Atomic Heterointerface Engineering of Nickel Selenide Confined Nickel Molybdenum Nitride for High‐Performance Solar‐Driven Water Splitting

A heterostructured electrocatalyst of small NiSe2 nanoparticles confined NiMoN nanorods (NiSe2–NPs/NiMoN–NRs) is prepared to accelerate both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in a same alkaline medium. The synergistic effects caused by the combination of merits derived from NiSe2 and NiMoN phases trigger an optimum electronic structure with high density of state at near Fermi level and enhance adsorption free energy, thereby resulting in excellent catalytic activities and strengthened working stability. The catalyst requires a low overpotential of 58 mV for HER and 241 mV for OER to reach 10 mA cm−2 in 1.0 M KOH electrolyte. A two‐electrode electrolyzer based on the developed catalyst shows outstanding cell voltage of 1.51 and 1.46 V to reach 10 mA cm−2 in 1.0 M and 30 wt% KOH solution at 25 °C for overall water splitting, respectively. In addition, the solar‐driven water splitting process delivers a high solar‐to‐H2 conversion efficiency of ∼18.4%, impressively unveiling that the developed bifunctional catalyst is highly potential for overall water splitting to produce green hydrogen fuel.

mechanical stability provide a way for the designing of cost-effective, highly active, and stable catalyst materials. [17,18] In this context, NiMonitride nanoplates, [19] NiMo-nitride nanowires, [20] and NiMo-nitride nanotubes [21] are of great significance in catalyst development to produce H 2 gas. As motivated by the abovementioned merits of the catalyst engineering approaches, a novel heterostructured catalyst was designed for the first time, which was derived from small nickel selenide nanoparticles (NPs) integrated with one-dimensional (1D) NiMonitride nanorods (NRs) (NiSe 2 -NPs/NiMoN-NRs). The catalyst results in superior catalytic activities for both HER and OER in alkaline medium. An electrolyzer based on NiSe 2 -NPs/NiMoN-NRs requires a small cell voltage of 1.51 V at 10 mA cm −2 , good stability, and a high solar-to-hydrogen (STH) conversion efficiency of ∼18.4% toward water splitting, superior to recently reported bifunctional electrocatalysts as well as commercial catalyst system.

Results and Discussion
Scheme 1 shows a schematic for the fabrication of NiSe 2 -NPs/ NiMoN-NRs material. In a specific process, light green NiMoO 4 NRs were initially grown on nickel foam (NF) surface by a simple hydrothermal method ( Figure S1a, Supporting Information). FE-SEM images in Figure S2a,b, Supporting Information, reveal that these NRs, with the length of several micrometers, an average diameter of around 100-200 nm, and a smooth surface, are densely attached to NF forming an interconnected network. The homogeneous dispersion of Ni, Mo, and O elements with their respective content identified by EDS is shown in Figure S2c,d, Supporting Information. Meanwhile, XRD pattern confirms the well-defined crystallinity belonging to NiMoO 4 structure (PDF#98-024-7435) ( Figure S3a, Supporting Information). The formation of Ni-NPs/NiMoN-NRs was then conducted by a nitridation step with the nitrogen source of dicyandiamide to avoid the use of hazardous and environment unfriendly ammonia gas. [22] As shown in Figure S3b, Supporting Information, the NiMoO 4 has an anorthic structure with Mo ions occupying a tetrahedral position (MoO 4 ) and Ni ions occupying an octahedral position (NiO 6 ). Consequently, on the basis of the crystal field theory, Ni in octahedral sites is more active than Mo in tetrahedral sites, [23] thus certain Ni atoms diffuse outward and then aggregate to form small and abundant nanoparticles uniformly distributed on the surface of