Nitrogen-doped carbon fiber sponges by using different nitrogen precursors: synthesis, characterization, and electrochemical activity

Abstract

Nitrogen-doped carbon fiber sponges (N-CFSs) were synthesized using the aerosol-assisted chemical vapor deposition (AACVD) method involving the decomposition of nitrogen precursors with a mixture of ferrocene (C10H10Fe), thiophene (C4H4S), and ethanol (C2H5OH) at 1020 °C under H2/Ar flow. As nitrogen precursors, pyridine (C5H5N), acetonitrile (CH3CN), urea (CH4N2O), and benzylamine (C7H9N) were used. The N-CFSs were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and thermogravimetric analysis. The nanocarbon material involved in N-CFS formation depended strongly on the nitrogen precursor used in the synthesis. The N-CFS synthesized from the benzylamine-ethanol precursor displayed corrugated carbon fibers with a bimodal diameter of ~190 nm and ~320 nm. The N-CFS obtained from the combination of benzylamine-urea was formed by carbon fibers with a zigzagging behavior also with a bimodal diameter of ~85 nm and ~190 nm. The N-CFS made from benzylamine-pyridine precursors exhibited highly entangled wavy carbon fibers and Fe-based nanoparticles surrounded by graphite materials. In this case, the diameters were ~270 nm and 390 nm. The N-CFS made from acetonitrile-ethanol favored the formation of large-diameter carbon fibers (~400 nm). Chemical surface analysis by XPS characterizations revealed the presence of different nitrogen doping (N-substitutional, N-pyridinic, and N-pyrrolic) and chemical functional groups (nitrogen oxide, amines, N2, and amides). The analysis also revealed that N-pyrrolic doping, quinone, ester, and ether groups were dominant in all samples. The nitrogen concentration contained in the sponges was 0.21–2%. The XRD characterization demonstrated the presence of the non-symmetric peak for the (002) crystallographic graphitic plane, suggesting the presence of slightly expanded graphite material. Voltammetry measurements showed a high surface activity owing to the presence of N-doping and several chemical species attached to the carbon fiber surface. Here, additional peaks to that of the quinone appeared.