Interface engineering induced NiFe/Ni-Mo-S@CC heterostructure with excellent OER and HER performance

The development of high-efficiency, low-cost and stable noble-metal-free electrocatalysts for water splitting is essential for the sustainable production of hydrogen. Herein, hierarchical Ni-Mo-S nanosheet decorated by NiFe alloy nanoparticle grown on carbon cloth (NiFe/Ni-Mo-S@CC) is designed as bifunctional electrocatalyst for overall water splitting. Benefiting from the strong synergistic effect between NiFe alloy and Ni-Mo-S, the NiFe/Ni-Mo-S@CC heterostructure exhibits superior electrocatalytic activity with extremely low overpotentials of 161 mV for oxygen evolution reaction (OER) at 10 mA cm−2 and 72 mV for hydrogen evolution reaction (HER) at 10 mA cm−2, respectively. In addition, the alkaline electrolyzer with NiFe/Ni-Mo-S@CC as both cathode and anode yield a low cell voltage of only 1.49 V to reach the current density of 10 mA cm−2. The formation of NiFe/Ni-Mo-S@CC heterostructure provides an efficient way to develop catalysts for water splitting.


Introduction
Energy crisis and the associated environmental issues have been stimulating global interest in alternative energy sources, leading to recent intensive research endeavors to explore clean and renewable energy sources [1][2][3][4][5].Hydrogen as renewable and eco-friendly energy with high energy density, is regarded as an alternative solution to the potential global energy crisis [6][7][8].Utilizing electrochemical strategy to convert the excess power produced by new energy into hydrogen has become an important research aspect in the new energy technology revolution [9,10].However, efficient and large-scale hydrogen production in industry has not been widely deployed at present due to the sluggish oxygen evolution reaction at anode [11].So far, noble-metals based catalysts such as Pt-based and Ir/Ru-based are still regarded as the only suitable electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) [12,13].Nevertheless, the scarcity and high cost of these noblemetals inhibit their large-scale commercial applications [14,15].From the perspective of practical application, the use of OER/HER bifunctional electrocatalysts can reduce costs in electrolytic water devices.Thus, it is of high desire to develop robust electrocatalysts based on earth-abundant elements as bifunctional electrocatalysts for overall water splitting.
Recently, the low-cost alternative alkaline water electrolysis, based on non-noble metal catalysts, including transition metal sulfides [16], oxides [17], hydroxides [18], phosphides [19], nitrides [20] and carbides [21] have received great attention in the field of alkaline water electrolysis.Among them, transition metal sulfides have been investigated extensively due to their distinctive structural features, rich active sites and tunable electronic properties [22,23].Molybdenum sulfide with rich under-coordinated Mo-S edges is considered as the most suitable candidate HER catalyst, but its poor OER activity makes it difficult to trigger the overall water splitting [24,25].Introduction of other transition metals into Molybdenum sulfide catalyst, termed as ternary metal sulfides, can further improve catalytic activities by controlling the surface adsorption energy of reaction intermediates [26].With the incorporation of Ni, the binary Ni-Mo-S catalyst exhibit excellent HER activity and slightly improved OER activity [27].Nevertheless, excellent OER electrocatalysts are crucial for overall water splitting due to the sluggish kinetics of OER [28].Therefore, to prepare hybrid-structured electrode materials by combing materials with high OER activity has been considered as an effective strategy.For instance, Zhao et al. prepared hybrid NiCo 2 O 4 @NiMo 2 S 4 nanosheet structures for overall water splitting [29].The as-fabricated products show the overpotential of 310 mV for OER at 20 mA cm −2 , 159 mV for HER at 10 mA cm −2 , and 1.63 V for cell voltage at 50 mA cm −2 , superior to single catalyst.Zhang et al. reported a 3D core-shell electrocatalyst consisting of Co(OH) 2 cavity array-encapsulated NiMo alloy on the flexible carbon cloth substrate [30].Due to the multiple synergistic effects of the active species from NiMo and Co(OH) 2 and their interfaces, the catalyst delivers a current density of 10 mA cm −2 at a low cell voltage of 1.52 V. NiFe alloy is one of the most promising candidates to expedite kinetically sluggish OER due to its low cost and intrinsic catalytic activity [31].The alloying effect may adjust the position of d-band center and change the adsorption energy of intermediate products to improve the intrinsic activity of catalysts [32,33].
Herein, the heterostructure electrocatalyst containing hierarchical Ni-Mo-S nanosheets decorated by NiFe alloy nanoparticles coated on the flexible carbon cloth substrate (NiFe/Ni-Mo-S@CC) is successfully fabricated as an efficient bifunctional electrocatalyst for overall water splitting.Benefiting from modulated electronic structure and synergistic effect on the heterointerface, the NiFe/Ni-Mo-S@CC composite electrode exhibits excellent HER performance in 1 M KOH solution, with low overpotential values of 72 mV and 182 mV to achieve 10 and 100 mA cm −2 , respectively.Meanwhile, it displays outstanding OER performance in 1 M KOH solution, requiring ultralow overpotentials of 161 mV and 260 mV to achieve 10 and 100 mA cm −2 , respectively.Furthermore, the overall water splitting with NiFe/Ni-Mo-S as both cathode and anode exhibits a low cell voltage of 1.49 V at 10 mA cm −2 .

