Assessing and measuring the active site density of PGM-free ORR catalysts

Fuel cells are already employed in commercial transportation even though their price is still too high to enable widespread production. A viable and promising pathway taken to lower this price is the replacement of expensive constitutes, namely the platinum-based catalysts at the cathode, by platinum group metal-free catalysts based on abundant materials, such as iron. This led to the development of iron-based catalysts that show high activity towards the oxygen reduction reaction. The extraction of the intrinsic catalytic activity of any catalyst is important both for finding relations between the chemical properties of the active sites and their activity, as well as a comparison measure between catalysts. An important parameter that has been elusive for many years is the turnover frequency, which is derived form the number of electrochemical active sites’ density (EASD). The ability to measure the EASD was very limited until the past few years, and several methods have been proposed to measure it. It is important for the investigation of catalysts’ stability and the development of durable catalysts. This review aims to critically analyze the current methodologies used for the quantification and analysis of the active sites.