Electrochemical behaviour of (nominally) iron disilicide electrodes in sulphuric acid

Materials

Chcmistr!,

and Ph!,sics,

ELECTROCRJHICAL IN SIJLPWRIC

19

215

( 1988) 2 I S-328

BEBAVIOUR OF (NOMINALLY)

IRON DISILICIDE

ELECTRODES

ACID

A.K. VIJH, G. BELANGER

and R. JACQUES

Institut

de recherche

d'Hydro-Qugbec,

Received

July 22, 1987; accepted

Varennes,

September

Quebec,

JOL 2PO (Canada)

8, 1987

ABSTRACT

Iron

disilicide

metallic

lustre

trode material

(FeSiq)

composition

to its electrochemical

electrocatalytic

metal

(see text)

displays

behaviour

of elec-

has been

examined

cathodic

typical

some

which

and its possibly

stability,

towards

properties

intermetallic

The electrochemical

and high conductivity. of this nominal

H$O,, with regards ing

is a transition

in 1N

interestanodic

and

reactions. The

remarkable

visible

of potentials, some

erosive

noted,

property

signs of electrode

side,

(>l A cm-*) without rates

anomalously

are

V

there

of

high

of a deposit

anodic

potentials

on the electrode

extremely

(2 1.5

high rates

no

(>5 V),

surface

of hydrogen

the overpotentials

damage; V),

Also

however.

side, up to 3.0 V, only a transition

reduction

galvanostatically behaviour

(or more), is

high

is observed;

or for the oxygen polarized

it permits

any surface

quite

On the anodic

40

formation

it displays

etc. in a wide range

is

required

the Tafel

evolution to sustain

slopes

have

high values -ca 5-6 F.

oxide growth,

type

and/or

is that

exfoliation

however.

On the cathodic

such

material

erosion,

-1.6 V to 3.0 V; at very

e.g., attack

electrode

of this corrosion,

also

no activity reaction

with

some

could

in

signs

concomitant

that of

the

evolution

growth

typical

these

at

of

reaction

the electrode

occurs

breakdown at

characteristic evolution

When

densities,

oxide

dielectric

oxygen

region,

the oxygen

be detected.

to very high current

is observed

some

for either

is

valve metal up

that

levels

to around potential; of

anodic

polarization.

0254-0584/88/$3.50

0 Elsevier Sequoia/Printed

in The Netherlands

216

In

solutions

passivation

containing

Tafel

line

.observed are extremely semiconducting)

organics

activity

the

transition

the chlorine

high (= 10 F),

surface

The possible cal

NaCl,

(for

region

evolution)

enters

around

characteristic

of

into

a

post-

4.0 V; the slopes non-conducting

(or

layers on the electrode.

activity

(CH$OO-,

of these electrodes HCOO-,

could be detected,

NH2-NH2

for the anodic

and

CH30H)

was

oxidation

also

of typi-

examined;

no such

however.

INTRODUCTION

Perhaps

the most-desired

surfaces

displaying

chemical

stability.

available,

coatings ting

to achieve

[l-3], chemical

oxides

involves

so

highly

both

with

conducting the early

with

e.g.,

that

does

of noble such

concomitant

be

not

based

have

to

of

electro-

on

readily-

rely

on

of approaches

based

on special

an

have alloy

[4,5] and the use of conducmetals

as

the

[6]. sodium

of these materials

electrochemical

is the discovery

A number

of surfaces

non-metals

to

one

and

must

electrocatalysts

traces

promise

regard

surfaces

noble metals.

modification

in conjuction

although

[7,gl, time

materials

this goal,

properties

such

use of rare and expensive

made

electrochemistry

electrocatalytic

Furthermore,

inexpensive

extensive been

good

goal in modern

Another

approach

tungsten

bronzes

did not stand the test of

stability

and

electrochemical

activity. Intermetallic some

cases,

occur

sive media

report

to

be

examine

namely,

formula

istics,

can

investigates

silicides, the

and

acids.

which

[9].

addition

another

to

the

attractive

inexpensive

can

have

Since

metallic

some

of

lustre

these

materials. of

aforementioned

The present one

desirable

in abundance

and

it is of

of

these giving

character-

from our point of view:

produced

in

compounds

stability,

behaviour

can,

many corro-

Fe 50%; Si 50%, by weight,

feature

material

silicides,

and chemical

as electrode

(nominally

e.g.

are inert towards

electrochemical

iron disilicide

available

hard

they behave general

In

metals,

The compounds

unusually

how

FeSi2).

solids

of high conductivity

the

it possesses

a readily

transition

as hard metallic

the requirements

interest

of some

such as strong

conductivity fulfill

compounds

it is

by the Quebec

mines.

