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Single-Step Electrodeposited Molybdenum Incorporated Nickel Sulfide Thin Films from Low-Cost Precursors as Highly Efficient Hydrogen Evolution Electrocatalysts in Acid Medium Arun Prasad Murthy, Jayaraman Theerthagiri, Kumar Premnath, Jagannathan Madhavan, and Kadarkarai Murugan J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 16 May 2017 Downloaded from http://pubs.acs.org on May 17, 2017

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The Journal of Physical Chemistry

Single-Step Electrodeposited Molybdenum Incorporated Nickel Sulfide Thin Films from Low-Cost Precursors as Highly Efficient Hydrogen Evolution Electrocatalysts in Acid Medium

Arun Prasad Murthy a, Jayaraman Theerthagiri a, Kumar Premnatha, Jagannathan Madhavan a,*, Kadarkarai Murugan b

a

Solar Energy Lab, Department of Chemistry, Thiruvalluvar University, Vellore-632 115, Tamilnadu, India. b

Department of Zoology, Bharathiar University, Coimbatore-641 046, Tamilnadu, India.

*Corresponding author. Tel.: +91 9585692101 E-mail address: [email protected] (Jagannathan Madhavan)

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Abstract Large-scale production of hydrogen - the energy carrier of the future - remains challenging on economy grounds. Low-cost synthetic designs are necessary to produce electrocatalysts for hydrogen evolution reaction (HER). Ternary nickel molybdenum sulfides Ni1-xMoxS (x = 0, 0.04, 0.08, 0.16) have been electrochemically grown on fluorine doped tin oxide substrate as highly active and stable HER electrocatalysts. The merits of this method are – 1) Electrochemical method is energy efficient at ambient conditions and can be easily scaled up on large surface area substrates. 2) Ni, Mo and S precursors used for the deposition are readily available and relatively cheaper compared to similar methods. 3) The synthesis procedure is simple, one-step and requires no further heat treatments. The deposited thin films have been examined using regular physical characterization techniques and their HER activity has been evaluated through electrochemical methods. Ni0.96Mo0.04S, especially, has exhibited a promising HER activity with a lowest Tafel slope of 46 mV/dec. The mechanistic aspects of HER on the thin films have been extensively studied by voltammetric and impedance methods. Tafel analysis and double layer capacitance measurements have revealed that the high activity of Ni0.96Mo0.04S can be attributed to the larger electrochemical surface area and the faster discharge of protons in the initial Volmer step. Moreover, stability tests performed on various thin films have shown that Mo doped thin films could retain more than 80 % of the initial activity whereas NiS has lost more than 50 % of the same.


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1. Introduction With world population running into several billions, there exists an expanding gap between the demand and supply of energy. The available fossil fuel is fast dwindling and will not meet the energy demands of rapidly growing population. Environmental pollution and carbon dioxide emission due to fossil fuel usage complicate the situation further leading to severe energy crisis. The solution to the above issues, proposed by the scientific community, is developing alternate energy sources that are clean, sustainable and economically viable.1-7 Renewable energies like sun light meet the above criteria, however, being intermittent in nature they need to be stored in high energy density sources. In this context, hydrogen offers an appealing way of storing harvested renewable energy in the form of chemical bonds8 and is rightly touted as energy carrier7,9, energy currency10,11 and energy economy of the future.12 Hydrogen, without accompanying

carbon

dioxide

emission,

can

be

produced

through

electrocatalytic/photoelectrocatalytic water splitting13 in which hydrogen evolution reaction (HER) forms the cathodic half reaction, 2H+ + 2e─ → H2

(1)

The state of the art electrocatalyst for HER is well-known platinum (Pt) and its high activity can be described through Sabatier’s principle.14 Present and future energy demands, however, require millions of kilograms of Pt as electrocatalyst which practically is not feasible due to paucity and prohibitive cost of Pt. Current research in the field of energy has been focusing towards exploring highly active and stable electrocatalysts that are cheaper enough for large scale production of hydrogen. In this context, various non-noble metal based electrocatalysts such as chalcogenides,15-17 phosphides,1820

carbides,21,22 alloys,23 etc., are actively pursued in the literature. Acid based electrocatalysts are 3 ACS Paragon Plus Environment


