Electrodeposited Nickel-Cobalt-Sulfide Catalyst for the Hydrogen

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Electrodeposited Nickel−Cobalt−Sulfide Catalyst for the Hydrogen Evolution Reaction Ahamed Irshad and Nookala Munichandraiah* Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India

ACS Appl. Mater. Interfaces 2017.9:19746-19755. Downloaded from pubs.acs.org by UNIV OF LOUISIANA AT LAFAYETTE on 01/23/19. For personal use only.

S Supporting Information *

ABSTRACT: A novel Ni−Co−S-based material prepared by the potentiodynamic deposition from an aqueous solution containing Ni2+, Co2+, and thiourea is studied as an electrocatalyst for the hydrogen evolution reaction (HER) in a neutral phosphate solution. The composition of the catalyst and the HER activity are tuned by varying the ratio of the concentrations of Ni2+ and Co2+ ions in the electrolytes. Under optimized deposition conditions, the bimetallic Ni−Co−S exhibits higher electrocatalytic activity than its monometallic counterparts. The Ni−Co−S catalyst requires an overpotential of 150 mV for the HER onset, and 10 mA cm−2 current density is obtained at 280 mV overpotential. The catalyst exhibits two different Tafel slopes (93 and 70 mV dec−1) indicating two dissimilar mechanisms. It is proposed that the catalyst comprises two types of catalytic active sites, and they contribute selectively toward HER in different potential regions. KEYWORDS: electrochemical deposition, electrolysis of water, hydrogen generation, amorphous catalysts, nickel cobalt sulfide, bimetallic catalysts, HER

1. INTRODUCTION Hydrogen is a promising fuel to substitute fossil fuels in the future, owing to its high energy density and pollution-free use.1 In addition, H2 is an important chemical feedstock in the petroleum refining industry and in the ammonia synthesis for fertilizers.2 Currently, H2 is produced on a large scale by the steam-methane reforming method

highly active, stable, economical, and earth-abundant catalysts for efficient H2 production is important. In recent years, several alternate materials have emerged as promising HER catalysts in acidic and alkaline electrolytes. They include metal alloys,7 dichalcogenides,8,9 nitrides,10,11 carbides,11,12 phosphides,13,14 and so forth. There are catalyst materials available for the HER in neutral electrolytes as well.15−20 It is desirable to carry out HER in neutral solutions to minimize the environmental impact and to prevent the corrosion of metallic parts of the electrolyzers. Also, the synthesis procedure for the catalyst must be easy, scalable, and environmental friendly. In this context, electrodeposited cobalt sulfide (Co−S) was reported as a highly active HER catalyst in neutral media.19 The catalyst exhibited a remarkable performance in terms of the onset potential (100 mV) and current density in the lower potential regions. On the other hand, Ni−S exhibits a lower Tafel slope of 77 mV dec−1, whereas it is 93 mV dec−1 for Co−S. It remains a challenge to develop an inexpensive HER catalyst exhibiting both lower HER onset potential and a smaller

CH4(g) + H 2O(g) → CO(g) + 3H 2(g) ΔH298 = −206 kJ mol−1

(1)

CO(g) + H 2O(g) → CO2(g) + H 2(g) ΔH298 = −41 kJ mol−1

(2)

As evident from the above chemical reactions, this process involves high energy (heat) input and releases a huge amount of carbon dioxide into the atmosphere. Hence, it is not considered as a favorable green method of H2 production.3 By contrast, electrolysis or photoelectrolysis of water produces pure H2 without greenhouse gas emission. This requires electrocatalysts for cathodic hydrogen evolution reaction (HER) and anodic oxygen evolution reaction (OER), to realize a high current density at moderate overpotentials.4 The most efficient catalysts for the HER are Pt and its composites.5,6 However, low abundance and high cost prohibit their use in the large-scale production of H2. Consequently, identifying and designing © 2017 American Chemical Society

