Experimental Campaign on PEMFC Catalyst Degradation: Enlightening the Impact of Automotive Operational Cycles Combined with Short-Stops

Abstract

Meeting long-term durability targets is still a significant challenge to achieve a wide-scale commercialization of PEM fuel cells in the transportation sector, for which the cathode catalyst layer is one of the most critical components1. Specifically, high potentials and potential cycling have been identified at the origin of its degradation. Indeed, they are responsible for platinum dissolution under the combined electrochemically oxidizing and acidic environment of catalyst layers. To limit ageing, vehicle systems adopt operating strategies that limit the operating voltage range, as well as they accurately control the start/stop procedures, avoiding both the open circuit voltage (OCV) and frequent air leaks into the anode compartment. In this perspective, passengers vehicles, that introduce frequent switching offs called short-stops2, exploit a procedure based on removing the air at the cathode side whilst supplying hydrogen at the anode.At a laboratory scale, accelerated ageing protocols (AST) are usually utilized for rapidly evaluating the degradation of materials, alternated with periodic performance testing and diagnostics. The conditions that promote Pt nanoparticle degradation have been studied over the past years3,4 mainly starting, and suitably modifying, the protocol proposed by the U.S. Department of Energy (DoE) in hydrogen/nitrogen. In this work, it was instead developed a complimentary approach in order to contribute to the study of the effect of the voltage cycles in a range relevant for the application. Different protocols in hydrogen/air or hydrogen/diluted air atmosphere were applied on a zero gradient cell, emulating the conditions of real operations5. The potential profiles under investigation were restricted to the voltage window accessible in real systems (i.e. ≤ 0.90 V), using commercial Catalyst Coated Membranes (CCM). The role both of the voltage range and of voltage transitions was clarified for a total of 40 tested Membrane Electrode Assembly (MEA) samples. Starting from the reference cycle included in Figure 1A, it was analyzed the influence of: (i) the upper potential limit (UPL); (ii) the lower potential limit (LPL), (iii) the impact of holding times, (iv) the role of short stops, (v) the short stop voltage, (vi) the short stop duration, (vii) scan rates during voltage transitions, (viii) the cathode gas feeding composition, (ix) the relative humidity during the short stop transient. Operando performance information indicates a clear recovery and boosting whenever the voltage was progressively reduced in the range 0.7 V–0 V. The irreversible performance decay recorded through polarization curves and impedance spectra (EIS) appeared dominated by the loss of the electrochemically active surface area (ECSA). The decay of this parameter was strictly connected to the Ostwald ripening mechanism, as corroborated by post mortem images obtained by transmission electron microscopy (TEM), while no Pt precipitation into the ionomer was observed. The average Pt nanoparticle diameter increased from 4.4 nm to 6.5 nm, at which value it almost stabilized. All the voltage profiles studied induced an ageing that was interpreted through the development of a newly formulated semi-empirical ECSA degradation model, which accounts for the role of the different stressors. The available data suggest that the Pt catalyst degradation is a strong function not only of UPL but also of LPL. Quite a large degradation was indeed observed in presence of short stops even in the typical system operational range (i.e. keeping UPL between 0.70 V and 0.90 V, Figures 1 B-D). It is suspected that the Pt nanoparticles are fully reduced at potentials significantly lower than 0.6 V and that their de-passivation enhances their growth. The work proves that attention must be paid on the transitions to low cell voltage (0÷0.4 V) in order to decelerate the catalyst ageing. The results are useful to orientate the selection of the PEMFC operational constraints for reaching the durability targets of the automotive sector (8’000 hours), but they can be also extended for matching the struggling requirements of heavy-duty transportation, namely 25’000 operating hours. This work received support from the Italian government (Progetto PERMANENT - BANDO MITE PNRR Missione 2 Investimento 3.5 A - RSH2A_0O0012). Borup, R. L. et al. Curr. Opin. Electrochem. 21, 192–200 (2020).Takahashi, T. et al. J. Electrochem. Soc. 169, 044523 (2022).Ahluwalia, R. K. et al. J. Electrochem. Soc. 168, 044518 (2021).Kneer, A. et al. J. Electrochem. Soc. 165, 805–812 (2018).Colombo, E. et al. J. Power Sources 553, 232246 (2023). Figure 1