Fig. 4. CV curves of (a) the non-hollow carbon nanoparticles and (b) hollow carbon nanobubbles between 1 and 3 V (vs. Na⁺/Na) at a potential sweeping rate of 5 mV s^–1. (c) CV curves of the hollow carbon nanobubbles between 1 and 3 V (vs. Na⁺/Na) at various rate in the range of 5–40 mV s^–1. (d) The dependence of anodic and cathodic current (at 2 V) on scanning rate. (e) Rate performance and (f) cycling performance of the hollow carbon nanobubbles and non-hollow carbon nanoparticles. The electrodes were firstly activated by charging/discharging at a current density of 50 mA g^–1 before testing at the current density of 10 000 mA g^–1, then tested between 0.001–3 V (vs. Na⁺/Na).