Anal. Chem. 1999, 71, 413-418
Charge-Selective Electrochemistry at High-Surface-Area Carbon Fibers Richard S. Kelly,† David J. Weiss,‡ Sing Hwa Chong,‡ and Theodore Kuwana*,‡
Department of Chemistry, Merrimack College, North Andover, Massachusetts 01845, and Department of Chemistry, University of Kansas, Lawrence, Kansas 66045
Carbon fibers (DuPont E120) subjected to high anodic current or potential undergo fracture whereby the capacitance can increase dramatically by 2 or 3 orders of magnitude. The capacitance, assumed to correspond to an increase in the surface area, increased from 2 to 8 to 1000-5000 µF/cm2, as normalized to the original fiber area. The calculated area-to-volume ratio is greater than 106 cm2/cm3, based on an observed diameter of 20 µm for a 2-cm-length fractured fiber. Interestingly, the shape and size of the cyclic voltammetric (CV) waves also changed dramatically, depending on the solution pH and the nature of the electroactive species. The pH dependence is related to the pKa of the carbon-oxygen groups, such as carboxylic acids, which can be deprotonated when pH > pKa to produce a negatively charged surface. Thus, negatively charged species such as ferro-/ferricyanide show small CV waves whereas, dopamine, which is positively charged, produces large peak-type waves. These observations support a model in which the penetration into the interior and its micropores by electroactive species is controlled by its charge and solution pH, which governs the charge on the carbon surface. The pretreatment of carbon electrode surfaces to achieve predictable and reproducible electrochemical behavior is of extreme importance in a variety of applications, from electrosynthesis to in vivo voltammetry.1,2 This goal is an elusive one, primarily due to the lack of a real understanding of the surface processes affecting electron transfer with carbon and the large number of treatment protocols available. This group, along with many others, has been involved for some time in the development of pretreatment procedures for carbon electrodes and the relationship between pretreatment and electrode performance.3-8 Some of the factors known to affect the * Corresponding author: (phone) 785-864-3015; (fax) 785-864-5396; (e-mail)
[email protected]. † Merrimack College. ‡ University of Kansas. (1) McCreery, R. L. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1991; Vol. 17, pp 221-374. (2) O’Neill, R. D. Analyst 1994, 119, 767-79. (3) Weisshaar, D. E.; Kuwana, T. Anal. Chem. 1985, 57, 378-9. (4) Karweik, D. H.; Hu, I.-F.; Weng, S.; Kuwana, T. In Catalyst Characterization Science; Deviney, M. L., Gland, J. L., Eds.; ACS Symposium Series 288; American Chemical Society: Washington, DC, 1985; pp 582-95. (5) Engstrom, R. C. Anal. Chem. 1982, 54, 2310-4. (6) Engstrom, R. C.; Strasser, V. A. Anal. Chem. 1984, 56, 136-41. 10.1021/ac980760g CCC: $18.00 Published on Web 12/15/1998
© 1999 American Chemical Society
electrochemical behavior of carbon include the cleanliness of the electrode surface, the kind and the extent of oxygen-containing functionalities on carbon, either as a result of pretreatment or subsequent exposure to air, and the actual form of carbon contained in the exposed electrode material.1,9 Carbon fibers offer a special challenge for activation, as mechanical polishing commonly employed for many electrode materials, like planar glassy carbon, is not possible with cylindrical fibers. As a result, a number of chemical and electrochemical pretreatments have appeared for these electrodes, which vary widely from lab to lab, and which are most often specific to a single fiber type.10-13 With this in mind, a systematic comparison of the properties of carbon fibers obtained from various manufacturers was undertaken in an attempt to identify the important activation parameters for each fiber and to determine whether the process was similar to that observed for glassy carbon.14 It was during this study that a particular high modulus fiber (10-µm diameter DuPont E120) exhibited extensive