EXPERIMENTAL

SECTION

Cell A

conventional

with gas inlets

three-compartment

and outlets

[LO]

Pyrex

electrochemical

was used for the

measurements.

cell

provided The three

217 could

compartments isolated

be

separated

from the ambient

by

atmosphere

solution

-

sealed

by water-filled

stopcocks

and

bubblers.

Electrodes Iron

disilicide

has

from 'SIDBEC DOSCO', The

real

analysis,

nominal

composition

would

FeSi, with

chromium

understood

composition,

Qugbec,

and

was

obtained

was

found,

by X-ray

diffraction

Si = 41.6 %; Fe = 56.4 %; Cr = 2.2 X.

correspond

[lo]

to a mixture

of FeSi2

In the context

as an impurity.

that

FeSig

Canada.

of the material

to be as follows:

composition

thus

the

Contrecoeur,

the composition

(or Fe$iS)

of this

of the electrode

This

paper,

is only

and it is

nominally

FeSip. The melted

small

lumps

in a vacuum

were

in

placed

The large

furnace.

a surface

of about 0.314 cm*, which

the piece

in Kel-F.

The working

600 grit on a silicon doubly-distilled The

counter

deionized

in heat-shrinkable Hydrogen

CORROSION

For

before

crucible

surface

was

and

was cut to get

to the solution

The electrodes

by sealing

polished

down

were then washed

to

with

using.

from a vitreous

graphite

rod mounted

tubing. were used.

employed

for the measurements

model

a Corrosion

332

System

Software

Hewlett-Packard

variable

power potentiostat

power

(PRT200-IX)

potentials

Princeton

Applied

for use with

were

printed

of

Apple

on an Epson

II.

galvanostatic supply

measurements

(50 V; 500 mA),

in a galvanostatic

were followed

EG/G

Diskette

the results

in line with an Apple

the high-current-density

in Figs. 1-8 consisted

(from

Program

(64 K ram from EG 6 G);

recorder

electrode

alumina

piece thus obtained

electrode

made

electrodes

SOFTWARE

using

II Computer FX-8W-

purity

and Measurements

The instruments a

water

high

was exposed

wheel.

were

Teflon

reference

Instruments

Research)

carbide

electrodes

a

(Fig. g-11),

a

or a Tacussel

high

mode were employed.

The

on an X-t Omega recorder

(Model 585).

Solutions The main ized,

solution

was made

doubly-distilled

(e.g., NaCl, HCOOK,

water.

CHBOH,

from Ultrex To

CHSCOOK

The rest of the procedures

this

sulphuric solution,

acid (IN)

and deion-

appropriate

additives

etc.) were added, as required.

were as in our other recent work

[11,12].

218 RESULTS MD

DISCUSSION

.Electrode Behaviour in the Potential Region, -1.6 V to 3.0 V

The most striking feature of the present electrode material was its shining, metallic appearance which remained completely unchanged even under drastic conditions of prolonged hydrogen evolution at high rates (1 1 A cm-') or on polarization to high (= 3 V) anodic potentials even in extremely aggressive media, e.g., containing NaCl; some electrode attack, however, is observed at extremely high ( >5 V) anodic potentials - see below.

There were

no

visible

signs of corrosion, erosion, growth, deposits, swelling, exfoliation, mechanical distortion or other changes usually seen on other electrode materials under these conditions, except at extremely high anodic potentials.

For example,

under conditions of gas evolution at such high rates, even high-pressure graphite shows visible erosion with the concomitant particle accumulation in the

solution; no

such disintegration was

noted for the iron disflicide,

however.

i$Ei~,, Fig. 1.

