Vishal Mittal

Membrane Degradation Mechanisms in PEMFCs

Nafion membrane degradation was studied in a polymer electrolyte membrane fuel cell (PEMFC) under accelerated decay conditions. Fuel cell effluent water was analyzed to determine the fluoride emission rate. Experimental findings show that formation of active oxygen species from H 2 O 2 decomposition or the direct formation of active oxygen species in the oxygen reduction reaction are not the dominating membrane degradation mechanisms in PEMFCs. Instead, membrane degradation occurs because molecular H 2 and O 2 react on the surface of the Pt catalyst to form the membrane-degrading species. The source of H 2 or O 2 is from reactant crossover through the membrane. The reaction mechanism is chemical in nature and depends upon the catalyst surface properties and the relative concentrations of H 2 and O 2 at the catalyst. The membrane degradation rate also depends on the residence time of active oxygen species in the membrane and volume of the membrane. The sulfonic acid groups in the Nafion side chain are key to the mechanism by which radical species attack the polymer.

Effect of Catalyst Properties on Membrane Degradation Rate and the Underlying Degradation Mechanism in PEMFCs

Nafion membrane degradation was studied in a polymer electrolyte membrane fuel cell (PEMFC) under accelerated decay conditions. Fluoride emission rate (FER) determined by fuel cell effluent water analysis was used to quantify the membrane degradation. Membrane degradation is most likely caused either directly or indirectly by the species formed as a result of the H 2 and O 2 reaction on the catalyst. To further understand the mechanism, the effects of the catalyst location, type, its interaction with O 2 and H 2 O, and cell current density on the FER were investigated and their implications on the underlying membrane degradation mechanism are discussed.

Is H2O2 Involved in the Membrane Degradation Mechanism in PEMFC

The involvement of H 2 O 2 in the membrane degradation mechanism in a polymer electrolyte membrane fuel cell (PEMFC) was investigated. Measurement of fluoride concentration in the effluent water was used as an indicator of the membrane degradation rate. It was found that H 2 O 2 is formed in the fuel cell in small concentrations but is not the main source of harmful species, which degrade the membrane. H 2 O 2 decomposition due to impurities or the catalyst leading to the possible formation of radical species would only account for a small fraction of the membrane degradation rate in a fuel cell.

Membrane Degradation Mechanisms and Accelerated Durability Testing of Proton Exchange Membrane Fuel Cells

Proton exchange membrane fuel cells (PEMFCs) have increasingly received worldwide attention as the technology that can lead to substantial energy savings and reductions in imported petroleum and carbon emissions. Cost, durability, performance, reliability, efficiency, and size, are some of the requirements that must be met before PEMFCs can be used commercially. The lifetime requirement for stationary applications is about 40,000 hours and for transportation applications 5,000 (cars) and 20,000 hours (buses) (1). Today, the typical operating temperature for both applications is between 60 – 80°C, but to meet the 2010 and 2015 Department of Energy targets, PEMFCs must operate at temperatures from below the freezing point to higher than 100°C (~120 °C maximum), humidity from ambient to saturated, and half-cell potentials from 0 to >1.5 V. Durability studies of proton exchange membrane fuel cells (PEMFC) show that, along with cost, the long-term stability of PEMFCs is a limiting factor in their commercialization (2-6). Degradation of PEM fuel cells is generally observed as slow, unrecoverable performance decay, followed by sudden failure. The gradual performance loss is typically associated with changes in the electrodes and the membrane. The degradation of electrodes is usually caused by catalyst degradation and carbon corrosion. Membrane chemical and mechanical degradation are related to reactant gas crossover, Pt dissolution and migration, transition metal ion contaminants, and hydroxyl radical formation, and cycling of relative humidity. The chemical decomposition of the side

Outstanding Student/Post-doc Presentation Award Recipient: Factors Accelerating Membrane Degradation Rate and the Underlying Degradation Mechanism in PEMFC

Membrane degradation and failure is one of the factors limiting the overall durability of the polymer electrolyte membrane fuel cell. The fluoride emission rate (FER), which was calculated from the fuel cell effluent water analysis, was used to quantify the membrane degradation. The FER was found to decrease with an increase in load in the range from open circuit voltage (OCV) to 150 mA/cm 2 . Since at OCV no current is drawn from the fuel cell the effect of location of Pt/C catalyst on the FER was evaluated by having only one layer of catalyst applied to the membrane. The membrane degradation rate was found to be very high at OCV irrespective of whether the catalyst is present at the anode, cathode or in the membrane (catalyst layer sandwiched between 2 membranes). Effect of Pt/C catalyst exposure to oxygen is shown to have a significant effect on the FER at OCV.