Catalytic reduction of O2 at polyaniline cathodes in the presence of CuII

Matwials

C%urnistrJ, cm11PIzJ~sI’cs,22

( 1989) 40 I 41 6

401

CATALYTIC REDUCTIONOF OP AT POLYANILINECATHODESIN THE PRESENCEOF CuII

L.M. DOUBOVA, G. MENGOLI,S VALCHERY AND G. ZOTTI Istituto Uniti

di Polarografia

ed Elettrochrmica

Preparatlva

de1 CNR, Corso Stati

4, 35020 Padua (Italy)

ABSTRACT The electrochemical polyaniline In the

modified first

reduction

of CuII in aqueous

glassy carbon electrodes

case, voltammetric

CuI1 -> GUI reduction

HCl has been investigated

under either

and potentiometric

mediated by the zitrinsic

at

N2 or 02. data

emphasize

a fast

emeraldine/leucoemerald~ne

couple

of the polymer film. In oxygenated solution reduction, CuII/CuI

is apparently

the catalytic enhanced

activity

of polyaniline

by the homogeneous

with respect

catalysis

focused

on the

respect to electronic semimetals

organic

metals,

conduction

A particular

class

prepared

polymerization

of

is, organic

for instance, exhibit

such

Anodic oxidation

compounds

by (chemical

of heterocycles

characteristics systems

the behaviour

which,

has with

of metals and

in a conductive

of

highly

conjugated

oxidative

coupling

hydrocarbons

[2-41.

molecules has often

for

state

resulted

because of oxidation

as synthesis

material,

with consequent

of the coherent

is a major advantage,

(Italy)

electrolyte:

by the precipitated

occur at the polymer surface,

* Dipartimento

aromatic

media of the parent

doping with anions of the background

and thickening

composed

route. In fact, when these polymers form and precipitate

onto the anode, they are already anode becomes shielded

is

or electrochemical)

or (substituted)

in suitable

in an improved synthetic

Bologna

that

[il.

macromolecules

fact

by the

couple.

The search for new materials with unusual physico-chemical lately

operated

to 02

possible

further

continues electron

molecular

di Chimica GCiamician,

untractable Universita

materials

transfers

weight increase

polymer layers bound to the substrate. quite

and

and the

This second

are thus

provided

di Bologna, Via Selml 2, 40100

402

solid-state

as

further

JWlCtl0l-S

investigations

As

regards

possible

electrocatalytic

now considered

[5-71

lorepared

by

electrochemical

properties.

m%_¶ified electrodes

polymer-modified

(or

electrodes)

suitable

for

or applications. Tl-ns

is

in a new light of aniline

such

junctions

may

for polyaniline,

(Fig. 1) and good stability

in aqueous H2SO4at constant

exhibit

an old material

since Diaz and Logan 183 have

with redox properties

oxidation

uses,

the case

or

shown that can easily

at

be

triangular

wave scanned potential.

Fig. 1. Voltamnetric pattern (100 mV1.s) of a polyaniline G.C. electrode filmed (area = 0. 2cm2) in 0. 5 M HCI. I/II : emeraldine - leucoemeralchne redox system (charge q 12E).

Photoreduction polyaniline reactive

are

substituted

polyaniline than

of Co, is thus made easier

anodes

anlllnes

cathode

on

at polyaniline-coated

the oxidative

substrates

cathodic

[9];

of

less

shown that

a

at several hundred xiV more positive

redox system (see Fig.11

reduced by

silicon

polymerization

Furthermore, we have recently

reduction

carbon (G. C. )

glassy

polyanillne

[lo].

operates 02

cycle bound to the first case:

seen to catalyze

[ill,

A chemical-electrochemical

appears to be

polarization

Involved in

is driven to the

this

emeraldine

form by 02, Hlhich is in turn reduced to H202. The present paper refocuses CJ1/cU1

system at polyaniline

on the above process: are investigated;

first,

then further

the features details

are

of

the given

reduction; lastly, occurrence of the possible both reactions of 0, sixultaneously is studied The scope of this worK is to assess if, through the intervention reduction

of

the

redox

can be accoxplished

inorganic

couple,

an

efficient

catalysis

for

02

403

WERIMENTAL

Materials and apparatus Aniline and all other by C. Erba, Italy.

