Group IV transition metal based [email protected]2 for electrochemical hydrogen evolution reaction over wide range of pH

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 4 6 2 8 e2 4 6 4 1

Available online at www.sciencedirect.com

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Group IV transition metal based phosphochalcogenides@MoTe2 for electrochemical hydrogen evolution reaction over wide range of pH Paramita Karfa*, Kartick Chandra Majhi, Rashmi Madhuri Indian Institute of Technology (Indian School of Mines), Dhanbad, Jharkhand, 826004, India

highlights
First time, group IV transition metal phosphochalcogenides was synthesized.
The nanocomposite of phosphochalcogenide@MoTe2 i.e. MP2S6@MoTe2 was used for HER.
Among the nanocomposites, TiP2S6@MoTe2 has the specific almond like morphology.
TiP2S6@MoTe2 elecrocatalyst shows extraordinary HER activity in the wide pH range.
TiP2S6@MoTe2 showed high Faradaic efficiency (~98%) and small onset potential.

article info

abstract

Article history:

Electrochemical water splitting proved to be one of the most attractive sources of green

Received 6 May 2019

fuel energy but the easy and time-consuming synthesis of highly active, low-priced, steady

Received in revised form

and earth-abundant metals electrocatalysts is still a challenge among the researcher. In

3 July 2019

this work, we have reported the fabrication of Group IV transition metal (M ¼ Ti, Hf, and Zr)

Accepted 25 July 2019

based phospho-chalcogenides and their composite with MoTe2 via simple hydrothermal

Available online 22 August 2019

synthesis process. The prepared nanocomposites (MP2S6@MoTe2) were compared with respect to their best performance towards HER over the entire pH range. Among the

Keywords:

different kind of prepared nanocomposite, TiP2S6@MoTe2 exhibited special almond like

Group IV transition metal

shape with self-layered morphology and henceforth selected as best catalyst on the basis

Phospho-chalcogenides

of their low Tafel slope, small onset potential, low charge transfer resistance and small

MoTe2

overpotential value over the wide range of pH. It was also found that nanocomposites

Hydrogen evolution reaction (HER)

modified working electrode showed long cycle durability in the acidic pH in controlled

Entire pH range of 0e14

potential electrolysis study. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction Fossil fuels can be replaced by new and renewable hydrogen energy source, which is used for various applications as a green and clean fuel [1]. The cleanest way to produce

hydrogen is by using biomass, but the yield of the process is very low. Another clean and sustainable source of hydrogen is water, which is abundant and easily available in the earth crust. However, to generate hydrogen from water, one has to perform the water splitting. But water splitting is a very

* Corresponding author. E-mail address: [email protected] (P. Karfa). https://doi.org/10.1016/j.ijhydene.2019.07.192 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 4 6 2 8 e2 4 6 4 1

tedious process. For example, production of hydrogen through thermal water splitting process requires very high temperature of about 2000  C, which is very difficult to perform and practically unfavorable [2]. Safe, clean, ecofriendly, easy production of hydrogen is possible through water-splitting process, which can be achieved through various techniques, like photo-electrochemical, photochemical, and electrochemical technique [3,4]. . Among them, the large-scale hydrogen production can be easily achieved through electrochemical water splitting process, which proceeds through two half-cell reactions i.e. oxygen evolution reaction (OER) occurs at anode and another one is hydrogen evolution reaction (HER) occurs at cathode [4,5]. The overall main reaction can be represented as: 2Hþ þ 2e / H2 [6]. Among these two reactions, HER requires multielectron and protons during the electron transfer process, which make the reaction kinetics very sluggish [7]. Therefore, electrocatalyst is needed to drive the HER at low potential value, which can use to produce large-scale hydrogen in lowcost. Till date; one of the pre-eminent catalyst designed for HER is Pt/C. But, due to the highly expensive nature and scarcely availability of Pt, Pt/C or any other platinum based catalyst cannot be used in large scale production of hydrogen [8]. So, now researchers are trying to discover good electrocatalysts, which should be highly efficient, noble metal free, easily available or earth abundant and can work over various electrochemical conditions. In brief, we need a new, stable, low-cost, noble metal free catalyst with 100% faradaic efficiency to generate pure hydrogen without any sophisticated and costly instrumentation [9]. A catalyst which can be applied in the wide working range starting from acidic to neutral to basic medium is of great demand now a days [10]. Proton exchange membrane (PER) fuel cell is the best suitable option for hydrogen production, working in acidic medium, thus a catalyst which can survive in acidic condition is of great demand. Similarly, catalysts which have high stability and efficient hydrogen evolution property in neutral medium are very important for microbial electrolysis cell (MEC) [11e13]. In addition, for electrochemical water splitting through basic electrochemical cell, we need a catalyst, which could be operated in highly basic condition. Therefore, based on the different needs and requirements, we require a catalyst which can work in complete pH range (i.e. from 0 to 14). Noble metal free catalyst, which are trending now a days are transition metal chalcogenides, metal phosphides, metal borides, and metal carbides, but their wide range pH activity for HER is seldom studied [14,15]. Few articles have been reported till now and intensive research is going on producing earth abundant catalyst, which can split water over entire pH range, possess high stability, involves easy method of synthesis and can be used in large scale [16]. Phosphides and chalcogenides of non-precious transition metals like nickel, cobalt, molybdenum, tungsten, are greatly studied for hydrogen evolution reaction but transition metals mainly from the group IV are rarely studied and investigated for energy application [17,18]. Till date very few number of articles are reported in the literature, where group IV metals have been used as electrocatalysts for HER, OER or oxygen reduction reaction (ORR), owing to the lack of knowledge about their electrochemical

