Electrodeposited NickelâCobaltâSulfide Catalyst for the Hydrogen

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Electrodeposited Nickel-Cobalt-Sulfide Catalyst for the Hydrogen Evolution Reaction Ahamed Irshad, and Nookala Munichandraiah ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 17 May 2017 Downloaded from http://pubs.acs.org on May 17, 2017

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ACS Applied Materials & Interfaces

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

- - - - - - - - - - - - - - - - - - - - - - - - -- - - - - - - - - - - - - - - - - - - - - - - - - - - - - * To whom correspondence should be addressed E-mail: [email protected]; Tel: +91-80-2293 3183 1 ACS Paragon Plus Environment


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Abstract A novel Ni-Co-S based material is prepared by potentiodynamic deposition from an aqueous solution containing Ni2+, Co2+ and thiourea, and is studied as an electrocatalyst for the hydrogen evolution reaction (HER) in neutral phosphate solution. The composition of the catalyst and HER activity are tuned by varying the ratio of 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 counter parts. 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 of two types of catalytic active sites and they contribute selectively towards HER in different potential regions.

Keywords: Electrochemical deposition, electrolysis of water, hydrogen generation, amorphous catalysts, nickel cobalt sulfide, bimetallic catalysts, HER.

2 ACS Paragon Plus Environment

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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 feed stock in petroleum refining industry and ammonia synthesis for fertilizers.2 Currently, H2 is produced in a large scale by steam-methane reforming method, CH4(g) + H2O(g)

CO(g) + 3 H2(g)

∆H298 = -206 kJ mol-1

(1)

CO(g) + H2O(g)

CO2(g) + H2 (g)

∆H298 = -41 kJ mol-1

(2)

As evident from the above chemical reactions, this process involves high energy (heat) input and releases huge amount of carbon dioxide into the atmosphere. Hence, it is not considered as a favorable green method of H2 production.3 In 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 large scale production of H2. Consequently, identifying and designing 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 etc. There are catalyst materials available for the HER in neutral electrolytes also.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, synthesis procedure for the catalyst must be easy, scalable and environmental friendly. In this context, an electrodeposited cobalt sulfide (Co-S) was reported as a highly active HER catalyst



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in neutral media.19 The catalyst exhibited a remarkable performance in terms of the onset potential (< 43 mV) and stability over 40 h. Following this report, an analogous electrodeposited Ni-S was also proposed.20 Ni is preferred owing to its low cost, high abundance and less toxicity compared to Co. However, catalytic activity of Ni-S is slightly inferior to Co-S in terms of the onset potential (> 100 mV) and current density at 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 Tafel slope. Among various approaches for improving the electrocatalytic activity, formation of bimetallic structure is interesting. It is observed that the bimetallic catalyst systems exhibit better performance than its individual metal counter parts.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) 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 at 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-WSx27 and Zn0.3Co2.7S4.28 The present work describes the electrochemical deposition of a mixed Ni-Co-S film, physical and electrochemical characterizations, and its application as a catalyst for the HER in 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.


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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 of phosphate solution was maintained at 7.4, unless otherwise stated. Fluorine doped tin oxide (FTO) glass (Technistro, TISXY 004, sheet resistance < 15 ohm 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 conductive silver paste. The unexposed area of the electrode was masked by Teflon tape. Two large Pt foil auxiliary electrodes on either side of the working electrode and a calibrated saturated calomel reference electrode (SCE) were used. 2.2. Physical characterizations and electrochemical studies The physical characterization studies of the electrodeposited materials were carried out by 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 EDX analysis, Ultra 55 scanning electron microscope equipped with EDXA system was used. TEM images were obtained using a FEI Tecnai T-20 transmission electron microscope at 200 kV. Powder X-ray diffraction (XRD) patterns were recorded in a Bruker D8 diffractometer using Cu Kα radiation. The surface chemical compositions of the materials were analyzed by X-ray photoelectron spectroscopy (XPS) using a SPECS GmbH spectrometer (Phoibos 100 MCD Energy Analyzer) with Mg Kα radiation (1253.6 eV). The peak of C1s at 284.6 eV was used as the reference energy position.



