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Oxygen reduction of glassy carbon in 0.1 M HCl: RDE and XPS study
161
J. Electroad. Chem., 220 (1987) 161-168 Ekevier !kquoia %A., Lausanne - Printed in The Netherlands
Preliminary
note
OXYGEN REDUCTION xP!!I STUDY
K.M. SUNDBERG,
ON GLASSY CARBON IN 0.1M HCI: RDE AND
Lj. ATANASOSKA *, R. ATANASOSKI
** and W.H. SMYRL
Corrosion Research Center and Department of Chemical Engineering and Materials Science, Universrv of Minnesota, Minneapolis, MN 55455 (U.S.A.) (Received 23rd December 1986) INTRODUCTION
Electrochemistry has been studied on glassy carbon (GC) for some time [1,2]. GC has become an indispensable indicating electrode in both aqueous and non-aqueous media, with a large electrochemical window [2-41. Like other carbonaceous materials, it is used as either a substrate for other catalysts or as a catalyst itself. The reduction of oxygen on glassy carbon has been studied extensively from the point of view of electrocatalysis [5-111, along with other oxidation/reduction reactions [3,12,13]. These interests led to fundamental studies in the double layer region and adsorption phenomena (14,151, and surface studies [l&18]. Recently, carbon has been introduced to the field of microelectronics as both a dielectric and as a protective coating, among other applications. Its use in microelectronics requires an understanding of oxygen reduction on carbon as a cathodic process, because in service this process may be galvanically coupled with anodic dissolution of metals in microstructured arrays. Our interest in the latter process has prompted the study described here. Due to the significance of chlorides in corrosion [19,20], it is important to study processes in an environment which includes chlorides. There is little data in the literature on the kinetics of oxygen reduction in chloride media in general, but some data are available for GC [ll] as an electrode material for oxygen reduction. Chloride media are also important for current and future use in the electrolytic industry. Thus, oxygen reduction in hydrochloric acid is of interest not only for understanding corrosion protection, which is the primary focus of the current study, but also for the broader purpose of understanding oxygen reactivity on a GC substrate with regards to other applications. * Institute of Technical Sciences, SASA, Belgrade, Yugoslavia. l
* Institute of Electrochemistry, ICTM, University of Belgrade, Belgrade, Yugoslavia
0022-0728/87/$03.50
8 1987 Elseker
Sequoia S.A.
162
In alkaline solutions the reduction of oxygen on GC was found to occur exclusively through a two-step process, first to peroxide, followed by further reduction of peroxide to water [5]. Fewer studies have been made in acidic electrolytes. In sulfuric acid, the number of electrons transferred was reported to be two [lo] and the first electron transfer to 0, was proposed to be the rate determining step [6]. It has been proposed that the surface states of GC are responsible for the irreproducibility and inconsistency of the experimental results described in the literature; surface treatment and characterization is therefore important for any glassy carbon study. Surface preparation has varied from one study to the next, but the technique most often used is mechanical polishing using alumina or diamond paste. In order to determine the effects of polishing, microscopy has been used [16,21]. XPS studies have been used to identify impurities from polishing [17,18]. Electrochemical pretreatment [5-7,13,14,18,22], as well as other forms of surface oxidation, including the use of oxygen plasma treatments [21] and chemical oxidizing agents [lo], were used to improve the electrocatalytic behavior of GC for oxidation/reduction reactions and to improve the reproducibility of electrode characteristics. The effects of electrochemical treatment on the surface state of GC have been studied with XPS [16,18]. It has been reported that the increased activity of electrode materials is due to functional groups which form on the surface, specifically oxygen-containing functional groups. In order to explore the role of chloride ion in both the electrochemical treatment of the surface and the cathodic reduction of oxygen, it was necessary during this study as well to complete an XPS analysis of the GC surface following the steps of the treatment in hydrochloric acid. EXPERIMENTAL.
