They were recorded at one selected scan rate (r= 10 mV s- 2, and after shape stabilization (ca 10 cycles). In particular, the cyclic curve shapes were found to.
Carbon Vol.33, Copyright
Pergamon
No. 9, pp. 125%1263,1995 0 1995 ElsevierScienceLtd
Printed in Great Britain.AI1rights reserved OOOS-6223/95 $9.50 + 0.00
0008-6223(95)00069-O
THE ELECTROCHEMICAL BEHAVIOUR OF CARBON FIBRE ELECTRODES IN VARIOUS ELECTROLYTES. DOUBLELAYER CAPACITANCE STANISLAW BINIAK, BO~ENA DZIELE~DZIAK and JANUSZ SIEDLEWSKI Faculty of Chemistry, Nicolaus Copernicus University, Gagarina 7,87-100 Toruti, Poland (Received 25 January 1994; accepted in revised form 7 March 1995)
Abstract-The electrochemical properties of carbon fibres (CFs) differentiated by heat treatment at temperatures up to 2400°C and oxidation in concentrated nitric acid were studied. Changes in the shapes of cyclic voltammetric curves for working electrodes prepared from carbon fibres (CFs) were analysed in aqueous and nonaqueous electrolytes. Possible ranges of the sweep potential were estimated for the electrochemical systems studied. Changes in the value of the double-layer capacitance of the carbon fibre electrodes are discussed. Key Words-Cyclic
voltammetry, carbon fibre, double-layer capacitance.
1. INTRODUCTION
of five PAN-based carbon fibres differentiated by heat-treatment temperatures (HTT) (produced by AGH, Krakbw, Poland)[8,9]. Samples of CFs were oxidized by treatment with 65% HNO, at 80°C for 1 hour and were identified as CFllOO, CF1250, CF1400, CF2000, CF2400 in unmodified form, and CFllOOox, CF12500x, CF14000x, CF20000x, CF2400ox after oxidative modification respectively. Basicity and acidity were measured for ca 0.1 g samples of CFs with 10 cm3 0.1 M electrolytes (HClO, and LiOH respectively). pH values were measured by bringing the glass electrode into contact with the fibre bundles in 0.1 M LiC104. The results obtained are set out in Table 1. The working electrode consisted of a fibre bundle (ca 10000 filaments, length 3 cm, plunged to 1 cm). The electrical resistance of the carbon fibre bundles was measured using a digital multimeter M-4650CR (Metex Instruments, Korea). The geometrical area (0.26-0.29 m2 g-‘) was calculated from the density (1.7-1.9 g cmm3) and diameter (8 pm) of the filaments, and the specific surface area (S,,,) determined by adsorption of N, at 77 K conducted volumetrically in an automatic Gemini 2364 Sorptomate (Micromeritics, U.S.A.).
Because carbon fibres exhibit an advantageous blend of chemical, electrical and thermal properties, they are very often considered for electrochemical applications. The double layer capacity and electrochemical activity of the carbon fibre electrolyte system depends not only on the surface area and pore structure, but also on the surface functionality[l-61. The influence of the bulk properties of carbon electrodes in electrochemistry is not clear and is highly dependent on the method of preparation. Therefore, the electrochemical properties of carbon fibres depend mainly on their thermal history and chemical pretreatment mode. The surface-active sites and surface functionalities participate in the creation of the double layer, but their role in this process is still open to question. The aim of this paper is to report on the cyclic voltammetric characteristics of carbon fibre electrodes manufactured at various temperatures. The influence exerted by oxidative treatment has also been taken into consideration. 2. EXPERIMENTAL 2.1 Chemiculs Reagent-grade LiOH, HC104, LiClO,, KPF, and tetrabutylammonium bromide (TBAB) were used as received. All aqueous solutions were prepared in laboratory-double-distilled water. All analyticalgrade organic solvents-acetonitrile (ACN), dimethylformamide (DMF), ethanol, and propylene carbonate (PCFwere dried in the manner described in the literature[7]. The concentration of all the electrolyte solutions used was 0.1 M (0.1 mol dmm3). 2.2
Working electrode
materials
In order to present a generalized picture, ten different types of carbon fibre materials are defined (see Table 1). They were obtained by the oxidation
2.3 Apparatus and cell The cyclic voltammetric experiments were performed with a typical set-up: a linear sweep generator (type EG20) and a potentiostat (type EP22), both from Elpan (Poland). Current-potential curves were recorded on an X-Y 4106 recorded (Laboratorni Pristroje, Czechoslovakia) as well as an IBM PC via an AD-DA (14 bit) converter. The data obtained were used to calculate the area enclosed by CV curves and to estimate average current values. The electrochemical cell was a 50 cm3 glass vessel with a teflon stopper, in which holes had been drilled for the inert gas purge and for the working and reference electrodes. The counter-electrode was a Pt-plate at
