As one of the most promising candidates to replace Pt-based electrocatalyst for oxygen reduction reaction in fuel cell, Metal-Nitrogen-Carbon (M-N-C) catalysts, especially Iron-Nitrogen-Carbon (Fe-N-C) materials, have attracted massive research interests in recent years. Typically, there are three main categories of synthesis approaches for Fe-N-C electrocatalyst, based on what chemicals are used as iron and nitrogen precursors: macrocyclic iron complexes, metal-organic frameworks, and various heterocyclic compounds. Despite these different approaches, it is found that numerous purified preliminary Fe-N-C products need further heat treatment to achieve better electrocatalysis performance. However, the reason why heat treatment is efficient, as well as the best parameters such as atmosphere, time and temperature, are still unclear and require intensive characterization for elucidation.

Here, we report in situ XPS for the investigation how carbon environment and nitrogen moieties change, in addition to in situ XRD, on a specific atomic-dispersed Fe-N-C material (made from nicarbazin via sacrificial support method^[1]) during high temperature heat treatment. Figure 1 shows the change of surface contents via XPS C 1s and N 1s under ultra-high vacuum. It is found by rising the temperature from room to 750 °C, the graphitic C with a characteristic peak around 284.5 eV decreases from ~50 atomic % to ~20 %, resulting in a more fragmented carbon structure, while sp³ carbon and nitrogen-carbon have an obvious increase. After flash heating to 950 °C (the typical point used in 2nd heat treatment for this electrocatalyst synthesis) and followed cooling back to 150 °C, there is no reversibility of graphitic carbon. For N 1s shown in Figure 1B, comparing initial 150 °C and final 150 °C after cooling, it is found that pyridinic-N content drops from ~20 atomic % to 15%, and pyrrolic-N decreases from ~40 % to 30 % respectively. Meanwhile, metallic/amine nitrogen increases from ~10% to ~15% while graphitic-N/N⁺ grows from ~7 % to 20 %, resulting in a more efficient Fe-Nx bonding and surface ionic conduction. In addition, in situ XRD shows that there is no obvious crystalline formed during high temperature treatment, indicating that there is no agglomeration of iron particles. These in situ XPS and XRD results elucidate that extra heat treatment of Fe-N-C electrocatalyst after preliminary synthesis and purification is an essential procedure and can improve the electrochemical performance efficiently.

[1] M. J. Workman, M. Dzara, C. Ngo, S. Pylypenko, A. Serov, S. McKinney, J. Gordon, P. Atanassov, K. Artyushkova, J. Power Sources 2017, 348, 30-39.