Capacitance Determination and Abusive Aging Studies of

Proceedings: The 16th International Seminar On Double Layer Capacitors,
Deerfield Beach, FL., U.S.A., December 4-6, 2006.
Capacitance Determination and Abusive Aging Studies of
Supercapacitors Based on Acetonitrile and Ionic Liquids
P. K URZWEIL 1 , M. C HWISTEK
1)
University of Applied Sciences, Kaiser-Wilhelm-Ring 23, D-92224 Amberg, Germany
R. G ALLAY 2
2)
Maxwell Technologies, CH-1728 Rossens, Switzerland
Abstract. a) We studied some safety aspects related to the operation of supercapacitors based on acetonitrile,
especially the decomposition products of electrodes and electrolytes during abusive testing at voltages of up to 6
V and temperatures up to 90 °C. Ethylene, volatile amines, intermediate amides, carboxylate anions of organic
acids, fluorinated and heterocyclic compounds are formed in the electrolyte. Nevertheless, life-times of several
months can be estimated even on destructive operating conditions (90°C). Inflammation or explosions were not
observed in any case of abusive testing. – b) Double-layer capacitors of different manufacturers were
characterized by ac impedance spectroscopy, cyclic voltammetry, and transient methods. We were able to gather
scaling factors for the conversion of ac series impedance capacitances at 1 Hz into values corresponding to dc
transient techniques: roughly a factor of 1.5 (acetronitrile) to 5 (propylencarbonate) for commercial capacitors in
the 100 F range. – c) State-of-the-art ionic liquids were characterized in supercapacitors. Without laborious and
costly purfication steps, however, the “best” ionic liquids in the test provided a 35% smaller potential window
and a tenfold lower conductivity than the acetonitrile system. Hence, acetonitrile cannot easily be replaced by
different low-cost electrolyes in near future.
1
Aging behaviour of supercapacitors on abusive conditions
1.1
Accelerated aging tests
F
Figure 1 shows that capacitance falls dramatically if
supercapacitors are stored at an ambient temperature
of 90°C and an applied voltage of 2.3 V. Nevertheless,
supercapacitors can be operated on these rigorous
conditions at least temporarily. All capacitors fail due
to the increasing internal resistance. The nearby
exponential decline of capacitance follows the
ARRHENIUS equation. With this, lifetime of a 50 F
Japanese supercapacitor B can be estimated by roughly
4.6 months (at 2.5 V) to 7.7 months (2,3 V) and 12.5
month (2.1 V) at 90°C, until the internal resistance
quadruples to a value of about 600 mΩ. Lowering
operating voltage by 0.4 V stretches lifetime by a
factor of about three (2.7).
Double-layer capacitors suffer a rise of the
equivalent series resistance by a factor of 0.5 to 5
during the first 1000 hours of operation at rated
voltage (2.5 V) and ambient temperatures between 70
and 90 °C (see Figure 2). The decrease of capacitance
by about 10% is far less affected by heat [1].
E
D
C
B
A
Fig. 1: Ratio of measured capacitance C and rated capacitance C0 (printed on the case) after storage at 90 °C and 2.3
V. A to F: different manufacturers from Asia, Europe and
America.
1
1.2
by electrolysis of a saturated solution of
[(CH3CH2)4N]BF4 in acetonitrile.
6. The active carbon electrodes (Figure 2) lose cyclic
siloxanes and aromatic contaminations even at
room temperature, and are destroyed mainly by
the anodic reaction. The electrodes catalyze the
thermal decompositon of the TEABF4 electrolyte.
7. The oxide layer on the etched aluminium support
material under the active carbon layer is destroyed
by fluorination. This process acts as a source of
oxygen.
8. Hydrogen is generated by electrolysis of water and
by the fluorination of carboxylic acids.
Decomposition products after abuse testing
With respect to the scarce knowledge in literature [4],
we were able to gather the first detailed insights into
the thermal and electrochemical decomposition
mechanisms of tetraethylammonium-tetrafluoroborate
in acetonitrile by help of GC-MS, UV und IR
spectroscopy, thermogravimetric analysis, and X-ray
diffraction [2,3]. Our results in short:
1. Unwanted traces of water in the electrolyte and
carbon electrodes cause the hydrolysis of
acetonitrile, whereby acetamide and organic acids
are formed (below 4 V). Above a cell voltage of 6
V derivates of fluoroacetic acid are found.
2. In the liquid phase heterocyclic compounds are
formed such as pyrazines. Water is generated by
condensation of amino-carbonyl compounds.
3. The alkylammonium cation is destroyed by the
elimination of ethylene. The trialkylamine byproduct can be oxidised further.
4. Tetrafluoroborate is a source of fluoride,
hydrogenfluoride and boric acid (metaborate).
5. Leaky capacitors lose a brownish crystalline mass,
which consists of residual supporting electrolyte
and active carbon, organic acids, acetamide, and
polyoles. We were able to reproduce the deposits
1.3
Salt residues in burst capacitors
The X-ray diffractogram (Figure 3) of the brownish
salt residues at burst ultracapacitors, which failed after
a long time of operation at high voltages, gives hints at
acetamide, aromatic and unsaturated organic acids,
fluoroacetic acid derivates and macromolecular
products. The unexpected variety of compounds in this
crystalline mass is partly due to several weeks of
storing in ambient air. The XRD results, indicating the
presence of amides, were verified by IR spectroscopy
(Figure 6) and thermal analysis (Figure 7).