Glassy carbon (Atomergic, grade VlO) rotating disk electrodes have been used throughout this study (diameter ranging from 0.645 cm to 1.2 cm). The disks were attached to a brass shaft and both were press-fitted into a Teflon sleeve. This assembly was attached to the rotating disk controller via a threaded joint. Contact between the carbon and the brass shaft was made using silver conducting epoxy. The GC disks were polished using silicon carbide paper, 420 and 600 grit, followed by alumina of 1.0 and 0.05 pm diameter, suspended in 18 MQ water on a polishing cloth. Alumina was removed from the surface by placing the electrode in an ultrasonic water bath for 2 min. Samples were checked for impurities due to polishing using XPS. The electrolyte was 0.1 M HCl prepared from reagent grade concentrated HCl and diluted in 18 MO water. The samples were immersed in deaerated electrolyte immediately after the polishing and rinsing procedure had been completed. The concentration of oxygen in the electrolyte was controlled by bubbling gas through the electrolyte. The potentials were measured with respect to a saturated calomel electrode. Oxygen and nitrogen gases of Ultra-pure carrier grade (total hydrocarbon
163
content < 0.5 ppm; Air Products, Inc.) were used without further purification. Electrochemical treatment of GC included holding the potential at 2.0 V for 15 min, followed by cycling the potential between - 1.0 V and 0.5 V, or holding the potential at - 1.0 V for 5 min. Samples with the same electrochemical treatment were prepared for XPS analysis. Linear sweep voltammetry was used to examine the reduction of oxygen in the potential range of -1.0 V to 0.5 V, with a sweep rate ranging from 10-100 mV/s, and a disk rotation rate of 400 to 3600 rpm. The sweeps began at 0.0 V, and the potential was swept initially in the negative direction. The XPS studies were carried out with a Physical Electronics 555 spectrometer (MgK, X-ray source) and a PDP-11 computer for data acquisition and analysis. RESULTS AND DISCUSSION
Oxygen reduction electrochemistry The current response for electrochemically treated samples is depicted in Fig. 1 along with the one before the electrochemical treatment. The reproducibility and quality of the data found on electrochemically treated polished surfaces was much better than for surfaces without the treatment. Holding the electrode at 2.0 V, followed by negative polarization, improved not only the reproducibility but the electrocatalytic performance of glassy carbon as well. Cycling the potential for 20 min between - 1000 mV and 500 mV at 20 mV/s or holding the potential at - 1000 mV for 5 min following the positive polarization yielded reproducible curves. This figure demonstrates two other important effects of the treatment. The curve for the reduction of oxygen was a single wave continuing with a well-defined limiting current density of 2.6 mA/d at 900 rpm. The potential at which the current is half that of the limiting current was - 320 mV. The shift of this value for the polished, untreated samples was around 0.5 V in the negative direction, to - 830
-301 -1000
'
' -800
'
' -600
'
iS;l
1'
' -200
'
0
E ImV Fig. 1. Oxygen reduction curves for electrochemically treated and polished glassy carbon electrode in 0.1 M HCl; 100%4; 900 rpm; 20 mV/s.
164
Fig. 2. The dependence of the oxygen reduction curves on the rotation rate; electrochemically treated glassy carbon electrode in 0.1 M HCl; 100% 0,; 20 mV/s.
mV, as compared to the treated samples, while the magnitude of the limiting current was slightly lower, 2.3 mA/cm*, on the untreated surface. The limiting currents were constant over a potential range of 0.5 V (Fig. 2) for the treated samples, at each rotation rate. The plot of i, vs. o’fl (o is the rotation rate) is linear (Fig. 3), in agreement with the Levich equation for diffusion transport control [22]. The latter allowed an accurate calculation of the number of electrons transferred; n was found to be 2.3. The value of n suggested that the “parallel’ mecharrism did not take place at a substantial rate in this case, while the absence of even a trace of the second wave up to - 1.1 V implied that hydrogen peroxide was the major final product of the reduction of oxygen. Examination of the literature reveals a variety of interpretations for oxygen reduction data as well as inconsistencies. Two waves were observed in H,SO, and KNO,, both before and after electrochemical treatment [6,16]; comparison of these
0
-112/ra&112
5112
Fig. 3. The dependence of the limiting currents of the oxygen reduction on the rotation rate; data from Fig. 2.
