Electrodeposited Ni–Co-oxide electrodes:characterization and kinetics of the oxygen evolution reaction

International Journal of Hydrogen Energy 25 (2000) 1163–1170www.elsevier.com/locate/ijhydene

ElectrodepositedNi–Co-oxide electrodes:characterization andkinetics of the oxygen evolution reaction

E.B. Castro∗, C.A. Gervasi 1

Instituto de Investigaciones Fisicoqu��micas Te�oricas y Aplicadas (INIFTA), Universidade Nacional de La Plata (UNLP),Suc. 4, C.C. 16, (1900) La Plata, Argentina

Abstract

In this paper we present results on the characterization of Ni–Co-oxide electrodes, prepared by anodic deposition fromCo(NO3)2 aqueous solutions on Ni substrates. The kinetics and mechanism of oxygen evolution was analysed. Tafel slopesclose to 40 mV per decade were measured. The reaction order with respect to [OH−] was found to be approximately 2 at25

◦C. A possible mechanism for oxygen evolution on these electrodes is presented, which accounts for the values of the

kinetic parameters experimentally obtained. ? 2000 International Association for Hydrogen Energy. Published by ElsevierScience Ltd. All rights reserved.

Keywords: Oxygen evolution reaction; Cobalt oxides; Anodic catalysts

1. Introduction

A signi�cant factor contributing to a good performance ofalkaline water electrolysis is the electrocatalytic activity ofthe anode material. The electrocatalysis of oxygen evolution(OER) on conductive metallic oxide electrodes has been atopic of great interest in electrochemistry for a long time,since these surface catalysts make it possible to signi�cantlyimprove the voltage balance of the cell.It is known that oxygen overpotentials are e�ciently de-

creased by application of mixed oxides containing Co indi�erent valence states [1]. Cobalt and nickel cobaltites areusually prepared by thermal decomposition, spray pyrolysisand sputtering [2–4]. Co(III) oxides can be electrodeposited,on di�erent metallic substrates, in aqueous solutions con-taining small concentrations of Co(II) ions [5–8]. Recently,

∗ Corresponding author. Fax: 54-221-4254642.E-mail address: [email protected] (E.B. Castro).1 CIC Researcher.

we reported on a preparative method of cobaltites byanodic deposition from Co(NO3)2 aqueous solutions, ren-dering oxide-coated substrates, which can be preparedreproducibly and exhibit high oxygen evolution currentdensities [5].The oxygen evolution reaction (OER) on Co3O4 and

NiCo2O4 �lms has been studied by several workers. It isgenerally accepted that before oxygen evolution the surfaceof cobaltites is oxidized to tetravalent oxidation states [9](namely, Co(IV) or Ni(IV)) and that no further oxida-tion takes place. However, depending on the preparationmethod, the experimental conditions and the nature of thesupports employed, di�erent values of the Tafel slope andof the reaction order with respect to [OH−] were found.Most relevant data in the literature, concerning this aspect,have been reviewed [10].In this work electrodeposited Co-oxide �lms, obtained

through potential cycling in solutions containing Co(II),were characterised by di�erent electrochemical and op-tical methods. The kinetics and mechanism of the OERin alkaline solutions were also investigated. For thesake of comparison we also present results correspond-ing to cobalt oxide �lms prepared by thermal decom-position.

0360-3199/00/$ 20.00 ? 2000 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.PII: S0360 -3199(00)00033 -1

1164 E.B. Castro, C.A. Gervasi / International Journal of Hydrogen Energy 25 (2000) 1163–1170

2. Experimental

All electrochemical experiments were performed inconventional three-compartment glass cells. The workingelectrodes consisted of high-purity Ni (“Specpure JohnsonMatthey Chemicals Ltd.) in the form of rotating discs,geometric area 0:1 cm2, embedded in PTFE handlers, Nifoils (Ar = 2:2 cm2) were used as substrates for the elec-trodes prepared by thermal decomposition. A high-area Ptfoil was used as counter electrode. Potentials in the text arereferred to the Hg=HgO 1 M OH− reference electrode. Allexperiments were performed at 25

◦C under N2 saturation.

2.1. Electrode preparation

Co-oxide electrodes were prepared by two methods.

