Oxygen electrocatalysis on thin porous coating rotating platinum electrodes

Oxygen electrocatalysis on thin porous coating rotatingplatinum electrodes

Joelma Perez, E.R. Gonzalez *, E.A. Ticianelli

Instituto de QuõÂmica de SaÄo Carlos, CP 780, 13560-970 SaÄo Carlos, SP, Brazil

Received 12 June 1998

Abstract

This work discusses the electrocatalysis of the oxygen reduction reaction on platinum on carbon thin porous

coating rotating disk electrodes in alkaline and acid media. The electrochemical techniques considered are cyclicvoltammetry, steady state polarization and impedance spectroscopy. Both the dc and ac polarization results areanalyzed and simulated with the thin ®lm/¯ooded agglomerate model. Cyclic voltammetry allows an inspection ofthe crystal facet structure of the Pt particles and the determination of e�ective surface areas which are lower than

those determined from X-ray di�raction techniques or transmission electron microscopy. The dc polarization andimpedance results show clearly a duplication of the Tafel slope due to structural e�ects on the porous electrode.Experiments in alkaline solutions show that in this medium there is an important contribution from the carbon to

the kinetics of the reaction, which is negligible in acid medium. The e�ect of Pt particle size on the oxygen reductionelectrocatalysis in both electrolytes is correlated with the predominant facets of the platinum crystallites. # 1998Elsevier Science Ltd. All rights reserved.

Keywords: Oxygen reduction; Electrocatalysis; Dispersed catalysts; Thin porous coating electrodes; Electrode modeling

1. List of symbols

b Tafel slope of the ORR in the catalyst (Vdecÿ1)

c * solubility of oxygen (mol cmÿ3)ca dimensionless capacitance for the agglomer-

ate networkC dl double layer capacitance (F)

Da di�usion coe�cient of oxygen in theagglomerate (cm2 sÿ1)

Df di�usion coe�cient of oxygen in the thin

®lm (cm2 sÿ1)E potential (V)

E0 standard potential (V)F Faraday constant (C/mol)

i current density (A cmÿ2)i0 exchange current density (nFk 0c *) (A cmÿ2)id limiting current density (A)

j��ÿ1p

J reduced current density (i/i0)k 0 e�ective rate constant for ORR at reversible

potential (cm sÿ1)

Ly thickness of the agglomerate (cm)n apparent number of electrons transferredRs resistance outside the TPC (solution resist-

ance) (O)R lf low frequency resistance (O)R1, R2 dimensionless resistances in the agglomerate

networkZ impedance in the agglomerate (O)Z electrode total impedance (O)f electrode rotation speed (rpm)

Electrochimica Acta 44 (1998) 1329±1339

0013-4686/98/$ - see front matter # 1998 Elsevier Science Ltd. All rights reserved.

PII: S0013-4686(98 )00255-2

PERGAMON

* Corresponding author. Tel.: +55-16-274-9208; Fax: +55-

16-274-9205.

G thin ®lm di�usion parameter (dk 0/Df )d thin ®lm thickness (cm)

Z overpotential (E 0ÿE)m reduced overpotential (Z/b)f dc potential-independent agglomerate di�u-

sion parameter (k 0Ly/Da)1/2

u kinematic viscosity (cm2 sÿ1)C ac potential dependent agglomerate di�u-

sion parametery angular frequency (rad sÿ1)oa characteristic agglomerate frequency (rad

sÿ1)of characteristic thin-®lm frequency (rad sÿ1)

2. Introduction

Dispersed catalysts of the type used in gas di�usionelectrodes have become key materials in electrochemi-cal technology. In spite of the work carried out to

date, it is well known that there are several di�cultiesin interpreting experimental results, both dc and ac, ongas di�usion electrodes. To overcome those di�cultiesseveral theoretical models have been considered, one of

the most successful being the thin ®lm/¯ooded agglom-erate model [1]. The thin porous coating rotating diskelectrode (TPC/RDE) technique has proved to be very

useful to study and evaluate dispersed catalysts of thetype used in gas di�usion electrodes for fuel cells [2±4].The advantage of the TPC/RDE is that it is an exper-

imental setup conceptually similar to a ¯oodedagglomerate, given by the electrode structure, in con-tact with a thin ®lm determined by the rotation of the

electrode.This work presents studies on the electrocatalysis of

the oxygen reduction reaction (ORR) on platinum oncarbon TPC/RDE in alkaline and acid media. The ma-

terials studied are dispersed platinum on carbon (Pt/C)with di�erent values of the ratio Pt/C. The electroche-mical techniques considered are cyclic voltammetry,

steady state polarization and impedance spectroscopy.Both the dc and ac results are analyzed and simulatedwith the thin ®lm/¯ooded agglomerate model.

