce high value-added ammonia. At −0.6 V vs RHE, the a-Cu clusters/1T′ WS₂ nanosheets achieve a selectivity of 97.2% for the production of ammonia from nitrate-containing waste water.

Additionally, s-Cu/1T′ MoS₂ demonstrates high current density and low overpotential in electrocatalytic hydrogen evolution reactions (HER). The s-Cu/1T′ MoS₂ nanostructures also perform well in HER, with low overpotentials and fast kinetics.

Compared to the s-Cu/1T′ WS₂ and c-Cu/1T′ WS₂, the significantly enhanced electrocatalytic performance of a-Cu clusters/1T′ WS₂ can be attributed to the maximum interaction between the isolated Cu and 1T′ WS₂ support, which facilitates the generation of adsorbed hydrogen, thus promoting the reduction of nitrate and facilitated the reaction kinetics.

The in-situ Raman and in-situ differential electrochemical mass spectrometry (DEMS) characterization unravel the tandem catalysis mechanism of the a-Cu clusters/1T′ WS₂ catalyst during nitrate reduction reaction. The deposited Cu preferentially stabilizes nitrogen intermediate species, whereas the 1T′ WS₂ support supplies the activated H species required for the surface hydrogenation to proceed. The adsorbed hydrogen species coming from the 1T′ WS₂ couple with the adsorbed nitrate molecules on Cu sites to form ammonia with lower energy barrier.

Moreover, the stability tests shows that the a-Cu clusters/1T′ WS₂ can be recycled more than 6 times without obvious performance decay, indicating the great potential of the catalysts for future industrial application.

1T′ MoS₂, 1T′ MoSe₂, and 1T′ WSe₂ are also unconventional-phase TMDs that exhibit unique electronic properties and structural advantages, making them excellent substrates for catalytic applications. 1T′ MoS₂ and 1T′ MoSe₂ have shown exceptional performance in HER, demonstrating low overpotentials and high current densities. 1T′ WSe₂ offers unique electronic interactions that are beneficial for electrochemical reactions requiring multi-electron transfer, such as nitrate reduction.

By employing the same electrochemical deposition techniques developed for 1T′ WS₂, these materials can support various Cu nanostructures, including single atoms, amorphous clusters, and nanoparticles.

In another aspect, the present invention also provides a facile, large-scale and cost-effective electrochemical method for the controlled growth of various metallic nanostructures (e.g., Cu) on unconventional-phase TMDs (e.g., 1T′ WS₂, 1T′ MoS₂, 1T′ MoSe₂, and 1T′ WSe₂) to construct novel unconventional-phase Cu/TMD composites, which can be used for the electrochemical conversion of waste nitrate ions to produce ammonia. This method possesses several advantages over the existing synthetic technologies for growing metal heterostructures on unconventional-phase TMDs support, including easy operation, fine controllability, broad applicability, non-damage to support, and mild experimental conditions.

The developed synthetic method is straightforward and easy to operate, reducing overall synthesis costs.

The synthetic method enables precise control over the composition and structure of the deposited Cu species on 1T′ TMDs. The controlled synthesis of the aforementioned unconventional-phase metal/TMD nanostructures can be achieved by finely tuning the operation parameters, including metal precursors, ligands, solvents, applied voltage, and deposition time. Thus, the size and dispersion of Cu nanostructures can be easily adjusted.

The method of the present invention achieves high purity and uniformity in the resulting metal species, including single atoms, clusters, or nanoparticles, under mild conditions such as room temperature and atmospheric pressure. This minimizes the formation of impurities and defects in the 1T′ TMDs substrate, ensuring high-quality metal/TMDs hybrids.

The upcoming description will explore the growth mechanism of metallic nanostructures on unconventional-phase TMDs, with an emphasis on the interfacial electronic interaction between metal and 1T′ WS₂, as well as the catalytic mechanism in different Cu/1T′ WS₂ nanosheets to enhance electrocatalytic performance. The insights gained from this research will help guide the controlled growth of other nanosized materials, such as single atom alloys, binary clusters, and high-entropy alloys, on unconventional-phase TMDs for various catalytic applications.

EXAMPLE

Example 1

Methods

Material Characterizations

Transmission electron microscopy (TEM) and high-resolution TEM images, selected area electron diffraction (SAED) patterns, STEM images and the corresponding energy dispersive X-ray spectroscopy (EDS) data are recorded on JEOL JEM-2100F (JEOL) at an acceleration voltage of 200 kV. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images are obtained on a JEOL ARM-200F (JEOL) operated at 200 kV with cold field emission gun and double-hexapole spherical aberration correctors (CEOS GmbH). Scanning electron microscopy images and the corresponding EDS spectra are recorded on JEOL JSM-7600F (JEOL). Atomic force microscopy (Cypher; Asylum Research) is used to characterize the thickness of 1T′-WS₂ nanosheets in tapping mode. ICP-OES is performed on a Dual-View Optima 5300 DV ICP-OES system. X-ray diffraction spectroscopy (XRD) is collected via an X-ray diffractometer of Japan Rigaku D/max-2500pc with monochromatic Cu Kα radiation (λ=1.5406 Å). Raman scattering spectra are obtained by using a Renishaw System 2000 spectrometer using the 514.5 nm line of an Ar⁺ laser for excitation. X-ray photoelectron spectroscopy (XPS) measurements are conducted on the ESCALAB 250Xi (Thermo Fisher Scientific) instrument. XANES and EXAFS spectra of Cu at the K-edge are carried out at the beamline BL14W station of the Shanghai Synchrotron Radiation Facility, China. The Cu K-edge XANES data are recorded in a fluorescence mode. Cu foils and CuS are used as the references. The acquired EXAFS data are extracted and processed according to the standard procedures using the ATHENA module. The k³-weighted EXAFS spectra are obtained by subtracting the post-edge background from the overall absorption and then normalizing with respect to the edge-jump step. Ultraviolet-visible absorption spectra are recorded on a UV-2700 (Shimadzu) with Suprasil quartz glass cuvettes (111-QS; Hellma Analytics) at room temperature.

