A comparative analysis of structural and surface effects in the electrochemical corrosion of carbons

Materials Chemistry and Physics, 21 (1989)

A COMPARATIVE CORROSION

ANALYSIS

495-506

OF STRUCTURAL

495

AND SURFACE

EFFECTS

IN THE ELECTROCHEMICAL

OF CARBONS

P.L. ANTONUCCI Facoltd

di Ingegneria,

Universita

di Reggio Calabria,

Reggio Calabria

(Italy)

dell'Energia,S. Lucia, Messina

(Italy)

L. PIN0 and N. GIORDANO Istituto

CNR, Trasformazione

e Accumulo

G. PINNA Dipartimento Received

di Scienze

October

Chimiche,

Universita

10, 1988, accepted

di Cagliari,

November

Cagliari

(Italy)

25, 1988

ABSTRACT Results

of

corrosion

surface

behavior

population

of

have

found

been

analysis

in

H3P04

are

oxygen-containing to

be

influence

of both

of carbon

is outlined.

of

blacks The

presented.

surface

related

surface

carbon

to

functional

their

and structural

and

graphitic groups

corrosion effects

their

electrochemical

character

and

of different

rates.

carbons

Evidence

in determining

the

the

of

the reactivity

INTRODUCTION The

reactivity

amongst it

which

has

been

is shown

graphitization, alkaline

a

I), It

previous

lower

works

when

subjected

to

found

shown

according

that

Moreover,

we

have

varying

in

be

of

parameters, oxidation

crystalline

achieved,

importance

either

to various

electrochemical

degree

can the

In

both

of

in thermal

examined surface

potentiostatic the

corrosion

to the equation

to be linearly

0254-0584/89/$3.50

the

rate

of reactivity

[8,9]

blacks

been

structure.

increasing

corrosion

is related

order

in

surface

acid

by and

parameters

or electro-oxidation

out [5-71.

carbon

has

by

[l-4].

several

on stream were

that,

pointed

materials

crystalline

the extent

has been widely

of

their

environments

in determining

In

of carbonaceous

related

the and

electrochemical

bulk

characteristics

corrosion

tests

currents

steadily

in

it= kt-" [lo]. Moreover

to surface

behaviour

H PO 3 4

(Table

at

decrease

170°C.

with

time

corrosion

currents

areas (up to 1500 m*/g).

Normali-

0 Elsevier Sequoia/Printed

in The Netherlands

496 zed

corrosion

surface)

currents

have

been

pH of the carbon having

Table

per

shown

unit

to

slurry,

surface

display

the maximum

area

of

carbon

the

a volcano-shaped

(mA/cm*

correlation

in the corrosion

occurring

with

real the

for slurries

pH = 8.

I. Synopsis

of carbon

Surface

Elemental

do Surface D

area

Sample

black characteristics

(mz/gr)

analysis

data

Corrosion

groups

(A) areas

H%

N%

S%

0%

(A.U.)* '% Monarch MogulL

1300

560 138

3.71 3.67

rbrco

Sz?L

650 233

Iknarch930

data

Corrosion k current lcx,min(mA/mg)&-l sn

928.2 901.2

81.74 0.67 0.12 0.45 12.03 89.90 0.46 0.86 4.75

0.25 0.11

67.3 2.72

3.64

942.6

6S.43 1.18 0.54

9.77

0.33

15.47

3.7l

8X1.0

92.93 0.47 0.22 0.66

4.25

0.12

5.44 2.78

-

Elftex

285

42

3.64

KU.7

97.16 0.45 0.10 1.20

1.31

0.04

Elftex

495

74

3.64

737.6

95.45 0.45 0.21

2.25

0.04

3.19

0.03

2.35

-

vulcan xC72R 245 3.67 817.3 95.92 0.26 0.25 1.05 1.05 * Sum of peak areas of oxygenated surface groups (see text), ted by integration selected

limiting

tremes

[8].

These

results

organization of surface [ll 1, to

and,

produce

is herewith

of distinctive wavenumbers,

have

now

(taking functional further, a

re-examined

do

spacing

groups, from

coherent

of the spectrum

with the baseline

been

the

portions

in

as

an

as derived

elemental

view

from

drawn to connect

relation index)

combines

with

(C, the

H,

between

crystalline

the

population

infrared

N, 0,

different

pre-

the two ex-

the

and with

our previous

analysis

which

each one calculatraced

S).

analysis

An

attempt

characteristics

outlined.