3. Experimental

The rotating electrode was made by adapting aPTFE cylinder with a 0.5 cm diameter cavity to a

PINE rotator system. A graphite rod was forced intothe cavity leaving a recess of 0.015 cm which was ®lledwith the catalyst material. This was prepared by mix-

ing the catalyst powders (Vulcan XC-72 carbon, 10±80w/w Pt/carbon and pure platinum (E-TEK, USA))with a dilute suspension (12% w/w) of a Te¯on emul-

sion (DuPont TM30). A conventional one compart-

ment electrochemical glass cell with a Luggin capillary

was used in the electrochemical experiments. A large

area platinized platinum foil served as the counter elec-

trode and a saturated calomel electrode (SCE) or a re-

versible hydrogen reference electrode (RHE) were

employed in the alkaline and acid electrolytes, respect-

ively. Except when mentioned, all potentials are

Fig. 1. Cyclic voltammograms of TPC electrodes prepared

with di�erent Pt/C catalysts in N2-saturated 1.0 M NaOH sol-

utions. v=10 mV sÿ1. Voltammograms were recorded after

cycling the electrode until a steady state was reached.

J. Perez et al. / Electrochimica Acta 44 (1998) 1329±13391330

referred to the RHE. All the experiments were carried

out in 1.0 M NaOH or 0.5 M H2SO4 solutions, pre-

pared from high purity reagents (Merck) and water

distilled and puri®ed in a Milll-Q (Millipore) system.

The electrolytes were saturated with puri®ed N2 or O2

gases. All the experiments were conducted at room

temperature (0258C).Cyclic voltammetry was performed with a PARC

Model 273 potentiostat. The ac impedance and rotat-

ing disk polarization measurements were carried outwith a Solartron 1286/1250 impedance system. The

electrode rotation speed was controlled by aAFMSRE/ASR Pine Instrument system. The oxygenreduction polarization potentials for the ac impedance

measurements were set point-by-point in the potentio-static mode. The ac perturbation was a 5 mV peak-to-peak sinusoidal signal, with the frequency varying

from 0.005 Hz to 10 kHz. In all cases, the dc steadystate currents at each electrode potential were recordedprior to the ac measurement.

Transmission electron microscopy (TEM) analysesof the Pt/C catalysts were conducted using a ZeizzEM912 equipment. X-ray di�raction (XRD) analysiswere made employing a X-URD, Carl Zeiss X-ray sys-

tem. The crystallite size of the Pt particles was calcu-lated from the full width at half-maximum of the (111)peak at 2y=39.88 [5]. Calculation of the Pt area from

these data was made using the standardprocedure [5, 6].

4. Results

4.1. Cyclic voltammetry

Cyclic voltammetry on the TPC/RDE allows the de-

termination of the e�ective electrochemical surfaceareas of the electrodes and an insight on the crystallinestructure of the catalysts. Figs. 1 and 2 present some

of the cyclic voltammograms obtained for the di�erentPt/C electrocatalysts in the alkaline and acid electro-lytes respectively. In these plots the currents were

divided by the mass of platinum in the TPC layer inorder to allow an easy comparison of the results. Avery good resolution of the platinum features is foundin the cyclic voltammograms. The active surface areas

per gram of platinum were evaluated using the hydro-gen adsorption/desorption peaks [7] and the results arepresented in Table 1, which also includes the mean

values obtained with TEM and XRD.

4.2. Steady state polarization

Fig. 3 shows the steady state oxygen reductionpolarization curves for several electrode rotation ratesobtained in 1.0 M NaOH solution, on the electrode

prepared with the 20 wt% Pt/C catalyst. The expectedlinear behavior of the di�usional limiting currents as afunction of the square root of the electrode rotation [8]

was observed for all catalysts and electrolytes, exceptfor pure carbon in acid media which is not active forthe ORR.

In order to understand the characteristics of the dcpolarization responses, the results were analyzed interms of the thin ®lm-¯ooded agglomerate model [1]

Fig. 2. Cyclic voltammograms of TPC electrodes prepared

with di�erent Pt/C catalysts in N2-saturated 0.5 M H2SO4 sol-

utions. v=10 mV sÿ1. Voltammograms were recorded after

cycling the electrode until a steady state was reached.

