J Solid State Electrochem DOI 10.1007/s10008-015-2866-z
ORIGINAL PAPER
Charge storage and capacitance-type properties of multi-walled carbon nanotubes modified with ruthenium analogue of Prussian Blue Magdalena Skunik-Nuckowska 1 & Pawel Bacal 1 & Pawel J. Kulesza 1
Received: 6 March 2015 / Revised: 17 April 2015 / Accepted: 18 April 2015 # Springer-Verlag Berlin Heidelberg 2015
Abstract The ruthenium analogue of Prussian Blue, ruthenium(II,III,IV) hexacyanoruthenate(II,III), was demonstrated to form ultra-thin films (deposits) of mixedv a l e n t r u t h e ni u m - o xo s p e ci e s c r o s s - l i n k e d w i t h cyanoruthenates on the surfaces of multi-walled carbon nanotubes. This polynuclear inorganic material was characterized by various redox transitions effectively adding to the double-layer-type capacitive properties of carbon nanotubes. Fabrication of the redox system (in a form of ultrathin deposits on carbon nanotube surfaces with the content of the oxocyanoruthenium deposit on the level of 15 wt%) was simply achieved by exposing nanostructured carbons to the mixture containing ruthenium(III) chloride and potassium hexacyanoruthenate(II) at pH 2. The composite (hybrid) material composed of carbon nanotubes and oxocyanoruthenium layers was considered here for the construction of symmetric electrochemical capacitor-type system. The specific capacitance, energy, and power densities were determined from the data of cyclic voltammetric, galvanostatic charging-discharging, and AC impedance measurements. The capacitor cell utilizing the composite material was characterized by increased specific capacitance (86 F g−1) which corresponds to the energy density of 2.9 Wh kg−1 without noticeable changes in This paper is dedicated to Professor Mikhail A. Vorotyntsev on the occasion of his 70th birthday. * Magdalena Skunik-Nuckowska
[email protected] * Pawel J. Kulesza
[email protected] 1
Faculty of Chemistry, University of Warsaw, Pasteura 1, 02-093 Warsaw, Poland
power performance in comparison to bare carbon nanotubes-type device. Keywords Multi-walled carbon nanotubes . Ruthenium-oxo species . Cyanoruthenate deposit . Double-layer capacitance . Charge storage systems
Introduction Electrochemical capacitors or supercapacitors [1] have received significant attention during recent years. Various kinds of nanostructured carbon materials that include activated carbons [2–4], carbon blacks [5, 6], aerogels [7, 8], and nanotubes [5, 9–11] have been widely investigated as electrode materials in such capacitors. In particular, multi-walled carbon nanotubes (MWNTs) are characterized by high electrical conductivity, exceptional mechanical stability, good resistance to corrosion, and significant durability during long-term operation. Their low internal resistance arises from the unique mesoporous structure formed by spaces between the entangled tubes in addition to the presence of opened central canals. Such morphology makes the nanotube-based interfaces highly accessible to electrolyte ions. On the other hand, MWNTs are typically characterized by a limited number of micropores which are believed to be responsible for efficient formation of electrical double layer on dispersed carbon surfaces (being in contact with electrolyte). Consequently, the capacitances obtained with MWNTs are moderately high and rarely exceed 100 F g−1 [12]. Obviously, electrical parameters of MWNTs are strongly dependent on methods of preparation and purification as well as on experimental conditions [13]. Chemical activation of MWNTs in alkali metal hydroxides leads to increase of the specific surface area (microporosity development) [14, 15] and to generation of oxygen-
J Solid State Electrochem
containing surface groups undergoing fast and reversible redox (Faradaic) reactions thus contributing to the capacitance-type behavior at the electrode/electrolyte interface. Similar effects have been achieved by exposing of MWNTs to treatments in acids and hydrogen peroxide [16, 17] or by derivatizing MWNTs through formation of nitrogen containing functionalities [12, 18]. Admixing of MWNTs with such redox active materials as conducting polymers or transition metal oxides has been also reported in many studies [19–28]. Ruthenium oxide is one of the most promising redox materials for redox-type supercapacitors (pseudocapacitors) i.e. high-energy electrochemical charge storage devices. Indeed, the specific capacitance of amorphous (sol–gel processed) hydrated RuO2 has been reported to be as high as 768 F g−1 [29, 30]. Because of high cost of ruthenium, further development of ruthenium-based systems would require minimizing the use of the metal while keeping the energy and power densities of devices at reasonable levels. A reasonable solution involves supporting