Effect of Ion Diffusion in Cobalt Molybdenum Bimetallic Sulfide toward

Research Article Cite This: ACS Appl. Mater. Interfaces 2019, 11, 21634−21644

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Effect of Ion Diffusion in Cobalt Molybdenum Bimetallic Sulfide toward Electrocatalytic Water Splitting Subhasis Shit,†,‡ Wooree Jang,§ Saikat Bolar,†,‡ Naresh Chandra Murmu,†,‡ Hyeyoung Koo,§ and Tapas Kuila*,†,‡

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†

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 S Supporting Information *

ABSTRACT: The electrocatalyst comprising two different metal atoms is found suitable for overall water splitting in alkaline medium. Hydrothermal synthesis is an 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 by electrochemical impedance spectroscopy. 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. KEYWORDS: catalysis, water splitting, bimetallic sulfide, Kirkendall effect, electrochemical impedance spectroscopy

1. INTRODUCTION Water electrolysis driven by renewable energy sources showed great potentiality toward the production of hydrogen, a carbon-neutral energy carrier.1 However, the 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-metalbased catalysts lack large-scale applicability due to their high cost, low stability, and scarcity.5,6 In addition, a single material that 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, © 2019 American Chemical Society

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 showing bifunctionality.12−17 Interestingly, most of the TMS-based bifunctional electrocatalysts comprise 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 toward both HER and OER increased synergistically.23−27 These studies suggested that the catalytic activity increased with increasing Co−S−Mo in the hybrid structure.23,26−28 On that perspective, cobalt molybdenum bimetallic sulfide with a greater Received: April 16, 2019 Accepted: May 28, 2019 Published: May 28, 2019 21634

DOI: 10.1021/acsami.9b06635 ACS Appl. Mater. Interfaces 2019, 11, 21634−21644

Research Article

ACS Applied Materials & Interfaces

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. Deionized (DI) water was used for synthesis and experiments. 2.2. Synthesis of Electrocatalysts. CoCl2 (5 mmol), Na2MoO4 (5 mmol), and thiourea (15 mmol) were dissolved in 80 mL of 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. Three other 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-α X-ray Photoelectron Spectrometer System (Thermo Fisher Scientific, USA). Field emission scanning electron microscopy (FE-SEM) images were captured using ∑igma 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 poly(vinylidene fluoride) (PVDF) binder in DMF. A measured amount of slurry was drop-cast 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. Electrocatalyst-loaded NF, Pt wire, and Ag/AgCl/saturated KCl were used as working, auxiliary, and reference electrodes, respectively, in a three-electrode setup. All recorded potentials were converted according to reversible hydrogen electrode (RHE) following the 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 s−1. All of 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. 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 and 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.

number of Co−S−Mo phase should be synthesized and its catalytic activity should be investigated. Recently, one-pot solvothermal synthesis has become a widely used synthetic procedure for the preparation of electrocatalysts.1,24,29,30 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, despite 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 radii 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, by varying the reaction time. The change in crystal structure over time was 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 toward the surface electronic structure were investigated by 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 toward HER, OER, and overall water splitting was investigated in 1.0 M KOH solution. The abovementioned changes modulated the dynamics of the electrode− electrolyte interface and charge transfer across it, as evidenced by 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 reactions. The product obtained after 12 h reaction was solely cobalt molybdenum bimetallic sulfide, which showed 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, too. This study will open up a new pathway toward the development of non-noble-metal-based bimetallic sulfide as electrocatalyst for water splitting application.

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 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 PXRD pattern of CMS-4h, which could be identified as the diffraction peaks of cobalt sulfides (CoS, CoS2).32,33 In

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 21635


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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 CMS12h, and a blue shift in the 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 to CMS-16h. This observation confirmed that the Co−S−Mo phase in the 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. 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 (Figures S2a,c, 1c and 2a) were Figure 1. (a) PXRD pattern [with standard diffraction patterns of 3RMoS2 (JCPDS No. 17-0744), 2H-MoS2 (JCPDS No. 37-1492), and CoMoS3.13 (JCPDS No. 16-0439)]. (b) Raman spectra of CMS-4h, CMS-8h, and CMS-12h & CMS-16h. (c) Co 2p and (d) Mo 3d XPS profiles of CMS-12h.

