epoxy

1000

Anal. Chem. 1990, 62, 1000-1003

( 5 ) Westerlund. D. Chromatograph& 1987. 2 4 , 155-164. (6) Shihabi. 2 . K. J . Liq. Chromatogr. 1988, 7 1 , 1579-1593. (7) Yoshida, H.; Morita, I.; Tarnai, G.; Masujima, T.; Tsuru. T.; Takai, N.: Imai, H. Chromatogmphia 1984, 79, 466-472. (8) Hagestam, I. H.; Pinkerton, T. C. Anal. Chem. 1985, 5 7 , 1757-1763. (9) Gish. D. J.; Hunter. B. T.; Feibush, B. J . Chromatogr. 1988, 433, 264-268. (10) Haginaka. J.; Yasuda, N.; Wakai, J.; Matsunaga, H.; Yasuda, H.; Kimura, Y. Anal. Chem. 1989, 6 1 , 2445-2448. (11) Bradford, M. M. Anal. Biochem. 1976, 72, 248-254. (12) Fujimura, K.; Kitagawa, M.: Takayanagi, H.; Ando, T. J . Li9. Chromatogr. 1986, 9 , 607-620. (13) Snyder. L. R.: Kirkland. J. J. An Introduction to Modern Liquid

Chromatography, 2nd ed.; Why-Interscience: New York. 1979; Chapter 5 . (14) Schmidt, D. E.; Giese, R. W.; Conron, D.; Karger, B. L. Anal. Chem. 1980, 52, 177-182. (15) Cook, S. E.; Pinkerton, T. C. J . Chromatogr. 1986, 368, 233-248. (16) Ward, T. J.; Armstrong. D. W. Chromatographic Chiral Separations; Zief. M., Crane. L. J., Eds.: Marcel Dekker, Inc.: New York, 1988: Chapter 5.

RECEIVED for review November 1,1989. Accepted February 22, 1990.

Electrochemical Characterization of a Microcellular Carbon Foam/Epoxy Composite Electrode Brian K. Davis and Stephen G . Weber* Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260

Alan P. Sylwester Sandia National Laboratories, Division 1811, Albuquerque, New Mexico 87185-5800

The construction of a microcellular carbon foamlepoxy resin composite electrode is described. The large perimeter-toarea ratio (P/A) of the resulting microelectrode array is desirable and represents an improvement in PIA over that of many composites presently in use. The array is characterized by using chronoamperometry and cyclic voltammetry. The results are compared to those of both a glassy carbon eiectrode and microelectrode array theory. The responses obtained are representative of expected behavior. The capacity of the electrode was 2.1 X lo3 pF/cm2, 2 orders of magnitude greater than that expected for carbon.

INTRODUCTION Arrays of microelectrodes have been shown to be useful analytical tools due to the combination of the signal to noise ratio (S/N) advantage of a microelectrode and the larger measured current from the array (1-4). In such an array, Cottrellian time behavior is theoretically expected at very short and very long times (,5-7). This is because the diffusion layer is parallel to the array within these time intervals. In the intermediate time range, the edge diffusion to the microelectrodes predominates and deviation from Cottrell is expected. Array geometry plays a key role in the absolute time scales of these phenomena (6-10). The short-time break from Cottrellian behavior depends on the perimeter-to-area ratio, P/A, of the individual array members and thus depends on the radius of the member, r. The larger the P/A, the sooner deviation from Cottrell occurs. The return to Cottrellian response at long times is determined by the interzonal spacing, I , between the members (6-10). A large I leads to a longer time to return to Cottrell. Therefore, to maximize the intermediate time, a minimum r and a maximum 1 are sought. The relationship between r a n d 1 or P I A and 1 depends on the array geometry. For disk electrodes, PIA = 2 / r . For ring electrodes we define the inner radius of the ring as a , the outer

as b, the average of those as R, and their difference as d. Then P I A is

P/A =

2da

+ b)

r ( b 2- a 2 )

