Synthesis and characterization of Ni-P-Ag composite coating as efficient electrocatalyst for alkaline hydrogen evolution reaction

Electrochimica Acta 219 (2016) 377–385

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Synthesis and characterization of Ni-P-Ag composite coating as efficient electrocatalyst for alkaline hydrogen evolution reaction Liju Elias, A. Chitharanjan Hegde* Electrochemistry Research Lab, Department of Chemistry, National Institute of Technology Karnataka, Surathkal, Srinivasnagar 575 025, India

A R T I C L E I N F O

Article history: Received 2 August 2016 Received in revised form 19 September 2016 Accepted 4 October 2016 Available online 5 October 2016 Keywords: Ni-P-Ag composite silver nanoparticle sol HER SEM

A B S T R A C T

The effect of addition of silver nanoparticle sol (SNS) into Ni-P plating bath was studied in terms of the variation in electrocatalytic behavior of the developed coatings in 1.0 M KOH. Ni-P-Ag composite coating was achieved through direct electrolysis by adding a known quantity of the conventionally prepared SNS into Ni-P bath. Ni-P-Ag coatings electrodeposited galvanostatically on copper under different conditions of the bath was used as electrode material for alkaline hydrogen evolution reaction (HER). The optimal concentration of the SNS required for maximum electrocatalytic activity towards HER was obtained by adding different volumes of SNS (from 0 to 50 mL L 1) into the bath. The HER efficiency of the test electrodes in 1.0 M KOH medium was examined using cyclic voltammetry (CV) and chronopotentiometry (CP) techniques. The kinetics of HER on the alloy and composite electrodes were established through Tafel polarization and electrochemical impedance spectroscopy (EIS) analyses. Energy dispersive spectroscopy (EDS) was used to confirm the incorporation of Ag nanoparticles into the Ni-P alloy matrix. The microstructure and morphology of the alloy and composite coatings were analyzed by Scanning Electron Microscopy (SEM). A significant improvement in the electrocatalytic property of nano-Ag derived composite coatings was found, and was attributed to the enhanced electroactive sites of Ag particles. Deposition conditions to maximize the electrocatalytic activity of Ni-P-Ag nanocomposite coatings in relation to traditional Ni-P alloy coatings was arrived, and results are discussed. ã 2016 Published by Elsevier Ltd.

1. Introduction The energy crisis and environmental concerns intensified the need for searching a clean and renewable alternate energy for the future [1]. In this regards, the global economy is looking into the possibilities of the utilization of the abundant, renewable and cleanest fuel, hydrogen, as a promising alternate energy carrier in the future [2,3]. Hydrogen has been promoted as the ideal clean energy source with zero-emission [4,5], and having excellent energy storage density [3]. Water is the most abundant source of hydrogen, free from carbon [2,6,7], and its splitting into H2 and O2 is the best choice for H2 production. Whereas the water electrolysis is an arduous process, suffers from practical overpotentials due to sluggish electrode kinetics [6,8–12]. The efficient water electrolysis for commercial applications can be implemented only through the development of electrode materials which can reduce the overpotential for hydrogen evolution reaction (HER) at the cathode and oxygen evolution reaction (OER) at the anode [13–17]. Even though

* Corresponding author. E-mail addresses: [email protected], [email protected] (A. C. Hegde). http://dx.doi.org/10.1016/j.electacta.2016.10.024 0013-4686/ã 2016 Published by Elsevier Ltd.

various materials has been reported as efficient and durable cathode materials for HER, noble metals (Pt, Ru based electrodes) are the best among them due to stability and high activity [5,18]. The utilization of these noble metal electrodes for industrial applications are impeded by their low natural abundance and high cost [19]. Thus, because of the availability and relatively cheaper rates of the active non-platinum metals like Fe, Ni, Co etc. and their alloys/composites have emerged as electrocatalysts for HER [1,4,18–20]. Many other material [21–24] and molecular [25–27] catalysts are also reported recently as efficient towards HER with low overvoltage. Although there are several methods available for the fabrication of electrode materials, the cost effectiveness and simplicity of electrodeposition making it more attractive and reliable [4,18,28]. A variety of coatings such as metal, alloy or composite can be developed with high precision through electrodeposition, and also it allows to have a close control over the desired properties such as composition, structure and properties [29,30]. In continuation of our early studies on the development of efficient electrode materials for HER [28], we are reporting the development of Ni-P alloy and Ni-P-Ag composite electrodes as an efficient electrode material for alkaline HER. There are some reports on

