Hydrogen peroxide as an intermediate in electrocatalytic
reduction of oxygen. A new method for the determination of
rate constants
Citation for published version (APA):
Brink, van den, F. T. B. J., Barendrecht, E., & Visscher, W. (1980). Hydrogen peroxide as an intermediate in
electrocatalytic reduction of oxygen. A new method for the determination of rate constants. Journal of the
Electrochemical Society, 127(9), 2003-2006. https://doi.org/10.1149/1.2130053
DOI:
10.1149/1.2130053
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Published: 01/01/1980
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Vol. 127,
No.
9
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Sect. B, 25, 925 (1969) ; ibid.,
26, 1076 (1970).Hydrogen Peroxide as an Intermediate in
Electrocatalytic Reduction of Oxygen.
A New Method for the Determination of
Rate Constants
F. van den Brink, E. Barendrecht,* and
W ,Visscher
Laboratory for Electrochemistry, University of Technology, 5600 MB Eindhoven, The Netherlands
A B S T R A C T On m o s t e l e c t r o c a t a l y s t s H202 is an i n t e r m e d i a t e in the e l e c t r o r e d u c t i o n of o x y g e n . H202 c a n d e c o m p o s e e i t h e r c h e m i c a l l y o r e l e c t r o c h e m i c a l l y a n d in assessing t h e p e r f o r m a n c e of t h e e l e c t r o c a t a l y s t i t is c r u c i a l to d i s t i n g u i s h b e - t w e e n t h e s e r e a c t i o n paths. A s i m p l e m e t h o d is p r o p o s e d to d e t e r m i n e w h e t h e r o r n o t t h e c h e m i c a l p a t h is followed. F o r t h e e l e c t r o c h e m i c a l r e d u c t i o n of o x y g e n a g e n - e r a l s c h e m e of r e a c t i o n s can be g i v e n as in Fig. 1. W h e t h e r t h e r e d u c t i o n of o x y g e n to w a t e r w i l l f o l l o w t h e d i r e c t p a t h 1 o r t h e consecutive p a t h 2-3, or b o t h t h e s e p a t h s i n p a r a l l e l , is d e t e r m i n e d b y the e l e c t r o - c a t a l y t i c p r o p e r t i e s of the e l e c t r o d e w h i c h a r e also c r i t i c a l for t h e f u r t h e r r e a c t i o n of H20~: e l e c t r o c h e m - i c a l r e d u c t i o n ( r e a c t i o n 3) a n d / o r c h e m i c a l d e c o m - p o s i t i o n ( r e a c t i o n 4). W i t h t h e i n t r o d u c t i o n of t h e r o t a t i n g r i n g - d i s k e l e c t r o d e t e c h n i q u e i t b e c a m e possible to d i s t i n g u i s h b e t w e e n t h e c o n s e c u t i v e a n d p a r a l l e l p a t h w a y s (1-4). Here, t h e d i s k c u r r e n t is m e a s u r e d a t v a r i o u s p o t e n - t i a l s a n d r o t a t i o n frequencies, w h i l e s i m u l t a n e o u s l y t h e r i n g c u r r e n t is m e a s u r e d at a p o t e n t i a l at w h i c h
* Electrochemical Society Active Member,
Key words: electrocatalysts, reduction, decomposition.
h y d r o g e n p e r o x i d e is o x i d i z e d q u a n t i t a t i v e l y . S e v e r a l m e t h o d s h a v e b e e n p r o p o s e d to a n a l y z e t h e d a t a o b t a i n e d f r o m t h e s e e x p e r i m e n t s ; t h e s e m e t h o d s h a v e b e e n r e v i e w e d b y P l e s k o v a n d F i l i n o v s k i i (5), w h i l e W r o b l o w a (6) has a d d e d a g e n e r a l c r i t e r i o n for d i s - t i n g u i s h i n g b e t w e e n the p a r a l l e l a n d consecutive p a t h - way. These a n a l y s e s s h o w that, in p r i n c i p l e , o n l y f o u r ( c o m b i n a t i o n s ) of t h e five r a t e c o n s t a n t s in t h e g e n - e r a l s c h e m e can be o b t a i n e d . A p r o b l e m w h i c h r e m a i n s to b e s o l v e d is t h a t of t h e f a t e of t h e h y d r o g e n p e r o x i d e , p r o d u c e d b y r e a c - tor 2 i n Fig. 1 : H 2 0 2 c a n b e e i t h e r r e d u c e d e l e c t r o - c h e m i c a l l y ( r e a c t i o n 3) o r c h e m i c a l l y d e c o m p o s e d ( r e a c t i o n 4). Moreover, t h e s o l u t i o n of this p r o b l e m is of v i t a l i m p o r t a n c e in t h e e v a l u a t i o n of t h e p e r f o r m - ance of e l e c t r o c a t a l y s t s for t h e o x y g e n electrode, since r e a c t i o n 4 i n v o l v e s no n e t e l e c t r o n t r a n s f e r and is
2004 J.
EIectrochem. Soc.:
E L E C T R O C H E M I C A L S C I E N C E A N D T E C H N O L O G YSeptember 1980
k
0 - _ . .
