Electrochemical Characterization of DSA®-Type ElectrodesUsing Niobium Substrate

Juliane C. Forti & Josimar Ribeiro &

Marcos R. V. Lanza & Adalgisa R. de Andrade &

Rodnei Bertazzoli

Published online: 3 July 2010# Springer 2010

Abstract This paper describes the electrochemical charac-terization of DSA®-type electrodes using niobium sub-strate, and the results were compared with traditionalDSA®-type oxide electrodes, i.e., using titanium substrate.The surface morphology, electrocatalytic activity, andstability of the coating were investigated by scanningelectron microscopy, energy dispersive X-ray spectrometry,cyclic voltammetry, electrochemical impedance spectrosco-py (EIS), and lifetime tests. EIS measurements wererecorded at a constant potential between 0.2 and 1.0 V vsAg/AgCl, in the frequency range of 5 mHz to 100 kHz,using the “single sine” method and a sine wave amplitudeof 5 mV (p/p). After testing a number of differentequivalent circuits, we found that the whole set of data inthe double layer domain of the electrodes can be fitted byassuming the circuits Rs(CPEfRf)(CdlRct), Rs(CPEfRf)(CPEdlRct), and RsCPEfG(CPEdlRct). The results suggestthe formation of a less conducting film on the Nb substratewhen compared to Ti substrate. The findings of this work,such as difficult adherence of coating on niobium, reductionof voltammetric charge, and short lifetime of electrodes

prepared on Nb substrate, suggest that the substitution oftitanium by niobium is unfeasible.

Keywords Niobium . Titanium . Dimensionally stableelectrodes . Oxide films . Impedance spectroscopy

Introduction

DSA®-type electrodes show good technological perfor-mance, and this success is due to desirable features such ashigh stability of the active coating, good overall perfor-mance under mild conditions, high conductivity, low cost,and commercial availability. Ceramic coatings are presentin a wide range of electrochemical applications, e.g., thechlor-alkali industry [1], oxygen production [2, 3], organicsynthesis applications [4–6], and wastewater treatment [7–10].

The electrochemical properties of thermally preparedoxides are highly dependent on the conditions of preparationand the chemical composition of the coating [11, 12]. Suitablemixtures are designed to optimize desirable characteristics atthe lowest possible cost. One of the advantages of usingDSA® is the geometry obtained in the construction ofindustrial cells [13]. The use of metallic titanium as supporthas enabled the construction of electrodes with differentshapes, allowing the gas produced to be released from theelectrochemical cell without completely blocking the surfaceof the electrodes [13–15]. Improved cell geometry hassuccessfully reduced the ohmic drop caused by saturationwith gas bubbles. The improved stability of DSA® electro-des has reduced the downtime of industrial cells formaintenance, diminishing operational costs [16].

Because of the formation of an isolating TiO2 oxidelayer originating from the base titanium, intensive research

J. C. Forti :M. R. V. Lanza : R. BertazzoliDepartamento de Engenharia de Materiais, Faculdade deEngenharia Mecânica, Universidade Estadual de Campinas,C. P. 6122, 13083-970 Campinas, São Paulo, Brazil

J. C. Forti (*) :A. R. de AndradeDepartamento de Química, Faculdade de Filosofia Ciências eLetras de Ribeirão Preto, Universidade de São Paulo,14040-901 Ribeirão Preto, São Paulo, Brazile-mail: juforti@gmail.com

J. RibeiroDepartamento de Química, Centro de Ciências Exatas,Universidade Federal do Espírito Santo,29075-910 Vitória, Espírito Santo, Brazil

Electrocatal (2010) 1:129–138DOI 10.1007/s12678-010-0020-3

(A) Ti/TiO2 (B) Ti/DSA®

(C) Nb/Nb2O5 (D) Nb/ DSA

(E) Nb/Pt/DSA

®

®

Fig. 1 SEM images of a Ti/TiO2, b Ti/DSA®, c Nb/Nb2O5, d Nb/DSA®, and e Nb/Pt/DSA®

130 Electrocatal (2010) 1:129–138

has focused on alternative substrates to increase the lifetimeof electrodes [17]. Tantalum as a substrate has theadvantage of being much less resistive (13 μΩ cm) thantitanium (42 μΩ cm) and of being highly corrosionresistant. However, its use as a support is economicallyunfeasible [17].

