The structure, morphology and electrochemical impedance study of the hydrogen evolution reaction on the modified nickel electrodes

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International Journal of Hydrogen Energy 29 (2004) 145 – 157

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The structure, morphology and electrochemical impedance study of the hydrogen evolution reaction on the modi(ed nickel electrodes a , A. Lasiab B. Losiewicza;∗ , A. Budnioka , E. R0owi0nskia , E. Lagiewka ; a Institute

of Physics and Chemistry of Metals, Silesian University, 40-007 Katowice, 12 Bankowa, Poland de chimie, Universit$e de Sherbrooke, Sherbrooke, Qu$ebec, J1K 2R1 Canada

b D$ epartement

Accepted 24 March 2003

Abstract Composite layers of modi(ed amorphous nickel were prepared by simultaneous electrodeposition of Ni and TiO2 on a Cu substrate from a solution containing TiO2 (anatase) particles suspended by stirring. Scanning electron microscopy, X-ray di:ractometry, Auger spectroscopy and absorption spectroscopy, were used for physical and chemical characterization of the layers. Obtained deposits exhibit an amorphous structure of the Ni-P matrix in which the crystalline component, TiO2 , is embedded. Additionally, the presence of non-stoichiometric oxide, Ti2 O3 , formed on a boundary of the TiO2 grain and nickel matrix in consequence of the reduction conditions during the electrodeposition, was revealed by auger electron spectroscopy (AES). The hydrogen evolution reaction (HER) was investigated on the Ni-P + TiO2 and compared with Ni-P electrode in 5 M KOH at 25◦ C using steady-state polarization and electrochemical impedance spectroscopy (EIS). In order to explain the electrochemical behaviour of the electrode materials, electrical equivalent circuits containing: (i) the constant-phase element (CPE), (ii) the porous electrode impedance, and (iii) two-CPE elements were compared and veri(ed. The ac impedance behaviour of the electrodes may be well described using the two-CPE or porous electrode model in case of the Ni-P + TiO2 , and a simple CPE model for the Ni-P. The results obtained from the EIS and steady-state measurements allowed for the determination of the mechanism and kinetics of the HER. It has been found that an increase in electrochemical activity of the Ni-P + TiO2 electrode is due to both the increase in the real surface area and the presence of titanium oxides TiO2 and Ti2 O3 , as compared with the Ni-P electrode. ? 2003 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. Keywords: Composite electrocoatings; Impedance; Hydrogen evolution; Porous electrodes; Titanium

1. Introduction Electrodeposited composite layers containing oxides are mainly used for the oxygen evolution reaction [1]. On the other hand, transition metal oxides embedded in a metallic matrix can lead to production of electrode materials which are interesting in the chlor-alkali industry because of their good electrocatalytic properties for the chlorine evolution reaction [2–10]. However, some of transition metal oxides ∗

Corresponding author. Fax: +48-32-596929. E-mail addresses: [email protected] (B. [email protected] (A. Lasia).

Losiewicz),

are also active for the hydrogen evolution reaction (HER) [2,7,11–18]. Oxidation of nickel surface results in increase in the activity towards the HER in alkaline solutions [11,12]. Lanthanum nikelate, LaNiO3 , obtained by calcinations of the corresponding salts and electrodeposited with Ni shows increased activity towards the HER, and it is more active than the sintered Ni [2,13]. Iridium [14–16] and cobalt [7] oxides can be used in alkaline and acidic solutions as it has been published yet in the literature. Electroactive Ni-RuO2 materials were obtained by electrodeposition of nickel from a Watts bath with RuO2 particles, which were suspended by intensive stirring [5,9,10]. The catalytic activity of those materials appeared to depend on the RuO2 content in the

0360-3199/03/$ 30.00 ? 2003 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/S0360-3199(03)00096-X

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B. Losiewicz et al. / International Journal of Hydrogen Energy 29 (2004) 145 – 157

Nomenclature A b Cdl EAES F f I I0 ITi j j0 kav ki kintr Rct

inverse of charge-transfer resistance, in J−1 cm−2 slope of Tafel plot, V dec−1 di:erential double-layer capacitance F cm−2 auger energy eV Faraday constant frequency of potential sinusoidal oscillation Hz intensity of auger line intensity of oxygen auger line intensity of titanium auger line total current assuming the Volmer–Heyrovsky reaction mechanism for the HER exchange current density in A cm−2 average rate constant mol cm−2 s−1 formal rate constant of step (i) mol cm−2 s−1 intrinsic rate constant of step (i) mol cm−2 s−1 charge-transfer resistance J cm2

layer, and the overpotential of hydrogen evolution on such electrode was small. In spite of that, there are only few papers in the literature dealing with quantitative information about the mechanism and kinetics of the HER on electrodes containing oxides [2,11,19,20]. In order to determine the origin of the high activity, to change the catalyst structure or composition, and to obtain higher-performance electrode materials the knowledge of the detailed mechanism and kinetics of the HER is necessary [2,21]. In our earlier paper, active composite Ni-P + TiO2 layers were prepared in the same manner [22–26]. Electrochemical characteristics of the layers were determined after prior cyclic activation by a saw-shaped polarization impulse in the direction of hydrogen evolution. Such cycling was carried out for 3 h and an increase in electrode activity has been observed, as indicated by the rising values of current after each cycle at comparable potentials. Subsequently, the cathodic polarization curves were recorded. It has been found that the introduction of TiO2 to the amorphous Ni-P layers results in the increase in the HER rate in comparison with conventional Ni-P layers and this was observed both in acid and alkaline solutions [22]. However, no quantitative information concerning the HER mechanisms and kinetics on these layers was given. In the present study, the amorphous Ni-P layers containing crystalline TiO2 were prepared by electrodeposition under optimal conditions which ensured a maximum titanium dioxide embedding in the layer. For comparison Ni-P coating were also prepared. The aim of the undertaken investigations was to determine the mechanism and kinetic parameters of the HER on such porous and rough electrodes using steady-state polarization and electrochemical

Rf R ; p Rs T vi x Z Z Z  i    100 !

roughness factor electrolyte resistance along the pore axis J cm2 solution resistance J cm2 capacity parameter F cm−2 s−1 reaction rate of step (i) content atoms in titanium oxides impedance J cm2 real component of impedance J cm2 imaginary component of impedance J cm2 symmetry coeLcient related to step (i) phase angle surface coverage by adsorbed hydrogen hydrogen overpotential V hydrogen overpotential at current density of 100 mA cm−2 , V angular frequency

impedance spectroscopy (EIS) techniques. In this paper, various electrical equivalent models are compared and veri(ed for the HER study on Ni-P + TiO2 and Ni-P electrodes in 5 M KOH at 25◦ C. 2. Theory Mechanisms and kinetics of the HER on oxides: The HER mechanism on oxides is di:erent from that on metallic electrodes [2,19,20]. It was suggested, that it proceeds through oxide reduction on the electrode surface in alkaline solutions [2,19]: M − OH + H2 O + eM ↔ M − OH2 + OH− ;

