Phosphorus–nitrogen dual doped carbon as an effective catalyst for oxygen reduction reaction in acidic media: effects of the amount of P-doping on the physical and electrochemical properties of carbon

Abstract

A new strategy for enhancing the oxygen reduction reaction (ORR) activity of carbon-based catalysts in acidic media is proposed and characterized; the strategy consists in modifying the ORR through dual doping of nitrogen and phosphorus into the carbon. The P, N-doped carbon is prepared via pyrolysis of a mixture composed of dicyandiamide (DCDA), phosphoric acid, cobalt chloride, and iron chloride at 900 °C under an Ar atmosphere. The P-doping induces an uneven surface with many open edged sites in the carbon morphology and increases the carbon surface area from 108.1 to 578.8 m² g⁻¹. The XRD, XPS-C_1s, and Raman spectroscopy results reveal that the crystallinity and degree of the sp²-carbon network decrease and the number of defect sites of the carbon increase as the amount of P-doping increases. All catalysts demonstrated similar proportions of N-doping types regardless of the P-doping amount: pyridinic-N and graphitic-N were dominant phases in the carbon lattice. In the ORR, the onset potential of the prepared catalysts was 0.6 V (vs. Ag/AgCl) in 1 M HClO₄. The N-doped carbon records −0.69 mA mg⁻¹ of mass activity at 0.5 V (vs. Ag/AgCl), but additional P-doping results in an increase of activity (−2.88 mA mg⁻¹) that is more than fourfold that produced without the additional P-doping. Moreover, additional P-doping also modifies the ORR pathway, as the N-doped carbon induces more than 10% of H₂O₂; however, the P, N-doped carbon produced below 4% of H₂O₂ during the ORR.