J. Perez et al. / Electrochimica Acta 44 (1998) 1329±1339 1331

from which it is derived that the reduced current den-

sity (J) passing from the agglomerate/thin ®lm into the

solution is given by the equation:

J � i=i0

� exp�m��tanh�f exp�m=2��=�f exp�m=2��1� G exp�m��tanh�f exp�m=2��=�f exp�m=2�� , �1�

where i is the actual current density and i0 is the

exchange current density. This expression contains par-ameters related to the current generation and the struc-

tural parameters of the system: the reduced

overpotential (m), the thickness (d) and di�usion of

oxygen in the thin ®lm (G), and the thickness (Ly) anddi�usion of oxygen in the agglomerate (f).Substitution of the expressions for m, f and G in

Eq. (1) gives the theoretical dependence of the current

density as a function of the electrode potential and the

several physicochemical and electrochemical par-

ameters of the system: Da, Df , Ly, d, c*, i0, b and n.

Parametrization of the theoretical equation was per-

formed using the depth value of the electrode cavity

(Ly=0.015 cm) and tabulated values for the di�usion

coe�cient and solubility of oxygen in the electrolytes

(here it was assumed Da=Df ).

The details of the entire ®tting procedure were pre-

sented elsewhere [3, 9]. Brie¯y, the method involved the

following steps: (i) Starting with standard values for

the other model parameters, i0 and b are adjusted with

the data at low overpotentials where the e�ects of fand G are negligible [4]. This is made taking into

account that the Tafel slope for the ORR is dependent

on the coverage of oxygen on the platinum surface (see

below). (ii) The values of n and d are obtained with

the data at high overpotentials where the dc polariz-

ation curve reaches the limiting current. The procedure

involved the iteration with the RDE convective-di�u-

sion equation [8, 10]

d � 0:62nFD2=3f uÿ1=6f 1=2c* �2�

until convergence of the n and d values. (iii) With the

values of i0, b, n and d already adjusted, the e�ect of

the agglomerate is considered (with Ly=0.015 cm,

Da3Df ) at intermediate overpotentials. (iv) Steps i to

Fig. 3. Steady-state polarization curves for di�erent electrode

rotation rates for oxygen reduction on 20% Pt/C TPC/RDE

in 1.0 M NaOH solution. T=258C.

Fig. 4. Experimental (symbols) and ®tted (Eq. (1), solid lines)

Tafel plots for oxygen reduction on di�erent Pt/C catalysts in

1.0 M NaOH solution f=2500 rpm.

Fig. 5. Experimental (symbols) and ®tted (Eq. (1), solid lines)

Tafel plots for oxygen reduction on di�erent Pt/C catalysts in

0.5 M H2SO4 solution. f=2500 rpm.

Table 1

Average particle size obtained from transmission electron mi-

croscopy and X-ray di�raction techniques. Active platinum

surface areas obtained from TEM/XRD and cyclic voltam-

metric (CV) experiments in alkaline and acid solutions. The

thickness of the TPC layer is 0.015 cm

Pt/C (%) Particle

size (nm)

Active area m2/g Pt

TEM/XRD CV/NaOH CV/H2SO4

10 2.4 114 45 65

20 2.8 100 52 58

30 3.8 74 29 31

40 4.5 62 28 30

60 9 31 13 15

80 16.5 17 7 7


iii are repeated starting with the latest set of ®tting

parameters until convergence of all values.Figs. 4 and 5 illustrate the e�ect of the catalyst com-

position ratio on the theoretical and experimental

polarization curves for a constant rotation speed of theelectrode ( f=2500 rpm) for both alkaline and acidelectrolytes. Tables 2 and 3 present the values of i0, band n resulting from the ®ttings in both electrolytes. In

the alkaline medium only one set of i0 and b values isfound for all electrode potentials for each catalyst,while in the acid medium the values of i0 and b were

di�erent in the regions of low and high overpotentials.

4.3. Impedance measurements

The interpretation of the shape and magnitude ofthe impedance responses of the porous electrode with

respect to the ORR as a function of potential wasmade following the expressions derived for the thin®lm/¯ooded agglomerate model [1]

Zelectrode � Rs � Z

1� joCdlZ, �3�

where

Z ��

em�i0=b��1� Gem�=R1

�1

�R1 � 1

�1=R2� � jca�o=o a��

ÿ �Gem=R1��1� �f4e2m=16��tanh C=C�2�tanh ��

j�o=o f�p

=� ��j�o=o f�

p �1� Gem�C2=f2em��tanh C=C��tanh ��

j�o=o f�p

=� ��j�o=o f�

p �

��ÿ1, �4�

o f � Df=d2, o a � Da=L

2y, �5�

ca � 1:216

�fem=2�2�1� 0:56�fem=2�2�1=2 , �6�

R1 � fem=2

tanh fem=2, �7�

R2 ��1

2

�sec h2fem=2 � 1

R1

�ÿ1�ÿ R1, �8�

and

C � �f2em � j�o=o a��1=2: �9�It should be emphasized that, while in the steady

state TPC/RDE measurements only the oxygen con-centration at the boundary of the porous electrode andthe external electrolyte can be controlled with the ro-

tation speed, in the ac experiment the concentrationinside the porous structure can be modulated depend-ing on the ac frequency.

