Synthesis and development of nano WO3 catalyst incorporated Ni–P coating for electrocatalytic hydrogen evolution reaction

Synthesis and development of nano WO3 catalyst incorporated Ni–P coating for electrocatalytic hydrogen evolution reaction

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Synthesis and development of nano WO3 catalyst incorporated NieP coating for electrocatalytic hydrogen evolution reaction S.M.A. Shibli*, V.R. Anupama, P.S. Arun, P. Jineesh, L. Suji Department of Chemistry, University of Kerala, Kariavattom Campus, Thiruvananthapuram, Kerala, 695 581, India

article info

abstract

Article history:

Monoclinic phased nano WO3 incorporated NieP coating has been developed and its

Received 3 March 2016

electrocatalytic activity towards HER has been investigated. The grains obtained in NieP

Received in revised form

matrix have been tuned to obtain a uniform and homogenous coating by incorporating

18 April 2016

nano WO3 catalyst. The improved electrocatalytic activity was evidenced from the low

Accepted 20 April 2016

overpotential, high exchange current density and improved double layer capacitance ob-

Available online 24 May 2016

tained from the Tafel polarization and EIS analysis of NiePeWO3 coating. The retention of catalytically active sites and surface homogeneity was evidenced from the intermittent

Keywords:

potential scan and EIS spectra obtained after HER of NiePeWO3 coating. The tolerance

Electroless coating

performance under vigorous conditions provided an insight into the sustained catalytic

WO3

activity and electrochemical stability of the internal layers of NiePeWO3 coating. The

Electrocatalysis

incorporation of nano WO3 catalyst into the NieP matrix has also improved the metal-

Hydrogen evolution

lurgical characteristics of electroless NieP coatings. © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction The Hydrogen Evolution Reaction [HER] has gained renewed attention now a days due to the necessity of a renewable energy carrier which is clean [1e7]. HER is technologically very important since it is one of the most investigated reactions in the photolytic and electrolytic water electrolysers, chlor-alkali industry, fuel cells etc [8e11]. The electrochemical hydrogen generation using alkaline water electrolysis utilising other renewable energy sources is a clean and renewable method for hydrogen production. The high energy consumption and lack of stability of the electrode materials restrains their large scale applications [1,12]. Electrode material with improved catalytic activity can bring down the cost of hydrogen

production. Catalytic activity of the electrode material can be improved by either increasing ratio between real and geometric surface area of the electrode or by the synergetic combination of electrocatalytic components [13,14]. The catalytic systems based on platinum and palladiums are widely accepted as hydrogen evolution catalysts [15]. But the high cost and less availability of these catalysts make water electrolysis an expensive process [2]. Thus in order to bring down the cost of water electrolysis, inexpensive electrode materials which possess low overpotential at high current densities and stability against highly corrosive cell electrolyte are very much essential [1,16]. Non noble metal alternatives like nickel or nickel based alloys and noble metal based composites which possess high initial electrocatalytic activity are widely accepted electrode materials for HER

* Corresponding author. Tel.: þ91 92498 63611 (mobile), þ91 4712308682 (work). E-mail address: [email protected] (S.M.A. Shibli). http://dx.doi.org/10.1016/j.ijhydene.2016.04.156 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

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[12,16e19]. Raney nickel with improved surface area was widely used as electrodes in conventional alkaline electrolysis [20]. But after various cycles of electrolysis the nickel electrodes experience extensive deactivation in the electrolyte solution and hence it loses catalytic activity [21]. The catalytic activity of nickel electrodes has been improved by adopting many methods. This is by alloying it with other transition metals or by improving the surface area by combining with other transition metal oxides or nonmetals [22e26]. Electroless Nickel e Phosphorous (NieP) alloys obtained by the co-deposition of a metal salt and phosphorous are well-known for the enhancement of corrosion resistance, wear resistance and hardness [27e29]. The catalytic role of Ni(OH)2/NiOOH redox couple in Ni electrodes [8,30] are well known and this can enhance the electrocatalytic activity of NieP system. A considerable improvement in the electrocatalytic activity and efficiency of electroless coatings occurs by the incorporation of active composite particles into the NieP matrix. The codeposition of composite particles of sufficient electronic conductivity induces a spontaneous increase in surface roughness of oxide e matrix composite with respect to the pure matrix [31]. This improved surface roughness increases the electrochemically active surface area by the efficient contact of the electrocatalyst with the electrolyte [32]. These codeposited particles act as a good catalytic support also. The support materials exhibit great influence in the electrocatalyst durability. According to the literature available, many transition metal oxides such as Fe2O3, TiO2, RuO2, ZrO2, CeO2 [24,33e35] incorporated into the NieP matrix has improved the electrocatalytic activity of the NieP coatings towards HER. The enhancement of catalytic activity due to the presence of a metal oxide support is called strong metal support interactions [SMSI] which are very important in the field of electrocatalyst [36]. According to the Brewer-Engel theory, when composite particles in the NieP matrix interact with Ni, it gave rise to a synergetic effect which improves the intrinsic activity of the electrode materials. Tungsten oxide is an n e type semiconductor with a band gap of about 2.6e2.8 eV. The high electrochemical stability, natural abundance and low cost make them a suitable catalyst for the conversion of solar energy into chemical fuels [37e39]. WO3 is also reported to be a very good electronic conductor with higher carrier concentration of 5  1019 cm3 and electron mobility of 6.5 cm2 V1 s1 [40,41]. Hou and Wang et al. has reported that WO3 stabilized on manganese based system acts as a good oxygen evolution catalyst [42]. Transition metal oxides such as WO3, TiO2, Nb2O5, ZrO2 [43e45] exhibits intrinsic proton transfer properties at the electrocatalytic interface. This results in the formation of the hydrated metal oxides. Introducing a proton conductor as a catalytic support, increase the effective utilisation of the catalyst which can improve the fuel cell performance [46]. It has been reported that the proton transfer property of the oxides not only improve the effective utilisation of catalytic metals, but also decrease the fuel cell cost by lowering the usage of nafion e ionomer [47]. There are several methods adopted for the synthesis of nano tungsten oxide like pyrolysis, hydrothermal routes, thermal evaporation [48] etc. We have adopted the synthesis of WO3 catalyst under relative mild condition by the wet

