ZnWO4 nanocomposite electrodes for electrocatalytic water splitting

international journal of hydrogen energy xxx (xxxx) xxx

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Chemically prepared Polypyrrole/ZnWO4 nanocomposite electrodes for electrocatalytic water splitting K. Brijesh**, K. Bindu, Dhanush Shanbhag, H.S. Nagaraja* Department of Physics, National Institute of Technology Karnataka, P.O. Srinivasnagar, Surathkal, Mangaluru 575025, India

article info

abstract

Article history:

ZnWO4, PPy, and PPy/ZnWO4 nanoparticles were prepared using chemical synthesis. The

Received 20 September 2018

structural, compositional and morphological properties of the prepared samples have been

Received in revised form

investigated using XRD, FTIR, SEM, and HRTEM respectively. The powder XRD reveals the

30 October 2018

monoclinic wolframite structure for both ZnWO4 and PPy/ZnWO4 nanocomposite. SEM

Accepted 1 November 2018

confirms the wrapping of ZnWO4 with PPy. The electrodes of ZnWO4, PPy, and PPy/ZnWO4

Available online xxx

have been tested as bifunctional electrocatalyst towards HER and OER using constant current chronopotentiometry (CP) and Linear Sweep Voltammetry (LSV). The electro-

Keywords:

chemical surface area and the electrocatalytic activity PPy/ZnWO4 nanocomposite towards

ZnWO4/PPy

HER and OER are greater than that of pure ZnWO4 and PPy. The Tafel slope of PPy/ZnWO4

Water splitting

nanocomposite is 76 and 84 mV dec
1 in 0.5 M H2SO4 and 1 M KOH at room temperature for

Hydrogen evolution

HER and OER respectively. The results suggest that PPy/ZnWO4 nanocomposite is a good

Oxygen evolution

candidate for the bifunctional electrocatalyst for water splitting.

Electrochemical surface area

© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Specific site density

Introduction Increase in demand for power generation and the limited availability of fossil fuels, leading towards the new development of electrochemical energy conversion systems around the world [1e4]. Hydrogen, owing to its high gravimetric energy density than any other known fuels and because of its clean and sustainable features, is a promising energy carrier [5]. Water is a rich source of hydrogen. Water splitting is one of the efficient ways to produce the good quality of hydrogen and oxygen. Water electrolysis consist of two half reactions namely Oxygen Evolution Reaction (OER) and Hydrogen

Evolution Reaction (HER) [6]. The OER is a complex reaction because of its proton-coupled electron transfers and O]O bond formation leading to unfavoured kinetics and it requires a catalyst to speed up the reaction. This imposes limitations on the large-scale production of hydrogen [7]. The noble-metal oxides and the platinum-based materials are the catalysts for OER and HER respectively. Due to their less availability and high cost limit their wide application. Hence, development of novel electrocatalysts is an area of the research scope. From couple of decades, a significant amount of efforts are taken to develop efficient water splitting catalysts on the basis of oxides, sulfides, selenides, phosphides, and borides of metals (Fe, Co, Ni) etc. [8e12]. At present, various class of platinum-

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (K. Brijesh), [email protected] (H.S. Nagaraja). https://doi.org/10.1016/j.ijhydene.2018.11.022 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Brijesh K et al., Chemically prepared Polypyrrole/ZnWO4 nanocomposite electrodes for electrocatalytic water splitting, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.11.022

