Oxygen Reduction on Platinum in Borate-Buffered

J. Electrochem. Soc., Vol. 136, No. 1, January 1989 9 The EFectrochemical Society, Inc. pophosphite oxidation increased and the reduction potential of copper and nickel also shifted. We speculated that in alkaline solution polymeric ions of boric acid may enhance the electron transfer reaction (11, 12). Furthermore, boron has been detected b y ESCA analysis from the deposit of hypophosphite-reduced electroless nickel bath containing boric acid (at pH = 9). Therefore, we speculate that a trace a m o u n t of boric acid may codeposit with nickel as nickelboron alloy (13), which has lower energy state than pure nickel. Hence the reduction potential of nickel-boron alloy is higher (less negative). The effect of boric acid then is, on one hand, to enhance the reduction of nickel; on the other hand, to promote catalytic activity of nickel deposit on hypophosphite oxidation. Thus, indirectly, boric acid will affect the oxidation of hypophosphite.

Conclusions From studies of cylic voltammograms, plating rates, complexing ability, and coordinating structures of copper (II) complex ions in six different plating baths, the following conclusions can be drawn: 1. Only those copper (II) complex ions which form dimeric structures have a significant plating rate. 2. The rate-determining step in hypophosphite-reduced electroless copper is the reduction of copper (II) complex ion. 3. The nickel deposit can catalyze the oxidation ofhypophosphite. 4. Boric acid can enhance the reduction of nickel ion. Therefore, the detailed process of copper (II) complex ion reduction in electroless plating needs further investigation.

75

Acknowledgments The support provided by Ming-Hsin Engineering College and Industrial Technology Research Institute is gratefully acknowledged. The authors would also like to thank Professor Cheu~Pyeng Cheng for helpful discussions during the course of this work. Manuscript submitted March 22, 1988; revised manuscript received J u n e 30, 1988.

Ming-Hsin Engineering College assisted in meeting the publication costs of this article. REFERENCES 1. K. F. Blurton, Plat. Surf. Finish., 74, 52 (1986). 2. H. Marshall, Met. Finish., 7 (Sep. 1975). 3. P. E. Kukanski, J. J. Grunwald, D. R. Ferrier, and A. Sawaska, U.S. Pat. 4,209,331 (1980). 4. J. J. Grunwald, P. E. Kukanskis, R. A. Leitzc, D.A. Sawoska, and H. Rhodenizer, Proc. AES 2nd Electroless Plating Symp. (1984). 5. R. Goldstein, P. E. Kukanskis, and J. J. Grunwald, U.S. Pat. 4,265,943 (1981). 6. A. Hung, Plat. Surf. Finish,, 75, 62 (1988). 7. A. Hung, ibid., 75, 74 (1988). 8. A. E. Martell and R. E. Smith, "Critical Stability Constants," P l e n u m Publishing Corp., New York (1974). 9. F. Ammar and J. M. Saveant, J. Electroanal. Chem., 47, 215 (1973). 10. J. B. Flanagan, S. Margel, A. J. Bard, and F. C. Anson, J. Am. Chem. Soc., lO0, 4248 (1978). 11. F. A. Cotton and G. Wilkinson, "Advanced Inorganic Chemistry," John Wiley & Sons, New York (1980). 12. J. P. Hoare, This Journal, 134, 3102 (1987). 13. Y. Z. Chen, L. L. Shen, and M. H. Rei, Proceedings of the 4th Seminar on Science and Technology 1986.

Oxygen Reduction on Platinum in Borate-Buffered Saline Solutions Susan M. Kaska,1 S. Sarangapani,* and Josd Giner* Giner, Incorporated, Waltham, Massachusetts 02254 ABSTRACT Rotating ring disk electrode studies show that the polarization curves for 02 reduction on platinum in buffered O.5M NaC1 are shifted cathodically relative to those in buffered 0.5M NaC104 in the pH range of 8-10, presumably due to chloride ion adsorption. Increased H202 currents at the ring also provide evidence of C1--specific adsorption in this pH range. The calculated rate constant, kL, for oxygen reduction in weakly alkaline, borate-buffered solutions shows the inhibiting effect of C1- ion. Triangular sweep cyclic voltammetry studies provide supporting evidence of chloride adsorption on platinum in the pH range of 8-12. Hypochlorite solutions have proven to be most reliable decontaminants. Present methods of hypochlorite generation use dilute saline or sea water as the source of chlorine and yield large quantities of by-product hydrogen. The generation of hydrogen has several disadvantages: hydrogen embrittlement of the electrode, greater power consumption, and safety considerations. All these problems can be overcome by replacing the hydrogen evolution reaction with the oxygen reduction reaction. The kinetics and mechanism of electrochemical oxygen reduction on platinum have been extensively studied over the past decade. Most of these studies focus on oxygen reduction in strongly acid or strongly alkaline solutions (1-7). Sepa et al. (8-10) have investigated the pH dependence of the kinetics over the entire pH range of 0-14. However, very little information is available on oxygen reduction kinetics u n d e r the conditions expected to prevail in a reactor generating hypochlorite from saline solutions. *Electrochemical Society Active Member. 1Present address: Department of Chemical Engineering, MIT, Cambridge, Massachusetts 02139.

I n the present study, we have investigated the effect of chloride ion adsorption on the kinetics of oxygen reduction in moderately alkaline solutions. Although the specific adsorption of chloride ion on platinum and its inhibition of oxygen reduction have been thoroughly documented in acid solutions (11-14), the subject of chloride adsorption in neutral and alkaline solutions has not received m u c h attention. The work to date indicates that chloride adsorption is weaker in neutral and alkaline electrolytes than in acid. I n an ellipsometric study of chloride, bromide, and iodide adsorption of platinum in neutral NaF solutions (pH 6.5), Chiu and Genshaw (15) reported coverages much smaller than those observed in voltammetric work by Gilman (16) and by Bagotzky et al. (17) in 1M H2SO4.However, the two sets of results may not be directly comparable because different assumptions have been made concerning the effects of monolayer coverage. Thus, our investigation of oxygen reduction in moderately alkaline saline solutions begins with a study of chloride adsorption on platinum in the pH range of 7.3-11.3. We used cyclic voltammetry to look at changes in the charge for hydrogen adsorption/desorption and for platinum oxide formation and reduction as a function of chloride concentra-

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J. Electrochem. Soc., V o l . 136, N o . 1, J a n u a r y 1 9 8 9 9 The E l e c t r o c h e m i c a l Society, Inc.

76

tion (17-18). I n addition, we r e p o r t t h e results of a rotating ring-disk electrode (RRDE) study of the influence of chlor i d e adsorption on the kinetics of oxygen reduction in buffered, alkaline solutions.

sequent oxygen reduction polarization curves in alkaline, borate-buffered electrolytes.