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properties. For example, Meganathan et al. have used hafnium phosphide and hafnium sulphide nanosheets bonded with graphene oxide for OER and ORR [19]. Sanchez et al. have developed transition metal modified inorganic layered zirconium phosphate for OER. Here, different transition metals are incorporated in ZrP to fasten the OER [20]. Toh et al. have synthesized a series of group IV compounds i.e. ZrS2, ZrSe2, ZrTe2, HfS2, HfSe2, HfTe2, TiS2, TiSe2, and TiTe2 and studied their catalytic property towards HER [21]. They have reported that transition metal sulphides provide large surface area and large number of active sites with S-M-S sandwiched layered structure. Here, bulk transition metal sulphides are exfoliated to form atomically thin sheets of transition metal sulphides through various chemical and mechanical synthesis methods. It has been also observed by the authors that HER is mainly observed in the edges of the layered sulphides not in the basal plane. Instead of sulphides, according to the literature, transition metal phosphides give us the ‘P sites’, which help in hydrogen adsorption and desorption [22]. Metal phosphides possess high electrical conductivity, good synergistic effect, good durability, and no binding to H atoms at certain coverage, which made them excellent electrocatalyst for hydrogen evolution reaction [23]. So, based on these finding and facts, we can conclude that sulfides and phosphides have certain hereditary properties that can be used to design new and efficient electrocatalyst for HER, if we us both of them in a single material. Some of the work has been reported in the literature by some of the researchers using the similar idea. For example, Mukherjee et al. have synthesized layered MPS3 type compound, where M is earth abundant iron (Fe) [24]. They designated it as ternary semiconducting material having ABC-ABC like arrangement of hexagonal sulfur sheet along ‘c’ axis and layers are connected through Van der Waals forces. Their hybrid was also prepared using graphene oxide sheets (FePS3-rGO) and showed its application over HER in wide range of pH. Similarly, Xue et al. have synthesized layered NiPS3 nanosheet-graphene for high performance OER [25]. After preparing FePS3, Mukherjee and his group have also synthesized FePSe3, just by replacing sulphur with selenide. The prepared selenide material also exhibited low band gap and good trifunctional property towards OER, HER and ORR [26]. In recent time, hybrid of two different kinds of nanomaterials or their nanocomposite has gained attention of researchers, owing to their enhanced properties [27]. For example, Liu et al. have prepared a nanocomposite of metal phosphide (Ni2P) and transition metal dichalcogenides (TMDs) and proposed that the strong coupling between the TMDs and metal phosphide results in improved HER behaviour [27]. However, among the TMDs (like MoS2, MoSe2, MoTe2 etc.), MoTe2 has come to light in very recent time. According to Qiao et al. MoTe2 have small band gap, excellent carrier transport property, and possess large number of active sites, which improves their HER properties [28]. Inspired from these studies, for the first time, we have tried to synthesize Group IV transition metal phosphochalcogenides, which will have synergetic effect and will show better HER performance via simple hydrothermal technique and made their nanocomposite with MoTe2. For the synthesis of transition metal phospho-chalcogenides,

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titanium (Ti), hafnium (Hf) and zirconium elements (Zr) were used and the HER response of final nanocomposites i.e. ZrP2S6@MoTe2, HfP2S6@MoTe2 and TiP2S6@MoTe2 were tested. We have found that change in group IV elements, during nanocomposite synthesis; will change their morphologies also, which plays a major role towards HER activity. Herein, TiP2S6@MoTe2 has shown unique almond like self-layered structure, while other two nanocomposites failed to exhibit any specific shape or morphology. Afterwards, the prepared nanocomposite TiP2S6@MoTe2 showed significant hydrogen evolution reaction over the broad pH range 0e14. Among these, TiP2S6@MoTe2 having almond like self-layered structure showed excellent hydrogen evolution property than the other two with good onset potential (91 mV), low Tafel slope (53 mV dec1), and high current density (157 mA cm2) at 0.5 M H2SO4.

Experimental part Materials Chemicals used for the reaction purpose are of Analytical category. Sodium molybdate (Na2MoO4.2H2O), red phosphorus powder (98% metal basis), tellurium powder (Te), sulfur powder (S), hafnium chloride (HfCl4), titanium isopropoxide [Ti {OCH(CH3)2}4], zirconium(IV) sulfate tetrahydrate [Zr(SO4)2.4H2O] were purchased from Alfa Aesar. Ethanol, sulfuric acid, hydrazine and other solvents are procured from Spectrochem Pvt. Ltd and Merck (India). Sodium phosphate dibasic heptahydrate (Na2HPO4.7H2O), potassium hydroxide flakes (KOH), cetyltrimethylammonium bromide (CTAB), sodium phosphate monobasic monohydrate (NaH2PO4.H2O), and potassium ferrocyanide [K4{Fe(CN)6}] are procured from TCI Chemicals.

Synthesis of bulk MoTe2and MoTe2 nanoparticles For the synthesis of bulk MoTe2, firstly, tellurium powder (4.0 mmol) was dissolved in 10 mL of hydrazine hydrate solution for whole night that is about 12 h [29]. After that sodium molybdate (2.0 mmol) was added to the hydrazine tellurium solution and heated for 3 h at 100  C with vigorous stirring. The mixture was transferred to the stainless-steel Teflon autoclave and heated in oven for 24 h at 200  C. After the hydrothermal reaction was complete, the autoclave was cooled at room temperature, and the product was centrifuged and washed at 10000 rpm for 3 times with water and ethanol mixture and finally washed 2 times only with ethanol. The black colour MoTe2 was vacuum dried for 24 h. The bulk MoTe2 was now used as precursor for synthesis of MoTe2 nanoparticles, used to prepare the nanocomposite. For the synthesis of MoTe2 NPs, the synthesized bulk MoTe2 was dispersed in 10 ml of water, followed by addition of 0.05 g of CTAB and was sonicated for several hours to form MoTe2 nanoparticle.

