A stable single-phase mixed electron/proton conductor with excellent capacitance from oxygen-functionalized porous carbon (OPC) has been presented herein. Electrochemical study suggested that at room temperature and 90% RH the values of electronic and protonic conductivity of OPC are 1.68 x 10(-3) and 3.14 x 10(-5) Scm(-1), respectively. Moreover, the specific capacitance of 204 F g(-1) at 10 mV s(-1) is observed. The improved electrochemical performance of OPC mostly associated with transformation of high temperature treated inert carbon into an electrochemically active substance and ionic movement associated with oxygen functional groups on the carbon surface during oxidation.