Heteroatom and Crystallinity-Controlled Carbon Felts Based on Silk for Li-TEMPO Redox Flow Battery

Silk fiber, a precursor rich in heteroatom, was heat-treated at various temperatures from 800 to 2800 oC to prepare carbon felts that have different heteroatom contents and carbon microstructures. With the heat treatment temperatures, the heteroatom contents of the silk-derived carbon felts (SCFs) gradually decreased, their graphitic carbon structures were continuously developed, and their electrical conductivities were linearly improved. By comparing with electrochemical reactivities of SCFs as an electrode in the lithim-2,2,6,6-tetramethylpiperidine-1oxyl (Li-TEMPO) redox flow battery (Li-TEMPO RFB) system, it was found that a redox potential separation of the SCFs heat-treated at 2800 oC (SCF2800) was reduced by ~61 mV at 10 mV s-1 compared with the SCF heat-treated at 800 oC (SCF800). In addition, in a galvanostatic charge/discharge tests at current densities from 0.1 to 2 mA g-1, the capacity retention of SCF2800 is ~24% higher than that of SCF800. Moreover, in an impedance resistance measurement, the charge transfer resistance of SCF2800 which occurs in an interface between electrolyte and electrodes was much lower than that of SCF800. These results provide that the electrical conductivity of electrode materials is the key on the electrochemical performances of Li-TEMPO RFBs which is much more significant than the catalytic effect of heteroatoms on the surface of electrode materials.