Electrocatalytic Nitrate Reduction

Electrochemical measurements are carried out in a customized H-type glass cell separated by Nafion 117 membrane (DuPont) at room temperature. In a typical three-electrode system, an Ag/AgCl electrode and a Pt grid are used as the reference and counter electrode, respectively. All potentials in the present invention are measured against the Ag/AgCl and converted to the RHE reference scale by:

E ⁡ ( V ⁢ vs . RHE ) = E ⁡ ( V ⁢ vs . Ag / AgCl ) + 0.0591 × pH + 0.198 .

The working electrode is prepared as follows: 2 mg of a-Cu nanoclusters/1T′ WS₂ nanosheets are dispersed in 30 μL of Nafion (5 wt %) and 970 μL of isopropanol/H₂O (7:3) and sonicated for 30 min to obtain a well-dispersed catalyst ink. Then, 125 μl of catalyst ink is uniformly dropped onto a carbon paper substrate with an area of 0.5*0.5 cm². Electrochemical experiments are performed using a CHI 760e electrochemical workstation in a customized H-type cell separated by a nafion-117 proton exchange membrane. In a typical three-electrode system, an Ag/AgCl electrode and a 2*2 cm² Pt grid are used as the reference and counter electrode, respectively.

For the NO₃RR, a solution with 0.5 M K₂SO₄ and 0.1 M KNO₃ is used as the electrolyte unless otherwise specified and is evenly distributed to the cathode and anode compartment. The electrolyte volume in both sections of the H-cell is 10 mL, and it is purged with high-purity Ar for 30 minutes prior to measurement. Linear sweep voltammetry (LSV) tests are conducted at a scan rate of 5 mV s⁻¹ over a potential interval from 0 to −1.4 V vs RHE. Chronoamperometry tests are performed at different potentials ranged from −0.6 V to −1.0 V vs RHE for 2 hrs. High-purity Ar is continuously fed into the cathodic compartment during the experiments. The evolved gaseous product is analyzed by a gas chromatography (GC, Agilent 7890A) equipped with a thermal conductive detector using nitrogen as the carrier gas. The solution products are identified by ion chromatography (Metrohm 930) and NMR (Bruker 400M). Cyclic voltammetry (CV) tests at different scan rates are performed to determine the electrochemical active surface area (ECSA) of different catalysts under a potential window where no Faraday reaction occurs, and then the geometric double layer capacitance (C_dl) is determined by calculating the current density difference ΔJ=(J_anodic−J_cathodic)/² as a linear correlation with the scan rate at a given potential. The catalytic durability of a-Cu clusters/1T′ WS₂ is evaluated by 6 consecutive cycles at −0.8 V vs RHE.

Calculation of the FE, NH₃ Yield Rate, and Turnover Frequency (TOF)

The FE, yield rate (Y), partial current density (J) and TOF of NO₃RR for the products NH₃ and NO₂⁻ are calculated as follows:

FE = ( n × V × C × F ) / ( M × Q ) Y = ( C × V ) / ( M × t × A ) J = ( Q × FE ) / ( A × t ) TOF = ( J × A ) / ( n × N × F )

where n is the electron transfer number required to form the products, which is 8 for NH₃ and 2 for NO₂⁻, V is the catholyte volume, C represents the mass concentration of the generated products, F is the Faraday constant (96485 C mol⁻¹), M represents the molar mass of the products, which is 17 for NH₃ and 46 for NO₂⁻, t is the reaction time, N represents the molar amount of Cu atoms, A is the geometric area of working electrode and Q is the total charge passed through the electrodes during the electrolysis.

Product Determination

I. Ammonia Determination by Colorimetric Methods

Concentration of produced ammonia is spectrophotometrically determined by the indophenol blue method. In detail, the electrolyte after the 2 hrs reaction is first diluted with 0.5 M K₂SO₄ solution to ensure that the concentration of ammonia to be detected is within the linear range of the indophenol blue method. Then, 2 mL of the diluted solution is successively mixed with 2 mL of 0.5 M K₂SO₄ solution containing 5 wt % salicylic acid and 5 wt % sodium citrate, 1 mL of 0.05 M NaClO and 0.2 mL of 1 wt % C₅FeN₆Na₂O (sodium nitroferricyanide) aqueous solution and shaken well. After standing in the dark at room temperature for 1 h, the absorbance of ammonia is recorded with an UV-Vis spectrophotometer in the wavelength range of 550-750 nm. The calibration curve for the NH₃ determination is plotted by linearly fitting the concentrations of a range of NH₃ standard solutions (1.0, 2.0, 3.0, 4.0 and 5.0 g mL⁻¹) to their absorbance values at 655 nm.

