Flexible interdigital solid-state supercapacitors based on pencil-drawn graphite/polypyrrole composite paper electrodes via laser cutting

Flexible paper-based interdigital solid-state supercapacitors were fabricated through a facile device-integration strategy involving pencil-drawing of a graphite layer, interfacial polymerization of PPy, and laser cutting of interdigital electrodes on cellulose paper. A thin layer of graphite was coated on cellulose paper by a pencil-drawing method, on which polypyrrole (PPy) was grown through a classical interfacial polymerization to obtain paper/graphite/PPy. Then interdigital paper/graphite/PPy electrodes were shaped easily by laser cutting, and they exhibited outstanding flexibility, high electrical conductivity and an areal specific capacitance of 223.8 mF cm^-2 at 5 mV s^-1. The paper-based solid-state supercapacitor assembled with two interdigital paper/graphite/PPy electrodes and a sodium polyacrylate/Na₂SO₄ electrolyte exhibited an areal specific capacitance of 26.9 mF cm^-2 at 5 mV s^-1 and an energy density of 2.3 μWh cm^-2 at a power density of 74.0 μW cm^-2. Furthermore, its electrochemical performance was enhanced dramatically compared to the device with paper/graphite electrodes. In addition, three series-connected supercapacitors can successfully power a light-emitting diode (LED) even under bending conditions, demonstrating outstanding flexibility. This work demonstrates a straightforward route for integrating conductive and pseudocapacitive components with laser-defined planar patterning on cellulose paper for flexible energy-storage devices.