Indian Journal of Chemistry Vo l. 42A. A pril 2003. pp. 1)0 1-806

Hydrodynamic chronocoulometric estimation of diffusion coefficients and saturated concentrations of dioxygen in KOH solutions

D Zhang, J F Wu. L Q Mao, T Okajima, F Kitamura & T Ohsaka *

Dcpartment of Electronic Chemi stry. Interdi sc iplinary Graduate School of Science and Engineering. Tokyo Institutc of Tcchnology,A259 Nagatsuta. Midori -ku . Yokohama 226-8502, Japan ~

E-mail address: ohsaka @echcm.titcch.ac.jp: Fax: +81 -45-924-5489

and

T Sotomura

anostructure Dev ice Group. Nanotechnology Research Laboratory. Advanced Technology Research Laboratories. Matsushita Elcc tri c Industria l Co .. Ltd. 3-4 Hikaridai. Seika-cho, Soraku-gun. Kyoto 6 19-0237. Japan

Received 22 N();Ielllber 2002

Hydrodynamic chronocoulometry has been app lied to determine diffusion coe ffi cients and saturated concentrations of dioxygcn in 0. 1 to 15 M potass ium hydrox ide solutions. The values of diffusion coeffi cient and saturated concentration of dioxygen decrcase with incrcasing conccntration and kinematic viscosity of KOH solutions. Furthermore vari ous phys ico­chcmica l quantities of KOH so lutions have been estimated at 25.5°C. The product of diffusion coefficient of dioxygcn and viscos ity of KOH so lution shows a constant valuc, i .e .. ( 1.4 ± 0.3) x 10.7 g ern S·1, wi thin the experimental error for I to 8 M KOH solutions. The conducti vity of KOH so lutions show a quarti c relation w ith respect to KOH conccntration. and its high­est value is obtained at the S M KOH solution.

The reducti on o f dioxygen (0 2) in alkal ine media is one of the mos t fundamenta l electrode reacti ons and is very important in practica l appl ications (e.g. metal­air batteri es and fuel ce ll s). The diffusion coefficient (D02) and concentration (C02 ) of O2 in the bulk elec­troly te highly influence the mass transfer process in O2 reduction. Dav is el 0/.

1 measured the so lubility of O~ in aqueous KOH solutions using a Van Slyke appa­ratus and an adsorption technique developed by Hil ­debrand. They al so evaluated the Do~ va lue from the limiting current obtained for the O2 reduction at a ro­tating di sk electrode and from the result of measure­ment with a stagnant tube technique. Gubbins and Walker~ used gas chromatography to determine the so lubility of O~ in different concentrations of KOH solutions, and obtained the D02 va lue by use of a po­larographic method. Recently Marian el aI. ' used the transit time o f O2 from the di sk to the ring of a rot;1t­ing ring-d isk elec trode to determine the O2 diffusivity in 0. 1 M ( I M = I mol dm" ) NaOH at 25°C and in 11.1 M NaOH at 25 and 80°e. In these prev ious studi es, at leas t two separate measurements were re­quired and in some cases the standard solution must be prepared. These render the simultaneous measure­ment of D02 and CO2 diffi cult.

Hydrodynamic chronocoulometry has been suc­cessfully used to simultaneously determine of D and C of an elec troactive gas and an unstable spec ie. in aprotic solvents4

.8

. Because thi s method can be used to determine D and C simultaneously, accurateiy and rapidly, it is expected to be also use ful for simul­taneous determination of D02 and CO2 in KOH so lu­ti ons.

The aim of thi s study is to simultaneously deter­mine D02 and CO2 using hydrodynamic chronocou­lometry with rotating disk elec trodes in a w ide con­centration range o f KOH solutions from 0. 1 to 15 M . The conductivity (K) and molar conductivi ty (II) of these solutions were also measured.

Materials and Methods Potassium hydroxide was purchased from Kanto

Chemical Co. (Japan). All of the so lutions used in the experiments were prepared with Milli-Q water. Argon (99.9999%) and O2 (99.98%) gases were or ultra-hi gh purity and supplied from ' ippon Sanso Co .. Inc. (Ja­pan). Gold (Au , 2 mm diameter) and glassy carbon (GC, 6 mm diameter) rotating di sk electrodes pur­chased from Nikko Keisoku Co. (Japan) were used as the working elec trode in this study.

