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Article

Ru Nanoparticle Assemblies Modified with Single Mo Atoms for Hydrogen Evolution Reactions in Seawater Electrocatalysis

by
Shuhan Wang
1,2,
Jiani Qin
1,*,
Yong Zhang
3,
Shuai Chen
4,
Wenjun Yan
4,
Haiqing Zhou
3 and
Xiujun Fan
2,*
1
School of Environmental Science and Engineering, Shaanxi University of Science and Technology, Xi’an 710021, China
2
School of Chemistry, Xi’an Jiaotong University, Xi’an 710049, China
3
Key Laboratory of Low-Dimensional Quantum Structures and Quantum Control of Ministry of Education, Department of Physics, Hunan Normal University, Changsha 410081, China
4
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China
*
Authors to whom correspondence should be addressed.
Catalysts 2025, 15(5), 475; https://doi.org/10.3390/catal15050475
Submission received: 20 March 2025 / Revised: 23 April 2025 / Accepted: 25 April 2025 / Published: 12 May 2025
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Abstract

:
Ru-based catalysts manifest unparalleled hydrogen evolution reaction (HER) performance, but the hydrolysis of Ru species and the accumulation of corresponding reaction intermediates greatly limit HER activity and stability. Herein, Ru nanoparticle assemblies modified with single Mo atoms and supported on N-incorporated graphene (referred to as MoRu-NG) are compounded via hydrothermal and chemical vapor deposition (CVD) methods. The incorporation of single Mo atoms into Ru lattices modifies the local atomic milieu around Ru centers, significantly improving HER catalytic behavior and stability. More specifically, MoRu-NG achieves overpotentials of 53 mV and 28 mV at 10 mA cm−2, with exceptional stability in acidic and alkaline seawater solutions, respectively. In MoRu-NG, Ru atoms have a special electronic structure and thus possess optimal hydrogen adsorption energy, which indicates that excellent HER activity mainly hinges upon Ru centers. To be specific, the d-electron orbitals of Ru atoms are close to half full, giving Ru atoms moderate bond energy for the assimilation and release of hydrogen, which is beneficial for the conversion of reaction intermediates. Moreover, the incorporation of single Mo atoms facilitates the formation of O and O’-bidentate ligands, significantly enhancing the structural stability of MoRu-NG in universal-pH seawater electrolysis. This work advances a feasible construction method of hexagonal octahedral configuration (Ru-O-Mo-N-C) and provides a route to synthesize an efficient and stable catalyst for electrocatalytic HER in universal-pH seawater.

Graphical Abstract

1. Introduction

With the emergence of the global low-carbon circular economy, hydrogen produced by water splitting is increasingly gaining attention in society. Hydrogen energy is widely regarded as an optimal medium for energy hoarding and distribution in the future while also serving as a crucial chemical feedstock in modern industrial processes. However, due to the scarcity of freshwater resources and the energy consumption associated with seawater purification, the hydrogen evolution reaction (HER) may not achieve its ideal effect in freshwater.
Electrocatalytic seawater splitting for hydrogen generation can not only avoid the consumption of worldwide freshwater supplies but also improve the utilization rate of seawater resources. Due to its ample reserves and widespread availability, seawater is considered a latent feedstock for electrolysis, which results in popularization with the purpose of producing hydrogen via seawater electrolysis [1]. In terms of catalysts, Pt-based catalysts are considered to be effective electrocatalysts for HER, although the large-scale commercialization of Pt-based catalysts faces the problem of high costs. To date, Ru-based catalysts have been regarded as one of the most up-and-coming and cheapest replacements for Pt-based catalysts for HER implementations, owing to the strength of the Ru-H bond, which is comparable to that of Pt-H bonds [2,3,4,5,6]. However, there are numerous challenges in producing hydrogen by seawater electrolysis, such as the agglomeration of Ru atoms in Ru-based catalysts and the attack of metal ions in seawater. Given the substantial binding energy of Ru atoms, the irreversible agglomeration of Ru atoms occurs in the actual catalytic process, which leads to the unsatisfactory stability of Ru-based catalysts [7]. Furthermore, a large number of Cl− and insoluble precipitates from Ca2+ and Mg2+ in seawater tend to poison catalysts, thereby reducing both HER activity and the durability of catalysts [8,9,10]. Moreover, the innate poor conductivity of seawater and the sophisticated nature of the seawater framework are not conducive to HER kinetics and pose challenges to understanding the reaction process and mechanism of seawater electrolysis [11,12]. These obstacles have contributed to the relatively slow progression of seawater electrolysis industrialization.
In HER, Ru atoms in Ru-based catalysts tend to agglomerate, and various ions in seawater attack HER catalysts. Therefore, we introduce Mo atoms within the Ru lattice to improve the activity and stability of HER catalysts. In this work, Ru nanoparticle assemblies (Mo-Ru NPAs) modified with single Mo atoms and supported on N-incorporated graphene (defined as MoRu-NG) are fabricated via hydrothermal and chemical vapor deposition (CVD) methods. MoRu-NG with a Ru-O-Mo-N-C configuration delivers outstanding HER catalytic efficiency and stability, surpassing those of commercial Pt/C and other reported electrocatalysts [13,14,15,16]. Electron interactions exist between O and Ru sites in MoRu-NG, tuning the electronic structure of active sites to their peak state and increasing the stability of O-bridged Ru-Mo structures, which facilitates HER activity and stability [16,17,18]. Specifically, the p-band center of O sites within MoO42− exhibits a more significant overlap with the d-band center of Ru sites, thereby improving HER activity and stability [19,20]. Furthermore, O, O’-bidentate ligands in Mo-Ru NPAs are formed after the introduction of single Mo atoms (SAs), which can shorten the Ru-Ru bond lengths in Ru nanoparticles (NPs), increasing Ru’s metallic bond intensity and further promoting HER stability [19]. Importantly, the existence of Ru atoms in Mo-Ru NPAs introduces abundant Lewis acid sites that replenish hydroxyl radicals in order to induce negative charge accumulation, which can resist Cl− attacks in seawater [21]. Meanwhile, these acid sites intensely bind to hydroxide ions, which prevents the pH values of seawater from rising locally, thus inhibiting the formation of deposits of metal ions in the seawater [22,23]. In MoRu-NG, the characteristic Ru-O-Mo-N-C architecture with pronounced electron–pair bonds functions as a robust and top-notch catalytic structure toward HER in universal-pH seawater electrolysis.

