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Characterization of acoustic emission from an electrolysis cell
223
Analytica Chimica Acta, 254 (1991) 223-234 Elsevier Science Publishers B.V., Amsterdam
Characterization of acoustic emission from an electrolysis cell T.G. Crowther, A.P. Wade * and P.D. Wentzell ’ Laboratory
for Automated
Chemical
Analysis,
Department of Chemistry, V4T I Y6 (Canada)
University
of British
Columbia,
Vancouver,
BC,
R. Gopal Dow Chemical
Canaak,
P. 0. Box 3030,
Vi&l
Street
South,
Samia,
Ont.,
N7T
7MI
(Canada)
(Received 8th November 1990; revised manuscript received 7th May 1991)
AbStMCt Evolution of hydrogen and oxygen from electrodes in an electrolysis cell may be conveniently monitored in a non-intrusive manner via its ultrasonic acoustic emission. The apparatus used in this work was comprised of a nickel anode, a stainless steel cathode, and a saturated calomel reference electrode, all situated in a three chamber cell containing sodium hydroxide solution. The potential necessary for gas bubble evolution was conclusively detected by onset of bursts of acoustic emission. Individual acoustic emission signals, captured using a broadband transducer mounted on the working electrode, contained frequencies from the audible range to as high as 800 kHz. These were correlated with the release of bursts of bubbles from the electrode’s surface, both visually and via a chart recorder trace of peak acoustic power vs. time. Trends in several time-domain signal descriptors were observed with increase in the applied voltage. Acoustic power spectra were obtained by averaging spectra from many acoustic signals. Semiquantitative estimates of rate of emission were made by integration of the peak acoustic level. The effects of applied potential and electrolyte concentration on the multiple bursts of acoustic emission were characterized and are presented as a system response surface. Increasing the applied potential resulted in greater rates of bubble emission, which increased the intensity of acoustic emission, but produced an identical acoustic power spectrum. The extent of acoustic emission at high concentrations and applied potentials was less than expected, which suggested decreased efficiency under these conditions. Electrolysis start-up kinetics were also studied using a digitizer capable of large record lengths. Keywords: Acoustic emission; Electrolysis; Frequency-domain
Industrial electrolysis of water is achieved by passing an electrical current through an aqueous solution of an inorganic acid such as sulfuric acid or an alkali metal hydroxide such as sodium hydroxide or potassium hydroxide. This process was first studied in 1789 by van Troostwijk and Deimann [l], but it was Davy [2] who considered the stoichiometric aspects. Today there is enormous
’ Present address: Chemistry Department, Dalhousie University, Halifax, NS, B3H 453, Canada. 0003-2670/91/$03.50
representation; Tie-domain
representation; Waters
commercial interest in electrolysis and similar electrode processes such as electroplating, electroforming, and electromachining. Research into advanced water electrolysis systems has gained momentum in technologically advanced countries because of the suitability of hydrogen as a valuable, high-yield, energy source (both for storage and power). Environmental pressures have also played a role; if produced cleanly, hydrogen is essentially a non-polluting fuel, since its combustion results only in water and heat. There has been much discussion of the possibility of a global
0 1991 - Elsevier Science Publishers B.V. All rights reserved
T.G. CROWTHER
224
ET AL.
hydrogen-based economy, once fossil fuels reserves are sufficiently depleted; however, some technological barriers remain. A detailed knowledge of the factors which affect the energetics and kinetics of electrode processes is a prerequisite for optimization and efficient operation [3,4]. The rate of energy supply to the cell is simply the electrical power, P, which is the product of i, the current passed, and Ecel,, the applied potential. The total energy (which is the principal operating cost of the process) is obtained by integrating this over time. The efficiency of electrolyzers is perhaps only 70-80%, as E,,, is of necessity substantially higher than the theoretical value, E&, derived from the free energy of the process. Reasons for this unfavorable situation include the high internal electrical resistance of the electrolyzer and high overvoltages experienced at the anode and cathode. These contributions may be summarized as
might be achieved. The most obvious place to seek such a gain is in lowering the overpotentials, and much effort has gone into seeking out electrode materials and coatings which improve efficiency in this way (e.g., RuO,-coated titanium subtrates [5]). Typically, iron is used as a cathode because of its low hydrogen overvoltage, and nickel is used as an anode due to its low oxygen overvoltage. In the last 10 years some electrolyzers have been based on “acidic” solid polymer electrolytes [3]. Alkaline water electrolysis remains the most common method, in part because some electrode materials cannot withstand the aggressiveness of an acidic medium [l]. Commercial electrolyzers use non-reactive electrodes to transmit the current. Development of non-invasive, real-time sensor techniques which are capable of monitoring the extent and nature of processes occurring on the surfaces of electrodes is of significant industrial interest.
J%, = EcOattmc~e - EaOnode
as many chemical processes release heat and light, some also emit sound [6]; this phenomenon can usefully monitor their progress [7-91. Analytical use of acoustic emission is already well established in fields such as materials science [lo-121 and experimental botany [13]. Industrial process control applications [14-161 have taken advantage of the passive, non-invasive nature of the technique, not to mention the relatively low cost of the instrumentation. Acoustic emission is presently less well developed in analytical chemistry [17], although much of the technology is similar and a multitude of potential applications exists [6-9,18211. Acoustic emissions from chemical reactions typically take the form of multiple bursts of duration 0.1 to 1.0 ms, and contain frequency components from the audible to at least 1 MHz. As one might expect from the “fizz” that accompanies effervescence, gas evolution processes emit acoustic energy. Wentzell and Wade [17] investigated the high-frequency characteristics of several
Ultrasonic emission from chemical reactions. Just
Ece~l
=
J%,,
+
iR
+
qmode
+
%athode
and r&,&e are the overpotentials, where %node and R is the specific resistance per unit surface area. The resistance term includes the “ohmic drop” due to the electrolyte and diaphragm, and its initial increase due to bubble curtain formation [3]. The half-cell reactions for the electrolysis of water are given in Scheme 1. A major objective of industrial electrochemistry is to achieve the maximum yield of product (here, primarily HZ) per unit time, per unit cell volume, with the minimum energy consumption. Current efficiency, fractional conversion and rate of reaction must all be considered. It has been estimated that a 100-mV decrease in the potential which must be applied could decrease the energy requirements of one major industrial company such that a direct saving of perhaps $lM per annum per site Cathodic half-reaction 4H,O + 4e- + 4H + 40H4H -+ 2H, gthode = - 0.83 v Scheme
1.
Anodic half-reaction 40H-+40H+4e40H -+ 2H,O + 0, E:riw~e = + 0.40 v
Overall reaction 2H,O --) 2H, + 0, AG” = 474 kJ mol-’ E,P,, = - 1.23V
CHARACTERIZATION
OF ACOUSTIC
EMISSION
FROM
AN ELECTROLYSIS
225
CELL
in chemical reactions; their implosion creates momentarily, local pressures of up to several GPa and local temperatures from lo3 to perhaps lo6 K. The premise of this paper is that an electrochemical reaction can be followed from the release of ultrasonic energy caused by the collapse (or formation) of bubbles. Moriguchi has suggested that the water decomposition voltage of electrochemical processes can be reduced by ultrasound [25], thus the natural ultrasonic emission may assist in the process. Ultrasonic waves have been used to study electrolytic solutions in other ways and ultrasound-induced electrochemical WI, synthesis of various anions [27] has been reported. In this paper we study the utility of ultrasonic emission to help characterize and optimize a sim-
processes which involve gas evolution, and found substantial emission between 50 kHz and 250 kHz. Furthermore, the audible frequency emission was monitored to quantify and characterize the evolution of oxygen bubbles during decomposition of hydrogen peroxide by catalase [9]. Acoustic emission from electrolysis has been reported [22]. Lube&in [23] has used a low-frequency acoustic technique to measure bubble nucleation rates on various surfaces. Stimulation by injection of power ultrasound is becoming increasingly valuable in chemical processes [24]. This is of interest in this paper since reactions such as electrolysis are low power sources of ultrasonic waves of similar and higher frequencies. Microbubbles account for the effects
Individual
Signals
TEKTRONIX 2430A DIGITAL STORAGE
REFERENCE ELECTRODE
COUNTER ELECTRODE IN GLASS APPARATUS CONTAINING
Fig. 1. Experimental
apparatus
for monitoring
WORKING ELECTRODE
CH
NaOH acoustic
signals
from
an electrochemical
cell.
226
ple electrolytic cell. The frequency and time domain behavior of the acoustic emission are studied in detail, as are the effects of applied potential and electrolyte concentration on emission intensity and power spectrum. The ability of acoustic emission to monitor the start-up phase of electrolysis is also demonstrated.
