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Kinetics of the hydrogen evolution reaction on Ni-(Ebonex-supported Ru) composite coatings in alkaline solution
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Kinetics of the hydrogen evolution reaction on Ni-(Ebonex-supported Ru) composite coatings in alkaline solution Lacnjevac a,*, B.M. Jovic a, V.D. Jovic a, V.R. Radmilovic b, N.V. Krstajic b U.C. a b
Institute for Multidisciplinary Research, University of Belgrade, Kneza Viseslava 1, 11030 Belgrade, Serbia Faculty of Technology and Metallurgy, University of Belgrade, Karnegijeva 4, 11000 Belgrade, Serbia
article info
abstract
Article history:
The hydrogen evolution reaction (HER) was studied on Ni, Ni-Ebonex and Ni-(Ebonex-Ru)
Received 26 April 2013
coatings in 1 mol dm3 NaOH solution at 25 C. The composite coatings were electro-
Received in revised form
deposited from a nickel Watts-type bath containing suspended Ebonex (chemical compo-
1 June 2013
sition mainly Ti4O7) or Ebonex-supported Ru(10 wt.%) particles (0e10 g dm3) onto Ni 40
Accepted 8 June 2013
mesh substrate. The electrodes were investigated by cyclic voltammetry (CV), transmission
Available online 15 July 2013
electron microscopy (TEM) and scanning electron microscopy (SEM) in combination with energy dispersive X-ray spectroscopy (EDS) and electron energy loss spectroscopy (EELS),
Keywords:
electrochemical impedance spectroscopy (EIS) and polarization measurements. These in-
Magneli phases
vestigations showed that the roughness factor of the Ni-(Ebonex-Ru) and the Ni-Ebonex
Ni-composite coating
coating was 29 and 6 times higher than that of a pure Ni coating, respectively. In the
H2 evolution
whole potential range of the HER only one Tafel slope of about 120 mV was present at the
Kinetics
polarization curves of Ni and Ni-Ebonex electrodes, with increased activity of the latter
Electrocatalysis
being attributed only to the increase of the electrochemically active surface area. The Ni(Ebonex-Ru) electrodes exhibited the highest intrinsic catalytic activity with two Tafel slopes, indicating also that the HER takes place exclusively on Ru active sites. Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Nickel, its alloys and Ni-based compounds are the most widely studied cathode materials for the HER in alkaline solutions. Electrocatalytic activity of Ni is not high, but this metal exhibits high resistance to corrosion in concentrated alkaline solutions at elevated temperatures. The overpotential of the HER increases with time, especially in the conditions of intermittent electrolysis [1]. Enhancement of cathode activity of Ni has been achieved by increasing the real surface area or by increasing the intrinsic activity using alloys or composites
[2e7]. The increase of the real surface area can be accomplished in several ways: by alloying Ni with an active metal like Al or Zn, followed by the dissolution of the secondary component (Raney-Ni) [8e15], or by depositing Ni from ammonium chloride baths at high current densities [16e18]. In all cases porous structures were obtained, characterized by a large electrochemically active surface area. The intrinsic activity of Ni has been also increased by electrodeposition of Ni-based composite materials containing active solid particles (MoO2, MoO3 [19,20], Mo [3], CeO2 [21], Ni-rare earth compounds [22], RuO2 [18,23e25], IrO2 [23], and TiO2 [26]), by
* Corresponding author. Tel.: þ381 11 2085039; fax: þ381 11 3055289. Lac njevac). E-mail address: [email protected] (U.C. 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.06.037
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alloying Ni with some metals (NiCo [27e29], NiMo [30,31], NiW [30,32], NiFe [30,33], NiLa [34]) or by activation of Ni by spontaneous deposition of Ru [35,36] or Ir [36] from acid deaerated aqueous solutions of corresponding ions. Ni-based composite coatings have recently attracted significant attention because they combine many desirable properties of an electrocatalyst for the HER: large surface area, good intrinsic catalytic activity, and high corrosion stability. Among composite catalysts, the NieRuO2 (Ru) composite catalyst was one of the most investigated, due to the fact that ruthenium oxide and ruthenium exhibit one of the highest activities for the HER [25]. Thermodynamically, ruthenium oxide should not be stable during cathodic polarization, but it has been found experimentally that it possesses a considerable stability, which has been attributed to a sufficient electronic conductivity of RuO2, preventing its reduction. Composite NieRuO2 catalysts can be prepared by thermal deposition of RuO2 into the Raney-Ni micropores [37] or by cathodic codeposition of RuO2 particles within a porous Ni matrix [18]. Particular attention has been devoted to co-deposition of Ni and RuO2 particles suspended in a commercial Watts bath. Iwakura et al. [24] have first confirmed the increase in electrocatalytic activity for the HER on these coatings. However, RuO2 is expensive and its use in composite cathode catalysts is not economically favorable. Therefore, research efforts have also been devoted to further reduction of the amount of RuO2 in the composite Ni-based catalysts and improvement of their stability by incorporation of other metal oxides. It has been shown that incorporation of TiO2 improved the coatings stability without significant reduction in the electrocatalytic activity [38,39]. Various composite Ni-RuxTi1-xO2 coatings, with Ru contents ranging from 0 to 50 at.%, were investigated with respect to catalytic activity for the HER and their stability in alkaline solutions [40]. In the present work, in order to improve the electrocatalytic activity for the HER and reduce the amount of Ru, Ebonex particles decorated with Ru nanoparticles were incorporated in the electroplated Ni coatings. Ebonex is electrically conductive ceramic composite of several reduced titanium oxide phases, mainly Ti4O7 and Ti5O9, which form a part of a homologous series of crystallographic shear phases of the formula TinO2n1, with n ¼ 4 to 10, known as Magneli phases [41]. Non-stoichiometric Ti oxides are electrochemically stable in both acid and alkaline solutions. Ebonex also possesses high overpotentials for hydrogen and oxygen evolution in aqueous solutions and shows slow electron transfer kinetics [42], being a better candidate for the substrate than for the catalyst. The goal of this work was to investigate the possibility of using Ni coatings co-deposited with Ebonex-Ru particles as catalysts for the HER in alkaline solution. Based on the results of the Tafel slopes analysis, a mechanism of the HER on Ni(Ebonex-Ru) electrodes was proposed.
2.
Experimental
2.1.
Preparation of Ebonex-supported Ru particles
Powder of Magneli phases of general formula TinO2n1, or in average Ti4O7 (trade name Ebonex, Altraverda, U.K.), was used
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in these investigations. The starting Ebonex powder was ground in a Fritsch Planetary Mill (Pulverisette 5) using 6 mm diameter balls with maximum acceleration of 700 m s2 [43]. The ground powder was collected after passing through a 100 mesh sieve and used in a preparation procedure. Precipitation of the ruthenium catalyst on the Ebonex support was conducted by a conventional borohydride reduction method [44]. 1.8 g of the Ebonex powder was dispersed in 100 ml of deionized water in ultrasonic bath, and then mixed with the amount of RuCl3 equivalent to 0.2 g Ru. Ruthenium ions were completely reduced by adding a sodium borohydride solution in excess. The Ru precipitate on Ebonex was washed with deionized water and then dried at 80 C for 24 h. In such a way, powder with Ru loading of 10 wt.% was prepared for further examinations.
2.2.
Preparation of electrodes
The Ni/(Ebonex-Ru) cathodes were prepared by simultaneous electrodeposition of Ni and suspended Ebonex-Ru particles onto Ni 40 mesh substrate from a conventional Watts bath of the following composition: 330 g dm3 NiSO4,6H2O þ 45 g dm3 NiCl2,6H2O þ 38 g dm3 H3BO3; pH 3.2e4.0; t ¼ 40 C. The amount of suspended Ebonex-Ru particles in the bath varied between 0 and 10 g dm3. Electrodeposition was carried out at a constant current density j ¼ 25 mA cm2 in the beaker cell of a small volume of 0.1 dm3 with two vertical Ni mesh anodes and a Ni 40 mesh cathode between them. Thickness of the coatings was controlled through the deposition charge density, which was fixed at 18.6 C cm2. Mixing of the electrolyte was provided by the air flow (50 dm3 h1) through the glass pipe of the shape of spiral with small openings facing the bottom of the cell. The Watts bath was prepared from analytical grade chemicals dissolved in deionized water. The pH was initially adjusted at 3.8 by adding sulfuric acid. Before the deposition of Ni-(Ebonex) and Ni-(Ebonex-Ru) coatings, Ni 40 meshes were etched in 2:1 HNO3:H2O solution for 60 s [20].
2.3. Microstructural characterization of powder and coatings 2.3.1.
SEM in combination with EDS
The morphology and composition of the Ebonex-Ru powder and electrodeposited coatings were investigated by a Tescan VEGA TS 5130 MM scanning electron microscope equipped with an EDS detector INCAPentaFET-x3, Oxford Instruments [20].
2.3.2.
TEM in combination with EDS and EELS
Conventional transmission electron microscopy (CTEM) and EDS were performed using the CM200-FEG transmission electron microscope operated at 200 kV. High resolution transmission electron microscopy (HRTEM) and EELS were performed using the TEAM 0.5 aberration corrected transmission electron microscope, equipped with GIF Tridiem imaging filter, operated at 80 kV, in order to minimize radiation damage of investigated catalysts. CrystalKitX and MacTepas programs were used for structure modeling and image simulation of experimental high resolution transmission electron microscopy data.
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2.4.
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Electrochemical measurements and solutions
The polarization characteristics of the HER onto Ni, Ni-Ebonex and Ni-(Ebonex-Ru) samples were tested in 1.0 mol dm3 NaOH solution in extra pure UV water (Smart2Pure UV, TKA) at 25 C. A three-compartment cell was used: the working electrode of the surface area of 1 cm2 was placed in a central compartment together with the Luggin capillary; two Pt mesh counter electrodes of larger surface areas were each placed in separate compartments (parallel to the working electrode mesh), so that oxygen evolved at the counter electrodes could not enter the working electrode compartment. The saturated calomel electrode (SCE) was placed in a side compartment connected to the central one through a bridge and a Luggin capillary, and was kept at the room temperature. Experiments were performed using the potentiostat Reference 600 and the software PHE 200 and DC 105 (Gamry Instruments). All samples were first submitted to the HER at a constant current density j ¼ 0.3 A cm2 for 800 s (step 1), followed by the HER at a constant potential for 60 s (step 2). The value of potential was adjusted to produce cathodic current density slightly higher than 0.3 A cm2. After such pre-electrolysis, polarization curves were recorded by sweeping potential with 1 mV s1 from the potential applied in step 2 to the value of open circuit potential, while potential was automatically corrected for the IR drop using the current interrupt technique. All potentials in the text are given with respect to the SCE (ESCE ¼ 0.244 V vs. SHE). Cyclic voltammetric curves were recorded on a pure Ni coating and on the Ni-Ebonex and Ni-(Ebonex-Ru) composite coatings (deposited from a Watts bath containing 5 g dm3 of Ebonex or 5 g dm3 of Ebonex-Ru(10 wt.%) suspended particles, respectively) in 1.0 mol dm3 NaOH at sweep rates of 10e200 mV s1 and in the potential range from 1.15 to 0.55 V. The voltammetric charge was determined by means of a graphical integrator. The EIS measurements were performed with the same potentiostat and EIS 300 software, applying the amplitude of 5 mV RMS in the frequency range from 50 kHz to 0.01 Hz. The real (Z0 ) and imaginary (Z00 ) components of electrochemical impedance spectra in the Nyquist plot were analyzed using the complex nonlinear least squares (CNLS) fitting program (EIS 300) to simulate the equivalent resistances and capacitances.