Received: December 1, 2016 Accepted: May 17, 2017 Published: May 17, 2017 19746

DOI: 10.1021/acsami.6b15399 ACS Appl. Mater. Interfaces 2017, 9, 19746−19755

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ACS Applied Materials & Interfaces

versus SCE at 5 mV s−1. N2 gas was purged through the electrolyte 30 min before the deposition and the flow was maintained throughout the deposition process. The electrolytes were 0.5 M aqueous solutions of thiourea containing x mM Ni2+ + y mM Co2+ (x + y = 5). Pure Co−S and Ni−S were formed when x = 0 and y = 0, respectively. Similarly, Ni− Co−S films of different compositions, namely, Ni−Co−S-1, Ni−Co−S2, Ni−Co−S-3, and Ni−Co−S-4, were prepared when x = 1, 2, 3, and 4, respectively. The deposited electrodes were rinsed with a copious amount of water and dried overnight under vacuum. To study the effect of calcination, the electrodes were heated at different temperatures ranging from 100 to 700 °C for 3 h in a N2 atmosphere. The prepared electrodes were stored in a desiccator at room temperature.

Tafel slope. Among various approaches for improving the electrocatalytic activity, the formation of a bimetallic structure is interesting. It is observed that the bimetallic catalyst systems exhibit better performance than its individual metal counterparts.21−23 It is usually attributed to enhanced conductivity, formation of structural defects, change in the surface charge density, and increase in the number of electrochemically active sites. Therefore, it is anticipated that nickel cobalt sulfide (Ni−Co−S) catalyst can combine the favorable features of both Co−S and Ni−S and also improve its overall performance as a result of possible synergistic interaction on the molecular level. Surprisingly, only limited reports are available for the HER activity of bimetallic sulfides such as Cu2MoS4,24 FeNiSx,25 Fe/ Co/Ni−MoSx,26 Co/Ni−WSx,27 and Zn0.3Co2.7S4.28 The present work describes the electrochemical deposition of a mixed Ni−Co−S film, its physical and electrochemical characterization, and its application as a catalyst for the HER in a neutral phosphate solution. This is the first report on the potentiodynamic deposition of an amorphous Ni−Co−S catalyst film for the HER in a neutral aqueous electrolyte.

3. RESULTS AND DISCUSSION 3.1. Electrochemical Deposition and Characterization. Cyclic voltammograms of an FTO electrode recorded in an aqueous solution containing 2.5 mM NiCl2 + 2.5 mM CoCl2 + 0.5 M thiourea between 0.15 and −1.25 V are shown in Figure 1.

2. EXPERIMENTAL DETAILS 2.1. Reagents and Materials. Analytical grade NiCl2·6H2O, CoCl2·6H2O, KH2PO4, K2HPO4, and thiourea (all from Merck) were used as received. All solutions were prepared using double-distilled water. Phosphate buffer solution was prepared by mixing calculated volumes of 1 M KH2PO4 and 1 M K2HPO4 solutions. The pH value of the phosphate solution was maintained at 7.4, unless otherwise stated. Fluorine doped tin oxide (FTO) glass (Techinstro, TISXY 004, sheet resistance < 15 Ω sq−1) of thickness 2.2 mm was used as the working electrode. A section of 1 cm width and 3 cm length was cut from a large glass sheet, and 1 cm2 area was exposed to the electrolyte. A copper wire was attached using a conductive silver paste. The unexposed area of the electrode was masked by a Teflon tape. Two large Pt foil auxiliary electrodes on either side of the working electrode and a calibrated saturated calomel electrode (SCE) as reference were used. 2.2. Physical Characterization and Electrochemical Studies. The physical characterization studies of the electrodeposited materials were carried out using scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray analysis (EDXA), X-ray photoelectron spectroscopy (XPS), and powder X-ray diffraction (XRD). For SEM images and EDXA, an Ultra 55 scanning electron microscope equipped with an EDXA system was used. TEM images were obtained using a FEI Tecnai T-20 transmission electron microscope at 200 kV. Powder XRD patterns were recorded using a Bruker D8 diffractometer using Cu Kα radiation. The surface chemical compositions of the materials were analyzed by XPS using a SPECS GmbH spectrometer (Phoibos 100 MCD Energy Analyzer) with Mg Kα radiation (1253.6 eV). The peak of C 1s at 284.6 eV was used as the reference energy position. Electrochemical experiments were carried out using a CH Instruments potentiostat/galvanostat model 608C. Electrochemical impedance spectroscopy (EIS) measurements were conducted at an ac excitation signal of 10 mV over the frequency range of 100 kHz to 0.10 Hz. All potential values for H2 evolution studies are reported against reversible hydrogen electrode (RHE) as the reference, whereas those of depositions are against SCE as the reference. iR correction was applied for the Tafel polarization data. A typical value of 10−15 Ω was measured as the internal resistance. All electrochemical experiments were performed at 22 ± 1 °C. The current density values are reported on the basis of the geometrical area of the electrode. 2.3. Electrochemical Deposition of Ni−Co−S Films. Electrochemical deposition was carried out by the potentiodynamic method on FTO-coated glass electrodes. Prior to electrodeposition, FTO electrodes were well-cleaned by sonication for 15 min consecutively in water, acetone, and isopropanol. Subsequently, they were dried in a N2 flow and stored under vacuum at room temperature. Electrodeposition was carried out by repeated potential cycling between 0.15 and −1.25 V

