Zeolitic imidazolate framework derived Co3S4 hybridized MoS2–Ni3S2 heterointerface for electrochemical overall water splitting reactions

Journal Pre-proof Zeolitic imidazolate framework derived Co3S4 hybridized MoS2–Ni3S2 heterointerface for electrochemical overall water splitting reactions Alagan Muthurasu, Gunendra Prasad Ojha, Minju Lee, Hak Yong Kim PII:

S0013-4686(19)32409-0

DOI:

https://doi.org/10.1016/j.electacta.2019.135537

Reference:

EA 135537

To appear in:

Electrochimica Acta

Received Date: 18 November 2019 Revised Date:

13 December 2019

Accepted Date: 16 December 2019

Please cite this article as: A. Muthurasu, G.P. Ojha, M. Lee, H.Y. Kim, Zeolitic imidazolate framework derived Co3S4 hybridized MoS2–Ni3S2 heterointerface for electrochemical overall water splitting reactions, Electrochimica Acta (2020), doi: https://doi.org/10.1016/j.electacta.2019.135537. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Zeolitic Imidazolate Framework Derived Co3S4 Hybridized MoS2-Ni3S2 Heterointerface for Electrochemical Overall Water Splitting Reactions Alagan Muthurasu1, Gunendra Prasad Ojha1, Minju Lee1, Hak Yong Kim1,2* 1

Department of BIN Convergence Technology, Jeonbuk National University, Republic Korea Department of Organic Materials and Fiber Engineering, Jeonbuk National University, Jeonju 561-756, Republic of Korea Corresponding Author’s E-mail: [email protected]

2

Abstract The production of hydrogen and oxygen from water through water electrolysis is of significant interest to generate clean energy via overall water splitting reaction. Herein, we prepare metalorganic framework based, cobalt sulfide nanoleaves followed by hydrothermal growth of hierarchical MoS2−Ni3S2 nanorods on a three-dimensional nickel foam substrate. The heterostructure of Co3S4@ MoS2-Ni3S2 catalyst electrode was served directly as an electrocatalyst for overall water splitting reactions such as oxygen evolution (OER) reactions and hydrogen evolution reactions (HER). The as-prepared Co3S4@ MoS2-Ni3S2 nanorods display outstanding electrocatalytic performance towards OER and HER with overpotential (η) of 270 mV and 136 mV at a current density of 50 mA.cm-2 for OER, 10 mA.cm-2 for HER in 1 M KOH solutions respectively. These results mainly originate the unique architecture of Co3S4@MoS2Ni3S2 heterostructure, which leads to enhance electrical conductivity, exposing a number of active sites and assisting the transport of ion to the electrode and electrolyte interface. This work suggests that the rational design of hybrid architecture have possible to develop for highperformance energy conversion applications. Keywords: Hierarchical, three-dimensional, metal-organic framework, electrocatalyst, and heterostructure 1. Introduction To adequately describe the sudden demise of fossil fuels and the burning of fuel that can trigger major environmental issues, it is necessary to identify alternative renewable energy sources that are very cheap and clean [1,2]. In recent years, electrochemical water splitting has attracted a lot of attention due to the production of hydrogen is one of the most perspective

methods for next-generation energy storage devices [3,4]. However, the bottleneck of electrochemical oxidation of water is greatly hindered by means of sluggish kinetics because the oxygen evolution reaction is a multistep proton-coupled electron transfer process. So, in order to bypass these higher kinetic activation energy barriers, a highly active electrocatalyst is therefore essential for OER and HER [5]. Though benchmark electrocatalysts such as iridium oxide and Pt/C denote exceptional electrocatalysts for OER and HER, relative expense and scarcity of these materials have prevented the uses of large scale production [6,7]. Henceforth, substantial efforts have been made to replace precious metal electrocatalysts with cost-effective catalysts such as transition metal oxide, chalcogenides, and phosphate used as overall water splitting reaction [810]. On the other hand, coupling the OER and HER electrocatalysts process together in integrated practical electrolyzer is highly difficult because the dissimilar pH range of both HER and OER can affect the stability and activity [11]. As a result, to design the highly stable and more active bifunctional electrocatalysts in the same electrolyte is one of the most challenges and become essential. Due to the synergistic effect between the two-phase of nano heterostructure and different active sites promptly increase the electrocatalytic activity towards both OER and HER. An efficient way, therefore, is to build together valid bifunctional electrocatalysts that can promote the overall water splitting reactions. Recently, highly active OER (Ni3S2) and HER active (MoS2) electrocatalysts were constructed on a nickel foam substrate which synergistically facilitates the overall water splitting reaction such as OER and HER [12,13]. Similarly, metal-organic frameworks based on hallow heterointerface of Co9S8/MoS2 have proved versatile materials for bifunctional

electrocatalysts

[14,15].

