anode effect in aqueous electrolysis

8/10/2019 Anode Effect in Aqueous Electrolysis

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A n o d e E f f e c t i n A q u e o u s E l e c t r o l y s i s

HERBERT H. KELLOGG

Schoo l o f Mines Columbia Univers i t y Ne w York Ne w York

ABSTRACT

A phenomeno n which closely resembles anode effect in molten electrolysis can be

developed in the electrolysis of aqueous solutions at high current density. Normal

operation of the electrode ceases and a so-called tra nsit ion period begins when the

electrode temperat ure reaches the boiling point of the electrolyte. When the app lied

voltage is increased beyond a critical value the transition behavior suddenly changes

to the aqueous anode-effect. Duri ng this effect the surface tempe ratur e of the anode

rises far above the boiling point of the electrolyte. Evidence is provided which indicates

tha t the gaseous envelope surroun ding the anode duri ng the aqueous anode-effect is

main tain ed by the vapor ization of the electrolyte aga inst the hot anode surface. An

aqueous cathode-effect was also obtained. The relation between aqueous anode-effect

and anode effect in molten media is discussed.

INTRODUCTION

Since the earliest experiments with the electrolysis

of molten salts, the behavior called anode effect

has been reported and many investigators have at-

tempted explanations of this peculiar and trouble-

some phenomenon. The description of anode effect

by C. S. Taylor (1) is quoted below:

The anode, during t he normal course of electrol-

ysis, is surrounded with a large number of gas bub-

bles which are constantly escaping from it. These

small bubbles seem to form on the anode, and then

break away easily and escape from the electrolyte by

breaking through t he surface film. This smooth, even

evolution of gas around the anode is always a sign

of normal operation. The moment the anode effect

occurs, however, conditions are entirely different.

The anede appears to be entirely surrounded by a

film of gas, which, by covering the surface of the

anode, pushes the fused electrolyte away, and thus

produces the so-called 'non wetting' of the anode.

As the electrolyte is pushed away, small arcs form

between the electrolyte and the anode.

The lit erature on anode effect is rich in factual in-

formation on the occurrence and control of the

phenomenon in a variety of molten electrolyses (2, 3,

4). Noticeably lacking, however, is a satisfactory

hypothesis concerning the anode effect which will ex-

plain what forces are responsible for holding the

molten electrolyte away from the anode in the form

of a gaseous envelope despite the ever present hydro-

static forces which tend to collapse this envelope.

To the best of the author's knowledge, the idea that

this envelope is maintained by the rush of gases

evolved at the anode is the most widely held hypoth-

esis at the present time.

1 Manusc ript received August 2, 1949. This paper pre-

pared for delivery before the Cleveland Meeting, April 19

to

22

1950.

This paper shows that a phenomenon can be ob-

tained with electrodes which evolve gas in aqueous

electrolysis, which phenomenon closely fits the pre-

viously cited description of anode effect in molten

electrolysis. Furthermore, by analysis of pertinent

data on this aqueous anode-effect, an explanation

of the forces which form and maintain the gaseous

envelope has been arrived at. It seems very likely

that the explanations of aqueous anode-effect may

also be valid for the anode effect in molten electrol-

ysis, and it is contemplated to investigate this possi-

bility in a future paper.

133

EXPERIMENTAL

The experiments on aqueous anode-effect were

made with the following circuit: The cell was an 800-

ml Pyrex beaker in which were placed the anode to

be investigated (in most instances a platinum wire,

1.25 mm diameter, immersed to a depth of 8 mm)

and a platinum cathode (4 x 6 cm sheet). The elec-

trolyte (usually 1.0N H2S04) was added so as to

almost fill the cell. A glass stirring-rod, bent into a

loop at the bottom, was attached to a motor and

rotated at 450 rpm. A mercury thermometer was

used to record the temperature of the electrolyte.

Direct current was supplied to the electrodes by

means of a Voltage divider connected across a 115

volt source. An ammeter was placed in series with

the cell, and the total cell voltage was measured

across the electrodes with a voltmeter.

The measurements of electrode temperature were

made with an electrode constructed in the following

manner: A tube Of 25-20 chromium-nickel ste el,

closed atone end, 3.5 mm OD, 2.7 mm ID, and 50 mm

long comprised the electrode. A copper-constantan

thermocouple (30 gauge wire) was threaded through

a two-hoIe ceramic insulator and inserted into the

steel tube so that the junction was in contact with

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134 J O U R N A L O F T H E E L E C T R O C H E M I C A L S O C I E T Y April 1950

t h e c l o s e d e n d o f t h e t u b e . A n e f f i c i e n t t h e r m a l c o n -

t a c t b e t w e e n t h e t h e r m o c o u p l e j u n c t i o n a n d t h e t u b e

w a s m a d e b y p l a c i n g a b o u t 5 0 m g o f a l o w m e l t i n g

( 8 0 ~ a l l o y o f l e a d , t i n , a n d b i s m u t h in t h e b o t t o m

'

2

O 2

F L

o -o - - , ~

4 6 eO I o t2o

olts

FIG. 1. An ode effect with p latinu m-w ire anode (0.314 cm~

imm ersed area) in norm al H~SO4 at 66 ~ 4- 4 ~ C.

FIG. 2. Normal operation of platinum-wire anode. 1N

H2S04 at 40 ~ C, 27 vo lts, 5.4 ampe res, a bou t 8 X.

o f t u b e a n d t h e n g e n t l y h e a t in g t h e t u b e w i t h t h e

t h e r m o c o u p l e i n pl a ce . T h e r m M e m f w a s m e a s u r e d

w i t h a p o t e n t i o m e t e r .

