Constructing Ni3Se2‐Nanoisland‐Confined Pt1Mo1 Dual‐Atom Catalyst for Efficient Hydrogen Evolution in Basic Media

Constructing efficient and stable catalysts is the key to achieving green hydrogen production through electrolysis of water. Atomically dispersed catalysts have received widespread attention due to their high atomic utilization and catalytic efficiency. Herein, Pt1Mo1 dual‐atom catalysts anchored on the nickel selenide nanoisland (Pt1Mo1/Ni3Se2) are prepared by a two‐step method. It only needs 53 mV to deliver the current density of 10 mA cm−2 in 1 M KOH media, and the mass activity at 200 mV is approximately 4.13 times higher than that of Pt/C. In addition, the Pt1Mo1/Ni3Se2 also exhibits electrochemical stability of nearly 60 h at 20 mA cm−2. It is shown in the studies that the synergistic effect between Pt and Mo atoms enables the migration of electrons around Mo atoms toward Pt, thus realizing charge redistribution. Further density functional theory calculations verify that synergistic effect of Pt and Mo atoms could optimize the adsorption of H*, enhancing the hydrogen evolution reaction activity. Moreover, the Ni3Se2 nanoisland prevents the aggregation of Pt and Mo dual atom, effectively improving the stability of the catalyst. In this work, a nanoisland confined strategy is provided to construct atomically dispersed catalysts with high activity and stability for water splitting.

modulate the electronic structure of the catalyst to promote the reaction. [35,36]However, for atomically dispersed catalysts, the increased free energy of the metal surface inevitably makes it easier to aggregate into larger clusters or nanoparticles during the reaction. [37,38]The aggregation of metal atoms reduces the number of active sites, [39,40] which leads to the decline of catalyst activity and stability, thus largely limiting the practical application of atomic-scale catalysts in electrocatalysis.Remarkably, the confinement strategy could effectively anchor the atoms to prevent their aggregation process, which is a valid way to improve the stability of catalysts.Zeng's group designed a "single-atom nano-islands (SANIs)" catalyst that confines the dispersed Pt metal atoms to isolated and defective CeO x nanoislands, ensuring Pt single atoms move around their respective "island" without aggregating. [41]The concept of "limiting effect of nanoislands" realizes the dynamic constraint on single-atom, resulting in the catalyst still maintains high stability under harsh reaction conditions.However, there are very few reports on the construction of "nanoislands"-confined DACs to enhance their HER activity and stability under alkaline conditions.Therefore, it is essential to develop a simple strategy to develop confined DACs for efficient and stable hydrogen evolution.
Herein, we successfully constructed a nickel-selenidenanoisland-confined Pt and Mo DAC (Pt 1 Mo 1 /Ni 3 Se 2 ) by a simple two-step method.Remarkably, Pt 1 Mo 1 /Ni 3 Se 2 exhibits excellent HER catalytic activity in alkaline solution with an overpotential of 53 mV at 10 mA cm À2 , which is superior to many reported noble metal-based SACs.Furthermore, the mass activity of Pt 1 Mo 1 /Ni 3 Se 2 at 200 mV is 0.33 A mg À1 , which is 4.13 times higher than that of Pt/C.Meanwhile, the obtained catalyst also maintains superb stability for nearly 60 h.The further mechanism analyses show that the excellent HER activity is attributed to the synergistic effect between Pt and Mo atoms.The introduce of Mo atom effectively regulates the electronic structure of the catalyst and promotes the transfer of electrons from Mo atoms to Pt atoms, thus enhancing the reaction kinetics.Moreover, the nanoisland structure of Ni 3 Se 2 could effectively anchor Pt and Mo atoms to form highly stable active centers, thereby improving the intrinsic activity and stability of Pt 1 Mo 1 /Ni 3 Se 2 .The density-functional theory (DFT) calculation results show that Pt 1 Mo 1 /Ni 3 Se 2 shows a Gibbs free energy of H adsorption (ΔG H* ) closer to 0 than Pt 1 /Ni 3 Se 2 .It verified that the synergistic effect of dual-atom sites leads to the catalyst show a more appropriate hydrogen-adsorption energy, which is the key factor to enhance the HER activity.This work provides a new strategy for constructing efficient and stable DACs for hydrogen evolution.