Likewise,

Co3O4@MoS2

heterostructures

have

demonstrated mutually enhanced the sluggish kinetics of both OER and HER. In addition, there are several reports have been ascertained transition metal atom doped MoS2 not only modulate the electronic structure of heterostructure but also boosts the exchange current density that can mutually assist to improve the overall activity to the electrolyzer [16]. Thus, coupling of well known HER active MoS2 with OER active transition metals leading to enhancement of overall water splitting performance. On the other hand, the synthesis of active electrocatalyst is based on the perfect engineering of heterostructure and these electrocatalysts can promote the ion as well as electron transport between heterointerface [17,18].

Consideration of the above perspective, Herein, we prepare MOF based Co3S4 nanoleaves were grown on nickel foam and then one dimensional hierarchical MoS2− Ni3S2 nanorods was further integrated over MOF Co3S4 supported by the nickel foam substrate. The obtained multi-phase heterostructure was subjected to bifunctional electrocatalysts for overall water splitting reaction, including OER and HER (Scheme 1). The as-prepared Co3S4@ MoS2−Ni3S2 heterointerface has proved as an outstanding electrocatalytic performance towards OER and HER. The corresponding overpotential of 270 mV for OER and 136 mV for HER mV at a current density of 50 mA.cm-2 and 10 mA.cm-2 in 1M KOH solutions respectively. The highest electrocatalytic activity is mainly endorsed by the multiphase active site of Co and Mo−Ni atom and efficient electron and ion transport between the interface during the electrochemical water oxidation. This work opens up to construct multiphase hybrid heterointerface have provides more active electrocatalysts for high-performance energy conversion applications.

Scheme 1: Schematic illustration of Co3S4@ MoS2−Ni3S2 heterointerface for overall water splitting reactions 2. Experimental Section 2.1. Chemicals Cobalt nitrate hexahydrate (Co(NO3)2.6H2O), and 2-methylimidazole (Hmim) were procured from Sigma Aldrich. Sodium molybdate (Na₂MoO₄·2H₂O.) was acquired from Showa Chemical Co. LTD. Thiourea (CH4N2S, TU) was bought from Samchun Chemicals. Before modification, the Ni foam electrode (2 × 2 cm2) was pre-cleaned with 6 M HCl for 30 min to

remove the surface NiO layer and then washed with deionized water and absolute ethanol for 3 to 5 times respectively. All other chemicals were used without any further purification. 2.2. Synthesis of Co3S4@ MoS2-Ni3S2 heterostructure The typical multi-phase electrocatalyst was prepared by using two-step procedure. At first, the precleaned nickel foam was immersed in an aqueous solution containing 1.3 g of 2−methylimidazoles (Hmim) and 0.58 g of Co(NO3)2. 6H2O for 1 h. After the formation of Co MOF nanoleaves, the Ni foam was washed with deionized water and dried at 60 °C overnight. Then, as grown Co MOF nanoleaves are converted into cobalt sulfide while at the same time one-dimensional Ni3S3 nanorod was grown on cobalt MOF nanoleaves by using hydrothermal treatment. Typically, 2 mM sodium molybdate (Na2MoO4·2H2O) and 8 mM thiourea (CH4N2S) dissolved in 30 ml water and the resulting mixture was transferred into Teflon-lined autoclave. Then, as prepared Co MOF nanoleaves modified nickel foam was immersed in the above solution. The autoclave was sealed and kept at 200 oC for 24 h. After cooling down to room temperature, the as-grown of Co3S4@MoS2−Ni3S2 nanorods supported nickel foam was thoroughly washed with water and ethanol for plenty of time and it's dried for overnight with electrical oven. For comparison, Co3S4@ Ni3S2 is prepared by using Co MOF nanoleaves used as substrate. The as-prepared Co MOF nanoleaves was immersed in an ethanol solution containing thiourea (0.122g) that was being transferred to a Teflon-lined autoclave and heated 120 oC for 4 h. Black colored Co3S4@ Ni3S2 was obtained after the reaction completion and then the product was washed several times with water and ethanol and dried overnight at 60 oC. The Co3S4@ Ni3S2 weight load was about 2.8 mg.cm-2. The synthesis of MoS2−Ni3S2: The precleaned nickel foam was soaked in 30 ml of an aqueous solution containing 2 mM sodium molybdate (Na2MoO4·2H2O) and 8 mM thiourea (CH4N2S) and this solution was added into Teflon-lined autoclave. The MoS2−Ni3S2 heterostructure was formed after a 24 h reaction at 200 o