3.
Results and discussion
3.1.
Electron microscopy characterization
Morphology of the composite Ebonex-Ru powder is presented in SEM images in Fig. 1, while the results of the EDS analysis performed at positions marked in Fig. 1 are given in Table 1. Elemental composition analysis of gray and white areas observed in the backscattered electron (BSE) micrograph shown in Fig. 1a confirmed that the Ru phase (white area) was non-uniformly distributed across the surface of the Ebonex substrate (gray area). The Ebonex powder consisted of characteristic flattened particles in the size range from 500 nm to 10 mm, whereas the Ru precipitate was composed of
agglomerated nanosized particles (Fig. 1b). In order to find the average content of each component of the Ebonex-Ru powder, EDS analysis was applied upon a larger surface area of 250 200 mm (Fig. 1c). As can be seen from Table 1, the average content of Ru in this case practically coincides with the value calculated from the initial weights of components used in the powder preparation procedure (2.9 at.%). TEM analysis of the Ebonex-Ru powder confirmed that the microstructure was characterized by the presence of pure metallic Ru regions deposited on Ebonex. In addition, as shown in high angle annular dark field (HAADF) image in Fig. 2a, the pure metallic Ru regions were locally characterized by the presence RuO2 nanorods. The presence of RuO2 nanorods was confirmed by the EELS spectrum shown in Fig. 2a, which clearly indicates the presence of RuM4,5, RuM2,3, and OK edges. The dark spots in Ru region indicate the presence of nanopores. High resolution bright field (BF) image of a RuO2 nanorod, taken close to 110 zone axis, is shown in Fig. 2b. It is clear that RuO2 nanorods are strongly faceted with an angle of 134 between 110 and 111 facets. The atomic ratio between Ti and Ru was measured by EDS. It was found, as expected, to vary depending on the local region where the measurement was done, and it was in the range: Ti:Ru w0.98 to 0.02, which indicates non-uniform distribution of Ru on Ebonex support. Typical top views of the Ni coating and the Ni-Ebonex and Ni-(Ebonex-Ru) composite coatings electrodeposited from a Watts bath with concentration of suspended Ebonex and Ebonex-Ru particles of 5 g dm3 are shown in Fig. 3. The presented SEM micrographs indicate that in comparison to a pure Ni deposit (Fig. 3a) surface roughness was distinctly increased after incorporation of Ebonex (Fig. 3b) and especially EbonexRu particles (Fig. 3c) in the Ni matrix. In the BSE mode micrographs corresponding to the Ni-Ebonex and the Ni-(Ebonex-Ru) coating, Fig. 3b and c, respectively, the Ni phase is visible as a white area. The Ni deposit in the Ni-Ebonex coating had a compact crystalline structure (Fig. 3b), while in the Ni-(EbonexRu) coating the Ni deposit consisted of agglomerates of the spherical grains with a diameter of about 500 nm (Fig. 3c). Considering the fact that the both coatings were prepared under identical deposition conditions ( jdep ¼ 25 mA cm2; sdep ¼ 39 min), it can be concluded that the presence of suspended Ebonex-Ru particles in the bath affects the mechanism of Ni deposition, producing a dispersed, fine-grained microstructure of the composite. This is a commonly reported effect when electrically conductive particles are employed for electrodeposition of composites [45]. Gray, flattened particles observed in Fig. 3b and c can be easily recognized as the Ebonex supporting particles. As can be seen in Fig. 3c, distribution of Ebonex-Ru particles embedded in a Ni matrix was relatively uniform, although some of the Ebonex-Ru particles on the surface were partially covered with a Ni deposit. Fig. 3b and c also show that the surface inhomogeneity was more pronounced on Ni-(Ebonex-Ru) than on Ni-Ebonex. Elemental composition of the deposits was determined by EDS analysis. The EDS spectra were acquired from areas of about 40 40 mm at 5 different positions on the surface of each coating. The average results are presented in Table 2. As can be seen, the content of Ru and Ti in the coatings increased with increasing the concentration of the suspended EbonexRu particles in the bath from 0 to 5 g dm3. With further
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Fig. 1 e SEM micrographs of the Ebonex-Ru powder taken at different magnifications: (a) 5.000 3 (BSE), (b) 10.000 3 and (c) 500 3 (BSE). The positions from which EDS spectra were acquired are marked in (a) and (c).
increase in the concentration of Ebonex-Ru in the bath, the content of Ru in the coatings varied insignificantly. For comparison, the content of RuO2 in Ni þ RuO2 composite coatings deposited from a Watts bath reached a saturation value at 10 g dm3 of suspended RuO2 [18]. The two times higher concentration of suspended particles corresponding to saturation in the case of Ni þ RuO2 deposition can be associated with more vigorous mixing of suspension applied in Ref. [18] (magnetic stirrer in combination with a rotating disc electrode)
Table 1 e Elemental composition of the Ebonex-Ru powder at positions marked in Fig. 1, determined by the EDS analysis. Spectrum Fig. 1a 1 2 3 4 Fig. 1c 1
At.% O
At.% Ti
At.% Ru
Remark
62.4 72.7 72.3 63.6
16.1 26.8 17.2 34.5
21.5 0.50 10.5 1.9
White area Gray area White area Gray area
70.2
27.1
2.7
Average
promoting the incorporation of particles in a deposit, but also with a smaller size of suspended RuO2 particles (100e200 nm). It is interesting to note that the ratio of atomic percentages of Ru/Ti in the coatings was found to be nearly constant irrespective of the bath composition and equal to 1:4, which is about 2.5 times higher than the value of the Ru/Ti ratio for the as-prepared Ebonex-Ru powder. Taking into account the prevailing content of Ni in the composite coatings, it can be estimated that the penetration depth of the electron probe used in EDS analysis decreased from 2.1 mm for the asprepared Ebonex-Ru powder to 1.0 mm for the Ni-(EbonexRu) coatings [46]. Since most of the incorporated Ebonex particles were larger than 1.0 mm and the Ru particles were nanosized, the relative amount of Ti found in the Ni-(EbonexRu) coatings was effectively lower compared to the Ebonex-Ru powder.
3.2.
Electrochemical characterization
3.2.1.
Cyclic voltammetry
The typical cyclic voltammetry (CV) curves recorded on a pure Ni coating and the Ni-Ebonex and Ni-(Ebonex-Ru) composite
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Fig. 2 e (a) HAADF image of Ebonex-supported Ru structure with a clearly visible RuO2 nanorod growing out of a Ru layer; Composition was confirmed by the EELS spectrum obtained at the red circle; (b) High resolution bright field (BF) image of a RuO2 nanorod with a fast Fourier transform (FFT, inset) showing crystallographic orientation close to 110 zone axis; [001] is growth direction. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
coatings in 1.0 mol dm3 NaOH at different sweep rates are shown in Fig. 4. The corresponding composite coatings were deposited from a Watts bath containing 5 g dm3 of Ebonex, or 5 g dm3 of Ebonex-Ru(10 wt.%) particles. The shapes of CV’s of the composite coatings are quite similar to that of a pure Ni electrode. In all cases reproducible voltammograms were obtained. All electrodes are characterized by a highly reversible pair of peaks in the so-called “oxyhydroxide region” or the “Ni(III) region” [47] in the potential range of 0.2e0.4 V. The reactions occurring at potentials in the range of both the anodic and the cathodic current peaks are apparently complementary processes, which are represented by the overall reaction: b NiðOHÞ2 5NiOOH þ Hþ þ ee
(1)
The anodic and cathodic processes referred to Eq. (1) are highly reversible processes in which the solid state diffusion of the proton is the rate determining step. The introduction of Ebonex and Ebonex-Ru particles into a Ni deposit increases the height of the peaks with no significant change in the peak position. This can be associated with enhancement of the surface roughness. Ruthenium particles present at the surface of the Ni-(Ebonex-Ru) composite coatings could contribute to some extent to the increase of the cathodic and anodic charges on the corresponding CV curves taking into account the fact that Ru (RuOx) exhibits pseudocapacitive behavior in a wide range of potentials. However, probably due to the low surface concentration of Ru particles (about 2 at. %), their influence on the anodic or the cathodic charge was negligible. The ratio between the active surface areas of the investigated electrodes can be estimated from the ratio between the corresponding cathodic voltammetric charges for the
reduction of NiOOH to b-Ni(OH)2 (Fig. 5). The anodic and cathodic peak currents are proportional to a square root of the sweep rate (v), because both reactions (Eqn. (1)) are diffusion controlled. In this case, the cathodic voltammetric charge (q) should be a linear function of v1/2 and the reciprocal voltammetric charge (q1) should be a linear function of v1/2 [48]. As seen in Fig. 5a and b, that linearity is indeed observed. Extrapolation of the q e v1/2 line to v1/2 / 0 gives the fraction of the total charge at the infinitely large sweep rate (qs), when the charging or discharging processes reach equilibrium only on the "outer" surface of the coating. On the other hand, extrapolation of q1 e v1/2 line to v1/2 / 0 gives the total charge at the infinitely slow sweep rate (qtot), when the charging process is able to reach equilibrium not only on the "outer" surface area, but also inside both meso- and micropores that represent the so-called "inner" part of the total surface area [48]. However, the obtained results undoubtedly show that the total charge is practically equal to the charge of the "outer" surface area, indicating that the Ni-Ebonex and Ni(Ebonex-Ru) coatings possess well-developed active surface areas without the presence of meso- and micro-pores. According to Fig. 5a, the values of voltammetric charges qs corresponding to the cathodic peaks for Ni, Ni-Ebonex and Ni(Ebonex-Ru) electrodes are about 1.7, 8.7 and 50 mC cm2, respectively, which gives the ratio of the active surface areas of about 1:5:29. Significantly higher roughness of the Ni-(Ebonex-Ru) composite coating in comparison with that for Ni and NiEbonex coatings is most likely due to embedded Ebonex particles decorated with Ru nanosized particles, which further contribute to enhancement of surface roughness of the Ni deposit.
