272 _Acta Chim. Slov. 2014, 61, 272-279
Scientific paper
Some Factors Influencing Power and Energy Capabilities
of RuO2 Supercapacitors
Suzana Sop~i}, Zoran Mandi}* and Marijana Kralji} Rokovi}
Faculty of Chemical Engineering and Technology, University of Zagreb, Maruli}ev trg 19, HR-10000 Zagreb, Croatia
* Corresponding author: E-mail: zmandic@fkit.hr
Received: 10-02-2014
Paper based on a presentation at the 4th RSE-SEE 2013 Symposium on Electrochemistry in Ljubljana, Slovenia
Abstract
Ruthenium oxide electrodes prepared by different routes were studied and the results discussed in terms of the possibility of using these electrodes in high power/high energy supercapacitors. The supercapacitor electrodes were prepared by mixing RuO2 particles with a binder (Nafion® or polyvinilydenfluoride) in various ratios. The results show that charging/discharging reaction of RuO2 consists of at least two redox reactions taking place simultaneously at different rates. The contribution of each reaction in the overall process depends on the hydration of RuO2 as well as on the type of binder and binder/RuO2 ratio. From both energy and power capability of supercapacitors the best electrode composition would be hydrous RuO2 with -20% Nafion® as a binder. Asymmetric supercapacitors assembled with RuO2 and activated carbon as a counter electrode gave 26 and 12 W h kg-1 at average specific power of 5 W g -1 for RuO2/Nafion® and RuO2/polyvinylidene fluoride, respectively.
Keywords: RuO2; Supercapacitors; specific energy; Nafion®; PVDF
1. Introduction
Materials exhibiting pseudocapacitive behaviour are in the focus of research aiming to develop active, reversible and efficient electrodes for supercapacitor applications. Such materials must be able to store high charge and release it fast enough upon demand to ensure driving high power applications.1 There are generally two classes of materials meeting such criteria: transition metal oxides and conducting polymers. Among them ruthenium oxide exhibits the most promising properties. It can be reversibly and repro-ducibly charged and discharged in a wide potential range and shows extraordinary chemical and morphological stability to undergo several thousands and in some cases even hundred thousands of charging/discharging cycles, without significant deterioration of its performance. When in amorphous and hydrous form, it can reach practical specific capacitance of up to 720 F/g2 which is one of the highest specific capacitances ever observed in real applications.
In spite of these outstanding properties, ruthenium oxide supercapacitors have still not achieved widespread commercial applications. The main obstacle is its high price and as a consequence, the ruthenium oxides supercapacitors
can nowadays be found only in some niche applications where the reliability and performance are more important than costs. In order to overcome these problems, many research efforts were made to design the electrodes with maximal utilization of ruthenium oxide and to increase charge content as much as possible. One direction of the improvement is the optimization of electronic conductivity and proton availability of the bulk ruthenium oxide since both quantities are necessary for its good performance.3-10 It is usually done by controlling the hydration of the ruthenium oxide particles annealing them at the temperatures around 150 °C where the good electron conductivity is achieved by removing the physically bound water present in the material and at the same time, preserving microcrystalline water enough to enable sufficiently fast proton diffusion.
Other directions of the increase of ruthenium oxide utilization were attempted by different preparation methods such as electrochemical deposition,6,11-13 oxidative chemical synthesis,5 sol-gel process,2,14-18 electrostatic spray deposition,7,19 electrophoretic deposition8 and hydrothermal synthesis.20 Further improvement was made by the preparation of various composite materials usually with different forms of carbon.3,14,15,21-25 Such com-
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posites effectively combined double layer capacitance of carbon with RuO2 pseudocapacitance and at low RuO2 loadings the capacitances as high as 1500 F/g,3'4'10 the values close to the theoretical capacitances of RuO2, were achieved. In real applications, however, where substantial energy content is to be ensured, such low RuO2 loadings are not practical and the bulk RuO2, either in the form of thin film or as micro- to nano-sized particle assembly, must be used.