Figure 1. Potentiodynamic scans (10 cycles) at 5 mV s−1 during the deposition of the Ni−Co−S film on an FTO electrode from 2.5 mM CoCl2 + 2.5 mM NiCl2 + 0.5 M thiourea solution.

The voltammograms exhibit broad oxidation and reduction peaks centered at −0.20 and −0.60 V, respectively. These peaks are attributed to the one-electron oxidation of thiourea to formamidine disulfide and its corresponding reduction according to eq 3.29

The peak current increases on repeated cycling. During this process, the exposed area of the working electrode becomes gray initially and turns black after several cycles. The overall reactions involved in the deposition of the Ni−Co−S film are given below.30 2H 2O + 2e− → 2OH− + H 2 −

2−

SC(NH 2)2 + 2OH → S

+ OC(NH 2)2 + H 2O

Ni 2 + + Co2 + + 2S2 − → NiCoS2

(4) (5) (6)

The cyclic voltammograms during the depositions of Ni−S and Co−S also exhibit similar features but with different rates of thiourea decomposition (Figure S1). The composition of the 19747


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Figure 2. SEM images of (a) Ni−Co−S-3/FTO, (b) Co−S/FTO, and (c) Ni−S/FTO. (d) TEM image, (e) high-resolution TEM (HR-TEM) image, and (f) SAED pattern of the Ni−Co−S-3 deposit.

final Ni−Co−S deposit was controlled by adjusting the ratio of concentrations of Ni2+ to Co2+ in the electrolyte but maintaining the total metal ion concentration at 5 mM. Four different composites, hereafter referred to as Ni−Co−S-1 to Ni−Co−S-4, are studied. It is found that the electrocatalytic activity of Ni− Co−S is higher than Ni−S and Co−S, and Ni−Co−S-3 provides the best performance among the catalysts studied. Thus, we have studied Ni−Co−S-3 composition in detail. The surface morphology of the electrodeposit was studied using SEM and TEM (Figure 2). The SEM image of the Ni− Co−S-3 film on FTO (Figure 2a) shows a honeycomb-like structure with wide open pores. The macroporous features are favorable for the effective contact of the electrolyte throughout the material, resulting in an extensive contact area. After five potential cycles, the FTO substrate was entirely covered with the electrodeposit, and a uniform deposition of Ni−Co−S-3 was observed. No agglomeration or structural collapse was noticed. In the case of pure Co−S (Figure 2b), a dense array of nanosheets that are interconnected to form islands are seen. Porosity is expected because of interlocking of the nanosheets. By contrast, the Ni−S film exhibits a totally different morphology (Figure 2c) with small particles and lumps. It is expected that Ni−S deposits as tiny particles initially, and then, they grow gradually to form microsized clusters. The other deposits such as Ni−Co−S-1, Ni−Co−S-2, and Ni−Co−S-4 exhibit morphological features intermediate to those of Co−S and Ni−S (Figure S2). To further study the morphology of the Ni−Co−S-3 deposit, TEM analysis was carried out. The TEM image of Ni− Co−S-3 (Figure 2d) shows that the material preserves the interwoven features of the honeycomb structure. However, a slight difference in the morphology compared with the SEM image could be due to structural collapse caused by sonication during the sample preparation. Lattice fringes of Ni−Co−S are seen in the high-resolution images (Figure 2e). Lack of wellarranged diffraction spots and appearance of diffused circles in the selected area diffraction (SAED) pattern (Figure 2f) suggest poor crystallinity of the electrodeposited Ni−Co−S-3. Powder XRD patterns were recorded for the as-deposited Ni− Co−S-3 film and also after heating at different temperatures in an inert atmosphere. Figure 3a shows the XRD pattern for the conducting side of a clean FTO glass. It is seen from Figure 3b that the pattern of the as-deposited Ni−Co−S-3 shows only the peaks from the FTO substrate, and no other peaks from the

