Effect of ion diffusion in cobalt molybdenum bimetallic sulfide towards

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Effect of ion diffusion in cobalt molybdenum bimetallic sulfide towards electrocatalytic water splitting Subhasis Shit, Wooree Jang, Saikat Bolar, Naresh Chandra Murmu, Hyeyoung Koo, and Tapas Kuila ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b06635 • Publication Date (Web): 28 May 2019 Downloaded from http://pubs.acs.org on May 29, 2019

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ACS Applied Materials & Interfaces

Effect of ion diffusion in cobalt molybdenum bimetallic sulfide towards electrocatalytic water splitting Subhasis Shit,†,‡ Wooree Jang,# Saikat Bolar, †,‡Naresh Chandra Murmu, †,‡ Hyeyoung Koo,# and Tapas Kuila†,‡,* †

Surface Engineering & Tribology Division, Council of Scientific and Industrial

Research-Central Mechanical Engineering Research Institute, Durgapur -713209, India ‡ Academy

of Scientific and Innovative Research (AcSIR), Ghaziabad- 201002, India

#Functional

Composite Materials Research Center, Institute of Advanced Composite

Materials, Korea Institute of Science and Technology (KIST), Jeonbuk - 565905, South Korea KEYWORDS: Catalysis, Water splitting, Bimetallic sulfide, Kirkendall effect, Electrochemical impedance spectroscopy.

ABSTRACT: The electrocatalyst comprising of two different metal atoms is found suitable for overall water splitting in alkaline medium. Hydrothermal synthesis is an

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extensively used technique for the synthesis of various metal sulfides. Time-dependent diffusion of the constituting ions during hydrothermal synthesis can affect the crystal and electronic structure of the product, which in turn would modulate its electrocatalytic activity. Herein, cobalt molybdenum bimetallic sulfide was prepared via hydrothermal method after varying the duration of reaction. The change in crystal structure, amount of Co-S-Mo moiety and electronic structure of the synthesized materials were thoroughly investigated using different analytical techniques. These changes modulated the charge transfer at the electrode-electrolyte interface, as evidenced from the electrochemical impedance spectroscopy (EIS). The Tafel plots for the prepared materials were investigated considering a less explored approach and it was found that different materials facilitated different electrocatalytic pathways. The product obtained after 12 h reaction showed superior catalytic activity in comparison to the products obtained from 4, 8 and 16 h reaction and it surpassed the overall water splitting activity of the RuO2-Pt/C couple. This study demonstrated the ion diffusion within the bimetallic sulfide during hydrothermal synthesis and change in its electrocatalytic activity due to ion diffusion.

1. INTRODUCTION Water electrolysis driven by renewable energy sources showed great potentiality towards the production of hydrogen, a carbon-neutral energy carrier.1 However, the


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bottleneck of the water splitting is sluggish kinetics of both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER).2 An electrocatalyst is employed to amplify the rate of these electrochemical reactions.3 Currently, Pt-based and Ru/Ir-based materials are considered to be the state-of-the-art catalysts for HER and OER, respectively.4-5 These noble-metal based catalysts are lack of large-scale applicability due to their high cost, low stability and scarcity.5-6 In addition, a single material which can effectively catalyze both HER and OER in the same medium is highly desirable to achieve greater competency in overall water splitting.7 Enormous attempts are therefore being made on developing stable and efficient non-noble metal-based bifunctional electrocatalysts.7-9 The OER is more sluggish compared to HER as it involves proton-coupled four electron transfer processes and therefore; alkaline water electrolysis shows more efficiency compared to acid water electrolysis.10-11 Prior investigations revealed that transition metal sulfides (TMS) can efficiently catalyze either HER or OER in alkaline medium, however; there are a few reports of transition metal sulfides which show bifunctionality.12-17 Interestingly, most of the TMS based bifunctional electrocatalysts comprise of two metal atoms.1,18-22 Therefore exploring the catalytic activity of bimetallic sulfide is an attractive strategy to achieve the desired bifunctional electrocatalyst.22 Previous studies revealed that when CoSx formed heterostructure with MoSx, catalytic activity towards both HER and OER increased synergistically.23-27 These


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studies suggested that the catalytic activity increased with increasing the Co-S-Mo in the hybrid structure.23,26-28 On that perspective, cobalt molybdenum bimetallic sulfide with a greater number of Co-S-Mo phase should be synthesized and its catalytic activity should be investigated. Recently, one-pot solvothermal synthesis is a widely used synthetic procedure for the preparation of electrocatalysts.1,24,29-30 The previous reports suggested that the use of ethylene glycol (EG) in solvothermal synthesis facilitates easy phase separation between CoSx and MoSx leading to the formation of the heterostructure, despiteof bimetallic sulfide (CoMoSx).25,27 Therefore, the use of EG should be avoided and rather a hydrothermal route should be followed for the preparation of CoMoSx. In addition, the precursor ions used for materials synthesis have different ionic radius and therefore show different diffusion rates at a higher temperature. As a consequence, at first an intermediate crystal would be precipitated out and then, time-dependent ion diffusion would occur within the crystal. The diffusion could alter the crystal and electronic structure and the amount of Co-S-Mo phase in the synthesized CoMoSx, causing variation in electrocatalytic activity. The above-mentioned ion diffusion mechanism could be demonstrated by the Kirkendall effect. Hence, the effect of Kirkendall effect to modulate the electrocatalytic activity of CoMoSx is of great interest. Herein, cobalt molybdenum sulfide (CoMoSx) was prepared via hydrothermal method, with varying the reaction time. The change in crystal structure over time was


