Superelastic Pseudocapacitors from Freestanding MnO₂-Decorated Graphene-Coated Carbon Nanotube Aerogels

In recent years, the demand for emerging electronic devices has driven efforts to develop electrochemical capacitors with high power and energy densities that can preserve capacitance under and after recovery from mechanical deformation. We have developed superelastic pseudocapacitors using ∼1.5 mm thick graphene-coated single-walled carbon nanotube (SWCNT) aerogels decorated with manganese oxide (MnO₂) as freestanding electrodes that retain high volumetric capacitance and electrochemical stability before, under, and after recovery from 50% compression. Graphene-coated SWCNT aerogels are superelastic and fatigue-resistant with high specific surface area and electrical conductivity. Electrodeposition of MnO₂ onto these aerogels does not alter their superelasticity, with full shape recovery even after 10 000 compression-release cycles to 50% strain. Total (utilized) gravimetric capacitances of these aerogels before compression are similar to those under and after recovery from 50% compression over a wide range of scan rates with capacitances reaching 98 (468), 106 (522), and 128 F/g (626 F/g) at a scan rate of 2 mV/s, respectively. These gravimetric capacitances are preserved even after 10 000 compression-release cycles to 50% strain. Further, 50% compression of these aerogels increases the volumetric capacitance from 1.5 to 3.3 F/cm³. Before compression, the lifetime performances of these aerogels remain largely stable, with capacitance degrading by only ∼14% over the first 2000 charge-discharge cycles and remains constant for further 8000 cycles. Under 50% compression, capacitance displays a similar trend over 10 000 charge-discharge cycles. After recovery from 10 000 compression-release cycles to 50% strain, the aerogels show slightly greater capacitance loss of ∼28% over the first 2000 charge-discharge cycles and an additional ∼10% loss over the subsequent 8000 charge-discharge cycles. Finally, substantially higher gravimetric capacitance is achieved through greater MnO₂ deposition, facilitated by the large porosity of these aerogels, albeit at a loss of capacitance retention upon compression. These capacitors display the feasibility of coating graphene-coated SWCNT aerogels with various pseudocapacitive materials to create superelastic energy-storage devices.