It is generally accepted that the electrochemical reaction accompanying the charging/discharging process involves simultaneous proton and electron exchange:
RuOx(OH)y + ze- + zH+ « RuOx-z(OH)y+z
(1)
However, a lot of experimental evidences accumulated over the years demonstrated that the charging/discharging reaction of RuO2 is more complex than it could be described by equation (1) and discerned from its cyclic voltammograms. This was explained by at least two different electrochemical reactions taking place simultaneously at different rates during charging/discharging process. Taking into account the reaction (1) it was proposed that the fast reaction was associated with the charging of outer surface area, whereas the slow process was due to the charging of deeper and less accessible oxide layers.26 27 Recent investigation by simultaneous cyclic voltammetry and QCM gravime-try110 28 showed that second reaction taking place in parallel to reaction (1) could be conveniently described by equation (2):
2RuO2 + 2e- + 2H+ « Ru2O3 + H2O
(2)
In contrast to the first mechanism which rate is limited by the diffusion of both electrons and protons in the hydrous oxide network, second reaction has more defined stoichiometry and proceeds through the incorporation of oxygen from water into the Ru(III) oxide during oxidation reaction and vice versa during reduction. Depending on whether water molecules stem from the solution side or from inside of hydrous layer, the process might be diffu-sionally or kinetically controlled. The RuO2 charging/discharging mechanism of the reaction (2) type was recently termed dissociative adsorption of water.29
In this work we focused on the preparation of RuO2 electrodes with the intention to investigate the factors influencing relative rates of the two charging/discharging reactions and how their relative contributions affects su-percapacitor performance from the high power/high energy standpoints. In order to be as close as possible to the practical applications for the preparation of the electrodes two binders, Nafion® and poly(vinylidene fluoride) (PVDF), were used. These binders have already shown significant impact on the electrochemical reaction of RuO2.30
2. Experimental
2. 1. Chemicals and Materials
RuO2 in hydrated and amorphous form, Nafion® 117 (5% solution in isopropyl alcohol) and poly(vinylidene fluoride) (PVDF) were purchased from Aldrich Chemie, N-methyl pyrrolidone (NMP) from Merck, isopropyl alcohol from T.T.T. (Croatia) and H2SO4 from Fluka. All solutions were prepared from bi-distilled water. X-ray diffraction and thermogravimetric analysis of the RuO2 sample were published elsewhere.10
2. 2. Preparation of the Electrodes
The electrodes for EQCM measurements were prepared by attaching the RuO2 particles by cotton swab containing the material, and by using the drop of distilled water to fix the particles. In that way the particles were glued to electrode and randomly distributed assembly of particles on gold (A = 1.22 cm2) with more or less uniform distribution, was obtained.3132 The RuO2 used in these measurements was either thermally untreated or it was annealed at various temperatures from 70 to 200 °C. The amount of RuO2 was determined by the resonant frequency changes caused by deposition of RuO2 in dry state. The deposited mass of Ru-O2 used in these studies was between 6-20 ^g.
RuO2/binder electrodes were prepared by casting of homogenized dispersions of RuO2 particles in the solutions of appropriate binder on the glassy carbon electrode (A = 0.07 cm2). Two binders were used: Nafion® in iso-propyl alcohol and PVDF in N-methyl pyrrolidone (NMP). The electrodes were dried for 24 hours at room temperature in the open air, in the case of Nafion®, and in the vacuum at the temperature of 40 °C, in the case of PVDF. The masses of RuO2 used for preparation of such composites were in the range of 0.04 to 0.46 mg whereas masses of binder were in the range of 0.025 to 0.4 mg, resulting in binder/RuO2 ratio ranging between 0.25 and 4.
2. 3. Assembling of Asymmetric Capacitor
The glassy carbon electrode was used as a substrate for both positive and negative electrodes. The positive electrode was electrode with RuO2/Nafion® or RuO2/PVDF composite, while the negative electrode was activated carbon mixed with PVDF binder. Electrodes were separated with cellulose membrane soaked in 0.5 mol dm-3 H2SO4 electrolyte.