Experimental Methods All reagents u s e d i n the measurements Were ultrapure or electronics grade. The borate buffers were prepared from Aesar Puratronic grade: H3BO3 or Na2B407 and Alfa ultrapure 30% NaOH. NaC104 st0ck solutions were made u p under nitrogen from Baker Ultrex H c l o 4 and Alfa ultrapure 30~/~NaOH, High Purity water of 16 M~rcm resistivity was prepared in a Millipore "Mfi!i-Q" filtration system with an additional cartridge for organics removal. The flag e!ectrodes Were fabricated from Johnson-MattheY c o m pany 99.998% platinum fo~l and wire. For the cyclic voltammetrY experiments, the e!ec~rdiyte Solutions were deaerated with Airco low oxygen grade nitrogen. In the oxygen r e d u c t i o n experiments, the electrolytes Were aerated With preanalyzed mixtures of Airco grade 4.3 iultrapure) oxygen a~d grade 5 nitrogen, An Orion Model 801A meter and Model 191-04 all-glass combination electrode were used to measure the pH of all electrolyte solutions. The experiments were Carried out in a conventional glass and PTFE electrochemical cell, With a platinum gauze counterelectrode and a double junction standard calomel reference electrode separated from the main cell by a closed glass stopcock and Luggin capillary arrangement. The design of the cell and its auxiliaries ensured that there was always a positive hydraulic pressure from the working electrode compartment to the reference electrode compartment, The electrode potential was controlled by a Pine Instruments RDE-3 bipotentiostat. All potentials are referred to SCE unless otherwise stated. Current interruption measurements indicated that the IR drop was negligible. A Pine ASR2 analytical rotator and speed control were used for the RRDE experiments. The RRDE was a Pine Instruments platinum i'ing/platinum disk electrode with disk diameter of 0.765 cm, ring id of 0.797 cm, and ring od of 0.843 cm. The theoretical ring collection efficiency was 0.183 as calculated by the method of Albery and Bruckenstein (19), and as measured using the ferri/ferrocyanide system. The RRDE was prepared for use by polishing sequentially with 1, 0.3, and 0.05 ~m alumina powders. Both the flag electrodes and the RRDE were degreased with acetone for lh, cleaned in concentrated H2SO4 for 4-5h, rinsed thoroughly, and soaked overnight in high purity water before each experiment. The roughness factor of electrodes cleaned and polished in this manner was between 1 and 2. To obtain a reproducible electrode surface before each scan, the ring and disk electrodes were pretreated in a separate cell in 1M H2SO4 (reagent grade) with eight alternating anodic and cathodic pulses between + 1.16 and -0.24V, followed by rinsing with high purity water. We found that ,this pretreatment improved the reproducibility of the sub-

cyclic voltammetry to study chlor}de adsorption on stationary platinum flag electrodes in borate buffers. The chloride ' concentration was increased by adding aliquots of a concentrated stock solution using a micropipettor. NaC104 Was used as a nonadsorbing supporting electrolyte because the conductivity, of the borate buffers was quite low. R e p r o d u c i b l e vol.tammograms were generally obtained within five to ten cycles.

I

I

I

l

I

1

OD2

Cyclic voltammetry in borate buffers.--Figure 1 represents the cyclic voltammogram of a polished platinum RDE in pH 8.9 borate buffer with 0.5M NaC104 as supporting electrolyte. The cyclic voltammetry of platinum in alkaline borate buffers is similar to that in 1M NaOH or in alkaline NaC104 solutions; the absence of extraneous peaks indicates that the borate species present [chiefly B(OH)3 and B(OH)(] do not undergo any redox reactions in this potential r a n g e . T h e i n o d i c Peak at -0.050V vs. SCE corresponds to the reverSible "Pt-OH" adsorption peak identified by Bag0tzky and other authors in KOH, HCtO4, and phosphate buffer solutions (20-21). Effect of chloride adsorption.--The effects of chloride adsorption on the cyclic voltammetry of platinum are much smaller in neutral and weakly alkaline solutions than in acid solutions. The most prominent changes upon addition of chloride to 1M NaC104 at pH 11.2 include an increase in the size of the hydrogen adsorption and desorption peaks and a decrease in the height of the cathodic Pt-O reduction peak. At this pH, no significant changes in the voltammograms were observed at chloride concentrations less than 5 x 10-2M. In the weakly alkaline pH range, the effects of chloride adsorption become more prominent in the platinum oxide formation region as well as at hydrogen adsorption/ 0.10

/

t

t

t

(

t

(

f

0.08

0.06

0.04

0.02

0

-0.02

AREA : 2 cmz

-0,04

I

ELECTROLYTE : 0.1M BORATE BUFFER

ELECTROLYTE : 0,1M BORATE BUFFER 0,5 ~ RatiO4 pH : 8.9 ATMOSPHERE : N 2 TEMP : 25~ S~EEP RATE. 20 rev/s AREA 0.459 cm 2

0,03

Results Chloride adsorption studies.--We used triangular sweep

o

& 0.5 M NaCIO 4 pH : 7 . 3 ATMOSPHERE : N 2 TEMPERATURE : 25 ~

-0.06

-0.08

SWEEP RATE : 20 mV/sec--

O.Ol -O.lO

SYMBOL

~o

[CI-]

........ -0.12

--

-O J4

--

, M

--

0 10- 4

. . . . .

i 0 -B 5X I0 - 2

-o.ol

-O J6

I -08

-06

I -04

b -0.2

]

I

I

I

0

0.2

0,4

06

POTENTIAL . V vs. SCE

Fig. 1: Cyclic voltammetry of platinum (disk electrode, stationary) in 0.1M borate buffer, pH 8.9, with 0.5M NaCI04 as supporting electrolyte.

-0.8

I

I

I

I

-0.6

-0.4

-0.2

0

POTENTIAL

I 0.2

I 0.4

I 0.6

I 0.8

LO

o V vs. S C E

Fig. 2. Effect of chloride anion on cyclic voltammetry of Pt (flag electrode) in 0.1M borate buffer, pH 7.3, with 0.5M NaCI04 as supporting electrolyte.


J. Electrochem. Soc., Vol. 136, No. 1, January 1989 9 The Electrochemical Society, Inc. o -Ol

77

ELECTROLYTE : 0 I_M_ BORATE ond 0.5 M_ NoCIO4 pH : 8,2 ROTATION RATE, rp~

400

< -02

~

-

* ~

E-_O5

ATMOSPHERE : 16 4 % 02 / BQI, N 2

1.6

ER = + 0 . 6 2 0 V AREA : 0 0 5 9

-

Cmz

1.4

ROTATION RATE, rprn

....

~

6400 4900

~

-05

~

-06

~REA 90.459 cm2 ELECTROLYTE OAMBORATE BUFFER

4900

AND 05 M NaClO 4 -07

6400

ATMOSPHERE,

t-,Z 1.0

#

I1c

16.4% o2 /BALANCE Nz OB

SWEEP RATE ' io my/s

//L/

-08

E -09 !

I -08

_017

I -06

I -05

I -04

~ -0~

I -02

I -OI

I 0

I OJ

oF2

I O~

I 04

I 05

I l,

0.6

/ .

POTENTIAL , V vS SCE

Fig. 3. Current-potential curves for oxygen reduction on platinum RRDE in 0.1M borate buffer and 0.5M NaCI04.

