A Facile In Situ Template Method to Fabricate Hollow Polypyrrole Nanoparticles for Asymmetric Electrochemical Capacitors with Enhanced Cyclic Stability

A novel, template-assisted synthesis strategy for producing hollow polypyrrole (PPy) nanoparticles (H-PPy) with an average diameter of ≈70 nm is reported. Unlike conventional methods, the approach uniquely exploits an in situ reaction between FeCl₃ and CaCO₃ to simultaneously generate Fe(OH)₃ nanoparticles that act as dynamic, self-decomposing templates for PPy deposition. This concurrent template formation and polymer growth restricts Fe(OH)₃ particle size via rapid PPy surface encapsulation, facilitating the formation of uniformly dispersed PPy-coated Fe(OH)₃ particles (T-PPy). Subsequent removal of unreacted CaCO₃ and Fe(OH)₃ yields hollow PPy nanoparticles (H-PPy) with a 30% size reduction due to contraction of the soft PPy, resulting in a high surface area morphology. The H-PPy exhibits excellent electrochemical performance as a cathode material in both pouch-type and all-solid-state asymmetric electrochemical capacitors. The specific capacitance of H-PPy calculated by using three electrode electrochemical cell is found to be 158.2 Fg^-1 at 1 Ag^-1 which is much higher than that of bulk PPy and T-PPy. Areal capacitances of 40.2 and 9.6 mF cm^-2 at 0.2 mA cm^-2 are obtained for the pouch and solid-state cells, respectively, where activated carbon electrodes are used as anode. The pouch cell demonstrates remarkable cycling stability, retaining 104.6% of its initial capacitance after 15 000 cycles.