C. The loading weight of nickel foam was held at 2.1 mg.cm-2. Preparation of Co3S4 nanoleaves:

1.3 g of 2−methylimidazoles (Hmim) and 0.58 g of Co(NO3)2. 6H2O were dissolved in 50 ml of water, then the mixing solution was stored at room temperature for 1h. The precipitate was then collected and washed with plenty of water and it is dried at 60oC for overnight. Further, Co MOF was sulfurized as follows, 80 mg of Co-MOF and 120 mg of thiourea were completely dispersed in 40 ml of ethanol and then the solution was transferred into Teflon-lined autoclave and heated

120 oC for 6h. The catalyst was collected after washed with water and dried at 60oC for overnight (MOF Co3S4 catalyst mass load 2.5 mg.cm-2). MoS2 synthesis: 30 ml aqueous solution containing 2 mM sodium molybdate (Na2MoO4·2H2O) and 8 mM thiourea (CH4N2S) and then the mixture was transferred into Teflon-lined autoclave and kept it at 200oC for 24 h to obtain MoS2 nanosheet (MoS2 catalyst mass load 2.5 mg.cm-2). 2.3. Material characterizations The as-prepared electrocatalysts were characterized using a scanning electron microscope (SEM) (Hitachi S−7400, Japan) to identify The surface morphology, elemental composition (energy dispersive X−ray (EDX)) and elemental mapping. TEM images were analyzed with a JEOL JEM 2010 with an operating of 200 kV (JEOL Ltd, Japan). The crystal structure of the samples was identified by using a Rigaku X-ray diffractometer (Rigaku Co., Japan) using Cu Ka radiation with a wavelength of 1.540 Å and a 2θ value ranging from 10 to 80⁰. X−ray photoelectron spectroscopy (XPS) data were obtained by Al Kα radiation (Thermo Scientific KA 1066). The electrochemical tests were carried out using VersaSTAT 4 analyzer at room temperature using a standard three-electrode configuration electrochemical setup. 2.4. Electrochemical measurements of electrocatalysts All the electrochemical experiments such as cyclic voltammetry (CV) electrochemical impedance spectroscopy (EIS) linear sweep voltammetry (LSV) and chronoamperometry (CA) were recorded using the standard three-electrode setup in 1 M KOH. A platinum wire and sat Ag/AgCl

used

as

a

counter

and

reference

electrode

respectively.

The

obtained

Co3S4@MoS2−Ni3S2 (1 cm × 1 cm) nanorod directly performed as a working electrode. For benchmark mark catalysts such as IrO2 and Pt/C were dissolved in N-methyl−2−pyrrolidone solvent (NMP) and 5% polyvinylidene difluoride (PVDF) binder for making homogeneous ink solution by using ultrasonication and then the resultant ink solution was loaded onto Ni foam using the drop coating method. After catalyst preparation, the geometrical mass loading on the Ni foam substrate was calculated around 2.5 mg.cm-2. All the current values were normalized with a geometric surface area in order to obtain their corresponding current density. The following Nernst equation ERHE = EAg/AgCl + 0.059 × pH + 0.197 V assigned all the potential to the

reversible

hydrogen

electrode

(RHE).

All

the

polarization

curves

were

iR

corrected. Electrochemical Impedance spectra were performed with the desired overpotential of OER

and

HER

from

100

kHz

to

0.01

Hz

using

an

amplitude

signal of

5

mV. Chronoamperometry (CA) measurement was used to carry out long term stability. Turn over frequency (TOF) of catalysts considered at overpotential of 270 mV for OER and 136 mV for HER respectively, using the relation of TOF = (J×S/4n×F), where J is the current density in A cm-2, S (cm2) is the geometric surface area, n is the number of moles of active sites (mole/cm2). 4 is a number of electron transfer for OER and 2 is used for HER respectively, and F is the Faraday constant (96485 C mol-1). 3. Results and Discussion Hierarchical multiphase MOF Co3S4 supported MoS2−Ni3S2 nanorods were synthesized by two-step solution deposition and subsequent hydrothermal process. As shown by equation 1, the reactive species of sulfur ion will be produced while reacting with thiourea. Consequently, MOF Co nanoleaves supported nickel foam react with sulfur ion simultaneously to generate MOF Co3S4 and Ni3S2 nanoparticles (equation 2). Meanwhile, MoS2 nanosheet was also formed during the reaction condition due to the precursor of MoO42- ion reacts with sulfur to form MoS2 nanosheet (3). Owing to the complex reaction mechanism, Ni3S2 nanoparticles develop in nanorods of Ni3S2 and continue to behave as one of the backbones of the preferable deposition of MoS2 nanosheet. It will be important to observe that within this hydrothermal process, nickel foam substrate carries a duel response. It functions as the core for orienting nanostructure assemblies as well as being the origin of Ni3S2 nanorods progression [19,20]. NH2CSNH2 + 2H2O 3Co + 3Ni + 6H2S 2-