2. Results and Discussion

2.1. Synthesis of MoRu-NG

MoRu-NG electrocatalysts were synthesized through the following steps, as portrayed in the schematic representation (Figure 1a). The first step was to prepare a precursor solution using sonication Ru salt (as Ru source), molybdate (as Mo source), and graphene oxide in water. After thorough mixing, the precursor solution underwent hydrothermal treatment, in which RuMo3 groups were anchored onto partially reduced GO (p-RGO), leading to the formation of a Ru-O-Mo-N-C structure. After this step, freeze-drying was applied to the mixture to reduce p-RGO sheet restacking and the mixture was generated by CVD treatment in the NH3/Ar atmosphere at 800 °C.
The formation mechanism of MoRu-NG electrocatalysts was based on electrostatic self-assembly, which produces an intensive coupling function between Mo and Ru atoms [24,25] (Figure 1b). Particularly, RuCl3·3H2O and (NH4)6Mo7O24·4H2O were blended with the GO suspension inside an ultrasonic bath where the pH value ranges from 6.1 to 6.3. After that, in the course of the hydrothermal process, protonation took place for the carboxyl anions. Then, the elimination of protonated carbonyl and epoxy groups on graphene oxide appeared, and the delocalized π-electron system and H+ underwent protonation to obtain positively charged p-RGO [26]. The p-band center of the O sites in MoO42− exhibited a more pronounced overlap with the d-band center of the Ru sites, which enhanced Ru-O-Mo covalency [27]. Initially, Ru3+ formed aquo ligands ([Ru(H2O)6]3+) in deionized water, which then underwent hydrolysis to produce [Ru(OH)6]3− under high-temperature and mild acidic conditions. In a mild acidic solution, simultaneously, protonation facilitated the transformation of Mo(VI) oxo species in the MoO42− segment. The Mo(VI) oxo species firstly changed into Mo hydroxo species and ultimately evolved into Mo aquo ligands [MoO4(H2O)2]2− [28]. [MoO4(H2O)2]2− replaced two hydroxyl bonds in [Ru(OH)6]3− to form [Ru(MoO4)3(H2O)6]3− (RuMo3) groups with an O-bridged Ru-Mo structure, where [MoO4(H2O)2]2− acts as O,O’-bidentate ligands [20]. The specific formula is as follows: Ru3+ + 6H2O ⇌ [Ru(H2O)6]3+, [Ru(H2O)6]3+ + 6H2O ⇌ [Ru(OH)6]3− + 6H3O+; MoO42− + 2H+ + 2H2O ⇌ [MoO4(H2O)2]2−; [Ru(OH)6]3− + 3[MoO4(H2O)2]2− + 6H+ ⇌ [Ru(MoO4)3(H2O)6]3− + 6H2O. As a result of the vigorous coupling between negatively charged RuMo3 groups [20,24] and positively charged protonated p-RGO sheets, O-coordinated RuMo3 groups could be electrostatically attracted to p-RGO sheets [29,30]. During the CVD process, nitrogen doping and p-RGO underwent additional reduction simultaneously, resulting in the formation of RuMo3 groups anchored within p-RGO sheets, which subsequently constructed a Ru-O-Mo-N-C structure. For the purpose of comparison, a sequence of composite materials with various mass percentages of Mo atoms relative to Ru atoms, together with Mo species and Ru species supported on several NG sheets (denoted as Mo-NG and Ru-NG, respectively), were also synthesized (Table S1).
The reason why MoRu-NG was successfully synthesized is due to the interaction of the coordination nature between Ru and Mo species, as well as the electronegative difference between protonated p-RGO sheets and RuMo3 groups during the hydrothermal process with a mild acidic solution. In MoRu-NG, Ru centers possess suitable hydrogen adsorption energy and a special electronic structure, which has a pivotal function in improving HER activity. Additionally, the introduction of an O,O’-bidentate ligand promotes HER stability by increasing the Ru metallic bond intensity [31,32].