the NRs during the nitridation process at high temperature ( Figure S3c, Supporting Information). Concurrently, O atoms in NiMoO 4 are also substituted by N, resulting in the NiMoN structure. These phenomena are well confirmed by SEM images in Figure 1a and EDS analysis in Figure S4, Supporting Information. Finally, the formation of NiSe 2 NPs integrated with NiMoN NRs via a selenidization step is observed in Figure 1b, which indicates an increase in nanoparticle size along with coarser characteristics. SEM-EDS results in Figure S5, Supporting Information, show the homogeneous distribution of Ni, Mo, N, and Se elements throughout the material structure. The XRD pattern of the Ni-NPs/NiMoN-NRs material is shown in Figure [24][25][26][27] Regarding the XRD pattern of the NiSe 2 -NPs/NiMoN-NRs, Figure 1c shows a series of diffraction peaks appearing at 29.8°, 33.4°, 36.8°, 42.7°, 44.8°, 50.6°, 55.4°, 57.7°, and 62.1°that are associated with the (200), (210), (211), (220), (221), (311), (230), (321), and (400) crystal plane, respectively, of the NiSe 2 (PDF#00-041-1495), [28] confirming the successful synthesis of NiSe 2 NPs on surface of the NiMoN NRs. However, the crystalline feature of the Ni 0.2 Mo 0.8 N phase is observable with unclear weak specific signals because it is critically affected by the interference phenomenon of background noise caused by the strong intensity of multi diffraction peaks derived from the crystalline NiSe 2 phase, as similarly seen in previous reports. [29,30] Figure 1h also confirms a high-quality crystalline state of the particle. In addition, STEM-EDS analysis in Figure S7, Supporting Information, shows a list of strong Ni signals well consistent with the position of specific nanoparticles at outer side of the nanorod, as well as uniform spreading of Ni, Mo, and N signals over the nanorod's own structure, further indicating the formation of Ni NPs integrated with NiMoN rod, as similarly observed by the earlier report. [31] To verify the atomic Ni/Mo/N ratio in structure of NiMoN-NRs,  Figure 1i,j indicate a good maintenance of microstructure with dense NPs having well-defined shape and distinct crystallinity homogeneously attaching to the nanorod structure. HR-TEM image obtained from a selected region of a nanoparticle in Figure 1k shows a lattice fringe spacing of 0.27 nm consistent with the feature of (210) plane from the NiSe 2 crystals. FFT image obtained in Figure 1l also verifies the high-quality crystallinity of such NiSe 2 nanostructures. STEM-EDS line profile across the protruding NPs on the nanorod's surface in Figure 1m displays strong intensities of Ni and Se elements, implying the formation of NiSe 2 NPs after the material suffers a selenidization process. Meanwhile, STEM-EDS element mapping images in Figure 1n clearly show a respective dispersion of Ni, Mo, and N signals over the nanorod structure, suggesting that a unique heterostructure of NiSe 2 NPs integrated 1D NiMoN NRs is successfully synthesized. Raman analysis was used to study chemical properties of the NiMoO 4 -NRs, Ni-NPs/NiMoN-NRs, and NiSe 2 -NPs/NiMoN-NRs materials ( Figure 2a). There is a strong peak at 951 cm −1 assigned to the symmetric stretch of Mo=O bonding while two small peaks appearing at 872 and 827 cm −1 are originated from the asymmetric stretching modes of Mo=O bonding (Figure 2b). In addition, a peak at 354 cm −1 is also recognized as the bending mode of Mo-O bonding. [32,33] A significant decrease in Mo-O/Mo=O intensity in Ni-NPs/NiMoN-NRs and NiSe 2 -NPs/NiMoN-NRs resulted from the replacement of O by N after an uncompleted nitridation step. [23] Particularly, the Raman spectrum of the NiSe 2 -NPs/NiMoN-NRs shows a new peak at 197 cm −1 that corresponds to the