Once the value of RΩ is measured, the value of Rct can be calculated from the impedance spectra using either equivalent-circuit analysis or graphical methods. Since the symmetric cell contains two WEs, the measured value of Rct is divided by two to give the contribution of a single electrode (Fig. ). For equivalent-circuit analysis, the PEIS spectra are fitted to a circuit of Z = RΩ + Rct/Q, with the value of RΩ determined from PEIS (Fig. ). Alternatively, Morasch's method can be used to calculate Rct by subtracting the measured ohmic resistance from the real impedance value at the touchdown point of the semicircle . A more precise measurement of Rct can be obtained with this method by a quadratic interpolation of the Nyquist data near the touchdown point (Fig. ). Specifically, the data point nearest the touchdown point is identified by finding the minimum value of dZIm/dZRe, then a quadratic equation is fit to the 7 data points centered around this point, and the root of the quadratic equation gives the interpolated ZRe value of the touchdown point. This graphical method requires fewer data points than equivalent circuit fitting, which can reduce the total measurement time. This graphical method is also advantageous because the data analysis is simpler to automate and involves less subjectivity in selecting the data range for fitting. However, the touchdown point interpolation method may only give accurate results for very well-resolved semicircles, and therefore may not be suitable for many systems.