A Study on the Electrochemical Impedance Analysis of PEMFC Under the Low Humidity Condition

Abstract

Polymer electrolyte membrane fuel cells (PEMFC) have received much attention as a next generation energy conversion system due to the pollutant-free byproduct (water), and high efficiency. PEMFC fundamentally consists of many components such as membrane electrode assembly (MEA) gas diffusion layers (GDLs), and bipolar plates (GDLs). Among them, MEA is directly correlated to the performance and durability of PEMFC since the MEA components such as catalyst materials, supporting carbon, and an ionomer (binder) can be degraded under corrosive environment during operating Hence, it is important to develop the meticulous analysis method to diagnose the degraded condition of PEMFC; thus, fault detection have to be properly identified before severe deterioration of MEA happens. Among an in situ diagnosis tool to investigate the degraded components of MEA, the electrochemical impedance spectroscopy (EIS) technique is well-known electrochemical method to classify each resistance such as high frequency resistance (HFR, Ohmic resistance), charge transfer resistance, and Warburg impedance, separately; thus, it makes it possible to trace the degraded components of MEA. Among them, the performance of PEMFC is closely related to the activity of catalyst that is reflected as a charge transfer resistance, and therefore, usual impedance studies are mostly focused on the catalyst [1]. In fact, however, the degradation behavior of a fuel cell system is very complicated since it originated from a complex chain and combination of corrosion issues [2], such as ionomer degradation, membrane thinning, corrosion of the supporting carbon, and Pt corrosion, which are the crucial degradation factors that affect both performance and durability of PEMFC. Hence, to elaborate the impedance measurement, it is necessary to focus on the inherent properties of all components of MEA.In this context, we performed the impedance measurements under low humidity (RH 30 %) condition to emphasize the ionic resistance during PEMFC operation. By deliberately limiting the ionic conductivity, the dehydrated ionomer accentuated the influence of ionic resistance among various resistance components inside the MEA, and thus the degraded MEA can be analyzed with respect to the ionomer surrounding catalyst layer (CL), as shown in Figure 1. Moreover, the complex capacitance adopting transmission line model (TLM) circuit under non-faradaic process facilitates to investigate the double layer capacitance (EDLC) of degrading CL [3, 4] as shown in Figure 2. To verify the feasibility of this approach, we studied degradation mechanism of MEA under dehydration and cathode flooding condition, respectively, since these problems related to water management are typical issues to be solved in systemic point of view [5].As a result, under the dehydration degradation, the increase of gas permeation induced the mixed potential formation at the anode, causing the cathode CL deterioration with surface variation of cathode CL. It could be monitored though the volume fraction of supporting carbon and ionomer. In addition, under the flooding degradation, the unbalanced current distribution caused the irregular anode deterioration, and phase differences between transported protons were reflected at horizontal aspect at low frequency region in Warburg impedance spectra. The detailed mechanism based on the aforementioned EIS measurement will be discussed in this presentation.