0.4

\,... 0,2

desorption potentials. Figure 2 depicts the changes in the cyclic v o l t a m m e t r y of p l a t i n u m in 0.1M b o r a t e b u f f e r p l u s 0.5M NaC104, p H 7.3, as t h e c h l o r i d e c o n c e n t r a t i o n is increm e n t a l l y r a i s e d f r o m 0 to 5 • 10 2M. I n t h e e q u i v a l e n t buffer at p H 8.3, t h e r e s u l t s are s i m i l a r q u a l i t a t i v e l y b u t s o m e w h a t less p r o n o u n c e d . F o r c h l o r i d e c o n c e n t r a t i o n s of a p p r o x i m a t e l y 10-3M or m o r e i n e i t h e r electrolyte, t h e a r e a of t h e Hb p e a k s ( - 0 . 2 5 to - 0 . 4 V in Fig. 2) d e c r e a s e s w h i l e t h a t of t h e Ha p e a k s i n c r e a s e s ( - 0 . 4 to -0.6V). T h e s e p e a k s h a v e b e e n s h o w n to c o r r e s p o n d to m u l t i p l e e n e r g y s t a t e s of h y d r o g e n c h e m i s o r p t i o n o n Pt. T h e c h a n g e in a r e a s c a n b e i n t e r p r e t e d as a s h i f t in h y d r o g e n a d s o r p t i o n f r o m a m o r e s t r o n g l y b o u n d s t a t e to a l o w e r e n e r g y state. T h i s s h i f t also c h a r a c t e r i z e s c h l o r i d e a d s o r p t i o n i n 1N H2SO4, a l t h o u g h in t h a t e l e c t r o l y t e t h e n u m b e r of d i s t i n c t s t a t e s of a d s o r b e d h y d r o g e n is g r e a t e r (22). I n t h e p l a t i n u m o x i d e f o r m a t i o n r e g i o n ( - 0 . 2 V a n d up), c h l o r i d e a d s o r p t i o n m a n ifests itselfin a reduction in area of the Pt-OH adsorption peak, a slight decrease in charge in the region +0.35 to +0.75V vs. SCE, and the appearance of a small shoulder at approximately +0.3V at high Cl concentrations. The loss of area in the Pt-OH peak increases with chloride concentration. Hence, the decrease in charge in this potential region can be attributed to competition for adsorption between chloride and hydroxide ions. At p H 8.3, the area under the Pt-OH peaks decreased by 9.5% at 5 x 10-EM NaCl, while at p H 7.3, the charge in the same region decreased by 12.2%. O x y g e n reduction studies in buffered a n d u n b u f f e r e d a l k a l i n e s o d i u m perchlorate solutions.--For t h e R R D E s t u d y , w e v a r i e d t h e d i s k p o t e n t i a l u s i n g cyclic or l i n e a r s w e e p v o l t a m m e t r y w h i l e h o l d i n g t h e r i n g at a c o n s t a n t p o t e n t i a l i n t h e l i m i t i n g c u r r e n t r e g i o n for H202 o x i d a t i o n (23-24). T h e d i s k p o l a r i z a t i o n c u r v e s w e r e r e c o r d e d i n t h e c a t h o d i c d i r e c t i o n f r o m a n a n o d i c l i m i t of +0.92V vs. N H E . T h e c a t h o d i c - g o i n g c u r v e s w e r e s e l e c t e d to a l l o w c o m p a r i s o n w i t h r e s u l t s (not r e p o r t e d here) i n s o l u t i o n s c o n t a i n i n g NaC10. I n h y p o c h l o r i t e s o l u t i o n s , t h e a n o d i c - g o i n g c u r v e s s h o w c o n s i d e r a b l e h y s t e r e s i s (25), w h i c h m a k e s c o m p a r a t i v e a n a l y s i s w i t h b a s e e l e c t r o l y t e e x p e r i m e n t s difficult. F i g u r e 3 a n d 4 s h o w t h e disk polarization c u r v e s as a funct i o n of co for 02 r e d u c t i o n in b o r a t e - b u f f e r e d 0.5M NaC104 at p H 8.2 a n d t h e c o r r e s p o n d i n g ring c u r r e n t s for H202 oxidation. A t t h e a n o d i c e x t r e m e of t h e c u r v e s in Fig. 3, t h e disk c u r r e n t i s k i n e t i c a l l y limited a n d s h o w s n o d e p e n d e n c e o n

"/

1600

~

I

I -0.6

~

I

I -0.4

I

J

r

-02

DISK POTENTIAL,

900

f

I

0

~

I

0.2

I 0,4

V vS, SCE

Fig. 4. Ring electrode currents for H202 oxidation during oxygen reduction on Pt RRDE in borate-buffered 0.5M NaCI04, pH 8.2.

r o t a t i o n rate. A t m o r e c a t h o d i c p o t e n t i a l s , t y p i c a l l i m i t i n g c u r r e n t b e h a v i o r is o b s e r v e d . T h e s l i g h t d o w n t u r n in t h e c u r v e s at p o t e n t i a l s b e l o w - 0 . 6 5 V vs. S C E m a r k s t h e o n s e t of h y d r o g e n e v o l u t i o n . O n t h e r i n g c u r r e n t c u r v e s (Fig. 4), t h i s e v e n t is m a r k e d b y a c o r r e s p o n d i n g i n c r e a s e in curr e n t d u e to H2 o x i d a t i o n . T h e r i n g c u r r e n t s are c o n s i d e r a b l y s m a l l e r t h a n t h o s e m e a s u r e d i n a n a l o g o u s experim e n t s i n 0.8-1.0M NaOH. T a b l e I lists t h e p e a k r i n g c u r r e n t s , c o r r e c t e d b y c o l l e c t i o n efficiency, as a f r a c t i o n o f t h e d i s k c u r r e n t a t t h e s a m e p o t e n t i a l in a series of b o r a t e b u f f e r e d electrolytes. T h e q u a n t i t y of h y d r o g e n p e r o x i d e g e n e r a t e d ( i n d i c a t e d b y IR/NID) i n c r e a s e s w i t h i n c r e a s i n g pH. F o r t h e s a m e pH, c h l o r i d e s o l u t i o n s t e n d to g e n e r a t e m o r e p e r o x i d e t h a n do p e r c h l o r a t e solutions. T h e l i n e a r i t y of t h e K o u t e c k y - L e v i c h p l o t s (for e x a m p l e , Fig. 5) o v e r a w i d e p o t e n t i a l r a n g e i n d i c a t e s t h a t o x y g e n red u c t i o n in b o r a t e - b u f f e r e d 0.5M NaC104 at p H 8-10 is firsto r d e r in o x y g e n c o n c e n t r a t i o n (24). I n t h i s series o f buffers, t h e a p p a r e n t l i m i t i n g c u r r e n t for 02 r e d u c t i o n dec r e a s e s s l i g h t l y w i t h i n c r e a s i n g p H (5% b e t w e e n p H 7.9 a n d 9.7). S i n c e t h i s d e c r e a s e c o i n c i d e s w i t h a rise i n solut i o n c o n d u c t i v i t y , w e h a v e i n t e r p r e t e d it as a " s a l t i n g o u t " effect. I n b o r a t e b u f f e r s y s t e m s at c o n s t a n t t o t a l b o r a t e c o n c e n t r a t i o n , t h e i o n i c s t r e n g t h i n c r e a s e s w i t h p H bec a u s e t h e e q u i l i b r i u m shifts f r o m B(OH)3 to B(OH)4-. A n overall e l e c t r o n t r a n s f e r n u m b e r , n, c a n b e c a l c u l a t e d f r o m t h e s l o p e of t h e K o u t e c k y - L e v i c h p l o t s u s i n g t h e following equation [1]

1/ID = 1/Ik + 1/B~o112

w h e r e Ik is t h e k i n e t i c c u r r e n t in a m p e r e s at a c o n s t a n t potential, co is t h e r o t a t i o n rate in r p m , a n d B, t h e r e c i p r o c a l of t h e slope, is g i v e n b y (26) B = 0.201 n F A y -116 C% Do22j3

[2]

TaMe I. Peak ring currents for H202 oxidation in various electrolytes Electrolyte

pH

0.1M Borate & 0.5M NaC104

7.9 8.2 8.9 9.7 11.1 8.0 9.0 14

0.5M NaC104 + NaOH (100% 02) 0.1M Borate & 0.5M NaC1 1.0M NaOH (100% Oa)

Peak Ia at 2500 rpm (A) 1.26 • 10-6 8.32-8.86 • 10-7 1.40 x 10 ~ 1.17-1.52 x 10 6 1.05-1.07 • 10-5 2.43 • 10-6 4.0 • 10-6 5.4 x 10-5

(IR/NID) •

100%

1.6 1.6-1.7 1.9 1.7-2.2 3.1-3.2 3.5-3.9 5.9 21.0


J. Electrochern. Soc., Vol. 136, No. 1, January 1989 9 The Electrochemical Society, Inc.

78

v

+ 2 8 mV + 1 0 mV

+ D

/

~ 0.20

- ~ 0 mV - 2 0 mV --4O mV - 6 0 mV - 8 0 mY

x

B.08

/

-

0,~5

+

pH 8.0

A

pH 8.B

r

pH 9.7

[]

pH 11,1, Na Borate

8.10

m

o

0.0B ~

o.o0

508-

-8.85 2.08 -0.10 1.o0 -OA5 0.00

I

i

f

0.040

0.020

0.000

60 .N

-0.20

,

[3

pH B.2, 0 1 M Borate buffer + 0,5 M NaC]O 4 pH

. . . . . . .