The electrical conductivity of niobium is higher than thatof titanium, and the final result of the substrate/coating setdepends on the resistivity of the oxide that is formedspontaneously after substrate treatment [18]. In view ofthese facts and since Brazil is the worldwide leader in theglobal niobium market [19], studies that investigate the useof this material as a substrate for DSA®-type electrodes arehighly relevant.

Experimental

Rectangular plates of titanium or niobium (1.6 cm2) wereused as substrate. These plates were sandblasted to increasetheir roughness in order to improve the adherence of oxidefilms. Before applying the coating, the titanium plates werewashed in a hot solution of oxalic acid, and the niobiumplates were chemically stripped for 60 s in 40% HF at roomtemperature [20]. Both plates were rinsed with pure waterand dried.

For purposes of comparison, the titanium and niobiumplates were held at 200 °C for 30 min to obtain Ti/TiO2 andNb/Nb2O5 electrodes. The electrodes with DSA® coating(Ru0.3Ti0.7O2) were prepared by standard thermal decom-position (tcalcination 400 °C). Briefly, the precursor solution(0.2 mol L−1 solutions of RuCl3 (Aldrich) and TiCl3 10% inHCl (Aldrich)) was dissolved in HCl/H2O 1:1 (v/v) and wasapplied by brushing upon one side of the titanium orniobium substrate. The resulting material was thermallydecomposed at 400 °C for 5 min in a preheated oven,followed by cooling. This procedure was repeated until thedesired oxide thickness was achieved (20 μm). The finalmaterial was post-treated for 1 h at 400 °C.

Another electrode composition studied here was Nb/Pt/DSA®, which presented a thin layer of deposited Pt beforethe coating was applied. Platinum was electrodepositedusing a controlled current (30 mA) for 15 min fromH2PtCl6 solution dissolved in 0.5 mol L−1 H2SO4.

The surface morphology and elemental composition ofthe deposited oxide films were analyzed by scanningelectron microcopy (SEM) and energy dispersive X-rayspectroscopy (EDS), using a Leica-Zeiss LEO model 440SEM coupled to an Oxford model 7060 analyzer.

A three-electrode arrangement was used in the electro-chemical cell (one compartment, 100 mL) with the workingelectrode, a platinum foil served as the counter electrode,and Ag/AgCl (KCl sat.) was used as the reference

electrode. The supporting electrolyte used throughout theelectrochemical experiments was 0.1 mol L−1 H2SO4.

Electrochemical experiments were carried out withAUTOLAB model PGSTAT30 (GPES/FRA) instrumenta-tion. Voltammetric curves were recorded in the potentialrange of 0.2–1.4 V vs Ag/AgCl at a 50 mV s−1 scan rate.Impedance spectra were recorded at a constant potentialbetween 0.2 and 1.0 V vs Ag/AgCl. Electrochemicalimpedance spectroscopy (EIS) measurements were obtainedin the 5 mHz–100 kHz frequency interval, using the “singlesine” method and a sine wave amplitude of 5 mV (p/p). AnAUTOLAB software program (FRA analyzer—version 4.9)was used to analyze the impedance data. The lifetime (LT)scale of the working electrode was defined under agalvanostatic condition (400 mA cm−2).

Results and Discussion

SEM and EDS Analyses

The morphology of oxide layers is highly dependent on thepreparation procedure, the physicochemical properties ofthe oxides, and the nature of the precursors. Figure 1 showsrepresentative SEM micrographs of the thin films prepared.

The micrographs of the Ti/TiO2 (a) and Ti/DSA® (b)electrodes show structures of apparent roughness and a fewcracks, which is typical for this type of electrode, accordingto the literature. Figure 1c reveals that the Nb/Nb2O5

electrode presents fewer cracks than the others. The micro-graphs of the Nb/DSA® (d) and Nb/Pt/DSA® (e) electrodesindicate a distinct morphology for Ti/DSA®. Note thepresence of some plates with apparently smoother surfaces.

