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Cubic Mo6S8-Efficient Electrocatalyst Towards Hydrogen Evolution Over Wide pH Range
Electrochimica Acta 252 (2017) 408–415
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Research Paper
Cubic Mo6S8-Efficient Electrocatalyst Towards Hydrogen Evolution Over Wide pH Range Keerti M. Naik, S. Sampath* Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India
A R T I C L E I N F O
Article history: Received 26 June 2017 Received in revised form 2 September 2017 Accepted 3 September 2017 Available online 5 September 2017 Keywords: Chevrel phase Mo6S8 nanocubes electrocatalyst HER
A B S T R A C T
Hydrogen evolution from aqueous medium is one of the fundamental processes in electrochemistry and is an important aspect of energy conversion and storage systems. Tremendous efforts to find cheap, efficient and durable electrocatalysts for the hydrogen evolution reaction (HER) have so far, resulted in various transition metal-based catalysts. In this direction, herein, we report the use of chevral Mo6S8 nanocubes synthesized by a simple precipitation method that exhibits high catalytic activity. The catalyst is very active in aqueous media of different pH, 0.5 M H2SO4, 0.1 M KOH and 3% NaCl with good durability and stability. © 2017 Elsevier Ltd. All rights reserved.
1. Introduction Hydrogen is frequently advocated as an alternate energy carrier due to its high energy density and importantly its environment friendliness of the combustion product [1–4]. Conventionally, Pt group metals are used as efficient catalysts for the hydrogen evolution reaction (HER) but the cost and limited resource hinder their continuous use and commercial exploitation [5,6]. It is desirable to find inexpensive and efficient alternates that are earth-abundant and possess high electrocatalytic activity. Towards this end, transition metal chalcogenides have recently attracted enormous attention [7–9]. Their layered nature akin to graphite and consequently interesting properties shown by few-layer to single layer [7–9] material have aroused enormous interest in the scientific community. Molybdenum-sulfur clusters have attracted attention for various studies during the last several decades [10– 30]. There are few reports on polymetallic molybdenum-sulfide clusters, such as [Mo3S13]2, [Mo3S4]4+, and [Mo2S12]2 for electrocatalytic hydrogen generation [28–30]. The amorphous [Mo3S13]2 cluster has been shown to be efficient towards HER and the bridging S in the structure is implicated to be responsible based on favourable free energy of adsorption. Similar arguments have been put forward to explain the activity of dimeric [Mo2S12]2
* Corresponding author. Tel: +91-80-22933315; Fax: +91-80-23600085 E-mail addresses: [email protected], [email protected] (S. Sampath). http://dx.doi.org/10.1016/j.electacta.2017.09.015 0013-4686/© 2017 Elsevier Ltd. All rights reserved.
clusters. The cubane-type [Mo3S4]4+ clusters are shown to be similar to edge site MoS2 in its HER activity. Mixed-valent nature of the cationic component is given as a possible reason for the observed activity. Chevrel phase molybdenum chalcogenides with general formula, Mo6T8 or MxMo6T8 (M = metal, T = S, Se, or Te) [10] forms an important class of inorganic materials having unique structural and physical properties. Several chevral phase compounds are superconducting at high temperatures and at fairly high magnetic fields [11,12]. Other areas wherein this class of materials has been studied include magnetism, catalysis, thermoelectric behaviour etc. [13–22]. Their crystal structure can be looked at, as a threedimensional array of Mo6C8 units that lead to channels in three directions containing the metal and each Mo6C8 unit consists of distorted Mo6 octahedra surrounded by a C8 cube as explained in the literature [16]. Several years ago, Aurbach and co workers proposed the use of chevral phase compounds for rechargeable magnesium batteries [23]. Since then there have been a spurt of publications in energy storage studies, using this cluster compound [17–26]. The compound possesses Mo and S centres that might serve well for hydrogen adsorption similar to metal dichalcogenides (MoS2) [27]. Hence, it is expected that chevrel phase Mo6S8 will be a good HER catalyst and in the nanostructured morphology, the activity is expected to be high. Here, we use Mo6S8 in the nanocube morphology prepared via a simple precipitation method for HER in aqueous media of varying wide pH. Substantial current densities and favourable Tafel slopes are observed.
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2. Experimental
2.5. Electrode preparation and electrochemical measurements
2.1. Materials
Electrochemical measurements were carried out in a standard three-electrode set-up with saturated calomel electrode (SCE) (in 0.5 M H2SO4 and 3% aqueous NaCl) or Hg/HgO (in 0.1 M aqueous KOH solution) as the reference electrode, large area platinum foil or carbon rod as the counter electrode and the glassy carbon (GC) surface modified with the catalyst as the working electrode. The working electrode was fabricated as follows: 4 mg of the asprepared catalyst (Mo6S8) and 10 mL of nafion solution were dispersed in 1 mL of deionized water and isopropanol (4:1 in volume ratio). After ultrasonication for 1 h, 5 mL of the solution was drop-casted onto the top of a glassy carbon electrode of area 0.07 cm2. The catalyst-coated GC electrode was dried at 80 C to obtain a catalyst loading of 0.28 mg/cm2. Linear sweep voltammetry (LSV) at a scan rate of 1 mV/s was performed in electrolytes of different pH. The electrochemical impedance spectroscopy (EIS) was recorded in the same configuration but with a ac amplitude of 5 mV peak to peak. All the potentials are referred to reversible hydrogen electrode (RHE) according to the relation, E (RHE) = E (SCE) + 0.265 V for acidic or E (RHE) = E (Hg/HgO) + 0.938 V for alkaline or E (RHE) = E (SCE) + 0.907 V for 3% NaCl solutions based on the calibration of reference electrodes against RHE. The polarization curves were plotted as potential (E) versus log current density (log j) to obtain Tafel plots for assessing the HER kinetics of the catalyst. Electrochemical active surface area (ECSA) was determined from cyclic voltammetry data to various scan rates ranging from 20 to 300 mV s1.