EXPERIMENTAL The ments of

experimental have

the

blacks

distances filtered

been

procedures

previously

was carried

(do)

were

the

reported out using

derived

MO K, radiation

for

from

i.r.

in

detail

a Carlo X-ray

analysis

and corrosion

measure-

Elemental

analysis

[8,111 .

Erba C-H-N

analyzer.

Interlayer

diffraction

measurements

on

of commercial

using

Zr

(b = 0.7107 i).

RESULTS Results (Table

I)

of have

previous been

experiments re-plotted

in

[8] Fig.

1

a series (as

corrosion

currents,

carbons mA/mg,

497

measured

after

asit is well

100 min)

as

a function

determines

known,

the

of

the

interlayer

distance

graphitic

character

of

do which,

the

material.

l

3.40

3.50

3.45

3.55

3.60

3.65

3.75

3.70

d,,(i) Fig.

1. Cgrrosion

spacing

The

plot

the

as

demonstrates

received

varies

with

as the

do

by

current

100 min (mA/mg) of carbon blacks vs. do interlayer blacks, o , Heat -treated Vulcan XC72R (data [4]).

'As received'

(A)..,

that,

carbons

irrespective

(i.e.

the order-disorder value

Taylor [4]

decreases.

for

not

subjected

degree

Also

graphitized

of

of the

plotted

Vulcan

their to

XC72R

previous

black,

in Fig.

origin,

corrosion

heat

showing

treatment)

a sharp decrease

1 are the results

samples

of

heat-treated

reported

at various

temperatures. Reexamination coherent

of

relationship

oxygen-containing

vibration

between the

2923

ascribed

3600 and to

i.r.

transmission

between

corrosion

surface functional

in [8]the following thing

our

functional

around

3435

groups cm

-1

groups

spectra and

the

111

has

I

relative

also amount

(Fig. 2a). As discussed

have been singled (through

out:

integration

shown of

a

the

in detail

(1) thev OH stret-

of

the

broad band

and

3000 cm-') attributable to weak H-bridged OH groups; (2) -1 -1 2860 cm (in overlap) as a part of the band 3000-2784 cm

v

asymm

CH

and

u

symm-CH

vibrations

in CH

3

and

CH2

groups;

(3) the

1600 cm-'

conjugated frequencies groups; ching

CO

as part of the band

in

whose

a

group frequencies

configuration

intensity

(4) a broad frequencies

quinone

is enhanced

band from

of the

COOH

(phenols,

1712-1527

1335 to group

epoxides)

or

cm to

by the

-1

attributable

aromatic

presence

1000 cm-' mainly but

in which

of

ring

to highly stretching

phenol

or ether

due to the

CO stret-

the contribution

of other

should be included.

(a)

800

600

400

200

coOH+aO+oH

(b)

2

4

6

8

10

12

14

16 o/c.10-2

Fig. 2. (a) Corrosion functional

groups

current

in carbon

100 min (mA/mg) vs. amount of oxygen-containing - Values fr%[8,11]. (b) Corrosion current

blacks

100 min (mA/mg) -vs. O/C ratio of carbon

blacks.

499

The

plot

bands

to

that

found

the

of

be

well for

density

showing

of

2a

shows

related

the

do

to

the the

parameter

oxygenated

groups

total

of

all

corrosion

rates

(Fig.

The

is

1).

further

of

higher

Notably, carbons

surface

as an almost and

areas

their and

linear dependence

O/C

increased

ratios

(Fig.31,

oxygen

oxygen-containing with

content

a shape

functional

proven

increased corrosioncurrentson carbonsofhigher

analysis). areas

Fig.

from

groups

similar

dependence plots

of

to

upon

Fig.

2b

O/C ratios

(from elemental

was found

also for surface

it

clearly

appears

are in some ways

that

directly

related. Furthermore -vs. the oxygen found coherent

the

kinetic

content

term

of carbons

in the corrosion (O/C ratios)

rates,

k, has been

in Fig. 4, and the correlation

with the above.

500

0 4

8

12

16 -2

o/c-10 Fig. 3. Surface

area values

plotted

(m2/gr) -vs. O/C ratio of carbon

blacks.

500

60

Y

2

4

6

8

10

12

14

16

18 o/c.lo-2

Fig. 4. Kinetic

constant

(Ag-'5")

values

(from [8])vs -.

results

clearly

indicates

O/C ratio of carbonblacks.

