Fig shows the ion concentration change in the
EIS analyses were performed to compare the ohmic and charge transfer resistances of the CEM and FO membrane in MECs. The EC represents resistances of a MEC caused by electrodes, electrolytes, membranes, and interfaces. Based on an EC of electrical double layer on the electrode surface (Park and Yoo, 2003) and operated MEC configuration, a circuit for the two chamber MEC is proposed in Fig. 3(a). The regression of obtained data from both the CEM–MEC and FO–MEC were conducted by using the EC to determine the ohmic resistance, charge transfer resistance, and capacitance values. Using the circuit the impedance spectrum is expected to comprise three parts: two semi-circles of ohmic resistance, and charge-transfer, and a Warburg G-15 region. The measured data as a Nyquist plot and fitted results from the proposed EC were depicted in Fig. 3(b). In this study, the ohmic and charge-transfer regions of both MECs were overlapped showing one semi-circle since ohmic resistance for the membrane was too small compared with the resistance of electrodes (Kim et al., 2014). The value of ohmic resistance (R1), which consists of electrolyte and membrane resistances, was higher in the FO–MEC (10.13 Ω) than the CEM–MEC (6.16 Ω). When electrolyte conditions are the same, the ohmic resistance can represent the membrane resistance, except for bipolar membranes ( Harnisch and Schröder, 2009). The resistance of the FO membrane was 1.5 times higher than for the CEM in the MECs, but this is caused by a physical difference between the FO membrane and CEM. The FO membrane is composed of two polymer layers having different pore sizes (active and support layers), whereas the CEM is a symmetric polymer membrane.