A g stretching mode of Se-Se bonding as result of the successful selenidization to form NiSe 2 material. [34,35] Surface chemistry of the NiSe 2 -NPs/NiMoN-NRs was analyzed by XPS technique in Figure S9, Supporting Information, that indicates the existence of binding energies from Ni 2p at 855.8 eV, Mo 3d at 232.5 eV, N 1 s at 399.1 eV, and Se 3d at 54.7 eV. High-resolution Ni 2p spectrum of the NiSe 2 -NPs/NiMoN-NRs is deconvoluted into two valence states associated with metallic Ni 0 at (852.9 and 869.7) eV and Ni 2+ at (855.8 and 873.6) eV, along with a corresponding satellite component at (861.1 and 879.6) eV ( Figure 2c). [36] High-resolution Mo 3d spectrum of the NiSe 2 -NPs/NiMoN-NRs shows a diverse valence states of Mo, including Mo 2+/3+ at (229.3 and 232.5) eV, Mo 4+ at (230.3 and 233.7) eV, and Mo 6+ at (231.6 and 235.6) eV ( Figure 2d). [37,38] As compared to Ni 2p spectrum of the Ni-NPs/NiMoN-NRs, the NiSe 2 -NPs/NiMoN-NRs shows a decrease in Ni 0 intensity along with a positive shifting of binding energy, implying an increased valance state of Ni at surface of the NiSe 2 material after selenium incorporation to transform metallic Ni into nickel selenide. [39] In addition, a positive shifting of Mo 3d binding energy in the NiSe 2 -NPs/NiMoN-NRs implies that the selenidization step also causes an effective charge redistribution for Mo 3d due to the interaction between NiSe 2 and NiMoN with a favorite electron transfer to Se. High-resolution N 1 s spectrum of the NiSe 2 -NPs/NiMoN-NRs in Figure 2e indicates two main peaks of metal-N and N-H at 398.1 and 400.4 eV, respectively, along with the appearance of a small peak at 394.8 eV assumed to characteristic of Mo 3p binding energy. [40] High-resolution Se 3d spectrum of the NiSe 2 -NPs/ NiMoN-NRs can be deconvoluted into two peaks of Se 3d 5/2 at 54.5 eV and Se 3d 3/2 at 55.3 eV from Se 2− species of NiSe 2 and a broad peak of Se-O at 58.9 eV caused by its surface oxidation phenomenon as exposed to air (Figure 2f). [41,42] Such above results well confirmed the successful formation of the heterogeneous structure derived from NiSe 2 integrated NiMoN on NF. Particularly, certain modulation of electronic properties on surface of the NiSe 2 -NPs/NiMoN-NR observed by the shifting of Ni 2p and Mo 3d binding energies may trigger its catalytic activities toward electrochemical reactions. [43]  The HER performance of the materials was studied by LSV technique at a scan rate of 2.0 mV s −1 in 1.0 M KOH medium ( Figure 3a). It can be realized that the NiSe 2 -NPs/NiMoN-NRs shows the best catalytic behavior with a small required overpotential (η) of only 58 mV to reach 10 mA cm −2 as compared to other surveyed candidates, such as NiMoO 4 -NRs (268 mV), NiMoN-NRs (221 mV), Ni-NPs/ NiMoN-NRs (134 mV), and NiSe 2 -NPs (162 mV) ( Figure S10a, Supporting Information). Such η value of the NiSe 2 -NPs/NiMoN-NRs is also superior to that of various previously reported HER catalysts (Figure S10b, Supporting Information). Tafel slope of the synthesized catalysts was investigated to understand their HER kinetics. Generally, there are two steps occurring on the surface of a catalyst during HER in an alkaline environment. The first step is known as the Volmer step that refers to the dissociation of water molecules to result in the absorbed hydrogen (H * ) at active sites, with a Tafel slope of 120 mV dec −1 (H 2 O +e − → H * + OH − ). The second step is the evolution of hydrogen molecules via a Heyrovsky reaction with a Tafel slope of 40 mV dec −1 (H 2 O + e − + H * → H 2 + OH − ), or via a Tafel reaction with a Tafel slope of 30 mV dec −1 (H * + H * → H 2 ). [44] Figure 3b shows a Tafel slope of 68.7 mV dec −1 for the NiSe 2 -NPs/NiMoN-NRs material, much