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free electrocatalysts for HER and OER have emerged, which includes transition metal oxides [13e16] metal chalcogenides [15,17e19] carbon materials [20e23] and Transition metal tungstates [24e26]. The metal tungstates of inorganic materials has wide range of applications such as X-rays and g-rays scintillator [27], solid state laser host [28], and photoanodes [29], supercapacitor [30], lithium ion battery [31e33] photocatalyst and photo electrocatalyst [34,35] since tungsten atom has high oxidation [36]. The compounds having metal tungstates crystalizes into either wolframite form of structure or into scheelite structure [37]. Metal tungstate is one of the important class of materials, which shows good rate capability, superior super capacitive performance, excellent cyclic performance and large specific capacitance [38]. Among them, ZnWO4 is the study of interest as it shows good electrochemical properties [39]. The immobilizations of the dispersed metal catalysts are often aided by conducting Polymers. Owing to the high surface area and porous structure, the conducting polymers favour as the ancillary material for the synthesis of novel electrocatalysts. The comparatively good electrical conductivity of the polymers helps to transport the electrons in an electrocatalytic reaction between the electrodes and scattered metal particles in the polymer chain. Thus, an effective electrocatalysis can be achieved by these composite materials. The sufficient amount of attention is drawn to use the conducting polymers as secondary matrices as catalytically active particles for the immobilization [40]. Based on their controllable electrical conductivity and redox states, conducting polymers can be used as supercapacitor electrode material. Among the conducting polymers, polypyrrole (PPy) has gained particular interest since it has high conductivity, environmental stability, better redox property and commercial availability of initial monomers [41]. Recently researches are focused on ZnWO4 in the field of photoelectrochemical water splitting and electrochemical water splitting. Yuan et al. [42], prepared and reported the photoelectrochemical water splitting property of the WO3@ZnWO4@ZnOeZnO hierarchical nanocactus arrays and proposed photoanode has an efficient PEC performance. Rani et al. [43]. prepared a XWO4 (X ¼ Co, Cu, Mn, Zn) nanostructures by solvothermal method and studied for electrochemical water splitting application. It was reported that ZnWO4 has a poor performance towards the electrocatalytic activity than other tungstate materials. In order to increase the performance of ZnWO4 as an efficient electrocatalyst in the water splitting applications, we focused on combining the material with conducting polymer and achieved better performance of ZnWO4 in the presence of polymer composite. Herein, we report the synthesis of PPy/ZnWO4 nanocomposite which has good electrocatalytic activity for water splitting, with comparatively lower over-potential for HER and OER than reported metal tungstates and their composites.

Experimental Preparation of ZnWO4 nanoparticles The chemicals in the present investigations are procured from Sigma Aldrich and are used as obtained. Zinc tungstate

(ZnWO4) nanomaterial was synthesized by single step hydrothermal method. Zinc nitrate and sodium tungstate are used as raw materials. An aqueous solution of 0.3 M Zn(NO3)2$6H2O and Na2WO4$2H2O were mixed together and stirred well to get a homogeneous mixture. To the obtained mixture, about 0.5 ml of ammonia solution is added and stirred well for about 30 min. Then, the reaction mixture was transferred to a Teflon jar of 120 ml capacity and kept in hydrothermal autoclave at 180  C for 24 h. The precipitate was collected by centrifugation and washed with distilled water and acetone to remove impurities. The sample was dried at 60  C for 12 h in hot air oven.

Preparation of polypyrrole (PPy) and PPy/ZnWO4 nanocomposite To prepare Polypyrrole, 0.4 ml of pyrrole monomer was dispersed in 50 ml of distilled water and stirred for an hour to get good dispersion. Then, 50 ml aqueous solution of anhydrous ferric chloride (2.125 g) was added to the pyrrole dispersion dropwise with constant stirring. Reaction was allowed to proceed for 8 h. Obtained solution was filtered and the precipitate was separated using a Buchner funnel. The residue was then washed with distilled water and acetone several times and kept for drying at 60  C for 12 h in a hot air oven. To synthesize PPy/ZnWO4 nanocomposite, the procedure for the preparation of polypyrrole was repeated with the addition of 50 mg of prepared ZnWO4 into the reaction mixture.

Electrode preparation The working electrodes for HER and OER were prepared using active material coated nickel foams (2l  1w). About 20 mg of active materials (PPy, ZnWO4, and PPy/ZnWO4 nanocomposite) were added in the mixture of ethanol (100 ml), distilled water (300 ml) and Nafion resin (50 ml), by 20 min continuous ultra-sonication. Then, 100 ml of the obtained ink was drop-casted on 1 cm2 nickel foam using a micropipette and dried overnight at room temperature.

Characterization The crystal structure of the prepared samples was analyzed by X-ray diffractometer (XRD, Rigakuminiflex 600 XRD instrument) at the scanning speed of 2 /min. The compositional study was carried out by Fourier Transform Infrared Spectroscopy (FTIR) (FTIR, Bruker Alfa FTIR spectrometer). The morphological and microstructure investigated using Scanning Electron Microscope (SEM) (JEOL, JSM 6380 LA) with an electron beam of kinetic energy 20 kV. High-Resolution Transmission Electron Microscope (HRTEM) analyses were performed using a JEOL JEM-2100 microscope with an acceleration voltage of 200 kV. With the help of Brunauer-EmmettTeller (BET) (Quanta Chrome Nova- 1000 Instrument), the specific surface area, the pore diameter and pore volume of the samples were evaluated. The electrochemical properties of the materials were studied using biologic SP-150 electrochemical workstation at room temperature.