Synthesis of group IV element phosphochalcogenide@MoTe2 nanocomposite Herein, we have synthesized three phospho-chalcogenide nanocomposites using three different transition elements

belong to group (IV). For the synthesis of nanocomposite, at first solution A, which is 6 mmol of red phosphorous, was stirred in 20 mL of water for 2 h. In another beaker 18 mmol of sulfur powder was added to 5 mL of hydrazine hydrate and stirred vigorously overnight, designated as solution B. Simultaneously, 3 mmol of group IV metal salts precursor (Zr, Hf, Ti) dissolved in 10 mL of water was taken in another beaker and marked as solution C. Now, solution A, B, and C were mixed together with 0.1 g of previously prepared MoTe2 nanoparticle and stirred vigorously for 12 h. The whole mixture was finally transferred to a stainless-steel Teflon autoclave and heated for 12 h at 150  C. The solution was cooled to room temperature and centrifuged at 10000 rpm, washed with ethanol for several times. The resulting product was calcined at 500  C in air atmosphere for 6 h and resulting nanocomposite MP2S6@MoTe2 (M ¼ Zr, Hf, Ti) was stored in vacuum desiccator. For comparative study, Ti based phospho-chalcogenide nanomaterial i.e. TiP2S6 was also prepared using the abovementioned procedure, in the absence of MoTe2 nanoparticles.

Instrumentation Powder X-ray diffraction (XRD) pattern was recorded on Bruker AXS D8 Advance having configuration vertical, theta/2 theta geometry diffractometer using Cu radiation source (l ¼ 1.5406 A ) at Sophisticated Analytical Instrument Facility (SAIF) at STIC, Kochi. Micromeritics 3Flex surface characterization analyzer was used for the N2 adsorption desorption isotherm BET surface area analysis in central research facility of IIT (ISM), Dhanbad. For obtaining the morphological study Zeiss model Supra 55 of Field-emission scanning electron microscope (FE-SEM) was utilized from central research facility of IIT (ISM), Dhanbad. CH instrument (USA, model number 440C) was used for doing all the electrochemical analysis and all of them were performed at ambient temperature (25 ± 1  C) in IIT (ISM), Dhanbad at Department of Applied Chemistry.

Electrochemical measurement and calculations All the electrochemical measurements were done in an air tight two chamber electrochemical cell having three electrode system i.e. nanocomposite modified pencil graphite electrode (PGE) as the working electrode, Pt wire as the counter electrode, and the reference electrode is Ag/AgCl (3.0 M KCl electrolyte). The schematic representation of electrochemical cell used in this work is shown in Scheme 1. For preparation of nanocomposite modified PGE, at first pencil rod was washed 2 to 3 times by plunging it in 6.0 M HNO3 for few minutes then rubbed with cotton to clean the PGE surface. The cleaned pencil lead was dried in oven at 60  C and the same process was repeated thrice, so that PGE gets free from all the impurities, if present any. For catalyst loading on the PGE, 13 mg of nanocomposite is mixed with Nafion solution (5 wt%), dispersed in 1:1 vol ratio of water-ethanol mixture solution and whole mixture was ultrasonicated for 1 h to form a homogeneous ink. Electrocatalytic performance of prepared nanocomposite was evaluated, mainly through linear sweep voltammetry (LSV), cyclic voltammetry (CV), bulk electrolysis, and

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nanocomposites, only TiP2S6@MoTe2 have shown a specific and unique shape/morphology TiP2S6@MoTe2 have shown almond nuts type morphology with self-layered structure. At higher magnification the layers in between the almond shaped single TiP2S6@MoTe2 can be clearly visible. The layers inside one almond shaped TiP2S6@MoTe2 have thickness around 20 nm. The layers are arranged one above the other to form the almond like arrangement. These layered structure of TiP2S6@MoTe2, probably help the nanocomposite towards their HER activity. Contrary to this, HfP2S6@MoTe2 and ZrP2S6@MoTe2 failed to show any unique morphology and exhibited flakes like structures (Fig. 1D and E, respectively). The flakes are randomly arranged and agglomerated to form clumped structure. In between these two nanocomposites, ZrP2S6@MoTe2 is consists of flakes bigger in sizes in comparison to the HfP2S6@MoTe2. From the FE-SEM image, we can conclude that TiP2S6@MoTe2 have a very different and uniform morphology with a striking internal layered almond nut like structure, on the other hand ZrP2S6@MoTe2 and HfP2S6@MoTe2 have similar type of flake like structure with different sizes. The change in morphology of these three nanocomposites could be attributed to the selection of transition metal elements only, because other parameters are same for all three nanocomposites.