products

were reagent-grade

S.I.A.D., Italy. The medium of all experiments in distilled in cells

H20. Electrochemical

with

The

generally

with

supplied

by

at room temperature

( 20°C)

S.c.2. as reference

and Pt

using

electrodes,

electrode

filmed

used as supplied

(99.99)

was composed of 0.5 M HCl dissolved

configuration,

as counter

WOrlclng

purity

run s were performed

three-electrode

coils or sheets

chemicals,

The gases, 02 and N2, were high

was a rotating

polyaniline:

disc

glassy

(supplied

carbon

by Tacussel,

(area

France),

x 0.07 cm?), Pt and Au

(area 0 0.03 cm2) were the substrates. The

electrochemical

potentiostat,

apparatus

a mod. 567 waveform

60.000 Gould Brians x-y recorder.

consisted

(AMEL, Italy) generator,

The kinetic

cof a mod.

a mod. 7?1 integrator data were analyzed

Computer using programs writ,t.en in Microsoft

OllVettl PerWnal

551

and a mod.

by a rn~~~d.M 24

GWBASIC.

Procedure Polyanlline continuous range

films cyclic

-0.05/0.80

generally during

were

at, t.he

(C.V.)

scans

V) in a 0.35 M solution

the

glassy

synthesis,

carbon

so that

and spreading

elsewere

[ll])

was not

intrinsic

redox charge

After synthesis,

disc,

more than

film s exhibited

IO-l%

solution

and

rate,

by

in

area the

(0 : 225 rpm) very

the

rotation

good adhesion

of t-he disc

thickest,

t.?

(as observed

deposit

electrode, = were transferred

scanned

0.00 / 0.50 V) for at least 3 hours. complete activation

substrate

(26 mC of

- see below).

polyanlline-modified

HCl test

for

disc sweep

in 0.5 M HCl. The electrode,

at moderate

of t-he geometrical

content

rcttating

(20 mV/sec

of aniline

was kept

the resulting

the substrate,

aqueous

synthesized

voltammetry

by

C.V. (20 mV/sec

This procedure

of the redox properties

- in

was necessary

into a 0.5 M the

range

to achieve

the

of t.he polymer.

RESULTSAND DISCUSSION Reduction of CulI at polyaniline Figure 2a shows the potentiodynamic a rotating

pOlyanlllne

electrode

(scan rat.e 1 mV/sec) i/E curves recorded

in 0.5 M HCl after

the addition

the redUCtlOn Of CUH here overlaps with the emeraldine -> leucoemeraldine of the

film

(26 mC of charge),

shown

comparing potentials

and currents

and Pt, the process

was identified

being stabilized

by the chloride

indicated

electrochemical

a fast

by the dotted

curve

as CulI - > Cul reduct>ion, electrolyte reaction.

[12]. The shape

process

in the figure.

of Fig. 2a with the parameters

at

of 0.01 M CuC12 : On

tested at bare Au the cuprous

stage

of the i/E curves

404

Indeed,

according

to

the

are straight

lines, which start

+0.2OV, -Le. performed

potentiostatically

charge

at the

foot

of

data

plotted

in

from the origin the

Fig. 2b, i vs &2 diagrams L * even for currents measured at

voltammetric using

pattern.

a thinner

This

film

experiment

electrode

was

(intrinsic

: 12.4 mC).

5 ovs/

0

15 10 rad’l2 .sec-112

Fig.2. Reduction of CulI (10m2 M) at rotating polyaniline filmed G.C. electrodes: a) potentiodynamlc (lmV/sec) curves for rotation speed of 225, 625 and 1600 rpm; the dotted curve was recorded in the absence of CuII. b) potentiostatic increasing w . currents at E = 0.20, 0.10, 0.00, - 0.10 V (SCE), with The fact predicted - the

that

a priori, the

leucoemeraldine Additional gathered

considering

potentials

of

conversion

the transition

conductor

data

A quantitative

containing *

with

Flg.2a,

reactivity

G.C. substrate polyaniline

activated negative

is far

undergoes

less reversible; the

emeraldlne

at least in the solid-state

->

[7], to

of polyaniline

with

CulI were therefore

reaction reaction

Kinetics

has been

[11,13]. polyaniline

CulI was maintained

Not fully either

bare

measure of leucoemeraldine/Cull rotating

could not be

route,

by potentiometry

The selected

control

that:

which corresponds,

on the

by a different

diffusion

-> insulator.

