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 x x x ( 2 0 1 7 ) 1 e1 2 Available online at www.sciencedirect.com ScienceDirect journal homepage: www.elsevier.com/locate/he Electrodeposition and characterization of NiMoW alloy as electrode material for hydrogen evolution in alkaline water electrolysis Mahdi Allam a, Mohamed Benaicha b,*, Achour Dakhouche a a Inorganic Materials Laboratory, Department of Chemistry, Faculty of Sciences, Mohamed Boudiaf University, M'sila, 28000, Algeria b Energetic and Solid-State Electrochemistry Laboratory, Faculty of Technology, Ferhat Abbas-Setif1 University, Setif, 19000, Algeria article info abstract Article history: The electrodeposition of ternary NiWMo alloys films from citrate ammonia-free electrolyte Received 31 January 2017 at room temperature was studied in an effort to evaluate the effect of applied potential on Received in revised form the composition limits, corrosion resistance and the electrocatalytic properties of the de- 20 July 2017 posits towards the hydrogen evolution reaction (HER) in concentrated alkaline solution. Accepted 5 August 2017 The alloys were potentiostatically electrodeposited onto pure copper sheet substrates. The Available online xxx electrodeposits were characterized by means of field-emission scanning microscopy (FESEM) and energy dispersive X-ray analysis (EDXA). In an electrolyte where Keywords: 2 MoO2 4 =WO4 ¼ 1 : 1, at a given deposition potential, there is more Mo than W in the de- Electrodeposition posits, indicating an advantageous induced co-deposition of Mo compared to W. The NiWMo alloy nucleation mechanism, studied according to Scharifker-Hills theoretical model, revealed Hydrogen evolution an instantaneous nucleation followed by a three-dimensional growth. On the hand, Nucleation mechanism 2 ratio in the electrolyte under the same deposition potential increasing MoO2 4 =WO4 Impedance spectroscopy reduced both Ni and W content in the deposits. A different trend was observed in an Cyclic voltammetry equimolar solution, when applying more negative potentials, both Mo and W contents decreased leading to the enhancement of Ni amount. The stability in corrosive media and the catalytic performances of the coatings depended mainly on the applied overpotentials, A mechanism of induced co-deposition of molybdenum and tungsten with nickel is proposed and discussed. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Introduction Hydrogen production by water electrolysis is one of the most promising techniques for the global future energy needs as an entirely clean and powerful alternative to climate -altering fossil fuels processes, particularly when renewable energy sources such as solar and wind energy are used [1]. Unfortunately, compared to technologies available on the market such as the steam reforming of methane gas or partial oxidation of oil, alkaline water electrolysis is not at present cost-effective due to the high amount of electricity used in the process which restrains its large-scale utilization. * Corresponding author. E-mail address: [email protected] (M. Benaicha). http://dx.doi.org/10.1016/j.ijhydene.2017.08.012 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Allam M, et al., Electrodeposition and characterization of NiMoW alloy as electrode material for hydrogen evolution in alkaline water electrolysis, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.08.012 2 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 x x x ( 2 0 1 7 ) 1 e1 2 Nomenclature AC B В C CSL Edep EDS EEC EIS Erev F FESEM HER I0 ic imax K K0 n ha hc OCP Rct Rs SCE tmax Zi Zr Alternative current Tafel slope, log2.303RT/aF Charge transfer coefficient Electrode capacitance Cathodic scan limit Deposition potential Dispersive X-ray spectroscopy Electrochemical Equivalent Circuit Electrochemical impedance spectroscopy Reversible thermodynamic potential Faraday constant, 96500 C Field emission scanning electron microscope hydrogen evolution reaction Apparent exchange current density Cathodic current density Current coordinate of the peak Electrochemical reaction rate constant Rate constant at Ei¼0 Number of electrons transferred Anodic overpotential Cathodic overpotential Open circuit potential Charge transfer resistance Solution resistance Saturated calomel electrode Time coordinate of the peak Imaginary part of Z Real part of Z In order to make water electrolysis more efficient and at affordable cost, reduction of the cell voltage is indispensable. The applied potential to drive the electrochemical cell at an applied current, I, includes a thermodynamic and a kinetic (dissipation) contribution due to inefficiencies in the system: DECell ¼ DErev þ ha þ jhc j þ IR þ DEst Where DErev is the reversible thermodynamic value (z1.23 V) which depends on the nature of the electrochemical reactions, ha and hc the overpotentials at anode and cathode, IR the inter-electrode resistance, and DEst the in time stability characteristic expressing the tendency of the actual cell voltage to increase as a consequence of the electrolyzer performance degradation [2]. It is worth mentioning that while the inter-electrode ohmic losses could be reduced by enhancing the bath conductivity and minimizing the space between the electrodes, the cathodic and anodic overpotentials depend mainly on the electrode material properties. During the last few years, significant improvements have been made in alkaline water electrolysis through the development of advanced alkaline water electrolysis systems with the so-called “zero gap cell” design (no space between anodes, diaphragms, and cathodes in the cell units) [3]. On the other hand, some research works on water vapor electrolysis have been carried out. Ganley [4] and Boll and co-workers [5] reported that increasing the operating steam electrolysis temperature and pressure up to 400 C and z10 MPa or 500 C and 400 MPa respectively, improved the reaction kinetics at the electrode surface and lowered the applied potentials to provide high current densities. However, increasing bath temperature and pressure drastically decreased the terminal potentials and strongly affected the stability of electrode materials in concentrated alkaline corrosive media. This fact motivates the extensive research efforts that have