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investigated from the Powder X-ray diffraction (PXRD) patterns of the reaction products. The Raman spectra of the samples were studied to understand the change in bonding between the atoms due to ion diffusion. Field emission scanning electron microscopy (FE-SEM) images were explored to understand the morphological change and to support the inference obtained from the PXRD. The high-resolution transmission electron microscopy (HR-TEM) images were examined to comprehend the microstructural features. The valence states of the constituting elements and the influence of ion diffusion towards the surface electronic structure were investigated from the X-Ray photoelectron spectroscopy (XPS).These analytical techniques confirmed that the crystal structure, amount of Co-S-Mo moiety and electronic structure of the synthesized materials changed due to time dependent ion diffusion. The electrocatalytic activity of the prepared materials towards HER, OER and overall water splitting was investigated in 1.0 M KOH solution. The above-mentioned changes modulated the dynamics of the electrode-electrolyte interface and charge transfer across it, as evidenced from the electrochemical impedance spectroscopy (EIS). On analyzing the Tafel plots via a less explored approach it was found that, different materials facilitated different electrocatalytic pathways for hydrogen evolution and oxygen evolution reaction. The product obtained after 12 h reaction was solely cobalt molybdenum bimetallic sulfide and it showed the superior catalytic activity in comparison to 4, 8 and 16 h synthesis products. The 12 h synthesis product surpassed the overall water splitting activity of the RuO2-Pt/C couple, also. This study will open up a new


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pathway towards the development of non-noble metal-based bimetallic sulfide as electrocatalyst for water splitting application. 2. EXPERIMENTAL SECTION 2.1 Materials Cobalt (II) chloride hexahydrate (CoCl2.6H2O), sodium molybdate dihydrate (Na2MoO4.2H2O), N, N-dimethylformamide (DMF), potassium hydroxide (KOH) pellets and hydrochloric acid (HCl, ~35%) were purchased from Merck Specialties Pvt. Ltd., India. Thiourea (NH2CSNH2), platinum (10 wt%) on activated carbon (Pt/C) and ruthenium oxide (RuO2) were acquired from Sigma-Aldrich Co., USA. Ethanol (C2H5OH, 99.9%) was obtained from Honyon International Inc., China. Nickel foam (NF) was bought from Shanghai Winfay New Material Co. Ltd., China. No further purification of the chemicals was done unless mentioned. De-ionized (DI) water was used for synthesis and experiments. 2.2 Synthesis of electrocatalysts 5 mmol of CoCl2, 5 mmol of Na2MoO4 and 15 mmol of thiourea were dissolved in 80 mL DI water by means of magnetic stirring. The mixture was transferred into a 100 mL Teflon-lined stainless-steel autoclave and the autoclave was locked properly. The autoclave was kept inside an oven preheated at 180 °C for 4 h. Another three


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hydrothermal reactions were performed taking the same compositions for 8, 12 and 16 h. The reaction products were filtered washed and vacuum dried at 60 °C for 16 h. The obtained products are designated as CMS-Xh where X denotes the reaction time. 2.3 Physicochemical characterizations Powder X-ray diffraction (PXRD) patterns of the synthesized materials were recorded with D2 PHASER (Bruker, Germany) using Cu Kα radiation (λ = 0.15418 nm). Raman spectra of the materials were acquired with alpha300 R (WITec, Germany) using a laser light source (λ = 532 nm). X-Ray photoelectron spectroscopy (XPS) of the materials was performed using K-Alpha X-ray Photoelectron Spectrometer System (Thermo Fisher Scientific, USA). Field emission scanning electron microscopy (FE-SEM) images were captured using Σigma HD (Carl Zeiss, Germany) at 5.00 kV. High-resolution transmission electron microscopy (HR-TEM) images and selected area electron diffraction (SAED) patterns were acquired using Tecnai G2-F20 (FEI, USA) at 200 kV. 2.4 Electrochemical experiments The electrocatalyst slurries were prepared by dispersing the synthesized material and polyvinylidene fluoride (PVDF) binder in DMF. A measured amount of slurry was drop casted onto the pretreated NF pieces so that in each case, the electrocatalyst loading was ~0.5 mg cm-2. PARSTAT 4000 electrochemical workstation (Princeton Applied Research, USA) was used for all electrochemical experiments in 1.0 M KOH electrolyte.


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Electrocatalyst loaded NF, Pt wire and Ag/AgCl/saturated KCl were used as working, auxiliary and reference electrode, respectively in three-electrode setup. All recorded potentials were converted according to reversible hydrogen electrode (RHE) following Nernst equation and all current responses were reported after geometric area normalization. Polarization curves were recorded by performing linear sweep voltammetry (LSV) at a scan rate of 5 mV sec-1. All the polarization curves were ~100% iR corrected to avoid the influence of uncompensated resistance. The Tafel plots were obtained from the iR corrected polarization curves. The electrochemical impedance spectroscopy (EIS) was performed at biased condition using sinusoidal alternating current (AC) of 10 mV (RMS) amplitude and frequency ranging between 100 mHz – 100 kHz. The electrochemically active surface area (ECSA) was calculated from the curves obtained after performing cyclic voltammetry (CV) at various scan rates. The overall water splitting efficiency was investigated in a two-electrode setup after constructing a symmetrical cell. The robustness of the electrocatalyst in operational condition was checked using CV and chronoamperometry. 3. RESULTS AND DISCUSSION 3.1 Physicochemical analysis PXRD patterns of the synthesized materials (Figure 1a) were analyzed to understand the structural transformations happening over time in the hydrothermal condition. The