1

~

-3 B

i

1

--36

,

,

-3,4

~

,

--3 2

,

,

-3.8

i

,

-2.e

,

1

" -2.5

,

,

-2.4

,

1

-2 2

,

-3

-2

IDIL/(ICID),h~PS

Fig. 7. Mass transport-corrected Tafel plots for 02 reduction on Pt RRDE in 0.1M borate buffers with 0.5M NaCI04 os supporting electrolyte, and in unbuffered alkaline 0.5M NoCI04. All data at 2500 rpm and 20% 02.

E =-

-O.201 -4.0

,

-4

r = 2500 r p m . To o b t a i n t h e m a s s - t r a n s f e r c o r r e c t e d Tafel p l o t s (Fig. 7), t h e c u r r e n t - p o t e n t i a l d a t a at r = 2500 r p m w e r e a n a l y z e d a c c o r d i n g to t h e e q u a t i o n

B,2, 8.1 M Borate buffer + 0.5 M NoCI

010

-0.30

,

i

t~G

9

ooo~ ' ~ - ~ - ~ ~ , , . ~

~

-5

(RpM)-~

Fig. 5. Koutecky-Levich plots as a function of potential for 02 reduction on Pt RRDE in pH 8.2 borate buffer with O.5M NaCI04 as supporting electrolyte.

0.20 /

,

-6

v~

-2.8

LOG I K, AMPS

Fig. 6. Tafel plot of kinetic currents for 02 reduction on Pt RRDE, derived from Kautecky-Levich plots. 0.1M borate buffer with O.5M supporting electrolyte. I n o u r case, t h e e l e c t r o d e area, A, is 0.459 c m 2, v is 0.9-1.0 • 10 -2 cm2/s, a n d Do2 is 2.5 x 10 -~ cm2/s (27-28). Ass u m i n g t h a t o x y g e n s o l u b i l i t y d e p e n d s m o r e o n t o t a l ionic s t r e n g t h t h a n o n t h e specific i d e n t i t y of t h e i o n i c s p e c i e s i n s o l u t i o n , w e e s t i m a t e d Co2 u s i n g t h e m e a s u r e d v a l u e s i n s e a w a t e r (28) a n d in s i m p l e KC1 or NaC1 s o l u t i o n s (29-31) at t h e s a m e t e m p e r a t u r e a n d c o n d u c t i v i t y as t h e b o r a t e b u f f e r e d NaC104 electrolytes. T h e vah~es of n c a l c u l a t e d in t h i s m a n n e r r a n g e d f r o m 3.2 to 3.4 e l e c t r o n s p e r o x y g e n molecule. F i g u r e 5 s h o w s a t y p i c a l plot of 1/ID VS. 1/(o1/2as a f u n c t i o n of p o t e n t i a l for O2 r e d u c t i o n in b o r a t e - b u f f e r e d 0.5M NaC104. U s i n g Eq. [2], w e h a v e e v a l u a t e d t h e k i n e t i c curr e n t s , Ik, o v e r t h e p o t e n t i a l r a n g e in w h i c h t h e 1/ID VS. 1/e~1/2 l i n e s are n e a r l y parallel. F i g u r e 6 s h o w s t h e s e r e s u l t s for p H 8.2. As e x p e c t e d , t h e slopes (&E/6 log Ik) o b t a i n e d f r o m t h i s figure a g r e e closely w i t h t h o s e c a l c u l a t e d f r o m a Tafel p l o t of t h e m a s s t r a n s p o r t - c o r r e c t e d d i s k c u r r e n t s at

RT 2.3~nF

log (i0A)

-

RT

[

ILID

log 2.3~nF L ~ J

]

[3]

T a b l e II lists t h e Tafel slopes in b o r a t e - b u f f e r e d 0.5M NaC104 as a f u n c t i o n of p H a n d c u r r e n t d e n s i t y . As pict u r e d i n Fig. 7, t h e Tafel p l o t s y i e l d t w o l i n e a r r e g i o n s of d i f f e r e n t slope. T h e s e t w o r e g i o n s c o r r e s p o n d to t h o s e obs e r v e d b y S e p a et al. (8-10), in t h e i r i n v e s t i g a t i o n of o x y g e n r e d u c t i o n in v a r i o u s a l k a l i n e electrolytes. I n t h e b o r a t e b u f f e r e d e l e c t r o l y t e s w h i c h w e e x a m i n e d , t h e Tafel slopes w e r e close to t h e e x p e c t e d - 2 . 3 (RT/F) for t h e l o w c u r r e n t d e n s i t y r e g i o n a n d - 2 . 3 (2 RT/F) at h i g h e r c u r r e n t d e n s i ties. T h e Tafel plot for o x y g e n r e d u c t i o n in u n b u f f e r e d a l k a l i n e 0.5M NaC104 at pt-I 11.1 also d i s p l a y e d s i m i l a r behavior. I n b o r a t e - b u f f e r e d a l k a l i n e NaC104, t h e p H dep e n d e n c e of t h e p o t e n t i a l at c o n s t a n t c u r r e n t (&E/BpH), is a p p r o x i m a t e l y - 5 8 m V / p H i n t h e r e g i o n of l o w e r Tafel s l o p e a n d - 3 4 m V / p H i n t h e r e g i o n of h i g h e r slope. To g a i n i n s i g h t i n t o t h e m e c h a n i s m o f o x y g e n r e d u c t i o n o n P t in b o r a t e - b u f f e r e d a l k a l i n e NaC104, w e a n a l y z e d t h e r i n g c u r r e n t s u s i n g t h e m e c h a n i s t i c criteria of W r o b l o w a et at. (32). T h e n o n z e r o r i n g c u r r e n t s d e m o n s t r a t e t h a t at l e a s t s o m e f r a c t i o n of t h e 02 b e i n g r e d u c e d goes t h r o u g h t h e s e q u e n t i a l p a t h w a y w i t h H 2 Q as a n i n t e r m e d i a t e . P l o t s Of ID/IR VS. 1/(o1/2s u c h as t h o s e s h o w n in Fig. 8 yield s t r a i g h t l i n e s w i t h s l o p e s g r e a t e r t h a n zero, t h u s i n d i c a t i n g t h a t t h e H202 g e n e r a t e d is f u r t h e r r e d u c e d to O H - at t h e d i s k [the slopes w o u l d b e 0 if H202 w e r e s t a b l e at t h e d i s k (20)]. T h e i n t e r c e p t s of t h e ID/II~ VS. 1/(o1/2 l i n e s v a r y w i t h pH, b u t are a l m o s t i n d e p e n d e n t of p o t e n t i a l at a g i v e n pH.