= -2R = Rd

2

2

With this in mind, previously studied composites can be examined. In KelGraf composite electrodes the small carbon particles aggregate, leading to a conductive particle size of -50 I.tm ( 1 , 2 ) , or r 25 pm. If the members of the array are considered to be disks, this yields a P I A = 2 / r of -800 cm-'. Epoxy filled reticulated vitreous carbon (RVC) electrodes have also been used successfully (3)and possess a maximum P I A value of -600-700 cm-'. Carbon fibers have been placed in organized arrays and possess a PIA of -4 x 103cm-' ( 4 ) . Here we report on the use of a microcellular foam/epoxy composite in which the P I A is -8 x lo4 cm-', if the electrodes are assumed to be disks, an increase in P/A of an order of magnitude. Electrodes constructed of a single piece of material, such as RVC, have an advantage of good electrical contact to each conducting surface. They possess the attendant disadvantage that the magnitude of 1 is limited by the requirement that the material be dimensionally stable. With this constraint in mind, one can see that 1 increases as the void fraction of the foam increases. The microcellular foams are made of carbonized poly(acrylonitrile) foams ( 1I ) . These foams are prepared via a thermally induced phase separation that allows control of density, pore size, and morphology. The carbonization of these polymer foams forms a conductive carbon matrix of very small pore size (2-10 pm) and large void volume (97%). By incorporation of an insulator into this matrix, an array can be made of average conducting particle size of 1 pm or smaller. Chronoamperometry and cyclic voltammetry of such arrays will be described and the array response characterized to

-

0003-2700/90/0362-1000$02.50/0G 1990 American Chemical Society

ANALYTICAL CHEMISTRY, VOL. 62, NO. 10, MAY 15, 1990

1001

determine if the filled foam obeys theory. EXPERIMENTAL SECTION Materials. The carbonized microcellular poly(acrylonitri1e) (PAN) foams were synthesized according to the procedure described in refs 11-13. The foam used had a density of 0.055 g/cm3 and a pore size of 5 pm. Small quantities of this material are available from one of the authors (A.P.S.). All experiments were carried out with 1 mM ferrocene solution in 0.1 M NaClO, in acetonitrile. Ferrocene (Aldrich) and acetonitrile (Fisher) were used as received. Sodium perchlorate (GFS Chemicals) was further purified by recrystallization from hot methanol. The epoxy used to fill the foam and construct the electrode was 353-ND from Epoxy Technology. This epoxy was chosen because casual experiments have shown that it does not soften in many common solvents, the notorious DMF among them. The reference electrode used in all experiments was Ag/AgC1/3 M NaC1, while the auxiliary electrode was platinum. Electrode Construction. Cylindrical samples of the PAN carbon foam were fashioned by using a simple drill press composed of a hand drill and a lab jack. A cutting edge was made on the end of a steel tube and placed in the drill as a bit. The drill was suspended over the jack, and the sample placed on the jack’s platform. As the jack was raised, the drill was slowly turned, cutting the fragile foam structure. In this way cylinders of foam with similar dimensions were constructed. These cylinders were then cleaned by immersion in liquid NP. This rids the foam of carbon particles created by the machining process. The filling of the foam with epoxy was accomplished in a vacuum oven by simply setting the foam in an excess of degassed epoxy resin under reduced pressure. The capillary action of the pores as well as the air evacuation resulted in the penetration of the epoxy throughout the carbon matrix. Once air bubbles no longer formed from the evacuation, the sample was heated slowly to 90-100 “C and the epoxy allowed to cure following the manufacturer’s instructions. After hardening, excess epoxy was removed by filing. The two faces of the cylindrical epoxy/carbon composite were filed to expose the carbon surface. One face was then polished with a polishing wheel (Leco) through a series of particle sizes: 600 grit sand paper, 1000 grit silicon carbide (Buelher), and 1pm alumina (Fisher). The composite was ultrasonicated in distilled water between polishing steps. The resulting polished carbon/ epoxy composite was housed in a glass tube using 353-ND epoxy as a sealant. Electrical contact was made with a mercury drop and a copper wire. The electrode was sheathed in heat-shrink Teflon tubing that extended several millimeters beyond the electrode surface. This was to shield the edges of the electrode and minimize edge diffusion effects at the array. The tubing was selectively shrunk in order to allow its removal for polishing. Cyclic Voltammetry. Ferrocene, 0.987 mM in 0.1 M sodium perchlorate/acetonitrile, was used. A Bioanalytical Systems CV-1B and a Houston Instruments XY recorder were used. The composite electrode was hand polished between voltammograms with 1-pm alumina for 1min, rinsed with distilled water, and dried. The glassy carbon electrode was polished only before the first voltammogram with 0.05-wm alumina because the voltammograms were very reproducible throughout the study. The Ag/AgC1/3 M NaCl reference electrode was used after being equipped with a Luggin probe. Voltammograms at various scan rates within the range of 10-600 mV/s were obtained for both the composite and the glassy carbon electrodes. The order of the sweep rates was randomized to avoid confounding of time-dependent effects and effects due to sweep rate. As a result, the scatter seen in the plots is a good measure of the experimental reproducibility over a several hour period. Chronoamperometry. Both glassy carbon and composite electrodes were studied. An Asyst computer program coupled with an IBM-PC and a Data Translation DT2801 A/D, D/A converter were used for potential control and data acquisition. The potential was stepped from 0.0 to 0.6 or 0.8 V. The data acquisition time increments were 3.5 and 10.0 ms. Two hundred points were taken before each potential step while eight hundred were taken after it. Blanks consisting of supporting electrolyte and solvent were run and the trials at each set of conditions were