The ®tting procedure for the ac method followedessentially the same guidelines as for the dc case. Thevalue of the solution resistance is obtained from thehigh frequency region of the impedance spectra and the

capacity of the double layer by adjusting the frequen-cies used in the ®tting with those used experimentally.Examples of the ®ttings of Eqs. (3)±(9) to the exper-

imental ac impedance results of the ORR on the TPC/RDE made with 40 wt% Pt/C are presented in Figs. 6

and 7 for several electrode potentials in the alkalineand acid electrolytes, respectively. In all cases the elec-

Table 2

Kinetic parameters obtained from the ®ttings of the dc results

with Eq. (1) for oxygen reduction on the various Pt/C cata-

lysts in 1.0 M NaOH at 258C

Pt/C i0(A cmÿ2)

b

(V decÿ1)

n

0 0.9�10ÿ13 0.036 1.8

10 1.2�10ÿ9 0.045 2.8

20 1.5�10ÿ8 0.05 3.6

30 5.5�10ÿ8 0.05 3.7

40 8.0�10ÿ8 0.056 3.8

60 1.1�10ÿ7 0.057 3.8

80 1.5�10ÿ7 0.057 3.8

100 8.2�10ÿ7 0.06 4

Ly=0.015 cm; d=0.0011 cm; Da=Df =1.65� 10ÿ5 cm2

sÿ1, c *=1.1�10ÿ5 g mol cmÿ3.

Table 3

Kinetic parameters obtained from the ®ttings of the dc results

with Eq. (1) for oxygen reduction on several Pt/C catalysts in

0.5 M H2SO4 at 258C

Pt/C (%) Low current density High current density n

i0(A cmÿ2)

b

(V decÿ1)

i0(A cmÿ2)

b

(V decÿ1)

10 3.4�10ÿ10 0.056 2.1�10ÿ5 0.120 4

20 5.1�10ÿ10 0.057 4.6�10ÿ5 0.120 4

30 1.5�10ÿ9 0.056 4.0�10ÿ5 0.120 4

40 2.3�10ÿ9 0.057 5.6�10ÿ5 0.120 4

60 5.6�10ÿ9 0.060 7.6�10ÿ5 0.120 4

80 2.6�10ÿ9 0.060 5.7�10ÿ5 0.120 4

100 5.0�10ÿ9 0.060 4.6�10ÿ5 0.120 4

Ly=0.015 cm; d=0.0010 cm; Da=Df =1.8� 10ÿ5 cm2 sÿ1,

c *=1.1� 10ÿ5 g mol cmÿ3.


trode was rotating at 2500 rpm. A summary of thevalues of i0, b, n and C dl resulting from the analyses onboth electrolytes is presented in Tables 4 and 5.

In Figs. 6 and 7 it is observed that the agreement

between the experimental results and the theoretical

lines is quite satisfactory, specially at the higher over-

Fig. 7. Impedance spectra at several potentials for oxygen re-

duction reaction on 40% Pt/C in 0.5 M H2SO4: (a) (.) 0.900,(w) 0.875, (Q) 0.850, (q) 0.825 and (R) 0.800 V vs. RHE; (b)

(r) 0.750; (R) 0.725; (w) 0.700; (.) 0.675 and (q) 0.650 V vs.

RHE. The frequency range is 0.01 Hz (®rst low frequency

point) to 10 kHz (10 points/dec). (± � ±) ®tted (Eqs. (3) and

(4)) impedance spectra. f=2500 rpm, T=258C.