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of great importance since research efforts have been geared toward proton exchange membrane (PEM) water electrolyzers and the PEM technology is well suited for mobile and portable devices.9 Recently, various molybdenum disulfide/carbon nano composites have been synthesized and evaluated as highly active alternative HER electrocatalysts in acid medium in our laboratory.16 It has been noted recently in the literature that nickel sulfides (NixSy) are rarely studied as HER catalysts despite their extensive use in the field of technology.24 NiS activity is limited due to relatively low conductivity which can be improved by incorporation of cheaper metals such as iron.24 In the present work, molybdenum (Mo) incorporated nickel sulphides (NiS) have been electrochemically grown on fluorine doped tin oxide (FTO) with an objective to improve HER activity as well as stability in acid medium. Electrochemical and hydrothermal methods are generally followed to synthesize NixSy, and various phases of nickel sulfides can be obtained by employing suitable synthetic conditions. Yang et al, synthesized iron incorporated nickel sulfide through a topotactic conversion reaction.24 Sun et al., prepared amorphous Ni-S films as HER electrocatalyst through potentiodynamic deposition method using nickel sulfate and thiourea as nickel and sulfur sources in aqueous medium.25 NiS/Ni3S4 composites with various compositions synthesized through hydrothermal method were reported by Guo et al.26 NiS and Ni3S2 nanoparticles as HER electrocatalysts were chemically synthesized under reflux conditions by Sung et al.27 Lou et al., described synthesis of cubic NiS nanoframes through template-engaged structure-induced anisotropic chemical etching/anion exchange strategy.28 Hydrothermal method involves high energy input therefore may not be suitable for large scale synthesis of cheaper electrocatalysts. Hence, in this paper we employed electrochemical deposition method to grow Ni1-xMoxS (x = 0, 0.04, 0.08 and 0.16) thin films on FTO. The merits


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of our method are as follows: 1) Electrochemical deposition method is energy effective, needs only ambient conditions and can be scaled up easily on large surface area substrates. 2) Ni, Mo and S precursors (nickel (II) chloride hexahydrate, sodium molybdate dihydrate and thiourea) used for the deposition are readily available and relatively cheaper. 3) The deposition procedure is simple, one-step and requires no further heat treatments. Addition of Mo greatly enhanced the HER activity of the thin films even at lower contents in acid medium. Specifically, Ni0.96Mo0.04S thin film exhibited highest electrocatalytic current and lowest Tafel slope among investigated thin films. Accelerated durability test (ADT) and long-term electrolysis performed on Ni1-xMoxS thin films indicated that Mo incorporation significantly improved the stability of the thin films. 2. Experimental Electrochemical experiments were performed using a CH Instruments potentiostat in a conventional single compartment three electrode cell assembly with Ag/AgCl (3 M KCl) reference electrode. However, the potentials were referenced with respect to reversible hydrogen electrode (RHE) in this paper using the following equation,29 ERHE = EAg/AgCl + 0.0591 pH + 0.1976

(2)

For HER experiments, a graphite rod and electrochemically deposited Ni1-xMoxS thin films served as a counter30 and working electrodes respectively. The procedure for electrochemical deposition of Ni1-xMoxS thin films is similar to our earlier report.31 Various compositions of Ni1xMoxS

(x = 0, 0.04, 0.08, 0.16) were electrochemically deposited on FTO conducting glass

substrate (Sigma Aldrich, India) using cyclic voltammetry. The electrochemical deposition was carried out in a three electrode system with precleaned FTO conducting glass substrate and a platinum wire serving as working and counter electrodes respectively while an aqueous Ag/AgCl 5 ACS Paragon Plus Environment