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Fig. 3 e SEM micrographs of the samples deposited on a Ni mesh 40 support from a Watts bath containing: (a) 0 g dmL3 of Ebonex particles (pure Ni); (b) 5 g dmL3 of Ebonex particles; (c) 5 g dmL3 of Ebonex-Ru(10 wt.%) particles. Note that micrographs (b) and (c) were taken in the BSE mode.
3.2.2.
EIS measurements
EIS measurements were performed at several potentials covering a current density range from about 1 mA cm2 to about 100 mA cm2 on each examined electrode. In Fig. 6 are presented the Nyquist plots for the Ni (a), Ni-Ebonex (5 g dm3) (b) and the Ni-(Ebonex-Ru) (5 g dm3) (c) electrode.
Table 2 e Average elemental composition of the Ni(Ebonex-Ru) composite coatings electrodeposited from a Watts bath containing different concentrations of suspended Ebonex-Ru particles. Conc. of Ebonex-Ru/g dm3 0 1 2 3 5 10
Average content of elements/at.% O
Ni
Ti
Ru
2.8 6.0 13.5 17.0 19.2 17.9
97.2 91.0 80.7 75.7 71.6 73.2
2.4 4.7 5.7 7.1 7.0
0.6 1.1 1.6 2.1 1.9
Ru/Ti
0.25 0.23 0.28 0.29 0.27
Experimental points are presented with symbols (squares, circles, triangles, etc.), while fitted curves are presented with solid lines. Results of the fitting procedure are given in Table 3. The impedance spectra for the HER on the Ni (Fig. 6a) and the Ni-Ebonex (Fig. 6b) electrode exhibit a single distorted semicircle on the Nyquist diagram, so the best fit was obtained with a classical Randles equivalent electrical circuit in which the double-layer capacitance is replaced by a constant phase element, CPEdl (Fig. 7a). The Nyquist plots recorded at the Ni(Ebonex-Ru) composite electrode consist of two overlapping distorted semicircles forming an arch (Fig. 6c). In order to fit the impedance spectra recorded on this electrode, a modified Armstrong’s equivalent circuit was applied (Fig. 7b). It consists of the solution resistance, Rs, in series with a set of two nested R jj CPE elements. This equivalent circuit predicts maximum two semicircles in the Nyquist diagram: the first at high frequencies corresponding to Rct jj CPEdl, where Rct is the charge transfer resistance, and the second at low frequencies corresponding to Rp jj CPEp, where Rp and CPEp are the elements related to relaxation of surface coverage of the reaction intermediate Hads, QH, following the potential perturbation.
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Fig. 4 e Part of the CV’s of: (a) the Ni, (b) Ni-Ebonex and (c) the Ni-(Ebonex-Ru) electrode recorded at various sweep rates (marked in the figures in mV sL1) in 1.0 mol dmL3 NaOH solution at 25 C, corresponding to the “Ni(III) region” of potentials. Inserts show CV’s recorded in the whole applied potential range at v [ 100 mV sL1. The examined composite coatings were deposited on a Ni mesh 40 support from a Watts bath containing 5 g dmL3 of Ebonex or 5 g dmL3 of Ebonex-Ru(10 wt.%) particles.
The impedance of a CPE element is given by ZCPE ¼
1 a YðjuÞ
(2)
where Y is a capacitance parameter (in U1 cm2 sa) and a is a parameter related to the constant phase angle 4 ¼ e(90a) . A value of the parameter adl that is less than unity indicates a deviation from the ideal capacitive behavior. As can be seen from Table 3, the values of adl for the composite Ni-(Ebonex-Ru) electrode were between 0.79 and 0.85, whereas for the NiEbonex and the Ni electrode adl was about 0.94 and 0.99, respectively. The obtained values confirm that the surface of composite Ni-(Ebonex-Ru) coatings was much more inhomogeneous than the surface of the Ni-Ebonex or the pure Ni coating, as was stated by the BSE SEM analysis (see Section 3.1.). Due to large differences in activity for the HER of the examined electrodes, EIS analysis of each electrode was performed in a different potential range. From comparison of EIS results of the electrodes at approximately the same potentials (Table 3) it is apparent that the Ni-(Ebonex-Ru) composite electrode is the most active for the HER since it exhibited the lowest values of Rct. Values of Cdl were calculated using the equation [49] i1 h ð1a Þ a Cdl ¼ Ydl Rct dl dl
(3)
and are included in Table 3. As can be seen, the values of Cdl corresponding to the three electrodes are markedly different. By comparing the average values of Cdl for the Ni, Ni-Ebonex and the Ni-(Ebonex-Ru) electrode, the ratio of their real surface areas was found to be 1:7:29, which is close to that obtained by analysis of the charge under cathodic peaks on the CV’s (1:5:29). Finally, it is worth noting that at high frequency ends of the Nyquist plots for the Ni-Ebonex and the Ni-(Ebonex-Ru) electrode no potential-independent feature can be observed,
indicating that effects related to electrode porosity can be considered negligible [50]. This is in accordance with the results of voltammetric charge analysis (see Section 3.2.1.).
3.2.3. Polarization measurements for the HER 3.2.3.1. Ni and Ni-Ebonex cathodes. Fig. 8 shows quasistationary potentiodynamic polarization curves for the HER on Ni and Ni-Ebonex electrodes in 1.0 mol dm3 NaOH solution at 25 C recorded at a sweep rate of 1 mV s1. In the whole potential range of the HER only one Tafel slope of about 120 V dec1 is present at the polarization curves of both electrodes. It has been recently shown [51] that the reaction mechanism of the HER on Ni is a consecutive combination of the Volmer and Heyrovsky steps and the reaction rate is controlled by the Heyrovsky reaction with the almost full surface coverage by Hads. Since Ebonex, as electrode material, exhibits very low activity for the electron transfer reactions, increase in the rate of the HER at the Ni-Ebonex electrode can be attributed only to the increase of the electrochemically active surface area (geometric effect) of this electrode in comparison to a Ni electrode. Comparing the corresponding values of the current densities at E ¼ 1.5 V, it can be concluded that the surface roughness of the Ni-Ebonex composite coating is almost 6 times higher than that of Ni. This is in good agreement with the ratio of the electrochemically active surface areas of these electrodes determined by the previous analysis of the corresponding cathodic voltammetric charges and Cdl values.
3.2.3.2. Ni-(Ebonex-Ru) cathodes. The polarization characteristics of composite Ni-(Ebonex-Ru) coatings electrodeposited from a Watts bath with different concentrations of suspended Ebonex-Ru particles (1e10 g dm3) are compared in Fig. 9. The kinetic parameters and overpotentials at an arbitrary current density of 100 mA cm2 were determined and presented in Table 4.
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Fig. 5 e (a) Dependence of the cathodic voltammetric charges (q) on the reciprocal of the square root of the sweep rate (vL1/2) for Ni, Ni-Ebonex and Ni-(Ebonex-Ru) composite electrodes; (b) Dependance of the reciprocal voltammetric charges (qL1) on the square root of the sweep rate (v1/2) for the same electrodes in 1.0 mol dmL3 NaOH solution at 25 C.
The amount of incorporated Ru clearly affected the activity for the HER of Ni-(Ebonex-Ru) electrodes, but the shape of polarization curves recorded on these electrodes remained essentially the same (Fig. 9). Unlike Ni and Ni-Ebonex electrodes, two Tafel slopes were observed in the whole potential range studied at Ni-(Ebonex-Ru) electrodes. Table 4 shows that in the range of concentrations of suspended Ebonex-Ru particles from 2 to 10 g dm3 the Tafel slope at lower current densities (b1) was about 0.065 V dec1 and the Tafel slope at higher current densities (b2) was about 0.205 V dec1 irrespectively of the Ru amount. It is evident from Table 4 that the Ni-(Ebonex-Ru) electrodes outperformed the Ni and the Ni-Ebonex electrode in activity for the HER. The most active Ni-(Ebonex-Ru) coatings possess for two orders of magnitude higher current density at an arbitrarily chosen potential of 1.38 V and for about 420 and 300 mV lower overpotential for the HER at j ¼ 100 mA cm2 compared to Ni and Ni-Ebonex, respectively. By comparing the polarization curve for the HER on the least active Ni-(Ebonex-Ru) electrode (curve 1 in Fig. 9) and the polarization curve for the Ni-Ebonex electrode that was previously corrected for the roughness by multiplying the values of current density by six (the average ratio of active surface areas of these electrodes), all assembled in Fig. 10, it can be
Fig. 6 e Results of the EIS measurements at different potentials (marked in the figures) for (a) Ni, (b) Ni-Ebonex (5 g dmL3) and (c) Ni-(Ebonex-Ru) (5 g dmL3) electrodes. Experimental points are presented with symbols (squares, circles, triangles etc.), while solid lines represent the fitting results.
seen that Ni-(Ebonex-Ru) electrodes possess significantly larger intrinsic catalytic activity for the HER. However, it is more important to notice that in the whole studied potential range the polarization curves for the Ni-Ebonex and Ni-(Ebonex-Ru) electrodes do not overlap. This observation leads to a conclusion that the HER on Ni-(Ebonex-Ru) electrodes proceeds through a different mechanism, i.e. the HER takes place not on Ni, but exclusively on Ru active sites. The Tafel slope of about 60 mV dec1 obtained at lower current densities is an often referred value for the HER on RuO2 [25,52] and suggests that this electrode reaction is limited by a chemical step after the primary discharge step. Ru nanoparticles deposited on Ebonex particles can be treated as metal oxide particles due to easy formation of RuOx on the Ru surface in alkaline solutions [35]. However, the Tafel slope of about 200 mV dec1 obtained at higher current densities is unusual and generally can be explained in two ways: (a) by the
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Table 3 e Parameters obtained by fitting the EIS results presented in Fig. 6. E/V
Rs/U cm2
Ni 1.44 1.52 1.58 1.64 Ni-Ebonex 1.345 1.40 1.43 1.48 Ni-(Ebonex-Ru) 1.04 1.08 1.12 1.15 1.22 1.26 1.30 1.34
Rct/U cm2
Rp/U cm2
Yp/U1 cm2 sap
Ydl/U1 cm2 sadl
adl
Cdl/F cm2
0.60 0.60 0.61 0.61
32.3 9.93 4.65 2.23
53 54 55 68
106 106 106 106
0.99 0.99 0.99 0.97
50 50 50 52
106 106 106 106
1.08 1.04 1.01 0.96
10.1 4.21 2.76 1.60
0.57 0.52 0.51 0.56
103 103 103 103
0.93 0.94 0.94 0.93
0.39 0.36 0.35 0.32
103 103 103 103
1.12 1.12 1.13 1.14 1.14 1.14 1.18 1.15
7.7 6.7 4.5 3.0 1.6 1.3 1.0 0.82
4.3 5.6 5.2 4.7 3.0 2.6 2.3 2.3
103 103 103 103 103 103 103 103
0.83 0.79 0.79 0.80 0.83 0.84 0.85 0.85
2.2 2.3 2.0 1.6 1.0 0.87 0.79 0.74
103 103 103 103 103 103 103 103
89.7 7.2 0.70 0.20 62 53 58 63
103 103 103 103
so-called “dual barrier model”, which assumes the presence of an additional space charge region formed at the electrode/ electrolyte interface [53] besides the double-layer region, causing additional polarization and, consequently, the increase of the corresponding Tafel slopes for the HER; (b) by doubling of the Tafel slope due to the ohmic effect inside the pores [54] present in the coating. However, both explanations may be rejected in this case. The polarization curve for the HER recorded on Ni-Ebonex electrode shows that the unique Tafel slope of about 120 mV dec1 is present in the whole potential range, which can be explained by formal kinetics and, therefore, there is no reason for Ni-(Ebonex-Ru) electrode to behave as a semiconductor. Also, the previous
Fig. 7 e Equivalent circuits for fitting EIS results obtained for (a) Ni and Ni-Ebonex electrodes and (b) Ni-(Ebonex-Ru) electrode.
