Investigation of charging, transport and reactionprocesses in Ti/1r02-Nb2O5 anodes prepared by a

sol-gel derived method

Ailton J. Terezol 3, Valterley A. Moura2, Mauro M. Costa2 and Ernesto C. Pereira31. Departamento de Fisica, Universidade Federal do Mato Grosso, CEP 78060-900, Cuiaba, MT, Brazil

2. Centro Federal de Educaq5o Tecnologica do Mato Grosso, CEP 78060-900, Cuiaba, MT, Brazil3. Laborat6rio Interdisciplinar de Eletroquimica e Ceramica, Universidade Federal de Sao Carlos, CEP 78060-900,

Sao Carlos, SP, Brazil

Abstract - In the present work we investigate the charging,transport and reaction processes in Ti/Ir02-Nb205 usingelectrochemical impedance spectroscopy (EIS). Theelectrochemical impedance response of the electrodes wasmodeled using the theory of porous electrodes. The spectrafor the electrodes in a wide range of composition haveshown the typical behavior of a porous structure,exhibiting a straight line with slope of 450 at highfrequency and a CPE in the low frequency domain,depending on the applied dc-potential. The spectra wereadjusted to the model and the impedance parameters wereextracted and analyzed as function of the electrodecomposition and applied dc voltage.

I. INTRODUCTION

Dimensionally stable anodes (DSA) containing IrO2 areexcellent electrodes for oxygen evolving reaction (OER).These electrodes have both electrochemical activity anddimensional stability, and are for these reasons intensivelyused in industrial applications[ 1]. In general Ir02-basedelectrodes are prepared by thermal decomposition of anappropriate precursor solution onto a metallic titaniumsupport, employing different methodologies[2]. Independentlyof the method, the so-called "mud-cracked" coating isformed. These coatings exhibit a polycrystalline and roughsurface, presenting microcracks and pores[2]. From atechnological point of view this morphologic configuration isdesirable since it implies a high surface area, for example.However, in the fundamental investigations and modelling theporous surface consists on an additional problem to beconsidered.The model used in this work emphasises the porosity as the

dominant characteristic in the electrochemical behaviour ofthe electrodes and, is based on de Levie model[3,4]. In thismodel one simplifies the inner region of the electrode, whichin the reality consists on a complex structure with two phases(the metal oxide and the electrolyte), to a single effectivemacrohomogeneous medium. Therefore a suitable averageover small volumes removes any spatial distinction betweenthe two phases, but at each value of the coordinate normal to

the electrode plane, the averaging procedure gives distinctvalues for the electrical potential and current in the twophases, so that a local overpotential can be defined as thedifference of potentials in the two phases. Then, obtaining acomplete model for the electrochemical behaviour of theelectrode is a matter of adopting suitable assumptions for thetransport in the two phases and the reaction at the surface.Here we assume that the transport is by drift currents, and thatreaction obeys a Tafel kinetics at the inner surface. Inaddition, the resistivity of the electrolyte is neglected withrespect to that of the metal oxide. On the basis of theseassumptions, the distribution of potential into the electroderegion can be viewed as the combination of an ohmic dropdistributed in the metal oxide phase and an overpotentialdistributed in the inner surface. The background for this typeof model is given in Refs.[3-6]Fig. 1 shows the transmission line representing the

distributed equivalent circuit proposed to describe theimpedance response of porous IrO2 electrode[5].

r, r, r, r, solid

rj1-;W-g r .r r r

solutiorssubstrate

Fig. 1. Schematic representation of the inner region (O < x < L) of a porouselectrode consisting on a porous metal oxide film deposited on a conducting

substrate and in contact with an electrolyte.

In the model of Fig. 1, the resistivity of the electrolyte isneglected and the resistance of electronic transport in the solidphase is described by a distributed resistance per unit length r,(corresponding to the whole electrode area), and the interfacialimpedance between the solution and the pore wall has theform a CPE (constant phase element) as in (1):

r3 (1)+ r3 q3 (iw)6

This CPE is a phenomenological description of complexpolarisation processes occurring at the inner solid/liquid

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interface, though the case /--1 gives the perfect capacitorwhich is normally associated with polarisation at the electricdouble layer. q3 is the pre-factor of the CPE and, denotes thecharging of the porous electrode and, r3 is the charge transferresistance at the inner surface.The impedance of that transmission line in Fig. 1 has the

form as in (2):

Z = ((r1)112 coth(L/I) (2)

where ,=(i-/r)I/2 is the penetration length of the ac

perturbation and L is the pore length.