2.1.1. Potentiodynamic electrodeposition (electrodes A)The preparation method is described in detail in Ref. [5].

Ni disk electrodes were subjected to repetitive triangularpotential scans (RTPS), in 1 M NaOH(200 ml) + 10 mlof (0.05 M Co(NO3)2 · 6H2O + 0:5 M Na(NO3)) (aque-ous solution), between Esc = −0:1 V and Esa = 0:7 V, atv = 0:05 V s−1 during 60 min, ! = 2000 rpm. Previous tothe application of the RTPS the electrode was polarised atE = −1:2 V for 5 min, in 1 M NaOH, in order to reduceair-formed �lms. Immediately after the RTPS was applied,the solution containing Co(NO3)2 · 6H2O was added. At theend of the RTPS the electrode surface was covered by abright-golden �lm.

2.1.2. Thermal decomposition (electrodes B)These electrodes were prepared as described in

[11]: Ni foils were immersed in a solution of 0.5 MNi(NO3)2 · 6H2O + 1 M Co(NO3)2 · 6H2O dissolved inethanol. After each immersion the electrode was heatedfor 4 min at T = 400

◦C, �nally after four immersions the

electrodes were cured at TF = 400◦C for 1 h, in order to

complete the decomposition of the nitrates and the forma-tion of the NiCo2O4 oxide. The substrate was covered by ablack rough deposit.

2.2. Characterisation of the samples

The electrochemical behaviour of electrodes A and B wascharacterised in 1 M NaOH solutions. Accordingly, cyclicvoltammograms were run between −0:1 V and a poten-tial value set in the oxygen evolution region. Unless other-wise stated all potentials in �gures and text are referred toHg=HgO=1 M OH− reference electrode.Steady-state potentiostatic polarisation and electrochem-

ical impedance spectroscopy (EIS) were used to study thekinetics of the oxygen evolution reaction (OER).Potentiostatic polarisation curves and cyclic voltammetric

measurements were performed with an EG& G M273 po-tentiostat combined with EG &GM270 software on an IBM

compatible PC. Electrochemical impedance measurementswere performed using the above-mentioned potentiostat anda Schlumberger SI 1255 HF frequency response analyzer.X-ray photoelectron spectroscopy (XPS) and X-ray

di�raction (XRD) techniques, were employed to study thestructure and composition of the electrodeposits.XPS measurements were performed using an ESCA 3

Mark II instrument, with Mg K� X-ray radiation (h� =1253:6 eV) as the excitation source. XPS core spectra werecollected for O 1s, Ni 2p and Co 2p shells corresponding toelectrodes A. The information was obtained from the sur-face and also from an in-depth pro�le analysis after argonion sputter etching. The measured XPS spectra of the elec-trodes were compared with literature data for similar �lmsand standards for chemically prepared Ni and Co-oxide andhydroxide compounds.X-ray di�raction studies, of electrodes A and B, were per-

formed with a Phillips (PW 1700) instrument, using Cu K�radiation (�=0:154184 nm). The incident beam reached thesample at an angle of ca. 2

◦.

3. Results

3.1. Characterisation

3.1.1. Electrochemical behaviourFig. 1(a) depicts the voltammetric behaviour of a

Co-oxide electrode, prepared by potentiodynamic deposi-tion on Ni (electrode A). The voltammetric response ofthese electrodes, in 1 M NaOH, exhibit good reproducibil-ity. No changes have been observed in the charge or thepotential of the current maxima with further cycling, orwith immersion time at open circuit potential.The voltammograms are similar to those reported for

NixCo3−xO4 oxide electrodes, with spinel structure, pre-pared by spray pyrolysis [9]. The redox couple located atca. E ≈ 0:2 V has been assigned to the Co(II)=Co(III) re-dox process [12]. The small intensity of these peaks maybe explained assuming that the anodic process correspondsto the oxidation of Co(II) cations in the tetrahedral posi-tions of the spinel oxide, these sites being more di�cult tooxidize or reduce than cations in octahedral sites [13]. Theoverlapped anodic current maxima located at Ea1 = 0:42 Vand Ea2 = 0:48 V may be assigned to Co(III)=Co(IV) andNi(III)=Ni(IV) transitions [9,10]. In many publications onlyone broad anodic peak, previous to the OER potential range,has been reported [12,14]. The cathodic pro�le depicts abroad peak at Ec ≈ 0:33 V corresponding to the reduction ofCo(IV),Ni(IV) species. The corresponding cathodic charge,q ≈ 3 mC cm−2, is equal to the sum of the charge densitiesinvolved in the overlapped anodic maxima.Fig. 1(b) shows a cyclic voltammogram corresponding

to a NiCo2O4 electrode prepared by thermal decompositionon a Ni substrate (electrode B), in 1 M NaOH, recordedat a sweep rate of v = 25 mV s−1. The voltammetric

