Ferrum-doped nickel selenide @tri-nickel diselenide heterostructure electrocatalysts with efficient and stable water splitting for hydrogen and oxygen production

To meet the increasing demand for clean energy, environmentally friendly and efficient transition metal selenides (TMSes) electrocatalysts for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) are being developed. There is an urgent need for a rational design of bifunctional non-precious metal catalysts with advanced structure and superior composition. In water splitting for the production of clean hydrogen energy, transition metal selenides have promising applications. We prepare a catalyst by a two-step hydrothermal method, and the crystal structure of the catalysts can be easily adjusted by adjusting the concentration of the selenizing agent; when the concentration of the selenizing agent (Na2SeO3) is 0.6 mmol, a phase transition occurred, forming the NiSe@Ni3Se2 heterostructure, reaching a current density of 10 mA cm−2 at an overpotential of 214 mV with a low Tafel slope of 41 mV dec−1. When the concentration of selenide is increased to 0.6 mmol, the prepared NiFeSe0.6-MOF (metal organic framework) demonstrates excellent HER performance. At 10 mA cm−2 current density, the overpotential is only 156 mV. Moreover, the monolithic hydrolysis electrolyzer assemble with NiFeSe0.6-MOF as the anode and cathode electrodes shows a low cell voltage of 1.7 V at a current density of 10 mA cm−2, and almost no attenuation is observed after a 72-h stability test. The excellent electrocatalytic performance of the prepared catalysts is attributed to the formation of nickel selenide heterostructures and the synergistic effect of two-dimensional ferrum-doped MOF, which provide abundant active sites. This study provides a good idea for the development of high activity and high stability catalysts.


Introduction
The majority of global energy in the past few years has been derived from traditional fossil fuels [1,2], represented by coal and oil.The combustion of these fossil fuels results in the emission of greenhouse gases or harmful gases [3] (SO x , CO 2 ), which causes several environmental issues.To alleviate this problem, renewable energy must replace conventional fossil fuels.Solar [4][5][6] and wind energy [7] are considered alternatives to traditional fossil fuels, but they 159 Page 2 of 13 have several restrictions, such as regionalism and low efficiency, and cannot be used on a large scale.Hydrogen (H 2 ) is an efficient energy carrier, including high energy ratio, nonpolluting, safe, efficient, and low cost [8,9].Furthermore, it is a renewable energy source that can be extracted from seawater and air and it can be regenerated.Steam reformation [10], coal gasification, and hydrogen production from water splitting [11] are all current methods of producing hydrogen (photo-catalytic hydrolysis [12] and water splitting [13]); although hydrogen production [14,15] by hydrolysis only accounts for 4% of total hydrogen sought, it is the cleanest method of hydrogen preparation.The electrolysis of water is the more advantageous of the two methods of producing hydrogen by hydrolysis.The H 2 and O 2 produced by the electrolysis of water need to overcome a certain energy barrier, and the theoretical voltage of the H 2 and O 2 production is respectively 0 V and 1.23 V, and electrolyze hydrogen is divided into two half-reactions, respectively HER and OER, but OER is a four-electron transfer process, with a very slow reaction dynamics, so electrolytic water needs over-power to be completed; the most current study is the use of catalysts [16] to accelerate water decomposition [17,18].So in order to reduce the overpotential of water splitting, it is crucial to develop catalysts that are both efficient and durable.
