In capacitive deionization (CDI), co-ions will be repelled simultaneously from the electrodes when counterions are adsorbed onto the electrodes, leading to a diminished CDI performance. To reduce this so-called co-ion effect, carboxylic groups were grafted onto the surface of commercial activated carbon fiber (ACF) using a simple nitric acid treatment (ACF-HNO3). ACF-HNO3 exhibited good cation selectivity. An asymmetric CDI cell (N-CDI) was assembled, in which ACF-HNO3 was used as the cathode and pristine ACF as the anode. The removal capacity of N-CDI at 1.2 V can be as high as 12.8 mg/g, whereas that of the symmetric CDI cell composed of pristine ACF (0-CDI) is only 8.0 mg/g. Accordingly, the charge efficiency of N-CDI stays above 74%, whereas that of 0-CDI is below 40% at various applied voltages. Moreover, N-CDI exhibited better cyclic stability than 0-CDI. These results indicate that use of ACF-HNO3 as the cathode can effectively reduce co-ion expulsion, thereby enhancing the CDI performance. (C) 2015 Elsevier Ltd. All rights reserved.