T h e p h o t o g r a p h s w e r e m a d e w i t h a b e n c h c a m e r a

e q u i p p e d w i t h a 4 2 - m m M i c r o - S u m m a r l e n s . T h e

i l l u m i n a n t w a s a n I d e n t i f i c a t i o n F l a s h O u t f i t u s e d b y

t h e S i g n a l C o r p s , a n d c o n s i s t ed o f a p o w e r s u p p l y

a n d t w o g a s - d i s c h a r g e f l a s h - l a m p s . T h e f l a s h i n -

t e n s i t y i s r e p o r t e d a s e l e v e n t i m e s m o r e i n t e n s e t h a n

s u n l i g h t a n d t h e d u r a t i o n a b o u t 1 / 1 0 , 0 0 0 s e c o n d .

D I s c u s s i O N

Description of Aqueous A node Effect

T h e d i s c u s s io n o f t h e m a i n b o d y o f t h e d a t a a n d

t h e d e v e l o p m e n t o f a h y p o t h e s i s f o r t h e a q u e o u s

anode-e f fec t wi l l be fo l lowed wi th g rea te r ea se i f p re -

ceded by a desc r ip t io n o f a typ ica l e lec t ro lys i s which

resu l t s in the anode e f fec t .

F i g . 1 r e c o r d s in g r a p h i c a l f o r m t h e v o l t - a m p e r e

re la t ionsh ips fo r a cel l cons i s t ing o f a shee t - p la t inum

ca thode (4 x 4 e ra ) and a p la t inum -wire anode (0 . 31

e m 2 i m m e r s e d a r e a ) , w i t h a n e l e c t r o l y t e o f n o r m a l

H=S 04 . The fo l lowing desc r ip t ion app l ie s to the be -

h a v i o r o f th e w i r e a n o de , w h e n t h e b u l k t e m p e r a t u r e

i s m a i n t a i n e d a t 6 6 ~ 4 - 4 ~ W i t h a lo w v o l t a g e i m -

pres sed on the ce l l , bubb les o f oxygen a re evo lved

a t t h e a n o d e i n a p e r f e c t l y n o r m a l m a n n e r . T h e

v o l t - a m p e r e r e l a t i o n f o r t h e c e l l i s t h a t g i v e n b y t h e

reg ion A-B in F ig . 1 ; the cur ren t inc reases a lm os t

l inea r ly wi th inc reased vo l tage . F ig . 2 i s a pho to-

g r a p h o f t h e g a s e v o l u t i o n a t t h e w i r e a n o d e d u r i n g

t h i s n o r m a l o p e r a t i o n o f t h e c e ll ( c o m p a r e w i t h F i g .

3 w h i c h s h o w s t h e w i r e a n o d e w h e n n o c u r r e n t i s

f l o w i n g ) . T h e o x y g e n b u b b l e s f o r m r a p i d l y , b r e a k

a w a y f r o m t h e e l e c t r o d e , a n d r i s e q u i c k l y t o t h e s u r -

f a c e o f t h e e l e c t r o l y t e w h e r e t h e y b r e a k . T h e o p e r -

a t ion o f the ce l l i s qu ie t and s teady .

W h e n a c u r r e n t d e n s i t y r e p r e s e n t e d b y t h e p o i n t

B on F ig . l i s reached , a new behav ior beg ins . The

v o l t - a m p e r e r e l a t i o n o f t h e c e l l i s e r r a t i c . S p i t t i n g

a n d h i s s i n g n o i s e s a r i s e f r o m t h e a n o d e , a n d m a n y

o f t h e g a s b u b b l e s a r e p r o j e c t e d d o w n a n d a w a y f r o m

t h e a n o d e b y t h e s u d d e n s p i t s . T h e e l e c t r o l y t e

c l o s e t o t h e a n o d e i s h o t a s e v i d e n c e d b y t h e p r e s -

e n c e o f c o n d e n s e d w a t e r v a p o r i n t h e e v o l v e d g a s e s .

A f u r t h e r i n c r e a s e in i m p r e s s e d v o l t a g e c a u s e s n o i n -

c r e a s e ( o r e v e n a d e c r e a s e ) i n c u r r e n t t h r o u g h t h e

ce l l , a s shown by B-C on F ig . 1 . F ig . 4 i s a pho to-

g r a p h o f t h e w i r e a n o d e d u r i n g t h i s b e h a v i o r , w h i c h

t h e w r i t e r h a s c a l l ed t h e t r a n s i t i o n p e r i o d .

When the vo l t age i s ra i s ed to a c r i t i ca l va lue (nea r

C in F ig . 1 ) , an ins tan taneous change t akes p lace .

T h e v o l t a g e r i s e s s u d d e n l y a n d t h e c u r r e n t d r o p s

to a low va lue (po in t D in F ig . 1 ) . The loud sp i t t ing

a n d h i s s i n g , w h i c h a c c o m p a n i e d t h e p r e v i o u s s t a g e ,

c e a s e . T h e w i r e a n o d e i s c o m p l e t e l y s u r r o u n d e d b y a

g a s e o u s f i l m . O c c a s i o n a l t i n y s p a r k s c a n b e s e e n t o

f o r m i n t h e f i lm . F ig . 5 i s a p h o t o g r a p h o f t h e a n o d e

d u r i n g t h i s b e h a v i o r . T h i s is t h e p h e n o m e n o n w h i c h

t h e w r i t e r h a s c a l l ed a q u e o u s a n o d e - e f f e c t .

I n c r e a s e o f v o l t a g e t o p o i n t E o n F i g . 1 d o e s n o t



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Vol. 97 No. 4 ANODE EF FEC T IN AQUEOUS ELECTR OLYSI S 135

alter the behavior described for point D, except tha t

a dull red glow can be seen at the lower tip of the

anode. The writer has taken this glow to indicate

that the temperature of the electrode is high--proba-

bly 750~ Fur the r indication th at the electrode is

hot is that, if the circuit is suddenly opened while

the anode is operating anywhere in the E-F region

an elect rolyte of normal NaOH 2. Fig. 6, 7, and 8

summarize the volt-ampere relations and the anode

temperature for three different bulk-electrolyte tem-

peratures. The behavior for each electrolyte temper-

ature can again be conveniently divided into three

periods: (a) normal operation, A-B in Fig. 6, 7, and

8; (b) transition period, B-C; (c) anode effect, F-D-E.

