Advances in the development of Pt-group metal-free (PGM-free) catalysts for oxygen reduction reaction in fuel cells produced active catalysts that allow to reduce the performance gap to the incumbent Pt-based materials. However, the utilization of the state-of-the-art PGM-free catalysts in commercial applications is currently impeded by their relatively low durability. Methods designed to study catalyst degradation in operating fuel cells are critical for the understanding and ultimately solving the durability issues. This work is the first report on the use of Fourier-transformed alternating current voltammetry (FTacV) as an electrochemical method for accurately quantifying the electrochemical site density of PGM-free ORR catalysts, and following their degradation during the operation of polymer electrolyte fuel cells. Using this method we were capable of detecting changes in performance of electrochemically active species (electrocatalytic centers in this case), allowing us, for the first time, to calculate the electrochemical active site density (EASD) which is necessary for the elucidation of the degradation mechanisms of PGM-free ORR catalysts in situ fuel cells.