1255
S. BINIAK etal.
1256
Table 1. Characteristics
S BET CF
Crystallographic parameters” [nm]
Functionality Electrical
Cwollgl
resistance
MOO2)
InI
pHb
Basic
Acidic
7.08 15.3
0.36 _
1.0
1.05 2.10
0.35 _
1.5 _
0.345 _
2.0
0.342 _
3.6
127 92 103 72 70 36.3 11.0 6.15 3.10 0.98
5.88 5.64 6.22 6.14 6.22 6.16 6.36 6.26 6.23 6.23
193 103 178 92 130 89 115 72 62 50
30 126 25 113 24 95 16 50 10 44
0.59 1.81
a From Ref. [9]. b In 0.1 M LiClO,
(blank
pH ~6.08).
the bottom of the vessel. All experiments were carried out in helium-saturated solutions. Potentials were measured and are quoted with respect to a saturated calomel electrode (SCE). A salt bridge was used for the non-aqueous solutions. Cyclic voltammograms were recorded after the shape of the recorded curves had been established.
3. RESULTS
fibres investigated
d(002)
[m* g-l]
1100 11000x 1250 12500x 1400 14000x 2000 20000x 2400 24000x
of carbon
AND DISCUSSION
3.1 Aqueous electrolytes The carbon fibre electrodes were prepared at five different temperatures, as a result of which they had different chemical structure, textures, graphitization stages and conductivity (Table 1). Additionally, they differed from each other in chemical resistance to oxidizing agents and the structure of the surface oxides produced. When the surface of a carbon material is oxidized, a series of functional groups is formed thereon. The oxidation of different carbon materials creates surface oxides and also threedimensional oxide defects, consisting mainly of carboxylic and phenolic groups[ l-61. In aqueous solutions of electrolytes, electrochemical activity may be attributed to the quasi-quinone-hydroquinone surface systems. The results of studies of such systems with respect to the carbon fibres used in this work have already been reported [ lo]. The characteristics of the physico-chemical properties of CF electrodes in aqueous solutions of different electrolytes are shown in Table 1. Negligible changes in the pH of the neutral solution during carbon fibre immersion and the values of acid and base adsorption from aqueous solutions point to the insignificant development of the carbon fibre surface and the slight acidic-basic properties of surface oxides[ l,lO-201. We observed a decrease in the reactivity of carbon fibre surfaces with increasing carbonization-graphitization temperatures. A greater degree of surface functionality is developed for materials heat-treated at lower temperatures (both before and after oxidizing treatment). Similar effects were
observed for low-modulus carbon fibres in relation to high modulus fibres after anodic oxidation (detected by XPS)[ 191. The very significant effect of HTT and oxidative modification of carbon electrode materials on the shape of the cyclic voltammograms can be also confirmed. Figures l-3 show the CV curves recorded for all the carbon fibres in aqueous solutions of acidic, neutral and basic electrolytes respectively. They were recorded at one selected scan rate (r= 10 mV s- 2, and after shape stabilization (ca 10 cycles). In particular, the cyclic curve shapes were found to be variable for as-received (nonmodified) carbon fibres graphitized at temperatures above 2000°C. By way of example, the first cyclic run (dashed line) is marked on Fig. l(g). Such behaviour appears to be typical of freshly received, heat-treated carbon materials[20&23] and may be explained by the ageing processes on carbon surfaces as a result of irreversible chemisorption of both oxygen and water, and by the products of electrochemical processes. Ageing yields results analogous to a mild oxidation[23], which