Kinetics of leucoemeraldine/CuI1 attempted

is always under

of CulI at the

reduction

- within

CulI reduction

under

polyaniline or positive

electrode cathodic electrodes intercepts.

inmersed polarization could

give

in the

test

SolutiOn

at 0.00 V; (the

yellow

1 vs o iI2 diagrams

405

colour,

visible

when

leucoemeraldine

the

film

opened, the potential-time oxidation had

(E/t)

to be transformed

into

are not univocal potentiodynamic

onto

This phenomenon slow C.V. driving

due to leucoemeraldine (Q/t)

of the polymer

and cathodic

data

(explained

by the

how Q/E plots

analysis

then

Known charge(obtained

redox

by a

for the

respectively.

to electrical

t-o the first

was

film. However, Q/E relationships

directions

is not restricted

that

-> emeraldine

for a Kinetic

scan at 1 mV/s in the range 0.00/0.50 V) differ

limited

intensity

Pt, indicated

As soon as the circuit

E/t data useful

[II]: the dots of Fig.3 illustrate triangular

same film in the anodic

properties

system

as due to a first-order

only [14]: during

of polyaniline,

transition)

a similar

was noted

for

ESR

[15].

120

0

deposited

charge-time

(Q/E) characteristics

hysteresis

been

response

by CulI was then recorded.

potential

signal

had

was the main form of the material).

2k

360E/fn”

480

Fig.3. Charge - potential characteristics of a PolYanlline filmed (6.1 mC1 G.C. electrode. Dots: experimental points; continuous lines: eqns. 1 and 2.

According similar

to Kaneto and Yoshino

to those of Fig.3, which

band model: electron top of the valence respectively. following Q = Kl/(i (going

(going

[16], polythiophene been explained

(oxidation)

and injection

band and at the bottom

In the present

empirical

equations

case the Q/E plots

(reduction)

Q/E plots levels

of a

occur

at the

(Interband)

level

of Fig.3 were found

to fit

the

[II]: (11

and

+ exp(E - K2)/K3) + K'QE + K'5

negative).

also exhibits by the energy

of the polaronlc

+ exP (E - KSl/K31 + K4Eg + KgE + Kg

positive)

Q = Ki/(l

ejection

have

(2)

406 In both equations the other

the exponential

terms may account

the set of constant

for

terms represent (pseudo]

K has been determined

of E/t data into the respective

were (predictably) transfer

Q/t sigmoidal obtained.

(for

60

120

film,

while

[11,17]: once

the transformation

curves

(initially

direction

t/set

were used for

slow and then may indicate

slowed down, does not agree

into much more reliable

0

in the positive

E < < 0.00 V), emphasized

Instead, E/t data are converted direction

each

This shape, which

with CuH is initially

reduction

for

charge,

contributions

Q/t plots is straightforward.

However, when the Q/E characteristics the transformation,

the Nernstian

capacitive

accelerating)

that with

the charge

the fast

Cur1

by Fig.2.

by the Q/E characteristics Ninetic curves,

in the negative

as shown in Fig 4a.

160

gob 7

0

I

I

I

0.16 0.20 a-%? rad-V2 .sech /

005

al0

Fig.4. Leucoemeraldine oxidation by Cu? Redox charge of film 6.1 mC; [CuII] : 5~10~~ M. a) Q/t characteristics for rotation speed equal to: 225, 400, 1600 and 2500 rpm. b) Plots of the reciprocal derivatives (for t -> 0) vs &2.

