CV and charging-discharging curves, corresponding specific capacitance, and capacitance retention of Mn
/Ni foam electrode. (a) CV curves of the Mn3O4/Ni foam electrode at different scanning rates. (b) Charging-discharging curves of the Mn3O4/Ni foam electrode at different current densities. (c) The corresponding specific capacitance as a function of current density. (d) Capacitance retention of the Mn3O4/Ni foam electrode as a function of cycle number. The insert shows the charging-discharging profiles of the first ten cycles recorded with a current density of 1 A · g-1.