explains the similarity in electrochemical behaviour of oxidized and nonmodified carbon materials. The presence of surface species influences the formation of the double electric layer, mainly by the specific adsorption of ions of electrolyte and solvent molecules. In aqueous solutions, some processes connected with the hydrolysis of surface oxides (anhydride-, lactoneand ester-like) also take place[6]. The cyclic voltammetry studies of carbon fibre electrodes in aqueous, neutral solutions indicate that the double layer capacity of oxidized carbon increases at negative potentials, while at positive potentials it decreases relative to that of the unoxidized sample[24]. Our experiments (Fig. 2) only partially confirmed these observations (in fibres carbonized at 11OOG14OO”C). It seems that a faradic process (or processes) with a broad, overlapping potential range(s) take(s) place in aqueous electrolytes which makes it impossible to assess accurately the double-
The electrochemical behaviour of carbon fibre electrodes
I,, _ O,Od
-0.I
/
+o.s
E,”
I -0,s
Fig. 1. Cyclic voltammograms of carbon fibres in aqueous HCIO, (0.1 M): (a) CFllOO, (b) CFllOOox, (c) CF1250, (d) CF1250ox, (e) CF1400, (f) CF14000x, (g) CF2000, (h) CF20OOox,(i) CF2400, (j) CF2400ox. Scan rate: 10 mV s-l.
layer capacity of the carbon electrodes in these environments. Because the electroactive species are bonded to or adsorbed on the carbon surface, only summed values of the double-layer capacity (C,) and “surface pseudo-capacity” (C,) can be obtained from the experimental relationships between total currents and scan rates. This may explain the experimental
difficulty in obtaining identical values of the doublelayer capacity for carbon fibre electrodes received from different sources. Additionally, with increasing HTT, the shape of the cyclic voltammograms gradually becomes more skewed for all the aqueous electrolytes studied. This indicates that the effective resistance of the electrode
S. BINIAK~U~
il
Fig. 2. Cyclic voltammograms of carbon fibres in aqueous LiCIO, (0.1 M):carbon fibres as in Fig. 1
system decreases and the interphase becomes more nonpolarizable. The electrical resistance of the tibre bundles (see Table 1) decreased with increasing HTT[ l,S]. These differences have been attributed to the degree of perfection of the graphitic layer structure, because the defects (e.g. vacancies, boundaries, etc.) affect the
electronic structure and the degree of preferred orientation (conduction is predominantly along the layers). The electrical conductivity increased appreciably after treatment with concentrated nitric acid, which is probably due to partially nonreversible intercalation of CFs and surface oxidation with the formation of a graphite oxide layer [ 25,261.
The electrochemical
Fig. 3. Cyclic voltammograms
of carbon
behaviour
of carbon
fibres in aqueous
Thus, the more complex shape of the cyclic curves recorded, especially for fibres graphitized at temperatures above 2000°C (additional cathodic and anodic peaks present) is probably due to the formation of intercalation compounds during the oxidizing pro-
fibre electrodes
LiOH
(0.1 M): carbon
1259
fibres as in Fig. 1.
cedure and during cycling. These processes additionally complicated the systems studied and made it impossible to find the potential limits with “pure” coulombic current in a simple way. Thus, the double layer capacitances obtained by CV methods for
S. BINIAK etal.
1260
Fig. 4. Cyclic voltammograms of carbon fibres in ethanol containing (b) CFllOOox, (c) CF1400, (d) CF14000x, (e) CF2400, (f) CF2400ox.
