Reduced graphene oxide/Mn3O4 nanocrystals hybrid fiber for flexible all-solid-state supercapacitor with excellent volumetric energy density

- Large size graphene nanosheets and Mn3O4 nanocrystals were used as assembled units for hybrid fiber.
- The hybrid fiber electrode delivers good mechanism property and high capacitance.
- Flexible all-solid-state fiber supercapacitor based on graphene/Mn3O4 was assembled.
- The assembled device shows excellent volumetric energy density and flexibility.

To solve balance problem between flexibility and energy density for graphene-based fiber electrodes, graphene oxide (GO) with nanosheet size larger than 20 μm and Mn3O4 nanocrystals with a size of about 5 nm were used as assembled units, reduced graphene oxide (RGO)/Mn3O4 hybrid fibers with excellent flexibility and high capacitance were prepared by a simple and scalable wet-spinning approach and followed by treating in a solution of hydrazine hydrate, in which GO could be reduced into RGO, while Mn3O4 nanocrystals were maintained after hydrazine vapor reduction treatment. A suitable amount of Mn3O4 nanocrystals not only maintained the tensile strength and flexibility of the obtained fiber electrode, but also obviously improved its capacitance. RGO/Mn3O4-30 fiber electrode showed a maximum volumetric capacitance of 311 F cm−3 at a current density of 300 mA cm−3 on the basis of maintaining the electrode good flexibility and mechanical strength. By intertwining two as-prepared RGO/Mn3O4-30 flexible hybrid fiber electrodes, flexible symmetric all-solid-state fiber supercapacitor was assembled by using PVA/H3PO4 as the gel electrolyte. The assembled device showed a maximum volumetric capacitance of 45.5 F cm−3 at a current density of 50 mA cm−3, and remarkable flexibility and mechanical property to tolerate long-term and repeated bending. Moreover, the assembled device exhibited a maximum volumetric energy density of 4.05 mWh cm−3 and a maximum volumetric power density of 268 mW cm−3. When three flexible symmetric all-solid-state fiber supercapacitors were connected in series, the series device could be used to power a red light emission diode (LED) lamp after being fully charged, demonstrating its potential application as an efficient energy storage component for flexible electronics.

The balance problem between flexibility and energy density for graphene-based fiber electrodes is solved by using GO with nanosheet size larger than 20 μm and Mn3O4 nanocrystals with a size of about 5 nm. The assembled device shows excellent volumetric capacitance of 45.5 F cm−3 and a maximum volumetric energy density of 4.05 mWh cm−3 in maintaining remarkable flexibility and mechanism property.112