ORR Activity of Pristine Graphite|Fe3c Interfaces
Authors: Hansen Heine A. , Reda Mateusz , Vegge Tejs 
Published: February 2020

Carbon based catalysts containing N, Fe or Co have shown activity for the oxygen reduction reaction (ORR) comparable to platinum in acid electrolytes at low current densities.(1–3) While promising, these catalysts require high loading to achieve satisfactory activity resulting in a thick catalyst layer and significant mass transport limitations at the high current densities desired in fuel cells. Further improvements in volumetric activity are therefore needed. Furthermore, long-term stability and suppression of H2O2 selectivity need to be addressed. While the active site of Fe and N containing graphene is believed to be an FeN4C12 site,(4) a nitrogen-free active site was recently proposed for graphite encapsulated Fe3C, which showed promising durability activity and durability in acid electrolyte.(2)
Here we use atomic-scale density functional theory to investigate whether it is possible for Fe3C to enhance the activity of nitrogen-free graphite for the 4e- reduction of O2 to H2O. Activity and selectivity of various extended model surfaces are investigated within the framework of the computational hydrogen electrode model,(5) which previously has been shown to capture trends in 4e- and 2e- reduction of O2.(6, 7)
We find significant activity enhancement of graphite zigzag edges and a single graphene layer on Fe3C(001), while activity enhancement of multiple layers of graphite is not observed. Comparing the activity of graphite zigzag edges anchored on Fe3C(001) to Pt(111), we predict the edge sites are less active for the 4e- ORR and significantly more selective for the 2e-ORR than Pt. This is caused by weaker binding of ORR intermediates on the graphite step edge compared to Pt, as show in the figure below.
It is, however, possible to increase activity and selectivity for the 4e- ORR through expansive strain. Although Fe3C significantly facilitate O2 activation on isolated graphene sheets, we find the activation is in sufficient to catalyze the 4e- reduction. Instead H2O2 formation is predicted. These results alludes to the necessity of adding dopants such as nitrogen or boron to further enhance reactivity and suppress H2O2 production on graphite-Fe3C based catalysts for the ORR.

Acknowledgements

This work is funded by Innovation Fund Denmark through 4106-00012A (NonPrecious) Initiative Towards Non-Precious Metal Polymer Fuel Cells.
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Figure (a) Free energy diagram at 0.9 V vs RHE for the 4e- ORR on Pt(111) and graphite zigzag edges anchored to Fe3C(001). (b) Free energy diagram for H2O2 production at 0.69 V vs RHE.

Figure 1