A study on the electrode degradation in polymer electrolyte membrane fuel cells using the electrochemical impedance spectroscopy analysis

Abstract

With regard to increasing energy consumption, a polymer electrolyte membrane fuel cell (PEMFC) technologies have been rapidly developed to meet the global energy paradigm shift away from fossil to fuel as for renewable energy sources in the future hydrogen society. In compliance with the development of hydrogen production and storage systems, there has been remarkably increasing demand for durable PEMFC with high energy conversion efficiency for automobile and power generation systems as well. Nevertheless, due to the undesired corrosion reaction during fuel cell operations, the durability is still limited, which is directly bearing on the cost efficiency and lifespan of PEMFC. Moreover, it is difficult to identify the correlation between degraded factors of membrane electrode assembly (MEA) and performance decay in PEMFC since the fuel cell system is a combination of multiple principles based on electrochemical, mechanism, and material engineering. In addition, the convoluted degradation mechanism in PEM systems results in problematic degradation issues of MEA, which are the major obstacles to its commercialization. Therefore, it is essential to understand the degradation mechanism of PEMFC in the systemic point of view, and based on the mechanism studies, degradation behaviors should be identified and differentiated in accordance to the diverse operating conditions. In PEMFC degradation issues, one of the most problematic form of PEMFC degradation is carbon corrosion reaction, resulting in structural deterioration and functional group generation on the surface of the supporting carbon. In particular, structural characteristics of porous electrode would influence on performance and functionality of supporting carbon, and therefore, structure components, such as the pore circumference and depth, should be specified during carbon corrosion reaction to understand the major factors affecting the performance decays in PEMFC. In here, different type of accelerated stress tests (ASTs) protocols, mimicking on/off cycling and load cycling conditions, are adopted to simulate the high potential induced-electrochemical oxidation of carbon support and Pt-catalyzed carbon corrosion, respectively. To diagnose the current status of degrading electrode, the electrochemical impedance spectroscopy (EIS) analyzed are performed, through which the impacts of deformation behaviors of supporting carbon on fuel cell performance are discussed. Typically, in EIS analysis, the complex capacitance analyzes are adopted to characterize the degrading electrode since the structural factors of porous electrode are directly related to the internal resistance such as ionic resistance, and charge transfer resistance during fuel cell operations. Moreover, as for diagnostic signal for electrode degradation, distribution of relaxation time (DRT) analyzes, which has been investigated to deconvolute the internal resistance factors, are applied.