of well-dispersed particles of ruthenium oxide onto nanostructured carbons [31–36]. In this respect, a choice of the material preparation method is crucial. For example, RuO2 nanoparticles (loading, 70 wt%), while dispersed via microwave-polyol process on carbon nanotubes, exhibited the capacitance of 450 F g−1 [36]; on the other hand, the chemically obtained Ru oxide (loading, 13 wt%) yielded the capacitances of only 70 and 120 F g−1 in the presence of hydrophobic and hydrophilic nanotubes, respectively [35]. In the present work, a ruthenium analogue of Prussian Blue, namely polynuclear mixed-valent ruthenium(II,III,IV) hexacyanoruthenate(II,III), abbreviated here as Ru-O/RuCN, has been considered and utilized to modify MWNTs surfaces. Historically, the system was considered in electrocatalysis, namely for electrooxidation of inert analytically or biologically important reactants such as arsenic (III) [39, 44], aliphatic alcohols [38, 40–45], aldehydes [37, 41, 45, 46], hydrazine [47, 48], NADH [47], dopamine [48], insulin [49], glucose [45], cysteine [50, 51], and thiocyanates [52]. Despite numerous similarities to Prussian Blue, i.e., iron(II, III) hexacyanoferrate(II,III), the ruthenium analogue [37–53] exhibits properties of both ruthenium oxide and polymeric cyanometalates. By analogy to Prussian Blue, Ru-O/Ru-CN forms ultra-thin stable films on common electrode surfaces. Contrary to iron(II) in Prussian Blue, the ionic ruthenium component in Ru-O/Ru-CN can be readily oxidized to Ru(III,IV) or Ru(IV) oxo species. In other words, Ru-O/RuCN features ruthenium-oxo species existing within the negatively charged cyanide-bridged network [37–41]. Under certain conditions, the ruthenium ionic sites may undergo further oxidation to even higher (V-VII) oxidation states [37, 43, 53]; but their stability is uncertain and their use is of limited practical importance. Depending on pH and a choice of electrolyte, Ru-oxo species may agglomerate to Ru3O26+ and Ru2O5+; on the other hand, hexacyanoruthenate anionic sites, while stable in
strong acids (1 mol dm−3 H2SO4) [42], tend to form the cyanidebridged ruthenium dimers, (CN)5-RuII-CN-RuIII-(CN)56− in moderately acidic chloride-containing media [44]. Here, modification of carbon nanotube surfaces with Ru-O/ Ru-CN has been achieved by chemical deposition directly on carbon nanotube supports. Such composite or hybrid material (Ru-O/Ru-CN_MWNTs) has been further considered for electrochemical charge storage in 1 mol dm−3 H2SO4 electrolyte. Capacitance properties of the system have been found to reflect combination of charge accumulation in electrical double layers on porous MWNTs and electrochemical charging originating from redox activity of ultra-thin Ru-O/Ru-CN films overcoating MWNTs. Various diagnostic experiments have also been performed to elucidate relative importance of those charging processes and to determine such electrical parameters as specific capacitance, energy, and power densities, as well as equivalent series resistance.
Experimental Materials and preparative procedures Multi-walled carbon nanotubes, MWNTs (from SigmaAldrich), had the following parameters: outer diameter 10– 30 nm, inner diameter 3–10 nm; length 1–10 μm; specific surface area 240 m2 g−1. The 5 % Nafion® 1100 solution (in a mixture of lower aliphatic alcohols and water) was also from Sigma-Aldrich. Hydrated RuCl3 (40.89 % Ru) and potassium hexacyanoruthenate (II) were from Alfa Aesar. Other chemicals were purchased from POCH (Polish Chemical Reagents, Gliwice). Triply distilled subsequently deionized water (Millipore Milli-Q) was used for preparation of solutions. Carbon nanotubes were purified before use under reflux conditions by boiling in 12 mol dm−3 HCl for 1 h (to remove metal catalysts) and in 3 mol dm−3 HNO3 for 8 h (to remove amorphous carbon particles and to enhance the wettability of carbon surfaces). After each treatment, samples were washed with large amount of water until pH became neutral. All other reagents were used as received. To fabricate carbon nanotubes (MWNTs) modified with ruthenium analogue of Prussian Blue, Ru-O/Ru-CN, first, a green-brown mixture containing 50 mg of RuCl3 and 32 mg of K4Ru(CN)6 in 0.25 mol dm−3 K2SO4 (at pH 2) was prepared. In the next step, 200 mg of MWNTs were dispersed in 10 cm3 of the solution described above and subjected to ultrasonic bath for 15 min. Thus, prepared suspension was left for a week to allow for precipitation and spontaneous adsorption of Ru-O/Ru-CN onto carbon (MWNTs) surfaces. The formation of the ruthenium analogue of Prussian Blue was confirmed with a change of the color of the modification solution from green-brown to navy blue. In the next step, the suspension was centrifuged, and the supernatant solution was decanted.