(1)

M − OH2 + H2 O + eM ↔ M − OH + H2 + OH− :

(2)

An alternative mechanism, involving formation of a direct M-H bond, has been also proposed in the literature [2,20]. In general, it is accepted that the HER in aqueous electrolytes proceeds via the Volmer–Heyrovsk0y or the Volmer–Tafel reaction mechanism, so to determine of the HER kinetics, only three rate constants and two symmetry coeLcients must be determined [2,9,11,14,21]. Assuming the Volmer–Heyrovsk0y reaction mechanism for the HER [2,21], the total current is expressed as j = −F(v1 + v2 ) = −Fr0 ←

←

= F{˜k 1 (1 − ) − k −1  + ˜k 2  − k −2 (1 − )};

(3)

where ˜k i = ki exp(±i f) are the potential-dependent rate constants of the Volmer (index 1) and Heyrovsk0y (index 2) reactions, vi are the reaction rates, i are the symmetry

B. Losiewicz et al. / International Journal of Hydrogen Energy 29 (2004) 145 – 157

coeLcients,  is the overpotential, and  is the surface coverage by adsorbed hydrogen. Because the kinetic parameters of the individual reactions could not be determined using steady-state polarization technique, the additional information concerning the double-layer capacitance, Cdl , and the charge-transfer resistance, Rct , is supplied by the EIS [2,21]. The inverse of the charge-transfer resistance, parameter A, is de(ned in Eq. (4):
 1 @r0 A= = −F Rct @  =

← F2 ˜ {k 1 1 (1 − ) + k −1 (1 − 1 ) + ˜k 2 2  RT ←

+ k −2 (1 − 2 )(1 − )}:

(4)

The kinetic parameters together with their standard deviations are obtained by simultaneously (tting the steady-state polarisation curves and the charge-transfer resistances from EIS to the corresponding kinetic equations using non-linear least-squares approximation (NLS) [2,9,11,14,21].

3. Experimental Bath composition and conditions for electrodes preparation: Modi(ed amorphous nickel layers were prepared by electrodeposition from the following nickel plating bath (concentrations in g dm−3 ): 51—NiSO4 · 7H2 O, 10.7—NH4 Cl, 29—NaH2 PO2 · H2 O, 10—CH3 COONa, 8—H3 BO3 , to which 99 g dm−3 of TiO2 (anatase, LOBA FEINCHEMIE, Lot. Nr 17508, 100 mesh) was added and maintained in suspension by stirring with the rate of 300 rev=min. For comparison, the Ni-P layers were obtained under the same conditions from a nickel plating bath without TiO2 . The solution had a pH of 4.8–5.1 (CP 101 Elmetron pH-meter). Deionized water and analytical purity reagents (Merck, POCH Gliwice, Poland) were used for the solutions preparation. The layers were deposited on a copper plate substrate of surface area 1 cm2 . The other side and all side walls of the plates were covered by non-conducting epoxy resin (Distal), leaving only one surface of the plate exposed. Before deposition the copper substrate was mechanically polished with an abrasive paper (LabsoftJ, P 800, SiC) and using diamond pastes (LabsoftJ, DP max/min grain size: 75 m), next it was treated for a few seconds with diluted HNO3 (v/v 1:3) in order to remove impurities, rinsed with water and degreased with acetone. Then the substrate surface was activated for a few seconds in a dilute HCl solution (v/v 1:3), washed again with water, and immediately introduced into the cell for electrodeposition. After deposition the layers were rinsed with water and dried. The mass of the deposit was determined from the mass increase during deposition. Cell and electrodes: The copper plate working electrode was placed parallel to the vessel bottom (V =400 cm3 ). The

147

vessel diameter was 8 cm. The distance between the plate and the solution surface was 5 cm. A platinum mesh with the geometric area of 1 dm2 was served as a counter electrode. Electrodeposition process was carried out at 25◦ C, at the current density of j = 250 mA cm−2 for 25 min. The layers thickness was estimated by the microscopic method to be approximately 150 and 25 m for the Ni-P + TiO2 and Ni-P layers, respectively. Instruments: The surface morphology and cross-section studies of the layers were carried out using a scanning electron microscope (SEM, ZEISS 940, Germany). Structural investigations were conducted by XRD method using a Philips di:ractometer and CuK radiation. Quantitative chemical analysis of the layers was determined by means of atomic absorption method (AAS) using a Perkin–Elmer spectrometer. For this purpose the layers were dissolved in a hydrochloric acid solution (v/v 1:1) with the addition of a few drops of nitric acid and after suitable dilution, the content was determined as described in details in Refs. [22–24]. The Auger electron spectroscopy (AES) was used for the determination of the layers surface composition. Measurements were carried out in the vacuum system SP-2000 1/M using stationary Auger spectrometer SEA 02 with single-pass CMA within the di:erential system (f = 5000 Hz). The applied modulation was 1 Vp−p . The di:erential Auger spectra were taken out using 0:2 mm diameter electron beam with energy of Ep = 3 keV and with current equal to Jp = 3 A. The clean surfaces of the investigated layers were obtained by removing C and O atoms by means of in situ method. The surface were additionally etched by Ej = 4 keV Ar + beam and current density of Jj = 0:72 A mm−2 . Such obtained surfaces were activated in air under the pressure of 3 × 109 hPa at room temperature during up to 12 h. The spectrograms were registered every 5 min during the (rst 210 min and after 8:5 h. Electrochemical measurements conditions: All the electrochemical measurements were carried out at 25◦ C in 5 M KOH solutions to which 10 g dm−3 of sodium salt of EDTA (C10 H14 O8 N2 Na2 ·2H2 O) as a complexing agent was added in order to avoid the electrode deactivation through cathode poisoning. The presence of such agent did not inTuence the HER kinetics and the stationary measurements conditions were maintained [27]. Oxygen was removed by bubbling argon. The HER was studied in a PyrexJ glass cell (Radiometer No. 1734). The counter electrode was a platinum mesh (∼ 1 dm2 ). The external Hg=HgO=5 M KOH electrode, maintained at a room temperature and connected to the cell through a bridge (lled with 5 M KOH and a Luggin capillary, served as a reference electrode. The experimentally determined reversible potential for the HER was time independent and was equal to −0:930 V. The steady-state and ac impedance measurements were carried out using an Autolab 20 Potentiostat/Galvanostat Frequency Response Analyser (FRA) and Di:erential