II. Nitrite Quantification and Nitrate Quantification by Ion Chromatography

The concentration of NO₂⁻ in the solution after electrolysis is first diluted to the detection range and then quantified by ion chromatography (Metrohm 930) equipped with an anion column (Metrosep A Supp 10-250/4.0). To plot the corresponding calibration curve, a series of solutions with known NO₂ concentrations of 0.1, 0.2, 0.4, 0.8, 2.0 and 4.0 μg mL⁻¹ are used as standard solutions. The concentration of NO₃⁻ in the solution is diluted and quantified by ion chromatography as described above. To plot the corresponding calibration curve, a series of solutions with known NO₃⁻ concentrations of 0.1, 0.2, 0.4, 0.8, and 1.0 μg mL⁻¹ are used as standard solutions.

III. Isotope Labeling Experiments

The NH₃ concentration is also quantitatively determined by ¹H nuclear magnetic resonance (NMR, 400 MHz) with using DMSO-d₆ as a solvent and maleic acid (C₄H₄O₄) as the internal standard. Specifically, after electrolysis at −0.8 V vs RHE for 1 hr, the collected electrolyte is first diluted to the detection range with 0.5 M K₂SO₄ solution and acidified to pH 2 by adding 0.01 M hydrochloric acid. After that, 0.5 mL of the standard solution is mixed with 0.1 mL DMSO-d₆ (with 0.04 wt % C₄H₄O₄) for NMR measurements. The calibration curve is achieved using the peak area ratio between NH₄⁺ and C₄H₄O₄ because the NH₄⁺ concentration and the area ratio are positively correlated. All analyses are performed under water-suppressed conditions.

For the ¹⁵N isotope-labeling experiment, 0.50 M K₂SO₄/0.10 M K¹⁵NO₃ (98 atom % ¹⁵N) mixed solution are used as the electrolyte to clarify the source of NH₃. After ¹⁵NO₃⁻ electroreduction for 1 h at −0.8 V (vs. RHE), the obtained ¹⁵NH₄⁺ is tested by ¹H NMR. The NMR test method of ¹⁵NH₄⁺ is the same to that of ¹⁴NH₄⁺.

In Situ Spectroscopy Characterizations

The in situ Raman spectroscopy measurements are conducted on the Horiba LabRam HR Evolution Raman spectrometer equipped with a 633 nm excitation laser, and the power is controlled at 2.5 mW. Before the experiments, calibration is carried out based on the peak at 520 cm⁻¹ of a silicon wafer standard. A typical three-electrode system in which Ag/AgCl and a platinum wire serves as the reference and counter electrodes are applied to obtain in situ Raman spectra of catalysts at different potentials from 0 V to −1.0 V vs RHE.

In situ ATR-FTIR tests are performed in an H-type three-electrode electrochemical cell, where the cathode and anode chambers are separated by a Nafion membrane. The sample ink is drop-coated on the surface of a silicon crystal chemically deposited with a gold film, and the whole is used as the working electrode. The counter electrode and reference electrode consist of platinum wire and saturated Ag/AgCl, respectively. The cell is integrated into a FTIR spectrometer equipped with a liquid nitrogen-cooled MCT detector. LSV tests are conducted by a CHI 760E electrochemical workstation between 0 and −1.0 V at a scan rate of 5 mV s⁻¹. Each spectrum consists of 32 single beams with a resolution of 8 cm⁻¹.

Example 2

Synthesis of Single-Atomically Dispersed Cu on 1T′ WS₂ Nanosheets

All the chemicals including solvents, precursors and surfactants were available from commercial sources. The deposition is carried out by the operation of electrochemical workstation. Briefly, the method used for the synthesis of unconventional-phase metal/TMDs hybrids includes the synthesis of 1T′ TMDs nanosheets as templates, Cu salts as metal precursors, acidic solution as electrolyte, and penicillamine as surfactant. Through varying the potential applied on the catalysts-loaded electrode, the s-Cu, a-Cu nanoclusters, and c-Cu nanoparticles can successfully grow on the 1T′ WS₂ substrate. In addition, the synthetic method used for other unconventional-phase Cu/1T′ TMDs nanostructures includes the 1T′ MoS₂, 1T′ MoSe₂, and 1T′ WSe₂ as template, Cu-containing solution as electrolyte, and the applied voltage as regulator.

1. Synthesis of 1T′ WS₂ Nanosheets

Before the deposition of metal, pure 1T′ WS₂ nanosheets are firstly prepared by the electrochemical exfoliation of bulk 1T′ WS₂ crystals. The electrochemical intercalation process is conducted in a two-electrode electrochemical cell.

The K_xWS₂ crystals are synthesized according to the previously reported method⁴. The electrochemical intercalation process is conducted in a two-electrode electrochemical cell. Firstly, the K_xWS₂ crystals and PVDF as a binder are mixed in DMF, the mixture is uniformly coated on a copper foil and dried under vacuum, which is then used as the cathode. The mass ratio of K_xWS₂ crystals, PVDF and DMF is 8:1:80. The graphite rod is used as the anode. The tetraheptylammonium bromide, which is dissolved in acetonitrile with a concentration of 5 mg mL⁻¹, served as the electrolyte. The intercalation process is performed at an applied voltage of 10 V for 1 h. The intercalated sample is then transferred into a centrifuge tube followed by sonication in 5 mL of DMF for less than 5 s. The dispersion is centrifuged at 6,000 rpm for 10 min. The obtained precipitate is redispersed in 5 mL of Milli-Q water. The final 1T′ WS₂ nanosheets is collected by centrifugation at 6,000 rpm for 10 mins and redispersed in Milli-Q water for further usage.