Scheme 1 e Graphical representation showing role of prepared nanocomposite in hydrogen evolution reaction (HER).

multistep chronoamperometric analyses were performed in N2 (99.9%) saturated air tight electrochemical cell. To avoid Pt contamination, prior to the electrochemical analysis, the electrochemical cell was purged with pure nitrogen gas for 1 h to nullify any oxygen present in the cell, which may interrupt with the hydrogen evolution reaction. In addition, the platinum electrode was cleaned prior to the reaction with chromic acid solution after polishing with alumina powder [30]. The HER activity was executed in various electrolyte i.e. 2.0 M PBS, 1.0 KOH, and 0.5H2SO4, to cover the wide pH range. Every potential has been recorded with respect to the Ag/AgCl electrode as reference and converted to reversible hydrogen electrode (RHE), following the equation given below: ERHE ¼ EqAg=AgCl þEAg=AgCl þ 0:059  pH

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(1)

Here, EqAg/AgCl is the standard potential at 25  C for Ag/AgCl which is þ0.210 V for 3.0 M KCl and EAg/AgCl is the experimental potential acquired taking Ag/AgCl as the reference electrode [31].

Result and discussions Different characterization methods Structural characterization of all the MP2S6@MoTe2 nanocomposites Here in this work, we have synthesized three different kinds of nanocomposites, namely TiP2S6@MoTe2, HfP2S6@MoTe2, and ZrP2S6@MoTe2, using three transition elements of group (IV). To determine the morphology of the nanoparticle FE-SEM analysis was performed and the corresponding FE-SEM images of nanocomposites are shown in Fig. 1(AeC). From the FE-SEM image, one can easily observe that among the three

Powder X-ray diffraction (XRD) characterization of the nanocomposites The X-ray powder diffraction pattern of different shaped phosphochalcogenide@MoTe2 of group IV transition metal (Zr, Hf, Ti) was recorded and shown in Fig. 2. As shown in Fig. 2A, TiP2S6@MoTe2 has XRD peak at 15.43 , 16.18 , 24.89 , 32.19 , 37.74 , 41.52 , 57.14 intended for the plane (111), (004), (115), (117), (131), (420), (606) for Titanium phospho-chalcogenide (TiP2S6) corresponding to the JCPDS no 72-0936 having face centered orthorhombic lattice parameter. While other peaks of the nanocomposite present at 26.27 , 29 , 35.47 , 44.0 ,49.7 can be assigned to (004), (100), (103), (105), (106) planes for MoTe2 according to the JCPDS no 73-1650. XRD pattern of ZrP2S6@MoTe2consist of peak at 15.8 , 21.1 , 23.60 , 41.38 , 45.34 , 48.08 , 56.14 considered for (101), (103), (102), (123), (302), (223), (304) plane of Zirconium phosphochalcogenide (ZrP2S6), which match up to the JCPDS number76-1206, having tetragonal primitive lattice parameter (Fig. 2B). The other peaks at 29.76 , 43.7 , 51.9 , 65.16 present in the XRD pattern could be attributed to the (100), (105), (110), (203) planes corresponds to the JCPDS number 73-1650. For Hafnium phospho-chalcogenide (HfP2S6) the peaks in XRD at 15.3 , 23.4 , 41.47 , 45.4 , 48.15 , 56.10 can be assigned to the (111), (115), (422), (333), (040), (606) plane, matched to the JCPDS no77-1322, which confirms their orthorhombic face centered lattice (Fig. 2C). The peaks of MoTe2 are present at 24.9 , 29.7 , 43.8 ,51.77 , 61.61 , 65.36 correspond to (004), (100), (105), (110), (108), (203) plane, which are confirmed after matching with JCPDS card number 73-1650. The assignment of powder XRD peaks for the entire nanocomposite clearly abides their successful synthesis.

Surface area determination of the nanocomposites N2 adsorption and desorption isotherm was studied for analyzing the BET specific surface area for all the

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Fig. 1 e FE-SEM images of TiP2S6@MoTe2 at lower magnification (A) and (B) and higher magnifications (C); FE-SEM images of (D) HfP2S6@MoTe2 and (E) ZrP2S6@MoTe2 nanocomposites.

nanocomposites. The BET isotherm for all the nanocomposites are shown in Fig. 2D. The surface area of the nanocomposites is found in the given order: TiP2S6@MoTe2 (131.5173 m2/g), HfP2S6@MoTe2 (94.5754 m2/g), and ZrP2S6@MoTe2 (93.0976 m2/g). The pore volume of the prepared nanocomposites was also found to be for TiP2S6@MoTe2 (0.004 cm3/g) HfP2S6@MoTe2 (0.004 cm3/g) and ZrP2S6@MoTe2 (0.0008 cm3/g) and TiP2S6@MoTe2 showed considerable pore volume. Additionally, the pore size distribution of the nanocomposites was also calculated from the desorption section of the N2 isotherm with the help of BJH method and the value are A), HfP2S6@MoTe2 found to be TiP2S6@MoTe2 (236.119  (289.511  A), and ZrP2S6@MoTe2 (277.112  A). From the BET surface area study, we can see that TiP2S6@MoTe2 possess the highest surface area, large pore volume and large pore size, which may help in electrolyte ion transport through the pores, provides large number of active sites and high surface area, which may result in improved performance of the nanocomposites towards HER. After BET surface area study, the electrochemical surface area and roughness factor of the nanocomposites were also measured by recording their electrocatalytic activity against potassium ferrocyanide. For the measurement, 5.0 mM of ferrocyanide solution was taken in the electrochemical cell containing 10.0 mL of 1.0 M KCl as supporting electrolyte and cyclic voltammetry (CV) runs were recorded (Fig. 2E). The electroactive surface area and roughness factor of each modified electrode was estimated by Randles-Sevcik equation [32]:


3 2

1 2

1 2

Ip ¼ 2:687  10 n w D AC 5

0

(2)

