Bimetallic transition metal chalcogenide nanowire array: An effective catalyst for overall water splitting

Electrochimica Acta 318 (2019) 901e912

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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Bimetallic transition metal chalcogenide nanowire array: An effective catalyst for overall water splitting Kartick Chandra Majhi*, Paramita Karfa, Rashmi Madhuri Department of Applied Chemistry, Indian Institute of Technology (Indian School of Mines), Dhanbad, Jharkhand, 826 004, India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 6 March 2019 Received in revised form 17 June 2019 Accepted 17 June 2019 Available online 25 June 2019

The development of highly active, durable, and inexpensive bifunctional catalysts towards both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is one of the most challenging tasks in the term of renewable energy. Keeping in mind the current scenario, herein, we have synthesized bimetallic transition metal chalcogenides (CoTe2@CdTe and CoSe2@CdSe) by a very easy, single step hydrothermal process. During the synthesis, two different morphologies (i.e. spherical and wire) were obtained depending on the selection of chalcogen. It was found that Te based nanocomposite i.e. CoTe2@CdTe showed a symmetrical nanowire array morphology with a high electrocatalytic surface area and good electrocatalytic activity towards both HER and OER. The structural, morphological and electrochemical features of synthesized nanocomposites were characterized by various useful techniques (like field emission scanning electron microscopy, high resolution transmission electron microscopy, powder X-ray diffraction analysis etc.) to confirm their successful synthesis. The CoTe2@CdTe nanowire array has shown a small onset potential value, high current density, low Tafel slope value along with high stability towards overall water splitting. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Nanowire array Bifunctional electrocatalysts OER HER Overall water splitting

1. Introduction

OER at anode : 4OH / O2 þ 2H2 O þ 4e

Presently, tremendous increase in the population over the whole world and constant consumption of fossil fuels has drawn our attention towards search of new and suitable energy generating as well as storage techniques/methods [1]. In recent time, electrolysis of water or water splitting by electrochemical means has become very popular to solve this problem producing hydrogen (H2) as a green energy carrier along with oxygen (O2) as by-product. The electrolysis of water by the electrochemical process possesses two reactions i.e. oxygen evolution reaction (OER) occurred at anode and hydrogen evolution reaction (HER) at cathode. At the cathode, the water molecule splits and produces hydrogen and hydroxyl ions. These hydroxyl ions move to the anode, resulting in formation of oxygen and water [2]. The overall water splitting can be represented by two reactions (shown below): 1. Basic medium:

HER at cathode : 2H2 O þ 2e / H2 þ 2OH

* Corresponding author. E-mail address: [email protected] (K. Chandra Majhi). https://doi.org/10.1016/j.electacta.2019.06.106 0013-4686/© 2019 Elsevier Ltd. All rights reserved.

(1)

(2)

2. Acidic medium:

HER at cathode compartment : 2Hþ þ 2e / H2

(3)

OER at anode compartment : 2H2 O / O2 þ 4Hþ þ 4e

(4)

The reactions look very simple at first sight, but splitting of water is actually a sluggish process and thermodynamically unfavorable. For example, for the OER, four electrons are required to form one oxygen molecule, which means OeH bond breaking followed by the formation of OeO bond demands large amounts of energy, making this step kinetically sluggish [3]. Thermodynamically and kinetically unfavored steps involved in overall water splitting are the major drawbacks for large-scale production of hydrogen from water by electrolysis methods. Therefore, in order to accelerate the overall water splitting through electrochemical processes, efficient electrocatalysts are required, which can complete the reaction with low energy requirement. Noble metal based electrocatalysts like iridium dioxide (IrO2), Pt/C and ruthenium dioxide (RuO2) are the benchmark