FIG. 3. Platinum-wire anode. No current flowing, im-

mersion 0.8 cm, about 87


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136 J O U R N A L O F T H E E L E C T R O C H E M I C A L S O C I E T Y Apr i l 1950

p a s s a g e o f c u r r e n t f r o m t h e b u l k o f t h e e l e c t r o l y t e

t o t h e e l e c t r o d e s u r f a c e a r e t h e b u b b l e w a l l s o f t h e

e v o l v i n g g a s . T h e c u r r e n t d e n s i t y i n t h e s e b u b b l e

w a l l s i s c o r r e s p o n d i n g l y h i g h , a n d t h e h e a t d i s s i p a t e d

b y I2R ' ' h e a t i n g i n t h e b u b b l e w a l l s i s h i g h e r t h a n

e l sewhere in the e lec t ro ly te .

T h e t e m p e r a t u r e o f t h e b u l k e l e c t r o l y te h a s o n l y

a m i n o r e f f e c t o n t h e v o l t - a m p e r e r e l a t i o n s i n t h i s

n o r m a l o p e r a t i o n p e r io d . F o r a g i v e n v o l t a g e , t h e

c u r r e n t i s s l i g h t l y h i g h e r f o r t h e h o t e l e c t r o l y t e t h a n

f o r t h e c o o l , a s w o u l d b e e x p e c t e d f r o m t h e h i g h e r

c o n d u c t i v i t y o f t h e h o t e l e c t r o l y t e .

e l e c t r o l y t e t e m p e r a t u r e . T h i s i s r e a d i l y e x p l a i n e d

s ince the co ld e lec t ro l y te wi l l o f fe r be t t e r cond i t ions

FIG. 5. Aque ous anode effect with platinum -wire

anode. IN H2SO4 at 40 ~ C, 70 volts, 1.[ am peres, abo ut 8X .

2. Tran sition Period]

F o r a l l t h r e e e l e c t r o l y t e t e m p e r a t u r e s t h e c h a n g e

f r o m t h e n o r m a l o p e r a t i o n t o t h e t r a n s i t i o n

p e r i o d i s c o i n c i d e n t w i t h t h e p o i n t a t w h i c h t h e

e l e c t r o d e t e m p e r a t u r e r e a c h e s 10 0 ~ 2 ~ T h e

r e g i o n c l o s e t o t h e e l e c t r o d e h a s b e c o m e s u f f i c i e n t l y

h o t t o v a p o r i z e t h e e l e c t r o l y t e , a n d t h e s p i t t i n g a n d

h i s s in g n o i s es w h i c h a c c o m p a n y t h e t r a n s i t i o n

p e r i o d a r e e v i d e n ce t h a t v a p o r i z a t i o n d o e s t a k e

p lace .

F i g . 6 , 7 , a n d 8 s h o w t h a t t h e c u r r e n t d e n s i t y a t

w h i c h t h e n o r m a l o p e r a t i o n e n d s a n d t h e t r a n s i -

t i o n p e r i o d b e g i n s is h i g h e r t h e l o w e r i s t h e b u l k -

FIG. a. Aqueous anode-effect with l)latinum-wire

anode. 1N ]|2SO4 at 90 ~ C, 74 volts, 0.12 ampe res, abou t 8X .

10

E

le ~

i

s

/ 50C

/

/

I

i

, j , . 4 0

. ~

Amp

~ ? 1

F D

2 o 4 0 6 0 8 0 /0 0 1 2 0

Vo l t s

FIG. 6. Anode effect with alloy~steel anod e (0.956 cm2

immersed area) in normal NaOH at 39~177~ C.

f o r h e a t t r a n s f e r a w a y f r o m t h e e l e c t r o d e , a n d , c o n se -

q u e n t l y , a h i g h e r c u r r e n t d e n s i t y w i l l b e r e q u i r e d t o



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Vol. 97, 25o. 4 A N O D E E F F E C T I N A Q U E O U S E L E C T R O L Y S I S 13 7

r a i s e t h e e l e c t r o d e t e m p e r a t u r e t o t h e b o i l i n g p o i n t

o f t h e e l e c t r o l y t e .

F i g . 6 , 7 , a n d 8 s h o w t h a t d u r i n g t h e t r a n s i t i o n

p e r i o d t h e c u r r e n t f a i ls t o i n c r e a s e a n d e v e n f a ll s

a s t h e v o l t a g e is f u r t h e r i n c r e a s e d . T h e e x p l a n a t i o n

o f t h i s b e h a v i o r i s f o u n d b y e x a m i n a t i o n o f t h e

e l e c t r o d e s u r f a c e d u r i n g t h e t r a n s i t i o n p e r i o d . F i g .

4 s h o w s t h a t t h e v a p o r i z a t i o n o f t h e b u b b l e w a l l s

d u r i ng t h e t r a n s i t i o n p e r i o d r e s u l ts i n t h e f o r m a -

t i o n o f a w i d e v a p o r f i l m w h i c h e n c l o s e s a l a r g e s e c -

t i o n o f t h e e l e c t r o d e . D u r i n g t h e t r a n s i t i o n p e r i o d

t h e s e f il m s h a v e a v e r y s h o r t l if e a n d a r e c o n s t a n t l y

I0

Amos

4OO

p

6 ~ 4 ' x 3 g

/ ,.

,,

9 1 0 (

. * - '

0 20 40 6 0 80 /oo Igo

Volts

Fro. 7 . Anode effect with alloy- steel a node (0.956 em 2

imme rsed area) in norm al N aOH at 66 ~ 4 - 4 ~ C .