148

B. Losiewicz et al. / International Journal of Hydrogen Energy 29 (2004) 145 – 157

Electrometer Ampli(er (Eco Chemie B.V., Netherlands), combined with one of the software packages, and computer-controlled electrochemical measurement system. The EIS measurements were performed after obtaining of reproducible polarisation curves (Tafel plots). Those conditions were obtained approximately after 24 h of the HER at a constant current density of j = 100 mA cm−2 . The Tafel plots were recorded galvanostatically (45 s after a constant current application) at cathodic current densities ranging from j = −100 mA cm2 to j = −1 A cm−2 . At the steady-state, the electrode potentials were corrected for jRs drop, determined by the EIS technique at various electrode potentials. Alternating current impedance measurements were carried out in the frequency range of 10 kHz to 0:1 Hz. Ten frequencies per decade were scanned using a sinusoidal signal of 5 mV peak-to-peak. The complex impedances were analysed using a modi(ed version of the complex non-linear least-squares (tting program (CNLS) of Macdonald et al. [28], from which the experimental parameters were determined. The approximations of the ac results were obtained using three typical models for the HER on solid, rough, or porous electrodes [2,21,27,29–35]: (i) the constant phase element (CPE) model, (ii) the porous electrode model, and (iii) the two-CPE model. Details of the calculation procedures are given with the experimental results. The HER mechanism was determined on the basis of the dependence of both the A parameter which is the inverse of the charge-transfer resistance, and the total current density j registered in the steady-state polarisation measurements on overpotential  [2,21,33,35]. The rate constants were evaluated using the NLS method [35,36]. 4. Results and discussion AAS and SEM studies: Composite Ni-P+TiO2 and Ni-P layers showed good adherence to the copper substrate and good physical stability. The deposits were hard, and diLcult to remove with an abrasive paper (SiC). The results of chemical composition analysis obtained by AAS method revealed that the Ni-P + TiO2 layers contain 52.2% Ni, 17.5% P, and 30.3% TiO2 . The Ni-P layers obtained under the same conditions contain 71.2% Ni and 28.8% P. The composition of the layers is given in weight percents. The scanning electron (SEM) micrographs of the Ni-P + TiO2 layer are presented in Fig. 1. The obtained layer had a mat-grey rough metallic surface with white tarnish visible to the naked eye. In the SEM of the Ni-P + TiO2 layer surface morphology (Fig. 1a) it can be seen that on a surface of titanium dioxide grains a great number of valleys is observed. It was also found that the presence of TiO2 grains embedded into the amorphous nickel matrix distinctly enlarges surface development of the Ni-P+TiO2 layer in comparison with Ni-P (see Ref. [22], Fig. 2a, b for details). The

Fig. 1. Scanning electron micrographs of the surface (a) and microscopic cross-section (b) of the Ni-P + TiO2 layer.

approximate thickness of the Ni-P + TiO2 layer was determined from the SEM of the cross-section (Fig. 1b) to be about 150 m. It should be stressed that no changes in the surface composition and morphology was observed after the hydrogen evolution. XRD measurements: Immediately after the layer electrodeposition, XRD measurements were carried out. The XRD pattern of the Ni-P + TiO2 layer (Fig. 2a) showed that the structure of this type of layers exhibits a nickel matrix in the range of 2 angles corresponding to the amorphous matrix of comparative Ni-P layer (Fig. 2b) prepared under the same conditions [22–26]. The structure of such composite layers additionally di:ers from the Ni-P deposit by the presence of crystalline TiO2 (anatase). The Ni-P and Ni-P + TiO2 layers being the object of our structural investigations, are amorphous from the r0ontgenographic point of view. Such a de(nition of amorphous material is used in this paper. In agreement with the SEM results shown above and in Ref. [22], the XRD patterns con(rmed presence of the TiO2 particles embedded into the nickel matrix. This

B. Losiewicz et al. / International Journal of Hydrogen Energy 29 (2004) 145 – 157

149

Fig. 3. A part of the Auger spectrogram registered from the Ni-P + TiO2 layer surface.

Table 1 The ratio of oxygen and titanium spectral lines intensities (IO =ITi ) for TiO [40], TiO2 [41], and from the Ni-P + TiO2 layer surface Electron spectroscopy

TiO

TiO2

The surface of the Ni-P + TiO2 layer

Auger electron spectroscopy, AES

1.25 1.25

1.84 1.84

1.57 1.57

Fig. 2. XRD patterns of the Ni-P + TiO2 (a) and comparative Ni-P (b) layer.

con(rms that a crystalline composite component could be embedded into the Ni-P during the electrodeposition. Di:raction lines corresponding to non-stoichiometric titanium oxides phases [22] were not observed. AES studies: Fig. 3 presents a part of the Auger spectrogram registered from the Ni-P + TiO2 layer surface. Identi(cation of spectral lines was carried out on the basis of the spectrogram standard published in the catalogue [37] and in Refs. [38] and [39]. The smoothing method of the registered AES spectrum (Fig. 3) was applied as described in Ref. [39]. The Auger line intensity measured by peak-to-peak method was analysed, and the chemical analysis based on elements identi(cation using the Auger peak parameters like the energy (EAES ) corresponding to the kinetic energy of emitted Auger electrons of a particular chemical element, and the line intensity (I ) proportional to an atoms number, was applied. The energies of the analysed element Auger lines are: 418 and 510 eV for Ti and O, respectively. From the analysis of the AES spectrum registered from the Ni-P + TiO2 layer surface (Fig. 3) a ratio of oxygen (IO ) and titanium (ITi ) spectral lines intensities was evaluated to

be approximately 1.57 (Table 1). According to Refs. [40] and [41] the ratio of IO =ITi for TiO and TiO2 is equal 1.25 and 1.84, respectively. Assuming a linear dependence of IO /ITi on parameter x (TiO1+x , where x stands for the oxygen atoms content parameter in titanium oxides in the range of 0 6 x 6 1 [38]) and the experimentally determined ratio IO =ITi =1:57, parameter x could be evaluated. The determined value of x = 0:57 points the presence on the layer surface of non-stoichiometric titanium oxide with formula TiO1:57 which approximately corresponds to Ti2 O3 . The AES results proved the non-stoichiometric titanium oxide, Ti2 O3 , is formed on the surface of Ni-P + TiO2 layer during the electrodeposition process. This may be connected with the presence of adsorbed Ni2+ ions on the surface of TiO2 particles in the electroplating bath. Nickel ions in the bath could be partially devoid of their hydration layer in consequence of adsorption on TiO2 particles, and readily reduced on the cathode surface. Electroreduction of TiO2 particles with adsorbed Ni2+ ions, which could be the charge-transfer carrier in the electrodeposition process, might simultaneously proceed. This way, the non-stoichiometric oxide particles, Ti2 O3 , are embedded into the layer. However, in the XRD analysis that phase

150

B. Losiewicz et al. / International Journal of Hydrogen Energy 29 (2004) 145 – 157

-1

-2

log (j / A cm )