lower than that of the NiMoO 4 -NRs (187.3 mV dec −1 ), NiMoN-NRs (178.1 mV dec −1 ), Ni-NPs/ NiMoN-NRs (137.8 mV dec −1 ), and NiSe 2 -NPs (165.9 mV dec −1 ), suggesting that it's HER process obeys an accelerated Volmer-Heyrovsky mechanism with a rate-determining Heyrovsky step. [45] Such rapid reaction kinetics of the NiSe 2 -NPs/NiMoN-NRs may result from its unique electronic structure with specific valence states of Ni and Mo elements that can effectively improve charge transferability for a fast HER process. This is well supported by the EIS analysis results shown in Figure S10c and Table S1, Supporting Information, in which the Nyquist plot of NiSe 2 -NPs/NiMoN-NRs exhibits a small semicircle part consistent with a charge transfer resistance (R ct ) of only 12.2 Ω, demonstrating its superior charge transfer ability as compared to other synthesized materials. In addition, the highest exchange current density (J 0 ) of the NiSe 2 -NPs/NiMoN-NRs material was also obtained in Figure S10d, Supporting Information, indicating the NiSe 2 -NPs/NiMoN-NRs possesses a favorable reaction path toward HER process with a great decrease in activation energy barrier. [46,47] To evaluate practicability of the NiSe 2 -NPs/NiMoN-NRs catalyst, the HER stability was measured by chronopotentiometry for a long-term operation in alkaline medium at a high constant current density of 50 mA cm −2 . Figure 3c exhibits a good retention of potential with a potential increase in about 27 mV after continuously working for a period of 48 h. LSV profiles before and after stability test also reveal negligible deterioration of the overpotential (inset in Figure 3c), confirming good stability of the material for a long-term HER operation in alkaline medium. To clarify above achievements, microstructure of the post-HER sample was investigated by SEM and TEM analyses. SEM ( Figure S11a,b, Supporting Information) and TEM ( Figure S11c, Supporting Information) images indicate that the sample still retains its original morphological features with abundant small NPs uniformly distributing on 1D NiMoN surface. In addition, the good crystallinity is confirmed by the HR-TEM image and FFT spectrum in Figure S11d,e, Supporting Information. Furthermore, STEM-EDS mapping images still show a consistency of element distribution over structure of the post-HER samples ( Figure S11f, Supporting Information), evidencing that the NiSe 2 -NPs/NiMoN-NRs has respectable stability of activity and structure for a long-term HER operation. The XPS analysis was also used to illustrate the stability of the NiSe 2 -NPs/NiMoN-NRs during HER process ( Figure S12, Supporting Information). The post-HER sample indicates no significant changes of the Ni 2p, Mo 3d, N 1s, and Se 3d binding energies as compared to those of the pristine sample, confirming the good valence-state maintenance of the elements at material's surface during the HER process.
The OER performance of the materials was then also studied by LSV in 1.0 M KOH medium (Figure 3d). Impressively, the NiSe 2 -NPs/ NiMoN-NRs still exhibits the best OER activities, in which it requires a low η value of 241 mV to reach a current density of 10 mA cm −2 , much smaller than that of the NiMoO 4 -NRs (331 mV), NiMoN-NRs (305 mV), Ni-NPs/NiMoN-NRs (298 mV), NiSe 2 -NPs (252 mV), and even commercial RuO 2 catalyst (302 mV) ( Figure S13b, Supporting Information). In addition, such η value also reveals its catalytic superiority as compared to that of various OER catalysts recently reported elsewhere ( Figure S13b, Supporting Information). Tafel slope was subsequently studied to evaluate OER kinetics of the materials. , and RuO 2 (146.5 mV dec −1 ), suggesting its better OER kinetics occurring in