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the electrocatalytic activity of Ni-P thin films [24,31] and nickel phosphide nanomaterials [32–35], focusing on their bifunctional behavior. Apart from that, many literatures reported the improvement in electrocatalytic efficiency after the incorporation of colloidal particles into the metal/alloy matrix through composite electrodeposition [19,36–38]. Whereas in composite electrodeposition, the main limitation is to achieve homogeneous dispersion of the colloidal particle within the metal/alloy matrix. In this regards, the present study focuses on overcoming that limitation by adopting sol-enhanced electrodeposition technique for the preparation of Ni-P-Ag composite electrode for HER. 2. Experimental 2.1. Electrodeposition of Ni-P alloy coatings The Ni-P coatings were achieved from a Hull cell optimized alkaline citrate bath [39,40] containing NiSO4. 6H2O (28.0 g L 1), NaPO2H2. H2O (51.0 g L 1), Na3C6H5O7. 2H2O (54.0 g L 1), NH4Cl (20 g L 1), H3BO3 (10 g L 1) and C3H8O3 (20 mL L 1). The development of alloy electrodes was carried out in a custom made deposition setup at a pH = 8, using copper rod of 1 cm2 exposed surface as cathode and nickel rod of same surface area as anode. To ensure good quality alloy coatings, prior to plating the copper substrates were cleaned through mechanical polishing to achieve mirror finish, followed by electro-cleaning and acid pickling. All the plating process were carried out from the same bath for an equal time duration (10 min), for comparison purpose. A computer controlled DC power source, DC Power Analyzer, Agilent, N6705A was used for carrying out the plating process in PVC chamber of 250 mL solution capacity, by keeping the Ni anode and copper cathode parallel, 5 cm apart. The developed coatings were characterized for phase structure, surface morphology and elemental composition respectively by X-ray diffraction analysis (XRD, JDX–8P, JEOL, Japan, with CuKl radiation (l = 1.5418 Å) as the X–ray source), scanning electron microscope (SEM, JSM–7610F from JEOL, USA) and energy dispersive spectroscopy (EDS) analyses. 2.2. Synthesis of Ni-P-Ag nanocomposite coating A stable yellow colloidal dispersion of silver nanoparticles or silver nanoparticle sol (SNS) was prepared through the procedure reported by Mulfinger et al. [41], using silver nitrate ( > 99% AgNO3, Sigma-Aldrich chemical company), sodium borohydride (99% NaBH4, Sigma Aldrich chemical company) and double distilled water. The possibility of aggregation was controlled by the use of glycerol (as a capping agent) in the present study, other than the usual polyvinylpyrrolidone (PVP) as reported [42–44]. Glycerol adsorption has a major role in the formation of the particle surface charge and thereby the stabilization of growing silver nanoparticles as shown in Fig. 1. The obtained Ag-nanoparticles were

Fig. 2. The schematic of homemade three-electrode glass setup with facility for collecting the gasses evolved during analysis to ascertain the practical efficiency of the electrode material for alkaline water electrolysis.

characterized for its size, morphology and optical properties. The size and morphological analysis were carried out using Transmission Electron Microscope (TEM, JEM-2100, JEOL) and the optical properties were studied using UV–vis spectroscopy (Analytik Jena Specord S 600, Spectrophotometer) technique. Further, the obtained stable yellow transparent SNS was added into the asoptimized Ni-P alloy plating bath (in varying quantities such as 10, 20, 30, 40 and 50 mL L 1) for the deposition of Ni-P-Ag coating through sol-enhanced composite electrodeposition. Composite coatings were developed on the copper substrate at an optimal deposition current density (c.d.) of 4.0 A dm 2 [40], immediately after the addition of SNS into the plating solution with constant stirring. The amount of SNS required to achieve a highly dispersed Ag nanoparticles within the Ni-P alloy matrix was also optimized. The surface morphology and composition of the composite coating was observed using SEM and EDS analysis tool. 2.3. Electrochemical measurements Electrocatalytic characterization of both Ni-P alloy and Ni-P-Ag composite coatings were made by depositing the coatings on copper rod with 1 cm2 exposed surface area, using a custom made three-electrode glass setup as illustrated in Fig. 2. The electrochemical measurements were made by using electrodeposited alloy/composite coatings, platinized platinum and saturated calomel electrode (SCE), respectively, as working, counter and reference electrodes, in 1.0 M alkaline KOH medium. The electrocatalytic behavior of the alloy and composite coatings were evaluated through cyclic voltammetry (CV) and chronopotentiometry (CP) techniques. The CV measurements were made by cycling the potential within a potential ramp of 0.0 to 1.6 V under electrocatalytic test conditions at a scan rate of 50 mV s 1, for 50 cycles. The CP analysis was carried out by applying a constant current density of 300 mA cm 2 (cathodic), for a duration of 1800 s. The practical activity of the test electrodes towards HER was

