Graphite anode reaction in the KF2HF melt

Journal of Fluorine

Chemistry, 40 (1988) 407-417

407

GRAPHITE ANODE REACTION IN THE KFZHF MELT* T. NAKAJIMA, T. OGAWA and N. WATANABE Department

of Industrial

**

Chemistry,

Faculty

of Engineering,

Kyoto University, Sakyo-ku, Kyoto, 606 (Japan)

SUMMARY Effects of traces of water on the graphite have

been

investigated

voltammetry

in the KF2HF

melt

anode

at

reaction

100 OC. Cyclic

shows that with increasing water content from 0.01%

to O-OS%, anode potentials

for the formation

of graphite

and graphite fluoride films on graphite electrodes

are

oxide shifted

to lower potentials. This may be ascribed to increase in the reaction of discharged oxygen with graphite and the subsequent decomposition

of graphite

oxide film by attack of discharged

fluorine, which gives ultimately

graphite

fluoride

film

on a

graphite electrode. When the water content is 0.05 %, the anode effect is thus caused in a short time by graphite fluoride film with a low surface stage

energy. However,

4 intercalation

prepared.

Addition

compound

when

it is O.Ol-0.02%, a

of graphite,

C,+HF~-

was

of 3-6 wt% LiF to the melt gave a stage 3

CxF(HFjy without occurrence of anode effect. INTRODUCTION Fluorine gas is produced by the electrolysis of KF2HF melt at around 100 *C using carbon anodes. Only carbon electrodes are effective as anode materials in the melt. difficult

However, it is

,because of'the. so-called anodec;effect to electrolyze

KFZHF melt at a high current density. The anode effect is caused * Dedicated to Emeritus Professor W.K.R. Musgranre on the occasion of his 70th birthday. **Present address: Applied Science Research Institute, 49 Ooicho, Tanaka, Sakyo-ku, Kyoto, 606 (Japan). 0022-1139/88/$3.50

0 ElsevierSequoia/F'rinted inTheNetherlands

408 bY

the carbon melt

surface

low

the

graphite metal

markedly

fluoride

fluoride

melt

order

we

CuF2

or AgF[l-31.

fluoride

having Among

atmosphere[21.

with

metal

effect

electrical

a high

the wettability

investigation a trace

this graphite

the intercalation

on

compounds,

conductivity

contained

reaction[6].

of water

of fluorine

It is that the

In

this

in graphite

the

by

forma-

ionic bonding, carbon

anode

the KF2HF

melt.

that not only

in the melt

on the graphite

of

interpre-

a new

by

revealed

shows

chemical

formation

to the

surface

AgF

strong

compound(GIC)with

of its

in fluorine

so far,

the

to

as a higher

ability

of the

Based

such

is considered

examined

on this problem

of water

anode

of a trace

fluoride

in graphite

may be suppressed

which

effect

has not

fluorine-graphite

for the role of solid LiF[5].

intercalation

Recent

but the role

in the anode

new

because

tion of a graphite

but also

of a KFZHF

to the

effect,

oxidative

intercalation

of the anode

gives

the

of

that addition

fluoride

fluorides

fluorine[41.

was proposed

and ensures

by KFZHF

energy

of a metal

intercalation

ability

such fluorine-graphite tation

The

a strong

metal

catalytic

interaction

a

in the presence

to the fluorine

occurrence

this anode

or impregnated

film on

surface

a low solubility

synthesized

compound

highest

having

for preventing

Recently

contribute

of anode

to the low surface

has long been known

in the melt

fluoride

previously.

intercalation

the

film.It

is effective

of LiF suspended

as LiF,

owing

such as LiF

been clarified

of a thin graphite

i.e. the wettability

anode,

decreases

energy

seriously

LiF

affect

paper,

we report

the

anode

reaction

and

in the KFZHF

melt.

EXPERIMENTAL The electrolytic

cell

is

and polytrifluorochloroethylene. electrode with was

were

a large

grafoil(graphite

surface

a platinum

wire.

area,

made

of polytetrafluoroethylene

Working sheet

electrode

respectively.