alkaline medium among the surveyed materials. Such results are well consistent with the EIS measurement, which shows a lower R ct value of 2.2 Ω for the NiSe 2 -NPs/NiMoN-NRs than other prepared materials ( Figure S13c and Table S2, Supporting Information) along with its highest J 0 value ( Figure S13d, Supporting Information) as compared to those of other ones. The OER stability of the NiSe 2 -NPs/NiMoN-NRs was investigated by chronopotentiometry for a long-term continuous operation at a current density of 50 mA cm −2 . As indicated in Figure 3f, there is only a slight potential increase in about 30 mV the after 48 h testing. In addition, LSV profiles also exhibit good overlapping behavior without significant degradation of overpotential even after a long-term operation at relatively high current response (Inset in Figure 3f), demonstrating good stability of the material toward OER in alkaline medium. To understand the above achievements, microstructure of the post-OER sample was investigated by FE-SEM ( Figure S14a, Supporting Information) and TEM analyses ( Figure S14b, Supporting Information). A good maintenance of the morphology, in which numerous 0D NPs homogeneously attach over 1D NiMoN surface, is clearly observed. HR-TEM image in Figure S14c, Supporting Information, indicates the appearance of different lattice fringes with 0.27, 0.21, and 0.32 nm, corresponding to the (210), (105), and (210) planes of NiSe 2 , NiOOH, and MoO 3 , respectively, suggesting an increase in valence state for Ni and Mo during the OER process, and STEM-EDS mapping images of the post-OER sample show a wellpreserved distribution of elements in the structure ( Figure S14d, Supporting Information). In addition, the appearance of new crystalline phases relating to NiOOH and MoO 3 oxides is clearly confirmed by the XRD analysis in Figure S15, Supporting Information. As a result, XPS analysis of the post-OER sample also exhibits a positive shifting of Ni 2p and Mo 3d binding energies ( Figure S16a,b, Supporting Information). The Ni 0 /Ni 2+ /Ni 3+ ratio is changed from 7.5%/39.5%/53% to 0/40.7%/59.3% and the Mo 2+/3+ /Mo 4+ /Mo 6+ ratio is changed from 44.5%/21.5%/34.0% to 0/42.7%/57.3%, implying an increase in valence state for Ni and Mo elements at its surface. In addition, a reduction in N 1 s binding energy (Figure S16c, Supporting Information) and an increase in SeO x intensity in Se 3d binding energy (Figure S16d, Supporting Information) additionally suggest oxide/hydroxide surface formation during the OER process. These results demonstrate that the high-valence metal sites generates hydroxide/oxide forms on the catalyst surface during OER and they may act as important active centers to promote the OER catalysis in alkaline medium.
The NiSe 2 NPs amount is dependent on the density of the Ni NPs generated on NiMoN surface during the first nitridation step, which can be relatively controlled by changing the nitridation temperature. Therefore, to explore the effect of NiSe 2 to NiMoN ratio on the HER/ OER properties, different nitridation temperatures of 350, 450, and 550°C were applied to prepare various NiSe 2 -NPs/NiMoN-NRs samples. As shown in Figure S17, Supporting Information, the density and size of the Ni NPs/NiSe 2 NPs on NiMoN surface increase with the increase in nitridation temperature. The NiSe 2 -NPs/NiMoN-NRs sample obtained at 450°C nitridation has NiSe 2 NPs with optimal size, high density, good uniformity as compared to the samples achieved at 350°C and 550°C. Furthermore, it is consistent with the fact that such sample also contains a suitable NiSe 2 /NiMoN ratio for producing high HER and OER catalytic performances superior to two other samples ( Figure S18, Supporting Information).