Increase of internal resistance
(Loss of capacitance)
240
0.005 Hz
232 F
220
200
Absorbed water
180
160
C/F
140
120
Aging 1000 h at 70°C
100
80
40
20
R c
1 or
re
R c
1 or
recte
d
cted
60
Loss of electrolyte
after testing > 2.5 V
100 Hz
0
0,000
0,002
0,004
0,006
0,008
0,010
0,012
0,014
R/Ω
Fig. 2: Left: Increase of resistance after thermal aging. Right: Salt deposits on burst supercapacitors after several thousand
hours of operation including abusive high temperatures (up to 90°C) and temporary voltag peaks (up to 3...5 V).
Above: Active carbon/aluminium electrode.
2
1, 15
9000
1
9
6
8000
3
3
7000
1
3
Counts
6000
4 14
1
5000
3
4000
6
13
3000
4
1
11
2000
10
8
Ammonium-tetrafluoroborate
Maleic acid
Hexahydroanthracene
Dimethylglyoxime
2-Fluorophenoxyacetic acid
Norgestrel C21H28O2
1,4-Naphthoquinone
p-Toluic acid
Poly(1,3-propanediol)
Acetamide
Carbon
Quinolinic acid
Terephthalic acid
1-Methylthymine
2-Tertbutyl-anthraquinone
p-Glyoxyl-anisidine-acetyl-oxime
5 14
1
12
2
1000
1
4
1
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
12
12 12
1
0
10
20
30
2Φ
40
50
60
Fig. 3: X-ray diffractogram (XRD) of the salt residue on the ultracapacitor in Figure 2. The products, mainly organic
amides and acids, are not considered as harmful. Toxic cyanides were not found.
Accelerated aging by electrolysis. We reproduced a
similar crystalline mass by electrolysis of the
electrolyte at 4 V (see Figure 4). The formation of
fluorocarboxylic acids requires a cell voltage of about
6 V. Acid gases escape. In the residual electrolyte we
could identify fluoride and borates by precipitation
with calcium and silver ions. Obviously hydrogenfluoride is formed. Heating the crystals in a glass vial,
the characteristic oily gas bubbles creep up the etched
glass walls. At a decomposition voltage of 4 V,
considerable gas formation was not observed.
Pure acetonitrile, and the electrolyte solution do not
absorb UV/VIS light below 800 nm (Figure 5). After
10 hours of electrolysis in a gas-tight vessel, flooded
with nitrogen, however, marked absorption peaks
arise, which are similar to those of the crystals on the
burst ultracapacitor, dissolved in acetonitrile. The
strong peak at 220 nm is also found in acetamide,
bands of organic acids occur at somewhat lower
wavelength. The absorption peak at 260 nm is typical
of the chromophoric group C=N, and the π→π*
transition in aromatic double bonds [5].
The IR spectra of the anode and cathode films in
Figure 6 give clear hints at the formation of
intermediate amides and carboxylate ions. Tiny
shoulders at ~1700, 1636 and 1435 cm–1 indicate
unsaturated organic acids, although they are covered
by stronger bands. A small portion of acetonitrile
seems to dimerisate and to form maleic acid by
hydrolysis. The quite similar cathodic conversion of
(6 V, 10 h)
Carbon/Al
electrodes
Headspace
filled with N2
Inner absorption
vessel filled with
CH3CN
Electrolyte
[(C2H5)4N]BF4
in CH3CN
Fig. 4: Model experiment in a reaction bottle containing two
active carbon electrodes in acetonitrile/TEABF4. The screwcap contains a polymer septum to allow headspace GC-MS
analysis.
3
acrylonitrile to adiponitrile is well-known in organic
electrosynthesis. The peaks at 1340 and 1616 cm–1,
typical of fluoroacetate, disappear after several weeks
of storing at ambient air.
Acetamide and organic acid derivates are generated
by hydrolysis of acetonitrile, a well-known reaction in
organic chemistry. Derivates of fluoroacetic acid stem
from a different reaction route by hydrohalogenation,
in which aromatic hydrocarbons in the carbon
electrodes play a role ( Figure 6b). This is consistent to
the fact, that the electrochemical fluorination of
carboxylic acids in hydrogenfluoride by the SIMONS’
process requires cell voltages of 5 to 8 V.
CnH2n+1COOH + (2n+1) HF → CnFn+1COOH + (2n+1) H2
With respect to the polymer-like characteristics of
the deposits, we assume the formation of polyamides
during electrolysis. IR spectra, not shown here, might
indicate polyamides by the shoulder at 3050 cm–1, and
the IR bands at 1650 cm–1 and 1560 cm–1, related to
NH-vibrations (superimposed by water).
The thermogravimetric curves of the electrolytic
deposits in Figure 7 exhibit a step at 225°C which can
be reproduced by the decomposition of acetamide. The
main step at 495°C is due to the decomposition of
mainly the alkylammonium ion followed by the more
stable tetrafluoroborate. Dry active carbon electrodes
show a linear decay up to 450 °C due to desorbing
water, followed by two steps at 620°C and 840°C. The
transition at 620°C is also found in the anodic residue
of the electrolysis experiment.
