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• A novel supercapacitor MFC was constructed using CNFs composite cathode. • The internal resistance of CNTs/CNFs cathode was only 0.18 Ω·cm−2. • The P max of CNTs/CNFs cathode was 140% higher than that of Pt/C cathode. • MFCs with CNTs/CNFs cathode had a apparent capacitance of 0.68 ± 0.11 F·cm−2. • Both faradaic and non-faradaic processes were involved during GLV discharge. Microbial fuel cells (MFCs) as a novel, environmental-friendly wastewater treatment technology have received increasing attention. One of the major challenges for its large-scale application lies in the sluggish oxygen reduction reaction (ORR) kinetics and high catalyst costs. To address this issue, carbon nanofibers (CNFs) composite electrodes that combine both ORR catalytic activity and electrochemical capacitance behavior were fabricated in this study. Results showed that the interconnectivity of fiber aggregates and thorns-like structure of the as-prepared electrodes exhibited a high ORR catalytic activity with low internal resistance (0.18 Ω·cm−2) and high exchange current density (13.68 A·m−2). When MFCs were equipped with the carbon nanotubes (CNTs)/CNFs electrode, a maximum power density of 306 ± 14 mW·m−2 was obtained, which was 140% higher than that with Pt/C. Moreover, the CNTs/CNFs electrode showed stable catalytic activity without decline in voltage for 60 d. Both faradaic and non-faradaic processes were involved during GLV discharge. The high apparent capacitance (0.68 ± 0.11 F·cm−2) and prolonged discharge time were achieved by the CNTs/CNFs electrode, which could be attributed to the simultaneous effects of electrochemical double layer capacitance and pseudo-capacitance behavior. Together we demonstrated that the great promise of the CNTs/CNFs electrode for MFCs applications. [ABSTRACT FROM AUTHOR]

Freestanding carbon nanotube/carbon nanofiber (CNT/CNF) composites are prepared using electrospun CNFs as skeletons in a tubular chemical vapor deposition system. The obtained CNT/CNF composites show a hierarchical structure with a high special surface area, a high conductance (1250 S cm −1 for a 10 mm × 20 mm sample), and a high flexibility. After coated with manganese dioxide (MnO 2 ) via an in-situ redox deposition for 0.5 h (∼0.33 mg), the CNT/CNF/MnO 2 electrodes show a high specific capacitance of 517 F g −1 at a scan rate of 5 mV s −1 , which is about 5.6 time higher than that CNF/MnO 2 -0.25 h ones with MnO 2 of ∼0.36 mg. Moreover, this CNT/CNF/MnO 2 electrodes show a much higher rate capability (57% at current density of 14 A g −1 ) than CNF/MnO 2 ones (24% at 14 A g −1 ), and also show a higher cycling stability (maintaining 75% of the initial capacitance at cycle number of 1000). In addition, the symmetric supercapacitor assembled using two piece CNT/CNF/MnO 2 electrodes shows their maximum energy density of 3.88 Wh kg −1 at power density of 7000 W kg −1 . The capacitance of the assembled capacitor maintains 70% after 100 bending cycles, indicating a good flexibility. These enhancements in EC performances should be due to our designed hierarchical structures. It is suggested that such freestanding flexible CNFs/CNTs/MnO 2 hierarchical composites are highly promising for high-performance flexible supercapacitors. [ABSTRACT FROM AUTHOR]