Indeed, this relaxation

choice

processes,

appears

to have a physical

explanation

for the same redox state, the potential

the charge is layered: when leucoemeraldine (HtAlso the hypothesis of a first-order case should talte place from the outer of eqn.2.

is oxidized

‘* I Owing to slow response

from the solution

feels

how

side (by

phase transition, which in the present side of the polymer, may justify the use

407

CuII), the charge (cathodically)

1s probably

reduced

The Icinet.ic

curves

(5~10~~ M), for

with

I - dQ,ed/dt

w

represent

the

= charge + Q,,)

oxidation

CulI concentration

bound

to emeraldine,

: intrinsic

rates

of the curves

charge

v : Fig.4a

for

Qred : charge content

of

the

shows how v increases

of

mixed

provided

for the currents

Unetic-diffusive

control

at a rotating

[18,19], the

disc,

following

A

l’. &ed]”

&I] l/v

+ values

line through

second

term

obtained

the origin

120

t/ set

(31

[($I]

from

the

is obtained

chemical

fast, and always under diffusion

60

.w i/2

$13

of eqn.3) vanishes

obtained, the heterogeneous

0

at a fixed

The derivatives

can be written:

On plotting the

(Q,, (Q,,d

1

straight

speeds.

to v the treatment

conditions

equation l/V r

initial

is

side [ll].

.

On adjusting for

rotating

to leucoemeraldine,

polymer)

in the polymer, as when emeraldine

of Fig.lCa were obtained

increasing

t -> 0, dQ,x/dt bound

distributed

from the substrate

at high

curves

(see Fig.4bl.

of

Fig.4a

In other

o and a negligible

leucoemeraldine/CuII

reaction

vs m112 a words, since intercept

is

must be very

control.

180

6 0 0

60

120 t/se:’

Fig.5. Varlatlon of leucoemeraldlne intrinsic charge of films : a) and 12.75 mC; b) tentative Kinetic

oxidation rate by CulI (5~10~~ M) with from bottom, charges of 0.81, 1.85, 6.1 analysis of previous data by eqn. 5.

408

This finding cathode

throws

some light

on the good performance

for Cur1 -> Cul reduction,

predictably

in the less conductive

rate by CuII is so fast

that

even

of polyaniline

when (E i 0.00 V)

leucoemeraldine

enough positive

the

form: the chemical

charge

is injected

as a

material

is

oxidation

into the polymer

to ensure conduction. Figure

5a shows Q/t curves

polyaniline

electrodes

increasing The

thickness

obtained

having of

these

appear to vary slightly

zero-order

with rt?SpeCt to +,a.

This fact is not so surprising

- a relatively

also means

curves). of

reaction

a

that:

initially

depend

mainly

on the

area of the solid; slow redistribution

been suggested Indeed,

but with

which

for

when we consider

of the heterogeneous

exposed surface

(2500 rpm)

charge,

the initial rates t -> 0, Le. with the charge of the films, thus indicating

curves

oxidation,

rotation

intrinsic

(from lowest to highest

derivatives

- the kinetics

at fixed

increasing

to explain

if leucoemeraldine

process should not differ

of the charge

the electrical

the polymer has already

oxidation

by Cur1 goes on mainly

so much from a diffusion-controlled

solid and a gas, for instance,

the conversion

simplest

then

kinetic

inside

hysteresis.

treatment

takes

by layers,

reaction

of a metal into an oxide by 02.

the

form

the

between a The

[20]:

dCx1RJ.t = k/ [xl

where

(4)

is the thickness of the product layer;

x

whence

(5)

[Xl2 = 2kt

(the constant Figure parabolic

should account

5b gives

an idea

for diffusion of

law, when the forming

substituted normalization,

for x. Figure

the film).

how much of the emeraldine

(Q,,),

data

of Fig.5a)

is in a first

5b may have only a qualitative

layers of different

However, it must be said that root time dependence)

thickness

kinetic

have already

been tested

H20 uptake [21] or non steady-state

are probably

diffusional for

limits

such

a

without

any

characteristics,

built

up.

(providing

polyaniline

electrochemical

fit

approximation,

meaning:

no comparison can be made among films of different

in which diffusion

either

inside

a square

in the cases of

experiments

1221.

Catalytic After

reduction

of 02 at rotating

potentiostatic

reactions

of

stationary

the

runs

polymer

currents

therefore

under

at any w

lasting

several

N2 saturation

become

negligible:

were

.I

20-

Or 0

These

under 02 saturation,

the

experiments

so that

total were

only currents

the features

of 02 reduction

at two films of different

-0.1 v . . .. a : . .

n

l

’

-0.2v

11.