"- H O
= H
2
b
2 2
Fig. 1. Scheme of reactions for reduction of oxygen
electrochemically w h o l l y u n p r o d u c t i v e . I n this p a p e r we will p r e s e n t a simple method to d e t e r m i n e the rate constant of reaction 4, so that, i n pJ:inciple, it becomes possibIe to calculate all five i n d i v i d u a l rate constants of Fig. 1.
Theory
I n the r i n g - d i s k electrode (RRDE) e x p e r i m e n t , r e - f e r r e d to in the i n t r o d u c t i o n , sets of disk c u r r e n t s (ID) a n d l i m i t i n g r i n g c u r r e n t s (IR) are obtained as functions of rotation f r e q u e n c y (~) a n d disk potential, ED. These data are a n a l y z e d by p l o t t i n g at c o n s t a n t E D according to (5)
--NolD/IR -- A + B / A / ~
[1] a n d- - N o (ID,limH20 - - I D ) / I R = C -~
Dxr
[2] w h e r e ID,limH20 is the l i m i t i n g c u r r e n t at the disk for the r e d u c t i o n of o x y g e n to w a t e r a n d No is the RRDE's collection efficiency (7). A, B, C, a n d D are functions of the five rate constants, from which kl, k2 f, 2k2 b --}- k4,a n d 2k3 -~ k4 can be o b t a i n e d (5).
We wish to point out that the value of k4 can be f o u n d i n m a n y practical cases b y m e a s u r i n g r i n g c u r r e n t s i n a solution c o n t a i n i n g only H 2 0 2 a n d no 02, while the disk c u r r e n t is kept zero.
Relations b e t w e e n c u r r e n t s a n d r o t a t i o n frequencies i n H_~O2 c o n t a i n i n g solutions have been derived b y Bagotskii (1). F r o m his e q u a t i o n s the c o m b i n a t i o n s 2ku b + k~, 2k3 + k4, a n d k2 f -- k3 can be obtained. A f u r t h e r extension of this method was given b y Tarasevich and R a d y u s h k i n a (8) who applied Bagot- skii's equations u n d e r the condition that ID = 0. They, again, f o u n d kl, k2 f, 2k2 b ~- k4 ( d e s c r i b i n g the rate with which H202 gives O2), a n d 2k3 + k~ (describing the rate with which H.~O2 gives HoO). We will show t h a t they lost v a l u a b l e i n f o r m a t i o n ,
viz.,
that o b t a i n e d from the value o.f the disk's o p e n - c i r c u i t potential, b y s u b s t i t u t i n g the zero c u r r e n t condition (ID = 0) only ~n Bagotskii's r e s u l t i n g equations (1).I n a solution c o n t a i n i n g h y d r o g e n peroxide, b u t no oxygen, the h y d r o g e n peroxide will react electro- chemically (reactions 2 a n d 3) a n d chemically (reac- tion 4), giving o x y g e n a n d / o r water; o x y g e n m a y react f u r t h e r (reactions 1 a n d 2). The q u a n t i t i e s of f o r m e d o x y g e n a n d of decomposed h y d r o g e n peroxide can be d e t e r m i n e d b y m e a s u r i n g the anodic and cathodic l i m i t i n g c u r r e n t s on the r i n g electrode. If, moreover, the disk c u r r e n t is zero, the n e t a m o u n t of o x y g e n formed b y the electrochemical reaction is k n o w n if the ratio of the rates of p r o d u c t i o n a n d of c o n s u m p t i o n of O2 by the electrochemical reactions,
i.e.,
k2 b --k3/2kz ~-
k2 f is k n o w n at the disk's r e s t - potential. This m e a n s that the a m o u n t of o x y g e n formed b y reaction 4 can be found,i.e.,
that the v a l u e of k4 can be d e t e r m i n e d .Thus, on the disk electrode
k3
H202 -}- 2H + -}- 2 e - --> 2H20 w i t h I3
k2t H2Of~2bOf-- -]- 2H + -{- 2 e - w i t h I2 with I1 -F I2 + I3 k 1 " - ID = 0 O2 -{- 4H+ + 4 e - -~ 2H20 w i t h Ii k4 H202 ~ 1~ Oe § HfOThe a s s u m p t i o n s are t h a t reaction 4 is p o t e n t i a l i n d e p e n d e n t , t h a t the p e r t i n e n t reactions are first order i n O2 or i n H202, a n d t h a t adsorption to a n d desorption f r o m the electrocatalyst of 02 a n d H202 are fast.