Results and Discussion
Figure 1 shows the preparation process of Pt 1 Mo 1 /Ni 3 Se 2 electrocatalyst.First, Mo-doped nickel selenide precursor (Mo/Ni 3 Se 2 ) was grown on nickel foam (NF) by hydrothermal method.NF serves as both a conductive substrate and nickel source.Pt atoms were then loaded on Mo/Ni 3 Se 2 by electrochemical deposition to form Pt 1 Mo 1 /Ni 3 Se 2 dual-atom nanoisland catalyst.From the scanning electron microscopy (SEM), it could be seen that the morphology of Pt 1 Mo 1 /Ni 3 Se 2 is a tightly arranged nanorod array (Figure 2a).Compared with the original Ni 3 Se 2 carriers (Figure S1, Supporting Information), the staggered nanorod array provides a larger specific surface area, which is conducive to exposing more active sites. [42]Transmission electron microscopy (TEM) image and scanning transmission electron microscopy (STEM) image further reveal the homogeneous nanorod morphology of Pt 1 Mo 1 /Ni 3 Se 2 , and no obvious nanoparticles are observed (Figure S2, Supporting Information).In addition, the highresolution transmission electron microscopy (HRTEM) image clearly displays the surface structural information of the Pt 1 Mo 1 /Ni 3 Se 2 catalyst (Figure 2b).In particular, the lattice spacing of 0.135 and 0.191 nm corresponds to the (312) and ( 211 In addition, aberration-corrected high-angle annular dark-field scanning TEM (AC-HAADF-STEM) shows that high density of small bright spots is anchored on nanoclusters with island-like structures (Figure 2d and S3, Supporting Information).Among them, the "nanoislands" (marked by orange ring) with a diameter less than 5 nm are nanoclusters composed of Ni 3 Se 2 species.The adjacent small bright spots verify the existence of Pt-Mo dual atom on the Ni 3 Se 2 nanoisland.The brighter and darker spots are marked with red and yellow circles, respectively.Next, the intensity profile analysis of the atom pairs in region 1-4 shows that the Pt and Mo atoms can be roughly distinguished (Figure 2e and S4, Supporting Information).The brighter points correspond to Pt atoms (red circle), while the darker points correspond to Mo atoms (yellow circle), because Pt atoms show a stronger peak intensity than Mo atoms.Meanwhile, Figure 2e and S4, Supporting Information, also reveal that the distance between Pt-Mo dual atom is approximately 2.85 AE 0.05 Å.It indicates that the strong electronic interaction exists between the two atoms, and the Pt and Mo atoms may be coupled. [43,44]In addition, it could be directly observed from the elemental mapping images and energy dispersive spectroscopy (EDS) spectrum that Pt, Mo, Ni, Se, and O elements are uniformly dispersed on the surface of catalyst (Figure 2f, S5, and S6, Supporting Information).Particularly, the O element is mainly derived from oxygencontaining species in electrolyte during the electrochemical deposition process.Inductively coupled plasma-optical emission spectrometry (ICP-OES) also reveals that the Pt and Mo metal contents in Pt 1 Mo 1 /Ni 3 Se 2 are 3.80 and 0.91 wt%, respectively (Table S1, Supporting Information).In addition, Figure 2g   ICP-OES is approximately 3.53 wt%, which is similar to Pt 1 Mo 1 / Ni 3 Se 2 with high loading capacity (Table S2, Supporting Information).This may be attributed to the effective anchoring of Ni 3 Se 2 nanoisland.As a comparison, the corresponding characterizations of Mo/Ni 3 Se 2 and Ni 3 Se 2 are shown in Figure S8 and S9, Supporting Information.