165
data, however, shows a difference in current density by a factor of two. A study of GC following treatment in chromic sulfuric acid [lo] reported a single wave, an n value of two, and a limiting current density which corresponds well with ours. Shifts of a similar magnitude to those found here in hydrochloric acid were observed in RDE studies of this reaction in sulfuric acid [6] and in potassium nitrate solutions [16], while slightly smaller changes of the oxygen reduction peak potentials in the cyclic voltammograms of untreated samples were found in hydrochloric acid
WIAn increase of the background current similar to that reported in the literature (see e.g. ref. 14) was observed after electrochemical treatment. Although the surface roughness may have increased, the fact that the change did not correspond to an equivalent increase in the limiting current for oxygen reduction (Fig. 1) suggests strongly that the true surface area remains the same. XPS surface ana&ses The XPS data for “as received” polished GC samples, prior to any contact with the solution, revealed the presence of trace amounts of polishing material in the form of Sic (l-2 at.%). No alumina was found on the polished glassy carbon surface. This finding is in agreement with Kamau et al. [17], who have demonstrated that contamination due to polishing can be minim&d by elaborate procedures, and with Cabaniss et al. [18], where polishing with alumina in deionized water was followed by treatment in an ultrasonic bath of deionized water. The high resolution photoelectron spectra of the Cls region of “as received” polished glassy carbon samples were reproducible. The average oxygen to carbon ratio derived from the XPS spectra of polished GC is about 0.23 f 0.04, which is close to the ratio (0.17) reported by Kamau et al. [17], and between the values (0.08 and 0.33) found elsewhere [16,18]. The full width at half maximum of the Cls photoelectron line, was 2.2 eV. On the basis of a curvefitting analysis, the surface of polished GC was composed of only three resolvable forms of carbon associated with graphitic, phenolic and carboxylic functional groups, at average concentrations of 68%, 25%, and 7% (atomic), respectively. This is again in good agreement with the data reported elsewhere [17,18]. The anodization of GC for 15 min at 2 V resulted in an obvious increase in the oxygen photoelectron peak intensity and in an additional peak at 199.9 eV due to the incorporation of chloride. The average chloride content in the surface layer was found to be = 7%. The atomic O/C ratio increased from 0.23 for untreated to 0.40 f 0.04 for the treated samples. This value falls between the ones reported for a similar treatment: 0.22 after anodic treatment at 1.75 V for 5 min in KNO, [16] and 0.89 for 30 mm anodization at 1.8 V in 0.1 M H,SO, [18]. The extent of oxidation caused by the anodization is larger than that on GC electrodes that have been subjected to oxygen plasma treatments (O/C ratio 0.33) [21]. The Cls high resolution spectrum for an anodized sample is shown in Fig. 4. The full width at half maximum of the carbon photoelectron peak was 4.0 eV in comparison to 2.2 eV as given above for untreated samples. This is an excellent
166
J
BlNDING ENERGY /eV Fig. 4. Cls XPS spectrum for &ssy carbon anodii in 0.1 M HCl (2 V, 15 min); (1) graphitic, (2) phenol&,(3) carbonyl, and (4) carboxylic type of carbon.
indication of the presence of more than one carbon functional group on the surface. Also, it could suggest the creation of additional oxygen-containing carbon functional groups during the anodization period. The major peak in this spectrum is located towards lower binding energies, at 284.4 eV, which is assigned to a graphitic or tetrahedral form of carbon. A distinct shoulder at higher binding energies, apparent in this spectrum, was more prominent than for polished, untreated samples. Three additional peaks were found by deconvolution of the shoulder in the case of anodized GC, as compared to two on the untreated ones. The noticeable buildup of oxygen on the CC surface upon anodization resulted in the appearance of a new, carbonyl functional group, in addition to the already existing groups. Relative contributions of graphitic, phenolic, carbonyl and carboxylic types of carbon to a total carbon photoemission are 46%, 16% 26% and 12% (atomic), respectively. A description of the peaks (see Fig. 4) which characterize the groups is as follows: the first, at 284.4 eV, is assigned to a graphitic carbon, the second, at 285.6 eV, is ascribed to a type of carbon in phenol, alcohol or ether, the third, at 287.0 eV, to a carbonyl or quinone type, and the fourth, at 288.6 eV, to carboxyl. We found the shifts of oxygen-containing carbon functional groups corresponding to the position of graphitic carbon to be, within the error introduced by the curve-fitting routine, in acceptable agreement with the vaIues obtained from the studies of native and pl~rna-~~~ polymers [21]. There is no report in the literature of a predominance of carbonyl groups produced by anodic treatment of glassy carbon. The use of chloride media in our study suggests some role for the chloride ion (or chlorine evolved during anodization) in the formation of carbonyl functional groups. Elemental chlorine present at the GC surface at the positive potential of 2 V could facilitate the formation of carbonyl groups via oxygen transfer [23] rather than increasing the content of phenolic carbon. Cabaniss et al. [18] reported an O/C atomic ratio of 0.89 in H,SO,. In spite of a high oxygen content, ascribed to three of the same functional groups as in our case, it was found that the major contribution was coming from the phenolic carbon.
