In-Situ High-Temperature X-Ray Absorption Spectroscopy of Fe-N-C Electrocatalysts during Pyrolysis

Abstract

Deeper understanding of the formation of active sites during the pyrolysis of iron-nitrogen-carbon (Fe-N-C) electrocatalysts for the oxygen reduction reaction (ORR) is required. Here, we present a nitrogen-doped carbon foam support treated at high temperature (1600 °C) to stabilise the material properties. Iron-containing precursors are then adsorbed onto the surface, followed by heat treatment. This changes the chemical state of the adsorbed iron, whilst the material properties of the support are effectively unchanged. The two-step synthesis method used to create these model electrocatalysts decouples porosity, microstructure, and conductivity from the electrochemistry, which can instead be attributed mainly to changes in the chemical environment of iron.Advanced characterisation techniques such as in-situ high-temperature X-ray absorption spectroscopy (HT-XAS) and high-temperature near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) are employed to gain insights into the state of the adsorbed iron as it decomposes during pyrolysis to form active sites. Transmission electron microscopy (TEM) is used to track the microstructure for different types of precursor heated at different temperatures. Meanwhile, in-situ electrochemical XAS is used to understand how the chemical environment of the catalysts change during the ORR.The results are discussed by relating different chemical environment with the electrochemical ORR activity. Despite the two-step synthesis technique, these model Fe-N-C electrocatalysts are highly active. The load-cycling durability and start-stop durability are measured, and the degradation mechanisms are discussed.