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(3 M KCl) served as the reference electrode. The deposition solution, typically 50 mL, contained 0.05 M of nickel (II) chloride hexahydrate (NiCl2.6H2O; SDFCL, India), various amounts (0.04, 0.08 and 0.16 mole ratios of Mo with respect to Ni) of sodium molybdate dihydrate (Na2MoO4.2H2O; SDFCL, India) and 1 M of thiourea (CH4N2S; SDFCL, India). The potential was scanned between -1.0 and 0.4 V at a rate of 0.005 V/s for 25 cycles as shown in Figure S1. The FTO glass plate colour gradually turned into darker with increasing deposition cycles indicating formation of thin film. Finally, the thin films were rinsed with water and dried in air for 3 hours. It may be mentioned that pure MoS (Ni1-xMoxS, x = 1) film could not be deposited under similar experimental conditions for comparison studies. Furthermore, the HER activity of the thin films was influenced by the number of deposition cycles. A highest activity was observed for 25 cycles (data not shown), therefore, 25 cycles were used for depositing all Ni1xMoxS

thin films having a mass loading of ca. 0.1 mg cm-2. Ni1-xMoxS thin films were evaluated for HER activity by voltammetric method at a scan

rate of 0.002 V/s in 0.5 M H2SO4 electrolyte at ambient temperature (298 K). Electrochemical impedance spectroscopy (EIS) experiments were conducted in a CH Instruments electrochemical works station in the frequency range between 10 mHz to 100 kHz using single sinusoidal AC potential of 5 mV amplitude to avoid nonlinearity. ZView software (Scribner Associates) was used for analyzing and fitting experimental impedance data. A simple Randles equivalent circuit with a constant phase element (CPE) in place of double layer capacitance (Figure S2) was used to fit experimental impedance spectra.32,33 Rs and Rct in Figure S2 denote uncompensated series and recombination resistances respectively. Electrochemical data were transferred to Matlab workspace for further computation of Tafel and capacitance parameters of the thin films and all


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voltammetric data were reported without iR compensation. Capacitance experiments were conducted in a non-faradaic potential region between 0.05 – 0.15 V vs RHE at various scan rates. Powder XRD characterization was performed on X-ray diffractometer Mini Flex II, Japan with Cu Kα radiation (λ= 0.154 nm) at a scan speed of 4º/min between 20º − 80º. The surface morphology of the thin films were analyzed on high resolution scanning electron microscopy (HRSEM) FEI Quanta FEG 200 at an accelerating voltage of 20 kV. Energy dispersive X-ray spectroscopy (EDXS) attached with HRSEM was used for the elemental analysis of the thin films. Compositions of Ni1-xMoxS samples were estimated by Perkin Elmer inductively coupled plasma optical emission spectrometer (ICP-OES). 3. Results and Discussion The XRD patterns of electrochemically deposited various Ni1-xMoxS thin films are depicted in Figure 1. It can be observed that the diffraction peaks at 30.2º, 45.5º and 53.5º of NiS (Figure 1a) correspond to (100), (102) and (110) diffraction planes of hexagonal NiS. The observed diffraction peaks are consistent with the standard hexagonal NiS data (JCPDS No. 750613). All other major peaks belong to pristine FTO substrate in agreement with our earlier report.31 The diffraction patterns of Ni0.96Mo0.04S, Ni0.92Mo0.08S and Ni0.84Mo0.16S thin films (Figure 1b-d) revealed that there is no change in the XRD pattern by the addition of Mo which might be due to the presence of Mo content below the detection limit of XRD. However, it can be noted in Figure 1 that the peak intensity of NiS decreases with Mo content confirming the incorporation of Mo atoms into interstitial or substitutional sites of NiS leading to significant contraction and expansion of the lattice parameters. The surface morphology of as-deposited NiS and Ni0.96Mo0.04S thin films was analysed by HRSEM and the images are shown in Figure 2. The surface morphology of pure NiS (Figure 7 ACS Paragon Plus Environment


