Mesoporosity and nitrogen doping: The leading effect in oxygen reduction reaction activity and selectivity at nitrogen-doped carbons prepared by using polyethylene oxide-block-polystyrene as a sacrificial template

Abstract

Four mesoporous carbons (MCs) with tunable pore size were synthesized by soft template synthesis, employing a resorcinol-formaldehyde resin as a carbon precursor and a polyethylene oxide-block-polystyrene block copolymer as a sacrificial template in which the length of the polystyrene block (165, 300, 500, and 1150 units) allowed the modulation of the surface area of MCs (567, 582, 718 and 840 m² g⁻¹, respectively). The complete set of MCs was also doped with nitrogen by ball milling in the presence of cyanamide and stabilized in a second thermal treatment at 750°C, leading to nitrogen content of ∼2.65% in all samples. The two sets of MCs were used for evaluating both the effect of textural properties and nitrogen doping in the electrochemical reduction of oxygen in acid electrolytes. Each catalyst was characterized by means of elemental analysis and N₂ physisorption analysis, whereas the selected series of samples were also characterized by transmission electron microscopy, scanning electron microscopy, X-ray photoemission spectroscopy, inductively coupled plasma mass spectroscopy (ICP-MS), and Raman analysis. Voltammetric rotating ring-disk measurements in 0.5 M H₂SO₄ demonstrated that the catalytic activity for the O₂ reduction scales with the surface area in the non-doped series, and also the selectivity for the two-electron process leading to H₂O₂ increases in the samples having wider pores and higher surface area, even if the leading mechanism is the tetraelectronic process leading to H₂O. The doping with nitrogen leads to a general increase of the catalytic activity with a shift of the O₂ peak potential to more positive values of 75–150 mV. In the doped series, nitrogen doping prevails on the textural properties for guiding the selectivity toward the two- or four-electron process, since a similar H₂O₂ yield was observed for all N-MC samples. The possible presence of FeN_x sites derived from the ball milling fixation of nitrogen was evaluated by using the NO-stripping technique.