2.2. Structural Characterization of MoRu-NG

The crystalline phase and morphology of MoRu-NG were initially investigated by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HR-TEM), high-angle annular dark-field scanning TEM (HAADF-STEM), and aberration-corrected high-angle annular dark-field scanning TEM (AC HAADF-STEM).
The details of the XRD calibration using Si are an integral part of this work. The following are brief details of the XRD calibration using Si: Firstly, high-purity and well-crystallized monocrystalline silicon is selected as the standard sample, and its crystal structure and diffraction data are accurately known. Secondly, according to the requirements of the XRD instrument, appropriate parameters are set, such as the tube voltage, tube current, scanning speed, step size, etc., in order to obtain high-quality diffraction patterns. Thirdly, the Si standard sample is installed on the sample table to ensure that the sample surface is flat and parallel to the instrument axis. XRD scans are performed to obtain the diffraction pattern of Si. Then, the Si diffraction peak position obtained by the experiment is compared with the known standard diffraction peak position, and the difference between the two is calculated. The angle scale of the instrument is adjusted according to the difference so that the measured values match the standard values. Finally, accuracy verification: the diffraction peaks of the Si standard samples are measured several times to verify the accuracy and repeatability of the calibrated instrument and to ensure the accuracy and reliability of the instrument measurement.
In XRD patterns, the typical diffraction peaks of MoRu-NG reveal the presence of a Ru crystalline (PDF#06-0663) phase, indicating the successful formation of Ru crystals after thermal nitridation. Additionally, no peaks related to Mo species are observed, which suggests that Mo species exist as an amorphous phase after the nitridation reaction under thermal conditions (Figure 2a). Notably, the diffraction peaks of MoRu-NG exhibit negative shifts relative to that of Ru/NG (selected areas in (Figure 2a)), indicating that lattice expansion occurs after the introduction of Mo SAs. In addition, the XRD patterns of A-MR and C-MR also exhibit the successful formation of Ru crystals, similar to that of MoRu-NG (Figure S1). As demonstrated by HR-TEM images (Figure 2b and Figure S2), Mo-Ru NPAs show homogeneous dispersion in NG sheets. In Figure 2b and Figure S3, Mo-Ru NPAs and Ru NPs with about 1.5 nm and 3.2 nm particle sizes are formed in the NG sheets, respectively. This observation indicates that Mo SAs in Mo-Ru NPAs can effectively suppress the particle size of Ru-NG, leading to higher specific surface area and stronger electrical conductivity [33]. As depicted in the HR-TEM image at relatively high magnification, the lattice fringes of Mo-Ru NPAs are clear and the interplanar distance of 0.26 nm can be correlated with the (100) facets of Mo-Ru NPAs (Figure 2c). Additionally, the HR-TEM images of Mo-NG and Ru-NG display the state of the amorphous phase and the crystal face distance of 0.24 nm corresponds to the (100) planes of Ru crystals (Figure S4). Moreover, lattice distortion and edge dislocation can be observed in the white circles and the yellow T-shaped marks (Figure 2c, inset), respectively [34]. The results illustrate that introducing Mo SAs increases the specific surface area values and further exposes more active sites, improving the conductivity of MoRu-NG. After introducing Mo SAs into Ru crystal lattices, the crystal face spacing of MoRu-NG is larger than that of Ru-NG, which indicates that lattice expansion appears in MoRu-NG, consistent with the results of XRD.
The AC HAADF-STEM images show that Mo-Ru NPAs are distributed on NG sheets, validating the interaction and electronic coupling of Mo and Ru atoms (Figure 2d,e). The energy-dispersive x-ray spectroscopy (EDS) mapping analysis indicates the well-distributed presence of C, N, O, Mo, and Ru elements (Figure 2f). The TEM-element energy dispersive spectroscopy (EDS) mapping analysis reveals that the Ru and Mo content in the synthesized Mo-Ru NPAs is close to 0.41 atom % and 0.69 atom %, respectively (Figure S5, Table S2). These findings demonstrate that densely packed Mo-Ru NPAs are effectively fabricated on NG sheets via the two methods.
To gain insights into the valence states of MoRu-NG, x-ray photoelectron spectroscopy (XPS) was carried out. The XPS survey spectrum of MoRu-NG confirms the characteristic peaks of Ru, Mo, C, N, and O species in agreement with the EDS elemental mapping (Figure 3f and Figure S6). The Mo 3d spectra of MoRu-NG exhibit three pairs of peaks, manifesting Mo(VI), Mo(V), and Mo-N species [35] (Figure 3a). For Ru 3p spectra of MoRu-NG, Ru species display two pairs of peaks that correspond to Ru(0) and Ru(IV), respectively [36], indicating the existence of Ru crystals on the MoRu-NG surface (Figure 3b). Compared to the control samples, positive displacements of 0.3 and 0.4 eV are observed on Mo(V) and Ru(0) in MoRu-NG, respectively, which demonstrates the diminution of electron density around Mo-Ru NPAs and the formation of M-O/M-N bonds (M refers to Mo/Ru). As revealed in Figure 3c, the deconvoluted peaks around 531.0 and 530.3 eV are assigned to Mo-O and Ru-O bonds [37,38,39], respectively, and the Mo-O and Ru-O bonds in MoRu-NG exhibit a 0.3 eV shift toward higher binding energies relative to those of Mo-NG and Ru-NG. These results confirm that Mo-Ru NPAs are O-coordinated within MoRu-NG, indicating that the formation of the Ru-O-Mo structure redistributes the Ru d-orbital electron density and affects the Mo electron cloud distribution.
Moreover, the comparison samples with various mass percentage of Mo atoms relative to Ru atoms were also characterized via XPS measurements. For the Mo 3d and Ru 3p spectra, the varying mass percentage of Mo atoms relative to Ru atoms catalyzes electron migration at a specific Mo loading (Figures S7 and S8). For the N 1s spectrum, pyridinic N (398.3 eV), pyrrolic N (399.8 eV), graphitic N (401.7 eV), and N-O (404.3 eV) species are clearly visible in all catalysts, while Mo-N (394.6 eV) only appears in MoRu-NG and Mo-NG, demonstrating Mo atoms coordinating with N atoms (Figure 3d). Compared to Mo-NG, a negative shift of 0.6 eV is observed on Mo-N in MoRu-NG, exhibiting an electron density increase around Mo-N bonds, confirming that electron transfer exists between Mo-Ru NPAs and NG sheets. For the O 1s and N 1s spectra, N-O bonds disappear, while Mo loading gradually increases to certain values, representing that excessive Mo loading affects the formation of N-O bonds (Figures S9 and S10). Meanwhile, the C 1s spectra are resolved into four characteristic peaks, corresponding to C-C (284.8 eV), C-N (285.6 eV), C-O (286.4 eV), and O=C-O (287.3 eV) species (Figures S11 and S12), which represents that N atoms have a coordination relationship with C atoms. Distinctly, the metal centers in MoRu-NG are O-coordinated, while Mo SAs in Mo-Ru NPAs are anchored in NG with a Mo-N-C structure.
The electron paramilitary resonance spectroscopy (EPR) was used to solve MoRu-NG surface defects (Figure 3e). The signals of g = 2.17 and g = 2.13 are assigned to the resonance on conduction electrons in MoRu-NG [40]. MoRu-NG and Mo-NG both display EPR signals at g = 1.95 (Mo-N species [41]), g = 1.89, and g = 2.07 (Mo5+ species [42]), followed by g = 2.01 (oxygen vacancies), suggesting that MoRu-NG has higher concentrations of Ov, Mo5+, and Mo-N species than those in Mo-NG. Moreover, the reference samples with a heterogeneous mass percentage of Mo atoms relative to Ru atoms display analogous EPR spectra compared with MoRu-NG in the Figure S13. As Mo loading increases gradually, the signal intensity is more and more obvious, demonstrating that the concentration of free radicals is related to Mo loading. In order to identify the varieties of chemical groups, Fourier transform infrared spectroscopy (FT-IR) was conducted. The FT-IR spectra of MoRu-NG include the vibration of C-OH, O=C=O, C=C, and C-N groups in Figure 3f. Compared with MoRu-NG, the control samples with various atom mass ratios exhibit similar characteristic bands (Figure S14). As exhibited in the Raman spectra, the intensity ratio of the D band to the G band (ID/IG) for MoRu-NG (1.15) is higher than those of Mo-NG (1.05) and Ru-NG (1.06) (Figure 3g). This result indicates that a structural defect is introduced into MoRu-NG via the synergistic effect between Mo and Ru atoms. In addition, the ID/IG for MoRu-NG (1.15) is higher than those of A-MR (1.05) and C-MR (1.02), which demonstrates that adjusting the atom mass ratio to appropriate values can introduce more structural defects (Figure S15).
Moreover, MoRu-NG affords a higher air contact angle (CA, 164.4°) and water contact angle (CA, 115.7°) than those of Mo-NG and Ru-NG, signifying that it is hydrophobic, which promotes the diffusion of hydrogen gas (Figure 3i, Figures S16 and S17). Furthermore, the Brunauer–Emmett–Teller (BET) surface areas of MoRu-NG, Mo-NG, and Ru-NG are 504.7, 237.1, and 150 m2 g−1, respectively, and the main pore size of MoRu-NG is centered at 1.6 nm (Figure 3h, Figures S18 and S19). The results suggest that introducing Mo SAs into Ru crystals can effectively inhibit the agglomeration of Ru atoms during the CVD process. Inserting Mo atoms into Ru lattices generates a considerable amount of holes and defects, which contributes to the adsorption of reactant molecules and increases the number of accessible active sites [43].
In this work, RuMo3 groups are synthesized using a Ru-O-Mo configuration through the facile self-assembly of Mo species and Ru species, followed by the CVD procedure. Usually, SAs about transition metals are spread over N-doped carbon supports as M-N-C structures (M refers to transition metals). Therefore, Mo SAs in RuMo3 groups were dispersed onto N-doped carbon supports in the form of Mo-N-C in the CVD procedure [44,45]. In MoRu-NG, O, O’-bidentate ligands and the chelation function between Mo and Ru atoms provide the possibility to modulate the electronic structure, thereby improving electrocatalytic activity and stability.