To further determine the crystal structure of the catalyst, X-ray diffraction (XRD) experiments were conducted.Figure 3a shows the XRD spectrums of Ni 3 Se 2 , Pt 1 /Ni 3 Se 2 , Mo/Ni 3 Se 2 , and Pt 1 Mo 1 /Ni 3 Se 2 .It can be seen that all samples have the same crystal phase, mainly composed of Ni 3 Se 2 (PDF#19-0841).It is consistent with the results of HRTEM and SAED.Among these peaks, the obvious peaks located at 20.93°, 29.97°, 36.54°,37.17°, 42.63°, 47.70°, 52.75°, 53.48°, 61.43°, 69.88°, and 74.81°belong to the (101), ( 012), ( 021), ( 003), ( 202), ( 211), ( 122), ( 104), ( 220), (312), and (205) crystal planes of Ni 3 Se 2 , respectively.Moreover, no obvious diffraction peaks of Pt and Mo species were observed from the XRD spectrum, which also confirms that Pt and Mo are not nanoparticles.The elemental composition and chemical valence state of the catalyst were determined by X-ray photoelectron spectroscopy (XPS).As shown in Figure 3b, the binding energies of Pt 4f 7/2 and Pt 4f 5/2 in Pt 1 Mo 1 /Ni 3 Se 2 are located at 71.2 and 74.2 eV, respectively, showing an oxidation state between Pt (0) and Pt (þ4). [45,46]It is worth noting that the binding energy of Pt (Figure 3c).The two peaks located at 228.9 and 232.2 eV can be attributed to Mo 4þ 3d 5/2 and Mo 4þ 3d 3/2 , [47,48] while the peaks at 233.5 and 235.4 eV can be attributed to Mo 6þ 3d 5/2 and Mo 6þ 3d 3/2 . [49]In particular, compared with Mo/Ni 3 Se 2 , the binding energy of Mo 4þ 3d 5/2 and Mo 6þ 3d 5/2 orbitals in Pt 1 Mo 1 / Ni 3 Se 2 exhibits positive shifts of about 0.15 and 0.4 eV, respectively.It further indicates the synergistic effect of electron transfer from Mo to Pt between Pt/Mo dual atom, which is the reason why Pt 1 Mo 1 /Ni 3 Se 2 catalyst has excellent HER performance.In addition, in the Ni 2p spectrum shown in Figure 3d, the peaks at 855.7 and 873.3 eV belong to Ni 2p 3/2 and Ni 2p 1/2 , indicating the existence of Ni 2þ . [50]At the same time, two corresponding satellite peaks are located at 861.1 and 879.1 eV, respectively.Similarly, the wider peak located at 58.7 eV in the Se 3d spectrum is due to the Se-O bond generated by surface oxidation (Figure 3e), while the Se 3d 5/2 and Se 3d 3/2 at 54.1 and 55.0 eV could be attributed to the presence of Se 2À . [51]Figure 3f shows the O 1s spectrum, in which two main peaks belong to metal oxide and adsorbed H 2 O, respectively. [52]It can be concluded that the strong interaction between Pt and Mo atoms in Pt 1 Mo 1 / Ni 3 Se 2 effectively regulates the electronic structure of the catalyst and significantly improves the electrocatalytic activity.
The electronic structure and chemical coordination environment of Pt and Mo in Pt   S3, Supporting Information).At the same time, a smaller coordination shell layer of Pt-O is also present at 2.28 Å.More importantly, the coordination peak at 2.58 Å corresponds to the Pt-Mo bond with coordination number of 1.45 [53] (Table S3, Supporting Information), indicating that Pt atoms could directly bond with Mo atoms, and the distance between Pt and Mo atoms is close to the STEM image results (Figure 2d), further proving the formation of Pt-Mo atomic pairs.Notably, the Pt-Pt coordination peak (2.64 Å) was not observed in the Pt  4e).The main peak located at 1.41 Å is close to the peak of Mo-O bonds in MoO 2 and MoO 3 , while the peak at 2.16 Å can be attributed to the Mo-Se scattering characteristics. [54]The weak peak at 2.87 Å corresponds to Mo-Pt bond with a coordination number of 1.45 (Table S4, Supporting Information), indicating the presence of electronic interactions between Pt and Mo atoms.Additionally, no obvious Mo-Mo coordination peak (2.40 Å) and other prominent peaks were detected, strongly proving the existence of atomically dispersed Mo sites.Finally, due to the different intensities of k 3 χ(k), the k 3 χ(k) oscillation curve of Mo in Pt 1 Mo 1 /Ni 3 Se 2 shown in Figure 4f shows a different spectral shape from that of Mo foil.The previous results confirm the existence of Pt/Mo dual atom, which is identical to the previous conclusion of AC-HAADF-STEM.