fracturing as a result of either galvanostatic or potentiostatic treatment.15,16 Scanning electron microscopy (SEM) confirmed that these fibers were fractured along their principal axis. The resulting deep fissures and crevices seemingly followed tiny surface striations that were left by the manufacturing process. The fissures in the fractured fibers varied in width from one-tenth to several micrometers, as measured from SEM photomicrographs. Most significantly, this fracturing treatment caused a dramatic change in double-layer capacitance of more than 3 orders of magnitude! Thus, the observed range of capacitances as measured by cyclic voltammetry (CV) changed from 2 to 8 µF/cm2 before fracturing to 2000-8000 µF/cm2, as normalized to the surface area of the original, unfractured fiber.16 Besides dramatic changes in (7) Rice, R. J.; Pontikos, N. M.; McCreery, R. L. J. Am. Chem. Soc. 1990, 112, 4617-22. (8) Chen, P.; Fryling, M. A.; McCreery, R. L. Anal. Chem. 1995, 67, 3115-22. (9) Kovach, P. M.; Deakin, M. R.; Wightman, R. M. J. Phys. Chem. 1986, 90, 4612-7. (10) Feng, J.-X.; Brazell, M.; Renner, K.; Kasser, R.; Adams, R. N. Anal. Chem. 1987, 59, 1863-7. (11) Gonon, F.; Buda, M.; Cespuglio, R.; Jouvet, M.; Pujol, J.-F. Nature 1980, 286, 902-4. (12) Sujaritvanichpong, S.; Aoki, K.; Tokuda, K.; Matsuda, H. J. Electroanal. Chem. 1986, 198, 195-203. (13) Netchiporouk, L. I.; Sul’ga, A. A.; Jaffrezic-Renault, N.; Martelet, C.; Olier, R.; Cespuglio, R. Anal. Chim. Acta 1995, 303, 275-83. (14) Swain, G. M. Activation Studies of Carbon Fiber Electrodes. Ph.D. Dissertation, University of Kansas, 1991. (15) Swain, G. M.; Kuwana, T. Anal. Chem. 1991, 63, 517-9. (16) Swain, G. M.; Kuwana, T. Anal. Chem. 1992, 64, 565-8.
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the capacitance, the calculated value for the area-to-mass ratio for these fractured fibers is on the order of 800 m2/g while the areato-volume ratio is greater than 106 cm2/cm3! This value is based on an observed diameter of 20 µm (2-cm length) after fracture and a capacitance of 10 µF/cm2 for a clean glassy carbon electrode.1 The large area-to-volume ratio can potentially magnify any surface-dependent electrochemistry. Although the structural rationale for such a large value of areato-volume is not yet fully understood, no other electrochemical or mechanical process is known that can produce such a dramatic increase in this ratio. For comparison, the open, spongelike structure of 100 pore/in. reticulated vitreous carbon is reported17 to have an area-to-volume ratio of only 66 cm2/cm3. The manner of fracture is thought to be a consequence of the structure of this particular fiber. The 10-µm-diameter DuPont E120 fiber is known to have a two-phase structure.16 There is a sheath of graphitized material on the periphery. The inner structure is composed of graphitized planes that extend parallel to the fiber axis and, if viewed end-on, appear to grow outward from the center of the fiber, intersecting with the outer sheath. This structure is similar to that described by Bonnamy and co-workers, who made a pitch-based fiber by heat treatment to 2600 °C with a spinneret method.18 For fracturing to cause such large increases in capacitance and surface area, an extensive network of internal channels and pores must be accessible to the solvent/electrolyte. Carbon fibers have been reported to have an average porosity of 16-18% and are known to contain needle-shaped pores 1-2 nm wide and 20-30 nm long.19 Interconnectivity of these channels is certainly necessary to achieve capacitive values of the magnitude observed for the fractured fibers. The voltammetric properties of these ultrahigh-surface-area carbon fibers (UHSACFs) are markedly different from those of unfractured fibers.16 Typically, the unfractured fibers give oxidative CV waves characteristic of diffusion to a microcylindrical electrode. As an example, the oxidations of 1.0 mM dopamine at pH 2.2 in phosphate buffer and of 1.0 mM ferrocyanide in 0.1 M KNO3 at a scan rate of 