II. EXPERIMETAL SECTION

The electrodes were prepared using a lOx 1 0xO.5 mmtitanium plate (99.7%) as metallic support. The precursorsolutions were prepared dissolving citric acid (CA) in ethyleneglycol (EG) at 60°C. At this temperature the precursor salts(PS) NH3H2[NbO(C204)34.H20 and IrCl4.nH2O was added.The molar ratio among the reactants was PS:CA:EG = 1:6:36.After that, the precursor solutions were painted over themetallic titanium and polymerised at 1300C for 30 minutes,followed by treatment at 250'C for 20 minutes, to improve theadhesion of the oxide. Finally, the electrodes were treated at4500C, for 10 minutes. This procedure was repeated 10 timesand 1.7 mg.cm-2 of the oxide loading was obtained. Thecalcination was carried out in static air atmosphere.Impedance measurements were carried out with an Autolab

potentiostat/ganvanostat model PGSTAT30 with frequencyresponse analyser FRA2 connected to a PC, which acquiresthe data and controls the equipment. Impedance spectra in 1.0mol dm-3 HC104 were done in the double layer region (0.15-1.15 V) as well as in the OER covering the dc-potential rangebetween 1.16-1.44 V. The spectra were registered in range of105 to 10-3 Hz superimposing an alternated perturbation of 10mV pp. A single compartment cell was used with a platinumsheet as counter electrode and the saturated calomel(Hg/Hg2Cl2) as reference electrode. All the electrodes werepre-treated by cycling 10 times in the potential range of -0.44to +1.44 V at 50 mVs-' in order to stabilise theelectrochemical response.

III. RESULTS AND DISCUSSION

Figure 2a shows results of impedance measurements in theTi/Ir02 electrode taken at different values of dc-potential. Thecomplex plane representations are characterised by an almoststraight line with slope slightly less than unity in the highfrequency wing, while distinct shapes occur in the lowfrequency part depending on the stationary potential applied.The various shapes of spectra predicted for the different stagesof the measurement, are indeed observed.Figs. 2b-d show the fittings of several spectra of the Ti/Ir02

electrode to transmission line model. In Fig. 2b the lowfrequency part shows a straight line with inclination closed to90 while the complex plane plots shown in figures 2c and 2dhave the arc-shaped feature in the low frequency wing. The

complex plane plot shown in figure 2b was taken at 0.75 V,where the charging process occurs and, obviously, theinterfacial impedance (4) assumes the capacitive formrepresented as in (1), with the heterogeneous charge-transferresistance being neglected (R3-1c'o). In case of impedancemeasurements taken at OER potentials (Figs. 2c and 2d),where a charge-transfer process takes place on the interface,the complex plane plots exhibit the classical semi-circles. InFig. 2c a straight line followed by an semi-circle, well fitted tothe model, is typical of the case where the charge-transferresistance is higher than the transport resistance (R3>R,). Onthe other side, when R,>R3 the complex plane shows a semi-circle distorted at low frequency region. As matter of fact, allthe possible electrochemical behaviours in this system areobserved in Fig. 2 and, moreover, the fittings were in excellentagreement with the experimental data, as is demonstrated inthe complex plane plots.

1606- 0.75V 1.24 a) (b)

1 2 3 4 5 6 7 0 40 80 10120

*0.53 2 1 40 80

2 0000.0 ....... 40 0.8 1 1.4

1015 20 2.5 30 091.A.1.2 1.3 1.4

1 30V

N 0 01 2 34 56 7 40 80 120 160

1.2

0.9 (c) (d(

0u6 0.2-

0.3 0.10.0

Fig. 2. Complex plane plots of the impedance of Ti/1r02 electrode. (a)Experimental data at different potentials, the inset shows details of high

frequency region o the spectra. Fitting of the data recorded at (b) 0.75 V, (c)1.30 V, and (d) 1.40 V. (o) Experimental and (+) Simulated

The shapes and behaviour described for IrO2 electrode (Fig.2) was found for binary electrode of Ti/lrO2-Nb2O5. For thecase of double layer charging (0.85 V) the capacitive feature isobserved, although the low frequency inclination is less thanthat for the Ti/IrO2 electrode, indicating that the binaryelectrode shows a higher deviation from the perfect capacitor.Impedance measured in the OER region also shown the arc atlow frequency region exhibiting the characteristic shape ofporous electrode, and the very good quality of the fittings wasobtained.The results shown above indicate that the transmission line

model obtained from the porous two-phase model approachdescribes the impedance spectra of these electrodes in a verysatisfactory manner. However, a quantitative interpretation ofthe parameters obtained from fittings is essential in order toestablish the physical significance of the model anddistinguish the mechanisms involved during the double layerand the OER taking place over such electrodes.Figs. 3a-f show the variations of the impedance parameters

against the dc-potential during the OER taking place overTi/1rO2 (Fig. 3a-c) and Ti/lrO2-Nb2O5 20:80 mol% (Fig. 3d-e). The impedance parameters exhibit a pattern independently

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of the electrode composition. In figs. 3a and 3d one canobserve that the charge-transfer resistance (R3) is one orderhigher for the Ti/1rO2-Nb2O5 (20:80 mol%) electrode,denoting the poor electrocatalytic activity of this composition.As expected for the faradic processes for the reaction beingstudied, R3 shows an exponential decrease with increase of theapplied potential. In fact, this is true in a range of potentialsdepending on the electrode. In general, for high positivepotentials (E > 1.30 V) a deviation from an exponentialvariation is observed and, is probably owing to a change in theOER mechanism.