Buffered a~kaline sodium chloride solutions.--To d e t e r m i n e t h e effect of c h l o r i d e specific a d s o r p t i o n o n t h e kin e t i c s of 02 r e d u c t i o n o n P t i n a l k a l i n e b o r a t e buffers, w e r e p l a c e d t h e 0.5M NaC104 s u p p o r t i n g e l e c t r o l y t e w i t h 0.5M NaC1 a n d c o m p a r e d r e s u l t s for t h e s a m e series of p H

Table II. Tafel slopes for 02 reduction in borate-buffered electrolytes Tafel slope (mV/dec) Electrolyte

pH

Region I Low c.d.'s


7.9 8.0 8.2 8.9 9.0 9.7 8.0 8.2 9.0 9.7

-77 -84 -84 -75 -68 -71 to -74 -89 -86 -77 to 78 -66 to -76


Region II High c.d.'s 129 -127 -144 118 -110 -113 -145 -150 -133 to -141 -120


,~176

J. Electrochem. Soc., Vol. 136, No. 1, J a n u a r y 1989 9 The Electrochemical Society, Inc

7OO

79 SYMBOL

ROTATIONR A T E , r p m

V -40mV- x -20mV +20mV

cm2

600o +~omV

/

/

,RATE 0

/

500]ALANCE N8

~0~[}0-

200 -OB

-0.6

-0.4

-0.2

0

DISK POTENTIAL,

0.2

V v s . SCE

Fig. lO. Ring electrode currents for H202 oxidation during oxygen reduction on Pt RRDE in borate-buffered 0.5M NaCI, pH 9.0. 0200

0.020

0.040

O) -~, (RPM)-89

Fig. 8. Disk-to-ring current ratio as a function of potential and reciprocal square root of rotation rate, for 02 reduction at a Pt RRDE in pH 7.9 borate buffer with 0.5M NaCI04.

values. F i g u r e s 9 and 10 s h o w typical e x a m p l e s of the disk polarization curves as a f u n c t i o n of rotation rate and the c o r r e s p o n d i n g ring currents for H20~ o x i d a t i o n in borateb u f f e r e d 0.5M NaC1, in this case at p H 9.0. As the values listed in Table I demonstrate, t h e ring currents in borateb u f f e r e d 0.5M NaC1 are r o u g h l y two to three t i m e s as large as t h o s e m e a s u r e d in borate-buffered 0.5M NaC10~ at the s a m e pH, rotation rate, and ring potential. In contrast, the l i m i t i n g currents for o x y g e n r e d u c t i o n agree to w i t h i n 3-5% b e t w e e n analogous electrolytes in t h e two series. As in the case of borate-buffered 0.5M NaC10~, t h e linearity of the K o u t e c k y - L e v i c h plots for O2 r e d u c t i o n in borate-buffered NaC1 (for e x a m p l e , Fig. 11) indicates that the reaction is first order in o x y g e n (24). The values of n calculated from t h e s e plots are v e r y close to t h o s e o b t a i n e d f r o m the b u f f e r e d NaC10~ data, in the range 2.7-3.3 for pH values b e t w e e n 8 and 9. F i g u r e 11 r e p r e s e n t s a typical plot of 1lID VS. 1/co~/2o v e r a r a n g e of potentials in alkaline borate-buffered 0.5M NaC1. F i g u r e 6 shows t h e kinetic currents derived f r o m the intercepts of the 1/ID VS. 1/r u2 lines as a f u n c t i o n of potential. A c o m p a r i s o n of t h e s e data w i t h the c o r r e s p o n d i n g results in b o r a t e - b u f f e r e d NaC104 reveals that, at t h e s a m e potential and pH, t h e kinetic currents for O~ r e d u c t i o n are considerably smaller in the buffered chloride t h a n in the b u f f e r e d perchlorate. The Tafel plots in Fig. 12 d e m o n s t r a t e the

s a m e behavior. In the p H range b e t w e e n 8 and 10, the POlarization curves for O2 r e d u c t i o n in borate-buffered 0.5M NaC1 are shifted cathodically relative to t h o s e in borateb u f f e r e d 0.5M NaC104 at the s a m e pH. The curves are displaced b y 30-40 m V at p H 8-9 and to a s o m e w h a t lesser ext e n t at h i g h e r pH. Despite the shift in potentials, t h e shape of t h e Tafel curves remains nearly the same; as in the borate-buffered NaC104, there are still two linear regions having different slopes. At a c o n s t a n t current d e n s i t y of 2 x 10 -5 A / c m 2, the p H d e p e n d e n c e of the potential, (aE/apH), is a p p r o x i m a t e l y - 5 9 m V / p H in the borate-buffered 0.5M NaC1 and NaC104 solutions (Fig. 13).

v A

^

--30 mV --40 mV

~ _-20~mv

v

o.oo

. . . . . . .

~2o

oLo

O) -89 (RpM}-~

Fig. ] 1. Koutecky-Levich plots as a function of potential for 02 reduction on Pt RRDE in pH 8.2 borate buffer with 0.5M NaCI as supporting electrolyte.

o -o~ -o2

-as -04 -~ -o5

0.20 goo

% X

0.15 -

16oo

D A + 0

pH B.O pH B,2 pHgO pH 9,7

0.10 -

....

0,05 0.00 ~

-o.7

~4OO

.........

-o.e

:2::%:2

-0.05 .....

-0.Io

pH 9 ~ o

-o s

ATVOSPSERE : ZO ~Vo02 /BALANCE N TEMPeRATURe Z 4 9 " C

-io

-0.20

-i.i -i 2

-0.15

SWr RAVE. ,V ~ W S AREA. O a~9 =m~

-6 I . I -08 -0.7

I -06

I I .013 I -0,5 - 0 4 -02 P O T E N T I A L , V vs SCE

_Oil

I 0

I 0.i

[ 02

Fig. 9. Current-potential curves for oxygen reduction on Pt RRDE in 0.1M borate buffer and 0.5M NaCI, pH 9.0.

-5

-~

LOGIDIL/(IL-ID),

-3

-2

~s

Fig. 12. Mass transport-corrected Tafel plots for 02 reduction on Pt RRDE in 0.1M borate buffers with 0.SM NaCI as supporting electrolyte. All data are at 2500 rpm and 20% 02.


J. Electrochem. Soc., Vol. 136, No. 1, J a n u a r y 1989 9 The Electrochemical Society, Inc.

80

As n o t e d a b o v e , t h e r i n g c u r r e n t s m e a s u r e d in b o r a t e b u f f e r e d 0.5M NaC1 s o l u t i o n s are c o n s i s t e n t l y l a r g e r t h a n t h o s e o b s e r v e d i n b o r a t e - b u f f e r e d NaC104 at t h e s a m e p H a n d r o t a t i o n rate. F i g u r e 14 r e p r e s e n t s t y p i c a l ID/IR VS. 1ko1/2 p l o t s for alkaline, b o r a t e - b u f f e r e d NaC1. T h e slope v a r i e s with potential and the intercepts converge within a narrow range. A t a c o n s t a n t potential, t h e i n t e r c e p t d e c r e a s e s w i t h i n c r e a s i n g p H (i.e., t h e a m o u n t of p e r o x i d e d e t e c t e d at t h e r i n g increases), b u t in t h e e l e c t r o l y t e s w h i c h w e s t u d i e d it w a s a l w a y s g r e a t e r t h a n 1/N. T h e slopes are c o n s i d e r a b l y l o w e r t h a n t h o s e o b s e r v e d i n b o r a t e - b u f f e r e d NaC104 at t h e s a m e p H a n d potential.

0.250

0.150 -

_vv~ ~

9

-0.050

-

-o.100

-

H202"1 H2~2,b

~

l

-0.150

9 +0.5 M NoCI, 1E-4 A

~, .

~

~~,

~

~

g

11 PH

Fig. 13. pH dependence of O2 reduction on Pt RRDE in borate-buffered electrolytes. Data derived from mass transport-corrected Tafel plots.

400j • -120rnV -90rnV

3501

/ /

o -70 rnV + -50 rnV ,}00o

>

//

-30 mV

200

150

IO0

!