Table 5

Kinetic parameters and double layer capacity obtained from

the ®ttings of ac results with Eqs. (3) and (4) for oxygen re-

duction on several Pt/C catalysts in 0.5 M H2SO4 at 258C

Pt/c

(%)

Low current density High current density n C dl

i0(A cmÿ2)

b

(V decÿ1)

i0(A cmÿ2)

b

(V decÿ1)

10 2.6�10ÿ10 0.056 1.1� 10ÿ5 0.120 4 0.016

20 3.8�10ÿ10 0.057 3.5� 10ÿ5 0.120 4 0.028

30 1.6�10ÿ9 0.057 1.0� 10ÿ5 0.120 4 0.031

40 1.7�10ÿ9 0.057 2.6� 10ÿ5 0.120 4 0.045

60 5.6�10ÿ9 0.060 2.8� 10ÿ5 0.120 4 0.060

80 2.6�10ÿ9 0.060 2.7� 10ÿ5 0.120 4 0.060

100 5.0�10ÿ9 0.060 2.6� 10ÿ5 0.120 4 0.070

Ly=0.015 cm; d=0.0010 cm; Da=Df =1.80� 10ÿ5 cm2

sÿ1, c *=1.1� 10ÿ5 g mol cmÿ3, Rs=2 O.

Table 4

Kinetic parameters and double layer capacity obtained from

the ®ttings of the ac results with Eqs. (3) and (4) for oxygen re-

duction on the various Pt/C catalysts in 1.0 M NaOH at 258C

Pt/C

(%)

i0(A cmÿ2)

b

(V decÿ1)

n C dl

(F)

0 1.4� 10ÿ13 0.036 1.8 0.009

10 1.3� 10ÿ9 0.045 2.8 0.023

20 1.3� 10ÿ8 0.05 3.6 0.040

30 1.7� 10ÿ8 0.05 3.7 0.050

40 1.4� 10ÿ8 0.057 3.8 0.056

60 2.3� 10ÿ7 0.06 4 0.068

80 2.3� 10ÿ7 0.06 4 0.071

100 2.4� 10ÿ7 0.06 4 0.120

Ly=0.015 cm; d=0.0011 cm; Da=Df =1.65� 10ÿ5 cm2

sÿ1, c *=1.1�10ÿ5 g mol cmÿ1, Rs=4 O.

Fig. 6. Impedance spectra at several potentials for oxygen re-

duction on 40% Pt/C in 1.0 M NaOH: (a) (.) 1.020, (w)

0.995, (Q) 0.970, (q) 0.945 and (R) 0.920 V vs. RHE; (b) (W)

0.870, (r) 0.845, (.) 0.820, (w) 0.795 and (Q) 0.770 V vs.

RHE. Frequency range is 0.01 Hz (®rst low frequency point)

to 5 kHz (15 points/dec). (± � ±) ®tted (Eqs. (3) and (4))

impedance spectra. f=2500 rpm, T=258C.


potentials. The higher deviations observed at low fre-

quencies and low overpotentials can be explained by

the presence of some drift in the experimental responsecaused by the presence of impurities in the high surface

area carbon substrate. At low overpotentials thephenomenon should be more apparent because the far-

adaic currents for oxygen reduction are much smallerthan the capacitive currents involved in double layer

charging. In Figs. 6 and 7 it is also seen that the mag-nitude of both the theoretical and the experimental

arcs ®rst decreases and then increases with the decrease

of the electrode potential. When the kinetics of theORR is the only factor considered previous studies [2±

4] have shown that the diameter of the impedance arcsdecreases exponentially with the decrease of the elec-

trode potential. However, for higher overpotentials thesize of the arc increases and those same studies have

shown that this is due to the di�usion of oxygen in thethin ®lm.

The impedance and the steady state polarization data

were compared in terms of mass transport in the exter-nal electrolyte corrected Tafel diagrams. The exper-

imental dc data were plotted as E vs. log[(i� id)/(idÿ i)], where id is the convective-di�usion limiting cur-

rent density [8]. The theoretical diagrams corrected forthe thin ®lm di�usion e�ect by taking G 4 0, were

obtained by plotting E vs. log i (Eq. (1), where

m=(E 0ÿE)/b) and i= i0J) for the dc case and E vs.log(1/R lf) (Eq. (4)) for the ac case, where R lf (low fre-

quency resistance) corresponds to the diameter of theimpedance arc, that is the value of Z when o 4 0.

Examples of these results are presented in Figs. 8 and 9.

The speci®c e�ect of the Pt particle size on the elec-trocatalysis of ORR at low overpotentials was ana-

lyzed in terms of the values of 1/R lf calculated for aconstant electrode potential (E=0.9 V vs. RHE in

acid media and E=1.02 V vs. RHE in alkalinemedia), maintaining the electrochemical parameters i0,

b, n and C dl the same as those used in the ®ttings, andconsidering two situations: (1) G(d) and f(Ly) tendingto zero; (2) G(d) tending to zero, while maintainingf(Ly) the same as that used in the ®ttings. Figs. 10±12

present the results of the Pt activity as a function ofparticle size. Figs. 10 and 12 present the speci®c ac-tivity (1/R lf per unit surface area of Pt catalyst,

obtained by TEM/XRD) for the acid and alkalinemedia, respectively. Fig. 11 presents the mass activity(1/R lf per unit mass of Pt catalyst) for the alkaline

medium.