Fig. 1. The scheme showing the formation of glycerol stabilized Ag-nanoparticles through NaBH4 reduction.

L. Elias, A.C. Hegde / Electrochimica Acta 219 (2016) 377–385 Table 1 The variation in Ni-P alloy coating properties with deposition current density under optimal deposition conditions. c.d. (A dm 2.0 4.0 6.0

2

)

Wt.% of P in the deposit

Vickers micro hardness (V100) (GPa)

Thickness (mm)

Appearance

4.3 9.0 13.4

2.3 2.6 2.5

10.6 16.4 19.2

Bright Bright Bright

estimated by measuring the amount of H2 gas liberated during the analysis [28]. The kinetics of HER on the alloy and composite electrodes were established through Tafel polarization and electrochemical impedance spectroscopy (EIS) analyses. EIS was recorded at constant overpotentials of 100, 150 and 250 mV,

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in the frequency range from 100 kHz to 10 mHz by using an alternating current amplitude of 10 mV, to study the processes that occur at the electrode/electrolyte interface. All the electrochemical measurements were carried out using an electrochemical workstation Biologic SP-150, France. The potential recorded in CV and CP with respect to SCE is converted to versus reversible hydrogen electrode (RHE), for better comparison. 3. Results and discussion 3.1. Characterization of Ni-P alloy coatings The obtained Ni-P alloy coatings from alkaline citrate bath at different c.d.’s (2.0, 4.0 and 6.0 A dm 2) were characterized for its composition, micro-hardness, coating thickness, surface

Fig. 3. Morphological and structural characterization of Ni-P alloy coatings; a) SEM images of the alloy coatings developed at different current densities and, b) XRD pattern of the alloy coatings developed at different current densities.

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Fig. 4. TEM image of the Ag nanoparticles dispersed on a copper grid along with the TEM size distribution and Gaussian fitting of the Ag nanoparticles.

Fig. 5. Schematic of nanoparticle mixing in the plating bath; a) addition of the nanoparticle sol into the plating bath and, b) direct addition of nanoparticles into the plating bath.

morphology and phase structure. All the developed coatings were found to be bright, the thickness and P content of the coatings were found to be increased with deposition c.d. as given in Table 1. Whereas, the alloy coating developed at 4.0 A dm 2 was found to have the maximum microhardness value irrespective of the P content (Table 1), attributed to its surface morphology and phase structure [29]. The microstructure analysis of the coatings showed a remarkable variation with deposition c.d.’s as shown in Fig. 3(a). The micro-cracks present in the coatings were attributed to the inherent brittle nature of P and also due to the coating strain developed as a result of the evolution of H2 during deposition [45]. The X-ray diffraction pattern of the alloy coatings deposited at different c.d.’s are shown in Fig. 3(b). The variation in phase structure with increasing c.d. and hence with P content is evident from the obtained XRD pattern. The formation of P rich Ni3P phases at high c.d., with high P content, is attributed to the variation in the crystal structure with deposition c.d. [46]. Further, the Ni-P alloy coatings deposited at different c.d.’s (test electrodes) such as 2.0, 4.0 and 6.0 A dm 2 were analyzed for their