2. Preparation of s-Cu on 1T′ WS₂ Nanosheets

The s-Cu/1T′-WS₂ composites are prepared using a three-electrode configuration. The 1T′ WS₂ substrate serves as the working electrode, a platinum wire is used as the counter electrode, and an Ag/AgCl electrode functions as the reference electrode. Cu underpotential deposition is performed via varying the applied voltage from 0.1 V to 0.6 V vs RHE in an Ar-saturated 5 mM CuSO₄ in 0.1 M H₂SO₄. After the deposition, the sample is thoroughly rinsed with deionized water and dried under a gentle stream of nitrogen.

3. Preparation of a-Cu Nanoclusters on 1T′ WS₂ Nanosheets

For synthesis of a-Cu/1T′ WS₂ composites, appropriate amounts of penicillamine are introduced in the electrolyte before the Cu deposition. Then, Cu overpotential deposition is performed on 1T′ WS₂ nanosheets via varying the applied voltage from −0.1 V to 0.6 V vs RHE in an Ar-saturated 10 mM CuSO₄ in 0.1 M H₂SO₄. After the deposition, the sample is thoroughly rinsed with deionized water and dried under a gentle stream of nitrogen.

4. Preparation of c-Cu Nanoparticles on 1T′ WS₂Nanosheets

For synthesis of c-Cu/1T′ WS₂ composites, Cu underpotential deposition is performed via varying the applied voltage from −0.1 V to 0.6 V vs RHE in an Ar-saturated 10 mM CuSO₄ in 0.1 M H₂SO₄. After the deposition, the sample is thoroughly rinsed with deionized water and dried under a gentle stream of nitrogen.

FIG. 1 shows the preparation of Cu nanostructures on unconventional-phase TMDs through a modified electrochemical method. First, few-layer 1T′ WS₂ nanosheets are obtained through the electrochemical exfoliation of high-purity 1T′ WS₂ bulk materials (FIGS. 2A-2C). Subsequently, three different types of Cu nanostructures (s-Cu, a-Cu and c-Cu), are deposited onto the surface of the 1T′ WS₂ nanosheets by adjusting the deposition overpotential or adding functional ligands. The underpotential deposition (UPD) technique is employed to facilitate the formation of favorable metal-support bonds, resulting in the growth of s-Cu. Additionally, a-Cu nanoclusters and c-Cu nanoparticles are deposited on the 1T′ WS₂ nanosheets using overpotential deposition (OPD) methods, with or without modification of ligands.

As determined by the inductive coupled plasma-optical emission spectroscopy (ICP-OES), the Cu loadings on s-Cu/1T′ WS₂ are calculated to be approximately 4.1%.

Example 3

Characterization of Single-Atomically Dispersed Cu on 1T′ WS₂ Nanosheets

The morphology of the prepared s-Cu/1T′ WS₂ sample is characterized using transmission electron microscopy (TEM). FIG. 3A illustrates the presence of large, semitransparent nanosheets with a characteristic ultrathin, planar-like morphology. No discernible Cu-containing clusters/nanoparticles are observed on the 1T′ WS₂ nanosheets. High-resolution TEM (HRTEM) images reveal the lattice fringe of s-Cu/1T′ WS₂, with some bright spots appearing on the surface of the s-Cu/1T′ WS₂ (FIG. 3B). The SAED pattern exhibits the distorted octahedral coordination feature of the 1T′ structure, with no diffraction spots assigned to Cu (FIG. 3C). Aberration-corrected high-angle annular dark-field-scanning TEM (HAADF-STEM) images confirm the presence of atomically dispersed Cu atoms (bright spots) on the 1T′ WS₂ nanosheets (FIG. 3D). These spots are identified as single-atom Cu due to the enhanced Z-contrast via the deposition of Cu on the substrate. The magnified STEM images reveal that atomically dispersed Cu atoms occupy three different locations: Cu adsorbed on the top site of S (Cu_ads—S), Cu adsorbed on the hollow site of W—S(Cu_hollow), and Cu adsorbed on the top site of W (Cu_ads—W) (FIGS. 3E-3G). Turning to FIG. 3H, the corresponding intensity profiles from the highlighted region in the STEM images align well with the two typical configurations of Cu on s-Cu/1T′ WS₂, that is the Cu_ads-W site and the Cu_hollow site. Furthermore, the uniform distribution of Cu signals observed in the EDS spectra and elemental mappings suggests the homogeneous dispersion of atomic Cu on the 1T′ WS₂ matrix (FIG. 3I and FIG. 4).

The a-Cu subnanoclusters are grown on 1T′ WS₂ nanosheets through the deposition of Cu atoms on the 1T′ WS₂ nanosheets with ligand modification, obtaining a-Cu/1T′ WS₂. TEM image illustrates the formation of high-dense dispersed Cu clusters on the surface of 1T′ WS₂ nanosheets (FIG. 5A). In FIG. 5B, the magnified TEM image displays the size distribution of the Cu subnanoclusters, with an average size of approximately 4 nm.