MoO4 + 3H2S

2NH3 + H2S + CO2 Ni3S2 + Co3S4 + 6H2 MoS2 + 3H2+ SO4

2-

(1) (2) (3)

The morphology of prepared samples was examined by employing FESEM and the respective FESEM images are shown in Figure 1A displays the compact alignment of several Co−MOFs nanoleaves on the nickel foam. It can be found that the nickel foam substrate facilitates the nucleation of Co−MOF and favored propagation through the b axis to comprise the nanosheet structure [21,22]. The nanosheet surface is very smooth and the width is about

200−300 nm. After sulfurization with thiourea, the leaf-like structure slightly changes and consists of aggregated particles (Figure 1B) and the surface of nanoleaves seems to be too rough. However, the thickness of each nanoleaves approximately similar to that of the former Co−MOF nanoleaves. Figure 1C reveals the FESEM image of Ni3S2@MoS2 nanorod on the surface of MOF Co3S4. The formation of Ni3S2 nanorod is clearly viewed as having been consistently covered to the MOF Co3S4 surface. The diameter of vertically grown Ni3S2 nanorod on MOF Co3S4 supported nickel foam is around 100 to 200 nm and a length of 5 µM. Additionally, the narrow FESEM image of the Ni3S2 nanorod shows MoS2 nanosheet uniformly covered on Ni3S2 nanorod and the nanosheet of MoS2 shells is closely intertwined and cover almost all Ni3S2 domains (Figure S1 in ESI). As depicted in Figure 1C corresponding elemental mapping analysis of Co3S4@MoS2−Ni3S2 heterostructure and these images show the characteristic of Ni, Co, Mo, S, O, and C that are evenly spread on Co3S4@MoS2−Ni3S2 heterostructure. The energy dispersive spectrum (EDX) test explores the possible elements, such as Ni, Co, Mo, S, O, and C present on the Co3S4@MoS2−Ni3S2 materials (Figure S2 in ESI). Moreover, the transmission electron microscope (TEM) examines more closely at the materials of Co3S4@MoS2−Ni3S2 heterostructure. Scraped off Co3S4@MoS2−Ni3S2 catalysts from the nickel foam structural feature as indicated in Figure 1E, shows (marked region) leaf-like porous Co3S4 structure surrounded with MoS2 nanosheets and it can also be noticed that enormous MoS2 nanosheets evenly cover the Ni3S2 nanorods. This finding results clearly indicate that Ni3S3 nanorod and Co3S4 nanoleaf are fully embedded in successive MoS2 nanosheets. Figure 1E inset, selected area electron diffraction (SAED) of Co3S4@MoS2−Ni3S2 catalysts, indicates the polycrystalline nature. The high-resolution TEM images of Figure F, G, and H provide more insight into the inner framework. It is interesting to note that hierarchical heterostructure of Co3S4@MoS2−Ni3S2 clearly demonstrates noticeable lattice springes with lattice distances of 0.285 nm, 0.28 nm, and 0.63 nm, which is concurrent with the d-spacing of (311) facets of (F) MOF Co3S4, (110) facets of (G) Ni3S2, and adjoining (002) plane of (H) MoS2 respectively.

Figure 1. FESEM image (A) Co MOF, (B) MOF Co3S4−Ni3S2 and (C) Co3S4@MoS2−Ni3S2 nanorods. (D) Elemental mapping Ni, Co, Mo, S, O, and C of Co3S4@MoS2−Ni3S2 nanorods. (E) TEM and selected area diffraction pattern (inset) image of Co3S4@MoS2−Ni3S2 nanorods. (F, G, H) the high-resolution TEM image of MOF Co3S4@MoS2−Ni3S2 nanorods. The crystal structure of the different products was identified by using X-ray diffraction (XRD) techniques. All prepared samples show three major diffraction patterns at 2θ values of 44.5°, 51.8°, and 76.4°, which indicate Ni peaks from nickel foam. Figure 2a, display XRD diffraction peaks at 14.05o, 33.28o, 35.8o 39.0o and 58.50o for MoS2 (002), (100), (102), (103) and (110) planes (JCPDS No. 37−1492). Conversely, it could also be observed that peaks at 2θ values of 16.1o, 26.6o, 31.2o, 38.0o, 50.1o, and 55.0o (Figure 2b) are assigned to the respective (111), (220), (311), (400), (511) and (440) planes of Co3S4 (JCPDS No. 42−1448). Figure 2c exhibits the diffraction peaks at 2θ values of 21.8o, 31.3o, 37.8o, 44.6o, 49.7o, 50.9o, 55.2o, 69.2o and 73.01o related to (101), (110), (003), (202), (113), (211), (122) (131) and (214) planes of Ni3S2 (JCPDS no. 44−1418).

