Effect of ruthenium on superior performance of cellulose nanofibers/poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate)/ruthenium oxide/ionic liquid actuators

Highlights

• Novel electrode actuators were designed based on cellulose nanofibres (CNFs).

• The CNF framework was contained in a polymer base with dispersed RuO₂ particles.

• Two ionic liquids with different species were compared.

• The actuator operates mainly by a faradaic capacitor mechanism.

Abstract

This study proposes novel actuators based on cellulose nanofibre/poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate)/hydrous ruthenium oxide/ionic liquid (CNF/PEDOT:PSS/RuO₂/IL) electrodes and the poly(vinylidene fluoride-co-hexafluoropropylene)/IL (PVdF(HFP)/IL) electrolyte, and investigates the effect of hydrous ruthenium oxide on the electrochemical and electromechanical properties of the proposed actuators. The proposed system contains an electrochemical capacitor electrode, which acts as both a faradaic capacitor (FC) and a small electrostatic double layer capacitor (EDLC). This hybrid capacitor is based on the FC (RuO₂) mechanism, and a base polymer (PEDOT:PSS) and CNF skeleton replace carbon nanotubes (CNTs). Therefore, this device functions differently from traditional CNT/PVdF(HFP))/IL electrode actuators, which are only used as EDLC units, and CNF/PEDOT:PSS/IL (without RuO₂) actuators, where PEDOT:PSS plays the role of both FC and base polymer. Devices built using the proposed actuators exhibit higher strains and greater maximum generated stresses than those exhibited by devices based on CNF/PEDOT:PSS/IL (i.e. without RuO₂). Surprisingly, the synergism obtained by combining RuO₂ and PEDOT:PSS is considerably greater than the enhancement achieved using PEDOT:PSS. The frequency dependences of the displacement responses of the CNF/PEDOT:PSS/RuO₂/IL electrode actuators were then measured and successfully simulated using a double layered charging kinetic model. The developed films are novel, robust, and flexible, and they exhibit potential for use as actuator materials.

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.