(Invited) Three-Dimensional Freestanding Nanofiber Electrodes for Electric Double Layer and Pseudo-Capacitors
We have developed porous carbon nanofibers (PCNF) that exhibit specific surface area of >1500 m2/g and multi-levels of pore sizes in the range of micro (< 2 nm), meso (< 50 nm) and macropores (> 50 nm). We studied these materials as freestanding electrodes in 1) electric double layer capacitors (EDLC) using liquid (aqueous and ionic liquid), as well as solid-state electrolytes and 2) as platforms for polyanilene (PANi) electrodeposition to develop hybrid supercapacitors that integrate electric double layer and pseudocapacitive energy storage. The unique combination of pores at different length scales allows us to achieve a battery-like energy density of up to 80 Wh/kg, while retaining high power and cycle life in EDLCs. In-operando infrared spectroelectrochemistry was conducted to understand the transport of ions through the carbon electrode pores and its effect on performance in ionic liquid-based EDLCs. In the hybrid supercapacitor devices, we demonstrated that the PCNF mats retain their EDLC behavior post PANi coating providing a 366 F g–1 gravimetric capacitance, 140 F cm–3 volumetric capacitance, and up to 2.3 F cm–2 areal capacitance at 100 mV s-1 scan rate based on the weight, volume and area of the complete hybrid electrode – polyaniline coating and carbon substrate.