J Solid State Electrochem
Composite material was washed out with large amount of distilled water and dried in 80 °C in air followed by heating for 2 h at 120 °C. From the difference in masses of MWNTs before and after modification, the amount of Ru-O/Ru-CN was estimated to be on the level of 15 wt%. To prepare materials for electrodes to be operated in the two-electrode capacitor cell and for diagnostic experiments, the respective colloidal Bink^ suspensions were obtained by mixing a known amount of MWNTs or Ru-O/Ru-CN-modified MWNTs with ethyl alcohol. The mixture was left under magnetic stirring for 2 h, and then Nafion® ionomer was added (20 wt%) to act as a proton-conductive binding agent. The suspension was stirred again overnight to become homogeneous. Actual preparation of the capacitor electrodes involved brushing of the material onto carbon paper (2 cm× 2 cm) surfaces (Toray, thickness 190 μm; from Electrochem. Inc., USA). Following drying at room temperature, thus, modified carbon paper electrodes were sandwiched in parallel between two stainless steel current collectors. A glassy fibrous separator (GF/C, thickness 260 μm, Whatman) soaked with 1 mol dm−3 H2SO4 as electrolyte was inserted between the electrodes to ensure the ionic (proton) conductivity within the cell. Masses of composite materials on carbon paper supports (including binder) were ca. 5–6 mg for both MWNTs and RuO/Ru-CN_MWNTs.
Electrochemical and structural characterization Electrochemical characterization of two-electrode cells was accomplished with CH 660C Workstation (Austin, USA) by considering cyclic voltammetry (at scan rate 5–100 mV s−1), galvanostatic charging-discharging (at constant current 100– 4000 mA g−1), and AC impedance (amplitude 10 mV; voltage 0 V; frequency range 10 mHz–100 kHz). In order to determine the potential ranges of positive and negative electrodes, an additional saturated silver/silver chloride (KCl, Ag/AgCl) reference electrode was inserted into the measurement cell. The potentials were monitored using two independent Appa (Taiwan) multimeters connected to the computer. The specific capacitance values were expressed in F per active mass of one electrode whereas the energy and power densities were calculated per mass of both electrodes (as it is generally recommended). To execute diagnostic three-electrode-type experiments, saturated silver/silver chloride electrode and a graphite rod were used as the reference and counter electrodes, respectively. A glassy carbon (GC) working electrode (geometric area, 0.07 cm2) was utilized following activation by polishing its surface with aqueous alumina slurries (size 0.05–1 μm) on a polishing cloth (Buehler, USA). The Bink^ suspension of MWNTs or Ru-O/Ru-CN_MWNTs was simply drop-casted onto the surface of glassy carbon electrode and left to dry
under ambient conditions. The material loading was equal to 0.5 mg cm−2. The morphology of investigated materials was examined using Philips CM 10 transmission electron microscope (TEM) operating at 100 kV. The samples were prepared by placing a drop of bare or modified MWNTs dispersed in water (without Nafion®) onto a nickel mesh grid (Agar Scientific, UK) and, subsequently, dried. To determine the quantitative composition and elemental distribution in the Ru-O/Ru-CN containing sample, a pellet pressed under 500 kg cm−2 prepared from RuO/Ru-CN_MWNTs was subjected to scanning electron microscopic measurements (Carl Zeiss Microscope) combined with the energy dispersive X-ray analysis (EDX). Fourier transform infrared (FTIR) examination was performed with Nicolet Magma-IR 550 spectrophotometer. The investigated materials were prepared as KBr pellets which w e r e a dm i xe d w i t h sm a l l am ou n t s of R u - O / R u CN_MWNTs or MWNTs powders or soaked with Ru-O/RuCN solution.