1.500 -2.000 -1.500 -1,000

-0.500 0.000

0.500

1.1 10

Log(Ti me)

Figure 1. log current density (pA/cm2) vs log time (seconds) plots for both the solid electrode (straight line) and the composite (curve). Points denote experimental data while lines denote prediction of theory. Data acquisition time is 10 ms/point. Solution is 0.987 mM ferrocene. Particle size radius used for theory is 0.6 Fm. Note: After 1 s only every tenth point is shown for clarity in the convergence region.

-t

t-

Figure 2. Interfacial impedance from diffusion.

averaged. This average background was subtracted from each individual chronoamperometric experiment at like conditions. The averaging and subtraction were done with an Asyst program. The diffusion coefficient for the ferrocene in acetonitrile was found experimentally from the glassy carbon data to be 1.8 X cmz/s. This is close to a value obtained under similar conditions (14). The chronoamperograms obtained were reproducible. RESULTS AND DISCUSSION Chronoamperometry. Typical log plots of chronoamperograms obtained experimentally are shown in Figure 1. The current density for the composite was calculated by using the total area, active plus inactive. At the shortest times recorded, the composite response is curved while that of the glassy carbon remains linear. At times longer than 1 s, the two converge. The slopes of the two data sets after the first 100 points (after 1s) are identical, -0,495 f 0.006 (95% confidence interval). This response represents the conclusion of the intermediate time range where overlapping depletion zones finally coalesce. This can be explained physically by considering the diffusion layers and their mutual interactions within these time intervals. The shortest time increments obtainable using our current system is 3.5 ms. This is not fast enough to view the short time Cottrell behavior but falls within the intermediate range. Here, the diffusion layes have grown far enough away from the electroactive surface of each individual microelectrode to allow edge diffusion to the surface to dominate (3-7). Deviation from Cottrell is the result. At long times, the diffusion layers overlap and cover the entire electrode area (5-7, 10). Diffusion to the surface is predominantly perpendicular. A general model for the impedance a t a n electrochemical interface is illustrated in Figure 1 of ref 8. A simpler model, which does not include double layer capacitance, ohmic resistance, or electrode kinetics, is shown in Figure 2. There is a Warburg impedance, W,, for diffusion in a direction perpendicular to the plane of the electrode to the electroactive part of the array, a steady-state term for diffusion from the insulating region t o the edge of the electroactive region, and a Warburg term for diffusion perpendicular t o the insulating regions. A second steady-state term for diffusion from the

