Encapsulation of cobalt-iron Prussian blue analog nanocubes using a redox polymer for advanced supercapacitor cathode materials

Prussian blue analogs (PBAs) are promising cathode materials for aqueous supercapacitors because of their open 3D framework, low cost, and large theoretical capacitance. Nevertheless, their poor electrical conductivity and unavoidable dissolution during cycling result in a low rate capability and cycle life. Herein, a facile in situ polymerization encapsulation strategy, which can increase the energy storage performance of cobalt-iron PBA nanocubes via the use of a redox polymer nanoskin (CFP@PTMT), is demonstrated for aqueous supercapacitors. The poly(trimethyl thionine) (PTMT) nanoskin serves a triple-functional role as a conductive skeleton, electroactive protection layer, and structural stabilizer to increase the electrical conductivity, pseudocapacitance contribution, and structural stability of CFP. Interestingly, CFP@PTMT delivers a high capacitance of 984 F g⁻¹ at 1 A g⁻¹ and a superior rate capability of 87.19 % capacitance retention at 10 A g⁻¹, with a 99.34 % capacitance retention over 5,000 cycles. Notably, an asymmetric supercapacitor is assembled using a CFP@PTMT cathode and an activated carbon anode, which results in a high capacitance of 287 F g⁻¹, a large energy density of 45.26 Wh kg⁻¹, and an excellent power density of up to 8,000 W kg⁻¹ with 99.00 % capacitance retention after 5,000 cycles. This work establishes that redox polymer nanoskin encapsulation not only addresses the intrinsic drawbacks of PBAs but also creates a new design paradigm for integrating conductivity, pseudocapacitance, and structural stability at a single nanoscale interface. This strategy paves the way for next-generation aqueous energy storage devices with balanced high performance and durability.