2,0
Reactions in the liquid electrolyte
In the headspace GC-MS analysis of the electrolysis
experiment we found the masses 41 (acetonitrile), 29
(ethyl), 20 (HF) and hints at complicated products.
The brown deposits on the electrodes and on the
burst ultracapacitor, existing after electrolysis only, led
us to the analogy of the MAILLARD reaction shown in
Figure 8. We found pyrazine structures and oxidation
products of triethylamine in the liquid and gaseous
phase of our closed model reactor (Figure 4). It is
noteworthy that the deposits smell of 2-acetylpyrazine, similar to the taste of popcorn. Certainly,
further investigations must be conducted to clarify the
nature of the heterocyclic compounds and fluorinated
aromatic hydrocarbons, which are generated by the
electrolysis of acetonitrile/N(CH2CH3)4BF4 at carbon
electrodes at cell voltages above 4 – 6 V.
A certain portion of the electrolytic decomposition
products at 4 and 6 V is not soluble in acetonitrile,
contains active carbon particles, attracts water, and the
taste of acetic acid is perceptible, when it is dried at
air. By help of GC-FID, we detected 0.2% to 0.6% of
acetic acid in the acidified deposits on the active
carbon anode. Free linear carboxylic acids higher than
acetic acid play no role. In the cathodic residues,
organic acid could not be detected at all.
Aromatic hydrocarbons, carboxylic acids, and
quinones cannot be found in thermally aged electrolyte
samples. Obviously they are generated by the catalytic
activity of the carbon electrodes.
Amides
After electrolysis at 6 V (10 h)
1: Electrolyte
in CH3CN)
Organic
acids(0.22%
(carboxylates)
2:
Crystals
on
burst
ultracapacitor
(0,22% in CH3CN)
Amines
3: Gaseous products absorbed in CH3CN
Aromatics and C=N compounds
4: Deposit on cathode (2,2% in CH3CN)
1
A
1.4
1,5
6 V (10 h)
2
1,0
2
3
3
4
200
220
π,π* C=C Ar
n,π* CONH2
n,π* COO
0,0
n,σ* NR3
-
0,5
4
240
260
1
280
300
320
340
360
380
400
λ / nm
Fig. 5: UV/VIS spectrum of the residual electrolyte 1 (after electrolysis at 6 V, 10 h), absorbed gaseous products 3, deposit
on the cathode 4, and crystals of the burst ultracapacitor 2.
4
ν(CO)
a)
Hydrolysis
H3C
of acetonitrile
C
O
H2O
N
H3C
O
H2O
C
C-N
NH2
H3C
C
B-F
O[NH4]
N-C=O
ν(CH)
3370
1657
6V
ν(OH)
Deposit
1,5
δ(NH2)
rock
3619
2995
4V
A
ν as(NH2)
1,0
δ(NH2)
CO
in amide
νs(NH2)
1050
Electrolyte
1390
1660
0,5
1616
Amide
Acetamide
Carbon
electrode
[NEt4]BF4 / Acetonitrile
H 2O
Deposit
0,0
4200 4000 3800 3600 3400 3200 3000 2800 2600 2400 2200 2000 1800 1600 1400 1200 1000 800
-1
1/λ (cm )
b)
N
HF
H3C
C
NH
HF H3C
F
F
N-C=O and δ(NH2)
in Amide
C
NH2 -2 HF H3C
NH2
ν(C-N) B-F
CH3COOH + HF → FCH2-COOH + H2
Deposit at cathode
(6 V, 2.5 h)
cathode
ν (COO )
FCH2COONa
1409
1570
A
[NR4]BF4 ν(C-O)
O
H2O
--
*
1,5
F
C
-O-CO-O-
*
B-O
* B-F
+
1,0
B-O
C
FCH2COOH
H3C
6 V (2.5 h)
Deposit at anode
(6 V, 2.5 h)
anode
+
ν(CO)
cathode
Deposit at cathode
(4 V, 30 h)
4 V (30 h)
in Amide
0,5
CH3CN
1340
1380
1616
Deposit at anode
anode
(4 V, 30 h)
Crystals on
ultracap
0,0
1700 1650 1600 1550 1500 1450 1400 1350 1300 1250 1200 1150 1100 1050 1000 950 900 850 800
-1
1/λ (cm )
Fig. 6: a) FTIR spectra of the cathode deposit after electrolysis (1 = 6 V, 2.5 h; 2 = 4 V, 30 h), in comparision to acetamide,
fresh electrolyte and pure acetonitrile. – b) IR bands of amides and fluorocarboxylates in the electrolytic deposits and the
crystals on the burst ultracapacitor.
5
1.5
In the headspace of a supercapacitor which was
continously charged at 2.5 V and 70 °C, mass 44 and
fluoride (mass 19) could be detected in significant
larger amount, whereas mass 43 (from salt) dropped.
Obviously, CO2 is formed by the oxidation of carbon
and the decarboxylation of organic acids.
We identified the fragments of metaboric acid and
alkyl boron compounds – obviously generated by
hydrolysis of BF4-anions. As proof, IR bands of B-O
vibrations in inorganic borates appear at 1460. 1054
and 780 cm–1 (see Figure 5).