Ammonium tetrathiomolybdate (NH4)2MoS4 (99.97%) and anhydrous copper (II) chloride CuCl2 (99.995%) were purchased from Sigma, USA. N,N,dimethylformamide, (DMF), and tetrahydrofuran (THF) were products of S.D.Fine chemicals, India. Distilled water with 18 M V cm resistivity was used throughout the study. 2.2. Synthesis of Cu2Mo6S8 In a typical synthetic procedure,Cu2Mo6S8 (Cu2CP) was prepared based on the method outlined by Nanjundaswamy et al. [31] [Scheme S1, supporting information]. Typically, 2 g of (NH4)2MoS4 and 0.35 g CuCl2 were dissolved in 66 mL of DMF and resulting solution was heated at 90 C for 6 h in an atmosphere of N2. The choice of DMF is based on its high boiling point and good solubilizing characteristics. The deep red mixture was cooled to room temperature, and excess THF (approximately 5 times the volume) was added to initiate precipitation. The as-formed precipitate was allowed to settle overnight, centrifuged and washed with THF followed by methanol and dried at 65 C for 12 h [24]. The dried, powdered solid was heated at 1000 C for 7 h. under Ar + 5% H2 atmosphere that yielded the desired ternary chevrel phase, Cu2Mo6S8 [31]. The reaction involved the formation of chevral phase with simultaneous release of ammonia, hydrogen and hydrogen sulphide as given below. 2CuðNH4 ÞMo3 S9 þ 10H2 ! Cu2 Mo6 S8 þ 10H2 S þ 2NH3 þ H2
3. Results and Discussion 3.1. Structural and compositional characterization of Mo6S8 catalyst
2.3. Preparation of Mo6S8 nanocubes Copper form the parent compound is leached out by acid treatment and the reaction involved is as follows: 2
Cu2 Mo6 S8 þ 8HCl þ O2 ! Mo6 S8 þ 2H2 O þ 2½CuCl4
þ 4Hþ
The leaching procedure involved stirring the powders in presence of 6 M hydrochloric acid (HCl) in presence of oxygen (bubbled for 8 h.) under ambient conditions [25]. After complete removal of copper from Cu2Mo6S8, the acid solution containing the residue was centrifuged, washed and dried before use. 2.4. Characterization of the electrocatalyst X-ray diffraction (XRD) patterns were carried out using Philips diffractometer (PANanlytical with Cu-Ka radiation, l = 1.5406 Å). Infrared spectra were taken in the transmittance mode (Perkin Elmer, Spectrum one model FT-IR) by preparing the sample with with KBr in the pellet form. Surface morphology was followed by scanning electron microscopy (SEM) using Carl Zeiss ultra 55 SEM equipped with energy dispersive X-ray analysis accessory (EDAX). The operating voltage was kept at 5 kV. Samples for SEM were prepared by spreading the powder on a conducting surface. Transmission electron microscopy (TEM), HRTEM and selected area electron diffraction (SAED) were obtained using JEOL 2200F TEM operated at 200 kV. The TEM samples were prepared by sonicating the catalyst in isopropanol, followed by drop casting on carbon coated copper grid followed by drying. X-ray photoelectron spectroscopic (XPS) analysis were performed (multi technique X-ray photoelectron spectrometer, Axis Ultra, UK) with Al Ka (1486.6 eV) source. The data analysis was carried out using XPSpeakfit 4.1 software with Shirley algorithm.
The as-obtained precursor powders show broad diffraction lines (Figs. S1 and S2) revealing the formation of amorphous material under ambient conditions. Based on the literature report [24], the precursor has the formula Cu(NH4)qMo3S9 which is confirmed by XRD and infrared spectroscopy analysis (Fig.S1 and S2). The amorphous powders heated at 1000 C under Ar/H2 mixture for 7 h. results in the formation of the desired chevral phase. Fig. 1 shows the XRD pattern of the heat treated Cu(NH4)qMo3S9 precursor that shows the formation of copper containing Mo6S8 phase. The diffraction pattern is indexed to a hexagonalrhombohedral symmetry unit cell of Cu2Mo6S8 based on the standard pattern (JCPDS-00-047-1519) [24,25] and is observed to be a single phase compound without any impurity phase present. Fig. 2 shows the XRD pattern of copper leached Mo6S8 phase using aqueous hydrochloric acid. The diffraction pattern with hexagonal-
Fig 1. XRD pattern of the annealed Cu2Mo6S8 along with the reported data (JCPDS00-047-1519).
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Fig. 2. XRD pattern of the as-synthesized Mo6S8 along with the standard pattern (JCPDS-01-082-1709).
rhombohedral symmetry unit cell of Mo6S8 is very evident (JCPDS01-08-1709). Table 1 shows the calculated structural parameters of Cu2Mo6S8 and Mo6S8 along with that obtained from standard XRD patterns. The crystallite size determined using the Scherrer formula, is 1.35 nm for both Cu2Mo6S8 and Mo6S8. This shows that the removal of copper does not alter the crystallite size though the shape is unaccounted for, in the calculations. Morphology of the Mo6S8 catalyst is analyzed using scanning electron microscopy (SEM) (Fig. 3) and high resolution transmission electron microscopy (HRTEM). The data clearly reveals cubic morphology along with high crystallinity. The crystal structure observed for the nanoparticles correspond well with that of the bulk Mo6S8 structure where Mo-Mo intercluster spacing of 2.67 A has been reported [32,33]. The EDAX analysis reveals evidence for the expected composition of Mo and S in the Mo6S8 catalyst with a S/Mo atomic ratio of 1.4. X-ray photoelectron spectroscopy (XPS) measurements show the composition and oxidation states of elements in Mo6S8 (Fig. 4). Fig. 4B and C show the high resolution spectra of Mo 3d and S 2p regions of the as-synthesized Mo6S8. Six deconvoluted peaks appear for Mo 3d level and the peak positioned at 226.2 eV is corresponds to S 2 s of Mo6S8. The characteristic peaks with binding energy values of 235.8 eV, 232.6 eV correspond to Mo6+, 231.4 eV, 228.0 eV corresponds to Mo4+and 229.3 eV corresponds to Mo5+. The high oxidation states of Mo may arise from the surface oxidation of Mo6S8 due to air contact. As shown in Fig. 4C, the S 2p region can be deconvoluted into two peaks. The peak positioned at 162.0 eV and 163.1 eV corresponds to S 2p3/2 and S 2p1/2 of Mo6S8 [34,35] due to spin orbit coupling. It is found that the oxidation states of the elements are different and Table 2 gives the atomic percentage of various species obtained from the deconvolution of XPS spectra. 3.2. Electrocatalytic HER activity The electrocatalytic properties of the chevrel phase Mo6S8 are evaluated using a standard three electrode electrochemical cell in N2 saturated 0.5 M H2SO4/0.1 M KOH or 3% NaCl electrolytes. The Table 1 Structural parameters obtained for Cu2Mo6S8 and Mo6S8 along with standard values. Samples
a (nm)
Cu2Mo6S8 Mo6S8
Unit cell volume (nm3)
Lattice parameters c (nm)
Expt.
Std.
Expt.
Std.
Expt.
Std.
