Elucidating the Formation Mechanisms and the Catalytic Reaction Mechanisms of Fe-N-C Electrocatalysts

Albert Mufundirwa, Postdoctral Researcher
Research Centre for Synchrotron Light Applications (RCSLA)

Stephen M. Lyth, Associate Professor
Platform of Inter/Transdisciplinary Energy Research (Q-PIT)

研究内容

 The oxygen reduction reaction (ORR) (Eq. 1) at the cathode side in polymer electrolyte fuel cells (PEFCs) (Fig. 1) is sluggish and requires the use of expensive platinum catalysts (Fig. 2). Currently, cheaper iron-decorated nitrogen-doped carbon (Fe-N-C) materials (Fig. 3) are gaining prominence and could replace platinum catalysts at the cathode side. However, advances in their development is hindered by lack of understanding of the formation mechanisms and catalytic mechanisms of active sites in these materials. Therefore, there is need to link the structure to function relationship in order to improve design criteria.

O₂ + 4e^- + 4H⁺ → 2H₂O       Eq.1

Here, we utilise a two-step synthesis method (Fig. 4a) which involves: (i) making a sodium ethoxide-derived nitrogen-doped carbon foam support with optimised porosity, surface area, and conductivity, (ii) then this support is infiltrated with iron precursors and heat treated (to make model Fe-N-C electrocatalysts), changing the chemical state of the adsorbed iron, whilst the support properties are relatively unchanged. 

 We then use: (i) in-situ high temperature XAFS to determine the real-time changes in the oxidation state of iron and its coordination chemistry with neighbouring atoms (Fig. 5), and (ii) electrochemical XAFS to visualize the catalytic mechanisms of the active sites towards oxygen reduction reaction in acid and alkaline media as well as to examine the degradation mechanisms (Fig. 6). The results from XAFS combined with other techniques such as near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS), quadrupole mass spectrometry (QMS), small angle X-ray scattering (SAXS), transmission electron microscopy (TEM) and Mössbauer spectroscopy are essential in giving meaningful insights of the active site chemistry. Thereby paving way for improving the design of the catalysts.