Simplified Ftacv Model to Quantify the Electrochemically Active Sitedensity in PGM-Free ORR Catalysts

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The development of platinum group metal-free catalysts is considered the most prominent path for reducing thecost of low-temperature fuel cells (LTFC). Despite the great advancement in the field, its further progress iscurrently limited by the ability to understand and mitigate the catalysts’ degradation mechanisms, which up torecent years was limited by the lack of activity descriptors. Recent work showed that this could be solved usingFourier-transformed alternating current voltammetry that enables to deconvolute Faradaic currents arising fromthe redox reaction of the active sites from the capacitive currents, and by that accurately measure the electrochemicallyactive site density of these catalysts in situ fuel cells. However, the analysis of the results can becomplex, requiring simulation software for accurate parameter extraction. Herein, a simplified analysis ofFourier-transformed alternating current voltammetry is presented. This was done by mapping the influence ofvarious variables, such as resistivity, capacitance, and thermodynamic and kinetic dispersion, on the accuracy ofelectrochemically active site density measurements. Under specific, yet realistic, operating conditions, a singleequation with the peak current of the 5th harmonic as the only variable can be used to quantify electrochemicallyactive site density with high accuracy. This approach was demonstrated using molecular catalysts, iron phthalocyanine,as a model molecule, and a commercial, heat-treated catalyst.