0.91 0.91
0.96 0.92
1.11 1.11
1.02 1.08
796 103 800 103
818 103 797 103
catalyst mass loading on a glassy carbon electrode used is 0.28 mg/ cm2. Fig. 5A shows the linear sweep voltammogramms (LSVs) at a scan rate of 1 mV/s. The 40% commercial Pt/C catalystis also included as a reference point (Fig. S3 S5). The featureless polarization curve for bare glassy carbon guarantees a minimal background. The voltammetry reveals the onset potential for H2 evolution occurs at 0.17, 0.19 and 0.21 V versus RHE in the acidic, neutral and alkaline media respectively (Fig. 5A). The onset potential is more positive in the acidic medium than that of neutral and alkaline solutions. Though the onset potentials do not different greatly, there is a trend observed. Similar behaviour has been reported in the case of other chalcogenides such as CoS2 [36]. The overpotentials required to achieve 10 mAcm2 current follow the order acidic, neutral and alkaline media as expected. The dependence of the logarithmic current density on overpotential (h, log j h) is used to probe the HER kinetics. Tafel slopes (Fig. 5B) of 67, 127 and 145 mV/dec are obtained in different media and the values are smaller than those of pristine Mo-based catalysts. The fast kinetics enables high current density of 10 mAcm2 at h values of 0.32, 0.49 and 0.52 V for acidic, NaCl and alkaline pH respectively. This is compared with the reported data for similar systems in Tables S1 and S2. Exchange current density (j ) obtained in 0.5 M H2SO4, 0.1 M KOH and 3% NaCl are 8 106 A/cm2, 6 106 A/cm2 and 4 106 A/cm2 respectively (Fig. 5D). There are three possible reaction steps have been suggested for the HER [37]. First is a primary discharge step (Volmer reaction): H3O+ + e ! Hads + H2O followed by either an electrochemical desorption step (Heyrovsky reaction), Hads + H3O+ + e ! H2 + H2O or a recombination step (Tafel reaction), Hads + Hads ! H2 The Tafel slope of the catalyst that is determined by the ratelimiting step of the HER. Volmer-Heyrovsky mechanism leads to a Tafel slope of 120 mV dec1, while the slope is expected to be 30 mV dec1 if the mechanism follows Volmer-Tafel steps [37]. The Tafel slopes obtained in the present studies suggest that the HER on the Mo6S8 catalyst proceeds via Volmer-Heyrovsky mechanism, with initial proton adsorption (Volmer reaction) as the rate determining step. It is also possible that spill over mechanism wherein the hydrogen atom formed on the catalyst spills over to the support before combining with another to form molecular hydrogen [38]. This leads to Tafel slopes in between that of the two pathways mentioned above. Electrochemical impedance spectroscopic (EIS) technique provides additional insight into the HER process. Fig. 5C shows the Nyquist plots for the catalyst Mo6S8 measured at onset potentials 0.17, 0.19 and 0.21 V versus RHE in the acidic, neutral and alkaline media respectively. The plots possess the smallest charge transfer resistance (Rct) in acidic medium as compared to that observed in other electrolytes. The impedance data are fitted to an equivalent circuit (Fig. 5C) and the corresponding Rs, Rct and Cdl parameters of solution resistance, charge transfer resistance, double layer capacitance and Warburg exponent (W) are extracted and presented in Table 3. 3.3. Stability Stability of the electrocatalyst is an important issue when gas evolution occurs on the electrode surface. To evaluate the stability of Mo6S8 catalyst, continuouscycling studies are carried out in all the electrolytes, i.e, 0.5 M H2SO4, 0.1 M KOH and 3% NaCl. As shown
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Fig. 3. (A) SEM image of the as-prepared Mo6S8 particles, (B) Corresponding SEM-EDS, (C) HRTEM image, (D) Contrast intensity profile and (E) selected area electron diffraction (SAED) pattern.
Fig. 4. High resolution XPS spectra (A) survey scan, (B) Mo 3d and (C) S 2p regions of Mo6S8. The deconvoluted peaks along with the experimental data are shown.
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Table 2 Atomic percentage of different species obtained from the deconvolution XPS spectral data.
Table 3 Electrochemical impedance parameters of Mo6S8 obtained under different conditions.
Elements
Binding energy in eV
Atomic percentage in %
Conditions
Rs in V
RCT in V
Cdl in mF
W
Mo6+ Mo6+ Mo4+ Mo4+ Mo5+ S 2s S 2p1/2 S 2p3/2
235.8 232.6 231.4 228.0 229.3 226.2 163.1 162.0
17.10 34.01 12.84 9.69 17.74 8.62 39.44 60.56
0.5 M H2SO4 0.1 M KOH 3% NaCl
6.562 115.4 28.87
3070 5819 3212
0.1044 0.2877 0.166
0.0006069 0.001907 0.00118
in Fig. 6A and Fig. S6, the cathodic polarization curves obtained after 1,000 continuous cyclic voltammogramms (with a scan rate 100 mVs1) shows negligible decrease in current density compared as a function of cycle number indicating the excellent stability of Mo6S8 at all pH values. Stability of the catalysts is also investigated by chronoamperomerty at constant DC voltage. As shown in Fig. 6B and Fig. S6, the Mo6S8 exhibits stable HER current density for several hours under all the conditions employed. The XRD patterns of the electrode material examined before and after 1000 cycles reveal no changes in the structure and composition during the catalytic process (Fig. S7). Experimental results observed using carbon rod and Pt as counter electrode show similar data. It confirms that Pt dissolution in solution and deposition on electrocatalyst does not contribute to HER. The substantial long term- and chemical stability observed in wide pH range suggests great promise for the Mo6S8 nanocubes catalyst as earth abundant, cost-effective viable material
3.4. Faradaic efficiency The amount of hydrogen gas evolved is quantified as a function of time. Chronoamperometry experiment is carried out at – 0.40 V vs. RHE, using H-shaped electrochemical cell (Fig. S8). The large area counter electrode used is separated from the working electrode through a porous glass frit. The Mo6S8 modified glassy carbon electrode and reference electrode (saturated calomel electrode) are kept in one compartment of the H-shaped cell. The quantity of gas evolved measured using inverse burette method shows that the Mo6S8 nanocubes produce around 2 mL gases in 1 h duration at the applied bias mentioned above. The amount of gas evolved during the reaction is in good agreement with the calculated value based on Faraday's law, suggesting nearly 100% faradaic efficiency for the HER. Fig. 7 compares the amount of H2 generated from the HER over Mo6S8 versus the calculated volume based on the charge. Qualitatively, the gas is analyzed using gas chromatography to confirm hydrogen evolution. 3.5. Electrochemical active surface area Cyclic voltammetry at non-faradaic region of potentials helps in extracting the double-layer capacitance (Cdl) of the catalyst which is proportional to the effective electrochemically active surface
Fig. 5. (A) HER polarization plots of Mo6S8 in different electrolytes (Black: 0.5 M H2SO4, Red: 0.1 M KOH and Blue: 3% NaCl), (B) corresponding Tafel plots, (C) Nyquist plots along with the equivalent circuit used and (D) exchange current densities and Tafel slopes of Mo6S8 under different conditions employed. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