16 103 32 103 0.20 1.8 4.6 4.3 3.2 2.6
voltammetric investigations showed that Ni-Ebonex and Ni(Ebonex-Ru) coatings possess well-developed active surface areas, but without the presence of meso and micro-pores. So, it could be concluded that a high value of the Tafel slope (w205 mV dec1) can only be explained using formal kinetics approach. Kodintsev and Trasatti [52] proposed the following general mechanism for the HER at metal oxide electrodes accounting for the change in Tafel slope with oxide composition, which can be applied for Ni-(Ebonex-Ru) electrodes: Ru OH þ H2 O þ e /Ru OH2 þ OH
(4)
Ru OH2 /Ru OH2
(5)
Fig. 8 e Quasi-stationary polarization curves for the HER recorded on Ni (B) and Ni-Ebonex (5 g dmL3) (,) composite electrode in 1.0 mol dmL3 NaOH solution at 25 C.
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Since the change of linear terms of the equation containing QH with the potential are much smaller then that in exponential terms, they can be ignored and the Eqn. (8) can be simplified: b rQH b FE ð1 bad ÞrQH ¼ k2 ,exp exp 1 k1 ,exp ad RT RT RT
(9)
and by rearranging the Eqn. (9) one obtains: rQH k1 b FE ¼ ,exp 1 exp RT RT k2
(10)
Grading both sides of the Eqn. (10) by (1-bad), the following dependence is obtained:
Fig. 9 e Quasi-stationary polarization curves for the HER recorded in 1.0 mol dmL3 NaOH solution at 25 C on Ni(Ebonex-Ru) composite electrodes electrodeposited from a Watts bath containing different concentrations of EbonexRu particles (marked in the figure).
ð1bad Þ ð1 bad ÞrQH k1 ð1 bad Þ,b1 FE ¼ ,exp exp RT RT k2 The overall reaction rate is: j ¼ 2F,k2 ,exp
Ru OH2 þ e /R H þ OH
(6)
H[2
(7)
Ru H þ H2 O/R OH þ
Step (5) represents a possible spill-over effect (surface chemical rearrangements) and if this elemental step controls the overall reaction rate, then the Tafel slope of 60 mV dec1 is observed on the log j e E polarization curve (if the coverage by Hads is low), as was the case in the low current densities region for Ni-(Ebonex-Ru) electrodes. However, if both steps, the primary discharge step (4) and the chemical step (5) control the overall reaction rate, assuming Temkin conditions of hydrogen adsorption, the following equations can be written for steady-state conditions: b rQH b FE exp 1 k1 ð1 QH Þ,exp ad RT RT ð1 bad Þr,QH ¼ k2 ,QH exp RT
(8)
where r(RT )1 represents the Temkin adsorption parameter, while bad is a symmetry factor for the adsorption.
ð1 bad Þ,rQH RT
(12)
By substituting the Eqn. (11) into Eqn. (12), the dependence of the overall reaction rate on potential could be obtained as: j ¼ 2F,k2
(11)
ð1bad Þ k1 ð1 bad Þ,b1 FE ,exp RT k2
(13)
The Tafel slope is defined as: b¼
2:303RT ð1 bad Þ,b1 F
(14)
For bad ¼ b1 ¼ 0.5 and T ¼ 298 K, b z 240 mV dec1. Thus, unusually high value of the Tafel slope for the HER obtained at high current densities at Ni-(Ebonex-Ru) electrodes can be explained by the formal kinetics approach, assuming that the reaction is controlled simultaneously by two elemental steps under Temkin conditions for the adsorption of the reaction intermediate (Hads). It is also interesting to note that the presence of an anomalously high Tafel slope for the HER was observed also by other authors at RuO2 and commercial DSA electrodes [55,56]. However, until now, there have been no attempts to explain this phenomenon with the formal kinetics approach. At the end, it would be interesting to compare the catalytic performance of the Ni-(Ebonex-Ru) composite coatings in alkaline solution with the Ni þ RuO2 composites, a class of materials that have been successfully employed as cathode
Table 4 e Kinetic parameters for the HER on Ni-(Ebonex-Ru) composite electrodes electrodeposited from a Watts bath containing different concentrations of Ebonex-Ru particles, recorded in 1.0 mol dmL3 NaOH solution at 25 C. Kinetic parameters
Electrode Ni
3
Ni-Ebonex (5 g dm )
Ni-(Ebonex-Ru) 3
b1/V dec1 b2/V dec1 j/A cm2 (E ¼ 1.38 V) h/V ( j ¼ 0.1 A cm2)
0.116
0.128
0.0012 0.583
0.0060 0.460
3
1 g dm
2 g dm
0.113 0.228 0.127 0.300
0.066 0.202 0.286 0.214
3 g dm3
5 g dm3
10 g dm3
0.067 0.212 0.385 0.190
0.068 0.204 0.574 0.165
0.071 0.207 0.597 0.156
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The activity for the HER of Ni-(Ebonex-Ru) composites should be further improved by optimization of the bath for Ni deposition, size of the Ebonex particles, electrodeposition parameters and the hydrodynamic conditions established in a deposition cell, which is going to be the subject of our future work.
4.
Fig. 10 e Tafel plots of the polarization curves recorded in 1.0 mol dmL3 NaOH at 25 C at the Ni-Ebonex electrode (B), Ni-Ebonex electrode corrected for roughness effects (6) and Ni-(Ebonex-Ru) electrode (C e curve 1 in Fig. 9).
electrocatalysts in chlor-alkali industry. Various data on the activity of Ni þ RuO2 composites have been reported in literature depending on conditions of their electrodeposition [18,24,25]. The overpotential for the HER on Ni-(Ebonex-Ru) is appreciably lower in comparison to the Ni þ RuO2 electrodes reported by Tavares et al. [25] (jDhj > 100 mV at j ¼ 10 mA cm2), while Ni-(Ebonex-Ru) and the Ni þ RuO2 electrodes prepared from a Watts bath by Musiani et al. [18] displayed similar activity for the HER in 1 M NaOH, e.g. the both composites achieved the current density of j ¼ 100 mA cm2 at E ¼ 1.21 V. On the other hand, highly porous Ni þ RuO2 electrodes deposited at large cathodic currents from an ammonium chloride Ni bath [18] or Ni þ RuO2 active cathodes prepared from a vigorously stirred Watts bath [24] exhibited for about 50 mV lower overpotential than Ni-(Ebonex-Ru) at j ¼ 100 mA cm2. Taking all into account, the catalytic behavior of Ni-(Ebonex-Ru) can be considered fairly well in view of industrial employment. In relation to that, it should be emphasized that the main advantage of the Ni-(Ebonex-Ru) cathodes lies in their potentially cost effective production. For an illustration, the optimum Ni-(Ebonex-Ru) electrode in this study was deposited from a Watts bath with 5 g dm3 of suspended EbonexRu(10 wt.%), whereas the maximum activity for Ni þ RuO2 electrodes obtained from a Watts bath in Ref. [18] was reached at 10 g dm3 of suspended RuO2, i.e. the concentration of Ru in a deposition bath for the optimum Ni-(EbonexRu) was 15 times lower. This remarkable reduction of the amount of high-priced Ru used in a preparation procedure for the Ni-(Ebonex-Ru) composite coatings can be attributed to the employment of Ebonex supporting particles, which provide high utility efficiency of the Ru nanosized electrocatalyst precipitated at the surface. Exactly these characteristics make the Ni-(Ebonex-Ru) cathodes promising candidates for industrial exploitation in water or chlor-alkali electrolysis.
Conclusions
1. Codeposition of nickel with suspended Ebonex-Ru particles produces composite coatings. The microstructural characterization of the suspended Ebonex-Ru particles by SEM and TEM in combination with EDS and EELS compositional analysis reveals that ruthenium is present in two forms, the pure Ru nanoparticles and highly faceted RuO2 nanorods. 2. The obtained Ni-(Ebonex-Ru) composite coatings demonstrate high electrocatalytic activity for the HER. Maximum activity is attained already at 5 g dm3 of Ebonex-Ru particles in a Watts bath (suspension). 3. Ebonex (Ti4O7) acts as an effective catalyst support for Ru. Low percentages of Ru (w10 wt.%) on Ebonex are sufficient to give high catalytic activity for the HER. 4. Decoration of Ebonex particles with Ru nanoparticles allows maximal utility efficiency of Ru catalyst, leading to a significant reduction in the amount of Ru (about 10 times lower than that in a commercial bath) for achieving the required activity for the HER. 5. Codeposition of Ni and Ebonex-Ru particles results in increased surface roughness compared to a pure Ni deposit. Beside the roughness effect, Ni-(Ebonex-Ru) electrodes also exhibit higher intrinsic catalytic activity for the HER, which takes place exclusively on Ru (RuOx) active sites. 6. An unusually high value of the Tafel slope (w200 mV) for the HER obtained at high current densities at Ni-(EbonexRu) electrodes can be explained by the formal kinetics approach, assuming that the reaction is controlled simultaneously by two elemental steps under Temkin conditions for the adsorption of the reaction intermediate (Hads).
Acknowledgment This work was financially supported by the Ministry of Education, Science and Technological Development of the Government of the Republic of Serbia through the Project No. 172054. The authors would also like to express their gratitude to National Center of Electron Microscopy, LBNL, University of California, Berkeley, for TEM characterization.
