Electrochemistry Communications 6 (2004) 313–318
www.elsevier.com/locate/elecom
Electrochemical characteristics of nanoporous carbide-derived
carbon materials in non-aqueous electrolyte solutions
A. J€
anes
a
a,b
, L. Permann
a,b
, M. Arulepp
a,b
, E. Lust
a,*
Institute of Physical Chemistry, University of Tartu, 2 Jakobi Street, 51014 Tartu, Estonia
b
Tartu Technologies Ltd., 185 Riia Street, 51014 Tartu, Estonia
Received 17 December 2003; accepted 8 January 2004
Published online: 3 February 2004
Abstract
Electrochemical characteristics for the 1 M (C2 H5 )3 CH3 NBF4 + acetonitrile (AN)jcarbide-derived carbon nanoporous (prepared
from TiC, a-SiC, Mo2 C, Al4 C3 and B4 C) interface have been established by cyclic voltammetry and electrochemical impedance
spectroscopy. The gas adsorption measurements have been used for the obtaining the specific surface area, pore size distribution,
nanopore volume and other characteristics, dependent on the precursor carbide used (nanopores are pores in the range of 2 nm and
below – i.e. micropores according to IUPAC classification). The region of ideal polarizability, values of series and parallel capacitances, parallel resistance and capacitance and other characteristics dependent on the precursor carbide have been established.
Ó 2004 Elsevier B.V. All rights reserved.
Keywords: Carbide-derived carbon; Microporous carbon; Nanoporous carbon; Double layer characteristics
1. Introduction
2. Experimental
Electrical double layer capacitors (EDLCs) are the
very important energy storage and conversation systems, which can be used in the various pulse energy
generation systems. The EDLC are based mainly on the
electrostatic interactions determining the very good reversibility and cyclability of EDLC systems [1–6]. For
the optimal energy density–power density regime, the
nanoporous carbon material characteristics have to be
optimised [1–10]. This paper reports the results of the
systematic studies of the interface: non-aqueous electrolytejnanoporous carbide-derived carbon, prepared
from different precursor carbides, TiC, Mo2 C, a-SiC,
Al4 C3 and B4 C. More detailed theoretical background
and analysis of EDLC is given in [1–11].
The electrodes were constituted from an aluminium
foil current collector and the active material layer. The
active material used consists of nanoporous carbon
prepared from TiC, Mo2 C, a-SiC, Al4 C3 and B4 C (noted as C(TiC), C(Mo2 C), C(a-SiC), C(Al4 C3 ) and
C(B4 C), respectively) by the chlorination method according to the preparation scheme presented in [1–6]), of
the mixture of binder (poly-tetra-fluoro-ethylene –
PTFE, 60% solution in H2 O ‘‘Aldrich’’) and of the
carbon black (‘‘Aldrich’’). This mixture was laminated
on the Ni foil and pressed together to form a very
flexible layer of the active electrode material. After
drying and plating under vacuum, the pure Al layer has
been vacuum-spray evaporated onto the one side of the
carbon material [1–6]. After that the Al-covered nanoporous carbon layer was spot-welded in the Ar atmosphere to the Al foil current collector.
The electrolytes used were prepared from the pure
acetonitrile (AN) (H2 O < 0.003%) (‘‘Riedel-de Ha€en’’)
stored over molecular sieves before use, and from very
*
Corresponding author. Tel.: +372-7-375-165; fax: +372-7-375-160.
E-mail address: [email protected] (E. Lust).
