Engineering the volumetric effect of Polypyrrole for auto-deformable supercapacitor

Polypyrrole (PPy) is one of the most extensively studied conducting polymers and it has promising application prospects in supercapacitors. However, PPy-based supercapacitors generally suffer from poor cycling stability due to the volumetric effect induced by ion doping/de-doping during the redox reactions. Therefore, the volumetric effect has usually been considered deleterious for supercapacitor applications. Herein, we demonstrate that the volumetric effect of PPy can be collected by synergy of flexible electrodes and solid-sate electrolytes. Furthermore, the accumulative volumetric effect is utilized in auto-deformable supercapacitors for the first time. The as-designed supercapacitor consists of flexible electrodes (PPy film, reduced graphene oxide (rGO) film) and a polyvinyl alcohol (PVA) gel electrolyte. When the redox reaction starts, the volumetric change of the PPy film induced by Na+ ion insertion is captured by the PVA gel electrolyte, resulting in mechanical deformation of the supercapacitor. As the accumulative volumetric effect drives greater deformation of the device, more electrochemically active sites are generated in the PPy film, leading to a higher energy storage capacity of the supercapacitor. Specifically, a certain correlation exists between the deformation degree and the energy storage capacity. As the deformation degree approaches 100% from 0%, the discharge capacity increases to 359 mF/cm(3) from 101 mF/cm(3). This auto-deformation characteristic endows the supercapacitor with subjective initiative, which establishes an important foundation for a smart energy storage device.