been recently focused on the improvement of the electrolysis cell performances through the devel opment of efficient and lowcost catalyst materials. According to the Brewer-Engel valence-bond theory [6,7], alloying transition metals with hypo-hyper-d - electronic structure, i.e., combination of metals of the left half of the transition elements in the Periodic Table having empty or half-filled vacant d-orbitals with metals of the right half of the series, having internally paired delectrons, leads to pronounced increase in the electronic density of states and consequently to advanced synergetic effect in electrocatalysis for the hydrogen evolution reaction (HER). Alloys of nickel ([Ar] 3d8 4s2) with tungsten, W ([Xe] 4f14 5d4 6s2) and/or molybdenum, Mo ([Kr] 4d5 5s1) are among the materials fulfilling these requirements [8e11]. Moreover, the Pt-group metals are known to be the best electrocatalysts for hydrogen evolution reaction (DG z 0.1 eV). The next ideal HER catalysts are nickel and cobalt (DG z 0.28 eV) followed by molybdenum (DG z 0.36 eV) and tungsten (DG z 0.42 eV) [12]. NieW and NieMo alloys are known to possess outstanding functional properties such as high corrosion resistance in many aggressive environments [13] and excellent electrocatalytic activity for hydrogen evolution. The electroplating process of NiMo and NiW alloys is classified as an induced alloy deposition type since tungsten and molybdenum cannot be deposited alone from their aqueous solutions, but are codeposited in the presence of nickel, forming an alloy [14]. Commonly, a complexing agent is needed to codeposit these metals, otherwise, the amount of Mo or W does not exceed 2 wt% in the alloy. Although a great number of different complexing agents have been reported in the literature, citrate has been among the most popular, particularly in the deposition of nickel alloys [15]. Sodium citrate forms stable complexes enough to bring closer the potentials of the alloy constituents and prevent deposition of hydroxides of the metals. Moreover, citrate aqueous solutions are environmentally friendly and can function as a complexing, buffering, brightening and levelling agent [16] in electroplating of metals and alloys. Several investigations have been devoted to the deposition of NiMo and NiW alloys. Sanches et al. [17] investigated the electrodeposition of the binary Nie Mo alloys using different molar ratios of Ni:Mo, in a sodium citrate electrolyte at pH 4.0. They have found that high Ni:Mo molar ratio favors deposition of the metallic molybdenum, while deposits coated from molybdate-rich bath contained higher amount of Mo, a mixture of polyvalent molybdenum oxides or hydroxides, mainly in the form of the Mo(IV) and Mo(V)) in addition to metallic molybdenum. Krstajic et al. [9] reported that NiMo alloy deposited from a pyrophosphatebased bath onto Ni mesh exhibited better catalytic activity for hydrogen evolution in 1 M NaOH solution than pure Ni electrode. However, the stability of the film in concentrated solution (33 wt%) was very poor and the coating was scaled from the substrate after 2 h of hydrogen evolution. Xu and co- Please cite this article in press as: Allam M, et al., Electrodeposition and characterization of NiMoW alloy as electrode material for hydrogen evolution in alkaline water electrolysis, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.08.012 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 x x x ( 2 0 1 7 ) 1 e1 2 workers [18] thought that the high electrochemical activity for the hydrogen evolution of the NieMo coating could be attributed to both exchange current density and larger real electrode area. Navarro-Flores et al. [19] studied the influence of alloying nickel by Fe, Mo and Won the electrocatalytic activity towards HER in an acidic environment. They claimed that NiW, NiMo and NiFe electrodes were more efficient for hydrogen generation that pure nickel. NieMo was found to yield the highest overall electrocatalytic activity, mainly due the highest surface roughness, while NieW yielded the highest intrinsic activity as a result of the modification of electron density in d-orbitals upon alloying nickel with tungsten. Conway and co-workers [20] investigated the catalytic activity of electrodeposited ternary NiMoCd alloys containg 1 at% Cd. They claimed that the resulting low Tafel slope (26e30 mV) obtained at elevated temperatures can be explained of the formation of a hydride phase. Eliaz and Gileadi [21] made a review on Mo and W alloys with transition metals. The citrate concentration increase was reported to decrease overall current efficiency. Citrate baths provided higher W content in comparison with those containing tartrate or malate. The effect of temperature was proved to depend on solution composition and increasing the current density led to higher W content for most used electrolytes. Tasic et al. [22] studied the electrodeposition of NieW alloys from ammoniacalcitrate bath containing different concentrations of sodium tungstate, on electrocatalytic activity towards HER. They found that the films obtained at higher deposition current densities had the lowest overvoltage for the HER and claimed that the surface roughness of the coatings is responsible for their electrocatalytic activity. If the electrodeposition of NiMo and NiW binary alloys was already approached, to the best of our knowledge, there are very few research works on the synthesis of ternary NiMoW alloys by electrodeposition that have been conducted. Cesiulis and co-workers [23] investigated the effect the ratio of the Na2MoO4 and Na2WO4 concentrations in a pyrophosphate bath on the content of the ternary NieMoeW alloys. They reported that the deposits were crack-free for a W content less than 5e6 at% and in all cases, the sum of Mo and W amounts in the alloys does not exceed 15 at%. More recently, Sun et al. [24] studied the effect of current density on the composition of NiWMo alloys electrodeposited onto rotating cylinder electrodes in citrate-boric acid electrolyte. They found that (but without clearly explaining why) when depositing NiWMo alloys in an equimolar solution, the Mo content in the deposit is significantly higher than W content. However, the influence of deposition parameters (electrolyte