E.B. Castro, C.A. Gervasi / International Journal of Hydrogen Energy 25 (2000) 1163–1170 1165

Fig. 1. Cyclic voltammograms corresponding to electrodes A andB, in 1 M NaOH.

pro�le is comparable to those reported by Trasatti et al. [15],and Barendrecht et al. [11]. The anodic current maximalocated at Ea1=0:35 V and Ea2=0:53 V have been assignedto Co(II)=Co(III) and Co(III),Ni(III)=Co(IV),Ni(IV) tran-sitions, respectively [11]. The cathodic pro�le exhibits abroad peak at Ec ≈ 0:25 V. The shift in the anodic andcathodic peaks towards more irreversible potentials, incomparison to Fig. 1(a), is probably related to the existenceof a resistive oxide layer, formed during the thermal treat-ment, between the metal substrate and the Co-oxide deposit[15]. The charge involved in the second anodic peak isq ≈ 0:14 C cm−2.

3.1.2. X-ray photoelectron spectroscopyIn-depth detailed spectra of the metal photoelectron lines,

corresponding to electrodes A, present a weak Ni 2p sig-nal, at the surface of the sample, which becomes stronger indepth. As the sputter etching proceeds, a signal with a peakat BE 855.2 eV can be detected (Fig. 2a), resulting from theNi 2p3=2 principal contribution. No shift of the C 1s (284.6eV) line was observed, which indicates that no sample charg-ing occurred during these experiments. The position of theNi 2p1=2 line corresponds to 872.6 eV. These values were

Fig. 2.XPS spectra ofNi 2p andCo 2p, corresponding to electrodeA.

Table 1Binding energies of Co 2p3=2 transition for di�erent cobalt oxides

Co 2p3=2 Co 2p1=2 Satellite CoII

(BE=eV) (BE=eV) intensity

Electrode A 779.91 795.5 IntermediateNiCo2O4 [3] 779.85 794.85 IntermediateCo3O4 [15] 779.5 794.5 WeakCoO [15] 781.8 796.3 Strong

attributed to Ni in NiCo2O4 [3]. A prominent satellite struc-ture as indicated in Ref. [3] can also be observed.The Co 2p spectra show two main peaks with BE of

779.91 eV (Co 2p3=2) and 795.5 eV (Co 2p1=2) that comparewell with those reported for NiCo2O4 [3]. These spectra alsoreveal a relatively weak shake-up satellite at ca. 786 eVdue to paramagnetic divalent high-spin cobalt. It is worthnoticing that weak satellite lines in the Co 2p spectrum arealso present in a cobalt spinel oxide, i.e. Co3O4 but theposition of the peaks in the BE, scale are closer to those forNiCo2O4 (Table 1).Regarding oxygen, the O 1s spectra exhibit two contri-

butions. One at 529.7 eV and a shoulder at 531.5 eV. Theformer value can be assigned to lattice oxygen ions O2−

and the latter to surface oxygen, including adsorbed oxygenspecies as hydroxyl, carbonate groups, etc. With increas-ing sputter time the O2− contribution increases, while the


Fig. 3. XRD spectra corresponding to electrodes A and B di�erentRTPS times (curve b1=5 min, curve b2=60 min), v=0:05 V s−1).

surface oxygen contribution drops. In general, oxides of re-lated metals which have identical crystallographic structureshave very similar O 1 s binding energies.