-2 -3 -4 -5 -6 0.00

-0.05

-0.10

-0.15

-0.20

-0.25

-0.30

-0.35

-0.40

η/V Fig. 4. The steady-state polarization curve on the Ni-P + TiO2 (
) and Ni-P ( ) electrode in the HER in 5 M KOH at 25◦ C. Linear approximation of the experimental Tafel curve: continuous (—) using the two-CPE model for Ni-P + TiO2 and CPE model for Ni-P, and dotted (- - -) line using the porous model for Ni-P + TiO2 .

was not found which indicates that Ti2 O3 is either amorphous or its amount is too small to be detected by XRD method as it was suggested in Ref. [22]. Probably, the non-stoichiometric Ti oxide, Ti2 O3 , is formed on a boundary of the TiO2 grains and nickel matrix in consequence of the reducing conditions during the electrodeposition process. Nevertheless, a system of non-stoichiometric titanium oxides and TiO2 coexists on the layer surface. This fact may be of considerable signi(cance for the HER. Steady-state polarisation: The steady-state polarisation curve obtained on the Ni-P + TiO2 electrode is presented in Fig. 4. At low current densities there is a di:erence between two jR-corrected Tafel plots for the HER in 5 M KOH at 25◦ C. A shift in the open-circuit potential of approximately 50 mV toward more positive values for the electrode Ni-P+ TiO2 as compared with the Ni-P electrode, was observed. It may be connected with di:erent chemical composition of the layers as a consequence of titanium dioxide embedding into an amorphous nickel matrix. At the low currents the electrode potential may be determined by the corrosion of electrode elements. In spite of fact that two slopes of Tafel plots usually observed on oxides and amorphous nickel electrode [4,11,14,15,42], only one slope was observed in the case of the Ni-P + TiO2 and Ni-P electrode. The Tafel kinetic parameters as slope b de(ned as dE=d log j, exchange current density, j0 , and the overpotential at j = 100 mA cm−2 , 100 , are displayed in Table 2. These parameters were calculated taking into account the layer geometric surface area. The Tafel slope on the Ni-P + TiO2 electrode is equal −0:197 V dec−1 . The activity of such electrode is worse than that of the Ni-P electrode prepared under the same con-

Table 2 Kinetic parameters obtained from the steady-state polarization curves for the HER on Ni-P + TiO2 and comparative Ni-P layers in 5 M KOH at 25◦ C Electrode Ni-P+TiO2 Ni-P

b (V dec−1 ) −0:197 −0:087

j0 (A cm−2 ) 10−3

1:43 × 1:88 × 10−4

100 (V) −0:363 −0:248

ditions with b=−0:087 V dec−1 . Similarly, a comparison of 100 values for the layer containing TiO2 (100 = −0:363 V) and the Ni-P layer (100 =−0:248 V) indicates the lower activity in the HER of composite Ni-P + TiO2 layer. However, further analysis of kinetic parameters (Table 2) showed that an increase in the exchange current density, j0 , from 1:88 × 10−4 A cm−2 for the Ni-P electrode to 1:43×10−3 A cm−2 for the Ni-P + TiO2 layer is observed after embedding TiO2 into an amorphous nickel matrix. These results are in good agreement with the SEM results shown above, where an increase in the Ni-P + TiO2 layer surface area is evident, resulting in a higher exchange current density. This may also suggest the participation of titanium oxides in the electrochemical surface reactions taking place during the HER in 5 M KOH. The present discussion, similarly to that mentioned in the literature, leads to the conclusion that a mechanistic interpretation of the HER studies on porous and rough electrodes on the basis of the steady-state results only could be questionable [21,35,43,44]. EIS measurements: The EIS data were obtained on the Ni-P + TiO2 and Ni-P electrodes in the potential range

B. Losiewicz et al. / International Journal of Hydrogen Energy 29 (2004) 145 – 157 0.6

2.5 2

-Z" / Ω cm

-Z" / Ω cm

2

η = -101 mV

0.0 0.0

2.5 2

η = -187mV

0.0 0.0

5.0

Z' / Ω cm

(a)

η = -92 mV

-Z" / Ω cm

2

-Z" / Ω cm

2

0.5

0

1.0 2

Z' / Ω cm

(d)

2.5

2.5

2

-Z" / Ω cm

2

-Z" / Ω cm (e)

0

12

6 2

Z' / Ω cm

1.6

η = -132 mV

0.0 0.0

1.2 2

6

η = -269 mV

(c)

0.6

Z' / Ω cm

(b)

0.5

0.0 0.0

151

5.0 2

Z' / Ω cm

0.0 0.0

(f )

η = -172 mV

0.8

1.6

2.4

3.2

2

Z' / Ω cm

Fig. 5. Experimental complex-plane plots (
) obtained on Ni-P + TiO2 (a–c) and Ni-P (d–f) electrodes in 5 M KOH at 25◦ C at various overpotentials. The lines indicate the (tted data using CNLS method. The following electrical equivalent models were used: continuous line (—) for two-CPE, dotted line (- - -) for porous, and dash-dotted line (- —) for CPE.

corresponding to the linear part of the Tafel plots in which the steady-state conditions were ensured and kinetic parameters from the Tafel equation could be calculated (see Fig. 4 and Table 2). Examples of the experimental complex-plane plots (Z  vs. Z  ) and the dependence of Z  and −Z  vs. logarithm of the angular frequency obtained at various overpotentials are displayed in Figs. 5 and 6, respectively. The Ni-P + TiO2 electrode displayed ac behaviour characteristic of the porous and rough electrode containing pear-shape pores [2,27,29,30,32–34]. In Fig. 5a–c two semicircles are visible on the complex-plane plots for the Ni-P + TiO2 electrode, where the diameter of the (rst one was constant in the whole range of overpotentials studied. The ac data obtained on the comparative Ni-P electrode are presented in Fig. 5d–f. That electrode showed only one semicircle in the whole frequency range. This behaviour is similar to that found for smooth electrodes covered with Tat pores. It is in a very good agreement with results obtained by Shervedani and Lasia for Ni-P electrodes [30]. Three equivalent electrical models described earlier in details [11,21,29–35,45,46] were used to explain ac impedance

of these electrodes: (i) two-CPE, (ii) CPE and (iii) porous model. Appearance of the CPE is related to the distribution of the time constants [21]. It has been suggested in the literature that the appearance of the CPE on rough electrodes is related to capacitance dispersion [46]. The CPE model describes a simple process of hydrogen evolution, found usually on polycrystalline Ni and some Ni based porous electrodes [11,45,47]. On the other hand on several porous electrodes high frequency part of the complex plane plots shows either straight line at 45◦ or a semicircle for cylindrical or pear shape pores [21]. We have tested these possible models and tried to statistically distinguish between them. The real and imaginary components, Z  and Z  , of the electrodes impedance, registered in the whole frequency range at various overpotentials, were analysed using a modi(ed version of the complex non-linear least squares (CNLS) program [28]. The approximations of Nyquist (Fig. 5) and dependence of Z  and −Z  vs. logarithm of the angular frequency (Fig. 6) using the appropriate models were very good. A very good approximation to the experimental data for the Ni-P + TiO2 electrode was obtained using the two-CPE

