Understanding Pyrolysis of PGM-Free Electrocatalysts with X-Ray Computed Tomography

Abstract

Atomically dispersed Fe-N-C electrocatalyst is the emerging advanced type of PGM-free catalyst. Its activity arises from a multitude of active sites, often viewed as co-catalytic (bifunctional sites), such as atomically-dispersed iron in nitrogen coordination (FeNx) and pyridinic and pyrrolic nitrogen moieties in graphene structures. The synthesis of Fe-N-C electrocatalysts involves a pyrolysis step, for which physical and chemical transformations are not well understood. Various ramps and temperature holds are currently used to evaporate precursors and solvents and to achieve the electrocatalyst’s high electric conductivity. An additional pyrolysis step, termed second pyrolysis, is sometimes used. Recipes exist with the pyrolysis of various precursors to arrive at catalyst that contains a high content of nitrogen. Here, Fe(NO3)3 .9 H2O and an aromatic amine charge-transfer organic salt, nicarbazin, was used. Custom pyrolysis furnace built by the Advanced Light Source’s (ALS) 8.3.2 beamline scientists was used that was capable of having gas circulation, high temperature and, at the same time, a rotating stage for x-ray computed tomography (CT) imaging. After ball-milling, the sample was placed on a ceramic stage of the vertical furnace with a circulating gas mixture of nitrogen and 7 % hydrogen. The catalyst was inserted at 525°C and then furnace temperature was increased; first to 900°C in 12 min (31.25 °C/min) and then to 975°C in 8 min (9.375 °C/min). After a 45 min hold time, the catalyst was removed from the furnace. Seven X-ray CT scans were performed during this pyrolysis process. This is the first study to use micro X-ray CT to observe the morphological changes of a catalyst powder during a pyrolysis process. Figure 1 shows the schematic of the stage, the quartz tube used to seal the gases and the ceramic stage for the catalyst placement. A photograph of our mounted catalyst within the pyrolysis chamber on the rotating stage is shown by Figure 1b and a temperature profile by Figure 1c. We have found that most of the changes in the catalyst morphology happen during the temperature ramp to 975oC and during cool down. The temperature hold at 975°C did not result in significant powder morphological changes. A follow-up study was conducted where the temperature ramp was studied in more detail with X-ray CT scans taken every 100°C. Furthermore, a second pyrolysis study was conducted where morphological changes were not observed. Figure 1. a) Schematic of pyrolysis stage and gases circulation, b) a photograph of our sample mounted within the pyrolysis chamber at 8.3.2 ALS during imaging, c) the temperature ramp during pyrolysis and d) the evolution of catalyst morphology during temperature ramp. Figure 1