Thermal decomposition products
Pure acetonitrile, and TEABF4/acetonitrile electrolyte
were aged in headspace vials for more than 500 hours
at 70°C. We did not find any significant contaminations in the aged acetonitrile besides some butanenitrile. In the electrolyte sample, we detected
negligible concentrations of fluorinated 2,3-butanediimine. Traces of hydrogen fluoride (mass 20) were
present both in the pure and in the aged electrolyte.
As shown in Figure 9, in the liquid phase of
acetonitrile and finished electrolyte, which were in
contact to the electrode material, we found significant
amounts of alkyl substituted aromatics, and cyclic
siloxanes, which are dissolved from the electrode
material already at room temperature. Triethylamine
and N-alkyl-amines could be detected only in the
electrolyte aged at 70 °C. Obviously the thermal
decomposition of the TEABF4 electrolye requires the
catalytic activity of the electrode material. Pure
acetonitrile behaves thermally stable.
1.6
1.7
The role of water
Commercial electrolytes contain less than 10 ppm of
water. Absorbed water at carbon electrodes cannot
completeley be removed even by drying at
temperatures above 150 °C, because the organic
binder between carbon particles and aluminium
support is destroyed. The question is whether
hydrogen gas, which is formed by electrolysis of water
traces or the fluorination of carboxylic acids (SIMONS’
process), is able to cause a drastical increase of
pressure in the capacitor.
The electrolytical generaton of 1 ml hydrogen requires
the electric charge of 8,6 C (2,4 mAh) and the volume
of 0,8 ml water. 10 ppm water equal about 8,5 ml in
one liter of electrolyte solution. Hence, 100 ml of
electrolyte, containing 0,85 mg water, are enough to
produce about 1 ml hydrogen gas (1 mg H2O = 1,245
ml H2) to expand the case.
Gaseous products in supercapacitors
In the headspace of the vials in Figure 8 we identified
ethylene (mass 28), formed by splitting of the
quarternary ammonium salt by a HOFFMANN
elimination. The byproduct triethylamine and the
tertiary amine radical CH3(C2H5)N=CH2+ (mass 72)
were found, too. As a result, a proton is set free which
can form HF (mass 20) with excess fluoride.
In the gas space of real supercapacitors, which
were aged uncharged at 70 °C for 500 hours, we found
excess acetonitrile, water vapor, traces of oxygen,
carbon dioxide and ethylene. In a volume of 100 µL,
after chromatography at 120 °C at an unpolar column,
the following masses were recorded.
1.8
Corrosion products at the electrodes
The carbon/aluminium electrodes were investigated by
TOF-SIMS after the aging experiment in headspace
vials at 70 °C for 550 hours. White spots appeared on
the part of the aluminium foil which was immersed in
the electrolyte. The mass spectrum in Figure 10
shows deposits of [(C2H5)4N]BF4 and a mixture of
hydrocarbons with long chains (mass 300 to 700).
Besides the expected Al+ signal, fluoroaluminate and
fluorooxy aluminate complexes of the kind AlF4–,
AlF4[AlOF]n–, AlF4AlF3(AlOF)n–, and AlF2O(AlOF)n–
are present. The white spots on the part of the
electrodes, which was in contact to air, show the
highest intensities of the Al/F/O clusters. Signals of
the kind [Al2O3]nOH–, which are usually found on
oxydized aluminium surfaces, are completely missing.
Obviously the aluminium support under the carbon
layer is attacked by aggressive fluoride. It is known
from literature that aluminium fluorides are more
stable than fluoroboron compounds. Hence, we
consider the dissolving Al2O3 layer on the etched
aluminium support below the active carbon layer as a
possible source of oxygen for the borate formation.
Table 1: Fragement ions in the headspace of a commercial
supercapacitor after aging at 70°C/500 h
Acetonitrile: 14 CH2+·, 26 CN+, 40/41 CH2-3CN+,
less important: 15 CH3+, 27 HCN+.
Gases:
18 H2O+, 28 N2+/C2H4+, 32 O2+, 40 Ar,
44 CO2+ (besides C2H6N+ from amine,
CONH2+ from amides)
Fragments of the salt: 16 NH2+, 17 NH3+, 19 F+;
traces of 20 HF+, 29 C2H5+, 30 BF+/CH2NH2+,
39 K+/C3H3+, 43 (CH2)2NH+/CONH+,
interestingly: 96 CF3CONH fragment.
Mass 44 plays no role in the pure electrolyte, and is
thus an indicator for the presence of triethylamine,
already found in the liquid phase. The difference 44
C2H6N+ → 16 NH2+ corresponds to the elimination of
CH2=CH2 due to the onium reaction of N-ethylamines.
The fluoroacetamide fragment (mass 96) supports our
IR study for the model reactor.
6
100
225
90
80
60
2 20
Deposit at cathode
after electrolysis
(6 V, 2.5 h)
Acetamide
Acetamide
added
82°C
b. p. of
CH3CN
70
∆m/m in %
°C
°C
onium
Amm e
t
aceta
50
Burst capacitor
40
495 °C
140
30
[(C2H5)4N]BF4 in
acetonitrile
Acetonitrile p. a.
°C
20
(4 V, 30 h)
10
melting point of NEt4BF4
0
0
100
200
300
400
500
600
700
T / °C
Fig. 7: Thermogravimetric analysis (TGA) of the electrolytic deposits (see Figure 4) in comparison with the bloom on the
burst ultracapacitor (dashed line).
