context: Electrospun CNFs/Nb2O5 composite nanofibers were prepared by electrospinning a solution containing the precursors of Nb(C2O4H)5, PAN and TEOS, followed by carbonization and SiO2-etching.Fig. 7f and g show the structural features of the prepared CNFs/Nb2O5 nanocomposites, which reveal a continuous mesoporous network architecture. Compared with solid CNFs/Nb2O5 composites without using TEOS in spinnable solution and bulk C/Nb2O5 prepared via directly annealing the Nb(C2O4H)5, the mesoporous CNFs/Nb2O5 composites revealed much superior electrochemical reactivity. In detail, the mesoporous Nb2O5/CNF showed outstanding cyclability (94% retention after 10000 cycles at 100C, 1C = 200 mA g−1), and superior rate capability with specific capacities of 287 mA h g−1 at 0.5C and 172 mA h g−1 at 150C, which was much higher than that of pure CNFs (159.1 mA h g−1 at 0.5C and 64.9 mA h g−1 at 100C). In addition, a hybrid Na-ion capacitor was fabricated by employing mesoporous CNFs/Nb2O5 as the anode and CNFs/graphene as the cathode. This device showed both large energy densities (124 W h kg−1, 11.2 mW h cm−3) and impressive power densities (60 kW kg−1, 5.4 W h cm−3), indicating that both the introducing of Nb2O5 and the unique continuous mesoporous network architecture played a crucial role in the enhancement of the electrochemical properties.
answer: CNFs/Nb2O5