Chemicals
All the chemicals were used as received without further purification.Nickel(II) chloride hexahydrate (NiCl

Preparation of Ni-Mo-S@CC
The hierarchical Ni-Mo-S nanosheets were grown on the cleaned carbon fiber cloth by hydrothermal synthetic route.Briefly, 0.25 mmol NiCl 2 •6H 2 O and 0.07 mmol (NH 4 ) 6 Mo 7 O 24 •4H 2 O were used as Ni and Mo sources, respectively, whereas 1.2 mmol C 2 H 5 NS was used as S source and reducing agent.Dissolved the three raw materials into 35 mL deionized water and stirred for 30 min to obtain a homogeneous dispersion, and then transferred into a 60 mL Teflon-lining containing the carbon fiber cloth.The autoclave was heated at 180 ℃ for 12 h.After naturally cooling down to room temperature, the sample was thoroughly washed with DI water and ethanol to remove impurities and dried at 60 ℃.

Fabrication of NiFe/Ni-Mo-S@CC
The NiFe alloy nanoparticles were electrodeposited on the hierarchical Ni-Mo-S nanosheets using a standard threeelectrode electrochemical system.The electrolyte solution for electrodeposition was composed of 0.1 M NiCl 2 •6H 2 O and 0.05 M FeCl 3 •6H 2 O.A chronopotentiometric method was carried out for 100 s under a cathodic current density of 50 mA cm −2 with Ni-Mo-S@CC as the working electrode, graphite rod as the counter electrode, and Ag/AgCl electrode as reference electrode.After that, the working electrode was washed with ethanol and DI water several times and then dried at room temperature to obtain NiFe/Ni-Mo-S@CC electrode.For comparison, NiFe@CC electrode was synthesized with CC as working electrode under the same electrodeposition procedure.Ni/Ni-Mo-S@CC electrode and Fe/Ni-Mo-S@CC electrode were synthesized by the same procedure except that the electrolyte consists of only

Characterization
X-ray diffraction (XRD, Empyrean PANalytical) with Cu-Kα radiation was used to investigate the phase structures of the as-prepared samples.The morphology and elemental composition were investigated using field emission scanning electron microscopy (SEM, ZEISS Gemini 300), energy-dispersive X-ray spectroscopy (EDS, Zeiss Smart EDX).The X-ray photoelectron spectrometry (XPS) analyses were performed by Thermo Scientific K-Alpha + , the standard C 1 s peak at 284.8 eV was used as a reference to correct the other peak data.

Electrochemical measurements
Electrochemical studies were accomplished by using a CHI 760E workstation with three-electrode system, while graphite rod as counter electrode, Ag/AgCl electrode (saturated KCl) as the reference electrode and the as-prepared electrodes as the working electrode.Linear sweep voltammetry (LSV) was measured with scan rate of 5 mV s −1 with 85% iR compensation.Electrochemical impedance spectroscopy (EIS) measurements were performed using the frequency range of 0.01 to

