Effect of ruthenium on superior performance of cellulose nanofibers/poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate)/ruthenium oxide/ionic liquid actuators
Graphical abstract
Introduction
Recently, electrochemical capacitors (ECs) have attracted considerable research attention because of their long lifetimes, high power densities, and the ability to bridge the power and energy gaps that exist between fuel cells or batteries and standard dielectric capacitors [1,2]. ECs can be divided into two main types: faradaic capacitors (FCs) and electrostatic double layer capacitors (EDLCs). The electrodes in EDLCs are electrochemically inactive substances, such as carbon-based materials. Therefore, no electrochemical reactions are observed at the electrode during charging and discharging. FCs, on the other hand, use electrodes that contain electrochemically active substances, such as metal oxides; furthermore, they can store charge during the charging and discharging processes [3,4]. Both the aforementioned types of capacitor have some basic requirements; for example, the electrode materials must be extremely conductive to achieve high capacitance. It is possible to fabricate hybrid capacitors that simultaneously exhibit the characteristics of both FCs and EDLCs, with one of these two capacitor types responsible for the primary mechanism [5].
Various metal oxides have been investigated as possible electrode materials to develop high-power ECs [6]. Among them, ruthenium oxide (RuO2) has been widely used for various applications because of its good catalytic properties in electrochemical and photochemical processes as well as in high charge storage capacity devices [[7], [8], [9]].
The use of RuO2 as an electrode material for ECs presents several advantages. RuO2 exhibits a high capacitance of ∼150–260 μF/cm2 [10], which is approximately ten times higher than that of carbon [11]; this is considered to be due to pseudocapacitance resulting from surface reactions between the Ru and H ions. The resistivity of RuO2 is on the order of 10−5 Ω cm [12], which is approximately two orders of magnitude lower than that of carbon [13].
Furthermore, the use of amorphous hydrous ruthenium oxide (RuO2·xH2O) as an electrode material has been demonstrated to lead to a considerably higher specific capacitance than that recorded for RuO2 [14]. The charge-storage mechanism of RuO2·xH2O is believed to be different to that of RuO2 because the charge is stored beyond, at, or near a solid electrode surface. For example, cellulose nanofibre/poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate)/hydrous ruthenium oxide/ionic liquid (CNF/PEDOT:PSS/RuO2(xH2O)/IL) electrodes would be expected to exhibit higher specific capacitances than CNF/PEDOT:PSS/IL (devoid of RuO2(xH2O)) electrodes; thus, CNF/PEDOT:PSS/RuO2(xH2O)/IL actuators would exhibit higher strains and greater generated stresses than those using only CNF/PEDOT:PSS/IL (without RuO2(xH2O)).
Increasing environmental concerns have focused greater attention on fundamental research into bionanofibres and their applications [15]. Cellulose—a crystalline polysaccharide mainly obtained from wood pulp—is the most abundant biopolymer. It exhibits a unique hierarchical structure comprising linear glucan chains that form 3–4-nm wide crystalline cellulose microfibrils consisting of 30–40 cellulose chains; these chains further become microfibril bundles that form cell walls → fibres → plant tissue → trees or other plants; they contain the hemicellulose and lignin necessary for reinforcing living plants. In addition to natural cellulose fibres, chemically and mechanically modified cellulose fibres have been used for decades in various fields, ranging from commodities, such as paper and textiles, to high-tech materials. Therefore, CNFs are expected to replace carbon nanofibres, as they can form highly entangled CNF open mesopore networks in electrodes [15]. However, CNFs are not electroconductive, which is a limitation of the material.
The doping of PEDOT with PSS results in the formation of PEDOT:PSS, which is an important type of conductive polymer with the ability to form colloidal particulate dispersions in water. PEDOT:PSS has been employed in numerous organic and polymeric electronic/optical devices [[16], [17], [18], [19]], and PEDOT:PSS-based electrodes have been studied [[20], [21], [22]] and confirmed to be capable of converting electrical energy to mechanical energy [23,24]. Hence, it is expected that insulator CNFs will become electroconductive when modified with PEDOT:PSS.