Where, Ip ¼ current obtained for ferrocyanide on different electrodes, n ¼ number of electrons transfer during the reaction, v ¼ scan rate, D ¼ diffusion coefficient of

ferrocyanide ¼ 0.76  105 cm2 s1, A ¼ surface area and C0 ¼ concentration of ferrocyanide used. Further, the roughness factor (Rf) was calculated using the equation given below:  Rf ¼ A Ageom

(3)

Here, A ¼ obtained surface area from above equation and Ageom ¼ geometric surface area of the PGE. The calculated values for electroactive surface area and roughness factor were depicted in the Table 1. Electrochemical surface area of other nanomaterials was calculated using CV current recorded for potassium ferrocyanide and are found in the order given below: TiP2S6@MoTe2>HfP2S6@MoTe2>ZrP2S6@MoTe2. TiP2S6@MoTe2 showed the highest electrochemical surface area (0.518 cm2) and the roughness factor (5.76), among the other prepared nanocomposite, which could be attributed to their porous layered structure and high surface area.

Electrochemical impedance spectroscopy (EIS) study EIS study was also performed to evaluate the solution resistance and electrocatalytic kinetics of the prepared nanocomposites and shown in Fig. 2F. The Nyquist plots were recorded for each nanocomposite at the frequency of 100 KHz to 1 Hz. As shown in the Figure, the semicircle region designates the charge transfer resistance (Rct) and forTiP2S6@MoTe2 it is found to be 22 U, implies best mass transfer and lower reaction resistance. The lower Rct value for TiP2S6@MoTe2 recommends the fast electron transfer and favorable HER kinetics at the TiP2S6@MoTe2 modified electrodes surface.

Electrocatalytic activity of prepared nanocomposites towards HER The electrocatalytic activity of prepared nanocomposites towards HER was performed in 0.5 M H2SO4 N2-saturated solution. The electrochemical cell used for the analysis was

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Fig. 2 e XRD pattern of (A) TiP2S6@MoTe2, (B) ZrP2S6@MoTe2 and (C) HfP2S6@MoTe2 nanocomposites; (D) BET adsorption desorption isotherm of all the nanocomposites; (E) Electrocatalytic activity study for all the nanocomposites modified PGEs and bare PGE using potassium ferrocyanide as an electroactive electrolyte; (F) EIS spectra study of the all nanocomposite.

Table 1 e Electrocatalyic surface area using potassium ferrocyanide as an electroactive electrolyte for all the nanocomposites modified and bare PGEs. S. N. 1. 2. 3. 4.

Electrode fabricated

CV current for ferricyanide(mA)

A (cm2)

Roughness factor (Rf)

TiP2S6@MoTe2 HfP2S6@MoTe2 ZrP2S6@MoTe2 Bare electrode

568 464 280 180

0.518 0.423 0.255 0.168

5.76 4.70 2.83 1.86

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separated in two chambers, therefore, in one compartment HER will occur, while in another compartment OER will take place (Scheme 1). Before recording the HER activity of the prepared nanocomposite modified electrodes, some crucial parameters have to be studied and optimized like catalyst loading amount, scan rate, concentration of the supporting electrolyte, etc. Herein also, all the parameters have been optimized and corresponding electrochemical graphs were shown in Fig. 3(AeC). It was found that using TiP2S6@MoTe2 modified electrode, appreciable HER performances with high current density, smaller onset potential, low over potential value was observed at 5.0 mVs1 scan rate using 13 mg of catalyst loading at the PGE in 0.5 H2SO4. Linear sweep voltammetry (LSV) and CV runs of all the nanocomposites of Group IV element were evidenced in 0.5 M acidic H2SO4 electrolyte (Fig. 3D and E). For the assessment of the nanocomposite as a better HER catalyst, LSV run of stateof-art catalyst i.e. commercially accessible Pt/C having 20 wt% Pt in it was also obtained having similar optimization condition (Fig. 3D). We can see that among the three nanocomposites, TiP2S6@MoTe2 demonstrates good HER performance having smaller onset potential (91 mV), low overpotential value 144 mV at a current density of 10 mA cm2, the highest current density obtained for TiP2S6@MoTe2 is 157 mA cm2 at 0.5 V. As reported in the literature, Pt/C possess ~0 V onset potential and our catalyst (TiP2S6@MoTe2) has acquired the value of 0.091 V, which is very near and comparable to the value of standard catalyst reported for HER [33]. In addition, TiP2S6@MoTe2 has also shown comparable current density to that of Pt/C. Other nanocomposite shows inferior HER performance than TiP2S6@MoTe2 and the onset potential of HfP2S6@MoTe2 is found to be 140 mV with respective overpotential value of 204 mV. Similarly, ZrP2S6@MoTe2 has onset potential of 163 mV and overpotential value of 299 mV. The maximum current density of HfP2S6@MoTe2 and ZrP2S6@MoTe2 is found to be 128 mA cm2 and 67 mA cm2, respectively. In addition, we have compared the HER properties of TiP2S6 and MoTe2 in the Fig. 3D. It is observed from the LSV plot that TiP2S6 has the onset potential of 626 mV and the MoTe2 has the onset potential of 467 mV, which is better than pristine TiP2S6. However, if we compare the value of these individual nanomaterials with their nanocomposite, the prepared TiP2S6@MoTe2 has shown smallest onset potential of 91 mV, owing to the increase in active sites and the synergetic effect between TiP2S6 and MoTe2 [34]. According to the literature, Tafel plot shows the performances of the catalyst headed for HER. The main equation for Tafel plot is [35]: h¼a þ

2:3RT logðjÞ anF

(4)

