Quantifying the Degradation of the Cathode Catalyst Layer Under Voltage Cycling Conditions Using Transmission Electron Microscopy Techniques

Abstract The large-scale commercialization of polymer electrolyte membrane fuel cells can significantly reduce greenhouse gas emissions from medium- and heavy-duty vehicle fleets. However, it is essential to enhance the long-term durability of the cathode catalyst layer (CCL), which degrades under operating conditions. Scanning transmission electron microscopy coupled with energy dispersive spectroscopy was used to investigate microstructural changes in four membrane electrode assemblies subjected to voltage cycling under different combination of stressors, including relative humidity (RH), upper and lower potential limits (UPL and LPL, respectively), dwell time, potential step, and cell temperature. The fluorine-to-platinum ratio was introduced to quantify the spatial distribution of platinum relative to the ionomer, both through-plane and in-plane. This metric, combined with nanoparticle size analysis, was used to assess initial heterogeneities and evaluate platinum losses. The main degradation modes were linked primarily to RH, dwell time, and potential step between UPL and LPL. High RH and a narrow potential step aided in the formation of a Pt depletion region near the membrane, excessive Pt band, and resulted in > 50 nm agglomerates within the CCL. Moreover, longer dwell times resulted in enhanced NP growth from electrochemical Ostwald ripening.