Fluoride

voltanunetry and galvanostatic

evolution

and

counter-

35x6 mm) and nickel The reference was monitored

electrolysis

at 33 mAcm

plate

electrode by -2

.

cyclic The

water content in the melt was estimated by the method previously[6]. After electrolysis,

the

graphite

reported

anode

was

analyzed by X-ray diffraction.

RESULTS AND DISCUSSION Effect of a Trace of Water on the Graphite Anode Reaction Figure graphite

1 shows

anodes

the cyclic

were

voltammograms

obtained

used in KF2HF melts containing different

amounts of water. All the voltammograms in Fig. 1 were as first scans. It

has

when

recorded

been pointed out in a previous papert

that a small amount of water less than 0.1% significantly influences the fluorine evolution reaction on graphite anodes. It known that peak A corresponds to the oxygen

evolution

is

reaction

by electrolysis of water contained in the melt[7]. Peak A: H20 -1/202

+ 2H+ + 2e

(1)

With decreasing water content from 0.05% to O.Ol%, the intensity of peak A decreases. In any electrolytic bath, the peak A is not anode

observed in the second scan because the wettability of an

by the KFZHF melt considerably decreases due to the partial foranode

mation of graphite fluoride films on the graphite

during

the first scan. Peaks B and C, however, move to higher potentials with creasing water content. The peak B, observed as

a shoulder

content is

relatively

large, say 0.05%, the anode effect occurs immediately, i.e. graphite fluoride film is quite easily formed on

in been

most cases, also disappears in the second scan. It has not identified previously. When the water

ae-

the

anode[61. It has been also found that graphite oxide

the

graphite is

easily

fluorinated to graphite fluoride at around 100 V[8],

probably

because graphite oxide is more unstable than graphite

fluoride,

therefore substitution of oxygen or hydroxyl by fluorine is very easy. From these facts, the peak B may be assigned to the formation of graphite oxide film on the anode surface.

0

2

4

6

8

10

4

6

8

10

400 ’ 300 200 looA o-

./‘

. 2 Anode

pOtentiaj/v

Fig. 1. Cyclic voltammograms for graphite anode in the KFZHF melt. Water content: (a)0.05%, (b)0.03% (c)0.01%. Scan rate: 10 mV/sec.

411

Peak B: XC + H20 ------LCXO(graphiteoxide film) + 2H+ + 2e Graphite oxide has a covalent bond,

as

has

(2)

graphite fluoride,

therefore it is almost an electric insulator(91. After peak B, since fluorine evolution

occurs vigorously,

graphite oxide is attacked by discharged fluorine, which would give the anode pristine

a

more active and larger surface than that of This facilitates the reaction of graphite

graphite.

anode with discharged

fluorine, i.e. the formation of graphite fluoride film. Thus the following reactions would occur between

peak B and peak C: F--l/2F2

+ e

C,O + 2F- ----*x'C x’C

+

F-e

(3) + COF2 + 2e

(4)

C,IF(graphite fluoride film) + e-

(5)

Then with increasing water content in the melt, the formation of graphite oxide and graphite fluoride are both accelerated. This would be the reason why the peaks B and C are shifted to lower potentials with increasing water content.

Intercalation of Fluorine in Graphite Based on the above result,anodic polarization of the graphWhen the water ite electrode was made by galvanostatic method. content in the melt is 0.05%, the anode effect occurs in a short time, i.e.the anode surface ii covered with graphite fluoride at least to some extent without formation of ionic intercalation compound of graphite(61. This is almost the same even when LiF is suspended patterns

in the melt[61. In these cases, X-ray diffraction

show

only

(002) line of graphite

because

graphite

fluoride film is very thin. Graphite fluoride film is detected only by ESCA measurements(l01. However, with decreasing water prepared GIC having ionic bonds are content to O.Ol-0.02%,

412

(A) graphite

(004) A ,

4

20

10

30

40

50

60

70

60

70

60

70

(6) graphite

(004) I

I

10

20

30

40

50

(C)stage

,

4-6

<

I

10

20

30

40

50

Fig. 2. Change in X-ray diffraction patterns obtained in the melt containing 0.01 -0.02% water as a function of quantity of electricity. Quantity of electricity/C*cm-2:

(AjO, (B120, (C)60, (D)lOO, (E)200, (F)380.