Please cite this article as: Brijesh K et al., Chemically prepared Polypyrrole/ZnWO4 nanocomposite electrodes for electrocatalytic water splitting, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.11.022

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Calculations The electrochemical surface area is used for charge transfer and/or storage. The electrocatalytic activity is attributed to their intrinsic catalytic nature and surface area effects. The Electrochemical surface area (ESA) calculation can be done using equation (1) [44],
ESA cm2 g
1 ¼

Catalyst ChargeðCÞ 2mðgÞ Charge of nickel foam ðCÞ

(1)

where, m is the active mass of the material. Specific site density is calculated using equation (2) [45], SDm ¼

Integrated CV Area ðA VÞ N ðsites=molÞ n scan rate ðV=sÞ F ðC=molÞ m ðgÞ

(2)

where, n is the number of electrons, m is the active mass of the material, N is the Avogadro number (6.023  1023) and F is Faraday Constant (96485 C mol
1). The turn over frequency (TOF) is the rate of electron delivery per surface metal atom per second [46]. TOF is one of the good parameters to measure the activity of the catalyst [47]. The TOF is calculated using equation (3), TOF ¼

ImðfixedÞ e SDm

(3)

where, SDm is the Specific Site Density and e is the charge of an electron.

Results and discussion Structural and morphological properties Structural properties Fig. 1(a) shows the powder XRD patterns of PPy, ZnWO4, and PPy/ZnWO4 nanocomposite. The diffraction pattern of PPy with no significant peaks indicates the amorphous nature of PPy. XRD patterns of ZnWO4 and PPy/ZnWO4 were indexed using Xpert high score plus software. Both ZnWO4 and PPy/ ZnWO4 have monoclinic Wolframite structure and match the JCPDS reference # 00-015-0774. A broad hump in the range of 18 to 35 in the PPy/ZnWO4 pattern is attributed to the incorporation of PPy into the composite. Fig. 1(b) depicts the IR spectra of the Pure PPy, ZnWO4 and PPy/ZnWO4 nanocomposite. The FT-IR spectrum confirms the

3

existence of PPy in the PPy/ZnWO4 composites as shown in the figure. For the PPy, the band at 1542 cm
1 is assigned for anti-symmetric stretching and 1454 cm
1 is assigned to symmetric stretching vibration mode of the ring. The peak at 1297 cm
1 is assigned to CeN stretching vibration of the ring and 1040 cm
1 is of fundamental vibration of the pyrrole ring [48]. The bands at 426, 458 and 495 cm
1 are due to the stretching modes of ZneO bands. The band at 787 cm
1 assigned to stretching vibration of WeO. Also, the bands at 924 cm
1 and 964 cm
1 are due to bending and stretching modes of vibrations of ZneWeO respectively [49,50]. The characteristic peaks of PPy in the PPy/ZnWO4 nanocomposite indicate the incorporation of ZnWO4 nanowires in PPy. We can observe that the stretching vibrations of PPy/ZnWO4 are moved towards the higher frequency side which can be attributed to the van der Waals force between Polypyrrole chain and ZnWO4, and the homogeneous distribution of ZnWO4 on the polymeric chain [51e53].

Morphological properties The morphological features of the prepared PPy, ZnWO4 and PPy/ZnWO4 nanocomposite were studied using SEM analysis. The morphology of the nanoparticles is influenced by synthesis method, reaction time, reaction temperature, precursors. Fig. 2. Shows the SEM and TEM images of PPy, ZnWO4, and PPy/ZnWO4 nanocomposite. SEM image of PPy shows agglomerated spherical shaped particles with elongated chain pattern. The TEM image of the PPy shows a spongy kind of amorphous-like structure. ZnWO4 has nanorods kind of structure and it can be seen clearly from TEM image of ZnWO4 in Fig. 2(g). The HRTEM of the ZnWO4 in Fig. 2(h) shows the atomic arrangement pattern of ZnWO4. The SEAD image of ZnWO4 in Fig. 2(i) shows the crystalline nature of ZnWO4 while the SEAD of PPy shows the amorphous nature. The morphology of composite is same as the morphology of PPy. It is because of the ZnWO4 nanorods are embedded into the PPy. From Fig. 2(j), we can observe that the ZnWO4 nanorods are distributed in the matrix of spongy PPy.