152

B. Losiewicz et al. / International Journal of Hydrogen Energy 29 (2004) 145 – 157 1.6

3

2

η = -101 mV

-Z" / Ω cm

Z' / Ω cm

2

4

2 1 0 -1

0

1

2

3

4

5

log ω / s

2

η = -269 mV

-Z" / Ω cm

2

Z' / Ω cm

0

1

2

3

4

5

6

-1

log ω / s 0.20

0.6

0.3

0

1

2

3

4

5

log ω / s

(c)

η = -269 mV

0.15 0.10 0.05 0.00 -1

6

-1

0

1

2

3

4

5

6

-1

log ω / s

(d)

9

6

-Z" / Ω cm

6

2

η = -92 mV

2

Z' / Ω cm

0.4

(b)

0.9

0.0 -1

0.8

0.0 -1

6

-1

(a)

η = - 101 mV

1.2

3

5

η = -92 mV

4 3 2 1

0

0

2

4 -1

log ω / s

(e)

0

6

4

6

log ω / s 0.8

-Z" / Ω cm

2

2

η = -171 mV

2

Z' / Ω cm

(g)

2 -1

(f )

3

1

0

0

0

1

2

3

4

5

log ω / s

0.4 0.2 0.0

6

-1

(h)

η = -171 mV

0.6

0

2

4

6

-1

log ω / s

Fig. 6. Experimental (
) and simulated (lines) dependence of Z  and Z  vs. log ! plots for Ni-P + TiO2 (a–d) and Ni-P (e–h) electrodes in 5 M KOH at 25◦ C at various overpotentials. The following electrical equivalent models were used: continuous line (—) for two-CPE, dotted line (- - -) for porous, and dash-dotted line (- —) for CPE.

electrical equivalent model consisting of the solution resistance, Rs , in series with two parallel CPE-R elements, similarly as it was observed for Ni+Zn powder or porous lanthanum-phosphate-bonded nickel (LPBN) electrodes [2,35]. The two-CPE model produces two semicircles on the complex-plane plot, with the high frequency semicircle related to the surface porosity, and the low-frequency semicircle related to the charge-transfer process [2,29,31,32]. From the high frequency part of the impedance diagram parameters Rs , A1 , T1 and 1 could be determined, and from the second part, connected with the faradaic process,

parameters A = 1=Rct , T and  were found (Table 3). The detailed de(nition of these parameters is given in Ref. [2]. It was suggested that the second semicircle (corresponding to the parallel connection of the HER charge-transfer resistance, Rct , and the di:erential double-layer capacitance, Cdl , is not a:ected by the pore texture [35,48,49]. Taking into account inhomogeneity of the Ni-P + TiO2 electrode surface, the parameter  (Table 3) could be considered as an empirical parameter related to physical, chemical or geometrical inhomogeneities [35,50,51]. The  value decreased from approximately 0:99 ± 0:007 at less negative

B. Losiewicz et al. / International Journal of Hydrogen Energy 29 (2004) 145 – 157

153

Table 3 Parameters values and their standard deviations obtained using the two-CPE equivalent circuit model to approximate experimental ac impedance diagrams for a Ni-P + TiO2 electrode in 5 M KOH at 25◦ C. For all overpotentials Rs was approximately equal to 0:3 J cm2 . Parameter  was calculated from the second semicircle (details in text)  (V)

A1 (J−1 cm−2 )

T1 (F cm−2 s−1 )

1

A (J−1 cm−2 )

T (F cm−2 s−1 )



−0.101 −0.111 −0.120 −0.130 −0.139 −0.149 −0.158 −0.168 −0.177 −0.187 −0.196 −0.205 −0.215 −0.224 −0.233 −0.242 −0.251 −0.260 −0.269 −0.278

2:535 ± 0:232 3:001 ± 0:320 2:547 ± 0:223 3:059 ± 0:214 2:859 ± 0:194 2:553 ± 0:153 2:561 ± 0:106 3:126 ± 0:246 2:605 ± 0:193 2:516 ± 0:102 2:565 ± 0:115 2:791 ± 0:250 2:565 ± 0:240 2:567 ± 0:233 2:569 ± 0:261 2:566 ± 0:301 2:715 ± 0:297 2:854 ± 0:211 2:581 ± 0:196 2:569 ± 0:210

0:114 ± 0:013 0:126 ± 0:014 0:113 ± 0:013 0:114 ± 0:011 0:188 ± 0:008 0:112 ± 0:018 0:112 ± 0:018 0:165 ± 0:025 0:113 ± 0:020 0:112 ± 0:018 0:112 ± 0:018 0:118 ± 0:017 0:112 ± 0:016 0:112 ± 0:017 0:113 ± 0:018 0:112 ± 0:019 0:147 ± 0:023 0:109 ± 0:013 0:126 ± 0:014 0:112 ± 0:017

0:803 ± 0:013 0:877 ± 0:018 0:831 ± 0:012 0:802 ± 0:013 0:802 ± 0:012 0:802 ± 0:011 0:803 ± 0:010 0:813 ± 0:015 0:802 ± 0:011 0:864 ± 0:023 0:800 ± 0:028 0:869 ± 0:033 0:803 ± 0:014 0:804 ± 0:012 0:844 ± 0:017 0:848 ± 0:015 0:806 ± 0:013 0:834 ± 0:011 0:808 ± 0:012 0:828 ± 0:015

0:055 ± 0:006 0:062 ± 0:007 0:069 ± 0:008 0:078 ± 0:009 0:087 ± 0:007 0:098 ± 0:008 0:109 ± 0:008 0:123 ± 0:009 0:137 ± 0:009 0:154 ± 0:011 0:172 ± 0:013 0:192 ± 0:019 0:216 ± 0:015 0:241 ± 0:028 0:268 ± 0:063 0:299 ± 0:121 0:333 ± 0:175 0:371 ± 0:227 0:413 ± 0:260 0:461 ± 0:283

0:042 ± 0:017 0:039 ± 0:015 0:038 ± 0:015 0:037 ± 0:014 0:037 ± 0:015 0:036 ± 0:016 0:037 ± 0:014 0:038 ± 0:015 0:037 ± 0:016 0:037 ± 0:017 0:037 ± 0:017 0:037 ± 0:018 0:037 ± 0:019 0:037 ± 0:017 0:037 ± 0:016 0:037 ± 0:017 0:037 ± 0:016 0:036 ± 0:018 0:036 ± 0:020 0:036 ± 0:022