Recently, researchers have extensively investigated soft materials that are capable of generating mechanical energy from electrical energy. These materials have been used for several applications, such as in robotics for prosthetics, tactile and optical displays, microelectromechanical systems, and medical devices [25]. In this context, electroactive polymers (EAPs) have many attractive characteristics that render them suitable for these applications, particularly for use in actuators and sensors. Previously, the production of a dry actuator [[26], [27], [28]] was reported via a facile process in which a so-called ‘bucky gel’ underwent layer-by-layer casting [29]. This material was composed of IL-containing single-walled CNTs (SWCNTs) and possessed a gel-like consistency at room temperature. More specifically, the bimorph-structured dry device was composed of an IL gel electrolyte layer supported on a polymer and positioned between polymer-supported bucky gel electrode layers. This configuration permitted rapid, long-lasting movement in response to minimal applied voltages in air under ambient conditions. In addition, we recently found that the electrochemical and electromechanical characteristics of these devices can be affected by the cation and anion of the IL as well as by the specific polymer and nanocarbon employed [28,[30], [31], [32], [33]]. Furthermore, in a previous study, a CNF/PEDOT:PSS/IL actuator was developed that exhibited high strain and maximum generated stress [34].
In view of the aforementioned information, novel CNT-free, hybrid, self-standing actuators with the CNF/PEDOT:PSS/RuO2(xH2O)/IL electrode and PVdF(HFP)/IL electrolyte structure were developed herein by exploiting the advantages of the component materials. In addition, we investigated the effects of RuO2(xH2O) on the electrochemical and electromechanical properties of the capacitors and actuators.
Section snippets
Materials
The CNFs were prepared using the aqueous counter collision method. RuO2·xH2O, PEDOT:PSS (1:2.5 w/w polyion complex, number 768618), methyl pentanone (MP), and propylene carbonate (PC) were purchased from Sigma-Aldrich. Bamboo-bleached kraft pulp (BB) (ca. 10 wt%) was purchased from the Chuetsu Pulp & Paper Co., Ltd. Two ILs, 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI[BF4]) and 1-ethyl-3-methylimidazolium triflate (EMI[CF3SO3]), were purchased from the Kanto Chemical Co., Inc. The
Results and discussion
CNF/PEDOT:PSS/RuO2/EMI[BF4] and CNF/PEDOT:PSS/RuO2/EMI[CF3SO3] electrodes with CNF/PEDOT/RuO2 ratios of 50/100/50, 50/100/100, 50/100/200, and 50/100/0 were synthesised according to the procedure in Section 2.2. Fig. S2 shows the cyclic voltammogram of the CNF/PEDOT:PSS/Ru2O/EMI[BF4] (50/100/200) electrode and PVdF(HFP)/IL electrolyte (applied triangular voltage: ±0.5 V, sweep rate =1 mV s−1 (0.0005 Hz)). The specific capacitance C (C1 scaled to the weight of PEDOT:PSS) of each electrode was
Conclusions
Here, we successfully developed a novel CNT-free, self-standing, and CNF-coated PEDOT:PSS electrode actuator containing RuO2 (CNF/PEDOT:PSS/RuO2/IL). The operation of the actuator is mainly attributed to an FC mechanism. By comparing the electrochemical and electromechanical properties of the CNF/PEDOT:PSS/RuO2/IL and CNF/PEDOT:PSS/IL (without RuO2) actuators, it was found that the FC mechanism can predominantly explain the specific capacitance values of the CNF/PEDOT:PSS/RuO2/IL electrodes.
Declaration of Competing Interest
There are no conflicts of interest to declare.
Acknowledgements
This work was partly supported by the KAKENHI Grant-in Aid for Scientific Research C from JSPS [grant number 17K05983].
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