167 7 6-
2-
lo
-260
-266
BINDING
-266
ENERGY
-2.94
-282
leV
Fig. 5. Cls XPS spectrum for reduced glassy carbon surface; same notation as in Fig. 4.
The high resolution Cls XPS spectrum for glassy carbon which has been oxidized and subsequently reduced at - 1 V is presented in Fig. 5. There is a striking resemblance of this spectrum to the one for untreated polished GC. The excess amount of oxygen introduced on the glassy carbon surface by anodization was almost completely removed with 5 min of reduction (O/C = 0.27 f 0.02). The chloride content was decreased substantially, as well, to less than 4%. The electrochemical reduction at - 400 mV, in the region of the oxygen half-wave potential, led to the same decrease in the number of oxidized surface sites, reversing the spectrum almost back to the original spectrum of a polished unactivated surface. The carbon peak was again composed of only three resolvable forms of carbon. The peak attributed to the presence of a carbonyl group disappeared as a consequence of the electrochemical reduction and the relative contributions of graphitic, phenolic, and carboxylic carbon to a total carbon photoemission for the reduced GC changed to 678, 23%, and 10% (at.) respectively. Similar responses to electrochemical reduction of the GC surface were observed in different solutions and for significantly shorter reduction times [16,17]. CONCLUDING
REMARKS
The discussion given along with the presentation of the experimental results included several important concepts. Three distinctive electrochemical features of the oxygen reduction curves in hydrochloric acid on electrochemically treated glassy carbon are: (i) the single wave followed by (ii) a wide horizontal plateau and (iii) the magnitude of the limiting current that obeyed the Levich equation. The latter allowed an accurate determination of the number of electrons transferred (n = 2.3), but the place of glassy carbon among materials characterized by a two-electron process in acid media [24] has yet to be established. It is tempting to correlate the electrocatalytic effect with the appearance of the carbonyl surface group during the positive polarization, regardless of its absence
168
from the XPS spectra at the reduced surfaces. The spectra, giving almost the same picture of the untreated and the reduced glassy carbon surface quantitatively, offer no other clue for distinguishing the significant changes due to the treatment-changes that lead to the catalysis of the electrochemical reduction of oxygen. Finally, there was no role that could be assigned to the presence of chlorides at the surface of glassy carbon during the oxygen reduction easily. One may speculate that the mechanism for the generation of carbonyl surface groups during the anodization involves elemental chlorine as a mediator for oxygen transfer to the carbon atoms. The correlation between the XPS data and the role of chlorides in modifying the electrochemistry of oxygen reduction seems to be a promising starting point for further study of the mechanism of oxygen reduction in acidic chloride environments. ACKNOWLEDGEMENTS
This work has been performed under an IBM (Rochester, Minnesota) University Program Grant. Lj.A. and R.A. are grateful for the support of the Yugoslav-American Fund for Scientific Cooperation, D.O.E. Grant 675. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