Figure 3. XRD patterns: (a) FTO; (b) as-deposited Ni−Co−S-3/FTO; and Ni−Co−S-3/FTO heated at (c) 300 and (d) 500 °C. Samples were heated in a N2 atmosphere for 3 h.

electrodeposit are present. It indicates the amorphous nature of the electrodeposited Ni−Co−S. However, after heating at 300 °C, additional peaks emerge at 72.6°, 49.1°, and 88.5° (Figure 3c). Further heating to 500 °C enhances the intensity of these peaks (Figure 3d). Moreover, a new shoulder peak at 37.1° and another triplet centered at 42.9° are also identified in the spectrum (Figure 3d). However, the XRD pattern is different from the one reported for the crystalline phase of Ni−Co−S.31 The new diffraction peaks at 49.1° and 72.6° are assigned to Ni3S2, whereas the peaks at 42.9° and 37.1° match with the data of Ni7S6.32,33 The weak one at 88.6° is usually observed in NiS.32 These results indicate that the electrodeposited Ni−Co−S-3 is amorphous in nature, and it decomposes at a temperature higher than 300 °C. Thermogravimetric analysis (TGA) also shows a mass loss due to the removal of S and formation of lower S content metal sulfide phases upon heating in an inert atmosphere (Figure S3). The chemical composition of the deposit was studied using EDXA and XPS. Figure 4a shows the EDXA of Ni−Co−S-3 on FTO, which identifies Ni, Co, S, and O as the major elements 19748


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Figure 4. EDXA of (a) Ni−Co−S-3/FTO, (b) Co−S/FTO, and (c) Ni−S/FTO. (d) SEM image and the corresponding X-ray maps for the distribution of (e) Ni, (f) Co, and (g) S in Ni−Co−S-3/FTO.

present. The other peaks of Sn and Si are from the glass substrate. Oxygen is from the substrate and the deposit. It is found that in the Ni−Co−S-3 deposit, the average atomic ratio of Co, Ni, and S is 3:2:6. Similarly, EDX spectra were also recorded for monometallic Co−S (Figure 4b) and Ni−S (Figure 4c), which confirm the formation of Co−S and Ni−S during electrochemical deposition. For Co−S, the atomic ratio between the metal ion and S is 0.6, whereas for Ni−S, it is around 6. The difference in the metal/S ratio could be due to the difference in the rate of formation of S2− in the presence of Ni2+ and Co2+ during electrochemical deposition. In general, crystalline Ni− Co−S exists as either Ni2CoS4 or NiCo2S4.31 By contrast, the phase diagrams of both Co−S and Ni−S are fairly complex, containing several different phases. The reported phases of Ni−S include NiS, NiS2, Ni3S2, Ni3S4, Ni7S6, Ni9S8, and so forth, whereas those of Co−S include CoS, CoS2, Co2S3, Co3S4, Co9S8, and so forth.34,35 Nevertheless, the chemical compositions of the electrodeposited Co−S, Ni−S, and Ni−Co−S do not match with the compositions of these crystalline forms, indicating the existence of more than one phase in the deposit. It was further intended to image the distribution of elements on the surface using X-ray maps. It is seen that in the case of Ni−Co−S-3 (Figure 4e−g), the constituent elements, Ni, Co, and S, are distributed uniformly. Elements can be found all around the surface, including along the walls and deep inside of the pores. As evident from the EDXA spectra, the ratio of Ni to Co ion in the electrodeposit is not the same as that in the deposition bath. For instance, Ni and Co ratio in the Ni−Co−S-3 material is 2:3 (Figure 4a), whereas the corresponding deposition electrolyte contains Ni and Co in the ratio of 3:2. Similar results were observed for other electrodes also. Hence, it was intended to make a correlation plot between the percent of each metal ion in the deposit against the percent of Ni in the electrolyte, considering the total Ni and Co ion concentration as 100%. Such a plot is helpful to predict the composition of the deposit from the composition of the electrolyte itself. Results based on EDXA are shown in Figure 5. Although not strictly linear, the percent of Ni or Co in the deposit is found to increase with an increase in the percent of the corresponding metal ion in the electrolyte. XPS spectra were also recorded to get additional information about the chemical state and surface composition of Ni−Co−S-

Figure 5. Correlation plot between the percentages of (i) Co and (ii) Ni in the electrodeposit against the percentage of Ni in the electrolyte.