Where, h ¼ is the overpotential, j ¼ current density and 2.3RT/ anF ¼ Tafel slope. Tafel slopes are calculated from the LSV curve following equation h ¼ a þ b log j, of the respective nanocomposite by taking linear portion of the LSV plot, where log of current density is in the X axis and overpotential in the Y axis. Tafel slope having small value is known to show good reaction kinetics with small overpotential value and higher

current density. On the basis of Tafel slope values, in acidic medium three reaction step has been reported for HER [36]. The first one is Volmer step, where the Tafel slope value is 120 and the reaction follows pathway: mVdec1 Hþ þ e þ Cat/CatHads, where, CatHads is the adsorption of hydrogen in the active catalytic sites of the surface of catalyst. Second step is called as Heyrovsky reaction pathway, having Tafel slope of 40 mVdec1 and the plausible reaction is pathway is: Cat Hads þ e þ Hþ/ Cat þ H2. Third step, possess the Tafel slope of 30 mVdec1, called as Tafel reaction step and follow the reaction: 2 Cat Hads/ 2 Cat þ H2. In the first step or pathway, hydrogen gets adsorbed in the surface of the catalyst which boosts discharge of proton from the acidic electrolyte, unite with electron and proton to release one hydrogen molecule, this complete reaction pathway is called VolmerHeyrovsky pathway (VeH pathway). In the next step, two adsorbed hydrogen atom on the catalytic surface comes together to form one hydrogen molecule and the catalytic active sites gets free to produce another H2 molecule, this pathway is called Volmer-Tafel pathway (V-T pathway) [37]. Herein, we have also calculated the Tafel slopes for all the nanocomposites and presented in given brackets: TiP2S6@MoTe2 (53 mV dec1), HfP2S6@MoTe2 (79 mV dec1), ZrP2S6@MoTe2 (100 mV dec1), MoTe2 (153 mV dec1) and Pt/C (30 mV dec1) (Fig. 3F). From the Tafel slope, one can found that TiP2S6@MoTe2 has Tafel slope of 53 mVdec1 that means it follows Volmer-Heyrovsky pathway (VeH pathway) with the values of a ¼ 0.55 and b ¼ 0.45. It is very clear from the Tafel slope that in the present HER study, Volmer reaction pathway is the rate determining step for TiP2S6@MoTe2.

Role of TiP2S6@MoTe2nanocomposite towards HER in pH range (0-14) The HER activity of the TiP2S6@MoTe2nanocomposite was studied over the complete pH range. Before the HER study, optimization of KOH and PBS electrolyte concentration was performed and shown in Fig. 4A and B. At optimized concentration of electrolyte, individual LSV plot were recorded at different pH solutions like for basic medium, 1 M KOH (pH ¼ 14) was used. For neutral solution, 2.0 PBS (pH ¼ 7) was used. The respective LSV and CV curves in different pH solutions for TiP2S6@MoTe2 are shown in Fig. 4C and D. The respective onset potential for 1.0 M KOH having pH ¼ 14 and 2.0 phosphate buffer saline having pH ¼ 7 is found to be 220 mV and 280 mV. From the polarization curve the respective overpotential value (Dh ¼ h0 e h10) were also calculated and found to be 270 mV for basic electrolyte and 330 mV for neutral electrolyte at 10 mA cm2. As shown in the figure and portrayed data, all the values are found proficient and comparable to that of the currently reported phospho-chalcogenides in acidic pH. The values are also found comparable or superior than other electrode materials reported so far in the literatures towards HER for entire pH range. The Tafel slopes of the TiP2S6@MoTe2 nanocomposite was also calculated in different pH electrolyte, for pH ¼ 14 it was found to be 71 mV dec1 and for pH ¼ 7 it was found to be 104 mV dec1 (Fig. 4E). From the Tafel slope, it is very clear that in entire pH range TiP2S6@MoTe2 nanocomposite has followed same reaction pathway i.e. Volmer-Heyrosky pathway. The proposed nanocomposite

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Fig. 3 e Optimization of analytical parameters viz. (A) scan rate, (B) loading amount of catalyst, (C) concentration of supporting electrolytes H2SO4 through LSV study; (D) LSV runs of all the prepared nanocomposites and Pt/C in 0.5 M H2SO4; (E) CV runs of all the prepared nanocomposites in 0.5 M H2SO4; (F) Tafel plots of all the nanocomposites including Pt/C. has a considerable current density about ~112 mA cm2 attending 0.70 V potential at pH ¼ 14 and of ~45 mA cm2 under potential 0.95 V at pH ¼ 7. In addition, a comparative study was also performed to equate the performance of proposed catalyst with respect to the other reported catalysts in the literature, in the wide pH range and shown in Table 2 [38e49]. It was found that proposed catalyst has shown comparable or better HER activity in all pH values in comparison to the other reported catalysts. From the different pH (0e14) electrochemical study, it was found that the prepared almond shaped TiP2S6@MoTe2 nanocomposite can be used for both microbial electrolysis cell

(MEC) and the proton exchange membrane (PER) with good stability, high current density, and smaller onset potential. It has also been proved that proposed nanocomposite possess capability to work as an electrode material of universal pH and can be used for various purposes.