-400

-200

, t * f , , ,~ 0

200 400 600 E vs RHE (mV1

800

WOO

A cyclic voltammogram on nominal FeSi* in IN H2S04 with nitrogen bubbling in the working compartment; scan rate is 100 @I/s.

The

anodie peak seen in this first scan, at around 600 mV disappears in the subsequent scans to give a hysteresis region.

A general potentiodynamic profile on a freshly-polished Fe-Si electrode is shown in Fig. 1; a vestigial anodic peak at around 0.7 V is seen on the first sweep.

On repeated cycling this peak disappears giving rise to a small hyster-

esis between the ascending and the descending sweeps. A steady state, potentiostatic current-potentialrelationship for the hydrogen evolution is presented in Fig. 2.

An excellent Tafel region over several

219

IO-*

Fig.

2.

A

106

cathodic

obtained a long

decades being

Tafel

of very

Tafel

region

high Viz. of

by

hydrogen

evolution

the

the

down to

meet

is

potential

the

observed

A similar

current

3)

stayed

nearly

a behavfour

valve-metal

like

behaviour

is

trode

at

formed the these

Tafel

of

a

is

only

at

the

potential;

current

reversible A cme2. cathodic

‘mfxed

current

slope generally nature

density

the

value for

the

The potential potential)

corrosion’

and

is

potential

the

high

Tafel

slope

semiconducting

1 x 1C5

potential

anodlc

at

curves (this

current

meet)

mV.

oxygen

evolution

a long

‘transition’

around be

the

The exchange to

the

-136

would 1131;

value phases

cathodic

constant

Such

Given

bubbling)

electrode

with

around

&,

the

map the

that

potential.

Si02

of

analogue,

to

of

of

line

(commenced

condLtions

in

IN H+SO,+ (N2

this

case.

Tafef

a value

which

attempt (Fig.

in

changes

obtained

surface

present

line

anodic

these

of

the

Tafel

at

5 and 6 $;

the

yields

its

under

successful

in

FeSi2

step-wise

is

presence

extrapolating

nominal

may be noted.

between

cathodic

on W/s)

density

[131, e.g., a hydride obtained

which

(0.5

current

characteristic

plot

by slow

2 x

10q4

consfstent

this

is

not

viz.

1.0

-

region A CIII-~ with with

excluded

that if

2.0

V was

less

was observed; the of

the

increase

an

of

‘incipient’

a surface

layer

of

on anadization. lack

of

anodic

visible potentials

corrosion it

or was

surface

naturally

disintegration of

interest

of to

the

examine

elecIts

An anodlc

3.

Fig.

Tafel

bubbling) would of

lU7l fO26-

plot

normally

oxide

I

(0.3

covering

potential

I

is

1

on nominal regfon

A long

be observed.

growth

1

mV/s steps)

the

observed,

in

FeSi2

which

‘transition’

in IN H2S0, oxygen

region

(N2

evolution

characteristtc

however.

I

I

’

I

I

1

I

’

I

I

962937-

200 Fig.

4.

400 A

cyclic

possible sweep working

(-)

no

evidence from

activity taken

voltammogram and

N2

drawn

600 800 E vs RHE (mV)

first

electrode

then

for the

for

(100

oxygen

oxygen

loo0

(----)

with

oxygen nitrogen

chamber,

is

Tafel

reduction and

virtually

on

nominal

bubbling

reduction

steady-state

the

mV/s)

42co

then no

in

FeS12 the

working

Similar

present.

In

H$O,,,

with

compartment;

conclusions

also

relationships.

In a slow

reaction. with

oxygen

difference

potentiodynamtc

bubbling

was

observed

through (Fig.

the 4).

