The most promising class of PGM-free materials for oxygen reduction reaction (ORR) is based on graphene-like carbon containing nitrogen and transition metal (MNC). They show promise as replacement of Pt in two different technological platforms - alkaline exchange membrane fuel cells (AEMFCs) and proton exchange membrane fuel cells (PEMFC). [1-3] It is well established that nitrogen coordination with metal in the carbon network of MNC materials is directly related to ORR activity; however, the exact nature of the active sites is still debated even after over 50 years of research. Understanding the specific roles of nitrogen and metal in the properties/activity/stability/durability of MNC-based catalytic materials is a prerequisite for the rational design of ORR electrocatalysts with improved performance.

The key component in elucidating the relationship between the chemistry of active sites and activity is a better understanding of the formation of adsorbates, intermediates, and products during reactions occurring within the fuel cell.

In situ monitoring reaction steps under realistic conditions in metal-free and metal-containing building blocks will shed light onto the reaction mechanism that is essential for developing active and durable PGM-free catalyst for ORR. [4]

We will report on AP-XPS analysis for series of electrocatalysts belonging to Fe-N-carbon families based on sacrificial support method (SSM) [1, 2] and Metal-organic frameworks (MOF) [5]. The effect the nitrogen chemistry and the type of iron have on the oxygen binding was investigated by ambient pressure X-ray Photoelectron Spectroscopy (XPS) and X-ray Adsorption Spectroscopy (XAS) under an O₂ environment at operating temperature of the fuel cell. The effect of the relative abundance of different types of nitrogens, such as pyridinic, coordinated to iron and hydrogenated nitrogens (pyrrolic and hydrogenated pyridine) on the preference of oxygen binding is studied by high-resolution nitrogen photoelectron spectra. The role of metallic and atomically dispersed iron is investigated by a combination of XAS and XPS.

Pyridinic N and N-coordinated to metal have been reported as active sites promoting the reduction of oxygen to water while hydrogenated N was hypothesized to reduce oxygen only partially, forming hydrogen peroxide. Figure 1a and 1c shows a difference between N 1s spectra acquired at 200 mTorr total pressure of oxygen/water mixture and that at UHV conditions for two different types of Fe-N-C electrocatalysts. For the catalyst Fe-N-C SSM, the largest change happens in the area of hydrogenated nitrogen while for the catalysts Fe-N-C MOF the largest change is occurring in the region of pyridinic nitrogen. This is also confirmed by different degree of change observed in Fe L-edge upon oxygen binding shown in Figure 1 b and 1d). Rearrangement of Fe(II) and Fe(III) intensity upon oxygen binding is observed only one of the catalysts. Linking differences in oxygen binding to the differences in the chemistry of the electrocatalysts are of ultimate importance for elucidating the oxygen reduction reaction mechanism.

 

1. Workman, M.J., et al., Platinum group metal-free electrocatalysts: Effects of synthesis on structure and performance in proton-exchange membrane fuel cell cathodes. Journal of Power Sources, 2017. 348: p. 30-39.

2. Gokhale, R., et al., Direct synthesis of platinum group metal-free Fe-N-C catalyst for oxygen reduction reaction in alkaline media. Electrochemistry Communications, 2016. 72: p. 140-143.

3. Strickland, K., et al., Highly active oxygen reduction non-platinum group metal electrocatalyst without direct metal–nitrogen coordination. Nature Communications, 2015. 6: p. 7343.

4. Artyushkova, K., et al., Oxygen Binding to Active Sites of Fe–N–C ORR Electrocatalysts Observed by Ambient-Pressure XPS. The Journal of Physical Chemistry C, 2017. 121(5): p. 2836-2843.

5. Li, J., et al., Structural and mechanistic basis for the high activity of Fe-N-C catalysts toward oxygen reduction. Energy & Environmental Science, 2016. 9(7): p. 2418-2432.