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electrocatalysts for HER and OER [4]. However, owing to the low abundance and high cost, these catalysts have now tended toward replacement by new-generation energy materials having compatibility for large scale industrial application [5]. Currently, numerous researchers are working to develop an inexpensive, efficient and sustainable electrocatalysts for overall water splitting as an alternative to these expensive electrocatalysts. In the last few years, transition metal oxides [6], nitrides [7], selenide [3], telluride [8], phosphides [9], sulfides [10], borides [11], carbides [12] have gained lots of interest towards OER and HER [13]. According to the literature, the electrocatalytic activity of these transition metal oxides or chalcogenide (sulfide, selenide, and telluride) can be attributed to their electronic band structure and alignment of conduction and valence edges [14]. We all are aware that electronegativity of the chalcogen series decreases with an increasing atomic number, which results in the decrease in the ionic character or increase in the covalent character in the metal chalcogenide bond (M-X, where X ¼ S, Se, and Te). According to Masud et al. [15], more covalent character in the M-X bonds, facilitates appropriate band alignment of the conduction and valence band boundaries with the water oxidation/reduction levels and also promote the redox reactions of the transition metal center [15]. This results in higher electrocatalytic property of metal chalcogenides. Based on this, it was expected that transition metal telluride materials would exhibit higher electrocatalytic activity (owing to the lower electronegativity of telluride i.e. 2.1 or higher covalent character) than selenide materials (having an electronegativity of 2.5) [14]. Moreover, from band structure analysis, it has also been found that the telluride has more band alignment than selenides [15]. Based on these fundamental studies, in recent time, a huge number of articles have been reported in literature with transition metal telluride such as NiTe2 [16], CoTe2 [17], CoTe [18], CdTe [13], WTe2 [19], and transition metal selenide such as NiSe [20], NiSe2 [21], CuSe [22], Co7Se8 [23], CoSe2 [21], CdSe [24] for OER and HER. Following the similar trend, herein, we have reported a new class of bimetallic transition metal chalcogenide nanocomposite using Co, Cd, Se, and Te elements. We have synthesized here the CoTe2@CdTe and CoSe2@CdSe nanocomposites by the simple onestep hydrothermal route. From the basic concepts, here, for the first time we have tried to develop the nanocomposite of Co and Cd metals along with chalcogens. It was found that the nanocomposite with tellurium elements showed good morphology (nanowire array like structure) and exhibited better performance towards OER and HER, in comparison to their selenium analogue. The prepared CoTe2@CdTe and CoSe2@CdSe catalyst showed overpotential value of 140 mV@10 mA cm2 and 329 mV@10 mA cm2, respectively for OER and 110 mV@10 mA cm2 and 147 mV@10 mA cm2, respectively for HER. In all electrochemical studies, the prepared CoTe2@CdTe nanocomposite exhibit better and stable electrocatalytic performance than CoSe2@CdSe, towards both OER and HER. In addition, the CoTe2@CdTe nanocomposite is successfully used as bifunctional electrocatalyst towards overall water splitting and showed overpotential of 1.51 V at the current density of 10 mA cm2. The prepared nanocomposite exhibited several benefits over the traditional electrocatalysts: (1) it needs low cost in preparation (without using any noble or rare metal), (2) can be synthesized in the simple one-step hydrothermal process and (3) have good stability over the wide time duration. More significantly, we have shown here the role of using Se and Te for modifying or upgrading the fundamental electrocatalytic activity of nanocomposites and compared their properties.

2. Experimental section 2.1. Chemicals and apparatus All the reagents were bought from different sources and used as such for the experiment. Cadmium sulfate monohydrate (CdSO4$H2O), cobalt acetate tetrahydrate [Co(OCOCH3)2$4H2O], selenium (Se) powder, potassium hexacyanoferrate (II) trihydrate [K4Fe(CN)6$3H2O], platinum carbon (Pt/C) and tellurium (Te) powder were purchased from Merck (India). All the solvents like ethanol (C2H5OH), conc. HNO3, conc. H2SO4, dimethyl sulfoxide (DMSO) were purchased from Merck (India) and Spectrochem (India). The powder X-ray diffraction (XRD) analysis of the synthesized nanocomposites were conducted using Xpert Pro MPD, (Pananalytical, The Netherlands, TIFR, Mumbai) and Philips X'pert (The Netherlands, IACS, Kolkata). Zeiss-SUPRA 55 and ZEISS MERLIN VP Compact was used for field emission scanning electron microscopy (FE-SEM) study at IIT (ISM) Dhanbad. Energy dispersive X-ray spectroscopy (EDS) study of prepared nanocomposites were performed Oxford EDS detector (POLARONSC7620) of Zeiss EVO 60 Scanning Electron Microscope at IIT Kharagpur (India). High-Resolution Transmission Electron Microscope (HR-TEM) analysis was carried out using Jeol/JEM 2100 model, with LaB6 source and 200 kV voltage (at Sophisticated Test and Instrumentation Center, Cochin INDIA). The electrochemical analyses were recorded on the CH instrument (Electrochemical workstation, USA, model number 440C). BET analysis was carried out using Quantachrome instruments, NOVA 3200e Surface Area & Pore Size Analyzer [at IIT (ISM), Dhanbad]. 2.2. One pot, single step synthesis of CoSe2@CdSe and CoTe2@CdTe nanocomposites The synthesis was done by previously reported hydrothermal methods with slight modification [10]. In brief, for the synthesis of CoSe2@CdSe nanocomposite, 2.0 mmol (0.354 g) of cobalt acetate and 1.0 mmol (0.208 g) of cadmium sulfate were taken in 100 mL breaker, followed by addition of 30 mL double distilled water. The solution was homogenously mixed by a stirrer for 15 min, and after that 4 mmol (0.315 g) of selenium powder (dissolved in 10 mL hydrazine hydrate, 98% purity) was added to the pink color metal solution and the solution color changed from pink to black. After that, the black color homogenous mixture was further stirred for 15 min followed by addition of 2.0 mL of ethylene-diamine solution. The final mixture was further stirred for 10 min, after that the final black solution was taken in a 50 mL Teflon-lined autoclave and placed in an air oven at 180  C for 12 h. The resulting reaction mixture was allowed to cool at room temperature, centrifuged at 9000 rpm, washed several times with distilled water and two times with ethanol followed by drying at 58  C for 6 h. The obtained solid product was stored for characterization and further electrochemical studies. Similarly, CoTe2@CdTe nanocomposite was also synthesized, using the same synthesis protocol, where tellurium powder was used instead of selenium powder. 2.3. Electrochemical measurement All the electrochemical measurements were performed using three electrode systems consist of a reference electrode (Ag/AgCl), a counter electrode (Pt) and nanocomposite modified pencil graphite electrode (PGE) as a working electrode, at room temperature. For the fabrication of PGE, 0.7 mm diameter pencil lead and micropipette tip were purchased from Cello (India) and Tarsons Pvt. Ltd.

