context: In comparison with MnOx, NiO and Co3O4, TiO2 with lower theoretical capacitance has also been tested as an electrode in SCs. For instance, by using TiO2 nanofibers, activated carbon and a PVA–H3PO4 membrane, a solid-state SC was fabricated, which delivered a specific capacitance of 310 F g−1. Recently, Ta-doped TiO2 nanofibers were synthesized through the electrospinning technique. In 1 M H2SO4, the 2% Ta-doped TiO2 nanofibers could deliver nearly two times higher specific capacitance than the undoped TiO2 counterpart (111 F g−1), which was attributed to the enhanced electronic conductivity of TiO2 stemming from the replacement of Ti4+ by Ta5+ in the lattice after doping. In addition to the most studied metal oxides mentioned above, Fe2O3, V2O5 and CuO have also been applied to SCs as electrodes. The first report on a pure iron oxide electrode composed of particles with sizes of 53 nm has demonstrated a specific capacitance of 256 F g−1 in 1 M LiOH. Recently, both interconnected Fe2O3 and V2O5 nanofibers with rich meso-/macro-pores were obtained via electrospinning and subsequent heat treatment. Owing to the hierarchical porous structure that acts as an excellent conductive highway for efficient electronic transfer and ion permeation, the resultant binder-free Fe2O3 and V2O5 electrodes gave specific capacitances of 255 F g−1 and 256 F g−1, respectively, at 2 mV s−1 in 1 M Na2SO4. Moreover, an all-solid-state ASC based on an Fe2O3 negative electrode and V2O5 positive electrode could be operated at up to 1.8 V and displayed a high energy density of 32.2 W h kg−1 at an average power density of 128.7 W kg−1. In 2012, pure electrospun V2O5 nanofibers with different structures have been synthesized and tested in two studies. These pure electrospun V2O5 nanofibers displayed a capacitance of around 200 F g−1 in both the studies. A high specific capacitance of 620 F g−1 (accounting for ∼35% of their theoretical capacitance) at 2 A g−1 in 6 M KOH was reported by Jose et al. for electrospun CuO nanowires in 2014. In another reference, an ASC device based on electrospun CuO nanowires as the anode and AC as the cathode was constructed and provided an enhanced voltage window (1.6 V), specific capacitance (83 F g−1), and threefold higher energy density (29.5 W h kg−1) than the AC-based symmetric capacitor (1.4 V, 33 F g−1, 11 W h kg−1).
answer: AC