We report the effect of acid treatment on porous carbon materials on their double layer capacitance and electrosorption of ions in capacitive deionization systems. The as-received activated carbon cloth (ACC) had the capacitance value of 210 F/g and was markedly (∼175%) improved when treated with acid solution. Surface functional groups such as hydroxyl, quinines, lactones and ethers on carbon after modification were increased by 40% and 29% with HNO3 and H2SO4 respectively. In capacitive deionization test of NaCl solution, the ACC treated with HNO3 had more than two times removal efficiency when charged with a 1.5 V current than the untreated one. This is attributed to activation and changes of surface functional groups when charged with a current.

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