Results and discussion Characterization of Ru-O/Ru-CN_MWNTs hybrid material Figure 1 shows scanning (Fig 1a, b) and transmission (Fig 1c, d) electron microscope images illustrating morphology of the investigated materials, i.e., MWNTs before (Fig 1a, c) and after modification with Ru-O/Ru-CN (Fig 1b, d). The results obtained here are consistent with the formation of the ruthenium analogue of Prussian Blue (Ru-O/Ru-CN) in a form of small spots of the 5 to 20 nm diameter anchored to the surface of carbon nanotubes. Although population of Ru-O/Ru-CN deposits is not dense, they are uniformly distributed on all nanotubes. The loadings of Ru-O/Ru-CN seem to be rather low. It should be remembered that formation of Ru-O/Ru-CN is induced by mild oxidation of ruthenium(III) to ruthenium-oxo species (here by oxygen from atmospheric air rather than by oxidative electrodeposition, as it was done before [42, 44]. It is also likely that rather inert graphitic structures of MWNTs do not favor excessive precipitation of Ru-O/Ru-CN on their surfaces. We have also performed FTIR measurements of pellets containing bare and Ru-O/Ru-CN-modified MWNTs to confirm the presence of the ruthenium analogue of Prussian Blue on carbon nanostructures (Fig. 2). A typical infrared spectrum of Ru-O/Ru-CN (generated as navy blue colloidal precipitate in the solution for modification of Ru(III) and Ru(CN)64−) indicates the presence of water [46] via its characteristic peaks at 1630 and 3500 cm−1 (the latter, which is much larger and corresponds to OH longitudinal vibration mode, is for simplicity not shown here in Fig. 2a). The characteristic peak occurring at 2075 cm−1 in the spectrum of Ru-O/Ru-CN (Fig. 2a) is
J Solid State Electrochem Fig. 1 SEM and TEM images of multi-walled carbon nanotubes, MWNTs: (a, c) bare and (b, d) overcoated with Ru-O/Ru-CN
typical for stretching vibrations of cyanides bridging the metal (ruthenium) ionic sites. The fact that the peak of almost identical wave number (though significantly lower) appears in RuO/Ru-CN_MWNTs (Fig. 2b) but it does not exist in the spectrum of bare MWNTs (Fig. 2c) is consistent with the presence of Ru-O/Ru-CN deposits (at rather low loadings) on carbon nanotubes. No peak splitting and absence of any sizeable shift (only from 2074 cm−1 in Fig. 2a to 2072 cm−1 in Fig. 2b) in the position of that peak upon immobilization of Ru-O/Ru-CN o n M W N Ts p r e c l u d e s s t r o n g e r i n t e r a c t i o n s o f oxocyanoruthenate species with carbon nanotube support.
The qualitative and quantitative analysis conducted with SEM-EDX permitted also to comment on the elemental distribution within Ru-O/Ru-CN_MWNTs hybrid material. To avoid problems originating from the non-uniform topography of the sample and adulteration related to uneven distribution of elements, the material was prepared as a densely packed pellet. Further, the determinations were performed in several independent points at different magnifications and with various primary beam energies. While Fig 3a–d illustrate mapping distribution of individual elements, Fig. 3e refers to the data after superposition of the signals. As expected, such elements as Ru, C, O, and N were present and uniformly distributed in the composite sample. On the basis of the measurements performed for five independent regions of the sample, the wt% amount of ruthenium was found to be on the level of ca. 20 wt% which corresponds to 3.0 at.% (Table 1). Although the peaks characteristic of ruthenium cover all major L-level emission lines for this element, the presence of Ru was additionally confirmed by the presence of K-level lines appearing in the measurement utilizing higher (25 keV) primary beam energy. The atomic ratio of Ru to O was found to be roughly equal to 1.1. It is also evident from the EDX spectrum collected up to 15 keV that, besides the signals related to O, C, N, and Ru, no other peaks were present. Basic electrochemical characteristics of Ru-O/Ru-CN_MWNTs