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Kinetics of the hydrogen evolution reaction on Ni-(Ebonex-supported Ru) composite coatings in alkaline solution Lacnjevac a,*, B.M. Jovic a, V.D. Jovic a, V.R. Radmilovic b, N.V. Krstajic b U.C. a b
Institute for Multidisciplinary Research, University of Belgrade, Kneza Viseslava 1, 11030 Belgrade, Serbia Faculty of Technology and Metallurgy, University of Belgrade, Karnegijeva 4, 11000 Belgrade, Serbia
article info
abstract
Article history:
The hydrogen evolution reaction (HER) was studied on Ni, Ni-Ebonex and Ni-(Ebonex-Ru)
Received 26 April 2013
coatings in 1 mol dm3 NaOH solution at 25 C. The composite coatings were electro-
Received in revised form
deposited from a nickel Watts-type bath containing suspended Ebonex (chemical compo-
1 June 2013
sition mainly Ti4O7) or Ebonex-supported Ru(10 wt.%) particles (0e10 g dm3) onto Ni 40
Accepted 8 June 2013
mesh substrate. The electrodes were investigated by cyclic voltammetry (CV), transmission
Available online 15 July 2013
electron microscopy (TEM) and scanning electron microscopy (SEM) in combination with energy dispersive X-ray spectroscopy (EDS) and electron energy loss spectroscopy (EELS),
Keywords:
electrochemical impedance spectroscopy (EIS) and polarization measurements. These in-
Magneli phases
vestigations showed that the roughness factor of the Ni-(Ebonex-Ru) and the Ni-Ebonex
Ni-composite coating
coating was 29 and 6 times higher than that of a pure Ni coating, respectively. In the
H2 evolution
whole potential range of the HER only one Tafel slope of about 120 mV was present at the
Kinetics
polarization curves of Ni and Ni-Ebonex electrodes, with increased activity of the latter
Electrocatalysis
being attributed only to the increase of the electrochemically active surface area. The Ni(Ebonex-Ru) electrodes exhibited the highest intrinsic catalytic activity with two Tafel slopes, indicating also that the HER takes place exclusively on Ru active sites. Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Nickel, its alloys and Ni-based compounds are the most widely studied cathode materials for the HER in alkaline solutions. Electrocatalytic activity of Ni is not high, but this metal exhibits high resistance to corrosion in concentrated alkaline solutions at elevated temperatures. The overpotential of the HER increases with time, especially in the conditions of intermittent electrolysis [1]. Enhancement of cathode activity of Ni has been achieved by increasing the real surface area or by increasing the intrinsic activity using alloys or composites
[2e7]. The increase of the real surface area can be accomplished in several ways: by alloying Ni with an active metal like Al or Zn, followed by the dissolution of the secondary component (Raney-Ni) [8e15], or by depositing Ni from ammonium chloride baths at high current densities [16e18]. In all cases porous structures were obtained, characterized by a large electrochemically active surface area. The intrinsic activity of Ni has been also increased by electrodeposition of Ni-based composite materials containing active solid particles (MoO2, MoO3 [19,20], Mo [3], CeO2 [21], Ni-rare earth compounds [22], RuO2 [18,23e25], IrO2 [23], and TiO2 [26]), by
* Corresponding author. Tel.: þ381 11 2085039; fax: þ381 11 3055289. Lac njevac). E-mail address: [email protected] (U.C. 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.06.037
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alloying Ni with some metals (NiCo [27e29], NiMo [30,31], NiW [30,32], NiFe [30,33], NiLa [34]) or by activation of Ni by spontaneous deposition of Ru [35,36] or Ir [36] from acid deaerated aqueous solutions of corresponding ions. Ni-based composite coatings have recently attracted significant attention because they combine many desirable properties of an electrocatalyst for the HER: large surface area, good intrinsic catalytic activity, and high corrosion stability. Among composite catalysts, the NieRuO2 (Ru) composite catalyst was one of the most investigated, due to the fact that ruthenium oxide and ruthenium exhibit one of the highest activities for the HER [25]. Thermodynamically, ruthenium oxide should not be stable during cathodic polarization, but it has been found experimentally that it possesses a considerable stability, which has been attributed to a sufficient electronic conductivity of RuO2, preventing its reduction. Composite NieRuO2 catalysts can be prepared by thermal deposition of RuO2 into the Raney-Ni micropores [37] or by cathodic codeposition of RuO2 particles within a porous Ni matrix [18]. Particular attention has been devoted to co-deposition of Ni and RuO2 particles suspended in a commercial Watts bath. Iwakura et al. [24] have first confirmed the increase in electrocatalytic activity for the HER on these coatings. However, RuO2 is expensive and its use in composite cathode catalysts is not economically favorable. Therefore, research efforts have also been devoted to further reduction of the amount of RuO2 in the composite Ni-based catalysts and improvement of their stability by incorporation of other metal oxides. It has been shown that incorporation of TiO2 improved the coatings stability without significant reduction in the electrocatalytic activity [38,39]. Various composite Ni-RuxTi1-xO2 coatings, with Ru contents ranging from 0 to 50 at.%, were investigated with respect to catalytic activity for the HER and their stability in alkaline solutions [40]. In the present work, in order to improve the electrocatalytic activity for the HER and reduce the amount of Ru, Ebonex particles decorated with Ru nanoparticles were incorporated in the electroplated Ni coatings. Ebonex is electrically conductive ceramic composite of several reduced titanium oxide phases, mainly Ti4O7 and Ti5O9, which form a part of a homologous series of crystallographic shear phases of the formula TinO2n1, with n ¼ 4 to 10, known as Magneli phases [41]. Non-stoichiometric Ti oxides are electrochemically stable in both acid and alkaline solutions. Ebonex also possesses high overpotentials for hydrogen and oxygen evolution in aqueous solutions and shows slow electron transfer kinetics [42], being a better candidate for the substrate than for the catalyst. The goal of this work was to investigate the possibility of using Ni coatings co-deposited with Ebonex-Ru particles as catalysts for the HER in alkaline solution. Based on the results of the Tafel slopes analysis, a mechanism of the HER on Ni(Ebonex-Ru) electrodes was proposed.
2.
Experimental
2.1.
Preparation of Ebonex-supported Ru particles
Powder of Magneli phases of general formula TinO2n1, or in average Ti4O7 (trade name Ebonex, Altraverda, U.K.), was used
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in these investigations. The starting Ebonex powder was ground in a Fritsch Planetary Mill (Pulverisette 5) using 6 mm diameter balls with maximum acceleration of 700 m s2 [43]. The ground powder was collected after passing through a 100 mesh sieve and used in a preparation procedure. Precipitation of the ruthenium catalyst on the Ebonex support was conducted by a conventional borohydride reduction method [44]. 1.8 g of the Ebonex powder was dispersed in 100 ml of deionized water in ultrasonic bath, and then mixed with the amount of RuCl3 equivalent to 0.2 g Ru. Ruthenium ions were completely reduced by adding a sodium borohydride solution in excess. The Ru precipitate on Ebonex was washed with deionized water and then dried at 80 C for 24 h. In such a way, powder with Ru loading of 10 wt.% was prepared for further examinations.
2.2.
Preparation of electrodes
The Ni/(Ebonex-Ru) cathodes were prepared by simultaneous electrodeposition of Ni and suspended Ebonex-Ru particles onto Ni 40 mesh substrate from a conventional Watts bath of the following composition: 330 g dm3 NiSO4,6H2O þ 45 g dm3 NiCl2,6H2O þ 38 g dm3 H3BO3; pH 3.2e4.0; t ¼ 40 C. The amount of suspended Ebonex-Ru particles in the bath varied between 0 and 10 g dm3. Electrodeposition was carried out at a constant current density j ¼ 25 mA cm2 in the beaker cell of a small volume of 0.1 dm3 with two vertical Ni mesh anodes and a Ni 40 mesh cathode between them. Thickness of the coatings was controlled through the deposition charge density, which was fixed at 18.6 C cm2. Mixing of the electrolyte was provided by the air flow (50 dm3 h1) through the glass pipe of the shape of spiral with small openings facing the bottom of the cell. The Watts bath was prepared from analytical grade chemicals dissolved in deionized water. The pH was initially adjusted at 3.8 by adding sulfuric acid. Before the deposition of Ni-(Ebonex) and Ni-(Ebonex-Ru) coatings, Ni 40 meshes were etched in 2:1 HNO3:H2O solution for 60 s [20].
2.3. Microstructural characterization of powder and coatings 2.3.1.
SEM in combination with EDS
The morphology and composition of the Ebonex-Ru powder and electrodeposited coatings were investigated by a Tescan VEGA TS 5130 MM scanning electron microscope equipped with an EDS detector INCAPentaFET-x3, Oxford Instruments [20].
2.3.2.
TEM in combination with EDS and EELS
Conventional transmission electron microscopy (CTEM) and EDS were performed using the CM200-FEG transmission electron microscope operated at 200 kV. High resolution transmission electron microscopy (HRTEM) and EELS were performed using the TEAM 0.5 aberration corrected transmission electron microscope, equipped with GIF Tridiem imaging filter, operated at 80 kV, in order to minimize radiation damage of investigated catalysts. CrystalKitX and MacTepas programs were used for structure modeling and image simulation of experimental high resolution transmission electron microscopy data.
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2.4.
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Electrochemical measurements and solutions
The polarization characteristics of the HER onto Ni, Ni-Ebonex and Ni-(Ebonex-Ru) samples were tested in 1.0 mol dm3 NaOH solution in extra pure UV water (Smart2Pure UV, TKA) at 25 C. A three-compartment cell was used: the working electrode of the surface area of 1 cm2 was placed in a central compartment together with the Luggin capillary; two Pt mesh counter electrodes of larger surface areas were each placed in separate compartments (parallel to the working electrode mesh), so that oxygen evolved at the counter electrodes could not enter the working electrode compartment. The saturated calomel electrode (SCE) was placed in a side compartment connected to the central one through a bridge and a Luggin capillary, and was kept at the room temperature. Experiments were performed using the potentiostat Reference 600 and the software PHE 200 and DC 105 (Gamry Instruments). All samples were first submitted to the HER at a constant current density j ¼ 0.3 A cm2 for 800 s (step 1), followed by the HER at a constant potential for 60 s (step 2). The value of potential was adjusted to produce cathodic current density slightly higher than 0.3 A cm2. After such pre-electrolysis, polarization curves were recorded by sweeping potential with 1 mV s1 from the potential applied in step 2 to the value of open circuit potential, while potential was automatically corrected for the IR drop using the current interrupt technique. All potentials in the text are given with respect to the SCE (ESCE ¼ 0.244 V vs. SHE). Cyclic voltammetric curves were recorded on a pure Ni coating and on the Ni-Ebonex and Ni-(Ebonex-Ru) composite coatings (deposited from a Watts bath containing 5 g dm3 of Ebonex or 5 g dm3 of Ebonex-Ru(10 wt.%) suspended particles, respectively) in 1.0 mol dm3 NaOH at sweep rates of 10e200 mV s1 and in the potential range from 1.15 to 0.55 V. The voltammetric charge was determined by means of a graphical integrator. The EIS measurements were performed with the same potentiostat and EIS 300 software, applying the amplitude of 5 mV RMS in the frequency range from 50 kHz to 0.01 Hz. The real (Z0 ) and imaginary (Z00 ) components of electrochemical impedance spectra in the Nyquist plot were analyzed using the complex nonlinear least squares (CNLS) fitting program (EIS 300) to simulate the equivalent resistances and capacitances.