<

i-2 PA.

the

(26 mC and 10.7 mC respectively).

a

a40

below

minutes,

process were measured.

Figure gaeb illustrates redox charge

electrodes

polarizations

repeated by this procedure

due to the catalytic

60

polyaniline

with

o.ov

.

.*_..

. . . .

..-..

O.lV . . . . . . 1

5

10 radh

OJ!? I 20

n

15

. sec-v2

-0.2v -0.3v _ _ _ -

b a 15 _-Cl-r

-

-O.lV

< - 10 _--+w

o.ov

5

0.1v 0

5

.10 0’12

15

rad’l2 .sec-1/2

I Fig.6. Catalytic reduction of 02 at rotating polyaniline filmed G.C. electrodes. a) Thiclter film (26 mC) : potentiostatic currents at E of (in order) 0.10, 0.00, -0.20, -0.010 V (SCE). b) Thinner film (10.7 mC) : potentiostatic currents at E of (in order) 0.10, 0.00, -0.10, -0.30, -0.20 V (SCE).

It may be noted that: - the currents

soon become independent

at all potentials - the current at

for

density

the thinner observed

of w : this

trend

is particularly

with the thiclter

film

(Fig.6a), e.g. > 0.5 mA/cm2

E = -0.100 V and w = 400 rpm, is by no means negligible:

conditions times

it reaches

higher

reduction

about

at Pt, which

of 02;

the same value is the

evident

film (Fig.6b);

optimum

at Au, while catalyst

for

for

similar

it becomes only 4-5 the

four-electron

410 - the currents increase with the potential,achieving a maximum Fig.Ba , and E

q

(at E z -0.1V,

-0.2 V , Fig.6b , respectively),and then tend to decline.

This behaviour was alsoobserved wlth other films.

,::I $ -

o.l-

o.ov

_

_

-0.1;0.2 v 01

0

I

I

0.2

a1

c&/2

/

0.3

rack’/2 .sec’h

I Fle.7.Kinetic analysis of data in Fig. 6a as l/i vs "-1/2 plots.

Figure 7, in which

the reciprocal currents

(from Fie.6a] are plotted

aeainst"-1/2, shows that 02 reduction at polyaniline exhiblts the features of a slow electrodereaction the rate of which increases with the overpotential. An alternativeway of dealingwith the data is to consider that the polyaniline electrodeis no different from a typical modified electrode whose active surface layer consistsof an lnsulatlnepolymer matrlx bonding a redox catalyst (generally an inorganic couple). It has been suggested that 02 reduction at polyaniline is mediated by the leucoemeraldlne/emeraldine redox system according to the followingmechanism [ii]:

Thisindeed may flt wlth the general schema of electrocatalysls of irreversible reactionsat redox polymer modified electrodesdiscussed by Andrleux et

1233

411

P + e- ->

Q

Q + A ->

( redox

P + B

A + e- -_)

catalyst

(A = depolarizing

B

(direct

The kinetic

model developed foresees

symbolized by letters the cross exchange diffusion

inside

respectively. situations but

the

substrate potentials

the

)

such as rotating

current

[23].

the nature of the rate determining electron

of the substrate of

transfer),

(SR, ER ...)

disc

These

(electron

indicate

are

steps: R for

S and E for

and the charge

letters

the

hopping)

pure

kinetic

from mutual compensation of the said processes.

The present results are not useful lines,

the

techniques for

(the mediated

Combinations arising

B : products)

at more negative

behaviour

underlining

the film

of

for stationary

limiting

reaction

substrate;

reduction

occurring

voltammetry

)

actual

kinetic

for a quantitative

situation

can

be

analysis

focused

*** along these

by

the

following

considerations: I) previous

voltammetric

data on CU~~/CU~ reduction

with the same process at bare metals) indicate electron

diffusion

inside

II) the currents III) IV)

value

increase

l/i

vs

plots

conversely

The increase

stationary

02 reduction

conditions

limitations;

the film thickness,

but

is observed,

not even at

linear.

