Thin Layer Electrochemistry Minimization of Uncompensated Resistance G . M. Tom and A. T. Hubbard Department of Chemistry, University of Hawaii, Honolulu, Hawaii 96822
A new thin layer electrode design, in which the reference and auxiliary electrodes are connected to the working electrode through separate, perpendicular current paths, results in substantially smaller ohmic potentials in the measuring circuit and more nearly uniform current densities than conventional designs. This new design should extend the range of applicability of thin layer techniques to include solutions with low conductivity or high depolarizer concentration, precise measurements of current density and potential, and thin layer methods involving ac polarization.
A than at B. The supply of reactant is altered more rapidly at A than at B with the result that the average length of the current path increases with time. Since the current density and the surface concentrations of the reactants are complicated functions of position and time, ohmic potentials at thin layer electrodes generally cannot be compensated by electronic feedback or accurately determined by ac impedance measurements.
THESUITABILITY of thin layer electrodes for the study of electrode kinetics at solid electrodes (1-4), and for general characterization of systems marked by adsorption of reactants at the electrode surface (5-7), complex stoichiometry ( 8 , 9 ) or coupled chemical, and electrochemical reactions (10-14) has been the subject of a number of recent papers. A disadvantage of thin layer electrode designs described in a number of recent communications (10, 15-21), however, is that current flowing between the thin layer and auxiliary electrodes traverses the lengrh of the solution layer (Figure l), introducing large ohmic potential differences into the measuring circuit. Although these ohmic potentials could often be minimized suitably by adjusting the current density and solution resistance to the lowest practical values, they could not be totally eliminated without sacrificing the lesser concentration polarization inherent in thin layer cells having a solution cm) and of comparatively layer which is ultra-thin ( l = large height (h = 1 cm). As can be seen from Figure 1, charge flowing from the capillary to point A , near the capillary, encounters less resistance than charge flowing to some point B, more distant from the capillary, causing the current density to be greater at
Schematic diagrams of thin layer electrodes having the porous-boundary feature appear in Figures 2 and 3. The micrometer cell of Figure 2 is less convenient for routine applications than the immersible cell of Figure 3, but is advantageous whenever its greater flexibility can be exploited. In construction of the micrometer cell, considerable care was taken to minimize the exchange of solution between the thin layer of solution and the auxiliary electrode compartment when the spinnerette porous boundary (Figure 2) was used; the use of a mercury-mercurous sulfate auxiliary electrode avoided bubble-formation in the auxiliary electrode compartment, which was liquid-tight and of sufficiently small total volume (about 0.05 ml) that changes in the solution volume accompanying typical fluctuations in apparatus temperature were negligible. Such precautions proved unnecessary when a small porous glass disk was employed instead of the spinnerette. Starrett Model No. TI263 RL 0-1 inch stainless steel and Model No. T63 RL 0-2 inch micrometer heads (L. S. Starrett Co., Athol, Mass.) are suited to this application. The mechanical accuracy and precision obtained with this electrode design are about i1 %. Numerous materials are available from which to construct the porous boundary. Although two types have been successfully employed in this work, many attractive alternatives remain untried and no claim is made to having found the best ones. The conventional electrode configuration was achieved by using the platinum-wire auxiliary electrode at the left of the Figure. When in use as a “porous-boundary” electrode, the Hg/Hg2S04 auxiliary was employed. The platinum spinnerette porous boundary frequently employed with this electrode design is shown in the inset. Construction of the porous boundary from the electrode metal avoids the necessity of putting a material other than the electrode metal in contact with the solution (Figure 2, inset). Metal disks suitable for this purpose, perforated by 21 evenly spaced, microscopic holes, are obtainable commercially at nominal cost (Spinnerette Division, Engelhard Industries, 207 Grant Ave., East Newark, N.J. 07029). Diffusion of reactant through the disk is negligible owing to the very small crosssectional area of the holes, the spacing and number of which were selected to adequately minimize the uncompensated resistance. Reaming of the perforation to within 0.002 cm of the working surface simplified construction of the disk, and also greatly reduced the ohmic drop across the disk and the possibility of blockage during use. Insulation of the face opposite the solution layer, required if electronic conduction through the disk is to be prevented, was accomplished by drawing a solution of casting or laminating resin, or asphalt wax (“Apiezon W black wax,” James G . Biddle Co., Plymouth Meeting, Pa. 19462) through the disk after mounting on the