10'

10'

a IV

10'

10f'

607

E

aY

50

0.8

0.6

a

Z 0.4

0.2

(a)

X

(b)

(c)

0 . .

1.15 1.20 1.25 1.30 1.35 140 1.45

E / V(sce)

103

10,

10'

(d)

A9\\Xw\o\ o\ ° r}\ v 0

(e}o,

X

(f)

11

10

9

8

120

100

80

60

40

2(t ........-.....-..... ...1.20 1.25 1.30 1.35 1.40 1.45

E / V(sce)

Fig. 3. Impedance parameters (R3; Q3 and R,) in the region ofOER for theelectrodes Ti/IrO2 (left side) and Ti/lrO2-Nb2O5 (right side).

Figs. 3b and 3e show the double-layer capacitance (Q3)variation in the range of OER potentials, for both electrodes.The maximum capacitance value was 61 mF for Ti/IrO2 whilefor the binary electrode it was 10 mF, clearly owing to theactive site concentration in the surface of the oxide coating.Similar behavior is observed for both the electrodes,consisting of an increase up to a maximum plateau around1.30-1.36 V, followed by a decrease in the capacitance, whichcould be related to the blocking of active site by the 02bubbles formed at high overpotential.

Figs. 3c and 3f show the variation in the Ri parameteragainst the applied potential, for both electrodes. It is observedthat the Ri parameter decreases as the applied potentialincreases. According to our initial assumptions the parameterRi is associated with the resistivity to the electronic transportthroughout the catalytic coating, however, the symmetry ofmodel does not exclude a diffusion of protons in theelectrolyte.The impedance parameters related to charging (Q3) and

transport (RI) was analyzed in function of electrodecomposition. Fig. 4a shows that the capacitance increases asfinction of the molar fraction of IrO2 in the coating reachingmaximum values for the electrode containing 70 mol% of the

noble-metal oxide. This behavior is explained with base inchanges in microstructure and morphology of the coating.

15 .(a) (b)

1212

E - \a8~~~~~~~~~~~~~~~~~1

0 0.1020 40 60 80 100 20 40 60 80 100

X0,02 J mol% X,02 mol%

Fig. 4. Impedance parameters (a) capacitance - Q3 and (b) transport - RI forthe Ti/1rO2-Nb2O5 electrodes in different compositions measured in

1.0 mol.L]' HC104. Ed. = 0.75 V(sce)

The impedance parameter RI, associated to the transport,was also analyzed as function of the electrode composition.One can see in Fig. 4b that the resistance of transportdecreases exponentially as function of IrO2 content, reachingminimum values after 50 mol% of Ir02. In the modelproposed, we neglected the resistance of transport in theelectrolyte channel of the transmission line. The resultsobtained for R, as function of the electrode composition (Fig.4b) demonstrate that this parameter are related to theelectronic transport into the oxide coating.

IV. CONCLUSIONS

The results obtained show that the porous model candescribes the electrochemical impedance response of Ti/1r02-Nb2O5 in any binary composition and in a wide range ofstationary potential, covering the regions of charging andcharge transfer reaction in acid media. The impedanceparameter related to charging, transport and reaction processeswas separated and analyzed.

ACKNOWLEDGMENT

We want to acknowledge FAPESP, CNPq and FAPEMATfor financial support.

REFERENCES

[1] S. Trasatti, "Transition Metal Oxides" in: The Electrochemistry ofNovelMaterials, J. Lipkowski and P.N. Ross, Eds., Weirheim: VCH, 1994,pp. 207-295.

[2] S. Trasatti, "Physical Electrochemistry of Ceramic Oxides,"Electrochim. Acta, vol. 36, pp. 225-241, 1991.

[3] R. de Levie, "On Porous Electrode in Electrolyte Solutions,"Electrochim. Acta, vol. 8, pp. 751-780, 1963.

[4] R. de Levie, "Electrochemical Response of Porous and RoughElectrodes," in: Advances in Electrochemisty and ElectrochemicalEngineering, P. Delahay, Ed., New York: Interscience, 1967, pp. 329-397.

[5] A. J. Terezo, J. Bisquert, G. Garcia-Belmonte, and E. C. Pereira,"Separation of Transport, Charge Storage and Reaction Processes ofPorous Electrocatalytic IrO2 and IrO2/Nb2O5 Electrodes," J. Electroanal.Chem., vol. 508, pp. 59-69, 2001.

[6] J. Bisquert, et al., "Doubling Exponent Models for the Analysis ofPorous Film Electrodes by Impedance. Relaxation of TiO2 Nanoporousin Aqueous Solution," J. Phys. Chem. B, vol. 104, pp. 2287-2298, 2000.

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