100 mV/s showed typical limiting currents of ∼1 and ∼2 µA, respectively. The factor of 2 in the limiting currents reflects the difference in the number of electrons in the electrode reactions. In marked contrast, at UHSACF electrodes (i.e., after fracturing), dopamine exhibited sharp voltammetric waves typical of either thin layer or adsorption, with greatly enhanced peak currents (∼50 µA) over what would be expected for diffusion to a microcylinder. This behavior was not found, however, for ferrocyanide, which showed only a current of ∼1.5 µA, barely discernible above the large background capacitive current. Although Wightman and co-workers9 reported enhanced currents for electrochemically treated carbon fibers, the enhancements were not the same order of magnitude as seen above for dopamine at UHSACF. Further, Wightman20 and Engstrom5 observed inhibi(17) Wang, J. Analytical Electrochemistry; VCH Publishers: New York, 1994; p 84. (18) Lafdi, K.; Bonnamy, S.; Oberlin, A. Carbon 1993, 31, 29-34. (19) Donnet, J.-B.; Bansal, R. C. Carbon Fibers, 2nd ed.; Marcel Dekker: New York, 1990; p 107. (20) Deakin, M. R.; Kovach, Paul M.; Stutts, K. J.; Wightman, R. M. Anal. Chem. 1986, 58, 1474-80.
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tion of the electron transfer of ferrocyanide at highly oxidized electrodes and attributed their results to electrostatic repulsion. The voltammetric behavior of several standard compounds at electrochemically pretreated carbon fibers has been reported.9 Of special interest to us are the differences in the extent of adsorption and the magnitude of observed currents related to the ionic charge of the electroactive species. For example, dopamine at pH 6.4 was found to strongly adsorb to the treated surface, leading to enhanced currents over those expected, while ferricyanide was not adsorbed and displayed currents smaller than predicted. The reasons for the large differences in the shapes and sizes of voltammetric waves recorded at UHSACFs when compared to unfractured fibers were not clear to us at the time of our previous report. Since then, we have conducted a more detailed study of the conditions necessary to produce the UHSACF and have investigated the effects of the charge of the electroactive species and the solution pH on the observed voltammetry. It was discovered that the voltammetric responses for cations and neutrals were enhanced at acidic pH, apparently as the result of penetration into the interior of the fractured fiber and a possible thin-layer or adsorption contribution to the observed current. Further, the behavior of anions at pH values sufficient to deprotonate acidic functional groups at the carbon surface suggests that they are excluded from the interior due to surface repulsion. EXPERIMENTAL SECTION Reagents and Materials. High-modulus, pitch-based carbon fibers (type E120 obtained from DuPont, Chattanooga, TN) with a nominal diameter of 10-12 µm were used. Dibasic sodium phosphate, phosphoric acid, and potassium nitrate were all reagent grade. Dopamine, ascorbic acid, 3,4-dihydroxybenzoic acid, potassium ferrocyanide, and potassium hexachloroplatinate(IV) were used as received from Aldrich. Potassium hexachloroiridate, hexammineruthenium(III) chloride, and tetrachlorodiammine platinum(IV) were purchased from Strem Chemicals (Newburyport, MA) and used as received without further purification. All solutions were prepared with water from a Nanopure water system (Barnstead/Thermolyne Corp., Dubuque, IA), and solutions, except those used to fracture, were thoroughly degassed with argon immediately before use. Apparatus. Experiments were conducted with a standard three-electrode system. Potentials were referenced to a commercial Ag/AgCl electrode, with either a platinum foil or a wire serving as the auxiliary electrode. Cyclic voltammetry was performed with either a Huntington Instruments model 200A (Yellow Springs, OH) or a Cypress Systems Omni 90 potentiostat. The resulting voltammograms were collected on an Omnigraphic 2000 X-Y