The most effective electrocatalysts for HER and OER are precious metal catalysts [19] such as Pt-based catalysts [20] and Ru-based catalysts, but they are not widely used due to the high cost and poor scarcity tolerance of precious metals.Therefore, it is necessary to develop non-precious metal [21] electrocatalysts with high stability and low cost.The development of efficient transition metal catalysts to replace electrocatalysts based on noble metals for hydrogen precipitation has garnered considerable interest.Metal organic framework (MOF) [22] is an organic-inorganic porous material composed of organic ligands and metal centers with crystalline morphology, representing a promising new class of efficient electrocatalysts.MOF materials have a high specific surface area (1000 to 6000 m 2 g −1 ) and play a crucial role in HER, due to their extremely high specific active area and abundance of metal active sites; MOFs have piqued the interest of researchers as potential electrocatalysts.Carbon-based materials derived from MOF have the following advantages in OER/HER electrocatalysis [23]: (1) The morphology and porosity of carbon-based materials can be modified to expose active sites and facilitate material transport in electrocatalytic reactions; (2) Interfacial electron-coupling interactions in MOF have been considered as one of the important factors to improve the efficiency of redox reactions during water cracking.The rational combination of specific redox-active metal nodes with organic ligands can provide a pathway to connect electroactive components and provide desirable electrocatalytic functions for MOF.Materials with a porous structure [24] have a low density, which helps to reduce their equivalent refractive index and improve catalytic performance.MOF are worthy of research for the development of new functional carbon materials due to the benefits listed above.Furthermore, efforts are ongoing to reduce the cost of MOF and promote their wide-scale application.
However, MOF has poor stability, electrical conductivity, and OER performance, limiting their application as efficient electrocatalysts, and they are not suitable for bifunctional catalysts.In general, MOF-derived functional nanomaterials can retain some of the precursor properties with high specific surface area and uniform elemental distribution under certain conditions.Several studies have used MOF materials as precursors to produce a range of TMSes materials for electrocatalysis.So MOF is often used as a precursor to prepare porous metal compounds (oxides, selenides, sulfides, and phosphides) by pyrolysis or chemical treatment.Transition metal compounds are becoming more popular in a variety of fields due to their high elemental earth abundance, low cost, and high structural property tunability.So far, significant advances have been made in the design and development of transition metal compound catalysts.Transition metals and their compounds, such as transition metalsulfides [25], phosphides [26], nitrides [27], hydroxides [28], and selenides [29], have received considerable attention.Transition metal selenides (TMSes), as one of the transition metal sulfur compounds (TMDCs), are a newly discover family of electrocatalysts with many advantageous properties, such as tunable band gap, special layer structure, unique morphology, and low cost, which can replace existing noble metals or serve as alternative materials.Base on this understanding, transition metal selenides are an excellent choice of electrocatalysts for the entire hydrolysis process.
The 3d orbital of Se may be involved in bonding with metal atoms because of its energy levels close to the 3 s and 3p orbitals.This electronic structure leads to the higher metallicity of transition metal selenides, which facilitates electron transport and reactions.In addition, transition metal selenides have the advantages of simple preparation, good catalytic activity, and stability, which make them ideal candidates for the electrochemical decomposition of water.In the metal selenide structure, the negative charge located on Se attracts protons and promotes the discharge process to accelerate HER.In addition, TMSes have a weaker Se-H bond than transition metal phosphides and sulfides, which accelerates hydrogen desorption from the active site and thus enhances the HER activity.For TMSes in OER, the transport of molecular oxygen molecules can be faster to accelerate the OER kinetics caused by the negative charge on the selenium site and the accumulated 3d-2p exclusion of the d-band center between the transition metal and selenium; the tunable electronic