802 INDIAN J C HEM . SEC A, APRIL 2003

A computer-controlled e lectrochemica l system (BAS 100B/W, Bioana lytica l Systems Inc .) and a ro­tating e lec trode system (Nikko Keisoku Co.) were used for hydrodynamic chronocoulometric experi­ments . The potential was stepped from an initial value where no current flows to a value where the electrode reac ti on proceeds at a mass transfer controlled rate . The chronocoulograms for 0 2-saturated KOH solu­tions were corrected for the res idual current by sub­tract ing the corresponding chronocou logram recorded with deoxygenated KOH solution .

The e lectrochemical cell used is a conventional two-compartment Pyrex® g lass containe r with Au or GC working e lectrode, a spiral platinum (Pt) wire auxiliary e lec trode and a potass ium chloride-saturated s il ver/s il ver chl oride (Ag/AgCI, KCI-sat. ) reference e lec trode. The Au and GC electrodes were succes­sive ly po lished with #2000 emery papers, and J and 0.06 ~tm alum ina powder on microc lo th wetted with Milli-Q water before use. The e lectrodes we re then carefully soni cated in Mi ll i-Q water for 10 mi n and rinsed with Milli-Q water. The Au e lectrode was c lean ed by scanning potenti a l repeatedly be tween -0.2 V and + 1.5 V liS Ag/ AgCl . KCI-sat. at a scan rate of 10 Vs·1 in 0.05 M H2S04 so lution for 10 min . O 2

gas was bubbl ed directl y into the ce ll to obtain an Or saturated so lution , and during the measurements, O2

gas was flu shed over the ce ll so lutio n. Argon gas was bubb led into the solutio n for deaeration , and then background measurements were carri ed out under Ar atmosphere. All the e lec trochemical measure ments

were performed at 25 .5 ± 1°C. Kinematic viscosity (II) of the soluti o ns was meas­

ured using an Ubbelohde viscometer in a the rmostated wa te r bath (Cool nics modl e CTR/CTE- 120 , Ko matsu­Yamato, Japan). The temperature of the so lutions was

con trolled at 25.5 ± O. I° C. The values of II for the vari ous concentrations o f KOH so lutio ns used in th is study are shown in Table I .

Conducti vity (x:) of the KOH soluti o ns with differ­ent concentratio ns was measured using EC (e lectri c conduc tivity) mete r (C M60G , conductivity cell: CT 57101A, TOA Electro nics Ltd , Japan) in a the rmo­stated water bath. The temperature of each solutio n

was kept at 25 .5 ± 0 . 1 0c. Results and Discussion

The typical cyc lic voltammograms for O 2 reduction in KOH sol utions with the concentrations ranging from 0.1 to 15 M were obtained at Au and GC elec trodes . Two reduction peaks were obta ined for O2

Table I- Kinematic viscosity ( v). viscosity (;1) and density (p) of KOH so lutions at va rious co ncentrations o f KOH (CKOH)

CKo,i M v "Vc m2 s" p ,,)/g cm ') fl h)/g c m" s"

0.10 0.008977 1.003 0.009002

0.57 0.009 189 1.0 17 0.00934 1

1.00 0.00947 1 1.034 0.009793

2.50 0.01039 1.089 0.0 11 32

5.00 0.01259 1.179 0.01484

8.00 0.0 1705 1.267 0.02 160

10.00 0.02117 1.340 0.02836

12.00 0.02778 1.405 0.03902

15.00 0.04709 1.474 0.06939

"The measurements of v and p of cac h solutio n we re performed at

25.5 ± 0.1 °e. Each ex periment was repeated 3 to 5 times. un til the errors of the measureme nt were less than l 'Yo . ''The va lues of vi scosity (fl) were ca lcul ated fro lll the data o f v and p using the equation o f /l = v p.

o .. .. .-...----

- 20 -< ::1.

-40 1

2 3 4 ---60 5 6

-0.8 -0.6 -0.4 -0.2 0

E / V vs. Ag! AgCI, KCl-sat.

