Automotive electrode degradation

This chapter was focused on electrode degradation in PEMFCs for automotive application. It has been pointed out that undesired side reactions are responsible for the limited long-term stabihty of a fuel-cell system. These reactions occur at high electrode potentials which therefore have to be avoided. 

While most modeling efforts have focused on steady-state operation, the dynamic behavior is of paramount importance for fuel-cell transportation appHcations due to the inherent load variation involved. Transient phenomena in automotive fuel cells are not yet fully understood. In addition to the complex dynamic response involving various time scales, severe degradation of membrane electrode assemblies... 

The performanee and durability of a membrane electrode assembly (MEA) is affected signifieantly by the eathode eleetrode eomposition and structure, due to the poor kineties of oxygen reduetion and reaetant transport limitations. Utilization and stability of platinum or its alloys in the PEMFC play important roles in fuel cell efficiency, durability, and the drive for eost reduction through reduced Pt loadings. Cathode catalyst layer degradation is a critical issue for fuel cell durability to meet the requirement of > 5000 hours for automotive applications and > 40,000 for stationary applications. 

Phosphoric acid-doped (PA) poly-benzimidazole (PBI) films attain proton conductivities >0.25 S cm at temperatures above 150°C (Xiao et al 2005). The shift to higher temperatures of fuel cell operation, enabled by the use of these membranes, improves the fuel tolerance to carbon monoxide impurities, enhances the electrode kinetics, and alleviates humidification requirements. The problem is that the higher temperature not only accelerates the kinetics of desired electrode reactions, but also those of degradation processes. Moreover, PBI membranes function poorly under ambient conditions. PEFCs using PBI PEMs are suitable for residential applications, but they are currently not envisaged for automotive applications. 

This chapter presents a summary of the systematic EIS modeling of industrial and automotive lubricants. Initially EIS data interpretation for fresh lubricant, influenced by the effects of the chemical composition, temperature, electrochemical potential, AC frequency, and electrode geometry, is presented. Another important practical aspect is determination of changes in the lubricant s bulk and interfacial impedance model parameters as a result of its degradation by time-dependent oxidation and contamination with soot, fuel, and water at different stages in the exploitation cycle. 

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