Development of Short Time Impedance Measurement Method for PEM Electrolysis Cell

Abstract

Electrochemical impedance spectroscopy (EIS) is expected as a method to evaluate a polymer electrolyte membrane (PEM) water electrolysis cell. The internal state of electrolysis cell can be investigated by EIS without destruction. However, it may be difficult to measure stable impedance during electrolysis at high voltage since the large amount of oxygen is generated on the anode. Therefore, a short time impedance measurement is required. The purpose of this study is to develop short time impedance measurement method1,2) that can estimate the electrolyte membrane resistance R M and the charge transfer resistance R ct in a short time.A Japan Automobile Research Institute (JARI) standard cell with an electrode area of 25 cm2 was used as the test cell. The impedance measurement was performed in a frequency range from 100 mHz to 1 kHz and started from high frequency toward low frequency in the logarithmic scan.The Nyquist plot of impedance spectrum shows inductive behavior caused by wiring in the high frequency region and capacitive semicircle related to the anode in the middle frequency region. Under the condition of high current density, it was found that impedance spectrum was scattered in the low frequency range. The scattering originated from oxygen evolution at the anode when the measurement time is long. The measurement time was about 80 s to obtain an impedance spectrum, thus it is necessary to measure in a shorter time. Therefore, we obtained R ct in a short time by the two-point frequency method proposed by Haruyama et al.1,2) The equation (1) is used to calculate R ct. R ct = Re [Z] - R M + Im [Z]2/( Re [Z] - R M) (1)In the equation (1), R M is the electrolyte membrane resistance, which is the real component of the impedance at the high frequency. The real and imaginary parts of the impedance at the low frequency are represented by Re [Z] and Im [Z]. The measurement was performed by fixing the frequency at 1 kHz as a high frequency value and changing the low frequency value from 100 Hz to 10 mHz. R ct obtained from the measurement results by the two-point frequency method was compared with R ct obtained from the curve fitting of the impedance spectrum with the equivalent circuit. As a result, the relative error of the R ct calculated from the measurements at two frequencies 1 kHz and 10 Hz was -2.15% from the fitted result, and the measurement time was 5.33 s. From these results, it was found that the two-point frequency method was able to reduce the measurement time significantly compared to the normal EIS and to estimate R ct accurately. References 1) S. Haruyama, T. Tsuru, ASTM STP 727, ASTM Philadelphia, p.167 (1981).2) T. Tsuru, S. Haruyama, Boshoku Gijutsu, 33, p.579-582 (1984).