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Electrochemical experiments were carried out using 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 the potential values for H2 evolution studies are reported against RHE reference, whereas those of depositions are against SCE 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±1ºC. Current density values are reported on the basis of geometrical area of the electrode. 2.3. Electrochemical deposition of Ni-Co-S films Electrochemical deposition was carried out by 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 iso-propanol. Subsequently, they were dried in N2 flow and stored under vacuum at room temperature. Electrodeposition was carried out by repeated potential cycling between 0.15 and -1.25 V vs. SCE at 5 mV s-1. N2 gas was purged through the electrolyte 30 min before the deposition and maintained the flow throughout the deposition process. 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-S-2, 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 copious amount of water and dried overnight under vacuum. To study the effect of calcination, electrodes were heated at different temperatures ranging from 100 to 700 ºC for 3 h in N2 atmosphere. The prepared electrodes were stored in a desiccator at room temperature.


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3. RESULTS AND DISCUSSION 3.1. Electrochemical deposition and characterization Cyclic voltammograms of a 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 Fig. 1. 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 equation (3).29

(3)

The peak current increases on repeated cycling. During this process, the exposed area of the working electrode becomes grey initially and turns black after several cycles. The overall reactions involved in the deposition of Ni-Co-S film are given below.30 2 H2O + 2 e-

2 OH- + H2

(4)

SC(NH2)2 + 2 OH-

S2- + OC(NH2)2 + H2O

(5)

Ni2+ + Co2+ + 2 S2-

NiCoS2

(6)

The cyclic voltammograms during the depositions of Ni-S and Co-S also exhibit similar features but with different rates of thiourea decomposition (Fig. S1 of the supporting information). The composition of the 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. 7 ACS Paragon Plus Environment

Current density / mA cm

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-2


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1.0

0.5

0.0

-0.5

-1.0

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

Potential / V vs. SCE

Figure 1: Potentiodynamic scans (10 cycles) at 5 mV s-1 during the deposition of Ni-Co-S film on FTO electrode from 2.5 mM CoCl2 + 2.5 mM NiCl2 + 0.5 M thiourea solution. The surface morphology of the electrodeposit was studied by SEM and TEM (Fig. 2). SEM image of Ni-Co-S-3 film on FTO (Fig. 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 (Fig. 2b), a dense array of nanosheets that are interconnected to form islands are seen. Porosity is expected due to interlocking of the nanosheets. In contrast, Ni-S film exhibits a totally different morphology (Fig. 2c) with small particles as well as lumps. It is expected that Ni-S deposits as tiny particles initially and then they grow gradually to form micro sized 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 (Fig. S2). To further study the morphology of Ni-Co-S-3 deposit, TEM analysis was carried out. TEM image of Ni-Co-S-3 (Fig. 2d) shows that the material preserves the interwoven features of the honey comb structure. However, a slight 8 ACS Paragon Plus Environment

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difference in the morphology compared to the SEM image could be due to structural collapse caused by sonication during sample preparation. Lattice fringes of Ni-Co-S are seen in the high resolution images (Fig. 2e). Lack of well-arranged diffraction spots and appearance of diffused circles in the SAED pattern (Fig. 2f) suggest poor crystallinity of electrodeposited Ni-Co-S-3.

[a]

[b]

[c]

[d]

[e]

[f]

Figure 2: SEM images of (a) Ni-Co-S-3/FTO, (b) Co-S/FTO and (c) Ni-S/FTO. (d) TEM image, (e) HR-TEM image and (f) SAED pattern of Ni-Co-S-3 deposit. Powder XRD patterns were recorded for the as-deposited Ni-Co-S-3 film, and also after heating at different temperatures in inert atmosphere. Fig. 3a shows the XRD pattern for the conducting side of a clean FTO glass. It is seen from Fig. 3b that the pattern of the as-deposited Ni-Co-S-3 shows only the peaks from FTO substrate and no other peaks from the electrodeposit are present. It indicates the amorphous nature of the electrodeposited Ni-Co-S. However, after heating at 300 ºC, additional peaks emerge at 72.6, 49.1 and 88.5º (Fig. 3c). Further heating to 500 ºC enhances the intensity of these peaks (Fig. 3d). Moreover, a new shoulder peak at 37.1º and another triplet centered at 42.9º are also identified in the spectrum (Fig. 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 72.6º are assigned to Ni3S2 whereas the peaks at 42.9 and 37.1º



match with the data of Ni7S6.32,33 The weak one at 88.6º 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 ºC. TGA analysis also shows mass loss due to removal of S and formation of lower S content metal sulfide phases upon heating in inert atmosphere (Fig. S3).