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chemical process [49]. In this process, the precursor is aqueous tungstic acid solution obtained by the hydrolysis of tungstate salt solution in acid medium. The monoclinic WO3 nanoparticles were obtained by the controlled thermal treatment of this tungstic acid precursor solution. The effect on electrocatalytic activity of electroless NieP coating by the incorporation of monoclinic WO3 nanoparticles synthesised is investigated and reported in this paper.

Materials and methods Synthesis and characterisation of WO3 nano particles Preparation of tungsten trioxide nano particles: Tungsten Oxide nanoparticles were prepared by the wet precipitation method. The materials used were Sodium tungstate di hydrate (ACS reagent 99% SigmaeAldrich) and Hydrochloric acid (Merck). Sodium tungstate dihydrate (Na2WO4$2H2O) was dissolved in 100 ml distilled water and heated to a temperature of 80  C. Hydrochloric acid (6 N, 30 ml) was added dropwise into the sodium tungstate solution while stirring and the resultant solution was kept 80  C on stirring for 120 min. Thus obtained bright yellow coloured tungstic acid precursor was collected and washed several times with distilled water and absolute ethanol. It was then dried in an air oven at 120  C for 1 h. After drying, the precipitate was calcined in air at temperatures ranging from 250 to 450  C for 2 h. The crystallographic analyses of the powder samples were done using Shimadzu, XRD-6000. The composites were scanned using Cu Ka radiation at a voltage of 40 kV and a current of 30 mA in the 2- theta range 10e80 , at a rate of 2 /min and at a step size of 0.02 . The TEM analysis was carried out using JEOL JEM 2100, Japan instrument equipped with a Kevex energy-dispersive Xray detector operated at 200 kV. The vibrational modes WO3 was studied using Shimadzu FTIR spectrometer in the mid IR range from 350 to 4000 cm1 with KBr pellets. The Micro Raman spectrum of the sample was analysed using Labram HR-800 spectrometer (Horiba Jobin Yvon), excited by a 784.8 nm diode laser. TG and DSC of the sample was recorded on a SDT Q600 V8.3 build 101.

Preparation of nano WO3 incorporated electroless nickelphosphorous coating Preparation and pre treatment of the substrate: Mild steel (MS) substrate (Fe-98.71%, C-0.128%, Mn-0.0942%, Si-0.12%, P0.020%, S-0.017%, Cr-0.013%, Mo-0.02% and Ni-0.011% (in at. %)) was used for the present study. The mild steel strips of dimension 3  2.5  0.1 cm3 were mechanically polished to mirror finish and cleaned thoroughly with distilled water. The cleaned MS was treated with 5% NaOH solution for 5 min to remove any abrasive, dirt, grese and oil from its surface. After rinsing with distilled water, the substrate as etched in 3% HCl for 5 min and then cleaned with trichloroethylene to remove oxides and improve adhesion (ASTM B 656). This was followed by a two step activation process of sensitization and nucleation. The sensitization process include the immersion of the substrate in a solution containing SnCl2 (10 g/L) (Merck India, assay 99%) and HCL (40 ml/L). The nucleation process was

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carried out by immersing the sensitized substrates in the nucleating solution for 2 min. Thus obtained activated MS was used for the coating process.

Electroless nickel coating process The composition of electroless bath was Nickel Sulphate (30 g/ L Sigma Aldrich, assay: 99.9%), Succinic acid (25 g/L, Merck India, assay: 99.5%) and Sodium hypophosphite (Merck India, assay: 99.5%). The bath pH was adjusted to 4.5 by adding ammonia solution. Temperature of the bath during the coating process was maintained at 85 ± 2  C by using a controlled hot plate. Different amounts of WO3 nanoparticles were added into the bath and stirred constantly during the coating process to get WO3 incorporated NieP coatings. The process parameters were standardised to ensure reproducibility using different batches of the specimens. The variation in deposition potential of the substrate in the electroless plating bath during coating process was monitored as a function of time with respect to a standard calomel electrode (SCE). The plating bath was connected to another beaker containing saturated KCl solution by means of a salt bridge. The potential values were then measured with respect to SCE placed in KCl solution.