2. 4. Simultaneous Cyclic Voltammetry (CV) and Electrochemical Quartz Crystal Microbalance (EQCM) Measurements
Simultaneous cyclic voltammetry and EQCM measurements were carried out using Quartz Crystal Microbalance Digital Controller QCM200 (SRS, USA) connected to EG&G M263A potentiostat. The three-electrode sys-
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tem was used with the working gold coated quartz crystal electrode made of 5 MHz AT-cut crystals of one inch diameter (Stanford Research Systems, SRS, USA). Each side of crystals was coated with titanium underlayer and gold. The surface area of electrode side which is in contact to an electrolyte was 1.22 cm2 and inner surface area, so called piezoelectric area was 0.427 cm2. The reference electrode was Ag/AgCl (3M KCl) while the counter electrode was Pt-foil. All measurements were performed in 0.5 M sulphuric acid in the potential range from -0.1 V to 1 V with scan rates of 0.05 and 0.2 V s-1. Due to the good reproducibility of cyclic voltammograms only one cycle was taken for each measurement.
2.	5. Electrochemical Impedance Spectroscopy
Electrochemical impedance spectroscopy measurements were performed in the same three-electrode system with lock-in amplifier PAR 5210 which was connected to M263 potentiostat. The frequency range from 100 kHz to 10 mHz was scanned at the dc potential of 0.45 V vs. Ag/AgCl, with the amplitude of 5 mV.
3. Results and Discussion
3.	1. Cyclic Voltammetry and EQCM
Investigations
Investigations of the pseudocapacitive reaction of Ru-O2 was undertaken by cyclic voltammetry and simultaneous monitoring of mass change by EQCM on the hydrous samples as well as on samples annealed at various temperatures in the range from 70 to 200 °C. Fig. 1 shows the comparison of the results of the two samples, thermally untreated (Fig. 1a) and one treated at 130 °C (Fig. 1b). The cyclic voltammograms look similar showing almost rectangular
shape and reversible electrochemical behaviour as would be expected for material having a potential to be used in su-percapacitors. A broad anodic peak at about 0.5 V and its cathodic counterpart are dominant part of the cyclic voltammograms for the thermally untreated RuO2. The peaks appear in the acidic media only and can be assigned to the reversible redox process accompanied by the sorption of protons and associated water molecules.33'34 Uneven current distribution throughout the investigated potential range points out the potential dependent capacitance of RuO2. Low currents at more negative potentials of the cyclic vol-tammograms are explained by increased resistances caused by the bound water between oxide particles.3'4
Thermal treatment of RuO2 particles before the measurements has the profound effect on their electrochemical performance. Heating the samples removes almost all physically bound water resulting in the flattened cyclic voltammograms with more uniform current profiles. Overall charge increases with the annealing temperature
Figure 2. Dependence of specific capacitance of RuO2 on the temperature pre-treatment. Specific capacitances were calculated by the integration of the cyclic voltammograms recorded at 0.05 V.
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and the maximum charge can be extracted from the samples annealed at the temperatures around 130 °C (Fig. 2).
Another striking effect of the temperature pre-treat-ment is the change of the charging/discharging mechanism. Fig. 1 also shows simultaneous mass changes during the course of electrochemical oxidation and reduction of the two samples. Oxidation of the thermally untreated sample is accompanied by almost linear mass decrease while during reduction mass increase is observed. However, the sample annealed at 130 °C shows opposite behaviour, mass increase during oxidation and mass decrease during reduction. These results clearly indicate that electrochemical reaction of RuO2 is a complex process consisting of at least two simultaneous reactions. As confirmed in the previous papers10,28 the two processes could be conveniently described by reactions (1) and (2). A dominant feature of the reaction mechanism described by equation (1) is a simultaneous proton and electron ingress during reduction and their expulsion during oxidation reaction. Since the reaction (1) involves the diffusion of protons and electrons which could limit the discharge rate of the oxide, high electronic and ionic conductivities are of the crucial importance in the design of high power supercapacitors based on RuO2. On the other hand, reaction (2) proceeds with the oxygen atom incorporation in the oxide matrix during oxidation which is manifested as the overall mass increase of the electrode.
