Fourier Transform Alternating Current Voltammetry (FTacV) As a Tool for in-Operando Study of PGM-Free ORR Catalysts' Durability in Pemfcs
Authors: Elbaz Lior , Friedman Ariel 
Published: December 2020

The activity of Pt-group metal-free (PGM-free) ORR catalysts, has been increasing significantly in the past decade, and reached a level where it is comparable to Pt-based catalysts, proving their viability. Nowadays, the advancement in their development matured to the point where their stability and durability under fuel cells operating conditions must be determined, and the activity of the catalyst must be studied in-situ in-operando, in order to monitor the catalyst performance. While the advancement in activity is impressive, the next necessary improvement, durability, is impeded by the lack of a procedure that is specific towards the catalyst’s activity, and informative with respect to its degradation rate during fuel cell testing, undermining the ability of understanding durability issues. Currently, the degradation of the catalyst is described quantitatively by using the Tafel region of polarization curves. While an indispensable measurement, it is not specific towards the processes that the catalyst undergoes, as its output is influenced by many factors, such as pore structure, membrane conductivity, and especially, the activity of various active sites. This lowers the usefulness of this method, as it is not specific towards any active site in particular, limiting any inferences on possible routes that may be taken towards understanding and mitigating the catalyst’s degradation. Hence, there is a critical need to develop new analytical tools that have high specificity towards different aspects of the catalyst’s performance.

With PGM catalysts, this problem issue was easily solved using an electrochemical activity descriptor that is specific to the active site: electrochemical active surface area (ECSA). This descriptor, which can be determined during an accelerated stress test (AST), helps quantify the degradation of Pt-based active sites, and has been playing an important role in the development of durable PGM ORR catalysts.

In this work, Fourier-Transform Alternating Current Voltammetry (FTacV), was used as a method for describing the PGM-free ORR catalyst degradation. FTacV has shown great applicability in unraveling complex mechanisms and in discerning low signal faradaic reactions from high capacitive currents and noise. It is backed by an extensive theoretical background. Recently, FTacV has been used to probe the electron transfer in surface bound catalysts and enzymes, providing invaluable information on the electron transfer mechanisms that underpin the electrocatalytic reactions, thus, enabling further development of the catalysts and better understanding of these reaction.

In this talk we will present the use of Fourier Transform ac voltammetry (FTacV) as an electrochemical, catalyst-specific, method for following the PGM-free degradation during fuel cell operation. For the first time, FTacV was employed during the operation of a polymer electrolyte membrane fuel cell before, during, and after stability tests. FTacV was found to be a highly sensitive technique to follow the activity of electrochemically active species, opening the path towards the accurate quantification of electrochemical active site density (EASD) of PGM-free ORR catalysts.