1002

ANALYTICAL CHEMISTRY, VOL. 62, NO. 10, MAY 15, 1990

~

~~

Table I. Mass Transfer Coefficients process

-75

ha,b

+

0

I

0

w,

(1 -

w2

nfl =

B)(D/Tt)1’2

4(1 - B)D/rr, B(D/rt)’i2

SSI

fraction of surface of the array that is insulating (blocked).

* r , = radius of circular microelectrode in the array.

-200

edge of the array as a whole is not shown because the experiments were done with shielded electrodes. Table I shows the mass transport coefficients, h, for the three processes to the array as a whole. Recall that apparent current density i is related to h by

I= nFC”h

(3)

In this equation, 5 is calculated by using the entire array area, including both blocked and electroactive regions. The overall mass transfer coefficient is then, assuming that the electrodes are disks

(4) 406’(1 - 6’)

8xrl

+ (1- 8)( ”)”’ xt + 4(1 - 0 ) ( ~ D t ) ’ / ~

(5)

The physical characteristics of the foam can be used to estimate rl, the radius of an individual microelectrode. Let us assume that the conducting portions of the array can be treated as circles. If there are, in the area A, n microelectrodes of area xr12,one can write

n w I 2= (1 - 8 ) A

(6)

The “pore size” of the material is known and can be used as an approximate measure of the mean intermicroelectrode distance, 1

n / A = microelectrode concentration

(7)

1 E (A/n)’/2 Combining eqs 8 and 6, one arrives a t

r =

(1 - 8)’/‘

1

For the material used in this study 1 is about 5 pm (estimated from SEM of the native carbon foam) and 0 is 0.97, so r is about 0.5 pm. In,Figure 1, the theoretical curve drawn through the array data has been calculated by using the model shown in Figure 2. The input parameters used were 6 = 0.97 and rl = 0.6 pm; the concentration and diffusion coefficient of ferrocene stated earlier were also used. The value for r , which agrees reasonably well with the approximate calculation, indicates that the electroactive fraction consists of domains with a characteristic size of about 1 pm. This is in substantial agreement with what one can infer from inspection of a scanning electron micrograph (15, 16) of the foam. Repeated attempts to visualize the epoxy-filled foam have failed. It is instructive to consider the conclusion that would be reached from the data had a different model been chosen to represent the array. A foam may be expected to have a geometry that yields ring electrodes; the cutting and polishing would yield the cross sections of foam cells. In this case the relationship between 1 and P I A (or d, recall definitions in the introduction) can be determined if one assumes an ordered array of rings and calculates the fractional area occupied by the ring. For example, in a hexagonal array of rings the unit cell is an equilateral triangle with one-sixth of a ring sub-

+

0

4

-225 -2.50

-2.00

-1.50

-1.00

-0.50

0.00

Lodv)

Figure 3. Plot of A€ (mV) vs log Y (Vls) for both the glassy carbon and carbon foam/epoxy composite electrodes: glassy carbon, 0; composite, 0 .

tending each corner. We define the equilateral triangle side length as 1, then

1 - 8 = 2aRd/12

(9)

To proceed further one needs more information. There are actually two interelectrode radii in this case: one across an individual ring and the other between rings. We will take the reasonable model that 2R = I , that is, the rings are effectively tangent, reminiscent of the symbol of the International Olympics. Then one has

d = (1 - O ) l / x

(10)

