Quantifying the electrochemical active site density of precious metal-free catalysts in situ in fuel cells

Abstract

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 Ptbased 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 PGMfree 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.

Identification of the catalytically dominant iron environment in iron- and nitrogen-doped carbon catalysts for the oxygen reduction reaction

“…33,37 Herein, we identify a new ORR intermediate (D4) in an FeNC catalyst based on operando Mössbauer data at RT and quenched to 1.6 K, where operando refers to ORR catalysis conditions in acidic environment. The selected catalyst 34 shows similar ORR activity as the commercial catalyst from Pajarito powders recently reported in Snitkoff-Sol et al, 44 illustrating its technological relevance. In combination with DFT, we identify a spectroscopically and thermodynamically consistent structural and electronic model for the dominant ORR active site in FeNC catalysts, namely an FeN4C12 active site with pyrrolic N-donor atoms.…”

Exploring Durable Single-Atom Catalysts for Proton Exchange Membrane Fuel Cells

“…While the stability conundrum remains unsolved, the ultimate solution depends on an accurate understanding of the structure–stability relationship. The use of advanced in situ/operando characterization techniques, such as in situ 57 Fe Mössbauer spectroscopy, operando XAS, and the latest in situ active site quantification method (FTacV), has allowed us to monitor the changes of active sites under realistic reaction conditions and gain a deeper understanding of the degradation mechanism. − On the basis of this, the most significant achievement in recent years is the identification of the ubiquitous D2 site (FeN 4 C 10 ) as the intrinsically stable active site. This is really encouraging if we could find a method to enrich D2 sites in the catalysts.…”

Identification of the Catalytically Dominant Iron Environment in Iron- and Nitrogen-Doped Carbon Catalysts for the Oxygen Reduction Reaction

“…Herein, we identify a new ORR intermediate (D4) in an FeNC catalyst based on operando Mössbauer data at RT and quenched to 1.6 K, where operando refers to ORR catalysis conditions in an acidic environment. The selected catalyst shows similar ORR activity to the commercial catalyst from Pajarito powders recently reported in Snitkoff-Sol et al., illustrating its technological relevance. In combination with DFT, we identify a spectroscopically and thermodynamically consistent structural and electronic model for the dominant ORR active site in FeNC catalysts, namely, an FeN 4 C 12 active site with pyrrolic N-donor atoms.…”