Reason for higher HER activity of the nanocomposite From the earlier studies (BET surface area, pore volume, electroactive surface area) we achieved that higher surface area with large pore size could be the reason for better performance of TiP2S6@MoTe2 nanocomposite. However, to

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Fig. 4 e HER LSV plot for optimization of concentration of supporting electrolyte: (A) KOH, and (B) Phosphate buffer; (C) LSV and (D) CV run of TiP2S6@MoTe2 over the entire pH range (0e14); (E) Tafel plot of TiP2S6@MoTe2 at all pH ¼ 0, 7, 14; (F) Graph between calculated and experimentally generated H2 from TiP2S6@MoTe2 versus time for Faradaic efficiency calculation.

further explore the reason behind the improved HER activity of TiP2S6@MoTe2 nanocomposite among others, we have also calculated the electrochemical surface area (ECSA) of all the nanocomposites. For this, the electrochemical double layer capacitance (Cdl) was calculated, after division with specific capacitance (Cs) value gives ECSA value (i.e. ECSA ¼ Cdl/Cs). ECSA was calculated by calculating the capacitive current of non-Faradaic region through the double-layer charging and plotting scan rate reliant cyclic voltammogram over the potential region of 0.0 to þ0.8 V (vs RHE). It is a known fact that double layer charging occurs in non-faradaic potential

window only [50]. CV runs for each nanocomposite were recorded at different scan rates (i.e. 20, 40, 60, 100 and 200 mV s1) and are shown in Fig. 5(AeC). To calculate the value of Cdl, the anodic (Ja) and cathodic (Jc) current density were calculated from CV at 0.20 V, which falls in the nonFaradaic region. After that, the difference between anodic and cathodic current density i.e. DJ (¼Ja - Jc) for all the scan rate was plotted against scan rate (n). The graph must be a linear one, whose slope gives the value of Cdl (Fig. 5D). From the double layer capacitance value, the maximum Cdl was obtained as 238 mF cm2, 86 mF cm2, and 5 mF cm2 for

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Table 2 e Comparative study of performance of HER catalysts reported over entire pH range on the basis of overpotential and tafel slope values with proposed TiP2S6@MoTe2 nanocomposite. S. N.

Electrode Materials

D hAcidic (mV)

bAcidic (mV/dec)

D hneutral (mV)

bneutral (mV/dec)

D hbasic (mV)

bbasic (mV/dec)

Ref.

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12 11.

WP2 NPs/W FePNPs@NPC Cubic Mo6S8 Cu4Mo5O17 film on Cu CoMoS4-H Mo2C@MoS2 Co2P@NiCo2O4 Co3W3C NSs/G Co-NRCNTs CoeNieB Ni-N-MoCx Co-NC-800 TiP2S6@MoTe2

223 130 320 e 143 67 e 64 260 209 163 290 144

66 67 67 e e 37 246 35 69 71 69.32 92 53

210 386 490 270 211 121 e 115 500 83 297 e 330

95 136 127 e e 46 240 107 e e 106.74 e 104

214 214 520 252 193 87 e 84 370 45 124 242 270

92 82 145 e e 39 331 84 e e 82.09 94 71

[38] [39] [40] [41] [42] [43] [44] [45] [46] [47] [48] [49] This work

WP2 NPs/W ¼ Tungsten diphosphide nanoparticles on tungsten foil, FePNPs@NPC ¼ Phytic acid-derivative iron phosphides incorporated in in N,P-codoped carbon, Co3W3C NSs/G ¼ Co3W3C nanosheets on graphene nanocomposite, Co-NRCNTs ¼ Cobalt-embedded nitrogen-rich carbon nanotubes; rGO ¼ reduced graphene oxide; NieN-MoCx ¼ Ni, N-codoped MoCx; Co-NC-800 ¼ Cobalt-embedded nitrogen-doped porous carbon sheet.

TiP2S6@MoTe2, HfP2S6@MoTe2 and ZrP2S6@MoTe2, respectively. The respective ECSA values are also calculated and found as 7 cm2, 2.15 cm2, and 0.125 cm2 for TiP2S6@MoTe2, HfP2S6@MoTe2 and ZrP2S6@MoTe2, respectively. So, it can be concluded from the above calculations that TiP2S6@MoTe2 has the highest electrochemical surface area with large number of exposed edges exposing S and P active sites more available for HER than other nanocomposite. As evident by the SEM study of prepared nanocomposites, selection of metal salt during their synthesis has enormous effect on the morphology of resulting nanocomposite. We have obtained different morphology for different metal-based nanocomposites like TiP2S6@MoTe2 have shown almond nuts type morphology with self-layered structure, while, contrary to this, HfP2S6@MoTe2 and ZrP2S6@MoTe2 failed to show any unique morphology and exhibited flakes like structures. Depending upon different morphology and structurers, the prepared nanocomposites have shown different electrocatalytic activity and ECSA values. Similarly, it has also been reported in the literature that with change in the metal ions the intrinsic properties of the

Faradaic efficiency ¼

improvement and better performance. According to Balandin et al. titanium has the variable valence with increased acid properties, which might be the reason for higher catalytic property of the Ti than Hf and Zr [52].

Faradaic efficiency of TiP2S6@MoTe2 Furthermore, exhibiting best HER performance in all the studies for TiP2S6@MoTe2, the faradaic efficiency of TiP2S6@MoTe2 was also calculated from the amount of hydrogen gas evolved using constant potential bulk electrolysis method. Water displacement method is used to calculate the amount of hydrogen gas evolved during the HER process. For this, a burette was inverted and placed in the HER compartment of electrochemical cell and hydrogen gas collected inside it was calculated after certain interval of time, via displaced amount of water in the burette. The amount of H2 collected in the burette is linearly proportional to the time. Faradaic efficiency was measured by dividing the theoretical value with that of the experimental value. The used equation for calculation of Faradaic efficiency is given below [53]:

Experimental amount of obtained hydrogen  100 Theoretical amount of obtained hydrogen

nanomaterials and composition of active sites also get changed, which results in their change in electrocatalytic activity. Constructing a new morphology through change in metal species, which possess high surface area and fast electron transport rate may improve the activity of the catalyst towards HER [51]. In this work also, the prepared all three kind of nanocomposite materials have shown good HER performance but among them TiP2S6@MoTe2 have shown more

In the two-compartment electrochemical cell, at the working electrode, H2 gas was evolved and the steady current was calculated though long-term bulk electrolysis. The plot between the experimental H2 evolved and theoretical value of H2 evolved was plotted and shown in Fig. 4F. The value of maximum Faradaic efficiency was found to be 98%, which implicates that the TiP2S6@MoTe2 can be used as electrocatalyst for real time and large-scale production of hydrogen.