3.
Results and discussion
3.1.
Electron microscopy characterization
Morphology of the composite Ebonex-Ru powder is presented in SEM images in Fig. 1, while the results of the EDS analysis performed at positions marked in Fig. 1 are given in Table 1. Elemental composition analysis of gray and white areas observed in the backscattered electron (BSE) micrograph shown in Fig. 1a confirmed that the Ru phase (white area) was non-uniformly distributed across the surface of the Ebonex substrate (gray area). The Ebonex powder consisted of characteristic flattened particles in the size range from 500 nm to 10 mm, whereas the Ru precipitate was composed of
agglomerated nanosized particles (Fig. 1b). In order to find the average content of each component of the Ebonex-Ru powder, EDS analysis was applied upon a larger surface area of 250 200 mm (Fig. 1c). As can be seen from Table 1, the average content of Ru in this case practically coincides with the value calculated from the initial weights of components used in the powder preparation procedure (2.9 at.%). TEM analysis of the Ebonex-Ru powder confirmed that the microstructure was characterized by the presence of pure metallic Ru regions deposited on Ebonex. In addition, as shown in high angle annular dark field (HAADF) image in Fig. 2a, the pure metallic Ru regions were locally characterized by the presence RuO2 nanorods. The presence of RuO2 nanorods was confirmed by the EELS spectrum shown in Fig. 2a, which clearly indicates the presence of RuM4,5, RuM2,3, and OK edges. The dark spots in Ru region indicate the presence of nanopores. High resolution bright field (BF) image of a RuO2 nanorod, taken close to 110 zone axis, is shown in Fig. 2b. It is clear that RuO2 nanorods are strongly faceted with an angle of 134 between 110 and 111 facets. The atomic ratio between Ti and Ru was measured by EDS. It was found, as expected, to vary depending on the local region where the measurement was done, and it was in the range: Ti:Ru w0.98 to 0.02, which indicates non-uniform distribution of Ru on Ebonex support. Typical top views of the Ni coating and the Ni-Ebonex and Ni-(Ebonex-Ru) composite coatings electrodeposited from a Watts bath with concentration of suspended Ebonex and Ebonex-Ru particles of 5 g dm3 are shown in Fig. 3. The presented SEM micrographs indicate that in comparison to a pure Ni deposit (Fig. 3a) surface roughness was distinctly increased after incorporation of Ebonex (Fig. 3b) and especially EbonexRu particles (Fig. 3c) in the Ni matrix. In the BSE mode micrographs corresponding to the Ni-Ebonex and the Ni-(Ebonex-Ru) coating, Fig. 3b and c, respectively, the Ni phase is visible as a white area. The Ni deposit in the Ni-Ebonex coating had a compact crystalline structure (Fig. 3b), while in the Ni-(EbonexRu) coating the Ni deposit consisted of agglomerates of the spherical grains with a diameter of about 500 nm (Fig. 3c). Considering the fact that the both coatings were prepared under identical deposition conditions ( jdep ¼ 25 mA cm2; sdep ¼ 39 min), it can be concluded that the presence of suspended Ebonex-Ru particles in the bath affects the mechanism of Ni deposition, producing a dispersed, fine-grained microstructure of the composite. This is a commonly reported effect when electrically conductive particles are employed for electrodeposition of composites [45]. Gray, flattened particles observed in Fig. 3b and c can be easily recognized as the Ebonex supporting particles. As can be seen in Fig. 3c, distribution of Ebonex-Ru particles embedded in a Ni matrix was relatively uniform, although some of the Ebonex-Ru particles on the surface were partially covered with a Ni deposit. Fig. 3b and c also show that the surface inhomogeneity was more pronounced on Ni-(Ebonex-Ru) than on Ni-Ebonex. Elemental composition of the deposits was determined by EDS analysis. The EDS spectra were acquired from areas of about 40 40 mm at 5 different positions on the surface of each coating. The average results are presented in Table 2. As can be seen, the content of Ru and Ti in the coatings increased with increasing the concentration of the suspended EbonexRu particles in the bath from 0 to 5 g dm3. With further
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Fig. 1 e SEM micrographs of the Ebonex-Ru powder taken at different magnifications: (a) 5.000 3 (BSE), (b) 10.000 3 and (c) 500 3 (BSE). The positions from which EDS spectra were acquired are marked in (a) and (c).
increase in the concentration of Ebonex-Ru in the bath, the content of Ru in the coatings varied insignificantly. For comparison, the content of RuO2 in Ni þ RuO2 composite coatings deposited from a Watts bath reached a saturation value at 10 g dm3 of suspended RuO2 [18]. The two times higher concentration of suspended particles corresponding to saturation in the case of Ni þ RuO2 deposition can be associated with more vigorous mixing of suspension applied in Ref. [18] (magnetic stirrer in combination with a rotating disc electrode)
Table 1 e Elemental composition of the Ebonex-Ru powder at positions marked in Fig. 1, determined by the EDS analysis. Spectrum Fig. 1a 1 2 3 4 Fig. 1c 1
At.% O
At.% Ti
At.% Ru
Remark
62.4 72.7 72.3 63.6
16.1 26.8 17.2 34.5
21.5 0.50 10.5 1.9
White area Gray area White area Gray area
70.2
27.1
2.7
Average
promoting the incorporation of particles in a deposit, but also with a smaller size of suspended RuO2 particles (100e200 nm). It is interesting to note that the ratio of atomic percentages of Ru/Ti in the coatings was found to be nearly constant irrespective of the bath composition and equal to 1:4, which is about 2.5 times higher than the value of the Ru/Ti ratio for the as-prepared Ebonex-Ru powder. Taking into account the prevailing content of Ni in the composite coatings, it can be estimated that the penetration depth of the electron probe used in EDS analysis decreased from 2.1 mm for the asprepared Ebonex-Ru powder to 1.0 mm for the Ni-(EbonexRu) coatings [46]. Since most of the incorporated Ebonex particles were larger than 1.0 mm and the Ru particles were nanosized, the relative amount of Ti found in the Ni-(EbonexRu) coatings was effectively lower compared to the Ebonex-Ru powder.
3.2.
Electrochemical characterization
3.2.1.
Cyclic voltammetry
The typical cyclic voltammetry (CV) curves recorded on a pure Ni coating and the Ni-Ebonex and Ni-(Ebonex-Ru) composite
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Fig. 2 e (a) HAADF image of Ebonex-supported Ru structure with a clearly visible RuO2 nanorod growing out of a Ru layer; Composition was confirmed by the EELS spectrum obtained at the red circle; (b) High resolution bright field (BF) image of a RuO2 nanorod with a fast Fourier transform (FFT, inset) showing crystallographic orientation close to 110 zone axis; [001] is growth direction. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
coatings in 1.0 mol dm3 NaOH at different sweep rates are shown in Fig. 4. The corresponding composite coatings were deposited from a Watts bath containing 5 g dm3 of Ebonex, or 5 g dm3 of Ebonex-Ru(10 wt.%) particles. The shapes of CV’s of the composite coatings are quite similar to that of a pure Ni electrode. In all cases reproducible voltammograms were obtained. All electrodes are characterized by a highly reversible pair of peaks in the so-called “oxyhydroxide region” or the “Ni(III) region” [47] in the potential range of 0.2e0.4 V. The reactions occurring at potentials in the range of both the anodic and the cathodic current peaks are apparently complementary processes, which are represented by the overall reaction: b NiðOHÞ2 5NiOOH þ Hþ þ ee
(1)
The anodic and cathodic processes referred to Eq. (1) are highly reversible processes in which the solid state diffusion of the proton is the rate determining step. The introduction of Ebonex and Ebonex-Ru particles into a Ni deposit increases the height of the peaks with no significant change in the peak position. This can be associated with enhancement of the surface roughness. Ruthenium particles present at the surface of the Ni-(Ebonex-Ru) composite coatings could contribute to some extent to the increase of the cathodic and anodic charges on the corresponding CV curves taking into account the fact that Ru (RuOx) exhibits pseudocapacitive behavior in a wide range of potentials. However, probably due to the low surface concentration of Ru particles (about 2 at. %), their influence on the anodic or the cathodic charge was negligible. The ratio between the active surface areas of the investigated electrodes can be estimated from the ratio between the corresponding cathodic voltammetric charges for the
reduction of NiOOH to b-Ni(OH)2 (Fig. 5). The anodic and cathodic peak currents are proportional to a square root of the sweep rate (v), because both reactions (Eqn. (1)) are diffusion controlled. In this case, the cathodic voltammetric charge (q) should be a linear function of v1/2 and the reciprocal voltammetric charge (q1) should be a linear function of v1/2 [48]. As seen in Fig. 5a and b, that linearity is indeed observed. Extrapolation of the q e v1/2 line to v1/2 / 0 gives the fraction of the total charge at the infinitely large sweep rate (qs), when the charging or discharging processes reach equilibrium only on the "outer" surface of the coating. On the other hand, extrapolation of q1 e v1/2 line to v1/2 / 0 gives the total charge at the infinitely slow sweep rate (qtot), when the charging process is able to reach equilibrium not only on the "outer" surface area, but also inside both meso- and micropores that represent the so-called "inner" part of the total surface area [48]. However, the obtained results undoubtedly show that the total charge is practically equal to the charge of the "outer" surface area, indicating that the Ni-Ebonex and Ni(Ebonex-Ru) coatings possess well-developed active surface areas without the presence of meso- and micro-pores. According to Fig. 5a, the values of voltammetric charges qs corresponding to the cathodic peaks for Ni, Ni-Ebonex and Ni(Ebonex-Ru) electrodes are about 1.7, 8.7 and 50 mC cm2, respectively, which gives the ratio of the active surface areas of about 1:5:29. Significantly higher roughness of the Ni-(Ebonex-Ru) composite coating in comparison with that for Ni and NiEbonex coatings is most likely due to embedded Ebonex particles decorated with Ru nanosized particles, which further contribute to enhancement of surface roughness of the Ni deposit.