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Fig. 6. (A) Polarization curves for the Mo6S8 catalyst before and after 1,000 cyclic voltammogramms at 100 mV s1 scan rate in 0.5 M H2SO4 (Black: 1st cycle and Red: 1000th cycle) and (B) Chronoamperometry (i t curve) of Mo6S8 at 0.25 V in 0.5 M H2SO4. The gas evolution on the electrode surface is responsible for the noise-like behaviour. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
area (ECSA) (Fig. 8). A plot of the current density (at 0.15 V vs RHE) against the scan rate is linear and its slope corresponds to Cdl. As shown in Fig. 8 the Cdl of Mo6S8 is 1.14 mF, which indicates that of Mo6S8 possesses large effective surface area as compared to other materials [38,39]. The HER catalytic activity in molybdenum based chalcogenides such as MoS2 are quite well studied [34,37,38,40–43]. Hu and co workers hypothesized that coordinatively unsaturated S atoms in MoSx effectively adsorb hydrogen and promote recombination to H2 [44]. Recently, Yeo and co-workers have proposed a linear correlation between turnover frequency of H2 production and the availability of S atoms with high electron binding energies [45,46]. It is clear from Fig. S9 shows that Mo6S8 shows higher activity than that of bulk MoS2 under similar conditions. This may be attributed to the presence of large number of adsorption sites based on Mo and S atoms in the catalyst. It should however be pointed out that MoS2 when exfoliated into few layers, turns out to be an excellent HER catalyst [27]. From the band structure calculations [47] of Mo6S8 show that the conduction band is extremely narrow and consists of primarily Mo- 4d and S-3d orbitals. The density of states at the Fermi energy is found to be large supposedly due to intercluster distances observed in the material. In addition, the [48] resistivity temperature (r T) behaviour of Mo6S8 in the range 77–300 K reveals metallic conductivity. The above observations are somewhat different from layered MoS2 which is known to be a semiconductor [49]. It is suggested that the adsorption sites
Fig. 7. Amount of H2 gas evolved with time at – 0.40 V bias along with the expected volume (Red: obtained and Blue: calculated). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
for hydrogen are quite enhanced in Mo6S8 thus leading to good HER activity. 3.6. Calculation of turnover frequency (TOF) The turnover frequency (TOF) is determined by quantifying the active sites through an electrochemical approach [50] as reported earlier using voltammetry. The corresponding voltammograms are given in Fig. 8. Assuming a one electron redox process, the number of active sites and subsequently the corresponding TOF (supporting information) determined from the total charge, to be 0.092 s1 @ 0.35 V vs. RHE. It is comparable with the TOF of 0.07 and 0.02 and 0.044 s1 reported for [Mo3S4]4+ and MoS2 nanoparticles and
Fig. 8. Cyclic voltammograms of Mo6S8 at different scan rates (top) and current density difference {Dj = 1/2 (Ja Jc)} versus scan rate (bottom).
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Cu2MoS4 respectively [29,51,52]. Hence, it is possible that the active sites for HER available on Mo6S8 are as substantial as that of other Mo-S clusters. 4. Summary In summary, electrochemical activity of Mo6S8 synthesized by a simple precipitation method is studied in aquesous media of varying pH values. The measured HER activity and stability of Mo6S8 shows promise as a good catalyst in acidic, alkaline and neutral conditions. Availability of several adsorption sites based on Mo and S possibly helps in good and stable catalytic activity. Acknowledgement Dr. Keerti M. Naik acknowledges the financial support provided by the UGC, Govt. of India through the D. S. Kothari postdoctoral fellowship. The authors thank DST, New Delhi for research funds. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.electacta. 2017.09.015. References [1] M.S. Dresselhaus, I.L. Thomas, Alternative energy technologies, Nature 414 (2001) 332–337. [2] J.A. Turner, Sustainable hydrogen production, Science 305 (2004) 972–974. [3] N.S. Lewis, D.G. Nocera, Powering the planet: chemical challenges in solar energy utilization, Proc Natl Acad Sci USA 103 (2006) 15729–15735. [4] M.G. Walter, E.L. Warren, J.R. McKone, S.W. Boettcher, Q. Mi, E.A. Santori, N.S. Lewis, Solar water splitting cells, Chem. Rev. 110 (2010) 6446–6473. [5] D.Y. Chung, J.W. Han, D.H. Lim, J.H. Jo, S.J. Yoo, H. Lee, Y.E. Sung, Structure dependent active sites of NixSy as electrocatalysts for hydrogen evolution reaction, Nanoscale 7 (2015) 5157–5163. [6] B. Fang, J.H. Kim, J.S. Yu, Colloid-imprinted carbon with superb nanostructure as an efficient cathode electrocatalyst support in proton exchange membrane fuel cell, Electrochem. Commun. 10 (2008) 659–662. [7] M. Chhowalla, H.S. Shin, G. Eda, L.J. Li, K.P. Loh, H. Zhang, The chemistry of twodimensional layered transition metal dichalcogenide nanosheets, Nat. Chem. 5 (2013) 263–275. [8] M. Xu, T. Liang, M. Shi, H. Chen, Graphene-like two-dimensional materials, Chem. Rev. 113 (2013) 3766–3798. [9] M. Pumera, Z. Sofer, A. Ambrosi, Layered transition metal dichalcogenides for electrochemical energy generation and storage, J. Mater. Chem. A 2 (2014) 8981–8987. [10] R. Chevrel, M. Sergent, J. Prigent, New molybdenum ternary sulfide phases, J. Solid State Chem. 3 (1971) 515–519. [11] O.K. Andersen, W. Klose, H. Nohl, Electronic structure of Chevrel-phase highcritical-field superconductors, Phys. Rev. B 17 (1978) 1209–1237. [12] O. Fischer, Chevrel phases: Superconducting and normal state properties, Appl Phys. 16 (1978) 1–28. [13] T. Caillat, J.P. Fleurial, G. Synde, Potential of Chevrel phases to thermoelectric applications, Solid State Sci. 1 (1999) 535–544. [14] K.F. McCarty, J.W. Anderegg, G.L. Schrader, Hydrodesulfurization catalysis by Chevrel phase compounds, J. Catal. 93 (1985) 375–387. [15] E. Levi, G. Gershinsky, D. Aurbach, O. Isnard, G. Ceder, New insight on the unusually high ionic mobility in Chevrel phases, Chem. Mater. 21 (2009) 1390– 1399. [16] T. Hughbanks, R. Hoffmann, Molybdenum chalcogenides: clusters, chains, and extended solids. The approach to binding in three dimensions, J. Am. Chem. Soc 105 (1983) 1150–1162. [17] B. Pan, J. Zhang, J. Huang, J.T. Vaughey, L. Zhang, S.D. Han, A.K. Burrell, Z. Zhang, C. Liao, A lewis acid-free and phenolate-based magnesium electrolyte for rechargeable magnesium batteries, Chem. Commun. 51 (2015) 6214–6217. [18] L. Suo, F. Han, X. Fan, H. Liu, K. Xu, C. Wang, Water-in-Salt electrolytes enable green and safe Li-ion batteries for large scale electric energy storage applications, J Mater Chem. A 4 (2016) 6639–6644. [19] L. Geng, G. Lv, X. Xing, J. Guo, Reversible electrochemical intercalation of aluminum in Mo6S8, Chem. Mater. 27 (2015) 4926–4929. [20] Y. Cheng, L. Luo, L. Zhong, J. Chen, B. Li, W. Wang, S.X. Mao, C. Wang, V.L. Sprenkle, G. Li, J. Liu, Highly reversible zinc-ion intercalation into Chevrel Phase Mo6S8 nanocubes and applications for advanced zinc-ion batteries, ACS Appl. Mater interfaces 8 (2016) 13673–13677.