This is in agreement with the determination of Amatore et al. for “stripes” as expected (10). In the experiments performed, 1 has been determined and r (or d ) inferred. The assumption of a ring geometry would lead to an estimation of d that is about 1 order of magnitude smaller than r and a concomitantly larger PIA. Inspection of the electron micrographs of the native foam shows a geometry that is well described as random loci connected to each other by struts reminiscent of Tinker Toys. A cross section of this would lead to a more disklike geometry than ringlike geometry. Cyclic Voltammetry. The shape of the voltammogram changes with sweep rate due to the variations in diffusion layer characteristics (7). At low scan rates, long time effects are manifested and the diffusion to the composite is more characteristic of a solid electrode with the same geometric area. On the other hand, a sigmoidal shape is expected a t higher scan rates as the edge diffusion becomes dominant. The shape changes are not apparent within the range of scan rates studied (0.010-0.600 V/s); although differences between the voltammograms of the composite and the glassy carbon can be seen, these are subtle. In their work on the digital simulation of ensembles of microelectrodes (7), Reller et al. examined the relationship between peak separation (LIE,) and scan rate (v) at both an array and a solid electrode. At high scan rates and low scan rates, the values of AE,,for the two electrode types converge for reversible reactions. At intermediate scan rates, the values deviate to some maximum value. Figure 3 presents voltammetric data for both the glassy carbon and the composite electrodes. Note that difference between the data sets decreases as scan rate increases. This behavior mimics that predicted by Reller et al. Theoretically, one should see the convergence at very low scan rates. However, due to convection, this limit is not obtainable with the composite. Note that the noise in the composite data is large a t high scan rate. This is attributed to the inability to obtain a reproducible surface due to the need to polish between voltammograms. While the polishing may clean some elec-

ANALYTICAL CHEMISTRY, VOL. 62, NO. 10, MAY 15, 1990

2.0 -2.50

-2.00

-1.50

-1.00

-0.50

1

0.00

Logb)

Flgure 4. Dependence of peak current density (lA/cm2) on scan rate (VIS) for a glassy carbon electrode, 0, and a carbon foam/epoxy composite electrode, 0 .

troactive areas, it may also expose or remove others. This polishing was necessitated because the voltammogram quality suffered after several trials, possibly due to electrode fouling. The fouling problem may be more severe with arrays in general due to the higher current density of the microelectrode and overall geometry of a macroelectrode. At any rate, caution should be taken when interpreting Figure 3. I t might be argued that the surface of the composite is sufficiently different from glassy carbon to make direct comparison of kinetics meaningless. We were led by such considerations to plot the data according to theory for quasireversible reactions ( 1 7 ) . If the composite data were different from the glassy carbon data by virtue of a lower k , only, it would have been clear from such a plot. Such a plot revealed that the two sets of data were not explicable in terms of a varying 12, only. The dependence of log (peak current) on log (scan rate) was examined. Plots of log (ip/A) vs log ( v ) are shown in Figure 4 ( A is the geometric electrode area). The data obtained by using a glassy carbon macroelectrode are linear with a slope of 0.47, which is close to the expected value of 0.5 (18). The data obtained by using a composite electrode are also linear with a slope of 0.37. At low scan rates the current densities obtained from both electrodes are similar in magnitude while at higher scan rates the values for the composite are less than that for the glassy carbon. This behavior is consistent with the fact that at long times (low scan rates), the diffusion layers of the array members have overlapped and diffusion becomes linear to the total geometric area. At shorter times, higher scan rates, this overlap is diminished and radial diffusion contributes to mass transport. Similar behavior at arrays of microelectrodes has been shown previously (3, 19). While the array shows deviation from purely linear diffusion behavior, it does not show the scan rate independent peak current indicative of purely radial diffusion. In the time domain explored, the array utilizes a combination of the two modes of mass transfer. The data illustrate the transition from linear dominated toward radial dominated diffusion with increasing scan rate. One would expect that a t higher scan rates, behavior approaching that of pure radial diffusion would be seen. As of yet, this study has not been done by using these composites.