On the r i n g electrode the anodic l i m i t i n g c u r r e n t
iS IR,lim a f o r the r e a c t i o n
H20~-> 02 ~ 2H + -{- 2 e -
a n d IR,lim c the cathodic l i m i t i n g c u r r e n t for the reac- tions
H202 + 2H + + 2 e - --> 2H20 a n d
02 +
4H + -{- 4 e - --> 2H~OAt the disk we have the mass balances, for h y d r o g e n peroxide-
72xr s - - C2 ~ = (k2,r b -~- k3,r + k 4 ) c 2 ~ - - k2,rfCl ~ [3]
a n d for o x y g e n
71~V/WCl ~ = ( k f , r b -~- 1/2 k 4 ) c 2 ~ - - ( k l , r - ~ k f , r f ) c l ~ [4]
Because of the zero c u r r e n t condition
ID
- - " = (k2,r b -- k3,r)C2 ~ -- (2kl,r + k2,rf)cl ~ = 0 [5] 2FA
where the c's are the c o n c e n t r a t i o n s of O2 (subscript 1) a n d of H202 (subscript 2). The superscripts 0 a n d s on the c's denote the electrode surface a n d the b u l k of the solution, respectively ,while a subscript r is attached to kz, k2 f, k2 b, a n d k~ because the disk elec- trode is at its rest potential. F u r t h e r , 7~/~- = 0.62D 2/~ ~-~/6~v/~ is the rate constant for diffusion a n d A the disk area.
At the ring we have the l i m i t i n g c u r r e n t s cathodic
~2/3
IR,lim c _ 4 7 1 ~ C l O - - 2k4c2 o -~-
NoAF
NoAF
. ID,limH202[6]
w h e r e IR,lim c k4 ( K fl2/3 '~ __2NoAFc2 s
1 ~- K
- 1 ~ - ~ ~- -~o J
72V~ [8] a n d --IR,lim a k4 fl'2/3 __ 2NoAFc - z - 7 2 W 27z k2,r b - - k3,r K = - - 72 2 k l , r -~- k2,r f[9]
anodic/R, lima
~2/3 - - - - 2k4c2 ~ -~ - - ID,limH202 [7]NoAF
NoAF
w h e r e ID,limH202 is the absolute v a l u e of the l i m i t i n g c u r r e n t at the disk for the oxidation of H202 to 02 a n d ~2/3 is a geometrical factor, defining the shielding factor (1 -- No~ -2/3) of the RRDE (7).
E l i m i n a t i o n of cz o a n d c2 o b e t w e e n the sets of Eq. [3], [4], a n d [6], a n d [3], [4], a n d [7], respectively, together w i t h the condition [5] gives
Vol. 127, No. 9 E L E C T R O C A T A L Y T I C R E D U C T I O N The pseudo c o n s t a n t k can be o b t a i n e d b y e x t r a p o l a -
t i o n of the values for the k's f o u n d with Eq. [1] a n d [2] to the rest p o t e n t i a l at each r o t a t i o n frequency.
If we m a y assume t h a t the rest p o t e n t i a l w i l l n o t v a r y s t r o n g l y w i t h the RRDE's r o t a t i o n f r e q u e n c y , K will be a constant. T h e n the plots of the l i m i t i n g r i n g c u r r e n t s vs. V"J will give straight lines. Two l i m i t i n g cases can be considered (Fig. 2). F i r s t K < < 1, i.e., H202 is, electrochemically, m u c h less reactive t h a n 02 at the disk electrode. Therefore the anodic a n d cathodic l i m i t i n g r i n g c u r r e n t s become equal, while r e a c t i o n 4 is responsible for e x t r a shielding of the r i n g c u r r e n t b y the disk electrode. The second case K > > 1, i.e., H202 is electrochemically m o r e reactive. 02 f o r m e d o n the disk diffuses away, t h e r e b y i n c r e a s - i n g the cathodic l i m i t i n g r i n g current. Now the i n - fluence of r e a c t i o n 4 is m u c h less t h a n i n the first case, because of the low H202 c o n c e n t r a t i o n n e a r the disk surface. I n Fig. 2, schematic diagrams are g i v e n for these two cases.