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Research Paper
Cubic Mo6S8-Efficient Electrocatalyst Towards Hydrogen Evolution Over Wide pH Range Keerti M. Naik, S. Sampath* Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India
A R T I C L E I N F O
Article history: Received 26 June 2017 Received in revised form 2 September 2017 Accepted 3 September 2017 Available online 5 September 2017 Keywords: Chevrel phase Mo6S8 nanocubes electrocatalyst HER
A B S T R A C T
Hydrogen evolution from aqueous medium is one of the fundamental processes in electrochemistry and is an important aspect of energy conversion and storage systems. Tremendous efforts to find cheap, efficient and durable electrocatalysts for the hydrogen evolution reaction (HER) have so far, resulted in various transition metal-based catalysts. In this direction, herein, we report the use of chevral Mo6S8 nanocubes synthesized by a simple precipitation method that exhibits high catalytic activity. The catalyst is very active in aqueous media of different pH, 0.5 M H2SO4, 0.1 M KOH and 3% NaCl with good durability and stability. © 2017 Elsevier Ltd. All rights reserved.
1. Introduction Hydrogen is frequently advocated as an alternate energy carrier due to its high energy density and importantly its environment friendliness of the combustion product [1–4]. Conventionally, Pt group metals are used as efficient catalysts for the hydrogen evolution reaction (HER) but the cost and limited resource hinder their continuous use and commercial exploitation [5,6]. It is desirable to find inexpensive and efficient alternates that are earth-abundant and possess high electrocatalytic activity. Towards this end, transition metal chalcogenides have recently attracted enormous attention [7–9]. Their layered nature akin to graphite and consequently interesting properties shown by few-layer to single layer [7–9] material have aroused enormous interest in the scientific community. Molybdenum-sulfur clusters have attracted attention for various studies during the last several decades [10– 30]. There are few reports on polymetallic molybdenum-sulfide clusters, such as [Mo3S13]2, [Mo3S4]4+, and [Mo2S12]2 for electrocatalytic hydrogen generation [28–30]. The amorphous [Mo3S13]2 cluster has been shown to be efficient towards HER and the bridging S in the structure is implicated to be responsible based on favourable free energy of adsorption. Similar arguments have been put forward to explain the activity of dimeric [Mo2S12]2
* Corresponding author. Tel: +91-80-22933315; Fax: +91-80-23600085 E-mail addresses: [email protected], [email protected] (S. Sampath). http://dx.doi.org/10.1016/j.electacta.2017.09.015 0013-4686/© 2017 Elsevier Ltd. All rights reserved.
clusters. The cubane-type [Mo3S4]4+ clusters are shown to be similar to edge site MoS2 in its HER activity. Mixed-valent nature of the cationic component is given as a possible reason for the observed activity. Chevrel phase molybdenum chalcogenides with general formula, Mo6T8 or MxMo6T8 (M = metal, T = S, Se, or Te) [10] forms an important class of inorganic materials having unique structural and physical properties. Several chevral phase compounds are superconducting at high temperatures and at fairly high magnetic fields [11,12]. Other areas wherein this class of materials has been studied include magnetism, catalysis, thermoelectric behaviour etc. [13–22]. Their crystal structure can be looked at, as a threedimensional array of Mo6C8 units that lead to channels in three directions containing the metal and each Mo6C8 unit consists of distorted Mo6 octahedra surrounded by a C8 cube as explained in the literature [16]. Several years ago, Aurbach and co workers proposed the use of chevral phase compounds for rechargeable magnesium batteries [23]. Since then there have been a spurt of publications in energy storage studies, using this cluster compound [17–26]. The compound possesses Mo and S centres that might serve well for hydrogen adsorption similar to metal dichalcogenides (MoS2) [27]. Hence, it is expected that chevrel phase Mo6S8 will be a good HER catalyst and in the nanostructured morphology, the activity is expected to be high. Here, we use Mo6S8 in the nanocube morphology prepared via a simple precipitation method for HER in aqueous media of varying wide pH. Substantial current densities and favourable Tafel slopes are observed.
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2. Experimental
2.5. Electrode preparation and electrochemical measurements
2.1. Materials
Electrochemical measurements were carried out in a standard three-electrode set-up with saturated calomel electrode (SCE) (in 0.5 M H2SO4 and 3% aqueous NaCl) or Hg/HgO (in 0.1 M aqueous KOH solution) as the reference electrode, large area platinum foil or carbon rod as the counter electrode and the glassy carbon (GC) surface modified with the catalyst as the working electrode. The working electrode was fabricated as follows: 4 mg of the asprepared catalyst (Mo6S8) and 10 mL of nafion solution were dispersed in 1 mL of deionized water and isopropanol (4:1 in volume ratio). After ultrasonication for 1 h, 5 mL of the solution was drop-casted onto the top of a glassy carbon electrode of area 0.07 cm2. The catalyst-coated GC electrode was dried at 80 C to obtain a catalyst loading of 0.28 mg/cm2. Linear sweep voltammetry (LSV) at a scan rate of 1 mV/s was performed in electrolytes of different pH. The electrochemical impedance spectroscopy (EIS) was recorded in the same configuration but with a ac amplitude of 5 mV peak to peak. All the potentials are referred to reversible hydrogen electrode (RHE) according to the relation, E (RHE) = E (SCE) + 0.265 V for acidic or E (RHE) = E (Hg/HgO) + 0.938 V for alkaline or E (RHE) = E (SCE) + 0.907 V for 3% NaCl solutions based on the calibration of reference electrodes against RHE. The polarization curves were plotted as potential (E) versus log current density (log j) to obtain Tafel plots for assessing the HER kinetics of the catalyst. Electrochemical active surface area (ECSA) was determined from cyclic voltammetry data to various scan rates ranging from 20 to 300 mV s1.