Figure 2. XRD pattern of (a) MOF Co3S4@MoS2-Ni3S2 nanorods, (b) Co3S4-Ni3S2, and (c) MoS2-Ni3S2 on nickel foam. Furthermore, X-ray photoelectron spectroscopy (XPS) defined the chemical state and composition of the element in the Co3S4@MoS2−Ni3S2 catalysts. The XPS survey spectrum in Figure S3 in ESI highlighted the existence of possible Ni, Co, Mo, S, O, and C elements in the heterostructure. The high-resolution spectrum of Ni 2p area (Figure 3A) could be split from the Ni3S2 region by binding energy of Ni 2p3/2 and Ni 2p1/2 peaks located at 855.7 and 873.5 eV respectively, and the energy variation around Ni 2p3/2 (855.7 eV) and Ni 2p1/2 (873.5 eV) is 17.8 eV, implying Ni2+ and Ni3+ coexistence compared to pure Ni3S3 catalysts (856.3 and 874.2 eV), Co3S4@MoS2−Ni3S2 catalyst is blue-shifted and besides, broad shakeup peaks confirm the spinorbital coupling of Ni 2p [13,23]. The shifting of binding energy strongly supports strong electronic interaction between MoS2 and Ni3S2 results charge can be redistributed over the heterointerface [13,16]. The high-resolution XPS is depicted in Figure 2B, reveals deconvoluted Co 2p region spin-orbital coupling and the corresponding spin-orbital coupling values of Co 2p3/2

and Co 2p1/2 are observed to be 15.1 eV assigned to Co2+ and Co3+ respectively. The two peaks centered appeared at 778.5 eV for Co 2p3/2 and 793.6 eV for Co 2p1/2 contrast to XPS spectra of bare Co3S4 (778.5 and 793.9 eV). In the meantime, a broad peak formed at 786.8 eV and 805 eV is ascribed to the shakeup peaks of Co 2p [24-26]. High-resolution Mo region of MoS2 presented in Figure 3C, Mo 3d5/2, and Mo 3d3/2 are found at 229.1 eV and 232.0 eV respectively. Similarly, the binding energy of Mo 3d also shifts to that of pure MoS2 (229.4 and 232.6 eV) suggesting that Mo is present in Co3S4@MoS2−Ni3S2 heterostructure [27]. The adjacent deconvoluted S 2s peaks at 225.4 eV and 223.7 eV, indicate the sulfur species binding with two chemical states of Mo and Co ions [12]. The high-resolution spectrum of S 2p is shown in Figure 3D, the deconvoluted spectra of S 2p can be further split into 2p3/2, and 2p1/2 at a binding energy of 162.3 and 163.4 eV respectively [28]. The high intensity of the residual SO42- ion at 168.2 eV corresponds to the oxidation of surface sulfur through molybdates [13]. In Figure S2 demonstrates that the high-resolution O 1s spectrum and the deconvoluted peaks located at 533.7 eV connected to surface lattice oxygen and physisorbed oxygen. The XPS spectra also show the effective development of Co3S4@MoS2−Ni3S2 heterostructure and imply a strong interaction of heterointerface.

Figure 3. High-resolution XPS spectra of (A) Ni 2p, (B) Co 2p, (C) Mo 3d, and (D) S 2p in MOF Co3S4@MoS2−Ni3S2 nanorods. To better understand the MOF based transition metal architecture with electrochemical activity against overall water splitting reaction, Co3S4@MoS2-Ni3S2 catalyst is first checked electrochemical water oxidation to OER in 1 M KOH using three‐electrode electrochemical assembly. The electrocatalytic OER activities of reference materials, including nickel foam Co3S4@Ni3S2, MoS2-Ni3S2, Co3S4, MoS2, and commercial IrO2 were also investigated under the identical condition for comparison. Figure 4A shows the typical polarization curve of the Co3S4@MoS2−Ni3S2 catalyst, there can be a clear noticeable peak at 0.45 V with respect to RHE (Figure S4 in ESI), that could be related to Co3S4/Ni3S2 oxidation [29,30]. Certainly, as synthesized Co3S4@MoS2−Ni3S2 catalyst exhibits excellent OER activity with a lower overpotential (η) of 270 mV at a current density of 50 mA.cm−2. The obtained overpotential value is lower than that of Co3S4@Ni3S2 (320 mV), MoS2−Ni3S2 (350 mV), Co3S4 (400 mV) and