Liquid phase
H2N
5 µL, 60 – 280 °C, 10 K/min
after 6 V (2.5 h, N2)
O
+
R
O
N
N
H2N
R
- 2 H2O
R
N
R
N
R'CHO
- H2O
R
R'
N
R
R'
Maillard reaction: condensation of α-amino-carbonyl compounds
R
N
R
pyrazine
R
N
+
N
Acetamide
derivates
NH2
Boric
acid ester
O
N
R
N
N
CH3
quinoline
NH
N
CH3
R
H
N
R
N
H
R
N
R
quinazoline
Fig. 8: GC-MS analysis (70 to 280 °C at 10 °C/min) of the electrolysis products in the liquid [(C2H5)4N]BF4/acetonitrile
phase (6 V, 2.5 h, N2, see Figure 4).
7
Air
Contamination
due to active carbon electrodes
Electrode
¦ Alkyl substituted aromates
Products due to
catalytic activity
¦ Amines + ethene
(Hoffmann elimination)
¦ Cyclic siloxanes
H3C-H 2C
1) Acetonitrile 2) Electrolyte
70 °C / 550 h
CH2CH3
N+
H3C-H 2C
CH 2CH 3
↓
(C2H5)3N + H2C=CH2 + HX
and
isomers
NEt3
NEt3
dimerisation
product
O
Si
O
Si
Si
O
O
Si
O
Si
Fig. 9: GC-MS analysis of the thermal decomposition of TEABF4/acetonitrile in presence of an active carbon electrode (70
°C, 550 hours).
Positive secondary ions
Negative secondary ions
mass / u
Electrolyte salt
70°C/550 h
a)
Al/F/O clusters
Electrolyte
air
b)
hydrocarbons
X–
Fig. 10: a) TOF-SIMS spectrum of the white spots on the aluminium support below the active carbon layer after aging in
(C2H5)4NBF4/acetonitrile. – b) Pure aluminium support: tenfold lower intensities of the Al/F/O clusters.
8
2
Ionic liquids and nanomaterials
2.1
Frequency response of ionic liquids
systems we have known so far. AMMOENG, a
commercial product, excels at an enjoyably small
frequency-dependence of capacitance, but its
resistance is 200 times higher compared with
TEABF4/acetonitrile.
Electrolytes based on acetonitrile and propylene
carbonate are limited to a potential window of about
2.7 V, but their conductivity of about 100 mS/cm
comes close to inorganic electrolytes such as sulfuric
acid (850 mS/cm) and potassium hydroxide (620
mS/cm).
a) We investigated different ionic liquids [6] with
respect to their use in supercapacitors by ac
impedance spectroscopy in a frequeny range between
10 kHz to 0.1 Hz. The measuring cell, shown in
Figure 11, comprised two gilt copper stamps in a
fixed distance of 2 mm. The vacuum capacitance of C0
≈ 3 pF at 1 kHz was verified by a LCR bridge. The
cell constant K = R·κ ≈ 0.027 cm–1 was determined by
help of 0.1 molar KCl solution (κ = 0.01215 S/cm,
22°C) and the measured impedance real part R at 1
kHz. Capacitance at any circular frequency f was
calculated from the imaginary part of impedance with
reference to a simple R–C series combination.
C = εK =
−1
2πf ⋅ Im Z (ω)
2.2
Voltage window
Wide voltage windows reported in literature [6] refer
to highly cleaned and dried samples of ionic liquids.
The voltammograms in Figure 12 show acetonitrile/
TEABF4 as the most stable solution. Traces of air and
moisture were allowed with respect to the technical
applications and aging. Thus, supercapacitor cells built
of commercial active carbon electrodes and ionic
liquids, absorbed by polymer separators, did not reveal
any advantage compared to (C2H5)4NBF4/acetonitrile.
The decomposition voltage of [BMIM]BF4 equals
about 2.5 V at anodic potentials, and about –4 V at
cathodic potentials. Unfortunately, ionic liquids tend
to decompose and polymerize after several hundred
charge/discharge cyles. The ionic liquids in the test
showed an unwanted leakage current due to
decomposition reactions already at cell voltages
around 1.5 to 2 V, especially [BMIM]OcSO4,
AMMOENGTM102, and [MMIM]Me2PO4. The formation of gas bubbles, white polymer-like strings, and a
brownish tinge were observed.
(2.1)
Figure 11 exhibits the low resistance and high
capacitance of [(C2H5)4N]BF4 in acetonitrile in
comparison to novel ionic liquids.
b) The temperature dependence of conductivity and
permittivity – shown in Figure 13 – was investigated
by help of an temperature controlled glass vessel filled
with 25 ml of ionic liquid, a glassy carbon working
electrode, and a platinum counter electrode. The cell
constants K were 0.742 (25°C), 0.790 (40°C), 0.850
(60°C), 0.905 (70°C), 1.204 (85°C), based on
literature values for the conductivity of a 1-molar
potassium chloride solution,
κ(T) = 0,00193 T/°C + 0,0634 (in S/cm),
at a frequency of 10 kHz. Conductivity, calculated by
help of the real parts of impedance according to κ(ω) =
K/R(ω), increases with temperature, which was to be
expected for ionic conductors. Arrhenius plots were
constructed by help of the resistance values at 1 kHz.