Structure and morphology analysis
Schematic presentation of the synthesis process of NiFe/ Ni-Mo-S@CC is illustrated in Fig. 1a.First, hierarchical Ni-Mo-S nanosheets were synthesized via hydrothermal reaction.Subsequently, NiFe alloy nanoparticles were electrodeposited on the surface of Ni-Mo-S nanosheets to obtain the NiFe/Ni-Mo-S@CC electrode.The XRD patterns shown in Fig. 1b provided the information on the crystal structure of powder Ni-Mo-S collected from the precipitate after hydrothermal reaction, Ni-Mo-S@CC, NiFe/Ni-Mo-S@CC, Ni/Ni-Mo-S@CC and Fe/Ni-Mo-S@CC.The predominant diffraction peak at 26°can be indexed to the (002) of CC.A broad peak centered at about 43° in Ni-Mo-S@CC, NiFe/Ni-Mo-S@CC, Ni/Ni-Mo-S@ CC and Fe/Ni-Mo-S@CC may be attributed to the (101) planes of CC.The diffraction peaks at 18.5° can be indexed to the second-order reflections from the (002) plane of layered Ni-Mo-S [34].Two diffraction peaks centered at  2f).The rough surface with highly open structure provides unobstructure path for the diffusion of gas, promotes the diffusion of ions and electrons, and exposes numerous active sites, allowing for the increased electrocatalytic performance [35][36][37][38][39]. Therefore, the wetting properties and bubble contact angle of NiFe/Ni-Mo-S@CC was characterized by measuring the contact angle.As shown in Fig. S2, the heterostructure shows typical superhydrophilic and superaerophobic features with contact angle of near 0°, and the bubble contact angle of NiFe/Ni-Mo-S@CC is 150.5°,indicating its weak adhesion force to bubble.Which ensures that the electrolyte enables rapid contact with the active site and the generated gas bubble quickly escape form the electrode surface, thereby make a contribution to the high-efficient electrochemistry activity and excellent stability [40].
XPS analysis was carried out to reveal the surface composition and chemical state of the as-prepared samples.Fig. S3 shows the XPS full spectra of Ni-Mo-S@CC, NiFe/Ni-Mo-S@CC, Ni/Ni-Mo-S@CC and Fe/Ni-Mo-S@CC samples, which reveals the co-existence of Ni, Mo and S elements in all the samples.While Fe only exists in the NiFe/Ni-Mo-S@CC and Fe/Ni-Mo-S@CC samples.The high-resolution XPS spectra of Ni 2p in Fig. 3a shows the strong peaks at 858.38 and 875.88 eV with the binding energy difference of 17.5 eV in Ni-Mo-S@CC, corresponding to the low band of Ni 2p 3/2 and high band of Ni 2p 1/2 , respectively, consistent with the previous reports [41].The intense satellite peak suggested that the Ni 2+ state was predominant in the Ni 2p spectra [42].After  3b. the peaks with lower binding energy and higher binding energy are related to Mo 4+ 3d 3/2 and Mo 6+ , respectively [27,34].It can be found that Fe/Ni-Mo-S@CC displays similar peak positions to Ni-Mo-S@CC, indicating that the electrodeposition of Fe metal has no effect on the surface electrons of Mo.However, compared with the peaks of Mo 3d in Ni-Mo-S@CC, a slight negative shift of about 0.3 eV can be found in Mo 3d of NiFe/Ni-Mo-S@CC.It is further confirmed that the electrons are transferred from NiFe alloy to Ni-Mo-S [43,44].For the Fe 2p spectrum of Fe/ Ni-Mo-S@CC (Fig. 3c), the peaks at 711.2 and 724.8 eV with the binding energy difference of 13.6 eV are related to Fe 3+ 2p 3/2 and Fe 3+ 2p 1/2 , respectively [45].Compared to Fe/Ni-Mo-S@CC, the peaks obtained a 1.55 eV positive shift in NiFe/Ni-Mo-S@CC, demonstrating that the deposition of Ni contributes to the formation of high valence of Fe, which is conducive to the improvement of catalytic activity.XPS results manifest that there is a strong electron interaction between NiFe alloy and Ni-Mo-S, which could trigger effective electron transfer between NiFe alloy and Ni-Mo-S, thereby promoting the improvement of the catalytic activity of NiFe/Ni-Mo-S@CC.