Characterisation of the electroless nickel coating Hardness of electroless nickel coatings was measured using Vicker's micro hardness indenter (as per ASTM e E 384 - 05) on Shimadzu HMV e 2000. The hardness test was done in a load of 50gf and an indentation time of 12 s. Thickness of the coating was obtained from metallographic Microsection using ISOMET 1000 grinder/polisher and Leica Metallux 3 microscope. The porosity of the coating was analysed by ferroxyl reagent. A solution of potassium ferricyanide, sodium chloride and agareagar in hot water was used as the ferroxyl reagent. The plates after immersing into the reagent for 5 min were inspected for any appearances of Prussian blue colour. The surface morphology and composition of the composite coating was analysed using Scanning Electron Microscope (SEM) [JEOL JSM-840A] and Energy Dispersive X-ray (EDX) spectroscope attached to the SEM instrument. Topographical image of the coating were obtained using Atomic Force Microscop (Bruker-Dimension edge with Scan asyst) in Tapping mode. The current-potential measurements, electrochemical impedance spectroscopic analysis and anodic stripping measurements were carried out using Princeton Applied Research make VERSASTAT 3 potentiostat. A three electrode configuration was used in which the working electrode was fixed in a Teflon holder with an exposed geometric area of 1 cm2, Platinum guaze as the counter electrode and Ag/AgCl the reference electrode. The analysis was carried out in 32% NaOH at a temperature of 30 ± 1  C. Steady state polarization curves were obtained under the potentiodynamic condition of 250 mV above the equilibrium potential. This was followed by an insitu anodic stripping analysis by implying an anodic current density of 50 mA/cm2 on the working electrodes. The AC impedance analyses of the electrodes were obtained immediately after the potentiodynamic polarization and anodic

stripping measurements. The corrosion behaviour of the composite coating was investigated in 32% NaOH after 120 h of immersion time. EIS experiments were conducted in the frequency range of 100 Hze0.01 Hz. All chemicals used were of reagent grade and the solutions were made in double distilled water.

Results and discussion Crystallographic evidence for the formation of monoclinic phased WO3 catalyst The X-ray diffraction pattern of the product obtained by the acid precipitation reaction of sodium tungstate solution was identified as crystalline tungstic acid hydrate (WO3$H2O) having orthorhombic structure [Fig. 1a] JCPDS No: 084 0886 [50]. The orthorhombic WO3$H2O didnot evidence any phase change upto a calcination temperature of 250  C. Further an increase in calcination temperature from 250  C to 350  C has resulted in mixed phased WO3 [Fig. 1c]. This WO3 comprising of orthorhombic, triclinic and hexagonal phases upon calcination at 450  C yielded single phased monoclinic WO3 [Fig. 1d]. All the peaks in this pattern could be indexed as pure monoclinic WO3 (space group: P21/n) with calculated lattice constants of a ¼ 7.29 Ao, b ¼ 7.53 Ao, c ¼ 7.68 Ao (JCPDS No: 0431035) [51,52]. Two characteristic triplets of WO3 comprising of (002), (020), (200) and (022), (202), (220) reflections are seen in the diffraction angle in the ranges 23 e 24.5 and 33 e 34.5 [53]. It has already reported that the facets (200), (020) and (002) of monoclinic facet are good catalysts for the photo oxidation of water [53]. The crystalline monoclinic facet of WO3 are well known for its stability at room temperatures [54,55]. No peaks of tungstic acid hydrate or other facets of WO3 are detected after calcination in the spectrum, revealing the absence of impurities in the product. The effects of annealing temperatures on the crystalline phase of WO3 catalyst at different annealing temperatures were investigated. On annealing, the peak intensity of monoclinic WO3 was found to increase as in the spectra given

(020) (002) (200)

(202) (112) (022) (120) (122)

Intensity(a.u.)

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(220)

(004) (240) (321) (040)

d

c b (111) (200) (040) (002)

20

30

(202)

40

50

a 60

2θ (degree) Fig. 1 e XRD pattern of WO3 catalyst obtained by the thermal treatment of tungstic acid at different temperatures [a) 100  C, b) 250  C, c) 350  C and d) 450  C].

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in Fig. 1 in supplementary information. During annealing a crystallite aggregation process occurred which lead to the formation of a bigger crystalline dimension resulting an increase in grain size [56,57].

Phase stability of the prepared WO3 catalyst TGA e DSC curve was obtained for the tungstic acid hydrate to study the chemical or physical changes taking place during the heating process and hence to improve the stability of calcined product [Fig. 2 in supplementary information]. According to TGA, initially a weight loss of 4.01% was observed at 60  C and a corresponding exothermic peak was observed in the DSC curve. This may be due to the evaporation of physically adsorbed water molecules. The major weight loss of 5.42% was observed at temperatures ranging from 150 to 230  C, in which the major loss occurred at 230  C. Correspondingly an endothermic peak was obtained at about 218  C. This could be ascribed to the loss of structural water of WO3 [58e60]. In addition, a slight weight loss at a temperature of 360  C and a diffused endotherm of DSC curves accounts for the final decomposition during the crystallisation of the amorphous WO3 [52,60]. There was no significant change in mass with further heating upto 1000  C which confirmed the phase stability of WO3 catalyst obtained over the wide range of temperature between 400 and 1000  C [69].

Spectroscopic evidence for the formation of WO3 catalyst

Intensity (a.u.)

