Platinum-free electrocatalysts for oxygen reduction reaction: Fe-Nx modified mesoporous carbon prepared from biosources

Nitrogen-doped mesoporous carbons containing a small amount of Fe (Fe-N-MC) are prepared from a low-cost carbon source (agarose) together with nitrogen and iron precursors. The synthesis consists in the formation of a hydrogel embedding FeCl2 or tris(1,10-phenanthroline)iron(II)dichloride (Fe(Phen)3Cl2), which after a freeze-drying treatment, is pyrolyzed at 400 °C to give a crude product, which is further graphitized at high temperature. The formation of a hydrogel assures an optimal Fe dispersion and an optimal surface area and pore network after pyrolysis, as evaluated by N2 adsorption-desorption isotherms. The final product is further activated by treatment in KOH at 750 °C or in H2SO4 at 100 °C followed by H2 reduction at 700 °C. Both treatments allow to increase the surface area and pore volume, and to expose Fe-Nx active sites, as confirmed by both XPS and Mössbauer spectroscopies. The Fe-N-MC catalytic performances towards oxygen reduction reaction are investigated by electrochemical techniques in 0.1 M HClO4, attesting that O2 is reduced following an almost 4e- pathway at very positive potentials (0.8 V vs RHE). The effect most influencing the catalytic activity is found to be the employment of Fe(Phen)3Cl2 instead of FeCl2 +1,10-phenanthroline for the generation of Fe-Nx active sites.