Compositing redox-rich Co–Co@Ni–Fe PBA nanocubes into cauliflower-like conducting polypyrrole as an electrode material in supercapacitors

Abstract

Supercapacitors (SCs) coupled with redox materials have been extensively investigated to maximize the energy density of SCs. Common metal–organic frameworks explicitly used for this purpose are Prussian blue analogs (PBAs). However, their low conductivity and number of external electroactive sites hinder their capacitance and reaction rate. Our work focuses on a material-level optimization strategy, including morphology modification and superstructure fabrication of PBA. We report the synthesis of Co–Co@Ni–Fe trimetallic core–shell PBA nanocubes containing an Fe^II/III redox couple in the Ni–Fe PBA shell over the conductive Co–Co PBA core. This complex underwent a reversible redox reaction and was further encapsulated in a polypyrrole (PPy) network as a redox additive to prepare a novel polymer-encapsulated double-PBA nanocube composite (Co–Co@Ni–Fe PBA–PPy). The composite exhibited a faradaic non-capacitive diffusion-dominated charge storage ability. It yielded an improved specific capacity of 318.1 C g⁻¹ at 1 A g⁻¹ with a capacity retention of 90% over 2000 cycles @15 A g⁻¹. Furthermore, an asymmetric supercapacitor based on Co–Co@Ni–Fe PBA–PPy and activated carbon electrodes delivered a maximum energy density of 20 W h kg⁻¹ at a power density of 808.9 W kg⁻¹ within a 1.6 V voltage window. The electrochemical analysis demonstrated a considerable improvement in the charge-storage performance due to an increase in electron transfer, electrolyte diffusion, and electroactive area via strong electronic coupling between the Co–Co@Ni–Fe core–shell PBA and the PPy. Furthermore, this work helps to differentiate the composite's current contributions from the PBA and PPy. The detailed electrochemical characterization steps of these methods concerning redox additives integrated into conducting polymers are provided in this work.