Recoverable Performance Loss due to Membrane Chemical Degradation in PEM Fuel Cells

Abstract

Recoverable voltage loss was observed for a PEM fuel cell due to membrane chemical degradation under open circuit voltage (OCV) conditions. The anion analysis of the fuel cell effluent water, collected during both the OCV hold and voltage recovery stages, indicates that sulfate release rate is much higher during recovery than that during the OCV hold. The surge in sulfate anion release occurs simultaneously with the recovery of fuel cell voltage. It was found that the reversible voltage loss, and concurrent sulfate release rate during voltage recovery, is lower for Ce-mitigated NRE211 membrane compared to as-received membrane. The recoverable voltage loss is proposed to be mainly due to the sulfate anion generated by membrane chemical degradation adsorbing onto the platinum catalyst surface. IntroductionIt has been well known that operating at an open circuit voltage (OCV) condition can accelerate polymer electrolyte membrane chemical degradation in PEM fuel cells (1-3). This degradation leads to thinning of the membrane, eventually forming pin holes as characterized by a significant increase of hydrogen and oxygen gas crossover.Membrane decomposition generates various compounds including fluoride, sulfate, and other low-molecular-weight organic acids (3-5). Using direct gas mass spectroscopy, Teranishi et al reported the detection of a series of fragments of sulfuric acid in addition to HF, H 2 O 2 , and CO 2 in the cathode outlet gas produced during OCV operation (3). No additional high molecular weight products were observed. In the water extracts of severely degraded fuel cell MEAs, Healy et al found the existence of perfluoro(3-oxapentane)-1-sulfonic-4-carboxylic diacid through F19 NMR and mass spectroscopy analysis, which is believed to be a degradation product from the side chain of the PFSA membrane (5). Kabasawa et al investigated the impact of potential membrane chemical degradation products on PEM fuel cell performance using model compounds. Of the several compounds evaluated, including sulfuric acid, perflurocarboxylic acids and perfluorosufonic acids, sulfuric acid poses the most poisoning effect on the cathode catalyst, leading to a large mass activity reduction and performance drop (4). The impact of these chemical degradation products on fuel cell performance at practical operating conditions has rarely been systematically studied (4, 6).Kundu et al observed that the open circuit voltage loss is partially recoverable due to unintentional interruption of the OCV tests (7). The authors did not provide further mechanistic analysis for this reversible voltage decay phenomenon. However, they highlighted the necessity of understanding the difference between reversible and irreversible voltage decay modes. Sugawara et al conducted a detailed analysis on the performance decay under an open circuit voltage condition (6). Using exhaust water analysis, the loss in performance is tentatively attributed to the contamination of the catalyst surface by membrane chemical degradation products, such as sulfate anio...

Toward a Comprehensive Understanding of Cation Effects in Proton Exchange Membrane Fuel Cells

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Scalable nanoporous carbon films allow line-of-sight 3D atomic layer deposition of Pt: towards a new generation catalyst layer for PEM fuel cells

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“…In the subsequent electrochemical activation via potential cycling to 1.0–1.4 V, the Pt surface restructures while any lingering contaminants are oxidized away . In MEAs, activation procedures generally involve a series of potential holds/cycles accompanied by a “voltage recovery” step in which the cathode is subjected to a long, low potential hold at 40 °C and >100% RH. , This voltage recovery step removes fluoride and sulfonate species from the cathode and can increase activity by as much as threefold . Still, it is conceivable that this process does not remove contaminants as completely as the preparation methods employed for RDE testing.…”