Hierarchical porous N-P-coupled carbons as metal-free bifunctional electro-catalysts for oxygen conversion

Abstract

Hierarchical porous N and P co-doped carbons were facilely synthesized by employing phenoxycycloposphazenes (PCPZs) as multi-source of nitrogen, phosphorus and carbon. The pyrolysis of PCPZs that supported on monodispersed silica spheres produced high-surface graphitized carbons (N,P-GCs) with abundant surface N-P coupling sites. The N,P-GCs were demonstrated as efficient metal-free electro-catalysts for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). The pore structures of these carbons for exposing more active sites could be adjusted by importing varisized SiO2 spheres (10–50 nm) as hard templates. Typically, N, P-GC-1000 using 30 nm SiO2 as templates exhibits the ORR half-wave potential of 0.85 V in 0.1 M KOH solution, with a loading density of 0.35 mg cm−2 on the electrodes, which is comparable to commercial Pt/C catalyst (0.1 mg cm−2, 20 wt%, JM). Moreover, the OER over N, P-GC-1000 electrodes (0.65 mg cm−2) could also reach 10 mA cm−2 current density at a lower potential of 1.56 V, close to typical RuO2 electrode (1.59 V, 0.2 mg cm−2, Premetek). This work not only suggested a simple and effective pathway to prepare N-P-doped carbon with N,P-coupled sites, but also provided a very efficient metal-free bifunctional carbon electro-catalyst for ORR and OER.