recorder (Houston Instruments, Austin, TX). The Cypress Omni 90 poteniostat was also employed in the fracturing process, with current- or potential-time traces recorded on the analog X-Y recorder. Digital instrumentation was not used for these measurements due to the way the voltammogram is collected. In digital staircase voltammetry, the current is sampled at some time following a potential step, allowing a portion of the capacitive or charging current to decay. Since we have used the capacitive current observed at the UHSACF upon fracture as being proportional to the exposed surface area of the fiber, we believe
the direct analog measurement to be a more accurate indicator of electrode area. Procedures. Carbon fibers were prepared either by mounting one end of an individual fiber onto copper wires with colloidal silver epoxy or by aspiration into glass capillaries that were pulled to a taper and sealed as previously described.21 Cylindrical electrodes with nominal areas of 0.0031 and 0.0063 cm2 were the result of cutting to a final length of 1 or 2 cm, or by submerging only the desired length into solution. Galvanostatic and potentiostatic methods were employed to fracture the fibers. In the galvanostatic method, a controlled constant current was passed through the fiber, with fracturing accomplished by the application of 1 mA for 35 s or 3 mA for 5 s. The current was controlled by placing a resistor between the working and reference electrode connections to the potentiostat. A 0.1 M KNO3 solution was used as the electrolyte. Application of +1.0 or +0.330 V across a 330 S resistor provided constant currents of 3 and 1 mA, respectively, to the fiber. In the potentiostatic method, fibers were fractured by the application of +2.5 V for 35 s in a solution of pH 2.2 phosphate buffer or for 2 min in 0.1 M KNO3. After fracturing by either method, the electrodes were scanned at 0.100 V/s in the same electrolyte solution until a steady-state voltammogram was obtained. The apparent electrode capacitance was calculated from the current envelope as previously described,16 using C ) I/(2Aν), where C is the capacitance (in µF/cm2), I is the current (in µA), A is the unfractured geometric area (in cm2) of the fiber, and ν is the scan rate (in V/s). The current, I, was measured in the potential region near 0.00 V where surface faradaic processes are minimal or absent, and the current remained essentially independent of potential under the conditions used for fracture. The UHSACF electrodes used for cyclic voltammetric measurements reported here were produced by galvanostatic fracture in KNO3 to a capacitance near 1600 µF/cm2. RESULTS AND DISCUSSION Creation of UHSACF Electrodes. The extent and reproducibility of fracture for “as-received” DuPont E-120 fibers were investigated using several fracture protocols. A comparison was made of the potentiostatic treatment in pH 2.2 phosphate buffer (0.1 M) and the galvanostatic treatment in 0.1 M KNO3 for a large number of fibers. After 35 s at +2.5 V, the fibers that were treated potentiostatically showed an average value for double-layer capacitance of 4.3 ((1.1) × 103 µF/cm2 (N ) 142). It was observed that the capacitance was approximately a Gaussian distribution, with 38% of the total appearing between 4000 and 5000 µF/cm2. In contrast, fibers that were treated with the galvanostatic method had an average capacitance value after fracture of 1.6 ((0.2) × 103 µF/cm2 (N ) 80). More than 85% of the fibers treated in this way had double-layer capacitance values between 1000 and 2000 µF/cm2, with the overall distribution being roughly Gaussian from 1000 to 3000 µF/cm2. Fibers fractured potentiostatically in 0.1 M KNO3 for 2 min showed results similar to those obtained in phosphate buffer. The average double-layer capacitance for these fibers was calculated to be 5.8 ((1.5) × 103 µF/cm2 (N ) 12). The scatter in results obtained for potentiostatic treatments, when compared to gal(21) Furbee, J. W., Jr.; Kuwana, T.; Kelly, R. S. Anal. Chem. 1994, 66, 1575-7.
