J . E l e c t r o c h e m . S o c . Vo l . 13 8, No. 2, F eb ru ar y 1991 9 The Elect rochemical Society , Inc.

4 4 5

4. C. K. Baker a nd J. R. Re ynolds , J. Elec t roana l . Chem.

251, 307 (1988).

5. D. Orata and D. A. But try, J. Am. Chem. Soc . 109, 3574

(1987).

6. (a) P. T. Vari neau an d D. A. Buttry, J.

Phys . Chem.

91,

1292 (1987); (b) M. D. Ward, 92, 2049 (1988).

7. T. Osaka, S. Ogano, K. Naoi, and N. Oyama,

This Jour -

n a l 136, 306 (1989).

8. (a) K. Naoi and T. Osaka, ib id . 134, 2479 (1988); (b) T.

Osaka and K. Naoi, in Primary and Secondary Am-

bient Temperat ure Lithiu m Batteries, PV 88-6, J. P.

Gaba no, Z. Takehara , a nd P. Bro, Editors, p. 770,

The Electrochemical Society Softbound Proceed-

ings Series, P enn ing ton , NJ (1988).

9. (a) K. Naoi, M. M. Lien, an d W. H. Smy rl, J. Electro-

ana l . Chem. In press: (b) K. Naoi and W. H. Smyrl,

Submitted to

T h i s J o u r n a l .

10. K. Naoi, B. B. Owens a nd W. H. Smyrl, in Recha rge-

able Lith ium Batteries, PV 90-5, S. Subba rao, V. R.

Koch, B.B. Owens, and W.H. Smyrl, Editors,

pp. 176-204, The Ele ctroc hemic al Society Softb ound

Proc eed ings Series, Pen nin gto n, NJ (1990).

11. C.J. Chande r, J.-B. Ju, R. Atana sosk i and W. H. Smyrl,

Corr osi on '89, p. 37, NACE, N ew Orlean s, April 1989.

12. G. Sauerbrey, Z. Phys . 155, 206 (1959).

13. (a) T. Shi midzu , A. Ohtani, T. Iyoda, and K. Honda, J.

C h e m . S o c . C h e m . C o m m u n .

1986, 1415 (1986). (b) T.

Iyoda, A. Ohtani, T. Shimidzu and K. Honda, Chem.

Let t . 1986, 687 (1986). (c) T. Shimidzu, A. Ohtani, T.

Iyoda, and K. Honda,

J . E lec troana l . C hem.

224, 123

(1987).

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S y n t h .

Met . 18, 7 (1987); (b) J. Tanguy and N. Mermilloid,

ib id . 21, 129 (1987); (c) J. T ang uy, N. Mermi lloid, a nd

M. Hoclet,

T h i s J o u r n a l

134, 795 (1987).

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16. B. J. Feldman, P. Burgermayer, and R. W. Murray,

ib id . 107,872 (1985).

Linear S wee p Vol tamm etry Th eo ry for I rrevers ib le Electrode

Reactions w i th an O rde r of O ne or H igher

I I E x p e r i m e n t a l R e s u l ts

D . L . P i ro n , H . K o h l e r , a n d N . M a s s ~

D ~ p a r t e m e n t d e G ~ n i e M ~ t a l l u rg i q u e E c o le P o l y t e c h n i q u e d e M o n t r e a l M o n t r e a l Q u e b e c C a n a d a H 3 C 3 A 7

ABSTRACT

Applyin g a new theory developed in a previous paper, linear sweep voltam metry was used to determine the reaction

order a nd th e tra nsfer coefficient for the oxida tion reactio n of SO~ to H2SO4, usin g plat inu m and lead oxid e as substra tes.

Solutions co ntaini ng up to 50 weight percent H2SO4 were studied. Results show that the oxidation reaction is of the sec-

ond order wh en lead oxide is used and of the first order with the plati num substrate. These observations will be used in a

sub sequ ent paper to de termin e the mech anis m involved at the lead oxide substrate. The results were verified by meas-

uring hyd rogen cathodically evolved and comparing it with the values calculated in this study. Thus, the pr esent work

shows the applicability of the new generalized theory of linear sweep vol tammetry for irreversible electrode processes.