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intense peak located at the 2θ value of 27.8 in the PXRD pattern of CMS-4h, was attributed to the Bragg diffraction (004) plane in 3R-MoS2 (JCPDS No. 17-0744).31 Another two intense peaks appeared due to the diffraction from (100) and (101) planes of 2H-MoS2 (JCPDS No. 37-1492).31 Some less intense peaks were also observed in the XRD pattern of CMS-4h which could be identified as the diffraction peaks of cobalt sulfides (CoS, CoS2).32-33 In addition to the lower intensity, the peaks related to cobalt sulfides were broad in nature inferring their poor crystallinity. The above-mentioned peaks related to MoS2were present in the PXRD pattern of CMS-8h also. However, the full width at half maxima values of these peaks increased, which inferred that the crystallinity of MoS2 started to deteriorate. The peaks related to cobalt sulfides became less prominent; rather a peak related to CoMoS3.13 (JCPDS No. 16-0439) emerged at 2θ ≈ 54.86° in the PXRD pattern of CMS-8h.34 Initially, the MoS2 got precipitated out as it has lower solubility product value compared to that of CoS or CoS2.35-36 The cobalt sulfide particles with poor crystallinity got precipitated out with the fairly crystalline MoS2 at the initial stage. After that, the Kirkendall effect i.e., diffusion of the constituting ions started to take place and MoS2 particles started to get sacrificed. The Mo6+ ions started to diffuse outwards at a higher rate than that of Co2+ ions. The bimetallic CoMoS3.13also started to grow due to the ion diffusion. The PXRD pattern of CMS-12h matched well with the diffraction pattern of CoMoS3.13.34 Thus, it could be concluded that further ion diffusion took place in later stage and after 12 h the whole product got converted into


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CoMoS3.13. The peaks related to the CoMoS3.13 were present in the PXRD pattern of CMS-18h. The FWHM of those peaks changed in comparison to that in CMS-12h. Thus, it could be inferred that the crystallinity of the previously formed CoMoS3.13 has changed over time due to the Kirkendall effect. The other peaks present in the PXRD pattern of CMS-16h were indexed to the Bragg diffraction peaks of Co3S4.37 Further ion diffusion took place after 12 h reaction and at the end of 16 h; the Co3S4 was formed alongside CoMoS3.13. Rotational-vibrational spectroscopy is a crucial experimental technique to understand the chemical bonding present in the sample. Thus, Raman spectra of all the prepared samples were obtained (Figure 1b). The bands related to E1g vibration of Mo, Co-S-Mo symmetric stretching, two S asymmetric stretching and Mo-S symmetric stretching is observed at the Raman shift values of 280, 366, 812, 858 and 932 cm-1, respectively.38-39 The band related to Mo-S symmetric stretching was found in all four samples however a gradual red-shift of this band was observed from CMS-4h up to CMS-12h and a blueshift in case of CMS-16h (Figure S1). The strain at the Mo-S bond was changed due to the Kirkendall effect, which was reflected in the change of Raman shift values. The FWHM of the same band also changed, which confirmed the change in crystallinity over time in hydrothermal condition. The relative intensity of the bands related to S asymmetric stretching (of S-Mo-S moiety) and Co-S-Mo was found to increase from CMS-4h till CMS-16h. This observation confirmed that the Co-S-Mo phase in the


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hydrothermal product increased over time. The presence of a very weak E1g vibrational band of Mo was identified in the Raman spectra of CMS-8h and CMS-16h only.

Figure 1.(a) PXRD pattern [with standard diffraction patterns of 3R-MoS2 (JCPDS No. 17-0744), 2H-MoS2 (JCPDS No. 37-1492), CoMoS3.13 (JCPDS No. 16-0439)] and (b) Raman spectra of CMS-4h, CMS-8h, CMS-12h & CMS-16h; (c) Co 2p and (d) Mo 3d XPS profile of CMS-12h. The valence state of the constituting elements can be studied from XPS and therefore, the high-resolution XPS profiles of Co 2p, Mo 3d and S 2p for all CMS-Xh samples were acquired. The Co 2p XPS profiles (Figure S2a, S2c, 1c and 2a)were deconvoluted after


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constraining the FWHM of the peaks = 2.20 eV and spin-orbit splitting = 16 eV. The Co having 2+ and 3+ oxidation states were present in all four samples.40 Two extra peaks at 776.85 and 791.73 were found to be present in the deconvoluted Co 2p XPS profile of CMS-16h (Figure 2a). These peaks appeared due to the presence of Co0 in CMS-16 h. The binding energy (BE) of the peaks related to Co 2p was found to increase in CMS-8h as compared to CMS-4h and then got decreased in CMS-12h. The Co2+ : Co3+ ratio increased from CMS-4h to CMS-8h and then further decreased till CMS-16h. The Mo 3d XPS profiles (Figure S2b, S2d, 1d and 2b) were deconvoluted after constraining the FWHM of the peaks = 2.00 eV and spin-orbit splitting = 3 eV. Both Mo6+ and Mo4+ were found to be present in all samples, except CMS-8h.41 An extra peak at higher B.E. (denoted as “X”) emerged in case of CMS-4h and CMS-12h. That “X” peak might be shake-up type satellite peak (Figure S2b and 2b). The Mo4+ peaks were found to be absent in the Mo 3d XPS profile of CMS-8h (Figure S2d). The Mo4+ : Mo6+ ratio for CMS4h was lower than that for CMS-12h and it further declined in CMS-16h. Therefore, as the density of Co with higher oxidation state increased in the material, the density of Mo with higher oxidation state got reduced. Along with Mo 3d XPS profile, a broad peak at ~226.71 was found to be present in XPS profile of CMS-16h (Figure 2b) which could be indexed to the S 2s peak.42 A peak located at ~168 eV B.E. appeared in the S 2p XPS profile of all the materials (Figure 2c). This peak emerged due to the bond formation between S and both Co and Mo atoms. A blue shift was observed in this peak