The Raman spectra given in Fig. 2 confirmed the XRD results. The most intense band obtained at 806 cm1 and 716 cm1 corresponds to the stretching modes of W6þeO bonds (OeWeO stretching). The bands observed at 329 cm1 and 272 cm1 corresponds to the OeWeO bending modes of the bridging oxide. Thus the four bands obtained from the Raman spectrum corresponds to the wavenumbers of the strongest modes of monoclinic phase of tungsten oxide [61e63] which is the stable form at room temperature. The peaks observed in the wavenumber region of 900 cm1 e 950 cm1 were assigned to the symmetric stretching vibrations of WeO bonds [49,63]. There are two weak bands around 435 cm1 and 450 cm1.

200

400

600

-1

800

1000

Raman shift (cm ) Fig. 2 e Raman spectra confirming the crystalline nature of the WO3 catalyst prepared.

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These are the characteristic bands that accounts for the crystalline nature of tungsten oxide [64]. The structural details of monoclinic WO3 catalyst were further investigated using FTIR spectra [Fig. 3 in supplemen tary information]. FTIR spectrum of monoclinic WO3 was recorded in the region 400e4000 cm1. A broad band located between 570 cm1 and 970 cm1 is attributed to the WeO terminal vibration mode stretching and bending modes [65,66]. A sharp band at 1602 cm1 and broad band at 3480 cm1 arises due to the bending (dH2O) and stretching (gH2O) vibrations of water molecules in the structure.

Confirmation of the formation of nano WO3 catalyst The TEM images of the WO3 catalyst provided an insight into the structure of the crystallised nanoparticles [Fig. 4 in supplementary information]. The average particle size of the WO3 sample was in the range of 90e110 nm, when the distribution statistics was applied for about 200 particles. From the high resolution TEM image of the WO3 nanoparticles [Fig. 5B in supplementary information], the interplanar distance value was calculated as 0.37 nm corresponding to the reflections from (200) plane of WO3 [67e69]. The ring like arrangement of spots obtained from the electron diffraction pattern in Fig. 5A in supplementary information revealed the polycrystalline nature of the composite particle. The particles were of heterogeneously shaped with different orientation and were nanofaceted [70,71].

Development of nano WO3 catalyst incorporated NieP coating Electrochemical and crystallographic evidence for the incorporation of nano WO3 catalyst into the NieP matrix The catalyst prepared after characterising with reproducibility was incorporated into the NieP matrix. The variation in potential with time againt standard calomel electrode was monitored during the deposition process and the trend is given in Fig. 6 in supplementary information. This gave information regarding the nature of substrate surface at the time of deposition and about the rate of deposition. Any change in the deposit character would reflect by the change in potential curve. Initially a sharp variation in the potential towards anodic region was observed for all the coatings. The deposition potential obtained for nano WO3 catalyst incorporated NieP coatings were found to be more anodic than the pure NieP and was in the potential range of 0.623 V to 0.570 V. This shift in equilibrium potential of the surface towards a nobler region suggests the insitu adsorption of the WO3 nano particles on the substrate surface [24]. These codeposited nano WO3 catalyst can act as a catalytic nucleation site for further deposition of the catalyst. The XRD pattern of the inner layer of NiePeWO3 is compared with that of the pure NieP in Fig. 3. A microcrystalline peak of Nickel at a diffraction angle of 44 [JCPDS no: (00-045-1022)] was observed as a common phase in both the patterns [72]. There occurs a peak broadening of Ni for the NiePeWO3 coating revealing the amorphous nature of the coating. The effective entrapment of nano WO3 catalyst into

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electrodes for HER. Also the HER in alkaline media are very sensitive to the surface structure of the electrodes [12]. Thus the electrocatalytic activity of the electrodes can be enhanced by improving its microstructure and surface composition.

Tuning the grain size and morphological characteristics of NieP coating by the incorporation of nano WO3 catalyst

Fig. 3 e XRD pattern confirming the incorporation of nano WO3 catalyst in the NieP matrix [a) pure NieP and b) NiePeWO3 coating].

the NieP matrix was also evidenced from the XRD pattern of NiePeWO3 coating.

Physico-chemical characteristics of the nano WO3 catalyst incorporated NieP coating Stability of the developed NiePeWO3 coating was assessed by evaluating various physicochemical characteristics such as hardness, thickness and porosity. The hardness value for NieP coatings were found to increase with increase in composite content as given in Table 1. The hardness value was in the range of 513e516 VHN for NiePeWO3 coating. Thickness of the coating was obtained from the metallographical crosssectional images Fig. 7 in supplementary information. The thickness of pure NieP and NiePeWO3 coatings for 120 min deposition time was observed to be 10e12 mm and 14e16 mm respectively. The coarse interface existing between the coating and MS substrate might be due to the pretreatments of the substrate. The cross-sectional morphology of pure NieP and NiePeWO3 coating appears to be same. This reveals that there was no appreciable change in cross-sectional morphology of NieP coating by the entrapment of WO3 nano particles. This might be due to the small size and relatively low content of WO3 nanoparticles [73]. The Prussian blue coloration observed during the Ferroxyl reagent test reveals about the porous nature of NiePeWO3 coatings. Thus the developed NiePeWO3 coating has characteristic uniform and porous surface which are the essential features of the

Table 1 e Comparison of physico chemical parameters of NiePeWO3 coating. Sl. No.

