A Study of the Electrochemical Behavior of Electrodes in Operating Solid State Super Capacitors

8/3/2019 A Study of the Electrochemical Behavior of Electrodes in Operating Solid State Super Capacitors

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Electrochimica Acta 53 (2007) 710–719

A study of the electrochemical behaviour of electrodes inoperating solid-state supercapacitors

P. Staiti ∗, F. Lufrano

CNR-ITAE, Istituto di Tecnologie Avanzate per l’Energia “Nicola Giordano”, Via Salita S. Lucia Sopra Contesse n.5,

98126 S. Lucia, Messina, Italy

Received 16 March 2007; received in revised form 20 July 2007; accepted 21 July 2007

Available online 26 July 2007

Abstract

The electrochemical behaviour of electrodes and of complete solid-state supercapacitors has been studied by cyclic voltammetry (CV) and

galvanostatic charge/discharge (CD) measurements using two independent electrochemical equipments. The first one controlled the execution of

the test and recorded the voltage and current values of the complete supercapacitor while the other one recorded the potential changes of the single

electrodes. In this work, two different types of capacitors were studied: (a) a symmetric supercapacitor using carbon electrodes, and (b) a hybrid

(asymmetric) supercapacitor with ruthenium oxide/carbon in the positive electrode and carbon in the negative electrode. The studies evidenced that

in the symmetric capacitors the positive electrode controlled the capacitive performance and an optimal mass ratio from 1.2:1 to 1.3:1 between the

positive and the negative electrodes was found in the investigated conditions. For the hybrid supercapacitor it was observed that the ruthenium-based

positive electrode influenced the capacitive performance of carbon-based negative electrode and that an accurate balance of carbon loading in the

negative electrode was necessary.

© 2007 Elsevier Ltd. All rights reserved.

Keywords: Electrochemical characterization; Polymer electrolyte; Positive electrode; Negative electrode; Supercapacitor

1. Introduction

The electrochemical features of the electrodes of a super-

capacitor are usually studied in a three-electrode cell, which

includes a working electrode, a reference electrode, and a

counter electrode [1–9]. The electrochemical measurements car-

ried out in this type of cell allow studying the properties of

the active materials contained in the working electrode and

the electrochemical stability of electrolyte and solvent. On the

other hand, the complete supercapacitors are usually studied

in a two-electrode cell, in which both of the electrodes act as

working electrodes [10–14]. In this latter cell type, it is not

possible to know the ranges of the effective working potentials

of negative and positive electrodes. Since negative and positive

electrodescan have differentelectrochemical behaviourin work-

ing supercapacitor, it would be more useful to know both the

electrochemical characteristics of the supercapacitor and those

∗ Corresponding author.

E-mail address: [email protected] (P. Staiti).

of the single electrodes. It is known that electrochemical per-

formances and potential windows of electrodes investigated in

three-electrode cells can differ from those of the same elec-

trodes investigated in supercapacitor configuration. This type of

observation was also reported from Khomenko et al. [15], who

evidenced that the values of specific capacitance of electrodes

with electronically conducting polymer (ECP), obtained by a

three-electrode test cell were not comparable to those measured

in a two-electrode system. Moreover, they found that the capac-

itance of ECP electrode was strictly dependent from the ranges

of potential window in which the electrode was investigated.

In this paper, we show as by means of an analytical proce-

dure, that includes the use of two electrochemical equipments, it

is possible to study with simple experiments the electrochemical

behaviour of single electrodes and of complete supercapacitors.

Some similar studies on supercapacitors, in which were simul-

taneously investigated negative and positive electrodes, and

complete devices, were reported in literature [16–19]. Laforgue

et al. [16] showed results obtained by hybrid supercapacitors,

with carbon in the negative electrode and an electronically con-

ducting polymer (ECP) in the positive electrode. The hybrid

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P. Staiti, F. Lufrano / Electrochimica Acta 53 (2007) 710–719 711

supercapacitors were tested in a non-aqueous electrolyte and

used two different ECPs as active materials in the positive elec-

trode, and an activated carbon in the negative electrode. The

voltage profiles of these supercapacitors and of their positive and

negative electrodes obtained by galvanostatic charge/discharge

tests were shown by the authors for evidencing the impor-

tance of this type of study to establish which of the capacitors

and/or of electrodes was presenting the better electrochem-

ical performances. Arbizzani et al. [17] reported a similar

approach to investigate the specific capacitance of three differ-

ent types of supercapacitors based on carbon/carbon, ECP/ECP

and carbon/ECP. They showed the voltage profiles of complete

devices and of each single electrode as obtained by galvano-

static charge/discharge measurements. Similarly, Wang et al.