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from CMS-4h to CMS-8h and after that, it was red-shifted till CMS-16h. This phenomenon occurred due to push-pull of electronic density around the metal atoms and S atom. An extra peak at ~162 eV B.E. emerged in the S 2p XPS profile of CMS-16h which was due to the presence of S2- ions in the material.43

Figure 2. (a) Co 2p and (b) Mo 3d XPS profile of CMS-16h; (c) S 2p XPS profile and (d) valence band spectra of CMS-4h, CMS-8h, CMS-12h & CMS-16h. The valence band plays a crucial role in the electrocatalytic process and therefore understanding the valence band electronic structure is important. The valence band XPS (VBXPS) profiles of CMS-Xh (Figure 2d) were acquired. The Fermi energy level (EF)


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was set as the 0 B.E. The valence band of CMS-4h was extended beyond the EF, however, for the other three materials the valence band edge (VBmax) was situated at positive B.E. The VBmax gradually shifted towards higher B.E. from CMS-8h to CMS-16h and the electron density near EF decreased accordingly. The S 3p peak was blue-shifted for CMS-8h as compared to CMS-4h and after that, it gradually red-shifted until CMS16h. This observation fully agrees with the inference obtained from the S 2p spectra of the materials. Distinct Co 3d and Mo 4d peaks were observed for CMS-12 h; however, the merger of those peaks resulted in a single peak in the other three cases. The peaks related to Co and Mo red-shifted in CMS-8h as compared to CMS-4h and thereafter blue-shifted, which was just opposite of the shifts observed in S 3p peak. The electron density push-pull around S and metal atoms, as inferred previously, was justified from these observations. Thus, the time-dependent ion diffusion in bimetallic sulfide altered the core-level as well as the valence band electronic structure of the prepared bimetallic sulfides. The morphological features of the synthesized materials were investigated from the FESEM images (Figure 3 and S3), as acquired. Flake-like structures, identified to be cobalt sulfides, were present in the CMS-4h (Figure S3a-b). Sheet-like MoS2 were found to be present along with those flake-like cobalt sulfides. Similar type of morphological features was recorded for CMS-8h also; however, more structured MoS2 sheets were found in it (Figure 3aand S3c). The amount of cobalt sulfide flakes decreased in CMS-


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8h. These observations were in accordance with the inference obtained from the PXRD patterns. Rod-like structures were found to be present in CMS-12h (Figure 3band S3d). The TEM image of CMS-12h (Figure S4) also confirmed the presence of rod-like structure. Along with those structures, particles with random morphology were also present in CMS-12h. A crumpled sheet-like structure with irregular morphology was found in the FE-SEM images of CMS-18h as shown in Figure 3c-d. The FE-SEM images also showed that various polyhedrons, mainly octahedrons (shown by an arrow in Figure 3d) were distributed throughout the surfaces, which was identified to be the Co3S4 particles. The previous reports suggested that stable octahedrons of Co3S4 were formed in hydrothermal condition.37 PXRD patterns of CMS-16h also suggested the formation of Co3S4. The ion diffusion not only changed the structure of the hydrothermal product, but it also changed its morphology, over time. The FE-SEM images validated the growth mechanism of the hydrothermal product, as predicted from the PXRD patterns.


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Figure 3.FE-SEM images of (a) CMS-8h, (b) CMS-12h & (c,d) CMS-16h. The HR-TEM images of CMS-12h (Figure 4a-c) and CMS-16h (Figure 4i) were acquired and fast Fourier transformation (FFT) followed by inverse fast Fourier transformation (IFFT) were performed at the R1, R2, R3 and R4 regions to obtain the images depicted in Figure 4d, 4e, 4fand 4j, respectively. The HR-TEM images of CMS-12h also showed the presence of rod-like structures. The d-spacing values calculated from the IFFT images of CMS-12h corroborated very well with its XRD peaks located at 2θ values of 26.98, 29.51 and 54.78°. However, only the lattice fringes with d-spacing value related to XRD peak at 29.51° was found in the HR-TEM image of CMS-16h. The presence of bright spots in the SAED pattern of CMS-12h (Figure 4g) confirmed its good crystallinity. The d-spacing related to the diffraction spots were calculated (Figure


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4g). The background of the SAED pattern was subtracted via “Rolling ball method” using Fiji and then intensity vs. d-spacing plot was calculated from it (Figure 4h).44 That plot showed close agreement with the same plot obtained from its PXRD pattern, further validating the crystal structure of CMS-12h. Lesser number of bright diffraction spot was observed in the SAED pattern of CMS-16h (Figure S5) indicating the deterioration of crystallinity.

Figure 4. (a-c) HR-TEM images of CMS-12h; IFFT images of (d) R1, (e) R2 & (f) R3 (with FFT pattern at inset) regions; (g) SAED pattern of CMS-12h; (h) intensity vs. d-spacing plot obtained from SAED pattern and PXRD pattern for CMS-12h; (i) HR-TEM image of CMS-16h and (j) IFFT image of R4 region.