structure of TMSes-based catalysts provides the opportunity to enhance the HER and OER activity of bifunctional catalysts because the Gibbs free energy of hydrogen adsorption can be tuned energy and can balance the metal-H and metal-OH bonds.The excellent physicochemical properties of TMSes-based catalysts, such as high electrical conductivity and unique electronic structure, are responsible for the better electrochemical hydrolysis performance.Heteroatom-doped selenides (polymetallic selenides) and TMSes-based composites have superior electrocatalytic performance than monometallic selenides due to their tunable electronic structure and electrical conductivity.The tunable electronic structure of TMSes-based catalysts provides an opportunity to improve the HER and OER activities of bifunctional catalysts because the Gibbs free energy of hydrogen adsorption can be tuned and metal-H and metal-OH bonds can be balanced.Secondly, the chemical and electrochemical stability of bifunctional catalysts in alkaline media is the key to their practical application.X. Huakai et al. [30] construct co-doped core-shell NiSebased catalysts as efficient HER catalysts using an ultrathin NFM precursor and a two-step method.Chen et al. [31] encapsulate ZnSe/NiSe heterojunction nanoparticles in a nitrogen-doped carbon framework (ZnSe/NiSe@NC) using a selenization process, and the resulting nanoparticles exhibit significant hydrogen precipitation catalytic activity.The HER's performance is markedly enhanced.Xinqiang Wang et al. [32] use a co-precipitation and annealing process to grow Co-N-doped carbon nanosheet arrays embedded with CoSe 2 nanoparticles on carbon cloth (CC).The synergistic effect of CoSe 2 nanoparticles and CNC nanosheet arrays not only provides abundant active sites for easy access to the electrolyte and diffusion of H 2 bubbles but also ensures their high electrical conductivity and structural stability.Wenchang Zhuang et al. [33] report a metal organic framework (MOF) derivatization strategy for making Fe-NiSe 2 nanocatalysts.When experimental data and theoretical analysis are combined, it can be seen that Fe doping can change the electronic structure of NiSe 2 in a big way, which makes it much better for electrocatalytic OER.More importantly, iron is frequently used as a doping element for the OER activity of nickel-based and cobalt-based electrocatalysts, and its abundance exceeds that of other transition metal elements by several orders of magnitude.Because selenides reorganize in OER under alkaline conditions, exposing more electrocatalytic active sites, iron-doped selenides have become a hot topic in recent catalyst research.
In this study, the electrocatalysts are successfully synthesized by a two-step hydrothermal [34] method employing FeCl 2 , NiCl 2 , and Na 2 SeO 3 as the Fe, Ni, and Se sources, respectively, and nickel foam (NF) as the substrate.The concentration of selenization can be easily adjusted to change the grain size; different degrees of selenization exhibit good catalytic activity for both HER and OER, with the best catalytic activity for HER and OER being selenizer concentration of 0.6 mmol; at this point, the crystalline phase transition from NiSe to Ni 3 Se 2 occurs, resulting in the formation of the NiSe@Ni 3 Se 2 heterogeneous structure.This heterostructure [35] enhances the stability and activity of the oxygen evolution reactions (OER) and hydrogen evolution reactions (HER).It demonstrates outstanding electrocatalytic performance for both OER and HER in alkaline media, and it exhibits overpotentials of 214 mV at 10 mA cm −2 OER current densities and the HER overpotential is only 156 mV when the overpotential reaches a current density of 10 mA cm −2 .This study provides a useful attempt to the design of transition metal selenide electrocatalysts by a simple two-step hydrothermal method, as well as some design ideas for the design of inexpensive and long-lasting water splitting electrocatalysts.

Synthesis of NiFe-MOF
Nickel foam (NF) (2 cm × 3 cm) is ultrasonically cleaned for 15 min with 3 M HCl, deionized water, and ethanol, respectively, and then dried in a vacuum oven at 60 °C for 6 h.FeCl 2`6 H 2 O (AR), NiCl 2 6H 2 O (AR), and BDC (AR) are dissolved in 16 mL of DMF (AR) containing 1 mL of absolute ethanol and 1 mL of deionized water and stirred for 30 min.The cleaned NF is then placed in a 50-mL PTFElined stainless steel autoclave and heated at 120 °C for 12 h at 120 °C.The resulting samples are washed three times with ultrasound with deionized water and ethanol before being dried under vacuum at 60 °C for 6 h and named as NFM.