EXPERIMENTAL
Reagents Electrolyte solutions of sodium hydroxide (0.1, 0.2, 0.3, 0.4, 0.5, 0.9, 1.0, 1.2, 1.5, 1.8, 2.0 and 2.1 M) were prepared from sodium hydroxide pellets (AnalaR grade, BDH, Toronto, Ont.). Nickel chloride solution (1.0 M) was prepared from the solid (BDH, Poole, UK). Apparatus Experiments to investigate the electrolysis startup kinetics, measure steady state emission rates and capture individual signals used the apparatus shown in Fig. 1. The anode was a stainless-steel rod of circular cross-section and of 120 mm length and 7 mm diameter. The cathode was a nickel rod of the same dimensions. The two electrodes were immersed in 1.0 M sodium hydroxide solution to a depth of 1.5 cm. Both had a 4 mm diameter hole drilled transversely through their upper end to form a socket suitable for good connection to the electrical power source, a laboratory power supply (Model LXD20, Xantrex, North Vancouver, BC). The top 80 mm of each electrode was planed flat to allow good contact with the acoustic sensor, which was mounted on the working electrode with insulating tape. A layer of stopcock grease (Apiezon Type L, Fisher Scientific, Richmond, BC) was applied between the working electrode and the transducer to enhance acoustic coupling. Unless otherwise stated, a thin layer of PTFE tape was used to prevent any sinking of current from the working electrode through the transducer to ground, and so obviate any electronic interference that this might cause. A saturated calomel reference electrode (Fisher Scientific) completed the apparatus. The applied potential was controlled
T.G. CROWTHER
ET AL.
through a simple power amplification circuit designed in this laboratory. The target voltage was set using one of the two digital-to-analog converters of the Data Acquisition and Control Adapter[(DACA), IBM Instruments, Boca Raton, FL], situated in an Intel 80286-based PC/AT microcomputer (NORA Systems, Vancouver, BC). To ensure accuracy, this 1Zbit resolution card also re-read the supplied voltage via an analog-todigital convertor. Additional current and potential readings were made with a digital multimeter (Model DM 25L, Beckmann, Brea, CA). All studies were done in a three-compartment glass cell constructed in this department (Fig. 1). Each compartment was of 125 mm depth and 70 mm internal diameter. The central chamber was connected to the counter electrode chamber by a glass conduit into which had been set a coarse porous frit of 2.5 cm diameter and 5 mm width. This arrangement was to allow free flow of electrons and ions between the two electrodes, but prevent inter-involvement of the gases formed. The reference electrode cell was partitioned from the working electrode compartment using a l-mm i.d. capillary tube. Initially, the centre chamber contained the working electrode, and the other two chambers contained the counter electrode and reference electrode, respectively. In later experiments, both working and counter electrodes were present in the middle compartment, but were separated by a polyvinylchloride dividing piece of 125 mm length, 70 mm width and 7 mm thickness. This partition had a 35-mm diameter porous glass frit mounted into its center, and was sealed into the center compartment with silicone rubber (RTV, General Electric Company, New York). This modification allowed closer proximity of the working and counter electrodes, and higher currents. Transducer and signal conditioning electronics. The ultrasonic transducer used was a broadband piezoelectric device (Model 8312, Bruel and Kjaer, Naerum, Denmark). This has its own built-in 34 dB preamplifier, and is designed specifically for acoustic emission studies. Its operation has been discussed elsewhere [17]. It derived its power from and delivered its output to a high quality signal conditioning amplifier (Type 2316, Bruel and Kjaer). This provided a switch-selectable gain of 0
CHARACTERIZATION
OF ACOUSTIC
EMISSION
FROM
AN ELECTROLYSIS
to 60 dB in 1-dB steps, and several bandpass filter ranges. A 50 kHz-2 MHz bandpass and an amplifier gain of 40 dB were chosen. The conditioning amplifier provided a d-c. peak level detector output which was connected to a high-impedance input of a dual-channel chart recorder (Model SE 120, BBC Goerz Metrawatt, Vienna, Austria). The a.c. output from the conditioning amplifier presented an amplified, filtered version of the acoustic signal to the input of the digitizer. Data acquisition systems. The digitizer used for monitoring electrolysis start-up was a commercially available unit (Model SDA 2000, Soltec, San Fernando, CA). It was controlled across an IEEE488 interface (Model PC-IIA, National Instruments, Austin, TX) by a PC/AT compatible microcomputer (Nora Systems, Vancouver, BC). Data acquisition software was as provided by the digitizer’s manufacturer. Steady-state emission rate measurements were made using a fast data acquisition board (Model RTI-815F, Analog Devices, Norwood, MA) mounted in a portable PC/XT compatible microcomputer (Compaq Model II, Houston, TX). A digital-to-analog converter on the RTI-815F board was used to relay the computed integration of the acoustic activity to the second channel of the chart recorder. Details of the direct memory access method of programming used have been published elsewhere [28]. The digitizer used to capture individual burst signals was a digital storage oscilloscope (Model 2430A, Tektronix, Beaverton, OR). The digital data were transferred to a PC/AT class personal computer via an IEEE-488 interface and stored on a hard disk for later processing. Data analysis software was written in this laboratory in Microsoft Quick-Basic (ver. 4.OOB, Microsoft Corporation, Richmond, WA). A commercial spreadsheet program was used for some exploratory data analysis. Commercially available graphics programs, Sigmaplot (ver. 3.10, Jandel Scientific, Sausatito, CA) was used to generate graphs, and Surfer (ver. 4.10, Golden Software, Golden, CO) to obtain three-dimensional plots. Method Monitoring the start-up of electrolysis. The soltec
CELL
221
digitizer used provided a single-shot record length of up to 2 MSamples, with 1Zbit resolution. The start of sampling was automatically triggered by the opening of the power supply switch which applied the working potential. Typical conditions, a sampling interval of 0.2 ms and record length of 65535 samples, allowed 12.85 s of obsenration. Studies were done at applied potentials of 0.0, 2.0, 3.0 and 5.0 V. Initial data analysis was done largely using Soltec’s operating software. A data format conversion program allowed further postprocess data analysis using routines written in this laboratory, and the commercial spreadsheet. Measurement of steady-state acoustic emission rates. Studies were carried out at applied potentials of 1.4, 1.6, 1.8, 1.9, 2.0, 2.1, 2.2, 2.4, 2.6, 2.8, 3.0, 3.5, 4.0 and 5.0 V. In each set of experiments the electrodes were placed into the solution at the same depth (1.5 cm) so as to maintain the same electrode surface areas. Before and after each individual experiment the current was measured by setting the digital multimeter in series with the working electrode, and the voltages of both the working electrode and the counter electrode were measured with respect to the reference electrode. First, the level of background noise was established by collecting background signals, in the abscence of an applied potential, for typically 1 min. Then, a voltage was applied, and readings were made over a period of 10 min. Capture of individual emissions during steadystate operation. The digitizer used allowed multiple 1024-point by 8-bit resolution records to be acquired, one per emission signal. The sampling frequency was 2.5 MHz. Each waveform contained 100 points collected before the trigger point. The maximum throughput of this system (capture and transfer) in its NORMAL acquisition mode [29] was ca. 2 signals s-i. Typically 200 signals were collected at each voltage using either continuous triggering or a trigger level set at 250 mV, just above background noise. Data analysis software written in this laboratory facilitated the calculation and viewing of individual and average power spectra, application of pattern recognition and descriptor analysis techniques [8] to the signals captured, and correlation of acoustic parameters with applied potential and
228
T.G. CROWTHER
current drawn. A Kalman filter based approach [30] was used to obtain a response surface equation to represent the effects on acoustic emission intensity caused by the changes in concentration and voltage. Direct volumetric measurements of gas evolution. The apparatus was set up as shown in Fig. 2. Gas volumes were collected over a period of 10 min at potentials of 1.5, 2.0, 2.5, 3.0, 3.5 and 4.0 V.
RESULTS
AND
expense of sensitivity to low-frequency emissions from the cell. It did not prevent the transducer from detecting close proximity high-frequency sounds, nor make it immune to any sudden local low-frequency mechanical vibrations, or electrical interruptions. The upper bandpass limit of the conditioning amplifier was 2 MHz. The maximum emission frequency observable was limited to about 1.2 MHz by the response of the transducer. Initial experiments, conducted in 1 M sodium hydroxide solution in a 250~ml beaker, showed that gas evolution started to occur at applied potentials of between 1.8 and 2.0 V and that this was accompanied by very rich acoustic emission. As might be expected, hydrogen and oxygen bub-
DISCUSSION
The bandpass filter selected prevented audio interference from the rest of the laboratory, at the PC-AT
DATA BUS COUNTER ELECTRODE lRBnREhCEELE”RBOE
WORKING ELECTRODE
IBM DACA -BOARD GLASS APPARATUS CONTAINING NaOH VOLTAGE CONTROL Fig. 2. Experimental
apparatus
used for volumetric
gas studies.