4

2

L ,.:

g

2 0

40 go

V o l t s

~ ~

A m p ~

~o~

I0

80 I00 120

FIG. 8. Anode effe ct with alloy- steel anode (0.95g em 2

im m ersed a rea ) i n n o rm a l N aO H a t 8 9 ~ 2 ~ C .

f o r m i n g a n d b r e a k i n g - - a b e h a v i o r w h ic h i s r es p o n s i-

b l e f o r t h e u n s t e a d y r e a d i n g s o f v o l t a g e a n d c u r r e n t

d u r i n g t h i s p e r i o d . T h e p r e s e n c e o f t h e s e v a p o r f i l m s

w h i c h p a r t i a l l y e n c l o s e t h e a n o d e r e s u l t s i n a l a r g e

i n c r e a s e i n t h e r e s i s t a n c e o f t h e e l e c t r o l y t e p a t h n e a r

t h e a n o d e a n d , h e n c e , t h e c u r r e n t m a y d r o p e v e n

t h o u g h t h e v o l t a g e i n c r e a s e s .

D u r i n g t h e t r a n s i t i o n p e r i o d t h e e l e c t r o d e t e m -

p e r a t u r e r e m a i n s c o n s t a n t a t 1 00 ~ 4 - 2 ~ T h e r e

r e m a i n s s o m e d ir e c t c o n t a c t b e t w e e n t h e a n o d e a n d

t h e e l e c t r o l y te w i t h t h e r e s u l t t h a t t h e e l e c t r o d e

t e m p e r a t u r e i s p r e v e n t e d f r o m r i si n g a b o v e t h e b o i l -

i n g t e m p e r a t u r e o f t h e e l e c t r o l y te . T h e g a s e v o l v e d

b y t h e e l e c t r o l y s i s c o n t i n u e s t o e v o l v e as b u b b l e s ,

t h o u g h t h e f o r c e of t h e s u d d e n v a p o r i z a t i o n s o f t e n

p r o j e c t s t h e g a s b u b b l e s f a r a w a y f r o m t h e e l e c t ro d e .

3. Anode-EffectRegion

T h e c h a i n o f e v e n t s t h a t l e a d s t o t h e i n s t a n t a n e o u s

c h a n g e f r o m t h e t r a n s i t i o n p e r i o d t o t h e a n o d e

e f f e c t w i ll b e b e t t e r u n d e r s t o o d i f t h e c h a r a c t e r i s t i c s

o f t h e a l r e a d y f o r m e d a n o d e e f f e c t f i l m a r e f i r s t d i s -

c u s s e d .

T h e m o s t s t r i k i n g f a c t s , s h o w n i n F i g . 6 , 7 , a n d

8 , a r e t h e v e r y h i g h e l e c t r od e t e m p e r a t u r e s w h i c h

p r e v a i l w h e n t h e a n o d e - e f f e c t f il m i s p r e s e n t . T h e

l o w e s t a n o d e t e m p e r a t u r e r e c o r d e d u n d e r t h e s e c i r -

c u m s t a n c e s w a s 1 6 5 ~ ; t h e h i gh e s t w a s a b o u t 6 2 0 ~

T h e s e h i g h a n o d e t e m p e r a t u r e s , t o g e t h e r w i t h t h e

f a c t s o n t h e v a p o r i z a t i o n o f t h e e l e c t r o l y t e d u r i n g

t h e t r a n s i t i o n p e r i o d , s u g g e s t a s i m p l e a n d c o m p e l -

l in g e x p l a n a t i o n f o r t h e f o r c e s w h i c h m a i n t a i n t h e

g a s e o u s e n v e l o p e d u r i n g t h e a n o d e e f f e c t .

The anode-effect ilm is prima rily a water-vapor film

surrounding a hot wire. T h e e l e c t r o ly t e is p u s h e d b a c k

f r o m t h e e l e c t r o d e s u r f a c e b y t h e v a p o r p r e s s u r e o f

t h e e l e c t r o l y t e , w h i c h e x c e e d s o n e a t m o s p h e r e a s a

r e s u l t o f t h e h i g h e l e c t r o d e t e m p e r a t u r e . I f t h e f il m

a t t e m p t s t o c o l la p s e u n d e r t h e i n f lu e n c e of h y d r o -

s t a t ic f o r c e s w h e n t h e e l e c t r o l y t e s u rf a c e a p p r o a c h e s

c l os e t o t h e a n o d e , f u r t h e r v a p o r i z a t i o n w i l l t a k e

p l a c e a n d p u s h t h e e l e c t r o l y t e b a c k .

T h i s h y p o t h e s i s o f t h e f o r c e s w h i c h m a i n t a i n t h e

f i l m i s i n a c c o r d a n c e w i t h t h e v i s u a l o b s e r v a t i o n o f

t h e a n o d e - e f f e c t f i l m . T h e s u r f a c e o f t h e f i l m i s n o t

s t a t io n a r y , b u t v i b r a t e s r a p i d ly t o w a r d a n d a w a y

f r o m t h e e l e c t r o d e s u r f a c e ( s ee F i g . 5 ). S i n c e t h e

e l e c t ro d e c a n lo s e h e a t b y c o n d u c t i o n u p t h r o u g h i t s

l e n g th , t h e b o t t o m t i p o f t h e w i r e i s t h e h o t t e s t p a r t

( th i s w a s e s t a b l i s h e d e x p e r i m e n t a l l y b y t h e o b s e r v a -

t i o n o n t h e p l a t i n u m - w i r e a n o d e t h a t t h e b o t t o m

s e c t i o n c o u l d b e m a d e t o g l o w a t a r e d h e a t , w h i l e

t h e t o p g a v e n o v i s i b l e r a d i a t i o n ) .

