Open Circuit Voltage Hold Induced Degradation Under Dry Conditions and Recovery Strategies in PEMFC Electrodes
Authors: Weber Konstantin A. , Gasteiger Hubert Andreas 
Published: August 2024

With regards to the decarbonization of the transportation sector in the near future, the importance of proton exchange membrane (PEM) fuel cells is shifting more and more from light-duty vehicles to heavy-duty vehicles, as well as to an application for maritime and aviation applications. With this change in the field of application, the requirements of the fuel cell system need to be adjusted towards an increased fuel efficiency and lifetime.1 Especially the targeted durability of 25,000 h for heavy-duty vehicles requires a detailed understanding of the degradation processes within the stack to be able to minimize performance losses or possible failure of the components.2 During the life cycle of a fuel cell electric vehicle, the stack encounters operation under increased temperature as well as under low relative humidity (RH). Both conditions are well known to lead to a degradation of the membrane electrode assembly (MEA).3,4 Deliberate operation under those conditions and at open circuit voltage (OCV) is commonly used to study the chemical degradation of the membrane. Here it is believed that gas crossover can lead to the formation of radicals, which attack the perfluoro sulfonic acid based ionomer chains and result in a variety of degradation products.5,6 Especially sulfonate group containing degradation species strongly adsorb on the Pt surface and reduce the oxygen reduction reaction (ORR) activity of the cathode catalyst. It was already shown that a suitable recovery step can restore of most of the observed performance losses.3,5

This work focusses on the chemical degradation and subsequent recovery strategies of a commercial MEA with an active area of 5 cm2 and a cathode loading of 0.4 mgPt/cm2. Although it is known that radical scavengers, like Ce3+ cations, minimize the chemical degradation within the membrane, we decided to use an MEA with a non-mitigated membrane (i.e., free of radical scavengers) in order to facilitate the degradation processes.5 As seen in the upper panel of Figure 1, the applied OCV hold for 72 h under H2/air (1000/1000 nccm) at 95 °C and at 30 % RH induces a voltage loss of ~115 mV (at 2.5 A/cm²) in the polarization curve after this accelerated stress test (AST), i.e., at end-of-degradation (EOD; red line/symbols), compared to the initial MEA performance at beginning-of-test (BOT; black line/symbols). In order to distinguish between reversible and irreversible losses, a subsequent recovery step at fully humidified conditions is implemented. The polarization curve at end-of-test (EOT; green line/symbols), i.e., after the recovery step, reveals that the majority of the observed losses is reversible. Provided that the applied recovery step accounts for all reversible effects, the remaining losses of ~45 mV (at 2.5 A/cm²) are of irreversible nature. On the one hand, those irreversible losses partially arise from a small increase in the high frequency resistance (HFR), which rises during the degradation period by ~2 mΩcm² (lower panel of Figure 1. On the other hand, the electrochemically active surface area (ECSA) is reduced by ~20 % over the course of the experiment, which leads to another unavoidable loss in performance. The comparison of the catalyst’s specific activity in units of mA/cm2
Pt, hence, accounting for the loss in ECSA, strongly suggests that the Pt surface at EOT approaches the initial state at BOT, which indicates a reversible degradation mechanism.

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Acknowledgements

We gratefully acknowledge funding from the German Federal Ministry for Digital and Transport (BMDV) under the funding scheme H2Sky (funding code 03B10706).

Figure 1