BET surface area analysis The specific surface area (SSA) of the samples were estimated using BET adsorption-desorption isotherm. The adsorption and desorption isotherms of PPy, ZnWO4 and PPy/ZnWO4 nanocomposite are given in Fig. 3. Further, the porosity of the

Fig. 1 e (a) XRD patterns and (b) FTIR of PPy/ZnWO4 nanocomposite, Pure ZnWO4 and Pure PPy. Please cite this article as: Brijesh K et al., Chemically prepared Polypyrrole/ZnWO4 nanocomposite electrodes for electrocatalytic water splitting, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.11.022

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Fig. 2 e SEM images of (a) PPy, (b) ZnWO4 and (c) PPy/ZnWO4 nanocomposite. TEM and HRTEM images of (d and e) PPY, (g and h) ZnWO4 and (j and k) PPy/ZnWO4 nanocomposite at different magnifications respectively and SAED patterns of (f) PPy, (i) ZnWO4 and (l) PPy/ZnWO4 nanocomposite. prepared samples was found out using BJH pore size distribution analysis. The pore diameter, pore volume and SSA of the samples are tabulated in Table 1. The SSA of the PPy/ ZnWO4 composite is 10 times greater than that of PPy and

ZnWO4. The pore volume is two times greater than that of the ZnWO4. From the SEM micrograph, it was evident that ZnWO4 nanorods are embedded into the PPy polymer chains which

Please cite this article as: Brijesh K et al., Chemically prepared Polypyrrole/ZnWO4 nanocomposite electrodes for electrocatalytic water splitting, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.11.022

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Fig. 3 e Nitrogen absorption-desorption isotherms of (a) PPy, (b) ZnWO4 and (c) PPy/ZnWO4 nanocomposite (inset are the corresponding pore size distribution curves).

also increased the pore diameter and pore volume of the composite with good amount of polymer content surrounding the ZnWO4 nanorods. Resulting in increased surface area of the composite. The presence of polymer matrix contributes to the increase in pore diameter and pore volume. This in turn increases the specific surface area of the composite. On composite formation, PPy grows around ZnWO4 nanorods which is confirmed in TEM images, without changing its volume since it possesses spongy type morphology. Surfaces of both ZnWO4 and PPy will be available for the reaction because of the porous structure of the PPy even though the PPy surrounded on ZnWO4. So, the specific surface area in BET and electrochemical surface area of the composite is more than that of pure ZnWO4 and PPy.

Electrochemical measurements for water splitting application The electrocatalytic behaviour of the prepared electrodes for both HER and OER were studied using three electrode systems with coated nickel foams as working electrode. Platinum

Table 1 e BET surface area of the PPy, ZnWO4 and PPy/ ZnWO4 nanocomposite. Samples

Surface Area (m2g
1)

Pore Volume (ccg
1)

Pore diameter (nm)

PPy ZnWO4 PPy/ ZnWO4

10.838 24.209 135.552

0.035 0.118 0.225

1.478 1.250 6.359

electrodes and saturated calomel electrode (SCE) were used as counter and reference electrodes respectively. 0.5 M H2SO4 and 1 M KOH were used as electrolytes for HER and OER respectively. Cyclic voltammetry (CV), chronopotentiometry (CP) electrochemical impedance spectroscopy (EIS) and linear sweep voltammetry (LSV) techniques were carried out to investigate the electrocatalytic properties of the prepared electrodes at room temperature.

