Technical Notes Anal. Chem. 1994, 66,4560-4563
Channel Flow Cell For UV/Visible Spectroelectrochemistry Zhenghao Wang, Ming Zhao, and Daniel A. Scherson"
Department of Chemistry, Case Western Reserve University, Cleveland, Ohio 44 106
A channel-type spectroelectrochemical cell has been designed and constructed for conducting in situ transmission UV/visible spectroscopic measurements of solution phase electrogenerated species. Excellent agreement between theory and experiment was obtained using the oxidation of ferrocyanide in aqueous electrolytes as a probe system. In particular, plots of the (relative)absorbance of the solutionat 420 nm (the absorption maximum of ferricyanide in the visible region) measured along the axis normal to the fluid flow downstream from the electrode edge, versus V-l13, where V is the flow rate, yielded a straight line with a close to zero intercept. Furthermore, a linear relationship was also found between the diffusion-limitedcurrent and VI3,as expected on the basis of hydrodynamic and electrochemical considerations. The development and implementation of in situ spectroscopic techniques under well-defined laminar flow may be expected to find application in studies of the mechanism of a variety of electrochemical reactions, as well as to provide a means of controlling critical operating parameters in electrosynthetic processes. Earlier efforts in this area focused on the use of both tube and channel configurationsin conjunctionwith electron spin resonance (ESR) for the quantitative analysis of electrogenerated radicals.' More recently, attention has been centered on the coupling of rotating disk electrodes (RDEs) with UV/visible spectroscopy, in which light is either transmitted through a transparent ring electrode2 or reflected from the surface of the RDE under near-normal incidence conditions? In particular, reflection-type experiments involving potential modulation in the case of single wavelength measurements4 or rotation-wavelength synchronizationfor rapid scanning spectrophotometry5have been found to yield sufficient sensitivity to observe, in certain cases, species in amounts equivalent to a single molecular monolayer. The present contribution describes a channel-type spectroelecM.; Compton, R G. In Chemical Kinetics; Compton, R G., Ed.; Elsevier: New York, 1989; Chapter 7, Vol. 29 (Hamnett A, coeditor) and references therein. (a) Debrodt, H.; Heusler, IC E. Bey. Bunsen-&. Phys. Chem. 1977,81, 1172. b) Dorr, R; Grabner, E. W. Bey. Bunsen-Ges. Phys. Chem. 1978, 82, 164. Zhao, M.; Scherson, D. A.Anal. Chem. 1992,64, 3064. Zhao, M.; Scherson, D. A J. Electrochem. SOC.1993,140, 2877. Wang, Z.; Zhao, M.; Scherson, D. A. Anal. Chem. 1994,66, 1993.
(1) Waller, A.
(2) (3) (4)
(5)
4560 Analytical Chemistry, Vol. 66,No. 24, December 15, 1994
trochemical cell which enables the quantitative detection of solution phase absorbing species produced at an electrode surface by transmission W/visible spectroscopy. THEORETICAL SECTION 1. Hydrodynamics. Various aspects of the fluid dynamics of channel electrodes under conditions of laminar flow have been thoroughly reviewed in the literature.'j The mathematical formalism to be followed in this work assumes that (i) the diffusion along the direction of fluid flow 0 is negligible compared to the convective contribution to mass transport; (ii) the width of the channel (a) is much larger than its thickness (2h), hence edge effects can be ignored; and (ii) k is much larger than the diffusion boundary layer so that the fluid velocity along the direction of flow (vJ is equal to (3U/h)y, where U is the mean flow velocity and y is the distance normal to the fluid flow measured from the plane of the electrode. Consider a situation in which a certain solution phase reactant R is consumed electrochemically at a channel-type electrode to yield quantitatively a solution phase product 0. If the concentration of 0 is assumed to be constant over the whole surface of the electrode, its integrated profile along y (0 may be given by6
In this equation, &J(E) = co (E)/mb is the dimensionless concentration of 0, where co (J3)and C R ~are the actual concentrations of 0 (mol/cm3) at a specific distance y and of the reactant R in the bulk of the solution, respectively, and c 0 ' 0 = coS(Q/c#, where cos(@ is the actual concentration of 0 at y = 0. As indicated, lo(@, COO, c o ' Q , and cos(@ are functions of the applied potential (E). The variable 5 = ( 3 U / h D 0 l ) ~ /is~ ya dimensionless distance normal to the fluid flow, where U as before is the mean fluid velocity (in cm/s), h is half the channel thickness (in cm) , DO is the diffusion coefficient of 0 (in cm2/s), and 1is the length of the working electrode along x (in cm). The variables X I = x l / l and xz = x2/l represent dimensionless distances, where X I and xz are the actual distances from the upstream and downstream edges (6) (a) Albery, W. J.; Coles, B. A; Couper, A. M. J. Electroanal. Chem. 1975, 65, 901. (b) Coles, B. A; Compton, R G.J. Electroanal. Chem. 1983,144, 87.