composition, deposition potential or current density etc) on the corrosion properties and electrocatalytic activities of electrodeposited ternary NiMoW coatings towards the hydrogen evolution reaction have not been investigated. Moreover, the nucleation/growth mechanism of these ternary alloys has never been treated. In the present work, we report the results regarding the electrochemical aspects of the deposition process and the mechanisms of nucleation and growth of NiMoW alloys from slightly acidic citrate ammonia-free electrolyte. Particular attention was paid to the effects of both Mo/W ratio in the plating bath and deposition potential on the corrosion resistance and 3 electrocatalytic activity of the alloys for the hydrogen evolution reaction (HER) in concentrated alkaline media. Experimental The electrochemical measurements were carried out with a Voltalab 40 potentiostat-galvanostat (Model PGZ 301) controlled by VoltaMaster 4 software (Hach Lange GmbH, Germany) and using a conventional three electrode cell assembly at ambient (laboratory) temperature. Platinum wire and pure copper sheets sized 1 2 cm2 were used as working electrode, a platinum foil having a large-area to prevent polarization of the anode was used as counter electrode and saturated calomel electrode (SCE) served as reference electrode. To obtain reproducible results, the substrate surface was polished with successive grades of sand paper to obtain a mirror finish. The samples were then treated in 10% (v/v) hydrochloric acid (HCl) for 2 min to remove any adherent oxide layer on the surface and rinsed with distilled water before every experiment. The plating solutions were made from analytical-grade chemicals (Sigma-Aldrich, USA) and doubly distilled water and contained 0.25 M NiSO4$6H2O, 0.05 M NiCl2$6H2O, Na2WO4, Na2MoO4. 2H2O as metal sources (with W/Mo ratio ranging from 0.5 to 2) and 0.4 M Na3C6H5O7$2H2O as complexing agent. After preparation, all solutions were deoxygenated with a stream of nitrogen within the cell before each experiment. The plating bath pH was quasi-neutral (6.25) adjusted by addition of potassium hydroxide or sulfuric acid as needed and monitored with a calibrated pH-meter (WTWinoLab pH 7310). No agitation was utilized in all experiments. To investigate the corrosion resistance and catalytic activities of electrodeposited NiMoW films for hydrogen evolution reaction (HER) in a 3.5% NaCl and 30% KOH solutions respectively, Tafel curves (5 mV/s scan rate from 350 mV to þ350 mV vs OCP), cyclic voltammograms and EIS (Nyquist and Bode) plots were performed at room temperature. The same three electrode electrochemical cell used for electrodeposition study was used for the characterization of the coatings. EIS measurements at a given applied potential and an alternative current (AC) sine wave of 10 mV amplitude were taken in the frequency range of 100 KHz to 10 mHz. The impedance data, such as the electrolyte resistance Re, the charge-transfer resistance (Rct) and the double layer capacity of the interface (Cdl) were determined from the Nyquist and Bode plots. All the chosen specimens were activated in 10 vol% HCl for 30 s prior to electrochemical testing. The corrosion rate occurring at the electrochemical interface (mm/y), corrosion potential (mV/SCE) and exchange current density (mA/cm2) were calculated automatically by using Tafel extrapolation method provided by Voltamaster 4 software [25,26]. The structural quality of the NiMoW samples was examined using X-ray diffraction (XRD) analysis, carried out on a powder diffractometer (Philips X'Pert Pro Multipurpose X-ray diffractometer) with copper Ka radiation source (lKa ¼ 1.5418 A) in the (2q) range of 20e100. The compositional and morphological properties were investigated using a field-emission scanning electron microscope (FESEM, JSM-7100F) with an accelerating voltage of 10 kV and acquisition time of 90 s, and equipped with a high resolution silicon drift detector (SDD) for X-ray Energy Dispersive Spectroscope (EDS) microanalysis. Please cite this article in press as: Allam M, et al., Electrodeposition and characterization of NiMoW alloy as electrode material for hydrogen evolution in alkaline water electrolysis, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.08.012 4 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 x x x ( 2 0 1 7 ) 1 e1 2 Results and discussion Electrochemical aspects of electrodeposition Cyclic voltammetry (CV) was used to investigate the electrochemical processes and define suitable potential regions for alloys deposition. A series of cyclic voltammograms for Ni, NieMo, NiW and NieMoeW recorded in plating solution with Pt working electrode at a sweep rate of 50 mV. s1 and 25 C are presented in Fig. 1. It is well established that in quasi-neutral citrate electrolyte (рНz6), nickel and citrate ions form complexes mainly of the type NiCit with stability constant log(K) ¼ 5.379. The voltammogram for pure Ni recorded between þ1.0 and 1.05 V/SCE, beginning from the open circuit potential (OCP) firstly towards cathodic direction, exhibits one main reduction peak at approximately 0.8 V/SCE ascribed to the reduction of Ni2þ ions: NiCit þ 2e /Ni þ Cit3 (1) and two anodic peaks at 0.075 and þ 0.6 V/SCE corresponding to the formation of nickel hydroxide (Eq. (2)) and nickel oxyhydroxide (Eq. (3)) respectively. Ni þ 2OH /NiðOHÞ2 þ 2e (2) NiðOHÞ2 /NiOOH þ Hþ þ e (3) With addition molybdate ions to the nickel plating bath (Fig. 1-a), the CV is somewhat different since the cathodic part of the CV moved towards more negative values, indicating additional difficulties for the reduction process. The anodic scan exhibits one main broad peak between the two oxidation responses of nickel. This behavior suggests that fundamental changes occurred at the electrode surface with a slower charge transfer due most probably to the formation of molybdenum oxides. It should be noted that during the reversal scan for Ni and NiMo, the presence of a crossover is characteristic of a nucleation process on Pt substrate. The shape of the CVs for NiW process (Fig. 1-b), doesn't differ substantially from that of pure nickel, excepted a slight shift of the oxidation peaks to the more positive potentials and the appearance of an additional anodic peak located at around þ0.05 