2.3. Electrocatalytic HER Performance

The electrocatalytic hydrogen evolution reaction (HER) performance of MoRu-NG was explored in 0.5 M H2SO4, 1.0 M KOH, and 1.0 M PBS seawater solutions by utilizing a three-electrode configuration. For the purpose of comparison, Mo-NG, Ru-NG, NG, and commercial Pt/C were also tested. In this study, all the potentials are based on a reversible hydrogen electrode (RHE), and it should be noted that all the data obtained are without IR compensation. Adjusting the acidity and alkalinity of seawater can change the charge distribution and chemical environment on the electrode surface, which is conducive to the adsorption of reactants and the desorption of products, thus speeding up the electrode reaction rate. In addition, by adjusting the acidity and alkalinity of seawater, the reaction intermediates on the surface of the electrode material can be made more stable, thereby reducing the overpotential and improving the electrolysis efficiency. HER in seawater is always under neutral or alkaline conditions. The reason for acidic HER testing is the existence of special ocean areas and acidic pollutants. Some special ocean areas (such as those near seafloor hydrothermal vents or those affected by specific geological activities) may have local seawater pH levels that are significantly lower. By testing HER in a mixed sulfuric acid/seawater system, we were able to simulate these special marine environments and study the performance of the catalyst in acidic seawater, which we are likely to encounter. In addition, with the increasing severity of marine environmental problems, seawater is increasingly affected by acidic pollutants (such as industrial wastewater discharge and atmospheric acid gas deposition). We tested acidic HER to assess the adaptability and stability of Ru-based catalysts in the presence of acidic contamination of seawater. This will help to prepare technical reserves for possible changes in seawater quality in advance and ensure that seawater hydrogen production technology can still operate reliably in the complex and changing marine environment.
In a 0.5 M H2SO4 seawater solution, MoRu-NG has the ability to provide a geometric current density of −10 mA cm−2 at an extraordinarily low overpotential of 53 mV (η10 = 53 mV) (Figure 4a). This overpotential is significantly lower than those of NG (>300 mV), Mo-NG (292 mV), and Ru-NG (153 mV), as well as other counterparts (Figure S20), suggesting that MoRu-NG serves as a first-rate HER catalyst. In order to acquire more profound knowledge about the reaction kinetics of MoRu-NG, the Tafel slope was scrutinized. In the 0.5 M H2SO4 seawater solution, MoRu-NG delivers a small Tafel slope of 30 mV dec−1, which is far lower than those of NG (183 mV dec−1), Mo-NG (162 mV dec−1), Ru-NG (139 mV dec−1), and other counterparts, as well as close to that of the benchmark Pt/C (30 mV dec−1) (Figure 4b and Figure S20) The results reflect that MoRu-NG exhibits a top-level HER kinetic process in accordance with the Volmer–Tafel mechanism, where the surface recombination step belongs to the rate-limiting step. Compared to Mo-NG, Ru-NG, NG, and Pt/C, MoRu-NG shows the best activity, featuring high current density at low overpotentials in the 1.0 M KOH seawater solution (Figure 4c). MoRu-NG merely requires overpotentials of 28 mV to reach −10 mA cm−2, which is lower than those of Mo-NG (290 mV), Ru-NG (270 mV), NG (>300 mV), and other catalysts of different atom mass ratios, as well as even better than that of the state-of-the-art Pt/C (37 mV) (Figure S20). In a corresponding fashion, the reaction kinetics within MoRu-NG undergoes a great leap forward, which is demonstrated by the unusually low Tafel slope of 37 mV dec−1 observed in the 1.0 M KOH seawater solution (Figure 4d). The Tafel slope (37 mV dec−1) of MoRu-NG is superior to those of Mo-NG (128 mV dec−1), Ru-NG (133 mV dec−1), NG (140 mV dec−1), and other catalysts of different atom mass ratios, as well as close to that of the commercial Pt/C (34 mV dec−1) (Figure S20). In 1.0 M KOH seawater solution, such a low Tafel slope of MoRu-NG suggests that the introduction of Mo SAs favors the generation of H* from H2O. In the 1.0 M PBS seawater solution, MoRu-NG shows optimal activity, characterized by high current density even at low overpotentials and small Tafel slopes compared to Mo-NG, Ru-NG, NG, and Pt/C (Figure S21).
In addition, double-layer electrochemical capacity (Cdl) technology was applied at different scanning rates via cyclic voltammetry (CV). In the 0.5 M H2SO4 and 1.0 M KOH seawater solutions, MoRu-NG has Cdl values of 18.3 and 11.8 mF cm−2, respectively. These values are higher than those of Mo-NG (15.8 and 8.3 mF cm−2) and Ru-NG (12.3 and 10.8 mF cm−2) (Figure 4e,f and Figure S22). The result suggests that the chelation function between Mo and Ru atoms can subjoin active surface areas of catalysts. Another point is that the Cdl values for MoRu-NG (18.3, 11.8 mF cm−2) are higher than those of NG (7.61, 5.2 mF cm−2), A-MR (15.8, 6.0 mF cm−2), and C-MR (11.2, 9.0 mF cm−2) in the 0.5 M H2SO4 and 1.0 M KOH seawater solutions, respectively (Figures S23 and S24). This result demonstrates that deficient and overly high Mo loadings reduce the number of accessible active sites, indicating that electrochemical activity depends on rational Mo content [46,47]. By using pure water to formulate acidic and basic solutions, the polarization curves along with their respective Tafel plots of MoRu-NG are shown in the Figure S25. In the same way, MoRu-NG has Cdl values of 13.2 and 9.6 mF cm−2 in the 0.5 M H2SO4 and 1.0 M KOH pure water solutions, respectively, which are similar to those of MoRu-NG in acidic and alkaline seawater solutions (Figure S26). The results explain that MoRu-NG can resist poison and erosion by anions and cations in seawater compared with pure water [48], which can prevent the wastage of fresh water and enhance the utilization efficiency of seawater.