Table 1 shows EDS analyses of the micrographs inFig. 1. These analyses were performed on random areas of

Table 1 EDS analyses of the electrodes

Nominal composition Experimental composition

Ti/TiO2 30.31% of Ti

69.69% of Ti combined with O2

Ti/Ru0.3Ti0.7O2 20.23% of Ru

79.77% of Ti

Nb/Nb2O5 25.64% of Nb

74.36% of Nb combined with O2

Nb/Ru0.3Ti0.7O2 13.40% of Ru

19.95% of Ti

66.65% of Nb

Nb/Pt/Ru0.3Ti0.7O2 12.15% of Ru

16.76% of Ti

2.3% of Pt

68.79% of Nb

Electrocatal (2010) 1:129–138 131

the film (average of three analyses). The electrodes withnominal compositions of Ti/TiO2 and Nb/Nb2O5 weresubjected to tests of the specific element and of thiselement combined with oxygen to verify the formation ofoxide. Substantial oxide was formed, especially in the caseof films deposited on niobium substrate.

The EDS analysis of the Ti/DSA® electrode indicated agood correlation between the experimental and nominalcomposition, which was not the case with the Nb/DSA®electrode. A significant increase in the Nb content wasobtained, corresponding to the growth of a non-conductivelayer (Nb2O5) between the substrate and the oxide coating.The Nb/Pt/DSA® electrode presented similar results tothose of the Nb/DSA® electrode.

Cyclic Voltammetry

As expected, the Ti/TiO2 and Nb/Nb2O5 electrodes pre-sented a resistive cyclic voltammetry (CV) with lowercurrent values. Figure 2 shows the CVof the Nb/Nb2O5 andNb/DSA® electrodes. Without the deposition of the oxidelayer, the voltammogram presented almost no currentresponse; however, the voltammetric charge increased withthe deposition of the layer, indicating the efficiency of thispreparation method.

Figure 3 shows the i/E curve obtained during cyclicvoltammetric experiments with Ti/DSA®, Nb/DSA®, andNb/Pt/DSA® electrode compositions. This profile is typicalof thermally prepared oxide layer electrodes [9, 21] and ischaracteristic of the DSA® electrodes. The figure shows ablurred peak at around 0.5 V vs Ag/AgCl associated with Ru(III)/Ru(IV) redox transition (2RuO2+2H

++2e⇆Ru2O3+H2O) [22]. The broadness of the peaks can be interpretedas indicative of a considerable heterogeneity in the surfacesites, as proposed by Trasatti and Kurwell [23]. All thecompositions studied here showed oxygen evolution reaction(OER) beginning around at 1.2 V vs Ag/AgCl.

The voltammetric charge, which is related to the realsurface area, was strongly affected by the support material.The lower current of the Nb/DSA® electrode than that ofthe Ti/DSA® was due to the higher oxygen content in thesample, as revealed by the EDS analysis, indicating thatNb2O5 was formed upon the clean Nb substrate. Over time,this may impair the electrode’s activity, since the Nb2O5

layer causes passivation of the electrode. The Nb/Pt/DSA®electrode displayed the same characteristics.

The voltammetric charge furnishes an estimation of theoverall catalytic activity of the material and reflects both themorphological and catalytic effects [22, 24]. However, tocompare different material compositions is important toeliminate the contribution due to changes in the surfacearea. This can be done by dividing the faradaic voltam-metric current for the OER by the voltammetric charge(q*). Table 2 shows the normalized current (iOER) measure

0.0 0.2 0.4 0.6 0.8 1.0 1.2

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2I /

mA

E / V (Ag/AgCl)

Nb/Nb2O5 Nb/DSA

Fig. 2 Cyclic voltammograms of (dashed line) Nb/Nb2O5 and (solidline) Nb/DSA® electrodes in 0.1 mol L−1 H2SO4, ν=50 mV s–1

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

-6

-3

0

3

6

9

12

15

I / m

A

E / V (Ag/AgCl)

Ti/DSA

Nb/DSA

Nb/Pt/DSA

Fig. 3 Cyclic voltammograms of Ti/DSA® (solid line), Nb/DSA®(dashed line), and Nb/Pt/DSA® (dotted line) electrodes in 0.1 mol L−1

H2SO4, ν=50 mV s−1

Fig. 4 Nyquist diagrams as a function of the applied potential. 0.2 V(plus sign), 0.4 V (empty triangle), 0.6 V (filled star), 0.8 V (emptycircle), and 1.0 V (filled square) vs Ag/AgCl. Electrodes: a Ti/TiO2, bTi/DSA®, c Nb/Nb2O5, d Nb/DSA®, and e Nb/Pt/DSA®

�

Table 2 Faradaic voltammetric current for the OER (iOER) measuredat 1.25 V vs Ag/AgCl, voltammetric charge (q*), and catalytic activity(iOER/q*) of the electrodes

iOER (mA) q* (mC) iOER/q* (mA/mC)