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3.2 Electrochemical analysis 3.2.1. HER activity measurements Polarization curve analysis: HER catalyzing activity of the CMS-Xh samples was examined from the polarization curves (Figure 5a). It was found that the catalytic activity gradually increased from CMS-4h to CMS-8h to CMS-12h. However, the activity got diminished in CMS-16h. CMS-12h required 190 mV overpotential to achieve the benchmarking current density of 10 mA cm-2 (η10). The η10 values for CMS-4h and CMS-16h were 225 and 196 mV, respectively. Both CMS-4h and CMS-8h showed comparable catalytic activity till 325 mV overpotential but, CMS-8h showed superiority after that overpotential. Pt/C showed the highest catalytic activity at lower current density, however, both CMS-12h and CMS-16h surpassed its activity at higher current density. CMS-16h achieved 400 mA cm-2 current density at overpotential of 364 mV. Furthermore, the catalytic activity of CMS-12h is either comparable or superior compared to the state-of-the-art non-noble metal-based electrocatalysts (Table S1). The Tafel slope suggests how fast current density can increase on changing the overpotential and therefore is an important parameter to investigate the electrocatalytic activity of any material.45 The Tafel plots (Figure5b and S6) were analyzed to have a better insight into the catalysis process. Interestingly, two linear parts were found to be present in each Tafel plot (Figure5b and S6). The slope values of each linear part were


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calculated to predict the underlying reaction path. The Tafel slope values suggested that the reaction precedes via Volmer‐Heyrovsky mechanism in every case.46 Both the Volmer and Heyrovsky mechanism hypothesizes simultaneous movement of nucleus and electron, which is a violation of the Frank-Condon principle. Therefore, each mechanism could be subdivided into two steps as suggested by Fletcher.47 H2O

↔

H+ads

+

OH-

(1) H+ads + e- → Hads

(2)

H+ads + H2O ↔ HadsH+ads + OH-

(3)

HadsH+ads + e- → H2

(4)

Equation 1 and 3 are chemical steps and Equation 2 and 4 are electrochemical steps. The Tafel slope value = [59.2/(np + nqβf)] mV dec-1, where, np and nq are the number of electron transfer before and during the rate determining step (rds), respectively.47 The symmetry factor of the forward reaction, βf = ½(1-Fη/2λm) = ½(1-∆) where, F, η and λm are Faraday constant, overpotential and reorganization energy per mole, respectively. When step 1, 2, 3 and 4 are the rds, the Tafel slope value would be ∞, 118.4/(1-∆), 118.4/(3-∆) and 29.6 mV dec-1, respectively. The calculated Tafel slope values suggested that the electrocatalytic HER proceeds via a complex mechanism for all the synthesized


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materials. The rds for CMS-8h and CMS-12h catalysis was the 2nd step, throughout the course of the reaction. The Tafel slope value might increase at the higher overpotential region due to increment in the surface coverage; however, the reverse happened for CMS-8h.46The 3rd step might become sluggish at higher overpotential which caused the decrement in the Tafel slope value. The 2nd step was the rds for HER catalysis by Pt/C, also. The 3rd step was the rds for CMS-4h at lower overpotential however, the slope value abruptly increased at the higher overpotential region. A sharp increase in Tafel slope value was observed for the CMS-16h and Pt/C, also. These too high Tafel slope values for CMS-4h, CMS-16h and Pt/C at higher overpotential region suggested that the 1st step had an influence towards 2nd step in these cases. The above discussion further validated the superiority of CMS-12h as HER electrocatalyst in comparison to the other three materials.


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Figure 5. (a) HER polarization curves for CMS-4h, CMS-8h, CMS-12h, CMS-16h, Pt/C & bare NF; (b) Tafel plot, (c) Nyquist plot and (d) DRT plot for CMS-4h, CMS-8h, CMS12h & CMS-16h. EIS analysis: EIS is a potent technique to have a better insight into the mechanistic steps occurring during the catalysis process. Nyquist plots for the synthesized materials (Figure 5c) were obtained from EIS study performed at DC bias equivalent to -0.2 V vs. RHE. The Nyquist plots for CMS-4h and CMS-8h were fitted with the equivalent circuit depicted in Figure S7aand for CMS-12h and CMS-16h were fitted with the circuit depicted in Figure S7b. The uncompensated and charge transfer resistance are denoted as Ru and RCT, respectively. The CPE1-Rf and CPE3-Rp couples are related to the porosity of the electrocatalyst and substrate adsorption-desorption at the electrode-electrolyte


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interface.48,49 The values obtained after fitting are tabulated in Table S2. The Ru was found to be lowest for CMS-12h among all four materials. The diffusion of the ions increased from CMS-4h to CMS-16h as evidenced from the decrement in Rf value. The charge transfer efficiency at the interface was found to be highest for CMS-12h among all the materials. The circuit elements related to the substrate adsorption-desorption was absent for CMS-4h and CMS-8h, rather an inductive nature was evidenced at the lower frequency. This inductive nature appeared due to slow adsorption-desorption kinetics at the interfacial region. The active substrate adsorption-desorption at the interface was faster for CMS-12 h as compared to CMS-16 h. The above analysis suggested that the catalytic activity of CMS-12 h was superior compared to others, which was in accordance with the inference obtained from polarization curve analysis. Thereafter, the distribution of relaxation time (DRT) plots were obtained (Figure 5d) from the EIS data using Gaussian basis function and 0.01 regularization parameter.50 Each peak present in the DRT plot signifies different physical processes occurring at the electrode-electrolyte interface. The peak located at the medium and lower frequency region could be attributed to the porosity of the material and charge transfer process. A less prominent peak observed at higher frequency region, which might appear due to the adsorption of active species. The γ(τ) values of two peaks located at the high and medium frequency region were lowest for CMS-12h. The γ(τ) value of the lowfrequency peak was lowest in CMS-4h compared to the other three materials, however,


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the frequency related to that peak was higher compared to the other three materials. The position of the low-frequency peak was comparable for CMS-8h, CMS-12h and CMS-16h, however, the γ(τ) was higher for CMS-8h. Another peak at lower frequency value was observed for CMS-8h and CMS-12h, which might be due to the formation of inductive nature at the interfacial region. γ(τ) value of that peak is lower for CMS-12h in comparison to that for CMS-8h. The DRT plot also supported that the CMS-12h was superior among the prepared materials, which was in accordance with the conclusion obtained from Nyquist plot and polarization curves.