DISCUSSION The

bulk

of

in an electrooxidative stic

as

reflected

1)

and

the

as

determined

clear

from an

by

prevalence

by

relative

Accordingly, manifested

a

our

behaviours

their density

either

increase

disclosure

of

untreated

mechanism

which

links

corrosion

behaviour.

and

similar

carbons,

for

both

Further

for

of

their (Fig.

the the

oxygenated

the

atoms

(Fig.1).

are,

character

of

carbons [4]

such as

of

of carbon

.?a, 2b

the

groups

(Fig.

do

values) due

to

2b). is the

It thus becomes how

similar

the

a common

any carbons

to their

correlation well

as

blacks irrespective

identify

(Fig.

suggesting

that do might be used in practice

screening

characteri-

functional

evidently

question

blacks

character,

(lower

carbons

1 shows

carbon

analysis

currents,

of carbon

Fig.

graphitic

character

corrosion

of

a structural

or elemental

graphitic

order-disorder

graphitized

as Figs

2a)

heat-treated

in fact

their

surface

to

corrosion

by both

by

’ 0 =*

contributes

the As

is ruled

fractions

and

it is suggested

characteristic history.

in

ordered

d

i.r. in

decrease

of more

that

environment

that

oxygenated

for

to

be valid

as-received

as a discriminating of their

previous

functional

groups,

501

originally

present

it

is

of

a carbon

and

obvious

to

are

surface

on

the

regard,

carbon

carbon there

are

of

carbons

to

the

with

has

several has

been

very few

and the relative behaviour

environment. the

role

in

of

the

first

Most

of

the

role

of

and

Hoogland

postulated below The

-

400

carbons

et

mV) oxide

al

in

grew of

to

a graphite

area carbon More oxygen

distinctive

one

was

two

first of

has

media

influence

been

paid

[2,3,10,16,] of the nature

to corrosion seem

does

in a PAFC

to

strenghten

primary

not

literature,

have

attack

find,

also

on

a

carbons.

carbon

at

a

account

on top

reported

a monolayer.

CO

the

only

for

the

carbons

in

thermal

patterns:

surface

oxygen

in HCl,

(reducible

1700

of

oxide.

mV.

of

Also

several

two

oxide

of C02,the

solid which

formation

high

surface

exists.

the desorption

activated

Janssen

stable

surface

onset

exception

Thus,

corrosion

formation

structure

studying

at

the

disregarding

oxide,

of the

reducible After

the

mainly

electrodes

protective

and

a

analyzed

of the reaction,

electrochemical

with

oxidation

groups on the electrooxida-

of

CO2

being

et a1.[13],

aqueous

as-received

little graphitized

Billinge

attention

evidence

formed

KOH,

thermal

adopted.

oxides:

the

on the

[ 7,13,14,15] but

while

in fact,

to

reaction)

authors

reported

previous

oxide,

proposed,

blacks, where

desorption

such

the

the

as sites of the

electrolysis

and

the

tenth

coal-based

complexes

on

of

in relation

herewith

be decomposed

H3P04

present

on the

in the course

the

surface

in

conditions

investigating

and

oxide

recently, from

could

species,

rapidly

of

a

[ 191,

H2S04

carbon-oxygen

and

them

but

groups

formation

elemental

(either originally

less

functional

in the

formed

original

features

related

Moreover,

investigations,

[18 1, studying the

stable

Binder

the

surface

questions

several

carbon

results

experimental

groups

centers,

and

the many

course

published

of

process,

previous

of functional

the

none

unanimous support varying

of

oxygenated

electrooxidation

glance,

the by

of the surface

surface

reactive

of structural,

groups

extensively,

[10,17]

experimental

the

of the widely

role

studies

of carbons, The

in

of uncertainty.

oxidation

amount

in solving

stressed

investigated

electrochemical

only

tion

points

main

structural

of surface

produced

been

the

the

of carbons.

influence or

as

the combination

be of help

oxidation

surface

reactivity

both

and that

might

the

surface,

that

important

to the electrochemical

on

carbon

conclude

analysis

In this

the

of chemisorbed

oxidation,

complexes

were

found

two

released

502

initially

as CO2

most

of which

ash

impurities

surface

resulting by

from

the

with

acidic

in thermal the

from

516 Kl followed

a carbon-ash

carbon)

rather

at Tb1123

interaction

than

by

K by CO evolution,

(reduction

the

direct

Boehm

[14

of intrinsic

decomposition

of

oxygen complexes.