One of the promisi ng sources of portable energy avail-

able is hy drog en gas (1, 2). It could be elec trolytically ex-

tracted from water an d stocked or piped to sites where it

could be re-oxidized to pro duce electricity, in fuel cells, for

instance (3). A hybr id sulfur process was developed by

West ingho use Electric Co mpa ny (4-6) to replace ano dic ox-

ygen evolution as the counterpart of the cathodic evolu-

tion of hydrogen. In this process, sulfur dioxide in the

anolyte is electrochemically oxidized to sulfuric acid

(H2SO4) at t he ano de

SO2 + 2H20 ~ 2H + + H2SO4 + 2e- [1]

while hydrog en gas is si multane ously evolved at the cath-

ode

2H § + 2e- -~ H2 [2]

The full cycle is completed by a thermochemical reac-

tion in wh ich aqu eous H2SO4 is cracke d b ack into SO2,

water, and oxy gen

H2SO4-~ SO2 + H20 + O~ [3]

Interest in avo iding the oxygen process is related to the

high cell voltage that it demands. The economic attraction

of the cycle is the possibility of performing the electro-

chemical step with less con sump tion of electrical power. It

has be en note d (7) that the therm odyn amic reversible po-

ten tia l for reac tio n [1] is 0.29V (in 50 wei ght per cen t [w/o] of

H2SO4 at 25~ as com pa red to 1.23V for wate r electrolysi s.

It is then expected that the above process would have the

same o utput of hydrogen as the direct electrolysis of water

but at a lower potential.

* Electrochemical Society Active Member.

** Electrochemical Society Student Member.

The electrochemical step of the hybrid sulfur process in

concentra ted acid solution has received relatively little at-

ten tio n (3, 4, 8, 9). However, high conc entr atio n of sulfuric

acid solutio n are of crucial importance for maximizat ion of

the overall ener gy efficien cy of the cycle (10, 11).

The objective of the pres ent st udy is to underst and the

kinetics of the a nodic reaction in high acid concentrations.

Through linear sweep voltamm etry (LSV) experimen ts the

parameters involved in the rate equation of the process

(12, 13) can be determined using a generalized theory for

the polarographic waves developed by the authors (14).

Two substrates were chosen for the electrode. It was ex-

pected that changing the substrate would result in a

change of the reaction me chanis m, since SO2 oxidation is

strongly sub strate -depen dent (4). The first electrode is lead

oxide (~-PbO2), chosen for its chemical resistance to the

sulfuric media, its electric conduc tivity , and its electrocat-

alytical behavi or with respe ct to the oxida tion of SO2 (15).

The second electrode is platinized platinum, selected for

its k no wn energeti c efficiency in reacti on [1] (16, 17).

E x p e r i m e n t a l

E l e c t r o n i c e q u i p m e n t . - - T h e type of experiment under-

taken in this study basically involved plotting the current

with respect to a linearly varying potential, measur ed with

respec t to a nonpola rizab te reference electrode. To per-

form this, a Tacussel potentiostat (PTR-20 2X) was con-

trolled by a Tacusse l triangular signal generator (GSTP 2)

and con nected to a Radiometer mercury-merc urous sul-

fate reference electrode (K6112) placed in an electrolytic

cell in the to p of a Lugg in capillary. Becau se of the use of a

high concentration of sulfuric acid and the placement of

the capillary closer than 1 mm from the electrode, the

ohmic drop was negligible and IR correction was not nec-

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446 d. E lec t rochem. Soc . Vol. 1 38, No. 2, Feb ruary 1991 9 The Elect rochemica l Socie ty Inc.

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

w e r e t w o K e i t h l e y e l e c t r o m e t e r s (6 02 ). T h e c u r r e n t a n d p o -

t e n t i a l c u r v e s w e r e o b t a i n e d t h r o u g h a T e k t r o n i x o s ci l l o -

s c o p e ( 16 2 3A ) e q u i p p e d w i t h a P o l a r o i d c a m e r a ( C 30 ) t h a t

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

a x i a l c a b l e s a n d B N C p l u g s t o a v o i d n o i s e p i c k - u p .

E l e c t r o l y t i c c e i L - - T h e

e l e c t r o l y t i c c e l l is s h o w n i n F i g . 1 .

I t w a s c l o s e d b y m e a n s o f w a t e r s e a ls , w i t h a t h e r m o m e t e r

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

a n o d i c c h a m b e r b y a f ri t te d g l a ss w i n d o w . T o e n s u r e a

c o n s t a n t w o r k i n g t e m p e r a t u r e , a H a a k e t h e r m o s t a t ( R -2 0)

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

n e t i c a g it a to r . T h e c a t h o d i c c h a m b e r u s e d a p l a t in i z e d

p l a t i n u m s h e e t a n d a w a t e r s e a l ( F ig . 2).