Synthesis of NiFeSe X -MOF
First, dissolve Na 2 SeO 3 (98%) in 10 mL of deionized water.After 20 min of vigorous stirring, hydrazine, N 2 H 4 •H 2 O (80%), is added to the mixture and stirred for another 20 min.Following the addition of the previous NFM, the mixture is sealed in a 50-mL PTFE-lined stainless steel autoclave and heated at 145 °C for 24 h and named as FNS/X.X is the concentration of the selenide Na 2 SeO 3 in mMol, and 159 Page 4 of 13 in this study, X is 0.2, 0.4, 0.6, and 0.8 representing different mMol of Na 2 SeO 3 .

Materials characterization
The samples' morphology is determined using a scanning electron microscope (SEM, Hitachi Regulus8100) and a transmission electron microscope (TEM, JEOL JEM 2100F).X-ray photoelectron spectroscopy (XPS) is carried out on an XPS-7000 spectrometer (Rigaku) using Mg K radiation.The crystal structures of the materials are investigated using an X-ray diffractometer (XRD, B X'Pert PRO MPD).

Electrochemical measurements
All electrochemical tests are performed using a three-electrode system on an electrochemical workstation (Bio-logic, SP-200, France).The experiments are conducted with a three-electrode system and a 1 M KOH electrolyte solution.As the working electrode, a 0.25 cm −2 electrocatalyst electrode is used, a platinum sheet is used as the auxiliary electrode, and a Hg/HgO electrode is used as the reference electrode.All potential transformations utilized the Nernst equation.
where E Hg/HgO is the potential of Hg/HgO for the test with the reference electrode, V; E @Hg/HgO is the standard electrode potential for the Hg/HgO reference electrode, and its value is 0.098 V; and the acid-base value of the test solution is represented by pH.
The following equations [36] are used to figure out the overpotentials: (1.1) All curves are not compensated by the IR potential.Tafel equation: where η, a, b, and j denote the overpotential, Tafel constant, Tafel slope, and current density, respectively.The Tafel plot is obtained by plotting the logarithm of the current density lg(j) against the overpotential (η) through the polarization curve, and the Tafel slope is obtained by fitting the linear part of the Tafel plot.Because the Tafel slope is inversely proportional to the charge transfer coefficient, it can be used as one of the parameters for evaluating electrocatalyst performance.
At a scan rate of 1-5 mV/s, linear scanning voltammetry (LSV) is performed.Electrochemical impedance spectroscopy (EIS) is performed in the frequency range of 106 to 0.01 Hz.CV can measure electrochemical double-layer capacitance (C dl ) in the range of − 0.30 to − 0.40 V at scan rates of 10, 20, 30, 40, and 50 mV/s.C dl derived from the linear slope reveals ECSA.
All experiments are repeated at least three times to ensure reproducibility.

Morphologies and structures of samples
Scheme 1 describes the preparation of composite catalysts using several steps of the hydrothermal method in a nickel foam (NF) substrate.For convenience, the sample generated after the first step of hydrothermal synthesis is labeled as NFM and the sample generated after the second step of synthesis is labeled as FNS/X, where X represents the degree of selenization.First, the organic ligand BDC, NiFe metal salt, and various solvents are added to the Teflon liner, and then, the acid-washed NF is put in to grow the NFM (NiFe-MOF) nanosheets by a simple hydrothermal method; after that, we prepare NFM nanosheets which are selenized by Na 2 SeO 3 and N 2 H 4 with different degrees of selenization; then, the NFM nanosheets are generated in situ on the surface by hydrothermal method with FNS/X compounds with different grain sizes and crystalline phases are then generated in situ on the surface of NFM nanosheets by hydrothermal method.In order to understand the surface morphology of our prepared materials, as well as the deep microscopic morphology, we use a scanning electron microscope (SEM) and transmission electron microscope (TEM) to examine the morphology and composition of the catalysts.
