Abstract: An energy storage device, such as a sodium ion capacitor, including an anode and a cathode, at least one of the anode and the cathode including a nitrogen and oxygen functionalized carbon (NOFC). The NOFC has a nitrogen content greater than 4 wt %, such as 13 wt %, an oxygen content greater than 8 wt %, such as 11 wt %, and a surface area greater than 800 m2g?1, such as 945 m2g?1. The energy storage device has favorable reversible and rate capability, such as 437 mAhg?1 at 100 mAg?1, and 185 mAhg?1 at 1600 mA g?1.