(1) A. T. Hubbard, J . Electroatzal. Chem., 22, 165 (1969). (2) J. R. Cushing and A. T. Hubbard, ibid., 23, 183 (1969). (3) A. L. Y . Lau and A. T. Hubbard, ibid., 24,237 (1970). (4) A. L. Y.Lau and A. T. Hubbard, unpublished data, 1971. (5) A. T. Hubbard and F. C. Anson, J. Electroanal. Chem., 9, 163 (1965). (6) A. T. Hubbard, R. A. Osteryoung, and F. C. Anson, ANAL. CHEM., 38,692 (1966). (7) A . T. Hubbard and F. C. Anson, ibid., p 58. (8) Zbid., p 1601. (9) Zbid., p 1887. (10) C. R. Christensen and F. C. Anson, ibid., 35,205 (1963). (11) Zbid., 36,495 (1964). (12) L. B. Anderson and C. N. Reilley, J. Electroatzal. Chem., 10, 295 (1965). (13) B. McDuffie, L. B. Anderson, and C. N. Reilley, ANAL. CHEM.,38,883 (1966). (14) D. M. Oglesby, J. D. Johnson, and C. N. Reilley, ibid., p 385. (15) E. Schmidt and H. R. Gygax, Clzimia, 16,165 (1962). (16) A. T. Hubbard and F. C. Anson, ANAL. CHEM.,36,723 (1964). (17) D. M. Oglesby, S. H. Omang, and C. N. Reilley, ibid., 37, 1312 (1965). (18) J. C. McClure and D . L. Maricle, ibid., 39,236 (1967). (19) R. W. Murray, W. R . Heineman, and G. W. O’Dom, ibid., p 1666. (20) A. T. Hubbard and F. C. Anson, ibid., 40,615 (1968). (21) A. Yildiz, P. T. Kissinger, and C. N. Reilley, ibid., p 1018.
EXPERIMENTAL
ANALYTICAL CHEMISTRY, VOL. 43, NO. 6, MAY 1971
671
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Figure 1. Schematic diagram of thin-layer electrode Not to scale. Points A and B are described in text
Pt
I
/
SCREEN
J
/
A‘ FRAME
I
POROUS CAPILLARY
\
SOFT G L A S S FASTENER
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Figure 3. Immersible porous-boundary thin layer electrode
--cJ. Figure 2. Micrometer porous-boundary thin layer electrode WORKING
SURFACE
I
I
.OZmm
I
__-A
Figure 2. Inset (see text). Spinnerette porous disk
micrometer, followed by thorough cleaning of the working surface of the disk. Construction of the porous boundary from a porous insulating material is perhaps simpler than from metal and offers the advantage of inherently greater resistance to solution-flow. The extreme solution-flow resistance of one such 672
ANALYTICAL CHEMISTRY, VOL. 43, NO. 6, MAY 1971
material, porous Vycor (“Thirsty Glass,” No. 7930, Corning Glass Company, Corning, N. Y . 14830), is exploited in the construction of the simple immersible ( 2 ) porous-boundary cell of Figure 3. The electrode cavity filled itself because of capillary action. Spent solution was removed by application of pressurized nitrogen. The platinum screen, shown in the enlargement, served as the auxiliary electrode. The solution layer thickness, about 2.5 X 10-3 cm, was established by a series of ridges lathed onto the electrode surface during its construction. The unused surfaces of the platinum working electrode were enclosed in Corning 0120 soft glass. The precision typically obtained with this electrode for the Fe(II1)Fe(I1) system was about f1 %; this uncertainty level is probably established by chemical rather than mechanical considerations. The tendency of porous Vycor to break when submitted to thermal shock was minimized by heating it slowly to about 975 “C; shrinkage occurred and the resulting glass was quite durable (22). Shrinkage also had the highly desirable effect of decreasing the rate of diffusion of electrolyte through the glass. The cell shown in Figure 3 was obtained on special order from the Corning Glass Company. Later units were constructed from lengths of porous Vycor tubing, heattreated as above, polished to the desired dimensions, and cemented or joined to common fused quartz tubing as required. This design appears to be the most universally effective thin layer cell introduced to date. Reversible adsorption of reactant at the surface of the porous Vycor, in competition with the solvent, which might be important in specific instances, has not been investigated in this work, apart from the observation that no such effect was detected for Fe(Hz0)63tin 1FHC104solutions. Electrode potentials were measured and reported relative to a 1N NaC1-calomel electrode (NaCE). Correction for the junction potential (1F NaClIlF HC104),about 27 mV, was not (22) I. Altug and M. L.Hair, J . Phys. Chem., 72,599 (1968).
