Na_0.35MnO₂/CNT Nanocomposite from a Hydrothermal Method as Electrode Material for Aqueous Supercapacitors

A Na_0.35MnO₂/CNT nanocomposite is prepared by a simple and low energy consumption hydrothermal method. Its electrochemical performance as an electrode material for asymmetric supercapacitors with aqueous Na₂SO₄ solution is investigated. In this nanocomposite less than 2 wt-% of CNTs are intermingled with the Na_0.35MnO₂ nanowires. Quite obviously the introduced CNTs efficiently improve the rate performance of the composite. When the current density increases from 0.1 to 10 A·g^-1, the capacitance decreases only slightly from 163 to 125 F·g^-1. When assembled into an asymmetric aqueous supercapacitor using activated carbon as the counter electrode and an aqueous electrolyte solution of 0.5 mol·L^-1 Na₂SO₄, Na_0.35MnO₂/CNT shows an energy density of 33.5 Wh·kg^-1 at a power density of 3 kW·kg^-1 based on the weights of the two electrode materials, higher than those for simple Na_0.35MnO₂, with an energy density of 28.7 Wh·kg^-1 at a power density of 3 kW·kg^-1 only. The Na_0.35MnO₂/CNT nanocomposite presents excellent cycling behavior, no capacitance fading after 10000 cycles, even when dissolved oxygen is not removed from the electrolyte solution. The results are very promising for practical applications of this electrode material since sodium is much cheaper than lithium and its natural resources are rich.