1003

The correlation of the hE, and i, data deserves comment. As long as 12, is in the correct range, at low scan rates the current density ratio between the two electrodes is a t its maximum. Thus, while i, values are identical at low scan rates because both electrodes collect material from the entire surface in front of them, the difference in AE, is exacerbated because the current density, calculated as current per electroactiue area, is roughly 30 times as great for the array. At higher scan rates, the i, values diverge. Material from less than the entire surface in frant of the array is collected. Because there is a negative correlation between the fraction of material electrolyzed in a plane parallel to the array and scan rate, the In contrast, dependence of i, on scan rate is lower than the current density ratio between the array and the uniform electrode decreases as the scan rate increases, so the difference in AE, is lower. Both effects, i, and Up, are small, and the AE, data are only suggestive. Together the i, and AE, data make a stronger case for this behavior. Unfortunately, the capacitive current at the array is larger than expected. The capacity for the array was measured by cyclic voltammetry in electrolyte and was found to be 2.1 x lo3 FF per square centimeter of calculated electrode area, or 63 pF/cm2 of the array (v = 100 mV/s). This remarkably large value is probably caused by a poor seal between epoxy and carbon. Frequency-dependent capacitance measurements may shed some light on the extent of this problem. There may also be a factor from surface roughness. Improvements in these mechanical aspects will make the electrode more useful analytically.

LITERATURE CITED (1) Weisshar, D. E.; Tallman, D. E.; Anderson, J. L. Anal. Chem. 1981, 53. 1809. (2) Chesney, D. J.; Anderson, J. L.; Weisshar. D. E.; Tallman, D. E. Anal. Chim. Acta 1981, 124, 321. (3) Sleszynski, N.; Osteryoung, J.; Carter, M. Anal. Chem. 1984, 5 6 , 130. (4) Caudill, W. L.; Howell, J. 0.; Wightman, R. M. Anal. Chem. 1982, 5 4 , 2532. (5) Wightman, R. M.; Wipf, D. 0.I n Nectroanalytical Chemistry; Bard, A. J., Ed.; Dekker: New York, 1986; pp 229. (6) Reller. H.; Kirowa-Eisner, E.; Gileadi, E. J . Electroanal. Chem. Interfacial Nectrochem. 1982, 738,65. (7) Reller, H.; Kirowa-Eisner, E.; Gileadi, E. J . Electroanal. Chem. Inferfacia/ Electrochem. 1984, 161, 247. (8) Weber, S.G. Anal. Cbem. 1989, 61, 295. (9) Amatore, C.; Saveant, J. M.; Tessier. D. J. Electroanal. Chem. Interfacial Electrochem. 1983, 147. 39. (10) Scharifker, B. R. J . Electroanal. Chem. Interfacial Electrochem. 1988, 240, 61. (11) Sylwester, A. P.; Aubert, J. H.; Rand, P. B.; Arnold. C. Jr.; Clough, L. R. Polym. Mater. Sci. Eng. 1987, 5 7 , 113. (12) US. Patent, 4,832,881, 1989. (13) Roncohlor, C. L.; Sylwester, A. P. Novel Forms of Carbon from Poly(acrylonitrile): Films and Foams. I n Amorphous Hydrogenated Carbon; Pouch, J. J., Alterovitz, s. A,, Eds.; Trans Tech, in press. (14) Kuwana. T.; Bublitz, D. E.; Hoh, G. J . Am. Chem. SOC. 1980, 8 2 , 5811. (15) Sylwester. A. P.; Clough, R. L. Synth. Met. 1989, 2 9 , F253. (16) U.S. Patent 4,832,870, 1989. (17) Matsuda, H.; Ayabe, V. 2.Nectrochem. 1955, 5 9 , 494. (18) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications; Wiley: New York, 1980; Chapter 6. (19) Penner, R.; Martin, C. R . Anal. Chem. 1987, 5 9 , 2625.

--.

RECEIVED for review November 17, 1989. Accepted February 6,1990. This work was supported through Sandia National Laboratories under DOE Contract DE-AC0476DP00789.