Investigation of bubble evolution with a quartz crystal microbalance

515

J. Electroanal. Chem., 297 (1991) 515-522 Elsevier Sequoia !%A., Lausanne

Preliminary

note

Investigation microbalance C. Gabrielli,

F. Huet, M. Keddam

UPR 15 du CNRS, 05 (France) (Received

of bubble

evolution

and R. Torresi

“Physique des Liquides et Electrochimie”,

12 November

with a quartz

crystal

* Tour 22-4, Place Jussieu,

75252 Paris Gdex

1990)

Numerous electrochemical processes of practical interest involve gas production at an electrode surface. The subsequent bubble evolution leads to a random behaviour of the observed potential when the electrode is current controlled. It has been shown that these potential fluctuations are, at least partially, related to a screening effect of the surface by the gas bubbles [l] which gives rise to an additional electrolyte resistance and hence to ohmic drop fluctuations. At low current densities it was shown that the ohmic drop and the overvoltage increase linearly when the bubbles grow and decrease suddenly when a bubble detaches from the surface [2]. The sizes and times of occurrence of the potential jumps are random and the observed potential is a random signal which was analyzed by spectral analysis [3]. For high values of the electrolysis current the measurement of the power spectral density (p.s.d.) of the potential fluctuations leads to information on the dynamics of the bubble evolution regime, i.e. the mean number of bubbles evolved per time unit, the average bubble detachment radius and the gas evolution efficiency [3]. For low values of the electrolysis current an estimate of the detachment size of the bubbles can be obtained only from the measurement of the power spectral density of the electrolyte resistance fluctuations [4]. However, information concerning the physico-chemical details of the bubble detachment is still lacking. Recently, it has been proposed that an electrochemical quartz crystal microbalante be used in order to detect electrolytically generated bubbles [5]. The response of the quartz resonance frequency to polarisation current steps has been observed. But probably due to the narrow bandwidth and the low amplification of the electronic equipment the individual transients related to the bubble growth and

l Post-doctoral Argentina.

fellow of the Consejo

W22-0728/91/$03.50

National

de Investigaciones

0 1991 - Elsevier Sequoia

S.A.

Cientificas

y Tecnicas

de la Republica

516

response is plotted. Hence this detachment are not visible and only a “smooth” response is related to a global resonance frequency change. In this note, we report some results proving that by using large bandwidth equipment and sufficient amplification of the measurement channel it is possible to detect the modulation of the resonance frequency of the quartz by the growth and detachment sequence of the bubbles. The equipment allows, on the one hand, the individual electrode mass transients to be recorded, and on the other hand, the power spectral density of the mass fluctuations to be measured. EXPERIMENTAL

The evolution of hydrogen in acid solution at nickel and gold electrodes was chosen as an example of an electrolytic gaseous evolution. One of the circular gold contacts (5 mm in diameter) to the quartz crystal (6 MHz, AT-cut, Copelec, France) is used directly, or after electrolytic nickel coating, as the working electrode of a standard three-electrode cell. The counter electrode is a platinum gauze and the reference electrode is of the saturated sulfate type. The oscillator and the current control are identical to those already described in ref. 6. In this case for small mass changes Am (less than a few per cent of the mass of the quartz crystal + electrodes device), the frequency shift Af is proportional to Am: Af=

-KAm

where K is the integral sensitivity of the quartz microbalance. Nevertheless, when a localized mass change, such as the growth and detachment of a bubble, is produced at a radial position r, K depends on r (the electrochemical quartz crystal microbal-

I 4lJ

I

fr - f,

Fre-quency difference

*

, workhg oscillator

I

Fig.

Schematic

J

I

representation

ti

Fourter

*?Af

Low pass ffltcr 1

fTi

I

-

Frequency to voltage converter

of the experimental

ARe

’ ‘YARe

517

ante is more sensitive in the center than at the edge of the electrode) [7,8]. As the location of the bubble detachment is not known exactly, the following results will be given in frequency change units rather than in mass change units. The mass and electrolyte resistance transients were analyzed using the experimental arrangement depicted in Fig. 1. The mass changes can be measured directly through a frequency counter (Schlumberger 2721) and recorded on a x-t plotter. The equivalent bandwidth of this measurement channel is rather narrow (a few Hz only) and the mass transients are not very well defined. On the other hand, the difference between the resonance frequency of a reference quartz crystal oscillating in air and the frequency of the working electrode, proportional to the mass changes, is converted to a voltage in a frequency to voltage converter, and then sampled in a Fourier analyzer (Hewlett Packard 5451C) together with the voltage proportional to the electrolyte resistance changes. This device can analyze over a sufficiently large bandwidth (40 Hz for time recording, in order to avoid parasitic influence of the mains, and 500 Hz for spectral analysis) the random signals either by recording them versus time or by plotting their power spectral density versus frequency. The electrolyte resistance changes are processed in a similar manner, for helpful comparison. RESULTS AND DISCUSSION