NieP0 NieP1 NieP2 NieP3

Weight Hardness Thickness percentage of (VHN) (mm) WO3 in NieP bath 0 2 5 10

g g g g

436 513 514 516

10e12 14e15 14e15 15e16

The morphology of developed NiePeWO3 coating compared with pure NieP coating is given in Fig. 4. The appearances of cauliflower like nodules which are typical of amorphous materials [73] are observed in the developed coatings. This distribution of nodular boundaries was even throughout. The pure NieP coating has large uniform grains of 5e7 mm size with clear facets and well defined grain boundaries. Similarly for the NiePeWO3, the nodules become more prominent with small grains which resembled that of the arrangement of pebbles in a definite pattern. The cluster like appearance on NiePeWO3 arises due to the agglomeration of WO3 nanoparticles deposited from an electroless bath [74]. Thus the obtained NiePeWO3 coatings were compact and continuous due to the uniform distribution of nucleating sites on the activated substrate by the effective surface pre-treatment [72]. As evident from the XRD, the obtained coatings were microcrystalline or amorphous in nature. Surface topography of the NieP coatings was imaged by AFM. The AFM images of nano WO3 incorporated NieP coating are compared with the pure NieP coating in Fig. 5. NiePeWO3 coating appeared to be more crystalline with non-uniform grains possessing small and clear grain boundaries. The slight agglomeration of WO3 particles in NiePeWO3 coating resulted in a large nodular like appearance with deep valleys. This was evident from the SEM results of NiePeWO3 coatings. But in pure NieP large hemispherical nodules occured and it appeared to be smoother. In electroless NieP coating, as the deposition time increased there arised a growth mount due to the overlapping of growing hemispherical grains. This surface appeared to be smoother due to the surface diffusion of both Ni and P species [75,76]. The nodules of NiePeWO3 coating appear to be more crystalline with sharp and prominent edges. The average roughness value of NiePeWO3 and pure NieP coating are 219 nm and 126 nm respectively. Thus the incorporation of nano WO3 particles had improved the surface roughness of NieP coating considerably. In Fig. 8 in supplementary information the elemental composition of the pure NieP and NiePeWO3 coating are compared. The effective incorporation of nano WO3 catalyst into the NieP matrix was evidenced from the EDS spectrum. Thus the EDS analysis of the coating revealed that the intended catalyst has been incorporated into the NieP matrix. Having ensured about the presence of the catalyst in the coating and also about their composition, it was proposed to evaluate the enhanced catalytic activity of the well known catalytic NieP coating by the incorporation of WO3 catalyst.

Porosity

Efficiency and practical feasibility of nano WO3 incorporated NieP coatings for electrochemical HER Less porous Porous Porous Porous

The electrocatalytic activity of NiePeWO3 coating was evaluated under different experimental conditions. The concentration of catalyst content in the NieP matrix was optimised from the cathodic polarisation measurements of NiePeWO3

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Fig. 4 e The SEM images evidenced the improved grain size and effective distribution of nano WO3 in the NieP matrix [pure NieP at magnifications A) X 1000, B) X 3000, C) X 5000 and NiePeWO3 coating at magnifications D) X 1000, E) X 3000 and F) X 5000].

coating. The trend of polarisation behaviour of different amounts (1 g/L, 2 g/L, 5 g/L and 10 g/L) of nano WO3 incorporated NieP coating in 32% NaOH are compared in Fig. 6. The ratio as well as the range of the content of WO3 nanoparticles was preliminarily fixed based on their performance with some short term experiments. A very small cathodic shift was observed for 2 g/L WO3 nanoparticles incorporated NieP coating. The observed overpotential for NiePeWO3 coatings

was much less than that of the pure NieP. This evidenced the enhanced catalytic activity of WO3 nanoparticles incorporated NieP coating. The information obtained from the cathodic polarisation curves revealed that 2 g/L WO3 nanoparticles incorporated NieP coating had higher catalytic activity. Further increase in concentration of WO3 content in the NieP coating has lead to a higher potential shift. Thus the optimum content of the WO3 nanoparticles in bath yielded a coating of

Fig. 5 e AFM images of pure NieP coating [A) 3D and B) 2D images] and NiePeWO3 coating [C) 3D and D) 2D images].

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Potential / V vs SCE

-1.40

d c b a

-1.35 -1.30

Coating

NieP0 NiePeWO3

-1.25

Tafel slope b (mV/decade)

Exchange current density J0 (mA/cm2)

Overpotential at 250 mV h 250 (mV)

120 108

4.26  105 6.943  105

300 165

-1.20 -1.15 1.0

1.2

1.4

1.6

log i / mAcm-2

1.8

2.0

Fig. 6 e The cathodic polarization curve evidenced the tuning of nano WO3 catalyst composition in the electroless NieP bath [ a) 2 g/L, b) 5 g/L, c) 1 g/L and 10 g/Ld) nano WO3 catalyst incorporated NieP coating in 32% NaOH solution].

optimum composite content with more electrochemically active area and good morphology.