1388-2481/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.elecom.2004.01.009
314
A. J€anes et al. / Electrochemistry Communications 6 (2004) 313–318
dry (C2 H5 )3 CH3 NBF4 (Stella Chemifa Corporation),
additionally purified and tested [1–6]. The three electrode standard glass cell with a large counter electrode
(apparent area 30 cm2 ), prepared from the carbon
cloth, was used. The reference electrode was an aqueous
saturated calomel electrode (SCE) connected through
the electrolytic salt bridge (0.1 M LiClO4 in H2 Oj1 M
(C2 H5 )3 CH3 NBF4 in AN) with the measurement system
[7]. Pure Ar (99.9999%, AGA) was used for saturating
the solutions. Specific surface area (SQ ) (Table 1), pore
size distribution (Fig. 1), micropore volume (Vm ), micropore area (Sm ) and other parameters were measured
using the Gemini 2375 (Micromeritics, Inc.) system and
calculated according to the density functional theory
(DFT) and other methods [8]. According to the data in
Table 1 and Fig. 1, the specific surface area and the
nanopore size distribution depend noticeably on the
carbide used. For C(a-SiC) there are practically only
nanopores at the surface but for C(Mo2 C) there are no
nanopores on the surface at all. Thus, the total pore
volume Vtot and micropore volume Vm values depend
noticeably on the precursor carbide used. These results
obtained are in a good agreement with the data of [9].
The cyclic voltammetry (j; E)-curves for C(TiC),
C(Al4 C3 ), C(Mo2 C) and C(B4 C) electrodes in 1 M
(C2 H5 )3 CH3 NBF4 solution (Fig. 2(a)), obtained at small
scan rates of potential v ¼ dE=dt 6 5 mV s1 , have
nearly mirror image symmetry of the current responses
about the zero current line (j – current density, obtained
using the flat cross section (geometrical) surface area).
The current density values at E > 0:5 V increase in the
order of materials C(B4 C) < C(Mo2 C) < C(Al4 C3 ) 6
C(TiC) < C(a-SiC). The current densities obtained for
C(a-SiC) are very small at E < 0:3 V and there are
very well-expressed distortion effects [10,11] caused by
the very small micropore diameter DDFT values for this
electrode material (Table 1). Thus, it can be concluded
that the micropores with medium diameter DDFT 6 0:7
nm are to small for the effective adsorption of big
(C2 H5 )3 CH3 Nþ cations into the nanopores. The experimental data show that the shape of j; E-curves is independent of the number of current cycles, n, if n P 3.
Fig. 1. Pore size distribution for nanoporous carbon prepared from
different precursor carbides [6], noted in figure.
Thus, the nanoporous carbon electrodes demonstrate
the stable electrochemical properties in the region of
ideal polarizability from )1.4 to 1.4 V (SCE). With increasing v ¼ 50 mV s1 (Fig. 2(b)) the cyclic voltammograms for C(a-SiC), C(TiC), C(Al4 C3 ) become
distorted from the mirror image symmetry [1–4]. More
symmetrical j; E-curves have been established for
C(Mo2 C) and C(B4 C) materials. Thus, at higher scan
rates there are the very well-expressed distortion effects
in the j; E-curves, caused probably by the significant
resistivity of the electrolyte in the porous material, as
well as by the high resistivity of porous electrode material discussed later (i.e. by the significant series resistance of the experimental system) and in [1–6]. Thus
comparison of data in Fig. 2(a) indicates that at v P 20
mV s1 there is noticeable dependence of j; E-curves on
the precursor carbides from which the nanoporous
carbon material was prepared. Analysis of experimental
data [1–3] demonstrates that the solution resistance is
important but, on the other hand, the establishment of
the adsorption equilibrium in the nanopores is a very
slow process (see discussion and results presented later),
which is caused by the very small ‘‘effective’’ diffusion
coefficient values of ions in nanopores influenced by the
effective diffuse layer thickness at the electrode surface,
as well as by the increase of the effective Debye screening
length with the dilution of the electrolyte [1–4].
Table 1
Pore characteristics of selected carbon materials [5,6]
Parameter
Crystal structure of
precursor carbide
Sa (m2 g1 )
Sm (m2 g1 )
Vm (cm3 g1 )
Vtot (cm3 g1 )
DDFT (nm)
Precursor carbide
a-SiC
TiC
Mo2 C
B4 C
Al4 C3
Rhombohedral
Cubic
Hexagonal
Rhombohedral
Rhombohedral
1085
1030
0.46
0.49
0.7
1505
1205
0.60
0.75
0.8
1490
0
0
1.50
4.0
1525
930
0.43
0.99
1.3
1470
1130
0.57
0.74
0.8
Sa , BET surface area; Sm , micropore surface area; Vm , micropore volume; Vtot , total pore volume; DDFT , median pore diameter calculated
according to density functional theory method.