Ti/DSA® 7.5 4.5 1.67

Nb/DSA® 1.5 0.71 2.11

Nb/Pt/DSA® 0.6 0.35 1.71

132 Electrocatal (2010) 1:129–138

Ti_DSA

D

0 50 100 150 200 250 300 350 400

0

50

100

150

200

250

300

350

400

Nb_Pt_DSA

0.2 V 0.4 V 0.6 V 0.8 V 1.0 V

E

A B

C

0

50

100

150

200

-Z''

/ Ohm

0.2 V 0.4 V 0.6 V 0.8 V 1.0 V

-Z''

/ Ohm

0

200

400

600

800Nb_DSA

0 200 400 600 800

-Z' / Ohm

0.2 V 0.4 V 0.6 V 0.8 V 1.0 V

0 50 100 150 200

-Z' / Ohm

Z' / Ohm

-Z''

/ Ohm

Electrocatal (2010) 1:129–138 133

at 1.25 V vs Ag/AgCl. The catalytic activity is higher forNb/DSA® electrode than the Ti/DSA®. This findingcorroborates with the finding reported before, whichshowed that the introduction of Nb2O5 increases theactivity for OER [25].

Electrochemical Impedance Spectroscopy

The AC impedance behavior of the freshly preparedelectrodes was investigated to further characterize thedifferent Ti/TiO2, Nb/Nb2O5, Ti/DSA®, Nb/DSA®, andNb/Pt/DSA® systems. The Nyquist diagram (Z′ vs −Z″) ofthe electrodes obtained between 0.2 and 1.0 V vs Ag/AgClis shown in Fig. 4. In the low frequency domain, the Ti/TiO2 (a) and Nb/Nb2O5 (c) electrodes exhibited a well-defined semicircle, which was attributed to the oxide/solution interface. The electrode mixture containingRuO2–TiO2 (Fig. 4b, d, and e) displays a straight line inthe high frequency domain, followed by a deformedsemicircle in the low frequency domain. This behaviorcan be explained by the fact that the charge transferresistance related to the reaction RuOx(OH)y+δH

++δe−⇋RuOx−δ(OH)y+δ 0≤δ≤2 is high, and the magnitude of thetotal capacitance increases due to the pseudocapacitivebehavior of the interface [26]. Recent studies have shownthat the formation of a straight line parallel to Z″ in RuO2-rich materials is simply indicative of an ideal metalelectrode, characteristic of an ideally polarizable electrode,and a slight deviation from the straight line along the Z″suggests a non-ideally polarizable electrode [27, 28]. Theshift from the ideal capacitor behavior is a consequence ofthe material’s porosity [29].

The Bode plot (θ vs log f) obtained at 0.6 V vs Ag/AgClfor electrodes of different compositions is shown in Fig. 5.

An analysis of this figure indicates that the main feature ofthese electrodes is the appearance of a well-defined timeconstant (τ) for the Ti/TiO2 and Nb/Nb2O5 electrodes,which is characterized by a maximum phase angle (65–77°)ranging from 10 to 100 Hz. Such behavior has also beenreported for TiO2 electrodes with low Ru loads and is theresult of the interposition of various time constants withrespect to frequency [26]. The electrodes containing DSA®material presented the same behavior; however, the maxi-mum phase angle (53–61°) ranged from 0.01 to 1 Hz. Thisbehavior can be ascribed to the large number of RuO2

transition states contributing to the charging system [26,30].

After testing a number of different equivalent circuits,we found that the whole set of data in the double layerdomain of the electrodes can be fitted by assuming thecircuits presented in Fig. 6(a, b), i.e., Rs(CPEfRf)(CdlRct)and Rs(CPEfRf)(CPEdlRct). At low frequencies, the filmcharging process involves mainly the oxide/solution inter-face. The (CdlRct) or (CPEdlRct) components introduced inthe scheme represent the double layer process (Cdl orCPEdl) coupled to the charge transfer resistance (Rct). Theuse of a CPE (constant phase element) in the equivalentcircuit notation instead of a C (pure capacitor) is due to thehigh porosity and degree of roughness of the oxide layers,which contribute to the film’s inhomogeneity [31]. As thefrequency is enhanced, the response of the electrode isgoverned by the metal/inner oxide region interface. Thecharge process in the high frequency domain (CPEfRf)takes into account how the film resistance (Rf) andcapacitance affect the film oxide (CPEf).