[19] reported a study for three hybrid supercapacitors based

on activated carbon at the negative electrode and lithium-ion

intercalated material at the positive electrode. The stability of

the positive electrode was evidenced by the shape of the gal-

vanostatic charge/discharge curve recorded for this electrode.

All the cited papers reported the voltage profiles of positiveand negative electrodes and those of supercapacitors as obtained

from galvanostatic charge/discharge measurements. Our paper

is different from previous studies because we report an electro-

chemical study performed on a particular type of supercapacitor

using a solid polymer electrolyte [20]. The behaviour of elec-

trodes and supercapacitors was studied by cyclic voltammetry

and galvanostatic charge–discharge measurements. Two types

of supercapacitors were investigated: (a) a symmetric capaci-

tor with electrodes based on carbon material, and (b) a hybrid

capacitor with negative electrode based on carbon material and

positive electrode based on ruthenium oxide/carbon material.

A solid polymer electrolyte was used as electrolyte separatorbetween the electrodes of the supercapacitors. The influence of

the loading of the carbon material in the electrodes, and that of

the presence of ruthenium oxide/carbon material in the positive

electrode, on the specific capacitances of single electrodes and

complete supercapacitors at various charge/discharge rates was

studied.

2. Experimental

2.1. Preparation of electrodes and capacitors

The composite electrodes used in the experiments were pre-

pared by a casting technique [21] that consisted in spreading on

a flat glass plate an ink formed with the active material (carbon

or RuO x·nH2O/carbon), the graphite fibres, the solid polymer

electrolyte and the solvent. The activated carbon used in the

preparation of electrodes was Norit A Supra Eur furnished by

Norit Italia S.p.A (Ravenna, Italy). The measured BET surface

area of carbon was 1530 m2 /g, the pore volume 0.775 cm3 /g

at P / P0 = 0.95 and the micropore volume 0.56 cm3 /g. Ruthe-

nium oxide on carbon material was prepared by precipitating

ruthenium oxide from an acidic solution of ruthenium chloride,

impregnated for 72 h in activated carbon (Norit A Supra Eur),

with a 0.1N NaOH solution. The obtained material was rinsed

with distilled water and dried at 160 ◦C for 90 min. The RuO x

loading in the final material was 40 wt%. The solid polymer

electrolyte in solution form, used for the preparation of elec-

trodes, was furnished by Fumatech GmbH, Germany and the

additional N , N -dimethylacetamide solvent (purity 99.8%) for

the ink preparation was purchased from Aldrich and used as

supplied.

After casting on the glass-plate, the solvent is evaporated

from the ink at the temperature of 50 ◦C for 15 h. The formed

electrode layer was peeled off from the glass, dried in oven at

80 ◦C and then washed in distilled water. The membrane andelectrodes assemblies (MEAs), formed with the polymer elec-

trolyte membrane sandwiched between two 4 cm2 samples of

electrodes, were prepared by a hot-pressing procedure at 130◦C

and 10 kg/cm2 for 10 min. The MEAs, before their insertion

into the test cell, were treated with 1 M H2SO4 for 2 h and then

repeatedly washed with distilled water until pH 5 was reached.

The electrodes investigated in this work with their respective

compositions are reported in Table 1.

The polymer electrolyte membrane used for the preparation

of assemblies was a per-fluorinated sulfonic acid/PTFE copoly-

mer with EW= 900 g/equiv. furnished by Fumatech GmbH. The

size of membrane (thickness∼

150m) in the MEA was largerthan that of electrodes and this allowed to a saturated calomel

electrode (SCE) to be contacted with membrane border in the

test cell (see Fig. 1).