5. Discussion

From the results in Table 1 it is observed that the cv

surface area per gram of platinum presents a tendency

Fig. 8. Mass-transport corrected Tafel plots for oxygen re-

duction on 40% Pt/C in 1.0 M NaOH. (± � ±) log(1/R lf) vs.

E; (w) log[(Id� I)/(IdÿI)] vs. E; (full line) ®tted plot using

Eq. (1) with G 4 0. f=2500 rpm.

Fig. 9. Mass-transport corrected Tafel plots for oxygen re-

duction on 40% Pt/C in 0.5 M H2SO4. (± � ±) log(1/R lf) vs.

E; (q) log[(Id� I)/(IdÿI)] vs. E; (full line) ®tted plot using

Eq. (1) with G4 0. f=2500 rpm.

Fig. 10. Dependence of the speci®c activity for oxygen re-

duction on Pt particle size in 0.5 M H2SO4 solution: (.)Ly=0.0 cm and (r) Ly=0.015 cm.


to decrease with the increase of the Pt/C ratio. This

e�ect can be attributed to an increase of the Pt particle

size with the increase of Pt/C ratio, as observed from

the TEM and XRD measurements. It is also seen that

the speci®c areas obtained from cv in alkaline or acid

media are smaller than those determined with the

other techniques. The smaller values of the areas

obtained by cv compared with TEM/XRD, should be

explained mainly by a low catalyst utilization and/or

the wetting characteristics related to the presence of

PTFE in the TPC layer. Other contributing factors

may be: (i) the presence of impurities in the Pt/C par-

ticles, leading to a partial blocking of the hydrogen

adsorption/desorption process in the Pt catalyst, (ii)

the fact that a polycrystalline structure equivalent to a

smooth surface (which correspond to a hydrogen

adsorption/desorption charge of 0.210 mC cmÿ2) is

assumed for the Pt crystallites (see below) and (iii) the

fact that in a TEM or XRD calculations a spherical

structure is assumed for the Pt particles and that the

total area of Pt loaded on the electrode is considered

in the calculation, therefore including the area partially

blocked as a consequence of anchoring the Pt on the

carbon substrate and the area of Pt which is not in

electrical contact with the current collector.

From the voltammograms in Figs. 1 and 2 it is seen

that in both, the acid and alkaline electrolytes, there

are some di�erences on the features of the hydrogen

desorption process, when the responses of the low and

high Pt/C ratio catalysts are compared. In the sulfuric

acid solution the middle hydrogen oxidation peak, cor-

responding to the desorption of the hydrogen atoms

from the edge and corner sites of the platinum

crystallites [5, 11], is particularly pronounced for the

smaller platinum particles. This phenomenon was also

observed in other works using the same electrolyte and

has been attributed to the fact that small platinum par-

ticles show an increased amount of edge and corner

sites in the crystals, which is a consequence of the

decrease of the atomic surface fraction (ASF) of the

(111) and (100) faces [5, 11, 12].

In the results of Figs. 1 and 2 it is seen that the

background current (corresponding to the desorption

of hydrogen from the (111) face [13, 14], over which

the hydrogen desorption peaks on the (110) and (100)

sites are superimposed, is particularly large for small

particles in the acid electrolyte. This is consistent with

the fact that for the smaller particles (2±4 nm), the

ASF of Pt atoms in the (111) face is higher than those

for the other orientations [11].

In both, alkaline and acid electrolytes, the dimin-

ution of particle size 16±2 nm (Table 1) leads to a

decrease of the more anodic hydrogen desorption peak

which corresponds to the desorption of hydrogen

atoms from the (100) face [11, 13]. As pointed above,

this is a consequence of a decrease in the ASF of Pt

atoms in that face with the decrease of particle size.

The ®rst peak in the voltammograms includes the

contribution of hydrogen desorption from the (110)

face. The (110) sites are likely to form on particles that

do not contain the exact number of atoms to produce

a complete cubo-octahedral structure, which was pro-

posed by Kinoshita [11] as a model of small platinum

crystallites. These sites correspond to a maximum

mass-average fraction of 15% in Pt particles with a di-

ameter of about 2 nm [11, 12], decreases for larger par-

ticles and becomes almost zero for particles with

diameter above ca. 8 nm [11, 12]. However, the results

in Figs. 1 and 2 show that the e�ect of the (110) face

predominates and the ®rst hydrogen desorption peak

increases as a function of particle size. This fact indi-

cates that a reconstruction process takes place during

the potential cycling, as observed for single

crystals [13, 14] when the upper potential limit is raised

above10.8 V vs. RHE.