0:991 ± 0:007 0:987 ± 0:006 0:978 ± 0:007 0:957 ± 0:007 0:951 ± 0:006 0:939 ± 0:006 0:916 ± 0:007 0:910 ± 0:009 0:902 ± 0:010 0:900 ± 0:011 0:885 ± 0:010 0:870 ± 0:016 0:858 ± 0:016 0:835 ± 0:019 0:803 ± 0:023 0:790 ± 0:031 0:746 ± 0:035 0:705 ± 0:037 0:665 ± 0:042 0:629 ± 0:049

to 0:63 ± 0:049 at more negative overpotentials. It means that an increase of  to more negative values, causes a strong change of the surface roughness. The sense of parameter values of 1 , T1 and A1 shown in Table 3 is not discussed in this paper. Using parameters from Table 3 (second semicircle), the charge-transfer resistance, the T parameter values and the double-layer capacitance were determined (Fig. 7). The Cdl values were calculated according to the formula [52]: 1− T = CM dl (R−1 : s + A)

(5)

The ratio of the estimated Cdl values and assumed capacitance of a smooth metallic electrode of 20 F cm−2 [53] leads to determination of the roughness factor, Rf . The double-layer capacities for the Ni-P + TiO2 electrode in 5 M KOH at 25◦ C determined from two-CPE model are linearly dependent on the overpotential (Fig. 7b). The Cdl value decreases from 40 mF cm−2 at less negative potentials to 3 mF cm−2 at more negative , and Rf changes from 2000 to 150, respectively. It may suggest that an increase in the Cdl value in a direction of more negative potentials could inTuence the change of electrochemical active surface area. Considering the AES results shown above, the reasons of activity change could be connected with the presence of Ti2 O3 on the surface of the Ni-P + TiO2 electrode, which might e:ect the activity towards the HER. The CPE model was also used to (t of the experimental EIS data for the Ni-P + TiO2 electrode [2,29,30,32,33].

In Fig. 5a–c and Fig. 6a–c the worst (t from among applied models was observed for that model. It is evident that the electrical equivalent model consisting of the solution resistance in series with the parallel connection of the charge-transfer resistance and a single constant-phase element instead of electrode capacitance cannot be used to describe the experimental ac impedance data over the entire frequency range. The experimental ac impedance data also agree well with the Levie porous model modi(ed by using the CPE element for the double layer capacitance [2,35,49]. That model predicts formation on a complex-plane plot of a straight line at 45◦ at high frequencies and a semicircle at lower frequencies, and has (ve adjustable parameters: Rs ; Ap ; Bp ;  and R ; p [2,35]. All parameters were de(ned in Ref. [2] and their values with standard deviations are shown in Table 4. That model explains the impedance behaviour of the electrode covered with deep cylindrical pores and it was successfully applied in the literature [27,31–34] for porous Ni based materials. De Levie’s model predicts that a linear part of the Nyquist plot at high frequencies is connected with the solution resistance-double layer capacitance coupling in semi-in(nite pores. The penetration depth of the ac signal depends on frequency, and at the lowest frequencies the pores respond like a Tat electrode [35,54]. The disadvantage of this model is that it includes speci(c pore parameters diLcult to determine [35]. The  parameter, similarly to those found using the two-CPE model, decreased from approximately

154

B. Losiewicz et al. / International Journal of Hydrogen Energy 29 (2004) 145 – 157

Fig. 7. Comparison of dependence of logarithm of the charge-transfer resistance, Rct , (a) and double-layer capacities, Cdl , (b) on overpotential, , obtained using the two-CPE and porous models for the HER on the Ni-P + TiO2 electrode in 5 M KOH at 25◦ C.

1:000 ± 0:004 at more positive to 0:70 ± 0:02 at more negative overpotentials. It may be expected that the change from a semi-in(nite to (nite length pore model takes place. It also means that on Ni-P + TiO2 electrode the ac impedance data can be (tted to the porous model in which the impedance of the pore walls resembles at low overpotentials that of a Tat surface (values of phase angles  are close to unity). As can be seen in Table 4, the values of capacitance parameter, Bp , are almost constant at overpotentials higher than  = −139 mV. The parameter R ; p is not constant in the overpotential range studied, it decreases from 9:12 ± 0:03 to 5:40 ± 0:11 with the increase in more negative overpotentials. As far as R ; p values are considered, the porous model validity could not be con(rmed. Using parameters from Table 4, another parameters of the porous model like Rct , T and Cdl could be determined (Fig. 7). The sum of squares and the average standard deviations of the parameters obtained using the two-CPE and porous models are of the same order of magnitude, therefore, on the basis of statistical comparison [55] it is impossible to determine which model gives the better approximation.

The values of the charge-transfer resistance, Rct , (Fig. 7a) and di:erential double-layer capacitance, Cdl , (Fig. 7b), determined from the porous model, are potential-dependent and similar to those determined from the two-CPE electrode model. The charge-transfer resistances of the HER are similar in the whole range of overpotentials, except that at the lowest ; the Rct value from the two-CPE model is slightly larger than that determined from porous model. The di:erence between charge-transfer resistances over the whole overpotentials studied, does not exceed 5% for two models used, with  values being higher than −101 mV. This fact can be explained by comparison of the complex plane plots obtained for the two-CPE and porous models. The semicircle registered at low frequencies for the two-CPE model, assuming a presence of pear-shape pores on the electrode surface, yields a larger radius, i.e. Rct , than that corresponding to the semicircle obtained for the porous model which predicts the electrode surface covered with deep cylindrical pores (larger real surface area). Overpotential dependence of Cdl for both models, displayed in Fig. 7b, shows that the di:erences in the double-layer capacities calculated from both models are lower than 29% over the whole range of . The conclusion is that both the two-CPE and porous models are appropriate for approximation of ac data obtained on the Ni-P + TiO2 electrode in 5 M KOH at 25◦ C. The approximations of the ac impedance data for the Ni-P electrode are presented in Figs. 5 and 6 (d–f). The best approximation was obtained for the CPE model. Using the CNLS approximation method, the di:erential double-layer capacitance, Cdl , was estimated according to the formula proposed by Brug (Eq. (5)) [52]. The double-layer capacities for the Ni-P electrode in 5 M KOH at 25◦ C determined from the CPE model are linearly dependent on the overpotential. The Cdl value decreases from 19 mF cm−2 at less negative potentials to 5 mF cm−2 at more negative , and Rf changes from 951 to 252, respectively. The roughness factor, Rf , for the Ni-P electrode in 5 M KOH at 25◦ C at the chosen  = −205 mV approximately equal 317 which is over 17 times higher than the value obtained by Shervedani et al. [30] on Ni70 P30 in 1 M NaOH at 70◦ C, and simultaneously over four times lower than that obtained on the Ni-P + TiO2 electrode investigated in this study. Our results con(rm the earlier studies that Ni-P alloys are not very active for the HER [2,21,22,30]. Modi(cation of the amorphous nickel layer with a crystalline titanium dioxide results in the increase of the real relative surface area. Electrodeposited Ni-P + TiO2 layers have larger electrochemically active surface area as compared with the Ni-P ones. The increase of the number of active sites per unit of surface area can be also expected. The plots of the logarithm of the inverse charge-transfer resistance, A = 1=Rct , vs. overpotential, , for both Ni-P + TiO2 and Ni-P layers, are presented in Fig. 8. These dependences are linear with the slope, b, of −197 mV and −87 mV for the Ni-P + TiO2 and Ni-P electrode,