To access the HER activity of the catalyst, a standard threeelectrode system was used for electrochemical testing in 1.0 M KOH solution at a scanning rate of 5 mV s À1 .First, the activity of Pt 1 Mo 1 /Ni 3 Se 2 was compared with other catalysts by linear sweep voltammetry (LSV) (Figure 5a).From the LSV curve, it could be seen that Pt   Additionally, electrochemical impedance spectroscopy reflects the conductivity of the catalyst.From the fitted Nyquist plot (Figure 5e), it can be seen that the charge-transfer resistance of Pt 1 Mo 1 /Ni 3 Se 2 is about 3.1 Ω, which is much smaller than that of Pt 1 /Ni 3 Se 2 (6.6 Ω), Mo/Ni 3 Se 2 (5.7 Ω), and Ni 3 Se 2 (7.7 Ω).It is suggested that the Pt 1 Mo 1 /Ni 3 Se 2 catalyst significantly enhances electron transfer due to the synergistic effect between dual atom and its unique nanoisland structure.The inset in Figure 5e shows the fitted equivalent circuit diagram, R 1 and R 2 represent solution resistance (R s ) and charge-transfer resistance (R ct ), respectively, and CPE1 is a constant phase element.The experimental results show that the fitted equivalent circuit diagram model matches the actual parameters.The electrochemical active surface area (ECSA) is proportional to the electric double-layer capacitance (C dl ). [55]Thus, the electric double-layer capacitance method can be used to reflect ECSA (Figure S11, Supporting Information) by fitting the cyclic voltammetry curves at different scanning speeds (20-120 mV s À1 ) in the non-Faradaic interval.As shown in Figure S12, Supporting Information, the C dl of Pt 1 Mo 1 /Ni 3 Se 2 is 35.3 mF cm À2 , which is larger than Pt 1 /Ni 3 Se 2 (12.2 mF cm À2 ) and Ni 3 Se 2 (10.1 mF cm À2 ), and only slightly lower than that of Pt/C (53.1 mF cm À2 ) and  S5, Supporting Information.
Generally, the sensitivity of the catalyst surface structure is much higher in alkaline media than in acidic media. [56]herefore, to verify that Pt 1 Mo 1 /Ni 3 Se 2 has super structural stability, a series of characterizations were carried out on the catalyst after the alkaline test.As shown in Figure S13, Supporting Information, the SEM results indicate that Pt 1 Mo 1 /Ni 3 Se 2 can maintain a relatively stable nanoarray structure after electrochemical testing.The lattice distance of 0.191 nm in HRTEM corresponds to the Ni 3 Se 2 (211) crystal plane (Figure 6a).Meanwhile, SAED also confirmed the corresponding crystal planes of Ni 3 Se 2 (Figure 6b).This is consistent with the characterization results before electrochemical test, indicating that Ni 3 Se 2 have stable crystal structures.In addition, the distribution of Pt, Mo, Ni, Se, and O elements can also be seen from the elemental mapping images (Figure 6c and S14, Supporting Information) and EDS spectrum (Figure S15, Supporting Information).Among them, Pt and Mo atoms are uniformly dispersed in the catalyst without obvious particle aggregation.The phase composition of Pt 1 Mo 1 /Ni 3 Se 2 after electrochemical testing was further determined by XRD (Figure 6d).The analysis results show that the tested Pt 1 Mo 1 /Ni 3 Se 2 is still mainly composed of the crystal peaks of Ni 3 Se 2 , which proves that Pt 1 Mo 1 / Ni 3 Se 2 has superior structural stability.Finally, XPS testing was conducted to reveal the changes of electronic valence state of the catalyst before and after scanning.As shown in Figure 6e, there are also spectral peaks corresponding to Pt, Mo, Ni, Se, and O elements in the tested Pt 1 Mo 1 /Ni 3 Se 2 .The high-resolution spectrum of Pt 4f (Figure 6f ) shows that the peaks located at 71.3 and 74.3 eV belong to Pt 4f 7/2 and Pt 4f 5/2 .Compared with the initial Pt 4f spectrum, only a slight positive shift occurred.Meanwhile, in the XPS spectrum of Mo 3d (Figure 6g), there are still two valence states of Mo 4þ and Mo 6þ .Among them, the peaks of Mo 4þ 3d 5/2 and Mo 4þ 3d 3/2 are located at 228.9 and 232.2 eV, while the peaks at 233.5 and 235.4 eV are attributed to Mo 6þ .The orbital binding energy of Mo 3d has barely shifted compared to that of Mo 3d before the test.It shows that the electronic structures of Pt and Mo have not changed significantly.Furthermore, as shown in Figure 6h, the main peak attributed to Ni 2þ 2p in Pt 1 Mo 1 /Ni 3 Se 2 has no obvious shift after electrochemical testing.Similarly, the binding energy of Se 3d orbit has only a slight change of À0.03 eV compared with that before the test (Figure 6i), which can be ignored.Figure S16, Supporting Information, also exhibits negligible changes in the electronic states of oxygen species on the catalyst surface.Combined with XPS, XRD, and other series of characterization, it is proved that Pt and Mo metals are still uniformly distributed on the surface of Ni 3 Se 2 carrier after electrochemical tests, verifying that Pt 1 Mo 1 /Ni 3 Se 2 could exhibit excellent structural stability.