To research intrinsic catalytic activity, the electrochemical surface area (ECSA) determined by deriving the electrochemical double-layer capacitance (Cdl) was measured. The ECSAs of MoRu-NG, Mo-NG, and Ru-NG are 457.5, 395.0, and 307.5 cm2 in the 0.5 M H2SO4 seawater solution and 295.0, 207.5, and 270.0 cm2 in the 1.0 M KOH seawater solution, respectively. Additionally, to exclude the effect of surface area and compare the inherent HER activity, geometric current density (j) was normalized by ECSA (jECSA) in the universal-pH seawater [49,50]. As shown in Figure S27, MoRu-NG gives the jECSA values of 110.0 and 341.7 μA cm−2 at η = 100 mV, which are evidently higher than those of Mo-NG (2.7, 4.2 μA cm−2) and Ru-NG (14.1, 1.8 μA cm−2) in 0.5 M H2SO4 and 1.0 M KOH seawater solutions, respectively. The outcome reveals that MoRu-NG possesses better intrinsic HER activity than those of Mo-NG and Ru-NG in 0.5 M H2SO4 and 1.0 M KOH seawater solutions, respectively. For the sake of an uncomplicated comparison, the η10 and Tafel slope values of MoRu-NG are further compared with other recently reported Mo- or Ru-based metal electrocatalysts (Figures S28 and S29, Tables S3 and S4). The small η10 and Tafel slope of MoRu-NG are competitive with these electrocatalysts, such as Ni2P/MoS2/N:CNT [51], CoRu-1 [52], and other catalysts [9,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67], suggesting that MoRu-NG is a top HER electrocatalyst in universal-pH seawater.
In addition, in order to obtain more in-depth understanding of the intrinsic catalytic activity, the turnover frequency (TOF) per active site was determined. Thus, the number of active sites was titrated from the integrated charge of anodic CV cycles in a phosphate buffer solution (pH = 7) [68] (Figure S30). Evidently, MoRu-NG affords the highest TOF values of 3.85 and 7.8 H2 s−1 at η = 100 mV, significantly surpassing those of Mo-NG (0.15 and 0.12 H2 s−1) and Ru-NG (0.76 and 0.09 H2 s−1) in 0.5 M H2SO4 and 1.0 M KOH seawater solutions, respectively (Figure 4g). These results demonstrate that MoRu-NG delivers a significantly enhanced catalytic hydrogen production capacity compared to Mo-NG and Ru-NG. This superior catalytic performance in HER can be ascribed to the distinctive Ru-O-Mo-N-C configuration [69]. This configuration gives rise to a unique electronic structure and improves electrical conductivity [70]. Additionally, the electrochemical impedance spectroscopy (EIS) technique was also carried out. Among them, MoRu-NG yields small charge-transfer resistance (Rct) (3 and 5 ohms) in 0.5 M H2SO4 and 1.0 M KOH seawater solutions, which are much lower than those of Mo-NG (9 and 7 ohms) and Ru-NG (6 and 7 ohms), respectively (Figure S31). MoRu-NG exhibits lower Rct than those of Mo-NG and Ru-NG, indicating that the atom doping of foreign metal facilitates electron transport at the electrode or electrolyte interface. The chemical coupling between Mo and Ru atoms provides an interconnected conductive network to enhance the electrical conductivity between Mo-Ru NPAs and NG sheets [71].
The following research results are about the stability and durability of MoRu-NG electrocatalysts. As illustrated in Figure S32, the polarization curve of MoRu-NG exhibits minimal discrepancies compared with the initial curve after 10,000, even 50,000, CV cycles in the 0.5 M H2SO4 seawater solution. This is the case even after 10,000 or, remarkably, 50,000 cyclic voltammetry (CV) cycles in the 1.0 M KOH seawater solution (Figure S33). Furthermore, several long-term potential stability tests were conducted on MoRu-NG using galvanostatic measurement at current densities of −100, −200, and −300 mA cm−2 under room temperature (25 °C) (Figure 4h). After 100,000 s of operation at j = −100, −200, and −300 mA cm−2, the potentials of MoRu-NG only increase by 24, 20, and 33 mV in the 0.5 M H2SO4 seawater solution and 14, 10, and 42 mV in the 1.0 M KOH seawater solution. In contrast, the potentials of Pt/C catalysts at j = −100 mA cm−2 increase by 142 and 110 mV in the 0.5 M H2SO4 and 1.0 M KOH seawater solutions, respectively. These outcomes demonstrate that MoRu-NG has exceptional stability and durability in acidic and alkaline electrolytes. In XRD patterns, the typical diffraction peaks of MoRu-NG still reveal a Ru crystalline (PDF#06-0663) phase after electrochemical cycling (Figure S34). And, beyond that, TEM characterization displays that Mo-Ru NPAs in MoRu-NG still maintain a fine nanoparticle state after electrochemical cycling (Figure S35). In addition, the chemical valence states of Mo and Ru species remain almost unchanged, further signifying the robustness of MoRu-NG catalysts in the electrocatalytic HER process (Figures S36 and S37). The structure of O, O’-bidentate ligands in MoRu-NG shortens the Ru-Ru bond length and provides stronger Ru metallic bonds to enhance the electrocatalytic stability [20]. In this work, the excellent physicochemical stability of MoRu-NG is attributed to the particular coordination structure between Mo and Ru atoms in NG sheets through electrostatic self-assembly [72,73].