I n o r d e r t o p r o v e t h a t a h o t w i r e i s c a p a b l e o f s u p -

p o r t i n g a fi l m , s u c h a s i s f o r m e d d u r i n g t h e a n o d e

e f f e c t , t h e f o l l o w i n g e x p e r i m e n t w a s p e r f o r m e d : A

n i e h r o m e w i r e , 0 . 1 0 c m d i a m e t e r a n d 1 2 c m l o n g ,

w a s f o r m e d i n to a l o o p a n d c o n n e c t e d t o a d i r e c t -

c u r r e n t s o u r c e ; a c u r r e n t o f 3 9 a m p e r e s w a s p a s s e d

t h r o u g h t h e w i r e . A s s o o n a s t h e w i r e w a s r e d h o t i t

w a s p l u n g e d i n t o a b e a k e r o f w a t e r , w i t h o u t d i s-

c o n n e c t i n g t h e c u r r e n t s o u rc e . A v a p o r f i lm w a s s e e n

t o s u r r o u n d t h e e n t i r e l e n g t h o f t h e w i r e . F i g . 9 is a

p h o t o g r a p h o f t h a t p a r t o f t h e w i r e c lo s e t o t h e s u r -

f a c e o f t h e w a t e r .

a By use o f a n ickel wire, 0 .81 ma t d iam eter , as an anode

the au thor was ab le to ob tain s ' ach a h igh elect rode tem-

pera tu re the n ickel mel ted ( rap - - 1452 C) .



6/10


I n t h i s e x p e r i m e n t t h e r e i s n o q u e s t i on b u t t h a t

t h e g a s e o u s e n v e l o p e a r o u n d t h e h o t w i r e i s a w a t e r

v a p o r f i l m . T h e r e i s n o e l e c t r o l y s i s d u r i n g t h i s e x -

p e r i m e n t a n d n o o t h e r p o s s i b l e s o u r c e o f l a r g e q u a n -

t i t i e s o f g a s o t h e r t h a n w a t e r v a p o r . T h e s t r i k i n g

s i m i l a r i t y b e t w e e n t h e h o t - w i r e f i l m ( F i g . 9 ) a n d

the aqu eous anode -e f fec t fi lm (F ig . 5 ) i s s t ron g ev i -

d e n c e t h a t t h e a n o d e - e f f e c t f i l m i s a l s o m a i n t a i n e d

b y t h e v a p o r i z a t i o n o f w a t e r .

FIG. 9. Vapor fihn surrounding hot wire (about 8X). A

nichrome wire (0.10 cm diam) was heate d to red heat b y

passing 29 amperes through it; the red hot wire was then

plunged into distilled w ater at 72 ~ C, witho ut disconnecting

the current source. Wire passes through the surface of the

water at the top of the picture.

A n o t h e r p o i n t o f si m i l a r it y b e t w e e n t h e a n o d e -

e f f e c t f i l m a n d t h e h o t - w i r e f i l m i s t h a t t h e t h e r m a l

e n e r g y w h i c h m u s t b e d i s s i p at e d i n o r d e r t o m a i n -

t a i n t h e s e f i lm s is o f t h e s a m e o r d e r o f m a g n i t u d e

f o r b o t h f i l m s. W i t h w a t e r a t 8 7 ~ t h e c r it i c al cu r -

r e n t r e q u i r e d t o m a i n t a i n t h e h o t - w i r e f i lm i s a b o u t

2 9 a m p e r e s . T h i s c o r r e s p o n d s t o a h e a t d i s s i p a t i o n

o f a b o u t 4 0 w a t t s p e r s q u a r e c e n t i m e t e r o f s u r f a c e

a r e a o f t h e w i re . T o j u s t m a i n t a i n t h e a n o d e e f f e ct ,

w i t h a n e l e c t r o l y t e t e m p e r a t u r e of 8 7 ~ r e q u i r e s

a b o u t 4 5 v o l t s a n d 0 . 3 5 a m p e r e s , w h e n t h e p l a t i n u m -

wire anode , i m m ers ed to a d ep t h o f 0 . 8 cm , is used .

N o t a l l o f t h i s v o l t a g e d r o p o c c u r s a t t h e a n o d e f i l m ,

h o w e v e r . T h e a p p r o x i m a t e v o l t a g e d r o p a t t h e f i l m

c a n b e o b t a i n e d b y s u b t r a c t i n g f r o m t h e t o t a l v o l t -

a g e d r o p , t h e v o l t a g e d r o p o f th e c e ll w h e n i t i s

o p e r a t i n g w i t h o u t a n o d e e f f e c t a t t h e s a m e c u r r e n t .

Thus , wi th norm a l H2S O4 a t 87~

T o t a l v o l t a g e d r o p t o j u s t m a i n t a i n a n o d e e f f e c t

= 45 vo l t s .

V o l t a g e d r o p f o r n o r m a l o p e r a t i o n a t 0 .3 5 a m -

pe res = 3 . 5 vo l t s .

Vol ta ge d rop a t f i lm = 45 - 3 . 5 = 41 . 5 vo l t s .

I m m e r s e d a r e a o f e l e c t r o d e = 0 . 31 4 c m 2 ( P t w i r e

i m m e r s e d 0 . 8 c m ) .

41.5 X 0.35

He a t d i s s ipa te d by f i lm = - = 46

0.314

w a t t s / c m 2.

T h e c l o s e a g r e e m e n t o f t h e s e t w o f i g u r e s f o r t h e

p o w e r r e q u i r e d t o m a i n t a i n t h e t w o k i n d s o f f i l m

a l so s u p p o r t s t h e w a t e r - v a p o r t h e o r y o f t h e a n o d e -

effect f i lm.

A p r o p e r t y o f t h e a n o d e - e f f e c t f i l m w h i c h i s o f

c o n s i d e r a b l e in t e r e s t a n d i m p o r t a n c e i s i t s a b i l i t y t o

c o n d u c t e l e ct r i c c u r r e n t. T h e a n o d e e f f e ct w o u l d b e

im poss ib le i f the f ilm were a non cond uc tor , s ince i t i s

t h e h e a t d i s s ip a t e d b y t h e c u r r e n t t h a t h e a t s t h e

e l e c t r o d e s u r f a c e a n d m a i n t a i n s t h e v a p o r f i l m . T h e

m e c h a n i s m o f c o n d u c t i o n t h r o u g h t h e v a p o r f i lm i s

b y n o m e a n s c le ar , b u t t h e f o l l o w i ng o b s e r v a t i o n s

and d i s cus s ion th row som e l igh t on i t .