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India, respectively. Prior to the modification of PGE with nanocomposite, the pencil rod was immersed in 3 (M) HNO3 solution for 10 min and carefully rubbed by cotton to remove the surface impurity on the pencil lead, if present. The pencil rod was housed in a pipette tip, keeping 0.56 cm length of the rod, outside the tip. For the modification of as-fabricated PGE, the definite weight amount (1, 5, 10 and 15 mg) of the nanocomposite was dispersed in 0.5 mL DMSO by ultra-sonication for 40 min. After that, 15.0 mL of dispersed solution was coated on the extended lower portion of the PGE (0.56 cm length) and dried at 50  C for overnight. For the electrochemical study, electrochemical cell of 10 mL capacity was used with different assistive electrolytes, such as KOH for OER and H2SO4 for HER study. All the mentioned potentials in this article were reported with respect to the reversible hydrogen electrode (RHE) as a reference, using the equation given below:

ERHE ¼ EqAg=AgCl þ 0:059  pH þ EAg=AgCl

(5)

where EqAg/AgCl is the standard Ag/AgCl electrode potential and have potential value ¼ þ0.201 V and EAg/AgCl is the potential, measured during the experiment, against Ag/AgCl reference electrode. The geometrical surface area of PGE was kept fixed i.e. 0.06 cm2, throughout the experiment. For OER measurement, 1.0 (M) KOH (pH ¼ 14) and for HER measurement 1.0 (M) H2SO4 was used, therefore, the equation (5) is modified to the equations (6) and (7) for different studies:

ERHE ðFor OERÞ ¼ 1:036 þ EAg=AgCl

(6)

ERHE ðFor HERÞ ¼ 0:210 þ EAg=AgCl

(7)

The catalytic activity of the prepared nanocomposites towards OER and HER was estimated by Tafel plot, following the equation (8), given below:

h¼a þ

2:3RT logðjÞ anF

(8)

where h and j represent overpotential and current density and other terms possess their usual significant. The value of 2:3RT anF corresponds to the Tafel slope, used to compare the electrocatalytic properties of prepared nanocomposite. Another parameter used to compare the electrocatalytic properties of the prepare nanocomposite is onset overpotential, which can be defined as a potential at the current density of zero mA cm2. For the OER and HER studies, linear sweep voltammetry (LSV) and cyclic voltammetric (CV) analysis were performed. The LSV runs were recorded in 1.0 (M) KOH and 1.0 (M) H2SO4 at the scan rate of 50 and 2 mV s1. The LSV runs were taken after 15 cycles of CV runs, to obtain stable current during the measurement. In order to calculate the double layer capacitances, CV runs were recorded at different scan rates (i.e. 200, 50 and 2 mV s1) in the potential range of 0e0.6 V (vs. Ag/AgCl). The long-term stability study of nanocomposite modified PGE was studied by chronoamperometry for 8.33 h. Among the prepared nanocomposite, the catalytic performance of CoTe2@CdTe was studied towards overall water splitting. For this, CoTe2@CdTe modified PGEs were used as both anode and cathode in 1.0 (M) KOH in two-electrode system. In the twoelectrode system, LSV, cyclic stability, and long-term stability studies were performed.

903

3. Result and discussion 3.1. X-ray diffraction analysis (XRD) The structural composition of synthesized nanocomposites was studied by powder XRD analysis. The XRD pattern of the prepared nanocomposites are shown in Fig. 1 (A and B). As shown in the XRD pattern of CoTe2@CdTe nanocomposite (Fig. 1 A), the diffraction peaks were obtained at 2q values of 22.91, 23.74 , 27.49 , 38.32 , 39.36 , 40.40 , 43.31, 45.81 46.44 , 49.56 , 52.06 , 56.85 , 62.89 , 63.72 , 65.80 , 67.68 , 71.22 , 72.05 , and 75.59 , which can be assigned to (100), (002), (111), (121), (110), (211), (103), (200), (130), (131), (202), (222), (321), (141), (105), (232), (300), (410), and (303) planes, attributed to the presence of CoTe2 and CdTe (from the JCPDS card no 89-1258 and 19-0193, respectively). On the other hand, diffraction peaks were observed at 2q values of 25.74 , 29.07, 30.95 , 34.49 , 35.94 , 42.196 , 43.86 , 50.10 , 51.95 , 56.98 , 63.22 , and 76.34 for CoSe2@CdSe nanocomposite (Fig. 1 B) corresponding to the (111), (200), (200), (210), (211), (220), (220), (311), (311), (023), (400), and (422) planes, confirms the presence of CoSe2 and CdSe from the JCPDS card no 88-1712 and 88-2346, respectively. 3.2. Surface morphology of the nanocomposites The surface morphology of prepared nanocomposites was studied by FE-SEM and shown in Fig. 1 (C to F). As shown in Fig. 1 (C and D), the CoSe2@CdSe nanocomposite did not reveal any specific shape and looks like composed of agglomerated nanoparticles having particle size ~30 nm. However, CdTe2@CdTe nanocomposite exhibited nanowire array like morphology with particle size ~20 nm [Fig. 1 (E and F)]. It is very clear from the FE-SEM study that variation in the chalcogen selection during the synthesis of bimetallic nanocomposite results in the variation of their morphological features also. Furthermore, the morphological feature of the CoTe2@CdTe nanowire array was also studied via high-resolution transmission electron microscopy (HR-TEM). HR-TEM analysis further confirmed the nanowire structure of the CoTe2@CdTe and clearly supports the results obtained from FE-SEM study (Fig. 2AeC). The low and high magnification images also suggest that nanowire array like morphology of CoTe2@CdTe was generated due to the agglomeration of small nanoparticles in the array shape. Additionally, the surface composition of prepared electrocatalysts was also studied by energy dispersive X-ray spectroscopy (EDS) study and shown in Supporting Information (Figure S1- A to D). The elemental analysis of catalysts was carried by energy dispersive X-ray spectroscopy (EDS) study. The weight and atomic percentage of the fresh electrocatalysts i.e. CoSe2@CdSe and CoTe2@CdTe are shown in Fig. S1 (A and C). It is clearly visible in the Figure that cobalt, cadmium, selenium/tellurium elements are present in the prepared catalysts. Here, the EDS study was performed on the used catalysts also i.e. after electrochemical measurements, to study the stability of catalyst, after HER and OER. As shown in Fig. S1 (B and D), the weight and atomic percentage of elements are almost same in both the electrocatalysts, before and after electrochemical study. In addition, no extra element (other than Co, Cd, Se/Te) was observed in the EDS study of both the electrocatalysts, which confirmed that the designed catalysts showed good stability during the electrochemical study. As we can see, the elemental percentage is almost same for each nanocomposite, before and after the electrochemical study. Therefore, their EDS spectra also looks exactly same with similar number, shape and size of the peaks. Furthermore, to confirm the structure and composition of the prepared nanocomposites and support the XRD shown in Fig. 1A,