413

(D)stage

4-6

10

20

30

40

50

60

70

10

20

30

40

50

60

70

60

I 70

(005) _

,L&;

:c1:l%

, 10

20

40

30 281

(Cu Kd

50

yto,

414

(A)

grap+hite

high stage

(6) stage 4 +,5 1,=16.40 A, 19.74 i (0 010) (O&I 10

20

30

40

50

60

70

10

20

30

40

50

60

70

Fig.

3. Change

in X-ray

obtained -0.02%

diffraction

in the melt

water

and

patterns

containing

3-6 wt% LiF as a

function

of quantity

Quantity

of electricity/C-cm-':

(B)60,

(C)100,

(G)4000.

0.01

of electricity.

(D)200,

(E)400,

(~)20, (F)lOOO,

415

,

I

40

50

60

70

60

70

60

70

60

70

10

20

30

10

20

30

40

50

10

20

30

40

50

c

*

1

(G) stage

10

20

30 2 e/g

40 (Cu Kod

50

4

3

416

while

electrochemically fluoride

film

becomes

Figure

2

+ e

shows

and

the

GIG's

(6)

X-ray

prepared

0.02% water.

The GIC

Ccmw2 (Fig.2

(C), (D)), after

until

the anode

polarization improved

effect

as shown

intercalation

patterns melt

was

KFZHF

a mixture

which

the

a stage

crystallinity

of

the

O.Ol-

4-6 till

4 GIC

was

With

(F)). of

pristine

containing

of stage

occurred(Fig.2(E),

hand,

when

of fluorine

of the anode

intercalation

diffraction in the

LiF was

added

observed increasing

stage

4 GIC

to the melt

was considerably Thie

effect.

of fluorine

as reported

was

without

accelerated

to the catalytic

is due

in graphite

(3-6 wt%)

by solid

LiF

suspended

In this

(6) and

a mixture A

stage

the

of

GIC

GIC

When

stage

3 GIC

remained

the

The

dimension fluoride

It(i)=

of the GIC

a main

distance

equation expanded ion(2.7

6.05

for

expressed

was

increased

component.

should

the

stage

(C)-(F)), though

with

reached Figure

be

as CxF(HFly.

(B)) and

Ccm-'(Fig.3

of electricity

3 GIC obtained

repeat

following

therefore

1000

intercalation

the product

observed(Fig.3

4 till

quantity

became

stage

first

and chemical

together,

and C,+F-,

was

crystallinity

time.

typical

(7), proceed

of Cn+HF24+5

number

(7)

as the electrochemical

case,

in

previously[l,2,61.

nC + l/2 F2bCnl'F-

reactions,

100

in the figure.

On the other

the melt

formed

time,

occurrence

of graphite

slow.

nC + HF2- jC,+HF2-

grafoil

rate

formation

the

polarization 3000

Ccmm2,

3 (G) shows

a the

at 4000 CcmS2.

along both

c-axis Cn+HF2-

by intercalation

is expressed and

C,+F-

is determined

by the

because

the

by the size of

A).

+ 3.35(n

- 1)

(8)

417

shown

As

above,

the main

initial

graphite

fluoride

shows

higher

[1,2,ll],and which

with

product

to the GIC with

electrical

permits

water

continuous

surface

in the melt,

is converted

bonding.This

graphite

than graphite

evolution

from

kind of GIC

than the pristine

energy

fluorine

content

anode

ionic

conductivity

has a higher

of the anode

decreasing

in the graphite

fluoride, occurrence

without

effect.

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