to point

points that

I) the

kinetic

of the current towards

point

must

currents

situation

E should

II-IV), which agree with the same diagnostic (6) is likely

never

be

criteria

to be an SR process.

at more negative

that the polymer becomes entirely

form, reactive A last

were

according

[23], would indicate considering

stationary

potentials; ~-l/~

Therefore, present;

by increasing

(compared

[ll];

no second wave (see below) of direct

very negative

under

the polymer does not cause kinetic

of 02 reduction

only up to a limiting

that

at polyanillne,

potential

converted

may be explained

by

into the leucoemeraldine

02, be at

considered, the

most

that

negative

is

the

possible

potentials:

the

decline

of

the

potentiodynamic

experiments of Fig.8 may account for the phenomenon. The voltammograms (Fig.Ba) were obtained cathodic

direction

of a polyanlllne

electrode

by slow (1 mV/s) polarizations (charge

content

in the

: 10 mC): the number

*** It is not possible to change significantly the experimental parameters of the system: the currents become very soon independent of w, the concentration of redox sites in the polymer cannot be varied and the concentration of 02 in solution cannot be increased to affect significantly the kinetic behaviour.

412

of each curve indicates rotation

speeds

saturation, process

the sequence of the runs. Thus l-2 were recorded

(w 2 100 and

It is observed

bound to 02 occurs:

declines. current

1600 rpm respectively)

(l-2) that

after

it strongly

l-2 is due to a further

polyaniline

increases

The voltammogram 3 however

curve

I

during

N2

the catalytic and then

and decrease likely

sucessive

refers

in to a

scans under N2

4).

b 10 /LA

the rise

of the polymer: this

metastable system as the wave tends to disappear (the plain

reduction

that

for two

02, 3 under

at between -O.l/-0.2V

indicates

reduction

under

-.H /-

/ _ .A. 0.7%0- 0.00

t

-0.45

Fig.8. Potentiodynamic (1 mV/s) i p E curves of 02 reduction at rotating polyaniline (10 mc) filmed G.C. electrode. a) Sequence of voltammograms under 02 (1,2,5) and under 52 (3,4) saturation ; b) Clean current of catalytic 02 reduction obtained from a) as the difference between 5 and 4. The low bacKground current was recorded at the bare G.C. electrode under 02 saturation. When the conversion, under

polymer

is led to show only

a limiting

potential-independent

02 (curve

cathodic

5). Figure

the

emeraldine current

gb shows the resulting

range of the figure

was restricted

was observed

The described different

thickness

experiments limitations reduces

the

very

current most

but relates 02

reversibility. interact

at

was confirmed

(5mC and 1OmCof charge the

decline

negative

to a metastable

efficiently

with

the current

plateau

the value

of direct

polyaniline

film electrodes

of

content).

which

may be observed

potentials

is

not

due

in stationary

to other

Kinetic

form of the polymer which catalytically

although

This form may in part

with 02.

current:

from -0.6V on).

behaviour

In conclusion,

net catalytic

achieved

to -0.45 V but a steady-current

was kept as far as -l.OV (at the bare glassy carbon 02 reduction

-> leucoemeraldine

is eventually

exhibitlng

be restored

after

poor leavlng

electrochemical the polymer

to

413

Reduction of CuII/O, at rotating The electrochemical

reduction

process

of Kyoto cells

current

densities

for

the

reactions

033,-W)

+ e-

of 02 catalyzed

-->

according

route, the primary

CuClm-(ml)

noticeably

controlled

product

initially,

of

increased

of the chemical

CulI

+

of the

reduction

(8)

l/w20

each elementary

reduced

reaction

step [12].

by the four-electron

is probably

H202, which

is

step 1251.

as a whole an electrochemxal at

graphite

for

catalytic

system [19]

t 1 60°C are

in fact

by 02 [25].

system has also been tested

but for the milder temperature

were similar

by the occurrence

(7)

through

to eqn.8, 02 is eventually

7-8 constitute

currents

This catalytic

cathodic

Remarltable

+ (n-m)Cl-

then reduced by cuprous ions in a further Reactions

of HCl [24].

are made possible

+ WC2 + H+ + (n-m)Cl- --> 03x,-(n-2)

Although,

the

by CuC12 is the basic

electrolysis

[241:

which are both very fast, diffusion

and

electrodes

industrial

at low overvoltages

following

Cuclm-(“1)

polvaniline

at both

metals: details

here, at bare Au and Pt electrodes

(20°C) adopted

are given

only

in this for

work. The effects

Pt.

