To characterize the hydroxide and proton crossover flux independently for each membrane under an electric field, we constructed a two-chamber supercapacitor cell, with one chamber having an alkaline (or acidic) initial state as crossover source and the other chamber having a near-neutral pH as acceptor chamber (Fig. ). The two sides were supported by NaCl solutions and separated by a CEM (or AEM) for hydroxides (or protons) crossover testing. Supercapacitors can be charged and discharged through ion adsorption, without introducing redox active molecules that could interfere with the measurements. Ideally, only Na + (or Cl -) ions should cross the membrane when the cell is subjected to an alternating current. However, ion-selective membranes cannot perfectly reject hydroxide ions and protons, leading to a crossover that can be observed on the initially pHneutral side. Depending on their selectivity and molecular structure, different ion-selective membranes exhibit varying rates of crossover. We selected well-established and commonly used ion-selective membranes: Fumasep E-620K as the CEM and Selemion DSV-N as the AEM, for evaluation of crossover fluxes, which were estimated by tracking the pH change of the initially neutral acceptor solution (Fig. ). To determine the effect of proton/hydroxide concentration and applied current density on the crossover fluxes, we varied the initial pH for the source chamber side and applied different alternating current densities in the constructed supercapacitor cell, mimicking the chargedischarge cycles in during real battery operation. A typical voltage and current density profile is shown in Fig. . The operation parameters were chosen to avoid water splitting so that the pH change is affected only by crossover. Then the pH change was normalized by solution volume and membrane area into crossover flux. Fig. and 2d show the measured crossover fluxes for protons and hydroxides, respectively. In both cases, the applied current density has only a minor effect on the magnitude of the crossover fluxes. Conversely, the initial concentration of ions in the source chamber has a significant, almost linear, impact on the crossover flux, in agreement with the relevant terms in Eqs. ( )- (2).