V/SCE indicating the possible presence of Ni, W in addition to Ni þ W oxides. The electrochemical process for NieMoeW in co-existence is represented in Fig. (1-C). It can be clearly seen that total cathodic current rises substantially compared to that of Ni, NiMo and NiW systems, as a consequence of catalytic effect of the ternary alloy on the HER. The reduction current beginning for nickel ions, molybdate, tungstate and hydrogen evolution are approximately overlapped. On the other hand, when the cathodic scan limit is progressively enhanced, additional anodic peaks appeared with enhanced areas. Usually, the height of any peak is proportional to the quantity of the deposited phases. The multiple anodic peaks seen during the Fig. 1 e Cyclic voltammograms recorded in citrate electrolyte at room temperature and pH ¼ 6.25 for: Ni and NieMo (a), Ni and NieW (b) and NieNieMoeW with variable cathodic scan limits-as indicated in the figure- (c). Comparative cathodic parts of CVs plots are shown in figure (d). Please cite this article in press as: Allam M, et al., Electrodeposition and characterization of NiMoW alloy as electrode material for hydrogen evolution in alkaline water electrolysis, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.08.012 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 x x x ( 2 0 1 7 ) 1 e1 2 reverse scan can be attributed to electrochemical dissolution of the metals in the alloy from different intermetallic phases [27]. The total current remained positive over the entire anodic scan, suggesting the presence of mixture of molybdenum and tungsten oxides as reported by Chassaing [28] and Obradovic [29] for related binary alloys respectively, in parallel with Ni, Mo and W elements in metallic state. For the sake of comparison, the cathodic parts of CV scan readings of the working electrode (Pt), were recorded in Ni, NiMo, NiW and NiMoW electroplating baths at room temperature (Fig. 1-d). As can be seen, both the current density and potential for NiMoW are enhanced compared to Ni, NiMo and NiW deposition processes. For further characterization of the electrochemical deposition process, EIS experiments were carried out. A family of EIS (Nyquist and Bode) plots for the different coatings is shown in Fig. 2(aec). The applied potentials at which complex-plane impedance was measured and the frequencies corresponding to the top of the Nyquist plot semicircle are indicated in the figure. The Nyquist impedance plots are quite similar for all the samples and exhibit one main loop in the high frequency region in the form of a depressed semicircle resulting from of combination of charge transfer resistance and capacitance and indicating that the electrodeposition process is mainly under activation control [30], followed by a small sloping line in the low frequency domain representing the Warburg impedance (Zw) as a result of diffusion phenomenon of various active species (such as hydrogen, Ni, Mo and W species) from the bulk of the solution to the electrode- 5 electrolyte interface. The fluctuations observed in the EIS plots at very low frequencies can be attributed to the hydrogen evolution reaction in concurrence with the deposition process. The analysis of the impedance spectra in Bode-plan, indicate the presence of a single time constant, suggesting the existence of only one prevailing mechanism in the electroplating process. All phase-angles (Fig. 2c) are less than 20 and decreases when the deposition potential is lowered further, which also confirms an accelerated charge transfer process. The charge transfer resistance (Rct) values were deduced from the intercept segment of the semicircle with the real axis in Nyquist plots. Fig. 2-(d) shows the variation of charge transfer resistance (Rct) with plating potential. The experimental data are presented as scatter and the fitted results using ORIGIN software as continuous line. The resulting equation is as follows: Rct ¼ 290:8*exp 3:6686*Edep The decrease of charge transfer resistance values from 3.6 U cm2 to 0.85 U cm2 (fitted: from 3.562 to 0.821 U cm2) respectively with increasing polarization in the potential range 1.2 to 1.6 V vs SCE should indicate that the nucleation/growth process of NiMoW coating onto a pre-formed homogenous film is much easier that onto the heterogeneous copper substrate surface. The reduction reaction rate in terms of the cathodic current density ic, for large activation overpotentials, h can be expressed according to the ButlereVolmer equation [31]: Fig. 2 e EIS plots for NiMoW alloy electroplating at different applied potentials (aec) and variations of Rct with corresponding potential Edep (d); experimental (symbols) and fitted (solid lines). Please cite this article in press as: Allam M, et al., Electrodeposition and characterization of NiMoW alloy as electrode material for hydrogen evolution in alkaline water electrolysis, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.08.012 6 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 x x x ( 2 0 1 7 ) 1 e1 2 bnFh ic ¼ i0 exp RT (4) where i0, b, F, R and T are the exchange current density, the cathodic transfer coefficient, the “Faraday constant”, the “gas constant” and the “absolute temperature” respectively. On the other hand, the formation rate of a product at the cathode-electrolyte interface, related to the electrode area, A can be expressed by differentiating the Faraday's law for electrolysis (m ¼ Aic t=nF) with respect to time: dm=Adt ¼ i=nF (5) By combining eq (x) and eq (x), the electrochemical reaction rate constant, K of the reduction reaction at a given overpotential h can be obtained: bnFh K ¼ K0 exp RT (6) where K0 is the rate constant for reduction at Ei¼0, with respect to the reference electrode. Therefore, the electrochemical reaction rate constant increases exponentially with the applied overpotential h, which causes the exponential decay of Rt with the applied cathodic potential (Fig. 2d). A similar behavior has been reported by Chen et al. [32] during electrodeposition of CoNiFe ternary alloy. Chronoamperometry is a powerful tool to study electrochemical nucleation mechanisms in deposition processes. The formation and growth of a stable nucleon can be directly observed by plotting the current versus transition-time evolution [33]. Scharifker and Hills [34] used the experimental current transients to derive the mathematical models describing the nucleation mechanisms during the first stages of electrodeposition. Depending on the manner and rate at which nuclei appear, the nucleation process during the first few seconds of deposition may be classified as “instantaneous” or “progressive” and the growth mechanism is termed as three-dimensional (3D) growth [35]. The instantaneous nucleation corresponds to immediate and parallel formation and growth of nuclei on active sites at the same rate. The progressive nucleation refers to a mechanism in which new nuclei are consecutively formed during the course of deposition. The mathematical expressions in a non-dimensional form for instantaneous and progressive nucleation are given by Eqs. (7) and (8), respectively, i2 i2max ¼ 1:9542 t 1 exp 1:2564 t=tmax tmax 2 (7) for instantaneous nucleation and i2 i2max ¼ ( " 2 #)2 1:2254 t 1 exp 2:3367 t=tmax tmax (8) for progressive nucleation, where imax and tmax are current and time coordinates of the peak, respectively. Fig. 3a shows experimental current density-time transient measurements carried out under potentiostatic regime with five applied potential values, Edep ¼ 1.26, 1.28, 1.30, 1.32 and 1.34 V versus SCE. As seen, the shapes of these current density transients are similar. An abrupt decrease in the current density value at the beginning means that, in the early stages Fig. 3 e Effect of deposition potential on chronoamperometry of NieMoeW plating process from citrate electrolyte at room temperature: (a) Potentiostatic currentetime transients and (b) dimensionless plots of the normalized experimental currentetime transients: instantaneous (solid line) and progressive (dashed line). of the transient, current density is mainly utilized for doublelayer charging. With the passage of time, the current density increases gradually until a maximum value (Im) corresponding to the maximum time (tm) necessary for the growth of the germs and boost of active sites number at the electrode surface. After that, the current is lowered again to a constant value corresponding to a diffusion mode which controls the process [36]. Increasing the applied potential further increases the current density characteristic Im and decreases tm value. This behavior indicate that the electrocrystallization process rate increases as the applied potential value becomes more electronegative, as a result of an enhanced surface concentration of active species which increase both the nucleation and the growth rates associated with the deposition process. The results obtained suggest a typical behavior for a threedimensional (3D) nucleation and growth under diffusion control [34]. The experimental data from Fig. 3-(a) were used to obtain dimensionless currents versus time plots, according to Eqs. (3) and (4). The curves for instantaneous mechanism are depicted as solid lines and the ones for progressive nucleation Please cite this article in press as: Allam M, et al., Electrodeposition and characterization of NiMoW alloy as electrode material for hydrogen evolution in alkaline water electrolysis, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.08.012 7 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 x x x ( 2 0 1 7 ) 1 e1 2 as dashed lines. As can be seen on the graphs in Fig. 3b, in the mixed activation-masse-transport region, the initial stages of NiMoW nucleation occurred according to the instantaneous nucleation mechanism, regardless of deposition potential value; i.e., the formation of nuclei of the same size, which advantageous criteria for plating homogenous films. Composition and structural investigation The amounts of metallic elements in the deposits were measured using the energy dispersive X-ray spectroscopy (EDS). According to the results reported in Table 1, at a given deposition potential, increasing Mo/W salts ratio in the electrolyte reduced both Ni and W content in the deposits. A different trend was observed in an equimolar solution 2 (WO2 4 =MoO4 ¼ 1 : 1), where at a given potential, there is more Mo than W in the deposits, indicating a favored induced codeposition of Mo compared to W. When applying more negative potentials, both Mo and W content decreased leading to the enhancement of Ni amount. According to Cruywagen et al. [37,38], in presence of citrate, 2 the tungstate ðWO2 4 Þ and molybdate ðMoO4 Þ ions form complexes in the type of ½ðWO4 Þp ðCitÞq Hr ð2pþ3qrÞ and ½ðMoO4 Þp ðCitÞq Hr ð2pþ3qrÞ even though both are negatively charged, where the number of protons, r, depends on the pH. At pH 6, the predominant complexes are ½ðWO4 ÞðCitÞðHÞ2 3 and ½ðMoO4 ÞðCitÞðHÞ2 3 respectively. In the same period, Murase et al. [39] reported that on the addition of citrate to NieMo plating bath, electrochemically active Hr MoO4 Citð5rÞ and NiCit complexes are formed. More recently, Eliaz and Gileadi [28] postulated that induced codeposition of NiW alloys occurs through the reduction of a ternary mixed-metal complexed species formed in citrate electrolyte: 4 2 ½ðNiÞðCitÞ þ ½ðHWO4 ÞðCitÞ /½ðNiÞðHWO4 ÞðCitÞ 2 ½ðNiÞðHWO4 ÞðCitÞ þ Cit3 þ 8e þ 3H2 O/NiW þ 7ðOHÞ þ Cit3 (9) (10) The chemical properties of tungsten are substantially similar to that of molybdenum and their alloys with Ni are commonly obtained from electrolytes of the same type. Moreover, Brenner [14] has suggested that there is an analogous behavior of Mo induced codeposition that can also hold for W induced codeposition. Accordingly, the mechanism proposed by Eliaz and Geleadi [40] for NiW deposition would be valid for NiMo alloy: Firstly, Nickel-citrate reacts with molybdate-citrate to form a ternary complex: 4 2 ½ðNiÞðCitÞ þ ½ðHMoO4 ÞðCitÞ /½ðNiÞðHMoO4 ÞðCitÞ þ Cit3 (11) The reduction of this precursor leads to the formation of the Ni/Mo alloy: 2 ½ðNiÞðHMoO4 ÞðCitÞ þ 8e þ 3H2 O /NiMo þ 7ðOHÞ þ Cit3 (12) However, for NiMo deposition, in addition to the scheme proposed by Eliaz and Geleadi, a parallel route for depositing Mo as Ni4MoO2 compound is expected. This could explain the presence of higher amount of Mo in the alloy compared to W (Table 1). Thermodynamically, the molybdate anions MoO2 4 can be reduced to molybdenum oxide MoO2 at 1.2 V vs SCE [41]: MoO2 4 þ 2H2 O þ 2e /MoO2 þ 4OH (13) On