fibre materials in aqueous solutions (i.e. for carbon clothing[27,28]) are probably only of an approximate character. carbon
3.2 Nonaqueous
electrolytes
The polarization potential ranges of the CFs studied (cathodic and anodic background limits) obtained for different solvents and various supporting electrolytes do not conform to the values obtained for such systems with other electrode materials (metals)[29]. For example, typical voltammograms recorded for selected CFs in the ethanol/LiClO, system are shown in Fig. 4. The presence of electroactive systems (peaks) in the -0.5-0.7 V potential range for CFs in alcoholic solvents was observed[ lo] and they were described as quais-quinonehydroquinone systems. Only in aprotic environments was no electroactivity observed over these potential ranges. This fact was confirmed for all the systems examined. Table 2 summarizes the estimated potential ranges when no Faraday processes occur (charging
0.1 M LiClO,: (a) CFllOO. Scan rate: 10 mV SKI.
of double layer potentials). The procedure to estimate these values is shown as an example in Fig. 5. The gradual increase of the scan potential range caused the appearance of a cathodic wave (for E, near -0.5 V) and gives a response to the anodic potential in range where an anodic wave appears (for Ea near + 1.7 V). Because the working range of the electrolyte solution used (0.1 M KPF, in PC) with metallic electrodes is significantly wider[ 291, the formation of intercalation compounds is probable. Their nature requires further study. The dependence of anodic and cathodic charge currents on scan rates was investigated for all the potential ranges collected in Table 2. Figure 6 shows typical CV curves for one selected electrochemical systems (TBAB in DMF). Ideally, the box representing the anodic and cathodic charging currents should be symmetrical around zero. The cyclic curves presented in Fig. 6 (for different scan rates) show the real interphase, with a finite faradic current. Figure 7 shows the relation between the average
The electrochemical
behaviour
of carbon
fibre electrodes
1261
Table 2. The estimated potential ranges of double-layer charging in the electrochemical systems studied (J’ vs SCE)
0.1 M Lic104 CF/solvent 1100 11000x
ACN
PC
DMF
DMF
PC
-0.5-1.0
-0.8-1.4 -0.8-1.4 - 0.5-0.7 -0.5-0.7 -0.5-0.7 - 0.4-0.6 - 0.4-0.7 - 0.4-0.7 -0.5-0.6 -0.6-0.6
0.2-1.0 0.5-1.0 - 0.5-0.5 -0.3-0.5 - 0.4-0.4 - 0.5-0.4 -0.4-0.3 -0.1-0.4 -0.5-0.4 -0.5-0.4
- 0.9-0.5 - 1.1-0.6 - 0.6-0.5 -0.6-0.5 -0.4-0.5 - 0.4-0.5 - 0.4-0.5 - 0.2-0.5 -0.1-0.5 -0.2-0.5
-0.9-1.5 -0.9-1.3 - 0.5-0.8 - 0.5-0.7 -0.5-0.7 - 0.7-0.9 -0.5-0.7 - 0.4-0.7 - 0.4-0.7 - 0.4-0.7
-0.5-1.0 -0.4-1.0 -0.4-1.0 -0.4-1.0 -0.3-0.9 0.0-0.8 0.1-0.7 -0.3-0.7 - 0.3-0.5
1250 12500x 1400 14000X
2ooo 2oooox 2400 24000x
0.1 M KPF,
0.1 M TBAB
a-
il
b
-.-
c
---
d
..___
ii
ri
e -..-
I
SOpA
._._-- --__ _..__...:~.~;/
-0,s I.c-
__ ~
4:
.+j$
,.= .. ,_+z+=1 .._._.--.--*.-++;
;
I ,I
Fig. 5. Voltammetric curves recorded for CF2000 in PC/KPF6 (0.1 M) for different scan amplitudes: (a) 0.5-0.7, (b) -0.5-1.2, (c) -0.5-1.7, (d) -0.7-1.7, (e) -0.5-1.8. Scan rate: 10 mV s-r.
I w _.___ . (VmA
__--
_.._a--_
. .
.
...--.’ .’