P t a n d P b 0 2 a n o d e s . - - L e a d o x i d e a n d p l a t i n i z e d p l a t i-

n u m w e r e u s e d a s e le c t r o d e s f o r th e a n o d i c r e a c t i o n o f S O2

o x i d a t i o n . T h e le a d o x i d e e l e c t r o d e w a s p r e p a r e d t h r o u g h

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

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

a n d l e a d n i t r a t e s n e u t r a l i z e d b y c o p p e r a c e t a t e ( p H = 3 .1 0)

a c c o r d i n g t o S h i b a s a k i (1 8). T o o b t a i n t h e p l a t i n i z e d p l a t i -

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

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

I t w a s i n t h e n p l a t i n iz e d w i t h t h e s t a n d a r d c h l o r o p l a t in i c

s o l u t i o n ( Y S I . R N o . 3 1 4 0 ) .

E l e c t r o l y t e . - - T h e

w a t e r u s e d w a s f r o m a d e i o n i z e d a n d

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

w e r e a n a l y t i c a l g r ad e . T h e i r c o m p o s i t i o n i n t h e w o r k i n g

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

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

b e f o r e e a c h e x p e r i m e n t .

T h e o r y o f L i n e a r S w e e p V o l t a m m e t r y in th e C a s e o f

S 0 2 O x i d a t i o n

T o a n a l y z e t h e k i n e t i c s o f th e S O 2 o x i d a t io n , t h e L S V

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

m a t i c a l t o o l s to q u a n t i f y t h e r e l a t i o n b e t w e e n t h e m e a s -

u r e d c u r r e n t p e a k a n d p o t e n t i a l a t t h is p e a k w i t h t h e c ha r -

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

p r e v i o u s p a p e r (1 4) t h e a u t h o r s d e v e l o p e d a m e t h o d p e r-

m i t t i n g L S V k i n e t i c a n a l ys i s w i t h r e s p e c t t o th e u n r e s t r ic t -

e d r e a c t i o n o r d e r o f t h e e l e c t r o d e r e a c t i o n .

T h e b a s i c e q u a t i o n s . - - T h e t h e o r y i s b a s e d o n t h r e e f u n -

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

s p e c i f i c c a s e o f t h e a n o d i c o x i d a t i o n o f SO 2 ( aq ). F i r s t t h e

r a t e o f r e a c t i o n e q u a t i o n

-rso2 = ke(t)[C(0, t)]~ [4]

I n t h i s e q u a t i o n r so 2 i s t h e r a t e o f r e a c t i o n i n w h i c h S O ~ i s

c o n v e r t e d i n t o H 2 S O 4, C(0 .t) i s t h e c o n c e n t r a t i o n o f SO 2 a t

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

~ r e f e r e n e

cap~ e

Fig. 1. Gen eral view of the electrolyt ic cel l

c thode

Fig. 2. Specif ic view o f the cathodic chamber used n the electrolytic

cell.

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

p r e s s e d f o r a n a n o d i c p r o c e s s a s

f o r

a n d

ke t) = Kch

e x p ( ~ t ) [ 5]

= a n ~ F v / R T [6]

K c h = k 0 e x p { ( - h G * + ~naFeo)/RT} [7]

I n t h e s e e q u a t i o n s a i s t h e t r a n s f e r c o e f f i c i e n t, na i s t h e

n u m b e r o f e l e c t r o n s t r a n s fe r r e d i n th e R D S , v t h e s c a n r a te

f o r t h e L S V , a n d t h e r e s t o f t h e s y m b o l s b e a r t h e i r u s u a l

s i g n i f i c a n c e .

T h e s e c o n d f u n d a m e n t a l e q u a t i o n i s F i c k s fi rs t l a w

J s o 2 a q ) = - D V C o , t) [8]

w h e r e J so 2(a q) i s t h e m a s s f l u x t o w a r d t h e e l e c t r o d e , D t h e

d i f f u s i o n c o e f f i c i e n t o f t h i s s p e c i e s , a n d V C(0, ) t h e g r a d i e n t

o f c o n c e n t r a t i o n e x h i b i t e d b y t h e e l e c t r o l y t e n e a r th e s u r -

f a c e o f t h e e l e c t r o d e .

T h e t h i r d e q u a t i o n i s F i c k s s e c o n d l a w

~ C o , t> / ~ t = D { ~ 2 C o , t ~ O x 2 } [9]

S o l v i n g t h e e q u a t i o n s f o r d e t e r m i n a t i o n o f t h e r e a c t io n

o r d e r . - - U s i n g

E q . [ 4 ], [ 5] , [ 8] , a n d [ 9] i t i s p o s s i b l e t o o b t a i n

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

r a m e t e r s . A c c o r d i n g t o t h e a u t h o r s ( 14 ) t h i s c a n b e d o n e

t h r o u g h a s pe c i a l n o r m a l i z e d c o n c e n t r a t i o n f u n c t i o n ~ , d e -

f i n e d a s f o l l o w s

@(y) = [C(0, ~C~] ex p {(y - J) /~}

[10]

