Statistical Analysis of Salt Deposition inside PEFC GDL from X-Ray Tomography

Abstract

X-ray Computed Tomography (XCT) has been utilized extensively in the past decade to examine the internal features of PEMFC components, including extensive examination of the GDL structure [1-3]. Through the use of optics, micro-scale (and nano-scale) resolution has been achieved allowing for investigation of GDL’s, MPL’s and MEA’s for both in-situ [2] and ex-situ[1,3] studies without the need to cut into and damage the sample to obtain internal views. Published research with respect to GDL’s has produced information ranging from porosity distributions [1] and transport properties[3] to liquid water distribution inside the pores [2,3]. This study examines the salt deposition within a PEMFC cathode GDL using micro-XCT. The GDL in question came from a cell which was run on air that contained a calcium sulfate impurity[4]. Following a break-in period of 24 hours, a nebulizer was used to inject calcium sulfate solution (in DI water) at a 5 ppm concentration (based on dry air flow rate) into the cathode air stream. Galvanostatic performance data was recorded at 200mA/cm2, which showed a steady voltage loss rate more than 10 times compared to a non-contaminated baseline cell. During this time, salt deposits began to build up within the GDL and flow channels. This salt build up, combined with ionic conductivity changes in the CCL and changes in flooding behavior eventually led to the end of the experiment, because the cell was unable to hold the 200mA/cm2 [4]. Following the experiment, the cell was disassembled and components separated. At this time, the salt build up on the GDL surface and on the gas flow channels was visually observed to block the air flow pathways. Tomographies of the salt deposited GDL were obtained in three regions: a) near the inlet b) near the center c) and near the outlet. The resulting tomographies are provided in the 3D images presented in Figure 1. These tomographs show the different deposition patterns through the cell. Near the inlet, where the highest amount of water is produced, salt deposition appears to be uniformly distributed with no preferential nucleation site. Alternatively, near the center and the exit of the cell, the salt deposits in and on the GDL follow a distinct precipitation pattern, which are characterized by salt deposits with a crystalline structure that have the potential to grow out of the GDL. These deposits were observed as the flow channel obstructions that contributed to the end of the experiment. Through close examination of the 3D reconstruction (and shown in Figure 1), it is seen that in the center and exit regions of the GDL, most of the salt deposition appears to be flush with the GDL surface, which indicates that salt precipitates appear to be restricted from growing in that direction (out of the GDL), which provides evidence that salts nucleate under the lands (ribs). These salt crystals then have the ability to grow in the in-plane direction and out from under the lands (ribs) where salts can grow unrestricted out of the GDL and into the flow channel grooves. This process is in good agreement with the conclusions presented by Wang et al.[4], that low water concentrations under the flow channel ribs precipitates the formation of salt deposits. Statistical analysis is performed on the exported salt deposited GDL tomographs to examine the changes in void space (porosity) within the GDL in the through-plane direction. As salts appear to nucleate under the lands, this examination of the deposition profiles is presented for both regions of salt build up under the grooves and under the lands (ribs). These results are extended to comment on the changes to porosity under cell compression. Finally, we examine the effect of salt contaminant concentration and cell operating current on the salt precipitation patterns.