Mechanism of HER The enhanced catalytic activity of NiePeWO3 was also confirmed from the tafel parameters obtained from the polarization curves given in Fig. 7. Electrochemical parameters like overpotential h, exchange current density Jo and Tafel slope b were determined from the polarization curves. The measured tafel parameters are listed in Table 2. The tafel parameters were measured at ±250 mV from the equilibrium potential during which, a considerable decrease in the overpotential and increase in exchange current density was observed for the NiePeWO3 coating. The Tafel parameters

-0.3 -0.4

Potential / V vs (SCE)

Table 2 e The electrochemical parameters of Tafel plots of NiePeWO3 coating for HER in 32% NaOH at 298 K.

-0.5 -0.6

b

-0.7 -0.8 -0.9

a

-1.0 -1.1 -1.2

-9.0 -8.5 -8.0 -7.5 -7.0 -6.5 -6.0 -5.5 -5.0 -2

log i / mAcm

Fig. 7 e The Tafel polarization curve evidenced the improved electrocatalytic performance of NiePeWO3 coating [ a) pure NieP and b) 2 g/L nano WO3 catalyst incorporated NieP coating in 32% NaOH solution].

obtained revealed that the HER on pure NieP and NiePeWO3 coating are kinetically controlled reaction. Tafel slope, which determines the rate of change of current density with the overpotential, observed for NieP coating was 120 mV/dec. However the incorporation of WO3 nanoparticles in the NieP matrix has resulted in the lowering of tafel slope value to 108 mV/decade. This phenomenon of lowering of tafel slope value by the incorporation of ionic conducting solid metal oxides has been reported early [32,77]. The electrochemical HER mechanism model in alkaline solutions involves three reactions. The formation of an adsorbed hydrogen atom intermediate, MHads (Volmer reaction), the electrodic deposition of hydrogen into the solution (Heyrovsky reaction) and a chemical desorption resulting in hydrogen evolution (tafel reaction) [21] M þ H2 O þ e /MHads þ OH

ðVolmerÞ

MHads þ H2 O þ e/H2 þ M þ OH MHads þ MHads /H2 þ 2M ðTafelÞ

ðHeyrovskyÞ

(1) (2) (3)

In alkaline media, the mechanism of HER is controlled by the Tafel slope. Unlike in acid media, the hydrogen discharge in alkaline solution occurs from hydronium ions (H3Oþ). Thus the dissociative adsorption of water molecules plays a major role in hydrogen discharge from alkaline solution and thus generally Volmer reaction becomes the rate determining step for HER in alkaline solution [78]. In the present work, the symmetric coefficient b which determines the charge transfer kinetics of HER calculated from the tafel slope was nearly 0.5. The charge transfer kinetics of HER coupled with the tafel slope value reveals that the rate determining step of NiePeWO3 coatings involves the coupling of Volmer with Heyrovsky reaction [79]. It has already been reported that, an electrocatalyst suitable for hydrogen evolution reaction in alkaline medium should always possess optimal hydrogen adsorption energy [80]. At the same time Strminik et al. [81] has reported that the presence of species that bound the hydroxyl molecules (OHads) improves the reactivity of the adsorbed hydrogen intermediates (Hads) on the catalyst surface. Thus materials like WO3 that are oxophilic in nature, offer a strong interaction with hydroxyl group resulting in an improved electrocatalytic activity towards HER in alkaline media. Among the hyper d electronic transition elements, the maximum activity is exhibited by the d8 system due to its highest d-electronic density of sate and maximum d-orbital exposure [82]. According to the Brewer-Engel valence bond theory, when elements of left half of the transition series

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200

-2

160

The electrode/electrolyte interfacial behaviour and the electrocatalytic activity of NieP and NiePeWO3 coating towards HER are investigated with the help of EIS technique. The typical Nyquist plots at equilibrium potential were obtained for pure NieP and NiePeWO3 coating and are compared in Fig. 8A. The electrocatalytic activity of the coating surface towards HER was studied with the help of the EIS technique. EIS measurements were carried out in the potential range corresponding to the linear part of the currentepotential curve of NiePeWO3 (Fig. 8B). The improved electrocatalytic activity of NiePeWO3 towards HER was evident from the depressed semicircle obtained at an overpotential of 250 mV. Again the tolerance of the internal layers of NiePeWO3 coating after HER has been analysed by EIS after the insitu anodic stripping of the electrolysed catalytic surface and the results are compared in Fig. 8C. Thus the stability and the

120 80

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Zre / ohmcm 140

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Zre / ohmcm 45

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Zim /ohmcm

Confirmation of the catalytic activity and tolerance of the internal layers of NiePeWO3 coating from the polarization resistance measurements

140

40

Tolerance of NiePeWO3 coating The variation of potential with time of NieP and NiePeWO3 coating on implying an anodic current density of 50 mA/cm2 are compared in Fig. 9 supplementary information. During cathodic HER for pure NieP coating, there occurs a slight dissolution of nickel from the active sites of the coating leading to the development of a phosphorous rich layer at the coating/solution interface [76]. The insitu anodic stripping of the NieP coating immediately after HER leads to the forceful dissolution of the top layer. This results in the appearance of a phosphorous rich layer, which acted as a barrier layer that hinders the further dissolution of Ni atom from the coating surface. This passive nature of the inner layer blocks the contact of coating with electrolyte preventing the further hydration of nickel [73,76]. During the anodic stripping of NiePeWO3 coating, there was an initial positive shift in potential which further remained constant. This shift in potential might be due to the preferential dissolution of soluble WO3 species from the coating surface as WO2 4 ions in to the bulk electrolyte [85]. The WO3 layer formed on the NieP surface possess large number of grain boundaries (Fig. 4) which also enhanced the dissolution rate [86].

