Analysis of Electrode Degradation By High-Potential Environment-Using Electrochemical Impedance Spectroscopy in Polymer Electrolyte Membrane Fuel Cells

Abstract

A Polymer electrolyte membrane fuel cell (PEMFC) is one of the most important applications for decarbonization of energy using hydrogen as an energy source and is considered a promising future power source due to its high efficiency, low operating temperature, fast startup, and low noxious emissions. In particular, improvement in durability of PEMFC is one of the major issues to spur the commercialization of the PEMFC system. Therefore, to improve the durability of PEMFC systems, diagnostic strategies are required to identify faulty conditions of PEMFC systems such as fuel starvation, dehydration, flooding, load cycling, and startup-shutdown (SU/SD). The problematic form of the membrane electrode assembly (MEA) that causes a decrease in the performance of PEMFCs in steady-state and dynamic operating conditions includes Pt dissolution, ionomer degradation, carbon corrosion [1] , and mechanical problems of the membrane in some circumstances. Among them, the undesired corrosion reaction, namely, the electrochemical carbon oxidation reaction, results in a drastic decrease in electrochemical double layer capacitance (EDLC), generation of oxygen functional groups in carbon supports, and severe electrochemical surface area (ECSA) fading [2, 3], followed by deteriorated PEMFC performance. Typically, the high potential inducing carbon corrosion reactions would occur during SU/SD and fuel starvation conditions. In detail, as for fuel starvation, a temporary shortage of H2 supply to one or several cells in a PEMFC stack causes the cell voltage reversal, accompanied by a potential strike over 1.0 V vs. RHE (Figure 1) with significant electrode degradation [4 , 5]. Also, the generation of hydrogen/air boundary at the anode due to the gas crossover through the membrane results in a high potential (~1.4 V vs. RHE), accelerating the decomposition of carbon support [6]. Therefore, in this work, to investigate the effects of high potential inducing electrode degradation on PEMFC performance, we conducted accelerated stress tests (AST), and in particular, we studied the correlation between electrode degradation behaviors as well as structural properties of porous carbon such as ionic resistance, EDLC with PEMFC performance in the systemic point of view. Specially, we could see the contrasting degradation behaviors and various performance decay rates using electrochemical impedance spectroscopy (EIS) analysis. EIS can be used to isolate the contribution of many processes to performance loss, allowing investigation of the effect of carbon corrosion-induced catalyst layer changes on fuel cell performance [ 7]. The measured EIS data is evaluated through the parametric fitting of the transmission line model (TLM) to the EIS spectrum. In addition, relaxation time distribution (DRT) analysis for direct analysis of internal resistance factors by reinforcing the TLM-based impedance analysis results was applied as a diagnostic method to evaluate electrode degradation. During the ASTs, electrochemical measurements (i-V, CV, LSV, and EIS) were performed to estimate the degree of PEMFC degradation, we also conducted ex-situ surface analysis of MEA using SEM, TEM, and XPS to elucidate the specific degradation components.