3.1.3. X-ray di�ractionX-ray di�ractograms corresponding to electrodes A and

B are depicted in Fig. 3(a) and (b), respectively.The X-ray di�raction spectrum corresponding to elec-

trodes A (Fig. 3(a)), indicates that a well-crystallized phaseis formed on the Ni substrate. Some peaks may be readilyattributed to a spinel-type oxide, i.e., 2� = 36:7, 44.7 and59:7

◦[16]. Peaks corresponding to the Ni substrate are also

detected. Electrodeposition on Ni results on strong orienta-tion e�ects, thus the peak located at 44:7

◦, corresponding to

the (4 0 0) plane, shows the highest relative intensity. Somecharacteristic peaks are absent, probably due to the smallthickness of the deposit.Fig. 3(b) shows a di�ractogram corresponding to

NiCo2O4 prepared by thermal decomposition on Ni (elec-trode B). The peaks corresponding to the spinel phase maybe readily observed: (2� = 19, 36.2, 44.1, 59, 65, 77.9 and93

◦) together with peaks related to the Ni substrate. Again

the peak located at 2� ≈ 44:1◦exhibits the highest relative

intensity.The lattice parameter a0 was calculated through the

d-spacing of the plane 400 [3]. A value of a0 = 0:808 nmwas calculated for the oxides prepared by potentiodynamicdeposition (electrodes A). a0 = 0:819 nm was determined

Fig. 4. Polarisation plots for oxygen evolution in 1 M NaOH, 25◦C,

on electrodes A, B and metallic Ni.

for NiCo2O4 prepared by thermal decomposition on Ni(electrodes B).It has been reported that the incorporation of Ni into a

Co3O4 spinel oxide, causes an expansion of the elementaryspinel lattice [10]. In this sense the a0 values calculated forboth types of electrodes indicate a higher content of Ni inelectrodes B.

3.2. The oxygen evolution reaction

3.2.1. Polarisation plotsPotentiostatic measurements of electrodes A showed

enhanced OER currents with respect to the correspondingbare Ni substrate [5,6]. Moreover, OER current densities,evaluated in terms of geometric area in 1 M NaOH and25

◦C are higher than those reported for Co3O4 and NiCo2O4

�lms obtained by conventional methods and similar toTi=RuO2 + TiO2 [5].Fig. 4 shows polarisation plots corresponding to elec-

trodes A, B and clean Ni in 1 M NaOH, All polarisa-tion curves were corrected for ohmic drop e�ects, withresistance values obtained from impedance measurements.For electrodes A, a Tafel region of bt = 40 mV dec−1,is observed at relatively low OER current densities, i.e.j¡ 100 mA cm−2. An increase of the Tafel slope for higheroverpotentials is observed, together with a profuse evolutionof oxygen bubbles.bT ≈ 80 mV dec−1, may be calculated for electrodes B.

Tafel slope values ranging from 40 to 120 mV dec−1 havebeen reported, for thermally prepared cobaltites [9,10,17],this kinetic parameter being highly dependent on preparationprocedures.Fig. 5 shows log j vs. E (Hg=HgO=1 M OH−) plots cor-

responding to electrodes A in solutions with di�erent con-centrations of OH−. Tafel slopes of ca. 40 mV dec−1 wereobtained within the whole pH range studied in this work,indicating no changes in the reaction mechanism of the OER.


Fig. 5. Polarisation plots for oxygen evolution on electrodes A inNaOH solutions of di�erent concentration.

Fig. 6. log j vs. log [OH−] plots at 25◦C.

Plots of log j vs. log[OH−] recorded at E=0:63 and 0:61 V(vs. Hg=HgO=1M OH−) are depicted in Fig. 6. The reactionorder p was found to be very close to 2 (p ≈ 1:85).Reaction orders, with respect to OH−, close to 2, have also

been reported by Singh et al. [9], for NiCo2O4=Ti electrodesand by Nikolov et al. [10] for spinel cobaltites prepared bythermal decomposition.

3.2.2. EIS dataIn order to obtain mechanistic information about the OER,

impedance measurements were performed on electrodes A,in varying NaOH concentrations and within the potentialrange 0:5 V 6 E 6 0:7 V (vs. Hg=HgO=1 M OH−).Impedance data corresponding to electrodes A in 1 and 3

M NaOH are depicted in Figs. 7 and 8, respectively. Thesedata exhibit similar features as those reported for Co3O4prepared by thermal decomposition on Ni substrates [18].Two overlapped capacitive contributions may be detected,

one of them being related to the interface double-layer

Fig. 7. Nyquist and Bode plots corresponding to electrodes A in 1M NaOH. (a) 0.54 and (b) 0.6 V.