Figures 2a and 2b show the mass changes and the electrolyte resistance changes recorded versus time for hydrogen evolution at a nickel electrode at a 0.5 mA current. It was already shown that the slow increase of the electrolyte resistance is related to the global growth of the bubbles on the whole surface and each sudden decrease is related to the detachment of one single bubble. Obviously the electrolytic resistance measurement is sensitive only to relatively large bubbles generated on active sites which are finite in number. The transients related to the numerous small bubbles are not detected because they are masked by the surrounding noise. However, the electrochemical quartz crystal microbalance is sensitive to much smaller bubbles. The recording of the frequency transient related to the mass changes shows that when the bubbles grow the mass decreases slowly. When a bubble detaches a sudden increase of mass is detected (possibly preceded by a fast negative transient of mass hardly detectable because of the ambient noise), due to the replacement of the gas of the bubble by the electrolyte. After a short decay of mass due to the bubble growth a steep decrease is observed. The origin of this sharp mass decrease is without a doubt related to the site where the previous detachment has just occurred, since this negative mass jump is never observed without a previous bubble departure. This feature can be related tentatively to the rearrangement of the contact angle of the gas bubble on the surface. In order to calculate the radius of the evolving bubble producing the 25 Hz frequency decrease (first part of the frequency transient in Fig. 2a), the dependence of K on the radial position is neglected and an integral sensitivity K= 5.2 X 10’ Hz

518

Time/s

c \

1.

cj

0.

“2

I

-l. -2. -3 -4. -5

0

9

6

3

12

15

Time/s Fig. 2. Hydrogen evolution reaction on a nickel electrode; i = - 0.5 mA; 1 M H,SO,. frequency shift vs. time; (b) electrolyte resistance change vs. time.

[7]. The mass change g -I cm* is considered frequency jump Af, in Hz, is equal to Am/g

= - (Af/Hz)/26

The volume of the bubble,

X

Am, in g, which

(a) Resonance

corresponds

to the

10’

of gas implied in the frequency change Af is related to the radius R the contact angle 6’ and the thickness h of the modulation layer

519

a

b

Fig. 3. Schematic diagrams of bubbles attached to the electrochemical quartz microbalance with (a) a contact angle of O” (R2 is the radius of the bubble); (b) a contact angle of 90° (R, is the radius of the bubble). In each case, h is the penetration depth of the ultrasonic wave (about 300 nm).

which corresponds to the penetration length of the ultrasonic wave in the electrolyte. Two limiting cases can be examined. Firstly, for a contact angle of 90” (Fig. 3b) the “active” volume Vi which is included in the modulation layer (here h = 300 nm), is roughly equal to Vi = 77R$ i.e.

i.e. for a p = 1 g cme3 density decrease, R, is equal to:

of the displaced

electrolyte

and a 25 Hz frequency

R, = 300 pm Secondly,

for a contact

angle of 0“ (Fig. 3a) the “active”

volume

V, is equal to

V, = mh2( R, - h/3) For Af = -25 Hz and p= 1 g cme3 the detachment radius would then be R 2 = - Af/vh2Kp i.e. R 2 = 30 cm, which is not reasonable for a bubble size. It is noticeable that at a given radius R z++h the mass detected by the quartz microbalance is very small for a contact angle of 0 ’ compared to that detected for a contact angle of 90 O, the ratio being

where Am,,0 and Am,0 are the masses detected for a bubble of radius R and a contact angle of 90 o and 0 O, respectively. Hence, under the experimental conditions of Fig. 2a the contact angle of the evolving bubble must be much greater than OO. In order to explain the last part of the frequency transient (frequency increase corresponding to a mass decrease) it is supposed that as soon as an active site is free after a bubble departure, a new bubble starts growing with a contact angle of 0 O. Then at some size (reached after about 2 s in the case of the first event observed in

520

Fig. 2a) the contact larger contact angle contact angle from volume, the bubble frequency jump Af2

angle changes suddenly as the largest bubbles certainly need a to continue to adhere to the electrode. If a quick change of the without any change of the bubble 0” to 90” is considered radius is increased in the ratio 2t13 = 1.26 and the resulting - Af, can be calculated from the frequency changes observed

10

100

Frequency / Hz

10‘5

‘N

X

1o-7 .

