Nanoscale Imaging of Voltage Cycled Cathode Catalyst Layers in Polymer Electrolyte Fuel Cells

Abstract

Widespread commercialization of polymer electrolyte fuel cells (PEFCs) requires improvement in the performance and durability of its key components. Catalyst layer degradation is a major factor contributing to the performance loss [1]. The relatively slower oxygen reduction reaction (ORR) kinetics and mass transport in the cathode contribute to a significant overpotential and thus requires noble metal catalysts to be used. The cathode catalyst layer (CCL) must maintain the heterogeneous catalyst structure for the species transport to/from the reaction sites which facilitates durable operation and low Pt loading. During operation, the heterogeneous CCL structures may experience localized non-uniform potential loads, fuel starvation, development of hot spots, and potential cycling that eventually leads to carbon corrosion, Pt agglomeration, ionomer redistribution, Pt oxidation, and Pt isolation in nano holes [2]. The affected structure may result in reduced catalytic activity, reduced Pt utilization and fuel utilization, inaccessible active sites, inefficient water management, increased electronic resistance, and increased mass transport resistance [3]. Hence, spatially resolved imaging of the catalyst layer and its constituents is required to unravel the effects of elemental and structural degradation. The present study aims to investigate the effects of high voltage excursions, which are typically encountered during start up/shut down conditions, on the heterogeneous nanoscale structure of the cathode catalyst layer. Membrane electrode assemblies (MEAs) with CCL comprising of a commercial 50:50 wt.% Pt/C catalyst and Nafion ionomer were subjected to a voltage cycling accelerated stress test (AST) with upper and lower potential limits of 1.3 and 0.6 V, respectively. The AST was operated until 4,700 cycles. Further details on the MEAs and AST can be found elsewhere [4]. The potential cycling is intended to induce CCL degradation, similar to that observed during field operation of fuel cells. The low potential operation leads to a higher current to be drawn from the fuel cell and causes temperature rise and flooding of the CCL. High voltage operation on the other hand triggers Pt oxidation and dissolution, carbon corrosion, and ionomer dehydration. Beginning-of-life (BOL) and end-of-test (EOT) MEAs extracted from in-situ AST were analyzed using transmission electron microscopy (TEM). The samples were embedded in epoxy resin and sliced to thin films (~70-90 nm) using ultramicrotome to reveal the cross-section and collected on copper grids for imaging. The nanoscale distribution of solid/void phases and the elemental maps were recorded at high resolution using a Tecnai Osiris TEM instrument from FEI. Elemental mapping of the CCLs was carried out by selectively capturing the generated X-rays with characteristic energy due to electron interaction with the sample. The X-ray energy distinguishes different elements present in the sample. The energy dispersive X-ray (EDX) analysis offers the sophistication of imaging the ionomer phase through the fluorine elemental map and carbon and platinum through their respective elemental maps. The BOL catalyst layer exhibiting uniformly distributed fluorine is presented in Figure 1(A). In contrast, the fluorine map of the EOT CCL (Figure 1(B)) reveals regions of increased concentration and closely compacted ionomer. The carbon and platinum elemental maps (not shown) at EOT reveal vacant carbon and aggregated platinum regions as a result of voltage cycling. The solid/void morphology of CCLs seen from the dark field images also revealed an increased solid content at EOT. Images indicate that the catalyst layer experienced compaction which will affect the porosity and triple-phase regions promoting ORR. The obtained results are in good qualitative agreement with the lower resolution structure visualized by X-ray computed-tomography and the present images confirm the trends observed in the 2-D virtual slices of ionomer dominated and Pt/C dominated catalyst layer regions [5]. Overall, the observed effects of compositional, structural, and morphological degradation of the CCL during voltage cycling contributes to the fundamental understanding of its complex degradation process and can aid development of catalyst layers with improved performance and extended durability. ACKNOWLEDGMENTS Research funding provided by Automotive Partnership Canada (APC), Natural Sciences and Engineering Research Council of Canada (NSERC) and Ballard Power Systems is gratefully acknowledged. We also thank Ballard Power Systems for experimental support. This work made use of the 4D LABS shared facilities supported by the Canada Foundation for Innovation, British Columbia Knowledge Development Fund, Western Economic Diversification Canada, and Simon Fraser University. REFERENCES [1] M. K. Debe, Nature, 486 43 (2012). [2] T. Saida et al., Angew. Chem., 124 10457 (2012). [3] R. Borup et al., Chem. Rev., 107 3904 (2007). [4] A. P. Hitchcock et al., J. Power Sources, 266 66 (2014) [5] A. Pokhrel et al., Meeting s. No. 37. 1357, The Electrochemical Society (2015). Figure 1