The cathode catalyst layer within a proton-exchange-membrane fuel cell is the most complex and critical, yet least understood, layer within the cell. The exact method and equations for modeling this layer are still being revised and will be discussed in this paper, including a 0.8 reaction order, existence of Pt oxides, possible non-isopotential agglomerates, and the impact of a film resistance towards oxygen transport. While the former assumptions are relatively straightforward to understand and implement, the latter film resistance is shown to be critically important in explaining increased mass-transport limitations with low Pt-loading catalyst layers. Model results demonstrate agreement with experimental data that the increased oxygen flux and/or diffusion pathway through the film can substantially decrease performance. Also, some scale-up concepts from the agglomerate scale to the more macroscopic porous-electrode scale are discussed and the resulting optimization scenarios investigated. IntroductionThe catalyst layer (CL) is a very complex chemical and geometric environment for electrochemical reactions in proton-exchange-membrane fuel cells (PEMFCs). It is composed of supported catalyst particles, ionomer, and gas pores. The reaction occurs at sites where various reacting species such as protons, electrons, and gases meet. Modeling the structure has been approached by various means 1 , and a rigorous mathematical model of the CL is required to capture transport within the different phases, electrochemical reaction, and heat and water generation. Among previous models, one of the most accepted models is an agglomerate particle composed of the ionomer, gas voids, liquid water, and catalyst that is covered by a thin film of ionomer . This idea is supported by various experimental observations such as scanningelectron-and transmission-electron-microscopy studies. In this model, oxygen is dissolved in the ionomer film surrounding the agglomerate, and the dissolved oxygen diffuses to the agglomerate where simultaneous transport and reaction occur. Typically, the agglomerate model is embedded (i.e., distributed uniformly across the CL) into a porous-electrode model to describe the CL fully 1 . _ENREF_29Optimization of the CL for enhancing PEMFC performance is of great interest for researchers and industry. Parametric studies of CLs were accomplished by Yin 14 using an agglomerate model, with the model predicting the general polarization-curve trend as a function of parameters such as gas void fraction 7,16 and ionomer 4,5,10,29 and catalyst loadings 5,10,[30][31][32] within the CL.Similarly, several optimization studies 16,22,30 using an agglomerate model were conducted to obtain optimum design parameters such as catalyst loading, CL thickness, and Pt/C ratio for best performance at a given potential. However, all the studies were based on several assumptions 4 and issues that are not correct validated or possibly correct including a lack of ionomer films and a first-order oxygen dependence for the oxygen redu...

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Accelerated degradation tests for polymer electrolyte membrane fuel cells are frequently conducted under open-circuit voltage (OCV) conditions at low relative humidity and high temperature. Comparative experimental studies were carried out to identify the localization of membrane degradation through the thickness direction of the membrane under the OCV hold condition and the effect of platinum band location on the membrane degradation behavior. A bilayer configuration of the membrane electrode assemblies (MEAs) was used for this study. In one test, the fuel cell with the bilayer MEA was exposed to hydrogen and air (regular test); in another test, the bilayer MEA was exposed to 4% hydrogen (balance nitrogen) and pure oxygen to induce the platinum band formation close to the anode side. Hydrogen crossover current, electrochemical areas (ECAs), and polarization curves were evaluated pre- and post-test. After the OCV hold test, the bilayer MEA was separated into two pieces of one-side coated membranes; each piece was characterized by uniaxial mechanical testing, IR spectroscopy, scanning electron microscopy, and energy-dispersive X-ray spectroscopy (EDS) analysis. The findings suggest highly localized degradation or a defect band formed through the thickness direction of the MEA, and the defect band shifts with the shift of the Pt band.

The DOE Fuel Cell technical team recommended ASTs were performed on 2 different MEAs (designated P5 and HD6) from Ballard Power Systems. These MEAs were also incorporated into stacks and operated in fuel cell bus modules that were either operated in the field (three P5 buses) in Hamburg, or on an Orange county transit authority drive cycle in the laboratory (HD6 bus module). Qualitative agreement was found in the degradation mechanisms and rates observed in the AST and in the field. The HD6 based MEAs exhibited lower voltage degradation rates (due to catalyst corrosion) and slower membrane degradation rates in the field as reflected by their superior performance in the high potential hold and open-circuit potential AST tests. The quantitative correlation of the degradation rates will have to take into account the various stressors in the field including temperature, relative humidity, start/stops and voltage cycles.

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