I n E q . [ 10 ] y is t h e n o r m a l i z e d t i m e d e f i n e d a s

y = ~ t

[ ]

C | i s t h e b u l k c o n c e n t r a t i o n i n S O 2 ( a q ) a n d f is t h e r e l a x -

a t i on q u o t i e n t d e f i n e d a s

f = I n

{X/-~D-~/KchC=~-1}

[12]

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J . E l e c t r o c h e m . S o c . Vol . 138, No. 2, Fe bru ary 1991 9 The Electrochemical Society , Inc. 447

The solution of the above basic equa tions in terms of this

function, according to [10], gives the following relation

for the current of the electrode

i * = ( n F A C ~ ~# )[ s ~ [13]

In [13] the paramet er s represent s the max imu m of the

function ~, which corresponds to the reduced time y* in

which the elect rode also exhibits the max imum current i*.

Further, as demons trat ed by the authors (14), ~* is only a

functio n of X when f- 6 with the following relation

X = - 3.67 + 3.03[$.*]~ + 1.08/[~*] ~

for 0.2 - [~*]~ ~ 0.35 [14]

Now exp ress ing ~ as in Eq. [6] and rear rang ing [13] to ob-

tai n t he val ue of [4~*]~, the following equat ion is obtained

[4~*]~ = X / R T I ' ~ D ~ n a F " ( i * I V v ) I n F A C | [15]

Equat ion [15] shows clearly that ( i * / X / v ) depends on the

reaction order because of Eq. [14], as well as on expe rimen-

tal conditions.

P r o c e d u r e t o f i n d t h e v a l u e o f ~n~.--The only unknown

par amet er in [15] is C~na. To find its value, the pro ced ure is

the follo wing for an anodi c sweep (19, 20)

e* = e i + v t* [16]

Here, e~ is the ini tial pote ntia l of the po tent ial swe ep, e* the

potential at which the current exhibits a maximum, and t*

the corresponding t ime. Replacing v by the corresponding

val ue o f Eq. [6] and [11] and rea rran ging [16] gives Eq. [17]

e*

e i

+ ( R T l e n , F ) . y * [17]

Addin g and subtracti ng the relaxation quoti ent f (Eq. [ 12])

from Eq. [17] gives

e * = el + ( R T / e n ~ F ) 9 ( y * - f ) + ( R T / e n ~ F ) f [18]

Using Eq. [12] and [18] it is possible to obtain a semilo-

garithmic relation between the potential at which the cur-

rent exhibits a maximum e* and the scan rate v

e* =S + Ul og v [19]

with

S = ei + ( R T M n , F ) [ ( y * - f )

+ 2.303 log { K ~h -I C | t ` x . ~ / = D ~ n a F / R T } ] [20]

and

U = 2 . 3 0 3 R T / 2 e n , F [21]

It is recalled that in Eq. [19], S is constant and independ-

ent of the scan rate (v) because (y* - f) is a function of X

alone w hen f>- 6. From Eq. [21], it is then possi ble to ob-

tain an~ and thus X.

R e s u l t s a n d D i s c u s s i o n

As ment ione d in a previ ous work of the authors (14), the

value of f (the relaxa tion quoti ent of Eq. [12]) must be

greater or equal to six in order to apply the new general-

ized theory of l inear sweep voltammetry to irreversible

electrode processes. It means that high potential sweep

values (relative to the value of Kc~) are applie d to t he elec-

trode. It causes fast consu mpti on of the reacti ng species

which does not allow for its replacement; therefore, no

steady state can be reached. High scan rates were chosen

in this study to satisfy the theory.

U s i n g a P b O ~ s u b s t r a t e . - - T e n sets of experiments were

conduct ed, co vering a range bet ween 23 ~ and 60~ scan

rates from 750 to 6530 mV/s, and electrolyte compositions

va ryi ng f rom 0.035 to 0.09M fo r SO~ and 14 to 50 w/o for

H~SO~. These expe rimen tal conditions are listed in Table I.