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 MoS2 were present in the PXRD pattern of CMS-8h also. However, the full width at half-maximum (FWHM) 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 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 outward at a higher rate than Co2+ ions. The bimetallic CoMoS3.13 also 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 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, 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 of 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 are observed at the

Figure 2. (a) Co 2p and (b) Mo 3d XPS profiles of CMS-16h; (c) S 2p XPS profile; and (d) valence band spectra of CMS-4h, CMS-8h, CMS-12h, and CMS-16h.

deconvoluted after constraining the FWHM of the peaks = 2.20 eV and spin−orbit splitting = 16 eV. 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 (B.E.) of the peaks related to Co 2p was found to increase in CMS-8h compared to CMS-4h and then decreased in CMS-12h. The Co2+:Co3+ ratio increased from CMS-4h to CMS-8h and then further decreased up to CMS16h. The Mo 3d XPS profiles (Figures S2b,d, 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 the case of CMS-4h and CMS-12h. That “X” peak might be shake-uptype satellite peak (Figures S2b and 2b). The Mo4+ peaks were found to be absent in the Mo 3d XPS profile of CMS-8h 21636


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Figure 3. FE-SEM images of (a) CMS-8h, (b) CMS-12h, and (c,d) CMS-16h.

(Figure S2d). The Mo4+:Mo6+ ratio for CMS-4h 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 the 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 of 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 from CMS-4h to CMS-8h and after that, it was red-shifted up to 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 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) 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 toward higher B.E. from CMS-8h to CMS-16h, and the electron density near EF decreased accordingly. The S 3p peak was blueshifted for CMS-8h compared to CMS-4h and after that, it gradually red-shifted until CMS-16h. 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 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 FE-SEM images (Figure 3 and S3), as acquired. Flake-like structures, identified to be cobalt sulfides, were present in the CMS-4h (Figure S3a,b). Sheetlike MoS2 were found to be present along with those flakelike cobalt sulfides. A similar type of morphological features was recorded for CMS-8h also; however, more structured MoS2 sheets were found in it (Figures 3a and S3c). The amount of cobalt sulfide flakes decreased in CMS-8h. These observations were in accordance with the inference obtained from the PXRD patterns. Rodlike structures were found to be present in CMS-12h (Figures 3b and S3d). The TEM image of CMS-12h (Figure S4) also confirmed the presence of rodlike structure. Along with those structures, particles with random morphology were also present in CMS-12h. A crumpled sheetlike 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. 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 in the R1, R2, R3, and R4 regions to obtain the images depicted in Figure 4d−f,j, respectively. The HR-TEM images of CMS-12h also showed the presence of rodlike structures. The d-spacing values calculated from the IFFT images of CMS-12h corroborated very well with its PXRD peaks located at 2θ values of 26.98, 29.51, and 54.78°. However, only the lattice fringes with dspacing value related to PXRD peak at 29.51° was found in the HR-TEM image of CMS-16h. The presence of bright spots in the SAED pattern of CMS12h (Figure 4g) confirmed its good crystallinity. The d-spacing 21637


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to CMS-12h. However, the activity 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 activities up to 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-8h and CMS-12h surpassed its activity at higher current density. CMS-12h achieved 400 mA cm−2 current density at an overpotential of 364 mV. Furthermore, the catalytic activity of CMS-12h is either comparable or superior 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 (Figures 5b 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 (Figures 5b and S6). The slope values of each linear part were calculated to predict the underlying reaction path. The Tafel slope values suggested that the reaction precedes via the Volmer−Heyrovsky mechanism in every case.46 Both the Volmer and Heyrovsky mechanisms hypothesize simultaneous movement of nucleus and electron, which is a violation of the Franck−Condon principle. Therefore, each mechanism could be subdivided into two steps as suggested by Fletcher.47

Figure 4. (a−c) HR-TEM images of CMS-12h; IFFT images of (d) R1, (e) R2, and (f) R3 (inset: FFT pattern) 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 the R4 region.

related to the diffraction spots was calculated (Figure 4g). The background of the SAED pattern was subtracted via the “rolling ball method” using Fiji and then intensity vs d-spacing plot was calculated from it (Figure 4h),44 which showed a close agreement with the same plot obtained from its PXRD pattern, further validating the crystal structure of CMS-12h. Lesser number of bright diffraction spots were observed in the SAED pattern of CMS-16h (Figure S5), indicating the deterioration of crystallinity. 3.2. Electrochemical Analysis. 3.2.1. HER Activity Measurements. 3.2.1.1. 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

H 2O ↔ H+ads + OH−

(1)

H+ads + e− → Hads

(2)

H+ads + H 2O ↔ HadsH+ads + OH−

(3)