ET AL.
CONTAINING POROUS FRIT
CHARAC’I-ERlZATION
OF ACOUSTIC
EMISSION
FROM
AN ELECTROLYSIS
ble evolution were both acoustically active, with more emission coming from the hydrogen electrode due to the larger volume of gas being produced. The acoustic power spectra revealed no obvious differences between the emission spectra of hydrogen and oxygen evolution. Currents of 0 to 300 mA were drawn when potentials of 0.0 to 5.0 V were applied across the working and counter electrodes situated in separate compartments of the three-chamber cell. The individual electrode potential and the total potential across the cell correlated linearly. The relationship between current and voltage at lower voltages followed the normal exponential Tafel relationship. However, the current at higher potentials became reasonably constant. At higher potentials there was a difference between the value specified by the digital-to-analog convertor, and the actual potential measured across the cell. Current requirements were increased by situating both electrodes in the central compartment of the three-chamber cell, thus bringing the two electrodes closer together. Currents of 0 to 500 mA were then drawn when potentials of 0.0 to 5.0 V were applied. Start-up kinetics. Data shown in Fig. 3 indicate that rate of bubble emission increases with applied potential. The traces above the raw time-domain signals in Fig. 3 plot the squared emission intensity, integrated from the start of acquisition, and are as generated by the Soltec data analysis software. The O-V trace is due solely to background noise from transducer (e.g. thermal noise) and amplifier electronics. The increase in intensity seen early in the traces for potentials of 2.0 V and above is due to the onset of gas evolution. This reveals that emission occurs rapidly after raising the potential, and agrees with visual observations of a rise to full steady-state operation within 2 s. Although the increase in signal intensity is not greatly above background, Fig. 4 shows that the emission is readily quantifiable. The acquisition frequency was less than the ca. 2.0 MHz needed for complete spectral estimation; even this digitizer had insufficient memory for more than 1 s of sampling at such a rate. Under the conditions used, a true frequency analysis would not be possible due to aliasing.
229
CELL
POINT
AT WHICH
VOLTAGE
WAS
APPLIED
0.0 VOLTS
./
_.,~._
vwr..-r--
--12.05
0
TIME (S)
Fig. 3. Extended raw emission traces at 0.0, 2.0, 3.0 and 5.0 V. Shown above each signal is the integration of the signal squared.
Quantitation of steady-state emission rates. As shown in Fig. 4, plots of integrated acoustic emission vs. time were highly linear at various voltages. This behaviour is very different from the more pseudo-first-order curves obtained from many other chemical reactions, and the electrolysis system is therefore described as a “linear, driven emitter”. The linearity indicated that the general rate of evolution of bubbles from the electrode was effectively constant during operation. The spikes in the time domain signal shown in Fig. 4 were visually identified as being due to the release of localized bursts of small bubbles or large individual bubbles. The acoustic emission intensity was found to increase with applied potential in a highly characteristic and repeatable manner, and this was accompanied by greater rates of gas evolution as would be expected, since the rate is dependent on the current (Faraday’s law) by the following relation i, = zrF where i, is the current density, z is the number of electrons involved in the reaction, r is the rate of reaction, and F is Faraday’s constant. The acoustic emission measurements provided a more than adequate signal-to-noise ratio for
230
T.G. CUOWTHER
ET AL.
typical values used in industry, which can vary from 1.8 to 2.2 V [3], and are considerably above the theoretical cell voltage of 1.23 V for 0, and H, evolution reactions. No acoustic signals were detected at lower values of applied potential, thus the acoustic approach would appear to be a useful way of automatically monitoring the potential required for beginning and sustaining gas bubble evolution. y , 0
so
Spectral analysis of electrolysis acoustic emission. 100
150
260
2io
TIME(S)
Fig. 4. Chart recorder trace of raw signal, showing the effect of changes rate of emission.
and integrated acoustic in applied potential on
these experiments. At 1 kHz, the data acquisition rate was sufficiently greater than the 200-ms time constant of the d.c. peak level output of the conditioning amplifier for all fluctuations in this output to be noticed and quantified. Least-squares fitting provided values for rates of emission. An applied potential of between 2.0 and 2.2 V was found to produce a maximum rate of emission per unit of applied potential (calculated using the area under the power curve), and as such would indicate conditions of optimum process efficiency and economy. Confirmatory measurements to correlate the volumetric rate of gas production with acoustic activity used the closed cell shown in Fig. 2. These suggested excellent correlation between the gas volume produced and acoustic emission observed over the range of applied potentials. This behaviour held true for electrolyte concentrations of up to 1.5 M. At NaOH concentrations of 2.0 M and above, the acoustic emission intensity dropped. The decrease could be due to a particular physical property of NaOH solution at high potentials due to the increased conductance of the electrolyte, or the inability of our apparatus to supply adequate current under such conditions, or simply the fact that the current departs from a simple exponential relationship at higher potentials. Acoustic monitoring and visual observation revealed that the experimental potential needed for electrolysis to begin was in good agreement with
Acoustic power spectra [7] given in Fig. 5 show that the acoustic emission and background noise are spectrally quite different. Subtraction of the background spectrum was quite straightforward and resulted in a superior spectrum for electrolysis with a flat background and well defined peaks as shown in Fig. 6. The acoustic emission from electrolysis was broadband in nature and occurred largely between 100 and 800 kHz. Emission between 50 and 100 kHz was largely occluded by the noise, and frequencies below 50 kHz (including ambient audible noise) had been filtered out electronically. The higher electrolysis emission frequencies were substantially greater than was expected from other studies [17,20]. In sonochemistry, the Minneart equation can be used to predict the resonant frequencies (f,) of a bubble of a particular radius (R), in a liquid medium of density ( p), with surface tension (a), and specific heat ratio (y), at atmosheric pressure (P), as defined by f,=
I/(~~~{(~Y/P)[(P
+
w/4 Y2
This equation suggests that the frequencies would be a result of bubbles which were in the range of 10e4 to lop6 m in radius. This would not be an unreasonable estimate for what is visually observed. Considerable structure is evident in the power spectrum obtained. Figure 5 shows that variation of the applied potential resulted only in a change in the magnitude of the observed spectrum, and not in shifting of the emission bands. Correlation coefficients of pairs of spectra taken at consecutive applied potentials were always close to 1.00. Distinct, repeating, inter-band spacings are evident in the power spectrum.
CHARACTERIZATION
OF ACOUSTIC
EMISSION
FROM
AN ELECTROLYSIS
231
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0.040 2 or5 gG 22 P
0.020
s
0 000 0 040 2 Ef 3% XE P
0.020
Q
0 000
0 040 g n: Yk a? E a
2.4 ”
E Ir: 9& ?i?P
0.020
0 020
Q
0 000
0 000
300
600
900
1200
FREOUENCY(kHz)
Fig. 5. Comparison acoustic emission evolution).
of raw average power spectra (computed from an electrolysis cell. Applied potential
The effect of applied potential and electrolyte concentration on emission intensity were visualized by plotting a response surface, shown in Fig. 7. This covered cell behaviour for applied potentials of 1.5 to 4.0 V and electrolyte concentrations of 0.1 to 2.0 M. The surface again reveals the steady increase in total acoustic power for electrolyte concentrations of 0.1 to 1.5 M, followed by the noticeable drop in total acoustic power at concentrations of 2.0 M and above. The surface was then modelled using a Kalman filter and a basis set which allowed least squares fitting of the function shown below [30].
