Boosting the Capacitance of an Aqueous Zinc-Ion Hybrid Energy Storage Device by Using Poly(3,3′-dihydroxybenzidine)-Modified Nanoporous Carbon Cathode

Aqueous electrochemical energy storage devices are highly safe, low cost, and environmentally benign, yet suffer from low energy storage capacity. Here, we devise a novel cathode material for making aqueous Zn-ion hybrid energy storage devices with high areal capacitance. A pseudocapacitive polymer, poly(3,3′-dihydroxybenzidine, DHB), is electrodeposited onto the surfaces of porous active carbon (AC) granules to increase the capacitance. This composite coating has high mass loading, leading to high areal capacitance in F cm^-2 scale. The flexible sandwich-structured cell made by combing the poly(3,3′-DHB)/AC cathode and the Zn foil anode shows stable electrochemical performance upon bending. The areal capacitance of this cell is up to 1.3 F cm^-2, and the maximum energy and power densities are 0.18 mWh cm^-2 and 4.01 mW cm^-2, respectively. Moreover, a Zn-ion micro cell is fabricated by combing two sets of carbon-paper-based finger electrodes, one is plated with Zn and the other is coated with the poly(3,3′-DHB)/AC composite. The in-plane micro cell shows a high areal capacitance of 1.1 F cm^-2 and a high areal energy density of 152 μWh cm^-2. Our research suggests a new approach to make high-capacitance Zn-ion hybrid energy storage devices with different forms to meet various applications.