221

Exactly

similar

runs

where,

from

that

results

again, in

electrode

nitrogen

displays

To examine more

the tion’

were

num descends

‘mirror

down to

the

rise of

current

to

the

the

and

for

curves

potential

region), (=

extremely the

available

It

is

oxygen

reduction

these

electrodes

of

at

are

of

the

region, 358 mV.

followed

for

vigorous

gas

fluctuations

bubble-free

V,

line

the

for

atomic

5)

and

on platibecomes almost

the in the

and the

and

a ‘transi-

is

fluctuations

in

somewhat

then

(which

evolution

at

1.6

The current

are

the

anodic

example,

region

consequent

in

end (Fig.

at

by a Tafel there

that reaction.

large

anodic

state

in oxygen

therefore

commenced

transition

1 A cm-‘),

(&,

clear

covering

passivation

cathodic

steady

indistinguishable

the

by starting

a long

anodic

potentiostatic,

curves

‘mixed’

with the

both

surface

densities

the

region

behaviour

When the

the

to

formation/collapse, potential

6).

in

observed.

no activity

potentiostatic

to

image’ high

owing

again

and cathodic

similar

giving

obtained transition

examined,

end (Fig.

region

very

anodic

regions

cathodic

was

steady-state

cathodic

also

absolutely

the

detail

cathodic

were

a nondescript

real

n

HER; at current bubble

electrode

level)

electrode

surface.

t600-

800% w

358 O-

-800-

Fig.

5.

A

steady-state

tionship anodic potential

(3.3

on nominal and

cathodic

(1.6

V).

mV/s), FeSi2

in

region;

potentiostatic

current

IN H$?O,, (N2 bubbling), the

plot

was

commenced

potential covering

at

the

relaa broad anodfc

-800 t

Fig.

6.

Same conditions cathodic

When one examines the

cathodic (Fig.

anodic

side

experiment formed to

6)

is

around

also

supported

the

two cases;

(Fig.

the

6)

concluded. found

cathodic It

as

hydride

but

it

is

it

is

A final (Fig.

6)

potentials, was also

electrodes

is

the

is

probably that

fact

that

on the on point

even

is

cathodic

side

anodic

when the

the

so

(Fig.

indicate

(Fig.

a normal

are

no

the

corrosion’

current-potential

are

when

the

phase

is rise

run is

presumably transition

transition

‘bump’

structure/composition conclusions

potential

is

hydrides

‘surface

transition

the

giving

cathodic

These

surface

on

when the

small

in the

where

thus

oxides’

involved.

where

that

51,

side,

transition

oxide’

However,

that

6);

cathodic

curve

are

oxides’

region was

are

different

on the

in

indicated have

been

anodic

side

commenced

at

the

be expected.

interest anodic

side

highest

started

‘surface

‘surface

due to a change

‘mixed

as would of

any

probably

that

side

region

the

the

a

regions.

case

the

anodic

run was

would

cathodic

transition

phases

thought

towards

together

Tafel

at

the

the

when the

potentials,

potentials,

indeed

by the

observed

on the

run from

although

anodic

cathodic

1 x 10e3 A cm-* of

is

the

the

results

the

reduced

run was commenced V).

disappears

that

These at

cathodic

arise,

etc.

as

the

(-1.4

by starting

nearly

and cathodic

in

5 but plot

same curve

5).

high

need

the

steep

slowly

reduced

region

as

anodic

at

Fig.

in

region

commenced

which

started

is

(Fig. is

clear-cut

already

the

transition

region

as in

potential

to

explore

oxidation

of

the

possible

organic

activity

molecules

and

of

these

chloride

223

ions.

If potassium

(cf. -

Figs.

observed

3,

(Fig.

acetate

5 and 7).

6)

is added to the sulphuric

extending

over

a long

no indications

Similarly,

acid, a transition

range

of anodic

suggesting

anodic

region

potentials

is

oxidation

of

FeSi2

IN

2400-

1800 5 w Coo-

600-

I ( pAlcm2) Fig. 7.

Anodic H2S04

polarization

possibly

associated

only a transition

either

methanol

curves

taken

of

potential

the transition that

the

properties zinc. up

or

in

V;

shifted region

or hydrazine

to less anodic

penetrate to the

novel

potentials

are discussed

conceptual

origins

step)

curve

on

nominal

No evidence

evolution

and/or

of a Tafel the Kolbe

the

conclusions

separately

quite

noted

similar

values

to somewhat

cases

features

were

in

in

region

reaction;

current-potential

of

Even

higher values; oxide'

acetate,

the case

(Fig. 8) except

and the current

'surface

in the next section,

and

formate

polarization high owing

( >5

the

at which

this would

methanol,

at extremely

that

density

present

refer to the anodic observed

of the experimental

the

or potentiodynamically.

ions was

is observed

Lens

contrast

some

oxygen

potentiostatically

All the foregoing

to 3.0

with

of chloride

chloride

mV/s

(N2 bubbling).

region is seen.

formate

either

the oxidation

mixed

(0.83

+ 0.1 N CH$OOK

suggest

modify

its

or hydravalues of V)

anodic

to the different

data at these high anodic

potentials.