The catalysts are subjected to scanning electron microscopy (SEM) to analyze their surface morphology.Prior to selenization, the NFM material exhibits a smooth needlelike structure, as depicted in Fig. 1a.In contrast, the FNS/0.2material displays a surface that is enveloped by numerous fine and uniform nanoparticles, as shown in Fig. 1b.Additionally, spherical particles are observed concurrently.Moving on to the FNS/0.4material, the surface is predominantly characterized by spherical particles, with an accompanying increase in the diameter of the encapsulate particles (Fig. 1c).Furthermore, the FNS/0.6 material exhibits further particle enlargement and adhesion on its surface, ultimately resulting in the formation of plateletized clumps (Fig. 1d).The nanosphere structure of FNS/0.8 underwent a gradual reduction as selenization increases (see Fig. 1e).The stability of the FNS/0.6 structure is determined to be higher in comparison to the other structures, as evidence by scanning electron microscopy (SEM) observations.Therefore, in order to further investigate the morphology and crystal structure, we use TEM to observe the microscopic morphology and electron diffraction pattern to determine the crystal plane spacing.FNS/0.6 is investigated by field emission TEM (Fig. 1f), and the lattice spacing of NiSe and Ni 3 Se 2 is observed.Figure 1g and h show TEM images with different magnifications.From the TEM images of Fig. 1g and h, it can be seen that the prepared FNS/0.6 is porous, which is beneficial to improve the charge transfer efficiency and mass transport of electrocatalysis, and the SAED images (Fig. 1i, j) show regular dif FNS/0.6.The high-resolution TEM (HRTEM) images (Fig. 1f-h) show that it has two crystal structures, from which it can be clearly distinguished that the lattice spacing of (012) and (021) planes of Ni 3 Se 2 is 0.296 nm and 0.246 nm, respectively, and the lattice spacing of ( 101) and (300) planes of NiSe is 0.308 nm and 0.288 nm, respectively.It can be seen that there is a certain spacing between these two crystalline phases, which proves the formation of NiSe@Ni 3 Se 2 heterostructure.In the form of an intertwined structure, this heterostructure maximizes the interfacial area.This interface is distinguished by strong interactions and low interfacial energy, allowing electrons to move quickly between interfacial boundaries.At the same time, numerous lattice defects can be seen.The intrinsic activity of NiSe@Ni 3 Se 2 will be significantly influenced by these newly formed heterostructure interfaces and lattice defects, and it is beneficial to the electron transport, which is also helpful to improve the electrocatalytic performance of the catalyst.The energy-dispersive X-ray (EDX) spectroscopy image in Fig. S4 shows the content of Ni, Se, and Fe in the NiSe@ Ni 3 Se 2 catalyst.Energy-dispersive X-ray (EDX) spectroscopy elemental mapping reveals the exposed atoms (e.g., Ni, Fe, Se) on the region as shown in Fig. 1k.These maps show that Ni, Fe, and Se atoms are uniformly distributed in the select region of FNS/0.6.In addition, we also test the pore size distribution and nitrogen adsorption/desorption curve of FNS/0.6, as shown in Fig. S7.The Brunauer-Emmett-Teller (BET) method is used to estimate the specific surface area of FNS/0.6 to be 7.5647 m 2 /g.Nanomaterials with high specific surface area and rich pore size can provide rich active sites, shorten ion/electron diffusion paths, and improve catalytic performance.In addition, the conductivity of the sample is tested to be 19.82S/m, as shown in Fig. S8.
To find out the type of crystal substance composition of the prepare material, the crystal structures of FNS/X and pure NFM are also analyzed by XRD diffractometry (see Fig. 2).It can be seen that when the concentration of selenide is less than 0.6 mM, the characteristic peaks of NFM have disappeared by this time and many new peaks appear, indicating that NFM has been completely converted to selenide, notably, 20.9 °, 29.6 °, 30.0 °, 36.5 °, 37.2 °, 42.6 °, 47.7 °, 48.2 °, 52.7 °, and 53.5 °.The diffraction peaks correspond to the cubic phase (101), ( 110), ( 012), ( 021), ( 003), ( 202), ( 211), ( 113 300), ( 021), ( 211), ( 131), ( 321), (330), and (012), when a crystalline phase transition of Ni 3 Se 2 occurs and a NiSe@Ni 3 Se 2 heterostructure is formed, which is well confirmed by the TEM results when the concentration of selenide is 0.8 mM, when the crystalline peak of Ni 3 Se 2 disappears and completely transforms into NiSe, and the intensity of the peak also increases, and the crystallinity of the material decreases at this time.The XRD diffraction peaks of the catalysts were obvious, which indicate that the catalysts had good crystallinity and purity.