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Fig. 3 e SEM micrographs of the samples deposited on a Ni mesh 40 support from a Watts bath containing: (a) 0 g dmL3 of Ebonex particles (pure Ni); (b) 5 g dmL3 of Ebonex particles; (c) 5 g dmL3 of Ebonex-Ru(10 wt.%) particles. Note that micrographs (b) and (c) were taken in the BSE mode.
3.2.2.
EIS measurements
EIS measurements were performed at several potentials covering a current density range from about 1 mA cm2 to about 100 mA cm2 on each examined electrode. In Fig. 6 are presented the Nyquist plots for the Ni (a), Ni-Ebonex (5 g dm3) (b) and the Ni-(Ebonex-Ru) (5 g dm3) (c) electrode.
Table 2 e Average elemental composition of the Ni(Ebonex-Ru) composite coatings electrodeposited from a Watts bath containing different concentrations of suspended Ebonex-Ru particles. Conc. of Ebonex-Ru/g dm3 0 1 2 3 5 10
Average content of elements/at.% O
Ni
Ti
Ru
2.8 6.0 13.5 17.0 19.2 17.9
97.2 91.0 80.7 75.7 71.6 73.2
2.4 4.7 5.7 7.1 7.0
0.6 1.1 1.6 2.1 1.9
Ru/Ti
0.25 0.23 0.28 0.29 0.27
Experimental points are presented with symbols (squares, circles, triangles, etc.), while fitted curves are presented with solid lines. Results of the fitting procedure are given in Table 3. The impedance spectra for the HER on the Ni (Fig. 6a) and the Ni-Ebonex (Fig. 6b) electrode exhibit a single distorted semicircle on the Nyquist diagram, so the best fit was obtained with a classical Randles equivalent electrical circuit in which the double-layer capacitance is replaced by a constant phase element, CPEdl (Fig. 7a). The Nyquist plots recorded at the Ni(Ebonex-Ru) composite electrode consist of two overlapping distorted semicircles forming an arch (Fig. 6c). In order to fit the impedance spectra recorded on this electrode, a modified Armstrong’s equivalent circuit was applied (Fig. 7b). It consists of the solution resistance, Rs, in series with a set of two nested R jj CPE elements. This equivalent circuit predicts maximum two semicircles in the Nyquist diagram: the first at high frequencies corresponding to Rct jj CPEdl, where Rct is the charge transfer resistance, and the second at low frequencies corresponding to Rp jj CPEp, where Rp and CPEp are the elements related to relaxation of surface coverage of the reaction intermediate Hads, QH, following the potential perturbation.
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Fig. 4 e Part of the CV’s of: (a) the Ni, (b) Ni-Ebonex and (c) the Ni-(Ebonex-Ru) electrode recorded at various sweep rates (marked in the figures in mV sL1) in 1.0 mol dmL3 NaOH solution at 25 C, corresponding to the “Ni(III) region” of potentials. Inserts show CV’s recorded in the whole applied potential range at v [ 100 mV sL1. The examined composite coatings were deposited on a Ni mesh 40 support from a Watts bath containing 5 g dmL3 of Ebonex or 5 g dmL3 of Ebonex-Ru(10 wt.%) particles.
The impedance of a CPE element is given by ZCPE ¼
1 a YðjuÞ
(2)
where Y is a capacitance parameter (in U1 cm2 sa) and a is a parameter related to the constant phase angle 4 ¼ e(90a) . A value of the parameter adl that is less than unity indicates a deviation from the ideal capacitive behavior. As can be seen from Table 3, the values of adl for the composite Ni-(Ebonex-Ru) electrode were between 0.79 and 0.85, whereas for the NiEbonex and the Ni electrode adl was about 0.94 and 0.99, respectively. The obtained values confirm that the surface of composite Ni-(Ebonex-Ru) coatings was much more inhomogeneous than the surface of the Ni-Ebonex or the pure Ni coating, as was stated by the BSE SEM analysis (see Section 3.1.). Due to large differences in activity for the HER of the examined electrodes, EIS analysis of each electrode was performed in a different potential range. From comparison of EIS results of the electrodes at approximately the same potentials (Table 3) it is apparent that the Ni-(Ebonex-Ru) composite electrode is the most active for the HER since it exhibited the lowest values of Rct. Values of Cdl were calculated using the equation [49] i1 h ð1a Þ a Cdl ¼ Ydl Rct dl dl
(3)
and are included in Table 3. As can be seen, the values of Cdl corresponding to the three electrodes are markedly different. By comparing the average values of Cdl for the Ni, Ni-Ebonex and the Ni-(Ebonex-Ru) electrode, the ratio of their real surface areas was found to be 1:7:29, which is close to that obtained by analysis of the charge under cathodic peaks on the CV’s (1:5:29). Finally, it is worth noting that at high frequency ends of the Nyquist plots for the Ni-Ebonex and the Ni-(Ebonex-Ru) electrode no potential-independent feature can be observed,
indicating that effects related to electrode porosity can be considered negligible [50]. This is in accordance with the results of voltammetric charge analysis (see Section 3.2.1.).
3.2.3. Polarization measurements for the HER 3.2.3.1. Ni and Ni-Ebonex cathodes. Fig. 8 shows quasistationary potentiodynamic polarization curves for the HER on Ni and Ni-Ebonex electrodes in 1.0 mol dm3 NaOH solution at 25 C recorded at a sweep rate of 1 mV s1. In the whole potential range of the HER only one Tafel slope of about 120 V dec1 is present at the polarization curves of both electrodes. It has been recently shown [51] that the reaction mechanism of the HER on Ni is a consecutive combination of the Volmer and Heyrovsky steps and the reaction rate is controlled by the Heyrovsky reaction with the almost full surface coverage by Hads. Since Ebonex, as electrode material, exhibits very low activity for the electron transfer reactions, increase in the rate of the HER at the Ni-Ebonex electrode can be attributed only to the increase of the electrochemically active surface area (geometric effect) of this electrode in comparison to a Ni electrode. Comparing the corresponding values of the current densities at E ¼ 1.5 V, it can be concluded that the surface roughness of the Ni-Ebonex composite coating is almost 6 times higher than that of Ni. This is in good agreement with the ratio of the electrochemically active surface areas of these electrodes determined by the previous analysis of the corresponding cathodic voltammetric charges and Cdl values.
3.2.3.2. Ni-(Ebonex-Ru) cathodes. The polarization characteristics of composite Ni-(Ebonex-Ru) coatings electrodeposited from a Watts bath with different concentrations of suspended Ebonex-Ru particles (1e10 g dm3) are compared in Fig. 9. The kinetic parameters and overpotentials at an arbitrary current density of 100 mA cm2 were determined and presented in Table 4.
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Fig. 5 e (a) Dependence of the cathodic voltammetric charges (q) on the reciprocal of the square root of the sweep rate (vL1/2) for Ni, Ni-Ebonex and Ni-(Ebonex-Ru) composite electrodes; (b) Dependance of the reciprocal voltammetric charges (qL1) on the square root of the sweep rate (v1/2) for the same electrodes in 1.0 mol dmL3 NaOH solution at 25 C.
The amount of incorporated Ru clearly affected the activity for the HER of Ni-(Ebonex-Ru) electrodes, but the shape of polarization curves recorded on these electrodes remained essentially the same (Fig. 9). Unlike Ni and Ni-Ebonex electrodes, two Tafel slopes were observed in the whole potential range studied at Ni-(Ebonex-Ru) electrodes. Table 4 shows that in the range of concentrations of suspended Ebonex-Ru particles from 2 to 10 g dm3 the Tafel slope at lower current densities (b1) was about 0.065 V dec1 and the Tafel slope at higher current densities (b2) was about 0.205 V dec1 irrespectively of the Ru amount. It is evident from Table 4 that the Ni-(Ebonex-Ru) electrodes outperformed the Ni and the Ni-Ebonex electrode in activity for the HER. The most active Ni-(Ebonex-Ru) coatings possess for two orders of magnitude higher current density at an arbitrarily chosen potential of 1.38 V and for about 420 and 300 mV lower overpotential for the HER at j ¼ 100 mA cm2 compared to Ni and Ni-Ebonex, respectively. By comparing the polarization curve for the HER on the least active Ni-(Ebonex-Ru) electrode (curve 1 in Fig. 9) and the polarization curve for the Ni-Ebonex electrode that was previously corrected for the roughness by multiplying the values of current density by six (the average ratio of active surface areas of these electrodes), all assembled in Fig. 10, it can be
Fig. 6 e Results of the EIS measurements at different potentials (marked in the figures) for (a) Ni, (b) Ni-Ebonex (5 g dmL3) and (c) Ni-(Ebonex-Ru) (5 g dmL3) electrodes. Experimental points are presented with symbols (squares, circles, triangles etc.), while solid lines represent the fitting results.
seen that Ni-(Ebonex-Ru) electrodes possess significantly larger intrinsic catalytic activity for the HER. However, it is more important to notice that in the whole studied potential range the polarization curves for the Ni-Ebonex and Ni-(Ebonex-Ru) electrodes do not overlap. This observation leads to a conclusion that the HER on Ni-(Ebonex-Ru) electrodes proceeds through a different mechanism, i.e. the HER takes place not on Ni, but exclusively on Ru active sites. The Tafel slope of about 60 mV dec1 obtained at lower current densities is an often referred value for the HER on RuO2 [25,52] and suggests that this electrode reaction is limited by a chemical step after the primary discharge step. Ru nanoparticles deposited on Ebonex particles can be treated as metal oxide particles due to easy formation of RuOx on the Ru surface in alkaline solutions [35]. However, the Tafel slope of about 200 mV dec1 obtained at higher current densities is unusual and generally can be explained in two ways: (a) by the
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Table 3 e Parameters obtained by fitting the EIS results presented in Fig. 6. E/V
Rs/U cm2
Ni 1.44 1.52 1.58 1.64 Ni-Ebonex 1.345 1.40 1.43 1.48 Ni-(Ebonex-Ru) 1.04 1.08 1.12 1.15 1.22 1.26 1.30 1.34
Rct/U cm2
Rp/U cm2
Yp/U1 cm2 sap
Ydl/U1 cm2 sadl
adl
Cdl/F cm2
0.60 0.60 0.61 0.61
32.3 9.93 4.65 2.23
53 54 55 68
106 106 106 106
0.99 0.99 0.99 0.97
50 50 50 52
106 106 106 106
1.08 1.04 1.01 0.96
10.1 4.21 2.76 1.60
0.57 0.52 0.51 0.56
103 103 103 103
0.93 0.94 0.94 0.93
0.39 0.36 0.35 0.32
103 103 103 103
1.12 1.12 1.13 1.14 1.14 1.14 1.18 1.15
7.7 6.7 4.5 3.0 1.6 1.3 1.0 0.82
4.3 5.6 5.2 4.7 3.0 2.6 2.3 2.3
103 103 103 103 103 103 103 103
0.83 0.79 0.79 0.80 0.83 0.84 0.85 0.85
2.2 2.3 2.0 1.6 1.0 0.87 0.79 0.74
103 103 103 103 103 103 103 103
89.7 7.2 0.70 0.20 62 53 58 63
103 103 103 103
so-called “dual barrier model”, which assumes the presence of an additional space charge region formed at the electrode/ electrolyte interface [53] besides the double-layer region, causing additional polarization and, consequently, the increase of the corresponding Tafel slopes for the HER; (b) by doubling of the Tafel slope due to the ohmic effect inside the pores [54] present in the coating. However, both explanations may be rejected in this case. The polarization curve for the HER recorded on Ni-Ebonex electrode shows that the unique Tafel slope of about 120 mV dec1 is present in the whole potential range, which can be explained by formal kinetics and, therefore, there is no reason for Ni-(Ebonex-Ru) electrode to behave as a semiconductor. Also, the previous
Fig. 7 e Equivalent circuits for fitting EIS results obtained for (a) Ni and Ni-Ebonex electrodes and (b) Ni-(Ebonex-Ru) electrode.