To further clarify the influence of the synergistic effect of Pt/Mo dual atom on the catalytic activity of HER, DFT calculations were carried out.According to the experimental results, we selected the Ni 3 Se 2 (202) crystal plane as the substrate to construct Pt 1 Mo 1 /Ni 3 Se 2 and Pt 1 /Ni 3 Se 2 structural models (Figure 7a,b).It is worth noting that the Gibbs free energy of H adsorption (ΔG H* ) is considered as the evaluation standard for HER catalytic activity.The closer the ΔG H* of the active sites to zero, the more conducive to achieving rapid HER kinetics by balancing the adsorption and desorption of H* on the surface. [2,57,58]As shown in Figure 7c

Conclusion
In summary, we successfully prepared nickel-selenidenanoisland-confined Pt and Mo DAC (Pt 1 Mo 1 /Ni 3 Se 2 ) through a simple two-step method.Electrochemical experiments showed that the synthesized catalyst exhibited excellent HER activity, which is even comparable to commercial Pt/C in alkaline media.Specifically, Pt 1 Mo 1 /Ni 3 Se 2 requires an overpotential of only 53 mV to reach the current density of 10 mA cm À2 , and the Tafel slope is only 49.6 mV dec À1 .Meanwhile, Pt 1 Mo 1 /Ni 3 Se 2 exhibits outstanding durability, almost no potential decay after stability tests for nearly 60 h at the current density of 20 mA cm À2 .Further experimental studies and DFT calculations suggest that the excellent HER activity is attributed to the synergistic effect between Pt and Mo atoms.It effectively modulates the transfer of electrons around Mo atoms to Pt atoms in the catalyst, thus optimizing the adsorption strength of the reaction intermediates.In addition, the nickel selenide nanoisland could effectively confine Pt and Mo atoms, thereby avoiding the aggregation of metal atoms, which is essential to improve the activity and stability of Pt 1 Mo 1 /Ni 3 Se 2 catalyst.This work provides a new strategy for the design and construction of highly active atomically dispersed HER electrocatalysts.