2.4. Mechanism of HER and Schematic Representation of Seawater Electrocatalysis

MoRu-NG has a higher HER kinetic performance than other electrocatalysts through the Volmer–Tafel mechanism. MoRu-NG has the higher ability to obtain values of η10 = 53 mV and η10 = 28 mV in the 0.5 M H2SO4 and 1.0 KOH seawater solutions, respectively, which surpasses those of other reported electrocatalysts [13,14,15,16]. In fact, HER occurs based on the Volmer–Heyrovsky mechanism or the Volmer–Tafel mechanism, while the evolutionary steps of HER are similar in both alkaline and acidic media (Figure 5a). For both acidic and alkaline conditions, the first step of HER is the Volmer reaction, which produces H* intermediates on MoRu-NG. In this work, Ru sites on MoRu-NG were chosen as active metal sites and potential adsorption sites for H* intermediates. The second step is the rate determination step (RDS), belonging to the surface recombination step, which indicates that the process of RDS depends on the activity of the catalysts. Consequently, Pt-based catalysts undergo the Tafel reaction in order to adsorb double H* intermediates to produce H2, while MoRu-NG as the transition metal catalyst typically reacts with H2O (in alkaline conditions) or H+ (in acidic conditions) to produce H2. A schematic representation of seawater electrocatalysis is shown in Figure 5b. In the actual overall seawater splitting test, the assembled MoRu-NG || RuO2 system exhibits better performance than the commercial Pt/C || RuO2 electrode pair (Figure S38). It is evident that the assembled MoRu-NG || RuO2 system requires lower voltages to achieve the required current density compared to the commercial electrode sets, demonstrating that the catalyst has the high activity for full seawater splitting. Meanwhile, the assembled MoRu-NG electrode pairs can also operate stably for 100,000 s at a current density of 300 mA cm−2, which indicates its good durability for full seawater splitting (Figure S39).

3. Experiments

3.1. Preparation of the MoRu-NG Composite

The precursors, namely (NH4)6Mo7O24·4H2O and RuCl3·3H2O, were added to an aqueous suspension of GO to form a homogeneous solution. Subsequently, the resulting mixture was transferred to a Teflon-lined autoclave and heated at 180 °C for 12 h. After the heating process, the as-prepared product underwent freeze-drying, which resulted in the formation of a spongy column [74]. The sponge column of p-RGO with RuMo3 groups was obtained by freeze-drying the product after hydrothermal synthesis. Finally, the composite was reacted with NH3/Ar at 800 °C in the CVD apparatus to obtain MoRu-NG [75,76].

3.2. Preparation of Mo-NG and Ru-NG Composites

(NH4)6Mo7O24·4H2O and RuCl3·3H2O precursors were added to two copies of aqueous suspension of GO to form two copies of a homogeneous solution, respectively. Following the same steps as in Section 2.1, Mo-NG and Ru-NG catalysts were obtained.

3.3. Preparation of A-MR, B-MR, and C-MR Composites

Following the same steps as in Section 2.1, A-MR, B-MR and C-MR catalysts were obtained via different atomic mass ratios. Details are shown in Table S1.

3.4. Material Characterization

X-ray powder diffraction was performed using a Rigaku D/Max Ultima IV (Rigaku Corporation, Tokyo, Japan) diffractometer configured with a Cu-Kα radiation source (λ = 1.5418 Å) and graphite monochromator at 40 kV voltage and 40 mA current. A JEOL-JSM-7001F SEM and JEOL 2100 (Rigaku Corporation, Tokyo, Japan) field emission gun TEM were used to examine the morphology. AC HAADF-STEM images and EDS elemental mappings were carried out by a Cs-corrected FEI Titan G2 60-300 equipped with a Super-X EDS detector (Thermo Fisher Scientific, Hillsboro, OR, USA) and operated at 300 kV. Chemical compositions and elemental oxidation states were checked by XPS spectra PHI-5702 (Waltham, MA, USA). Raman spectra were recorded on a confocal micro-Raman spectrometer (Horiba Jobin Yvon, Stanmore, UK) (Horiba Jobin Yvon, LabRAM HR Evolution, λ = 532 nm). N2 adsorption–desorption isotherms were measured by a Quantachrome autosorb iQ2 analyzer (Quantachrome Instruments, Boynton Beach, FL, USA). The specific area was obtained using the BET method. The pore size was calculated based on the Barret–Joyner–Halenda (BJH) method. Water and air bubble CA were recorded using a DSA100 contact angle analyzer (Kruss, Germany). FT-IR spectra were collected on a Thermo Nicolet (Waltham, MA, USA). Room-temperature EPR spectra were obtained using an EMXPLUS10/12 paramagnetic resonance spectrometer (Bruker Corporation, Billerica, MA, USA).

3.5. HER Electrochemical Setup

Electrochemical measurements for HER were performed using a standard three-electrode system at an electrochemical workstation (CHI 660E) (Chenhua Instrument Co., Ltd., Shanghai, China). To prepare the working electrode, a 2 mg catalyst and 40 μL of 5 wt% Nafion solution were dispersed in 0.5 mL of 4:1 v/v water/ethanol followed by about 2 h bath-sonication until a homogeneous suspension was formed. Then, 10 μL of the above catalyst suspension was loaded onto carbon fiber paper (CFP, 1 cm × 1 cm). The electrode was allowed to dry at room temperature at least 24 h before measurement. After drying, a catalyst mass loading of 0.823 mg cm−2 was obtained.
The CFP was used with various catalysts as the working electrode, graphite rod as the counter electrode, and saturated calomel electrode (SCE) as the reference electrode. The polarization curves were obtained in the 0.5 M H2SO4 and 1.0 M KOH seawater solutions with a scan rate of 50 mV s−1 at room temperature. The potentials were referenced to an RHE: E (RHE) = E (SCE) + (0.242 + 0.059 pH) V. To measure electrochemical capacitance, the CV curves were swept from 0.15 to 0.35 V at different scan rates in 0.5 M H2SO4 and 1.0 M KOH. The electrochemical EIS was performed from 10−2 to 106 Hz with an AC voltage of −5 mV. The electrolyte solution was purged with H2 for 30 min prior to the experiment.