Ammonium tetrathiomolybdate (NH4)2MoS4 (99.97%) and anhydrous copper (II) chloride CuCl2 (99.995%) were purchased from Sigma, USA. N,N,dimethylformamide, (DMF), and tetrahydrofuran (THF) were products of S.D.Fine chemicals, India. Distilled water with 18 M V cm resistivity was used throughout the study. 2.2. Synthesis of Cu2Mo6S8 In a typical synthetic procedure,Cu2Mo6S8 (Cu2CP) was prepared based on the method outlined by Nanjundaswamy et al. [31] [Scheme S1, supporting information]. Typically, 2 g of (NH4)2MoS4 and 0.35 g CuCl2 were dissolved in 66 mL of DMF and resulting solution was heated at 90 C for 6 h in an atmosphere of N2. The choice of DMF is based on its high boiling point and good solubilizing characteristics. The deep red mixture was cooled to room temperature, and excess THF (approximately 5 times the volume) was added to initiate precipitation. The as-formed precipitate was allowed to settle overnight, centrifuged and washed with THF followed by methanol and dried at 65 C for 12 h [24]. The dried, powdered solid was heated at 1000 C for 7 h. under Ar + 5% H2 atmosphere that yielded the desired ternary chevrel phase, Cu2Mo6S8 [31]. The reaction involved the formation of chevral phase with simultaneous release of ammonia, hydrogen and hydrogen sulphide as given below. 2CuðNH4 ÞMo3 S9 þ 10H2 ! Cu2 Mo6 S8 þ 10H2 S þ 2NH3 þ H2
3. Results and Discussion 3.1. Structural and compositional characterization of Mo6S8 catalyst
2.3. Preparation of Mo6S8 nanocubes Copper form the parent compound is leached out by acid treatment and the reaction involved is as follows: 2
Cu2 Mo6 S8 þ 8HCl þ O2 ! Mo6 S8 þ 2H2 O þ 2½CuCl4
þ 4Hþ
The leaching procedure involved stirring the powders in presence of 6 M hydrochloric acid (HCl) in presence of oxygen (bubbled for 8 h.) under ambient conditions [25]. After complete removal of copper from Cu2Mo6S8, the acid solution containing the residue was centrifuged, washed and dried before use. 2.4. Characterization of the electrocatalyst X-ray diffraction (XRD) patterns were carried out using Philips diffractometer (PANanlytical with Cu-Ka radiation, l = 1.5406 Å). Infrared spectra were taken in the transmittance mode (Perkin Elmer, Spectrum one model FT-IR) by preparing the sample with with KBr in the pellet form. Surface morphology was followed by scanning electron microscopy (SEM) using Carl Zeiss ultra 55 SEM equipped with energy dispersive X-ray analysis accessory (EDAX). The operating voltage was kept at 5 kV. Samples for SEM were prepared by spreading the powder on a conducting surface. Transmission electron microscopy (TEM), HRTEM and selected area electron diffraction (SAED) were obtained using JEOL 2200F TEM operated at 200 kV. The TEM samples were prepared by sonicating the catalyst in isopropanol, followed by drop casting on carbon coated copper grid followed by drying. X-ray photoelectron spectroscopic (XPS) analysis were performed (multi technique X-ray photoelectron spectrometer, Axis Ultra, UK) with Al Ka (1486.6 eV) source. The data analysis was carried out using XPSpeakfit 4.1 software with Shirley algorithm.
The as-obtained precursor powders show broad diffraction lines (Figs. S1 and S2) revealing the formation of amorphous material under ambient conditions. Based on the literature report [24], the precursor has the formula Cu(NH4)qMo3S9 which is confirmed by XRD and infrared spectroscopy analysis (Fig.S1 and S2). The amorphous powders heated at 1000 C under Ar/H2 mixture for 7 h. results in the formation of the desired chevral phase. Fig. 1 shows the XRD pattern of the heat treated Cu(NH4)qMo3S9 precursor that shows the formation of copper containing Mo6S8 phase. The diffraction pattern is indexed to a hexagonalrhombohedral symmetry unit cell of Cu2Mo6S8 based on the standard pattern (JCPDS-00-047-1519) [24,25] and is observed to be a single phase compound without any impurity phase present. Fig. 2 shows the XRD pattern of copper leached Mo6S8 phase using aqueous hydrochloric acid. The diffraction pattern with hexagonal-
Fig 1. XRD pattern of the annealed Cu2Mo6S8 along with the reported data (JCPDS00-047-1519).
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Fig. 2. XRD pattern of the as-synthesized Mo6S8 along with the standard pattern (JCPDS-01-082-1709).
rhombohedral symmetry unit cell of Mo6S8 is very evident (JCPDS01-08-1709). Table 1 shows the calculated structural parameters of Cu2Mo6S8 and Mo6S8 along with that obtained from standard XRD patterns. The crystallite size determined using the Scherrer formula, is 1.35 nm for both Cu2Mo6S8 and Mo6S8. This shows that the removal of copper does not alter the crystallite size though the shape is unaccounted for, in the calculations. Morphology of the Mo6S8 catalyst is analyzed using scanning electron microscopy (SEM) (Fig. 3) and high resolution transmission electron microscopy (HRTEM). The data clearly reveals cubic morphology along with high crystallinity. The crystal structure observed for the nanoparticles correspond well with that of the bulk Mo6S8 structure where Mo-Mo intercluster spacing of 2.67 A has been reported [32,33]. The EDAX analysis reveals evidence for the expected composition of Mo and S in the Mo6S8 catalyst with a S/Mo atomic ratio of 1.4. X-ray photoelectron spectroscopy (XPS) measurements show the composition and oxidation states of elements in Mo6S8 (Fig. 4). Fig. 4B and C show the high resolution spectra of Mo 3d and S 2p regions of the as-synthesized Mo6S8. Six deconvoluted peaks appear for Mo 3d level and the peak positioned at 226.2 eV is corresponds to S 2 s of Mo6S8. The characteristic peaks with binding energy values of 235.8 eV, 232.6 eV correspond to Mo6+, 231.4 eV, 228.0 eV corresponds to Mo4+and 229.3 eV corresponds to Mo5+. The high oxidation states of Mo may arise from the surface oxidation of Mo6S8 due to air contact. As shown in Fig. 4C, the S 2p region can be deconvoluted into two peaks. The peak positioned at 162.0 eV and 163.1 eV corresponds to S 2p3/2 and S 2p1/2 of Mo6S8 [34,35] due to spin orbit coupling. It is found that the oxidation states of the elements are different and Table 2 gives the atomic percentage of various species obtained from the deconvolution of XPS spectra. 3.2. Electrocatalytic HER activity The electrocatalytic properties of the chevrel phase Mo6S8 are evaluated using a standard three electrode electrochemical cell in N2 saturated 0.5 M H2SO4/0.1 M KOH or 3% NaCl electrolytes. The Table 1 Structural parameters obtained for Cu2Mo6S8 and Mo6S8 along with standard values. Samples
a (nm)
Cu2Mo6S8 Mo6S8
Unit cell volume (nm3)
Lattice parameters c (nm)
Expt.
Std.
Expt.
Std.
Expt.
Std.