3. Photoelectron peaks and Auger lines in the survey spectrum (Figure 6a) confirm the presence of Ni, Co, S, and O. Other peaks are due to the glass substrate and adventitious carbon. The high-resolution spectrum in the Ni 2p region (Figure 6b) shows a spin−orbit doublet and two satellite peaks. The peaks at 872.2 and 854.7 eV correspond to Ni 2p1/2 and 2p3/2, respectively.36 Similarly, the Co 2p spectrum (Figure 6c) shows peaks due to Co 2p1/2 and 2p3/2 at 796.1 and 780.2 eV, respectively.37,38 In the case of S 2p region (Figure 6d), the broad peak centered at 161 eV is due to S2− in the metal sulfide, whereas the peak in the higher energy side corresponds to the sulfur ion with a higher oxide state.39 It is found that the sulfates on the surface dissolve in the electrolyte under HER, leading to a reduced activity (Figure S4). Liu et al. proposed that the partial substitution of S with P can reduce the formation of sulfates.40 3.2. Electrocatalytic Activity toward HER. At first, several electrodes were made by the potentiodynamic deposition in the range of 0.15 to −1.25 V at 5 mV s−1 for five cycles. The deposition bath was 0.5 M aqueous solution of thiourea containing x mM Ni2+ + y mM Co2+ (x + y = 5 always). For instance, when x = 0, the electrolyte had 5 mM Co2+, and hence, pure Co−S was formed. Similarly, when x = 1, the electrolyte 19749


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Figure 6. XPS spectra of Ni−Co−S-3/FTO: (a) survey and high-resolution spectra of (b) Ni 2p, (c) Co 2p, and (d) S 2p.

Figure 7. (a) Linear sweep voltammograms (without iR compensation) at 1 mV s−1 in 1 M phosphate solutions (pH 7.4) using(i) Ni−S/FTO, (ii) Co− S/FTO, and (iii) Ni−Co−S-3/FTO electrodes. (b) Variation in the HER current density at (i) −0.50 and (ii) −0.40 V during LSV at 1 mV s−1 in 1 M phosphate solutions (pH 7.4) as a function of concentration of Ni2+ in the deposition bath. Electrode from 3 mM Ni2+ + 2 mM Co2+ + 0.5 M thiourea (Ni−Co−S-3/FTO) shows the best performance. The electrodes were prepared by five potential cycles.

contained 1 mM Ni2+ + 4 mM Co2+, and thus, Ni−Co−S-1 was deposited (Experimental Details). After deposition, the electrodes were thoroughly rinsed with a copious amount of water and dried at 100 °C in vacuum for 12 h. The HER activity was studied using linear sweep voltammetry (LSV) at 1 mV s−1 in 1 M potassium phosphate solution (pH 7.4). It is seen from Figure 7a that all electrodes exhibit high catalytic activity toward HER. In the case of Ni−S [Figure 7a(i)], hydrogen evolution starts at −0.23 V, and the current density reaches −6.2 mA cm−2 at −0.50

V. Similarly, pure Co−S [Figure 7a(ii)] shows the HER onset at −0.21 V, and a current density of −6.8 mA cm−2 is obtained at −0.50 V. Interestingly, Ni−Co−S-3 shows a better performance than both Ni−S and Co−S [Figure 7a(iii)]. In this case, hydrogen evolution commences at −0.18 V, and a high current density of −11.3 mA cm−2 is attained at 500 mV overpotential. This is almost 2 times higher than the current density obtained for pure Co−S and Ni−S under identical experimental conditions. Similar experiments were conducted for other 19750


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Figure 8. Variation in the HER current density of the Ni−Co−S-3/FTO electrode at −0.50 V during LSV at 1 mV s−1 in 1 M phosphate solutions (pH 7.4) as a function of (a) the number of deposition cycles and (b) the heating temperature. In all cases, electrodes were deposited at 5 mV s−1 in 0.15 to −1.25 V vs SCE from 3 mM Ni2+ + 2 mM Co2+ + 0.5 M thiourea solution. The electrodes were prepared by 10 consecutive potential cycles for temperature studies.