Coherently most

(starting

the

findings

oxygen-containing oxidation

acidic

groups

of

species

conditions

are

BiTlinge,

bound

to

on

the

give

surface

CO2,

exclusively

] claims

(carboxylsl

and moreover

to

the

that

the

decompose

he observed

peripheral carbon

that

atoms

of

each layer. Also on

Kinoshita

thermal

desorption

which functional The

according

to

of

stable

Thomas

1221,

surface

decompose

at

additional

studies

of

groups containing

formation

active

1 agrees with this view, supported

[16

area; a

it

rate

gaseous

surface the

is

than

products.

thereby

these

to

further

oxides

powder

more

also

The

process

difficulty process

mentioned

some

block

active

experiments and

may

The main of

be

the

experi~ntally from

complexes

are

formed

complex

pointed

out

these

the

to

in turn

give

in turn

reported the

the out

that

contact

and

makes

more

[3]

formation

carbon

pointed

and

the

may

been

facilitates

more

the

a

range and

before

oxygen

enter

of

originally

by Ahmed

by Ahmed

of

and

its

removal

of

the

the

of

carbon

rate

the

of

Back

complexes

thermal

[15],

are

reaction

able

1.6,231.

into the oxidation processes

however,

these

groups

present binding

on

has

who to

Other

process [24,26].

in the difficulty on

the

the

energies

Back [15]indicate reaction,

in

and

surface

to reside,

effects

a wide

groups

carbon-oxygen

series

appears

surface,

surface

also

complexes

complexes

with

of

that

the

in

the

reported on

claiming

inhibit

of uncertainty

apart

the

role

intermediates

[27,30 1. Results

increasing

the

work

and to

in that,

of

been

shown that

factor

testing

therefore

previous

have

been

oxides

of

inhibits,

by decreasing

has

area

interparticle

defining

has

sites

the

has

phase

it

C02.

oxidation

CO2

to

[3].

in

oxidation

It

hand,

surface

produce

processes,

reaction

the

corrosion.

conductive,

the oxidation

the

increasing

improves

other

phase and

that

oxidation

the

of

CO

however,

the

On

of

studies

[ 20,213 , according

preferentially

in gas

production

surfazeawides during

liability

oxides

likely,

slower

carbons

two oxygens

th~t~~rno~l~ micropores,

oxidized

by previous

reaction,

surface, and

that

other

mobilities

the

a significant

presence effect

503

on

the

the

reaction

overall

stable the

rate.

of which

reaction

our

(either

rate)

for

factor.

more

CO

or

formation lead

is

CO2.

to

of

Evidence

a

surface

a

other

asserts

onset

groups

the

to

kinetic

influenced

by

results

and

then

accelerating

an

intermediate

according

the

formation

of the anodic

hand,

CO21

are

part

in the

-et

of

the

case

of

following

product

site

and, in this

Laine

to the mechanism

each

a free

decreasing

to

formed

according of

of

that

and

concentration

2a and 4 appears the

of

molecule, subsequent

complex

sense,

of

can

the complex

oxygenated increase

of

compounds

the

only

in the an

the

as

above

develops

only

accumulates corrosion

as

both

on samples by

reported

in

density

wherein i.r. Fig.

is 2a

compounds,

of oxygenated

that also k, the experimentally equation[lO]

determined

oxygenated

on the

(Fig. 4) increase.

determined

a critical

of carbon

(Fig. 1) is thus further

only

the

nature of the oxygenated

when

independent

pre-existing

in

relationship

Kinoshita's

oxidation

groups

are detectable

oxygenated

with

oxidation

2a) and the O/C ratio

densities

shape

to agree well

electrochemical

(Fig.

of

41,

role

on

other

of

Indeed the mere observation

(Fig.

only

and

small

occurs

the

the

occur

reconciled

as the main discriminating

play

formation

exponential

groups

constant

be

and,

the

a

could

might

surface,

Thus

interpretations

carbon

the

the reaction

kinetic

oxidation)

(CO

of the complex

the do character

as

On

irrespective

by

and

(the more

[15].

the

exponential

O/C

reaction

regeneration

as

as current

is present.

determined related

of

[15]; by

far

presence

the that,

Back

on

concentration

denoted

significant

in the

then

products

of product

so

the

can

complex

in Figs.

oxygenated

observed,

the

in

words,

increasing

complexes

sites

oxides

of

complex.

amount

significant

functional

In

and

reported

as

and

active

phase

complexes.

as a catalyst

considerations after

the

another

may be regarded

the

accompanied

a large

period

surface

of these oxides

stable (ASA)

gaseous

oxides,

by Ahmed

gas

reaction

by

of

or

more

covered

accumulation

described

the

contradictory

stability

area

oxidation

not

directly

apparently

the

surface

unstable

the

these

In fact,

surface

induction

author,

leaving

participate

the relative

the

the

desorb

electrochemical

active

-al. [61,

to

the

of products.

view,

by invoking

the

According

probably

in the formation In

shortening

kinetics,

is exponentially

quantity,

confirms

in an acid medium groups.

strengthened.