a

A

180

Zim / ohmcm

which are hypo d-electronic is combined with the elements of the right half of the series with more than half filled d-orbitals leads to the synergism that imparts high bond strength and stability to the intermetallic phase [83]. Thus for HER, the stronger the ded interelectronic interaction, the d orbitals will be more exposed and strengthened while the intermediate MH or MeOH bond strength will be less and this further enhances the overall rate of HER. The combination of hyper d-electronic elements of high electronic density at the Fermi level with hypo d-oxide of tungsten, niobium, molybdenum possesses extended d e orbitals which are oxophilic in nature causes a high SMSI effect and lead to a primary oxide transport and effusion [83,84]. So, in metal catalyst with a metal oxide support, the water dissociation occurs at active site of the metal oxide while the adsorption of atomic H and association to H2 molecule occurs at the metal surface [78].

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Fig. 8 e Nyquist plots of A) a) NiePeWO3 and b) NieP at OCP B) a) NiePeWO3 at OCP and b) NiePeWO3 at 250 mV from OCP and C) a) NiePeWO3 at 250 mV from OCP and b) NiePeWO3 at anodic current density of 50 mA/cm2.

catalytic effect of nano WO3 catalyst in the phosphorous enriched inner layers of NiePeWO3 coating had been investigated. The circuit element analysis in the impedance spectroscopy was carried out by measuring the data using non linear-

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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 1 ( 2 0 1 6 ) 1 0 0 9 0 e1 0 1 0 2

least-square fitting analysis (NLS) software. The CPE model has been chosen as the suitable circuit by considering all the fit criteria for the present investigated coating. The electrical circuit model chosen for the present subsystem are shown in Fig. 14 in supplementary information. It consists of a solution resistance Rs, in series with the parallel connection of the CPE element and the charge transfer resistance Rct. Rct is equal to the faradaic impedance of the single semicircle [87]. The impedance of CPE is given as ZCPE ¼ 1/T(ju)4. In the equation u is the angular frequency in rad/s of the AC voltage and 4 corresponds to the depression angle 90(14) of the semicircle. The time constant T, related to the double layer capacitance Cdl is given as [23] T ¼ C4 dl Rs 1 þ RCT 1

14

The electrochemical impedance parameters obtained from the analysis of impedance spectra are reported in Table 3. The Cdl value of pure NieP coating has been increased from 4.62  105 F/cm2 to 2.79  104 F/cm2 by the incorporation of nano WO3 catalyst. Since the Cdl value obtained corresponds to the electrochemically accessible surface area of the catalytic coating, it can be stated that nano WO3 catalyst has considerably enhanced the surface area of NieP amorphous coating. The decrease in charge transfer resistance accounts for the improved current density of NiePeWO3 coating during the polarisation measurements. The parameter 4 gave information regarding the homogeneity of the coating surface and 4 value assigned for a smooth electrode is unity [33,79]. Comparison of 4 parameter in Table 3 showed that there was only a slight change in 4 value for NiePeWO3 coating surface during HER. This evidenced the retention of surface homogeneity of the NiePeWO3 coating even after vigorous HER. But their occurred a considerable decrease in 4 value for the anodically stripped NiePeWO3 surface. This evidenced the reduction in homogeneity of the coating surface as a result of anodic stripping or the coating surface has been modified through the stripping process. From Fig. 8B, the Nyquist plots revealed a slightly depressed capacitance semi-circular loop indicating the surface inhomogenities [21]. The single loop in Nyquist plots evidenced that HER is mainly controlled by a charge transfer process. The onset of Faradaic reactions of hydrogen evolution which accounts for the improved electrocatalytic performance of NiePeWO3 coating was evident from its increase in capacitance value [88]. The electrocatalytic activity of a coating directly depends on its electrochemically active area. In order to get information regarding the number active sites per unit area of the investigated coating surface, a comparison of the double layer capacitance of a smooth electrode (20 mF/ cm2) with that of NiePeWO3 has been done. Thus the surface roughness value of the coating has been obtained from the