A. J€anes et al. / Electrochemistry Communications 6 (2004) 313–318
Fig. 2. Current density vs. potential curves for nanoporous carbon
(prepared from different precursor carbides: 1-C(a-SiC), 2-C(TiC),
3-C(Al4 C3 ), 4-C(Mo2 C) and 5-C(B4 C)) in 1 M (C2 H5 )3 CH3 NBF4 +
AN solution at potential scan rates 5 (a) and 50 mV s1 (b).
Fig. 2(a) shows that, in the j; E-curves obtained, there
is a minimum of current density in the region of potentials 0 6 E 6 0:5 V (SCE), increasing with the dilution
of electrolyte explained by the diffuse nature of the
electrical double layer in the region of zero free charge
[1–4,7,13,14]. The potential of this minimum is practically independent of the direction of potential scan if
v 6 5 mV s1 and thus for 1 M (C2 H5 )3 CH3 NBF4 this
potential Emin characterises the adsorption equilibrium
at the conditions of zero surface charge density r 0.
The C; E-curves calculated from j; E-curves have a very
well expressed capacitance minimum Cmin with the po-
315
tential of minimum Emin , given in Table 2. According to
the experimental data for 1 M (C2 H5 )3 CH3 NBF4 + AN
solution for all materials studied, the capacitance is
practically independent of v in the limited region of Emin
potential (corresponding to the zero free charge potential
Er¼0 [1–4,7,15–17]). At E > 0:5 V and E < 0:5 V (SCE)
there is a noticeable dependence of C on v, increasing in
the order of nanoporous carbons C(B4 C) < C(Mo2 C) <
C(TiC) < C(Al4 C3 ) < C(a-SiC). At E < 0:5 V, where
the adsorption of very large (C2 H5 )3 CH3 Nþ cations have
to take place, the very low capacitance values have been
established for C(a-SiC). The unsymmetrical shape of
C; E-curves indicates that the specific adsorption of BF
4
anions is very well expressed in comparison with the
(C2 H5 )3 CH3 Nþ cation. According to the experimental
data, the series differential capacitance is independent of f
only at f 6 0:01 Hz (Fig. 3), demonstrating that the almost equilibrium capacitance values (given in Table 2)
have been established. It should be noted that the values
of C established from j; E-curves at v 6 5 mV s1 are in a
good agreement with the Cs values measured at ac frequency f ¼ 5 mHz (Fig. 3 and Table 2) (Cs is the series
capacitance, calculated from the complex impedance
plane (Z 00 ; Z 0 ) plots shown in Fig. 4). At higher frequency, there is a very big dependence of Cs on f , which is
caused by the very small values of ac penetration depth
[1–4] compared with the pore length [2,10,11], as well as by
the essential IR drop at v > 10 mV s1 . The higher values
of Cs at E Emin that at E 6 Emin indicate the weak specific adsorption of BF
4 anions at NPC electrode. The
more pronounced dependence of Cs on f as well as C on v
at E > Emin indicates that the specific adsorption process
of BF
4 anions is very slow on NPCE. The Cs values increase in the order of carbons C(a-SiC) < C(B4 C) <
C(Al4 C3 ) < C(Mo2 C) < C(TiC) as the nanopore volume
increases, except C(Mo2 C) having very big total pore
volume values. Thus the nanopore volume values are
important but the very large total pore volume values can
compensate the absence of nanopores at the carbon
electrode surface.