The

that

is strongly

relationship

with

504

Although to

be

not

existence

associated

seem

to

surface been

the

be

area

taken

with the

(SA)

into

investigation

the

only

on

we

the

oxygen-containing

initiation determining

the

account

[8],

of

rate

and

area

and measured

corrosion

between

iodine

absorption

and corrosion

tized carbon

blacks

if

not

it

does

could

present

in alkaline

appear

a

unifying,

ce of the pointed

out

presence index

active

by Laine

of

oxygen

that

al.

atoms,

on

the

Fig.3

centers

seems

related

of attack

[6].

The

above

complexes

the

above

phenomena,

bulk

and

surface

that

a concluding

Nevertheless, of carbons

groups towards

the reactivity of surface

oxidation

the transformations

the

are

results

should occurring

between

a correlation and ungraphi[27].

the

to

numbers

preferential

reaction,

as already

inferred

of

that

surface,

confirm

surface

the

is

oxygenated

environment.

Even

the importan-

here unequivocally

of carbons

an

groups

such

a view

as the

prefe-

In this regard,

area to be directly

is determined

as demonstrated, are of

present the

gas

whole

phase

group concentration useful

at the carbon

in

the

matter

and

by a number

influence

contemporarily,

of these materials.

prove

relation

carbon

made seem

be of great help in establishing

of the evolution chemical

between

previous

(O/C) on the blacks.

which,

rationalization

similarities

could

describe

as it shows the

characteristics

In our

absorption

reasonably

to

ASA

appear

behaviour

in

has

it strenghtens

in an electrooxidative

the electrooxidation

concurrent

bound

of

to the oxygen concentration

Since

to

It is then

variously

surface.

to confirm

parameter

iodine

nevertheless

portions

the oxygen-containing

rential

of

et

of carbon

has been demonstrated

whether

does the

recently,

area (ASA) in the carbon-oxygen

significative

accumulated and

surface

still of

rate of both graphitized

clear

attack on sites in electrooxidation,

linear

More

appears

influence

reactivity

a

currents.

specific

the

it

[ 5,6,8,9,31,32].

environments

entirely

of

demonstrated

surface

then

corrosion,

Indeed,

extent

authors

clearly

for

factor. the

by several

have

sites

groups

of

it

is

both clear

is still

premature.

electrochemical

oxidation

the role of surface In this respect, in the course

obtaining

complex

an analysis

of the electro-

greater

insight

on

surface.

CONCLUSIONS Data results

presented on

the

here constitute electrochemical

a set of information, corrosion

behaviour

of

based on experimental several

carbon

black

505

sample tal

in an aqueous

Evidence

analysis.

information groups of

feature

This

on

of

the

evidence Our

(from

i.r.)

points

of

strict

correlation

It

character)

has (i.e. -

and attack

functionalities

in

terms

(from

show

the

of the

demonstrated

priori

the

analysis), conditions

structural

the

the

of

their

basis

of

can be envisaged. on heat-treated

oxygen

denoting are

a

on

behaviour,

with

of

behaviour

expression

parameter,

a relationship

sets

oxygen-containing

already well established,

at the carbon

two

that

value,

of corrosion

elemental

and elemen-

electro-oxidation

spacing

a

electrooxidative

present

between

and

-

an

to that,

further

O/C

do

in terms of surface

combination

clearly

the

represents

is congruent

the

surface been

prevision,in

results

itemized

from

carbon

blacks

a warranted

blacks.

the

(H3PO4),

derived

a

materials.

graphitic which

suggests

present

these

medium

surface

groups

definitely

that

oxygen-containing

surface.

ACKNOWLEDGEMENTS The authors

gratefully

ne) for elemental

acknowledge

Dr. B. Colombo

(Carlo Erba Strumentazio-

analysis.

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