,_*em .-_ - __----_-_
‘/ I L_*_._.-.-
-i,o
__--_
_;; ,‘I
.-.-._.__
_ ._ ._-. -.-.+y& _ / 7:’/ ~~___------~_~__...-I __.._-.-__.’ : _.____.---L-. 4 -
b)
-.-
cl---
dj
E,V
._.___
Fig. 6. Current-potential curves for CFllOO electrode in DMF,/TBAB (0.1 M) solution. (a) 3.33 mV s-r, (b) 10 mV s-r, (c) 33.3 mV s-r, (d) 66.7 mV s-r. Scan amplitude: -0.9-0.5
double-layer charge current and the scan rate for selected CF electrodes and aprotic electrolyte solutions. The average current (i) was calculated as the U\R33:9-o
Scan rates: V (vs SCE).
quotient of the area enclosed by the curves (S,,) and the doubled scan amplitude (AE), (i=S,,AE-‘). For the majority of the electrochemical systems
S. BINIAK
1262 iXE
[m Al
a,
E=/O-’ -
.
b,
E=fP
-
0
Cj E=fOP -
a -
dj
E=2.10-Y
+
‘?
1
0,oz
0.06
”
[vs-‘1
Fig. 7. Sweep rate dependence of CFs in different electrochemical systems: (a) CFllOO in DMF/TBAB (0.1 M), scan amplitude -0.9-0.5 V; (b) CF1400 in ACN/LiCl04 (0.1 M), scan amplitude -0.4-1.0 V;(c) CF1400in PC/KPF, (0.1 M), scan amplitude -0.4-0.7 V; (d) CF2400 in ACNjLiCIO, (0.1 M), scan amplitude -0.3-0.7 V.
a more or less linear deviation with increasing scan rate is observed. Figure 8 shows the change in the double-layer capacitance of CF electrode (in ACN/LiClO,) as their heat-treatment temperature increases. The capacitances have been estimated for scan rates below 10 mV s- ’ (linearity of i vs c-see Fig. 7). The obtained double-layer capacitance values of the tested carbon fibres depends on their carbonization/graphitization temperatures. In relation to other carbon materials, the estimated capacitances of carbon hbres heat treated at llOO-1400-C are similar to the values obtained in aqueous solutions for pyrolytic graphite electrodes (AC impedance technique)[ 30,3 I ]. Carbon fibres graphitized at high temperatures (above 2000’C) exhibit a double layer capacitance comparable with graphite cloth electrodes in aqueous solutions of electrolytes[27,28]. The observed difference in capacitance of oxidized carbon fibres in relation to nonmodified fibres (higher for low-temperature investigated,
et al
samples) can be attributed partly to the increase in specific surface area and partly to the formation of a surface oxide layer. These studies suggest that a relationship exists between the heat treatment temperature of the precursor material and the double-layer capacity in nonprotonic electrolytes. The determination of Cd for a carbon fibre electrode yields a parameter that allows the comparison of different electrodes and provides information about their thermal history. We consider that the method described is a simple way of obtaining well-characterized carbon fibre electrodes, which can be very useful when used as analytical sensors and for adsorption experiments.
4. CONCLUSIONS
The behaviour of carbon fibre electrodes, especially in nonaqueous electrolytes, strongly depends on their thermal history. The observed decrease in double-layer capacitance with increasing heattreatment temperature is probably caused by structural changes in the carbon materials (e.g. graphitization) and changes in their surface chemical structure. The presence of electrochemically active species and the slow rate of charge homogenization in electrode materials may be responsible for the observed effects. According to [ 321, the low double-layer capacitance is due to the low density of electronic states around the Fermi level of graphite. The electronic properties of different carbon fibres as a function of HTT indicate that heat-treatment around 175O’C is a more significant temperature for affecting changes in the electronic structure and in the development of electronic properties. The observed effects may also due to the electroadsorption of ions and molecules on the surface of the electrode materials as well as to the formation of various intercalation compounds. These problems require further study and will be the subject of subsequent papers. Acknowlr&cments -We thank Dr S. Blaiewicz.
Institute of Material Science, Academy of Mining and Metallurgy, Cracow, Poland for supplying the carbon hbre samples. This work was supported by the Committee for Scientific Research (KBN) (Grant No. 208629101).
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Fig. 8. Double-layer capacitance for carbon fibres in 0.1 M LiClO,/ACN solution as a function of their heat-treatment temperature: (a) unmodified CFs, (b) oxidized CFs.
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