0003-2700/94/0366-4560$04.50/0 Q 1994 American Chemical Society
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B
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.
Figure 1 Comparison of the integrated dimensionlessconcentration profiles along x as a function of 9 obtained from a numerical evaluation of the functions in eq 1 (dashed line) and a straightforward integration of the goveming differential equation (see text for other details). The thick solid line (see right ordinate) represents the ratio of the numerical and (semi)analytical solutions.
---1
light + path=
I'
of the electrode to the point of interest, respectively, Ai is the 2h
Airy function, and
..''.
C In direct analogy with ESR applications to the study of electrogenerated radicals, the technique described in this work is sensitive to the total amount of solution phase absorbing material along y and therefore proportional to the integral in eq 1. Figure 1 compares the integrated dimensionless concentration profiles along 6 at the diffusion-limited current as a function of x 2 obtained from: (i) a numerical evaluation of the functions in eq 1 (Imd, dashed line) and (ii) a straightforward integration of the governing differential equation (subject to the same boundary conditions) using computationalmethods detailed elsewhere (I,,,,,,,, solid line)? Based on the magnitude of the ratio of the values calculated from these two methods (see thick solid curve and right axis in this figure), the (semi)analytical solution appears to provide satisfactory results, especially for distances close to the electrode edge, i.e., small 22 values. From an electrochemical viewpoint, the steady state current flowing through a channel electrode may be shown to be proportional to cos@), i.e.,
lh
expressed in terms of the integrated concentration profiles as follows:
i = 1.165FD~/3(U/h)'/3~P/3[~~ - c,"(E)] A = X,eigci dy = 1.165FD~/3(U/h)1/3wZ2/3c~(E)
where F is Faraday's constant, DRis the diffusion coefficient of the reactant R, c$(Q is the actual concentration of R at y = 0, and w is the width of the electrode (see panel B, Figure 2). At the diffusion-limited current (ib,,J, this equation reduces to
ilim= 1.165FD:/3(U/h)1/3w12/3c R
(5)
If it is assumed, for simplicity, that only one of the redox species, e.g., 0, absorbs light appreciably in the wavelength at which the measurements are performed, eq 5 reduces to A = ~ ~ J dy i c ~
(4)
2. Spectroscopy. The optical absorbance of the fluid in the channel measured along and denoted hereafter as A can be (7) Wang, 2.;Wu,Y.J. Electroanal. Chem. 1993,360,283.
i = 0,R
(3)
where €0 is the molar absorptivity of the product and 2h is the full thickness of the channel (in cm). Therefore, for measure ments conducted at two different potentials, the change in absorbance [A(A@ = A (El) - A (Ed J can be written in terms of the difference of two integrals of the form given in eq 6. In Analytical Chemistry, Vol. 66, No. 24, December 75, 1994
4561
particular, if E2 and E1 are chosen so that 0 is formed only at E1 (and not at Ez), then
35
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-28
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.
.
