Multiple heteroatom-doped few-layer carbons for the electrochemical oxygen reduction reaction

Abstract

Heteroatom-doped two-dimensional (2D) carbon materials have been recognized as promising metal-free catalysts for the oxygen reduction reaction (ORR) due to their impressive surface activity. Such materials are often achieved by conventional chemical vapour deposition (CVD) growth followed by post-treatment of heteroatom-doping or obtained by template-guided polymerization and carbonization of organic substances. In this contribution, we report a new strategy that allows a direct conversion of three-dimensional (3D) aggregates of guanine into naturally N/O-doped ultrathin few-layer carbon nanosheets without the use of any template or guiding agent. Moreover, by pre-ionizing guanine with H₂SO₄/H₃PO₄, a group of multiple heteroatom-doped (N/O/S, N/O/P, and N/O/S/P) 2D carbons can also be readily realized. Considering the low cost of precursors and the simplicity of the synthetic procedure, large-scale production of such heteroatom-doped 2D carbons is potentially available. The best catalytic performance is obtained from the N/O/S/P-doped carbons which show a half-wave potential (E_1/2) of 0.84 V vs. RHE and a diffusion-limited current density (j_L) of 5.40 mA cm⁻² in 0.1 M KOH, better than those of most graphene-based catalysts and even the commercial Pt/C catalyst. Systematic studies indicate that the observed superior ORR performance arises from a combined effect of the multiple heteroatom-doping, high surface area and abundant structural defects of the electrocatalyst.