HadsH+ads + e− → H 2

(4)

Equations 1 and 3 are chemical steps, and eqs 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 electrons transferred before and during the rate-determining step (rds), respectively.47 The symmetry factor of the forward reaction is βf = 1/2(1 − Fη/2λm) = 1/2(1 − Δ), where, F, η, and λm are the Faraday constant, overpotential, and reorganization energy per mole, respectively. When steps 1, 2, 3, and 4 are the rds, the Tafel slope values would be ∞, 118.4/(1 − Δ), 118.4/(3 − Δ), and 29.6 mV dec−1, respectively. The calculated Tafel slope values suggested that the electrocatalytic HER proceeds via a complex mechanism for all of the synthesized materials. The rds for CMS-8h and CMS-12h catalysis was the second step throughout the course of the reaction. The Tafel slope value might increase in the higher-overpotential region due to increment in the surface coverage; however, the reverse happened for CMS-8h.46 The third step might become sluggish at higher overpotential, which caused the decrement in the Tafel slope value. The second step was the rds for HER catalysis by Pt/C, too. The third step was the rds for CMS-4h at lower overpotential; however, the slope value abruptly increased in the higher-overpotential region. A sharp increase in the Tafel slope value was observed for the CMS-16h and Pt/ C, too. These too high Tafel slope values for CMS-4h, CMS16h, and Pt/C in the higher-overpotential region suggested that the first step had an influence toward the second step in these cases. The above discussion further validated the

Figure 5. (a) HER polarization curves for CMS-4h, CMS-8h, CMS12h, CMS-16h, Pt/C, and bare NF. (b) Tafel plot, (c) Nyquist plot, and (d) DRT plot for CMS-4h, CMS-8h, CMS-12h, and CMS-16h. 21638


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Figure 6. Three-dimensional (3D) DRT plot (with top contours in the inset) for (a) CMS-4h, (b) CMS-8h, (c) CMS-12h, and (d) CMS-16h.

medium- and lower-frequency regions could be attributed to the porosity of the material and charge transfer process. A less prominent peak was observed in the higher-frequency region, which might appear due to the adsorption of active species. The γ(τ) values of two peaks located in the high- and mediumfrequency regions were the lowest for CMS-12h. The γ(τ) value of the low-frequency peak was the lowest in CMS-4h compared to the other three materials; however, 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) value 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. The γ(τ) value of that peak is lower for CMS-12h compared to that for CMS-8h. The DRT plot also supported that CMS-12h was superior among the prepared materials, which was in accordance with the conclusion obtained from Nyquist plot and polarization curves. Three-dimensional (3D) DRT plots (Figure 6) explain the variation of peak related to charge transfer process at different overpotential values. The γ(τ) value of that peak decreased with increasing overpotential in the 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 CMS4h. The shift toward 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. 3.2.2.1. 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

superiority of CMS-12h as HER electrocatalyst in comparison to the other three materials. 3.2.1.2. 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 CMS4h and CMS-8h were fitted with the equivalent circuit depicted in Figure S7a, and for CMS-12h and CMS-16h, they were fitted with the circuit depicted in Figure S7b. The uncompensated and charge transfer resistances 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 interface.48,49 The values obtained after fitting are tabulated in Table S2. The Ru was found to be the 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 the Rf value. The charge transfer efficiency at the interface was found to be the highest for CMS-12h among all of the materials. The circuit elements related to the substrate adsorption−desorption were absent for CMS-4h and CMS8h, 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 compared to CMS-16 h. The above analysis suggested that the catalytic activity of CMS-12 h was superior 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 in the 21639


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current density of 400 mA cm −2 . CMS-12h showed comparable or superior activity toward OER compared to recently reported electrocatalysts (Table S3). The Tafel slopes as obtained from the polarization curves (Figures 7b and 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 the 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 follows10,47 OH− ↔ OH−ads

Figure 7. (a) OER polarization curves for CMS-4h, CMS-8h, CMS12h, CMS-16h, RuO2, and bare NF. (b) Tafel plot, (c) Nyquist plot, and (d) DRT plot for CMS-4h, CMS-8h, CMS-12h, and CMS-16h.

