Ion-Beam Modulation of Nitrogen Defect Sites in Carbon Nanotubes for Controlled ORR Selectivity in Alkaline Media

Abstract

The oxygen reduction reaction (ORR) on metal-free carbon catalysts is relevant both for fuel cells and metal-air batteries, where peroxide suppression is desired, and for decentralized H2O2 electrosynthesis, where partial reduction is beneficial. Here, multiwalled carbon nanotubes (MWCNTs) were modified by N+ ion irradiation at 20 keV for 5, 10, and 15 min, corresponding to fluences of about 1.4 × 1016, 2.8 × 1016, and 4.3 × 1016 cm–2 for samples N1, N2 and N3. The treatment preserved the overall nanotube scaffold while introducing progressively higher defect density and nitrogen content. X-ray diffraction and Raman spectroscopy showed expansion of interlayer spacing from 3.46 to 3.51 Å and an increase of ID/IG from 1.61 to 2.31. X-ray photoelectron spectroscopy revealed growth of total nitrogen from 0.7 to 1.4 atom % and of pyridinic plus pyrrolic nitrogen from 0.61 to 1.17 atom %, while conductivity changed nonmonotonically from 0.32 S ·cm–1 for pristine MWCNTs to 0.45, 0.28, and 0.22 ·S·cm–1 for N1, N2 and N3. Electrochemical measurements in alkaline media showed that ion-beam treatment tunes both ORR activity and selectivity. Sample N2 displayed the most positive onset potential, 0.82 V versus the reversible hydrogen electrode, and a lower peroxide-related response under the tested conditions, whereas N3 gave the highest disk currents and a more pronounced peroxide pathway. These results show that ORR selectivity on a single metal-free carbon scaffold can be adjusted through a balance between nitrogen incorporation, defect accumulation, and charge transport, with the intermediate irradiation regime providing the most favorable compromise for low-peroxide oxygen reduction route.