Figure 9. (a) Linear sweep voltammograms (after iR compensation) of the Ni−Co−S-3/FTO electrode (10 cycles of deposition) at 1 mV s−1 in 1 M phosphate solutions (pH 7.4) and (b) the corresponding Tafel plot in the linear region.

presence of Ni3+ and Co3+ in the Ni−Co−S deposit will lead to higher conductivity and better electrochemical performance in comparison with Ni−S and Co−S deposits. Electrocatalytic activity depends on the number of active sites on the surface. Hence, it is anticipated that the HER current density can be improved by increasing the loading level. To study the effect of loading level, Ni−Co−S-3 was deposited on FTO at different potential cycles. After deposition, the electrodes were cleaned, dried in vacuum at 100 °C, and finally subjected to LSV at 1 mV s−1 in 1 M phosphate solution (pH 7.4). The current density obtained at −0.50 V is plotted against the number of potential cycles of deposition, as shown in Figure 8a. It is seen from Figure 8a that the current density of −8.8 mA cm−2 is obtained for two cycles. It increases to −11.3 mA cm−2 and −15 mA cm−2 after five and seven cycles of deposition, respectively. The highest activity is obtained after 10 cycles of deposition, and the HER current density at −0.50 V is −16 mA cm−2. Further increase in the thickness causes a gradual fall in the activity. The current density values of −12.2 and −11.1 mA cm−2 are obtained after 15 and 20 cycles, respectively. This could be due to an increase in the electronic resistance of the film at higher thickness. Also, a compact layer of the catalyst blocks the diffusion of the electrolyte across the electrode material, resulting in a decreased accessible area for the catalysis. Thus, the performance of the electrode prepared by 10 potential cycles at 5

electrodes (Ni−Co−S-1, Ni−Co−S-2, etc.), and the results are summarized in Figure 7b. Figure 7b(i) shows the variation in the HER current density at −0.50 V during LSV at 1 mV s−1 in 1 M phosphate solution (pH 7.4) as a function of concentration of Ni2+ in the electrolyte during deposition. As stated above, the current density of −6.2 mA cm−2 is obtained for the electrode deposited at 5 mM Ni2+. It gradually increases to −9.8 mA cm−2 for Ni−Co−S-4 and attains a maximum of −11.3 mA cm−2 for Ni−Co−S-3. Further decrease in the concentration of Ni2+ causes a decline in the HER performance. As evident from the curve, the current density falls to −8.5 mA cm−2 for Ni−Co−S-2 and then to −7.8 mA cm−2 for Ni−Co−S-1. However, these values are still higher than −6.8 mA cm−2 obtained for pure Co− S. A similar trend was also observed for other potentials as well. For example, Figure 7b(ii) shows similar results at −0.40 V. Here also, Ni−Co−S-3 exhibits a superior catalytic activity compared with both Co−S and Ni−S. The superior HER catalytic activity of Ni−Co−S to Ni−S and Co−S is attributed to its higher electronic conductivity due to fast electron hopping between the metal cations with mixed valences and richer redox chemistry due to the presence of both Ni and Co in different oxidation states.41 Recently, the higher activity of bimetallic sulfide has also been explained by the concept of self-doping.42 Accordingly, Ni3+ in the catalyst will provide extra electrons as n-type doping, whereas Co2+ will result in extra holes as p-type doping. Therefore, the 19751


