“Wrapped” nitrogen-doped defective reduced graphene oxide (ND-rGO): A virtual electron bed for enhanced supercapacitive charge storage in stepped-surfaced-NiCo2O4/ND-rGO||Bi2O3 asymmetric device

Abstract

In the context of low impedance electron transfer in electrochemical charge storage devices, stepped-surfaced solids are important because they provide low work function and suitable interface for electron transfer. Contextually, in this work, for the first time, a carbonate-assisted synthesis method is adopted to fabricate severely stepped-surfaced porous NiCo2O4. Further, N-doped defective rGO (ND-rGO) wrapped NiCo2O4 is assembled, and the electron bedding effect of ND-rGO on the supercapacitive charge storage performance of NiCo2O4 is thoroughly studied. The physicochemical studies authenticate restricted crystal growth of NiCo2O4 on ND-rGO and chemical bonding between ND-rGO and NiCo2O4. Detailed electrochemical studies using cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) techniques show redox-mediated charge storage in NiCo2O4, ND-rGO-induced enhanced high-rate supercapacitive charge storage and lower charge-transfer resistance, equivalent series resistance and Warburg resistance in NiCo2O4/ND-rGO heteronanocomposite. Further, for the first time, Bi2O3 was used as a super compatible negative electrode material to fabricate asymmetric supercapacitor (ASC) devices based on pristine NiCo2O4 and NiCo2O4/ND-rGO, and in depth electrochemical characterizations show redox-capacitive charge storage behaviour in both the devices. The NiCo2O4/ND-rGO||Bi2O3 demonstrates 2-times more areal capacitance at extreme rate condition, lower iR drop, very low charge transfer resistance, solution resistance and ion-diffusion resistance, as compared to NiCo2O4||Bi2O3 ASC device. The NiCo2O4/ND-rGO||Bi2O3 ASC device also retains a very high 93.1% of specific capacitance after 5000 GCD cycles at an extreme applied current density of 32 mA cm−2. Further, the NiCo2O4/ND-rGO||Bi2O3 offers twice the energy density as compared to NiCo2O4||Bi2O3 ASC device, at elevated power density up to ∼10,000 Wh kg−1. The enhanced supercapacitive performance of NiCo2O4/ND-rGO||Bi2O3 ASC device is ascribed to electron bedding effect of wrapped ND-rGO which provides a continuous conducting network, ND-rGO induced improved wettability, activated NiCo2O4|ND-rGO interface, and excellent electromechanical properties of NiCo2O4/ND-rGO heteronanocomposite.