For each experiment, a current peak value (i*) was meas-

ured corre spond ing to a given pot ential (e*). Figure 3

shows typical curves of current evolution during potential

sweep. It shows that increasing the speed of the sweep (v)

results in a shorter time to reach the current peak (i*) but

T a b l e I , E x p e r i m e n t a l c o n d i t i o n s a n d r e s u l t s f o r P b O 2 s u b s t r at e

Sweep Surface of

Exp. Temperature rate SO2 H~S O4 electrode

No. (~ (mV/s) (M) (w/o) (cm 2)

1 23 1030-3240 0.051 14 0.186

2 23 1230-3170 0.043 14 0.166

3 23 870-2130 0.035 14 0.186

4 31 760-3030 0.045 50 0.337

5 39 770-3090 0.045 50 0.337

6 50 750-3010 0.045 50 0.337

7 60 760-3030 0.045 50 0.337

8 30 10 50 -3 16 0 0.090 50 0.176

9 40 760-3960 0.090 50 0.176

10 30 21 10 -6 53 0 0.059 50 0.337

Slope of Slope o_f

Exp. e* vs . log (v) i vs. ~/v

No. (V ) c~na (Al*v/-v-~) [r

1 0.0307 0.415 0.01245 0.2481

2 0.0300 0.425 0.01002 0.2341

3 0.0268 0.476 0.00721 0.1956

4 0.0296 0.442 0.01564 0.2357

5 0.0263 0.511 0.01661 0.2212

6 0.0295 0.472 0.01632 0.2048

7 0.0246 0.583 0.01850 0.1928

8 0.0237 0.551 0.01538 0.2005

9 0.0285 0.473 0.01710 0.2221

10 0.0238 0.548 0.01917 0.1995

also increases the value of the cu rrent peak. Fro m Eq. [19]

and [21] the slope of e*

v s . I n ( v )

can be used to obtain the

value of ~n~. Typical cu rves are shown in Fig. 4 and val ues

of ~n~ are given in Table I for the different experimental

conditions. The averag e result of the transfer coefficient

group over PbO2 is

~n~ = 0.490 (-+ 10% on a confident interval of 99%)

The ne xt step is to find the reacti on order (~). This is

done by using Eq. [14] and [15]. The un known s in these

0.022

0.020

0.018

0.016

~ 0.014

0.012

0.010

o 0.008

0.006

0.004

0.002

0.000

/ / /A 0/

/ 0 A / A / A B / O /

/ / " / . * ' j [ ] "

/ 0 9 / A / D /

0 .0 0 .2 0 ,4 0 .6 0 .8 1 .0 1 .2

T i m e ( s )

o 7 ~ 7 6

/ / ~ m ~ e ~ , ~

1.4 1.6 1.8 2.0

F i g . 3 . C u r v e s o f t h e c u r r e n t re s p o n s e o f e x p e r i m e n t N o . 4 ( T a b l e I ) .

S w e e p r a t e s a r e ( [ ~ ) 7 6 0 m V / s , ( A ) 8 7 0 m V / s , ( ~ _ ) 1 0 0 0 m V / s , ( )

1 2 1 0 m V / s , a n d ( 2 ) 1 5 4 0 m V / s .

2.020

C

u~ 1,980

>

1.940

z~

1.900 7

0.5 1,0 2.0 5.0

Scan ra te (V/s )

F i g . 4 . G r a p h i c o f t h e p o t e n t i a l c o r r es p o n d i n g t o c u r r e n t p e a k ( e * )

v s . t h e s c a n ra t e o f a n o d i c p o t e n t i a l: ( 0 ) e x p e r i m e n t N o . 4 , ( [ - I e x p e r i -

m e n t N o . 5 , ( _ ~ ) e x p e r i m e n t N o . 7 , a n d ( ~ _ ) e x p e r i m e n t N o . 8 . ( E x p e r i -

m e n t n u m b e r s r e f e r t o T a b l e I . )

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448

J . E l e c t r o c h e m . S o c . Vol. 13 8, No. 2, F eb rua ry 19 91 9 The Electrochemical Society, Inc.

0.030

0 025

0.020

0.015

0.010

0.0o.5

0 000

0.0

S q u a r e r o o t o f t h e s c a n r a t e V s - 1 ) ~ /z

0.2 0.4 0.6 0 8 1.0 1.2 1.4 1.6 1.8

F ig . 5 . Graph ic o f the cur ren t peak i * ) v s the squ are roo t o f the scan

ra te : 2 ) exper iment No . 4 , ~ ] ) exper iment No . 5 , A ) exper iment

No . 7 , and _V) exper iment N o . 8 . Exper iment numbers re fe r to

T a b l e I . )

equations are the slope of i * v s . X/-v(U), he diffusion coef-

ficien t (D), and the surfac e area of the e lec trod e (A). The

slopes of

i * v s . X / v

are given in Table I, and typical curves

are shown in Fig. 5.

The va lue of the diffusion coefficient is given by the re-

lation

p . D I T =

5.27 x 10 -8 cm 2

c S / s ~

[20]

where ~t is the kinematic viscosity. The latter value was

calculated from Table II. As shown in this table, only the

conce ntrat ion of H2SO4 was tak en into ac count for calcula-

tion of ~. The range of concentration of SO2 used in this

study was so small that it did not significantly affect the

kinematic viscosity.