S. Yamada and H. Sate, Nature, 193 (1962) 261. H.E. Zittel and F.J. Miller, Anal. Chem., 37 (1%5) 200. W.E. Van Der Linden and J.W. Dieker, Anal. Chim. Acta, 119 (1980) 1. D.C. Thornton, K.T. Corby, V.A. Spendel, J. Jordan, A. Robbat, Jr., D.J. Rutstrom, M. Gross and G. Ritsler, Anal. Chem., 57 (1985) 150. R.J. Taylor and A.A. Humffray, J. Electroanal. Chem., 64 (1975) 63. R.J. Taylor and A.A. Humffray, J. Electroanal. Chem., 64 (1975) 85. R.J. Taylor and A.A. Humffray, J. Electroanal. Chem., 64 (1975) 95. F.Z. Sabirov and M.R. Tarasevich, Sov. Etectrcchem., 8 (1969) 564. M.Z. Brezina, Anal. Chem., 224 (1967) 74. C. Mayer and K. Juttner, Electrochim. Acta, 27 (1982) 1609. T. Nagaoka, T. Sakai, K. Ogura and T. Yoshino, Anal. Chem., 58 (1986) 1953. W.J. Blaedel and R.A. Jenkins, Anal. Chem., 46 (1974) 1952. R.J. Taylor and A.A. Humffray, J. Electroanal. Chem., 42 (1973) 347. L. Bjelica, R. Parsons and R.M. Reeves, Croat. Chem. Acta, 53 (1980) 211. J. Randin and E. Yeager, J Electroanal. Chem., 58 (1975) 313. R.C. Engstrom and V.A. Strasser, Anal. C&em., 56 (1984) 136. G.N. Kamau, W.S. Willis and J.F. Rusling, Anal. Chem., 57 (1985) 545. G.E. Cabaniss, A.A. Diamantis, W.R. Murphy, Jr., R.W. Linton and T.J. Meyer, J. Am. Chem. Sot., 107 (1985) 1845. H.H. Uhlig, Corrosion and Corrosion Control, Wiley, New York, 1971, p. 76. C.G. Munger, Corrosion Prevention by Protective Coatings, NACE, Houston, 1984. C.W. Miller, D.H. Karweik and T. Kuwana, Anal. Chem., 53 (1981) 2319. V.G. Levi& Physiccchemical Hydrodynamics, Prentice Hall, En&wood Cliffs, 1%2. A.H. Haines, Methods for the Oxidation of Organic Compounds, Academic Press, London, 1985. M.R. Tarasevich, A. Sadkowski and E. Yeager in B.E. Conway, J.O’M. Bockris, E. Yeager, S.U.M. Kahn and R.E. White (Eds.), Comprehensive Treatise of Electrochemistry, Vol. 7, Plenum Press, New York, 1983, pp. 361-362.
J. Electroad. Chem., 220 (1987) 161-168 Ekevier !kquoia %A., Lausanne - Printed in The Netherlands
Preliminary
note
OXYGEN REDUCTION xP!!I STUDY
K.M. SUNDBERG,
ON GLASSY CARBON IN 0.1M HCI: RDE AND
Lj. ATANASOSKA *, R. ATANASOSKI
** and W.H. SMYRL
Corrosion Research Center and Department of Chemical Engineering and Materials Science, Universrv of Minnesota, Minneapolis, MN 55455 (U.S.A.) (Received 23rd December 1986) INTRODUCTION
Electrochemistry has been studied on glassy carbon (GC) for some time [1,2]. GC has become an indispensable indicating electrode in both aqueous and non-aqueous media, with a large electrochemical window [2-41. Like other carbonaceous materials, it is used as either a substrate for other catalysts or as a catalyst itself. The reduction of oxygen on glassy carbon has been studied extensively from the point of view of electrocatalysis [5-111, along with other oxidation/reduction reactions [3,12,13]. These interests led to fundamental studies in the double layer region and adsorption phenomena (14,151, and surface studies [l&18]. Recently, carbon has been introduced to the field of microelectronics as both a dielectric and as a protective coating, among other applications. Its use in microelectronics requires an understanding of oxygen reduction on carbon as a cathodic process, because in service this process may be galvanically coupled with anodic dissolution of metals in microstructured arrays. Our interest in the latter process has prompted the study described here. Due to the significance of chlorides in corrosion [19,20], it is important to study processes in an environment which includes chlorides. There is little data in the literature on the kinetics of oxygen reduction in chloride media in general, but some data are available for GC [ll] as an electrode material for oxygen reduction. Chloride media are also important for current and future use in the electrolytic industry. Thus, oxygen reduction in hydrochloric acid is of interest not only for understanding corrosion protection, which is the primary focus of the current study, but also for the broader purpose of understanding oxygen reactivity on a GC substrate with regards to other applications. * Institute of Technical Sciences, SASA, Belgrade, Yugoslavia. l
* Institute of Electrochemistry, ICTM, University of Belgrade, Belgrade, Yugoslavia
0022-0728/87/$03.50
8 1987 Elseker
Sequoia S.A.