the other hand, Chassaing et al. [28] reported that when an electrode is polarized at low overpotentials (in the range of 1.35 to 1.45 Vvs SSE), the presence of Ni2þ ions allows the possible transformation of such Mo oxide MoO2 into a mixed NieMo oxide according: MoO2 þ 4Ni2þ þ 8e /Ni4 MoO2 (14) This possible additional route for depositing Mo would not be applicable to W codeposition since, according to Pourbaix [42], tungsten oxides (WO3, WO2 and W2O5) are not stable in the solution pH used here (pH ¼ 6.25). It should be noted that the effective existence of all these compounds was not confirmed since X-ray diffraction experiments (not reported here) revealed the amorphous structure of all examined coatings. Table 1 shows the variations of Mo and W contents in the coatings with Mo/W molar ratio in the solution and plating potential. At a given applied potential (1.3 V/SCE), varying the Mo/W ratio from 0.5 to 2, increases the Mo content in the deposit from 28.52 at% to 36.59 at%, while the W one decreases down to 1.06 at %. This behavior is due to the mass transport effect, more expected for tungstate ions due to their low bulk concentration than for molybdate and nickel species. Guettaf [25] thought that presence of molybdate ions could reduce substantially the deposition overpotential of Ni. On the other hand, in an equimolar solution (Mo/W ¼ 1), there is more Mo than W in the deposits. The main factor that may influence this behavior is that the citrate ligand may complex with WO2 4 stronger than that of MoO2 4 . Increasing the cathodic polarization from activation control region (1.1 V/SCE) up to mass-transport control zone (1.4 V/SCE, Fig. 1d) causes a drop of both Mo and W contents in the deposit. At low overpotential, the adsorption of Mo and W species inhibits subsequent discharge of nickel ions and leads to the preferential deposition of Mo and at less extent W, although Ni deposition is not completely blocked. In slightly acidic citrate solution (pH ¼ 6.25), even though the concentrations of Table 1 e Elemental composition deduced from EDX analysis of NieMoeW coatings obtained under various deposition potentials and Mo/W salts ratio in the plating bath. Element (at%) Ni Mo W 2 ðMoO2 4 =WO4 Þ ðEdep ðV=SCEÞ ¼ 1:3) 2 Edep ðV=SCEÞ ðMoO2 4 =WO4 ¼ 1Þ 0.5 1.0 2.0 1.1 1.2 1.3 1.4 68.16 28.52 3.32 64.41 33.70 1.89 62.35 36.59 1.06 18.13 73.34 8.53 60.47 35.87 3.67 61.63 35.88 2.49 64.41 33.7 1.89 Please cite this article in press as: Allam M, et al., Electrodeposition and characterization of NiMoW alloy as electrode material for hydrogen evolution in alkaline water electrolysis, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.08.012 8 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 x x x ( 2 0 1 7 ) 1 e1 2 ½ðMoO4 ÞðHCitÞðHÞ2 3 and ½ðWO4 ÞðHCitÞðHÞ2 3 active complexes reach their maximum [37] and [38], increasing the deposition overpotential further causes an increase in the nickel content in the deposit until it reaches its maximum (64.4 at%) at around 1.4 V vs SCE, while Mo and W contents are lowered respectively from 73.34 at% and 8.53 at% for a deposition potential of 1.1 V vs SCE to 33.7 at% for a more cathodic potential (1.4 V vs SCE. This could result from lower cathodic current efficiency of both NiW and NiMo than pure Ni deposition and attributed to a partial restraint of Mo and W active species by nickel ions. A similar phenomenon has been observed by Xu et al. [18] who reported that the deposition current efficiency of NiMo alloy was lower than pure Ni as the potential moved negatively. Morphological study The surface morphological features of deposited NiMoW alloys were studied by field emission scanning electron microscopy. A film plated onto copper substrate from a solution with a 2 ratio ¼ 2 contains micro-cracks (Fig. 4-a) due MoO2 4 =WO4 the relaxation of internal tensile stress after hydrogen trapping into the coatings, which is the characteristic of the Mo-rich electrodeposited NieMo alloys [43], while for a ratio 2 MoO2 4 =WO4 ¼ 0.5, the cracks disappeared (Fig. 4-b). The topview images (Fig. 4a-b) show large real area composed of spheroid grains sized 0.5e1 mm, which confirms the amorphousness structure of the coatings. Cross-sectional images of 2 NiMoW alloy coated from an equimolar bath (MoO2 4 =WO4 ¼ 1 ) are presented in Fig. 4-(ced), indicating crack-free and amorphous uniform layers with 0.5 and 1 mm thickness were obtained for plating potentials of 1.2 and 1.4 V vs SCE respectively. Electrochemical corrosion behavior The anti-corrosion performance of the coatings in 3.5% NaCl solution was investigated using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) methods. Tafel polarization plots for NiMoW alloys plated at pH z 6.2 and variable applied potentials are shown in Fig. 5 (a). The corrosion parameters reported in Table 2 show clearly that the charge transfer resistance (Rct) of NiMoW coatings increased with increasing applied cathodic potential in the range of 1.2 to 1.6 V/SCE while their corrosion rate (CR) decreased. Therefore, it can be concluded that NieMoeW alloys, deposited at low overpotentials and containg high amounts of Mo and W deduced for EDX analysis are more corrosion resistant compared to alloys deposited at high overpotentials. Furthermore, for the sake of comparison, data for pure Ni [15], NiW [15] and NiMo alloys and inserted in Table 2, show clearly an enhancement of corrosion resistance of the ternary alloy, particularly for films coated at 1.2 and 1.3 V/SCE, where the corrosion rate (102.mm y1) and the current density (mA.cm2) couples are reduced from 156.4/134, 73,95/63 and 163.6/139.9 for Ni, NiW and NiMo respectively to 55.56/4.75 for NiMoW. The electrochemical impedance spectroscopy study revealed the same trend as Tafel polarization. As can be seen in Fig. 5b, it is clear that the diameter of the semicircle which describes the electron-transfer resistance of the coating considerably decreases with increasing the cathodic deposition potential, which indicates faster corrosion reaction kinetics at the electrode surface for films coated at high overpotentials. This is consistent with the Tafel plots results. For low overpotentials, the small inductive loop observed at low frequencies domain (notably between 10 and 71 mHz), as a