electrocatalytic efficiency for alkaline HER in 1.0 M KOH medium using electrochemical methods of analysis (supplementary information, Fig. S1). The study showed that the alloy test electrode developed at 4.0 A dm 2 as the best coating for HER (supplementary information, Table S1). Thus, the coating obtained at 4.0 A dm 2 was selected as the optimal coating, with unique morphology, structure, composition and microhardness, for the development of electrode material for HER. 3.2. Characterization of Ag nanoparticles The Ag-nanoparticles obtained through glycerol mediated NaBH4 reduction of AgNO3 were characterized for its size, morphology and optical properties. The TEM image of the nanoparticles has been taken by drying a drop of the freshly prepared SNS on a copper grid. The TEM micrograph shows the synthesized Ag nanoparticles as spherical in shape, with an average particle size of 10 nm. The obtained TEM image of a region of the sample along with the particle size distribution are shown in Fig. 4.

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Fig. 6. a) SEM image and, b) EDS spectra of the Ni-P-Ag composite coating developed at an optimal c.d. of 4.0 A dm dispersed Ag nanoparticles.

The optical activity of the Ag-nanoparticles in the visible region is attributed to the excitation of the d-band electrons to states above Fermi level and subsequent relaxation resulting in the photoluminescent radiative recombination [47,48]. The plasmon absorbance phenomenon is resulting in the yellow color to the colloidal SNS [47–50]. The UV–vis absorption spectra of the clear yellow colloidal SNS (diluted with distilled water) was recorded to relate its optical activity with particle size (supplementary information Fig. S2). Thus the average particle size was again confirmed as 10 nm from the plasmon resonance peak obtained near to 395 nm, with a full-width half maximum (FWHM) value of 56 [41,48,50]. 3.3. Characterization of Ni-P-Ag composite coating The Ni-P-Ag composite coating developed through SNS enhanced electrodeposition at optimal deposition c.d. (4.0 A dm 2) was characterized using SEM and EDS analyses. The limitation over the possibility for agglomeration of the nanoparticles within the plating bath during deposition in conventional composite electrodeposition, by directly adding the nanoparticles into the plating bath, has been overcome by the sol-enhanced electrodeposition. As shown in the schematic diagram (Fig. 5), the addition of nanosol is allowing the homogeneous mixing and dispersion of the nanoparticles within the plating solution [51]. Whereas, direct addition of nanoparticles into the plating bath can lead agglomeration of the nanoparticles by interacting with the electrolyte. In the present case, the used glycerol as nanoparticle stabilizer can also enhance the homogeneous dispersion of the Ag-nanoparticles in the plating bath. Since glycerol was used as an additive in the alloy plating bath, the same can act as the dispersed nanoparticle stabilizer in the bath and thereby to prevent its further agglomeration during the deposition. The microstructure of the obtained Ni-P-Ag composite coating was analyzed using SEM, the incorporation of Ag within the alloy matrix and its composition in the coating were established through EDS analysis (Fig. 6). The compositional analysis showed the presence of about 4.1 wt.% of Ag, 12.6 wt.% of P and remaining Ni, in the developed Ni-P-Ag nanocomposite coating. A small decrease in P content in the composite coating, compared to alloy coating (developed at same c.d., 4.0 A dm 2) may be attributed to peculiar variation of the reluctant metal (P) observed in induced type of codeposition in the presence of Ag-nanoparticles in the bath [46,52].

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2

from the alloy plating bath having homogeneously

3.4. Hydrogen evolution reaction 3.4.1. Cyclic voltammetry analysis The stability and activity of the electrode material towards chemical reactions can be established from the sweep reversal method, called cyclic voltammetry. The electrode–ion interaction in the electrode/electrolyte interface can be obtained from the exceptionally potent and the most widely used CV technique [53,54]. The current response of the CV can give a direct measure of the surface adsorbed ionic species of the electrolyte [55] and hence its efficiency towards catalytic reduction of adsorbed H+ ions i.e., for HER. The cathodic peak current density (ipc) and onset potential for HER were noted from the CV responses of the alloy and composite test electrodes. The CV curves of the alloy and composite electrode developed at an applied c.d. of 4.0 A dm 2 are shown in Fig. 7. The obtained CV response of the composite electrode shows a remarkable variation in onset potential value as compared with the optimal alloy electrode, as given in Table 2. The data given in Table 2 shows that the composite coating as the best electrode material towards HER with least overvoltage ( 135 mV) compared with alloy electrode ( 475 mV). The large variation in ipc

Fig. 7. Cyclic voltammograms for Ni-P alloy and Ni-P-Ag composite test electrodes developed at a deposition current density (c.d.) of 4.0 A dm 2, in alkaline 1.0 M KOH medium showing large cathodic peak current density (ipc) and least onset potential of Ni-P-Ag composite electrode towards HER.