162
In alkaline solutions the reduction of oxygen on GC was found to occur exclusively through a two-step process, first to peroxide, followed by further reduction of peroxide to water [5]. Fewer studies have been made in acidic electrolytes. In sulfuric acid, the number of electrons transferred was reported to be two [lo] and the first electron transfer to 0, was proposed to be the rate determining step [6]. It has been proposed that the surface states of GC are responsible for the irreproducibility and inconsistency of the experimental results described in the literature; surface treatment and characterization is therefore important for any glassy carbon study. Surface preparation has varied from one study to the next, but the technique most often used is mechanical polishing using alumina or diamond paste. In order to determine the effects of polishing, microscopy has been used [16,21]. XPS studies have been used to identify impurities from polishing [17,18]. Electrochemical pretreatment [5-7,13,14,18,22], as well as other forms of surface oxidation, including the use of oxygen plasma treatments [21] and chemical oxidizing agents [lo], were used to improve the electrocatalytic behavior of GC for oxidation/reduction reactions and to improve the reproducibility of electrode characteristics. The effects of electrochemical treatment on the surface state of GC have been studied with XPS [16,18]. It has been reported that the increased activity of electrode materials is due to functional groups which form on the surface, specifically oxygen-containing functional groups. In order to explore the role of chloride ion in both the electrochemical treatment of the surface and the cathodic reduction of oxygen, it was necessary during this study as well to complete an XPS analysis of the GC surface following the steps of the treatment in hydrochloric acid. EXPERIMENTAL.
Glassy carbon (Atomergic, grade VlO) rotating disk electrodes have been used throughout this study (diameter ranging from 0.645 cm to 1.2 cm). The disks were attached to a brass shaft and both were press-fitted into a Teflon sleeve. This assembly was attached to the rotating disk controller via a threaded joint. Contact between the carbon and the brass shaft was made using silver conducting epoxy. The GC disks were polished using silicon carbide paper, 420 and 600 grit, followed by alumina of 1.0 and 0.05 pm diameter, suspended in 18 MQ water on a polishing cloth. Alumina was removed from the surface by placing the electrode in an ultrasonic water bath for 2 min. Samples were checked for impurities due to polishing using XPS. The electrolyte was 0.1 M HCl prepared from reagent grade concentrated HCl and diluted in 18 MO water. The samples were immersed in deaerated electrolyte immediately after the polishing and rinsing procedure had been completed. The concentration of oxygen in the electrolyte was controlled by bubbling gas through the electrolyte. The potentials were measured with respect to a saturated calomel electrode. Oxygen and nitrogen gases of Ultra-pure carrier grade (total hydrocarbon
163
content < 0.5 ppm; Air Products, Inc.) were used without further purification. Electrochemical treatment of GC included holding the potential at 2.0 V for 15 min, followed by cycling the potential between - 1.0 V and 0.5 V, or holding the potential at - 1.0 V for 5 min. Samples with the same electrochemical treatment were prepared for XPS analysis. Linear sweep voltammetry was used to examine the reduction of oxygen in the potential range of -1.0 V to 0.5 V, with a sweep rate ranging from 10-100 mV/s, and a disk rotation rate of 400 to 3600 rpm. The sweeps began at 0.0 V, and the potential was swept initially in the negative direction. The XPS studies were carried out with a Physical Electronics 555 spectrometer (MgK, X-ray source) and a PDP-11 computer for data acquisition and analysis. RESULTS AND DISCUSSION
Oxygen reduction electrochemistry The current response for electrochemically treated samples is depicted in Fig. 1 along with the one before the electrochemical treatment. The reproducibility and quality of the data found on electrochemically treated polished surfaces was much better than for surfaces without the treatment. Holding the electrode at 2.0 V, followed by negative polarization, improved not only the reproducibility but the electrocatalytic performance of glassy carbon as well. Cycling the potential for 20 min between - 1000 mV and 500 mV at 20 mV/s or holding the potential at - 1000 mV for 5 min following the positive polarization yielded reproducible curves. This figure demonstrates two other important effects of the treatment. The curve for the reduction of oxygen was a single wave continuing with a well-defined limiting current density of 2.6 mA/d at 900 rpm. The potential at which the current is half that of the limiting current was - 320 mV. The shift of this value for the polished, untreated samples was around 0.5 V in the negative direction, to - 830
-301 -1000
'
' -800
'
' -600
'
iS;l
1'
' -200
'
0
E ImV Fig. 1. Oxygen reduction curves for electrochemically treated and polished glassy carbon electrode in 0.1 M HCl; 100%4; 900 rpm; 20 mV/s.