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2a) shows spherical particles of ca.100 − 200 nm diameter along with some aggregated grains. In the case of Ni0.96Mo0.04S (Figure 2b), the surface morphology is similar to that of NiS indicating no significant change in the surface morphology occurred by the incorporation of Mo. However, particle sizes of Ni0.96Mo0.04S are relatively smaller (ca. 50 – 100 nm) with less agglomeration. Hence, Ni0.96Mo0.04S nano particles can be expected to have relatively high surface area, consequently, high HER activity. Figures 2c-d show TEM images of NiS and Ni0.96Mo0.04S nanoparticles concurring the results of HRSEM analysis. The elemental analysis of NiS and Ni0.96Mo0.04S thin films was examined by EDXS and the results are presented in Figure 3. As shown in Figure 3a, pure NiS is composed of Ni and S, while Ni0.96Mo0.04S sample (Figure 3b) is consisted of Ni, Mo and S elements. The additional peaks of Sn, O, Si and F in Figure 3 can be attributed to the FTO substrate.25 The compositions of Ni1-xMoxS samples were further established by ICP-OES and compared with the results of EDXS as shown in Table S1. Electrochemical studies on Ni1-xMoxS thin films were conducted using a three electrode cell configuration in 0.5 M H2SO4 electrolyte. Cathodic polarization curves of various Ni1-xMoxS thin films are shown in Figure 4a indicating highest HER activity for Ni0.96Mo0.04S followed by Ni0.92Mo0.08S, Ni0.84Mo0.16S and NiS, with FTO itself showing negligible HER activity in the investigated potential domain (not shown). The standard metric for evaluating and comparing HER activities of different electrocatalysts is the overpotential (η10) to achieve 10 mA/cm2(geometric) current density (j) because of its relevance in solar hydrogen production.34 NiS achieved 10 mA/cm2 current density at a moderate η10 of 0.23 V as expected for NiS thin films.27 Ni1-xMoxS (x ≠ 0) thin films achieved much better electrocatalytic performance compared to NiS as tabulated in Table 1 in order of decreasing activity. Ni0.96Mo0.04S showed the lowest η10 of 0.18 V which can be placed among various highly active NiS based


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electrocatalysts.25 The above order in the HER activity was established through four sets of independent experiments. Tafel analysis of the voltammetric data (Figure 4a) was conducted adhering to the directions for correct analysis35 as follows: 1) The potential regime selected was neither under diffusion control (high overpotential) nor at low overpotential where backward reaction was comparable to HER. It can be noted that Tafel analysis requires HER rate far exceeding the backward reaction rate and kinetics under steady or quasi-steady state condition. 2) At least one order of linearity in the Tafel plot was used for Tafel analysis. 3) Voltammetric data were free from background currents. Figure 4b shows linear parts of variation of logarithmic current density of various Ni1-xMoxS thin films over a range of overpotential. The above plots can be fit into the following Tafel equation,36 η = b log (j) + a

(3)

where b denotes Tafel slope and a is the exchange current density. Tafel equation is an important analytical tool for identifying possible HER mechanism operating at the surface of the thin films. Tafel slope b is independent of the catalytic current, vis-a-vis, surface area and assumes a unique value characteristic of the thin film at the given experimental condition. Various elementary steps that lead to hydrogen evolution are:

Initial discharge or Volmer step H3O+ + e− → Hads + H2O

(4)

Atom + ion or Heyrovsky step Hads + H3O+ + e− → H2 + H2O Atom + atom or Tafel step


(5)


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Hads + Hads → H2

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(6)