Fig. 2 FTIR spectra recorded for: a Ru-O/Ru-CN, b Ru-O/Ru-CNmodified MWNTs, and c bare MWNTs
To get insight into electrochemical behavior of Ru-O/Ru-CN attached to surfaces of MWNTs, at first, cyclic voltammetry
J Solid State Electrochem Fig. 3 Elemental mapping distribution for Ru-O/Ru-CNmodified MWNTs obtained from SEM-EDX
experiments have been performed in a conventional threeelectrode cell. Figure 4b illustrates a voltammetric response of glassy carbon electrode overcoated with Ru-O/RuCN_MWNTs composite material. For comparison, a response of bare MWNTs (deposited on glassy carbon) is also shown (Fig. 4a). In view of previous reports [37, 38, 42, 44, 46], the system acts as ruthenium(II,III,IV) hexacyanoruthenate(II,III), and it can be considered as the ruthenium analogue of Prussian Blue. Our present results with the spontaneously deposited RuO/Ru-CN (Fig. 4b) also support this view. The voltammetric peaks appearing at the potentials around 0 Vand in the range of 0.8–1.2 V originate from the electrochemical activity of Ru-O/ Ru-CN, and the behavior is consistent with that reported earlier [42, 51]. The nature of the responses can be explained in terms of overlapping reactions involving electron transfers in the redox couple -RuII(CN)6/-RuIII(CN)6 and reversible oxidations of ionic RuIII to RuIII,IV or RuIV oxo species. The redox transitions Table 1 Elemental composition of Ru-O/ Ru-CN_MWNTs material
Element
wt%
at%
Carbon Ruthenium Nitrogen Oxygen
74.7 20.4 2.1 2.9
92.1 2.9 2.2 2.7
of ruthenium-oxo species [38, 42, 44] can be described as follows: −RuIII −OH þ −RuIII −OH → −RuIV −O−RuIII −OH þ Hþ þ e−
ð1Þ
−RuIV −O−RuIII −OH → −RuIV −O−RuIV −O þ Hþ þ e−
ð2Þ
Fig. 4 Cyclic voltammetric characteristics of a bare and b Ru-O/Ru-CNmodified MWNTs obtained in conventional, three-electrode configuration; scan rate 10 mV s−1; electrolyte 1 mol dm−3 H2SO4
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It is believed that by decreasing the pH of the electrolyte to ca. 0 (1 mol dm−3 H2SO4), as in the present case, direct formation of ruthenium(IV) oxo centers is favored over partial oxidation to mixed oxo/hydroxyl RuIII,IV redox centers [42]. The most negative set of peaks appearing at about 0 V originates from RuII/RuIII transitions; they are analogous to electron transfers between the FeII and FeIII ionic sites in Prussian Blue [39]. Contrary to Ru-O/Ru-CN_MWNTs, bare nanotubes exhibit only pure double-layer capacitive behavior as expected for porous carbon materials with a slight redox signal appearing at ca. 0.35 V related to the redox activity of oxygen-containing surface groups, such as quinones, and hydroquinones (Fig. 4a). Also, carbon nanotube supports (for Ru-O/Ru-CN) may function as an additional source of capacitive-type behavior, but its relative contribution is not dominating, simply upon judging from the relative sizes of voltammetric currents characteristic of curves a and b in Fig. 4. Another important issue is durability of Ru-O/Ru-CN deposit on MWNTs. Practically, no decrease in peak currents was noticed after 400 continuous voltammetric cycles at 10 mV s −1 in 1 mol dm−3 sulfuric acid. In other words, the composite of MWNTs with polynuclear Ru-O/Ru-CN acts as the stable and well-behaved electron conductive material and, therefore, it can be considered for application in electrochemical charge storage devices operating in acidic electrolytes.