164
Fig. 2. The dependence of the oxygen reduction curves on the rotation rate; electrochemically treated glassy carbon electrode in 0.1 M HCl; 100% 0,; 20 mV/s.
mV, as compared to the treated samples, while the magnitude of the limiting current was slightly lower, 2.3 mA/cm*, on the untreated surface. The limiting currents were constant over a potential range of 0.5 V (Fig. 2) for the treated samples, at each rotation rate. The plot of i, vs. o’fl (o is the rotation rate) is linear (Fig. 3), in agreement with the Levich equation for diffusion transport control [22]. The latter allowed an accurate calculation of the number of electrons transferred; n was found to be 2.3. The value of n suggested that the “parallel’ mecharrism did not take place at a substantial rate in this case, while the absence of even a trace of the second wave up to - 1.1 V implied that hydrogen peroxide was the major final product of the reduction of oxygen. Examination of the literature reveals a variety of interpretations for oxygen reduction data as well as inconsistencies. Two waves were observed in H,SO, and KNO,, both before and after electrochemical treatment [6,16]; comparison of these
0
-112/ra&112
5112
Fig. 3. The dependence of the limiting currents of the oxygen reduction on the rotation rate; data from Fig. 2.
165
data, however, shows a difference in current density by a factor of two. A study of GC following treatment in chromic sulfuric acid [lo] reported a single wave, an n value of two, and a limiting current density which corresponds well with ours. Shifts of a similar magnitude to those found here in hydrochloric acid were observed in RDE studies of this reaction in sulfuric acid [6] and in potassium nitrate solutions [16], while slightly smaller changes of the oxygen reduction peak potentials in the cyclic voltammograms of untreated samples were found in hydrochloric acid
WIAn increase of the background current similar to that reported in the literature (see e.g. ref. 14) was observed after electrochemical treatment. Although the surface roughness may have increased, the fact that the change did not correspond to an equivalent increase in the limiting current for oxygen reduction (Fig. 1) suggests strongly that the true surface area remains the same. XPS surface ana&ses The XPS data for “as received” polished GC samples, prior to any contact with the solution, revealed the presence of trace amounts of polishing material in the form of Sic (l-2 at.%). No alumina was found on the polished glassy carbon surface. This finding is in agreement with Kamau et al. [17], who have demonstrated that contamination due to polishing can be minim&d by elaborate procedures, and with Cabaniss et al. [18], where polishing with alumina in deionized water was followed by treatment in an ultrasonic bath of deionized water. The high resolution photoelectron spectra of the Cls region of “as received” polished glassy carbon samples were reproducible. The average oxygen to carbon ratio derived from the XPS spectra of polished GC is about 0.23 f 0.04, which is close to the ratio (0.17) reported by Kamau et al. [17], and between the values (0.08 and 0.33) found elsewhere [16,18]. The full width at half maximum of the Cls photoelectron line, was 2.2 eV. On the basis of a curvefitting analysis, the surface of polished GC was composed of only three resolvable forms of carbon associated with graphitic, phenolic and carboxylic functional groups, at average concentrations of 68%, 25%, and 7% (atomic), respectively. This is again in good agreement with the data reported elsewhere [17,18]. The anodization of GC for 15 min at 2 V resulted in an obvious increase in the oxygen photoelectron peak intensity and in an additional peak at 199.9 eV due to the incorporation of chloride. The average chloride content in the surface layer was found to be = 7%. The atomic O/C ratio increased from 0.23 for untreated to 0.40 f 0.04 for the treated samples. This value falls between the ones reported for a similar treatment: 0.22 after anodic treatment at 1.75 V for 5 min in KNO, [16] and 0.89 for 30 mm anodization at 1.8 V in 0.1 M H,SO, [18]. The extent of oxidation caused by the anodization is larger than that on GC electrodes that have been subjected to oxygen plasma treatments (O/C ratio 0.33) [21]. The Cls high resolution spectrum for an anodized sample is shown in Fig. 4. The full width at half maximum of the carbon photoelectron peak was 4.0 eV in comparison to 2.2 eV as given above for untreated samples. This is an excellent
166
J
BlNDING ENERGY /eV Fig. 4. Cls XPS spectrum for &ssy carbon anodii in 0.1 M HCl (2 V, 15 min); (1) graphitic, (2) phenol&,(3) carbonyl, and (4) carboxylic type of carbon.