Fig. I- T ypical steady-state vo ltammograms for reductio n o f O2

to H0 2' at rota ting GC electrodes (6 mill dia le te r) in 0 2-saturated (so lid lines) and Ar-saturated (dotted line ) 5 M KO H soluti ons. E lectrode rotat ion rate : ( I ) 300. (2) 400. (3) 500. (4) 600. (5) 700 and (6) 800 rpm. Scan rate : 5 mV s· ' .

reduction, wh ich is similar to those reported by Yea-9· 11 l d F' h d H . b 12 Y ger et a . an ISC er an . e lt aum . eager ef

al. 9 concluded that the react ions proceeded in the fol-low ing two processes : .

O2 + H20 + 2e' ~ H02' + OH '

H02- + H20 + 2e-~ 30H -

( I )

(2)

Z HANG el af.: CHRONOCOULOMETRIC ESTIMATION OF DIFFUSION COEFFIC IENTS 803

6 5 4 3

100 2 1

U ::1. --C)l

50

o o 0.5 1.5

l i s

Fig. 2- Typical hydrodynamic chronocoulometric data for O2 reduction at GC electrodes (6 mm diameter) in 0 2-saturated (solid lines) and Ar-saturated (dotted line) 5 M KOH solutions. The electrode rotatio n rate: ( I) 300, (2) 400, (3) 500, (4) 600, (5) 700 and (6) 800 rpm . The potentia l o f the work ing electrode was stepped from - 0 . 15 to - 0.6 V vs Agi AgCl, KCI-sat.

In thi s study, the first two-elec tron reduction step of O2 to H02- was used to determine D02 and CO2 by hydrodynamic chronocoulometry . Typical steady­state vo ltammograms for O2 reduction to H02- in the 0 2-saturated and Ar-saturated 5 M KOH solutions are shown in Fig. I. As expected the linear Levich plo t pass ing through the ori g in was obtained, indicating that the limiting current is controlled by mass transfer of O2 from the bulk of solution to the e lectrode sur­face . The reduct ion current obtained with both cyc lic vo ltammetry and rotating di sk e lectrode vo ltammetry decreases wi th increasing concentrati on of KOH so­luti on (CKOH ) .

Typical hydrodynami c chronocoulo metric curves obtained for the reduct ion o f O 2 to H02- in the O 2-

saturated and Ar-saturated 5 M KOH solutio ns are shown in Fi g. 2. A linear regress ion fo r each curve was carri ed o ut over the e lectrol ys is time between 1.2 to 1.5 S, where a steady-s tate current was reached . The re latio nship between the charge (Q) and time (/) is g iven by Eqs (3)-(6/':

Q = QiJ + ill

iL = III-A DCI()

Q6= 0.3764nFACo

(3)

(4)

(5)

0= 1.6 10 DI I3VI /6(J) .1I2 ( I +0.29805("- 1/3+0. 145 1 45c·213 )

. . . (6)

100

A

•

• 1 50 • -- 0 . .:::

30

B

• 20 •

•• • • U ::t

--~ 10

0. 1 0.2

w -In / (fad s·lr ln

Fig. 3- Plots o f (A) - i L vs win and (8 ) - Qo vs W ·1/2 obtained

from hydrodynam ic chronocoulometri c data shown in Fig. 2. (e ): Without background correction, (0 ): with background cor­rectio n.

where iL is the Levich limiting current, Q/i is the charge passed in e lectro lys is of O2 contained initi a lly in the hydrodynamic bo undary layer, wh ich has the thi ckness 0, II is the number of e lectrons in vo lved in the e lectrode reactio n, F is the Faraday constant , A is the e lectrode surface area, v is the kinematic viscosity of the so lutio n. 5(" is the Schmidt number (vlD), and w is the e lectrode ang ul ar ve locity (w = 2n: I, where I is the frequency of the e lectrode rotat ion).