The above elementary steps result in two mechanisms – Volmer-Heyrovsky and Volmer-Tafel, and three possible rate determining steps (RDS), viz. Volmer, Heyrovsky and Tafel (equations 4 – 6). In both the mechanisms, the electrocatalytic current increases exponentially with overpotential. The rate of increase, however, is unique to each RDS hence possible mechanism operating at the thin film surface can be identified.16 Volmer reaction is the initial common step to both the mechanisms and a Tafel slope of 120 mV/dec ensues when it becomes rate determining. Similarly, Tafel slopes of 40 and 30 mV/dec result when either Heyrovsky or Tafel step becomes RDS of the corresponding mechanisms.36 A Tafel slope of 80 mV/dec was obtained for NiS which is similar to a reported slope for Ni-S/FTO electrocatalyst.25 A lowest Tafel slope of 50 mV/dec was achieved for Ni0.96Mo0.04S while the slopes found for Ni0.92Mo0.08S and Ni0.84Mo0.16S are 66 and 72 mV/dec respectively (Table 1). Generally, lower Tafel slopes are highly desirable since relatively higher catalytic currents can be attained at lower overpotentials. The above Tafel analysis based on voltammetric method suffers from the fact that it includes parasitic resistances arising from several sources such as FTO plate, bulk solution, thin film, wiring etc., forming the components of series resistance Rs in Figure S2.37 Different ways of iR drop corrections are followed in the literature13 and it may be mentioned that ohmic corrections were not applied in this study. The above limitation can be circumvented by impedance method of Tafel analysis as reported by Vrubel et al.13 Tafel analysis based on the impedance method consists of estimation of charge transfer resistance (Rct) at various overpotentials employing suitable equivalent circuit models. Subsequently, Tafel plot is obtained by plotting inverse of logarithmic Rct versus corresponding overpotentials. In this method of analysis, 1/Rct reflects charge transfer current and further ohmic 10 ACS Paragon Plus Environment

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corrections need not be applied. Impedance method of Tafel analysis has been reported for various HER electrocatalysts in the literature.15,18,21,23,38 Impedance analysis of various Ni1-xMoxS thin films was conducted in the overpotential window of ca. 0.1 – 0.2 V (the linear region in the polarization Tafel plots in Figure 4b) between 10 mHz to 100 kHz. Nyquist plots as a function of overpotential in the case of NiS are shown in Figure 5a. These complex plane plots are characterized by single potential dependent semicircles representing charge transfer process. A simple Randels equivalent circuit (Figure S2) was used for fitting (solid circles, Figure 5a) the experimental data (solid lines) in order to obtain Rct at various overpotentials. In the case of Ni0.96Mo0.04S (Figure 5b), the impedance response is similar albeit with smaller semi circles − the smaller the semi-circle, the larger the 1/Rct and hence faster the HER kinetics. Nyquist plots of Ni0.96Mo0.04S were also fitted using Randels equivalent circuit and an excellent fitting can be observed in Figure 5b. Rct data obtained from the impedance method (Figures 5a and 5b) were used in the Tafel plots shown in Figures 5c and 5d. A linear variation between η and log (1/Rct) yielded the Tafel slopes of 60 and 46 mV/dec respectively for NiS and Ni0.96Mo0.04S thin films. A significant variation in the Tafel slopes between the voltammetric and impedance methods is observed in NiS (80 and 60 mV/dec; Table 1) while only a slight variation between the slopes is observed (50 and 46 mV/dec; Table 1) in the case of Ni0.96Mo0.04S. Analogous impedance behavior was observed in Ni0.92Mo0.08S and Ni0.84Mo0.16S thin films – potential dependent semicircles that can be fitted using Randels equivalent circuit (Figures S3 and S4). As can be seen in Table 1, Tafel slopes of 48 and 55 mV/dec were obtained from the impedance method for Ni0.92Mo0.08S and Ni0.84Mo0.16S thin films respectively. Again, significant differences in the Tafel slopes between the voltammetric and the impedance methods are observed in Ni0.92Mo0.08S and Ni0.84Mo0.16S



