Supercapacitive Properties of 3D-Arrayed Polyaniline Hollow Nanospheres Encaging RuO₂ Nanoparticles

A major limitation of polyaniline (PANi) electrodes for supercapacitors is the slow rate of ion transport during redox reactions and the resultant easy saturation of areal capacitance with film thickness. In this study, three-dimensionally (3D)-arrayed PANi nanospheres with highly roughened surface nanomorphology were fabricated to overcome this limitation. A hierarchical nanostructure was obtained by polymerizing aniline monomers on a template of 3D-arrayed polystyrene (PS) nanospheres and appropriate oxidative acid doping. The structure provided dramatically increased surface area and porosity that led to the efficient diffusion of ions. Thus, the specific capacitance (C_sp) reached 1570 F g^-1, thereby approaching a theoretical capacitance of PANi. In addition, the retention at a high scan rate of 100 mV s^-1 was 77.6% of the C_sp at a scan rate of 10 mV s^-1. Furthermore, 3D-arrayed hollow PANi (H-PANi) nanospheres could be obtained by dissolving the inner PS part of the PS/PANi core/shell nanospheres with tetrahydrofuran. The ruthenium oxide (RuO₂) nanoparticles (NPs) were also encaged in the H-PANi nanospheres by embedding RuO₂ NPs on the PS nanospheres prior to polymerization of PANi. The combination of the two active electrode materials indicated synergetic effects. The areal capacitance of the RuO₂-encaged PANi electrode was significantly larger than that of the RuO₂-free PANi electrode and could be controlled by varying the amount of encaged RuO₂ nanoparticles. The encagement could also solve the problem of detachment of RuO₂ electrodes from the current collector. The effects of the nanostructuring and RuO₂ encagement were also quantitatively analyzed by deconvoluting the total capacitance into the surface capacitive and insertion elements.