Reversible electrochemical actuators

Reversible electrochemical actuators comprise of an anode, a cathode and a separating electrolyte. Either electrode or both can be conducting polymers. However, other redox materials such as graphite have also been used as electromechanical electrodes. The nature of redox processes for conducting polymer electrodes has been found to be rate dependent. For instance, an electrode that is anion doped can be reduced either during the addition of cations or the removal of anions. 

Anode, cathode, and electrolyte are the three main elements in reversible electrochemical actuators and the conducting polymer actuators can he classified as extensional actuators and hydrostatic actuators. In order to perform mechanical work, linear or biaxial dimensional changes are used for the extensional actuators as illustrated in Figure 8.81 [134]. On the other hand, during the electrochemical redox processes, the overall volume change of the anode, cathode, and electrolyte is considered to perform the mechanical work in the hydrostatic actuators. 

Suzuki H, Yoneyama R (2002) A reversible electrochemical nanosyiinge pump and some considerations to realize low-power consumption. Sens Actuators B-Chem 86 242-250... 

Takada K, lida T, Kawanishi Y, Yasui T, Yuchi A (2011) An electrochemical actuator based on reversible changes in volume of poly(acrylic acid) gel induced by quinone redox. Sens Actuators B 160 1586-1592... 

Fig. 31 Mechanical actuation of a gold-coated microcantilever by molecular muscles [227]. (a) Structural formula of a palindromic, bistable [3]rotaxane with gold-binding dithiolane groups attached to the cyclophanes. (b) Reversible bending up and down of a cantilever by actuation of a monolayer ( 8 billion molecules) of the rotaxanes on its surface. The gold surface bends when the rotaxanes contract under the influence of an electrochemical oxidation that causes the cyclophanes to shuttle inward from the periphery of the molecule, (c) Electrochemical cell (Ag/AgCl, Pt, and the cantilever are the reference, counter, and working electrodes, respectively) and combined AFM device used to measure the bending by detecting a laser beam reflected off of the cantilever s surface...

Without using ions, electric field or heat, electrochemically synthesized PPy films may exhibit fast and intensive bending in ambient air on the basis of a reversible van der Waals adsorption of polar, organic or water molecules in the vapor state [139-141]. As shown in Table 8.13, the chemomechanical response of the actuator strongly depended on the adsorbate [139]. It was reported that PPy film exhibited contraction when an electric field was applied in ambient air because of the contraction caused by desorption of water vapor and thermal expansion of polymer chains [142]. 

So, the process for a polypyrrole film in a solution containing CP anions can be envisaged as stated above by Equation 16.1. The oxidized polymer is a gel and the electrons are transferred from, or toward, the metal in contact with the conducting polymer. The oxidized polymer [(PPy" )s(Cl ) (H20)m]gei is a nonstoichiometric compound the Cl content can be increased (or decreased) under control of anodic (or cathodic) charges (ne in Equation 16.1). The electrochemical equipment allows a continuous, reversible, and infinitesimal control of the oxidized material composition (a unique fact related to electromechanical actuators) by the flow of constant anodic or constant cathodic currents (constant charge per time unit), by reversing the direction of the current flow, or by the flow of infinitesimal charges, respectively. 

FIGURE 16.3 Schematic representation of the reversible volume change associated with the electrochemical reactions of polypyrrole in electrolytes. (Modified from Otero, T.F., Polymer Sensors and Actuators, ed. D. de Rossi and Y. Osada, Springer-Verlag, Berlin, 2000. With permission.)... 

For pH-responsive electrochemical systems, on the other hand, the electrical field indirectly influences the polymer conformation to induce a reversible volume transition. The electricity is used to electrolyze water, generating hydrogen ions at the anode and hydroxyl ions at the cathode. This produces a pH gradient across the solution, which can be used to influence the conformation of pH-responsive polymers. Strategic placement of the polymer in the solution can cause expansion, contraction, or even bending (when placed directly between the electrodes) of a polymer actuator. Switching the direction of the current creates the opposite effect. This second method of actuation... 

They are studied for actuation tasks too. For this purpose, they are used as components of an electrochemical cell, whose basic structure includes two electrodes immersed in an electrol d e. The conducting polymer material constitutes of one or both of the electrodes of the cell. By applying a potential difference between them, red-ox reactions cause strongly anisotropic and reversible volume variations of the material [219], which can be used for actuation [220-242]. 

Actuation in ICPs occurs through their reversible redox chemistry. The electrochemical reaction of polypyrrole, one of the most widely studied conjugated polymers, is illustrated... 

FIGURE 1.6 (a) Setup of intermittent-contact SECM with a piezoelectric bender actuator, (b) Forward and (c) reverse scan images of the edge of a gold band on glass obtained with a 160-nm-radius recessed Pt tip in 1 mM FcTMA+. (Reprinted from Lazenby, R. et al., J. Solid State Electrochem., Vol. 17, 2013 pp. 2979-2987.)... 

Warren MR, Madden JDW (2006a) A structural, electronic and electrochemical study of polypyrrole as a function of oxidation state. Synth Met 156(9-10) 724—730 Warren MR, Madden JDW (2006b) Electrochemical switching of conducting polymers a variable resistance transmission line model. J Electroanal Chem 590(1) 76-81 Wing Yu Lam J (2011) Influences of growth conditions and porosity on polypyrrole for supercapacitor electrode performance. UBC, Vancouver, BC, Canada Wu Y et al (2007) Soft mechanical sensors through reverse actuation in polypyrrole. Adv Funct Mater 17(16) 3216-3222... 

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