Tunable Capacitive Behavior in Metallopolymer-based Electrochromic Thin Film Supercapacitors

Volumetric capacitance is a more critical performance parameter for rechargeable power supply in lightweight and microelectronic devices as compared to gravimetric capacitance in larger devices. To this end, we report three electrochromic metallopolymer-based electrode materials containing Fe2+ as the coordinating metal ion with high volumetric capacitance and energy densities in a symmetric two-electrode supercapacitor setup. These metallopolymers exhibited volumetric capacitance up to 866.2 F cm–3 at a constant current density of 0.25 A g–1. The volumetric capacitance (poly-Fe-L2: 544.6 F cm–3 > poly-Fe-L1: 313.8 F cm–3 > poly-Fe-L3: 230.8 F cm–3 at 1 A g–1) and energy densities (poly-Fe-L2: 75.5 mWh cm–3 > poly-Fe-L1: 43.6 mWh cm–3 > poly-Fe-L3: 31.2 mWh cm–3) followed the order of the electrical conductivity of the metallopolymers and are among the best values reported for metal–organic systems. The variation in the ligand structure was key toward achieving different electrical conductivities in these metallopolymers with excellent operational stability under continuous cycling. High volumetric capacitances and energy densities combined with tunable electro-optical properties and electrochromic behavior of these metallopolymers are expected to contribute to high performance and compact microenergy storage systems. We envision that the integration of smart functionalities with thin film supercapacitors would warrant the surge of miniaturized on-chip microsupercapacitors integrated in-plane with other microelectronic devices for wearable applications.