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Table 2 The comparison of hydrogen evolution reaction parameters of Ni-P alloy, Ni-P-TiO2 composite and Ni-P-Ag composite test electrodes achieved through electrodeposition, composite electrodeposition and sol-enhanced electrodeposition, respectively, from the same Ni-P alloy plating bath. Electrodes developed at a deposition c.d. of 4.0 A dm Ni-P alloy electrode Ni-P-TiO2 composite electrode Ni-P-Ag composite electrode

2

Onset potential of H2 evolution (mV vs RHE) 475 148 135

value for the composite electrode is attributed to the enhanced adsorption of the H+ ions on the electrode surface, influenced by the increased surface area and the no of active sites affected from the incorporation of Ag nanoparticles into the alloy matrix [36–38]. 3.4.2. Chronopotentiometry analysis Steady current/constant current chronopotentiometry is a valuable tool to study the HER efficiency of the electrode materials under working conditions. The applied constant current (–300 mA cm 2) leads to the continuous reduction of the H+ ions at the cathode surface and thereby the continuous evolution of H2. The potential of the electrode attains values according to the features of the electrode material and the availability of the redox couple within the medium, and may vary with time [53,54]. The ‘E-t’ curve showing the potential of the electrode can change either due to the instability of the electrode material under working conditions or due to the unavailability or decrease in concentration of the H+ ions in the electrolyte with time [56]. At the same time, with increasing the efficiency of the electrode material towards HER, the potential value becomes nobler (moves close to zero) and therefore, from the obtained ‘E-t’ response at a particular electrode potential, it is possible to establish the robustness of the electrode material under HER working conditions [28,57]. Apart from that, during the chronopotentiometry analysis, hydrogen evolution takes place continuously at the same rate, at a particular potential (overvoltage) depending on the stability and efficiency of the electrode material [54]. Hence, the practical efficiency of the electrode material can be reasoned by measuring the amount of H2 liberated during the analysis.

Fig. 8. Chronopotentiograms of Ni-P alloy electrode and Ni-P-Ag composite electrode developed at an optimal deposition current density (c.d.) of 4.0 A dm 2, recorded under a constant impressed current density of 300 mA cm 2 along with the amount of H2 gas evolved during the first 300 s of the analysis.

Volume of H2 evolved in 300 s (cm3) 9.6 15.2 18.8

The chronopotentiometry responses of Ni-P and Ni-P-Ag test electrodes developed at 4.0 A dm 2 along with the amount of H2 gas evolved during the analysis (for first 300 s) are shown in Fig. 8. The corresponding amount of H2 gas liberated during the analysis is listed in Table 2. The obtained results show the composite electrode as the best material for alkaline HER with good stability and maximum amount of H2 gas produced during the analysis. 3.5. Kinetics of HER on N-P alloy and Ni-P-Ag composite electrodes The kinetics of HER on test electrodes were established from the Tafel slope measurements (supplementary information Fig. S3). The composite electrode was found to have the lowest Tafel slope (b = 98 mV dec 1) compared with the alloy electrode (b = 112 mV dec 1). The obtained Tafel slopes ranging from 98 to 112 mV dec 1 for the test electrodes indicate the charge transfer (Volmer) reaction as the rate determining step in HER on both the electrodes [18,31,58]. In order to further investigate the processes at the electrode/ electrolyte interface, EIS analysis was carried out at different constant overpotentials as obtained from the CV analysis. The composite electrode was found to show a minimum onset overvoltage of 135 mV, and hence three overpotentials such as 100, 150 and 250 mV were selected for the EIS measurements. The Nyquist plots (plot of imaginary values Zimag vs real values Zreal) recorded at different overpotentials for Ni-P alloy and Ni-PAg composite coatings along with the electrical equivalent circuit (EEC) are shown in Fig. 9. The depressed semicircle shape of the obtained Nyquist plots (Fig. 9) for alloy and composite electrodes further supports the HER is controlled by charge transfer reaction [18,35,58]. Further, the variation in polarization resistance and double layer capacitance of the electrode materials, and there by the electrocatalytic efficiency of the materials were established from the EEC fitting parameters, as given in Table 3. The EEC was found to be a simple Randles circuit with solution resistance (Rs), charge transfer resistance (Rct) and a constant phase element (QCPE). The obtained fitting parameters as given in Table 3 shows that the Rct values decreases with increase in overpotential for both the electrodes, indicating the enhancement of HER kinetics [18,31]. The much lower Rct values of Ni-P-Ag electrode compared with that of the Ni-P alloy electrode confirms the improvement in HER activity with the incorporation of Ag nanoparticles. Further, the QCPE value, associated with the effective surface area for cathodic reaction [18,58], was found to be increased with increase in overvoltage for both alloy and composite coatings. The large variation of QCPE values of the NiP-Ag composite electrode compared with the Ni-P alloy electrode suggests the improvement in electrocatalytic efficiency towards HER, and is attributed to its increase in active surface area. The HER activity of the Ni-P-Ag composite electrode in comparison with the earlier reported Ni-P-TiO2 composite electrode [52] as given in Table 2 (supplementary information Fig. S4) confirms the enhancement in electrocatalytic efficiency after the incorporation of Ag nanoparticles. The observed intensification in electrocatalytic activity of the Ni-P-Ag composite

