Aqueous redox electrochemical capacitors (redox ECs) generally suffer from low energy density and uncontrolled cross-diffusion of soluble redox components, causing serious self-discharge and capacity fade. We have designed a membraneless potassium bromide electrochemical capacitor (ML-PBEC) single cell generating high energy density with reduced self-discharge. From the perspective of CO2 utilization, highly porous carbon with a hierarchical structure was synthesized from CO2 gas and the change of the electrochemical performance by doping nitrogen on the CO2-derived carbon materials was investigated. Using the synthesized carbon as electrodes with a Br−/Br3− redox active aqueous electrolyte, the designed ML-PBEC single cell produced enhanced energy density due to suppressed cross-diffusion of soluble bromine molecules through electrostatic interaction between the protons of N-doped sites and polybromide anions even without membrane. The designed single cell provided a high energy density equivalent to the highest energy density of membrane contained aqueous electrochemical capacitors, 14.3 Wh kg−1 at 2 A g−1 over 10,000 cycles.