Figure 1. Current-time profile recorded during the potentiostatic fracture of a DuPont E120 carbon fiber (length 2 cm). Fiber was fractured for 2 min at +2.5 V vs Ag/AgCl in 0.1 M KNO3.
vanostatic treatments, was also observed by Sherwood for fiber bundles fractured in HNO3.22 The extent of fracture is dependent on the current density imposed on the fibers during the process. Since the current density is constant for galvanostatic treatment, the extent of fracture should be more reproducible. The potential measured during these treatments usually varied between +2.6 and +2.8 V.15 The current density for a fiber at constant potential is no doubt dependent on the surface conditions of each fiber with respect to its ability to act as a conductor or insulator, leading to high variability in the extent of fracture. This behavior is governed in part by the oxygen evolution observed at the surface of the fiber during the fracturing process and the resulting functional groups formed (vide infra). In fracturing, it appears that initially the outer layer, which has few reactive graphitic edges, is disrupted at high anodic potentials. Monitoring of the potential during galvanostatic treatment indicates that the fracture of the outer skin begins at a threshold value of ∼+1.8 V vs Ag/AgCl. This fracture allows entry of the solvent/electrolyte into the interior of the fiber, and oxidation proceeds simultaneously along the exposed reactive edge planes. Such concurrent entry and oxidation may disrupt basal plane alignment in graphitic microcrystalline regions, further opening the interior of the fiber. Video microscopic recordings during fracture showed vigorous oxygen evolution emanating from various surface fissures. Oxygen generation may also contribute to the extensive structural damage from the intercalation of electrolyte ions and water molecules between graphitic planes.23 The current-time behavior in chronoamperometry during fracture shows evidence of plateaulike regions that are thought to be steps in the surface and subsurface reactions occurring at the fiber.24 Figure 1 shows a typical current-time trace recorded for a 2-min potentiostatic treatment at +2.5 V in 0.1 M KNO3. It is characterized by an initial increase in oxidative current, presumably as reactive surface sites are functionalized. Following a normal, exponential decay in current for ∼10 s, a significant increase in anodic current then occurs, which is thought to signal the onset of fracture, when interior reactive sites are exposed to solution. One or more steps are then observed as layers of exposed carbon (22) Xie, Y.; Sherwood, P. M. A. Appl. Spectrosc. 1991, 45, 1158-65. (23) Xie, Y.; Sherwood, P. M. A. Appl. Spectrosc. 1990, 44, 1621-8. (24) Surmann, P.; Peter, B. Electroanalysis 1996, 8, 692-7.
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Figure 2. pH dependence of double-layer capacitance measured at galvanostatically fractured fibers: (a) fibers pretreated for 1 min at +1.0 V prior to fracture; (b) untreated, as received fibers. Fracture conditions: 1 mA for 35 s in 0.1 M KNO3.
become oxidized to the maximum extent. Surmann reported that the last plateau observed for anodic treatment of multiple fibers of a different type embedded in an epoxy resin matrix indicates the point where the current due to oxygen production reaches a constant value.24 In our case, the length of the potential step was limited by breaking of the fiber at times longer than 2-2.5 min so it is not clear whether a steady-state current due only to oxygen production has been reached. At any rate, the chronoamperometric trace supports the existence of specific steps to the fracture process. Although not shown here, the potential-time profile recorded during galvanostatic treatment was similarly stepped. Role of pH at the Surface of UHSACFs. Xie and Sherwood demonstrated that oxidation of the DuPont E120 fibers immersed in 1 M HNO3 at potentials greater than +2.0 V results in the formation of a number of oxygen-containing functionalities at the surface of the fiber.23 Examples of these structures include hydroxide and carbonyl groups that were identified by XPS. Because such groups are known to be involved in the electrochemical behavior observed at oxidatively treated electrodes, and since groups such as these are sensitive to changes in pH, we investigated the effect of changing pH on the double-layer capacitance for the UHSACF. Figure 2 shows the pH dependence of double-layer charge measured for fibers that were fractured