B. Losiewicz et al. / International Journal of Hydrogen Energy 29 (2004) 145 – 157

155

Table 4 Parameters values and their standard deviations obtained using the porous model equivalent circuit model to approximate experimental ac impedance diagrams for a Ni-P + TiO2 electrode in 5 M KOH at 25◦ C. The parameter are de(ned as Ap = aRct and Bp = T=a, a = r=2#l, R ; p = #l=%r 2 , r—pore radius, l—pore length, and #—speci(c resistance of the solution [2]. For all overpotentials Rs was approximately equal to 0:3 J cm2  (V)

Ap

Bp (s )



R ; p (J · cm2 )

−0.101 −0.111 −0.120 −0.130 −0.139 −0.149 −0.158 −0.168 −0.177 −0.187 −0.196 −0.205 −0.215 −0.224 −0.233 −0.242 −0.251 −0.260 −0.269 −0.278

1:90 ± 0:03 1:79 ± 0:04 1:50 ± 0:04 1:49 ± 0:06 1:17 ± 0:06 1:00 ± 0:06 0:94 ± 0:05 0:89 ± 0:04 0:80 ± 0:06 0:74 ± 0:05 0:76 ± 0:06 0:71 ± 0:05 0:68 ± 0:07 0:63 ± 0:07 0:61 ± 0:09 0:57 ± 0:10 0:56 ± 0:12 0:50 ± 0:12 0:48 ± 0:12 0:40 ± 0:23

0:433 ± 0:006 0:426 ± 0:005 0:419 ± 0:003 0:411 ± 0:005 0:404 ± 0:003 0:399 ± 0:010 0:394 ± 0:034 0:388 ± 0:043 0:383 ± 0:055 0:380 ± 0:056 0:377 ± 0:060 0:375 ± 0:058 0:372 ± 0:045 0:369 ± 0:043 0:361 ± 0:056 0:354 ± 0:051 0:346 ± 0:058 0:338 ± 0:045 0:331 ± 0:068 0:323 ± 0:070

0:998 ± 0:004 0:994 ± 0:003 0:987 ± 0:006 0:963 ± 0:007 0:956 ± 0:007 0:947 ± 0:007 0:925 ± 0:009 0:914 ± 0:006 0:909 ± 0:009 0:905 ± 0:008 0:893 ± 0:010 0:881 ± 0:011 0:871 ± 0:010 0:844 ± 0:012 0:818 ± 0:012 0:808 ± 0:014 0:766 ± 0:016 0:724 ± 0:019 0:685 ± 0:020 0:696 ± 0:017

9:147 ± 0:025 8:687 ± 0:029 9:248 ± 0:035 8:361 ± 0:042 9:504 ± 0:048 9:874 ± 0:047 9:411 ± 0:040 8:894 ± 0:038 8:814 ± 0:049 8:534 ± 0:045 7:544 ± 0:052 7:179 ± 0:041 6:666 ± 0:063 6:554 ± 0:065 6:003 ± 0:073 5:809 ± 0:086 5:281 ± 0:102 5:411 ± 0:109 5:061 ± 0:207 5:403 ± 0:112

-0.2

-0.6

-1

-2

log (A / Ω cm )

-0.4

-0.8

-1.0

-1.2

-1.4 -0.32

-0.28

-0.24

-0.20

-0.16

-0.12

-0.08

η/V Fig. 8. Dependence of the logarithm of the inverse charge-transfer resistance, A=1=Rct , on overpotential, , for the HER on the Ni-P+TiO2 (
) and Ni-P ( ) electrode in 5 M at 25◦ C. The calculated values of the logarithm of parameters A for the Ni-P + TiO2 electrode: continuous line (—) for Table 3 and dotted line (- - -) for Table 4. The calculated values of the logarithm of parameters A for the Ni-P electrode: dash-dotted line (- - -) using the CPE model.

respectively. The values of b are identical with the slope calculated for the Tafel slopes registered in steady-state measurements (see Figs. 4 and Table 2). An approximation of the parameter A vs. overpotential and j registered from the steady-state polarization vs over-

potential, was carried out by adjusting the average rate constant, kav , and transfer coeLcient  by the NLS method [28,35,51]. A good approximation was observed (see Figs. 4 and 8). It was suLcient to assume the Volmer-Heyrovsk0y mechanism to explain the experimental data. The deter-

156

B. Losiewicz et al. / International Journal of Hydrogen Energy 29 (2004) 145 – 157

Table 5 Comparison of the apparent and the real kinetic parameters of the HER obtained from steady-state and ac impedance experiments on the Ni-P + TiO2 and Ni-P electrode at the overpotential of −205 mV in 5 M KOH at 25◦ C Electrode

kav (mol cm−2 s−1 )



Rf

kintr = kav =Rf (mol cm−2 s−1 )

j0 =Rf (A cm−2 )

