(Invited) Linking Microporous Structure and Surface Properties with Energy Storage Device Performance of Well-Aligned Electrospun Carbon Fiber Electrodes

In an effort to more extensively utilize abundant US coal reserves, a zero-waste approach to depolymerize raw Powder River Basin (PRB) coal was invented using environmentally friendly ionic liquids. Coal-derived carbon nanofibers (CCNFs) mats were electrospun using extracted coal char as feedstock. It was revealed that the surface of resulting CCNFs mats was enriched with oxygen, while detailed porous structure characterization via Brunauer-Emmett-Teller (BET) and Dubinin-Radushkevish (D-R) methods showed significantly enhanced porosity in the microporous regime (micro- (pores < 2 nm). Consequently, it was observed that electrical conductivity also increased significantly, compared to carbon fibers without PRB coal. To investigate how fiber alignment affects physiochemical properties of CCNF, well-aligned CCNFs was fabricated by optimizing process parameters in electrospinning process. Both conductivity and microporous pore volume further increased in the well-aligned fibers, with respect to fibers randomly aligned. Based on electrochemical impedance spectroscopy (EIS), the well-aligned CCNFs also demonstrated excellent electrochemical properties, resulting from a cumulative result of the shortened ion transfer path, facilitated wettability from high surface oxygen concentration. Specifically, well-aligned CCNFs reached the specific capacitance of 1590 mF cm-2 at a current density of 4 mA cm-2 in 6M KOH solution. Moreover, a symmetrical carbon/carbon prototype supercapacitor cell, consisting of well-aligned CCNFs mats as electrodes, generated the energy density of 15 µWh cm-2 at a power density of 0.3 mW cm-2, corresponding to a current density of 1 mA cm-2, a gravimetric energy density of 30 Wh kg-1 at a power density of 0.6 kW kg-1 and volumetric energy density of 18.2 mWh cm-3 at a scan rate of 50 mV s-1. Finally, 93.0% specific capacitance remained after 10,000 cycles of galvanostatic charge-discharge, demonstrating that the CCNFs mats possess desired durability. Preliminary results showing CCNF performance as electrodes in sodium ion coin batteries will be briefly discussed.