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Fig. 1. XRD pattern of (A) CoTe2@CdTe and (B) CoSe2@CdSe nanocomposites; High and low magnification FE-SEM images of CoSe2@CdSe (C and D) and CoTe2@CdTe (E and F) nanocomposites.

the selected area energy diffraction (SAED) analysis of the nanocomposite was also performed and shown in Fig. 2D. In the SAED pattern of nanowire CoTe2@CdTe, diffraction rings are clearly visible, which confirms the crystalline nature of prepared nanocomposite. With the help of first and second SAED diffraction rings, the d spacing values for CoTe2@CdTe was calculated. The d spacing value of first and second diffraction ring was well matched and corresponds to the (100) and (111) planes of CdTe and CoTe2, respectively. The study confirms the successful synthesis of the nanocomposite.

3.3. Comparison of electrocatalytic activity and surface area of prepared nanocomposites For

the

measurement

of

electrocatalytic

activity

and

electroactive surface area of the synthesized nanocomposites, CV runs were recorded at 50 mV s1 scan rate within the potential range of 0.8e1.2 V. For this experiment, 1.0 (M) potassium chloride and 0.1 (M) potassium ferrocyanide was used as a supporting electrolyte and electroactive probe molecule, respectively. The electrocatalytic activity of the prepared nanocomposites is shown in Fig. S2. As shown in figure, CoTe2@CdTe showed much higher current (302.87 mA) response than CoSe2@CdSe (197 mA). In addition, the ferrocyanide reversible peaks are more symmetrical with less gap between anodic and cathodic potential (DE ¼ EpaEpc ¼ 0.426e0.205 ¼ 0.221 V) at CoTe2@CdTe than CoSe2@CdSe modified PGE. This suggest the high electrocatalytic activity of CoTe2@CdTe in comparison to the CoSe2@CdSe nanocomposite. The electrocatalytic surface area of the prepared

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Fig. 2. (A-C) HR-TEM images and (D) SAED pattern of CoTe2@CdTe nanocomposite.

nanocomposites has also been calculated by the Randles-Sevcik equation [25].

Ip ¼



 3 1 1 2:687  105 n2 D2 v2 Co A

(9)

where Ip corresponds to peak current, h corresponds to the number of electrons participated in the reaction, D corresponds to diffusion coefficient, v corresponds to the scan rate, C corresponds to the concentration of potassium ferrocyanide and A corresponds to the electrocatalytic surface area, respectively. In this study, the number of electrons participated in the reaction is one, because Fe2þ converts to Fe3þ, the scan rate is kept at 50 mV s1, with C ¼ 0.1 M. After placing all the values in equation (9), the electrocatalytic surface area (A) of the nanocomposite modified electrodes can be calculated. Additionally, the roughness factor of the modified electrodes is also calculated using the equation (10) [25]:

 Rf ¼ A Ageo

(10)

where Rf, A and Ageo referred to roughness factor, electrocatalytic surface area and geometric surface area of the modified electrodes, respectively. Using equations (9) and (10), electrocatalytic surface area and roughness factor of the nanocomposite modified

electrodes were calculated. A comparison between the electrocatalytic activities of nanocomposite modified electrodes in terms of peak current, electrocatalytic surface area, and roughness factor is shown in Table S1. As shown in Table S1, the telluride containing nanocomposite possesses a higher electrocatalytic surface area (i.e. 18.28 cm2) than selenide containing nanocomposite (11.89 cm2). 3.4. Oxygen evolution reaction (OER) study Prior to study the OER behavior of prepared nanocomposite, the analytical parameters have been optimized. 3.4.1. Optimization of analytical parameters towards OER study Optimization of analytical parameters, such as concentration of supporting electrolyte, scan rate, and mass loading is one of the primary steps, prior to the OER study. For this experiment, we have recorded LSV runs, varying different parameters. Prior to the measurements, oxygen gas (purity 99.999%) was purged into the electrolyte (KOH) for 35 min. Firstly, scan rate has been optimized and for this, LSV runs were recorded at different scan rate i.e. 5, 50, 100 mV s1 in the potential range of þ1.0 to þ3.0 V vs. RHE using CoTe2@CdTe nanocomposite modified PGEs. As shown in Figs. S3eA, runs recorded at 50 mV s1 scan rate shows low onset