-

~

v

50-

0

i

i

0.000

M e c h a n i s m o f o x y g e n r e d u c t i o n . - - W e will c o n s i d e r a v e r y g e n e r a l r e a c t i o n s c h e m e for 02 r e d u c t i o n o n P t a n d t r y to d e t e r m i n e w h e t h e r t h e d a t a allow u s to m a k e a n y simplifications -kt

2e --_k 4-

a &

[:1

0.050-

>~ u3

Discussion

4e-

D

0.100 -

Values of n.--The biggest problem in estimating the n v a l u e s ( f r o m 1 / I D V S . 1/(o 1/2 plots) is d e c i d i n g w h o s e solubil-

i t y / d i f f u s i v i t y d a t a to use. J o v a n c i c e v i c a n d B o c k r i s (33) cite v a l u e s of 5 x 10 -6 cm~/s for Doz a n d 4 x 10 4 m o l / c m s for Co2 in 0.15M b o r a t e buffers. U s i n g t h e s e values, w i t h Co2 c o r r e c t e d for Po2 = 0.2 a t m , w e c o m e u p w i t h n v a l u e s of 2.74.8 e l e c t r o n s / m o l e c u l e . H o w e v e r , if Co2 a n d Do2 are c o r r e c t e d a c c o r d i n g to t h e i r f i n d i n g t h a t Co2 d e c r e a s e s b y 50% a n d Dd2 d e c r e a s e s b y 25% for a s u p p o r t i n g e l e c t r o l y t e (SO42-) c o n c e n t r a t i o n of 0.5M, w e get n v a l u e s >6. T h e i r n u m b e r s s e e m to b e s o m e w h a t o u t of line With o t h e r literat u r e v a l u e s for Co2 a n d Do2 i n a q u e o u s s o l u t i o n s , b u t w e h a v e n o t y e t f o u n d a n y b o r a t e d a t a for d i r e c t c o m p a r i s o n . I n p u r e H20, r e p o r t e d Do2 v a l u e s (25~ range from 1.9 • 10 4 cm2/s to 2.9 x 10 4 cm2/s. (27-28). R o t t h a u w e (28a) also r e p o r t s v a l u e s for sea w a t e r at v a r i o u s c o n c e n t r a t i o n s (2.7-2.8 • 10 -5 cm2/s). W e h a v e s u m m a r i z e d t h e s e results, t o g e t h e r w i t h s o m e r e p o r t e d v a l u e s for Co~, i n T a b l e III. O u r o w n a t t e m p t at m e a s u r i n g Co2 ( u s i n g t h e L a z a r O2 e l e c t r o d e ) g a v e v a l u e s of 1.0-1.2 x 10 -6 m o l / c m 3.

+0.5 ~ #loCl0~, IE--5 k A +0 5 M NoC;, 1E--5 A

[]

0.200 -

I

0.000

* = surface

species

b = bulk species

W r o b l o w a et aL (32), h a v e a n a l y z e d t h i s s c h e m e a n d der i v e d e x p r e s s i o n s for t h e d i s k c u r r e n t / r i n g c u r r e n t ratio as a f u n c t i o n of r o t a t i o n rate. I n t h i s a n a l y s i s t h e i n t e r c e p t s (J) a n d slopes (S) of t h e ID/IR VS. r 4/2 p l o t s are u s e d to i n f e r t h e mechanistic pathway. T h e J vs. S p l o t s d e r i v e d f r o m o u r d a t a are n o n l i n e a r for b o t h b u f f e r e d c h l o r i d e a n d p e r c h l o r a t e electrolytes. D u e to t h i s n o n l i n e a r i t y , t h e J. vs. S p l o t s for b o r a t e - b u f f e r e d c h l o r i d e a n d p e r c h l o r a t e s o l u t i o n s c a n n o t b e u s e d to dist i n g u i s h b e t w e e n t h e series m e c h a n i s m a n d t h e parallel m e c h a n i s m . N o n l i n e a r J vs. S p l o t s c o u l d m e a n t h a t t h e W r o b l o w a m o d e l is n o t a p p l i c a b l e (32). To d e t e r m i n e w h e t h e r catalytic d e c o m p o s i t i o n of H~O2 is i m p o r t a n t u n d e r t h e g i v e n c o n d i t i o n s , w e also a n a l y z e d

I

i

0020 ( 1 ) -'=' 9

I

0.040

(RP~I) 4

Fig. 14. Disk-to-ring current ratio as a function of potential end reciprocal square root of rotation rate, for 02 reduction at a Pt RRDE in pH 8.0 borate buffer with 0.5M NaCI. t h e d i s k c u r r e n t d a t a u s i n g t h e d i a g n o s t i c c r i t e r i o n dev e l o p e d b y H s u e h et al. (2). T h e s e a u t h o r s r e p o r t e d t h a t p l o t s of IL/(IL -- ID) VS. r -1~2 at v a r i o u s p o t e n t i a l s w o u l d b e l i n e a r if k4 w e r e n e g l i g i b l e a n d n o n l i n e a r if catalytic H202 d e c o m p o s i t i o n w a s a s i g n i f i c a n t side reaction. As Fig. 15 a n d 16 d e m o n s t r a t e , o u r r e s u l t s y i e l d e d p l o t s of IL/(IL - ID) vs. ~o-1/2 w h i c h are s u b s t a n t i a l l y l i n e a r o v e r a fairly w i d e pot e n t i a l range. T h u s , to a first a p p r o x i m a t i o n , w e c a n simplify t h e g e n e r a l r e a c t i o n s c h e m e [4] b y e l i m i n a t i n g k4. Since the results of our oxygen reduction experiments in b o r a t e - b u f f e r e d e l e c t r o l y t e s c o u l d n o t b e r e a d i l y interp r e t e d in t e r m s of t h e m o d e l of W r o b l o w a et al. (32), w e

Table III. Oxygen solubility and diffusivity data Author(s)

Solution

Jovancicevic & Bockris (1986) Kreuzer (1950) Rotthauwe (1958) Rotthauwe (1958) Gubbins & Walker (1965) Gubbins & Walker (1965) Eucken & Hertzberg (1956) Yasunishi (1978) Whitfield & Jagner (1981) Kaska

0.15M borate buffers H20 H20 Sea water H20 5 wl% KOH 0.63M NaC1 0.5 M KCI Sea water (25% salinity) 0.1M borate + 0.5M NaCI, 0.1M borate + 0.5M NaC104

Do2, 25~ (crnZ/s) 5 x 10-6 1.9 • 10 5 2.9 • 10-5 2.7-2.8 x 10-5 1.9 x 104 1.6 x 10-5 -----

Co2, 25~ mol/cm 3 (for 1 atm O2) 4 x 10-6

_ --

1.2 x 10-~9 x 10-~ to 1 x 10-8 1.12 x 10_8 1.24 • 10-8 1.1 x 104 1.0-1.2 x 10-6


J. Electrochem. Soc.,

V o l . 1 3 6 , N o . 1, J a n u a r y

1989 9 The Electrochemical

I0.0

S o c i e t y , Inc.

81

o.~oo

V -80mV

0.080

-60mV

0.060

X

~.0"

0.040

-40 mV 0 -20 mV + 0 mV 0 § mV

8.0

0.020 o.000 -0.020 -0.040

7,0

-0.060 -0,080

6.0 "T J

-0.1oo -0.120

52

-d.l~o

k% 0.5 M ~aO

-0.160

k2, 0.5 M NOCI

-0.180

,L0-

--0.200

'

--4

"~

'

-'2

10 -

'

",

'

LOG Ki , Cars

Fig. 17. Potential dependence of rate constants kl and k2 for oxygen reduction on Pt in borate-buffered O.5M NaCI04 at pH 7.9 and in borate-buffered 0.5M NaCI at pH 8.0. These rate constants correspond to the simplified reaction scheme in Eq. [7].

tO0.~

O.DI)O

O.D2~}

rate constants to be calculated from the intercepts and slopes of plots of ID/IR VS. r 112 a n d IL/(IL -- ID) VS. CO Y2. Using t h e s e e x p r e s s i o n s (2), we calculated the rate constants kl, k2, and k3

