Catalytic Reactions on the Open-Edge Sites of Nitrogen-Doped Carbon Nanotubes as Cathode Catalyst for Hydrogen Fuel Cells

Recent experimental reports proposed that pyridinic-type sites on the open edges of carbon nanotubes (CNTs) may contribute to the high catalytic activity for oxygen reduction reaction (ORR) on nitrogen-doped CNTs (N-CNTs). Herein, we performed first-principles spin-polarized density functional theory calculations to examine the catalytic steps for ORR and water formation reaction (WFR) on the open edges of N-CNTs. For half-N doping on the open edge of CNTs (HN-CNTs), O-2 and OOH can be chemisorbed and partially reduced on the C-N bridge site without an energy barrier. The subsequent WFR for reduced O-2/OOH with ambient H+ and additional electrons can be finished without energy barrier for the formation of two H2O molecules. The second H2O molecule needs an energy of similar to 0.49 eV to be desorbed from the catalytic site, which completes an electrocatalytic reaction cycle on the cathode catalyst for hydrogen fuel cells (HFCs). For H-saturated open-edge sites of HN-CNT, ORR and WFR can also be completed energetically. For full-N doping on the open edge of CNTs (FN-CNTs), O-2 can be reduced and dissociated on the N-N bridge site with an energy barrier of 0.81 eV during the ORR. The WFR steps can then be finished spontaneously. OOH can also be adsorbed and reduced on the N-N bridge site of FN-CNTs, and the subsequent WFR steps can be completed spontaneously. The rate-limiting steps for the full electrocatalytic reactions on N-CNTs as cathode catalyst for HFCs are determined.