2.2. Cyclic voltammetry and charge/discharge

measurements

Cyclic voltammetry (CV) measurements were carried out in

the complete supercapacitors with an AMEL electrochemical

apparatus connected to the negative and positive electrodes. The

negative electrode was also in contact with the reference elec-

trode terminal of apparatus, thus allowing the control on voltage

scan parameters. Through these connections, the electrochemi-

cal instrument also measured the voltage difference between the

electrodes but did not give any information on the potential of

Table 1

Characteristics of the electrodes tested in the supercapacitors

Electrode Composition Thickness () Loading of the active material (mg/cm2)

AC1 65% carbon–5% graphite fibres–30% polymer electrolyte 155 5.5 (carbon)




ACR1 65% RuO x /carbon–5% graphite fibers–30% polymer electrolyte 150 7.8 (4.5 carbon–3.3 RuO x·nH2O)



712 P. Staiti, F. Lufrano / Electrochimica Acta 53 (2007) 710–719

Fig. 1. Scheme of the supercapacitor test station: (1) main electrochemical

equipment; (2) auxilary equipment; (3) graphite plates; (4) positive and neg-

ative electrodes; (5) polymer electrolyte membrane; (6) gasket; (7) reference

electrode.

each electrode. CVs were carried out within the voltage window

0–1 V and at the voltage scan rates of 5 and 20 mV/s for the

supercapacitors.

Galvanostatic charge/discharge (CD) measurements were

carried out on the complete supercapacitors at the constant

currents of ± 2.5 mA/cm2 and± 10 mA/cm2 through continu-

ous cycling tests within the voltage window 0–1 V. The test in

this case was based on the control of the electric current flow-

ing through the supercapacitor that was obtained by connecting

positive and negative poles to the apparatus.

In addition to the main equipment, a second electrochemi-

cal apparatus was connected to the cell and had the function

of measuring the potentials of the negative and positive elec-

trodes in the CV and CD experiments. For these measurements,

a reference electrode (a saturated calomel electrode—SCE) in

contact with the membrane of the supercapacitor was used. The

potential values of the electrodes were acquired when the CV

or CD curves of supercapacitors were perfectly reproducible.

Fig. 1 shows in detail the electrical connections of electrodes

and reference electrode with the electrochemical equipments.

The specific capacitance (C s) was calculated for super-

capacitor (or electrode) applying the following formula:C s = Q(V w)−1, in which, Q, is the amount of charge discharged

from the supercapacitor (or electrode) during the variation of

voltage (or electrode potential) from V max to V min (V) and w the

weight of the active material contained in both the electrodes

(or electrode). In the CV tests, Q was measured by an integra-

tor connected to the supercapacitor, whereas in CD tests it was

calculated by multiplying current by discharge time.

3. Results and discussion

The electrochemical measurements were carried out on sym-

metric supercapacitors with activated carbon in the negative and

positive electrodes, and on hybrid supercapacitors with activated

carbon in the negative electrode and ruthenium oxide/carbon in

the positive electrode.

3.1. Study of symmetric supercapacitors and discussion

Some characteristic parameters of the two studied symmet-

ric supercapacitors are reported in Table 1. The carbon loadings

for the electrodes were 5.5 mg/cm2 for the AC1 electrode and

13.8 mg/cm2 for the AC4 electrode. Fig. 2(a)–(d) shows the

characteristic voltammograms of negative electrodes, positive

electrodes and supercapacitors as obtained by the test station

of Fig. 1. The differences of potential for negative and positive

electrodes and the specific capacitances for complete superca-

pacitors as well as for each electrode are summarized in Table 2.

In Fig. 2, the values of potential difference of electrodes and

voltage difference of supercapacitors are always displayed with

the positive sign and, the current is for all that measured for

the supercapacitor. Analysing the voltammograms in Fig. 2 it is

observed that the supercapacitor current is increasing almostproportionally with the increase of voltage scan rate, which

should indicate a purely capacitive process that occurs at the

electrodes. The specific capacitances of symmetric AC1/AC1

supercapacitor are 26.3 F/g at 5 mV/s and 24.5 F/g at 20 mV/s.

The corresponding capacitances at 5 and 20 mV/s (vsr of super-

capacitor) are varying from 117 to 113 F/g for the negative

electrode and from 95.7 to 86.7 F/g for the positive electrode

(see Table 2). As can be observed the specific capacitance of

each electrode presents a slight variation as a function of the

voltage scan rate of supercapacitor and, the capacitance of posi-

tive electrode is lower than that of negative electrode. Therefore,

for a correct balance of the capacitance of AC1/AC1 capacitorit would be necessary to increase the mass of positive electrode

of a factor of about 1.23 at 5 mV/s and 1.3 times at 20 mV/s.