Ni-P+TiO2 (two-CPE) Ni-P+TiO2 (porous) Ni-P

(7:0 ± 0:12) × 10−9 (7:7 ± 0:31) × 10−9 (9:6 ± 1:30) × 10−10

0:309 ± 0:002 0:302 ± 0:004 0:679 ± 0:024

950 1500 317

7:4 × 10−12 5:1 × 10−12 3:0 × 10−12

1:5 × 10−6 9:5 × 10−7 5:9 × 10−7

mined rate constants values k−1 and k2 were much grater than k1 rate constant, where k1; −1 and k2 are the rate constants for the Volmer and Heyrovsk0y step, respectively (see Eq. (3)). Thus to explain the EIS results, the average rate constant kMav were determined according to the formula: 1 : (6) kMav = 1 1 + k1 k2 Intrinsic activity of the electrodes was evaluated on the basis of the apparent experimental values of the exchange current densities, j0 , (Table 2) and the values of the kinetic parameters, kav , divided by the roughness factor, Rf , for the chosen  = −205 mV. The apparent and real (per unit of a real surface area) results, are presented in Table 5. The values of the average apparent rate constant determined for the Ni-P + TiO2 electrode is one order of magnitude larger than that for Ni-P electrode. It should be noted that the average rate constant and transfer coeLcient shown in Table 5 and calculated using parameter A from the two-CPE model (A = 1=Rct ) and the porous model (A = 1=Ap R ; p ) are very close. The determined logarithmic dependence both the j (Fig. 4) and A (Fig. 8) on  are also very similar. Higher intrinsic activity for the HER was also observed on the electrode containing titanium dioxide as compared with the Ni-P electrode. The intrinsic activity, kintr , of the Ni-P + TiO2 electrode has the same order of magnitude as the Ni-P electrode, but the values of the real rate constant are larger in case of the Ni-P electrode containing TiO2 . From Tafel experiments (see Fig. 4) there is no evidence that Ni-P+TiO2 presents a better performance than Ni-P besides the apparent exchange current density (that is strongly inTuenced by the real surface area). Therefore, the inTuence of real structure was taken into account. The values of Rf were considered, and next, the real exchange current density was lower (Table 5). Comparison of the real activities, j0 =Rf , of the electrodes studied indicates that the real exchange current density of the Ni-P + TiO2 electrode (two-CPE model) is only two times larger than that of Ni-P while the apparent exchange current density is ∼ 8 times larger. Large values of the observed apparent exchange current density are mainly due to the real surface area. On this basis, the synergetic e:ect of above-mentioned factors on the HER kinetics on the Ni-P + TiO2 electrode can be ascertained. It seems that the synergetic e:ect, responsible for that electrode activity, arises not only from

the presence of TiO2 . Additionally, as it was revealed in our AES sensitive-surface measurements (see Fig. 3), the thin monomolecular layer of the non-stoichiometric titanium oxide, Ti2 O3 , formed on a boundary of the TiO2 grain and nickel matrix, exists on the surface electrode. The presence of such phases, evokes the electrocatalytic e:ect on the HER according to equations: Ti2 O3 + 3H2 O + 3e− ↔ Ti2 O3 · H3 + 3OH−

Volmer step;

(7)

Ti2 O3 · H3 + 3H2 O + 3e− ↔ Ti2 O3 + 3H2 ↑ +3OH−

Herovsky step:

(8)

The proposed mechanism is based on the aLnity of the hydrogen atoms to free electron pairs of oxygen. The nonstoichiometric titanium oxide and dioxide on the Ni-P + TiO2 surface have catalytic activity towards H reduction/adsorption and hence, can e:ect on the HER kinetics. This is a suggestion being in agreement with our earlier papers [22] and Vasudevan reports [56–59]. 5. Conclusions Composite Ni-P + TiO2 layers can be electrodeposited from a nickel bath containing TiO2 powder suspension. Obtained layers exhibit amorphous structure of the nickel matrix in which the crystalline TiO2 is embedded. Additionally, the non-stoichiometric Ti oxide, Ti2 O3 , formed on a boundary of the TiO2 grain and nickel matrix in consequence of the reduction conditions during the electrodeposition, is present on the Ni-P + TiO2 layer surface. Modi(cation of the nickel matrix with titanium dioxide results in the change of the electrochemical behaviour from a simple CPE (electrode Ni-P) to the porous or two-CPE (electrode Ni-P + TiO2 ) model. Ac behaviour of the Ni-P + TiO2 electrode can be successfully explained using the two-CPE and porous electrical equivalent models. The steady-state and EIS measurements allowed to determine the HER mechanism and kinetics quantitatively. It was established that the HER on both types of investigated electrodes in 5 M KOH at 25◦ C proceeds via the Volmer-Heyrovsk0y reaction mechanism. It has been found that the increase in electrochemical activity of the

B. Losiewicz et al. / International Journal of Hydrogen Energy 29 (2004) 145 – 157

Ni-P+TiO2 electrode is due to both the presence of titanium oxides and the increase in the real surface area, as compared with the Ni-P electrode. Acknowledgements Financial support from the Polish Committee for Scienti(c Research (Project 7 T08A 046 19) is gratefully acknowledged. The authors also thank Dr. H. Jehn and Dr. A. Zielonka from Forschungsinstitut fXur Edelmetalle und Metallchemie, (SchwXabisch GmXund, Germany) for their assistance with the SEM studies. References [1] Trasatti S. In: Lipkowski J, Ross PN, editors. Electrochemistry of novel materials. New York: VCH, 1994. p. 207. [2] Lasia A. Applications of the electrochemical impedance spectroscopy to hydrogen adsorption, Evolution and absorption into metals. In: Conway BE, White RE, editors Modern Aspects of Electrochemistry, vol. 35. New York: Kluwer/Plenum, 2002. p. 1. [3] Trasatti, S. In: Gerischer H, Tobias CW. editors. Advances in electrochemical science and engineering, vol. 2. Weinheim, VCH, 1992. p. 2. [4] Spataru N, Le Helloco JG. Rev Roum Chimie 1995;40:505. [5] Iwakura C, Tanaka M, Nakamatsu S, Inoue H, Matsuoka M, Furukawa N. Electrochim Acta 1995;40:977. [6] Kolotyrkin YM, Losev VV, Shub DM. Electrokhimiya 1979;15:291. [7] Trasatti S. In: Wellington TC. editor. Modern chlor-alkali technology. Elsevier, Amsterdam, 1992. p. 281. [8] KXotz ER, Stucki S. J Appl Electrochem 1987;17:1190. [9] Miousse D, Lasia A. J New Mat Electrochem Systems 1999;2:71. [10] Tavares AC, Trasatti S. Electrochim Acta 2000;45:4195. [11] Lasia A, Rami A. J Electroanal 1990;294:123. [12] Diard JP, Le Gorrec B, Maximovich S. Electrochim Acta 1990;35:1099. [13] Anani A, Mao Z, Srinivasan S, Appleby AJ. J Appl Electrochem 1991;21:683. [14] Chen L, Guay D, Lasia A. J Electrochem Soc 1996;143:3576. [15] Chen H, Trasatti S. J Electroanal Chem 1993;357:91. [16] Boots JCF, Trasatti S. J Appl Electrochem 1989;19:255. [17] Jaccaund M, Leroux F, Millet JC. Mater Chem Phys 1989;22:105. [18] Kleijn M, Van Leeumen HP. J Electroanal Chem 1988;247:253. [19] Pauport0e T, Andolfatto F, Durand R. Electrochim Acta 1999;45:431. [20] Kondintsev IM, Trasatti S. Electrochim Acta 1994;39:1803. [21] Lasia A. Electrochemical impedance spectroscopy and its applications. In: Conway BE, Bockris J, White RE, editors. Modern aspects of electrochemistry, vol. 32. New York: Kluwer Academic/Plenum Publishers, 1999. p. 143. ; E. J Appl [22] Gierlotka D, R0owi0nski E, Budniok A, Lagiewka Electrochem 1997;27:1. [23] Gruszka A, Budniok A. Adv Perform Mater 1999;6:141.

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