Fig. 11. (Q) Dependence of the mass activity for oxygen re-

duction on Pt particle size in 1.0 M NaOH solution. (- - -)

AVF (111) as a function of particle size from Ref. [11].

Fig. 12. (Q) Dependence of the speci®c activity for oxygen re-

duction on Pt particle size in 1.0 M NaOH solution. (- - -)

ASF (111) as a function of particle size from Ref. [11], solid

line: mass activity (solid line, Fig. 11) divided by the Pt

speci®c area.


Tables 2±5 show that the values of i0, b and n

obtained from either the dc or the ac simulations aretotally consistent with each other. In Tables 2±5 it isalso seen that the C dl, and i0 values increase with the

increase of the Pt/C weight ratio in both electrolytes.This is due to the increase in active area in the electro-des resulting from the increase of Pt loading on the

thin porous coating layer with the increase of Pt/Cratio.

It is well known that the ORR can proceed via a 4-electron pathway to give water as the ®nal product ora 2-electron pathway to form peroxide ions, depending

on the electrode material [15±18]. Smooth electrodesformed by polycrystalline gold, graphite and carbon

catalyze the reaction through the two electronmechanism [19±29], while gold single crystal withorientation (100), silver, platinum and several metals

of the platinum group promote the four electronpathway [15±18, 20, 30±35].For smooth polycrystalline platinum electrodes, two

Tafel regions, with slopes of 60 and 120 mV for lowand high overpotentials, respectively, have been

observed for the ORR in acid or alkaline solutions.The existence of two slopes is explained in terms of thecoverage of the electrode surface by adsorbed oxygen,

which follows a Temkin isotherm at low overpotentialsand a Langmuir isotherm at higher overpotentials [31±33]. It is interesting to note that the same character-

istics of the Tafel slope have been found for platinumsingle crystals in sulfuric acid [14].

Tables 2 and 4 show that in alkaline solution theTafel slopes obtained from the ®ttings (Eqs. (1) and(4)) for the several Pt/C catalysts increase from 36 to

60 mV decÿ1 as the Pt weight ratio increases from 0to 100% It is also seen that the number of electrons

involved in the ORR changes from a value close totwo to a value equal to four. These results show thatin this medium there is an important contribution

from the carbon to the kinetics of the reaction, onwhich the ORR takes place by a two electron re-duction process, even in the presence of small amounts

of Pt. In contrast, the results in Tables 3 and 5 showthat in acid medium, where the contribution from the

carbon to the ORR is negligible, the simulated valuesof b and n are practically the same for all catalysts andpresent the expected values for single crystal or poly-

crystalline platinum.In Figs. 8 and 9 it is observed that both the dc and

the ac results lead to Tafel plots that show a doubling

of the slope in alkaline medium and a duplication ®rstand then a quadruplication of the slope in acid med-

ium. The duplication phenomenon takes place at po-tentials which are not compatible with those alreadyobserved many times on smooth platinum surfaces and

explained on the basis of the oxidation state of thesurface [31±33]. Simulation of the Tafel plots with the

theoretical model shows clearly that the duplication in

alkaline medium and the ®rst duplication in acid sol-ution are due to structural e�ects in the porous elec-trode especially associated with the thickness of the

¯ooded agglomerate zones (Ly).On the basis of the above picture it is seen that the

structural characteristics of the TPC layer are respon-sible for a strong e�ect on the polarization behavior ofthe electrode. To compensate for this e�ect, the value

of R lf was calculated at low overpotentials, where oxy-gen adsorption follows the Temkin isotherm, taking Gand f tending to zero for all catalysts. In this way, the

calculated values of 1/R lf are equivalent to those of alarge area ¯at plate electrode composed of Pt/C with-

out di�usion limitations or structural contributions.Thus, the value of 1/R lf corrected in this way, which isproportional to the current generated by the electro-

chemical reaction, is very adequate to discuss thespeci®c e�ect of the Pt particle size on the electrocata-lysis of the ORR.

This problem has been treated by several authors,for the reaction occurring on gas di�usion electrodes

in acid media, under several di�erent experimentalconditions [5, 11±13, 36]. The analyses are usually madein terms of the speci®c activity (current at 0.9 V vs.

