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

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.