To measure the surface area, the differential capacitance

was used. The changing values for the surface area noted

in Table I are due to the different electrodes used in this

study.

Knowing the slope of i * v s . V~, D, and A, it is possible to

calc ulate ($*)x usin g Eq. [15]. The m ean valu e of this func-

tion (listed in Table I) is equal to

(~*)~ = 0.2154 (_+ 8% on a confident interval of 99%)

From this valu e and using Eq. [14] the reac tion order is

equal to

~, = 2.00 (-+ 18%)

Thus, the reaction order of the o xidation of SO2 into

H~SO4 over a s ubst rate of PbO~ is equa l to 2. Since a reac-

tion order equal to one was found in the literature in the

case of platinum for a range of anodic overpotenti als com-

parable to values used in this study, different substrates

result in different reaction mechanisms.

U s i n g a p l a t i n u m s u b s t r a t e . - - S t u d y

of the plat inum sub-

strate was undert aken here to verify the validity of the new

theory on LSV. Others have already studied and discussed

the mechanism of SO2 oxidation based on classical meth-

ods of gathering data. Thus, in the case of Pt substrate, ex-

periments were c onducted using the same method of data

acquisi tion as for the PbO2 substrate. For th e study of the

platinum substrate, nine sets of experiment s were con-

ducted. The experimental conditions are l isted in

Table III. Once again, the first step consists of calculating

Tab le I I . D e te rmina t ion o f the k inem at ic v iscos ity

Specific Kinematic

HzS O4 Temperature gravity Viscosity viscosity

(atom percent) (~ (g/cm~) (cp)

cS)

14 23 1.093 1.181 1.062

50 30-60

A B B I A

Wh er eA = 1.4109 - 8.05 x 10-4T + 5 x l0 ~T2 (Ti n ~ and

B = d 1 3398-0 01645 T) T in ~

Tab le I I I . Exper imenta l cond i t ions and resu l ts fo r P t subs tra te

Sweep Surface of

Exp. Temperature rate SO2 H2SO4 electrode

No. (~ (mV/s) (M) (w/o) (cm2)

1 23 1630-3030 0.113 4.0 1.55

2 23 280-750 0.113 4.0 1.55

3 22.5 800-950 0.109 7.5 1.55

4 24 370-660 0.113 7.5 1.55

5 27 490-2260 0.088 7.5 1.55

6 20 1790-6250 0.049 4.5 1.65

7 50 140-300 0.125 1.5 1.65

8 43.9 260-470 0.071 3.0 1.65

9 46 190-480 0.121 2.0 1.65

Slope of Slope of

Exp. e* vs. log v) i vs . ~/v

No. (V)

A/~F~s)

[~*]~

1 0.1168 0.1797 0.3457

2 0.1168 0.1571 0.3022

3 0.1161 0.1559 0.3106

4 0.1070 0.1584 0.3052

5 0.1141 0.1066 0.2775

6 0.1083 0.0564 0.2498

7 0.1172 0.1787 0.3260

8 0.1119 0.0912 0.2912

9 0.1276 9.1466 0.2750

the transfer coefficient group using Eq. [19] and [21] and

the slope of e* v s . In (v). From this value and t he sl ope of i*

v s . V v it is possi ble to calculate th e function [~*]~ by

Eq. [15]. Finally, the reaction order is calculated using

Eq. [14]. The l atter v alu e was equ al to on e in the ca se of oxi-

dation of SO~ to H2SO4 over platinum substrate. Results

confirm that the theory pr oposed by the authors in a previ-

ous stud y (14) and use d in this paper fits well with observa-

tions mad e b y othe rs (9, 16, 21, 22) using othe r meth ods.

To verify the validity of the results obtained in this

study, calculation of the estimated volu me of hydrogen at

the cathode was done using parameters found in the pres-

ent work. It is mathe matic ally possible to relate the anodic

overpotential to the volume of hydrogen evolved during

potentiostatic experiments. A few experimental trials

were undertaken and i t follows that hydrogen evolution at

the ca thode fits the pr edicti on well. This makes it possible

to assu me tha t t he transfer coefficient group (an~) is equal

to 0.5 and the reaction order (X) is equal to 2 in the case of

SO2 oxidation over PbO2 substrate.

C o n c l u s i o n

The generalized theory of l inear sweep voltammetry

(LSV) for irreversibl e electrode processe s has practical ap-

plications. The present study shows that it is possible to

obtain reproducible results using other meth ods to verify

its validity. Furtherm ore, it was used i n the case of plati-

num to find a reaction order of one, which fits with results

obtained by researchers using other methods.