224

00

10

IO0 I(pA/cm2)

Same as in Fig. 7 except

Fig. 8.

+

1M NaCl.

growth

Electrode

V,

one

curiosFty

only

at Extremely

observes

as to what,

PAR system

is not designed

acidic

passivation'

NaCl Tafel

of a transition electrolytes,

the

around

trode

occurring

reactions

electrodes the absence

covered

of oxide for oxygen

in the

slopes

V;

layers

(z LOP)

enters of

supply

(50 V;

this

mode.

into

a

'post-

line

is

around

of elec-

with

semicon-

evolution

in anhydrous

since

are characteristic

electrodes

presumably

The

out these experi-

power

covered

[13] and have been previously

e.g., anodic

layer,

ions, which

slope

to

up

region

potentials;

in a galvanostatic

region

the

that

growth.

post-transition

we carried

(PRT 200-1X)

(Figs. 3, 5, 6, clear

of an oxide

a Hewlett-Packard

transition 4.0

processes,

by a NiF2

of chloride

place

region

at very high anodic

conducting

on

thick surface

in many electrochemical

with

potentiostat

Such high Tafel

ducting/insulating

takes

for this type of work,

600 mV (Fig. 9).

line

it was

characteristic

of FeSiR

either

at

Tafel

Polarizations

in various

solutions, line

characteristic

occurrence

if anything,

galvanostatically,

region

of a possible

High Anodic

the behaviour

now is 1N H$O,,

solution

evolution.

a behavtour

500 mA) or with a Tacussel In

a transition

no evidence

polarization

led us to explore

ments,

only

of the persistent

7, 8) on anodic

I$

that the electrolyte

and/or chlorine

Behaviour

Because

Again

is observed;

evolution

3.0

lo2

of fluorine

hydrofluoric

penetrate

acid

observed on nickel [Lb].

into the surface

In

oxide

225

7-

I

I

111111~

I

I

I

I

IllIll

I

I

I

lllll~

lllll

6-

KP

0'

Iv Current density , A/cm2

Fig. 9.

Galvanostatic

anodic

NaCl solutions chlorine

making high

typical

on

Nb,

enough

metal

Zn

Hf,

gets

evolution.

At

Si

etc.

converted low

line appears

cates

that it is an oxide evolution.

build-up the

oxide

metal

growth

type

11, where function tions;

CH30H;

the magnitude

data

(Fig.

of behaviour

(Fig.

build-up

curves acid

HCOOK;

clearly

sol"tio"s

CH3COOK;

NH*-NH2;

waiting

with

region

clearly

one

of

In solutions

potential

sense

The general

indicated, oxide

metal

tndi-

in the sense

in this

only.

however,

in a variety

characteristics following

containing

of

these valve-

in Fig.

has been followed

the

a

line, for example,

for the termination

of 1 A cm-'

valve

oxygen

resembling

the initial

density;

a

high

on Ta,

(3.5 V/decade)

after

at

iron-silicon,

concomitantly

interest

without

NaCe.

value

the growing

classic or

IM

to

very

growth

the

than a Tafel

current

polarization

have with

oxide

a linear

without

10) is more across

up

10) are not steady-state

density

at the chosen

build

case,

of its slope

rather

(Fig. 10) are of qualitative

the voltage

the

region

anodic

oxides

(albeit

IN HZSO,, leads

can

present

although

over lo-15 minutes) process

the

iron-silicon

of time at an anodic

sulphuric

In

densities,

These

+

line for the

evolution

in

one

for the case

to the next current

(occurring

relationships

as

growth

in IN H$O,+

a Tafel

chlorine

polarization

lo), a,

[13].

