Additional doping of phosphorus into polypyrrole functionalized nitrogenous carbon nanotubes as novel metal-free oxygen reduction electrocatalyst in alkaline solution

Abstract

Novel phosphorus-doped polypyrrole functionalized nitrogenous carbon nanotubes (P/NCNTs) was developed for the first time as metal-free electrocatalyst for enhancing the oxygen reduction reaction (ORR) activity in alkaline medium. The P/NCNTs was successfully synthesized by pyrolyzing PPy and triphenylphosphane (TPP) under N2, using pyrrole as carbon and nitrogen precursors, TPP as phosphorus precursor. Various characterizations such as transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectra, Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) reveal that the P/NCNTs material has covalently bound P atoms with carbon framework, which can introduce defect sites and can induce uneven charge distribution. Moreover, the content of pyridine N increased after P-doping, which is of great significance in improving the ORR activity. The electrochemical behavior of the resultant material shows that the P/NCNTs has much enhanced electroactivity and better stability for ORR. Additionally, a direct four-electron pathway occurred efficiently on P/NCNTs modified electrode. These enhanced performances indicate that P/NCNTs catalyst may be an excellent cathode catalyst for ORR.