High-precision identification of polarization processes of distribution of relaxation times by polarization curve model for proton exchange membrane fuel cell

Abstract

The distribution of relaxation times (DRT) is a promising impedance analysis method that is often used to decompose polarization processes with different time constants. However, uncertainty in interpreting polarization processes by DRT arises from two main sources: (1) overlap of the DRT peaks and (2) lack of peak identification methods. Thus, herein, a high-precision polarization process identification method based on polarization curve model involving electrochemical processes of proton exchange membrane fuel cell (PEMFC) was proposed by establishing the intrinsic relationship between polarization curve and electrochemical impedance spectroscopy (EIS) and comparing the quantitative relationship between the steady-state resistance obtained by the inverse slope of polarization curve model and each DRT peak. Then, an improved DRT method based on the characteristic frequency resolution optimization was suggested to avoid the overlap of the DRT peaks in case of the narrow interval time constants of the electrochemical system. Based on these two methods, each polarization resistance of PEMFC and DRT peak was quantitatively analyzed. First, it was determined that the total steady-state resistance has a resistance recovery compared to the low-frequency intercept resistance obtained by the improved DRT method, because of the ultra-low-frequency pseudo-inductive behavior arising mainly from dissolved water transport. Second, the peak with a characteristic frequency in the range of 7–54 Hz corresponds to two processes: mass transport and dissolved water transport. The peak with a characteristic frequency in the range of 3–20 Hz represents charge transfer. Interestingly, the two DRT peaks in the low-frequency region overlapped at 400 mA cm−2 because their characteristic frequencies changed in opposite directions with the current density. Moreover, the peak with a high characteristic frequency in the range of 250–300 Hz corresponds to proton transport of the catalyst layer ionomer. The methodology based on the model lays the foundation for identifying the distribution of the relaxation times for PEMFCs.