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0
z
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where the values of the integral can be obtained by either one of the methods described in the previous section. EXPERIMENTAL SECTION A schematic diagram of the channel-typespectroelectrochemical cell used in these experiments is shown in panel A, Figure 2. The working and counter electrodes were made by cutting with a diamond saw a small gold cylinder (5 mm diameter, 6 mm long, 99.99%purity, Alfa Aesar, Ward Hill,MA) along its main axis. Each of these pieces was then cast in Kel-F to expose rectangular (flat) areas of 5 x 6 mm2. The Kel-F pieces were then machined in the form of parallelpipeds (15 x 20 x 10 mm3) and precision polished particularly along the side closer and parallel to the long side of the electrodes (see below) to yield very sharp edges. As indicated in the figure, the working electrode was placed very close to the edge in contact with the quartz segment to gain spectroscopic access to small values of XZ. Two holes were drilled in these Kel-F pieces to fit the electrolyte inlet and outlet Teflon tubes (not shown in this panel). The cell was assembled by laying down a silicon rubber gasket (Buckeye Rubber & Packing Co., Cleveland, OH,silicon duro) of a thickness of about 0.7 mm of the form shown in the figure on top of a large rectangular quartz plate (100 x 25 x 3 mm, Buck Scientific, Inc., East Norwalk, 0. The channel was then formed by placing side by side the Kel-F cast working electrode, a precision polished quartz plate (25 x 25 x 5 mm, Buck Scient&, Inc.), and the Kel-F cast counter electrode directly onto the gasket. The whole assembly was compressed along y with a set of Plexiglas brackets and along x (the direction of fluid flow) by a custom-made aluminum bracket (see panel B, Figure 2). The thickness of the channel (2h) was determined by filling the cell with a solution of known concentration of potassium ferricyanide and measuring the absorbance at 420 nm. Based on the molar absorptivity of the ferricyanide in this medium determined in this laboratory (€(a = 420 nm) = 1.14 x lo6 mol-' cmz),2h was found to be 0.535 0.005 mm. A variable speed pump (Fluid Metering Inc., Oyster Bay, NY, Model RHV) interposed between the channel cell and a 500 mL glass reservoir was used to flow about 0.5 L of solution through the cell at rates in the range 1.3-15 mWmin (see panel C, Figure 2). The pump was calibrated by allowing the liquid to flow out into a graduated cylinder, enabling volumes as a function of time to be measured in a straightforward fashion. The optical system employed in these experiments was the same as that described in an earlier paper.3 The focused light (a rectangle of 1.5 x 1mm2,with the longer side normal to the fluid flow) illuminated to the channel about an average x value of 1.1 cm from the downstream edge of the electrode, Le., to xz = 2.2. The size of the beam along x is sufficiently small for the integral of the dimensionless concentration profle to be regarded as a linear function of xz; hence, the integral evaluated at xz = 2.2 (0.547 numerical; 0.552 (semi)analytical) is accurate enough for the quantitative analysis of the experimental data.
*
4562 Analytical Chemistry, Vol. 66, No. 24, December 15, 1994
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POTENTIAL / V vs. SCE Figure 3. Combined plots of i (left ordinate, solid circles) and A (right ordinate, open circles) vs €for a solution of 0.01 M &Fe(CN)d 0.25 M K2S04. These data were collected simultaneously as € was scanned at 1 mV/s at a flow rate of 1.74 mumin. The ordinates of the two curves have been scaled using an appropriate conversion factor (see text for details).