For the Nb/DSA® electrodes, the best fit was obtainedusing another circuit RsCPEfG(CPEdlRct) (Fig. 6c). The newelement, G, was introduced into the circuit to simulate

Fig. 5 Bode plot as a function of the electrodes with nominalcomposition at 0.6 V vs Ag/AgCl

Fig. 6 Equivalent circuit (EC) used to fit the experimental resultspresented in Fig. 4. a Ti/TiO2 and Nb/Nb2O5, b Ti/DSA® and Nb/Pt/DSA®, and c Nb/DSA® electrodes

134 Electrocatal (2010) 1:129–138

experimental data. The Gerischer impedance, G,ðG ¼ Goðkaþ iwÞ�1=2, where ka represents the effectivetransfer rate of the chemical reaction [32–34], can beexplained easily for bulk processes in mixed conductingmaterials, by the formation of immobile complexes to oneof the mobile species [32].

Figure 7 shows the film resistance, Rf, as a function ofthe electrode composition and applied potential. The resultsuggests that a poor conducting film was formed on the Nb/Nb2O5 and Nb/Pt/DSA® electrodes. Charge carrier mobilitythrough the film increased when Pt was introduced into thecoating (Nb/Pt/DSA®). On the other hand, the Rf observedfor the Ti/TiO2 and Ti/DSA® electrodes are at least twofoldlower, and it ranges from 157 to 287 Ω as the potentialincreases. The potential dependence observed here may beattributed to the tunneling resistance associated with thehigh-energy barrier that is necessary for the electron to go

through the metal/oxide interface [35]. The Rs valuesobtained by simulation using the EC shown in Fig. 6ranged from 5 to 8 Ω and did not change with the electrodecomposition.

The Rct values changes with the electrode composition.For example, the Ti and Ti/DSA® electrodes showed lowerRct values than the Nb/Nb2O5 and Nb/DSA® electrodes. On

0.2 0.4 0.6 0.8 1.00.0

0.2

0.4

0.6

0.8

1.0

10

20

30

40

50

A

Cdl /

mF

CP

Edl /

mF

s-n

E vs. Ag/AgCl / V

Ti Ti_DSA Nb Nb_DSA Nb_Pt_DSA

0.2 0.4 0.6 0.8 1.00.0

0.2

0.4

0.6

0.8

20

40

60

Ti Ti_DSA Nb Nb_DSA Nb_Pt_DSA

B

CP

Efil

m /

mF

s-n

E vs. Ag/AgCl / V

Fig. 8 a Cdl or CPEdl and b CPEfilm as a function of the potentialsapplied to the different electrodes

Table 3 Lifetime (LT) test of the electrodes evaluated by theapplication of 400 mA cm−2 in a medium of 0.1 mol L−1 H2SO4

Electrodes E initial (V) E final (V) LT

Ti/DSA® 5.6 6.0 23 h

Nb/DSA® 7.6 13.5 20 min

Nb/Pt/DSA® 7.1 13.1 32 min

0.2 0.4 0.6 0.8 1.0

200

400

600

800

1000

1200

1400

A

Rfil

m /

Ohm

E vs. Ag/AgCl / V

Ti Ti_DSA Nb Nb_Pt_DSA

0.2 0.4 0.6 0.8 1.0

0

20

500

1000

1500

2000

B

Rct /

Ohm

E vs. Ag/AgCl / V

Ti Ti_DSA Nb Nb_DSA Nb_Pt_DSA

Fig. 7 The Rfilm (a) and Rct (b) as a function of the potentials appliedto the different electrodes

Electrocatal (2010) 1:129–138 135

the other hand, the DSA® coating applied upon the Nbsubstrate caused the Rct to decrease.

Figure 8 shows the Cdl or CPEdl (a) and CPEfilm (b) as afunction of the potentials applied to the different electrodes.Note that the electrodes with DSA® coating presented atleast fivefold lower capacitance values than those withoutDSA®. This behavior can be explained by the double layercharge allied to the faradic process, i.e., the reversibleexchange of charge/protons with the active ruthenium sites[353].