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Fig. 5 e CV runs at different scan rates in the potential range of þ0.0 to þ0.8 V for (A) TiP2S6@MoTe2, (B) HfP2S6@MoTe2, and (C) ZrP2S6@MoTe2; (D) Plot between current density (D J) versus scan rate for all the nanocomposites at þ0.2 V; CV runs at different cycle to examine the stability of TiP2S6@MoTe2 in (E) acidic and (F) basic medium.

Stability of the TiP2S6@MoTe2 nanocomposite The stability of the nanocomposite is very important because it ensure the real time and industrial/commercial application of the prepared nanocomposite. It is essential that the prepared nanocomposites must be physically and chemically stable, have cost effective preparation, enough stability at different pH, different electrolytic condition and can be used for prolong studies. Herein also, we have tested the stability of TiP2S6@MoTe2 by various studies like controlled potential

electrolysis, multistep chronoamperometry, cyclic voltammetry, and linear sweep voltammetry. Firstly, to examine the stability of TiP2S6@MoTe2 multiple cycle of CV runs were recorded in the potential window of 0.2 to 1.5 V at constant scan rate of 5 mV s1 (Fig. 5E). The current density was measured after different cycles (i.e. 12000) are found similar. It can be clearly seen in the figure that after different cyclic runs, almost negligible alteration in onset potential and current density was observed. Similar study was performed in basic (Fig. 5F), as well as in neutral medium

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Fig. 6 e (A) Cyclic CV stability at neutral pH for TiP2S6@MoTe2; (B) Bulk electrolysis to observe the long-term stability of TiP2S6@MoTe2; (C) LSV run before and after performing all the electrochemical studies using the same TiP2S6@MoTe2 modified electrode; (D) Storage stability of the nanocomposite TiP2S6@MoTe2 for several months; (E) Multi-step chronoamperometric runs for TiP2S6@MoTe2 and (F) Corresponding LSV runs after Multi-step chronoamperometric runs.

(Fig. 6A) also, and extraordinary stability of prepared nanocomposite is found which suggest the wide pH range stability of prepared nanocomposite. Controlled potential electrolysis was done at a constant potential of 1.4 V in 0.5 M H2SO4 for 15000 s (Fig. 6B). No observable variation in the current density for such a long-term analysis, suggest the stable and constant performance of the proposed catalyst. The superior stability of nanocomposite maybe supported from the unaffected performance of nanocomposite, subsequent to the overall electrochemical HER study. To prove this,

the LSV runs taken on unused (before electrochemical studies) and used (after the entire electrochemical performance studies) catalyst were observed (Fig. 6C). More or less no alteration in the current density and onset potential value was observed. Furthermore, the TiP2S6@MoTe2 customized electrode was kept in storage at normal temperature for 9 months. After each 15e30 days, linear sweep voltammetry runs were recorded to check their stability. Small change in onset potential with minimal change in current density was noticed after nine months of storage of nanocomposite (Fig. 6D).

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Multistep chronoamperometric run was performed by the prepared nanocatalyst followed by recording of LSV on the same electrode. Any obvious change in the onset potential and current density is not observed in any one of the studies (Fig. 6E and F) and nanocatalyst shows long term multistep chronoamperometric stability. The overall study clearly suggests the higher stability and robustness of the nanocomposite in various aspects.

Conclusion This work reports the synthesis of Group IV transition metal phospho-chalcogenides nanocomposite with MoTe2 i.e. TiP2S6@MoTe2, HfP2S6@MoTe2 and ZrP2S6@MoTe2. Among the prepared nanocomposite, TiP2S6@MoTe2 shows the best electrocatalytic property towards HER. The prepared TiP2S6@MoTe2 has unique almond like self-layered structure with high BET surface area, high electrochemical and electrocatalytic surface area, and exhibited high stability for several months and for several cycle in CV and LSV studies. The prepared catalyst shows high Faradaic efficiency value of 98% towards production of hydrogen. The HER activity of the best suited catalyst TiP2S6@MoTe2 was also examined in wide range of pH from 0 to 14 and their stability study was also done in the wide pH range, in which the catalyst gives acceptable performances. The electrocatalyst TiP2S6@MoTe2 shows very good onset potential 91 mV and a small Tafel slope of 53 mV dec1 with small overpotential value of 144 mV towards HER at 0.5 M H2SO4. Beholding the excellent catalytic activity, unique morphology, high BET and electrochemical surface area, high Faradaic efficiency, it is very clear that TiP2S6@MoTe2 have potential to be used in commercial purposes, energy application as well for real time purposes.

Acknowledgment Authors are thankful to DST for sponsoring the research project to R.M. (Ref. No.: SERB/F/2798/2016-17). The experimental work has been carried out by Ms. Karfa with the help of Mr. Majhi and she is responsible for all the data presented in this work.

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