Interestingly, the SAED patterns of a-Cu/1T′ WS₂ show the absence of Cu-related patterns, indicating the formation of amorphous structure (FIG. 5C). HAADF-STEM is used to further investigate the different forms of Cu on 1T′ WS₂ by exploiting contrast differences. As shown in FIG. 5D, numerous tiny Cu nanoclusters are observed on the surface of the a-Cu/1T′ WS₂ nanosheets. These Cu nanoclusters are highly dispersed without the formation of obvious lattice fringe.

To gain a deeper understanding of the structures of a-Cu/1T′ WS₂, in FIG. 5E, the simulated HAADF-STEM image of a typical Cu cluster shows the lattice spacing of Cu, which closely matches that of 1T′ WS₂, suggesting the epitaxial growth of the Cu layer on the 1T′ WS₂. The intensity profiles from the marked region in the STEM images (FIG. 5F) exhibit a significant increase in intensity following the deposition of Cu, confirming the presence of Cu on the 1T′ WS₂(FIG. 5G). The corresponding elemental mapping reveals the dispersion of Cu across the entire 1T′ WS₂ nanosheets. The Cu content, as determined by EDS and ICP-OES, is in good agreement, with values of 9.9% and 9.5%, respectively (FIG. 6).

As shown in FIG. 7A, in the absence of ligand modification, direct electrodeposition results in the formation of Cu nanoparticles on the 1T′ WS₂ nanosheets. There is a characteristic lattice fringe of 0.21 nm, corresponding to Cu (100) plane. The lattice fringe can be distinguished from the characteristic lattice spacing of 0.32 nm, which are attributed to (100) facets of 1T′ WS₂ (FIG. 7B). The SAED patterns of c-Cu/1T′ WS₂ clearly exhibit distinct reflections corresponding to the (100) facets of Cu (FIG. 7C). EDS mapping reveals a homogenous distribution of Cu across the entire surface of the 1T′ WS₂ nanosheets (FIG. 7D). The corresponding EDS spectra displays a Cu content of 12.8%, which is in close agreement with the ICP-OES value of 12.3% (FIG. 8).

The structures of Cu/1T′ WS₂ samples are further analyzed through the combination of XRD, Raman spectroscopy, and XPS. As shown in FIG. 9, the XRD patterns of both s-Cu/1T′ WS₂ and a-Cu/1T′ WS₂ exhibit a characteristic peak that matches the signature of pure 1T′ WS₂. Along with the prominent 1T′ WS₂ peaks, the XRD patterns of a-Cu/1T′ WS₂ also display distinct diffraction peaks at 26.8° and 34.5°, corresponding to the (111) and (100) planes of metallic Cu.

Turning to FIG. 10A, Raman spectra of the as-prepared 1T′ WS₂ shows six distinctive peaks located at 124, 182, 244, 271, 317, and 409 cm⁻¹, respectively, which are consistent with those of previous reported 1T′ WS₂. Notably, the peaks at 127.4, 244, 271, 317, and 409 cm⁻¹ exhibit a significant enhancement after the deposition of Cu, which could be attributed to Cu-induced surface-enhanced Raman scattering (SERS) effects.

As shown in FIG. 10B, the surface chemical environment of Cu/1T′ WS₂ nanostructures are analyzed by XPS. The high-resolution Cu 2p spectra of s-Cu/1T′ WS₂ and a-Cu/1T′ WS₂ show prominent peaks centered at 932.2 eV and 952.0 eV, which are associated with metallic/monovalent Cu. For the c-Cu/1T′ WS₂, the dominant peaks at 934 eV and 953.8 eV are assigned to divalent Cu due to oxidation. Due to the close binding energies of Cu(0) and Cu(I) in the Cu 2p XPS spectrum, the LMM Auger spectrum is characterized (FIG. 11). Both peaks centered at 916.3 eV demonstrate the predominance of Cu(I) in Cu/1T′ WS₂. The binding energies of W 4f and S 2p in the 1T′ WS₂ supports exhibit a negligible shift after the deposition of Cu (FIGS. 12A-12B).

X-ray absorption spectroscopy (XAS), including X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS), are employed to investigate the local electronic structure of Cu in Cu/1T′ WS₂. The white line of Cu K-edge for a-Cu/1T′ WS₂ shifts to higher energy compared to metallic Cu, positioning itself between CuS (FIG. 13A), indicating a positive oxidation state of Cu in a-Cu/1T′ WS₂. Furthermore, the first derivative of the XANES curve for Cu/1T′ WS₂ exhibits a distinct peak closest to CuS, confirming the nature of Cu in Cu/1T′ WS₂, which aligns with the XPS analysis.