0
300
600
900
1200
FREOUENCY(kHz)
from transients captured at an acquisition shown vary from 1.4 V (no gas evolution)
rate of 2.5 MHz) of the to 2.6 V (copious gas
the total acoustic power. Values for the six coefficients were calculated. The goodness of fit was indicated by values for standard error of estimation (SEE) and standard error of prediction (SEP). The SEE is defined as
SEE= [c (yi- ~calcd )‘I( %a,- Npar ,]1’2 where yi is the observed response, y,,, is the calculated response, NC, is the number of points in the calibration set (NC, = 48), and NPar is the number of fitted parameters ( NPar = 6). The SEP is defined by
y = a, + b,x, + b,x, + qx; + c2x; + dx,x,
SEP =[c (Yj - YcaIcd)2/N,es*] 1’2
The two x parameters are applied potential and electrolyte concentration, and the response, y, is
where y, is the measured response of the test point, ycalcdis the calculated response, and N,,,, is
T.G. CROWTHER
232
FREOUENCY(kHz)
Fig. 6. Background-subtracted operation at 2.6 V.
average
power
spectrum
for
the number of points in the test set. The SEE was 0.05 and the SEP 0.103; together, these values indicated that the model equation provided a close fit to the experimental data. The fit coefficients obtained were: aa = 2.90, b, = 0.0935, b, = -0.200, cl = -0.200, c2 = 0.244 and d, = 0.511. Use of squares fitting traditional methods of least squares fitting would have been equally feasible. Variation of signal characteristics with applied potential. Sets of 200 signals were captured at voltages between 2.0 and 4.0 V and were analyzed
Fig. 7. Response surface from the electrolysis combined effects of electrolyte concentration potential.
cell showing the and applied
ET AL.
by means of multiple descriptive statistical factors, as per the method described elsewhere [S]. Figure 8 shows that the mean and median frequency of the acoustic power spectrum are either constant or decrease slightly as potential is increased (the standard deviation for these data points is = 10 kHz). Table 1 indicates the good correlation of the intensity of the mean and median acoustic emission frequencies with individual electrode potential. Useful time-domain descriptors were the root mean square (RMS) voltage, area, crest, and kurtosis [8]. The mean and median values of each of these were found to correlate linearly with applied potential. The descriptors RMS and AREA relate directly to the magnitude of the signal, and increasing the potential leads to an increase in their magnitude. The distribution of their magnitudes is broadened at high applied potentials. Outlier identification methods proposed elsewhere [8] were applied to these data sets, and it was confirmed that as the applied potential increases a greater number of signals could be classified as outliers. Visual observation of cell operation at the higher potentials suggested that sporadic spurts of bubbles from the base and sides of the electrodes, with en route combination of small bubbles into larger ones, were the cause of these emissions. There is intense clouding of the electrode surfaces at higher voltages, and this “avalanche behaviour” is made possible by the large number density of small bubbles in the region of the electrodes. The
SINGLE ELECTRODE POTENTIAL(V)
Fig. 8. Correlation frequency median
of mean values of frequency signal descriptors with applied
mean potential.
and
CHARACTERIZATION
TABLE Correlation
OF ACOUSTIC
EMISSION
FROM
AN ELECTROLYSIS
233
CELL
1 of signal
descriptors
Electrode potential
Freq. mean
(v)
W-W
with
Mean values for descriptor distributions - 0.22 512.9 0.56 505.1 0.72 497.9 1.00 495.9 1.38 489.9 1.82 475.3 Correlation coefficient - 0.991 Median values for descriptor distributions - 0.22 512.2 0.59 504.0 0.72 502.4 1.00 496.4 1.38 488.3 1.82 479.9 Correlation coefficient - 0.992
applied
potential
(using
all signals)
Freq. median W-W
RMS
Area
442.5 432.9 421.7 416.7 412.5 391.3
0.1037 0.1088 0.1134 0.1156 0.1244 0.1343
83.03 87.03 91.43 93.00 100.8 109.1
- 0.986
0.986
439.5 432.1 432.1 419.9 411.4 400.4
0.1038 0.1087 0.1087 0.1127 0.1208 0.1308
- 0.976
0.975
increased turbulence and/or resistance caused by this change to chaotic behaviour is likely one of the causes of decreased efficiency at high potentials. Acoustic detection of this phenomenon may therefore be a valuable tool in design of improved electrolysers. Crest and kurtosis are descriptors related to the decay rate of the acoustic signal. The higher their value the faster the decay of the acoustic signals. Table 1 indicates that higher potentials result in a slower rate of signal decay, which would also be expected from the time required to form the larger bubbles observed. Injection of power ultrasound into various heterogeneous chemical systems is known to have a homogenizing effect. It is therefore likely that one of the primary mechanisms by which power ultrasound assists electrolysis efficiency [25] is in the destruction or minimization of these large bubbles and localized bubble bursts. Sources of acoustic emission. The above studies indicated unequivocally that the ultrasonic emissions detected were due to gas bubble evolution. Other types of chemical reaction are also acoustically active [6,7,9,18]. The apparatus design prevented the possibility of an acoustically active
0.985
83.23 86.15 87.56 90.56 97.76 106.2 0.971
Crest
4.572 4.406 4.294 4.158 3.867 3.644 - 0.983
4.553 4.373 4.369 4.216 3.942 3.653 - 0.980
Kurtosis
1.007 0.8527 0.8294 0.6747 0.4167 0.2874 - 0.973
0.9392 0.8341 0.8459 0.6521 0.4019 0.2874 - 0.953
recombination of the oxygen and hydrogen gases. Confirmatory experiments were run to prove if the formation of a nickel oxyhydroxy layer on the surface of the nickel electrode was itself acoustically active. The layer can be seen as a brown/ black discoloration, and can be removed by reversing the applied potential. It is conductive and may be responsible for electrode-kinetic changes during the reaction. No acoustic signals were detected from deposition or removal of the layer. Further studies carried out in a 50-ml beaker sought to determine if the reaction of nickel (as chloride) with sodium hydroxide to form nickel hydroxide (initially a gel) had any observable acoustic emissions. Again no acoustic signal was detected. An oxide layer (probably chromium oxide) is formed on the surface of the stainless-steel electrode, but again no signals were detectable. From this we conclude that the chemical acoustic emissions being monitored were exclusively from the formation of hydrogen and oxygen bubbles. Later studies using the same apparatus and well-cleaned electrode surfaces resulted in collection of a very small number of signals which were significantly different from those from gas evolu-
234
T.G. CROWTHER
tion [31]. These occured only early on during initial coating of well-cleaned electrodes. This observation suggests that formation of certain species on the surfaces may be acoustically emissive to a small extent; this is worthy of further attention.
Conclusions
Acoustic emission monitoring has been shown to be a useful tool for optimization and characterization of the operation of gas producing electrolysers. Electrolysis is perhaps particularly suited to being monitored by acoustic emission (electrosonimetry, c.f. [21]) because close contact between the electrode and transducer is possible. Both timeand frequency-domain representations of the acoustic emission from electrolysis reliably show the onset of gas evolution, and the effect that electrolyte concentration and applied potential has on emission intensity and other signal characteristics. The authors thank Oliver Lee, Kevin Soulsbury, and Stephen VanSlyke for their assistance with programming and hardware. This work was supported by Grant 5-50886 from the Institute for Chemical Science and Technology, and matching Grant 5-80389 from the Cooperative Research and Development Program of the Natural Sciences and Engineering Research Council of Canada.
ET AL.
5 M. Vukovic, Electrochimica Acta, 34 (1989) 287. D. Betteridge, M.T. Joshn and T. Lilley, Anal. Chem., 53 (1981) 1064. 7 0. Lee, Y. Koga and A.P. Wade, Talanta, 37 (1990) 861. 8 A.P. Wade, K.A. Soulsbury, P.Y.T. Chow and I.H. Brock, Anal. Chim Acta, 246 (1991) 23. 9 P.D. Wentzell, K.J. Bateman and S.J. Vanslyke, Anal. Chim. Acta, 246 (1991) 43. 10 A.G. Beattie, J. Acoust. Emission, 2 (1983) 95. 11 D. Betteridge, P.A. Connors, T. Lilley, N.R. Shoko, M.E.A. Cudby and D.G.M. Wood, Polymer, 24 (1983) 1207. 12 R.M. Belchamber, D. Betteridge, Y.T. Chow, T.J. Sly and A.P. Wade, Anal. Chim. Acta, 150 (1983) 115. 13 K.T. Ritman and J.A. Milbum, J. Exp. Bot., 39 (1988) 1237. 14 G.A. Dumont, A.P. Wade and 0. Lee, Acoustic Emission Monitoring of Wood Chip Refiners, Patent application No. PCT/CA/90/00015,1990. 15 R.M. Belchamber and M.P. Collins, Patent application No. WPI#89-152794/21, 1989. 16 M. Noguchi, S. Kitayama, K. Satoyoshi and J. Umetsu, Forest Prod. J., 37 (1987) 28. 17 P.D. Wentzell and A.P. Wade, Anal. Chem., 61 (1989) 6
2638.
J.A.C. van Ooijen, E. van Tooren and J. Reedijk, J. Am. Chem. Sot., 100 (1978) 5569. 19 T. Sawada, Y. Gohshi, C. Abe and K. Fumya, Anal Chem.,
18
57 (1985) 20
1743.