galvanostatically in KNO3. Two sets of data are presented. The first set, for which capacitance remains constant at pH values between 4 and 12, was collected at UHSACF electrodes that had been electrochemically “precleaned” for 1 min at +1.0 V prior to fracture in KNO3. The average capacitance for between five and nine electrodes was obtained at each pH value. The capacitance measurements for this set were made in the fracture solution, with pH adjustment being accomplished with additions of HCl or NaOH. A sharp drop in observed capacitance is seen for pH values below 4, with the midpoint of the break at pH ∼3. The second set of data was collected using UHSACF electrodes that were not pretreated at +1.0 V (N ) 2 at each pH). Capacitance measurements were made in solutions of phosphate buffer 416 Analytical Chemistry, Vol. 71, No. 2, January 15, 1999
prepared for each individual value of pH. These data show a similar plateau at pH values greater than 6 and a midpoint in the curve at pH ∼5. The electrodes used for this experiment showed a higher value for maximum capacitance than the average of the electrodes used in the collection of the first data set. Acidic surface oxides, including carboxylic acids, phenols, and quinones, are known to form when carbon reacts with an oxidizing solution at room temperature.25 Carboxylic acids have pKa values in the range of 1-4,26 which would lead to the formation of a charged surface upon deprotonation. While such graphitic groups are expected to not behave exactly as their counterparts in solution,27 this range of pKa’s can be used in predicting similar pH properties for surface-bound functionalities. Furthermore, a pKa for the carboxyl functionality at carbon has been reported to be near 4.28 The pH at which the capacitance increases dramatically for UHSACFs covers the range of approximately 2.5-5 (shaded area in Figure 2). A significant portion of the surface groups will therefore be deprotonated in this pH range and most or all of the surface functional groups deprotonated at higher pH. An increase in a charged surface state has been shown previously to result in an increase in the observed capacitance at glassy carbon electrodes.29 The increase in observed capacitance as a result of increasing pH at UHSACFs is most likely due to an increase in surface charge. In effect, as the pH increases, a resultant increase in interior volume of the fiber is expected as like charges repel each other, and the exposed electrode surface increases. Once all acidic surface sites have been deprotonated, the capacitance should stop increasing, which is consistent with the observed behavior above pH 5-6. Voltammetry of 3,4-Dihydroxybenzoic Acid at UHSACF. Figure 3 presents cyclic voltammograms for 1.0 mM 3,4-dihydroxybenzoic acid (DHBA) in phosphate buffer at pH 2.2 recorded at (a) an unfractured fiber and (b) an UHSACF. The voltammograms are presented on the same current scale and are shown with corresponding background scans recorded in the same buffer. The background current envelope recorded at the unfractured fiber, measured at a more sensitive current setting, was ∼2.0 nA. Following fracture, the background current was found to be ∼2.0 µA, showing a 1000-fold increase in the capacitance, which is interpreted to be proportional to the increase in the exposed surface area. At the unfractured fiber, DHBA exhibits voltammetry consistent with diffusion to a microcylinder, with the limiting current reaching essentially a steady-state value. The half-peak potential, Ep/2, was found to be 0.67 V, with a backgroundcorrected current height of 6.1 µA at a scan rate of 0.100 V/s. The voltammetry of DHBA recorded at the UHSACF (Figure 3, trace b) is dramatically different, becoming peak shaped and exhibiting a greatly enhanced current above that observed at the unfractured fiber. Further, the sharpness of the peak suggests that there are contributions to the current other than diffusion to the “outer” surface, perhaps adsorptive or thin-layer contributions. (25) Deakin, M. R.; Wightman, R. M. J. Electroanal. Chem. 1986, 206, 167-77. (26) Perrin, D. D.; Dempsey, B.; Sergeant, E. P. pKa Prediction for Organic Acids and Bases; Chapman and Hall: London, 1981; p 16. (27) Nagaoka, T.; Fukunaga, T.; Yoshino, T.; Watanabe, I.; Nakayama, T.; Okazaki, S. Anal. Chem. 1988, 60, 2766-9. (28) Deakin, M. R.; Stutts, K. J.; Wightman, R. M. J. Electroanal. Chem. 1985, 182, 113-22. (29) Anjo, D. M.; Kahr, M.; Khodabakhsh, M. M.; Nowinski, S.; Wanger, M. Anal. Chem. 1989, 61, 2603-8.