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potential along with higher current density. For the optimization of concentration of the assistive electrolyte, similar study (LSV) was performed at 50 mV s1 scan rate, in different KOH concentration i.e. 0.5, 1, and 2 M in the same potential range. From Figs. S3eB, it is clearly visible that the low onset potential, as well as high current density, was observed for 1.0 (M) KOH. At last, a similar experiment was performed for the optimization of mass loading i.e. amount of nanocomposite used for the electrocatalytic activity. For this, LSV was carried out at a scan rate of 50 mV s1 in 1.0 (M) KOH using different electrodes modified with the different amount of nanocomposite i.e. 1e3 mg. The corresponding LSV runs were shown in Figs. S3eC; where it can be very easily observed that with the 2 mg loading mass of the nanocomposite, the electrode has shown low onset potential and

higher current density. The optimized parameters i.e. 1.0 (M) KOH as an electrolyte, scan rate of 50 mV s1 with 2 mg mass loading were used for further OER studies.

3.4.2. Electrocatalytic activity of the prepared nanocomposites towards OER The oxygen evolution study of the prepared nanocomposite was performed by recording LSV and CV runs at optimized parameters. CV runs were recorded in the potential range of þ1.0 to þ3.0 V (vs. RHE) (Fig. 3A). As shown in the figure, the CoTe2@CdTe nanowire modified PGEs exhibited low onset potential with similar current density, in comparison to the CoSe2@CdSe modified PGEs. The LSV was recorded in the potential range of þ1.0 V to þ2.7 V (vs. RHE) using nanocomposite modified PGEs and shown in Fig. 3B. From the

Fig. 3. (A) CV, (B) LSV, and (C) Tafel plot of prepared nanocomposites in 1.0 (M) KOH towards OER activity; (D) cyclic stability, (E) monthly stability and (F) controlled electrolysis study showing stability of CoTe2@CdTe nanocomposite.

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LSV runs, it was clearly observed that the CoTe2@CdTe nanowire array exhibited higher current density (i.e. 163.6 mA cm2) at þ2.69 V potential (vs. RHE) and small onset potential value of þ1.38 V vs. RHE at zero mA cm2 current density. Whereas, CoSe2@CdSe shows onset potential of þ1.54 V vs. RHE (at zero mA cm2 current density) and shows the current density of 146.1 mA cm2 only, at the same potential i.e. þ2.69 V (vs. RHE). Additionally, the overpotential (Dh) value of the two nanocomposites was also calculated, taking the difference in potential at zero current density (h0) and the current density at 10 mA cm2 (h10). The overpotential values of CoTe2@CdTe and CoSe2@CdSe are found 140 mV@10 mA cm2 and 329 mV@10 mA cm2, respectively (Figs. S4eA). The Tafel slope study was performed to check the reaction rate of OER and best catalyst always gives low Tafel slope value. The Tafel slope value of the prepared nanocomposites is represented in Fig. 3C. The nanowire CoTe2@CdTe nanocomposite gave a low Tafel slope value (68 mV/dec) than the CoSe2@CdSe nanocomposite (128 mV/dec). So, from Tafel slope value it is confirmed that the nanowire CoTe2@CdTe nanocomposite has enhanced performance towards OER in comparison to the selenium nanocomposite. Our prepared CoTe2@CdTe nanowire array as a catalyst exhibits the better OER performance than the previously reported similar type of nanomaterials. A comparative table summarizing the OER activity of proposed nanocomposite along with earlier reported materials confirms the same (Table 1 [18,26e32]). 3.4.3. Stability study towards OER Our prepared catalysts show very good results towards OER, but this is not enough to reach the status of the good catalyst, a prolonged period of stability is also required. For this, CV and LSV runs were recorded to test their performance against the cyclic stability test and the monthly stability test. The LSV runs were performed in the potential range of þ1.0 V to þ2.8 V (vs. RHE) at scan rate 50 mV s1. From Fig. 3D, it was observed that our prepared catalyst shows very good cyclic stability i.e. constant current and potential value up to 500th cycles. Similarly, the catalyst showed high stability under storage condition and exhibited almost negligible change in the onset potential as well as current density, after storage of three months (Fig. 3E). Furthermore, the bulk electrolysis study was also performed using the prepared CoTe2@CdTe nanocomposite in 1.0 (M) KOH for 30000 s and the obtained result is shown in Fig. 3F. From the bulk electrolysis study, it was observed that the nanowire array-based catalyst showed a stable and constant current density up to 30000 s, which suggest the constant performance and high durability of the prepared catalyst. Similarly, a comparison has also been