It is discovered by TEM and XRD that FNS/0.6 forms a heterostructure, and the microscopic morphology observed by SEM is excellent, indicating that FNS/0.6 had significant potential as a good catalyst.So in order to further investigate the valence and chemical structure of FNS/0.6, we analyze the X-ray photoelectron spectra (XPS) and highresolution peaks of various elements.The measure full spectrum of XPS Fig. 3a reveals the presence of Ni, Se, and Fe elements in FNS/0.6, which is consistent with the XRD results.To study the composition and chemical bonding states of FNS/0.6, we give the XPS spectra and highresolution peaks of different elements; as shown in Fig. 3a, the measure XPS spectra shows the presence of Fe, Ni, and Se elements in FNS/0.6, which is consistent with the XRD and EDX results; and as shown in Fig. 3b, the high-resolution Ni 2p spectra of 0.6 shows typical peaks centered at 873.09 eV [37] and 855.41 eV, attributed to Ni 2p1/2 and Ni 2p3/2, respectively, and at 879.5 eV and 861.5 eV [38] for the satellite peaks.In Fig. 3c, the positions of the Se 3d5/2 and 3d3/2 peaks are located at 54 eV and 54.9 eV [39], respectively.This shows the interfacial interaction between the FNS/0.6 heterostructures, which effectively shortens and accelerates the electron transfer pathway and rate during catalysis.In addition, the high-resolution Se 3d5/2 and Se 3d3/2 peaks reveal the successful synthesis of selenides.The Fe 2p deconvolution high-resolution spectrum reveals two spin-orbit peaks at 726.6 eV and 712.84 eV [40] (Fig. 3e), which correspond to the Fe2p3/2 and Fe2p1/2 spin-orbit peaks of Fe 2+ .

Electrocatalytic performance of OER
The OER performance of the catalysts is evaluated in an alkaline solution (1 M KOH) using an electrochemical three-electrode system in order to determine the effect of varying degrees of selenization on the OER properties of NFM.Under the same conditions, electrochemical tests are performed on samples FNS/0.8,FNS/0.6,FNS/0.4,FNS/0.2, and pure NFM.To exclude any potential catalytic current effect due to catalyst oxidation, the cathodic scan records all LSV curves as inverses curves.
As shown in Fig. 4a, the FNS/0.6OER performance is best, when a current density of 10 mA cm −2 can be obtained with a 214-mV overpotential, which is significantly lower than that of pure NFM (324 mV), FNS/0.8 (334 mV), FNS/0.4 (334 mV), and FNS/0.2 (314 mV). Figure 4b depicts a comparison between the histograms of overpotentials at current densities of 20 and 50 mA cm −2 .FNS/0.6 requires low overpotentials of 374 and 464 mV to drive current densities of 20 and 50 mA cm −2 , which reduces energy consumption in the oxygen process and demonstrates that it has a promising future application.Figure 4c depicts the NFM with varying degrees of selenization and the pure NFM corresponding Tafel curves that are utilized to evaluate the OER kinetics.The FNS/0.6 still demonstrates good Tafel slope (41 dec mV −1 ), which is significantly lower than the NFM (171 dec mV −1 ), FNS/0.8 (66 dec mV −1 ), FNS/0.4 (65 dec mV −1 ), and FNS/0.2 (57 dec mV −1 ).In comparison to other samples, FNS/0.6 exhibits faster OER kinetics and mass transfer rate.