MoS2 (520 mV). Among the other individual catalysts, the Co3S4@MoS2−Ni3S2 catalyst shows lower overpotential and higher current density. This can be provoked by exposing the interconnected active site of Co−Mo−Ni atom. Further, the Tafel slope assessed the corresponding OER kinetic of all the materials, as can be seen in Figure 4B the Co3S4@MoS2−Ni3S2 was measured small Tafel slope of 69 mV·dec–1. However, Co3S4@Ni3S2, MoS2−Ni3S2, Co3S4, MoS2, and bare nickel foam display a larger Tafel slope of 84, 89, 94, 222 and 205 mV·dec–1respectively. The fast electron transfer kinetic was further assessed by electrochemical impedance spectroscopy (EIS) respective Nyquist plots are shown in Figure 4C the Nyquist plots show much lower charge transfer resistance (Rct) for Co3S4@MoS2−Ni3S2 catalysts among all samples, signifying that excellent conductivity and fast charge transfer during the OER process. Long term stability is another most vital role for evaluation for practical application in the OER process. As illustrated in inset Figure 4D, it shows the stable chronoamperometric (CA) curve of Co3S4@MoS2−Ni3S2 catalysts in the OER process with no observable current change after the 30 h, indicating excellent stability and OER performance is almost maintained at 97%. In addition, durability was also studied for Co3S4@MoS2−Ni3S2 catalysts and the corresponding CV curve shown in Figure 4D, there was little change in overpotential after the 500 cycles. This result reveals the outstanding catalytic performance of Co3S4@MoS2−Ni3S2 catalysts towards the OER process. The correlation of both surface area and OER activity of the different catalysts were also explored using the CV test. The electrochemical surface area (ESCA) is equal to the double-layer capacitance (Cdl) that could be obtained from the CV curve of the modified electrode. These CV curves were measured in the 1 to 1.25 V vs RHE with a different scan rate of 10 to 100 mV.s-1 obtained a quasi-rectangular form which is the characteristic performance of electrical double layer capacitance (Cdl) (Figure S5 in ESI). The slope which can be seen in Figure 4 F is used to measure the Cdl, and thus higher the Cdl serve to show the numerous active sits greatly enhance to OER activity by Co3S4@MoS2−Ni3S2 catalysts. It is worth noting that Co3S4@MoS2−Ni3S2 catalysts should have the highest Cdl value of 35.2 mF.cm-2 especially compared to all other references electrocatalysts. The above finding indicates that the OER activity depends greatly on its ESCA. Turn over frequency (TOF) of prepared catalysts was evaluated at an overpotential of 270 mV for OER and 136 mV for HER. The corresponding TOF values are summarized in Table S1 in ESI. From the results, TOF of

Co3S4@MoS2−Ni3S2 catalysts exhibit higher values than individual catalysts, indicate greater catalytic activity toward water-splitting reaction [31].

Figure 4. (A) LSV curve of MOF Co3S4@MoS2−Ni3S2nanorods, Co3S4@Ni3S2, MoS2−Ni3S2, Co3S4, MoS2, commercial IrO2, and nickel foam in 1 M KOH at a sweep rate of 5 mV.s-1 for OER. (B) Tafel slope of corresponding all the samples. (C) EIS plot all the prepared catalysts