907

made between freshly prepared nanocomposites modified PGE and used PGE, after all OER measurements. It was found that the used PGE shows almost the same onset potential and current density, as recorded with the freshly prepared electrode (Figs. S3eD). 3.5. Hydrogen evolution reaction (HER) study Before studying the behavior of prepared nanocomposite towards HER, the analytical parameters have also been optimized. 3.5.1. Optimization of analytical parameters towards HER study Before studying the HER activities of the fabricated nanocomposite, some important analytical parameters were also optimized including scan rate, electrolyte concentration and mass loading. As shown in Figure S5-A and B, with 2 mg loading mass of the nanocomposite at 2 mV s1 scan rate gave the superior results with smaller onset potential and higher current density. In addition, the best HER behavior in terms of onset potential and the current density was obtained using 1.0 (M) H2SO4 as a supporting electrolyte (Figs. S5eC). Therefore, for the further HER study, these optimized analytical parameters were used. 3.5.2. Electrocatalytic performance of the prepared nanocomposites towards HER The HER activity of prepared nanocomposites was studied by recording and comparing their onset potential and current density in the CV and LSV runs. The CV and LSV runs were recorded in the potential range of þ0.2 to 1.3 V (vs. RHE) and 0.2 Ve1.6 V (vs. RHE) respectively, under the optimized analytical condition. Additionally, the HER performance of the nanocomposites was also compared with the standard Pt/C catalyst (20 wt% platinum on carbon black) available in the market, in the same reaction conditions. The corresponding CV and LSV runs are shown in Fig. 4-A and B. Among the two nanocomposites, CoTe2@CdTe showed the best HER activity in LSV with smaller onset potential (0.0789 V vs. RHE) and higher current density (67.12 mA cm2 at 0.6 V). As we can see in the Figures, the onset potential value of nanowire CoTe2@CdTe is very near to the standard Pt/C catalyst, whereas the other nanocomposite i.e. CoSe2@CdSe showed comparatively poor HER activity. The onset potential and the current density of CoSe2@CdSe is found to be 0.2 V (vs. RHE) and 51.0 mA cm2 at 0.6 V, respectively. Additionally, overpotential values of CoTe2@CdTe and CoSe2@CdSe are found 110 mV@10 mA cm2 and 178 mV@10 mA cm2, respectively (Figs. S4eB). Therefore, it is very clear from these studies that CoTe2@CdTe nanowire array exhibited low overpotential value than another nanocomposite as well as previously reported similar kind of nanomaterials [Table 2 [32e36]].

Table 1 Comparison of OER activity of CoTe2@CdTe nanowire array with recently reported similar type of nanomaterials in alkaline medium. S$N. Electrode material and morphology

Method of preparation

Onset potential (V)

Overpotential (mV)

Tafel slope (mV/dec)

Ref.

Anoin exchange tellurization Hydrothermal

1.45

340

e

[18]

1.52

340

67

[26]

300 360 320 183 Neutral pH ¼ 402 Alkaline pH ¼ 260 152 340 140

90 102 78 97 Neutral pH ¼ 123 Alkaline pH ¼ 72 124 43.2 68

[27] [27] [28] [29] [30]

1.

CoTe nanotabular flim

2. 3. 4. 5. 6. 7.

Hierarchical CoTe2 nanowire array on Ti mesh CoTe2@N-GC CoTe2 powder Ni0$88Co1$22Se4 NiCoSe2/Ni foam NiCo2S4 nanosheet/carbon cloth

Chemical conversion Chemical conversion e Electrodeposition e

1.55 1.62 1.57 1.51 1.49

8. 9. 10.

CoSe2 Zn0$1Co0$9Se2 CoTe2@CdTe nanowire array

Hydrothermal Hydrothermal Hydrothermal

1.48 e 1.38

[31] [32] This work

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Fig. 4. (A) CV, (B) LSV, and (C) Tafel plot of prepared nanocomposites in 1.0 (N) H2SO4 towards HER activity; (D) cyclic stability, (E) monthly stability and (F) controlled electrolysis study showing stability of CoTe2@CdTe nanocomposite. Table 2 Comparison of HER activity of CoTe2@CdTe nanowire array with recently reported similar type of nanomaterials in acidic medium. S. N.

Electrode material

Method of preparation

Onset potential (V)

Overpotential (mV)

Tafel slope (mV/dec)

Ref.

1. 2. 3. 4. 5. 6. 7. 8.

Zn0$1Co0$9Se2 Co1$11Te2/C CoNiSe2@ nickel foam Box-in-box structured Co/(NiCo)Se2 CoTe/carbon fiber paper CoSe2 P doped CoSe2 CoTe2@CdTe nanowire array

Hydrothermal Chemical vapor deposition Hydrothermal e Hydrothermal Solvothermal e Hydrothermal

e 0.15 0.127 0.17 0.12 0.14 0.031 0.078

140 178 192 200 at 17.5 mA/cm2 230 90 at 4 mA/cm2 104 110

49.2 77.3 98 39.8 57.1 42.4 69 90

[32] [33] [34] [35] [36] [37] [38] This work

The Tafel slope of the prepared nanocomposites was also calculated and shown in Fig. 4C. It is very clear that CoTe2@CdTe nanowire shows a low Tafel slope value than their selenium analogue. However, in HER Tafel slope not only predict the catalytic

property of electrocatalyst but also used to predict the probable reaction mechanism followed during HER process. In general, three types of reaction mechanisms steps are involved in HER, based on their Tafel slope value [11]:

K. Chandra Majhi et al. / Electrochimica Acta 318 (2019) 901e912

1. Volmer reaction mechanism: H þ þe þ M/M  H * (Tafel slop ¼ 120 mV/dec) 2. Heyrovsky reaction mechanism: M H * þ e þ H þ /H2 þ M (Tafel slop ¼ 40 mV/dec) 3. Tafel reaction mechanism: 2M H * /H2 þ 2M (Tafel slop ¼ 30 mV/dec) Herein, the prepared nanocomposite has shown the Tafel value of 90 mV/dec, suggest that Heyrovsky reaction mechanism was followed by CoTe2@CdTe nanowire, during the HER process. Our prepared CoTe2@CdTe nanowire array as a catalyst exhibits the enhanced HER performance than the previously reported similar type of nanomaterials, shown in the Table 2 [32e38]. 3.5.3. Stability study towards HER Stability of CoTe2@CdTe modified electrode was also checked because stability is very crucial for making a cost-effective and efficient electrocatalyst for real-time application. Here, we have studied the cyclic and storage stability of the nanocomposite using the LSV method, recorded within the potential range of þ0.2 V to 1.3 V (vs. RHE) at 2 mV s1 scan rate (Fig. 4 D and E). After 500 cycles, there was no visual change has been observed in onset potential and current density values of the electrocatalyst. In the case of storage stability, after 90 days of storage, our material showed a slight change in current density and almost same onset potential value. This suggests that prepared electrocatalyst is highly stable, even after multiple cyclic runs and three months of storage. Additionally, a bulk-electrolysis study of nanowire CoTe2@CdTe was carried out for 30000 s in 1.0 (M) H2SO4 at 0.5 V (Fig. 4 F). Almost constant current response was obtained throughout the experiment, which clearly suggest the stable and constant

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performance of prepared nanocomposite towards HER. Similarly, HER study was carried out for the fresh and used nanocomposites (as discussed in OER) and almost same onset potential and the current density were observed on the fresh as well as used electrocatalyst modified PGEs (Figs. S5eD). 3.6. Reason for the superior performance of nanowire array-based electrocatalyst In order to find the reason behind the superior performance of Te based nanowire array as an electrocatalyst in comparison to the Se based electrocatalyst, the surface area of prepared nanocomposites were studied. 3.6.1. Calculation of an electrochemically active surface area (ECSA) In general, ECSA was calculated for a better understanding of HER activity of proposed electrocatalysts. For this, electrochemical double-layer capacitance (Cdl) was calculated taking the help of CV runs. Herein, CV runs were recorded and their non-Faradaic region is used to calculate ECSA of the nanocomposites, which is actually proportional to the Cdl of the solid-liquid interface [39]. Fig. 5 (A-B) shows the CV runs recorded in 1.0 (M) KCl (supporting electrolyte) at different scan rates of 2, 50, and 200 mV s1 using CoSe2@CdSe and CoTe2@CdTe modified PGEs. The difference (Dj) between anodic (ja) and cathodic (jc) current density at the center of the potential range was calculated for all the scan rates. To calculate the Cdl, Dj values were plotted against the scan rate (n) and the slope of this linear fitting plot gives the value of Cdl according to the equation given below [26]:

Cdl ¼ Dj=V

(11)

Fig. 5. CV study at different scan rate (200, 50, and 2 mV s1) in the potential range of þ0.0 to þ0.6 V for (A) CoTe2@CdTe and (B) CoSe2@CdSe nanocomposites; (C) plot of current density versus scan rate for prepared nanocomposites; (D). BET surface area curve of prepared nanocomposite.

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ECSA of the electrocatalysts were calculated using the equation (12) (given below), with the help of Cdl value obtained from equation (11) [26],:

ECSA ¼ Cdl =Cs

(12)

Here, Cs is the standard specific capacitance of the surface having 1 cm2 surface area i.e. 0.04 mF cm2. The nanowire CoTe2@CdTe nanocomposite showed greater Cdl value than selenium nanocomposite (Fig. 4-C). With the help of equation (12), the ECSA values for nanocomposites was calculated and found as 1.77 and 1.09 cm2 for CoTe2@CdTe are and CoSe2@CdSe, respectively. This data offered that a large number of active sites is present in CoTe2@CdTe, which may accelerate their electrocatalytic activity in comparison to the CoSe2@CdSe. 3.6.2. Calculation of Brunauer-Emmett-Teller (BET) surface area The BET analysis was performed using the 80-point method, after 5 h degassing at 200  C degassing temperature. Both the nanocomposites exhibited similar types of isotherm (i.e. Type IV adsorption-desorption isotherm) with the distinct hysteresis loops in the N2 pressure range of 0.0e1.0, which indicates their mesoporous characteristic (Fig. 5-D) [27]. The BET surface area, Barrett-Joyner-Halenda (BJH) absorption and desorption average diameter of both nanocomposites are summarized in Table S2. As shown in figure and table, CoTe2@CdTe showed much higher surface area value (approximately 9 times higher) (17.4304 m2/g) than that of CoSe2@CdSe (2.0127 m2/g). Similarly, the CoTe2@CdTe nanowire array has also exhibited relatively higher BJH adsorption average pore diameter than CoSe2@CdSe. The higher surface area and the BJH adsorption average pore diameter of CoTe2@CdTe can be the reason for the