Figure1shows the preparation process of Pt 1 Mo 1 /Ni 3 Se 2 electrocatalyst.First, Mo-doped nickel selenide precursor (Mo/Ni 3 Se 2 ) was grown on nickel foam (NF) by hydrothermal method.NF serves as both a conductive substrate and nickel source.Pt atoms were then loaded on Mo/Ni 3 Se 2 by electrochemical deposition to form Pt 1 Mo 1 /Ni 3 Se 2 dual-atom nanoisland catalyst.From the scanning electron microscopy (SEM), it could be seen that the morphology of Pt 1 Mo 1 /Ni 3 Se 2 is a tightly arranged nanorod array (Figure2a).Compared with the original Ni 3 Se 2 carriers (FigureS1, Supporting Information), the staggered nanorod array provides a larger specific surface area, which is conducive to exposing more active sites.[42]Transmission electron microscopy (TEM) image and scanning transmission electron microscopy (STEM) image further reveal the homogeneous nanorod morphology of Pt 1 Mo 1 /Ni 3 Se 2 , and no obvious nanoparticles are observed (FigureS2, Supporting Information).In addition, the highresolution transmission electron microscopy (HRTEM) image clearly displays the surface structural information of the Pt 1 Mo 1 /Ni 3 Se 2 catalyst (Figure2b).In particular, the lattice spacing of 0.135 and 0.191 nm corresponds to the (312) and (211) crystal planes of Ni 3 Se 2 .The selected area electron diffraction (SAED) in Figure 2c is composed of a series of concentric rings with different radii, indicating the polycrystalline structure of Pt 1 Mo 1 /Ni 3 Se 2 catalyst.Notably, SAED image definitely proves the existence of Ni 3 Se 2 (202) and (205) crystal planes.In addition, aberration-corrected high-angle annular dark-field scanning TEM (AC-HAADF-STEM) shows that high density of small bright spots is anchored on nanoclusters with island-like structures (Figure2dand S3, Supporting Information).Among them, the "nanoislands" (marked by orange ring) with a diameter less than 5 nm are nanoclusters composed of Ni 3 Se 2 species.The adjacent small bright spots verify the existence of Pt-Mo dual atom on the Ni 3 Se 2 nanoisland.The brighter and darker spots are marked with red and yellow circles, respectively.Next, the intensity profile analysis of the atom pairs in region 1-4 shows

Figure 1 .
Figure 1.Synthesis route of nickel selenide nanoisland confined Pt and Mo dual-atom catalyst.
is the SEM image of the comparison sample Pt 1 /Ni 3 Se 2 , which shows a relatively disordered nanoarray structure.The HRTEM of Pt 1 /Ni 3 Se 2 reveals that the lattice spacing of 0.212 nm corresponds to Ni 3 Se 2 (202) crystal plane (Figure S7c, Supporting Information).The inset in Figure S7c, Supporting Information, further illustrates that only crystal planes of Ni 3 Se 2 exist in the catalyst.In Figure 2h, it can be clearly seen that the nanoisland in Pt 1 /Ni 3 Se 2 is also composed of Ni 3 Se 2 nanoclusters.Many bright spots corresponding to Pt single atoms are scattered on the nanoisland.These bright spots are evenly anchored on the highly dispersed nanoisland, thus effectively avoiding the aggregation of Pt single atoms (Figure S7d,e, Supporting Information).The element mapping images in Figure 2i also reveals the uniform distribution of Pt, Ni, Se, and O elements.The metal content of Pt in Pt 1 /Ni 3 Se 2 determined by

Figure 2 .
Figure 2. The electron microscopic characterization of Pt 1 Mo 1 /Ni 3 Se 2 : a) Scanning electron microscopy (SEM) image, b) high-resolution transmission electron microscopy (HRTEM) image, c) selected area electron diffraction (SAED) image, and d) aberration-corrected high-angle annular dark-field scanning TEM (AC-HAADF-STEM) image (red and yellow circles represent Pt and Mo atoms, respectively).e) Intensity profiles obtained in area 1 shows Pt-Mo projection distances on the visual plane, and f ) elemental mapping images.The electron microscopic characterization of Pt 1 /Ni 3 Se 2 : g) SEM images, h) AC-HAADF-STEM image, and i) elemental mapping images.