Hydrogen evolution reaction (HER) The electrocatalytic activity of the prepared samples for hydrogen evolution reaction was performed using a threeelectrode system in 0.5 M H2SO4 at room temperature. CV with scan rate of 50 mVs
1 and LSV with scan rate of 5 mVs
1 were run in the potential range of
1.2 to 0 V. By applying iR compensation, the background currents and ohmic losses were removed. E (RHE) ¼ E (SCE) þ 0.244 V [6] was used to calibrate the resulting data to the reversible Hydrogen electrode (RHE). Fig. 4(a) and (b) show the CV curves and LSV respectively, of PPy, ZnWO4, and PPy/ZnWO4 nanocomposite in the potential range of
1.2 to 0 V. CV curves clearly show the high current in the recorded potential range. The onset potential of PPy, ZnWO4, and PPy/ZnWO4 nanocomposite from LSV curves at the current density of 10 mA cm
2 are
556 mV,
372 mV and
543 mV respectively. ZnWO4 has the lowest onset potential, whereas the current density of the PPy/ZnWO4 nanocomposite increases rapidly with the potential than that of the ZnWO4. The current density of the composite reaches ~600 mA cm
2 at
1.2 V, whereas for PPy and ZnWO4 it reaches to ~400 and ~500 mA cm
2 respectively.

Please cite this article as: Brijesh K et al., Chemically prepared Polypyrrole/ZnWO4 nanocomposite electrodes for electrocatalytic water splitting, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.11.022

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Fig. 4 e (a) Cyclic voltammetry (50 mVs¡1) and (b) iR corrected LSV (5 mVs¡1) of the PPy, ZnWO4 and PPy/ZnWO4 nanocomposite in the potential range of ¡1.2 to 0 V (c) Tafel plot from iR corrected LSV curves (d) Chronopotentiometric curves of prepared electrodes at an applied current of ¡30 mA for 13 h towards HER.

Kinetics of electrodes and possible mechanism involved in the HER reaction was inspected by the Tafel slope. According to the theory, the mechanism of hydrogen conversion takes place in the acidic medium is classified into three steps [54], 1. Volmer step, H3O þ e
/ Hads þ H2O which is an electrochemical hydrogen adsorption reaction, 2. Heyrovsky step, Hads þ H3O þ e
/ H2 þ H2O which is an electrochemical desorption path and finally 3. Tafel reaction, Hads þ Hads / H2 which is a recombination path. The standard Tafel polarization slopes for Volmer, Heyrovsky, and Tafel are ˂120 mV dec
1, ˂40 mV dec
1 and ˂30 mV dec
1 respectively. Tafel plots were obtained from the LSV curves of PPy, ZnWO4 and PPy/ZnWO4 electrodes as revealed in Fig. 4(c), which has the Tafel slope of 132 mV dec
1, 118 mV dec
1 and 76 mV dec
1. The Tafel slopes of the samples represent the combination of Volmer-Heyrovsky reaction and Volmer-Tafel reaction [55e57]. The lower value of Tafel slope and high current density of the composite PPy/ZnWO4, compared to the pristine materials indicates the enhancement of electrocatalytic activity of composite towards HER. The enhanced electrocatalytic activity of composite towards the HER can be explained on the basis of morphology; specific and electrochemical surface area, and turnover frequency. The specific Surface area (135.552 cm2g-1), electrochemical surface area (131.016 cm2g-1) and specific site density (3.045  1021 Active site-g
1) of PPy/ZnWO4 nanocomposite are higher than that of PPy and ZnWO4 (Values are

given in Table S1 of Supplementary Information), which indicates the enhanced electrocatalytic activity of the composite. The spongy nature morphology of the composite provides the large specific surface area, and the large pore volume, which is responsible for the highest electrochemical surface area for the composite. The stability of an electrode plays an important role in the water splitting reactions. The long-term stability of the samples was studied using chronopotentiometry at a constant current of
30 mA. Fig. 4(d) Shows the chronopotentiometry curves of PPy, ZnWO4, and PPy/ZnWO4 nanocomposite for continuous 13 h and the corresponding potentials are
0.59 V,
0.44 V and
0.52 V respectively. It is observed from Fig. 4(d), PPy/ZnWO4 nanocomposite has more stability than PPy and ZnWO4 electrodes. Further, there is no substantial change in the LSV curves both before and after 13 h chronopotentiometry as show in Fig. F1 (Supplementary Information). To obtain more information about the intrinsic catalytic activity, the turnover frequency (TOF) is calculated using equation (3) and tabulated in Table S1 of Supplementary data. ZnWO4 has the highest TOF (5.906  10
3 h
1), which indicates the highest catalytic activity and PPy has the lowest activity of 2.027  10
3 h
1. On incorporation of ZnWO4 to PPy we have achieved a TOF of 4.085  10
3 h
1, which has a higher activity along with a good stability. The kinetics of the electrode materials is studied using EIS. EIS measurements are carried out using 0.5 M H2SO4 electrolyte in the frequency range of 1 MHze0.1 Hz at the onset potentials, 0 mV,
300 mV,
400 mV and - 500 mV as given in Fig. 5 [58]. Nyquist plots of the samples reveal the semicircles,