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ACS Applied Materials & Interfaces mV s−1 in the potential range of 0.15 to −1.25 V versus SCE is the optimum. The best electrodes prepared by 10 potential cycles were heated at different temperatures in a N2 atmosphere, and then, the HER activity was studied using LSV. Variation in the current density obtained at −0.50 V is plotted against the calcination temperature (Figure 8b). The HER current density of −16 mA cm−2 is obtained for the electrode heated at 100 °C. On increasing the temperature to 300 °C, the current density slightly falls to −14.6 mA cm−2. However, further heating of the electrode above 400 °C causes an abrupt fall in the HER current density. Current density values as low as −0.7 and −0.2 mA cm−2 are obtained after heating at 500 and 700 °C, respectively. On heating at high temperatures, probably Ni−Co−S is decomposed to various monometallic sulfides of Ni and Co. This is in accordance with the XRD results (Figure 3). Also, heating reduces the active sites because of morphological changes. Tafel polarization experiment was performed for gaining insight into the kinetics and mechanism of the HER on Ni−Co− S-3/FTO electrodes in a neutral phosphate solution. The electrodes were prepared by 10 potential cycles in 0.15 to −1.25 V range at 5 mV s−1 in an aqueous solution of 0.5 M thiourea containing 3 mM Ni2+ + 2 mM Co2+. The electrodes were rinsed with water, dried in vacuum at 100 °C, and subjected to steadystate LSV at 1 mV s−1 under stirring conditions. iR correction was applied to account for the potential drop across the working and reference electrodes because of internal resistance of the cell. The resultant voltammogram is shown in Figure 9a. The catalytic current increases at an overpotential of 150 mV with a concomitant evolution of H2 bubbles from the electrode surface. To reach the current densities of 10 and 20 mA cm−2, the catalyst requires overpotentials of 280 and 300 mV, respectively. These values are comparable with the performance of other similar earth-abundant heterogeneous catalysts in neutral solution.17,18 To account for the mechanism of the HER, Tafel plot is carefully analyzed. It is seen in Figure 9b that there are two Tafel regions, indicating two different mechanisms for the HER on Ni−Co−S3/FTO. In the lower overpotential region, a Tafel slope of 93 mV dec−1 is obtained. This value is in agreement with the reported Tafel slope on the Co−S/FTO electrode.19 On the other hand, in the higher overpotential zone, a Tafel slope of 70 mV dec−1 is observed, which is close to the Tafel slope observed for the electrodeposited Ni−S/FTO electrode.20 From these observations, it is concluded that there are two different potential dependent mechanisms for the HER on Ni−Co−S-3/FTO in the neutral phosphate solution. In the low overpotential region, the reaction is primarily governed by Co sites of the Ni−Co−S deposit, whereas at higher overpotential, the reaction proceeds mainly on Ni sites. Thus, the two different Tafel slopes are due to the presence of two types of catalytic active sites (Ni and Co sites) in the catalyst film, which contribute selectively toward the HER in different potential regions. The generally accepted mechanism of HER in acidic media involves43

or Tafel reaction: Hads + Hads → H 2 Tafel slope = 2.3RT /2F = 30 mV

The Tafel slopes obtained in the present case (93 and 70 mV dec−1) do not match with any of the above three steps. However, these values are comparable with the values reported for electrodeposited Co−S,19 Ni−S,20 and other amorphous catalysts such as Cu2MoS4,24 and MoS3.26 Significant deviation from the Tafel slopes predicted according to the above reaction scheme signifies the complexity of the HER mechanism on these amorphous catalysts. The superior HER activity of Ni−Co−S-3/FTO over Ni−S/ FTO and Co−S/FTO is further evident from the EIS measurements. Nyquist plots during the HER at −0.20 V in 1 M phosphate electrolytes (pH 7.4) are shown in Figure 10. In all

Figure 10. Nyquist plots at −0.20 V vs RHE for (i) Ni−Co−S-3/FTO, (ii) Co−S/FTO, and (iii) Ni−S/FTO electrodes in 1 M phosphate solution (pH 7.4). Electrodes were prepared by five potential cycles.