higher electrocatalytic activity of nanowire array. It is very obvious that high surface area provides a large number of active sites and supports to adsorb more electrolyte molecules and therefore enhances the catalytic activity of the material. The BET surface area result is in well-accordance with the surface area obtained from electrochemical measurement (using ferrocynide as probe molecule). Both the studies have shown that CoTe2@CdTe possesses high surface area than their respective selenide catalyst i.e. CoSe2@CdSe (Table S1 and Table S2). From the electrochemical measurement, ECSA and BET surface area calculation, it is very clear that Te based nanocomposite possess superior electrocatalytic performance than Se based nanocomposites on the basis of their higher surface area and pore volume/diameter. However, the higher surface area of Te-nanocomposite may be attributed to the higher covalent character of metal telluride bond, than the corresponding metal selenide bond [14], results in facile binding, high surface area and better performance of CoTe2@CdTe nanocomposite. 3.7. Overall water splitting Nanowire CoTe2@CdTe exhibited better performance for both HER and OER in acidic and alkaline medium, respectively, with low onset potential, low overpotential and high current density than the previously reported similar nanomaterials shown in Tables 1 and 2 Therefore, from the HER and OER study, we can expect that the nanowire CoTe2@CdTe nanocomposite can also perform the good catalytic activity towards overall water splitting. To test this, we have performed the two electrodes based LSV study in 1.0 (M) KOH, where CoTe2@CdTe nanowire array modified PGEs were used as both cathode and anode, at room temperature. The polarization curve of the two electrode system for overall splitting is

Fig. 6. (A) LSV plot showing over all water splitting in the two electrode system; (B) camera picture of two-electrode system, showing O2 and H2 bubbles; (C) Cyclic stability and (D) constant current stability study in the two electrode system.

K. Chandra Majhi et al. / Electrochimica Acta 318 (2019) 901e912

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Table 3 Comparison of overall water splitting activity of CoTe2@CdTe nanowire array with recently reported similar type of nanomaterials in alkaline medium. S. N.

Electrocatalyst

Morphology

Potential at current density 10 mA cm2 (V)

Electrolyte

Ref.

1. 2. 3. 4. 5. 6. 7.

Ni0$85Se EG/Ni3Se2/Co9S8 Co7Se8 NiCo2S4 on carbon cloth CoSe2/carbon cloth CoSe2 nanoparticles N-doped carbon CoTe2@CdTe

e Nanosheet Nanoflake Nanosheet Nanosheet array Polyhedral Nanowire array

1.73 1.62 1.6 1.53 1.62 1.73 1.51

1.0 1.0 1.0 1.0 1.0 1.0 1.0

[2] [5] [40] [41] [42] [43] This work

represented in Fig. 6-A. As depicted in the figure, the LSV polarization curve shows low onset potential (1.51 V) than earlier reported electrocatalyst used for overall water splitting (shown in the Table 3). It means, CoTe2@CdTe catalyst only needs 1.51 V to attain the current density of 10 mA cm2 (Figs. S4eC) and a constant potential 1.51 V is actually needed or sufficient enough to accelerate the water electrolysis or overall water splitting. It is very obvious that H2 and O2 gas bubbles will be released from the corresponding electrodes. The video of H2 and O2 evolution at anode and cathode was also recorded and shown in Video S1. The corresponding camera picture of the electrochemical cell was also captured and shown in Fig. 6-B. In the video and camera picture, the evolution of oxygen and hydrogen gas bubbles from anode and cathode can be clearly visualized. Supplementary video related to this article can be found at https://doi.org/10.1016/j.electacta.2019.06.106. Additionally, the cyclic stability and bulk electrolysis of water were also measured. For this, LSV runs were recorded, after 1, 100 and 1500 cycles (Fig. 6-C). Similarly, the current density was also studied by long term bulk electrolysis in the duration of 30000 s (Fig. 6-D). As shown in the figures, after 1500 cycles of study or 8.33 h of bulk electrolysis, no change in polarization curve behavior or current density was observed, which exhibited the excellent performance and stability of prepared electrocatalyst in twoelectrode system also. The above results clear the application of prepared nanocomposite for overall water splitting as a bifunctional electrocatalyst. 4. Conclusion In summary, we have successfully developed nanowire shaped CoTe2@CdTe and flakes like CoSe2@CdSe nanocomposite by the simple, one-step hydrothermal method. Remarkably, nanowire CoTe2@CdTe nanocomposite revealed high electrocatalytic activity for both OER and HER with long term durability. Especially, CoTe2@CdTe showed small onset potential value 1.38 V and 0.078 V for OER and HER, respectively and also have low overpotential (140@10 mA cm2 and 110 mV@10 mA cm2 for OER and HER, respectively) with a large current density for both OER and HER with small Tafel slope values 68 and 90 mV/dec, respectively. The optimized nanowire CoTe2@CdTe nanocomposite has also showed a long period of storage stability for three months and cyclic stability (up to 500th cycles) without significant change in onset potential and/or current density. Furthermore, optimized electrocatalyst has also offered good overall water splitting in 1.0 (M) KOH and requires the small potential of 1.51 V to achieve 10 mA cm2 current density and exhibited outstanding stability of 8.33 h in bulk electrolysis study. Acknowledgment Authors are thankful to DST for sponsoring the research project to R.M. (Ref. No.: SERB/F/2798/2016-17). The experimental work

(M) (M) (M) (M) (M) (M) (M)

NaOH KOH KOH KOH KOH KOH KOH

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