R.M. Belchamber, D. Betteridge, M.P. Collins, T. Lilley, C.Z. Marczewski and A.P. Wade, Anal. Chem., 58 (1986) 1873.
K. Lonvik, Thermochim. Acta, 110 (1987) 253. F. Mansfeld and P.J. Stocker, Sol. Energy Res. Inst., 2 (1978) 652. 23 S.D. Lube&in, J. Appl. Electrochem., 19 (1989) 668. 24 D. Bremner, Chem. Br., 22 (1986) 633. 25 H.D. Dewald and B.A. Peterson, Anal. Chem., 62 (1990) 21 22
779. 26
REFERENCES 1 C.A. Hampel, The Encyclopedia of Electrochemistry, Reinhold New York, 1964. 2 C.L. Mantell, Sparks from the Electrode, Williams & Wilkins, Baltimore, MD, 1933. 3 H. Wendt and G. Imarisio, J. Appl. Electrochem., 18 (1988) 1. 4 E. Heitz and G. Kreysa, Principles of Electrochemical Engineering, Weinheim, New York, 1986.
E. Yeager, J. Bugosh and F.J. Hovorka, J. Chem. Phys., 17 (1949)
27
28 29 30 31
411.
B. Gautheron, G. Tainturier and C. Degrand, J. Am. Chem. Sot., 107 (1985) 5579. 0. Lee, P.D. Wentzell, D.A. Boyd and A.P. Wade, Trends Anal. Chem., 9 (1990) 217. P.D. Wentzell, S.J. Vanslyke and A.P. Wade, Trends Anal. Chem., 9 (1990) 3. P.D. Wentzell, A.P. Wade and S.R. Crouch, Anal. Chem., 60 (1988) 905. T.G. Crowther, M.Sc. Thesis, University of British Columbia. 1991.
Analytica Chimica Acta, 254 (1991) 223-234 Elsevier Science Publishers B.V., Amsterdam
Characterization of acoustic emission from an electrolysis cell T.G. Crowther, A.P. Wade * and P.D. Wentzell ’ Laboratory
for Automated
Chemical
Analysis,
Department of Chemistry, V4T I Y6 (Canada)
University
of British
Columbia,
Vancouver,
BC,
R. Gopal Dow Chemical
Canaak,
P. 0. Box 3030,
Vi&l
Street
South,
Samia,
Ont.,
N7T
7MI
(Canada)
(Received 8th November 1990; revised manuscript received 7th May 1991)
AbStMCt Evolution of hydrogen and oxygen from electrodes in an electrolysis cell may be conveniently monitored in a non-intrusive manner via its ultrasonic acoustic emission. The apparatus used in this work was comprised of a nickel anode, a stainless steel cathode, and a saturated calomel reference electrode, all situated in a three chamber cell containing sodium hydroxide solution. The potential necessary for gas bubble evolution was conclusively detected by onset of bursts of acoustic emission. Individual acoustic emission signals, captured using a broadband transducer mounted on the working electrode, contained frequencies from the audible range to as high as 800 kHz. These were correlated with the release of bursts of bubbles from the electrode’s surface, both visually and via a chart recorder trace of peak acoustic power vs. time. Trends in several time-domain signal descriptors were observed with increase in the applied voltage. Acoustic power spectra were obtained by averaging spectra from many acoustic signals. Semiquantitative estimates of rate of emission were made by integration of the peak acoustic level. The effects of applied potential and electrolyte concentration on the multiple bursts of acoustic emission were characterized and are presented as a system response surface. Increasing the applied potential resulted in greater rates of bubble emission, which increased the intensity of acoustic emission, but produced an identical acoustic power spectrum. The extent of acoustic emission at high concentrations and applied potentials was less than expected, which suggested decreased efficiency under these conditions. Electrolysis start-up kinetics were also studied using a digitizer capable of large record lengths. Keywords: Acoustic emission; Electrolysis; Frequency-domain
Industrial electrolysis of water is achieved by passing an electrical current through an aqueous solution of an inorganic acid such as sulfuric acid or an alkali metal hydroxide such as sodium hydroxide or potassium hydroxide. This process was first studied in 1789 by van Troostwijk and Deimann [l], but it was Davy [2] who considered the stoichiometric aspects. Today there is enormous
’ Present address: Chemistry Department, Dalhousie University, Halifax, NS, B3H 453, Canada. 0003-2670/91/$03.50
representation; Tie-domain
representation; Waters
commercial interest in electrolysis and similar electrode processes such as electroplating, electroforming, and electromachining. Research into advanced water electrolysis systems has gained momentum in technologically advanced countries because of the suitability of hydrogen as a valuable, high-yield, energy source (both for storage and power). Environmental pressures have also played a role; if produced cleanly, hydrogen is essentially a non-polluting fuel, since its combustion results only in water and heat. There has been much discussion of the possibility of a global
0 1991 - Elsevier Science Publishers B.V. All rights reserved
T.G. CROWTHER
224
ET AL.
hydrogen-based economy, once fossil fuels reserves are sufficiently depleted; however, some technological barriers remain. A detailed knowledge of the factors which affect the energetics and kinetics of electrode processes is a prerequisite for optimization and efficient operation [3,4]. The rate of energy supply to the cell is simply the electrical power, P, which is the product of i, the current passed, and Ecel,, the applied potential. The total energy (which is the principal operating cost of the process) is obtained by integrating this over time. The efficiency of electrolyzers is perhaps only 70-80%, as E,,, is of necessity substantially higher than the theoretical value, E&, derived from the free energy of the process. Reasons for this unfavorable situation include the high internal electrical resistance of the electrolyzer and high overvoltages experienced at the anode and cathode. These contributions may be summarized as
might be achieved. The most obvious place to seek such a gain is in lowering the overpotentials, and much effort has gone into seeking out electrode materials and coatings which improve efficiency in this way (e.g., RuO,-coated titanium subtrates [5]). Typically, iron is used as a cathode because of its low hydrogen overvoltage, and nickel is used as an anode due to its low oxygen overvoltage. In the last 10 years some electrolyzers have been based on “acidic” solid polymer electrolytes [3]. Alkaline water electrolysis remains the most common method, in part because some electrode materials cannot withstand the aggressiveness of an acidic medium [l]. Commercial electrolyzers use non-reactive electrodes to transmit the current. Development of non-invasive, real-time sensor techniques which are capable of monitoring the extent and nature of processes occurring on the surfaces of electrodes is of significant industrial interest.
J%, = EcOattmc~e - EaOnode
as many chemical processes release heat and light, some also emit sound [6]; this phenomenon can usefully monitor their progress [7-91. Analytical use of acoustic emission is already well established in fields such as materials science [lo-121 and experimental botany [13]. Industrial process control applications [14-161 have taken advantage of the passive, non-invasive nature of the technique, not to mention the relatively low cost of the instrumentation. Acoustic emission is presently less well developed in analytical chemistry [17], although much of the technology is similar and a multitude of potential applications exists [6-9,18211. Acoustic emissions from chemical reactions typically take the form of multiple bursts of duration 0.1 to 1.0 ms, and contain frequency components from the audible to at least 1 MHz. As one might expect from the “fizz” that accompanies effervescence, gas evolution processes emit acoustic energy. Wentzell and Wade [17] investigated the high-frequency characteristics of several
Ultrasonic emission from chemical reactions. Just
Ece~l
=
J%,,
+
iR
+
qmode
+
%athode
and r&,&e are the overpotentials, where %node and R is the specific resistance per unit surface area. The resistance term includes the “ohmic drop” due to the electrolyte and diaphragm, and its initial increase due to bubble curtain formation [3]. The half-cell reactions for the electrolysis of water are given in Scheme 1. A major objective of industrial electrochemistry is to achieve the maximum yield of product (here, primarily HZ) per unit time, per unit cell volume, with the minimum energy consumption. Current efficiency, fractional conversion and rate of reaction must all be considered. It has been estimated that a 100-mV decrease in the potential which must be applied could decrease the energy requirements of one major industrial company such that a direct saving of perhaps $lM per annum per site Cathodic half-reaction 4H,O + 4e- + 4H + 40H4H -+ 2H, gthode = - 0.83 v Scheme
1.
