Anal. Chem. 1986, 58, 1493-1497
1493
Characteristics of a Full Edge Current Flow Thin-Layer Electrochemical Cell That Uses Both Internal (Real) and External (Auxiliary) Reference Points X. Q. Lin and K. M. Kadish*
Department of Chemistry, The University of Houston-University
from outside of the thin-layer chamber to inside the chaniber. After these modifications, symmetrical cyclic voltammograms with negligible peak separations can be obtained a t low scan rates in organic solvents. In addition, minimal peak separations can be obtained at scan rates as high as 10 V/s. This is illustrated for the ferrocene/ferrocenium couple in MeCN and CH2C12. To further demonstrate the characteristics of the designed cell, an equivalent circuit is proposed that is analyzed in terms of the dc potential distribution and the control of ac oscillation. A simplified dummy cell was also constructed and analyzed.
The constructlon and characteristics of a full edge current flow thin-layer electrochemical cell that uses both an Internal dc reference polnt and an external ac reference point are descrlbed. Thls cell Is a modlfled deslgn of a vacuum-tlght thin-layer spectroelectrochemlcal cell wlth a doublet platlnum gauze worklng electrode, whlch has been previously reported. Symmetrlcal thln-layer cycllc voltammograms wlth negligible peak-to-peak separatlons were obtalned with thls cell for the ferrocene/ferrocenlum couple at low scan rates In organic solvents. Small peak-to-peak separatlons were also malntalned at scan rates as high as 10 V/s. A correspondlng equlvalent clrcult for the thin-layer cell Is proposed and analyzed.
Thin-layer electrochemistry has now become a standard method in electroanalytical chemistry ( I ) , and various kinds of thin-layer cells and electrode designs have been reported in the literature (2-21). This technique allows the complete electrolysis of a thin-layer solution to be achieved in a very short time period. In addition, the in situ generated species can be detected and characterized by either electrochemical or spectrophotometric methods (2-5). The theoretical peak potential (E,) of a thin-layer voltammogram is located a t the E l j z of the system for reversible oxidations or reductions ( 4 , 5 ) . However, in practice, this is not observed with existing thin-layer cell designs, and often E, is significantly shifted from the half-wave potential. This shift of Ep is usually attributed to the effect of iR drop across the thin-layer chamber. Many attempts have been made to reduce iR drop in a thin-layer cell. One type of cell utilizes a perpendicular current flow design in which the iR drop across the thin-layer chamber is expected to be zero. However, only small improvements in reducing the peak separation are actually achieved, and potential sweep rates larger than 5 mV/s generally cannot be used with these thin-layer cells (3-5,9). All other published thin-layer cell designs utilize parallel current flow arrangements (2,6-8, 10-21). This is because the cell construction is relatively simple, but more importantly, these cells are easily combined with spectrophotometric measurement techniques. The iR drop in cells with parallel current flow arrangements is usually significant. Recently we reported the construction and characteristics of a full edge current flow (FECF) thin-layer spectroelectrochemical cell design (6). Voltammograms with this cell have peak separations that can be as low as 20 mV in organic media. The effective resistance of this cell is 200 fl in MeCN/0.4 M TBAP, and the 20-mV peak separation at low scan rates comes mainly from an edge diffusional contribution (22). This paper reports a newly modified FECF cell design. This design has all the advantages of the original cell, but edge diffusional contributions and iR effects have both been minimized by using an edge eliminator to minimize the edge area and by moving the reference point of the electrode potential
Park, Houston, Texas 77004
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EXPERIMENTAL SECTION Cell Construction. A schematic illustration of the designed cell is shown in Figure 1. The construction of the cell is similar to that described in a previous paper (6). The cell body is made from Pyrex glass by glassblowing and does not involve the use of adhesive spacers or epoxy. A 0.25-mm-thickthin-layer chamber, a, is open to the bulk solution at all four edges. It is formed by sucking down the walls against a specially prepared iron or graphite mold during red-hot heating. Three platinum posts (for the counter electrode, the auxiliary reference electrode, and the working electrode) are connected to the cell body through a spot of cobalt-glass or uranium-glass to form vacuum-tight joints. A large expanded platinum metal counter electrode, CE, is positioned around the thin-layer chamber in order to equally collect current flow from either side of the chamber. A platinum button auxiliary referenceelectrode, ARE, is placed close to the thin-layer chamber. The tip of the ARE, e, functions as the external reference point, ERP, for ac signal control. k seen in the side view of Figure 