Redox-Active Polypyrrole: Toward Polymer-Based Batteries

TLDR: In this paper, the authors describe a battery consisting of two electrodes coated with polypyrrole (pPy) doped with different redox-active compounds: indigo carmine (IC) or 2,2′-azinobis(3-ethylbenzothiazoline-6-sulfonate) (ABTS) (Fig. 1).

Abstract: One of the most important challenges in energy research is the development of an energy-storage device that can deliver electricity for longer periods of time at higher power demand. A governing principle, as illustrated in Ragone plots of energy density (ED) versus power density (PD), is that the deliverable energy stored in a device decreases with an increasing demand for power or current. At high power demands, the inefficiency of a device (i.e., loss of energy) is due to limitations involving the mass transfer of ions or sluggish reaction kinetics, or ionic or electric resistance involved within materials or at interfaces between different phases or materials. Batteries and electric double-layer capacitors (EDLCs) are both ends of a wide spectrum of devices used to deliver power. Batteries are based on faradaic reactions (e.g., electrochemical reduction and oxidation); EDLCs are based on nonfaradaic reactions (e.g., charging and discharging the electric double layer). Batteries are known for their high ED but low PD (ED = 20–100 W h kg and PD = 50–200 W kg), whereas ultracapacitors are the opposite (ED = 1–10 W h kg and PD = 1000–2000 W kg). The faradaic reactions on which batteries are based enable high EDs but charge-transfer reactions (e.g., from Li to LiCoOx) that accompany a phase change (e.g., from ions in electrolyte to solid) are kinetically slow processes, making batteries inappropriate for applications that require high PDs. In contrast, the non-faradaic processes (i.e., charging) on which EDLCs are based involve the formation of an electric double layer at the interface between the electrolyte and the porous electrodes. This process is fast and facile, making EDLCs ideal for applications that require high PDs. We describe herein a device for storing energy, the performance parameters of which reside between a rechargeable battery and an ultracapacitor. This system consists of two electrodes coated with polypyrrole (pPy) doped with different redox-active compounds: indigo carmine (IC) or 2,2′-azinobis(3-ethylbenzothiazoline-6-sulfonate) (ABTS) (Fig. 1). The resulting redox-active conducting polymers (pPy[IC] and pPy[ABTS]) form the basis of a battery that depends on the faradaic reactions of the redox-active dopants. This feature is uniquely different from batteries or electrochemical capacitors that depend solely on redox reactions or doping/dedoping of conducting polymers. The battery consisting of pPy[IC]  pPy[ABTS] shows significant enhancement in performance at high PDs (e.g., ED = 8 W h kg at PD = 10 to 10 W kg). This enhanced performance derives from a combination of merits found in batteries and EDLCs. The principle of energy storage is based on faradaic processes of redoxactive dopants (battery-like) but the electrochemical reactions are surface confined without diffusion of the electroactive materials. Instead, the counterions in the electrolyte neutralize the charge on the electrode (EDLC-like). The porous structure of the conducting polymer provides an electrode with high surface area and enables counterions to access the redoxactive dopants. In addition, the polymer matrix provides an environment that is conductive, leading to enhanced electron transfer between the base electrodes and the electroactive dopants. The cationic charge that develops in pPy during the electropolymerization of pyrrole requires an influx of anions from the electrolyte to maintain charge neutrality. Because the C O M M U N IC A TI O N S