Please cite this article as: Brijesh K et al., Chemically prepared Polypyrrole/ZnWO4 nanocomposite electrodes for electrocatalytic water splitting, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.11.022

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Fig. 5 e Nyquist plots of (a) PPy, (b) ZnWO4 and (c) PPy/ZnWO4 nanocomposite with 0 mV, ¡300 mV, ¡400 mV and ¡500 mV Potential Vs RHE towards HER.

Table 2 e Over-potential and Tafel slopes of different tungsten oxides and PPy composites towards HER. Catalyst PPy NiWO4 CuWO4 PPy Ni/PPy Ag/PPy PPy ZnWO4 PPy/ZnWO4

Activity HER HER HER HER HER HER HER HER HER

Potential (mV)@ 10 mA cm
2
587
574 724
558
500
556
372
543

Tafel Slope (mV dec
1)

Solution

Reference

79.1

0.5 M H2SO4 1 M H2SO4 1 M H2SO4 0.1 M H2SO4 0.5 M H2SO4 0.5 M H2SO4 0.5 M H2SO4 0.5 M H2SO4 0.5 M H2SO4

[60] [61] [61] [62] [63] [63] Present work Present work Present work

404 45 173 132 118 76

Fig. 6 e (a). Cyclic voltammetry (50 mVs¡1) (b) iR corrected LSV curves (5 mVs¡1) of PPy, ZnWO4, and PPy/ZnWO4 nanocomposite in the potential range of 1.2 to 0 V. (c) Tafel plot from iR corrected LSV curves (d) Chronopotentiometric curves of prepared electrodes at an applied current of ¡30 mA for 13 h towards OER.

Please cite this article as: Brijesh K et al., Chemically prepared Polypyrrole/ZnWO4 nanocomposite electrodes for electrocatalytic water splitting, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.11.022

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which is due to the charge transfer resistance at the electrode and electrolyte interface. The diameter of the semicircles decreases with increase in the bias voltage, which indicates that the charge transfer resistance has decreased [16,59]. Prepared electrodes have the good electrocatalytic activities towards HER compared to other reported tungsten oxides and PPy composites (Table 2).

Oxygen Evolution Reaction (OER) The electrocatalytic activity of PPy, ZnWO4, and PPy/ZnWO4 nanoparticles for OER was performed in 1 M KOH at room temperature. CV and LSV curves were recorded in the positive potential range of 0 Ve1.2 V with scan rate of 50 mVs
1 and 5 mVs
1 respectively. Fig. 6(b) shows the typical LSV curves of the ZnWO4, PPy and PPy/ZnWO4 nanocomposite with the over-potential of 537 mV, 636 mV, and 507 mV respectively. The over-potential of PPy/ZnWO4 for OER is lower than the over-potential of ZnWO4 and PPy electrodes. Fig. 6(c) shows the Tafel plot of ZnWO4, PPy and PPy/ZnWO4 nanocomposite towards OER, with a Tafel slope of 89 mV dec
1, 155 mV dec
1 and 84 mV dec
1 respectively. The lower values of overpotential and Tafel plot for composite indicates the enhanced electrocatalytic activity [64]. The enhanced performance of composite is attributed to the morphology and specific surface area of the composite as discussed in HER section. The estimated Specific site density per unit gram of KOH treated catalyst is tabulated in Table S2 (Supplementary Information). The Specific site density of the PPy, ZnWO4, and

