Study of Non-PGM ORR Catalyst Degradation Using Synchrotron Techniques

Several recent reports on the development of oxygen reduction reaction (ORR) non-platinum group metal (non-PGM) catalysts have shown an impressive progress towards high ORR activity [1-2]. By now, in RDE testing, these catalysts are quite routinely reaching E_½values of 0.8 V versus reversed hydrogen electrode [1]. The high initial performance notwithstanding, truly durable non-PGM catalysts, capable of maintaining ORR activity for hundreds or thousands of hours at low pH values of the polymer electrolyte fuel cell (PEFC), are yet to be developed. The development of such catalysts must involve a carefully thought-out experimental design and the use of reliable multiple techniques to provide answers to the quarter-of-a-century-old questions about the structure of the ORR active site(s) in metal-nitrogen-carbon (M-N-C) catalysts.

According to some previous in-situ XAS studies [3-4], a vast majority of non-PGM catalysts consist of two forms of transition metal, iron coordinated by nitrogen species (Fe-N_x) and non-coordinated iron nanoparticles (Fe_NP). While the existence of these species has been detected using different methods, the in-situ XAS reviled a specific redox behavior accompanied with spin switching behavior of the Fe-N_x species when subjected to potential bias simulating PEFC environment [2-4]. There is still a large ambiguity, however, about the actual nature of the M-N-C active sites and their degradation modes. Among many challenges facing this research, it is important to take a closer look at other elements present in such catalysts, such as nitrogen and carbon. This is especially needed for non-PGM catalysts without nitrogen-coordinated Fe (e.g. Fe_NP) with very high activity [5].

Herein, we present a study of a Fe-based non-PGM catalyst derived from multiple nitrogen precursors. The study has been performed using synchrotron X-ray absorption techniques coupled with standard electrochemical methods, including RRDE and MEA testing. We look at the structures involving all elements in the catalyst by employing high-energy photons at Argonne National Laboratory (to monitor in-situ Fe-containing species) and low-energy photons at Stanford Synchrotron Lightsource to monitor carbon and nitrogen species (C and N K-edge), claimed to play an important part in the ORR active site sites. With better understanding of the structure-to-function relationship in Fe-N-C species as the main objective, we study the effect of durability cycling (Figure1) on the catalyst performance and structural changes involving all elements in Fe-N-C species.