W h e n t h e p l a t i n u m - w i r e a n o d e i s u s e d i n a n e l ec -

t ro ly te o f su l fu r ic ac id , the on ly v i s ib le s igns o f

c u r r e n t c o n d u c t i o n a r e r a t h e r i n f r e q u e n t a n d t i n y

s p a r k s a c r o s s t h e a n o d e f i l m . O n t h e o t h e r h a n d , i f

a l i t t l e sod ium su l fa te i s added to the e lec t ro ly te (o r

i f s o d i u m h y r o x i d e i s t h e e l e c t r o l y t e ) , t h e a n o d e f i lm

is s een to em i t a ye l low g low, cha rac te r i s t i c o f so -

d i u m e m i s s i o n . W i t h a c e l l v o l t a g e o f 7 0 v o l t s a n d

a n e l e c t r o l y t e t e m p e r a t u r e o f 4 0 ~ t h e a n o d e s u r -

f a c e i s c o v e r e d w i t h a g r e a t m a n y ( p e r h a p s 1 0 0 )

br igh t ye l low spo t s , bu t the re i s no gene ra l g low.

W h e n t h e v o l t a g e i s r a i s e d t o 1 10 v o l t s a y e l l o w

g l o w p e r v a d e s m o s t o f t h e a n o d e s u r f a c e . T h e i n -

t e n s i t y o f t h e g l o w v a r i e s o n a n y o n e p o s i t i o n o f t h e

s u r f a c e w i t h a p e r i o d i c i t y t h a t i s s i m i l a r t o t h e v i -

b r a t i o n o f t h e v a p o r f i lm d e s c r i b e d e a r li e r . T h e g l o w

does no t ex i s t ou t in the wide por t ions o f the f i lm ;

i t i s conf ined c lose to the anode sur face .

B a s e d u p o n t h e a b o v e o b s e r v a t i o n s , a n d w i t h o u t

r i g o r o u s p r o o f , t h e f o l l o w i n g h y p o t h e s i s r e g a r d i n g

c u r r e n t c o n d u c t i o n a c r o s s t h e f i l m i s o f f e r e d :

V e r y l i t t l e c o n d u c t i o n c a n t a k e p l a c e a c r o s s t h e

wide por t ions (0 .2 to 2. 0 m m th ick) o f the f i lm , a s

e v i d e n c e d b y t h e l a c k o f s o d i u m g l o w i n t h e w i d e

p o r t i o n o f t h e f il m . I n t h o s e p o r t i o n s o f t h e f i lm



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Vol. 97 No. 4 ANODE EF FE CT IN AQUEOUS ELECT ROLYS IS 139

which are sufficiently thin (perhaps 0.2 mm judging

from the photograph in Fig. 5), the potential gradient

is sufficiently high to cause ionization of the gases

within the film and c urren t conducti on will take place

by migrat ion of the gaseous ions. This ionization may

be sufficiently intense to cause a visible discharge in

the gas, as evidenced by the sodium glow noted

above. Since any one position on the film is periodi-

cally approaching closer to the electrode and then

receding, the conductio n across the film at th at point

will also var y and the intens ity of th e glow will vary,

thus, the flickering aspect of the glow is described.

Since appreciable current still flows during the

anode effect, there must be some electrolytic reaction

taking place at the anode. On the other hand, visual

observation of the anode during the anode effect

shows no formation of gas bubbles. The seat of the

electrolytic reaction must now be the electrolyte-gas

interface. This type of electrolysis, where the metal

electrode is separated from the electrolyte by a

gaseous region, has been described as long ago as

1887 by Gubkin (5) and studied at len gth by Klemenc

(6). However, these studies involved low pressures

(5-15 mm of Hg) in the gas phase and real glow-

discharge conduction was obtained.

To prov e that during the anode effect oxygen gas

is evolved into the water-vapor envelope and exits

through the neck of this envolope to the atmosphere,

the following experiment was performed: The upper

portion of the platinum-wire anode was sheathed

with a close-fitting ceramic insulator, so that a tip

about 0.5 cm long of platinum was exposed. The up-

per end of the sheath was cemented to the wire to

prevent gas leakage. The electrode was immersed so

that the 0.5 cm of bare platinum and about 0.2 cm

of the sheath were below the electrolyte surface.

When the anode effect was developed with this elec-

trode a large bubble of noneondensable gas formed

at the top of the bare platinum and broke off from

time to time when it grew too large. Tests proved

the gas to be oxygen. Fig. 10 shows the anode effect

under these conditions.

Fig. 6, 7, and 8 show that the el ectrolyt e temper -

ature has a marked effect on both the current and

the electrode temperat ure during the anode effect.

A low electrolyte temperature gives rise to a high

current and a high electrode tempera ture during the

anode effect. The explanation of this peculiar re-

lationship is to be found in the thickness of the vapor

films formed at different electrolyte temperatures.

Fig. 5 and Fig. 5a show the anode film at electrolyte

tempe ratu res of 40~ and 90~ respectively. With

the high electrol yte temp erat ure the film is quite uni-

form and it vibrates only slightly. With the low elec-

trolyte temperature the film vibrates violently and

is very wide (1-2 mm) in some places and extremely

thin in others. It is reasonable to expect that the

cooler the electrolyte, the more closely the film can

approach the electrode before it is heated sufficiently

by the hot surface to cause vaporization. As dis-

cussed previously, the main conduction through the

film occurs at the very thin sections; thus, the cold

electrolyte makes possible a larger current because

the film approaches closer to the electrode surface.

As would be expected, the intensity of agitation

of the electrolyte also affects the current during the

anode effect. At a given electrolyte temperature, in-

FIG. 10. Aqueous anode-effect with par tly insulated

platinum wire. The top part of the electrode is sheathed

with a ceramic insulator. The large bubble breaks off

periodically when it grows too large. 1N H2SO4 at 67~ C,

77 volts, 0.15 amperes, about 8X.

creased agitation causes an increased current to flow.