Since the presence of crystallized Cu species has been excluded by TEM and XRD, Cu(I) should be attributed to single-atom Cu, with its oxidation state resulting from partial electron transfer to the coordinated atoms of 1T′ WS₂ To further investigate the coordination environment of Cu, the local atomic structures of c-Cu/1T′ WS₂, a-Cu/1T′ WS₂, ands-Cu/1T′ WS₂ are evaluated using EXAFS (FIG. 13B). The main peak of s-Cu/1T′ WS₂ and a-Cu/1T′ WS₂ is centered at 1.8 Å in the R space, corresponding to the Cu—S bond. No obvious peaks assigned to Cu—Cu coordination are observed, indicating that the Cu atoms are atomically dispersed, consistent with the HAADF-STEM observation. Consistent with the Cu K-edge FT-EXAFS spectra, the Cu K-edge wavelet transform EXAFS analysis further confirmed the successful stabilization of isolated Cu sites through S coordination on the s-Cu/1T′ WS₂ and a-Cu/1T′ WS₂ supports (FIG. 13C). In contrast, the c-Cu/1T′ WS₂ exhibits a distinct Cu—Cu bond at 2.2 Å, indicating the formation of metallic Cu. Together with the structural and spectroscopic characterizations, these results clearly confirm the successful stabilization of isolated Cu sites on the surface of electrophilic 1T′ WS₂ through the Cu—S interaction.

Example 4

Cu/1T′ WS₂Nanosheets as Highly Efficient Catalysts for Nitrate Reduction to Ammonia

As a proof-of-concept, the Cu/1T′ WS₂ nanosheets are utilized as catalysts for the NO₃RR in a 0.5 M K₂SO₄ solution using a standard three-electrode electrochemical system. For comparison, the NO₃RR activities of pure 1T′ WS₂ and other control samples are also measured under identical experimental and electrochemical conditions. FIG. 14 shows the LSV curves on the Cu/1T′ WS₂ nanosheets with and without the addition of 0.1 M KNO₃ as nitrogen source. The current density of Cu/1T′ WS₂ nanosheets exhibits a drastic increase in the presence of nitrate.

Next, chronoamperometry tests are performed over a potential ranging from −0.6 to −1.0 V vs RHE. Indophenol blue spectrophotometry is used to determine the concentration of NH₄⁺, and the possible by product NO₂⁻ is quantified via ion chromatography (FIGS. 15A-15B). To evaluate the selectivity for specific products, the potential-dependent faradaic efficiencies (FEs) of NH₄⁺ are presented in FIG. 16A. Specifically, at −0.8 V vs RHE, a-Cu/1T′ WS₂ demonstrates a maximum FE of 99.2% for NH₄⁺, rendering it as a promising catalyst for NO₃RR. The decrease in FE at more negative potentials can be attributed to the competing HER. Compared to pure 1T′ WS₂, s-Cu/1T′ WS₂ and c-Cu/1T′ WS₂, the FE of a-Cu/1T′ WS₂ exhibits a significant improvement. Furthermore, the yield rate (Y_NH3) and partial current density (J_NH3) of NH₃ on the Cu/1T′ WS₂ nanosheets are calculated, as shown in FIG. 16B. Impressively, the a-Cu/1T′ WS₂ sample exhibits a superior NH₃ yield rate and partial current density, reaching up to 253.7 mol h⁻¹ cm⁻² at −0.8 V vs RHE, significantly surpassing those of c-Cu/1T′ WS₂ and s-Cu/1T′ WS₂. These values render a-Cu/1T′ WS₂ nanosheets as one of the most efficient catalysts for nitrate-to-ammonia conversion among all reported Cu-based catalysts (Table 1).

The durability of the a-Cu/1T′ WS₂ catalyst is assessed by performing continuous NO₃RR electrolysis at −0.8 V vs RHE. As shown in FIG. 17, the NH₃ yield rate and FE remain stable over 6 consecutive rounds of the reaction, with no apparent decay trend. When the electrolytes after electrolysis are stained with an indophenol blue indicator, they all show an almost identical color, further demonstrating the excellent catalytic stability of the Cu-based material. The FE of 99.2% for ammonia on a-Cu/1T′ WS₂ nanosheets surpasses that of the state-of-the-art NO₃RR catalysts. The long-term and continuous electrocatalytic NO₃RR of a-Cu/1T′ WS₂ is also explored by continuously flowing electrolyte to replenish the constantly consumed NO₃—, and the FE can still maintain 92% after a 10-hour test, demonstrating excellent stability (FIG. 18). The TEM image of a-Cu/1T′ WS₂ after NO₃RR for 10 h proves the stability of the structure (FIGS. 19A-19B).

To determine the source of the nitrogen incorporated into the synthesized ammonia, several control experiments are performed. Initially, FIG. 20 shows that no NH₃ is detected in the ¹H NMR spectra when blank K₂SO₄ solutions are used for electrolysis, which confirms that NH₃ is exclusively produced through the electroreduction of NO₃—.

To unambiguously verify that ammonia originates from NO₃—, isotope labelling experiments are carried out, where NO₃RR is performed in the presence of ¹⁴NO₃⁻ or “NO₃⁻, followed by product identification and quantifications via ¹H NMR. As shown in the ¹H NMR spectra (FIG. 21A), when ¹⁴NO₃⁻ is used as the reactant for the electrolysis reaction, the ¹H NMR spectrum of the detected NH₄⁺ exhibits a typical three-peak feature with a coupling constant of 52.0 Hz. When ¹⁴NO₃⁻ is replaced by ¹⁵NO₃⁻, the ¹H NMR spectrum of the detected is NH₄⁺ exhibits a typical double-peak feature with a coupling constant of 73.1 Hz. The ¹⁵N isotope-labeling results strongly suggest that the NH₃ detected in the electrolyte originates from the NO₃RR. The concentrations of detected ¹⁴NH₄⁺ and ¹⁵NH₄⁺ using ¹⁴NO₃⁻ or ¹⁵NO₃⁻ as nitrogen sources are found to be nearly the same (FIG. 21B). Moreover, the calculated FEs of ammonia measured by colorimetric method and ¹H NMR spectra method are compared, with consistent results (FIG. 21C). These data further corroborate that NO₃⁻ in solution is the only source of nitrogen during the electrochemical synthesis of ammonia.