To clarify advantages of the NiSe 2 -NPs/NiMoN-NRs toward HER and OER, specific surface area and porosity of the synthesized materials were evaluated. In general, a large surface area is relatively consistent with the number of electroactive sites to well promote the catalytic performance of catalyst. Figure 3g shows that the NiSe 2 -NPs/NiMoN-NRs has a large specific surface area of 48.6 m 2 g −1 and high pore volume, better than those of the others. In another regard, C dl values of the materials obtained by running CV at different scan rates were also studied to verify their consistent ECSA ( Figure S19, Supporting Information). C dl value of the NiSe 2 -NPs/NiMoN-NRs are found to be 84.5 mF cm −2 , remarkably higher than that of the NiMoO 4 -NRs observed by its high turnover frequency (TOF) values toward HER and OER in Figure 3i, evidencing the great intrinsic catalytic activities of the NiSe 2 -NPs/NiMoN-NRs as compared to other catalysts. Insight into the original reasons of the prospective catalytic behaviors occurred on the NiSe 2 -NPs/NiMoN-NRs was further studied by carrying out density functional theory (DFT) calculation. According to the three structural models of the NiSe 2 , NiMoN, and NiSe 2 /NiMoN heterostructure in Figure 4a, the density of states (DOS) and Gibbs free energy of the adsorbed H* intermediate species (ΔG H* ) were calculated. As compared to pure NiSe 2 or NiMoN, the formation of the NiSe 2 /NiMoN heterostructure exhibits a superior DOS value near the Fermi level (Figure 4b), consistent with a better electronic conductivity that is highly beneficial to improve the catalytic activities. [48] In addition, ΔG H* value of the NiSe 2 /NiMoN is found to be −0.102 eV, closer to 0 than that of the NiSe 2 (−0.238 eV) and NiMoN (−0.450 eV) (Figure 4c), suggesting such heterostructure possesses an optimum ΔG H* value among two others for obtaining its excellent HER performance. Furthermore, to verify which atoms in structure of the NiSe 2 /NiMoN are the main centers to promote HER activities, ΔG H* values at different sites were assessed ( Figure S20, Supporting Information). Figure 4d indicates that Mo sites possess the lowest ΔG H* (−0.102 eV), followed by Ni-1(at the NiMoN phase) (0.176 eV), Ni-2 (at the NiSe 2 phase) (−0.199 eV), N (−0.222 eV), and Se site (−0.230 eV), suggesting Mo sites act as the most vital role in accelerating the reaction kinetics of the HER process. The DFT calculation was also carried out for the NiSe 2 /NiMoN to identify the mechanism and the main active sites toward OER process ( Figure S21, Supporting Information). In this context, the OER is assumed to proceed via a four elementary steps with the formation of the adsorbed species, such as OH*, O*, and OOH* on active surface of the material under alkaline condition, [49] consistent with the formation of metal oxides/hydroxides (NiOOH and MoO 3 ) along with the valence-state change in elements as well recognized by above TEM and XPS analyses, respectively. The achieved results in Figure S22 and Table S3, Supporting Information, exhibit that the OH* to O* conversion is the rate-determining step for Ni-2 (at the NiSe 2 phase), Ni-1(at the NiMoN phase), Mo, and N site while the OOH* → O 2 conversion is the rate-determining step for the Se site. Impressively, the smallest overpotential is found to be 0.43 V for the Se site, followed by Ni-2 (at the NiSe 2 phase) (0.54 V), Ni-1(at the NiMoN phase) (0.60 V), Mo (0.62 V), and N site (0.86 V). This confirms that the Se and the Ni sites in NiSe 2 phase are the main active centers for catalyzing the OER process.
Encouraged by the excellent HER and OER performances of the NiSe 2 -NPs/NiMoN-NRs material, we directly used it as self-supported cathodic and anodic electrodes to design a twoelectrode electrolyzer for overall water splitting in 1.0 M KOH. Figure 4e shows that the device requires a small cell voltage of 1.51 V to reach a current density of 10 mA cm −2 at 25°C, superior to that of a Pt/C (−) //RuO 2(+) cell (1.54 V), as well as surpassing performance of many bifunctional electrocatalysts recently reported elsewhere (Figure 4f). [50][51][52][53][54][55][56][57][58][59][60][61][62][63][64][65][66][67][68] Excitingly, the NiSe 2 -NPs/NiMoN-NRs (+,−) cell can steadily operate at a high current density of 50 mA cm −2 with only a slight cell voltage change of +30 mV after 50 h continuous working (Figure 4g). In addition, LSV responses before and after the stability test also reveal a good overlapping phenomenon with a cell voltage difference of about 30 mV at 50 mA cm −2 (Figure 4h), further validating that the electrolyzer cell is stable for long-term overall water splitting operation in alkaline medium. The faradaic efficiency (ηF) of the NiSe 2 -NPs/NiMoN-NRs (+,−) cell was evaluated by recording the evolved volume of H 2 and O 2 gases with a gas collection cylinder at an operation current value of 50 mA cm −1 . Figure 4i indicates a volume ratio of evolved H 2 to O 2 gas around 2:1, well consistent with the theoretical value. The ηF of the HER and OER is found to be 97.4% and 95.8%, respectively, confirming excellent efficiency of the catalyst for overall water splitting.