0.91 0.91
0.96 0.92
1.11 1.11
1.02 1.08
796 103 800 103
818 103 797 103
catalyst mass loading on a glassy carbon electrode used is 0.28 mg/ cm2. Fig. 5A shows the linear sweep voltammogramms (LSVs) at a scan rate of 1 mV/s. The 40% commercial Pt/C catalystis also included as a reference point (Fig. S3 S5). The featureless polarization curve for bare glassy carbon guarantees a minimal background. The voltammetry reveals the onset potential for H2 evolution occurs at 0.17, 0.19 and 0.21 V versus RHE in the acidic, neutral and alkaline media respectively (Fig. 5A). The onset potential is more positive in the acidic medium than that of neutral and alkaline solutions. Though the onset potentials do not different greatly, there is a trend observed. Similar behaviour has been reported in the case of other chalcogenides such as CoS2 [36]. The overpotentials required to achieve 10 mAcm2 current follow the order acidic, neutral and alkaline media as expected. The dependence of the logarithmic current density on overpotential (h, log j h) is used to probe the HER kinetics. Tafel slopes (Fig. 5B) of 67, 127 and 145 mV/dec are obtained in different media and the values are smaller than those of pristine Mo-based catalysts. The fast kinetics enables high current density of 10 mAcm2 at h values of 0.32, 0.49 and 0.52 V for acidic, NaCl and alkaline pH respectively. This is compared with the reported data for similar systems in Tables S1 and S2. Exchange current density (j ) obtained in 0.5 M H2SO4, 0.1 M KOH and 3% NaCl are 8 106 A/cm2, 6 106 A/cm2 and 4 106 A/cm2 respectively (Fig. 5D). There are three possible reaction steps have been suggested for the HER [37]. First is a primary discharge step (Volmer reaction): H3O+ + e ! Hads + H2O followed by either an electrochemical desorption step (Heyrovsky reaction), Hads + H3O+ + e ! H2 + H2O or a recombination step (Tafel reaction), Hads + Hads ! H2 The Tafel slope of the catalyst that is determined by the ratelimiting step of the HER. Volmer-Heyrovsky mechanism leads to a Tafel slope of 120 mV dec1, while the slope is expected to be 30 mV dec1 if the mechanism follows Volmer-Tafel steps [37]. The Tafel slopes obtained in the present studies suggest that the HER on the Mo6S8 catalyst proceeds via Volmer-Heyrovsky mechanism, with initial proton adsorption (Volmer reaction) as the rate determining step. It is also possible that spill over mechanism wherein the hydrogen atom formed on the catalyst spills over to the support before combining with another to form molecular hydrogen [38]. This leads to Tafel slopes in between that of the two pathways mentioned above. Electrochemical impedance spectroscopic (EIS) technique provides additional insight into the HER process. Fig. 5C shows the Nyquist plots for the catalyst Mo6S8 measured at onset potentials 0.17, 0.19 and 0.21 V versus RHE in the acidic, neutral and alkaline media respectively. The plots possess the smallest charge transfer resistance (Rct) in acidic medium as compared to that observed in other electrolytes. The impedance data are fitted to an equivalent circuit (Fig. 5C) and the corresponding Rs, Rct and Cdl parameters of solution resistance, charge transfer resistance, double layer capacitance and Warburg exponent (W) are extracted and presented in Table 3. 3.3. Stability Stability of the electrocatalyst is an important issue when gas evolution occurs on the electrode surface. To evaluate the stability of Mo6S8 catalyst, continuouscycling studies are carried out in all the electrolytes, i.e, 0.5 M H2SO4, 0.1 M KOH and 3% NaCl. As shown
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Fig. 3. (A) SEM image of the as-prepared Mo6S8 particles, (B) Corresponding SEM-EDS, (C) HRTEM image, (D) Contrast intensity profile and (E) selected area electron diffraction (SAED) pattern.
Fig. 4. High resolution XPS spectra (A) survey scan, (B) Mo 3d and (C) S 2p regions of Mo6S8. The deconvoluted peaks along with the experimental data are shown.
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Table 2 Atomic percentage of different species obtained from the deconvolution XPS spectral data.
Table 3 Electrochemical impedance parameters of Mo6S8 obtained under different conditions.
Elements
Binding energy in eV
Atomic percentage in %
Conditions
Rs in V
RCT in V
Cdl in mF
W
Mo6+ Mo6+ Mo4+ Mo4+ Mo5+ S 2s S 2p1/2 S 2p3/2
235.8 232.6 231.4 228.0 229.3 226.2 163.1 162.0
17.10 34.01 12.84 9.69 17.74 8.62 39.44 60.56
0.5 M H2SO4 0.1 M KOH 3% NaCl
6.562 115.4 28.87
3070 5819 3212
0.1044 0.2877 0.166
0.0006069 0.001907 0.00118
in Fig. 6A and Fig. S6, the cathodic polarization curves obtained after 1,000 continuous cyclic voltammogramms (with a scan rate 100 mVs1) shows negligible decrease in current density compared as a function of cycle number indicating the excellent stability of Mo6S8 at all pH values. Stability of the catalysts is also investigated by chronoamperomerty at constant DC voltage. As shown in Fig. 6B and Fig. S6, the Mo6S8 exhibits stable HER current density for several hours under all the conditions employed. The XRD patterns of the electrode material examined before and after 1000 cycles reveal no changes in the structure and composition during the catalytic process (Fig. S7). Experimental results observed using carbon rod and Pt as counter electrode show similar data. It confirms that Pt dissolution in solution and deposition on electrocatalyst does not contribute to HER. The substantial long term- and chemical stability observed in wide pH range suggests great promise for the Mo6S8 nanocubes catalyst as earth abundant, cost-effective viable material
3.4. Faradaic efficiency The amount of hydrogen gas evolved is quantified as a function of time. Chronoamperometry experiment is carried out at – 0.40 V vs. RHE, using H-shaped electrochemical cell (Fig. S8). The large area counter electrode used is separated from the working electrode through a porous glass frit. The Mo6S8 modified glassy carbon electrode and reference electrode (saturated calomel electrode) are kept in one compartment of the H-shaped cell. The quantity of gas evolved measured using inverse burette method shows that the Mo6S8 nanocubes produce around 2 mL gases in 1 h duration at the applied bias mentioned above. The amount of gas evolved during the reaction is in good agreement with the calculated value based on Faraday's law, suggesting nearly 100% faradaic efficiency for the HER. Fig. 7 compares the amount of H2 generated from the HER over Mo6S8 versus the calculated volume based on the charge. Qualitatively, the gas is analyzed using gas chromatography to confirm hydrogen evolution. 3.5. Electrochemical active surface area Cyclic voltammetry at non-faradaic region of potentials helps in extracting the double-layer capacitance (Cdl) of the catalyst which is proportional to the effective electrochemically active surface
Fig. 5. (A) HER polarization plots of Mo6S8 in different electrolytes (Black: 0.5 M H2SO4, Red: 0.1 M KOH and Blue: 3% NaCl), (B) corresponding Tafel plots, (C) Nyquist plots along with the equivalent circuit used and (D) exchange current densities and Tafel slopes of Mo6S8 under different conditions employed. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
