Supercapacitors with cotton shell-derived activated carbons and porous polymer electrolyte films†

Abstract

Some of the well-known challenges in the field of supercapacitors (SCs), or more specifically, electrical double-layer capacitors (EDLCs), such as low energy density and high cost, have proven to be major barriers to their widespread market success despite having some excellent electrochemical merits such as high-power density and good cyclic stability. In this work, efforts have been made to overcome these gaps and eventually enhance the performance of EDLCs via a cost-effective and eco-friendly approach. To fabricate these EDLCs, a bio-waste, namely, cotton-shell-derived activated carbons (ZnACs) (activated with ZnCl₂), was used in a mass ratio of 1 : 2 for cotton shell to ZnCl₂. This resulted in a large BET surface area of 2031 m² g⁻¹ and a hierarchical porous structure, which contributed to faster diffusion of electrolyte ions. These two features ultimately resulted in a high specific capacitance of 247.82 F g⁻¹ at a current density of 0.52 A g⁻¹ of the cell with a porous polymer electrolyte (PPE) film made from polycaprolactone and poly (vinylidene fluoride-co-hexafluoropropylene), which offered the advantages of a wider potential window (∼7.22 V vs. Ag) and high conductivity (1.51 mS cm⁻¹). A comparison was then made with another cell using commercial activated carbon powder and the same PPE film. The ZnAC-based EDLC cells showed better performances, such as a high energy density (∼22.58 W h kg⁻¹) and high Coulombic efficiency (∼83.6%) without compromising the effective power density (∼0.42 kW kg⁻¹). EDLC cells exhibited only ∼3% capacitance fading at the end of 10 000 charge–discharge cycles. Thus, the incorporation of cotton shell-biowaste resulted in a two-way advantage of reducing environmental pollution caused by their large-scale burning practices and delivering substantial electrochemical performance, ultimately opening new avenues in the field of green energy technology.