Electrocatalytic performance for OER
The electrocatalytic OER activities of as-prepared electrocatalysts are shown in Fig. 4. As shown in Fig. 4a, b and Fig. S4, Ni-Mo-S@CC shows inferior activity and requires an overpotential of 294 mV to offer a current density of 10 mA cm −2 .After incorporation of NiFe alloy, the overpotential of NiFe/Ni-Mo-S@CC decreases to 161 mV to achieve the same current density, better than the commercial RuO 2 catalyst (385 mV), Fe/Ni-Mo-S@CC (262 mV), Ni/ Ni-Mo-S@CC (271 mV) and NiFe@CC (215 mV).Strikingly, even at high current density of 100 and 300 mA cm −2 , only 240 mV and 300 mV are required for NiFe/Ni-Mo-S@ CC, respectively, demonstrating the high OER activity of NiFe/Ni-Mo-S@CC, which may be due to the synergistic effect of NiFe alloy and Ni-Mo-S.Tafel plots of the as-prepared electrocatalysts are collected according to the LSV curves to in-depth understand the OER kinetics mechanism.As shown in Fig. 4c, NiFe/Ni-Mo-S@CC shows low Tafel slope of 70.7 mV dec −1 , confirming the rapid OER kinetic process of NiFe/Ni-Mo-S@CC.The OER activity of NiFe/ Ni-Mo-S@CC is comparable or even better than those of previously published transition metal-based OER electrocatalysts (Table S1).EIS was performed to evaluate the charge transfer resistance during the electrocatalytic reaction.As shown in Fig. 4d, the smaller semicircle of NiFe/Ni-Mo-S@ CC heterostructure means the lower charge transfer resistance and faster reaction rate than other catalysts, leading to a smaller overpotential [43,46,47].Furthermore, the electrochemical surface area (ESCA) was used to evaluated the structure activity relationship of the above mentioned electrocatalysts in promoting the OER process.In general, higher C dl value means the larger ESCA, which shows a large number of active catalytic sites and promotes electrocatalytic activity.Fig. S5 provides the CV curves of these electrocatalysts with various scan rates.In Fig. 4e, the NiFe/ Ni-Mo-S@CC (2.33 mF cm −2 ) show higher C dl than other catalysts except NiFe@CC, which may be due to the lowest C dl (0.84 mF cm −2 ).To identify the durability of the NiFe/Ni-Mo-S@CC catalyst, the time-dependent current density curve is recorded, and the current at 10 mA cm −2   shows a slightly decrease after 24 h (Fig. 4f).The potential shows little change from 1.47 to 1.50 V at 100 mA cm −2 (Fig. S6).In addition, LSV curves before and after 2000 CVs only slightly shifted backward, indicating the excellent stability of NiFe/Ni-Mo-S@CC.The higher activity of NiFe/ Ni-Mo-S@CC demonstrates that the combination between Ni-Mo-S and NiFe alloy can improve the number of active sites, accelerate OER kinetics, reduce charge transfer resistance and improve the intrinsic activity of the catalyst.

Electrocatalytic performance for HER
The HER performance of the electrocatalysts include commercial Pt/C@CC electrode for comparison was also evaluated under the same conditions as OER.As expected, the commercial Pt/C@CC exhibits the highest activity for HER, only needs negligible voltage to start HER (Fig. 5a,  b).Encouragingly, the NiFe/Ni-Mo-S@CC shows remarkable HER activity, only required an overpotential of 72 mV at the current density of 10 mA cm −2 , outperforming Ni-Mo-S@CC (140 mV), Ni/Ni-Mo-S@CC (125 mV), Fe/Ni-Mo-S@CC (225 mV) and NiFe@CC (112 mV).Strikingly, even at high current density of 100 mA cm −2 , only 182 mV is required for NiFe/Ni-Mo-S@CC, indicating its excellent HER activity.The HER activity of NiFe/Ni-Mo-S@ CC is comparable or even better than those of previously published transition metal-based HER electrocatalysts in alkaline solution (Table S2).Although NiFe@CC exhibits higher activity than Ni/Ni-Mo-S@CC and Fe/Ni-Mo-S@CC at low current density, it exhibits lower activity at high current density.This is due to the higher Tafel slope of NiFe@ CC (166.0 mV dec −1 ), compared with Ni/Ni-Mo-S@CC (116.0 mV dec −1 ) and Fe/Ni-Mo-S@CC (123.0 mV dec −1 ) (Fig. 5c).As expected, the NiFe/Ni-Mo-S@CC exhibits the lowest Tafel slope of 104.2 mV dec −1 for HER among all the samples, demonstrating the fastest electron-transfer kinetics during HER reaction [48].The low overpotential and rapid HER kinetics of NiFe/Ni-Mo-S@CC heterostructure may be attributed to the negative shift of Ni 2p in XPS spectrum, indicating that the numerous dangling bonds around the Ni centers provide abundant empty d orbitals, which enhances the binding with H proton, thereby improving the HER performance [49].On the other hand, the Fe/Ni-Mo-S@CC is a worse HER catalyst than others, indicating the Fe neither provides active sites in our cases.Therefore, the HER activity should be primarily attributed to the electron donation and tuned electronic structure of NiFe alloy by Fe and the synergy effect between heterojunction interfaces [50][51][52][53][54].Moreover, the current density depicts a tiny potential increase with long-term stability of NiFe/Ni-Mo-S@CC for 24 h, indicating its superior activity and stability for HER (Fig. 5d).