Figure 3. Cyclic voltammetric response for DHBA at (a) an unfractured carbon fiber and (b) an ultrahigh-surface-area carbon fiber (UHSACF) in pH 2.2 phosphate buffer. Scan rate, 0.100 V/s; DHBA concentration, 1.0 mM. Shown with each scan are the currentpotential background scans recorded in pH 2.2 phosphate buffer.
Figure 4. Cyclic voltammetric response for DHBA at an UHSACF in pH 7.0 phosphate buffer. Scan rate, 0.100 V/s; DHBA concentration, 1.0 mM.
The formal potential, E°′, taken as the average of the anodic and cathodic peak potentials, was +0.58 V at the UHSACF (∆Ep ) 88 mV). The value for the oxidative peak current for DHBA was found to be 16.6 µA at this electrode. For comparison, the calculated current for diffusion to a cylindrical electrode with a nominal geometric area equal to that of the unfractured fiber is 4.9 µA, using a value of 6.0 × 10-6 cm2/s for the diffusion coefficient.30 Effects of Charge on Observed Voltammetry at UHSACF. As indicated in an earlier section, the electrode surface, both exterior and interior, of the UHSACF is thought to be fully protonated at pH 2.2. Thus, any charge selectivity that might govern the inclusion into or exclusion from the interior for electroactive species as the surface functionalities deprotonate should be eliminated. The effects of a charged, fully deprotonated electrode surface on the voltammetry of DHBA will now be examined. At pH 7.0, DHBA is negatively charged as a result of deprotonation due to its pKa value of 4.5.31 The electrode surface is also negatively charged at this pH. Figure 4 shows the cyclic voltammogram of DHBA in pH 7.0 phosphate buffer, obtained at an UHSACF. At the negatively charged surface, DHBA exhibits quite different voltammetric (30) Gerhardt, G.; Adams, R. N. Anal. Chem. 1982, 54, 2618-20. (31) Dean, J. A., Ed. Lange’s Handbook of Chemistry, 14th ed.; McGraw-Hill: New York, 1992; p 838.
Figure 5. Sweep rate dependence of the cyclic voltammetric response for potassium ferrocyanide at an UHSACF in 0.1 M KNO3 at pH 4.0. Scan rates of (a) 0.010, (b) 0.020, (c) 0.050, (d) 0.100, and (e) 0.200 V/s are shown. Concentration of Fe(CN6)4-, 1.0 mM; pH adjusted with HCl.
behavior than it did at the neutral electrode surface at pH 2.2. The formal potential, E°′, is shifted cathodically at this pH to a value of +0.26 V as predicted by the Nernst equation for a 2-electron, 2-proton process.32 Most striking is the drastic reduction in observed current from that observed in acidic solution. The measured anodic current of 3.7 µA is now less than that calculated for diffusion to a microcylinder. The sharp, peak shape observed at pH 2.2 has been replaced with what appears to result mainly from a diffusion-dominated process. These results clearly show the effect of charge repulsion between the negatively charged DHBA and the like-charged surface of the electrode. The charge also appears to prevent this species from entering the interior of the fiber, thereby limiting the current to that resulting primarily from diffusion to the outer electrode surface. The effects of charge selectivity and anionic exclusion behavior of UHSACF electrodes are clearly seen in the slope values obtained from plots of peak current as a function of square root of the scan rate. For DHBA, the slope obtained at pH 2.2, where the species is neutral, and entry into the interior of the fiber is not inhibited, was 52.6 µA V-1/2 s1/2 (N ) 4). Conversely, at pH 7.0, where anionic exclusion from the interior of the fiber is operable, the slope obtained for DHBA was only 10.7 µA V-1/2 s1/2 (N ) 3). Charge selectivity at an UHSACF electrode was also demonstrated in the cyclic voltammetry of potassium ferrocyanide in 0.1 M KNO3 at pH 4.0. Figure 5 shows a series of cyclic voltammograms recorded between 0.010 and 0.200 V/s. At each of the scan rates, a limiting current indicative of diffusion to a microcylinder was observed. The value for this current at 0.100 V/s was 3.4 µA, just slightly larger than the value of 2.5 µA, which was calculated for a microcylinder 2 cm in length. The half-wave potential, Ep/2, was found to be +0.24 V under these conditions. As the scan rate is increased, the capacitive component of the total current increases at a much faster rate than the current due to the electrochemistry of ferro-/ferricyanide. This increase can be explained by considering that the nonfaradaic capacitive current (32) Laitinen, H. A. Chemical Analysis: An Advanced Text and Reference; McGrawHill Inc.: New York, 1960; p 291.
