Charge storage and capacitance-type properties of multi-walled carbon nanotubes modified with ruthenium analogue of Prussian Blueby Magdalena Skunik-Nuckowska, Pawel Bacal, Pawel J. Kulesza

J Solid State Electrochem

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ORIGINAL PAPER

Charge storage and capacitance-type properties of multi-walled carbon nanotubes modified with ruthenium analogue of Prussian Blue

Magdalena Skunik-Nuckowska1 & Pawel Bacal1 & Pawel J. Kulesza1

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 mixedvalent ruthenium-oxo species cross- l inked with 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 dens i t ies were de termined from the data of cyc l ic 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 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 oxygenThis paper is dedicated to Professor Mikhail A. Vorotyntsev on the occasion of his 70th birthday. * Magdalena Skunik-Nuckowska mskunik@chem.uw.edu.pl * Pawel J. Kulesza pkulesza@chem.uw.edu.pl 1 Faculty of Chemistry, University of Warsaw, Pasteura 1, 02-093 Warsaw, Poland

J Solid State Electrochem

DOI 10.1007/s10008-015-2866-z 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.