Figure 6. 3D DRT plot (with top contours at inset) for (a) CMS-4h, (b) CMS-8h, (c) CMS12h & (d) CMS-16h.


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3D DRT plots (Figure 6) explain the variation of peak related charge transfer process at different overpotential values. The γ(τ) value of that peak decreased with increasing overpotential in case of all four CMS-Xh. The γ(τ) value for the peak was lowest for CMS-12h compared to the other three. Interestingly, the peak related to charge transfer shifted to lower τ value for every material except CMS-4h. The shift towards lower τ was larger for CMS-12h. An extra peak appeared in CMS-4h and CMS-8h, which might be attributed to the porosity of the material, which was in accordance with the inference from Nyquist plot analysis. The above inferences were in line with the previous findings. 3.2.2 OER activity measurements Polarization curve analysis: OER catalytic activity of the synthesized materials was investigated from the polarization curves as shown in Figure 7a. All synthesized electrocatalysts except CMS-4h showed superior electrocatalytic activity towards OER in comparison to the RuO2. The catalytic activity of CMS-4h was comparable with the activity of RuO2. The electrocatalytic activity gradually increased till CMS-12h and then got suppressed in case of CMS-16h. The CMS-12h required 280 mV overpotential to achieve a current density of 10 mA cm-2, which was 10 mV less than that required for RuO2. η10 values for CMS-4h, CMS-8h and CMS-16h was 302, 307 and 290 mV respectively. CMS-4h showed superiority in comparison to CMS-8h till 17 mA cm-2


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current density however, after that current density, CMS-4h showed inferiority. The CMS-8h, CMS-12 and CMS-16 required 416, 336 and 381 mV overpotential, respectively to achieve a current density of 400 mA cm-2. The CMS-12h showed comparable or superior activity towards OER as compared to recently reported electrocatalysts (Table S3). The Tafel slopes as obtained from the polarization curves (Figure 7band S8) were analyzed to have an idea about the mechanistic pathway.51 Interestingly, two linear portions were found in the Tafel plots for CMS-4h, CMS-8h and RuO2. The Tafel slope value at the higher overpotential region is considered to predict the superiority of a material as an electrocatalyst.52 Herein, the Tafel slope value was lowest for CMS-12h confirming it to be the superior OER catalyst among the prepared materials. The OER proceeds via four proton coupled electron transfer steps, which makes the catalytic mechanism more complex in nature. Conway and Bourgault proposed the mechanistic pathway for OER and Fletcher subdivided the mechanism into multiple steps as follows:10,47 OH- ↔ OH-ads (5) OH-ads → OHads + e(6)


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OHads + OH-ads ↔ OHadsOH-ads

(7)

OHadsOH-ads → OHOHads + e-

(8)

OHOHads

Oads

→

+

H2O

(9) Oads + OHads ↔ O2Hads

(10)

O2Hads + OHads → H2O + O2

(11)

The symmetry factor for these reactions is βb = 1-βf = ½(1+∆), as oxidation is considered here. Therefore, the Tafel slope value will be ∞, 118.4/(1+∆), 59.2 and 236.8/(3+∆) mV dec-1 when the rds is 1st, 2nd, 3rdand 4th step, respectively. However, the Tafel slope value is 29.6 mV dec-1 when the rds is either 5th or 6th or 7th step. At the lower overpotential region, the rds for CMS-4h was 3rd step and for CMS-8h rds is either the 5th, 6th or 7th step. The 2nd step became the rds for CMS-4h and CMS-8h, at the higher overpotential region. However, the influence of the 1st step towards the rds was observed for CMS-4h. The influence of both 1st step and 2nd step towards the rds was found for RuO2 catalysis process and the Tafel slope value increased at higher overpotential region due to increased surface coverage.46 The rds for CMS-12h and CMS-16 was the 3rd step however, an influence of 4th step and 2nd step was found for CMS-12h and CMS-16h, respectively.


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Figure 7. (a) OER polarization curves for CMS-4h, CMS-8h, CMS-12h, CMS-16h, RuO2& bare NF; (b) Tafel plot, (c) Nyquist plot and (d) DRT plot for CMS-4h, CMS-8h, CMS12h & CMS-16h. EIS analysis: EIS measurement for synthesized materials was performed after biasing the system at 1.48 V vs. RHE and the Nyquist plots (Figure 7c) were obtained. All Nyquist plots were fitted with the equivalent circuit as shown in Figure S9. The values of the circuit elements obtained after fitting were presented in the Table S4. The Ru value for CMS-12h was the lowest among all four materials. The ion diffusion proficiency gradually increased from CMS-4h till CMS-16h. The charge transfer efficiency increased from CMS-4h till CMS-12h; however, it got diminished in CMS-16h. Above analysis confirmed that the electrocatalytic activity of CMS-12h towards OER was superior


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compared to other synthesized materials. The DRT plots (Figure 7d) suggested that three distinct physical processes occurred at the electrode-electrolyte interface. The three peaks present at higher, medium and lower frequency region was related to terminal group formation, the porosity of the material (which is related to diffusion of ions) and charge transfer, respectively. The peak related to terminal group formation had equivalent γ(τ) for CMS-4h and CMS-8h. That peak was absent in the DRT plot of CMS-12h; however, the γ(τ) of the same was highest for CMS-16h. The peak related to porosity and charge transfer gradually blue-shifted from CMS-4h till CMS-12h and then red-shifted in case of CMS-16h. The γ(τ) for those two peaks were lowest in case of CMS-12h. These observations suggested that the ion diffusion, as well as the charge transfer kinetics, was faster when CMS-12h acted as electrocatalyst, which was in accordance with the inference obtained from the Nyquist plot.