Both reactions occur simultaneously during charging and discharging of RuO2. However, their relative rates and their contributions to the overall charge vary according to the sample treatment and experimental conditions. In highly hydrated and thermally untreated samples the reaction (1) is a dominant reaction in the overall process. In order to maximally utilize the material and to achieve capacitance and consequently specific energy as high as possible, the water content should be optimized. Removing the physically bound water enables less resistive electron pathways extending the depth of reaction layer and improving both available energy and power at the same time. However, decreasing the water content is less favourable for the proton diffusion. Consequently the contribution of the reaction (1) starts to decline and at pre-treatment temperatures above 130 °C the performance of charging/discharging process is determined in a great extent by reaction (2). By increase of temperature, the overall capacitance and energy start to decrease but on the other hand, since the diffusion as a slow process is eliminated, it should have favourable influence on the available power. The evidence for power improvement is more reversible cyclic voltammogram (Fig. 1b) obtained by temperature pre-treatment of RuO2.
3. 2. Preparation and Properties of RuO2 Electrodes
To confirm the conclusions made from simultaneous cyclic voltammetry and EQCM measurements in the previous section, bulk RuO2 electrodes were prepa-
red and tested in the sulphuric acid electrolyte. The electrodes were prepared by mixing certain quantities of RuO2 particles with a binder in order to keep mechanical integrity of the electrodes and to be as close to the practical applications as possible. Two binders, PVDF and Nafion®, possessing different physico chemical properties were used since they were already shown to exert a considerable effect on the overall performance of RuO2 electrodes.30 The comparison of the cyclic vol-tammograms of RuO2 electrodes prepared with Na-fion® and PVDF as binders is given in Figure 3. Due to different influence of binder on RuO2 particles, somehow higher specific current and slight peak separation was obtained in RuO2 electrode with Nafion® as binder. The cyclic voltammograms exhibit similar features to those recorded at the EQCM electrodes. The values of specific currents achieved at EQCM measurement setup are much higher due to thinner layer and better electric contact between of RuO2 particles with substrate as well as better contact of RuO2 particles with electrolyte.
Figure 3. Cyclic voltammograms RuO2/Nafion (full line) and Ru-O2 /PVDF (dash line) electrodes with binder/RuO2 ratio of 0.25, m(RuO2 = 0.1 mg), examined in 0.5 M H2SO4 electrolyte. Scan rate: 0.05 V s-1.
Although two cyclic voltammograms look similar, the complexity of the charging/discharging reaction of RuO2 was revealed by dependence of specific capacitances of RuO2 electrodes on the type and the quantity of the binder used and by the impedance spectroscopy measurements. Figure 4 shows the dependence of specific capacitances, Cs, of RuO2 electrode on its composition. While specific capacitance in the presence of PVDF increases steadily with its mass increase, specific capacitances of RuO2 electrodes in the presence of Nafion® follow more complex relationship. They increase with mass of Nafion® up to binder/RuO2 ratio of approx. 0.25 and at higher ratios, specific capacitances start to decline.
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Bode plots for two different compositions of RuO2 /Nafion® and RuO2 /PVDF electrodes are shown in Figs. 5a and 5b, respectively. As expected for the electrodes with good capacitive performance, the Bode plots of all samples consist of resistive behaviour at high and capacitive response at low frequencies. Depending on the sample composition and temperature pre-treatment, diffusion-like behaviour is observed in some cases in the middle frequency range. Diffusion-like behaviour is manifested in close to 45° phase angle and the slope of log Z - log o of -0.5 in the middle frequency region. Such impedance responses were usually observed for the porous electrodes involving RuO2 pseudocapacitive beha-viour.6,35,36
Figure 4. Characteristic dependence of the specific capacitance, Cs, on the mass of binder (squares - Nafion®, circles - PVDF) in the composite electrodes with m(RuO2) = 0.1 mg. Specific capacitances were calculated by the integration of the cyclic voltammo-grams recorded at 0.05 V.
Diffusion characteristics can be discerned in all impedance plots of RuO2 /PVDF electrodes and for Ru-O2/Nafion® electrodes but only with low Nafion® content. Observed diffusion processes are frequently explained by the rate-limiting transport of ions, mainly protons, into the deeper layers of porous and hydrous RuO2. By increasing the amount of Nafion® content diffusion process disappears from the middle frequency ranges. Taking into account higher capacitance obtained with RuO2/Nafion® electrodes compared to RuO2/PVDF electrodes (Fig. 3), it is evident that the presence of Nafion® facilitates the diffusion. In terms of two different electrochemical reactions represented by equations (1) and (2) it seems that Nafion® facilitates the contribution of reaction (1) in the overall capacitance of RuO2 electrodes. Such composite electrode could be favourable for preparation of high power super-capacitors.