Example 5

Mechanistic Insights into the Enhanced Performance of a-Cu/1T′ WS₂ Electrocatalyst for NO₃RR

The origin of the remarkable performance of the a-Cu/1T′ WS₂ electrocatalyst for NO₃RR is further investigated. The comparison of catalytic performance between a-Cu/1T′ WS₂ and pure 1T′ WS₂ confirms that the introduction of appropriate amounts of atomically dispersed Cu on 1T′ WS₂ can promote ammonia production, while pure 1T′ WS₂ nanosheets are considered effective HER catalysts. It is speculated that the 1T′ WS₂ supports promote the hydrogenation step, which may then contribute to the subsequent formation of ammonia during NO₃RR. The kinetic isotope experiments indicate that when H₂O solvent is replaced by D₂O for the NO₃RR, the current density of a-Cu/1T′ WS₂ varies slightly compared to that on pure 1T′ WS₂ (FIGS. 22A-22B). This indicates that the hydrogenation step is the rate-limiting step for 1T′ WS₂, while a-Cu/1T′ WS₂ is less affected. These results suggest that the introduction of 1T′ WS₂ support promotes the hydrogenation step of a-Cu/1T′ WS₂ in NO₃RR.

In situ Raman spectroscopy is employed to identify the potential intermediates and reaction pathways of catalysts in NO₃RR. The Raman spectrum of pure 1T′ WS₂ and a-Cu/1T′ WS₂ at different potentials are shown in FIGS. 23A-23B. The peaks near 982 cm⁻¹ and 1049 cm⁻¹ correspond to SO₄²⁻ and NO₃⁻, which originate from the applied electrolyte in the tests. The peak at 1036 cm⁻¹ can be classified as the symmetric NO₃⁻ stretching and that at 1317 cm⁻¹ assigned to the symmetric N—O stretches of NO₂ moiety in adsorbed NO₃⁻. The peak at 1449 cm⁻¹ can be attributed to the antisymmetric NH₂ deformation of NH₄⁺, while the peak at 1586 cm⁻¹ corresponds to the δ_a(HNH) of NH₃ and δ_(E) antisymmetric NH₂ deformation of NH₄⁺. Several NH₃-related intermediates adsorptions are observed on the a-Cu/1T′ WS₂ nanosheets, while no corresponding signals are detected on the pure 1T′ WS₂ nanosheets. This confirms the deposition of Cu, which enhances the performance of 1T′ WS₂ in ammonia production.

To construct a comprehensive description of the reaction mechanism, on-line DEMS was used to detect the intermediate species generated during the NO₃RR. FIG. 24 display the mass-to-charge ratio (m/z) signals recorded as a function of time while performing 5 subsequent voltammetry scan cycles (each cycle involves a scan from 0 V to −1.4 V vs. RHE). Notably, the m/z signals at 46, 33, 31, and 17, corresponding to the adsorbed NO₂, NH₂OH, NOH, and NH₃, respectively, are detected and tracked, providing direct evidence of crucial intermediates involved in the reduction of nitrate to ammonia. Based on the kinetic isotope experiments, in situ Raman results, and on-line DEMS, a tandem catalysis mechanism for a-Cu/1T′ WS₂ towards NO₃RR is proposed, as shown in FIG. 25. Considering that 1T′ WS₂ supports possess appropriate binding with H* species and Cu shows a strong adsorption for NO₃⁻. Once an efficient Cu/1T′ WS₂ interface is formed, the adsorbed NO₃⁻ on Cu surface couples with adjacent H* species from the 1T′ WS₂ supports, promoting the multi-step proton-coupled electron transfer process for the conversion of NO₃⁻ into ammonia.

In summary, a facile electrochemical method is developed to grow various Cu nanostructures on 1T′ WS₂ supports, yielding an active and selective electrocatalyst for reducing nitrate to ammonia. The as-prepared a-Cu clusters/1T′ WS₂ nanosheets achieve a superior activity and selectivity for ammonia, with the FE of 99.2%, surpassing state-of-the-art NO₃RR catalysts. Mechanistic studies revealed that the transfer of adsorbed *H from 1T′ WS₂ to *NO intermediates on Cu sites facilitate the hydrogenation step, enhancing ammonia synthesis. This work demonstrates that metal deposition on unconventional-phase TMDs supports is a promising strategy for advancing electrocatalysis and designing efficient catalysts for various applications.

The foregoing description of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many modifications and variations will be apparent to the practitioner skilled in the art.

The embodiments were chosen and described in order to best explain the principles of the invention and its practical application, thereby enabling others skilled in the art to understand the invention for various embodiments and with various modifications that are suited to the particular use contemplated.

Definition

Throughout this specification, unless the context requires otherwise, the word “comprise” or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers. It is also noted that in this disclosure and particularly in the claims and/or paragraphs, terms such as “comprises”, “comprised”, “comprising” and the like can have the meaning attributed to it in U.S. Patent law; e.g., they allow for elements not explicitly recited, but exclude elements that are found in the prior art or that affect a basic or novel characteristic of the present invention.

