Thinning of Cathode Catalyst Layer in Polymer Electrolyte Fuel Cells Due to Foreign Cation Contamination

Abstract

Four hundred hour fuel cell tests were performed on commercial, as-received and Ca 2+ contaminated catalyst coated membranes (CCMs) to evaluate the effects of long term exposure of foreign cations on fuel cell performance and degradation. Following testing, significant thinning of the cathode catalyst layer was observed across the entire active area in the contaminated cell, while only localized thinning was observed in the as-received baseline CCM. Analysis of the elemental maps and line intensity profiles of platinum (Pt) obtained from energy dispersive X-ray spectrum (EDX) shows minimal change in total platinum content across the thickness of the catalyst layer, and hence loss of carbon is suspected to be the cause of the thinning. A numerical model is employed to show the foreign cation redistribution during testing, which shows proton depletion in the cathode catalyst layer when the CCM is contaminated with foreign cations. We hypothesize that this lack of protons leads to accelerated carbon corrosion in the cathode to supply protons to oxygen reduction where foreign cations block transport of protons. A carbon corrosion scheme is presented which shows that under the operating potentials of the cell, carbon oxidation can occur, leading to the observed thinning. The polymer electrolyte fuel cell (PEFC), a promising clean energy source for automobile application, still needs to overcome durability issues originating from various sources.1 Cationic impurities originated from either ambient air (e.g. roadside contaminants) or from the corrosion of stack and balance of plant components can significantly decrease the performance and life time of PEFCs. [1][2][3][4] In this paper, we report on the influence of foreign cationic contamination on the thinning of the cathode catalyst layer during non-accelerated testing. Long duration (400hr) testing was conducted using commercial catalyst coated membranes which showed significant catalyst layer thinning in a calcium cation contaminated cell, this thinning of the cathode catalyst layer was not observed in a non-contaminated baseline. While foreign cations are known to cause several degradation mechanisms in PEFCs, no study has examined the influence of cationic contamination on carbon corrosion, which we believe causes the thinning of the catalyst layer.To understand the impact and mechanism of cation contamination in PEFC, several modeling 5-11 and experimental studies 12-33 have been conducted, including ex-situ contamination of the polymer membrane with various cations before the test as well as injecting cations into the air or fuel stream during the cell test. Okada and co-workers extensively investigated the effect of various metal cations (Li + , Na + , Ca 2+ , Fe 2+ , Ni 2+ , Cu 2+ , Rb 2+ , Cs + ) on the transport properties of perfluorosulfonic acid (PFSA) membranes, thermodynamics and oxygen reduction reaction (ORR) kinetics. 5,[13][14][15][16][17][18] They reported that, with the exception of Li + , all foreign cations have higher affinity toward the sulfo...

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