0.040

( x ) - 8 9 (RPM)-L~

Fig. 15. Diagnostic plots for determining the relative importance of catalytic H202 decomposition (rate constant/(4) in the generalized reaction scheme for 02 reduction on Pt (Eq. [4]), in borate-buffered O.SM NaCI04 at pH 8.9.

kl = 0.201 Do22/3v-1/6 S 2 ( J N - 1 ) / ( J N + 1) k2 = 2(0.201 D%2m~-u6) S 2 / ( J N + 1) k3 = 0.201 DH2o22/~v-1/6 N S / ( J N + 1)

also analyzed the data using the s i m p l e r reaction s c h e m e originally p r o p o s e d by D a m j a n o v i c e t al. (5) _k 1

4e%,b

-->

%*

-k22e_-> H a i 2 *

1

k.~

2e -> H2

[5] * = surface

H202'b

b = bulk

species species

This reaction m o d e l a s s u m e s that h y d r o g e n p e r o x i d e ads o r b e d on t h e e l e c t r o d e surface is in e q u i l i b r i u m w i t h pero x i d e in the b u l k solution and that the rate of catalytic H202 d e c o m p o s i t i o n is negligible (k4 ~ 0). The latter ass u m p t i o n appears to be b o r n e out by t h e linearity of plots of IL/(IL -- ID) VS. CO-]/2. F u r t h e r m o r e , the simplified m o d e l a s s u m e s k2 > > k_~; that is, the electrode potential is in the Tafel r e g i o n for o x y g e n r e d u c t i o n to H202, so that the back r e a c t i o n can be neglected. B a s e d on this reaction scheme, H s u e h e t a t . (2) d e r i v e d e x p r e s s i o n s w h i c h allow all three 10.0

9.0t 8,0

7,0

-140mV X -100mV ,~ -60rnV 0 -20rnV + OmV D +20mV

w h e r e $2 is the slope of a plot of IL/(IL -- ID) VS. co-lj2 and the c o n s t a n t 0.201 c o r r e s p o n d s to ~o in rpm. We u s e d values of DH2O2 = 1.6 • 10 -5 cm2/s, based on the data of P r a b h u e t a L (34), and Do2 = 2.5 • 10 -8 cm2/s. F i g u r e s 17 and 18 p r e s e n t the rate constants k~, k2, and k3 as f u n c t i o n s of potential. The data in Fig. 18 indicate that in t h e b u f f e r e d chloride solutions ks is nearly i n d e p e n d e n t of potential o v e r the range shown. This lack of potential d e p e n d e n c e m a y signify that the rate of H202 r e d u c t i o n is controlled by a c h e m i c a l step p r e c e d i n g t h e first e l e c t r o n transfer. In contrast, kl and k2 are potential d e p e n d e n t b o t h in b u f f e r e d perchlorate and in buffered chloride solutions. T h e s e rate constants d e m o n s t r a t e b e h a v i o r s o m e w h a t similar to that o b s e r v e d in the Tafel plots of Fig. 7 and 12, alt h o u g h the transition b e t w e e n the regions of l o w e r overpotential and h i g h e r o v e r p o t e n t i a l is not as clearcut. The results for kl and k2 qualitatively r e s e m b l e t h o s e of H s u e h e t al. for o x y g e n r e d u c t i o n on P t in acid solutions (2, 35). The calculated rate c o n s t a n t for the 4e- process, kl, is generally two to eight t i m e s as large as that for the 2e process, k2. H o w e v e r , in borate-buffered NaC104 at p H 8.9, kl app e a r e d to reach a limiting v a l u e at potentials b e l o w -0.04V v s . S C E (+0.73V v s . RHE); at this point, the calculated values of kl and k2 b e c a m e comparable. H s u e h e t a l . , also o b s e r v e d limiting b e h a v i o r in k~, but only at potentials O.lOO o,o8o

LOT ..~

?

o.o60 o.o~,o

5.@.

0.020 o.o00

4.0.

-0.020 -0.040

0.0.

-0.060

-o.o~0

ZO

-O.lOO -o.12o

A

+ 0.5 M NoCIO4, pH 7.9

LO.

-o.16o

O

+ 0.5 M Noel, pH ~,0

-o.16o

O

+ 0.5 M NaCI, pH 9.0

o.18o

0.0 0.000

i

i 0.020 (jJ-~

t

I 0.040

(RPM)-89

Fig. 16. Diagnostic plots for determining the relative importance of rate constant k4 in the generalized reaction scheme for 02 reduction on Pt, in borate-buffered O.SM NaCI at pH 9.0.

-0.200

I

--3.0

J

--2.6

~

I

--2.2

~

f

--%8

I

I

--1,4

I

I

--1.0

I

I

--0.6

I

I

--0.2

~ % , cws

Fig. 18. Potential dependence of rote constant k3 for oxygen reduction on Pt in 0.1M borate buffers with O.5M NaCI04 or 0.5M NaCI as supporting electrolyte.


J. Electrochem. Soc., Vol. 136, No. 1, January 1989 9 The Electrochemical Society, Inc.

82

b e l o w a b o u t 0.55-0.60V vs. R H E in 0.55M H2SO4 and in 0.7M H~PO4. T h e s e authors attributed the p h e n o m e n o n to a rate-limiting c h e m i c a l step, possibly 02 adsorption, prior to t h e first electron transfer (2, 35). In any case, the simple reaction m o d e l of D a m j a n o v i c et al., clearly will no longer hold u n d e r t h e s e conditions, sinc@ it does not t a k e into acc o u n t t h e possibility of a c h e m i c a l rate-limiting step prec e d i n g the series of steps l u m p e d u n d e r kl. F u r t h e r m o r e , the analysis begins to break d o w n w h e n kl -= ks, b e c a u s e t h e intercepts (J) of the ID/IR VS. CO:1/2 plots c a n n o t be determ i n e d w i t h sufficient a c c u r a c y (i.e., at m o r e cathodic potentials, the plots m a y yield intercepts w h i c h are less t h a n 1/N). To date, w e h a v e not b e e n able to ascertain w h e t h e r t h e a p p a r e n t limitation in k, is a p H d e p e n d e n t effect, because t h e ring current data in buffered NaC104 at p H v a l u e s greater t h a n 9 did not yield satisfactory plots of ID/IR VS.

In this case, the analogous e x p r e s s i o n for the fractional c o v e r a g e w i t h a d s o r b e d i n t e r m e d i a t e will be O0 = Keq_ Co2OM. S i n c e the rate e x p r e s s i o n d e r i v e d f r o m this analysis is only first-order in v a c a n t sites, any decrease in (~M d u e to specific a d s o r p t i o n of anions will h a v e a smaller effect on k2 t h a n on kl. K i n e t i c p a r a m e t e r e s t i m a t e s . - - T h e s e are s u m m a r i z e d in Table IV. The linearity of the K o u t e c k y - L e v i c h plots o v e r a w i d e potential range indicates that the c o n c e n t r a t i o n dep e n d e n c e , a log i/a log Po2, is 1 for b o t h chloride and perchlorate solutions. C o m p a r i n g our e s t i m a t e d kinetic p a r a m e t e r s to t h o s e t a b u l a t e d by J o v a n c i c e v i c and Bockris (33) for various prop o s e d m e c h a n i s m s , w e find that our results agree m o s t closely with t h o s e p r e d i c t e d for the following m e c h a n i s m , w h e r e the subscript " a d s " indicates an a d s o r b e d species

(D-1/2.

