Effects of Cathode Corrosion on Through-Plane Water Transport in Proton Exchange Membrane Fuel Cells

Abstract

The corrosion of carbon in the cathodes of proton-exchange-membrane fuel cells leads to electrode collapse, reduced active catalyst area, and increased surface hydrophilicity. While these effects have been linked to performance degradation over cell lifetime, the role of corrosion in the evolving water balance has not been clear. In this study, neutron imaging was used to evaluate the through-plane water distribution within several cells over the course of accelerated stress testing using potential holds and square-wave cycling. A dramatic decrease in water retention was observed in each cell after the cathode was severely corroded. The increasing hydrophilic effect of carbon surface oxidation (quantified by ex situ X-ray photoelectron spectroscopy) was overwhelmed by the drying effect of increased internal heat generation. To evaluate this mechanism, the various observed electrode changes are included in a multiphase, non-isothermal one-dimensional cell model, and the simulated alterations to cell performance and water content are compared with those observed experimentally. Simulation results are consistent with the idea that collapse and compaction of the catalyst layer is the dominant limitation to cell performance and not the lower amounts of active Pt surface area, and that higher temperature gradients result in drying out of the cell.The corrosion of carbonaceous components in proton-exchangemembrane fuel cells (PEMFCs) is a concern for long-term durability. Carbon corrosion can cause performance losses through several mechanisms. 1 As carbon catalyst support is lost, catalyst activity is lost due to particle detachment, agglomeration and/or isolation. The pore space is collapsed, blocking transport pathways for oxygen to the active catalyst sites. Surfaces can be made more hydrophilic by roughening and addition of oxide surface groups, 2,3 thereby increasing the propensity of flooding. While the kinetics of carbon corrosion are usually slow at PEMFC operating temperatures and normal potentials, regional reversals and starvation lead to high local potentials and accelerated oxidation. 4,5 Moreover, carbon corrosion is catalyzed by contact with platinum in the electrodes. 6,7 Automotive applications of PEMFCs require an estimated 5500 h of operation including 38,500 startup/shutdown cycles. 8 Each of these cycles involves brief high potential excursions that can accelerate corrosion. 4,5 During operational periods, transient power demands and localized water blockage 9 can result in bulk or regional fuel starvation, 10-12 also leading to more corrosive conditions. Altogether, this is a demanding environment for carbonaceous components, and requires a combination of corrosion-resistant materials (typically graphitized carbons) 13 and careful system management strategies 14 to achieve acceptable durability. To evaluate rapidly the suitability of different support materials for a PEMFC cathode, accelerated stress tests (ASTs) have been developed which greatly accelerate corrosion. These ASTs commonly raise the...

3D Printed Flow Field and Fixture for Visualization of Water Distribution in Fuel Cells by X-ray Computed Tomography

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