Highly active Fe-N-reduced graphene oxide electrocatalysts using sustainable amino acids as nitrogen source.

Abstract

This work reports the preparation of oxygen reduction reaction catalysts by pyrolysis of a mixture of Fe(NO3)3 and graphene oxide (GO) functionalized with three different amino acids, Arginine (Arg), Cysteine (Cys), Histidine (His). The combination of TPD, FTIR and XPS revealed that the functionalization of GO rendered a reduction of epoxide and carboxyl surface groups at along with the preferential generation of amide bridges. XPS confirmed the incorporation of nitrogen in edge and quaternary positions of the graphene layers after the pyrolysis at 800 °C of the amino acid functionalized GO mainly in form of pyridines (Arg), pyrroles (Cys) and quaternary nitrogen (His). XRD and XPS showed that most iron is retained after pyrolysis in the form of crystalline Fe3O4 and in a lesser extend as Fe2O3. The amount, distribution, and particle size of iron species on the pyrolyzed samples relies on the amino acid used for the modification, with histidine rendering the lower retained amount and higher dispersion. Raman analyses pointed out that the structural order seems to follow the order His > Cys > Arg. The electrochemical characterization revealed that, in spite of having a lower iron content, Fe-rGO-His shows a higher double layer capacitance, improved conductivity and enhanced ORR activity than their counterparts, delivering onset and half-wave potentials only 30 mV behind the current state-of-the-art platinum–carbon black catalyst. These results confirm that amino acids, and specially Histidine, can be interesting N-dopant agents for the preparation of nitrogen-iron graphene electrocatalysts with excellent ORR activities.