The quantity ε = 70 · C / 0.77 mF was calculated as
a rough estimate of dissociation and interionic forces.
The measured capacitance C was divided by the
capacitance of 1-molar KCl solution at 0.5 Hz and
multiplied by its permittivity of 70 at 25°C [7], which
slightly decreases at higher temperatures.
Summarizing, organic electrolytes based on
acetonitrile show the best conductivity of all solvent
2.3
Carbon nanomaterials
Active carbon electrodes bound on aluminium foils
have been the most successful commercial materials
so far.
Figure 14 shows the ac impedance spectra
(capacitance versus resistance) of aluminium foils
which were coated with nanomaterials in a thin and a
thick layer. The thicker the active layer is, the more
increases the resistance of the supercapacitor cell.
However, nanofibres show an advantage in dc
capacitance in contrast to active carbon.
Due to the lower contact resistance between active
material and aluminium foil, the commercial electrode
(as shown in Figure 2) exhibits the best frequency
response at the left upper side at low resistance and
high capacitance [8].
9
-1
10
-2
10
1 Hz
4
4
BF 4
M] ]EtSO e 3PO
I
SO 4
M
M
]
B
Oc
M
[
]
I
M
I
N
IM
[E [MM
MO 2
[BM
AM G 10
EN
10 Hz
-3
100 Hz
C / F cm
-2
10
-4
10
1 kHz
-5
10
10 kHz
-6
10
-7
10
F4
t 4]B N
[NE H 3C
C
in
state-ofthe-art
2,5 cm x 2,5 cm x 0,2 cm
-8
10
10
100
1000
10000
K = 0,032 cm-1
2
R / Ω cm
Fig. 11.: Left: Frequency response of capacitance and resistance of different ionic liquids in camparsion with
TEABF4/acetonitrile at room temperature (22°C). – Right: Cell of two gilt copper stamps, polymer spacer and seals.
[BMIM][OcSO4]
= 1-Butyl-3-methyl-imidazolium octylsulfate
[BMIM][BF4]
= 1-Butyl-3-methyl-Imidazolium tetrafluoroborate
[MMIM][Me2PO4] = 1,3-Dimethyl-imidazolium dimethylphosphate
[EMIM][EtSO4]
= 1-Ethyl-3-methyl-imidazolium ethylsulfate
AMMOENG
= commercial ionic fluid by “Solvent Innovation”, Cologne, Germany
2,5
1,5
I [A] / A
Current
Active carbon
electrode (Gore)
TEABF4 AN
BMIM OcSO 4
BMIM BF4
MMIM Me2PO4
EMIM EtSO4
2,0
TM
[BMIM]OcSO4
PA-Separator
copper contact
AMMOENG 102
AMMOENGTM 102
1,0
[MMiM]Me2PO4
[EMIM]EtSO4
[BMIM]BF4
0,5
TEABF4/CH3CN
0,0
-0,5
0,0
0,5
1,0
1,5
2,0
2,5
state-ofthe-art
3,0
Cell
voltage / V
U [V]
Fig. 12: Cyclic voltammograms of ionic liquids (without preceding cleaning processes) in capacitor cells at 22°C.
10
F
H3C
B
N
+
CH3
N
F
N
-
H3C
N
O
H3C
+
CH3
100
O
-
O
0,5 Hz
εr
S
O
F
F
100
[BMIM]BF4
εr
1 Hz
[EMIM]EtSO4
0,5 Hz
1 Hz
10
10
10 Hz
10 Hz
100 Hz
1
1 kHz
1 kHz
10
kH
z
1
100 Hz
z
kH
10
25°C
0,1
40
85°C
60
70
0,1
0,1
1
10
κ / mS cm
a)
100
1
10
100
-1
κ / mS cm
-1
100
εr
b)
0.5 Hz
εr
[MMIM]Me2PO4
100
70 85°C
60
40
25°C
[BMIM]OcO4
1 Hz
10
10
Hz
z
0H
10
10
1
1 kHz
1
0,1
40
25°C
60
70
85°C
0,1
0,1
1
10
40°C
25°C
0,01
0,5
10
-1
κ / mS cm
1
85°C
60°C
κ / mS cm
kH
z
-1
25°C
5
10
c)
d)
εr
100
AMMOENG 102
0,5 Hz
1 Hz
10
Fig. 13: Frequency response of
conductivity and permittivity of selected
ionc liquids at different temperatures.