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Figure 8. 3D DRT plot (with top contours at inset) for (a) CMS-4h, (b) CMS-8h, (c) CMS12h & (d) CMS-16h. 3D DRT plots for every material were obtained (Figure 8) to understand how the charge transferability changes with changing overpotential. The γ(τ) related to the peak decreased on increasing potential value. The γ(τ) of that peak was the lowest for CMS12h confirming the better charge transferability of it. The charge transfer peak shifted towards lower τ value foreach synthesized material. However, the shift was higher for the CMS-12h compared to the other three compositions. The decrement of both γ(τ) and τ is an indication of increment in catalytically active sites. An extra peak at lower overpotential was observed for CMS-4h, CMS-8h and CMS-16h, which might be due to the presence of inductive nature. The charge transfer at the electrode-electrolyte


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interface was higher for CMS-12h and therefore the accumulation of ions at the interface was lower. In case of other three materials, the chance of charge accumulation is high which caused inductive behavior at the interface. Mott-Schottky analysis: The Mott-Schottky (M-S) analysis is a suitable technique to understand the electrode-electrolyte interfacial dynamics and to probe the change in carrier density at the valence band. The M-S plot is governed by the following equation. 1/C2 = (2/εε0N)(Eapp – Efb – kT/q)

(12)

where C, q, ε0, ε, N and Eapp are the capacitance, electric charge, permittivity in vacuum, dielectric constant, carrier density and applied potential, respectively.53The modulation of the valence band structure changes the N value and as a consequence, the slope of the 1/C2vs. potential plot changes. It was found that the slope of linear part in the M-S plot was different for different CMS-Xh (Figure S10). The absolute slope value of the linear region in the M-S plot was found to decrease till CMS-12 from CMS-4h and then increase in case of CMS-16h. This confirmed that the ion diffusion modulated the charge carrier density in the valence band of the prepared materials, which was in accordance with the inference obtained from the XPS analysis. The M-S analysis also confirmed that the donor density was the highest for CMS-12h, which was the reason behind superior charge transfer kinetics across the electrode-electrolyte interface and electrocatalytic activity. Electrochemically active surface area (ECSA) analysis: The abundance of ion accessible and catalytically active sites could be predicted qualitatively from the ECSA measurements.


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The double layer capacitance (Cdl) was calculated from the difference between cathodic and anodic current density (∆J) at the potential of 1.15 V vs. RHE (Figure S12a). The Cdl as well as ECSA increased gradually from CMS-4h to CMS-12h and then decreased for CMS-16h (Table S5). The Cdl was calculated also using the approach suggested by Diaz et al. (Figure S12b), to avoid the influence of Faradaic component towards the capacitive charging.54 The ECSA was found to be higher for CMS-12h compared to the other three materials, in this approach also. Interestingly, the Cdl value did not follow the similar trend as that found in the previous approach (Table S5). The Cdl value was found to be lowest for CMS-8h, which might be due to the fact that the influence of Faradaic component towards overall current response was very high for it. Both the approaches confirmed the highest ECSA for CMS-12h among all the prepared materials, which was in line with the above findings. Thereafter, the polarization curves for HER and OER were normalized with the ECSA (Figure S13) to understand the intrinsic catalytic activity of the prepared material. The ECSA normalized polarization curve do not follow the similar trend as that is observed in the area normalized polarization curve however, the CMS-12h showed superiority in both the cases.


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Figure 9. (a) Polarization curves of CMS-4h II CMS-4h, CMS-8h II CMS-8h, CMS-12h II CMS-12h, CMS-16h II CMS-16h, RuO2IIPt/C & NF II NF for overall water splitting; (b) HER polarization curves & (c) OER polarization curves of CMS-12h, before and after 1000 CV cycles; (d) chronoamperometric curves of CMS-12h for HER, OER & overall water slitting. 3.2.3 Overall water splitting activity measurements Polarization curve analysis: The overall water splitting proficiency of the prepared materials was investigated in 1.0 M KOH after constructing a symmetrical twoelectrode setup. The water splitting proficiency of the two-electrode setup consisting of


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RuO2 as anode and Pt/C as cathode was studied for comparison. The overall water splitting efficiency of CMS-12h and CMS-16h was superior compared to RuO2-Pt/C couple. The CMS-8h showed efficiency comparable with RuO2-Pt/C couple until 1.75 V however after that, it surpassed the activity of RuO2-Pt/C couple. Although the CMS-4h showed inferior performance in comparison to RuO2-Pt/C couple until 1.88 V, it surpassed the activity of RuO2-Pt/C couple thereafter. When the electrocatalytic activity of the prepared materials was investigated, it was found that the activity increased from CMS-4h till CMS-12h and then decreased for CMS-16. CMS-4h II CMS-4h, CMS-8h II CMS-8h, CMS-12h II CMS-12h, CMS-16h II CMS-16h system required 1.782, 1.746, 1.574, 1.710 V potential, respectively, to achieve the benchmarking current density of 10 mA cm-2 (Figure 9a). The CMS-12h showed superior or comparable overall water splitting efficiency as compared to recently reported bifunctional electrocatalysts (Table S6). Moreover, the CMS-12h II CMS-12h achieved a current density of 100 mA cm-2 at 1.998 V potential. 2.2.4 Stability investigation Stability and robustness in catalyzing condition is an important feature of any electrocatalyst. About1000 CV cycles were carried out between 0 to -0.35 and 1.23 to 1.58 V at a scan rate of 100 mV sec-1 taking CMS-12h as catalyst and thereafter polarizations curves were acquired by means of LSV. The final polarization curves were compared