4. Conclusions

In summary, MoRu-NG has been successfully synthesized by facile hydrothermal and CVD methods, in which Mo-Ru NPAs are O-coordinated. Additionally, MoRu-NG exhibits a well-defined Ru-O-Mo-N-C structure characterized by the presence of O,O’-bidentate ligands. In MoRu-NG, Ru atoms have half full d-electron orbitals and thus possess optimal hydrogen adsorption energy, which indicates that the excellent HER activity is determined by Ru centers. The O,O’-bidentate ligands in MoRu-NG play a vital role in HER performance, shortening the Ru-Ru bond length in Ru NPs, which increases Ru metallic bond intensity and further promotes HER stability. The high HER activity of MoRu-NG mainly depends on Ru atoms as the active center, and the excellent HER stability is attributed to the structure of O,O’-bidentate ligands after the introduction of Mo SAs. Therefore, the pursuit of knowledge and understanding regarding strong electron coupling offer favorable pathways for the development of cost-effective, high-performance, and durable catalysts within the renewable energy economy.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal15050475/s1, Catalysts synthesis. Material characterization. Electrode preparation. HER test. ECSA determination. HER mechanism. Figure S1. XRD of A-MR and C-MR; Figure S2. TEM images of MoRu-NG; Figure S3. Particle size distribution image of Ru-MR; Figure S4. TEM images of Mo-NG (A–C) and Ru-NG (D–F); Figure S5. TEM–element energy dispersive spectroscopy (EDS) mapping of MoRu-NG; Figure S6. XPS survey spectrum of MoRu-NG, Mo-NG, Ru-NG, A-MR, and C-MR; Figure S7. The Mo 3d spectra of A-MR and C-MR; Figure S8. The Ru 3p spectra of A-MR and C-MR; Figure S9. The N 1s spectra of A-MR and C-MR; Figure S10. The O 1s spectra of A-MR and C-MR; Figure S11. The C 1s + Ru 3d spectra of MoRu-NG, Mo-NG, and Ru-NG; Figure S12. The C 1s + Ru 3d spectra of A-MR and C-MR; Figure S13. The electron paramagnetic resonance (EPR) spectra of A-MR, B-MR, and C-MR; Figure S14. The Fourier transform infrared spectroscopy (FT-IR) spectra of A-MR, B-MR, and C-MR; Figure S15. The Raman spectra of A-MR, B-MR, and C-MR; Figure S16. The water contact angle (CA) of MoRu-NG, Mo-NG, and Ru-NG; Figure S17. The air contact angle (CA) of Mo-NG and Ru-NG; Figure S18. The Brunauer–Emmett–Teller (BET) surface areas of Mo-NG and Ru-NG; Figure S19. The Brunauer–Emmett–Teller (BET) surface areas of A-MR and C-MR; Figure S20. The Polarization curves and the corresponding Tafel plots of A-MR, B-MR, and C-MR in (A,B) 0.5 M H2SO4 and (C,D) 1.0 M KOH seawater solutions; Figure S21. The Polarization curves (A) and the corresponding Tafel plots (B) of MoRu-NG, Mo-NG, and Ru-NG in 1.0 M PBS seawater solutions; Figure S22. The cyclic voltammetry (CV) of Mo-NG (A,C) and Ru-NG (B,D) at various scan rates in 0.5 M H2SO4 and 1.0 M KOH seawater solutions, respectively; Figure S23. The cyclic voltammetry (CV) at various scan rates and double-layer capacitance (Cdl) of NG (A,B), A-MR (C,D), and C-MR (E,F) in 0.5 M H2SO4 seawater solutions, respectively; Figure S24. The cyclic voltammetry (CV) at various scan rates and double-layer capacitance (Cdl) of NG (A,B), A-MR (C,D), and C-MR (E,F) in 1.0 M KOH seawater solutions, respectively; Figure S25. The polarization curves and the corresponding Tafel plots of MoRu-NG in (A,C) 0.5 M H2SO4 and (B,D) 1.0 M KOH pure water solutions, respectively; Figure S26. The cyclic voltammetry (CV) at various scan rates and double-layer capacitance (Cdl) of MoRu-NG in 0.5 M H2SO4 and 1.0 M KOH pure water solutions, respectively; Figure S27. Geometric current density (j) was normalized by the ECSA (jECSA) of MoRu-NG, Mo-NG, and Ru-NG in 0.5 M H2SO4 (A) and 1.0 M KOH (B) seawater solutions; Figure S28. The small η10 and Tafel slope of other reported Mo- or Ru-based metal electrocatalysts in an acidic solution; Figure S29. The small η10 and Tafel slope of other reported Mo- or Ru-based metal electrocatalysts in an alkaline solution; Figure S30. The integrated charge of anodic CV cycles in a phosphate buffer solution (pH = 7); Figure S31. EIS Nyquist plots of MoRu-NG-, Mo-NG-, and Ru-NG-catalyzed HER at an overpotential of 100 mV. The inset shows an equivalent circuit for the MoRu-NG electrode. Rs, Rct, CPE, and W represent the bulk resistance of the electrolyte and electrodes, the charge transfer resistance, the chemical capacitance, and the Warburg impedance, respectively; Figure S32. The curve after 10,000, even 50,000, CV cycles in 0.5 M H2SO4 seawater solution; Figure S33. The curve after 10,000, even 50,000, CV cycles in 1.0 M KOH seawater solution; Figure S34. XRD patterns of MoRu-NG after electrochemical cycling; Figure S35. TEM characterization after electrochemical cycling; Figure S36. The Ru 3p spectra of MoRu-NG after electrochemical cycling; Figure S37. The Mo 3d spectra of MoRu-NG after electrochemical cycling; Figure S38. Comparison of the LSV curves; Figure S39. Chronopotentiometric curve of the MoRu-NG || RuO2 system for overall water splitting with a constant current density at 300 mA cm−2; Table S1. CVD processing time, CVD processing temperature, and composition of NG-supported electrocatalysts; Table S2. The Ru and Mo content in the MoRu-NG; Table S3. Summary of the HER performance of the reported Mo-based or Ru-based metal catalysts in a 0.5 M H2SO4 electrolyte; Table S4. Summary of the HER performance of the reported Mo-based or Ru-based metal catalysts in a 1.0 M KOH electrolyte.