60

u -z -40

Fig.9. Potentiostatic electrode for CulI lines: under 02

currents (E : 0.00 V) of Cu11/02 system and 5~10~~ M. Dotted lines: under

: 10m3 M

Thus, Fig.9 shows the potentiostatic CuII concentrations increasing

currents

(E = 0.00 V) achieved

(10m3 M and 5~10~~ M respectively)

w . Although

at this

potential

a very

at rotating Pt N2; continuous

effective

under direct

wlth

two

N2 or 02 with reduction

414

of 02 should be attained, by the

this process in fact appears

CulI -> Cul reaction:

probably

poison

the catalytic

either activity

restores

CulI concentration In

contrast

intrinsic

the current

increment

.but fades

with o 112, according

to provisions

disc electrode

with

such

used,

reaction

is proportional for

to

a catalytic

[19].

behaviour,

were

increase

to the chemical

: in fact, the current

(Two films charge

ascribed

inhibited

Cu or Cul adsorption

C@

mechanism at a rotating polyaniline.

of

of Pt. Therefore,

over the values tested under N2 must be totally 8, which rapidly

to be practically

microdeposits

Fig.10 a and

illustrates

b, with

what

happens

10.7 mC and

at

5.7 mc of

respectively),

/ 5 J/2

/ lo rad’f2 .se&S

I’ Fig.10. Potentiostatic currents of Cu11/02 system at rotating polyaniline filmed G.C. electrodes. Dotted lines: under N2; continuous lines: under 02. a) [CUDS] = at E = 0.00 V for :;;I M, E of 0.20, 0.00, -0.20, -0.10 V (SCE). b) Currents and iOm2 M. 10-3, 5x10-3 The data at the first concentration the currents similar

for w -> 0 with

potential

making a similar several only

film elecrode

(Fig.lOa) were achieved

(lo-3 M) but with increasing dependence, comparison

the values noticeable with

8.

with one single CuII

polarizations.

On comparing

of Fig. 6b (the film was the same) current

increases

Fig.9 (the current

times lower) it becomes clear that

be due to reaction

cathodic

are observed.

densities

the increments

at bare

at polyaniline

at On

Pt are cannot

415

On the other underlined

hand,

Figure

processes

7 and 8, is also (E = 0.00

V)

CulI concentrations.

10 reveals with w

that the current increment

(pointing

and the overall

of catalytic

achieved at the same potential

by the data of Fig 10b,

but with increasing slightly

the occurrence

effects

out the significant

soon become

caused by

02

decreases

weight of reaction

6

very

over

7 - 8

independent of film thickness.

CONCLUSION The

data

~scussed

so far e-size

cathode for the fast reduction The rapid

(chemical)

that polyaniline

behaves as a

of CulI to Cul in hydrochloric

iniection

of charge into the

very

good

medium

material

by

may

CulI

account for this performance. When 02 and cUII are sirmltaneously activity

of

conversely

polyaniline

last

thiclu-iess

cycle

and

effect

low dependence on

charged

corrlplexes

Pt. Since

w),

of the polymer

On recalling

unaltered

as

of

the

possibly of

O2 at

on

of film

the

that a steady-state

occurs during the process:

this may involve a

outer emeraldine layer doped by the strongly

negative

This layer may in fact be the electrochemical

[W.

all,

observations

dependence

it may be speculated

or

reduction

the final

which shows low

at

the efficiency

appears

system operates an efficient

(catalytic

catalytic

the intrinstic

6) is not hindered

7-8) also

and at low overvoltage.

section

surface modification positively

(reactions

the polyaniline/cUCl2

room terature the

(reaction

happens at the noble metal catalyst

CXiC12/02 catalytic enhanced,

led to reduction,

towards 0,

catalyst

CuClq3of

O2

reduction

‘Ihe

authors

would

1iRe

to

thank Mrs A

Randi

of

CNR for

experimental

assistance.

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