Intensity / a.u

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(d) (c) (b) (a) 20

30

40

50

60

70

80

90

2θ / deg

Figure 3: XRD pattern of (a) FTO, (b) as-deposited Ni-Co-S-3/FTO and Ni-Co-S-3/FTO heated at (c) 300 ºC and (d) 500 ºC. Samples were heated in N2 atmosphere for 3 h. Chemical composition of the deposit was studies using EDXA and XPS. Fig. 4a shows the EDXA of Ni-Co-S-3 on FTO, which identifies Ni, Co, S and O as the major elements present. The other peaks of Sn and Si are from the glass substrate. Oxygen is from the substrate as well as the deposit. It is found that in the Ni-Co-S-3 deposit, the average atomic ratio of Co:Ni:S is 3:2:6. Similarly, EDX spectra was also recorded for monometallic Co-S (Fig. 4b) and Ni-S (Fig. 4c), which confirm the formation of Co-S and Ni-S during electrochemical deposition. In the case of Co-S, the atomic ratio between metal ion and S is 0.6 while that in the case of Ni-S is around 6. The difference in metal/S ratio could be due to 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 In contrast, the phase diagrams of both Co-S and


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Ni-S are fairly complex containing several different phases. The reported phases of Ni-S include NiS, NiS2, Ni3S2, Ni3S4, Ni7S6, Ni9S8, etc. whereas those of Co-S are CoS, CoS2, Co2S3, Co3S4, Co9S8, etc.34,35 Nevertheless, the chemical compositions of the electrodeposited Co-S, Ni-S and Ni-Co-S do not match with 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-CoS-3 (Fig. 4e - g), the constituent elements, Ni, Co and S are distributed uniformly. Elements can be found all around the surface, including along the walls as well as deep inside the pores.

[a]

[d]

[c]

[b]

[e]

[f]

[g]

Figure 4: EDXA of (a) Ni-Co-S-3/FTO, (b) Co-S/FTO and (c) Ni-S/FTO. (d) SEM image, and corresponding X-ray maps for the distribution of (e) Ni, (f) Co and (g) S in Ni-Co-S-3/FTO. As evident from 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 (Fig. 4a) whereas the corresponding deposition electrolyte contains Ni and Co in a ratio of 3:2. Similar results were observed for other electrodes also. Hence, it was intended to make a correlation plot between the % of each metal ion in the deposit against the % of Ni in the 11 ACS Paragon Plus Environment


electrolyte, considering 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 Fig. 5. Although not strictly linear, the % of Ni or Co in the deposit is found to increase with increase in the % of corresponding metal ion in the electrolyte. 100

100

(ii)

(i)

80

80

60

60

40

40

20

20

0

0 0

20

40

60

80

% of Co in the deposit

% of Ni in the deposit

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100

% of Ni in the electrolyte

Figure 5: Correlation plot between the percentages of (i) Co and (ii) Ni in the electrodeposit against the percentage of Ni in the electrolyte. XPS spectra were also recorded to get additional information about the chemical state and surface composition of the Ni-Co-S-3. Photoelectron peaks and Auger lines in the survey spectrum (Fig. 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 Ni 2p region (Fig. 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, Co 2p spectrum (Fig. 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 (Fig. 6d), the broad peak centered at 161 eV is due to S2- in the metal sulfide, while the peak in the higher energy side corresponds to the sulfur ion with a higher oxide state.39 It is found that the sulfates 12 ACS Paragon Plus Environment

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at the surface dissolve in the electrolyte under HER leading to a reduced activity (Fig. S4). Liu et al., proposed that the partial substitution of S with P can reduce the formation of sulfates.40

[a]

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. 3.2. Electrocatalytic activity towards HER At first, several electrodes were made by potentiodynamic deposition in the range of 0.15 to -1.25 V at 5 mV s-1 for 5 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, electrolyte had 5 mM Co2+ and hence pure Co-S was formed. Similarly, when x=1, the electrolyte contained 1 mM Ni2+ + 4 mM Co2+ and thus Ni-Co-S-1 was deposited (experimental section). After deposition, electrodes were thoroughly rinsed with copious amount of water and



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dried at 100 ºC in vacuum for 12 h. HER activity was studied by linear sweep voltammetry (LSV) at 1 mV s-1 in 1 M potassium phosphate solution (pH 7.4). It is seen from Fig. 7a that all the electrodes exhibit high catalytic activity towards HER. In the case of Ni-S (Fig. 7a(i)), hydrogen evolution starts at -0.23 V and current reaches -6.2 mA cm-2 at -0.50 V. Similarly, pure Co-S (Fig. 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 (Fig. 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 two times higher than the current obtained in the case of pure Co-S and Ni-S under identical experimental conditions. Similar experiments were conducted in the case of other electrodes (Ni-Co-S-1, Ni-Co-S-2, etc.) and the results are summarized in Fig. 7b. Fig. 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 the function of concentration of Ni2+ in the electrolyte during the deposition. As stated above, 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 in the case of Ni-Co-S-4 and attains maximum of -11.3 mA cm-2 for Ni-Co-S-3. Further decreasing the concentration of Ni2+, cause a decline in the HER performance. As evident from the curve, current 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 in the case of pure Co-S. Similar trend was also observed for other potentials as well. For example, Fig. 7b(ii) shows the similar results at -0.40 V. Here also, Ni-Co-S-3 exhibits superior catalytic activity compared to 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