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Fig. 6. (A) Polarization curves for the Mo6S8 catalyst before and after 1,000 cyclic voltammogramms at 100 mV s1 scan rate in 0.5 M H2SO4 (Black: 1st cycle and Red: 1000th cycle) and (B) Chronoamperometry (i t curve) of Mo6S8 at 0.25 V in 0.5 M H2SO4. The gas evolution on the electrode surface is responsible for the noise-like behaviour. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
area (ECSA) (Fig. 8). A plot of the current density (at 0.15 V vs RHE) against the scan rate is linear and its slope corresponds to Cdl. As shown in Fig. 8 the Cdl of Mo6S8 is 1.14 mF, which indicates that of Mo6S8 possesses large effective surface area as compared to other materials [38,39]. The HER catalytic activity in molybdenum based chalcogenides such as MoS2 are quite well studied [34,37,38,40–43]. Hu and co workers hypothesized that coordinatively unsaturated S atoms in MoSx effectively adsorb hydrogen and promote recombination to H2 [44]. Recently, Yeo and co-workers have proposed a linear correlation between turnover frequency of H2 production and the availability of S atoms with high electron binding energies [45,46]. It is clear from Fig. S9 shows that Mo6S8 shows higher activity than that of bulk MoS2 under similar conditions. This may be attributed to the presence of large number of adsorption sites based on Mo and S atoms in the catalyst. It should however be pointed out that MoS2 when exfoliated into few layers, turns out to be an excellent HER catalyst [27]. From the band structure calculations [47] of Mo6S8 show that the conduction band is extremely narrow and consists of primarily Mo- 4d and S-3d orbitals. The density of states at the Fermi energy is found to be large supposedly due to intercluster distances observed in the material. In addition, the [48] resistivity temperature (r T) behaviour of Mo6S8 in the range 77–300 K reveals metallic conductivity. The above observations are somewhat different from layered MoS2 which is known to be a semiconductor [49]. It is suggested that the adsorption sites
Fig. 7. Amount of H2 gas evolved with time at – 0.40 V bias along with the expected volume (Red: obtained and Blue: calculated). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
for hydrogen are quite enhanced in Mo6S8 thus leading to good HER activity. 3.6. Calculation of turnover frequency (TOF) The turnover frequency (TOF) is determined by quantifying the active sites through an electrochemical approach [50] as reported earlier using voltammetry. The corresponding voltammograms are given in Fig. 8. Assuming a one electron redox process, the number of active sites and subsequently the corresponding TOF (supporting information) determined from the total charge, to be 0.092 s1 @ 0.35 V vs. RHE. It is comparable with the TOF of 0.07 and 0.02 and 0.044 s1 reported for [Mo3S4]4+ and MoS2 nanoparticles and
Fig. 8. Cyclic voltammograms of Mo6S8 at different scan rates (top) and current density difference {Dj = 1/2 (Ja Jc)} versus scan rate (bottom).
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Cu2MoS4 respectively [29,51,52]. Hence, it is possible that the active sites for HER available on Mo6S8 are as substantial as that of other Mo-S clusters. 4. Summary In summary, electrochemical activity of Mo6S8 synthesized by a simple precipitation method is studied in aquesous media of varying pH values. The measured HER activity and stability of Mo6S8 shows promise as a good catalyst in acidic, alkaline and neutral conditions. Availability of several adsorption sites based on Mo and S possibly helps in good and stable catalytic activity. Acknowledgement Dr. Keerti M. Naik acknowledges the financial support provided by the UGC, Govt. of India through the D. S. Kothari postdoctoral fellowship. The authors thank DST, New Delhi for research funds. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.electacta. 2017.09.015. References [1] M.S. Dresselhaus, I.L. Thomas, Alternative energy technologies, Nature 414 (2001) 332–337. [2] J.A. Turner, Sustainable hydrogen production, Science 305 (2004) 972–974. [3] N.S. Lewis, D.G. Nocera, Powering the planet: chemical challenges in solar energy utilization, Proc Natl Acad Sci USA 103 (2006) 15729–15735. [4] M.G. Walter, E.L. Warren, J.R. McKone, S.W. Boettcher, Q. Mi, E.A. Santori, N.S. Lewis, Solar water splitting cells, Chem. Rev. 110 (2010) 6446–6473. [5] D.Y. Chung, J.W. Han, D.H. Lim, J.H. Jo, S.J. Yoo, H. Lee, Y.E. Sung, Structure dependent active sites of NixSy as electrocatalysts for hydrogen evolution reaction, Nanoscale 7 (2015) 5157–5163. [6] B. Fang, J.H. Kim, J.S. Yu, Colloid-imprinted carbon with superb nanostructure as an efficient cathode electrocatalyst support in proton exchange membrane fuel cell, Electrochem. Commun. 10 (2008) 659–662. [7] M. Chhowalla, H.S. Shin, G. Eda, L.J. Li, K.P. Loh, H. Zhang, The chemistry of twodimensional layered transition metal dichalcogenide nanosheets, Nat. Chem. 5 (2013) 263–275. [8] M. Xu, T. Liang, M. Shi, H. Chen, Graphene-like two-dimensional materials, Chem. Rev. 113 (2013) 3766–3798. [9] M. Pumera, Z. Sofer, A. Ambrosi, Layered transition metal dichalcogenides for electrochemical energy generation and storage, J. Mater. Chem. A 2 (2014) 8981–8987. [10] R. Chevrel, M. Sergent, J. Prigent, New molybdenum ternary sulfide phases, J. Solid State Chem. 3 (1971) 515–519. [11] O.K. Andersen, W. Klose, H. Nohl, Electronic structure of Chevrel-phase highcritical-field superconductors, Phys. Rev. B 17 (1978) 1209–1237. [12] O. Fischer, Chevrel phases: Superconducting and normal state properties, Appl Phys. 16 (1978) 1–28. [13] T. Caillat, J.P. Fleurial, G. Synde, Potential of Chevrel phases to thermoelectric applications, Solid State Sci. 1 (1999) 535–544. [14] K.F. McCarty, J.W. Anderegg, G.L. Schrader, Hydrodesulfurization catalysis by Chevrel phase compounds, J. Catal. 93 (1985) 375–387. [15] E. Levi, G. Gershinsky, D. Aurbach, O. Isnard, G. Ceder, New insight on the unusually high ionic mobility in Chevrel phases, Chem. Mater. 21 (2009) 1390– 1399. [16] T. Hughbanks, R. Hoffmann, Molybdenum chalcogenides: clusters, chains, and extended solids. The approach to binding in three dimensions, J. Am. Chem. Soc 105 (1983) 1150–1162. [17] B. Pan, J. Zhang, J. Huang, J.T. Vaughey, L. Zhang, S.D. Han, A.K. Burrell, Z. Zhang, C. Liao, A lewis acid-free and phenolate-based magnesium electrolyte for rechargeable magnesium batteries, Chem. Commun. 51 (2015) 6214–6217. [18] L. Suo, F. Han, X. Fan, H. Liu, K. Xu, C. Wang, Water-in-Salt electrolytes enable green and safe Li-ion batteries for large scale electric energy storage applications, J Mater Chem. A 4 (2016) 6639–6644. [19] L. Geng, G. Lv, X. Xing, J. Guo, Reversible electrochemical intercalation of aluminum in Mo6S8, Chem. Mater. 27 (2015) 4926–4929. [20] Y. Cheng, L. Luo, L. Zhong, J. Chen, B. Li, W. Wang, S.X. Mao, C. Wang, V.L. Sprenkle, G. Li, J. Liu, Highly reversible zinc-ion intercalation into Chevrel Phase Mo6S8 nanocubes and applications for advanced zinc-ion batteries, ACS Appl. Mater interfaces 8 (2016) 13673–13677.
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