Interconnected and high cycling stability polypyrrole supercapacitors using cellulose nanocrystals and commonly used inorganic salts as dopants

Polypyrrole (PPy) is wildly used as electrode material in supercapacitors due to its high conductivity, low cost, ease of handling, and ease of fabrication. However, limited capacitance and poor cycling stability hinder its practical application. After developing carboxylated cellulose nanocrystals (CNC-COO−) as immobile dopants for PPy to improve its cycling stability, we investigated the effect of different commonly used salts (KCl, NaCl, KBr, and NaClO4) as dopants during electrode fabrication by electropolymerization. The film's capacitance increased from 160.6 to 183.4 F g−1 after adding a combination of KCl and NaClO4 into the electrodeposition electrolyte. More importantly, the porous and interconnected PPy/CNC-COO−-Cl−(ClO4−)_0.5 electrode film exhibited an excellent capacitance of 125.0 F g−1 (0.78 F cm−2) at a high current density of 2.0 A g−1 (20 mA cm−2, allowing charging in less than 1 min), increasing almost 204% over PPy/CNC-COO− films. A symmetric PPy/CNC-COO−-Cl−(ClO4−)_0.5 supercapacitor retained its full capacitance after 5000 cycles, and displayed a high energy density of 5.2 Wh kg−1 at a power density of 25.4 W kg−1 (34.5 μWh cm−2 at 1752.3 μW cm−2). These results reveal that the porous structure formed by doping with CNC-COO− and inorganic salts opens up more active reaction areas to store charges in PPy-based films as the stiff and ribbon-like CNC-COO− as permanent dopants improve the strength and stability of PPy-based films. Our demonstration provides a simple and practical way to deposit PPy based supercapacitors with high capacitance, fast charging, and excellent cycling stability.