Moreover, when the scan rate is increased from 5 to 20 mV/s

in the supercapacitor, an increase of the potential difference

for the positive electrode (AC1) is observed. These effects may

be ascribed to an increased difficulty at the charges redistribu-

tion into the positive electrode that occurs at the higher voltage

scan rate. This behaviour becomes more accentuated for the

AC4/AC4 capacitor, with high carbon loading in the electrodes

(Fig. 2(c) and (d)), that also show a more pronounced distortion

from rectangular shape of the voltammogram of supercapacitor.

Thedistortion is more evident at thevoltagescanrate of 20 mV/s.

For this capacitor, the optimal mass ratio, between positive andnegative electrodes, is found to be 1.33 at 5 mV/s and 1.78 at

20 mV/s. In this supercapacitor, the increase of equivalent series

resistance in the positive electrode is evidenced by the slow

variation of the changing of current sign after inversion of volt-

age scan rate at 0 and 1 V. This increased difficulty in the charge

redistribution into the positive electrode may be due to the lower

rate in accepting/releasing anions at the positive electrode than

in accepting/releasing cations at the negative electrode. The cal-

culated specific capacitances for the AC4/AC4 supercapacitor

are 25.2 and 21.4 F/g at 5 and 20 mV/s, respectively, whereas

the values of specific capacitance of the negative and positive

electrodes are respectively 118 and 88 F/g at 5 mV/s and 119 and




Fig. 2. Cyclic voltammograms obtained from symmetric supercapacitors at the voltage scan rates of 5 mV/s (a) and (c) and 20 mV/s (b) and (d). The carbon loading

in the electrodes is 5.5 mg/cm2

(a) and (b) and 13.8 mg/cm2

(c) and (d).

67 F/g at 20mV/s (see Table 2). If, from one side, the compar-

ative study between the AC1/AC1 capacitor and the AC4/AC4

capacitor show relatively high differences of capacities, from

the other side, the negative electrodes of both capacitors do not

show substantial variation of specific capacitance in function of

carbon loading and voltage scan rate. Thus, the main differences

between the two supercapacitors are to attribute to the perfor-

mances of positive electrodes. Therefore, even if this study is

performed on a particular type of supercapacitor, i.e. a solid-

state supercapacitor, the results confirm that it is essential to

do an accurate mass balance of materials of electrodes for the

enhancement of capacitive performance. A direct comparison

of the present results with others reported in the current litera-

ture is quite difficult to do because it is not easy to find similar

studies. For examples, the behaviour of electrodes for symmet-

ric supercapacitors was studied by Mastragostino et al. [22] who

reported values of specific capacitance of 125 and 158 F/g for

the positive and negative electrodes, respectively for carbon-

based supercapacitors and using 1 M NEt4BF4 in acetonitrile

as electrolyte. Wang et al. [23] showed difference of 16% of

the specific capacitance between the positive electrode (87 F/g)

and the negative electrode (102 F/g) for an AC/AC capacitor

Table 2

Electrochemical data of positive electrodes, negative electrodes and supercapacitors obtained by cyclic voltammetry tests

Supercapacitor

configuration (− /+)

Cyclic voltammetry at 5 mV/s Cyclic voltammetry at 20 mV/s

V (V) Specific supercap.

capacitance (F/g)

Specific

capacitance (F/g)

V (V) Specific supercap.

capacitance (F/g)

Specific

capacitance (F/g)

(−) pole (+) pole (−) pole (+) pole (−) pole (+) pole (−) pole (+) pole

AC1/AC1 0.449 0.551 26.3 117.4 95.7 0.434 0.566 24.5 113.1 86.7

AC4/AC4 0.428 0.572 25.2 117.7 88.0 0.360 0.640 21.4 119.2 67.0

AC2/ACR1 0.714 0.286 28.0 100.1 161.3 0.704 0.296 25.3 91.6 140.6

AC3/ACR1 0.609 0.391 33.1 103.8 177.4 0.570 0.430 29.5 99.1 144.0

AC4/ACR1 0.456 0.544 39.5 135.6 201.6 0.627 0.373 33.2 82.9 247.3




in 1.5 M TEMAPF6 electrolyte in propylene carbonate (PC).