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thin films (Table 1) probably indicating further addition of Mo is inimical to the HER. Figure S5 shows difference in Tafel slopes between voltammetric and impedance methods as a function of mole fraction of Mo. Since impedance method of Tafel analysis reflects pure charge transfer kinetics, it can be deduced from Figure S5 that a better electrical integration was achieved in the thin film with least difference. The difference is minimum in the case of Ni0.96Mo0.04S therefore this composition might be the optimal ternary composition of Ni1-xMoxS for HER. Tafel slopes for HER estimated from the impedance method fall between 46 – 60 mV/dec (Table 1) indicating similar surface chemistry at Ni1-xMoxS thin films. As discussed earlier (equations 4 − 6), the major HER path follows Volmer-Heyrovsky mechanism with a faster Volmer step and a rate determining Heyrovsky step at Ni1-xMoxS thin film surfaces. It may be noted that Volmer-Tafel mechanism with a rate determining Tafel step (b = 30 mV/dec) operates in Pt and other earth-abundant elements based electrocatalysts.12,15,39 On the other hand, Volmer step becomes rate determining (b = 120 mV/dec) in electrocatalysts such as bulk molybdenum disulfide.29 According to Volmer-Heyrovsky mechanism with Heyrovsky step as RDS, adsorption of H is thermodynamically favorable even at lower overpotentials.16 Hydrogen coverage increases with overpotential on the active sites and subsequently forms molecular hydrogen (H2) through atom + ion step (equation 5). Consequently, H2 formation results in lowering of H coverage and this cycle continues at faster rates (higher frequency) with η. The increase in the activity of Ni1-xMoxS thin films, especially Ni0.96Mo0.04S, could be due to 1) increase in the electrochemical surface area (ECSA) and/or 2) increase in the inherent activity of the thin film. Measurement of double layer capacitance (Cdl) at non-faradaic current region can be used to evaluate relative ECSA of the thin films. Voltammetric experiments were conducted in a potential window of 0.05 – 0.15 V vs RHE at various scan rates and Figures 6a


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and b show typical plots for NiS and Ni0.96Mo0.04S thin films respectively. In general, halves of anodic and cathodic current density differences, ∆j =

ja - jc 2

, at certain overpotential are plotted

against corresponding scan rates to obtain Cdl from the slope. However, deviation from a linear variation may be observed when a minor faradaic component contributes to the charging current. Since charging current varies linearly with υ and faradaic current varies linearly with υ1/2,40,41 the faradaic component can be deconvoluted employing a procedure reported by Diaz et al.42 Accordingly, the slope of the linear variation between ∆jυ-1/2 and υ1/2 provides pure Cdl as per the following equation, ∆jυ-1/2 = Kc υ1/2 + KF

(7)

where Kc and KF denote capacitive and faradaic components independent of scan rate. Figure 6c shows variation of ∆jυ-1/2 with υ1/2, and Cdl for NiS and Ni0.96Mo0.04S were estimated to be 1.7 and 2.6 mF/cm2 respectively. Cdl for various Ni1-xMoxS thin films are given in Table 1. It is clear from Figure 6c that ECSA of Ni0.96Mo0.04S is higher by a factor of 1.5 relative to NiS resulting in a higher catalytic current. However, the increase in ECSA alone cannot be presumed for the observed high electrocatalytic activity of Ni0.96Mo0.04S thin film. Doping of a second metal increases the activity of the electrocatalyst as reported in the case of Fe-doped CoP nanoarray.43 It can be observed in the Table 1 that with increase in activity, Tafel slope moves closer to a value of 40 mV/dec indicating that rate determining step moves closer to Heyrovsky step from Volmer step (equations 4−6). In other words, the initial discharge of protons is relatively faster in Ni0.96Mo0.04S compared to NiS. Furthermore, the above observation also indicates that the bond between the adsorbed hydrogen and the active site is stronger in Ni0.96Mo0.04S (equation 4) hence its removal as molecular hydrogen in the subsequent Heyrovsky