(Fig. lo),

FeSiZ

densities;

vigorous

anodic (Fig.

to

current

Tafel

that one shifted

the

polarization,

and

of nominal

around 4.0 V may be noted.

to sustain

behaviour

anodic

presumably,

for oxygen

starting

polarizations),

valve

voltages Al,

evolution

it conducting anodic

polarization

up to very high current

as a

of solu[13]

in

additives:

chloride

ions,

Current density Fig. 10. Galvanostatic very

high

layers, the

I

densities;

the

to the oxygen

'electrode

potentials'

attained,

layers)

behaviour 6@

current

polarization

concomitant

high

oxide

anodic

, A/cm2

so

of nominal build-up

FeSip

of

evolution, (actually

up to

insulating

oxide

thick

is clearly voltage

characteristic

in 1N HpS04

of

indicated

drops

a

across

valve-metal

by the type

[13].

I

I

I

I

I

I

I

I

*?/A-A-

A

4 A-A

,,,; .

.-.

A do-mOo

I to

I 20

I

I

30

40

Fig. 11. The potential-time nominal

FeSi,

&lM

NH2-NH 2;

l,lM

I

50 60 Time 1s) curve observed

in H$O,,

(where applicable):

I

solutions

70

on anodic

90

100

polarization

containing

x, none; A, 1M CH30H; NaCl.

60

the

(1 A cmq2) of

following

0, 1M HCOOK;

A, 1M

additives CH3COOK;

227 the oxide growth stops at around 4.0 V (Fig. 11) with chlorine evolution as the preferred reaction (Fig. 9); in the presence of the organic molecules, the valve-metal type of characteristics, as indicated by the oxide growth up to quite high voltages, are maintained however.

At these very high anodizations

there are some visible signs of electrode attack, with erosion and/or formation of a whitish deposit is some cases; also, at the higher end of oxide growth voltages, potential oscillations, characteristic of dielectric breakdown in valve metal oxides [15], were also noted in some cases.

CONCLUSIONS

Electrodes of (nominally) iron disilicide composition display a remarkable electrochemical stability and can sustain some electrodic reactions at quite high rates, e.g., hydrogen evolution reaction.

RElWU3NCl3S

D.E. Brown, M.N. Mahmood, A.K. Turner, S.M. Hall and P.O. Fogarty, Int. J. Hydrogen Energy, 1 (1982)

405.

D.E. Brown and M.N. Mahmood, Eur. Pat. 0 009

406

(1980).

A.K. Vijh, G. Bclanger and R. Jacques, Prog. Bat. Solar Cells, 2 (1984) 255; JEC Press, Cleveland, Ohio.

R.W. Murray, Acct. Chem. Res., -13 (1980) 135; e, (London), A302 (1981) 253.

I. Rubinstein and A.J. Bard, J. Am.

Phil. Trans. R. Sot.

Chem. Sot., -102 (1980) 6641.

J.P. Randin in J.O'M. Bockris, B.E. Conway, E. Yeager and R.E. White, (eds.), Comprehensive Treatise

of

Electrochemistry, Vol,

4,

Plenum

Publishing Corporation, New York, 1981, p. 473.

J.P. Randin, J. Electrochem. Sot., -121 (1974) 1029; a, 742.

J.P. Randin, J. Electroanal. Chem., 51 (1974) 471.

ibid, 122 (1975)

228 9

J.C.

Bailar, H.J.

Emel&s,

R.

Nyholm

and

A.F.

Trotman-Dickenson,

Comprehensive Inorganic Chemistry Vol. 1, Pergamon Press, New York, 1973, p. 1356.

10

R.L. Rickett in T. Lyman, (Ed.) Metals Handbook, Amer. Sot. for Metals, Novelty, Ohio, 1960, p. 1217.

11

A.K. Vijh, G. Bglanger and R. Jacques, J. Power Sources, fi (1981) 229; A.K. Vijh, R. Jacques and G. Bblanger, J. Power Sources, 2 (1983) 10.

12

A. B'elangerand A.K. Vijh, Surface Tech., -15 (1982) 59.

13

A.K. Vijh, Electrochemistry of Metals and Semiconductors, Marcel Dekker, New York, 1973, p. 167.

14

A.K. Vijh, Surface Tech., 4 (1976) 401.

15

A.K. Vijh, Corr. Sci., 11 (1971) 411.