Lozano-Castello et al. [24] reported higher capacitance for neg-

ative electrodes than for positive electrodes. The authors were

using activated carbon materials in the preparation of both elec-

trodes, and a standard three-electrode cell with 1 M LiClO4

electrolyte in propylene carbonate (PC) to perform the elec-

trochemical measurements. Shiraishi et al. [25] showed higher

capacitance for the positive electrodes than for the negative

electrodes when studied electrodes based on different activated

carbon fibers (ACFs) in a standard three-electrode cell with 1 M

LiClO4 electrolyte in PC. Chmiola et al. [26] reported higher

specific capacitance for the positive electrodes than for the neg-

ative electrodes, although they found for the positive electrodes

a remarkable dependence of capacitance from the pores size of

carbons derived from metal carbide precursors. In the paper,

the authors were using the tetraethylamonium tetrafluoroborate

salt in acetonitrile as electrolyte. Wen et al. [27] showed that

the specific capacitance of the positive electrode was lower than

that of the negative electrode. They studied the electrochem-

ical characteristics of supercapacitors with nanoporous glassycarbons-based electrodes in aqueous KOH electrolyte.

The analysis of our results and those of literature allows to

evidence the difficulty in obtaining clear and univocal findings.

Generally, the reported results [22–24,27] are in agreement with

our own, in the attribution of higher capacitance at the negative

electrode and in the affirmation that the adsorption/release of

cations is favourite, but in some cases an opposite behaviour was

observed [25,26]. These results show that the AC/AC symmet-

ric supercapacitors, which normally are made with electrodes of

equal weight and size, need to be optimised for the weight of

the active material. The results above reported also evidence that

the accessibility of pores to the ionic species (ion-sieving) could

not be the only parameter that controls the specific capacitance

of positive and negative electrodes because controversial results

have been found by analysing different systems of supercapac-

itors. Our studied solid-state supercapacitors, at difference of

those of literature, use a solid polymer electrolyte that is charac-

terized to have a mobile cation (proton) in hydrated form, while

the anion group (R–SO3−) is anchored to the polymer back-

bone. For such reason and also because the polymer micelles

are of dimension in the range of 100–200 nm, the fixed anion

species are not able to reach directly the internal surface of

micro- and mesoporous carbons. Thus, the values of capacitance

of 100–120F/g (e.g. 6.5–8F/cm2), which are comparable tothose currently reported in literature for liquid electrolyte-based

supercapacitors should be explained by a different mechanism.

We suppose that at the positive electrode (in solid-state super-

capacitors) others weak anion species could participate at the

Fig. 3. Galvanostatic charge/discharge curves obtained from symmetric supercapacitors at the current of ± 2.5 mA/cm2 (a) and (b) and ± 10 mA/cm2 (c) and (d).

The carbon loading in the electrodes is 5.5 mg/cm2 (a) and (b) and 13.8mg/cm2 (c) and (d).




formation of the double layer capacitance. These should be orig-

inate by charge exchanges between R–SO3− groups and H2O

molecules. However, the strength of these charges or electric

dipoles could never be equivalent to that of anions present in

aqueous or non-aqueous electrolyte. Therefore, the high level of

double layer capacitance measured in our solid-state superca-

pacitor remains, at this time, a not fully understood subject. In

spite of our previous studies [10,11,20,21], in this work we are

able to separate the capacitance of single electrodes from that

of complete supercapacitor but the mechanism through which

the cations and anions influence the double layer capacitance

remains at the moment without answer.

Fig. 3(a)–(d) shows the galvanostatic charge/discharge (CD)

curves for the same reported supercapacitors performed at the

current densities of ± 2.5 and ± 10 mA/cm2. The experimen-

tal results are summarized in Table 3. It is evident from the

profiles of curves that the positive electrode is also in these

tests the performance-limiting electrode. In fact, it always

presents higher polarization during the charging process and

its polarization increases with the increasing of the carbonloading and the current density. The calculated specific capac-

itances for the AC1/AC1 capacitor are 27.9 and 24.5 F/g at

the currents of ± 2.5 and ± 10 mA/cm2, respectively. The spe-

cific capacitances for negative electrode and positive electrode

are respectively 116 and 107 F/g at ± 2.5 mA/cm2 and 121

and 82F/g at ± 10 mA/cm2. The specific capacitances for the

AC4/AC4 capacitor are 27.5 and 22.7 F/g at the currents of

± 2.5 and± 10 mA/cm2, respectively. The specific capacitances

for negative and positive electrodes are respectively 122 and

100 F/g, at± 2.5 mA/cm2 and, 136 and 68 F/g, at± 10 mA/cm2

(see Table 3). The electrochemical characteristics of electrodes

and capacitors follow the same trend of those reported in Table 2related at CV tests.