Agitation makes possible a more rapid rate of heat

transfer away from the electrolyte-gas interface, with

the result that this interface remains cooler, can ap-

proach more closely to the electrode surface, and can

allow a larger current to flow.

The high electrode temperature obtained with the

cold electrolyte is a secondary effect. Since the cold

electrolyte allows a large current to flow, and the

large current will dissipate more heat than the small

one, the electrode will become hotter. In brief, the



8/10


e l e c t r o d e t e m p e r a t u r e i s a f u n c t i o n o f, a m o n g o t h e r

fac tors , t he r a t e of hea t d i s s ipa tion .

4. Change rom Transition Period to Anode-Effect

With the foregoing d i s cus s ion of the anode-e f fec t

f i lm after i t i s formed, i t i s now poss ible to discuss

the c r i t i ca l f ac tors which re su l t i n the change f rom

t h e t r a n s i t i o n p e r i o d t o t h e a n o d e e f fe c t.

I t i s e v i d e n t t h a t t h e a q u e o u s a n o d e - e f f e c t b e g i n s

a t a c r i t i ca l vo l t age ra the r tha n a t a c r i t i ca l cur re n t

dens i ty . F ig . 1 , 6 , 7 , and 8 show tha t fo r the evolu-

t i o n o f o x y g e n t h e c r i t i ca l v o l t a g e i s a b o u t 4 5 - 5 0

v o l t s. I f h y d r o c h l o r i c a c i d i s t h e e l e c t r o l y t e a n d

chlor ine i s evolved , t he c r i t i ca l vo l t age for anode

e f fec t i s abo ut 30-35 vol t s . App aren t ly , t he re fore , t he

n a t u r e o f t h e g a s e v o l v e d a t t h e a n o d e h a s s o m e

bear ing on the c r i t i ca l vo l t age . These fac t s sugges t

the fo l lowing hypothes i s for the ch a in of event s which

l e a d f r o m th e t r a n s i t i o n p e r i o d t o t h e a n o d e e f fe c t.

D u r i n g t h e t r a n s i t i o n p e r i o d t h e r e i s s u ff i ci e nt

h e a t d i s s i p a t e d t o v a p o r i z e m u c h o f t h e e l e c t r o l y t e ;

howev er , an anode-e f fec t f i lm canno t form unless the

20 40 ~ 0 ~0 /00 f20

olts

FIG. II. C ~thode effect w ith I)l~tinu m-wire catho de

(0.314 cm 2 im me rs ed are ~) in no rm fl H,,SO4 at 66 ~ -4- 3 ~ C.

a n o d e t e m p e r a t u r e r i s e s a p p r e c i a b l y a b o v e t h e b o i l -

i n g p o i n t o f t h e e l e c t r o l y t e . T h e t y p e o f c o n d u c t i o n

d u r i n g t h e t r a n s i t i o n p e r i o d t h a t r es u l t s i n t h e

vapo r i za t ion of the bu bble wa l l s can neve r g ive a

t e m p e r a t u r e a b o v e t h e b o i l i n g p o i n t o f t h e e l e c t r o -

ly t e , s ince a s soon as a pa r t i cu la r bubble wa l l i s

v a p o r i z e d c o m p l e t e l y th e c i r c ui t is b r o k e n a t t h a t

p o i n t a n d t h e c u r r e n t c e as e s f o r t h a t p a r t i c u l a r p a t h .

However , a s the vo l t age i s p rogres s ive ly inc reased ,

a po in t i s r eached where the re i s suf f i c i en t po ten t i a l

drop ac ros s some th in s ec t ion of one of the t rans i en t

f i lms to cause conduc t ion th rough the gas f i lm. As

soon as th i s occurs , t he hea t d i s s ipa ted by th i s cur -

r e n t c a n r e s u l t i n a l o c a l e l e c t r o d e - t e m p e r a t u r e i n

exces s of 100~ Th e hea t can be cond uc ted a long

t h e m e t a l e l e c t r o d e a n d c a u s e t h e v a p o r f il m t o s p r e ad

and be s t ab le a s a comple te enve lope . In shor t , i t i s

s u g g e s te d t h a t f o r a q u e o u s a n o d e - e f fe c t u n d e r t h e

condi t ions c i t ed , t he c r i t i ca l f ac tor for the onse t o f

anode e f fec t i s a suf fi c i en t po te n t i a l d rop ac ros s a

t r a n s i e n t v a p o r f il m t o c a u s e a n a p p r e c i a b l e c o n -

d u c t i o n t h r o u g h t h e g a s p h a s e .

Aqueous Cathode-effect

In a l l o f t he exp e r iment s and h ypoth eses d i s cus sed

above the re i s no fac tor which i s pecul i a r t o anodes

a lone . I f t he hypotheses a re cor rec t , t hen one could

p r e d i c t t h a t a c a t h o d e w h i c h e v o l v e d g a s, a n d w h i c h

i s ope ra t ed a t a h igh cur ren t dens i ty should show a

s imi l a r behavior and deve lop a vapor f i lm under

proper condi t ions . Such i s found to be the case . I f

the p l a t inum-wi re e l ec t rode i s made a ca thode in a

1N su l fur i c -ac id e l ec t ro ly te , i t fo l lows a behavior

exac t ly s imi l a r t o th a t o f t he anod e . F ig . 11 records

FIG. 12. Aqueous cathode- effec t with platinum- wire

cath ode. 1N H2SO4 at 49 ~ C, 70 volts, 1.0 ampe res, about 8X.

t h e v o l t - a m p e r e c h a r a c t e r i s t i c s f o r t h e p l a t i n u m

c a t h o d e a n d F i g . 1 2 i s a p h o t o g r a p h o f a q u e o u s

c a t h o d e - e f f e c t . T h e c a t h o d e e m i t s a b r i g h t b l u e

glow, cha rac te r i s t i c o f hyd roge n emis s ion , if t he

vol t age i s ra i s ed to 110 vol t s .

