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.