The behaviour of the negative and positive electrodes

observed through the voltammograms and CD curves

(Figs. 2 and 3) suggests that a more functional symmetric super-

capacitor should contain a higher mass of carbon in the positive

electrode than in the negative electrode to balance its lower

capacitance.

3.2. Studies of hybrid supercapacitors and discussion

The hybrid supercapacitors are formed of negative electrodes

based on carbon (AC2, AC3, AC4) and positive electrodes based

on ruthenium oxide/carbon (ACR1). The compositions of theelectrodes are reported in Table 1 and the electrochemical results

obtained from the supercapacitors by CV and CD tests are sum-

marized in Tables 2 and 3, respectively.

The behaviour of hybrid supercapacitors in dependence of

the carbon loading in the negative electrodes (from AC2 to

AC4) is more complex to explain compared to that of the

symmetric supercapacitors. It is well known that redox phe-

nomena associated with the ruthenium-based positive electrodes

influence the functionality of hybrid capacitors. The origin of

pseudo-capacitance in RuO2· xH20 electrode has been attributed

at different redox processes [2,28,29] in which protons and

electrons are exchanged by ruthenium compound through theT a b l e 3

E l e c t r o c h e m i c a l d a t a o f p o s i t i v e e l e c t r o d e s , n e g

a t i v e e l e c t r o d e s a n d s u p e r c a p a c i t o r s o b t a i n e d b y g a l v a n o s t a t i c c h a r g e / d i s c h a r g e t e s t s

S u p e r c a p a c i t o r

c o n fi g u r a t i o n ( − / + )

C h a r g e / d i s c h a r g

e a t 2 . 5 m A / c m

2

C h a r g e / d i s c h a r g e a t 1 0 m A / c m

2

V

( V )

S p e c i fi c s u p e r c a p .

c a p a c i t a n c e ( F / g )

S p e c i fi c c a

p a c i t a n c e ( F / g )

V

( V )

S p e c i fi c s u p e r c a p .

c a p a c i t a n c e ( F / g )

S p e c i fi c c a p a c i t a n c e ( F / g )

( − ) p o l e ( + ) p o l e

( − ) p o l e

( + ) p o l e

( − ) p o l e ( + ) p

o l e

( − ) p o l e

( + ) p o l e

A C 1 / A C 1

0 . 4

8 0

0 . 5

2 0

2 7 . 9

1 1 6 . 5

1 0 7 . 5

0 . 4

0 4

0 . 5 9

6

2 4 . 5

1 2 1 . 5

8 2 . 4

A C 4 / A C 4

0 . 4

5 2

0 . 5

4 8

2 7 . 5

1 2 1 . 9

1 0 0 . 5

0 . 3

3 4

0 . 6 6

6

2 2 . 7

1 3 6 . 1

6 8 . 2

A C 2 / A C R 1

0 . 6

9 1

0 . 3

0 9

3 2 . 9

1 2 1 . 5

1 7 5 . 4

0 . 6

7 3

0 . 3 2

7

3 0 . 6

1 1 5 . 9

1 5 3 . 9

A C 3 / A C R 1

0 . 6

0 3

0 . 3

9 7

3 1 . 2

9 9 . 0

1 6 4 . 9

0 . 5

9 5

0 . 4 0

5

2 8 . 9

9 2 . 9

1 4 9 . 7

A C 4 / A C R 1

0 . 4

9 8

0 . 5

0 2

4 1 . 5

1 3 0 . 3

2 2 9 . 4

0 . 4

2 1

0 . 5 7

9

3 5 . 8

1 3 3 . 0

1 7 1 . 6




following mechanism:

RuOx(OH)y +wH++we−⇔ RuOx−w(OH)y+w

(0 ≤w≤ 2)

The reaction indicates that the symmetric redox pairs can be

observed by appearance of current peaks in the voltammograms,

which are due to reversible gradual changes of ruthenium oxi-

dation state from Ru2+ to Ru4+ with the variation of potential

of the electrode. The capacitance is therefore influenced from

proton and electron conduction in the bulk and/or on surface

of the material as well as from the particle sizes and the struc-

ture of ruthenium oxide. The water content plays an important

role in H+ transport, while the electron conduction is mainly

influenced by the crystalline degree of RuO x structure. Opti-

mal water content and crystalline degree should be realized

for a balanced mixing of protonic–electronic conduction, which

should maximize the pseudo-capacitance delivered from ruthe-

nium oxide. Further, the specific capacitance could be improved

by supporting the oxide on carbon. This latter, in fact, should

increase the available active surface area and should function as

an electrolyte reservoir [30]. Recently, has been reported that the

specific capacitance of RuO2· xH20 isgivenfromthree maincon-

tributes, which are: (a) electric double layer capacitance (C DL),

(b) reversible redox reaction due to electro-adsorption effects

(C AD), and (c) irreversible redox related charge (C IRR) [31,32].