The complex impedance plane plots (so-called
Nyquist plots, Fig. 4) were measured for nanoporous
carbonj1 M (C2 H5 )3 CH3 NBF4 electrolyte solution interface in the ranges of ac frequency from 5 103 to
Table 2
Electrochemical characteristics of different nanoporous carbons in 1 M (C2 H5 )3 CH3 NBF4 acetonitrile solution
Nanoporous carbon
Emin (V) vs. SCE
Cs (F cm2 )
Cs (F g1 )
Cs (F cm3 )
fmax (Hz)
a
C(TiC)
C(a-SiC)
C(Mo2 C)
C(Al4 C3 )
C(B4 C)
0.31 0.02
0.30 0.02
0.31 0.02
0.30 0.02
0.24 0.02
0.69 0.05
0.14 0.03
0.63 0.05
0.60 0.05
0.47 0.04
98.3 0.8
16.3 0.4
120.0 0.8
82.3 0.5
70.9 0.5
62.2 0.5
12.8 0.3
52.0 0.5
53.0 0.5
41.8 0.4
1990
630
790
1255
1255
0.95
0.73
0.99
0.86
0.82
Emin , minimum potential in C; E-curves; Cs , series differential capacitance at E ¼ 1:4 V (vs. SCE) and at f ¼ 10 mHz; fmax , characteristic
frequency; a, fractional exponent obtained from the slope of the Z 00 ; Z 0 plots in the region of frequencies 0:005 < f < 1:0 Hz.
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A. J€anes et al. / Electrochemistry Communications 6 (2004) 313–318
Fig. 3. Cs ; f -curves for nanoporous carbon (prepared from different
precursor carbides, noted in figure) in 1 M (C2 H5 )3 CH3 NBF4 + AN
solution at potential )1.4 V vs. SCE.
Fig. 4. Complex plane plots for nanoporous carbon (prepared from
different precursor carbides, noted in figure) in 1 M
(C2 H5 )3 CH3 NBF4 + AN solution at potential )1.4 V vs. SCE (a), and
for nanoporous carbon, prepared from Al4 C3 , at different potentials
(V vs SCE, noted in figure) (b).
1 105 Hz and potentials from )1.4 to 1.4 V (SCE) (at
various fixed potentials). The results obtained demonstrate that the shape of Z 00 ; Z 0 -plots depends noticeably
on E as well as on the nanoporous carbon characteristics. The complex impedance plane plot consists mainly
of three parts: (1) of the very noticeably depressed
semicircle at higher ac frequencies (f P 40 Hz) with the
characteristic frequency fmax (given in Table 2), (2) of
the so-called double layer capacitance region at very low
frequencies (f < 1 Hz), obtained by the finite length
effect [1,4,7, 10,11,18–22], and of the not very well expressed so-called porous region in the Z 00 ; Z 0 -plots with
the slope of a0 ¼ 45°, characteristic of the diffusion
limited process in the homogeneous macroporous electrode [1–4,18–22]. The shape of Z 00 ; Z 0 -plots (Fig. 4) at
f > 40 Hz indicates that there are two limiting processes
(the slow diffusion and slow heterogeneous adsorption
steps), obtaining the rate of adsorption process of the
electrolyte at the nanoporous carbonjsolution interface
at f > 40 Hz [1–6,10,11,18–21]. The value of so-called
charge transfer resistance Rct depends on the electrode
potential and increases in the order Rct ðE ¼ 1:4 VÞ <
Rct ðE ¼ 1:4 VÞ < Rct ðEmin Þ:
According to the data in Table 2 the relaxation frequency for the kinetically mixed process depends noticeably on the nanoporous carbon and fmax decreases in
the order C(TiC) > C(B4 C) P C(Al4 C3 ) > C(a-SiC) >
C(Mo2 C). Thus the characteristic relaxation time
sexp ¼ ð2pfmax Þ1 increases in the presented order of the
nanoporous carbon materials.
