Role of conducting carbon in electrodes for electric double layer capacitors

Six electrodes with a varying amount (5, 10, and 15 wt.%) of conducting carbon nanotubes (CNT) and carbon nanofibers (CNF) were fabricated and their performance evaluated against a control sample that was devoid of any conducting material. The goal of this work was to determine the correlation between electrode conductivity and capacitance in 1 M tetraethyl ammonium tetrafluoroborate (TEABF4) in propylene carbonate (PC) electrolyte. CNT electrodes exhibit the lowest electrical resistance, while CNF electrodes had the highest capacitance. The specific capacitance (120–140 F/g) increased monotonically up to 2.5 V. An inverse correlation between electrical resistance and capacitance was observed for various concentrations. The electrodes were characterized using CV, EIS, SEM, and BET analysis.

Research Highlights
► This paper investigates the role of conductivity enhancers on the capacitance of a double layer capacitor. Electrostatics governs the phenomenon in the electric double layer capacitance, and hence a highly conducting electrode is expected to yield better performance. An extremely high capacitance is reported in a carbide derived carbon material (conductivity ~100 – 140 S/cm) as compared with activated carbons (~10 – 12 S/cm). Unfortunately, the prohibitive costs of these specialized carbons render them impractical for commercial usage. This letter investigates the role of conductivity enhancing additives and their role in double layer capacitance enhancement, as an inexpensive alternative to high performance carbons.
► Lowest ESR (0.4 – 0.55 Ω) and double layer capacitance behavior were observed in CNT (120 – 125 F/g) electrodes, while a higher capacitance was observed in CNF electrodes (125 – 140 F/g). The ESR reduced with increasing CNT content, while the opposite trend is observed in CNF electrodes.
► We could not establish any direct correlation between electrode conductivity and ESR due to lack of dispersion of conductivity enhancers. Nevertheless, the presence of CNF does lead to a better electrolyte penetration, and hence a higher capacitance, while CNT electrodes led to an enhanced electrode conductivity. We believe both of these are artifacts of surface functionalities (on CNT and CNF) rather than conductivity effect.
► The BET surface area of all electrodes was in the range 1000 – 1200 m2/g. We observed a monotonic increase in the capacitance with increasing voltage up to 2.5 V, and then it reduced at 3.0 V. We noted the maximum capacitance value of 120 – 140 F/g at 2.5 V, typical in organic electrolytes.