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(Invited) Decoupling the Contributions of Different Instability Mechanisms to the Fuel Cell Performance Decay of Fe-Based O2-Reduction Catalysts

The last decade of research on Fe-based O2-reduction single atom catalysts (SACs) has led to the development of SACs with an initial polymer electrolyte fuel cell (PEFC) performance close to that of Pt-based catalyst layers (CLs). However, these inexpensive materials generally suffer from a fast performance decay that has been ascribed to several mechanisms that can be simultaneously at play during device operation. These include (i) the demetallation of the SACs’ active sites; (ii) the potential-induced corrosion of the carbonaceous matrix that hosts these active centers; and/or (iii) the chemical degradation of the CL-ionomer, active sites and/or carbon support by radicals derived from the H2O2 produced as an O2-reduction by-product. Unfortunately, little is known regarding the relative contributions of these mechanisms to the overall PEFC-performance loss and as a function of the operative conditions – a missing understanding of pivotal importance for the design of strategies to mitigate this instability.
 With this motivation, this contribution will start with a comparison between the device stability in the course of 30 min holds at various currents of two type of SACs featuring similar beginning-of-life PEFC-performance. Particular attention will be paid to the effect of the catalyst loading on the observed degradation, which will be linked to the operando mapping of the distribution of liquid water within these materials’ CLs based on neutron imaging. In a subsequent step, we will present the results derived from a stability protocol in which the combination of different cathode gas feeds (i.e., air vs. N2) and potential hold durations allow decoupling the relative contributions of the above deactivation mechanisms to the overall performance decay. This requires a careful assessment of the kinetic, ohmic and mass transport overpotentials (and changes thereof in the course of the stability measurements) based on Tafel analyses and electrochemical impedance spectroscopy measurements. Moreover, these results are again complemented by an assessment of the protocol-induced changes in catalyst morphology and surface chemistry based on transmission electron microscopy and X-ray photoelectron spectroscopy measurements.
 In summary, this contribution will showcase our efforts to deconvolute the relative effects of various deactivation mechanisms to the overall PEFC-performance loss of SAC-CLs.