The dependence of the slope of the Z 00 ; Z 0 -plot on the
electrode potential at f < 1 Hz (so-called finite length
effect region) is mainly caused by the decrease of the
effective screening length of the electrolyte ions with the
increase of the surface charge density at the electrodejelectrolyte interface [1–7,12–14]. It should be noted that the effective diffuse layer thickness as well as the
inverse Debye length of the electrolyte ions as a function
of the electrode rational potential ER ¼ E Er¼0 is
given as jeff ðER Þ ¼ 2=j cosðebER Þ, where jeff ðER Þ and j
are the effective and usual Gouy lengths. According to
the experimental data for (C2 H5 )3 CH3 NBF4 solutions
in AN, the effective size of (C2 H5 )3 CH3 Nþ ion in AN
solution can be taken equal to the crystallographic radii
of this ion (rCR ¼ 0:327 0:002 nm and rCR ¼
0:229 0:002 nm for BF
4 ) [23,24]. Thus, the dependence of the shape of the Z 00 ; Z 0 -plot on E, to a first
approximation, indicates that the effective pore dimension is of the same order of magnitude as the effective
Debye length for the 1 M (C2 H5 )3 CH3 NBF4 + AN system [1–4,21]. According to the results in Fig. 4 the slope
values for the Z 00 ; Z 0 -plots at E P Er¼0 are noticeably
lower than at E 6 Emin , indicating the more pronounced
weak specific adsorption of anions at nanoporous carbon electrode (NPCE) from the AN solutions [1–
4,7,10,11]. The values of a depend on the nanoporous
A. J€anes et al. / Electrochemistry Communications 6 (2004) 313–318
317
carbon used for the preparation of the electrodes and a
decreases in the order of materials C(Mo2 C) >
C(TiC) > C(Al4 C3 ) > C(B4 C) > C(a-SiC) as the total
pore volume decreases, except C(B4 C).
The Z 00 ; Z 0 -plots were used for the calculation of the
values of differential series capacitance Cp (Fig. 5(a)),
(using the parallel Cp ; Rp – equivalent circuit as the classical approximation [1–4,11,18]) series resistance Rs and
parallel resistance Rp values. The values of parallel capacitance only very weakly depend on the nanoporous
carbon material at f > 1 Hz, but at very low frequency
(f 5 mHz) Cp increases in the order C(a-SiC) <
C(B4 C) < C(Al4 C3 ) < C(Mo2 C) < C(TiC). The ratio of
Cp =Cs (Fig. 5(b)) being established very close to unity for
nanoporous carbon at f < 0:05 Hz (except C(a-SiC))
indicates that the nearly ideally polarisable materials and
systems have been developed and practically the limiting
capacitance values have been received. The inflection
frequency for the Cs ; f and Cp ; f plots depends on the
nanoporous carbon studied, and the value of inflection
frequency decreases in the order C(TiC) > C(Mo2 C) >
C(Al4 C3 ) > C(B4 C) > C(a-SiC). Comparatively high Rp
values, increasing exponentially with decreasing ac frequency f , indicate that there is no quick faradaic processes at the NPC surface and the rise of Rp is mainly
caused by the increase of adsorption and diffusion resistance at f < 0:1 Hz.
The dependence of the phase angle d on log f for
different nanoporous carbons and at various fixed electrode potentials are given in Fig. 6. Only at very low
frequency f < 1 102 Hz, d approaches to )90° (except C(a-SiC)), which is characteristic of purely heterogeneous adsorption limited process, i.e. characteristic
of the so-called finite capacitive effects [1–4,10,11,18–21].
The values of jdj decrease in the order C(Mo2 C) P
C(TiC) > C(B4 C) > C(Al4 C3 ) > C(a-SiC), and for the
Fig. 5. The dependences of Cp (a) and ratio Cp =Cs (b) on ac frequency
for nanoporous carbon (prepared from different precursor carbides,
noted in figure) in 1 M (C2 H5 )3 CH3 NBF4 + AN solutions at potential
)1.4 V vs. SCE.
Fig. 6. The phase angle vs. ac frequency dependences for nanoporous
carbon, prepared from different precursor carbides (noted in figure), in
1 M (C2 H5 )3 CH3 NBF4 + AN solutions at potential )1.4 V vs. SCE (a),
and for nanoporous carbon, prepared from B4 C, at different potentials
(V vs. SCE, noted in figure) (b).
318
A. J€anes et al. / Electrochemistry Communications 6 (2004) 313–318
same nanoporous carbon the values of jdj are higher in
the potential region E 6 Emin than at E > Emin (indicating the slower adsorption of BF
4 anions at NPCE
compared with (C2 H5 )3 CH3 Nþ cations). The d; log f plot for C(a-SiC) indicates that there is a kinetically
mixed process (slow diffusion and slow heterogeneous
adsorption steps) and the adsorption equilibrium has
not been established at f P 5 103 Hz. The results of
more detailed analysis of complex impedance plane
plots will be given in our future publication.
Acknowledgements
This work was supported in part by the Estonian
Science Foundation under Project No. 4568.
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