Electrocatalytic performance for overall water splitting
Inspired by the excellent catalytic activity toward both HER and OER, the NiFe/Ni-Mo-S@CC was further used as bifunctional catalyst for overall water splitting in 1 M KOH.Pt/C@CC‖RuO 2 @CC and Ni-Mo-S@CC‖Ni-Mo-S@CC were also measured under the same conditions for comparison.As shown in Fig. 6a, to afford the current densities of 10 and 100 mA cm −2 , the NiFe/Ni-Mo-S@ CC‖NiFe/Ni-Mo-S@CC needs cell voltages of 1.49 V and 1.71 V, respectively, superior to Pt/C@CC‖RuO 2 @ CC (1.66 V, 1.93 V) and Ni-Mo-S@CC‖Ni-Mo-S@CC (1.69 V, 2.02 V), suggesting its excellent overall water splitting activity.Notably, the overall water splitting performance of NiFe/Ni-Mo-S@CC is comparable or even outperforms most of the recently reported bifunctional transition metal-based electrocatalysts (Table S3).As a further demonstration, the commercial AA battery (1.5 V) was used as the energy source to drive the NiFe/Ni-Mo-S@CC‖NiFe/Ni-Mo-S@CC cell.The gas bubbling for overall water splitting confirms the high efficiency of NiFe/Ni-Mo-S@CC electrode (Fig. 6b).The amount of H 2 /O 2 produced in the H-type electrolytic cell based on NiFe/NiFeCH/CC catalyst was accumulated by the water drainage method (Fig. 6c, d).As shown in Fig. 6d, the faradaic efficiency is determined to be almost 100% with the volume ration of 2:1 for H 2 to O 2 .Additionally, the current density keeps almost unchanged for the NiFe/Ni-Mo-S@CC after 24 h test at a constant potential of 1.49 V (Fig. 6e).LSV curves in the inset of Fig. 6e illustrates a negligible difference in overpotential before and after stability tests.However, LSV curves of Ni-Mo-S@CC catalyst show an undeniable difference in overpotential before and after stability testing of 60 mV at 10 mA cm −2 (Fig. S7).These results confirm the superior electrocatalytic activity and high stability of NiFe/Ni-Mo-S as both cathode and anode for overall water splitting.

Conclusions
In summary, a novel NiFe/Ni-Mo-S@CC heterostructure electrocatalyst is successfully prepared through hydrothermal and electrodeposition methods.Due to the superhydrophilic and superaerophobic features, strong synergistic effect and interface effect of heterogeneous structure, the NiFe/Ni-Mo-S@CC heterostructure exhibits excellent activity and stability towards the HER, OER and overall water splitting in 1 M KOH.Extremely low overpotentials of 161 mV and 260 mV are required for OER to reach the current density of 10 and 100 mA cm −2 , respectively.When the current density is 10 and 100 mA cm −2 , only a tiny overpotential of 72 mV and 182 mV for HER are needed, respectively.More importantly, the NiFe/Ni-Mo-S@CC‖NiFe/Ni-Mo-S@CC cell requires a very low voltage of only 1.49 V to afford the current density of 10 mA cm −2 with excellent stability over 24 h.

10 5
Hz with a 5 mV AC voltage amplitude.The double-layer capacitance (C dl ) was calculated through the CV curves with different scan rates in the non-faradaic potential region.The long-time durability was measured by chronoamperometry at a fixed potential.The recorded potentials were normalized according to the Nernst equation: E(RHE) = E(Ag/AgCl) + (0.059 pH + 0.197) V.The overpotential was calculated through the equation: η = E(RHE) -1.23 V.

Fig. 1
Fig. 1 (a) Illustration scheme for the preparation process of NiFe/Ni-Mo-S@CC; (b) XRD patterns of the prepared samples; (c) EDS spectrum of Ni-Mo-S@CC; (d) EDS spectra of NiFe/Ni-Mo-S@CC

Fig. 6
Fig. 6 Over water splitting performances.(a) LSV curves; (b) Photograph of NiFe/Ni-Mo-S@CC‖NiFe/Ni-Mo-S@ CC cell powered by 1.5 V AA battery; (c) Gas collection device of water splitting in 1 M KOH aqueous solution; (d) Corresponding levels of oxygen and hydrogen gas generated at 0, 500, 1000, 1500, 2000 and 2500 s; (e) Faradaic efficiency measurement; (f) Time-dependent current density curves for NiFe/Ni-Mo-S@CC‖NiFe/Ni-Mo-S@CC cell and corresponding polarization curves before and after stability test