Since specific capacitance of RuO2 /Nafion® electrodes increases with Nafion® content (Fig. 4), as long as its mass does not exceed the critical value, it can be concluded that Nafion® has a positive impact on the Ru-O2 overall performance.36 The positive influence of Na-fion® is demonstrated on energy but also on the power content, depending on its amount in overall mass of composite electrode. Obtained results indicate the ability of composite materials to give high specific energy with smaller Nafion® content or high specific power with higher amount of Nafion®. Higher Nafion® loadings are not so effective for energy probably due to increased ohmic resistances among RuO2 particles and decreased utilization of RuO2.
Difference of the impedance behaviour between the electrodes prepared with Nafion® and PVDF can be explained by different physico-chemical properties of these two macromolecules.30 With its sulphonic groups attached
a) -ï
90
b) 2-5
90
60
- 30 &
a ta
Figure 5. Bode plots of (a) RuO2 /Nafion electrodes with binder/RuO2 ratio of 4, m(RuO2) = 0.1 mg (black curve) and 0.86, m(RuO2) = 0.46 mg (red curve); (b) RuO2 /PVDF electrodes with binder/RuO2 ratio of 0.25, m(RuO2) = 0.1 mg (black curve) and 0.1, m(RuO2) = 0.04 mg (red curve).
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on the polymer backbone, Nafion® is attracted toward hydrous surface of RuO2 effectively enveloping the particles interfering with the exchange of ions during charging and discharging process.
On the other hand, PVDF as a binder due to its hydrophobicity tends to form separate clusters within Ru-O2 electrode leaving the surface of RuO2 particles exposed to the solution. As a consequence, the pseudocapaci-tance reactions are not affected and reaction (1), contributes to the overall charge of the electrode displaying diffu-sional process in the impedance spectra (Fig. 5b).
As explained in the previous section, temperature pre-treatment of RuO2 particles prior assembling the electrodes removes water from the hydrous layers of oxide and hinders proton diffusion decreasing the importance of the reaction (1) in the overall capacitance and leaving reaction (2) as the dominant reaction in the charging/discharging reaction. This behaviour is clearly visible in the impedance spectra of the RuO2 /PVDF electrodes where the process in the middle frequency range which is assigned to diffusion disappears (Figure 6).
3,0
2,4
~ 1,2
0,6
0,0
		—•—untreated
		-»-130 oC
y		
\		
\		
		
	> V	
	V \ -ri-1—I i " i ill-1—i i i i i m 1-1—1 i t iTii "—T—f"	t.._______
10'
10
10
10
10'
10'
10-
10
f/Hz
Figure 6. Bode plots of the thermally untreated RuO2 /PVDF sample (black curve) and one treated at 130 °C (red curve); binder/Ru-
These results are in accordance with the results obtained by the simultaneous CV and EQCM measurements and they unambiguously demonstrate the existence of at least two types of charge in the pseudocapacitive reaction of RuO2. Although total reaction mechanism is probably even more complex and composed of several simultaneous reactions including double layer char-ging,33 two reactions described by equations (1) and (2) can reasonably explain the behaviour of RuO2 electrodes. In order to achieve high energy RuO2 supercapaci-tor it would be of a key importance to enable both reactions, taking place at the surface and in the deeper layer of oxide, to proceed at sufficiently high rates. In the case when Nafion®/RuO ratio does not exceed the value
of 0.25, Nafion® promotes proton conducting pathways enhancing significantly charging/discharging capacitance. Nafion®/RuO2 ratio of 0.25 consists 20% of Nafion® in the overall mass of the electrode and it would be the composition of choice for achieving excellent superca-pacitor performance. If higher energy is required only, then RuO2/PVDF electrode and RuO2/Nafion® with smaller loading of Nafion® would be optimal compositions. However, if only high power is required then Ru-O2/Nafion® with higher amounts of binder would be advisable.