PPy/ZnWO4 nanocomposite, are listed as 4.141  1021, 1.25  1021 and 7.0046  1021 Active sites-g
1 respectively. Specific site density of PPy/ZnWO4 is comparable with the value of PPy. The electrochemical Surface area of the PPy, ZnWO4 and PPy/ZnWO4 nanocomposite are 127.51 cm2g-1, 39.12 cm2g-1, and 104.99 cm2g-1 respectively. The specific surface area from BET and Electrochemical surface area are comparable and are tabulated in Table S2 (Supplementary Information). The surface area of the PPy/ZnWO4 nanocomposite is higher than the PPy and ZnWO4. The stability of the prepared electrodes (ZnWO4, PPy, and PPy/ZnWO4 nanocomposite) studied in the similar way as mentioned in HER study, at the applied current of 30 mA. Fig. 6(d) shows the chronopotentiometric curves of PPy, ZnWO4 and PPy/ZnWO4 nanocomposite for continuous 13 h at an applied current of 30 mA and the corresponding potentials are 0.56 V, 0.57 V, and 0.53 V respectively. The constant potential with time confirms the continuous evolution of oxygen, which indicates the stability of electrodes. It is observed from Fig. 6(d), PPy/ZnWO4 nanocomposite has more stability than PPy and ZnWO4 electrodes. Further, there are no substantial changes in the LSV curve before and after 13 h chronopotentiometry as show in Fig. F2. (Supplementary Information), which is indicating PPy/ZnWO4 nanocomposite is more stable than PPy and ZnWO4 electrodes towards OER. The prepared electrodes have the good electrocatalytic activities towards OER compared to other reported tungstates and PPy composites. Further, the OER kinetics of the prepared PPy, ZnWO4 and PPy/ZnWO4 nanocomposite has been studied using EIS

Fig. 7 e Nyquist plots of (a) PPy, (b) ZnWO4 and (c) PPy/ZnWO4 nanocomposite with 0 mV, 300 mV, 400 mV and 500 mV Potential Vs RHE towards OER.

Table 3 e Over-potential and Tafel slopes of different tungsten oxide and PPy composites toward OER. Catalyst RGO/ZnWO4/Fe3O4 PPy/IL nanoparticles ZnWO4Nbs Co0.5Mn0.5WO4 CoNiMn-LDH/PPy/RGO CoWO4/Ni NiWO4/Ni PPy ZnWO4 PPy/ZnWO4

Activity

Potential (mV) @ 10 mA cm
2

Tafel Slope (mV dec
1)

Solution

Reference

OER OER OER OER OER OER OER OER OER OER

619 583 475 400 369 336 363 537 636 507

90 55 140 84 77 e e 89 155 84

0.1 M KOH 1 M KOH 1.0 MKOH 0.1 MKOH 1 M KOH 1 M KOH 1 M KOH 1 M KOH 1 M KOH 1 M KOH

[26] [64] [25] [24] [65] [66] [66] Present work Present work Present work

Please cite this article as: Brijesh K et al., Chemically prepared Polypyrrole/ZnWO4 nanocomposite electrodes for electrocatalytic water splitting, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.11.022

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technique. From EIS, the Nyquist plots of PPy, ZnWO4 and PPy/ ZnWO4 nanocomposite have been studied at the vicinity of the bias voltage, 0 mV, 300 mV, 400 mV and 500 mV as given in Fig. 7. As explained in the HER section, the diameter of the semicircle in the low-frequency range vary with the applied bias voltage. The diameter of the semicircles decreases with the increase in the bias voltage, which indicates the decrease in the charge transfer resistance. A comparative value for the OER activity of tungsten oxides and PPy composites are provided in Table 3.

Conclusions Pristine and composites of PPy/ZnWO4 have been synthesized using chemical polymerization method. XRD analysis confirmed the monoclinic wolframite structure of the PPy/ ZnWO4 nanocomposite. SEM and HRTEM analysis show the incorporation of ZnWO4 nanorods into the PPy matrix. The enhanced electrocatalytic activity of the composite towards the HER and OER are attributed to the factors, such as good electrical conductivity, large surface area and porosity on incorporation of ZnWO4 into PPy. PPy/ZnWO4 nanocomposite shows specific surface area of 135.552 m2g-1 and electrochemical surface area of 131.016 cm2g-1, which is higher than that of pure PPy and ZnWO4. The tafel slope of PPy/ZnWO4 is much smaller than PPy and ZnWO4, which is 76 and 84 mV dec
1 for HER and OER respectively. PPy/ZnWO4 nanocomposite has a good stability in HER and OER than the pristine material. The electrochemical results suggest that PPy/ZnWO4 nanocomposite is a good candidate for the bifunctional electrocatalyst for water splitting.

Acknowledgement Authors would like to thank DST SERB project (SB/S2/CMP105/2013 Dated: 20/10/2014) for providing financial support to carry out the research. And also, we would like to thank SAIF Cochin for providing TEM Facility.

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Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2018.11.022.

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