RHE divided by the platinum active area) and themass activity (current at E=0.9 V vs. RHE dividedby the platinum loading). In general there is some dis-

agreement on the results and conclusions, and thecauses were recently analyzed by Tamizhmani et al. [5]

and Markovic et al. [13], who attributed the discrepan-cies to the anion adsorption characteristics of thedi�erent acid electrolytes used in di�erent studies. On

the other hand, it is highly probable that in some ofthese works the reason for the discrepancy is related tothe uncompensated e�ects introduced by the electrode

structure, as those mentioned above for the TPC sys-tem.

The results in Fig. 10 show that in the sulfuric acidelectrolyte the speci®c activity of the platinum par-ticles, calculated without compensation for the struc-

tural e�ects of the active layer, is practicallyindependent of Pt particle size. On the other hand the

values obtained after compensation for the structurale�ects present a marked dependence on particle size.These results show that, at least for the TPC electro-

des, the structural e�ects are very important in deter-mining the activity pro®les.The behavior of the structurally corrected speci®c

activity (Fig. 10) as a function of the Pt particle size insulfuric acid has also been studied by other authors

using di�erent kinds of active layers [5, 13, 36]. Theshape of the speci®c activity curve has been explainedin terms of the shape of the curve representing the

ASF made up of (100) facets as a function of particlesize [11, 13], because the pro®les of both curves present


exactly the same tendency. This proposal has been

made under the assumption that only the (100) and

(111) facets are present in the Pt crystallites [11] and

that only the atoms on the (100) orientation are active

for the ORR [13]. However, the results of the present

work clearly show that the (110) facet is also present

and thus its contribution to the ORR

electrocatalysis [13, 14] can not be disregarded.

Figs. 11 and 12 present the structurally corrected

mass and speci®c activities, respectively, in the NaOH

electrolyte plotted as a function of Pt particle size. In

this electrolyte the (111) face is the more active for the

ORR [13]. So, the mass activity should present the

same pro®le as the atomic volume fraction (AVF)

made up of (111) facets, as represented by the dashed

line in Fig. 11. Analogously, the speci®c activity should

present the same pro®le as that of the ASF made up

of (111) facets (dashed line in Fig. 12).

In Fig. 11 it is seen that, although the shape of the

mass activity and the AVF lines are similar, the pos-

itions of the maxima do not coincide. It is observed

that the maximum of the AVF appears for particle

sizes smaller than those employed in the experimental

investigation. Based on this fact it is expected that, in

the range of 2.5±5.0 nm, the trend on the experimental

curve would be a monotonic decrease of the mass ac-

tivity with the increase in particle size. The deviation

from this behavior can be attributed to the fact that

for the smaller Pt/C ratio catalysts (smaller Pt particle

size) there is a strong participation of the carbon in

the catalysis of the ORR (a 2eÿ process), which is

apparent until the Pt/C ratio reaches 30/40 wt% (3.5/

4.5 nm). Thus the increase in the mass activity in the

region of small particle sizes, can be understood as an

increase of the platinum contribution to the catalysis

of ORR (a 4eÿ process). Beyond 5 nm, the decay in

the mass activity is strongly governed by the decrease

of the AVF of the Pt crystallite (111) facets.

The comparison of the speci®c activity and the ASF

lines in Fig. 12 is more complex, because of the dis-

persion in the experimental data. To facilitate the

analysis, the speci®c activities of the particles were also

calculated from the mass activity line (solid line in

Fig. 11), taking the speci®c activity as the mass activity

divided by the Pt speci®c area (m2 gÿ1). This result

corresponds to the solid line in Fig. 12. It is seen that

there is a good agreement between the two sets of ex-

perimental data, indicating that the pro®le is indeed

realistic, although it does not follow the same trend as

the ASF lines. As observed from cyclic voltammetry

and the ORR polarization data, the causes for the dis-

crepancy are probably related to the reconstruction of

the crystallite facets and/or to the contribution of the

carbon particles to the catalysis of the ORR.

Acknowledgements

The authors wish to thank Fundac° aÄ o de Amparo aÁ

Pesquisa do Estado de SaÄ o Paulo (FAPESP), ConselhoNacional de Desenvolvimento CientõÂ fõÂ co e TecnoloÂ gico

(CNPq), Fundac° aÄ o Coordenac° aÄ o de Aperfeic° oamentode Pessoal de NõÂ vel Superior (CAPES) andFinanciadora de Estudos e Projetos (FINEP), Brazil,

for ®nancial assistance.

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Oxygen electrocatalysis on thin porous coating rotating platinum electrodes

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