Effects of Membrane- and Catalyst-Layer-Thickness Nonuniformities in Polymer-Electrolyte Fuel Cells

Abstract

In this paper, results from mathematical, pseudo 2-D simulations are shown for four different along-the-channel thickness distributions of both the membrane and cathode catalyst layer. The results and subsequent analysis clearly demonstrate that for the membrane thickness distributions, cell performance is affected a few percent under low relative-humidity conditions and that the position along the gas channel is more important than the local thickness variations.However, for the catalyst-layer thickness distributions, global performance is not impacted, although for saturated conditions there is a large variability in the local temperature and performance depending on the thickness. * Electrochemical Society Member ** Electrochemical Society Fellow z E-mail: azweber@lbl.gov 2 Introduction and ApproachAs polymer-electrolyte fuel cells (PEFCs) make the transfer from demonstration to production, manufacturing issues begin to become important. One such issue is that of materialproperty tolerances, and specifically layer thicknesses. While some thickness variation may be acceptable, the limits are not known. The variation or nonuniformity of layer thicknesses also brings fundamental questions that impact water and thermal management on the cell or global level, and it provides clues as to how durability and degradation may be initiated and proceed on the local level. In this paper, the effect of local variations in the membrane and cathode-catalystlayer thicknesses in terms of both the local and global performances is investigated.It is known that manufacturing and production processes inherently result in nonuniform material properties, especially thickness. This variability can be seen in scanning-electron micrographs of the membrane-electrode assembly such as that shown in Figure 1. From multiple micrographs taken from various parts of a PEFC, thickness distributions can be obtained. Figure 2 gives three distributions for both the membrane and cathode catalyst layer, hereby referred to as the catalyst layer, taken from three different PEFCs. A fourth distribution is that of uniform thickness (the solid lines in Figure 2) with values of 13.5 and 30 μm for the catalyst layer and membrane, respectively. In terms of the distributions, for the mathematical analysis, the gas channel was discretized into 32 segments, with every two segments having the same thickness.The distributions clearly show that there is a great deal of variability and randomness in the local thickness values; however, the average values for the different distributions are almost identical as seen in Table I. Table I also gives the standard deviations for each distribution and the cumulative ones, which are essentially normal distributions. 3The distributions are for virgin materials; however, it is not expected that they will change in relation to each other upon cell assembly and operation. This is because the gas-diffusion layers are the most compressible, and a simple stress analysis demonstrates that upon membrane swelling and hydration, it...

A Critical Review of Modeling Transport Phenomena in Polymer-Electrolyte Fuel Cells

“…This is something that has only minimally been studied in the modeling literature. 529,530 In the same fashion, in order to accelerate the components development it is necessary to correlate the measurable properties to the performance effect on a PEFC over its lifetime. This is a place where modeling has a possible significant role to play, since experiments require substantial times to enact.…”

Modeling Water Management in Polymer-Electrolyte Fuel Cells

“…Although the crossover is normally only a small efficiency loss, it does limit the thickness of the membrane, 78 and can become important if pinholes or membrane thinning occur. Furthermore, crossover is attributed to carbon corrosion during fuel starvation, 49 platinum band formation, 45 and peroxide generation.…”

“…This situation was looked at by Weber and Newman,78 through the use of a 1-D model that is run in a 2-D array of node points, where each point was connected to each other through simultaneous mass and energy balances. They found that crossflow increases the average humidity and provides around a 30% increase in current density for 25 % relativehumidity feeds than the coflow case.…”

The Influence of Catalyst Layer Thickness on the Performance and Degradation of PEM Fuel Cell Cathodes with Constant Catalyst Loading

“…Given the limited availability and high cost of precious metals such as Pt, optimization of catalyst layer structure is of particular importance for large-scale PEMFC deployment. Evidence exists to show that catalyst layer imperfections such as a spot with restricted metal loading have a substantial effect on both membrane-electrode assembly (MEA) performance and durability [7,8,9]. This shows the importance of studies on catalyst layers in the larger context of PEMFC commercialization.…”