The slope (il) and intercept (Q6) o f these straight I· I d . I /~ d . 1/' regressIo n Ines are p o tte agaInst w - an w -

804 INDI AN J C HEM, SEC A. APRIL 2003

A .1 M

80 B 6M

60

1 ..... .,2 40

M

20

5 10 15

CJ) 112 / (rad S· I)1 12

Fi g. 4- Plo ts of - iL vs w l12 obtained from hydrody namic chrono­cou lometric da ta. (A) shows the results obtained on the Au rotat· ing disk e lec trode (2 mm diameter) in 0. 1. 0.57. I and 2.5 M KO H so lutions and (8) shows the results obtained on the GC rotating di sk elec trode (6 mm d iamete r) in 5, 8. 10. 12 and ISM KO H solutions.

as shown in Fig. 3 (A) and (B), respecti vely. From Fig. 3, we find that both pl ots o f - iL VS w l12 and - Qo vs W ·

I/2 are linear and pass th rough the orig in after

background correctio n. With increas ing CKOH, the charg ing current increases, and the reducti on current of O2 decreases . Thi s made the measurement diffi ­cult with increas ing CKOI·I .

10-3 ~ II

~ ~ ........ 10-4 ~ 0 0

c3 '" ~

• 10--5 '"

o 10

Fi g. 5- Dependence of satu ra ted concentration of dioxygen (Co~) on KO H concentrati on (CKOH)' (0 ): Measured with the Au elec· trode in thi s work. (0 ): measured wi th the GC elec trode in this work, ( A ): cited from Ref. I , and (~): ci ted fro lll Re f. 2.

Th I f · 1/2 . C ' e p ots 0 - IL VS w at van ous KOH s a re shown in Fig . 4. In concentrated KOH so lutions fro m 5 to 15 M, O2 gas bubble adsorbed on the surface of Au e lectrode, which made the measure ment di fficul t, and so the GC rotating di sk e lectrode with larger surface area was used in solutio ns of more th an 5 M KOH in this study. In Fi g. 4, as expected fro m Eq . (4 ), each plo t passes through the orig in and is linear. Al so as expected fro m Eq. (5) each p lo t o f Qo versus w - 112 passed through the orig in and was linear. Therefore, based o n these Qo and iL va lues, the va lues of 0 02 and CO2 in KOH soluti ons of di fferen t concentrati o ns were es timated using Eqs (3)-(6) and are summari zed in Table 2. Bo th values of D 02 and CO2 decrease with increas ing CKOH , resulting in the decrease in the reduction current o f O2 in concentrated KOH soluti ons as observed with both cyc lic vo ltammetry and ro tating d isk e lectrode vo ltammetry.

The values of CO2 and 0 02 as a functi on of CKOH

are sho wn in Fig. 5, respecti vely. For compari son, the data reported by Davis el al. l

, and G ubbins and Walker 2 are a lso g iven. The va lues of CO2 are in a good agreement with the results reported by G ub bins and Walker for I to 10M KOH soluti o ns. The re la­tionship between CO2 and C KO II in less than 5 M KOH so lutio ns was fOLind to be expressed by Eq. (7):

log (C02) = log ( 1. 11 x 10.3)-0. 16 1 (CKO H) .. . (7)

with the corre latio n coe ffic ient of 0.9979. But in mo re

ZHANG el al.: C HRONOCOULOMETRIC ESTIMATION OF DIFFUSION COEFFIC IENTS 805

Table 2- Saturated concentrations and diffusion coeffi c ients of O2 in KOH so lutions with concentrati ons ranging from 0.1 to 15 M

CKo,IM Co2"/mM 0 02" x I O' /cm2s" Co2 b/mM 0 02 h x IO',cm2s" Cm ' /mM 0 02 ' x IO',c m2s-'

O. 10 1.03±0. 12 1.95±0.40 1.20 1.90 1. 23 1.83

0.57 0.86±0. 17 1.66±0.30 0 .93 1.63 0.98 1.7 1

1.00 0.82±0. 10 1.44±0.30 0.80 1.45 0.81 1.6 1

2.50 0.45±0.02 1.25±0. 10 0.42 1.07 0.44 1.27

5.00 0.17±0.01 1.1 8±0.03 0.14 0.77 0.19 0.82

8.00 0.093±0.01 0.73±0.08 0 .048 0.55 0 .081 0.41

10.00 0.063±0.01 0.62±0.0 1 0 .010 0 .052 0.26

12.00 O.081±0.01 0.34±0.02 0 .009 0.15

15.00 0.077±0.01 0. 15±0.03

"Va lues simultaneously measured by hydrodynami c chronocou lometry at 25.5± I DC in thi s work. bValues c ited fro m Ref. I . "Va lues c ited from Ref. 2.