This theory also permits th e stud y of electrochemica l re-

actions of any order. It was fou nd that t he react ion order is

equal to two if a lead ox ide is used as a substrate for the ox-

idat ion of SO2 into H2SO4. Other su bstra tes can al so be an-

alyzed following the proc edure used here.

Utilization of the SOJI-I2SO4 hybrid cycle for hydrogen

production has to be evaluated with respect to its

strongest competitor, the d irect electrolysis of water. Also,

a detailed stu dy of possible separator memb rane s is

critical, being necessary to separate the anodic and ca-

thodic chambers due to the detrimental effects of inter-

mixing hydrogen gas and sulfurous acid.

Manuscrip t submi tted May 14, 1990; revised manusc ript

received Aug. 21, 1990.

E c o le P o l y t e c h n i q u e d e M o n t r e a l a s s i s t e d i n m e e t i n g t h e

p u b l i c a t i o n c o s ts o f t h i s a r ti c l e.

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2. R. Shinnar, D. Shaphira , and S. Zakai,

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Vo l . 138, No. 2, F eb ru ary 1991 9 The Electrochemical Society , Inc.

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(1980).

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347 (1981).

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D u j k a , I n t . J . H y d r o g e n E n e r g y , 7, 43 (1982).

7 . P. W . T . L u a n d R . L . A m m o n , i n H y d r o g e n E n e r g y

P r o c e s s , T . N . V e z i r o g l u , K . F u e k i , a n d T . O h t a , E d -

i t o r s , V o l . 1, p . 4 39 , P e r g a m o n P r e s s , N e w Y o r k

(1980).

8 . A . J . A p p l e b y a n d B . P i n c h o n , I n t . J . H y d r o g e n

E n e r g y , 5, 253 (1980).

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95, 59 (1979).

1 0. G . H . F a r b m a n a n d L . E . B r e c h e r , i n P r o c e e d i n g s

T e n t h I n t e r s o c i e t y E n e r g y C o n v e r s i o n E n g i n e e r i n g

C o n f e r e n c e , p . 1 19 9, N e w a r k , D e l a w a r e ( 19 75 ).

1 1. G . H . F a r b m a n a n d G . H . P a r k e r , i n H y d r o g e n : P r o -

d u c t i o n a n d M a r k e t i n g , M . W . S m i t h a n d J . G . S a n -

t a g e l o , E d i t o r s , p . 3 59 , A m e r i c a n C h e m i c a l S o c i e t y ,

W a s h i n g t o n , D C ( 19 80 ).

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N e w Y o r k ( 19 69 ).

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(1980).

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(1964).

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t e m s , T . N . V e z i r o g l u a n d W . S e i f ri t z , E d i t o r s ,

V o l . 2, p . 68 7, P e r g a m o n P r e s s , N e w Y o r k ( 1 97 8) .

2 2 . K . W i e s e n e r , E l e c t r o c h i m . A c t a , 18, 185 (1973).

Synthes is and Ch arac te r iza t ion o f a New Co nduc t ing

E lec tropo lymer ized F i lm f rom 1 -Naph tho l

M i n h C h a u P h a m J a m a l M o s l i h a n d P i e rr e -C a m i l le L a c a z e

I n s t i t u t d e T o p o l o g i e e t d e D y n a m i q u e d e s S y s t ~ m e s d e l U n i v e r s i t ~ P a r i s 7 , a s so c i ~ a u C . N . R . S . - U R A 3 4 , 7 50 05 P a r i s ,

C e d e x , F r a n c e

A B S T R A C T

A c o n d u c t i n g a n d e l e c t r o a c t i v e f il m , p o l y ( N A P - 1 ) , h a s b e e n e l e c t r o c h e m i c a l l y s y n t h e s i z e d i n a c e t o n i t r i l e s o lu t i o n . T h e

p o l y m e r s t r u c tu r e , t h e e l e c t r o p o l y m e r i z a t i o n m e c h a n i s m , a n d t h e e l e c t r o c h e m i c a l p r o p e r ti e s w e r e s t u d i e d u s i n g i n s i t u

I R , X P S , a n d S E M s p e c t r o s c o p y .

I n a p r e l i m i n a r y s t u d y ( 1), w e h a v e r e p o r t e d t h e p r e p a r a -

t i o n o f a n e w c o n d u c t i n g p o l y m e r f il m , p o l y ( N A P - l ), b y

e l e c t r o c h e m i c a l o x i d a t i o n o f 1 - n a p h t h o l i n a c e t o n i t ri l e .