measured at an overpotential of 300 mV. (D) LSV stability studies of MOF Co3S4@MoS2−Ni3S2 nanorods before and after 500 cycles. Inset: CA curve at a fixed overpotential of 270 mV for 30 h. (E) Histogram image of η @ 50 mA.cm-2 against Tafel slope. (F) Capacitive current obtained at 1.125 V vs RHE with respect to different scan rates. HER activity has been evaluated in the same electrolyte as the above mention efficient alkaline OER catalysts. The LSV curve of MOF Co3S4@MoS2−Ni3S2 catalysts for HER activity demonstrates in the 1M KOH electrolyte in a three-electrode electrochemical setup. HER performance of bare nickel foam, Pt/C, Co3S4@Ni3S2, MoS2−Ni3S2, Co3S4, and MoS2 have also been explored in comparison. The polarization curve of all the catalysts was evaluated in a low scan rate of 5 mV.s-1 and the corresponding iR corrected LSV plots as shown in Figure 5A it is noticed that the relatively insignificant HER activity for bare nickel foam, whilst the Pt/C shows highest catalytic activity to an almost very low overpotential of 70 mV at a current density of 10 mA.cm-2 due to their expensiveness and scarcity of these materials limits practical utilization. However, the HER performance of Co3S4@MoS2-Ni3S2 catalysts is superior to that of the other catalysts and needing overpotential of 136 mV to accomplish the current density of 10 mA.cm-2. But, all other reference catalysts took the overpotential of 183 mV, 250 mV, 317 mV, 279 mV, and 406 mV at the same current density of 10 mA.cm-2 for Co3S4@Ni3S2, MoS2−Ni3S2, Co3S4, and MoS2 and bare nickel foam respectively. As shown in Figure 4B the Tafel slopes are evaluated to the electrocatalytic kinetics of HER. Obviously, Co3S4@MoS2−Ni3S2 catalysts have a lower Tafel slope of 72 mV.dec-1 contrast to Co3S4@Ni3S2 (99 mV.dec-1), MoS2-Ni3S23 (109 mV.dec-1), Co3S4 (132 mV.dec-1), and MoS2 (128 mV.dec-1) and bare nickel foam (179 mV.dec1

), implies fast reaction kinetics and rate of production of hydrogen very large during the HER.

In order to further investigate the kinetics of HER for all the catalysts, the EIS experiment was performed at an overpotential of 200 mV. As can be seen in Figure 5C EIS plot of all the catalysts exhibits semicircle, a smaller semicircle diameter indicates faster the electron transfer between the catalyst and electrolyte results the charge transfer resistance (Rct) value will be lower. Interestingly, the Rct value of Co3S4@MoS2−Ni3S2 catalysts much lower when compared to other catalysts suggesting that electron transfer between electrode and electrolyte is very fast. The polarization CV test of 500 cycles at the potential range of 0 to −0.5 v vs RHE (Figure 5D) examines the stability of Co3S4@MoS2-Ni3S2 catalysts. The LSV curve displays minimal activity of degradation, then after the 500 cycles similar to the very first cycle. The long term stability

test was carried out by chronoamperometric studies at the fixed potential of 136 mV for 30 h. The Co3S4@MoS2−Ni3S2 catalysts show better stability and there is no large current density variation with an almost straight line by an overpotential of 136 mV for 30 h, speculating excellent long term stability in HER performance. It should always be noted that Co3S4@MoS2−Ni3S2 catalysts outperformed activities for both OER and HER when compared with previously described electrocatalysts as can be depicted in Table S2 [32-41].

Figure 5. (A) Polarization curve of MOF Co3S4@MoS2−Ni3S2 nanorods, Co3S4@Ni3S2, MoS2−Ni3S2, Co3S4, MoS2, commercial IrO2, and nickel foam in 1 M KOH at a sweep rate of 5 mV.s-1 for HER. (B) Consequent Tafel plot of all the samples. (C) EIS plot all the prepared catalysts measured at an overpotential of 200 mV. (D) LSV stability studies of MOF Co3S4@MoS2−Ni3S2 nanorods before and after 500 cycles. Inset: CA curve at a fixed overpotential of 136 mV for 30 h.

The electrocatalytic water splitting for both HER and OER analyses was evaluated in 1M KOH solution as shown in Figure 6A. The 1.72 V overpotential could generate at a current density of 50 mA.cm-2. For the overall water splitting reaction, an electrolyzer has been constructed using the Co3S4@MoS2−Ni3S2 nanorods as both anode and cathode. Figure 6B the Co3S4@MoS2−Ni3S2 nanorods pair shows that the overall water splitting is about 1.72 V at a current density of 50 mA.cm-2. The photographic image in Figure 6B shows a substantial amount of H2 and O2 bubbles which may form on cathode and anode, respectively. Furthermore, the electrocatalytic efficiency of electrode materials was influenced by structural changes during the OER and HER analysis. Non-oxide transition metal catalyst undertakes in situ electrochemical conversion and produces hydroxide/oxyhydroxide under the medium. Compared to pristine TM hydroxide/(oxy)hydroxide, electrochemically generated TM hydroxide/(oxy)hydroxide, it is suggested that electrochemically formed hydroxide /(oxy)hydroxide is the valid catalytically active species for the OER [42]. Upon OER and HER stability test and the corresponding PXRD of Co3S4@MoS2−Ni3S2 nanorods were shown in Figure S6 in ESI. The closely related PXRD pattern has been attained to endorse unaffected catalytic activities. Figure S7 in ESI shows the SEM images of Co3S4@MoS2−Ni3S2 nanorods after long OER and HER stability analyses, indicating that there is no change of morphology. And the subsequent SEM elemental mapping supports the possible elements, including Ni, Co, Mo, S, C, and O that are present after the cyclic test.