10 Hz
The ARRHENIUS equations read (with
temperature T given in Kelvins and
conductivity κ at 1 kHz in mS/cm):
100 Hz
1
60
0,1
a) log κ = –1589 (1/T) + 3.08
b) log κ = –1509 (1/T) + 2.79
c) log κ = –1914 (1/T) + 3.88
d) log κ = –1513 (1/T) + 2.12
1 kHz
40
80°C
e)
25°C
0,01
0,5
70
5 kHz
5
1
κ / mS cm
10
-1
11
Copper conductor
Nanotubes/PTFE
on aluminium
support (12 cm²)
PA separator,
[NEt4]BF4/CH3CN
CNF: Herringbone-Nanofiber + Cu / Ni
0,01
Commercial
Gore electrode
1E-3
Cell capacitance
C [F/cm²g]
/ F cm-2g-1
Active
carbon
1 Hz
CNT: Multiwall carbon nanotube
+ MgO-Co catalyst
¦ High contact
resistance between
aluminium support
and carbon nanotubes
1E-4
Vulcan
Vulcan 2XC72R (0,21 g + 2,16 g binder)
CNF1112
CNF (0,212g + 3,0 g binder)
CNF1314
CNF (1,1 g2 + 0,51 g binder)
CNT (0,222g + 3,0 g binder)
CNT3132
CNT (1,462g + 1,0 g binder)
CNT3334
1E-5
10 kHz
Binder: contains
Dyneon TF
5035R,
62% PTFE
Binder
62%
PTFE
1E-6
10
100
1000
10000
100000
2
Cell resistance
R [ Ω/cm²g] / Ω cm-2g-1
Fig. 14: Frequency response of specific capacitance (per unit area and gram of active mass) and resistance of supercapacitor
cells: nanomaterials coated on aluminium foil in comparision to active carbon “Vulcan” – and commercial active carbon
electrodes (see Fig. 2) . Temperature 22°C.
Fig. 15: Capacitance of a MAXWELL 350 F supercapacitor (BCAP0350) determined by fast and slow techniques working in
different voltage ranges and states-of-charge of the supercapacitor.
12
3
Determination of rated capacitance
3.1
Capacitance measurement in practice
Electric charge Q was determined by help of the
trapezoidal rule at each point in time. In the special
case that the average value of discharge current is
considered as constant, the simple IEC-method can be
derived if equation 3.4 is reduced to 2 points.
Industrial quality assurance during the fabrication of
supercapacitors requires reliable and time-saving
methods to determine rated capacitances. As we
stressed in the last year seminar [1], capacitance is a
function of voltage and the state-of-charge of the
supercapacitor, respectively.
■ Capacitance determined by electrochemical
techniques working in different time domains yield
basically different values of capacitance. Figure 15
illustrates that ac impedance spectroscopy,
chronoamperometry, and constant current discharge
reflect different capacitive processes at the outer
surface (if the excitation signal is fast) and the inner
surface (at dc measurements) of the porous
electrode/electrolyte interface.
■ Capacitance appears as a dynamic quantity that can
be divided up in a double-layer capacitance at the
“outer” electrode surface and a pseudo-capacitance
due to slow faradaic processes depending on
voltage: C = C0 + kU, whereby C0 is the capacitance
at 0 V. Transient techniques, which generate fast
voltage changes, reflect the fast charge/discharge
processes with “frozen” diffusion at the outer
electrode surface. At slow voltage alteration the
reactions controlled by diffusion in the inner
electrode surface (strongly depending on voltage)
contribute to the measured capacitance values.
■ Constant current discharge according to IEC
40/1378 [8] yields the highest capacitance values.
By this method, the linear drop of the rated voltage
from 80% to 40% is evaluated (see Figure 15).
Q ( tn ) =
C=
I (t )
dU (t ) / dt
3.2
Scaling factors for ac impedance spectroscopy
The scaling factor depends primarily on the electrode/
electrolyte system. Size and geometry of the capacitor
play a secondary role. The scaling factor is obtained
by dividing the average value of dc capacitance by the
average value of ac capacitance at 1 Hz.
Table 2: Rated capacitance (in F) and series resistance
determined for a lot of supercacitors. The given error equals
the experimental standard deviation.
Aceonitrile
Propylencarbonate
Rated value
50 F
100 F
50 F
100 F
Cyclic voltammetry
at 0.02 V/s
51 ± 1.7
ac-impedance spectroscopy
at 0.01 Hz
50
at 1 Hz
33.7 ± 2.3
Scaling factor
C0.01Hz/C1Hz
1.5
CCV/C1Hz
1.5
Resistance
14.8 ±
at 100 Hz
1.2 mΩ
t
∫
(3.4)
1.5 ⋅ C1Hz (acetonitrile)
Cdc = f ⋅ C1Hz = 
(3.5)
 5 ⋅ C1Hz (propylene carbonate)
(3.1)
Q (t )
1
=
I (t ' ) dt '
U 0 − U (t ) U 0 − U (t )
0
Qi − Qi −1 I ⋅ (ti − ti −1 )
=
U i − U i −1
U i − U i −1
(3.3)
Figure 17 shows that ac impedance spectroscopy [9]
at frequencies below 0.1 Hz is able to provide rated
capacitance of supercapacitors. Interestingly, the
capacitance values printed on the capacitor cases
cannot be confirmed for every manufacturer.
Fast ac impedance measurements allow to gather
series resistance at 100 Hz and capacitance at 1 Hz
within about 30 seconds. Rated dc capacitance must
be extrapolated by multiplying the 1 Hz capacitance
with a scaling factor f according to Table 2.
Due to the measurement error of the voltage values
∆U(t) ≈ 10 µV, and the small time intervals of less
than 1 ms, the derivative dU/dt was smoothed by an
FFT algorithm.
Integral capacitance calculated according to equation
3.2 shows a bit lower values.
C (t ) =
∑
The IEC method provides a capacitance value (46.8 F)
that is higher than the average value of differential
capacitance in the full voltage window of discharge
(45.6 F). This result affirms Figure 15.
Figure 16 shows that the IEC-value is a more or less
good approximation of average differential
capacitance in the constant current discharge test.