I n general, b y p l o t t i n g fR,lim a VS. %v]~, or, m o r e p r e - cisely, (1 -5 K)IR.t~m a vs. (1 -5 K ) V rJ (Eq. [9]), c2 s c a n be found. T h e n k4 c a n be calculated at each v a l u e of ~ f r o m
k 4 . ( 1 - 5 K ) [ I R , l i m O
+(K---+-~,)+I'21~"
]2NoAFC + 1 + . , . + . o -, [10a]
#+]3 I
--IR'uma -5 --~-o "y~v~-J
[lOb]
-- (1
-5
K ) 2NoAFc2 sThe results c a n be a v e r a g e d to give a final v a l u e of k4. A l t e r n a t i v e l y , k4 can be d e t e r m i n e d from the i n t e r - cepts of (1 -5 K ) IR.limC/2NoAFc2 s a n d - - ( 1 -5 K) IR.uma/2NoAFc2 s vs. (1 -5 K ) ? 2 ~ . The r e s u l t i n g v a l u e of k4 is t h e n s u b t r a c t e d from 2k2 h -5 k4 a n d 2k3 -5 k4, o b t a i n e d f r o m Eq. [1] a n d [2], to give the separate values for all five rate constants at each p o t e n t i a l for which r i n g a n d disk c u r r e n t s are available.
E x p e r i m e n t a l
As a n example, a n e x p e r i m e n t according to the l i n e of r e a s o n i n g described above was carried out, u s i n g a n RRDE with a n A u disk a n d a P t r i n g in 1M K O H at 298~ The c o n s t r u c t i o n of the RRDE was as described before (9). Its d i m e n s i o n s w e r e such t h a t A ---- 51.5 X 10 -6 m 2, No ~ 0.1437, a n d fl ---- 0.1515 (as calculated f r o m the radii rl0 : 4.047 ram, r20 ---- 4.260 mm, a n d r30 =- 4.437 m m ) . A s t a n d a r d electrochemical cell was used, w i t h a r e v e r s i b l e h y d r o g e n electrode (RHE) as reference a n d a p l a t i n u m wire as c o u n t e r - electrode. The P t r i n g was slightly p l a t i n i z e d (5 •
10i'11R,tim t / m . s -1 cathodic +3.0 2 2 9 + C + 2.0 / ., '4 . 4 / o
,r
2
+1.o/ ' / ~ /
/y+"
o/e
fiR,tim I
Q
/ /
a,c
IzR,
ti,..l
2005 l I I 0 + 0.5 + 1.0 + 1.5~. 10/*
~I'HzozV-~/m.s'I
Fig. 3. Limiting ring currents vs. ~ / ~ ; for an Au/Pt-RRDE in 1N
KOH at 298~ plotted according to Eq. [8] and [9]. a ~ anodic,
c ~ cathodic.
10-6 m o l e . m - S ) . The m e a s u r e m e n t s were c a r r i e d out u s i n g a Tacussel bipotentiostat, Type BIPAD.
The K O H solution ( p r e p a r e d w i t h p.a. q u a l i t y K O H a n d d o u b l y distilled w a t e r ) was purified b y preelec- trolysis, made o x y g e n - f r e e , a n d kept i n A r a t m o - sphere. H y d r o g e n peroxide was added 1 a n d anodic a n d cathodic l i m i t i n g c u r r e n t s at the r i n g were m e a - s u r e d w h i l e the disk c u r r e n t was zero. To avoid poisoning of the r i n g b y the stabilizer from the H202 solution used, the c u r r e n t s were m e a s u r e d b y cyclic v o l t a m m e t r y w i t h scan rates of 100-500 m V . s e c -1. The rest p o t e n t i a l at the disk was m e a s u r e d to be 889 m V vs. a r e v e r s i b l e h y d r o g e n electrode i n the same solution a n d was i n d e p e n d e n t of the r o t a t i o n frequency. Therefore it was possible to o b t a i n the
H 2 0 2 c o n c e n t r a t i o n from a IR,lim a VS. ~ - p l o t . The
slope of this plot gave c2 s _-- 5.3 mM, which v a l u e was verified b y t i t r a t i o n w i t h KMnO4. The results, p l o t t e d as IR,lim/2NoAFc2 s vs. ~ 2 ~ , are given i n Fig. 3. The
z B r o c a c e s 30% v / w , P h . E u x .
