The ions present in the electrolyte in which a conjugated polymer actuator is cycled are known to affect performance. Understanding how force, response time, and strain are affected by ion size and other ion characteristics is critical to applications, but is not yet well understood. In this paper, we present the effect of alkali cation size on transport velocity and volume change in polypyrrole doped with dodecylbenzenesulfonate, PPy(DBS), which is a cation- transporting material. Volume change measured by mechanical profilometry is greatest for Li<sup>+</sup> and decreases in order of atomic mass: Li<sup>+</sup> > Na<sup>+</sup> > K<sup>+</sup> > Rb<sup>+</sup> > Cs<sup>+</sup>. Ion transport, measured by phase front propagation experiments, is also fastest for Li<sup>+</sup>, contradicting the expectation that larger species would move more slowly.
It is important to increase the switching speed of conjugated polymers between oxidized and reduced states for a wide range of devices, including capacitors, electrochromic displays, and actuators. In this paper, we compare the in-plane and the out-of- plane ion transport speed during electrochemical reduction of a conjugated polymer, polypyrrole doped with dodecylbenzenesulfonate. Results show that the in-plane ion transport is approximately 50 times faster than out-of-plane transport. The anisotropy is likely induced by the dodecylbenzenesulfonate, which has been shown previously to form layers parallel to the surface. An engineering method is presented to enhance the in-plane ion transport by etching pores into the polymer.
Previously, we presented a model for ion transport in conjugated polymers during electrochemical reduction. In this paper, we will present a more advanced model that includes hole transport, which was neglected in the first-cut model. This addition takes into account the interactions between holes and cations during transport. The result is that the front between oxidized and reduced material now propagates with constant velocity, instead of slowing down over time. Also, an electrolyte layer has been added to the model, and as a result the ion concentration behind the phase front is more accurately predicted.
The transport of charged species, including both polarons/bipolarons and charge-compensating ions, occurs when conjugated polymers switch between oxidized and reduced states. However, physics-based models of the charge transport processes have not yet been developed. Previously, we presented an electrochromic device that made the path for ion transport much longer than that for electrons, ensuring that ion transport was the rate-limiting step so that the constitutive equation for ion transport could be formulated. Ion concentration profiles and velocities could be tracked by color changes. In this paper, we present the correlation between ion transport and volume change, measured in this device using a mechanical profilometer to scan height profiles during electrochemical reduction. In addition, the effects of electrolyte concentration, electrolyte temperature, film thickness, and ion barrier stiffness on ion transport velocities are explored.
Electron transport and ion transport are two critical processes taking place during electrochemical oxidation/reduction of conjugated polymers. Because they accompany and depend on each other, research on the individual processes is difficult. We present a device that allows us to measure ion transport directly and independently from electron transport in conjugated polymers. The device geometry makes the ion path much longer than the electron path, ensuring that ion transport is the rate-limiting step. Ion transport is also visualized directly through the color change of the film (electrochromism) as the electrochemical reaction proceeds, allowing one to precisely and quantitatively track the ion velocity. During reduction at sufficiently negative potentials, a phase front between the oxidized and reduced states was observed to travel into the film, the speed of which was proportional to the applied voltage, demonstrating that migration (rather than diffusion) is the key driving force. At less negative reducing potentials, the film gradually and more uniformly changed color, indicating that diffusion plays a large role. A simple first-cut model with drift and diffusion terms is presented. The simulated ion concentration profile matched the experimentally measured intensity profile strikingly well.