Toward more practical site density determination in Fe-N-C catalysts for oxygen reduction reaction: NO-stripping at gas diffusion electrode setup

Evaluation of the site density of single-site iron catalysts has always been a challenge due to the complexity of those materials, and the difficulty of finding selective and simple probing methodologies. Since iron coordinating nitrogen (FeN4) is recognized as the main active site for oxygen reduction reaction (ORR), it is fundamental to develop a method to assess the content in catalysts. Several bulk-probing and surface-probing methods have been developed in the last decade each one with its pros and cons. Among others, with certain limitations, NO stripping performed at a rotating disc electrode (RDE) has become a common characterization to evaluate site density and turnover frequency of Fe-N-C due to the relatively simple procedure. At the same time, gaining information on the activity on more practical devices has brought to the spreading of gas diffusion electrodes (GDE) in half-cell configuration to mimic the cathodic compartment of a fuel cell, a leading device for those materials applications. Here we proposed a transposition of NO-stripping from RDE to GDE to evaluate the site density under more practical devices. Several parameters have been considered (i.e., pH, loading, FeN4 selec- tiveness) and comparisons with RDE have been done to validate the methods. A homemade Fe-N-C catalyst has been used as a benchmark and compared to Pajarito Powder catalyst and others (including metal-free) showing the validity and selectivity of the method.