Understanding dynamic behavior of proton exchange membrane fuel cell in the view of internal dynamics based on impedance

The transient characteristic of the proton exchange membrane (PEM) fuel cell is an imperative factor for vehicular applications. Currently, a substantial part of previous experimental study regarding the transient response of the PEM fuel cell only focuses on voltage output, while the dynamic internal polarization process is usually ignored. Considering that the electrochemical impedance spectroscopy can represent internal dynamics at different time scales, in this paper, for the first time, in addition to voltage as an indicator of dynamic response performance, the fixed frequency impedance is also applied as an assist signal for voltage response interpretation and internal mechanism analysis. First, a transmission line model is introduced for polarization loss calculation of measured EIS in various conditions. Through quantitative analysis of each polarization loss and each frequency impedance, it is determined that 2500 Hz and 10 Hz impedance can describe the variation trend of proton transfer loss and charge transfer loss, respectively. Based on this, the effects of different step-current, air stoichiometry, backpressure, air humidity, and temperature on the dynamic response of the PEM fuel cell are investigated. Moreover, to fully reveal the internal dynamic mechanism, the transient variation of 2500 Hz and 10 Hz impedance under a broad range of current density is studied. The results indicated that voltage response is strictly related to internal hydration state and oxygen transfer. Especially, poor internal humidification may lead to local starvation. These works offer a more profound understanding of PEM fuel cell transient response, which is significant for system controller design.