1, a reference frit, c, is directly welded onto the wall of the thin-layer chamber. The small hole between the chamber and the frit is sealed by a spot of asbestos, d, which passes through the wall at the tip of the frit. The hole is positioned between the center and the edge of the chamber, b, and serves as the internal reference point, IRP, for dc potential control. A small homemade reference electrode (Ag/AgCl, KC1) is set in the frit so that the bulk solution together with the solution in the frit can be degassed for vacuum operation. The light-transparent thin-layer working electrode, WE, is a 104 mesh doublet platinum gauze with dimensions 2.5 x 7.8 x 0.25 mm3. This electrode is constructed as described in a previous paper (6). An improvement has been made in the earlier design in that a 0.24-mm-thick Tefzel film is welded onto the edges of the electrode for use as the edge eliminator, g. This is shown in Figure 2. In this configuration the edge area of the thin-layer chamber is reduced to about 5% of its original value after the working electrode is inserted. The thin-layer chamber contains about 3 p L of solution. Reagents. Dicyclopentadienyliron (ferrocene) was obtained from Aldrich Chemical Co. and was directly used wi€hout further purification. The supporting electrolyte, tetrabutylammonium perchlorate (TBAP) (Fulka Chemical Co.) was recrystallized from ethanol and dried in vacuo. Analytical grade dichloromethane (CH2Cl2) (J. T. Baker Chemical Co.) was freshly distilled from P20sbefore use. Acetonitrile (MeCN) (UV, Burdick and Jackson Lab, Inc.) was stored over 5-A molecular sieves before use. Instrumentation. Thin-layer cyclic voltammograms were obtained with a PAR Model 174A polarographic analyzer that was coupled with a PAR Model 175 Universal Programmer. For scan rates lower than 500 mV/s, an Omnigraphic 2000 X-Y
0003-2700/86/0358-1493$01.50/00 1986 American Chemlcal Society
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 7, JUNE 1986
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Figure 1. Schematic illustration of the vacuum-tight thin-layer elec-
trochemical cell wlth an Internal reference point. The side view shows how the cell works In acquiring spectral data. The top view shows the connection of the external RC circuit. The relevant parts of the cell are as follows: a, thin-layer chamber wlth sandwiched doublet platinum gauze working electrode and edge eliminator; b, position of the Internal reference point (IRP); c, reference frit; d, asbestos spot; e, position of the external reference point (ERP); CE, expanded platinum metal counter electrode; ARE, platinum button auxlllary reference electrode; RE, reference electrode; C, variable Capacitor; R, variable resistor.
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electrode wlth edge eliminator. Components are as follows: e, doublet platinum gauze; f, stainless-steel rlbbon; Q, Tefzel film edge eliminator; h, Tefzel film cover. The dimensions of the components are as foliows: a = 2.5 mm, b = 7.8 mm, c = 0.25 mm, and d = 0.24 mm.
recorder was used to record the voltammograms. A Tektronix Model 5111 storage oscilloscope and Model C-5A oscilloscope camera were used for high-scan-rate voltammograms. A YSI Model 31 conductivity bridge was used for conductivity measurements. The dummy cell and the external circuit of the thin-layer cell were constructed from commercially available components. All reported potentials are referenced to an Ag/AgCl reference electrode. The potential of this electrode was measured as -38 mV vs. a commercial SCE. Operational Procedure. Nitrogen deaeration was used to remove oxygen from both the bulk solution and the solution in the frit. The asbestos spot at the tip of the frit is diffusionally conductive. Thus, the frit can be considered as partially “open” to the thin-layer chamber, and deoxygenation in the frit solution is necessary. The water content of the solution is also important. The frit solution should be frequently replaced with a fresh solution whenever an aqueous reference electrode is used for long-term experiments. To prevent chloride ions from diffusing into the thin-layer chamber, a double frit can be used. The position of the working electrode related to the IRP (point b) is important, and this should be carefully calibrated in the solution containing a reversible electron transfer system (which in this case is the ferrocene/ferrocenium couple). In a well-adjusted system, minimal peak separation of thin-layer cyclic voltammograms should be obtained over a wide range of scan rates. The ideal position of the IRP is approximately half way between the center and the edge of the working electrode. A damped electronic oscillation from the cell system occurs when the external RC circuit is opened. This oscillation shows that the electrode potential is out of control. This can be corrected
in CH2C12/0.1 M TBAP with potential scan rates of (i)2 mV/s, (il) 5 mV/s, (iii) 10 mV/s, (lv) 20 mV/s, and (v) 30 mV/s; (b) 2.5 mM ferrocene in MeCNl0.4 M TBAP wlth potential scan rates of (i) 1 V/s, (ii) 2 V/s, (iil) 5 V/s, (iv) 10 V/s, (v) 20 V/s, and (vi) 50 V/s.