Anodic half-reaction 40H-+40H+4e40H -+ 2H,O + 0, E:riw~e = + 0.40 v
Overall reaction 2H,O --) 2H, + 0, AG” = 474 kJ mol-’ E,P,, = - 1.23V
CHARACTERIZATION
OF ACOUSTIC
EMISSION
FROM
AN ELECTROLYSIS
225
CELL
in chemical reactions; their implosion creates momentarily, local pressures of up to several GPa and local temperatures from lo3 to perhaps lo6 K. The premise of this paper is that an electrochemical reaction can be followed from the release of ultrasonic energy caused by the collapse (or formation) of bubbles. Moriguchi has suggested that the water decomposition voltage of electrochemical processes can be reduced by ultrasound [25], thus the natural ultrasonic emission may assist in the process. Ultrasonic waves have been used to study electrolytic solutions in other ways and ultrasound-induced electrochemical WI, synthesis of various anions [27] has been reported. In this paper we study the utility of ultrasonic emission to help characterize and optimize a sim-
processes which involve gas evolution, and found substantial emission between 50 kHz and 250 kHz. Furthermore, the audible frequency emission was monitored to quantify and characterize the evolution of oxygen bubbles during decomposition of hydrogen peroxide by catalase [9]. Acoustic emission from electrolysis has been reported [22]. Lube&in [23] has used a low-frequency acoustic technique to measure bubble nucleation rates on various surfaces. Stimulation by injection of power ultrasound is becoming increasingly valuable in chemical processes [24]. This is of interest in this paper since reactions such as electrolysis are low power sources of ultrasonic waves of similar and higher frequencies. Microbubbles account for the effects
Individual
Signals
TEKTRONIX 2430A DIGITAL STORAGE
REFERENCE ELECTRODE
COUNTER ELECTRODE IN GLASS APPARATUS CONTAINING
Fig. 1. Experimental
apparatus
for monitoring
WORKING ELECTRODE
CH
NaOH acoustic
signals
from
an electrochemical
cell.
226
ple electrolytic cell. The frequency and time domain behavior of the acoustic emission are studied in detail, as are the effects of applied potential and electrolyte concentration on emission intensity and power spectrum. The ability of acoustic emission to monitor the start-up phase of electrolysis is also demonstrated.
EXPERIMENTAL
Reagents Electrolyte solutions of sodium hydroxide (0.1, 0.2, 0.3, 0.4, 0.5, 0.9, 1.0, 1.2, 1.5, 1.8, 2.0 and 2.1 M) were prepared from sodium hydroxide pellets (AnalaR grade, BDH, Toronto, Ont.). Nickel chloride solution (1.0 M) was prepared from the solid (BDH, Poole, UK). Apparatus Experiments to investigate the electrolysis startup kinetics, measure steady state emission rates and capture individual signals used the apparatus shown in Fig. 1. The anode was a stainless-steel rod of circular cross-section and of 120 mm length and 7 mm diameter. The cathode was a nickel rod of the same dimensions. The two electrodes were immersed in 1.0 M sodium hydroxide solution to a depth of 1.5 cm. Both had a 4 mm diameter hole drilled transversely through their upper end to form a socket suitable for good connection to the electrical power source, a laboratory power supply (Model LXD20, Xantrex, North Vancouver, BC). The top 80 mm of each electrode was planed flat to allow good contact with the acoustic sensor, which was mounted on the working electrode with insulating tape. A layer of stopcock grease (Apiezon Type L, Fisher Scientific, Richmond, BC) was applied between the working electrode and the transducer to enhance acoustic coupling. Unless otherwise stated, a thin layer of PTFE tape was used to prevent any sinking of current from the working electrode through the transducer to ground, and so obviate any electronic interference that this might cause. A saturated calomel reference electrode (Fisher Scientific) completed the apparatus. The applied potential was controlled
T.G. CROWTHER
ET AL.
through a simple power amplification circuit designed in this laboratory. The target voltage was set using one of the two digital-to-analog converters of the Data Acquisition and Control Adapter[(DACA), IBM Instruments, Boca Raton, FL], situated in an Intel 80286-based PC/AT microcomputer (NORA Systems, Vancouver, BC). To ensure accuracy, this 1Zbit resolution card also re-read the supplied voltage via an analog-todigital convertor. Additional current and potential readings were made with a digital multimeter (Model DM 25L, Beckmann, Brea, CA). All studies were done in a three-compartment glass cell constructed in this department (Fig. 1). Each compartment was of 125 mm depth and 70 mm internal diameter. The central chamber was connected to the counter electrode chamber by a glass conduit into which had been set a coarse porous frit of 2.5 cm diameter and 5 mm width. This arrangement was to allow free flow of electrons and ions between the two electrodes, but prevent inter-involvement of the gases formed. The reference electrode cell was partitioned from the working electrode compartment using a l-mm i.d. capillary tube. Initially, the centre chamber contained the working electrode, and the other two chambers contained the counter electrode and reference electrode, respectively. In later experiments, both working and counter electrodes were present in the middle compartment, but were separated by a polyvinylchloride dividing piece of 125 mm length, 70 mm width and 7 mm thickness. This partition had a 35-mm diameter porous glass frit mounted into its center, and was sealed into the center compartment with silicone rubber (RTV, General Electric Company, New York). This modification allowed closer proximity of the working and counter electrodes, and higher currents. Transducer and signal conditioning electronics. The ultrasonic transducer used was a broadband piezoelectric device (Model 8312, Bruel and Kjaer, Naerum, Denmark). This has its own built-in 34 dB preamplifier, and is designed specifically for acoustic emission studies. Its operation has been discussed elsewhere [17]. It derived its power from and delivered its output to a high quality signal conditioning amplifier (Type 2316, Bruel and Kjaer). This provided a switch-selectable gain of 0
CHARACTERIZATION
OF ACOUSTIC
EMISSION
FROM
AN ELECTROLYSIS
to 60 dB in 1-dB steps, and several bandpass filter ranges. A 50 kHz-2 MHz bandpass and an amplifier gain of 40 dB were chosen. The conditioning amplifier provided a d-c. peak level detector output which was connected to a high-impedance input of a dual-channel chart recorder (Model SE 120, BBC Goerz Metrawatt, Vienna, Austria). The a.c. output from the conditioning amplifier presented an amplified, filtered version of the acoustic signal to the input of the digitizer. Data acquisition systems. The digitizer used for monitoring electrolysis start-up was a commercially available unit (Model SDA 2000, Soltec, San Fernando, CA). It was controlled across an IEEE488 interface (Model PC-IIA, National Instruments, Austin, TX) by a PC/AT compatible microcomputer (Nora Systems, Vancouver, BC). Data acquisition software was as provided by the digitizer’s manufacturer. Steady-state emission rate measurements were made using a fast data acquisition board (Model RTI-815F, Analog Devices, Norwood, MA) mounted in a portable PC/XT compatible microcomputer (Compaq Model II, Houston, TX). A digital-to-analog converter on the RTI-815F board was used to relay the computed integration of the acoustic activity to the second channel of the chart recorder. Details of the direct memory access method of programming used have been published elsewhere [28]. The digitizer used to capture individual burst signals was a digital storage oscilloscope (Model 2430A, Tektronix, Beaverton, OR). The digital data were transferred to a PC/AT class personal computer via an IEEE-488 interface and stored on a hard disk for later processing. Data analysis software was written in this laboratory in Microsoft Quick-Basic (ver. 4.OOB, Microsoft Corporation, Richmond, WA). A commercial spreadsheet program was used for some exploratory data analysis. Commercially available graphics programs, Sigmaplot (ver. 3.10, Jandel Scientific, Sausatito, CA) was used to generate graphs, and Surfer (ver. 4.10, Golden Software, Golden, CO) to obtain three-dimensional plots. Method Monitoring the start-up of electrolysis. The soltec
CELL
221
digitizer used provided a single-shot record length of up to 2 MSamples, with 1Zbit resolution. The start of sampling was automatically triggered by the opening of the power supply switch which applied the working potential. Typical conditions, a sampling interval of 0.2 ms and record length of 65535 samples, allowed 12.85 s of obsenration. Studies were done at applied potentials of 0.0, 2.0, 3.0 and 5.0 V. Initial data analysis was done largely using Soltec’s operating software. A data format conversion program allowed further postprocess data analysis using routines written in this laboratory, and the commercial spreadsheet. Measurement of steady-state acoustic emission rates. Studies were carried out at applied potentials of 1.4, 1.6, 1.8, 1.9, 2.0, 2.1, 2.2, 2.4, 2.6, 2.8, 3.0, 3.5, 4.0 and 5.0 V. In each set of experiments the electrodes were placed into the solution at the same depth (1.5 cm) so as to maintain the same electrode surface areas. Before and after each individual experiment the current was measured by setting the digital multimeter in series with the working electrode, and the voltages of both the working electrode and the counter electrode were measured with respect to the reference electrode. First, the level of background noise was established by collecting background signals, in the abscence of an applied potential, for typically 1 min. Then, a voltage was applied, and readings were made over a period of 10 min. Capture of individual emissions during steadystate operation. The digitizer used allowed multiple 1024-point by 8-bit resolution records to be acquired, one per emission signal. The sampling frequency was 2.5 MHz. Each waveform contained 100 points collected before the trigger point. The maximum throughput of this system (capture and transfer) in its NORMAL acquisition mode [29] was ca. 2 signals s-i. Typically 200 signals were collected at each voltage using either continuous triggering or a trigger level set at 250 mV, just above background noise. Data analysis software written in this laboratory facilitated the calculation and viewing of individual and average power spectra, application of pattern recognition and descriptor analysis techniques [8] to the signals captured, and correlation of acoustic parameters with applied potential and
228
T.G. CROWTHER
current drawn. A Kalman filter based approach [30] was used to obtain a response surface equation to represent the effects on acoustic emission intensity caused by the changes in concentration and voltage. Direct volumetric measurements of gas evolution. The apparatus was set up as shown in Fig. 2. Gas volumes were collected over a period of 10 min at potentials of 1.5, 2.0, 2.5, 3.0, 3.5 and 4.0 V.
