Methods for assessment and measurement of the active site density in platinum group metal–free oxygen reduction reaction catalysts

The recent rise in the activity of platinum group metal–free oxygen reduction reaction catalysts for polymer electrolyte fuel cells, made them very interesting alternative for Pt-based catalysts. Currently, the application of this family of catalysts in fuel cells is impeded by their relatively low durability and stability. The lack of simple methods to follow the specific degradation of these catalysts in situ and in operando fuel cells inhibits durability and stability studies, where the key catalyst performance descriptor is the turnover frequency, for which the electrochemical site density needs to be accurately measured. Herein, the most commonly available techniques for measuring the active site density were surveyed, along with their advantages and disadvantages, leading to the conclusion that only electrochemical techniques such as square wave voltammetry and Fourier transform alternating current voltammetry can be used as standard measurements.