E .2
VOLT NaCE
1
I
0
20
80
60
40
7
100
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140
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Figure 5. Chronopotentiometric curve for Fe(IIItFe(I1) at conventional and porous-boundarythin layer electrodes A. Conventional micrometer electrode configuration (please refer to Figure 2) B. Porous-boundary micrometer electrode configuration 300
i
Figure 4. Cyclic current potential curve for Fe(II1)-Fe(I1) at conventional and porousboundary thin layer electrodes
Q millicoulomb
201
A
0
Conventional micrometer electrode configuration (please refer to Figure 2) B. Porous-boundary micrometer electrode configuration A.
made because only relative values of the potential were required. RESULTS AND DISCUSSION The departure from ideal behavior observed for reduction of Fe3+ at large current densities is illustrated in Figures 4-6. The effect of ohmic polarization on linear potential sweep current-potential curves (21) is to decrease the currents to smaller values than expected from the experimental equations (1, 21) and shift the curves to less reversible potentials. Figure 4, curve A, shows the current-potential curve obtained for 0.1F Fe3+in lFHCIOl using a micrometer electrode similar to the ones described in References (16) and (17). It is typical of the result obtained with various electrode designs at relatively large current densities (10, 15-21). The peak current was 165 5 microampere in Curve A and 285 =k 5 microampere in Curve B. The anodic and cathodic peak potentials were 0.57 and 0.25 volt NaCE, respectively, in Curve A, and 0.43 and 0.41 in Curve B. Experimental conditions: The reactant solution was 0.1F Fe(C10& in 1 F HCIO,. The cell volume V = 1.628 microliter; sweep rate r = 2 mV sec-I; temperature T = 296 i 2 O K ; cross-sectional electrode area = 0.3175 cm2; solution layer thickness 1 = 4.57 x 10-3 cm. The platinum spinnerette porous boundary described in detail in Figure 2 was employed. In order to illustrate the effect, the reactant concentration (0.1F) was made larger than would usually be necessary; but, in fact, this pronounced distortion is typical of the results obtained for supporting electrolytes of lesser conductance than 1 F HCIOl, such as encountered in the study of organic electrolytes or of aqueous solutions at low salt concentrations. It can be seen from Figure 5, curve A, that ohmic polarization shifts the chronopotentiometric curve to less reversible potentials; in fact, when the constant current is made sufficiently large, the chronopotentiometric reduction of Fe(II1)
*
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0
20
40
60
80
100
120
140
.-_ I60
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Figure 6. Potential-step coulometric curve for Fe(II1)-Fe(I1) at conventional and porous-boundary thin layer electrodes Conventional micrometer electrode configuration (please refer to Figure 2). B. Porous-boundary micrometer electrode configuration A.