indication of the presence of more than one carbon functional group on the surface. Also, it could suggest the creation of additional oxygen-containing carbon functional groups during the anodization period. The major peak in this spectrum is located towards lower binding energies, at 284.4 eV, which is assigned to a graphitic or tetrahedral form of carbon. A distinct shoulder at higher binding energies, apparent in this spectrum, was more prominent than for polished, untreated samples. Three additional peaks were found by deconvolution of the shoulder in the case of anodized GC, as compared to two on the untreated ones. The noticeable buildup of oxygen on the CC surface upon anodization resulted in the appearance of a new, carbonyl functional group, in addition to the already existing groups. Relative contributions of graphitic, phenolic, carbonyl and carboxylic types of carbon to a total carbon photoemission are 46%, 16% 26% and 12% (atomic), respectively. A description of the peaks (see Fig. 4) which characterize the groups is as follows: the first, at 284.4 eV, is assigned to a graphitic carbon, the second, at 285.6 eV, is ascribed to a type of carbon in phenol, alcohol or ether, the third, at 287.0 eV, to a carbonyl or quinone type, and the fourth, at 288.6 eV, to carboxyl. We found the shifts of oxygen-containing carbon functional groups corresponding to the position of graphitic carbon to be, within the error introduced by the curve-fitting routine, in acceptable agreement with the vaIues obtained from the studies of native and pl~rna-~~~ polymers [21]. There is no report in the literature of a predominance of carbonyl groups produced by anodic treatment of glassy carbon. The use of chloride media in our study suggests some role for the chloride ion (or chlorine evolved during anodization) in the formation of carbonyl functional groups. Elemental chlorine present at the GC surface at the positive potential of 2 V could facilitate the formation of carbonyl groups via oxygen transfer [23] rather than increasing the content of phenolic carbon. Cabaniss et al. [18] reported an O/C atomic ratio of 0.89 in H,SO,. In spite of a high oxygen content, ascribed to three of the same functional groups as in our case, it was found that the major contribution was coming from the phenolic carbon.
167 7 6-
2-
lo
-260
-266
BINDING
-266
ENERGY
-2.94
-282
leV
Fig. 5. Cls XPS spectrum for reduced glassy carbon surface; same notation as in Fig. 4.
The high resolution Cls XPS spectrum for glassy carbon which has been oxidized and subsequently reduced at - 1 V is presented in Fig. 5. There is a striking resemblance of this spectrum to the one for untreated polished GC. The excess amount of oxygen introduced on the glassy carbon surface by anodization was almost completely removed with 5 min of reduction (O/C = 0.27 f 0.02). The chloride content was decreased substantially, as well, to less than 4%. The electrochemical reduction at - 400 mV, in the region of the oxygen half-wave potential, led to the same decrease in the number of oxidized surface sites, reversing the spectrum almost back to the original spectrum of a polished unactivated surface. The carbon peak was again composed of only three resolvable forms of carbon. The peak attributed to the presence of a carbonyl group disappeared as a consequence of the electrochemical reduction and the relative contributions of graphitic, phenolic, and carboxylic carbon to a total carbon photoemission for the reduced GC changed to 678, 23%, and 10% (at.) respectively. Similar responses to electrochemical reduction of the GC surface were observed in different solutions and for significantly shorter reduction times [16,17]. CONCLUDING
REMARKS
The discussion given along with the presentation of the experimental results included several important concepts. Three distinctive electrochemical features of the oxygen reduction curves in hydrochloric acid on electrochemically treated glassy carbon are: (i) the single wave followed by (ii) a wide horizontal plateau and (iii) the magnitude of the limiting current that obeyed the Levich equation. The latter allowed an accurate determination of the number of electrons transferred (n = 2.3), but the place of glassy carbon among materials characterized by a two-electron process in acid media [24] has yet to be established. It is tempting to correlate the electrocatalytic effect with the appearance of the carbonyl surface group during the positive polarization, regardless of its absence
168
from the XPS spectra at the reduced surfaces. The spectra, giving almost the same picture of the untreated and the reduced glassy carbon surface quantitatively, offer no other clue for distinguishing the significant changes due to the treatment-changes that lead to the catalysis of the electrochemical reduction of oxygen. Finally, there was no role that could be assigned to the presence of chlorides at the surface of glassy carbon during the oxygen reduction easily. One may speculate that the mechanism for the generation of carbonyl surface groups during the anodization involves elemental chlorine as a mediator for oxygen transfer to the carbon atoms. The correlation between the XPS data and the role of chlorides in modifying the electrochemistry of oxygen reduction seems to be a promising starting point for further study of the mechanism of oxygen reduction in acidic chloride environments. ACKNOWLEDGEMENTS
This work has been performed under an IBM (Rochester, Minnesota) University Program Grant. Lj.A. and R.A. are grateful for the support of the Yugoslav-American Fund for Scientific Cooperation, D.O.E. Grant 675. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
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