Growing iron oxide nanosheets on highly compressible carbon sponge for enhanced capacitive performance

Abstract

Compressible supercapacitor is a promising flexible energy storage device in view of its excellent capacitive performance, which is recoverable at different compression states. The compressible electrode constitutes the core component that largely determines the performance of a compressible supercapacitor. Commercial polymer sponges are highly compressible materials because most of them are composed of elastic and interconnected polyurethane fibers. However, polymer sponges cannot be directly used as supercapacitor electrodes due to their non-conductive polymer framework. In contrast, carbon sponge (CS) derived from melamine sponge has superior compressible property and exhibits substantially improved conductivity compared to commercial polymer sponge. However, the low specific surface area of CS leads to low specific capacitance, which severely limits its application as compressible supercapacitor electrodes. Currently, pseudocapacitive materials are grown on the conductive CS framework to form hybrid electrodes with improved specific capacitance. Among various pseudocapacitive electrode, iron oxides have attracted considerable attentions due to their natural abundance, high theoretical specific capacitance, and negative working potential. Moreover, the much higher specific capacitance than that of carbon electrodes makes iron oxides one of promising negative candidates for configuring an asymmetric supercapacitor. Herein we report the successful growth of α-Fe2O3 nanosheets on CS by electrodeposition followed by low-temperature thermal annealing. The α-Fe2O3 on CS displays typical nanosheet morphology with mass loading ranging from 3.4 to 6.7 mg•·cm-3 that can be facilely controlled by extending the deposition time from 4 to 16 h. The CS-Fe2O3 electrode retains 90% of its geometric height even after manual compression for 100 cycles. Moreover, the CS-Fe2O3 can withstand 60% strain even at Fe2O3 mass loading as high as 6.5 mg·•cm-3. The performance of the CS-Fe2O3 electrode at different strains was systematically investigated in 3.0 mol•·L-1 KOH aqueous electrolyte by cyclic voltammetry (CV), galvanostatic charge-discharge, and electrochemical impedance spectroscopy (EIS) in a three-electrode system. Our results show that the CS-Fe2O3 composite electrode produces lower specific capacitance at lower strain. The EIS characterization and IR drop results indicate that this is due to the larger internal resistance arising from looser contact of electrode with the current collector and longer ion diffusion length. Particularly, the CS-Fe2O3-12 electrode delivers a maximum specific capacitance of 294 F·•g-1 at a current density of 1.0 A•·g-1, 1.7-times higher than that of CS substrate. Assuming that the specific capacitance of CS-Fe2O3-12 is derived from the double-layer capacitance of CS and the pseudocapacitance of Fe2O3, the capacitance of Fe2O3 nanosheets in the hybrid is calculated to be as high as 421 F•·g-1, much higher than most of recently reported results, showing that the sheetlike structure with more exposed active sites and short ion pathways could dramatically improve the utilization efficiency of the electrode for reversible faradaic reactions. More importantly, the CS-Fe2O3-12 electrode retains 87% of its initial capacitance after continuous charge-discharge at 5.0 A·•g-1 for 10000 cycles, showing promising application as a stable and compressible supercapacitor electrode.