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step (equation 5) becomes rate determining. Scheme 1 illustrates the above condition wherein a strongly adsorbed hydrogen in the initial Volmer step results in lowering of the Tafel slope and vice versa. Sung et al., confirmed the above deduction through DFT analysis in the cases of NiS and Ni3S2 where strongly adsorbed hydrogen (Had) led to decreasing Tafel slope.27 Hence, the high activity of Ni0.96Mo0.04S among other Ni1-xMoxS thin films can be attributed to both the larger ECSA and the faster discharge of protons in the initial step. Electrochemical long-term stability of HER electrocatalysts is one of the major constraints of commercializing PEM based electrolyzers.44-46 Electrocatalysts that are robust47,48 and stable at alkaline49-50 and all pH conditions51 are reported recently. An accelerated durability test52,53 was conducted to evaluate the long-term stability of Ni1-xMoxS thin films. The potential was cycled 3000 times between 0 to -0.3 V vs RHE at a rate of 0.05 V/s in 0.5 M H2SO4 electrolyte. Figures 7a and 7b show polarization curves before and after ADT for NiS and Ni0.96Mo0.04S thin films respectively. The HER activity decreased to about 50 % at -0.25 mV in NiS after 3000 cycles while Ni0.96Mo0.04S retained 80 % of the initial activity. Figure 7c graphically shows the results of ADT of various Ni1-xMoxS thin films. Ni1-xMoxS (x ≠ 0) thin films consistently retained more than 80 % of the initial activity whereas NiS lost more than 50 % of the initial activity. A similar result was reported for NiS electrocatalyst by Sung et al in the durability test.27 Long-term stability of the Ni0.96Mo0.04S thin film was also analysed by electrolysis at a constant potential of -200 mV vs RHE for continuous 24 h in 0.5 M H2SO4. As can be seen in Figure 7d the current was slightly increasing till 16 h and then started decreasing. At the end of 24 h about 13 % of the initial activity was lost. This is consistent with the voltammetric experiment where the loss in activity was about 20 %. After the electrolysis, Ni0.96Mo0.04S thin film was analysed by HR-SEM as shown in Figure 7e. The thin film could


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retain overall initial morphology other than that the particles are a little more aggregated and bigger. This could explain the loss in HER activity of Ni0.96Mo0.04S thin film by about 20 % in the voltammetric experiments and 13 % in the electrolysis experiment. The above results clearly show that incorporation of Mo in to NiS enhances the stability of the thin films towards HER in the acid medium. 4. Summary While hydrogen is considered as energy carrier and energy economy of the future, its large-scale production remains challenging on economy grounds. Hydrogen production through HER, the cathodic half reaction of water splitting, requires efficient and stable electrocatalyst that is synthesized through inexpensive methods and precursors. Electrochemical deposition method was chosen in this study to grow various Ni1-xMoxS thin films on FTO and the advantages of this method are: 1) Electrochemical deposition method is energy efficient, requires only ambient conditions and can be scaled up easily on large surface area substrates. 2) Compared to other contemporary methods Ni, Mo and S precursors used in the electrochemical cell are readily available and relatively cheaper. 3) The deposition procedure is easy, one-step and requires no further heat treatments. The deposited Ni1-xMoxS thin films were studied using XRD, HRSEM and EDXS. The electrochemical studies clearly showed that incorporation of Mo into NiS, even at lower contents, significantly enhanced the HER activity. Particularly, Ni0.96Mo0.04S exhibited highest HER activity among the Ni1-xMoxS thin films. Extensive voltammetric and impedance methods of Tafel analysis were carried out to further understand the mechanistic aspects of HER. Through double layer capacitance measurements, it was deduced that the increase in ECSA alone could not account for high activity of Ni1-xMoxS relative to NiS. Tafel analysis from the mechanistic perspective revealed that the higher activity 15 ACS Paragon Plus Environment