3. 3. Performance of the Assembled RuO2 Supercapacitors
RuO2 electrodes were tested in the two-electrode configuration of the asymmetric supercapacitor with the counter electrode consisting of activated carbon. The quantities of activated carbon were carefully selected to match the energies of the two electrodes. Activated carbon was loaded on the glassy carbon support with PVDF as a binder and it was used for the negative electrode in the capacitor. Positive electrode was RuO2/Na-fion® or RuO2/PVDF with binder/RuO2 ratio of20.25 and 0.5, respectively. In all cases the cell was charged up to 1 V while the current used for charging and discharging of such capacitors was 2 mA. Figure 7 shows the comparison of the performances of the supercapaci-tors based on RuO2 electrodes made with Nafion® and PVDF as binders.
Cyclic voltammograms and constant current charging/discharging profiles of investigated supercapaci-tors are shown in Figs. 7a and 7b. Cyclic voltammo-grams are rectangular and, as expected, the higher specific current was obtained for RuO2/Nafion® in comparison to RuO2/PVDF capacitor (Fig. 7a). Specific energies of 26 and 12 W h kg-1 were obtained for Nafion® and PVDF based RuO2 electrodes, respectively, at average specific power of 5 W g-1. During 1000 charging/discharging cycles the specific energy practically did not change for RuO2/Nafion® capacitor while it slightly increased in the case of RuO2/PVDF capacitor (Fig. 7c). Such increase can be explained by the progressive hydration enabling enhanced diffusion of protons at the both electrodes.
4. Conclusions
Charging/ discharging of RuO2 is a complex process consisting of at least two reactions taking place simultaneously. Relative contributions of these two reactions depend on the hydrous state of RuO2 and exact electrode composition. It has been found that the presence of Nafion® has favourable influence on the performance of RuO2 electrodes but only up to 20% of overall content.
O2 ratio = 0.1.
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C) 35 3025
T™
J? 20£
S 15
■ ■ ■.
•u.
1050
Figure 7. Electrochemical characterization of asymmetric RuO2/AC capacitors. (a) Cyclic voltammograms of supercapacitors based on RuO2 /Na-fion® (binder/RuO2 ratio = 0.25) and RuO2 /PVDF (binder/RuO2 ratio = 0.5) electrodes, m(RuO2) = 0.1 mg; (b) galvanostatic charge/discharge curves for the two asammbled capacitors, (c) dependence of the specific energies during 1000 charge/discharge cycles.
-■-RuO/NafWi/AC • RuO/Nafior)RWAC
0 200 400 600 800 1000 number of cycles
Higher loadings are less effective. The RuO2 /Nafion® electrodes with 20% content are found to have the optimal composition for both high power and high energy supercapacitors. For higher energy RuO2/PVDF compositions and for higher power RuO2/Nafion® with higher Nafion® loadings are preferred.
5. Acknowledgements
The financial support for the Ministry of Science, Education and Sport (project 125-1252973-2576) is greatly appreciated.
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Povzetek
Elektrode rutenijevega oksida smo pripravili na različne načine. Dobljene rezultate smo diskutirali glede možnosti uporabe teh materialov v visokozmogljivih superkondenzatorjih. Superkondenzatorske elektrode smo pripravili z mešanjem RuO2 delcev z veznim sredstvom (Nafion® ali polivinilidenfluorid) v različnih razmerjih. Rezultati kažejo, da je reakcija nabijanja in pražnjenja sestavljena iz vsaj dve redoks reakciji, ki se odvijata istočasno z različno hitrostjo. Prispevek vsake izmed reakcij je odvisen od termične obdelave RuO2 kot tudi vrste veznega sredstva in razmerja med veznim sredstvm in RuO2. Na podlagi energije in delovne moči superkondenzatorja smo kot optimalno sestavo elektrode določili RuO2, ki ni bil termično obdelan, z dodatkom ~20% Nafion® kot veznega sredstva. Pri povprečni specifični moči 5 W g-1 proizvede asimetrični superkondenzator, ki je sestavljen in RuO2 in aktiviranega ogljika kot števne elektrode, 26 in 12 W h kg-1 za RuO2/Nafion® oziroma RuO2/polivinilidenfluorid.
Sopcic et al.: Some Factors Influencing Power and Energy Capabilities