Fig. 4. Cyclic voltammograms obtained from the hybrid supercapacitors at the voltage scan rate of 5 mV/s (a), (c), and (e) and 20 mV/s (b), (d), and (f). The carbon

loading in the negative electrodes is 5 mg/cm2 (a) and (b); 8.5 mg/cm2 (c) and (d); 13.8 mg/cm2 (e) and (f).




The contribution of redox processes on the specific capacitance

may depend from the characteristics of ruthenium oxide materi-

als, e.g. anhydrous, amorphous, crystalline and/or porous form.

Moreover, the electrochemical utilization of ruthenium oxide

material is usually limited to very thin layer, while in the ACR1

electrode we have thickness as high as 150m. However, being

the ruthenium oxide supported on activated carbon material high

particles dispersion can be obtained.

Also forhybridsupercapacitors,the mass balance of thenega-

tive electrode has to be optimised for maximizing the capacitive

performance. In fact, at a low (AC2) and medium (AC3) car-

bon loading in the negative electrode, it is this electrode to limit

the capacitive performance, as it is shown in Table 3. Further,

the voltammograms of supercapacitor do not display rectangular

shape because they present the effects of faradaic process occur-

ring at thepositive electrode. Nevertheless,the redox peaks in the

voltammograms of supercapacitor result attenuated because at

the positive electrode, with pseudo-capacitive property, it is cou-

pled the negative electrode with pure double layer capacitance.

Therefore, in hybrid supercapacitors the shapes of the voltam-

mograms show that there is a reciprocal influence between the

positive and negative electrodes and that is important to make

a correct balancing of materials for enhancing the capacitive

performances. The specific capacitances of AC2/ACR1 capaci-

Fig. 5. Galvanostatic charge/discharge curves obtained from the hybrid supercapacitors at the current of ± 2.5 mA/cm2 (a), (c), and (e) and ± 10 mA/cm2 (b), (d)

and (f). The carbon loading in the negative electrodes is 5 mg/cm2 (a) and (b); 8.5 mg/cm2 (c) and (d); 13.8 mg/cm2 (e) and (f).




tor at 5 and 20 mV/s are 28.0 and 25.3 F/g, respectively. Those

of the negative and positive electrodes are 100.1 and 161.3 F/g

at 5 mV/s and 91.6 and 140.6 F/g at 20 mV/s, respectively. By

increasing the carbon loading of the negative electrode (carbon

loading isin electrode AC2< AC3 < AC4, see Table 1), the polar-

izations of the negative electrodes, which are measured through

the CVs at 5 mV/s, gradually decrease (Fig. 4(a), (c), and (e)).

The AC4/ACR1 supercapacitor with the highest carbon loading

achieves the highest value of specific capacitance of 39.5 F/g,

and the respective correspondent values for the negative and

the positive electrode are 135.6 and 201.6 F/g. In such better

situation, the negative electrode efficiently redistributes a large

amount of charges and presents lower polarization than that of

positive electrode. At the voltage scan rate of 20 mV/s (Fig. 4e),

the behaviour of AC4/ACR1 supercapacitor presents some dif-

ference from that expected because the negative electrode turns

to be the more polarised electrode because it is affected by high

ionic resistance that contrasts the rate of charge redistribution.