Differential capacitance was calculated by help of the
inifinitesimal voltage drop at each point in time.
C (t ) =
1 n
( I i + I i −1 )(ti − ti −1 ) = I ⋅ tn
2 i =1
(3.2)
13
112 ± 1.8
41 ± 0.7
80 ± 0.8
110
34
65.0 ± 8.2 7.14 ± 0.56
66
14.7 ± 0.54
1.7
1.7
13.0 ±
2.2 mΩ
4.5
5.4
22.2 ±
6.5 mΩ
4.8
5.7
31.2 ±
2.0 mΩ
IEC 40/1378 method
Capacitance during discharge
2,0
Nesscap 50 F / 2.7 V
2,0
U0
1,8
1,6
1,8
0.8 U0
1,6
1,4
1,4
1,2
Q / (U0-U)
1,0
0,8
I/v
1,0
-1
C = 2 A / 0.0437 V s = 45.8 F
0.4 U0
0,8
0,6
0,6
0,4
0,4
8.92 s
0,2
0,0
0
5
10
IEC-value
27.23 s
U/V
1,2
15
20
25
30
35
0
0,2
0,0
10
40 20 30 40 50 60 70 80
C/F
t/s
Fig. 16: Capacitance of a 50 F supercapacitor, charged at 2.0 V, determined by the IEC method at a discharge
current of 2 A (left) and “true” differential capacitance (right, see text).
Fig. 17: Frequency response of capacitance C = –[2πf· Im Z]–1 of supercapacitors with different electrolyte
systems (A: acetonitrile, B: propylencarbonate) rated at 1 = 50 F, and 2 = 100 F by the manufacturers.
14
3.3
Reliable dc capacitance by CV
Cyclic voltammetry [1] at low scan rates proved as a
most precise, robust and uncorrupible method to
determine rated capacitance. Charge and discharge
cycle were averaged in the usable voltage window ∆U
between 0 and 2.7 V to yield the values in Table 2.
C=
The horizontal line g = 1 marks “average quality”. The
best capacitors lie clearly above 1, and the worst
samples lie below 1. The A2 type capacitors really
provide the highest capacitance and the lowest
resistance useful for the technical application, although
the B2 type was designated with the identical rated
values by a different manufacturer.
∆U
0

Qin + Qex
1 
 I in (U ) d U + I ex (U ) d U 
=
2 ⋅ ∆U
2 v ⋅ ∆U 

∆U
 0

∫
and
3.4
v=
dU
= 0.02 V/s
dt
∫
(3.6)
A novel quality criterion
In order to simplify industrial quality control by fast ac
impedance spectroscopy, we propose the following
dimensionless quantity.
C
R
g = 1 ⋅ n >> 1
(3.7)
R100 C n
C1
R100
Rn
Cn
Capacitor sample in the lot
measured series ac capacitance at 1 Hz
measured series resistance at 100 Hz
rated resistance, average of the R100-values
of all capacitors in the test
rated capacitance, e. g. average value measured
by slow cyclic voltammetry
Fig. 18: Quality quantity g for good and bad lots of
supercapacitors of different lots (A and B), rated at 1 = 50 F,
and 2 = 100 F.
The quantity g gets a big value for capacitors with high
capacitance and slow resistance. The example in
Figure 18 shows four lots of supercapacitors.
4
Acknowledgement – The authors wish to thank our student
A. ROTHÄUGER for valuable measurements.
References
[1] P. KURZWEIL, B. FRENZEL, R. GALLAY, Capacitance Characterization Methods and Ageing Behaviour of
Supercapacitors Proc. 15th International Seminar On Double Layer Capacitors, Deerfield Beach, U.S.A., December 57, 2005.
[2] P. KURZWEIL, M. CHWISTEK, Electrochemical Stability Of Organic Electrolytes In Supercapacitors: Spectroscopy And
Gas Analysis Of Decomposition Products, Proc. 10th Ulm Electrochemical Talks (UECT), June 27-28, 2006; to be
published in J. Power Sources.
[3] P. KURZWEIL, M. CHWISTEK, R. GALLAY, Electrochemical and Spectroscopic Studies on Rated Capacitance and Aging
Mechanisms of Supercapacitors Based on Acetonitrile, Proc. 2nd European Symposium on Super Capacitors &
Applications (ESSCAP), Lausanne, 2006.
[4] Literature listed by www.scirus.com.
[5] Spectral data in digital databases.
[6] P. C. TRULOVE, R. A. MANTZ, Electrochemical properties of ionic liquids, in: P. WASSERSCHEID, T. WELTON (Eds.),
Ionic Liquids in Synthesis, Wiley-VCH, Weinheim
[7] G. KORTÜM, Lehrbuch der Elektrochemie, Verlag Chemie, Weinheim 1970, p. 140.
[8] IEC-40/1378 / DIN IEC 62391-1, Fixed electric double layer capacitors for use in electronic equipment. Part I: Generic
specification, June 2004. IEC-40/1379 / DIN IEC 62391-2, Fixed electric double layer capacitors for use in electronic
equipment. Part I: Sectional specification: Electric double layer capacitors for power application, June 2004.
[9] P. KURZWEIL, H.-J. FISCHLE, A new monitoring method for electrochemical aggregates by impedance spectroscopy, J.
Power Sources 127 (2004) 331–340.
15