C
|
/Y
/ / /
Fig. 2. Limiting cases of Eq. [8] and [9] (arbitary units). I: K < < 1; Ih K ~ > 1; a ~ anodic; c ~ cathodic limiting current; - - - for
k4 ~ 0; for k4 --~ 0.
2006 J. Electrochem. Soc.: E L E C T R O C H E M I C A L S C I E N C E A N D T E C H N O L O G Y S e p t e m b e r I980 slope of the anodic b r a n c h is calculated to be 1.98
-~ 0.06, i n v e r y good a g r e e m e n t with the theoretical v a l u e of
-- 1.978 No
F r o m the slope of the cathodic b r a n c h (2.89 _ 0.07), the v a l u e of K ~ 10.1 can be calculated. This m e a n s that H202 is, electrochemically a p p r o x i m a t e l y five times more reactive at a gold electrode t h a n 02.
The intercepts of the anodic a n d cathodic b r a n c h e s are, respectively, --0.12 X 10 -4 msec -1 a n d --0.17 X 10 -4 msec -1. This gives, b y application of Eq. [8] a n d [9], k4 ---- (1.6 __+ 0.3) X 10 -4 msec -1. C a l c u l a t i o n according to Eq. [10] gives k4 ~- (1.6 _+ 0.2) X 10 -4 m s e c - 1.
Conclusion
As s h o w n i n the introduction, there are two nodal points i n the scheme of reactions for 02 r e d u c t i o n w h e r e a distinction is to be m a d e b e t w e e n possible paths. T h e first is w h e r e o x y g e n can give either H20 or H202. Here, RRDE e x p e r i m e n t s i n O2-containing solutions e n a b l e us to m a k e the distinction. The sec- ond is w h e r e h y d r o g e n peroxide can react either electrochemically or chemically. The m e t h o d proposed here makes it possible to d e t e r m i n e w h e t h e r or not
H202
decomposes chemically. I n those cases w h e r e it m a y be a s s u m e d that k4 does n o t depend on the disk electrode potential, it is even possible to calculate the m a g n i t u d e of k4. This a s s u m p t i o n is valid as long as the catalytic properties of the electrode surface for the decomposition of H202 do n o t change significantly w h e n the electrode p o t e n t i a l is changed from the rest potential.F u r t h e r m o r e , the m e t h o d described here is s i m p l e r t h a n t h a t g i v e n r e c e n t l y b y A p p l e b y a n d S a v y (10). T h e y use a n RRDE w i t h a pyrolytic graphite ring, at which HeO2 is stable, to find k4. This m e a n s that they need two different RRDE's, one to find kl, k2 f,
k2 b,
a n d k3 i n o x y g e n - c o n t a i n i n g solutions a n d a n o t h e r one for use in solutions c o n t a i n i n g H2Oe to find k4.Acknowledgment
This work has b e e n carried out w i t h financial s u p - port from the Nether]ands O r g a n i z a t i o n for the A d - v a n c e m e n t of P u r e Research (ZWO).
M a n u s c r i p t received Nov. 7, 1979.
A n y discussion of this paper will a p p e a r in a Dis- cussion Section to be p u b l i s h e d in the J u n e 1981 JOURNAL. All discussions for the J u n e 1981 Discussion Section should be s u b m i t t e d b y Feb. 1, 1981.
Publication costs of this article were assisted by the University of Technology.
L I S T OF SYMBOLS A disk area ( m ~)
co c o n c e n t r a t i o n n e a r disk surface (mole m - a : m M )
c s c o n c e n t r a t i o n i n solution (mole m -3 ~_ mM) D diffusion coefficient ( m 2 sec - 1 )
ED disk p o t e n t i a l (V) F F a r a d a y c o n s t a n t ID disk c u r r e n t (A)
ID,limH20
l i m i t i n g disk c u r r e n t for reaction 02 --> H20 (A)ID,limH202
l i m i t i n g disk c u r r e n t for reactionH202 --> 02
or H202 --> H20 (A) Ir~ r i n g c u r r e n t
irR.lim l i m i t i n g c u r r e n t (A)
k reaction rate c o n s t a n t (see Fig. 1) (msec - i ) K pseudo constant, defined in f o r m u l a [9] No collection efficiency of RRDE
geometric factor of RRDE 1~ 0.62 D2/3~ -1/6 (msec-V2) v k i n e m a t i c viscosity (m 2 sec -1) a n g u l a r r o t a t i o n f r e q u e n c y of RRDE (sec - 1 ) Superscripts a anodic c cathodic Subscripts 1 02 2 H202 r refers to rest p o t e n t i a l REFERENCES
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