Fig. 8. Nyquist and Bode plots corresponding to electrodes A in 3M NaOH. (a) 0.47 and (b) 0.55 V.

capacitance Cdl, in parallel with the OER charge transferresistance, Rct [5], the other one describing the potentialdependence of the surface concentration, �, of some re-action intermediate in the OER process [5,18]. The Tafelslope bT calculated by bT = 2:303Rpi (where i is the currentdensity and Rp = lim!→0 Zf ) was bT = 40 mV dec−1 forE¡ 0:7 V in accordance with steady-state data.

4. Discussion

The kinetics of the OER, on electrodes A, in the pH rangeanalysed in the present work, is characterised by a Tafelslope of 40 mV dec−1, at low overpotentials, and a reactionorder with respect OH− close to 2. EIS spectra exhibit only


one capacitive time constant related to the OER, this capac-itive loop being related to the potential-dependent surfacecoverage of only one adsorbed intermediate.As previously indicated in the literature, for these types

of deposits, it is probable that tetravalent oxidation states ofCo and=or Ni metal ions, are involved as intermediate states(mediator sites), in the OER, together with OH and other Ospecies at the reaction interphase [5,9,18].A simpli�ed reaction scheme can be put forward as

follows:

Surface oxidation, previous to the OER, gives rise toCo(IV) and Ni(IV) surface states

Co(III)sCo(IV)s+e− or Ni(III)sNi(IV)s+e−;(R1)

where Co(III)s, etc., denote atoms at the oxide surface.Then the OER can proceed through

Co(IV)s + OH− Co(IV)(OH−)ad Co(III)(OH)ad

Co(IV)(OH)ad + e−:

(Ni(IV) species may participate in the same way as Co(IV).)This series of very reversible steps can be written as

Co(IV)s + OH− Co(IV)(OH)ad + e

−; (R2)

followed by

Co(IV)(OH)ad + OH− → Co(IV)s

+1=2 O2 + H2O + e−: (rds) (R3)

In process (R3) the charge transfer rds is probably fol-lowed by other fast steps leading to O2 evolution. The mech-anism represented by processes (R2) and (R3) may explainthe value of bt =40 mV dec−1, and the reaction order p=2with respect to OH−.Assuming that the adsorption–desorption of the inter-

mediate species, Co(IV)(OH)ad, follows a Langmuir-typeisotherm, the fundamental equations relating charge andmass balances are given by the following IF(t) and g(t)functions:

IF(t) = 2F(k2[OH−](1− �)− k−2� + k3�[OH−]); (1)

g(t) = �max d�=dt = k2[OH−](1− �)

−k−2�− k3�[OH−]; (2)

where ki = koi exp(biE); E corresponds to the appliedpotential, bi=(1−�i)F=RT and b−i=−�i F=RT; �=�=�maxis the fractional coverage of the adsorbed species (� isthe number of covered electroactive sites and �max is thesaturation number of electroactive sites).

4.1. Steady-state conditions

In this case, d�=dt = 0:

� s =k2[OH−]

k2[OH−] + k−2 + k3[OH−]: (3)

For low overpotentials k−2/k2[OH−] and k−2/k3[OH−];so �s.1 and

I sF =4Fko2k

o3 [OH

−]2

ko−2exp[(b2 + b3 − b−2)Es]: (4)

For �i = 0:5 and T = 298 K, Eq. (4) predicts a Tafel slope,bT = 40 mV dec−1, and a reaction order, p = 2, with re-spect to OH−, in accordance with experimental results.At higher overpotentials the more general Eq. (1) must beused.

4.2. Faradaic impedance analysis

Taking into account that IF = IF(E; �); the faradaicimpedance ZF may be calculated from Eqs. (1) and (2)after a Taylor series expansion limited to the �rst-orderterms and subsequent Fourier transform (14):

1ZF=�IF(!)�E(!)

=(@IF@E

)Es ;�s

+(@IF@�

)Es ;�s

��(!)�E(!)

; (5)

�max j!��(!)�E(!)

=(@g@E

)Es ;�s

+(@g@�

)Es ;�s

��(!)�E(!)