result Fig. 4 e FESEM images of NiMoW samples deposited at pH 6.2, room temperature and ¡1.4 V vs. SCE, with Mo/W molar ratio in plating bath: (a) 2; (b) 0.5; (c) 1 and (d) 1, coated at ¡1.2 V vs. SCE. Please cite this article in press as: Allam M, et al., Electrodeposition and characterization of NiMoW alloy as electrode material for hydrogen evolution in alkaline water electrolysis, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.08.012 9 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 x x x ( 2 0 1 7 ) 1 e1 2 Fig. 5 e Potentiodynamic polarization (a) and EIS (Nyquist and Bode, b-d) plots recorded in 3.5 wt% NaCl solution for NiMoW deposits obtained at different applied potentials (as indicated in the figures). For comparison, curves for NiW [15] and NiMo alloy coated at ¡1.4 V/SCE are also added. The inset in (b) shows the EEC for NiMoW coating. Table 2 e Kinetics parameters in 3.5 wt% NaCl deduced from Tafel and Nyquist plots for NiMoW deposits obtained at different applied potentials. Ni, NiW and NiMo alloys were coated form a citrate-based solution at pH ¼ 6.25 and 25 C. Sample Edep (V/SCE) Jcorr (mA.cm2) Ei¼0 (mV/SCE) Rct (kU.cm2) Cdl (mF.cm2) CR (102.mm y1) NiMoW 1.2 1.3 1.4 1.5 1.6 1.2 1.2 1.4 4.75 4.90 29.1 32.8 81.8 134 63 139.9 781.0 735.1 832.7 865.5 903.0 644.3 572.1 970 10.91 9.699 7.409 4.462 3.903 3.022 1.825 2.915 36.46 36.75 48.11 49.92 513.6 258.2 503.9 611.3 55.56 50.50 339.9 383.2 956.2 156.4 73,95 163.6 Ni [15] NiW [15] NiMo of adsorbed species at the electrode/electrolyte [44], is converted to a sloped line due the diffusion of active ions from the bulk of the solution to electrode surface. The EIS spectra are also presented in Bode-plan. The number of the slopes (Fig. 5c) and the phase angle peaks (Fig. 5d) in Bode plot show the existence of only one time constant, and indicating that only main mechanism is prevailing during the exposition of the deposits to aggressive aqueous media. Meanwhile, all the phase-angles for NiMoW coatings (Fig. 5c) are higher than those for NiMo and NiW binaries. The results show a gradual increase of the corresponding frequency of phase-angle peak when the deposition potential is lowered, which confirms the enhanced resistive behavior of the ternary alloys. Consequently, the high-to-medium frequency EIS data were fitted using the Randles circuit [45]. The corresponding electrochemical equivalent circuit is shown in Fig. 5b.), where R1 represents the solution resistance, C is attributed to the electrode capacitance and R2, the charge transfer resistance. Based on these results, it seems likely that during electrodeposition at low applied overvoltages, the resulting local pH increase and the presence of Ni2þ species induce the formation of a tungsten and molybdenum oxides or hydroxides (NiO, Ni(OH)2, WO3 and MoO2). The incorporation of such oxides into the alloy structure enhances the corrosion resistance of the deposits. A similar behavior has been observed by Chassaing [28] for the electrodeposition of NieMo and attributed to the formation of a molybdenum oxide. For higher polarization, these oxides are more readily reduced to the metal form in the presence of adsorbed atomic hydrogen at the electrode surface, using the unpaired 3d subband electrons of Ni, as suggested by Fukushima et al. [46], leading to less protected coatings in corrosive media. This is in line with Please cite this article in press as: Allam M, et al., Electrodeposition and characterization of NiMoW alloy as electrode material for hydrogen evolution in alkaline water electrolysis, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.08.012 10 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 x x x ( 2 0 1 7 ) 1 e1 2 the views of Guettaf and co-workers [25] who studied the effect of current density on the performances of NiMo alloy electrodeposited from ammonium-based electrolyte and reported that the corrosion resistance decreases with increasing deposition current density, The nobler NieMo sample, deposited at low a current density (10 mA/cm2), contained around 13 wt% Mo. According to Obradovic et al. [47], the presence of Ni(I) intermediate could enhance the reduction of metal-oxides formed during the deposition process. The formation of corrosion products can be described through the following possible reaction pathways: in neutral aqueous solutions, the anodic reactions imply strong interactions between H2O and Ni-based alloy substrate, yielding primary adsorbates Ni(H20)ad in equilibrium with (NiOH)ad adsorbate [48]. The dissolution of nickel proceeds as follows [42]: reaction (HER). The kinetic parameters for the HER were derived from Tafel equation [50]: h ¼ a þ blogj where a (V), b (V.dec1), h (V) and j (A.cm2) represent the curve intercept, the Tafel slope, the applied overpotential, and the resulting current density respectively. The generally accepted mechanism for the hydrogen evolution reaction HER in alkaline media involves the formation of adsorbed hydrogen atoms, MHads at the electrode surface, known as Volmer reaction (Eq. (11)), followed by either electrochemical (Heyrovsky step, Eq. (12)) or chemical (Tafel step, Eq.z) desorption: H2 O þ M þ e /MHads þ OH (25) Ni þ H2 O⇔NiðH2 OÞad ⇔NiðOHÞad þ Hþ þ e (15) H2 O þ MHads þ e /H2 þ M þ OH (26) NiðOHÞad þ H2 O⇔NiðOHÞ2 þ Hþ þ e (16) MHads þ MHads /H2 þ 2M (27) NiðOHÞ2 ⇔NiO þ H2 O In chloride containing solution, an additional pathway involving competitive adsorption between Cl and OH ions at the electrode surface can be expected: NiðH2 OÞad þ Cl ⇔NiðClOHÞ þ Hþ þ 2e (17) NiðClOHÞ⇔NiO þ Cl þ Hþ (18) The total oxidation reaction of nickel can be written as: Ni þ H2 O⇔NiO þ 2Hþ þ 2e ; E0 ¼ 0:11 0:0591pH (19) The oxidation reactions of metallic Mo can result from the following reaction sequence [42]: Mo þ 2H2 O⇔MoO2 þ 4Hþ þ 4e ; E0 ¼ 0:072 0:0591pH (20) þ MoO2 þ 2H2 O⇔MoO2 4 þ 4H þ 2e (21) With E0 ¼ 0:606 0:1182pH þ And for W [42]: 0:0295logðMoO2 4 Þ W þ 2H2 O⇔WO2 þ 4Hþ þ 4e ; E0 ¼ 0:119 0:0591pH (22) þ WO2 þ 2H2 O⇔WO2 4 þ 4H þ 2e (23) 0:0295logðMoO2 4 Þ With E0 ¼ 0:386 0:1182pH þ The common cathodic reactions in practical cases of corrosion are by far reductions of hydrogen ion to hydrogen, or of oxygen to hydroxyl ion. In neutral-aqueous solution, the cathodic reduction of molecular oxygen is expected [49]: O2 þ 2H2 O þ 4e /4OH (24) Molecular oxygen is continually replaced from the gas phase by diffusion and/or convection through the liquid to the electrode surface. By plotting the experimental dependence of the potential (E) on the logarithm of the current density (logi), known as the Tafel polarization curves, one can deduce which reaction of these is the rate determining-step. At 25 C, a cathodic slope b (b ¼ 2:3RT=aF, where a is the charge transfer coefficient) of 120, 40 or 30 mV$dec1 is ascribed to Volmer, Heyrowsky or Tafel reaction respectively [51]. Generally, an electrode material showing low Tafel slope is indication of favoring hydrogen evolution. The experimental kinetic parameters, indicative of electrocatalytic performances of NiMoW deposits coated at different potentials using an equimolar 2 (MoO2 4 =WO4 ¼ 1 : 1) electrolyte were determined automatically from the Tafel plots recorded in 30% KOH solution. The results are summarized in Table 3. As can be seen, the exchange current density increases gradually with increasing deposition potentials, indicating faster hydrogen gas generation i.e highest activity for HER. Furthermore, the experimental Tafel slope values for all NiMoW alloys changed between 60 and 77 mV$dec1 s for low-to-high deposition potentials respectively, suggesting gradual change from Heyrowsky to Volmer step rate-determining mechanism for the HER. For example, the magnitude of the Tafel slope (63 mV/dec) obtained on NiMoW coated at 1.5 V vs. SCE is much lower than that obtained on NiMoFe (165 mV/dec) reported by Raj [52]. This result reflects an enhancement in the Table 3 e Kinetic parameters determined from Tafel curves in 30% KOH solution for NiMo, NiW and NiMoW coatings obtained at different potentials using an equimolar Mo/W electrolyte. Sample NiMoW Electrocatalytic activities of NiMoW films for HER Linear polarization method was used to estimate the electrocatalytic activity of NiMoW coatings for hydrogen evolution NiMo NiW Edep Eeq jo bc (V/SCE) (mV/SCE) (102 mA cm2) (mV.dec1) 1.2 1.3 1.4 1.5 1.6 1.4 1.4 1008.7 1017.2 1007.1 997.7 1023.8 1025 1068.4 10.38 18.22 36.45 54.11 13.04 16.61 30.07 60 47 55 63 77 150 112.5 Please cite this article in press as: Allam M, et al., Electrodeposition and characterization of NiMoW alloy as electrode material for hydrogen evolution in alkaline water electrolysis, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.08.012 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 x x x ( 2 0 1 7 ) 1 e1 2 -1,3 NiMo NiW E (V/SCE) -1,2 NiMoW -1,1 -Edep: -1,0 1,2 V 1,3 V 1,4 V 1,5 V 1,6 V -0,9 -3 -2 -1 log i(mA/cm²) 0 1 2 Fig. 6 e Tafel polarization curves recorded in 30 wt% KOH at 25 C on NiMoW films coated at various potentials. NiMo and NiW electrodes were coated at ¡1.4 V/SCE. surface coverage of the adsorbed hydrogen on these cathodes. For the sake of comparison, the cathodic part of Tafel plots for NiMo and NiW alloys, coated at 1.4 V vs SCE from the same citrate-based electrolyte at pH ¼ 6.25 and room temperature, are shown in the same figure. It is clearly seen that the NiMoW alloy electrode obtained at 1.5 V/SCE had a much better catalytic activity than the binary NiMo and NiW electrodes, since Tafel slope for NiMo and NiW samples were larger (150 and 112.5 mV/dec) with lower exchange current densities. The decrease of Tafel slope value for the ternary alloys compared to binary ones could be explained by terms of increased surface area. For any applied potential, when the electrode surface area increases, the real current density is higher than the apparent current density, which causes the decrease of Tafel slope value [22]. Sum up, the good electrocatalytic activity of NiMoW alloy coated at high overpotential could be the result of synergetic effect of Mo and W in the films, increased electrode surface area and reduction of their oxides by atomic hydrogen adsorbed on freshly deposited Ni atoms. It is worth noting that the coatings having the best catalytic activity towards hydrogen evolution are les noble compared to films coated at low overvoltages which revealed better corrosion resistance in aggressive media, most probably due the presence of molybdenum and tungsten oxides on the electrode surface [19]. (see Fig. 6). Conclusion Ternary NiMoW alloys were electrodeposited onto copper sheet substrates from citrate-ammonia free aqueous electrolyte at room temperature. In an electrolyte where 2 WO2 4 =MoO4 ¼ 1 : 1, there was more Mo than W in the deposits, indicating advantageous induced codeposition of Mo compared to W. With applying low overpotentials (i.e deposition current densities), the linear polarization resistance and electrochemical impedance study revealed the possible formation and incorporation of molybdenum-tungsten oxides allowing a better corrosion resistance to the alloys in corrosive 11 media but decreasing their catalytic activity for the hydrogen evolution reaction. Films coated at higher voltages exhibited higher exchange current densities in 30 wt% KOH solution, indicating better electrocatalytic properties towards HER as a result of synergetic effect of the three elements in the coatings and high real area. The application of ScharifkereHills theoretical models to study the first instants of the electrodeposition of NiMoW film suggested instantaneous nucleation mechanism for all specimen, regardless of the metal ions concentrations and the applied potentials; i.e., the formation of nuclei of the same size, which represents an advantageous criteria for plating homogenous films. 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Chichester: Ellis Horwood Ltd; 1985. [51] Tsyntsaru N, Cesiulis H, Donten M, Sort J, Pellicer E, PodlahaMurphy EJ. Modern trends in tungsten alloys electrodeposition with iron group metals. Surf Eng Appl Electrochem 2012;48:491e520. [52] Arul Raj I, Vasu KI. Transition metal-based cathodes for hydrogen evolution in alkaline solution: electrocatalysis on nickel-based ternary electrolytic codeposits. J Appl Electrochem 1992;22(5):471e7. Please cite this article in press as: Allam M, et al., Electrodeposition and characterization of NiMoW alloy as electrode material for hydrogen evolution in alkaline water electrolysis, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.08.012