Furthermore, throughout the specification and claims, unless the context requires otherwise, the word “include” or variations such as “includes” or “including”, will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

References in the specification to “one embodiment”, “an embodiment”, “an example embodiment”, etc., indicate that the embodiment described can include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is submitted that it is within the knowledge of one skilled in the art to affect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described.

As used herein, terms “approximately”, “basically”, “substantially”, and “about” are used for describing and explaining a small variation. When being used in combination with an event or circumstance, the term may refer to a case in which the event or circumstance occurs precisely, and a case in which the event or circumstance occurs approximately. As used herein with respect to a given value or range, the term “about” generally means in the range of ±10%, ±5%, ±1%, or ±0.5% of the given value or range. The range may be indicated herein as from one endpoint to another endpoint or between two endpoints. Unless otherwise specified, all the ranges disclosed in the present disclosure include endpoints. When reference is made to “substantially” the same numerical value or characteristic, the term may refer to a value within ±10%, ±5%, ±1%, or ±0.5% of the average of the values.

In the methods of preparation described herein, the steps can be carried out in any order without departing from the principles of the invention, except when a temporal or operational sequence is explicitly recited. Recitation in a claim to the effect that first a step is performed, and then several other steps are subsequently performed, shall be taken to mean that the first step is performed before any of the other steps, but the other steps can be performed in any suitable sequence, unless a sequence is further recited within the other steps. For example, claim elements that recite “Step A, Step B, Step C, Step D, and Step E” shall be construed to mean step A is carried out first, step E is carried out last, and steps B, C, and D can be carried out in any sequence between steps A and E, and that the sequence still falls within the literal scope of the claimed process. A given step or sub-set of steps can also be repeated. Furthermore, specified steps can be carried out concurrently unless explicit claim language recites that they be carried out separately.

The term “unconventional-phase transition metal dichalcogenides” refers to a subclass of TMDs that possess a phase distinct from the commonly observed thermodynamically stable phase, often exhibiting unique electronic, optical, or catalytic properties.

The term “electrocatalytic reduction” refers to a process where an electrochemical reaction reduces a species by the application of an external electrical potential, resulting in the conversion of the species into a desired product.

The term “aqueous electrolyte” refers to a solution, typically water-based, that contains dissolved ions and is used to facilitate the flow of current in electrochemical reactions. The electrolyte often includes salts or other species to enhance conductivity and promote specific reactions.

The term “activated hydrogen species” refers to highly reactive forms of hydrogen, such as protons (H⁺) or hydrogen atoms (H⋅), that are generated on the catalyst surface and can readily participate in chemical reactions, such as surface hydrogenation.

The term “surface hydrogenation” refers to chemical process where hydrogen is added to unsaturated compounds at the surface of a catalyst, leading to the formation of hydrogenated products.

The term “electrochemical exfoliation” refers to a method used to separate layered materials into thin nanosheets by applying an electrochemical potential to a bulk material in a suitable electrolyte, resulting in the detachment of individual layers.

The term “underpotential deposition (UPD)” refers to a technique in electrochemistry where a metal is deposited onto a substrate at a potential more positive than the equilibrium potential for the metal, resulting in a highly ordered, single-atom metal layer.

The term “overpotential deposition (OPD)” refers to a method where a metal is deposited at a potential more negative than the equilibrium potential for the metal, resulting in the formation of nanoclusters or amorphous structures due to the higher applied voltage.

Other definitions for selected terms used herein may be found within the detailed description of the present invention and apply throughout. Unless otherwise defined, all other technical terms used herein have the same meaning as commonly understood to one of ordinary skill in the art to which the present invention belongs.

INDUSTRIAL APPLICABILITY

The ammonia production market is vital to industries like agriculture, pharmaceuticals, and chemicals, with ammonia primarily used as fertilizer and a chemical precursor. This invention introduces a heterogeneous electrocatalyst, which efficiently converts waste nitrate ions into valuable ammonia with 99.2% selectivity. The catalyst shows excellent stability, being recyclable over six cycles without performance loss, highlighting its industrial potential.

Furthermore, this invention could revolutionize electrocatalysis for HER, a promising solution to the energy crisis. H₂, a clean energy source, has applications in fuel cells and electric vehicles. Current HER catalysts, such as Pt/C and Pd/C, are expensive and less stable. The s-Cu/1T′ MoS₂ catalysts in this invention offer improved catalytic performance, low overpotential, fast kinetics, and durability, making them ideal for large-scale hydrogen production and related catalytic reactions.

References: The disclosures of the following references are incorporated by reference ADDIN EN.REFLIST

• 1. U.S. Ser. No. 11/652,206B2.
• 2. US20210080419A1.
• 3. Sun, Y., Wang, Y., Chen, J. Y., Fujisawa, K., Holder, C. F., Miller, J. T., . . . & Schaak, R. E. (2020). Interface-mediated noble metal deposition on transition metal dichalcogenide nanostructures. Nature chemistry, 12(3), 284-293.
• 4. Lai, Z. et al. Salt-Assisted 2H-to-1T′ Phase Transformation of Transition Metal Dichalcogenides. Adv. Mater. 34, 2201194 (2022).