W i t h c a t h o d es , h o w e v e r , t h e r e i s y e t a n o t h e r p h e -

n o m e n o n t h a t t a k e s p l a c e . I f t h e s o l u t i o n c o n t a i n s

sodium su l fa t e , o r i f sod ium hydroxide i s used a s the

e l e c t r o ly t e , t h e c a t h o d e ef f e c t i s n o t o b t a i ne d , i ~ -

s t ead , t he ca tho de sur face i s cove red wi th a mul t i -

r u d e o f w h a t a p p e a r t o b e s p a r k d i s c h a r ge s ( b lu e

color ) , no wide vap or f i lm deve lops , and the sur -



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Vol . 97 No . 4

ANODE EF FEC T IN AQUEOUS ELECTROL YSIS 141

face of the catho de remains at 100~ or slightly

lower. Appare ntly some kind of film does form around

the cathode because the meniscus at the electrolyte

surface dips down, just as during the "cat hode effect"

or anode effect. However, the wide vapor-envelope

which can be clearly seen during the true "cathode

effect" is absent in this case. The explanation of

this phenomenon lies out of the scope of this paper,

but the writer feels that a different mechanism of

electric conduction between electrolyte and the elec-

trode surface is probably responsible for this phe-

nomenon.

Rela t i on Between Anode Ef f ec t i n Aqueous

and Mo l ten Electrolysis

The writer does not claim that the explanations

found valid for aqueous anode-effect are necessarily

valid for anode effect in molten media. I n p articular,

the chain of events which leads to molten anode-

effect is very likely different. In molten media with

a graphite anode, the e lectrol yte usually has a high

contact-angle against graphite (7, 8). On the other

hand, platinum and the alloy steel used in this paper

are completely wet (have 0 ~ conta ct angle) by the

electrolyte used. The non-wettability of the graphite

anode by the molten media may contribute to the

incidence of anode effect by making it possible for

gas bubbles to adhere strongly to the anode. More-

over, the high temperatures found in molten elec-

trolysis undoubtably have some effect upon the ease

with which the gas film can ionize and thus con-

duet current.

The writer does feel, however, that the concept of

the gaseous envelope stabilized by the vaporization

of the electrolyte close to the surface of a hot anode

should be closely investigated for molten anode-

effect. The explanations for the stability of the

gaseous envelope to be found in the literature all

center about a

gaseous f i lm- -one

stabilized by a

rush of noneo ndensable gases (02, CO, CO2) (9).

There are at least two considerations that make such

an explanation untenable. In the first place, in a

laboratory cell, when the anode effect starts, the

current will usually drop to ~ or ~ao of its previous

value. This means that during anode effect only 89

or ~o as much gas is being evolved as before anode

effect. Thus, one is forced to explain how a small

amount of gas will cause a gaseous envelope to form,

where five or ten times that amount of gas was un-

able to do so. Second, it is not hard to show by

means of hydrodynamics th at the velocity of gas re-

quired to hold back the electrolyte from the anode,

at any reasonable depth in the electrolyte, is very

large and far more than one could obtain from molten

electrolysis.

The writer is planning a future paper which will

attempt to apply the vapor-film theory of aqueous

anode-effect to anode effect in molten electrolysis.

At this time, however, it can be pointed out that it

is entirely possible that the electrolyte in the Hall

aluminum cell could vaporize if in contact with

a hot anode. Waddington and Pearson (10) have

recently pointed out that the current carriers in

cryolite are Na + and possibly AIF~. The tra nsfer-

ence of these ions will result in an anode layer of

electro lyte which is impoverish ed in NaF and rich

in A1Fa. A1Fa is a relativ ely unstabl e compou nd a nd

volatilizes with decomposition ar ound 1000 to 1100 ~

C (11). Thus, the anode may be surrounded with an

electrolyte which can volatilize at a temperature

about 100~ higher than the operating tempera ture

of the cell (1000~ and it is possible tha t a vapor

film could be formed if the anode became over-

heated.

CONCLUSIONS

1. A phenomenon which occurs when electrodes

which evolve gas in aqueous media are operated at

high current densities has been described and named

"aqueous anode-effect" because of its similarity to

anode effect in molten electrolysis.

2. Normal operation of the anode was found to

cease when the electrode temperature reached the

boiling point of the electrolyte. Under these condi-

tions, the anode enters a so-called "transition period"

where vaporization of the bubble walls leads to an

increased electrical resistance at the anode and the

current through the cell falls as the voltage is in-

creased.

3. The "transition period" behavior instantane-

ously changes to the "aqeueous anode-effect" when

the voltage is raised to a critical value that will per-

mit conduction through the gas phase.

4. The gaseous envelope which encloses the anode

during "aqueous anode-effect" is maintained by the

vaporization of the electrolyte close to the surface of

tile anode, which surface was found to be at a tem-

perature far above the boiling point of the electro-

lyte.

5. It was also shown that an "aqueous cathode-

effect," entirely similar to the anode effect, could be

obtained.

6. The relation between "aqueous anode-effect"

and anode effect in molten media was discussed and

it was pointed out that there is a possibility that the

anode-effect film in molten media is also stabilized

by vaporization of the electrolyte.

CKNOWLEDGEMENTS

The writer is indebted to Professors M. D. Hass-

ialis, T. A. Read, and A. F. Taggart for helpful

criticisms and suggestions. Mr. M. A. Kaei, Asso-



10/10

42

J O U R N A L OF T H E E L E C T R O C H E M I C A L S O C I E T Y

A p r i l 1 9 5 0

ciate in Metal lurgy, aided with much of the experi-

menta l work .

Any discussion of this paper will appear in a Discussion

Section, to be published in the December 1950 issue of the

JOURNAL.

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