“c\ d 10-a : 9 1o‘g ; I,

I

doll

10 Frequency

100

11

/ Hz

Fig. 4. Hydrogen evolution reaction on a nickel electrode; i = -5 mA; 1 M H,SO,. (a) Power spectral density of the resonance frequency fluctuations vs. frequency; (b) power spectral density of the electrolyte resistance fluctuations vs. frequency.

521

for two bubbles Afi = -26

which have contact

angles of 0” and 90” and the same volume:

x 10’ ?rh2( R, - h/3)p

and x 10’ ?rR;hp

Af, = -26

which corresponds to the frequency shifts having a radius R, and R, = 1.26 R 2. Hence Af2-Afi

= -26x107prh[h(RZ-h/3)

For an observed

frequency

jump

Af2 and

Afr

related

to the bubbles

- (1.26R2)2]

Af2- Afl= 5 Hz (see Fig. 2a), R, is equal to

R 2 = 112 pm Hence for these limiting contact angles the change of 0 from 0” to 90” would occur at a 112 pm radius. This change is detected by the microbalance but not by the measurement of the electrolyte resistance as the change of contact angle does not sufficiently modify the screening effect on the electrode surface. On a gold quartz electrode the elementary transients are not separable as too many small bubbles evolve simultaneously at the surface.

100

16’

1o-8

c,e

10-9 $ \ SW

16’0

-11 10

w 10-12 1O-y

1o-z

10-l

1

log(i/A)

Fig. 5. Power (m) Resonance

spectral density at 10 Hz for hydrogen evolution frequency fluctuations; (0) potential fluctuations.

on a gold electrode

in 1 M H,SO,.

522

When many bubbles evolve at the electrode surface the elementary growth-detachment transients overlap and a spectral analysis is very helpful. In Fig. 4 are plotted the power spectral densities of the random mass changes expressed in frequency changes and the concomittant random electrolyte resistance changes on a nickel electrode. The results concerning hydrogen evolution on a gold electrode are very similar. Although the p.s.d. of the electrolyte resistance fluctuations decreases continuously with f-* in the frequency range (1 Hz-20 Hz) (the low frequency white noise plateau could be observed in the lower frequency range) the p.s.d. of the “mass” fluctuations shows a frequency vs. frequency dependence with a slope changing from -1 down to -2 in the same frequency range. Furthermore, although the potential fluctuation p.s.d. at 10 Hz increases regularly with the current, the “mass” fluctuation p.s.d. increases up to a 10 mA current and then decreases when the current increases (Fig. 5). These complicated experimental features show that several processes interact during bubble detachment. The replacement of the gas by the electrolyte may be preceded as in Fig. 2a by a depression occurring when the bubble leaves the electrode surface rapdily, which could be detected by a fast decrease of the mass. At high current density when many bubbles are generated, some change of the average viscosity of the layer close to the electrode may be invoked in order to explain the variation of the “mass” p.s.d. versus the current and the change of slope of the p.s.d. versus frequency. In addition, the sensitivity of the quartz microbalance depends on the position of the bubble on the electrode. REFERENCES 1 2 3 4 5 6 7 8

C. Gabrielli, F. Huet, M. Keddam, A. Macias and A. Sahar, J. Appl. Electrochem., 19 (1989) 617. C. Gabrielli, F. Huet and M. Keddam, J. Appl. Electrochem., 15 (1985) 503. C. Gabrielli, F. Huet, M. Keddam and A. Sahar, J. Appl. Electrochem., 19 (1989) 683. C. Gabrielli, F. Huet and M. Keddam, to be published. N.W. Carr, A.R. Hillman, S.D. Lube&in and M.J. Swann, J. Electroanal. Chem., 267 (1989) 313. S. Bourkane, C. Gabrielli and M. Keddam, Electrochim. Acta, 34 (1989) 1081. C. Gabrielli, M. Keddam and R. Torresi, J. Electrochem. Sot., submitted. B.A. Martin and H.E. Hager, J. Appl. Phys., 65 (1989) 2630.