OH

− ads

→ OHads + e

OHads + OH

OHadsOH

CMS-4h showed superior electrocatalytic activity to OER in comparison to the RuO2. The catalytic activity of CMS-4h was comparable to the activity of RuO2. The electrocatalytic activity gradually increased up to CMS-12h and then got suppressed in the case of CMS-16h. 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. The η10 values for CMS-4h, CMS-8h, and CMS-16h were 302, 307, and 290 mV, respectively. CMS-4h showed superiority to CMS-8h up to 17 mA cm−2 current density, beyond which it showed inferiority. CMS-8h, CMS-12, and CMS-16 required 416, 336, and 381 mV overpotential, respectively, to achieve a

(5)

−

− ads

ads

−

(6)

↔ OHadsOH

−

→ OHOHads + e

OHOHads → Oads + H 2O

ads

−

(7) (8) (9)

Oads + OHads ↔ O2 Hads

(10)

O2 Hads + OHads → H 2O + O2

(11)

The symmetry factor for these reactions is βb = 1 − βf = 1/2(1 + Δ), as oxidation is considered here. Therefore, the Tafel slope values will be ∞, 118.4/(1 + Δ), 59.2, and 236.8/(3 + Δ) mV dec−1 when the rds is the first, second, third, and fourth steps, respectively. However, the Tafel slope value is 29.6 mV

Figure 8. Three-dimensional (3D) DRT plot (with top contours in the inset) for (a) CMS-4h, (b) CMS-8h, (c) CMS-12h, and (d) CMS-16h. 21640


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ACS Applied Materials & Interfaces dec−1 when the rds is the fifth, sixth, or seventh step. In the lower-overpotential region, the rds for CMS-4h was the third step and for CMS-8h, rds is the fifth, sixth, or seventh step. The second step became the rds for CMS-4h and CMS-8h, in the higher-overpotential region. However, the influence of the first step on the rds was observed for CMS-4h. The influence of both the first and second steps on the rds was found for RuO2 catalysis process, and the Tafel slope value increased in the higher-overpotential region due to increased surface coverage.46 The rds for CMS-12h and CMS-16 was the third step; however, an influence of the fourth and second steps was found for CMS-12h and CMS-16h, respectively. 3.2.2.2. 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 are presented in Table S4. The Ru value for CMS-12h was the lowest among all four materials. The ion diffusion proficiency gradually increased from CMS-4h to CMS-16h. The charge transfer efficiency increased from CMS-4h to CMS-12h; however, it got diminished in CMS-16h. The above analysis confirmed that the electrocatalytic activity of CMS12h toward OER was superior 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 in the higher-, medium-, and lower-frequency regions were 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 the highest for CMS-16h. The peak related to porosity and charge transfer gradually blue-shifted from CMS-4h to CMS-12h and then red-shifted in the case of CMS-16h. The γ(τ) values for those two peaks were lowest in the 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. Three-dimensional (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 the potential value. The γ(τ) of that peak was the lowest for CMS-12h, confirming its better charge transferability. The charge transfer peak-shifted toward lower τ value for each 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 interface was higher for CMS-12h and therefore, the accumulation of ions at the interface was lower. In the case of other three materials, the chance of charge accumulation is high, which caused inductive behavior at the interface. 3.2.2.3. 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/C 2 = (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.53 The modulation of the valence band structure changes the N value, and as a consequence, the slope of the 1/C2 vs potential plot changes. It was found that the slope of the 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 the 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 CMS12h, which was the reason behind superior charge transfer kinetics across the electrode−electrolyte interface and electrocatalytic activity. 3.2.2.4. Electrochemically Active Surface Area (ECSA) Analysis. The abundance of ion-accessible and catalytically active sites could be predicted qualitatively from the ECSA measurements. The double-layer capacitance (Cdl) was calculated from the difference between cathodic and anodic current densities (ΔJ) 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 faradic component toward 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 to that found in the previous approach (Table S5). The Cdl value was found to be the lowest for CMS-8h, which might be due to the fact that the influence of faradic component toward overall current response was very high for it. Both the approaches confirmed the highest ECSA for CMS-12h among all of 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 does not follow a similar trend to that observed in the areanormalized polarization curve; however, CMS-12h showed superiority in both the cases. 3.2.3. Overall Water Splitting Activity Measurements. 3.2.3.1. Polarization Curve Analysis. The overall water splitting proficiency of the prepared materials was investigated in 1.0 M KOH after constructing a symmetrical two-electrode setup. The water splitting proficiency of the two-electrode setup consisting of 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 to the RuO2−Pt/C couple. The CMS-8h showed efficiency comparable to the RuO2−Pt/C couple up to 1.75 V; however after that, it surpassed the activity of the RuO2−Pt/C couple. Although the CMS-4h showed inferior performance in comparison to RuO2Pt/C couple up to 1.88 V, it surpassed the activity of the RuO2−Pt/C couple thereafter. When the electrocatalytic activity of the prepared materials was investigated, it was 21641


