Nanofiber-Based Electrodes for High Power Supercapacitors

Abstract

Fabrication of novel three-dimensional material architectures is essential for successful development of energy storage devices that allow high rate operation along with sufficient energy capacity. To this end, we report a facile methodology to fabricate hierarchically-porous nanofiber-based electrodes for electric double layer capacitors. In particular, blends of polyacrylonitrile and a sacrificial polymer at different compositions were electrospun into non-woven nanofiber mats with the fiber diameter in the range of 200–400 nm and the mat thickness of 150-200 µm. Electrospun nanofiber mats were then subjected to carbonization to obtain porous carbon nanofibers (CNFs) as polyacrylonitrile converted to carbon and the sacrificial polymer decomposed out creating intra-fiber pores resulting in high specific surface areas of upto 1600 m2/g. No additional chemical or physical activation process was used. Electrochemical measurements in aqueous electrolyte showed large specific capacitance of up to 210 F g−1 at a high cyclic voltammetry scan rate of 100 mV s−1. These materials retain 75% capacitance at a large current density of 20 A g−1 indicating excellent power handling capability. These porous CNFs provided an ideal three-dimensional, free standing (without any binders) electrode design with multi-levels of pore sizes; through-connected Macropores (~500nm-1 µm), created due to inter-fiber spacing in the nanofiber mat & Micro (d <2 nm) and/or Meso pores (2<d<50 nm) created in each nanofiber due to decomposition of sacrificial polymer.While the intra-fiber pores provided large specific surface areas and hence large capacity, the inter-fiber pores facilitated the permeation of electrolyte to the carbon surfaces to allow fast charge adsorption/desorption and enhanced power handling capability.