For further evaluating practicability of the NiSe 2 -NPs/NiMoN-NRs (+,−) device, a 30 wt% KOH electrolyte solution was used for its operation toward overall water splitting. Figure 5a shows the use of high KOH concentration impressively facilities the kinetics of water dissociation, thus leading a significant reduction in cell voltage for the device. Specifically, the NiSe 2 -NPs/NiMoN-NRs (+,−) device only requires a cell voltage of 1.46 V to reach 10 mA cm −2 in 30 wt% KOH electrolyte (Figure 5b). The potential utilization of solar-driven overall water splitting system to produce green H 2 gas was then also studied via integrating a commercial solar cell panel (AM 1.5 G 100 mW/cm 2 , open-circuit voltage of 2.35 V, short circuit J of 15.07 mA cm −2 , and maximum power conversion efficiency of 26.3%) with the NiSe 2 -NPs/NiMoN-NRs (+,−) device (Figure 5c). The J-V curves derived from the commercial solar cell and the NiSe 2 -NPs/ NiMoN-NRs (+,−) device toward water splitting shows a high solar-tohydrogen (STH) conversion efficiency of ∼18.4%, which is superior to most of the catalyst materials reported so far (Figure 5d). [69][70][71] It can be realized that the NiSe 2 -NPs/NiMoN-NRs (+,−) device can effectively work with high yields according to the power supply of solar cell panel by controlling its light/dark environment (Figure 5e,f). In addition, the NiSe 2 -NPs/NiMoN-NRs (+,−) device exhibits a good periodical current response under the intermittent working condition, indicating the satisfying tolerance of the NiSe 2 -NPs/NiMoN-NRs catalyst to current variation (Figure 5g). These observations suggest that the use of the NiSe 2 -NPs/NiMoN-NRs (+,−) device with the help of an integrated solar system is an efficient route to produce perfect green H 2 fuel via water splitting.
From all above achievements, the excellent catalytic HER and OER performances of the NiSe 2 -NPs/NiMoN-NRs catalyst can be ascribed to the synergistic effects caused by the following main reasons: (1) The 1D NiMoN NRs vertically in-situ grown on 3D NF display as excellent supporting nanostructures that have high conductivity and form a unique architecture with abundant open channels for promoting electron transmission and ion/gas diffusion ability. (2) The binder-free nature of the resulting electrode can effectively prevent the inactive dead volume caused by nonconductive polymer binder, thus maintaining good conductivity and improving electroactive sites of the material.
(3) The combination of the 0D NiSe 2 and 1D NiMoN generates abundant heterogeneous interfaces with decent lattice matching and a welladjusted electron distribution, thereby effectively optimizing DOS and Gibbs free energy for promoting the HER and OER. In addition, an increase in multiple active sites in the NiSe 2 /NiMoN heterostructure creates a highly efficient bifunctional catalytic ability to promote both the HER and OER processes.

Conclusion
In this study, a unique heterogeneous structure of NiSe 2 -NPs/NiMoN-NRs in which abundant small 0D NiSe 2 NPs are uniformly distributed on 1D NiMoN NRs is successfully synthesized and used as an effective bifunctional catalyst for high-performance water splitting. The catalyst shows good catalytic ability toward both HER and OER in the alkaline electrolyte, due to the synergistic effects arising from its optimized morphology and electronic structure. The electrolyzer of NiSe 2 -NPs/ NiMoN-NRs (+,−) exhibits small voltage of only 1.51 and 1.46 V to obtain a current density of 10 mA cm −2 in 1.0 M and 30 wt% KOH solution at 25°C, respectively. Impressively, when the NiSe 2 -NPs/ NiMoN-NRs (+,−) device is connected with a solar cell system for a solar-driven water splitting process, it achieves a high STH conversion efficiency of ∼18.4%. These results suggest that the use of NiSe 2 -NPs/ NiMoN-NRs catalyst for a solar-driven overall water splitting process is an effective and robust approach to produce green H 2 fuel. Preparation of NiSe 2 -NPs/NiMoN-NRs hybrid-The NiMoO 4 -NRs was initially synthesized on NF substrate by a facile hydrothermal