: (6)

So from Eqs. (5) and (6):

1ZF=(@IF@E

)+

(@IF=@�)(@g=@E)(�max j!− (@g=@�)) =

1Rct

+A

(�max j!+ B); (7)

where

Rct =1

2F�((k3[OH−] + k−2)b2 − k−2b−2 + k3[OH−]b3);

A= 2F�(−k2[OH−]− k−2 + k3[OH−])[(k3[OH−]

+k−2)b2 − k−2b−2 − k3[OH−]b3];

B = k2[OH−] + k−2 + k3[OH

−]:

Considering the double-layer capacitance Cdl in parallelconnection with Zf the interfacial impedance ZT can be writ-ten as1ZT=1Zf+ j!Cdl: (8)

Experimental steady-state and impedance data, corre-sponding to electrodes A in 1 and 3 M NaOH, were simu-lated in terms of Eqs. (1) and (8), respectively. Results ofthe corresponding simulation routines for the steady-statepolarisation plots and impedance diagrams are shown in


Fig. 9. Experimental (�) and simulated (O) steady-state andimpedance plots corresponding to electrodes A in 1 M NaOH.

Fig. 10. Experimental (�) and simulated (O) steady-state andimpedance plots corresponding to electrodes A in 3 M NaOH.

Table 2Parameters used for the simulated curves of Figs. 9 and 10. Kinetic constants are expressed per unit of active area

[OH−] Cdl Areal �max Ak k02 k0−2 k03 b2 b−2 b3(Farad) (cm2) (mol) (cm2) (cm s−1) (mol s−1 cm−2) (cm s−1) (V−1) (V−1) (V−1)

1 M 2× 10−4 3 3× 10−9 1.64 2:7× 10−16 1:54× 10−4 9:7× 10−16 26.7 −12:2 26.93 M 5× 10−16 3:3× 10−4 3× 10−15

Figs. 9 and 10. Optimum �t parameters are assembledin Table 2. The real area, Ar , was calculated from thebest �t values for Cdl, assuming a double-layer capaci-tance of Codl = 60 �F cm−2 for a smooth electrode [10].A roughness factor of R = 30 was determined by thisprocedure.In accordance with Nikolov et al. [10], the charge in-

volved in the overlapped anodic peaks, previous to theoxygen evolution potential (Fig. 1(a)), Qap, was usedto evaluate the active area, Ak , with the assumption that inthis potential range, Co ions (and also Ni ions) are oxi-dized to Co(IV) giving rise to the electroactive sites for theOER, i.e.

Ak = NQap=SeF;

where Se is the number of oxygen evolution sites per unitarea, i.e. 1:1 × 1015 sites cm−2 [19], N the Avogadro’snumber and F is the Faraday constant.The parameter �max corresponds to

�max = SeAk=N:

A good agreement between experimental and simulateddata is observed, for steady-state and impedance data. In thecase of 3 M NaOH the values of the kinetic constants aresomewhat higher than expected.For electrodes B, real area, Ar , values were deter-

mined from high-frequency impedance data (1×102 Hz¿f¿ 1 × 105 Hz). Experimental high-frequency data, corre-sponding to these rough surfaces, are best described in termsof a series (RQ) circuit according to the transfer function:

Z = R + (i!)−(1−�)Q;

where R is the electrolyte resistance and Q is a constantrepresentative for a constant phase element (CPE) [20]. Cdlvalues may be derived from the best-�t values for Q and �,as described by Brug et al. [20]:

C1−�dl = Q−1R�:

Accordingly, Cdl = 0:3 F was calculated for electrode B.Assuming, Codl =60×10−6 F cm−2, for a smooth electrode,then Ar ≈ 5000 cm2 may be calculated, this corresponds toa roughness factor R=2270. Active area values were deter-mined from the charge involved in the second anodic peak(Fig. 1b), as described above. In this case, Ak = 1700 cm2,was calculated.