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of Ni0.96Mo0.04S can be attributed not only to the larger ECSA but also to the relatively faster discharge of protons in the initial Volmer step. ADT experiments performed on various Ni1-xMoxS thin films indicated that NiS lost about 50 % of the initial activity while Ni1-xMoxS (x ≠ 0) thin films consistently retained more than 80 % of the initial activity. Moreover, long-term electrolysis of Ni0.96Mo0.04S thin film showed that only 13 % of the initial activity was lost. Our principle of synthesising highly active and stable HER electrocatalysts through simpler, energy and cost efficient methods as described in this study may contribute to the contemporary synthetic designs and strategies for producing advanced electrocatalysts for HER.

Supporting information available: Additional voltammetric and impedance data.

Acknowledgements: The authors A. P. M, J. T and J. M are grateful to the authorities of Thiruvalluvar University for the support.


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Figures

Figure 1. XRD patterns of electrochemically deposited (a) NiS, (b) Ni0.96Mo0.04S, (c) Ni0.92Mo0.08S and (d) Ni0.84Mo0.16S thin films on FTO.



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C

d

Figure 2. HR-SEM images of electrochemically deposited a) NiS and b) Ni0.96Mo0.04S thin films. TEM images of c) NiS and d) Ni0.96Mo0.04S nanoparticles.



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Figure 3. EDXS spectra of (a) NiS and (b) Ni0.96Mo0.04S thin films.


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a

b



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Figure 4. a) Linear sweep voltammograms of NiS, Ni0.96Mo0.04S, Ni0.92Mo0.08S and Ni0.84Mo0.16S thin films showing HER activity in 0.5 M H2SO4. Scan rate = 0.002 V/s. b) Tafel plots of respective Ni1-xMoxS thin films.

a

b


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c

d



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Figure 5. Nyquist plots of a) NiS, b) Ni0.96Mo0.04S thin films as a function of η from 0.2 V (smallest semi-circle) to 0.1 V (largest semi-circle) in increments of 0.01 V. Solid lines denote experimental data while circles represent fitting. Tafel plots of c) NiS and d) Ni0.96Mo0.04S thin films obtained employing Rct data of respective impedance plots.

a

b


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c

Figure 6. Cyclic voltammograms of a) NiS, b) Ni0.96Mo0.04S thin films in the non-faradaic potential region at various scan rates and c) ∆jυ-1/2 vs υ1/2 plots of NiS and Ni0.96Mo0.04S thin films.



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a

b


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c

d



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e

Figure 7. Accelerated durability test for evaluating electrochemical stability of a) NiS, b) Ni0.96Mo0.04S thin films in 0.5 M H2SO4 at a scan rate of 0.05 V/s. c) A bar graph showing activity loss of various Ni1-xMoxS thin films after ADT. Current density is normalized to initial value and sampled at -0.25 V vs RHE. D). Long-term controlled potential electrolysis of Ni0.96Mo0.04S thin film in 0.5 M H2SO4 at -0.2 V vs RHE. e) HR-SEM image of electrochemically deposited Ni0.96Mo0.04S thin film after electrolysis.

Increasing Bond Strength

H

120 mV/dec

H

H

H

H

H

H

Decreasing Tafel Slope 28 ACS Paragon Plus Environment

H

H

40 mV/dec

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Scheme 1. A schematic relationship between Tafel slope and the bond strength of Had. Increase in bond strength of Had results in concomitant decrease in Tafel slope.



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Table 1. HER parameters pertaining to Ni1-xMoxS thin films

Ni1-xMoxS

η (mV) @ j = 10 mA/cm2

NiS Ni0.96Mo0.04S Ni0.92Mo0.08S Ni0.84Mo0.16S

230 180 210 220

Tafel slope from voltammetric method (mV/dec) 80 50 66 72

Tafel slope from impedance method (mV/dec) 60 46 48 55


Cdl (mF/cm2) 1.70 2.55 1.87 2.04

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