This effect was not observed in the experiments conducted at

5 mV/s because probably it would appear at higher values of carbon loading than those here investigated. The hybrid super-

capacitors were also studied by galvanostatic CD tests. The

respective experimental curves are reported in Fig. 5(a)–(f) and

the results summarized in Table 3. From the tests performed

at ± 2.5 and ± 10 mA/cm2, we have found at each current a

gradual decrease of polarization of negative electrode with the

increase of the carbon loading. The comparison of the CD curves

of supercapacitors at the same carbon loading and at different

charge-discharge currents evidences that for the low (Fig. 5(a)

and (b)) and medium carbon loading (Fig. 5(c) and (d)), there

is not sensible variation in the behaviour of electrodes, apart of

a slight decrease of capacitance by increasing the CD current.With the AC4/ACR1 supercapacitor at high carbon loading, the

increase of the CD current from ± 2.5 to± 10 mA/cm2 causes

a pronounced lowering of the polarization of the negative elec-

trode (Fig. 5(e) and (f)) that produces a sensible increase of

the specific capacitance of this electrode. In such condition, the

calculated specific capacitances are 133.0 F/g for the negative

electrode, 171.6F/g for the positive electrode and 35.8 F/g for

the supercapacitor. The highest capacitance performance in CD

tests, with a value of 41.5F/g, is obtained for the capacitor with

the highest carbonloading in thenegative electrode at thecurrent

density of ± 2.5 mA/cm2. The specific capacitances for negative

and positive electrodes are 130.3 and 229.4 F/g, respectively.

The behaviour of negative electrode of the same supercapaci-tor previously observed in the voltammograms of Fig. 4(d) and

(f) varying from medium to high the carbon loading is appar-

ently different from that observed in Fig. 5(d) and (f) referred

to CD tests. In fact, in the CD curves a regular decrease of the

polarization is evidenced for the negative electrode varying from

low, to medium, to high the carbon loading, while this effect is

not observed in the voltammograms. A possible explanation for

this different behaviour is that in the CD curves a lower current

(±10 mA/cm2) flows in the cell respect to that measured during

the CV tests at 20 mV/s (about ± 15 mA/cm2). It is reasonable

to expect also in CD tests that a higher polarization for the neg-

ative electrode would be obtained at higher currents than those

here investigated, when substantial effects of equivalent series

resistance in negative electrode would be apparent. The analysis

of Tables 2 and 3 and Figs. 4 and 5 shows that in the investigated

hybrid capacitors the electrodes are reciprocally influenced and

that their specific capacitances are function of parameters such

as the active material loading and the electric current flow.

The analysis of the results reported in this paper evidence,

for example, that the values of specific capacitance of nega-

tive electrodes for symmetric and hybrid supercapacitors were

found to vary up to 35% (from 100 to 135.6 F/g). This means

that for any given carbon the specific double layer capaci-

tance is not an invariant property of material, but rather it is

a characteristic value that is valid in the specific condition

of measurement. This study also demonstrates that the spe-

cific capacitance of an electrode obtained in a three-electrode

system may be much different from that measured in a real

supercapacitor because of the reciprocal influence between the

electrodes.

4. Conclusions

In this work, we report a study performed on symmetric and

hybrid solid-state supercapacitors in which the electrochemical

features of the complete capacitors and of the positive and nega-

tive electrodes were studied by two electrochemical equipments

connected to the test cell. In particular, the study showed that

an optimisation mass ratio, between the positive and negative

electrode was essential for maximizing the capacitance perfor-

mances of symmetric solid-state supercapacitors. We found that

the mass of carbon in the positive electrode must be 1.2–1.3

times than that of negative electrode in determined operative

conditions. Moreover, it was found that the positive electrode isthat controlling the capacitive performance of symmetric super-

capacitor because it showed higher polarizations. However, high

value of specific capacitance of 26.3 F/g at 5 mV/s were found

for the symmetric AC1/AC1 supercapacitor and specific capac-

itances of negative and positive electrodes of 117 and 95.7 F/g,

respectively. The study of hybrid supercapacitors has demon-

strated that an optimal capacitance performance may be obtained

by optimising the carbon loading in the carbon based negative

electrode. In fact,it was evidenced that thebehaviour of oneelec-

trode, influences that of other electrode and that of complete

supercapacitor. Furthermore, we have found that the specific

capacitance for a given carbon material may vary very much

in dependence of composition of electrodes and working con-ditions, and a variation up to 35% was recorded in our tests.

This is meaning that the specific capacitance of an active carbon

material may not be considered as a characteristic property, but

a characteristic value that is valid for those specific experimental

conditions.

Acknowledgements

The authors acknowledge Fumatech GmbH (Germany) that

has supplied the polymer electrolyte solution and membrane,

and NoritSpa Ravenna (Italy) thathas furnished activated carbon

material for our experiments.




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A Study of the Electrochemical Behavior of Electrodes in Operating Solid State Super Capacitors

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Transcript of A Study of the Electrochemical Behavior of Electrodes in Operating Solid State Super Capacitors