by having a capacitor, C, and a resistor, R, in the external circuit (see Figure 1,top view). The best values of R and C are dependent on the experimental conditions. In the present electrode configuration, a variable resistance of 0-800 kQ and a variable capacitance of 0-1 pF were used. The oscillation was stopped by increasing the capacitance and/or decreasing the resistance to a point where the damped oscillation went to attenuation.
RESULTS AND DISCUSSION Cyclic Voltammetric Characterization. As shown in Figure 3a (curve i), a 1.9 mM ferrocene solution gives symmetrical oxidation-reduction peaks with a flat base line at a scan rate of 2 mV/s. The peak separation is actually zero. The experimentally obtained Eljzin CH2Cl2/0.1M TBAP was +0.525 V vs. Ag/AgCl. An increase of scan rate leads to an increase of the peak width in both potential scanning directions; however, the peak potentials remain close to the halfwave potential of 0.525 V, and at 30 mV/s the peak separation, AEp, is 18 mV. A reference electrode chamber that opens to the thin-layer compartment has been used in long-path-length thin-layer cell designs (17, 18) and leads to better defined thin-layer voltammograms. However, the free diffusion of the electroactive species in and out of the reference frit to the thin-layer chamber moves the real reference point of the electrode potential away from the thin-layer chamber. Thus, these cells can only be operated at low scan rates. Because the tip of the reference frit is sealed by an asbestos spot (see Figure 1))the internal reference point (IRP) of this cell is positioned in the thin-layer chamber just at the surface of the working electrode. The iR drop between the working electrode and the IRP can be expected to be zero for a small working electrode area. However, iR drop does exist for that part of the working electrode which is far from the IRP, but this is minimized by the small size of the work electrode. A full edge current flow design also minimizes the iR drop, which can only be built up across a short distance, i.e., 1.25 mm of the thin-layer solution. The edge diffusional current contribution is eliminated in the described thin-layer cell. However, the diffusional contribution inside the thin-layer chamber is expected to increase with increasing potential scan rate (3-5). The higher the scan rate the larger the diffusion gradient that will be built up in the thin-layer chamber. This diffusional contribution leads to a larger deviation of the peak current from ideal thin-layer current behavior. This also leads to shifts in the peak potentials that give an increase in the peak-to-peak separation. The experimentally obtained 18-mV peak separation at 30 mV/s can be seen as a combination of the diffusional contribution effect and the unbalanced iR drop. Figure 3b shows thin-layer cyclic voltammograms at high potential scan rates in MeCNl0.4 M TBAP. A peak separation of 15 mV can actually be maintained at a scan rate as high as 10 V/s (curve iv). At this scan rate the peak current
ANALYTICAL CHEMISTRY, VOL. 58, NO. 7, JUNE 1986
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maximum reaches about 2 mA. Only when the scan rate is higher than 10 V/s does the peak separation increase significantly. At 50 V/s, the peak separation reaches about 100 mV, and the peak-to-peak current reaches about 12 mA. However, as seen in the figure, the voltammetric peaks are still well-defined. A low resistance of the thin-layer solution is essential for obtaining well-behaved thin-layer voltammograms. In the specific case a specific resistance of 675 0 cm was measured in CH2C12/0.1M TBAP, while 52 Q cm was measured in MeCN/0.4 M TBAP. Using this specific resistance and the equation R = p ( L / S ) ,where L = (1/2)a and S = 2(a b)c (a, b, and c are as defined in Figure 2) enables one to estimate the resistance of the thin-layer solution from the center to the edges. For the given cell an estimate of 1600 0 was obtained in CH2C12/0.1M TBAP and 130 Q in MeCNf0.4 M TBAP. Thus, only an 8-mV iR drop can be expected to occur in CHzC12for a cell current of 10 bA, and less than a 5-mV iR drop can be expected to occur in MeCN for a cell current of 77 MA. However, the 130-0 resistance in the MeCN solution