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presence of both Ni and Co in different oxidation states.41 Recently, higher activity of bimetallic sulfide is also explained by the concept of self-doping.42 Accordingly, Ni3+ in the catalyst will provide extra electrons as n-type doping while Co2+ will result in extra holes as p-type doping. Therefore, 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.

[a]

[b]

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 V and (ii) -0.40 V during LSV at 1 mV s-1 in 1 M phosphate solutions (pH 7.4) as the function of concentrations 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. Electrodes were prepared by five potential cycles. 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, electrodes were cleaned, dried in vacuum at 100 ºC and finally subjected to LSV at 1 mV s-1 in 1 M phosphate solution (pH 7.4). The current obtained at -0.50 V is plotted against the number of potential cycles of deposition in Fig. 8a. It is seen from Fig. 8a that the current density of -8.8 mA cm-2 is obtained for 2 cycles. It increases to -11.3 mA cm-2 and



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-15 mA cm-2 after 5 and 7 cycles of deposition, respectively. Highest activity is obtained for 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. Current density values of -12.2 mA cm-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 electrode prepared by 10 potential cycling at 5 mV s-1 in the potential range of 0.15 to -1.25 V vs. SCE is the optimum. The best electrodes prepared by 10 potential cycles were heated at different temperatures in N2 atmosphere and then the HER activity was studied by LSV. Variation in the current density obtained at -0.50 V is plotted against the calcination temperature (Fig. 8b). The HER current density of -16 mA cm-2 is obtained for the electrode heated at 100 ºC. On increasing the temperature to 300 ºC, current density slightly falls to -14.6 mA cm-2. However, further heating the electrode above 400 ºC causes an abrupt fall in the HER current density. Current density values as low as -0.7 mA cm-2 and -0.2 mA cm-2 are obtained after heating at 500 ºC and 700 ºC, 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 XRD results (Fig. 3). Also, heating reduces the active sites due to morphological changes.


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[a]

[b]

-15 -2

-15


-2

-16


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-14 -13 -12 -11 -10 -9

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300

400

500

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Figure 8: Variation in the HER current density of 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 the function of (a) number of deposition cycles and (b) heating temperature. In all the cases, electrodes were deposited at 5 mV s-1 in 0.15 V to -1.25 V vs. SCE from 3 mM Ni2+ + 2 mM Co2+ + 0.5 M thiourea solution. Electrodes were prepared by 10 consecutive potential cycling for temperature studies. Tafel polarization experiment was performed for gaining insight into the kinetics and mechanism of HER on Ni-Co-S-3/FTO electrodes in neutral phosphate solution. 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+. Electrodes were rinsed with water, dried in vacuum at 100 ºC and subjected to steady state linear sweep voltammetry at 1 mV s-1 under stirring condition. IR correction was applied to account for the potential drop across the working and reference electrodes due to internal resistance of the cell. The resultant voltammogram is shown in Fig. 9a. The catalytic current increases at an overpotential of 150 mV with concomitant evolution of H2 bubbles from the electrode surface. To reach the current densities of 10 mA cm-2 and 20 mA cm-2, the catalyst requires overpotentials of 280 mV 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 HER, Tafel plot is carefully analyzed. It is seen in Fig. 9b that there are two Tafel regions, indicating two different mechanisms for the HER on Ni-Co-S-3/FTO. In the lower overpotential region, a Tafel slope of 17 ACS Paragon Plus Environment


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93 mV dec-1 is obtained. This value is in agreement with the reported Tafel slope on 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 neutral phosphate solution. In the low overpotential region, reaction is primarily governed by Co sites of Ni-Co-S deposit, whereas at higher overpotential, reaction proceeds mainly on Ni sites. Thus, 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 towards HER in different potential regions. The generally accepted mechanism of HER in acidic media involves,43 Volmer reaction: H3O+ + e-

Hads + H2O

(7)

Tafel slope = 2.3RT/αF = 120 mV Heyrovsky reaction: Hads + H3O+ + e-

H2 + H2O

(8)

Tafel slope = 2.3RT/(1+α)F = 40 mV or Tafel reaction: Hads + Hads

H2

(9)

Tafel slope = 2.3RT/2F = 30 mV The Tafel slopes obtained in the present case (93 mV dec-1 and 70 mV dec-1) do not match with any of the above threes steps. However, these values are comparable with the values reported for electrodeposited Co-S19, Ni-S20 and other amorphous catalysts such as Cu2MoS4,24 MoS3,26 etc. Significant deviation from the Tafel slopes predicted according the above reaction scheme signifies the complexity of the HER mechanism on theses amorphous catalysts.