The electrochemical measurements were performed using a PAR 173potentiostat,a PAR 175 signal generator, and aYokowaga XY-recorder (Model 3036). A saturated calomel electrode (SCE) placed upstream in the reservoir (see panel C, Figure 2) was used as a reference electrode. Unless otherwise specified, the spectroelectrochemical data, i.e., preamplified photomultiplier response, and the current were digitized using the MD channel of a lock in amplifier (Stanford Research Systems, Sunnyvale, CA, SR 510) and subsequently transferred to a PC for further analysis. All measurements presented in this work were carried out at a wavelength of 420 nm using 0.01 M &Fe(cN)6 (Fisher, reagent grade)/0.25 M & SO4 (Baker analyzed reagent) aqueous solutions at room temperature made with ultrapure water obtained from a modified Gilmont distillation system. RESULTS AND DISCUSSION According to eqs 3 and 7, both the current and the absorbance are proportional to cos, the concentration of 0 at the electrode surface, regardless of the value of the applied potential E. This behavior is clearly illustrated in Figure 3, which displays a combined plot of i (left ordinate, solid circles) and A (right ordinate, open circles) data for a solution 0.01M&Fe(CN)6/0.25 M K&04flowing at a rate of 1.74 mL/min collected simultaneously as E was scanned at 1 mV/s. As independent measurements indicate, this scan rate is sufficiently slow for the system to achieve effectively steady state conditions at each potential. In this plot, the ordinates of the two curves have been scaled to enable a direct comparison between the electrochemical and optical responses. The conversion factor was calculated on the basis of eqs 3 and 7 using DR= 6.4 x cm2/s; DO = 7.3 x cm2/s; the width of the electrode, w = 0.6 cm; 1 = 0.5 cm; the channel width, a = 1.2 cm; xz = 2.2; and EO = 1.14 x lo6 mol-' cm2,yielding a value of 14.3 mA. A few strictly electrochemical experiments were conducted with the same solution, in which the current was monitored with an XY recorder (as opposed to digital acquisition) while the potential was scanned at 5 mV/s at various flow rates. Panel A in Figure 4 shows the results obtained for V = 1.3 (curve a) and 13 mWmin (curve b). Due to mass transport effects, the observed
positive for the oxidation of ferrocyanide to occur under pure diffusion control, while monitoring both A and i for different values of V. As predicted by the theory, a plot of ili, vs PI3 involving four independent runs yielded a straight line with essentiallyzero intercept (y = a bx; a = 0.0284; b = 0.295; R = 0.9943, SD = 0.0124). Furthermore, the thickness of the cell as calculated from the average value of the slope using D[Fe(CN),j4-1 = 6.4 x cm2/s and the actual dimensions of the working electrode and width of the channel yielded a value of 0.28 mm, which is essentially identical to that obtained from the spectroscopic measurements described in the Experimental Section above. It may be concluded on the basis of these results that the spectrcelectrochemical cell from the hydrodynamics viewpoint is wellbehaved. Also in agreement with theoretical expectations, a plot of the absorbance recorded during the acquisition of three of the four runs specified above versus W 3 yielded excellent linear behavior (U = -0.OOO 962; b = 0.0318; R = 0.9957; SD = 0.OOO 443).
a
+
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\
POTENTIAL / V vs. SCE
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CONCLUSIONS
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00
02
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06
POTENTIAL / V vs. SCE Figure 4. Dynamic polarizationcurve obtained in the channel-type spectroelectrochemicalcell using the solution described in Figure 3 at a scan rate of 5 mV/s for flow rates of 1.3 (curve a) and 13 mUmin (curve b). Curve c (dashed line) shows the effect of a faster scan rate (40 mV/s) on the observed current, using otherwise the same conditions as those in curve a in this figure. For comparison, the cyclic voltammetry obtained in the same cell under stagnant conditions is given in panel B in this figure.
current at faster scan rates (40 mV/s) can reach transitorilyvalues larger than the d~sion-limitedcurrent as shown in curve c in this panel. Also included in this figure (see panel B) for comparison is the cyclic voltammetry obtained in the cell under stagnant solution conditions. A series of measurements was then conducted in which the electrode was polarized at 0.45 V vs SCE, a potential sufficiently
The results presented in this paper indicate that a spectroelectrochemical channel cell system of the type shown in Figure 2 behaves both electrochemically and spectroscopically according to theoretical predictions, and therefore it may be employed as a quantitative tool for studies of certain types of reactions involving absorbing reactants, medium-to-long-lived intermediates, and products. Although not discussed in this work, the use of a large quartz window facing both electrodes enables measurements to be made in the reflection mode, making it, in principle, possible to detect species adsorbed on the electrode surface. One possible means of enhancing the sensitivity of this technique is to use a doublebeam instrument so that the fluid can be probed simultaneously before and after flowing past the electrode. This would require replacing a section of the Kel-F piece by another quartz window. Such cell modihtions are presently behg implemented, and the results obtained will be reported in due course. ACKNOWLEDGMENT
This work was supported by ARPA Grant N-0001492-J-1848. Received for review April 22, 1994. Accepted September 19, 1994.a @Abstractpublished in Advance ACS Abstructs, October 15, 1994.
Analytical Chemistry, Vol.
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