The C dl of the samples is measured in a voltage window in the non-Faraday region [41] using the CV (as shown in Fig. S1) method at varying scanning speeds.This is to analyze the catalytic activity of each sample.As can be seen in Fig. 4d, FNS/0.6 exhibits the highest bilayer capacitance (4.27 mF cm −2 ), which is significantly higher than NFM (0.414 mF cm −2 ) as well as the value of FNS/0.8 (2.27 mF cm −2 ), FNS/0.4 (1.99 mF cm −2 ), and FNS/0.2 (1.48 mF cm −2 ).This suggests that FNS/0.6 exposes more catalytically active sites during the OER.The fast electrode kinetics of FNS/X is made more evident by electrochemical impedance spectroscopy (EIS); the Nyquist plots (Fig. 4e) reveal that the charge transfer resistance (R ct ) of FNS/0.6 is only 3.3 g, which is significantly lower than that of NFM (7.7 s, FNS/0.8 (14.77fi,FNS/0.4 (9.6377, and FNS/0.2 (11.5an.This suggests that FNS/0.6 has high electrical conductivity and an excellent capacity for the transport of electrons, and FNS/0.6 demonstrates superior OER performance compared to non-precious metal OER catalysts in recent years.We then test its stability using chronoamperometry.The catalyst maintains a stable cell potential at a current density of 10 mA cm −2 for 72 h without significant amplification (Fig. 4f).These electrochemical tests demonstrate that FNS/0.6 has a great deal of potential for practical applications in driving electrochemical water splitting.Combining the characterization and electrochemical tests, several factors contribute to the exciting catalytic activity and stability of FNS/0.6 catalysts, according to experimental and structural analyses.For starters, it has a large number of easily accessible catalytic active sites.Second, due to Fe doping and heterostructure formation, FNS/0.6 has a high specific surface area and a good transition metal system electronic environment, which can improve OER catalytic performance.To further investigate the change of the structure and composition of the catalyst during the OER process, we analyze the appearance of FNS/0.6 after OER testing; as shown in Fig. S5, the granular core-shell structure of FNS/0.6 remains well maintained without structural collapse, indicating good structural stability.Comparing the high-resolution Ni 2p and Fe 2p spectra after the OER test (Fig. S2a, b), the peaks as a whole underwent some slight shifts, and the peak of Se 3d (Fig. S2c) at the newly emerged 58.98 eV is a Se-O bond, indicating that the catalyst surface underwent oxidation during the OER test.These prove that FNS/0.6 is an excellent pre-catalyst for OER.Additionally, a comparison [42][43][44][45][46][47][48][49][50][51][52][53][54][55][56][57] is made between the overpotentials and Tafel slopes that are reported in the literature (Fig. S6).The findings indicate that the samples we examine exhibit superior performance compared to the majority of electrocatalysts that are not composed of precious metals, specifically in relation to the oxygen evolution reaction (OER).

Electrocatalytic performance of HER
Similarly, in a three-electrode system with 1 M KOH solution, we analyze the HER performance of the resulting electrocatalysts.Figure 5a depicts the linear scanning voltammetry (LSV) of FNS/0.6.The LSV curves demonstrate that the FNS/0.6 has higher HER activity.At a current density of 10 mA cm −2 , it can be seen that FNS/0.6 demonstrates the highest HER activity with an overpotential of 156 mV.This value is significantly lower when compared to that of pure NFM.It indicates that selenization is a significant improvement for the HER activity of NFM. Figure 5b shows the histogram of overpotential at different current densities of 20 mA cm −2 and 50 mA cm −2 .From the figure, we can see that the effect of selenization degree is not as obvious as the OER.We can see from Fig. 5a that the effect of different selenization levels on HER activity is small.Although this effect is small, its overpotential is still much smaller than that of the non-precious metal catalysts.Despite this, it still demonstrates the beneficial effect of selenization.