equation, Rf ¼ [Cdl/(20 mF/cm2)] [87]. The improved roughness factor revealed an improved electrochemically active area of NiePeWO3 coating. There was a slight decrease in the surface roughness of the NiePeWO3 coating after HER at h250. This might be due to the exposure of inner amorphous phosphorous enriched layer of NiePeWO3 coating. The application of an anodic current density of 50 mA/cm2 resulted in a capacitance loop not only in the high frequency domain but also a low frequency capacitive response in the form of an inductive loop as evidenced in Fig. 8C. The high frequency loop can be ascribed to the parallel combination of charge transfer resistance and double layer capacitance and the low frequency loop to the potential dependent coverage of the electrode surface by adsorbed hydrogen species. The appearance of two capacitive loops in the impedance curve revealed the rough or porous nature of the electrode [23,89]. Hitz and Lasia [11] in their study of porous Ni electrodes obtained by leaching of NieZn and NieAl alloys reported that the high frequency loops revealed the porous structure of the electrode and the low frequency loop to the parallel combination of charge transfer resistance and double layer capacity. A considerable decrease in charge transfer resistance, RCT value (2.32 mU) after anodic stripping suggests that the activation of the coating surface has been occurred by the removal of adsorbed less stable hydroxide layer. WO3 catalysts in highly alkaline pH experience a considerable resistance against the OH ion diffusion [85]. This resistance was offered by the already existing hydroxide layer on the coating surface. Thus the small size of the low frequency capacitive loop might be due to the less stability of hydrated layer formed on the coating surface. The SEM images of pure NieP coating are compared with the NiePeWO3 coating after cathodic electrolysis (HER) in Fig. 10 supporting information. The figure demonstrated the occurrences of slight alkaline corrosion on both the coating surface. This might be due to the combined effect of H2 scouring and alkaline corrosion on the coating surface during the electrolysis. The SEM image of NiePeWO3 coating after electrolysis and stripping shows the considerable retention of WO3 nanoparticles on the dense and smooth NieP surface. This supports the better tolerance performance of NiePeWO3 coatings under vigorous conditions. The stability of catalytic activity of NiePeWO3 coating was observed by implying potentials at different sweep rates as given in Fig. 11 supporting information. The polarisations trends of NiePeWO3 coating are found to be independent of the scan rates such as 5 mV/s, 10 mV/s and 15 mV/s [8]. No considerable deviations or potential breakdown were observed for the polarisation curve of NiePeWO3 coating during the intermittent change in the potential scan confirmed the catalytic stability of NiePeWO3 coating.

Table 3 e Comparison of electrochemical impedance parameters of the NieP and NiePeWO3 coatings. Rct (U)

Coating NieP (OCP) NiePeWO3

OCP Overpotential (200 mV) Anodic current density (50 mA/cm2)

4.08 5.69 3.86 1.84

 103  103  103  103

F 0.780 0.765 0.801 0.690

Cdl (F) 3.12  2.79  2.05  4.25 

105 104 104 103

Rf 15.60 139.5 102.5 212.5

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 1 ( 2 0 1 6 ) 1 0 0 9 0 e1 0 1 0 2

Long term stability of the coating in aggressive alkaline environment The long term stability of NiePeWO3 coating in 32% NaOH was studied by open circuit potential (OCP) analysis. The OCP trends of NiePeWO3 coatings obtained by the incorporation of different wt % of nano WO3 in the NieP matrix are compared with that of the pure NieP in Fig. 12 supporting information. The OCP variations are monitored in a static aggressive medium of 32% NaOH solution for 40 days at 30 ± 2  C with respect to Hg/HgO, OH reference electrode. There occurred an initial decrease in potential for all the coatings as evidenced in Fig. 12 supporting information. This might be due to the dissolution of the surface oxide layer of the coatings due to localised corrosion. Then slowly as the time increased, the decrease in potential of the coating was found to be decreased. This evidenced the tendency of the coating to attain the equilibrium potential. The 2 g/L WO3 incorporated NieP coating experienced a lower cathodic shift in potential. This was attributed to the higher stability of composite incorporated NieP coatings in 32% NaOH even after for a period of 40 days. The electrochemical stability under long term use was determined by the tafel analysis of NiePeWO3 coating after 1 h and 240 h exposure at open circuit conditions in highly aggressive alkaline medium was obtained as shown in Fig. 13 supporting information. The growth of surface protective layer of Ni(OH)2 and Ni(OOH) during longer immersion time was evidenced from the occurrence of a curve in the anodic region of the Tafel plot in the potential range of 0.6 V to 0.1 V. The corrosion potential of NiePeWO3 coating shifted slightly to a nobler region and their occurred a decrease in current density value after prolonged exposure which accounts for long-term stability of the coating in aggressive alkaline electrolyte.

Conclusions NieP coating with improved electrocatalytic activity towards HER could be achieved by means of incorporating the coatings with nano WO3 catalyst. Efficiency and practical feasibility of the developed nano WO3 incorporated NieP coating was confirmed electrochemically. Incorporation of nano WO3 catalyst in the NieP matrix has improved the number of catalytically active sites of NieP coating as evidenced from the increment in Cdl 26.32 mF/cm2 for pure NieP to 38.52 mF/cm2 NiePeWO3 coatings. This accounts for the improved electrocatalytic activity of NiePeWO3 coatings towards HER. Thus developed NiePeWO3 coating could retain the surface homogeneity even after vigorous HER. The same surface when subjected to anodic stripping to check the internal tolerance resulted in slightly porous surface as evidenced from EIS with improved catalytic activity. A considerable retention of WO3 particles in the NieP matrix could be observed even after HER followed by anodic stripping in the aggressive medium. When compared to the pure NieP hydrogen scouring and alkaline corrosion on the coating surface was observed to be less for NiePeWO3. This supports the better tolerance performance of NiePeWO3 coatings under potential reversals in highly

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aggressive alkaline environment. From the Tafel polarization curves and the EIS data the mechanism of HER on NiePeWO3 coating was predicted to be controlled by the combination of VolmereHeyrovsky reactions. The EIS and polarization measurements evidenced the formation of a passive layer on long term immersion in the alkaline medium. Thus the developed nano WO3 catalyst incorporated NieP coatings were found to be stable under highly aggressive alkaline environment.

Acknowledgements The authors are grateful to the Prof & Head, Department of Chemistry, University of Kerala for providing facilities to carry out this work.

Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2016.04.156.

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