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symmetric stretching, Mo−O−Mo, and MoO asymmetric stretching were found to be present in the spectra.38,55,56 It 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 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 two-electrode setup. CMS-12h retained ∼88.20% current density after ∼25 h long overall water splitting performance. The above investigations confirmed that CMS-12h also had long-term water splitting competency in tandem with its efficiency.

found that the activity increased from CMS-4h up to CMS-12h and then decreased for CMS-16. CMS-4h II CMS-4h, CMS-8h II CMS-8h, CMS-12h II CMS-12h, and CMS-16h II CMS-16h systems required 1.782, 1.746, 1.574, and 1.710 V potentials, respectively, to achieve the benchmarking current density of 10 mA cm−2 (Figure 9a). CMS-12h showed superior or

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 structures changed due to such ion diffusion. The change in electronic structure resulted in modulation of the electron density near EF. The change in the amount of Co−S−Mo moiety and the electronic structure of the electrocatalyst regulated the efficiency of charge transfer toward the active species. CMS-12h showed superior electrocatalytic activity toward 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. The Tafel slope analysis suggested that the product obtained at different times facilitated different electrocatalytic pathways. Along with efficiency, CMS-12h showed long-term operational stability, which made CMS-12h a potent candidate for the non-noblemetal-based electrocatalyst for the water splitting application. This study will shed light on the new path of developing transition-metal-based bimetallic sulfide for water splitting application.

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, RuO2 II Pt/ C, and NF II NF for overall water splitting. (b) HER polarization curves and (c) OER polarization curves of CMS-12h, before and after 1000 CV cycles. (d) Chronoamperometric curves of CMS-12h for HER, OER, and overall water splitting.

comparable overall water splitting efficiency compared to the recently reported bifunctional electrocatalysts (Table S6). Moreover, 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. About 1000 CV cycles were carried out between 0 to −0.35 and 1.23 to 1.58 V at a scan rate of 100 mV s−1 taking CMS-12h as catalyst, and thereafter, polarization curves were acquired by means of LSV. The final polarization curves were compared to the initial polarization curves. No significant change in the initial and final polarization curves 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 ∼5 h in the case of HER and then did not change much for the next 5 h. In the case of OER, a decrease in current density was observed and it was found that the electrocatalyst retained ∼87.7% current density for ∼10 h. 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 post-HER sample compared to the original spectra recorded before HER (Figure S14a) was observed, confirming the retention of its chemical structure after long-term HER catalysis. The intensity of the bands appeared in the precatalysis sample was found to decrease when the Raman spectra of the post-OER sample were investigated. Rather, four intense bands related to E1g vibration of Mo, Co−O−Mo

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b06635. Raman band related to Mo−S symmetric stretching in CMS-4h, CMS-8h, CMS-12h, and CMS-16h; Co 2p and Mo 3d XPS profiles of CMS-4h and CMS-8h; FE-SEM images of CMS-4h, CMS-8h, and CMS-12h; TEM image of CMS-12h; SAED pattern of CMS-16h; Tafel plots for Pt/C and RuO2; equivalent circuits used to fit the Nyquist plots obtained for HER and OER; Nyquist plots fitted data; Mott−Schottky plots for CMS-4h, CMS-8h, CMS-12h, and CMS-16h; cyclic voltammograms obtained at different scan rates for CMS-4h, CMS-8h, CMS-12h, and CMS-16h; ΔJ vs scan rate plot and ΔJν−1/2 vs ν1/2 plot for CMS-4h, CMS-8h, CMS12h, and CMS-16h; calculated Cdl, ECSA values for 21642


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prepared materials; ECSA-normalized HER and OER polarization curves for CMS-4h, CMS-8h, CMS-12h, and CMS-16h; FE-SEM and Raman spectra of CMS-12h after HER and OER; and comparison tables of recently reported HER, OER, and bifunctional electrocatalysts in alkaline medium (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +91-9647205077. Fax: 91343-2548204. ORCID

Subhasis Shit: 0000-0003-2019-2487 Tapas Kuila: 0000-0003-0976-3285 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS 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 CSIR-CMERI, Durgapur.

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Effect of Ion Diffusion in Cobalt Molybdenum Bimetallic Sulfide toward

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