Moreover, the HER kinetics of the above electrocatalysts is investigated by extracting the slopes from the Tafel plots in Fig. 5c. Figure 5c depicts the Tafel slopes of the electrocatalysts obtained in Fig. 5a, where the Tafel slope of FNS/0.6 is 193 mV dec −1 , which is significantly lower than that of pure NFM (241 mV dec −1 ).It indicates that FNS/0.6 has favorable electron transfer kinetics.
In addition, an EIS analysis is performed to investigate the HER kinetics of each sample.Figure 5d shows that all samples have relatively small solution resistance.Compared to NFM (14.5 a, FNS/0.2 (17.5 a, FNS/0.4 (77., and FNS/0.8 (10.35sa,FNS/0.6 has a charge transfer (R ct ) of 3.15a, which is significantly lower than that of other samples.The lower R ct value indicates that FNS/0.6 has better electron conductivity and faster charge transfer.As a result, electrons can be transferred directly and efficiently between the electrocatalyst and the electrolyte during the HER process, contributing to increase HER activity.This phenomenon can be explained by the fact that FNS/0.6 forms a heterostructure, resulting in a lower charge transfer resistance, and a lower apparent activation energy, as well as better electronic conductivity.The stability of FNS/0.6 is tested using chronoamperometry (CP) (Fig. 5e) to evaluate its potential use, which illustrates that no significant decay of the current density occurs after 72 h of stability testing, which also indicates its excellent during HER.

Water splitting
In light of the remarkable OER and HER performance of FNS/0.6, we are of the opinion that this catalyst possesses a significant amount of untapped potential for use in contexts involving total water splitting.After that, the two electrodes are combined into a single unit to create a monolithic water splitting apparatus, with FNS/0.6 functioning as both the anode and cathode.The hydrolysis device requires only 1.7 V to achieve a current density of 10 mA cm −2 , as shown in Fig. 6a, and it can operate for 72 h with minimal voltage decay, as shown in Fig. 6b.This is due to the fact that the device can operate for so long.These demonstrate the potential application of the catalyst for water cracking, which further proves that the catalyst could also be used for water splitting.

Conclusions
In summary, a two-part hydrothermal method for the preparation of efficient water electrolysis catalysts is proposed in this paper.The results indicate that selenization has a favorable impact on the NFM.Fe doping can modulate the electronic structure of the catalyst, and the crystalline phase transition occurs at a selenization level of 0.6, forming a NiSe@Ni 3 Se 2 heterostructure; TEM and XRD confirm the heterostructure's existence.The special heterojunction enlarges the area of the interface and further enhances the electronic interactions.As a result, FNS/0.6 shows significant OER and HER activity.The catalyst has an overpotential of 214 mV at 10 mA cm −2 and a Tafel slope of 41 mV dec −1 at 1.0 M KOH, as well as outstanding catalytic OER performance and high cycling stability.When the voltage is 1.7 V, the maximum current density is 10 mA cm −2 , and the catalytic performance shows almost no degradation after 72 h of stability testing, demonstrating excellent electrocatalytic activity and stability.This study demonstrates that heterostructure formation is an effective strategy for enhancing the electrocatalytic OER and HER activities of transition metal-based materials, we use a simple operation method and construction of a heterogeneous structure, enhanced electron transport, and excellent performance.It provides a concept for the development of highly active electrocatalysts.

Acknowledgements
The authors would like to thank Shiyanjia Lab (www.shiya njia.com) for the support of XPS and TEM tests.

Fig. 3 a
Fig. 3 a XPS survey spectra; high-resolution XPS of b Ni 2p, c Se 3d, and d Fe 2p of FNS/0.6 and NFM

Fig. 6 aFunding
Fig. 6 a LSV curves of FNS/0.6;b chronopotentiometry curve of FNS/0.6 at a potential of 1.7 V