Figure 6. (A) Polarization curve of MOF Co3S4@MoS2−Ni3S2 nanorods in 1 M KOH at a sweep rate of 5 mV.s-1 for HER and OER. (A) LSV curve of F Co3S4@MoS2−Ni3S2 nanorods in 1 M

KOH at a sweep rate of 5 mV.s-1 for overall water splitting (Inset: a digital photographic image of the overall water splitting reaction captured by during measurements). Based on the detailed experimental results and overall water splitting performance studies, the mechanism is developed to emphasize the largest catalytic activity of Co3S4@MoS2−Ni3S2 catalysts can be ascribed to the following aspects. (1) The successful hybridization of tricomponent transition metal sulfide (Co−Ni−Mo) driven the excellent electrochemical output, which can reasonably be considered through the synergistic effect of each element. In addition to integrating the inherent characteristic of multiple (MoS2, Ni3S2, and Co3S4) hetero-architectures also build efficient interface architecture between these three phases, to promote powerful electron transfer between adjacent phase and much more active sites, and this has played a crucial role in bifunctional electrocatalysts [43,44]. (2) Transition metal chalcogenides such as MoS2, Ni3S2, and Co3S4 can be provided a large number of active sites, which improve the catalytic efficiency towards overall water splitting [25,28]. (3) The hetero-architectures of Co3S4@MoS2−Ni3S2 catalysts with rich interfaces have an outstanding chemisorption efficiency both

hydrogen

(H−chemisorption

of

MoS2)

and

oxygen-containing

intermediate

(HO−chemisorption of Ni3S2 and Co3S4) leading in superb electrocatalytic OER and HER in alkaline medium [15,13]. (4) Recently. Experiments and simulation studies have also shown that catalyzes of uncoordinated Mo-S sites are very active along the edges of MoS2. Due to the greater ability of chemisorption for oxygen-containing intermediates, the active sites are important for OER. It is, therefore, necessary to increase the density of active sites in order to improve the OER activity of MoS2. Based on the fact that our 1D Ni3S2 nanorods integrated with the MoS2 nanosheet, which can result in an increase in specific surface area, and extending the active site coverage, thereby enhancing the activity towards the OER process. Besides this, a calculation based on density functional theory confirms that the OER behavior of MoS2 originates from the edge site instead of basal plane sites. M-edge (M=Mo) and S-edge in 2H phase with the number of sulfur covering and 1T phase edge (only one edge from in 1T structure). The edges of the 1T phase and the most active edge sites in the 2H phase containing 50% sulfur coverage. The overpotential emerges from order to overcome the limiting step barrier of O=O bond formation, which in most case splits a second water molecule through a one-proton transfer method (O* + H2O → *OOH + H+ + e−), for which the associative step of forming a molecule of oxygen via OOH* (OOH* → * + O2 + H+ + e−) has the greatest difference in the

free energy [45,46]. Moreover, sulfur in the heterostructure often leads to some interesting multiatom interaction and facilitates electron transfer around multiple sites, and thus decreasing the intermediate reaction energy barrier. Besides, sulfur in heterostructure can alter the electronic structure of metal active sites, reactive species in it’s also changing, thereby overall reactivity increased [47]. 4. Conclusion In summary, we created an entirely new strategy for the fabrication of highly abundant and more active electrocatalysts for overall electrochemical water splitting electrocatalysts through interface engineering. The Ni3S2 nanorods successfully embedded with a layer of MoS2 nanosheet on the MOF Co3S4 nanoleaves supported on the nickel foam substrate, providing excellently

exposed

heterointerfaces.

The

investigation

of

water

splitting,

MOF

Co3S4@MoS2−Ni3S2 exhibits outstanding electrocatalytic properties owing to unique heteroarchitectures of MOF Co3S4@MoS2−Ni3S2 hybrids show substantial improvement in both HER and OER performance and thus enhance the long term stability. This work could provide a new idea for understanding and rational designing of heterointerfaces to develop low cost and extremely effective electrocatalyst for overall water splitting reaction. Acknowledgment Authors acknowledge National Research Foundation of Korea (NRF) grant funded by the Korea government (MISP) (Grant number 2014R1A4A1008140) and the program for fostering next-generation researchers in the engineering of National Research Foundation of Korea (NRF) funded by the Ministry of Education ICT (Grant number 2017H1D8A2030449). References [1]

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The corresponding author is responsible for ensuring that the descriptions are accurate and agreed by all authors.

Declaration of interests

☒ The authors declare that they have no known competing for financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: