In this lecture, the most efficient working cell voltage of a capacitive deionization (CDI) system can be precisely determined by a newly designed cell voltage-programming method when the maximum working potential windows of both positive and negative electrodes have been correctly determined. For conducting the cell voltage-programming method, the CDI cell was initially set under the open-circuit state to measure the initial conductivity. Then, 100 mV increase in the cell voltage is applied to the cell for 3-6 min with the conductivity measurement. The cell voltage is repeatedly increased to obtain the ion-removal capacity against cell voltage curve. Also, a novel strategy for both positive and negative electrodes in a CDI cell, consisting of (1) selecting the charge/discharge time ratio close to 1, (2) finding the potential windows, and (3) using the charge-balanced method for determining the optimal mass ratio (denoted as (m+)/(m-)), is proposed to find the optimal cell voltage of a capacitive deionization (CDI) system with a high desalination efficiency and low energy consumption. Finally, a type of nitrogen-doped activated carbons (NACs) is prepared by a combination of acid pre-treatment and thermal nitrogen doping for the positive electrode of asymmetric capacitive deionization (a-CDI) cells. The oxygen content in activated carbon (AC) controlled by the acid pre-treatment significantly affects the doping amount of N atoms from melamine, which enhances the surface negative charge in NACs to promote the salt adsorption capacity (SAC). The NAC with 30% HNO3 pre-treatment (NAC30) possesses a highly negatively charged surface to exhibit a fast ion desorption rate during discharging. The NAC30 shows the maximum SAC of 24.7 mg/g in the NAC30//AC asymmetrical assembly.