Lifetime

The stability of an electrode in response to any reactionalprocess is often defined as its ability to keep the potentialconstant over a long period of time. On a laboratory scale,the electrode’s stability is evaluated based on a lifetime test,which involves subjecting the electrode to experimentalconditions that lead to its destruction. The electrodes’stability was evaluated by applying 400 mA cm−2 in a 0.1-mol L−1 H2SO4 medium. The useful life of the Ti/DSA®electrode was considered the time required to reach 6 V.Therefore, above this potential value, the electrode isconsidered inactive to OER due to the total destruction ofthe oxide layer or the formation of an isolating oxide layer,such as TiO2, for example [36]. In the case of the Nb/DSA®and Nb/Pt/DSA® electrodes, the initial potential was 7.0 Vvs Ag/AgCl, so a cutoff potential could not be determined.The following procedure was therefore adopted: the LT ofthe Ti/DSA® electrode was analyzed, and according to thetime obtained, the Nb/DSA® and Nb/Pt/DSA® electrodeswere analyzed, adopting as cutoff potential the LT obtainedfor the Ti/DSA® electrode.

The measured LT values (average of two sets ofelectrodes) are listed in Table 3. A significant decreasewas observed in the LT of electrodes supported on Nb, dueto the growth of a non-conductive layer (Nb2O5) betweenthe substrate and the oxide coating.

(A) Ti/DSA

(B) Nb/ DSA

(C) Nb/Pt/ DSA

®

®

®

Fig. 9 SEM images after lifetime (LT) tests of a Ti/DSA®, b Nb/DSA®, and c Nb/Pt/DSA®

Table 4 EDS analyses after LT test

Nominal composition Experimental composition

Ti/Ru0.3Ti0.7O2 4.41% of Ru

95.59% of Ti

Nb/Ru0.3Ti0.7O2 7.33% of Ru

11.85% of Ti

80.82% of Nb

Nb/Pt/Ru0.3Ti0.7O2 8.25% of Ru

12.45% of Ti

1.1% of Pt

78.2% of Nb

136 Electrocatal (2010) 1:129–138

These electrodes were analyzed after the LT test, usingthe SEM and EDS techniques. Figure 9 shows themorphological changes in the electrodes after beingsubjected to the drastic conditions of this experiment. Notethe worn structures with some erosion of the active layer,mainly in the electrodes supported on Nb. The EDS results(Table 4) revealed a decrease in the quantity of Ru,confirming the loss of the catalytic agent from the electrodecomposition.

The LT of electrode oxides is correlated directly withtwo factors. The first factor is passivation due topenetration of the electrolyte through the pores or crackstowards the substrate, resulting in the oxidation of themetallic support and forming a non-conductive layerbetween the substrate and the oxide coating [37–39].The second factor is dissolution of the coating, whichinvolves loss of electroactive material (erosion or dissolu-tion) and results in a gradual reduction of the voltammetriccharge. This may occur due to the pores in the layer and therapid evolution of gas on the surface, inducing theseparation of weakly bound parts of the active layer [37,40, 41].

During the LT experiments, the electrode potential wasrecorded as a function of time, and the voltammetriccharge (q*) was measured by recording the CV curves atregular time intervals. The q* value was found to remainconstant up to 85% of the electrode’s LT, suggesting thatthe deactivation is caused by the presence of a non-conductive layer between the substrate and the oxidecoating. Therefore, the instability of the Nb/DSA andNb/Pt/DSA electrodes is caused by the growth of a thin,highly resistive Nb2O5 film (see EIS results) in the earlystages of preparation. Moreover, the EDS analysis alsoindicated that there was loss of the active layer after the LTtest.

Conclusions

We have shown that the equivalent circuits that match ourexperimental data in terms of impedance measurements areRs(CPEfRf)(CdlRct), Rs(CPEfRf)(CPEdlRct), and RsCPEfG(CPEdlRct). In the range of potentials investigated here,the behavior of impedance in the low frequency domainwas attributed mainly to the system’s capacitive behavior.Moreover, our impedance results showed that the electrodesprepared on Ti substrate presented less resistance than thoseon Nb. This characteristic is probably due to theformation of a non-conductive Nb2O5 layer between thesubstrate and the oxide coating of the electrodes onniobium substrate. The findings of this work, such asdifficult adherence of the coating on niobium, reduction ofcharge voltammetry, and short lifetime of electrodes prepared

on Nb substrate, suggest that the substitution of titanium byniobium is unfeasible.

Acknowledgments The authors thank FAPESP and Carbocloro S.A.Indústrias Químicas for funding this work and the supply of niobium.

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