2 • ";- • '" .... E 1.5 • u

'0 •• x

8 • Q

• 0.5

• •

00 0.025 0.05 0.075

j.1. / g cm-' sol

Fig. 6- Pl ot of diffusion coefficient (002) liS viscosity (ji) of KOH soluti ons.

than 5 M KOH solutions, the values of CO2 level off to a constant value (i.e. 0.074 ± 0.0 I I mM). This result is diffe rent from the results of Davis el at. , but is similar to th at reported by Gubbin s and Walker 2.

Co mparing the va lues of D02 obta ined in thi s study with those in the references ,.2, it becomes obvious that the di sparity is at mos t 2.6 % in KOH solut ions of low concentration s « 2.5 M), and that the values of 0 02 obta ined in thi s s tudy are lata 34 % larger than those reported by Davis el at. in concentrated KOH solu­ti ons (:::: 2 .5 M).

The pl ot of 0 02 liS J.I. (v iscos ity) in th is study is shown in Fi g . 6. The 0 02 J.I. product remains constant within the experimental error for I to 8 M KOH solu­tions at 25.5 °C: 0 02 J.I. = ( 1.4 ± 0 .3) x 10-7 g cm S-2 .

60 0 5 10 15

200

•

40

-; a

r/J

lo/ 20

O~L-______ ~ __ ~ __ ~ __ ~ ____ ~O o 5 10 15

CKO~ / M

Fig. 7- KOH concentration dependences of conducti vi ty (K . • ) and molar conduct ivity (;1 , 0).

The values o f conductivity (K) of the KOH solu­tions of various concentrations were determined and in addition the values of molar conductivities (;/) were calculated using the K va lues obta ined . The pl ots of K liS C KOH and II liS C KOH are shown in Fig. 7. II de­creases with increasing CKOH , but K shows its max i­mum at 8 M KOH solution . In the KOH solutions, the conductivity significantly depends on CKOH . The K

va lue increases with increas ing CKOH in less than 8 M KOH solutions because of an increase in the total number of ions present and reach its maximum at 8 M KOH solution, and then the K value decreases with increasing CKOH in more th an 8 M KOH soluti ons be­cause of an increase of flowing resistance of ions de­duced by the increase of number of ions. The re lation between K and CKOH can be expressed by a quartic

806 INDIAN J CHEM , SEC A. APRIL 2003

equation [Eq. (8)] w ith the correlati on coefficien t of 0.9997.

K = - 0.002060 (C"OH )~ + 0.1093 (CKOH)' - 2.307 (CKOH)2 + 19.34 (CKOH ) + 0.3270 .. . (8)

Conclusions Hydrodynamic ch ronocoulometry has been suc­

cessfu ll y app li ed to determine D 02 and CO2 in 0. 1 to 15 M KOH so lutions. In less than 5 M KOH solutions, the CKOII dependence of CO2 can be expressed by log (Co:!) = log ( 1.11 x 10-') - 0.161 (CKOH), but in more than 5 M KOH solutions, the va lues of CO2 level off to a constant va lue. D 02 decreases with increas ing CKOH.

The value of Do:! P. product remains constant w ithin the experimental error for I to 8 M KOH solutions. The K v:1 lues of various KOH so lutions show a quartic relation with respect to CK01 h and the highest value is obtai ned at the 8 M KOH solution.

Acknowledgement Thi s work was supported by Grant-in-Aids for Sci­

enti f ic Research (A) (No. 10305064), Scientific Re­search (No. 12875 164) and Scientific Research on Pri­ority Area (No. 417) from the Ministry of Education,

Culture, Sports, Science and Technology, Japan. 0 Zhang gratefull y ackonowledges the Saneyoshi Foun­dati on for the scholarship.

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