Hydroxide (or proton) crossover rate across CEM (or AEM) was evaluated in a standard 5 cm 2 flow cell with two chambers (receiving and source chambers). The receiver chamber contains 1 M NaCl and its initial pH was set at 7. The source chamber contains 1 M NaCl and its pH was varied by NaOH (or HCl) for each crossover test (1 M NaOH for pH 14 and 1 M HCl for pH 0, without NaCl). Each cell was built with two layers of high surface area porous carbon coated supercapacitor electrodes in the anode and the cathode that are separated by an ion exchange membrane (Selemion® DSV-N for proton crossover test and E620 K for hydroxide crossover test). The membrane was soaked in 1 M NaCl overnight before use. During each crossover test, alternating constant current was applied with a restricted cell voltage of 1.1 V to avoid water splitting. During alternating current cycling, the pH of both the source and receiving chambers were tracked by pH sensors in real-time.