cases, two semicircles are seen. The corresponding electrical equivalent circuit is shown in the inset of Figure 10. The electrolyte resistance is indicated by Rs. Rf and Qf denote the resistance and constant phase elements (CPE) of the deposit, whereas Rct and Qdl represent the charge transfer resistance of HER and the double-layer capacitance, respectively. During the curve fitting, the capacitive elements are replaced by CPE, denoted by Q to account for the surface nonuniformity and porous structure.44 The values of the parameters obtained by fitting the impedance spectra are given in Table 1. Particularly, the Rct value obtained for the Ni−Co−S-3/FTO electrode is 104 Ω, which is smaller than the Rct values obtained for Ni−S/FTO (259 Ω) and Co−S/FTO (153 Ω) electrodes. Because the electrocatalytic activity and the charge transfer resistance are inversely related, it predicts a higher HER catalytic activity for the Ni−Co−S-3/FTO electrode in comparison with others. These results are well in agreement with the voltammetric studies. Stability of the electrode is a major concern for practical applications. Therefore, the Ni−Co−S-3/FTO electrode was subjected to a long-term controlled potential electrolysis at 200 mV overpotential. During this experiment, vigorous gas evolution was visually observed from the electrode surface. The resulting current density is plotted against the electrolysis time in Figure 11a. The HER current is almost stable at around 1 mA cm−2 throughout the electrolysis. No significant change in the electrode performance was observed. This implies a high

Volmer reaction: H3O+ + e− → Hads + H 2O Tafel slope = 2.3RT / αF = 120 mV

(9)

(7)

Heyrovsky reaction: Hads + H3O+ + e− → H 2 + H 2O Tafel slope = 2.3RT /(1 + α)F = 40 mV (8) 19752


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ACS Applied Materials & Interfaces Table 1. Impedance Parameters Ni−Co−S-3

Ni−S

Co−S

parameters

value

error (%)

value

error (%)

value

error (%)

Rs (Ω) Rf (Ω) Qf − Y0 (F sn−1) Qf − n Rct (Ω) Qdl − Y0 (F sn−1) Qdl − n

1.9 36 1.7 × 10−7 0.92 104 0.003 0.79

3.9 10.1 2.1 3.9 2.4 1.2 1.4

2.1 24 2.3 × 10−7 0.90 259 0.0006 0.80

3.1 15.1 1.6 1.9 1.8 0.5 0.8

1.7 17 2.8 × 10−7 0.92 154 0.003 0.85

2.4 14.3 2.5 5 1.5 0.6 1.2

Figure 11. (a) Chronoamperogram at −0.20 V and (b) volume of H2 produced as a function of time during electrolysis at −0.50 V, using Ni−Co−S-3/ FTO electrodes in 1 M phosphate solution (pH 7.4).

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stability of the catalyst film during the HER in the neutral phosphate solution. It was also intended to quantify the amount of H2 gas evolved and to calculate the faradaic efficiency. For this, Ni−Co−S-3 was deposited under optimized conditions, and electrolysis was carried out at a relatively high overpotential (500 mV). The evolved H2 was quantitatively measured by water displacement in an inverted burette. As evident from Figure 11b, the volume of H2 collected in the burette varies linearly with time, indicating a constant rate of hydrogen evolution and a high stability of the electrocatalyst. After 110 min, the amount of H2 produced is 180 μmol, which theoretically corresponds to 34.7 C. The actual charge consumed during the reaction is 36.5 C, suggesting a faradic efficiency close to 95%.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b15399. Cyclic voltammograms during the deposition of Co−S, Ni−S, and Ni−Co−S; morphology of all Ni−Co−S catalysts; TGA of Ni−Co−S-3; and study on the effect of catalyst storage (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +91-80-22933183. ORCID

4. CONCLUSIONS

Ahamed Irshad: 0000-0001-7107-9623

Novel Ni−Co−S amorphous films are prepared by potentiodynamic deposition and are studied as catalysts for the HER in a neutral phosphate solution. The composition of the deposit and the electrocatalytic activity are tuned by changing the ratio of the concentrations of Ni2+ and Co2+ ions in the electrolyte. The catalyst film deposited from an aqueous solution containing 3 mM Ni2+ + 2 mM Co2+ + 0.5 M thiourea, namely, Ni−Co−S-3, provides the best performance and is found to be superior to its monometallic sulfides. This catalyst requires an overpotential of 150 mV for the HER onset, and a current density of 10 mA cm−2 is obtained at 280 mV overpotential. Two different Tafel slopes (93 and 70 mV dec−1) are observed, indicating two different potential-dependent HER mechanisms on the Ni−Co−S film. It is proposed that there are two types of active sites in the catalyst, and they contribute selectively toward the HER in different overpotential regions.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the CSIR, India for the financial support and the CeNSe at IISc, Bangalore for the instrumental facilities.

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Electrodeposited Nickel-Cobalt-Sulfide Catalyst for the Hydrogen

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