Charging-discharging of symmetric cells utilizing Ru-O/Ru-CN_MWNTs In the next step, we have addressed the charge storage properties originating from the redox reactions of Ru-O/Ru-CN examined in the Breal^ two-electrode configuration. The voltammetric-type (current vs. potential difference, U) characteristics of the symmetric capacitor cell utilizing Ru-O/RuCN-covered MWNTs as positive and negative electrodes is presented in Fig. 5b (solid line). For comparison, the dotted line (Fig. 5a) shows a response recorded under the same experimental conditions (5 mV s−1 scan rate) but for pristine (bare) MWNTs. The voltammetric currents of Fig. 5 refer to the specific capacitances (of single electrodes). In the calculations, the generally accepted formula has been used: C¼2
i v⋅m
ð3Þ
where C is specific capacitance [F g −1 ], i stands for voltammetric current [A], v is scan rate [V s−1], and m refers to mass of the active material in one electrode [g]. As it can be seen from Fig. 5a, a capacitor-type cell built of bare nanotubes is characterized by voltammetric curves of proper rectangular-like shapes which remain unchanged even at scan rates up to at least 500 mV s−1. This behavior is
Fig. 5 Comparative cyclic voltammetric curves of a MWNTs and b RuO/Ru-CN_MWNTs symmetric supercapacitors recorded at 5 mV s−1; electrolyte 1 mol dm−3 H2SO4
consistent with fast charging/discharging rate as well as with low internal resistance that is generally expected for mesoporous carbon materials easily accessible to electrolyte ions in the whole volume of electrode. Some current increases that are observed in the range from 0 to 0.2 V should be assigned to electroactivity of quinone/hydroquinone surface groups. However, it must be remembered that other oxygen functional groups such as carboxyl, hydroxyl (phenolic) or lactonic, could also exist on acid-treated nanotubes, and their presence may lead to enhanced wettability and hydrophilicity of the surfaces. A shape of the voltammetric-type curve recorded for a capacitor utilizing Ru-O/Ru-CN_MWNTs (Fig. 5b) differs somewhat from the ideal box-like response characteristic of materials in which accumulation of charge undergoes according to the double-layer-type mechanism. The Birregular^ shape of the voltammogram of Fig. 5b reflects contributions originating from the redox activity of various ruthenium ionic centers of Ru-O/Ru-CN appearing at 0.4 and 1 V. By integrating charges released from the cell during the discharging cycle (at 5 mV s−1), a value of 80 F g−1 has been obtained for the capacitor utilizing Ru-O/Ru-CN modified MWNTs supercapacitor (vs. 23 F g−1 for the system with bare nanotubes investigated under the same experimental conditions). To comment on the charging-discharging dynamics of RuO/Ru-CN_MWNTs introduced to the symmetric capacitor cell, a series of voltammetric experiments (Fig. 6) has been performed at different scan rates (5–100 mV s−1). It can be observed that following the increase of scan rate from 5 up to 20 mV s−1, only a slight drop in specific capacitance has been observed. Further increase of scan rates (e.g., to 100 mV s−1) leads to more sizeable capacitance drops, especially at the potentials where ruthenium ions of Ru-O/Ru-CN are electroactive and are responsible for the capacitive-type effects. These observations should be attributed to the limitations in kinetics of redox reactions as well in dynamics of
J Solid State Electrochem
Fig. 7 Galvanostatic charging-discharging curves of symmetric twoelectrode cells containing a MWNTs and b Ru-O/Ru-CN_MWNTs at constant current of 100 mA g−1
Fig. 6 Current (a) and specific capacitance (b) as a function of voltage changes recorded for Ru-O/Ru-CN_MWNTs supercapacitor during application of increasing scan rates 5, 10, 20, 50, 70, and 100 mV s−1
charge propagation at the electrode/electrolyte interface. The existence of redox transitions, which may significantly improve such electrical parameters as specific capacitance and energy density, could also lead to the increased resistances. Although, Ru-O/Ru-CN is considered as a well-behaved redox system, and it is often described as kinetically fast in electrocatalysis [42, 44], a simple diagnostic experiment, in which the dependence of a peak current (appearing at ca. 1 V in Fig. 4) is plotted vs. scan rate, shows linearity only up to 50 mV s−1. At higher scan rates, the system starts to behave as the quasi-reversible one. These observations seem to explain limitations in the dynamics of charging-discharging phenomena of the capacitor cell at faster scan rates. To get better insight into performance of investigated systems, a series of galvanostatic experiments has been performed at different charging-discharging currents (100– 4000 mA g−1). Figure 7 illustrates the charging-discharging curves for the capacitor cells built of both bare nanotubes (A) and nanotubes modified with Ru-O/Ru-CN (B) recorded at the current as low as 100 mA g−1. The specific capacitance (expressed per one electrode) has been calculated using the following equation:
C¼2
i⋅t U ⋅m
ð4Þ
where i is discharging current [A], t is discharging time [s], and U is applied voltage decreased by the ohmic drop [V]. The obtained values were equal to 24 and 86 F g −1 for supercapacitors utilizing bare (pristine) and the Ru-O/Ru-CN modified MWNTs, respectively. The triangular shape of the galvanostatic curves suggests almost ideal capacitive-type behavior of both systems. However, at current densities higher than 1000 mA g−1, some ohmic drops could be noticed upon changing the electrode polarization. Also, no rapid decrease in specific capacitance during galvanostatic experiments is observed upon increasing current densities. In the whole range of applied currents, the capacitor cell utilizing RuO/RuCN_MWNTs shows much higher capacitance values in comparison to the cell based on unmodified nanotubes. Upon increasing the charging-discharging current from 100 to 4000 mA g−1, only a drop of 15 F g−1 in capacitance has been produced. The cell responses upon application of high external loads seem to be of particular importance when it comes to evaluation of such capacitor’s parameters as energy and power densities. Therefore, the galvanostatic method was applied here to estimate values (per mass of active materials in both electrodes) of the energy (E) and power (P) densities of investigated systems. In the calculations, the generally accepted equations were used: 1 CU 2 2 E P¼ t E¼
ð5Þ ð6Þ
The dependencies of E as a function of P (Ragone plots) were plotted in Fig. 8. As expected from the analysis of capacitance data, the capacitor utilizing Ru-O/Ru-CN_MCNTs was characterized by significantly higher values of energy densities in comparison to those determined for the
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Fig. 8 Dependencies of energy density on power density (Ragone plots) for supercapacitors utilizing a MWNTs and b Ru-O/Ru-CN_MWNTs
MWNTs-containing cell. On the other hand, the power density was comparable for both systems. The maximum of energy was found upon discharging at 100 mA g−1, and it was equal to 2.9 Wh kg−1, which is 3.6 times higher in comparison to the value obtained for unmodified carbon nanotubes. But the power density (at the discharging current of 4000 mA g−1) was equal to 1.14 kW kg−1; this value was a bit lower but still comparable to that characteristic of bare MWNTs. To comment about ranges of operating potentials of positive and negative electrodes during charging-discharging of capacitor symmetric cell (in two-electrode configuration), diagnostic experiments utilizing the additional reference electrode were performed. The open circuit potential of both electrodes (Fig. 9) corresponding to application of the 0 V potential difference between electrodes (discharged capacitor) was equal to 0.43 V (vs. Ag/AgCl). This value was similar to that observed for the cell containing only bare nanotubes. Upon
Fig. 9 Variation of positive (a) and negative (b) electrode potentials (vs. Ag/AgCl reference electrode) during charging/discharging of Ru-O/RuCN_MWNTs supercapacitor up to 1 V at constant current of 200 mA g−1
Fig. 10 Capacitance retention in Ru-O/Ru-CN_MWNTs supercapacitor upon continuous voltammetric cycling at 5 mV s−1
charging up to 1 V, the potentials of both electrodes were polarized in opposite directions up to 0.98 and −0.02 V for the positive and negative electrodes, respectively. Upon application of this potential window, neither carbon materials existing at both electrodes nor the supporting electrolyte are expected to undergo undesirable decomposition. On the other hand, redox processes characteristic of Ru-O/Ru-CN are almost completely utilized (Fig 4) during electrochemical charging-discharging under conditions mentioned above. To comment on the stability of the composite material during continuous operation in the two-electrode mode, both
Fig. 11 Impedance characteristics (Nyquist plots) for a MWNTs and b Ru-O/Ru-CN_MWNTs-based supercapacitors in the frequency range of a 10 mHz–100 kHz and b 1 Hz–100 kHz
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investigated systems were subjected to 1000 voltammetric charging-discharging cycles at 5 mV s−1 (Fig. 10). While excellent durability of the performance of the MWNT-based electrical double-layer-type capacitor was not surprising (for simplicity not shown here), it is noteworthy that the system utilizing Ru-O/Ru-CN_MWNTs was also stable because it exhibited the capacitance loss of only 7 % after 1000 cycles of uninterrupted operation. Finally, the AC impedance diagnostic experiments were performed to comment on the resistance parameters characteristic of the investigated cells. The data was collected for discharged capacitors within the range of frequencies as wide as required to observe pure resistive (high frequency region) and pure capacitive (low frequency region) components. At intermediate frequencies, the system’s behavior can be referred to the