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Fig. 9. Nyquist responses recorded under different overpotentials of obtained electrical equivalent circuit (EEC).

100,

150 and

Table 3 Electrochemical equivalent circuit (EEC) parameters obtained by fitting the experimental EIS data recorded under different overpotentials on the alloy and composite test electrodes. Test electrode

Rs (Ohm)

Rct (Ohm)

QCPE (mF)

Ni-P electrode at h = 100 mV Ni-P electrode at h = 150 mV Ni-P electrode at h = 250 mV Ni-P-Ag electrode at h = 100 mV Ni-P-Ag electrode at h = 150 mV Ni-P-Ag electrode at h = 250 mV

0.92 0.91 0.95 0.91 0.89 0.89

766 572 241 192 124 52

73 97 114 237 536 896

electrode for alkaline HER is accredited to the increased surface area and the increased number of active sites for hydrogen adsorption affected by the homogeneous dispersion of the Ag nanoparticles in the alloy matrix, achieved through sol-enhanced electroplating technique [19,59]. 4. Conclusions The following conclusions are established based on the systematic study on synthesis, characterization and electrocatalytic study of the Ni-P alloy and Ni-P-Ag composite coating as electrode materials for alkaline HER. 1. The alloy coatings with varying P content, achieved from the optimal plating bath by controlling the deposition conditions showed that the coating developed at 4.0 A dm 2 as the best alloy electrode for alkaline HER with 9.0 wt.% of P content. 2. Silver nanoparticles of spherical shape and an average particle of 10 nm were successfully synthesized through glycerol mediated reduction of AgNO3 using NaBH4, and characterized.

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250 mV for HER of a) Ni-P alloy, b) Ni-P-Ag composite test electrodes, and c) the

3. Sol-enhanced electroplating has been successfully employed for the deposition of Ni-P-Ag composite coating with a homogeneously dispersed Ag nanoparticles to intensify the electrocatalytic efficiency of the Ni-P alloy electrodes for HER. 4. The highest electrocatalytic efficiency of the Ni-P-Ag composite electrode was ascertained from the least onset potential for H2 evolution, large ipc value and the maximum amount of H2 gas evolved during the analysis, in comparison with the Ni-P alloy and Ni-P-TiO2 composite electrodes. 5. The observed high efficiency of the Ni-P-Ag composite electrode for alkaline HER is accredited to the increased surface area and the increased number of active sites for hydrogen adsorption affected by the homogeneous dispersion of the Ag-nanoparticles in the alloy matrix, achieved through sol-enhanced electroplating technique.

Acknowledgement Mr. Liju Elias acknowledges National Institute of Technology Karnataka, Surathkal, India for proving the laboratory facilities for the present study. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. electacta.2016.10.024. References [1] Z. Chen, Z. Ma, J. Song, L. Wang, G. Shao, Novel one-step synthesis of wool-balllike Ni-carbon nanotubes composite cathodes with favorable electrocatalytic

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