Nitrogen-doped graphene fiber electrodes with optimal micro-/meso-/macro-porosity ratios for high-performance flexible supercapacitors

The introduction of graphene fiber-based supercapacitors as important power storage components in wearable electronic products has attracted increasing attention because of their lightweight, good flexibility, high power density, long lifecycle and excellent charge/discharge capacity. However, their capacitance and energy density are greatly confined as a result of the dense graphene stacking, hydrophobicity and poor electrical conductivity of graphene fiber electrodes. Here we report a nitrogen-doped graphene fiber electrode with optimal micro-/meso-/macro-porosity ratios to improve the utilization efficiency of fiber inner interface for accumulated charge storage and boosted ion transport. Urea is employed as both nitrogen doping source and self-removed template for graphene fibers to tune the hierarchically porous architecture, wettability, nitrogen content, and electrical conductivity for improved electrochemical performance. The resulting supercapacitors display superior areal capacitance of 1,217 mF/cm² (486.3 F/g) and high energy density of 27 μW h/cm² (10.8 W h/Kg) in polyvinyl alcohol/H₃PO₄ gel electrolyte. These metrics represent the highest values to date among existing all-solid-state fiber-based supercapacitors based on inorganic electrolyte. Moreover, the fiber-based supercapacitors show good rate capability and excellent cyclic performance. Our work also provides an understanding of the effect of fiber structure on electrochemical activity and highlights the importance of full utilization of graphene interior interface.