Self-Discharge Characteristics of Vapor Deposited Polymer Electrodes in an All-Textile Supercapacitor
Authors: Viola Wesley , Andrew Trisha L. 
Published: May 2020

Despite the many efforts put toward developing flexible supercapacitors for wearable technology, few studies have focused on self-discharge, the spontaneous voltage decay of devices stored in their charged state. In this work, we study and seek to improve the self-discharge behavior of a high-loading, all-textile charge storage device comprised of two conducting polymer-coated prewoven fabrics as the active electrodes. The enabling pseudocapacitive polymer coating on these fabric electrodes is a persistently p-doped conducting polymer, poly(3,4-ethylenedioxythiophene)-chloride (PEDOT-Cl), that we vapor deposit onto untreated commercial textiles using a custom-built hot wall reactor, following a previously reported process.

We find that a significant portion of apparent self-discharge in this vapor-deposited pseudocapacitive soft material is due to charge redistribution, a physical process involving the rearrangement of ions within the electrode material. Primarily observed for highly porous carbon materials, charge redistribution arises when, during charging, ions accumulate at high-low resistance interfaces (i.e., at pore mouths) and subsequently redistribute throughout the material. A higher density of charge yields a higher electrode surface potential that diminishes as the charges diffuse through higher resistance regions. A hold step used during charging pre-conditions our polymer electrodes, reducing self-discharge losses by uniformly diffusing charges throughout the film. Among the scarce studies on self-discharge of conducting polymers, none to our knowledge have identified charge redistribution as a mechanism for this behavior.

The magnitude of the observed charge redistribution effects was modeled using a circuit model with two RC branches, representing the slowest and fastest responding regions. The resistance of the slow branch was found to be about three orders of magnitude higher than that of the fast branch. The ratio of these resistances was quite similar to those reported for porous carbon electrodes. However, since the polymer films contained essentially no micropores (BET surface area = 6.7 m2/g), an alternative explanation was needed for the observed charge redistribution effects.

A distinguishing feature between electrochemical double-layer capacitors (EDLC’s) and pseudocapacitors is that charge storage is a surface phenomenon in the former and a bulk phenomenon in the latter. We propose, in analogy, that the charge redistribution we observed in PEDOT-Cl electrodes arose not due to pore characteristics, but to varying crystallinity (and the presence of crystal domain walls, in particular) across the bulk polymer film. During a redox cycle, chloride ions must travel through these crystal interstices with dimensions comparable to their hydrated diameter. We anticipate dramatically higher resistance to transport in these crystalline regions than in the free-volume-rich disordered regions of the polymer. Such morphology-dependent ion effects have been observed for other conjugated polymers. At short charge times, we expect that ions build up at the interface of ordered and disordered regions before uniformly redistributing throughout. Judicious control of polymer morphology, namely the degree of crystallinity and orientation of crystallites, may be used to facilitate ion transport and minimize the ion accumulation associated with charge redistribution. Indeed, the variation of crystallite orientation in vapor-deposited PEDOT-Cl films has previously been shown to influence resulting electrochemical properties via ion transport effects.

The remainder of self-discharge losses are found to be associated with a diffusion-limited process, likely involving a low-concentration impurity. Iron oxidant residues (a reagent used in the polymer deposition) were ruled out as a potential contribution, while oxygen reduction remains a likely possibility.

As a matter of practical importance for integrating flexible energy storage devices into functional systems, self-discharge of conjugated polymer-based electrodes was investigated, and crystallinity-dependent ion dynamics were proposed to be responsible for a portion of the observed voltage decay. Future work directed at charge redistribution in polymers might focus on control of morphology to provide amorphous networks for ions to more efficiently percolate through.

Figure 1