W e p r e s e n t i n t h i s p a p e r d e t a i l s c o n c e r n i n g t h e p o l y m e r

s t r u c tu r e , t h e e l e c t r o p o l y m e r i z a t i o n m e c h a n i s m a n a l y z e d

b y i n s i t u I R a n d X P S s p e c t r o s c o p y , a n d t h e e l e c tr o c h e m i -

c a l p r o p e r t i e s o f t h i s n e w t y p e o f p o l y m e r f il m .

Experimental

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

P A R 1 7 3 p o t e n t i o s t a t c o n n e c t e d t o a P A R 1 7 5 p r o -

g r a m m e r .

T h e w o r k i n g e l e c t r o d e w a s a P t o r g l a s s y c a r b o n d i s k

s e a l e d i n T e fl o n , a P t p l a t e f o r X P S e x p e r i m e n t s , o r g e r -

m a n i u m c r y s t a l c o a t e d w i t h a t h in l a y e r o f P t d e p o s i t e d b y

s p u t t e r i n g ( B a l z e r s M o d e l S p u t r o n I I) fo r i n s i t u I R a n a l y -

s i s b y t h e m u l t i p l e i n t e r n a l r e f l e ct i o n F o u r i e r t r a n s f o r m i n -

f r a r e d ( M I R F T I R S ) m e t h o d .

F i l m s c o u l d b e p r o d u c e d a t c o n s t a n t c u r r e n t o r c o n s t an t

p o t e n t i a l (e.g. , + 1 . 3 V vs . A g / A gC 1 ) o r b y p o t e n t i a l c y c l i n g

b e t w e e n + 0 .2 a n d + l . 3 V . F o r e x a m p l e , a f il m f o rm e d b y

t e n c y c l e s h a s a t h i c k n e s s o f 1.5 ~t m.

M I R F T I R S s p e c t r a w e r e r e c o r d e d o n a N i c o l e t 6 0 S X

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

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

v i o u s p a p e r ( 2 ).

I n s i t u M I R F T I R S s p e c t r a a t a n in d i c a t e d p o t e n t i a l a r e

t r a n s m i t t a n c e d i f f e r e nc e s p e c tr a . F o r e a c h s p e c t r u m , t h e

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

( t h e r e f e r e n c e s p e c t r u m ) i s s u b t r a c t e d f r o m t h a t o f t h e s y s -

t e m a t a n i n d i c a t e d v o l t a g e .

X P S s p e c t r a w e r e r e c o r d e d o n a V a c u u m G e n e r a t o r s E s -

c a la b M K 1 S p e c t r o m e t e r , w i t h a n u n m o n o c h r o m a t e d

M g K s x - r a y s o u r c e ( p o w e r a p p l i e d t o t h e a n o d e = 1 00W )

u n d e r p r e s s u r e s i n t h e 1 0 - 8 m b a r r a n g e . T h e a n a l y z e r w a s

o p e r a t e d a t c o n s t a n t p a s s e n e r g y ( 2 0 e V ). T h e s p e c t r a w e r e

d i g i ti z e d , s u m m e d , s m o o t h e r , a n d r e c o n s t r u c t e d u s i n g

G a u s s i a n - s h a p e d c o m p o n e n t s . B i n d i n g e n e r g i e s a re re -

f e r r e d t o C l s 2 8 5 e V .

Results and Discussion

E l e c t r o c h e m i c a l S y n t h e s i s . - - P o l y m e r f i l m s w e r e

e l a b o r a t e d o n t o p l a t i n u m , g r a p h i te , o r g e r m a n i u m / p l a -

t i n u m e l e c t r o d e s b y e l e c t r o c h e m i c a l o x i d a t i o n o f 1 n a p h -

t h o l i n a c e t o n i t r i l e s o l u t i o n c o n t a i n i n g 0 .1 M o f t h e e l e c t r o -

l y t e ( N B u4 C 10 4 , L i A s F s , L i C 1 0 4 , N B u 4 P F 6 ) . S a t i s f a c t o r y

f i lm s c o u l d b e p r o d u c e d a t c o n s t a n t c u r r e n t o r c o n s t a n t

p o t e n t i a l (e.g. , + 1.3V vs . A g /A g C 1 ) o r b y p o t e n t i a l c y c l i n g

b e t w e e n + 0 .2 a n d + l . 3 V ( F i g . 1 ).

Structure and Polymer iza t ion Mechanism

P o l y m e r s t r u c t u r e .- - T h e p o l y ( N A P - l ) f i l m i n i t s n e u t r a l

u n d o p e d s t at e w as d e m o n s t r a t e d b y i n s i t u I R a n a l y s i s (1 )

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

r i n g s .

0 fOH

) unless CC License in place (see abstract).ecsdl.org/site/terms_useaddress. Redistribution subject to ECS terms of use (see  143.117.44.138Downloaded on 2015-01-07 to IP