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Figure 9: (a). Linear sweep voltammogram (after iR compensation) of 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) corresponding Tafel plot in the linear region. The superior HER activity of Ni-Co-S-3/FTO over Ni-S/FTO and Co-S/FTO is further evident from electrochemical impedance spectroscopy (EIS) measurements. Nyquist plots during HER at -0.20 V in 1 M phosphate electrolytes (pH 7.4) are shown in Fig. 10. In all the cases, two semicircles are seen. The corresponding electrical equivalent circuit is shown in the inset of Fig. 10. The electrolyte resistance is indicated by Rs. Rf and Qf denote the resistance and CPE of the deposit whereas Rct and Qdl represent the charge transfer resistance of HER and double layer capacitance, respectively. During the curve fitting, capacitive elements are replaced by constant phase elements (CPE), denoted by Q to account for the surface non-uniformity and porous structure.44 The values of parameters obtained by fitting the impedance spectra are given in Table 1. Particularly, the Rct value obtained for 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. Since the electrocatalytic activity and charge transfer resistance are inversely related, it predicts a higher HER catalytic activity for Ni-Co-S-3/FTO electrode in comparison with others. These results are well in agreement with the voltammetric studies.



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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.

Table 1: Impedance parameters Stability of the electrode is a major concern for practical application. Therefore, 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 Fig. 11a. The HER current is almost stable at around 1 mA cm-2 throughout the electrolysis. 20 ACS Paragon Plus Environment

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No significant change in the electrode performance was observed. This implies a high stability of the catalyst film during HER in neutral phosphate solution. It was also intended to quantify the amount of H2 gas evolved and calculate the Faradaic efficiency. For this Ni-Co-S-3 was deposited under the 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 Fig. 11b, 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 %.

-2

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[a]

180

[b]

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H2 produced / µ mol


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120

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60

30

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40

60

80

100

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Time / min

Figure 11: (a). Chronoamperogram at -0.20 V and (b) volume of H2 produced as the function of time during electrolysis at -0.50 V, using Ni-Co-S-3/FTO electrodes in 1 M phosphate solution (pH 7.4). 4. CONCLUSIONS Novel Ni-Co-S amorphous films are prepared by potentiodynamic deposition, and are studied as catalysts for the hydrogen evolution reaction (HER) in neutral phosphate solution. The composition of the deposit and the electrocatalytic activity are tuned by changing the ratio of concentrations of Ni2+ and Co2+ ions in the electrolyte. The catalyst film deposited from aqueous 21 ACS Paragon Plus Environment


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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 mAcm-2 is obtained at 280 mV overpotential. Two different Tafel slopes (93 mV dec-1 and 70 mV dec-1) are observed, indicating two different potential dependent HER mechanisms on Ni-Co-S film. It is proposed that there are two types of active sites in the catalyst and they contribute selectively towards HER in different overpotential regions. ASSOCIATED CONTENT Supporting Information: Cyclic voltammograms during the deposition of Co-S, Ni-S and Ni-Co-S (Fig. S1), Morphology of all Ni-Co-S catalysts (Fig. S2), TGA of Ni-Co-S-3 (Fig. S3) and study on the effect of catalyst storage (Fig. S4). This material is available free of charge via the internet at http:// pubs.acs.org AUTHOR INFORMATION Corresponding Author * E-mail: [email protected], Tel: +91-80-22933183 NOTES These authors declare no competing financial interest ACKNOWLEDGMENTS The authors thank CSIR, India for the financial support and CeNSe at IISc, Bangalore for the instrumental facilities. REFERENCES (1). Lubitz, W.; Tumas, W. Hydrogen: An Overview. Chem. Rev. 2007, 107, 3900 - 3903. (2). Ball, M.; M. Weeda. The Hydrogen Economy Vision or Reality. Int. J. Hydrogen Energy 2015, 40, 7903 - 7919.


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Graphic for the manuscript (TOC graphic):


Electrodeposited NickelâCobaltâSulfide Catalyst for the Hydrogen

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