4f 7/2 in Pt 1 Mo 1 /Ni 3 Se 2 is about 0.3 eV lower than that in Pt 1 /Ni 3 Se 2 , indicating a trend of electron transfer from Mo to Pt after the introduction of Mo atoms.The Mo 3d spectrum shows two valence states of Mo 4þ and Mo 6þ

1
Mo 1 /Ni 3 Se 2 were further clarified by X-ray absorption spectrum.It can be seen from the L-edge X-ray absorption near-edge structure (XANES) in Figure 4a that the absorption edge of Pt in Pt 1 Mo 1 /Ni 3 Se 2 is located between Pt foil and PtO 2 .It indicates that Pt atoms exhibit an oxidation state

1
Mo 1 /Ni 3 Se 2 and Pt 1 /Ni 3 Se 2 catalysts, suggesting that no Pt clusters or nanoparticles were formed.Furthermore, Pt-L-edge-extended XANES oscillation k 3 χ(k) is shown in Figure 4c.The coordination shell of Pt in Pt 1 Mo 1 /Ni 3 Se 2 and Pt 1 /Ni 3 Se 2 is different from the Pt atom in Pt foil, again verifying the atomic level dispersion state of Pt.Similarly, Figure 4d displays the Mo-K-edge XANES spectrum of Pt 1 Mo 1 /Ni 3 Se 2 .It can be clearly observed that the absorption edge position of Mo K-edge in Pt 1 Mo 1 /Ni 3 Se 2 is between Mo foil and MoO 3 .It manifests that the average oxidation state is between Mo (0) and Mo (þ6), which is consistent with the conclusion analysis of Mo orbital in XPS.The Mo atom in Pt 1 Mo 1 /Ni 3 Se 2 exhibits three different coordination peaks (Figure

1
Mo 1 /Ni 3 Se 2 has excellent activity, requiring only 53 mV overpotential at a current density of 10 mA cm À2 , which is much better than Pt 1 /Ni 3 Se 2 (108 mV), Mo/Ni 3 Se 2 (125 mV), and Ni 3 Se 2 (176 mV).It is worth noting that Pt 1 Mo 1 /Ni 3 Se 2 (η 110 = 159 mV) displays better hydrogen evolution activity than commercial Pt/C (η 110 = 160 mV) under the current densities of over 110 mA cm À2 (Figure S10, Supporting Information).The results indicate that the strong interaction between Pt and Mo atoms in Pt 1 Mo 1 /Ni 3 Se 2 catalyst could significantly improve the HER activity.Moreover, the kinetic process of the reaction was evaluated by Tafel slope.Figure 5b clearly manifests that the Tafel slopes of Pt 1 Mo 1 / Ni 3 Se 2 , Pt 1 /Ni 3 Se 2 , Mo/Ni 3 Se 2 , Ni 3 Se 2 , and Pt/C correspond
to 49.6, 58.8, 74.4,79.2, and 32.3 mV dec À1 , respectively.Therefore, Pt 1 Mo 1 /Ni 3 Se 2 with a smaller Tafel slope exhibits quicker reaction kinetics.Furthermore, according to Tafel slope value, it can be inferred that Pt 1 Mo 1 /Ni 3 Se 2 follows the Volmer-Heyrovsky mechanism.Figure 5c shows the mass activity of Pt/C, Pt 1 /Ni 3 Se 2 , and Pt 1 Mo 1 /Ni 3 Se 2 catalysts are 0.08, 0.10, and 0.33 A mg À1 at 200 mV overpotential, respectively.Among them, the mass activity of Pt 1 Mo 1 /Ni 3 Se 2 is approximately 4.13 times higher than that of Pt/C.The turnover frequency (TOF) is also an important parameter for measuring the intrinsic activity of the catalyst.As shown in Figure 5d, Pt 1 Mo 1 /Ni 3 Se 2 shows the highest TOF of 0.34 s À1 (at À0.2 V), which is 3.40 and 4.25 times higher than Pt 1 /Ni 3 Se 2 (0.10 s À1 ) and Pt/C (0.08 s À1 ), so it exhibits faster HER kinetics.
, we calculated the ΔG H* of Pt, Mo, and Ni sites in the DAC Pt 1 Mo 1 /Ni 3 Se 2 , as well as the Pt sites in Pt 1 /Ni 3 Se 2 .The ΔG H* of Pt sites in Pt 1 Mo 1 / Ni 3 Se 2 is À0.18 eV, showing a ΔG H* value closer to 0 than that of Pt 1 /Ni 3 Se 2 (À0.33 eV).This indicates that the introduction of Mo atoms can effectively regulate the hydrogen-adsorption energy of Pt sites and thus improve the HER activity.In addition, the ΔG H* of Mo site in Pt 1 Mo 1 /Ni 3 Se 2 (0.73 eV) is much lower than the Ni site (1.28 eV), which proves that the Mo site also has certain catalytic activity.Based on the previous results, the enhanced HER activity of Pt 1 Mo 1 /Ni 3 Se 2 catalyst is attributed to the synergistic effect between Pt and Mo atoms, which makes the catalyst have more suitable hydrogen-adsorption energy to further improve the HER activity.