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Table 1. Peak Current as a Function of the Square Root of Scan Rate for Test Compounds
compound, conditionsa
charge
slope, ip vs ν1/2 plots (µA V-1/2 s1/2)
dopamine, phosphate, pH 2.2 ascorbic acid, phosphate, pH 2.2 ascorbate, phosphate, pH 5.5 3,4-DHBA, KNO3, pH 4 3,4-DHBA, KNO3, pH 6 Ru(NH3)63+/2+, KNO3, pH 7 Fe(CN)63-/4-, KNO3, pH 7 Ir(Cl)62-/3-, KNO3, pH 7 Pt(NH3)2Cl4 in KNO3 PtCl62-
+1 0 -1 0 -1 3+/2+ 3-/42-/30 2-/0
29.1 21.7 6.2 34.2 3.5 14.2 5.9 3.6 reduction to Ptb reduction to Ptb
a All solutions are 1.0 mM in electroactive species. b The amount of Pt metal deposited is greater with Pt(NH3)63+ than with PtCl63-.
is due to charging the total surface of the fiber, both exterior and interior, and increases as a function of scan rate. The faradaic current, on the other hand, results only from the diffusion of the electroactive species to the external, microcylindrical area of the fiber and is dependent on the square root of the scan rate. The magnitude of the faradaic current at UHSACFs is found to be dependent on the solution pH and the charge of the electroactive species. The results for several species are summarized in Table 1. The slopes from the plots of the Ip vs the square root of scan rate are used as a diagnostic indicator of the electrochemistry, recognizing that the slopes become nonlinear at longer times at microelectrodes. Only the linear portions of the plots were used for diagnostic purposes. For the positively charged species, the slopes are much greater than those with
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negative charges, especially at pH values where the fiber surface is also negatively charged. For example, the slopes for dopamine of 29.1 at pH 2.2, ascorbic acid of 21.7 at pH 2.2, and Ru(NH3)63+/2+ of 14.2 at pH 7 are much greater than those for ascorbic acid of 6.2 at pH 5.5, DHBA of 3.5 at pH 6, ferro-/ferricyanide of 5.9 at pH 7, and IrCl62-/3- of 3.6 at pH 7. In the cases of Pt metal deposition, the amount of Pt deposited onto UHSACFs was much greater for the neutral tetrachlorodiammineplatinum(IV) than for the negatively charged platinum hexachloro(-2) species. These results clearly show the effect of species and surface charge on the magnitude of the faradaic current. CONCLUSION A conclusion can be drawn that the total faradaic current must be a composite of contributions from species diffusion to the outer microcylindrical fiber and from species penetration into the interior of the UHSACF. The latter can result in both adsorptive and thinlayer contributions, the extent of which will be resolved by computer simulating various models and matching to experimental results. These results, in progress, will be reported soon. ACKNOWLEDGMENT We gratefully acknowledge support for this project from the National Science Foundation (University of Kansas, Macro-ROA Program; T.K., R.S.K.) and from a William and Flora Hewlett Foundation Award of Research Corp. (R.S.K.). Experimental contributions from Sharon Dalton and Maura Fitzpatrick (Merrimack College) are also acknowledged. Received for review July 13, 1998. Accepted November 6, 1998. AC980760G