Author Contributions

S.W.: conceptualization, data curation, formal analysis, investigation, writing—review and editing; J.Q.: conceptualization, writing—review and editing, supervision, resources; Y.Z.: conceptualization, writing—review and editing, supervision, resources; S.C.: conceptualization, investigation, writing—review and editing; W.Y.: writing—review and editing, resources; H.Z.: conceptualization, writing—review and editing, supervision, resources; X.F.: funding acquisition, writing—review and editing, project administration, resources, supervision. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (No. 22175109) and the “Young Talent Support Plan” of Xi’an Jiaotong University.

Data Availability Statement

The original contributions presented in this study are included in this article/its Supplementary Materials. Further inquiries can be directed to the corresponding authors.

Acknowledgments

The authors are thankful to the Instrument Analysis Center of Xi’an Jiaotong University for their help with the XRD, XPS, and TEM analyses.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Synthesis of MoRu-NG. (a) Schematic illustration of the synthetic procedure. (b) The proposed chemical mechanism.
Figure 1. Synthesis of MoRu-NG. (a) Schematic illustration of the synthetic procedure. (b) The proposed chemical mechanism.
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Figure 2. (a) XRD patterns and selected range of XRD patterns. (b,c) HR-TEM images of MoRu-NG; inset of (b) indicates particle size distribution diagram; inset of (c) indicates lattice fringe pattern. (d,e) AC HAADF-STEM images of MoRu-NG. The image displays Mo-Ru NPAs (yellow circles). (f) HAADF-STEM image of MoRu-NG and corresponding EDS elemental mapping images.
Figure 2. (a) XRD patterns and selected range of XRD patterns. (b,c) HR-TEM images of MoRu-NG; inset of (b) indicates particle size distribution diagram; inset of (c) indicates lattice fringe pattern. (d,e) AC HAADF-STEM images of MoRu-NG. The image displays Mo-Ru NPAs (yellow circles). (f) HAADF-STEM image of MoRu-NG and corresponding EDS elemental mapping images.
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Figure 3. The high-resolution XPS spectra of (a) Mo 3d, (b) Ru 3p, (c) O 1s, and (d) N 1s and the (e) EPR spectra, (f) FTIR spectra, and (g) Raman spectra of MoRu-NG, Mo-NG, and Ru-NG. (h) N2 adsorption–desorption isotherm of MoRu-NG; the inset is a pore size distribution diagram. (i) Air bubble CA of MoRu-NG.
Figure 3. The high-resolution XPS spectra of (a) Mo 3d, (b) Ru 3p, (c) O 1s, and (d) N 1s and the (e) EPR spectra, (f) FTIR spectra, and (g) Raman spectra of MoRu-NG, Mo-NG, and Ru-NG. (h) N2 adsorption–desorption isotherm of MoRu-NG; the inset is a pore size distribution diagram. (i) Air bubble CA of MoRu-NG.
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Figure 4. The Polarization curves and the corresponding Tafel plots of MoRu-NG, Mo-NG, Ru-NG, NG, and Pt/C in (a,b) 0.5 M H2SO4 and (c,d) 1.0 M KOH seawater solutions, respectively. Cdl measurement with varying scan rates from 20 to 300 mV s−1 in (e) 0.5 M H2SO4 and (f) 1.0 M KOH seawater solutions; inset is the CV cycle of MoRu-NG. (g) TOF values of MoRu-NG, Mo-NG, and Ru-NG in 0.5 M H2SO4 (left) and 1.0 M KOH (right) seawater solutions. (h) Galvanostatic measurement of MoRu-NG in 0.5 M H2SO4 and 1.0 M KOH at j = −100, −200, and −300 mA cm−2.
Figure 4. The Polarization curves and the corresponding Tafel plots of MoRu-NG, Mo-NG, Ru-NG, NG, and Pt/C in (a,b) 0.5 M H2SO4 and (c,d) 1.0 M KOH seawater solutions, respectively. Cdl measurement with varying scan rates from 20 to 300 mV s−1 in (e) 0.5 M H2SO4 and (f) 1.0 M KOH seawater solutions; inset is the CV cycle of MoRu-NG. (g) TOF values of MoRu-NG, Mo-NG, and Ru-NG in 0.5 M H2SO4 (left) and 1.0 M KOH (right) seawater solutions. (h) Galvanostatic measurement of MoRu-NG in 0.5 M H2SO4 and 1.0 M KOH at j = −100, −200, and −300 mA cm−2.
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Figure 5. Mechanism of HER (a) and schematic representation (b) of seawater electrocatalysis.
Figure 5. Mechanism of HER (a) and schematic representation (b) of seawater electrocatalysis.
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MDPI and ACS Style

Wang, S.; Qin, J.; Zhang, Y.; Chen, S.; Yan, W.; Zhou, H.; Fan, X. Ru Nanoparticle Assemblies Modified with Single Mo Atoms for Hydrogen Evolution Reactions in Seawater Electrocatalysis. Catalysts 2025, 15, 475. https://doi.org/10.3390/catal15050475

AMA Style

Wang S, Qin J, Zhang Y, Chen S, Yan W, Zhou H, Fan X. Ru Nanoparticle Assemblies Modified with Single Mo Atoms for Hydrogen Evolution Reactions in Seawater Electrocatalysis. Catalysts. 2025; 15(5):475. https://doi.org/10.3390/catal15050475

Chicago/Turabian Style

Wang, Shuhan, Jiani Qin, Yong Zhang, Shuai Chen, Wenjun Yan, Haiqing Zhou, and Xiujun Fan. 2025. "Ru Nanoparticle Assemblies Modified with Single Mo Atoms for Hydrogen Evolution Reactions in Seawater Electrocatalysis" Catalysts 15, no. 5: 475. https://doi.org/10.3390/catal15050475

APA Style

Wang, S., Qin, J., Zhang, Y., Chen, S., Yan, W., Zhou, H., & Fan, X. (2025). Ru Nanoparticle Assemblies Modified with Single Mo Atoms for Hydrogen Evolution Reactions in Seawater Electrocatalysis. Catalysts, 15(5), 475. https://doi.org/10.3390/catal15050475

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