combination of resistor and capacitor. Here, the electrode porosity and the thickness of electroactive materials determine the capacitance. Typical Nyquist plots illustrating the dependence of real impedance as a function of imaginary impedance are shown in Fig. 11. For more convenient comparison, the results are shown in two frequency ranges, namely 10 mHz–100 kHz (Fig. 11a) and 1 Hz–100 kHz (Fig. 11b). The shapes of impedance spectra (Fig. 11, curve a) obtained for the cells utilizing simply bare carbon nanotubes are typical for the systems operating as the carbon-based electrical double layer capacitors characterized by a small value of real impedance at the intercept of the plot with X axis and a straight line almost parallel to imaginary impedance axis. Such behavior is consistent with low equivalent series resistance (ESR) due to high conductivity of electrode material and electrolyte as well as lack of parasitic contact resistances. ESR at the intercept of Z′ with x-axis was equal to 0.35 Ω. However, as it is usually expected for porous electrodes, ESR here is affected by the presence of slight equivalent distributed resistance (EDR) in the middle frequency region (Fig. 11b) related with the diffusion of ions through the pores of different geometry. It can be seen that the shape of the Nyquist plot obtained for Ru-O/Ru-CN-modified carbon nanotubes capacitor is similar. In the low frequency region (Fig. 11a), a typical capacitive response was observed with a spike (vertical line) parallel to imaginary impedance axis. However, what is important, in the middle frequency range (Fig. 11b), no semi-circle related with parasitic charge transfer, usually attributed to kinetic limitations in faradaic reaction was observed. It can be easily explained by the fact that the voltage of 0 V (discharged capacitor) corresponds to zero charge potential at both electrodes equal to ca. 0.4 V (Fig. 9) and as can be seen in Fig. 4; at that potential value, there is no any specific faradaic reaction. Therefore, such shape of the impedance plot reflects typical porous electrode behavior. It should also be noticed that the value of ESR for the Ru-O/Ru-CN_MWNTs based system was only slightly higher (0.4 Ω) than for the capacitor utilizing
unmodified nanotubes (0.34 Ω). Having in mind that the electrolyte composition as well as current collectors and the cell assembly were the same for both systems, it might be concluded that the value measured for Ru-O/Ru-CN_MWNTs reflects the internal resistance of the active material which is somewhat higher because of the presence of redox conducting system in addition to carbon nanotubes. On the basis of impedance data, the specific capacitance was calculated using the following formula: C¼−
1 π f Z 00 m
ð7Þ
where f is the lowest applied frequency (10 mHz), and Z″ is the imaginary impedance at 10 mHz. The values of 24 and 87 F g−1 were obtained for capacitors utilizing MWNTs and Ru-O/Ru-CN_MWNTs, respectively; this result is in good agreement with the voltammetric and galvanostatic data.
Conclusions We demonstrate here attractive capacitive properties of multiwalled carbon nanotubes (MWNTs) modified with ultra-thin polynuclear deposits of mixed-valent ruthenium(II,III,IV) hexacyanoruthenate(II,III). The presence of the cyanidebridged ruthenium-oxo material was confirmed by means of microscopic (TEM, SEM-EDX), spectroscopic (FTIR), and electrochemical measurements. The uniform distribution of elements, namely ruthenium, oxygen, nitrogen, and carbon was also found. The material was further applied in symmetric electrochemical capacitor operating in acidic medium (1 mol dm−3 sulfuric acid). We also show that the charge storage mechanism can be understood in terms of the combination of charging/discharging of electrical double layers existing on porous carbon nanotubes with the reasonably high contribution from redox reactions of ruthenium analogue of Prussian Blue. Consequently, such parameters as specific capacitance and energy density significantly increased from of 24 F g−1 and 0.8 Wh kg−1 (characteristic of bare MWNTs) to 86 F g−1 and 2.9 Wh kg−1 following modification of carbon nanotubes with oxocyanoruthenium redox system. On the whole, the composite material (once introduced to the capacitor cell) behaved stable and reversibly upon application of continuous galvanostatic charging/discharging up to 1 V. The presented material can be considered as alternative to those utilizing low content hydrous ruthenium oxide-doped nanostructured carbon materials.
Acknowledgments The support from Ministry of Science and Higher Education—National Science Centre (Poland) under Project no. N N507 322040 is highly appreciated. The SEM-EDX analysis was obtained using the equipment purchased within CePT Project No. POIG.
J Solid State Electrochem 02.02.00-14-024/08-00. The authors are also grateful to Dr. Marianna Gniadek for her technical assistance during TEM analysis.
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