is partially masked by solvent reduction, making an accurate determination of the transition time from the experimental curve more difficult (16). The anodic cathodic half-wave potentials used were 0.54 and 0.33 volt NaCE, respectively, in curve A, and 0.425 and 0.445 in curve B. Experimental conditions: The constant current was 200 microampere. Other conditions were as in Figure 4. When a potential step with electronic integration of the current (thin layer coulometry) is employed as the measuring technique, the time required for the current-integral to become constant may be lengthened by ohmic polarization (Figure 6, curve A). The time required for the current in a four-electrode amperometric circuit (23) to become constant is also lengthened. The initial and final potentials were 0.650 and 0.250 volt NaCE, respectively. Other conditions were as in Figure 2. The two-electrode amperometric cell is expected to suffer virtually no ohmic polarization, even during establishment of the steady state (24). Minimization of Ohmic Potentials at Thin Layer Electrodes. It is often possible to keep ohmic polarization within ac(23) L. B. Anderson, B. McDuffie, and C. N. Reilley, J. Electround. Chem., 12,477 (1966). (24) A. T. Hubbard and F. C . Anson, in “ElectroanalyticalChemistry,’’ Vol. 5 , A. J. Bard, Ed., Marcel Dekker, New York, N. Y . , 1970. ANALYTICAL CHEMISTRY, VOL. 43, NO. 6, MAY 1971
0
673
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Figure 7. Schematic diagram of porous-boundarythin layer cell Not to scale. Details of the electrode design are given in Figures 2 and 3
ceptable limits by careful manipulation of the experimental conditions. Referring to Figure 1, the cavity height should be decreased to the minimum value for which diffusion between the thin layer cavity and the adjoining solution can still be disregarded on the time scale of the measurements (commonly 1 cm and 10-100 seconds, respectively). The reactant concentration, rate of polarization, and solution resistance should be kept to the practical minimum. However, a more general approach which places less stringent limits on the experimental conditions is described in the following. Relatively complete elimination of ohmic polarization is possible by means of a radical modification in electrode design. One of the parallel faces of the thin layer electrode is constructed from a porous material and the auxiliary electrode is positioned so that the current path passes perpendicularly through the porous boundary of the solution layer (Figure 7). The reference electrode is connected to the solution layer by means of a capillary, as usual. Since the current path is perpendicular to the plane of the solution layer and traverses only the thinnest dimension of the solution layer, the ohmic potential in the measuring circuit is expected to remain vanishingly small even when electrolytes of rather high specific resistivity, p , are employed. For example, if the solution cm and the current density, I, is layer thickness, 1, is 1 mA/cm* (the largest current density commonly encountered in thin layer experiments), the maximum ohmic potential anticipated for properly constructed porous-boundary cells can be estimated by means of Equation 1, with the results shown below: electrolyte, N 1 0.1
IR = I l p typical p , ohm cm 10
0.01
100 loo0
0.001
1o,oo0
IR, volt
(1)
10-5 10-4
10-3 10-2
The degree of porosity is adjusted so that diffusion of reactant between the thin layer and the auxiliary electrode compartment is not appreciable. When the porous boundary is
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ANALYTICAL CHEMISTRY, VOL. 43, NO. 6, MAY 1971
constructed of the electrode metal, the surface not in contact with the solution layer must be coated with an insulating material, in order to minimize stray currents stemming from electronic conduction through the metal boundary. Typical porous-boundary thin layer electrode designs are shown in Figures 2 and 3. The details of their construction are given in the Experimental Section. Experimental curves, obtained using the porous-boundary thin layer electrode of Figure 2, exhibit virtually no distortion due to ohmic polarization (Curves B of Figures 4-6), in contrast to the result obtained under similar circumstances with the conventional configuration (Curves A ) . The mechanical accuracy and precision achievable, about 1 %, as determined by thin-layer coulometry, are comparable to that of earlier designs, so that the appreciable increase in the uniformity of current density and electrode potential has been achieved without sacrificing mechanical reliability. Porous-boundary cells appear to be particularly advantageous for studies of electrode kinetics ( 2 ) and thermodynamics (25) requiring a precise knowledge of current-density and potential. The equations describing thin layer electrochemical experiments are relatively simple ( I , 24) as are the physical and electronic considerations, even for complicated systems. Studies of the thermodynamic properties of reversible systems, such as the electrodeposition of metallic monolayers (25), should yield readily to porous-boundary cells. Rates of fast chemical and electrochemical reactions could be investigated by applying ac polarization to porous boundary cells. Undoubtedly, much can be learned by applying thin-layer techniques to the study of electrode kinetics in solutions of Iow ionic strength, for which double-layer effects are pronounced (2). The particular suitability of thin-layer electrodes as a diagnostic tool can now be exploited to investigate numerous nonpolar solvent systems : organic electrode reactions; reactions in aprotic media; solutions of variable dielectric constant ; electrochemistry of liquified gases; electrogeneration of unstable species, such as free radicals, at low temperatures; and studies of electrode reactions under conditions of minimal thermal disorder, such as liquid-crystal or cryogenic solutions. Porous boundary cells offer advantages in the upper extremes of concentration as well. Studies of ionic association, solubility and composition in relatively concentrated solutions, such as molten salts, might be expedited by these cells. RECEIVED for review June 19, 1970. Accepted January 25, 1971. This work was supported in part by the Petroleum Research Fund of the American Chemical Society (Grant No. PRF 1265 G-2,3) and the National Science Foundation (Contract No. G P 8880).
( 2 5 ) E. Schmidt and H. R. Gygax, Helc. Chim. Acta, 50, 2058 (1 9 67).