should lead to an iR drop of 160 mV a t 10 V/s when the current is 2.4 mA, but only a 15-mV peak separation is observed. An iR drop of 390 mV is expected for the same solution at 50 V/s, but in this case only a 100-mV peak separation is experimentally observed. Further analysis shows that this is the effect of the IRP setting, which minimizes the effective iR drop. The peak current for oxidation of ferrocene at different scan rates is shown in Figure 4. As seen in this figure, the peak current is directly proportional to scan rate, u, only at low scan rates. A t scan rates higher than 20 mV/s, the peak current becomes proportional to u1I2. The transition between purely thin-layer behavior and purely seminfinite diffusion control is expected to be a continuous process that will lead to a larger current bias at higher scan rates (3-5). The peak potential and the shape of the voltammogram should also be shifted with increasing scan rate. The broader voltammetric peaks in Figure 3b and the diffusional characteristic of the peak current behavior a t higher scan rates (Figure 4) may lead to a suggestion that conventional but distorted cyclic voltammograms are obtained at high scan rates. This distortion is also related to the setting of the IRP. This is because the IRP changes the nature of the iR drop in the thin-layer chamber. Equivalent Circuit. To further characterize both dc and ac properties of the designed cell and to identify the function
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Flgure 5. Diagram of (a) equivalent circuit of the thin-layer cell and (b) enlarged ith division. Symbols are as follows: R,,,, resistance of the thin-layer solution; IRP, internal reference point; ERP, external reference point; $,, potential of the thin-layer solution at the ith division; c, and r,, capacitive and resistive impedances of the ith divisions, i E [0, n - 11 and n m; total current of the cell; I,, current of the ith division; WE, RE, ARE, and CE, post of the working, reference, auxiliary reference, and the counter electrode, respectively.
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of the IRP, an equivalent circuit analysis is necessary. However, no equivalent circuit of a thin-layer cell has ever been discussed in any detail. This is because complications arise from the significant resistance of thin-layer solutions (20, 21). A simplified representation is possible for a cell that has edge current flow. The thin-layer section can be represented as a set of parallel c,r, circuits ( i E [0, n - 11, where n is the number of equipotential divisions of the thin-layer chamber) that are conneded with the resistance of the thin-layer solution, R&. This is shown in Figure 5a. Assuming the diffusion process is eliminated at low scan rates, ri can be considered only from the charge transfer resistance (R,) and c, only from the double-layer capacity (&)
The internal reference point, IRP, is positioned somewhere in the thin-layer solution. The resistance of the frit, Rfrit, mainly comes from the tip of the frit. Rsolis the resistance of the bulk solution between the counter electrode and the edge of the thin-layer electrode. The external reference point, ERP, is positioned in the bulk solution. R and C are the resistance and capacitance in the external circuit. It is clear that this circuit can be reduced to the equivalent circuit proposed from a conventional electrochemical cell if the resistance of the thin-layer solution, &,in, is zero ( I ) . If one assumes that the thin-layer chamber is rectangular with a small width and a very large length, and that the thin-layer working electrode can be seen as a uniform metal plate, such that the electrolysis current uniformly goes out of the chamber only through two long edges, then the thinlayer chamber can be divided into n ( n m) equipotential divisions that are parallel to the edges. Each of the divisions is physically equivalent and consists of the same electrode surface area and the same volume of thin-layer solution. Thus, in this ideal case, each division consists of the same values of c,, ri, and d(Rt,in). This gives
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 7, JUNE 1986
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