RESULTS
AND
expense of sensitivity to low-frequency emissions from the cell. It did not prevent the transducer from detecting close proximity high-frequency sounds, nor make it immune to any sudden local low-frequency mechanical vibrations, or electrical interruptions. The upper bandpass limit of the conditioning amplifier was 2 MHz. The maximum emission frequency observable was limited to about 1.2 MHz by the response of the transducer. Initial experiments, conducted in 1 M sodium hydroxide solution in a 250~ml beaker, showed that gas evolution started to occur at applied potentials of between 1.8 and 2.0 V and that this was accompanied by very rich acoustic emission. As might be expected, hydrogen and oxygen bub-
DISCUSSION
The bandpass filter selected prevented audio interference from the rest of the laboratory, at the PC-AT
DATA BUS COUNTER ELECTRODE lRBnREhCEELE”RBOE
WORKING ELECTRODE
IBM DACA -BOARD GLASS APPARATUS CONTAINING NaOH VOLTAGE CONTROL Fig. 2. Experimental
apparatus
used for volumetric
gas studies.
ET AL.
CONTAINING POROUS FRIT
CHARAC’I-ERlZATION
OF ACOUSTIC
EMISSION
FROM
AN ELECTROLYSIS
ble evolution were both acoustically active, with more emission coming from the hydrogen electrode due to the larger volume of gas being produced. The acoustic power spectra revealed no obvious differences between the emission spectra of hydrogen and oxygen evolution. Currents of 0 to 300 mA were drawn when potentials of 0.0 to 5.0 V were applied across the working and counter electrodes situated in separate compartments of the three-chamber cell. The individual electrode potential and the total potential across the cell correlated linearly. The relationship between current and voltage at lower voltages followed the normal exponential Tafel relationship. However, the current at higher potentials became reasonably constant. At higher potentials there was a difference between the value specified by the digital-to-analog convertor, and the actual potential measured across the cell. Current requirements were increased by situating both electrodes in the central compartment of the three-chamber cell, thus bringing the two electrodes closer together. Currents of 0 to 500 mA were then drawn when potentials of 0.0 to 5.0 V were applied. Start-up kinetics. Data shown in Fig. 3 indicate that rate of bubble emission increases with applied potential. The traces above the raw time-domain signals in Fig. 3 plot the squared emission intensity, integrated from the start of acquisition, and are as generated by the Soltec data analysis software. The O-V trace is due solely to background noise from transducer (e.g. thermal noise) and amplifier electronics. The increase in intensity seen early in the traces for potentials of 2.0 V and above is due to the onset of gas evolution. This reveals that emission occurs rapidly after raising the potential, and agrees with visual observations of a rise to full steady-state operation within 2 s. Although the increase in signal intensity is not greatly above background, Fig. 4 shows that the emission is readily quantifiable. The acquisition frequency was less than the ca. 2.0 MHz needed for complete spectral estimation; even this digitizer had insufficient memory for more than 1 s of sampling at such a rate. Under the conditions used, a true frequency analysis would not be possible due to aliasing.
229
CELL
POINT
AT WHICH
VOLTAGE
WAS
APPLIED
0.0 VOLTS
./
_.,~._
vwr..-r--
--12.05
0
TIME (S)
Fig. 3. Extended raw emission traces at 0.0, 2.0, 3.0 and 5.0 V. Shown above each signal is the integration of the signal squared.
Quantitation of steady-state emission rates. As shown in Fig. 4, plots of integrated acoustic emission vs. time were highly linear at various voltages. This behaviour is very different from the more pseudo-first-order curves obtained from many other chemical reactions, and the electrolysis system is therefore described as a “linear, driven emitter”. The linearity indicated that the general rate of evolution of bubbles from the electrode was effectively constant during operation. The spikes in the time domain signal shown in Fig. 4 were visually identified as being due to the release of localized bursts of small bubbles or large individual bubbles. The acoustic emission intensity was found to increase with applied potential in a highly characteristic and repeatable manner, and this was accompanied by greater rates of gas evolution as would be expected, since the rate is dependent on the current (Faraday’s law) by the following relation i, = zrF where i, is the current density, z is the number of electrons involved in the reaction, r is the rate of reaction, and F is Faraday’s constant. The acoustic emission measurements provided a more than adequate signal-to-noise ratio for
230
T.G. CUOWTHER
ET AL.
typical values used in industry, which can vary from 1.8 to 2.2 V [3], and are considerably above the theoretical cell voltage of 1.23 V for 0, and H, evolution reactions. No acoustic signals were detected at lower values of applied potential, thus the acoustic approach would appear to be a useful way of automatically monitoring the potential required for beginning and sustaining gas bubble evolution. y , 0
so
Spectral analysis of electrolysis acoustic emission. 100
150
260
2io
TIME(S)
Fig. 4. Chart recorder trace of raw signal, showing the effect of changes rate of emission.
and integrated acoustic in applied potential on
these experiments. At 1 kHz, the data acquisition rate was sufficiently greater than the 200-ms time constant of the d.c. peak level output of the conditioning amplifier for all fluctuations in this output to be noticed and quantified. Least-squares fitting provided values for rates of emission. An applied potential of between 2.0 and 2.2 V was found to produce a maximum rate of emission per unit of applied potential (calculated using the area under the power curve), and as such would indicate conditions of optimum process efficiency and economy. Confirmatory measurements to correlate the volumetric rate of gas production with acoustic activity used the closed cell shown in Fig. 2. These suggested excellent correlation between the gas volume produced and acoustic emission observed over the range of applied potentials. This behaviour held true for electrolyte concentrations of up to 1.5 M. At NaOH concentrations of 2.0 M and above, the acoustic emission intensity dropped. The decrease could be due to a particular physical property of NaOH solution at high potentials due to the increased conductance of the electrolyte, or the inability of our apparatus to supply adequate current under such conditions, or simply the fact that the current departs from a simple exponential relationship at higher potentials. Acoustic monitoring and visual observation revealed that the experimental potential needed for electrolysis to begin was in good agreement with
Acoustic power spectra [7] given in Fig. 5 show that the acoustic emission and background noise are spectrally quite different. Subtraction of the background spectrum was quite straightforward and resulted in a superior spectrum for electrolysis with a flat background and well defined peaks as shown in Fig. 6. The acoustic emission from electrolysis was broadband in nature and occurred largely between 100 and 800 kHz. Emission between 50 and 100 kHz was largely occluded by the noise, and frequencies below 50 kHz (including ambient audible noise) had been filtered out electronically. The higher electrolysis emission frequencies were substantially greater than was expected from other studies [17,20]. In sonochemistry, the Minneart equation can be used to predict the resonant frequencies (f,) of a bubble of a particular radius (R), in a liquid medium of density ( p), with surface tension (a), and specific heat ratio (y), at atmosheric pressure (P), as defined by f,=
I/(~~~{(~Y/P)[(P
+
w/4 Y2
This equation suggests that the frequencies would be a result of bubbles which were in the range of 10e4 to lop6 m in radius. This would not be an unreasonable estimate for what is visually observed. Considerable structure is evident in the power spectrum obtained. Figure 5 shows that variation of the applied potential resulted only in a change in the magnitude of the observed spectrum, and not in shifting of the emission bands. Correlation coefficients of pairs of spectra taken at consecutive applied potentials were always close to 1.00. Distinct, repeating, inter-band spacings are evident in the power spectrum.