16 103 32 103 0.20 1.8 4.6 4.3 3.2 2.6
voltammetric investigations showed that Ni-Ebonex and Ni(Ebonex-Ru) coatings possess well-developed active surface areas, but without the presence of meso and micro-pores. So, it could be concluded that a high value of the Tafel slope (w205 mV dec1) can only be explained using formal kinetics approach. Kodintsev and Trasatti [52] proposed the following general mechanism for the HER at metal oxide electrodes accounting for the change in Tafel slope with oxide composition, which can be applied for Ni-(Ebonex-Ru) electrodes: Ru OH þ H2 O þ e /Ru OH2 þ OH
(4)
Ru OH2 /Ru OH2
(5)
Fig. 8 e Quasi-stationary polarization curves for the HER recorded on Ni (B) and Ni-Ebonex (5 g dmL3) (,) composite electrode in 1.0 mol dmL3 NaOH solution at 25 C.
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Since the change of linear terms of the equation containing QH with the potential are much smaller then that in exponential terms, they can be ignored and the Eqn. (8) can be simplified: b rQH b FE ð1 bad ÞrQH ¼ k2 ,exp exp 1 k1 ,exp ad RT RT RT
(9)
and by rearranging the Eqn. (9) one obtains: rQH k1 b FE ¼ ,exp 1 exp RT RT k2
(10)
Grading both sides of the Eqn. (10) by (1-bad), the following dependence is obtained:
Fig. 9 e Quasi-stationary polarization curves for the HER recorded in 1.0 mol dmL3 NaOH solution at 25 C on Ni(Ebonex-Ru) composite electrodes electrodeposited from a Watts bath containing different concentrations of EbonexRu particles (marked in the figure).
ð1bad Þ ð1 bad ÞrQH k1 ð1 bad Þ,b1 FE ¼ ,exp exp RT RT k2 The overall reaction rate is: j ¼ 2F,k2 ,exp
Ru OH2 þ e /R H þ OH
(6)
H[2
(7)
Ru H þ H2 O/R OH þ
Step (5) represents a possible spill-over effect (surface chemical rearrangements) and if this elemental step controls the overall reaction rate, then the Tafel slope of 60 mV dec1 is observed on the log j e E polarization curve (if the coverage by Hads is low), as was the case in the low current densities region for Ni-(Ebonex-Ru) electrodes. However, if both steps, the primary discharge step (4) and the chemical step (5) control the overall reaction rate, assuming Temkin conditions of hydrogen adsorption, the following equations can be written for steady-state conditions: b rQH b FE exp 1 k1 ð1 QH Þ,exp ad RT RT ð1 bad Þr,QH ¼ k2 ,QH exp RT
(8)
where r(RT )1 represents the Temkin adsorption parameter, while bad is a symmetry factor for the adsorption.
ð1 bad Þ,rQH RT
(12)
By substituting the Eqn. (11) into Eqn. (12), the dependence of the overall reaction rate on potential could be obtained as: j ¼ 2F,k2
(11)
ð1bad Þ k1 ð1 bad Þ,b1 FE ,exp RT k2
(13)
The Tafel slope is defined as: b¼
2:303RT ð1 bad Þ,b1 F
(14)
For bad ¼ b1 ¼ 0.5 and T ¼ 298 K, b z 240 mV dec1. Thus, unusually high value of the Tafel slope for the HER obtained at high current densities at Ni-(Ebonex-Ru) electrodes can be explained by the formal kinetics approach, assuming that the reaction is controlled simultaneously by two elemental steps under Temkin conditions for the adsorption of the reaction intermediate (Hads). It is also interesting to note that the presence of an anomalously high Tafel slope for the HER was observed also by other authors at RuO2 and commercial DSA electrodes [55,56]. However, until now, there have been no attempts to explain this phenomenon with the formal kinetics approach. At the end, it would be interesting to compare the catalytic performance of the Ni-(Ebonex-Ru) composite coatings in alkaline solution with the Ni þ RuO2 composites, a class of materials that have been successfully employed as cathode
Table 4 e Kinetic parameters for the HER on Ni-(Ebonex-Ru) composite electrodes electrodeposited from a Watts bath containing different concentrations of Ebonex-Ru particles, recorded in 1.0 mol dmL3 NaOH solution at 25 C. Kinetic parameters
Electrode Ni
3
Ni-Ebonex (5 g dm )
Ni-(Ebonex-Ru) 3
b1/V dec1 b2/V dec1 j/A cm2 (E ¼ 1.38 V) h/V ( j ¼ 0.1 A cm2)
0.116
0.128
0.0012 0.583
0.0060 0.460
3
1 g dm
2 g dm
0.113 0.228 0.127 0.300
0.066 0.202 0.286 0.214
3 g dm3
5 g dm3
10 g dm3
0.067 0.212 0.385 0.190
0.068 0.204 0.574 0.165
0.071 0.207 0.597 0.156
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The activity for the HER of Ni-(Ebonex-Ru) composites should be further improved by optimization of the bath for Ni deposition, size of the Ebonex particles, electrodeposition parameters and the hydrodynamic conditions established in a deposition cell, which is going to be the subject of our future work.
4.
Fig. 10 e Tafel plots of the polarization curves recorded in 1.0 mol dmL3 NaOH at 25 C at the Ni-Ebonex electrode (B), Ni-Ebonex electrode corrected for roughness effects (6) and Ni-(Ebonex-Ru) electrode (C e curve 1 in Fig. 9).
electrocatalysts in chlor-alkali industry. Various data on the activity of Ni þ RuO2 composites have been reported in literature depending on conditions of their electrodeposition [18,24,25]. The overpotential for the HER on Ni-(Ebonex-Ru) is appreciably lower in comparison to the Ni þ RuO2 electrodes reported by Tavares et al. [25] (jDhj > 100 mV at j ¼ 10 mA cm2), while Ni-(Ebonex-Ru) and the Ni þ RuO2 electrodes prepared from a Watts bath by Musiani et al. [18] displayed similar activity for the HER in 1 M NaOH, e.g. the both composites achieved the current density of j ¼ 100 mA cm2 at E ¼ 1.21 V. On the other hand, highly porous Ni þ RuO2 electrodes deposited at large cathodic currents from an ammonium chloride Ni bath [18] or Ni þ RuO2 active cathodes prepared from a vigorously stirred Watts bath [24] exhibited for about 50 mV lower overpotential than Ni-(Ebonex-Ru) at j ¼ 100 mA cm2. Taking all into account, the catalytic behavior of Ni-(Ebonex-Ru) can be considered fairly well in view of industrial employment. In relation to that, it should be emphasized that the main advantage of the Ni-(Ebonex-Ru) cathodes lies in their potentially cost effective production. For an illustration, the optimum Ni-(Ebonex-Ru) electrode in this study was deposited from a Watts bath with 5 g dm3 of suspended EbonexRu(10 wt.%), whereas the maximum activity for Ni þ RuO2 electrodes obtained from a Watts bath in Ref. [18] was reached at 10 g dm3 of suspended RuO2, i.e. the concentration of Ru in a deposition bath for the optimum Ni-(EbonexRu) was 15 times lower. This remarkable reduction of the amount of high-priced Ru used in a preparation procedure for the Ni-(Ebonex-Ru) composite coatings can be attributed to the employment of Ebonex supporting particles, which provide high utility efficiency of the Ru nanosized electrocatalyst precipitated at the surface. Exactly these characteristics make the Ni-(Ebonex-Ru) cathodes promising candidates for industrial exploitation in water or chlor-alkali electrolysis.
Conclusions
1. Codeposition of nickel with suspended Ebonex-Ru particles produces composite coatings. The microstructural characterization of the suspended Ebonex-Ru particles by SEM and TEM in combination with EDS and EELS compositional analysis reveals that ruthenium is present in two forms, the pure Ru nanoparticles and highly faceted RuO2 nanorods. 2. The obtained Ni-(Ebonex-Ru) composite coatings demonstrate high electrocatalytic activity for the HER. Maximum activity is attained already at 5 g dm3 of Ebonex-Ru particles in a Watts bath (suspension). 3. Ebonex (Ti4O7) acts as an effective catalyst support for Ru. Low percentages of Ru (w10 wt.%) on Ebonex are sufficient to give high catalytic activity for the HER. 4. Decoration of Ebonex particles with Ru nanoparticles allows maximal utility efficiency of Ru catalyst, leading to a significant reduction in the amount of Ru (about 10 times lower than that in a commercial bath) for achieving the required activity for the HER. 5. Codeposition of Ni and Ebonex-Ru particles results in increased surface roughness compared to a pure Ni deposit. Beside the roughness effect, Ni-(Ebonex-Ru) electrodes also exhibit higher intrinsic catalytic activity for the HER, which takes place exclusively on Ru (RuOx) active sites. 6. An unusually high value of the Tafel slope (w200 mV) for the HER obtained at high current densities at Ni-(EbonexRu) electrodes can be explained by the formal kinetics approach, assuming that the reaction is controlled simultaneously by two elemental steps under Temkin conditions for the adsorption of the reaction intermediate (Hads).
Acknowledgment This work was financially supported by the Ministry of Education, Science and Technological Development of the Government of the Republic of Serbia through the Project No. 172054. The authors would also like to express their gratitude to National Center of Electron Microscopy, LBNL, University of California, Berkeley, for TEM characterization.
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