T h e calculated rate constants in borate-buffered electrolytes at p H 8 (Fig. 17) clearly d e m o n s t r a t e the effect of chloride a d s o r p t i o n on the o x y g e n r e d u c t i o n process. In the region of h i g h e r overpotentials (roughly b e l o w 0 V vs. S C E at this pH), b o t h k~ and k2 are smaller in solutions containing chloride t h a n in b u f f e r e d NaC104, b u t k, decreases to a c o n s i d e r a b l y greater e x t e n t t h a n does k2. In the r e g i o n of l o w e r overpotentials, k2 appears to be nearly u n a f f e c t e d by t h e p r e s e n c e of C1 . T h e s e results can be i n t e r p r e t e d in t e r m s of a difference in a d s o r b e d i n t e r m e d i a t e s in the 4eand 2e- pathways. If the 4e- process occurs t h r o u g h oxygen a d s o r p t i o n in the bridged configuration (1, 4), t h e n each a d s o r b e d m o l e c u l e requires two v a c a n t m e t a l sites ka

2 M + 02

M/

0-0

\M

(02,

ads)

._kd At equilibrium, setting the rates of a d s o r p t i o n and desorption e q u a l gives kaCo2OM2 = kdO0 or Oo = KeqCo2 0M 2 w h e r e Keq = kJkd Thus, the rate of o x y g e n r e d u c t i o n t h r o u g h t h e 4e- pathw a y will be first-order in t h e c o n c e n t r a t i o n of dissolved 02 (or first-order in Po2 as long as H e n r y ' s law applies) but seco n d - o r d e r in t h e fractional c o v e r a g e of e m p t y m e t a l sites. By contrast, o x y g e n r e d u c t i o n to p e r o x i d e along the serial (2e-) p a t h w a y is e x p e c t e d to p r o c e e d t h r o u g h an intermediate a d s o r b e d in the end-on (Pauling) configuration (1, 4, 32), in w h i c h the m o l e c u l e occupies only one P t site

/

O

O

I

M + 0 2 N kd

02 r

O2ads

O2~d~ + HeO + e -~ O2H~ds + O H -

(rds)

in w h i c h t h e first electron transfer is always rate d e t e r m i n ing b u t the c o v e r a g e w i t h o x y g e n i n t e r m e d i a t e s goes f r o m T e m k i n conditions at m o r e anodic potentials to L a n g m u i r conditions at l o w e r potentials. This m e c h a n i s m predicts the o b s e r v e d c h a n g e in Tafel slope f r o m -2.3 (RT/F) to -2.3 (2 RT/F) as the potential decreases. S e p a et al. (8, 9) also p r o p o s e a m e c h a n i s m in w h i c h the initial step involves o x y g e n adsorption and the s e c o n d step is t h e r a t e - d e t e r m i n i n g first electron transfer. The p H d e p e n d e n c e p r e d i c t e d by this t y p e of m e c h a n i s m does not, however, c o i n c i d e w i t h w h a t w e have o b s e r v e d (see Table IV; t h e p r e d i c t e d values for alkaline solutions are -0.5 in the low cd region and 0 in the high cd region, w h i l e for acid solutions, the p r e d i c t e d values are -1.5 and -1.0, respectively). The data of S e p a et al. (8, 9) in alkaline solutions also s h o w a small p H d e p e n d e n c e in the high cd region, alt h o u g h these authors fit their results to a m e c h a n i s m for w h i c h (aE/3pH)i = O. In a recent publication (10), S e p a et al. h a v e t a k e n s o m e pains to show, furthermore, that the m e c h a n i s m p r o p o s e d by Tarasevich et aL w o u l d not yield a p H d e p e n d e n c e of - 6 0 m V / p H [i.e., (~ log i/SpH)E = --1 in the low cd region] as claimed. Nevertheless, the data p r e s e n t e d in Fig. 9 and 10 of Ref. (9) at least suggest the possibility of a s m o o t h transition in (~E/apH)i in going f r o m acid to alkaline solutions. A l t h o u g h we h a v e investigated the p H d e p e n d e n c e o v e r a rather limited range (pH 7.9-11.7), our results w o u l d s e e m to indicate that the case is not yet closed and that further careful m e a s u r e m e n t s in b u f f e r e d solutions are warranted. Conclusions I n s u m m a r y , we h a v e reached the following conclusions c o n c e r n i n g chloride anion adsorption on p l a t i n u m and its influence on o x y g e n r e d u c t i o n kinetics. 1. Chloride anion adsorbs on p l a t i n u m to a m u c h lesser e x t e n t in m o d e r a t e l y alkaline solutions t h a n in acid solu-

Table IV. Kinetic parameters for 02 reduction in borate-buffered electrolytes

C1-

High cd region Supporting electrolyte ClO4

C1

70-80 mV/dec 2.3 (RT/F) to 2.3 (3 RT/2F)

70-90 mV/dec 2.3 (RT/F) to 2.3 (3 RT/2F)

110-140 mV/dec 2.3 (2 RT/F) to 2.3 (5 RT/2F)

130-150 mV/dec 2.3 (2 RTfF) to 2.3 (5 RT/2F)

I-~E1 - [~ppHJ

60 mV/pH 2.3 (RT/F)

60 mV/pH (60)~ 2.3 (RT/F)

30 mV/pH (33-34)~ 2.3 (RT/2F)

40 mV/pH (36-41)a 2.3 (RT/2F) to 2.3 (2 RT/3F)

_ plogi 1 L ~p--Hp~ J~

0.67-1

0.67-1

0.25

0.2-0.3

Parameter [ 5a~oEg ] i

Low cd region Supporting electrolyte C10(

(Estimated from above values) aValues of slopes within ( ) are experimentally determined. Downloaded on 2015-03-12 to IP 129.252.86.83 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

J. Electrochem. Soc., Vol. 136, No. 1, January 1989 9 The Electrochemical Society, Inc. tions. Nevertheless, the effects of chloride adsorption can be detected up to pH 11. 2. I n moderately alkaline solutions (pH 7-9), chloride specific adsorption inhibits Pt-OH formation. 3. Chloride ion adsorption shifts the polarization curves for oxygen reduction to more cathodic potentials, by 30-40 mV for 0.5M C]- in pH 8.2 borate buffer. The extent of this inhibition diminishes with increasing pH. 4. During oxygen reduction, chloride ion adsorption also gives rise to increased hydrogen peroxide generation. 5. In terms of mechanism, the first electron transfer is the rate-determining step for oxygen reduction on Pt, both in the presence and in the absence of adsorbed chloride.

Acknowledgment This work was supported by the U.S. Office of Naval Research, Contract no. N00014-84-C-0521. Manuscript submitted Aug. 5, 1987; revised manuscript received May 10~ 1988. This was in part Paper 568 presented at the Boston, MA, Meeting of the Society, May 4-9, 1986.

Giner, Incorporated, assisted in meeting the publication costs of this article. LIST OF SYMBOLS electrode area, cm 2 reciprocal of the slope of a plot of 1/ID VS. (o-1/2 Co2 dissolved 02 concentration in the bulk solution, mol/cm 3 Do2 diffusion coefficient of 02, cm2/s DH202 diffusion coefficient of H202, cm2/s disk electrode potential, V vs. SCE, unless otherwise E specified ER ring electrode potential, V vs. SCE Faraday constant, 9.6485 • 104 C/mol F current density, A/cm 2 i exchange current density, A/cm 2 disk electrode current at a given potential, A ID kinetic current, A diffusion limited disk electrode current, A IL ring electrode current at a given potential, A IR intercept of a plot of [D//R V S . (0-1/2 J adsorption rate constant ka desorption rate constant, kd rate constant for ith reaction in 02 reduction pathki way, cm/s [M] bulk concentration of species M electron transfer n u m b e r n ring electrode collection efficiency N Po2 partial pressure of 02, atm gas constant, 8.3144 J/tool 9K R slope of a plot of ID/IR VS. r -1~2 S $2 slope of a plot of IL/(IL - ID) VS. 0)-1/2 temperature, K T transfer coefficient p kinematic viscosity, cm2/s OM fractional surface coverage with empty sites fractional surface coverage with adsorbed oxygen O0 electrode rotation rate, rpm

A

B

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