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with the initial polarization curves. No significant change in the initial and final polarization curve was observed for both HER and OER (Figure 9b-c), which confirmed the robustness of CMS-12h as electrocatalyst. The long-term operational stability of CMS-12h was investigated by performing chronoamperometry at -0.40 and 1.63 V vs. RHE, for HER and OER, respectively (Figure 9d). The current density gradually increased for ~5h in case of HER and then the current density did not change much for the next 5h. In case of OER, a decrease in current density was observed and it was found that the electrocatalyst retained ~87.7% current density for ~10h. The change in chemical features of CMS-12h after electrochemical performances was investigated from Raman spectroscopy. There was no obvious change in the Raman spectra in the postHER sample as compared with the original spectra recorded before HER (Figure S14a), confirming the retention of its chemical structure after long-term HER catalysis. The intensity of the bands appeared in the pre-catalysis sample was found to decrease when Raman spectra of the post-OER sample were investigated. Rather, four intense bands related to E1g vibration of Mo, Co-O-Mo symmetric stretching, Mo-O-Moand Mo=O asymmetric stretching was found to be present in the spectra.38,55-56It can be inferred that partial oxidation of the electrocatalyst occurred during long-term OER catalysis process.57 The change in morphology of CMS-12h after HER and OER catalysis was investigated from the FE-SEM images. There was no significant change in morphology observed after long-term HER performance (Figure S14b). However, the morphology of


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CMS-12h changed after OER performance as evidenced from the FE-SEM image of the post-OER material (Figure S14c). The partial oxidation of the electrocatalyst due to long-term

OER

catalysis

caused

the

morphological

changes.

Thereafter

chronoamperometry was carried out at 1.8 V potential taking CMS-12h II CMS-12h twoelectrode setup. The CMS-12h retained ~ 88.20% current density after ~25h long overall water splitting performance. The above investigations confirmed that CMS-12h also had long-term water splitting competency in tandem with its efficiency. 4. CONCLUSIONS In summary, the diffusion of the constituting ions during hydrothermal synthesis of bimetallic cobalt molybdenum sulfide was studied. It was evidenced that with time the diffusion of ions modulated the amount of Co-S-Mo moieties in the reaction product. The core-level and the valence band electronic structure changed due to such ion diffusion. The change in electronic structure resulted in modulation of the electron density near the EF. The change in the amount of Co-S-Mo moiety and the electronic structure of the electrocatalyst regulated the efficiency of charge transfer towards the active species. The CMS-12h showed superior electrocatalytic activity towards HER, OER and overall water splitting. Moreover, CMS-12h surpassed the overall water splitting activity of state-of-the-art RuO2-Pt/C couple. Therefore, when solely the bimetallic sulfide (CoMoS3.13) was formed, the electrocatalytic activity was the highest.


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The Tafel slope analysis suggested that the product obtained at different time facilitated different electrocatalytic pathways. Along with efficiency, the CMS-12h showed longterm operational stability which made CMS-12h a potent candidate for the non-noble metal based electrocatalyst for the water splitting application. This study will shed light upon the new path of developing transition-metal based bimetallic sulfide for water splitting application. ASSOCIATED CONTENT Supporting Information Raman band related to Mo-S symmetric stretching in CMS-4h, CMS-8h, CMS-12h &CMS-16h; Co 2p & Mo 3d XPS profile of CMS-4h & CMS-8h;FE-SEM images CMS-4h, CMS-8h & CMS-12h; TEM image of CMS-12h; SAED pattern of CMS-16h; Tafel plots for Pt/C & RuO2; equivalent circuits used to fit the Nyquist plots obtained for HER & OER; Nyquist plots fitted data; Mott-Schottky plots for CMS-4h, CMS-8h, CMS-12h & CMS-16h; cyclic voltammograms obtained at different scan rates for CMS-4h, CMS-8h, CMS-12h & CMS-16h; ∆J vs. scan rate plot & ∆Jν-1/2 vs. ν1/2 plot for CMS-4h, CMS-8h, CMS-12h & CMS-16h; calculated Cdl, ECSA values for prepared materials; ECSA normalized HER & OER polarization curves for CMS-4h, CMS-8h, CMS-12h & CMS-16h; FE-SEM and Raman spectra of CMS-12h after HER & OER; comparison tables of recently reported HER, OER & bifunctional electrocatalysts in alkaline medium. AUTHOR INFORMATION


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Corresponding Author *Correspondence to Tapas Kuila +91-9647205077; Fax: 91-343-2548204; E-mail: [email protected]. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT The authors acknowledge the financial support from the Department of Science and Technology, New Delhi, India (GAP215312). This study was supported by the KIST Institutional Program (2Z05430). The authors are also thankful to the Director of CSIRCMERI, Durgapur. REFERENCES [1] Zhong, X.; Tang, J.; Wang, J.; Shao, M.; Chai, J.; Wang, S.; Yang, M.; Yang, Y.; Wang, N.; Wang, S.; Xu, B.; Pan, H., 3D Heterostructured Pure and N-Doped Ni3S2/VS2 Nanosheets for High Efficient Overall Water Splitting. Electrochim. Acta, 2018, 269, 55-61. [2] Ning, Y.; Ma, D.; Shen, Y.; Wang, F.; Zhang, X., Constructing Hierarchical Mushroom-Like BifunctionalNiCo/NiCo2S4@NiCo/Ni Foam Electrocatalysts For


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Effect of ion diffusion in cobalt molybdenum bimetallic sulfide towards

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