CHARACTERIZATION
OF ACOUSTIC
EMISSION
FROM
AN ELECTROLYSIS
231
CELL
0.040 2 or5 gG 22 P
0.020
s
0 000 0 040 2 Ef 3% XE P
0.020
Q
0 000
0 040 g n: Yk a? E a
2.4 ”
E Ir: 9& ?i?P
0.020
0 020
Q
0 000
0 000
300
600
900
1200
FREOUENCY(kHz)
Fig. 5. Comparison acoustic emission evolution).
of raw average power spectra (computed from an electrolysis cell. Applied potential
The effect of applied potential and electrolyte concentration on emission intensity were visualized by plotting a response surface, shown in Fig. 7. This covered cell behaviour for applied potentials of 1.5 to 4.0 V and electrolyte concentrations of 0.1 to 2.0 M. The surface again reveals the steady increase in total acoustic power for electrolyte concentrations of 0.1 to 1.5 M, followed by the noticeable drop in total acoustic power at concentrations of 2.0 M and above. The surface was then modelled using a Kalman filter and a basis set which allowed least squares fitting of the function shown below [30].
0
300
600
900
1200
FREOUENCY(kHz)
from transients captured at an acquisition shown vary from 1.4 V (no gas evolution)
rate of 2.5 MHz) of the to 2.6 V (copious gas
the total acoustic power. Values for the six coefficients were calculated. The goodness of fit was indicated by values for standard error of estimation (SEE) and standard error of prediction (SEP). The SEE is defined as
SEE= [c (yi- ~calcd )‘I( %a,- Npar ,]1’2 where yi is the observed response, y,,, is the calculated response, NC, is the number of points in the calibration set (NC, = 48), and NPar is the number of fitted parameters ( NPar = 6). The SEP is defined by
y = a, + b,x, + b,x, + qx; + c2x; + dx,x,
SEP =[c (Yj - YcaIcd)2/N,es*] 1’2
The two x parameters are applied potential and electrolyte concentration, and the response, y, is
where y, is the measured response of the test point, ycalcdis the calculated response, and N,,,, is
T.G. CROWTHER
232
FREOUENCY(kHz)
Fig. 6. Background-subtracted operation at 2.6 V.
average
power
spectrum
for
the number of points in the test set. The SEE was 0.05 and the SEP 0.103; together, these values indicated that the model equation provided a close fit to the experimental data. The fit coefficients obtained were: aa = 2.90, b, = 0.0935, b, = -0.200, cl = -0.200, c2 = 0.244 and d, = 0.511. Use of squares fitting traditional methods of least squares fitting would have been equally feasible. Variation of signal characteristics with applied potential. Sets of 200 signals were captured at voltages between 2.0 and 4.0 V and were analyzed
Fig. 7. Response surface from the electrolysis combined effects of electrolyte concentration potential.
cell showing the and applied
ET AL.
by means of multiple descriptive statistical factors, as per the method described elsewhere [S]. Figure 8 shows that the mean and median frequency of the acoustic power spectrum are either constant or decrease slightly as potential is increased (the standard deviation for these data points is = 10 kHz). Table 1 indicates the good correlation of the intensity of the mean and median acoustic emission frequencies with individual electrode potential. Useful time-domain descriptors were the root mean square (RMS) voltage, area, crest, and kurtosis [8]. The mean and median values of each of these were found to correlate linearly with applied potential. The descriptors RMS and AREA relate directly to the magnitude of the signal, and increasing the potential leads to an increase in their magnitude. The distribution of their magnitudes is broadened at high applied potentials. Outlier identification methods proposed elsewhere [8] were applied to these data sets, and it was confirmed that as the applied potential increases a greater number of signals could be classified as outliers. Visual observation of cell operation at the higher potentials suggested that sporadic spurts of bubbles from the base and sides of the electrodes, with en route combination of small bubbles into larger ones, were the cause of these emissions. There is intense clouding of the electrode surfaces at higher voltages, and this “avalanche behaviour” is made possible by the large number density of small bubbles in the region of the electrodes. The
SINGLE ELECTRODE POTENTIAL(V)
Fig. 8. Correlation frequency median
of mean values of frequency signal descriptors with applied
mean potential.
and
CHARACTERIZATION
TABLE Correlation
OF ACOUSTIC
EMISSION
FROM
AN ELECTROLYSIS
233
CELL
1 of signal
descriptors
Electrode potential
Freq. mean
(v)
W-W
with
Mean values for descriptor distributions - 0.22 512.9 0.56 505.1 0.72 497.9 1.00 495.9 1.38 489.9 1.82 475.3 Correlation coefficient - 0.991 Median values for descriptor distributions - 0.22 512.2 0.59 504.0 0.72 502.4 1.00 496.4 1.38 488.3 1.82 479.9 Correlation coefficient - 0.992
applied
potential
(using
all signals)
Freq. median W-W
RMS
Area
442.5 432.9 421.7 416.7 412.5 391.3
0.1037 0.1088 0.1134 0.1156 0.1244 0.1343
83.03 87.03 91.43 93.00 100.8 109.1
- 0.986
0.986
439.5 432.1 432.1 419.9 411.4 400.4
0.1038 0.1087 0.1087 0.1127 0.1208 0.1308
- 0.976
0.975
increased turbulence and/or resistance caused by this change to chaotic behaviour is likely one of the causes of decreased efficiency at high potentials. Acoustic detection of this phenomenon may therefore be a valuable tool in design of improved electrolysers. Crest and kurtosis are descriptors related to the decay rate of the acoustic signal. The higher their value the faster the decay of the acoustic signals. Table 1 indicates that higher potentials result in a slower rate of signal decay, which would also be expected from the time required to form the larger bubbles observed. Injection of power ultrasound into various heterogeneous chemical systems is known to have a homogenizing effect. It is therefore likely that one of the primary mechanisms by which power ultrasound assists electrolysis efficiency [25] is in the destruction or minimization of these large bubbles and localized bubble bursts. Sources of acoustic emission. The above studies indicated unequivocally that the ultrasonic emissions detected were due to gas bubble evolution. Other types of chemical reaction are also acoustically active [6,7,9,18]. The apparatus design prevented the possibility of an acoustically active
0.985
83.23 86.15 87.56 90.56 97.76 106.2 0.971
Crest
4.572 4.406 4.294 4.158 3.867 3.644 - 0.983
4.553 4.373 4.369 4.216 3.942 3.653 - 0.980
Kurtosis
1.007 0.8527 0.8294 0.6747 0.4167 0.2874 - 0.973
0.9392 0.8341 0.8459 0.6521 0.4019 0.2874 - 0.953
recombination of the oxygen and hydrogen gases. Confirmatory experiments were run to prove if the formation of a nickel oxyhydroxy layer on the surface of the nickel electrode was itself acoustically active. The layer can be seen as a brown/ black discoloration, and can be removed by reversing the applied potential. It is conductive and may be responsible for electrode-kinetic changes during the reaction. No acoustic signals were detected from deposition or removal of the layer. Further studies carried out in a 50-ml beaker sought to determine if the reaction of nickel (as chloride) with sodium hydroxide to form nickel hydroxide (initially a gel) had any observable acoustic emissions. Again no acoustic signal was detected. An oxide layer (probably chromium oxide) is formed on the surface of the stainless-steel electrode, but again no signals were detectable. From this we conclude that the chemical acoustic emissions being monitored were exclusively from the formation of hydrogen and oxygen bubbles. Later studies using the same apparatus and well-cleaned electrode surfaces resulted in collection of a very small number of signals which were significantly different from those from gas evolu-
234
T.G. CROWTHER
tion [31]. These occured only early on during initial coating of well-cleaned electrodes. This observation suggests that formation of certain species on the surfaces may be acoustically emissive to a small extent; this is worthy of further attention.
Conclusions
Acoustic emission monitoring has been shown to be a useful tool for optimization and characterization of the operation of gas producing electrolysers. Electrolysis is perhaps particularly suited to being monitored by acoustic emission (electrosonimetry, c.f. [21]) because close contact between the electrode and transducer is possible. Both timeand frequency-domain representations of the acoustic emission from electrolysis reliably show the onset of gas evolution, and the effect that electrolyte concentration and applied potential has on emission intensity and other signal characteristics. The authors thank Oliver Lee, Kevin Soulsbury, and Stephen VanSlyke for their assistance with programming and hardware. This work was supported by Grant 5-50886 from the Institute for Chemical Science and Technology, and matching Grant 5-80389 from the Cooperative Research and Development Program of the Natural Sciences and Engineering Research Council of Canada.
ET AL.
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