5. Conclusions

Co-oxide electrodes, prepared by potentiodynamic depo-sition on Ni substrates (electrodes A), were characterisedby in situ electrochemical methods and by XPS and XRDtechniques.XPS spectra indicate the existence of a cobaltite

spinel oxide. The intercalation of Ni(III) into thecobaltite matrix leads to an inhomogeneous distribu-tion of these cations. The electrodeposited material canbe described as a Co3O4 matrix which su�ers a par-tial substitution of Co ions for Ni ions. The sameconclusion can be drawn from the voltammetric pro-�les after comparison with literature data for electrodescoated with NixCo3−xO4. Finally, the X-ray di�ractionpattern also con�rmed the existence of a spinel-typeoxide.The O2 evolution kinetics on electrodes A in alkaline so-

lutions was studied by means of stationary measurementsand ac impedance. The results indicate that the OER is char-acterised by a multi-step reaction pathway involving at leastone intermediate species adsorbed at the oxide surface. Inturn, the oxidation state of the surface region of the electro-deposit governs its catalytic behaviour.Tafel slopes close to 40 mV per decade and a re-

action order with respect to [OH−] equal to 2 are themain features of the reaction mechanism. Theoretical ex-pressions were derived for the steady-state case and forthe faradaic impedance according to a proposed reac-tion model, which describes properly the experimentaldata.The roughness factors and active area values determined

for electrodes B are close to 2 orders higher than those cal-culated for electrodes A, accordingly, the enhanced OERcurrent densities measured with electrodes A in comparisonwith thermally prepared oxides (electrodes B) may be re-lated to a true electrocatalytic e�ect. The interpretation ofthis catalytic e�ect requires a comparative analysis of thekinetic parameters related to the OER on electrodes A, Band clean Ni.Perhaps the most relevant feature of the impedance anal-

ysis, carried out in this work, is the successful determinationof real surface areas.

Acknowledgements

Grateful acknowledgment is made to the Consejo Na-cional de Investigaciones Cient��cas y T�ecnicas (CONCET)and to the Comisi�on Nacional de Investigaciones Cient��cas(CIC) for �nancial support of this work.

References

[1] Wendt H, Plzak V. Electrochim Acta 1988;28:27.[2] Haenen J, Visscher W, Barendrecht E. J Electroanal Chem

1986;208:273.[3] Haenen J, Visscher W, Barendrecht E. J Electroanal Chem

1986;208:297.[4] Haenen J, Visscher W, Barendrecht E. J Electroanal Chem

1986;208:323.[5] Castro EB, Gervasi CA, Vilche JR. J Appl Electrochem

1998;28:835.[6] Castor EB, Gervasi CA. Electrodeposited cobalt oxides for

alkalyne water electrolysis. In: Bolcich JC, Veziroglu TN,editors. Hydrogen energy progress XII. Editoraial Asociaci�onArgentina de Hidr�ogeno, 1998, p. 767.

[7] Chen Y-WD, Nou� RN. J Electrochem Soc 1984;131:1447.[8] Brossard L. J Appl Electrochem 1991;21:612.[9] Singh RN, Koenig J-F, Poillerat G, Chartier P. J Electrochem

Soc 1990;137:1408.[10] Nikolov X, Darkaoui R, Zhecheva E, Stoyanova R, Dimitrov

N, Vitanov T. J Electroanal Chem 1997;429:157.[11] Haenen JD, Visscher W, Barendrecht E. J Appl Electrochem

1985;15:29.[12] Nkeng P, Poillerat G, Koenig JF, Chartier P, Lefez B, Lopitaux

J, Lenglet M. J Electrochem Soc 1995;142:1777.[13] Belova ID, Roginskaya YuE, Shifrina RR, Gagarin SG,

Plekhanov YuV, Venevtsev YuN. Solid State Commun1983;47:577.

[14] Gennero de Chialvo MR, Chialvo AC. Electrochim Acta1993;38:2247.

[15] Baronetto D, Kodintsev IM, Trasatti S. J Appl Electrochem1994;24:189.

[16] JCPDS:ICDD �les Nos. 43-1003 and 20-0781M.[17] Singh RN, Hamdani M, Koenig JF, Poillerat G, Gautier JL,

Chartier P. J Appl Electrochem 1990;20:442.[18] Conway BE, Liu TC. Ber Bunsenges Phys Chem 1987;91:461.[19] Pope P, Walker D, Whalley L, Moss RC. J Catal 1973;31:335.[20] Brug GJ, Van den Eeden ALG, Sluyters-Rehbach M, Sluyters

JH. J Electroanal Chem 1984;176:275.

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Electrodeposited Ni–Co-oxide electrodes:characterization and kinetics of the oxygen evolution reaction