technique. Typically, a piece of NF (5 cm × 2 cm) was cleaned by immersion in 3.0 M HCl solution under ultrasonication for 20 min to remove oxide impurities, followed by washing in ethanol, acetone, and DI water, respectively. After that, the NF substrate was put into an autoclave containing 50 mL DI water with 0.605 g Na 2 MoO 4 Á2H 2 O and 0.727 g Ni(NO 3 ) 2 ‧6H 2 O to perform a hydrothermal reaction at 150°C for 6 h. After the reaction finished, the obtained NiMoO 4 -NRs on NF was cleaned with DI water and then dried in a vacuum oven. Subsequently, the NiMoO 4 -NRs/NF sample and 0.75 g dicyandiamide were put into a porcelain boat for calcination in an Ar atmosphere at 450°C for 4 h with a heating rate of 3°C min −1 . The Ni-NPs/NiMoN-NRs on NF was then obtained after the system was naturally cooled down to room temperature. The sample of Ni-NPs/NiMoN-NRs on NF and 0.3 g Se powder were finally put into a porcelain boat to conduct calcination in an Ar atmosphere at 350°C for 2 h with a heating rate of 5°C min −1 . After the reaction process finished, the furnace was naturally cooled down to room temperature. The NiSe 2 -NPs/NiMoN-NRs material was obtained on NF with a mass loading of 4.2 mg cm −2 on 1.0 cm −2 of NF. For a comparison, the NiMoN-NRs material and NiSe 2 -NPs material on NF substrate were also prepared by the specific methods as discussed in Supporting Information.

Experimental Section
Material characterizations-The micro-morphology and structure of materials were observed by field-emission scanning electron microscopy (FE-SEM) and energy dispersive X-ray analysis (EDS) on a Supra 40 VP instrument (Zeiss Co., Germany). Transmission electron microscopy (TEM) and high-resolution TEM (HR-TEM) on an H-7650 instrument (Hitachi Ltd., Japan) were used to obtain insight into the microstructure characteristics of materials. The crystallinity of the synthesized materials was also analyzed by X-ray diffraction (XRD) (Cu Kα radiation (λ = 0.154 nm) Rigaku Co., Japan). The surface element composition and valence state were investigated by X-ray photoelectron spectroscopic (XPS) analysis (Thermo Fisher Scientific, Inc., USA). The specific surface area and pore distribution of materials were measured by N 2 adsorption-desorption isotherms (ASAP 2020, Micromeritics Instrument Corp., USA). Raman analysis was studied by HR800 UV micro-Raman spectrometry.
Electrochemical measurements-Electrochemical workstation (CHI 660D) was used to study the electrochemical properties of the electrocatalyst with a three-electrode system at room temperature. The synthesized material, Hg/HgO electrode, and graphite rod were applied as working electrode, reference electrode, and counter electrode, respectively. Linear sweep voltammetry (LSV) technique was used to study the catalytic ability of synthesized material for HER, OER, and overall water splitting in 1.0 M KOH at a scan rate of 2.0 mV s −1 . The conductivity of the synthesized materials was evaluated by electrochemical impedance spectrum (EIS). Chronopotentiometry was applied to evaluate the durability of the materials under long-term operation. To evaluate the electrochemically active surface area (ECSA), cyclic voltammetry (CV) curves were recorded at different scan rates of 5-50 mV s −1 in a non-Faradaic region to calculate the double-layer capacitance (C dl ) values. Pt/C-based and RuO 2 -based working electrodes were also prepared for comparison. In a specific procedure, 4.2 mg of Pt/C (RuO 2 ) powder was dispersed in a solution containing 475 μL ethanol and 25 μL Nafion (5%) and ultrasonicated for 15 min, followed by depositing the achieved slurry on the surface of the NF (1 cm × 1 cm), and drying at 60°C for 6 h.
All measured potentials of the electrochemical results were calibrated to the reversible hydrogen electrode (RHE) according to the following equation: The potential of LSV measurement was corrected by iR-compensation as following equation: where E measured and I measured are the measured potential and current, respectively; R s is the interface resistance measured by EIS.